3 The Heterogeneously Catalysed Hydrogenation of Carbon Monoxide

~ ~~ ~

BY P. J. DENNY AND D. A. WHAN

1 Introduction, History, and Industrial Background An examination of the scientific literature on the subject of the hydrogenation of carbon monoxide reveals that there has been a significant growth in the number of papers published on this topic during the last six or seven years. As in the past, the amount and type of publication have been strongly influenced by industrial interest. The recent interest was stimulated first by the imminent shortage of natural gas, particularly in North America, and the desirability of producing a substitute natural gas from petroleum based naphtha. This con- version is normally carried out by a steam reforming step yielding carbon oxides and hydrogen followed by the hydrogenation of the carbon oxides to methane. Secondly, more recent escalation in the price of crude oil and the ever-present possibility of another oil embargo have resulted in considerable increases in the price of oil-based chemicals. Such price rises, coupled with the inescapable conclusion that natural petroleum resources will, sooner or later, become exhausted, have driven chemists to reconsider coal as a source of carbon for both energy needs and petrochemical requirements.

With the exception of South Africa, where the utilization of coal as a substitute for crude oil has received much attention, the subject had remained relatively dormant since Germany was confronted with similar oil shortages and the necessity to manufacture hydrocarbons from coal approximately fifty years ago. The first book on this subject was published in 1924 and research effort in this area peaked during the second world war, although some important work was carried out later by the U.S. Bureau of Mines.

Coal is a dirty, inconvenient to handle material with an ash byproduct but this, coupled with the fact that approximately 1+ tons of coal are required to supply the same energy as 1 ton of oil, is at present a consideration of little significance when it is remembered that the world reserves of coal are very large in comparison to those of oil. It has been estimated2 that coal reserves are an order of magnitude greater than those of oil (see Table 1).

It is also worthy of note at this point that the majority of hydrocarbons produced by the hydrogenation of carbon monoxide are aliphatic in character, but this is no disadvantage in the manufacture of diesel and aviation jet fuels. Furthermore, the conversion of methanol, which can readily be derived from

F. Fischer, ‘Herstellung flussiger Kraftstoffe aus Kohle’, Borntrager, Berlin, 1924. P. H. Spitz, Chemtech, 1977,295.

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Heterogeneously Catnlysed Hydrogenation of Carbon Monoxide 47 carbon monoxide and hydrogen with high selectivity, to aromatic compounds in a single step using zeolite catalysts has been shown to be po~sible.~ This opens up a fairly direct route to aromatic hydrocarbons from synthesis gas.

Table 1 World energy reserves (1973 world energy production = 1)

Prouen recoverable Estimated total Gas 8 46 Oil 17 73 Shale and tar sands 47 250 Coal 66 730

Apart from the synthesis of hydrocarbons for fuel purposes, the other major products from carbon monoxide and hydrogen have been oxygen-containing molecules, principally alcohols. The synthesis of methanol over zinc oxide- chromia catalysts, the synthesis of higher alcohols and the iso-synthesis were all discovered and developed in the early 192O’s, about the same time as the Fischer-Tropsch reaction. Industrial development of the methanol synthesis process has resulted in the current use of the more active copper-containing catalysts. The direct synthesis of higher alcohols, mainly iso-butanol, was used industrially for a time, but was rendered uneconomic by the 0 x 0 process in which alcohols are formed from olefins, carbon monoxide, and hydrogen, all derived from petroleum feedstocks. The direct synthesis of higher alcohols from carbon monoxide and hydrogen has attracted some recent attention as the products are suitable as gasoline extenders.

The values of oxygenated products, such as alcohols and acids, are generally higher than those of hydrocarbons. Moreover, since oxygen is included in the product, the fraction of synthesis gas feed converted into waste carbon dioxide or water is reduced. It is therefore likely that the future developments in the hydrogenation of carbon monoxide will be directed towards the synthesis of oxygen-containing molecules rather than hydrocarbons.

2 Thermodynamics of Possible Reactions In principle any hydrocarbon may be produced by the reaction between carbon

monoxide and hydrogen yielding either carbon dioxide or, more commonly, water as the byproduct. These processes may be represented by

Alkanes: (n + 1)H, + 2nCO e, CnH2,6+2 + K O , (1)

(2n + 1)H, + K O CnHZn+, + nH20 (2)

Alkenes: nH, + 2nCO C,H,, + nCO, (3)

2nH, + nCO C,H,, + nH,O (4)

Alkynes: (n - 1)H, + 2nCO C,H,,-, + nCO, ( 5 )

(2n - 1)H, + IZCO -- C,H,,-, + nH,O (6)

With n = 1 reactions (1) and (2) correspond to methanation.

C. D. Chang and A. J. Silvestri, J. Catalysis, 1977, 47, 249.

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48 Catalysis The formation of oxygenated products, considering only the case where water

is produced, may be represented by

Alcohols and ethers: 2nHz + K O E. C,H2,+2O + (n - 1)HZO (7)

2(n - 1)HZ + K O e. CnH2,0, + (n - 1)HzO (8) Aldehydes and ketones :

An overall picture of the thermodynamics of the hydrogenation of carbon monoxide, with the production of water, is given in the first figure. The yield per pass through a reactor will be small for compounds which have a positive standard free energy change for formation and, because of this, alkynes would not be expected among the products.

The formation of all compounds illustrated in Figure 1 is exothermic and thus all become less thermodynamically probable as the temperature is raised. The standard free energy change associated with the formation of alcohols increases rapidly with temperature predicting that, if possible kinetically, alcohol synthesis should be carried out at low temperatures. Also, solely on a thermodynamic basis, the degree of unsaturation of the products would be expected to increase with temperature.

Fortunately, the yields from chemical reactions may be controlled by kinetic considerations, within the overall scope of the thermodynamics, and thus the products obtained may be selected, to a greater or lesser extent, by the choice of suitable catalysts and the appropriate conditions of temperature and pressure.

In practice the products which have been obtained over heterogeneous catalysts cover almost all possible classes of compounds. These include hydrocarbons, both saturated and unsaturated, branched and unbranched, and oxygenated species which are principally alcohols with some aldehydes, acids, and ketones. The products from any synthesis are usually complex mixtures of widely varying chain lengths up to, in certain circumstances, 100000 carbon atoms. The only exceptions to this are the single-carbon products, methane and methanol, which can be obtained with high selectivity.

The nature of the product distribution depends on the type of catalyst used and the temperature and pressure at which the process is operated. Catalysts can be broadly divided into two categories, transition metals which, under synthesis conditions, are considered to be predominantly in the reduced state, and oxide catalysts which are essentially The first type includes metals such as iron, nickel, cobalt, ruthenium, and rhodium which are used either unsupported or supported on refractory oxides with the addition of promoters such as alkali metal oxides. Fischer-Tropsch synthesis, and variations thereon, would fall into this classification. Although the majority of

4 R. B. Anderson, J. Felchman, and H. H. Storch, Ind. Eng. Chem., 1952,44,2418. 5 H. Pichler, Adv. in Catalysis, 1952, 6, 272.

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Heterogeneously Catalysed Hydrogenation of Carbon Monoxide 49 work on metal catalysts was concerned primarily with the formation of hydro- carbons, in which the yields of alcohols were kept deliberately low, it was found that, under suitable conditions over an iron catalyst similar to that used for

I 1 I I 400 600 800 1000

T I K Figure 1 Standard free energies of formation for synthesis of hydrocarbons and alcohols

from carbon monoxide and hydrogen with water as byproduct

ammonia synthesis, alcohols could be the predominant product. This was the so-called Synol process which yielded liquid condensates containing comparable amounts of alcohols and olefins. Later research' also showed that a nitrided iron catalyst could give a product containing a substantial amount of alcohols.

H. H. Storch, H. Golumbic, and R. B. Anderson, 'Fischer-Tropsch and Related Syntheses', Wiley, Chichester, 1951.

7 R. B. Anderson, J. F. Schultz, B. Seligman, W. K. Hall, and H. H. Storch,J. Amer. Chem. SOC., 1950,72, 3502.

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50

very high m.w. p a r a f f i n s

Ru/alkaline. media

a l coho l s

Catalysis

Ru I Pd/SiO* me t hano 1

Rh/Si02 e thano l , acetaldehyde a c e t i c a c i d

Fe/K

and o l e f i n s

Ni me thrine

Rh, Pd me thane

500 600

Temperature/K \

7(

Figure 2 Products from the reaction of carbon monoxide with hydrogen over transition metals at various temperatures and pressures

In certain circumstances significant amounts of alcohols could be obtained over conventional co bal t-thoria-magnesia Fisher-Tropsch catalysts and more recent patents 9 9 lo have claimed selectivities to oxygenated materials of 65-80% over iron/copper-based catalysts.

The conditions which favour alcohol formation at the expense of hydro- carbons are low temperatures, high pressures, high space velocities, and high

D. Gall, E. J. Gibson, and C. C. Hall, J. Appl. Chem., 1952,2, 371. W. Rottig, Ger. P., 911 848 (1954).

lo W. Rottig, Ger. P., 923 127 (1955).

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Heterogeneously Catalysed Hydrogenation of Carbon Monoxide 51

I ooc

I oc

k

$ s [o : 10

1

L

'9 22'3 9 very high m.w. p a r a f f i n s

I I

ZnO/A120j/Cr20j/Cu

methanol I methanol

i sobu tano l I

I I 500 600 700

Ternperature/K

I I 500 600 700

Ternperature/K

Figure 3 Products from the reaction of carbon monoxide with hydrogen over oxide catalysts at various temperatures and pressures

CO : H2 ratios. These observations lead to suggestions that alcohols are the primary products of Fischer-Tropsch reaction * and that hydrocarbons are formed by further hydrogenation of the alcoholic species if contact times are prolonged. Appreciable formation of oxygenated products has never been observed over nickel,4 but there are recent indications of oxygenated products using rhodium catalysts at high pressures.11

l1 M. M. Bhasin, Ger. P., 2 503 233 (1975).

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52 Catalysis The products from the hydrogenation of carbon monoxide consist mostly of

molecules with straight chains and any branching which does occur is limited to single methyl groups at random intervals along the chain. Alcohols or olefins are almost exclusively 1 -hydroxy or 1 -ene species.12$ l3

A summary of the main products observed on metal catalysts, together with details of temperature and pressures are given in Figure 2.

Reactions on oxides include methanol synthesis, higher alcohol synthesis, and iso-synthesis.14 Oxides commonly used are ZnO, Thoz, and Zr02.1S Although the products over oxide catalysts are mainly alcohols, at higher temperatures over thoria the iso-synthesis gives predominantly iso-paraffins. Also it has been reported16s17 that high molecular weight paraffins (poly- ethylene) can be made over phosphomolybdate catalysts under approximately the same conditions of high pressure and low temperature required for the similar reaction on ruthenium. The distribution of alcohols from oxide-catalysed reactions is substantially different from that over transition metals and generally consists of mostly methanol and isobutanol. This provides justification for considering that the reaction mechanism is probably different for oxide- and metal-catalysed reactions and they will be treated separately in this review, A summary of the main reactions on oxide catalysts is given in Figure 3.

Recent informative reviews on the synthesis of hydrocarbons from carbon monoxide and hydrogen are by Mills and Steffgen,18 Eidus,1° Vannice,20 and Vlasenko and Yuzefovich.21 Few reviews of the synthesis of oxygen-containing products are available, one of the most recent being that of Natta l4 in 1957.

3 Reactions on Transition Metals Mechanism of Reaction Yielding Hydrocarbons.-The bulk of the work on the hydrogenation of carbon monoxide over transition metals has been concerned with the production of hydrocarbons. Theories of chain growth have thus concentrated mainly on hydrocarbon formation also but, in the majority of cases with the notable exception of the carbide theory, the theories are equally applicable to the formation of alcohols.

Postulated mechanisms by which hydrocarbons may be formed from carbon monoxide and hydrogen have been numerous, each having risen and fallen in popularity with the advent of new experimental observations. Much effort has been expended on investigating the mechanism of the formation of higher hydrocarbons despite the fact that methane formation is intrinsically simpler, as has been pointed out by Vlasenko and Yuzefovich,21 because it does not involve the formation of carbon-to-carbon bonds. l2 A. W. Weithamp, H. S. Seeling, N. 5. Bowman, and W. E. Cady, Ind. Eng. Chem., 1953, 45,

343. l 3 A. Seitz and D. K. Barnes, Ind. Eng. Chem., 1953, 45, 353. l4 G. Natta, ‘Catalysis’, ed. P. H. Emmett, Reinhold, New York, Vol. 5, Chapter 3. 15 W. Himmler, U.S. P., 2 787 628, (1957). l6 J. H. Balthes, U.S. P., 2 817 577, (1957). l7 H. R. Arnold, U.S. P., 2 900 235, (1959).

G. A. Mills and F. W. Steffgen, Catalysis Rev., 1973, 8, 159. lo Ya T. Eidus, Russ. Chem. Rev., 1967, 36, 338. 2o M. A. Vannice, Catalysis Rev., 1976, 14, 153.

V. M. Vlasenko and G. E. Yuzefovich, Rum. Chem. Rev., 1969, 38, 728.

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Heterogeneously Catalysed Hydrogenation of Carbon Monoxide 53 The first theory of the hydrogenation of carbon monoxide was that of

Boudouard22 who proposed that carbon and metal oxide were involved. This was followed by the ‘carbide’ theory of Fischer and TropschZ3 which suggested the involvement of bulk metal carbide. The carbide theory was subsequently modified by Craxford and Rideal 249 25 to include surface rather than bulk metal carbide. The various forms of the carbide theory all assumed that chemisorbed carbon monoxide was hydrogenated to chemisorbed carbon and water, the carbide in turn being further hydrogenated to methylene groups. Chain growth, as postulated by this mechanism, is accomplished by the linking of adjacent methylene groups with the final step being the hydrogenolysis of the remaining metal to carbon bonds. The carbide theory would account for the formation of branched chains, including quaternary carbon atoms, via direct incorporation of surface carbide, or of partially hydrogenated carbide, into the growing chains. Experimental observations show that most chain branching is through tertiary rather than quaternary carbon atoms. For this reason, and because it makes no provision for the synthesis of oxygen-containing products, the carbide theory has been much ~riticized.~

The involvement of oxygenated intermediates was proposed by Storch, Golumbic, and Anderson6 and for many years has been widely accepted. They assumed that adsorbed carbon monoxide is initially hydrogenated to hydroxylated species [reaction (lo)], and that such species link by condensation [reaction (1 l)],

OH H&, ,OH C It

M

H\ / H2 .+ c-c II It

M M

and are hydrogenated away from the surface at one of the carbon atoms thus giving chain growth [reaction (12)]. Further hydrogenolysis of the remaining metal to carbon bond yields alcohols and hydrogenolysis of both the metal-to- carbon amd metal-to-oxygen bonds gives hydrocarbons, possibly via olefinic intermediates.26 Condensations involving a partially hydrogenated intermediate 22 0. Boudouard, J. Chem. SOC., 1901, 383. 23 F. Fischer and H. Tropsch, Brennstof-Chem., 1926, 7, 97. 2 4 S. R. Craxford and E. K. Rideal, J. Chem. Soc., 1939, 1604. 26 S. R. Craxford and E. K. Rideal, Trans. Faraday SOC., 1946,42, 576. 26 H. Koch and H. Hilberath, Brennstof-Chem., 1941, 22, 145.

3

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54 Cataiysis can explain the experimental observation that side-groups longer than methyl are not commonly found [reactions (13) and (14)].

Elegant experiments by Emmett and his c o - w o r k e r ~ , ~ ~ ~ 28 using primary alcohols labelled with carbon-14 in the presence of hydrogen and carbon monoxide on an iron Fischer-Tropsch catalyst, showed that alcohols can act as starting nuclei for chain growth but not as building units in the hydrocarbon chains. Labelled methanol was found to appear mostly as methane, carbon dioxide, or carbon monoxide although some was incorporated in higher hydro- carbons. Labelled isopropanol was converted to isobutane or isobutene, and t-butanol was not incorporated at all. Similar conclusions have been reported by Hall et aZ.29

I M

I I M

I M

CH,

C I II M M

R\ /CH3 /OH + C H-C-C (14)

H\ /OH R,l ,OH

I I M

Infrared studies of the interaction of carbon monoxide and hydrogen on silica-supported iron catalysts by Blyholder and Neff 30 gave evidence for adsorbed alcoholic species. Blyholder and Emmett 319 32 conducted experiments in which radioactive ketene, labelled selectively in the methylene or carbonyl group, was added to synthesis gas on an iron catalyst. When the methylene group was labelled the hydrocarbon products had a constant radioactivity per mole equal to that of the ketene; when the carbonyl group was labelled the products had only a small degree of radioactivity which was proportional to the number of carbon atoms in the molecule. These results have been interpreted on the assumption that methylene may function as a chain initiator or, alternatively, that the reaction intermediates may be held by metal-oxygen as well as metal-carbon bonds. The reaction of adsorbed ketene with carbon monoxide may be represented by Scheme 1. The formation of the carbon- carbon bond displaces the original carbon-oxygen bond of the complex. If, instead of the carbon-oxygen bond, either the carbon-hydrogen or carbon- metal bond is displaced and the carbon-oxygen bond of the added carbon monoxide is broken then branched-chain hydrocarbons can be produced.

More recently there have been suggestions by Le Roux 33 and Pichler 34 that Fischer-Tropsch synthesis may take place by a mechanism involving chain 27 J. T. Kummer and P. H. Emmett, J . Amer. Chem. Soc., 1953,75, 5177. ** J. T. Kummer, W. B. Spencer, H. H. Podgurski, and P. H. Emmett, J. Amer. Chem. SOC.,

29 W. K. Hall, R. J. Kokes, and P. H. Emmett, J. Amer. Chem. Suc., 1960, 82, 1027. 3 0 G. Blyholder and L. D. Neff, J. Phys. Chem., 1962, 66, 1664. 31 G. Blyholder and P. H. Emmett, J . Phys. Chem., 1959, 63, 962. 32 G. Blyholder and P. H. Emmett, J. Phys. Chem., 1960, 64,470. 33 J. H. Le ROUX, J. Appl. Chem. Biotechnol., 1972, 22, 719. 34 H. Pichler and G. Kruger, ‘Herstellung flussiger Kraftstoffe aus Kohle’, Gersbach und Sohn,

Munchen, 1973, and references therein.

1951,73, 564.

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Heterogeneously Catalysed Hydrogenation of Carbon Monoxide 55

R\ /H H

I I I

M M

--+ 0-c---c-0 R--C-.O 3 C - 0 - I I I I h1 M M M

I I M M

M M Scheme 1

growth out from a single surface site rather than by a process requiring several adjacent sites on the surface of a catalyst. Le R O L I X ~ ~ notes that the products of Fischer-Tropsch synthesis are usually characterized by ;

(i) branches situated randomly along chains ; (ii) branching does not increase with increase in molecular weight; (iii) single methyl groups are by far the most predominant form of branching; (iv) branching is increased by raising the temperature or lowering the space

To accommodate these facts Le Roux has postulated a mechanism based on a single surface site and involving hydrogen transfer by which chain branching could arise [reaction (15)].

velocity.

Henrici-OlivC and OlivC 35 have also advocated a mechanism for Fischer- Tropsch-type synthesis which is based on ‘active centre’ rather than ‘active surface’ arguments. They have developed a reaction scheme based on individual steps which are well established in the chemistry of homogeneous catalysis by soluble transition metal complexes. Their suggested mechanism is shown in Scheme 2, the final products being identified by enclosure in double-ruled boxes. It is assumed that the metal hydride is formed during activation of the catalyst and that carbon monoxide, co-ordinated with the metal, is then inserted into the metal hydrogen bond. Although the insertion of carbon monoxide into a metal to hydrogen bond has never been observed, the reverse process has been reported36 and it is not unreasonable to expect significant amounts of the carbonylated metal hydride on the surface. Since aldehydes are not primary products of Fischer-Tropsch synthesis it is necessary to assume that the formaldehyde postulated does not leave the metal centre, but remains co- ordinatively bonded via the carbonyl group. This can lead to either an adsorbed alcohol species or, by elimination of water, an adsorbed carbenoid ligand which

31 G. Henrici-OlivC and S. OlivC, Angew. Chem. Infernaf. Ed., 1976, 15, 136. 36 J. P. Collman and S. R. Winter, J. Amer. Chern. SOC., 1973, 95, 4098.

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56 Catalysis

H H I H co I H, > co H M i H,l,

I - co M M

H + I

M H\ /H

T/H M

OH y 3 3 I H co CH3 I I M

H-C-H 4 C=O f-- I

M M

OH CH3 I - H O I co I

H,C-C-H ---& YHz 4 propagation

H-M-H M

Scheme 2

rearranges to a a-bonded methyl group. Carbon monoxide insertion can then take place into the carbon to metal bond in a fashion similar to that into the original hydrogen to metal bond with subsequent chain growth to alcohols or, by 8-hydrogen abstraction, to olefins.

It is possible that the metal hydride can participate in secondary reactions such as hydrogenation or incorporation of the initially formed a-olefins. Two possibilities exist (Scheme 3), leading to either linear or methyl-branched

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Heterogeneously Catalysed Hydrogenation of Carbon Monoxide 57

R

H R--CH=CH, -I- I

M

I H-C-H

I M

Scheme 3

hydrocarbons. This suggestion is in good agreement with the experimental observation that branches other than methyl are relatively uncommon among the products of Fischer-Tropsch synthesis.

Henrici-Olive and OlivC 35 themselves ask why, if the proposed mechanism is acceptable, there are no reports of Fischer-Tropsch-type reactions in solutions of iron or cobalt organometallic compounds. They suggest that the answer may be related to the fact that heterogeneous catalysts have on their surfaces low- valent co-ordinatively unsaturated metal species whereas in solution solvent molecules tend to block such sites. This explanation would appear to be less than convincing, bearing in mind that the proposed mechanism is based entirely on reactions which are well known in homogeneous chemistry where the metal species are indeed surrounded by solvent molecules.

Ponec3' has carried out elegant experiments on nickel which suggest that carbon deposits on the surface of the catalyst, or carbide, may be important in the synthesis of hydrocarbons from carbon monoxide and hydrogen, and Joyner 38 has presented evidence that dissociatively adsorbed carbon monoxide is involved. The majority of authors have18 previously rejected the direct hydrogenation of surface carbide to methane because carbides have been out of favour as intermediates in the Fischer-Tropsch synthesis, despite the observation 39

as long ago as 1948 that synthesis on surfaces carbided with radioactive carbon yielded predominantly radioactive methane. The principal experiment reported by Ponec3' is presented in Figure 4. It is seen that, for the hydrogenation of carbon monoxide on a fresh nickel film, the rate of formation of methane was initially low, but increased rapidly with reaction time: carbon dioxide was the main reaction product at the beginning of the experiment. In subsequent runs on the used film the rate of methane formation was greater, and the rate of production of carbon dioxide was less, than on a fresh film. This indicates that the presence of a reaction mixture causes modification of the surface. Experiments conducted in the absence of hydrogen showed that the carbon dioxide was formed by the disproportionation of carbon monoxide with accumulation of carbon at the 37 M. Araki and V. Ponec, J. Catalysis, 1976, 44, 439. 38 R. W. Joyner, J. Catalysis, 1977, 50, 176. as J. T. Kummer, T. W. Dewitt, and P. H. Emmett, J. Amer. Chem. SOC., 1948, 70, 3632.

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58 Catalysis

surface. It was confirmed that the amount of deposited carbon was equal to the quantity of carbon dioxide produced and that the deposited carbon could be hydrogenated off the surface as methane. The rate of removal by hydrogenation of previously deposited carbon was suppressed by carbon monoxide, suggesting

I I I I I

1 3 5 7 9 Run number

Figure 4 Initial rates of formation of carbon dioxide and methane from carbon monoxide and hydrogen for repeated reactions on a nickel film at 523 K

that carbon monoxide and hydrogen compete for the same surface sites. This observation correlates well with reports 21y 40 that the methanation reaction is usually of negative order with respect to carbon monoxide.

Confirmation that methane was produced from surface carbon was obtained by laying down radioactively labelled carbon and then treating the surface with a mixture of hydrogen and unlabelled carbon m~noxide.~' The results of this experiment are illustrated in Figure 5 . It is clear that the first product to appear was I3CH4, substantial production of 12CH4 and 12C02 being accompanied by an induction period of over 20 min. Q0 M. A. Vannice, J. Catalysis, 1975, 37, 462.

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Heterogeneously Catalysed Hydrogenation of Carbon Monoxide 59 Ponec 37 summarizes the results obtained from his kinetic studies by concluding

that the primary process in methane formation is dissociative and that adsorbed hydrogen reacts with adsorbed carbon. His results would appear to contradict

OO Time I min

Figure 5 Number of molecules of methane and carbon dioxide formed at 523 K by the reaction of l2C0 with hydrogen on a nickel film previously saturated with 13C formed by the disproportionation of 13C0

the importance in methanation of oxygen-containing species, such as are still accepted in the Fischer-Tropsch type synthesis of higher hydrocarbons.

The kinetics of the methanation reaction, according to the surface carbon theory, can be interpreted if it is assumed that the hydrogenation of carbon is the rate-determining step. Tho rate of methanation, Y, may be given by equation (16), where 8c is the surface coverage in carbon, which is independent of the carbon monoxide pressure, and 8B; is the surface coverage of adsorbed hydrogen

r = k.&.8E

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60 Catalysis

atoms. As carbon monoxide and hydrogen compete for the same surface sites, with carbon monoxide being more strongly held than hydrogen, equation (17)

r = k.p%.&

applies, where /I < 0 < a. The kinetic equation is equal to equation (18), where m is a constant and the kinetics are in agreement with those reported at higher pressures.23, 41* 4a

Recent indications, based on the absence of a kinetic hydrogen/deuterium isotope effect, that hydrogen is not involved in the rate-determining step43 on nickel, ruthenium, or platinum would appear to be at variance with the mechanism proposed.

The role of surface carbon in the production of methane from carbon monoxide has also been investigated by Wentrcek, Wood, and Wise44 using a pulse microreactor and a catalyst consisting of 25% nickel on alumina. At temperatures in excess of 553 K the formation of carbon dioxide was confirmed by its presence in the effluent from the reactor when pulses of carbon monoxide were passed over the catalyst. The carbon dioxide formation was accompanied by the deposition of carbon on the surface of the catalyst and the reactivity of the carbon was measured by the passage of hydrogen over the used catalyst to give methane. In agreement with the experiments of Ponec 3B the surface carbon was found to be important in methanation, although at particularly high surface carbon densities there was evidence of some carbon present in a non-reactive form which could block sites to carbon monoxide hydrogenation.

Analysis by Auger spectroscopy of the surface of the nickel catalyst 44 after it had been exposed to carbon monoxide at 553 K revealed the presence of carbidic type species. Additional support for the existence of carbidic carbon is provided by the loss in reactivity of the carbon on heating to 723 K, a process which is known to convert carbide to graphite.46 The graphite does not react with hydrogen.46 If it is assumed that the formation of carbidic carbon is accompanied by the formation of nickel to oxygen bonds [reaction (19)] then,

CO + 2Ni - NiC,,, + NiO(,, (1 9)

using known values for the nickel-oxygen bond strength, the nickel to carbon binding energy must be greater than 167 kJ rno1-l to make possible the dissociative chemisorption of carbon monoxide. A strength for the nickel- carbon bond which is in accord with this value has been rep~rted.~' It is perhaps worth emphasizing that the active carbidic species proposed by Ponec39 and Wise 44 can not be bulk Ni,C which is known to be relatively inert in the presence of hydrogen.48 41 D. F. Ollis and M. A. Vannice, J. Catalysis, 1975, 38, 514. 42 R. A. Dalla Betta, A. G. Piken, and M. Shelef, J. Catalysis, 1974, 35, 54. 43 R. A. Dalla Betta and M. Shelef, J. Catalysis, 1977, 49, 383. 44 P. R. Wentrcek, B. J. Wood, and H. Wise, J . Catalysis, 1976, 43, 363. O5 S. Oketani, S. Nagakura, and K. Tsuchiya, Nippon Kinzoku Gakkaishi, 1954, 18, 325. 46 P. Breisacher and P. C. Marx, J. Amer. Chem. Soc., 1963, 85, 3518. 47 L. C. Issett and J. M. Blakely, Surface Sci., 1975, 47, 645.

A. K. Galwey, J. Catalysis, 1962, 1, 227.

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Heterogeneously Catalysed Hydrogenation of Carbon Monoxide 61 Further support for the dissociation of carbon monoxide as a prerequisite

for the Fischer-Tropsch reaction comes from a comparison of the Fischer- Tropsch synthesis and the Haber synthesis of ammonia. Jones and McNicol 49

have listed the similarities between the Fischer-Tropsch hydrogenation of carbon monoxide (in its simplest form to yield methane) and the hydrogenation of the isoelectronic molecule nitrogen to give ammonia. The most efficient catalysts for both processes are the group VIII metals combined with a promoter, such as potassium oxide and often supported on an inert oxide. Both reactions are exothermic and accompanied by a decrease in the number of molecules; they are commonly operated under conditions of high temperature and high pressure.

Evidence that at low temperatures both carbon monoxide and nitrogen are associatively adsorbed, while at high temperatures both can be dissociatively adsorbed, is summarized by Jones and McNico149 for metals active in hydro- carbon and ammonia synthesis. It is normally accepted that the dissociative adsorption of nitrogen is an essential step in the synthesis of ammonia.60s61 In the Haber process the formation of ammonia is then considered to proceed by stepwise hydrogenation of nitrogen atoms into imino and amino radicals. This can be compared with the synthesis of hydrocarbons, given the existence of the species C, CH, CH2, CH3, H, and OH arising from the hydrogenation of carbon and oxygen on the surface. Simple addition reactions can account for products containing more than one carbon atom. Also the proposals that higher molecular weight species are obtained by an insertion mechanism34 are in agreement with the presence on the surface of CH and CH, groups. The para- magnetic or ferromagnetic nature of the catalyst could well align unpaired spins to yield triplet species of essentially free radical character.

One significant difference between ammonia synthesis and the formation of hydrocarbons from carbon monoxide and hydrogen is that no higher molecular weight products are produced in the case of nitrogen, This disparity is readily explicable on simple thermodynamic grounds where the formation of hydrazine from a molecule of nitrogen and two molecules of hydrogen is endothermic to the extent of 95 kJ mol-l. It is perhaps worthy of note that both ammonia and hydrocarbon syntheses can proceed simultaneously on a doubly promoted iron cataly~t.~,

Joyner 53 has reviewed the electron spectroscopic information relating to the adsorption of carbon monoxide on metals. Kishi and Roberts54 have un- ambiguously shown, by means of He1* ultraviolet photoelectron spectroyopy, that on iron at low temperatures carbon monoxide is associatively adsorbed. On raising the temperature to 295 K dissociative adsorption becomes apparent and above 350 K adsorption is totally dissociative.

In the case of associative adsorption at low temperatures the adsorbate resonances are closely similar to the gas phase spectrum of carbon monoxide. 4B A. Jones and B. D. McNicol, J. Catalysis, 1977, 47, 388. 5 0 M. Tempkin and V. Pyzhev, Acta Physicochim., U.S.S.R., 1940, 12, 327. 51 K. Aika and A. Ozaki, J. Catalysis, 1960, 16, 97. 5 2 C. Bokhoven, Proc. 2nd Radioisotope Conference, 1954, 53. 53 R. W. Joyner, Surface Sci., 1977, 63, 291. 54 K. Kishi and M. W. Roberts, Surface Sci., 1977, 62, 252.

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62 Catalysis For dissociative adsorption at higher temperatures the peak characteristics of molecular carbon monoxide are replaced by a single broad peak about 7eV below the Fermi level. It is also possible to differentiate between associatively and dissociatively held carbon monoxide by an X-ray photoelectron spectro- scopic determination of the oxygen and carbon 1s binding energies. In the case of high temperature adsorption the XPS peak positions, with the oxygen 1s and carbon 1s values being similar to those found in chemisorbed oxygen and metal carbides, show clearly that the molecular integrity of the carbon monoxide molecule has been lost.

The factors influencing the dissociation of carbon monoxide on metal surfaces have also been examined by J ~ y n e r . ~ ~ He has concluded that, for dissociative adsorption to take place, the metal must be capable of forming a stable bulk carbide. If bulk carbide formation is not possible then desorption rather than dissociation takes place as the temperature is raised ; for example, dissociation does not take place on platinum. The threshold temperature, above which dissociation proceeds at measurable rates on metals where it is thermo- dynamically possible, gives an insight into the kinetic parameters of dissociative adsorption. For metals such as tungsten and molybdenum the threshold is below room temperature while for nickel and iron it is of the order of 300-400 K. The threshold temperature is inversely related lo the heat of adsorption, and the activation energy for dissociation may also be estimated from a knowledge of this temperature. In the cases of tungsten and molybdenum the activation energy is extremely small and this, together with a correspondingly large heat of adsorption, may imply that these metals are not effective Fischer-Tropsch catalysts because the carbon species on the surface are too strongly held. Kishi and Roberts54 have found that the activation energy for the dissociative adsorption of carbon monoxide on iron may be modified by the presence of other adsorbed species, in particular sulphur which was found to inhibit dissociation of molecularly adsorbed carbon monoxide at 300 K. Carbon Number Distribution in Products.-The most recent analysis of the products of Fischer-Tropsch synthesis by Henrici-OlivC and OlivC 35 is based on molecular weight and hydrocarbon type distributions as well as on the extent of chain branching. Their ideas are based on the conclusions of Pichler 6 5 9 s6 and Anderson5' that alcohols are the primary products of the synthesis. The evidence for the primary nature of these species is twofold. Firstly, the con- centration of a-olefins is much higher than that predicted from thermodynamic arguments and, secondly, neither the a-olefin nor the primary alcohol contents of the products falls with increasing space velocity. There may, of course, be a common precursor to linear a-olefins and alcohols.

Schulz et aZ.58 have added carbon-14 labelled terminal olefins in an attempt to examine the fate of the primary products. They found that over 90% of the added olefins underwent reaction on cobalt catalysts although the amount on iron was somewhat less. This incorporation of olefins is in marked contrast to 55 H. Pichler, H. Schulz, and F. Hojabri, Brennstofl-Chem., 1964, 45, 215. 58 H. Pichler, H. Schulz, and M. Elstner, Brennstof-Chem., 1967, 48, 78. 57 R. A. Friedel and R. B. Anderson, J. Amer. Chem. Soc., 1950, 72, 1212. 58 H. Schulz, B. R. Rao, and M. Elstner, Erdoel Kohle, 1970, 23, 651 and references therein.

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Heterogeneously Catalysed Hydrogenation of Carbon Monoxide 63 paraffins. Isotopically labelled butane was recovered unchanged to an extent of almost 100%. Most of the added a-olefins underwent hydrogenation to the corresponding alkane but a significant proportion was also incorporated into the growing chain. With ethylene and propylene the extent of incorporation decreased with molecular weight, in accordance with expected polymerization chemistry, and the incorporation of propylene produced methyl branching. It was also observed that the double bond in an a-olefin could be split by hydrogenolysis to give methane. Labelled methane was found from experiments in which the central carbon atom in propylene was labelled. As well as chain growth, the following reactions (20)-(22) are thus relevant to the fate of a-olefins :

CH,=CH, - 2CH4 (20)

R-CH=CH2 - R-CH, + CH4 (21)

CH,-CH=CH, - CH,-CH, + CH, or 3CH4 (22)

A typical molecular weight distribution of hydrocarbons formed from a Fischer-Tropsch reaction on a cobalt catalyst 56 is shown in Figure 6 . Henrici- OlivC and OlivC point out that, with the exception of excess methane, the curve is similar to a Schulz-Flory59 or ‘most probable’ distribution of molecular weights commonly found in oligomerization or polymerization process. They suggest that Fischer-Tropsch synthesis may be treated as a polymerization in which the monomer is (CO + H,) and that the kinetics are similar to those which have been published 6o for ethylene polymerization. Under Fischer-Tropsch synthesis conditions it may be assumed that the rate of chain propagation remains constant and may be defined as in equation (23). The constancy of the

rate of propagation implies that the reaction interrupting chain growth must be a chain transfer rather than a chain termination and, since this theory assumes the primary products to be a-olefins, the carrier must be a metal hydride [reaction (24)]. The rate of chain transfer is given by equation (25). The Schulz-Flory

R-CH,-CH, R-CH=CH2tg, + H (24) I I M M

d [alkene] rt = - at

m, = (In2 a) cac (26)

molecular weight distribution may be represented by equation (26), where m, is the weight fraction of the oligomers of degree of polymerization c and c is

69 G. Henrici-Olive and S. Olive, ‘Polymerization’, Verlag Chemie, Weinheim, 1969. 6 o G. Henrici-Olive and S. Olive, Polymer Sci., 1974, 1, 15.

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64 Catalysis

0.1 5

0.10

C 0

c)

.- -c,

2 rc

0,OE cn .- :

Figure 6 Chain length distribution of hydrocarbons formed by the hydrogenation of carbon monoxide on a cobalt on thoria catalyst

equal to the number of carbon atoms in the chain; 01 is a measure of the probability of chain growth defined as in equation (27). The Schulz-Flory

ra + rt

equation may be expressed in the more convenient form of equation (28). For any experimentally obtained product distribution linearity of a plot of log(m,/c) vs c would imply obedience to the Schulz-Flory type of kinetics.

Henrici-OlivC and OlivC 35 have analysed the distribution of hydrocarbons from a Fischer-Tropsch synthesis according to the Schulz-Flory equation and their

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Heterogeneously Catalysed Hydrogenation of Carbon Monoxide 65 results, for the experiment illustrated in Figure 6, are presented in Figure 7. The weight fraction in Figure 7 is the sum of the alkanes and alkenes at each carbon number, since a-olefins can be isomerized into internal olefins under reaction conditions. Agreement between the line in Figure 8, calculated using 01 = 0.81, and the experimental points is good with the exceptions of the point

0

2 4 6 8 10 12 14 1 o j

0 Carbon number

Figure 7 Plot of the carbon number distribution in Figure 7 according to the Schulz- Ffory equation (a = 0.81)

for C, being too high and the C2--3 and C13-14 values being slightly low. The deviations in the Cl-3 range have been attributed to the conversion of ethylene and propylene into methane, simultaneously with Fischer-Tropsch synthesis. Discrepancies at high carbon numbers may be due to under- estimation by the gas chromatographic technique used in the analysis.

A similar examination of the alcohols produced on an Fe304 catalyst containing alumina and potassium oxide35 is also shown to obey accurately the Schulz- Flory distribution.

It is worthy of emphasis that, if the synthesis of hydrocarbons or alcohols from carbon monoxide and hydrogen does obey the Schulz-Flory law, the only products which can be obtained with 100% selectivity are the C, species methane and methanol. Formation of Oxygen-containing Products.-As has already been mentioned, the products from the reaction of carbon monoxide with hydrogen on metals are

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66 Catalysis

predominantly hydrocarbons. However, alcohols can be formed in substantial yield on iron catalysts. The differences in catalytic behaviour between iron, cobalt, and nickel have been rationalized by Kobe1 and Tillmetz61 who have calculated the binding energies and electronic structures of the possible chemi- sorption complexes by means of the extended Huckel theory followed by a Mulliken 62 population analysis. They have confirmed that oxygenated com- plexes are possible on iron and cobalt and suggest that the complex on iron has a higher stability and more marked enolic behaviour than that on cobalt. This is

Figure 8 The chemical composition of an ‘iron’ catalyst used for the synthesis of higher alcohols from carbon monoxide and hydrogen as a function of time. Pressure, 200 atm; temperature, 417 K; CO : €I2 ratio 1 : 6

consistent with the production of longer hydrocarbon chains with a higher content of oxygenated species in the product spectrum on iron. Since iron is known to adsorb molecular hydrogen, as well as atomic hydrogen,63 the hydro- genation rate of the complex should be lower than that on cobalt. This, together with the higher stability of the complex on iron, explains the considerable amount of olefins in the products on this metal. On nickel it is concluded that there is only a very low probability of an oxygenated complex and a methylene type of intermediate is favoured. Thus, taking into account the excellent hydrogenating properties of nickel, only paraffinic products of short chain length would be expected.

H. Kolbel and K. D. Tillmetz, J. Catalysis, 1974, 34, 307. 62 R. S. Mulliken, J . Chem. Phys., 1965, 23, 1833. 63 R. Brill, D. Kleiner, and H. Schafer, Ber. Bunsengesellschaft phys. Chem., 1969,73, 267.

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Heterogeneously Catatysed Hydrogenation of Carbon Monoxide 67 It has been shown6* that, although nominally iron, catalysts active for the

synthesis of alcohols also contain substantial amounts of carbide and oxide under reaction conditions. The relative amounts of metallic iron, iron carbide, and iron oxides in a catalyst used for the hydrogenation of carbon monoxide is shown in Figure 8. After an initial slight fall in the activity the catalyst yielded products at a constant rate throughout the experiment indicated. It has also been confirmed by thermogravimetric analysis 65 that there is no correlation between the bulk phase composition of an iron-based alcohol synthesis catalyst and its activity or selectivity.

Blyholder and Goodsel 66 have investigated the insertion reaction of carbon monoxide with a surface alkyl group on evaporated iron by infrared spectroscopy. This is a direct analogy with insertion reactions in co-ordination complexes. They postulate chain growth to proceed on iron by the following mechanism (Scheme 4) to produce n-propoxide species or by a somewhat more complicated

CH, 0 CH2CM3 CH,CH,CH, c\H3 P I II I I CH C + CH2---C + C=O + 0

\ / I I Fe Fe Fe

, =// Fe

Scheme 4

H,C CH3 \ / CH

I 0 / \

Fe Fe Scheme 5

process (Scheme 5 ) to yield iso-propoxide which was their principal product. It is likely that the presence of alkyl groups bonded through oxygen to the metal is responsible for the production of alcohols rather than hydrocarbons on iron.

A recent patent 67 claims that palladium supported on silica yields alcohols, mostly methanol, at high pressure. Although ruthenium is noted for the

S. M. Loktev, A. N. Bashkirov, E. V. Slivinskii, L. I. Zvezdkina, and Yu. B. Kagan, Kinetica i Kataliz, 1973, 14, 214.

65 S. M. Loktev, L. I. Makarenkova, E. V. Slivinskii, and S. D. Eritin, Kinetica i Kutaliz, 1972, 13, 1042.

68 G. Blyholder and A. J. Goodsel, J . Catalysis, 1971, 23, 374. 67 M. L. Poutsma, J. A. Rabo, and A. P. Risch, Belg. P., 849 121 (1977).

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68 Catalysis

production of very long-chain hydrocarbons, ruthenium catalysts suspended in an alkaline aqueous or alcoholic medium have been found 68 to give alcohols in the range C-2 to C-30 with high selectivity. Substantial yields of oxygenated products (ethanol up to 30%) can also be attained over rhodium supported on ~ i l ica .*~~ 70 It would appear that nickel is the only metal where significant yields of oxygenated products have not been observed.

Bimetallic Catalysts.-Experiments conducted by Ponec 37 on a series of nickel- copper alloys suggests that the rate constant for the hydrogenation of surface carbon, predominantly to methane, is not much affected by the presence of copper. Alloying did, however, produce a significant effect on the number of carbon atoms which the surface could accommodate. This indicated that the deposition of surface carbon was possible only on sites containing ensembles of contiguous nickel atoms, the results being explicable on purely geometric grounds.

An investigation of the nickel-copper system by infrared spectroscopy 71 has yielded important information about the surface sites on which carbon monoxide may be dissociatively adsorbed. The catalysts used in this study were prepared by impregnation of aerosil by a solution of the metal nitrates 7 2 followed by drying and reduction in flowing hydrogen for at least 16 h. These conditions would be expected to give single-phase alloys with a surface composition similar to that of the copper-rich phase stable under conditions where two phases are present at e q ~ i l i b r i u m . ~ ~ ~ 74 With repeated adsorption and desorption of carbon monoxide on the same catalyst sample the amount of surface carbon builds up and the infrared spectra revealed changes in the adsorption of carbon monoxide. The normal infrared spectrum of carbon monoxide on nickel shows two bands, the first ‘My band corresponding to adsorption with strong back donation (bridged or multiple adsorption) appearing at 1925 cm-l and the second ‘L’ band (low co-ordinated) at 2055 cm-l. Repeated treatment of the surface with carbon monoxide caused the ‘My band to decrease and the ‘L’ band to at first increase and then decrease in intensity. A slight shift to lower frequency was also observed with the ‘L’ band.

From these results Ponec 71 concluded that carbon is deposited at sites where carbon monoxide is adsorbed in its bridged form and subsequently dissociates. Using information reported by Gwathmey and C ~ n n i n g h a m , ~ ~ he further deduced that the place where carbon monoxide is adsorbed in its multiply bound form with strong back donation can be visualized as the surface hole on the (111) plane among three nickel atoms. The dramatic decrease in the rate of methanation when copper is added to nickel37 may thus be rationalized by the observation that the addition of copper eliminates carbon monoxide adsorption of the multiply co-ordinated type.72

B. W. Howk and G. F. Hager, U.S. P., 2 549 470 (1951). 89 M. M. Bashin, Ger. Offen., 2 503 233; 2 268 463; 2 628 576 (all 1975). ‘O M. M. Bhasin, Belg. P., 824 822 (1975).

W. L. van Dijk, J. A. Groenewegan, and V. Ponec, J. Catalysis, 1976,45, 277. 72 Y. Soma-Noto and W. M. H. Sachtler, J . Catalysis, 1974, 34, 162. 73 J. H. Sinfelt, J. L. Carter, and D. J. C. Yates, J. Catalysis, 1972, 24, 283. 7 4 P. E. C. Franken and V. Ponec, J. Catalysis, 1976, 42. 398. 75 A. T. Gwathmey and R. E. Cunningham,-‘Advances incatalysis’, Academic Press, New York,

1958, Vol. 10.

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Heterogeneously Catalysed Hydrogenation of Carbon Monoxide 69 The thermal desorption of carbon monoxide and hydrogen from the (110)

face of a single crystal of a copper-nickel alloy has been studied by Yu, Ling, and Spi~er .?~ A novel method of preparation of alloy surfaces of different compositions from the same bulk sample, referred to by the authors as interrupted annealing, is reported. The technique involves the sputtering of the surface by argon ion bombardment, thus causing depletion of the surface copper

-...I

b C m v)

0 0

.-

'10 Ni

100

82

75

65

60

52

35

0 200 400

Temp. / K Figure 9 Thermal desorption spectra of carbon monoxide on a series of copper-nickel

alloys. Rate of heating, 8 K s-l

content, followed by annealing in the temperature range 710-920 K when the surface composition is controlled by the rate of diffusion of copper atoms from the bulk of the sample to the surface. At these temperatures confirmation by LEED shows that, because of rapid surface diffusion, surface defects are healed and the expected diffraction pattern is seen. The elemental composition of the surface was determined by Auger spectroscopy. Thermal desorption results obtained from a series of (110) surfaces of different copper to nickel ratios are presented 78 in Figure 9.

Yu et al.76 interpret these results, in which four desorption peaks are detected for the alloy surfaces, on the basis of non-dissociative adsorption of carbon monoxide and the ensemble theory of surface composition. The highest tempera- ture peak is correlated with desorption from a pure nickel ensemble, since, as the 76 K. Y. Yu, D. T. Ling, and W. E. Spicer, J. Catalysis, 1976, 44, 373.

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70 Catalysis nickel content increases, its desorption temperature approaches that of pure nickel. The lowest temperature peak is similarly associated with desorption from a pure copper ensemble leaving the peaks labelled A and B in the figure as being due to adsorption on sites involving both copper and nickel. Evidence for peaks A and B being due to mixed sites comes from the fact that such sites would be expected to have binding energies intermediate between the values for the pure metals. The magnitudes of peaks A and B are similar to those of the pure metals, indicating that mixed sites are as numerous as those on the pure metals. This observation suggests that the surface compositions are uniform and that significant clustering or islanding does not take place. Due to geometric constraints, a site on which carbon monoxide can adsorb cannot comprise more than four surface atoms and the three most obvious binding sites are head-on, bridged, and fourfold with one, two, or four atoms respectively in contact with the carbon monoxide molecule. On the pure metals any of the three types of adsorption is possible but adsorption on a mixed site can only be bridged or fourfold configurations. Sites giving rise to the A and B desorption peaks must thus be due to such bridged and fourfold adsorptions. The observation that the activation energy of the pure nickel binding state decreases linearly with increasing surface copper concentration is taken as evidence that, to some extent, long- range effects are present in the chemisorptive bonding.

The desorption kinetics for hydrogen on the same series of alloy surfaces76 were second order and were thus more difficult to interpret than the first-order spectra obtained from carbon monoxide. The binding states were, however, resolved for hydrogen and both were associated with pure nickel. The activation energy of the higher temperature state was constant even though the surface composition was changed.

Other studies of the adsorption and desorption of carbon monoxide on nickel- copper alloys 77 confirm significant differences from hydrogen adsorption. This has been interpreted as being caused by gas-induced surface enrichment of the alloys and such a possibility must always be borne in mind when comparing seemingly similar catalysts in different environments.

It would appear that further studies on bimetallic systems, particularly those systems in which a metal active in the formation of hydrocarbons from mixtures of carbon monoxide and hydrogen is diluted in a matrix of an inactive metal, will be helpful in the elucidation of the mechanism of the Fischer-Tropsch reaction. In particular, such experiments should be able to distinguish between mechanisms where several contiguous sites are required (hydrogenation of surface carbide or condensation of adjacent oxygenated species) and those where chain growth takes place outwards from a metal site (carbon monoxide or carbene insertion). In the former case the addition of copper, for example, might be expected to reduce the quantity of higher hydrocarbons produced while it should not, on simple geometric arguments, necessarily do so if growth is from an isolated surface site. Novel Catalysts.-A recent paper by Luengo, Cabrera, McKay, and Maple reports on the methanation of carbon monoxide catalysed by the three 77 J. C. M. Harberts, A. F. Bourgonje, J. J. Stephan, and V. Ponec, J . Catalysis, 1977, 47, 92. 78 C. A. Luengo, A. L. Cabrera, H. B. MacKay, and M. B. Maple, J. Cutulysis, 1977,47, 1.

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Heterogeneously Catalysed Hydrogenation of Carbon Monoxide 71 iso-structural cubic phase compounds CeAI,, CeCo,, and CeNi, in a micro-flow reactor at temperatures between 373 and 1073 K. On exposure to reaction conditions it was found that CeCo, and CeNi, yielded CeO, and free cobalt or nickel respectively. Their catalytic activities for methanation were found to be comparable to those of pure cobalt and pure nickel, except that they exhibited an increased selectivity towards the production of higher hydrocarbons. The CeAl, compound, which was stable under reaction conditions, exhibited only moderate activity for methanation, in comparison with pure nickel or cobalt, but it was particularly resistant to loss of activity due to sintering at temperatures up to 970 K.

Rare earth transition metal intermetallic compounds, in particular mischmetal- nickel alloys, have been reported'O as being more active for the production of methane from carbon monoxide and hydrogen than a commercially available supported nickel methanation catalyst. The catalysts were prepared by arc- melting the metal constituents, the mischmetal having a composition of 67% Ce, 13% La, 9% Pr, and 11% Nd in terms of weight. As e~pected,'~ the intermetallic compounds were not stable under methane synthesis conditions but, in this case, an X-ray analysis of the spent catalyst showed the presence of rare earth oxide and nickel carbide rather than nickel metal. Exposure of the catalyst to the synthesis gas caused an increase in surface area from a few square metres to as high as 37 m2 g-l for a material containing 50% of mischmetal. The trend in activity for methane formation as a function of mischmetal content shows that there is a pronounced maximum at 30% mischmetal, this catalyst displaying an activity more than an order of magnitude greater than the reference pure nickel catalyst. It is suggested that the optimum combination of specific activity and surface area occurs at an alloy composition corresponding to five nickel atoms per rare-earth atom. The catalytic activity and stability of the rare earth-nickel intermetallic compounds suggests that this area may be worthy of further study.

Kinetics.-The majority of the investigations of the kinetics of hydrocarbon synthesis from carbon monoxide and hydrogen have been concerned with methanation, rather than the formation of higher hydrocarbons. Nickel has been studied to a much greater extent than any other metal.

Early work on the kinetics of the hydrogenation of carbon monoxide has been tabulated by Vannice 2o and reviewed by Mills and Steffgen l8 thus it will not be discussed in detail here. It is perhaps worth emphasizing the diversity of information obtained from the early studies. This diversity stemmed largely from the fact that the surface area of catalysts was not measured and it is thus not generally possible to compare results on the basis of specific activities. In addition, problems of data interpretation arise from results obtained at high extents of conversion, and pretreatment conditions differed markedly between various laboratories. In general terms, the early studies suggested that the methanation reaction is approximately first order in hydrogen and fractionally negative in order with respect to carbon monoxide. In contrast, the Fischer- Tropsch reaction tends to be nearer zero order in carbon monoxide whilst remaining first order in hydrogen.

G. B. Atkinson and L. J. Nicks, J. Catalysis, 1977, 46, 417.

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72 Catalysis In a recent series of papers Vannice40s80-83 has determined the specific

activities of alumina-supported group VIII metals for the synthesis of hydro- carbons. The relative activities of the supported metals, based on the number of surface metal atoms, were found to be significantly different from values obtained from previous work which did not correct for variations in surface area. Vannice’s kinetic studies were conducted in a flow micro-reactor system operated at a total pressure of 1 atm and run at high space velocities, in the range 2500- 10 000 h-l, with carbon monoxide conversions of less than 5%. These conditions ensured that heat and mass transfer effects were minimized, as were effects due to product inhibition, and initial rates could be determined free from the effects of secondary reactions. The initial product distributions show that ruthenium, cobalt, and irridium are the metals which give the highest yields of olefins. The larger extents of production of species with more than four carbon atoms on cobalt and iron are in accordance with the commercial use of these metals in the manufacture of liquid hydrocarbons, while the selectivity of nickel to produce predominantly methane is clearly evident. Ruthenium is unique in its ability to form paraffin waxes at high pressures and, at atmospheric pressure, shows the highest average molecular weight distribution.

The expected behaviour, with regard to the hydrogen : carbon monoxide feed ratio, was observed with all catalysts. Increasing the ratio caused enhanced hydrogenation and favoured ethane production at the expense of higher molecular weight species.

The turnover numbers of the group VIII metals for methanation40,80 are . given in Table 2.

Table 2 Turnover numbers for methanation at 548 K Catalyst

15% Fe/Al,O, 5% Ni/Al,O, 2% Co/Al,O, 1% Rh/A1203 2% Pd/A1203 1.75% Pt/Al,O,

5% Ru/AI,O,

2% Ir/A1203

Turnover number x lo3 181 57 32 20 13 12 2.7 1.8

Vannice40 has also shown that the activity of the group eight metals for methanation may be correlated with the heat of adsorption of carbon monoxide on the metals. Such a correlation is presented in Figure 10. As carbon monoxide is known to chemisorb strongly on transition metals,84 it is reasonable to assume that the metal surface is saturated with carbon monoxide, or with a partially hydrogenated form of carbon monoxide adsorbed with a bond strength paralleling the strength of adsorption of carbon monoxide itself. It is thus reasonable to

M. A. Vannice, J. Catalysis, 1975, 37, 449. D. F. Ollis and M. A. Vannice, J. Catalysis, 1975, 38, 514.

82 M. A. Vannice, J. Catalysis, 1975, 40, 129. - 83 M. A. Vannice, J. Catalysis, 1976, 44, 152.

s4 D. 0. Hayward, ‘Chemisorption and Reactions on Metal Films’, ed. J. R. Anderson, Academic Press, London, 1971, Vol. 1.

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Heterogeneously Catalysed Hydrogenation of Carbon Monoxide 73 expect that the rate of methanation will be dependent upon the strength of the metal to adsorbate bond as determined experimentally.

Expressing the methanation reaction rate in the form of a power law [equation (29)] and on the assumption that the rate-determining step in methanation is

R h i 0 0

Pt 0 ' Ir

100 150 200 Heat of adsn./ kJ rnolD1

Figure 10 CorreZation between the activity for methanation and heat of adsorption of carbon monoxide

the hydrogenation of an adsorbed CHOH species, Vannice 40s has been able to determine the number of hydrogen atoms interacting in the rate-determining step. It has been shown that the number of hydrogen atoms involved can lie between one and four and that the number increases with increasing ability of the metal to produce higher molecular weight species. Ruthenium is unique in requiring four hydrogen atoms, consistent with its unique wax-forming ability.

The activity of platinum and palladium catalysts for the methanation of carbon monoxide has been shownE2 to depend markedly on the nature of the material on which the metals are supported. In the case of platinum the most dramatic change was apparent when the metal dispersion was altered while, in contrast, palladium catalysts did not show such an obvious dependence on particle size. In the case of palladium the most active catalysts were those utilizing the most acidic supports, This suggests that a direct metal-support

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74 Catalysis

interaction, possibly involving electron transfer, takes place with palladium catalysts and is relatively independent of metal particle size. For both palladium and platinum it is suggested that the effects of the support cause an increase in the concentration of the more weakly held carbon monoxide species, thus enhancing the rate of methanation. This conclusion is in agreement with suggestions by Figueras, Mencier, and Primet85 and by Dalla Betta and Boudart,8s each of which assumes a reduction in the electron levels in the metal due to transfer to the support.

The effects of changes in the support and in metal loadings have been investi- gated in detail for nickel.83 Both commercial and laboratory-prepared samples of catalysts were examined and it was found that, although large differences were not apparent, specific activities for methanation did vary by factors of up to five. This reduced effect of the support, when compared with platinum or palladium, may be due to the relatively weak adsorption of carbon monoxide on nickel. In the case of nickel the support had an observable effect on the distribution of products, the presence of any support tending to reduce the selectivity of methane production at the expense of higher hydrocarbons, in particular species with more than two carbon atoms. The support also influenced the quantity of carbon dioxide produced. A summary of the results obtained on supported nickel 83 is given in Table 3.

Table 3 Selectivity of nickel catalysts

Catalyst Bulk Ni 42% Ni/a-Al,O, 30% Ni/a-Al,O, 8.8% Ni/q-A120, 5% Ni/q-A1203 16.7% NiISiO, 20% Ni/graphite

TIK 502 509 502 503 508 493 49 1

Conversion/% Hydrocarbons CO,

2.8 0.039 2.1 0.030 8.2 0.320 3.1 0.058 4.9 0.054 3.3 0.01 1 7.0 0.130

Hydrocarbonslmole :< 90 10 - 76 14 9 81 11 8 81 14 5 87 9 4 92 3 4 88 9 3.5

c, c2 c,+

Pressure, 103 kPa; H,/CO, 3.

The kinetics of the methanation of carbon monoxide and carbon dioxide under conditions applicable in the purification of ammonia synthesis gas, where the total concentration of carbon oxides is of the order of 0.5%, have been reported by Van Herwijnen, Van Doesburg, and De J ~ n g . ~ ’ In agreement with previously published data 88, 89 they found that carbon monoxide, in concen- trations greater than 200 p.p.m., acts as a poison for the methanation of carbon dioxide while the presence of carbon dioxide does not significantly affect the hydrogenation of carbon monoxide.

85 F. Figueras, R. Gomez, and M. Primet, A h . in Chem. Ser., 1973, 121, 480. 86 R. A. Dalla Betta and M. Boudart, ‘Catalysis’, ed. J. W. Hightower, North Holland,

Amsterdam, 1973, p. 1329. T. van Herwijnen, H. van Doesburg, and W. A. de Jong, J . Catalysis, 1973, 28, 391. V. M. Vlasenko, G. E. Yuzefovich, and M. T. RUSOV, Kinetica i Kataliz, 1965, 6, 938. J. S. Campbell, P. Craven, and P. W. Young, ‘Catalyst Handbook’, Wolfe Scientific Books, London, 1970, p. 1 17.

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Heterogeneously Catalysed Hydrogenation of Carbon Monoxide 75 The initial rates of the reaction of carbon monoxide with hydrogen on supported

nickel and ruthenium catalysts have been determined and compared by Dalla Betta, Piken, and Shelef.42 Although the rate of reaction decreased rapidly with time on a clean supported ruthenium surface, it was found that the initial rate was independent of ruthenium particle size for crystallites in the range < l o to 90 A. Nickel exhibited turnover numbers which were twice as high as those for ruthenium at 553 K. Activation energies were found to be 84 and 1 1 7 kJ mol-1 for total hydrocarbon and methane formation on nickel compared with 71 and 100 kJ mol-1 respectively on ruthenium.

The kinetics of the interaction of carbon monoxide with supported cobalt catalysts have been determined by Sastri, Balaji Gupta, and Viswanathan 91

and from these conclusions have been drawn 92 about the nature of the Fischer- Tropsch synthesis on cobalt. They found that the adsorption of carbon monoxide on a cobalt-thoria-kieselguhr catalyst was enhanced by the presence of hydrogen and that the uptake of hydrogen was enhanced by the presence of carbon monoxide at temperatures above 329 K. The mutual enhancement of adsorption is attributed to the formation of a surface complex and, as the surface composition approximated to one carbon monoxide molecule per two hydrogen atoms, regardless of the gas-phase composition, this is considered to be a COHa species. The activation energy found for the interaction of hydrogen with chemisorbed carbon monoxide was 28 kJmol-I and, as this was very much lower than the 105 kJmol-l activation energy for hydrocarbon synthesis, it is concluded that the initial hydrogenation step is not rate determining in Fischer- Tropsch reaction. This conclusion is in agreement with the mechanism proposed by V a n n i ~ e . ~ ~ Sastrig2 is of the opinion that the rate-determining step for Fischer-Tropsch reaction on cobalt is the final hydrogenolysis of the carbon to oxygen bond in the adsorbed alcoholic species formed by successive condensations of surface complexes. On this basis he distinguishes between the Fischer- Tropsch synthesis and other simpler reactions of carbon monoxide with hydrogen, such as methanation or methanol synthesis. In an attempt to isolate this step and determine its energy of activation, Sastri considers it analogous to the hydrogenolysis of ethanol to ethane and water and has investigated this latter reaction. The activation energy determined for ethane production from ethanol was found to be 79 kJ mol-1 which, although somewhat lower than the 105 kJ mol-1 found for Fischer-Tropsch synthesis, is more comparable to it than the activation energies measured for steps preceding the final hydrogenolysis.

In an extensive study of the hydrogenation of both carbon monoxide and carbon dioxide over polycrystalline rhodium, Sexton and Somorjai 93 have correlated the surface composition with the reaction kinetics and the distribution of products. Rhodium, in the form of a small piece of foil of surface area about 1 cm2, was used to study reaction at pressures of and 700 Torr in the same apparatus, the surface composition being monitored by Auger electron spectro- scopy. The binding states of carbon monoxide on the catalyst were studied by

R. Balaji Gupta, B. Viswanathan, and M. V. C. Sastri, J. Catalysis, 1972, 26, 212. 91 M. V. C. Sastri, R. Balaji Gupta, and B. Viswanathan, J. Catalysis, 1974, 32, 325. 92 M. V. C. Sastri, R. Balaji Gupta, and B. Viswanathan, J. Indian Chem. SOC., 1974, L1, 140. 93 B. A. Sexton and G. A. Somorjai, J. Catalysis, 1977, 46, 167.

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76 Catalysis

thermal desorption. No surface oxygen was detected after rejection, indicating that its removal by carbon monoxide to form carbon dioxide or by hydrogen to form water was rapid at low temperatures. Under reaction conditions, however, in the temperature range 525-625 K some oxygen was located below the surface while the surface itself was covered with a catalytically active carbonaceous deposit; no surface oxygen was found. In a manner similar to that observed by Sastrigl on cobalt, the presence of hydrogen augmented the uptake of carbon monoxide, but no changes in its binding energy were observed. It was found that carbon monoxide adsorbed in molecular form on clean rhodium, but partially dissociated on surfaces which had been pretreated by heating in the presence of carbon monoxide. In the higher pressure range the rates of reaction on the rhodium foil, together with the activation energy and product distributions, were almost identical to those obtained on supported Pre- treatment of the surface with acetylene, which caused the deposition of carbonaceous material, reduced methanation activity, but not chain growth thus yielding a higher percentage of C , and C3 species in the products. Pre- treatment of the surface with oxygen caused dissolution of oxygen in the bulk metal and an increase in the methanation activity by a factor of five in comparison with the clean surface. It is suggested that the presence of oxygen causes the formation of active rhodium-carbon-oxygen complexes which rehydrogenate to give the reaction products.

The kinetics and mechanism of the hydrogenation of carbon monoxide over silica-supported catalysts containing ruthenium and copper have been reported by Bond and Turnham 94 with the object of assessing the validity of the arguments of Vannice 8 2 y 83 on bimetallic cluster catalysts of the type employed by Sinfelt.95 It was found that the activities of the ruthenium-copper catalysts decreased by a factor of about fifty on passing from pure ruthenium to a catalyst containing equal amounts of ruthenium and copper and that the differences in activities were due to the pre-exponential factors, rather than the activation energies which remained at 85 kJ mol-l for all catalysts studied. These observations suggest that the mechanism of the reaction on ruthenium is essentially unaffected by the addition of copper but that the number of active sites is thereby diminished. Simultaneously, however, the order of reaction in hydrogen became less positive and that in carbon monoxide more positive. Bond 94 analyses the kinetics of the reaction using a scheme similar to that proposed by Vannice 40s *l except that surface hydrogen coverage is discussed in terms of hydrogen atoms rather than molecules. This interpretation leads to the number of hydrogen atoms in the rate-determining step on ruthenium being three, rather than the value of four found by Vannice. Three is, in some respects, more reasonable as it is hard to conceive a mechanism by which an adsorbed CHOH species could be converted directly to methane in a single rate-determining step.

As well as influencing the activity and orders of reaction, the addition of copper to ruthenium also changed 94 the selectivity for the formation of higher hydro- carbons. Because of the relatively high temperatures used in this study only C, and C3 hydrocarbons were detected but, in general, the addition of copper to

G. C. Bond and D. Turnham, J. Catalysis, 1976, 45, 126. s5 J. H. Sinfelt, J. Catalysis, 1973, 29, 308.

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Heterogeneously Catalysed Hydrogenation of Carbon Monoxide 77 ruthenium reduced the initial yields of ethane and propane. Bond interprets his results on the assumption that the catalytic properties of ruthenium are modified by the presence of copper but that the copper atoms themselves are quite inert, with all the relevant adsorbed species residing solely on the ruthenium atoms. Furthermore, it is concluded that the hydrogenation of carbon monoxide on ruthenium requires an ensemble of ruthenium atoms, perhaps as many as four or five, with an inner atom bonding a CHOH complex and adjacent ones holding the hydrogen atoms. Progressive dilution of the surface of the bimetallic particles with copper atoms is seen as reducing the average number of ruthenium atoms surrounding the one holding the CHOH complex, thus altering the number of hydrogen atoms involved in the rate-determining step and hence the kinetics. The experimental results also suggest that a larger ensemble of ruthenium atoms is required for the production of a higher hydrocarbon than is needed to form methane.

Dautzenberg et aLQS have reported pulse technique analysis of the kinetics of the Fischer-Tropsch reaction on ruthenium supported on y-alumina. Transient operating conditions have been shown to influence the molecular weight distribution of the product with inhibition of high molecular weight compounds. It is concluded that the low overall activity of Fischer-Tropsch catalysts appears to be due to the very low intrinsic activity of the exposed metal atoms rather than to a low number of active surface atoms.

Promoters and Poisons.-The enhancement of the activity of hydrocarbon synthesis catalysts caused by the incorporation of promoters may be either structural or electronic in nature, although in many instances these two effects are not readily separable. While it is undoubtedly true that the irreducible oxides act as structural promoters by hindering the sintering of metals such as iron, cobalt, and nickel, such oxides may cause changes in the product distributions as well as increasing the overall activity of the catalyst. For example, thoria tends to increase the yield of higher molecular weight species in Fischer-Tropsch synthesis on cobalt while magnesia has the opposite effect. Similarly Q7 lanthanum oxide promoted cobalt was substantially better at producing high molecular weight compounds than was a thoria promoted catalyst.

The action of some promoters is unlikely to be structural and must thus be electronic in nature. Such promoters are exemplified by the alkali metals which are well known to be beneficial in the synthesis of hydrocarbons from carbon monoxide and hydrogen. Although the mechanism by which electronic promotion operates has not been confirmed, analogy with ammonia synthesis from the isoelectronic nitrogen molecule, where alkali metals are also used as promoters, suggests that electron donation from an alkali metal atom may increase either the chemisorption of carbon monoxide or the tendency for the molecule, once adsorbed, to dissociate. The adsorption of carbon monoxide on nickel is known Q* to be accompanied by an increase in the work function and hence a charge transfer from the metal to the carbon monoxide takes place. An increased

86 F. M. Dautzenberg, J. N. Helle, R. A. van Santen, and V. Verbeek, J . Catalysis, 1977, 50, 8. 97 E. E. Sense1 and R. A. Beck, U.S. P., 2 517 035 (1950).

E. K. Enikeev and V. A. Krylova, Kinetica i Kataliz, 1962, 3, 116.

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78 Catalysis electron density on the metal, such as provided by donation from an alkali metal, would be expected to facilitate chemisorption of carbon monoxide. It has been shou7ngs that when a bond is formed between linearly adsorbed carbon monoxide and a metal it is the lone pair of electrons on the carbon atom which is involved. There is also substantial back bonding from the metal into the lowest unfilled orbital of the carbon monoxide which is antibonding. As has been pointed out by Dalla Betta,loO increased back donation from the metal, such as would be enhanced by the presence of electron-donating promoters, would decrease the strength of the carbon-oxygen bond thus causing an increase in the ability of the catalyst to dissociate carbon monoxide. Weakening of the carbon to oxygen bond as a result of the back donation has been detectedlo0 as a decrease in the frequency of the carbon-oxygen stretching vibration in the infrared spectrum.

Since the methanation reaction is often required to operate using coal-derived synthesis gas several investigations have been made into the effects of poisons, particularly sulphur-containing species, on the activity of hydrocarbon synthesis catalysts. This area has been reviewed recently by Madon and Shaw lol and thus requires only brief mention here.

The steady rates of carbon monoxide hydrogenation have been measured lo2

on supported nickel, ruthenium, and rhenium in the presence of hydrogen sulphide. Initial specific rates of reaction were found to be similar, in the absence of hydrogen sulphide, on nickel supported on alumina or zirconia but, as a general rule, it was found that the steady rate under sulphur-free conditions was a factor of twenty-five lower than the initial rate required to satisfy the extrapolated Arrhenius data 42 on sulphided catalysts. This difference was attributed to the greater extent of deposition of carbon on the non-sulphided catalysts. The presence of 1 p.p.m. of hydrogen sulphide was found to reduce the activity of a 1.5% ruthenium on alumina catalyst by a factor of thirty-six while an increase to 10 p.p.m. resulted in only a further halving of the activity, thus emphasizing the non-linear effect of sulphur addition. Hydrogen sulphide had an even greater effect on a 2% nickel on alumina catalyst where the addition of 1 p.p.m. of hydrogen sulphide reduced the methanation activity by a factor of sixty-nine.

It is particularly interesting that in the absence of sulphur the highest steady- state activity in the cases of both ruthenium and nickel was associated with the lowest formation of higher hydrocarbons.lo2 For all catalysts examined the addition of sulphur altered the product distribution to increase the relative content of higher hydrocarbons. This has been interpreted as sulphur poisoning the ability of the surface to hydrogenate carbon to a much greater extent than it poisons the ability to form carbon-carbon bonds. When sulphur was removed from the gas stream the recovery of methanation activity was steeper than the recovery in the synthesis of higher hydrocarbons. Similar observations by Herington and Woodward lo3 have been explained by assuming that hydrogen 99 R. L. Gerlach and T. N. Rhodin, Surface Sci., 1970, 6, 133.

loo R. A. Dalla Betta, J. Phys. Chem., 1975, 79, 2519. lol R. J. Moden and H. Shaw, Catalysis Rev., 1977, 15, 69. loa R. A. Della Batta, A. G. Piken, and M. Shelef, J . Catalysis, 1975, 40, 173. lo3 E. F. G. Herington and L. A. Woodward, Trans. Faraday SOC., 1939,35,958.

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Heterogeneously Catalysed Hydrogenation of Carbon Monoxide 79 sulphide adsorbed on sites which would be normally occupied by hydrogen, thus reducing the hydrogenation activity to a greater extent than the activity for carbon-carbon bond formation.

Kishi and Roberts 54 have found that the activation energy for the dissociative adsorption of carbon monoxide on iron was increased by the presence of sulphur on the surface to such an extent that dissociation of molecularly held carbon monoxide was greatly inhibited. It is thus reasonable to suggest that poisons such as sulphur, which are generally considered to reduce the electron density at the surface, may hinder dissociation of carbon monoxide by lowering the extent of back donation from the metal and thus prevent weakening of the carbon to oxygen bond.

4 Reactions On Oxide Catalysts (including those Promoted by Copper) Introduction.-Apart from the methanol synthesis reaction over catalysts con- taining zinc oxide, recent publication about reaction over other oxides has been minimal. The methanol synthesis reaction has continued to receive attention because of its increasing industrial importance and freedom from complicating side-reactions. Russian work has made up the majority of recent reports. Although many methanol synthesis catalysts contain high proportions of copper, as well as other oxides, they will be reviewed here together with unpromoted oxides as the mechanisms are likely to have much in common.

Methanol Synthesis.-Catalyst Types and Selectivities. Catalysts used for the synthesis of methanol from carbon monoxide and hydrogen are of two main types. The first is based on zinc oxide and these catalysts are mainly used with chromia as a promoter. The second comprises catalysts containing copper and zinc oxide, together with structural stabilizers such as chromium and aluminium

Copper-promoted catalysts have the higher activities and can be used at lower temperatures where the thermodynamics of methanol synthesis are more favourable. Little definite information has been published about the method of preparation of either type of catalyst apart from a paper105 on precipitation processes in the production of zinc oxide-chromia catalysts.

It has recently been reported 67 that 5% palladium on silica is a good methanol synthesis catalyst with a selectivity in excess of 95% at temperatures of 548- 598 K and pressures in the range 100-1100 bar. Also, according to Lunev and Rusov lo6 the selectivity and rate of methanol formation were increased appreciably by the addition of 6% palladium to an alkalized zinc oxide-chromia catalyst. Addition of alkali to catalysts based on zinc and chromium decreases the selectivity to methanol and causes appreciable amounts of higher alcohols, principally isobutanol, to be produced. The selectivities to various products as a function of pore size distributions and degrees of utilization of the internal catalyst surface are also reported.106

Industrially, methanol synthesis over copper-containing catalysts is now carried out at temperatures of 493-573 K and pressures of 50-100 bar. Over lo* G. Natta, ‘Catalysis” Reinhold, New York, 1955, Vol. 3, p. 349. lo5 Z . V. Komova and V. M. Vlasenko, Trudy Nauch-Issled Prekot. Inst. Azotn., Prom. Prod. Org.

loe N. Lunev and M. RUSOV, Kinerica i Kataliz, 1969, 10, 1052. Sint., 1971, 10, 255.

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80 Catalysis

the last ten years this process has largely replaced that using zinc oxide-chromia catalysts which was carried out at 573-673 K and pressures of 200-350 lo8

Although both types of catalyst offer high selectivity, that with the copper- containing catalyst is in excess of 99%. A recent paper by Vlasenko et aI.lo9 suggests that this greater selectivity may be due to its lower temperature of operation. It was observed that the parallel formation of methane over both types of catalyst increased rapidly with temperature above 623 K and was higher over copper-promoted zinc oxide-chromia than a similar formulation without copper. Vlasenko log proposes that the reactions yielding methane and methanol occur on different active centres, viz. metal and oxide. The initial adsorptions of carbon monoxide and hydrogen yield a HCOH complex on the surface,llo~lll the subsequent conversion of which is determined by the properties of the catalyst. On metal catalysts the oxygen-carbon bond is cleaved, followed by the formation of a methylene radical which is further hydrogenated to methane [reaction (30)] On oxide catalysts the carbon-oxygen bond remains intact and the surface complex undergoes hydrogenation to methanol [reaction (3 l)]. This difference has been attributed by Zasukha and Roev 112 to the fact that on adsorption on metals the carbon-oxygen bond in carbon monoxide is weakened while adsorption on oxides causes it to be strengthened. It is possible that zinc oxide-chromia catalysts may, under operating conditions, contain some zinc metal which could act as a centre for methane formation 113 and the increased methane production observed upon addition of copper is probably due to the increased number of active metal centres.

HZ > CH, + * (30) H\ /H

C + H 2 0 H\ /OH H2

C II II *

CH30H + * (31) H, C /OH HZ > II *

Kinetics and Mechanism. The kinetics of methanol synthesis have been studied over a wide range of catalysts at differing conditions of temperature and pressure as indicated in Table 4. Attempts have been made to elucidate the mechanism of the reaction by considering a series of steps involving adsorption, reaction, and desorption and comparing a theoretical composite rate expression with the experimental data. Expressions of this type, which are extremely valuable in designing reactors, are summarized in Table 4. The overall kinetic equations 107 P. L. Rogerson, Her. Zngenieursblad, 1971, 21, 657. lo8 P. L. Rogerson, Chem. Eng., 1973, 112. log V. M. Vlasenko, V. L. Chernobrivets, N. K. Lunev, and A. I. Melchevskii, React. Kinet.

Catalysis Letters, 1977, 6, 195. T. Saida and A. Ozaki, Bull. Cliem. Japan, 1964, 37, 1817. G. M. Kozub and M. T. RUSOV, ‘Catalysis and Catalysts’, Naukova Dunka, Kiev, 1965. V. A. Zasukha and L. M. Roev, Teor. ieksp. Khim., 1970, 6, 607.

113 G. M. Kozub, M. T. RUSOV, V. L. Chernobrivets, and L. F. Antonyuk, ‘Catalysis and Catalysts’, 12th edition, Naukova Dunka, Kiev, 1974.

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Heterogeneously Catalysed Hydrogenation of Carbon Monoxide 81

always contain a large number of arbitrary parameters and the experimental results can usually be fitted to more than one model. Each of the following has been postulated as rate determining and all have been claimed to give acceptable fits to the experimental data for the conditions studied, (i) adsorption of h y d r ~ g e n , ~ l * - ~ ~ ~ (ii) trimolecular reaction between adsorbed carbon monoxide and adsorbed h y d r ~ g e n , l ~ l - l ~ ~ (iii) bimolecular reaction between adsorbed carbon monoxide and adsorbed (iv) reaction between an adsorbed surface complex (CH30) and an adsorbed hydrogen atom,llo (v) Rideal type reaction between a surface complex (HCHO) and gaseous hydrogen,ll0$ 12*

(vi) two-step hydrogenation of adsorbed carbon monoxide by adsorbed hydrogen,12& (vii) desorption of methanol.f26 In addition it has also been claimed that the reaction proceeds by hydrogenation of carbon dioxide rather than carbon monoxide 12’ and it has been established that, under some conditions with large-sized catalyst pellets, the rate is pore diffusion controlled.128

Analysis of kinetic data does thus not permit an unambiguous conclusion as to the reaction mechanism. This is well exemplified by a recent paper 123 where the experimental data of Natta and his co-workers were reanalysed using a computer for best-fit. Seventeen possible kinetic models were considered and three of them proved to be equally acceptable. It has also been claimed 116 that Natta’s data will fit alternative kinetic expressions.

Most kinetic studies have been carried out using mixtures of carbon monoxide and hydrogen but, in some cases 119-127, 129 the synthesis gas also contained carbon dioxide, conditions more relevant to the situation pertaining in industrial methanol manufacture. According to one using 14C tracers over a copper-containing catalyst, all methanol produced came from carbon dioxide rather than carbon monoxide. Scheme 6 was proposed and the conditions used

Scheme 6

114 V. Cherednichenko and M. Tempkin, Zhur. Fiz. Khim., 1957, 31, 1072. ll6 V. Pomerentsev, I. P. Mukhlenov, and D. G. Traber, Zhur. priklad. Khim., 1963, 36, 754. 116 I. P. Mukhlenov, V. M. Pomerantsev, and M. L. Syrkina, Zhur. Priklad. Khim., 1970,43, 362. 117 V. M. Vlasenko, R. Rosenfeld, and M. RUSOV, Znternat. Chem. Eng., 1965, 5 , 195. 118 V. I. Atroshchenko, V. E. Leonov, and M. M. Karavaev, Kinetica i Kataliz, 1971, 12, 160. llS V. E. Leonov, M. M. Karabaev, E. N. Tsybina, and G. S. Petrischeva, Kinetica i Kataliz,

1973, 14, 970. lZo Yu. V. Lender, L. S. Parfenova, and K. N. Tel’nykh, Khim. Prom., 1973,49, 654. 121 G. Natta, P. Pino, G. Mazzanti, and I. Pasquon, Chimica e Industria, 1953, 35, 705. lZ2 G. Natta, G. Mazzanti, and I. Pasquon, Chimica e Industria, 1955, 37, 1015. 123 G . B. Ferraris and G. Donati, Zng. Chim. Ztal., 1977, 7 , 53. 124 S. Tzuchiya and T. Shiba, Bull. Chem. Japan, 1965, 38, 1726. 125 I. Pasquon, Chimica e Industria, 1960, 42, 352. 126 H. Uchida and Y. Ogino, Bull. Chem. Japan, 1958, 31, 45. 12’ Yu. B. Kagan. L. G. Liberov, E. V. Slivinskii, S. M. Loktev, G. I. Lin, A. Ya. Rozovskii,

and A. B. Bashkirov, Doklady Akad. Nauk S.S.S.R., 1975, 221, 1093. lZ8 C. Brown and C. Bennett, A.Ch.E. Journal, 1970, 16, 817. lZ9 E. Blasiak and W. Cotowski, Prezemysl Chemiczny, 1964, 43, 657.

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Tab

le 4

M

etha

nol

synt

hesi

s C

ondi

tions

Pr

essu

re/

Cat

alys

t T/

K

bar

Zin

c ox

ide-

57

3-67

3 20

0-30

0

Zin

c ox

ide-

57

2-60

4 20

0-31

6 ch

rom

ia a

chro

mia

- co

pper

(0

.6-0

.9

mm

ir

anul

es) b

Zin

c ox

ide-

ch

rom

ia

(as

in re

f. 12

1)

parti

cles

)

Zin

c ox

ide-

ch

rom

ia

(2

4 mni

pa

rticl

es)

Zin

c ox

ide-

ch

rom

ia-

copp

er

(1-2

m

m

parti

cles

) Z

inc

oxid

e-

chro

mia

- co

pper

f

Zin

c ox

ide-

ch

rom

ia g

Zin

c ox

ide-

ch

rom

ia

(0.5

-1

mm

an

d 4-

5 m

m

parti

cles

)

573-

673

200-

300

573-

673

200-

300

453

1

523

<1

573-

673

1

593-

673

200-

350

Kin

etic

exp

ress

ion for

rate

&a

+C

O - +

CH,O

H/K

eq

(A +

B+C

O +

c+H

, -k

D

+CH

80H

)3

As

abov

e

As

abov

e

kP

E P

co

UP&

, PCO

)O.~

~ or

(whe

re A

is a

mou

nt o

f C

O a

dsor

bed)

b

H,

ACO

bH

,

bH

, P8

8 PC

H,O

H

PCH

,OH

00

h,

Ded

uced

mec

hani

sm a

nd r

ate-

co

ntro

lling

step

Tr

imol

ecul

ar s

urfa

ce r

eact

ion

As

abov

e, a

lso

Bim

olec

ular

rea

ctio

n

Bim

olec

ular

sur

face

rea

ctio

n

Des

orpt

ion

of m

etha

nol

CO

(a) +

H2(

a) + C

HaO

H(a

)

HC

HO

(a) +

H2c

a) +

C&

OH

(a)

Surf

ace

reac

tion

Surf

ace

reac

tion

Rid

eal

CH

2O(a

) + H

a) +

CH

&H

(a)

HC

HO

,,, +

H2(g

) + CH

3OH

Ads

orpt

ion

of H,

P A

dsor

ptio

n of

H,

a" F

Pore

diff

usio

n w

ith la

rger

par

ticle

s E'

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LII

IC;

UA

lUG

-

chro

mia

(g

ranu

les)

C

oppe

r-zi

nc

oxid

e-

alum

ina

(5 x

5m

m

pelle

ts) f

Cop

per-

zinc

ox

ide-

ch

rom

ia

(5 x

5m

m

pelle

ts)

Cop

per-

zinc

ox

ide-

ch

rom

ia

Zin

c ox

ide-

ch

rom

ia

(0.3

mm

and

-13

-33

3

1

493-

533

4&60

(6

% C

OZ)

473-

573

50

453

and

44-5

9 49 1

573-

673

207

6.3

x 6.

3 m

m

pelle

ts)

chro

mia

chro

mia

- co

pper

0 =

fuga

city

. p

= p

artia

l pre

ssur

e,

Zin

c ox

ide-

60

8-65

8 25

0

Zin

c ox

ide-

53

3 25

0

No

kine

tic e

xpre

ssio

n bu

t tr

acer

stu

dies

sho

w a

ll m

etha

nol

deriv

es fr

om C

O,

No

kine

tic e

xpre

ssio

n bu

t ef

fect

of

cata

lyst

par

ticle

si

ze re

port

ed

Ref

eren

ce a

nd d

ate:

a 1

21 (1

953)

; 12

2 (1

955)

; f

124

(196

5);

114

(195

7);

120

(197

3);

' 127

(19

75); No

kine

tic e

xpre

ssio

n bu

t ra

tes

depe

nden

t on

carb

on

diox

ide

cont

ent

123

(197

2);

126

(195

8);

110

(196

4);

117

(196

5);

118

(197

1); ' 11

9 (1

973)

; *

128

(197

0);

129

(196

4);

nu

au

lyr

lull

w1

"2

Ads

orpt

ion

of H

,

Ads

orpt

ion

of H

,

* M

echa

nism

inv

olve

s C

02

only

, L

ra

ther

tha

n C

O

%

with

larg

er c

atal

yst

pelle

ts

5-

% 9 b

% s Po

re d

iffus

ion

unde

r so

me

cond

ition

s $ ;3

Mec

hani

sm i

nvol

ves

CO

, as

wel

l as

CO

&

0 3

Cat

alys

t cla

imed

to

be C

u20

00

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84 Catalysis were temperatures of around 473 K and pressures of 44-59 bar in a recycle system with a CO, : CO ratio of 20 : 1. These conditions are somewhat different from those used in most other studies and it would be of interest to carry out similar experiments under more usual conditions.

Blasiak and Kotowski 120 have also reported that carbon dioxide pressure has a marked effect on the rate of methanol synthesis on copper-containing oxide catalysts. The rates were found to pass through a maximum at 5% carbon dioxide, 533 K, and a total pressure of 250 bar. No such maxima were found with zinc oxide-chromia catalysts, where the effect of increased carbon dioxide pressure was to reduce the synthesis rate. In a more recent paper 130 Lender et al. also state that the synthesis of methanol over copper-containing catalysts, in contrast to that on zinc oxide-chromia, proceeds only in the presence of carbon dioxide, the quantity of which should not be less than 5%. They point out that such carbon dioxide levels are higher than those afforded by carbon monoxide shift equilibrium (CO + H 2 0 + CO, + H,) under reaction conditions. As the shift reaction is endothermic the temperature rise through a catalyst bed, during methanol synthesis under adiabatic conditions, will be reduced by the reverse carbon monoxide shift reaction. They assume the kinetic expression for methanol synthesis proposed by Pomerantsev 115 and show that the temperature profiles calculated from the combination of the reverse shift reaction and methanol synthesis agree well with their experimentally determined temperature profiles in an adiabatic converter.

In contradiction of Blasiak and K o t o ~ s k i , ~ ~ ~ Atroschenko l1** 130 reports that carbon dioxide addition to synthesis mixtures resulted in a sharp increase in the rate of methanol formation over a zinc oxide-chromia catalyst. Undoubtedly more work is required to clarify the effect of carbon dioxide on methanol synthesis and any complete kinetic expression should include a term involving the partial pressure of carbon dioxide.

The kinetics of methanol synthesis are also complicated by the fact that the equilibrium conversion is small, even at high pressures, and the kinetics of the reverse reaction must be considered. Hence many of the expressions listed in Table 4 contain a methanol decomposition term.llO~ l2O, 121-123 A necessary constraint on a kinetic expression intended to cover a range of methanol concen- trations is that the net rate should become zero at the point where equilibrium is established and this criterion explains why the equilibrium constant is incorporated in many of the kinetic equations. Experimental determinations of the rate of methanol decomposition have been made over zinc oxide, zinc oxide- chromia, and zinc oxide-chromia-copper catalysts; most show that the rate is zero order in These measurements were conducted with pure methanol vapour and thus the catalyst was exposed to only negligible amounts of carbon monoxide and hydrogen as decomposition products. A paper,116 which is perhaps more relevant to industrial synthesis, reports that methanol decomposition rates near equilibrium over a catalyst containing zinc oxide, chromia, and copper at 1 bar pressure are approximately 0.25 order in methanol 130 V. E. Leonov, V. I. Atroshchenko, and M. M. Kararaev, Khim. Tekhnol., 1970,20, 10. 131 F. Morelli, M. Giorgini, and R. Tartarelli, J. Caralpis, 1972, 26, 106. lSa A. Dandy, J. Chem. SOC., 1963, 5956.

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Heterogeneously Catalysed Hydrogenation of Carbon Monoxide 85

and -0.25 order in carbon monoxide. The same authors also conclude that the equation coincides with that derived theoretically115 and is the same as the decomposition term quoted in ref. 120 (see Table 4).

Other studies on methanol decomposition have provided information which has a bearing on methanol synthesis. Morelli et a1.l3l report that the rate of methanol decomposition over zinc oxide depends on the compacting pressure of the pellets. The activation energy changed from 171 to 322 kJmol-l, passing through a maximum, as pelleting pressures were increased. Other workers 133, 13*

confirm that pelleting pressure affects the rate of methanol decomposition. Such changes have been attributed to variations in lattice spacing134 or to the generation of lattice defects which form new types of active centres of different activation energy. Changes in the energy of activation of methanol decomposition with temperature 1 3 1 9 135 have been observed and have variously been attributed to a change in the rate-determining step, changes in catalyst semiconductivity, and to diffusional changes within the catalyst pellet.

Brown and Bennett lZ8 have studied the effect of particle size on the rate of methanol formation over a commercial zinc oxide-chromia catalyst in a stirred tank reactor. They found that the rates with 6.3 x 6.3 mm pellets were lower than those over fine particles and established that this was due to pore diffusion limitation and calculated effectiveness factors for varying temperatures and gas compositions. A similar particle-size effect has been noted by Vlasenko et aL117

As attempts to elucidate the mechanism from kinetic studies have lead to a variety of conflicting opinions concerning the rate determining step, alternative approaches have been attempted. These have usually consisted of examinations of the individual steps of the reaction under conditions which allow them to be isolated or distinguished from the others. One main result of investigations of this type has been the recognition of a rapid interaction between adsorbed carbon monoxide and adsorbed hydrogen. Tsuchiya and Shiba,124, 136 working with a zinc oxide-chromia-copper catalyst at 523 K, found that the adsorption of each of the reactants is strongly enhanced by the presence of the other. It was assumed that this implied the formation of a surface compound, especially as the ratio of adsorbed carbon monoxide to adsorbed hydrogen was relatively independent of the partial pressures of the gases. They took the average composition of the adsorbate (1.5H2 to 1C0, corresponding to a methoxy radical) as the stoicheiometric composition of the surface compound and other workers have observed similar effects on pure zinc 0~ide . l~ ' Aharoni and Tompkins 138 compared the kinetics of the mixed adsorptions with those of the separate reactants at room temperature and concluded that the composition of the surface compound was 1H2 to 1CO (corresponding to HCOH) under these conditions. The possibility that the mutual enhancement of adsorption may be due to a surface potential effect has been considered by Nagarjuram et aZ.13'

133 Y. Takamyama, T. Ono, and Y. Ogino, Kogyo Kagaki Zasshi, 1967, 70, 161 1. 13Q Y. Ogino and S. Nakajima, J. Catalysis, 1967, 9, 251. 135 P. Fuderer-Leutic and I. Sviben, J. Catalysis, 1965, 4, 109. 136 S. Tzuchiya and T. Shiba, J. Catalysis, 1965, 4, 116. 13' T. Nagarjuran, M. V. Sastri, and J. C. Kuriacose, J. Catalysis, 1963, 2, 223. 138 C. Aharoni and F. C. Tompkins, Trans. Faraduy SOC., 1970, 66,434.

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86 Catalysis In an attempt to discover the fate of the surface compound, Aharoni and

Starer 139 performed a series of experiments in which carbon monoxide and hydrogen were adsorbed at low pressures and the reaction products desorbed and analysed at temperatures between 470 and 570 K. The main reaction product was methane, although methanol was obtained in small amounts under conditions of high temperature, low pressure and low CO : H2 ratio. It was concluded that hydrogen and carbon monoxide first formed an intermediate surface compound which could be converted into either methane or methanol by reaction with adsorbed hydrogen, the selectivity to methane or methanol depending on the conditions. The first stage of the process was found to take place fairly rapidly whereas the second stage was more difficult. A basically similar conclusion was reached by Borowitz I4O who studied the hydrogen-deuterium kinetic isotope effect in the reaction. He formed the surface compound by coadsorption of carbon monoxide and hydrogen and then removed the gas phase by pumping. Carbon monoxide and deuterium were added at 520 K and a total pressure of 4 bar and the methanol formed was analysed for hydrogen/deuterium distribution in both the methyl and hydroxyl groups. From the results obtained he concluded that the step in which the hydroxyl hydrogen is added to the methoxy radical, with desorption of the product, was rate determining.

It would be reasonable to conclude that most mechanistic studies tend to show that the hydrogenation of the surface complex, or its subsequent desorption, is rate determining in the synthesis of methanol although there remains considerable scope for further work in this area. 139 C. Aharoni and H. Starer, Canad. J. Chem., 1974,52,4044. 140 J. Borowitz, J. Catalysis, 1969, 13, 106.

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