Porosity and Structure of Hierarchically Porous Ni/Al2O3 Catalysts for CO2 Methanation

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.

Sample Environment for Operando Hard X-ray Tomography—An Enabling Technology for Multimodal Characterization in Heterogeneous Catalysis

“…The diffractograms measured under reaction conditions clearly differ significantly from the original ex situ measurement of the dry gel. Prior to activation, the Ni/Al 2 O 3 catalyst shows reflections probably originating from an Al-oxide-hydroxide precursor phase forming the dry gel [45]. The broad feature at 5 • present in all patterns is from the quartz glass capillary sample holder.…”

“…A Ni/Al 2 O 3 catalyst previously reported in the literature was studied in the form of a dry gel received directly after synthesis [45]. The catalyst is active in CO 2 methanation to CH 4 , according to Equation (1):…”

“…500 min, Figure 2-blue). These originate from the decomposition of organic compounds used as porogenic agents (e.g., dodecylamine), which are still present in the dry gel after synthesis [45]. Switching from activation conditions to reaction conditions (ca.…”

“…The aRCTIC setup was installed at the microprobe end station of beamline P06 of PETRA III (DESY, Hamburg, Germany). Approximately 2.4 mg of 20 wt % Ni/Al 2 O 3 (dried gel after synthesis) [45] was loaded into a quartz capillary of 1 mm outer diameter (particle size 100-200 µm, bed length ca. 6 mm) and held in position using quartz wool plugs.…”

Harte Röntgen‐Nanotomographie zur 3D‐Analyse der Verkokung in Nickel‐basierten Katalysatoren

“…In dieser Studie demonstrieren wir das Potential von PXCT zur 3D‐Analyse der Verkokung von Ni/Al 2 O 3 ‐Katalysatoren, welche für die CO 2 ‐Methanisierung oder CH 4 ‐Trockenreformierung verwendet werden können. Die Katalysatoren wurden über zwei verschiedene Routen hergestellt, um durch eine homogene Abscheidungs‐Ausfällung [27] ein mesoporöses (Ni/Al 2 O 3 ‐m) und durch eine modifizierte Sol‐Gel‐Methode [17, 28] ein hierarchisch meso‐/markoporöses Material (Ni/Al 2 O 3 ‐h) zu erhalten. Die Porosität der Proben wurde in vorangegangen Studien untersucht, welche die Anwesenheit von Makroporen im Bereich von 50 bis 100 nm ausschließen [17, 27] .…”