Glycerol Steam Reforming over Bimetallic Co-Ni/Al2O3

Glycerol steam reforming has been performed in a fixed-bed microreactor containing bimetallic Co-Nil Al2O3 catalyst using a wide range of steam-to-glycerol ratios (3 <= STGR <= 12) for reaction temperatures between 773 and 823 K at atmospheric pressure. Physicochemical characterization revealed the presence of both Lewis and Bronsted acid sites on the catalyst although the catalyst appears to have a net surface acidity (acid:basic site concentration ratio = 9.0). Co and Ni oxides as well as the metal aluminates were identified from XRD pattern with crystallite size (131.5 nm) similar to that obtained from H-2 chemisorption experiments (136.0 nm). Glycerol consumption rate data analysis implicates fractional orders with respect to both glycerol (0.25) and steam (0.36) with an activation energy of 63.3 kJ mol(-1). Similar treatment for H-2, Ca-2, CO2, and CH4 production rate evinced positive fractional orders for both reactants with the exception of CO which has mild inhibition by steam (-0.065). Mechanistic considerations and associated Langmuir-Hinshelwood and Eley-Rideal kinetic models were derived for both single- and dual-site adsorption modes. However, statistical discrimination as well as thermodynamic evaluation of the associated parameter estimates suggest that the most adequate representation involved molecular adsorption of glycerol and steam on two different sites with surface reaction as the rate-controlling step consistent with the presence of both Bronsted acid and basic sites on the catalyst. Carbon deposition during reaction appeared to be responsible for the loss in surface area and pore volume of the used catalysts. However, these attributes were nearly recovered after regeneration (>90%) using TPR-TPO-TPR-TPO cycles. Significantly, carbon deposition is a strong function of glycerol partial pressure but somewhat insensitive to the presence of steam, suggesting that the carbon residue was probably unreactive with steam under the reaction conditions. indeed, temperature-programmed heat treatment (TPO-TPR-TPO-TPR and TPR-TPO-TPR-TPO) revealed at least two types of carbonaceous deposits. However, one of these carbon pools was resistant to removal with H-2.