Specific Surface Sites Research Articles

Exploiting Non-Classical Nucleation for Improved Gas Bubble Mitigation on Nickel Electrocatalysts for the Oxygen Evolution Reaction

The efficient transport of gas poses a significant challenge in the design of high-performance gas evolving electrocatalysts. Inefficiencies due to mass transport limitations become more pronounced at the high current densities required in industrial electrolyzers, largely as a result of gas bubbles that obstruct a portion of the electrochemically active surface area of the anode and/or cathode, while also hindering electrolyte transport to the electrode surfaces. One promising approach to mitigate these sources of inefficiency is to engineer micro-to-nanoscale surface textures that promote the favorable growth and release of these bubbles. A range of structures have been shown to improve electrocatalyst performance under both stagnant and induced convection conditions, particularly in applications for hydrogen production via electrocatalytic water splitting in alkaline water electrolyzers.1 Trends between surface texture and electrocatalyst performance have been observed for a range of features, such as pillars,2,3 ridges,4 dimples,5 “cracked” structures,6 and electrodeposited crystalline structures,7 however substantial work is still needed to optimize the designs of these materials. These textured surfaces can influence bubble evolution in various ways, such as by altering the wetting properties of a surface, or by providing preferential nucleation sites. Previous studies have noted that surface structures that provide good control over bubble nucleation can often be adversely affected by increased bubble retention.3–5 The present study investigates a method of fabricating patterned electrocatalysts that provides a fine, tunable control over the distribution of bubble nucleation sites on the surfaces of planar Ni electrocatalysts for the oxygen evolution reaction (OER). These textured surfaces are designed to take advantage of so-called “non-classical nucleation” or “type IV nucleation”, a scenario in which gas is trapped at specific surface sites with an effective radius larger than the critical radius for classical nucleation, thereby allowing spontaneous bubble growth without being required to overcome an activation energy barrier.8 The separation between these sites on the surface of the OER active material (i.e., Ni) is then varied to obtain the most favorable distribution of these non-classical nucleation sites. Chronoamperometry is used to compare the current densities achieved by these patterned Ni electrodes to that of a planar Ni electrode. High speed videography is used to directly measure how these textures influence processes such as bubble growth and detachment, and to determine how these processes influence performance. A better understanding of these gas evolution processes can help lower the energy requirements for electrolyzers and may have more general implications for the design of energy efficient gas evolving electrocatalysts relevant to a variety of industrially important reactions.

Multi-Enzyme Cascade Nanoreactors for High-Throughput Immunoassay: Transitioning Concept in Lab to Application in Community.

In this work, we reported a cholesterol oxidase (Chox)-loaded platinum (Pt) nanozyme with the collaborative cascade nanoreactor for the construction of nanozyme-enzyme-linked immunosorbent assay (N-ELSA) models to realize high-throughput rapid evaluation of cancer markers. Considering the high specific surface area and manipulable surface sites, ZIF-8 was used as a substrate for natural enzyme and nanozyme loading. The constructed ZIF-8-Pt nanozyme platform exhibited efficient enzyme-like catalytic efficiency with a standard corrected activity of 60.59 U mg-1, which was 12 times higher than that of the ZIF-8 precursor, and highly efficient photothermal conversion efficiency (∼35.49%). In N-ELISA testing, developed multienzyme photothermal probes were immobilized in microplates based on antigen-antibody-specific reactions. Cholesterol was reacted in a cascade to reactive oxygen radicals, which attacked 3,3',5,5'-tetramethylbenzidine, causing it to oxidize and color change, thus exhibiting highly enhanced efficient photothermal properties. Systematic temperature evaluations were performed by a hand-held microelectromechanical system thermal imager under the excitation of an 808 nm surface light source to determine the cancer antigen 15-3 (CA15-3) profiles in the samples. Encouragingly, the temperature signal from the microwells increased with increasing CA15-3, with a linear range of 2 mU mL-1 to 100 U mL-1, considering it to be the sensor with the widest working range for visualization and portability available. This work provides new horizons for the development of efficient multienzyme portable colorimetric-photothermal platforms to help advance the community-based process of early cancer detection.

Nascent chains derived from a foldable protein sequence interact with specific ribosomal surface sites near the exit tunnel

In order to become bioactive, proteins must be translated and protected from aggregation during biosynthesis. The ribosome and molecular chaperones play a key role in this process. Ribosome-bound nascent chains (RNCs) of intrinsically disordered proteins and RNCs bearing a signal/arrest sequence are known to interact with ribosomal proteins. However, in the case of RNCs bearing foldable protein sequences, not much information is available on these interactions. Here, via a combination of chemical crosslinking and time-resolved fluorescence-anisotropy, we find that nascent chains of the foldable globin apoHmp1–140 interact with ribosomal protein L23 and have a freely-tumbling non-interacting N-terminal compact region comprising 63–94 residues. Longer RNCs (apoHmp1–189) also interact with an additional yet unidentified ribosomal protein, as well as with chaperones. Surprisingly, the apparent strength of RNC/r-protein interactions does not depend on nascent-chain sequence. Overall, foldable nascent chains establish and expand interactions with selected ribosomal proteins and chaperones, as they get longer. These data are significant because they reveal the interplay between independent conformational sampling and nascent-protein interactions with the ribosomal surface.

Nanomaterials Based Multifunctional Bioactivities of V2O5 and Mesoporous Carbon@V2O5 Composite: Preparation and Characterization.

Nanocarriers have attracted considerable interest due to their prospective applications in the delivery of anticancer medications and their distinct bioactivities. Biogenic nanostructures can be effective nanocarriers for delivering drugs as a consequence of sustainable and biodegradable biomass-derived nanostructures that perform specific functions. In this case, a vanadium oxide (V2O5) and mesoporous carbon@V2O5 (C@V) composite was developed as a possible drug delivery system, and its bioactivities, including antioxidant, antibacterial, and anticancer, were investigated. Doxorubicin (DOX), an anticancer drug, was introduced to the nanoparticles, and the loading and release investigation was conducted. Strong interfacial interactions between mesoporous carbon (MC) and V2O5 nanostructures have been found to improve performance in drug loading and release studies and bioactivities. After incubation, the potent anticancer effectiveness was seen based on C@V nanocomposite. This sample was also utilized to research potential biomedical uses as an antioxidant, antibacterial, and anticancer. The most effective antioxidant, the C@V sample (61.2%), exhibited a higher antioxidant activity than the V-2 sample (44.61%). The C@V sample ultimately attained a high DOX loading efficacy of 88%, in comparison to a pure V2O5 sample (V-2) loading efficacy of 80%. Due to the combination of mesoporous carbon and V2O5, which increases specific surface area and surface sites of action as well as the morphology, it proved that the mesoporous carbon@V2O5 composite (C@V) sample demonstrated greater efficacy.

Site-specific reactivity of stepped Pt surfaces driven by stress release.

Heterogeneous catalysts are widely used to promote chemical reactions. Although it is known that chemical reactions usually happen on catalyst surfaces, only specific surface sites have high catalytic activity. Thus, identifying active sites and maximizing their presence lies at the heart of catalysis research1-4, in which the classic model is to categorize active sites in terms of distinct surface motifs, such as terraces and steps1,5-10. However, such a simple categorization often leads to orders of magnitude errors in catalyst activity predictions and qualitative uncertainties of active sites7,8,11,12, thus limiting opportunities for catalyst design. Here, using stepped Pt(111) surfaces and the electrochemical oxygen reduction reaction (ORR) as examples, we demonstrate that the root cause of larger errors and uncertainties is a simplified categorization thatoverlooks atomic site-specific reactivity driven by surface stress release. Specifically, surface stress release at steps introduces inhomogeneous strain fields, with up to 5.5% compression, leading to distinct electronic structures and reactivity for terrace atoms with identical local coordination, and resulting in atomic site-specific enhancement of ORR activity. For the terrace atoms flanking both sides of the step edge, the enhancement is up to 50 times higher than that of the atoms in the middle of the terrace, which permits control of ORR reactivity by either varying terrace widths or controlling external stress. Thus, the discovery of the above synergy provides a new perspective for both fundamental understanding of catalytically active atomic sites and design principles of heterogeneous catalysts.

Controlling the Phase Transformation of Alumina for Enhanced Stability and Catalytic Properties

AbstractCurrent transition alumina catalysts require the presence of significant amounts of toxic, environmentally deleterious dopants for their stabilization. Herein, we report a simple and novel strategy to engineer transition aluminas to withstand aging temperatures up to 1200 °C without inducing the transformation to low‐surface‐area α‐Al2O3 and without requiring dopants. By judiciously optimizing the abundance of dominant facets and the interparticle distance, we can control the temperature of the phase transformation from θ‐Al2O3 to α‐Al2O3 and the specific surface sites on the latter. These specific surface sites provide favorable interactions with supported metal catalysts, leading to improved metal dispersion and greatly enhanced catalytic activity for hydrocarbon oxidation. The results presented herein not only provide molecular‐level insights into the critical factors causing deactivation and phase transformation of aluminas but also pave the way for the development of catalysts with improved activity for catalytic hydrocarbon oxidation.

Controlling the Phase Transformation of Alumina for Enhanced Stability and Catalytic Properties.

Current transition alumina catalysts require the presence of significant amounts of toxic, environmentally deleterious dopants for their stabilization. Herein, we report a simple and novel strategy to engineer transition aluminas to withstand aging temperatures up to 1200 °C without inducing the transformation to low-surface-area α-Al2O3 and without requiring dopants. By judiciously optimizing the abundance of dominant facets and the interparticle distance, we can control the temperature of the phase transformation from θ-Al2O3 to α-Al2O3 and the specific surface sites on the latter. These specific surface sites provide favorable interactions with supported metal catalysts, leading to improved metal dispersion and greatly enhanced catalytic activity for hydrocarbon oxidation. The results presented herein not only provide molecular-level insights into the critical factors causing deactivation and phase transformation of aluminas but also pave the way for the development of catalysts with improved activity for catalytic hydrocarbon oxidation.

The effects of NaCl addition on the particle-bubble interactions of galena in the presence of xanthate

This work investigated the effects of NaCl addition on galena flotation in the presence of xanthate. The micro-flotation experiments were performed using NaCl solutions which also included xanthate, at pH 9 (±0.1). Our results indicated that galena recovery improved for higher NaCl as well as higher xanthate concentrations.A pH-dependent chemisorption model for the galena surface, with the addition of xanthate adsorption was calibrated using measured zeta potential values. We propose that xanthate adsorption on galena can take place via two separate mechanisms. The first mechanism involves direct xanthate chemisorption to specific surface sites. The second mechanism involves lead/xanthate complexes formed in the bulk solution. These lead/xanthate complexes attach on the galena surface as hydrophobic lead xanthate salts.The galena-air bubble interactions are repulsive in 1 mM NaCl, with or without xanthate, consistent with the lower galena recovery measured experimentally. An increase to 100 mM NaCl, irrespective of the xanthate addition, resulted in attractive galena-air bubble total interaction energies. The agreement with the experimental results shows the effectiveness of the charge regulated model for estimating the galena and air bubble behaviours during flotation in NaCl solutions.

Designed fabrication of MoS2 hollow structures with different geometries and the comparative investigation toward capacitive properties.

Hollow MoS2 cubes and spheres were synthesized by a one-step hydrothermal method with the hard template method. The structure and morphology were characterized, and their electrochemical properties were studied. It is concluded that the specific capacitance of the hollow MoS2 cubes (335.7 F g-1) is higher than that of the hollow MoS2 spheres (256.1 F g-1). The symmetrical supercapacitors were assembled, and the results indicate that the specific capacitance of the device composed of hollow MoS2 spheres (32.9 F g-1) is slightly lower than that of the hollow MoS2 cube (37.4 F g-1) device. Furthermore, the symmetrical supercapacitor (MoS2-cube//MoS2-cube) provides a maximum energy density of 4.93 W h kg-1, which is greater than that of the symmetrical capacitor (MoS2-sphere//MoS2-sphere, 3.65 W h kg-1). This may indicate that hollow molybdenum disulfide cubes with substructures have more efficient charge transfer capabilities and better capacitance characteristics than hollow spheres. After 8000 cycles, the coulombic efficiency of the two symmetrical capacitors is close to 100%. The capacity retention of the MoS2 sphere device (95.2%) is slightly higher than that of the MoS2 cube device (90.1%). These results show that the pore structure, specific surface, and active site of MoS2 with different hollow structures have a greater impact on its electrochemical properties.

Gas-Phase Pd Clusters-Modified Mesoporous Copper Oxide Hollow Spheres As Electrocatalysts for CO2 Reduction to Ethylene

Renewable energy-driven electrochemical CO2 conversion to value-added chemicals is a prospective strategy for addressing global carbon emission and energy consumption issues worldwide. Until today, only copper-based electrocatalysts can successfully transform CO2 into C2H4 or other desirable C2+ products, but their stability and product selectivity remain insufficient.1 Oxide-derived Cu mesoporous foam catalysts currently show the best selectivity toward C2+ product formation at particularly low overpotentials due to the availability of specific surface sites for C−C coupling in their structure and to the temporal trapping of gaseous intermediates inside the mesoporous catalyst material during CO2 electrolysis.2 To further improve their stability and product selectivity their surface can be modified with metallic clusters that can promote the adsorption of CO2 and the subsequent formation of intermediates. In particular, Pd clusters provide a favorable surface for the initial adsorption of CO2,3 while inducing a continuous restructuring of the Cu surface that maintains its catalytic properties for CO2 reduction to hydrocarbons.4 Herein, we report a novel highly efficient electrocatalyst for CO2 conversion in C2+ products based on mesoporous oxygen-rich copper hollow spheres prepared by a colloid templating method, whose surface is uniformly modified by the deposition of different loadings of well-defined Pd clusters of ca. 3 nm diameter using the laser ablation cluster beam deposition (CBD) technology.5 Primary electrochemical results show that these electrodes are able to reduce CO2 to ethylene with a faradaic efficiency of more than three times higher than that of commercial Cu2O nanoparticles under the same reaction conditions. A clear phase transition from CuO to Cu2O and metallic Cu is occurring under CO2 electro-reduction conditions as highlighted by XRD.These remarkable performances are likely originating from the facile gas charge transport via the mesoporous structure of the oxygen-rich copper spheres as imaged by SEM (Figure 1) as well as from their high surface area, which allows a high catalytic activity and a uniform accommodation of the metallic clusters.As CBD is a versatile technique that allows the deposition of virtually any type of well-defined cluster on a large variety of support, this work provides an attractive avenue to achieve stable selective multicarbon products via rational electrode design. References Kuhl KP, et al., Energy and Environmental Science, 2012;5 (5):7050-7059.Abhijit Dutta, et al., ACS Catal. 2016, 6, 3804−3814Sichao Ma, et al., J. Am. Soc. 2017, 139, 47−50Zhe Weng, et al., Angew. Int. Ed. 2017, 56, 13135 –13139Chinnabathini V. C., et al., Nanoscale, 2023, DOI: 10.1039/D2NR07287D. Figure 1

Electrochemical surface functionalization engineering of carbon cloth for ultrasensitive electrochemical detection of nitrite

In this work, the commercial carbon cloth modified with a large number of oxygen-containing functional groups (C-OH, COOH and CO) was fabricated by simple constant current electrochemical treatment (EFCC electrode) for the ultrasensitive electrochemical detection of nitrite. Due to the combination of enhanced superhydrophilicity, significantly improved conductivity and dramatically increased active specific surface area and sites, the EFCC electrode exhibited excellent nitrite detection performance, including wide detection ranges of 0.1–3500 μM divided into three ranges, low detection limit of 0.016 μM (S/N = 3) and ultra-high sensitivities of 12310, 3515, 1877 μA mM−1 cm−2. This work provided an important reference for the application of functionalized modification strategies to construct high-performance electrochemical sensors.

Isotope effect suggests site‐specific nonadiabaticity on Ge(111)c(2×8)

AbstractEnergy transferred in atom‐surface collisions typically depends strongly on projectile mass, an effect that can be experimentally detected by isotopic substitution. In this work, we present measurements of inelastic H and D atom scattering from a semiconducting Ge(111)c(2×8) surface exhibiting two scattering channels. The first channel shows the expected isotope effect and is quantitatively reproduced by electronically adiabatic molecular dynamics simulations. The second channel involves electronic excitations of the solid and, surprisingly, exhibits almost no isotope effect. We attribute these observations to scattering dynamics, wherein the likelihood of electronic excitation varies with the impact site engaged in the interaction.Key Points Previous work revealed that H atoms with sufficient translational energy can excite electrons over the band gap of a semiconductor in a surface collision. We studied the isotope effect of the energy transfer by H/D substitution and performed band structure calculations to elucidate the underlying excitation mechanism. Our results suggest a site‐specific mechanism that requires the atom to hit a specific surface site to excite an electron‐hole pair.

Building Surfaces with Controlled Site-Density of Anchored Human Serum Albumin.

Stable and uniform layers of protein molecules at the surface are important to build passive devices as well as active constructs for smart biointerfaces for a large number of biomedical applications. In this context, a strategy to build-up surfaces able to anchor protein molecules on specific and controlled surface sites has been developed. Human serum albumin (HSA) has been chosen as a model protein due to its important antithrombogenic properties and its features in cell response highly valuable for in vivo devices. Uniform self-assembled monolayers of 2,2':6'2″-terpyridines (SAM), whose sites were further employed to chelate copper and iron ions, forming SAM-Cu(II) and SAM-Fe(II) complexes, have been developed. The effect of two metal cations on the physicochemical features of SAM, including thickness, Young's modulus, and tip-monolayer adhesion factors, has been investigated. Protein adsorption at different concentrations showed that the copper ion-templated surfaces exhibit highly specific mass uptake, kinetic behavior, and recognition and anchoring of HSA molecules owing to the coordination sphere of the different cations. The results pave the way to the development of a more general strategy to obtain ordered and density-tuned arrays of specific metal cations, which in turn would drive the anchoring of precise proteins for different biological functions.

Carboxyl-based deep eutectic solvent modified magnetic graphene oxide as a novel adsorbent for fast enrichment and extraction of estrogens in milk prior to HPLC-UV analysis

In this study, a magnetic graphene oxide modified with green carboxyl-based deep eutectic solvent (MGO@DES) was synthesized for the first time and employed as a highly efficient adsorbent for the magnetic solid-phase extraction (MSPE) of three estrogens in milk samples. With large specific surface, good hydrophobicity and abundant adsorption sites, MGO@DES exhibited excellent extraction performance for estrogens. The adsorption kinetic experiment revealed that 5 mg of MGO@DES reached the equilibrium of estrogen adsorption within 3 min. MGO@DES based MSPE coupled with HPLC-UV method was developed and applied to the determination of three estrogens in milk samples. Good linearity (R2 > 0.9953) was obtained in the range of 1–200 ng mL−1 and the low limits of detection were 0.20–0.28 ng mL−1. The spiked recoveries were in the range of 90.3–105.7%, with RSDs less than 7.9%.

Phosphate adsorption on amorphous aluminum hydroxide-yielded transition aluminas: Insights on fundamental chemistry and making engineering adsorbents

Transition aluminas (TAs) are evolved alumina polymorphs during mineral phase transformation from amorphous to crystalline structure, which have been extensively used as engineering adsorbents or platforms of functional materials for pollutants (e.g. eutrophication nutrient PO43−) mitigation in water remediation. A well-understanding of structure-binding activity of pollutants on transition aluminas is essential for developing promising water remediation materials. In this work, TAs were synthesized through annealing amorphous aluminum (oxy)hydroxide, and a decreased trend of phosphate (PO43−) adsorption was observed, which was not in pace with the changes of specific surface areas and site densities. The structure disorders of TAs deteriorated continuously peaked at 700 °C and TA at 700 °C has the largest extent of structural disorder, but does not present the highest adsorption capacity, which is not consistent with the consensus that oxides' structural disorder facilitate adsorption. Multiple techniques including pair distribution function, selective chemical extraction, 27Al NMR and quantum chemistry calculations were employed to reveal the underlying mechanism differences of phosphate adsorption on TAs. The results indicated the fraction of amorphous contents is the key feature governing PO43− adsorption, regardless of mineral phase, structural disorder, AlO4/AlO6 arrangement, and Gibbs free energy of adsorption. Three strategic measures are suggested on making promising alumina engineering adsorbents, including tuning transition temperature, introducing non-tetrahedral and non-octahedral nanostructured grains, and employing more potential methods for synthesizing amorphous nanocrystalline composite. This work sheds lights on understanding the fundamental chemistry of phosphate on transition alumina and producing prospective alumina remediation materials.

Overall Surfacial Antenna Immobilization Boosts Carrier Migration for Visible Light Superoxide Generation.

The artificial photosynthesis efficiency is restricted for the covering of introduced light-absorbing antennas, where they could generally be immobilized at specific surface sites of semiconductors. Here, we highlight the emerging π-π interaction strategies in realizing overall surfacial antenna immobilization onto semiconductor supports, during which large-scale built-in electric fields are established, strongly driving photocarrier migration to catalytic centers accompanied with enhanced light absorption. By taking porphyrin and ZIF-8 as a prototype, both theoretical calculation and experimental results show that the photocatalytic systems with overall surfacial antennas could absorb broader visible light and form abundant photocarrier transport pathways from porphyrin to ZIF-8, boosting the generation of superoxide and organic synthesis efficiency. This work develops a universal strategy for design of efficient artificial photosynthetic systems.

Electrochemical Catalysis Generated By an External Electric Field in Conjunction with a Low Density-of-States Conductor

In electrochemistry the reaction rate depends strongly on the voltage difference applied between the two electrodes. A voltage increase at the electrodes will generate a larger electric field in the electric double layer (EDL), which increases the reaction rate, but this will also cause additional power to be dissipated. We show both theoretically and experimentally that one can increase the electrochemical reaction rate, at no additional cost in power, by utilizing an external electric field in the proximity of a material catalyst. This is not surprising since, although the exact mechanism for catalysis due to a material compound can be quite intricate, the basic underlying physical processes for catalysis can often be shown to be due to the molecular electric field at a specific catalyst surface site. Here we generalize this observation and show how the application of a ‘simple external’ electrostatic field in conjunction with a low density-of-states conductor such as graphene can be used to intensify the already large electric field in the EDL layer - thus accelerating the reaction rate without additional power dissipation. We focus on water electrolysis (more specifically, the hydrogen evolution reaction - HER) in our electrochemical reaction study. Based on our observed experimental results as well as our theoretical analysis we show how an electrode incorporating trapped charge at the interface between two dielectrics will generate a large electrostatic field which, in conjunction with the quantum capacitance of a low density-of-states conductor, allows penetration of this field into the electric double layer (EDL), thereby adding to the original EDL field and thus, functioning as a reaction rate accelerant or a catalyst. Our experiment shows a 100% increase in the hydrogen generation rate for the HER reaction when employing our external electrostatic ‘catalyst’ technique. Since this method depends primarily on the enhancement of the EDL electric field for the reaction rate increase, it can be universally applied to all electrochemical reactions without requiring a detailed molecular level understanding of the reaction of interest.

Hollow porous GaN nanofibers gas sensor for superior stability and sub-ppb-level NO2 gas detection

Accurate detection of NO2 gas with low concentration and high stability is of great significance to ecological protection and human health. In this work, hollow porous GaN nanofibers were fabricated by a low-cost solvothermal method and controlled nitridation process. Systematic characterizations results demonstrate large specific surface and abundant adsorpation sites of the GaN nanofibers. Excellent NO2 sensing performances were tested at 200 °C, including the high sensitivity of 22.9 (200 ppm), rapid response time within 11 s, high selectivity, and low detection limit (0.5 ppb). Furthermore, the anti-interference ability, cross-sensitivity characteristics, and long-term stability of the sensor in the actual working environment were also systematically investigated. The surface depletion layer model and the change of the potential barrier were used to explain the enhanced gas-sensing performance. The fabricated GaN sensor offered a promising candidate for real-time monitoring of sub-ppb-level NO2 gas in actual environments.

(Digital Presentation) High-Resolution Imaging of Active Sites Under Reaction Conditions for Carbon-Based Electrocatalysis

Due to the abundance and the electrochemical versatility of carbon, it is becoming an increasingly popular material for electrocatalytic applications. Whether it is used as a support material, as a catalyst, or even as a bifunctional catalyst, insights into the reaction processes are of fundamental value. In this light, we employ electrochemical scanning tunneling microscopy (EC-STM) to in-situ evaluate electrode surfaces' behavior and identify the nature of the active sites.[1] The experimental distinction between inactive and active sites of a catalytic system can be achieved by comparing the noise level of surface sites in the EC-STM signal while a reaction is ‘Off’ or ‘On’, respectively. The tunneling current will be stable under both conditions if the scanning tip is positioned over an inactive site. Over an active site, reactions occurring within the tunneling gap will influence the EC-STM signal, which can be observed as locally confined noise features superimposed on the surface morphology.Here, we examine highly ordered pyrolytic graphite (HOPG) in alkaline and acidic media as a model system for carbon-based structures. In an alkaline medium, we compare the activity of specific surface sites under oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) conditions (Figure 1a).[2] In both cases, predominantly steps and defects are active. However, in the case of the OER, the terraces also play a role. For the hydrogen evolution reaction (HER) in acidic media, it was possible to identify individual active sites on the ‘honeycomb’ structured surface with down to atomic resolution (Figure 1b).[3] Apart from HOPG, the technique will be demonstrated for metal-organic frameworks, another class of promising catalysts for ORR and OER.

Atomic-Site-Specific Surface Valence-Band Structure from X-Ray Standing-Wave Excited Photoemission.

X-ray standing-wave (XSW) excited photoelectron emission was used to measure the site-specific valence band (VB) for ½ monolayer (ML) Pt grown on a SrTiO_{3} (001) surface. The XSW induced modulations in the core level (CL), and VB photoemission from the surface and substrate atoms were monitored for three hkl substrate Bragg reflections. The XSW CL analysis shows the Pt to have a face-centered-cubic-like cube-on-cube epitaxy with the substrate. The XSW VB information compares well to a density functional theory calculated projected density of states from the surface and substrate atoms. Overall, this Letter represents a novel method for determining the contribution to the density of states by valence electrons from specific atomic surface sites.