Microplastics in European sea salts - An example of exposure through consumer choice and of interstudy methodological discrepancies.

Microplastics are contaminants of emerging concern, not least due to their global presence in marine surface waters. Unsurprisingly, microplastics have been reported in salts harvested from numerous locations. We extracted microplastics from 13 European sea salts through 30% H2O2 digestion and filtration over 5-µm filters. Filters were visually inspected at magnifications to x100. A subsample of potential microplastics was subjected to Raman spectroscopy. Particle mass was estimated, and human dose exposure calculated. After blank corrections, median concentrations were 466 ± 152 microplastics kg-1 ranging from 74 to 1155 items kg-1. Traditionally harvested salts contained fewer microplastics than most industrially harvested ones (t-test, p < 0.01). Approximately 14 µg of microplastics (< 12 particles) may be absorbed by the human body annually, of which a quarter may derive from a consumer choosing sea salt. We reviewed existing studies, showing that targeting different particle sizes and incomplete filtrations hinder interstudy comparison, indicating the importance of method harmonisation for future studies. Excess salt consumption is detrimental to human health; the hazardousness of ingesting microplastics on the other hand has yet to be shown. A portion of microplastics may enter sea salts through production processes rather than source materials.

Contrasting the EXAFS obtained under air and H2 environments to reveal details of the surface structure of Pt-Sn nanoparticles.

Understanding the surface structure of bimetallic nanoparticles is crucial for heterogeneous catalysis. Although surface contraction has been established in monometallic systems, less is known for bimetallic systems, especially of nanoparticles. In this work, the bond length contraction on the surface of bimetallic nanoparticles is revealed by XAS in H2 at room temperature on dealloyed Pt-Sn nanoparticles, where most Sn atoms were oxidized and segregated to the surface when measured in air. The average Sn-Pt bond length is found to be ∼0.09 Šshorter than observed in the bulk. To ascertain the effect of the Sn location on the decrease of the average bond length, Pt-Sn samples with lower surface-to-bulk Sn ratios than the dealloyed Pt-Sn were studied. The structural information specifically from the surface was extracted from the averaged XAS results using an improved fitting model combining the data measured in H2 and in air. Two samples prepared so as to ensure the absence of Sn in the bulk were also studied in the same fashion. The bond length of surface Sn-Pt and the corresponding coordination number obtained in this study show a nearly linear correlation, the origin of which is discussed and attributed to the poor overlap between the Sn 5p orbitals and the available orbitals of the Pt surface atoms.

An in situ XAS study of high surface-area IrO2 produced by the polymeric precursor synthesis.

Iridium oxide powders with a surface area of more than 1 m2 g-1 (4 m2 g-2 from the H-UPD charge) and iridium-oxide crystallites less than 10 nm across were synthesized by heat treating gels formed from citric acid, ethylene glycol and dihydrogen hexachloroiridate(iv) in air. The characteristics of the resulting material was found to be strongly dependent on the heat-treatment step in the synthesis. A single heat-treatment of the gel resulted in a material with a substantial fraction of elemental iridium metal, i.e. iridium in oxidation state zero (Ir0). Post-synthesis modification of the powder by potential cycling resulted in oxidation peaks consistent with the conversion of the metal phase to iridium oxide. Linear combination of the near-edge part of the X-ray absorption data (X-ray absorption near-edge spectroscopy, XANES) collected in situ during potential cycling and an analysis of the extended X-ray fine-structure (EXAFS) part of the spectrum showed that the overall metal fraction was not significantly affected by the cycling. The oxidation of the metal phase is therefore limited to a thin layer of oxide at the metal surface, and a significant part of the iridium is left inactive. A modification of the heat treatment procedure of the sample resulted in iridium oxide containing only insignificant amounts of elemental iridium metal.

In situ XAFS of acid-resilient iridate pyrochlore oxygen evolution electrocatalysts under operating conditions.

Pyrochlore iridates (Na,Ca)2-xIr2O6·H2O are acid-stable electrocatalysts that are candidates for use in electrolysers and fuel cells. Ir LIII-edge X-ray absorption fine structure spectroscopy in 1 M H2SO4 at oxygen evolution conditions suggests the involvement of the electrons from the conduction band of the metallic particles, rather than just surface iridium reacting.

Strategies for the analysis of the elemental metal fraction of Ir and Ru oxides via XRD, XANES, and EXAFS.

Iridium and ruthenium oxide are active electrocatalysts for oxygen evolution. The relation between preparation method, structure, and behavior of mixed oxides of iridium and ruthenium are of interest in order to obtain active and stable catalysts. In this work the structure of mixed Ru-Ir oxides synthesized by the polymeric precursor method, which involves the formation of a gel containing the metal precursors and subsequent heat-treatment in air, was studied for the IrxRu1-xO2 system. An in-depth analysis of X-ray diffraction (XRD) and X-ray absorption (XAS) data, including EXAFS and linear combination of XANES, shows that the polymeric precursor synthesis method is capable of providing an intimate mixing of Ir and Ru in the catalyst. In addition to the oxide phase, metal phases, i.e. with Ru or Ir or both in oxidation state zero (Ir(fcc) and Ru(hcp)), were also found in the product materials. Facing complex structures such as some of those synthesized here, we have shown that a representation of shells with more than one atom type are efficiently represented using mixed sites, i.e. including scattering contributions from several elements in a site corresponding to a partial occupancy of the site by these elements, this method forming a very efficient basis for analyzing EXAFS data.

Effects of heat treatment atmosphere on the structure and activity of Pt3Sn nanoparticle electrocatalysts: a characterisation case study.

Comprehensive identification of the phases and atomic configurations of bimetallic nanoparticle catalysts are critical in understanding structure-property relationships in catalysis. However, control of the structure, whilst retaining the same composition, is challenging. Here, the same carbon supported Pt3Sn catalyst is annealed under air, Ar and H2 resulting in variation of the extent of alloying of the two components. The atmosphere-induced extent of alloying is characterised using a variety of methods including TEM, XRD, XPS, XANES and EXAFS and is defined as the fraction of Sn present as Sn0 (XPS and XANES) or the ratio of the calculated composition of the bimetallic particle to the nominal composition according to the stoichiometric ratio of the preparation (TEM, XRD and EXAFS). The values obtained depend on the structural method used, but the trend air < Ar < H2 annealed samples is consistent. These results are then used to provide insights regarding the electrocatalytic activity of Pt3Sn catalysts for CO, methanol, ethanol and 1-butanol oxidation and the roles of alloyed Sn and SnO2.

The borohydride oxidation reaction on La-Ni-based hydrogen-storage alloys.

This work provides insights into the processes involved in the borohydride oxidation reaction (BOR) in alkaline media on metal hydride alloys formed by LaNi(4.7)Sn(0.2)Cu(0.1) and LaNi(4.78)Al(0.22) with and without deposited Pt, Pd, and Au. The results confirm the occurrence of hydrolysis of the borohydride ions when the materials are exposed to BH(4)(-) and a continuous hydriding of the alloys during BH(4)(-) oxidation measurements at low current densities. The activity for the direct BOR is low in both bare metal hydride alloys, but the rate of the BH(4)(-) hydrolysis and the hydrogen-storage capacity are higher, while the rate of H diffusion is slower for bare LaNi(4.78) Al(0.22). The addition of Pt and Pd to both alloys results in an increase of the BH(4)(-) hydrolysis, but the H(2) formed is rapidly oxidized at the Pt-modified catalysts. In the case of Au modification, a small increase in the BH(4)(-) hydrolysis is observed as compared to the bare alloys. The presence of Au and Pd also leads to a reduction of the rates of alloy hydriding/de-hydriding.

Time-resolved, in situ DRIFTS/EDE/MS studies on alumina-supported rhodium catalysts: effects of ceriation and zirconiation on rhodium-CO interactions.

The effects of ceria and zirconia on the structure-function properties of supported rhodium catalysts (1.6 and 4 wt % Rh/γ-Al2O3) during CO exposure are described. Ceria and zirconia are introduced through two preparation methods: 1) ceria is deposited on γ-Al2O3 from [Ce(acac)3] and rhodium metal is subsequently added, and 2) through the controlled surface modification (CSM) technique, which involves the decomposition of [M(acac)x] (M=Ce, x=3; M=Zr, x=4) on Rh/γ-Al2O3. The structure-function correlations of ceria and/or zirconia-doped rhodium catalysts are investigated by diffuse reflectance infrared Fourier-transform spectroscopy/energy-dispersive extended X-ray absorption spectroscopy/mass spectrometry (DRIFTS/EDE/MS) under time-resolved, in situ conditions. CeOx and ZrO2 facilitate the protection of Rh particles against extensive oxidation in air and CO. Larger Rh core particles of ceriated and zirconiated Rh catalysts prepared by CSM are observed and compared with Rh/γ-Al2O3 samples, whereas supported Rh particles are easily disrupted by CO forming mononuclear Rh geminal dicarbonyl species. DRIFTS results indicate that, through the interaction of CO with ceriated Rh particles, a significantly larger amount of linear CO species form; this suggests the predominance of a metallic Rh phase.

Water-splitting electrocatalysis in acid conditions using ruthenate-iridate pyrochlores.

The pyrochlore solid solution (Na(0.33)Ce(0.67))2(Ir(1-x)Ru(x))2O7 (0≤x≤1), containing B-site Ru(IV) and Ir(IV) is prepared by hydrothermal synthesis and used as a catalyst layer for electrochemical oxygen evolution from water at pH<7. The materials have atomically mixed Ru and Ir and their nanocrystalline form allows effective fabrication of electrode coatings with improved charge densities over a typical (Ru,Ir)O2 catalyst. An in situ study of the catalyst layers using XANES spectroscopy at the Ir L(III) and Ru K edges shows that both Ru and Ir participate in redox chemistry at oxygen evolution conditions and that Ru is more active than Ir, being oxidized by almost one oxidation state at maximum applied potential, with no evidence for ruthenate or iridate in +6 or higher oxidation states.

Structural characterization of alumina-supported Rh catalysts: effects of ceriation and zirconiation by using metal-organic precursors.

The effects of the addition of ceria and zirconia on the structural properties of supported rhodium catalysts (1.6 and 4 wt % Rh/γ-Al2O3) are studied. Ceria and zirconia are deposited by using two preparation methods. Method I involves the deposition of ceria on γ-Al2O3 from Ce(acac)3, and the rhodium metal is subsequently added, whereas method II is based on a controlled surface reaction technique, that is, the decomposition of metal-organic M(acac)x (in which M=Ce, x=3 and M=Zr, x=4) on Rh/γ-Al2O3. The structures of the prepared catalyst materials are characterized ex situ by using N2 physisorption, transmission electron microscopy, high-angle annular dark-field scanning transmission election microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy (XPS), and X-ray absorption fine structure spectroscopy (XAFS). All supported rhodium systems readily oxidize in air at room temperature. By using ceriated and zirconiated precursors, a larger rhodium-based metallic core fraction is obtained in comparison to the undoped rhodium catalysts, suggesting that ceria and zirconia protect the rhodium particles against extensive oxidation. XPS results indicate that after the calcination and reduction treatments, a small amount of chlorine is retained on the support of all rhodium catalysts. EXAFS analysis shows significant Rh-Cl interactions for Rh/Al2O3 and Rh/CeOx /Al2O3 (method I) catalysts. After reaction with H2/He in situ, for series of samples with 1.6 wt % Rh, the EXAFS first shell analysis affords a mean size of approximately 30 atoms. A broader spread is evident with a 4 wt % rhodium loading (ca. 30-110 atoms), with the incorporation of zirconium providing the largest particle sizes.

Dressing plasmons in particle-in-cavity architectures.

Placing metallic nanoparticles inside cavities, rather than in dimers, greatly improves their plasmonic response. Such particle-in-cavity (PIC) hybrid architectures are shown to produce extremely strong field enhancement at the particle-cavity junctions, arising from the cascaded focusing of large optical cross sections into small gaps. These simply constructed PIC structures produce the strongest field enhancement for coupled nanoparticles, up to 90% stronger than for a dimer. The coupling is found to follow a universal power law with particle-surface separation, both for field enhancements and resonant wavelength shifts. Significantly enhanced Raman signals are experimentally observed for molecules adsorbed in such PIC structures, in quantitive agreement with theoretical calculations. PIC architectures may have important implications in many applications, such as reliable single molecule sensing and light harvesting in plasmonic photovoltaic devices.

Insight into the Activity and Selectivity of Nanostructured Copper Titanates during Electrochemical Conversion of CO2 at Neutral pH via In Situ X-ray Absorption Spectroscopy.

The electrochemical conversion of carbon dioxide (CO2) to useful chemical fuels is a promising route toward the achievement of carbon neutral and carbon negative energy technologies. Copper (Cu)- and Cu oxide-derived surfaces are known to electrochemically convert CO2 to high-value and energy-dense products. However, the nature and stability of oxidized Cu species under reaction conditions are the subject of much debate in the literature. Herein, we present the synthesis and characterization of copper-titanate nanocatalysts, with discrete Cu-O coordination environments, for the electrochemical CO2 reduction reaction (CO2RR). We employ real-time in situ X-ray absorption spectroscopy (XAS) to monitor Cu species under neutral-pH CO2RR conditions. Combination of voltammetry and on-line electrochemical mass spectrometry with XAS results demonstrates that the titanate motif promotes the retention of oxidized Cu species under reducing conditions for extended periods, without itself possessing any CO2RR activity. Additionally, we demonstrate that the specific nature of the Cu-O environment and the size of the catalyst dictate the long-term stability of the oxidized Cu species and, subsequently, the product selectivity.

Exploring the first steps in core-shell electrocatalyst preparation: in situ characterization of the underpotential deposition of Cu on supported Au nanoparticles.

The underpotential deposition (upd) of a Cu shell on a non-Pt nanoparticle core followed by galvanic displacement of the Cu template shell to form core-shell electrocatalyst materials is one means by which the Pt-based mass activity targets required for commercialization of PEM fuel cells may be reached. In situ EXAFS measurements were conducted at both the Au L(3) and the Cu K absorption edges during deposition of Cu onto a carbon-supported Au electrocatalyst to study the initial stages of formation of such a core-shell electrocatalyst. The Au L(3) EXAFS data obtained in 0.5 mol dm(-3) H(2)SO(4) show that the shape of the Au core is potential dependent, from a flattened to a round spherical shape as the Cu upd potential is approached. Following the addition of 2 mmol dm(-3) Cu, the structure was also measured as a function of the applied potential. At +0.2 V vs Hg/Hg(2)SO(4), the Cu(2+) species was found to be a hydrated octahedron. As the potential was made more negative, single-crystal studies predict an ordered bilayer of sulfate anions and partially discharged Cu ions, followed by a complete/uniform layer of Cu atoms. In contrast, the model obtained by fitting the Au L(3) and Cu K EXAFS data corresponds first to partially discharged Cu ions deposited at the defect sites in the outer shell of the Au nanoparticles at -0.42 V, followed by the growth of clusters of Cu atoms at -0.51 V. The absence of a uniform/complete Cu shell, even at the most negative potentials investigated, has implications for the structure, and the activity and/or stability, of the core-shell catalyst that would be subsequently formed following galvanic displacement of the Cu shell.

The effect of Bi adsorption on CO oxidation inside 1.8 nm Pt pores.

Modification of the surface of H(1)-e Pt with Bi causes significant changes in the CO stripping voltammetry; the pre-wave disappears and CO and Bi oxidation peaks appear. The absence of the pre-wave suggests that Bi preferentially adsorbs on the trough sites of the concave 1.8 nm diameter pore walls preventing oxygenated species from nucleating there.

SERS from two-tier sphere segment void substrates.

Sphere segment void, or inverse opal, films prepared from a variety of coinage metals have shown promise as reliable and reproducible substrates for surface enhanced Raman spectroscopy (SERS). Sphere segment void substrates are prepared from colloidal templates consisting of one or more layers of polystyrene spheres. In this paper, we investigate the reflection spectra and SERS for a gold film consisting of two-tiers of spherical cavities, and show that the best SERS enhancements are obtained from substrates consisting of just a single layer of sphere segment voids.

Microplastics in fish and fishmeal: an emerging environmental challenge?

Microplastics are contaminants of emerging concern; they are ingested by marine biota. About a quarter of global marine fish landings is used to produce fishmeal for animal and aquaculture feed. To provide a knowledge foundation for this matrix we reviewed the existing literature for studies of microplastics in fishmeal-relevant species. 55% of studies were deemed unsuitable due to focus on large microplastics (> 1 mm), lack of, or limited contamination control and polymer testing techniques. Overall, fishmeal-relevant species exhibit 0.72 microplastics/individual, with studies generally only assessing digestive organs. We validated a density separation method for effectiveness of microplastic extraction from this medium and assessed two commercial products for microplastics. Recovery rates of a range of dosed microplastics from whitefish fishmeal samples were 71.3 ± 1.2%. Commercial samples contained 123.9 ± 16.5 microplastics per kg of fishmeal-mainly polyethylene-including 52.0 ± 14.0 microfibres-mainly rayon. Concentrations in processed fishmeal seem higher than in captured fish, suggesting potential augmentation during the production process. Based on conservative estimates, over 300 million microplastic particles (mostly < 1 mm) could be released annually to the oceans through marine aquaculture alone. Fishmeal is both a source of microplastics to the environment, and directly exposes organisms for human consumption to these particles.

Study of carbon monoxide oxidation on mesoporous platinum.

H(1) mesoporous platinum surfaces formed by electrodeposition from lyotropic liquid crystalline templates have high electroactive surface areas (up to 60 m(2) g(-1)) provided by the concave surface within their narrow (≈2 nm diameter) pores. In this respect, they are fundamentally different from the flat surfaces of ordinary Pt electrodes or the convex surfaces of high-surface-area Pt nanoparticles. Cyclic voltammetry of H(1) mesoporous Pt films in acid solution is identical to that for polycrystalline Pt, suggesting that the surfaces of the pores are made up of low-index Pt faces. In contrast, CO stripping voltammetry on H(1) mesoporous Pt is significantly different from the corresponding voltammetry on polycrystalline Pt and shows a clear prewave for CO oxidation and the oxidation CO at lower overpotential. These differences in CO stripping are related to the presence of trough sites where the low-index Pt faces that make up the concave surface of the pore walls meet.

Identification of tidal trapping of microplastics in a temperate salt marsh system using sea surface microlayer sampling.

Microplastics are contaminants of increasing global environmental concern. Estuaries are a major transport pathway for land-derived plastics to the open ocean but are relatively understudied compared to coastal and open marine environments. The role of the "estuarine filter", by which the supply of sediments and contaminants to the sea is moderated by processes including vegetative trapping and particle flocculation, remains poorly defined for microplastics land to sea transfer. Here, we focus on the sea surface microlayer (SML) as a vector for microplastics, and use SML sampling to assess microplastic trapping in a temperate marsh system in Southampton Water, UK. The SML is known to concentrate microplastics relative to the underlying water and is the first part of rising tidal waters to traverse intertidal and upper tidal surfaces. Sampling a salt marsh creek at high temporal resolution allowed assessment of microplastics in-wash and outflow from the salt marsh, and its relationship with tidal state and bulk suspended sediment concentrations (SSC), over spring and neap tides. A statistically significant decrease in microplastics abundance from the flood tide to the ebb tide was found, and a weak positive relationship with SSC observed.

A quick and versatile one step metal-organic chemical deposition method for supported Pt and Pt-alloy catalysts.

A simple, modified Metal-Organic Chemical Deposition (MOCD) method for Pt, PtRu and PtCo nanoparticle deposition onto a variety of support materials, including C, SiC, B4C, LaB6, TiB2, TiN and a ceramic/carbon nanofiber, is described. Pt deposition using Pt(acac)2 as a precursor is shown to occur via a mixed solid/liquid/vapour precursor phase which results in a high Pt yield of 90-92% on the support material. Pt and Pt alloy nanoparticles range 1.5-6.2 nm, and are well dispersed on all support materials, in a one-step method, with a total catalyst preparation time of ∼10 hours (2.4-4× quicker than conventional methods). The MOCD preparation method includes moderate temperatures of 350 °C in a tubular furnace with an inert gas supply at 2 bar, a high pressure (2-4 bar) compared to typical MOCVD methods (∼0.02-10 mbar). Pt/C catalysts with Pt loadings of 20, 40 and 60 wt% were synthesised, physically characterised, electrochemically characterised and compared to commercial Pt/C catalysts. TEM, XRD and ex situ EXAFS show similar Pt particle sizes and Pt particle shape identifiers, namely the ratio of the third to first Pt coordination numbers modelled from ex situ EXAFS, between the MOCD prepared catalysts and commercial catalysts. Moreover, electrochemical characterisation of the Pt/C MOCD catalysts obtained ORR mass activities with a maximum of 428 A gPt -1 at 0.9 V, which has similar mass activities to the commercial catalysts (80-160% compared to the commercial Pt/C catalysts).

Evaluation of existing methods to extract microplastics from bivalve tissue: Adapted KOH digestion protocol improves filtration at single-digit pore size.

Methods standardisation in microplastics research is needed. Apart from reagent-dependent effects on microplastics, varying target particle sizes can hinder result comparison between studies. Human health concerns warrant recovery of small microplastics. We compared existing techniques using hydrogen peroxide, Proteinase-K, Trypsin and potassium hydroxide to digest bivalve tissue. Filterability, digestion efficacy, recoverability of microplastics and subsequent polymer identification using Raman spectroscopy and a matching software were assessed. Only KOH allowed filtration at ≤25 µm. When adding a neutralisation step prior to filtration, KOH digestates were filterable using 1.2-µm filters. Digestion efficacies were >95.0% for oysters, but lower for clams. KOH destroyed rayon at 60 °C but not at 40 °C. Acrylic fibre identification was affected due to changes in Raman spectra peaks. Despite those effects, we recommend KOH as the most viable extraction method for exposure risk studies, due to microplastics recovery from bivalve tissues of single-digit micrometre size.