Hard X-Ray Nanotomography for 3D Analysis of Coking in Nickel-Based Catalysts.

Understanding catalyst deactivation by coking is crucial for knowledge-based catalyst and process design in reactions with carbonaceous species. Post-mortem analysis of catalyst coking is often performed by bulk characterization methods. Here, hard X-ray ptychographic computed tomography (PXCT) was used to study Ni/Al2 O3 catalysts for CO2 methanation and CH4 dry reforming after artificial coking treatment. PXCT generated quantitative 3D maps of local electron density at ca. 80 nm resolution, allowing to visualize and evaluate the severity of coking in entire catalyst particles of ca. 40 µm diameter. Coking was primarily revealed in the nanoporous solid, which was not detectable in resolved macropores. Coke formation was independently confirmed by operando Raman spectroscopy. PXCT is highlighted as an emerging characterization tool for nanoscale identification, co-localization, and potentially quantification of deactivation phenomena in 3D space within entire catalyst particles.

Correction: Thermally stable mesoporous tetragonal zirconia through surfactant-controlled synthesis and Si-stabilization.

[This corrects the article DOI: 10.1039/D2RA01459A.].

Thermally stable mesoporous tetragonal zirconia through surfactant-controlled synthesis and Si-stabilization.

Thermally stable, highly mesoporous Si-stabilized ZrO2 was prepared by sol-gel-synthesis. By utilizing the surfactant dodecylamine (DDA), large mesopores with a pore width of ∼9.4 nm are formed. Combined with an NH3-treatment on the hydrogel, a high specific surface area of up to 225 m2 g-1 and pore volume up to 0.46 cm3 g-1 are obtained after calcination at 973 K. The individual contributions of Si-addition, DDA surfactant and the NH3-treatment on the resulting pore system were studied by inductively coupled plasma with optical emission spectrometry (ICP-OES), X-ray diffraction (XRD), N2 sorption, and transmission electron microscopy (TEM). Electron tomography was applied to visualize and investigate the mesopore network in 3D space. While Si prevents the growth of ZrO2 crystallites and stabilizes the t-ZrO2 phase, DDA generates a homogeneous mesopore network within the zirconia. The NH3-treatment unblocks inaccessible pores, thereby increasing specific surface area and pore volume while retaining the pore width distribution.

Evolution of Hierarchically Porous Nickel Alumina Catalysts Studied by X-Ray Ptychography.

The synthesis of hierarchically porous materials usually requires complex experimental procedures, often based around extensive trial and error approaches. One common synthesis strategy is the sol-gel method, although the relation between synthesis parameters, material structure and function has not been widely explored. Here, in situ 2D hard X-ray ptychography (XRP) and 3D ptychographic X-ray computed tomography (PXCT) are applied to monitor the development of hierarchical porosity in Ni/Al2 O3 and Al2 O3 catalysts with connected meso- and macropore networks. In situ XRP allows to follow textural changes of a dried gel Ni/Al2 O3 sample as a function of temperature during calcination, activation and CO2 methanation reaction. Complementary PXCT studies on dried gel particles of Ni/Al2 O3 and Al2 O3 provide quantitative information on pore structure, size distribution, and shape with 3D spatial resolution approaching 50 nm, while identical particles are imaged ex situ before and after calcination. The X-ray imaging results are correlated with N2 -sorption, Hg porosimetry and He pycnometry pore characterization. Hard X-ray nanotomography is highlighted to derive fine structural details including tortuosity, branching nodes, and closed pores, which are relevant in understanding transport phenomena during chemical reactions. XRP and PXCT are enabling technologies to understand complex synthesis pathways of porous materials.

Electrolysis of Natural Waters Contaminated with Transition-Metal Ions: Identification of A Metastable FePb-Based Oxygen-Evolution Catalyst Operating in Weakly Acidic Solutions.

The possibility of efficient water electrooxidation sustained by continuous (re)generation of catalysts derived from the oxidative electrodeposition of transition-metal contaminants is examined herein for three natural water samples from Australia and China. The metal composition of the solutions has been determined by inductively coupled plasma optical emission spectrometry, and a range of strategies to produce water-splitting catalysts by means of in situ electrodeposition have been applied. The performance of the resulting electrocatalysts is below the state-of-the-art level owing to large amounts of impurities in the solutions and non-optimal concentrations of naturally available catalyst precursors. Nevertheless, these studies have identified the FePb-based system as a rare example of an electrocatalyst for water oxidation that forms in situ and maintains reasonable activity (≥4.5 mA cm-2 at an overpotential of 0.8 V) in weakly acidic solutions (pH 2.9).