Abstract
Over the past decade, single-atom alloys (SAAs) have been a lively topic of research due to their potential for achieving novel catalytic properties and circumventing some known limitations of heterogeneous catalysts, such as scaling relationships. In researching SAAs, it is important to recognize experimental evidence of peculiarities in their electronic structure. When an isolated atom is embedded in a matrix of foreign atoms, it exhibits spectroscopic signatures that reflect its surrounding chemical environment. In the present work, using photoemission spectroscopy and computational chemistry, we discuss the experimental evidence from Ag0.98Pd0.02 SAAs that show free-atom-like characteristics in their electronic structure. In particular, the broad Pd4d valence band states of the bulk Pd metal become a narrow band in the alloy. The measured photoemission spectra were compared with the calculated photoemission signal of a free Pd atom in the gas phase with very good agreement, suggesting that the Pd4d states in the alloy exhibit very weak hybridization with their surroundings and are therefore electronically isolated. Since AgPd alloys are known for their superior performance in the industrially relevant semi-hydrogenation of acetylene, we considered whether it is worthwhile to drive the dilution of Pd in the inert Ag host to the single-atom level. We conclude that although site-isolation provides beneficial electronic structure changes to the Pd centers due to the difficulty in activating H2 on Ag, utilizing such SAAs in acetylene semi-hydrogenation would require either a higher Pd concentration to bring isolated sites sufficiently close together or an H2-activating support.
Highlights
In the present work, using photoemission spectroscopy and computational chemistry, we discuss the experimental evidence from Ag0.98Pd0.02 SAAs that show free-atom-like characteristics in their electronic structure
We demonstrate the spectroscopic characteristics of an Ag0.98Pd0.02 SAA and discuss how they are a consequence of its electronic structure and coordination environment by comparing with the computed photoemission signal (PES) of a free Pd atom
AgPd SAAs were computed by substituting a single Ag atom with a Pd atom in (2 × 2 × 2) and (3 × 3 × 3) crystallographic supercells of face-centered-cubic Ag, and relaxing all atom positions and lattice vectors until forces dropped below 10−3 a.u., the cell-pressure dropped below 0.5 kbar, and the change in total energy was below 10−4 Ry at the scalar relativistic level
Summary
Single-atom alloys (SAAs) have gained considerable attention in heterogeneous catalysis research,[1] especially in selective hydrogenation reactions.[2,3,4,5,6,7,8,9] Usually, in a SAA, a dilute active metal of group (Ni, Pd, Pt) is substituted in a noble metal host of group (Cu, Ag, Au).[1,2,3,4,5,6,7,8,9,10,11] By this approach, only small amounts of the expensive active metal are used.[10]. Scitation.org/journal/jcp the semi-hydrogenation of acetylene can be referred to their electronic structure, and the question arises how the catalytic properties of AgPd change when the dilution is pushed to the Pd site-isolation limit in a SAA. In some SAAs, the solute exhibits a very weak interaction with the matrix element, resulting in an electronically isolated metal site, where the solute’s valence states resemble a free-atom state. This behavior can be seen in the d-band of the solute. We discuss whether the geometric and electronic site-isolation of Pd in the Ag0.98Pd0.02 model catalyst can manifest themselves in the catalytic behavior in the semi-hydrogenation of acetylene This discussion contributes to an improved understanding of the nature of active sites in SAAs and single-atom catalysts
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