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The Promotional Effect of CeO2 on Rh-based Catalysts in CH4/CO2 Reforming

Author: WangRui
Tutor: XuHengYong;LiWenZuo
School: Dalian Institute of Chemical Physics
Course: Industrial Catalysis
Keywords: CH4/CO2 reforming Rh-CeO2 interaction active sites of redox couples mechanism of carbon deposition
CLC: O643.32
Type: PhD thesis
Year: 2007
Downloads: 370
Quote: 2
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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/Rhδ+ redox couples were found to coexist in Rh-CeO2/Al2O3 catalyst. Moreover, the presence of Ce4+/Ce3+ and Rh0/Rhδ+ 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/Rhδ+ 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/Rhδ+. It was found that the electronic effects in Rh-CeO2 interaction on different supports were different. The Ce4+/Ce3+ and Rh0/Rhδ+ redox couples were found to coexist in Rh-CeO2/Al2O3 catalyst, while Ce4+/Ce3+ and Rh0/Rhδ- redox couples were found to coexist in Rh-CeO2/SiO2 catalyst, which were associated with the acidity of support.

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CLC: > Mathematical sciences and chemical > Chemistry > Physical Chemistry ( theoretical chemistry ),chemical physics > Chemical kinetics,catalysis > Catalytic > Catalytic reaction
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