Theoretical Investigation of Small Transition-Metal Clusters Supported on the CeO₂(111) Surface

We have performed a systematic investigation of 4-atom transition-metal (TM) clusters (TM = Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au) supported on the unreduced CeO₂(111) surface using density functional theory calculations within the Perdew–Burke–Ernzerhof functional and on-site Coulomb interactions for the Ce <i>f</i>-states. We found two structure TM₄ patterns on CeO₂(111), namely, two-dimensional (2D) arrays with zigzag orientation for Ru, Rh, Os, and Ir and tetrahedral three-dimensional (3D) configurations for Cu, Pd, Ag, Pt, and Au. Our analyses indicate that the occupation of the antibonding <i>d</i>-states and the hybridization of the TM <i>d</i>-states with O <i>p</i>-states play a crucial role in the magnitude of the TM–TM and TM–O interactions and determine the formation of the 2D and 3D configurations on CeO₂(111). The interaction of TM₄ with the CeO₂(111) surface changes the nature of the occupied Ce <i>f</i>-states from itinerant (Ce^IV in the clean surface) to localized (Ce^III) states; hence, it increases the atomic size of Ce^III compared with Ce^IV by 4.4%, which plays a crucial role in building in a lateral tensile strain in the topmost Ce layer in the surface. Furthermore, we found an enhancement of the electron localization of the TM <i>d</i>-states upon the adsorption of TM₄ on CeO₂(111). We found that the number of Ce atoms in the Ce^III oxidation state depends on the TM element and structure. For Ru, Rh, Os, and Ir on CeO₂(111), all the Ce atoms in the topmost Ce layer change the oxidation state from IV to III (i.e., 100%), while for (Pd, Pt) and (Cu, Ag, Au) on CeO₂(111), only 25% and 50% of Ce atoms, respectively, convert the oxidation state from IV to III

Ab Initio Investigation of the Role of Atomic Radius in the Structural Formation of Pt_<i>n</i>TM_{55–<i>n</i>} (TM = Y, Zr, Nb, Mo, and Tc) Nanoclusters

Platinum-based nanoclusters have been widely studied because of the possibility to tune the physical and chemical properties as a function of shape, size, chemical composition, and so forth. Although the Pt composition can be experimentally controllable, the location of the Pt species is a challenge as several physical parameters might play a role, for example, surface energy, segregation energy, atomic radius, charge transfer, strain, and so forth. Here, we report density functional theory calculations for 55-atom Pt_<i>n</i>TM_{55–<i>n</i>} (TM = Y, Zr, Nb, Mo, and Tc) nanoalloys, which provides new insights. In general, the replacement of TM by Pt atoms increases the relative stability of the nanoalloys, and the maximum stability is reached at Pt-rich compositions (<i>n</i> = 35–42). From our analysis, an increase in the number of heterobonds maximizes the charge transfer among the Pt–TM species, and its magnitude depends on the electronegativity difference, coordination, and location (core or surface) of both species. For most cases, there is an electron density flow from the core to the surface region, and hence, the core is cationic, whereas the surface is anionic; however, there are few exceptions, in particular, for PtY. Thus, it yields an attractive Coulomb interaction among the chemical species and minimizes the total energy. For PtTM in which Pt is slightly larger or has a similar size as the TM atoms (Tc, Mo, and Nb), Pt atoms prefer the surface sites (smaller species are located on the core), which helps to release the strain energy. However, the charge transfer from TM to Pt helps to increase the attractive interaction between the surface and core, increasing the pressure on the core region. For PtTM in which TM (Zr and Y) is larger than Pt, contrary to what would be expected, there are some Pt atoms in the core region (including at the geometric center), resulting in a cationic surface. The release of the strain energy is obtained by symmetry breaking

Theoretical Investigation of the Adsorption Properties of CO, NO, and OH on Monometallic and Bimetallic 13-Atom Clusters: The Example of Cu₁₃, Pt₇Cu₆, and Pt₁₃

We report a density functional theory investigation of the adsorption properties of CO, NO, and OH on the Cu₁₃, Pt₇Cu₆, and Pt₁₃ clusters in the cationic, neutral, and anionic states with the aim to improve our atomistic understanding of the adsorption properties on bimetallic clusters compared with monometallic clusters. The adsorption energy of CO and NO are substantially stronger on Pt₁₃ than on Cu₁₃, and hence, CO and NO bind preferentially on Pt sites on Pt₇Cu₆. Thus, it can contribute to drive the migration of the Pt atoms from the core to the surface region in large PtCu nanoalloys. The CO and NO adsorption energies on the bimetallic cluster are enhanced by a few percent compared with the energies of the monometallic clusters, which shows that the Pt–Cu interaction can contribute to an increase in the adsorption energy. In contrast with CO and NO trends, the OH adsorption energies on Cu₁₃, Pt₇Cu₆, and Pt₁₃ deviates only up to 0.31 eV, and hence, there is no clear preference for Cu or Pt sites on Pt₇Cu₆ or an enhancement of the adsorption energy on the bimetallic systems. We found a reduction of the CO and NO vibrational frequencies upon adsorption, which indicates a weakening of the CO and NO binding energies, and it is supported by a slight increase in the bond lengths. However, the OH vibrational frequency increases upon adsorption, which indicates an enhancement of the OH binding energy, which is supported by a slight decrease in the bond length by about 0.01 Å. It can be explained by the large charge transfer from the clusters to the O atom, which enhances the electrostatic interaction in the O–H bonding

The Role of Charge States in the Atomic Structure of Cu_<i>n</i> and Pt_<i>n</i> (<i>n</i> = 2–14 atoms) Clusters: A DFT Investigation

In general, because of the high computational demand, most theoretical studies addressing cationic and anionic clusters assume structural relaxation from the ground state neutral geometries. Such approach has its limits as some clusters could undergo a drastic structural deformation upon gaining or losing one electron. By engaging symmetry-unrestricted density functional calculations with an extensive search among various structures for each size and state of charge, we addressed the investigation of the technologically relevant Cu_<i>n</i> and Pt_<i>n</i> clusters for <i>n</i> = 2–14 atoms in the cationic, neutral, and anionic states to analyze the behavior of the structural, electronic, and energetic properties as a function of size and charge state. Moreover, we considered potentially high-energy isomers allowing foresight comparison with experimental results. Considering fixed cluster sizes, we found that distinct charge states lead to different structural geometries, revealing a clear tendency of decreasing average coordination as the electron density is increased. This behavior prompts significant changes in all considered properties, namely, energy gaps between occupied and unoccupied states, magnetic moment, detachment energy, ionization potential, center of gravity and “bandwidth” of occupied d-states, stability function, binding energy, electric dipole moment and sd hybridization. Furthermore, we identified a strong correlation between magic Pt clusters with peaks in sd hybridization index, allowing us to conclude that sd hybridization is one of the mechanisms for stabilization for Pt_<i>n</i> clusters. Our results form a well-established basis upon which a deeper understanding of the stability and reactivity of metal clusters can be built, as well as the possibility to tune and exploit cluster properties as a function of size and charge