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

Authors: 
Delgass, W. N., Purdue University
Thomson, K. T., Purdue University


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 AunO2.
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
AunH2 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.