Thorsten Ressler

Implication of the microstructure of binary Cu/ZnO catalysts for their catalytic activity in methanol synthesis

Binary Cu/ZnO catalysts with varying molar ratios (90/10 through 10/90) were studied under methanol synthesis conditions at 493 K and at atmospheric pressure. The methanol synthesis activity of the catalysts was correlated to their specific Cu surface area (N2O reactive frontal chromatography, N2O RFC) after reduction in 2 vol% H2 at 513 K. Activity data were supplemented with a detailed analysis of the microstructure, i.e., crystallite size and strain of the reduced Cu and the ZnO phases after reduction using X-ray diffraction line profile analysis. The estimated copper surface area based on a spherical shape of the copper crystallites is in good agreement with data determined by N2O RFC. A positive correlation of the turnover frequency for methanol production with the observed microstrain of copper in the Cu/ZnO system was found. The results indicate a mutual structural interaction of both components (copper and zinc oxide) in the sense that strained copper particles are stabilized by the unstrained state of the zinc oxide microcrystallites. The observed structural deformation of ZnO in samples with higher Cu loading can originate, for instance, from epitaxial bonding of the oxide lattice to the copper metal, insufficient reduction or residual carbonate due to incomplete thermal decomposition during reduction. Additional EXAFS measurements at the Cu K and the Zn K edge show that about 5% ZnO are dissolved in the CuO matrix of the calcined precursors. Furthermore, it is shown that the microstructural changes (e.g., size and strain) of copper can be traced back to the phase composition of the corresponding hydroxycarbonate precursors.

Nanostructured Manganese Oxide Supported on Carbon Nanotubes for Electrocatalytic Water Splitting

Incipient wetness impregnation and a novel deposition symproportionation precipitation were used for the preparation of MnOx/CNT electrocatalysts for efficient water splitting. Nanostructured manganese oxides have been dispersed on commercial carbon nanotubes as a result of both preparation methods. A strong influence of the preparation history on the electrocatalytic performance was observed. The as‐prepared state of a 6.5 wt. % MnOx/CNT sample could be comprehensively characterized by comparison to an unsupported MnOx reference sample. Various characterization techniques revealed distinct differences in the oxidation state of the Mn centers in the as‐prepared samples as a result of the two different preparation methods. As expected, the oxidation state is higher and near +4 for the symproportionated MnOx compared to the impregnated sample, where +2 was found. In both cases an easy adjustability of the oxidation state of Mn by post‐treatment of the catalysts was observed as a function of oxygen partial pressure and temperature. Similar adjustments of the oxidation state are also expected to happen under water splitting conditions. In particular, the 5 wt. % MnO/CNT sample obtained by conventional impregnation was identified as a promising catalytic anode material for water electrolysis at neutral pH showing high activity and stability. Importantly, this catalytic material is comparable to state‐of‐art MnOx catalyst operating in strongly alkaline solutions and, therefore, offers advantages for hydrogen production from waste and sea water under neutral, hence, environmentally benign conditions.

Performance Improvement of Nanocatalysts by Promoter-Induced Defects in the Support Material : Methanol Synthesis over Cu/ZnO:Al

Addition of small amounts of promoters to solid catalysts can cause pronounced improvement in the catalytic properties. For the complex catalysts employed in industrial processes, the fate and mode of operation of promoters is often not well understood, which hinders a more rational optimization of these important materials. Herein we show for the example of the industrial Cu/ZnO/Al2O3 catalyst for methanol synthesis how structure-performance relationships can deliver such insights and shed light on the role of the Al promoter in this system. We were able to discriminate a structural effect and an electronic promoting effect, identify the relevant Al species as a dopant in ZnO, and determine the optimal Al content of improved Cu/ZnO:Al catalysts. By analogy to Ga- and Cr-promoted samples, we conclude that there is a general effect of promoter-induced defects in ZnO on the metal-support interactions and propose the relevance of this promotion mechanism for other metal/oxide catalysts also.

Understanding the Role of Gold Nanoparticles in Enhancing the Catalytic Activity of Manganese Oxides in Water Oxidation Reactions

The Earth-abundant and inexpensive manganese oxides (MnOx) have emerged as an intriguing type of catalysts for the water oxidation reaction. However, the overall turnover frequencies of MnOx catalysts are still much lower than that of nanostructured IrO2 and RuO2 catalysts. Herein, we demonstrate that doping MnOx polymorphs with gold nanoparticles (AuNPs) can result in a strong enhancement of catalytic activity for the water oxidation reaction. It is observed that, for the first time, the catalytic activity of MnOx/AuNPs catalysts correlates strongly with the initial valence of the Mn centers. By promoting the formation of Mn(3+) species, a small amount of AuNPs (<5%) in α-MnO2/AuNP catalysts significantly improved the catalytic activity up to 8.2 times in the photochemical and 6 times in the electrochemical system, compared with the activity of pure α-MnO2.

Chemically Induced Fast Solid-State Transitions of ω-VOPO4 in Vanadium Phosphate Catalysts

Vanadium phosphates are important catalysts for the oxidation of alkanes, and commercial catalysts comprise a complex range of V4+ and V5+ phosphates. We used three complementary in situ characterization methodologies—powder x-ray diffraction and laser Raman and electron paramagnetic resonance spectroscopies—to show that the metastable phase ω-VOPO4 is very sensitive to many of the reactants and products of butane oxidation. A rapid transformation from ω-VOPO4 to δ-VOPO4 occurs on exposure to butane at the reaction temperature, and hence the metastable ω-VOPO4 may play a role in the formation of commercial catalysts.

Manganese speciation in exhaust particulates of automobiles using MMT-containing gasoline.

Manganese speciation in exhaust particulates of automobiles using MMT-containing gasoline monochromator, 50% detuned, was used. Multiple scans varying from 4 sweeps for XANES and 8 sweeps for EXAFS were performed to yield optimal S/N ratios. All spectra were collected in the QEXAFS mode (Frahm, 1988). A manganese metal standard foil located in front of a reference ion chamber was measured simultaneously with each sample for in-situ energy calibration.

Sulfated zirconia with ordered mesopores as an active catalyst for n-butane isomerization

Zirconia/surfactant composites were hydrothermally synthesized in aqueous sulfuric acid at 373 K using Zr(O-nPr)4 as oxide precursor and hexadecyl-trimethyl-ammonium bromide as template. Mesostructural features similar to those of MCM-41 were detected by X-ray diffractometry, with d=4.6 nm. A sample obtained from a starting mixture with Zr:S:CTAB = 2:2:1 was stable enough for removal of occluded organics. After calcination at 813 K, the d-value was 3.6 nm, the surface area 200 m2/g, and the mean pore diameter estimated by the BJH method 2.2 nm. Extended X-ray absorption fine structure analysis suggests Zr to be in a short-range structure (<4 Å) similar to that of Zr in monoclinic ZrO2. Scanning electron microscopy including energy dispersive X-ray analysis showed 1-5 μm sulfur-containing ZrO2 spheres. The material catalyzes the isomerization of n-butane to i-butane at 378 K with a steady activity in the order of magnitude of commercial sulfated ZrO2.

In situ analysis of metal-oxide systems used for selective oxidation catalysis: how essential is chemical complexity?

The mode of operation of selective oxidation reactions is described by a series of chemical rules defining the catalyst and some reaction intermediates. In contrast to catalytic processes over metallic elements, little is known, however, about the atomistic details of selective oxidation. In particular, the participation of the subsurface region of the catalyst in the kinetically relevant elementary steps (Mars–van Krevelen mechanism) is not positively verified. Using in situ X-ray absorption techniques to study binary and ternary molybdenum oxides the present contribution shows that it is possible to tackle some of the problems in selective oxidation by direct experimental observation. The modification of the Mo–O local bonding interaction upon thermal reduction of MoO3to MoO3-xis illustrated. This was also found for mixed Mo–V oxides in which the chemical state of the vanadium seemed unaffected by the reaction but the surface Mo : V ratio varied substantially with the gas phase composition. It is further shown that the solid-state phase transformation between reduced and oxidised forms of molybdenum oxides occur so rapidly, that possibly relevant suboxide cannot be identified by ex situ phase analysis. Observation of the time-law of redox transformations showed that lattice oxygen is only available for selective oxidation if the associated solid-state transformation occurs in the kinetic regime of reaction control and not in that of diffusion control.

The structure of molybdenum-heteropoly acids under conditions of gas-phase selective oxidation catalysis: a multi-method in situ study

Abstract The present study focuses on the evidence about the existence of Keggin ions under various reactive conditions. The stability of the hydrated parent heteropoly acid (HPA) phases is probed in water, by thermal methods in the gas phase, by in situ X-ray diffraction and in situ EXAFS. An extensive analysis of the in situ optical spectra as UV-Vis-near-IR (NIR) in diffuse reflectance yields detailed information about the activated species that are clearly different from Keggin ions but are also clearly no fragments of binary oxides in crystalline or amorphous form. Infrared spectroscopy with CO as probe molecule is used to investigate active sites for their acidity. Besides OH groups evidence for electron-rich Lewis acid sites was found in activated HPA. All information fit into a picture of a metastable defective polyoxometallate anion that is oligomerised to prevent crystallisation of binary oxides as the true nature of the “active HPA” catalyst. The as-synthesized HPA crystal is thus a precatalyst and the precursor oxide mixture is the final deactivated state of the catalyst.

Formation of Bronzes during Temperature-programmed Reduction of MoO3 with Hydrogen - An In situ XRD and XAFS Study

The temperature-programmed reduction of MoO3 from 300 K to 773 K in 50 vol-% hydrogen in He (10 5 Pa) at different heating rates (0.1, 0.2, and 5 K/min) was investigated by in situ XRD and XAFS. At heating rates of ~ 0.1 and ~ 0.2 K/min the formation of the molybdenum bronze H0.34 MoO3 was observed by in situ XRD in the early stage of the reduction of MoO3. At a heating rate of 5 K/min the formation of a more disordered bronze (HxMoO3 with x ~ 0.07) prior to the detection of the product phase MoO2 was observed by in situ XAFS. In both studies the consumption of the bronze was found prior to the complete reduction of MoO3. A simplified mechanism for the temperature-programmed reduction of MoO3 in hydrogen is proposed that includes (i) in corporation of H2 in the MoO3 bulk and formation of a more or less ordered bronze, (ii) consumption of the bronze and formation of nucleation site for MoO2, (iii) nucleation of MoO2 and nuclei growth.