Have you tried turning the light off and on again? - Mechanistic studies on the impact of light on the hydrogen evolution reactivity of thiomolybdates

Abstract

As the global population continues to grow, energy demand is increasing rapidly. At present, energy production is still largely associated with greenhouse gas emissions, which contribute significantly to climate change. Therefore, the development of sustainable and environmentally friendly energy technologies has become increasingly urgent. Hydrogen is widely regarded as a promising energy carrier, as it enables the controlled storage and transport of energy. However, its current production is itself often associated with substantial greenhouse gas emissions and high energy consumption. Consequently, new strategies for hydrogen production based on renewable energy sources and sustainable materials are required. In recent years, thiomolybdates have attracted considerable attention as catalysts for the light-driven hydrogen evolution reaction (HER). This work focuses on the reactivity of thiomolybdates and their reaction mechanism with an emphasis on the impact of light. Prominent representatives of this compound class are the [Mo3S13]2- ({Mo3}) and the [Mo2S12]2- ({Mo2}) clusters. While the intrinsic reaction conditions of homogeneous light-driven HER systems employing these clusters have already been extensively studied and optimized in the literature, extrinsic parameters such as irradiation conditions have often been neglected. In this work, the influence of dynamic irradiation on these systems was investigated using a Design of Experiments (DoE) approach. Three parameters were selected: frequency, duty cycle, and LED current, the latter determining the irradiation power. The individual effects of these parameters, as well as their interactions, were evaluated. Based on the resulting data analysis, optimized parameter settings for maximizing the turnover number (TON) were identified. In addition, their influence on photonic efficiency was assessed. Furthermore, degradation of both the photosensitizer (PS) and the catalyst under optimized irradiation conditions was investigated in order to rationalize the observed trends. Time-dependent density functional theory (TD-DFT) calculations were used to interpret UV-Vis spectroscopic data obtained during the catalyst degradation process. Thio-oxo-molybdates have also been proposed in the literature as promising catalysts for the light-driven HER, although this field has so far received comparatively little attention. To address this knowledge gap, initial photocatalytic and mechanistic studies on the thio-oxo-molybdate [Mo2O2S6]2− as prototype were carried out in a homogeneous system. Possible catalyst degradation via ligand exchange, as previously reported for thiomolybdates, was investigated using Raman and UV-Vis spectroscopy. In addition, TD-DFT calculations were performed to evaluate the catalytic activity of the different species formed during this process, and a plausible reaction mechanism was proposed.