Low-coordinated Surface Atoms Research Articles

Structures and Properties of As(OH)3 Adsorption Complexes on Hydrated Mackinawite (FeS) Surfaces: A DFT-D2 Study.

Reactive mineral–water interfaces exert control on the bioavailability of contaminant arsenic species in natural aqueous systems. However, the ability to accurately predict As surface complexation is limited by the lack of molecular-level understanding of As–water–mineral interactions. In the present study, we report the structures and properties of the adsorption complexes of arsenous acid (As(OH)3) on hydrated mackinawite (FeS) surfaces, obtained from density functional theory (DFT) calculations. The fundamental aspects of the adsorption, including the registries of the adsorption complexes, adsorption energies, and structural parameters are presented. The FeS surfaces are shown to be stabilized by hydration, as is perhaps to be expected because the adsorbed water molecules stabilize the low-coordinated surface atoms. As(OH)3 adsorbs weakly at the water–FeS(001) interface through a network of hydrogen-bonded interactions with water molecules on the surface, with the lowest-energy structure calculated to be an As–up outer-sphere complex. Compared to the water–FeS(001) interface, stronger adsorption was calculated for As(OH)3 on the water–FeS(011) and water–FeS(111) interfaces, characterized by strong hybridization between the S-p and O-p states of As(OH)3 and the surface Fe-d states. The As(OH)3 molecule displayed a variety of chemisorption geometries on the water–FeS(011) and water–FeS(111) interfaces, where the most stable configuration at the water–FeS(011) interface is a bidentate Fe–AsO–Fe complex, but on the water–FeS(111) interface, a monodentate Fe–O–Fe complex was found. Detailed information regarding the adsorption mechanisms has been obtained via projected density of states (PDOS) and electron density difference iso-surface analyses and vibrational frequency assignments of the adsorbed As(OH)3 molecule.

Step-Edge Assisted Direct Linear Alkane Coupling.

Direct coupling of alkanes via C-H activation of terminal methyl groups has acquired tremendous interests both scientifically and technically. Herein we present the results of linear alkane-coupling at the step edges of Cu surfaces at modulated temperatures. Combining the observations of scanning tunneling microscopy (STM) with density functional theory plus dispersion (DFT-D) calculations, we elucidate the mechanism of the reaction and demonstrate that the low activation barrier relies on heterogeneous catalysis at the upper step edges, where low-coordinated surface atoms are present. We further reveal the generality of the reaction, so that it can be applied on the step edges of different facets of surfaces.

Graphene-like holey Co3O4 nanosheets as a highly efficient catalyst for oxygen evolution reaction

Co3O4 nanosheets with a graphene-like holey structure are successfully synthesized through a bottom-up self-assembly approach and utilized as a catalyst for the oxygen evolution reaction (OER). This unique nanostructure possesses a large fraction of low-coordinated surface atoms and highly accessible surface areas due to its atomic thickness and mesoporosity, which could provide abundant active sites and facilitate the electrode/electrolyte contact for OER catalysis. In addition, density functional theory (DFT) calculations reveal that the loss of the neighboring layers gives the ultrathin nanostructure remarkable lattice distortion, which leads to decreased energy barriers for facile mass conversion and transfer on the surface of the catalyst. As a result, the graphene-like holey Co3O4 nanosheets exhibit excellent OER catalytic performance with low onset potential of 0.617 V vs. Hg/HgO, high current density of 12.26 mA cm-2 at 0.8 V vs. Hg/HgO, and long-term stability with negligible fading in current density after 2000 cycles, significantly outperforming the performances of conventional Co3O4 nanostructures and commercial IrO2. This unique graphene-like holey structure should be of great benefit for applications ranging from electronic devices to energy conversion and storage systems.

Hexagonal FeS nanosheets with high-energy (001) facets: Counter electrode materials superior to platinum for dye-sensitized solar cells

The catalytic activity of materials is highly dependent on their composition and surface structure, especially the density of low-coordinated surface atoms. In this work, we have prepared two-dimensional hexagonal FeS with high-energy (001) facets (FeS-HE-001) via a solution-phase chemical method. Nanosheets (NSs) with exposed high-energy planes usually possess better reaction activity, so FeS-HE-001 was used as a counter electrode (CE) material for dye-sensitized solar cells (DSSCs). FeS-HE-001 achieved an average power conversion efficiency (PCE) of 8.88% (with the PCE of champion cells being 9.10%), which was almost 1.15 times higher than that of the Pt-based DSSCs (7.73%) measured in parallel. Cyclic voltammetry and Tafel polarization measurements revealed the excellent electrocatalytic activities of FeS-HE-001 towards the I3–/I– redox reaction. This can be attributed to the promotion of photoelectron transfer, which was measured by electrochemical impedance spectroscopy and scanning Kelvin probe, and the strong I3– adsorption and reduction activities, which were investigated using first-principles calculations. The presence of high-energy (001) facets in the NSs was an important factor for improving the catalytic reduction of I3–. We believe that our method is a promising way for the design and synthesis of advanced CE materials for energy harvesting.

Identification of step-edge sites on Rh nanoparticles for facile CO dissociation

Understanding the dependence of the rate of catalytic reactions on metal nanoparticle size remains one of the great challenges in heterogeneous catalysis. Especially, methods to probe step-edge sites on technical supported nanoparticle catalysts are needed to put structure–activity relations on a surer footing. Herein, we demonstrate that N2 is a useful IR probe for the semi-quantitative identification of step-edge sites on zirconia-supported metallic Rh nanoparticles. The intensity of the strongly perturbed band at 2205cm−1 correlates with the CO bond dissociation rate under conditions relevant to the Fischer–Tropsch reaction. Due to the intermediate reactivity of Rh, step-edge sites are required to dissociate the strong CO bond. DFT calculations show that N2 prefers to adsorb on top of low-coordinated surface atoms such as steps, corners and edges. The occurrence of the intensity maximum at intermediate particle size is explained by the presence of surface overlayers on terraces that give rise to step-edges. These step-edge sites are important in the dissociation of di-atomic molecules such as CO, NO and N2.

Low-coordinated surface atoms of CuPt alloy cocatalysts on TiO2 for enhanced photocatalytic conversion of CO2.

We report the photocatalytic conversion of CO2 to CH4 using CuPt alloy nanoclusters anchored on TiO2. As the size of CuPt alloy nanoclusters decreases, the photocatalytic activity improves significantly. Small CuPt nanoclusters strongly bind CO2 intermediates and have a stronger interaction with the TiO2 support, which also contributes to an increased CH4 generation rate. The alloying and size effects prove to be the key to efficient CO2 reduction, highlighting a strategic platform for the design of photocatalysts for CO2 conversion.

Mechanistic Insights into the Electrochemical Reduction of CO2 to CO on Nanostructured Ag Surfaces

Electroreduction of CO2 in a highly selective and efficient manner is a crucial step toward CO2 utilization. Nanostructured Ag catalysts have been found to be effective candidates for CO2 to CO conversion. In this report, we combine experimental and computational efforts to explore the electrocatalytic reaction mechanism of CO2 reduction on nanostructured Ag catalyst surfaces in an aqueous electrolyte. In contrast to bulk Ag catalysts, both nanoparticle and nanoporous Ag catalysts show enhanced ability to reduce the activation energy of the CO2 to COOHads intermediate step through the low-coordinated Ag surface atoms, resulting in a reaction mechanism involving a fast first electron and proton transfer followed by a slow second proton transfer as the rate-limiting step.

Influence of Step Geometry on the Reconstruction of Stepped Platinum Surfaces under Coadsorption of Ethylene and CO.

We demonstrate the critical role of the specific atomic arrangement at step sites in the restructuring processes of low-coordinated surface atoms at high adsorbate coverage. By using high-pressure scanning tunneling microscopy (HP-STM) and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), we have investigated the reconstruction of Pt(332) (with (111)-oriented triangular steps) and Pt(557) surfaces (with (100)-oriented square steps) in the mixture of CO and C2H4 in the Torr pressure range at room temperature. CO creates Pt clusters at the step edges on both surfaces, although the clusters have different shapes and densities. A subsequent exposure to a similar partial pressure of C2H4 partially reverts the clusters on Pt(332). In contrast, the cluster structure is barely changed on Pt(557). These different reconstruction phenomena are attributed to the fact that the 3-fold (111)-step sites on Pt(332) allows for adsorption of ethylidyne-a strong adsorbate formed from ethylene-that does not form on the 4-fold (100)-step sites on Pt(557).

Catalysis by Unsupported Skeletal Gold Catalysts

Catalysis is one of the key technologies for the 21st century for achieving the required sustainability of chemical processes. Critical improvements are based on the development of new catalysts and catalytic concepts. In this context, gold holds great promise because it is more active and selective than other precious metal catalysts at low temperatures. However, gold becomes only chemically and catalytically active when it is nanostructured. Since the 1970s and 1980s, the first type of gold catalysts that chemists studied were small nanoparticles on oxidic supports. With the later onset of nanotechnology, a variety of nanostructured materials not requiring a support or organic stabilizers became available within about the last 10 years. Among these are gold nanofoams generated by combustion of gold compounds, nanotube membranes prepared by electroless deposition of gold inside a template, and corrosion-derived nanoporous gold. Even though these materials are macroscopic in their geometric dimensions (e.g., disks, cubes, and membranes with dimensions of millimeters), they are comprised of gold nanostructures, for example, in the form of ligaments as small as 15 nm in diameter (nanoporous gold, npAu). The nanostructure brings about a high surface to volume ratio and a large fraction of low coordinated surface atoms. In this Account, we discuss how unsupported materials are active catalysts for aerobic oxidation reaction in gas phase (oxidation of CO and primary alcohols), as well as liquid phase oxidation and reduction reactions. It turns out that the bonding and activation of molecular oxygen for gas phase oxidations strongly profits from trace amounts of an ad-metal residue such as silver. It is noteworthy that these catalysts still exhibit the special gold type chemistry, characterized by activity at very low temperatures and high selectivity for partial oxidations. For example, we can oxidize CO over these unsupported catalysts (npAu, nanotubes, and powder) at temperatures well below water's freezing point (-30 °C) and with turnover frequencies up to 0.5 s(-1) (at 30 °C). Yet, we can anticipate the surface chemistry of these unsupported and extended gold surfaces based on model experiments under UHV conditions. We have demonstrated this for the selective oxidation of primary alcohols at low temperatures employing npAu catalysts. Chemists have paid growing interest to oxidation and reduction reactions in liquid phase catalysis, most suitable for synthetic organic chemistry. Early work on the aerobic oxidation of d-glucose in 2008 using Raney type npAu already showed the potential of this type of catalyst for liquid phase reactions. Since then, researchers have investigated further oxidation reactions (silanes to silanols) and reduction reactions of alkynes, as well as C-C coupling reactions ([4 + 2] benzannulation) and azo compound decomposition, with likely several more reactions to be reported in the next years. The advantage of this unsupported skeletal type of catalyst is its recyclability and retrievability without leaching of gold into the reaction medium, owing to its monolithic structure. Even though these materials contain nanoscopic structures, they are macroscopic in their geometric dimensions and pose no threat to the environment or health as discussed for other nanomaterials.

Enhanced electro-oxidation of alcohols at electrochemically treated polycrystalline palladium surface

The present study demonstrates the effectiveness of an electrochemical treatment consisting in cycles of constant potential oxidation and reduction of polycrystalline palladium surface in the enhancement of the electro-oxidation of ethanol, ethylene glycol and glycerol. The rise of the activity after the treatment has been ascribed to the increase of both surface area and density of low coordination surface atoms. FTIR spectra showed that a change in the reaction products distribution also occurs, resulting, in some cases, in an increased tendency to cleave the C–C bond.

Spin-polarized ground states and ferromagnetic order induced by low-coordinated surface atoms and defects in nanoscale magnesium oxide

We investigate the effect of surface defects and the related low-coordinated surface atoms on the defect-induced magnetism in MgO nanocrystallites using hybrid density functional theory calculations. It has been demonstrated that when Mg vacancies are introduced at the surface or near surface of cubelike MgO clusters, a magnetic state ($S$ $\ensuremath{\ge}$ 1) becomes lower in total energy than the nonmagnetic singlet state ($S$ = 0) by several electron volts, resulting in the robust spin-polarized ground state. The total spin $S$ of the clusters in their ground state is equal to the number of the surface Mg vacancies introduced. The resulting spin density is not only located at the surrounding O atoms neighbor to the Mg vacancy site but is also extended to the low-coordinated surface O atoms along the $\ensuremath{\langle}110\ensuremath{\rangle}$ direction, forming ferromagneticlike domains. This directional spin delocalization allows a remote (\ensuremath{\sim}1 nm or longer) vacancy-vacancy interaction, eventually leading to a long-range ferromagnetic interaction.

Microkinetics of steam methane reforming on platinum and rhodium metal surfaces

We have investigated the most important elementary reaction steps in the steam methane reforming (SMR) process for planar and stepped Pt surfaces (dissociative CH4 adsorption, CHads–Oads recombination, H2O activation) and compared activation barriers for Rh surfaces. Compared to Rh, the lower reactivity of Pt results in (i) higher barriers for dissociative CH4 adsorption and (ii) endothermic formation of OHads and Oads. Microkinetic simulations show that Rh nanoparticle catalysts will be more active than Pt ones. The rate-controlling step is dissociative CH4 adsorption occurring on low-coordinated surface atoms (edges, corners, step-edges). The stepped surfaces are much more reactive than planar surfaces of the corresponding metals. For stepped Pt surfaces, CO formation via recombination of Cads+OHads is favored because of the low Oads coverage. At higher temperatures, deactivation may occur due to poisoning by carbonaceous species because the rate of OHads/Oads formation becomes too low compared to the rate of CHads formation. This occurs at lower temperature for Pt than for Rh because of the lower Pt–O bond energy.

On the Effect of Atomic Structure on the Activity and Deactivation of Catalytic Gold Nanoparticles

AbstractWe present studies of the structure and stability of catalytic gold nanoparticles through in situ aberration‐corrected electron microscopy to investigate the effect of heating on the nature of the identified atomic active sites. Low coordination surface atoms are replaced by atomically clean surface facets through local rearrangements to minimise surface energy. The associated movement of surface atoms is proposed to directly precede Ostwald ripening. Expansive surface strain resulting from inherently strained structures, such as the decahedra, is shown to diminish with increasing particle size and the associated elastic energy is reduced through a shifting of the disclination axis towards the particle surface. At elevated temperatures a reduction in surface energy anisotropy may lead to energetically favourable morphologies with minimal intrinsic strain. Such processes will act as structural deactivation mechanisms, resulting in a loss of active sites without any necessary associated loss of surface area or change in particle size through traditional sintering mechanisms. Considerations of the active site stability and the particle size according to the reaction conditions are described.

Thermodynamic properties of Pt nanoparticles: Size, shape, support, and adsorbate effects

This study presents a systematic investigation of the thermodynamic properties of free and \ensuremath{\gamma}-Al${}_{2}$O${}_{3}$-supported size-controlled Pt nanoparticles (NPs) and their evolution with decreasing NP size. A combination of in situ extended x-ray absorption fine-structure spectroscopy (EXAFS), ex situ transmission electron microscopy (TEM) measurements, and NP shape modeling revealed (i) a cross over from positive to negative thermal expansion with decreasing particle size, (ii) size- and shape-dependent changes in the mean square bond-projected bond-length fluctuations, and (iii) enhanced Debye temperatures (\ensuremath{\Theta}${}_{D}$, relative to bulk Pt) with a bimodal size-dependence for NPs in the size range of \ensuremath{\sim}0.8--5.4 nm. For large NP sizes (diameter $d$ >1.5 nm) \ensuremath{\Theta}${}_{D}$ was found to decrease toward \ensuremath{\Theta}${}_{D}$ of bulk Pt with increasing NP size. For NPs \ensuremath{\le} 1 nm, a monotonic decrease of \ensuremath{\Theta}${}_{D}$ was observed with decreasing NP size and increasing number of low-coordinated surface atoms. Our density functional theory calculations confirm the size- and shape-dependence of the vibrational properties of our smallest NPs and show how their behavior may be tuned by H desorption from the NPs. The experimental results can be partly attributed to thermally induced changes in the coverage of the adsorbate (${H}_{2}$) used during the EXAFS measurements, bearing in mind that the interaction of the Pt NPs with the stiff, high-melting temperature \ensuremath{\gamma}-Al${}_{2}$O${}_{3}$ support may also play a role. The calculations also provide good qualitative agreement with the trends in the mean square bond-projected bond-length fluctuations measured via EXAFS. Furthermore, they revealed that part of the \ensuremath{\Theta}${}_{D}$ enhancement observed experimentally for the smallest NPs ($d$ \ensuremath{\le} 1 nm) might be assigned to the specific sensitivity of EXAFS, which is intrinsically limited to bond-projected bond-length fluctuations.

The role of promoters for Ni catalysts in low temperature (membrane) steam methane reforming

In the search for active and stable Ni-based catalysts for steam methane reforming in membrane reactors, the effect of three different promoters La, B and Rh was compared. Promoted and unpromoted Ni catalysts were characterized by TEM, TPR and X-ray absorption spectroscopy. The average Ni particle size is between 4 and 10 nm. Promoters affected both dispersion and reducibility of Ni. Smaller particles were found to be more difficult to reduce than larger ones. The use of B resulted in very small Ni particles. The degree of Ni reduction strongly increased by use of La and Rh promoters, whereas B strongly impeded Ni reduction. The initial intrinsic rate per surface metal atom was found to increase linearly with the Ni metal dispersion, suggesting that the rate is controlled by dissociative methane adsorption over low-coordinated surface atoms. The data indicate that Rh and La act as structural promoters to the activity. Catalysts modified by B show a much higher activity of the Ni surface atoms. Catalyst stability was investigated by using feed compositions representing the inlet of the membrane reactor and the hydrogen lean reformate towards its outlet. Stability increases in the order La < Rh < B. Deactivation of the catalysts is caused by insufficient removal of carbon species from the surface of Ni particles and the formation of stable, graphitic carbon deposits, most likely covering the surface of metal. This is substantially suppressed when the Ni particles are small. B is an excellent structural promoter to obtain small Ni particles, Rh stabilizes metallic Ni and La aids in the removal of some of the carbon deposits more effectively by gasification.

Amine-Assisted Synthesis of Concave Polyhedral Platinum Nanocrystals Having {411} High-Index Facets

High-index surfaces of a face-centered cubic metal (e.g., Pd, Pt) have a high density of low-coordinated surface atoms and therefore possess enhanced catalysis activity in comparison with low-index faces. However, because of their high surface energy, the challenge of chemically preparing metal nanocrystals having high-index facets remains. We demonstrate in this work that introducing amines as the surface controller allows concave Pt nanocrystals having {411} high-index facets to be prepared through a facile wet-chemical route. The as-prepared Pt nanocrystals display a unique octapod morphology with {411} facets. The presence of high-index {411} exposed facets endows the concave Pt nanocrystals with excellent electrocatalytic activity in the oxidation of both formic acid and ethanol.

Dynamics of low-coordinated surface atoms on gold nanocrystallites

The authors highlight the importance of transient configurations of atoms on the surface of nanocrystallites, and present methodologies for their investigation. A Monte Carlo method has been developed and is used to simulate the thermodynamic equilibrium of nanometer sized Au nanocrystallites, both free and supported on a MgO(100) surface. The authors find that appreciable numbers of atoms transiently occupy adatom positions on Au(111) facets, even at room temperature. This type of dynamically appearing site is usually neglected in relation to catalysis but may have a significant activity (for CO oxidation, for example). They also observe a complex solid-solid roughening transition which involves a variety of transient local atom configurations on the surface of nanocrystallites.

Mechanism of NO x decomposition

The mechanism of NO x decomposition is reviewed. This reaction is first studied on metal or oxide single crystals, in order to get solid basis from catalysis on well-defined surface structures. If we understand few things about this reaction, at least the catalytic surface is known. This statement is not 100% correct, as the surface is reconstructed during the catalytic reaction; and this point is considered in the manuscript. Then we analyse this reaction on metallic catalysts supported on metal oxides. The crystallites are more or less dispersed, and the support can play the role of a modifier or of a promoter. A general consensus emerges on (i) the chemisorption of the reactant(s) which causes adsorbate-induced restructuring of the surfaces, (ii) the substrate, which is influenced by the redox properties of the support and by the particle sizes of the metallic aggregates, (iii) the fact that low-coordinated surface atoms are the more active in NO bond breaking, and (iv) the general pathways of the reaction, where redox mechanisms involving nitrosyl species, as intermediates, moving to nitrite-nitrosyl or dinitrosyl species are proposed. All these points are discussed.

Catalysis by Gold

Despite occasional references in the older literature to the ability of gold to catalyze certain reactions, the metal has until recently had the reputation of being one of the least catalytically useful. The recent discovery that some supported gold catalysts can affect the oxidation of carbon monoxide at or below ambient temperature has, however, focused attention on the metal's ability in this respect. For oxidation of carbon monoxide at low temperature, catalysts comprising small (<5 nm) gold particles supported preferably on an oxide of the first transition series (e.g., TiO2, α-Fe2O3) are needed. Deposition–precipitation and coprecipitation are better methods than impregnation for this purpose and provide the desired intimacy of contact between metal and support. High activity may well originate at sites at the gold–support interface, with the support making a vital contribution. Stable activity can result by optimizing aging in solution during the preparation, and low calcination temperatures are generally desired. Gold catalysts also have potential for both selective and nonselective oxidation of hydrocarbons, for methanol synthesis by hydrogenation of carbon monoxide or dioxide, for the water–gas shift, and for the reduction of nitric oxide by hydrogen, propene, or carbon monoxide. The hydrogenation of unsaturated hydrocarbons also occurs on highly dispersed gold catalysts. Supported gold chloride is the most active catalyst for the hydrochlorination of ethyne, the activity being correlated with the metal's high standard electrode potential. The remarkable catalytic behavior shown by gold depends on forming it into very small particles. This is because the massive metal and large particles cannot chemisorb typical reactant molecules to any useful extent; this only occurs when an adequate number of low-coordination surface atoms are present, ideally on particles so small that they lack full metallic character. The long neglect of gold as a catalyst is chiefly due to the failure to appreciate the necessity of creating particles that are sufficiently small and, for oxidations, of selecting a helpful support. Other relevant factors may be the likely high mobility of surface atoms on small particles and the electronegative character of gold, both stemming from the relativistic contraction of the s-electron orbitals. Attention is drawn to the uncertainties concerning reaction mechanisms and to areas for possible future development, such as use of promoters and bimetallic compositions and the extension of concepts already revealed to other reactions and metals (e.g., the platinum group metals). The practical application of gold catalysts in oxidations and other processes can be confidently expected.

On the evolution of cluster to bulk properties: a theoretical LCGTO-LDF study of free and coordinated Ni n clusters ( n=6-147)

The electronic properties of Ni clusters containing up to 147 atoms have been determined by all-electron LCGTO-LDF calculations. Both icosahedral and octahedral packings have been considered, the icosahedral being always more stable than the cuboctahedral. The cohesive energy of the clusters shows a linear dependence on n − sol1 3 , where n is the number of atoms in the cluster; the slope of the curve is the same as for experimental cohesive energies of gas-phase Ni clusters. The first ionization potential and the electron affinity of Ni clusters qualitatively follow the prediction of the classical droplet model. The Ni clusters exhibit average magnetic moments per atom which are higher than that of metallic Ni. This enhanced magnetization is due to the presence of many low-coordinated surface atoms in the clusters. The effect of a ligand shell on the electronic structure of the Ni clusters has also been considered. He atoms have been used to simulate one of the electronic effects induced by closed shell ligands like CO, i.e. the quenching of the cluster magnetic moment. It is shown that this magnetic quenching is due to the repulsive interaction with the ligand shell and is essentially restricted to the metal atoms on the cluster surface.