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ESCA STUDIES ON CATALYSTS
Electron Spectroscopy for Chemical Analysis (ESCA) Studies on Catalysts. Rhodium on Charcoal by J. S. Brinen* Central Research Division, American Cyanamid Company, Stamford, Connecticut 06904
and A. Melera Scientific Instrument Division, Hewlett Packard, Palo Alto, California (Received April IO, 107@ Publication costs assisted by American Cyanamid Company
ESCA (electron spectroscopy for chemical analysis) spectra have been obtained for rhodium foil, rhodium sesquioxide, and several rhodium catalysts. The spectra reveal the presence of both metal and oxide on the catalyst surface. Catalysts with high activity are characterized by a meta1:oxide ratio of less than unity while for poor catalysts the meta1:oxide ratio is greater than unity.
Introduction
Results and Discussion
I n a recent review article,’ the potential application of ESCA (electron spectroscopy for chemical analysis) or photoelectron spectroscopy to catalyst studies was discussed. Since ESCA measurements reveal the chemical composition and chemical states of the atoms on the surface of solids it provides a new probe for catalytic research. Important questions dealing with chemical composition of the catalyst surface and chemical differences, if any, between catalysts of high and low activity may be directly investigated. I n this comunication some preliminary results of such a study on hydrogenation catalysts composed of rhodium on charcoal2 are presented. Examination of the rhodium 3d electron lines of some standard rhodium materials as well as several catalysts provides a clue to the chemistry of the catalyst surface. The spectra clearly differentiate betwen catalysts established to have high and low activity.
Figure 1 shows the 3d5,, and 3d8,, photoelectron lines observed from rhodium foil. I n addition to the lines from the metal (307.1 and 311.9 eV), lines from a surface oxide are also observed. By graphical curve resolution4 the surface rhodium oxide 3d5/, electron line is at -308.4 eV as indicated in the figure. A weak oxygen Is photoelectron line is also observed in support of the assignment of a surface oxide coating. The spectra in Figure 2 show the 3d electron lines from rhodium foil and from rhodium sesquioxide, Rh203. A chemical shift of 1.6 eV is observed between rhodium metal and Rh203. A significant difference in line width is observed between the oxide and the metal. By graphical analysis the line width of the 3db/,electron line for metallic rhodium and for Rh203is 0.7 and 1.6 eV, respectively, while for the surface oxide a line width of -1.8 eV is estimated. Spectra of three catalyst samples examined by ESCA are shown in Figure 3. The catalytic acitivity with respect to the efficiency of selective hydrogenation by each catalyst was independently determined. The spectra of cata1ysts.A and B are quite similar. Two lines were observed indicating the presence of a t least two distinct rhodium species on the catalytic surface. The lowest binding energy peak, -307.1 eV, corresponds to the presence of metallic rhodium. The
Experimental Section The catalysts, rhodium on charcoal, as well as the rhodium foil were obtained from Englehard Industries. Rhodium sesquioxide was obtained from Pfaltz and Bauer, Inc. ESCA measurements were performed on a Hewlett Packard 59508 spectrometer using monochromatic A1 K a radiation. The binding energies reported here are not absolute in the sense that we have not corrected for shifts which may be attributable to charging effects. Such measurements using the gold deposition technique described by Perlman, et u Z . , ~ will be performed in the near future. However, a knowledge of the absolute binding energies does not alter the results regarding the relative efficienciesof the catalysts under investigation.
(1) W. N. Delgass, T. R. Hughes, and C. S. Fadley, Catal. Rev., 4, 179 (1970). (2) R. L. Augustine, “Catalytic Hydrogenation,” Marcel Dekker, New York, N. Y.,1965,p 39. (3) D. J. Hnatowich, J. Hudis, M. L. Perlman, and R. C. Ragaini, J. Appl. Phys., 42, 4883 (1971). (4) The low binding energy edge of the Rh 3d5/%line was folded about the peak maximum and was then subtracted from the observed spectrum to obtain the line positions and line widths for the metal and surface oxide.
The Journal of Physical Chemistry, Vol. 76, No. 18, 1072
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J. S. BRINENAND A. MELERA I
I
1
I
Rh
-..-
Rh CATALYSTS
3d
A
Rh 3d
106
B
307.1
c
'E 8
$
6x104
1)
8
z x 104
315
310 BINDING ENERGY IeVJ
305
I 319.9
Figure 1. ESCA spectrum of rhodium 3d electrons obtained from rhodium foil.
I
I
I
I
---
I
I
309.9
I 289.9
BINDING ENERGY (eVJ
Figure 3. ESCA spectra of rhodium 3d electrons obtained from three different catalyst samples. Spectra A and B are from catalysts of high activity while C is from a catalyst with low activity.
Rh 3d
Rh FOIL
20,00(
RhrOO
v)
J 8 10,000
sities. For C, the intensity of the line corresponding to metallic rhodium is greater than the intensity of the oxide line, which is the reverse of situation existing in Catalysts A and B. Catalysts A and B are highperformance catalysts while C exhibits low performance. The bulk chemical analysis of all these catalysts is the same.6 The desirable catalytic properties may then be attributable to the higher concentration of surface oxide(s) of rhodium in catalysts A and B.6
Summary I I
315
306 BINDING ENERGY ( e 4
Figure 2. ESCA spectra of rhodium 3d electrons obtained from foil (-) and RhlOs (- - -).
second 3d6,, electron line, at -308.4 eV, corresponds to either the surface oxide observed on the metal foil or to Rh203. Absolute binding energy measurements, as discussed above, are required to make a more definitive statement regarding the nature of this species. The photoelectron spectrum obtained from Catalyst C is different from that of Catalysts A and B. The same lines are observed with different relative inten-
The Journal of Physical Chemistry, Vol. 76,No. 18,I N #
ESCA measurements have been performed on three rhodium catalyst systems. The spectra reveal the presence of multiple rhodium species on the catalyst surface. A direct correlation exists between the catalytic activity and the oxide t o metal ratio. This investigation demonstrates the potential power of ESCA in relating performance and surface composition of catalyst sytems. ( 5 ) The surface areas of the carbon substrate for these catalysts are similar. The dispersion of rhodium on the carbon surface is not known. X-Ray measurements show broad lines indicating the presence of small rhodium particles. (6) It should be kept in mind, however, that these differences are observed in the precursors to actual catalysts in their working states. Direct ESCA measurements of catalysts in the course of a reaction may never be realizable. Nonetheless, the correlation which exists between catalytic activity and oxide to metal ratio still retains its importance.
