Unraveling the Carbon Footprint: How Ti 3 C 2 T x MXene Stability Affects CO 2 Photoreduction on TiO 2

MXenes are promising cocatalysts for CO 2 photoreduction (CO 2 RR) on semiconductors like TiO 2 . 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 Ti 3 C 2 T x /TiO 2 system. We first demonstrate that aqueous Ti 3 C 2 T x ink undergoes continuous hydrolysis under ambient conditions, releasing CH 4, CO, CO 2, and C 2 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 TiO 2 . Photocatalytic testing revealed a pivotal finding. CO 2 not only failed to increase the rate of CH 4 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 Ti 3 C 2 T x 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 CO 2 reduction from catalyst degradation, providing an essential framework for evaluating stable MXene-based photocatalysts.