Acidic and basic molecular hardness in LCAO approximation

Hardness of a molecule has been formulated as an average of a set of terms representing various types of reactivity. Hardness parameters for molecules and bonded atoms have been calculated by the ab initio Hartree-Fock self-consistent field (HF SCF) method. An analysis has been presented of the correlation between the relative magnitude of contributions to the total hardness and experimentally observed acidic or basic reactivity of molecules. © 1997 John Wiley & Sons, Inc.     

Temperature-programmed decompositions of NH4VO3 and NH4VO3-TiO2 investigated by mass-spectrometry and DTA methods

The thermal decompositions of NH₄VO₃ and NH₄VO₃-TiO₂ mixtures were investigated by mass-spectrometry and DTA. Three stages of decomposition were distinguished, in which the contribution of redox-type reactions increased successively. The bulk of the decomposition was independent of the nature of the atmosphere; only the third stage displayed a variation. In vacuo and in argon there was continuous reduction of the vanadium oxide system, but in air its reoxidation took place.

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Zusammenfassung Die thermische Zersetzung von NH₄VO₃ und NH₄VO₃- TiO₂ Mischungen wurde durch Massenspektrometrie und DTA untersucht. Drei Zersetzungsstufen wurden unterschieden, bei denen die Beteiligung der Reaktionen vom Redox-Typ der Reihe nach zunahm. Der grö\ere Teil der Zersetzungsreaktionen war unabhÄngig von der Art der AtmosphÄre, nur die dritte Stufe Änderte sich hierbei. Im Vakuum und in Argon erfolgte eine kontinuerliche Reduktion des Vanadiumoxid-Systems, jedoch in Luft eine Reoxidierung.

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Résumé On a étudié, par spectrométrie de masse et par ATD, la décomposition thermique de NH₄VO₃ et des mélanges de NH₄VO₃-TiO₂. On a distingué trois étapes de décomposition, dans lesquelles la contribution des réactions du type redox augmentait successsivement. La plus grande part de la décomposition est indépendante de la qualité de l'atmosphère, seule la troisième étape est influencée. Dans le vide dans l'argon la réduction du système d'oxyde de vanadium est continue mais dans l'air la réoxydation de celui-ci a lieu.

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- NH₄VO₃ NH₄VO₃- TiO₂. , - . - . , .

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The authors wish to express their gratitude to Professor A. Bielaski for his interest in this work and for valuable discussions.     

CO2 adsorption over Si-MCM-41 materials having basic sites created by postmodification with La2O3

Moderate basic sites could be created onto mesoporous Si-MCM-41 materials by postsynthesis modification with highly dispersed La2O3. The La2O3-modified MCM-41 materials (designated here as LaM) have been characterized by Fourier transform infrared spectroscopy, temperature-programmed desorption, X-ray photoelectron spectroscopy, X-ray diffraction (XRD), and N2 adsorption/desorption and have been tested as model adsorbents for CO2 adsorption. XRD and N2 adsorption results showed that all LaM materials still maintained their uniform hexagonal mesoporous structure even after postsynthesis modification with La2O3 loading up to 20 wt %. Although the surface area, pore size, and pore volume of LaM materials decreased with increasing La2O3 loading, their capacity for CO2 storage could be significantly improved when La2O3 loading was increased from 0 to 10 wt %. Unidentate and bidentate carbonates have been identified by in situ FTIR as the two types of CO2 species adsorbed on LaM surface. The LaM material also possesses good thermal stability, allowing the model adsorbent to be regenerated at high temperature and recyclable.     

Cation migration upon adsorption of methanol in NaY and NaX faujasite systems: a molecular dynamics approach

Molecular dynamics simulations have been carried out to address the question of cation migration upon adsorption of methanol in NaY and NaX faujasite systems as a function of the loading. For NaY, it has been shown that, at low and intermediate loadings, SII cations can migrate toward the center of the supercage due to strong interactions with the adsorbates, followed by a hopping of SI' from the sodalite cage into the supercage to fill the vacant SII site. A SI' cation can also migrate across the double six ring and takes a SI' vacant position. SI cations mainly remain trapped in their initial sites whatever the loading. At high loading, only limited motions are observed for SII cations due to steric effects induced by the presence of adsorbates within the supercage. For NaX, the SIII' cations which occupy the most accessible adsorption sites are significantly moving upon coordination to the methanol molecules; the extent of this mobility exhibits a maximum for 48 methanol molecules per unit cell before decreasing at higher loadings due to steric hindrance. In addition, the SI' and SII cations remain almost trapped in their initial sites whatever the loading. Indeed, the most probable migration mechanism involves SIII' cation displacements into nearby SIII' sites.     

Enzymatic Electrosynthesis of Glycine from CO2 and NH3

Enzymatic electrosynthesis has gained more and more interest as an emerging green synthesis platform, particularly for the fixation of CO₂. However, the simultaneous utilization of CO₂ and a nitrogenous molecule for the enzymatic electrosynthesis of value-added products has never been reported. In this study, we constructed an in vitro multienzymatic cascade based on the reductive glycine pathway and demonstrated an enzymatic electrocatalytic system that allowed the simultaneous conversion of CO₂ and NH₃ as the sole carbon and nitrogen sources to synthesize glycine. Through effective coupling and the optimization of electrochemical cofactor regeneration and the multienzymatic cascade reaction, 0.81 mM glycine was yielded with a highest reaction rate of 8.69 mg L⁻¹ h⁻¹ and faradaic efficiency of 96.8 %. These results imply a promising alternative for enzymatic CO₂ electroreduction and expand its products to nitrogenous chemicals.     

CO2 adsorption in LiY and NaY at high temperature: molecular simulations compared to experiments

Grand Canonical Monte Carlo simulations combined with adsorption measurements have been carried out to gain further insight into the CO₂ adsorption process at the microscopic scale in both LiY and NaY faujasites at various temperatures. A new Li⁺−CO₂ force field derived by ab initio calculations was validated by a reasonable agreement between the simulated isotherms and those obtained by experiments in a wide range of temperature (from 323 K to 473 K). In addition, the microscopic mechanisms of CO₂ adsorption in both systems, consistent with the trends observed for the simulated differential enthalpies of adsorption as a function of the loading, were proposed. It was observed that two different types of adsorption behaviour exist for NaY and LiY at 323 K and 373 K, mainly caused by the significant more exposed position of the SII Na⁺ from the six-ring plane of the supercage compared to those occupied by the SII Li⁺, whereas at higher temperature, both faujasites exhibit the same flat profile for the differential enthalpy of adsorption as a function of loading.     

Thermal decomposition of the [Pt(NH3)4]2+ complex in NaX zeolite

The effect of the calcination procedure on the decomposition of the [Pt(NH₃)₄]²⁺ complex in a cesium-containing NaX zeolite was studied by thermal decomposition accompanied by mass spectrometry and diffuse reflectance spectroscopy, as well as electron paramagnetic resonance and infrared spectroscopy. The decomposition of the complex took place in two steps. Under oxygen, the [Pt(NH₃)₄]²⁺ complex was first converted into the [Pt(NH₃)₂]²⁺ complex in the first step, with predominant nitrogen release. In the second step, corresponding to the decomposition of the remaining two amine ligands, NO was also formed and adsorbed. Oxygen paramagnetic species were also observed. Under He, the decomposition also occurred in two steps with H₂ release.     

Favoring CO Intermediate Stabilization and Protonation by Crown Ether for CO2 Electromethanation in Acidic Media

The large-scale deployment of CO₂ electroreduction is hampered by deficient carbon utilization in neutral and alkaline electrolytes due to CO₂ loss into (bi)carbonates. Switching to acidic media mitigates carbonation, but suffers from low product selectivity because of hydrogen evolution. Here we report a crown ether decoration strategy on a Cu catalyst to enhance carbon utilization and selectivity of CO₂ methanation under acidic conditions. Macrocyclic 18-Crown-6 is found to enrich potassium cations near the Cu electrode surface, simultaneously enhancing the interfacial electric field to stabilize the *CO intermediate and accelerate water dissociation to boost *CO protonation. Remarkably, the mixture of 18-Crown-6 and Cu nanoparticles affords a CH₄ Faradaic efficiency of 51.2 % and a single pass carbon efficiency of 43.0 % toward CO₂ electroreduction in electrolyte with pH=2. This study provides a facile strategy to promote CH₄ selectivity and carbon utilization by modifying Cu catalysts with supramolecules.     

Heats of adsorption of CO2 and NH3 on natural mordenites

The heats of adsorption of CO₂ and NH₃ were studied for sodium, lithium, calcium and decationized forms of natural mordenites. The exchange of Na⁺ for Ca²⁺ leads to increasing adsorption heats at low coverages. For CO₂ the region of elevated heats corresponds to the formation of Ca²⁺ ... CO₂ adsorption complexes. Two molecules of NH₃ probably interact with each Ca²⁺ ion (or with the Li⁺ ions in Li-mordenite). The heats of adsorption of NH₃ on decationized mordenite are much higher than those on calcium mordenite.

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Zusammenfassung Adsorptionswärmen von CO₂ und NH₃ an der Na-, Li-, Ca- und dekationisierten Form von natürlichem Mordenit wurden bestimmt. Der Austausch von Na⁺ gegen Ca²⁺ führt zu einer Erhöhung der Adsorptionswärmen bei geringen Bedeckungen. Bei CO₂ erfolgt im Bereich hoher Adsorptionswärmen die Bildung des Adsorptionskomplexes Ca²⁺ ...CO₂. Mit jedem Ca²⁺-Ion (bzw. Li⁺-Ion im Li-Mordenit) treten wahrscheinlich zwei NH₃-Moleküle in Wechselwirkung. An dekationisiertem Mordenit sind die Adsorptionswärmen von NH₃ höher als an Ca-Mordenit.

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, , . . Ca²⁺ ... CO₂. B , , ( -). , -.

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The authors would like to thank Dr. I. A. Belitsky for supplying the sample of natural mordenite. They also thank Professor S. P. Zhdanov for his interest and encouragement of this work.     

Platinum species in the pores of NaX, NaY and NaA zeolites studied using EPR, XAS and FTIR spectroscopies

The results of X-band EPR, X-ray absorption and Fourier transform infrared spectroscopy on Pt(NH(3))(4)(2+) exchanged NaX, NaY and NaA zeolites reveal after oxygen calcination at 573 K that diamagnetic Pt(2+) is not the only product. Calcination provides Pt(3+) cations, but depending on the heating rate, the decomposition of amino groups during calcination also produces hydrogen that reduces Pt(3+) to Pt(2+) and Pt(+). NaX (Si/Al = 1.23) has a more negative framework charge than NaY (Si/Al = 2.31), so Pt(3+) can be stabilized only in NaX, whereas lower oxidation states of Pt such as Pt(+) can be stabilized in both, NaX and NaY, and neither of the paramagnetic Pt cations are stabilized in NaUSY (Si/Al = 3). The autoreduction process allows controlling the number of Pt(3+) and Pt(+) in the NaX zeolite by changing the calcination heating rate: a heating rate of 1.25 K min(-1) gives only Pt(+), but 0.5 K min(-1) gives a Pt(3+)/Pt(+) ratio close to 1. The structure of the support is also important for the synthesis of Pt species. While isolated paramagnetic Pt ions were stabilized in faujasite zeolites (NaX and NaY), a paramagnetic Pt dimer was obtained in a Linde type A zeolite (LTA, Si/Al = 1) by applying the same preparation methods. The fraction of paramagnetic Pt species which were characterized by X-band EPR spectroscopy amounts to 2-18% of the total Pt in the zeolites, the remaining Pt must be diamagnetic.     

Synthesis,characterization and CO2 adsorption of NaA,NaX and NaZSM-5 from rice husk ash

NaA, NaX and NaZSM-5 zeolites were prepared by using silica extracted from rice hull ash as a raw material, and they were investigated for CO₂ adsorption performance as an adsorbent in order to solve the problem of suppressing the global warming. Three zeolites were synthesized by hydrothermal methods with seed technology, and a series of characterization methods, including XRD, FTIR, nitrogen adsorption-desorption and SEM, were used to demonstrate their advantages compared to traditional hydrothermal methods. The maximum equilibrium adsorption capacity of NaA-RS, NaX-RS and NaZSM-5-RS was 1.46, 3.12 and 2.20 mmol/g at 0 °C and 101.3 kPa, respectively. The CO₂ and N₂ adsorption isotherms recorded at different temperatures were perfectly fitted by the Dual-site Langmuir model. The CO₂/N₂ selectivity and Henry's law constants were calculated to demonstrate that the samples have a stronger affinity for CO₂, especially at low pressures. The isosteric heat of CO₂ and N₂ adsorption of the three zeolites was calculated, which was indicated that they were in an excellent potential for adsorption and separation of CO₂ in industrial flue gas.     

Micropore filling of NO,SO2, NH3, and CO2 onα-FeOOH dispersed activated carbon fibers

Adsorption isotherms of NO, SO₂, NH₃, and CO₂ on-FeOOH dispersed activated carbon fibers at 303 K were examined to determine the role of surface modification in micropore filling. The parameters on micropore structures were obtained from both nitrogen adsorption at 77 K and benzene adsorption at 303 K; both isotherms were of BDDTI type and gave the same micropore volume. The preoxidation conditions of ACF, prior to the deposition of-FeOOH against the NO adsorptivity were examined. The dispersion of-FeOOH on ACF was effective in enhancement of micropore filling, irrespective of the adsorbate molecule. We determined the degree of volume filling for each gas by the use of a DR plot. The modified DR plot for an NO gas, of which the critical temperature was much lower than 303 K, was proposed. The degree of volume filling for various gases was correlated with the deviation of each boiling point from 303 K and with the van der Waalsa constant.     

Synthesis of alkyl-substituted pyridines from CO,C2H4, H2, and NH3 on heterogeneous catalysts

The reaction of preparation of alkyl-substituted pyridines by hydroformylation of ethylene in the presence of ammonia on heterogeneous Rh-containing catalysts with the formation of 2-ethyl-3,5-dimethylpyridine (I) as the basic product with up to 70–80% conversion of gases and up to an 80 wt. % concentration of the basic product was investigated. The dependence of the activity and selectivity of the catalysts on the concentration of Rh and nature of the support was determined. A correlation between the activity of the catalysts and their acid properties was established by IR spectroscopy.We would like to thank M. M. Savel'ev for assistance in developing the method of analysis of the gases.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2481–2486, November, 1990.     

Method/basis set dependence of the traceless quadrupole moment calculation for N2, CO2, SO2, HCl,CO, NH3, PH3, HF,and H2O

Dipole moments and traceless quadrupole moments for a training set of nine molecules—N₂, CO₂, SO₂, HCl, CO, NH₃, PH₃, HF, and H₂O—were deduced from the final optimized density matrices of a series of ab initio calculations, thereby employing different levels of theory with varying basis set quality. All the results were obtained in a true ab initio sense, meaning that at a certain combination of method/basis set, the configurations of the molecules were first subjected to a geometry optimization and only afterwards the dipole and traceless quadrupole moments were derived for the final relaxed structures. All results could be compared to experimental data, thus the quality of the different models with respect to applicability and reliability of calculating molecular moments without any knowledge of structural details of the equilibrium geometries could be evaluated. Generally, a combination of methods that take into account correlation effects, together with a high‐level basis set, was found to yield molecular moments close to the experimental values. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Adsorption Behaviors of CO2 and NH3 on Chemically Surface-Treated Activated Carbons

The adsorption characteristics of activated carbon treated with 30 wt% HCl and 30 wt% NaOH were investigated. The acid and base values were determined by Boehm's method and the surface structures were studied by the BET method with N2 adsorption and iodine adsorption capacity. Also the adsorption properties of the activated carbons treated with acid and base chemical solutions were investigated with CO2 and NH3 adsorptions. Different adsorption behaviors of CO2 and NH3 on the modified activated carbons were observed, even though the physical surfaces of the activated carbons (i.e., specific surface area, pore size, and pore volume) were not significantly changed. Copyright 1999 Academic Press.     

The effect of gas molecule affinities on CO2 separation from the CO2/N2 gas mixture using inorganic membranes as investigated by molecular dynamics simulation

Applying molecular dynamics simulation and computer graphics methods we have investigated the dynamic behavior of the separation process of CO₂ from the CO₂/N₂ gas mixture in inorganic membranes at high temperatures. We have demonstrated that the permeation dynamics follows the Knudsen diffusion mechanism in our model system that has a slit-like pore of 6.3 Å. We have analyzed the effect of affinities of gas molecules for the membrane wall on the permeation to predict the optimal affinity strength for high selectivity of CO₂. Our results indicate that in the model with the 600 K and 200 K affinities for CO₂ and N₂, respectively, we can obtain a high selectivity of CO₂ even if the temperature is 1073 K. It is also shown that there is an optimal range for the CO₂ affinity for the membrane wall to achieve good separation, which was estimated as the range of 400–600 K in our system, if the affinity of N₂ is always weaker than that of CO₂.