Surface Speciation Research Articles

Surface Speciation in Microwave-Assisted CO Oxidation over Perovskites─The Role of Water and Activation Pretreatment.

As a model for the energy-efficient aftertreatment of exhaust gas components, we studied microwave-assisted (MW) CO oxidation over a (La,Sr)CoO3-δ (LSC) perovskite oxide catalyst under dry and humidified conditions. We found that the use of a MW-based process can offer multiple advantages over traditional thermocatalysis in this scenario, as the nature of the MW-solid interaction offers quick, adaptive, and energy-efficient heating as well as improved yield and lower light-off temperatures. As found by combined CO and water MW-desorption experiments, the presence of technically relevant amounts of water leads to a competition for surface active sites and thus slows the reaction rate without indications for a fundamental change in the mechanism. Remarkably, the performance loss related to the presence of water was less pronounced in the MW-assisted process. Additionally, while we recorded a temperature-dependent degradation of the reaction rate in extended MW-catalysis experiments both in dry and humidified conditions, it quickly recovered after a short reactivation MW-treatment. Our study confirms that surface reaction can be driven by the use of MW-radiation in a similar magnitude that can be achieved by thermal activation at significantly higher temperatures. The nature of the effect of the MW-treatment on the structural and electronic surface properties of the LSC material was investigated by X-ray absorption (XAS) and X-ray photoelectron spectroscopy (XPS). We found evidence of a significant structural, chemical, and electronic reorganization of the oxide surface, possibly consistent with the occurrence of overheated surfaces or "hotspots" during MW-exposure, which may explain the increased catalytic and heating properties of the LSC after the MW-pretreatment. The good catalytic performance, quick response to MW-heating, and long-term stability of the catalyst all indicate the promising potential of a MW-based process for the energy-efficient exhaust aftertreatment using noble-metal-free oxide catalysts.

Speciation studies at the Illite - solution interface: Part 2 – Co-sorption of uranyl and phosphate ions

Although it is well known that clays and phosphate ions (hereafter referred to as “P”) can contribute to the long term uptake of uranium (U) in a variety of geochemical systems, work is still needed to document the surface speciation of U(VI) and P sorbed on relevant clay minerals such as illite. Following on from previous work showing strong sorption of phosphate ligands on the surface of a homoionic Na-illite ([1]), we addressed the (competitive/synergistic) mechanisms of (co)sorption of trace levels of uranyl ions (1–10 µM) and phosphate ligands (100 µM) onto this clay, and the identity of the surface species formed, by in situ spectroscopy monitoring of the clay-solution interface along sorption. We also performed complementary batch sorption experiments and electrophoretic mobility (EM) measurements. Macroscopic data indicated a uranyl sorption dependent on pH, aqueous U concentration and clay-to-solution ratio, a signature of P on the U sorption, and a promotion of P sorption with U concentration. Macroscopic and EM data also suggested mostly reversible mechanisms of P and U co-sorption that confer negative charges on the mineral surface and involve several types of uranyl phosphate surface species and/or surface sites present on the clay edges. FTIR interface spectra confirmed that several types of inner sphere uranyl phosphate surface species formed at acidic pH at the illite-solution interface, as a function of U surface coverage and/or reaction time. An inner-sphere uranyl phosphate surface complex, likely with a U-bridging structure, was formed rapidly on high-affinity surface sites existing in limited quantities on the clay edges. Another U-P surface complex (with a possible P-bridging structure) was progressively formed in increasing amounts with U surface coverage and time on low affinity clay edge sites. Finally, a third U-P surface species having an autunite-like structure (probably a U-P polynuclear surface species) was formed on the illite surface at high U concentration (10 µM). The two later species competed with the existence on the illite surface of inner sphere surface complexes and outer-sphere surface complex of phosphate, which were identified in the absence of U. Information on uranyl surface speciation in the presence of phosphate ligands presented here is novel and provides for the first time in situ spectroscopic evidence of synergistic U-P sorption process leading in the formation of several types of uranyl phosphate sorption species on the surface of illite. These results provide a sound basis for better understanding / prediction of U sorption in the environment, including in clayey formations being considered as long term barriers to radionuclide migration in far field of high level radioactive waste repository.

Stability and Speciation of Hydrated Magnetite {111} Surfaces from Ab Initio Simulations with Relevance for Geochemical Redox Processes.

Magnetite is a common mixed Fe(II,III) iron oxide in mineral deposits and the product of (anaerobic) iron corrosion. In various Earth systems, magnetite surfaces participate in surface-mediated redox reactions. The reactivity and redox properties of the magnetite surface depend on the surface speciation, which varies with environmental conditions. In this study, Kohn-Sham density functional theory (DFT + U method) was used to examine the stability and speciation of the prevalent magnetite crystal face {111} in a wide range of pH and Eh conditions. The simulations reveal that the oxidation state and speciation of the surface depend strongly on imposed redox conditions and, in general, may differ from those of the bulk state. Corresponding predominant phase diagrams for the surface speciation and structure were calculated from first principles. Furthermore, classical molecular dynamics simulations were conducted investigating the mobility of water near the magnetite surface. The obtained knowledge of the surface structure and oxidation state of iron is essential for modeling retention of redox-sensitive nuclides.

Speciation studies at the Illite - solution interface: Part 1 – Sorption of phosphate ions

Although phosphate ion (hereafter “P”) sorption is known to influence clay surface reactivity and trace metal behavior in the environment, there is still a lack of in situ spectroscopic evidence on the identity of phosphate sorption species formed at the interface between solution and clays with a 2:1 layered structure (2 silica tetrahedral: 1 octahedral layers), which are ubiquitous in soils and clay formations. We studied the surface speciation of phosphate ions by in situ spectroscopic monitoring along P sorption at the interface between a solution and a homoionic Na-illite, as a function of time or aqueous phosphate concentration (20–250 µM) leading to low/moderate P surface coverage. We also combined them with batch sorption experiments and electrophoretic mobility (EM) measurements. Macroscopic data indicated that the percentage of P sorption depends on pH, aqueous phosphate concentration, and the clay/solution ratio of the experiment, i.e., the coverage of the clay surface by P, with a maximal sorption capacity of the clay equal to ca. 6 µmol.g−1 at pH 4. Macroscopic and EM data further suggest that P sorption mechanisms onto illite confer negative charges on the surface and involve several sorption species and/or surface sites present on the clay edges. In situ FTIR spectra showed that, at acidic pH, three types of phosphate surface species exist at the illite-solution interface: an outer-sphere surface complex (OSSC) of phosphate (ΞSOH2+…H2PO4-, with S referring to a sorption site on the clay), an inner-sphere surface complex (ISSC) of phosphate, probably a monodentate binuclear surface complex ((ΞSO)2PO2) formed by surface ligand exchange with high-affinity aluminol sites and, to a lesser extent, low-affinity sites on the clay edges, and, in limited amounts, an Al-phosphate surface precipitate. Phosphate ion sorption occurs through the initial formation of the OSSC of phosphate, which dominates phosphate surface speciation at short reaction times and low P concentrations and transforms over time into the ISSC of phosphate. This study presents the first published in situ surface speciation of phosphate ions sorbed at the interface between a solution and an illite. The evidence of a strong sorption of phosphate ions at this clay surface provides new and sound knowledge to better understand the environmental fate of phosphate ions and trace metals, particularly in agricultural soil - groundwater continua or in clay host rocks considered for high-level radioactive waste disposal.

Infrared and Raman Diagnostic Modeling of Phosphate Adsorption on Ceria Nanoparticles.

Cerium dioxide (CeO2; ceria) nanoparticles (CeNPs) are promising nanozymes that show a variety of biological activity. Effective nanozymes need to retain their activity in the face of surface speciation in biological environments, and characterizing surface speciation is therefore critical to understanding and controlling the therapeutic capabilities of CeNPs. In particular, adsorbed phosphates can impact the enzymatic activity exploited to convert phosphate prodrugs into therapeutics in vivo and also define the early stages of the phosphate-scavenging processes that lead to the transformation of active CeO2 into inactive CePO4. In this work, we utilize ab initio lattice-dynamics calculations to study the interaction of phosphates with the three major surfaces of ceria and to predict the infrared (IR) and Raman spectral signatures of adsorbed phosphate species. We find that phosphates adsorb strongly to CeO2 surfaces in a range of stable binding configurations, of which 5-fold coordinated P species in a trigonal bipyramidal coordination may represent a stable intermediate in the early stages of phosphate scavenging. We find that the phosphate species show characteristic spectral fingerprints between 500 and 1500 cm-1, whereas the bare CeO2 surfaces show no active modes above 600 cm-1, and the 5-fold coordinated P species in particular show potential diagnostic P-O stretching modes between 650 and 700 cm-1 in both IR and Raman spectra. This comprehensive exploration of different binding modes for phosphates on CeO2 and the set of reference spectra provides an important step toward the experimental characterization of phosphate speciation and, ultimately, control of its impact on the performance of ceria nanozymes.

Plant-substrate biochar properties critical for mediating reductive debromination of 1,2-dibromoethane

Dibromoethane is a widespread, persistent organic pollutant. Biochars are known mediators of reductive dehalogenation by layered FeII-FeIII hydroxides (green rust), which can reduce 1,2-dibromoethane to innocuous bromide and ethylene. However, the critical characteristics that determine mediator functionality are lesser known. Fifteen biochar substrates were pyrolyzed at 600 °C and 800 °C, characterized by elemental analysis, X-ray photo spectrometry C and N surface speciation, X-ray powder diffraction, specific surface area analysis, and tested for mediation of reductive debromination of 1,2-dibromoethane by a green rust reductant under anoxic conditions. A statistical analysis was performed to determine the biochar properties, critical for debromination kinetics and total debromination extent. It was shown that selected plant based biochars can mediate debromination of 1,2-dibromoethane, that the highest first order rate constant was 0.082/hr, and the highest debromination extent was 27% in reactivity experiments with 0.1 µmol (20 µmol/L) 1,2-dibromoethane, ≈ 22 mmol/L FeIIGR, and 0.12 g/L soybean meal biochar (7 days). Contents of Ni, Zn, N, and P, and the relative contribution of quinone surface functional groups were significantly (p < 0.05) positively correlated with 1,2-dibromoethane debromination, while adsorption, specific surface area, and the relative contribution of pyridinic N oxide surface groups were significantly negatively correlated with debromination.

Potential Use of Precipitates from Acid Mine Drainage (AMD) as Arsenic Adsorbents

The role of precipitates from acid mine drainage (AMD) in arsenic removal in water is a process to be investigated in more detail. The present study is focused on the potential use of two AMD precipitates using oxidation and Ca(OH)2 (OxPFe1) or CaCO3 (OxPFe2) as As(V) adsorbents and the comparison of their performance with two commercial adsorbents (nanohematite and Bayoxide®). The AMD’s supernatants and precipitates were characterized using several techniques and assessed with theoretical speciation and mass balance methods. Gypsum was identified by XRD and assessed as the main component of the precipitates. Amorphous iron hydroxide was assessed as the second component (22% in mass), and jurbanite or aluminum hydroxide were present in the third likely phase. The equilibrium adsorption of As(V) in water at a pH between 4 and 6 was tested with the four adsorbents, and the Langmuir model correlated well. The maximum adsorption capacity (qmax) had the highest value for OxPFe1 and the lowest value for nanohematite (that could be explained in terms of the adsorbent surface speciation). The two precipitates have limited application to the adsorption of very low concentrations of arsenic because they have a binding constant (b) lower than the commercial adsorbents and could release a small amount of the arsenic contained in the precipitate.

Retention of trivalent actinides (Am, Cm) and lanthanides (Eu) by Ca feldspars

Abstract. The transport of radionuclides in the environment is a major problem for the safety assessment of radioactive waste repositories. Storage in deep geological repositories is considered a safe disposal strategy because of their ability to isolate hazardous components from the biosphere for hundreds of thousands of years. Minor actinides (i.e. Am, Cm and Np) dominate the radiotoxicity of spent nuclear fuel over geological timescales. In underground repositories, reducing conditions are expected, and therefore the trivalent oxidation state is dominant for Am and Cm as well as possibly for Pu. For investigations of the mobility of the trivalent actinides Am3+ and Cm3+, the less toxic trivalent rare earth elements, in particular Eu3+, are commonly used. Besides clay and salt, crystalline rock is considered a possible host rock for deep geological repositories. Crystalline rock (e.g. granite) consists mainly of quartz, mica and feldspar. The latter forms common aluminosilicates making up ∼60 vol. % of the earth's crust, but their sorption behaviour is not well understood, especially for the Ca-bearing members of the group. Here, we study the sorption of trivalent actinides and their rare earth element homologues on plagioclase which are Ca-bearing feldspars, quantitatively and mechanistically. Zeta potentials of various Ca feldspars show an unexpected increase at pH 4–7, which becomes more pronounced as the amount of Ca in the crystal lattice increases. This can be interpreted by assuming uptake of Al3+ and/or the precipitation of an Al phase, where Al originates from feldspar dissolution at different pH values. Nevertheless, only minor differences were found in the retention and surface speciation of Cm3+ on Ca and K feldspars (Neumann et al., 2021). Ca feldspar has a slightly higher potential to retain trivalent metal ions compared to K feldspar. An inner-sphere (IS) complex and its two hydrolysis forms have been identified on both minerals, but the hydrolysis of the IS complex is stronger in the Ca-rich mineral. A surface complexation model for Ca feldspar was developed by combining the batch sorption data and the spectroscopically identified surface complexes to describe the experimental data. These data will be the basis for the improvement of transport simulations for a reliable safety assessment of potential radioactive waste repositories in crystalline rock.

(Invited) Understanding Photoelectrocatalysis on Epitaxial Oxide Surfaces

The intermittent nature of renewable energy sources requires a clean, scalable means of converting and storing energy. One Earth abundant storage option is water electrolysis, storing energy in the bonds of O2 and H2. Photoelectrochemical (PEC) cells based on semiconductor/liquid interfaces can convert sunlight to chemical fuels without external circuitry, such as “splitting” water into O2 and H2 upon illumination. The efficacy of conversion depends in part on the location of the semiconductor band edges, dictating absorption, and also the rectifying properties of semiconductor–electrolyte junctions, which drives the separation of electron–hole pairs.We will present studies of model oxide La(1-x)SrxFeO3 photoelectrodes grown by molecular beam epitaxy (MBE) and pulsed laser deposition (PLD) that display a known crystallographic orientation, surface area, path for charge transport, and strain. Aliovalent doping can tune the transition metal redox properties as well as the material band gap and conductivity, resulting in a rich phase space of possible water-oxidation photoanodes. Comparing the photooxidation of a fast redox couple to the slugging water oxidation reaction demonstrates the potential of electronic states that must be filled prior to oxygen evolution. Measurements of the surface speciation in a humid environment demonstrate that Sr incorporation also promotes hydroxylation, suggesting that water oxidation proceeds through the oxidation of FeIII-OH states, which is facilitated by Sr incorporation and results in an increased photovoltage. Surface-sensitive measurement of the valence band and unoccupied states (via O K-edge X-ray absorption spectroscopy) under humid conditions further support this picture. This fundamental insight builds understanding necessary for the design of active, earth-abundant photocatalysts that can be integrated into PEC devices for efficient conversion of solar energy into chemical fuels.

Efficient antibacterial activity of ternary nanocomposites containing hydroxyapatite, Co3O4, and cerium oxide

Metal oxide nanoparticles are routinely utilized in the biomedical field due to their diverse functionality. In particular, hybrid nanocomposites containing metal oxides possess a significant tunability of their chemical properties, as well as modifiable grain size and morphology. Hydroxyapatite (HA) and graphene oxide (GO) are already widely used in bone tissue engineering. We hypothesized that the addition of cobalt oxide or cerium oxide to form a hybrid nanocomposite with HA and GO can further enhance their properties toward cell viability and biodegradability. In this work, cobalt oxide/cerium oxide were individually combined with HA and GO to fabricate a ternary nanocomposite - HA/Co3O4/GO and HA/CeO2/GO for its potential use as a bone replacement hybrid material. The crystal structure of the resulting HA/Co3O4/GO composite was elucidated with XRD. The surface composition of the composite was analyzed using X-ray Photoelectron Spectroscopy and complex surface speciation was observed and assigned to Co(OH)2 and Co3O4. The particle length of HA in the composite was 35.5 ± 7.0 nm while the particle size of the cubic Co3O4 was 49.3 ± 10.8 nm, as revealed in TEM images. The antibacterial properties of HA/CeO2/GO were measured with the resulting inhibition area for E. coli and S. aureus of 15.9 ± 0.3 mm and 16.4 ± 0.2 mm, respectively. The ternary composite of HA/CeO2/GO and HA/Co3O4/GO showed an improvement in hardness of 4.1 ± 0.2 GPa and 3.2 ± 0.2. Summarily, HA/Co3O4/GO exhibited improved porosity, cell viability, and biodegradability compared with pristine HA. Overall, HA/Co3O4/GO nanocomposite possessed a porous structure and showed excellent antibacterial properties as well as controlled biodegradability and can be proposed as an improved bone-implant biomaterial.

Unintended cation crossover influences CO2 reduction selectivity in Cu-based zero-gap electrolysers

Membrane electrode assemblies enable CO2 electrolysis at industrially relevant rates, yet their operational stability is often limited by formation of solid precipitates in the cathode pores, triggered by cation crossover from the anolyte due to imperfect ion exclusion by anion exchange membranes. Here we show that anolyte concentration affects the degree of cation movement through the membranes, and this substantially influences the behaviors of copper catalysts in catholyte-free CO2 electrolysers. Systematic variation of the anolyte (KOH or KHCO3) ionic strength produced a distinct switch in selectivity between either predominantly CO or C2+ products (mainly C2H4) which closely correlated with the quantity of alkali metal cation (K+) crossover, suggesting cations play a key role in C-C coupling reaction pathways even in cells without discrete liquid catholytes. Operando X-ray absorption and quasi in situ X-ray photoelectron spectroscopy revealed that the Cu surface speciation showed a strong dependence on the anolyte concentration, wherein dilute anolytes resulted in a mixture of Cu+ and Cu0 surface species, while concentrated anolytes led to exclusively Cu0 under similar testing conditions. These results show that even in catholyte-free cells, cation effects (including unintentional ones) significantly influence reaction pathways, important to consider in future development of catalysts and devices.

Building high-capacity mesoporous adsorbents for fluoride removal through increased surface oxygen anions using organogel-assisted synthesis

In this work, mesoporous calcium-based adsorbents are prepared with the assistance of an organogel, 1,3:24-Bis (3,4-dimethylobenzylideno) sorbitol (DMDBS), which can self-assemble into nanoscale fibrillar networks and act as soft template to create mesopores. Particularly, it is found that the specific surface area of adsorbent is increased about 25 times by adding an extremely low concentration of DMDBS template (0.4 wt%) during the adsorbent production, which is much more efficient than common template materials. Additionally, the physical property and adsorption capability of calcium-based adsorbents can be tuned by control over calcination temperatures and DMDBS loadings. Under optimal preparation conditions (700 °C and 0.4 wt% DMDBS), the synthesized mesoporous adsorbent can achieve a Langmuir adsorption capacity of 48.8 mmol g−1 at pH of 7 and a removal efficiency of 70% in a mimicking natural water with the existence of NaCl. Thermodynamic analysis of equilibriums indicates that the adsorption reaction of fluoride with synthesized adsorbents from water is endothermic and spontaneous in nature. Most significantly, the results demonstrate that adsorbents with surface mesopores possess enriched ionized surface hydroxyl groups, CaO-, which act as the dominant species for fluoride attraction via electrostatic force at a neutral environment. This finding can be supported by the zeta potential analysis and surface speciation calculation. Moreover, the fluoride adsorption capacities of DMDBS-synthesized adsorbents are stable over neutral pH range of 6–8, revealing a promising potential for practical applications in fluoride-contaminated groundwater.

Speciation of Oxygen Functional Groups on the Carbon Support Controls the Electrocatalytic Activity of Cobalt Oxide Nanoparticles in the Oxygen Evolution Reaction.

The effective use of the active phase is the main goal of the optimization of supported catalysts. However, carbon supports do not interact strongly with metal oxides, thus, oxidative treatment is often used to enhance the number of anchoring sites for deposited particles. In this study, we set out to investigate whether the oxidation pretreatment of mesoporous carbon allows the depositing of a higher loading and a more dispersed cobalt active phase. We used graphitic ordered mesoporous carbon obtained by a hard-template method as active phase support. To obtain different surface concentrations and speciation of oxygen functional groups, we used a low-temperature oxygen plasma. The main methods used to characterize the studied materials were X-ray photoelectron spectroscopy, transmission electron microscopy, and electrocatalytic tests in the oxygen evolution reaction. We have found that the oxidative pretreatment of mesoporous carbon influences the speciation of the deposited cobalt oxide phase. Moreover, the activity of the electrocatalysts in oxygen evolution is positively correlated with the relative content of the COO-type groups and negatively correlated with the C═O-type groups on the carbon support. Furthermore, the high relative content of COO-type groups on the carbon support is correlated with the presence of well-dispersed Co3O4 nanoparticles. The results obtained indicate that to achieve a better dispersed and thus more catalytically active material, it is more important to control the speciation of the oxygen functional groups rather than to maximize their total concentration.

Comparative Spectroscopic Study Revealing Why the CO2 Electroreduction Selectivity Switches from CO to HCOO- at Cu-Sn- and Cu-In-Based Catalysts.

To address the challenge of selectivity toward single products in Cu-catalyzed electrochemical CO2 reduction, one strategy is to incorporate a second metal with the goal of tuning catalytic activity via synergy effects. In particular, catalysts based on Cu modified with post-transition metals (Sn or In) are known to reduce CO2 selectively to either CO or HCOO- depending on their composition. However, it remains unclear exactly which factors induce this switch in reaction pathways and whether these two related bimetal combinations follow similar general structure-activity trends. To investigate these questions systematically, Cu-In and Cu-Sn bimetallic catalysts were synthesized across a range of composition ratios and studied in detail. Compositional and morphological control was achieved via a simple electrochemical synthesis approach. A combination of operando and quasi-in situ spectroscopic techniques, including X-ray photoelectron, X-ray absorption, and Raman spectroscopy, was used to observe the dynamic behaviors of the catalysts' surface structure, composition, speciation, and local environment during CO2 electrolysis. The two systems exhibited similar selectivity dependency on their surface composition. Cu-rich catalysts produce mainly CO, while Cu-poor catalysts were found to mainly produce HCOO-. Despite these similarities, the speciation of Sn and In at the surface differed from each other and was found to be strongly dependent on the applied potential and the catalyst composition. For Cu-rich compositions optimized for CO production (Cu85In15 and Cu85Sn15), indium was present predominantly in the reduced metallic form (In0), whereas tin mainly existed as an oxidized species (Sn2/4+). Meanwhile, for the HCOO--selective compositions (Cu25In75 and Cu40Sn60), the indium exclusively exhibited In0 regardless of the applied potential, while the tin was reduced to metallic (Sn0) only at the most negative applied potential, which corresponds to the best HCOO- selectivity. Furthermore, while Cu40Sn60 enhances HCOO- selectivity by inhibiting H2 evolution, Cu25In75 improves the HCOO- selectivity at the expense of CO production. Due to these differences, we contend that identical mechanisms cannot be used to explain the behavior of these two bimetallic systems (Cu-In and Cu-Sn). Operando surface-enhanced Raman spectroscopy measurements provide direct evidence of the local alkalization and its impact on the dynamic transformation of oxidized Cu surface species (Cu2O/CuO) into a mixture of Cu(OH)2 and basic Cu carbonates [Cux(OH)y(CO3)y] rather than metallic Cu under CO2 electrolysis. This study provides unique insights into the origin of the switch in selectivity between CO and HCOO- pathways at Cu bimetallic catalysts and the nature of surface-active sites and key intermediates for both pathways.

Effect of the fuel on the surface VOx concentration, speciation and physico-chemical characteristics of solution combustion synthesised VOx/MgO catalysts for n-octane activation

VOx/MgO catalysts, synthesised via solution combustion synthesis were developed for ODH of n-octane using urea, citric acid, hydrazine hydrate, oxalyldihydrazide and glycine as fuels. The effect of these fuels on influencing defects, surface VOx concentration, basic properties and redox properties using various analytical methods were studied. Notable differences in surface and bulk characteristics of the vanadates were observed due to the fuel employed, affecting conversion of octane and selectivity to ODH products. Glycine as fuel gave best octene selectivity due to ease of reducibility of the VOx species, medium surface basicity, and relatively high concentration of surface oxygen vacancies.

Influence of Co:Fe:Ni ratio on cobalt Pentlandite’s electronic structure and surface speciation

X-ray photoemission spectroscopy (XPS) was used to investigate the electronic structure and oxidation state of transition metals in synthetic cobalt pentlandites (Fe,Co,Ni)9S8 with measured stoichiometries of Fe4.85Ni4.64S8, Co0.13Fe4.68Ni4.71S8, Co2.73Fe3.18Ni3.16S8, Co5.80Fe1.63Ni1.59S8 and Co8.70S8. The addition of cobalt was found to decrease the bond length and hence decrease the unit cell dimensions of the cobalt pentlandite crystal structure by up to 0.18 Å. High resolution XPS S 2p spectra show increases in the binding energies of bulk 5-coordinated (162.1 eV) and surface 3-coordinated (160.9 eV) sulfur (≈0.2 eV) with increasing Co concentration and decreasing bond lengths. As the Co concentration increases, variation in metal site occupation decreases, resulting in smaller S 2p FWHMs due to a dominant single Co-S state rather than mixed Fe-S, Ni-S and Co-S states with similar binding energies. The S 2p high binding energy tail, previously identified as a S 3p- Fe 3d ligand to metal transfer in Fe chalcogenides, shows a marked decrease in intensity as the concentration of Co increases, that is attributed to a decreased probability of ligand-to-metal charge transfer as the eg orbitals are filled. The transition metal XPS 2p spectra were modelled using CTM4XAS to investigate metal site occupation and ligand-to-metal charge transfer. Fe, Co, and Ni were all best simulated using a tetrahedral symmetry and 2+ oxidation state, their 2p3/2 and 2p1/2 peaks occurred at 706.9 and 719.9 eV, 778.2 and 793.1 eV, and 852.8 and 870.0 eV, respectively. A negative charge transfer energy confirms the high binding energy tail results from S3p-Fe3d ligand to metal charge transfer. This increased understanding of the pentlandite electronic structure will provide a basis for the refinement of mineral processing techniques and allow for a reduced environmental impact from limited efficiency.

Spectroscopic (XAS, FTIR) investigations into arsenic adsorption onto TiO2/Fe2O3 composites: Evaluation of the surface complexes, speciation and precipitation predicted by modelling

Over 50 million people in South Asia are exposed to groundwater contaminated with carcinogenic arsenic(III). Photocatalyst-adsorbent composite materials are popularly developed for removing arsenic in a single-step water treatment. Here, As(III) is oxidised to As(V), which is subsequently removed via adsorption. We previously developed a component additive surface complexation model (CA-SCM) to predict the speciation of arsenic adsorbed onto TiO2/Fe2O3 under different environmental conditions, using surface complexes taken from studies of single-phase minerals. In this work, we critically evaluate this approach, using experimental observations of the surface structures of arsenic adsorbed onto TiO2/Fe2O3. Extended X-ray absorption fine structure spectroscopy (EXAFS) indicates significant As(III) surface precipitation, and the possible formation of tridentate 3C complexes. EXAFS was unable to identify As binding modes for TiO2 and Fe2O3 surface complexes simultaneously, highlighting the challenge of analysing composite surfaces. FTIR and zeta potential analysis indicate that As(III)-Fe2O3 surface complexes are protonated at neutral pH, whilst As(III)-TiO2, As(V)-Fe2O3 and As(V)-TiO2 surface complexes are negatively charged. Our study confirms the speciation predicted by CA-SCM, particularly As(III) surface precipitation, but also introduces the possibility of tridentate As(III) at acidic pH. This study highlights how experiment and modelling can be combined to assess surface complexation on composite surfaces.

A deep look into the diverse surface speciation of the mono-molybdate/lepidocrocite system by ATR-IR and polarized ATR-IR spectroscopy

The present study is focused on the adsorption of monomeric molybdate anions on the surface of lepidocrocite investigated by several infrared spectroscopy methods. To study the surface speciation of molybdates on lepidocrocite, the behavior of the anion at the interface upon increasing anion concentration (fixed pH) or decreasing pH (fixed concentration) was analyzed in situ. The obtained results expand the knowledge for surface speciation by revealing a second molybdate complex, which is more stable, arising from the interaction with defects on the (010) facets of lepidocrocite. A curve fitting procedure enabled us to confirm what is observed from the pristine spectra, and outer-sphere complexes were probed as well. The evolution of the different bands upon increasing adsorbate concentration and decreasing pH, as obtained by curve fitting, shows a complex evolution for the different surface species with protonation, with the formation of strongly bonded complexes at low pH. The 2D synchronous and asynchronous spectra support the interpretation of the data. Furthermore, insights into the orientation of the complexes with respect to the lepidocrocite surface are obtained by using linearly polarized radiation. The results confirm most of the conclusions ascertained on the basis of the unpolarized spectra. The combination of the different methodologies yields a consistent picture for the complexity of this system and paves the way for subsequent utilization of this approach to study other systems.

Probing the role of surface speciation of tin oxide and tin catalysts on CO2 electroreduction combining in situ Raman spectroscopy and reactivity investigations

Electrochemical CO2 reduction to formate is a promising approach to store renewable electricity and utilize CO2. Tin oxide catalysts are efficient catalysts for this process, while the mechanisms underneath, especially the existence and role of oxidized tin species under CO2 electroreduction conditions remain unclear. In this work, we provide strong evidence on the presence of oxidized tin species on both SnO2 and Sn during CO2 reduction via in situ surface-enhanced Raman spectroscopy, while in different nature. Reactivity measurements show similar activity and selectivity to formate production on SnO2 and Sn catalysts. Combined analysis of Raman spectra and reactivity results suggests that Sn(IV) and Sn(II) oxide species are unlikely the catalytic species in CO2 electroreduction to formate.

Effect of Surface Nanostructures and Speciation on Undercooling for Low-Temperature Solder Alloys

Role of surface structure/composition in altering solidification behavior is demonstrated through undercooling of core–shell metal particles. Autonomous surface speciation in the passivating oxide plays a critical role in the relaxation of a molten metal due to divergence in the concentration and composition of oxidizing species across the thickness of the oxide layer. In an unreactive environment, surface speciation is dictated by flux, cohesive energy density, and surface energy minimization. Under oxidizing conditions (e.g., ambient), however, reduction potential, curvature, and surface plasticity dictate the spatial order and concentration(s) across thin passivating oxide layers. It is therefore important to redefine solubility beyond the limitations of Hume–Rothery rules by substituting electronegativity for redox potential and cohesive energy density. Increasing number of components in an alloy does not necessarily lead to increased undercooling, but a maximum is observed around two to three components. These results lead to an empirical observation that ΔGLS (Gibbs’ free energy for liquid–solid transition) around the freezing point can be understood from enthalpy and surface tension balance, with the degree of undercooling being a proportionality term for the enthalpic component. This work extends our understanding of classical nucleation theory by illustrating the importance of asymmetry in surface work in phase change, largely due to changes in structure of nanoscale passivating oxides. This phenomena is critical in development of low-temperature solders.