Systematic Evaluation of Reaction Phase Effects on Photocatalytic CO2 Reduction Using Cu-Doped SrTiO3.

Carbon dioxide conversion into value-added products offers a promising strategy to mitigate greenhouse gas emissions while enabling sustainable fuel production. Among the available approaches, photocatalytic CO2 reduction has attracted significant attention, particularly when combined with advanced catalyst design and reaction engineering. In this work, Cu-doped SrTiO3 is employed as a model photocatalyst to systematically investigate the influence of reaction phase on CO2 reduction performance across three distinct reaction configurations: solid-liquid (SL), solid-gas (SG), and pressurized solid-gas (PSG). The results demonstrate that the reaction phase strongly affects both activity and product selectivity. The SG configuration exhibits the highest performance, reaching accumulated yields of 11.13 µmol g-1 of CH4 and 2.32 µmol g-1 of CO after 4 h of reaction, owing to enhanced reactant accessibility and reduced mass-transfer limitations. In contrast, the SL configuration shows the lowest gaseous-product activity due to limited CO2 solubility and accumulation of dissolved intermediates, while favoring the formation of oxygenated liquid products such as methanol and ethanol. The PSG system displays intermediate performance, where increased CO2 availability is partially offset by hindered product desorption. These findings highlight reaction-phase engineering as a key factor governing photocatalytic CO2 conversion efficiency and selectivity.