(617e) Spin-Crossing in Heterogeneous Catalysis By Atomically Dispersed Transition Metals. an Example: Ethane Dehydrogenation By Co/SiO2

Supported atomically dispersed metals and sub-nanometer metal clusters have been garnering attention on account of the high activity and selectivity they impart to reactions. Of particular interest is the dehydrogenation (DH) of small alkanes to alkenes, as the latter are used in the synthesis of several commodity chemicals and the recent shale gas boom in the US has made dehydrogenation economically viable.

Common catalysts of the reaction are Pt-Sn alloys and alumina-supported chromium oxides, both of which have several shortcomings. The burgeoning activity in replacing supported Pt-group metal atom/sub-nanometer cluster catalysts with earth abundant metals is driven by the lower biological toxicity and cost of the latter. Experimental reports have demonstrated that highly dispersed Co(II) on am-SiO₂, both as single sites and sub-nanometer CoO_x clusters, exhibit high activity and selectivity for alkenes (>95%).¹

Here, we use DFT calculations and microkinetic modelling to explore the theoretical efficiencies in small alkane DH owed to spin-crossing kinetics exhibited by Co(II) sites of varying nuclearity. We develop reaction mechanisms and rank them in kinetic importance using micro-kinetic analysis for single-site and di-nuclear Co(II)/SiO₂ active sites. Microkinetic analysis shows that the reaction rate on the di-nuclear site is higher than that on the mononuclear site, which can be explained by the Ï€-donor strength of the ligands. We use insights from the mechanistic model to study the effect of am-SiO₂ heterogeneity on the reaction mechanism. To this end, we construct an ensemble of single- site models with varying degrees of distortion and employ electronic descriptors such as Fukui indices and NBO analysis to relate the active site coordination geometry to reaction energy barriers. Finally, we use ab-initio MD to understand Co’s local coordination geometry as a function of temperature.

• Estes, D. P. et al. Journal of the American Chemical Society 138, 14987-14997, (2016).