Single-Atom Alloys
A single-atom alloy is a dilute alloy in which individual atoms of a reactive metal are dispersed in the surface of a more inert host metal, so that every minority atom is surrounded only by host atoms. Such isolated sites can carry out a step the host cannot (for example, dissociating a molecule), while the host determines what happens to the products. They are a limiting case of bimetallic surfaces, where ensemble effects are pushed to the single-atom limit.
Isolated Pd atoms in Cu(111) and Au(111) (2009)
Tierney, Baber, Kitchin and Sykes made isolated Pd atoms in Cu(111) and Au(111) by depositing 0.01 monolayer of Pd, and imaged them with low-temperature STM; DFT showed that Pd alloys exothermically into both hosts and prefers to be surrounded by host atoms at low Pd fraction. The same Pd loading dissociated H2 on Cu but not on Au. DFT traced the difference to the host: H bound next to Pd is stable on Cu sites but not on Au sites, and the calculated H2 dissociation barrier over a Pd atom in Cu(111) was only about 0.02 eV, versus about 0.5 eV on Cu(111). They concluded that the host metal affects reactivity in a way that goes beyond dilution, electronic or geometric effects, and proposed computational surveys of host and dopant combinations to design such bifunctional catalysts (Tierney et al. 2009). Summary: tierney2009. See also Hydrogen spillover.
Electronic structure and probe reactions of Pd single atom alloys (2018)
Thirumalai and Kitchin used DFT to screen Cu, Ag and Au surfaces doped with single Pt, Pd, Ir, Rh or Ni atoms. In Au and Ag hosts the dopant’s d-band density of states shows a sharp, atom-like peak near the Fermi level, which they propose as the origin of the high reactivity; in Cu hosts this peak is absent, attributed to compressive strain. For water dissociation, NO dissociation and acetylene hydrogenation, AuPd and AgPd bound intermediates more strongly than their hosts (about 0.5 eV per step for water dissociation), while CuPd was less favorable than pure Cu. In acetylene hydrogenation the alloy pathway after vinyl formation tracked that of Pd(111), and ethylene desorbed with a negligible barrier, which the authors link to selectivity (Thirumalai 2018). Summary: thirumalai2018.
Stability of AgPd single-atom sites under acrolein (2021)
Dilute AgPd is a candidate single-atom alloy for selective C=O hydrogenation of acrolein, but Pd ensembles reduce selectivity. Using machine learning surrogates and semi-grand canonical Monte Carlo at 373 K, Liu, Yang and Kitchin found that adsorbed acrolein draws isolated Pd atoms to an otherwise Ag-terminated (111) surface and, from about 6.5% bulk Pd, induces Pd dimers, so the single-atom character is lost at fairly low Pd loadings under adsorbate cover (Liu et al. 2021). Summary: liu2021.