(49a) Identifying Optimal Active Sites for Heterogeneous Catalysis By Metal Alloys Based On Molecular Descriptors and Electronic Structure Engineering

While one metal might be chemically inert for a particular catalytic transformation, its first neighbor in the periodic table might be overly reactive and equally inefficient. The main reason of this is that neighboring metals in the periodic table exhibit very large differences in binding energies of various adsorbates (as large as ~1 eV (~100 kJ/mol)). Simply put, the periodic table does not provide enough flexibility in identifying optimal catalysts. One avenue to address this problem is through fine-tuning the chemical reactivity of surface atoms in metals by changing their local coordination, for example by creating metal alloys. The phase space of alloys is immense, and it is almost impossible to screen through even a small fraction of potential alloys using experimental studies or even quantum-chemical calculations on model systems.

In this talk we will discuss our work on developing predictive models of chemical reactivity for alloys based on detailed understanding of the molecular transformations that govern catalytic processes.  More specifically, we analyzed how the change in the local coordination of metal surface atoms due to alloying changes the local chemical reactivity of the metal site. The central finding of our studies is that it is possible to reliably predict the change in the local electronic structure of an active site induced by the formation of an alloy, and the change in the local chemical reactivity, based on very simple analytical models which employ only easily accessible physical characteristics of the elements that form the alloy (mainly their electronegativity, distance between neighboring atoms, and the spatial extent of metal orbitals). We show how these predictive structure-reactivity relationships can be employed to rapidly screen through large libraries of alloy compositions and structures to identify optimal alloy catalysts. We describe our findings by focusing on an example of the design of optimal Pt alloy electrocatalysts for the oxygen reduction reaction (ORR) in fuel cell cathodes. The model permits rapid screening through an enormous phase space of alloy structures and compositions using analytical expressions instead of expensive quantum-chemical calculations. Since the model is grounded on validated theories of chemisorption on metal surfaces, it can be used for the identification and targeted manipulation of the critical electronic structure descriptors of catalytic activity and propose how these features can be obtained based on electronic structure engineering.

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4. H. Xin, A. Holewinski, S. Linic, ACS Catal., 2, 12, (2012)
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6. A. Holewinski, H. Xin, E. Nikolla and S. Linic, Curr Opin Chem Eng., accepted (2013)