Influence of ensemble size on CO chemisorption and catalytic n-hexane conversion by Au-Pt(111) bimetallic single-crystal surfaces

Abstract

Epitaxial Au layers were prepared by vapor deposition of Au on a Pt(111) single-crystal surface. Surface alloys were formed by heating the Pt(111) crystal that was covered with a Au multilayer. The surfaces were characterized with Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), and temperature-programmed desorption (TPD) of CO. LEED showed that both gold overlayers and Pt-Au alloy surfaces were well ordered and had (1 × 1) surface structures. The temperature at the maximum of the TPD spectrum of CO was found to be sensitive to the distribution of Pt atoms in the surface. As a result, epitaxial surfaces were shown to have large Pt ensembles while much smaller ensembles were predominant on alloy surfaces. With TPD of CO it could also be demonstrated that the ligand effect of alloying is absent for CO adsorbed on these alloy surfaces. The differences in ensemble size were found to have profound effects on the skeletal reactions of n-hexane. This reaction was carried out in situ with 26.7 mbar (20 Torr) n-hexane in 267 mbar (200 Torr) hydrogen at 573 K, by utilizing a sample isolation cell. Surface alloys were found to be more active than pure Pt(111). Large increases in the isomerization rate of n-hexane and simultaneous exponential decreases of hydrogenolysis and aromatization rates with gold concentration led to high selectivity for isomerization. These effects are caused by changes in the bonding of organic molecules that are induced by structural alterations of the Pt(111) single-crystal surface. Epitaxial gold layers decreased the activity of the Pt(111) surface in proportion to the gold coverage because of the reduction of the available platinum surface area, without substantial selectivity changes.