Spin-echo mapping

While the techniques described above are widely applied in the study of metal oxide catalysts, they are not necessary suitable for systems that contain paramagnetic nuclei. Paramagnetic nuclei, as discussed in Section 5.3.1.2, are not observable by conventional NM R techniques and furthermore can increase the linewidths, sometimes to beyond observable limits, of neighboring nuclei. This poses a problem when studying catalytic metal oxides that contain paramagnetic centers. Vanadium phosphorus oxides (VPO), for instance, contain paramagnetic V.  

Spin-echo mapping allows the indirect observation of paramagnetic centers by discerning their effect on neighboring nuclei. The technique was developed 

Figu re 5.6 P NMR spin-echo mapping spectra ofVPOpi, VPOp2, VPOp3 and VPOp4 catalyst precursors. V species appear at 1725-1 770 ppm and dimers at -900 ppm. 

Adapted from ref [35] reprinted with permission from the Royal Society of Chemistry. 

900 ppm. The results show that the dimers formed by the latter route have a higher stability in water, in addition to characterizing vanadium sites, spin-echo mapping has also been used to provide similar information on paramagnetic cobalt centers [36]. 

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FIGURE 25 SEM micrographs (A), XRD spectrum (B) and spin-echo mapping NMR spectroscopy of active catalysts derived from VPA (row 1), VPO (row 2), and VPD (row 3) precursors. Reproduced with permission from Ref. (39). Copyright 1996 Elsevier. 

Applied to VPO catalysts, spin-echo mapping not only provides information on the oxidation state of different vanadium nuclei, but can also differentiate different phases with the same oxidation state. The development of a NMR technique to probe such materials has been extremely valuable, as their often poorly crystalline nature prevents characterization through X-ray diffraction. Additionally, variable-temperature spin-echo mapping has been shown capable of determining magnetic characteristics of materials, such as their Weiss temperature [34, 37]. 

Spin-echo mapping therefore provides valuable structural information on VPO catalysts, and can also be used, for example, to determine magnetic characteristics such as the Weiss temperature of pure phases. Additionally, standard MAS NMR techniques have successfully been employed to yield important information on VPO catalysts, in a similar manner as for supported catalysts discussed above [105, 106, 108, 112, 113]. 

The VPD catalyst appears more crystalline than the VPO catalyst. XRD and P NMR both show crystalline (VO)2P207, with a small amount of VOPO4 and disorganized (VO)2P207 also visible in the spin echo mapping spectrum. The TEM study shows the characteristic rosettes make up 95% of the catalyst, with a few flat platelets. Diffraction patterns showed the rosettes to be made up of (VO)2P207 (100) planes, while the flat plates can be indexed to an-VOP04. 

A spin-echo mapping technique has been used to investigate various CoAPO-re (288). The different NMR lines of the spectra are assigned to various P(reCo) environments in the structure, and their shift is found to be approximately proportional to the number (n) of the Co atoms in the first coordination sphere of the P atoms. Also, the presence of signals above 500 ppm is taken as a direct proof for the framework cobalt sitting. 

Use of 31p NMR by Spin Echo Mapping to prepare precursors of Vanadium Phosphate catalysts for n-Butane oxidation to Maleic Anhydride... 

We developed 3ip NMR by spin echo mapping to study VPO catalysts. With this technique, it is possible to discriminate between the phases of this system with vanadium in different valencies and corresponding to different environments (5-6). 

In this paper, we use the possibilities of Ip NMR by spin echo mapping to study the conditions of the formation of the hemihydrate VOHPO4, 05 H2O by reduction of VOPO4, 2 H2O with isobutanol. This method of preparation provides a new route for obtaining a precursor with a different morphology with a high development of the (220) X-rays line as compared to the (001) one (7). 

The advantage of ip NMR by spin echo mapping originates from the possibility to follow the V5 /V4+ reduction from the dihydrate (V5+) to the hemihydrate (V ) and thus to follow the process of preparation of this last phase. 

The 31p spin echo spectra were recorded under static conditions, using a 90°x-t-180°y-t- (acquire sequence). The 90° pulse was 4.2 ms and t was 20 ps. For each sample, the irradiation frequency was varied in increments of 100 kHz above and below the 3ip resonance of H3PO4. The number of spectra thus recorded was dictated by the frequency limits beyond which no spectral intensity was visible. The 3lp NMR Spin Echo Mapping information was then obtained by superposition of all spectra. 

Figure 3 shows the NMR spectrum by spin echo mapping of VOPO4,2 H2O. It presents a typical signal at 0 ppm indicative of P atoms bonded to V5+ atoms of this structure (5). Figure 4 shows the spectra of the samples depending on time of refluxing with isobutanol. 

Fgure 4 3lp NMR by spin echo mapping of the material after refluxing isobutanol for 2 hours (a), 4 hours (b), 8 hours (c), 16 hours (d) and 23 hours (e). 

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