Identification and Exclusion of Intermediates of Photocatalytic
CO2 Reduction on TiO2
Anna Pougin1, Martin Dilla2, and Jennifer Strunk2,
1Ruhr-University Bochum, Bochum/Germany,
2Max-Planck-Institute for Chemical Energy Conversion, Mülheim a.d. Ruhr/Germany
It would be the simplest and most desirable way to convert solar energy directly into energy carriers such as methane over semiconductor powders. However, in spite of more than thirty years of intense research, no system has been developed yet with sufficiently high yields for the industrial scale . The slow progress might potentially be attributed to little understanding of the reaction mechanism and the elementary processes, even for pure TiO2 . In this contribution, significant new insight into the reaction mechanism of photocatalytic CO2 reduction on TiO2 will be presented.
Under the conditions of highest purity applied in this study , CH4 and CO are found as main products of photocatalytic CO2 reduction with a ratio of 4:1. Under identical reaction conditions, CO was not converted at all within a time course of six hours. So, we can clearly rule out CO as a reaction intermediate to CH4 or any other product. Other previously suggested intermediates such as HCOOH, H2CO and CH3OH were added to the photocatalyst, but none of them led to methane formation. All C1 species were exclusively oxidized or degraded. Considering these observations, we strongly support a mechanism involving C-C bond formation and C2 intermediates as suggested previously . Studies of the reactivity of acetic acid and acetaldehyde on TiO2 yielded product distributions closely resembling those in photocatalytic CO2 reduction, which is significant evidence in favor of the C2 mechanism .
 E.V. Kondratenko, G. Mul, J. Baltrusaitis, G. O. Larrazábal, J. Pérez-Ramírez, Energy Environ. Sci. 6 (2013) 3112-3135.
 S. N. Habisreutinger, L. Schmidt-Mende, J. K. Stolarczyk, Angew. Chem. Int. Ed. 52 (2013) 7372–7408.
 B. Mei, A. Pougin, J. Strunk, J. Catal. 306 (2013) 184–189.
 I. A. Shkrob, T. W. Marin, H. He, P. Zapol, J. Phys. Chem. C 116 (2012) 9450–9460.
 A. Pougin, M. Dilla, J. Strunk, Phys. Chem. Chem. Phys., SI, in revision.