The definition of Lewis acid strength in the study of heterogeneous catalysts largely relies on the use of probe Lewis base molecules and spectroscopic methods that characterize the Lewis adduct. As a result, these Lewis adducts are not necessarily representative of the strength of Lewis acid sites as relevant for a catalyzed reaction of interest. In CO2-to-CH3OH reactions, Lewis acid sites that are sufficiently proximal to Cu nanoparticles stabilize surface intermediates and promote the formation of CH3OH. Catalysts composed of Cu nanoparticles dispersed on SiO2, which has been decorated with isolated group IV and group V metal centers (Ti, Zr, Hf, Nb, Ta), were synthesized using a surface organometallic chemistry approach. As a result of this synthetic strategy, these catalysts are nearly identical, with only the identity of the group IV or group V metal center differing. Enthalpies of adsorption for pyridine were used as a functional measure of Lewis acid strength of these dispersed d0 metal centers, and CH3OH formation rates increase as Lewis acid sites become stronger. Spectroscopic characterization indicates that formate and methoxy intermediates are stabilized by these acid sites. In fact, the 13C chemical shifts for methoxy intermediates in nuclear magnetic resonance spectroscopy experiments are sensitive to the strength of the Lewis acid sites of the catalyst, reflecting the energy of the empty d-orbital and its ability to accept lone pairs from the methoxy species. The same Lewis acid sites that stabilize these surface intermediates likely stabilize transition states to form CH3OH, resulting in the promotion of their formation rates. Crucially, the assessment of Lewis acid strength using the 13C chemical shift of a surface intermediate formed during reaction, an active complex analogous to a Lewis adduct, most precisely reflects the stabilization afforded by Lewis acid sites for CO2 hydrogenation to CH3OH.

Understanding Lewis acid strength effects in CO2 hydrogenation to CH3OH on Cu-based catalysts

ACS Spring Meeting 2021

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