Nitrogen cycle electrocatalysis

Silicon-doped graphene: an effective and metal-free catalyst for NO reduction to N2O?

Density functional theory (DFT) calculations were performed on the NO reduction on the silicon (Si)-doped graphene. The results showed that monomeric NO dissociation is subject to a high barrier and large endothermicity and thus is unlikely to occur. In contrast, it was found that NO can easily be converted into N2O through a dimer mechanism. In this process, a two-step mechanism was identified: (i) the coupling of two NO molecules into a (NO)2 dimer, followed by (ii) the dissociation of (NO)2 dimer into N2O + O(ad). In the energetically most favorable pathway, the trans-(NO)2 dimer was shown to be a necessary intermediate with a total energy barrier of 0.464 eV. The catalytic reactivity of Si-doped graphene to NO reduction was interpreted on the basis of the projected density of states and charge transfer.

Theory of the transition from sequential to concerted electrochemical proton-electron transfer

A theory for the calculation of potential energy surfaces of electrochemical proton-coupled electron transfer is considered and parameterized on the basis of thermodynamic relations. The paper discusses the qualitatively different potential energy surfaces predicted by the theory, and their relation to the existence of sequential and concerted proton-electron transfer pathways. The concomitant activation energies for sequential and concerted PET are calculated. The applied overpotential may change the qualitative shape of the PES and therefore the mechanism of the proton-coupled electron transfer reaction.

Nitrate Reduction on Platinum (111) Surfaces Modifiedl with Bi: Single Crystalsl and Nanoparticles

Nitrate reduction on well-oriented platinum surfaces modified with Bi adatoms has been studied. The quantification of the electrocatalytic enhancement of the reaction rate due to the presence of Bi at different coverages was made on Pt(111) and the vicinal surfaces Pt(554) and Pt(332). These contain 9 and 5 atoms-width (111) terraces, respectively, separated by (110) monoatomic steps. The study was then extended to preferentially {111}Pt oriented nanoparticles. In all cases, Bi catalyzes nitrate reduction at high potentials, but the catalytic current suddenly drops when Bi is reduced. The analysis of the variation of catalytic activity with Bi coverage reveals the participation of a third body effect, meaning that Bi impedes the NO formation on the surface that acts as a poison for the nitrate reduction.

The use of Ebonex electrodes for the electrochemical removal of nitrate ion from water

This work addresses the remediation of nitrate-contaminated water using electrodes made of Ebonex (a titanium oxide ceramic with a wide range of potential stability). The objective was the complete denitrification of solutions containing nitrate ion. Denitrification was achieved in about 50% yield with unreactive supporting electrolytes when Ebonex was used as both cathode and anode, the remaining product being ammonia. Ammonia could be re-oxidized at the Ebonex anode, but this was much less efficient than the reduction step. A more efficient electrolytic denitrification was possible for solutions containing chloride; this is oxidized anodically to hypochlorite, which then oxidizes ammonia chemically to N2. The overall rate of denitrification was highest at moderate concentrations of chloride ion, because hypochlorite also re-oxidizes reduction intermediates such as nitrite back to nitrate. Complete denitrification was achieved at all stages of the reaction using Ebonex cathode and a dimensionally stable anode based on Ti/IrO2 or Ti/RuO2, because the DSA oxidizes chloride ion more efficiently than Ebonex. Cathode fouling by water sources that are high in hardness cations can be prevented by using one DSA and a pair of Ebonex electrodes that undergo periodic polarity reversal.

Noble metal-free oxidative electrocatalysts: polyaniline and Co(II)-polyaniline nanostructures hosted in nanoporous silica

An efficient nanocomposite electrocatalyst composed of mesoporous silica (SBA-15) with polyaniline (PANI) nanostructures within its channel pores (PANI/SBA-15) is synthesized and characterized. The resulting PANI/SBA-15 is capable of chelating Co(II) ions, presumably via its nitrogen atoms on PANI/diamine groups. Both the metal-free (SBA-15/PANI) and the Co(II)-doped SBA-15/PANI nanocomposite materials showed high electrocatalytic activity for oxidation of L-ascorbic acid, with very low overpotential and high current density. The activity of PANI/SBA-15 toward oxidation of L-ascorbic acid is comparable to that obtained from a conventional Pt/C electrocatalyst.

Critical review of Pd-based catalytic treatment of priority contaminants in water

Catalytic reduction of water contaminants using palladium (Pd)-based catalysts and hydrogen gas as a reductant has been extensively studied at the bench-scale, but due to technical challenges it has only been limitedly applied at the field-scale. To motivate research that can overcome these technical challenges, this review critically analyzes the published research in the area of Pd-based catalytic reduction of priority drinking water contaminants (i.e., halogenated organics, oxyanions, and nitrosamines), and identifies key research areas that should be addressed. Specifically, the review summarizes the state of knowledge related to (1) proposed reaction pathways for important classes of contaminants, (2) rates of contaminant reduction with different catalyst formulations, (3) long-term sustainability of catalyst activity with respect to natural water foulants and regeneration strategies, and (4) technology applications. Critical barriers hindering implementation of the technology are related to catalyst activity (for some contaminants), stability, fouling, and regeneration. New developments overcoming these limitations will be needed for more extensive field-scale application of this technology.

Optimization of the cathode material for nitrate removal by a paired electrolysis process

Ni, Cu, Cu(90)Ni(10) and Cu(70)Ni(30) were evaluated as cathode materials for the conversion of nitrate to nitrogen by a paired electrolysis process using an undivided flow-through electrolyzer. Firstly, corrosion measurements revealed that Ni and Cu(70)Ni(30) electrodes have a much better corrosion resistance than Cu and Cu(90)Ni(10) in the presence of chloride, nitrate and ammonia. Secondly, nitrate electroreduction experiments showed that the cupro-nickel electrodes are the most efficient for reducing nitrate to ammonia with a selectivity of 100%. Finally, paired electrolysis experiments confirmed the efficiency of Cu(70)Ni(30) and Cu(90)Ni(10) cathodes for the conversion of nitrate to nitrogen. During a typical electrolysis, the concentration of nitrate varied from 620ppm to less than 50ppm NO(3)(-) with an N(2) selectivity of 100% and a mean energy consumption of 20kWh/kg NO(3)(-) (compared to ∼35 and ∼220kWh/kg NO(3)(-) with Cu and Ni cathodes, respectively).

The mechanism of hydrazine electro-oxidation revealed by platinum microelectrodes: role of residual oxides

The electrochemistry of hydrazine at platinum has been re-evaluated by an investigation using microelectrodes. Platinum oxides remaining from preceding oxidative scans results in hydrazine oxidation occurring up to ca. 400 mV more cathodic than at an oxide-free Pt electrode. The observed voltammetry at oxidised or 'activated' platinum electrodes was found to be a function of the immersion time (time since 'activation') and pH. Differences between phosphate, sulphate and acetate-based electrolytes are noted. The anodic hydrazine oxidation features at 'activated' electrodes occurred as a prewave or a prepeak, depending upon the electrolyte and scan rate employed. Although hydrazine is known to react with bulk Pt oxide, the loss of activation with time was found to be independent of hydrazine concentration and was instead a function of pH and supporting electrolyte, therefore the 'activation' corresponds to residual rather than bulk platinum oxide. The condition of platinum was examined by X-ray photoelectron spectroscopy (XPS), which demonstrated an increase in oxygen coverage with cycling and the absence of any strongly adsorbed or poisoning species. The facile oxidation of hydrazine has implications with regards to hydrogen storage, generation and fuel cells. The different effects corresponding to insufficient buffering, which has relevance to the electroanalytical detection of hydrazine, was also investigated.