Direct in-situ imaging of electrochemical corrosion of Pd-Pt core-shell electrocatalysts.

Corrosion of electrocatalysts during electrochemical operations, such as low potential - high potential cyclic swapping, can cause significant performance degradation. However, the electrochemical corrosion dynamics, including structural changes, especially site and composition specific ones, and their correlation with electrochemical processes are hidden due to the insufficient spatial-temporal resolution characterization methods. Using electrochemical liquid cell transmission electron microscopy, we visualize the electrochemical corrosion of Pd@Pt core-shell octahedral nanoparticles towards a Pt nanoframe. The potential-dependent surface reconstruction during multiple continuous in-situ cyclic voltammetry with clear redox peaks is captured, revealing an etching and deposition process of Pd that results in internal Pd atoms being relocated to external surface, followed by subsequent preferential corrosion of Pt (111) terraces rather than the edges or corners, simultaneously capturing the structure evolution also allows to attribute the site-specific Pt and Pd atomic dynamics to individual oxidation and reduction events. This work provides profound insights into the surface reconstruction of nanoparticles during complex electrochemical processes.

Distribution of Pt single atom coordination environments on anatase TiO2 supports controls reactivity.

Single-atom catalysts (SACs) offer efficient metal utilization and distinct reactivity compared to supported metal nanoparticles. Structure-function relationships for SACs often assume that active sites have uniform coordination environments at particular binding sites on support surfaces. Here, we investigate the distribution of coordination environments of Pt SAs dispersed on shape-controlled anatase TiO2 supports specifically exposing (001) and (101) surfaces. Pt SAs on (101) are found on the surface, consistent with existing structural models, whereas those on (001) are beneath the surface after calcination. Pt SAs under (001) surfaces exhibit lower reactivity for CO oxidation than those on (101) surfaces due to their limited accessibility to gas phase species. Pt SAs deposited on commercial-TiO2 are found both at the surface and in the bulk, posing challenges to structure-function relationship development. This study highlights heterogeneity in SA coordination environments on oxide supports, emphasizing a previously overlooked consideration in the design of SACs.

Dynamic Behavior of Platinum Atoms and Clusters in the Native Oxide Layer of Aluminum Nanocrystals.

Strong metal-support interactions (SMSIs) are well-known in the field of heterogeneous catalysis to induce the encapsulation of platinum (Pt) group metals by oxide supports through high temperature H2 reduction. However, demonstrations of SMSI overlayers have largely been limited to reducible oxides, such as TiO2 and Nb2O5. Here, we show that the amorphous native surface oxide of plasmonic aluminum nanocrystals (AlNCs) exhibits SMSI-induced encapsulation of Pt following reduction in H2 in a Pt structure dependent manner. Reductive treatment in H2 at 300 °C induces the formation of an AlOx SMSI overlayer on Pt clusters, leaving Pt single-atom sites (Ptiso) exposed available for catalysis. The remaining exposed Ptiso species possess a more uniform local coordination environment than has been observed on other forms of Al2O3, suggesting that the AlOx native oxide of AlNCs presents well-defined anchoring sites for individual Pt atoms. This observation extends our understanding of SMSIs by providing evidence that H2-induced encapsulation can occur for a wider variety of materials and should stimulate expanded studies of this effect to include nonreducible oxides with oxygen defects and the presence of disorder. It also suggests that the single-atom sites created in this manner, when combined with the plasmonic properties of the Al nanocrystal core, may allow for site-specific single-atom plasmonic photocatalysis, providing dynamic control over the light-driven reactivity in these systems.

Effect of Phosphorus Modulation in Iron Single-Atom Catalysts for Peroxidase Mimicking.

Fe-N-C single-atom catalysts (SACs) exhibit excellent peroxidase (POD)-like catalytic activity, owing to their well-defined isolated iron active sites on the carbon substrate, which effectively mimic the structure of natural peroxidase's active center. To further meet the requirements of diverse biosensing applications, SAC POD-like activity still needs to be continuously enhanced. Herein, a phosphorus (P) heteroatom is introduced to boost the POD-like activity of Fe-N-C SACs. A 1D carbon nanowire (FeNCP/NW) catalyst with enriched Fe-N4 active sites is designed and synthesized, and P atoms are doped in the carbon matrix to affect the Fe center through long-range interaction. The experimental results show that the P-doping process can boost the POD-like activity more than the non-P-doped one, with excellent selectivity and stability. The mechanism analysis results show that the introduction of P into SAC can greatly enhance POD-like activity initially, but its effect becomes insignificant with increasing amount of P. As a proof of concept, FeNCP/NW is employed in an enzyme cascade platform for highly sensitive colorimetric detection of the neurotransmitter acetylcholine.

NiFe Nanoparticle Nest Supported on Graphene as Electrocatalyst for Highly Efficient Oxygen Evolution Reaction.

Designing cost-efffective electrocatalysts for the oxygen evolution reaction (OER) holds significant importance in the progression of clean energy generation and efficient energy storage technologies, such as water splitting and rechargeable metal-air batteries. In this work, an OER electrocatalyst is developed using Ni and Fe precursors in combination with different proportions of graphene oxide. The catalyst synthesis involved a rapid reduction process, facilitated by adding sodium borohydride, which successfully formed NiFe nanoparticle nests on graphene support (NiFe NNG). The incorporation of graphene support enhances the catalytic activity, electron transferability, and electrical conductivity of the NiFe-based catalyst. The NiFe NNG catalyst exhibits outstanding performance, characterized by a low overpotential of 292.3 mV and a Tafel slope of 48 mV dec-1, achieved at a current density of 10 mA cm- 2. Moreover, the catalyst exhibits remarkable stability over extended durations. The OER performance of NiFe NNG is on par with that of commercial IrO2 in alkaline media. Such superb OER catalytic performance can be attributed to the synergistic effect between the NiFe nanoparticle nests and graphene, which arises from their large surface area and outstanding intrinsic catalytic activity. The excellent electrochemical properties of NiFe NNG hold great promise for further applications in energy storage and conversion devices.

How Pt Influences H2 Reactions on High Surface-Area Pt/CeO2 Powder Catalyst Surfaces.

The addition of platinum-group metals (PGMs, e.g., Pt) to CeO2 is used in heterogeneous catalysis to promote the rate of redox surface reactions. Well-defined model system studies have shown that PGMs facilitate H2 dissociation, H-spillover onto CeO2 surfaces, and CeO2 surface reduction. However, it remains unclear how the heterogeneous structures and interfaces that exist on powder catalysts influence the mechanistic picture of PGM-promoted H2 reactions on CeO2 surfaces developed from model system studies. Here, controlled catalyst synthesis, temperature-programmed reduction (TPR), in situ infrared spectroscopy (IR), and in situ electron energy loss spectroscopy (EELS) were used to interrogate the mechanisms of how Pt nanoclusters and single atoms influence H2 reactions on high-surface area Pt/CeO2 powder catalysts. TPR showed that Pt promotes H2 consumption rates on Pt/CeO2 even when Pt exists on a small fraction of CeO2 particles, suggesting that H-spillover proceeds far from Pt-CeO2 interfaces and across CeO2-CeO2 particle interfaces. IR and EELS measurements provided evidence that Pt changes the mechanism of H2 activation and the rate limiting step for Ce3+, oxygen vacancy, and water formation as compared to pure CeO2. As a result, higher-saturation surface hydroxyl coverages can be achieved on Pt/CeO2 compared to pure CeO2. Further, Ce3+ formed by spillover-H from Pt is heterogeneously distributed and localized at and around interparticle CeO2-CeO2 boundaries, while activated H2 on pure CeO2 results in homogeneously distributed Ce3+. Ce3+ localization at and around CeO2-CeO2 boundaries for Pt/CeO2 is accompanied by surface reconstruction that enables faster rates of H2 consumption. This study reconciles the materials gap between model structures and powder catalysts for H2 reactions with Pt/CeO2 and highlights how the spatial heterogeneity of powder catalysts dictates the influence of Pt on H2 reactions at CeO2 surfaces.

Selective Formation of Acetic Acid and Methanol by Direct Methane Oxidation Using Rhodium Single-Atom Catalysts.

Atomically dispersed catalysts such as single-atom catalysts have been shown to be effective in selectively oxidizing methane, promising a direct synthetic route to value-added oxygenates such as acetic acid or methanol. However, an important challenge of this approach has been that the loading of active sites by single-atom catalysts is low, leading to a low overall yield of the products. Here, we report an approach that can address this issue. It utilizes a metal-organic framework built with porphyrin as the linker, which provides high concentrations of binding sites to support atomically dispersed rhodium. It is shown that up to 5 wt% rhodium loading can be achieved with excellent dispersity. When used for acetic acid synthesis by methane oxidation, a new benchmark performance of 23.62 mmol·gcat-1·h-1 was measured. Furthermore, the catalyst exhibits a unique sensitivity to light, producing acetic acid (under illumination, up to 66.4% selectivity) or methanol (in the dark, up to 65.0% selectivity) under otherwise identical reaction conditions.

Selective Methane Oxidation by Heterogenized Iridium Catalysts.

Oxidative methane (CH4) carbonylation promises a direct route to the synthesis of value-added oxygenates such as acetic acid (CH3COOH). Here, we report a strategy to realize oxidative CH4 carbonylation through immobilized Ir complexes on an oxide support. Our immobilization approach not only enables direct CH4 activation but also allows for easy separation and reutilization of the catalyst. Furthermore, we show that a key step, methyl migration, that forms a C-C bond, is sensitive to the electrophilicity of carbonyl, which can be tuned by a gentle reduction to the Ir centers. While the as-prepared catalyst that mainly featured Ir(IV) preferred CH3COOH production, a reduced catalyst featuring predominantly Ir(III) led to a significant increase of CH3OH production at the expense of the reduced yield of CH3COOH.

Engineering Atomic Single Metal-FeN4Cl Sites with Enhanced Oxygen-Reduction Activity for High-Performance Proton Exchange Membrane Fuel Cells.

Fe-N-C single-atomic metal site catalysts (SACs) have garnered tremendous interest in the oxygen reduction reaction (ORR) to substitute Pt-based catalysts in proton exchange membrane fuel cells. Nowadays, efforts have been devoted to modulating the electronic structure of metal single-atomic sites for enhancing the catalytic activities of Fe-N-C SACs, like doping heteroatoms to modulate the electronic structure of the Fe-Nx active center. However, most strategies use uncontrolled long-range interactions with heteroatoms on the Fe-Nx substrate, and thus the effect may not precisely control near-range coordinated interactions. Herein, the chlorine (Cl) is used to adjust the Fe-Nx active center via a near-range coordinated interaction. The synthesized FeN4Cl SAC likely contains the FeN4Cl active sites in the carbon matrix. The additional Fe-Cl coordination improves the instrinsic ORR activity compared with normal FeNx SAC, evidenced by density functional theory calculations, the measured ORR half-wave potential (E1/2, 0.818 V), and excellent membrane electrode assembly performance.

Single-Atomic Site Catalyst Enhanced Lateral Flow Immunoassay for Point-of-Care Detection of Herbicide.

Point-of-care (POC) detection of herbicides is of great importance due to their impact on the environment and potential risks to human health. Here, we design a single-atomic site catalyst (SASC) with excellent peroxidase-like (POD-like) catalytic activity, which enhances the detection performance of corresponding lateral flow immunoassay (LFIA). The iron single-atomic site catalyst (Fe-SASC) is synthesized from hemin-doped ZIF-8, creating active sites that mimic the Fe active center coordination environment of natural enzyme and their functions. Due to its atomically dispersed iron active sites that result in maximum utilization of active metal atoms, the Fe-SASC exhibits superior POD-like activity, which has great potential to replace its natural counterparts. Also, the catalytic mechanism of Fe-SASC is systematically investigated. Utilizing its outstanding catalytic activity, the Fe-SASC is used as label to construct LFIA (Fe-SASC-LFIA) for herbicide detection. The 2,4-dichlorophenoxyacetic acid (2,4-D) is selected as a target here, since it is a commonly used herbicide as well as a biomarker for herbicide exposure evaluation. A linear detection range of 1-250 ng/mL with a low limit of detection (LOD) of 0.82 ng/mL has been achieved. Meanwhile, excellent specificity and selectivity towards 2,4-D have been obtained. The outstanding detection performance of the Fe-SASC-LFIA has also been demonstrated in the detection of human urine samples, indicating the practicability of this POC detection platform for analyzing the 2,4-D exposure level of a person. We believe this proposed Fe-SASC-LFIA has potential as a portable, rapid, and high-sensitive POC detection strategy for pesticide exposure evaluation.

Single-Atomic Iron Doped Carbon Dots with Both Photoluminescence and Oxidase-Like Activity.

Multifunctional nanozymes can benefit biochemical analysis via expanding sensing modes and enhancing analytical performance, but designing multifunctional nanozymes to realize the desired sensing of targets is challenging. In this work, single-atomic iron doped carbon dots (SA Fe-CDs) are designed and synthesized via a facile in situ pyrolysis process. The small-sized CDs not only maintain their tunable fluorescence, but also serve as a support for loading dispersed active sites. Monoatomic Fe offers SA Fe-CDs exceptional oxidase-mimetic activity to catalyze 3,3',5,5'-tetramethylbenzidine (TMB) oxidation with fast response (Vmax  = 10.4 nM s-1 ) and strong affinity (Km  = 168 µM). Meanwhile, their photoluminescence is quenched by the oxidation product of TMB due to inner filter effect. Phosphate ions (Pi) can suppress the oxidase-mimicking activity and restore the photoluminescence of SA Fe-CDs by interacting with Fe active sites. Based on this principle, a dual-mode colorimetric and fluorescence assay of Pi with high sensitivity, selectivity, and rapid response is established. This work paves a path to develop multifunctional enzyme-like catalysts, and offers a simple but efficient dual-mode method for phosphate monitoring, which will inspire the exploration of multi-mode sensing strategies based on nanozyme catalysis.

Directly Probing the Local Coordination, Charge State, and Stability of Single Atom Catalysts by Advanced Electron Microscopy: A Review.

The drive for atom efficient catalysts with carefully controlled properties has motivated the development of single atom catalysts (SACs), aided by a variety of synthetic methods, characterization techniques, and computational modeling. The distinct capabilities of SACs for oxidation, hydrogenation, and electrocatalytic reactions have led to the optimization of activity and selectivity through composition variation. However, characterization methods such as infrared and X-ray spectroscopy are incapable of direct observations at atomic scale. Advances in transmission electron microscopy (TEM) including aberration correction, monochromators, environmental TEM, and micro-electro-mechanical system based in situ holders have improved catalysis study, allowing researchers to peer into regimes previously unavailable, observing critical structural and chemical information at atomic scale. This review presents recent development and applications of TEM techniques to garner information about the location, bonding characteristics, homogeneity, and stability of SACs. Aberration corrected TEM imaging routinely achieves sub-Ångstrom resolution to reveal the atomic structure of materials. TEM spectroscopy provides complementary information about local composition, chemical bonding, electronic properties, and atomic/molecular vibration with superior spatial resolution. In situ/operando TEM directly observe the evolution of SACs under reaction conditions. This review concludes with remarks on the challenges and opportunities for further development of TEM to study SACs.

2D metal-organic framework for stable perovskite solar cells with minimized lead leakage.

Despite the notable progress in perovskite solar cells, maintaining long-term operational stability and minimizing potentially leaked lead (Pb2+) ions are two challenges that are yet to be resolved. Here we address these issues using a thiol-functionalized 2D conjugated metal-organic framework as an electron-extraction layer at the perovskite/cathode interface. The resultant devices exhibit high power conversion efficiency (22.02%) along with a substantially improved long-term operational stability. The perovskite solar cell modified with a metal-organic framework could retain more than 90% of its initial efficiency under accelerated testing conditions, that is continuous light irradiation at maximum power point tracking for 1,000 h at 85 °C. More importantly, the functionalized metal-organic framework could capture most of the Pb2+ leaked from the degraded perovskite solar cells by forming water-insoluble solids. Therefore, this method that simultaneously tackles the operational stability and lead contamination issues in perovskite solar cells could greatly improve the feasibility of large-scale deployment of perovskite photovoltaic technology.

Crystallinity after decarboxylation of a metal-carboxylate framework: indestructible porosity for catalysis.

We report a curious case study of a Zr(iv)-carboxylate framework, which retains significant crystalline order after cascade thermocyclization of its linker components, and - more notably - after the crucial carboxylate links were severed by heat. Vigorous heat treatment (e.g., 450 °C and above) benzannulates the multiple alkyne groups on the linker to generate linked nanographene blocks and to afford real stability. The resultant Zr oxide/nanographene hybrid solid is stable in saturated NaOH and concentrated H3PO4, allowing a convenient anchoring of H3PO4 into its porous matrix to enable size-selective heterogeneous acid catalysis. The Zr oxide components can also be removed by strong hydrofluoric acid to further enhance the surface area (up to 650 m2 g-1), without collapsing the nanographene scaffold. The crystallinity order and the extensive thermal transformations were characterized by X-ray diffraction, scanning transmission electron microscopy (STEM), IR, solid state NMR and other instrumental methods.

Proximity Effects of Methyl Group on Ligand Steric Interactions and Colloidal Stability of Palladium Nanoparticles.

Metal nanoparticle catalysts functionalized with small, well-defined organic ligands are important because such systems can provide a spatial control in the catalyst-substrate interactions. This article describes the synthesis, stability, and catalytic property evaluations of four different Pd nanoparticles capped with constitutional isomers of pentanethiolate ligands that have either a straight chain or an alkyl chain with one methyl group at different locations (α, ß, or γ from the surface-bound sulfur). The structure and composition analyses of Pd nanoparticles confirm that they have similar average core sizes and organic ligand contents. The presence of methyl group at α position is found to lower the capping ability of short ligands and thus make Pd nanoparticles to lose their colloidal stability during the catalytic reactions. The overall activity and selectivity for hydrogenation and isomerization of pentene and allylbenzene derivatives are investigated for each combination of ligand and substrate. Catalysis results indicate that steric interactions between the ligands on the metal catalyst surface and the alkene substrates are a factor in controlling the activity of Pd nanoparticles. In particular, Pd nanoparticles capped with pentanethiolate isomer having a methyl group at α position exhibit poor and inconsistent catalytic activity due to its low colloidal stability. The presence of a methyl group at ß position mildly interferes the adsorption of alkene group on the nanoparticle surface resulting in lower conversion yields. Interestingly, a methyl group at γ position only has a minimal effect on the catalytic property of Pd nanoparticles exhibiting similar catalysis results with Pd nanoparticles capped with straight chain pentanethiolate ligands. This indicates the proximity of steric group at the reactive site controls the nanoparticle activity for surface oriented reactions, such as hydrogenation and isomerization of alkenes in addition to their colloidal stability.

Probing the dynamics of nanoparticle formation from a precursor at atomic resolution.

Control of reduction kinetics and nucleation processes is key in materials synthesis. However, understanding of the reduction dynamics in the initial stages is limited by the difficulty of imaging chemical reactions at the atomic scale; the chemical precursors are prone to reduction by the electron beams needed to achieve atomic resolution. Here, we study the reduction of a solid-state Pt precursor compound in an aberration-corrected transmission electron microscope by combining low-dose and in situ imaging. The beam-sensitive Pt precursor, K2PtCl4, is imaged at atomic resolution, enabling determination of individual (K, Pt, Cl) atoms. The transformation to Pt nanoclusters is captured in real time, showing a three-stage reaction including the breaking of the ionic bond, formation of PtCl2, and the reduction of the dual-valent Pt to Pt metal. Deciphering the atomic-scale transformation of chemicals in real time using combined low-dose and in situ imaging brings new possibility to study reaction kinetics in general.