Kinetics of ammonia oxidation over Pt foil studied in a micro-structured quartz-reactor

The kinetics of Pt-catalyzed ammonia oxidation on polycrystalline Pt were investigated at partial pressures of ammonia and oxygen up to 6 kPa and temperatures between 286 and 385 °C, applying a micro-structured reactor that ascertained temperature control of the exothermic reaction. Using literature-based mechanistic models, a micro-kinetic model was derived based on parameter optimization and a model discrimination procedure. The model described the rates of formation of all nitrogen-containing products, i.e. N2, N2O and NO. Catalyst characterization of platinum samples by electron microscopy indicated that reaction-induced restructuring of the Pt surface limited the accuracy of derived kinetic parameters already at the low temperature applied in this work. Despite of necessary simplifications, the best-fitting kinetic model predicted reasonable product selectivities for the reaction conditions of an industrial ammonia burner

Future Challenges in Heterogeneous Catalysis: Understanding Catalysts under Dynamic Reaction Conditions

In the future, (electro-)chemical catalysts will have to be more tolerant towards a varying supply of energy and raw materials. This is mainly due to the fluctuating nature of renewable energies. For example, power-to-chemical processes require a shift from steady-state operation towards operation under dynamic reaction conditions. This brings along a number of demands for the design of both catalysts and reactors, because it is well-known that the structure of catalysts is very dynamic. However, in-depth studies of catalysts and catalytic reactors under such transient conditions have only started recently. This requires studies and advances in the fields of 1) operando spectroscopy including time-resolved methods, 2) theory with predictive quality, 3) kinetic modelling, 4) design of catalysts by appropriate preparation concepts, and 5) novel/modular reactor designs. An intensive exchange between these scientific disciplines will enable a substantial gain of fundamental knowledge which is urgently required. This concept article highlights recent developments, challenges, and future directions for understanding catalysts under dynamic reaction conditions

From a Molecular Single-Source Precursor to a Selective High- Performance RhMnOx Catalyst for the Conversion of Syngas to Ethanol

The first carbonyl RhMn cluster Na2[Rh3Mn3(CO)18] 2 has been synthesized and structurally characterized, resulting from the salt metathesis reaction of RhCl3 with Na[Mn(CO)5] 1 in 49% isolated yield. The dianionic Rh3Mn3 cluster core of 2 can serve as a molecular single‐source precursor (SSP) for the low temperature preparation of selective high‐performance RhMn catalysts for the conversion of syngas to ethanol (StE). Impregnation of 2 on silica (davisil) led to three different silica‐supported RhMnOx catalysts with dispersed Rh nanoparticles tightly surrounded by a MnOx matrix. With ethanol selectivities of up to 24.1%, the Rh3Mn3 cluster precursor‐derived catalysts show the highest reported selectivity and performance in the conversion of StE for silica‐supported RhMnOx catalysts

Electrochemically dealloyed platinum with hierarchical pore structure as highly active catalytic coating

Electrochemical dealloying of Pt–Si produces Pt films with hierarchical pore structure and superior performance in butadiene hydrogenation.</p

Atomic‐Scale Observation of the Metal‐Promoter Interaction in Rh‐Based Syngas Upgrading Catalysts

The direct conversion of syngas to ethanol is a cornerstone reaction in evolving technologies of CO2 utilization and hydrogen storage, which is typically performed using promoted Rh catalysts. A rational catalyst development requires a detailed structural understanding of the activated catalyst and in particular, the specific roles that promoters play in driving the chemoselectivity of this process. Herein, we report for the first time a comprehensive and comparative atomic‐scale study of metal‐promoter interaction in silica‐supported Rh, Rh‐Mn and Rh‐Mn‐Fe catalysts by aberration‐corrected transmission electron microscopy (AC‐TEM). We uncover that while the catalytic reaction leads to the formation of a Rh carbide phase in the Rh‐Mn/SiO2 catalyst, the addition of Fe results in the formation of bimetallic Rh‐Fe alloys. The latter further improves the selectivity and prevents the carbide formation. In all promoted catalysts, the Mn is present as oxide decorating the metal particles. Based on the atomic insight presented in this work, structural and electronic modifications induced by promoters are revealed and a basis for refined theoretical models is provided

Assessing Optical and Electrical Properties of Highly Active IrO_x Catalysts for the Electrochemical Oxygen Evolution Reaction via Spectroscopic Ellipsometry

Efficient water electrolysis requires highly active electrodes. The activity of corresponding catalytic coatings strongly depends on material properties such as film thickness, crystallinity, electrical conductivity, and chemical surface speciation. Measuring these properties with high accuracy in vacuum-free and non-destructive methods facilitates the elucidation of structure–activity relationships in realistic environments. Here, we report a novel approach to analyze the optical and electrical properties of highly active oxygen evolution reaction (OER) catalysts via spectroscopic ellipsometry (SE). Using a series of differently calcined, mesoporous, templated iridium oxide films as an example, we assess the film thickness, porosity, electrical resistivity, electron concentration, electron mobility, and interband and intraband transition energies by modeling of the optical spectra. Independently performed analyses using scanning electron microscopy, energy-dispersive X-ray spectroscopy, ellipsometric porosimetry, X-ray reflectometry, and absorption spectroscopy indicate a high accuracy of the deduced material properties. A comparison of the derived analytical data from SE, resonant photoemission spectroscopy, X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy with activity measurements of the OER suggests that the intrinsic activity of iridium oxides scales with a shift of the Ir 5d t2g sub-level and an increase of p–d interband transition energies caused by a transition of μ1-OH to μ3-O species

A micro-structured quartz reactor for kinetic and in situ spectroscopic studies in heterogeneous catalysis

A modular micro-structured quartz reactor was developed as a flexible device for controlled catalytic measurements in combination with in situ spectroscopic analysis. Depending on the actual reactor configuration, different shaped catalysts can be studied ex situ (e.g. by microscopy), or via in situ optical spectroscopy. The successful application of the reactor is demonstrated on two examples, the temperature-controlled ammonia oxidation over polycrystalline Pt catalyst, and the selective oxidation of propene over a supported molybdenum oxide-based thin film catalyst. The latter thin film catalyst was synthesized by a novel synthetic route using gelatine as wetting agent