1324
J. Phys. Chem. 1992, 96, 1324-1328
Temperaturs-Programmed Reduction of Silica-Supported Pt/NI Catalysts Studied by XANES Andreas Jentys; Brian J. McHugkt**Gary L. Heller,* and Jobannes A. Lercher*it Institut fiir Physikalische Chemie, Technische Universitiit Wien, Getreidemarkt 9, A - 1060 Vienna, Austria and Department of Chemical Engineering, Yale University, P.O. Box 21 59 Yale Station, New Haven, Connecticut 06520 (Received: April 2, 1991)
The reduction of bimetallic silica-supported Pt/Ni chloride catalyst precursors was followed by means of X-ray absorption spectroscopy. Quantitative analysis of the near-edge structures (XANES) of the Pt LIIIand the Ni K edge was used to follow the reduction kinetics of each metal phase. The reduction to the final material occurred in two steps: In the first, the reduction of Pt atoms and Ni atoms adjacent to Pt took place (forming a bimetallic phase); in the second, the reduction of the remaining Ni atoms occurred (forming a separate Ni phase). In the reduced bimetallic particles, XANES indicates electron transfer from Ni to Pt atoms compared to the monometallic particles.
Introduction Bi- or multimetallic supported catalysts are often preferred over monometallic ones, because they may combine or modify positive effects of constituent components. A large number of examples of the use of bimetallic catalysts span from applications for reforming catalysts1" to catalysts for automobile emission reduction.6 One of the difficulties in using these catalysts is that the presence of several elements may lead to very complex structures, which are dependent upon the carrier material used and the preparation procedure? Thus,it is crucial to be able to follow and characterize the interaction effects during preparation in order to assess the mutual influence of all catalysts components. The most important parameters in this respect are the oxidation state, the mutual influence on the electronic structure (of the metal components), and their geometrical arrangement in the catalyst. While there are numerous analytical techniques that provide information on the electronic nature of the metal,&" most of them require a high-vacuum environment, entirely different from the situation prevailing during the preparation of industrial cataly~ts.'~ This usually means that the preparation procedure has to be stopped at some point and the specimen has to be transferred into an ultra-high vacuum system for analysis. In contrast, X-ray absorption techniques may give useful information about the chemical nature (X-ray absorption near-edge structure, XANES) or the geometric arrangement (extended X-ray absorption fine structure, EXAFS) which can be derived without removing the catalyst or the catalyst precursor from its environment during ana1ysis.l3J4 XANES is particularly advantageous because it does not suffer from Debye-Waller limitations, although recently EXAFS has been demonstrated to provide useful information on the in situ reduction of R/Rh and &/Re catalyst^.'^ With both techniques, subtle effects on the photoelectrons ejected by X-ray photons from a core level into states around or about the vacuum level are observed. XANES arises from the transitions of the ejected photoelectron to the electronic levels close to the Fermi level.16 Therefore, XANES provides information on the density of the vacant states near the Fermi level of the absorber atom and, hence, ultimately on the oxidation state. With EXAFS the variations in the absorption coefficient arise from the interference of the outgoing electron wave function and that wave backscattered from surrounding metal at0ms.l' We have chosen the Pt/Ni system for investigation because both metals are important constituents of hydrogenation and reforming catalysts, but t'F and Ni exhibit very different catalytic and reductive18-20properties. Moreover, the system has been Technische Universitlt Wien. *Yale University. 1 Resent address: AT&T Bell Labs, 480 Red Hill Road, Middletown, NJ, 07756.
TABLE I: Composition of the Catalysts wt % atom % wt % wt % tot. catalyst Ni Ni Pt metal
A B C
100 90
D E F G H
70 50 30 10 0
a
80
2.30 2.05 1.81 1.58 1.11 0.66 0.22
0.00
0.00 0.86 1.71 2.55 4.20 5.81 7.38 8.15
2.30 2.92 3.53 4.13 5.31 6.47 7.59 8.15
10-'gaccessible
metal, atoms/g'
dispersion, %
3.83 3.86 4.15 4.92 4.70 3.08 3.94 4.84
16 16 20 21 21 14 18 22
Determined by H2chemisorption.
characterized previously by several group^,^^-^^ including ourselves.24 (1) Sinfelt, J. H. Bimetallic Catalysts; John Wiley & Sons: New York, 1983. (2) Gates, B. C.; Katzer, J. R.; Schuit, G. C. A. Chemistry of Catalytic Processes; McGraw-Hill Book Co.: New York, 1979; Chapter 3. (3) Ciapetta, F. G.; Debra, R. M.;Baker, R. W. Catalysis; Reinhold: New York, 1958; Vol. 6, Chapter 6. (4) Weisz, P. B. Advances in Catalysis; Academic Press: New York,1962; Vol. 13. ( 5 ) Ciapetta, F. G.;Wallace, D. N. Catal. Rev. 1971, 5, 67. (6) Irandoust, S.; Anderson, B. Catal. Rev.-Sci. Eng. 1988, 30, 341. (7) Foger, K. In Catalysis. Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer Verlag: Berlim/Heidelberg/New York, 1984; Vol. 6. (8) Lemmaitre, J. L.; Menon, P. G.; Delannay, F. Characterization of Bimetallic Catalysts; Marcel Dekker: New York, 1984. (9) Gasser, R. P. H. An Introduction to Chemisorprion and Catalysis by Metals; Clarendon Press: Oxford, U.K., 1985. (10) Madix, R. J. Advances in Catalysis; Academic Press: New York, 1980; Vol. 29, Chapter 1. (1 1) Ertl, G.; KBppens, J. Low Energy Electrons and Surface Chemistry; VCH: Weinheim, FRG, 1985. (12) Stiles, A. B. Curulysr Manufucture; Marcel Dekker: New York, 1983. (1 3) Koningsberger D. C.; Prins, R. Principles, Applications, Techniques of EXAFS. SEXAFS and XANES; John Wiley & Sons: New York/Chichester/Brisbane/Toronto/Singapore,1988. (14) Sinfelt, J. H.; Via, G.H.; Lytle, F. J. Catal. Reu.-Sci. Eng. 1984, 26, 81. (15) Bazin, D.; Dexpert, H.; Bournonville; J. P.; Lynch; J. J . Catal. 1990, 123, 86. (16) Bart, J. C. J. Advances in Catalysis; Academic Press: New York, 1986, Vol. 34, p 203. (17) Stern, E. A. Phys. Rev. 13. 1974, 10(8), 3027. (18) Dominguez, E. J. M.; Vasquez, S. A.; Renouprez, A. J.; Yacaman, M. J. J . Coral. 1982, 75, 101. (19) Wielers, A. F. H.; Dings, M. M. M.; van der Grift, C. J. G.; Geus, J. W. App. Catal. 1986, 24, 299. (20) Bommanavar, A. S.; Montano, P. A.; Yacaman, M. J. Surf. Sci. 1985, 156,426.
0022-3654/92/2096-1324%03.00/00 1992 American Chemical Society
TPR of Silica-Supported Pt/Ni Catalysts Studied by XANES In this paper the changes in the oxidation state of a Pt/Ni chloride precursor upon its transformation into a Pt/Ni bimetallic catalyst (supported on silica to exclude the effects of strong interactions of the metals with the support25) during temperature-programmed reduction (TPR) are reported.
The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 1325 r
1'
Experimental Section Chtdysts. A series of silica-supported Pt/Ni bimetallic catalysts with a constant metal loading of 4 X lo4 mol-g-l of silica were prepared using the incipient wetness technique? The compositions of the reduced samples are compiled in Table I. For further characterization of these materials, see ref 24. X-rayAbsorption Spectroscopy. The chloride precursors were pressed into self-supporting wafers and placed inside a stainless steel cell (1000-mL volume), which permitted collection of XAS in situ at temperatures between 100 and 773 K in an inert gas atmosphere (usually He) or hydrogen (during TPR). The cell had two sample positions, allowing two samples to be treated simultaneously and examined afterward. Usually the gas flow was 5 mL& (NTP). With treatment of the cell as a continuous stirred tank reactor (CSTR), the average residence time of the gas was 200 s. It should be noted that the gases were not forced to flow through the catalyst pellet (thickness of approximately 1 mm). The temperature was measured at the sample holder. Note that the half-widths of the TPR peaks are nearly identical with those obtained in a previous experiment in which H2 was forced through the catalyst bed.24This suggests that transport of H2to the catalyst precursor does not limit the rate of reduction and true reduction kinetics is monitored during TPR. The weight of the samples was selected to achieve an absorption intensity for the reduced sample of 2.5 to optimize the signalto-noise The intensities of the XAS were normalized to the mass areal loading of the edge metal by fitting the absorption of the background with a polynomial function and the absorption due to the ejection of the photoelectron with the victoreen coefficient~.~' (pt)tot,exp =A
8300 8525 8350 8375 11400 11425 0450
ENERQY [ev]
+ BX + cXz + DX3 + ... + NA"+ Z ( p / p &
The wavelength is designated by A, the absorption coefficient by 1,the sample thickness by t , and the density of the metal by p. The linear coefficients A through N were used to construct the background. Z represents the mass areal loading of the edge metal. After the wafers were dried in He at 373 K for 1 h, TPR of the sample was carried out in pure H2 with an increase of the temperature of 30 K*min-' up to the indicated temperature. After the sample was rapidly cooled to Iiquid nitrogen temperature to stop the reduction at a chosen temperature, X-ray absorption spectra were collected. In a second series of experiments, the reduction was followed in situ by collecting a XAS every 25 K during TPR (in this case the heating rate was 10 K-min-l). Because the energy scale was not recalibrated by an internal standard during the analysis, the edge positions of all X-ray absorption spectra were aligned with the edge of the bulk metal. ACI4 and NiC12 (diluted with sugar) were used as reference materials for the precursors, and thin metal foils were taken as references for the reduced catalysts. The X-ray absorption spectra were measured at the Beamline X18-B at the National Syn-
c.
(21) van Stiphout, P, M.; Geus, J. W. Appl. Caral. 1986, 25, 19. (22) Mookerje, A.; Singh, R. P.J. Phys. C: Solid Srore Phys. 1985, 18(22), 4261. (23) Mansour, A. N.; Cook, J. W.; Sayers, D. W.; Emrich, R. J.; Katzer, J. R.J. Caral. 1981, 89, 462. (24) Raab, Ch. G.; Lercher, J. A.; Goodwin; J. G.; S h y , J. Z. J. Caral. 1990,122,406. (25) Tauster, S.J.; Fung, S. C.; Garden, R. L. J . Chem. Soc., Perkin Trans. 1 1978, 170. (26) Lec,P. A,; Citrin, P. H.; Eisenbeger, P.; Kincaid, P. M. Reoiews of Modern Physics 1981,53(4), 769. (27) McHugh, B. J.; Larsen, G. L.; Haller, G. L. J . Phys. Chem. 1990, 94, 8621. (28) Outh, D. A.; StBhr, J. J. Chem. Phys. 1988,88(6), 3539.
11550
11575
1100
11625
11650
ENERQY [ev]
Figure 1. (a, top) The XANES of NiCI, (...) and Ni foil (-), and (b, bottom) the XANES of R C 4 (...) and F't foil (-).
chrotron Light Source, Brookhaven National Laboratory, N.Y.
Results The X-ray absorption spectra of the reference compounds are compiled in F i e 1. The spectra of the precursors are dominated by peaks with high intensity above the absorption edge, the socalled white line. These peaks have a very low intensity in the case of the Ni foil (peak I, Figure la) and the Pt foil (Peak 11, Figure lb). This difference in the white line intensities between the oxidized and the metallic states is understood in terms of the greater density of the unoccupied states near the Fermi level for the oxidized elements.I6 The XANES of a Ni-rich Pt/Ni catalyst (catalyst D, 7 0 atom 8 Ni) during TPR are plotted in Figure 2. The spectra (at Ni K and Pt L absorption edges) of the chlorine precursors show a peak (peak I and 11) with high intensity above the absorption edge. After TPR to 573 K, the intensity of the first peak beyond the Ni edge decreased moderately (50%) in comparison to that of the start of the experiment, while that beyond the Pt edge decreased markedly. After TPR up to 723 K, the intensity of the f i t peak beyond the Ni edge was also decreased. No further change in the structure of the Pt edge compared to that of TPR to 573 K was observed after this additional treatment. Also with a R-rich bimetallic catalyst precursor (catalyst F, 30 atom 5% Ni), the XANES showed an intense peak beyond the Pt and Ni absorption edges (Figure 3 peak I and 11, respectively). After TPR up to 573 K, both of these peah decreased in intensity in comparison with those of the starting material. TPR up to 723
Jentys et al.
1326 The Journal of Physical Chemistry, Vol. 96, No. 3, 1992
T
1'
1'
1
2 3
o300 0325 8350 8375 8400 8425 8450 ENERQY [ev
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1" i"
II F . 11550
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11600
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ENERGY [ow
Figure 2. The XANES during TPR of catalyst D (70 atom % Ni) at (a, top) the Ni K edge and (b, bottom) the Pt LIIIedge: (1) precursor; (2) after TPR up to 573 K (3) after TPR up to 773 K.
K did not induce any further changes. In order to characterize the time dependence of this observation shown in Figures 2 and 3, XANES were recorded in situ during the TPR. Typical examples for the Ni K and the Pt LIIIedges of the chlorine prawsor are shown in Figures 4 and 5. In general, the intensities of the peaks beyond the absorption edge (Figure 4a,peak I; Figure 5a, peak 11) decnasbd with increasing reduction temperature. A closer inspection of these data indicated that the changes in the peak height between two consecutive XANES showed two maxima for Ni (462 and 553 K), while only one maximum was observed for Pt (423 K). Note that this corresponds to the measurement of the rate of change in the partial electron density of states at the Fermi level at the absorber atom as a function of the reduction temperature. The X-ray absorption spectra of the reduced catalysts are plotted in Figures 6 and 7. The intensity of the absorption peak above the Ni edge (peak I) increased with decreasing Ni concentration, while the intensity of the peak above the Pt edge (peak 11) decreased with decreasing Pt concentration in the catalysts. Discussion The reduction of the NiC12 and PtC14 precursors during the TPR of the Ni-rich catalysts proceeded in two steps. At lower temperatures, the maximum in the rate of reduction was assigned to the reduction of platinum chloride or a mixed Ni/Pt chloride
'i I
25 cm2g.'
Y 11550
11575
1 1 m
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11650
ENERGY [oVJ
Figure 3. The XANES during TPR of catalyst F (30 atom 96 Ni) (a, top) the Ni K edge and (b, bottom) the Pt LIIledge: (1) precursor; (2) after TPR up to 573 K; (3) after TPR up to 773 K.
phase, while the high-temperature maximum was attributed to the reduction to a pure Ni phase.24 Because these assignments were based on the indirect evidence from the comparison of the relative importance of the two maxima with the composition of the bimetallic catalyst, the present XANES study was undertaken since it should provide a more direct method for probing the oxidation state of the catalyst components. For Pt, the Pt LIIIedge was used, which means that XANES results from the transition of the Pt 2p3/*state into partially filled 5d orbitals (with a small p-to-s contribution). In the case of Ni, XANES above the Ni K edge corresponds to the transition of the Ni 1s into the np symmetric portion of the density of the final ~tates.~~3" Parallel with the electron density around the absorber atom (as it becomes partially oxidized), the electron populations of the d and the s-p states decrease and hence the peak intensity increases. The sharper edge structure of the Pt LIIIedge (see Figure lb) relative to the Ni K edge (see Figure la) reflects in part the narrower bands of d states (of Pt) relative to the s-p states (of Ni) near the Fermi level. ( 2 9 ) Szmulowicz, F.; Pease, D. M.Phys. Reo. E . 1978, 17(8), 3341. (30) Lengeler, B.; Zeller, R. Solid Srare Commun. 1984, 51, 889.
TPR of Silica-Supported Pt/Ni Catalysts Studied by XANES
The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 1327 T
T
h\
8300
8325
TEMPERATURE
8350
8375
8400
ENERQY [ev
11550
11575
1100
1162s
ENERQY [eq 4
3
2 dl/dT
1
I" 350
0
450
550
660
TEMPERATURE
Figure 4. (a, top) Variations of the XANES observed at Ni K edge during TPR (80 atom % Ni), and (b, bottom) changes in the height of the peak beyond the Ni K absorption edge versus temperature.
We reported the presence of three phases detected by means of XRD in the reduced catalysts: pure Pt,pure Ni, and a PtNi alloy phase. We suggested that the bimetallic catalyst particles primarily consist of a stoichiometric PtNi phase and that the metal in excess is present, most likely, in the form of separate particles. Because the Pt XANES between TPR up to 573 and 723 K were identical for all samples investigated and hence only one maximum in the change of the XANES signal was observed, we conclude that all Pt atoms (in the pure Pt phase as well as in the bimetallic) are reduced below 573 K. The slightly lower temperature of the maximum in Figure 4 in comparison with that of the TPR maximum reported in ref 24 is caused by the lower heating rate in the present experiment, which was necessary to be able to record the XANES with appreciable time resolution. The variations in the Ni XANES depend subtly upon the molar ratio between Ni and Pt in the catalyst. For samples with 50 atom 96 Ni or less, the XANES after reduction up to 573 K and up to 723 K (Figure 3), respectively, were identical, suggesting again that all Ni atoms were reduced in one step. Moreover, the timeresolved XANES showed only one maximum in the change of the XANES. In contrast, for catalysts with more than 50 atom 96 Ni, we observed variations in the XANES of Ni after TPR to 573 or 723 K, respectively. Moreover,the variation in the intensity
350
450
550
(is0
TEMPERATURE [K]
Figure 5. (a, top) Variations of the XANES observed at the R LllIedge during TPR (80 atom W Ni), and (b, bottom) changes in the height of the peak beyond the Pt LIll absorption edge versus temperature.
of the XANES intensity as a function of the reduction temperature showed two maxima at those temperatures where maxima of the reduction have been observed by means of hydrogen consumption (see ref 24). Note that only after reduction to 723 K was the XANES identical to that of the reference Ni foil for the catalysts containing more than 50 atom % Ni. Thus, we conclude that Ni2+ present in a separate NiC12 phase is significantly more difficult to reduce to NiO than that very close to Pt. Let us focus on the possible reasons for the easier reduction of Ni in the presence of Pt. XRD showed that, in catalysts containing 50 atoms k Ni or less, by far largest the fraction of Ni atoms was affiliated with the bimetallic PtNi phase. Because of the close (inter- and intracrystalline) distance between Ni and Pt, already reduced Pt atoms catalyze the reduction of the neighboring Ni atoms. Support for this is given by the slightly lower temperature of the maximum of the reduction of Pt compared to that of Ni in the timeresolved XANFS experiment. This suggests that the presence of reduced Pt triggers the reduction of Ni, maybe by dissociation and spillover of hydrogen. The ability of Pt to reduce Ni atoms at a distance far from from Pt would appear to decrease markedly with the distance. However, the presence of Pt lowered the reduction temperature of Ni even when a separate Ni phase was formed. Because this aid in reduction is not constant but increases with decreasing average
Jentys et al.
1328 The Journal of Physical Chemistry, Vol. 96,No. 3, 1992 AVERAGE CHARGE OF NI 0.17'
0.075 O.'
T I
t 0
I
10 20 30 40 50 80 70 80
do 100
NI
Figure 8. The average charge of the Ni atoms inside the bimetallic catalysts with respect to Ni2+ of NiC12 (wet) and Nio of a Ni foil.
8300
8325
8350
8375
8400
8425
8450
ENERGY [ e q
Figure 6. The XANES of the reduced catalysts observed at the Ni L edge: (A) catalyst A (100 atom % Ni); (B) catalyst B (90 atom % Ni); (C) catalyst C (80 atom % Ni); (D) catalyst D (70 atom % Ni); (E) catalyst E (50 atom % Ni); (F) catalyst F (30 atom % Ni).
ED
11550
11575
11600
11825
11650
ENERGY [eq
Figure 7. The XANES of the reduced catalysts observed at the Pt Kill edge: (H) catalyst H (0 atom % Ni); (F) catalyst F (30 atom % Ni); (E)catalyst E (50 atom % Ni); (D) catalyst D (70 atom % Ni); (C) catalyst C (80 atom % Ni).
distance between Pt-containing and only Ni-containing phases, the transport of dissociated hydrogen seems to limit the reaction rate under our experimental conditions. In the next step, we used the intensity (area under the first peak after subtraction of the features of the edge using an arctangent function2*)on the first peak beyond the absorption edge to study the mutual electronic influence of Pt and Ni and to test the model of the separate Pt/Ni and Ni phases by quantitative analysis of that peak. Previously, it was suggested that variations in the XANES of the system indicate that there is a net electron transfer from Ni to Pt.zo The increase in the intensity of the first peak above the edge observed for the XANES of Ni and the decrease of that peak (peak I) for Pt with increasing concentration of Ni
in the catalyst confirms this. The formal oxidation state of Ni inside the bimetallic catalysts was estimated from the difference between the integral intensity of peak I in the XANES of NiC12 (Ni2+)and that of the Ni (NiO) reference foil. A direct proportionally between this difference and the partial charge was assumed. It especially should be mentioned that the values of the formal charges depend upon the nature of the reference substances used. The values determined with the use of NiC12 and the Ni foil as reference substances range from +0.013 (catalyst with 90 atom 3'% Ni) to +0.13 (catalysts with 30 and 50 atom 5% Ni). The variation as a function of the Ni concentration suggests (seeFigure 8) that indeed a constant value of +O. 13 exists for Pt-rich samples in which only the PtNi phase exists. Using the charge neutrality of the metal particle as the boundary condition, this implies that Pt atoms inside the bimetallic catalysts, compared to those in a metal foil, are negatively charged. The comparison between the formal charges estimated from a calculation using the assumption of additive behavior indeed suggests that both the assumption of the separate PtNi phase and the Ni phase and that of additive contribution to the XANES signal are justifiable. Conclusions It has been shown that time-resolved XANES can be used to characterize a catalyst during its preparation stages. Information on the oxidation state and the phases can be at least qualitatively derived. It was thus possible to directly attribute the TPR peaks of Pt/Ni chloride precursors to the reduction of distinct metal chloride phases. Three different processes occurred: (i) the reduction of Pt4+, which was independent of its environment (Le. of how much Ni was around), (ii) the reduction of Ni very close to Pt (Le. in the mixed-metal phase), and (iii) the reduction of Ni at a further distance from Pt. The results show that the driving force to lower the temperature of reduction of the non-noble metal (in this study Ni) by the presence of a noble metal (in this study Pt) is the easier availability of hydrogen in the presence of the noble metal. In agreement with the differences in the electronegativities, XANES of the bimetallic catalysts suggest an average positive charge for Ni and a negative one for Pt in the bimetallic catalysts. Acknowledgment. The work was supported by the Fonds zur Forderung der Wissenschaftlichen Forschung under project FWF 6912 CHE (NSF-FWF cooperative program). Research was carried out at the National Synchrotron Light Source (Beamline X18B), Brookhaven National Laboratory, which is supported by the U.S.Department of Energy, Division of Materials Sciences and Division of Chemical Sciences. Registry No. RCI.,, 13454-96-1; NiCI,, 7718-54-9; Ni, 7440-02-0; Pt, 7440-06-4.
