The Promotional Effect Of CeO₂ On Rh-based Catalysts In CH₄/CO₂ Reforming

Methane reforming by carbon dioxide has received much attention because of the beneficial utilization of both CH₄ and CO₂, two major greenhouse gases, and the chemical energy. Ceria has been intensively studied as an effective promoter or support, because of the unique function of ceria in redox behaviors of Ce⁴⁺/Ce³⁺ and the interaction between metal and ceria. In this thesis, the promotional effect of CeO₂ on the catalytic activity and coke resistance of Rh-based catalyst in CH₄/CO₂ reforming were investigated, and the emphasis was put on the electronic effect in the Rh-CeO₂ interaction.An unique Rh-CeO₂ interaction was induced by high temperature reduction, which resulted in the creation of CeO₂-x and oxygen vacancies in ceria. The d-electrons could transfer from Rh to CeO₂ on the Rh-ceria interface. As a result, the Ce⁴⁺/Ce³⁺ and Rh⁰/Rh^δ+ redox couples were found to coexist in Rh-CeO₂/Al₂O₃ catalyst. Moreover, the presence of Ce⁴⁺/Ce³⁺ and Rh⁰/Rh^δ+ redox couples is a reversible cycle process in CH₄/CO₂ atmosphere. A mechanism of CH₄/CO₂ reforming in the presence of active sites of Ce⁴⁺/Ce³⁺ and Rh⁰/Rh^δ+ was proposed, in which activation of both CH₄ and CO₂ was facilitated and consequently the activity and coke resistance of Rh-CeO₂/Al₂O₃ were enhanced.Three types of carbon species (α,β,γ) always existed on the reacted Rh/Al₂O₃ catalyst.α-carbon, as an active intermediate, was found to be related to the reforming activity.β-carbon had a lower reactivity toward CO₂ and most likely responsible for the deactivation of catalyst.γ-carbon seemed not to react with CO₂ during the reforming reaction. They were further proved to be located on highly dispersed Rh, Rh crystals and Al₂O₃ support, respectively. When CeO₂ was added, a new formed Rh-CeO₂ interface increased the relative amount as well as the reactivity ofα-carbon,while bothβ-carbon andγ-carbon were remarkably decreased. These result were associated with the spillover hydrogen adsorbed on Rh-CeO₂ interface, which inhibited the further dehydrogenation ofα-carbon toβ-carbon. A mechanism of carbon deposition on CeO₂-promoted Rh/Al₂O₃ catalyst was proposed, in conjunction with the presence of active sites of Ce⁴⁺/Ce³⁺ and Rh⁰/Rh^δ+. It was found that the electronic effects in Rh-CeO₂ interaction on different supports were different. The Ce⁴⁺/Ce³⁺ and Rh⁰/Rh^δ+ redox couples were found to coexist in Rh-CeO₂/Al₂O₃ catalyst, while Ce⁴⁺/Ce³⁺ and Rh⁰/Rh^δ- redox couples were found to coexist in Rh-CeO₂/SiO₂ catalyst, which were associated with the acidity of support.