Isolated Pd atoms in a silver matrix: Spectroscopic and chemical properties

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.

Liquid-Phase Hydrogenation of 1-Phenyl-1-propyne on the Pd1Ag3/Al2O3 Single-Atom Alloy Catalyst: Kinetic Modeling and the Reaction Mechanism

“…However, the mechanism proposed for Pd-Cu SAA is not valid for Pd-Ag and Pd-Au SAA catalysts. According to the current views spillover of hydrogen atoms over the surface of Pd-Ag SAAs is thermodynamically and kinetically unfavorable due to a low binding energy of Ag-H [33][34][35]. Similar conclusions were proposed for AuPd catalytic systems [36,37].…”

“…Unlike Pd-Cu SAA catalyst for Pd-Ag an alternative hydrogenation pathway was proposed, because adsorption of both alkyne or alkene on Ag surface is unfavorable. Therefore, the majority of authors suggested that H 2 and alkyne/alkene adsorption occurs on isolated Pd1 sites located preferably close to each other [20,35]. For this mechanism, one can expect competitive adsorption of H 2 and the alkyne, which inevitably affects kinetics of the entire process.…”

“…Taking into account that the Ag surface is not capable to adsorb and activate alkynes [39,40] most authors agree that both hydrogen and alkyne adsorption occurs on isolated palladium centers. Moreover, investigation of the surface structure and H 2 dissociation on PdAg single-atom alloy demonstrated that for successful partial hydrogenation of acetylene it is necessary that isolated Pd sites, on which the adsorption of hydrogen and alkyne molecules takes place, should be close enough to each other [35].…”

Understanding Single-Atom Catalysis in View of Theory

“…According to the experimentally measured valence photoemission spectra, the d state of isolated Cu atoms, which are embedded into a silver host at a very low concentration, shows a sharp peak close to the Fermi level, exhibiting a free-atom-like feature 115 , 116 as shown in Figure 7 c. It was experimentally verified that the dilute Pd dopant in the Ag host also shows similar free-atom-like peaks. 117 Kitchin et al revealed that the ineffective mixing of the electron densities between the host and the dopant atoms as well as an effective tensile strain is the reason for the emergence of such free-atom-like state. 118 It is expected that such the unusual state could play a special role in catalytic processes.…”

“…It was experimentally verified that the dilute Pd dopant in the Ag host also shows similar free-atom-like peaks. 117 Kitchin et al revealed that the ineffective mixing of the electron densities between the host and the dopant atoms as well as an effective tensile strain is the reason for the emergence of such free-atomlike state. 118 It is expected that such the unusual state could play a special role in catalytic processes.…”