Selective Methane Oxybromination over Nanostructured Ceria Catalysts

Abstract

This article studies the impact of the carrier type (MgO, SiO₂, SiC, Al₂O₃, and ZrO₂) and synthesis method (dry impregnation, coprecipitation, hydrothermal synthesis, and mechanochemical synthesis) on the structure, redox properties, and performance of supported CeO₂ catalysts for methane oxybromination. Major distinctions are evidenced in the product distribution with respect to bulk CeO₂, and the selectivity to methyl bromide (CH₃Br) varies in the following order: CeO₂/MgO (61–81%) > CeO₂/SiC (56–73%) ≈ CeO₂/SiO₂ (52–71%) > bulk CeO₂ (40%) ≈ CeO₂/ZrO₂ > CeO₂/Al₂O₃ (28–35%) at 6–40% methane conversion. The selectivity is primarily governed by the catalyst propensity to combust CH₃Br and byproduct dibromomethane (CH₂Br₂), which is strongly affected by the choice of the carrier. Specifically, the formation of carbon oxides is substantially suppressed over CeO₂ nanoparticles stabilized on basic MgO (CO_x selectivity <10%) with respect to bulk CeO₂ (CO_x selectivity ≥50%), whereas it is promoted by near atomic dispersions of CeO₂ on acidic ZrO₂ and Al₂O₃ carriers. Changes in the size and shape of CeO₂ nanoparticles on MgO had little impact on the selectivity, as they exhibit similar oxidation properties after exposure to the reaction environment. The synthesis–structure–performance relationships developed demonstrate the great potential of supporting CeO₂ to enhance the oxybromination performance.