Boosting Low-Temperature CO₂ Methanation Activity on Ru/Anatase-TiO₂ Via Mn Doping: Revealing the Crucial Role of CO₂ Dissociation

Abstract

A series of Ru/Ti1–xMnxO2 catalysts with varying Mn/(Ti + Mn) molar ratios (x = 0.10–0.25) were synthesized to investigate the CO2 methanation mechanism on anatase TiO2-supported Ru catalysts (Ru/a-TiO2) and develop high-performance catalysts at low temperatures. Among these catalysts, the Ru/Ti0.8Mn0.2O2 exhibited the highest activity, achieving approximately 65% CO2 conversion at 230 °C, which is markedly superior to the unmodified Ru/a-TiO2 catalyst yielding only about 15% CO2 conversion. The majority of Mn cations were incorporated into the lattice of a-TiO2 as Mn3+ cations, forming a solid solution structure in the Ti0.8Mn0.2O2 support. This modification resulted in a higher specific surface area, improved reducibility, and increased oxygen vacancy compared with pure a-TiO2. Consequently, Ru dispersion and electronic metal–support interactions were enhanced in Ru/Ti0.8Mn0.2O2 compared to those in Ru/a-TiO2. In-situ diffuse reflectance infrared Fourier transform spectroscopy combined with temperature-programmed surface reaction experiments revealed that CO2 methanation predominantly proceeded via the CO* route on the Ru/a-TiO2. The CO2 adsorption in the presence of decomposed H2 led to dissociation to linear CO*, followed by CO methanation where CO2 dissociation to CO* was identified as the rate-determining step (RDS). Mn cation doping induced the formation of oxygen vacancies, significantly enhancing CO2 dissociation on Ru/Ti0.8Mn0.2O2, thereby shifting the RDS to CO methanation. This mechanism explains the superior activity of Ru/Ti0.8Mn0.2O2 at low temperatures for CO2 methanation compared to the Ru/a-TiO2.