Chelating Nitrite Research Articles

Reaction Mechanism of H2-Assisted C3H6-SCR over Ag-CexZr Catalyst as Investigated by In situ FTIR

A series of silver-doped cerium zirconium oxide(Ag-CexZr) samples was synthesized successfully for selective catalytic reduction of nitric oxide(NO) with hydrogen and propene(H2/C3H6-SCR) under excess oxygen condition. The catalytic activity test proved that Ag-Ce0.4Zr exhibited the best C3H6-SCR activity. Hydrogen(H2) significantly enhanced NO conversion and widened the temperature window. Multi-technology characterizations were conducted to ascertain the properties of fabricated catalysts including X-ray diffraction(XRD), Fourier transform infrared spectrometry(FTIR), scanning electron microscopy(SEM) and H2 temperature programmed reduction (H2-TPR). In situ FTIR results demonstrated that various types of nitrates and chelating nitrite were generated on Ag-CexZr after introduction of NO. Besides, adding H2 could increase the concentration of bidentate nitrate and chelated bidentate nitrate dramatically, especially for Ag-Ce0.4Zr catalyst. Transient reaction between pre-adsorbing NO and C3H6/C3H6+H2 illuminated that the most active intermediate was chelating nitrite,which was promoted significantly by H2 participation. Furthermore, adding H2 improved the formation of organo-nitro(R-NO2), which was the key intermediate in C3H6-SCR. The reaction mechanism over Ag-CexZr catalysts was proposed at 200 °C as follows: nitric oxide(NO)+propene(C3H6)+hydrogen(H2)+oxygen(O2)→chelating nitrite(NO2−)+acrylate-type species(CxHyOz)→organo-nitro(R-NO2)→isocyanate(—NCO)+cyanide(—CN)→nitrogen(N2).

Promotional effects of Fe on manganese oxide octahedral molecular sieves for alkali-resistant catalytic reduction of NOx: XAFS and in situ DRIFTs study

Currently, selective catalytic reduction (SCR) of NOx with NH3 in the presence of alkali metal ions by using vanadium-free catalysts is still a big challenge for the removal of NOx for stationary sources. In this work, improved reduction of NOx with NH3 in the presence of alkali metal ions over novel Fe-doped manganese oxide octahedral molecular sieve (OMS-2) catalysts has been demonstrated and the promotional effects of Fe have been clarified. The Fe-doped OMS-2 (Fe-OMS-2) catalysts exhibited excellent NH3-SCR activity, improved N2 selectivity and enhanced alkali resistance. The X-ray absorption fine structure spectroscopy (XAFS) results indicated that the trapped K+ ions could make the catalyst structure more stable. In situ diffuse reflectance infrared transform spectroscopy (in situ DRIFTs) results revealed that the Fe doping could enhance adsorption of NH3 species and adsorb various types of NOx species namely monodentate nitrate, bridged nitrate, bidentate nitrate and chelating nitrite. These formed intermediates on the Fe-OMS-2 catalysts were more reactive and thus more effectively participated in the SCR reactions. Superior alkali resistance of the Fe-OMS-2 catalysts was due to improved redox species, more acid sites and stronger adsorption of NOx species. The present investigations in this work may lead to an alternative development of high-performance non-vanadium catalysts for alkali-resistant NOx reduction.

Selective catalytic reduction of NOx by H2 over Pd/TiO2 catalyst

Pd/TiO2 catalysts prepared by three different methods (impregnation, deposition-precipitation, and polyethylene glycol reduction) were investigated in the selective catalytic reduction of NOx by H2 (H2-SCR). It was found that the preparation method exerted a significant effect on the activity of the Pd/TiO2 catalyst, and that the catalyst prepared by the polyethylene glycol reduction method exhibited the highest activity in the reduction of NOx. Characterization of the catalyst showed that, in the Pd/TiO2 catalyst prepared by the polyethylene glycol reduction method, the existing Pd species was Pd0, which is the desirable species for the H2-SCR of NOx. In situ DRIFTS studies demonstrate d that over this catalyst, more chelating nitrite and monodentate nitrite species formed, both of which are reactive intermediates in the H2-SCR of NOx. All of these factors account for the high activity of Pd/TiO2 prepared by the polyethylene glycol reduction method.

Selective catalytic reduction of NO by H2/C3H6 over Pt/Ce1-xZrxO2-δ: The synergy effect studied by transient techniques

A series of Pt/CexZr1-xO2-δ (x=0.4–0.6) solids were synthesized and evaluated for the SCR of NO under lean burn conditions (2.5vol% O2) using C3H6 and H2 as reducing agents. SSITKA-Mass Spectrometry, SSITKA-DRIFTS and other in situ DRIFTS experiments were conducted for the first time to gather fundamental information in explaining the remarkable H2/C3H6 synergy effect towards steady-state selective reduction of NO into N2 at T>400°C. In particular, the chemical structure of adsorbed active and inactive (spectator) NOx species formed under C3H6-SCR, H2-SCR and H2/C3H6-SCR of NO and the surface coverage and site formation of active NOx were probed. The Pt/Ce1-xZrxO2-δ catalysts present significant differences in their H2-SCR performance (NO conversion and N2-selectivity) in the low-temperature range of 120–180°C but practically the same catalytic behavior at higher temperatures. It was proved that the active NOx of the H2-SCR path reside within a reactive zone around each Pt nanoparticle which extends to less than one lattice constant within the support surface. The chemical structure of the active intermediate was proved to be the chelating nitrite, whereas nitrosyl, monodentate and bidentate nitrates were considered as inactive species (spectators). It was illustrated for the first time that the presence of 15 vol% H2O in the H2-SCR feed stream applied over the 0.1wt% Pt/Ce0.5Zr0.5O2 catalyst results in a 25% decrease in the concentration of active NOx, thus partly explaining the drop in activity observed when compared to the H2-SCR in the absence of H2O. A remarkable activity and N2-selectivity enhancement was observed at T>400°C when both H2 and C3H6 reducing agents were used compared to H2-SCR or C3H6-SCR alone. This synergy effect was explained to arise mainly because of the increase of θΗ by the presence of –CHx species derived from adsorbed propylene decomposition on Pt, which block sites of oxygen chemisorption, and of the increase of surface oxygen vacant sites that promote the formation of a more active chelating nitrite (NO2−) species compared to the case of H2-SCR.

Three nickel(II) complexes derived from a tridentate NNO donor Schiff base ligand: Syntheses, crystal structures and magnetic properties

Three new Ni(II) complexes, [NiL(NO2)(H2O)] (1), [Ni2L2(PhCH2COO)2(H2O)] (2) and [Ni2L2(N(CN)2)2(H2O)] (3), have been synthesized using a tridentate Schiff base ligand, 1-[(3-dimethylamino-propylimino)-methyl]-naphthalen-2-ol (HL) and the polyatomic monoanions NO2−, PhCH2COO− or N(CN)2−. All three complexes (1–3) have been characterized by spectral analysis and X-ray crystallography. Structural analyses reveal that complex 1 is mononuclear, in which the Ni(II) ion is in a distorted octahedral environment, coordinated by the deprotonated tridentate Schiff base (L), a chelating nitrite and one water molecule. The structures of both 2 and 3 consist of dinuclear octahedral Ni(II) units with crystallographic C2 symmetry. In complex 2, two μ2-phenoxido and a water molecule bridge the two Ni(II) centers to make the complex a face sharing bioctahedron, whereas in 3 the Ni(II) atoms are bridged by two μ1,5-dicyanamide ligands. Variable temperature magnetic susceptibility measurements show the presence of ferromagnetic coupling in complex 2 with J=+26.20cm−1 and antiferromagnetic coupling in complex 3 with J=−1.90cm−1.

Pentacoordinated copper–sparteine complexes with chelating nitrite or nitrate ligand: Synthesis and catalytic aspects

Copper(II)–sparteine complexes, [CuII{(-)Sp}(NO2)Cl] (1) and [CuII{(−)Sp}(NO3)Cl] (2) (Sp=sparteine) with chelating nitrite and nitrate ligands, respectively, have been synthesized and structurally characterized. The penta-coordinated 1 or 2 exhibits distorted square pyramidal geometry and shows characteristic EPR spectra with g||: 2.27 and g⊥: 2.06. 1 and 2 behave similarly towards the catalytic epoxidation of alkenes as well as oxidation of alcohols. Though the epoxidation of cyclohexene using 1 or 2 as a catalyst and tertiarybutyl hydroperoxide (TBHP) as an oxidant at 298K in acetonitrile results in 100% cyclohexene oxide product, under identical reaction conditions styrene selectively transforms to benzaldehyde (∼90%) instead of any styrene oxide. However, at higher temperature (353K) the selectivity of cyclohexene to corresponding epoxide formation decreases appreciably and additional products, cyclohexanol and cyclohexanone are formed. Furthermore, 1 or 2 effectively catalyzes the oxidation of benzyl alcohol to benzoic acid and cyclohexanol to cyclohexanone in presence of molecular oxygen (O2) as an oxidant at 353K in acetonitrile.

The influence of reaction temperature on the chemical structure and surface concentration of active NO x in H 2-SCR over Pt/MgO [sbnd]CeO 2: SSITKA-DRIFTS and transient mass spectrometry studies

Steady-state isotopic transient kinetic analysis (SSITKA), transient isothermal, and temperature-programmed surface reaction in H 2 (H 2-TPSR) techniques coupled with online mass spectroscopy (MS) and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were used to study essential mechanistic and kinetic aspects of the selective catalytic reduction (SCR) of NO with the use of H 2 under strongly oxidizing conditions (H 2-SCR) over a novel Pt/MgO CeO 2 catalyst. The main focus was to study and report for the first time the effects of reaction temperature on the chemical structure and surface concentration of the active NO x intermediate species thereby formed. The information obtained is essential to understanding the volcano-type profile of the catalyst activity versus reaction temperature observed here and also reported previously. In the present work, two active NO x intermediate species identified by SSITKA-DRIFTS were found in the nitrogen-reaction path toward N 2 and N 2O formation, one species located in the vicinity of the Pt CeO 2 support interface region (nitrosyl [NO +] coadsorbed with a nitrate [NO − 3] species on an adjacent Ce 4+ O 2− site pair) and the second located in the vicinity of the Pt MgO support interface region. The chemical structure of the second kind of active NO x species was found to depend on reaction temperature. In particular, the chemical structure was that of bidentate or monodentate nitrate (NO − 3) at T < 200 ° C and that of chelating nitrite (NO − 2) at T > 200 ° C . The concentration of the active NO x intermediates that lead to N 2 formation was found to be practically independent of reaction temperature (120–300 °C) and significantly larger than 1 equivalent monolayer of surface Pt ( θ NO x = 2.4 – 2.6 ). The former result cannot be used to explain the volcano-type behavior of the catalytic activity versus the reaction temperature observed; alternative explanations are explored. The H-spillover process involved in the H 2-SCR mechanism was found to be limited within a support region of about a 4–5 Å radius around the Pt nanoparticles ( d Pt = 1.2 – 1.5 nm ).

Chelating ability and role of nitrite. Synthesis and structural properties of AgNO 2 with 3,3′-oxybispyridine

The reaction of silver(I) nitrite with 3,3′-oxybispyridine (Py 2O) produces a coordination polymer of [Ag(NO 2)Py 2O] unit. The Py 2O ligand connects two Ag(I) ions via the nitrogen donors of 3-pyridyl group to give a zigzag single strand. The nitrite anion acts as a chelating ligand rather than a counteranion. Thus, the Ag(I) ion is surrounded by N 2O 2 donors including the chelating nitrite. Thermal properties and anion exchange are consistent with the structural features such as the chelating ability of NO 2 − and the relatively weak bonds of Ag–N. The chelating ability of anion has a significant effect on the formation of skeletal structure in coordination polymerization.

Solid–gas interaction of nitrogen oxide adsorbed on MnOx–CeO2: a DRIFTS study

The formation of fluorite-type solid solutions with compositions (n)MnOx–(1 − n)CeO2 is effective in increasing adsorptive NO uptake. Solid–gas interactions for NO adsorbed onto preoxidized samples was studied by the use of diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) to elucidate the role of each oxide component. On exposure to a flowing gas containing 0.04% NO, several different types of nitrate and/or nitrite were yielded depending on temperature (25 and 150 °C), O2 concentration (0, 2, and 10%) in the gas feed, and composition of the binary oxide (n = 0, 0.25, 0.5, and 1.0). On CeO2 (n = 0), chelating nitrite (NO−2) prevailed, while only small amounts of bidentate and unidentate nitrates (NO−3) were formed at 150 °C. By contrast, adsorption onto MnOx (n = 1.0) at 150 °C produced bidentate nitrate even in the absence of O2, suggesting that the lattice oxygen takes part in the NO oxidation process. On MnOx–CeO2 (n = 0.25 and 0.5), chelating nitrite bound to Ce ions was increasingly converted to nitrate at higher O2 concentration and at elevated temperature. A surface reaction model on the basis of the DRIFTS results reveals that the redox of Mn ions with simultaneous oxygen equilibration with the gas phase should play a key role in facilitating the oxidative adsorption of NO.

The synthesis of cis-Re(O,O′-No2)(CO)2(PPh3)2 containing the unusual O,O′-chelating nitrite ligand and its reaction with CO. Evidence for a rhenium-nitrogen interaction

The synthesis of the bidentate nitrite complex, Re(O,O−-NO2)(Co)2(PPh3)2, from [Re(CH3CN)2(CO)2(PPh3)2]ClO4 is reported. The possibility of a ReN interaction in Re(O,O−NO2)(CO)2)(PPh3)2) is indicated by the derivative chemistry of the complex with CO, p-tolylisocyanide and HCl. A superior, high yield route to ReH(CO)2(PPh3)3 is also described.

The single crystal electronic spectra of cis-octahedral nickel(II) diamine complexes at 10 K

The single crystal polarised spectra, at 10 K, of a series of nickel(II) complexes containing the cis-chromophore NiN4O2 are reported. The four N ligands are provided by the diamines N,N′-diethylethylenediamine or meso-stilbenediamine, and the two O ligands by chelating nitrate or nitrite ion. The spectra are assigned in the point group C2v(III). The energies are calculated within the Normalised Spherical Harmonic Hamiltonian and the Orbital Angular Overlap formalisms. The parameters so derived are consistent with previous experience obtained with trans-analogs once the small bite of the chelating nitrite ligand is accounted for.

Structural and spectroscopic studies of transition metal nitrite complexes. II. Crystal structures of cis-Bis(ethane-1,2-diamine)(nitrito-O,O')zinc(II) nitrite and trans-Bis[N,N-dimethyl(ethane-1,2-diamine)]dinitritozinc(II)

The crystal structures of the complexes cis-bis(ethane-1,2-diamine)(nitrito-O,O')zinc(II) nitrite and trans-bis[N,N-dimethyl(ethane-l,2-diamine)]dinitritozinc(II) are described. The former compound contains one chelating nitrite, the second group being present as a counter ion. In this complex the coordination polyhedron about the metal ion may be described either as a distorted trigonal bi-pyramid or an octahedron, depending upon whether the chelated nitrite is considered to occupy one or two coordination sites. The second compound is a trans nitrito complex, having an octahedral ligand geometry, though with three markedly different metal-ligand bond lengths. The structures of the complexes are compared with those of analogous nickel(II) nitrite complexes, and the differences are discussed in terms of the electron configurations of the two metal ions.

Magnetic and spectral properties of nickel(II) complexes with bridging nitrite groups

The preparations and magnetic and spectral properties of the compounds NiD(NO2)2(D = ethylenediamine, and N-methyl- and NN-diethyl-ethylenediamine) and NiL2(NO2)2(L = pyridine, isoquinoline, and 3- and 4-methylpyridine), the latter as benzene solvates in some cases, are reported. The nitrite groups in the ethylenediamine (en) and N-methylethylenediamine (men) complexes are thought to be present as M–ON(O)–M [type (I)] bridges. The spectral properties of the rest of the compounds are similar to those of the trimer [Ni(3-Mepy)2(NO2)2]3, C6H6(Mepy = methylpyridine) which is known to contain M–O(NO)–M [type (II)] bridges, type (I) bridges, and chelating nitrite groups. The type (II) bridges give rise to ν(NO) bands in the regions 1007–1026 and 1470–1490 cm–1.Single-crystal polarized electronic spectra of [Ni(3-Mepy)2(NO2)2]3, C6H6 are reported and discussed. The magnetic properties of this compound over the range 90–300 K are interpreted in terms of J=–11 cm–1(for the coupling between adjacent metal atoms of the trimer). For the compound [Ni(en)2(NO2)]ClO4, which contains chains of nickel atoms connected by type (I) bridges, J=–18 cm–1.

Complexes of cobalt(II) nitrite with amine ligands

The preparations, i.r. and electronic spectra, and magnetic moments of a range of cobalt(II) nitrite-complexes with amine ligands are reported. In all cases the nitrite-groups co-ordinate through oxygen, as unidentate nitrite-groups in [Co(pyridine)4(ONO)2],2(pyridine), CoL4(ONO)2 where L = isoquinoline or 4-methylpyridine, and Co(diamine)2(ONO)2 where diamine =NN-dimethyl- or NN′-diethyl-ethylenediamine, but as chelating nitrite groups in Co(NNN′N′-tetramethylethylenediamine)(NO2)2 and in various 3:1 and 2:1 complexes with heterocyclic amines. Distorted seven co-ordination is suggested for the metal ion in the compounds CoL3(NO2)2 where L = pyridine or 3-methylpyridine.

Some new amine complexes of nickel nitrite and a mixed nitro-nitritocomplex

The preparation, electronic and infrared (2000–200 cm −1 spectra of several new amine complexes of nickel nitrite are reported. The solid compounds, NiL 4 (ONO) 2 (L  pyridine, 3-methylpyridine and 4-methylpyridine) are nitrito-complexes, but dissociate in benzene to give 2:1 complexes containing chelating anions. The complex Ni(o-phenylenediamine) 2(NO 2) 2 is a nitro-complex, while Ni(NNN′N′-tetraethylethylenediamine)(NO 2) 2 contains chelating nitrite groups. The manner of nitrite bonding in the solid complexes is apparently related to the steric requirements of the amines. With isoquinoline, nickel nitrite forms a mixed nitro-nitrito-complex of the type Ni(isoquinoline) 4(NO 2) 2 xNi(isoquinoline) 4(ONO) 2. Bands at 270-295 cm −1 for the nitrito-complexes are assigned as v(NiO).