Origins of the High Reactivity of Au Nanostructures Deduced from the Structure and Properties of Model Surfaces

In this chapter, experimental and theoretical studies on surface segregation in Ag-Au systems, including our own thermodynamic studies and molecular dynamics simulations of surface restructuring, on the basis of density functional theory are reviewed. The restructuring processes are triggered by adsorbed atomic O, which is supplied and consumed during catalysis. Experimental evidence points to the essential role of Ag impurities in nanoporous gold for activating O2. At the same time, increasing Ag concentration may be detrimental for the selectivity of partial oxidation. Understanding the role of silver requires a knowledge on its chemical state and distribution in the material. Recent studies using electron microscopy and photoelectron spectroscopy shed new light on this issue revealing a non-uniform distribution of residual Ag and co-existence of different chemical forms of Ag. We conclude by presenting an outlook on electromechanical coupling at Ag-Au surfaces, which shows a way to systematically tune the catalytic activity of bimetallic surfaces

Partial Oxidation of Methanol on Gold: How Selectivity Is Steered by Low-Coordinated Sites

Partial methanol oxidation proceeds with high selectivity to methyl formate (MeFo) on nanoporous gold (npAu) catalysts. As low-coordinated sites on npAu were suggested to affect the selectivity, we experimentally investigated their role in the isothermal selectivity for flat Au(111) and stepped Au(332) model surfaces using a molecular beam approach under well-defined conditions. Direct comparison shows that steps enhance desired MeFo formation and lower undesired overoxidation. DFT calculations reveal differences in oxygen distribution that enhance the barriers to overoxidation at steps. Thus, these results provide an atomic-level understanding of factors controlling the complex reaction network on gold catalysts, such as npAu

Nanoporous Gold: From Structure Evolution to Functional Properties in Catalysis and Electrochemistry

Nanoporous gold (NPG) is characterized by a bicontinuous network of nanometer-sized metallic struts and interconnected pores formed spontaneously by oxidative dissolution of the less noble element from gold alloys. The resulting material exhibits decent catalytic activity for low-temperature, aerobic total as well as partial oxidation reactions, the oxidative coupling of methanol to methyl formate being the prototypical example. This review not only provides a critical discussion of ways to tune the morphology and composition of this material and its implication for catalysis and electrocatalysis, but will also exemplarily review the current mechanistic understanding of the partial oxidation of methanol using information from quantum chemical studies, model studies on single-crystal surfaces, gas phase catalysis, aerobic liquid phase oxidation, and electrocatalysis. In this respect, a particular focus will be on mechanistic aspects not well understood, yet. Apart from the mechanistic aspects of catalysis, best practice examples with respect to material preparation and characterization will be discussed. These can improve the reproducibility of the materials property such as the catalytic activity and selectivity as well as the scope of reactions being identified as the main challenges for a broader application of NPG in target-oriented organic synthesis

First-Principles Study of the Electronic Properties and Thermal Expansivity of a Hybrid 2D Carbon and Boron Nitride Material

In an attempt to push the boundary of miniaturization, there has been a rising interest in two-dimensional (2D) semiconductors with superior electronic, mechanical, and thermal properties as alternatives for silicon-based devices. Due to their fascinating properties resulting from lowering dimensionality, hexagonal boron nitride (h-BN) and graphene are considered promising candidates to be used in the next generation of high-performance devices. However, neither h-BN nor graphene is a semiconductor due to a zero bandgap in the one case and a too large bandgap in the other case. Here, we demonstrate from first-principles calculations that a hybrid 2D material formed by cross-linking alternating chains of carbon and boron nitride (HCBN) shows promising characteristics combining the thermal merits of graphene and h-BN while possessing the electronic structure characteristic of a semiconductor. Our calculations demonstrate that the thermal properties of HCBN are comparable to those of h-BN and graphene (parent systems). HCBN is dynamically stable and has a bandgap of 2.43 eV. At low temperatures, it exhibits smaller thermal contraction than the parent systems. However, beyond room temperature, in contrast to the parent systems, it has a positive but finitely small linear-thermal expansion coefficient. The calculated isothermal bulk modulus indicates that at high temperatures, HCBN is less compressible, whereas at low temperatures it is more compressible relative to the parent systems. The results of our study are important for the rational design of a 2D semiconductor with good thermal properties

What Changes on the Inverse Catalyst? Insight From CO Oxidation on Au-Supported Ceria Nanoparticles Using Ab Initio Molecular Dynamics

Oxidation reactions catalyzed by Au nanoparticles supported on reducible oxides have been widely studied both experimentally and theoretically, whereas inverse catalysts, in which oxide nanoparticles are supported on metal surfaces, received considerably less attention. In both systems catalytic activity at metal – oxide interfaces can arise not only from each material contributing its functionality, but also from their interactions creating properties beyond the sum of individual components. Inverse catalysts may retain the synergy between the metal and oxide functionalities, while offering further specific advantages, e.g. a possibility to have better control over interfacial sites or to yield improved stability, activity, and selectivity. Our work provides the mechanism of O atom/vacancy diffusion-assisted Mars-van-Krevelen CO oxidation on gold-supported ceria nanoparticle through state-of-the-art ab initio molecular dynamic simulation studies

Carbon vacancy-assisted stabilization of individual Cu5 clusters on graphene. Insights from ab initio molecular dynamics

15 pags., 7 figs., 2 tabs. -- This article is part of the themed collection: Stability and properties of new-generation metal and metal-oxide clusters down to subnanometer scale . -- Supplementary movies: https://www.rsc.org/suppdata/d2/cp/d2cp05843j/d2cp05843j2.mp4 https://www.rsc.org/suppdata/d2/cp/d2cp05843j/d2cp05843j3.mp4 https://www.rsc.org/suppdata/d2/cp/d2cp05843j/d2cp05843j4.mp4 https://www.rsc.org/suppdata/d2/cp/d2cp05843j/d2cp05843j5.mp4 https://www.rsc.org/suppdata/d2/cp/d2cp05843j/d2cp05843j6.mp4 https://www.rsc.org/suppdata/d2/cp/d2cp05843j/d2cp05843j7.mp4Recent advances in synthesis and characterization methods have enabled the controllable fabrication of atomically precise metal clusters (AMCs) of subnanometer size that possess unique physical and chemical properties, yet to be explored. Such AMCs have potential applications in a wide range of fields, from luminescence and sensing to photocatalysis and bioimaging, making them highly desirable for further research. Therefore, there is a need to develop innovative methods to stabilize AMCs upon surface deposition, as their special properties are lost due to sintering into larger nanoparticles. To this end, dispersion-corrected density functional theory (DFT-D3) and ab initio molecular dynamics (AIMD) simulations have been employed. Benchmarking against high-level post-Hartree-Fock approaches revealed that the DFT-D3 scheme describes very well the lowest-energy states of clusters of five and ten atoms, Cu5 and Cu10. AIMD simulations performed at 400 K illustrate how intrinsic defects of graphene sheets, carbon vacancies, are capable of confining individual Cu5 clusters, thus allowing for their stabilization. Furthermore, AIMD simulations provide evidence on the dimerization of Cu5 clusters on defect-free graphene, in agreement with the ab initio predictions of (Cu5)n aggregation in the gas phase. The findings of this study demonstrate the potential of using graphene-based substrates as an effective platform for the stabilization of monodisperse atomically precise Cu5 clusters.This work has been partly supported by the Spanish Agencia Estatal de Investigacio´n (AEI) under Grant No. PID2020-117605GB-I00. This publication is also based upon work of COST Action CA21101 ‘‘Confined molecular systems: from a new generation of materials to the stars’’ (COSY) supported by COST (European Cooperation in Science and Technology). One of the authors (M. P. d. L. C.) is also greatly thankful for the support of the EU Doctoral Network PHYMOL 101073474 (project call reference HORIZON-MSCA-2021-DN-01). L. V. M. acknowledges the support from the South African National Research Foundation (NRF), grant 148775. The CESGA supercomputer center (Spain), the Spanish Supercomputing Network (RES), and the Centre for High Performance Computing (CHPC), South Africa, are acknowledged for having provided computational resources. We also acknowledge support of the publication fee by the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI).Peer reviewe

Ring-Opening Reactions of Methylcyclopentane over Metal Catalysts, M = Pt, Rh, Ir, and Pd: A Mechanistic Study from First-Principles Calculations

Using density functional calculations we studied the conversion of methylcyclopentane to its various ring-opening products, branched and unbranched hexanes, that is, 2-methylpentane and 3-methylpentane, as well as <i>n</i>-hexane. We examined four metal catalysts, M = Pt, Rh, Ir, and Pd, using slab models of flat M(111) and stepped M(211) surfaces, to describe terrace-rich large and defect-rich small M particles, respectively. As C–H bond activation and formation is rather independent of the particle structure, we focused on C–C bond scission which is expected to be structure sensitive. The barriers of C–C bond scission indeed vary from ∼20 kJ mol^–1 to ∼140 kJ mol^–1 on various sites of these metal surfaces. In general, lower activation energies were calculated for Rh and Ir surfaces, in agreement with the higher experimental activity of these two metals compared to Pt and Pd. From the calculated C–C bond breaking barriers, we were able to rationalize the selectivity toward different ring-opening products, as observed in experiments over the metal catalysts studied

Chemisorbed Oxygen on the Au(321) Surface Alloyed with Silver: A First-Principles Investigation

The adsorption of oxygen on kinked Au(321) slabs is investigated theoretically on the basis of density functional theory. On-surface, subsurface, and surface-oxide forms of O are analyzed and compared on pure gold and on the surfaces containing silver atoms. At low O coverage (0.1 ML) subsurface O species are shown to be unstable both thermodynamically and kinetically due to a low barrier for conversion to stronger bound on-surface chemisorbed oxygen. The presence of Ag in the near-surface region was shown to increase the binding strength of on-surface as well as subsurface O, but the activation barrier for releasing subsurface O to the surface remains essentially unaffected by the presence of Ag. At oxygen coverage 0.2 ML or higher, the most stable surface arrangements of O atoms are chain-like structures consisting of linear −O–Au–O– fragments. Subsurface O atoms being a part of such chains are significantly stabilized. We examine phase transitions between the clean surface and possible stable oxidized surface structures as a function of temperature and O₂ partial pressure. Ag atoms replacing Au on the Au(321) surface are shown to stabilize the O-covered surface with respect to the clean surface. Pre-existent chemisorbed atomic oxygen is predicted to facilitate the dissociation of molecular oxygen on the pure and alloyed gold surfaces

Tracking down the origin of peculiar vibrational spectra of aromatic self-assembled thiolate monolayers

Kankate L, Hamann T, Li S, et al. Tracking down the origin of peculiar vibrational spectra of aromatic self-assembled thiolate monolayers. PHYSICAL CHEMISTRY CHEMICAL PHYSICS. 2018;20(47):29918-29930.Several studies have previously observed surprisingly low frequencies for the C-H stretching modes of self-assembled monolayers (SAMs) prepared from aromatic thiols. The reason for this property has so far remained elusive. Therefore, we report a novel study of the vibrational spectra of SAMs prepared on Au from two different aromatic thiols, namely, 4-nitro-1,1-biphenyl-4-thiol (NBPT) and 4-aminothiophenol (ATP). The SAMs were prepared by vapor deposition (VD) in ultrahigh vacuum (UHV) as well as by the solution method (SM) and their quality was controlled by X-ray photoelectron spectroscopy (XPS). In addition, amino terminated SAMs were also obtained by electron irradiation and by chemical reduction of NBPT SAMs. Beside infrared reflection absorption spectroscopy (IRRAS), we have employed high resolution electron energy loss spectroscopy (HREELS), by which VD SAMs can be studied in situ, i.e. without exposing them to air. Hence, we can exclude possible contributions of solvent molecules to the vibrational spectra. Nonetheless, HREELS in fact reveals the same large red shift of the C-H stretching modes in the SAMs as also observed in ex situ IRRAS experiments. In contrast, HREELS for physisorbed ATP and ATP in a KBr pellet measured by transmission infrared spectroscopy exhibit the expected aromatic bands. Using a computational approach, we can exclude molecular packing effects as origin of this shift. Therefore, we propose chemical changes in the aromatic rings during SAM formation as an alternative explanation for the observed frequency shift. As another striking effect, the N-H stretching vibrational modes of the amino-terminated SAMs are extremely weak in both IRRAS and HREELS despite the fact that XPS confirms the presence of amino groups. A very weak signal is observed only in the case of an electron irradiated NBPT SAM. In contrast, an energy loss ascribed to the N-H stretching vibrations is clearly observed in HREELS of ATP physisorbed on an ATP SAM and on graphite as well as in the transmission infrared spectrum of ATP in KBr. The extremely low intensity of these vibrations in the SAM is traced back to the inherently low transition dipole moment for the excitation of N-H stretching modes in free N-H groups. Furthermore, the calculations suggest that the much stronger signals of N-H stretching modes involved in hydrogen-bonding with adjacent amino groups are suppressed because these vibrations are oriented parallel to the surface