In situ diffraction studies

As already mentioned at the end of Sec. 7.2, autoclaves in hydrothermal synthesis are black boxes , and all the attempts to explain the formation of the porous solids originated from ex situ experiments. The above hypothesis concerns reactions within the autoclave and requires for its analysis in situ measurements in hydrothermal conditions which were never performed so far. These experiments must give information both on the solution and on the solid. Moreover, in the systems chosen for such studies, the kinetics of formation must be adapted to the possibilities of observation of the chosen technique. Two of them are particularly useful for this purpose in situ diffraction studies using synchrotron radiation (SR) and in situ NMR (liquid and solid state). 

In situ diffraction studies during the release of hydrogen allowed a comprehensive insight into the phase transformation processes taking place during the reversible storage ofhydrogen [113,114]. The decomposition mechanism for sodium aluminum... 

Even if only two starting components are used, a system becomes complicated (multi-component) when hydrogen desorption begins. As a number of intermediate and decomposition products are involved, the system becomes multi-component, and thus it appears essential to know which components are involved in the crucial steps of hydrogen release. In several works an in-situ diffraction study of hydride mixtures was important in the identification of new phases and gives a sequence of intermediate compounds. 

I. C. Madsen, N. V. Y. Scarlett and B. I. Whittington, Pressure acid leaching of nickel laterite ores an in-situ diffraction study of the mechanism and rate of reaction, J. Appl. Crystallogr., 2005, 38, 927-933. 

Reimers JN, Dahn JR (1992) Electrochemical and in situ diffraction studies of hthium intercalation in LixCo02. J Electrochem Soc 139 2091-2097... 

Wang J, Ocko B M, Davenport A J and Isaacs H I 1992 In situ diffraction and reflectivity studies of the Au(111 )/electrolyte interface Reconstruction and anion adsorption Phys. Rev B 34 10 321-38... 

A number of ex situ spectroscopic techniques, multinuclear NMR, IR, EXAFS, UV-vis, have contributed to rationalise the overall mechanism of the copolymerisation as well as specific aspects related to the nature of the unsaturated monomer (ethene, 1-alkenes, vinyl aromatics, cyclic alkenes, allenes). Valuable information on the initiation, propagation and termination steps has been provided by end-group analysis of the polyketone products, by labelling experiments of the catalyst precursors and solvents either with deuterated compounds or with easily identifiable functional groups, by X-ray diffraction analysis of precursors, model compounds and products, and by kinetic and thermodynamic studies of model reactions. The structure of some catalysis resting states and several catalyst deactivation paths have been traced. There is little doubt, however, that the most spectacular mechanistic breakthroughs have been obtained from in situ spectroscopic studies. 

X-ray surface diffraction has been applied in situ to study several processes at the electrode solution interface [13, 14]. An important phenomenon in electrochemistry at Au is surface reconstruction in which a monolayer of atoms on the surface of a single crystal acquires a different arrangement from that of the... 

Despite the observation of profuse spectroscopic changes on compression and decompression of NaNs, the in situ high-pressure structures of the various phases remain unknown. The small sample size required by high pressure studies and large hysteresis due to the strain and stress make difficult in situ diffraction measurements. At high temperature conditions, however, the transformation from an azide to a polynitrogen phase may be more readily characterized as the requisite transformation pressures may be much lower and less subject to hysteresis. Here we describe the heating experiments on pure sodium azide and a... 

All samples showed diffraction pattern similar to that ofhydrotalcite without any discrete crystalline impurity phases. In situ PXRD studies revealed varying phase evolution processes depending on the concentration of magnesium, complemented by in situ DRIFT measurements. The thermal stability of the materials improved with an increase in magnesium concentration, as evidenced from TG-DTA measurements. Details of physicochemical properties of these samples arc disclosed elsewhere (94]. 

Recent decades have witnessed spectacular developments in in-situ diffraction and spectroscopic methods in electrochemistry. The synchrotron-based X-ray diffraction technique unraveled the structure of the electrode surface and the structure of adsorbed layers with unprecedented precision. In-situ IR spectroscopy became a powerfiil tool to study the orientation and conformation of adsorbed ions and molecules, to identify products and intermediates of electrode processes, and to investigate the kinetics of fast electrode reactions. UV-visible reflectance spectroscopy and epifluorescence measurements have provided a mass of new molecular-level information about thin organic films at electrode surfaces. Finally, new non-hnear spectroscopies such as second harmonics generation, sum frequency generation, and surface-enhanced Raman spectroscopy introduced unique surface specificity to electrochemical studies. 

In situ structural studies can also provide considerable insight into material properties and formation pathways. Koziej and co-workers have employed X-ray absorption spectroscopy (XAS) and diffraction for this purpose and have studied the formation of cobalt and cobalt oxide nanoparticles from a benzyl alcohol route.Here, the importance of the reaction temperature on the reactivity of the cobalt isopropoxide starting material with benzyl alcohol was noted. The first report of cobalt nanoparticles from a benzyl alcohol route via the reduction of Co " was established for reaction temperatures of 180 °C. Meanwhile, lower reaction temperatures (80 °C) lead to oxidation and afford cubic C03O4 nanoparticles. The in situ studies have been carried out on samples prepared at 140 °C in an effort to understand the mechanism behind such a complex reaction scheme. It was found that at this temperature both processes occur simultaneously reduction to Co° and oxidation to C03O4, followed by reduction to CoO. Studying the assembly of nanoparticles in detail can also provide an insight into how these processes could be potentially manipulated. 

In order to get answers to these questions, the ability to better characterize catalysts and electrocatalysts in situ under actual reactor or cell operating conditions (i.e., operando conditions) with element specificity and surface sensitivity is crucial. However, there are very few techniques that lend themselves to the rigorous requirements in electrochemical and in particular fuel cell studies (Fig. 1). With respect to structure, in-situ X-ray diffraction (XRD) could be the method of choice, but it has severe limitations for very small particles. Fourier transform infra red (FTTR), " and optical sum frequency generation (SFG) directly reveal the adsorption sites of such probe molecules as CO," but cannot provide much information on the adsorption of 0 and OH. To follow both structure and adsorbates at once (i.e., with extended X-ray absorption fine stmcture (EXAFS) and X-ray absorption near edge stmc-ture (XANES), respectively), only X-ray absorption spectroscopy (XAS) has proven to be an appropriate technique. This statement is supported by the comparatively large number of in situ XAS studies that have been published during the last decade. 16,17,18,19,20,21,22,23,24,25 highly Versatile, since in situ measme-... 

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