Metal species with different size (single atoms, nanoclusters, and nanoparticles) show different catalytic behavior for various heterogeneous catalytic reactions. It has been shown in the literature that many factors including the particle size, shape, chemical composition, metal–support interaction, and metal–reactant/solvent interaction can have significant influences on the catalytic properties of metal catalysts. The recent developments of well-controlled synthesis methodologies and advanced characterization tools allow one to correlate the relationships at the molecular level. In this Review, the electronic and geometric structures of single atoms, nanoclusters, and nanoparticles will be discussed. Furthermore, we will summarize the catalytic applications of single atoms, nanoclusters, and nanoparticles for different types of reactions, including CO oxidation, selective oxidation, selective hydrogenation, organic reactions, electrocatalytic, and photocatalytic reactions. We will compare the results o...

Metal Catalysts for Heterogeneous Catalysis: From Single Atoms to Nanoclusters and Nanoparticles.

We introduce density functional theory and review recent progress in its application to transition metal chemistry. Topics covered include local, meta, hybrid, hybrid meta, and range-separated functionals, band theory, software, validation tests, and applications to spin states, magnetic exchange coupling, spectra, structure, reactivity, and catalysis, including molecules, clusters, nanoparticles, surfaces, and solids.

Density functional theory for transition metals and transition metal chemistry

After prolonged annealing of a Ru(0001) sample in ultrahigh vacuum a superstructure with a periodicity of $\ensuremath{\sim}30\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ was observed by scanning tunneling microscopy (STM). Using x-ray photoelectron spectroscopy it was found that the surface is covered by graphitic carbon. Auger electron spectroscopy shows that between 1000 and $1400\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ carbon segregates to the surface. STM images recorded after annealing to increasing temperatures display islands of the superstructure, until, after annealing to $T\ensuremath{\geqslant}1400\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, it covers the entire surface. The morphology of the superstructure shows that it consists of a single graphene layer. Atomically resolved STM images and low-energy electron diffraction reveal an $(11\ifmmode\times\else\texttimes\fi{}11)$ structure or incommensurate structure close to this periodicity superimposed by $12\ifmmode\times\else\texttimes\fi{}12$ graphene cells. The lattice mismatch causes a moir\'e pattern. Unlike the common orientational disorder of adsorbed graphene, the graphene layer on Ru(0001) shows a single phase and very good rotational alignment. Misorientations near defects in the overlayer only amount to $\ensuremath{\sim}1\ifmmode^\circ\else\textdegree\fi{}$, and the periodicity of $\ensuremath{\sim}30\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ is unaffected. In contrast to bulk graphite both carbon atoms in the graphene unit cell were resolved by STM, with varying contrast depending on the position above the Ru atoms. The filled and empty state images of the moir\'e structure differ massively, and electronic states at $\ensuremath{-}0.4$ and $+0.2\phantom{\rule{0.3em}{0ex}}\mathrm{V}$ were detected by scanning tunneling spectroscopy. The data indicate a significantly stronger chemical interaction between graphene and the metal surface than between neighboring layers in bulk graphite. The uniformity of the structure and its stability at high temperatures and in air suggest an application as template for nanostructures.

Scanning tunneling microscopy of graphene on Ru(0001)

Whenever a compound crystal is cut normal to a randomly chosen direction, there is an overwhelming probability that the resulting surface corresponds to a polar termination and is highly unstable. Indeed, polar oxide surfaces are subject to complex stabilization processes that ultimately determine their physical and chemical properties. However, owing to recent advances in their preparation under controlled conditions and to improvements in the experimental techniques for their characterization, an impressive variety of structures have been investigated in the last few years. Recent progress in the fabrication of oxide nano-objects, which have been largely stimulated by a growing demand for new materials for applications ranging from micro-electronics to heterogeneous catalysis, also offer interesting examples of exotic polar structures. At odds with polar orientations of macroscopic samples, some smaller size polar nano-structures turn out to be perfectly stable. Others are subject to unusual processes of stabilization, which are absent or not effective in their extended counterparts. In this context, a thorough and comprehensive reflexion on the role that polarity plays at oxide surfaces, interfaces and in nano-objects seems timely.This review includes a first section which presents the theoretical concepts at the root of the polar electrostatic instability and its compensation and introduces a rigorous definition of polar terminations that encompasses previous theoretical treatments; a second section devoted to a summary of all experimental and theoretical results obtained since the first review paper by Noguera (2000 J. Phys.: Condens. Matter 12 R367); and finally a discussion section focusing on the relative strength of the stabilization mechanisms, with special emphasis on ternary compound surfaces and on polarity effects in ultra-thin films.

https://iopscience.iop.org/article/10.1088/0034-4885/71/1/016501/pdf

Polarity of oxide surfaces and nanostructures

Ministry of Science and Technology of China [2011CB932403]; National Nature Science Foundation of China [21131005, 20925103, 21373167, 21033006, 21333008]; Natural Sciences and Engineering Research Council of Canada

Interfacial Effects in Iron-Nickel Hydroxide–Platinum Nanoparticles Enhance Catalytic Oxidation

We have investigated the magnetic and electronic properties of a FeO film grown on Pt(111). Coupling first-principles density-functional theory calculations with scanning tunneling microscopy (STM) measurements we have identified the principal mechanisms for the structural and electronic characteristics observed in the Moire unit cell formed between oxide film and metal support. We show that both free and supported FeO(111) monolayers present an antiparallel alignment of the magnetic moments. Due to the lattice mismatch between Pt(111) and the FeO film, the interface properties are different in different regions of the supercell. The resulting modulation of the structural and electronic properties of the film is determined by the strength of the film-substrate interaction in various regions of the supercell. In particular, for the Fe-top site, where this interaction is weak, there is a small rumpling of the FeO layer which increases the interface separation. In correspondence of this structural modification, we observe a change in the work function, coherent with the most recent experimental findings. Our results show that a good agreement with the experimental interpretation of the work function modulation and of the STM images can be obtained only within the more robust, nonpseudomorphic computational models.

/pdf/interplay-between-structural-magnetic-and-electronic-3os1i0kbwf.pdf

Interplay between structural, magnetic, and electronic properties in a FeO/Pt(111) ultrathin film

The spatial distribution of single Au atoms on a thin FeO film has been investigated by low-temperature scanning tunneling microscopy and spectroscopy. The adatoms preferentially adsorb on distinct sites of the Moir\'e cell formed by the oxide layer and the Pt(111) support and arrange into a well-ordered hexagonal superlattice with 25 \AA{} lattice constant. The self-organization is the consequence of an inhomogeneous surface potential within the FeO Moir\'e cell and substantial electrostatic repulsion between the adatoms.

/pdf/self-organization-of-gold-atoms-on-a-polar-feo-111-surface-2quzvh7fcs.pdf

Self-organization of gold atoms on a polar FeO(111) surface.

We have studied a thin FeO film on Pt(111) using scanning tunneling microscopy. The corrugation of the Moir\'e pattern, that arises due to a lattice mismatch between the oxide film and the substrate, is found to increase dramatically when the microscope is operated in the field-emission regime. This contrast enhancement can be attributed to variations in the energy at which field-emission resonances are observed. We assign this effect to variations of the surface potential within the Moir\'e unit cell. Using a simple electrostatic model we show that in this oxide film with polar termination, such variations can be induced by very subtle differences in geometry.

Surface potential of a polar oxide film: FeO on Pt(111)

We present a combined experimental (STM/scanning tunneling spectroscopy) and theoretical (density functional theory) study on the deposition of Au and Pd metal atoms on FeO/Pt(111) ultrathin films. We show that while the Pd atoms are only slightly oxidized, the Au atoms form positive ions upon deposition, at variance to a charge transfer into the Au atoms as observed for MgO/Ag(100). The modulation of the adsorption properties within the surface Moire cell and the charging induce the formation a self-assembled array of gold adatoms on FeO/Pt(111), whereas Pd atoms are randomly distributed.

/pdf/charging-of-metal-adatoms-on-ultrathin-oxide-films-au-and-pd-3irqs3y1ea.pdf

Charging of metal adatoms on ultrathin oxide films: Au and Pd on FeO/Pt(111).

The electronic structure around the Fermi energy of a thin film of iron monoxide on Pt(111) is studied using low temperature scanning tunneling spectroscopy. The surface exhibits a coincidence structure (Moir\'e pattern) due to a lattice mismatch between the oxide thin film and the metal substrate. While the local density of states around the Fermi energy is featureless at most positions in the unit cell, a pronounced depression is observed at one particular site. The origin of this local zero-bias anomaly is discussed in relation to results from first-principle calculations. As a preliminary conclusion, it is suggested that the anomaly is a Kondo resonance due to a frustrated antiferromagnetic layer.

/pdf/local-zero-bias-anomaly-in-tunneling-spectra-of-a-transition-2fdyv8mkeh.pdf

Emile D. L. Rienks

Papers

Interplay between structural, magnetic, and electronic properties in a FeO/Pt(111) ultrathin film

Self-organization of gold atoms on a polar FeO(111) surface.

Surface potential of a polar oxide film: FeO on Pt(111)

Charging of metal adatoms on ultrathin oxide films: Au and Pd on FeO/Pt(111).

Local zero-bias anomaly in tunneling spectra of a transition-metal oxide thin film