NiO supported on Y2Ti2O7 pyrochlore for CO2 reforming of CH4: insight into the monolayer dispersion threshold effect on coking resistance
In this study, with the target to unravel the interaction between Ni/NiO and Y2Ti2O7 support, and eventually fabricate highly efficient catalysts for DRM reaction, NiO/Y2Ti2O7 catalysts with varied NiO loadings have been designed based on monolayer dispersion theory. With XRD and XPS extrapolation methods, it is revealed that the monolayer dispersion capacity of NiO on the Y2Ti2O7 support is 1.20 mmol NiO/ 100 m2 Y2Ti2O7, equalling to 2.4 (wt)% loading. All the catalysts exhibit similar and high DRM reaction when NiO loading is above 1%. TGA-DSC and Raman results indicate that the samples with NiO loadings around the monolayer dispersion capacity possess optimal anti-coking ability. On these samples, NiO species is present majorly in monolayer or sub-monolayer state, which has strong interaction with the support through electron donating from both Y3+ and Ti4+ in the support to Ni2+ cations. As a result, active metallic Ni with high surface area can be obtained and stablized. However, further increasing the NiO loadings leads to weaker Ni/NiO-Y2Ti2O7 interaction, and the formation of larger Ni grains devoting to coking. Therefore, the samples with NiO loadings around the monolayer dispersion capacity possess the best ability to activate the reactants and have considerable quantities of active Ni surface area, thus displaying optimal DRM reaction performance. In conclusion, an evident monolayer dispersion threshold effect is observed for NiO/Y2Ti2O7 on coking resistance. It is proposed that the best NiO/Y2Ti2O7 catalyst for DRM reaction can be fabricated by loading exactly the monolayer dispersion amount of NiO onto the Y2Ti2O7 support.