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.

Figure  SEQ Figure \* ARABIC 1 : Particle size distribution of sol immobilised AuPd catalysts: Dried (blue) vs.   Calcined at 800 °C (red).

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

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