Unraveling the Carbon Footprint: How Ti3C2Tx MXene Stability Affects CO2 Photoreduction on TiO2.

MXenes are promising cocatalysts for CO2 photoreduction (CO2RR) on semiconductors like TiO2. However, their inherent hydrolytic instability can produce carbonaceous gases that are identical to the target products. This study systematically investigates this interplay for a model Ti3C2Tx/TiO2 system. We first demonstrate that aqueous Ti3C2Tx ink undergoes continuous hydrolysis under ambient conditions, releasing CH4, CO, CO2, and C2 hydrocarbons, a process that is significantly accelerated by visible light via a photothermal mechanism. Composite catalysts were synthesized and characterized, confirming intimate contact between MXene and TiO2. Photocatalytic testing revealed a pivotal finding. CO2 not only failed to increase the rate of CH4 evolution but even suppressed it compared to that under an inert (Ar) atmosphere, whereas the CO evolution rate remained unchanged. Product formation scaled with MXene loading and dispersion state, with exfoliated flakes degrading faster. The total carbon evolved per hour accounted for only about 1% of the carbon initially present in the MXene, a rate consistent with many literature reports. Our results establish that the hydrolytic self-decomposition of Ti3C2Tx provides a dominant background signal that can confound the interpretation of photocatalytic performance. This work underscores the necessity of rigorous control experiments to distinguish genuine CO2 reduction from catalyst degradation, providing an essential framework for evaluating stable MXene-based photocatalysts.