Availability Of Surface Sites Research Articles

High Performance Flexible Supercapacitor Based on Single Precursor Derived NiFe2O4 Spinel with Tailored Cationic Distribution and Oxygen Vacancies in Acidic Medium

AbstractSpinel metal oxides are extensively studied for supercapacitors (SCs) in alkaline electrolytes, where charge storage capacity is limited by surface site availability due to surface reconstruction forming metal hydroxides/oxyhydroxides. The use of an acidic medium, which can boost the charge storage capacity of spinel oxides offering an additional channel of intercalation‐deintercalation of protons, is underexplored. Moreover, the impact of chemical compositions and the cationic distributions is crucial on the electrocatalysis performance of spinel oxides, however, such a correlation is first time reported for charge storage properties of spinel ferrite NiFe2O4 nanoparticles (NFO NPs). Besides, a low‐cost and scalable synthesis of NFO NPs involving the thermal decomposition of Ni‐Fe glycolate, followed by controlled air‐calcination is reported. Thus crafted NFO NPs‐based device shows impressive specific capacitance (2112 F g−1 at 10 A g−1) in half‐cell configuration. A flexible all‐solid‐state asymmetric device (full‐cell) configuration depicts impressive energy density (20.7 Wh kg−1), power density (4000 W kg−1), gravimetric capacitance (140 F g−1 at 2 A g−1), and retention of its performance (≈75% after 10,000 charging/discharging cycles). The results depict a new insight toward the tuning of electronic and charge storage properties in NFO, which otherwise are predominately attributed to only the crystallite size and morphological effects.

Nanotubes and other nanostructures of VS2, WS2, and MoS2: Structural effects on the hydrogen evolution reaction

Vanadium sulfide (VS2) is a layered transition metal dichalcogenide (TMD), comparable in crystal structure to the well-known MoS2 and WS2. Theoretical predictions attribute much potential to VS2, since it is metallic-like and has an active basal plane, essential for catalytic performance. However, it is much less studied than other members of the TMD family due to the difficulties in synthesizing specific structures with controlled properties. Here we present unique structures of VS2 nanotubes and conduct a comparative study with other well-known inorganic nanotubes and nanostructures of MoS2 and WS2. We evaluate the effect of the curvature and strain, the abundance of surface defects, and the availability of surface sites in various structures by electrochemical methods. We show that MoS2 has the best intrinsic activity, which is enhanced by an extensive electrochemical surface area. The woven-like structure of the MoS2 nanotube walls provides a combined effect of strain, crystallinity, and defects. For WS2 structures, the strained surface of the nanotubes results in sites with higher intrinsic activity than the edge sites, but structures such as the nano-triangles, which provide a higher number of edge sites, exhibit competing activity. As for the VS2 structures, although theoretical calculations predict optimal active sites for the hydrogen evolution reaction (HER), they are extremely sensitive to stoichiometry variations that hamper their catalytic activity. Our findings contribute insights to the improvement and design of VS2–based nanocatalysts for the HER and shed light on the general factors that govern the activity in the unique TMD nanotubes family.

Atomic-Scale Visualization of Heterolytic H2 Dissociation and COx Hydrogenation on ZnO under Ambient Conditions.

Studying catalytic hydrogenation reactions on oxide surfaces at the atomic scale has been challenging because of the typical occurrence of these processes at ambient or elevated pressures, rendering them less accessible to atomic-scale techniques. Here, we report an atomic-scale study on H2 dissociation and the hydrogenation of CO and CO2 on ZnO using ambient pressure scanning tunneling microscopy, ambient pressure X-ray photoelectron spectroscopy, and density functional theory (DFT) calculations. We directly visualized the heterolytic dissociation of H2 on ZnO(101̅0) under ambient pressure and found that dissociation reaction does not require the assistance of surface defects. The presence of CO or CO2 on ZnO at 300 K does not impede the availability of surface sites for H2 dissociation; instead, CO can even enhance the stability of coadsorbed hydride species, thereby facilitating their dissociative adsorption. Our results show that hydride is the active species for hydrogenation, while hydroxyl cannot hydrogenate CO/CO2 on ZnO. Both AP studies and DFT calculations showed that the hydrogenation of CO2 on ZnO is thermodynamically and kinetically more favorable compared to that of CO hydrogenation. Our results point toward a two-step mechanism for CO hydrogenation, involving initial oxidation to CO2 at step sites on ZnO followed by reaction with hydride to form formate. These findings provide molecular insights into the hydrogenation of CO/CO2 on ZnO and deepen our understanding of syngas conversion and oxide catalysis in general.

Understanding the Effects of Electrode Material, Single Crystal Facet, and Electrolyte Ion on the Helmholtz Capacitance of Metal/Aqueous Solution Interfaces.

The comprehensive interpretation of the measured differential Helmholtz capacitance curve is vital for advancing our understanding of the interfacial structure. While several possible physical effects contributing to the Helmholtz capacitance have been proposed theoretically, combining those factors to explain the experimentally observed potential-dependent capacitance profile remains a significant challenge. In this study, we employ ab initio molecular dynamics simulations to model various metal/solution interfaces. Our investigation primarily emphasizes the substantial effect of water chemisorption on the potential-dependent behavior of the Helmholtz capacitance. Additionally, we identify other critical factors that profoundly impact the Helmholtz capacitance: (1) Ions with low hydration energy hinder the availability of surface sites for water adsorption, resulting in a diminished enhancement of capacitance from water chemisorption. (2) Using large-sized ions leads to an expansion of the Helmholtz layer, causing a decrease in the Helmholtz capacitance. (3) Metal surfaces with higher affinity for water attract water adsorption at lower potentials, resulting in a lower peak potential for the differential Helmholtz capacitance curve.

Improved Oxygen Reduction Reaction (ORR) Kinetics Via Interfacial Engineering

Sluggish oxygen reduction reaction (ORR) kinetics, even on the best available catalyst (Pt), is one of the major challenges in widespread commercialization of polymer electrolyte membrane fuel cells (PEMFC).1 Significant advances have been made towards making highly active Pt catalysts through nanostructuring, alloying, size, and shape control to mitigate these kinetic losses.2 An additional strategy for ORR activity enhancement is though the targeted engineering of the electrochemical interface. For example, Ionic liquid (IL) interlayers improve the ORR kinetics on bulk and nanostructured catalysts for both Pt and alloyed-Pt materials.3 Along with the kinetic enhancement, ionic liquid interlayers also mitigate the sulfonate adsorption on Pt and improve the durability of Pt/C thin films.4,5 Despite remarkable improvement in performance at half-cell and MEA-level, the mechanism of this improvement is not completely understood. In this work, we combine single crystal voltammetry with microkinetic modeling to analyze the mechanism of ORR enhancement in presence of ionic liquids. With iterative feedback between single crystal voltammetry and microkinetic model output, we establish the importance of accounting for lateral interactions between adsorbed oxygenated species by using real data informed adsorption isotherms. With our experimentally validated model, we show that the mechanism of impact of ILs on ORR activity is through exclusion of water and reduction in electrochemical interface solvation, yielding lower spectator hydroxyl coverages and a reduced barrier for hydroxyl removal, resulting in higher surface site availability during the ORR. This improved understanding will help design Pt-interfaces to moderate the interaction of the interfacial water network with Pt active sites, providing another strategy for improving ORR kinetics.

Surface Fe clusters promote syngas reaction to oxygenates on Rh catalysts modified by atomic layer deposition

Converting syngas to higher oxygenates is a promising strategy for sustainably producing fuels and chemicals used every day. Rh-based catalysts promoted with metal oxides have long been of interest for oxygenate synthesis, but the role of promoters and the structure of the catalyst during reaction are often not well understood. In this work, we characterize a Rh/SiO2 catalyst modified by Fe2O3 deposited by atomic layer deposition (ALD). We show that Fe2O3 deposits selectively on the Rh nanoparticles and enhances both the turnover frequency and the oxygenate selectivity of the catalysts. A maximum 29% selectivity towards higher oxygenates is achieved with just 1 cycle of Fe2O3 ALD, and we relate the trends in selectivity to the changing availability of surface sites that drive higher alcohol (e.g., ethanol and propanol) formation. Further, we show that the ALD promoter mitigates catalyst sintering, improving the stability of these materials. In-situ X-ray absorption spectroscopy experiments reveal that the Fe2O3 undergoes reduction during the catalyst pretreatment and forms Fe clusters on the surface of the Rh that migrate but remain stable under syngas reaction conditions. We find that modification of the Rh with these surface Fe species is key to the enhanced alcohol production observed in these materials.

Al2O3 anchored silver and gold nanoparticles as accessible, stable, and re-usable catalysts

Nanoparticles of silver and gold have become widely researched as homogeneous catalysts for organic transformations. Silver systems have shown promising activity but are often hindered by their instability and difficulty in recycling. In this work we present nanoparticles of silver and gold, colloidally stabilized on aluminum oxide, and their performance as re-useable catalysts for the reduction of 4-nitrophenol. Compared with more standard citrate stabilized catalyst nanoparticles (k = 0.71 and 0.17 min−1 for Ag and Au respectively), the aluminum oxide stabilized systems are significantly more active (k = 1.82 and 0.63 min−1 for Ag and Au respectively) due to increased surface site availability. Importantly, Al2O3 supported silver nanoparticles exhibit excellent activity/re-usability as redox catalysts that are stable across a surprising variation of reaction media, including halide salts.

PH Dependence of Hydroxyl Radical, Ferryl, and/or Ferric Peroxo Species Generation in the Heterogeneous Fenton Process.

The heterogeneous Fenton process in the presence of Fe-containing minerals is ubiquitous in nature and widely deployed in wastewater treatment. While there have been extensive relevant studies, the dependence on pH of the nature and extent of oxidant generation and key reaction pathways remain unclear. Herein, the adsorption and decomposition of formate and H2O2 were quantified in the presence of ferrihydrite within the pH range of 3.0-6.0, and experiments with methyl phenyl sulfoxide were conducted to distinguish between HO• and weaker oxidant(s) which react via oxygen atom transfer including ferryl ion ([FeIVO]2+) and/or ferric hydroperoxo intermediates (≡FeIII(O2H)). Both HO• and [FeIVO]2+/≡FeIII(O2H) are concurrently produced on the surface over the acidic to near-neutral pH range. Despite the simultaneous formation of both oxidants, HO• is the major oxidant responsible for substrate oxidation in the interfacial boundary layer with [FeIVO]2+/≡FeIII(O2H) exhibiting limited exposure to substrates. With an increase of pH, the yield of both oxidants is inhibited by the decreasing availability of surface sites due to ferrihydrite particle aggregation. Increasing pH also favors the nonradical decay of H2O2 as evident from the consistent oxidant production rate relative to the surface area (SSA) despite an accelerated H2O2 decay rate relative to SSA with pH increase.

Solution 31P NMR Investigation of Inositol Hexakisphosphate Surface Complexes at the Amorphous Aluminum Oxyhydroxide-Water Interface.

Phytate (myo-inositol hexakisphosphate, myo-IHP) is one of the most common organic phosphorus (P) species in soils and sediments which can be mineralized to increase the concentration of dissolved phosphate in pore water. Because of its six phosphate functional groups, functional group-specific adsorption mechanisms in reactive soil minerals become important in predicting solubility. In this study, solution 31P NMR was used to elucidate the functional group-specific adsorption mechanisms of myo-IHP at the amorphous Al hydroxide (AAH)-water interface at pH 6.5 in conjunction with batch adsorption experiments and Zetasizer measurements. The adsorption maximum of myo-IHP with AAH was ∼312.50 mmol kg-1, and the charge reversal effects in IHP-reacted AAH particles suggested the presence of inner-sphere surface species. The upfield shifts of various phosphate groups in the NMR spectra further supported the formation of inner-sphere IHP complexes at the AAH-water interface. When the initial myo-IHP/AAH (mol kg-1) was decreased from 2.5 to 1.25-1.67, P1,3 and P4,6 functional groups were coordinated in addition to P2; P5 became reactive with the ratio being decreased to <0.84, P5. This multifunctional group coordination increased aggregate size. The study showed that the availability of surface sites of adsorbents influenced the functional group-specific myo-IHP adsorption.

Suppressing H2 Evolution and Promoting Selective CO2 Electroreduction to CO at Low Overpotentials by Alloying Au with Pd

CO 2 electroreduction is a promising technology to produce chemicals and fuels from renewable resources. Polycrystalline and nanostructured metals have been tested extensively while less effort has been spent on understanding the performance of bimetallic alloys. In this work, we study compositionally variant, smooth Au-Pd thin film alloys to discard any morphological or mesoscopic effect on the electrocatalytic performance. We find that the onset potential of CO formation exhibits a strong dependence on the Pd content of the alloys. Strikingly, palladium, a hydrogen evolution catalyst with reasonable exchange current density, suppresses hydrogen evolution when alloyed with gold in the presence of CO 2 . Cyclic voltammetry, in situ surface enhanced infrared absorption spectroscopy, and potential-dependent online product analysis strongly suggest that by alloying Au with Pd a significant increase in the surface coverage of adsorbed CO occurs with increasing Pd content at low overpotentials (e.g., approximately -0.35 V vs RHE). Such an increase in CO coverage suppresses H 2 evolution due to the lack of vacant active sites. Moreover, the overall increase in the binding energy with the CO 2 intermediates gained with the addition of Pd increases the CO production at low overpotentials, where polycrystalline Au suffers from poor CO 2 adsorption and poor selectivity for CO production. These results show that promising CO 2 reduction electrode materials (e.g., Au) can be alloyed not only to tune the catalyst's activity but also to deliberately decrease the availability of surface sites for competitive H 2 evolution.

Tailoring manganese oxide with atomic precision to increase surface site availability for oxygen reduction catalysis

Controlling the structure of catalysts at the atomic level provides an opportunity to establish detailed understanding of the catalytic form-to-function and realize new, non-equilibrium catalytic structures. Here, advanced thin-film deposition is used to control the atomic structure of La2/3Sr1/3MnO3, a well-known catalyst for the oxygen reduction reaction. The surface and sub-surface is customized, whereas the overall composition and d-electron configuration of the oxide is kept constant. Although the addition of SrMnO3 benefits the oxygen reduction reaction via electronic structure and conductivity improvements, SrMnO3 can react with ambient air to reduce the surface site availability. Placing SrMnO3 in the sub-surface underneath a LaMnO3 overlayer allows the catalyst to maintain the surface site availability while benefiting from improved electronic effects. The results show the promise of advanced thin-film deposition for realizing atomically precise catalysts, in which the surface and sub-surface structure and stoichiometry are tailored for functionality, over controlling only bulk compositions.

Effect of alkaline earth metal oxide (MO) Cu/MO/Al2O3 catalysts on methanol synthesis activity and selectivity via CO2 reduction

Heterogeneous copper/alumina catalysts with MgO, CaO, SrO, and BaO were prepared by co-precipitation synthesis method and characterized by means of N2-sorption, X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and subsequent H2 temperature-programmed reduction (H2 TPR) and H2/CO2 temperature-programmed desorption (H2 and CO2 TPD). The incorporation of oxides increases the metallic Cu surface area, as follows: Mg > Sr > Zn > Ca > Ba. These metal oxides thus increased the interaction between CuO and Al, resulting weaker reducibility of CuO. MgO increased the number of carbon dioxide and hydrogen adsorption active sites, whereas Ba augmented the basicity of the material. The activity with respect to methanol production matched the increase of Cu surface. As CH3OH selectivity monotonically decreased with the temperature, CO increased proportionally. Among the examined catalytic materials, Cu/MgO/Al2O3 exhibited a higher reactant conversion than the pelletized commercial analogues. Hydrogenation performance of the catalysts thus depended on both, the Cu surface site availability and the interaction between copper–metal oxides. While the industrial Cu/ZnO/Al2O3 has been extensively employed for quite some time, it is demonstrated herein that even upon the application of inexpensive non-critical raw material resources, such as MgO, an increment in bifunctional catalysis performance is attained. The Cu/Mg/Al catalyst showed a turnover frequency of 11.9 × 10−4 s−1, which is higher than the commercial HFR120 and LURGI catalysts.

Morpho-chemical characterization and surface properties of carcinogenic zeolite fibers

Erionite belonging to the zeolite family is a human health-hazard, since it was demonstrated to be carcinogenic. Conversely, offretite family zeolites were suspected carcinogenic. Mineralogical, morphological, chemical, and surface characterizations were performed on two erionites (GF1, MD8) and one offretite (BV12) fibrous samples and, for comparison, one scolecite (SC1) sample. The specific surface area analysis indicated a larger availability of surface sites for the adsorption onto GF1, while SC1 shows the lowest one and the presence of large pores in the poorly fibrous zeolite aggregates. Selected spin probes revealed a high adsorption capacity of GF1 compared to the other zeolites, but the polar/charged interacting sites were well distributed, intercalated by less polar sites (Si–O–Si). MD8 surface is less homogeneous and the polar/charged sites are more interacting and closer to each other compared to GF1. The interacting ability of BV12 surface is much lower than that found for GF1 and MD8 and the probes are trapped in small pores into the fibrous aggregates. In comparison with the other zeolites, the non-carcinogenic SC1 shows a poor interacting ability and a lower surface polarity. These results helped to clarify the chemical properties and the surface interacting ability of these zeolite fibers which may be related to their carcinogenicity.

Sludge-Derived Biochar for Arsenic(III) Immobilization: Effects of Solution Chemistry on Sorption Behavior.

Recycling sewage sludge by pyrolysis has attracted increasing attention for pollutant removal from wastewater and soils. This study scrutinized As(III) sorption behavior on sludge-derived biochar (SDBC) under different pyrolysis conditions and solution chemistry. The SDBC pyrolyzed at a higher temperature showed a lower As(III) sorption capacity and increasingly nonlinear isotherm due to loss of surface sites and deoxygenation-dehydrogenation. The Langmuir sorption capacity on SDBC (3.08-6.04 mg g) was comparable to other waste-derived sorbents, with the highest As(III) sorption on SDBC pyrolyzed at 400°C for 2 h. The As(III) sorption kinetics best fit with the pseudo-second-order equation, thus suggesting the significance of the availability of surface sites and initial concentration. Sorption of As(III) was faster than that of Cr(VI) but slower than that of Pb(II), which was attributed to their differences in molar volume (correlated to diffusion coefficients) and sorption mechanisms. The X-ray photoelectron spectra revealed an increase of oxide oxygen (O) with a decrease of sorbed water, indicative of ligand exchange with hydroxyl groups on SDBC surfaces. The As(III) sorption was not pH dependent in acidic-neutral range (pH < 8) due to the buffering capacity and surface characteristics of the SDBC; however, sorption was promoted by increasing pH in the alkaline range (pH > 8) because of As(III) speciation in solution. An increasing ionic strength (0.001-0.1 mol L) facilitated As(III) sorption, indicating the predominance of ligand exchange over electrostatic interactions, while high concentrations (0.1 mol L) of competing anions (fluoride, sulfate, carbonate, and phosphate) inhibited As(III) sorption. These results suggest that SDBC is applicable for As(III) immobilization in most environmentally relevant conditions.

Ciprofloxacin adsorption on graphene and granular activated carbon: kinetics, isotherms, and effects of solution chemistry

Ciprofloxacin (CIP) is a commonly used antibiotic and widely detected in wastewaters and farmlands nowadays. This study evaluated the efficacy of next-generation adsorbent (graphene) and conventional adsorbent (granular activated carbon, GAC) for CIP removal. Batch experiments and characterization tests were conducted to investigate the adsorption kinetics, equilibrium isotherms, thermodynamic properties, and the influences of solution chemistry (pH, ionic strength, natural organic matter (NOM), and water sources). Compared to GAC, graphene showed significantly faster adsorption and reached equilibrium within 3 min, confirming the rapid access of CIP into the macroporous network of high surface area of graphene as revealed by the Brunner–Emmet–Teller measurements analysis. The kinetics was better described by a pseudo-second-order model, suggesting the importance of the initial CIP concentration related to surface site availability of graphene. The adsorption isotherm on graphene followed Langmuir model with a maximum adsorption capacity of 323 mg/g, which was higher than other reported carbonaceous adsorbents. The CIP adsorption was thermodynamically favourable on graphene and primarily occurred through π − π interaction, according to the FTIR spectroscopy. While the adsorption capacity of graphene decreased with increasing solution pH due to the speciation change of CIP, the adverse effects of ionic strength (0.01–0.5 mol L−1), presence of NOM (5 mg L−1), and different water sources (river water or drinking water) were less significant on graphene than GAC. These results indicated that graphene can serve as an alternative adsorbent for CIP removal in commonly encountered field conditions, if proper separation and recovery is available in place.

Green Synthesis of Metal Nanoparticles via Natural Extracts: The Biogenic Nanoparticle Corona and Its Effects on Reactivity

The optical and catalytic properties of metal nanoparticles have attracted significant attention for applications in a wide variety of fields, thus prompting interest in developing sustainable synthetic strategies that leverage the redox properties of natural compounds or extracts. Here, we investigate the surface chemistry of nanoparticles synthesized using coffee as a biogenic reductant. Building on our previously developed synthetic protocols for the preparation of silver and palladium nanoparticle/carbon composite microspheres, a combination of thermogravimetric and spectroscopic methods was used to characterize the carbon microsphere and nanoparticle surfaces. Infrared reflectance spectroscopy and single particle surface enhanced Raman spectroscopy were used to characterize Pd and Ag metal surfaces, respectively, following synthesis. Strongly adsorbed organic layers were found to be present at metal nanoparticle surfaces after synthesis. The catalytic activity of Pd nanoparticles in hydrogenation reactions was leveraged to study the availability of surface sites, and coffee-synthesized nanomaterials were compared to commercial Pd-based hydrogenation catalysts. Our results demonstrate that biogenic adsorbates block catalytic surface sites and affect nanoparticle functionality. These findings highlight the need for careful analysis of surface chemistry as it relates to the specific applications of nanomaterials produced using greener or more sustainable methods.

Comparison of chemisorption close to ambient vs. under reaction conditions for Group VIII metal catalysts

A comparison of the relationship of H 2 or CO chemisorption measurements at 25–100 °C to similar results obtained under CO hydrogenation conditions by steady-state isotopic transient kinetic analysis (SSITKA) is made for the first time for a wide variety of Group VIII metal catalysts. The ratio N T ∗ / N chem (amount of chemisorption by SSITKA vs. by static chemisorptions) was found to be almost always ca. unity for Co catalysts. SSITKA can, thus, be used as a complementary characterization technique to TEM, XRD, and static chemisorption for better understanding of Co metal catalyst dispersion and metal surface site availability for Co catalysts with a wide variety of promoters/supports. Unfortunately, application of SSITKA chemisorption measurements under reaction conditions for characterizing metal dispersion for the other metals is limited at this time. However, the results do suggest some possibilities for Ru and Rh.

Surface Area Limited Model for Predicting Anisotropic Coarsening of Faceted Nanoparticles

The engineering and control of systems of monodispersed polyhedral nanoparticles are important for the development of devices based on size and shape-dependent properties. This includes a detailed understanding of how these designer nanoparticles are formed, and how they evolve and change over time. Although an experimental strategy or atom based theory may be used to observe different stages of this process, it is highly desirable to model the entire process, based on a limited set of physical or chemical parameters. Here we derive a kinetic model of nanoparticle shape evolution as a function of time, using specific concepts relating to surface diffusion, rate of step growth, and particularly surface site availability, referred to as the surface area limited (SAL) theory of growth and nanomorphology. This model is then applied to examples, including the axial growth of an arbitrary nanorod and the anisotropic growth of a nanogold truncated octahedron, restricted to either the AE111ae or AE100ae direction, each driven by the coarsening mechanism.

PH-dependent ciprofloxacin sorption to soils: Interaction mechanisms and soil factors influencing sorption

A study of ciprofloxacin sorption to 30 soils from the eastern United States revealed a statistically significant effect of pH on the solid–water distribution coefficient ( K d). Cation exchange capacity was the key soil factor influencing the extent of sorption at all pH values (3–8), with soil metal oxide content playing a smaller role at higher pH. Although, cation exchange, cation bridging and surface complexation mechanisms all appeared to contribute to the net extent of ciprofloxacin sorption to soils, the ciprofloxacin molecule appeared to be best suited for cation exchange. Notably, the ciprofloxacin cation (dominant at pH ≤ 5) exhibited a greater potential for cation exchange than the net neutral zwitterion (relevant at pH > 6). In addition to ciprofloxacin speciation in aqueous solution, the availability of surface sites for cation exchange (cation exchange capacity) and surface complexation (metal oxide content) played a key role in determining the shape of the pH edge and the potential contributions from individual sorption mechanisms to the net extent of sorption. Soils not limited by the availability of cation exchange sites and possessing a high effective cation exchange capacity (ECEC > 20 cmol c/kg), exhibited distinct pH edges (general decrease in sorption with pH) and the highest extent of sorption at all pH values. In these soils, cation exchange was found to be a dominant mechanism of sorption, with potential contributions from cation bridging and surface complexation being either unimportant or not discernable. In soils of lower cation exchange capacity (ECEC < 20 cmol c/kg), limited with respect to the availability of cation exchange sites, sorption initially increased with increasing pH, peaked at pH 5.5 and decreased at pH 7–8. In these soils, a specific contribution from surface complexation to the net extent of sorption was surmised at high pH values in metal oxide-rich soils, possessing low cation exchange capacities.

Catalytic reduction of N-nitrosodimethylamine with nanophase nickel–boron

Recent work by the authors shows that toxic N-nitrosamines, widely detected in disinfected wastewater and drinking water and recalcitrant towards conventional treatment processes, can be rapidly reduced to less hazardous products by treatment with hydrogen and Raney nickel catalysts. Unfortunately, Raney Ni catalysts are pyrophoric when dry and are readily deactivated when exposed to air, severely limiting their application for water treatment. In this work, a non-pyrophoric and air-tolerant nickel–boron (NiB) catalyst is described, and its reactivity with N-nitrosodimethylamine (NDMA) is quantified. The catalyst, prepared by aqueous reduction of NiCl2 with NaBH4, has an elemental composition of 85wt.% Ni and 7wt.% B. Nanophase primary catalyst particles are formed and aggregate in micrometer-sized assemblies in solution. The catalyst has a specific surface area of 18m2g−1, and the surface contains Ni(0) and B in a mixture of oxidation states (0/+III). Kinetic studies show that NDMA is rapidly reduced via hydrogenation to dimethylamine and ammonia in H2(g)-saturated (PH2=1 atm) aqueous NiB suspensions, with mass- and surface area-normalized first-order rate constants of 29.5±2.4LgNi−1h−1 and 1.38±0.11Lmcat−2h−1, respectively. Although the mass-normalized rate constant is less than that measured for Raney Ni, the surface area-normalized rate constant is greater, suggesting higher intrinsic reactivity of the NiB surface if the availability of surface sites for NDMA reactions on either catalyst surface is not significantly affected by aggregation of the catalyst nanoparticles. Kinetic studies show that reactivity of the NiB catalyst with NDMA is unaffected by solution pH and extended pre-exposure of the dry catalyst to air (Raney Ni is immediately deactivated by exposure to air). The NiB catalyst is also more tolerant of dissolved oxygen and low concentrations of dissolved (bi)sulfide than Raney Ni, suggesting potential for greater catalyst longevity during water treatment applications.