Active sites for methanol partial oxidation on nanoporous gold catalysts

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 O-2 and the subsequent reaction of methanol with the adsorbed atomic oxygen are critical steps in this selective oxidation. The density of sites for O-2 dissociation was determined to be 0.1% of the total surface (3 x 10(12) per cm(2)) using both transient and steady flow measurements. The activation energy for O-2 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 O-2 at 423 K is similar to 1 x 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 0 atom is 10(-4)-10(-5), which corresponds well with the measured turnover frequency for methanol conversion to form methylformate of similar to 160 s(-1) at 423 K. Taken together, these results strongly indicate that Ag is an integral part of the active site for O-2 activation and the subsequent activation of methanol. (C) 2016 Elsevier Inc. All rights reserved.