(253c) Radical Mechanism for Low Temperature C-O Bond Activation on Metal/Metal Oxide Catalysts

Selective C-O bond scission is a key step for a variety of industrial processes ranging from Fischer-Tropsch synthesis to valorization of biomass. Recently discovered multifunctional metal/metal oxide catalysts (e.g., Rh/MoO_x, Ir/VO_x) exhibit unusual activity and selectivity for C-O bond hydrogenolysis at temperatures < 200^oC. Here, we combine density functional theory calculations, electronic structure analysis and microkinetic modeling to develop comprehensive insights.

We describe a highly efficient mechanism of carbon-oxygen bond activation on metal/metal oxide multifunctional catalysts, which potentially reconciles the contradiction between low C-O bond scission activities, exposed by models and surface science experiments, and high experimentally observed reaction rates. We employ catalytic transfer hydrogenolysis chemistry of furfural to 2-methyl furan on a Ru/RuO₂ catalyst as an illustrative example due to its importance in the production of renewable fuels and chemicals. Unexpectedly, we discover an oxygen vacancy-mediated reverse Mars-van Krevelen-type radical mechanism that explains the high catalyst activity. Radical mechanisms are ubiquitous in various catalytic high temperature oxidations of, e.g. methane or propylene; yet, to our knowledge, this is the first study introducing the radical reduction mechanism in heterogeneous catalysis at low temperatures in the liquid phase. We describe electronic properties of both the catalyst and the key reaction intermediate that contribute to facile chemistry and overcome energy scalings. Finally, we find the mechanistic picture to be fully consistent with isotopic labeling experiments and kinetic studies. Given the importance of heterogeneous catalysis for tailored bond-breaking in energy production and storage, we believe that our discovered mechanism provides an important step toward development of low temperature processes catalyzed by surfaces.

The key findings are the following:

1. For the first time, we establish a consistent mechanistic picture of a vacancy formation mechanism on RuO₂ from first principles microkinetic modeling that explains all reactivity trends observed in surface science experiments.
2. We show that RuO₂ vacancies are by far the most reactive sites for activation of the C-O bond and the only sites that are consistent with low temperature activity of the catalysts.
3. Unexpectedly, we discover that the C-O bond scission on a vacancy proceeds via a furfuryl radical intermediate, allowing us to classify it as a reverse Mars-van Krevelen-type radical reduction mechanism, which has not been reported previously.
4. We elucidate a single reactivity factor that underlines the radical reduction mechanism and its generality to other catalytic systems. Unlike on metals, where the reaction barrier is lowered due to binding of the final products to the surface, here the low barrier is a result of the enhanced stability of the weakly bound radical due to conjugation.