AVS 59th Annual International Symposium and Exhibition
    Surface Science Thursday Sessions
       Session SS+EN+OX-ThM

Paper SS+EN+OX-ThM3
Multi-step Photooxidation of CO on TiO2(110)

Thursday, November 1, 2012, 8:40 am, Room 21

Session: Catalysis and Photocatalysis on Oxides
Presenter: G.A. Kimmel, Pacific Northwest National Laboratory
Authors: G.A. Kimmel, Pacific Northwest National Laboratory
N.G. Petrik, Pacific Northwest National Laboratory
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TiO2 is an important photocatalyst with many practical applications. However, a detailed understanding of the relevant physical and chemical processes for the photocatalysis remains elusive. We have studied the photooxidation of CO adsorbed on rutile TiO2(110) during UV irradiation with ~1 ms time resolution. Previous investigations with ~0.1 s resolution found that the maximum CO2 photon-stimulated desorption (PSD) signal occurred for the first data point and then decreased monotonically with increasing irradiation time. However our experiments with improved time resolution show that the initial rate of CO2 production is zero, and then increases smoothly to a maximum before decreasing at longer irradiation times. Experiments varying the UV photon flux show that the CO2 PSD kinetics are proportional to the photon fluence but are independent of the photon flux (for the range investigated). The photon fluence required to reach the maximum CO2 PSD signal increases as the initial coverage of chemisorbed O2 increases – an effect that we attribute to changes in the initial charge state of the chemisorbed O2. These results demonstrate that the production of CO2 proceeds through the formation of stable precursor. The angular distribution of the photodesorbing CO2, which is peaked at ~40° with respect to the surface normal perpendicular to the BBO rows, is also consistent with the production of CO2 from a precursor state. Previously, the photooxidation of CO on TiO2(110) was believed to occur in a single non-thermal reaction step: CO + O2 + hν -> CO2 + Oad. However, our results show that the photooxidation of CO requires at least two non-thermal reaction steps – one to form the precursor and a second to produce the CO2. We will compare the experimental results to DFT calculations and discuss the role of photo-generated electrons and holes in the photooxidation of CO. These results show that the photooxidation of CO on TiO2 is more complicated than previously appreciated.