Highly active Ru/TiO₂ nanostructures for total catalytic oxidation of propane

Ruthenium is a robust catalyst for a variety of applications in environmental heterogeneous catalysis. The catalytic performance of Ru/TiO₂ materials, synthesized by using the deposition precipitation with urea method, was assessed in the catalytic oxidation of C₃H₈, varying the ruthenium loading. The highest catalytic reactivity was obtained for a Ru loading of 2 wt. % in comparison with the 1, 1.5, 3, and 4 wt. % Ru catalysts. The physicochemical properties of the synthesized materials were investigated by XRD, N₂ adsorption, TEM, FT-IR pyridine, H₂-TPR, and XPS. The size of ruthenium particles was found to be greatly dependent on the pretreatment gas (air or hydrogen) and the catalytic activity was enhanced by the small-size ruthenium metal nanoparticles, leading to changes in the reduction degree of ruthenium, which also increased the Brönsted and Lewis acidity. Metal to support charge transfer enhanced the reactant adsorption sites while oxygen vacancies on the interface enabled the dissociation of O₂ molecules as revealed through DFT calculations. The outstanding catalytic activity of the 2Ru/TiO₂ catalysts allowed to convert C₃H₈ into CO₂ at reaction temperatures of about 100 °C. This high activity may be attributed to the metal/support interaction between Ru and TiO₂, which promoted the reducibility of Ti⁴⁺/Ti³⁺ and Ru⁴⁺/Ru⁰ species, and to the fast migration of TiO₂ lattice oxygen in the catalyst. Furthermore, the Ru/TiO₂ catalyst exhibited high stability and reusability for 30 h under reaction conditions, using a GHSV of 45,000 h^-1. The underlying alkane-metal interactions were explored theoretically in order to explain the C-H bond activation in propane by the catalyst.