Catalytic Hotspots Research Articles

Transient transition from Stable to Dissipative Assemblies in Response to the Spatiotemporal Availability of a Chemical Fuel

AbstractThe transition from inactive to active matter implies a transition from thermodynamically stable to energy‐dissipating structures. Here, we show how the spatiotemporal availability of a chemical fuel causes a thermodynamically stable self‐assembled structure to transiently pass to an energy‐dissipating state. The system relies on the local injection of a weak affinity phosphodiester substrate into an agarose hydrogel containing surfactant‐based structures templated by ATP. Injection of substrate leads to the inclusion of additional surfactant molecules in the assemblies leading to the formation of catalytic hotspots for substrate conversion. After the local disappearance of the substrate as a result of chemical conversion and diffusion the assemblies spontaneously return to the stable state, which can be reactivated upon the injection of a new batch of fuel. The study illustrates how a dissipating self‐assembled system can cope with the intermittent availability of chemical energy without compromising long‐term structural stability.

Transient transition from stable to dissipative assemblies in response to the spatiotemporal availability of a chemical fuel.

The transition from inactive to active matter implies a transition from thermodynamically stable to energy-dissipating structures. Here, we show how the spatiotemporal availability of a chemical fuel causes a thermodynamically stable self-assembled structure to transiently pass to an energy-dissipating state. The system relies on the local injection of a weak affinity phosphodiester substrate into an agarose hydrogel containing surfactant-based structures templated by ATP. Injection of substrate leads to the inclusion of additional surfactant molecules in the assemblies leading to the formation of catalytic hotspots for substrate conversion. After the local disappearance of the substrate as a result of chemical conversion and diffusion the assemblies spontaneously return to the stable state, which can be reactivated upon the injection of a new batch of fuel. The study illustrates how a dissipating self-assembled system can cope with the intermittent availability of chemical energy without compromising long-term structural stability.

Synergistic Microfluidic and Optical Performance Enhancements of Photoelectrodes for Space Applications

The realisation of long-term space missions as well as lunar habitats require reliable and efficient oxygen and chemical producing devices e.g., for life support, fuel generation and in-situ resource utilisation (ISRU). We have recently demonstrated that integrated semiconductor-electrocatalyst systems can be operated in microgravity environments generated for 9.2 s at the Bremen Drop Tower (Center of Applied Space Technology and Microgravity, ZARM) at terrestrial efficiencies, thus opening the possibility of utilising these photoelectrochemical (PEC) devices for chemical synthesis in space. 1,2 , 3 Our findings reveal that by altering the electrocatalyst surface morphology through nanostructuring, we can introduce catalytic hot-spots on the photoelectrode which - through their hydrophilic nature - can also enhance gas bubble detachment in microgravity and thus circumvent the near-absence of buoyancy.1,3 We have found that the surface hydrophilicity and electrocatalyst loading require however careful balancing as the most effective nanostructures for gas bubble desorption can also result in less catalyst material on the photoelectrode surface, which in turn leads to a non-optimal PEC device performance.Here, we present a combined theoretical and experimental microfluidic, optical and photoelectrocatalytic performance analysis of photoelectrodes with integrated, nanostructured electrocatalysts, aiming at providing a guideline for an optimal design for microgravity applications. Focusing on the hydrogen evolution reaction (HER) and a well-investigated photoabsorber for this reaction, p-type InP, we firstly built an optoelectronic model in COMSOL Multiphysics in pursuit of quantifying the optical performance enhancements of Rh and Pt electrocatalyst nanostructures such as nanopyramids, nanowires and sub-wavelength aperture holes.4 We then developed a microfluidic model using the same nanostructure geometries to predict the hydrophilicity of the device surface and subsequently, the gas bubble contact angle. The best-performing nanostructured photoelectrodes predicted in our models where then manufactured, photoelectrochemically tested and fully optically and spectroscopically characterised. Predicted local electric field enhancements through catalytic hotspot formation were moreover experimentally validated using scanning electrochemical cell microscopy (SECCM). Our findings indicate that our developed optoelectronic and microfluidic models can be utilised to design a theoretical framework for identifying ideal electrocatalyst nanostructures for microgravity applications.References Brinkert K, et al. Efficient solar hydrogen generation in microgravity environment. Nature Communications 9, (2018). Brinkert K, Mandin P. Fundamentals and future applications of electrochemical energy conversion in space. npj Microgravity 8, (2022). Akay Ö, et al. Releasing the Bubbles: Nanotopographical Electrocatalyst Design for Efficient Photoelectrochemical Hydrogen Production in Microgravity Environment. Advanced Science 9, 2105380 (2022). Tembhurne S, Haussener S. Integrated Photo-Electrochemical Solar Fuel Generators under Concentrated Irradiation. Journal of The Electrochemical Society 163, H988 (2016). Pan Y, Huang S, Li F, Zhao X, Wang W. Coexistence of superhydrophilicity and superoleophobicity: theory, experiments and applications in oil/water separation. Journal of Materials Chemistry A 6, 15057-15063 (2018).

Plasmonic Cavity-Catalysis by Standing Hot Carrier Waves.

Manipulating active sites of catalysts is crucial but challenging in catalysis science and engineering. Beyond the design of the composition and structure of catalysts, the confined electromagnetic field in optical cavities has recently become a promising method for catalyzing chemical reactions via strong light-matter interactions. Another form of confined electromagnetic field, the charge density wave in plasmonic cavities, however, still needs to be explored for catalysis. Here, we present an unprecedented catalytic mode based on plasmonic cavities, called plasmonic cavity-catalysis. We achieve direct control of catalytic sites in plasmonic cavities through standing hot carrier waves. Periodic catalytic hotspots are formed because of localized energy and carrier distribution and can be well tuned by cavity geometry, charge density, and excitation angle. We also found that the catalytic activity of the cavity mode increases several orders of magnitude compared with conventional plasmonic catalysis. We ultimately demonstrate that the locally concentrated long-lived hot carriers in the standing wave mode underlie the formation of the catalytic hotspots. Plasmonic cavity-catalysis provides a new approach to manipulate the catalytic sites and rates and may expand the frontier of heterogeneous catalysis.

Formation of Catalytic Hotspots in ATP-Templated Assemblies.

The self-assembly of surfactant-based structures that rely for their formation on the combination of a thermodynamically controlled and a dissipative pathway is described. Adenosine triphosphate (ATP) acts as a high-affinity template and triggers assembly formation at low surfactant concentrations. The presence of these assemblies creates the conditions for the activation of a dissipative self-assembly process by a weak-affinity substrate. The substrate-induced recruitment of additional surfactants leads to the spontaneous formation of catalytic hotspots in the ATP-stabilized assemblies that cleave the substrate. As a result of the two self-assembly processes, catalysis can be observed at a surfactant concentration at which low catalytic activity is observed in the absence of ATP.

Two-dimensional template-directed synthesis of one-dimensional kink-rich Pd3Pb nanowires for efficient oxygen reduction

Developing facile synthetic strategies toward ultrafine one-dimensional (1D) nanowires (NWs) with rich catalytic hot spots is pivotal for exploring effective heterogeneous catalysts. Herein, we demonstrate a two-dimensional (2D) template-directed strategy for synthesizing 1D kink-rich Pd3Pb NWs with abundant grain boundaries to serve as high-efficiency electrocatalysts toward oxygen reduction reaction (ORR). In this one-pot synthesis, ultrathin Pd nanosheets were initially generated, which then served as self-sacrificial 2D nano-templates. A dynamic equilibrium growth was subsequently established on the 2D Pd nanosheets through the center-selected etching of Pd atoms and edge-preferred co-deposition of Pd/Pb atoms. This was followed by the oriented attachment of the generated Pd/Pb alloy nanograins and fragments. Thus, kink-rich Pd3Pb NWs with rich grain boundary defects were obtained in high yield, and these NWs were used as electrocatalytic active catalysts. The surface electronic interaction between Pd and Pb atoms effectively decreased the surface d-band center to weaken the binding of oxygen-containing intermediates toward improved ORR kinetics. Specifically, the kink-rich Pd3Pb NWs/C catalyst delivered outstanding ORR mass activity and specific activity (2.26 A⋅mgPd−1 and 2.59 mA⋅cm−2, respectively) in an alkaline solution. These values were respectively 13.3 and 10.8 times those of state-of-the-art commercial Pt/C catalyst. This study provides an innovative strategy for fabricating defect-rich low-dimensional nanocatalysts for efficient energy conversion catalysis.

Water induced ultrathin Mo2C nanosheets with high-density grain boundaries for enhanced hydrogen evolution

Grain boundary controlling is an effective approach for manipulating the electronic structure of electrocatalysts to improve their hydrogen evolution reaction performance. However, probing the direct effect of grain boundaries as highly active catalytic hot spots is very challenging. Herein, we demonstrate a general water-assisted carbothermal reaction strategy for the construction of ultrathin Mo2C nanosheets with high-density grain boundaries supported on N-doped graphene. The polycrystalline Mo2C nanosheets are connected with N-doped graphene through Mo–C bonds, which affords an ultra-high density of active sites, giving excellent hydrogen evolution activity and superior electrocatalytic stability. Theoretical calculations reveal that the dz2 orbital energy level of Mo atoms is controlled by the MoC3 pyramid configuration, which plays a vital role in governing the hydrogen evolution activity. The dz2 orbital energy level of metal atoms exhibits an intrinsic relationship with the catalyst activity and is regarded as a descriptor for predicting the hydrogen evolution activity.

Cerium oxide stabilizes single-atom catalysts

By using a molecular “glue” to hold atoms in place, researchers have come up with a method that disperses individual metal atoms across a solid surface and prevents them from coalescing. The advance may lead to practical procedures for making commercial catalysts that maximize use of expensive metals such as platinum. Many industrial-scale chemical processes are driven by solid catalysts consisting of tiny chunks of a noble metal such as platinum or rhodium dispersed on a solid support—often a metal oxide. Known as heterogeneous catalysts because the catalytic material is solid and the reactants are gases and liquids, these catalysts drive petroleum refining, polymerization chemistry, emissions cleanup, and other processes. Smaller is better when it comes to supported metal catalysts. That’s because the catalytic hot spots that convert reactants to products reside on the surfaces of the particles. Reactant molecules cannot reach metal atoms in the interior of a metal

Non‐Stoichiometric NiFeMo Solid Solutions; Tuning the Hydrogen Adsorption Energy via Molybdenum Incorporation

AbstractSolution precursor plasma spraying is used to produce catalytic trimetallic coatings containing Ni, Fe and Mo directly onto stainless‐steel mesh, Ni foam and carbon paper. The resulting material is mostly comprised of face centered cubic FeNi3 alloy forming a highly porous coating with nanostructured features. The addition of Mo (up to ≈14 at%) generates no new crystal phases but only an increase in the lattice parameter, indicating the formation of FeNi3Mox solid solutions. The FeNi3Mox solid solutions are used as electrocatalyst for the hydrogen evolution reaction (HER) in alkaline media. The addition of Mo increases the HER activity significantly reaching an optimum performance at ≈9 at% Mo (FeNi3Mo0.40) with an overpotential at −10 mA cm−2 of 112 mV and a Tafel slope of 109 mV dec−1. The enhanced HER activity is attributed to the formation of a FeNi3Mox solid solution with an increased work function that is correlated to smaller hydrogen adsorption energies. Theoretical activity maps reveal that sites near superficial Mo atoms forms catalytic hot spots and are responsible for the observed activity.

A Phosphatase-Mimetic Nano-Stabilizer of Mast Cells for Long-Term Prevention of Allergic Disease.

Allergic diseases are pathological immune responses with significant morbidity, which are closely associated with allergic mediators as released by allergen‐stimulated mast cells (MCs). Prophylactic stabilization of MCs is regarded as a practical approach to prevent allergic diseases. However, most of the existing small molecular MC stabilizers exhibit a narrow therapeutic time window, failing to provide long‐term prevention of allergic diseases. Herein, ceria nanoparticle (CeNP‐) based phosphatase‐mimetic nano‐stabilizers (PMNSs) with a long‐term therapeutic time window are developed for allergic disease prevention. By virtue of the regenerable catalytic hotspots of oxygen vacancies on the surface of CeNPs, PMNSs exhibit sustainable phosphatase‐mimetic activity to dephosphorylate phosphoproteins in allergen‐stimulated MCs. Consequently, PMNSs constantly modulate intracellular phospho‐signaling cascades of MCs to inhibit the degranulation of allergic mediators, which prevents the initiation of allergic mediator‐associated pathological responses, eventually providing protection against allergic diseases with a long‐term therapeutic time window.

Controlling the Growth Locations of Ag Nanoparticles at Nanoscale by Shifting LSPR Hotspots.

Controlling chemical reactions by plasma is expected to be a new method for improving the structural properties of substrates. An Au nanojar array was prepared when Au was deposited onto a 2D polystyrene (PS) array. The site-selective chemical growth of Ag nanoparticle rings was realized around the Au nanojar necks by a local surface plasmon resonance (LSPR)-assisted chemical reaction. The catalytic hotspots in the nanostructure array could be controlled by both etching the nanojars and Au or TiO2 sputtering onto the nanojars, which were confirmed by the growth sites of the Ag nanoparticle in the LSPR-assisted chemical reaction. The structure of the nanojars and the electric field distributions of the growing nanoparticles were simulated and analyzed using Finite-Difference Time-Domain. FDTD simulations showed that the changes in the nanojar shape led to the changed hotspot distributions. At the same time, tracking the hotspot shifts in the process of structural change was also achieved by the observation of Ag growth. Nanoarray structure prepared by LSPR-assisted chemical reaction is one of the hot fields in current research and is also of great significance for the application of Surface-Enhanced Raman Scattering.

Oxygen Vacancy-Engineered PEGylated MoO3-x Nanoparticles with Superior Sulfite Oxidase Mimetic Activity for Vitamin B1 Detection.

Sulfite oxidase (SuOx ) is a molybdenum-dependent enzyme that catalyzes the oxidation of sulfite to sulfate to maintain the intracellular levels of sulfite at an appropriate low level. The deficiency of SuOx would cause severe neurological damage and infant diseases, which makes SuOx of tremendous biomedical importance. Herein, a SuOx mimic nanozyme of PEGylated (polyethylene glycol)-MoO3-x nanoparticles (P-MoO3-x NPs) with abundant oxygen vacancies created by vacancy-engineering is reported. Their level of SuOx -like activity is 12 times higher than that of bulk-MoO3 . It is also established that the superior increased enzyme mimetic activity is due to the introduction of the oxygen vacancies acting as catalytic hotspots, which allows better sulfite capture ability. It is found that vitamin B1 (VB1) inhibits the SuOx mimic activity of P-MoO3-x NPs through the irreversible cleavage by sulfite and the electrostatic interaction with P-MoO3-x NPs. A colorimetric platform is developed for the detection of VB1 with high sensitivity (the low detection limit is 0.46 µg mL-1 ) and good selectivity. These findings pave the way for further investigating the nanozyme which possess intrinsic SuOx mimicing activity and is thus a promising candidate for biomedical detection.

Simultaneous SECM-AFM-Ters Mapping of Electrocatalytic Reactions on a Nanometric Scale

We have developed and used Scanning Electrochemical Microscopy-Atomic Force Microscopy (SECM-AFM) imaging technique that can provide high resolution detailed electrochemical and spectral information of oxygen reduction reaction (ORR) reaction at a lateral resolution of less than 50 nm. The ORR activity of an individual unsupported Pt nanoparticle, carbon-supported Pt aggregates and Fe, N surface modified graphitic carbon was mapped under selected reduction conditions. This newly developed method utilizes a gold coated SiO2 tip imbedding a 50 nm diameter Pt electrode. positioned at a constant working distance of ~4-8 nm above the catalyst surface using force feedback control. A 532 nm Raman laser directed to the apex of the tip was applied to collect tip enhanced Raman spectral (TERS) signal of the reaction byproduct (H2O2). Thus, we simultaneously mapped the electrochemical oxygen reduction by the Pt tip electrode and measured the H2O2 oxidation currents on the gold coating secondary electrode and at the same time reordered the peroxide tip enhanced Raman signal under the same scanned area. A direct evidence was seen, showing for the first time that the same active ORR sites are also responsible for peroxide formation (figure 1). Moreover, on a nanometric level ORR proceed mostly via 2 electron reaction to hydrogen peroxide rather than the literature reported 4 electrons reduction of O2 to water, as widely accepted from other electrochemical techniques. Fig. 1. Nanoscale Mapping of Catalytic Hotspots on Fe, N -modified HOPG by Scanning Electrochemical Microscopy-Atomic Force Microscopy [1]. [1] Srikanth Kolagatla, Palaniappan Subramanian and Alex Schechter*, Nanoscale, 2018, 10, 6962 Figure 1

Polydopamine-assisted immobilization of Ag@AuNPs on cotton fabrics for sensitive and responsive SERS detection

Depositing anisotropic noble metal nanoparticles with high uniformity and yield on flexible substrates is the determining factor for surface enhanced Raman spectroscopy (SERS) detection. In this work, flexible, durable and sensitive SERS substrates were fabricated by in situ reduction of Ag nanoparticles on polydopamine templated cotton fabrics (CF) as catalytic hotspots to enhance the following deposition of Au nanoparticles. The coated CF were characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy and X-ray diffraction to understand the surface morphology, chemical composition and crystalline structure, respectively. The SEM images indicate that the nanoparticles are dispersed evenly on the CF. 4-Mercaptobenzoic acid (4-MBA) has been used as the probe molecule to evaluate the sensitive and reproducible SERS properties of the as-fabricated SERS substrate. The as-prepared SERS substrates were demonstrated to detect carbaryl pesticides on a cucumber, and carbaryl with the concentration of as low as 10−6 M (0.20 ppm) could be detected to ensure food safety.

Investigation on the phase transformation and lattice parameters of Sn2+, Cu2+, La3+ and Ce4+ ions doped titania: characterization and solar light activity study

Nanocrystalline titania and metal ion doped titania systems were prepared by combustion method. Titania exhibited bicrystalline phase consisting of both anatase and rutile. The bicrystalline framework in TiO2 powders can be conveniently altered by the incorporation of various bivalent/trivalent/tetravalent metal ions like Sn2+, Cu2+, La3+and Ce4+. The microstructure development and the influence of small amount of dopants on the resulting structural change have been investigated. p-XRD results confirms the stabilization of anatase phase in the case of Cu2+ and Sn2+ doped titania, while the polymorphic interfaces were retained in La3+and Ce4+ doped titania samples. The efficacy of the modified titania was studied on the degradation of Congo Red dye by harvesting solar light. Sn2+/TiO2 shows almost same photodegradation ability as that of pristine TiO2. Subsistence of catalytic hot spots at the interface of anatase-rutile and the synergistic effect of polymorphic phases explains the enhanced activity of La3+/TiO2 and Ce4+/TiO2. But the activity of Cu2+/TiO2 was found to be less due to high electron-hole recombination rate.

Coenzyme A-regulated Pd nanocatalysts for formic acid-mediated reduction of hexavalent chromium

Coenzyme A (CoA), as a cofactor of enzymes in bacteria, is involved in various reactions of intermediary metabolism owing to its thiol groups. Herein we develop a facile CoA-assisted approach for the green synthesis of nonspherical Pd nanoparticles (NPs) with grain boundaries, which potentially serve as the catalytic hot spots. As-prepared CoA-Pd nanocomposites were applied for the catalytic reduction of highly toxic chromium (VI) by using formic acid (FA) as the reductant with the highest reduction rate of 2.45 mmol mg−1 min−1 (50 °C), which is superior to recently reported homogenous and heterogeneous Pd-based catalysts. Through the characterizations by UV–vis, FTIR, TEM, XRD and XPS in combination with DFT calculations, CoA plays an essential role in regulating the growth of Pd NPs, resulting in the formation of nonspherical morphology at high [Na2PdCl4]/[CoA]. The catalytic activity of CoA-Pd was also assessed for the hydrogenation reduction of p-nitrophenol and the degradation of trichloroethylene (TCE) in the presence of formic acid. This strategy expands eco-friendly synthetic techniques to design highly efficient nanocatalysts to meet the requirement of catalytic reduction of Cr(VI) and other types of FA-mediated hydrogenation reactions.

Imaging Catalytic Hotspots on Single Plasmonic Nanostructures via Correlated Super-Resolution and Electron Microscopy.

Surface-plasmon (SP) enhanced catalysis on plasmonic nanostructures brings opportunities to increase catalytic efficiency and alter catalytic selectivity. Understanding the underlying mechanism requires quantitative measurements of catalytic enhancement on these nanostructures, whose intrinsic structural heterogeneity presents experimental challenges. Using correlated super-resolution fluorescence microscopy and electron microscopy, here we report a quantitative visualization of SP-enhanced catalytic activity at the nanoscale within single plasmonic nanostructures. We focus on two Au- and Ag-based linked nanostructures that present plasmonic hotspots at nanoscale gaps. Spatially localized higher reaction rates at these gaps vs nongap regions report the SP-induced catalytic enhancements, which show direct correlations with the nanostructure geometries and local electric field enhancements. Furthermore, the catalytic enhancement scales quadratically with the local actual light intensity, attributable to hot electron involvement in the catalytic enhancement mechanism. These discoveries highlight the effectiveness of correlated super-resolution and electron microscopy in interrogating nanoscale catalytic properties.

Surface activation of graphene nanoribbons for oxygen reduction reaction by nitrogen doping and defect engineering: An ab initio study

Introducing heteroatoms and creating structural defects on graphene is a common and rather successful strategy to transform its inert basal plane into an efficient metal-free electrocatalyst for oxygen reduction reaction (ORR). However, the intricate atomic configuration of defective graphenes difficult their optimization as ORR electrocatalysts, where not only a large density of active sites is desirable, but also excellent electrical conductivity is required. Therefore, we used density functional theory to investigate the current-voltage characteristics and the catalytic active sites towards ORR of nitrogen-doped and defective graphene by using 8 zig-zag graphene nanoribbons as model systems. Detailed ORR catalytic activity maps are created for ten different systems showing the distribution of catalytic hot spots generated by each defect. Subsequently, the use of both current-voltage characteristics and catalytic activity maps allow to exclude inefficient systems that exhibit either low electrical conductivity or have adsorption energies far from optimal. Our study highlights the importance of considering not only the interaction energy of reaction intermediates to design electrocatalysts, but also the electrical conductivity of such configurations. We believe that this work is important for future experimental studies by providing insights on the use of graphene as a catalyst towards the ORR reaction.

Nanoscale mapping of catalytic hotspots on Fe, N-modified HOPG by scanning electrochemical microscopy-atomic force microscopy.

The scanning electrochemical microscopy-atomic force microscopy (SECM-AFM) technique is used to map catalytic currents post Fe and N surface modification of graphitic carbon with an ultra-high resolution of 50 nm. The oxidation current of the partial reduction product, hydrogen peroxide, was also mapped in the same location in the graphitic carbon. The current mapping and ex situ spectroscopic evidence revealed that Fe-coordinated nitrogen sites formed both in the edge and basal planes of highly ordered pyrolytic graphite (HOPG) constitute the primary oxygen reduction catalytic sites in acid solutions of this important yet insufficiently understood class of catalysts.

Ligand-Assisted, One-Pot Synthesis of Rh-on-Cu Nanoscale Sea Urchins with High-Density Interfaces for Boosting CO Oxidation.

Predictable synthesis of bimetallic nanocrystals with spatially controlled metal distributions offers a versatile route to the development of highly efficient nanocatalysts. Here we report a one-pot synthesis of super branched Rh-on-Cu nanoscale sea urchins (Rh-Cu NSUrs) with a high density of Cu-Rh interfaces by manipulating the ligand coordination chemistry. Structural analysis and UV-vis spectra reveal that ascorbic acid can serve as a Rh-selective coordination ligand in the nonaqueous synthesis to reverse the reduction potentials of Rh3+ and Cu2+ cations. The sequential reduction of Cu2+ and then Rh3+ cations, as well as the island epitaxial growth of Rh atoms on Cu cores, leads to the formation of Rh-on-Cu nanostructures mimicking sea urchin. The size of the Cu cores and the density of Rh branches can both be facilely regulated by tuning the mole ratio of Cu to Rh. The Cu-Rh NSUrs show enhanced activity and stability in catalyzing CO oxidation, as the intrinsic Cu-Rh interfaces can act as catalytic hot spots through a bifunctional mechanism. The Cu-Rh two-component system can separate the adsorption and activation of CO and O2 on the Rh and Cu surfaces, respectively, accelerating the generation of CO2 at the interfaces.