Adsorption and reaction of acetaldehyde on Pt([formula omitted]) and Sn/Pt([formula omitted]) surface alloys

Abstract

Adsorption and reaction of acetaldehyde (CH3CHO) on Pt(111) and two ordered Pt–Sn alloys has been studied primarily by using temperature-programmed desorption (TPD) mass spectrometry and high-resolution electron-energy loss spectroscopy (HREELS). The two alloys investigated were the (2×2) and (√3×√3)R30°Sn/Pt(111) surface alloys with θSn=0.25 and θSn=0.33, respectively, as-prepared by vapor deposition of Sn on a Pt(111) single-crystal substrate. The desorption products in TPD experiments following CH3CHO exposures on the Pt(111) surface were CH3CHO, CO, H2, and CH4. Auger electron spectroscopy (AES) detected some carbon (θC∼0.1 ML) left on the surface following TPD experiments. At small coverages, CH3CHO adsorption on Pt(111) is completely irreversible and all CH3CHO decomposes during heating in TPD. At near monolayer coverages, CH3CHO is partially reversibly adsorbed, with desorption competing effectively with decomposition so that 52% of the adsorbed monolayer of acetaldehyde decomposed during heating in TPD. CH3CHO bonds molecularly, mainly in a η1(O) configuration on Pt(111) at 90 K. On both of the (2×2)Sn/Pt(111) and (√3×√3)R30°Sn/Pt(111) alloys, no CH3CHO decomposition took place during TPD and the adsorption–desorption behavior was entirely reversible. HREELS revealed that only η1(O)-CH3CHO was present on the two Sn/Pt(111) alloys. CH3CHO is adsorbed more weakly and the chemical reactivity of CH3CHO on these Sn/Pt alloys is decreased from that on the Pt(111) surface. Thus, the presence of Sn in the surface layer of these Pt–Sn alloys does not thermally activate acetaldehyde for reaction in UHV due to the thermodynamic driving force provided by the Sn–O interaction. We attribute this to be primarily because of kinetic barriers that arise from Pt–Sn bonding interactions in the alloy and acknowledge the important implications of this observation for the synthesis of organic molecules by selective oxidation over PtSn catalysts.