Jörg Wambach

CO2 hydrogenation over metal/zirconia catalysts

Metal/zirconia catalysts have attracted considerable interest for the hydrogenation of carbon dioxide. Various preparation methods such as impregnation, co-precipitation, sol–gel synthesis, and controlled oxidation of amorphous metal alloys have been used, leading to catalysts with very different properties. The catalytic behaviour of these materials is greatly influenced by the active metal and by the interfacial contact area between the metal and zirconia. Cu and Ag catalyse mainly methanol formation, while methane is formed over, e.g., Ni, Ru and Rh. Examples for less reactive metal constituents are Pd, Rh, Pt and Au, catalysing simultaneously the formation of methanol, carbon monoxide (by reverse water–gas shift reaction) and methane. The structural and catalytic properties of the various catalysts are compared and possible reaction pathways are discussed.

Methanol synthesis reactions over a CuZr based catalyst investigated using periodic variations of reactant concentrations

Abstract The application of the modulation concept to study the reaction pathway and the reaction rate constants of the methanol synthesis starting from CO and CO 2 over a CuZr based catalyst is described. Kinetic data of different reaction steps can be calculated by performing modulation experiments using different feed gas compositions. The dynamic variations in the concentrations of products, intermediates and reactants are monitored in situ in a DRIFT cell. The observed retardation (phase shift Δ ϕ ) is examined depending on the modulation frequency ω . This method is used to derive equilibrium and overall rate constants characterising the investigated reaction steps. By repeating the experiments at different reaction temperatures, constants such as Arrhenius activation energies can be assessed.

Structural changes of model Cu/ZnO catalysts during exposure to methanol reforming conditions

Cu/ZnO/Si model catalysts were prepared by resistive evaporation of Cu on ZnO thin films deposited on Si(1 0 0) by DC magnetron sputtering. Exposure of the Cu/ZnO/Si model catalysts to methanol reforming conditions at 550 K causes agglomeration of the supported Cu islands as shown by XPS, AFM and spectroscopic ellipsometry. The Cu remains in the metallic state for O2/CH3OH molar ratios 6 0.25 but is oxidised to Cu(I) for higher oxygen content. When Cu is in the metallic state the degree of agglomeration is larger in the presence of O2 in the gas phase. On the other hand oxidation of the Cu islands is associated with a less pronounced agglomeration upon methanol reforming treatment. The origin of this phenomenon is discussed. 2003 Elsevier Science B.V. All rights reserved.

Modelling Catalyst Surfaces Using DFT Cluster Calculations

We review our recent theoretical DFT cluster studies of a variety of industrially relevant catalysts such as TiO2, γ-Al2O3, V2O5-WO3-TiO2 and Ni/Al2O3. Aspects of the metal oxide surface structure and the stability and structure of metal clusters on the support are discussed as well as the reactivity of surfaces, including their behaviour upon poisoning. It is exemplarily demonstrated how such theoretical considerations can be combined with DRIFT and XPS results from experimental studies.

Use of periodic variations of reactant concentrations in time resolved DRIFT studies of heterogeneously catalysed reactions

Abstract For investigating the mechanism of solid-state catalysed reactions, sine wave modulation of feed gas concentrations was used to induce dynamic variations in the concentrations of products, intermediates and reactants, as monitored in situ in a DRIFT cell. The phase shift Δϕ between the external perturbation of the feed gas and the signals of products, intermediates and reactants was examined in dependence on the modulation frequency ω. A micro-kinetic model consisting of several simple steps was used to describe the process. A relationship between the phase shifts and the constants characterising the single reaction step can be derived from the applied micro-kinetic model. Therefore, the evolution of each phase shift as function of the modulation frequencies is characteristic for the individual reaction step. Repeating the experiment at different temperatures made it possible to assess the temperature dependent constants characterising the model, like the Arrhenius energy EA. This new method was tested for the CO oxidation over a Pd25Zr75 based catalyst. Possible reaction orders for the reactants CO and O2 together with reaction constants were estimated.

Ion Association in Hydrothermal Sodium Sulfate Solutions Studied by Modulated FT-IR-Raman Spectroscopy and Molecular Dynamics

Saline aqueous solutions at elevated pressures and temperatures play an important role in processes such as supercritical water oxidation (SCWO) and supercritical water gasification (SCWG), as well as in natural geochemical processes in Earth and planetary interiors. Some solutions exhibit a negative temperature coefficient of solubility at high temperatures, thereby leading to salt precipitation with increasing temperature. Using modulated FT-IR Raman spectroscopy and classical molecular dynamics simulations (MD), we studied the solute speciation in solutions of 10 wt % Na2SO4, at conditions close to the saturation limit. Our experiments reveal that ion pairing and cluster formation are favored as solid saturation is approached, and ionic clusters form prior to the precipitation of solid sulfate. The proportion of such clusters increases as the phase boundary is approached either by decreasing pressure or by increasing temperature in the vicinity of the three-phase (vapor-liquid-solid) curve.

On‐Stream Regeneration of a Sulfur‐Poisoned Ruthenium–Carbon Catalyst Under Hydrothermal Gasification Conditions

Catalytic processes that employ Ru catalysts in supercritical water are capable of converting organics, such as wood waste or biosolids, into synthetic natural gas (CH4) with high efficiencies at relatively moderate temperatures of around 400 °C. However, Ru catalysts are prone to S poisoning and are quickly deactivated. As S is ubiquitous in raw biomass and technologies to remove S from hydrothermal biomass feeds are lacking, regeneration protocols that efficiently reactivate S‐poisoned catalysts are required to realize efficient conversion processes and long catalyst lifetimes. In this work, we developed a method to remove S from a S‐poisoned Ru catalyst under hydrothermal conditions through an oxidative treatment in the aqueous phase. By using in situ X‐ray absorption spectroscopy under the reaction conditions, we show that Ru is oxidized by dilute H2O2 at low temperatures, which leads to the removal of adsorbed S species from the catalyst surface. By optimizing the regeneration conditions, it was possible to prevent oxidation of the catalyst carbon support, as revealed by ex situ TEM. This treatment led to a reactivation of the Ru catalyst with a significant increase in carbon‐to‐gas conversion and methane selectivity.

Structural properties of Cu/ZnO/Si methanol reforming catalysts: Influence of the composition of the reactant mixture and of the Cu island size

The structural properties of Cu/ZnO/Si model catalysts were investigated by X-ray photoelectron spectroscopy (XPS), Ar-ion XPS depth profiling and atomic force microscopy (AFM) after exposure to various methanol reforming conditions. The composition of the reactant mixture is found to influence drastically the chemical state and the morphology of the Cu overlayer. Oxidation of Cu to Cu(I) is observed upon increasing the oxygen content in the gas phase, with thinner Cu islands being less reactive, probably due to the stabilising influence of the support. In the absence of water in the reactant mixture the formation of Cu(I) is associated with Cu islands with greater lateral size covering a larger fraction of the ZnO surface. On the other hand, the formation of Cu(I) in the presence of water is accompanied by a drastic decrease of the dispersion of the Cu overlayer. The presence of water in the reactant mixture also favours the oxidation of Cu during methanol reforming and promotes the formation of Cu(II) on the surface of thin Cu islands (0.4 nm nominal thickness of the deposited Cu layer). On the other hand, no formation of Cu(II) was observed in the case of high Cu coverage Cu/ZnO/Si model catalysts (1.3 nm nominal thickness of the deposited Cu layer) for all tested compositions of the reactant mixture. The formation of Cu(II) is associated with an increase of the dispersion of the Cu overlayer and of the fraction of ZnO surface covered by Cu. The formation of a ZnO layer partially covering the surface of metallic Cu islands is observed in the presence of oxygen in the reactant mixture. This modification, which is inhibited by the presence of water in the gas phase, is more pronounced in the case of thicker Cu islands. The relevance of the observed structural changes for the behaviour of industrial Cu/ZnO-based catalysts and their influence on the catalytic activity for the production of hydrogen from methanol are discussed.