Buta-1,3-diene and but-1-ene chemisorption on Pt(111), Pd(111), Pd(110) and Pd50Cu50(111) as studied by UPS, NEXAFS and HREELS in relation to catalysis

Abstract

The chemisorption of both buta-1,3-diene and but-1-ene on Pt(111), Pd(111), Pd(110) and Pd₅₀Cu₅₀(111) samples has been characterized by near-edge X-ray absorption fine structure (NEXAFS), ultraviolet photoelectron spectroscopy (UPS) and high-resolution electron energy loss spectroscopy (HREELS). The buta-1,3-diene hydrogenation reaction, investigated on the same surfaces of Pd(111), Pd(110) and Pd₅₀Cu₅₀(111), displays a very good selectivity in butenes and a higher activity compared to Pt(111). Furthermore, the Pd activity greatly depends on the surface crystalline orientation and is also influenced by alloying effects. The origin of these effects has been sought in differences of chemisorption modes of buta-1,3-diene and but-1-ene on the various surfaces: at 95 K, buta-1,3-diene and but-1-ene are physisorbed on the different Pd-based single crystals. On Pt(111), buta-1,3-diene is π-bonded when but-1-ene is di-σ-bonded. At 300 K, buta-1-diene either dehydrogenates into butadiene on Pd(110) and Pd₅₀Cu₅₀(111) or very probably transforms into butylidyne on Pt(111) and on Pd(111) by analogy to ethene adsorbed on these (111) surfaces. Buta-1,3-diene leads to a di-σ mode on Pt(111) and to a di-π one on the different Pd surfaces. Moreover, the NEXAFS esperiments reveal that the π^*₁–π^*₂ splitting variations [2.0 eV on Pd(111), ca. 2.4 eV on Pd(110) and Pd₅₀Cu₅₀(111), cf. 2.7 eV on the condensed multilayer] agree with a decrease of the hydrocarbon–substrate interaction according to the sequence: Pd(111) > Pd(110) Pd₅₀Cu₅₀(111). The activity would therefore obey the reverse sequence in agreement with the reactivity results.