Реферат: Multinuclear 23Na, 39K, 133Cs,17O, 51V magnetic resonance studies of the M2S2O7-V2O5 (M=Na, K, Cs) systems in the temperature range 20–650°C have been performed for vanadium oxide mole fractions, X(V2O5), in the range 0–0.5. At ambient temperature the melt-quenched glassy samples exhibit a three-dimensional network of vanadium oxosulfate complexes. Octahedral coordination of vanadium atoms is found in the glassy samples at all compositions studied, in accordance with 51V NMR spectra. Alkali cations are distributed randomly within an anion network. At high vanadium concentration the structure of vanadium sites in the glasses is very similar to that found in Cs4(VO)2O(SO4)4, whereas for small vanadium contents the vanadium sites are separated by additional sulfate ligands. Heating to the glass-transition temperature, Tg, 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 the sulfate groups of the network. Addition/splitting mechanism involving SO3 has been proposed to be responsible for random fluctuations of the 51V nuclear quadrupole tensor at given vanadium network sites with characteristic correlation time τc. For 10−8<τc<10−6 s the 51V NMR line became unobservable. For Cs-containing samples the increase of the temperature is accompanied by fast crystallization. In this case a cooperative motion of the anion network, caused by bond breaking and bond formation, dominates at temperatures around Tg. 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. 17O, 23Na, 39K, 133Cs spectra recorded at 500°C point to the formation of different species and rapid exchange between them. A change of the local vanadium environment in melts takes place at X(V2O5)∼0.1 and 0.3 most probably due to the formation of dimeric and polymeric V(V) complexes, possibly (VO)2O(SO4)44− and (VO2SO4)nn−. Correlation time of 51V quadrupole tensor fluctuations for samples with X(V2O5)∼0.1–0.5 is higher than 10−8 s, which makes 51V NMR spectra unobservable in the region 400–500°C, whereas for more dilute samples, τc is determined mainly by the size of the vanadium-sulfate species making 51V spectra of these samples observable. The dependence of 51V chemical shift on the vanadium concentration indicates a change of coordination number in the system M2S2O7-V2O5 from tetrahedral in pure V2O5 to octahedral in dilute samples. The structure of supported catalysts is very similar to the structure of bulk melts (M2S2O7-V2O5), the main difference revealed is lower mobility of all structural units (such as metal cations, SO42− and SO3) for the supported melts.

Библиографическая ссылка: Lapina O.B. , Terskikh V.V. , Shubin A.A. , Eriksen K.M. , Fehrmann R.
High-Temperature Multinuclear Magnetic Resonance Studies of Vanadia Catalysts for SO2 Oxidation
Colloids and Surfaces A: Physicochemical and Engineering Aspects. 1999. V.158. N1-2. P.255-271. DOI: 10.1016/S0927-7757(99)00153-3 WOS Scopus РИНЦ CAPlusCA OpenAlex

Даты:

Опубликована в печати:	1 нояб. 1999 г.
Опубликована online:	15 февр. 2000 г.

Идентификаторы БД:

Web of science:	WOS:000083776500033
Scopus:	2-s2.0-0032732012
РИНЦ:	13308147
Chemical Abstracts:	1999:798754
Chemical Abstracts (print):	132:98680
OpenAlex:	W1995309784

Цитирование в БД:

БД	Цитирований
Web of science	9
Scopus	8
РИНЦ	8
OpenAlex	9

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