(555e) Computationally-Enhanced Spectroscopic Studies of Supported Vanadium Oxide

Computationally-Enhanced Spectroscopic Studies of Supported
Vanadium Oxide

Supported vanadium oxide materials are utilized as catalysts
for an expansive array of vital chemical transformations. Such reactions
include the oxidation of paraffins, olefins,
alcohols, and sulfur dioxide and the selective catalytic reduction (SCR) of NO_x.
The importance of these oxide catalysts has driven extensive efforts to
understand the nature of these catalysts at the atomic level; promoting a
greater understanding of the interactions between the catalyst and reactants.
Despite these efforts, many questions remain regarding the structural nature of
the vanadium centers, particularly with contrasting supporting materials and
under different vanadium environments. Previous investigations have suggested
the presence of VO₄ monomers at low surface coverages and
progressive oligomerization up to the monolayer limit on most oxide supports.
Less decisive evidence is available for silica-supported catalysts due to their
low surface reactivity and high surface area. Further ambiguity exists when considering
the environmental effects on the catalyst’s structure. Hydration has been shown
to alter the surface structure where contrasting claims of decavandate
and vandia gel formation exist. In this study,
solid-state nuclear magnetic resonance (NMR) spectroscopy and quantum chemistry
simulations of the NMR spectra are used to aid in understanding the nature of V₂O₅/SiO₂
and SCR V₂O₅(-WO₃)/TiO₂ catalysts
exposed to different environments (dry, hydrated, and ammonia/NO).

To meet the
challenge of collecting the ⁵¹V MAS NMR spectra, both a high-field
(19.98 T) Varian-Inova spectrometer and a 14.09 T Bruker
spectrometer were used. A sample spinning rate of 35 kHz was possible in
pencil-type 2.5 mm rotors. Chemical-shift calculations were performed with the
Amsterdam Density Functional software using the Gauge Independent Atomic
Orbital approach. Chemical shift calculations were verified by comparison to
known reference compounds. V₂O₅/SiO₂ catalysts
with coverages between 0.22θ
and 0.63θ were probed with
NMR to assess the potential for oligomerization upon increased loading.
Vanadium oxide catalysts were also studied under both dehydrated and ambient
environments to elucidate the structural changes that occur upon exposure to
water. Dehydrated V₂O₅/SiO₂ catalysts exhibited
one broad feature at -675 ppm while the hydrated form offered peaks at -566 ppm
and -610 ppm. Computational modeling of the ⁵¹V NMR chemical shifts
indicated that anchoring of vanadium oxide does not occur on three-ringed
silica and requires larger silica anchoring clusters such as 6-ringed support
anchoring positions. The results also show that, unlike titania-supported
vanadium oxide, the chemical shifts for monomeric and dimeric vanadium species
are similar. Cyclic trimer species may also resonate near the observed chemical
shift, but linear trimers, tetramers, and V₂O₅
crystallites are unlikely candidates. Hydrated structures were considered on
the basis of water addition and previously-proposed olation
mechanisms. Both di-grafted, isolated VO₄ units and di-grafted dimer
species demonstrated reasonable agreement between the calculated NMR parameters
and the main peak at -566 ppm observed in the spectra.

The results
suggest the possible presence of monomers, dimers, and cyclic trimers on the
dehydrated V₂O₅/SiO₂ samples. Upon exposure to
ambient moisture, the V-O-Support bridge bonds partially hydrolyze to form
di-grated species and a bulk-like species. No support for decavanadate
or vanadia gel formation is available from this
study. These results are compared with those obtained from SCR V₂O₅/TiO₂
catalysts where different surface vanadium species are clearly observed
by ⁵¹V MAS NMR. Understanding the structure of these materials and
how they respond to changes in chemical environment is essential for proposing
chemical reaction mechanisms and promoting the rational design of new
materials.