Theoretical Studies on the Synergetic Effects of Au-Pd Bimetallic Catalysts in the Selective Oxidation of Methanol

The selective oxidation of methanol to formaldehyde with molecular O₂ on Au-Pd alloy surfaces has been studied by using density functional theory (DFT). We show that the existence of Pd remarkably improves the adsorption and activation of O₂ on Au-Pd surfaces. In particular, the second-neighbor Pd monomer pair (Pd-SNMP) surrounded by gold atoms can provide unique active sites for the coadsorption of O₂···CH₃OH, thus facilitating the activation of O₂ via a hydroperoxyl radical (∗OOH). With the involvement of ∗OOH and its decomposed fragments (∗O and ∗OH), the oxidative dehydrogenation of methanol to formaldehyde is facilely achieved on bimetallic Au-Pd surfaces, the barriers of which are calculated to be 0.02-0.45 eV on Au₂Pd/(111) and AuPd/(100) surfaces. Importantly, we find that the unusual activation of O₂ via an OOH pathway instead of direct dissociation on Au-Pd catalysts is mainly responsible for the enhanced activity and selectivity in the selective oxidation of alcohols. This hydroperoxyl-based mechanism reveals the intrinsic synergy of Au-Pd bimetallic catalysts in the selective oxidation of alcohols and may provide insights for designing better bimetallic catalysts.