J . Phys. Chem. 1986, 90,4839-4843 same trend in specific activity. But whether this is an intrinsic geometric effect, an effect of preferential deactivation of certain sites, or due to metal-support interactions is a difficult matter to discern based on the data available to this point. That the support plays an important role is emphasized by the steady-state activity data in Table VI for poorly dispersed iron catalysts showing Fe/C to be about as active as unsupported Fe but significantly more active than Fe/AI2O3 and Fe/Si02. Effects of Dispersion and Support on the Selectivity Properties of Fe/C. The product distribution data in Table IV provide further evidence of crystallite size and/or support effects. Although the hydrocarbon product distributions (methane excepted) are nearly equivalent for the 3% and 10% catalysts, the production of C 0 2 and CH4 is higher on the 10% catalyst of lower dispersion. More importantly the increase in olefin to paraffin ratio (Table IV) with increasing crystallite size is significant and large enough that it probably outweighs any effect that differences in percentage C O conversion might have had on the olefin to paraffin ratio. Indeed, while increases in C O conversion have been shown to decrease the olefin to paraffin r a t i ~ , ~the . ~ data J ~ in Table I11 show that despite a higher C O conversion on the larger crystallites, the olefin to paraffin ratio increased. However, the trend observed in this study is contrary to that reported by Jung et al.,3 namely, decreasing olefin to paraffin ratio with increasing crystallite size. This may be due to the different reaction conditions in the two studies and the fact that the data of Jung et al. were not obtained at steady state; nevertheless, both sets of data indicate that crystallite size does affect olefin selectivity. While Jung et al.1-3determined product distributions for similar Fe/C catalysts at similar temperatures and pressures, since their H 2 / C 0 ratio of 3 was significantly higher from that of the present study and their data were not a steady state, the hydrocarbon selectivify data cannot be quantitatively compared. Nevertheless, comparison of the data of this study for Fe/C with steady-state data for other unpromoted Fe catalysts (Table VI and ref 37) (37) Rankin, J. L.; Bartholomew, C. H., in preparation.
4839
indicates that Fe/C is more selective for olefins than unsupported (unpromoted) iron, Fe/AI2O3, and Fe/Si02, in agreement with Jung et al.1-3
Conclusions 1. Fe/carbon catalysts are highly selective for olefins compared to unpromoted Fe/alumina and Fe/silica. The olefin/paraffin ratio increases with increasing crystallite size. 2. The specific activity of Fe/C decreases significantly with decreasing iron crystallite size and with decreasing iron metal loading both initially and at steady state. 3. Well-dispersed Fe/carbon catalysts deactivate rapidly at a H 2 / C 0 ratio of 2; indeed, the activity of 1% Fe/carbon is lowered by a factor of 50 within 24 h of reaction compared to a factor of 10 for 10% Fe/carbon. 4. The significantly lower activity of 1% and 3% Fe/carbon catalysts may be attributed at least in part to electronic modifications in the small iron clusters due to interaction of the metal and support. However, effects of crystallite decoration by the support, promotion by Fe oxides, or structure sensitivity cannot be ruled out.
Acknowledgment. We gratefully acknowledge financial support from the Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences (Contract No. DE-AC0281ER10855) and technical assistance by Richard D. Jones in obtaining and reducing the Mossbauer data and by Wilford M. Hess in obtaining TEM data. Registry No. Fe, 7439-89-6; C, 7440-44-0; CO, 630-08-0; H,, 133374-0. (38) In this paper the periodic group notation in parentheses is in accord with recent actions by IUPAC and ACS nomenclature committees. A and B notation is eliminated because of wide confusion. Groups IA and IIA become groups 1 and 2. The d-transition elements comprise groups 3 through 12, and the p-block elements comprise groups 1 3 through 18. (Note that the former Roman number designation is preserved in the last digit of the new numbering: e g , I11 3 and 13.)
CO Oxidation over Rh and Ru: A Comparatlve Studyt D. W. Goodman* and C. H. F. Pedent Surface Science Division, Sandia National Laboratories, Albuquerque, New Mexico 871 85 (Received: February 26, 1986)
The oxidation of CO over Ru and Rh single crystals has been studied in a high-pressurereaction-high-vacuum surface analysis apparatus. Steady-state catalytic activity as a function of temperature and partial pressure of CO and O2has been measured. Both the specific rates and the relative activity (Ru > Rh) obtained in this study compare very favorably with results found for high area supported catalysts. Surface concentrationsof oxygen were monitored following reaction and found to be dependent on the partial pressures of the reactants. For Ru, the highest rates of reaction correspond t o reaction on a Ru surface covered with a monolayer of oxygen as detected subsequent to reaction by Auger spectroscopy. For Rh, the reaction was found to be inhibited by an oxygen covered surface. These results suggest an explanation for the relative activities (Rh > Ru) observed in ultrahigh vacuum measurements for this reaction on clean surfaces of Ru and Rh.
Introduction F~~both practical and fundam&tal reasons, the heterogeneous catalytic oxidation of carbon monoxide has received much recent attention. The removal of CO as C 0 2 from automotive exhaust is accomplished by a catalytic converter utilizing the supported noble metals, Pt, Pd, and Rh. Because of the importance of this This work, performed at Sandia National Laboratories, was supported by the US.Department of Energy under Contract DE-AC04-76DP00789. *Present address: Inorganic Materials Chemistry Division, Sandia National Laboratories, Albuquerque, N M 87185.
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process, numerous studies of the kinetics as well as investigations into the effects of supports and additives on the kinetics of this reaction Over supported catalysts have been reported.'-* Fur(1) Cant, N. W.; Hicks, P. C.; Lemon, B. S. J . Catal. 1978, 54, 372. (2) Kiss, J. T.; Gonzalez, R. D. J. Phys. Chem. 1984, 88, 892. ( 3 ) Kiss, J. T.; Gonzalez, R. D. J. Phys. Chem. 1984, 88, 898. (4) Oh, S.H.; Carpenter, J. E. J . Catal. 1984, 80,472. (5) Yao, Y.-F. Y. J. Catal. 1984, 87, 152. (6) Okamoto, H.; Kawamura, G.; Kudo, T. J . C a r d . 1984, 87, 1 . (7) Su, E. C.; Watkins, W. L. H.; Gandhi, H. S. Appl. Catal. 1984, 12, 59.
0 1986 American Chemical Society
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thermore, the relative simplicity of this reaction on a metal surface makes it an ideal model system of a heterogeneous catalytic process, a process involving molecular and dissociative (atomic) adsorption, surface reaction, and desorption of products. Since each of these reaction steps are tractable for study by most modern surface probes, the full complement of surface science techniques has been directed intensively toward CO oxidation and was provided valuable information aiding in our understanding of the elementary surface processes related to this r e a ~ t i o n . ~ - * ~ A recent review of CO oxidation by Engel and Ert19 discusses the present understanding regarding the interaction of gaseous CO and 0, with a metal surface, mutual interactions among the adsorbates, and kinetic measurements of the CO oxidation reaction on well-defined single-crystal surfaces. The correlation between these idealized experiments at low pressure and studies on realistic systems at or near atmospheric pressures is also made. There remains, however, at least one anomaly in the comparison between the ultrahigh vacuum (UHV) single-crystal data and those data obtained for supported catalysts. In a survey study,' similar activation energies (-24 kcal/mol) and kinetic orders (+I in O2 and -1 in CO) were found for supported Pd, Rh, and Ir catalysts. (For Ru, however, a more complex kinetic behavior was observed.) Further, Ru showed the highest activity for this reaction of the several metals studied.] In contrast, UHV measurements of the relative activities of Pd, Pt, Rh, Ir, and Ru toward this reaction have shown Ru to exhibit the lowest activity.'~".'* The recent development of the surface science/kinetic hybrid approach for the study of heterogeneous catalysis has led to significant new insights into the nature and concentration of surface species present during catalytic reaction.23 This approach involves the coupling of a microcatalytic reactor for the measurement of reaction kinetics at realistic pressures with a traditional UHV apparatus for surface analysis. This paper described kinetic studies, using these techniques, of the CO oxidation reaction over single crystals of R u * and ~ Rh.*5326 The results for Rh and Ru are compared with each other and with the corresponding data for supported catalyst^^^^**^ under similar conditions. These comparisons verify the relevance of the single-crystal models as well as show the importance of coupling in the same apparatus kinetic studies with surface analytical techniques. The results of these studies resolve a long-standing question regarding the ordering of a~tivities'~'~.'~ in UHV studies (Rh > Ru) and in studies of supported catalysts' at atmospheric pressures (Ru > Rh). First, the kinetic data for the model systems are compared with the corresponding data for the supported catalysts. Secondly, the response of the kinetics over these two materials to changes in the partial pressures of CO and O2 will be discussed. Finally, (8) Bennet, C. 0.Catalysis under Transient Conditions, Bell, A. T., Hegedus, L. L., Ed.; American Chemical Society: Washington, DC. 1982; ACS Symp. Ser. No. 178, 1. (9) Engel, T.; Ertl, G. Ado. Catal. 1979, 28, 1. (IO) Creighton, J. R.: White, J. M. Catalysis under Transient Conditions, Bell, A. T.; Hegedus, L. L., Ed.; American Chemical Society, Washington, DC, 1982; ACS Symp. Ser. No. 178, 33. (11) Lee, H.-I.; White, J. M. J . Catal. 1980, 63, 261. (12) Savchenko, V. I.; Boreskov, G. K.; Kalinkin, A. V.; Salanov. A . N. Kinet. Catal. 1984, 24, 983. (13) Madey, T. E.; Engelhardt, H. a.; Menzel, D. Surf: Sci. 1975,48,304. (14) Fuggle, J. C.; Madey, T. E.; Steinkilberg, M.; Menzel, D. Surf. Sci. 1975, 52, 521. (15) Reed,P. D.; Comrie, C. M.; Lambert, R. M. Surf. Sci. 1977, 64, 603. (16) Thomas, G. E.; Weinberg, W. H. J . Chem. Phys. 1979, 70, 954. (17) Lee, H.-I.; Praline, G.; White, J. M. Surf. Sci. 1980, 9 / , 581. (18) Parrot, S. L.; Praline, G.; Koel, B. E.; White, J. M. Taylor, T. N. J . Chem. Phys. 1979, 71, 3352. (19) Praline, G.; Koel, B. E.; Lee, H.-I.; White. J. M. Appl. Surf. Sri. 1980, 5, 296. (20) Stuve, E. M.; Madix, R. J.; Brundle, C. R. Sufi. Sei. 1984, 146, 155. (21) Campbell, C. T.; Paffett, M. T. Surf. Sei. 1984, 143. 517. (22) Kellogg, G. L. J . Catal. 1985, 92, 162. (23) Goodman, D. W. Acc. Chem. Res. 1984, 17, 194. (24) Peden, C. H. F.; Goodman, D. W. J . Phys. Chern. 1986, 90, 1360. (25) Oh, S. E.; Fisher, G.B.; Carpenter, J. E.; Goodman, D. W. J . Catal., in press. (26) Peden, C. H. F.; Blair, D. S.; Goodman, D. W.; Fisher, G. B.; Oh, S. E., submitted for publication.
Goodman and Peden
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surface analytical data obtained following reaction will be described for Rh and Ru. These data, taken as a function of the partial pressure of reactants, show marked differences between Rh and Ru regarding the nature of the catalytic surface under optimum reaction conditions.
Experimental Section The studies to be discussed were carried out utilizing the specialized apparatus described in ref 23. This device consists of two distinct regions, a surface analysis chamber and a microcatalytic reactor. The custom-built reactor, contiguous to the surface analysis chamber, employs a retraction bellows that supports the metal single crystal and allows translation of the catalyst in vacuo from the reactor to the surface analysis region. Both regions are of ultrahigh vacuum construction, bakeable, and capable of ultimate pressures of less than 2 X Torr. Auger spectroscopy (AES) is used to characterize the sample before and after reaction. The single-crystal catalysts, 1 cm diameter X 1 mm thick, were aligned within '/," of the desired orientation. Thermocouples (W/5% R e W / 2 6 % Re for Ru and chromel-alumel for Rh) were spotwelded to the edge of the crystal for temperature measurement. Details of sample cleaning procedures are given in the references accompanying the relevant data. All reactants were initially of high purity; however, further purification procedures are used to improve the gas quality. These typically include multiple distillations for condensables and/or cryogenic scrubbing using a low conductance glass-wool-packed trap at 80 K. For the studies discussed here, the CO was cryogenically scubbed to remove any residual carbonyls and/or hydrocarbons. The kinetic data presented were obtained under steady-state reaction conditions and at low conversions (typically < 5 % ) . C 0 2 production was measured by a gas chromatograph equipped with a flame ionization detector (FID). CO and C 0 2 were first catalytically converted to methane before detection by the FID. Rates of reaction are expressed as turnover frquencies (TOF), i.e. C 0 2 molecules formed per surface metal atom per second. Since CO is known to decompose on transition metals when exposed to an electron beam,?' only surface oxygen could be measured by AES. This analysis was accomplished after flashing the crystal to 600 K to remove adsorbed CO.
-
Results and Discussion Figure 1 compares the CO oxidation rate measured over single crystals of Rh and Ru with supported Rh/A1,0,25 and Ru/Si02 c a t a 1 y ~ t s . I ~The ~ turnover frequencies for the Rh/A1,0, and Ru/SiO, catalysts were obtained by normalizing the measured reaction rates to the total number of Rh and Ru surface atoms. (27) Fuggle, J. C.; Umbach, E.; Feulner, P.; Menzel, D. Surf. Sci. 1977, 64, 69.
The Journal of Physical Chemistry, Vol. 90, No. 20, 1986 4841
CO Oxidation over Rh and Ru
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Notice that the turnover frequencies for the single-crystal catalysts traverse four orders of magnitude over a temperature range of 450-600 K. Kinetic measurements over such a wide temperature range with supported catalyst are not possible due to heat and mass transfer limitations encountered at high temperatures. Thus a direct comparison of the kinetic data between the two types of catalyst must necessarily be limited to a relatively small temperature range. Nevertheless, it is clear from Figure 1 that there is excellent agreement between the model and supported systems in both the specific reaction rates and apparent activation energies (26 kcal/mol for R h ( l l 1 ) vs. 30 kcal/mol for Rh/A120325and 20 kcal/mol for Ru(0001) vs. 22.5 kcal/mol for Ru/SiO:). These results indicate that the kinetics of CO oxidation on Rh and Ru are not sensitive to changes in catalyst surface morphology. Futher, it should be noted that the activity measured for the Ru single crystal, at T < 500 K, is significantly greater than that obtained for the Rh( 111 ) catalyst, in agreement with the relative activities of the corresponding supported metal catalysts.' Under the conditions of Figure 1, the surface is predominantly covered with COZ5so that the reaction rate is limited by the adsorption rate of oxygen. As the temperature is increased, the reaction rate increases because more vacant sites become available for oxygen adsorption as a result of the higher desorption rate. Therefore, it is not surprising to observe in Figure 1 that the CO oxidation rate increases with temperature with an apparent activation energy similar to that for CO desorption. Figure 2 shows the dependence of the rate of CO oxidation over the single-crystal Rh and Ru catalysts on the partial pressure of CO. For Rh, the reaction rate is observed to decrease linearly with increasing CO partial pressure reflecting the domination of reactant surface coverage by CO. For these reaction conditions, this behavior has been accurately modeled25with individual elementary reaction steps established from surface science studies of the interactions of CO and O2 with Rh. The results for Ru are more complex than those for Rh. At low CO partial pressures,
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pressure of CO (16 Torr) and temperature (500 K) were held constant. the dependence is approximately positive first order in CO. This changes to negative first order at CO partial pressures above approximately 16 Torr. The rates of CO oxidation as a function of oxygen partial pressure at the same catalyst temperatures as the measurements above and at a fixed CO partial pressure are shown in Figure 3. The rates for the Rh(ll1) catalyst exhibit a first-order dependence on the partial pressure of O2at Po2 < 100 Torr. At higher oxygen partial pressures, however, the rate is observed to roll over with a maximum activity occurring at approximately O2/CO = 30. In the case of Ru, at low oxygen partial pressures (Figure 3b), the reaction is positive order in 0,. The slope of the curve changes for 0, pressures above approximately 4 Torr, becoming zero order in 0,;that is, further increase in O2pressure has no effect on the rate of reaction. Surface concentrations of oxygen following reaction were monitored by AES and found to be dependent on the partial pressures of the reactants. This dependence is shown for Rh and Ru in Figure 4 for varying CO partial pressures. For Rh, the surface oxygen coverage remains near zero for CO partial pressures from 1 to 200 Torr. For Ru, the surface oxygen level is constant at low CO partial pressures, falling at pressures above of that observed at low about 16 Torr to a level approximately CO pressures. The dependence of the surface oxygen coverage on the partial pressure of oxygen is shown in Figure 5 . For Rh( 111) little change in the postreaction surface oxygen level with varying O2 pressure is observed with AES. The surface oxygen coverage, subsequent to the thermal desorption of CO, is essentially zero even at 0, pressures sufficient to attenuate the overall activity. Surface oxygen is believed to be present at high surface coverages under reaction conditions leading to activity roll over but is subsequently removed during the cooling period of the catalyst in the reaction mixture and the following CO flash off. Results for Rh(100) (Figure 4a), at O2pressures resulting in activity roll over, confirm that significant surface oxygen is present at these reaction conditions. In the case of Rh(100), the more open
The Journal of Physical Chemistry, Vol. 90, No. 20, 1986
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P I< a
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Ru) compard to elevated pressure measurements (Ru > Rh) on supported catalysts.
Acknowledgment. The authors thank the Department of Energy, Office of Basic Sciences, Division of Chemical Sciences, for partial support of this work. We also thank Kent Hoffman and Dianna Blair for their technical assistance. Registry No. CO, 630-08-0; Ru, 7440-18-8; Rh, 7440-16-6. (28) Zhu,
Y.;Schmidt, L.D.Surf. Sci.
1983, 129, 107.
ZEOLITE AND TECHNIQUES Gallozeolite Catalysts: Preparation, Characterization, and Performance John M. Thomas* and Xin-Sheng Liu Department of Physical Chemistry, Uhiuersity of Cambridge, Cambridge CB2 IEP, U. K . (Received: December 20, 1985; In Final Form: March 13, 1986)
Highly silic,eous porotectosilicates exemplified by silicalite-I1(the framework topology of which is structurally identical with that of ZSM-11) can be readily transformed into gallosilicate (zeolitic) catalysts. Proof that Ga3+ions replace Si4+in the parent framework when silicalite-I1is exposed to aqueous solutions of NaGa02 rests on XRD, IR, 29Siand ”Ga MASNMR, electron microscopy (TEM, SEM, electron diffraction, electron-induced X-ray emission), gas adsorption (N2and Xe) studies, and chemical analysis. Good yields of benzene, toluene, and xylene are obtained when a feedstock of n-butane is exposed to the gallosilicate catalysts at ca. 550 OC.
1. Appreciation Paul Emmett influenced the work of innumerable chemists from every corner of the globe: his scientific contributions, rich in their diversity and deep in their analysis, are of lasting importance. For a period of some 6 or so years in the mid-1970s we (PHE and JMT) met intermittently at the laboratories of the W. R. Grace Co. in Maryland. There are many reasons why I shall never forget those encounters. To meet the “E” of BET was itself a novelty; to be exposed to his encyclopedic knowledge of catalysis was to be reminded of the schoolmaster in Oliver Goldsmith’s Deserted Village: “And still they gazed, and still the wonder grew, that one small head could carry all he knew”. But, above all, it was 0022-3654/86/2090-4843$01.50/0
his gentle manner, his kindness, friendship, and his humor that shone through. I am grateful for the opportunity to contribute to this memorial issue; I believe that the work that my Chinese colleague (Xin-Sheng Liu) and I have been doing in the design and characterization of new catalysts would have interested Paul Emmett, who, toward the final stages of his life, showed a keen interest in the catalytic performance of zeolites. 2. Introduction Even though the significance of gallium in heterogeneous catalysis awaits fuller clarification, it is already apparent that it can play a key role in the conversion of light alkanes to gasoline of 0 1986 American Chemical Society
