(322c) Gold-Palladium Particle Size Effect on Methane Oxidation Activity

Effect of Gold-Palladium Particle Size on Methane Oxidation Activity

Christopher Williams^*[a], James
Carter^[a], Nicholas F. Dummer^[a],
Robert Armstrong^[a], Sara Yacob^[b],David J. Willock^[a],
Randall J. Meyer^[b], Stuart H. Taylor^[a], Graham J.
Hutchings^[a]

[a]Cardiff Catalysis Institute, School of Chemistry, Cardiff University,
CF10 3AT

[b] ExxonMobil Research and Engineering Company, Corporate Strategic
Research, Annandale, NJ 08801, USA

*Email:WilliamsCP1@cf.ac.uk

Natural gas, of which methane is a
major component (approx. 70-90%), is a naturally abundant gas with reserves of
ca. 190 trillion cubic meters.^[1] Both affordable and readily
available, natural gas may provide an alternative feedstock from the traditional
petroleum reserves in addition to providing a cleaner source of fossil energy. Directly
upgrading methane to oxygenates presents a global challenge complicated by the
chemistry of methane. Current utilization involves indirect routes via the
formation of synthesis gas (CO and H₂), requiring costly and energy
intensive processes. A direct methane-to-methanol process at low temperatures
provides an attractive and efficient method of producing an energy-dense fuel
and important chemical feedstock.

In addition to requiring the use of
preformed reactive oxygen species, current systems such as Periana^[2]
or Palkovits^[3] catalysts have required
highly acidic reaction media for the oxidation of methane. Alternatively,
Hutchings et al. have demonstrated the use of H₂O₂ as an
efficient oxidant for the low-temperature oxidation of methane, with noticeable
productivities with the application of zeolite catalysts and supported gold-palladium
nano-particulate catalysts.^[4,5] Gold-palladium
catalysts have also been shown to activate C-H bonds present in toluene^[6]
and in benzyl alcohol^[7], in addition to the direct synthesis of H₂O₂
from H₂ and O₂.^[8] The in situ generation of H₂O₂
provides a catalytic system for the oxidation of methane.

It has previously been reported that gold-palladium catalysts, prepared
by incipient wetness, produce materials with a bimodal distribution of particles
ranging from 5-20 nm and 100-200 nm.^[6,8] These 1- wt% AuPd/TiO₂
catalysts gave turnover frequencies of 6.5 h^-1 and methanol and
oxygenate selectivities of 12.1% and 85.4%,
respectively. Furthermore, methanol selectivity was improved with increasing
metal weight loading to 5 wt%, whilst retaining high oxygenate selectivity. The
bimodal particle size observed with incipient wetness is thought to inhibit
reactivity due to (i) small particles that are highly activation.

In this work, a particle size effect
is observed for methane oxidation using added H₂O₂ at 50
°C. 1 wt% AuPd/TiO₂
catalysts prepared by sol immobilization initially show relatively low rates of
methane activation, forming CO₂ only and decomposing all H₂O₂.
The AuPd catalysts are composed of a uniform size
distribution ranging 4-8 nm, with mean particle size of 4.9 nm. Heat treatment
of the dried catalyst produced a catalyst 3.6 times more active for the oxidation
of methane, with 80% oxygenate selectivity  and 50% of added H₂O₂
remaining after 0.5 h. Analysis by TEM showed that heat treatment at 800 °C yielded
materials with increased AuPd particle sizes, increasing
mean particle size from 4.9 nm to 20.3 nm (Fig.1). This particle size increase
correlated with an increase in TOF (1.3 h^-1 to 4.8 h^-1)
and oxygenate selectivity (0% to 80%). These catalysts demonstrated high
selectivity towards methyl hydroperoxide (64%), a key intermediate towards the
formation of methanol.

The effect of particle size is an
important consideration for the efficient utilization of H₂O₂.
Increased particle size results in reduced H₂O₂ decomposition
rates which facilitate the selective oxidation of methane to valuable oxygenates.
Further work to optimize the synthesis of gold-palladium particles of larger
size will be investigated to further improve the oxygenate selectivity.

References/ Bibliography

 (1)       BP
Statistical Review of World Energy June 2016, bp.com/statisticalreview

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Antonietti, M.; Kuhn, P.; Weber, J.; Thomas, A.; Schüth, F. ChemSusChem 2010, 3 (2), 277–282.

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