Adsorption on Rhodium

The adsorption of CH3NC on Rh/Al203 shows only T bonding. On Rh(lll) and a SERS-active Rh electrode, both T and some type of bridge-bonding are proposed. 

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Table 1 summarizes the most important surface complexes formed when NO and CO are adsorbed on noble metal catalysts. According to the literature NO and CO are adsorbed as nitrites, nitrates and carbonates on alumina [2]. The most important surface complexes for CO and NO adsorption on rhodium are a gem-dicarbonyl (Rh(CO)2) and a linear Rh-NO complex [1]. However, tricarbonyl and bridged Rhx-CO complexes have been proposed to be formed and different kinds of linear Rh nitrosyl complexes are possible [1-4]. The adsorption of CO on R and Pd catalysts depends much on the oxidation stage of R and Pd [5]. CO adsorption on R forms mostly linear and bridged carbonyls [6-11]. NO is adsorbed linearly on R [12]. In the case of Pd the most common surface complexes are linear carbonyls [13], strong multilaterally-bonded carbonyls, bridged carbonyls [5,14,15] and triply-bonded CO [5]. Isocyanate, nitrous oxide or nitrogen dioxide are proposed to be coimected to the reaction mechanism of the NO-CO reactions [2,16-19]. 

Wieckowski A, Sobkowski J, Zelenay P (1977) The potential dependence and kinetics of formic acid adsorption on rhodium electrodes. J Electroanal Chem Interfacial Electrochem... 

Wauquier and Jungers 110) have employed a similar treatment to abstract from kinetic data the relative adsorption constants of a number of aromatic compounds on a nickel catalyst. Rader and Smith 111) have extended the measurements to all the possible methyl-substituted benzenes on a platinum catalyst and Smith and Campbell 112) have. studied the same series on rhodium. 

The co-existence of at least two modes of ethylene adsorption has been clearly demonstrated in studies of 14C-ethylene adsorption on nickel films [62] and various alumina- and silica-supported metals [53,63—65] at ambient temperature and above. When 14C-ethylene is adsorbed on to alumina-supported palladium, platinum, ruthenium, rhodium, nickel and iridium catalysts [63], it is observed that only a fraction of the initially adsorbed ethylene can be removed by molecular exchange with non-radioactive ethylene, by evacuation or during the subsequent hydrogenation of ethylene—hydrogen mixtures (Fig. 6). While the adsorptive capacity of the catalysts decreases in the order Ni > Rh > Ru > Ir > Pt > Pd, the percentage of the initially adsorbed ethylene retained by the surface which was the same for each of the processes, decreased in the order... 

C-Tracer studies of acetylene adsorption on alumina- and silica-sup-ported palladium [53,65], platinum [66] and rhodium [53] show the coexistence of at least two adsorbed states, one of which is retained on the surface, the other being reactive undergoing molecular exchange and reaction with hydrogen. Acetylene adsorption exhibits the same general characteristics as those observed with ethylene (see Sect. 3.2). However, there are important differences. The extent of adsorption and retention is substantially greater with acetylene than with ethylene. Furthermore, the amounts of acetylene retained by clean and ethylene-precovered sur-... 

Fig. 21. Adsorption isotherm and composition of the gas phase on equilibrium with the surface for the adsorption of acetylene on rhodium—silica at 20°C. °, Total molecules adsorbed , gas phase acetylene , ethane.

The greater amount of retention observed with acetylene than with ethylene has been ascribed to the ability of the former to polymerise extensively. The existence of surface polymers following acetylene adsorption on alumina- and silica-supported platinum [60], evaporated palladium films [154] and silica-supported rhodium [67] has been demonstrated by thermal desorption studies. 

Thus we may conclude that the pre-adsorption of hydrogen had no effect on CO chemisorption on rhodium while both oxygen and carbon blocked many sites for CO chemisorption and weakened the metal-adsorbate interaction (Vjyj-c decreased, V( q increased,... 

With the temperature increase from 20 to 70°C the region of hydrogen adsorption on the Ir-Rh alloys, and on pure iridium and rhodium becomes narrower, and the potentials of the onset of hydrogen adsorption and of the maxima of the both types of adsorbed hydrogen shift towards less positive potential values. The hydrogen quantity adsorbed on the unit electrode surface actually remains almost the same in this case. 

Grishina T. M., Mestcheryakova E. V., Hydrogen adsorption on platinum, rhodium and their alloys at different temperatures. J. Phys. Chem. 60 (1986) pp. 1510-1513 (in Russian). 

Jia M., Korenovsky N. L., Meretsky A. M., The features of hydrogen adsorption on iridium- rhodium metallurgical alloys during electrolysis of sulphuric acid solutions. J. Phys. Chem. 71 (1997) pp. 2044-2047 (in Russian). 

Supported rhodium is used in many catalytic processes, and rhodium is an active component in the automobile catalytic converter (rhodium catalyzes the reduction of NO to N2, as well as the oxidization of CO to CO2) (1), which explains the large number of investigations of adsorption under UHV (e.g., references cited in Reference (403)). As rhodium surfaces are able to dissociate CO (373-375), CO adsorption may be accompanied by CO dissociation. CO dissociation on rhodium is... 

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. ... 

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