We have investigated the reaction mechanism of the electrochemical reduction of carbon dioxide to hydrocarbons on copper electrodes. This reaction occurs via two pathways: a C1 pathway leading to methane, and a C2 pathway leading to ethylene. To identify possible intermediates in the reduction of carbon dioxide we have studied the reduction of small C1 and C2 organic molecules containing oxygen. We followed the formation and consumption of intermediates during the reaction as a function of potential, using online mass spectrometry. For the C1 pathway we show that it is very likely that CHOads is the key intermediate towards the breaking of the C–O bond and, therefore, the formation of methane. For the C2 pathway we suggest that the first step is the formation of a CO dimer, followed by the formation of a surface-bonded enediol or enediolate, or the formation of an oxametallacycle. Both the enediol(ate) and the oxametallacycle would explain the selectivity of the C2 pathway towards ethylene. This new mechanism is significantly different from existing mechanisms but it is the most consistent with the available experimental data.

A new mechanism for the selectivity to C1 and C2 species in the electrochemical reduction of carbon dioxide on copper electrodes

. This study presents a modeling framework based on laboratory data to describe the kinetics of glyoxal reactions that form secondary organic aerosol (SOA) in aqueous aerosol particles. Recent laboratory results on glyoxal reactions are reviewed and a consistent set of empirical reaction rate constants is derived that captures the kinetics of glyoxal hydration and subsequent reversible and irreversible reactions in aqueous inorganic and water-soluble organic aerosol seeds. Products of these processes include (a) oligomers, (b) nitrogen-containing products, (c) photochemical oxidation products with high molecular weight. These additional aqueous phase processes enhance the SOA formation rate in particles and yield two to three orders of magnitude more SOA than predicted based on reaction schemes for dilute aqueous phase (cloud) chemistry for the same conditions (liquid water content, particle size). The application of the new module including detailed chemical processes in a box model demonstrates that both the time scale to reach aqueous phase equilibria and the choice of rate constants of irreversible reactions have a pronounced effect on the predicted atmospheric relevance of SOA formation from glyoxal. During day time, a photochemical (most likely radical-initiated) process is the major SOA formation pathway forming ∼5 μg m−3 SOA over 12 h (assuming a constant glyoxal mixing ratio of 300 ppt). During night time, reactions of nitrogen-containing compounds (ammonium, amines, amino acids) contribute most to the predicted SOA mass; however, the absolute predicted SOA masses are reduced by an order of magnitude as compared to day time production. The contribution of the ammonium reaction significantly increases in moderately acidic or neutral particles (5 Glyoxal uptake into ammonium sulfate seed under dark conditions can be represented with a single reaction parameter keffupt that does not depend on aerosol loading or water content, which indicates a possibly catalytic role of aerosol water in SOA formation. However, the reversible nature of uptake under dark conditions is not captured by keffupt, and can be parameterized by an effective Henry's law constant including an equilibrium constant Kolig = 1000 (in ammonium sulfate solution). Such reversible glyoxal oligomerization contributes Sensitivity tests reveal five parameters that strongly affect the predicted SOA mass from glyoxal: (1) time scales to reach equilibrium states (as opposed to assuming instantaneous equilibrium), (2) particle pH, (3) chemical composition of the bulk aerosol, (4) particle surface composition, and (5) particle liquid water content that is mostly determined by the amount and hygroscopicity of aerosol mass and to a lesser extent by the ambient relative humidity. Glyoxal serves as an example molecule, and the conclusions about SOA formation in aqueous particles can serve for comparative studies of other molecules that form SOA as the result of multiphase chemical processing in aerosol water. This SOA source is currently underrepresented in atmospheric models; if included it is likely to bring SOA predictions (mass and O/C ratio) into better agreement with field observations.

/pdf/glyoxal-processing-by-aerosol-multiphase-chemistry-towards-a-2199spoetk.pdf

Glyoxal processing by aerosol multiphase chemistry: towards a kinetic modeling framework of secondary organic aerosol formation in aqueous particles

Electrochemical reduction of halogenated organic compounds is gaining increasing attention as a strategy for the remediation of environmental pollutants. We begin this review by discussing key components (cells, electrodes, solvents, and electrolytes) in the design of a procedure for degrading a targeted pollutant, and we describe and contrast some experimental techniques used to explore and characterize the electrochemical behavior of that pollutant. Then, we describe how to probe various mechanistic features of the pertinent electrochemistry (including stepwise versus concerted carbon–halogen bond cleavage, identification of reaction intermediates, and elucidation of mechanisms). Knowing this information is vital to the successful development of a remediation procedure. Next, we outline techniques, instrumentation, and cell designs involved in scaling up a benchtop experiment to an industrial-scale system. Finally, the last and major part of this review is directed toward surveying electrochemical studi...

Electroreductive Remediation of Halogenated Environmental Pollutants.

Molecularly imprinted polymers (MIP) have been elucidated to work as artificial receptors. In our present study, a MIP was applied as a molecular recognition element to a chemical sensor. We have constructed an atrazine sensor based on a MIP layer selective for atrazine and its electrochemical reduction on gold electrode. The atrazine sensor was fabricated by directly polymerizing the atrazine-imprinted polymer composed from methacrylic acid and ethylene glycol dimethacrylate onto the surface of a gold electrode. By introducing LiCl into the MIP, atrazine was reduced below −800 mV vs Ag/AgCl reference electrode, at pH 3. The cathodic current of atrazine depended on the concentration of atrazine at the range of 1−10 μM. The sensor exhibited a selective response to atrazine. A nonimprinted polymer-modified electrode did not show selective response to atrazine, thus implying that the imprinted polymer acts as recognition element of atrazine sensor.

Atrazine sensor based on molecularly imprinted polymer-modified gold electrode.

Recent developments in the use of polarography and voltammetry at mercury electrodes in environmental analysis are reviewed and their combination with preliminary separation and pre-concentration using liquid or solid phase extraction is discussed. Attention is focused on ecotoxic nitrated polycyclic aromatic hydrocarbons, heterocyclic compounds, and pesticides. Advantages and limitations of mercury electrodes are critically evaluated and some recent applications of these techniques developed in our laboratory are given.

Mercury Electrodes–Possibilities and Limitations in Environmental Electroanalysis

EC(EE) process in the reduction of the herbicide clopyralid on mercury electrodes

A contribution to the study of the electroreduction of 2-chloro-4,6-di(ethylamino)-1,3,5-triazine (simazine) on mercury electrodes

On the electroreduction of 4-chloro-2,6-diisopropylamino-s-triazine (propazine) on mercury electrodes

CEC mechanisms in the electroreduction of α-dicarbonyl compounds on mercury electrodes

http://jes.ecsdl.org/content/160/4/H213.full.pdf

M. Ruiz Montoya

Papers

EC(EE) process in the reduction of the herbicide clopyralid on mercury electrodes

A contribution to the study of the electroreduction of 2-chloro-4,6-di(ethylamino)-1,3,5-triazine (simazine) on mercury electrodes

On the electroreduction of 4-chloro-2,6-diisopropylamino-s-triazine (propazine) on mercury electrodes

CEC mechanisms in the electroreduction of α-dicarbonyl compounds on mercury electrodes

Analysis of the Interaction of Radical Scavengers with ROS Electrogenerated from Hydrogen Peroxide