Hydrogen Production from Methane Using Oxygen-permeable Ceramic Membranes

Abstract

Non-porous ceramic membranes with mixed ionic and electronic conductivity have received significant interest as membrane reactor systems for the conversion of methane and higher hydrocarbons to higher value products like hydrogen. However, hydrogen generation by this method has not been commercialized yet and suffers from low membrane stability, low membrane oxygen flux, high membrane fabrication costs, and high reaction temperature requirements. In this dissertation, hydrogen production from methane on two different types of ceramic membranes (dense SFC and BSCF) has been investigated. The focus of this research is on the effects of different parameters (temperature, catalysis, membrane material, membrane thickness, pH, and feed ratio) on hydrogen production improvement in a membrane reactor. Pulse studies of reactants and products over physical mixtures of crushed membrane material and catalyst have clearly demonstrated that a synergy exists between the membrane and the catalyst under reaction conditions. The onset temperature of oxygen release for BSCF was observed to be lower than that for SFC while the amount of oxygen release was significantly greater, indicating that BSCF might be a better material for hydrogen production at medium temperatures. Pulse injections of CO over crushed membranes at 800 °C have shown more CO2 production on BSCF membrane than SFC membrane. It was found out that hydrogen consumption on BSCF particles is 24 times higher than that on SFC particles. CO2 reforming reactions on BSCF and SFC dense membranes in a membrane reactor, showed higher methane conversion and H2/CO ratio on BSCF than SFC in the presence of the Pt/CeZrO2 catalyst. This high conversion and H2/CO ratio could be ascribed to higher CO and H2 adsorption on BSCF than SFC, leading to more occurrence of steam reforming and CO oxidation on BSCF membrane. In an attempt to develop new catalysts for hydrogen production using ceramic membranes, Pt-Ni/Al2O3 and Pt-Ni/CeZrO2 catalysts were investigated. According to this study, platinum enhances the reducibility of Ni/Al2O3 and Ni/CeZrO2 catalysts and makes them to be better catalysts for H2 production at moderate temperatures. Varying the pH of the precursor solution during membrane preparation has no significant effect on the oxygen flux or the reaction. The BSCF membrane with low thickness demonstrates a higher methane conversion and H2:CO ratio than BSCF membrane with high thickness because membrane oxygen flux is inversely proportional to thickness. The CH4:CO2 feed ratio significantly affects the hydrogen production over the BSCF membrane. Altering the CH4:CO2 ratio has a direct impact on the oxygen flux, which in turn can influence the reaction pathway. Although Pt-Ni/Al2O3 catalyst shows high methane conversion in the presence of the BSCF membrane at 800°C, the activity of this catalyst is low at 600°C. Pt-Ni/CeZrO2 bimetallic catalyst demonstrates superior performance compared to Pt-Ni/Al2O3 catalyst at 600°C. The BSCF membrane can reduce the apparent activation energy of CO2 reforming reaction by changing the reaction pathway via more steam reforming.