Abstract
CO2 methanation is often performed on Ni/Al2O3 catalysts, which can suffer from mass transport limitations and, therefore, decreased efficiency. Here we show the application of a hierarchically porous Ni/Al2O3 catalyst for methanation of CO2. The material has a well-defined and connected meso- and macropore structure with a total porosity of 78%. The pore structure was thoroughly studied with conventional methods, i.e., N2 sorption, Hg porosimetry, and He pycnometry, and advanced imaging techniques, i.e., electron tomography and ptychographic X-ray computed tomography. Tomography can quantify the pore system in a manner that is not possible using conventional porosimetry. Macrokinetic simulations were performed based on the measures obtained by porosity analysis. These show the potential benefit of enhanced mass-transfer properties of the hierarchical pore system compared to a pure mesoporous catalyst at industrially relevant conditions. Besides the investigation of the pore system, the catalyst was studied by Rietveld refinement, diffuse reflectance ultraviolet-visible (DRUV/vis) spectroscopy, and H2-temperature programmed reduction (TPR), showing a high reduction temperature required for activation due to structural incorporation of Ni into the transition alumina. The reduced hierarchically porous Ni/Al2O3 catalyst is highly active in CO2 methanation, showing comparable conversion and selectivity for CH4 to an industrial reference catalyst.
Highlights
Carbon dioxide emissions must be reduced significantly to limit the negative consequences of climate change
We analyzed the crystallographic structure of the calcined Ni/Al2 O3 -h sample via Rietveld refinement of powder X-ray diffraction (PXRD) data to understand the influence of the co-gelation synthesis procedure on the resulting structure
A hierarchically porous Ni/Al2 O3 catalyst was shown to be active for CO2 methanation, having previously shown increased performance in dry reforming of CH4
Summary
Carbon dioxide emissions must be reduced significantly to limit the negative consequences of climate change. The potential of combining multiscale imaging techniques was shown for a study of the porous structure of Pt/Al2 O3 -based catalysts for exhaust aftertreatment [28] In these studies, ptychographic X-ray computed tomography (PXCT) was used to image length scales from tens of nanometer to several micrometer, covering large meso- and macropores. Ptychographic X-ray computed tomography (PXCT) was used to image length scales from tens of nanometer to several micrometer, covering large meso- and macropores This relatively new technique for 3D imaging requires the use of synchrotron radiation but provides information about the electron density within the material and allows resolutions smaller than the scanning beam size with resolutions. The combination of targeted synthesis of hierarchical pore structures, multimodal characterization of pore structures, and simulation is readily applicable to other hierarchically structured materials with complex porosity
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