Methane Oxidation on supported Gold Catalysts

Methane (CH4 ), a major compound of natural gas, has been suggested as a future energy carrier.
However, it is also known to be a strong greenhouse gas. The use of CH4 obtained from crude oil
as an associated gas is often uneconomical, and it is thus burned off. Avoiding flaring and making
the energy stored in the molecule available, is a major research challenge.

Extensive steady-state activity measurements were performed to obtain the reaction rates for CO
and H2 oxidation. These reactions were studied on three different gold particle sizes using either
O2 or N2O as oxidation agents. Using particle size distributions obtained from TEM analysis, it
was found that the CO oxidation rates follow the d−3 relationship proposed in [Nano Today 2, 14
(2007)]. To corroborate the experimental findings, density functional theory (DFT) calculations
on the Au{532} surface and a Au12 cluster, which model corner sites, were used in a microkinetic
model. This model reproduced the apparent activation energies for CO oxidation by both O2 and
N2 O. Interestingly, the apparent activation energy for small particles (< 5 nm) is independent on
both particle size and oxidation agent.

CH4 oxidation was studied comprehensively on nanoparticular gold under mild conditions (p =
1 bar, 30C – 250C). Different apparent activation energies lead to suggest that the barrier for
CH4 oxidation is dependent on the particle size. High-resolution TEM images clearly show that
irrespectively to the particles size of the catalysts used, the gold particles adhere to the support
via their (111) facet. In a similar fashion, CH4 oxidation was studied with respect to a support
effect. From both steady-state activity measurements and high-resolution TEM investigations on
the particle’s stability, TiO2 was found to be the most appropriate support material for nanopar-
ticulate gold. The experimental results are corroborated by further DFT calculations investigating
the thermodynamics of CH4 oxidation an a stepped Au{211} surface.
These findings show a decrease in the apparent activation energy for CH4 oxidation on gold as
the gold’s particle size decreases. For CO and H2 oxidation, the apparent activation energies were
found to be approximately the same. From this, it seems that the reaction of CH4 oxidation and the
oxidation of CO and H2 occurs on different active sites. Further, if this trend continues for smaller
particle sizes, the barrier for CO oxidation may become larger than that for CH4. This would open
a route for partial CH4 oxidation.