Role of Water in Low-Temperature CO₂ Reduction at Defect-Rich TiO₂

Point defects in titanium dioxide (TiO₂) are the most relevant reaction sites for the activation of oxygenates. Herein, we probe the activation of CO₂ on highly defective TiO₂ in situ using synchrotron-based near-ambient pressure X-ray photoelectron spectroscopy between 0.1 mbar and 2.6 mbar from room temperature to 700 K. Multiple carbon surface intermediates form upon CO₂ activation. The presence of key intermediates acts as a fingerprint for the population of different reaction pathways, that is, promotion or suppression of selected reaction steps in various gas environments. In the absence of potent H/OH donors, the formyl, glyoxal, formaldehyde and carbene pathways are populated simultaneously. However, aqueous atmospheres boost intermediate formation and promote oxygen-rich organic molecules, suppressing coke formation along the carbene pathway. Thermal loss of hydroxyls above ≈550 K triggers the population of such oxygen-lean routes, along with a decrease in carbon intermediates, converging to the chemistry under water-free conditions. Our results highlight reduced titania as a noble metal-free CO₂ activation catalyst and demonstrate how water can be used to favor the desired product distribution. The findings herein will guide the development of sustainable catalysts from heavily reduced oxides, for example, black titania, for platform chemicals based on CO₂ as a building block.