Solvation Of Ammonia Research Articles

Understanding enhancement of strong Copper-Yttrium interactions on catalytic properties of Cu/Y-SSZ-13 for NH3-SCR

Cu-SSZ-13 represents one of the most promising catalysts for selective catalytic reduction of NOx with ammonia (NH3-SCR), but it has remained challenging to gain superior low-temperature activity and hydrothermal stability for meeting the stringent emission regulations. Here, Cu/Yx-SSZ-13 have been prepared by in situ doping of Y and subsequent ion-exchanging with Cu ions. The combined analysis of various energy spectra techniques unveils that Y ions are mainly located in the six-membered rings (6MRs) of chabazite framework and have a strong interaction with Cu cations in 8MRs, resulting in the remarkable low-temperature NOx abatement efficiency and hydrothermal stability. Mechanism studies reveal that the interplay between Y and Cu not only enhances the electron transfer from Cu to Y ions that favors the ammonia solvation process and reduces the reaction barrier, but also inhibits the aggregation of Cu sites. This work paves the way for next generation of advanced NH3-SCR catalysts.

Ammonia Solvation vs Aqueous Solvation of Samarium Diiodide. A Theoretical and Experimental Approach to Understanding Bond Activation Upon Coordination to Sm(II).

Coordination-induced desolvation or ligand displacement by cosolvents and additives is a key feature responsible for the reactivity of Sm(II)-based reagent systems. High-affinity proton donor cosolvents such as water and glycols also demonstrate coordination-induced bond weakening of the O-H bond, facilitating reduction of a broad range of substrates. In the present work, the coordination of ammonia to SmI2 was examined using Born-Oppenheimer molecular dynamics simulations and mechanistic studies, and the SmI2-ammonia system is compared to the SmI2-water system. The coordination number and reactivity of the SmI2-ammonia solvent system were found to be similar to those of SmI2-water but exhibited an order of magnitude greater rate of arene reduction by SmI2-ammonia than by SmI2-water at the same concentrations of cosolvent. In addition, upon coordination of ammonia to SmI2, the Sm(II)-ammonia solvate demonstrates one of the largest degrees of N-H bond weakening reported in the literature compared to known low-valent transition metal ammonia complexes.

Kinetics and thermodynamics of ammonia solvation on Z2Cu, ZCuOH and ZCu sites in Cu-SSZ-13 – Implications for hydrothermal aging

Temperature and ammonia (NH3) pressure dependent surface coverage of Cu-amine complexes formed on Z2Cu, ZCuOH and ZCu sites in Cu-SSZ-13 are described using a kinetic model. Consistent with results from in-situ electron paramagnetic resonance (EPR) spectra upon NH3 solvation [47] and quantification from NH3 temperature programmed desorption (TPD) experiments [33], the model considers a maximum NH3/Cu ratio of 2 for Z2Cu sites, 1 for ZCuOH sites and 2 for ZCu sites. Experimentally determined Z2Cu and ZCuOH number densities are provided as initial inputs for the kinetic model, along with density functional theory (DFT) estimated NH3 binding energies [3]. These parameters are further modified to predict the rate of NH3 adsorption and desorption during TPD experiments on oxidized and reduced catalysts. Relative adsorption free energies of surface species based on equilibrium constants are consistent with the expected nature of NH3 solvated Cu sites. Relative mobility estimates based on the entropy of Cu-amine complexes are consistent with electrostatic considerations, first-principles calculations and observations from impedance spectroscopy [3,68,69]. Desorption pre-exponentials on CuII sites are validated via in-situ diffuse reflectance infrared fourier transform spectroscopic (DRIFTS) experiments during NH3 adsorption. The resulting kinetic model is incorporated into a formalism that combines all framework aluminum sites and treats mild hydrothermal aging as a reaction involving the transformation of ZCuOH sites to Z2Cu sites. NH3 storage at 200 °C on Cu sites increases upon reduction. Modifications to the NH3 coordination on Brønsted acid (ZH) sites in the reduced catalyst upon hydrothermal aging are predicted by the model and confirmed experimentally. Accurate predictions of the distribution of Z2Cu, ZCuOH and ZH sites as a function of catalyst aging, along with predictions of the extent of NH3 solvation of these sites is an important foundation to enable quantitative descriptions of the changes in the oxidation state and SCR redox reaction rates during real-world aging.

In situ X-ray absorption study of Cu species in Cu-CHA catalysts for NH3-SCR during temperature-programmed reduction in NO/NH3

Ammonia-mediated selective catalytic reduction (NH3-SCR) using Cu-exchanged chabazite zeolites as catalysts is one of the leading technologies for NOx removal from exhaust gases, with CuII/CuI redox cycles being the basis of the catalytic reaction. The amount of CuII ions reduced by NO/NH3 can be quantified by the consumption of NO during temperature-programmed reduction experiments (NO-TPR). In this article, we show the capabilities of in situ X-ray absorption near-edge spectroscopy (XANES), coupled with multivariate curve resolution (MCR) and principal component analysis (PCA) methods, in following CuII/CuI speciation during reduction in NO/NH3 after oxidation in NO/O2 at 50 °C on samples with different copper loading and pretreatment conditions. Our XANES results show that during the NO/NH3 ramp CuII ions are fully reduced to CuI in the 50–290 °C range. The number of species involved in the process, their XANES spectra and their concentration profiles as a function of the temperature were obtained by MCR and PCA. Mixed ligand ammonia solvated complexes [CuII(NH3)3(X)]+ (X = OH−/O− or NO3−) are present at the beginning of the experiment, and are transformed into mobile [CuI(NH3)2]+ complexes: these complexes lose an NH3 ligand and become framework-coordinated above 200 °C. In the process, multiple CuII/CuI reduction events are observed: the first one around 130 °C is identified with the reduction of [CuII(NH3)3(OH/O)]+ moieties, while the second one occurs around 220–240 °C and is associated with the reduction of the ammonia-solvated Cu-NO3− species. The nitrate concentration in the catalysts is found to be dependent on the zeolite Cu loading and on the applied pretreatment conditions. Ammonia solvation increases the number of CuII sites available for the formation of nitrates, as confirmed by infrared spectroscopy.

The interaction of ammonia with the protic ionic liquid ethylammonium nitrate: A simulation study

In this paper, we present a molecular dynamics study of ammonia solutions in a protic ionic liquid with a structurally similar cation, ethylammonium nitrate. Using two different models of the ammonia molecule, a conventional 4-site model and a 6-site model optimizing the dipolar and quadrupolar electron distributions, we evaluate their effect on the microstructure and single-particle dynamics of these mixtures, as well as on their behaviour at the electrochemical interface. Quite significant differences are found on the solvation of ammonia in these dense ionic solvents, the 6-atom model predicting a stronger affinity of solvent molecules for the cations of the solvent as well as a caging effect on NH3 solvation shells, not predicted by the 4-site model. Finally, we discuss the structure of the liquid layers near charged graphene walls. The structure in the layer next to the wall varies with the charge on the wall and varies with the model used for ammonia.

Ligation Kinetics as a Probe for Non-Covalent Electrostatic Bonding and Electron Solvation of Alkali and Alkaline Earth Cations with Ammonia.

Mono-ligation kinetics were measured for ammonia reacting with atomic cations in the first two groups of the periodic table (K+, Rb+, Cs+ and Ca+, Sr+, Ba+). Also, mono-ligation energies were computed using density functional theory (DFT) in an attempt to assess the role of non-covalent electrostatic interactions in these chemical reactions. The measurements were performed at room temperature in helium bath gas at 0.35Torr using an inductively coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer. Rate coefficients are reported for ammonia addition, the only reaction channel that was observed with all these cations. A systematic decrease in the rate of addition of NH3 was observed for both group 1 and 2 cations going down the periodic table. The computational studies predict a decrease in the adduct binding energy and an increase in the bond separation going down groups 1 and 2 of the periodic table and provide some insight into the role of the extra selectron in the group 2 radical cations in ligand bonding. A correlation is seen between the efficiency of ligation and the binding energy of the adduct ion and attributed to the lifetime of the intermediate encounter complex against back dissociation which is dependent on its well depth. Higher-order additions of ammonia were also observed. Remarkable differences in the extent and kinetics were seen between the group 1 and 2 cations, and these were attributed to the occurrence of ammonia solvation of the extra s electron in the higher-order adducts of the alkaline earth cations.

Comment on “Solubilities of ammonia in basic imidazolium ionic liquids” [Fluid Phase Equilib. 297 (2010) 34–39

The enthalpies of solvation of ammonia in imidazolium-based ionic liquids (and other associated thermodynamic properties) are re-analyzed based on the data of [Fluid Phase Equilib. 297 (2010) 34–39], correcting numerical errors in the original analysis. In contrast to the original conclusions, the re-analyzed data demonstrates that the enthalpy of solvation is not a strong function of alkyl chain length. Errors and uncertainties of the analysis procedure are also discussed, suggesting that the strong observed temperature dependence of enthalpy may in fact be an artifact of the systematic neglect of the non-ideality of the ammonia solute.

Aqueous Solvation of Ammonia and Ammonium: Probing Hydrogen Bond Motifs with FT-IR and Soft X-ray Spectroscopy.

In a multifaceted investigation combining local soft X-ray and vibrational spectroscopic probes with ab initio molecular dynamics simulations, hydrogen-bonding interactions of two key principal amine compounds in aqueous solution, ammonia (NH3) and ammonium ion (NH4+), are quantitatively assessed in terms of electronic structure, solvation structure, and dynamics. From the X-ray measurements and complementary determination of the IR-active hydrogen stretching and bending modes of NH3 and NH4+ in aqueous solution, the picture emerges of a comparatively strongly hydrogen-bonded NH4+ ion via N-H donating interactions, whereas NH3 has a strongly accepting hydrogen bond with one water molecule at the nitrogen lone pair but only weakly N-H donating hydrogen bonds. In contrast to the case of hydrogen bonding among solvent water molecules, we find that energy mismatch between occupied orbitals of both the solutes NH3 and NH4+ and the surrounding water prevents strong mixing between orbitals upon hydrogen bonding and, thus, inhibits substantial charge transfer between solute and solvent. A close inspection of the calculated unoccupied molecular orbitals, in conjunction with experimentally measured N K-edge absorption spectra, reveals the different nature of the electronic structural effects of these two key principal amine compounds imposed by hydrogen bonding to water, where a pH-dependent excitation energy appears to be an intrinsic property. These results provide a benchmark for hydrogen bonding of other nitrogen-containing acids and bases.

Undissociated versus Dissociated Structures for Water Clusters and Ammonia−Water Clusters: (H2O)n and NH3(H2O)n−1 (n = 5, 8, 9, 21). Theoretical Study

To understand the autoionization of pure water and the solvation of ammonia in water, we investigated the undissociated and dissociated (ion-pair) structures of (H2O) n and NH3(H2O)n-1 (n = 5, 8, 9, 21) using density functional theory (DFT) and second order Moller-Plesset perturbation theory (MP2). The stability, thermodynamic properties, and infrared spectra were also studied. The dissociated (ion-pair) form of the clusters tends to favor the solvent-separated ion-pair of H3O+/NH4+ and OH-. As for the NH3(H2O)20 cluster, the undissociated structure has the internal conformation, in contrast to the surface conformation for the (H2O)21 cluster, whereas the dissociated structure of NH3(H2O)20 has the surface conformation. As the cluster size of (H2O)n/NH3(H2O)n-1 increases, the difference in standard free energy between undissociated and dissociated (ion-pair) clusters is asymptotically well corroborated with the experimental free energy change at infinite dilution of H3O+/NH4+ and OH-. The predicted NH and OH stretching frequencies of the undissociated and dissociated (ion-pair) clusters are discussed.

Reactivity of anomeric 1alpha- and 1beta-pentofuranosyl azide derivatives in ammonia chemical ionization.

Electron impact (EI), fast atom bombardment (FAB) and ammonia chemical ionization [CI(NH3)] mass spectrometry were applied with the aim of differentiating between the anomeric 1alpha- and 1beta-azidopentofuranosyl derivatives. Calculated ammonium affinities [AA(M)] and proton affinities [PA(M)] show that beta-anomers have higher affinities for H+ and NH4+ ions than alpha-azides. Protonated molecules, obtained by CI(NH3) of azidofuranosyl derivatives, lose HN3 giving abundant furanosyl (S+) ions. Ammonia solvation of MH+ ions competes with the previous reaction producing the [SNHN2NH3]+ ion, a competitive product to the ammonium-attached [SN3NH4]+ ion. The fragmentation pathways of the stable and metastable [MNH4]+, MH+ ions, and several other important fragment ions, were determined using mass analyzed ion kinetic energy spectrometry (MIKES). The abundance of the [SN3NH4]+ and/or [SNHN2NH3]+ ions was found to correlate inversely with the exothermicity of ammonia solvation of the MH+ ion. The abundance of the fragment ions [SNHNH3]+, [SNH3]+ and SNH+ in some examples correlates with the exothermicity of the corresponding [MNH4]+ and MH+ parent ion formation. The fragment ions SNH3+ and SNHNH3+ can be formed, at least in part, in the ammonia solvation reaction of the S+ and SNH+ ions taking place within the high-pressure region of the CI ion source.

Ammonia adsorption by hydrogen bond on ice and its solvation

Regarding the solvation of molecules to water, the adsorption of molecules on the water surface has mostly been considered. Here we provide spectroscopic evidence for the adsorption and solvation behavior of ammonia on the ultra thin ice film surface formed on Ru(001) by the use of infrared reflection absorption and thermal desorption spectroscopies. Here we prove that the solvation of ammonia involves two steps. They are the hydrogen bond adsorption and the transfer into bulk. The hydrogen bonding adsorbed ammonia on ultra thin ice film in a NH3 form is evidenced for the first time. Upon heating, bulk transfer upon a conversion to a NH4+ form is observed for this hydrogen bonded species, however it is not for bilayer and multilayer species. © 2000 American Institute of Physics.

Ab Initio Studies for Geometrical Structures of Ammonia Cluster Cations

Structures of unprotonated (NH3)n+ (n = 1−6), protonated NH4+(NH3)n-1 (n = 1−6), and proton-transferred (NH4+−NH2)(NH3)n-2 (n = 3−7)] ammonia cluster cations have been optimized with ab initio Hartree−Fock (HF) and second-order Moller−Plesset (MP2)/6-31+G** levels, and the harmonic vibrational frequencies have also been evaluated. In ammonia cluster cations, NH3+, NH4+, and NH4+−NH2 form as a central core of the first ammonia solvation shell, respectively. In unprotonated dimer cation, the totally symmetric structure with the head-to-head interaction is optimized to be stable. In the hydrogen-bonded dimer cation, the unprotoned NH3···NH3+ cation is optimized to the protonated-transferred NH4+···NH2 cation. In unprotonated trimer cation, there are two types of isomers (hydrogen-bonded and head-to-head). The hydrogen-bonded type of unprotonated trimer cation is more stable. In unprotonated pentamers and hexamers, a NH3+ core has both interactions in a complex. On the other hand, in unprotonated tetramer, pr...

ChemInform Abstract: Sequential Solvation of Atomic Transition‐Metal Ions. The Second Solvent Molecule Can Bind More Strongly Than the First.

AbstractSequential water and ammonia solvation energies are determined for V+, Cr+, Mn+, Fe+, Co+, and Ni+ by means of collision‐induced dissociation in a triple quadrupole mass spectrometer.