Gas-phase oxidation of ethanol over Au/TiO2 catalysts to probe metal–support interactions

Abstract

Ethanol and oxygen were converted over titania and gold nanoparticles supported on titania to investigate the reactivity of the support, the influence of the metal, and the role of metal–support interactions. In addition to determining the degrees of conversion and the yields as a function of temperature, temperature-programmed desorption and diffuse reflectance infrared spectroscopy were performed in fixed-bed reactors under continuous flow conditions. Over pure TiO₂ mainly selective oxidative dehydrogenation to acetaldehyde and water and, to a minor extent, total oxidation to CO₂ and H₂O were found to occur above 500 K. The presence of Au nanoparticles additionally induced the selective oxidation to acetaldehyde and H₂O at temperatures below 400 K. Thus, the Au/TiO₂ catalyst shows bifunctional properties in oxygen activation needed for the selective oxidation of ethanol. Ethoxy species were detected by IR spectroscopy, which are identified as intermediate species in ethanol conversion. In contrast, strongly bound acetates and acetic acid acted as catalyst poisons for the selective low-temperature oxidation route but not for the high-temperature route. Selective low-temperature oxidation is assumed to occur at the perimeter of the Au nanoparticles, which additionally enhance the high-temperature oxidation route on TiO₂ pointing to a Mars–van Krevelen mechanism based on an enhanced reducibility of TiO₂.