Model studies of the chemisorption of hydrogen and oxygen on nickel surfaces

Atomic chemisorption of hydrogen and oxygen on the Ni(100) surface has been studied using an Effective Core Potential (ECP) approach described in a previous paper. Clusters of up to 50 nickel atoms have been used to model the surface. The computed chemisorption energies are 62 kcal/mol (exp. 63 kcal/mol) for hydrogen and 106 kcal/mol (exp. 115–130 kcal/mol) for oxygen. Correlating the adsorbate and the cluster-adsorbate bonds is extremely important for obtaining accceptable results, particularly for oxygen. Reasonable convergence of chemisorption energies is obtained with 40–50 cluster atoms for both hydrogen and oxygen. For hydrogen the addition of a third cluster layer stabilizes the results considerably. Both hydrogen and oxygen are adsorbed at (or close to) the four-fold hollow site. The calculated barriers for surface migration are also in good agreement with the experimental estimates. The calculated equilibrium heights above the surface are on the other hand too high compared with experiments. This disagreement is believed to be due to core-valence correlation effects, which are not incorporated in the present ECP. The cluster convergence for the height above the surface is much slower than for the chemisorption energy.     

Mass-spectrometric investigation of the kinetics of hydrolysis of methoxysilyl groups on the surface of silica under the conditions of linear temperature increase

The mass-spectrometric investigation of the kinetics of chemisorption under the conditions of linear temperature increase was tested on the example of the hydrolysis of methoxysilyl groups on the surface of aerosil; a constant vapor pressure of the modifier was maintained above the adsorbent. It has been established that the above-mentioned reaction proceeds in the kinetic region according to an impact mechanism, and that its rate is limited predominantly by the entropy factor.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 5, pp. 634–637, September–October, 1985.     

Plasma-Enhanced CVD of Organosilicate Glass (OSG) Films Deposited from Octamethyltrisiloxane,Bis(trimethylsiloxy)methylsilane,and 1,1,3,3-Tetramethyldisiloxane

Octamethyltrisiloxane (OMTS), bis(trimethylsiloxy)methylsilane (BTMS), or 1,1,3,3-tetramethyldisiloxane (TMDS) mixed with oxygen and argon produced organosilicate glass (OSG) films having high methyl content under pulse plasma-enhanced chemical vapor deposition (PECVD). The hydrogen concentration in films deposited from OMTS, BTMS, and TMDS can exceed twice the maximum concentration in films grown from methylsilane precursors. However, the refractive indices and dielectric constants of the films grown from OMTS, BTMS, and TMDS were close to that of minimum values observed for the films grown from methylsilane precursors. This suggests that at high concentrations, methyl incorporation does not result in further reduction of film density. ²⁹Si nuclear magnetic resonance (NMR) resolves the complex differences in the structure of the OSG films that are not apparent from Fourier transform infrared spectroscopy (FTIR) and reveals that the stability of OSG materials is improved by eliminating M groups and favoring the incorporation of T groups. These films exhibit low dielectric constants in the range of 2.4–2.6.     

Facile route to synthesis of functionalised poly(methylalkoxy)siloxane under mild and aerobic conditions in the presence of platinum catalysts

A facile and high yield method of synthesis of novel and functional poly(methylalkoxy) siloxanes is reported. The Si–H groups of poly(methylhydrogen) siloxanes (PMHS) were treated with various simple (primary, secondary, tertiary) alcohols (1a–10a) in the presence of platinum based catalysts (Speier’s and Karstedt’s catalysts). Also oxyethylene, aldehyde, epoxide, halogen and allyl grafted polysiloxane were smoothly and quantitatively prepared by the alcoholysis between linear siloxanes polymer and functional alcohols (11a–20a) with use of Karstedt’s catalysts. It is found that alcoholysis reaction in the presence of the Karstedt’s catalyst proceed faster than Speier’s catalyst .In addition, the rate of alcoholysis reaction is dependent on amount of the catalyst and reaction temperature. The polymers prepared were characterized by IR, NMR spectroscopy and GPC analysis.     

Quantum mechanical approach to the chemisorption of molecular hydrogen on defect magnesium oxide surfaces

Quantum mechanical theoretical calculations have been performed on the linear atomic chain in order to simulate the interaction of molecular hydrogen with the defects present at the surface of activated MgO. The total energy of the system, the relative energy of the various molecular orbitals, and the electronic charge distribution have been computed for various lattice parameters (d _O-O = 4.0–4.8 Å) as a function of the H-H (or O-H) separation. A symmetrical motion of the hydrogen nuclei with respect to the central Mg²⁺ vacancy was assumed. It is shown that chemisorption of hydrogen on surface O^–ions sites results in the formation of pseudo-hydroxyl groups. For a small lattice parameter (4.0 Å), no stable state of molecular hydrogen has been found while an increase in the lattice parameter results in a uniform increase of the calculated activation energy for the molecular hydrogen activation process. A mechanism is proposed which is not so different from that put forward for the hydrogen activation by transition metal complexes. Molecular hydrogen is found to act as an electron donor.     

Chemisorption of o-xylene on activated carbons

It was determined volumetrically that at temperature of gas-phase oxidative catalysis (473–573 K) o-xylene is chemisorbed on SKN and oxidized AG-3 carbons. The chemisorption is weak and reversible and is not accompanied by dissociation of o-xylene molecules, The active chemisorption centers are surface heteroatoms of nitrogen (in SKN carbon) or oxygen (in oxidized AG-3 carbon), promoting polarization of the C-H bond of the methyl group of o-xylene.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 3, pp. 370–374, May–June, 1986.     

Effect of intramolecular hydrogen bonds on the kinetics of proton exchange of thiocarboxylic acids with phenols

The kinetics and mechanism of proton exchange between thiocarboxylic acids and o-substituted phenols with intramolecular hydrogen bonds (IMHB) were studied by dynamic PMR spectroscopy. Fast uncatalyzed exchange in cyclic binary complexes is observed for phenols with weak IMHB. In systems with strong IMHB fast exchange occurs only in the presence of an alkaline catalyst, the reaction of which with the phenol leads to cleavage of the IMHB. Concerted transfer of two protons and a cation is realized in the final step of the reaction. According to the proposed mechanism, the measured energy of activation of proton exchange is the sum of the energies of activation of the steps involving cleavage of the IMHB and strictly proton transfer.Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 24, No. 1, pp. 49–56, January–February, 1988.     

Highly stable supramolecular hydrogels formed from 1,3,5-benzenetricarboxylic acid and hydroxyl pyridines

New supramolecular hydrogels with the maximal sol-gel transition temperature(T_(gel)) of 95℃were obtained by using gelators formed from 1,3,5-benzenetricarboxylic acid(BTA) and para-hydroxyl pyridine(PHP) or meta-hydroxyl pyridine(MHP).The single crystal structure of the complex formed from BTA and ortho-hydroxyl pyridine(OHP) indicated that the molecules assembled into superstructure via both hydrogen bonds andπ-πstacking interaction.     

Cleavage of di- and trioxacyclohexanes by trimethyliodosilane

Conclusions The reaction of Me₃SiI with 1,4-dioxane gives 1,2-diiodoethane, 1,2-bis(trimethylsiloxy)ethane, and hexamethyldisiloxane, while reaction with 1,3,5-trioxane and 2,4,6-trimethyl-1,3-5-trioxane respectively gives the,-diiodomethyl and ethyl ethers and HMDS.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1391–1392, June, 1981.     

Kinetics and mechanism of oxidation of hydrogen on metal-like nitrides,borides, and silicides

A stationary circulation method was used to study the kinetics of the oxidation of hydrogen on metal-like nitrides, borides, and suicides. At high occupancies of the surface by oxygen (), the reaction proceeds by the reaction of H₂ with oxygen adsorbed in atomic form. At low and in the case of excess hydrogen, the catalysis rate in the mixture is determined by the step involving the reaction of O₂ with adsorbed atomic hydrogen.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 23, No. 6, pp. 748–752, November–December, 1987.     

Association-Assisted Kinetics of Pentanol-1 and Heptanol-1 Oxyethylation: Kinetic Parameters of Reactions and Association Parameters of Alcohols in the Series Butanol-1–Heptanol-1

The kinetics of pentanol-1 and heptanol-1 oxyethylation in the absence and in the presence of solvents (dodecane, p-xylene, and 1,4-dioxane) is studied under the conditions of base catalysis at 80–150°C and the concentrations of the catalyst (the corresponding sodium alkoxide) and ethylene oxide in the starting mixture of 1 and 10^–3–10^–1 mol/l, respectively. The experimental results are adequately described by the rate law that takes into account the association of alcohol molecules via hydrogen bonds. A hypothesis is advanced that an associate consisting of n alcohol molecules acts as a kinetically independent species in this reaction. The kinetic and association parameters for alcohols in the C₄–C₇ series are compared with the published data.     

Reactivity of trimethylcyanosilane toward the surface of silica

The reaction of the surface of silica with (CH₃)₃SiCN vapor was studied by IR spectroscopy. It was established that the chemisorption of trimethylcyanosilane belongs to the processes of electrophilic substitution of a proton in free silanol groups. The activation energy, calculated from the kinetic curves, amounts to 22±1 kJ/mole of grafted trimethylsilyl groups.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 23, No. 1, pp. 117–120, January–February, 1987.     

Insertion of Ethyl Isocyanate at the Tungsten–Chlorine Bond. Crystal Structure of 1,3,5-Triethyl-1,3,5-triazine-2,4,6-(1H,3H,5H)-trione

The reaction of tungsten hexachloride with excess ethyl isocyanate in dichloroethane leads to the insertion of three ligand molecules at one of the tungsten–chlorine bonds. The data of elemental analysis and IR spectroscopy confirm that the thermolysis of the reaction mixture affords the WCl₄(L₃Cl) complex (I), where L = –N(Et)C(O)–. The structure of the chain of inserted molecules in I is established on the basis of IR and NMR data, and the results of X-ray diffraction analysis of crystals of an organic product (II) of the hydrolysis of I. According to the latter, compound (II) is a derivative of s-triazine—1,3,5-triethyl-1,3,5-triazine-2,4,6-(1H,3H,5H)-trione.     

A Mixed Quantum-Classical Approach for Computing Effects of Intramolecular Motion on the Low-Barrier Hydrogen Bond

The fractionation factor is defined as the equilibrium constant for the reaction: R – H + DOH R – D + HOH. Of interest are values of fractionation factors for reactions where reactants and/or products form intramolecular low-barrier hydrogen bonds. Experimentally measured isotopic fractionation factors are usually interpreted via a one-dimensional potential energy surface along the intrinsic proton hydrogen bond coordinate. Such a one-dimensional picture cannot be completely correct. Intramolecular motions, such as vibrations and librations, can modulate the underlying potential energy surface along the hydrogen bond coordinate and thus affect the isotopic fractionation factor. We have recently generated a picture of the motion of the proton in a low-barrier hydrogen bond as taking place in an effective single-dimensional potential, which we term the potential of mean force (PMF). In this paper, we compute the PMF for a molecule with an intramolecular hydrogen bond in order to quantify the effect of intramolecular motions on the fractionation factor. The PMF and isotopic fractionation factor are computed with a combination of high-level density functional theory and molecular dynamics simulations.     

Structure of surface compounds formed during chemisorption of BBr3 on dehydrated Aerosil

The chemisorption of boron bromide on the surface of Aerosil calcined at 870 K in vacuo was studied by IR spectroscopy combined with chemical and gravimetric analysis. It was determined that the chemical reaction of structural silanol groups with BBr₃ vapor occurs monofunctionally with the formation of SiOBBr₂ groups. This indicates uniform distribution of isolated silanol groups on the surface of dehydrated Aerosils.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 25, No. 3, pp. 374–377, May–June, 1989.     

Kinetics of methanation of carbon dioxide on a nickel catalyst

The kinetics of hydrogenation of CO₂ to methane on an NKM-4A nickel-containing catalyst was studied in a wide range of change of partial pressures of hydrogen and carbon dioxide at 498–543 K. It was shown that when hydrogen is replaced by deuterium in the reaction, a reverse kinetic isotope effect is observed. The kinetic equation of the reaction was derived and a stepwise scheme is proposed which includes the involvement of oxygen-containing intermediates in the process without preliminary dissociation of CO₂.Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 26, No. 5, pp. 620–624, September–October, 1990.     

Mechanism and kinetics of chemisorption of vinyl chloride on cobalt-chromium catalyst

The mechanism and kinetics of vinyl chloride adsorption on cobalt-chromium catalyst were investigated by means of a response method, gravimetry, and IR spectroscopy. The mechanism of chemisorption of the chlorohydrocarbon consists of its oxidative decomposition and the formation of surface compounds with various bond strengths.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 30, No. 5, pp. 283–288, September–October, 1994.     

The formation of three-center bonds in chemisorption

Summary It is proposed that molecular adsorption may be brought about by the formation of three-center bonds. In this two donor-acceptor bonds are formed between the molecule to be adsorbed and an atom in the adsorbent surface. The molecule to be adsorbed, for instance, ethylene, acts as a donor because of its -electron pair and the adsorbent atom because of its p-orbital. At the same time the adsorbent atom is a donor on account of its electron pair in a d_XZ-orbital and the molecule an acceptor on account of its antibonding ¹-²- orbital; by this a dative bond is formed. In the case of a metal without d-electrons (for instance, Al) a dative bond is impossible, but the metal can give an acceptor bond because of its empty p-orbital.Such a three-center chemisorption must proceed with a small activation energy, that is, be reversible, unactivated and rapid. Most often chemisorption at low temperature has these properties. The heat of such a chemisorption may vary between a few times ten or a few kcal.Three-center chemisorption may be of considerable importance in catalytic reactions and also in exchange processes on the surface.Only a considerable activation energy and a very high heat of adsorption is a reliable proof that the adsorbed substance occurs on the surface as atoms or in a state in which one adsorbent atom forms a stable chemical bond with only one adsorbate atom.Translated from Zhurnal Strukturnoi Khimii, Vol. 1. No. 2, pp. 189–199, July–August, 1960     

Kinetics of chemisorption of trimethylisothiocyanatosilane and trimethylisocyanatosilane on silica and properties of the surface compounds

The kinetics of the chemisorption of trimethylisothiocyanatosilane and trimethylisocyanatosilane on surfaces of dehydrated silica have been studied with the aid of infrared spectroscopy. The activation energies for the immobilization of the trimethylsilyl and isothiocyanate (or isocyanate) groups and the loss of the free silanol groups as a result of the chemisorption of the silanes have been calculated. The reactivity of the immobilized groups in their interactions with water and methanol has been studied.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 4, pp. 496–500, July–August, 1988.