Boosting Low-Temperature\nCO<sub>2</sub> Methanation\nActivity on Ru/Anatase-TiO<sub>2</sub> Via Mn Doping: Revealing the\nCrucial Role of CO<sub>2</sub> Dissociation

A series of Ru/Ti<sub>1–<i>x</i></sub>Mn<sub><i>x</i></sub>O<sub>2</sub> catalysts with varying\nMn/(Ti + Mn)\nmolar ratios (<i>x</i> = 0.10–0.25) were synthesized\nto investigate the CO<sub>2</sub> methanation mechanism on anatase\nTiO<sub>2</sub>-supported Ru catalysts (Ru/a-TiO<sub>2</sub>) and\ndevelop high-performance catalysts at low temperatures. Among these\ncatalysts, the Ru/Ti<sub>0.8</sub>Mn<sub>0.2</sub>O<sub>2</sub> exhibited\nthe highest activity, achieving approximately 65% CO<sub>2</sub> conversion\nat 230 °C, which is markedly superior to the unmodified Ru/a-TiO<sub>2</sub> catalyst yielding only about 15% CO<sub>2</sub> conversion.\nThe majority of Mn cations were incorporated into the lattice of a-TiO<sub>2</sub> as Mn<sup>3+</sup> cations, forming a solid solution structure\nin the Ti<sub>0.8</sub>Mn<sub>0.2</sub>O<sub>2</sub> support. This\nmodification resulted in a higher specific surface area, improved\nreducibility, and increased oxygen vacancy compared with pure a-TiO<sub>2</sub>. Consequently, Ru dispersion and electronic metal–support\ninteractions were enhanced in Ru/Ti<sub>0.8</sub>Mn<sub>0.2</sub>O<sub>2</sub> compared to those in Ru/a-TiO<sub>2</sub>. In-situ diffuse\nreflectance infrared Fourier transform spectroscopy combined with\ntemperature-programmed surface reaction experiments revealed that\nCO<sub>2</sub> methanation predominantly proceeded via the CO* route\non the Ru/a-TiO<sub>2</sub>. The CO<sub>2</sub> adsorption in the\npresence of decomposed H<sub>2</sub> led to dissociation to linear\nCO*, followed by CO methanation where CO<sub>2</sub> dissociation\nto CO* was identified as the rate-determining step (RDS). Mn cation\ndoping induced the formation of oxygen vacancies, significantly enhancing\nCO<sub>2</sub> dissociation on Ru/Ti<sub>0.8</sub>Mn<sub>0.2</sub>O<sub>2</sub>, thereby shifting the RDS to CO methanation. This mechanism\nexplains the superior activity of Ru/Ti<sub>0.8</sub>Mn<sub>0.2</sub>O<sub>2</sub> at low temperatures for CO<sub>2</sub> methanation\ncompared to the Ru/a-TiO<sub>2</sub>.