Noble Metal based Catalysts for Natural Gas Steam Reforming Activity, Endurance and Kinetics

This thesis illustrates catalytic activity, stability and intrinsic kinetics of methane steam reforming (MSR) reaction over noble metal catalysts. The main objective of this thesis is to evaluate a best performing catalyst based on the maximization of H2 production and minimization of CO in the synthesis gas produced from MSR reaction. The noble metal catalysts tested towards MSR reaction were Rh, Ru and Pt supported on different reducible and irreducible oxides. The oxides (CeO2, MgO and Al2O3) used in this work were synthesized from their nitrite precursor by Simultaneous combustion synthesis (SCS) while Nb2O5 was prepared by heat treatment of Niobic acid obtained from Companhia Brasileira de Metalurgia e Mineracão (CBMM, Brasil). In all the catalysts the noble metals were deposited on the support by wetness impregnation method, except Pt/CeO2 which was prepared by one shoot SCS method. All the prepared catalysts were calcined under different calcination regimes. The best performing catalysts were characterized by different techniques BET, CO chemisorption, porosiometery, XRD, XPS, ICP, TEM and SEM analyses. Efforts have been made to correlate the catalytic activity with the physical characterization. All the catalysts prepared were initially screened by MSR reaction in a tubular fixed bed quartz reactor of 4mm ID containing 30mg of catalyst diluted with 50mg of inert. For catalytic screening and stability test the feed was introduced at a weight hourly space velocity of 20 NLh-1g-1cat and steam to carbon ratio 3-4 depending upon the catalyst. The results obtained from basic screening of the catalysts were analyzed in terms of methane conversion, H2 produced in dry reformate and CO2 selectivity. Among all the catalysts tested towards MSR only two were chosen based on initial screening, Rh/CeO2 and Pt/CeO2, for the further test concerning catalyst stability. The stability of Rh/CeO2 and Pt/CeO2 catalysts was determined based on daily start up and shut down cycle (DSS) with a 6h performance period. The Pt/CeO2 catalyst was tested for a total of 150 h in which 100h performance was with DSS in N2 environment while 50h of catalyst activity with DSS in reaction environment. The Rh/CeO2 catalyst was tested for a total of 25 h catalyst activity with DSS in N2 environment. Additionally the Rh/CeO2 catalyst was also tested in 100h continuous ageing. Both the catalysts showed good results in terms of catalyst activity and stability during the time period. As Rh/CeO2 catalyst showed good activity during 100h continuous endurance this catalysts was chosen to evaluate the intrinsic kinetics of methane steam reforming. For the kinetics test firstly the heat and mass transfer limitations were evaluated both experimentally and theoretically. The reactor was operated in an integral mode and no inert was used in feed for the kinetic experiments. The effect of WHSV at constant S/C 3 on the methane conversion and product composition was also determined. The partial pressures of the reactants were varied by varying the steam-to-carbon ratio of the feed. An attempt was made to fit kinetic data obtained using the models available in literature. The kinetic data obtained was perfect fit for the model proposed by Berman, and the activation energy of Rh/CeO2 was found to be 38.6 kJ/mol.