C-O versus C-C Bond Cleavage in Ethylene Glycol Photochemistry on Rutile TiO2(110): Selectivity Depends on Excess Electrons

Excess electrons play a crucial role in surface reactions on transition metal oxides. However, the role of excess electrons in the selectivity of bond cleavage in photocatalytic reactions is rarely studied. Using ethylene glycol (EG) as a probe, the photochemistry of EG on reduced and O2 pre-dosed rutile-TiO2(110) surfaces has been investigated using the temperature-programmed desorption (TPD) method, which illustrates that excess electrons play an important role in determining the bond cleavage selectivity in EG photochemistry. Acetaldehyde (CH3CHO) formation via the C-O bond cleavage dominates on the reduced surface at high EG coverage. However, formaldehyde (CH2O) formation via the C-C bond cleavage is largely enhanced with increasing O2 exposure; conversely, the yield of the CH3CHO product remains nearly constant. By decreasing EG coverage to minimize the thermal reaction between EG and O2, enhanced CH2O production is still observed at 0.08 ML EG coverage, which is most likely due to the hindrance of excess electrons to the hole-mediated half-reaction of CH2O formation. This may be common for other hole-mediated half-reactions on oxides with excess electrons.