Non-oxidative calcination enhances the methane dry reforming performance of Ni/CeO2−x catalysts under thermal and photo-thermal conditions

Abstract

We analyzed the effect of the calcination atmosphere and visible-light contribution to an accelerated reaction rate and improved H₂ selectivity over 2 wt% Ni/CeO_2−x nanorod catalysts. Spectroscopic and structural characterization was performed by operando DRIFTS, in situ Raman, UV-vis and XAS techniques, which were complemented by DFT calculations. Calcination in an argon or H₂ atmosphere yields 15% more active catalysts in the thermally driven reaction, which are also more susceptible to light-induced rate acceleration compared to the catalyst calcined in air. The most active 2Ni/CeO₂ catalyst calcined in hydrogen converts methane with a rate of 7.5 mmol (g_cat min)⁻¹ and produces a H₂/CO ratio of 0.6 at 460 °C when stimulated by a combination of visible light and thermal energy. In the absence of visible light illumination and at an identical catalyst temperature, the achieved methane rate was 4.2 mmol (g_cat min)⁻¹ and the H₂/CO ratio was 0.49. The non-oxidative calcination improves nickel dispersion and the formation of subnanometer sized Ni clusters, together with a higher abundance of surface and bulk oxygen vacancies in ceria nanorods. The Ni–O_v–Ce³⁺components constitute the catalytically active sites under visible light illumination, which enable the DRM reaction to proceed with an E_a value of 20 kJ mol⁻¹. Visible light also induces the following changes in the 2Ni/CeO_2−x catalyst during the DRM reaction: (1) decomposition and desorption of carbonates from the nickel–ceria interface sites, (2) reduced population of nickel surface with carbonyl species and (3) promoted adsorption and dissociation of methane.