The reactivity, selectivity and structure of 2-butanol on clean and oxygen-covered Au/Pd(100) alloys

Abstract

The surface chemistry of 2-butanol is studied on clean and oxygen-saturated Au/Pd(100) alloys using a combination of temperature-programmed desorption (TPD) and reflection-absorption infrared spectroscopy (RAIRS) as a function of alloy composition. The alloys are synthesized by evaporating gold on a Pd(100) substrate and heating to various temperatures to obtain a range of alloy compositions. It is found that 2-butanol initially forms 2-butoxide species both on clean and oxygen-saturated Au/Pd(100) alloys and can react either to reform 2-butanol or undergo a β-hydride elimination reaction to form 2-butanone. No 2-butanone is formed on alloys with palladium coverages below ~0.5 ML, and the 2-butanone yield increases linearly with palladium coverage at high coverages as the proportion of Pd‒Pd bridge sites on the alloy increases. Some ethylene is formed on alloys with palladium coverages between ~0.4 and 0.6 ML. Saturating the gold-palladium alloy with oxygen for palladium coverages above ~0.5 ML, where oxygen can dissociatively adsorb on the surface at pressures attainable in ultrahigh vacuum, increases the selectivity to 2-butanone formation to 100% for palladium coverages above ~0.75 ML, when Pd‒Pd bridge sites are present, while the selectivity decreases for palladium coverages of ~0.6 ML, when both isolated and bridge sites are present. This indicates that alloy catalysts containing just bridge sites at gold coverages below ~0.35 ML should be the most selective, while still being able to dissociatively adsorb oxygen