Rational design of mesoporous N-doped TiO2 and ligand environment of Ru catalysts for CO2 hydrogenation to formic acid

Abstract

We developed a rational strategy to enhance the catalytic performance of Ru catalysts supported on mesoporous N-doped TiO2 for CO2 hydrogenation by modifying the morphological structure of metal oxide support and the tailoring the coordination environment of the active sites. The purposed TiO2 structure with mesoporosity and substitutinally doped nitrogen sites was successfully synthesized via alkali hydrothermal method and mechanochemical process. A ligand exchange process through NaOH treatment transformed Cl-ligated Ru catalysts into their OH-ligated catalysts, leading to a significant enhancement in catalytic stability. The OH-ligated Ru catalysts supported on mesoporous N-doped TiO2 retained 100 % of their initial activity after five recycling tests, whereas the Cl-ligated catalysts lost 32 % of their activity. Time-of-flight secondary ion mass spectrometry and X-ray absorption fine structure analyses confirmed the successful modification of the ligand structure at Ru active sites. Density functional theory calculations revealed that replacing Cl ligands with OH molecules increased the oxidation state of Ru, thereby strengthening its interaction with the support material. This enhanced bonding was identified as the key to improved catalytic stability. These findings provide valuable insight into the synergistic role of structural support design and coordination environment modulation in developing Ru catalyst with improved catalytic stability for CO2.