Ab Initio Chemical Kinetics for Oxidation of CH3OH by N2O4: Elucidation of the Mechanism for Major Product Formation and Its Relevancy to Tropospheric Chemistry

Next to CH4, CH3OH is the most abundant C1 organics in the troposphere. The redox reaction of CH3OH with N2O4 had been shown experimentally to produce CH3ONO, instead of CH3ONO2. The mechanism for the reaction remains unknown to date. We have investigated the reaction by ab initio MO calculations at the UCCSD(T)/6-311+G(3df,2p)//UB3LYP/6-311+G(3df,2p) level. The result indicates that the reaction takes place primarily by the isomerization of N2O4 to ONONO2 through a very loose transition state within the N2O4-CH3OH collision complex with a 14.3 kcal/mol barrier, followed by the rapid attack of ONONO2 at CH3OH producing CH3ONO and HNO3. The predicted mechanism for the redox reaction compares closely with the hydrolysis of N2O4. The computed rate constant, k1 = 1.43 × 10-8 T1.96 exp (-9092/T) (200-2000 K) cm3molecule-1s-1, for the formation of CH3ONO and HNO3 agrees reasonably with available low-temperature kinetic data and is found to be similar to that of the isoelectronic N2O4 + CH3NH2 reaction. We have also estimated the kinetics for the termolecular reaction, 2 NO2 + CH3OH, and compared it with the direct bimolecular process; the latter was found to be 4.4 × 105 times faster under the troposphere condition. On the basis of the known pollution levels of NO2, N2O4, and CH3OH, both processes were estimated to be of negligible importance to tropospheric chemistry, however.

Water-Catalyzed Formation of Reactive Oxygen Species from NO2 on a Weakly Hydrated Calcite Surface

The interfaces of weakly hydrated mineral substrates have been shown to serve as catalytic sites for chemical reactions that may not be accessible in the gas phase or under bulk conditions. Currently known mechanisms for the formation of reactive oxygen species (ROS) from nitrogen dioxide (NO2) involve NO2 dimerization. Here, we report the formation of the ROS HONO via a mechanism involving simple adsorption of a single NO2 molecule on a weakly hydrated calcite substrate. First-principles molecular dynamics simulations coupled with enhanced sampling techniques show how an adsorbed water sublayer can enhance NO2 adsorption on calcite compared to adsorption on a bare dry substrate. On the weakly hydrated calcite surface, an interfacial electric field facilitates proton extraction from water, thus allowing HONO formation from a single adsorbed NO2, i.e., without the need for the formation of a NO2 dimer precomplex. HONO formation on calcite is kinetically more favorable than that in the gas phase, with a reaction barrier of 14 kcal/mol on the weakly hydrated calcite surface compared to 27 kcal/mol in the gas phase. Further photocatalysed HONO production by visible light and HONO dissociation are hampered on calcite, unlike the process on silica. NO2 is a significant anthropogenic pollutant, and understanding its chemistry is crucial for explaining the high ROS levels and haze formation in polluted areas or prebiotic ROS generation. These findings emphasize how mineral substrates under water-restricted hydration conditions can trigger chemical pathways that are unexpected in the gas phase or under bulk conditions.

Uptake and reactivity of NO2 on the hydroxylated silica surface: A source of reactive oxygen species

We report state-of-the-art first-principles molecular dynamics results on the heterogeneous chemical uptake of NO2, a major anthropogenic pollutant, on the dry and wet hydroxylated surface of α-quartz, which is a significant component of silica-based catalysts and atmospheric dust aerosols. Our investigation spotlights an unexpected chemical pathway by which NO2 (i) can be adsorbed as HONO by deprotonation of interfacial silanols (i.e., -Si-OH group) on silica, (ii) can be barrierless converted to nitric acid, and (iii) can finally dissociated to surface bounded NO and hydroxyl gas phase radicals. This chemical pathway does not invoke any previously experimentally postulated NO2 dimerization, dimerization that is less likely to occur at low NO2 concentrations. Moreover, water significantly catalyzes the HONO formation and the dissociation of nitric acid into surface-bounded NO and OH radicals, while visible light adsorption can further promote these chemical transformations. This work highlights how water-restricted solvation regimes on common mineral substrates are likely to be a source of reactive oxygen species, and it offers a theoretical framework for further and desirable experimental efforts, aiming to better constrain trace gases/mineral interactions at different relative humidity conditions.

A WO3-TiO2 nanorod/CaCO3 photocatalyst with degradation-regeneration double sites for NO2-inhibited and durable photocatalytic NO

As a promising cleaner technology for nitric oxide degradation, photocatalysis has attracted extensive attention, while the main limitations of photocatalytic nitric oxide are that the toxic NO₂ is produced easily and the photocatalytic durability was inferior due to the accumulation of photocatalytic products. In this paper, a WO₃-TiO₂ nanorod/CaCO₃ (TCC) insulating heterojunction photocatalyst with degradation-regeneration double sites was prepared by simple grinding and calcining. The effects of CaCO₃ loading on the morphology, microstructure and composition of TCC photocatalyst were investigated by SEM, TEM, XRD, FT-IR and XPS etc. Also, TCC exhibits NO₂-inhibited and durable characteristics for NO degradation. DFT calculation, the detection of active radicals by EPR, capture test and the NO degradation pathway characterized by in-situ FT-IR spectra showed that the electron-rich region formed and the existence of regeneration sites are the main reasons for promoting the NO₂-inhibited and durable NO degradation. Furthermore, the mechanism of NO₂-inhibited and durable NO degradation by TCC was revealed. Finally, TCC superamphiphobic photocatalytic coating was prepared, which still exhibits similar NO₂-inhibited and durable characteristics for NO degradation to TCC photocatalyst. It may provide new application value and development prospects in the field of photocatalytic NO.

NOx Photooxidation over Different Noble Metals Modified TiO2

We compared the activity enhancement effect of noble metal deposited on TiO2 in photocatalytic nitrogen oxides oxidation. Titanium dioxide was decorated with Ag, Au, Pt or Pd in the sol-gel process. Synthesized catalysts were characterized by X-ray diffraction (XRD), Brunauer–Emmett–Teller measurement (BET), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and energy dispersive X-ray analysis (EDX). All catalysts together with pure TiO2 obtained by sol-gel (SG) technique were tested for their photocatalytic activity towards nitrogen oxide oxidation (high concentrations of 50, 150 and 250 ppm). FTIR spectrometry was used to determine the gas phase composition and identify TiO2 surface species. The Ag0.1 sample turned out to be deactivated within 60 min of UV/Vis irradiation. Photocatalytic oxidation rate towards NO2 turned to be the highest over SG (photocatalyst without metal deposition). NO2 formation was also observed for Au0.1, Au0.5, Pt0.1, Pt0.5 and Pd0.1. The best NOx removal, i.e., conversion to final product HNO3 was obtained with the Au0.5 photocatalyst.

Metal-free catalysis for the reaction of nitrogen dioxide dimer with phenol: An unexpected favorable source of nitrate and aerosol precursors in vehicle exhaust

Atmospheric reaction mechanism and dynamics of phenol with nitrogen dioxide dimer were explored by the density functional theory and high-level quantum chemistry combined with statistical kinetic calculations within 220-800 K. The nitric acid and phenyl nitrite, the typical aerosol precursors, are the preponderant products because of the low formation free energy barrier (∼8.7 kcal/mol) and fast rate constants (∼10^-15 cm³ molecule^-1 s^-1 at 298 K). Phenyl nitrate is the minor product and it would be also formed from the transformation of phenyl nitrite in NO₂-rich environment. More importantly, kinetic effects and catalytic mechanism of a series of metal-free catalysts (H₂O, NH₃, CH₃NH₂, CH₃NHCH₃, HCOOH, and CH₃COOH) on the title reaction were investigated at the same level. The results indicate that CH₃NH₂ and CH₃NHCH₃ can not only catalyze the title reaction by lowering the free energy barrier (about 1.4-6.5 kcal/mol) but also facilitate the production of organic ammonium nitrate via acting as a donor-acceptor of hydrogen. Conversely, the other species are non-catalytic upon the title reaction. The stabilization energies and donor-acceptor interactions in alkali-catalyzed product complexes were explored, which can provide new insights to the properties of aerosol precursors. Moreover, the lifetime of phenol determined by nitrogen dioxide dimer in the presence of dimethylamine may compete with that of determined by OH radicals, indicating that nitrogen dioxide dimer is responsible for the elimination of phenol in the polluted atmosphere. This work could help us thoroughly understand the removal of nitrogen oxides and phenol as well as new aerosol precursor aggregation in vehicle exhaust.

Metal-free Transformations of Nitrogen-Oxyanions to Ammonia via Oxoammonium Salt

Transformations of nitrogen-oxyanions (NO_x ^- ) to ammonia impart pivotal roles in sustainable biogeochemical processes. While metal-mediated reductions of NO_x ^- are relatively well known, this report illustrates proton-assisted transformations of NO_x ^- anions in the presence of electron-rich aromatics such as 1,3,5-trimethoxybenzene (TMB-H, 1 a) leading to the formation of diaryl oxoammonium salt [(TMB)₂ N⁺ =O][NO₃ ^- ] (2 a) via the intermediacy of nitrosonium cation (NO⁺ ). Detailed characterizations including UV/Vis, multinuclear NMR, FT-IR, HRMS, X-ray analyses on a set of closely related metastable diaryl oxoammonium [Ar₂ N⁺ =O] species disclose unambiguous structural and spectroscopic signatures. Oxoammonium salt 2 a exhibits 2 e^- oxidative reactivity in the presence of oxidizable substrates such as benzylamine, thiol, and ferrocene. Intriguingly, reaction of 2 a with water affords ammonia. Perhaps of broader significance, this work reveals a new metal-free route germane to the conversion of NO_x  to NH₃ .

A Computational Study on the Redox Reactions of Ammonia and Methylamine with Nitrogen Tetroxide

The redox reactions of NH₃ and CH₃NH₂ with N₂O₄ (NTO) have been studied by ab initio molecular orbital (MO) calculations at the UCCSD(T)∥UB3LYP/6-311+G(3df,2p) level of theory. These reactions are related to the well-known NTO-hydrazine(s) propellant systems. On the basis of the predicted potential energy surfaces, the mechanisms for these reactions were found to be similar to the hydrolysis of NTO and the hypergolic initiation reaction of the NTO-N₂H₄ mixture, primarily controlled by the conversion of NTO to ONONO₂ via very loose transition states (with NH₃ and CH₃NH₂ as spectators in the collision complexes) followed by the rapid attack of ONONO₂ at the spectating molecules producing HNO₃ and RNO (R = NH₂ and CH₃NH). The predicted mechanism for the NH₃ reaction compares closely with its isoelectronic process NTO + H₂O; similarly, the mechanism for the NTO + CH₃NH₂ reaction also compares closely with its isoelectronic NTO + NH₂NH₂ reaction. The kinetics for the formation of the final products, HNO₃ + RNO (R = NH₂, OH, CH₃NH, and N₂H₃), were found to be weakly pressure-dependent at low temperatures and affected by the strengths of H-NH₂ and H-OH but not in the RNH₂ case. We have also compared the predicted rate constant for the oxidation of NH₃ by N₂O₄ with that for the analogous NH₃ + N₂O₅ recently reported by Sarkar and Bandyopadhyay [J. Phys. Chem. A. 2020, 124, 3564-3572] under troposphere conditions. The rate of the latter reaction was estimated to be 2 orders of magnitude slower than that of the N₂O₄ reaction under troposphere conditions.

The use of halloysite functionalized with isothiouronium salts as an organic/inorganic hybrid adsorbent for uranium(VI) ions removal

Elimination of U(VI) from nuclear wastes and from the underground water near the uranium mines is the serious problem. Therefore search for new sorbents for U(VI) is still a big challenge for the scientists. This paper investigates of U(VI) ions sorption on halloysite modified with the isothiouronium salts: S-dodecaneisothiouronium bromide (ligand 1), S,S'-dodecane-1,12-diylbis(isothiouronium bromide) (ligand 2), S-hexadecaneisothiouronium chloride (ligand 3), S,S'-naphthalene-1,4-diylbis(methylisothiouronium) dichloride (ligand 4), and S,S'-2,5-dimethylbenzene-1,4-diylbis(methylisothiouronium) dichloride (ligand 5). It was established that halloysite modified by the ligands with four nitrogen atoms in their structure (ligand-5, 2 and 4) was characterized by higher sorption capacity compared with that modified by the ligands with two donor nitrogens (ligand-1 and 3). The maximum sorption capacity of halloysite-5 toward U(VI) was 157 mg U/g and this places the modified mineral among the most effective sorbents for U(VI) removal from wastes. As follows from ATR, XPS and thermal degradation spectra of the sorption products [R-S-C(NH)(NH₂)]_n = 1-2(UO₂²⁺) complexes are formed on the external surface of the halloysite whereas oligomeric hydroxy complexes (UO₂)₃(OH)₅⁺ and (UO₂)₄(OH)₇⁺ are present in the interior of halloysite structure and interact predominantly with aluminols.

The Role of Iron-Bearing Minerals in NO2 to HONO Conversion on Soil Surfaces

Nitrous acid (HONO) accumulates in the nocturnal boundary layer where it is an important source of daytime hydroxyl radicals. Although there is clear evidence for the involvement of heterogeneous reactions of NO2 on surfaces as a source of HONO, mechanisms remain poorly understood. We used coated-wall flow tube measurements of NO2 reactivity on environmentally relevant surfaces (Fe (hydr)oxides, clay minerals, and soil from Arizona and the Saharan Desert) and detailed mineralogical characterization of substrates to show that reduction of NO2 by Fe-bearing minerals in soil can be a more important source of HONO than the putative NO2 hydrolysis mechanism. The magnitude of NO2-to-HONO conversion depends on the amount of Fe(2+) present in substrates and soil surface acidity. Studies examining the dependence of HONO flux on substrate pH revealed that HONO is formed at soil pH < 5 from the reaction between NO2 and Fe(2+)(aq) present in thin films of water coating the surface, whereas in the range of pH 5-8 HONO stems from reaction of NO2 with structural iron or surface complexed Fe(2+) followed by protonation of nitrite via surface Fe-OH2(+) groups. Reduction of NO2 on ubiquitous Fe-bearing minerals in soil may explain HONO accumulation in the nocturnal boundary layer and the enhanced [HONO]/[NO2] ratios observed during dust storms in urban areas.

Investigations on HONO formation from photolysis of adsorbed HNO3 on quartz glass surfaces

HONO formation by photolysis of HNO₃ on clean surfaces is no significant source of HONO and NO_x in the atmosphere.

Release of nitrous acid and nitrogen dioxide from nitrate photolysis in acidic aqueous solutions

Nitrate (NO3(-)) is an abundant component of aerosols, boundary layer surface films, and surface water. Photolysis of NO3(-) leads to NO2 and HONO, both of which play important roles in tropospheric ozone and OH production. Field and laboratory studies suggest that NO3¯ photochemistry is a more important source of HONO than once thought, although a mechanistic understanding of the variables controlling this process is lacking. We present results of cavity-enhanced absorption spectroscopy measurements of NO2 and HONO emitted during photodegradation of aqueous NO3(-) under acidic conditions. Nitrous acid is formed in higher quantities at pH 2-4 than expected based on consideration of primary photochemical channels alone. Both experimental and modeled results indicate that the additional HONO is not due to enhanced NO3(-) absorption cross sections or effective quantum yields, but rather to secondary reactions of NO2 in solution. We find that NO2 is more efficiently hydrolyzed in solution when it is generated in situ during NO3(-) photolysis than for the heterogeneous system where mass transfer of gaseous NO2 into bulk solution is prohibitively slow. The presence of nonchromophoric OH scavengers that are naturally present in the environment increases HONO production 4-fold, and therefore play an important role in enhancing daytime HONO formation from NO3(-) photochemistry.

Ground-State Intermolecular Proton Transfer of N2O4 and H2O: An Important Source of Atmospheric Hydroxyl Radical?

To evaluate the significance of the generation of atmospheric hydroxyl radical from reaction of N2O4 with H2O, CASPT2//CASSCF as well as CASPT2//CASSCF/Amber QM/MM approaches were employed to map the minimum-energy profiles of sequential reactions, NO2 dimerization and ground-state intermolecular proton transfer of trans-ONONO2 as well as the photolysis of HONO. A highly efficient ground-state intermolecular proton transfer of trans-ONONO2 is found to dominate the generation of hydroxyl radical under atmospheric conditions. Although proton transfer occurs with high efficiency, the precursor reaction of dimerization producing trans-ONONO2 has to overcome a 17.1 kcal/mol barrier and cannot compete with the barrierless channel of symmetric O2N-NO2 formation from isolated NO2 monomers. Our computations reveal that the photolysis of HONO without a barrier definitely makes significant contributions to the concentration of the atmospheric hydroxyl radical, but its importance is influenced by the lack of trans-ONONO2 isomer in the atmospheric environment.