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Achieving the optimal balance between cost-efficiency and stability of oxygen reduction reaction (ORR) catalysts is currently among the key research focuses aiming at reaching a broader implementation of proton-exchange membrane fuel cells (PEMFCs). To address this challenge, we combine two well-established strategies to enhance both activity and stability of platinum-based ORR catalysts. Specifically, we prepare ternary PtNi-Au alloys, where each alloying element plays a distinct role: Ni reduces costs and boosts ORR activity, while Au enhances stability. A systematic comparative analysis of the activity-stability relationship for compositionally tuned PtNi-Au model layers, prepared by magnetron co-sputtering, was conducted using a diverse range of complementary characterization techniques and electrochemistry, supported by density functional theory calculations. Our study reveals that a progressive increase of the Au concentration in the Pt 50 Ni 50 alloy from 3 to 15 at % leads to opposing catalyst activity and stability trends. Specifically, we observe a decrease in the ORR activity accompanied by an increase in catalyst stability, manifested in the suppression of both Pt and Ni dissolution. Despite the reduced activity compared to PtNi, the PtNi-Au alloy with 15 at % Au still exhibits nearly three times the activity of monometallic Pt. It also demonstrates a significantly improved dissolution stability relative to that of the PtNi alloy and even monometallic Pt. These findings provide valuable insights into the intricate balance between activity and stability in multimetallic ORR catalysts, paving the way for the design of cost-effective and durable materials for PEMFCs., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

The energetically most favorable chemical ordering of bimetallic nanoparticles can be characterized by combining global optimization algorithms and surrogate energy models. The latter approximate the energy of nanoalloys relying on structural descriptors, training models, and data. Here, we systematically evaluate the performance of highly data-efficient topological descriptors [Kozlov et al., Chem. Sci. 6, 3868 (2015)] for predicting the energies of metal nanoalloys with different chemical orderings. We also introduce a new descriptor based on atomic coordination types, which results in a less data-efficient and interpretable approach, but improves the general accuracy and the quantification of orderings in the inner parts of nanoparticles. The capacity of both the original and new approaches in combination with a basin hopping algorithm is illustrated by generating convex hulls of PdZn nanoalloys and predicting the resulting active surface site distribution as a function of particle composition. Finally, we show how these approaches can be combined with machine-learning adsorption models in electrocatalysis studies for a fast evaluation of the reactivity landscape of targeted nanoalloys., (© 2024 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).)

Modification of CeO 2 (ceria) with 3d transition metals, particularly iron, has been proven to significantly enhance its catalytic efficiency in oxidation or combustion reactions. Although this phenomenon is widely reported, the nature of the iron-ceria interaction responsible for this improvement remains debated. To address this issue, we prepared well-defined model FeO x /CeO 2 (111) catalytic systems and studied their structure and interfacial electronic properties using photoelectron spectroscopy, scanning tunneling microscopy, and low-energy electron diffraction, coupled with density functional theory (DFT) calculations. Our results show that under ultrahigh vacuum conditions, Fe deposition leads to the formation of small FeO x clusters on the ceria surface. Subsequent annealing results in the growth of large amorphous FeO x particles and a 2D FeO x layer. Annealing in an oxygen-rich atmosphere further oxidizes iron up to the Fe 3+ state and improves the crystallinity of both the 2D layer and the 3D particles. Our DFT calculations indicate that the 2D FeO x layer interacts strongly with the ceria surface, exhibiting structural corrugations and transferred electrons between Fe 2+ /Fe 3+ and Ce 4+ /Ce 3+ redox pairs. The novel 2D FeO x /CeO 2 (111) phase may explain the enhancement of the catalytic properties of CeO 2 by iron. Moreover, the corrugated 2D FeO x layer can serve as a template for the ordered nucleation of other catalytically active metals, in which the redox properties of the 2D FeO x /CeO 2 (111) system are exploited to modulate the charge of the supported metals.

The orderings of atoms in bimetallic 1.6-2.1 nm-large CuCo nanoparticles, important as catalytic and magnetic materials, were studied using a combination of DFT calculations with a topological approach. The structure and magnetism of Cu 50 Co 151 , Cu 101 Co 100 , Cu 151 Co 50 , and Cu 303 Co 102 nanoparticles; their resistance to disintegrating into separate Cu and Co species; as well as the exposed surface sites, were quantified and analyzed, showing a clear preference for Cu atoms to occupy surface positions while the Co atoms tended to form a compact cluster in the interior of the nanoparticles. The surface segregation of Co atoms that are encapsulated by less-active Cu atoms, induced by the adsorption of CO molecules, was already enabled at a low coverage of adsorbed CO, providing the energy required to displace the entire compact Co species inside the Cu matrices due to a notable adsorption preference of CO for the Co sites over the Cu ones. The calculated adsorption energies and vibrational frequencies of adsorbed CO should be helpful indicators for experimentally monitoring the nature of the surface sites of CuCo nanoparticles, especially in the case of active Co surface sites emerging in the presence of CO.

Pt-based alloys, such as Pt3Ni, are among the best electrocatalysts for oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells. Doping of PtNi alloys with Zr was shown to enhance the durability of the operating ORR catalysts. Rationalizing these observations is hindered by the absence of atomic-level data for these tri-metallic materials, even when not exposed to the fuel cell operation conditions. This study aims at understanding structure-property relations in Zr-doped PtNi nanoparticles as a key to their ORR function. In particular, we calculated, using a method based on density functional theory, the most stable chemical orderings of pristine and Zr-doped Pt3Ni particles containing over 400 atoms. We thus clarify (i) preferential location and charge states of Zr atoms in the Pt3Ni NPs; (ii) effect of doping Zr atoms on the stability of the Pt skin of the Pt3Ni NPs; (iii) charge redistribution induced by Zr dopants; (iv) layer-by-layer atomic ordering in the Pt3Ni/Zr NPs with the increasing Zr content; and (v) effect of Zr atoms on the adsorption energies of O and OH species as indicators of the ORR activity., (© 2024 Author(s). Published under an exclusive license by AIP Publishing.)

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.

Electronic metal-support interactions affect the chemical and catalytic properties of metal particles supported on reducible metal oxides, but their characterization is challenging due to the complexity of the electronic structure of these systems. These interactions often involve different states with varying numbers and positions of strongly correlated d or f electrons and the corresponding polarons. In this work, we present an approach to characterize electronic metal-support interactions by means of computationally efficient density functional calculations within the projector augmented wave method. We describe Ce 3+ cations with potentials that include a Ce4 f electron in the frozen core, overcoming prevalent convergence and 4 f electron localization issues. We systematically explore the stability and chemical properties of different electronic states for a Pt 8 /CeO 2 (111) model system, revealing the predominant effect of electronic metal-support interactions on Pt atoms located directly at the metal-oxide interface. Adsorption energies and the reactivity of these interface Pt atoms vary significantly upon donation of electrons to the oxide support, pointing to a strategy to selectively activate interfacial sites of metal particles supported on reducible metal oxides., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

Stabilization of cathode catalysts in hydrogen-fueled proton-exchange membrane fuel cells (PEMFCs) is paramount to their widespread commercialization. Targeting that aim, Pt-Au alloy catalysts with various compositions (Pt 95 Au 5 , Pt 90 Au 10 , and Pt 80 Au 20 ) prepared by magnetron sputtering were investigated. The promising stability improvement of the Pt-Au catalyst, manifested in suppressed platinum dissolution with increasing Au content, was documented over an extended potential range up to 1.5 V RHE . On the other hand, at elevated concentrations, Au showed a detrimental effect on oxygen reduction reaction activity. A systematic study involving complementary characterization techniques, electrochemistry, and Monte Carlo simulations based on density functional theory data enabled us to gain a comprehensive understanding of the composition-activity-stability relationship to find optimal Pt-Au alloying for maintaining the activity of platinum and improving its resistance to dissolution. According to the results, Pt-Au alloy with 10% gold represent the most promising composition retaining the activity of monometallic Pt while suppressing Pt dissolution by 50% at the upper potential limit of 1.2 V RHE and by 20% at devastating 1.5 V RHE .

Understanding how reaction conditions affect metal-support interactions in catalytic materials is one of the most challenging tasks in heterogeneous catalysis research. Metal nanoparticles and their supports often undergo changes in structure and oxidation state when exposed to reactants, hindering a straightforward understanding of the structure-activity relations using only ex situ or ultrahigh vacuum techniques. Overcoming these limitations, we explored the metal-support interaction between gold nanoparticles and ceria supports in ultrahigh vacuum and after exposure to CO. A combination of in situ methods (on powder and model Au/CeO 2 samples) and theoretical calculations was applied to investigate the gold/ceria interface and its reactivity toward CO exposure. X-ray photoelectron spectroscopy measurements rationalized by first-principles calculations reveal a distinctly inhomogeneous charge distribution, with Au + atoms in contact with the ceria substrate and neutral Au 0 atoms at the surface of the Au nanoparticles. Exposure to CO partially reduces the ceria substrate, leading to electron transfer to the supported Au nanoparticles. Transferred electrons can delocalize among the neutral Au atoms of the particle or contribute to forming inert Au δ- atoms near oxygen vacancies at the ceria surface. This charge redistribution is consistent with the evolution of the vibrational frequencies of CO adsorbed on Au particles obtained using diffuse reflectance infrared Fourier transform spectroscopy., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)

Nanostructured materials based on CeO 2 and Pt play a fundamental role in catalyst design. However, their characterization is often challenging due to their structural complexity and the tendency of these materials to change under reaction conditions. In this work, we combine calculations based on the density functional theory, a machine-learning assisted global optimization method, and ab initio thermodynamics to characterize stable oxidation states of ceria-supported Pt y O x particles under different environmental conditions. The collection of global minima structures for different stoichiometries resulting from the global optimization effort is used to assess the effect of temperature, oxygen pressure, and support interactions on the phase diagrams, oxidation states, and geometries of the Pt y O x particles. We, thus, identify favored structural motifs and O:Pt ratios, revealing that oxidized states of freestanding and ceria-supported platinum particles are more stable than reduced ones under a wide range of conditions. These results indicate that studies rationalizing activity of ceria-supported Pt clusters must consider oxidized states and that previous understanding of such materials obtained only with fully reduced Pt clusters may be incomplete.

Metal nanoparticles (NPs) are ubiquitous in many fields, from nanotechnology to heterogeneous catalysis, with properties differing from those of single-crystal surfaces and bulks. A key aspect is the size-dependent evolution of NP properties toward the bulk limit, including the adoption of different NP shapes, which may bias the NP stability based on the NP size. Herein, the stability of different Pd n NPs ( n = 10-1504 atoms) considering a myriad of shapes is investigated by first-principles energy optimisation, leading to the determination that icosahedron shapes are the most stable up to a size of ca. 4 nm. In NPs larger than that size, truncated octahedron shapes become more stable, yet a presence of larger {001} facets than the Wulff construction is forecasted due to their increased stability, compared with (001) single-crystal surfaces, and the lower stability of {111} facets, compared with (111) single-crystal surfaces. The NP cohesive energy breakdown in terms of coordination numbers is found to be an excellent quantitative tool of the stability assessment, with mean absolute errors of solely 0.01 eV·atom -1 , while a geometry breakdown allows only for a qualitative stability screening.

Various CO x species formed upon the adsorption and oxidation of CO on palladium and silver single atoms supported on a model ceria nanoparticle (NP) have been studied using density functional calculations. For both metals M, the ceria-supported MCO x moieties are found to be stabilised in the order MCO < MCO 2 < MCO 3 , similar to the trend for CO x species adsorbed on M-free ceria NP. Nevertheless, the characteristics of the palladium and silver intermediates are different. Very weak CO adsorption and the small exothermicity of the CO to CO 2 transformation are found for O 4 Pd site of the Pd/Ce 21 O 42 model featuring a square-planar coordination of the Pd 2+ cation. The removal of one O atom and formation of the O 3 Pd site resulted in a notable strengthening of CO adsorption and increased the exothermicity of the CO to CO 2 reaction. For the analogous ceria models with atomic Ag instead of atomic Pd, these two energies became twice as small in magnitude and basically independent of the presence of an O vacancy near the Ag atom. CO 2 -species are strongly bound in palladium carboxylate complexes, whereas the CO 2 molecule easily desorbs from oxide-supported AgCO 2 moieties. Opposite to metal-free ceria particle, the formation of neither PdCO 3 nor AgCO 3 carbonate intermediates before CO 2 desorption is predicted. Overall, CO oxidation is concluded to be more favourable at Ag centres atomically dispersed on ceria nanostructures than at the corresponding Pd centres. Calculated vibrational fingerprints of surface CO x moieties allow us to distinguish between CO adsorption on bare ceria NP (blue frequency shifts) and ceria-supported metal atoms (red frequency shifts). However, discrimination between the CO 2 and CO 3 2- species anchored to M-containing and bare ceria particles based solely on vibrational spectroscopy seems problematic. This computational modelling study provides guidance for the knowledge-driven design of more efficient ceria-based single-atom catalysts for the environmentally important CO oxidation reaction.

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 L1 1 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 L1 1 and L1 0 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 L1 1 of inner atoms can be noticeably favored only in small AgPt particles and much less in AgPd ones, whereas this L1 1 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., Competing Interests: The authors declare no competing financial interest., (© 2021 American Chemical Society.)

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.

Structural properties and reducibility of zirconium-doped cerium dioxide systems were studied using periodic plane-wave calculations based on density functional theory. A systematic analysis of the results for nanoparticles of two sizes, Ce40-nZrnO80 ∼ 1.5 nm large and Ce140-nZrnO280 ∼ 2.4 nm large, in comparison with slab model data for Ce1-xZrxO2(111) surface has been performed focusing on specific nanoscale effects. Several loadings of Zr dopants ranging from 0.7 to 50 atomic metal percent have been considered. Subsurface cationic sites of ceria are calculated to be energetically most favourable for doping Zr4+ ions in all models. The system stability with several zirconium ions is defined by the relative stability of the occupied individual Zr4+ positions when only one zirconium ion is present. Data for the Ce70Zr70O280 nanoparticle with an equal number of Ce4+ and Zr4+ cations reveal that atomic orderings of neither separated oxide (Janus-type) particles nor randomly intermixed ones are more stable than the distribution of Zr atoms occupying all cationic positions inside the nanoparticle to minimize the presence of surface zirconium. The basicity of surface oxygen centers in nanoparticles is predicted to be decreased when Zr dopants are located in surface positions. The presence of Zr4+ dopants in CeO2 systems can notably lower the oxygen vacancy formation energy and shows interesting peculiarities at higher Zr loadings. Among them is the higher stability of inner oxygen vacancies in Zr-containing nanoparticles and enhanced oxygen mobility beneficial for application in catalysis and as a solid electrolyte with oxygen ions as charge carriers. Similar to pure ceria, Zr-doped ceria nanoparticles exhibit notably higher reducibility than the corresponding extended systems.

Platinum is the most widely used and best performing sole element for catalyzing the oxygen reduction reaction (ORR) in low-temperature fuel cells. Although recyclable, there is a need to reduce the amount used in current fuel cells for their extensive uptake in society. Alloying platinum with rare-earth elements such as yttrium can provide an increase in activity of more than seven times, reducing the amount of platinum and the total amount of catalyst material required for the ORR. As yttrium is easily oxidized, exposure of the Pt-Y catalyst layer to air causes the formation of an oxide layer that can be removed during acid treatment, leaving behind a highly active pure platinum overlayer. This paper presents an investigation of the overlayer composition and quality of Pt 3 Y films sputtered from an alloy target. The Pt 3 Y catalyst surface is investigated using synchrotron radiation X-ray photoelectron spectroscopy before and after acid treatment. A new substoichiometric oxide component is identified. The oxide layer extends into the alloy surface, and although it is not completely removed with acid treatment, the catalyst still achieves the expected high ORR activity. Other surface-sensitive techniques show that the sputtered films are smooth and bulk X-ray diffraction reveals many defects and high microstrain. Nevertheless, sputtered Pt 3 Y exhibits a very high activity regardless of the film's oxide content and imperfections, highlighting Pt 3 Y as a promising catalyst. The obtained results will help to support its integration into fuel cell systems.

We applied a method of plasma arc synthesis to study effects of modification of the fluorite phase of ceria by tin ions. By sputtering active components (Pt, Ce, Sn) together with carbon from a graphite electrode in a helium ambient we prepared samples of complex highly defective composite PtCeC and PtCeSnC oxide particles stabilized in a matrix of carbon. Subsequent high-temperature annealing of the samples in oxygen removes the carbon matrix and causes the formation of active catalysts Pt/CeO x and Pt/CeSnO x for CO oxidation. In the presence of Sn, X-Ray Diffraction (XRD) and High-Resolution Transmission Electron Microscopy (HRTEM) show formation of a mixed phase CeSnO x and stabilization of more dispersed species with a fluorite-type structure. These factors are essential for the observed high activity and thermic stability of the catalyst modified by Sn. X-Ray Photoelectron Spectroscopy (XPS) reveals the presence of both Pt 2+ and Pt 4+ ions in the catalyst Pt/CeO x , whereas only the state Pt 2+ of platinum could be detected in the Sn-modified catalyst Pt/CeSnO x . Insertion of Sn ions into the Pt/CeO x lattice destabilizes/reduces Pt 4+ cations in the Pt/CeSnO x catalyst and induces formation of strikingly high concentration (up to 50% at.) of lattice Ce 3+ ions. Our DFT calculations corroborate destabilization of Pt 4+ ions by incorporation of cationic Sn in Pt/CeO x . The presented results show that modification of the fluorite lattice of ceria by tin induces substantial amount of mobile reactive oxygen partly due to affecting geometric parameters of ceria by tin ions.

Using periodic density functional calculations, we studied the local structure and preferred locations of yttrium cations and oxygen vacancies in Y-doped cerium dioxide. We employed three kinds of models - a slab of the CeO 2 (111) surface and two ceria nanoparticles of different sizes and shapes. In the slab models, which represent the (111) surface of ceria and the corresponding extended terraces on the facets of its nanoparticles, Y 3+ cation dopants were calculated to be preferentially located close to each other. They tend to surround a subsurface oxygen vacancy that forms to maintain the charge balance. Such general behavior was not found for the nanoparticle models, in which structural flexibility and the presence of various low-coordinated surface centers seem to be crucial and suppress most of the trends. Configurations with four Y 3+ cations were calculated to be particularly stable when they combined two of the most stable configurations with two Y 3+ cations. However, no clear trend was found regarding the preferential spatial distribution of the Y 3+ pairs - they can be stable both in isolation and close to each other. In general, doping by yttrium does not notably change the reducibility of ceria systems but selectively facilitates the formation of oxygen vacancies at the ceria surface in comparison with pristine ceria. Yttrium cations also slightly increase the basicity of the nearby oxygen centers with respect to a stoichiometric ceria surface., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Pd is widely used to catalyse hydrogenation and dehydrogenation reactions. One of them is the hydrogenation of ethylene, which includes the transformation of ethyl species to ethane. Herein, by means of density-functional calculations we address several still insufficiently understood factors affecting the latter process. In particular, we shed light on the following aspects of hydrogenation of alkyls on Pd: (i) the mechanistic details of how subsurface H accelerates the reaction on a (111) surface; (ii) the role of nanoparticle edges; and (iii) the influence of a common spectator ethylidyne, [triple bond, length as m-dash]C-CH 3 . These factors are identified as significant for the height of the ethyl hydrogenation barrier on Pd. Moreover, we show that butyl hydrogenation on Pd is also governed by very similar interactions, which suggests a broader applicability of our conclusions. This study highlights the complexity of alkyl hydrogenation and analyses the factors that need to be taken into account for a more realistic description of the hydrogenation processes on metal surfaces.

Doping oxide materials by inserting atoms of a different element in their lattices is a common procedure for modifying properties of the host oxide. Using catalytically active, yet expensive noble metals as dopants allows synthesizing materials with atomically dispersed metal atoms, which can become cost-efficient catalysts. The stability and chemical properties of the resulting materials depend on the structure of the host oxide and on the position of the dopant atoms in it. In the present work we analyze by means of density functional calculations the relative stability and redox properties of cerium dioxide (ceria) nanoparticles doped with atoms of four technologically relevant transition metals - Pt, Pd, Ni and Cu. Our calculations indicate that the dopants are most stable at surface positions of ceria nanoparticles, highlighting the role of under-coordinated sites in the preparation and characterization of doped nanostructured oxides. The energies of two catalytically important reduction reactions - the formation of oxygen vacancies and homolytic dissociative adsorption of H 2 - are found to strongly depend on the position of the doping atoms in nanoparticulate ceria.

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 Pt(0), 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.

The reactivity of atomically dispersed Pt(2+) species on the surface of nanostructured CeO2 films and the mechanism of H2 activation on these sites have been investigated by means of synchrotron radiation photoelectron spectroscopy and resonant photoemission spectroscopy in combination with density functional calculations. Isolated Pt(2+) sites are found to be inactive towards H2 dissociation due to high activation energy required for H-H bond scission. Trace amounts of metallic Pt are necessary to initiate H2 dissociation on Pt-CeO2 films. H2 dissociation triggers the reduction of Ce(4+) cations which, in turn, is coupled with the reduction of Pt(2+) species. The mechanism of Pt(2+) reduction involves reverse oxygen spillover and formation of oxygen vacancies on Pt-CeO2 films. Our calculations suggest the existence of a threshold concentration of oxygen vacancies associated with the onset of Pt(2+) reduction.

Electronic interactions between metal nanoparticles and oxide supports control the functionality of nanomaterials, for example, the stability, the activity and the selectivity of catalysts. Such interactions involve electron transfer across the metal/support interface. In this work we quantify this charge transfer on a well-defined platinum/ceria catalyst at particle sizes relevant for heterogeneous catalysis. Combining synchrotron-radiation photoelectron spectroscopy, scanning tunnelling microscopy and density functional calculations we show that the charge transfer per Pt atom is largest for Pt particles of around 50 atoms. Here, approximately one electron is transferred per ten Pt atoms from the nanoparticle to the support. For larger particles, the charge transfer reaches its intrinsic limit set by the support. For smaller particles, charge transfer is partially suppressed by nucleation at defects. These mechanistic and quantitative insights into charge transfer will help to make better use of particle size effects and electronic metal-support interactions in metal/oxide nanomaterials.

The high catalytic activity of Pt-Co nanoalloys in oxygen reduction and other reactions is usually attributed to their Pt-rich surfaces. However, identification of the precise near-surface structure is by no means easily achievable experimentally. In this work we systematically analyzed the chemical ordering and surface composition of PtXCo(79-X) and PtXCo(140-X) bimetallic nanoparticles by means of a recently developed method based on topological energy expressions and electronic structure calculations. Pt is found to segregate on the surface, especially on corner and edge sites, forming a one atomic layer thick skin independent of the size and composition of the nanoparticle. In turn, the subsurface shell of the particle is composed mostly of Co, whereas the core area has a mixed composition, which depends on the overall stoichiometry. The formation of an outer Pt shell is corroborated by thoroughly analyzed data of X-ray photoelectron spectroscopy experiments performed with various photon energies on annealed Pt-Co particles prepared in vacuum by magnetron sputtering. The core-shell structure of Pt-Co particles is calculated to be more stable than the respective L10 structure. The obtained topological energy expressions are shown to depend only very moderately on the nanoparticle size, which allowed us to apply them to determine the ordering in ∼4 nm big PtXCo(1463-X) species. The presented results deepen our understanding of the intrinsic structure of Pt-Co nanoparticles depending on their size and composition.

Chemical and physical properties of binary metallic nanoparticles (nanoalloys) are to a great extent defined by their chemical ordering, i.e. the pattern in which atoms of the two elements are located in a given crystal lattice. The reliable determination of the lowest-energy chemical ordering is a challenge that impedes in-depth studies of several-nm large bimetallic particles. We propose a method to efficiently optimize the chemical ordering based solely on results of electronic structure (density functional) calculations. We show that the accuracy of this method is practically the same as the accuracy of the underlying quantum mechanical approach. This method, due to its simplicity, immediately reveals why one or another chemical ordering is preferred and unravels the nature of the binding within the nanoparticles. For instance, our results provide very intuitive understanding of why gold and silver segregate on low-coordinated sites in Pd 70 Au 70 and Pd 70 Ag 70 particles, while Pd 70 Cu 70 exhibits matryoshka-like structure and Pd 70 Zn 70 features Zn and Pd atoms arranged in layers. To illustrate the power of the new method we optimized the chemical ordering in much larger Pd 732 Au 731 , Pd 732 Ag 731 , Pd 732 Cu 731 , and Pd 732 Zn 731 nanocrystals, whose size ∼4.4 nm is common for catalytic applications.

We report our results for the structure and relative stability of mononuclear platinum species on a ceria nanoparticle Ce21O42 depending on reduction or oxidation of the system. The most stable platinum species is Pt(2+) at small {100} facets, where the ion is coordinated in a square-planar complex with four oxygen anions as ligands. Partial reduction of the system does not affect the state of platinum in this position but causes reduction of cerium ions. Atomic platinum species in all other modeled positions on the surface of the ceria nanoparticle are found to be in the oxidation state 0. Based on the calculated thermodynamic quantities we analyzed the formation of a preferable type of platinum species depending on the temperature and O2 pressure. Our thermodynamic model shows that the most stable species under standard conditions is PtO, while at the partial pressure of O2 below 100 Pa the stoichiometric complex Pt-Ce21O42 is formed. In both structures there is Pt(2+) located in a square-planar complex. The characteristics of these two structures fit well the available EXAFS and XPS data. These structures are energetically stable with respect to sintering, while the agglomeration to platinum clusters is exothermic for the neutral mononuclear Pt species located at {111} facets.

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.

Density functional theory calculations are performed to investigate oxygen dissociation on 116-atom truncated octahedron platinum particles. This work builds on results presented previously [Jennings et al., Nanoscale, 2014, 6, 1153], where it was shown that shell flexibility played an important role in facilitating fast oxygen dissociation. In this study, through investigation of the larger particle size, it is shown that oxygen dissociation on the (111) facet of pure platinum species is still aided by shell flexibility at larger sizes. Only the hollow sites close to the edges of the (111) facet mediate oxygen dissociation; oxygen is bound too weakly at other hollow sites for dissociation to occur. Further studies are performed on the (100) facet, which is larger for the Pt116 particle than for either the Pt38 or Pt79 ones. Much higher dissociation barriers are found on the (100) facet compared to the (111) facet, where the bridge sites are favourable for oxygen dissociation.

Heterogeneous catalysis is commonly governed by surface active sites. Yet, areas just below the surface can also influence catalytic activity, for instance, when fragmentation products of catalytic feeds penetrate into catalysts. In particular, H absorbed below the surface is required for certain hydrogenation reactions on metals. Herein, we show that a sufficient concentration of subsurface hydrogen, H(sub) , may either significantly increase or decrease the bond energy and the reactivity of the adsorbed hydrogen, H(ad) , depending on the metal. We predict a representative reaction, ethyl hydrogenation, to speed up on Pd and Pt, but to slow down on Ni and Rh in the presence of H(sub) , especially on metal nanoparticles. The identified effects of subsurface H on surface reactivity are indispensable for an atomistic understanding of hydrogenation processes on transition metals and interactions of hydrogen with metals in general., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Platinum is the most versatile element in catalysis, but it is rare and its high price limits large-scale applications, for example in fuel-cell technology. Still, conventional catalysts use only a small fraction of the Pt content, that is, those atoms located at the catalyst's surface. To maximize the noble-metal efficiency, the precious metal should be atomically dispersed and exclusively located within the outermost surface layer of the material. Such atomically dispersed Pt surface species can indeed be prepared with exceptionally high stability. Using DFT calculations we identify a specific structural element, a ceria "nanopocket", which binds Pt(2+) so strongly that it withstands sintering and bulk diffusion. On model catalysts we experimentally confirm the theoretically predicted stability, and on real Pt-CeO2 nanocomposites showing high Pt efficiency in fuel-cell catalysis we also identify these anchoring sites., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Cerium dioxide is a compound important for heterogeneous catalysis, energy technologies, biomedical applications, etc. One of its most remarkable properties is low O vacancy (Ovac) formation energy Ef. Nanostructuring of ceria was shown to decrease Ef and to make the oxide material more active in oxidative reactions. Here we investigate computationally formation of Ovac on CeO2(111) surfaces nanostructured by steps with experimentally observed structures. To facilitate the search for Ovac + 2Ce(3+) configurations that yield the lowest Ef values we proposed and employed an efficient computational scheme where DFT + U calculations were preceded by a pre-screening procedure based on the results of plain DFT calculations. Ef values on the steps were calculated to be up to 0.7 eV lower than on a regular CeO2(111) surface. Some energetically stable Ovac + 2Ce(3+) configurations were found to include subsurface Ce(3+) ions. The present results quantify to what extent the roughness of the CeO2(111) surface affects its reducibility.

Density functional theory calculations are performed on 38 and 79 metal atom truncated octahedron clusters to study oxygen dissociation as a model for the initial stage of the oxygen reduction reaction. Pure platinum and alloyed platinum-titanium core-shell systems are investigated. It is found that barrierless oxygen dissociation occurs on the (111) facet of the pure platinum clusters. A barrier of ~0.3 eV is observed on the (100) facet. For the alloyed cluster, dissociation barriers are found on both facets, typically ~0.6 eV. The differences between the two systems are attributed to the ability of oxygen to distort the (111) surface of the pure platinum clusters. We show that flexibility of the platinum shell is crucial in promotion of fast oxygen dissociation. However, the titanium core stabilises the platinum shell upon alloying, resulting in a less easily distortable surface. Therefore, whilst an alloyed platinum-titanium electrocatalyst has certain advantages over the pure platinum electrocatalyst, we suggest alloying with a more weakly interacting metal will be beneficial for facilitating oxygen dissociation.

Presently, density functional computational studies of nanostructures in heterogeneous catalysts consider either sufficiently big ("scalable with size") unsupported metal nanoparticles (NPs) or small supported metal clusters. Both models may not be sufficiently representative of a few nm in size supported transition metal NPs dealt with in experiment. As a first step in closing the gap between theoretical models and prepared systems, we investigate the effect of a rather chemically inert oxide support, MgO(100), on relative energies and various properties of Pd and Pt NPs that consist of 49-155 atoms (1.2-1.6 nm in size) and exhibit bulk-like fcc structural arrangements. Shapes and interface configurations of metal NPs on MgO were obtained as a result of thorough optimization within the fcc motif using interatomic potentials. Then the stability and properties of the NPs were studied with a density functional method. We comprehensively characterize interaction between the NPs and MgO(100) support, their interface and effect of the support on NP properties. While the effect of MgO on relative stabilities of NPs with different shapes is found to be significant, other properties of the NPs such as electronic structure and interatomic distances within NP do not notably change upon deposition. This work paves the way to large-scale first-principles computational studies of more realistic models of oxide-supported metal catalysts.

Palladium is crucial for industry-related applications such as heterogeneous catalysis, energy production, and hydrogen technologies. In many processes, atomic H and C species are proposed to be present in the surface/near-surface area of Pd, thus noticeably affecting its chemical activity. This study provides a detail and unified view on the interactions of the H and C species with Pd nanoparticles (NPs), which is indispensable for insight into their catalytic properties. Density functional calculations of the interplay of C and H atoms at various concentrations and sites on suitable Pd NPs have been performed, accompanied by catalysis-relevant experiments on oxide-supported bare and C-modified Pd NPs. It is shown that on a Pd(79) NP a subsurface C atom destabilizes nearby atoms H at low coverage. Our experiments confirm that H atoms bind more weakly on C-containing Pd NPs than on C-free NPs. Various factors related to the presence of both H and C atoms on a Pd(79) surface, which may influence the penetration of H atoms from the surface into the subsurface area, have been investigated. Carbon atoms facilitate the subsurface penetration of atomic H both thermodynamically and kinetically when the surface is densely covered by H atoms. Moreover, subsurface H atoms are also energetically favored, even in the absence of C atoms, when several facets of the NP are covered by H atoms., (Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

We predict that tetrahedral Ce(n)O(2n) nanoparticles <2 nm in size become more stable than those experimentally observed at larger sizes with truncated octahedral morphologies, based on global optimisation and density functional calculations.

Scanning tunneling microscopy (STM) combined with density functional theory (DFT) are used to analyze the structural and electronic properties of step edges on the surface of CeO(2)(111) films grown on Ru(0001). Depending on the preparation conditions, 211 or 110-oriented steps develop on the surface, which results in the formation of ceria ad-islands with hexagonal or triangular shapes. STM conductance spectroscopy reveals pronounced differences in the electronic properties of the step edges, as reflected in different onset positions of the ceria conduction band. The band shifts are related to the development of distinct edge electronic states that split-off from the ceria conduction band, as shown with DFT calculations. The separation of the edge states from the main band is governed by the atom-coordination and local charge-distribution along the edge, the latter giving rise to the development of electrostatic dipoles. We expect that the observed edge morphologies determine not only the electronic properties but also the adsorption behavior of step edges on the CeO(2)(111) surface.

The direct adsorption of Pt(2) dimers on CeO(2)(111) and their formation from isolated adsorbed Pt atoms have been studied using periodic slab model calculations based on density functional theory and including the so-called on-site Hubbard parameter (GGA + U). In the most stable configuration Pt(2) is found to be almost parallel to the surface; the electronic ground state is closed shell and there is no evidence of charge transfer towards or from the surface. The formation of Pt(2) from two single adsorbed Pt atoms involves a rather small energy barrier of ~0.10 eV only. On the contrary, dissociation of adsorbed Pt(2) requires to overcome a considerable barrier of ~1.43 eV. This indicates that once Pt(2) is formed it will remain on the surface, thus likely triggering the growth of larger supported Pt particles.

The interaction of Pt particles with the regular CeO(2)(111) surface has been studied using Pt(8) clusters as representative examples. The atomic and electronic structure of the resulting model systems have been obtained through periodic spin-polarized density functional calculations using the PW91 exchange-correlation potential corrected with the inclusion of a Hubbard U parameter. The focus is on the effect of the metal-support interaction on the surface reducibility of ceria. Several initial geometries and orientations of Pt(8) with respect to the ceria substrate have been explored. It has been found that deposition of Pt(8) over the ceria surface results in spontaneous oxidation of the supported particle with a concomitant reduction of up to two Ce(4+) cations to Ce(3+). Oxygen vacancy formation on the CeO(2)(111) surface and oxygen spillover to the adsorbed particle have also been considered. The presence of the supported Pt(8) particles has a rather small effect (∼0.2 eV) on the O vacancy formation energy. However, it is predicted that the spillover of atomic oxygen from the substrate to the metal particle greatly facilitates the formation of oxygen vacancies: the calculated energy required to transfer an oxygen atom from the CeO(2)(111) surface to the supported Pt(8) particle is only 1.00 eV, i.e. considerably smaller than 2.25 eV necessary to form an oxygen vacancy on the bare regular ceria surface. This strongly suggests that the propensity of ceria systems to store and release oxygen is directly affected by the presence of supported Pt particles.

Interactions of metal particles with oxide supports can radically enhance the performance of supported catalysts. At the microscopic level, the details of such metal-oxide interactions usually remain obscure. This study identifies two types of oxidative metal-oxide interaction on well-defined models of technologically important Pt-ceria catalysts: (1) electron transfer from the Pt nanoparticle to the support, and (2) oxygen transfer from ceria to Pt. The electron transfer is favourable on ceria supports, irrespective of their morphology. Remarkably, the oxygen transfer is shown to require the presence of nanostructured ceria in close contact with Pt and, thus, is inherently a nanoscale effect. Our findings enable us to detail the formation mechanism of the catalytically indispensable Pt-O species on ceria and to elucidate the extraordinary structure-activity dependence of ceria-based catalysts in general.

Recently, several forms of unsupported gold were shown to display a remarkable activity to catalyze oxidation reactions. Experimental evidence points to the crucial role of residual silver present in very small concentrations in these novel catalysts. We focus on the catalytic properties of nanoporous gold (np-Au) foams probed via CO and oxygen adsorption/co-adsorption. Experimental results are analyzed using theoretical models represented by the flat Au(111) and the kinked Au(321) slabs with Ag impurities. We show that Ag atoms incorporated into gold surfaces can facilitate the adsorption and dissociation of molecular oxygen on them. CO adsorbed on top of 6-fold coordinated Au atoms can in turn be stabilized by co-adsorbed atomic oxygen by up to 0.2 eV with respect to the clean unsubstituted gold surface. Our experiments suggest a linking of that most strongly bound CO adsorption state to the catalytic activity of np-Au. Thus, our results shed light on the role of silver admixtures in the striking catalytic activity of unsupported gold nanostructures.

The formation of oxygen vacancies in nanoparticles Ce(n)O(2n) (n < or = 80), studied using density-functional calculations, is found to be greatly facilitated compared to extended surfaces, which explains the observed spectacular reactivity of nanostructured ceria.