The kinetics of Ni/Al2O3 and Ni-Fe/Al2O3 catalysts for the CO2 methanation reaction and the reasons for promotion

Abstract

Replacing nickel with iron is known to promote supported nickel catalyst for CO2 methanation. This promotion is often attributed to the presence of Ni-Fe alloy. However, the effect of replacing Ni with Fe on a common rate expression of CO2 methanation and the specific causes leading to the promotion are not addressed. To achieve these objectives, we synthesize and characterize an alumina supported Ni, 15NiAl, and two alumina supported Ni-Fe catalysts, 15Ni7FeAl and 15Ni3FeAl. These catalysts contain 15 wt% of Ni or Ni + Fe. In 15Ni7FeAl and 15Ni3FeAl the Ni/Fe ratio = 7 and 3, respectively. Characterization studies reveal the presence of smaller Ni-Fe alloy particles in the reduced and spent 15Ni7FeAl and 15Ni3FeAl catalysts. Other characteristics of the catalysts are also identified. With these catalysts we carry out the CO2 methanation reaction under different operating conditions. The enhancement in rate of CH4 formation, rCH4, while using the two bimetallic supported catalysts strongly depends on the Ni/Fe ratio, reactant partial pressures and temperatures. We modify an existing rate expression to adequately represent the reaction data of the three catalysts. In this rate expression, the surface reaction between molecularly adsorbed reactant gases is the rate-determining-step. As Ni is progressively replaced with Fe in the bimetallic catalysts, the apparent rate constants and equilibrium constants for the adsorption of CO2 and H2 of the rate expression increases, which increases rCH4. Under all operating conditions the 15Ni3FeAl catalyst shows the best rCH4 and the apparent rate constants and equilibrium constants are the largest. It appears that the Ni-Fe alloy increases the number of adsorption-sites and the fractional coverage of adsorbed CO2 and H2, which then results in enhanced rCH4 values.