Highly Selective Methane Photooxidation to Formaldehyde by Constructing Symmetry-Breaking Sites

Solar-driven direct conversion of methane (CH4) and water to formaldehyde (HCHO) offers a sustainable route for hydrocarbon production under mild conditions. However, thermodynamic parallels between HCHO and methanol (CH3OH) formation limit product selectivity. Using titanium dioxide (TiO2), we have engineered symmetry-breaking sites to overcome this constraint. The studies reveal that symmetry-breaking sites generate surface hole-trapping states, enabling rapid hole migration (∼8 ps). This process combines with adsorbed hydroxyl groups to produce hydroxyl radicals (•OH), which cleave titanium-oxygen (Ti-O) bonds in methoxy intermediates. The resulting methoxy radicals (•OCH3) drive a single-step pathway that entirely bypasses CH3OH formation, achieving 93.7% HCHO selectivity with HCHO accounting for 100% of liquid products. In sharp contrast, symmetric sites follow a two-step pathway involving substantial CH3OH coproduction via slower electron transfer (>4 ns), yielding only ∼42.8% HCHO selectivity. This work demonstrates a breakthrough in sustainable HCHO production, with symmetry-breaking site engineering commandeering photoinduced charge dynamics to eliminate selectivity barriers.