Catalysis Today 78 (2003) 91104
Solid-state 51V NMR for characterization ofvanadium-containing systems
O.B. Lapina a,, A.A. Shubin a, D.F. Khabibulin a, V.V. Terskikh a,P.R. Bodart b, J.-P. Amoureux b
a Boreskov Institute of Catalysis, Prosp. Lavrentieva 5, 630090 Novosibirsk, Russiab Laboratoire de Dynamique et Structure des Matriaux Molculaire, Universite des Sciences et Technologies de Lille,
F-59655 Villeneuve dAscq, France
This overview paper includes both published and original data of the current state of the field of 51V NMR in solid-statechemistry. Advantages and shortcomings of different NMR techniques in their applications to vanadium are discussed onthe examples of their application to various vanadia based systems (including individual highly crystalline compounds, solidsolutions, glasses, catalysts). New correlations between local structure of vanadium atoms and NMR parameters allowingto discriminate at least seven different types of vanadium sites (tetrahedral sites of Q0, Q1 and Q2 types; trigonal pyramidsof 3 = 1 and 3 = 2 (V2O5 like) types; tetragonal pyramids of 4 = 1, 4 = 2 types) are proposed. It is demonstrated thatcompetent combination of different NMR approaches permits now not only to describe different vanadium sites in highlycrystalline and amorphous materials, but also to insight into the structural aspects of disorder in crystallinity as well as toreveal the behavior of different functional groups at elevated temperatures. The influence of low valence vanadium atoms on51V NMR spectra is also discussed. 2002 Published by Elsevier Science B.V.
Keywords: Vanadium; Solid-state nuclear magnetic resonance; 51V NMR; Ultra-high-speed MAS; MQMAS; SATRAS; STMAS; Individualcompounds; Solid solutions; Glasses; Catalysts
Among group V elements, vanadium is one of themost important element, widely used in solid-statechemistry, materials science, catalysis and engineer-ing. Nowadays 51V solid-state nuclear magneticresonance (NMR) spectroscopy became a keystonetechnique for characterization of local structure ofvanadium sites in different vanadium systems [1,2].Modern NMR techniques such as ultra-high-speedMAS (35 kHz and higher), MQMAS, SATRAS al-
Corresponding author.E-mail address: firstname.lastname@example.org (O.B. Lapina).
low to obtain direct and precise information on thelocal structure of vanadium sites: (i) the number ofnonequivalent vanadium sites, (ii) coordination num-bers, (iii) the nature of the atoms in the first coordi-nation sphere, (iv) the distortion of this coordinationsphere, (v) association of vanadiumoxygen poly-hedron. In addition, spin echo mapping spectra orultra-high-speed MAS experiments can highlight V5+atoms bounded via oxygen atom by V3+ or V4+; de-fects and distortions of the structure can be revealedby analysis of distributions of chemical shielding andquadrupolar tensor parameters. The main purpose ofthis work is to demonstrate current possibilities of51V NMR in solid-state chemistry, that is why there is
0920-5861/02/$ see front matter 2002 Published by Elsevier Science B.V.PII: S0 9 2 0 -5861 (02 )00299 -7
92 O.B. Lapina et al. / Catalysis Today 78 (2003) 91104
no deep historical excursus in this field, as well as nocomplete bibliography. At present the bibliography insolid-state 51V NMR applications counted hundredspapers, the review of them is for the future. In thispaper, we considering only some results obtained bySATRAS, high-speed MAS (35 kHz), MQMAS, andSTMAS, which demonstrate applicability of thesetechniques to various vanadia systems.
2. Modern solid-state 51V NMR technique.Polycrystalline V5+ oxide compounds
51V nucleus (natural abundance 99.76%) has a spinquantum number of 72 and an electric quadrupolemoment of 0.05 b, the relative intensity of 51V NMRsignal is 0.38 compared to an equal number of pro-tons. In presence of a magnetic field, each vanadiumnucleus of solid diamagnetic samples experiences, ingeneral, three different types of interaction: (i) dipoleinteraction of its magnetic moment with magneticmoments of other nuclei, (ii) quadrupole interactionof its electric quadrupole moment with the electricfield gradient, (iii) chemical shielding anisotropy(CSA) interaction. These interactions individuallybroaden and even shift (in the case of the quadrupo-lar interaction at second order) the observed lines, tosuch an extent that in static powdered solids, NMRhas for long been accepted as of moderate value,until magic angle spinning (MAS) technique was in-troduced. Indeed, the extensive applicability of NMRto solids relies heavily on MAS. This technique isable to narrow the lines by successful averagingdipolar, anisotropic chemical shielding and first orderquadrupolar effects. However, a residual line broaden-ing issued from the quadrupolar interaction at secondorder remains and it has been until 1995 the mainlimitation of MAS resolution. Nevertheless, in theparticular case of vanadium, the small value of elec-tric quadrupole moment moderates the quadrupolarinteraction and simple MAS technique, has proven tobe a very convenient technique for vanadium char-acterization. The SAtellite TRAnsition Spectroscopy(SATRAS) [3,4] method has rapidly taken advan-tage of the MAS high-resolution spectra availablefor vanadium-containing sample. This technique al-lows a simple determination of the isotropic chemicalshift (iso) and composite quadrupolar coupling con-
stant ( = CQ
1+ 2/3, CQ = e2qQ/h). Incombination with automatic analysis (refinement)of the intensities of well-resolved satellite spinningsidebands the complete set of quadrupolar and CSAtensor parameters as well as their relative orientationscan be measured. Thanks to SATRAS and spectralsimulation, precise NMR data have been obtained formost of the individual vanadium-oxide compounds ofthe system V2O5MxOy (M = mono-, di-, tri- andtetra-valent metals) .
Seven types of vanadium sites could be recog-nized based on the values of chemical shielding andquadrupolar tensors parameters obtained by SATRASmethod for the above mentioned highly crystallineindividual compounds with well known structures.
Tetrahedral vanadium sites of Q0, Q1 and Q2 typescould be revealed using correlation between the type(chemical shielding asymmetry parameter) andvalue (CSA) of chemical shielding anisotropy[1,2,11] (Fig. 1):
(i) Vanadium in regular tetrahedral oxygen environ-ment (Q0 type) has almost spherically symmet-ric chemical shielding tensor with small valueof anisotropy ( < 100 ppm); quadrupolarconstant CQ varies from 1 to 6 MHz; chemicalshielding asymmetry parameter varies from 0 upto 1 [3,4,10].
(ii) Vanadium in slightly distorted tetrahedral siteswith the adjacent tetrahedra sharing one com-mon oxygen atom (Q1 type) has an asymmetricchemical shielding tensor, but with larger value ofanisotropy (100 < < 200 ppm); quadrupo-lar constant varies from 2.5 to 10 MHz; chemicalshielding asymmetry parameter changes from 0.1to 0.9 [3,4,10].
(iii) Vanadium in strongly distorted tetrahedral siteswith adjacent tetrahedra sharing two commonoxygen atoms (Q2 type) has an asymmetricchemical shielding tensor with large value ofanisotropy (200 < < 500 ppm); quadrupo-lar constant varies from 2 to 7 MHz; chemicalshielding asymmetry parameters changes from0.6 to 0.8 [3,4,10].
Whereas it is clear that vanadium sites indifferent pyramid couldnt be determined fromFig. 1. These sites could be recognized using
O.B. Lapina et al. / Catalysis Today 78 (2003) 91104 93
Fig. 1. Correlation between 51V asymmetry ( ) and anisotropy parameters ( ) of chemical shielding tensor obtained for various vanadiacompounds.
the correlation between effective estimatedas ( 1/2(1 + 2), icomponents ofCS-tensor) and quadrupolar coupling constant(CQ) for the case of a large value of CSA(200 ppm
94 O.B. Lapina et al. / Catalysis Today 78 (2003) 91104
from 1 to 3 MHz; chemical shielding asymme-try parameters changes from 0 to 0.2, 200400 ppm .
(vi) Vanadium in trigonal pyramid of 3 = 1 type hasan axially symmetric chemical shielding tensorwith large value of anisotropy (200