Pd-Ga model SCALMS: Characterization and stability of Pd single atom sites

We have prepared model systems for Pd-Ga supported catalytically active liquid metal solutions (SCALMS) by physical vapor deposition of Pd and Ga in ultrahigh vacuum onto highly oriented pyrolytic graphite (HOPG) pre-modified by Ai(+) bombardment. Combining AFM and IRAS with CO as a probe molecule, we investigate the growth behavior, the morphology, the surface chemistry, and the stability of Pd-Ga alloys on HOPG. Our results prove the single atom character of the active sites in Ga-rich Pd-Ga alloys. We show that upon interaction with CO, Ga is displaced from the surface and CO adsorbs in on-top position on surface Pd atoms. Experimental data on the vibrational frequencies and binding energies of CO are compared to the results of DFT calculations for CO adsorption on four different Pd-Ga models, i.e. PdGa(1 00), PdGa (1 1 1), Pd-doped alpha-Ga(0 0 1), and Pd-doped fcc-Ga(1 0 0). The comparison suggests that widely separated Pd sites are formed in the Ga matrix. (C) 2018 Elsevier Inc. All rights reserved.