In this study we assess the general applicability of the widely used P25-TiO₂ in gas-phase photocatalytic CO₂ reduction based on experimentally determined reactivity descriptors from classical heterogeneous catalysis (productivity) and photochemistry (apparent quantum yield/AQY). A comparison of the results with reports on the use of P25 for thermodynamically more feasible reactions and our own previous studies on P25-TiO₂ as photocatalyst imply that the catalytic functionality of this material, rather than its properties as photoabsorber, limits its applicability in the heterogeneous photocatalytic CO₂ reduction in the gas phase. The AQY of IrO_x/TiO₂ in overall water splitting in a similar high-purity gas-solid process was four times as high, but still far from commercial viability.