Probing the Local Coordination Environment of Mononuclear Pd Ions Supported on Ceria Using CO Adsorption Infrared Vibrational Spectroscopy

The electronic structure and geometric configuration of oxide-supported metal ions are important coordination properties that can be related to catalytic activity and stability. Herein, we interrogate the coordination environment of mononuclear Pd ions supported on ceria using CO adsorption, infrared vibrational spectroscopy, and DFT modeling. We observed the 15 h continuous coevolution of a palladium- (2167 cm–1) and cerium-carbonyl (2177 cm–1) complex by monitoring the ν(CO) infrared region. The slow CO adsorption kinetics were caused by the reactive ligand exchange between an oxygen atom of the support and the CO adsorbate to yield an oxygen vacancy and adsorbed CO2. We hypothesize that the coevolved cerium-carbonyl complex was formed upon CO adsorption at or adjacent to this oxygen vacancy. Our hypothesis was experimentally supported by a dramatic attenuation of the cerium carbonyl signal upon preadsorption of water through an apparent competitive adsorption mechanism. The attenuation was also accompanied by a 6 cm–1 redshift of the palladium carbonyl band (2161 cm–1) attributed to hydrogen bonding between the carbonyl and a nearby hydroxyl. Characteristic ν(CO) stretch frequencies cataloged through CO adsorption onto single crystal ceria by Wöll et al. ( Phys. Rev. Lett., 2020, 125, 256101.) led us to index the cerium carbonyl to the {100} nanofacet of the polycrystalline ceria support. It follows from the observed coevolution of the two carbonyl complexes that Pd was also adsorbed at the {100} nanofacet. Redeployment of a previously developed DFT model by Ivanova-Shor et al. ( Materials, 2021, 14, 6888.) featuring square-planar coordination of Pd2+ at the {100} nanofacet (O4Pd) of a Ce21O42 nanoparticle model qualitatively reproduced several experimental observations.