Ethanol steam reforming over MgxNi1-xAl2O3 spinel oxide-supported Rh catalysts
Résumé
Two series of Rh/spinel catalysts supported on alumina were prepd. for use in prodn. of H for fuel use. In the first series, magnesium, nickel, and aluminum nitrates were co-impregnated over gamma-alumina beads (200 m2 g-1), dried, and further calcined at 1000° to obtain MgxNi1-xAl2O4 (x = 0-1) /Al2O3 supports (~100 m2 g-1). These supports were impregnated with aq. solns. of Rh nitrate to obtain 0.1-0.2% Rh catalysts. The second series was obtained by coating alumina beads with Mg acetate, drying and calcining at 1000°. The MgAl2O4 spinel was formed by solid-solid reaction between magnesia and alumina. MgAl2O4 was impregnated with Rh salts (nitrate, chloride, acetate) to obtain 0.2-0.8% Rh loading. Supports and catalysts were characterized by BET area, pore vol., x-ray diffraction, TEM and SEM, CO2 chemisorption (basicity), and dimethyl-3,3-but-1-ene isomerization (acidity). Rh dispersion was measured by H2 chemisorption. The second series of catalysts was also characterized by FTIR of adsorbed lutidine and DRIFT. The activity of the catalysts was evaluated in ethanol steam reforming at 700° under 1 or 11 atm (H2O/ethanol molar ratio of 4, space velocity 24,000 h-1). Acidity and basicity of the catalysts are crucial for control of selectivity for ethylene or acetaldehyde. To avoid ethylene formation, which leads to significant carbonaceous deposits, all acidic sites should be neutralized. The effects of precursor of support and catalyst were studied. The second prepn. method gave less acidic materials with high performances (activity and stability) in the ethanol steam reforming reaction. The morphol. of the support, with a Mg-deficient spinel layer (thickness of about 8-9 nm) intimately covering all of the alumina grains (around 40 nm in size), can explain the neutralization of most acidic sites. Nitrate precursors should be avoided, as the products show the most acidity and poorest stability. Rh acetate led to neutral catalysts with good performance in ethanol steam reforming, and Rh chloride allowed prepn. of well-dispersed Rh catalysts. Although the catalysts prepd. with Rh chloride were moderately acidic, they were active and stable.