Langmuir 1988,4, 1083-1090 This in turn suggests that adsorbed 0 or OH,rather than HzO, may be responsible for CO electrooxidation on these surfaces. This is consistent with interpretations of some electrochemical data.1° Interestingly, the SERS results differ from those observed for CO adsorbed on platinum and palladium films under similar conditions in that no high-frequency vc0 bands are obtained prior to CO electrooxidation on the latter surfaces. This difference is not entirely surprising since the rhodium and ruthenium surfaces are substantially more susceptible to oxidation and indeed enable CO electrooxidation to proceed at slightly lower overpotentials than on the platinum and palladium films.4 Generally speaking, then, although detailed interpretation of such spectra for electrochemical mechanistic purposes is speculative at best, we believe that these spectra point to the value of SERS for examining oxidizable species on transition-metal surfaces, especially in electrochemical systems. Obtaining time-resolved SERS by means of an optical multichannel analyzer (OMA) ar-
1083
rangement would be desirable in this regard since it would enable potential-dependent SERS to be obtained concurrently with cyclic voltammetric measurements. Although we have recently utilized this approach to examine a number of reactions involving adsorbed aromatic speci e ~less , ~satisfactory ~ data were obtained with the transition-metal surfaces as a result of the signal-to-noise restrictions with our present OMA arrangement. Most centrally, however, the results obtained so far indicate that the extension of SERS to transition-metal surfaces by electrodepositing them on thin films on gold is a tactic worthy of further consideration. Acknowledgment. This work is supported by the National Science Foundation. Registry NO. Au, 7440-57-5;Rh, 7440-16-6; Ru, 7440-18-8;CO, 630-08-0; HC104,7601-90-3; KOH, 1310-58-3. (24) Gao, P.; Gosztola, D.; Weaver, M. J. Anal. Chim. Acta, in press; J. Phys. Chem., submitted.
The Role of Preparative Variables on the Surface Composition of Supported Pt-Ru Bimetallic Clusters+ Saeed Alerasool, Dirk Boecker, Bahman Rejai, and Richard D. Gonzalez* Department of Chemical Engineering, University of Illinois at Chicago, Box 4348, Chicago, Illinois 60680
Gloria del Angel, Maximiliano Azomosa, and Ricardo Gomez Department of Chemistry, Universidad Autonoma Metropolitana, Iztapalapa, P.O.Box 55-534, Mexico 09340, D.F. Received December 9, 1987. In Final Form: February 10, 1988 The effect of catalyst pretreatment on the surface composition of silica-supportedPt-Ru bimetallic clusters has been studied. Surface enrichment in Pt is consistent with a model in which a mobile PtCls2-phase is envisioned to diffuse freely across the support. During pretreatment, the mobile Pt phase nucleates atop the immobile Ru surface phase. It was found that precalcination in air at 150 O C prior to reduction results in an increase in the surface concentration of Ru. The results of a differential scanning calorimetry study show that a bimetallic assisted coreduction process occurs in which both metals are simultaneously reduced at temperatures lower than those observed for the reduction of a Pt/Si02 catalyst. The reduction exotherms for the Pt/Si02 and Ru/Si02 monometallic catalysts are centered at 154 and 124 "C, respectively. The nature of the PtC&s41ica interaction was studied by using "in-situ" UV diffuse reflectance spectroscopy. Shifts in the position of the charge-transfer bands as a result of the adsorption of PtCb2-on silica are interpreted in terms of Cl- ligand exchange with the hydroxyl groups of the support. The hydrogenation of CO was studied over supported Pt-Ru bimetallic clusters of known surface composition. A statistical model based on a Ru ensemble size between 2 and 3 correctly predicts the rate of methanation as a function of surface composition. Introduction
It has been suggested that surface enrichment of one metal in preference to another in supported bimetallic clusters is strongly dependent on the relative heats of sublimation of the metals.lt2 Bimetallic clusters prepared from group Ib and group VIII metals generally substantiate these claims.- For example, supported bimetallic catalysts prepared by alloying either Ni or Ru with Cu have
* Author
to whom correspondence regarding this manuscript should be addressed. Presented at the symposium on 'Bimetallic Surface Chemistry and Catalysis", 194th National Meeting of the American Chemical Society, New Orleans, LA, Sept 1-3, 1987; B. E. Koel and C. T. Campbell, Chairmen.
been shown to have surface compositionsthat are strongly enriched in Cu. Recent studies performed by Wu et al.' using a Monte Carlo simulation method have shown that for particles having cubooctahedral symmetry the Cu atoms are preferentially located at sites that have low surface coordinations. These sites are identified with edges, steps, corners, and surface defects. The Ru atoms, on the other hand, are identified with high surface coordination sites (1) Gonzalez, R. D. Appl. Surf. Sci. 1984, 19, 113. (2) Williams, F. L.; Nason, D. Surf. Sci. 1974, 45, 377. (3) Sinfelt, J. L.; Garten, J. L.; Yates, D. J. C. J. Catal. 1972,24,283. (4) Haller, G. L.; Rasasco, D. E.; Wang J. Catal. 1983,84, 477. (5) Sinfelt, J. H. J. Catal. 1973, 9, 308. (6) King, T. S.; Donnelly, R. G. Surf. Sci. 1984,141, 417. (7) Wu, X.; Smale, M. W.; Gertstein, B. C.; King, T. S. Paper 15a; AIChE National Meeting, New York, November, 1987.
0743-7463/88/2404-l083$01.50/00 1988 American Chemical Society
1084 Langmuir, Vol. 4, No. 5, 1988
such as terraces. Because both Ru-Cu and Ni-Cu form endothermic alloys, Cu and Ru atoms are expected to form separated metal phases. The surface composition of these supported Ru-Cu bimetallic clusters has been experimentally determined by King using solid-state NMR spectroscopy.8 These results are in good agreement with studies performed by Sinfelt5and by Haller et al.? who suggest that supported Ru-Cu bimetallic clusters conform to a cherry model structure with an inner core consisting primarily of Ru and a peripheral skin consisting of Cu. Recent studies by McHugh et a1.: which make use of both EXAFS and the ethane hydrogenolysis reaction as catalyst characterization techniques, have found significant differences in the catalytic properties of the resulting Ru-Cu bimetallic clusters when different silicas are used as supporting materials. According to these authors these differences may, in some way, be linked to differences in the surface concentration of hydroxyl groups on the different silicas used. Catalyst characterization studies performed in our laboratory on a series of supported Pt-Ru bimetallic clusters prepared by coimpregnation of HzPtC16and RuC13 have revealed major differences in both the surface composition and particle morphology of these particles.'O These differences are linked primarily to the nature of the supporting material. On silica, Miura et al.'O have identified Pt-Ru bimetallic clusters which have a well-defined cherrylike structure and a core strongly enriched in Ru. Furthermore, the Ru content of the core is not a strong function of catalyst composition for overall compositions below 75 atom 9% Pt. When alumina was used as the support, the Pt content of the Ru core was shown to increase monotonically with the overall concentration of Pt in the catalyst. In a separate series of experiments, Miura et ala1'showed that when the supported Pt-Ru bimetallic clusters were prepared with HzPtClg6Hz0and RuC13-3Hz0 as metallic precursors the Pt phase was mobile with respect to the Ru surface phase. A model was developed in which Ru3+was anchored to the surface during the initial catalyst pretreatment while PtCls2- migrated freely across the support, nucleating atop Ru surface sites. Because PtChZinteracts more strongly with the surface of alumina than silica, the surface diffusion of the PtC16'- on alumina is more restricted. This restricted surface diffusion results in a decrease in the surface enrichment of Pt. Ru-Rh and Ru-Ir silica-supported bimetallic clusters prepared from the corresponding chlorides were found to have a homogeneous particle structure with little or no surface enrichment in either metallic component." It is noteworthy tha, like Ru, both Rh and Ir show little surface mobility during catalyst pretreatment. Presumably, this lack of surface mobility during the precursor state is linked to the cationic nature of the interaction between the metal precursor and the support. Because the interaction between PtC16'- and the support is anionic, and because it is also highly soluble in its water of hydration and the water formed as a result of the dehydroxylation of the support, its surface diffusivity is much higher than that observed for Ru, Rh, or Ir. Anhydrous RuC1, is nearly insoluble in water. These observations suggest that a second factor, related to the strength of metal-support interactions (8) King, T.S.,personal communication.
(9) McHugh, B.;Hong, A.; Resasco, D. E.; Haller, G. L. Paper 15b; AIChE National Meeting, New York, November, 1987. (10) Miura, H.; Suzuki, T.; Ushikubo, Y.; Sugiyama, K.; Matsuda, T.; Gonzalez, R. D.J. Catal. 1984,85, 331. (11) Miura, H.; Feng, S. S.; Saymeh, R.; Gonzalez, R. J. D. ACS Symp. Ser. 1985, 25,294.
Alerasool et al.
during catalyst pretreatment, may be as important as heats of sublimation in determining the surface composition and the morphology of supported bimetallic particles. It is noteworthy that the heat of sublimation of Pt is lower than that of Ru,12 and for this reason we may expect Pt to surface segregate. However, the heat of sublimation of Rh is nearly equal to that of Pt, and surface enrichment in Rh is not observed when it is alloyed with Ru." The heats of sublimation of Ir and Ru are nearly equal,lZ and, as expected, surface enrichment does not occur. In order to obtain further insight into the nature of the factors which influence surface enrichment in Pt, we have performed experiments aimed at probing the structure of the PtCh2-mobile precursor phase using in-situ diffuse UV reflectance spectroscopy. We have also varied catalyst pretreatment conditions in order to study their effect on the surface composition of the resulting silica-supported bimetallic clusters. Finally, we have performed experiments using both differential scanning calorimetry and gravimetric analysis to obtain a better understanding of the reduction sequence of the two metals which comprise the bimetallic cluster. Turnover frequencies for the rate of methanation over these well-defined bimetallic clusters will be discussed in terms of the Ru ensemble requirements on the surface of the cluster.
Experimental Section Catalyst Preparation. The silica-supported samples used in this study were prepared by direct impregnation. Initially the appropriate weights of H2PtClg4H20and RuCI3.3H20(Strem Chemical) were dissolved in an amount of doubly deionized water sufficient to ensure the complete wetting of the support. The solutions were mixed with Cab-0-Si1(Grade M-5, Cabot Corp.) in a dropwise manner by using a magnetic stirrer. The resulting slurry was either dried under vacuum or calcined in air at 150 "C prior to reduction. The surface area and the averagepore size of Cab-0-Sil, as reported by the manufacturer, were 200 m2/g and 14.0 nm, respectively. The dried catalyst was ground to a fine powder before use. Total metal loadings were 0.3 mmol of metal/g of Cal-0-Si1for both the monometallic and the bimetallic catalysts. This correspondsto a total loading of about 3 4 % by weight, ranging from monometallicRu to monometallicPt. On this basis, the total number of moles of metal are kept constant rather than the total metal weight loading. The supportedPt/Si02 catalysts characterized by diffuse UV reflectance spectroscopy were vacuum-dried at room temperature. Surface Composition Measurements. The apparatus and procedure were identical with those described in ref 13. Surface composition measurements were based on an 02-CO titration technique described by Miura and Gonzalez.ls The surface metal/Oz/C0 ratio was 1/1/1 for Ru/Si02 and 1/0.5/2 for Pt/Si02 These titration ratios were found to be independent of surface composition. Surface compositionsdetermined by the 02-C0 titration method have been verified by a variety of experimental techniq~es.'~Metal dispersions were obtained by the dynamic pulse method16using either H2,CO, or O2 chemisorption at 25 "C. Pretreatment conditions prior to the measurement of catalyst surface concentrationsby the 0,320titration technique were as follows: The catalyst was treated in flowing He (30 mL/min) while the temperature was increased to 120 O C at a rate of 10 "C/min for 0.5 h. The gas flow was then switched to H2at 30 mL/min, and the temperature was increased at a rate of 10 "C/min to the final reduction temperature of 400 "C. The catalyst was reduced at 400 OC in H2for 3 h. The catalyst was then cooled to room temperature in flowing He. It is important to stress that the initial pretreatment in He is essential to facilitate (12) Gates, B. C.;Katzer, J. R.; Shuit, G. C. A. Chemistry of Catalytic Processes; McGraw-Hill: New York, 1979; p 198. (13) Miura, H.; Gonzalez, R. D. J. Catal. 1982, 74, 216. (14) Miura, H.; Gonzalez, R. D. J . Phys. Ed. Sci. Instrum. 1983, 15, 373. (15) Sarkany, J.; Gonzalez, R. D. J. Catal. 1982, 76, 75.
Supported F't-Ru Bimetallic Clusters
Langmuir, Vol. 4, No. 5, 1988 1085
QJam window
I
Y
T m""ourvp1r
Sample Beam
Reference
m
0
20
40
M)
BO
100
Catalyst Composition. %Pt
Detector
Reference
&
-r3 Beam
Catalyst Bed Figure 1. (a, Top) UV reactor: top view. (h, Bottom) Diagram of the UV reactor and the integrating sphere.
the surface diffusion of the mobile phase consisting of the Pt precursor. Initial pretreatment in H, generally results in the reduction of the Pt precursor at temperatures as low as room temperature. Under these conditions, the surface diffusion of the Pt phase is strongly inhibited. Diffuse UV Reflectance Studies. A diagram of the UV reactor together with the integrating sphere attachment is shown in Figure 1. The reactor was specially designed for in situ measurements. It is made of stainless steel and has a gas inlet and outlet as well as a throughput for inserting an insulated thermocouple (diameter of 1.59 mm). The thermocouple was placed in direct contact with the catalyst surface. The reactor also featured a quartz window with a wavelength cutoff of 190 nm (Figure 1). A specially designed stand, equipped with a wpper block and a heating hand for heating the reactor to 500 "C, was constructed. The reactor was mounted on this stand throughout the course of the experiment. Because temperature control is crucial, a programming unit was used to accurately monitor the temperature as well as the heating rate. The integrating sphere was extemdy interfaced with the UV spectrophotometer(Perkin-Elmer, Model Lambda 5) by fiber optic cables. The reactor stand was designed to enable the mounting and securing of the intergrating sphere directly to the top of the reactor. Because of this geometrical arrangement the sphere was positioned directly above the reactor and the quartz window. This provided the essential capability of recording a spectrum at any time during the course of the reaction. The diffuse reflectance UV cell and stand were designed and built hy the Byron-Lambert Co. of Franklin Park, IL. The data were collected and stored in a Perkin-Elmer 3600 data station. This is a computer dedicated to the Perkin-Elmer Lambda 5 spectrophotometer. The data could he transferred to an IBM-AT personal computer for processing, using computer programs developed in our laboratory Differential Scanning Calorimetric Studies. A PerkinElmer DSC (Model DSC-7) was modified for continuous flow operation in order to study gas-solid reactions. These modifications were as follows: (1) the sample and reference outlet lines were separated, (2) the DSC was coupled to a quadrupole mass spectrometer (BalzersQMG-112)hy connecting the sample outlet to the mass spectrometerprobe, and (3) the Pt pan covers were perforated and the alumhum covers replaced by 80-mesh stainless steel screens in order to obtain a better gasaolid contact. It was found that optimum reduction conditionscould be achieved with
"
0
20
40
64
BO
100
Catalyst Composition, %Pt Fignre 2. (a,Top) Variation in surface composition with catalyst compwition. (h, Bottom) Variation in total dispersion with catalyst composition. a gas flow rate of 54 mL/min together with a heating rate of 10 "C/min. The concentrationof H, in the flow gas was 33.390,with the balance b e i i He. The baseline optimization and calibration of the temperature and the energy axes were performed under these conditions. Methanation Studies. Tumover frequencies for methane formation were measured at 227 'C by a procedure which was identical with that used in ref 13. Hydrogen and CO were premixed to a H,/CO partial pressure ratio of 3.
Results Effect of Pretreatment on Surface Composition. The results of this study are shown in Figure 2. Catalysts dried under strong vacuum conditions at room temperature showed a marked surface enrichment in Pt. The extent of this surface enrichment in Pt was far greater than that previously reported by Miura et al.,'O who had dried their bimetallic catalysts in a vacuum desiccator a t room temperature. When the catalysts were calcined in air at 150 "C prior to reduction, a much lower surface enrichment in Pt was observed. The effect of catalyst composition on total metal dispersion for both series of catalysts is shown in Figure 2h. It is apparent from this study that clustering as a result of alloy formation leads to a substantian decrease in particle size. This is particularly noticeable for the catalyst that had been dried in air a t 150 OC prior to reduction. The inhibition of Pt surface enrichment as a result of precalcination in air is interesting and, to our knowledge, has not been reported previously. It is well-known that
1086 Langmuir, Vol. 4 , No. 5, 1988
Alerasool et al. 5
0 3
E i
5
-5
2 -10
-15
40
80 120 160 200 240 280
40
Temperature, C
Temperature, C
Temperature, C
-15 40
80 120 160 200 240 280
40
80 120 160 200 240 280
80 120 160 200 240 280
I 40
80 120 160 200 240 280
Temperature, C
Temperature, C
Figure 3. DSC reduction exotherms for vacuum-dried bimetallic catalysts: (a) 100% Pt, (b) 75% Pt, (c) 50% Pt, (d) 25% Pt, (e) 100% Ru.
Ru is rapidly oxidized to Ru02 in the presence of air. Exposure of Pt to air at 150 "C will also result in the formation of an oxidic phase. However, it is not clear whether this oxide phase is PtO or simply chemisorbed OB Because it was possible that the transformation of the chloride precursor phase into an oxidic phase prior to reduction in H2may have had an important bearing on the surface composition of the resulting bimetallic clusters, a temperature-programmed reduction study using DSC was performed. The results of this study for the vacuum-dried catalysts are shown in Figure 3a-e. The reduction of Pt shows two exotherms centered at 104.3 and 154.2 "C. The reduction of Ru occurs at temperatures which are considerablylower than those observed for Pt. The high-temperature exotherm observed for Ru is centered at 124.3 "C. The reduction exotherms observed for the bimetallic clusters are shown in Figure 3b-d. From these reduction exotherms, it is clear that coreduction of both metals within the cluster occurs simultaneously. The reduction exotherm was observed to occur at a temperature well below that observed for Pt. In fact, for the catalyst which had a composition of 25% Pt the reduction exotherm was observed at 121.2 "C. This was 3 "C lower than the reduction temperature observed for Ru/SiOz. The position of the exotherms observed for the reduction of the supported Pt-Ru bimetallic clusters also shows that bimetallic clusters which contain dual sites are formed. If this were not the case, separate exotherms for the reduction of Pt and Ru would be expected. This is shown in Figure 4 for a mechanical mixture having a composition of 75 atom % Ru. The reduction exotherms clearly show the presence of distinct exotherms for Ru and Pt reduction at 127 and 152 "C, respectively.
--15 ' 40
.
.
.
O
L
80
120 160 200 Temperature, C
I
I
I
240
-
1
280
Figure 4. DSC reduction exotherms for a physical mixture of 25% Pt/Si02 and 75% Ru/SiOz.
The reduction exotherms obtained from the air-dried samples were very similar to those observed for the vacuum-dried samples. However, the position of the exotherms occurred at temperatures which were slightly lower than those observed for the vacuum-dried samples. The differential scanning calorimetric data for both sets of catalysts are summarized in Table I. Gravimetric Studies. In order to obtain additional insight regarding the reduction process, weight changes following reduction were obtained for both the pure precursors and the supported metallic and bimetallic catalysts. The weight loss for the pure precursors following reduction is based on the following reduction equations: H,PtC1,*4H20(~)+ 2Hz(g) Pt(s) 6HC1 4H20 (1) RuCly2.5HZO(s) + 1.5&(g) RU + 3HC1 + 2.5H20 (2)
-
-+
+
+
Langmuir, Vol. 4, No. 5, 1988 1087
Supported Pt-Ru Bimetallic Clusters 1
Table I. Reduction Temperatures and Compositions of Pt-Ru/Si02 Bimetallic Catalysts' overall comp, % Pt sets 1 and 2 100 75 50 25 0
surface comp, % Pt set 1 set 2 100 100 96.8 77.8 93.1 72.3 33.0 77.6 0 0
reduction set 1 154.2 139.9 138.6 121.2 124.3
T, O C set 2 150.5 139.9 132.4 121.2 111.2
'Set 1: dried under vacuum a t room temperature. Set 2: dried in flowing air at 150 O C . Table 11. Weight Changes of Supported Bimetallic Catalysts upon Reduction catalvst. % Pt Aw/w exDtl Aw/w theor 100 75 50 25 0
iao 75 50 25
0
450
0.075 0.067 0.059 0.049 0.039
Air-Dried Catalysts 0.069 0.039 0.038 0.070 0.049
0.008 0.009 0.009 0.009 0.009
510
530
550
570
590
Wavelength (nm)
UV reflectance spectra.
The number of waters of hydration for HZPtC&and RuC13was based on the manufacturer's reported Pt and Ru percentages for the compounds. For reactions 1and 2, the calculated weight loss (Awlw) following reduction should be 0.60 and 0.58, respectively. The experimental weight losses actually obtained were within 5% of the predicted values, clearly indicating complete reduction. Assuming complete conversion of Pt and Ru to the oxides following precalcination in air at 150 "C, the relevant reduction equations are PtOz(s) + 2H&) Pt + 2Hz0 (3) +
+
490
Figure 5. Effect of metal loading on the resolution of the diffuse
Vacuum-Dried Catalysts 0.090 0.080 0.099 0.080 0.073
RuOz(s) + 2Hz(g)
470
RU + 2HzO
(4)
For reactions 3 and 4, the calculated weight losses (Aw/w) following reduction are 0.14 and 0.24. The actual weight losses observed for the supported catalysts following the conclusion of each DSC experiment, together with their theoretical values, are given in Table 11. These results suggest that the vacuum-dried catalysts are completely reduced in accord with eq 1 and 2. The Catalysts which were precalcined at 150 "C could not have been completely converted to the oxides. If this had been the case, the weight loss upon reduction would have been substantially lower than that which was actually observed, i.e.., chlorinated species must still be present on the surface following precalcination in air at 150 "C. The rather large weight losses observed for the supported catalysts are undoubtedly due to additional weight losses arising from the dehydroxylation of the support. Diffuse UV Reflectance Studies. The results of the differential scanning calorimetry experiments taken together with the gravimetric measurements clearly suggest that precalcination in air at 150 "C does not convert the chloride precursors into the corresponding oxides. The positions of the reduction exotherms observed for the vacuum dried samples are only slightly different from those obtained from the air-dried samples. However, the surface composition measuremepts obtained following different catalyst pretreatments are very different. In order to gain more insight into the precursor-support interactions a UV
study of the mobile PtC162: phase was performed. In order to obtain meaningful UV diffuse reflectance spectra, it is important to resolve UV absorbance bands corresponding to both the d-d transition region and the charge-transfer region of the spectrum. Although the PtCb2- complex is one of the most investigated platinum compounds, results concerning ligand field energy level diagrams are rather uncertqin.16J7 The reason for this is that Pt(1V) complexes usually absorb at higher wavelengths. For this reason, d-d transitions are for the most part covered by strong charge-transfer bands. However, since in reflectance spectra smaller extinctions appear somewhat enhanced as compared with larger ones,l* the reflectance spectra should be more highly resolved in the d-d transition region compared to the solution spectra. Previous studies of PtC&* performed by Lieske et al.19and by Lietz et a1.20have failed to resolve absorbance bands in the d-d transition region. The effect of total metal loading on the resolution of the UV diffuse reflectance spectra is critical. The results shown in Figure 5 show improvements in the resolution of the d-d absorbance bands in the 450-600-nm spectral region when the Pt loading is reduced from 10 to 0.2 wt. %. It is apparent from these results that the most highly resolved diffuse reflectance spectrum was obtained for a 0.2% metal loading. In order to minimize specular reflectance relative to diffused reflectance, it was found best to record the spectrum at low metal loading corresponding to smaller particle sizes. A comparison of the UV spectral region of interest for PtCb2- adsorbed on silica to that of the solution spectrum of PtClG2-is shown in Figure 6. Two intense chargetransfer bands are observed for both the diffuse reflectance spectrum and the solution spectrum of PtC16'-. These bands are Laporte-allowed transitions from the ground lAl, state to the lTlustate and involve ligand-metal electronic transiti0ns.l' The transition giving rise to the 261-nm band is due to electrons involved in the bonding of the 7r orbitals of the ligand. The second band, centered at 201 nm, is due to ligand u 0rbita1s.l~ In the d-d transition region, six bands are observed in the reflectance spectra. The bands centered at 560,531, (16) Jorgensen, C. K. Acta Chem. Scand. 1956,10, 500. (17) Swihart, D.L.; Mason, W. R. Znorg. Chem. 1970, 9, 1749. (18) Kortum, G.Reflectance Spectroscopy;Springer-Verlag: New York, 1969. (19) Lieske, H.; Lietz, G.; Spindler, H.; Volter, R. J. Catal. 1983, 81, 8.
(20) Lietz, G.; Lieske, H.; Spindler, H.; Hanke, W.; Volter, R. J. Catal. 1983, 81, 17.
1088 Langmuir, Vol. 4, No. 5, 1988
Alerasool et al. 210 n m
+
-
191
291
Solution
+
NaOH ( 2 5 C )
Solution
+
NaOH ( 9 0 C )
Supported SolJtion
49 1
39 1 Wavelength (nm)
59 1
191
211
231
251
271
291
311
331
Wavelength (nm)
Figure 7. Change in the W spectmh as a result of adding NaOH to an aqueous solution of PtCls2-at 25 and 90 "C.
-1
09
---
1
08
2 s t. _. 73 c 125 C
I
I
v) 0
n Q
06
I
450
470
490
510
530
550
570
590
Wavelength (nm)
210
Figure 6. (a, Top) Full UV spectra of H2PtC&on silica and in solution. (b, Bottom) Part of the d-d transition region in the UV spectrum of H2PtC&on silica and in solution. Table 111. Band Assignments for PtCl,? solution solution + NaOH + NaOH (90 O C ) on silica solution (25 " C ) CT bands: 264 205
-
261 201
262 210
211
473 497
473 497
473 497
527
527
527
56 1
56 1
561
(nm) lAlh
-
assignment
d-d bands: 355
473 497 525" 531 560
05
Shoulder.
525, 497, and 473 nm are tentatively assigned to spinforbidden transitions from the lAl, to the triplet 3T1gand 3T, states. The remaining two transitions were observed on the silica-supported catalyst and were tentatively assigned to spin-allowed transitions to the ITlg.and ITzg states. These transitions are, approximately,10 times more intense than the other d-d transitions. The above assignments are in fairly good agreement with those found by Swihart et a1.l' The UV bands observed in the diffuse UV spectrum of PtC16'- adsorbed on silica and in the solution spectrum are summarized in Table 111. The broad band centered at 355 nm is absent in the solution spectrum. This can be attributed to improved resolution achieved for PtC16'-/Si02 and is in all likelihood due to the higher sensitivity obtained for d-d transitions in the
260
310
360
410
460
510
560
Wavelength (nm)
Figure 8. In situ UV diffuse reflectance spectra of PtCb2-/SiO2 as a function of temperature under oxygen flow. diffuse UV reflectance spectrum.'* A comparison of the solution spectrum of P€Cb2-to that of PtC16'-/SiOz shows an upward shift in the position of the charge-transfer bands. The two charge-transfer bands centered at 261 and 201 nm in the solution spectrum were observed to shift to 264 and 205 nm for PtCbZ-/Si02 The position of the d-d transitions was virtually identical for PtC16'- in solution and PtC&'-/SiOz. The shift in the position of the charge-transfer bands on PtC&2-/SiOzis strongly indicative of a metal precursor-support interaction. Because the most likely way in which PtCb2-/SiOz can interact with the silica support is through an exchange between C1- ligands on the metal and OH- groups on the support, an experiment was performed in which NaOH was added to a solution of PtCbz-. A UV absorption spectrum taken at room temperature showed a substantial shift in the position of the charge-transfer band centered at 201-210 nm. This shift was precisely in the same direction as that observed when PtCb2- was adsorbed on silica. When the solution was boiled at high pH, only one charge-transfer band centered at 210 nm was observed. The charge-transfer band centered at 261 nm completely disappeared from the solution spectrum, suggesting the complete replacement of C1- ligands by OH- in the coordination sphere of Pt.The d-d transitions were unchanged as a result of treating PtCbz- with NaOH. These results are shown in Figure 7. Diffuse UV reflectance spectra for PtCbZ-/SiOZtreated in flowing oxygen as a function of temperature are shown in Figure 8. The decomposition of the Pt precursor state appears to be nearly complete at 125 "C, which is well below the precalcination temperature of 150 "C.
Supported Pt-Ru Bimetallic Clusters
-
1.o
0.2
Langmuir, Vol. 4, No. 5, 1988 1089
w
-
0.0
0.0
0.2
0.4
0.6
0.8
1.o
Pt Surface Composition Figure 9. CO methanation: turnover frequency as a function of surface composition, normalized to the methanation rate on Ru at 227 "C. The ensemble size probability curves are superimposed.
The weak nature of the PtC162--silica interaction was verified by an experiment in which the catalyst was washed with distilled water prior to reduction. The UV spectra of the resulting wash solutions in addition to the diffuse reflectance of PtCbs/SiO2 catalyst confirmed that H2PtC& could be quantitatively removed by repeated washes in distilled water. Methanation Studies. The rate of CO methanation at 227 "C was studied over the well-characterized Pt-Ru bimetallic catalysts which had been vacuum-dried. The results of this study are summarized in Figure 9. Turnover frequencies for the rate of methane production observed for the total number of surface metal atoms were normalized to dimensionless Ru turnover frequencies, by subtracting the contribution from Pt and dividing by the turnover frequency for monometallic Ru. Superimposed on the normalized turnover frequency vs surface composition curve, the probability of finding n adjacent Ru surface sites is plotted. A reasonable fit to the experimental data is obtained for a Ru surface ensemble of between two and three.
Discussion The observation that precalcination of the supported Pt-Ru bimetallic clusters in air at 150 "C prior to reduction in Hz leads to a substantial decrease in the surface segregation of Pt is an important finding of this study. The differential scanning calorimetry experiments clearly show that Ru is reduced at a temperature considerably lower than that observed for Pt whether or not the catalysts had been precalcined in air prior to reduction in H2. Precalcination leads to a somewhat larger decrease in the temperature of reduction of Ru than pt. However, the gravimetric experiments show that the chlorides could have been only partially converted into the oxides as a result of exposure to air at 150 "C. This rather small decrease in the reduction temperature as a result of precalcination in air could not, in our opinion, lead to the sharp decrease in the surface segregation of Pt. The differential scanning calorimetry results also show that bimetallic clusters having dual metal sites are formed regardless of pretreatment condition. A multimetallic coreduction assisted process occurs in which both metals are reduced simultaneously. The sharp decrease in the reduction temperature of Pt as a result of cluster formation is an indication of a multimetallic-assisted reduction process. For this to occur, both metals must be in intimate contact with one another. Hzspillover across the support must be ruled out
as a possible mechanism for the bimetallic-assisted reduction process as mechanical mixtures consisting of Pt/Si02 and Ru/SiOz were reduced at temperatures nearly equal to those of the monometallic catalysts. The diffuse UV reflectance studies show that significant changes in the structure of adsorbed PtC16'- occur in flowing oxygen at temperatures below the precalcination temperature of 150 "C. The decomposition of the Pt precursor state appears to be nearly complete at 125 "C (Figure 8). We suggest that the surface diffusion of the precursor state following calcination at 150 "C is considerably lower than that observed for the catalysts prepared by vacuum drying at room temperature. Following precalcination at 150 "C, the resultant Pt particles interact more strongly with the support, and the barrier to surface migration is sharply increased. The "in situ" diffuse UV reflectance studies give considerable insight as to the nature of the interaction between the PtCb2- precursor state and the silica support. In order to obtain a better understanding of this interaction it is useful to consider the speciation of PtC@ in solution. In aqueous solution, the hydrolysis of PtC1G2- occurs in accordance with the following equations:21
+ HzO F? PtClE(H20)- + c1PtClb(H20)- + HzO e PtC14(H20)2 + C1PtClE(H20)- PtCl,(OH)'- + H+ PtC14(H20)2 2 PtCl,(OH)(H,O)- + H+ PtCl,(OH)(HZO)- e PtC14(OH)zZ-+ H+ PtC16'-
(5) (6)
(7) (8)
(9) The equilibrium constants for these reactions (at 37 "C) 2 X lob4, have been found to be 5.6 X 6.3 X lod, and 6.3 X lo-', respectively. According to Anderson?l at temperatures below 55 "C and in neutral or slightly acidic solutions, it is not possible for the hydrolysis to go beyond reaction 9. However, in basic solution and boiling temperatures, it is possible to obtain Pt(OH),2-.21 Therefore, the addition of concentrated OH- at high temperatures should lead to the formation of the Pt(OH),2complex. The results shown in Figure 7 are in accord with this interpretation. Treatment of an aqueous solution of PtCb2- with NaOH and heating to 90 "C resulted in only one charge-transfer band centered at 211 nm. This result is strongly suggestive of a complete C1- to OH- ligand-exchange process. A UV spectrum of an aqueous solution of PtCb2- recorded at high pH but at room temperature, before the solution was boiled, gave two charge-transfer bands centered at 210 and 262 nm. The position of these charge-transfer bands is suggestive of a partial OH--Clligand-exchange process. The charge-transfer band centered at 261 nm is due to electronic transitions between the p orbitals of C1- and the octahedrally located Pt atom. The charge-transfer band centered at 210 nm, on the other hand, is due to transitions which involve bonding electrons between an OH- ligand and the Pt atom. A similar UV absorption band is observed for the u bonding orbitals between C1- and Pt. This band was centered at 201 nm in the absence of NaOH. The UV absorption spectrum for PtCb2- adsorbed on silica shows a shift in the position of this charge-transfer band to 205 nm. We interpret this band to be a convolution of the band centered at 211 nm due to OH- ligands and at 201 nm due to C1- ligands. A possible interpretation of these results is that PtC162- interacts with the silica support by a Cl--OH- exchange (21) Anderson, 3. R. Structure of Metallic Catalysts;Academic: New York, 1975.
1090 Langmuir, Vol. 4, No. 5, 1988 process and that this OH- participation drives the hydrolysis reactions through reaction 7. The persistence of the charge-transfer band centered at 262 nm assigned to C1- ligands is a positive indication that the C1- ligands remain coordinated to the Pt atom in an octahedral configuration. The invariance in the position of the UV bands assigned to the d-d transitions is a strong indication that the octahedral symmetry has not been significantly distorted through the incorporation of the OH- groups of the silica support into the sphere of coordination of Pt. For this reason this interaction is quite weak, as expected for anion-silica interactions. Because of this mode of PtC&2--supportinteraction we propose a surface diffusion model in which the PtC1,2mobile phase diffuses across the support by a mechanism which may involve Cl--OH- exchange processes with the support. For a catalyst which has been thoroughly dried, we envision the diffusion process to be very fast. This is due to a decrease in the energy barrier to diffusion because of a decrease in the interaction between PtCs2- and silica. Catalysts dried under less severe conditions would interact more strongly with the PtCle2-support, and the surface diffusional process would be somewhat inhibited. When precalcination in air at 150 “C is performed, the octahedral structure of the complex is completely destroyed and surface migration does not occur. With this interpretation, the observed surface enrichment in Pt as a function of pretreatment is readily explained. The methanation studies shown in Figure 9 confirm the presence of bimetallic clusters that have mixed metal surface sites. Turnover frequencies for methane formation conform approximately to small surface ensembles consisting of two to three Ru surface atoms. These results are in fair agreement with the proton-induced reduction of CO to CH4 over a series of homonuclear and heteronuclear metal carbonyl^.^^*^^ These authors have shown that a minimum cluster size of four active metal atoms is required to catalyze the formation of CHI. For the reaction between Fe4(C0)13-and HS03CF3,Whitmire and ShriverZ3identified an intermediate consisting of a carbon atom bound to four Fe atoms in a “butterfly” configuration. The ensemble requirement of four appears, therefore, to have an analogue in the homogeneous reaction between polynuclear transition-metal carbonyls and hydrogen in strongly acidic solutions. 00 methanation studies performed over supported Cu-Ru bimetallic clusters by others4q5-‘dispute the surface structure sensitivity of the CO methanation reaction. When methanation reaction rates are expressed in terms of Ru turnover frequencies, a one-to-one site blocking of Ru by Cu appears to be established, suggesting the facile nature of the reaction. However, we should point out that
Alerasool e t al. there are important differences between supported Cu-Ru and Pt-Ru bimetallic clusters: (1) Cu-Ru forms endothermic alloys. For this reason extensive surface-phase segregation may occur on supported Ru-Cu bimetallic clusters. Monte Carlo simulations have shown that Cu atoms prefer to saturate low surface coordination sites.8 The Ru atoms, on the other hand, are preferentially located on terrace sites. (2) Methanation turnover frequencies over supported Ru are sensitive to metal dispersions. When comparing rates for very small RU particles with those of large Ru particles, there may be a difference as great as 2 orders of magnitude, the rate being faster for larger Ru particles.% (3) Pt-Ru form exothermic alloys. Extensive phase segregation does not occur, and there is strong evidence that dual Pt-Ru surface sites occur over supported Pt-Ru bimetallic clusters.25 These observations suggest that methanation rates over supported Cu-Ru bimetallic clusters would be less susceptible to ensemble requirements than methanation rates over supported Pt-Ru bimetallic clusters. In the case of Cu-Ru bimetallic clusters, the Cu atoms first poison the catalytically inactive edge sites while leaving the more active terrace sites relatively intact. Because the edge sites of Ru are rather inactive for CO methanation, the alloying effect of Cu is expected to be small. For supported Pt-Ru bimetallic clusters, the Pt atoms are located both on edge and terrace sites. If geometric effects are important in CO methanation, the incorporation of inactive Pt sites into active Ru terrace sites should have a pronounced effect on the turnover frequency as was actually observed.
Conclusions The following important conclusions emerge from this study: (1) Precalcination of silica-supported Pt-Ru bimetallic clusters in air at 150 OC inhibits the surface segregation of Pt. (2) DSC experiments suggest that Pt-Ru bimetallic clusters appear to undergo a bimetallic-assisted reduction process. (3) Diffuse UV reflectance studies suggest that PtC1,2- interacts weakly with silica through’ a ligand Cl--OH- exchange process. Precalcination in air destroys the structure of this complex and inhibits surface diffusion. (4) The formation of methane is promoted by Ru surface ensembles consisting of two to three adjacent Ru atoms. Acknowledgment. We acknowledge support from the
U.S. Department of Energy (Grant #DOEFG02-86ER1351) for this research. We also acknowledge the National Science Foundation and CONACYT for providing support in the form of a U.S.-Mexico CooperativeResearch Grant (NSF-INT-8607615)for this research. Registry No. PtC192-,16871-54-8;CO, 630-08-0;Pt, 1440-06-4;
Ru,7440-18-8. (22) Drezdon, M. A.; Whitmire, K. H.; Bhattachmyya, K. H.; Hsu,A. A.; Wen-Liang; Nape, C. C.; Shore, S. G.;Shriver, D. F. J.Am. Chem. SOC. 1984,104,5630.
(23)Whitmire, K.; Shriver, D. F. J. Am. Chem. SOC.1980,102, 1456.
(24) King, D. L. J. Catal. 1978, 51, 1886. (25) Bowman, R.; Sachtler, W. M. H. J. Catal. 1972, 26, 63.
