Isotopic effects of the N(2D) + H2 → NH + H reaction: a quantum time-dependent wave packet investigation

Based on the potential energy surface reported by Li and co-workers (J. Comput. Chem. 34 1686–1696 (2013)), the dynamics calculations of N(²D)?+?H₂(v ₀?=?0, j ₀?=?0) reaction and its isotopic variants HD and D₂ are studied using time-dependent wave packet method in the collision energy range of 0.01–1.0?eV. Dynamics properties such as reaction probability, differential cross section, and integral cross section are studied at state-to-state level of theory. Present values are compared with available theoretical and experimental results. The results indicate that the integral cross sections of N(²D)?+?D₂ reaction are in general good agreement with the experimental data at collision energy below 0.15?eV. The rotational state-resolved integral cross sections of N(²D)?+?H₂/HD/D₂ reactions are compared with experimental values for the first time, with the obtained values being in good agreement with the experimental data.     

Solid ionic conductor/semiconductor junctions for chemical sensors

The potential drop at solid ionic conductor/semiconductor contacts responds to partial pressure changes of gases since the Fermi level of the semiconductor depends on the adsorbed species and equilibrates with that of the ionic conductor. In an alternative consideration, the semiconductor acts as a catalyst for the reaction of the mobile ions of the solid ionic conductor with the adsorbed species. The junction is employed as a chemical sensor for the detection of CO₂ by using NASICON as solid electrolyte and doped SnO₂ as semiconductor. The device is applicable even at room temperature with fast response time. Mechanisms of the response of the junction to gases are discussed in detail. The principle of employing solid ionic conductor/semiconductor junctions for sensors is in general applicable for many gases.On leave from the Institute of Physics, Academy of Sciences, Chernogolovka, Russia     

Totale effektive Streuquerschnitte für die Streuung von He,HD und D2 an verschiedenen Molekülen

The measured velocity dependence of the total scattering cross section for the scattering of He, HD, and D₂ by the molecules CH₄, N₂, O₂, NO, CO, HD, and D₂ can be described relatively well by a Lennard-Jones-(12.6)-potential. Only for the systems with a CO₂ target are large deviations from the L. J.(12,6) predictions found: the measured glory amplitudes are considerably smaller. The deformation of the spherically symmetric potential which must be made in order to fit the CO₂ glory pattern is given. Furthermore, the CO₂ systems were calculated using an angle-dependent L.J.(12.6)-potential, taking into account only elastic processes. The agreement with the measurements is good.     

小分子在真空紫外波段从量子态到量子态的光解离动力学

本文主要描述小分子在真空紫外波段(VUV,6-20 eV)光解离动力学的最新实验和理论研究进展.得益于基于商业化激光器的真空紫外光源技术,以及离子速度成像、高分辨氢原子-里德堡态标记-飞行时间测量和VUV-VUV泵浦-探测等方法的发展,研究人员现在可以对很多小分子在真空紫外波段的光解离动力学进行量子态到量子态层面的测量和研究,本文重点综述H_2(D_2,HD),CO,N_2,NO,O_2,H_2O(D_2O,HOD),CO_2,N_2O以及一些多原子分子在真空紫外波段光解离动力学的最新研究进展.这些小分子在真空紫外波段的光解离在天体化学以及大气化学中有着非常重要的应用.分子吸收一个VUV光子以后,通常会被直接激发到比较高的电子激发态,解离过程会涉及到多个电子态势能面之间的复杂非绝热相互作用.在实验上对解离截面等参数进行从量子态到量子态层面的精细测量对于深入了解这些复杂的势能面之间的相互作用有非常重要的意义.最近建成的大连相干光源是目前世界上唯一一台在真空紫外波段工作的自由电子激光,具有脉冲能量高、扫描范围宽(50～150 nm)等优越的性能,它的建成必将会大大促进小分子真空紫外光解离研究的发展.     

Mechanism of CO2 hydrogenation over Cu/ZrO2(2̅12) interface from first-principles kinetics Monte Carlo simulations

It has been a goal consistently pursued by chemists to understand and control the catalytic process over composite materials. In order to provide deeper insight on complex interfacial catalysis at the experimental conditions, we performed an extensive analysis on CO₂ hydrogenation over a Cu/ZrO₂ model catalyst by employing density functional theory (DFT) calculations and kinetic Monte Carlo (kMC) simulations based on the continuous stirred tank model. The free energy profiles are determined for the reaction at the oxygen-rich Cu/m-ZrO₂ (2?12) interface, where all interfacial Zr are six-coordinated since the interface accumulates oxidative species at the reaction conditions. We show that not only methanol but also CO are produced through the formate pathway dominantly, whilst the reverse-water-gas-shift (RWGS) channel has only a minor contribution. H₂CO is a key intermediate species in the reaction pathway, the hydrogenation of which dictates the high temperature of CO₂ hydrogenation. The kinetics simulation shows that the CO₂ conversion is 1.20%, the selectivity towards methanol is 68% at 500 K and the activation energies for methanol and CO formation are 0.79 and 1.79 eV, respectively. The secondary reactions due to the product readsorption lower the overall turnover frequency (TOF) but increase the selectivity towards methanol by 16%. We also show that kMC is a more reliable tool for simulating heterogeneous catalytic processes compared to the microkinetics approach.     

ϑ-Sensors: A new concept for advanced solid-state ionic gas sensors

A new principle of solid state electrochemical sensors based on the kinetics of controlled chemical reactions of the gas with the electroactive species of a solid electrolyte is presented and demonstrated for the measurement of CO₂ partial pressures. The reaction may be for many gases modified by the formation of intermediate product phases.A periodic voltage or current signal is applied to the electrolyte. The current corresponds to alternating chemical reactions at the electrodes in contact with the gas. The new concept avoids the often complicated application of reference electrodes and shows characteristic signals for different types of gases and allows in this way to overcome problems of cross-sensitivity. The experimental results show high accuracy with an approximately linear relationship between the current and the CO₂ partial pressure and also show extraordinarily fast response.     

Electron impact cross sections of vibrationally and electronically excited molecules

It is well known that the electron impact cross sections for elastic and inelastic processes for the vibrationally and electronically excited molecules are predominantly different than those for molecules in the ground state. Collisions of low energy electrons with excited molecules play an important role in explaining the behavior of gas discharges in laser and plasma physics, in planetary atmospheres, stars, and interstellar medium and in plasmas widely used in the fabrication of microelectronics. This explains as to why there is a need for having validated sets of electron impact cross sections for different processes. This work reviews the subject of electron collisions with vibrationally and electronically excited molecules in a comprehensive way. The survey has been carried out for a few excited molecules such as H₂, D₂, T₂, HD, HT, DT, N₂, O₂, and CO₂.     

The broadening and shift of the ratational Raman lines for hydrogen isotopes at low temperatures

Collisional broadening of rotational Raman lines has been investigated for the gases nH₂, nD₂ and HD between 20 and 300 K. For H₂ and D₂ the dependence of the linewidth on ortho-para composition has been investigated. For a number of systems the pressure shift of the Raman lines has also been determined. The experimental results are compared with theory.     

Intensity dependence of multiphoton dissociation in formaldehyde

A new intensity-dependent measurement of multiple-photon dissociation (MPD) in H₂CO, HDCO, and D₂CO gases by the use of an intense pulsed CO₂TEA laser is reported. In this measurement the energy and duration of the laser pulses are kept constant, and the intensity is varied by irradiating the sample using concave mirrors of different focal lengths. A model calculation is used to analyze and fit the present and previous experimental MPD data of HDCO and D₂CO. In this model it is assumed that dissociation is obtained by a repeated mechanism in which coherent multiphoton excitation (CME) of the molecule to high vibration-rotation states |v, J〉 is followed by intramolecular transfer of the excitation energy (ITEE) to the other modes of the molecule. In the calculations the CME is described in the framework of the density matrix formalism with relaxations, and is used to calculate absorption from the ground state as well as absorption from excited states reached by the energy redistribution in the molecule. The ITEE process is assumed to be intensity independent and to cause a random energy distribution in each transferring process. It is found that the experimental results are consistent with the absorption of 14±4 and 17±5 photons per molecule for HDCO and D₂CO, respectively, and this is sufficient to cause their dissociation.     

Use of High-Resolution Mass Spectrometry to Identify Products from Microwave Discharges in COAL-D2O Mixtures

Reactions of carbonaceous materials and H O in microwave discharges are known to produce H₂, HCN, CO, CO₂, and light hydrocarbon gases (primarily C₁ and C₂) in varying amounts. To determine if the solid or the H₂O is the source of hydrogen in formation of the above products, Fu and Blaustein reacted coal and graphite with D₂O.¹ Low-resolution mass spectra of the gaseous products from the D₂O experiments indicated the possibility of non-deuterated and corresponding deuterated compounds in the reaction mixture. Conventional separation and analytical techniques are not applicable to mixtures of this type. This communication describes the use of a high-resolution mass spectrometer, operated at a resolution 35 percent less than theoretically required for separation of the H₂-D doublet, to electrically measure precise masses for mixtures containing micromole amounts of deuterated and non-deuterated light gases.     

Thermal release of trapped deuterium from silica injected by 80-keV D+ ion implantation and RF D2 plasma

When silica is irradiated by 80-keV D⁺ ions or RF plasma of D₂ gas, deuterium is trapped in the silica forming Si-OD bonds. The deuterium, trapped as OD bonds, is desorbed from the silica upon heating to form some release products. The thermal detrapping process corresponds to decrease of OD bonds and was studied by measurement of infrared Fourier transform spectroscopy (FTIR). The release products HDO, D₂O, HD, and D₂ were measured by quadrupole mass spectroscopy (QMS). The detrapping and release processes of trapped deuterium were studied by simultaneous measurement of FTIR and QMS. Since the release spectra of HDO, D₂O, HD, and/or D₂ correspond to the decrease spectra of OD bonds, these release products are formed by thermal decomposition of OD bonds. The formation of water (HDO, D₂O) and hydrogen (HD, D₂) depends upon concentration of pre-existing OH bonds and deuterium injection methods (80-keV D⁺ implantation or RF D₂ plasma irradiation).     

Isotope effects in the kinetics of simultaneous H and D thermal desorption from Pd

The kinetics of simultaneous hydrogen and deuterium thermal desorption from PdH_xD_y has been investigated. A novel experimental approach for the study of the transition state (TS) characteristics of the surface recombination reaction is proposed based on the analysis of the H and D partitioning into H₂, HD and D₂ molecules. It has been found that the hydrogen molecular isotopes distribution is determined by the energy differences of the corresponding TS of the atom-atom recombination reactions. On the other hand, the mechanisms and activation energies of the desorption process have been obtained. At 420 K, the desorption reaction changes from a surface recombination limiting mechanism during desorption from β-PdH_xD_y to a reaction limited by the rate of β to α phase transformation during the two phase coexistence. Surface recombination reaction becomes again rate limiting above 480 K, due to a change in the catalytic properties of the Pd surface. TS energies obtained from the kinetic analysis of the thermal desorption spectra are in good accordance with those obtained from the analysis of the H₂, HD and D₂ distributions. Anomalous TS energies have been observed for the H-D recombination reaction, which may be related to the heteronuclear character of this molecule.     

Carbon dioxide activation and alkali compound formation. I. Vibrational characterization of oxalate intermediates

The activation of CO₂ in thin potassium layers adsorbed on Cu(1 1 1) has been studied with time-evolved Fourier transform-infrared reflection absorption spectroscopy. The growth of thin layers by reactive evaporation of potassium in a CO₂ atmosphere permits control of the K:CO₂ stoichiometry, which strongly affects the selectivity in the formation of intermediates and the decomposition pathways of the layer. Layers grown in a CO₂ rich atmosphere exhibit the preferential growth of stoichiometric potassium oxalate K₂C₂O₄ (D_2h). The molecular identity of oxalate with D_2h symmetry is confirmed by vibrational spectra utilizing isotopic substitution methods (¹³CO₂ and C¹⁸O₂) and by the use of isotopic mixtures of CO₂/C¹⁸O₂ and CO₂/¹³CO₂. A comparison of the isotope data with theoretical calculations gives an estimated OCO bond angle in oxalate of 132°. Far-IR spectra obtained with synchrotron radiation indicate the equivalent interaction of all oxygen atoms with the potassium. A comparison of the vibrational data with theoretical ab initio calculations confirms the structural model of an oxalate species that is bulk coordinated with no strong directional bonding and all oxygen atoms equally interacting with potassium.At medium and low CO₂:K ratios, very complex vibrational spectra are observed, indicating the formation of an oxalate surface species with C_2v symmetry in addition to D_2h⁻ oxalate, CO₂⁻ and CO₂²⁻ species.     

Nitrogen release during reaction of coal char with O2, CO2, and H2O

The chemistry of char-N release and conversion to nitrogen-containing products has been probed by studying its release and reactions with O₂, CO₂, and H₂O. The experiments were performed in a fixed bed flow reactor at pressures of up to 1.0 MPa. The results show that the major nitrogen-containing products observed depend on the reactant gas; with O₂, NO, and N₂ being the major species observed. Char-N reaction with CO₂ produces N₂ with very high selectivity over a broad range of pressures and CO₂ concentrations, and reaction with H₂O gives rise to HCN, NH₃, and N₂. Observed distributions of nitrogen-containing products are little affected by pressure when O₂ and CO₂ are the reactant gases, but increasing pressures in the reaction with H₂O results in the formation of increasing proportions of NH₃. Formation of NH₃ is also promoted by increasing concentrations of H₂O in the feed gas. The results suggest that NO and HCN are primary products when O₂ and H₂O, respectively, are used as the reactant gases, and that the other observed products arise from interactions of these primary products with the char surface.     

Design and experimental characterisation of a DC-excited CW/repetitively pulsed sealed off CO2 laser

This paper reports the design procedure and experimental study of a sealed off CO₂ laser. Simple algorithms for threshold and steady state excitation voltage calculation, resonator design and its temperature dependent operation are presented. The sealed off CO₂ laser was operated both in CW and pulsed modes and found stable both thermodynamically and optically. Frequency limits for pulsed operation regarding maximum and minimum output energy ranges are determined. Different aspects of CO₂ laser studied include threshold excitation voltages, temperature dependent efficiency, optical power saturation limitations, pulsed and steady state discharge currents for optimum gases mixture combinations. The laser has successfully been constructed, operated and tested for different applications within the limits of its maximum output power of 14 W.     

Infrared synthesis of SO3 by homogeneous gas-phase CO2 laser-driven reactions

By using a CO₂ laser operated at 10.6μm, a bimolecular reaction was induced in a binary SO₂/O₂ gas mixture, with a sensitizer gas (SF₆) as photo-absorbing species.The infrared (IR) synthesis of SO₃ was performed in a flowing gas device equipped with a continuous trapping system of the reaction product. Vibrational energy transfer processes presumably should play a specific role in SO₂ excitation and reaction in the presence of SF₆.     

Carbonate formation on Al2O3 thin film model catalyst supports

The formation of carbonate species on alumina upon CO and CO₂ exposure was studied by PM-IRAS and XPS, utilizing the model system of an ultrathin alumina film grown on NiAl(110). No carbonates were detected under UHV conditions, even after exposures up to 100.000 L of the gasses. In contrast, in a 100 mbar CO₂ atmosphere the formation of monodentate carbonates was identified. The surface concentration of the carbonates increased after generating defects on the alumina film by Ar⁺ ion bombardment. It is suggested that this kind of carbonate species is produced by reaction of coordinatively unsaturated O^2? ions of alumina with the C-atom of the CO₂ molecule. This is corroborated by the observation that the amount of carbonates further increased when CO₂ and oxygen were dosed simultaneously. In agreement with the “water gas shift” mechanism previously proposed for carbonate formation on high surface area alumina powders, no carbonates were detected from CO even upon mbar exposure, consistent with the absence of the required OH-groups on the model alumina support.     

Shock-tube study on high-temperature CO formation during dry methane reforming

The use of natural-gas-fueled combustion engines at unusual operating conditions to provide electrical and/or chemical energy on demand emphasizes the need for fundamental research on decomposition and formation of base chemicals at these conditions. In this work, the CO formation behind reflected shock waves from the pyrolysis of CO₂/CH₄ mixtures was investigated for the first time in the context of engine-based dry methane reforming, to understand the interaction of CO₂ and CH₄ at high temperatures and to test the validity of literature reaction mechanisms. Different CO₂/CH₄ mixtures at atmospheric pressure and temperatures between 1900 K and 2700 K were investigated. Time-resolved CO measurements were performed by laser absorption using a quantum cascade laser.With increasing CO₂ addition later reaction onset was observed, showing a reduction in the overall reactivity. Rate of production and sensitivity analyses highlight competing reactions in the pyrolysis and oxidation pathways and that the number of available H radicals is limited, which is attributed to the reduced reactivity. However, the analysis shows that CO₂ is also a source for OH radicals (via CO₂ + H ⇌ CO + OH), which enhance methane decomposition. The comparison with literature reaction mechanisms showed that none of the tested mechanisms can perfectly predict the time-resolved CO formation, highlighting the need for the validation of detailed kinetics models under nontypical conditions.     

Single-walled carbon nanotubes formation with a continuous CO2-laser: experiments and theory

It has been proved [1] that the use of a CO₂-laser system operating in continuous wave mode (cw) can be efficiently used for the production of carbon single-walled nanotubes (SWNTs). In this article we first describe in detail the variable experimental conditions (different ambient gases, static gas pressure, and gas flow) for SWNT formation and summarize the results of the characterization studies of the synthesized materials. Second, we analyze the influence of the different experimental conditions on the SWNTs formation process. We show that the heat transport, kinetic, and diffusion processes allow us to explain seemingly different formation conditions in a qualitative and semi-quantitative agreement with the experimental results. The presented self-consistent scenario for nanotube formation in a gas phase allowed us to propose future experiments on testing the mechanism of nanotube formation.     

First realization of new ps-10 mm-CO2-OFID laser systems with far-infrared laser gases as spectral filters

We demonstrate the first operation of a new ps 10 mm CO₂ laser system based on optical free induction decay (OFID) in far-infrared (FIR) laser gases, i.e. CH₃F, D₂O and NH₃. New phenomena are discussed.