STK-PAPYR-125-ETHYLENE-in-CEC-V-44-ISS-1-6-PP-53-74-Y-1986

8/14/2019 STK-PAPYR-125-ETHYLENE-in-CEC-V-44-ISS-1-6-PP-53-74-Y-1986

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Chem. Eng. Commun. Vol. 44 pp. 53 740098-644518614406-0053 25.00/01986, Gordon and Breach Science Publishers S.A.Printed in the United S tates of America.

ETHYLENE OXIDATION ON SILVERCATALYSTS: EFFECT OF ETHYLENE OXIDE

AND OF EXTERNAL TRANSFERLIMITATIONS

M I C H A E L S T O U K I D ESDepartment of Chemical EngineeringTufts University Medford MA 02155

andS T A V R O S P A V L O UDepartment of Chemical EngineeringUniversity of PatrasPatras 261 10 Greece

Received June 10, 1985; in final form December 9, 1985)Ethylene oxidation to ethylene oxide and to carbon dioxide over silver catalysts was studied in aCST R. T he effects of two factors on th e catalyst performanc e were exam ined. The first was thepresence of excess ethylene oxide in the feed. A kinetic model was introduced which assumed thatethylene and ethylene oxide compete for the same sites on the catalyst surface. This model providedreasonable quantitative agreement with kinetic and potentiometric measurements. The second factorthat was studied was the presence of external heat and mass transfer limitations. It was found thatsuch limitations cause a significant decrease of the selectivity to ethylene oxide. This decrease is aresult of the te mpe rature difference between the catalyst surface and the bulk of the gas pha se and ofthe fact that the activation energy of ethy lene combustion is greater than tha t of ethylene epoxidation.The contribution of other factors such as inhibition by CO, or possible incomplete mixing in thereactor is shown to be insignificant.KEY WO RD S Catalytic ethylene epoxidation Ethylene oxide selectivitySolid electrolyte potentiometry Oxygen activity measurem entSelectivity of polycrystalline silver Exte rnal transfer limitations

I NT R ODUC T I ONEthylene oxide is produced commercially from ethylene oxidation ove r supportedsilver catalysts. Th ere is a general consensus Sachtler et al. 98 ; Voge andAdams, 1967 , that the kinetics can be described by means of a triangularscheme: C z h O5 hy y + 3 o JG H , 12 +301)- [ C O Z+ H@l

The activation energies and reaction orders for the three reactions have beeninvestigated by numerous groups and their results have been summarized by


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54 M . S T O U K ID E S A N D S . P A V L O UVoge and Adams (1967), Kilty and Sachtler (1974) and recently by Sachtler,Backx and Van Santen (1981) as well as by Barteau and Madix (1982). A numberof para mete rs that affect the selectivity to ethy lene oxide (moles of ethylene oxideproduced per mole of ethylene consumed) has been reported so far includinghalogen co mp oun ds (I(ilty and Sachtler, 1974; Kilty t al . 1972), crystal size andorientation (Kummer, 1956; Harriot, 1971; Johnson and Verykios, 1983) as wellas y-preirradiation (Carberry, 1972). The inhibiting effect of reaction productsand other species on the reaction yield and selectivity has also been studiedextensively (Voge and Adams, 1967; Sachtler l a l . 1981; Cant and Hall, 1978;Hayes, 1960). Nevertheless, despite the large number of investigators and somevery interesting recent expe rimental findings (Wachs and Ke lemen, 1981; Ghazalit al . 1983; Campbell and Paffett, 1984; Haul t al . 1984; Grant and Lambert ,1985(i), 1985(ii)) no generally accepted reaction mechanism h as been established.The very wide scatter of the published parameters make i t impossible to drawunambiguous conclusions on the mechanism from such da ta.The present communication is focusing attention on a) the effect of ethyleneoxide on the reaction rates and b) the effect of external diffusional effects on thecatalyst activity and selectivity. Conflicting results exist so far on the role ofethylene oxide on the re action selectivity. A linear dec rease of th e selectivity withincreasing ethylene oxideloxygen ratio has been reported by Temkin and hisco-workers (1962; 1979). Force and Bell (1975) however observed an increase inselectivity with increasing ethylene oxide concentration although this increase wasnot permanent.In the present work kinetics are combined with in situ electrochemicalmeasurement of the activity of oxygen adsorbed on the catalyst surface using thetechnique of Solid Electrolyte Potentiometry (SEP). This technique has alreadybeen used in conjunction with kinetic measurements in order to study themechanism of various catalytic oxidations (Vayenas t al . 1981; Stoukides and

Vayenas, 1980; Stoukides and Vayenas, 1981). Reactor cells similar to the oneused in this study have been used so far to enhanc e the rate of N O decom positionand C O hydrogenation (Pancharatnam t a l . 1975; Giir and Huggins, 1981), aswell as in high temperature fuel cells (Etsell and Flengas, 1971; Vayenas andFarr, 1980).

E X P E R I M E N T A L M E T H O D SA schematic diagram of the experimental apparatus is shown in Figure 1. Itconsists of the flow system, the reac tor cell and the analytical system. Th e reactorcell was a 8 yttria stabilized zirconia tube of 2cm 2 cross sectional area and30cm 3 volume. Th e A g catalyst film was deposited on the flat bottom of thezirconia tube. A similar Ag film was deposited on the outside bottom wall of thereactor as shown in Figure 2. This film was exposed to air and sewed as thereference electrode. To deposit the film, a few drops of a silver suspension inbutyl acetate (Stoukides and Vayenas, 1980) was used, followed by drying andcalcining at 400C. The Auger spectrum of the catalyst showed (Stoukides and


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ETHYLENE OXIDATION ON SILVER

S c h m t i c nlagran of A paraturIR Infrared Cn2 h a l y z e rGC: Gar ChmratnsraphT C : lcmpcraturr C o n t r o l l e rDV : Dlff~rentlal V o l t m e t e r

FIG UR E Schematic diagram of apparatusAg w i r e

FIG UR E Schematic diagram of reactor cell


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56 M . S T O U K ID E S A N D S . P A V L O UVayen as, 1980) that th e silver surface was fairly clean. Trac e impu rities includingsom e CI (less than 1 of a mono layer) might have been responsible for therelatively high selectivities obtained. The catalyst surface area was estimatedusing oxygen chemisorption followed by reaction with ethylene as it has beendescribed in detail elsewhere (Stoukides and Vayenas, 1980). It was thus foundthat the catalyst could adso rb approxim ately 1.8 moles of oxygen at 300C.A silver wire, enclosed in a pyrex tube was used to make contact with theinternal Ag film catalyst-electrode (Figure 2). The reactor temperature wascontrolled within 2C by means of a Leeds and Northrup temperature controller.J. Fluke voltmeter was used to m onitor th e open-circuit EM F of th e cell.Matheson certified gases (ethylene oxide, ethylene, oxygen) diluted in nitrogenwere used as reactants. The feed and product composition was determined bymeans of a Perkin-Elmer Gas Ch romatogra ph with a TC detector. A molecularsieve 5A column was used to separate N2 and O 2 and a Pora pak Q was used toseparate air, C 0 2 , ethylene and ethylene oxide. The CO, concentration in theproducts was also monitored by a Beckman 864 Infrared Analyzer.Measurement of Surface O xygen ActivityThe technique of Solid Electrolyte Potentiometry (SEP) provides a continuous insitu measurement of the thermodynamic activity of oxygen adsorbed on thecatalyst surface. It has been established (Wagner, 1970; Stoukides and Vayenas,1980), that the open circuit EMF of the cell reflects the difference in chemicalpotential of oxygen adsorbed on the two silver electrodes:

E po,(Ag catalyst) po,(Ag reference)]4 F 1)The chemical potential of oxygen adsorbed on the reference electrode which is incontact with air (Po, .21 ba r) is given by

po,(Ag reference) &(g) RT ln(0.21) (2)where &(g) is the standard chemical potential of oxygen at the temp eratu re ofintere st. Th e activity a of adsorbed oxygen can be defined by:

Thus a; expresses the partial pressure of gaseous oxygen that would be inthermody namic equilibrium with oxygen adsorbed on the catalyst surface, if suchan equilibrium were established. Therefore, combining Eqs. ( I ) , (2), and (3) oneobtains:a ( 0 . 2 1 ) ' ~ x p

The above equation is always valid, irrespective of whether thermodynamicequilibrium is established between gaseous and adsorbed oxygen or not. In thespecial case where thermodynamic equilibrium indeed exists between adsorbedand gaseous oxygen, then a: Po,.


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E T H Y L E N E O X I D A T I O N O N S I L V E R 57E XP E R I M E NT AL R E S UL T SThe catalytic oxidation of ethylene was studied in presence of excess ethyleneoxide in the feed stream at tem peratures 280-400C and atmospheric totalpressure. Under the flowrates employed in this study, it has been shownStoukide s and Vayenas, 1980) that the reactor is well mixed CS TR ). Absence ofexternal o r internal diffusional effects has been established in previous studiesusing the sam e reactor under similar tem pera ture and Bow conditions Stoukide sand Vayenas, 1981).Th e rate of net production of ethylene oxide in moles of ethylene oxidels) wascalculated from the raw kinetic data using the appropriate mass balance

where x ~ ~ 0 . i nd XETO,,,, are the mole fractions of ethylene oxide in the feedand outlet respectively and G is the total molar flowrate. Due to the dilution ofthe reactan ts in N, the chan ge in G from the stoichiometry of the reactions wasless than 1 and hence G was considered constant. The rate of CO, formationr o , was calculated from the steady state material balance:

Th e rates of ethylene oxide formation, ethylene deep oxidation and ethyleneoxide oxidation have already been studied-in absence of excess ethylene oxidein the feed-using the sam e appa ratus , reactor and catalyst preparation Stouk-ides and Vayenas, 1980; 1981). It was found that for excess air in the reactor thethree reaction rates could be described rather accurately by the equations:

Th e validity of Eqs. 7) through 9) was verified by repeating the experimen ts attem pera ture s 280-400C. Th e values of the rate constants K , , K , , K 3, K, , KETothat best fit the experimental data were:


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M . S T O U K I D E S A N D S. P A V L O U10600KETo 3 .3 lo- exp( bar-

i.e. very close to the values reported previously.Th e effect of excess ethylene oxide in the feed stream on the rate of C 0 2formation rc0 as well as on the net rate of ethylene oxide formation rET isshown in Table I. Each one of the data resulted from the average of twomeasurements. T he partial pressure of ethylene oxide in the reactor varied fromT BLE

ffect of ethylene oxide on reaction rates


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ETHYLENE OXIDATION ON SILVER 59

FIGURE 3 Rate of C O production vs . P . Po 0 . bar P . =O.OlOS bar

FIGURE 4 Rate of ethylene oxide formation vs. P . Po 10 .1 bar PET=0.0105bar.


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60 M . S T O U K l D E S A N D S . P A V L O U0.0006 to 0.024 bars while th e partial pressure of ethylene and oxygen were k eptconstant (within 5 -10 ). Figure 3 shows the effect of ethylene oxide on rcoOnly a slight increase is observed with increasing PETO t all temperaturesexamined. The corresponding effect of ethylene oxide on rETo is shown in Figure4. A considerable decrease on rETo with increasing PETOs observed, the effectbeing more important at high tem peratu res. Finally, the surface oxygen activity nwas found to depend on PETOn the way shown in Figure 5. At all temperaturesexamined, a seems t o depe nd very weakly (slightly decreasing) on PET .In a separate set of experiments the effect of external transport phenomena onthe selectivity t o ethylene oxide was studied. First, it was necessary to determ inethe range of flowrates within which external transport phenomena becamedominant. To this end, the rate of ethylene consumption was measured whilevarying the total volum etric flowrate between 160 and 720c m3/m in at 440C.Ethylene oxide was not introduced in the feed. The data are shown in Figure 6,

FIGURE Depend ence o f surface oxygen activity on P . P =O.l bar PET=0.0105 ar.


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E T H Y L E N E O X I D A T I O N O N S I L V E R

FIGURE 6 Effect of volumetric flowrate on the rate of ethylene consumption. T=440 C,Po 0 1 bar.

where the ratio APETIPETs plotted vs. the reciprocal flowrate l / Q . Here PET isthe partial pressure of ethylene in the reactor and APET PET,; PET ,where PET,iis the partial pressure of ethylene entering the reactor. A t 440C both rates rl andr2 Eqs. 7) and 8)) are practically first orde r in ethylene Stoukide s andVayenas, 1981). Henc e, on e may write

r, KIKETPETrz KZKETPET

o r

and consequently:

Therefore, in absence of diffusional effects, APETIPET lotted vs. Q - shouldgive a straight line. It can be see n in Figure 6 that this is true for flowrates greaterthan 250 cm3/min. At 440C and for Q values lower than 250cm3/min, transportphenomena have a significant effect on the measured reaction rates.Th e effect of external transport p henom ena on the selectivity S) to ethyleneoxide was studied by measuring reaction rates at various flowrates. Results aresummarized in Table I1 for 400 and 440C The rates r, and r2 were calculated


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M . S T O U K I D ES A N D S . P A V L O UT BLE

Etfect of flowrate on reaction rates and selectivity

from the equations:r , = r , - r - -P E TO- R T

and

The approximations in the above equations were based on kinetic datarepo rted previously Stoukide s and Vay enas, 1980, 1981), which indicate that therate r3 is negligible compared to the rates r , and r2.The selectivity S was calculated as

The effect of Q on r, and r2 as well as on S is shown in Figures 7 and 8. Withdecreasing flowrate, both rates r , and r2 decrease Figure 7 ) , but the decrease in r,is larger than the decrease in r2 and thus the selectivity decreases Figure 8 .


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ETHYLENE OXIDATION ON SILVER

FIG UR E 7 Effect of volumetric flowrate on rates of ethylene oxide and CO formation.P 0 005-0.0067 bar Po 0.085 bar.

FIGURE 8 Effect of volumetric flowrate on reaction selectivity. Po 0.085 bar P 0.005-0.007 bar.


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64 M. S T O U K ID E S A N D S . P A V L O UDISCUSSIONIn a ~ r e v io u s ommunication (Stoukides and V ayenas, 1981) a kinetic model forethylene oxidation on polycrystalline silver films was proposed. Th e experim entalresults resented here were obtained under similar temDerature and gas com~osi-tion and with the same reactor. Moreover, the catalyst preparation was identicalto tha t followed previously (Stoukides and Vayen as, 1980, 1981).According to the previously presented kinetic model two types of adsorbedoxygen are assumed, i.e. molecular, being responsible for the ethylene epoxida-t ion and atomic, being responsible for C 0 2 and H 2 0 formation. Ethylene andethylene oxide were assumed to compete for the same sites following Langmuiradsorption kinetics. Eth ylene oxide was considered to adso rb primarily as a dimer(Stoukides and Vayenas, 1980).How ever, since under the reaction condit ions the part ial pressure of ethyleneoxide was very small (PETo


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ETHYLE NE OXID ATIO N ON SILVER 65

where the values of K , , K , , K , , K E To and K , are the same as those shown inEqs. (10) through ( 1 4 ) .In presence of excess ethylene oxide the rates rco, and rETo can be written as:rco, rz r3 K 2 K ~ ~ P ~ ~K 3 K E T O P E T 0 ) 21 KETPET KETO (~ ETO),

and

FIGURE 9 Predicted rates of ethylene oxide formation ethylene oxide oxidation and ethylene deepoxidation at 400C. P 0.0105 bar. Po 0.1 bar.


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M STOUKIDES AND S. PAVLOU

FIGURE 1 Predicted rates of ethylene oxid e formation ethylene oxid e oxidat ion and ethylenedeep oxidation at 280C. P 0.01 15 bar Po 0. bar.

The values of r,, r,, r,, r and rco, predicted from Eqs. (7a), (8a), (9a),(23), and and (24) are shown in Figures 9 and 10 for 400 and 280 respectively.The experimentally observed rates rETo and r o , are also shown. A reasonableagreement between experimental and predicted values for r o , and rETo isobserved.In addition to the kinetic results the behavior of a should be in agreement withthat predicted by the model. Assuming that the measured open-circuit EMFreflects the activity of atomically adsorbed oxygen rather than any other type ofoxygen species present on the catalyst surface one can write a steady statematerial balance for atomic oxygen:~ ~ ( P o , ) ' ~ ( l -( 1 - E T ~ E T O )~ , e , ( l - ~ E T ~ E T O )

Y K ~ ~ E T ~ OK I ~ E T ~ O ~K ~ ~ O ~ E T O0 (25)


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M. STO U K I D ES A N D S . PA V LO U

FIGURE Depe ndenc e of surface oxygen activity on gas composition. P =0.0105 bar Po =0. bar.

mass and heat balance equations:

r, A H , r, AH, hA Tb T, (29)Th e subscripts b and s refer to values in the bulk and right above the surfacerespectively. Of course, the mass and heat transfer coefficients increase with theflowrate. If we assume the following dependence of the mass and heat transfercoefficients on the flowrate

we find that the best fit to the data is obtained for C , 1 78 q t 1.98


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E T H Y L E N E O X I D A T I O N O N SILVERT BLE

Predicted values in the range of external transfer limitations

C 3.41 x lo- and q2 1.96 when Q is expressed in cm3/min, K A is in m3/sand hA is in cal ls . K. Namely, it is found tha t both the mass and the hea t transfercoefficients are propo rtional t o th e squ are of th e flowrate. Th e predicted values ofrl r2 and S using the above expressions for K A and hA are shown in Table 111.Comparing these values with the ones obtained experimentally we may observethat in some cases there is a good agreement between the experimental and thepredicted values while in other cases the agreement is not so good. This may bedu e to a more co mplicated depe nde nce of th e mass and heat transfer coefficient,on the flowrate than th e on e expressed by Eqs. 30,3 1). Nevertheless, Eqs.30,31) can be used as approximations to derive qualitative conclusions on theeffect of external transfer limitations on the performance of the system. Figures12 and 13 show the reaction rates r and r th e selectivity S and the temperatureof the catalyst surface T as functions of the flowrate Q. Equations 28,29) and30,31) were used for computing these curves with the partial pressure ofethylene in the bulk being PET 6 bar. It can b e seen that as the flowratedecreases, the reaction rate rl decreases while r2 initially increases, goes through amaximum and then decreases. The temperature on the surface of the catalystincreases as the flowrate decreases and this causes the initial increase of r2 butthe partial pressure of ethylen e ne ar th e surface of the catalyst decreases and thisresults to decrease of r and r2 at low flowrates. Th e selectivity dec rease s at lowflowrates because of increase of th e temp erature on th e catalyst surface.


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M TOUKIDES ND S . PAVLOU

FIGURE 12 Predicted values of reaction rates r r in presence of significant diffusion limitations.

FIGURE 13 Effect of diffusion limitations on catalyst temperature and reaction selectiv~ty.


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E T H Y L E N E O X ID A T IO N O N S IL V E R 71Besides the temperature gradient formed between the bulk and the catalystsurface, one could think of some other phenomena that could contribute to thedrop in th e selectivity with decre asing flowrate. First of all, the approxim ations inEqs . (1 7,1 8) a re not necessarily valid in th e range of stron g diffusion limitations.T o that en d, th e rate of ethylene oxide oxidation r3 was measured ind ependentlyby introducing air and ethylene oxide in the reactor at flowrates and partialpressures of ethylene oxide similar to those shown in Table 11 The values of rfound were again two orders of magnitude lower than those of r and r2 under thesame flowrate conditions. Thus, the decrease in S shown in Figure 8 cannot beattributed to a relative increase in r in the region of low flowrates.Also at low flowrates the gas phase mixing in the reactor might be incompleteand th e assumption of C STR behavior may no longer hold. This could co ntributeto the decrease in r and r2. Using the infrared C 0 2 analyzer (Stoukides andVayenas, 1980) we obtained the residence time distribution function over theflowrate rang e of 40-300cm3/min. It was found that even for th e lowest flowrate

used, the reactor behavior was very close (< 5 deviation) to that of the CSTR.Thus, the observed phenomena could not be at tributed to deviation from idealCSTR behavior.An other phenom enon that could contribute to the drop of the selectivity withdecrease of th e flowrate is the inhibiting effect that C 0 2 has on t he r ate ofethylene epoxidation r, as reported by Stoukides and Vayenas (1981). As theflowrate decreases, the reaction conversion increases and thus the C 0 2 con-centration in the reactor increases. Moreover, the diffusion rate between thecatalyst surface and the bulk decreases upon decreasing the flowrate. This causesa further increase of the C 0 2 concentration near the catalyst surface. Due to theinhibiting effect of C 0 2 on r,, the selectivity is expected to decreas e. How ever,the inhibiting effect of C 0 2 becom es impo rtant at lower tempe rature s and is ofminor significance at temperatures greater than 400C. This implies that theinhibiting effect of C 0 2 contr ibutes very little to th e de crease in th e selectivity.Therefore, we may conclude that the external temperature gradient is mainlyresponsible fo r the ob served drop in the selectivity with decre ase of the flowrate.An internal temperature gradient may also have some effect in the present case,but as it has been stressed in various analyses (Carberry, 1975; Hutchings andCarberry, 1966) and experimental observations (Butt et al. 1977; Kehoe andBu tt, 1972) for gas-solid catalytic reactions, the predominan t tem pera turegradient occurs in the external film and the catalyst itself may be taken asinternally isothermal. Also, the pore size of the Ag catalyst used in the presentstudy was quite large 2 pm) and internal diffusion was found to be negligible(Stoukides and Vayenas, 1980, 1981).In conclusion, the present work shows that the observed phenomenon ofconsiderable decrease in the selectivity in the regime of low flowrates can bemainly attributed to the difference in activation energies of the two reactions r,and r2 and to th e tempera ture gradient that is formed between the bulk and th ecatalyst surface. T he inhibiting effect of C 0 2 on r and temperature gradientsinside the catalyst pores may also contribute to the selectivity drop to a minorextent. Due to the industrial importance of ethylene oxide production from


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72 M . S T O U K ID E S A N D S. P A V L O Uethylene, it is highly desirable to minimize the production of CO z, i.e. m aximizethe selectivity. Presence of external mass and heat transfer limitations causes asubstantial decrease in selectivity. Therefore, care should be taken to operate thereactor in the range of condit ions where external transport phenomena are notimportant.

N O M E N C L A T U R ESurface oxygen activityMass and heat transfer area, m2Electromotive force, voltsFaraday s con stant, 96,500 Cb gr equivalent-Total molar flowrate, molelsHe at trans fer coefficient, cal s- K- m-2Mass transfer coefficient, m -Kinetic rat e constants, mole s-Adsorption coefficients for ethylene and ethylene oxiderespectivelyAdsorption and desorption rate constants for atomic oxygenAdsorp tion coefficient for atomic oxygenPartial pressure of species i , barVolum etric flowrate, cm31minRate of reactions of ethylene epoxidation (i I) , ethylene deepoxidation i 2) and ethylene-oxide oxidation i = 3Ideal gas constant, 1.987 cal mole- K-Selectivity to ethylene oxide (moles of ethylene oxide producedper mole of ethylene consumed)Absolute temperature, K

reek Symbolsdimensionless coefficient , y(KzIKl)Z1 (Stoukides and Vayenas,1981)

PET PET i PET i.e. inlet minus outlet partial pressure of ethylene inthe reactori coverage of species i

Po2 chemical potential of oxygen~ck,(g) standard chemical potential of oxygen in gas phase


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ETHYLENE OXIDATION ON SILVER 73ACKNOWLEDGEMENTSWe gratefully acknowledge the National Science Foundation for financial supportthrough NSF Grant CPE 82-15085.We also thank Prof. C.G. Vayenas for hisuseful comme nts on this work.

REFERENCES1. Barteau , M.A ., and Ma dix, R.J., The C hemical Physics of Solid Surfaces and HererogeneourCatalysis , D.A. King and D.P. Woodruff, Editors. Vol. 4, (1982).2. B utt, J.B., Downing, D.M ., and Lee, J.W., Inter-Intraphase Tempe rature Gradients in Freshand Deactivated Catalyst Particles , Ind. Eng. Chem. Fund., 16 270 (1977).3. Cam pbell, C.T., and Paffett, M.T ., Model Studies of Ethylene Epoxidation Catalyzed by theAg .(llO ) Surface , Surf Sci. 139 96 (1984).4. C ant , N.W., and Hall, W.K., Oxidation of Labeled Olefins over Silver , I . Catal. 52 81 (1978).5. Carberry, J.J ., Kuczynski, G. C ., and Martinez, E ., On the Influence of y-Irradiation upon

Catalytic Selectivity . I Catal. 26 247 (1972).6. Carb erry, J.J., On th e Relative Importance of External-Internal Tem peratu re Gradients inHeterogeneous Catalysis , Ind. Eng. Chem. Fund. 14, 129 (1975).7. Etsell, T.H ., and Flengas, S.N ., Overpotential Behavio r of Stabilized Zirconia Solid ElectrolyteFuel Cells , I Electrochem. Soc. 118(12), 1890 (1971).8. Force, E.L., and Bell, A.T ., The Relation ship of Adsorbed Species Observed by InfraredSpectroscopy to the Mechanism of Ethylene Oxidation ove r Silver , I . Catal. 40 356 (1975).9. Ghazali, S., Park, D.W ., and Gau , G., Kinetics of Ethylene Epoxidation on a Silver Catalyst ,Appl. Caral. 6 195 (1983).10. Gran t, R.B ., and Lambert , R.M., (i) Ethylene O xide Isomerization on Single-Crystal Ag. (111)in Atomically Clean and Cs-Moderated Conditions , I Catal. 93 32 (1985). (ii) A S ingleCwstal Studv of the Silver-Catalned Selective Oxidation and Total Oxidation of Ethvlene , I.c i a / . 92 381 (1985).11. Giir, T.M ., and Huggins, R.A., Electrocatalytic Synthesis of Methane on Stabilized Zirconiafrom H X O , Mixtures . Solid Slate lonics 5. 563 (1981).12. H arr io t , P . , The ox ida tion of E thylene us ing h w b n Different Supports , I. Caral. 21 56(1971).13. Haul, R ., Neubauer, G., Fischer, D., Hoge, D., and Zeeck, U., Kinetic and TD S Studies on theSilver Catalyzed Ethene Oxidation , Proc. Int. Co ngress on Catalysis, Berlin, Vol. 111 265(1984).14. Hayes, K.E., The Role of Reaction Products in the Silver Catalyzed Oxidation of Ethylene ,Can. I. Chem. 38 2256 (1960).15. Hutchings. J. and Carberry, J.J ., The Influence of Surface Coverage on Catalytic Effectivenessand Selectivity. The Isothermal and Nonisothermal Cases , AIC hE lour nal U 0 (1966).16. John son, D.L ., and Verykios, X.E., Selectivity Enhanceme nt in Ethylene Oxidation EmployingPartially Impregnated Catalysts , I Catal. 79 156 (1983).17. Kehoe, J.P .G ., and Butt, J.B ., lnteractions of Inter- and Intraphase Gradients in a DiffusionLimited Catalytic Reaction , AIChE lournal 18, 347 (1972).18. Kilty, P.A ., and Sachtler, W.M .H., The Mechanism of the Selective Oxidation of Ethylene toEthylene Oxide , Cat. Rev.-Sci. Eng. 10 1 (1974).19. Kilty, P.A., Rol, N.C.. and Sachtler, W.M .H., Identification of Oxygen Complexes Adsorbedon Silver and their function in the Catalytic Oxidation of Ethylene , 5th Int. Congress onCatalysis, M iami, Paper 64 (1972).20. Kumm er, J.T., The Catalytic Oxidation of Ethylene to Ethylene Oxide Over Single Crystals ofSilver , I. Phys. Chem. 60 666 (1956).21. Ostrovskii, V.E., Kulkov a, N.V., Lopatin, V.L.. and Temk in, M.I., The M odifying Action ofAdditives to the Ethylene Oxidation Catalyst , Kiner. Karol. 3 183 (1962).22. Pancharatnam, S ., Huggins, R. A. , and Mason, D.M., Catalytic Decomposition of Nitric Oxideon Zirconia by Electrolytic Removal of Oxygen , I. Elecrroch. Soc. U2 69 (1975).23. Sachtler, W.M.H., Backx, C., and Van Santen, R.A., On the Mechanism of EthyleneEpoxidation , Car. Rev.-Sci. Eng. 23 127 (1981).


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74 M. STOUKIDES AND S. PAVLOU24. Stoukides, M., and Vayenas, C.G ., Solid Electrolyte Aided Study of the Ethylene OxideOxidation on Silver , I Catal. 64, 18 1980).25. Stoukides, M., and Vayenas, C.G. , Solid Electrolyte Aided Study of the Ethylene Ox idation onPolycrystalline Silver , I Carol. 69, 18 1981).26. Tem kin, M.I., Oxidation of Ethylene into Ethylene Oxide , Adu Car. 28, 230 1979).27. Vayenas, C.G., Georgakis, C., Michaels, J., and Tormo, J . , The Role of Pro, in the IsothermalRa te Oscillations of Ethylene Ox idation on Platinum , I Carol. 67, 348 1981).28. Vayenas, C.G., and Farr, R .D. , Cogeneration of Electric Energy and Nitric Oxide , Science208 533 1980).29. Voge, H.H., and Adam s, C.R., Catalytic Oxidation of Olefins , Adu Caral. 17, 154 1967).30. Wach s, I.E. , and Kelemen, S.R ., The Interaction s of Ethylene and Ethylene Oxide with SurfaceCarbonate and Hydroxide Intermediates on Silver , I Carol. 68, 213 1981).31. Wag ner, C., Adsorb ed Atom ic Species as Intermed iates in Heterogen eous Catalysis , AduCarol. 21 323 1970).

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STK-PAPYR-125-ETHYLENE-in-CEC-V-44-ISS-1-6-PP-53-74-Y-1986