Single atom and sub-nanometer copper clusters deposited on titania for hydrogen evolution reaction: A density functional study

We present a systematic theoretical study on the applicability of the hybrid Cu_n-TiO₂ systems (n = 1 – 7) for photo electrochemical hydrogen evolution reaction. The preferred binding sites for hydrogen atom are determined. The hydrogen adopts a Cu-H-Ti bridging position when bound to Cu₁-(TiO₂)₃₄ or Cu₇-(TiO₂)₃₄ or Cu-H-Cu when bound to Cu₂-(TiO₂)₃₄ or Cu₄-(TiO₂)₃₄. In Cu_n-(TiO₂)₃₄ with n = 3, 5, 6 the H atom sits in the three coordinate site, similar to that found for its favourable position while on Cu(111). The population analysis reveals that in the selected cases, n = 1, 2, 5, 7, the hydrogen binding exhibits three centre two electron bond character. In all the cases studied here the adsorbate has a hydride character. Among the studied clusters, those containing 1, 4, and 7 copper atoms exhibit moderate Gibbs free H binding energies indicating them as the best candidates for hydrogen evolution. The obtained results are compared with data available in literature for metallic copper and other materials having isolated single Cu atom sites. Considering Cu atom efficiency as well as wide availability and use of the titania as a support, these materials may be competitive for H₂ generation.