Photostimulated exoemission from the surface of magnesia

Photostimulated exoemission (PSE) from magnesia was studied along with the effect of the adsorption of active gases (H₂, O₂, CO₂, and water vapors) on the intensity and kinetics of the emission decay. Based on the literature data, it was shown that PSE was excited from the local surface centers including a pair of low-coordinated ions Mg_1c²⁺-O_1c²⁻ and V_s hole centers, which were also active centers of photosorption and photocatalytic oxidation of methane and some other hydrocarbons.     

The formation of alumina surface active in methanol decomposition and exoemission in thermocycling experiments

The nature of the alumina surface irradiated with electrons active in methanol dehydration was studied by exoemission in comparison with the initial surface. Methanol decomposition and thermal treatment in a vacuum were found to influence the formation of the active surface of the catalyst. Methanol dehydration occurred on centers formed with the participation of low-coordinate surface oxygen, O^δ?, which represented exoemission centers. The desorption of methanol and its decomposition products was accompanied by the emission of negative charges. A possible reaction mechanism is discussed.     

Surface and adsorption studies of high surface area oxides

Modern FT-IR spectroscopy offers an ample potential for the study of adsorption and desorption processes on high surface area materials. Three examples will be presented here: (i) the thermal desorption of H₂O from SiO₂, (ii) the chemisorption and physisorption of H₂O on SiO₂, (iii) the interaction of NH₃ with OH groups on MgO surfaces.     

Exoemission accompanying the decomposition of methanol on zinc oxide

The electronic phenomena accompanying the adsorption and dehydrogenation of methanol on zinc oxide were studied using the method of exoemission of negative charges. Postemission excited from ZnO by an electron beam was found to be suppressed by the adsorption of methanol vapor, which exhibited electron acceptor properties. Subsequent heating to temperatures close to the temperature of the beginning of methanol decomposition increased the intensity of exoemission, which was evidence of the participation of emission centers (O^δ?) in dehydrogenation. A possible mechanism of methanol decomposition with the participation of surface V _s hole centers (O^δ?) of zinc oxide was suggested.     

Adsorption properties and surface chemistry of MgO

The adsorption properties of MgO, which is used as a sorbent and catalyst support, were studied using gas chromatography. The test absorbents used were n-alkanes (which show only nonspecific dispersion interactions when physisorbed on any adsorbent) and adsorbates whose molecules are capable of specific interactions with the surface reactive sites of MgO. Adsorption isotherms were measured for CHCl₃, CH₃NO₂, CH₃CN, (CH₃)₂CO, CH₃COOC₂H₅, and (C₂H₅)₂O on MgO at 50–100°C. Differential molar enthalpy changes (?ΔH), equal to molar heats of adsorption, were determined. For polar adsorbates, contributions from dispersive and specific interactions into ?ΔH were determined. The electron-acceptor and electron-donor abilities of the MgO surface were estimated.     

The molecular and electronic structure of the Mo12S24 macromolecule as a model of the active component of a hydrodesulfurization catalyst

The density functional theory (DFT) with the B3P86 hybrid exchange-correlation functional was used to calculate the molecular and electronic structure of the Mo₁₂S₂₄ macromolecule as a single MoS₂ layered structure slab. Calculations with geometry optimization are indicative of insignificant relaxation of the coordinatively unsaturated Mo and S atoms, which corresponds with the literature DFT data on the MoS₂ single slab obtained with periodic boundary conditions. The calculated forbidden band width (0.85–0.98 eV) is comparable with its experimental value (1.30 eV) and the results of DFT calculations of MoS₂ with periodic boundary conditions (0.89 eV). An analysis of the electronic state of the surface Mo centers in the Mo₁₂S₂₄ macromolecule showed that these centers were reduced to a greater degree than the Mo(IV) atoms in the bulk. The adsorption complex between the Mo₁₂S₂₄ macromolecule and six H₂S molecules was calculated to characterize the adsorption ability of the coordinatively unsaturated Mo centers. The structure and energy characteristics of the adsorption complex corresponded to weak donor-acceptor interaction between the π lone pair of H₂S and the surface (reduced) Mo centers. The suggestion was made that the active center of the catalytic cycle of thiophene hydrodesulfurization should induce the oxidative addition of H₂ followed by the occlusion of hydrogen into the MoS₂ matrix.     

Thermal and calorimetric investigations of Mg(IO3)2·nH2O (n=10, 4) and their deuterated analogues

A DTA and DSC study was made of the thermal dehydration and decomposition of Mg(IO₃)₂·4H₂O and Mg(IO₃)₂·4D₂O. The data obtained show that the dehydration takes place in one stage. The ΔH _deh ^o obtained were used to calculate ΔH _f ^o , and comparisons were made with the literature data. It was confirmed that the thermal decomposition passes through an intermediate Mg₅(IO₆)₂, which is unstable and immediately decomposes to MgO. An isotope effect is observed in both DTA and DSC curves. Thermogravimetric data on Mg(IO₃)₂·10H₂O are also presented.     

Dehydration-dehydrogenation of 1-phenylethanol over acid-basic catalysts

1-Phenylethanol transformation over several oxide catalysts (MgO, MgO-B₂O₃, ZrO₂, AlPO₄-SiO₂ and a Spanish sepiolite of Vallecas) was used as test reaction to determine their acid-basic properties. Different kinds of surface sites are proposed for dehydration and dehydrogenation processes. Thus, strong basic sites are related to the dehydrogenation process while both weak acid and basic sites are responsible for that of dehydration.     

Polymerization with heterogeneous metalorganic catalysts. VI. Differences in polymerization activity of α-olefins and some kinetic results on butene-1 polymerization

Relative changes in polymerization activity of ethylene, propylene, and butene-1 in Ziegler-Natta polymerization were compared by use of TiCl₃ samples contaminated with O₂ and H₂O to various extents. Catalyst depletion varied for the three monomers which supported the existence of different active centers. In butene-1 polymerizations with the system Al(C₂H₅)₂Cl–TiCl₃, the formation of active centers involves an irreversible and a reversible (adsorption) reaction, the former pertaining to the formation of Al(C₂H₅)Cl₂ and dependent upon the purity of the TiCl₃. The kinetic treatment of the rate curves suggests a mixed order of catalyst deactivation and again points to the importance of Al(C₂H₅)Cl₂.     

MoS<Subscript>2</Subscript> single slab as a model for active component of hydrodesulfuration catalyst: a quantum chemical study 1. Molecular and electronic structure of Mo<Subscript>12</Subscript>S<Subscript>24</Subscript> macromolecule and its adsorption complex with H<Subscript>2</Subscript>S

The molecular and electronic structure of Mo₁₂S₂₄ macromolecule as the MoS₂ single slab structure was calculated by the density functional theory (DFT) method with the B3P86 hybrid exchange-correlation functional. The results of calculations point to slight relaxation of coordinatively unsaturated Mo and S atoms, which is consistent with the published data. The calculated width of the forbidden band (0.85–0.98 eV) is comparable with the experimental value (1.30 eV) and similar to that obtained from DFT calculations with periodic boundary conditions (0.89 eV). The surface Mo centers in the Mo₁₂S₂₄ macromolecule are more reduced than the internal (Mo^IV) atoms. In order to characterize the adsorption capacity of coordinatively unsaturated Mo centers, a Mo₁₂S₂₄·6H₂S adsorption complex was calculated. The structure and energy characteristics of the adsorption complex point to a weak donor-acceptor interaction of the π-lone pair of H₂S molecule with the surface (reduced) Mo centers. The active center of thiophene hydrodesulfuration catalysts is formed as a result of the oxidative addition of hydrogen followed by occlusion of hydrogen into the MoS₂ matrix. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2189–2193, October, 2005.     

Analysis of NH3-TPD Profiles for CuSSZ-13 SCR Catalyst of Controlled Al Distribution – Complexity Resolved by First Principles Thermodynamics of NH3 Desorption,IR and EPR Insight into Cu Speciation**

NH₃ temperature-programmed desorption (NH₃-TPD) is frequently used for probing the nature of the active sites in CuSSZ-13 zeolite for selective catalytic reduction (SCR) of NO_x. Herein, we propose an interpretation of NH₃-TPD results, which takes into account the temperature-induced dynamics of NH₃ interaction with the active centers. It is based on a comprehensive DFT/GGA+D and first-principles thermodynamic (FPT) modeling of NH₃ adsorption on single Cu²⁺, Cu⁺, [CuOH]⁺ centers, dimeric [Cu-O-Cu]²⁺, [Cu-O₂²⁻-Cu]² species, segregated CuO nanocrystals and Brønsted acid sites (BAS). Theoretical TPD profiles are compared with the experimental data measured for samples of various Si/Al ratios and distribution of Al within the zeolite framework. Copper reduction, its relocation, followed by the intrazeolite olation/oxolation processes, which are concomitant with NH₃ desorption, were revealed by electron paramagnetic resonance (EPR) and IR. DFT/FPT results show that the peaks in the desorption profiles cannot be assigned univocally to the particular Cu and BAS centers, since the observed low-, medium- and high-temperature desorption bands have contributions coming from several species, which dynamically change their speciation and redox states during NH₃-TPD experiment. Thus, a rigorous interpretation of the NH₃-TPD profiles of CuSSZ-13 in terms of the strength and concentration of the active centers of a particular type is problematic. Nonetheless, useful connections for molecular interpretation of TPD profiles can be established between the individual component peaks and the corresponding ensembles of the adsorption centers.     

Electronically excited oxygen states in exoemission and heterogeneous catalytic oxidation reactions

The role played by electronically excited oxygen in exoemission and heterogeneous catalysis is considered. The results obtained in a study of thermally stimulated exoemission from the surface of Al₂O₃ and SiO₂ are compared with data of temperature-programmed desorption of singlet oxygen O₂(¹Δ_g) from the surface of Al₂O₃ and HZSM-5 zeolites with different SiO₂/Al₂O₃ ratios. The role played by electronically excited oxygen states in heterogeneous catalysis is discussed on the basis of our own and literature data. Thermally stimulated exoemission after the action of an electron flow is considered taking into account electron-stimulated desorption and the available data on electron bombardment in catalysis.     

特定碱性MgO纳米晶暴露晶面上铁基催化剂及其费-托合成反应的研究

本研究采用等量浸渍法、化学沉淀法和超声浸渍法合成了一系列具有良好外露晶面的Fe/MgO催化剂。采用X射线粉末衍射、高分辨透射电子显微镜、CO₂程序升温脱附、H₂程序升温还原、X射线光谱学和N₂物理吸附等物理化学方法对催化剂进行了表征。MgO纳米晶载体的碱性会影响费-托合成产物的选择性。在超声浸渍过程中,MgO纳米晶载体的碱性得到了保持。研究结果显示,Fe/MgO催化剂的碱性会提高CO解离速率和产物中烯烃的选择性。此外,相比于MgO（100）晶面,MgO（111）晶面负载铁基催化剂具有更高的活性（TOF）和烯烃选择性。MgO（111）晶面上更有利于CO的吸附,抑制二次加氢反应,提高产物中烯烃的收率。     

Oxidative addition of dihydrogen as the key step of the active center formation in the HDS sulfide bimetallic catalysts: ab initio MO/MP2 study

The electronic structure of Ni in the sulfide bimetallic species (SBMS), which is the active component of the sulfide HDS catalysts, is studied with the ab initio molecular orbital calculations. In the previous paper [I.I. Zakharov, A.N. Startsev, G.M. Zhidomirov, J. Mol. Catal. 119 (1997) 437], we have shown that the d⁸ Ni(II) electronic state in the SBMS composition cannot be active in HDS reaction because of the lack of possibility to coordinate S-containing molecule. Therefore, this paper deals with the study of the possibility to stabilize d⁶ electron configuration with the formal Ni(IV) oxidation state. With this in mind, the reaction of oxidative addition of dihydrogen to square–planar complex Ni(II)Cl₂(PH₃)₂ has been studied, which allowed to predict a stabilization of the octahedral complex Ni(IV)H₂Cl₂(PH₃)₂ with d⁶ configuration. This allows us to assume a possibility of an oxidative adsorption of dihydrogen to the Ni atom entering the SBMS composition. Ab initio calculations have shown that such type of oxidative addition is thermodynamically favorable resulting in stabilization of the Ni(IV) d⁶ electronic state. Consequently, the dihydrogen molecule is assumed to dissociate on the Ni atom resulting in the formation of `surface' H<sub>s</sub> and `occluded' H_o hydrogen, which is located under the Ni atom in the center of the trigonal sulfur prism. The structure of the active centers is optimized and the stretching modes of the hydrogen atoms are calculated, which appear to be close to the literature data. The H₂S adsorption on the active center was also investigated and it was shown that the hydrogen disulfide molecule benefits to stabilization of the active Ni(IV) d⁶ state. The conclusion is drawn that the deciding factor in the formation of the active centers of sulfide HDS catalysts is the `occluded' hydrogen.     

Supramolecular Open‐Framework of a Bipyridinium‐Carboxylate Based Copper Complex with High and Reversible Water Uptake

The rigid zwitterionic ligand 1,1′‐bis(4‐carboxylatphenyl)‐(4,4′‐bipyridinium) (pc1) and copper(II) ions give rise to a linear complex [Cu(pc1)₂(H₂O)₄]²⁺, which self assembles in a pseudo tetragonal supramolecular arrangement leading to the supramolecular open‐framework [Cu(pc1)₂(H₂O)₄](Cl)₂ · 8H₂O exhibiting an open structure including water molecules and chlorides in pores. The dehydration of this material occurs at relatively low temperature (70 °C) and results in structure modification accompanied by shrinking of the crystals. Characterization of the obtained material by FT‐IR spectroscopy reveals appearance of coordinated carboxylates groups, which may indicate the formation of coordination polymer after dehydration. Water adsorption (maximum uptake 0.23 g H₂O per g) on dehydrated material allows to restore the initial structure. A large hysteresis in water adsorption‐desorption isotherm is characteristic of a significant modification of the structure during the hydration‐dehydration cycle which is in line with the structural transition determined from thermodiffractometry and FT‐IR spectroscopy.     

Theoretical studies of adsorption property on Ir_4/MgO and Ir_4/γ-Al_2O_3

The adsorption properties of atomic and molecular species on Ir4/MgO and Ir4/γ-Al2O3 have been systematically studied by means of planewave density functional theory(DFT)calculations using the periodic boundary conditions.The binding energies of these species were ordered as follows:H2O相似文献   

Study on the equilibrium in the MBr2?CH3OH?H2O System (M = Sr2+, Ba2+) at 25° C

The dehydration processes in the three-component system MBr₂? CH₃OH? H₂O (M = Sr²⁺, Ba²⁺) have been studied at 25°C by physico-chemical analyses. Crystallization fields for the lower crystal hydrates SrBr₂ · H₂O and BaBr₂ · H₂O have been found. The solubility curves exhibit complex-formation processes. The dehydration and solvation processes in three-component system such as MBr₂? CH₃OH? H₂O at 25°C with M = Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺ have been discussed in general terms.     

Photostimulated exoemission and reduction of CeO2

The thermovacuum and photochemical reduction of CeO₂ was studied by the method of photostimulated exoemission. Analysis of the kinetics and temperature dependence of photostimulated exoemission permits one to suggest a possible mechanism of electron and ion phenomena accompanying the reduction processes. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 838–844, May, 2000.     

CO<Subscript>2</Subscript> adsorption and dehydration behavior of LiNaX,KNaX, CaNaX and CeNaX zeolites

In this study, NaX synthetic zeolite was modified by following the conventional cation exchange method at 70°C. 82, 81, 79 and 48% of sodium were exchanged with Li⁺, K⁺, Ca2⁺ and Ce3⁺, respectively. Thermal analysis data obtained by TG/DSC was used to evaluate the dehydration behavior of the zeolites. The strongest interaction with water and the highest dehydration enthalpy (ΔH) value were found for Li-exchanged form and compared with the other forms. The temperature required for complete dehydration increased with decreasing cation size (cation size: K⁺>Ce³⁺>Ca2⁺>Na⁺>Li⁺). CO₂ adsorption at 5 and 25°C was also studied and the virial model equation was used to analyze the experimental data to calculate the Henry’s law constant, K _o and isosteric heat of adsorption at zero loading Q _st. K _o values decreased with increasing temperature and the highest Qst was obtained for K rich zeolite. It was observed that both dehydration and CO₂ adsorption properties are related to cation introduced into zeolite structure.