High-temperature multinuclear magnetic resonance studies of vanadia catalysts for SO2 oxidation

Multinuclear Na-23, K-39, Cs-133,O-17, V-51 magnetic resonance studies of t he M2S2O7-V2O5 (M = Na, K, Cs) systems in the temperature range 20-650 degr ees C have been performed for vanadium oxide mole fractions, X(V2O5), in th e range 0-0.5. At ambient temperature the melt-quenched glassy samples exhi bit a three-dimensional network of vanadium oxosulfate complexes. Octahedra l coordination of vanadium atoms is found in the glassy samples at all comp ositions studied, in accordance with V-51 NMR spectra. Alkali cations are d istributed randomly within an anion network. At high vanadium concentration the structure of vanadium sites in the glasses is very similar to that fou nd in Cs-4(VO)(2)O(SO4)(4), whereas for small vanadium contents the vanadiu m sites are separated by additional sulfate ligands. Heating to the glass-t ransition temperature, T-g, and above, leads to jumps of the alkali cations between different sites. The mobility of pyrosulfate groups is accompanied by dissociation to SO42- and SO3. At the elevated temperature the mobility of SO3 molecules is sufficient to participate in chemical exchange with th e sulfate groups of the network. Addition/splitting mechanism involving SO3 has been proposed to be responsible for random fluctuations of the V-51 nu clear quadrupole tensor at given vanadium network sites with characteristic correlation time tau(c). For 10(-8) < tau(c) < 10(-6) s the V-51 NMR line became unobservable. For Cs-containing samples the increase of the temperat ure is accompanied by fast crystallization. In this case a cooperative moti on of the anion network, caused by bond breaking and bond formation, domina tes at temperatures around T-g. The NMR spectra of alkali metals were found to be very characteristic for the structure of the network formed in melts between V2O5 and M2S2O7. O-17, Na-23, K-39, (CS)-C-133 spectra recorded at 500 degrees C point to the formation of different species and rapid exchan ge between them. A change of the local vanadium environment in melts takes place at X(V2O5) similar to 0.1 and 0.3 most probably due to the formation of dimeric and polymeric V(V) complexes, possibly (VO)(2)O(SO4)(4-)(4) and (VO2SO4)(n-)(n). Correlation time of V-51 quadrupole tensor fluctuations fo r samples with X(V2O5) similar to 0.1-0.5 is higher than 10(-8) s, which ma kes V-51 NMR spectra unobservable in the region 400-500 degrees C, whereas for more dilute samples, tau(c) is determined mainly by the size of the van adium-sulfate species making V-51 spectra of these samples observable. The dependence of V-51 chemical shift on the vanadium concentration indicates a change of coordination number in the system M2S2O7-V2O5 from tetrahedral i n pure V2O5 to octahedral in dilute samples. The structure of supported cat alysts is very similar to the