(447d) Hydrogen Peroxide Formation and Propylene Epoxidation on Gas-Phase Au Clusters

Abstract

We report a
detailed DFT (B3LYP) analysis of the gas-phase H₂O₂
formation from H₂ and O₂ on the Au₃, and Au₄⁺.
We find that H₂, which interacts only weakly with the Au clusters,
is dissociatively added in the Au-O bond, upon interaction with the Au_nO₂.
Once formed, the hydroperoxy (OOH) intermediate acts as a precursor for the
closed-loop catalytic cycle. The second H₂ addition to form H₂O₂
is the rate determining step (RDS) of the close loop catalytic cycle. The
H₂O₂ desorption is followed by O₂ addition to
Au_nH₂ to form the hydroperoxy intermediate, thus leading
to the closure of the cycle. Based on the Gibb's free energy of activation, Au₄⁺
is more active than Au₃ for the formation of the H₂O₂.
In the next step, we also studied the propylene epoxidation on the neutral Au₃,
with hydroperoxy intermediate as the precursor. The more electrophilic O atom
(proximal to the Au) of the OOH group attacks the C=C of the propylene to form PO,
with an activation barrier of 19.5 kcal/mole. Although the activation barrier
of the RDS in this mechanism (with no Ti involved) is somewhat higher than that
in the published olefin epoxidation mechanisms on the Ti site (with no Au
involved), our pathway is a potential PO formation channel, with only Au
playing a direct role in the reaction.