Enhanced photocatalytic CO₂ reduction to methanol by modulating valence states of copper in CuSrTiO₃

Manipulating the oxidation states of transition metals is an ideal solution to achieve selective generation of different products and copper is widely used in CO₂ photocatalytic reduction to methanol. However, the understanding on the effects of valence states of copper species in SrTiO₃ on its structure and the selectivity of the corresponding product in CO₂ photocatalysis are currently still limited. Here, copper-doped SrTiO₃ nanofibers with different controllable ratios of Cu⁺/Cu²⁺ and Cu⁺/Cu⁰ were prepared by the combination of the electrospinning with the in-situ reduction method. The proportion of copper with different valence states (Cu⁺/Cu²⁺ and Cu⁺/Cu⁰) was changed by rationally adjusting the time of hydrogen reduction. The presence of monovalent copper facilitated the generation of reactive intermediates, thereby enhancing its photocatalytic activity. The copper-doped SrTiO₃ nanofibers obtained after 3 h of hydrogen reduction (STCu_0.08-H-3 h) had the highest proportion of Cu⁺/Cu²⁺ and Cu⁺/Cu⁰ with the highest methanol yield (6.96 μmol·g⁻¹·h⁻¹). Methanol generation rate and Cu⁺/Cu²⁺ ratio of STCu_0.08-H-xh (x = 1, 2, 3, 4,5) with different hydrogen reduction time were proven to be linearly positively correlated. Furthermore, theoretical investigation and in-situ CO₂ infrared spectrum deeply understood the difference of CO₂ adsorption behavior and CO₂ activation over Cu²⁺ and Cu⁺ of STCu_0.08-H-3 h photocatalyst and the reaction intermediates involved in the photocatalytic CO₂ reduction to methanol.