Au Atoms and Dimers on the MgO(100) Surface: A DFT Study of Nucleation at Defects

Single-Atom Catalysis: Insights from Model Systems

The field of single-atom catalysis (SAC) has expanded greatly in recent years. While there has been much success developing new synthesis methods, a fundamental disconnect exists between most experiments and the theoretical computations used to model them. The real catalysts are based on powder supports, which inevitably contain a multitude of different facets, different surface sites, defects, hydroxyl groups, and other contaminants due to the environment. This makes it extremely difficult to determine the structure of the active SAC site using current techniques. To be tractable, computations aimed at modeling SAC utilize periodic boundary conditions and low-index facets of an idealized support. Thus, the reaction barriers and mechanisms determined computationally represent, at best, a plausibility argument, and there is a strong chance that some critical aspect is omitted. One way to better understand what is plausible is by experimental modeling, i.e., comparing the results of computations to experiments based on precisely defined single-crystalline supports prepared in an ultrahigh-vacuum (UHV) environment. In this review, we report the status of the surface-science literature as it pertains to SAC. We focus on experimental work on supports where the site of the metal atom are unambiguously determined from experiment, in particular, the surfaces of rutile and anatase TiO2, the iron oxides Fe2O3 and Fe3O4, as well as CeO2 and MgO. Much of this work is based on scanning probe microscopy in conjunction with spectroscopy, and we highlight the remarkably few studies in which metal atoms are stable on low-index surfaces of typical supports. In the Perspective section, we discuss the possibility for expanding such studies into other relevant supports.

Measuring and directing charge transfer in heterogenous catalysts

Precise control of charge transfer between catalyst nanoparticles and supports presents a unique opportunity to enhance the stability, activity, and selectivity of heterogeneous catalysts. While charge transfer is tunable using the atomic structure and chemistry of the catalyst-support interface, direct experimental evidence is missing for three-dimensional catalyst nanoparticles, primarily due to the lack of a high-resolution method that can probe and correlate both the charge distribution and atomic structure of catalyst/support interfaces in these structures. We demonstrate a robust scanning transmission electron microscopy (STEM) method that simultaneously visualizes the atomic-scale structure and sub-nanometer-scale charge distribution in heterogeneous catalysts using a model Au-catalyst/SrTiO₃-support system. Using this method, we further reveal the atomic-scale mechanisms responsible for the highly active perimeter sites and demonstrate that the charge transfer behavior can be readily controlled using post-synthesis treatments. This methodology provides a blueprint for better understanding the role of charge transfer in catalyst stability and performance and facilitates the future development of highly active advanced catalysts.

Growth-mode and interface structure of epitaxial ultrathin MgO/Ag(001) films

MgO ultrathin films are of great technological importance as electron tunneling barrier in electronics and spintronics, and as template for metallic clusters in catalysis and for molecular networks for 2D electronics. The wide band-gap of MgO allows for a very effective decoupling from the substrate. The films morphology and the detailed structure of the interface are crucial for applications, controlling the electronic transfer. Using surface x-ray diffraction, we studied the growth-mode and the structure of MgO/Ag(001) ultrathin films elaborated by reactive molecular beam epitaxy as function of the substrate temperature. We observed that deposition of about 1 monolayer results in an MgO(001) film in coherent epitaxy, with the oxygen atoms on top of silver as predicted by DFT calculations, and an interlayer distance at the interface of about 270 pm. Under well-defined conditions, a sharp MgO bilayer is formed covering a fraction of the substrate surface.

Boosting carbonyl sulfide catalytic hydrolysis performance over N-doped Mg-Al oxide derived from MgAl-layered double hydroxide

Carbonyl sulfide (COS), the organic sulfur generated in the chemical industry, has been receiving more attention due to its environmental and economic influence. In this study N-doped MgAl-LDO catalyst was successfully prepared and tested for the COS hydrolysis reaction at low temperature, it was observed that the N species can be formed both in surface and bulk. Moreover, the basicity property and the H₂O adsorption-desorption property were remarkably improved due to the N-doping. Besides, the hydroxyl group can be formed more easily and more abundantly on N modified catalyst surface, which was beneficial to the COS adsorption and the remarkable improvement of catalytic performance. The catalytic hydrolysis performance can proceed for almost 1440 min without any deactivation at 70 °C. However, further increase of temperature was not beneficial to improve the catalytic performance due to the occurrence of H₂S oxidation side reaction. Furthermore, it was revealed that the surface hydroxyl groups were responsible for the adsorption of COS and then the formed surface transitional species reacted with the H₂O molecules. Hydrogen thiocarbonate and bicarbonate were the main reaction intermediate. The rate-determining step was IM6→IM7 i.e., a type transformation of bicarbonate.

Support work function as a descriptor and predictor for the charge and morphology of deposited Au nanoparticles

We show, using density functional theory calculations, that the charge, magnetic moment, and morphology of deposited Au nanoclusters can be tuned widely by doping the oxide support with aliovalent cations and anions. As model systems, we have considered Au_n (n = 1, 2, or 20) deposited on doped MgO and MgO/Mo supports. The supports have been substitutionally doped with varying concentrations θ of F, Al, N, Na, or Li. At θ = 2.78%, by varying the dopant species, we are able to tune the charge of the Au monomer between -0.84e and +0.21e, the Au dimer between -0.87e and -0.16e, and, most interestingly, Au₂₀ between -3.97e and +0.49e. These ranges can be further extended by varying θ. These changes in charge are correlated with changes in adsorption and/or cluster geometry and magnetic moment. We find that the work function Φ of the bare support is a good predictor and descriptor of both the geometry and charge of the deposited Au cluster; it can, therefore, be used to quickly estimate which dopant species and concentration can result in a desired cluster morphology and charge state. This is of interest as these parameters are known to significantly impact cluster reactivity, with positively or negatively charged clusters being preferred as catalysts for different chemical reactions. It is particularly noteworthy that the Na-doped and Li-doped supports succeed in making Au₂₀ positively charged, given the high electronegativity of Au.

Large-Scale Molecular Dynamics Simulations of Homogeneous Nucleation of Pure Aluminium

Despite the continuous and remarkable development of experimental techniques for the investigation of microstructures and the growth of nuclei during the solidification of metals, there are still unknown territories around this topic. The solidification in nanoscale can be effectively investigated by means of molecular dynamics (MD) simulations which can provide a deep insight into the mechanisms of the formation of nuclei and the induced crystal structures. In this study, MD simulations were performed to investigate the solidification of pure Aluminium and the effects of the cooling rate on the final properties of the solidified material. A large number of Aluminium atoms were used in order to investigate the grain growth over time and the formation of stacking faults during solidification. The number of face-centred cubic (FCC), hexagonal close-packed (HCP) and body-centred cubic (BCC) was recorded during the evolution of the process to illustrate the nanoscale mechanisms initiating solidification. The current investigation also focuses on the exothermic nature of the solidification process which has been effectively captured by means of MD simulations using 3 dimensional representations of the kinetic energy across the simulation domain.

Extent of Spin Contamination Errors in DFT/Plane-wave Calculation of Surfaces: A Case of Au Atom Aggregation on a MgO Surface

The aggregation of Au atoms onto a Au dimer (Au₂) on a MgO (001) surface was calculated by restricted (spin-un-polarized) and unrestricted (spin-polarized) density functional theory calculations with a plane-wave basis and the approximate spin projection (AP) method. The unrestricted calculations included spin contamination errors of 0.0⁻0.1 eV, and the errors were removed using the AP method. The potential energy curves for the aggregation reaction estimated by the restricted and unrestricted calculations were different owing to the estimation of the open-shell structure by the unrestricted calculations. These results show the importance of the open-shell structure and correction of the spin contamination error for the calculation of small-cluster-aggregations and molecule dimerization on surfaces.

Cu dimer anchored on C2N monolayer: low-cost and efficient Bi-atom catalyst for CO oxidation

By means of density functional theory (DFT) computations, we systemically investigated CO/O2 adsorption and CO oxidation pathways on a bi-atom catalyst, namely, a copper dimer anchored on a C2N monolayer (Cu2@C2N), and we compared it with its monometallic counterpart Cu1@C2N. The Cu dimer could be stably embedded into the porous C2N monolayer. The reactions between the adsorbed O2 and CO via both bi-molecular and tri-molecular Langmuir-Hinshelwood (L-H) and Eley-Rideal (E-R) mechanisms were comparably studied, and we found that the bi-atom catalyst Cu2@C2N possessed superior performance toward CO oxidation as compared to the single-atom catalyst Cu1@C2N. Our comparative study suggested that the newly predicted bi-atom catalyst, i.e., a copper dimer anchored on a suitable support is highly active for CO oxidation, which can provide a useful guideline for further developing highly effective and low-cost green nanocatalysts.

Effect of surface vacancies on the adsorption of Pd and Pb on MgO(100)

Abstract

Theoretical quantum mechanical calculations have been carried out to establish the effect of surface vacancies on the adsorption of Pd and Pb atoms on the defective MgO(100) surface. The investigated defects included neutral, singly and doubly charged O and Mg vacancies on the (100) surface of MgO. These vacancies played the role of F_sⁿ⁺ and V_sⁿ⁻ (n = 0, 1, 2) adsorption centers for a single Pd or Pb atom. From the results of calculations, it is clear that the Pd- and Pb-atom adsorption at the F_sⁿ⁺ and V_sⁿ⁻ centers shows different characteristics than at the regular O²⁻ and Mg²⁺ centers. Drastic changes in geometric, energetic, and electronic parameters are evident in Pd/V_sⁿ⁻ and Pb/V_sⁿ⁻. The effect of F_s⁰ and F_s⁺, which in practice are the most important vacancies, is smaller, yet the adsorption of Pd and Pb at these centers is more energetically favorable than at the regular O²⁻ center. Of the two metals studied, the atom of Pd is bound by the F_s⁰ and F_s⁺ centers with higher adsorption energies.

Graphical abstract

Electronic supplementary material

The online version of this article (10.1007/s00706-018-2159-1) contains supplementary material, which is available to authorized users.

First-Principle Investigations of the Interaction between CO and O2 with Group 11 Atoms on a Defect-Free MgO(001) Surface

In this contribution, we investigate the interaction between CO and O₂ with metal atoms of group 11 deposited on a defect-free magnesium oxide surface using density functional theory with periodic point charge embedding. We present the first transversal study of the adsorption and coadsorption of CO and O₂ on coinage metal adatoms deposited on metal oxide surfaces from the perspective of single-atom catalysis. Various analysis tools shed light on the binding situation of the metal atoms to the substrate as well as on the situation of the two molecules on the different metal centers. Our analysis demonstrates that cooperative electronic effects enhance the stability of CO upon coadsorption with O₂ for all three metal centers. Our results also explain the lack of catalytic activity of group 11 metal atoms with respect to CO oxidation under thermal conditions as a competition between OC-O₂ bond activation and surface diffusion, leading to metal atom agglomeration. Additionally, it is shown how coadsorption of CO and O₂ on Au/Mg(001) could pave the way to single-atom photocatalysis.

Tunable reactivity of supported single metal atoms by impurity engineering of the MgO(001) support

The reactivity of single Pd and Au atoms supported by MgO(001) can be tuned by surface doping.

Identification of activity trends for CO oxidation on supported transition-metal single-atom catalysts

Identification of activity trends for CO oxidation on transition-metal single-atom catalysts by using E_ad(CO) and E_ad(O₂) as descriptors.

How surface reparation prevents catalytic oxidation of carbon monoxide on atomic gold at defective magnesium oxide surfaces

In this contribution, we study using first principles the co-adsorption and catalytic behaviors of CO and O2 on a single gold atom deposited at defective magnesium oxide surfaces. Using cluster models and point charge embedding within a density functional theory framework, we simulate the CO oxidation reaction for Au1 on differently charged oxygen vacancies of MgO(001) to rationalize its experimentally observed lack of catalytic activity. Our results show that: (1) co-adsorption is weakly supported at F(0) and F(2+) defects but not at F(1+) sites, (2) electron redistribution from the F(0) vacancy via the Au1 cluster to the adsorbed molecular oxygen weakens the O2 bond, as required for a sustainable catalytic cycle, (3) a metastable carbonate intermediate can form on defects of the F(0) type, (4) only a small activation barrier exists for the highly favorable dissociation of CO2 from F(0), and (5) the moderate adsorption energy of the gold atom on the F(0) defect cannot prevent insertion of molecular oxygen inside the defect. Due to the lack of protection of the color centers, the surface becomes invariably repaired by the surrounding oxygen and the catalytic cycle is irreversibly broken in the first oxidation step.

Diffusion Barriers Block Defect Occupation on Reduced CeO_{2}(111)

Surface defects are believed to govern the adsorption behavior of reducible oxides. We challenge this perception on the basis of a combined scanning-tunneling-microscopy and density-functional-theory study, addressing the Au adsorption on reduced CeO_{2-x}(111). Despite a clear thermodynamic preference for oxygen vacancies, individual Au atoms were found to bind mostly to regular surface sites. Even at an elevated temperature, aggregation at step edges and not decoration of defects turned out to be the main consequence of adatom diffusion. Our findings are explained with the polaronic nature of the Au-ceria system, which imprints a strong diabatic character onto the diffusive motion of adatoms. Diabatic barriers are generally higher than those in the adiabatic regime, especially if the hopping step couples to an electron transfer into the ad-gold. As the population of O vacancies always requires a charge exchange, defect decoration by Au atoms becomes kinetically hindered. Our study demonstrates that polaronic effects determine not only electron transport in reducible oxides but also the adsorption characteristics and therewith the surface chemistry.

Determination of the structures of small gold clusters on stepped magnesia by density functional calculations

The structural modifications of small supported gold clusters caused by realistic surface defects (steps) in the MgO(001) support are investigated by computational methods. The most stable gold cluster structures on a stepped MgO(001) surface are searched for in the size range up to 24 Au atoms, and locally optimized by density-functional calculations. Several structural motifs are found within energy differences of 1 eV: inclined leaflets, arched leaflets, pyramidal hollow cages and compact structures. We show that the interaction with the step clearly modifies the structures with respect to adsorption on the flat defect-free surface. We find that leaflet structures clearly dominate for smaller sizes. These leaflets are either inclined and quasi-horizontal, or arched, at variance with the case of the flat surface in which vertical leaflets prevail. With increasing cluster size pyramidal hollow cages begin to compete against leaflet structures. Cage structures become more and more favourable as size increases. The only exception is size 20, at which the tetrahedron is found as the most stable isomer. This tetrahedron is however quite distorted. The comparison of two different exchange-correlation functionals (Perdew-Burke-Ernzerhof and local density approximation) show the same qualitative trends.

SLIDE AND ROLLING MECHANISMS OF Pt CLUSTERS OUT OF OXYGEN VACANCY REGION ON MgO(100) SURFACE

We present the theoretical DFT/B3LYP investigations on the cluster nucleation process and moving mechanism for the Pt n (n = 1, 2, 3, 4, 8, and 12) clusters deposited on MgO surface with an embedded cluster model. The structures and energies of the Pt n clusters adsorbed on the perfect and oxygen vacancy MgO surfaces have been calculated. Based on the nucleation of the Pt n clusters on the oxygen vacancy sites of MgO surfaces, the two moving mechanisms of slide and rolling out of the vacancy region have been energetically investigated. The results show that the needed energies per atom moving out of the vacancy region decrease as the cluster sizes increase. Consequently, the big clusters move likely out of the vacancy sites by either slide or rolling model under a certain temperature condition.

Diffusion of adatoms and small clusters on magnesium oxide surfaces

The diffusion of isolated adatoms and small clusters is reviewed for transition and noble metals adsorbed on the (001) surface of magnesium oxide. While isolated adatoms diffuse by hopping among adsorption sites, small clusters such as dimers, trimers and tetramers already display a variety of diffusion mechanisms, from cluster hopping to rotation, sliding, leapfrog, walking, concertina, flipping, twisting, rolling and rocking. Since most of the available results are computational, the review is mostly related to theoretical work. Connection to experiments is discussed where possible, mostly by dealing with the consequences that adatom and small cluster mobility may have on the growth of larger aggregates on the MgO(001) surface.

Gold supported on thin oxide films: from single atoms to nanoparticles

[Figure: see text]. Historically, people have prized gold for its beauty and the durability that resulted from its chemical inertness. However, even the ancient Romans had noted that finely dispersed gold can give rise to particular optical phenomena. A decade ago, researchers found that highly dispersed gold supported on oxides exhibits high chemical activity in a number of reactions. These chemical and optical properties have recently prompted considerable interest in applications of nanodispersed gold. Despite their broad use, a microscopic understanding of these gold-metal oxide systems lags behind their application. Numerous studies are currently underway to understand why supported nanometer-sized gold particles show catalytic activity and to explore possible applications of their optical properties in photonics and biology. This Account focuses on a microscopic understanding of the gold-substrate interaction and its impact on the properties of the adsorbed gold. Our strategy uses model systems in which gold atoms and clusters are supported on well-ordered thin oxide films grown on metal single crystals. As a result, we can investigate the systems with the rigor of modern surface science techniques while incorporating some of the complexity found in technological applications. We use a variety of different experimental methods, namely, scanning probe techniques (scanning tunneling microscopy and spectroscopy, STM and STS), as well as infrared (IR), temperature-programmed desorption (TPD), and electron paramagnetic resonance (EPR) spectroscopy, to evaluate these interactions and combine these results with theoretical calculations. We examined the properties of supported gold with increasing complexity starting from single gold atoms to one- and two-dimensional clusters and three-dimensional particles. These investigations show that the binding of gold on oxide surfaces depends on the properties of the oxide, which leads to different electronic properties of the Au deposits. Changes in the electronic structure, namely, the charge state of Au atoms and clusters, can be induced by surface defects such as color centers. Interestingly, the film thickness can also serve as a parameter to alter the properties of Au. Thin MgO films (two to three monolayer thickness) stabilize negatively charged Au atoms and two-dimensional Au particles. In three dimensions, the properties of Au particles bigger than 2-3 nm in diameter are largely independent of the support. Smaller three-dimensional particles, however, showed differences based on the supporting oxide. Presumably, the oxide support stabilizes particular atomic configurations, charge states, or electronic properties of the ultrasmall Au aggregates, which are in turn responsible for this distinct chemical behavior.

Theory and simulation in heterogeneous gold catalysis

This critical review covers the application of quantum chemistry to the burgeoning area of the heterogeneous oxidation by Au. We focus on the most established reaction, the oxidation of CO at low temperature. The review begins with an overview of the methods available comparing the treatment of the electron-electron interaction and relativistic effects. The structure of Au particles and their interaction with oxide reviews is then discussed in detail. Calculations of the adsorption and reaction of CO and O2 are then considered and results from isolated and supported Au clusters compared (155 references).

Structure of Ag Clusters Grown on Fs-Defect Sites of an MgO(1 0 0) Surface

The structure of AgN clusters (N=1-4, 6, 8, 10), both in the gas phase and grown on the MgO(1 0 0) surface containing Fs-defects, has been investigated by a density functional basin-hopping (DF-BH) approach. In analogy with what observed in the case of gold clusters, it is found that the presence of the defect implies a double frustration and a cylindrical invariance of the metal-surface interaction, causing small Ag clusters growing around the Fs defect to be highly fluxional. Nevertheless, two different structural crossovers are found to be induced by the metal-defect interaction for the adsorbed clusters such that: 1) planar structures prevail for N<or=4 (as in the gas phase); 2) noncrystalline (fivefold symmetric) structures, which are the lowest energy ones in the gas phase for medium sized AgN clusters (N>or=7), prevail for N=6 and N=8; 3) distorted face-centered cubic (fcc) structures grown pseudomorphically on the defected surface prevail for N=10. The transition from fivefold to fcc motifs is rationalized in terms of the double-frustration effect, which increases the bond strain of the noncrystalline structures. Detrapping energies from the defect were also calculated; the lowest energy pathway corresponds to the detachment of a dimer.

Crossover from three-dimensional to two-dimensional geometries of Au nanostructures on thin MgO(001) films: a confirmation of theoretical predictions

In this Letter, we report a low-temperature scanning tunneling microscopy study aiming to explore the adsorption properties of Au with respect to the thickness of supported MgO films. For different MgO film thicknesses (3 ML and 8 ML), we find significant differences in the distribution of Au adsorption sites and in the Au cluster geometry, in line with recent calculations and electron paramagnetic resonance experiments. On the surface of thick MgO films or unsupported MgO, Au adsorbs on O sites [Phys. Rev. Lett. 96, 146804 (2006)], and the equilibrium cluster geometry is three-dimensional. In contrast, on thin MgO films, the calculations predicted (i) a change of the preferred Au nucleation site [Phys. Rev. Lett. 94, 226104 (2005)] and (ii) a stabilization of two-dimensional Au cluster geometries [Phys. Rev. Lett. 97, 036106 (2006)].