Atmospheric degradation of volatile organic compounds

Over the last two decades, our understanding of soot formation has evolved from an empirical, phenomenological description to an age of quantitative modeling for at least small fuel compounds. In this paper, we review the current state of knowledge of the fundamental sooting processes, including the chemistry of soot precursors, particle nucleation and mass/size growth. The discussion shows that though much progress has been made, critical gaps remain in many areas of our knowledge. We propose the roles of certain aromatic radicals resulting from localized π electron structures in particle nucleation and subsequent mass growth. The existence of these free radicals provides a rational explanation for the strong binding forces needed for forming initial clusters of polycyclic aromatic hydrocarbons. They may also explain a range of currently unexplained sooting phenomena, including the large amount of aliphatics observed in nascent soot formed in laminar premixed flames and the mass growth of soot in the absence of gas-phase H atoms. While the above suggestions are inspired, to an extent, by recent theoretical findings from the materials research community, this paper also demonstrates that the knowledge garnered through our longstanding interest in soot formation may well be carried over to flame synthesis of functional nanomaterials for clean and renewable energy applications. In particular, work on flame-synthesized thin films of nanocrystalline titania illustrates how our combustion knowledge might be useful for developing advanced yet inexpensive thin-film solar cells and chemical sensors for detecting gaseous air pollutants.

Formation of nascent soot and other condensed-phase materials in flames

Nanodiamonds exhibit great potential as green catalysts for remediation of organic contaminants. However, the specific active site and corresponding oxidative mechanism are unclear, which retard further developments of high-performance catalysts. Here, an annealing strategy is developed to accurately regulate the content of ketonic carbonyl groups on nanodiamonds; meanwhile other structural characteristics of nanodiamonds remain almost unchanged. The well-defined nanodiamonds with well-controlled ketonic carbonyl groups exhibit excellent catalytic activity in activation of peroxymonosulfate for oxidation of organic pollutants. Based on the semi-quantitative and quantitative correlations of ketonic carbonyl groups and the reaction rate constants, it is conclusively determined that ketonic carbonyl groups are the catalytically active sites. Different from conventional oxidative systems, reactive oxygen species in nanodiamonds@peroxymonosulfate system are revealed to be singlet oxygen with high selectivity, which can effectively oxidize and mineralize the target contaminants. Impressively, the singlet-oxygen-mediated oxidation system significantly outperforms the classical radicals-based oxidation system in remediation of actual wastewater. This work not only provides a valuable insight for the design of new nanocarbon catalysts with abundant active sites but also establishes a very promising catalytic oxidation system for the green remediation of actual contaminated water.

Identification and Regulation of Active Sites on Nanodiamonds: Establishing a Highly Efficient Catalytic System for Oxidation of Organic Contaminants

Primary intermediates of oxygen photoevolution (water photooxidation) reaction at the TiO2 (rutile)/aqueous solution interface were investigated by in situ multiple internal reflection infrared (MIRIR) absorption and photoluminescence (PL) measurements. UV irradiation of TiO2 in the presence of 10 mM Fe3+ in the solution caused the appearance of a new peak at 838 cm-1 and a shoulder at 812 cm-1. Detailed investigations of the effects of solution pH, the presence of methanol as a hole scavenger, and isotope exchange in water (H216O→H218O) on the spectra have shown that the 838- and 812-cm-1 bands can be assigned to the O−O stretching mode of surface TiOOH and TiOOTi, respectively, produced as primary intermediates of the oxygen photoevolution reaction. The results give strong support to our previously proposed mechanism that the oxygen photoevolution is initiated by a nucleophilic attack of a H2O molecule on a photogenerated hole at a surface lattice O site, not by oxidation of surface OH group by the hole...

Primary intermediates of oxygen photoevolution reaction on TiO2 (Rutile) particles, revealed by in situ FTIR absorption and photoluminescence measurements.

The tropospheric chemistry of sea salt: a molecular-level view of the chemistry of NaCl and NaBr.

Determination of phosphate/arsenate by a modified molybdenum blue method and reduction of arsenate by S2O42−

The pulse radiolysis of aqueous NO has been reinvestigated, the variances with the prior studies are discussed, and a mechanistic revision is suggested. Both the hydrated electron and the hydrogen atom reduce NO to yield the ground-state triplet (3)NO(-) and singlet (1)HNO, respectively, which further react with NO to produce the N(2)O(2)(-) radical, albeit with the very different specific rates, k((3)NO(-) + NO) = (3.0 +/- 0.8) x 10(9) and k((1)HNO + NO) = (5.8 +/- 0.2) x 10(6) M(-)(1) s(-)(1). These reactions occur much more rapidly than the spin-forbidden acid-base equilibration of (3)NO(-) and (1)HNO under all experimentally accessible conditions. As a result, (3)NO(-) and (1)HNO give rise to two reaction pathways that are well separated in time but lead to the same intermediates and products. The N(2)O(2)(-) radical extremely rapidly acquires another NO, k(N(2)O(2)(-) + NO) = (5.4 +/- 1.4) x 10(9) M(-)(1) s(-)(1), producing the closed-shell N(3)O(3)(-) anion, which unimolecularly decays to the final N(2)O + NO(2)(-) products with a rate constant of approximately 300 s(-)(1). Contrary to the previous belief, N(2)O(2)(-) is stable with respect to NO elimination, and so is N(3)O(3)(-). The optical spectra of all intermediates have also been reevaluated. The only intermediate whose spectrum can be cleanly observed in the pulse radiolysis experiments is the N(3)O(3)(-) anion (lambda(max) = 380 nm, epsilon(max) = 3.76 x 10(3) M(-)(1) cm(-)(1)). The spectra previously assigned to the NO(-) anion and to the N(2)O(2)(-) radical are due, in fact, to a mixture of species (mainly N(2)O(2)(-) and N(3)O(3)(-)) and to the N(3)O(3)(-) anion, respectively. Spectral and kinetic evidence suggests that the same reactions occur when (3)NO(-) and (1)HNO are generated by photolysis of the monoprotonated anion of Angeli's salt, HN(2)O(3)(-), in NO-containing solutions.

One-Electron Reduction of Aqueous Nitric Oxide: A Mechanistic Revision

Motor vehicle exhaust emissions are known to contain hydrogen cyanide (HCN), but emission rate data are scarce and, in the case of idling vehicles, date back over 20 years. For the first time, vehicular HCN exhaust emissions from a modern, in-use fleet at idle have been measured. The 14 tested light duty motor vehicles were operating at idle as these conditions are associated with the highest risk exposure scenarios (i.e., enclosed spaces). Vehicular HCN was detected in 89% of the sampled exhaust streams and did not correlate with instantaneous air-fuel-ratio or with any single, coemitted pollutant. However, a moderate correlation between HCN emissions and the product of carbon monoxide and nitric oxide emissions was observed under cold-start conditions. Fleet average, cold-start, undiluted HCN emissions were 105 ± 97 ppbV (maximum: 278 ppbV), whereas corresponding emissions from vehicles operating under stabilized conditions were 79 ± 71 ppbV (maximum: 245 ppbV); mean idle fleet HCN emission rates were 39 ± 35 and 21 ± 18 g-min-1 for cold-start and stabilized vehicles, respectively. The significance of these results is discussed in terms of HCN emissions inventories in the South Coast Air Basin of California and of health risks due to exposure to vehicular HCN. (A)

Hydrogen cyanide exhaust emissions from in-use motor vehicles.

All major properties of the aqueous hyponitrite radicals (ONNO- and ONNOH), the adducts of nitric oxide (NO) and nitroxyl (3NO- and 1HNO), are revised. In this work, the radicals are produced by oxidation of various hyponitrite species in the 2−14 pH range with the OH, N3, or SO4- radicals. The estimated rate constants with OH are 4 × 107, 4.2 × 109, and 8.8 × 109 M-1 s-1 for oxidations of HONNOH, HONNO-, and ONNO2-, respectively. The rate constants for N3 + ONNO2- and SO4- + HONNO- are 1.1 × 109 and 6.4 × 108 M-1 s-1, respectively. The ONNO- radical exhibits a strong characteristic absorption spectrum with maxima at 280 and 420 nm (e280 = 7.6 × 103 and e420 = 1.2 × 103 M-1 cm-1). This spectrum differs drastically from those reported, suggesting the radical misassignment in prior work. The ONNOH radical is weakly acidic; its pKa of 5.5 is obtained from the spectral changes with pH. Both ONNO- and ONNOH are shown to be over 3 orders of magnitude more stable with respect to elimination of NO than it has bee...

Hyponitrite Radical, a Stable Adduct of Nitric Oxide and Nitroxyl

The acid dissociation constant of the hydroxyl radical in aqueous solution was determined by pulse radiolysis from the [OH-] dependence of the rate constant for reaction of the •OH/O•- radical with benzoate ions. The rate constant and the pKa(•OH) values were determined over the temperature range 284−343 K. At 298 K, pKa 298(•OH) = 11.54 ± 0.04 and ΔionG°298(•OH) = (65.9 ± 0.3) kJ mol-1. From the temperature dependence of Ka(•OH), we calculate ΔionH°298(•OH) = (24.85 ± 0.5) kJ mol-1 and ΔionS°298(•OH) = (−139 ± 2) J mol-1 K-1. The activation energy for the reaction of •OH with C6H5CO2- was found to be (8.8 ± 0.6) kJ mol-1 and the Arrhenius preexponential factor (2.4 ± 0.6) × 1011 L mol-1 s-1.

G.A. Poskrebyshev

Papers

Determination of phosphate/arsenate by a modified molybdenum blue method and reduction of arsenate by S2O42−

One-Electron Reduction of Aqueous Nitric Oxide: A Mechanistic Revision

Hydrogen cyanide exhaust emissions from in-use motor vehicles.

Hyponitrite Radical, a Stable Adduct of Nitric Oxide and Nitroxyl

Temperature Dependence of the Acid Dissociation Constant of the Hydroxyl Radical