Deciphering the Phillips Catalyst by Orbital Analysis and Supervised Machine Learning from Cr Pre-edge XANES of Molecular Libraries

Unveiling the nature and distribution of surface sites in heterogeneous catalysts, for Phillips catalyst (CrO3/SiO2) particular, is still a grand challenge despite more than 60 years research. Commonly used references Cr K-edge XANES spectral analysis rely on bulk materials (Cr-foil, Cr2O3) or molecules (CrCl3) that significantly differ from actual sites. In this work, we built library spectra series tailored molecular complexes, varying oxidation state, local coordination environment, ligand strength. Quantitative pre-edge region revealed origin shape intensity distribution. characteristic splitting observed Cr(III) Cr(IV) complexes directly related to electronic exchange interactions frontier orbitals (spin-up -down transitions). The experimental was extended by theoretical potential active site structures training Extra Trees machine learning algorithm. most informative features (descriptors) were selected prediction states, mean interatomic distances first sphere, type ligands. This set descriptors applied uncover at three different stages process. freshly calcined consists mainly Cr(VI) CO-exposed contains Cr(II) silicates with minor fraction exposed ethylene highly coordinated along unreduced