Methane Conversion to Synthesis Gas over Platinum Supported on Rare Earth Oxides

Citation

Pantu, Piboon (2001) Methane Conversion to Synthesis Gas over Platinum Supported on Rare Earth Oxides. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/rksa-xc11. https://resolver.caltech.edu/CaltechTHESIS:10062010-111511765

Abstract

The central theme of this research is to study methane conversion to synthesis gas focusing on the redox capabilities of cerium oxide. Reaction of methane with platinum or ruthenium supported on Ce_(1-x)Zr_xO_2 (x=0, 0.2, 0.5) in the absence of gaseous oxygen was studied in a packed-bed reactor at 550-700°C. The oxidation of methane utilized lattice oxygen of the support, which was subsequently restored by reacting with oxygen in a separate step. Thus, by using theredox property of cerium oxide, methane oxidation can be carried out by air without diluting the product with nitrogen. Addition of ZrO_2 into CeO_2 increased the reducibility of the oxide as well as the rate of methane oxidation but decreased the selectivity to CO and H_2. The rate of oxidation was initially very fast but slowed down as the oxide support became progressively reduced. On the other hand, the selectivity increased with the reduction of the support and sharply rose to over 90% as the support attained 10%,40%, and 65% degree of reduction for the oxide compositions x=0, 0.2, and 0.5, respectively.

Partial oxidation of methane to synthesis gas over 0.5wt% Pt/Al_2O_3 and 0.5wt% PtfCeO_2 catalysts was studied in a packed-bed reactor. At temperatures up to 650°C, the PtfCeO_2 catalyst gave higher conversion and higher selectivity but the activity and selectivity became comparable to those of Pt/Al_2O_3 above 700°C. The Pt/CeO_2 catalyst also maintained high conversion and high selectivity when the CH_4:O_2 feed ratio varied from 1.7 to 2.3 while the Pt/Al_2O_3 catalyst had considerably lower selectivity under methane-rich conditions. The effect of reducibility of support on the catalytic activity was discussed. A multiple microreactor system for parallel testing of heterogeneous catalysts was developed. The reactor system was composed of nine tubular microreactors housed in a single wider tube and used a multiposition valve to conduct the reaction products sequentially from each microreactor to a mass spectrometer for analysis. The catalyst samples were prepared in the form of thin films coated on quartz rods for convenience of preparation and loading samples in the reactors. The system was tested with the reaction of methane reforming with carbon dioxide over Pt/Ce_(1-x)Gd_xO_(2-0.5x) and Pt/Ce_(1-x)Sm_xO_(2-0.5x) at 650°C and 700°C. The measurements showed that Pt/CeO_2 had the highest activity and, generally, the activity increased with cerium oxide content. After exposure to the feed stream for 2-3 hours at 700°C, most catalysts suffered significant deactivation with the exception of the mixed oxides with 25-85% samarium oxide that maintained relatively stable activity.

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