Raman experiments, selective oxidation

Figure 3 illustrates the concepts of Raman cells that can be used for experiments under reaction conditions. Several commercial cells are suitable for use in combination with Raman microscopy. The rotating sample design was modified by Wachs s group (Figure 3A, Banares et al., 1994) and used to investigate supported oxides during selective alkane oxidation (Banares et al., 2000C Guliants et al., 1995 Sim et al., 1997) and various catalysts...

Raman spectra of a bismuth molybdate catalyst obtained with this apparatus are shown in Figure 13 (Snyder and Hill, 1991) they represent the starting material and the catalyst after 24 h of proven operation (by online GC analysis) in propene oxidation to acrolein at 400 °C. The conversion values in the cell ranged from 20% to 40%, with the selectivity to acrolein reaching values between 90% and 50%. Long-term experiments did not show significant variations within 26 days of operation at 400 °C (Snyder and Hill, 1991) if the catalysts were properly annealed beforehand. 

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. 

Strength (FLS) empirical approach are discussed in Section 3 as methods for determining the molecular structures of metal-oxide species from their Raman spectra. The state-of-the-art in Raman instrumentation as well as new instrumental developments are discussed in Section 4. Sampling techniques typically employed in Raman spectroscopy experiments, ambient as well as in situ, are reviewed in Section S. The application of Raman spectroscopy to problems in heterogeneous catalysis (bulk mixed-oxide catalysts, supported metal-oxide catalysts, zeolites, and chemisorption studies) is discussed in depth in Section 6 by selecting a few recent examples from the literature. The future potential of Raman spectroscopy in heterogeneous catalysis is discussed in the fmal section. 

In this study, suboxides of vanadia catalysts were used in pentane, pentene, dicyclopentadiene and cyclopentane oxidation reactions. In the previous phase of the work [12,13], the role of alkali promoters on the catalyst selectivity was examined. The catalysts were reduced in situ at different temperatures and the effect of pre-reduction temperature was investigated. Controlled-atmosphere characterization of pre-reduction, post-reduction, and post-reaction catalysts were performed using X-ray diffraction. X-ray photoelectron spectroscopy, laser Raman spectroscopy and temperature-programmed desorption experiments. The objectives of this study were to determine the activity and selectivity of different suboxides of vanadia in... 

All vibrations present in the neat, ciystalline explosive seem to be enhanced by the same factor (non-selective enhancement). Moreover, the spectra resemble solid phase normal Raman (NR) spectra, with fully resolved, non-broadened peaks. The reproducibility in band intensities is slightly better than in SERS spectra ( 20-30%), but is prone to sample variation due to the lack of homogeneity in the sohd-solid mixtures. KBr-analyte pellets were used as reference, since no evidence of ERS was fotmd for this substrate. Experiments using cobalt (11) oxide did not result in observation of enhanced Raman signals under the experimental protocol followed (Figure 7). 

Recent reconstitution experiments open the possibility of selectively introducing other metallo bacteriochlorins (or chlorins) at the acceptor BChl or BPheo sites. Zinc substitutions are obvious choices since the optical and coordination properties of Zn complexes resemble those of Mg derivatives but the former are easier to reduce (and harder to oxidize) than the latter. Perhaps a less obvious candidate for metal replacement is nickel which offers several advantages Ni complexes are also easier to reduce and harder to oxidize than Mg compounds but, in addition, redox potentials of Ni porphyrins and chlorins are close to those of the metal-free derivatives the optical properties of Ni chlorins are similar to those of Mg ones they possess shortlived excited states as a result of which they have proved to be valuable for resonance Raman studies. A more practical advantage of Ni complexes is their enhanced stability towards demetallation, compared to Mg or Zn complexes, a property of merit if the chromophores are to be used in reconstitution reactions. (Ni protoporphyrins IX have been successfully exchanged for the histidine-bound hemes in hemoglobin.) ... 

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