Polymolybdates spectroscopy

The molybdenum dispersion also depends on the phosphorus content of the catalyst. Atanasova et al. (68, 87) reported that the dispersion of molybdenum and nickel, measured by X-ray photoelectron spectroscopy (XPS), shows a steep increase due to the presence of phosphorus at low loadings. The dispersion of molybdenum in NiMoP/Al catalysts increases further as a result of calcination, whereas that of nickel decreases. In contrast, Sajkowski et al. (83) reported, on the basis of an extended X-ray absorption fine structure (EXAFS) investigation, that phosphorus does not affect the size of the polymolybdate species, Mangnus et al. (31) inferred that the stacking of molybdates does not increase as a result of the addition of phosphorus since the height of a temperature-programmed reduction (TPR) peak at 400°C due to the reduction of deposited multilayered molybdenum oxo-species was found to be independent of the phosphorus content. However, Chadwick el al. (60) concluded from XPS measurements that the dispersion of molybdenum decreases upon addition of phosphorus. 

The [Mo04 ]j species have been detected as intermediates by Raman spectroscopy (see also Figure 16). They subsequently undergo condensation with formation of the surface polymolybdate provided their local concentration becomes sufficiently high. 

The formation of an intermediate, which is then reduced to form Mo—with either Ni or Fe acting as a catalyst, was also claimed, based on in-situ Raman spectroscopy studies.Although the exact composition of the intermediate was not identified in these studies, it was argued that at low cathodic polarization, the main species on the electrode surface were polymolybdates, that could be reduced to Mo(IV) at a higher cathodic polarization. The species of Mo(IV) could be further reduced to Mo atoms only when cations of the iron-group metal were present in the electrolyte. 

Three Co2Moio(Co) catalysts were prepared one on gamma alumina and two on supports modified by TMSiOMe respectively at 0.5 TMSi/nm and 0.9 TMSi/nm. Catalysts thus obtained on all three supports have a similar cobalt/molybdenum molar ratio of 0.5. Their respective Mo loadings expressed as wt% of M0O3 are 16.7 for catalyst prepared on alumina, 15 for catalyst prepared on 0.5-TMSi-alumina and 15.7 for catalyst prepared on 0.9-TMSi-alumina. Raman spectroscopy was used to characterize the nature of the supported oxomolybdate phase. In both cases, the spectra (not reported here) show the features of a well dispersed polymolybdate phase. In particular, no M0O3 nor C0M0O4 was evidenced. 

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