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RhCu/Si02 catalysts of various compositions were prepared from nitrate precursors and characterized by transmission electron microscopy and chemisorption. The catalytic properties were determined in a flow microreactor, at low conversion, for the hydrogenation of benzene, poisoned or not by thiophene, and the conversion of a series of alkanes: «-hexane («-H), 2-methylpentane (2MP), 2,2-dimethylbutane (22DMB), 2,2,3,3-tetramethylbutane (2233TMB), methylcyclopentane (MCP). The conversion of alkanes at 493 K gives mainly hydrogenolysis of C-C bonds. The rate of cleavage depends on the degree of substitution of the carbon atoms and on the Cu/Rh ratio. Cu inhibits all reactions; however, the magnitude of this effect depends on the structure of the reactant. The comparison of the alloying effect with the effect of Rh particle size on the same set of reactions (Coq et al.) shows that on the one hand 2233TMB has a lower sensitivity to alloying than to particle size; the selectivity to i-C4 remains high and characteristic of large particles of rhodium up to Cu/Rh = 1. On the other hand n-H, 2MP, and MCP are more sensitive to alloying than to change of particle size. These results suggest that Cu occupies preferentially the sites of low coordination, edges, and corners, and Rh the sites of high coordination on dense planes. This set of reactions, therefore, probes the extent of surface segregation on RhCu-supported particles. By addition of thiophene a monotonic decrease of activity in the hydrogenation of benzene occurs due to sulfurization of the surface. The rate of poisoning is faster when the local density of states at the Fermi level of Rh surface atoms is higher. Dilution of Rh by Cu has little effect on the thioresistance which remains low even at high Cu content. Very likely little electronic interaction occurs between Rh and Cu.

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