Formaldehyde, a major hazardous indoor pollutant, can be efficiently degraded by TiO2 photocatalysts, yet its surface mechanism under ambient conditions remains elusive. Here we use in situ sum-frequency vibrational spectroscopy to track formaldehyde adsorption and UV-driven photodegradation on rutile-TiO2 (110) in controlled formaldehyde and oxygen atmospheres. While dioxymethylene (DOM) is often considered the most stable adsorption configuration, we found that it coexists with molecular formaldehyde across a wide pressure range. Density-functional theory attributed this stabilization to a Lewis acid–base pairing effect. Under UV irradiation, elevated oxygen partial pressure unexpectedly retards DOM degradation, suggesting an oxygen-poisoning effect in ambient air that highlights the key role of oxygen vacancies on the surface.
Zhou et al. (2026) studied this question.