Density functional theory calculations were performed to investigate the adsorption and hydration of an ammonium ion (NH4+) confined in the interlayer space of montmorillonites (MMT). NH4+ is trapped in the six-oxygen-ring on the internal surface and forms a strong binding with the surface O atoms. The hydration of NH4+ is affected significantly by the surface. Water molecules prefer the surface sites, and do not bind with the NH4+ unless enough water molecules are supplied. Moreover, the water molecules involved in NH4+ hydration tend to bind with the surface simultaneously. The hydration energy increases with the intercalated water molecules, in contrast to that in gas phase. In addition, the hydration leads to the extension of MMT basal spacing.
In the synthesis of cyanuric acid from NH3 and CO2, urea and isocyanic acid OCNH are two pivotal intermediates. Based on density functional theory (DFT) calculations, the synthesis mechanism of cyanuric acid from NH3 + CO2 was investigated systematically. Urea can be synthesized from NH3 and CO2, and cyanuric acid can be obtained from urea or NH3 + CO2. In the stepwise mechanism of cyanuric acid from urea or NH3 + CO2, the energy barriers are relatively high, and the condition of high pressure and temperature does not decrease the energy barriers. Our theoretical model shows that cyanuric acid is actually acquired from OCNH via a one-step cycloaddition reaction.
Figure
The synthesis mechanism of cyanuric acid from NH3 and CO2 was revealed systematically with density functional theory methods relative to 3NH3 + 3CO2相似文献
The adsorption of CO onto Ni-doped boron nitride nanotubes (BNNTs) was investigated using density functional theory at the B3LYP/LanL2DZ level of theory. The structures of the Ni-doped BNNTs and their CO-adsorbed configurations were obtained. It was found that the strength of adsorption of CO onto Ni-doped perfect BNNTs is higher than that on defective BNNTs. The electronic properties of all of the adsorption configurations of CO on Ni-doped BNNTs are reported.
Figure
The optimized structures of CO adsorption on Ni-doped BNNTs 相似文献
Physical and chemical adsorption of CO2 on ZnO surfaces were studied by means of two different implementations of periodic density functional theory. Adsorption energies were computed and compared to values in the literature. In particular, it was found that the calculated equilibrium structure and internuclear distances are in agreement with previous work. CO2 adsorption was analyzed by inspection of the density of states and electron localization function. Valence bands, band gap and final states of adsorbed CO2 were investigated and the effect of atomic displacements analyzed. The partial density of states (PDOS) of chemical adsorption of CO2 on the ZnO(0001) surface show that the p orbitals of CO2 were mixed with the ZnO valence band state appearing at the top of the valence band and in regions of low-energy conduction band.
Figure
ELF analysis of bidentate and tridentate chemical adsorptions 相似文献
We have investigated the interaction between open-ended zig-zag single-walled carbon nanotube (SWCNT) and a few benzene derivatives using the first-principles van der Waals density functional (vdW-DF) method, involving full geometry optimization. Such sp2-like materials are typically investigated using conventional DFT methods, which significantly underestimate non-local dispersion forces (vdW interactions), therefore affecting interactions between respected molecules. Here, we considered the vdW forces for the interacting molecules that originate from the interacting π electrons of the two systems. The ?0.54 eV adsorption energy reveals that the interaction of benzene with the side wall of the SWCNT is typical of the strong physisorption and comparable with the experimental value for benzene adsorption onto the graphene sheet. It was found that aromatics are physisorbed on the sidewall of perfect SWCNTs, as well as at the edge site of the defective nanotube. Analysis of the electronic structures shows that no orbital hybridization between aromatics and nanotubes occurs in the adsorption process. The results are relevant in order to identify the potential applications of noncovalent functionalized systems.
Figure
First-principles van der Waals density functional (vdW-DF) calculations show that aromatics are physisorbed on the side wall of perfect single-walled carbon nanotubes (SWCNTs) as well as at the edge site of defective nanotubes 相似文献
The structure and properties of natural gas hydrates containing hydrocarbons, CO2, and N2 molecules were studied by using computational quantum chemistry methods via the density functional theory approach. All host cages involved in I, II, and H types structures where filled with hydrocarbons up to pentanes, CO2 and N2 molecules, depending on their size, and the structures of these host–guest systems optimized. Structural properties, vibrational spectra, and density of states were analyzed together with results from atoms-in-a-molecule and natural bond orbitals methods. The inclusion of dispersion terms in the used functional plays a vital role for obtaining reliable information, and thus, B97D functional was shown to be useful for these systems. Results showed remarkable interaction energies, not strongly affected by the type of host cage, with molecules tending to be placed at the center of the cavities when host cages and guest molecules cavities are of similar size, but with molecules approaching hexagonal faces for larger cages. Vibrational properties show remarkable features in certain regions, with shiftings rising from host-guest interactions, and useful patterns in the terahertz region rising from water surface vibrations strongly coupled with guest molecules. Likewise, calculations on crystal systems for the I and H types were carried out using a pseudopotential approach combined with Grimme’s method to take account of dispersion.
Figure
Density Funcional Theory methods were used to study structural, energetic and spectroscopical properties of natural gas hydrates. 相似文献
Vanadium-based catalysts are used in many technological processes, among which the removal of nitrogen oxides (NOx) from waste gases is one of the most important. The chemical reaction responsible for this selective catalytic reaction (SCR) is based on the reduction of NOx molecules to N2, and a possible reductant in this case is pre-adsorbed NH3. In this paper, NH3 adsorption on Brønsted OH acid centers on low-index surfaces of V2O5 (010, 100, 001) is studied using a theoretical DFT method with a gradient-corrected functional (RPBE) in the embedded cluster approximation model. The results of the calculations show that ammonia molecules are spontaneously stabilized on all low-index surfaces of the investigated catalyst, with adsorption energies ranging from ?0.34 to ?2 eV. Two different mechanisms of ammonia adsorption occur: the predominant mechanism involves the transfer of a proton from a surface OH group and the stabilization of ammonia as an NH4+ cation bonded to surface O atom(s), while an alternative mechanism involves the hydrogen bonding of NH3 to a surface OH moiety. The latter binding mode is present only in cases of stabilization over a doubly coordinated O(2) center at a (100) surface. The results of the calculations indicate that a nondirectional local electrostatic interaction with ammonia approaching a surface predetermines the mode of stabilization, whereas hydrogen-bonding interactions are the main force stabilizing the adsorbed ammonia. Utilizing the geometric features of the hydrogen bonds, the overall strength of these interactions was quantified and qualitatively correlated (R?=?0.93) with the magnitude of the stabilization effect (i.e., the adsorption energy).
Figure
Two different modes (NH3/NH4+) of ammonia adsorption on the (001)V2O5 net plane. 相似文献
To explore the adsorption mechanism of NO, NH3, N2 on a carbon surface, and the effect of basic and acidic functional groups, density functional theory was employed to investigate the interactions between these molecules and carbon surfaces. Molecular electrostatic potential, Mulliken population analyses, reduced density gradient, and Mayer bond order analyses were used to clarify the adsorption mechanism. The results indicate that van der Waals interactions are responsible for N2 physisorption, and N2 is the least likely to adsorb on a carbon surface. Modification of carbon materials to decorate basic or acidic functional groups could enhance the NH3 physisorption because of hydrogen bonding or electrostatic interactions, however, NO physisorption on a carbon surface is poor. Zig-zag sites are more reactive than armchair sites when these gas molecules absorb on the edge sites of carbon surface.
Molecular dynamic (MD) simulations have been performed to study the behaviors of ten kinds of cyclo-hexa-peptides (CHPs) composed of amino acids with the diverse hydrophilic/hydrophobic side chains at the water/cyclohexane interface. All the CHPs take the “horse-saddle” conformations at the interface and the hydrophilicity/hydrophobicity of the side chains influences the backbones’ structural deformations. The orientations and distributions of the CHPs at the interface and the differences of interaction energies (ΔΔE) between the CHPs and the two liquid phases have been determined. RDF analysis shows that the H-bonds were formed between the OC atoms of the CHPs’ backbones and Hw atoms of water molecules. N atoms of the CHPs’ backbones formed the H-bonds or van der Waals interactions with the water solvent. It was found that there is a parallel relationship between ΔΔE and the lateral diffusion coefficients (Dxy) of the CHPs at the interface. The movements of water molecules close to the interface are confined to some extent, indicating that the dynamics of the CHPs and interfacial water molecules are strongly coupled.
Figure
Scheme of the ten kinds of CHPs formed by even alternating D- and L- amino acids with the different hydrophilic/hydrophobic side chains. The letters in the parentheses stand for the abbreviations of the composed amino acids in the CHPs 相似文献
To assess the effects of atmospheric N deposition on the C budget of an alpine meadow ecosystem on the Qinghai–Tibetan Plateau, it is necessary to explore the responses of soil-atmosphere carbon dioxide (CO2) exchange to N addition.
Methods
Based on a multi-form, low-level N addition experiment, soil CO2 effluxes were monitored weekly using the static chamber and gas chromatograph technique. Soil variables and aboveground biomass were measured monthly to examine the key driving factors of soil CO2 efflux.
Results
The results showed that low-level N input tended to decrease soil moisture, whereas medium-level N input maintained soil moisture. Three-year N additions slightly increased soil inorganic N pools, especially the soil NH4+-N pool. N applications significantly increased aboveground biomass and soil CO2 efflux; moreover, this effect was more significant from NH4+-N than from NO3?-N fertilizer. In addition, the soil CO2 efflux was mainly driven by soil temperature, followed by aboveground biomass and NH4+-N pool.
Conclusions
These results suggest that chronic atmospheric N deposition will stimulate soil CO2 efflux in the alpine meadow on the Qinghai–Tibetan Plateau by increasing available N content and promoting plant growth. 相似文献
In this paper, we assessed the quantum mechanical level of theory for prediction of linear and nonlinear optical (NLO) properties of push-pull organic molecules. The electric dipole moment (μ), mean polarizability (〈α〉) and total static first hyperpolarizability (βt) were calculated for a set of benzene, styrene, biphenyl and stilbene derivatives using HF, MP2 and DFT (31 different functionals) levels and over 71 distinct basis sets. In addition, we propose two new basis sets, NLO-V and aNLO-V, for NLO properties calculations. As the main outcomes it is shown that long-range corrected DFT functionals such as M062X, ωB97, cam-B3LYP, LC-BLYP and LC-ωPBE work satisfactorily for NLO properties when appropriate basis sets such as those proposed here (NLO-V or aNLO-V) are used. For most molecules with β ranging from 0 to 190 esu, the average absolute deviation was 13.2 esu for NLO-V basis sets, compared to 27.2 esu for the standard 6-31 G(2d) basis set. Therefore, we conclude that the new basis sets proposed here (NLO-V and aNLO-V), together with the cam-B3LYP functional, make an affordable calculation scheme to predict NLO properties of large organic molecules.
Figure
Calculated values for total static first hyperpolarizability (βt) for 4-amino-4′-nitrostilbene at cam-B3LYP/basis set level. Experimental from Cheng et al. [1, 2]. 相似文献
The first step in the mechanism of n-butane oxidative dehydrogenation (ODH) on a V4O10 cluster and V4O10 supported SBA-15 is examined using DFT method. The activation and adsorption energies, oxidation state of V atoms are calculated. Over V4O10 the obtained results indicate that the activation of C-H bond of methylene group can occur at both the terminal and the bridging oxygen atoms with similar barrier (21.5–22.5 kcal mol?1). The role of SBA-15 (with and without modification by Al) in n-butane adsorption step has been studied in detail. SBA-15 itself has mild effect on the reaction process, but the substitution of silicon atoms by aluminum atoms results in an active supporter for V2O5 in ODH reaction. In that, the ratio of Si/Al will decide the direction of initial interaction steps between n-butane and catalyst surface and it will result in the selectivity of the reaction products.
Figure
Transition state of adsorption of n-C4H10 over V4O10/SBA-15(Si8Al) 相似文献
Density functional theory calculations were performed to examine the effect of a C vacancy on the physisorption of H2 onto Ti-functionalized C60 fullerene when H2 is oriented along the x-, y-, and z-axes of the fullerene. The effect of the C vacancy on the physisorption modes of H2 was investigated as a function of H2 binding energy within the energy window (?0.2 to ?0.6 eV) targeted by the Department of Energy (DOE), and as functions of a variety of other physicochemical properties. The results indicate that the preferential orientations of H2 in the defect-free (i.e., no C vacancy) C60TiH2 complex are along the x- and y-axes of C60 (with adsorption energies of ?0.23 and ?0.21 eV, respectively), making these orientations the most suitable ones for hydrogen storage, in contrast to the results obtained for defect-containing fullerenes. The defect-containing (i.e., containing a C vacancy) C59TiH2 complex do not exhibit adsorption energies within the targeted energy range. Charge transfer occurs from Ti 3d to C 2p of the fullerene. The binding of H2 is dominated by the pairwise support–metal interaction energy E(i)Cn...Ti, and the role of the fullerene is not restricted to supporting the metal. The C vacancy enhances the adsorption energy of Ti, in contrast to that of H2. A significant reduction in the energy gap of the pristine C60 fullerene is observed when TiH2 is adsorbed by it. While the Cn fullerene readily participates in nucleophilic processes, the adjacent TiH2 fragment is available for electrophilic processes.
Figure
The effect of a C vacancy on the interaction of H2 with Ti-functionalized C60 fullerene. H2 preferentially orients itself along the x- and y-axes of C60, yielding adsorption energies in the energy window targeted by the DOE. The C vacancy enhances the adsorption energy of Ti, in contrast to that of H2. The role of fullerene is not restricted to supporting the metal. The physicochemical properties investigated in the present work characterize the H2 interaction 相似文献
The objective of this study was to test the suitability of greenwaste biochar to aid nitrogen (N) retention in rehabilitated bauxite-processing residue sand (BRS).
Methods
Bauxite residue sand was collected from the Alcoa of Australia Pinjarra refinery. The pH of BRS was adjusted to values of 5, 7, 8 and 9 and subsequently amended with different rates (1, 5, 10 and 20 %, w/w) of greenwaste biochar. The loss of N via NH3 volatilization following addition of di-ammonium phosphate (DAP) was determined using an acid trapping method.
Results
At low pH (5), increasing pH rather than adsorption capacity, resulting from biochar addition, caused greater losses of N through volatilization from BRS. In BRS with medium pH (7, 8), increasing adsorption capacity, induced by biochar addition, played the more dominant role in enhancing adsorption of NH4+-N /NH3-N and lowering NH3 volatilization. In the BRS with high pH (9), the majority of NH4+-N /NH3-N pools was lost via NH3 volatilization due to the strong acid-base reaction at this pH.
Conclusions
It is concluded that the interaction of changes in pH and adsorption capacity induced by greenwaste biochar addition affects the availability and dynamics of NH4+-N/ NH3-N in BRS amended with DAP. 相似文献
Density functional theory (DFT) was used to investigate the ruthenium hydride-catalyzed regioselective addition reactions of benzaldehyde to isoprene leading to the branched β,γ-unsaturated ketone. All intermediates and the transition states were optimized completely at the B3LYP/6-31?G(d,p) level (LANL2DZ(f) for Ru, LANL2DZ(d) for P and Cl). Calculated results indicated that three catalysts RuHCl(CO)(PMe3)3 (1), RuH2(CO)(PMe3)3 (2), and RuHCl(PMe3)3 (3) exhibited different catalysis, and the first was the most excellent. The most favorable reaction pathway included the coordination of 1 to the less substituted olefin of isoprene, a hydrogen transfer reaction from ruthenium to the carbon atom C1, the complexation of benzaldehyde to ruthenium, the carbonyl addition, and the hydride elimination reaction. The carbonyl addition was the rate-determining step. The dominant product was the branched β,γ-unsaturated ketone. Furthermore, the presence of one toluene molecule lowered the activation free energy of the transition state of the carbonyl addition by hydrogen bonds between the protons of toluene and the chlorine, carbonyl oxygen of the ruthenium complex. On the whole, the solvent effect decreased the free energies of the species.
Figure
DFT study suggests that RuHCl(CO)(PMe3)3 has better catalysis than RuH2(CO)(PMe3)3 and RuHCl(PMe3)3 in the regioselective addition reactions of benzaldehyde to isoprene leading to the branched β,γ-unsaturated ketone. 相似文献
Mammalian glycosylated rhesus (Rh) proteins include the erythroid RhAG and the nonerythroid RhBG and RhCG. RhBG and RhCG are expressed in multiple tissues, including hepatocytes and the collecting duct (CD) of the kidney. Here, we expressed human RhAG, RhBG and RhCG in Xenopus oocytes (vs. H2O-injected control oocytes) and used microelectrodes to monitor the maximum transient change in surface pH (ΔpHS) caused by exposing the same oocyte to 5 % CO2/33 mM HCO3? (an increase) or 0.5 mM NH3/NH4+ (a decrease). Subtracting the respective values for day-matched, H2O-injected control oocytes yielded channel-specific values (*). $({\Updelta {\text{pH}}_{\text{S}}^{*} })_{{{\text{CO}}_{ 2} }}$ and $({ - \Updelta {\text{pH}}_{\text{S}}^{*} })_{{{\text{NH}}_{ 3} }}$ were each significantly >0 for all channels, indicating that RhBG and RhCG—like RhAG—can carry CO2 and NH3. We also investigated the role of a conserved aspartate residue, which was reported to inhibit NH3 transport. However, surface biotinylation experiments indicate the mutants RhBGD178N and RhCGD177N have at most a very low abundance in the oocyte plasma membrane. We demonstrate for the first time that RhBG and RhCG—like RhAG—have significant CO2 permeability, and we confirm that RhAG, RhBG and RhCG all have significant NH3 permeability. However, as evidenced by $({\Updelta {\text{pH}}_{\text{S}}^{*} })_{{{\text{CO}}_{ 2} }} /({ - \Updelta {\text{pH}}_{\text{S}}^{*} })_{{{\text{NH}}_{ 3} }}$ values, we could not distinguish among the CO2/NH3 permeability ratios for RhAG, RhBG and RhCG. Finally, we propose a mechanism whereby RhBG and RhCG contribute to acid secretion in the CD by enhancing the transport of not only NH3 but also CO2 across the membranes of CD cells. 相似文献