Oxygen and Hydroxyl Species Induce Multiple Reaction Pathways for the Partial Oxidation of Allyl Alcohol on Gold

Oxygen-Promoted on-Surface Synthesis of Polyboroxine Molecules

We present a protocol for the on-surface synthesis of polyboroxine molecules derived from boroxine molecules precursors. This process is promoted by oxygen species present on the Au(111) surface: oxygen atoms facilitate the detachment of naphthalene units of trinaphthyl-boroxine molecules and bridge two unsaturated boroxine centers to form a boroxine-O-boroxine chemical motif. X-ray spectroscopic characterization shows that, as the synthesis process proceeds, it progressively tunes the electronic properties of the interface, thus providing a promising route to control the electron level alignment.

Assembly and Healing: Capacitive and Conductive Plasmonic Interfacing via a Unified and Clean Wet Chemistry Route

Solution-based nanoparticle assembly represents a highly promising way to build functional metastructures based on a wealth of synthetic nanomaterial building blocks with well-controlled morphology and crystallinity. In particular, the involvement of DNA molecular programming in these bottom-up processes gradually helps the ambitious goal of customizable chemical nanofabrication. However, a fundamental challenge is to realize strong interunit coupling in an assembly toward emerging functions and applications. Herein, we present a unified and clean strategy to address this critical issue based on a H2O2-redox-driven "assembly and healing" process. This facile solution route is able to realize both capacitively coupled and conductively bridged colloidal boundaries, simply switchable by the reaction temperature, toward bottom-up nanoplasmonic engineering. In particular, such a "green" process does not cause surface contamination of nanoparticles by exogenous active metal ions or strongly passivating ligands, which, if it occurs, could obscure the intrinsic properties of as-formed structures. Accordingly, previously raised questions regarding the activities of strongly coupled plasmonic structures are clarified. The reported process is adaptable to DNA nanotechnology, offering molecular programmability of interparticle charge conductance. This work represents a new generation of methods to make strongly coupled nanoassemblies, offering great opportunities for functional colloidal technology and even metal self-healing.

Bimetallic TiO2 Nanoparticles for Lignin-Based Model Compounds Valorization by Integrating an Optocatalytic Flow-Microreactor

The challenge of improving the activity of TiO₂ by modifying it with metals and using it for targeted applications in microreactor environments is an active area of research. Recently, microreactors have emerged as successful candidates for many photocatalytic reactions, especially for the selective oxidation process. The current work introduces ultrasound-assisted catalyst deposition on the inner walls of a perfluoro-alkoxy alkane (PFA) microtube under mild conditions. We report Cu-Au/TiO₂ and Fe-Au/TiO₂ nanoparticles synthesized using the sol-gel method. The obtained photocatalysts were thoroughly characterized by UV-Vis diffuse-reflectance spectroscopy (DRS), high-resolution scanning electron microscopy (HR-SEM), high-resolution transmission electron microscopy (HR-TEM), X-ray diffraction analysis (XRD), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and N₂ physisorption. The photocatalytic activity under UV (375 nm) and visible light (515 nm) was estimated by the oxidation of lignin-based model aromatic alcohols in batch and fluoropolymer-based flow systems. The bimetallic catalyst exhibited improved photocatalytic selective oxidation. Herein, four aromatic alcohols were individually investigated and compared. In our experiments, the alcohols containing hydroxy and methoxy groups (coniferyl and vanillin alcohol) showed high conversion (93% and 52%, respectively) with 8% and 17% selectivity towards their respective aldehydes, with the formation of other side products. The results offer an insight into ligand-to-metal charge transfer (LMCT) complex formation, which was found to be the main reason for the activity of synthesized catalysts under visible light.

Insights into the electronic structure of hydroxyl on Ag(110) under near ambient conditions

Adsorbed hydroxyl is a key intermediate present in many catalytic reactions and electrochemical processes. In particular, hydroxyl adsorbed on noble metal surfaces has attracted attention due to its role in water-gas shift, selective oxidation of hydrocarbons and water splitting. In this work, from a well-defined oxygen covered Ag(110) surface with O-p(2 × 1) reconstruction, we prepared a fully hydroxylated surface phase in equilibrium with water and oxygen in the gas phase under near ambient conditions. In situ soft X-ray spectroscopy combined with density functional theory revealed distinctive modifications in the electronic structure of the adsorbate layer upon hydroxylation. We show that both the core and valence electronic states of OH adsorbates have higher binding energies relative to the Fermi level than the states for the O adsorbate. The OH orbitals interact with the d band of Ag giving rise to hybridized orbitals with bonding and anti-bonding symmetry, with larger energy splitting than the oxygen adsorbate.

Adsorption and oxidation of SO2 on the surface of TiO2 nanoparticles: the role of terminal hydroxyl and oxygen vacancy–Ti3+ states

The absorption and oxidation reactions of SO₂ on TiO₂ nanoparticles were investigated by using a flow chamber, synchrotron X-ray absorption near-edge structure and high resolution synchrotron X-ray photoelectron spectroscopy techniques.

In-situ graft-crosslinked gold nanoparticles with high-density surface defects and coated with a polytaurine membrane for the voltammetric determination of dopamine

Well-dispersed and graft-crosslinked gold nanoparticles (AuNPs) were synthesized by the reduction of tetrachloroaurate with hydrazine at room temperature. The AuNPs possess a high density of surface defects which is due to grafting of n-octanoic acid to polyvinylpyrrolidone. The physical and chemical properties of the resulting AuNPs were characterized by UV-vis, XRD, TEM/HRTEM, SAED, and XPS, respectively. The modified AuNPs were placed on a glassy carbon electrode (GCE) in an electropolymerized taurine layer to obtain a sensitive, selective, stable and rapid electrochemical dopamine sensor. The peak current, typically measured at 0.17 V (vs. SCE), increases linearly in the 1.0 to 120 μM dopamine concentration range, and the limit of detection (at S/N = 3) is 0.16 μM with a sensitivity of 2.94 μA·μM^-1·cm^-2. The sensor was successfully applied to the determination of dopamine in injections and spiked serum samples. The recoveries from spiked serum samples range from 97.5 to 102.4%, with RSDs ranging between 2.8 and 3.4%. Graphical abstract Schematic representation of a glassy carbon electrode modified with in-situ graft-crosslinked gold nanoparticles combined with an electropolymerized polytaurine membrane. The sensor exhibits excellent features towards dopamine determination.

Understanding Charge Transport in Carbon Nitride for Enhanced Photocatalytic Solar Fuel Production

Photocatalytic solar fuel production, for example, production of hydrogen via water-splitting, is an effective means of chemical storage of solar energy and provides a potential option for achieving a zero-emissions energy system. Conveniently, hydrogen can be converted back to electricity either via fuel cells or through combustion in gas turbines, or it can be mixed in low concentrations with natural gas or biogas for combustion in existing power plants. The cornerstone of a practical solar fuel production process is a stable, efficient, and scalable photocatalyst (a semiconductor material that accommodates photon absorption, charge carrier generation and transport, and catalytic reactions). Therefore, the quest for suitable photocatalyst materials is an ongoing process. Recently, carbon nitride (CN) has attracted widespread interest as a metal-free, earth-abundant, and highly stable photocatalyst. However, the catalytic efficiency of CN is not satisfactory because of its poor charge transport attributes. There is a direct relation between the photocatalytic efficiency and charge transport because the basic principle of light-promoted overall photodecomposition of water into H₂ and O₂ molecules (or, generally speaking, photoredox reactions) relies on separation and subsequent transfer of excited-state electron-hole pairs to relative redox couples. However, the excited states last for a very short time, typically nanoseconds to microseconds in liquids, and unless they are separated within this time frame, the excited-state electron-hole pairs undergo recombination with release of the captured light energy as heat or photon emission. To utilize light in a form other than heat or emitted photons by avoiding the recombination of excited-state electron-hole pairs, charged excitons must be scavenged before the absorption of subsequent photons to sustain a multielectron photoredox reaction. Otherwise, the extraction of charges becomes more difficult. This imposes a potential efficiency-limiting factor. An enhancement in water to hydrogen conversion efficiency in CN therefore requires the use of precious-metal cocatalysts (e.g., Pt) and sacrificial electron donor/acceptors to facilitate multielectron/multiproton transfers to overcome the high kinetic barriers. The use of Pt and sacrificial agents is not consistent with the notion of low-cost and sustainable hydrogen production from water. CN must overcome this dependence to stand out as a truly scalable photocatalyst. To make progress, the foremost requirement is to attain an in-depth understanding of the fundamental charge transport phenomena needed for the rational design of CN-based photocatalysts. In this Account, therefore, our aim is to provide a synopsis of current understanding and progress regarding charge-transport-related phenomena (e.g., recombination, trapping, transfer of charge carriers, etc.) and to discuss the effects of charge transport in enhancing the apparent quantum yield of hydrogen production in CN. This understanding is necessary to broaden the scope of CN for other catalytic applications, for example, efficient CO₂ reduction to methanol or methane, fixation of nitrogen to ammonia, and use as an active material in solar cells. We also identify research gaps and issues to be addressed for a more clear elucidation of charge-transport-related phenomena in CN. Thus, this Account may inspire new research opportunities for tuning the extrinsic/intrinsic photophysicochemical properties of CN by rational design to attain the most favorable properties for improved catalytic efficiency.

Theoretical Studies on the Direct Propylene Epoxidation Using Gold-Based Catalysts: A Mini-Review

Direct propylene epoxidation using Au-based catalysts is an important gas-phase reaction and is clearly a promising route for the future industrial production of propylene oxide (PO). For instance, gold nanoparticles or clusters that consist of a small number of atoms demonstrate unique and even unexpected properties, since the high ratio of surface to bulk atoms can provide new reaction pathways with lower activation barriers. Support materials can have a remarkable effect on Au nanoparticles or clusters due to charge transfer. Moreover, Au (or Au-based alloy, such as Au–Pd) can be loaded on supports to form active interfacial sites (or multiple interfaces). Model studies are needed to help probe the underlying mechanistic aspects and identify key factors controlling the activity and selectivity. The current theoretical/computational progress on this system is reviewed with respect to the molecular- and catalyst-level aspects (e.g., first-principles calculations and kinetic modeling) of propylene epoxidation over Au-based catalysts. This includes an analysis of H2 and O2 adsorption, H2O2 (OOH) species formation, epoxidation of propylene into PO, as well as possible byproduct formation. These studies have provided a better understanding of the nature of the active centers and the dominant reaction mechanisms, and thus, could potentially be used to design novel catalysts with improved efficiency.

Structural and electronic properties of bulk and low-index surfaces of zincblende PtC

Transition metal carbides have been extensively used in diverse applications over the past decade. Their versatility is in part thanks to their unique bonding, which displays a mixture of ionic, metallic and covalent character. While the bulk structure of zincblende (ZB) PtC has been investigated several times, a detailed understanding of the electronic and structural properties of its low-index surfaces is lacking. In this work, we present an ab initio investigation of the properties of five crystallographic ZB PtC surfaces (Pt/C-terminated PtC(1 0 0), PtC(1 1 0) and Pt/C-terminated PtC(1 1 1)). Upon geometry optimization, both polar and nonpolar surfaces undergo a mild interlayer relaxation, without extensive reconstructions. Calculated vacancy formation energies indicate facile C removal on the (1 1 1) surface while Pt-vacancy formation is endothermic. Finally, atomic O adsorption energies on all surfaces reveal a high affinity of the C-terminated surfaces towards this species.

Reliable and efficient reaction path and transition state finding for surface reactions with the growing string method

The computational challenge of fast and reliable transition state and reaction path optimization requires new methodological strategies to maintain low cost, high accuracy, and systematic searching capabilities. The growing string method using internal coordinates has proven to be highly effective for the study of molecular, gas phase reactions, but difficulties in choosing a suitable coordinate system for periodic systems has prevented its use for surface chemistry. New developments are therefore needed, and presented herein, to handle surface reactions which include atoms with large coordination numbers that cannot be treated using standard internal coordinates. The double-ended and single-ended growing string methods are implemented using a hybrid coordinate system, then benchmarked for a test set of 43 elementary reactions occurring on surfaces. These results show that the growing string method is at least 45% faster than the widely used climbing image-nudged elastic band method, which also fails to converge in several of the test cases. Additionally, the surface growing string method has a unique single-ended search method which can move outward from an initial structure to find the intermediates, transition states, and reaction paths simultaneously. This powerful explorative feature of single ended-growing string method is demonstrated to uncover, for the first time, the mechanism for atomic layer deposition of TiN on Cu(111) surface. This reaction is found to proceed through multiple hydrogen-transfer and ligand-exchange events, while formation of H-bonds stabilizes intermediates of the reaction. Purging gaseous products out of the reaction environment is the driving force for these reactions. © 2017 Wiley Periodicals, Inc.

Surface chemistry of group IB metals and related oxides

Catalytic surface chemistry of IB metals are reviewed with an attempt to bridge model catalysts and powder catalysts.

Selective oxidation of alcohols by supported gold nanoparticles: recent advances

The review presents a commercially important field of alcohol oxidation using gold nanoparticles. It systematically discusses scope and limitation of various supports on the activity and selectivity of catalyst.

Gold and gold-palladium alloy nanoparticles on heterostructured TiO2 nanobelts as plasmonic photocatalysts for benzyl alcohol oxidation

Plasmonic photocatalysts composed of Au and bimetallic Au-Pd alloy nanoparticles (NPs) on one-dimensional TiO2 nanobelts (TiO2-NBs) were used for the aerobic oxidation of benzyl alcohol under visible light irradiation. Remarkable light-promoted activity was observed for the as-synthesized M/TiO2-NB (M = Au, Au-Pd) nanostructures based on the TiO2(B)/anatase heterostructured nanobelt. The difference in band structure and the well matched interface between the TiO2(B) and anatase phases, coupled with the one-dimensional nanostructure, enable an enhanced charge transfer within the heterostructured nanobelt. This inter-phase charge transfer greatly facilitates the flow of hot electrons from the metal NPs to TiO2 and promotes benzyl alcohol oxidation. This efficient electron transfer was identified by the much higher photocurrent response measured for the Au/TiO2-NB nanostructure with the TiO2(B)/anatase heterojunction than those with either of the single phases under visible light irradiation. Alloying Au with Pd in Au-Pd/TiO2-NB results in a significant improvement in the visible light-promoted activity compared to the monometallic Au/TiO2-NB sample. It is supposed that the plasmon-mediated charge distribution within the alloy NPs is mainly responsible for the enhanced photocatalytic activity of the bimetallic nanostructures.

Au@UiO-66: a base free oxidation catalyst

We present the in situ synthesis of Au nanoparticles within the Zr based Metal Organic Framework, UiO-66.

Control of selectivity in allylic alcohol oxidation on gold surfaces: the role of oxygen adatoms and hydroxyl species

Gold catalysts display high activity and good selectivity for partial oxidation of a number of alcohol species.

Single-step preparation of two-dimensionally organized gold particles via ionic liquid/metal sputter deposition

Sputter deposition of Au onto an ionic liquid with a hydroxyl-functionalized cation produces a uniform monoparticle film on the liquid surface. The size of nanoparticles is controlled by the sputtering conditions.