Catalysis in membrane reformers: a high-performance catalytic system for hydrogen production from methane

A palladium membrane reactor has been designed and sized to be applied to the dry reforming of methane for pure hydrogen production at a small scale. Three different parameters affecting the reactor operation have been adjusted to tune the reformer and optimise its performance: the extraction conditions, the CO2/CH4 ratio in the reactant mixture composition, and the reactants feed flow rate. By forcing H2 extraction and adjusting the rates of hydrogen production in the reactor and hydrogen permeation through the Pd membrane, it has been possible to obtain very high CH4 conversion for mixtures with CO2/CH4 ratios above unity. Reactant mixtures with CO2/CH4 ratios close to 2 offered the best results: high H2 recovery yields (above 95%) and lower carbon deposition in the catalysts under the severe conditions imposed by the membrane reactor operation. Hydrogen extraction from the reaction side has been shown to enhance the carbon deposition rate on nickel-based reforming catalysts, in which the formation of low-reactivity carbon in the form of fibres has been observed. The dispersion of nickel on high oxygen mobility supports, such as Ce-Zr mixed oxides (Ce0.5Zr0.5O2), which are chemically stable under reaction conditions, results in highly efficient catalysts capable of keeping their surface free of inactive carbon deposits. This kind of oxide provides an extra source of oxygen that equilibrates the net rates of carbon deposition and removal on the nickel surface, thus avoiding the accumulation of carbon in the catalyst.