143
Applied Surface Science 29 (1987) 143-146 North-Holland, Amsterdam
LETTER SOME COADSORPTION J.H. CRAIG, Department Received
EFFECTS
ON Rh(ll1)
Jr.
of Physics, Uniwrsiry of Nebraska at Omaha, Omaha, NE 68182. USA 5 December
1986; accepted
for publication
20 April 1987
ESD energy analysis is used to study the reaction products produced during coadsorption of CO-O, and CH,-0, on Rh(ll1). Residence of CO, on the surface is confirmed by detection of a CO: ionic component with energy of 1.8 eV. Adsorption and dissociation of CH, on oxygencovered Rh(lll) is inferred as a result of a low energy component present in the energy spectra of desorbed O+.
The interaction between coadsorbed molecules on solid surfaces is of considerable interest because of its obvious relation to catalysis. Much interest currently exists in identification of surface reaction products or intermediates resulting from interaction of coadsorbed species. The approach to this problem pursued in this paper is use of ESD energy analysis to detect presence of surface species other than the parent adsorbed molecules. Electronically desorbed ions are known to be adsorption site specific. A given adsorbed atomic or molecular species is expected, by electron desorption, to release an array of ions with mass and energy characteristic of the adsorption state. In principle, it should be possible to correlate an ion of a given energy with a specific adsorption site. In a previous paper [l] the characteristic ions along with their desorption energies have been reported for CO and 0, adsorbed on Rh(ll1). In this paper, these data are used to detect reaction products for two different coadsorption systems. Since rhodium is known to be effective in CO oxidation, coadsorption of 0, and CO on Rh(ll1) has been examined from the point of view of ESD energy analysis. The second coadsorption system for which data is presented is 0, and CH,. Methane has an extremely small sticking coefficient on metal surfaces, estimated to be lop5 on rhodium [2]. However, significant enhancement of the ability to accommodate and dissociate methane by metal surfaces has been achieved by preadsorption of oxygen on W(lll) [3] and Ni(lOO) [4]. The energy spectra of fig. 1 were acquired after a 2 L exposure of the clean Rh(ll1) surface to a 1 : 1 gas mixture of CO and 0,. Thermal desorption studies showed generation of CO, from the surface at 375 K. The observation of a CO: (fig. la) component in the array of ions desorbed is significant since 0169-4332/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
J. H. Craig, Jr. / Some coudsorptron effects
,
on Rh(l I I)
,
r
1
4
8
I2
ENERGY
4
(eV)
12
8 ENERGY
I
(eV)
Fig. 1. ESD energy distribution for (a) COT. (b) 0 ‘. and (c) O- during wadsorption of CO and 0, on Rh( 111). The sample temperature was 300 K. The electron beam energy was 250 eV.
it confirms residence of some fraction of the CO2 on the surface after reaction. The yield of CO; was however very low. The yield ratio of CO-j+ to 0 ’ was approximately 1%. The small observed ionic cross section for release of CO-j+ is probably not entirely due to a small molecular desorption probability. Rather. because of the large mass of the molecular species, upon initial desorption its velocity near the surface is small. Consequently, one would expect a large probability for neutralization of the ion in the near-surface region. Fig. lb shows the energy distribution of 0 + during coadsorption of CO and 0,. There are several significant aspects associated with this distribution. The main peak at 7.5 eV corresponds to presence of adsorbed CO. Adsorbed oxygen on Rh(lll) exhibits an O+ energy peak at 5.6 eV. There is no evidence for an Ot component in this spectrum associated with adsorbed oxygen. Apparently all of the adsorbed oxygen, upon coadsorption with CO, was depleted in the oxidation process to CO?. leaving some residual CO and the
J.H. Craig, Jr. / Some coadsorption effecis on Rh(ll1)
145
product CO,. Furthermore, the low energy peak at 2.5 eV appears neither in the O+ spectrum of CO or O2 on Rh(ll1). It is likely that this species is an ion fragment resulting from dissociation of some fraction of the adsorbed CO, molecule by electron impact. If the surface is gently warmed to 375 K, the peak desorption temperature of CO,, and returned to room temperature, both the 2.5 eV Of peak and the CO: signal decrease, suggesting a common origin. The energy distribution for O- (fig. lc) during coadsorption of CO and 0, confirms the absence of chemisorbed oxygen. The 5.6 eV peak is associated with sample exposure to oxygen, but appears to be due to oxide formation, since it can only be eliminated by heating above the desorption temperature of oxygen. Chemisorbed oxygen on Rh(ll1) exhibits an O- energy peak at 7.7 eV, which is absent from fig. lc. As indicated in the figure, the high temperature shoulder appears at 9.2 eV which corresponds to the energy of O- ions released from adsorbed CO. Thus, the absence of a 7.7 eV peak confirms the absence of chemisorbed oxygen on the surface. The second case considered is sample exposure to methane with oxygen preadsorbed. As indicated above, the very small sticking probability of methane on clean metal surfaces has been significantly enhanced by preadsorption of oxygen on W(111) [3] and Ni(lOO) [4]. In the present case, the effect of preadsorbed oxygen on methane adsorption on Rh(ll1) was examined. When a clean Rh(ll1) surface was exposed to 3 L methane after pre-exposure to 1 L oxygen, upon heating, desorption of H,, CO,, CO and 0, was observed, indicating the probable adsorption and dissociation of CH,. The O+ energy distribution shown in fig. 2 was acquired before heating, after the oxygen and methane exposure conditions indicated above. The double peak structure observed is characteristic of the oxygen-bearing species on the surface. The high energy 7.6 eV peak may easily be associated with a CO reaction product
I
76
ENERGY
(eV)
Fig. 2. ESD energy distribution for O+ after sample exposure to 1 L 0, followed by 3 L CH,. Sample temperature during exposure was 300 K. The electron beam energy was 250 eV.
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J. H. C’rmg Jr. / Some coad.sorprron effects on
Rh(I I I)
adsorbed on the surface. The low energy 3.7 eV peak cannot be easily associated with a previously observed adsorbed species. In the previously referenced work on Ni(lOO) [4] formation of an intermediate species, tentatively identified as CH,O, was proposed based on an EELS analysis. Based on this interpretation it is very tempting to attribute the 3.7 eV peak to an intermediate surface species of the type proposed for Ni(lOO). Although a definitive answer is not available, the high sensitivity of ESD to loosely bound surface species makes this a reasonable suggestion. In summary, application of ESD energy analysis appears to be a rather useful technique in detection and identification of adsorbed surface species. Broadening of the data base for known desorption energies of adsorbed molecules could provide a means of identification of surface intermediates formed during catalytic reactions on surfaces. The author wishes to acknowledge the support of this work by the University Research Committee of the University of Nebraska at Omaha.
References [l] [2] [3] [4]
J.H. Craig, Jr., Appl. Surface Sci. 28 (1987) 323. C.N. Stewart and G. Ehrlich. J. Chem. Phys. 62 (1965) 4672. T.E. Madey, Surface Sci. 29 (1972) 571. H. Wise and M. Quinlan, in: Proc. Workshop on Basic Research Opportunities in Methane Activation Chemistry, February 4-6, Houston, TX. 1985, p. 51, Gas Research Institute Report No. GRI-85/0142.