Quantifying interactions on interfaces between metal partic¬les and oxide supports in catalytic nanomaterials

Metal-support interactions can dramatically affect the properties of nanocomposite materials. Nevertheless, comprehensive studies of the interfaces between metal nanoparticles and oxide supports remain scarce due to challenges in experimental characterization. A significant understanding of the interactions at such interfaces can be obtained by combining state-of-the-art experiments with density functional calculations. In particular, this Perspective illustrates how theory and experiment can be combined to study interfacial charge transfer, the short- or long-range natures of nanoparticle-support interactions and the effects of oxide nanostructuring on the properties of supported metal particles. These studies aid our understanding of the role of metal-oxide interactions in industrially employed nanocomposites and the design of interfaces with unique properties for future applications

Effects of electron transfer in model catalyst composed of Pt nanoparticles on CeO2(111) surface

Interactions between transition metal nanoparticles and reducible oxide supports are thought to significantly affect the performance of many catalysts. Usually, several metal-support effects act together and cannot be separated from each other. Herein, by means of density-functional calculations we succeeded to single out and quantify effects of the metal-support electron transfer on the structure and electronic properties of important model Pt-ceria catalysts. Namely, we considered ∼1.5 nm large Pt95 and Pt122 particles supported on CeO2(1 1 1). We show that Pt-ceria interactions notably reconstruct Pt nanofacets forming the interface and shift valence d-states of the Pt particles. These effects are rather insensitive to the Pt-ceria electron transfer, at variance with the electronic structure of oxygen anions at the interface, which is significantly affected by the electron transfer. The findings of this work and the special modeling approach applied pave the way for deeper analysis of electronic metal-support interactions in catalysis

Energetic stability of absorbed H in Pd and Pt nanoparticles in a more realistic environment

Absorbed hydrogen can dramatically increase hydrogenation activity of Pd nanoparticles and was predicted to do so also for Pt. This calls for investigations of the energetic stability of absorbed H in Pd and Pt using nanoparticle models as realistic as possible, i.e., (i) sufficiently large, (ii) supported, and (iii) precovered by hydrogen. Herein, hydrogen absorption is studied in MgO(100)-supported 1.6 nm large Pd and Pt nanoparticles with surfaces saturated by hydrogen. The effect of surface H on the stability of absorbed H is found to be significant and to exceed the effect of the support. H absorption is calculated to be endothermic in Pt, energy neutral in Pd(111) and bare Pd nanoparticles, and exothermic in H-covered Pd nanoparticles. Hence, we identify the abundance of surface H and the nanostructuring of Pd as prerequisites for facile absorption of hydrogen in Pd and for the concomitantly altered catalytic activity

Can the state of platinum species be unambiguously determined by the stretching frequency of adsorbed CO probe molecule?

The paper addresses possible ambiguities in the determination of the state of platinum species by the stretching frequency of a CO probe, which is a common technique for characterization of platinum-containing catalytic systems. We present a comprehensive comparison of the available experimental data with our theoretical modeling (density functional) results of pertinent systems - platinum surfaces, nanoparticles and clusters as well as reduced or oxidized platinum moieties on a ceria support. Our results for CO adsorbed on-top on metallic Pt0, with C-O vibrational frequencies in the region 2018-2077 cm−1, suggest that a decrease of the coordination number of the platinum atom, to which CO is bound, by one lowers the CO frequency by about 7 cm−1. This trend corroborates the Kappers-van der Maas correlation derived from the analysis of the experimental stretching frequency of CO adsorbed on platinum-containing samples on different supports. We also analyzed the effect of the charge of platinum species on the CO frequency. Based on the calculated vibrational frequencies of CO in various model systems, we concluded that the actual state of the platinum species may be mistaken based only on the measured value of the C-O vibrational frequency due to overlapping regions of frequencies corresponding to different types of species. In order to identify the actual state of platinum species one has to combine this powerful technique with other approaches

Reduced ceria nanofilms from structure prediction

Experimentally, Ce2O3 films are used to study cerium oxide in its fully or partially reduced state, as present in many applications. We have explored the space of low energy Ce2O3 nanofilms using structure prediction and density functional calculations, yielding more than 30 distinct nanofilm structures. First, our results help to rationalize the roles of thermodynamics and kinetics in the preparation of reduced ceria nanofilms with different bulk crystalline structures (e.g. A-type or bixbyite) depending on the support used. Second, we predict a novel, as yet experimentally unresolved, nanofilm which has a structure that does not correspond to any previously reported bulk A2B3 phase and which has an energetic stability between that of A-type and bixbyite. To assist identification and fabrication of this new Ce2O3 nanofilm we calculate some observable properties and propose supports for its epitaxial growth

Pd single-atom sites on the surface of PdAu nanoparticles: A DFT-based Topological search for suitable compositions

Structure of model bimetallic PdAu nanoparticles is analyzed aiming to find Pd:Au ratios optimal for existence of Pd1 single-atom surface sites inside outer Au atomic shell. The analysis is performed using density-functional theory (DFT) calculations and topological approach based on DFT-parameterized topological energy expression. The number of the surface Pd1 sites in the absence of adsorbates is calculated as a function of Pd concentration inside the particles. At low Pd contents none of the Pd atoms emerge on the surface in the lowest-energy chemical orderings. However, surface Pd1 sites become stable, when Pd content inside a Pd-Au particle reaches ca. 60%. Further Pd content increase up to almost pure Pd core is accompanied by increased concentration of surface Pd atoms, mostly as Pd1 sites, although larger Pd ensembles as dimers and linear trimers are formed as well. Analysis of the chemical orderings inside PdAu nanoparticles at different Pd contents revealed that enrichment of the subsurface shell by Pd with predominant occupation of its edge positions precedes emergence of Pd surface species

Towards stable single-atom catalysts: Strong binding of atomically dispersed transition metals on the surface of nanostructured ceria

The interaction of a series of different transition metal atoms with nanoparticulate CeO2 has been studied by means of density-functional calculations. Recently, we demonstrated the ability of sites exposed on {100} nanofacets of CeO2 to very strongly anchor atomic Pt, making the formed species exceptionally efficient single-atom anode catalysts for proton-exchange membrane fuel cells. Herein, we analyzed the capacity of these surface sites to accommodate all other group VIII-XI transition metal atoms M = Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Cu, Ag, and Au. The interaction of the M atoms with {100} nanofacets of ceria leads to oxidation of the former and such interaction is calculated to be stronger than the binding of the atoms in the corresponding metal nanoparticles. Comparing the stability of metal-metal and metal-oxide bonds allows one to establish which metals would more strongly resist agglomeration and hence allows the proposal of promising candidates for the design of single-atom catalysts. Indeed, the remarkable stability of these adsorption complexes (particularly for Pt, Pd, Ni, Fe, Co, and Os) strongly suggests that atomically dispersed transition metals anchored as cations on {100} facets of nanostructured ceria are stable against agglomeration into metal particles. Therefore, these sites appear to be of immediate relevance to the preparation of stable catalysts featuring the highest possible metal efficiency in nanocatalysis

O2 dissociation on M@Pt core-shell particles for 3d, 4d and 5d transition metals

Density functional theory calculations are performed to investigate oxygen dissociation on 38-atom truncated octahedron platinum-based particles. This study progresses our previous work (Jennings et al. Nanoscale, 2014, 6, 1153), where it was shown that flexibility of the outer Pt shell played a crucial role in facilitating fast oxygen dissociation. In this study, the effect of forming M@Pt (M core, Pt shell) particles for a range of metal cores (M = 3d, 4d, and 5d transition metals) is considered, with respect to O2 dissociation on the Pt(111) facets. We show that forming M@Pt particles with late transition metal cores results in favorable shell flexibility for very low O2 dissociation barriers. Conversely, alloying with early transition metals results in a more rigid Pt shell because of dominant M-Pt interactions, which prevent lowering of the dissociation barriers

Effects of Oxygen Adsorption on the Optical Properties of Ag Nanoparticles

Plasmonic metal nanoparticles are efficient light harvesters with a myriad of sensing- and energy-related applications. For such applications, the optical properties of nanoparticles of metals such as Cu, Ag, and Au can be tuned by controlling the composition, particle size, and shape, but less is known about the effects of oxidation on the plasmon resonances. In this work, we elucidate the effects of O adsorption on the optical properties of Ag particles by evaluating the thermodynamic properties of O-decorated Ag particles with calculations based on the density functional theory and subsequently computing the photoabsorption spectra with a computationally efficient time-dependent density functional theory approach. We identify stable Ag nanoparticle structures with oxidized edges and a quenching of the plasmonic character of the metal particles upon oxidation and trace back this effect to the sp orbitals (or bands) of Ag particles being involved both in the plasmonic excitation and in the hybridization to form bonds with the adsorbed O atoms. Our work has important implications for the understanding and application of plasmonic metal nanoparticles and plasmon-mediated processes under oxidizing environments

AgPd, AuPd, and AuPt nanoalloys with Ag- or Au-rich compositions: Modeling chemical ordering and optical properties

Bimetallic nanoparticles have a myriad of technological applications, but investigations of their chemical and physical properties are precluded due to their structural complexity. Here, the chemical ordering and optical properties of AgPd, AuPd, and AuPt nanoparticles have been studied computationally. One of the main aims was to clarify whether layered ordered phases similar to L11 one observed in the core of AgPt nanoparticles [Pirart, J.; Nat. Commun. 2019, 10, 1982] are also stabilized in other nanoalloys of coinage metals with platinum-group metals, or the remarkable ordering is a peculiarity only of AgPt nanoparticles. Furthermore, the effects of different chemical orderings and compositions of the nanoalloys on their optical properties have been explored. Particles with a truncated octahedral geometry containing 201 and 405 atoms have been modeled. For each particle, the studied stoichiometries of the Ag- or Au-rich compositions, ca. 4:1 for 201-atomic particles and ca. 3:1 for 405-atomic particles, corresponded to the layered structures L11 and L10 inside the monatomic coinage-metal skins. Density functional theory (DFT) calculations combined with a recently developed topological (TOP) approach [Kozlov, S. M.; Chem. Sci. 2015, 6, 3868−3880] have been performed to study the chemical ordering of the particles, whose optical properties have been investigated using the time-dependent DFT method. The obtained results revealed that the remarkable ordering L11 of inner atoms can be noticeably favored only in small AgPt particles and much less in AgPd ones, whereas this L11 ordering in analogous Au-containing nanoalloys is significantly less stable compared to other calculated lowest-energy orderings. Optical properties were found to be more dependent on the composition (concentration of two metals) than on the chemical ordering. Both Pt and Pd elements promote the quenching of the plasmon