PD1NI99 AND PD5NI95 - PD SURFACE SEGREGATION AND REACTIVITY FOR THE HYDROGENATION OF 1,3-BUTADIENE

The combined use of low-energy ion-scattering and angular X-ray photoemission spectroscopy experiments permitted determination of the surface concentration and the concentration depth profile of Pd1Ni99 and Pd5Ni95 alloys equilibrated at 870 K. There was evidence that the outermost surface is largely enriched in Pd: the Pd surface compositions of Pd1Ni99 and Pd5Ni95 were found to be as large as 20 and 50 at.%, respectively. Such results fit well with the predictions based on a recently developed thermodynamic model. Reactivity of both alloys towards 1,3-butadiene hydrogenation was also measured (at 300 K and under an excess of hydrogen pressure). The activity of Pd1Ni99 (5 x 10(15) molecules cm(-2) s(-1)) was found to be of the same order of magnitude as for (111) and (110) Pd single-crystal faces. Furthermore, a strong exaltation was observed for Pd5Ni95 (30 x 10(15) molecules cm(-2) s(-1)), the selectivity in butenes being always about unity. The results are discussed in terms of geometric andior electronic effects. A definition of the ''active site'' is proposed: the active site may involve a group of Pd atoms (probably two or a little more), but not mixed sites constituted of Pd and Ni atoms. These Pd atoms are found to be electronically modified by surrounding Ni atoms. (C) 1994 Academic Press, Inc.