Metal-oxygen vibrations

Raman spectroscopy has been successfully applied to the investigation of oxidic catalysts. According to Wachs, the number of Raman publications rose to about 80-100 per year at the end of the nineties, with typically two thirds of the papers devoted to oxides [41]. Raman spectroscopy provides insight into the structure of oxides, their crystallinity, the coordination of metal oxide sites, and even the spatial distribution of phases through a sample when the technique is used in microprobe mode. As the frequencies of metal-oxygen vibrations found in a lattice are typically between a few hundred and 1000 cm 1 and are thus difficult to investigate in infrared, Raman spectroscopy is clearly the indicated technique for this purpose. 

The internal vibrations of the MnOi ion seem to be influenced less by the cations than other metal-oxygen vibrations [see(705)]. For example, the isotypical potassium-, rubidium-, cesium-, and ammonium permanganates have practically the same vi and vz frequencies. The difference observed in the case of AgMn04 is explained in Ref. 83). By the large cations, such as tetraphenylarsonium and tetraphenylphosphonium, the vz band is very sharp and well defined. Since these vz bands are not spht as expected it can be concluded that the anion... 

Table Il-3c lists the vibrational frequencies of pyramidal XOj-type compounds. Rocchiciolli has measured the infrared spectra of a number of sulfites, selenites, chlorates, and bromates. Dasent and Waddington also measured the infrared spectra of metal iodates and suggested that extra bands at 480-420 cm" may be due to the metal-oxygen vibrations. Again the P3 and P4 vibrations may split into two bands because of lowering of symmetry in ihe crystalline state. Although Vi> V/ holds in all cases, ihe order of two stretching frequencies (pi and V3) depends on the nature of the central metal. Figure 11-5 illustrates ihc infrared spectra of KCIO3 and KIO3 obtained in ihe crystalline state.

Figure 12.12. Recorded Raman spectra of the terminal metal-oxygen vibration regime. Spectra have been normalized to the intense band of V2O5 at 990 cm for better visual comparison with the exception of the spectrum of the deactivated catalyst. Color code green and blue = pristine catalyst black = after calcination at 300°C red = after calcination at 410°C for 3 days light blue = after standard operation for 3 months pink = catalyst deactivated after high temperature stress test for 1 month.

Adams, R.W., Martin, R.L., and Winter, G. (1967) Possible ligand field effects in metal-oxygen vibrations of some first-row transition metal alkoxides. Aust J. Chem., 20,773-774. 

Measurements of supported catalysts in diffuse reflection and transmission mode are in practice limited to frequencies above those where the support absorbs (below about 1250 cm-1). Infrared Emission Spectroscopy (IRES) offers an alternative in this case. When a material is heated to about 100 °C or higher, it emits a spectrum of infrared radiation in which all the characteristic vibrations appear as clearly recognizable peaks. Although measuring in this mode offers the attractive advantage that low frequencies such as those of metal-oxygen or sulfur-sulfur bonds are easily accessible, the technique has hardly been explored for the purpose of catalyst characterization. An in situ cell for IRES measurements and some experiments on Mo-O-S clusters of interest for hydrodesulfurization catalysts have been described by Weber etal. [11],... 

The metal-oxygen and related vibrations occur in the far IR region and these vibrations have been studied only in a few cases. In the complexes of lanthanide perchlorates with PyO, i>Ln o occurs in the region of 270-370 cm-1 (148). Three i n-o... 

A variety of spectroscopic studies of metal cupferronates have been carried out by a number of investigators. The IR spectra of Cu2+, Hg2+, Al3+, Fe3+, Ga3+, Bi3+, Ti4 Zr4+, V4 Th4+, U4+, V5+ and Nb4+ cupferronates indicate a close parallelism in the spectra of different metals in the same oxidation state, indicating similarity of structural features. The absorption maxima in the 930-400 cm-1 region move to the lower frequency side, as expected, with increasing atomic weight of the cations and these, therefore, apparently correspond to the vibrations of the metal-oxygen bonds.104... 

Monodentate attachment to a metal ion lowers the symmetry of perchlorate to C3v and bidentate attachment to C2v (15-17). Consequently the number of vibrational modes should increase (Table I). In addition, a metal-oxygen stretching frequency would also be expected in the far-IR region and has been located in the range 360-290 cm-1 (18). These effects resulting on coordination, particularly the increase in the number of vibrational modes, may be used for identifying coordination of perchlorate. 

Observation of metallic ion populations have also led to speculation about vibrational temperatures. Without an elevated oxygen vibrational temperature at 110 km, Ferguson et al. [214] could not explain the observed atmospheric Si+/SiO+ ratio. However, as noted by Bauer et al. [70], it is difficult to reconcile an elevated oxygen vibrational temperature with known O deactivation rates by atomic oxygen. 

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