Copper as a sacrificial deposit on noble metallic catalysts

Abstract

Multimetallic catalysts exhibit some interesting properties which cannot be expected from the individual components alone. Some spectacular changes in activity and selectivity have been reported, resulting from the addition of a second metal to metallic catalysts [l-6]. The consequent changes were attributed in terms of “ligand (or electronic )” and “ensemble (or geometric)” effects. The concept of the “ensemble effect” is based on the assumption that adsorption or catalytic activation steps require more than one surface atom. On the other hand, the “ligand effect” concept is based on the assumption that the electronic properties, and so the catalytic properties, of a metallic surface atom are influenced by its surroundings and by the nature of the neighbouring metallic atoms. The frequency of the infrared band of adsorbed carbon monoxide can be altered by alloying and such shifts are considered to be evident of the occurrence of a ligand effect [ 71. Several surface species of carbon monoxide absorption on supported rhodium were detected by IR spectroscopy. The dominant ones are linearly bonded carbon monoxide (adsorbing at 2040-2076 cm-‘), the bridged for (adsorbing at 1845-1920 cm-l) and the gem dicarbonyl, Rh(CO),, (yielding bands at 2090-2105 and 2022-2032 cm-’ corresponding to the symmetrical and antisymmetrical vibrations [ 8-91. If the formation of linearly and bridged-bonded carbon monoxide species is ascribed to adsorption on a metallic rhodium surface, it is generally accepted that the gem dicarbonyl species is formed on isolated Rh’ sites. The most probable route for the formation of such positively charged rhodium is the oxidation of rhodium atoms by the OH groups of the support during carbonylation [lo]. The purpose of this note is to define by IR studies, the role of copper deposition on Rh/Al& catalysts, by studying the modifications induced by carbon monoxide adsorption on Rh/A1203 and Rh-Cu/AlzO, catalysts. A series of Rh/A1203 was prepared by exchanging the support (RhBne-Poulenc GFS C alumina) with an aqueous solution of rhodium chloride. The metal