Characterization of V–Mo–W Mixed Oxide Catalyst Surface Species by 51V Solid-State Dynamic Nuclear Polarization NMR

Abstract

The investigation of V–Mo–W mixed oxide catalysts by 51V DNP MAS NMR is reported. It is shown that the 51V NMR signal from surface near vanadium nuclei was enhanced by a factor of ≈50 by direct polarization transfer from unpaired electrons to 51V nuclei. The dependency of the DNP enhancement of the samples on different polarizing mixtures is investigated and complemented by EPR studies. V–Mo–W mixed oxide catalysts with different stoichiometry (V2Mo8W0.5Oy, V2Mo8W1Oy, and V2Mo8W1.5Oy) are compared, and a suitable assignment of 51V signals to different phases that are present near to the catalysts surfaces is given. Finally, the applicability of DNP solid-state NMR for the characterization of V–Mo–W mixed oxide catalysts is discussed in terms of the synthesis conditions (i.e., pH-value).