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

When Can Limited Randomness Be Used in Repeated Games?

The central result of classical game theory states that every finite normal form game has a Nash equilibrium, provided that players are allowed to use randomized (mixed) strategies. However, in practice, humans are known to be bad at generating random-like sequences, and true random bits may be unavailable. Even if the players have access to enough random bits for a single instance of the game their randomness might be insufficient if the game is played many times. In this work, we ask whether randomness is necessary for equilibria to exist in finitely repeated games. We show that for a large class of games containing arbitrary two-player zero-sum games, approximate Nash equilibria of the $n$-stage repeated version of the game exist if and only if both players have $\Omega(n)$ random bits. In contrast, we show that there exists a class of games for which no equilibrium exists in pure strategies, yet the $n$-stage repeated version of the game has an exact Nash equilibrium in which each player uses only a constant number of random bits. When the players are assumed to be computationally bounded, if cryptographic pseudorandom generators (or, equivalently, one-way functions) exist, then the players can base their strategies on "random-like" sequences derived from only a small number of truly random bits. We show that, in contrast, in repeated two-player zero-sum games, if pseudorandom generators \emph{do not} exist, then $\Omega(n)$ random bits remain necessary for equilibria to exist

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

Data-driven efficient score tests for deconvolution problems

We consider testing statistical hypotheses about densities of signals in deconvolution models. A new approach to this problem is proposed. We constructed score tests for the deconvolution with the known noise density and efficient score tests for the case of unknown density. The tests are incorporated with model selection rules to choose reasonable model dimensions automatically by the data. Consistency of the tests is proved

Limits and Confidence Intervals in the Presence of Nuisance Parameters

We study the frequentist properties of confidence intervals computed by the method known to statisticians as the Profile Likelihood. It is seen that the coverage of these intervals is surprisingly good over a wide range of possible parameter values for important classes of problems, in particular whenever there are additional nuisance parameters with statistical or systematic errors. Programs are available for calculating these intervals.Comment: 6 figure

Density-functional study of Cu atoms, monolayers, and coadsorbates on polar ZnO surfaces

The structure and electronic properties of single Cu atoms, copper monolayers and thin copper films on the polar oxygen and zinc terminated surfaces of ZnO are studied using periodic density-functional calculations. We find that the binding energy of Cu atoms sensitively depends on how charge neutrality of the polar surfaces is achieved. Bonding is very strong if the surfaces are stabilized by an electronic mechanism which leads to partially filled surface bands. As soon as the surface bands are filled (either by partial Cu coverage, by coadsorbates, or by the formation of defects), the binding energy decreases significantly. In this case, values very similar to those found for nonpolar surfaces and for copper on finite ZnO clusters are obtained. Possible implications of these observations concerning the growth mode of copper on polar ZnO surfaces and their importance in catalysis are discussed.Comment: 6 pages with 2 postscript figures embedded. Uses REVTEX and epsf macro

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

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

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