Thermally Assisted Photocatalytic Dehydrogenation of Methanol in Continuous Steady-State Operation

The direct dehydrogenation of methanol to produce formaldehyde has long been considered a dream reaction, because it promises higher product value, lower safety hazards, and lower CO₂ emissions compared to state-of-the-art methanol oxidation. However, thermodynamic equilibrium constraints prevent competitive yields under purely thermocatalytic conditions. This study presents thermally assisted photocatalytic dehydrogenation of methanol as a method enabling formaldehyde yields far beyond the limits of pure thermocatalysis. In continuous, stable operation over 70 h, formaldehyde selectivity of 95% and methanol conversions up to 60 times above thermodynamic equilibrium are achieved. The setup consists of a flow reactor with a catalytically active Pt/TiO₂-coated plate, which is irradiated with UV light from the top, heated from the backside, and continuously fed with methanol. Variation of temperature (78-120°C), residence time (6.1-23.5 s), and irradiation intensity (118.1-255.8 mW cm^{- 2}) shows that high values of all three parameters are required to maximize formaldehyde and hydrogen yields. Continuous co-dosing of oxygen at very low concentrations (≤1 mol%) significantly increases conversion and shifts selectivity toward methyl formate. Our findings suggest that oxygen microdosing allows replenishment of surface oxygen sites on TiO₂, which act as key adsorption centers in the thermally assisted photocatalytic process.