Surface reactive oxygen in Ni/CexZr1-xO2 catalysts may impede the CH4/CO2 reforming activity by stabilizing small Ni species

Dry reforming of methane (DRM) utilizes CO2 and CH4, the two major greenhouse gases, to produce value-added syngas, providing a viable route for combating global warming. Ceria–zirconia solid solution supported Ni catalyst is an often-used benchmark owing to its carbon oxidation function by the surface reactive oxygen (SRO). However, excessive SRO may over-stabilize the small Ni species at low surface density, resulting in the final deactivation by the carbon deposition. Still, the dual effects of SRO remain unclear and sometimes controversial. Herein, we synthesized CexZr1-xO2 supports with different Ce/Zr ratios to investigate the impact on the highly dispersed Ni species and DRM reactivity. Through a suite of DRM reaction studies, by monitoring Ni species and carbon deposition during the reaction, we found that 0.5Ni/Ce0.7Zr0.3O2 with the highest SRO concentration possesses a lower extent in-situ growth of Ni species (<3 nm), less graphitic carbon, but inferior activity compared to the Ni/CeO2 catalyst. Structural characterizations indicate that smaller Ni species are inclined to be completely covered by the coke. Moreso, surface basicity declines significantly when Ce/Zr reaches ∼ 0.5 which prevents CO2 activation and causes carbon deposition to a limited extent. Experimental and theoretical studies verified that carbon diffusion is more difficult on smaller Ni species, which prevents its further oxidation by SRO, leading to the complete coke coverage of Ni species and catalyst deactivation as the primary factor.