Reactivity of Bimetallic Systems Studied from First Principles

The Challenge of Electrochemical Ammonia Synthesis: A New Perspective on the Role of Nitrogen Scaling Relations

The electrochemical production of NH3 under ambient conditions represents an attractive prospect for sustainable agriculture, but electrocatalysts that selectively reduce N2 to NH3 remain elusive. In this work, we present insights from DFT calculations that describe limitations on the low-temperature electrocatalytic production of NH3 from N2 . In particular, we highlight the linear scaling relations of the adsorption energies of intermediates that can be used to model the overpotential requirements in this process. By using a two-variable description of the theoretical overpotential, we identify fundamental limitations on N2 reduction analogous to those present in processes such as oxygen evolution. Using these trends, we propose new strategies for catalyst design that may help guide the search for an electrocatalyst that can achieve selective N2 reduction.

Noble metal alloy complex nanostructures: controllable synthesis and their electrochemical property

From the perspective of noble metal alloy nanocrystals with complex structures, we highlight their controllable synthesis and improved electrochemical property.

CO oxidation catalysed by Pd-based bimetallic nanoalloys

Density functional theory based global geometry optimization has been used to demonstrate the crucial influence of the geometry of the catalytic cluster on the energy barriers for the CO oxidation reaction over Pd-based bimetallic nanoalloys.

Evidence for geometric effects in neopentane conversion on PdAu catalysts

Silica-supported Pd and Pd(shell)/Au(core) nanoparticles of a similar size were evaluated for neopentane conversion.

Bimetallic alloys in action: dynamic atomistic motifs for electrochemistry and catalysis

Multifarious structural motifs, dynamic surface morphologies and novel reaction mechanisms are essential aspects of bimetallic alloys, making them promising candidates for diverse applications in electrochemistry and heterogeneous catalysis.

Surface lattice-engineered bimetallic nanoparticles and their catalytic properties

When nanoparticles become small (ca. <5 nm), surface stress becomes significant and generates strain that results in a change of surface structures. In this regard, the surface lattice of nanoparticles can be engineered to create strains or other structural changes with atomic positions away from the normal lattice points. Such changes impact the electronic and catalytic properties of nanoparticles. Recently, several groups have reported the change of catalytic and electrocatalytic properties of bimetallic nanoparticles. In this tutorial review, we discuss the principles related to lattice strain and other distorted structures, and the catalytic properties of bimetallic nanostructures.

Interaction of CO and deuterium with bimetallic, monolayer Pt-island/film covered Ru(0001) surfaces

The adsorption properties of structurally well defined bimetallic Pt/Ru(0001) surfaces, consisting of a Ru(0001) substrate partly or fully covered by monolayer Pt islands or a monolayer Pt film, were studied by temperature programmed desorption (TPD) using CO and deuterium as probe molecules. Additionally, the adsorption of CO was investigated by infrared reflection absorption spectroscopy (IRAS). The presence of the pseudomorphic platinum islands or monolayer film leads to considerable modifications of the adsorption properties for both adsorbates, both on the Pt covered and, to a smaller extent, on the bare Ru part of the surfaces. In addition to distinct weakly bound adspecies, which are adsorbed on the monolayer Pt islands, we find unique contributions from island edge desorption, from spill-over processes during the desorption run, and a general down-shift of the peak related to desorption from Pt-free Ru(0001) areas with increasing Pt coverage. These effects, which we consider as characteristic for adsorption on bimetallic surfaces with large contiguous areas of the respective types, are discussed in detail.

Critical review of Pd-based catalytic treatment of priority contaminants in water

Catalytic reduction of water contaminants using palladium (Pd)-based catalysts and hydrogen gas as a reductant has been extensively studied at the bench-scale, but due to technical challenges it has only been limitedly applied at the field-scale. To motivate research that can overcome these technical challenges, this review critically analyzes the published research in the area of Pd-based catalytic reduction of priority drinking water contaminants (i.e., halogenated organics, oxyanions, and nitrosamines), and identifies key research areas that should be addressed. Specifically, the review summarizes the state of knowledge related to (1) proposed reaction pathways for important classes of contaminants, (2) rates of contaminant reduction with different catalyst formulations, (3) long-term sustainability of catalyst activity with respect to natural water foulants and regeneration strategies, and (4) technology applications. Critical barriers hindering implementation of the technology are related to catalyst activity (for some contaminants), stability, fouling, and regeneration. New developments overcoming these limitations will be needed for more extensive field-scale application of this technology.

Influence of water on the properties of an Au/Mpy/Pd metal/molecule/metal junction

The geometric and electronic structure of the metal-molecule interface in metal/molecule/metal junctions is of great interest since it affects the functionality of such units in possible nanoelectronic devices. We have investigated the interaction between water and a palladium monolayer of a Au(111)/4-mercaptopyridine/Pd junction by means of DFT calculations. A relatively strong bond between water and the palladium monolayer of the Au/Mpy/Pd complex is observed via a one-fold bond between the oxygen atom of the water molecule and a Pd atom. An isolated H(2)O molecule adsorbs preferentially in a flat-lying geometry on top of a palladium atom that is at the same time also bound to the nitrogen atom of a Mpy molecule of the underlying self-assembled monolayer. The electronic structure of these Pd atoms is considerably modified which is reflected in a reduced local density of states at the Fermi energy. At higher coverages, water can be arranged in a hexagonal ice-like bilayer structure in analogy to water on bulk metal surfaces, but with a much stronger binding which is dominated by O-Pd bonds.

The atomic structural dynamics of γ-Al2O3 supported Ir-Pt nanocluster catalysts prepared from a bimetallic molecular precursor: a study using aberration-corrected electron microscopy and X-ray absorption spectroscopy

This study describes a prototypical, bimetallic heterogeneous catalyst: compositionally well-defined Ir-Pt nanoclusters with sizes in the range of 1-2 nm supported on γ-Al(2)O(3). Deposition of the molecular bimetallic cluster [Ir(3)Pt(3)(μ-CO)(3)(CO)(3)(η-C(5)Me(5))(3)] on γ-Al(2)O(3), and its subsequent reduction with hydrogen, provides highly dispersed supported bimetallic Ir-Pt nanoparticles. Using spherical aberration-corrected scanning transmission electron microscopy (C(s)-STEM) and theoretical modeling of synchrotron-based X-ray absorption spectroscopy (XAS) measurements, our studies provide unambiguous structural assignments for this model catalytic system. The atomic resolution C(s)-STEM images reveal strong and specific lattice-directed strains in the clusters that follow local bonding configurations of the γ-Al(2)O(3) support. Combined nanobeam diffraction (NBD) and high-resolution transmission electron microscopy (HRTEM) data suggest the polycrystalline γ-Al(2)O(3) support material predominantly exposes (001) and (011) surface planes (ones commensurate with the zone axis orientations frequently exhibited by the bimetallic clusters). The data reveal that the supported bimetallic clusters exhibit complex patterns of structural dynamics, ones evidencing perturbations of an underlying oblate/hemispherical cuboctahedral cluster-core geometry with cores that are enriched in Ir (a result consistent with models based on surface energetics, which favor an ambient cluster termination by Pt) due to the dynamical responses of the M-M bonding to the specifics of the adsorbate and metal-support interactions. Taken together, the data demonstrate that strong temperature-dependent charge-transfer effects occur that are likely mediated variably by the cluster-support, cluster-adsorbate, and intermetallic bonding interactions.

A kinetic Monte Carlo study of Pt on Au(111) with applications to bimetallic catalysis

Pt-decorated Au nanostructures and bimetallic PtAu nanoparticles have been shown to act as catalysts. Consequently we investigate the formation of extended Pt decorations of an Au island edge on Au(111) as possible catalysts. The investigation is by simulation using the kinetic Monte Carlo method. The effects of varying the rate of deposition of Pt atoms and the simulation temperature on the morphology of the resulting Pt nanostructures were investigated. The thickness and roughness of the nanostructures are readily influenced, with temperature being the more important factor. A combination of both high temperature and low deposition rate was the most effective at reducing the roughness. PtAu alloying in the Au island edge was identified. This work is (to the best of our knowledge) the first kinetic Monte Carlo simulation study of the formation of Pt nanostructures on Au. We demonstrate how the morphology of the Pt nanostructures can be controlled. The nanostructures studied here, comprising an adjustable mix of Pt overlayers and novel 1D PtAu surface alloys, are expected to be of considerable interest as potential bimetallic nano-catalysts.

Tuning adsorption via strain and vertical ligand effects

We report on the structure and electrochemical adsorption properties of well-defined pseudomorphic Pt mono- and multilayers on Ru(0001). These act as model surfaces for Pt(111) with slightly decreased affinity to adsorbed hydrogen (H(ad)) and hydroxyl (OH(ad)). In cyclic voltammograms, this is reflected in more negative/positive potential regions for the reversible adsorption of upd-H(ad)/OH(ad), respectively, compared to Pt(111). For upd-H(ad), we show that the corresponding trends can be predicted with high accuracy by density functional theory (DFT). In particular, the upd-H(ad) onset regions can be precisely simulated using the H(ad) adsorption energies from DFT, the layer thickness distribution from STM, and the base voltammogram of Pt(111) as reference.

Entropy effects in atom distribution and electrochemical properties of Au(x)Pt(1-x)/Pt(111) surface alloys

We report on the structural and electrochemical properties of Au(x)Pt(1-x) surface alloys prepared by Au vapour deposition onto Pt(111) followed by annealing to 1000 K. Driven by configurational entropy, Pt and Au atoms are distributed homogeneously over the surface. On the nm scale, however, atomically resolved scanning tunnelling microscopy images with chemical contrast reveal the formation of nm-sized Pt-rich and Au-rich aggregates, similar to the behaviour recently reported for Pd(x)Ru(1-x)/Ru(0001) [H. Hartmann, T. Diemant, A. Bergbreiter, J. Bansmann, H. E. Hoster, R. J. Behm, Surf. Sci. 2009, 603, 1439]. Based on the STM data, we determine the abundance of specific adsorption sites for different Au contents, and we derive effective pair interaction parameters that allow reproducing the lateral distribution in Monte Carlo simulations. Cyclic voltammograms of the surface alloys have many similarities with Pt(111). H(ad) and OH(ad) related features both decrease with increasing amount of Au. Both seem to adsorb only on Pt sites, but H(ad) requires smaller ensembles of Pt atoms than OH(ad). The onset potential for H(ad)-formation decreases with increasing Au content. This is can be explained by an effect of the Au atoms on the entropy of adsorption.

Another way of looking at bonding on bimetallic surfaces: the role of spin polarization of surface metal d states

The nature of electronic and chemical properties of an unstrained Pt monolayer on a 3d transition metal substrate, M (M = Cr, Mn, Fe), is studied using spin-polarized density functional theory calculations. High spin polarization of Pt d states is noted, verifying the magnetization induced on Pt, which is observed to be responsible for redirecting the analysis of bond formation on a metal surface towards a different perspective. While the shift in the Pt d band center (the average energy of the Pt d band, commonly used to predict the reactivity of surfaces) does give the expected trend in adsorbate (oxygen) chemisorption energy across the bimetallic surfaces in this work, our results show that for spin-polarized Pt d states, the variation in strength of adsorption with respect to the Fermi level density of states is more predictive of Pt chemisorption properties. Hence, this study introduces a scheme for analyzing trends in reactivity of bimetallic surfaces where adsorption energies are used as reactivity parameters and where spin polarization effects cannot be neglected.

Interaction of Hydrogen with Bimetallic Surfaces

The interaction of hydrogen with bimetallic surfaces is assessed and discussed using structurally well defined PdAu and PtRu single crystal surfaces as model systems. The description is based on a correlation between the hydrogen adsorption behavior and the presence and abundance of specific bimetallic nanostructures. The latter are determined quantitatively by high resolution scanning tunneling microscopy. Following the changes in the hydrogen adsorption behavior on surfaces with systematically varied composition and thus varying concentrations of specific structural elements and adsorption ensembles allows a direct correlation between structural and chemical properties and thus to identify the adsorption characteristics of individual adsorption ensembles. This provides a structural basis for parallel theoretical studies, which are essential for a systematic understanding of the complex changes in the adsorption properties.Based on results from experimental studies and calculations, the role of electronic strain and ligand effects, including lateral and vertical ligands, as well the influence of geometric ensemble effects on the hydrogen adsorption characteristics are discussed. The distinct differences between surfaces modified by a monolayer or monolayer islands of a second metal and mixed surfaces such as surface alloys or alloy surfaces are discussed in these terms.

Tailoring the reactivity of bimetallic overlayer and surface alloy systems

Changing the composition and structure of a bimetallic surface system modifies its electronic properties and thus its catalytic activity. On the basis of density functional theory calculations, the electronic factors underlying the modified properties of bimetallic surfaces such as overlayer systems and, in particular, surface alloys will be discussed. It will be demonstrated that by mixing two metals a new metallic compound can result whose properties are not intermediate but beyond those of both constituents, so that for example by adding a relatively inert metal a more reactive surface can result. Besides the reactivity, the stability of the bimetallic systems will also be briefly discussed.

Complementary structure sensitive and insensitive catalytic relationships

The burgeoning field of nanoscience has stimulated an intense interest in properties that depend on particle size. For transition metal particles, one important property that depends on size is catalytic reactivity, in which bonds are broken or formed on the surface of the particles. Decreased particle size may increase, decrease, or have no effect on the reaction rates of a given catalytic system. This Account formulates a molecular theory of the structure sensitivity of catalytic reactions based on the computed activation energies of corresponding elementary reaction steps on transition metal surfaces. Recent progress in computational catalysis, surface science, and nanochemistry has significantly improved our theoretical understanding of particle-dependent reactivity changes in heterogeneous catalytic systems. Reactions that involve the cleavage or formation of molecular pi-bonds, as in CO or N(2), must be distinguished from reactions that involve the activation of sigma-bonds, such as CH bonds in methane. The activation of molecular pi-bonds requires a reaction center with a unique configuration of several metal atoms and step-edge sites, which can physically not be present on transition metal particles less than 2 nm. This is called class I surface sensitivity, and the rate of reaction will sharply decrease when particle size decreases below a critical size. The activation of sigma chemical bonds, in which the activation proceeds at a single metal atom, displays a markedly different size relationship. In this case, the dependence of reaction rate on coordinative unsaturation of reactive surface atoms is large in the forward direction of the reaction, but the activation energy of the reverse recombination reaction will not change. Dissociative adsorption with cleavage of a CH bond is strongly affected by the presence of surface atoms at the particle edges. This is class II surface sensitivity, and the rate will increase with decreasing particle size. Reverse reactions such as hydrogenation typically show particle-size-independent behavior. The rate-limiting step for these class III reactions is the recombination of an adsorbed hydrogen atom with the surface alkyl intermediate and the formation of a sigma-type bond. Herein is our molecular theory explaining the three classes of structure sensitivity. We describe how reactions with rates that are independent of particle size and reactions with a positive correlation between size and rate are in fact complementary phenomena. The elucidation of a complete theory explaining the size dependence of transition metal catalysts will assist in the rational design of new catalytic systems and accelerate the evolution of the field of nanotechnology.

Total Oxidation of Methanol on Cu(110): A Density Functional Theory Study

The partial and total oxidation of methanol on clean and oxygen-precovered Cu(110) has been studied by periodic density functional theory calculations within the generalized gradient approximation. Reaction paths including the geometry and the energetics of several reaction intermediates and the activation barriers between them have been determined, thus creating a complete scheme for methanol oxidation on copper. The calculations demonstrate that the specific structure of oxygen on copper plays an important role in both the partial and the total oxidation of methanol. For lower oxygen concentrations on the surface, the partial oxidation of methanol to formaldehyde is promoted by the presence of oxygen on the surface through the removal of hydrogen in the form of water, which prevents the recombinative desorption of methanol. At larger oxygen concentrations, the presence of isolated oxygen atoms reduces the C-H bond breaking barrier of adsorbed methoxy considerably, thus accelerating the formation of formaldehyde. Furthermore, oxygen also promotes the formation of dioxymethylene from formaldehyde, which then easily decays to formate. Formate is the most stable reaction intermediate in the total oxidation. Thus the formate decomposition represents the rate-limiting step in the total oxidation of methanol on copper.

Energetics driving the short-range order in CuxPd1-x/Ru(0001) monolayer surface alloys

The energetics determining the distinct short-range order in two-dimensional (2D) monolayer Cu(x)Pd(1-x) surface alloys on a Ru(0001) substrate were investigated by Monte Carlo simulations and density functional theory calculations. Using a 2D lattice gas Hamiltonian based on effective pair interaction (EPI) parameters, the EPIs were derived for different Cu concentrations with Monte Carlo (MC) simulations by comparing with the atomic distributions obtained from atomic resolution STM images and the related Warren-Cowley short-range order parameters (Hoster et al., Phys. Rev. B, 2006, 73 165413). The ground state structures and mixing energies at 0 K derived from these EPIs agree well with mixing energies determined from DFT calculations of different ordered surface alloys. Additional MC simulations yield rather low transition temperatures which explain the absence of ordered 2D phases in the experiments. The consequences of our findings for the use of alloy surfaces and surface alloys as model systems for adsorption and catalytic reaction studies are discussed.