Solid-State Kinetic Investigations of Nonisothermal Reduction of Iron Species Supported on SBA-15

Iron oxide catalysts supported on nanostructured silica SBA-15 were synthesized with various iron loadings using two different precursors. Structural characterization of the as-prepared Fe x O y /SBA-15 samples was performed by nitrogen physisorption, X-ray diffraction, DR-UV-Vis spectroscopy, and Mössbauer spectroscopy. An increasing size of the resulting iron species correlated with an increasing iron loading. Significantly smaller iron species were obtained from (Fe(III), NH4)-citrate precursors compared to Fe(III)-nitrate precursors. Moreover, smaller iron species resulted in a smoother surface of the support material. Temperature-programmed reduction (TPR) of the Fe x O y /SBA-15 samples with H2 revealed better reducibility of the samples originating from Fe(III)-nitrate precursors. Varying the iron loading led to a change in reduction mechanism. TPR traces were analyzed by model-independent Kissinger method, Ozawa, Flynn, and Wall (OFW) method, and model-dependent Coats-Redfern method. JMAK kinetic analysis afforded a one-dimensional reduction process for the Fe x O y /SBA-15 samples. The Kissinger method yielded the lowest apparent activation energy for the lowest loaded citrate sample (Ea ≈ 39 kJ/mol). Conversely, the lowest loaded nitrate sample possessed the highest apparent activation energy (Ea ≈ 88 kJ/mol). For samples obtained from Fe(III)-nitrate precursors, Ea decreased with increasing iron loading. Apparent activation energies from model-independent analysis methods agreed well with those from model-dependent methods. Nucleation as rate-determining step in the reduction of the iron oxide species was consistent with the Mampel solid-state reaction model.

Application of time-resolved in-situ X-ray absorption spectroscopy in solid-state chemistry

Time-resolved X-ray absorption spectroscopy (TR-XAS) possesses excellent capabilities to reveal quantitative phase composition and average valence together with the evolution of the local structure of a system under dynamic reaction conditions. The work discussed here focused on time-resolved in-situ XAS investigations aiming, first, at understanding structural evolution under dynamic conditions and, second, at revealing properties of the system studied not available from investigations under stationary conditions. Hence, not only was the local structure of a material studied under reaction conditions, but characteristic properties of the reaction, such as reaction intermediates or the kinetics of the reaction, were also elucidated. The solid-gas reactions presented here clearly demonstrate the potential of TR-XAS investigations to extend the suitability of XAS for in-situ studies in solid-state chemistry to investigations under dynamic conditions.

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.

In situ studies on the structure of copper oxide/zinc oxide catalysts

Cu/ZnO supported on alumina is a well-known catalyst for steam reforming of methanol. In this work it is attempted to assess the influence of elemental composition on the resulting active copper phase. XAFS measurements of calcined precursors were carried out at the Cu K edge and the Zn K edge. Corresponding RDF show that both copper oxide and zinc oxide exhibit considerable deviations from a linear dependence of their structure on the composition coinciding with changes in phase composition of hydroxycarbonate precursor. From time-resolved in situ experiments at the Cu K edge the degree of reduction can be monitored using a combination of factor analysis (PCA) and least-squares XANES fitting with suitable reference spectra (e.g. Cu metal, Cu2O, and CuO). It is shown that Cu2O forms prior to Cu. The extent of reduction to Cu exhibited a typical nucleation growth behavior with an enhanced reaction rate for more diluted samples. Adding oxygen to the feed gas leads to the formation of mixed Cu2+ and Cu+ phases accompanied by a complete loss of activity in methanol steam reforming. After switching back to steam reforming conditions the activity is regained.

Manganese, iron and sulfur K edge XAFS of promoted sulfated zirconia catalysts

Promoted sulfated zirconia samples were prepared by the incipient wetness technique to produce isomerization catalysts which were active for the conversion of n-butane to isobutane at 338K (up to 10% conversion of 1% n-butane, 1atm., 0.25 h(-1) WHSV). The local structure of Fe and Mn in promoted sulfated zirconia was investigated using fluorescence yield XAS. Spectra were taken of calcined samples, activated samples, and samples after reaction with n-butane (maximum activity and deactivated). Factor analysis reveals that the Mn K edge XANES can be described by a linear combination of the spectra of two separate components, and that the ratio of these components changes with activation of the catalyst, and during use in the n-butane isomerization reaction. The change in ratio of the Mn species during activation and reaction results in a reduction of the average Mn valence from 2.4 to 2.2. The Fe K edge XANES was not similarly affected by activation and reaction with n-butane.

Catalytic methanol oxidation over copper: observation of reaction-induced nanoscale restructuring by means of in situ time-resolved X-ray absorption spectroscopy

The catalytically active copper phase for the partial oxidation of methanol is studied by means of time-resolved extended X-ray absorption fine structure (EXAFS) spectroscopy combined with the detection of the catalytic turnover. It is found that the active form of the copper is a strained nanocrystalline form of the metal. The metal is no longer made up from large crystallites but contains a defect structure in which oxygen is already intercalated.

A scattering filter for energy-dispersive optics

A filtering technique to remove parasitic scattering from X-ray absorption spectra that are acquired in energy-dispersive mode has been developed and tested at the European Synchrotron Radiation Facility. The improved set-up removes small-angle scattering of the sample or the windows of sample cells which may spoil the energy resolution or reduce the intensity of prominent features in the absorption spectrum, such as the white line at the Pt L(III) edge. The sample is placed behind the curved monochromator and between two plane perfect crystals in the Bonse-Hart configuration. The dispersion of the Bonse-Hart double-crystal camera is matched to the dispersion of the curved monochromator by inclining the scattering planes of the two optical elements against each other.

WinXAS: a program for X-ray absorption spectroscopy data analysis under MS-Windows

WinXAS is a new X-ray absorption spectroscopy (XAS) data-analysis program. It runs under the operating system MS-Windows 95/NT and offers several unique features. It has a user-friendly graphical environment and is capable of reading a variety of data formats. It contains a number of useful numerical algorithms beyond those used in conventional XAS analysis and offers a simple interface to the ab-initio theoretical code FEFF. The availability of fast macros in WinXAS makes it particularly useful for on-line data examination at synchrotron radiation facilities during XAS experiments as well as for the analysis of multiple-scan data such as those from time-resolved experiments.

Curved Crystal Transmission Optics for Energy-Dispersive X-ray Absorption Spectroscopy

The development of a curved crystal monochromator of the Laue type for energy-dispersive X-ray absorption spectroscopy is presented. The quality of the X-ray absorption spectra at high photon energies is compared with spectra measured with silicon crystals in the more frequently used Bragg geometry. In the Bragg case, an asymmetric broadening of the reflectivity profile leads to strong distortions of the near-edge fine structure and to a reduction in spectral resolution. The reflectivity profiles of fiat and curved crystals for Laue and Bragg geometry have been calculated using dynamical theory and are compared with experimental data. The new optics have been used for in situ time-resolved X-ray absorption spectroscopy. An example of the application of the technique for the characterization of a Pd catalyst is given. The X-ray absorption fine structure at the Pd K-edge has been measured during the activation and during the heterogeneous catalytic oxidation of carbon monoxide.