4474
J. Phys. Chem. 1988, 92, 474-4478
mation of micelles and their subsequent growth as a function of surfactant concentration in an aqueous medium. Acknowledgment. E.Y.S. thanks Dr. J. S. Huang of Exxon Research & Engineering Co. for using his computer facility to analyze part of the SANS data presented here. He also thanks
Mr. J. Sung for purification of AOT surfactant for this study. This research is supported by a National Science Foundation grant administered through MIT Center for Material Science and Engineering. Registry No. SDS, 151-21-3; GOT, 577-11-7; DDAO, 1643-20-5.
Isotopic Identification of Surface Site Transfer on Ni/AI,O, Catalysts Paul G. Glugla, Keith M. Bailey, and John L. Falconer* Department of Chemical Engineering, University of Colorado, Boulder, Colorado 80309-0424 (Received: November 23, 1987; In Final Form: January 26, 1988)
Isotope labeling with temperature-programmed reaction (TPR) for CO hydrogenation was used to separate two distinct CO adsorption sites on a Ni/A1203 catalyst. One site is Ni metal and has the higher activity for CO hydrogenation. The less-reactive CO is on the A1203support. Only the Ni metal is occupied at 300 K, and transfer of CO between the two sites occurs at higher temperatures. In the presence of adsorbed Hz, CO that was adsorbed on the Ni metal moved to the A1203support. This is an activated process, and the only pathway to occupy the A1203sites is by adsorption on the Ni. The reverse transfer from A1203to Ni occurs if some of the H2 is desorbed; surface hydrogen inhibits this reverse process. These results show that during a typical TPR experiment, transfer between sites competes with reaction.
Introduction Nickel supported on y-alumina exhibits adsorption and reaction properties different from nickel on a silica support. For C O hydrogenation to methane, temperature-programmed reaction (TPR) has been used previously to observe two distinct types of sites on Ni/Al2O3 catalysts.',2 Only one type of site exists on Ni/Si02 for C O methanation. The sites on Ni/Si02 and one type of site on Ni/AlZO3are due to reduced Ni. Kester and Falconer' proposed that adsorbed CO could move between the reduced Ni sites (called A sites) and the second type of site (called B sites), and they proposed that this process was assisted by surface hydrogen. Recent TPR studies on Ni/AlZO3indicate that the B sites ~ studies for Ni,3,4R U , Rh,6 ,~ are on the A1203~ u p p o r t . ~Infrared and Pd7 on A1203are consistent with the T P R studies, and they show that C O and Hz are adsorbed on the A1203support as a formate or a methoxy species. Some of these studies also concluded that surface species move between the metal and the A1203.3*677 The purpose of this investigation was to examine the A1203sites on a Ni/A1203catalyst and study the transfer between sites. Temperature-programmed reaction, in combination with isotopic labeling, was found to be an effective way to study the properties of the A1203sites. Carbon monoxide on the Ni sites could be reacted away without reacting away the A1203sites. This was done by interrupting the TPR temperature ramp at the minimum point between the A and B peaks. A major innovation in this study was to cool the sample after interruption and then adsorb 13C0on the Ni sites while leaving the A 1 2 0 3 sites occupied by ITO. This technique allowed communication between the two reaction sites to be studied. This technique also showed that the sites were not saturated under normal experimental conditions, and adsorption at elevated temperatures, in the presence of H2, was found to be necessary to saturate the sites. Thus in previous TPR experiments, only a fraction of the catalyst surface was covered with CO. Moreover, in previous TPR studies, transfer between sites was occurring in competition with reaction to methane. (1) Kester, K. B.; Falconer, J. L. J . Coral. 1984, 89, 380. (2) Kester, K.B.; Zagli, E.; Falconer, J. L. Appl. Catal. 1986, 22, 31 1. (3) Lu, Y.;Xue,J.; Li, X.;Fu, G.; Zhang, D. Cuihuu Xuebao (Chinese J. Catal.) 1985, 6, 116. (4) Mirodatos, C.; Praliaud, H.; Primet, M. J . Catal. 1987, 207, 275. (5) Dalla-Betta, R. A.: Shelef. M. J. Caral. 1977. 48. 111. (6) Solymosi, F.; Bansagi, T.; Erdohelyi, A. J . Catal. 1981, 72, 166. (7) Palazov, A.; Kadinov, G.; Boney, C.; Shapov, S . J. Catul. 1982, 74,44.
The procedures described for isotope separation are general ones that can be applied to significantly improve our understanding of adsorbed species on surfaces. In a subsequent paper,* we have used isotope labeling in combination with temperature-programmed desorption of coadsorbed C O and H2to relate the reaction sites to adsorption sites.
Experimental Section The temperature-programmed reaction (TPR) system was similar to that described previo~sly.~A 100-mg catalyst sample was located on a frit in a 1-cm 0.d. quartz down-flow reactor. An electric furnace was used to heat the catalyst at 0.7 K/s for most of the TPR experiments. A constant rate of heating was maintained by feedback from a small thermocouple located in the catalyst bed. Hydrogen flowed over the catalyst at 1 atm of pressure, and the gas residence time in the bed was less than 0.07 s. The effluent from the reactor was analyzed with a quadrupole mass spectrometer. A computer system allowed multiplexing between mass peaks. Carbon monoxide (%O or I3CO) was adsorbed by injecting 0.2-mL pulses into a Hz or He carrier gas that flowed through the catalyst bed. Adsorption was done at 300 or 385 K by repeated injections; in most experiments pulses were continued until no additional adsorption was detected. Some Ni(C0)4 may form during CO exposure at 385 K, but not much Ni was removed since a series of repeated experiments yielded the same TPR spectra. A 10% C O in H e (UHP, Matheson) gas was used for I2CO adsorption. The 13C0was from Monsanto Research Corp. and was specified as 99.25% carbon monoxide, of which 99.6% was labeled with 13C. The Hz and He carrier gases were UHP grade (Matheson) and were further purified. A 5.1% Ni/Al2O3 catalyst was prepared with nickel nitrate on Kaiser A-201 y-alumina by the same wet impregnation as described in the literature.' The nickel nitrate was directly reduced in H2without first calcining. The Ni loading was measured by atomic absorption. Temperature-programmed reaction was carried out by adsorbing carbon monoxide on the surface and then heating the catalyst in Hz while detecting I2CH, (mass 15) or I3CH4(mass 17) and I2CO or 13C0with the mass spectrometer. The cracking fraction of 13CH4at mass 15 was subtracted from the mass 15 (8) Glugla, P. G.; Falconer, J. L.; Bailey, K. M. J . Cutal., accepted for publication. (9) Falconer, J. L.; Schwarz, J. A,, Catal. Reu. Sci. Eng. 1983, 25, 141.
0022-3654188 12092-4474%01.50/0 0 1988 American Chemical Societv
The Journal of Physical Chemistry, Vol. 92, No. 15, 1988 4475
Surface Site Transfer on Ni/Al2O3 Catalysts
Procedure Reduce, cool in H2 - Pulse '*CO at 300 K - Heat to 460 K, cool - Pulse'3COat300K
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Temperature (K) Figure 1. Methane TPR spectra for CO adsorbed at 300 and 385 K in H2 on 5.1% Ni/A120, catalyst. The insert shows, on an expanded y axis (and a reduced x axis), the CHI spectra for 300 K adsorption. The
heating rate was 0.7 K/s. signal to obtain the I2CH4signal. Since water was a product of methanation, the cracking fraction of water was subtracted from the mass 17 signal to obtain the I3CH4signal. The water product adsorbed on the Alz03, however, and only desorbed above 600 K, where the rate of methane formation was small, and thus the cracking correction was small over the range where methane formed. The adsorption temperature and the adsorbate gas (lZCO or I3CO) were varied, and interrupted reactions, in which some carbon monoxide was removed from the surface by hydrogenation, was used. The details of each type of experiment will be described in the Results to avoid duplication. Calibrations were done with high-purity CH4 and CO.
Results Routine TPR. The procedure for obtaining a standard TPR was as follows.' After a 2-h pretreatment in Hz at 785 K, the catalyst was rapidly cooled to 300 K in Hz by blowing air around the outside of the quartz reactor. The catalyst was exposed to C O by injecting 0.2-mL pulses of 10% C O in He over the catalyst. Almost all the CO in the first pulses adsorbed; the amount that did not adsorb was detected immediately downstream of the reactor with the mass spectrometer. The catalyst was assumed to be saturated when several pulses in succession produced the same mass spectrometer signal. Approximately 12 pulses (1 rmol/pulse) were used to saturate the catalyst at 300 K. The catalyst was then heated in the Hz carrier gas a t 0.7 K/s. The resulting CH, spectra is shown in Figure 1. This curve (seen more clearly in the insert of Figure 1) is identical with that previously reportedlo and quite similar to that seen for a catalyst with similar weight loadings.' Lowering the adsorption temperature to 263 K did not change the spectra. Two distinct CHI peaks were observed. The A peak is a t 443 K, and the B peak is a t 540 K. The B peak is significantly larger than the A peak, and the total amount of methane was 60 rmol/g of catalyst. Pumped TPR. When C O was adsorbed at 385 K instead of 300 K, the amount of adsorbed C O increased dramatically, and the resulting TPR (Figure 1) was quite different. Moreover, the adsorption process behaved quite differently. As observed at 300 (10) Bailey, K.M.;Chai, G.-Y.;Falconer, J. L. "Catalysis: Theory and Practice"; Proceedings of the 9th International Congress on Catalysis, 1988; Vol. 3 , p 1090.
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Figure 2. Methane (12CH4and I3CH4)TPR spectra for sequential adsorption of I2CO and I3CO in H2 on 5.1% Ni/A120, catalyst. The surface was saturated with 'VOat 300 K, and peak A was reacted away
before I3CO absorption at 300 K. The heating rate was 0.7 K/s.
K, approximately 12 pulses were required to saturate the surface at 385 K. After 4-5 min in Hz a t 385 K,however, the catalyst was no longer saturated. Note that this was well below the temperature where methane formed. The catalyst was exposed to a second series of 12 pulses, and again the surface appeared saturated. Again after 4-5 min the catalyst was not saturated. This is distinctly different from the behavior seen at 300 K, where the catalyst was saturated after 12 pulses, and it did not become unsaturated even after hours in Hz flow. Thus to saturate at 385 K, a new saturation procedure was adopted. The catalyst was exposed to 2 pulses/min until saturation was observed. This took about 40 min, Le., about 80 pulses. After the pulsing was finished, the catalyst was cooled, and the TPR proceeded in the standard fashion. The process of injecting 2 pulses/min will be referred to as pumping since CO appears to be pumped from a saturated to an unsaturated site. The large curve in Figure 1 is the result of the pumped TPR. Two peaks are seen, just as in the TPR for adsorption at 300 K. Both peaks grew, so that the total amount of CHI was 310 pmollg. That is, adsorption at 385 K increased the amount of CH4 by a factor of 5 . The A peak shifted to higher temperature and the B peak to lower temperature, so that the peaks overlapped and were difficult to distinguish. The use of isotopes, described in the next subsections, showed that two peaks are indeed present and that the distribution of methane in the peaks changed drastically to favor the B peak. Interrupted TPR. This experiment was done in an attempt to separate the peaks by filling site A with 13C0while leaving I2CO on the B site. The I2CO was pulsed over the catalyst until saturation was obtained at 300 K. The catalyst was heated to 460 K in Hz, to react away CO in the A site, and then quickly cooled to 300 K. The minimum between the A and B peaks occurs at approximately 460 K. Then, 13C0was pulsed until saturation was obtained. The resulting TPR yielded the IZCH4and I3CH4 peaks shown in Figure 2. The I2CH4exists almost exclusively in the B peak; only a small shoulder exists on the low-temperature side. That is, the I2CO on the B sites did not move to the A sites. In contrast, the 13C0, which was adsorbed on the sites that were reacted away during heating to 460 K (Le., the A sites), reacted to form I3CH4in both the A and B peaks. Moreover, the amount of 13C0that adsorbed was almost the same amount that adsorbed at 300 K on a freshly reduced surface. Thus, more total CH4 (40 pmol of I2CH4/g, 65 pmol of I3CH4/g) formed than for a routine TPR. Note also
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The Journal of Physical Chemistry, Vol. 92, No. 15, 1988 I
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I Procedure - Reduce - Switch to He, cool - Switch to H? at 300K - Pulse ' ~ C O - Heal to 460 K
Procedure - Reduce, cool in H2 - Pulse I2CO at 385 K - Heat to 460 K, cool - Pulse % O a t 385 K
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Temperature (K) Figure 3. Methane (I2CH4and "CHI) TPR spectra for sequential adsorption of I2CO and "CO in H2 on 5.1% Ni/A1203 catalyst. The surface was saturated with ' T O at 385 K, and peak A was reaated away before I3COadsorption at 300 K. Note that the rate scale is 10 times that of Figure 2. The heating rate was 0.7 K/s. that while the 13CH4appeared similar to the routine TPR, the B peak was smaller. Interrupted Pumped TPR. In this TPR, an attempt was made to separate the peaks by reacting off the A peak and filling it with I3CO after first saturating the catalyst at 385 K by the pumping procedure. One might expect, if the B site was truly full, that C O transfer from the A site would be impeded. After saturation of the surface at 385 K, the catalyst was heated to 460 K in H2 to react off the l2C0in the A peak, and the catalyst was quickly cooled to 300 K. Then, I3CO was pulsed to saturation. This required approximately 12 pulses. The results of this interrupted pumped TPR are shown in Figure 3. The I2CH4is found only in the B peak. The I3CH4is mostly contained within the A peak, but a small amount of bleed-over into the B peak occurred. Note that the scale in Figure 3 is 10 times that in Figure 2. The I2CH4amplitude is 10 times that in Figure 2, and the A peak is more than twice as large as in Figure 2. The I2CH4peak is 250 pmol/g, and the 13CH4,including the bleed-over, is 70 Fmol/g. Carbon monoxide and carbon dioxide signals were observed during this TPR, but less than 1% of the adsorbed CO desorbed as C O or C 0 2 . As a variant to this experiment, the I3CO was pulsed at 385 K, and the results were identical with those reported here. Interrupted Pumped TPR Cooled in Helium. In the previous experiments, no transport of I2CO from peak B to peak A was observed. Cooling in He, however, demonstrates that communication between the two peaks is indeed open in both directions. After pretreatment at 785 K, the carrier gas was switched to He before cooling the catalyst. Once at 300 K the carrier gas was switched to H2, and the normal pulsing sequence was performed in H,. The catalyst was heated to 460 K to remove site A. At 460 K however, the carrier gas was again changed to He before cooling. Once at 300 K, the carrier gas was switched to H2 and I3CO was pulsed to saturation. At this point, the normal TPR heating sequence was initiated. As shown in Figure 4, this TPR was drastically different from the interrupted TPR in Figure 2. The I2CH4was present in both the A and B peaks. Similar to Figure 2 the 13CH4was seen in both peaks. The ratio of the A peak to the B peak was also affected. In Figure 2 the B peak is substantially larger than the A peak. In Figure 4 the peak amplitudes are of comparable size. Note that for I2CH4the A peak is slightly smaller than the B peak, and for I3CH4the A peak is slightly larger than the B peak. That
Temperature (K) Figure 4. Methane (I2CH4and I3CH4)TPR spectra for sequential adsorption of I2CO and I3COin He on 5.1% Ni/AI2O3catalyst. The main
difference from Figure 2 is that the catalyst was cooled in He after pretreatment and after peak A had been reacted away. The heating rate was 0.7 K/s. is, the I2CO still has some preference for the B sites. The total I2CH4found in both peaks was 15 pmol/g, and the total I3CH4 was 40 pmol/g. Two variants of this experiment was carried out. First, the C O pulsing was done in H e at 300 K, and second, the C O pulsing was done at 385 K in He. Both of these variants produced TPRs similar to Figure 4. The second variant demonstrates that no pumping occurred in He. That is, at 385 K, in the absence of H2,the B site could not be saturated with CO. The size of the B peak is smaller than the B peak of the routine TPR, and the A peak is larger. This trend has been reported previously for C O adsorption following cooling in He.lJo The ratio of B to A peaks or Figure 4 is between that seen for a routine TPR and for a TPR following cooling in He.
Discussion The purpose of this study is to use temperature-programmed reaction with isotopic labeling to study the nature of the surface site transfer processes for CO and H2on Ni/A1203catalysts. Since the two sites can be filled with different isotopes without interchange of these isotopes, these experiments clearly show that the two TPR peaks are indeed due to the presence of two distinct sites on the Ni/A1203 surface. A recent study of Ni/A120, catalysts3 related the two TPR peaks to different structures in the IR spectra. Lu et aL3 also showed that two distinct sites were present. Their B peak, which was filled by heating the catalyst in a CO/H2 mixture, was seen in their TPR at approximately the same temperature as our B peak. They attributed the B peak to hydrogenation of a formate species, which may have formed on the metallic Ni and spilled over to the Al2O3. More recently, Mirdatos et al." made a similar assignment. Our TPD studies8 verify that the B peak is due to hydrogenation of a surface compound, but the stoichiometry is that of a methoxy group instead of a formate. Infrared studies on R u , ~Rh,6 and Pd7 have also observed the slow buildup of surface formate on the A1203 surface during exposure to CO/H2 mixtures at elevated temperatures. The formate was assigned to the A1203surface because of its low activity and because formate forms, at an even slower rate, on pure AI2O3.' On Pd/A1203, methoxy groups were detected on
Surface Site Transfer on Ni/A1203 Catalysts the A1203under some conditions and formate was seen under other conditions? Huang et al." attributed the B peak to nickel aluminate, which forms during catalyst preparation and was detected on Ni/Al2O3 by X-ray photoelectron spectroscopy. The fact that an aluminate was present on the Ni/A1203 is not sufficient justification, however, for concluding that the B site was the aluminate. The similarity between the IR spectra for Ni, Ru, Rh, and Pd on A1203argues that adsorption is not on the aluminate. Moreover, a B peak was also seen during TPR on Ru/A1203,12 and Ru does not form an aluminate. Thus, in agreement with ref 3 and 4, we conclude that the B sites are on the A1203 surface. Under the appropriate conditions we find that adsorbed C O can move between the A sites (Ni metal) and B sites (Alz03)on the surface. Carbon monoxide on Ni moves to A1203 in the presence of adsorbed hydrogen. The reverse process occurs only in the absence of hydrogen. Carbon monoxide adsorbs on the Ni at 300 K, and the A1203sites fill by transfer from the N i at a higher temperature. This transfer is activated because it occurs only in Hz and because H2 adsorption is activated. The following discussion will justify this description of the surface processes on Ni/A1203. W e will refer to CO as the surface species because our results indicate that the carbon is still bound to oxygen. The adsorbed CO appears to be present in other forms on the Al,03,g* but we will refer to C O on the surface in the rest of this discussion for simplicity. Surface Coverage. Previous ~tudies'J*'~,'~ assumed that the TPR experiment reflected saturated behavior when the catalyst was saturated with C O at 300 K. Figure 1 shows that this is not the case. For a routine TPR, the catalyst was less than 20% saturated a t 300 K. Pulsing C O at a temperature just below where the methanation rate was significant, Le., 385 K, produced much larger amounts of adsorbed CO. The isotope separation for adsorption at 385 K indicates that this procedure produces conditions close to saturation since little "CH4 was present in the B peak (Figure 3). Site Transfer. Figure 2 provides insight into how the sites fill. The l2C0, which was adsorbed first, was observed only on the A1203site (as lZCH4). In the process of reacting off the CO from the Ni by heating to 460 K, no lzCO from the A1203 sites transferred to the Ni. Furthermore, the same amount of 12CH4 was produced from the A1203sites in the interrupted TPR as was produced from the Alz03sites in the routine TPR. This suggests that not much l 2 C 0 was removed from the A1203sites during the process of reacting off the Ni. Instead, as shown by the 13CH4 spectra, the A1203sites were partially filled from the Ni during the heating to 460 K. The 13CH4produced in the interrupted TPR was observed in both the Ni and A1203sites. This may occur because the I3CO simply adsorbed on both Ni and A120, sites at 300 K. However, the l 2 C 0 was exposed to the catalyst at the same conditions and should have saturated the AI2O3 sites if this were the case. Furthermore, the amount of I3CH4seen in the interrupted TPR in Figure 2 is the same as the amount of 12CH4seen in the routine TPR. A consistent explanation is that only the Ni sites fill at 300 K,and during heating the CO on the Ni sites transfers to the Alz03 sites. That is, the process is activated; the rate is slow at 300 K but much faster at 385 K. This is not just an example of normal activated adsorption since waiting at constant temperature allows more adsorption to occur, as seen during adsorption for the pumped TPR. Thus, the Ni sites are the only source of C O for the Al,03 sites, and the number of the Ni sites limits the amount of methane seen for one filling. If this model is correct and if the number of A1203sites is much larger than the number of Ni sites, then the interrupted experiment could be repeated and each time more CO would transfer to the A1203sites. The same amount of C O should adsorb on the vacant (1 1) Huang, Y . J.; Schwarz, J . A,; Diehl, J. R.; Baltrus, J. P. Appl. Catal. 1988, 36, 163.
(12) Sen, B.; Falconer, J. L. J. Catal., in press. (13) Huang, Y.-J.; Schwarz, J. A. Appl. Catal. 1987, 30, 239. (14) Huang, Y.-J.;Schwarz, J . A. Appl. Catal. 1987, 32, 45.
The Journal of Physical Chemistry, Vol. 92, No. 15, 1988 4477
0
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Figure 5. Methane TPR spectra for CO adsorbed at 300 K in H2on 5.1% Ni/A1203catalyst. Conditions are the same as for Figure 1 insert except that the heating rate was 0.07 K/s.
Ni sites each time the experiment is repeated. Indeed, exactly this was observed. Moreover it should be possible to totally empty the N i sites into the A1203 sites if given enough time. Two experiments were carried out that confirmed this: (1) Carbon monoxide was pulsed at 300 K to saturation, and the catalyst was held at 385 K for a few minutes prior to the TPR. A temperature of 385 K is below the temperature where methane forms. During the subsequent TPR, no peak A methane was seen, and the B peak was the same size as the sum of the A and B peaks in a routine TPR. If heating to 385 K and subsequent adsorption at 300 K was carried out 2, 3, or 4 times before the TPR, then 2, 3, and 4 times as much CH4 formed as in a routine TPR. (2) A routine TPR was carried out at a lower heating rate (0.07 K/s). Only peak B was observed, as shown in Figure 5, and the amount of methane from the A1203sites was equal to the amount from the Ni and A1203sites in a routine TPR. The rate of transfer from Ni to A1203slows as the A1203sites become saturated. Figure 3 clearly demonstrates this point. At 385 K,most of the I2C0 that is adsorbed on the Ni sites transfers to the A1203sites. If one pulses slowly enough to allow the Ni sites to empty, then the A1203sites can be saturated. This was done by pulsing I2C0at 2 pulses/min for 40 min. At this point, the C O on the Ni sites was reacted off by heating to 460 K in Hz and the catalyst was cooled. The Ni sites were then refilled with ' 3 C 0at 300 K. The subsequent TPR (Figure 3) produced little transfer of 13C0from Ni to Alz03,Le., only a small I3CH4 peak was observed from the A1203sites. Since the A1203sites, when saturated, have a finite methanation rate at 460 K, the small amount of 13CH4seen is most likely due to the number of A1203 sites that were reacted off at 460 K. Also, the Ni peak was approximately the same size as the combination of the two peaks in the routine TPR (Figure 1). These experiments all indicate that the A1203sites in Figure 3 were close to saturation. These isotope studies clearly show that CO on Ni can transfer to the A1203support. Previous studies found that formate could form directly on A1203:-6 but the rate was found to be much faster when a metal was present.6 Most authors have concluded that the formate forms on the metal and transfers to the A1203.336*7 Our studies directly verify that the initial adsorption is on the Ni for Ni/AI2O3catalysts. Role of Hydrogen in Site Transfer. The experiments in which C O was pulsed in He are an interesting contrast to those for pulsing in Hz. The interrupted TPR, in which the catalyst was heated to 460 K and the carrier gas then switched to He so that H, could desorb from the surface,15 shows a quite different dis(15) Bailey, K. M.; Falconer, J . L., manuscript in preparation.
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The Journal of Physical Chemistry, Vol. 92, No. 15, 1988
tribution of isotopes, as shown in Figure 4. The I2COon the A1203 had the opportunity to move onto the N i while at a temperature above 300 K. Thus, unlike the experiments where the catalyst was cooled and exposed to CO in H2, some I2CO was on the Ni. Since the only source of the I2COis on the A1203 (all the I2CO originally on the Ni was either transferred to the A1203or reacted to I2CH4),this experiment clearly demonstates communication between the Ni and A1203sites. Indeed, the observed peaks appear to be the result of almost complete mixing between the sites. The absence of gas-phase H2 during cooling from 460 K must open new pathways for CO to transfer from A1203to Ni. The amount of I2CH4is smaller in Figure 4 than in Figure 2 for two reasons. Less I2COadsorbed in He than in H2,I0and the additional time at 460 K (while the carrier gas was switched from H2 to He) allowed additional l2C0 to be removed by both reaction and desorption.s An attempt was made to enlarge the A1203peak by pulsing CO in He at 385 K.N o enhancement of the A1203peak was seen. The fact that the A1203peak could not be pumped in He suggests that H2 must be present to fill the A1203site. This is in agreement with the IR studies3-' that concluded a formate or methoxy group is present on the A1203. Hydrogen must be present to form these groups. We believe that the A1203sites were filled to their observed level because of the H2present during the interrupted TPR and the H2 available during the final TPR. Note that unlike the interrupted TPR, for which subsequent adsorption was done in H2, the total amount of I3CH4in Figure 4 was significantly smaller than the number of Ni sites present on the surface. Note also that the total amount of methane (I2CH4plus I3CH4)was nearly the same as was seen in the routine TPR. It seems logical that the Ni sites will totally fill in both this experiment and in the routine TPR. The only way that the Ni sites can be filled, given the amount of methane that was observed, was to have all of the I2CO segregate to the Ni sites prior to pulsing I3CO. That is, the I2COon the A1203transferred back to the Ni almost completely prior to pulsing so that only 40 pmol/g of I3CO could be adsorbed. The H2 that was adsorbed while cooling was the H2 required for transfer. If the catalyst was cooled in H2, then adsorption at 300 K in either H2or He gave the same result, Le., Figure 1 insert. For T P R following cooling in He, less C O adsorbed and much less peak B was present during TPR. Moreover, if the catalyst was cooled in He, then subsequent adsorption in H2 or H e gave the same result. These results suggest a possible reason for the lack of transfer from A1203to Ni sites when the catalyst was continuously exposed to H2 after pretreatment at 785 K. When C O is removed from the Ni by reaction to methane, it is replaced with hydrogen. The C O on the A1203must exchange with this hydrogen or displace it to move to the Ni. Carbon monoxide from the gas phase is able to replace or react with the hydrogen on Ni; CO from A1203 apparently cannot. For the He-pulsed experiments, H2 desorbs at 460 K in He and the Ni sites are thus empty and available for
Glugla et al.
CO to occupy. Transfer from A1203 to Ni can occur without exchange or displacement. Summary The distribution between sites depends on H2 pressure and reaction temperature because the competing processes of site transfer and reaction have different activation energies. Without isotopic labeling, it would be difficult to verify the distinct nature of the sites, particularly if the activity of the sites were somewhat closer than on Ni/A1203. Indeed, we have observed on catalysts prepared by other methods that the two sites overlap sufficiently that they can only be distinguished with isotopes.16 Previous studies by Huang and SchwarzI4 observed that for some low-weight loading catalysts, only the A1,03 peak was present. They used a heating rate of 0.3 K/s, and as was shown here, at low heating rates the C O in the Ni sites can completely transfer to the A1203before the reaction on Ni reaches a significant rate. That is, the activation energy for transfer is smaller than that for methanation on the Nil since at 385 K transfer is faster and at 443 K methanation is faster. Thus the TPR studies of Huang and Schwarz are consistent with the present studies. We also carried out a few experiments on a 0.74% Ni/AI2O3 catalyst and observed the same pumping phenomena reported here for a 5.1% Ni/A1203 catalyst. Conclusions Isotopic labeling shows clearly that two distinct sites for C O adsorption (Ni and Al,03) are present on the surface of a Ni/ A1203catalyst. The number of A1203sites is 4 times that of Ni sites. At 300 K,only Ni sites are occupied when exposed to CO; the A1203sites are filled only by shuttling C O through the Ni sites. This transfer is an activated process. In previous TPR studies, the catalysts were only 20% saturated. During a typical TPR experiment, transfer of C O from Ni to A1203 competes with methanation of C O on Ni. Hydrogen must be present to fill A1203sites, and the reverse process, in which CO on A1203transfers to Ni, readily occurs in the absence of H2. Using isotopes to separate TPR peaks is a useful new technique that yields a rich spectrum of information about communication between surface sites. Acknowledgment. We gratefully acknowledge support by the National Science Foundation, Grant CBT 86-16494. We are grateful to Professor Keith B. Kester for carrying out some of the initial experiments that motivated these studies and to Professor Qi-Chang Shi for translating ref 3. Registry No. Ni, 7440-02-0; CO, 630-08-0; H2,1333-74-0. (16) Sen, B.; Kester, K. B.; Falconer, J. L., manuscript in preparation.
