Efficient UV-vis-IR photothermocatalytic selective ethanol oxidation on MnOx/TiO2 nanocomposites significantly enhanced by a novel photoactivation

Abstract

MnO_x/TiO₂ nanocomposites of amorphous manganese oxide supported on TiO₂ with Mn/(Mn + Ti) molar ratios of 0.1, 0.2, 0.3, and 0.4 were prepared and characterized. Based on these nanocomposites, a novel strategy of photothermocatalytic selective ethanol oxidation to acetaldehyde was developed. The MnO_x/TiO₂ nanocomposites with a Mn/(Mn + Ti) molar ratio of 0.3 exhibit efficient catalytic activity, high acetaldehyde selectivity, and excellent catalytic durability under UV-vis-IR irradiation. They also exhibit good catalytic activity and more than 92% acetaldehyde selectivity under vis-IR irradiation of λ > 420, 560, and 690 nm. Even under λ > 830 nm IR irradiation, they exhibit catalytic activity and 98.8% acetaldehyde selectivity. Selective ethanol oxidation on the MnO_x/TiO₂ nanocomposites follows a mechanism of light-driven thermocatalysis. A novel photoactivation is found to occur on amorphous manganese oxide in the MnO_x/TiO₂ nanocomposites: at the same reaction temperature, both UV-vis-IR and λ > 420 nm vis-IR irradiation significantly enhance the ethanol reaction rate (r_ethanol) and the acetaldehyde production rate (r_acetaldehyde) as compared to the corresponding rates in the dark. For example, at 150 °C, the r_ethanol and r_acetaldehyde values under UV-vis-IR are enhanced by 19.2 and 19.6 times as compared to the corresponding values in the dark, respectively. This catalytic enhancement upon UV-vis-IR and λ > 420 nm vis-IR irradiation is ascribed to the activation energy of selective ethanol oxidation being significantly reduced from 85.8 kJ mol⁻¹ in the dark to 14.2 and 13.5 kJ mol⁻¹, respectively. It is found that the novel photoactivation arises from the activity of oxygen of amorphous manganese oxide in MnO_x/TiO₂ being significantly promoted upon irradiation.