J. Phys. Chem. 1992, 96,9393-9400 of Energy (New Mexico WERC Program) for funding of this research. We appreciate the assistance of Kelly Brown and Ray Forrister (University of New Mexico) during various phases of this study. R W NO. Au, 7440-57-5; Nz,7727-37-9; H20,7732-18-5; HS(CH2),Si(OCH3)3, 4420-74-0; n-hexane, 1 10-54-3; isooctane, 5&84-1.
Refeteaces and Notea (1) (a) Bein, T.; Brown, K.;Frye, G. C.;Briukcr, C . J. J. Am. Chem. Soc. 1989, 111,7640. (b) &in, T.; Brown, K.;Frye, G. C.;Brinker, C. J. US. Patent Application 07/580,373; allowed on Jan 23, 1992, to be hued. (2) Ward, M.D.; Buttry, D. A. Science 1990,249, 1OOO. (3) For rccmt reviews. see: (a) Janata, J. Anal. Chem. 1990,62,33r. (b) Hughes,R.C.; Rim, A. J.; Butler, M.A.; Martin,S . J. Science 1991,254, 75.
(4) Win, T.; Brown, K.;Enztl, P.; Brinker, C. J. Mater. Res. Soc. Symp. Proc. 1988,121,761. (5) Bein, T.; Brown, K.; Brinker, C. J. In Studies in Surface Science Ekvicr: Amsterdam, 1989; Caralvsis: Jacob&. P. A... Van Santcn, R.A., W.; Vol. 49, p 887. (6) Yan, Y.; Bein, T. Chem. Mater. 1992, 4,975. (7) Somorjai, G. A. Chemistry in Two Dimensions-Surfaces; Cornell University Prcas: Ithaca, NY, 1982. (8) Bain, C. D.; Troughton, E.B.; Tao, Y. T.; Evall, J.; Whitaides, G. M.; N u w , R.G. J. Am. Chem. Soc. 1989,111, 321. (9) Wasscnnan, S.R.; Biebuyck, H.;Whitesides, G. M.J. Muter. Res. 1989. - - __ ,4. (4). ,.,, 886. - - -. (10) Porter, M.D.; Bright, T. B.; Allara, D. L.; Chidscy, C.E.D. J. Am. Chem. Soc. 1987,109, 3559. (11) Li. Z.: Lai. C.; Mallouk. T. E. lnorn. Chem. 1989. 28, 178. (12) For the 5-MHZ QCM used in this &pcriment, a frequency shift of 1 Hz correspondsto a mass change of 17.4 ng/cm2 on one face or 8.7 ng/cm2 when two face are used as in this study.
9393
(13) King, W. H.A d . Chem. 1964,36 (9), 1735. (14) Hlavay, J.; Guilbault, G. G. A d . Chem. 1977,49 (3), 1892. (15) Ballantine, D. S.; Wohltjen, H. A d . Chem. 1989,612 ( l l ) , 704A. (16) Tbia calculation b bowd on a nitrogenadsorption capacity of 0.239 g/g for CaA and a denrc monolayer packing of nitrogen (0.162 nm2) on the external surface at liquid nitrogen temperature. (17) Troughton, E. B.; Bain, C. B.; Whitesides, G. M.;Nuzzo, R. G.; Allara, D. L.; Porter, M.D. Lungmuir 1988, 4, 365. (18) Allara, D. L.; Swalen, J. D. J. Phys. Chem. 1982,86,2700. (19) Benziger, B.; Preston, R.E.;Schoofe, G. R.Appl. opl. 1987.26 (2), 343. (20) Brunauer, S.;Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938,60, 309. (21) Gmgg, S. J.; sing, K.S.W. Adsorption, Surface Arm and Pomity, 2nd ed.; Academic: New York, 1982. (22) Rioco, A. J.; Frye, G. C.; Martin,S. J. Lungmuir 1989, 5, 273. (23) Ishida, H.;Koeni& J. L. J. Colloid Interfuce Sci. 1978,64 (3), 555. (24) Chian& C. H.; Ishida, H.; Koeai& J. L. J. Colldd Intqface Sci. 1960, 74 (2), 396. (25) Schrader, M.E. J. Colloid Interfae Sci. 1984, 100, 512. (26) Janrmk, B.; Bialopiotrowicz, T.; Chibowski, E.; Dawidowicz, A.; Kliszcz, A. J. Mater. Sci. 1990, 25, 1682. (27) Szostak, R. Molecular Stews, Principles of Synthesis und Identiflcurion; Van Nostrand Reinhold New York, 1988. (28) Barn, R. M.Hyabthermal Chemistry of Zeoliteg Acadcmic: New York, 1982. (29) Grcgg, S. J.; S i , K.S . W. Adsorption, Surface Area and Porosiry, 2nd ed.;Academic Press: New York, 1982; Chapters 2 and 4. (30) Carrott, P. J. M.;Sing, K.S . W. Chem. Ind. 1986 (Nov 17), 786. (31) Breck, D. W. Zeolite Mohculur Sieves; Wiley: New York, 1974. (32) Camtt, P. J. M.;Roberts,R.A.; Sing,K.S.W. In Charactvirorion of Porous Solids; Unger, K.K.,Rouquerol, J., Sing,K.S. W., Kral, H.,Us.; Elsevier: Amsterdam,1988; p 88. (33) Kakci, K.;Ozeki, S.;Suzuki, T.; Kaneko, K.J. Chem. Soc., Faruday Trans. 1 1990,80 (2), 371.
Structure and Catalytic Activity of Alumina Supported Platlnum-Cobait Blmetalllc Catalysts. 3. Effect of Treatment on the Interface Layer zoltin Zsoldos and Liszl6 Cuczi* Surface Science and Catalysis Laboratory, Institute of Isotopes of the Hungarian Academy of Sciences, P.O. Box 77, H-1525 Budapest, Hungary (Received: February 25, 1992)
The structure of the interface between metal and alumina in a series of Ptl,cO,/AlzO3 bimetallic samples has keninvestigated by X-ray photoelectron spectrmcopy (XPS) after calcination in oxygen at 770 K with subsequent in situ reduction in hydrogen at 770 K. On all calcined samples platinum was found in the Pt(4+) valence state as an oxide species. Subsequent treatment in H2at 770 K resulted only in a partial reduction of ionic platinum into the m r d e n t state. The remaining platinum found in the Pt(2+) form is strong evidence of metal-support interaction affecting the platinum-alumina system. After calcination a04 and the cobalt surface phase (CSP), dependins upon the cobalt content of the samples, were found to be the predominant species. Starting from a pure cobalt sample (XQ = 1.O), the initial decrease in dispersion of the oxidized cobalt phases is interpreted by the effect of platinum (or chlorine). On thcsc bimetallic samples, the dispersion of the Co304phase forming three-dimensional particles was d e c r d , which also contributes to the enhanced cobalt reducibility. After reduction on the samples with low cobalt content, formation of COR3bimetallic particles was clearly demonstrated. Along with this result, a prediction of the possible catalytic behavior of Ptl-xChx/A1203 catalysts of various surface compositions is discusssd. The results are compared with those obtained earlier in the CBSC of treatments at 570 K (J. Phys. Chem. 1991, 95, 798).
Introdllctioll
Modification of supported cobalt catalyst has been a subject of several investigations. Thape studies are primarily concerned with the increased selectivity toward oxygenates in CO hydrogenation. Alkaline earth metals,' Ru and Re? as well as other transition metals? were found to promote silica supported cobalt catalysts for increased yield of C2+-alcohol. Supported cobalt, in which metallic Cooand the support were interfaced by a c+ balt(2+) oxide layer, was also observed to facilitate alcohol formation.' In addition to the effect of the support, even the various form of cobalt precursors (nitrate, chloride, acetate, and influenced this interface by modifying the carbonyl cluster) reducibility of cobalt, thereby controlling the Coo/CoOratio. 1*5v6
0022-3654/92/2096-9393$03.00/0
These studies indicated that selectivity t a very sensitive function of the Co/support interface. In addition to the effect of various additives, a dedicated balance among the various cobalt species depends also upon the reduction conditions. At low temperature reduction, only a small fraction of cobalt ions is transformed into zerovalent cobalt,whereas at a too high temperature the CoO/AlzO3and Cog04/A1203systems can be converted into an almost irreducible ChAlZO4 spinel phase.' On the other hand, addition of noble metals of group VI11 transition metals (Pt?-ll F'd,12 or Ir9 may help the reduction of cobalt;however, formation of the bimetallic particles has not unambiguously been proven. In earlier studies it was found that in the Ptl,C0~/Al2O3 ~ a m p l e s ' ~after . ' ~ calcination at 570 K followed by reduction at @ 1992 American Chemical Society
9394 The Journal of Physical Chemistry, Vol. 96, No. 23, 1992
the same temperature, platinum became zerovalent, while cobalt was present in several oxidation states. Three different forms of cobalt were idmtifed by X-ray photoelectron spectroscopy (XFS): (i) the almost irreducible cobalt surface phase (CSP) of monolayer thickness was the predominant one in all bimetallic catalysts, (ii) mainly COOwas found in monometallic cobalt sample, and (iii) zerovalent cobalt was also present in all cases in various amounts depending on the sample composition. At low cobalt contents, platinum was found to cover CSP. After the 570 K treatments, bimetallic particles were not detected. However, after reduction under more severe conditions,8 traces of bimetallic particles measured by XRD and EXAFS indicated an intimate contact between the two transition metals. It was further supported by the appearance of higher alcohols in CO hydrogenation." In the present work the main goal is to answer the question of how the structure of interface and its composition in a series of Pt-Co/AlZO3 bimetallic catalysts is modified by treatments under severe condition (calcination at 773 K followed by reduction at 773 K). The data obtained by X-ray photoelectron spectroscopy, along with the previous information,1b1shelp us to determine the chemical environment of the surface components (valence states, quantities) as well as to correlate these results to the species produced at reduction under mild conditions.
Zsoldos and Guczi
m
TABLE I: P h W Ira, 2
(BE)Vd- lftcr Redoctho at 770 K vs Sample
c.id..tiolld after &tieat Qtrlyst coepodtioll (xca) Pt 4d~pBE/eV xco calcined +reduced 0.0 0.2 0.4
317.7 317.3 317.7
Pt 4d~iiBE/eV
315.0 315.2 315.2
xco
calcined
+reduced
0.5 0.67 0.85
317.7 317.7 317.3
315.2 315.2 314.7
CO2~311BE/eV xco
calcined
0.2 0.4 0.5 0.67 0.85 1.o
781.9 78 1.9 78 1.9 781.9 78 1.6 780.3
+reduced 778.7 778.7 778.7 778.3 778.3 778.2
78 1.7 78 1.7 781.7 78 1.7 78 1.5 780.8
6000 7 P
Experimental Section The Ptl,Cox/A1203 samples were prepared by impregnation of yalumina (Woelm). Appropriate amounts of Hz[PtC&]and &(NO,), with 10 wt % total metal loading were dissolved in water and used for impregnation. The atomic fractions of cobalt, xcor 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 0 2 (40 cm3 m i d ) at 770 K for 1 h. The reduction was carried out in situ in flowing Hz (40 cm3 min-I) at 770 K for 1 h in a small reaction chamber attached directly to the ESCA machine. The arrangement allowed us to transfer the reduced samples into the analyzing chamber without exposure to air. More details of the sample preparation have been published e1~ewhere.l~ The XPS measurements were carried out with a KRATOS ES-300 type ESCA machine. The characteristic X-ray was generated with aluminum anode (K,= 1486.6 eV, 150 W).The hemispherical analyzer worked in fixed retarding ratio (FRR) mode, and the slits were set in the largest positions to increase sensitivity. During recording of the spectra, the pressure in the sample analyzing chamber (SAC) did not exceed lo4 Pa. All binding energies (BE) were referenced to the A1 2s peak (1 19.3 f 0.3-eV BEIS) as an internal standard because of the overlapping of the Al 2p and Pt 4f transitions. For the same reason Pt 4d and A1 2s peaks were used to characterize the platinum phases and to represent the alumina support, respectively. In order to determine the intensity of the different cobalt species, the recorded Co 2p regions were synthetized from Gaussian peaks with the help of a one-parameter fitting process. Prior to the fitting, X-ray satellitedg and the inelastic background (Sherwood-type) were subtracted. To calculate the ratio of the reduced cobalt (R)from the XFS results, a method worked out by Stranick et al." was applied, as has been described earlier.14 The method is based on the XPS intensity ratio of the supported monoatomic layer, ZMoN0 developed theoretically for supported cataly~ts.~ This value gives the obtainable XPS intensity ratio of the peaks r e p resenting the supported metal and the support provided that all supported materials are present in the phase of monolayer thickness (maximum dispersion). The values of the inelastic mean free pathz1and the photoelectronic cross sectionZZrequired for the calculations were taken from the literature. The atomic fractions of surface cobalt and platinum used in this work were calculated by normalizing the Co 2p/A12s and the Pt 4d/Al2s XPS intensity ratios to the appropriate relative sensitivity factors in which the energy corrections, because of the large energy differences between the transitions used, were also taken into account. The XPS relative sensitivity factor (S)used for the cobalt 2p line is based on the value found in the literat~re.2~ The sensitivity factor of the platinum 4d doublet was established
n X
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,
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Composition, X(Co) Flgwe 1. Changes in platinum dispersion (D(Pt)) after 770 K treatments vs composition: solid line, after calcination; dashed line, after subsequent
reduction. from the literature value of its 4f transitionlo and from our own measurement on a cleaned platinum foil. ReSdtS The Pt 4d512binding energy values obtained after calcination at 770 K are presented in Table I. At x~ = 0.2 and 0.85, the peaks are centered at 317.3 eV, while at the other compositions the appropriate value is 317.7 eV. In Table I the Pt BE values measured after the subsequent reduction at 770 K are also listed. These are all around 315.2 eV with the exception of that measured for xco = 0.85 composition (314.7 eV). Dispersion of the platinum-containing phases vs the atomic fraction of cobalt is represented by the Pt 4d/M 2s peak intensity ratios referred to a unit amount of platinum, and presented in Figure 1. The character of the curve obtained for the calcined as well as that for the reduced samples is nearly identical. At any compositions the apt)curve obtained after reduction is lower than that measured after calcination. Both curves reveal very high platinum dispersion for the sample with the lowest platinum content (xc, = 0.85). For other compositions the platinum dispersion is nearly constant except for the lowest cobalt content (XC, = 0.2) where a small maximum is observed. The Co 2p regions of the photoelectron spectra, recorded for the calcined and reduced samples of various compositions, are plotted in Figures 2 and 3, respectively. Binding energies of the corresponding Co lines for both the calcined and the subsequently reduced samples were determined and are presented in Table 11. For the calcined monometallic and bimetallic cobalt samples the Co 2p312BE are 780.3 and 78 1.9 eV, respectively, as indicated in Table 11. The slightly lower BE, 78 1.6 eV, belongs to the sample
The Journal of Physical Chemistry, Vol. 96, No. 23, 1992 93%
Alumina Supported R-Co Bimetallic Catalysts
xco= 0 . 2
c *
-
0.85
J
*
1.0
8 10
800
790
7 80
7 70 8 10
Binding Energy lev
Figure 2. Co 2p XPS regions for various sample compositions after calcination at 770 K.
with the lowest platinum content (xc0 = 0.85). In the Co 2p spectra the main XPS transitions are accompanied with strong shake-up satellites for all bimetallic samples (with slightly decreased intensity for the xco = 0.85 sample). However, the shake-up satellites are missing from the spectrum of the monometallic cobalt catalyst (Figure 2). For the reduced samples two different peaks are observed for all samples. The position of the peak of higher BE is 781.7 eV for the bimetallic catalysts while the respective value for the monometallic cobalt sample is 780.8 eV, as is shown in Table 11. In this latter case the Co 2p312peak is shifted 0.5 eV toward higher binding energies compared to that obtained after calcination. The position of the new peak component at lower binding energy is also shifted with compaition. At the lowest cobalt contents (xco = 0.2-0.5) the Co 2p3p BE is 778.7 eV, and this value is shifted to 778.2 eV with increasing cobalt fraction. The appearance of the new peak in the Co 2p spectra is clearly illustrated in Figure 3. Here the presence of the strong shake-up satellite in the spectrum of the monometallic cobalt sample must also be noted. Changes in the cobalt dispersion, D(Co), calculated by the method applied for D(Pt), are shown in Figure 4. Here the curve representing the samples after calcination at 770 K (solid line) has two features. First, for the composition range between xco = 1.O and 0.85, the cobalt dispersion decreased. From this point on, a monotonic increase in the D(Co) is observed as the cobalt content further decreased. Second, at the lowest Co atomic fractions, the dispersion of the cobalt surface phase (CSP) is nearly reached by D(Co). After reduction at 770 K the XPS data for the cobalt phases were quantitatively evaluated by the Stranick method detailed earlier.“ It was assumed17that (i) in the reduced samples the Co 2p peak with higher binding energy is solely assigned to the CSP interface, (ii) CSP is of monolayer thickness, and (iii) the Kerkhof-Moulijn calculationmodel of supported catalystsm is valid for the system. Using the Stranick method and peak synthesis, the relative amounts of different kinds of cobalt species being represented by low (778.2-778.7-eV) and by high (781.6-
790
800
7 80
770
Binding Energy lev
Figure 3. Co 2p XPS regions for various sample compositions after calcination followed by reduction at 770 K.
‘5003
* - - -* -* - - * - -
2.
.-
500
$ 0
1
-.
-calcined
---+
reduced
- b -CSP
,
, 0-
,
,
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0
. ,
e
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,
,
Co
-0-Metallic
Co
0
0.0
0.2
0.4
0.6
0.8
1.0
Corn p ositi on, X ( C o )
Figwe 4. Changes in cobalt dispersion (D(C0))after 770 K treatments vs composition: solid line, after calcination; dashed lie, after subsequent
reduction.
TABLE IIk Extat Of Cobdt R ~ ~ U C(XC, ~ ~ OMok I I h r c t i ~ lOl f Cob.lt; R , Reduced Fraction of Cobalt) md tbe CorrcspoadinO A”@of Metallic ( s w )md Ionic (acos) Cobalt Species As Calculated According to the S M c k Metbod‘
amounts of cobalt/(pmol.&.,,-’)
0.4
62
171
0.5 0.67 0.85
70 72 76
277 465 819
107 117
182 252
781.7-eV) BE could be calculated. The values-together with the extent of cobalt reduction (R)-are presented in Table 111. The amount of cobalt in CSP (ionic cobalt) monotonically increases with rising cobalt content, and it is far below the maximum value (320 pmol.&,,-’) obtained after 570 K treatm e n t ~ .In~ ~Figure 5 the extents of cobalt reduction (R) after
9396 The Journal of Physical Chemistry, Vol. 96, No. 23, 1992
Zsoldos and Guczi
0
7
BE/& A----4
1
material Pto
0
0.0
0.4
0.2
1 .o
0.0
0.6
Composition, X(Co) Figure 5. Extents of cobalt reduction after treatments carried out at 770 K (solid line) and at 570 K’* (dashed line), respectively.
f
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on
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,
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Composition, X( Co)
Flgm 6. Surface compositions determined by XPS vs bulk composition of the catalyst samplcs after different 770 K treatments. treatments at 770 and 570 KI4 displayed by solid and dashed lines, respectively, are compared. Here the solid curve reveals a monotonic increasc of the cobalt reducibility with increasing cobalt content in the 59-76% range. Comparing these results to those obtained after 570 K treatment^,'^ the cobalt reducibility changes in the opposite direction in the region of the lowest cobalt contents (xc, = 0.2-0.4) but exhibits a similar trend for the further range of cobalt. On the basis of the amounts of cobalt in different valence states listed in Table 111, the corresponding D(Co) values could be calculated for the reduced catalysts which are illustrated in Figure 4 (dashed lines). As a CoIIBtQuena of assumption ii accepted for the Stranick method, the surface phase (CSP)containing ionic cobalt species, is characterized by a constant, maximum dispersion value (monolayer). Simultaneously, reduced cobalt species form supported p h w with a much lower dispersion which slowly increases with their decreasing cobalt content. Using the appropriate XPS relative sensitivity factors for platinum and cobalt, the changes in surface vs bulk compositions after calcination and reduction at 770 K are shown in Figure 6. As can be seen, the ionic cobalt p h a s t a p a r t from the sample with x a = 0.2-dominates the surface after both calcination and subsequent reduction. Regarding the surface concentrations of the reduced platinum and cobalt phases, it can be established that the former always exceeds the latter one. This behavior differs from that observed after 570 K treatments,14 where the corresponding curves crossed each other in the medium composition range.
Discllssion In order to identify the platinum species by binding energies of the Pt 4d5ptransition, reference BE values for the relevant &-containing compounds are needed. The respective results from
Pt 4f1p 71.3 71.3 71.0 71.5 71.1 71.3 71.1 71.1 71.1 71.1 no shift (PtO) 72.2 72.3 72.4 72.2 72.5 73.6 73.2 73.0 73.4 74.0 74.1 74.2 74.3 73.7 74.2 75.5 75.7 75.3 75.5 75.4
Pt 4d5/2
314.8 314.8 314.2 314.5
315.9 315.3
317.0 316.9
ref 24 25 26 27 28 29 23 18 30 this work 31 24 26 29 18 32 33 34 33 35 26 27 18 30 29 26 33 34 36 33 this work
“All data are referenced to C 1s = 284.8-eV BE.23 *Data are listed in the order of appearance.
TABLE V: Summuy of tk XPS Data I.&ted h Tnbk IV for Phtinum-Codrw.p Reference Materials material
WO) PtO
WOW2 PtCI2, [PtC4]2-
Pt4d52
BE/ed
material
314.5 315.7 315.8 316.7
PtO2 Pt(OH14 PtCL, [PtC16I2-
Pt4d52
BE&
317.2 317.6 318.9
the literature and from our own measurements are presented in Table IV. Unfortunately, only a few data are available for the Pt 4ds12 XPS transition. However, accepting the “solid sphere model”according to which the binding energy shifts of an inner electron shell (Pt 4dSp) must be equal to the shifts of an outer shell (Pt 4f712)-the corresponding BE shifts of the Pt 4fTI2transition can also be used to determine the shifts of the Pt 4dSl2line. (The reliability of this simple correlation is clearly supported by the referred data.’*) The results obtained for the Pt-containing reference materials are summarized in Table V. Comparison of the Pt 4ds12binding energies established for the samples after calcination at 770 K with those listed in Table V demonstrates that platinum stays in Pt(4+) oxidation state and is surrounded by oxygen atoms (Pt-0 bonds). It means that after calcination at 770 K the chlorine ions around Pt(4+) are exchanged with oxygen. It is further confirmed by the simultaneous measurements of C12p lines in the XPS spectra. Since no correlation between platinum and chlorine contents could be established on the surface by XPS,the conclusion may be drawn that chlorine is mainly bound to the support. This complete ligand exchange is obviously due to the presence of support because at the applied temperature the unsupported HzPtCb transforms into pure Furthermore, two samples ( x a = 0.2 and OM), which are represented in Table I by a slightly lower BE, are the same as those having slightly higher platinum dispersion shown in Figure
Alumina Supported Pt-Co Bimetallic Catalysts TABLE VI: XPS h t a Of Cobrlt-CRdMatcrhkr*b materials CO 2 ~ 3 1 2BE/eV ref 42 778.2 coo 43 778.2 26 778.2 28 778.1 16 781.1 778.0 44 this work 778.1 31 +0.5 BE shift Copt3 45 779.6 COO 46 780.2 47 780.9 48 780.2 43 779.8 26 780.1 28 780.2 49 781.0 23 780.2 44 780.3 this work 780.1 45 780.7 44 781.1 780.9 this work this work 781.9 47 780.1 779.3 42 779.7 43 780.3 50 51 779.5 28 780.0 779.6 49 780.7 16 52 780.6 23 780.0 44 779.5 this work 780.5 779.6 45 780.9 26 45 779.7 26 780.5 781.4 53 49 782.0 782.2 7 78 1.8 52 45 782.1 781.7 this work this work 780.3 ZnCqO, 'All data are referenced to C 1s = 284.8-eVBE.23 *Data are listed in the order of appearance.
1. This coincidence might be interpreted by the fact that on these two samples R(4+) ions are located in a different environment. That is, on the samples with Pt 4d512 BE of 317.7 eV mostly bridgebor~ded~~ R-0-A1 species are formed, indicating metalsupport i n t e r a c t i ~ n , ~while, ~ ' ~ on those having lower BE for Pt 4d52, the predominant platinum phase resembles crystalline Pt02. t! is not known exactly why the platinum dispersion (D(Pt)) after 770 K calcination displays increased values in these latter cases. It might be related to the specific composition of these catalysts: xa = 0.2 and xa = 0.85 contain the highest amount of platinum and cobalt, respectively. For the latter composition the cobalt phase is also present with high dispersion, covering the alumina support to a nonnegligible extent; thus, the D(Pt) calculated by using the intensity of the Ala peak no longer displays the real Pt dispersion. Explaining the behavior of the catalyst at x a = 0.2 composition requires further investigation. On the subsequent reduction at 770 K the Pt 4 d 5 / 2 binding energies are shifted to lower values. The BE measured for the sample with % = 0.85 composition shows that here zerovalent data obplatinum is present (see Table V). The other Pt tained after reduction fall in the region between the values characteristicof metallic platinum and that of PtO. Unfortunately, Pt 4d spectra could not be synthetized because of their complex character. Thus, the observed line position of the samples in the x a = 0 . 6 7 composition range can be interpreted as a sum of
The Journal of Physical Chemistry, Vol. 96, No. 23, 1992 9397 TABLE W: Spmmur of Data Resented In Table M material Co 2 ~ 3 1 2BE/& reliability no. of data *O.l 7 coo 778.1 Copt3 +0.5 shift (to Coo) 1 COO 780.1 h0.9 11 A0.2 3 Co(OH)2 780.9 1 Co(NO3)2 781.9 k0.7 12 c0304 780.0 A0.7 2 C%03 780.2 CoOOH 780.1 k0.4 2 CoAl2O4 781.9 k0.5 6 ZnC%04 780.3 1 TABLE VIlk Chuacteristig of Cobalt 2p XPS Spectra Shown Q Figure 7 for Cobolt-Cont.ininsReference Materials shake-up satellite spin-orbit material BE/eV strong weak coupliig/eV 778.1 co 15.1 780.1 COO 15.5 780.9 16.0 Co(OH)2 16.0 781.9 + Co(NO3)2 781.7 CoAl2O4 15.5 15.0 780.5 cot04 780.3 + ZnCo204 15.0
+ + +
+
two peak components characteristic of Pt(0) and Pt(2+). The obtained BE shift cannot be ascribed to final-state effect since its change is not in correlation with that of the platinum content. As another possibility for explaining this chemical shift, the formation of bimetallic particles could be taken into account. However, this shift is also found for platinum alone, and, on the other hand, no shift of Pt 4f712XPS transition was reported for Pt-Co bimetallics in the l i t e r a t ~ r e . ~Accordingly, ~ it can be established that, after calcination and reduction carried out at 770 K on most samples, platinum is present in metallic form. In addition, platinum in the Pt(2+) valence state is also present on the surface to a significant extent. This is an unexpected result compared with that obtained after similar treatments at 570 K,where movalent platinum is the only species detectcd.4 consequently, after high temperature calcination P t U A l bonds are produced which are partially maintained after reduction resulting in higher BE for the Pt 4 5 1 2 transition. The full reduction of platinum appears only at x a = 0.85, which could be interpreted by assuming that the number of A l ( 3 + ) ions is strongly diminished on the surface. This is probably because the COO interface spreads out and fully covers the aluminum oxide. Nevertheless, it must be emphasized that the primary effect controlling the behavior of supported platinum phases is the metal-support interaction enhanced by the high temperature calcination. As a basis for the identification of the different cobalt phases by XPS,the binding energies of the Co 2p312transitions of the relevant Co-containing materials are listed in Table VI. The data presented in Table VI are summarized in Table VII, where the averages of the BE values are displayed together with the observed maximum difference (reliability) and with the number of data taken into account. As can be seen, the BE values found for the two different cobalt oxide forms (COO and Co304) are close to each other and their uncertainty does not allow us to distinguish between them. However, the structures of the Co 2p spectra of the reference materials, presented in Figure 7, are different for the two kinds of cobalt oxides. Thus, the secondary features (shake-up, spinorbit coupling) of the spectra can make distinction possible. The characteristics of the Co 2p spectra shown in Figure 7 are summarized in Table VIII. The results shown in Table I1 and in Figure 2 for the catalysts with various compositions along with those listed in Tables VI1 and VIII, indicate that after calcination at 770 K cobalt is predominantly present as Co304on the monometallic cobalt sample. For other compositions the highly dispersed CSP14J7is predominant. The 781.9-eV BE determined for the samples with low cobalt contents cannot be assigned to bulk CoAl2O4because
9398 The Journal of Physical Chemistry, Vol. 96, No. 23, 1992
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Figure 7. Co 2p region of the XPS spectra of some Co-containing reference materials.
solid-state diffusion of C0(2+) requires higher temperature than that applied here.' The transition is probably smooth, and at xco = 0.85 CSP and Co304 can be found together indicated by the lower BE of 781.6 eV. The monotonic increase of cobalt dispersion, D(Co),with decreasing cobalt content shown in Figure 4 is in agreement with general experience. The highest D(Co) values in the % = 0.4-0.2 composition range is very close to the maximum (CSP).It is in agreement with the results established earlier on the basis of Co 2~312binding energies (Table 11), i.e. here the CO-containingphase mainly consists of CSP with monolayer thickness. The lower cobalt dispersion found in the XC, = 0.5-0.85 atomic fraction clearly indicates the presence of the three-dimensional Co304 phase. However, the drop in cobalt dispersion observed between the monometallic and the first bimetallic (XC, = 0.85) catalyst samples is unambiguous proof that the presence of platinum (or possibly Cl) decreases the dispersion (Le. increases the particle size) of the Co304 phase. It should also mean that generally the enhancement in reducibility of Co by a noble metal is at least partly due to the decrease in dispersion of the oxide phase (Co304) to be reduced. After subsequent reduction at 770 K a new component in the Co 2p region occurred at the low binding energy side of the spectra, as is seen in Figure 3. A comparison of the CO 2p3/2peak pitions listed in Table I1 with the references in Tables VI1 and VI11 indicates that the peak component at 781.5-781.7-eV BE can be assigned to CSP in the case of the bimetallic samples. Here the absence of a peak component in the medium BE range (780.0-780.5 eV) indicates that ionic cobalt species are all included in CSP,which has been one of the assumptions (point i) for quantitative analysis of the reduced catalysts. However, on the monometallic cobalt sample the position of the component at 780.8-eV BE deviates from the medium BE range of cobalt oxides. Simultaneously, in the corresponding Co 2p region in Figure 3 the strong shake-up satellite also appears. All these mean that after reduction COO is the predominant ionic cobalt phase in the monometallic cobalt catalyst. In addition to that, CSP is also present to a lesser extent as indicated by the BE shift of the peak component from the BE of pure COO.Unfortunately, the similar but not well-defined structures of COOand
Zsoldos and Guczi of CSP spectra (shake-up satellites) do not allow one to make a clear distinction or a precise quantitative analysis. All Co 2p peak components with lower binding energy listed in Table I1 are located in the BE range of zerovalcnt cobalt (Tables VI and VII). Within this region two kinds of peak positions can be distinguished: (i) 778.7-eV BE at X~ = 0.2-0.5 sample compositions and (ii) 778.2-778.3-eV BE for the xco = 0.5-1.0 composition range. The lower BE value can be attributed to the presence of the metallic cobalt phase as compared to the data of Tables VI1 and VIII. The 0.6eV-BE shift toward higher BE could be interpreted by two kinds of phenomena: (i) the shift could be a result from the final-state effect, and (ii) it can be assigned to a new material (initial-state effect), viz., to the formation of &Pt3 intermetallics3' of the same Co 2p3/zBE value (Table VI). The positive BE shift from the final-state effect can be detected only in the case of very small metallic p a r t i ~ l e s . ~However, ~ 3 ~ ~ here the dispersion of the Co(0) phase presented in Figure 4 is too low to represent such small particles. Thus, it can be established that, in contrast to the case of the treatments carried out at 570 K temperat~re,'~ here the bimetallic Copt3phase is formed on the samples. It must be noted that the results obtained do not preclude the presence of bimetallics Over the whole composition range. The lack of the BE shift at xco= 0.67-0.85 indicates only that, instead of the bimetallic phase, monometallic Co(0)is predominant. The extent of Co reducibility (R), shown in Table I11 and Figure 5 (solid line), monotonically increases with rising cobalt content. displayed In comparison with the results of 570 K red~ction,'~ by the dashed line in Figure 5 , the smooth increase of R at the XC, = 0.2-0.4 compositions gives no reason to suppose that after the 770 K treatments the platinum phase covers the CSP interface. When the results in the xc0 = 0.5-0.85 composition range are compared, reduction at 770 K resulted in an increased cobalt reducibility. It can be attributed either to an enhanced formation of easily reducible particulate Co304phase during the calcination or to the partial reduction of almost nonreducible CSP under the more drastic conditions. Since the existence of the saturation coverage of the alumina surface by the CSP was revealed earlier after 570 K treatment^,'^ here the latter explanation is favored. It must also be noted that the monotonically increasing amount of ionic cobalt in CSP (see Table 111) does not point to a similar saturation at 770 K treatments, but the data here are all below those reported earlier.14 Nevertheless, even the highest R value does not exceed 80%, which indicates high stability of CSP. The change of the surface amounts of the reduced platinum and cobalt phases in Figure 6 reveals predominance of platinum over the whole composition range of the reduced bimetallic catalysts. It seems different from what was found after 570 K treatment^,'^ where the curves of the reduced cobalt and platinum crossed each other at the medium compositions. However, in the full knowledge of the results explained above, the values of the surface amounts of reduced components include different kinds of species. Thus,the interpretation relevant for p s i b l e catalytic behavior is complicated. In order to gain useful information about the surface composition, the following assumptions must be a g plied: (i) After reduction at 770 K the BE values of the Pt 4dy2peaks are nearly equal to the simple average of those characteristic of R(0) and of R(2+) (see Table V) in the xco = 0.0-0.67 composition range. Therefore, a Pt(O)/Pt(2+) = 1 ratio is assumed in the case of these catalysts. (ii) CoR3bimetallic particles, as was proven above for the x a = 0.2-0.5 compositions, must be present to as high extent as possible. (For the sake of simplicity the presence of the other possible kind of intermetallics, COR: is omitted.) The results 80 obtained are presented in Figure 8 where Co(0) and Pt(0) indicates the amounts of zeravalent species present in the corresponding monometallic phases. As can be seen in Figure 8 in the composition range of xc0 = 0.2-0.4, the monometallic platinum phase dominates the surface, while at high cobalt contents (xc0 = 0.67-0.85)monometallic cobalt and Copt3 are present in commensurable amounts on the surface. Around xco = 0.5, besides a small amount of Pt(O),
The Journal of Physical Chemistry, Vol. 96, NO. 23, 1992 9399
Alumina Supported Pt-Co Bimetallic Catalysts 0.020
catalytic character of cobalt metal and Copt3 bimetallics are operative together in the bimetallic catalysts of high cobalt contents (XC, = 0.67485). The unique catalytic character of the supported Copt3 is expected to be most purely detected on the catalyst of x a = 0.5 composition.
1 P
I
b
0 0
Acknowledgment. This work was supported by the Hungarian Science and Research Fund (Grant OTKA 1220). R e g l s q NO. CO, 7440-48-4; Pt, 7440-06-4.
0
References and Notes
h{-: ,
0.000 0.0
I
'T
0.2
I
-A- CI A
7
0.4
0.6
0.8
-
1o :
Composition, X(Co) Figm 8. Changes of surface composition vs bulk one according to presumptions detailed in the text.
bimetallics are predominant on the catalyst surface. Consequently, by increasing the cobalt content at first a metallic platinum-like catalytic behavior may be expected. At the higheat cobalt contents the catalytic character of the samples may simultaneously be determined by those of monometallic cobalt and of Copt3 bimetallics. Although Copt3may participate in catalytic processes in the cases of all bimetallic catalysts, its own catalytic character may be expected to be investigated primarily over the catalyst of XG = 0.5 composition.
Conclusions Increase of the pretreatment temperature from 570 to 770 K of Ptl,Cox/Alz03 bimetallic catalysts results in a distinctively different surface structure. (1) Pt(4+) species in the platinum-containing surface phases are only partially reduced into zerovalent platinum during 770 K reduction. This is clear evidence for the metal-support interaction in the Pt/alumina interface compared with the complete Pt reduction observed under 570 K treatments.l Thus, it can be established that, with increasing calcination and subsequent reduction temperatures, the Pt/alumina interface is present in the strong metal support interaction (SMSI)state. (2) After 770 K calcination in the samples with high (xco= 1.0) and low (xG = 0.2-0.5) cobalt contents, three-dimensional Co304phase and CSP in monolayer thickness, respectively, are present as predominant phases. In the monometallic cobalt sample the dispersion of the oxidized cobalt interface is higher than that for lower cobalt contents (XC, = 0.85). This result points out that, in the presence of platinum (or chlorine), dispersion of the particulate Co304phase is lower than in the monometallic cobalt sample, which can also contributeto the effect of enhanced cobalt reduction in the case of bimetallic catalysts. Similarly to what was found earlier," after reduction at 770 K the Co304 phase was transformed into COO only in the monometallic cobalt catalysts, while it was completely reduced into zerovalent cobalt over all bimetallic samples. (3) In contrast to the result of 570 K reduction,l4after the 770 K treatments CSP interface may not be covered by platinum phases as indicated by the smooth change of the cobalt reducibfity (R).The higher extent of cobalt reduction obtained at 770 K reduction may also include partial reduction of CSP. The characteristic difference between 570 and 770 K treatments is that in the latter case formation of the bimetallic (Copt,) phase was directly evidenced by XPS over the catalysts of the lowest cobalt contents (xC, = 0.2-0.5). (4) As a result, on the basii of modified surface compositions the possible catalytic behavior of Ptl-xC0x/Alz03 catalysts of various compositions could be predicted. Accordingly, on the catalysts of high platinum contents (xc, = 0.2-0.4) primarily metallic platinum-like catalytic behavior is expected while the
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Semiintegral Analysis in Cyclic Voltammetry: Determination of Surface Excess and Concentration In the Presence of Weak Adsorption and Thin Films Michael S. F r e d and Anna Brajter-Totb* Department of Chemistry, University of Florida, Gainesville, Florida 3261 1-2046 (Received: March 6, 1992; In Final Form: July 30, 1992) Semiintegral analysis is used as an effective method for determining surface coverage (Le., initial surface excess, r*)from can be determined from the voltammetry in the presence of weak adsorption. Both *'I and bulk concentration (e) semiintegration of a single voltammogram. The approach is illustrated using p-benzoquinone (pBQ)where the adsorption and diffusion response are indistinguishable,characteristicof weak adsorption. A surface ex- of 3 nmol/cm2was determined via semiintegration and confirmed using chronocoulometry. Advantages and disadvantages of the method relative to chronocoulometry are discussed. The semiintegralanalysis approach is extended to cases where the elwtrcdc surface is covered by a thin film where the thickness is sufficiently small so that mass transfer within the Nm is not significant (Le., thickness
