- The infrared spectrum of an adsorbate formed from NH3 on polycrystalline Pt (111)
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We have studied the infrared reflection-absorption spectrum of an adsorbate formed when an annealed polycrystalline Pt foil, presumably having a (111) surface structure, is exposed to ammonia gas at pressure in the range 0.1-10 torr.Isotopic substitution establishes that the adsorbate contains N2 in addition to NHx species.There is also CN present in the adsorbate, presumably formed by reaction of nitrogen with carbon impurities on the surface.
- Dunn, Douglas S.,Severson, Mark W.,Golden, W. G.,Overend, John
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- Direct observation of the symmetric bending vibrations (ν2) of NH3 physisorbed on a partially hydroxylated planar (0001) surface of ZnO
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For the first time the symmetric bending vibrational frequencies (ν2) of NH3 physisorbed on a partially hydroxylated metal oxide surface have been obtained with the coherent anti-Stokes Raman spectroscopy (CARS) using the planar optical waveguide geometry. The CARS signal from the adsorbed NH3 was enhanced by establishing an interference condition within the waveguide to reduce the CARS contribution from the oxide surface. The surface CARS spectra, in the investigated ν2 spectral region, provided evidence that the partially hydroxylated ZnO surface supported several sites for the physisorption of NH3. The physisorbed species had ν2 frequencies around 950, 963, 975 and 1012 cm-1 and were very quickly desorbed as soon as evacuation of the vacuum chamber began. In addition, the presence of NH3 which is coordinated to Zn2+ sites on the surface was detected.
- Wijekoon,Koenig,Hetherington III,Salzman
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p. L929 - L934,L929-L934
(1993)
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- Energy disposal in the reactions O(1D) + NH3 -> OH + NH2 and O(1D) + ND3 -> OD + ND2
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Further investigations of the reaction O(1D) + NH3 -> OH + NH2 and the first results on the reaction O(1D) + ND3 -> OD + ND2 are reported.The OH and OD rotational distributions have been found to be statistical.Hotter than statistical vibrational distributions are measured.The spin state distribution is statistical, with a strong preference to populate the energetically lower Λ component in both spin states.A preliminary study of the NH2 product shows very little rotational excitation.An analysis of this radical's vibrational hot bands was not carried out due to lack of detailed spectroscopic information.The total energy is found to be partitioned according to R(OH)> ca. 0.1, V(OH)> ca. 0.25, R(NH2)> ca. 0.04, and T> ca. 0.25.
- Cordova, Jose F.,Rettner, Charles T.,Kinsey, James L.
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- Mass-spectrometric experiments together with electronic structure calculations support the existence of the elusive ammonia oxide molecule and its radical cation
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Mass-spectrometric experiments were combined with ab initio calculations to explore the cationic and neutral [H3,N,O]?+/0 potential energy surfaces and relevant anionic species. The calculations predict the existence of three stable cationic and neutral [H3,N,O]?+/0 isomers, i.e. ammonia oxide H3NO?+/0 (1?+/0), hydroxylarnine H2NOH?+/0 (2?+/0) and the imine-water complex HNOH2?+/0 (3?+/0). Hydroxylamine 2 represents the most stable isomer on the neutral surface (Erel = 0), and the metastable isomers 1 (Erel = 24.8 kcal mol-1) and 3 (Erel = 61.4 kcal mol-1) are separated by barriers of 49.5 kcal mol-1 and 64.2 kcal mol-1, respectively. Adiabatic ionization of 2 (IEa = 9.15 eV) yields 2?+, which is 21.4 kcal mol-1 more stable than 1?+ and 36.4 kcal mol-1 more stable than 3?+. The barriers associated with the isomerizations of the cations are 58.6 kcal mol-1 for 2?+ → 1?+ and 71.4 kcal mol-1 for 2?+ → 3?+. Collisional activation (CA) and unimolecular decomposition (MI) experiments allow for a clear distinction of 1?+ from 2?+. Besides, neutralization/reionization (NR) experiments strongly support the gas-phase existence of the long-sought neutral ammonia oxide.
- Broenstrup, Mark,Schroeder, Detlef,Kretzschmar, Ilona,Schalley, Christoph A.,Schwarz, Helmut
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- Some thermodynamic properties of liquid ammonia and trideuteroammonia
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The orthobaric density of liquid NH3 has been measured from about 200 to 287 K, and for liquid ND3 from about 205 to 273 K.The molar volume of liquid NH3 exceeds that of ND3 by between 0.8 and 0.9 per cent.The difference of the vapour pressures of the two compounds has been measured from about 200 to 266 K, and the vapour pressure of NH3 from the triple-point temperature to 234 K.Liquid NH3 has the higher vapour pressure, the difference being relatively large for a pair of isotopic compounds.At 200 K, the ratio of the vapour pressure of NH3 to that of ND3 is about 1.2.The available vapour pressures for NH3 have been fitted to a Wagner equation.By combining vapour pressures derived from this equation with the differential measurements, values for the vapour pressure of ND3 have been obtained.These values have likewise been fitted to a Wagner equation.The material presented in this paper has been used to estimate the molar enthalpies of vaporization of NH3(l) and of ND3(l) from the triple-point temperatures to 290 K.The molar enthalpy of vaporization of ND3 exceeds that of NH3 throughout this range.The difference amounts to about 3.5 per cent at the triple-point temperatures, and decreases with rising temperature.
- Streatfeild, M. H.,Henderson, C.,Staveley, L. A. K.,Ferreira, A. G. M.,Fonseca, I. M. A.,Lobo, L. Q.
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- Bimetallic Cooperative Cleavage of Dinitrogen to Nitride and Tandem Frustrated Lewis Pair Hydrogenation to Ammonia
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Although reductive cleavage of dinitrogen (N2) to nitride (N3?) and hydrogenation with dihydrogen (H2) to yield ammonia (NH3) is accomplished in heterogeneous Haber–Bosch industrial processes on a vast scale, sequentially coupling these elementary reactions together with a single metal complex remains a major challenge for homogeneous molecular complexes. Herein, we report that the reaction of a chloro titanium triamidoamine complex with magnesium effects complete reductive cleavage of N2 to give a dinitride dititanium dimagnesium ditriamidoamine complex. Tandem H2 splitting by a phosphine–borane frustrated Lewis pair (FLP) shuttles H atoms to the N3?, evolving NH3. Isotope labelling experiments confirmed N2 and H2 fixation. Though not yet catalytic, these results give unprecedented insight into coupling N2 and H2 cleavage and N?H bond formation steps together, highlight the importance of heterobimetallic cooperativity in N2 activation, and establish FLPs in NH3 synthesis.
- Doyle, Laurence R.,Wooles, Ashley J.,Liddle, Stephen T.
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- Cluster size effects on hydrazine decomposition on Irn/Al 2O3/NiAl(1 1 0)
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A series of planar model catalysts were prepared by deposition of size-selected Irn+ on Al2O3/NiAl(1 1 0), and hydrazine decomposition chemistry was used to probe their size-dependent chemical properties. Small Irn (n ≤ 15) on Al2O 3/NiAl(1 1 0) are able to induce hydrazine decomposition at temperatures well below room temperature, with significant activity first appearing at Ir7. Both activity and product branching are strongly dependent on deposited cluster size, with these small clusters supporting only the simplest decomposition mechanism: dehydrogenation and N2 desorption at low temperatures, followed by H2 recombinative desorption at temperatures above 300 K. For Ir15, we begin to see ammonia production, signaling the onset of a transition to clusters able to support more complex chemistry.
- Fan, Chaoyang,Wu, Tianpin,Kaden, William E.,Anderson, Scott L.
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- Hydrogen Spillover to Oxygen Vacancy of TiO2- xHy/Fe: Breaking the Scaling Relationship of Ammonia Synthesis
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Optimizing kinetic barriers of ammonia synthesis to reduce the energy intensity has recently attracted significant research interest. The motivation for the research is to discover means by which activation barriers of N2 dissociation and NHz (z = 1-2, surface intermediates) destabilization can be reduced simultaneously, that is, breaking the scaling relationship . However, by far only a single success has been reported in 2016 based on the discovery of a strong-weak N-bonding pair: transition metals (nitrides)-LiH. Described herein is a second example that is counterintuitively founded upon a strong-strong N-bonding pair unveiled in a bifunctional nanoscale catalyst TiO2-xHy/Fe (where 0.02 ≤ x ≤ 0.03 and 0 y 0.03), in which hydrogen spillover (H) from Fe to cascade oxygen vacancies (OV-OV) results in the trapped form of OV-H on the TiO2-xHy component. The Fe component thus enables facile activation of N2, while the OV-H in TiO2-xHy hydrogenates the N or NHz to NH3 easily.
- Mao, Chengliang,Wang, Jiaxian,Zou, Yunjie,Qi, Guodong,Yang Loh, Joel Yi,Zhang, Tianhua,Xia, Meikun,Xu, Jun,Deng, Feng,Ghoussoub, Mireille,Kherani, Nazir P.,Wang, Lu,Shang, Huan,Li, Meiqi,Li, Jie,Liu, Xiao,Ai, Zhihui,Ozin, Geoffrey A.,Zhao, Jincai,Zhang, Lizhi
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- Dissociative and Associative Concerted Mechanism for Ammonia Synthesis over Co-Based Catalyst
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The current catalytic reaction mechanism for ammonia synthesis relies on either dissociative or associative routes, in which adsorbed N2 dissociates directly or is hydrogenated step-by-step until it is broken upon the release of NH3 through associative adsorption. Here, we propose a concerted mechanism of associative and dissociative routes for ammonia synthesis over a cobalt-loaded nitride catalyst. Isotope exchange experiments reveal that the adsorbed N2 can be activated on both Co metal and the nitride support, which leads to superior low-temperature catalytic performance. The cooperation of the surface low work function (2.6 eV) feature and the formation of surface nitrogen vacancies on the CeN support gives rise to a dual pathway for N2 activation with much reduced activation energy (45 kJ·mol-1) over that of Co-based catalysts reported so far, which results in efficient ammonia synthesis under mild conditions.
- Hosono, Hideo,Kitano, Masaaki,Li, Jiang,Lu, Yangfan,Park, Sang-Won,Sasase, Masato,Wu, Jiazhen,Ye, Tian-Nan
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supporting information
p. 12857 - 12866
(2021/08/30)
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- Catalytic Silylation of N2 and Synthesis of NH3 and N2H4 by Net Hydrogen Atom Transfer Reactions Using a Chromium P4 Macrocycle
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We report the first discrete molecular Cr-based catalysts for the reduction of N2. This study is focused on the reactivity of the Cr-N2 complex, trans-[Cr(N2)2(PPh4NBn4)] (P4Cr(N2)2), bearing a 16-membered tetraphosphine macrocycle. The architecture of the [16]-PPh4NBn4 ligand is critical to preserve the structural integrity of the catalyst. P4Cr(N2)2 was found to mediate the reduction of N2 at room temperature and 1 atm pressure by three complementary reaction pathways: (1) Cr-catalyzed reduction of N2 to N(SiMe3)3 by Na and Me3SiCl, affording up to 34 equiv N(SiMe3)3; (2) stoichiometric reduction of N2 by protons and electrons (for example, the reaction of cobaltocene and collidinium triflate at room temperature afforded 1.9 equiv of NH3, or at -78 °C afforded a mixture of NH3 and N2H4); and (3) the first example of NH3 formation from the reaction of a terminally bound N2 ligand with a traditional H atom source, TEMPOH (2,2,6,6-tetramethylpiperidine-1-ol). We found that trans-[Cr(15N2)2(PPh4NBn4)] reacts with excess TEMPOH to afford 1.4 equiv of 15NH3. Isotopic labeling studies using TEMPOD afforded ND3 as the product of N2 reduction, confirming that the H atoms are provided by TEMPOH.
- Kendall, Alexander J.,Johnson, Samantha I.,Bullock, R. Morris,Mock, Michael T.
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supporting information
p. 2528 - 2536
(2018/02/28)
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- Enrichment of Na and K or6Li and7Li by counter current flow of the phases of Na/K- Or6Li/7Li-ammonia solutions
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Metal-ammonia solutions (M-AS) have a miscibility gap, they separate at low temperatures into a diluted, electrolytic phase of high density and a concentrated, metallic phase of low density. In mixed sodium-potassium solutions (Na/K-AS) after phase separation sodium is enriched in the metallic phase, the separation factor αNa/K (Eq. (2)) is the larger, the lower the temperature relative to the critical temperature of the miscibility gap or the wider the miscibility gap. Therefore in deuteroammonia solutions (DAS) with higher critical temperatures the enrichment is more effective than in normal ammonia solutions. In Li-DAS with the two stable isotopes 6Li and 7Li the lighter one is enriched in the metallic phase. The separation factor (Eq. (4)) is αLi ≈ 1.010 at -73°C. By multiplication of the single equilibrium effect in a counter current column, which acts like a solvent-solvent extraction column, the species can be enriched. The counter current operation of the column was enforced by electrochemical means (application of a dc-current through the column), and the enrichment was tested around -74°C with equimolar Na/K-AS and Na/K-DAS for which αNa/K = 1.61 and 1.70 resp. and with Li-DAS of natural Li-isotope abundance. The height of the theoretical plate of the column for all three systems was HTP = 12.9±0.3cm. VCH Verlagsgesellschaft mbH 1996.
- Schindewolf,Reheis
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p. 1280 - 1285
(2007/10/03)
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- Mechanisms of the various nitric oxide reduction reactions on a platinum-rhodium (100) alloy single crystal surface
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The reduction of nitric oxide with hydrogen was studied over a Pt0.25-Rh0.75(100) alloy surface used as a model catalyst for the automotive three-way catalyst. This paper emphasizes the mechanisms of the different reactions leading to the products dinitrogen, ammonia and nitrous oxide. For this purpose the reaction was studied under various experimental conditions including reactivity measurements both in the 10-7 mbar range under steady-state conditions and in the 10 mbar range with varying NO/H2 ratio. In addition, the thermal decomposition of NO and the reactions of NO + NH3 were investigated. 15NO and 15NH3 were used in order to gather additional information concerning the mechanisms of the formation of the various N-containing products. The surface was characterized by using low-energy electron diffraction, Auger electron spectroscopy and thermal desorption spectroscopy. The main conclusions emerging from these studies are: (a) N2 can be formed by combination of 2 N adatoms in the whole temperature range used (350-1300 K), provided that sufficient N adatoms are available; (b) below 600 K the main contribution to N2 formation is via NOads + Nads → N2 + Oads; at higher temperatures the dominant mechanisms is 2Nads → N2; (c) N2O and NH3 are formed via Nads + NOads → N2O, and Nads + 3Hads → NH3, the contributions of which respectively decrease and increase with increasing temperature; (d) the selectivities to N2, NH3 and N2O are determined by the relative concentrations of NOads, Nads and Hads, which vary with the experimental conditions such as the temperature.
- Hirano, H.,Yamada, T.,Tanaka, K. I.,Siera, J.,Cobden, P.,Nieuwenhuys, B. E.
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- The structure of NH3*H2S and free internal rotation effects
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The structure of NH3*H2S has been determined from microwave and radiofrequency spectroscopy of this complex and its deuterated isotopomers, using molecular beam electric resonance techniques.Rotational constants, electric dipole moments and nitrogen quadrupole coupling constants were obtained from the spectra.The molecule was found to have a linear, hydrogen bonded structure with the ammonia as the proton acceptor.The NS distance is 3.639 Angstroem, the root-mean-square angular deviation of the NH3 axis from the N-S axis is 24.6 deg and the H2S C2 axis is 40.5 deg from the N-S axis.Although the molecule is an asymmetric rotor, first-order Stark effects were observed for K = 1 rotational levels.These Stark effects are caused by torsional oscillations which are essentially ammonia monomer free internal rotation.Similar effects were observed for NH3*H2O and are reported here.
- Herbine, Peter,Hu, T. A.,Johnson, G.,Dyke, T. R.
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p. 5485 - 5495
(2007/10/02)
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- NH3 reorientation in phases I, II, and III of Ni(NH3)6(NO3)2
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The inelastic neutron scattering (INS) spectra of rotational tunnelling and librations of NH3 ligands in phase III of Ni(NH3)6(NO3)2 are not consistent with a rotational hindrance potential containing only cos (3nΦ) terms owing to strong interaction between neighboring cations.This type of interaction, and motion of the whole cation, also influences the classical reorientational motions involving displacement of the H atoms, where the overall radius of rotation is consistently greater than the 0.9 Angstroem expected for isolated NH3 rotors.Quasielastic neutron scattering (QNS) suggests that in phase III there are two sublattices of cations, one of which becomes completely disordered (with respect to the NH3 rotors) at the III-II phase transition.Disorder of the second sublattice marks the II-I transformation where only a single type of rotational motion is found.
- Kearley, Gordon J.,Blank, Herma
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p. 692 - 697
(2007/10/02)
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- Mechanistic Details of the Heterogeneous Decomposition of Ammonia on Platinum
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Absolute reaction rates have been measured for the catalytic decomposition of NH3 and ND3 and for the NH3 + D2 exchange reaction over a polycrystalline platinum wire at pressures between 5 x 10-7 and 0.5 torr and temperatures between 400 and 1200 K in a continuous flow microreactor.At relatively low pressures and/or high temperatures, a primary isotope effect was observed for the decomposition of ND3, indicating that a surface reaction involving N-H bond cleavage is the rate-limiting step.Under these conditions, the order of the decomposition reaction is unity with respect to ammonia pressure with an apparent activation energy of 4.2 kcal/mol.A coverages increase, corresponding to relatively high pressures and/or low temperatures, the order of the decomposition reaction is zero with respect to ammonia, and the reaction rate becomes controlled by nitrogen desorption.In this case the apparent activation energy of the decomposition reaction is 22 kcal/mol.The kinetics of the NH3 + D2 exchange reaction have been used, together with data concerning the adsorption-desorption parameters of NH3, H2, and N2 as well as the reaction intermediates NH and NH2, to develop a mechanistic model which descibes the reaction rate over a wide range of experimental conditions and which includes the energetics of each intermediate step in the decomposition reaction.This model is discussed in terms of a potential energy diagram for ammonia decomposition on platinum.
- Vajo, J. J.,Tsai, W.,Weinberg, W. H.
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p. 3243 - 3251
(2007/10/02)
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- ENZYME-CATALYZED ORGANIC SYNTHESIS: REGENERATION OF DEUTERATED NICOTINAMIDE COFACTORS FOR USE IN LARGE-SCALE ENZYMATIC SYSNTHESIS OF DEUTERATED SUBSTANCES.
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Procedures were developed for in situ regeneration of deuterated nicotinamide cofactors based on ethanol-1,1,-d//2/alcohol dehydrogenase/aldehyde dehydrogenase, deuterioformate (DCO//2** minus )/formate dehydrogenase, and glycolaldehyde-1,2,2-d//3/aldehyde dehydrogenase. These procedures can be used in the enzyme-catalyzed synthesis of deuterated organic products; representative procedures are illustrated by syntheses of (R)-trifluoroethanol-1-d//1 and L-glutamic alpha -d//1 acid on 0. 1 mol scales. For most synthetic applications, the first of these procedures (CH//3CD//2OH/ADH/AldDH) seems to provide the most practical method for in situ regeneration of deuterated nicotinamide cofactors. The high turnover numbers observed for the nicotinamide cofactors in these syntheses make possible the preparation of deuterated chiral substances with high isotopic purity, regardless of the stereochemistry of the hydride transfers to and from the nicotinamide cofactors.
- Wong,Whitesides
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p. 5012 - 5014
(2007/10/02)
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