Rhodium-carbon monoxide adsorption

We have undertaken a series of experiments Involving thin film models of such powdered transition metal catalysts (13,14). In this paper we present a brief review of the results we have obtained to date Involving platinum and rhodium deposited on thin films of tltanla, the latter prepared by oxidation of a tltanliua single crystal. These systems are prepared and characterized under well-controlled conditions. We have used thermal desorption spectroscopy (TDS), Auger electron spectroscopy (AES) and static secondary Ion mass spectrometry (SSIMS). Our results Illustrate the power of SSIMS In understanding the processes that take place during thermal treatment of these thin films. Thermal desorption spectroscopy Is used to characterize the adsorption and desorption of small molecules, In particular, carbon monoxide. AES confirms the SSIMS results and was used to verify the surface cleanliness of the films as they were prepared. 

Figure 2.9 Thermal desorption of carbon monoxide from two rhodium surfaces in ultrahigh vacuum, as measured with the experimental set-up of Fig. 2,10. Each curve corresponds to a different surface coverage of CO. At low coverages CO desorbs in a single peak indicating that all CO molecules bind in a similar configuration to the surface. At higher coverages, an additional desorption peak appears, indicative of a different adsorption geometry (courtesy of M.J.P. Hopstaken and W.E. van Gennip [141).

Figure A.16 Energy diagram for the adsorption of carbon monoxide on the (100) plane of rhodium (from de Koster et al. [21]).

Carbon monoxide oxidation is a relatively simple reaction, and generally its structurally insensitive nature makes it an ideal model of heterogeneous catalytic reactions. Each of the important mechanistic steps of this reaction, such as reactant adsorption and desorption, surface reaction, and desorption of products, has been studied extensively using modem surface-science techniques.17 The structure insensitivity of this reaction is illustrated in Figure 10.4. Here, carbon dioxide turnover frequencies over Rh(l 11) and Rh(100) surfaces are compared with supported Rh catalysts.3 As with CO hydrogenation on nickel, it is readily apparent that, not only does the choice of surface plane matters, but also the size of the active species.18-21 Studies of this system also indicated that, under the reaction conditions of Figure 10.4, the rhodium surface was covered with CO. This means that the reaction is limited by the desorption of carbon monoxide and the adsorption of oxygen. 

Dichlorotetracarbonyldirhodium has been obtained by the action of carbon monoxide at high temperature and pressure on a mixture of anhydrous rhodium(III) chloride and finely divided copper powder and by reaction of rhodium(III) chloride 3-hydrate with carbon monoxide saturated with methanol at moderate temperatures and atmospheric pressure. The preparation described here is a modification of the latter method, without use of methanol. This procedure is considerably simpler than the recently described preparation which involves adsorption of rhodium chloride on silica gel, chlorination, and subsequent carbonylation. ... 

Al-Ammar AS, Webb G (1978) Hydrogenation of acetylene over supported metal catalysts Part 1 - Adsorption of [ C] Acetylene and [ C] ethylene on silica supported rhodium, iridium and palladium and alumina supported palladium. J Chem Soc Earaday Trans 74 195 Al-Ammar AS, Webb G (1979) Hydrogenation of acetylene over supported metal catalysts Part 3 - [ C] tracer studies of the effect of added ethylene and carbon monoxide on the reaction catalyzed by silica-supported palladium, rhodium and iridium. J Chem Soc Faraday Trans 75 1900... 

Leung, L.-W.H. and Weaver, M.J. (1988) Adsorption and electrooxidation of carbon monoxide on rhodium- and ruthenium-coated gold electrodes as probed by surface-enhanced... 

The dynamic phenomena associated with the rhodium-catalyzed oxidation of carbon monoxide, methane and propane have been studied by in-situ infrared thermography. High-resolution temperature maps of the reacting catalyst revealed the mobility of the reaction front during ignition and extinction of the CO oxidation, and the development of thermokinetic oscillations. The catalytic oxidation of methane and propane produced weaker dynamics. Chemisorption and kinetic experiments suggest that the competitive adsorption of the reactants and the occurrence of self-inhibition, represent key factors in the development of the observed transient effects. 

Many studies on carbon monoxide adsorbed on polycrystalline and single crystal Pt, Pd, and Rh electrodes have been carried out during recent years by means of electrochemical methods and IR spectroscopy (EMIRS, SNIFTIRS, IRRAS, etc.), potential-modulated reflectance spectroscopy and other methods.Electrochemical results show that the number of Pt adsorption sites per CO molecule is changing from 2 to 1 with increasing coverage in acidic solution. There is, however, a discussion in the literature about the evaluation of absolute saturation coverage on ordered low-index platinum (and rhodium) electrodes with particular reference to Pt(l 1... 

It is known that strong acids act as oxidants. Consequently, acidic surface hydroxyls can also be oxidizers. This reaction potential of surface OH groups is well expressed when they interact with supported metals. For example, it was reported that adsorption of carbon monoxide on dispersed supported metaUic rhodium leads to formation of geminal dicarbonyls of Rh (163). 

Fig. 4 Energy distribution function, (p(e t) (cmol/kJ/mol/), against the dimensionless product of the lateral interaction energy (P) and the local isotherm (0)P0, for carbon monoxide adsorption over a bimetalhc Pto.25-Rho.75 silica supported catalyst, at 698 K. Source From Gas chromatographic kinetic study of carbon monoxide oxidation over platinum-rhodium catalysts, in J. Chromatogr.

V.V. Gorodetskii, W.M.H. Sachtler, G.K. Boreskov, B.E. Nieuwenhuys, Adsorption of oxygen and its reactions with carbon-monoxide and hydrogen on rhodium surfaces— comparison with platinum and iridium. Appl. Surf. Sci. 7(4), 355-371 (1981)... 

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