Vanadium signal

The effect of etch time (30 sec vs. 180 sec) on the composition of a Ti-6-4 surface etched in hydrofluoric acid/ammonium orthophosphate solution was followed by two SIMS spectra shown in Figure 29.(33) The aluminum signal decreases with increasing etch time while the vanadium signal increases. Preferential etching of the aluminum-rich a phase relative to the vanadium-rich p phase by the etch leads to the decreased aluminum signal. 

Multi-cellular organisms with considerably increased development of the use of zinc, copper and selenium. New uses of minerals and reactions in controlled extracellular fluids lesser use of Co and Ni. Changed use of Se, halogens and vanadium more extensive use of Ca and oxidised organic molecule signalling (one billion years or so ago). 

The CL signal arises from the reaction of luminol with I2 produced by the latter reaction. Other reactions are also available including KIO3-KI [68], H202-KI [69], and H202-Na2S203 reactions [70] catalyzed by Mo(VI), among others. Similar reactions could also be used for the determination of trace vanadium(V) ions [71, 72], The developed methods were applied to the determination of trace elements in natural waters and other matrices with detection limits below the ng/mL levels for traces of molybdenum and vanadium. 

The Figures 10.1 and 10.3 present the TPO spectra of the samples with and without metals. For the sample impregnated with 4100 ppm vanadium, it was observed the appearance of a shoulder around 680°C that translates in a 10% increase in peak C area, compared to the metal-free catalyst as illustrated in Figure 10.3. Then, the signal C located around 61TC apparently corresponds to the contaminant coke produced by the hydro-dehydrogenation properties of vanadium. 

Table 10.6 summarizes data from the TPO profiles for the three samples with and without metals. This table clearly shows how the signal C increase as the concentration of nickel and vanadium increases and supports the hypothesis that this peak corresponds to contaminant coke. It is possible to support the theory that a higher content of vanadium in the catalyst results in a loss of activity because the peak area B, previously attributed to catalytic coke, decreases strongly with vanadium levels. 

In Figure 10.4 was plotted the contaminant coke yield as a function of Ni equivalent. In this graph it is observed that the signal C, expressed as grams of contaminant coke, is almost a linear function of Ni equivalent. When the vanadium factor is changed to 0.38 the ratio is completely linear. Then with this technique it is possible to find the real dehydrogenation factor of vanadium with respect to nickel. 

The technique of solid-state NMR used to characterize supported vanadium oxide catalysts has been recently identified as a powerful tool (22, 23). NMR is well suited for the structural analysis of disordered systems, such as the two-dimensional surface vanadium-oxygen complexes to be present on the surfaces, since only the local environment of the nucleus under study is probed by this method. The nucleus is very amenable to solid-state NMR investigations, because of its natural abundance (99.76%) and favourable relaxation characteristics. A good amount of work has already been reported on this technique (19, 20, 22, 23). Similarly, the development of MAS technique has made H NMR an another powerful tool for characterizing Br 6nsted acidity of zeolites and related catalysts. In addition to the structural information provided by this method direct proportionality of the signal intensity to the number of contributing nuclei makes it a very useful technique for quantitative studies. 

John P. Hunt So far we have not seen any organic free radicals in these experiments. We have looked mainly at the maleatopentammine Co(III) system s reacting with chromium(II) and vandium(II). In the chromium system this problem is complicated by a large chromium(III) ESR signal. In the case of vanadium, we sometimes see small traces of vandium(IV) form which we don t completely understand. 

Because of the superhyperfine interaction which arises when the OJ ion is formed on a cation with nonzero nuclear spin (see Section III,A,3) vanadium pentoxide, with 100% naturally abundant5 V isotope (I = ), has been of considerable interest. However, the presence of a superhyperfine splitting has created some difficulty in the assignment of the signals. V205 cannot be prepared with large surface area and most of the data refer to supported V205 systems. 

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