Dehydrogenation Of Ammonia Research Articles

Ruthenium Complexes with Cooperative PNP Ligands: Bifunctional Catalysts for the Dehydrogenation of Ammonia–Borane

G“Ru”vy reactivity: The new ruthenium(II) complex having cooperative PNP enamido ligand A reversibly activates two equivalents H2 under reversible hydrogenation of amido (B) and amino (C) complexes. B exhibits the highest reported activities in the dehydrocoupling of ammonia–borane.

NH3 Oxidation on Oxygen-Precovered Au(111): A Density Functional Theory Study on Selectivity

We have performed density functional theory simulations applied to slabs to study the dehydrogenation of ammonia on the surface of clean and atomic oxygen or hydroxyl precovered Au(111). Ammonia does not dissociate unimolecularly on the surface and needs a proton scavenger to drive decomposition. O atoms or hydroxyl groups on the surface are basic enough to attract the first H atom from the molecule and are also needed in the resting dehydrogenation steps. Recombination steps to form N2 and NO are hindered by barriers lower than those on Pt or Rh(111) due to the low interaction energies of molecular moieties with the Au(111) surface. The barrier for N2 formation is in any case lower than that of NO in agreement with the high selectivity observed toward N2. Finally, coverage effects are discussed for the competing reactions.

Catalyzed Dehydrogenation of Ammonia–Borane by Iridium Dihydrogen Pincer Complex Differs from Ethane Dehydrogenation

Catalyzed Dehydrogenation of Ammonia–Borane by Iridium Dihydrogen Pincer Complex Differs from Ethane Dehydrogenation

Catalyzed Dehydrogenation of Ammonia–Borane by Iridium Dihydrogen Pincer Complex Differs from Ethane Dehydrogenation

Kein bisschen ähnlich: Nach DFT-Studien verläuft die Ammoniak-Boran-Dehydrierung durch Iridium-Pinzettenkomplexe als konzertierter Hydrid- und Protonentransfer vom Ammoniak-Boran auf den Katalysator und nicht, wie früher vorgeschlagen, über eine B-H-Aktivierung und anschließende β-Hydrideliminierung vom Stickstoffatom. Ein derartiger konzertierter Mechanismus tritt bei Ethan nicht auf, das isoelektronisch mit Ammoniak-Boran ist. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2001/2007/z702886_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Density-Functional Theory Study of NHx Oxidation and Reverse Reactions on the Rh(111) Surface

The adsorption of NHx fragments and oxidation of them by O and OH on the Rh(111) crystal surface have been studied using first-principles density-functional calculations. The stability and configurations of OHx and NHx have been investigated and characterized using frequency analysis. Several paths of NHx (x = 1−3) oxidation with O and OH and reverse elementary processes have been determined. The transition states have been determined and analyzed in detail. The activation barriers and thermodynamic and kinetic data have been calculated for all of the elementary steps. The calculations have shown that atomic oxygen does not promote ammonia decomposition. The elementary reactions with O are endothermic, and they have significant barriers, comparable with ammonia dehydrogenation barriers [Popa, C.; Offermans, W. K.; van Santen, R. A.; Jansen, A. P. J. Phys. Rev. B 2006, 74 (15), 155428-1−155428-10].1 The OH fragment does promote ammonia decomposition. The elementary reactions are exothermic or slightly endo...

Computational Study of Titanocene-Catalyzed Dehydrocoupling of the Adduct Me2NH·BH3: An Intramolecular, Stepwise Mechanism

The titanocene-catalyzed dehydrocoupling of the adduct Me2NH·BH3 has been investigated by density functional theory (B3LYP). Unlike the thermal dehydrogenation of ammonia−borane, which was previously found to occur via a concerted mechanism, such a catalytic dehydrogenation of Me2NH·BH3 has been found to be a stepwise process. Both intra- and intermolecular dehydrogenation pathways have been computed. The entire intramolecular dehydrogenation process is exergonic by 7.4 kcal/mol, and the intermolecular dehydrogenation is an endergonic process. The solvation free-energy barriers for the intramolecular dehydrogenations are 14.1 and 7.9 kcal/mol, respectively, which are lower than those of the intermolecular dehydrogenation pathway (24.1 and 9.2 kcal/mol, respectively). These results suggest that the intramolecular pathway is preferable both kinetically and energetically. The resulting monomeric aminoborane species, Me2NBH2, may undergo dimerization leading to the cyclic (Me2N−BH2)2, which was experimentally...

Base Metal Catalyzed Dehydrogenation of Ammonia−Borane for Chemical Hydrogen Storage

We report here the first example of a homogeneous first row transition-metal-based catalyst which is active for dehydrogenation of ammonia−borane, H3NBH3, a promising chemical hydrogen storage material. Addition of ammonia−borane to an active catalyst formed in situ from the reaction of Ni(cod)2 and 2 equiv of an appropriate N-heterocyclic carbene (NHC) rapidly evolves hydrogen at 60 °C. Using a gas burette to quantify the gas evolved, 29 of a possible 31 mL of H2 for 3 equiv of H2 was produced, equating to >2.5 equiv of H2 from ammonia−borane. Kinetic isotope effects of deuterated derivatives of ammonia−borane suggest that both N−H and B−H bonds are being broken in the rate-determining step(s).

Mechanistic insights into the formation of N2O and N2 in NO reduction by NH3 over a polycrystalline platinum catalyst

The reaction pathways of N2 and N2O formation in the direct decomposition and reduction of NO by NH3 were investigated over a polycrystalline Pt catalyst between 323 and 973K by transient experiments using the temporal analysis of products (TAP-2) reactor. The interaction between nitric oxide and ammonia was studied in the sequential pulse mode applying 15NO. Differently labelled nitrogen and nitrous oxide molecules were detected. In both, direct NO decomposition and NH3–NO interaction, N2O formation was most marked between 573 and 673K, whereas N2 formation dominated at higher temperatures. An unusual interruption of nitrogen formation in the 15NO pulse at 473K was caused by an inhibiting effect of adsorbed NO species. The detailed analysis of the product distribution at this temperature clearly indicates different reaction pathways leading to the product formation. Nitrogen formation occurs via recombination of nitrogen atoms formed by dissociation of nitric oxide or/and complete dehydrogenation of ammonia. N2O is formed via recombination of adsorbed NO molecules. Additionally, both products are formed via interactions between adsorbed ammonia fragments and nitric oxide.

Thermodynamics of ammonia activation by iron cluster cations: Guided ion beam studies of the reactions of Fen+ (n=2–10,14) with ND3

The kinetic energy dependences of the reactions of Fen+ (n=2–10,14) with ND3 are studied in a guided ion beam tandem mass spectrometer over the energy range of 0–10 eV. Dehydrogenation of ammonia to form FenND+ is found to be efficient and exothermic for n=4 in agreement with previous FT-ICR studies. In contrast to the ICR studies, we also observe exothermic dehydrogenation for n=3 and 5, although these processes are much less efficient than for n=4. Other clusters also undergo this process but exhibit an energy threshold. A multitude of other primary products are observed including Fen−1ND3+ (n=2–4,9,10), FenND2+ (n=1,4–8), and Fen−1ND2+ (n=2–5), which all have reaction efficiencies that depend on cluster size. At high energies, FenN+ and FenD+ are observed along with products corresponding to Fe atom loss from the primary products. Thresholds for the various primary and secondary reactions are analyzed and bond energies for iron cluster cations bound to N, ND, ND2, and ND3 are determined. Comparisons of this thermochemistry to that for isoelectronic CD, CD2, and CD3 show that the binding of ND and ND2 is enhanced by dative interactions of the lone pair of electrons with the cluster. In contrast, the N atom is found to have a weaker bond than CD for most cluster sizes. On the basis of this thermochemistry, we find that there are barriers to the dehydrogenation reactions of about 1 eV for all the clusters, except n=3–5 exhibit an additional low energy pathway. Bond energies for larger clusters are used to estimate the bond energies of the ND and ND2 molecular fragments to iron surfaces.

Production of CO x-free hydrogen for fuel cells via step-wise hydrocarbon reforming and catalytic dehydrogenation of ammonia

The stringent CO x -free hydrogen requirement for the current low temperature fuel cells has motivated the development of CO x -free hydrogen production alternatives to the conventional hydrogen production technologies. Recently, our group has investigated step-wise reforming of hydrocarbons and catalytic decomposition of ammonia for CO x -free production of hydrogen. These investigations have employed conventional surface science techniques, model catalysts as well as high surface area supported metal catalysts. This paper presents an overview of the studies undertaken in our laboratory and highlights the important aspects of the proposed CO-free hydrogen production processes.

Adsorption and thermal dehydrogenation of ammonia on Ru(112̄1)

Adsorption and thermal dehydrogenation of NH3, adsorbed at 80 K on the open Ru(112̄1) surface, was studied using high-resolution electron energy-loss spectroscopy (HREELS) and thermal desorption spectroscopy (TDS). For the NH3 monolayer, a strong dipole-active mode is found at 15 meV which is newly assigned to Tz, the frustrated-translation mode perpendicular to the surface of NH3 bonding with the nitrogen atom to the Ru surface. Increasing the temperature, 70% of NH3 desorbs before a channel for dehydrogenation opens at about 280 K. The remaining 30% decomposes completely during further warming to 470 K. The dehydrogenation of NH3 gives rise to four peaks in the H2 TDS which are assigned to desorption of coadsorbed hydrogen at 220 K and three dehydrogenation reaction steps at 320, 360, and 420 K in accordance with HREELS. The reaction intermediates NH2 and NH are identified through HREELS. In a new interpretation NH2 is characterized by intense modes at 163 meV (rocking) and at 189 meV (scissoring). Using a maximum entropy algorithm six frequencies for ν(Ru–N) were resolved at 46, 50, 58, 61, 69, and 75 meV.

Electrical properties of V 2O 5–WO 3/TiO 2 EUROCAT catalysts evidence for redox process in selective catalytic reduction (SCR) deNO x reaction

Electrical conductivity measurements on EUROCAT V 2O 5–WO 3/TiO 2 catalyst and on its precursor without vanadia were performed at 300°C under pure oxygen to characterize the samples, under NO and under NH 3 to determine the mode of reactivity of these reactants and under two reaction mixtures ((i) 2000 ppm NO + 2000 ppm NH 3 without O 2, and (ii) 2000 ppm NO + 2000 ppm NH 3 + 500 ppm O 2) to put in evidence redox processes in SCR deNO x reaction. It was first demonstrated that titania support contains certain amounts of dissolved W 6+ and V 5+ ions, whose dissolution in the lattice of titania creates an n-type doping effect. Electrical conductivity revealed that the so-called reference pure titania monolith was highly doped by heterovalent cations whose valency was higher than +4. Subsequent chemical analyses revealed that so-called pure titania reference catalyst was actually the WO 3/TiO 2 precursor of V 2O 5–WO 3/TiO 2 EUROCAT catalyst. It contained an average amount of 0.37 at.% W 6+dissolved in titania, i.e. 1.07 × 10 20 W 6+ cations dissolved/cm 3 of titania. For the fresh catalyst, the mean amounts of W 6+ and V 5+ ions dissolved in titania were found to be equal to 1.07 × 10 20 and 4.47 × 10 20 cm −3, respectively. For the used catalyst, the mean amounts of W 6+ and V 5+ ions dissolved were found to be equal to 1.07 × 10 20 and 7.42 × 10 20 cm −3, respectively. Since fresh and used catalysts have similar compositions and similar catalytic behaviours, the only manifestation of ageing was a supplementary progressive dissolution of 2.9 × 10 20 additional V 5+ cations in titania. After a prompt removal of oxygen, it appeared that NO alone has an electron acceptor character, linked to its possible ionosorption as NO − and to the filling of anionic vacancies, mostly present on vanadia. Ammonia had a strong reducing behaviour with the formation of singly ionized vacancies. A subsequent introduction of NO indicated a donor character of this molecule, in opposition to its first adsorption. This was ascribed to its reaction with previously adsorbed ammonia strongly bound to acidic sites. Under NO + NH 3 reaction mixture in the absence of oxygen, the increase of electrical conductivity was ascribed to the formation of anionic vacancies, mainly on vanadia, created by dehydroxylation and dehydration of the surface. These anionic vacancies were initially subsequently filled by the oxygen atom of NO. N o atoms, resulting from the dissociation of NO and from ammonia dehydrogenation, recombined into dinitrogen molecules. The reaction corresponded to 3 NO+2 NH 3→ 5 2 N 2+3 H 2 O . In the presence of oxygen, NO did not exhibit anymore its electron acceptor character, since the filling of anionic vacancies was performed by oxygen from the gas phase. NO reacted directly with ammonia strongly bound on acidic sites. A tentative redox mechanism was proposed for both cases.

Dehydrogenation of ammonia by early transition metals: formation of metal nitride clusters

The dehydrogenation of NH 3 by Ti, Zr, and Nb cations, produced in a laser-induced plasma reactor, is found to form both neutral and cationic clusters. Depending on the conditions, either the production of neutral metal nitride clusters is favored or, the formation of cationic `precursor' metal nitride clusters containing additional intact NH 3 is observed. The neutral clusters are formed when full dehydrogenation of the NH 3 is achieved, while the cationic precursor species are formed under conditions where a large fraction of the NH 3 remains intact. Metastable dissociation studies show that intact NH 3 molecules are bound to the developing transition metal nitride clusters.

HCN Synthesis from Methane and Ammonia: Mechanisms of Pt+-Mediated C−N Coupling

The Pt+-mediated coupling of methane and ammonia has been studied both experimentally and computationally. This system serves as a model for the Degussa process for the industrial production of the valuable feedstock hydrogen cyanide. Mass spectrometric studies demonstrate that C−N bond formation is catalyzed efficiently by Pt+. Details of the experimentally observed reaction channels have been explored computationally using the B3LYP hybrid DFT/HF functional. In the first reaction step, Pt+ dehydrogenates CH4 to yield PtCH2+; in contrast, dehydrogenation of ammonia by Pt+ is endothermic and does not occur experimentally. Starting from PtCH2+ and NH3, C−N bond formation, which constitutes the crucial step in making HCN from CH4 and NH3, is achieved via two independent pathways. The major pathway is found to be exothermic by 23 kcal mol-1 and yields neutral PtH and CH2NH2+. The second pathway involves a dehydrogenation to yield the aminocarbene complex PtC(H)NH2+ (ΔrH = −36 kcal mol-1); dehydrogenation of ...

The thermal chemistry of ammonia on Ni(110)

The thermal chemistry of NH 3 on Ni(110) surfaces has been investigated using temperature-programmed desorption (TPD). The thermal activation of ammonia-saturated Ni(110) leads primarily to molecular desorption, but some decomposition is identified by the appearance of hydrogen and nitrogen TPD peaks ca 400 and 800 K, respectively. Previous reports have shown that ammonia dehydrogenation is readily induced by mild electron bombardment, but this study found dehydrogenation to persist despite the absence of any electron current at the surface, indicating that both thermal- and electron-induced dissociation processes are possible in the NH 3/Ni(110) system. Our experimental data also identified the onset of the thermally-activated ammonia dehydrogenation at about 300 K. Evidence is given to suggest that the surface intermediate formed subsequent to this dehydrogenation is NH 2,ads. Further activation of the NH 2,ads moiety by the Ni(110) surface results in its dehydrogenation to NH ads and N ads around 380 K, but also to some NH 2,ads+H ads recombination to NH 3(g) ca 360 K. After considering this recombination step, it was calculated that as much as 23% of adsorbed ammonia dissociates by thermal activation, a much greater value than previously reported.

Lanthanide (Eu or Yb)-promoted Ni, Cu and Ag-catalyzed transfer hydrogenation with NH 3 as a hydrogen donor

Novel lanthanide (Ln)-promoted Ni, Cu and Ag catalysts prepared by the reaction of Ni/SiO 2, Cu/SiO 2 and Ag/ZrO 2 with the dissolved Eu or Yb metals in liquid ammonia have been studied for the catalytic transfer hydrogenation of ethene, buta-1,3-diene and propyne. The transfer hydrogenation with the aid of ammonia as a hydrogen donor was greatly enhanced by the Ln-promoted catalysts. The catalytic activity for the transfer hydrogenation increased with increasing lanthanide content in the bimetallic catalysts, which evidently reflected synergetic effects between the lanthanide and transition metals. The transfer hydrogenation of ethene with ammonia over Eu–Ni/SiO 2 proceeded to form ethane and nitrogen, maintaining the following stoichiometric relationships: 3C 2H 4+2NH 3→3C 2H 6+N 2 The catalytic hydrogen transfer from ammonia to buta-1,3-diene or propyne similarly followed the stoichiometric relationships. The Ln-promoted catalysts showed substrate selectivity for the transfer hydrogenation using ammonia as the hydrogen source; Ln–Ni/SiO 2 was reactive for ethene and buta-1,3-diene, but not for propyne. Ln–Cu/SiO 2 was active for propyne and buta-1,3-diene. Ln–Ag/SiO 2 was relatively active for buta-1,3-diene. The major factors governing the catalytic transfer hydrogenation seem to be involved in the participation of the acceptor and donor molecules in the hydrogen transfer step rather than the ease of dehydrogenation of ammonia and the processes of hydrogen addition to the acceptor. The catalytic hydrogen transfer from ammonia to acceptor molecules occurs concertedly.

Site-specific reactivity of oxygen at Cu(110) step defects: an STM study of ammonia dehydrogenation

Defects on surfaces, such as steps, are known to influence catalytic activity considerably, but few microscopic observations have been made of such site-specific reactivity. Using atomic-resolution scanning tunneling microscopy (STM), we have observed the reactivity of oxygen adsorbed in the (2 × 1) structure at step sites on the Cu(110) surface with ammonia. It is found that the reactivity is high at both the top and bottom of a [11¯0] step and at the bottom of a [001] step, whereas almost no reaction occurs at the sites bordering the top of a [001] step. Reaction at the bottom of [001] steps appears to lead to deposition of copper atoms at the step edge.

Catalytic activities of polynaphthoquinone, containing various metal halides. Dehydrogenation of formic acid, cyclohexene, ammonia and alcohols, and isomerization of butene

The catalytic activities of polynaphthoquinone, containing various metal halides, such as FeCl3, VCl3, MoCl5 and TiCl4, were studied. The polynaphthoquinone was obtained by the oxidation of naphthalene-1,7-diol and successive heat-treatment. The dehydrogenation of formic acid and alcohols occurred in the temperature region 20 to 100°C over both polynaphthoquinone containing FeCl3 and metal free polynaphthoquinone. The selectivity of the dehydrogenation of isopropyl alcohol was characteristically related to the FeCl3 content of the catalyst. The dehydrogenation of cyclohexene at 200°C and the decomposition of ammonia at 300°C were also studied. The catalytic isomerization of cis-but-2-ene to trans-but-2-ene and to but-1-ene in the region 100 to 200°C was markedly affected by the nature and the quantity of the metal halides in the polynaphthoquinone.