Development Of Ionic Catalysts For The Water-gas Shift Reaction And Exhaust Gas Purification

Abstract

Treatment of fuel cell feed H2 for the removal of CO is important owing to the poisoning of the catalysts, thereby affecting the performance of the fuel cell. Strong and preferential adsorption of CO over the catalyst takes place resulting in a reduction of the power output of the cell. Therefore, it is important to treat the fuel cell feed H2 to reduce its CO content below the tolerable limit. Development of efficient catalysts for the treatment of synthesis gas for the removal of CO and and H2 enrichment of the gas to make it suitable for fuel cells is one of the two goals of this thesis. One of the various possible strategies for the removal of CO from the synthesis gas can be the use of the water-gas shift reaction. We have developed noble metal substituted ionic catalysts for catalyzing the water-gas shift reaction and have studied in detail the kinetics of the reactions by proposing the relevant reaction mechanisms. Solution combustion, a novel technique for synthesizing nanocrystalline materials, was used for the synthesis of all the catalysts. All the compounds synthesized were solid solutions of the noble metal ion and transition or rare earth metal oxide support. Three different supports were used, viz., CeO2, ZrO2 and TiO2. Substitution of Zr and Ti in CeO2 up to 15 at% was also carried out to obtain the compounds with enhanced oxygen storage capacity. All the compounds were characterized by X-ray diffraction, X-ray photoelectron spectroscopy and transmission electron microscopy. In some cases, where it was required, the use of FT-Raman spectroscopy was made for structural analysis. The compounds were nanocrystalline with metals substituted in ionic form in the support. The water-gas shift reaction was carried out over the synthesized catalysts with a reactant gas mixture that simulated the actual refinery gas composition. The variation of CO concentration with temperature was traced. The changes in the oxidation state of the metal showed the involvement of the various redox pairs over the reducible oxide like substituted CeO2 and TiO2. The mechanism of the reaction over ZrO2-based compounds was found to take place utilizing the surface hydroxyl groups. Rate expressions for the reactions over all the catalysts following different mechanisms were derived from the proposed elementary processes. Nonlinear regression was used for the estimation of various parameters describing the rate of reaction. Having established the high activity of Pt-ion substituted TiO 2 for the reactions, steam reforming of wood gas obtained from the gasification of Casuarina wood chips was carried out. The enrichment of the gas stream, which initially consisted of nearly 10% H 2 was carried out by steam reforming and H2-rich stream was obtained with H2 as high as 40% by volume in the treated gas. The second motive behind this thesis was to test the activity of the noble-metal substituted ionic catalysts for the treatment of the exhaust gas coming out of a fuel cell. In the fuel cell utilizing H2, the exhaust gases contain certain amount of unreacted H2, which can not be recovered or utilized economically. However, the gases are combustible and H 2 has to be removed in order to make the gas clean. We have shown high activity of the combustion-synthesized ionic compounds for catalytic combustion of H2. All the compounds showed high activity for H2 combustion and complete removal of H2 was possible. The rates were found to increase with an decrease in H2:O2 ratio and complete conversion of H2 was possible within 100 oC with air. A mathematical model was developed for the kinetics of catalytic H2 combustion based on the elementary processes that were proposed using the spectroscopic evidences. CO tolerant capacity of the catalysts was also tested. It was found that the temperature requirement for most of the catalysts increased with the introduction of CO. However, it was still possible to obtain complete conversions within 200 oC. To summarize, fuel cell processing systems utilizing H 2 remained central to the study. Treatment of the gases, both before and after reaction from the fuel cell was carried out over noble metal-substituted ionic catalyst, synthesized by solution combustion technique. Mechanisms of the reactions were proposed on the basis of spectroscopic evidences and the kinetic rate parameters were estimated using non-linear regression.