Effect of Ti dopants in Ce1−xTixO2−δ-supported Ni catalysts: structure, redox properties, and carbon resistance in DRM

Abstract

In this study, 5 wt% Ni catalysts over a series of Ce_1−xTi_xO_2−δ supports with controlled Ti dopant composition (x = 0–0.5) were synthesized by sol–gel and impregnation methods. Compositions, crystal structures, and surface properties were investigated to confirm the formation of Ce_1−xTi_xO_2−δ mixed oxides with low Ti compositions (e.g., Ce_0.9Ti_0.1O_2−δ). Ce⁴⁺ in Ce–O–Ti shows a lower reduction temperature compared to bulk Ce⁴⁺ in CeO_2−δ, and thus Ti-doped ceria exhibits better reducibility. TiO₂ is also formed over Ce_1−xTi_xO_2−δ with high Ti compositions (e.g., Ce_0.5Ti_0.5O_2−δ), suggesting the limited solubility of Ti in the ceria lattice. The amount of Ti in Ce_1−xTi_xO_2−δ plays a role in the formation of Ni species. NiO was found to be the major species over CeO_2−δ and Ce_0.9Ti_0.1O_2−δ. However, NiTiO₃ was observed over Ce_1−xTi_xO_2−δ (x ≥ 0.2). Compared to Ni/CeO_2−δ, Ni/Ce_0.9Ti_0.1O_2−δ delivers better CH₄ and CO₂ conversions in DRM. This can be attributed to the enhanced reducibility of Ce_0.9Ti_0.1O_2−δ and the stronger metal–support interaction by a small amount of Ti doping in ceria. The DRM activity of Ni decreases with the increased Ti composition in Ni/Ce_0.5Ti_0.5O_2−δ. This can be correlated with the formation of NiTiO₃, which produces significantly less metallic Ni as the active species for DRM compared to NiO that is formed over CeO_2−δ and Ce_0.9Ti_0.1O_2−δ. The TGA results of spent catalysts indicate a decrease in carbon deposition during DRM with increasing Ti composition in Ni/Ce_1−xTi_xO_2−δ. XRD data suggest the formation of a new Ce₂Ti₂O₇ phase in spent Ni/Ce_0.5Ti_0.5O_2−δ, which could better help remove carbon deposits. Doping Ti into the ceria lattice significantly helps mitigate the issue related to carbon deposition over the Ni catalyst during DRM. Similar behavior was also observed over Ce_1−xTi_xO_2−δ-supported Co catalysts. Our study clearly demonstrates that doping Ti in ceria can tune both the activity and stability of supported metal catalysts in DRM.