Water-mediated proton hopping on an iron oxide surface

Toward High-Performance Hydrogenation at Room Temperature Through Tailoring Nickel Catalysts Stable in Aqueous Solution

The development of highly active, reusable catalysts for aqueous-phase reactions is challenging. Herein, metallic nickel is encapsulated in a nitrogen-doped carbon-silica composite (SiO2@Ni@NC) as a catalyst for the selective hydrogenation of vanillin in aqueous media. The constructed catalyst achieved 99.8% vanillin conversion and 100% 4-hydroxymethyl-2-methoxyphenol selectivity at room temperature. Based on combined scanning transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman analyses, the satisfactory catalytic performance is attributed to the composite structure consisting of an active metal, carbon, and silica. The hydrophilic silica core promoted dispersion of the catalyst in aqueous media. Moreover, the external hydrophobic NC layer has multiple functions, including preventing oxidation or leaching of the internal metal, acting as a reducing agent to reduce the internal metal, regulating the active-site microenvironment by enriching the concentrations of H2 and organic reactants, and modifying the electronic structure of the active metal via metal-support interactions. Density functional theory calculations indicated that NC facilitates vanillin adsorption and hydrogen dissociation to promote aqueous-phase hydrogenation. This study provides an efficient strategy for constructing encapsulated Ni-based amphiphilic catalysts to upgrade biomass-derived compounds.

Probing Catalytic Sites and Adsorbate Spillover on Ultrathin FeO2-x Film on Ir(111) during CO Oxidation

The spatially resolved identification of active sites on the heterogeneous catalyst surface is an essential step toward directly visualizing a catalytic reaction with atomic scale. To date, ferrous centers on platinum group metals have shown promising potential for low-temperature CO catalytic oxidation, but the temporal and spatial distribution of active sites during the reaction and how molecular-scale structures develop at the interface are not fully understood. Here, we studied the catalytic CO oxidation and the effect of co-adsorbed hydrogen on the FeO2-x/Ir(111) surface. Combining scanning tunneling microscopy (STM), isotope-labeled pulse reaction measurements, and DFT calculations, we identified both FeO2/Ir and FeO2/FeO sites as active sites with different reactivity. The trilayer O-Fe-O structure with its Moiré pattern can be fully recovered after O2 exposure, where molecular O2 dissociates at the FeO/Ir interface. Additionally, as a competitor, dissociated hydrogen migrates onto the oxide film with the formation of surface hydroxyl and water clusters down to 150 K.

Crystal Plane Engineering to Boost Water Cluster Evaporation for Enhanced Solar Steam Generation

Polymer based low evaporation enthalpy materials have become a universal selection for improving the efficiency of solar steam generation. Although water cluster and intermediate water mechanisms have been proposed to explain the low evaporation enthalpy, the production process and microstructure of activated water are still unclear. Here, crystal plane engineering is used to investigate the intermediate water state and the water cluster activation mechanism. The unique open-closed coordination structure on the optimized crystal surface promotes the generation of firm water clusters by optimizing the intermediate water state. Under the similar solar energy absorption of all materials, crystal plane engineering increased the solar steam generation rate of the evaporator by 31.2% and increased the energy efficiency to 94.8%. Exploring the micro-evaporation process and activated water structure is expected to stimulate the development of the next generation low evaporation enthalpy materials.

Role of H2O in Catalytic Conversion of C1 Molecules

Due to their role in controlling global climate change, the selective conversion of C1 molecules such as CH4, CO, and CO2 has attracted widespread attention. Typically, H2O competes with the reactant molecules to adsorb on the active sites and therefore inhibits the reaction or causes catalyst deactivation. However, H2O can also participate in the catalytic conversion of C1 molecules as a reactant or a promoter. Herein, we provide a perspective on recent progress in the mechanistic studies of H2O-mediated conversion of C1 molecules. We aim to provide an in-depth and systematic understanding of H2O as a promoter, a proton-transfer agent, an oxidant, a direct source of hydrogen or oxygen, and its influence on the catalytic activity, selectivity, and stability. We also summarize strategies for modifying catalysts or catalytic microenvironments by chemical or physical means to optimize the positive effects and minimize the negative effects of H2O on the reactions of C1 molecules. Finally, we discuss challenges and opportunities in catalyst design, characterization techniques, and theoretical modeling of the H2O-mediated catalytic conversion of C1 molecules.

Proton Relay for the Rate Enhancement of Electrochemical Hydrogen Reactions at Heterogeneous Interfaces

Proton transfer is critically important to many electrocatalytic reactions, and directed proton delivery could open new avenues for the design of electrocatalysts. However, although this approach has been successful in molecular electrocatalysis, proton transfer has not received the same attention in heterogeneous electrocatalyst design. Here, we report that a metal oxide proton relay can be built within heterogeneous electrocatalyst architectures and improves the kinetics of electrochemical hydrogen evolution and oxidation reactions. The volcano-type relationship between activity enhancement and pKa of amine additives confirms this improvement; we observe maximum rate enhancement when the pKa of a proton relay matches the pH of the electrolyte solution. Density-functional-theory-based reactivity studies reveal a decreased proton transfer energy barrier with a metal oxide proton relay. These findings demonstrate the possibility of controlling the proton delivery and enhancing the reaction kinetics by tuning the chemical properties and structures at heterogeneous interfaces.

Water-assisted hydrogen spillover in Pt nanoparticle-based metal-organic framework composites

Hydrogen spillover is the migration of activated hydrogen atoms from a metal particle onto the surface of catalyst support, which has made significant progress in heterogeneous catalysis. The phenomenon has been well researched on oxide supports, yet its occurrence, detection method and mechanism on non-oxide supports such as metal-organic frameworks (MOFs) remain controversial. Herein, we develop a facile strategy for efficiency enhancement of hydrogen spillover on various MOFs with the aid of water molecules. By encapsulating platinum (Pt) nanoparticles in MOF-801 for activating hydrogen and hydrogenation of C=C in the MOF ligand as activated hydrogen detector, a research platform is built with Pt@MOF-801 to measure the hydrogenation region for quantifying the efficiency and spatial extent of hydrogen spillover. A water-assisted hydrogen spillover path is found with lower migration energy barrier than the traditional spillover path via ligand. The synergy of the two paths explains a significant boost of hydrogen spillover in MOF-801 from imperceptible existence to spanning at least 100-nm-diameter region. Moreover, such strategy shows universality in different MOF and covalent organic framework materials for efficiency promotion of hydrogen spillover and improvement of catalytic activity and antitoxicity, opening up new horizons for catalyst design in porous crystalline materials.

Revealing Intramolecular Isotope Effects with Chemical-Bond Precision

Isotope substitution of a molecule not only changes its vibrational frequencies but also changes its vibrational distributions in real-space. Quantitatively measuring the isotope effects inside a polyatomic molecule requires both energy and spatial resolutions at the single-bond level, which has been a long-lasting challenge in macroscopic techniques. By achieving ångström resolution in tip-enhanced Raman spectroscopy (TERS), we record the corresponding local vibrational modes of pentacene and its fully deuterated form, enabling us to identify and measure the isotope effect of each vibrational mode. The measured frequency ratio ν_H/ν_D varies from 1.02 to 1.33 in different vibrational modes, indicating different isotopic contributions of H/D atoms, which can be distinguished from TERS maps in real-space and well described by the potential energy distribution simulations. Our study demonstrates that TERS can serve as a non-destructive and highly sensitive methodology for isotope detection and recognition with chemical-bond precision.

Relationship between oxide identity and electrocatalytic activity of platinum for ethanol electrooxidation in perchlorate acidic solution

Water and its dissociated species at the solid‒liquid interface play critical roles in catalytic science; e.g., functions of oxygen species from water dissociation are gradually being recognized. Herein, the relationship between oxide identity (PtOH_ads, PtO_ads, and PtO₂) and electrocatalytic activity of platinum for ethanol electrooxidation was obtained in perchlorate acidic solution over a wide potential range with an upper potential of 1.5 V (reversible hydrogen electrode, RHE). PtOH_ads and α-PtO₂, rather than PtO_ads, act as catalytic centers promoting ethanol electrooxidation. This relationship was corroborated on Pt(111), Pt(110), and Pt(100) electrodes, respectively. A reaction mechanism of ethanol electrooxidation was developed with DFT calculations, in which platinum oxides-mediated dehydrogenation and hydrated reaction intermediate, geminal diol, can perfectly explain experimental results, including pH dependence of product selectivity and more active α-PtO₂ than PtOH_ads. This work can be generalized to the oxidation of other substances on other metal/alloy electrodes in energy conversion and electrochemical syntheses.

Ultralong Distance Hydrogen Spillover Enabled by Valence Changes in a Metal Oxide Surface

Hydrogen spillover is a phenomenon in which hydrogen atoms generated on metal catalysts diffuse onto catalyst supports. This phenomenon offers reaction routes for functional materials. However, due to difficulties in visualizing hydrogen, the fundamental nature of the phenomenon, such as how far hydrogen diffuses, has not been well understood. Here, in this study, we fabricated catalytic model systems based on Pd-loaded SrFeO_x (x ∼ 2.8) epitaxial films and investigated hydrogen spillover. We show that hydrogen spillover on the SrFeO_x support extends over long distances (∼600 μm). Furthermore, the hydrogen-spillover-induced reduction of Fe⁴⁺ in the support yields large energies (as large as 200 kJ/mol), leading to the spontaneous hydrogen transfer and driving the surprisingly ultralong hydrogen diffusion. These results show that the valence changes in the supports' surfaces are the primary factor determining the hydrogen spillover distance. Our study leads to a deeper understanding of the long-debated issue of hydrogen spillover and provides insight into designing catalyst systems with enhanced properties.

Aldehyde Hydrogenation by Pt/TiO2 Catalyst in Aqueous Phase: Synergistic Effect of Oxygen Vacancy and Solvent Water

The aldehyde hydrogenation for stabilizing and upgrading biomass is typically performed in aqueous phase with supported metal catalysts. By combining density functional theory calculations and ab initio molecular dynamics simulations, the model reaction of formaldehyde hydrogenation with a Pt/TiO₂ catalyst is investigated with explicit solvent water molecules. In aqueous phase, both the O vacancy (Ov) on support and solvent molecules could donate charges to a Pt cluster, where the Ov could dominantly reduce the Pt cluster from positive to negative. During the formaldehyde hydrogenation, the water molecules could spontaneously protonate the O in the aldehyde group by acid/base exchange, generating the OH* at the metal-support interface by long-range proton transfer. By comparing the stoichiometric and reduced TiO₂ support, it is found that the further hydrogenation of OH* is hard on the positively charged Pt cluster over stoichiometric TiO₂. However, with the presence of Ov on reduced support, the OH* hydrogenation could become not only exergonic but also kinetically more facile, which prohibits the catalyst from poisoning. This mechanism suggests that both the proton transfer from solvent water molecules and the easier OH* hydrogenation from Ov could synergistically promote aldehyde hydrogenation. That means, even for such simple hydrogenation in water, the catalytic mechanism could explicitly relate to all of the metal cluster, oxide support, and solvent waters. Considering the ubiquitous Ov defects in reducible oxide supports and the common aqueous environment, this synergistic effect may not be exclusive to Pt/TiO₂, which can be crucial for supported metal catalysts in biomass conversion.

High Photoreactivity on a Reconstructed Anatase TiO2(001) Surface Predicted by Ab Initio Nonadiabatic Molecular Dynamics

Anatase TiO₂(001) surface with (4 × 1) reconstruction is proposed to be a highly active catalytic surface. In this work, using time-domain ab initio nonadiabatic molecular dynamics, we reveal that the ridge structure formed by anatase(001) surface reconstruction is the photoreactive site for hole migration and trapping. Moreover, the ridge structure is destroyed by low-coverage CH₃OH adsorption, leading to the suppression of its high photoreactivity. However, when the CH₃OH coverage is increased and intermolecular hydrogen bonds (H-bonds) form, the ridge structure and its high photoreactivity are restored. Furthermore, the hole trapping dynamics is strongly coherent with intermolecular proton transfer in structures with intermolecular H-bonds. Our study proves that anatase TiO₂(001)-(4 × 1) is a highly photoreactive surface where the ridge is the photoreactive site for hole trapping, which is coherent with the proton transfer process.

A high-speed variable-temperature ultrahigh vacuum scanning tunneling microscope with spiral scan capabilities

We present the design and development of a variable-temperature high-speed scanning tunneling microscope (STM). The setup consists of a two-chamber ultra-high vacuum system, including a preparation and a main chamber. The preparation chamber is equipped with standard preparation tools for sample cleaning and film growth. The main chamber hosts the STM that is located within a continuous flow cryostat for counter-cooling during high-temperature measurements. The microscope body is compact, rigid, and highly symmetric to ensure vibrational stability and low thermal drift. We designed a hybrid scanner made of two independent tube piezos for slow and fast scanning, respectively. A commercial STM controller is used for slow scanning, while a high-speed Versa Module Eurocard bus system controls fast scanning. Here, we implement non-conventional spiral geometries for high-speed scanning, which consist of smooth sine and cosine signals created by an arbitrary waveform generator. The tip scans in a quasi-constant height mode, where the logarithm of the tunneling current signal can be regarded as roughly proportional to the surface topography. Scan control and data acquisition have been programmed in the experimental physics and industrial control system framework. With the spiral scans, we atomically resolved diffusion processes of oxygen atoms on the Ru(0001) surface and achieved a time resolution of 8.3 ms per frame at different temperatures. Variable-temperature measurements reveal an influence of the temperature on the oxygen diffusion rate.

Photocatalytic hydrogen evolution by degradation of organic pollutants over quantum dots doped nitrogen carbide

Semiconductor photocatalysts are of great importance for addressing current environmental and energy crises. In this study, we developed a simple exfoliation-sonication route to fabricate nitrogen carbide quantum dots (CNQDs) doped nitrogen carbide nanosheet (CNS) composite photocatalysts which were employed to produce hydrogen and degrade organic pollutants (methyl orange, acridine orange, aniline, and phenol) synchronously under visible light irradiation. The presence of acridine orange and aniline enhanced the hydrogen evolution efficiency from 8.8 mmol g^-1 h^-1 to 32.1 and 11.7 mmol g^-1 h^-1, respectively. On the contrary, methyl orange and phenol with the same concentration inhibited hydrogen evolution. Based on the proton chain and energy band analyses, the synchronous mechanism of photocatalytic hydrogen evolution and organic pollutant degradation on CNQDs/CNS was also proposed. On one side, the oxygen-containing functional groups on the surface of CNQDs and the surrounded water molecules constructed proton chains, increasing the combination probability between protons and photo-generated electrons. On the other side, the heterojunction of CNQDs/CNS induced the separation of photo-generated electron-hole pairs. The photo-generated electrons migrate to CNQDs, on which the protons were transformed into hydrogen molecules, while the holes migrated to CNS where the organic pollutants were oxidized synchronously.

Water dissociation on mixed Co-Fe oxide bilayer nanoislands on Au(111)

We investigate the hydroxylation behaviour of mixed Co-Fe oxide nanoislands synthesized on a Au(111) surface under exposure to water vapour at vacuum conditions. The pure Co and Fe bilayer oxides both become hydroxylated by water exposure in vacuum conditions, albeit to a very different extent. It is however an open question how mixed oxides, exposing sites with a mixed coordination to Fe and Co, behave. By forming surface O species with a mixed Fe/Co coordination, we can investigate the nature of such sites. By means of scanning tunnelling microscopy and x-ray photoelectron spectroscopy, we characterize a series of Co-Fe oxides samples with different Fe contents at the atomic scale and observe a scaling of the hydroxylation degree with the amount of Fe inside the Co-Fe oxides. Our results indicate that the Fe dopants within the Co-Fe oxides have opposing effects on edge and basal plane sites modifying the maximum hydroxylation degree of pure cobalt oxide, perturbing the original binding sites of H, releasing the absorbed H or blocking the diffusion pathway of H.

Unprecedentedly high activity and selectivity for hydrogenation of nitroarenes with single atomic Co1-N3P1 sites

Transition metal single atom catalysts (SACs) with M₁-N_x coordination configuration have shown outstanding activity and selectivity for hydrogenation of nitroarenes. Modulating the atomic coordination structure has emerged as a promising strategy to further improve the catalytic performance. Herein, we report an atomic Co₁/NPC catalyst with unsymmetrical single Co₁-N₃P₁ sites that displays unprecedentedly high activity and chemoselectivity for hydrogenation of functionalized nitroarenes. Compared to the most popular Co₁-N₄ coordination, the electron density of Co atom in Co₁-N₃P₁ is increased, which is more favorable for H₂ dissociation as verified by kinetic isotope effect and density functional theory calculation results. In nitrobenzene hydrogenation reaction, the as-synthesized Co₁-N₃P₁ SAC exhibits a turnover frequency of 6560 h⁻¹, which is 60-fold higher than that of Co₁-N₄ SAC and one order of magnitude higher than the state-of-the-art M₁-N_x-C SACs in literatures. Furthermore, Co₁-N₃P₁ SAC shows superior selectivity (>99%) toward many substituted nitroarenes with co-existence of other sensitive reducible groups. This work is an excellent example of relationship between catalytic performance and the coordination environment of SACs, and offers a potential practical catalyst for aromatic amine synthesis by hydrogenation of nitroarenes.

Modulating the atomic coordination structure has emerged as a promising strategy to further improve catalytic performance. Here, the authors report an atomic Co1/NPC catalyst with unsymmetrical single Co1N3P1 sites that displays high activity and chemoselectivity for hydrogenation of functionalized nitroarenes.

New monoclinic ruthenium dioxide with highly selective hydrogenation activity

HxRuO2 acts as a standalone catalyst exhibiting selective hydrogenation under mild conditions. Mobile protons embedded in the oxide lattice play an important role in stabilizing the distorted structure, and facile proton dynamics is key to improving catalytic properties.

Electrochemical Storage of Atomic Hydrogen on Single Layer Graphene

If hydrogen can be stored and carried safely at a high density, hydrogen-fuel cells offer effective solutions for vehicles. The stable chemisorption of atomic hydrogen on single layer graphene (SLG) seems a perfect solution in this regard, with a theoretical maximum storage capacity of 7.7 wt %. However, generating hydrogenated graphene from H₂ requires extreme temperatures and pressures. Alternatively, hydrogen adatoms can easily be produced under mild conditions by the electroreduction of protons in solid/liquid systems. Graphene is electrochemically inert for this reaction, but H-chemisorption on SLG can be carried out under mild conditions via a novel Pt-electrocatalyzed "spillover-surface diffusion-chemisorption" mechanism, as we demonstrate using dynamic electrochemistry and isotopic Raman spectroscopy. The apparent surface diffusion coefficient (∼10^-5 cm² s^-1), capacity (∼6.6 wt %, ∼85.7% surface coverage), and stability of hydrogen adatoms on SLG at room temperature and atmospheric pressure are significant, and they are perfectly suited for applications involving stored hydrogen atoms on graphene.

Dynamic Optical Visualization of Proton Transport Pathways at Water-Solid Interfaces

Probing proton transport is of vital importance for understanding cellular transport, surface catalysis and fuel cells. Conventional proton transport measurements rely on the use of electrochemical conductivity and do not allow for the direct visualization of proton transport pathways. The development of novel experimental techniques to spatiotemporally resolve proton transport is in high demand. Here, building upon the general conversion of aqueous proton flux into spatially resolved fluorescence signals, we optically visualize proton transport through nanopores and along hydrophilic interfaces. We observed that the fluorescence intensity increased at negative voltage due to lateral transport. Thanks to the temporal resolution of optical imaging, our technique further empowers the analysis of proton transport dynamics.

Selective Hydrogenation of the Carbonyls in Furfural and 5-Hydroxymethylfurfural Catalyzed by PtNi Alloy Supported on SBA-15 in Aqueous Solution Under Mild Conditions

Valuable furfuryl alcohol (FFA) and 2,5-dihydroxymethylfuran (DHMF) could be produced by selective hydrogenation of biomass-derived furfural (FF) and 5-hydroxymethylfurfural (HMF) with high atom economy. In this study, SBA-15 (a kind of mesoporous silica molecular sieve)-supported low metal loading (3 wt% total metal content) PtNi alloy catalyst (PtNi/SBA-15) was synthesized via two steps, including the generation of PtNi alloy by hydrothermal method, and the immobilization of PtNi alloy on SBA-15. PtNi/SBA-15 has ordered mesoporous structure with high surface area, and high dispersion of the PtNi alloy with the formation of Pt^δ--Ni^δ+ surface pairs on SBA-15, which benefit hydrogen activation and selective carbonyl hydrogenation. The selective hydrogenation of FF and HMF over PtNi/SBA-15 in water solvent at 303 K with 1.5 MPa H₂ within 2 h, could respectively yield 64.6% FFA with 77.0% selectivity, and 68.2% DHMF with 81.9% selectivity. Besides, PtNi/SBA-15 exhibited a satisfactory water resistance and stability after recycling at least five runs.

Spot the difference: hydrogen adsorption and dissociation on unsupported platinum and platinum-coated transition metal carbides

Hydrogenation reactions are involved in several processes in heterogeneous catalysis. Platinum is the best-known catalyst; however, there are limitations to its practical use. Therefore, it is necessary to explore alternative materials and transition metal carbides (TMCs) have emerged as potential candidates. We explore the possibility of using cheap TMCs as supports for a Pt monolayer, aiming to reduce the amount of the noble metal in the catalyst without a significant loss of its activity towards H₂ dissociation. Hence, analyzing H₂ dissociation from a fundamental point of view is a necessary step towards a further practical catalyst. By means of periodic DFT calculations, we analyze H₂ adsorption and dissociation on Pt/β-Mo₂C and Pt/α-WC surfaces, as a function of hydrogen surface coverage (Θ_H), resembling a more realistic model of a catalyst. H₂ dissociation rates were analyzed as a function of the reaction temperature. The results show that Pt/C-WC and Pt/Mo-Mo₂C have a Pt-like behavior for H₂ dissociation at Θ_H > 1/2 ML. At a particular temperature of 298 K, Pt/C-WC and Pt/Mo-Mo2C have low energy barriers for H₂* → 2H* (0.13 and 0.11 eV, respectively), close to the value of Pt (0.06 eV). For the highest coverage, i.e. Θ_H = 1, Pt/C-WC has a lower activation energy and a higher reaction rate than Pt. Finally, the H₂ dissociation rate is higher in Pt/Mo-Mo₂C than in Pt when increasing the temperature above 298 K. Our results put Pt/C-WC and Pt/Mo-Mo₂C under the spotlight as potential catalysts for H₂ dissociation, with a similar performance to Pt, paving the way for further experimental and/or theoretical studies, addressing the capability of Pt/TMC as practical catalysts in hydrogenation reactions.

Recognition of Water-Induced Effects toward Enhanced Interaction between Catalyst and Reactant in Alcohol Oxidation

Pickering emulsion stabilized by solid nanoparticles provides a diverse solvent microenvironment and enables to promote the phase transfer of reaction substrates/products in catalytic reactions, but the intrinsic role of solvent is still not clear. Herein, using benzyl alcohol (BA) as a model reactant, we demonstrate the nature of the water-promoted activity for alcohol oxidation over the Pd/MgAl-LDO catalyst. Depending on the water in the solvent, we observe different reactivities regarding the proportion of the water in the system. Kinetic isotope effects confirm the participation and positive effects of water for oxidation of BA. The water promotion effects are recognized and identified by the water vapor pulse adsorption coupled with temperature program desorption. Moreover, the adsorption behavior of BA or benzaldehyde at the interface of water and Pd/MgAl-LDO is also investigated by quasi-in-situ Raman spectroscopy. In addition, the mechanism of water-promoted alcohol oxidation is rationally proposed based on the Langmuir-Hinshelwood mechanism. The general applicability of the water promotion effects is further demonstrated over different supports and substrates, which well achieves excellent catalytic activity and selectivity in Pickering emulsion compared to that in the pure toluene system.

Kinetics of H2 Adsorption at the Metal-Support Interface of Au/TiO2 Catalysts Probed by Broad Background IR Absorbance

H₂ adsorption on Au catalysts is weak and reversible, making it difficult to quantitatively study. We demonstrate H₂ adsorption on Au/TiO₂ catalysts results in electron transfer to the support, inducing shifts in the FTIR background. This broad background absorbance (BBA) signal is used to quantify H₂ adsorption; adsorption equilibrium constants are comparable to volumetric adsorption measurements. H₂ adsorption kinetics measured with the BBA show a lower E_app value (23 kJ mol^-1 ) for H₂ adsorption than previously reported from proxy H/D exchange (33 kJ mol^-1 ). We also identify a previously unreported H-O-H bending vibration associated with proton adsorption on electronically distinct Ti-OH metal-support interface sites, providing new insight into the nature and dynamics of H₂ adsorption at the Au/TiO₂ interface.

A tailored multi-functional catalyst for ultra-efficient styrene production under a cyclic redox scheme

Styrene is an important commodity chemical that is highly energy and CO2 intensive to produce. We report a redox oxidative dehydrogenation (redox-ODH) strategy to efficiently produce styrene. Facilitated by a multifunctional (Ca/Mn)1-xO@KFeO2 core-shell redox catalyst which acts as (i) a heterogeneous catalyst, (ii) an oxygen separation agent, and (iii) a selective hydrogen combustion material, redox-ODH auto-thermally converts ethylbenzene to styrene with up to 97% single-pass conversion and >94% selectivity. This represents a 72% yield increase compared to commercial dehydrogenation on a relative basis, leading to 82% energy savings and 79% CO2 emission reduction. The redox catalyst is composed of a catalytically active KFeO2 shell and a (Ca/Mn)1-xO core for reversible lattice oxygen storage and donation. The lattice oxygen donation from (Ca/Mn)1-xO sacrificially stabilizes Fe3+ in the shell to maintain high catalytic activity and coke resistance. From a practical standpoint, the redox catalyst exhibits excellent long-term performance under industrially compatible conditions.

Design of Lewis Pairs via Interface Engineering of Oxide-Metal Composite Catalyst for Water Activation

The rational design and controlled construction of active centers remain grand challenges in heterogeneous catalysis, in particular for oxide catalysts with complex surface and interface structures. This work describes a facile way in the design of highly active Ni-O Lewis pairs for water activation where Ni and O sites act as Lewis acid and base, respectively. Surface science experiments indicate that dissociative adsorption of water occurs at edges of NiO_x nanoislands grown on Au(111) and NiO_x-Ni interfaces formed by further depositing metallic Ni layers along the edges of NiO_x nanoislands. Enhanced activity of Ni-O Lewis pairs at the NiO_x-Ni interface has been demonstrated by theoretical calculations, which are attributed to the higher Lewis acidity of metallic Ni sites and synergy of the metal and oxide components. Moreover, proton can migrate away from the NiO_x-Ni interface and refresh the O base sites, leading to further hydroxylation of the neighboring Ni acid sites.

Robust Ruthenium Catalysts Supported on Mesoporous Cyclodextrin-Templated TiO2-SiO2 Mixed Oxides for the Hydrogenation of Levulinic Acid to γ-Valerolactone

In this paper, we present a versatile template-directed colloidal self-assembly method for the fabrication in aqueous phase of composition-tuned mesoporous RuO2@TiO2-SiO2 catalysts. Randomly methylated β-cyclodextrin/Pluronic F127 supramolecular assemblies were used as soft templates, TiO2 colloids as building blocks, and tetraethyl orthosilicate as a silica source. Catalysts were characterized at different stages of their synthesis using dynamic light scattering, N2-adsorption analysis, powder X-ray diffraction, temperature programmed reduction, high-resolution transmission electron microscopy, high-angle annular bright-field and dark-field scanning transmission electron microscopy, together with EDS elemental mapping. Results revealed that both the supramolecular template and the silica loading had a strong impact on the pore characteristics and crystalline structure of the mixed oxides, as well as on the morphology of the RuO2 nanocrystals. Their catalytic performance was then evaluated in the aqueous phase hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL) under mild conditions (50 °C, 50 bar H2). Results showed that the cyclodextrin-derived catalyst displayed almost quantitative LA conversion and 99% GVL yield in less than one hour. Moreover, this catalyst could be reused at least five times without loss of activity. This work offers an effective approach to the utilization of cyclodextrins for engineering the surface morphology of Ru nanocrystals and pore characteristics of TiO2-based materials for catalytic applications in hydrogenation reactions.

Machine Learning in Computational Surface Science and Catalysis: Case Studies on Water and Metal-Oxide Interfaces

The goal of many computational physicists and chemists is the ability to bridge the gap between atomistic length scales of about a few multiples of an Ångström (Å), i. e., 10⁻¹⁰ m, and meso- or macroscopic length scales by virtue of simulations. The same applies to timescales. Machine learning techniques appear to bring this goal into reach. This work applies the recently published on-the-fly machine-learned force field techniques using a variant of the Gaussian approximation potentials combined with Bayesian regression and molecular dynamics as efficiently implemented in the Vienna ab initio simulation package, VASP. The generation of these force fields follows active-learning schemes. We apply these force fields to simple oxides such as MgO and more complex reducible oxides such as iron oxide, examine their generalizability, and further increase complexity by studying water adsorption on these metal oxide surfaces. We successfully examined surface properties of pristine and reconstructed MgO and Fe₃O₄ surfaces. However, the accurate description of water–oxide interfaces by machine-learned force fields, especially for iron oxides, remains a field offering plenty of research opportunities.

Understanding Hydrodechlorination of Chloromethanes. Past and Future of the Technology

Chloromethanes are a group of volatile organic compounds that are harmful to the environment and human health. Abundant studies have verified that hydrodechlorination might be an effective treatment to remove these chlorinated pollutants. The most outstanding advantages of this technique are the moderate operating conditions used and the possibility of obtaining less hazardous valuable products. This review presents a global analysis of experimental and theoretical studies regarding the hydrodechlorination of chloromethanes. The catalysts used and their synthesis methods are summarized. Their physicochemical properties are analyzed in order to deeply understand their influence on the catalytic performance. Moreover, the main causes of the catalyst deactivation are explained, and prevention and regeneration methods are suggested. The reaction systems used and the effect of the operating conditions on the catalytic activity are also analyzed. Besides, the mechanisms and kinetics of the process at the atomic level are reviewed. Finally, a new perspective for the upgrading of chloromethanes, via hydrodechlorination, to valuable hydrocarbons for industry, such as light olefins, is discussed.

Pt nanoparticles on Ti3C2Tx-based MXenes as efficient catalysts for the selective hydrogenation of nitroaromatic compounds to amines

The development of Pt nanocatalysts for the selective hydrogenation of nitroaromatic compounds to the corresponding amines is of great significance to solve the drawbacks associated with a low reserve of Pt. Herein, we develop a protocol for the preparation of a Pt/titanium carbide-based MXene heterostructure for the selective reduction of nitroaromatic compounds. In the heterostructure, well-defined and nano-sized metallic Pt crystallites are uniformly decorated on Ti3C2Tx nanosheets using a mild reducing agent of ammonia borane without additional stabilizing agents. The selective hydrogenation of p-chloronitrobenzene (p-CNB) to p-chloroaniline (p-CAN) was employed as a model reaction to investigate the catalytic performance of the as-synthesized heterostructure, denoted as Pt/Ti3C2Tx-D-AB. Notably, this catalyst can catalyze the complete conversion of p-CNB to p-CAN with 99.5% selectivity, superior to that of Pt/Ti3C2Tx-D-SB synthesized with sodium borohydride. The high performance of the present catalytic system can be ascribed to the well-dispersed Pt nanoparticles, the abundant surface electron-efficient Pt(0), and the synergistic catalysis between Pt/Ti3C2Tx-D-AB and water. This catalyst also shows generality toward the selective hydrogenation of a series of nitroaromatic compounds to the corresponding amines with high efficiency. The present study provides a strategy to synthesize efficient catalysts for catalytic applications.

Electrodes with Electrodeposited Water-excluding Polymer Coating Enable High-Voltage Aqueous Supercapacitors

Aqueous supercapacitors are powerful energy sources, but they are limited by energy density that is much lower than lithium-ion batteries. Since raising the voltage beyond the thermodynamic potential for water splitting (1.23 V) can boost the energy density, there has been much effort on water-stabilizing salvation additives such as Li₂SO₄ that can provide an aqueous electrolyte capable of withstanding ~1.8 V. Guided by the first-principles calculations that reveal water can promote hydrogen and oxygen evolution reactions, here, we pursue a new strategy of covering the electrode with a dense electroplated polymerized polyacrylic acid, which is an electron insulator but a proton conductor and proton reservoir. The combined effect of salvation and coating expands the electrochemical window throughout pH 3 to pH 10 to 2.4 V for both fast and slow proton-mediated redox reactions. This allows activated carbon to quadruple the energy density, a kilogram of nitrogen-doped graphene to provide 127 Watt-hour, and both to have improved endurance because of suppression of water-mediated corrosion. Therefore, aqueous supercapacitors can now achieve energy densities quite comparable to that of a lithium-ion battery, but at 100 times the charging/discharging speed and cycle durability.

Atomic-Scale Dynamic Interaction of H_{2}O Molecules with Cu Surface

Atomic-scale interaction of water vapor with metal surfaces beyond surface adsorption under technologically relevant conditions remains mostly unexplored. Using aberration-corrected environmental transmission electron microscopy, we reveal the dynamic surface activation of Cu by H_{2}O at elevated temperature and pressure. We find a structural transition from flat to corrugated surface for the Cu(011) under low water-vapor pressure. Increasing the water-vapor pressure leads to the surface reaction of Cu with dissociated H_{2}O, resulting in the formation of a metastable "bilayer" Cu─O─H phase. Corroborated by density functional theory and ab initio molecular dynamics calculations, the cooperative O and OH interaction with Cu is responsible for the formation and subsurface propagation of this phase.

In situ tuning of electronic structure of catalysts using controllable hydrogen spillover for enhanced selectivity

In situ tuning of the electronic structure of active sites is a long-standing challenge. Herein, we propose a strategy by controlling the hydrogen spillover distance to in situ tune the electronic structure. The strategy is demonstrated to be feasible with the assistance of CoO_x/Al₂O₃/Pt catalysts prepared by atomic layer deposition in which CoO_x and Pt nanoparticles are separated by hollow Al₂O₃ nanotubes. The strength of hydrogen spillover from Pt to CoO_x can be precisely tailored by varying the Al₂O₃ thickness. Using CoO_x/Al₂O₃ catalyzed styrene epoxidation as an example, the CoO_x/Al₂O₃/Pt with 7 nm Al₂O₃ layer exhibits greatly enhanced selectivity (from 74.3% to 94.8%) when H₂ is added. The enhanced selectivity is attributed to the introduction of controllable hydrogen spillover, resulting in the reduction of CoO_x during the reaction. Our method is also effective for the epoxidation of styrene derivatives. We anticipate this method is a general strategy for other reactions.

In situ tuning of the electronic structure of active sites is a long-standing challenge. Here, the authors report an approach to tune the electronic structure of cobalt species during the styrene epoxidation reaction by the introduction of controllable hydrogen spillover for enhanced selectivity.

Proton-assisted electron transfer and hydrogen-atom diffusion in a model system for photocatalytic hydrogen production

Solar energy can be converted into chemical energy by photocatalytic water splitting to produce molecular hydrogen. Details of the photo-induced reaction mechanism occurring on the surface of a semiconductor are not fully understood, however. Herein, we employ a model photocatalytic system consisting of single atoms deposited on quantum dots that are anchored on to a primary photocatalyst to explore fundamental aspects of photolytic hydrogen generation. Single platinum atoms (Pt₁) are anchored onto carbon nitride quantum dots (CNQDs), which are loaded onto graphitic carbon nitride nanosheets (CNS), forming a Pt₁@CNQDs/CNS composite. Pt₁@CNQDs/CNS provides a well-defined photocatalytic system in which the electron and proton transfer processes that lead to the formation of hydrogen gas can be investigated. Results suggest that hydrogen bonding between hydrophilic surface groups of the CNQDs and interfacial water molecules facilitates both proton-assisted electron transfer and sorption/desorption pathways. Surface bound hydrogen atoms appear to diffuse from CNQDs surface sites to the deposited Pt₁ catalytic sites leading to higher hydrogen-atom fugacity surrounding each isolated Pt₁ site. We identify a pathway that allows for hydrogen-atom recombination into molecular hydrogen and eventually to hydrogen bubble evolution.

Direct observation of water-mediated single-proton transport between hBN surface defects

Aqueous proton transport at interfaces is ubiquitous and crucial for a number of fields, ranging from cellular transport and signalling, to catalysis and membrane science. However, due to their light mass, small size and high chemical reactivity, uncovering the surface transport of single protons at room temperature and in an aqueous environment has so far remained out-of-reach of conventional atomic-scale surface science techniques, such as scanning tunnelling microscopy. Here, we use single-molecule localization microscopy to resolve optically the transport of individual excess protons at the interface of hexagonal boron nitride crystals and aqueous solutions at room temperature. Single excess proton trajectories are revealed by the successive protonation and activation of optically active defects at the surface of the crystal. Our observations demonstrate, at the single-molecule scale, that the solid/water interface provides a preferential pathway for lateral proton transport, with broad implications for molecular charge transport at liquid interfaces.