Integrated Experimental and Computational K‑Edge X‑ray Absorption Near-Edge Structure Analysis of Vanadium Catalysts

X-ray absorption near-edge structure (XANES) spectroscopy
is a
powerful tool to reveal key structural and electronic features of
isolated catalytic sites, yet insights into the molecular structure
and more detailed orbital analysis through a combination of experimental
and computed XANES analysis are necessary for accurate interpretation
of the spectra, especially when significant heterogeneity exists among
the catalytic sites. Herein, we present an integrated computational
and experimental strategy to determine both primary and secondary
bonding interactions within the XANES pre-edge region for organovanadium
complexes, which was developed using a series of well-defined molecular
vanadium complexes and then applied to the characterization of a supported
organovanadium olefin hydrogenation catalyst. Time-dependent density
functional theory is used to predict the energy of pre-edge XANES
features for a series of vanadium complexes with a variety of oxidation
states and local coordination environments. A calibration scheme incorporating
different density functionals and basis sets is established, resulting
in an optimized scheme that accurately predicts pre-edge energies
with a mean absolute error of 0.40 eV. Second-shell coordination (e.g.,
V···V) effects within XANES are identified through
the analysis of the computed dominant orbital contributions for multi-vanadium
complexes. Orbital analysis also provided confirmation that the vanadium-hydride
formation combined with the heterogeneity of the catalytic active
species in olefin hydrogenation caused the energy shift and broadening
of the pre-edge peak after hydrogen treatment of the silica-supported
organovanadium pre-catalyst. This work further elucidates computational
XANES simulations and techniques potentially guiding characterization
in surface organometallic chemistry.