Probing the Local\nCoordination Environment of\nMononuclear Pd Ions Supported\non Ceria Using CO Adsorption Infrared Vibrational Spectroscopy

The\nelectronic structure and geometric configuration\nof oxide-supported\nmetal ions are important coordination properties that can be related\nto catalytic activity and stability. Herein, we interrogate the coordination\nenvironment of mononuclear Pd ions supported on ceria using CO adsorption,\ninfrared vibrational spectroscopy, and DFT modeling. We observed the\n15 h continuous coevolution of a palladium- (2167 cm<sup>–1</sup>) and cerium-carbonyl (2177 cm<sup>–1</sup>) complex by monitoring\nthe ν(CO) infrared region. The slow CO adsorption kinetics were\ncaused by the reactive ligand exchange between an oxygen atom of the\nsupport and the CO adsorbate to yield an oxygen vacancy and adsorbed\nCO<sub>2</sub>. We hypothesize that the coevolved cerium-carbonyl\ncomplex was formed upon CO adsorption at or adjacent to this oxygen\nvacancy. Our hypothesis was experimentally supported by a dramatic\nattenuation of the cerium carbonyl signal upon preadsorption of water\nthrough an apparent competitive adsorption mechanism. The attenuation\nwas also accompanied by a 6 cm<sup>–1</sup> redshift of the\npalladium carbonyl band (2161 cm<sup>–1</sup>) attributed to\nhydrogen bonding between the carbonyl and a nearby hydroxyl. Characteristic\nν(CO) stretch frequencies cataloged through CO adsorption onto\nsingle crystal ceria by Wöll et al. (Phys. Rev. Lett., 2020, 125, 256101.) led us\nto index the cerium carbonyl to the {100} nanofacet of the polycrystalline\nceria support. It follows from the observed coevolution of the two\ncarbonyl complexes that Pd was also adsorbed at the {100} nanofacet.\nRedeployment of a previously developed DFT model by Ivanova-Shor et\nal. (Materials, 2021, 14, 6888.) featuring square-planar coordination of Pd<sup>2+</sup> at the\n{100} nanofacet (O<sub>4</sub>Pd) of a Ce<sub>21</sub>O<sub>42</sub> nanoparticle model qualitatively reproduced several experimental\nobservations.