In situ Raman experiments

The study of the mechanism of the fast SCR over V-W-Ti-0 catalysts was addressed first by Koebel and co-workers [65-68]. They suggested that (i) the reoxidation ofthe catalyst is rate determining at low temperature in the redox cycle of standard SCR catalyst, (ii) NO2 reoxidizes the catalyst faster than O2 the NO2-enhanced reoxidation of the catalyst was demonstrated by in situ Raman experiments, (hi) the reaction occurs via the nitrosamide intermediate in both standard and fast SCR and (iv) ammonium nitrate is considered an undesired side-product. 

Because of the extreme oxidation sensitivity of Fe(OH)2 and the likelihood of its formation, Odziemkowski et al. [106] performed in situ Raman experiments using an iron electrode in aqueous solution. NRS did not reveal the existence Fe(OH)2. However, Fe(OH)2 is known to form very thin films, and it was suspected that the sensitivity of NRS would not be sufficient to observe the film if present. Therefore, they deposited a discontinuous layer of Ag metal on the electrode, while still immersed in solution, in order to prepare a SERS-active surface. Spectra were then reacquired and peaks at 540 and 3416 cm both attributable to Fe(OH)2, were clearly observed (see Fig. 21). 

IN SITU RAMAN EXPERIMENTS ON POLYACETYLENE IN ELECTROCHEMICAL CELLS... 

In conclusion, in situ" Raman experiments, even if they are far from being easy to run, can give important information relevant to the conducting polymers domain. In the future, they will probably be used as a complementary technique to the other spectroscopic ones already widely used in solid state research implying electrochemical dopings or synthesis. 

A final experiment was completed with a very slow dose of the reducing agent in an attempt to monitor the formation of intermediate and product using the in-situ Raman probe. A... 

TPRS experiments for this reaction step. The two hydrogens released in the methanol adsorption and the surface methoxy decomposition steps are eventually converted to water. Spectroscopic details about the formation of water are presently not available, but the formation of water most likely proceeds via the condensation of two surface hydroxyl groups. The reduced surface vanadia site is readily reoxidized back to vanadium (+5) by gas phase oxygen as shown by in situ Raman measurements.42... 

Raman spectroscopy can offer vibrational information that is complementary to that obtained by IR. Furthermore, since the Raman spectrum reveals the backbone structure of a molecular entity [55], it is particularly useful in the examination of polymer film-coated electrodes. There are also some distinct advantages over in situ IR. For example, both the mid and far infrared spectral regions can be accessed with the same instrumental setup (in IR spectroscopy, these two regions typically require separate optics) [55]. Second, solvents such as water and acetonitrile are weak Raman scatterers thus the solvent medium does not optically obscure the electrode surface as it does in an in situ IR experiment. 

The transition from LDA to HDA Si was observed in the successive experiment by McMillan et al. [264]. In situ Raman spectra and electronic resistance measurements were performed with optical observation. After compression, the LDA form prepared by solid-state metathesis synthesis [10] was found to be transformed to the HDA form at 14 GPa. The electronic resistance exhibited a sharp decrease at 10-14 GPa (Fig. 15), which is consistent with the early experimental findings by Shimomura et al. [260], Optical micrographs show that HDA Si is highly reflective, whereas LDA Si is dark colored and nonreflective. This finding again supports that the LDA-HDA transition of Si is accompanied by a semiconductor-metal transition. Reverse transitions with large hysteresis were also observed LDA Si began to form from HDA Si at 4-6 GPa after decompression from --20 GPa. 

Hutchings et al. (118) carried out in situ Raman spectroscopy experiments with VPA precursors as they were being converted into the active catalyst. They foimd that during the activation there is a structural disordering at 370 °C, which corresponds to the appearance of MA in the catalytic reaction product. The disordering was foimd to occur at a lower temperature (300 °C) when MA was added to the butane/air reaction mixture. This result demonstrated that the presence of the products is important in controlling the structural transformations and that a highly disordered structure can be important in selective butane oxidation. 

Under reaction conditions with the coexistence of ozone and ethanol, the intensities of both these adsorbed species dramatically decreased, indicating that these two species reacted with each other on the catalyst surface. This was also supported by the transient experiment results. When ozone was introduced on a surhice preadsorhed with ethoxide species, the intend of the ethoxide ecies decreased gradually due to the reaction with ozone (gas phase or adsorbed), and that of the peroxide species increased with time due to the removal of ethoxide ecies from the surface. However, if the reaction of ethoxide ecies was mainly due to gas phase ozone, under steady state conditions, the surfrce ould he covered by adsorbed peroxide ecies. The in situ Raman ectra indicated that the reaction of ethoxide species was primarily due to reaction with adsorbed peroxide species because the concentratiorts of both adsorbed ecies decreased dramatically in the presence of both ethanol and ozone. Thus, a Langmuir-ICnshelwood type mechanism appears to be operating ... 

Ramem and IR spectroscopic experiments support these ideas [7]. Last but not least, in-situ XRD measurements showed reflections of NH4VO3 on equilibrated samples cooled down to r.t. after finishing the transformation process [8]. Otherwise, these reflections were absent if the transformed sample was flushed with a nitrogen flow before cooling down. Recently, in-situ Raman spectroscopy revealed the existence of NH4VO3 too [9]. Thus, it seems very likely that NH4VO3 could be formed from in-situ existing mixed-valent vanadium oxides and an excess of ammonium ions located on the surface. 

Earlier studies [24,25] have shown that in addition to the concentration of organic species, temperature is the most significant variable controlling the kinetics of the citric acid-H20 system. Cody et al. [25] further show that for a given citric acid concentration, the system exhibits a very complex behavior at high P and T. These studies have described the behavior of the system under hydrothermal conditions in detail and have formed the basis for interpretation of these diamond cell experiments. In this study, we have used in-situ Raman... 

Table I gives an overview of selected nitrogen systems thus far studied at high pressures and high or low temperatures by means of in situ DAC experiments. For each system, we list the initial molecular species loaded in the DAC, the range of pressures and temperatures examined, the data reported (R, IR, Opt, Dif, EC and NMR denote Raman, infrared, optical spectra, x-ray or neutron diffraction, electrical conductivity and magnetic nuclear resonance). We also indicate the phases observed, whether involving the initial molecular species (denoted by M) or a different species (denoted nonmolecular, NM, if not specifically identified) that becomes prominent at high P and T. The references cited include the original research reports that provide experimental data and most theoretical papers pertinent to these studies.

Pressure was generated with a diamond anvil cell (DAC) employing beveled anvils with central flats ranging from 20 to 100 jim and flat diamonds with 200-500 pm culets. Two types of DAC were used modified (to match a continuous flow He cryostat) Mao-Bell cell for operations at room and low temperatures [41] and a Mao-Bell high-T external heating cell [42]. The latter one is equipped with two heaters and thermocouples. Four experiments were performed at RT aiming to highest pressure and the final pressures varied from 180 to 268 GPa. For low-temperature measurements we used a continuous-flow He cryostat, which allowed infrared and in situ Raman/ fluorescence measurements. More details about our IR/Raman/fluorescence setup at the NSLS are published elsewhere [41]. 

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