New Insight into the Role of Electron Transfer to O₂ in Photocatalytic Oxidations of Acetone over TiO₂ and the Effect of Au Cocatalyst

Photocatalytic oxidation by semiconductors is a dominant way to eliminate toxic organic pollutants. Different from thermal-activated catalysis, it is generally considered that the rate of charge carrier transfer from semiconductors to reactants determined photocatalytic activity. However, how charge carrier transfer correlates with photocatalytic activity is not well known, especially in gaseous photocatalytic oxidations of organics. By means of photoconductances, the present research gains some novel insight into the electron transfer in the acetone photocatalysis over TiO₂. Because it is shown that the electron transfer from TiO₂ to O₂ is faster than the acetone conversion, our results point toward a fact that the electron transfer also contributes to the recombination via the O₂ sorption cycling in addition to the photocatalytic effect. The role of Au for a cocatalyst was also investigated in the acetone photocatalysis. It is found that the decoration of TiO₂ with Au nanoparticles indeed leads to an increase in the electron transfer from TiO₂ to O₂. Instead of a desirable increase, the photocatalytic rates however are decreased by Au decoration, independent on the methods to deposit Au, the Au nanoparticle size, and the Au amounts. The Au decoration also has no effect on the apparent activation energies of acetone conversion. These results lead to that the Au-induced increase in the electron transfer cannot contribute to the photocatalysis but can contribute to the recombination via the O₂ sorption cycling. Therefore, it is possible that the photoinduced holes tend to accumulate around the Au/TiO₂ perimeter and then recombine with the photoinduced electrons stored on Au at a faster rate, resulting in the decrease of photocatalytic activity. This research sheds some new light on the role of electron transfer in photocatalysis. The mere increase of the electron transfer could not promote the photocatalytic effect if the O₂ sorption-assisted recombination is not inhibited; this should be helpful in designing highly-efficient photocatalysts.