Influence of the dispersion of metallic particles on the reaction of triphenylarsine with alumina-supported nickel

The hydrogenolysis of triphenylarsine with alumina-supported nickel catalys ts with various particle sizes was studied at temperatures ranging from 303 to 443 K under 12 bar of hydrogen. The reaction initially takes place sele ctively on the surface of the nickel particles and leads to the successive hydrogenolysis of -As-Ph bonds with benzene and cyclohexane formation. At 3 03 K, the reaction stops when the Ni particles are completely covered with grafted-As-Ph fragments. The quantity of fixed arsenic increases with the d ispersion of the metal particles. It is proposed that more As-Ph fragments (per metallic atom) are grafted onto edge atoms than onto face atoms of the Ni particles. When the reaction is performed at higher temperature, the As atoms migrate inside the nickel particles easily and form an intermetallic compound. At 373 K, the Ni5As2 phase, very poorly crystallized, is obtaine d. At 443 K, the reaction leads to a well-crystallized phase NiAs. The disp ersion of the catalyst has no influence on the nature of the formed interme tallic species. However, the formation rate of these species increases with the dispersion of the catalysts. (C) 2001 Academic Press.