Photocatalytic Cleavage of C-H Bonds: A Case Study of Formaldehyde Photodegradation from Density Functional Theory

Photocatalysis can efficiently activate C-H bonds and achieve complete degradation of organic pollutants. However, there is still a lack of knowledge regarding the photodegradation mechanism at the atomic scale or the regulation principle for the rational design of novel catalysts. Herein, we took the photocatalytic degradation of formaldehyde to CO2 on the rutile TiO2(110) surface as an example and conducted the density functional theory calculations to clarify and compare the reaction pathways of the direct hole-trapping degradation and the (OH)-O-center dot radical-assisted indirect oxidation of formaldehyde featuring various adsorption and intermediate configurations. Our results clearly demonstrated the significant promotion effect of photogenerated holes and/or (OH)-O-center dot radicals on the C-H cleavage and revealed that the (OH)-O-center dot is more efficient for cleaving the first C-H bond of formaldehyde whereas the photogenerated holes for the second. Accordingly, an optimal formaldehyde photodegradation mechanism integrating the individual advantages of photogenerated holes and (OH)-O-center dot radicals was proposed. By investigating the C-H bond activation of common organic molecules such as alkanes, alcohols, aldehydes, and esters on the rutile TiO2(110) surface, we also found that both the photocatalytic and thermocatalytic reaction barriers of the C-H cleavage correlate well with the corresponding C-H bond polarity, exhibiting the antivolcanic and linear trends, respectively. Given the general significance of the (OH)-O-center dot radicals in organic photodegradation, we further suggested to promote water hydrolysis and (OH)-O-center dot radical formation by doping Pt into TiO2, and it may broaden the conventional understanding about the functional manners of Pt components in photocatalysis.