Disproportionation Of NO Research Articles

Electrocatalytic Disproportionation of Nitric Oxide Toward Efficient Nitrogen Fixation

AbstractElectrocatalytic conversion of waste nitric oxide into ammonia is a promising approach to achieve sustainable nitrogen fixation. Herein, a CoNi co‐oxides catalyst is designed for NH3electrosynthesis with the merits of facilitating NO adsorption and reducing the reaction energy barrier. By synergistic coupling with anodic NO oxidation, electrocatalytic disproportionation of NO is first realized to simultaneously synthesize value‐added double nitrogen products (NH3and nitrate) with increased total energy efficiency. Furthermore, decoupled acid–base asymmetric electrolyte design is proposed in a united assembled electrolyzer, enabling a high NH3production rate (26.27 mg h−1cm−2) with unit faradaic efficiency and a remarkable nitrate production rate of 68.41 mg h−1cm−2at the anode. A low cell voltage of 3.58 V is obtained by optimizing ion agglomeration within the membrane to promote the charge‐ion exchange and electrode kinetics. Technoeconomic analysis demonstrates the economic feasibility of recycling waste NO by the electrocatalytic disproportionation strategy.

IR-spectroscopic study of complex formation of nitrogen oxides (NO, N2O) with strong Lewis acid sites and the reactivity of adsorbed species in CO and CH4 oxidation

The formation of complexes and transformations of nitrogen oxides (NO, N2O) on strong Lewis acid sites of HZSM-5, H[Ga]ZSM-5, ZnO/HZSM-5 zeolites and dealuminated mordenites was investigated by diffuse-reflectance IR spectroscopy. Adsorbed N2O that is formed by disproportionation of NO is capable of oxidizing CO and CH4 molecules to CO2. The behaviour of strong Lewis acid sites in zeolites and cationic forms of zeolites in the NO disproportionation and CO or CH4 oxidation was comparatively studied.

Selective catalytic reduction of NOx with NH3 over Ce-Mn oxide and Cu-SSZ-13 composite catalysts – Low temperature enhancement

A composite catalyst comprised of ceria-manganese oxide and Cu-SSZ-13 components represents a potential solution for improving selective catalytic reduction (SCR) of NOx by ammonia. Here, clarity is provided to the pathway of promotion on these catalysts at low temperatures (< 200 °C), and it is shown that improved performance comes solely from interaction of the oxide phase with Brønsted acid bound NH3. We demonstrate that at low temperatures, the catalytic components interact via oxide-derived nitrites/nitrite-precursors (e.g., HONO, N2O3) reacting with Brønsted acid bound NH3 from the zeolite. Then, only at elevated temperatures is the influence of the oxide phase on Cu redox in the Cu-SSZ-13 observed. Furthermore, we show how the oxide component has the great potential to circumvent NH4NO3 inhibition on Cu-SSZ-13 in the composite catalysts at low temperatures by bypassing the formation of nitrates from NO2 disproportionation, thus providing a practical solution to improving low temperature SCR reactivity.

IR-Spectroscopic Study of Complex Formation of Nitrogen Oxides (NO, N2O) with Cationic Forms of Zeolites and the Reactivity of Adsorbed Species in CO and CH4 Oxidation.

The formation of complexes and disproportionation of nitrogen oxides (NO, N2O) on cationic forms of LTA, FAU, and MOR zeolites was investigated by diffuse-reflectance IR spectroscopy. N2O is adsorbed on the samples under study in the molecular form and the frequencies of the first overtone of the stretching vibrations ν10–2 and the combination bands of the stretching vibrations with other vibrational modes for N2O complexes with cationic sites in zeolites (ν30–1 + ν10–1, ν10–1 + δ0–2) are more significantly influenced by the nature of the zeolite. The presence of several IR bands in the region of 2400–2600 cm−1 (the ν10–1 + δ0–2 transitions) for different zeolite types was explained by the availability of different localization sites for cations in these zeolites. The frequencies in this region also depend on the nature of the cation (its charge and radius). The data can be explained by the specific geometry of the N2O complex formed, presumably two-point adsorption of N2O on a cation and a neighboring oxygen atom of the framework. Adsorption of CO or CH4 on the samples with preliminarily adsorbed N2O at 20–180 °C does not result in any oxidation of these molecules. NO+ and N2O3 species formed by disproportionation of NO are capable of oxidizing CO and CH4 molecules to CO2, whereas NOx is reduced simultaneously to N2 or N2O. The peculiarities in the behavior of cationic forms of different zeolites with respect to adsorbed nitrogen oxides determined by different density and localization of cations have been established.

Structure and Reactivity of NO/NO+/NO−Pincer and Porphyrin Complexes

AbstractThis review covers recent progress in the field of nitrosyl pincer and porphyrinate complexes, and how their reactivity depends on the redox state of the NO ligand. Synthetic methods, and the most significant structural information, spectroelectrochemical studies, and different reactivities of nitrosyl pincer complexes are presented. These include selective activation and functionalization of carbon‐halogen bonds, catalysis for a variety of specific organic reactions, mediation of NO disproportionation and reversible coordination of solvents and weakly‐coordinating anions. Regarding the porphyrinate complexes, the focus is on biorelevant nitroxyl (NO−/HNO) complexes, discussing pioneering studies along with preparation and common characterization methods. Their relative stability in different conditions is compared, and possible decomposition mechanisms are discussed in terms of their ability to act as biomimetic models for enzymatic systems.

NO Reduction to N2O Triggered by a Dinuclear Dinitrosyl Iron Complex via the Associated Pathways of Hyponitrite Formation and NO Disproportionation.

In spite of the comprehensive study of the metal-mediated conversion of NO to N2O disclosing the conceivable processes/mechanism in biological and biomimetic studies, in this study, the synthesis cycles and mechanism of NO reduction to N2O triggered by the electronically localized dinuclear {Fe(NO)2}10-{Fe(NO)2}9 dinitrosyl iron complex (DNIC) [Fe(NO)2(μ-bdmap)Fe(NO)2(THF)] (1) (bdmap = 1,3- bis(dimethylamino)-2-propanolate) were investigated in detail. Reductive conversion of NO to N2O triggered by complex 1 in the presence of exogenous ·NO occurs via the simultaneous formation of hyponitrite-bound {[Fe2(NO)4(μ-bdmap)]2(κ4-N2O2)} (2) and [NO2]--bridged [Fe2(NO)4(μ-bdmap)(μ-NO2)] (3) (NO disproportionation yielding N2O and complex 3). EPR/IR spectra, single-crystal X-ray diffraction, and the electrochemical study uncover the reversible redox transformation of {Fe(NO)2}9-{Fe(NO)2}9 [Fe2(NO)4(μ-bdmap)(μ-OC4H8)]+ (7) ↔ {Fe(NO)2}10-{Fe(NO)2}9 1 ↔ {Fe(NO)2}10-{Fe(NO)2}10 [Fe(NO)2(μ-bdmap)Fe(NO)2]- (6) and characterize the formation of complex 1. Also, the synthesis study and DFT computation feature the detailed mechanism of electronically localized {Fe(NO)2}10-{Fe(NO)2}9 DNIC 1 reducing NO to N2O via the associated hyponitrite-formation and NO-disproportionation pathways. Presumably, the THF-bound {Fe(NO)2}9 unit of electronically localized {Fe(NO)2}10-{Fe(NO)2}9 complex 1 served as an electron buffering reservoir for accommodating electron redistribution, and the {Fe(NO)2}10 unit of complex 1 acted as an electron-transfer channel to drive exogeneous ·NO coordination to yield proposed relay intermediate κ2-N,O-[NO]--bridged [Fe2(NO)4(μ-bdmap)(μ-NO)] (A) for NO reduction to N2O.

Mechanism of N2O generation over chromium poisoned γ-Fe2O3 catalyst during selective catalytic reduction of NOx with NH3

Poisoning effect of Cr in flue gas on the remarkable generation of greenhouse gas N2O over γ-Fe2O3 catalyst during selective catalytic reduction of NOx with ammonia was firstly observed in this work. At low temperature, N2O generation over Cr poisoned γ-Fe2O3 catalyst was attributed to non-selective catalytic reduction (NSCR) of NOx. At high temperature, NSCR of NOx and ammonia peroxidation accompanying by weak NO disproportionation induced a more generation of N2O. The seeming promotion effect of Cr on NOx conversion of γ-Fe2O3 catalyst at low temperature was partly resulted from the NSCR of NO which also contributed to NOx conversion. At high temperature, the reduction of NOx to innocent N2 was far inhibited after chromium poisoning in view of the low selectivity of NO to N2. The formed FeCr2O4 in chromium poisoned γ-Fe2O3 catalyst was the induction factor for large N2O production, which enhanced the acidity and reducibility of catalyst. Transient DRIFTS results indicated that SCR and NSCR of NOx over chromium poisoned γ-Fe2O3 catalyst predominantly happened via Eley-Rideal mechanism. Moreover, O2 in flue gas determined the activation of adsorbed NH3 and which dictated the NSCR of NOx and ammonia peroxidation.

Spectroscopic Evidence of Hyponitrite Radical Intermediate in NO Disproportionation at a MOF‐Supported Mononuclear Copper Site

AbstractDianionic hyponitrite (N2O22−) is often proposed, based on model complexes, as the key intermediate in reductive coupling of nitric oxide to nitrous oxide at the bimetallic active sites of heme‐copper oxidases and nitric oxide reductases. In this work, we examine the gas‐solid reaction of nitric oxide with the metal–organic framework CuI‐ZrTpmC* with a suite of in situ spectroscopies and density functional theory simulations, and identify an unusual chelating N2O2.− intermediate. These results highlight the advantage provided by site‐isolation in metal–organic frameworks (MOFs) for studying important reaction intermediates, and provide a mechanistic scenario compatible with the proposed one‐electron couple in these enzymes.

Spectroscopic Evidence of Hyponitrite Radical Intermediate in NO Disproportionation at a MOF-Supported Mononuclear Copper Site.

Dianionic hyponitrite (N2 O22- ) is often proposed, based on model complexes, as the key intermediate in reductive coupling of nitric oxide to nitrous oxide at the bimetallic active sites of heme-copper oxidases and nitric oxide reductases. In this work, we examine the gas-solid reaction of nitric oxide with the metal-organic framework CuI -ZrTpmC* with a suite of in situ spectroscopies and density functional theory simulations, and identify an unusual chelating N2 O2.- intermediate. These results highlight the advantage provided by site-isolation in metal-organic frameworks (MOFs) for studying important reaction intermediates, and provide a mechanistic scenario compatible with the proposed one-electron couple in these enzymes.

Thermal Cycling of a MOF-Based NO Disproportionation Catalyst.

The metal-organic framework CuI-MFU-4l reacts with NO, initially forming a copper(I)-nitrosyl at low pressure, and subsequently generates NO disproportionation products CuII-NO2 and N2O. The thermal stability of MFU-4l allows NOx to be released from the framework at temperatures greater than 200 °C. This treatment regenerates the original CuI-MFU-4l, which can engage in subsequent cycles of NO disproportionation.

Nickel-mediated N-N bond formation and N2O liberation via nitrogen oxyanion reduction.

The syntheses of (DIM)Ni(NO3)2 and (DIM)Ni(NO2)2, where DIM is a 1,4-diazadiene bidentate donor, are reported to enable testing of bis boryl reduced N-heterocycles for their ability to carry out stepwise deoxygenation of coordinated nitrate and nitrite, forming O(Bpin)2. Single deoxygenation of (DIM)Ni(NO2)2 yields the tetrahedral complex (DIM)Ni(NO)(ONO), with a linear nitrosyl and κ1-ONO. Further deoxygenation of (DIM)Ni(NO)(ONO) results in the formation of dimeric [(DIM)Ni(NO)]2, where the dimer is linked through a Ni–Ni bond. The lost reduced nitrogen byproduct is shown to be N2O, indicating N–N bond formation in the course of the reaction. Isotopic labelling studies establish that the N–N bond of N2O is formed in a bimetallic Ni2 intermediate and that the two nitrogen atoms of (DIM)Ni(NO)(ONO) become symmetry equivalent prior to N–N bond formation. The [(DIM)Ni(NO)]2 dimer is susceptible to oxidation by AgX (X = NO3−, NO2−, and OTf−) as well as nitric oxide, the latter of which undergoes nitric oxide disproportionation to yield N2O and (DIM)Ni(NO)(ONO). We show that the first step in the deoxygenation of (DIM)Ni(NO)(ONO) to liberate N2O is outer sphere electron transfer, providing insight into the organic reductants employed for deoxygenation. Lastly, we show that at elevated temperatures, deoxygenation is accompanied by loss of DIM to form either pyrazine or bipyridine bridged polymers, with retention of a BpinO− bridging ligand.

Influence of Water on the Oxidation of NO on Pd/TiO2 Photocatalysts.

Two series of new photocatalysts were synthesized based on modification with Pd of the commercial P25 photocatalyst (EVONIK®). Two techniques were employed to incorporate Pd nanoparticles on the P25 surface: photodeposition (series Pd-P) and impregnation (series Pd-I). Both series were characterized in depth using a variety of instrumental techniques: BET, DRS, XRD, XPS, TEM, FTIR and FESEM. The modified series exhibited a significant change in pore size distribution, but no differences compared to the original P25 with respect to crystalline phase ratio or particle size were observed. The Pd0 oxidation state was predominant in the Pd-P series, while the presence of the Pd2+ oxidation state was additionally observed in the Pd-I series. The photoactivity tests were performed in a continuous photoreactor with the photocatalysts deposited, by dip-coating, on borosilicate glass plates. A total of 500 ppb of NO was used as input flow at a volumetric flow rate of 1.2 L·min−1, and different relative humidities from 0 to 65% were tested. The results obtained show that under UV-vis or Vis radiation, the presence of Pd nanoparticles favors NO removal independently of the Pd incorporation method employed and independently of the tested relative humidity conditions. This improvement seems to be related to the different interaction of the water with the surface of the photocatalysts in the presence or absence of Pd. It was found in the catalyst without Pd that disproportionation of NO2 is favored through its reaction with water, with faster surface saturation. In contrast, in the catalysts with Pd, disproportionation took place through nitro-chelates and adsorbed NO2 formed from the photocatalytic oxidation of the NO. This different mechanism explains the greater efficiency in NOx removal in the catalysts with Pd. Comparing the two series of catalysts with Pd, Pd-P and Pd-I, greater activity of the Pd-P series was observed under both UV-vis and Vis radiation. It was shown that the Pd0 oxidation state is responsible for this greater activity as the Pd-I series improves its activity in successive cycles due to a reduction in Pd2+ species during the photoactivity tests.

A DFT study of healing the N vacancy in h-BN monolayer by NO molecules

During the synthesis of h-BN nano-material, there unavoidably exist some N vacancies that will largely affect its physicochemical properties. Therefore, it is full of necessity to repair these defects before further investigations and applications. In this paper, we propose the use of NO molecules to heal the N vacancy in a h-BN monolayer through the NO disproportionation based on DFT calculation. Our calculations indicate that the repairing processes are energetically favorable and the repaired h-BN monolayer, from both geometric and electronic manners, has good similarity with the perfect counterpart. This theoretical work exhibits a promising approach to heal the N vacancy in h-BN monolayer, and we assume it is also feasible for repairing the N defects in other 2D nano-materials.

NO disproportionation over defective 1T'-MoS2 monolayers.

The MoS2 monolayers are usually created with vacancies (most likely missing S atoms). At an S vacancy, the exposed and under-coordinated Mo atoms become reactive and can strongly bind to small molecules. Here, by using first-principles calculations, it is proved that 1T'-MoS2 monolayers are an efficient catalyst for NO disproportionation. The reaction starts with NO adsorptions at the exposed Mo atoms. Later the incoming NO molecules react with the ones already adsorbed to give NO2 molecules, which readily desorb. The remaining N-doped MoS2 sheets can then easily react with NO molecules to produce N2O, and can be heated to desorb them. Thus, the defective 1T'-MoS2 monolayers are recovered and the catalytic cycle is completed. The NO2 formation step has a relatively high activation barrier of 1.58 eV, but it can be lowered to 0.19 or 0.56 eV by applying biaxial -3% or 3% strain, respectively. The reaction mechanism is totally different from those catalyzed by metal-centered catalysts (complexes, clusters, or metal-organic frameworks), which feature the N2O formation as the rate-limiting step and the NO2 in the metal-nitrite complexes cannot be released. This work paves the way for strain engineering two-dimensional (2D) materials into efficient NO disproportionation catalysts.

The Occurrence of Putative Nitric Oxide Dismutase (Nod) in an Alpine Wetland with a New Dominant Subcluster and the Potential Ability for a Methane Sink.

Recently, a new oxygenic pathway has been proposed based on the disproportionation of NO with putative NO dismutase (Nod). In addition to a new process in nitrogen cycling, this process provides ecological advantages for the degradation of substrates in anaerobic conditions, which is of great significance for wastewater treatment. However, the Nod distribution in aquatic environments is rarely investigated. In this study, we obtained the nod genes with an abundance of 2.38 ± 0.96 × 105 copies per gram of dry soil from the Zoige wetland and aligned the molecular characteristics in the corresponding Nod sequences. These Nod sequences were not only found existing in NC10 bacteria, but were also found forming some other clusters with Nod sequences from a WWTP reactor or contaminated aquifers. Moreover, a new subcluster in the aquifer-similar cluster was even dominant in the Zoige wetland and was named the Z-aquifer subcluster. Additionally, soils from the Zoige wetland showed a high potential rate (10.97 ± 1.42 nmol of CO2 per gram of dry soil per day) for nitrite-dependent anaerobic methane oxidation (N-DAMO) with low abundance of NC10 bacteria, which may suggest a potential activity of Nod in other clusters when considering the dominance of the Z-aquifer subcluster Nod. In conclusion, we verified the occurrence of Nod in an alpine wetland for the first time and found a new subcluster to be dominant in the Zoige wetland. Moreover, this new subcluster of Nod may even be active in the N-DAMO process in this alpine wetland, which needs further study to confirm.

Species formed during NO adsorption and NO + O2 co-adsorption on ceria: A combined FTIR and DFT study

Adsorption of NO and co-adsorption of NO and O2 on ceria have been re-investigated by FTIR spectroscopy. To provide unambiguous assignments of the IR bands, adsorption of 15NO and co-adsorption of 14NO + 15NO isotopic mixtures have also been studied and DFT calculations performed. At the initial adsorption stages mainly symmetric nitrite species (2472, 1304, ca. 1160 and 822 cm‐1) and trans-hyponitrites ([N2O2]2−, ca. 1100 cm‐1) are simultaneously produced as a result of NO disproportionation. At further stages non-symmetric NO2− species (1393 and 1014 cm‐1), N2O (2240, 1252 cm‐1) and small amount of nitrates are formed. Nitrates are the only surface products of the thermal decomposition of the nitrites and hyponitrites. In the presence of small amounts of oxygen, the hyponitrites are very easily oxidized. In contrast, all nitrites increase in concentration after addition of small amount of O2 to the NO/CeO2 system but are gradually oxidized in the presence of higher amount of oxygen giving rise to different surface nitrates.

N2O Formation via Reductive Disproportionation of NO by Mononuclear Copper Complexes: A Mechanistic DFT Study

The mechanism of the copper(I)-mediated reductive disproportionation reaction of NO to form N2O was investigated for five different 3,5-substituted tris(pyrazolyl)borate copper complexes (CuTpR1,R2) by means of DFT calculations. A thorough search of the potential surface was performed, using the B3LYP functional with the def2-SVP basis set for optimization purposes and def2-TZVP single-point calculations for constructing the potential energy surface for two of these complexes. The results can be condensed into six competing reaction mechanisms, two of which were more closely investigated using full def2-TZVP optimized potential and free energies. The results consistently predict the same mechanism to have the lowest overall barrier. For all five different complexes, this is found to be in good agreement with the experimental reaction barriers. The key intermediate for the transition from the N-bound reactant to the O-bound product contains a stable (NO)3 unit with one N-Cu and one O-Cu bond, which was not included in the mechanistic considerations reported in the literature. Further analysis of the charge distribution and the spin density demonstrates the formation of a Cu(II)-(N2O2-) intermediate and the electronic influence of the different ligands.

Spontaneous and Selective Formation of HSNO, a Crucial Intermediate Linking H2S and Nitroso Chemistries.

Thionitrous acid (HSNO), a potential key intermediate in biological signaling pathways, has been proposed to link NO and H2S biochemistries, but its existence and stability in vivo remain controversial. We establish that HSNO is spontaneously formed in high concentration when NO and H2S gases are mixed at room temperature in the presence of metallic surfaces. Our measurements reveal that HSNO is formed by the reaction H2S + N2O3 → HSNO + HNO2, where N2O3 is a product of NO disproportionation. These studies also suggest that further reaction of HSNO with H2S may form HNO and HSSH. The length of the S-N bond has been derived to high precision and is found to be unusually long: 1.84 Å, the longest S-N bond reported to date for an R-SNO compound. The present structural and, particularly, reactivity investigations of this elusive molecule provide a firm foundation to better understand its potential physiological chemistry and propensity to undergo S-N bond cleavage in vivo.

Nitric oxide activation by distal redox modulation in tetranuclear iron nitrosyl complexes.

A series of tetranuclear iron complexes displaying a site-differentiated metal center was synthesized. Three of the metal centers are coordinated to our previously reported ligand, based on a 1,3,5-triarylbenzene motif with nitrogen and oxygen donors. The fourth (apical) iron center is coordinatively unsaturated and appended to the trinuclear core through three bridging pyrazolates and an interstitial μ4-oxide moiety. Electrochemical studies of complex [LFe3(PhPz)3OFe][OTf]2 revealed three reversible redox events assigned to the Fe(II)4/Fe(II)3Fe(III) (-1.733 V), Fe(II)3Fe(III)/Fe(II)2Fe(III)2 (-0.727 V), and Fe(II)2Fe(III)2/Fe(II)Fe(III)3 (0.018 V) redox couples. Combined Mössbauer and crystallographic studies indicate that the change in oxidation state is exclusively localized at the triiron core, without changing the oxidation state of the apical metal center. This phenomenon is assigned to differences in the coordination environment of the two metal sites and provides a strategy for storing electron and hole equivalents without affecting the oxidation state of the coordinatively unsaturated metal. The presence of a ligand-binding site allowed the effect of redox modulation on nitric oxide activation by an Fe(II) metal center to be studied. Treatment of the clusters with nitric oxide resulted in binding of NO to the apical iron center, generating a {FeNO}(7) moiety. As with the NO-free precursors, the three reversible redox events are localized at the iron centers distal from the NO ligand. Altering the redox state of the triiron core resulted in significant change in the NO stretching frequency, by as much as 100 cm(-1). The increased activation of NO is attributed to structural changes within the clusters, in particular, those related to the interaction of the metal centers with the interstitial atom. The differences in NO activation were further shown to lead to differential reactivity, with NO disproportionation and N2O formation performed by the more electron-rich cluster.

Evidence of aerosols as a media for rapid daytime HONO production over China.

Current knowledge of daytime HONO sources remains incomplete. A large missing daytime HONO source has been found in many places around the world, including polluted regions in China. Conventional understanding and recent studies attributed this missing source mainly to ground surface processes or gas-phase chemistry, while assuming aerosols to be an insignificant media for HONO production. We analyze in situ observations of HONO and its precursors at an urban site in Beijing, China, and report an apparent dependence of the missing HONO source strength on aerosol surface area and solar ultraviolet radiation. Based on extensive correlation analysis and process-modeling, we propose that the rapid daytime HONO production in Beijing can be explained by enhanced hydrolytic disproportionation of NO2 on aqueous aerosol surfaces due to catalysis by dicarboxylic acid anions. The combination of high abundance of NO2, aromatic hydrocarbons, and aerosols over broad regions in China likely leads to elevated HONO levels, rapid OH production, and enhanced oxidizing capacity on a regional basis. Our findings call for attention to aerosols as a media for daytime heterogeneous HONO production in polluted regions like Beijing. This study also highlights the complex and uncertain heterogeneous chemistry in China, which merits future efforts of reconciling regional modeling and laboratory experiments, in order to understand and mitigate the regional particulate and O3 pollutions over China.