Thermo-assisted photocatalytic dehydrogenation of methanol for COx-free hydrogen and formaldehyde production over Au confined in porous TiO2

Abstract

Methanol has great potential as a liquid organic hydrogen carrier (LOHC) and serves as a key feedstock for formaldehyde synthesis via the Formox (250–400 °C) and BASF (600–720 °C) processes. Developing low-temperature methods for methanol dehydrogenation has therefore significant practical interest. Herein, we present a thermo-assisted photocatalytic (TAPC) strategy for methanol dehydrogenation, enabling CO_x-free H₂ and HCHO production in equimolar amounts at a low thermal input (105 °C). Fluoride-etched TiO₂ microspheres (F-TMS) were synthesized, loaded with Au single atoms, fully characterized, and employed as catalysts. The TAPC methanol dehydrogenation was conducted in a continuous-flow reactor, with key parameters (Au loading, temperature, methanol concentration, and light intensity) optimized. A minimal Au loading (0.1 wt%) confined within F-TMS was sufficient to achieve the highest H₂ evolution rate at 105 °C, with no CO or CO₂ detected. Increasing the temperature above 105 °C led to undesirable byproducts (CO, CO₂, CH₄), emphasizing the need for an optimized low-temperature window. No thermocatalytic activity was observed at 105 °C, confirming the essential role of light, further supported by a linear increase in H₂ production rate with light intensity. Water played a crucial role in enhancing hydrogen production, either by providing a rich source of hydrogen ions or by facilitating the generation of ˙OH radicals. The introduction of Au single atoms reduced the apparent activation energy by half, greatly enhancing the kinetics of the methanol dehydrogenation reaction. The gas-phase TAPC process outperformed liquid-phase traditional photocatalysis in both activity and selectivity. Compared to the benchmark TiO₂ P25 photocatalyst, F-TMS exhibited 2.6-fold higher TAPC activity. These findings demonstrate that low-temperature TAPC methanol dehydrogenation over Au/F-TMS offers an efficient and selective route for CO_x-free hydrogen and HCHO production.