Single Atomic Species Research Articles

Stabilizing single atoms and a lower oxidation state of Cu by a ½[110]{100} edge dislocation in Cu−CeO2

Stabilizing atomically dispersed catalytic metal species at surfaces is a significant challenge for obtaining high-performance single atom catalysts. This is because of the strong tendency for the dispersed metal atoms to agglomerate. We propose that dislocations can provide a strong anchor for stabilizing single atoms. A \textonehalf{}[110]{100} edge dislocation in Cu doped ceria, $\mathrm{Cu}\text{\ensuremath{-}}\mathrm{Ce}{\mathrm{O}}_{2}$, is investigated as a model system with density functional theory. The defect formation energies are found to be lower at the dislocation core, with a large segregation energy ranging within 0.8--2.5 eV depending on the site and species at the dislocation core. The high segregation energy indicates that the edge dislocations can enrich Cu defects in an atomically sized area and, thus, have a potential to strongly anchor single atom species at surfaces. Moreover, the edge dislocation also stabilizes reduced cation species, Cu (1+) and Ce (3+). The more reduced dislocation core can offer high concentration of oxygen vacancy as well as in-gap electronic states which provide more reactivity for surface reactions.

Highlights of Major Progress on Single-Atom Catalysis in 2017

Single-atom catalysis has rapidly progressed during the last few years. In 2017, single-atom catalysts (SACs) were fabricated with higher metal loadings and designed into more delicate structures. SACs also found wide applications in C1 chemical conversion, such as selective oxidation of methane and conversion of carbon dioxide. Both experimental characterizations and computational modeling revealed the presence of tunable interactions between single atom species and their surrounding chemical environment, and thus SACs may be more effective and more stable than their nanoparticle counterparts. In this mini-review, we summarize the major achievements of SACs into three main aspects: (a) the advanced synthetic methodologies, (b) catalytic performance in C1 chemistry, and (c) strong metal-support interaction induced unexpected durability. These accomplishments will shed new light on the recognition of single-atom catalysis and encourage more efforts to explore potential applications of SACs.

Defect-Induced Pt-Co-Se Coordinated Sites with Highly Asymmetrical Electronic Distribution for Boosting Oxygen-Involving Electrocatalysis.

Rational design and synthesis of hetero-coordinated moieties at the atomic scale can significantly raise the performance of the catalyst and obtain mechanistic insight into the oxygen-involving electrocatalysis. Here, a facile plasma-photochemical strategy is applied to construct atomically coordinated Pt-Co-Se moieties in defective CoSe2 (CoSe2- x ) through filling the plasma-created Se vacancies in CoSe2- x with single Pt atomic species (CoSe2- x -Pt) under ultraviolet irradiation. The filling of single Pt can remarkably enhance the oxygen evolution reaction (OER) activity of CoSe2 . Optimal OER specific activity is achieved with a Pt content of 2.25 wt% in CoSe2- x -Pt, exceeding that of CoSe2- x by a factor of 9. CoSe2- x -Pt shows much better OER performance than CoSe2- x filled with single Ni and even Ru atomic species (CoSe2- x -Ni and CoSe2- x -Ru). Noticeably, it is general that Pt is not a good OER catalyst but Ru is; thus the design of active sites for electrocatalysis at an atomic level should follow a different intrinsic mechanism. Mechanism studies unravel that the single Pt can induce much higher electronic distribution asymmetry degree than both single Ni and Ru, and benefit the interaction between the Co sites and adsorbates (OH*, O*, and OOH*) during the OER process, leading to a better OER activity.

Plasma mass separation

This tutorial describes mechanisms for separating ions in a plasma device with respect to their atomic or molecular mass for practical applications. The focus here is not on separating isotopes of a single atomic species but rather on systems with a much lower mass resolution and a higher throughput. These separation mechanisms include ion gyro-orbit separation, drift-orbit separation, vacuum arc centrifugation, steady-state rotating plasmas, and several other geometries. Generic physics issues are discussed such as the ion charge state, neutrals and molecules, collisions, radiation loss, and electric fields and fluctuations. Generic technology issues are also discussed such as plasma sources and ion heating, and suggestions are made for future research.

Sexiphenyl on Cu(100): nc-AFM tip functionalization and identification

The contrast obtained in scanning tunneling microscopy (STM) and atomic force microscopy (AFM) images is determined by the tip termination and symmetry. Functionalizing the tip with a single metal atom, CO molecule or organic species has been shown to provide high spatial resolution and insights into tip-surface interactions. A topic where this concept is utilized is the adsorption of organic molecules at surfaces. With this work we aim to contribute to the growing database of organic molecules that allow assignment by intra-molecular imaging. We investigated the organic molecule para-sexiphenyl (C36H26, 6P) on Cu(100) using low-temperature STM and non-contact AFM with intra-molecular resolution. In the sub-monolayer regime we find a planar and flat adsorption with the 6P molecules rotated 10° off the 〈010〉 directions. In this configuration, four of six phenyl rings occupy almost equivalent sites on the surface. The 6P molecules are further investigated with CO-functionalized tips, in comparison to a single-atom metal and 6P-terminated tip. We also show that the procedure of using adsorbed CO to characterize tips introduced by Hofmann et al., Phys. Rev. B 112 (2014) 066101 is useful when the tip is terminated with an organic molecule.

Catalyst Architecture for Stable Single Atom Dispersion Enables Site-Specific Spectroscopic and Reactivity Measurements of CO Adsorbed to Pt Atoms, Oxidized Pt Clusters, and Metallic Pt Clusters on TiO2.

Oxide-supported precious metal nanoparticles are widely used industrial catalysts. Due to expense and rarity, developing synthetic protocols that reduce precious metal nanoparticle size and stabilize dispersed species is essential. Supported atomically dispersed, single precious metal atoms represent the most efficient metal utilization geometry, although debate regarding the catalytic activity of supported single precious atom species has arisen from difficulty in synthesizing homogeneous and stable single atom dispersions, and a lack of site-specific characterization approaches. We propose a catalyst architecture and characterization approach to overcome these limitations, by depositing ∼1 precious metal atom per support particle and characterizing structures by correlating scanning transmission electron microscopy imaging and CO probe molecule infrared spectroscopy. This is demonstrated for Pt supported on anatase TiO2. In these structures, isolated Pt atoms, Ptiso, remain stable through various conditions, and spectroscopic evidence suggests Ptiso species exist in homogeneous local environments. Comparing Ptiso to ∼1 nm preoxidized (Ptox) and prereduced (Ptmetal) Pt clusters on TiO2, we identify unique spectroscopic signatures of CO bound to each site and find CO adsorption energy is ordered: Ptiso ≪ Ptmetal < Ptox. Ptiso species exhibited a 2-fold greater turnover frequency for CO oxidation than 1 nm Ptmetal clusters but share an identical reaction mechanism. We propose the active catalytic sites are cationic interfacial Pt atoms bonded to TiO2 and that Ptiso exhibits optimal reactivity because every atom is exposed for catalysis and forms an interfacial site with TiO2. This approach should be generally useful for studying the behavior of supported precious metal atoms.

Atomically Dispersed Rhodium on Self-Assembled Phosphotungstic Acid: Structural Features and Catalytic CO Oxidation Properties

Atomically dispersed Rh catalysts supported on phosphotungstic acid (PTA) with Rh loading up to 0.9 wt % were prepared through a self-assembly method. Rh stays exclusively as single atom species coordinated to six oxygen atoms, plausibly located at the 4-fold hollow site on one phosphotungstic acid (PTA) molecule together with a chemically adsorbed O2 molecule. The catalyst is active in CO oxidation affording turnover frequencies between 0.2 and 1.7 s–1 from 165 to 195 °C, with an apparent activation energy of 127 kJ/mol. The catalyst is highly stable, well maintaining the Keggin structure of PTA as well as the single-atom identity of Rh after three catalytic cycles (50 to 400 °C). CO activation is mainly achieved on Rh via the formation of dicarbonyl species, while oxygen activation and transfer mainly occur through PTA. The proposed catalytic cycle consists of an alternation of Rh(CO)23+ species and Rh(CO)21+ species on PTA, during which CO transforms into CO2 with oxygen activation being rate-determining.

Direct aerobic oxidative homocoupling of benzene to biphenyl over functional porous organic polymer supported atomically dispersed palladium catalyst

Synthesis of biphenyl directly from the oxidative coupling of benzene with O2 as the sole oxident is an atom-efficiency and environmental-friendly route, which is however unattainable as yet due to the most inert nature of sp2 CH bond in benzene ring, especailly for recyclable heterogeneous catalysis. In this work, single atomic dispersed palladium(II)-porous organic polymer (POP) catalyst with a high loading (>2wt%) was constructed by anchoring Pd(II) species on the task-specifically designed POP support tethered with carboxyl acid and sulfonic acid groups. It exhibited efficient activity in the heterogeneous aerobic oxidative coupling of benzene with O2, giving the highest biphenyl yield of 26.1% so far. The high electrophilicity of thus anchored single atomic Pd(II) species is demonstrated, endowing the unprecedentedly maximum turnover number (TON) of 487 and turnover frequency (TOF) of 352h−1 that expletively exceeds 15 and 88 times of previous heterogeneous catalyst. The catalyst can be facilely recycled and reused, and readily extendable to the conversion of other nonactivated arenes into corresponding biaryls. The designing strategy of POP materials developed in the study may provide a platform towards stable single atomic dispersed noble metal species with desirable electrophilicity as efficient catalysts for more sustainable CC formation-involved organic transformations.

Vacuum Chromatography of Tl on SiO2 at the Single-Atom Level

An isothermal vacuum chromatography setup for superheavy element chemistry studies was developed and tested online at the one-atom-at-a-time level. As a model system, the adsorption behavior of thallium on quartz was chosen with respect to a future chemical characterization of its superheavy homologue, element 113 (E113, Z = 113), using the described setup. Short-lived 184Tl (t1/2 = 10.1(5) s) was produced in the reaction 152Gd(35Cl, 3n)184Tl and delivered as a mass-separated ion beam to the chemistry experiment: A subsurface implantation and a subsequent fast thermal release from a metal matrix was followed by isothermal vacuum chromatography as the chemical separation stage. Single atomic species passing this chromatographic separation were finally identified by time- and energy-resolved event-by-event α-spectroscopy using a diamond-based solid-state detector. The derived adsorption enthalpy of −ΔHadsSiO2(Tl) = 158 ± 3 kJ·mol–1 significantly exceeds available data but correlates well with the adsorption...

Dual Element Intercalation into 2D Layered Bi₂Se₃ Nanoribbons.

We demonstrate the intercalation of multiple zero-valent atomic species into two-dimensional (2D) layered Bi2Se3 nanoribbons. Intercalation is performed chemically through a stepwise combination of disproportionation redox reactions, hydrazine reduction, or carbonyl decomposition. Traditional intercalation is electrochemical thus limiting intercalant guests to a single atomic species. We show that multiple zero-valent atoms can be intercalated through this chemical route into the host lattice of a 2D crystal. Intermetallic species exhibit unique structural ordering demonstrated in a variety of superlattice diffraction patterns. We believe this method is general and can be used to achieve a wide variety of new 2D materials previously inaccessible.

A General Mechanism for Stabilizing the Small Sizes of Precious Metal Nanoparticles on Oxide Supports

We recently discovered that MgAl2O4 spinel (111) nanofacets optimally stabilize the small sizes of platinum nanoparticles even after severe high-temperature aging treatments. Here we report the thermal stabilities of other precious metals with various physical and chemical properties on the MgAl2O4 spinel (111) facets, providing important new insights into the stabilization mechanisms. Besides Pt, Rh, and Ir can also be successfully stabilized as small (1–3 nm) nanoparticles and even as single atomic species after extremely severe (800 °C, 1 week) oxidative aging. However, other metals either aggregate (Ru, Pd, Ag, and Au) or sublimate (Os), even during initial catalyst synthesis. On the basis of ab initio theoretical calculations and experimental observations, we rationalize that the exceptional stabilization originates from the epitaxially matched structure, i.e., lattice matching in geometry and the correspondingly strong electronic attractions at interfaces between the spinel (111) surface oxygens and epitaxial metals/metal oxides. On this basis, design principles for catalyst support oxide materials that are capable of stabilizing precious metals are proposed.

Collision assisted Zeeman cooling with multiple types of atoms

Through a combination of spin-exchange collisions in a magnetic field and optical pumping, it is possible to cool a gas of atoms without requiring the loss of atoms from the gas. This technique, collision assisted Zeeman cooling (CAZ), was developed theoretically assuming a single atomic species [G. Ferrari, Eur. Phys. J. D 13, 67 (2001)]. We have extended this cooling technique to a system of two atomic species rather than just one and have developed a simple analytic model describing the cooling rate. We find that the two-isotope CAZ cooling scheme has a clear theoretical advantage in systems that are reabsorption limited.

PT symmetry via electromagnetically induced transparency

We propose a scheme to realize parity-time (PT) symmetry via electromagnetically induced transparency (EIT). The system we consider is an ensemble of cold four-level atoms with an EIT core. We show that the cross-phase modulation contributed by an assisted field, the optical lattice potential provided by a far-detuned laser field, and the optical gain resulted from an incoherent pumping can be used to construct a PT-symmetric complex optical potential for probe field propagation in a controllable way. Comparing with previous study, the present scheme uses only a single atomic species and hence is easy for the physical realization of PT-symmetric Hamiltonian via atomic coherence.

Ordered Array of Single Adatoms with Remarkable Thermal Stability:Au/Fe3O4(001)

Gold deposited on the Fe3O4(001) surface at room temperature was studied using scanning tunneling microscopy (STM) and x-ray photoelectron spectroscopy (XPS). This surface forms a (√2 × √2)R45° reconstruction, where pairs of Fe and neighboring O ions are slightly displaced laterally producing undulating rows with "narrow" and "wide" hollow sites. At low coverages, single Au adatoms adsorb exclusively at the narrow sites, with no significant sintering up to annealing temperatures of 400 °C. We propose the strong site preference to be related to charge and orbital ordering within the first subsurface layer of Fe3O4(001)-(√2 × √2)R45°. Because of its high thermal stability, this could prove an ideal model system for probing the chemical reactivity of single atomic species.

High-dimensional neural-network potentials for multicomponent systems: Applications to zinc oxide

Artificial neural networks represent an accurate and efficient tool to construct high-dimensional potential-energy surfaces based on first-principles data. However, so far the main drawback of this method has been the limitation to a single atomic species. We present a generalization to compounds of arbitrary chemical composition, which now enables simulations of a wide range of systems containing large numbers of atoms. The required incorporation of long-range interactions is achieved by combining the numerical accuracy of neural networks with an electrostatic term based on environment-dependent charges. Using zinc oxide as a benchmark system we show that the neural network potential-energy surface is in excellent agreement with density-functional theory reference calculations, while the evaluation is many orders of magnitude faster.

Possibility of Stark-insensitive cotrapping of two atomic species in optical lattices

Much effort has been devoted to removing differential Stark shifts for atoms trapped in specially tailored ``magic'' optical lattices, but thus far work has focused on a single trapped atomic species. In this work, we extend these ideas to include two atomic species sharing the same optical lattice. We show qualitatively that, in particular, scalar $J=0$ divalent atoms paired with nonscalar state atoms have the necessary characteristics to achieve such Stark shift cancellation. We then present numerical results on ``magic'' trapping conditions for $^{27}\mathrm{Al}$ paired with $^{87}\mathrm{Sr}$, as well as several other divalent atoms.

Atomic and molecular effects on spherically convergent ion flow. I. Single atomic species

A formalism for analyzing the effect of ion-neutral gas interactions on the flow of ions between nearly transparent electrodes in spherical geometry has been developed for atomic ions in a weakly ionized plasma, so that the important atomic effects are charge exchange and ion impact ionization. The formalism is applied to spherical, gridded, inertial-electrostatic confinement (IEC) devices. The formalism yields detailed information about the energy spectra of the ions and fast neutral atoms, and the resulting fusion rate for H3e ions in a background H3e gas. The results are illustrated with an example calculation for the Wisconsin IEC device operating on H3e.

Analytical Model of Heterogeneous Atomic Recombination on Silicalike Surfaces

An analytic model to describe the heterogeneous recombination of a single atomic species on silicalike surfaces is developed. The theoretical investigation herein presented provides ready-to-use expressions for the surface atomic recombination probability γ obtained as a function of surface characteristics such as the densities of adsorption sites and the activation energies for the different elementary surface processes. The model takes into account physisorption, chemisorption, thermal desorption, surface diffusion, and both Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) recombination mechanisms. The results are applied to the cases of nitrogen and oxygen recombination on silica and to oxygen recombination on Pyrex. However, since the derivation is kept in a very general form, it allows the exploration of several distinct limit cases and provides a deeper understanding of the underlaying surface kinetics. The dependence of the recombination probability with the wall temperature and with the gas pressure is studied in detail. It is found that γ can have a complex nonmonotonic behavior with the wall temperature, as a result of the competition between E-R and L-H recombination processes. The transition from first- to second-order recombinations (and vice-versa) with pressure is studied and debated.

Magnetron sputtering of gold nanoparticles onto WO 3 and activated carbon

In this paper we describe the production and investigation of two supported gold catalyst systems prepared by magnetron sputtering: Au on WO 3 and Au on activated carbon. The magnetron sputtering technique entails using an argon plasma to sputter a high purity gold target producing a flux of gold atoms which are deposited onto a constantly tumbling support material. This technique offers a number of advantages over conventional chemical preparation methods. One advantage is the ability to create gold nanoparticles (diameters <3 nm) on unusual support materials, such as WO 3 and carbon, which are generally not accessible using the ubiquitous deposition-precipitation technique. We present data demonstrating the formation of catalytic gold nanoparticles with average diameters of 1.7 nm (Au/C) and 2.1 nm (Au/WO 3), as well as a substantial number of single atom species on the Au/C sample. Prototypical carbon monoxide oxidation (Au/WO 3) and glycerol oxidation (Au/C) reactions were performed in order to gauge the activity of these catalysts. The WO 3 supported catalyst exhibits substantial catalytic activity from room temperature to 135 °C (0.0018–0.082 mol CO/mol Au s) with an activation energy near 23 kJ/mol. The activity of the Au/C catalyst was compared to a Au/C catalyst prepared from a poly(vinyl alcohol) (PVA) sol. The smaller catalysts prepared by sputtering are more active than the large gold particles prepared using the PVA sol, however the larger gold nanoparticles are substantially more selective towards the production of intermediate products from the oxidation of glycerol.

Defect structure in nanoalloys

The defect structure of bimetallic nanoparticles differs from that of particles made of a single atomic species. Using high resolution TEM imaging along with molecular dynamics simulations, it is possible to investigate the nature and features of these defects. The definition of a local order parameter allows one to locate regions with different kinds of stacking on the simulated nanoparticles and thus to make a direct comparison with the experimental observations.