Is the CO2 methanation on highly loaded Ni-Al2O3 catalysts really structure-sensitive?

Highly loaded Ni-Al2O3 catalysts (14-88 wt.-% Ni) were prepared by dry impregnation, wet impregnation, deposition precipitation and coprecipitation and tested in the CO2 methanation. By means of ICP-OES, XRD, TPR, H-2 chemisorption, N-2 physisorption and thermogravimetry, different Ni species were identified, and their dependence on the synthesis method and the Ni loading was investigated. The catalysts prepared had Ni particle sizes of 5-91 nm and Ni surface areas of 5-51 m(2) g(-1). It could be shown that the turnover frequency (TOF) does not depend on metal-support interactions, the metal-support interface or the particle size. Hence, the CO2 methanation on Ni-Al2O3 catalysts is clearly a structure-insensitive reaction. There is a linear correlation between the conversion and the Ni surface area at constant modified residence time. Hence, catalysts with the highest Ni surfaces, i.e., the samples prepared by deposition precipitation and coprecipitation, reached the highest weight time yields. The prerequisites for high Ni surface areas at high Ni loadings could be elucidated: During the synthesis, Ni has to be able to completely penetrate the mesopores of the support. Furthermore, a linear correlation between the selectivity to CO and the weight percentage of the impurity Na could be revealed. The stability of the supported catalysts was highly dependent on the spatial distribution of Ni and the stability of the support: A high dispersion or the presence of unsupported Ni reduced the stability. Furthermore, the stability of the supported catalysts was smaller than the one of the coprecipitated catalysts, where the stability decreased with increasing n(Ni)/n(Al).