(700c) Effects of d-Band Shape on the Surface Reactivity of Transition-Metal Alloys

Transition-metal alloys represent an important class of materials that are used in many commercial catalytic processes. Rational design of alloy catalysts with optimized performance and minimal cost relies on a fundamental understanding of the electronic structure of an ensemble of surface atoms and its relationship with energetics of adsorbate-surface interactions. For that matter, identification of simple reactivity descriptors from complex electronic properties of materials becomes tremendously important. The d-band model, developed in the 90s', is the standard model in surface chemistry for understanding variations in chemisorption energies of adsorbates from one metal surface to another. In general, a metal site with higher d-band center exhibits stronger affinity to adsorbates. This simple design principle has proven to be extremely useful in search of optimal catalytic materials in many chemical and electrochemical reactions. However, outliers exist in many technological relevant alloy systems, i.e., increasing (decreasing) the d-band center of a metal site is not always associated with stronger (weaker) chemical bonding, which indicates the need for better electronic descriptors.

In this talk, I will start with a brief introduction to some conceptual aspects of the theory of surface chemisorption, followed by the discussion of alloy systems where the d-band center failed to describe the trend of surface reactivity. In particular, I will emphasize the effects of the shape of the d-band of a metal site, represented by higher moments of the d-band, on energetics of surface chemical bonding. The upper d-band edge, defined as the highest peak position of the Hilbert transform of the density of states projected onto d-orbitals of an active metal site, is identified as an electronic descriptor for the surface reactivity of transition metals and their alloys, regardless of variations in the d-band shape. The utilization of the upper d-band edge with scaling relations enables a considerable reduction of the parameter space in search of improved alloy catalysts and further extends our understanding of the relationship between the electronic structure and chemical reactivity of metal surfaces.