Active sites for methanol partial oxidation on nanoporous gold catalysts

Abstract

Nanoporous gold is a complex material comprised of a small amount of silver that is the residual from the dealloying process used in its formation. This material activates dioxygen and selectively self-couples methanol. The dissociative adsorption of O2 and the subsequent reaction of methanol with the adsorbed atomic oxygen are critical steps in this selective oxidation. The density of sites for O2 dissociation was determined to be 0.1% of the total surface (3 × 1012 per cm2) using both transient and steady flow measurements. The activation energy for O2 dissociation was measured to be 5.0 kcal/mol and is similar in magnitude to that on metallic Ag and much lower than expected for Au surfaces. The area-averaged dissociation probability of O2 at 423 K is ∼1 × 10−7, commensurate with the active site density and the activation barrier to dissociation. The reactive oxygen is immobile under reaction conditions. The collisional reaction probability of methanol striking an adsorbed O atom is 10−4–10−5, which corresponds well with the measured turnover frequency for methanol conversion to form methylformate of ∼160 s−1 at 423 K. Taken together, these results strongly indicate that Ag is an integral part of the active site for O2 activation and the subsequent activation of methanol.