Abstract
AbstractIn promoted catalyst systems, the location of dopants is of very high interest to investigate promoter effects. A Ni/Al2O3 catalyst (wNi=11 wt. %) prepared by deposition‐precipitation and a co‐precipitated NiAlOx (nNi/nAl=1) catalyst are modified with Fe by means of the surface redox reaction technique and tested for activity under differential and integral conditions and for thermal stability (aging at 500 °C, 8 bar, 32 h) in the methanation reaction of CO2. By applying detailed material characterization studies comprising H2 and CO2 chemisorption, ICP‐OES, XRD, STEM‐EDX, FMR and BET, it is shown that the surface deposition techniques can be used to selectively deposit Fe in the vicinity of Ni nanoparticles. Doping with Fe leads to an increase of the catalytic activity, attributed to electronic effects through the formation of surface Ni−Fe alloys, and, for the co‐precipitated Ni−Al catalyst, to an enhancement of the apparent thermal stability at higher Fe loadings, which is assumed to be caused by a dynamic variation of Ni, Fe, and Al interactions depending on the reaction conditions.
Highlights
Fe has been claimed to enhance the activity of Ni-based catalyst systems by electronic modification of the active Ni centers, forming Ni Fe alloy particles.[10]
Besides the positive effect of Fe on the methanation activity, we recently proved an enhancement of the apparent thermal stability under aging conditions for co-precipitated NiFeAlOx catalysts at sufficiently high Ni/Fe ratios.[10a] the reasons for the stability improvement are not clear yet
A Ni/Al2O3 catalyst prepared by deposition-precipitation and a co-precipitated NiAlOx catalyst were taken as the template catalysts for the surface redox reaction
Summary
(ΔNFe/ΔNNi = 2/3), which is deposited on top or the perimeter of the Ni particle, or to Fe2 +, which may either be deposited on perimeter sites (ΔNFe/ΔNNi = 2), or stay in solution The latter pathway seems to be the prominent one in our approach, since, for all catalysts, ΔNFe/ΔNNi is lower than the expected minimum value of 2/3. The central areas in Figure SI 3 A and SI 3B feature a very homogenous distribution of both Al and Ni. No distinct NiO clusters can be observed, which highlights the different morphologies of a supported Ni/Al2O3 catalyst and a coprecipitated NiAlOx catalyst. Figure SI 3C indicates that different phases exist, one rich in Ni2 + and one that is rich Due to their strong ferromagnetic character after reduction (compare chapter on FMR studies), no STEM images or EDX data of the activated or aged catalyst samples could be collected. Ni27xFe9 samples and on the strong correlation of ΔNFe/ΔNNi, one can conclude that the replacement mechanism proposed in Scheme 1 is valid
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