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

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


Williams, C. - Presenter, Cardiff Catalysis Institute
Carter, J. H., Cardiff University
Dummer, N. F., Cardiff University
Armstrong, R., Cardiff Catalysis Institute
Yacob, S., ExxonMobil Research and Engineering
Willock, D., Cardiff University
Meyer, R., Exxonmobil
Taylor, S. H., Cardiff Catalysis Institute
Hutchings, G. J., Cardiff Catalysis Institute

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.

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

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


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

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

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

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