Photocatalytic carbon dioxide reduction

Dr. Tim Peppel

It is already known that photocatalytic reaction of CO2 and H2O to products such as methane and carbon monoxide is in principle feasible. However, achievable yields are far too low for industrial application, and a deeper insight into the reaction mechanism is still lacking.

Our work is particularly focused on the implementation of reliable, reproducible experimental tests for photocatalytic CO2 reduction as a basis to unravel the mechanistic details of the reaction pathway. The main issue to take care of is the complete exclusion of hydrocarbon impurities in any reactor system: Since CO2 is a thermodynamically extremely stable molecule, any impurity, for example residual solvents from catalyst synthesis or outgassing from greased connectors, would react faster to (apparent) products than the CO2 itself. Yields would then be overestimated, and wrong conclusions on the product distribution would be drawn.

Thus, we operate a high-purity gas-phase photoreactor, consisting entirely of parts suitable for high-vacuum set-up. All connectors are grease-free, and metal seals are used whenever possible. The gas supply contains exclusively gases of highest available purity. Combined with high-end trace gas analysis by a gas chromatograph equipped with a barrier discharge ionization detector (BID), we can characterize the complete product spectrum of the reaction with a detection limit of ~0.1 ppm. Our reactor that can be operated in batch and flow mode provides an ideal basis for the study of the influence of reaction conditions or additives on yields and product selectivity.

Under those controlled conditions, we find that on different TiO2 materials methane is always the main product. Carbon monoxide, formed as byproduct, is not an intermediate on the way to methane. Instead, the formation of methane requires the intermediate formation of C-C bonds, possibly resulting in intermediates such as acetic acid and acetaldehyde. Any potential C1 intermediate (e.g. methanol, formic acid) is preferably oxidized or degraded instead of reacting on to methane.

Current studies target the fate of oxygen in the reaction, the nature of the catalytic active sites, and the improvement of the availability of photogenerated charge carriers at the surface. In the BMBF projects PROPHECY and PRODIGY. We are part of a consortium aiming at the development of alternative catalysts and process conditions in order to increase the yields by orders of magnitude. The R&D projects are combined with sustainability considerations to guide research and development towards ecological and economic feasibility early on.