Study by 31P NMR spin echo mapping of vanadium phosphorus oxide catalysts.
Vanadium phosphorus oxides (VPO) containing vanadium ions in the +3 and +4 oxidation states, namely VPO4, (VO)2P2O7, VOHPO4.0.5H2O and VO(H2PO4)2 have been characterized using 31P solid state NMR spectroscopy. Because of couplings between the unpaired electrons of V4+ or V3+ ions and 31P nuclei, 31P NMR lines are drastically broadened and shifted by more than 4500 ppm in the case of the V3+ phases. It is therefore necessary to use the Spin Echo Mapping technique to obtain the complete NMR information. The technique has proved to be particularly interesting as it permits us to distinguish between various V4+ phases. The observed frequency shift is proportional to the atomic magnetic susceptibility of the material and, therefore, changes with temperature. For V4+ phases, we have observed that the inverse shift was proportional to temperature in the working temperature range and that good estimations of the Weiss temperatures could be obtained. Moreover, at low temperature, the 31P NMR spin echo mapping of the pyrophosphate (VO)2P2O7 showed four distinct peaks that could be assigned to various unpaired electron spin densities on the phosphorus atoms. This has been correlated with previous structural determinations, particularly the oxidation state of vanadium ions in these solids. We have also observed that the inverse of the line width increased linearly with temperature in the working range. Special attention was given to VPO4 where both the evolution with temperature of the observed shift and the line width were very different from those observed on V4+ phases.
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Institut de Recherches sur la Catalyse, C.N.R.S., Villeurbanne, France.
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D002384:Catalysis; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D015394:Molecular Structure; D010084:Oxidation-Reduction; D010758:Phosphorus / Q000737:chemistry; D017976:Phosphorus Acids / Q000737:chemistry; D017968:Vanadium Compounds / Q000737:chemistry*
D017976:Phosphorus Acids; D017968:Vanadium Compounds; D010758:Phosphorus