H2 Atoms Research Articles

Promoting effect of Au on Pd/TiO2 catalyst for the selective catalytic reduction of NOx by H2

Au promoted Pd/TiO2 catalysts with different Pd loading were investigated for the selective catalytic reduction of NOx by hydrogen (H2-SCR). The results showed that 1%Pd-0.5%Au/TiO2 catalyst exhibited the highest NOx conversion and N2 selectivity, as well as a wide activity temperature window. Moreover, the 1%Pd-0.5%Au/TiO2 catalyst offered remarkable resistance to high gas hourly space velocity (GHSV). XRD, XPS and TEM results illustrated that the excellent H2-SCR performance of 1%Pd-0.5%Au/TiO2 catalyst was closely related to the formed PdAu alloy and the higher dispersion of active sites compared with 1%Pd/TiO2. In-situ DRIFTS results revealed that more and new adsorbed nitrogen oxides species, as well as more adsorbed NH3 and NH4+ active species generated over 1%Pd-0.5%Au/TiO2 catalyst, thus accelerating the ability to active H2 atom and the subsequent reaction with adsorbed nitrogen oxides species.

Another Unprecedented Wieland Mechanism Confirmed: Hydrogen Formation from Hydrogen Peroxide, Formaldehyde, and Sodium Hydroxide.

In 1923, Wieland and Wingler reported that in the molecular hydrogen producing reaction of hydrogen peroxide with formaldehyde in basic solution, free hydrogen atoms (H. ) are not involved. They postulated that bis(hydroxymethyl)peroxide, HOCH2 OOCH2 OH, is the intermediate, which decomposes to yield H2 and formate, proposing a mechanism that would nowadays be considered as a "concerted process". Since then, several other (conflicting) "mechanisms" have been suggested. Our NMR and Raman spectroscopic and kinetic studies, particularly the determination of the deuterium kinetic isotope effect (DKIE), now confirm that in this base-dependent reaction, both H atoms of H2 derive from the CH2 hydrogen atoms of formaldehyde, and not from the OH groups of HOCH2 OOCH2 OH or from water. Quantum-chemical CBS-QB3 and W1BD computations show that H2 release proceeds through a concerted process, which is strongly accelerated by double deprotonation of HOCH2 OOCH2 OH, thereby ruling out a free radical pathway.

Half-Sandwich Ruthenium Carbene Complexes Link trans-Hydrogenation and gem-Hydrogenation of Internal Alkynes.

The hydrogenation of internal alkynes with [Cp*Ru]-based catalysts is distinguished by an unorthodox stereochemical course in that E-alkenes are formed by trans-delivery of the two H atoms of H2. A combined experimental and computational study now provides a comprehensive mechanistic picture: a metallacyclopropene (η2-vinyl complex) is primarily formed, which either evolves into the E-alkene via a concerted process or reacts to give a half-sandwich ruthenium carbene; in this case, one of the C atoms of the starting alkyne is converted into a methylene group. This transformation represents a formal gem-hydrogenation of a π-bond, which has hardly any precedent. The barriers for trans-hydrogenation and gem-hydrogenation are similar: whereas DFT predicts a preference for trans-hydrogenation, CCSD(T) finds gem-hydrogenation slightly more facile. The carbene, once formed, will bind a second H2 molecule and evolve to the desired E-alkene, a positional alkene isomer or the corresponding alkane; this associative pathway explains why double bond isomerization and over-reduction compete with trans-hydrogenation. The computed scenario concurs with para-hydrogen-induced polarization transfer (PHIP) NMR data, which confirm direct trans-delivery of H2, the formation of carbene intermediates by gem-hydrogenation, and their evolution into product and side products alike. Propargylic -OR (R = H, Me) groups exert a strong directing and stabilizing effect, such that several carbene intermediates could be isolated and characterized by X-ray diffraction. The gathered information spurred significant preparative advances: specifically, highly selective trans-hydrogenations of propargylic alcohols are reported, which are compatible with many other reducible functional groups. Moreover, the ability to generate metal carbenes by gem-hydrogenation paved the way for noncanonical hydrogenative cyclopropanations, ring expansions, and cycloadditions.

The effect of the diameter of cyclic peptide nanotube on its chirality discrimination

In this work, the transport behaviors of the enantiomers of lactic acid (LA) in two cyclic peptide nanotubes (CPNTs) with different diameters were studied using steered molecular dynamic (SMD) simulation to investigate the effect of the diameter of CPNT on the discrimination of the enantiomers of LA. For this purpose, two cyclic peptides with two different sizes ([Ala-D-Ala-L]5 and [Ala-D-Ala-L]4) were used for constructing two CPNTs so that each CPNT was composed of eight cyclic peptide units. The docking calculations were performed to obtain the appropriate position of each enantiomer at the lumen of each CPNT. The variation of the pulling force versus time, exerted on the enantiomers moving in the CPNTs was calculated using the SMD simulations with two different strategies (positional and directional).The obtained results showed that the diameter of CPNT has considerable effect on the discrimination of the LA enantiomers so that the increase of the diameter of CPNT, increased the velocity difference between two enantiomers and improved the performance of CPNT for the chirality discrimination. The SMD simulations indicated that the velocity of S-enantiomer became more than R-enantiomer and its motion became more comfortable than R-enantiomer when the diameter of CNPT increased. The RDFs of the H and O atoms of the LA enantiomers relative to the O atoms of CPNT were calculated and it was found that the increase of the diameter of CPNT creates the significant changes in the RDFs of H1, H2 and H3 atoms of the enantiomers.

Cross-sections and rate coefficients calculations for rotational excitation of cyanoethynylide ions () induced by collision with He atoms at low temperature

The focal points of the present paper are the calculations of cross-sections and rate coefficients for rotational (de-)excitation of C3N− induced by collision with He and H2 atoms for energies of up to 2000 cm−1 and temperatures ranging from 1 K to 300 K. For this study, the new two-dimensional potential energy surface (PES) of the C3N−−He complex has been calculated at the ab initio restricted coupled cluster level of theory, including single, double and perturbative triple excitation [RCCSD(T)-F12a] with the cc-pVTZ-F12 basis set augmented by mid-bond functions. Basis set superposition errors were taken into account. To allow for the dynamical calculations, the PES was then fitted to a global analytical form, the main features of which are discussed. Collisional cross-sections among the first 17 rotational levels were calculated and employed to compute downward and upward rate coefficients. A propensity towards was observed.

Molecular mechanics and dynamics simulation of hydrogen diffusion in aluminum melt

The main impurities in aluminum melt are hydrogen and Al2O3, which can deteriorate melt quality and materials performance. However, the diffusion process of H atoms in aluminum melt and the interactions among Al atoms, Al2O3 and hydrogen have been studied rarely. Molecular mechanics and dynamics simulations are employed to study the diffusion behaviors of different types of hydrogen, such as free H atoms, H atoms in H2 and H+ ions in H2O using COMPASS force field. Correspondingly, force field types h, h1h and h1o are used to describe different types of hydrogen which are labeled as Hh, Hh1h and Hh1o. The results show that the adsorption areas are maximum for Hh1o, followed by Hh1h and Hh. The diffusion ability of Hh1o is the strongest whereas Hh is hard to diffuse in aluminum melt because of the differences in radius and potential well depth of various types of hydrogen. Al2O3 cluster makes the Al atoms array disordered, creating the energy conditions for hydrogen diffusion in aluminum melt. Al2O3 improves the diffusion of Hh and Hh1o, and constrains Hh1h which accumulates around it and forms gas porosities in aluminum. Hh1o is the most dispersive in aluminum melt, moreover, the distance of Al-Hh1o is shorter than that of Al-Hh1h, both of which are detrimental to the removal of Hh1o. The simulation results indicate that the gas porosities can be eliminated by the removal of Al2O3 inclusions, and the dispersive hydrogen can be removed by adsorption function of gas bubbles or molten fluxes.

Isotope analysis for understanding the hydrogen transfer mechanism in direct liquefaction of Bulianta coal

Direct coal liquefaction (DCL) was investigated using a deuterium tracer method which makes it possible to determine which structural positions in the coal react with H2 or hydrogen-donor solvents, i.e. hydrogen-transfer mechanism. D2 and/or 1,2,3,4-d4-tetralin (tetralin-d4) were used in DCL of Bulianta coal at 7.0MPa (initial charge pressure) and 445°C for 0 or 60min in a tubing-bomb microreactor. 2H NMR and isotope ratio mass spectrometry (IRMS) were used to determine the distribution of deuterium in liquefaction solvent and products qualitatively and quantitatively. The 2H NMR of solvents after DCL under a variety of conditions showed that the α-radical of tetralin is an important intermediate in hydrogen transfer to and exchange with coal, and the de/re-hydrogenation cycle of tetralin is not a dominating approach of hydrogen transfer in DCL. The distribution of deuterium in each structural position and the D/1H in preasphaltenes and asphaltenes (PAA) under a variety of conditions showed that both hydrogen atoms in tetralin and H2 could enter alkyl groups remote from aromatic nucleus of PAA in preference to aromatic nucleus and α to aromatic groups of PAA. The hydrogen atoms in tetralin are more easily to stabilize radicals from coal cracking to form PAA than those in H2 during the heating-up stage of DCL. Both the kinetic isotope effect and electrophilic aromatic substitution reaction in DCL, indicating the existence of ionic reaction process besides a radical reaction process in DCL.

Mo3VOx catalyst in biomass conversion: A review in structural evolution and reaction pathways

Mo3VOx heterogeneous catalyst is among the complex metal oxides that have piqued recent interest due to its superior ýcatalytic properties. Its synthesis via sol gel or hydrothermal methods focused on the production of pentagonal polyoxomolybdate unit in the reaction precursor assembles with other V and Mo ions to form crystalline MoV oxides with pentagonal, hexagonal, and heptagonal units in either trigonal or orthorhombic phases. Moreover, the orthorhombic structure of Mo3VOx catalyst exhibited particularly high catalytic activity for selective oxidation of ethane, propane, and acrolein, whereby the reaction can take place not only in the mouth of heptagonal channels, but also in the entirety of the Mo3VO4 channel. Indeed, the Vanadyl groups with vanadium at the crystallographic positions are accountable for C–H activation of propane in the rate determining abstraction of the first H2 atoms. The molybdenum surface enrichment is detrimental with respect to the selectivity of the reaction. With that advantage, Mo3VOx has been applied as a catalyst for the acrolein oxidation to acrylic acid, selective oxidative activation of alkanes, ammoxidation of propane to acrylonitrile, or to acrylic acid, oxidation of n-butane to maleic acid or anhydride, oxidative of dehydrogenation of ethane to ethylene, or oxidation of ethane to acetic acid and partial oxidation of alcohols/aromatic hydrocarbon. Abundant metal components, such as Fe, W, Cu, Nb, and Te, have been used with the intention of optimizing the structure and catalytic performance, elucidating structural peculiarities, and improving catalytic performance.

Monte Carlo simulation of the effect of “hot” atoms on active species kinetics in combustible mixtures excited by high-voltage pulsed discharges

The energy degradation of fast H and O atoms in H2:O2, CH4:O2 and CH4:air mixtures is studied using Monte Carlo simulations, taking into account elastic collisions and chemical reactions. Based on the simulated results, the effect of high-energy atoms on the amount and composition of chemically active species produced in high-voltage pulsed discharges is estimated. The obtained results are used to consider the contribution of “hot” atom production to ignition and oxidation of fuel in combustible mixtures excited by nonequilibrium plasmas. It is shown that the presence of high-energy atoms leads to a decrease in the specific deposited energy required for plasma-assisted ignition and an increase in the density of products of plasma-assisted oxidation at gas room temperature.

Hydrogen sensors based on Pt-loaded WO3 sensing layers

In this letter we report the enhanced sensing of platinum (Pt)-loaded tungsten oxide (WO3) sensor compared to pure WO3 sensor towards hydrogen (H2) gas at low operating temperature of 200 °C. The hydrogen sensing of pure and Pt-loaded WO3 sensors is reported at operating temperatures 200, 300 and 400 °C. The presence of Pt promotes the spillover mechanism and the dissociated H2 atoms can react with adsorbed/lattice oxygen atoms to release electrons which in turn increases the conductivity of the WO3 film. The H2 sensing mechanism of Pt-loaded WO3 sensor is investigated using micro-Raman spectroscopy. The Raman spectrum of Pt-loaded sensing layer shows a shift of symmetric stretching band from 796 to 804 cm−1 with the introduction of H2 gas.

Hydrogenation induced structure and property changes in GdGa

Hydrides GdGaHx were obtained by exposing the Zintl phase GdGa with the CrB structure to a hydrogen atmosphere at pressures from 1.5 to 50bar and temperatures from 50 to 500°C. Structural analysis by powder X-ray diffraction suggests that conditions with hydrogen pressures in a range between 15 and 50bar and temperatures below 500°C afford a uniform hydride phase with the NdGaH1.66 structure (Cmcm, a=3.9867(7) Å, b=12.024(2) Å, c=4.1009(6) Å) which hosts H in two distinct positions, H1 and H2. H1 is coordinated in a tetrahedral fashion by Gd atoms, whereas H2 atoms are inserted between Ga atoms. The assignment of the NdGaH1.66 structure is corroborated by first principles DFT calculations. Modeling of phase and structure stability as a function of composition resulted in excellent agreement with experimental lattice parameters when x=1.66 and revealed the presence of five-atom moieties Ga-H2-Ga-H2-Ga in GdGaH1.66. From in situ powder X-ray diffraction using synchrotron radiation it was established that hydrogenation at temperatures above 200°C affords a hydride with x≈1.3, which is stable up to 500°C, and that additional H absorption, yielding GdGaH1.66, takes place at lower temperatures. Consequently, GdGaH1.66 desorbs H above T=200°C. Without the presence of hydrogen, hydrides GdGaHx decompose at temperatures above 300°C into GdH2 and an unidentified Gd-Ga intermetallics. Thus the hydrogenation of GdGa is not reversible. From magnetic measurements the Curie-Weiss constant and effective magnetic moment of GdGaH1.66 were obtained. The former indicates antiferromagnetic interactions, the latter attains a value of ~8 μB which is typical for compounds containing Gd3+ions.

Effect of the partial charge distributions in the ZIF-8 framework on the adsorption of CO2 as assessed by Gibbs Ensemble Monte Carlo simulations

The effect of the partial charge distributions on the adsorption of CO2 molecules in ZIF-8 framework has been obtained using GEMC simulations. The charge models are derived from the ab initio electrostatic potential surfaces of framework fragments. The signs of the atomic charges obtained with the MK model are more consistent with respect to the basis set than those obtained from the other models. The charges on the H1 and H2 atoms are found to have a notable effect on the amount of absorbed CO2, in agreement with a previous mention that these hydrogen sites are the favored absorption sites.

Tunneling chemical exchange reaction D + HD → D2 + H in solid HD and D2 at temperatures below 1 K

We report on a study of the exchange tunneling reaction D + HD → D2 + H in a pure solid HD matrix and in a D2 matrix with a 0.23% HD admixture at temperatures between 130 mK and 1.5 K. We found that the exchange reaction rates, kexHD ∼ 3 × 10-27 cm3 s-1 in the pure HD matrix, and kexD2 = 9(4) × 10-28 cm3 s-1 in the D2 matrix, are nearly independent of temperature within this range. This confirms the quantum tunnelling nature of these reactions, and their ability to proceed at temperatures down to absolute zero. Based on these observations we concluded that exchange tunneling reaction H + H2 → H2 + H should also proceed in a H2 matrix at the lowest temperatures. On the other hand, the recombination of H atoms in solid H2 and D atoms in solid D2 is substantially suppressed at the lowest temperatures as a result of a decreased probability of resonant tunneling of atoms when they approach each other.

Hydrogenation-Induced Structure and Property Changes in the Rare-Earth Metal Gallide NdGa: Evolution of a [GaH]2- Polyanion Containing Peierls-like Ga-H Chains.

The hydride NdGaH1+x (x ≈ 0.66) and its deuterized analogue were obtained by sintering the Zintl phase NdGa with the CrB structure in a hydrogen atmosphere at pressures of 10-20 bar and temperatures near 300 °C. The system NdGa/NdGaH1+x exhibits reversible H storage capability. H uptake and release were investigated by kinetic absorption measurements and thermal desorption mass spectroscopy, which showed a maximum H concentration corresponding to "NdGaH2" (0.93 wt % H) and a two-step desorption process, respectively. The crystal structure of NdGaH1+x was characterized by neutron diffraction (P21/m, a = 4.1103(7), b = 4.1662(7), c = 6.464(1) Å, β = 108.61(1)° Z = 2). H incorporates in NdGa by occupying two distinct positions, H1 and H2. H1 is coordinated in a tetrahedral fashion by Nd atoms. The H2 position displays flexible occupancy, and H2 atoms attain a trigonal bipyramidal coordination by centering a triangle of Nd atoms and bridging two Ga atoms. The phase stability and electronic structure of NdGaH1+x were analyzed by first-principles DFT calculations. NdGaH1H2 (NdGaH2) may be expressed as Nd(3+)(H1(-))[GaH2](2-). The two-dimensional polyanion [GaH](2-) features linear -H-Ga-H-Ga- chains with alternating short (1.8 Å) and long (2.4 Å) Ga-H distances, which resembles a Peierls distortion. H2 deficiency (x < 1) results in the fragmentation of chains. For x = 0.66 arrangements with five-atom moieties, Ga-H-Ga-H-Ga are energetically most favorable. From magnetic measurements, the Curie-Weiss constant and effective magnetic moment of NdGaH1.66 were obtained. The former indicates antiferromagnetic interactions, and the latter attains a value of ∼3.6 μB, which is typical for compounds containing Nd(3+) ions.

Formation of Ruthenium Carbenes by gem‐Hydrogen Transfer to Internal Alkynes: Implications for Alkyne trans‐Hydrogenation

Insights into the mechanism of the unusual trans-hydrogenation of internal alkynes catalyzed by {Cp*Ru} complexes were gained by para-hydrogen (p-H2) induced polarization (PHIP) transfer NMR spectroscopy. It was found that the productive trans-reduction competes with a pathway in which both H atoms of H2 are delivered to a single alkyne C atom of the substrate while the second alkyne C atom is converted into a metal carbene. This “geminal hydrogenation” mode seems unprecedented; it was independently confirmed by the isolation and structural characterization of a ruthenium carbene complex stabilized by secondary inter-ligand interactions. A detailed DFT study shows that the trans alkene and the carbene complex originate from a common metallacyclopropene intermediate. Furthermore, the computational analysis and the PHIP NMR data concur in that the metal carbene is the major gateway to olefin isomerization and over-reduction, which frequently interfere with regular alkyne trans-hydrogenation.

Formation of Ruthenium Carbenes by gem-Hydrogen Transfer to Internal Alkynes: Implications for Alkyne trans-Hydrogenation.

Insights into the mechanism of the unusual trans‐hydrogenation of internal alkynes catalyzed by {Cp*Ru} complexes were gained by para‐hydrogen (p‐H2) induced polarization (PHIP) transfer NMR spectroscopy. It was found that the productive trans‐reduction competes with a pathway in which both H atoms of H2 are delivered to a single alkyne C atom of the substrate while the second alkyne C atom is converted into a metal carbene. This “geminal hydrogenation” mode seems unprecedented; it was independently confirmed by the isolation and structural characterization of a ruthenium carbene complex stabilized by secondary inter‐ligand interactions. A detailed DFT study shows that the trans alkene and the carbene complex originate from a common metallacyclopropene intermediate. Furthermore, the computational analysis and the PHIP NMR data concur in that the metal carbene is the major gateway to olefin isomerization and over‐reduction, which frequently interfere with regular alkyne trans‐hydrogenation.

Controllable hydrogen adsorption and desorption by strain modulation on Ti decorated defective graphene

Hydrogen storage with Ti decorated nano-materials is attributed to the d levels of Ti with unsaturated bonding, whose configuration significantly affects the system's stability and activity. Using first-principles calculations, we have investigated hydrogen adsorption and desorption on the Ti decorated defective graphene under various strains (from 0% to 15%), in which Ti atoms' dispersing are energetically stable. According to the phase diagram, we showed that hydrogen uptake can be modulated as a function of chemical potential and strain, since the strain modifies the configuration of d levels, and consequently affects the binding between H2 and Ti atom. Remarkably, Ti decorated defective graphene under 15% strain could be considered as an ideal media of hydrogen storage, in which the desorption temperature of H2 is expected to be ∼300 K at 0.5 atm. The control of strain is found to be dominant to the H2 uptake, besides the temperature and pressure.

Lattice dynamics study of the ferroelectric glycinium hydrogenphosphite (GPI) crystal

AbstractLattice dynamics simulation of NH3CH2COOHH2PO3 crystal is performed in paraelectric P21/a and ferroelectric P21 phases within the semi‐empirical model. The detailed assignment of phonon modes is done in Brillouin zone centre. Density of phonon states and partial density of states of all constituent atoms are calculated in both structural phases. The O2 and H2 atoms in – hydrogen bonds presumably play an essential role in the mechanism of the phase transition.

Theoretical Investigation of Inner and Outer Surface Interaction of H2 and Cl2 Adsorption with C20 (cage) Fullerene

In this research, the structure of C20 cage fullerene which has carbon-bound to H2 atoms is designed and quantum computations is conducted through using density function theory (DFT) at the level of B3LYP and basis set of 6-31G. By modeling the structure of H2@ C20, H2 adsorption on the H2@C20 surface at angles of 90 and 180 degrees is the appropriate optimized location for adsorption. To study the type of gas interaction with surface, and changes of electronic properties of fullerenes, the AIM calculations are performed. Therefore, the results show that the binding power of exterior hydrogen (H23-H24) is stronger than interior hydrogen (H21-H22) in H2@C20, H2@C20-H2-90o, H2@C20 -H2-180o molecules. Also, the results are obtained from the chlorine adsorption in the surface structure of H2@C20-90o and 180o and compared with H2. The results about chlorine show that the joint power of exterior chlorine is stronger than interior hydrogen in the H2@C20-H2-90o and H2@C20-H2-180o molecules. Also, the comparison of the joint power of exterior chlorine for these molecules shows that the binding power of H2@C20-Cl2-180o is stronger than H2@C20-Cl2-90o.

Hydrogen isotope systematics among H2–H2O–CH4 during the growth of the hydrogenotrophic methanogen Methanothermobacter thermautotrophicus strain ΔH

Stable hydrogen isotope systematics among H2, H2O, and CH4 during hydrogenotrophic methanogenesis were investigated by growing a thermophilic methanogen, Methanothermobacter thermautotrophicus strain ΔH, in batch cultures spiked with deuterium-labeled H2 and/or H2O. The hydrogen isotope ratio of the product, CH4, reflected not only the isotope ratio of the H2O in the medium but also that of the substrate, H2. The D/H ratios of the CH4 were highest during the early phase of growth, and the growth-phase-dependent changes were greatest in the deuterium-enriched H2 cultures. The hydrogen isotope systematics among H2, H2O, and CH4 during growth of the methanogen could be described with the following equations: δDCH4=a×δDH2O+b×δDH2-ca=0.71-0.55×b0.17⩽b⩽0.38c=1000×(a+b-1) The greatest effect of δDH2 on δDCH4 (b=0.38) was observed during the earliest phase of growth. In contrast to this study, the possible disappearance of the effect (b=0) has been suggested in a previous study (Valentine et al., 2004a) in which Methanothermobacter marburgensis was cultured and hydrogen isotope systematics during growth was monitored, as was the case in this study. The close phylogenetic relationship between M. thermautotrophicus and M. marburgensis, which likely have similar biochemical pathways, suggests a possibly broad range of the b value, 0–0.38. To explain the observed hydrogen isotope systematics, two cellular mechanisms were proposed. One is that the hydrogen atoms of both H2 and H2O are directly incorporated into the product, CH4. The other is that all four hydrogen atoms in the product, CH4, are derived from intracellular H2O, which consists of a mixture of medium-derived pristine H2O and isotopically distinct H2O derived from methanogenic H2 oxidation. Although we attempted to evaluate the feasibility of these mechanisms, both cellular mechanisms remain hypothetical. The hydrogen isotope systematics shown here contribute to put forward utility of δDCH4 value as a geochemical tracer of the origins of environmental CH4.