Unifying the ORR and OER with surface oxygen and extracting their intrinsic activities on platinum

Common half-cell measurements deliver oxygen reduction activities linked to the chosen scan rate, limiting their value for fundamental and comparative studies on platinum. Here we show a deconvolution of the intrinsic kinetics from the effect of surface oxygen on platinum. We find an electronic effect of the surface oxygen, substantiate a Tafel slope of ~120 mV/decade, obtain an exchange current density of 13 ± 4 µA/cm2, and an activity of 7 mA/cm2 at 900 mV. Eventually, we broaden the scope of this analysis to the effects of surface rearrangement, alloying, and supported Pt nanoparticles, the latter providing insight into discrepancies between half-cell and fuel cell measurements. We find through computational methods that binding energies of intermediates would be weakened by the presence of highly coordinated oxygen atoms. Finally, we obtain a phenomenological rate equation for the oxygen reduction and evolution reaction, suggesting that both reactions follow a shared mechanism.

Alkali Metal Cation-Sulfate Anion Ion Pairs Promoted the Cleavage of C-C Bond During Ethanol Electrooxidation

Direct ethanol fuel cells show great promise as a means of converting biomass ethanol derived from biomass into electricity. However, the efficiency of complete conversion is hindered by the low selectivity in breaking the C-C bond. This selectivity is determined by factors such as the material structure and reaction conditions, including the nature of the supporting electrolyte. Cations serve not only as facilitators of electricity conduction through ion migration but also as influencers of the reaction pathways. In this study, we utilized differential electrochemical mass spectrometry to track the in situ generation of CO2 during potential scanning. The presence of alkali cations led to an enhancement in the CO2 selectivity. In addition, in situ Raman spectroscopy provided evidence of the formation of alkali metal cation-sulfate anion ion pairs. The catalytic activity and CO2 selectivity were found to be directly correlated to the ionic strength of these ion pairs.

Development of Electrochemical Cells and Their Application for Spatially Resolved Analysis Using a Multitechnique Approach: From Conventional Experiments to X-Ray Nanoprobe Beamlines

Real (electro)catalysts are often heterogeneous, and their activity and selectivity depend on the properties of specific active sites. Therefore, unveiling the so-called structure-activity relationship is essential for a rational search for better materials and, consequently, for the development of the field of (electro-)catalysis. Thus, spatially resolved techniques are powerful tools as they allow us to characterize and/or measure the activity and selectivity of different regions of heterogeneous catalysts. To take full advantage of that, we have developed spectroelectrochemical cells to perform spatially resolved analysis using X-ray nanoprobe synchrotron beamlines and conventional pieces of equipment. Here, we describe the techniques available at the Carnaúba beamline at the Sirius-LNLS storage ring, and then we show how our cells enable obtaining X-ray (XRF, XRD, XAS, etc.) and vibrational spectroscopy (FTIR and Raman) contrast images. Through some proof-of-concept experiments, we demonstrate how using a multi-technique approach could render a complete and detailed analysis of an (electro)catalyst overall performance.

Terrace-Rich Ultrathin PtCu Surface on Earth-Abundant Metal for Oxygen Reduction Reaction

The activity and stability of the platinum electrode toward the oxygen reduction reaction are size-dependent. Although small nanoparticles have high Pt utilization, the undercoordinated Pt sites on their surface are assumed to have too strong oxygen binding strength, thus often leading to compromised activity and surface instability. Herein, we report an extended nanostructured PtCu ultrathin surface to reduce the number of low-coordination sites without sacrificing the electrochemical active surface area (ECSA). The surface shows (111)-oriented characteristics, as proven by electrochemical probe reactions and spectroscopies. The PtCu surface brings over an order of magnitude increase in specific activity relative to commercial Pt/C and nearly 4-fold enhancement in ECSA compared to traditional thin films. Moreover, due to the weak absorption of air impurities (e.g., SO2, NO, CO) on highly coordinated sites, the catalyst displays enhanced contaminant tolerance compared with nanoparticulate Pt/C. This work promises a broad screening of extended nanostructured surface catalysts for electrochemical conversions.

Design of Pt-Sn-Zn Nanomaterials for Successful Methanol Electrooxidation Reaction

This work highlights the potential for the synthesis of new PtSnZn catalysts with enhanced efficiency and durability for methanol oxidation reaction (MOR) in low-temperature fuel cells. In this research, PtZn and PtSnZn nanoparticles deposited on high surface area Vulcan XC-72R Carbon support were created by a microwave-assisted polyol method. The electrochemical performances of synthesized catalysts were analyzed by cyclic voltammetry and by the electrooxidation of adsorbed CO and the chronoamperometric method. The physicochemical properties of obtained catalysts were characterized by transmission electron microscopy (TEM), thermogravimetric (TGA) analysis, energy dispersive spectroscopy (EDS) and by X-ray diffraction (XRD). The obtained findings showed the successful synthesis of platinum-based catalysts. It was established that PtSnZn/C and PtZn/C catalysts have high electrocatalytic performance in methanol oxidation reactions. Catalysts stability tests were obtained by chronoamperometry. Stability tests also confirmed decreased poisoning and indicated improved stability and better tolerance to CO-like intermediate species. According to activity and stability measurements, the PtSnZn/C catalyst possesses the best electrochemical properties for the methanol oxidation reaction. The observed great electrocatalytic activity in the methanol oxidation reaction of synthesized catalysts can be attributed to the beneficial effects of microwave synthesis and the well-balanced addition of alloying metals in PtSnZn/C catalysts.

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

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

N-Doped Carbon Shells Encapsulated Ru-Ni Nanoalloys for Efficient Hydrogen Evolution Reaction

The alkaline hydrogen evolution reaction (HER) is of great significance for the large-scale green H₂ production. Currently, pressing challenges in the fabrication of cost-effective HER electrocatalysts are related to their sluggish water dissociation kinetics. Herein, a facile strategy to accelerate the desorption of HER intermediates and water dissociation is proposed. RuNi nanoalloy confined within N-doped carbon shells (Ru₇ Ni₃ @NC/C) with optimized Ru/Ni ratio and the dicyandiamide dosage was prepared. It displays an overpotential (η₁₀ ) of 16 mV, Tafel slope of 29.9 mV dec^-1 , and long-term stability over 10 000 cycles. The decent HER performance on Ru₇ Ni₃ @NC/C stems from the core-shell structure that is favoring the exposure of dispersed active sites, and the synergistic effect to promote water capture and dissociation. This work provides insight into the relationship between the HER performance and the electrochemical behavior of the intermediate adsorbed state, and paves an avenue toward rational design efficient electrocatalysts for HER.

Screening of Carbon-Supported Platinum Electrocatalysts Using Frumkin Adsorption Isotherms

An important stage in the development of platinum electrocatalysts on carbon support is the analysis of their basic parameters. Cyclic voltammetry is an effective tool for analyzing the structural and electrochemical properties of such electrocatalysts. Using Frumkin adsorption isotherms, the contribution of the platinum surface to the hydrogen adsorption region was well described by three peaks corresponding to different crystal structures. The screening was carried out for platinum black and platinum electrocatalysts supported by carbon black, reduced graphene oxide (RGO), carbon nanotubes (CNTs), and nanofibers (CNFs). For most samples, the peak contribution to the electrochemical surface area (ESA) and corresponding hydrogen adsorption energies had close values, but the parameters deviated for Pt black and RGO-based samples was observed. The dependence of the calculated peak parameters on the number of accelerated stress test cycles was used to evaluate the effect of the type of carbon support on the stability of the electrocatalyst and the structure of platinum nanoparticles. The experimental results indicate a high degree of stability and differences in the degradation mechanisms of electrocatalysts based on nanostructured carbon compared to carbon black, which are explained by differences in the metal-support interaction and corrosion resistance of nanostructured carbon supports.

Preparation of Cu/Sn-Organic Nano-Composite Catalysts for Potential Use in Hydrogen Evolution Reaction and Electrochemical Characterization

In this work, the solvothermal solidification method has been used to be prepared as a homogenous CuSn-organic nano-composite (CuSn-OC) to use as a catalyst for alkaline water electrolysis for cost-effective H2 generation. FT-IR, XRD, and SEM techniques were used to characterize the CuSn-OC which confirmed the formation of CuSn-OC with a terephthalic acid linker as well as Cu-OC and Sn-OC. The electrochemical investigation of CuSn-OC onto a glassy carbon electrode (GCE) was evaluated using the cyclic voltammetry (CV) method in 0.1 M KOH at room temperature. The thermal stability was examined using TGA methods, and the Cu-OC recorded a 91.4% weight loss after 800 °C whereas the Sn-OC and CuSn-OC recorded 16.5 and 62.4%, respectively. The results of the electroactive surface area (ECSA) were 0.5, 0.42, and 0.33 m2 g-1 for the CuSn-OC, Cu-OC, and Sn-OC, respectively, and the onset potentials for HER were -420, -900, and -430 mV vs. the RHE for the Cu-OC, Sn-OC, and CuSn-OC, respectively. LSV was used to evaluate the electrode kinetics, and the Tafel slope for the bimetallic catalyst CuSn-OC was 190 mV dec-1, which was less than for both the monometallic catalysts, Cu-OC and Sn-OC, while the overpotential was -0.7 vs. the RHE at a current density of -10 mA cm-2.

Bacterial nanocellulose as green support of platinum nanoparticles for effective methanol oxidation

Bacterial nanocellulose, BNC, has emerged as a new class of nanomaterials recognized as renewable, biodegradable, biocompatible and material for versatile applications. BNC also proved as a perfect support matrix for metallic nanoparticle synthesis and appeared as suitable alternative for widely used carbon based materials. Following the idea to replace commonly used carbon based materials for platinum supports with the green and sustainable one, BNC appeared as an excellent candidate. Herein, microwave assisted synthesis has been reported for the first time for platinum nanoparticles supported on BNC as green material. Bacterial nanocelullose-platinum catalyst, Pt/BNC, was investigated by Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), atomic force microscopy (AFM), X-ray diffractometry (XRD) and transmission-electron microscopy (TEM) analysis. The obtained results confirmed successful synthesis of new Pt-based catalyst. It was found that Pt/BNC catalyst has high electrocatalytic performance in methanol oxidation reaction. Green/sustainable catalytic system is highly desirable and provided by the elegant microwave assisted synthesis of Pt/BNC will pave the way for a larger scale application and expedite the market penetration of such fuel cells.

Ultrasmall, Coating-Free, Pyramidal Platinum Nanoparticles for High Stability Fuel Cell Oxygen Reduction

Ultrasmall (<5 nm diameter) noble metal nanoparticles with a high fraction of {111} surface domains are of fundamental and practical interest as electrocatalysts, especially in fuel cells; the nanomaterial surface structure dictates its catalytic properties, including kinetics and stability. However, the synthesis of size-controlled, pure Pt-shaped nanocatalysts has remained a formidable chemical challenge. There is an urgent need for an industrially scalable method for their production. Here, a one-step approach is presented for the preparation of single-crystal pyramidal nanocatalysts with a high fraction of {111} surface domains and a diameter below 4 nm. This is achieved by harnessing the shape-directing effect of citrate molecules, together with the strict control of oxidative etching while avoiding polymers, surfactants, and organic solvents. These catalysts exhibit significantly enhanced durability while, providing equivalent current and power densities to highly optimized commercial Pt/C catalysts at the beginning of life (BOL). This is even the case when they are tested in full polymer electrolyte membrane fuel cells (PEMFCs), as opposed to rotating disk experiments that artificially enhance electrode kinetics and minimize degradation. This demonstrates that the {111} surface domains in pyramidal Pt nanoparticles (as opposed to spherical Pt nanoparticles) can improve aggregation/corrosion resistance in realistic fuel cell conditions, leading to a significant improvement in membrane electrode assembly (MEA) stability and lifetime.

In Situ Growth of Pt-Co Nanocrystals on Different Types of Carbon Supports and Their Electrochemical Performance toward Oxygen Reduction

Carbon-supported Pt-M (M = Co, Ni, and Fe) alloy nanocrystals are widely used as catalysts toward oxygen reduction, a reaction key to the operation of proton-exchange membrane fuel cells. Here we report a colloidal method for the in situ growth of Pt-Co nanocrystals on various commercial carbon supports. The use of different carbon supports resulted in not only variations in size and composition for the nanocrystals but also their catalytic activity and durability toward oxygen reduction in acidic media. Among the nanocrystals, those grown on Vulcan XC72 and Ketjenblack EC300J showed the highest specific and mass activities in the 0.1 M HClO4 and 0.05 M H2SO4 electrolytes, respectively. Additionally, the catalysts also showed different durability depending on the strength of the interaction between the nanocrystals and the carbon support. Our analysis demonstrated that the difference in catalytic performance could be ascribed to the distinct effects of carbon support on both the synthetic and catalytic processes.

Binary and ternary Pt-based clusters grown in a plasma multimagnetron-based gas aggregation source: electrocatalytic evaluation towards glycerol oxidation

Platinum (Pt), platinum-bismuth (PtBi), platinum-copper (PtCu) and platinum-bismuth-copper (PtCuBi) clusters were grown in a gas aggregation source (GAS) equipped with three in-plane plasma magnetrons located in a single region of the gas aggregation zone. The X-ray diffraction results have shown that PtCu clusters form alloys as the decrease of the lattice parameter occurs when the Cu atomic content increases. PtBi clusters do not form alloys, but the presence of secondary Bi oxide phases was detected. Scanning transmission electron microscope mapping images revealed that simultaneously adding Bi and Cu to Pt leads to PtCu alloyed clusters decorated with Bi or CuBi species on the surface. The electrochemical results indicated that the shell might be composed of a metastable CuBi phase. Electrochemical measurements have shown that the addition of Bi or Cu to the Pt clusters enhances the catalytic activity for glycerol oxidation by decreasing the oxidation onset potential.

Nanoscale morphological evolution of monocrystalline Pt surfaces during cathodic corrosion

Cathodic corrosion is a relatively unexplored but highly enigmatic electrochemical phenomenon that transforms, roughens, and dissolves metal surfaces under cathodic polarization. We show how cathodic corrosion of a platinum spherical single-crystal electrode in an aqueous alkaline electrolyte leads initially to the formation of etch pits that reflect the local symmetry of the surface and subsequently develop into a growth regime in which self-similar diffusion-limited patterns emerge. These are unique observations that may eventually open the door to controlled surface patterning and nanoparticle preparation.

This paper studies the cathodic corrosion of a spherical single crystal of platinum in an aqueous alkaline electrolyte, to map out the detailed facet dependence of the corrosion structures forming during this still largely unexplored electrochemical phenomenon. We find that anisotropic corrosion of the platinum electrode takes place in different stages. Initially, corrosion etch pits are formed, which reflect the local symmetry of the surface: square pits on (100) facets, triangular pits on (111) facets, and rectangular pits on (110) facets. We hypothesize that these etch pits are formed through a ternary metal hydride corrosion intermediate. In contrast to anodic corrosion, the (111) facet corrodes the fastest, and the (110) facet corrodes the slowest. For cathodic corrosion on the (100) facet and on higher-index surfaces close to the (100) plane, the etch pit destabilizes in a second growth stage, by etching faster in the (111) direction, leading to arms in the etch pit, yielding a concave octagon-shaped pit. In a third growth stage, these arms develop side arms, leading to a structure that strongly resembles a self-similar diffusion-limited growth pattern, with strongly preferred growth directions.

Single Crystal Electrochemistry as an In Situ Analytical Characterization Tool

The electrochemical behavior of platinum single crystal surfaces can be taken as a model response for the interpretation of the activity of heterogeneous electrodes. The cyclic voltammogram of a given platinum electrode can be considered a fingerprint characteristic of the distribution of sites on its surface. We start this review by providing some simple mathematical descriptions of the voltammetric response in the presence of adsorption processes. We then describe the voltammogram of platinum basal planes, followed by the response of stepped surfaces. The voltammogram of polycrystalline materials can be understood as a composition of the response of the different basal contributions. Further resolution in the discrimination of different surface sites can be achieved with the aid of surface modification using adatoms such as bismuth or germanium. The application of these ideas is exemplified with the consideration of real catalysts composed of platinum nanoparticles with preferential shapes.

Imaging the Heterogeneity of the Oxygen Evolution Reaction on Gold Electrodes Operando: Activity is Highly Local

Understanding the mechanism of the oxygen evolution reaction (OER), the oxidative half of electrolytic water splitting, has proven challenging. Perhaps the largest hurdle has been gaining experimental insight into the active site of the electrocatalyst used to facilitate this chemistry. Decades of study have clarified that a range of transition-metal oxides have particularly high catalytic activity for the OER. Unfortunately, for virtually all of these materials, metal oxidation and the OER occur at similar potentials. As a result, catalyst surface topography and electronic structure are expected to continuously evolve under reactive conditions. Gaining experimental insight into the OER mechanism on such materials thus requires a tool that allows spatially resolved characterization of the OER activity. In this study, we overcome this formidable experimental challenge using second harmonic microscopy and electrochemical methods to characterize the spatial heterogeneity of OER activity on polycrystalline Au working electrodes. At moderately anodic potentials, we find that the OER activity of the electrode is dominated by <1% of the surface area and that there are two types of active sites. The first is observed at potentials positive of the OER onset and is stable under potential cycling (and thus presumably extends multiple layers into the bulk gold electrode). The second occurs at potentials negative of the OER onset and is removed by potential cycling (suggesting that it involves a structural motif only 1–2 Au layers deep). This type of active site is most easily understood as the catalytically active species (hydrous oxide) in the so-called incipient hydrous oxide/adatom mediator model of electrocatalysis. Combining the ability we demonstrate here to characterize the spatial heterogeneity of OER activity with a systematic program of electrode surface structural modification offers the possibility of creating a generation of OER electrocatalysts with unusually high activity.

Hydrogen peroxide reduction on single platinum nanoparticles

Understanding oxygen reduction, key to much of electrochemical energy transformation technology, crucially requires exploration of the role of hydrogen peroxide as a possible intermediate especially on catalysts such as Pt which can bring about the 4e reduction of O₂ to water. We reveal that at the single nanoparticle scale the direct platinum catalysed reduction of hydrogen peroxide is found - even at high overpotentials - not to be controlled by the rate mass-transport of the reagents to the interface but by a surface limited process. Further under alkaline (pH 12.3) and near mass-transport free conditions, the single nanoparticle hydrogen peroxide reduction rate goes through a maximum at potentials comparable to the surface deposition of hydrogen (H _upd) with the highest reaction rate occurring when the surface is partially covered in hydrogen.

Advances and challenges for experiment and theory for multi-electron multi-proton transfer at electrified solid–liquid interfaces

Understanding microscopic mechanism of multi-electron multi-proton transfer reactions at complexed systems is important for advancing electrochemistry-oriented science in the 21st century.

An introduction to electrochemical energy conversion

This paper is meant to provide a basic introduction to electrochemical energy conversion. It should be a low-barrier entry point for reading the relevant literature and understanding the basic phenomena, approaches and techniques. Starting with some basics of electrochemistry to establish the most important techniques, I will touch upon established electrochemical processes which are carried out today on industrial scale to finish with an outline of state-of-the art research on proton exchange membrane fuel cells and electrolysers for water splitting.

Atomic-Scale Identification of the Electrochemical Roughening of Platinum

Electrode degradation under oxidizing conditions is a major drawback for large-scale applications of platinum electrocatalysts. Subjecting Pt(111) to oxidation-reduction cycles is known to lead to the growth of nanoislands. We study this phenomenon using a combination of simultaneous in situ electrochemical scanning tunneling microscopy and cyclic voltammetry. Here, we present a detailed analysis of the formed islands, deriving the (evolution of the) average island growth shape. From the island shapes, we determine the densities of atomic-scale defect sites, e.g., steps and facets, which show an excellent correlation with the different voltammetric hydrogen adsorption peaks. Based on this combination of electrochemical scanning tunneling microscopy (EC-STM) and CV data, we derive a detailed atomistic picture of the nanoisland evolution during potential cycling, delivering new insights into the initial stages of platinum electrode degradation.

Hydrogen-Induced Step-Edge Roughening of Platinum Electrode Surfaces

Electrode surfaces may change their surface structure as a result of the adsorption of chemical species, impacting their catalytic activity. Using density functional theory, we find that the strong adsorption of hydrogen at low electrode potentials promotes the thermodynamics and kinetics of a unique type of roughening of 110-type Pt step edges. This change in surface structure causes the appearance of the so-called "third hydrogen peak" in voltammograms measured on Pt electrodes, an observation that has eluded explanation for over 50 years. Understanding this roughening process is important for structure-sensitive (electro)catalysis and the development of active and stable catalysts.

Electrocatalysis of Oxygen Reduction Reaction on Shape-Controlled Pt and Pd Nanoparticles-Importance of Surface Cleanliness and Reconstruction

Shape-controlled precious metal nanoparticles have attracted significant research interest in the recent past due to their fundamental and scientific importance. Because of their crystallographic-orientation-dependent properties, these metal nanoparticles have tremendous implications in electrocatalysis. This review aims to discuss the strategies for synthesis of shape-controlled platinum (Pt) and palladium (Pd) nanoparticles and procedures for the surfactant removal, without compromising their surface structural integrity. In particular, the electrocatalysis of oxygen reduction reaction (ORR) on shape-controlled nanoparticles (Pt and Pd) is discussed and the results are analyzed in the context of that reported with single crystal electrodes. Accepted theories on the stability of precious metal nanoparticle surfaces under electrochemical conditions are revisited. Dissolution, reconstruction, and comprehensive views on the factors that contribute to the loss of electrochemically active surface area (ESA) of nanoparticles leading to an inevitable decrease in ORR activity are presented. The contribution of adsorbed electrolyte anions, in-situ generated adsorbates and contaminants toward the ESA reduction are also discussed. Methods for the revival of activity of surfaces contaminated with adsorbed impurities without perturbing the surface structure and its implications to electrocatalysis are reviewed.

Effect of Ionic Liquid Modification on the ORR Performance and Degradation Mechanism of Trimetallic PtNiMo/C Catalysts

Ionic liquids (ILs) modification, following the concept of “solid catalyst with ionic liquid layer (SCILL)”, has been demonstrated to be an effective approach to improving both activity and stability of Pt-based catalysts for the oxygen reduction reaction. In this work, the SCILL concept has been applied to a trimetallic PtNiMo/C system, which has been documented recently to be significantly advantageous over the benchmark PtNi-based catalysts for oxygen reduction. To achieve this, two hydrophobic ILs ([BMIM][NTF2] and [MTBD][BETI]) were used to modify PtNiMo/C with four IL-loading amounts between 7 and 38 wt %. We found that the Pt mass activity (@0.9 V) could be improved by up to 50% with [BMIM][NTF2] and even 70% when [MTBD][BETI] is used. Exceeding a specific IL loading amount, however, leads to a mass transport related activity drop. Moreover, it is also disclosed that both ILs can effectively suppress the formation of nonreactive oxygenated species, while at the same time imposing little effect on the electrochemical active surface area. For a deeper understanding of the degradation mechanism of pristine and IL modified PtNiMo/C, we applied identical location transmission electron microscopy and in situ scanning flow cell coupled to inductively coupled plasma mass spectrometry techniques. It is disclosed that the presence of ILs has selectively accelerated the dissolution of Mo and eventually results in a more severe degradation of PtNiMo/C. This shows that future research needs to identify ILs that prevent the Mo dissolution to leverage the potential of the IL modification of PtNiMo catalysts.

Surface Structure Characterization of Shape and Size Controlled Pd Nanoparticles by Cu UPD: A Quantitative Approach

The search for new surface sensitive probes that characterize the surface structure of shape and size-controlled nanoparticles is an interesting topic to properly understand the correlations between electrocatalytic properties and surface structure at the nanoscale. Herein, we report the use of Cu UPD to characterize, not only qualitatively but also quantitatively, the surface structure of different Pd nanoparticles with controlled particle shape and size. Thus, Pd nanoparticles with cubic, octahedral and rhombic dodecahedral shapes, that is, with preferential {100}, {111}, and {110} surface structures, respectively, were prepared. In addition, cubic Pd nanoparticles with different particles sizes and spherical (2–3 nm) Pd nanoparticles were also synthesized. Based on the Cu UPD results on Pd single crystals, a new approach is proposed to qualitatively and quantitatively determine the percentages of {100}, {111}, and {110} surface domains present at the surface of the different shape and size controlled Pd nanoparticles. The results reported clearly show the benefits of this Cu UPD to get detailed information of the surface structure of the nanoparticles according to their particle shape and size.

Pd-Shaped Nanoparticles Modified by Gold ad-Atoms: Effects on Surface Structure and Activity Toward Glucose Electrooxidation

Palladium nanoparticles (Pd-NPs) with controlled distributions of sizes and shapes (nanospheres-Pd-NS-, nanocubes -Pd-NC-, and nanooctahedrons -Pd-NO-) are synthesized by wet chemistry methods and characterized by TEM/HRTEM. The surfaces of Pd-NPs are modified by spontaneous adsorption of gold and characterized by cyclic voltammetry in acidic medium. It is shown that the modification of Pd-NPs by dipping in HAuCl4 solutions of different concentrations allows controlling the surface coverage by gold. It is also shown that the modification of Pd-NPs surfaces involves first the formation of PdAu surface alloys. For higher coverages, both PdAu surface alloys and pure Au structures are formed. The activity toward the glucose electrooxidation reaction is determined by linear scan voltammetry (LSV). Higher activity is observed on pure Pd-NC presenting extended (100) surfaces than on Pd-NO with mainly (111) surface orientation and on Pd-NS without preferential surface orientation, both these latter Pd-NPs displaying almost the same activity. The modification of the surface by spontaneous adsorption of gold greatly improves the activity of all Pd-NPs. However, Au-modified Pd-NC materials remain the most active catalysts. PdAu surface alloys seem to be involved in the improvement of the catalytic activity at low potentials, although the role of pure gold structures on Pd-NPs toward the enhancement of the catalytic activity cannot be excluded for high gold coverage. The study allows a better understanding of the material structure/electrocatalytic behavior relationship.

Cu Modified Pt Nanoflowers with Preferential (100) Surfaces for Selective Electroreduction of Nitrate

Improving surface selectivity and maximizing electrode surface area are critical needs for the electroreduction of nitrate. Herein, preferential (100) oriented Pt nanoflowers with an extended surface area were prepared by potentiostatic deposition on carbon cloth (Pt NFs/CC), and then Cu atoms were adsorbed on the Pt NFs (Cu/Pt NFs/CC) for application of nitrate electroreduction. The results reveal that Cu/Pt NFs/CC with 8.7% Cu coverage exhibits a high selectivity for nitrate electroreduction to N2 following two steps: Nitrate firstly converts into nitrite on Cu sites adsorbed on Pt NFs, then nitrite subsequently selective reduction and ammonia oxidation to N2 occur on the large exposed (100) terraces in Pt NFs. In addition, electrocatalytic activity and selectivity of nitrate reduction strongly rely on the Cu surface coverage on Pt NFs, the lower activity of nitrate reduction is displayed with increase of Cu coverage. Accordingly, the selective reduction of nitrate to N2 is feasible at such nanostructured Pt nanoflowers modified with Cu.

Evaluation of the Cadmium Accumulation in Tamarillo Cells (Solanum betaceum) by Indirect Electrochemical Detection of Cysteine-Rich Peptides

Long-term cadmium intake can be very dangerous to human health due to its toxic effects. Although people can be contaminated with this element from different sources, contaminated food is probably the most important one. Foods such as vegetables and fruits can become contaminated with cadmium existing in soils, irrigation water, or chemical fertilizers. Some plants produce an excess of cysteine-rich peptides (CRp) when affected by high concentrations of heavy metals such as cadmium, thus indicating the presence of this type of contamination. Among these plants is tamarillo (Solanum betaceum), which is locally known as “tree tomato”. This is a native plant widely consumed in the Ecuadorian Andes because of its abundance, low cost, and high content of vitamin C and fiber. The fact that Solanum betaceum produces CRp upon contamination with heavy metals means that this plant may be able to accumulate heavy metals. If this is the case, the plant can possibly be used as an indicator of metal pollution. The main goals of the present work were to evaluate the possibility of using Solanum betaceum as an indicator of metal contamination in plants and to examine its capability to accumulate metals. Both goals were met by determination of the amounts of CRp produced by Solanum betaceum cells cultivated in vitro in the laboratory under controlled conditions in the presence of different concentrations of cadmium. The CRp determination was carried out by means of electrogeneration of iodine in an iodide solution containing reduced glutathione as a biological thiol model. Solanum betaceum cells were grown in a Murashige and Skoog solution enriched with a 30 g L⁻¹ sugar aqueous solution and 1 mg L⁻¹ 2,4-dichlorophenoxyacetic acid. The results of these experiments confirmed the following: (1) CRp production is a function of the amount of cadmium present as a contaminant up to a limiting value after which cell apoptosis occurs; (2) Solanum betaceum accumulates cadmium; (3) the analytical method used is appropriate for CRp determination; and (4) CRp determination is a valid alternative to detect contamination by heavy metals in plants.