Exploring optimal total metal loading of Ni3Fe/Al2O3 catalyst for CO2 methanation and its kinetic model

Abstract

A series of Al2O3-supported bimetallic Ni-Fe catalyst with Ni/Fe = 3 are prepared, characterized, and examined for CO2 methanation to identify the total metal loading (Ni+Fe) that yields the maximum activity. The XRD patterns of all catalysts exhibit a noticeable peak shift, which suggests alloy formation and XPS studies reveal that the near-surface concentration of the reduced catalysts possess Ni/Fe ratios similar to those targeted. Analysis of the particle size distribution from TEM and degree of reduction from TPR clearly shows that an increase in metal loading results in a gradual decrease in effective dispersion. However, the catalyst with 55% total metal loading of Ni+Fe, 55Ni3FeAl, possessed the maximum surface metal sites and effective surface metal area, which is reflected in the highest activity of this catalyst during CO2 methanation. Kinetic measurements of the CO2 methanation reaction are obtained on 55Ni3FeAl at temperatures ranging from 513 to 533 K and ambient pressure under differential reactor conditions and in the absence of heat and mass transfer resistance. Several kinetic models are then considered to fit the reaction data for CO2 methanation. The parameters of the models are determined by non-linear regression and analyzed for their thermodynamic consistency and statistical validity (R2, R2adj, NRMSE and F - test). The kinetic data of CO2 methanation on 55Ni3FeAl is adequately represented by a reaction model, where the surface reaction between molecularly adsorbed CO2 and adsorbed H2 appears to be the rate-determining step. The activation energy of this rate determining step was 136 kJ/mol. This kinetic model can then be used to design, analyze, and control different reactors where the most active Al2O3 supported Ni-Fe catalyst with Ni/Fe = 3, 55Ni3FeAl, is used for the CO2 methanation reaction.