The influence of mass transfer conditions on the stability of molybdenum carbide for dry methane reforming

The effect of mass transfer conditions on the stability of a low surface area Mo2C catalyst for dry (CO2) methane reforming has been studied in a packed bed reactor at temperatures up to 1000 degreesC and pressures up to 8.3 bar using stoichiometric feeds (CO2:CH4 = 1). Because stoichiometric feeds can be net oxidizing at typical reforming temperatures, the carbide is oxidized to MoO2, forming a moving oxidation front, which will eventually deactivate the entire bed. Based on experiments which isolated the effects of space velocity and molar feed velocity, it is concluded that these catalysts are stable under conditions where gas-solid mass transfer coefficients are low; that is, at high pressures and low mass flow rates. In these situations CO and H-2 product gases remain near the catalyst surface, preventing oxidation. Experimental results are also presented which show the temperature above which the carburizing rates are greater than oxidation rates, and this temperature is dependent on molar feed velocity and pressure. These results explain the observations made by previous researchers who observed that stability was greater at higher pressures and that the catalyst oxidized when space velocities were raised. Finally, it is concluded that Mo2C catalysts can be stabilized with respect to oxidation by either operating at low mass velocities, higher temperatures, or by recycling product gases. (C) 2004 Published by Elsevier B.V.