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.