Monodisperse Au nanoparticles for selective electrocatalytic reduction of CO2 to CO

Rational Design of a Hierarchical Tin Dendrite Electrode for Efficient Electrochemical Reduction of CO2

Catalysis is a key technology for the synthesis of renewable fuels through electrochemical reduction of CO2 . However, successful CO2 reduction still suffers from the lack of affordable catalyst design and understanding the factors governing catalysis. Herein, we demonstrate that the CO2 conversion selectivity on Sn (or SnOx /Sn) electrodes is correlated to the native oxygen content at the subsurface. Electrochemical analyses show that the reduced Sn electrode with abundant oxygen species effectively stabilizes a CO2 (.-) intermediate rather than the clean Sn surface, and consequently results in enhanced formate production in the CO2 reduction. Based on this design strategy, a hierarchical Sn dendrite electrode with high oxygen content, consisting of a multi-branched conifer-like structure with an enlarged surface area, was synthesized. The electrode exhibits a superior formate production rate (228.6 μmol h(-1)  cm(-2) ) at -1.36 VRHE without any considerable catalytic degradation over 18 h of operation.

Hybrid bioinorganic approach to solar-to-chemical conversion

Natural photosynthesis harnesses solar energy to convert CO2 and water to value-added chemical products for sustaining life. We present a hybrid bioinorganic approach to solar-to-chemical conversion in which sustainable electrical and/or solar input drives production of hydrogen from water splitting using biocompatible inorganic catalysts. The hydrogen is then used by living cells as a source of reducing equivalents for conversion of CO2 to the value-added chemical product methane. Using platinum or an earth-abundant substitute, α-NiS, as biocompatible hydrogen evolution reaction (HER) electrocatalysts and Methanosarcina barkeri as a biocatalyst for CO2 fixation, we demonstrate robust and efficient electrochemical CO2 to CH4 conversion at up to 86% overall Faradaic efficiency for ≥ 7 d. Introduction of indium phosphide photocathodes and titanium dioxide photoanodes affords a fully solar-driven system for methane generation from water and CO2, establishing that compatible inorganic and biological components can synergistically couple light-harvesting and catalytic functions for solar-to-chemical conversion.

Concave Rhombic Dodecahedral Au Nanocatalyst with Multiple High-Index Facets for CO2 Reduction

A concave rhombic dodecahedron (RD) gold nanoparticle was synthesized by adding 4-aminothiophenol (4-ATP) during growth from seeds. This shape is enclosed by stabilized facets of various high-indexes, such as (331), (221), and (553). Because it is driven thermodynamically and stabilized by 4-ATP ligands, the concave RD maintains its structure over a few months, even after rigorous electrochemical reactions. We discussed the mechanism of the shape evolution controlled by 4-ATP and found that both the binding energy of Au-S and the aromatic geometry of 4-ATP are major determinants of Au atom deposition during growth. As a possible application, we demonstrated that the concave RD exhibits superior electrocatalytic performance for the selective conversion of CO2 to CO in aqueous solution.

A convenient phase transfer protocol to functionalize gold nanoparticles with short alkylamine ligands

HYPOTHESIS

Aqueous citrate-stabilized gold nanoparticles (Au NPs) cannot be directly transferred from water to an immiscible organic solution using short alkyl ligands. However, Au NPs can be transferred from water to a water-organic interface if chemical and mechanical inputs are used to modify the interfacial energy and interfacial area. Ligand exchange can then take place at this interface. After separating the particles from the liquids, they can be transferred to a different organic phase.

EXPERIMENTS

Hexane, alkylamine, and acetone were added to aqueous citrate-stabilized Au NPs to form a film at the system interfaces. After removing the liquid phases, Au NPs were readily redispersed into tetrahydrofuran (THF). The size and shape of the transferred Au NPs were evaluated by transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS).

FINDINGS

Au NPs with 13nm diameter are readily segregated from water with the aid of short alkylamine ligands. They form a thin film at the water/organic solvent interface, rendering them easy to separate from the liquid phases and possible to redisperse into another organic solvent. After the phase transfer process, Au NPs were functionalized with short amine ligands. In addition, the shape and size of Au NPs were preserved. The short amine-protected Au NPs in THF can stay stable for up to 27days or longer.

Covalent organic frameworks comprising cobalt porphyrins for catalytic CO₂ reduction in water

Conversion of carbon dioxide (CO2) to carbon monoxide (CO) and other value-added carbon products is an important challenge for clean energy research. Here we report modular optimization of covalent organic frameworks (COFs), in which the building units are cobalt porphyrin catalysts linked by organic struts through imine bonds, to prepare a catalytic material for aqueous electrochemical reduction of CO2 to CO. The catalysts exhibit high Faradaic efficiency (90%) and turnover numbers (up to 290,000, with initial turnover frequency of 9400 hour(-1)) at pH 7 with an overpotential of -0.55 volts, equivalent to a 26-fold improvement in activity compared with the molecular cobalt complex, with no degradation over 24 hours. X-ray absorption data reveal the influence of the COF environment on the electronic structure of the catalytic cobalt centers.

Overpotential for CO2 electroreduction lowered on strained penta-twinned Cu nanowires

Based on first-principles calculations, we predict that penta-twinned Cu nanowires (NWs) are superior to conventional Cu catalysts for CO₂ electroreduction. The penta-twinned NWs possess a combination of ultrahigh mechanical strength, large surface-to-volume ratios and an abundance of undercoordinated adsorption sites, all desirable for CO₂ electroreduction. In particular, we show that the penta-twinned Cu NWs can withstand elastic strains orders of magnitude higher than their conventional counterpart, and as a result their CO₂ electroreduction activities can be significantly enhanced by elastic tensile strains. With a moderate tensile strain, the bias potential for methane production at a decent current density (2 mA cm^-2) can be reduced by 50%. On the other hand, the competing hydrogen evolution reaction can be suppressed by the tensile strains. The presence of H at the NW surface is found to have a minor effect on CO₂ electroreduction. Finally, we propose to use graphene as a substrate to stretch deposited Cu NWs.

Efficient electrochemical CO2 conversion powered by renewable energy

The catalytic conversion of CO2 into industrially relevant chemicals is one strategy for mitigating greenhouse gas emissions. Along these lines, electrochemical CO2 conversion technologies are attractive because they can operate with high reaction rates at ambient conditions. However, electrochemical systems require electricity, and CO2 conversion processes must integrate with carbon-free, renewable-energy sources to be viable on larger scales. We utilize Au25 nanoclusters as renewably powered CO2 conversion electrocatalysts with CO2 → CO reaction rates between 400 and 800 L of CO2 per gram of catalytic metal per hour and product selectivities between 80 and 95%. These performance metrics correspond to conversion rates approaching 0.8-1.6 kg of CO2 per gram of catalytic metal per hour. We also present data showing CO2 conversion rates and product selectivity strongly depend on catalyst loading. Optimized systems demonstrate stable operation and reaction turnover numbers (TONs) approaching 6 × 10(6) molCO2 molcatalyst(-1) during a multiday (36 h total hours) CO2 electrolysis experiment containing multiple start/stop cycles. TONs between 1 × 10(6) and 4 × 10(6) molCO2 molcatalyst(-1) were obtained when our system was powered by consumer-grade renewable-energy sources. Daytime photovoltaic-powered CO2 conversion was demonstrated for 12 h and we mimicked low-light or nighttime operation for 24 h with a solar-rechargeable battery. This proof-of-principle study provides some of the initial performance data necessary for assessing the scalability and technical viability of electrochemical CO2 conversion technologies. Specifically, we show the following: (1) all electrochemical CO2 conversion systems will produce a net increase in CO2 emissions if they do not integrate with renewable-energy sources, (2) catalyst loading vs activity trends can be used to tune process rates and product distributions, and (3) state-of-the-art renewable-energy technologies are sufficient to power larger-scale, tonne per day CO2 conversion systems.

Chemical looping of metal nitride catalysts: low-pressure ammonia synthesis for energy storage

The activity of many heterogeneous catalysts is limited by strong correlations between activation energies and adsorption energies of reaction intermediates. Although the reaction is thermodynamically favourable at ambient temperature and pressure, the catalytic synthesis of ammonia (NH3), a fertilizer and chemical fuel, from N2 and H2 requires some of the most extreme conditions of the chemical industry. We demonstrate how ammonia can be produced at ambient pressure from air, water, and concentrated sunlight as renewable source of process heat via nitrogen reduction with a looped metal nitride, followed by separate hydrogenation of the lattice nitrogen into ammonia. Separating ammonia synthesis into two reaction steps introduces an additional degree of freedom when designing catalysts with desirable activation and adsorption energies. We discuss the hydrogenation of alkali and alkaline earth metal nitrides and the reduction of transition metal nitrides to outline a promoting role of lattice hydrogen in ammonia evolution. This is rationalized via electronic structure calculations with the activity of nitrogen vacancies controlling the redox-intercalation of hydrogen and the formation and hydrogenation of adsorbed nitrogen species. The predicted trends are confirmed experimentally with evolution of 56.3, 80.7, and 128 μmol NH3 per mol metal per min at 1 bar and above 550 °C via reduction of Mn6N2.58 to Mn4N and hydrogenation of Ca3N2 and Sr2N to Ca2NH and SrH2, respectively.

Face the Edges: Catalytic Active Sites of Nanomaterials

Edges are special sites in nanomaterials. The atoms residing on the edges have different environments compared to those in other parts of a nanomaterial and, therefore, they may have different properties. Here, recent progress in nanomaterial fields is summarized from the viewpoint of the edges. Typically, edge sites in MoS2 or metals, other than surface atoms, can perform as active centers for catalytic reactions, so the method to enhance performance lies in the optimization of the edge structures. The edges of multicomponent interfaces present even more possibilities to enhance the activities of nanomaterials. Nanoframes and ultrathin nanowires have similarities to conventional edges of nanoparticles, the application of which as catalysts can help to reduce the use of costly materials. Looking beyond this, the edge structures of graphene are also essential for their properties. In short, the edge structure can influence many properties of materials.

Computational Design of Strain in Core-Shell Nanoparticles for Optimizing Catalytic Activity

Surface strains in core-shell nanoparticles modify catalytic activity. Here, a continuum-based strategy enables accurate surface-strain-based screening and design of core-shell systems using minimal input as a means to enhance catalytic activity. The approach is validated here for Pt shells on Cu(x)Pt(1-x) cores and used to interpret experimental results on the oxygen reduction reaction in the same system. The analysis shows that precise control of particle sizes and shell thicknesses is required to achieve peak activity, rationalizing the limited increases in activity observed in experiments. The method is also applied to core-shell nanorods to demonstrate its wide applicability.

Heterogeneous catalytic conversion of CO2: a comprehensive theoretical review

The conversion of CO2 into fuels and useful chemicals has been intensively pursued for renewable, sustainable and green energy. However, due to the negative adiabatic electron affinity (EA) and large ionization potential (IP), the CO2 molecule is chemically inert, thus making the conversion difficult under normal conditions. Novel catalysts, which have high stability, superior efficiency and low cost, are urgently needed to facilitate the conversion. As the first step to design such catalysts, understanding the mechanisms involved in CO2 conversion is absolutely indispensable. In this review, we have summarized the recent theoretical progress in mechanistic studies based on density functional theory, kinetic Monte Carlo simulation, and microkinetics modeling. We focus on reaction channels, intermediate products, the key factors determining the conversion of CO2 in solid-gas interface thermocatalytic reduction and solid-liquid interface electrocatalytic reduction. Furthermore, we have proposed some possible strategies for improving CO2 electrocatalysis and also discussed the challenges in theory, model construction, and future research directions.

Selective reduction of CO2 to formate through bicarbonate reduction on metal electrodes: new insights gained from SG/TC mode of SECM

We discovered using SECM of the electro-reduction of CO2 on a Au substrate in CO2-saturated KHCO3 solutions that (i) formate comes solely from the direct reduction of bicarbonate; and (ii) CO forms only from CO2 reduction (under low pH conditions) and at higher applied potentials. The results point to the possibility of the selective reduction of CO2 to the formate product.

A New Core/Shell NiAu/Au Nanoparticle Catalyst with Pt-like Activity for Hydrogen Evolution Reaction

We report a general approach to NiAu alloy nanoparticles (NPs) by co-reduction of Ni(acac)2 (acac = acetylacetonate) and HAuCl4·3H2O at 220 °C in the presence of oleylamine and oleic acid. Subject to potential cycling between 0.6 and 1.0 V (vs reversible hydrogen electrode) in 0.5 M H2SO4, the NiAu NPs are transformed into core/shell NiAu/Au NPs that show much enhanced catalysis for hydrogen evolution reaction (HER) with Pt-like activity and much robust durability. The first-principles calculations suggest that the high activity arises from the formation of Au sites with low coordination numbers around the shell. Our synthesis is not limited to NiAu but can be extended to FeAu and CoAu as well, providing a general approach to MAu/Au NPs as a class of new catalyst superior to Pt for water splitting and hydrogen generation.

Fabrication and robotization of ultrasensitive plasmonic nanosensors for molecule detection with Raman scattering

In this work, we introduce the history and mechanisms of surface enhanced Raman scattering (SERS), discuss various techniques for fabrication of state-of-the-art SERS substrates, and review recent work on robotizing plasmonic nanoparticles, especially, the efforts we made on fabrication, characterization, and robotization of Raman nanosensors by design. Our nanosensors, consisting of tri-layer nanocapsule structures, are ultrasensitive, well reproducible, and can be robotized by either electric or magnetic tweezers. Three applications using such SERS nanosensors were demonstrated, including location predictable detection, single-cell bioanalysis, and tunable molecule release and monitoring. The integration of SERS and nanoelectromechanical system (NEMS) devices is innovative in both device concept and fabrication, and could potentially inspire a new device scheme for various bio-relevant applications.

A DFT-based genetic algorithm search for AuCu nanoalloy electrocatalysts for CO₂ reduction

Using a DFT-based genetic algorithm (GA) approach, we have determined the most stable structure and stoichiometry of a 309-atom icosahedral AuCu nanoalloy, for potential use as an electrocatalyst for CO2 reduction. The identified core-shell nano-particle consists of a copper core interspersed with gold atoms having only copper neighbors and a gold surface with a few copper atoms in the terraces. We also present an adsorbate-dependent correction scheme, which enables an accurate determination of adsorption energies using a computationally fast, localized LCAO-basis set. These show that it is possible to use the LCAO mode to obtain a realistic estimate of the molecular chemisorption energy for systems where the computation in normal grid mode is not computationally feasible. These corrections are employed when calculating adsorption energies on the Cu, Au and most stable mixed particles. This shows that the mixed Cu135@Au174 core-shell nanoalloy has a similar adsorption energy, for the most favorable site, as a pure gold nano-particle. Cu, however, has the effect of stabilizing the icosahedral structure because Au particles are easily distorted when adding adsorbates.

Grain-boundary-dependent CO2 electroreduction activity

Uncovering new structure-activity relationships for metal nanoparticle (NP) electrocatalysts is crucial for advancing many energy conversion technologies. Grain boundaries (GBs) could be used to stabilize unique active surfaces, but a quantitative correlation between GBs and catalytic activity has not been established. Here we use vapor deposition to prepare Au NPs on carbon nanotubes (Au/CNT). As deposited, the Au NPs have a relatively high density of GBs that are readily imaged by transmission electron microscopy (TEM); thermal annealing lowers the density in a controlled manner. We show that the surface-area-normalized activity for CO2 reduction is linearly correlated with GB surface density on Au/CNT, demonstrating that GB engineering is a powerful approach to improving the catalytic activity of metal NPs.

Dumbbell-like Au-Fe3O4 nanoparticles: a new nanostructure for supercapacitors

Monodispersed dumbbell-like Au-Fe3O4 nanoparticles (NPs) were synthesized and studied for use in supercapacitors. These dumbbell NPs show Au/Fe3O4-size dependent capacitive behaviors and the 7-14 nm Au-Fe3O4 NPs have the best specific capacitance of 464 F g(-1) at 1 A g(-1) and capacity retention of 86.4% after 1000 cycles, much larger than pure Fe3O4 NPs (160 F g(-1) and 72.8% retention). This capacitive enhancement is believed to arise from the Au-induced increase in electron transfer across the dumbbell-like NPs. Thise report demonstrates a new strategy to enhance oxide NP capacitance for applications in high performance supercapacitors.

Highly porous non-precious bimetallic electrocatalysts for efficient hydrogen evolution

A robust and efficient non-precious metal catalyst for hydrogen evolution reaction is one of the key components for carbon dioxide-free hydrogen production. Here we report that a hierarchical nanoporous copper-titanium bimetallic electrocatalyst is able to produce hydrogen from water under a mild overpotential at more than twice the rate of state-of-the-art carbon-supported platinum catalyst. Although both copper and titanium are known to be poor hydrogen evolution catalysts, the combination of these two elements creates unique copper-copper-titanium hollow sites, which have a hydrogen-binding energy very similar to that of platinum, resulting in an exceptional hydrogen evolution activity. In addition, the hierarchical porosity of the nanoporous copper-titanium catalyst also contributes to its high hydrogen evolution activity, because it provides a large-surface area for electrocatalytic hydrogen evolution, and improves the mass transport properties. Moreover, the catalyst is self-supported, eliminating the overpotential associated with the catalyst/support interface.

Investigations into non-precious metal catalysts for hydrogen evolution are ongoing. Here, the authors report a hierarchical, nanoporous copper-titanium electrocatalyst, and demonstrate that it catalyses hydrogen production at twice the over-all rate of commercial platinum-based catalysts.

Electrocatalytic carbon dioxide reduction by using cationic pentamethylcyclopentadienyl-iridium complexes with unsymmetrically substituted bipyridine ligands

Eight [Ir(bpy)Cp*Cl](+) -type complexes (bpy= bipyridine, Cp*=1,2,3,4,5-pentamethylcyclopentadienyl) containing differently substituted bipyridine ligands were synthesized and characterized. Cyclic voltammetry (CV) of the complexes in Ar-saturated acetonitrile solutions showed that the redox behavior of the complexes could be fine tuned by the electronic properties of the substituted bipyridine ligands. Further CV in CO2 -saturated MeCN/H2 O (9:1, v/v) solutions showed catalytic currents for CO2 reduction. In controlled potential electrolysis experiments (MeCN/MeOH (1:1, v/v), Eapp =-1.80 V vs Ag/AgCl), all of the complexes showed moderate activity in the electrocatalytic reduction of CO2 with good stability over at least 15 hours. This electrocatalytic process was selective toward formic acid, with only traces of dihydrogen or carbon monoxide and occasionally formaldehyde as byproducts. However, the turnover frequencies and current efficiencies were quite low. No direct correlation between the redox potentials of the complexes and their catalytic activity was observed.

Synergistic effect in bimetallic copper–silver (CuxAg) nanoparticles enhances silicon conversion in Rochow reaction

The prepared bimetallic Cu_xAg nanoparticles exhibit a better silicon conversion for the Rochow reaction than monometallic Cu and Ag nanoparticles with the synergistic effect.

Enhanced catalytic activity of Au nanoparticles self-assembled on thiophenol functionalized graphene

A new catalyst containing Au nanoparticles anchored to thiophenol functionalized graphene sheets (Au/TP-GS) was fabricated. The resulting Au/TP-GS exhibited excellent catalytic activity for both the reduction of 4-nitrophenol and the photodegradation of Rhodamine.

Composition dependent activity of Cu–Pt nanocrystals for electrochemical reduction of CO2

The Cu–Pt nanocrystal (NC) samples with different atomic ratios (Cu : Pt) show composition dependent activities toward electrocatalytic CO₂ reduction.

Oxygen-induced changes to selectivity-determining steps in electrocatalytic CO2 reduction

The state of the electrocatalyst surface—including the oxidation state of the catalyst and the presence of spectator species—is investigated on Cu surfaces with density functional theory in order to understand predicted ramifications on the selectivity of CO₂ reduction between CH₄ and CH₃OH.

Exceptional size-dependent activity enhancement in the electroreduction of CO2 over Au nanoparticles

The electrocatalytic reduction of CO2 to industrial chemicals and fuels is a promising pathway to sustainable electrical energy storage and to an artificial carbon cycle, but it is currently hindered by the low energy efficiency and low activity displayed by traditional electrode materials. We report here the size-dependent catalytic activity of micelle-synthesized Au nanoparticles (NPs) in the size range of ∼1-8 nm for the electroreduction of CO2 to CO in 0.1 M KHCO3. A drastic increase in current density was observed with decreasing NP size, along with a decrease in Faradaic selectivity toward CO. Density functional theory calculations showed that these trends are related to the increase in the number of low-coordinated sites on small NPs, which favor the evolution of H2 over CO2 reduction to CO. We show here that the H2/CO product ratio can be specifically tailored for different industrial processes by tuning the size of the catalyst particles.

Active and selective conversion of CO2 to CO on ultrathin Au nanowires

In this communication, we show that ultrathin Au nanowires (NWs) with dominant edge sites on their surface are active and selective for electrochemical reduction of CO2 to CO. We first develop a facile seed-mediated growth method to synthesize these ultrathin (2 nm wide) Au NWs in high yield (95%) by reducing HAuCl4 in the presence of 2 nm Au nanoparticles (NPs). These NWs catalyze CO2 reduction to CO in aqueous 0.5 M KHCO3 at an onset potential of -0.2 V (vs reversible hydrogen electrode). At -0.35 V, the reduction Faradaic efficiency (FE) reaches 94% (mass activity 1.84 A/g Au) and stays at this level for 6 h without any noticeable activity change. Density functional theory (DFT) calculations suggest that the excellent catalytic performance of these Au NWs is attributed both to their high mass density of reactive edge sites (≥16%) and to the weak CO binding on these sites. These ultrathin Au NWs are the most efficient nanocatalyst ever reported for electrochemical reduction of CO2 to CO.

A strategy for fabricating nanoporous gold films through chemical dealloying of electrochemically deposited Au-Sn alloys

We report a novel strategy for the fabrication of nanoporous gold (NPG) films. The fabrication process involves the electrodeposition of a gold-tin alloy, followed by subsequent chemical dealloying of tin. Scanning electron microscopy (SEM) images show a bicontinuous nanoporous structure formed on the substrates after chemical dealloying. Energy dispersive x-ray (EDX) analysis indicates that there are no impurities in the Au-Sn alloy film with an average composition of 58 at. % Au and 42 at. % Sn. After dealloying, only gold remains in the NPG film indicating the effectiveness of this technique. X-ray diffraction (XRD) results reveal that the as-prepared Au-Sn alloy film is composed of two phases (Au5Sn and AuSn), while the NPG film is composed of a single phase (Au). We demonstrate that this approach enables the fabrication of NPG films, either freestanding or supported on various conductive substrates such as copper foil, stainless steel sheet and nickel foam. The resulting NPG electrode exhibits enhanced electrocatalytic activity toward both H2O2 reduction and methanol oxidation compared to the polished Au disc electrode. Our strategy provides a general method to fabricate high quality NPG films on conductive substrates, which will broaden the application potential of NPG or NPG-based materials in various fields such as catalysis, optics and sensor technology.

High sintering resistance of size-selected platinum cluster catalysts by suppressed Ostwald ripening

Employing rationally designed model systems with precise atom-by-atom particle size control, we demonstrate by means of combining noninvasive in situ indirect nanoplasmonic sensing and ex situ scanning transmission electron microscopy that monomodal size-selected platinum cluster catalysts on different supports exhibit remarkable intrinsic sintering resistance even under reaction conditions. The observed stability is related to suppression of Ostwald ripening by elimination of its main driving force via size-selection. This study thus constitutes a general blueprint for the rational design of sintering resistant catalyst systems and for efficient experimental strategies to determine sintering mechanisms. Moreover, this is the first systematic experimental investigation of sintering processes in nanoparticle systems with an initially perfectly monomodal size distribution under ambient conditions.

Selective visible-light-driven CO2 reduction on a p-type dye-sensitized NiO photocathode

We present a photocathode assembly for the visible-light-driven selective reduction of CO2 to CO at potentials below the thermodynamic equilibrium in the dark. The photoelectrode comprises a porous p-type semiconducting NiO electrode modified with the visible-light-responsive organic dye P1 and the reversible CO2 cycling enzyme carbon monoxide dehydrogenase. The direct electrochemistry of the enzymatic electrocatalyst on NiO shows that in the dark the electrocatalytic behavior is rectified toward CO oxidation, with the reactivity being governed by the carrier availability at the semiconductor-catalyst interface.