Abstract
AbstractOn a Ni/Al2O3 and a Ni−Fe/Al2O3 catalyst spatial concentration and temperature profiles were determined that occur along the axial direction of the catalyst bed. They were correlated to structural gradients under reaction conditions to elucidate the local dependency of catalyst structure on reaction‐induced changes of the gas phase. The quantitative concentration and temperature profiles revealed a hotspot in the first third part of the fixed‐bed, which led to by‐product formation of CO. Complementary structural information obtained by spatially‐resolved quick X‐ray absorption spectroscopy unraveled a strong impact of reaction‐induced gradients in gas phase on the oxidation state of Fe with a higher oxidation state towards the end of the catalyst bed, while Ni was only slightly affected. Diffuse reflectance infrared Fourier transform spectroscopy further revealed that addition of Fe to a Ni/Al2O3 catalyst reduces the amount of adsorbed CO species. Hence, Fe hampers blocking of active Ni0 sites by CO and preserves a high fraction of reduced Ni species. Furthermore, an alternative reaction pathway observed on Ni−Fe provided locally a higher activity for CO2 hydrogenation. Overall, the importance of considering local gradients in catalytic reactors is demonstrated.
Highlights
The depletion of fossil energy sources and reduction of CO2 emissions, with the aim to limit global warming, drive the need for new and efficient renewable energy systems
Local activity and structure profiling was conducted for Nibased CO2 methanation catalysts with the aim to investigate changes in the catalyst structure with respect to catalytic activity and local composition of the gas phase
Gradients in gas phase composition and temperature along a monometallic 17 wt % Ni/γ-Al2O3 and a bimetallic 17 wt % Ni3.2Fe/γ-Al2O3 catalyst were resolved by local activity profiling using a fixedbed reactor setup
Summary
The concentration of water changes along the reactor and this raises new questions regarding results obtained by the integral X-ray based in situ and operando studies, usually recorded at one fixed position of the catalyst bed.[19,21,28] Apart from variation of the reactant and product concentration along the fixed-bed reactor, which may result in gradients in oxidation state and catalytic performance, gradients in temperature might occur due to the exothermic nature of the Sabatier reaction [cf Eq (1)] Such gradients might influence the structural composition of the catalysts, in case of the highly dynamic NiÀ Fe system. We aim at correlating the performance, temperature, and structure data to gain deeper insights into local relationships between structure and activity of the Ni- and NiÀ Fe-based catalysts during CO2 methanation
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