Pd{sub 1}Ni{sub 99} and Pd{sub 5}Ni{sub 95}: Pd surface segregation and reactivity for the hydrogenation of 1,3-butadiene
- Institut de Recherches sur la Catalyse, Cedex (France); and others
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 Pd{sub 1}Ni{sub 99} and Pd{sub 5}Ni{sub 95} alloys equilibrated at 870 K. There was evidence that the outermost surface is largely enriched in Pd: the Pd surface compositions of Pd{sub 1}Ni{sub 99} and Pd{sub 5}Ni{sub 95} 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 Pd{sub 1}Ni{sub 99} (5 x 10{sup 15} molecules cm{sup -2} s{sup -1}) was found to be of the same order of magnitudes as for (111) and (110) Pd single-crystal faces. Furthermore, a strong exaltation was observed for Pd{sub 5}Ni{sub 95} (30 x 10{sup 15} molecules cm{sup -2} s{sup -1}), the selectivity in butenes being always about unity. The results are discussed in terms of geometric and/or electronic effects. A definition of the {open_quotes}active site{close_quotes} 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. The Pd atoms are found to be electronically modified by surrounding Ni atoms. 45 refs., 8 figs., 1 tab.
- OSTI ID:
- 98933
- Journal Information:
- Journal of Catalysis, Vol. 149, Issue 2; Other Information: PBD: Oct 1994
- Country of Publication:
- United States
- Language:
- English
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