NNNS Chemistry blog
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Methane activation by Gallium
05 June 2014 - Catalysis
A previous episode highlighted the conversion of methane to ethylene via iron. Li et al. report here on the conversion of the same methane to benzene now catalysed by gallium. The lede explains that the odds are not in favour of this transition. The reaction 6CH4 -> C6H6 + 9H2 requires 434 KJ/mole. This reaction can take place assisted by heterogeneous catalysis but only with temperatures in excess of 500°C. The new strategy reported about involves photocatalysis and gallium nitride (GaN). It helps that GaN is employed as a nanowire (800 nm length, 6 m2/g surface) that when deposited on silicon orientates itself perpendicular to the surface.
The results look impressive. With a 300 W xenon lamp conversion now takes place at 5°C and benzene and hydrogen are produced in the expected 1 to 9 ratio. The reaction is characterised by an induction period suggesting that it takes time for the methane to absorb to the GaN surface. On one of the GaN surfaces Ga3+ and N3- ions alternate and this arrangement can polarize the C-H bonds in methane. Because the Ga to N distance is longer the C-H bond can be weakened even further. Methyl radicals form first and ethane is the initial reaction product.
Reported conversion rate is 325 micromole h-1 g-1 with 35 mg of catalyst in a 36 hour run. Reported quantum efficiency 0.72%. But hold on. The reported conversion is stated as 0.6 micromole at best. But based on what amount of starting material? This number can only be found in the supplementary info and is: 150 micromole. That puts a dent in the performance. It also does not help (again from the supplementary info) that in order to desorb the reaction product from the catalyst it is heated at 200°C for one hour. Reviewer do something.