Multiscale modelling of CO2 hydrogenation of TiO2-supported Ni8 clusters: on the influence of anatase and rutile polymorphs†

The selection of TiO₂ phase, whether anatase or rutile, for supporting small Ni clusters significantly influences the activity and selectivity in CO₂ hydrogenation to methane. To model and understand these variances, we developed a hierarchical multiscale catalytic model. Utilizing a hybrid approach combining genetic algorithms and density functional theory, we identified the putative global minimum structures of Ni₈ clusters supported on anatase (Ni₈/TiO₂-a) and rutile (Ni₈/TiO₂-r), which are morphologically distinct. Microkinetics simulations based on the energetics derived from DFT calculations over these distinct clusters reveal the mechanism of CO₂ hydrogenation to CO, CH₄ and CH₃OH. On both Ni₈/TiO₂-a and Ni₈/TiO₂-r, CH₄ is the main product at low temperature while a shift to CO occurs with increasing temperature. In comparison to Ni₈/TiO₂-r, Ni₈/TiO₂-a exhibits a higher activity and keeps a higher selectivity towards CH₄ with increasing temperature. Using a sensitivity analysis, we identify the steps responsible for the observed selectivity difference and rationalize the observed barrier differences for these steps between the different clusters by means of detailed electronic structure analysis.