Iron−Ceria Aerogels Doped with Palladium as Water−Gas Shift Catalysts for the Production of Hydrogen

Abstract

Mixed 4.5% iron oxide−95.5% cerium oxide aerogels doped with 1% and 2% palladium (Pd) by weight have been synthesized, and their activities for the catalysis of water−gas shift (WGS) reaction have been determined. The aerogels were synthesized using propylene oxide as the proton scavenger for the initiation of hydrolysis and polycondensation of a homogeneous alcoholic solution of cerium(III) chloride heptahydrate and iron(III) chloride hexahydrate precursor. Palladium was doped onto some of these materials by gas-phase incorporation (GPI) using (η3-allyl)(η5-cyclopentadienyl)palladium as the volatile Pd precursor. Water−gas shift catalytic activities were evaluated in a six-channel fixed-bed reactor at atmospheric pressure and reaction temperatures ranging from 150 to 350 °C. Both 1% and 2% Pd-doped 4.5% iron oxide−95.5% cerium oxide aerogels showed WGS activities that increased significantly from 150 to 350 °C. The activities of 1% Pd-doped 4.5% iron oxide−95.5% cerium oxide aerogels were also compared with that of the 1% Pd-doped ceria aerogel without iron. The WGS activity of 1% Pd on 4.5% iron oxide−95.5% cerium oxide aerogels is substantially higher (∼5 times) than the activity of 1% Pd-doped ceria aerogel without iron. The gas-phase incorporation results in a better Pd dispersion. Ceria aerogel provides a nonrigid structure wherein iron is not significantly incorporated inside the matrix, thereby resulting in better contact between the Fe and Pd and thus enhancing the WGS activity. Further, neither Fe nor Pd is reduced during the ceria-aerogel-catalyzed WGS reaction. This behavior contrasts with that noted for other Fe-based WGS catalysts, in which the original ferric oxide is typically reduced to a nonstoichiometric magnetite form.