Julia Wienold

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.

Kinetics of solid-state reactions in heterogeneous catalysis from time-resolved X-ray absorption spectroscopy

The potentials of X-ray absorption spectroscopy (XAS) (quantitative phase composition and average valence together with a short-range order structure analysis) combined with a time-resolution in the second range make time-resolved (TR-) XAS a powerful tool for investigating the reactivity of solids in catalysis and solid-state chemistry. General aspects of TR-XAS investigations are discussed (i.e., instrumentation, data analysis). In addition, some experiments illustrate how the kinetics of solid-state reactions in heterogeneous catalysis can be elucidated from TR-XAS studies.

Structure–activity relationships of heterogeneous catalysts from time-resolved X-ray absorption spectroscopy

Knowing the composition and the evolution of the bulk structure of a heterogeneous catalyst under working conditions (in situ) is a pre-requisite for understanding structure–activity relationships. X-ray absorption spectroscopy can be employed to study a catalytically active material in situ. In addition to steady-state investigations, the technique permits experiments with a time-resolution in the sub-second range to elucidate the solid-state kinetics of the reactions involved. Combined with mass spectrometry, the evolution of the short-range order structure of a heterogeneous catalyst, the average valence of the constituent metals, and the phase composition can be obtained. Here we present results obtained from time-resolved studies on the reduction of MoO3 in propene and in propene and oxygen.

Solid-state kinetics from time-resolved in situ XAFS investigations: reduction and oxidation of molybdenum oxides

The reduction of MoO3 with hydrogen was studied by in situ X-ray absorption spectroscopy. The experiments performed focused on elucidating phase composition and evolution with time under isothermal reduction conditions. From temperature programmed experiments short-range structural details about the early stage of the reduction were obtained.

Phase formation during the decomposition of ammonium heptamolybdate – an in situ XAFS and XRD Investigation

The decomposition of ammonium heptamolybdate was investigated by in situ XAFS and in situ XRD to elucidate the influence of different atmospheres on the products formed the formation of different decomposition products and various intermediates is described in detail.