Adsorption Of Oxalate Research Articles

Ab Initio Molecular Dynamics Simulations of Phosphocholine Interactions with a Calcium Oxalate Dihydrate (110) Surface.

We use ab initio modeling (CASTEP) to help elucidate the crystallization phenomena and chemistry behind kidney stone composition and formation. To explore the stone formation process, we have constructed a surface model of calcium oxalate dihydrate-the mineral most commonly found in patients with hypercalciuria and modeled stone growth, by simulating further calcium oxalate adsorption onto the surface (-7.446 eV, -0.065 eV/atom). Furthermore, urine analysis of kidney stone patients has previously revealed that their urine contains higher concentrations of phospholipids compared to healthy individuals. Therefore, to investigate the interactions between urinary macromolecules and the growing crystal surfaces at an atomic level, we have performed ab initio molecular dynamics simulations of phosphocholine adsorption on calcium oxalate surfaces. We have shown that the phosphocholine headgroups become entrapped within the growing crystal and the lowest energy structures (-18.008 eV, -0.0396 eV/atom) are those where the calcium oxalate dihydrate surfaces have become disrupted, with reorganization of their crystallographic structure. Urinary calculi (kidney stones) are a common ailment affecting around 10% of the world's population and resulting in nearly 90,000 finished consultant episodes (FCE) each year in the United Kingdom [Hospital Episode Statistics, Admitted Patient Care-England, 2011-12 NHS Digital, 2021-2022. https://digital.nhs.uk/data-and-information/publications/statistical/hospital-admitted-patient-care-activity/hospital-episode-statistics-admitted-patient-care-england-2011-12].

Utilization of argan nut shells as a bioresource for efficient malachite green oxalate adsorption

Utilization of argan nut shells as a bioresource for efficient malachite green oxalate adsorption

Effect of Adsorbed Carboxylates on the Dissolution of Boehmite Nanoplates in Highly Alkaline Solutions.

Understanding the dissolution of boehmite in highly alkaline solutions is important to processing complex nuclear waste stored at the Hanford (WA) and Savannah River (SC) sites in the United States. Here, we report the adsorption of model carboxylates on boehmite nanoplates in alkaline solutions and their effects on boehmite dissolution in 3 M NaOH at 80 °C. Although expectedly lower than at circumneutral pH, adsorption of oxalate occurred at pH 13, with adsorption decreasing linearly to 3 M NaOH. Classical molecular dynamics simulations suggest that the adsorption of oxalate dianions onto the boehmite surface under high pH can occur through either inner- or outer-sphere complexation mechanisms depending on adsorption sites. However, both adsorption models indicate relatively weak binding, with an energy preference of 1.26 to 2.10 kcal/mol. By preloading boehmite nanoplates with oxalate or acetate, we observed suppression of dissolution rates by 23 or 10%, respectively, compared to pure solids. Scanning electron microscopy and transmission electron microscopy characterizations revealed no detectable difference in the morphologic evolution of the dissolving boehmite materials. We conclude that preadsorbed carboxylates can persist on boehmite surfaces, decreasing the density of dissolution-active sites and thereby adding extrinsic controls on dissolution rates.

Construction of an OCP-ATR-FTIR Spectroscopy Device to In Situ Monitor the Interfacial Reaction of Contaminants: Competitive Adsorption of Cr(VI) and Oxalate on Hematite.

The comprehensive understanding of contaminant interfacial behavior strongly depends on the in situ characterization technique, which is still a great challenge. In this study, we constructed a device integrated with open-circuit potentialand attenuated total reflectance Fourier transform infrared (OCP-ATR-FTIR) spectroscopy to simultaneously monitor the electrochemical and infrared spectral information on the interfacial reaction for the process analysis, taking the competitive adsorption of hexavalent chromium (Cr(VI)) and oxalate on hematite nanocubes (HNC) as an example. The synchronous OCP and infrared results revealed that Cr(VI) interacted with HNC via bidentate binuclear inner-sphere coordination, accompanied by electron transfer from HNC to Cr(VI), while oxalate was adsorbed on HNC through bidentate mononuclear side-on inner-sphere coordination with electron transfer from HNC to oxalate, and also outer-sphere coordination with negative charge accumulation. When oxalate was added to HNC with preadsorbed Cr(VI), oxalate would occupy the inner-sphere adsorption sites and thus cause the detaching of preadsorbed Cr(VI) from HNC. This study provides a promising in situ characterization technique for real-time interfacial reaction monitoring and also sheds light on the competitive adsorption mechanism of oxalate and Cr(VI) on the mineral surface.

Silicon Incorporation Reduces the Reactivity of Short-range Ordered Aluminosilicates Toward Organic Acids

The structure and composition of short-range ordered aluminosilicates (SROAS) may control their affinity for organic acids with potential effects on soil organic matter stabilization. Adsorption mechanisms of model organic acids were studied to resolve the effect of Si incorporation. Adsorption of oxalic, salicylic, and octanoic acid on Al-rich (Al:Si = 3.7) and Si-rich (Al:Si = 1.4) SROAS was quantified by analyses of dissolved organic carbon using catalytic high-temperature combustion. The initial pH of 5 and 6.5 increased to 6.3–8.2 during adsorption of oxalic and salicylic acid, demonstrating hydroxyl release by ligand exchange. Minor changes in pH indicated weak interactions of octanoic acid with both SROAS. Adsorbates were characterized by Fourier-transform infrared spectroscopy. Asymmetric stretching of carboxylate groups at 1720 and 1700 cm–1, and symmetric stretching at 1430 cm–1 evinced the formation of chelate complexes for oxalic acid. An absorption band centered at 1545 cm–1 indicated partial inner-sphere binding of salicylic acid on both SROAS. Silicon-rich SROAS adsorbed 80–90% less than Al-rich SROAS, suggesting that adsorption of oxalic and salicylic acid was controlled by surface aluminol groups. Fast kinetics of oxalate adsorption on Al sites was studied by a conductivity-based stopped-flow technique. Ligand exchange proceeded at a rate constant of 3.5 s–1 (25°C), similar to solute Al complexation, with an activation energy of up to 34.1 kJ mol–1. A slow process with a rate constant of 0.13 s–1 (25°C) was attributed to diffusion of oxalate at the surface or into SROAS particles. As supported by structural characterization of Si-rich SROAS, the much lower susceptibility of Si-rich SROAS to ligand exchange relates to Al speciation. The formation of tetrahedral Al precludes its complexation by carboxyl groups.

Stimulation of oxalate root exudate in arsenic speciation and fluctuation with phosphate and iron in anoxic mangrove sediment

Mutual transformations of rhizospheric arsenic (As) in pollution-prone mangrove sediments affected by root exudate oxalate were simulated. This study focuses on the effect of oxalate on As release, mobilization, and phase speciation associated with P and Fe was examined under anoxic conditions in time-dependent changes. Results showed that oxalate addition significantly facilitated As–Fe–P release from As-contaminated mangrove sediments. Sediment As formed the adsorptive and the carbonate-binding fractionations, facilitating the re-adsorption processes. Solution As and As5+ correlated with NaOH–P positively but with NaHCO3–P and HCl–P negatively. Dominant Fe3+ (>84 %) from the amorphous Fe regulated suspension changes and then time-dependent co-precipitation with As and P. Sediment P formed strong complexes with Fe oxides and could be substituted for As via STEM analysis. Oxalate ligand exchange, competitive adsorption of oxalate, and Fe-reduced dissolution are confirmed to involve, allowing for an insight As/P/Fe mobilization and fate in mangrove wetland.

K+-induced formation of granular and dense copper phyllosilicate precursor converts dimethyl oxalate to ethylene glycol in absence of H2

The reducibility of heterogeneous Cu-based precursor greatly influences the activity and stability of the final catalysts because the excessive reduction to metallic Cu significantly deactivates the catalysts. Herein, we develop a crystallization inhibition strategy to fabricate the granular and densely stacked copper phyllosilicate precursor which possesses stronger resistance to H2 reduction. The reduction-resistant CuSiO3 precursor contains more abundant Cu+ species and oxygen deficiencies, which was verified by O2-TPD, TGA, O XPS and Cu XAES measurements, and is beneficial for methanol dehydrogenation and dimethyl oxalate (DMO) adsorption. A high yield of 85.4% of ethylene glycol (EG) was achieved from DMO hydrogenation at 240 °C, with methanol as both the solvent and hydrogen supplier. The delicately designed synthesis of specific Cu precursor with unique properties resulted in an excellent multi-functional Cu catalyst for coupled methanol dehydrogenation and DMO hydrogenation to EG.

Adsorption of small organic acids and polyphenols on hematite surfaces: Density Functional Theory + thermodynamics analysis

HypothesisThe interactions of organic molecules with mineral surfaces are influenced by several factors such as adsorbate speciation, surface atomic and electronic structure, and environmental conditions. When coupled with thermodynamic techniques, energetics from atomistic modeling can provide a molecular-level picture of which factors determine reactivity. This is paramount for evaluating the chemical processes which control the fate of these species in the environment. ExperimentsInner-sphere adsorption of oxalate and pyrocatechol on (001), (110), and (012) α-Fe2O3 surfaces was modeled using Density Functional Theory (DFT). Unique bidentate binding modes were sampled along each facet to study how different adsorbate and surface factors govern site preference. Adsorption energetics were then calculated using a DFT + thermodynamics approach which combines DFT energies with tabulated data and Nernst-based corrective terms to incorporate different experimental parameters. FindingsInstead of a universal trend, each facet displays a unique factor that dominates site preference based on either strain (001), functional groups (110), or topography (012). Adsorption energies predict favorable inner-sphere adsorption for both molecules but opposite energetic trends with varying pH. Additionally, vibrational analysis was conducted for each system and compared to experimental IR data. The work presented here provides an effective, computational methodology to study numerous adsorption processes occurring at the surface-aqueous interface.

Understanding Reaction Pathways in High Dielectric Electrolytes Using β-Mo2C as a Catalyst for Li-CO2 Batteries.

The rechargeable Li-CO2 battery has attracted considerable attention in recent years because of its carbon dioxide (CO2) utilization and because it represents a practical Li-air battery. As with other battery systems such as the Li-ion, Li-O2, and Li-S battery systems, understanding the reaction pathway is the first step to achieving high battery performance because the performance is strongly affected by reaction intermediates. Despite intensive efforts in this area, the effect of material parameters (e.g., the electrolyte, the cathode, and the catalyst) on the reaction pathway in Li-CO2 batteries is not yet fully understood. Here, we show for the first time that the discharge reaction pathway of a Li-CO2 battery composed of graphene nanoplatelets/beta phase of molybdenum carbide (GNPs/β-Mo2C) is strongly influenced by the dielectric constant of its electrolyte. Calculations using the continuum solvents model show that the energy of adsorption of oxalate (C2O42-) onto Mo2C under the low-dielectric electrolyte tetraethylene glycol dimethyl ether is lower than that under the high-dielectric electrolyte N,N-dimethylacetamide (DMA), indicating that the electrolyte plays a critical role in determining the reaction pathway. The experimental results show that under the high-dielectric DMA electrolyte, the formation of lithium carbonate (Li2CO3) as a discharge product is favorable because of the instability of the oxalate species, confirming that the dielectric properties of the electrolyte play an important role in the formation of the discharge product. The resulting Li-CO2 battery exhibits improved battery performance, including a reduced overpotential and a remarkable discharge capacity as high as 14,000 mA h g-1 because of its lower internal resistance. We believe that this work provides insights for the design of Li-CO2 batteries with enhanced performance for practical Li-air battery applications.

Oxalic Acid Adsorption on Rutile: Molecular Dynamics and ab Initio Calculations.

Detailed analysis of the adsorption of oxalic acid ions, that is, oxalate and hydrogenoxalate, on the rutile (110) surface was carried out using molecular dynamics augmented by free energy calculations and supported by ab initio calculations. The predicted adsorption on perfect nonhydroxylated and hydroxylated surfaces with surface charge density from neutral to +0.208 C/m2 corresponding to pH values of about 6 and 3.7, respectively, agrees with experimental adsorption data and charge-distribution multisite ion complexation model predictions obtained using the most favorable surface complexes identified in our simulations. We found that outer-sphere complexes are the most favorable, owing to strong hydrogen binding of oxalic acid ions with surface hydroxyls and physisorbed water. The monodentate complex, the most stable among inner-sphere complexes, was about 15 kJ/mol higher in energy, but separated by a large energy barrier. Other inner-sphere complexes, including some previously suggested in the literature as likely adsorption structures such as bidentate and chelate complexes, were found to be unstable both by classical and by ab initio modeling. Both the surfaces and (hydrogen)oxalate ions were modeled using charges scaled to 75% of the nominal values in accord with the electronic continuum theory and our earlier parameterization of (hydrogen)oxalate ions, which showed that nominal charges exaggerate ion-water interactions.

Oxalic Acid Adsorption on Rutile: Experiments and Surface Complexation Modeling to 150 °C.

Here, we characterize oxalate adsorption by rutile in NaCl media (0.03 and 0.30 m) and between pH 3 and 10 over a wide temperature range which includes the near hydrothermal regime (10-150 °C). Oxalate adsorption increases with decreasing pH (as is typical for anion binding by metal oxides), but systematic trends with respect to ionic strength or temperature are absent. Surface complexation modeling (SCM) following the CD-MUSIC formalism, and as constrained by molecular modeling simulations and IR spectroscopic results from the literature, is used to interpret the adsorption data. The molecular modeling simulations, which include molecular dynamics simulations supported by free-energy and ab initio calculations, reveal that oxalate binding is outer-sphere, albeit via strong hydrogen bonds. Conversely, previous IR spectroscopic results conclude that various types of inner-sphere complexes often predominate. SCMs constrained by both the molecular modeling results and the IR spectroscopic data were developed, and both fit the adsorption data equally well. We conjecture that the discrepancy between the molecular simulation and IR spectroscopic results is due to the nature of the rutile surfaces investigated, that is, the perfect (110) crystal faces for the molecular simulations and various rutile powders for the IR spectroscopy studies. Although the (110) surface plane is most often dominant for rutile powders, a variety of steps, kinks, and other types of surface defects are also invariably present. Hence, we speculate that surface defect sites may be primarily responsible for inner-sphere oxalate adsorption, although further study is necessary to prove or disprove this hypothesis.

Oxalate-TiO2 complex-mediated oxidation of pharmaceutical pollutants through ligand-to-metal charge transfer under visible light

Oxalate-adsorbed TiO2 shows visible activity for the oxidation of ranitidine, although neither oxalate nor pure TiO2 alone absorbs visible light. The formation of an oxalate-TiO2 complex facilitates electron transfer from oxalate to the TiO2 conduction band (CB) (i.e., a ligand-to-metal charge transfer (LMCT)) and generates superoxide/hydroperoxyl radicals (O2−/HO2), which are primarily responsible for ranitidine oxidation, under visible light. The attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra of oxalate-adsorbed TiO2 indicates that the formation of a LMCT complex between oxalate and TiO2 occurs through bidentate carboxylate linkages. The visible light-induced generation of photocurrent (Iph) on the TiO2/FTO electrode in the presence of oxalate confirms the LMCT mechanism in the oxalate-TiO2 complex under visible light. Kinetic studies with varying oxalate concentrations, initial pH values, and TiO2 types demonstrate that the oxidation efficiency increases as the adsorption of oxalate and the molar fraction of O2− increase. Not only ranitidine but also other pharmaceutical pollutants, such as cimetidine, propranolol, imidazole, and nizatidine, were oxidized in the pure TiO2/oxalate/visible light system. The oxidation efficiency of the oxalate-TiO2 complex was higher than that of other organic ligand-TiO2 complexes (i.e., citrate-, EDTA-, malonate-, acetate-, and glucose-TiO2 complexes). In this regard, the pure TiO2/oxalate/visible light system can be proposed as a practical method for the treatment of pharmaceutical-contaminated water and wastewater.

A density functional theory investigation of oxalate and Fe(II) adsorption onto the (010) goethite surface with implications for ligand- and reduction-promoted dissolution

Oxalic acid is an important, biologically-produced species in the natural environment. The deprotonated form, oxalate, is dominant in aqueous solutions under circumneutral pH conditions and is a strong ligand for Fe(III). The high affinity of oxalate for Fe(III) means that Fe(III)-oxalate surface and aqueous complexes are common and can lead to ligand-enhanced dissolution. Fe(II) adsorption onto goethite (α-FeOOH) has been shown to enhance dissolution-recrystallization reactions. The goethite (010) face is one of the more common and reactive surfaces on this environmentally critical Fe-hydroxide phase. Hence, this study models both separate and coordinated adsorption of oxalate and Fe(II) onto the (010) face of goethite in order to test for synergistic effects of ligand-promoted and reductive dissolution. Periodic and cluster density functional theory (DFT) energy minimizations were performed to determine the structure, vibrational frequencies and energies of various configurations. The adsorption mechanism of oxalate is verified via comparison to observed IR spectra. The potential roles of oxalate and Fe(II) in ligand-enhanced reductive dissolution of goethite are discussed.

Mechanochemical and chemical activation of lignocellulosic material to prepare powdered activated carbons for adsorption applications

This work presents a solvent-free mechanochemical approach toward controlled preparation of powdered activated carbons from lignocellulosic residue (horse chestnut shell, CS). The physicochemical changes of the biomass during mechanochemical and chemical activation with K2CO3 and the properties of the final materials obtained after pyrolysis at 700°C have been investigated by means of thermogravimetric analysis (TG–DTG), scanning electron microscopy (SEM), X-ray diffraction (XRD), N2 adsorption–desorption, infrared and micro-Raman spectroscopy. The results indicated that the mechanochemical activation is superior to the traditional chemical activation in terms of specific surface area, pore development and the degree of structural order of the activated carbons. Furthermore, it has been observed that the later parameters depend on the milling time and the maximal BET surface area (1040m2/g) and total pore volume (1.01cm3/g) of the resulting mesoporous material can be obtained after 3h milling of the raw material/K2CO3 mixture. In addition, malachite green oxalate (MGO) adsorption capacities of the carbons were also examined and adsorption uptake up to 250mg/g is observed. This study demonstrates the potential use of CS as a precursor in the preparation of activated carbon for adsorption applications and opens up new ways for activation of lignocellulosic materials under ball-milling conditions.

Remediation of arsenic contaminated soil by coupling oxalate washing with subsequent ZVI/Air treatment.

The application of a novel coupled process with oxalate washing and subsequent zero-valent iron (ZVI)/Air treatment for remediation of arsenic contaminated soil was investigated in the present study. Oxalate is biodegradable and widely present in the environment. With addition of 0.1 mol L(-1) oxalate under circumneutral condition, 83.7% and 52.6% of arsenic could be removed from a spiked kaolin and an actual contaminated soil respectively. Much more oxalate adsorption on the actual soil was attributed to the higher soil organic matter and clay content. Interestingly, oxalate retained in the washing effluent could act as an organic ligand to promote the oxidation efficiency of ZVI/Air at near neutral pH. Compared with the absence of oxalate, much more As(III) was oxidized. Arsenic was effectively adsorbed on iron (hydr)oxides as the consumption of oxalate and the increase of pH value. For the actual soil washing effluent, about 94.9% of total arsenic was removed after 120 min's treatment without pH adjustment. It has been demonstrated that As(V) was the dominant arsenic speciation adsorbed on iron (hydr)oxides. This study provides a promising alternative for remediation of arsenic contaminated soil in view of its low cost and environmental benign.

Modeling the Adsorption of Oxalate onto Montmorillonite.

In this work, a multiscale modeling of the interaction of oxalate with clay mineral surfaces from macroscale thermodynamic equilibria simulations to atomistic calculations is presented. Previous results from macroscopic adsorption data of oxalate on montmorillonite in 0.01 M KNO3 media at 25 °C within the pH range from 2.5 to 9 have been used to develop a surface complexation model. The experimental adsorption edge data were fitted using the triple-layer model (TLM) with the aid of the FITEQL 4.0 computer program. Surface complexation of oxalate is described by two reactions: >AlOH + Ox(2-) + 2H(+) = >AlOxH + H2O (log K = 14.39) and >AlOH + Ox(2-) + H(+) = >AlOx(-) + H2O (log K = 10.39). The monodentate complex >AlOxH dominated adsorption below pH 4, and the bidentate complex >AlOx(-) was predominant at higher pH values. Both of the proposed inner-sphere oxalate species are qualitatively consistent with previously published diffuse reflectance FTIR spectroscopic results for oxalate on montmorillonite edge surface (Chem. Geol. 2014, 363, 283-292). Atomistic computational studies have been performed to understand the interactions at the molecular level between adsorbates and mineral surface, showing the atomic structures and IR frequency shifts of the adsorption complexes of oxalate with the edge surface of a periodic montmorillonite model.

The Formation of Surface Oxides on Nickel in Oxalate-Containing Alkaline Media

The electrochemical formation of α-Ni(OH)2 and NiOOH in the presence of adsorbed oxalate in alkaline media is studied under well-controlled experimental conditions that include the variation of the system temperature (T = −10 to 20 °C), the scan rate (v = 20, 150, and 200 mV s−1), and the concentration of supporting electrolyte (0.10 and 0.50 M KOH). The studies are carried out using cyclic voltammetry (CV) with polycrystalline bulk nickel and nickel foams. In situ infrared spectroscopy with voltammetry confirms the adsorption of oxalate to the surface of nickel in the 0.10 to 0.30 V potential window, concurrent with the formation of the α-Ni(OH)2 species. The presence of oxalate in the system increases the charge density (Q) for the formation of both the α-Ni(OH)2 and NiOOH surface oxides. The Q values calculated under various conditions indicate that the presence of oxalate in the system encourages the formation of a full single monolayer (ML) of NiOOH in the first CV scan. Measurements carried out at room temperature demonstrate that an increase in v decreases the Q values for NiOOH in the presence of oxalate to minimum values achieved at v ≥150 mV s−1. An increase of KOH concentration results in the formation of a thicker layer of NiOOH both in the presence and absence of oxalate. The Q values of NiOOH reduction in conditions that favor the formation of one complete monolayer of NiOOH are used to calculate the specific surface areas of open-cell nickel foams. The calculation of electrochemical surface area using this method is discussed and evaluated with respect to calculations based on the charge of α-Ni(OH)2 formation.

Hybrid Biotic/Abiotic Catalysts for Oxidation of Complex Biofuels

The electrochemical oxidation of oxalic acid on platinum is one of the main methods explored so far in the design of amperometric sensors for quantitative determination of oxalate and a commonly used approach for wastewater purification and energy production. Apart from that, herein we report the application of various platinum alloys and platinum nanoclusters as catalyst for oxidation of oxalic acid as a final step in the multistep oxidation of various organic compounds, such as fructose, glycerol, glyceric acid, glyceraldehyde, etc. This multistep oxidation processes can involve purely biotic or abiotic catalysts or a hybrid organo/metalo/enzyme catalysts, where the latter along with traditional electrocatalysis takes advantage of bioelectrocatalysis as well. Various PtSn and PtRu alloys were electrochemically evaluated towards the oxidation of oxalic acid in neutral and acidic media (Fig. 1). The onset potential of oxalate oxidation was significantly shifted towards lower potentials for all platinum alloys tested indicating decreased activation energy for the rate-limiting step especially at low pH. Along with the improved thermodynamics of the process, the kinetic of oxalate oxidation was also enhanced when PtSn (1:1) was employed. Density Functional Theory (DFT) calculations were explored to gain a better understanding of the reactivity of the platinum alloys [1]. The mechanism in which two electrons are transferred directly from oxalic acid molecule to the catalyst surface was considered: C2O4H2→*C2O4H + H+ + e-→*CO2H+CO2+H++e-→2CO2+2H++2e- where * represents the active site on the catalyst’s surface. Other mechanisms were also considered, which involve participation of adsorbed species such as OH or O. The free energy diagrams for the oxidation of oxalic acid on Pt based on these mechanisms were calculated using Perdew-Burke-Ernzerhof functional [1]. The results indicate that the adsorption of oxalic acid, which is accompanied by the release of H+and transfer of one electron to the electrode, requires the largest change in the free energy and can be considered as thermodynamically limiting step. Since the adsorption of oxalic acid is the potential-limiting step, an increase in the adsorption energy of oxalic acid on the different platinum alloys can explain the differences in the overpotential observed during the electrochemical evaluation. Based on the DFT calculations it was established that the presence of Sn and/or Ru increases oxalate adsorption and leads to decrease in the overpotential of oxalate oxidation in accordance to the experimentally observed results. DFT also predicted that oxalate oxidation is highly sensitive to the size of the Pt clusters used as a catalyst, imposing the conclusion that decreasing the size of Pt would provide higher activity. Thus, platinum nanoclusters were synthesized using DNA as a template that provides improved cluster stability and were evaluated towards oxidation of oxalic acid. These nanoclusters could be further assembled with aldehyde dehydrogenase to provide multistep catalysis of glyoxalic acid to CO2. In addition, Pt alloys were explored in combination with TEMPO for the subsequent and complete oxidation of glycerol. [1] I. Matanovic, S. Babanova, A. Perry III, A. Serov, K. Artyushkova and P. Atanassov, Bio-inspired design of electrcatalysis for oxalate oxidation: a combined experimental and computational study of Mn-N-C catalyst, Phys. Chem. Chem. Phys., 2015, DOI: 10.1039/C5CP00676G Figure 1

Probing the Surface Area of Nickel Catalyst Materials: The Effect of Oxalate Adsorption on the Electrochemistry of Nickel in Basic Media

INTRODUCTIONNickel and nickel-based materials find widespread applications as secondary battery anodes, supercapacitors, electro-catalysts, photocatalysts, and photovoltaic cell components (e.g., [1]-[3]). In alkaline media, nickel-based electrodes are effective catalysts for the hydrogen and oxygen evolution reactions (HER, OER), by which hydrogen may be renewably produced as fuel or industrial feedstock. However, accurate methods to measure the electrochemically active surface area (ECSA) for nickel-based electrodes have not yet been established [4]. The ECSA directly affects the catalytic activity and electrode capacitance, hence, a reliable method for the measurement of the ESCA is desirable.In this work, a new method to probe the surface area of metallic nickel electrodes utilizing voltammetry will be presented. The results are compared to alternative ECSA measurement techniques in terms of accuracy, precision and practical applications for nickel-based catalyst materials.RESULTS AND DISCUSSIONNi electrodes were mechanically polished using 320 grit SiC paper, 9 and 3 μm polycrystalline diamond pastes and a 50 nm Al2O3 suspension. Electrochemical measurements were collected in 0.1 or 1 M KOH with and without the addition of 0.001 M – 0.1 M K2C2O4.In comparison with a forward voltammetric sweep in an electrolyte containing only KOH, the addition of oxalate anions to the solution has no observed effect on the voltammetry of metallic nickel in the hydrogen evolution reaction (HER) or the Ni/Ni(II) oxidation potential regions (< 1.3 VRHE). This result is similar to those reported for several other organic additives [5]. However, whereas literature evidence shows that at higher potentials many organic additives oxidize at a diffusion-limited rate, oxalate anions do not show the expected oxidation current. Instead, we observe that the Ni(II)/Ni(III) oxidation peak, normally at ~1.45 VRHE, occurs at lower potentials (1.35 – 1.40 VRHE). Further, in the presence of sufficiently high concentrations of oxalate anions, the Ni(II)/Ni(III) oxidation peak is very sharp, with full widths at half maxima (FWHMs) less than 20 mV. The voltammetric peak characteristics, i.e., the peak position, width and area, have been measured at various concentrations of oxalate and at various potential sweep rates. The peak position is very sensitive to the electrode’s surface state. However, the peak width and area are very reproducible.The practical result of the present work is that we now have a sharp voltammetric peak that is well-separated from concurrent electrochemical processes, in particular the OER, which typically onsets near 1.45 - 1.5 VRHE. The electrical charge associated with this oxidation peak is consistent with a process involving a monolayer. The implementation of these results for measuring the ECSA of metallic nickel electrodes has been tested by conducting voltammetry on different electrode preparation regimes, which ranged from a rough polish to a mirror finish.The validity of this characterization method will be discussed and the results will be compared with alternative methods [6]. The extension of this work to characterizing nickel-based catalyst materials will be discussed.ACKNOWLEDGEMENTSThis work was supported by National Research Council Canada (NRC), the Natural Sciences and Engineering Research Council of Canada (NSERC) and the University of Ottawa.REFERENCES1. D. Chade, L. Berlouis, D. Infield, A. Cruden, P. T. Nielsen and T. Mathiesen, Int. J. Hydrogen Energy, 38, 14380 (2013).2. Y. Miao, L. Ouyang, S. Zhou, L. Xu, Z. Yang, M. Xiao and R. Ouyang, Biosens. Bioelectron., 53, 428 (2014).3. Y. Wang, S. Gai, N. Niu, F. He and P. Yang, J. Mater. Chem. A, 1, 9083 (2013).4. D. S. Hall, C. Bock and B. R. MacDougall, J. Electrochem. Soc., 160, F235 (2013).5. M. Fleischmann, K. Korinek and D. Pletcher, J. Electroanal. Chem. Interfacial Electrochem., 31, 39 (1971).6. S. Trasatti and O. A. Petrii, J. Electroanal. Chem., 327, 353 (1992).

An Oxalate Method for Measuring the Surface Area of Nickel Electrodes

This work establishes a new method to measure the electrochemically active surface area (AECSA) of nickel electrodes, in situ. The addition of 0.08 mol L−1 of an oxalate salt to an alkaline electrolyte solution shifts the half-wave potential of the Ni(II)/Ni(III) redox pair by about −80 mV. Further, these peaks are very narrow. The sharpest peak in this work has a full width at half maximum (FWHM) of just 11 mV. This unusual sharpness is attributed to the layered structure of nickel hydroxides and the adsorption of oxalate from the solution on the (001) surface. This is supported by attenuated total reflectance infrared (ATR-IR) peaks measured at 1265 cm−1, 1655 cm−1, and 1713 cm−1, which correspond to mononuclear bidentate oxalate bonded to nickel. At sufficiently fast potential scan rates (≥150 mV s−1), the adsorbed oxalate limits growth of the surface hydroxide to a single layer. During the reverse scan, the surface NiOOH/Ni(OH)2 reduction peak is well-separated from other electrochemical processes and may be used to accurately and precisely measure the AECSA. The error of this method is estimated at < 10%.