Plasma-Induced Catalyst Support Defects for the Photothermal Methanation of Carbon Dioxide

The presence of defects in a catalyst support is known to benefit catalytic activity. In this work, a He-plasma treatment-based strategy for introducing and stabilising defects on a Ni/TiO₂ catalyst for photothermal CO₂ hydrogenation was established. The impact of pretreatment step sequence—which comprised He-plasma treatment and reduction/passivation—on defect generation and stabilisation within the support was evaluated. Characterisation of the Ni/TiO₂ catalysts indicated that defects created in the TiO₂ support during the initial plasma treatment stage were then stabilised by the reduction/passivation process, (P-R)Ni/TiO₂. Conversely, performing reduction/passivation first, (R-P)Ni/TiO₂, invoked a resistance to subsequent defect formation upon plasma treatment and consequently, poorer photothermal catalytic activity. The plasma treatment altered the metal-support interaction and ease of catalyst reduction. Under photothermal conditions, (P-R)Ni/TiO₂ reached the highest methane production in 75 min, while (R-P)Ni/TiO₂ required 165 min. Decoupling the impacts of light and heat indicated thermal dominance of the reaction with CO₂ conversion observed from 200 °C onwards. Methane was the primary product with carbon monoxide detected at 350 °C (~2%) and 400 °C (~5%). Overall, the findings demonstrate the importance of pretreatment step sequence when utilising plasma treatment to generate active defect sites in a catalyst support.