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Abstract

Identifying catalytically active structures or intermediates in homogeneous and heterogeneous catalysis is a formidable challenge. However, to obtain experimentally verified insight into the active species in heterogeneous catalysis is a tremendously challenging problem. Many highly advanced spectroscopic and microscopic methods have been developed to probe surfaces. However, developing the full information content of the wealth of experimental information that is available through these methods has been proven to be ambitious. At least three key issues must be addressed: a) sample heterogeneity, b) interpretation of complex spectroscopic patterns in terms of geometric and electronic structure and c) cross-correlation between different experimental methods. All three challenges must be addressed simultaneously through careful experiments. Key insights can be obtained by combining rate measurements with spectroscopic measurements. Such in-situ experiments require dedicated experimental setups and frequently will also require to simplify the catalytic system as much as possible in order to render a coherent interpretation of the data conceivable. This implies the necessity for a more immediate connection between theory and experiment. It is the aim of this work to emphasize that strong correlation between theory and experiment can be uniquely established by combining a range of spectroscopic methods with the results of carefully calibrated theoretical spectroscopy. In this account we employ a combination of spectroscopic methods to study two closely related systems from the heterogeneous (the silica-supported vanadium oxide VOx/SBA-15) and homogeneous (the complex K[VO(O₂)Hheida]) domains. Spectroscopic measurements were conducted strictly parallel for both systems and consisted of oxygen K-edge and vanadium L-edge X-ray absorption measurements in conjunction to resonance Raman spectroscopy. It is shown that the full information content of the spectra can be developed through advanced quantum chemical calculations that directly address the sought after structure-spectra relationships. To this end we employ the recently developed restricted open shell configuration interaction theory together with the time-dependent theory of electronic spectroscopy to calculate XAS and rR spectra respectively. The results of the study demonstrate that: a) a combination of several spectroscopic techniques is of paramount importance in identifying signature structural motifs and b) quantum chemistry is an extremely powerful guide in cross connecting theory and experiment as well as the homogeneous and heterogeneous catalysis fields. It is emphasized that the calculation of spectroscopic observables provides an excellent way for the critical experimental validation of the theoretical results.