Carbon dioxide plus methane

22 August 2015 - And hot electrons

carbon dioxide reforming and hot electrons.PNGNews from the field of carbon dioxide reforming, the experimental process of taking carbon dioxide and methane directly to syngas (carbon monoxide and hydrogen). Methane is already used commercially in syngas production via steam reforming so a modification would get rid of a lot of greenhouse gas carbon dioxide in the same effort. Huimin Liu et al report on a new catalyst system in the Angewandte DOI involving photochemistry and notably hot electrons!.

Catalyst recipe: take a poly(ethylene glycol)-poly(propylene glycol)-poly(ethylene glycol) (P123) triblock copolyer, dissolve in water, add HCl and add tetraethyl orthosilicate. Stir for 24 hours. Then add chloroauric acid to water solution of urea (forces the gold to precipitate), stir 4 hours at 80°C and then calcinate at 400°C for another 4 hours. This process gives you mesoporous silica (SBA-15) a well-known support for nanoparticles, in this case gold nanoparticles. To this configuration can be added rhodium nanoparticles by adding rhodium chloride in water to suspension of Au/SBA-15 with stirring and again calcination. Rhodium loading is 1% and gold loading is between 1% and 5%.

The catalyst was tested by exposing 5 mg in a alumina cell to a methane and carbon dioxide flow in 1:1 molar ratio at 500°C and with visible-light irradiation. GC was the monitoring tool. Result 1: without light Au/SBA-15 did nothing but identical catalytic activity was observed for Rh/SBA-15 and Rh-Au/SBA-15. Conclusion 1: gold does not have anything to contribute. Result 2: with irradiation Rh-Au/SBA-15 outperformed Rh/SBA-15 although increase is less than twofold. Conclusion 2: with irradiation gold does something.

At the selected temperature range photocatalysis alone cannot explain the observed reactivity but the report offers a rationale based on a concept called surface plasmon resonance that alone the gold particles can provide. This resonance creates the "hot electrons" required for the reaction taking place. How? For gold this resonance takes place in the visible range (the UV range for rhodium) creating a plasma of ions and electrons. This creates a local magnetic field which in turn accelerate (?) the electrons into "hot electrons". In simulations local magnetic fields can be observed between the different surface-bound nanoparticles. The hot electrons then are able to polarise the otherwise inert carbon dioxide and methane molecules and hence the enhanced catalytic activity. Hot electrons!