Ce0.9Zr0.1O2 nanocatalyst: Influence of synthesis conditions in the reducibility and catalytic activity

Total Oxidation of Methane on Oxide and Mixed Oxide Ceria-Containing Catalysts

Methane, discovered in 1766 by Alessandro Volta, is an attractive energy source because of its high heat of combustion per mole of carbon dioxide. However, methane is the most abundant hydrocarbon in the atmosphere and is an important greenhouse gas, with a 21-fold greater relative radiative effectiveness than CO2 on a per-molecule basis. To avoid or limit the formation of pollutants that are dangerous for both human health and the atmospheric environment, the catalytic combustion of methane appears to be one of the most promising alternatives to thermal combustion. Total oxidation of methane, which is environmentally friendly at much lower temperatures, is believed to be an efficient and economically feasible way to eliminate pollutants. This work presents a literature review, a statu quo, on catalytic methane oxidation on transition metal oxide-modified ceria catalysts (MOx/CeO2). Methane was used for this study since it is of great interest as a model compound for understanding the mechanisms of oxidation and catalytic combustion on metal oxides. The objective was to evaluate the conceptual ideas of oxygen vacancy formation through doping to increase the catalytic activity for methane oxidation over CeO2. Oxygen vacancies were created through the formation of solid solutions, and their catalytic activities were compared to the catalytic activity of an undoped CeO2 sample. The reaction conditions, the type of catalysts, the morphology and crystallographic facets exposing the role of oxygen vacancies, the deactivation mechanism, the stability of the catalysts, the reaction mechanism and kinetic characteristics are summarized.

In situ X-ray absorption spectroscopy study of CuO–NiO/CeO2–ZrO2 oxides: redox characterization and its effect in catalytic performance for partial oxidation of methane

In this work we analyze the effect of adding CuO to a NiO/Ce_0.9Zr_0.1O₂ oxide by in situ X-ray absorption near-edge structure XANES technique in Ce L3, Ni K and Cu K absorption edges in terms of sample reducibility and catalytic activity. The oxidation states of Ce, Ni and Cu cations are followed up during temperature programmed reduction (TPR) experiments in diluted hydrogen and during catalytic tests for partial oxidation of methane (POM) reaction. Redox behavior was correlated to conventional fixed bed reactor results. The effect of firing temperature, crystallite size, CeO₂–ZrO₂ support and the presence of Cu and/or Ni as an active phase is also analyzed. Results showed a beneficial effect of CuO addition in terms of Ce and Ni reduction. A stronger interaction of NiO species with the support was revealed upon analysis of XANES reduction profiles in sample NiO/ZDC in contrast to bimetallic CuO–NiO/ZDC sample. Reduction onset temperature was found to depend on Ni crystallite size, being markedly promoted when samples exhibited low values of crystallite size both in supported and non-supported CuO–NiO species. In situ catalytic experiments for partial oxidation of methane showed a clear interplay between the redox behavior from the Ce in the CeO₂–ZrO₂ support and the Ni from the active phase. Sample NiO/ZDC exhibited a continuous reduction of Ce cations in CH₄ : O₂ feed flow, carbon formation was detected in X-ray Powder Diffraction (XPD) patterns and Ni re-oxidation was found to take place, clear indications of catalyst deactivation. In contrast, sample CuO–NiO/Ce_0.9Zr_0.1O₂ displayed a slight re-oxidation of Ce and no re-oxidation of Ni altogether with the suppression of carbon formation.

In situ X-Ray Absorption (XAS) experiments in reducing atmospheres (H₂ and CH₄ : O₂) uncovered Ce, Ni and Cu redox interplay during catalytic experiments.