CO2 Hydrogenation on Rh/TiO2 Previously Reduced at Different Temperatures

CO2 hydrogenation on Rh/TiO2 previously reduced at different

temperatures

E. Novak, K. Fodor, T. Szailer, A. Oszko and A. Erdohelyi

Institute of Solid State and Radiochemistry, University of Szeged, PO Box 168, H-701 Szeged, Hungary

E-mail: [email protected]

Hydrogenation of CO2 was studied on 1% Rh/TiO2 reduced at different temperatures. The interaction of CO2 with the catalyst

and that of the CO2 þH2 mixture was also studied. FTIR and TPD measurements revealed that CO2 dissociation depends on the

reduction temperature of the catalyst. In the surface reaction, besides Rh carbonyl hydride, formate groups and different carbonates

and surface formyl species were also formed. The surface concentration of the formyl group depended on the reduction temperature.

The initial rate of CO2 hydrogenation significantly increased with increasing reduction temperature but after some time it drastically

decreased. The promotion effect of the reduction temperature was explained by the formation of oxygen vacancies on the perimeter of

the Rh/TiO2 interface, which can be re-oxidized by the adsorption of CO2 and H2O.

KEY WORDS: carbon dioxide; Rh/TiO2; hydrogenation of CO2; effect of reduction temperature; oxygen vacancy

1. Introduction

The conversion of CO2, one of the cheapest carbonsources in nature, into more valuable compounds is agreat challenge for both academic and industrialresearch.The catalytic hydrogenation of CO2 has been widely

studied. It has been demonstrated that the turnover ratesfor CH4 formation decreased in the orderRu > Rh > Pt � Ir � Pd [1]. In the case of supportedRh catalysts the rate of methane formation in CO2hydrogenation was higher and the activation energy waslower than those of CO hydrogenation [1–3]. On Rh foilthe reaction produces exclusively methane and also has alower activation energy than for the CO–H2 reaction [4].It was found that the support exerted a marked

influence on the specific activity of the Rh. The mosteffective support was TiO2 and the least effective onewas SiO2 [3,5,6]. On Rh/TiO2 the turnover rate wasabout 50 times higher than on Rh/SiO2 [5].When hydrogenation of CO2 was carried out on

supported Rh at higher pressure (10 atm), not onlymethane but also methanol was detected [7]. It wasfound that when Rh/TiO2 was reduced at low tem-perature (533K) the activity was high, but the methanolselectivity was low. As the reduction temperatureincreased, CO2 conversion decreased but the methanolselectivity was enhanced. It was supposed that theactivity change with reduction temperature was not onlydue to the change in the metal surface area but also tothe change in the electronic state of Rh, since the dif-ference in the metal surface area when the sample wasreduced at 533 and 673K was fivefold, whereas theactivity difference was more than tenfold [7]. A smallpositive effect was observed in the CH4 formation ratefollowing high-temperature reduction of Rh/TiO2, while

the H2 adsorption capacity of the sample decreasedsignificantly [8].The effect of sub-monolayer deposits of titania on the

hydrogenation of CO2 on Rh foil has also been inves-tigated [9–11]. The rate increases with the addition oftitania up to approximately 0.5 monolayer then theactivity declines. The maximum rate of CO2 hydro-genation is 15 times higher than that of the bare Rhsurface. An increase in the selectivity for higher hydro-carbons is also observed in the presence of the oxides.The effects of titania on this reaction are attributed to aninteraction between C–O bonds and Ti3þ ions located atthe perimeter of titania islands [8].The catalytic hydrogenation of CO2 has been inves-

tigated as a function of the electric properties of theTiO2 support [12–16], doping TiO2 with lower- andhigher-valence cations. It was demonstrated that varia-tion of the electric conductivity of TiO2 influences thecatalytic properties of Rh. The turnover rate of methaneformation increased significantly due to the incorpora-tion of W6þ ions into the TiO2 increasing its electricconductivity by one or two orders of magnitude [12]. Itwas found that the specific activity of CO2 methanationis increased by up to 24 times upon doping the TiO2support with less than 1% W6þ cations [15]. In contrastto these results, when Rh was dispersed on DegussaTiO2 of high surface area [5] the selectivity of CH4formation was 95–98%; in the present case it varied inthe range 30–90%. Doping TiO2 with lower-valence ions(Mg2þ, Al3þ), which did not much influence the electricconductivity of the support, caused only little alterationin the specific activity of the Rh [12].Rh/SiO2 promoted with CeO2 also showed high

activity in CO2 methanation [17]. This is probablyrelated to the presence of vacancies at the interface

Topics in Catalysis Vol. 20, Nos. 1–4, July 2002 (# 2002) 107

1022-5528/02/0700-0107/0 # 2002 Plenum Publishing Corporation

between Rh and reduced CeO2. High-temperaturereduction of Rh/CeO2 and Rh/CeO2=SiO2 had a posi-tive effect on the catalytic activity in CO2 methanation.The presence of bulk vacancies, which are formed in thelarge CeO2 crystallites after reduction at 773K, isbelieved to be the driving force leading to CO2 activa-tion [17]. The mechanisms of interaction between theceria-supported metals and CO2 and the activation ofCO2 in the presence of H2 to CH4 are strongly influ-enced by the reduction temperature. It was suggestedthat by increasing the reduction temperature a pro-gressive reduction of bulk CeO2 takes place, which is notpromoted by the presence of metal [18]. The interactionmechanism suggested involves activation of CO2 onsurface Ce3þ sites with formation of CO followed byoxidation of Ce3þ to Ce4þ. The presence of oxygen bulkvacancies will create the additional driving force for thereduction of the CO2 to CO and/or surface carbon-aceous species, which then rapidly hydrogenate to CH4over the supported metal [18].In the present paper the catalytic activity of Rh/TiO2

reduced at different temperatures is studied in the CO2methanation reaction.

2. Experimental

The catalysts were prepared by impregnating the sup-port with an aqueous solution (Rh content was 4 g/100mL)ofRhCl3�3H2Osalt (JohnsonMatthey, 99.99%metal basis) to yield a nominal 1% metal content. SiO2(Cab-O-Sil, 198m2=g) and TiO2 (Degussa P25, 50m

2=g)were used as the support. The impregnated powders weredried at 383K for 3 h and the fragments of catalyst pelletswere oxidized at 473K for 30min and reduced in ahydrogen stream (40mL/min) at different temperatures(473, 573, 673K) in the catalytic reactor for 1 h.The gases used were initially of commercial purity.

CO2 was further purified by fractional distillation; CH4(99.995%), CO (99.995%), and Ar (99.996%) were usedas received. He (99.995%) and H2 (99.995 %) werepurified by passage at liquid nitrogen temperaturethrough a trap filled with molecular sieve. Great care wasto ensure dry conditions and to eliminate traces of water.The catalytic reactions were carried out in a fixed bed

continuous-flow reactor (100� 27mm outer diameter).The amount of catalysts used was usually 0.3–0.5 g.Analyses of the gases were performed with a Chrom-pack 9001 gas chromatograph using a Porapak QScolumn. The products were detected simultaneously byTCD and FID detectors. In other experiments thereaction products were also analyzed using an on-linemass spectrometer (Balzers QMS 200). The ratio of H2/CO2 in the reacting gas mixture was four. The systemwas operated at a total pressure of 1 atm. The spacevelocities were 6000–9000/h. The CO2 conversion in thesteady state was in general less than 10–15%.

The TPD experiments were carried out in an 8mminner diameter quartz reactor heated by an externaloven. The amount of catalyst used was 0.3 g. Theheating rate was usually 20K/min. The carrier gas wasHe and the desorbed gases were analyzed using an on-line mass spectrometer (Balzers QMS 200).Infrared (IR) spectra were recorded with a Bio-Rad

FTS-135 type FTIR spectrometer, to which a diffusereflectance attachment (Harrick Type, with CHC-CHAlow-temperature reaction chamber) was connected witha wavenumber accuracy of 2 cm�1. Some IR spectro-scopic studies were made in a vacuum cell using a self-supporting wafer, which underwent the same pretreat-ment as the catalysts. Typically 32 scans were recorded.All spectra presented in this study are difference spectra.The amount of surface carbon formed in the catalytic

reaction was determined by temperature-programmedreduction. After the catalytic run the reactor was flushedwith Ar, the sample was cooled to room temperature,then the Ar flow was changed to H2 and the sample washeated up to 1100K and the hydrocarbons were detec-ted.X-ray photoelectron spectroscopy (XPS) measure-

ments were obtained using a Kratos XSAM 800instrument using AlK� (1486.6 eV) primary radiation.To compensate for possible charging effects the bindingenergies were normalized with respect to the position ofC (1s), this value being assumed constant at 285.1 eV.The samples were pretreated in separate equipment,which was connected directly to the analyzing chamber.The dispersity of the supported Rh was determined

by hydrogen adsorption in a conventional gas volu-metric apparatus at room temperature after the samepretreatment mentioned above [5,24].

3. Results

3.1. Characterization of the catalysts

The XP spectra of titania-supported rhodium areshown in figure 1. After oxidation of Rh/TiO2 at 473Kfor 30min the binding energy of the Rhð3d5=2Þ peak was309.65 eV, which is nearly the same as that of Rh3þ ions.The shape and width of the doublet (full width at halfmaximum ðFWHMÞ ¼ 2:45 eV) indicate the coexistenceof more than one Rh oxidation state. The deconvolutionof the two Rh(3d) peaks—either by Gaussians or byusing the Rh(3d) spectrum of a pure Rh single crystal asa model—leads to a similar result: two states can bedistinguished in the spectrum. The binding energy of thefirst state’s Rhð3d5=2Þ peak is at 309.8 eV, characteristicfor Rh3þ, while the second, smaller state’s correspond-ing binding energy is around 308.0 eV, showing theexistence of a lower oxidation state. The peak area ratioof the two states is about 3:1.

108 E. Novak et al./Hydrogenation of CO2 on Rh/TiO2

After reduction at 473K for 1 h the peak shifted tolower binding energy; the Rhð3d5=2Þ orbital was centeredat 307.75 eV with 4.75 eV separation from its spin–orbital coupled Rhð3d3=2Þ pair. The FWHM was 1.75 eVfor the Rhð3d5=2Þ orbital. When the sample was reducedat 573K for 1 h, practically the same XP spectrum wasrecorded. After hydrogen treatment at 673K the bind-ing energy of the Rh(3d) doublet shifted further to lowerenergy, the binding energy of the 3d5=2 peak appeared at307.45 eV with FWHM ¼ 1:6 eV. This peak position liesabout 0.5 eV higher than that for metallic bulk rhodium.The gradually narrowing peaks indicate that elevatedreduction temperatures result in cleaner rhodium states,as could be expected. A similar feature was observedafter reduction of different supported Rh [19] or whenTiOx was deposited on Rh [20]. According to the con-sideration of Mason [21] the higher binding energyreflects the fact that the particles sizes are small. In thedispersed system there are less neighboring atoms thanin bulk and therefore there are less screening electrons.As a consequence the core–hole screening is less effective

and the binding energy of the orbital shifts to higherenergies.As to the TiO2 substrate, we could not point out any

characteristic or significant changes in the Ti(2p) spec-tra. The measured binding energy of the Tið2p3=2Þ orbitalin 1% Rh/TiO2 reduced at different temperature variedbetween 458.95 and 459.15 eV but these values did notshow a trend. The FWHMs of the peaks also remainedpractically the same (1.47–1.50 eV).The dispersity of the catalysts decreased with

increasing the reduction temperature. The values were56, 49, and 41% when the Rh/TiO2 was reduced at 473,573, and 673K, respectively. These changes are alsoreflected on the XP spectra: the Rh(3d) peak areasdecrease upon increasing the reduction temperature.

3.2. Surface interaction of CO2 with Rh/TiO2

The adsorption of CO2 on a clean Rh surface is awell-studied process; it is weak and non-dissociative.

Figure 1. XP spectra of 1% Rh/TiO2 reduced at different temperatures. The inset shows the deconvoluted spectrum of the oxidized sample.

E. Novak et al./Hydrogenation of CO2 on Rh/TiO2 109

Under ultrahigh vacuum (UHV) conditions CO2,adsorbed at 100K, completely desorbs below 300Kwithout dissociation. The results obtained in this fieldare summarized in two excellent reviews [22,23]. Onsupported Rh the situation is basically different. In thiscase the dissociation of CO2 occurs, but it also dependson the nature of the support. The most effective carrierwas TiO2, but even on these samples the CO2 dissocia-tion was detected only above 373K [24,25]. Recently itwas found that on 5% Rh/TiO2 reduced at 673K COformation was observed already at 230K, but when thissample was reduced at 473K the dissociation of CO2was not detected until 290K [26].The most sensitive method for following the inter-

action of CO2 and the supported metal catalysts is IRspectroscopy, as CO, the primary product of this pro-cess, is strongly bonded to the Rh. Figure 2 showsDRIFT spectra for 1% Rh/TiO2—reduced at 473 or673K—in a CO2 flow. We have found that independentof the reduction temperature the CO band indicatingCO2 dissociation was observed only at 423K. When thesample was reduced at 673K after 1min the CO bandwas found at 2021 cm�1 and its intensity increased intime. For the sample that was reduced at 573K, the COband was first detected after 10min of CO2 flow, but onthe catalyst reduced at 473K it appeared only after20min.Further increase of the temperature of the catalyst

causes an increase of the CO band intensities up to

473K; independent of the reduction temperature abovethis temperature they decreased. For the sample reducedat 673K, CO formed in the CO2 flow was detected evenup to 598K (figure 3).The interaction of CO2 and Rh/TiO2 was also studied

by temperature-programmed desorption. When the CO2was adsorbed at room temperature on Rh/TiO2 reducedeither at 673 or at 473K, CO2 desorbed in one stage atabout 373K (figure 4). CO desorption was not observedðm=e ¼ 28 was not greater than that of the fragment ofCO2 ðm=e ¼ 44Þ at m=e ¼ 28Þ. In contrast to this, whenCO2 was added to the Rh/TiO2 (reduced at 673K) at513K and cooled to room temperature in CO2 flowbesides the low-temperature CO2 desorption, a sig-nificant amount of CO was detected above 600K in abroad peak (Tmax ¼ 723K), while CO2 desorption wasnot observed. The high-temperature CO evolution afteradsorption of CO2 on the sample reduced at 473Kunder the same experimental conditions was not detec-ted (figure 4).

3.3. Surface interaction of H2 and CO2 on Rh/TiO2

The surface interaction of CO2 with H2 takes placeeven at room temperature. Previous IR spectroscopicmeasurements revealed that chemisorbed CO and for-mate ions are formed in this process [24,27].

Figure 2. DRIFT spectra recorded on Rh/TiO2 reduced at (A) 473K and (B) 673K in CO2 flow: at room temperature (1); at 373K (2) and 398K

(3) after 20min; and at 423K for 1min (4), 10min (5), and 20min (6).


For comparison we have studied the adsorption ofCO on Rh/TiO2 reduced at different temperatures. Itseems that the pretreatment of the catalyst did notinfluence significantly the IR spectra of adsorbed CO.Introducing CO onto the catalyst bands at 2103, 2072and 2040 cm�1 appeared on the spectra which can beattributed to the twin (2103 and 2040 cm�1) and linearlybonded CO (2072 cm�1).Temperature-programmed desorption studies of

adsorbed CO on Rh/TiO2 have shown that there wereno significant differences as a function of the reductiontemperature of the catalyst. It was found that the des-orption temperature of CO, Tmax ¼ 463–473K waspractically the same in all cases. Simultaneously withCO desorption, however, CO2 was also detected. Ahigh-temperature stage of CO (Tmax � 550K) was alsodetected. These results agree well with previous findings[28].In the presence of H2 þ CO2 at room temperature the

following changes occurred on the IR spectra of Rh/TiO2. The general features are shown for the samplereduced at 523K (figure 5). Even at the beginning of theadsorption new bands were identified at 2056 cm�1 andat about 1900 cm�1. In addition, in the low-frequencyregion bands appeared at 1585, 1444, and 1377 cm�1,and a weak one at about 1720 cm�1 (figure 5). In timetheir intensities, especially in the CO region, grew andbands at 1555, 1536, and 1362 cm�1 also appeared onthe spectra. When the cell was evacuated at room tem-

perature the spectra above 1800 cm�1 only slightlychanged. Below 1750 cm�1 significant changes wereobserved. In this case the band at 1720 cm�1 dis-appeared and bands were detected at 1615, 1555, 1540,and 1362 cm�1. On increasing the evacuation tempera-ture the intensities of these bands decreased. At 423Konly peaks at 2043, 1555 (with a shoulder at 1540), and1362 cm�1 were observed (figure 5).When the catalyst was reduced at different tempera-

tures the spectral features detected after the adsorptionof CO2 þH2 mixture at room temperature were nearlythe same as discussed above; only the peak intensity at1720 cm�1 changed significantly upon increasing thereduction temperature to 523K (figure 6).To help the assignment of the bands below

1800 cm�1, the adsorption of HCOOH on TiO2 and onRh/TiO2 was also studied. Introducing 10

�2 mbar ofHCOOH gave intense bands at 2951, 2870, 1555, 1377,and 1362 cm�1. These results agree well with previousobservations on TiO2 and on TiO2-supported noblemetals [29,30].

3.4. Hydrogenation of CO2

Previously it was found that the hydrogenation ofCO2 on alumina-supported Rh occurred at a measurablerate at above 443K [5]. When the alumina was replacedby some other oxide, the temperature range of measur-

Figure 3. DRIFT spectra of Rh/TiO2 reduced at (A) 473K and (B) 673K recorded in CO2 flow at 423 (1), 448 (2), 473 (3), 498 (4), 523 (5), 548 (6),

573 (7), and 598K (8) for 20min.


able activity changed considerably: this range was thelowest for Rh/TiO2 and the highest for Rh/SiO2 [5]. Theselectivity of methane formation approached 99–100%,and only a trace amount of ethane was formed.In the present work, when the Rh/TiO2 was reduced

at 473K, the conversion of CO2 at 473K slightlydecreased in the first minutes of the reaction and thenremained the same for a longer time. For the samplesreduced at higher temperature (573 or 673K), the initialactivity decreased significantly (figure 7(A)). The CO2conversion measured in the first minutes decreased fromabout 43 to 11% when the sample was reduced at 673K.The activity changes of the catalysts in the first

minute of the reaction were also followed by an on-linemass spectrometer (figure 7(B)). After introducing theCO2 þH2 mixture onto the catalyst the CH4 signalsuddenly increased and then decreased; after about 50 sthe CH4 formation rate remained nearly constant. The

maximum value of the methane formation rate in thefirst seconds of the reaction decreased with decreasingreduction temperature. For the sample reduced at 673Kthe maximum value of the methane formation rate wasabout twice that in the presence of Rh/TiO2 reduced at473K, while in the steady state the CH4 production wasnearly the same. Calculating the amount of excessmethane from the peak areas of the CH4 vs. time curves,we found that on the sample reduced at 673K this valueis about 130�mol/g, while when the catalyst was treatedwith H2 at only 473K, only 73�mol/g methane surpluswas detected.When the sample reduced at 673K was treated with

water (1�L, 55�mol) or with CO2 (1mL, 41.2�mol) atthe reaction temperature (513K) before introducingCO2 þH2 mixture, the initial excess in the methaneformation is completely missing, the methane formationdescribes a saturation curve in time.

Figure 4. CO and CO2 desorption after adsorption of CO2 at room temperature or at 513K on 1% Rh/TiO2 reduced at 673 and 473K.


Comparing the methane formation rates related tothe number of surface metal atoms obtained in thesteady state we can conclude that the activity of Rh/TiO2 slightly increased with increasing the reductiontemperature of the catalysts. This difference is morepronounced when the initial activities are compared(table 1). Not only the turnover rate of methane for-mation but also the conversion of CO2 is much higherfor the sample reduced at 673K than on the catalyst thatwas hydrogenated at 473K.The conversion of CO2 on Rh/SiO2 at 548K was only

2–3%. The activities of these samples only slightlychanged in the first hour of the reaction.To compare the turnover rate of methane formation

obtained on Rh/TiO2 and on Rh/SiO2 we have toextrapolate the rate measured on Rh/TiO2 to highertemperature using the Arrhenius diagram. We concludethat the activity of Rh/TiO2 is higher by at least oneorder of magnitude than that of Rh/SiO2. This observ-ation is in good agreement with previous findings [5].

4. Discussion

4.1. Some properties of TiO2-supported noble metals

Much effort has been made in the last three decadesto understand the effect of the support on the supported

metal, i.e. the nature of the metal–support interaction[31,32]. Particular attention has been paid to titania-supported metals, which exhibit high activity in severalcatalytic reactions. It has long been recognized thatsupports not only provide a high surface area for metalcatalysts, but also drastically modify the catalyticproperties of metals. One of the first explanations wasthat there is an electronic interaction at the metal sup-port interface proposed by Schwab [31] and Solymosi[32]. This concept was based on the finding that anelectronic interaction develops between two solids withdifferent Fermi levels [32]. The different theories ofmetal–support interaction have been summarized indifferent reviews [33,34]. Earlier assumptions expressedthe view that the C atoms of the CO may be attached tothe metal and the O atom to the Mnþ ion of the oxidesupport [35]. It was assumed that active sites in the COhydrogenation to methane were situated on the metal–metal oxide borderline. This effect was confirmed by theresults obtained for Rh foil decorated with differentoxides (e.g. AlOx; TiOx, WOx) [11,36,37]. The max-imum of the methane formation was observed when theoxide coverage on the Rh was about one-half of themonolayer. In these cases the perimeter of the oxideislands are the longest. When the TiO2 support wasdoped with WO3 in the Rh/TiO2 catalyst the turnoverrate increase of methane formation in the CO or CO2hydrogenation was 16-fold, but the addition of MgO or

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1540

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5

Figure 5. IR spectra of H2 þ CO2 gas mixture adsorbed on Rh/TiO2 reduced at 523K at room temperature: (A) for 1 (1), 5 (2), 10 (3), 30 (4), and

60min (5); (B) evacuated at room temperature for 30 (1) and 60min (2); and at 323 (3), 373 (4), and 423K (5); for 15min.


Al2O3 produced minor TOF changes [12], showing thatthe electronic interaction may be more important herethan merely increasing the number of perimeter sites. Itwas found that when titania- or other reducible oxide-supported metal catalysts were reduced at higher tem-perature (>700K) the adsorption capacity of the sam-ples for hydrogen or CO strongly decreased, but thesecatalysts show increased activity for different reactionsamong others for CO hydrogenation. It was concludedthat after high-temperature reduction of Rh/TiO2 at theperimeter the support is reduced and the TiOx speciesmigrate onto the metal [38]. This process upon high-temperature reduction (>773K), also well known asencapsulation or decoration, has been evidenced byhigh-resolution electron microscopy [39]. Recently inscanning tunneling microscopy studies Berko et al. [40]

obtained strong evidences for TiOx encapsulation of Rhparticles deposited onto TiO2 single-crystal surfacesafter annealing it in UHV to 500–700K.Somorjai and co-workers [36] found that a sub-

monolayer deposit of titania on Rh foil increases therate of CO2 hydrogenation. This was attributed to aninteraction between the adsorbed CO, released by CO2dissociation, and Ti3þ ions located at the edge of TiOxislands covering the surface. CO molecules adsorbed onRh sites lying at the adlineation between the Rh surfaceand the titania islands can interact with the exposed Ti3þ

ions via a Lewis acid–base interaction. This mechanismhas been proposed by Sachtler and Ichikawa [41] toexplain the promotion of Rh catalysts by TiOx for COhydrogenation. This theory is in agreement with thesuggestion of Rasko [42] who supposed that the noble

Figure 6. IR spectra of H2 þ CO2 gas mixture adsorbed at room temperature for 1min on Rh/TiO2 reduced at 473 (1), 523 (3), 623 (5), and 673K

(7); and for 20min on the sample reduced at 473 (2), 523 (4), 623 (6), and 673K (8).


metal–TiO2 interface is active in CO2 dissociation whilethe carbon atom of the molecule is linked to a noblemetal atom and one of the oxygens is bonded to theoxygen vacancy of titania.

4.2. Surface interaction of CO2 with Rh/TiO2

IR spectroscopic and TPD measurements revealedthat CO2 dissociation occurred on the surface of Rh/TiO2 (figures 2–4). We have found that the higher thereduction temperature, the greater the rate of CO evo-lution; while CO could be detected in the first minute ofCO2 adsorption on the sample reduced at 673K, on thecatalyst, pretreated with hydrogen at 473K, this hap-pened only after 20min of CO2 adsorption. Thisobservation is in agreement with previous results [26,42]

and supports the speculation that the oxygen vacancyproduced by the reduction of the catalysts promotes thedissociation of CO2. When the sample was reduced athigher temperature the higher amount of defects formedresulted in the higher dissociation rates. We haveobserved that the IR spectra of adsorbed CO and thecharacteristics of CO TPD did not change significantlywith the reduction temperature so the differencesobserved in the CO2 TPD (figure 4) were caused only bythe higher CO formation rate for the sample reduced at673K.

4.3. Surface interaction of H2 and CO2 on Rh/TiO2

The CO2 þH2 interaction has also been studiedpreviously [24,27,43] on different supported Rh cata-

Figure 7. (A) CO2 conversion in the CO2 þH2 reaction at 473K on 1% Rh/TiO2 reduced at various temperatures. (B) Rate of CH4 formation at

513K on 1% Rh/TiO2 reduced at various temperatures.

Table 1

Some kinetic data for the hydrogenation of CO2 on 1% Rh/TiO2 catalysts at 473K.

Catalyst Reduction CH4 formation rate Turnover number for CH4temperature ð�mol=gsÞ formation (103/s)

(K) s�1

Initial Steady state Initial Steady state

1% Rh/TiO2 473 0.343 0.308 6.3 5.62

at 473 K 573 1.926 0.667 40.4 14.00

673 2.956 0.648 74.2 16.27


lysts; adsorbed CO and formate species were identifiedat 300–423K, without any indication of the evolution ofgaseous CO. Strong evidence has been presented thatthe formate species is located not on the Rh but ratheron the support [24,27]. The spectral features of adsorbedCO produced in the H2 þ CO2 interaction differedbasically from those observed during the adsorption ofgaseous CO on the same sample: the doublet due toRh(CO)2 was missing and the band due to linearlybonded CO appeared at lower frequencies. It was sup-posed that Rh carbonyl hydride was formed [24,27,43].Figures 5 and 6 clearly show that in the CO region onlyone peak was developed indicating carbonyl hydrideformation.In the H2 þ CO2 interaction many absorption bands

were detected by IR spectroscopy in the low-frequencyregion below 1800 cm�1 (figures 5 and 6). The adsorp-tion of HCOOH on TiO2 or on Rh/TiO2 producesintense bands at 1555 and 1362 cm�1 which can beassigned as the �as and �sym frequencies of the O–C–Ovibration of the formate ion. The bands at 2951, 2870,and 1377 cm�1 can be attributed to the C–H frequencies[29,30]. Fisher and Bell [2] found that adsorbed CO andCO2 could be hydrogenated to surface formaldehyde onRh/SiO2, which absorb at 1710 cm

�1. This species hasbeen observed rarely during the hydrogenation of CO orCO2 because it decomposes under the reaction condi-tions as was found on Rh/Al2O3 [44]. Bradford andVannice [30] who detected a band at 1690� 10 cm�1

after methane adsorption assigned this peak as theH2C ¼ O species bonded to a strongly coordinatedLewis acid Tin+ site. Taking into account these resultswe can assign the band at 1720 cm�1 as adsorbed for-maldehyde and bands at 1555, 1377, and 1362 cm�1 asformate groups on the surface of the titania.We have found that the intensity of the band at

1720 cm�1 due to the adsorbed formyl group shows amaximum as a function of the reduction temperature ofthe catalysts (figure 6). Bradford and Vannice [45] sug-gested that interfacial sites in TiOx/Pt catalysts couldpromote decomposition of CHxO intermediates. Ourobservation could be explained by this proposal, but itseems that since higher reduction temperature increasesboth the formation and the decomposition rate of for-myl groups the resultant is the maximum in the surfaceconcentration.All the other IR bands below 1700 cm�1 belong to

different carbonate species.

4.4. Hydrogenation of CO2

Numerous papers have reported on the catalyticreaction of CO2 þH2 on different supported Rh cata-lysts. In Section 1 we mentioned some of the mostimportant ones. In all this work it was stated that theRh/TiO2 has an outstanding activity in this reaction.

This high efficiency was explained either by the elec-tronic effect of TiO2 [5,12] or the oxygen vacancyformed in the reduction of the catalysts [36]. In thiswork and in our earlier paper we have found that thesteady-state activity of Rh/TiO2 is much higher thanthat of Rh/SiO2 [5].Our XPS results (figure 1) show that independent of

the reduction temperature the oxidation state of Rh wasnearly the same. The binding energy of Rh(3d) orbitalswas only slightly higher than that of bulk Rh. Despitethe 673K reduction of the sample the binding energiesof Ti(2p) orbitals do not indicate the presence of Ti3þ

species on the surface.In spite of this XPS result our present experiments

show that the efficiencies of these samples in the firstseconds depend sensitively on the reduction temperatureof the catalysts but after a few minutes the rate ofmethane formation or CO2 conversion were nearly thesame in all cases (figure 7). When the reduced sampleswere pretreated with water or with CO2 before thereaction the high initial activities were not observed.Previously it was shown that H2O [46] and CO2 [47]

dissociation involves the oxidation of Ti3þ to Ti4þ, i.e.this process may play role in the disappearance of theoxygen vacancy of titania. Taking into account theseresults we may conclude that the high initial efficiency ofRh/TiO2 can be attributed to the oxygen vacancyformed in the reduction of the catalysts. Higher reduc-tion temperature caused higher amounts of vacancies,resulting in higher initial rates of CH4 formation or CO2conversion. CO2 as the reaction partner or H2O as aproduct can remove these defects by re-oxidizing of thesample and this is the reason why the reaction ratedecreased suddenly in the first seconds of the reaction.Kramer and co-workers [48] have recently found that

the water formed in the reduction is in equilibrium withTi3þ species:

Ti4þ þO2� þH2()VO þ Ti3þ þ e� þH2O:

On the basis of these results we suppose that the roleof the oxygen vacancy cannot be ruled out in the steady-state condition of the catalyst, because it can also beformed during the reaction, but the high efficiency of thetitania-supported metal cannot be attributed only to thisspecies.The differences in the initial activity of Rh/TiO2

could also be explained with the particle size effect. Ingeneral the turnover rate of CO methanation decreaseswith increasing number of surface metal atoms [49]. Inthe CO2 hydrogenation on Pd/Al2O3 catalyst we havefound [50] results as mentioned above but our resultsclearly show that on smaller particles (initial dispersionwas 40%) the selectivity of methane formation was near100%, but when the dispersion was low (4%) the mainproduct of the reaction was CO. In the present casethere is no great difference in the dispersion of the metal,


i.e. in the metal particle size of different Rh/TiO2 sam-ples and in all cases the CH4 selectivity was near 100%.These observations do not support the idea that theparticle size influences the rate of methane formation.For the interpretation of the higher efficiency of

titania-supported metal catalysts the electronic interac-tion between the metal and the support [32] has to takeninto consideration.

5. Conclusions

1. By means of FTIR and TPD measurements it wasfound that CO2 dissociation probability increased withincreasing the reduction temperature of the catalyst.2. In the surface interaction of CO2 þH2 mixture

besides Rh carbonyl hydride, formate groups, differentcarbonates and formyl species were also formed. Thesurface concentration of formyl groups depended on thereduction temperature.3. The initial rate of CO2 hydrogenation, CO2 con-

version and the rate of methane formation significantlyincreased with increasing reduction temperature, butafter some seconds it drastically decreased due to thepoisoning effect of water and/or CO2 and in the steadystate activity was nearly the same in all cases.4. The promotion effect of the reduction temperature

was explained by the formation of oxygen vacancies onthe perimeter of the Rh/TiO2 interface, which can be re-oxidized by the adsorption of CO2 and H2O.

Acknowledgement

Financial support of this work by OTKA (contractnumber T034270) is gratefully acknowledged.

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Documents

CO2 Hydrogenation on Rh/TiO2 Previously Reduced at Different Temperatures