In situ X-ray absorption spectroscopy to study redox dynamics in Ni and Cu co-catalysts on porous TiO2 photocatalyst films during H2 evolution

Bimetallic NiCu co-catalysts are consistently reported to be more active than monometallic Ni and Cu counterparts for photocatalytic H2 evolution. There is consensus in the literature that this effect is due to the NiCu electronic structure, which provides an optimal adsorption/desorption energy landscape for faster H2 evolution kinetics compared to monometallic co-catalysts. Less is known, however, about the oxidation state of Ni and Cu co-catalysts under photocatalytic conditions, both in the case of mono- and bi-metallic systems. Red-ox dynamics for Ni and Cu are particularly complex in liquid aqueous media where, in addition to changes in oxidation state induced by photogenerated charge carriers, Ni and Cu can undergo dissolution, redeposition and surface reconstruction. Several diverging hypotheses on Ni and Cu oxidation states have been formed in recent years, most of which were based primarily on results of ex situ characterization techniques. Herein, we use in situ X-ray Absorption Spectroscopy to investigate red-ox dynamics in Ni-, Cu-, and NiCu-TiO2 photocatalysts in plain water and water/methanol solutions under hydrogen evolution conditions. This enables us to monitor changes in Ni and Cu oxidation state over time and identify the active phase of Ni and Cu co-catalysts “at work”. It is proposed that the observed synergy in NiCu/TiO2 arises from a division of roles, where Cu serves as an efficient electron sink for hydrogen evolution, while Ni acts as a dynamic hole buffer through reversible oxidation, thereby enhancing charge separation and suppressing self-deactivation pathways.