Mechanochemistry, as a synthesis tool for inorganic materials, became an ever-growing field in material chemistry. The direct energy transfer by collision of the educts with the milling media gives the possibility to design environmentalfriendly reactions. Nevertheless, the underlying process of energy transfer and hence the kinetics of mechanosynthesis remain unclear. Herein, we present in situ synchrotron X-ray diffraction studies coupled with pressure measurements performed during the formation of ZnS and the subsequent phase transition (PT) from the hexagonal to the cubic modification. Milling Zn and S 8 results in the sublimation of S 8 , observed by a sudden pressure increase. Simultaneously, the hexagonal metastable ZnS-modification (wurtzite) forms. Via detection of the pressure maximum, the exact start of the wurtzite formation can be determined. Immediately after the formation of wurtzite, the structural PT to the thermodynamic stable cubic modification sphalerite takes place. This PT can be described by the Prout-Tompkins equation for autocatalytic reactions, similar to thermally induced PT in sulfur vapor at high temperatures (T > 1133 K). The increase in the reactivity of the wurtzite formation is explained by the reaction in sulfur vapor and the induction of defect structures by the collisions with the milling media.
Pt–Re bimetallic catalysts have many applications, ranging from catalytic reforming to the reduction of carboxylic acid derivatives. However, the exact role of Re in these systems has remained a matter of discussion, partly due to the plethora of suggested synthesis protocols and analysis conditions. This study presents an extensive comparison of such literature protocols and the resulting materials. In detail, characterization by N2 physisorption, X-ray diffraction, temperature-programmed reduction, CO pulse chemisorption, Fourier-transform infrared spectroscopy of adsorbed CO, scanning transmission electron microscopy, energy-dispersive X-ray spectroscopy, and in situ X-ray photoelectron spectroscopy is combined with catalytic testing to yield synthesis–structure–activity correlations. Accordingly, the investigated catalysts share common features, such as Pt0 nanoparticles (1–4 nm) decorated with partially reduced Re species (ReO x–y ). The remaining rhenium oxide is spread over the TiO2 support and enhances Pt dispersion in sequential impregnation protocols. While differences in the number of active sites (Pt0/ReO x–y ) mostly explain catalytic results, small variations in the extent of Re reduction and site composition cause additional modulations. The optimal bimetallic catalyst outperforms Ru/C (previous benchmark) in the reduction of N-(2-hydroxyethyl)succinimide, an important step in the production of a bio-based polyvinylpyrrolidone polymer.
Cracking of ammonia, a hydrogen carrier with high storage capacity, gains increasing attention for fuel cell systems for heavy load transportation. In this work, we studied the influence of metal loading and synthesis temperatures on the properties of Co@Al2O3 catalysts. The combination of in situ bulk characterization methods with in situ surface spectroscopy provides insights into the structure‐property relation of the Co catalyst on the γ‐Al2O3 support. At too high temperatures, the formation of CoAl2O4 during synthesis or during the catalytic reaction itself results in inactive mixed metal aluminium spinels which do not contribute to the catalytic reaction. The amount of ‘active’ Co catalyst thus varies significantly as well as its catalytic activity. The latter is correlated to the size of the reduced Co particles on the alumina support. The experiments also highlight that the state of the catalyst changes after reaction which strongly emphasizes the necessity of in situ studies.
Mechanochemistry has proven to be an excellent green synthesis method for preparing organic, pharmaceutical, and inorganic materials. Mechanocatalysis, inducing a catalytic reaction by mechanical forces, is an emerging field because neither external temperature nor pressure inputs are required. Previous studies reported enhanced catalytic activity during the mechanical treatment of supported gold catalysts for CO oxidation. So far, the processes inside the milling vessel during mechanocatalysis could not be monitored. In this work, the results of high-energy operando X-ray powder diffraction experiments and online gas analysis will be reported. A specific milling setup with a custom-made vessel and gas dosing system was developed. To prove the feasibility of the experimental setup for operando diffraction studies during mechanocatalysis, the CO oxidation with Au@Fe 2 O 3 as a catalyst was selected as a well-known model reaction. The operando studies enabled monitoring morphology changes of the support as well as changes in the crystallite size of the gold catalyst. The change of the crystal size is directly correlated to changes in the active surface area and thus to the CO 2 yield. The studies confirm the successful implementation of the operando setup, and its potential to be applied to other catalytic reactions.
What prompted you to investigate this topic/problem?Mechanochemistry has become an ever-growing research field over the last decades. In particular, the direct energy transfer from the milling media to the reactant leads to the possibility to design environmentally friendly processes by reducing solvents and activation energies. This direct local energy transfer leads to significantly different reaction pathways and different product compositions compared to classical (thermal and pressure-induced) synthesis routes. The mechanical synthesis of zinc sulfide from its elements results in the formation of the metastable hexagonal modification instead of the thermodynamically more stable cubic phase. Thus, this reaction was a perfect showcase to investigate the influence of the direct local energy transfer on the reaction mechanism via in situ monitoring of the reactants.
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