Mossbauer characterization, catalytic

Reductive Activation of Aerobically Purified Desulfovibrio vulgaris Hydrogenase Mossbauer Characterization of the Catalytic H Cluster... 

In this section, we summarize results characterizing catalytic materials by Mossbauer spectroscopy. An analysis of the literature leads to the following classifications of the objectives of this work ... 

Three series of Au nanoparticles on oxidic iron catalysts were prepared by coprecipitation, characterized by Au Mossbauer spectroscopy, and tested for their catalytic activity in the room-temperature oxidation of CO. Evidence was found that the most active catalyst comprises a combination of a noncrys-taUine and possibly hydrated gold oxyhydroxide, AUOOH XH2O, and poorly crystalhzed ferrihydrate, FeH0g-4H20 [421]. This work represents the first study to positively identify gold oxyhydroxide as an active phase for CO oxidation. Later, it was confirmed that the activity in CO2 production is related with the presence of-OH species on the support [422]. 

In situ characterization. Catalysts should preferably be investigated under the conditions under which they are active in the reaction. Various reasons exist why this may not be possible, however. For example, lattice vibrations often impede the use of EXAFS, XRD and Mossbauer spectroscopy at reaction temperatures the mean free path of electrons and ions dictates that XPS, SIMS and LEIS are carried out in vacuum, etc. Nevertheless, one should strive to choose the conditions as close as possible to those of the catalytic reaction. This means that the catalyst is kept under reaction gases or inert atmosphere at low temperature to be studied by EXAFS and Mossbauer spectroscopy or that it is transferred to the vacuum spectrometers under conditions preserving the chemical state of the surface. 

Since that time, Mossbauer spectroscopy has been used widely to characterize catalysts in reactive atmospheres, leading to continuous progress in the understanding of structure/catalytic property relationships (180-194). 

Volume 50 of Advances in Catalysis, published in 2006, was the hrst of a set of three focused on physical characterization of solid catalysts in the functioning state. This volume is the second in the set. The hrst four chapters are devoted to vibrational spectroscopies, including Fourier transform infrared (Lamberti et al.), ultraviolet Raman (Stair), inelastic neutron scattering (Albers and Parker), and infrared-visible sum frequency generation and polarization-modulation infrared rehection absorption (Rupprechter). Additional chapters deal with electron paramagnetic resonance (EPR) (Bruckner) and Mossbauer spectroscopies (Millet) and oscillating microbalance catalytic reactors (Chen et al.). 

It seems, therefore, that the recent characterization of the phase compensation and structural properties of tin-antimony oxides may readily be correlated with the Mossbauer determination of cationic oxidation states and lattice distortion, especially in materials containing a high concentration of antimony. The charge compensation mechanism, however, remains an intriguing aspect of this material and the nonlocalization of electron density may well be considered as a feature of potential catalytic relevance. In this respect a technique which is well suited to the study of the dependence of catalytic performance on the presence of any spin free species and semiconducting properties is ESR. 

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