First-principles Study On Photocatalytic Water Splitting And Carbon Oxides Redox Processes On TiO₂ Surface

The conversion of water and carbon dioxide into hydrogen,methanol and other fuels using catalysts under sunlight irradiation presents a promising way to convert solar energy into chemical energy,addressing the global warming and energy crises.However,the low conversion efficiency hinders practical applications.A deeper understanding of photochemical fundamental reactions can help identify drawbacks and facilitate catalyst improvements or new designs.Here,we employ first-principles calculations based on density functional theory（DFT）to explore the photochemical reaction mechanisms of water photolysis,carbon dioxide reduction,and carbon monoxide oxidation on TiO₂ surfaces at the atomic scale,in order to deepen the understanding.The photocatalytic oxygen evolution reaction（OER）process on the rutile TiO₂（110）surface has been detailed investigated.We establish a more comprehensive picture of OER,which includes previously unreported reaction pathways.The energy changes and corresponding energy barriers for each elementary reaction step in these pathways are provided using the high-level HSE06hybrid functional.Charge analysis shows that various oxygen and hydroxyl chains formed during the reaction absorb different amounts of charge from their surroundings.This property determines whether a photo-excited hole is needed to complete a step with favorable energy change;i.e.,with the hole,a step becomes exothermic rather than endothermic.By analyzing the energy change and activation energy of each elementary reaction,we find that OER can be easily preceded on this surface,and therefore the generation,separation,and transport of photogenerated carriers may be the key to improving efficiency.The photo-induced activation of CO₂ on the rutile TiO₂（110）surface has been investigated.So far,the efficiency of CO₂ photocatalytic reduction to value-added chemicals remains low and far from commercial applications.The initial activation of CO₂ is one of the major bottlenecks,as CO₂ is an extremely stable and inert molecule.This process was proposed to proceed mainly by CO₂ accepting a photo-electron to form a CO₂·^-radical,or by CO₂ accepting two photo-electrons and a proton to form HCOO^-anion on the TiO₂ surface.Here,we reveal a new and more favorable reaction mechanism,where CO₂ is directly cleaved to CO and adsorbed O^2-anion under the action of two photo-electrons.Our results deepen the understanding of CO₂ photoinduced activation and point to the deficiency of photoelectrons on the catalyst surface is a potential reason for the current low efficiency of CO₂ photoreduction.The photocatalytic oxidation process of CO on the rutile TiO₂（110）surface has been investigated.Unlike the reactions of photocatalytic water splitting and CO₂reduction,the photo-catalytic oxidation of CO is an exothermic reaction that does not need to convert solar energy into bond energy in the product.This reaction is usually believed to start with the adsorption and activation of O₂ at oxygen vacancies on the surface,followed by the combination of an O in O₂ with CO to form CO₂.Therefore,a defective surface is necessary.However,this pathway is not a sustainable catalytic process because the oxygen vacancy will be filled with the remaining O after the reaction.Here,we reveal a new mechanism in which the photo-oxidation of CO can be sustained on a defect-free surface.Interestingly,we found that the hole plays a catalytic role here,significantly lowering the activation energy of the reaction and not being consumed after a complete catalytic cycle.This is meaningful because it indicates that we can provide holes by p-type doping.Our findings not only deepen the understanding of CO photo-oxidation but also contribute to the understanding of other exothermic photocatalytic reactions.