Properties of supported Pd-Ni catalysts prepared by coexchange and organometallic chemistry .2. Correlation between the reactivity for butadiene hydrogenation and the surface properties studied by XPS and LEIS

The preparation and bulk characterization of a series of silica-supported Pd-Ni catalysts obtained either by coexchange or by organometallic chemistry have been described in Part I (J. F. Faudon, F. Senocq, G. Bergeret, B. Moraweck, G. Clugnet, C. Nicot, and A. Renouprez, J. Catal. 144, 460 (1993)). The present work describes their catalytic activity in the hydrogenation of 1,3-butadiene, in which palladium is one order of magnitude more active than nickel, in relation with their surface composition measured by LEIS, a powerful physical method, still seldom applied to study the surface of real catalysts. Both types of Pd-Ni catalysts exhibit an activity increasing monotonically with the palladium concentration, without any synergy effect, A more rapid increase of initial activity with the concentration is, however, observed for the catalysts with organometallic precursors than for those prepared by exchange. This difference is explained by the migration of palladium from the bulk to the surface of the particles, shown by XPS and LEIS measurements, which is more pronounced on the large particles of the catalysts prepared by organometallic chemistry. Indeed, the theoretical predictions for bulk alloys, based on thermodynamic calculations but corrected for the effect of particle size, are in agreement with the LEIS results and do conclude to more enrichment on large particles. Compared to nickel, the Pd-Ni alloys undergo less deactivation. A larger proportion of l-butene and trans-2-butene is formed on the alloys than on pure nickel. This variation of selectivity is explained by the presence on nickel of a strongly adsorbed di-sigma metallocyclobutene intermediate coexisting with the di-pi adsorbed form of the diene normally present on palladium. This strongly adsorbed species can also explain the low activity of nickel and its large deactivation. (C) 1997 Academic Press.