Beyond Methane Formation: Product Dilution and the Conditional Relevance of Methane-Selective Extraction in Batch Photocatalytic CO2 Reduction

Photocatalytic CO2-to-CH4 conversion is usually assessed through catalyst activity, selectivity, and methane formation rate, although the practical usefulness of the resulting gas stream also depends on methane recovery from CO2-rich mixtures. Here, a phenomenological Langmuir–Hinshelwood reactor model was used to evaluate batch gas-phase CO2 photoreduction under different reactor thicknesses, water-availability conditions, photocatalyst activities, and idealized methane-selective extraction configurations. The model considered competitive adsorption, transient gas-phase balances, finite or buffered water supply, and a lumped selective CH4 extraction term. Under finite vapor inventory, water depletion limited reaction progress, particularly in thin reactors. Buffered-water operation increased CO2 conversion and methane formation, while reactor thickness produced a trade-off: thin reactors favored apparent conversion and methane enrichment, whereas thicker reactors provided a larger CO2 reservoir and higher cumulative methane formation. At the baseline kinetic condition, methane extraction strongly decreased the in-reactor CH4 fraction but only modestly increased methane formation. When photocatalyst activity was increased, the non-membrane thin reactor reached a product-accumulation-limited regime, and methane-selective extraction became kinetically relevant. These results indicate that methane-selective extraction concepts should be evaluated not only as separation devices, but as conditional reactor-intensification tools whose relevance depends on water availability, reactor geometry, catalyst productivity, and product dilution.