(618d) Towards the Speciation and Reactivity of Facet-Controlled Vanadium Oxide Catalysts

Towards the Speciation and Reactivity of Facet-Controlled Vanadium
Oxide Catalysts

Nicholas R. Jaegers,^1,2 Lu Zhang,²
Berlin Sudduth,¹ Eric D. Walter,² Mark Engelhard,²
Libor Kovarik,² Mary Y. Hu,² Feng Gao,² Huamin
Wang,² Yong Wang,^1,2* and Jian Z. Hu^2*

¹The Gene and Linda Voiland School of Chemical
Engineering and Bioengineering, Washington State University, Pullman,
Washington 99163

²Pacific Northwest National Laboratory, Richland,
Washington 99354

*Yong.Wang@pnnl.gov, Jianzhi.Hu@pnnl.gov

Titania-supported vanadium oxide catalysts are widely utilized materials
for an extensive library of vital chemical transformations, such as the
oxidation of paraffins, olefins, alcohols, and sulfur dioxide and the selective
catalytic reduction (SCR) of NO_x. Due to their wide employment,
efforts to better understand their structure and function are expansive, which
help to better identify the atomic-level interactions between the catalyst and
reactants. Identifying potential methods to tune the surface properties in a way
that promotes catalytic performance. Herein, we explore the structure and
function of titania-supported vanadium oxide catalysts that possess differing
dominant facet exposure. Both plate-like and square bipyramidal materials with
a range of vanadium loadings are utilized to discriminate the impacts that the
anchoring surface has on catalytic structure and reactivity for the oxidation
of methanol. We employ a range of techniques to better understand these
materials including Raman spectroscopy, density functional theory (DFT), and
nuclear magnetic resonance (NMR).

The results of our characterization confirm the increase in
oligomerization upon enhanced loading on each support. ⁵¹V MAS NMR
provided the most specific information regarding the surface structure of
vanadium oxide on the titania surfaces, whereby more detailed speciation is
possible. NMR shows that our {001}-dominant surface promotes the formation of
larger polymeric vanadium domains compared to the commercial and {101}-dominant
samples. Changes in the observed methanol oxidation activity are attributed to
specific species present on the surface of these materials. Chemical-shift
calculations were performed with the Amsterdam Density Functional software and used
the Gauge Independent Atomic Orbital approach to verify the identity of the
observed NRM signals. Paramagnetic vanadium species are likewise given
consideration, and further characterization conducted at the end of the
reaction demonstrates the migration of vanadium upon exposure to reaction
conditions.

The potential reasons these materials exhibit such contrast in their
vanadium signatures is explored, where differences in the support hydroxyl
groups are directly observable by NMR, infrared, and thermogravimetric
analysis. Variances in the surface acidity can be considered as well. The
results provide a framework for exploring vanadium oxide to better understand
their speciation on supports with preferentially exposed facets. Understanding
the factors that contribute to the surface directing the structure of vanadium
is essential to a deep understanding of such an important catalytic system.

Acknowledgements

This work is supported by the US Department of
Energy, Offices of Basic Energy Sciences and used resources in the William R.
Wiley Environmental Molecular Sciences Laboratory at Pacific Northwest National
Laboratory.