New perspectives on the nature and imaging of active site in small metallic particles: II. Electronic effects

Abstract

The science of heterogeneous catalysis is primarily based on surface phenomena, which occur on nanoscale surfaces on active sites with sub-Å dimensions. On these surfaces, only some atomic sites which have a unsaturated valence feature (under-coordinated or coordinately unsaturated) – typically located at steps, edges, kinks, corners, and other discontinuities, are discerned to be the active sites, i.e., active for surface processes, including binding and adsorption, surface reactions, and desorption of products. These sites have a profound effect on surface reactivity, on account of their “under-coordinated” feature, and a highly electronic character, with a high electron density in the valence band, evidenced by its location around the Fermi level. In part I of this Series, we offered new knowledge-based analysis of the geometric effects/properties of the nanoclusters and active sites, which are governed by a specific size (also termed as “quantum size,” at nm scale) and shape/morphology. Here, in part II, we elaborate on the electronic properties which are evident as a result of typical metallic to nonmetallic transition of small metallic particles at the quantum size of 2–5 nm. With several practical research examples/case studies from recent literature, we illustrate how the techniques of core-level XPS and its analogs, AP-XPS and NAP-XPS, and STM/STS can be utilized to investigate and discern the electronic structure of catalyst surfaces and interfaces, including electronic band gaps, onset of valence and conduction band transitions, assessment of cluster size effects on band gaps, topographical features with atomic-scale resolution, presence of defects/charge centers at the metal-support interface, and chemical imaging via chemical contrast. We also introduce and discuss new concepts such as “Brønsted–Evans–Polanyi (B.E.P.) relationships” and “volcano curves,” and illustrate how the information gathered from surface-sensitive characterization techniques and first-principles calculations can be used to design more effective and efficient catalysts with increased chemical selectivity. We will also illustrate the efficacy of two interrelated techniques, STEM-EELS and STXM-XAS, which can provide insights into the atomic-scale (sub-Å) features of these nanoclusters, and gather corroborative evidences for other studies. This information can provide novel insights into the scientific basis for first-principles design of new catalysts and nanostructured materials and adds to the current understanding about behavior of these nanomaterials under in-situ conditions.

Keywords:

• Active site
• confinement
• electron energy loss spectroscopy (EELS)
• electronic effects
• heterogeneous catalysis
• Kubo gap
• metal-to-insulator transition
• nanocluster
• quantum size
• scanning tunneling microscopy (STM)
• scanning tunneling spectroscopy (STS)
• tip-enhanced Raman spectroscopy (TERS)
• transmission electron microscopy (TEM)
• X-ray photoelectron spectroscopy (XPS)

Disclosure statement

No potential conflict of interest was reported by the author(s).