Exploring Cu oxidation state on TiO2 and its transformation during photocatalytic hydrogen evolution
•Cu oxidation state was tailored by heat treatment in a reducing (H2/Ar) atmosphere.•Initially present CuO was reduced to Cu2O by photoexcited electrons.•H2-treated metallic Cu was retained over the entire photocatalytic reaction.•Both Cu2O and metallic Cu on TiO2 are able to photocatalytically generate hydrogen, albeit by distinctly different mechanisms.
In-depth characteristic studies with H2 activity and theoretical calculations were used to reveal the copper oxidation states most effective for photocatalytic hydrogen production when loaded on TiO2. When the copper was originally present as CuO, photogenerated electrons initially reduced the Cu2+ to Cu+ in preference to proton reduction. The resulting Cu2O then behaved as a secondary photocatalyst on the TiO2 surface acting to improve the hydrogen production rate (1.4 times greater than neat TiO2). When the copper was originally present as Cu0, an improved hydrogen generation rate was also evident (2.4 times greater than Cu2O/TiO2) and the metallic state was retained over the course of the reaction. In this case, the Cu0 deposits function as a co-catalyst for proton reduction. The findings reconcile past disagreements associated with this system, demonstrating both Cu+ (following reduction from Cu2+) and Cu0 are able to photocatalytically generate hydrogen, albeit by distinctly different mechanisms.
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Journal: Applied Catalysis A: General - Volume 521, 5 July 2016, Pages 190–201