(684d) DFT Study of Trends in Reactivity at Bifunctional Interfaces: A Case Study of Water Gas Shift on Doped Au/MgO

Metal nanoparticles supported on oxides are ubiquitous in their usage as heterogeneous catalysts. Such materials often have bifunctional properties whereby the interface in the supported system allows for higher turnovers than either the oxide or the metal alone. Catalysis using supported Au nanoparticle is an example where this effect is often observed [1]. While bifunctional effects have been shown for specific cases both theoretically and experimentally [1-4], a systematic theoretical design paradigm for understanding trends in reactivity due to changes in electronic properties at bifunctional interfaces is lacking. In this work, we used Density Functional Theory (DFT) to study model Au nanowire supported on MgO, wherein the electronic properties at the Au/MgO interface was tuned through substitutional doping of MgO. The effect of dopants on reactivity at the perimeter sites of the supported nanowire was studied using Water Gas Shift (WGS) as a probe reaction, which is known to depend on support properties [1]. Using extensive DFT calculations, it was found that scaling relations for atomic as well as bifunctionally bound species exist at the doped Au/MgO interface, with slopes different from those identified for extended surfaces [5]. The scaling properties originate from electrostatic interactions between adsorbate and redistributed charges at the interface of doped Au/MgO. In addition, Bronsted-Evans-Polanyi relations were also identified for reaction steps that involved both Au and MgO sites. These trends were then combined with a dual site microkinetic model to develop a volcano plot, relating WGS turnover rates with descriptor for adsorption energies at the interface. Our results show that doping the oxide can change turnover rates at the interface even when the reactive intermediates are not in direct contact with the dopant. In addition, there exists an optimum descriptor value at which the WGS rate of the doped Au/MgO is maximized, with the volcano behavior likely arising from different rate limiting steps at different ranges of descriptor values. These results lay the groundwork for constructing a systematic design framework for bifunctional interfaces, and suggest a possible means to enhance activity of Au/oxide catalysts through doping, if they can be experimentally synthesized.

References

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