(572d) CO2 Methanation Over Supported Bimetallic Ni-Fe Catalysts: Effect of Support and Total Metal Loading

The hydrogenation reactions of CO and CO₂ to methane and higher molecular weight hydrocarbons are important in purification of coal-derived gases, methanation of ammonia feeds and Fischer-Tropsch synthesis [1]. All Group VIII metals were tested for the hydrogenation of CO₂ and nickel is preferred due to its high activity, selectivity, long life and relatively low cost. Iron, however, is usually much less active and more prone to carbon deposition. Furthermore, it has been shown that besides enhancing surface area for proper dispersion of active metal, the support affects activity and selectivity of the catalysts [2].

It has been suggested [3, 4 and 5] that there are two important properties of the catalyst surface that determine the activity of some catalytic reaction: the barrier for reactant dissociation (E_diss) and the stability of the main intermediates on the surface. One of the approaches towards catalyst optimization could then be to construct a surface (active sites) with the desired E_diss. This can also be achieved by combining two metals: one with a high and one with low dissociation energy in an alloy catalyst [3]. It is important that both metals are present in a close proximity at the surface so that the active sites of the mixed-metal catalyst have the interpolated properties. This is not always the case since often one of the components can segregate to the surface or there may be alloying. It was furthermore concluded that the specific interactions between metal species in bimetallic catalysts often results in a completely different adsorption properties of the reactants [3]. It was also observed that in many cases, the reduction of bimetallic catalyst, for example Ni–Fe or Co–Fe, proceeds at significantly lower temperatures than for monometallic iron catalysts [6].

In the present work, a number of supported bimetallic Ni–Fe catalysts with different Ni/Fe ratios and total metal loadings were prepared and tested for the CO2 methanation reaction. The supports considered were Al2O3, SiO2, TiO2, ZrO2 and Nb2O5. The series of catalyst was characterized with different techniques, such as for their surface area and X-ray diffraction (XRD). The CO2 hydrogenation reaction was performed in a down flow tubular quartz reactor operating at atmospheric pressure and the temperatures ranging from 448 to 548 K. For reaction studies, the catalyst was first reduced at 773 K for 4 hrs in a H2 stream, then cooled to room temperature in a nitrogen stream and then the temperature was increased to reaction condition in a H2+CO2 mixture. The results were compared with those of the pure Ni and Fe based catalysts. Bimetallic catalysts with compositions 75Ni25Fe supported on Al2O3, SiO2, TiO2, and ZrO2 showed better activity and higher selectivity to methane in comparison with the monometallic Ni and Fe supported catalysts. It was also found that enhancement in activity decreases on increasing the total metal loading from 10 to 30 wt.% for alumina supported Ni-Fe catalysts. For Nb2O5 supported catalysts, however, the activity monotonically decreases on increasing the Fe content. The results of this work suggest that it is possible to substantially increase the efficiency of traditional Ni-based methanation catalyst by mixing it with Fe and at the same time lower the catalyst cost. Additional studies are required to ascertain the reason for this increase in methane yield for the bimetallic system.

References

[1]   Weatherbee, G. D. and Bartholomew, C. H. (1981), “Hydrogenation of CO₂ on group VIII metals: I. Specific activity of Ni/SiO₂”, Journal of Catalysis, Vol. 68, pp. 67-76.

[2]   Chen, S. L., Zhang, H. L., Hul, J., Contescuz, C. and Schwarz, J. A. (1991), “Effect of alumina supports on the properties of supported nickel catalysts”, Applied Catalysis, Vol. 73, pp.289-312.

[3]   Andersson, M. P., Bligaard, T., Kustov, A. L., Larsen, K. E., Greeley, J., Johannessen, T., Christensen, C. H. and Norskov, J. K. (2006), “Toward computational screening in heterogeneous catalysis: Pareto-optimal methanation catalysts”, Journal of Catalysis, Vol. 239, pp. 501-506.

[4]   Bligaard, T., Norskov, J. K., Dahl, S., Matthiesen, J., Christensen, C. H. and Sehested, J. (2004), “The Bronsted–Evans–Polanyi relation and the volcano curve in heterogeneous catalysis”, Journal of Catalysis, Vol. 224, pp. 206-217.

[5]   Norskov, J. K., Bligaard, T., Logadottir, A., Bahn, S., Hansen, L. B., Bollinger, M., Bengaard, H., Hammer, B., Sljivancanin, Z., Mavrikakis, M., Xu, Y., Dahl, S. and Jacobsen, C. J. H. (2002), “Universality in heterogeneous catalysis”, Journal of Catalysis, Vol. 209, pp. 275-278.

[6]   Tsipouriari, V. A. and Verykios, X. E. (1998), “Catalytic partial oxidation of methane to synthesis gas over Ni-based catalysts”, Journal of Catalysis, Vol. 179 pp. 292-299.