798
J. Phys. Chem. 1991, 95, 798-801
bands attributable to partially protonated J-aggregate in Figure 5D are much like those observed for J-aggregate at high pH and high negative electrode potential in Figure 6D,again suggesting that the resultant adsorbate structure is similar. Since the FT Raman spectrum attributed to J-aggregate is not drastically altered by partial protonation, also recognizing that J-aggregate must expand in volume upon protonation, and that J-aggregated ions apparently do not exchange rapidly with solution-phase ions (the mean enthalpy barrier for dissolution of monomers from J-aggregate is found to be 42 kJ/mol),20 it is likely that protonation occurs primarily at J-aggregate chain ends, as suggested in our earlier publication!
IV. Conclusion The merging of near-infrared FT Raman and visible Raman studies for 2,2’-cyanine adsorbed onto a smooth silver electrode in various chemical environments and under different excitation
frequency and applied surface conditions leads to the following conclusions: (1) Adsorption of 2,2’-cyanine from high-pH solution onto a silver electrode at negative potentials to -1.0 V vs SCE results in an adsorbate that is primarily polycrystalline and Jaggregated materials. (2) Lowering the pH of the supernatant to near 0 with an electrode potential ca. -0.8 V vs SCE leads to dissolution of most of the crystalline and part of the J-aggregated adsorbate, and the J-aggregate that remains on the electrode is partially protonated. (3) Variations in the Raman spectra of 2,2’-cyanine as a function of solution makeup and electrode potential can be explained in terms of adsorbate composition and partial protonation. (4) Increasingly negative electrode potential increases both J-aggregation and protonation.
Acknowledgment. Support for this research by the National Science Foundation (NSF) under Grants RII-8504995 and RII-8802964 is gratefully acknowledged.
Structure and Catalytic Activity of Alumina-Supported Pt-Co Bimetallic Catalysts. 1. Charactorlzation by X-ray Photoelectron Spectroscopy %Itin Zsoldos, TamPs Hoffer, and Liszlo C u d * Institute of Isotopes of the Hungarian Academy of Sciences, P.O.Box 77, H-1525 Budapest, Hungary (Received: January 30, 1990; In Final Form: June 25, 1990) A series of Ptl-,CoX/Al2O3 bimetallic catalysts has been studied by X-ray photoelectron spectroscopy following in situ hydrogen reduction of the samples previously calcined in oxygen at 570 K. Platinum was found to be in the zerovalent state for the entire composition range and its dispersion increased with decreasing platinum loading. When combined with platinum, cobalt stayed partly in a hardly reducible cobalt oxide surface phase (denoted by CSP) and partly in the metallic state, whereas without platinum COOwas the predominant form. At low cobalt concentration the variation of the XPS intensity changes of the cobalt 2pYz line in the different states was interpreted by assuming that the cobalt surface phase was being covered by platinum. On the other hand, at high cobalt contents saturation of the alumina surface by the cobalt surface phase was concluded from the Occurrence of a constant, high amount of cobalt in the surface phase. On the basis of the comparison of surface platinum and cobalt contents first platinum-like properties and then cobalt-like is predicted for these catalysts with increasing cobalt content.
Introduction Addition of second metal to a supported metallic catalyst may affect its catalytic behavior via various effects.’ Nowadays, bimetallic catalysts such as Pt-Re, Pt-Ir, Pt-Sn are being successfully utilized in some industrially feasible catalytic processes, e.g., in naphtha reforming, and Pd-Cu and Pd-Ag are used in selective hydrogenations. The bimetallic catalysts studied so far revealed different effects, such as improved stability or retarded deactivation. For instance, rhenium in Pt-Re/AI2O3 catalyst maintained its catalytic activity during naphtha reforming at lower pressure and temperature by hindering deactivation processes, whereby the catalyst lifetime increased.2 In other cases the catalytic activity was also enhanced3 or the selectivity was modified? Little is available on the Pt-Co bimetallic catalystss.6 which have high activity in methanol formation at certain compositions. Extensive studies on the surface characteristics of both (1) In ‘Metal-Support and Metal-Additive Effects in Catalysis”, Imelik,
and catalysts supported on alumina started only a few years ago. In the present work the main goal is to determine the surface composition and the possible structure of a series of Pt-Co/A1203 bimetallic catalysts by X-ray photoelectron spectroscopy and to obtain information about the chemical environment of the surface components (valence states, quantities). In the second part of this series the surface structures found here at different compositions will be related to the catalytic properties observed. Experimental Section
The Ptl-xC~,/A1203samples were prepared by the incipient wetness method using y-alumina (Woelm). Appropriate amounts of H,PtCI, and C O ( N O ~with ) ~ 10 wt Q total metal loading were dissolved in water and used for impregnation. The atomic fractions of cobalt, X,,, were 0, 0.2, 0.4, 0.5, 0.67,0.85, and 1.0. After drying in air at room temperature the samples were calcined in flowing O2(40 cm3 m i d ) at 570 K for 1 h. The reduction was
B.,Naccack, C., Coudurier, G.,Praliaud, H., Meriaudeau, P., Gallezot, P.,
Martin, 0. H., Vedrine, J. C., Eds.; Elsevier: Amsterdam, 1982. (2) Coughlin, R. W.; Kawakami, K.; Hasan, A.; Buu, P. in ref 1, p 307. (3) Pan, W. X.; Cao, R.; Griffin, G.L. J . Carol. 3988, 114, 447. (4) GUCZI,L. In Catalysis 1987; Ward, J. W., Ed.;Elsevier: Amsterdam, 1988. (5) Niemantsverdriet, J. W.; Louwers, S.P. A.; van Grondelle, J.; van der Kraan, A. M.; Kampers, F. W. H.; Kocnigsberger,D. C. In Proceedings of rhr 9th Inrernarional Conwss on Catalysis;Phillips, M.J., Ternan, M., Eds.; The Chemical Institute: Ottawa, Canada, 1988; Vol. 2, p 674. (6) Zyade, S.; Garin, F.; Maire, G. New J. Chem. 1987, 1 1 , 429. (7) Bcrdala. J.; Freund, E.; Lynch, J. P. J . Phys. 1986, 47, (3-265. (8) Shushumna, I.; Ruckenstein, E. J . Caral. 1988, 109, 433.
0022-3654/91/2095-0798$02.50/0
(9) Shushumna, I.; Ruckenstein, E. J . Catal. 1987, 108, 77. (10) Shyu, J. Z.; Otto, K. Appl. Surf. Sci. 1988. 32, 246. (1 1) Chin, R. L.; Hercules, D. M. J . Phys. Chem. 1982,86, 360. (12) Arnoldy, P.; Moulijn, J. A. J . Carol. 1985, 93, 38. (13) Alstrup. I.; Chorkendorf, 1.; Candia, R.; Clausen, B. S.;T o p e , H. J . Catal. 1982. 77, 397. (14) Wivel, C.; Clausen, B. S.;Candia, R.; Morup, S.;Topoc, H. J . Carol. 1984,87,497. (15) Stranick, M. A.; Hualla, M.; Hercules, D. M. J . Carol. 1987, 103, 151.
(16) Sexton, B. A.; Hughes, A. E.; Turney, T. W. J . Caral. 1986,97,390.
0 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 799
Alumina-Supported Pt-Co Catalysts
TABLE I: The XPS Intensity Ratios of Pt Id and 0 1s Regions and Their Values Related to Unit Platinum Loading after Hydrogen Reduction
0.2 0.4 0.5 0.67 0.85
477 426 394 317 189
0.093 0.055 0.092 0.103 0.135
t
195.8 129.0 233.6 324.8 714.9
C
.-0
En fn
6
OMole fraction of cobalt. bPlatinum content.
carried out in situ in flowing H2(40 cm3 m i d ) at 570 K for 1 h in a small reaction chamber attached directly to the ESCA machine. Thus, after reduction the sample could be introduced into the analyzer chamber without exposure to air. An ES-300type ESCA machine manufactured by KRATOS was used to perform the XPS measurements. Analysis were performed in the fixed retarding ratio (FRR) mode with an aluminum target (1486.8 eV) at 15 kV and 10 mA. All samples were thoroughly crushed and pressed into a copper grid and then mounted on the sample transfer rod. The sample was first positioned into the reaction chamber for in situ hydrogen reduction, and then the chamber was evacuated and the sample transferred into the analysis chamber. The pressure was lo-' Pa during recording of the spectra. In all cases the C Is, 0 Is, A1 2p, Co 2p lines were measured. The t'F 4d line was used for analysis due to the overlapping of the Pt 4f line with the A1 2p peak. All binding energies (BE) were referenced to the C 1s peak (BE = 285.0 eVI7) applied as internal standard. Because of the overlapping of the Pt 4f and AI 2p transitions, the intensity of the 0 1s peak at BE = 53 1.3 f 0.3 eV was applied to represent the A1203 support for all samples. In order to determine the intensity of the different cobalt species, Co 2p regions were deconvoluted with the help of a one-parameter fitting process. Prior to the fitting, X-ray satellites'*J9 and inelastic background (Sherwd-type2O) were subtracted in all cases. During the fitting Gaussian peaks were used. The XPS sensitivity factor (S)used for cobalt 2p line is based on the value given in the 1iterat~re.I~ The sensitivity factor of the platinum 4d doublet was established from the literature value of its 4f lineI7 and from the work presented here. In all samples the Pt 4d5/2line position was found at BE = 313.7 f 0.3 eV with a spin-orbit coupling of 16.9 f 0.2 eV. On the basis of these values platinum is assumed to be present in the zerovalent state on alumina under our experimental conditions. Separate measurements on pure platinum foil resulted in the same binding energy value and coupling for the Pt line. The intensity ratios of the Pt 4d doublet and 0 Is line are presented in Table I and plotted vs the total amount of platinum as illustrated in Figure l a for the samples with different compositions. From this graph, an inverse proportionality for the I(Pt(4d))/f(O( Is)) peak intensity ratio vs Pt loading can be recognized. The ratio must be proportional to the platinum dispersion, DR,provided that it is normalized to the same unit amount of platinum. This, indeed, shows the same trend as the I(Pt(4d))/(O( Is)) ratios themselves. As far as the Co 2p region is concerned three kinds of line position can be distinguished: at BE = 782.0 f 0.3 eV (spin-orbit coupling: 15.4 f 0.1 eV), at BE = 778.1 f 0.2 eV (15.1 f 0.1 eV), and at BE = 780.5 f 0.3 eV (15.5 f 0.2 eV). The first and the last peaks are accompanied by strong shake up satellites at 787.3 f 0.5 eV BE. The first two species can be found on the bimetallic samples while on the sample containing cobalt alone (17) Pructicul Surfme Anulysis; Briggs, D., Seah, M. P., Eds.;Wiley: Chichester, U.K.,1983. (18) Krause. 0.; Ferreira, J. G. J . Phys. B 1975, 8. 2007. (19) Hricovini, K.; Dientka, P. Vacuum 1986, 36, 453. (20) Proctor, A.; S h e r w d , M. A. Anal. Chrm. 1982, 54, 13.
z
I
'
)
i
l
I
I
~
l
Binding Energy IeVI
Figure 2. XPS Co 2p spectra obtained over 10 wt W Co (a), Pb.15C%.85 (b)*pb.33c%.67 (c), b . S c % . S (d), b.6C%.4(e), and Pb.8c%.2 (r) catslysts supported on alumina after pretreatments.
the Co 2~312peak is centered at BE = 780.5 eV. Considering the Co 2p spectra measured on the various samples illustrated in Figure 2, no metallic component can be observed on the pto.sC~.2 sample and on the pure cobalt after the treatments mentioned. After evaluation of the data obtained by the XPS in the Co 2p region, the I(C0(2p))/I(O( 1s)) ratios for the different phases are presented in Figure 3. First, the relative intensity of ionic cobalt starts increasing sharply and then it slightly decreases with increasing cobalt loading. Simultaneously, the corresponding value of metallic cobalt continuously increases. The variation in the absolute amounts of the different elements and species on the surface measured by XPS are displayed in Figure 4. Here the relative intensity values are referred to the appropriate sensitivity factors of platinum 4d and cobalt 2p XPS lines. As can be seen, the relative intensity curves shown in Figure 1 and Figure 3 are not affected in shapes. However, the changes in the surface amounts of each species presented in Figure 4 make their comparison possible. As is demonstrated in Figure 4, the surface amounts of ionic cobalt are far larger on all bimetallic
800 The Journal of Physical Chemistry, Vol. 95, No. 2, 1991
Zsoldos et al. C o 3 p Chisq=I2. 388
810
6
.
,
.
.
,
560
.
,
.
lobo'
'
'
' 1i o 0
,
,
Co Content/pmoI Figure 3. I(Co(2p))/I(O(ls)) peak intensity ratios denoted by (Co/O) of ionic (a) and metallic (b) cobalt measured by XPS vs cobalt loading
after hydrogen reduction.
\
0.b
0.i
0.2
oh
. 0.b
1
.b
Co Atomic Fraction Figure 4. Relative XPS intensity ratios normalized to the appropriate sensitivity factors (S)being characteristic of the surface amounts of platinum (a), metallic cobalt (b), ionic cobalt (c) species vs cobalt loading after hydrogen reduction. samples than that of other surface species except in the case of the sample containing cobalt in 0.2 atomic fraction. Simultaneously, the surface amount of metallic platinum on the surface is larger than that of metallic cobalt up to the Pt0,6c00,4composition from which point this relation is reversed.
Discussion According to the binding energy value (BE) obtained for the platinum 4dY2peaks, platinum is present in metallic form on each sample. Binding energies of the Pt peak measured and here as 314.2 f 0.3 and 313.7 f 0.3 eV, respectively, agree within the experimental error with those in the literature.I0 Metallic platinum on alumina is not unexpected because during the reduction in H2 at 570 K most of the chlorine must have been removed.I0 On the other hand, the increase in D R found during calcination in oxygen a t 770 K8v9does not take place under the treatment at mild conditions applied here (570 K). Platinum in the ionic state*-1° was not observed by XPS within the limits of detection. The inverse proportionality obtained for the relative platinum intensity vs platinum content indicated in Figure l a as well as in Table I can be interpreted in the following way. Let us suppose a constant dispersion for platinum on alumina. If it is so, I(Pt(4d))/(I(O(1s)) peak ratios should increase with increasing metal loading. Conversely, at constant metal loading an increase in the
800 810281 1:
790 780 Binding anergy (PV)
770
Figure 5. Evaluated Co 2p spectrum of the Pb,33Cg,,7/A1203sample measured by XPS after hydrogen reduction.
same ratio can be experienced only if Pt dispersion rises. So D R can be related to the intensity ratio provided that this latter value is divided by the amount of platinum. Thus, the strong dispersion effect operating for metallic platinum when cobalt is also present on alumina can be described by a nearly exponential function. The cobalt species represented by the Co 2p XPS line located at 778.1 eV BE can be unambiguously assigned to zerovalent metal in good agreement with the literature data.16*'7*21The shift of the Co 2p line to a value 3.9 eV higher than 778.1 eV along with strong shake up satellite peaks is assigned to an unreduced surface phase,I5 surface spinel," or mixed oxide phase12containing Co3+ and Co2+ ions in octahedral lattice sites14 in close contact with the support surface. It is further denoted as 'Co surface phase" (CSP) and it is present in all bimetallic catalysts containing platinum. However, on the pure cobalt sample the CO 2~312peak is centered at BE = 780.5 eV which is characteristic of cobalt monoxide as indicated by the literature.16J7 Nevertheless, it is inferred that in the Co 2p region the COO signal overlaps with that from the CSP, and therefore, these two components cannot be surely distinguished. The presence of the cobalt monoxide phase is further supported by a change in the cobalt dispersion as discussed later. Cobalt monoxide is also present on pure Co/A1203 as a result of the first step of the Co304reductionI6 formed during calcination, and it is unreducible in the absence of platinum under our conditions. However, it is of primary importance that, in the samples containing platinum and cobalt, cobalt is present solely in the CPS and metallic forms as demonstrated by the deconvoluted Co 2p region shown for the Pt0,33C00,67/A1203 catalyst (Figure 5). The absence of the characteristic 780-eV BE peak of Co304as well as of COO is an indirect proof of the complete reduction of the COO phase in the presence of platinum after reduction in hydrogen under mild conditions. From these facts the following scenario is proposed. During mild calcination the original nitrate is decomposedI2 and Co3+ oxide is produced from the oxidation of Co2+ (either by NO2 evolved in the decomposition of CO(NO,)~or by O2during the calcination). Part of the Co2+ oxide spreads over the support surface forming the hardly reducible surface phase (CSP) with alumina. In the absence of solid-state diffusion into the support matrix,"*'2this su:face phase is highly dispersed and is presumably present in monolayer t h i c k n e ~ . 'As ~ the cobalt loading is raised, Co304 crystallites are formed to an increasing degree. In the presence of platinum this oxide phase is completely transformed into metallic cobalt during hydrogen reduction, as illustrated in Figure 5 . However, in the absence of platinum, cobalt monoxide crystallites are the only species formed during reduction in hydrogen at 570 K as was demonstrated for the 10 wt % cobalt on alumina sample. (21) Wagner, C. D.; Riggs, W. N.; Davis, L. E.; Moulder, J. R.;Mullenberg, G. E. Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Co.: Eden-Prarie, Minnesota, 1979.
Alumina-Supported Pt-Co Catalysts
The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 801
TABLE II: The Extent of Cobalt Reduction and the Corresponding Amounts of Metallic and Ionic Cobalt Species As Calculated by Stranick's M e t M I S
I
amounts of cobalt X IOd, mol (g of cat.)-' 0.2 0.4 0.5 0.67 0.85
77 78 32 50 70
92 223 I28 324 755
27 61 266 323 317
"Mole fraction of cobalt. bReduced fraction of cobalt. 'Metallic. Ionic.
These observations indicate the promotional effect of platinum in the reduction of cobalt oxide by hydrogen, whereas the surface phase strongly bound to the alumina support is unreducible even in the presence of platinum similarly to that demonstrated for Pt-Re/A1203 systemaZ A further question is whether the reduced cobalt forms bimetallic particles with zerovalent platinum. Although evidence from XPS measurements is not available, XRD and EXAFS measurements indicate the presence of Pt and R-Co bimetallic particles (Pt,Co or PtCo).6 In addition to the observed changes in dispersion, the relative intensity values shown in Figure 3 are also subject to the influence of the increasing cobalt content. To eliminate this effect, the corresponding intensity ratios, as was calculated for Pt, should be normalized to the amount of cobalt present in the different phases. However, since the dispersions of the various cobalt phases are all probably different, a method of calculation bypassing this difficulty has been adopted from Stranick et al.ls First, some assumptions must be made: (i) In the case of the bimetallic catalysts the cobalt 2p peak at higher binding energy (782.0 eV) must be assigned to the surface phase alone; that is, in the presence of platinum the oxide phases (excluded CSP) formed on calcination are completely reduced under our conditions. (ii) The CPS is of a monolayer thickness. (iii) The XPS intensity ratio for the total cobalt being in monolayer can be estimated by the model of Kerkhof and Moulijn (IMONO) .23 The extent of reduction (R) calculated by the Stranick method and the amounts of cobalt in the different states are listed in Table 11. Here the reducibilities obtained for the samples with the two lowest cobalt contents seem to be unexpectedly high. The other striking feature is the constant, high-amount CSP phase in the last two rows. This may point to a saturation of alumina surface by CSP which is not expected.I4 Applying the values presented in Table 11, the changes in dispersion are calculated and are illustrated in Figure 6. Three particular regions can be distinguished. In regions I (Xco = 0.2 and 0.4) and I1 (X, = 0.5,0.67, and 0.85) the high and constant dispersion of the ionic cobalt species is the consequence of the starting assumptions, while in region I11 (Xco = 1.O) the large drop in dispersion is the result of the presence of cobalt oxide phase with low dispersion. In region I1 the slight, continuous drop in the dispersion of Coo with increasing cobalt loading is in agreement with the general trend, Le., the higher the loading, the lower the dispersion. Here, the formation of the platinum-balt bimetallic particles is supported by the other e~idences.6.~~ However, in region I, very low values of dispersion are obtained. Here the only straightforward explanation appears to be that the platinum is covering a large part of the cobalt phase. The I(Co(2p)) signal is much lower than it would be without platinum. (22) Augustine, S. M.; Sachtler, W. M. H. J . Carol. 1989, 116, 184. (23) Kerkhof, F. P. J. M.; Moulijn, J. A. J . Phys. Chem. 1979,83, 1612. (24) Guczi, L.; Hoffer, T.; Zsoldos, 2.;Zyade, S.; Maire, G.; Garin, F. In press.
0
1
500
~
1
~
~
1000
1
1
~
1
1500
Co Content/pmoI Figure 6. XPS I(Co(2p))/I(O(ls)) peak intensity ratios, denoted by Co dispersion, of ionic (a) and metallic (b) cobalt normalized to the corresponding amounts of cobalt species vs cobalt loading after hydrogen reduction.
The amounts of the different species shown in Figure 4, give further crucial information about the catalyst surface. Here two interesting features should be emphasized. First, in the bimetallic samples the amount of ionic cobalt exposed to the surface is much larger than that observed for both metallic (Co and Pt) species. The only exception is found for the Pb.sCo.2 sample where it is nearly the same as the amount of surface platinum. It means that, apart from the samples with lowest cobalt content, the surface is monopolized by the CSP. However, in the case of the pure cobalt sample, in addition to CSP, ionic cobalt species is also present in the form of cobalt monoxide. The most interesting feature displayed in Figure 4 is the intersection of the curves representing the two surface metallic components. Before this crossing point the character of the sample is controlled by platinum and after the crossing point by cobalt. Accordingly the samples in the range of X, = 0.0.4are basically platinum-like, while for the higher value of X,, the bimetallic samples the catalysts reveal cobalt-like character. In the middle range of composition (Xco = 0.5, 0.67, and 0.85) bimetallic particles are presumed to be present.
Conclusions The state of a bimetallic catalyst highly depends upon the valence of the metallic components to which extent the various surface species are exposed. It has been shown that platinum is always present in the metallic form on the surface under our pretreatment conditions. Cobalt exists partly in the well-dispersed "Co surface phase" (CSP) as an ionic species as well as in the zerovalent state. The Co304 formed during calcination can be easily reduced to COO. Platinum promotes the complete reduction of the reducible oxide phases even under our mild conditions. Dispersion of the metallic platinum rises with decreasing platinum loading. Evaluating the variation in the dispersion of cobalt it is shown that CSP is covered by Pt at the two lowest cobalt contents. The results also point to the possible existence of a saturation coverage of the alumina surface by CSP. Concerning the type of the predominant surface species it was established that the amount of ionic cobalt in the CSP is enhanced with increasing cobalt content. On the other other band, metallic cobalt species form Pt-Co bimetallic particles which are primarily platinum-like for the samples possessing the lowest cobalt fractions, while their properties are shifted toward cobalt-like for the samples with high cobalt loadings. Registry No. Pt, 7440-06-4; Co, 7440-48-4.
1
1
~
(
