Kinetics of isoleucine chemisorption on the modified silica surface

Russian Journal of General Chemistry - Kinetics of isoleucine chemisorption on the modified silica surface involving an amide bond formation was studied. The grafted amino groups acting as...     

Periodic surface calculations on chemisorption of H2 on a Ni surface

The periodic model of finite surfaces has been employed to investigate the localizability of the chemisorption interaction and to examine the validity of the cluster model. The adsorbate—surface interaction is estimated by the usual perturbation method. The model is applied to chemisorption of hydrogen molecule on a Ni(100) surface. As for the present calculations, the period model is shown to be advantageous over the cluster model in representing both the adsorbate-free surface state and the adsorbate—surface interaction.     

Heat of chemisorption of 1-propanol on α-Al2O3 at 25–200°C

We have studied by means of differential microcalorimetry the adsorption process of 1-propanol on α-Al₂O₃ at the temperatures of 25, 50, 100, 150 and 200°C, respectively. Both amounts of adsorbed alcohol and heats released decrease as the temperature of adsorption increases. At very low coverage, the high value of differential heat shows a strong irreversible chemisorption on active sites (Lewis acid sites) (qdiff>200 kJ·mol^?1). Moreover, we carried out some thermokinetic investigations on heat emission peaks at different coverage degree (different equilibrium pressure of 1-propanol vapour as a function of time) and at different temperatures of adsorption, at same coverage.     

Calculation of molecular chemisorption on a crystal surface

Theoretical and Experimental Chemistry -     

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.     

Grafting of chains organo-silane on silica surface: a quantum chemical investigation

Theoretical calculations have been carried out on the grafting of two chains organo-silane compounds on SiO₂ hydroxylated solid surfaces. Considering two different silylated coupling agents, two grafting stable complexes are obtained. These complexes are stabilized by two interactions: (i) the chain is grafted to the cluster with a covalent bond SiOSi; (ii) the chain interacts with the cluster via an hydrogen bond HOO in the other side of the chain. The electronic, geometrical and vibrational properties of these systems are analysed. These results give new insight about the grafting of long chains organo-silane on silica surfaces.     

Potentiometric study of the protonation and distribution equilibria of 2-chlorophenol in NaCl medium at 25 degrees C. Construction of a thermodynamic model

The potentiometric determination of the protonation constant of 2-chlorophenol in NaCl media at different ionic strengths and its distribution coefficient between these media and two organic solvents at 25 degrees C are presented. An automated potentiometric system was used, and the determination of the constants was carried out using both graphical and numerical methods. A thermodynamic model using the modified Bromley methodology has been constructed for the prediction of protonation constants and distribution coefficients in NaCl media. The relevance of this study to the development of supported liquid membrane recovery systems is discussed.     

Theoretical study of the hydrogen chemisorption on a ZnO surface

Reactions of a hydrogen molecule with a ZnO surface are studied by an ab initio method. For simulating the ZnO (10 1 0) surface, one ZnO molecule both with and without a Madelung potential is used. Since the electrostatic potential due to the ionic layer decreases exponentially, the effect of the layers deeper than the second one can be neglected. The Madelung potential is, therefore, expressed by the 32 point charges of ±0.5 situated on the first and second layers. Several low-lying states of ZnO and the ZnO + H₂ system have been calculated by the symmetry-adapted cluster (SAC ) and SAC –CI methods. It is found that the ¹Σ⁺ state of ZnO is the ground state and catalytic active and the other states are inactive. ZnO (¹Σ⁺) reacts with H₂ and dissociatively adsorbs it with making Zn? H and O? H bonds. This occurs both with and without the Madelung potential. Without the Madelung potential, the heat of reaction is 81.3 kcal/mol and the reaction barrier is 14.0 kcal/mol. With the Madelung potential, the heat of reaction decreases to 73.5 kcal/mol and the barrier decreases to 11.5 kcal/mol. The mechanism of this reaction is the electron donation from the 2pπ orbital of O to the antibonding σ_u MO of H₂ and the back-donation from the bonding σ_g MO of H₂ to the LUMO of ZnO. In the intermediate stage of the reaction, the dipole of ZnO works to increase the overlap of the active MOS to make the reaction easier. Throughout the reaction, the in-plane 2pπ orbital of O and the HOMO of ZnO are inactive and work to keep the ZnO bond stable during the catalytic process.     

Quantum chemical investigation of formation of polychlorodibenzo-p-dioxins and dibenzofurans from oxidation and pyrolysis of 2-chlorophenol

Density functional theory (DFT) calculations have been used to obtain thermochemical parameters for formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/PCDF) from the oxidation of 2-chlorophenol. Formation mechanisms of PCDD through radical-radical coupling have been investigated in detail. The sequence of 2-chlorophenoxy radical coupling has been studied. The formation of chlorinated bis keto dimers which results from cross coupling of 2-chlorophenoxy at the ortho carbon bearing hydrogen (a known direct route for PCDF formation) passes through a tight transition structure whose barrier is 9.4 kcal/mol (0 K). Three routes for the formation of the most abundant PCDD/PCDF species (viz., 4,6-dichlorodibenzofuran, 4,6-DCDF, and 1-monochlorodibenzo-p-dioxin, 1-MCDD) in oxidation and pyrolysis of 2-chlorophenol are discussed. In the case of 4,6-DCDF, formation through H or HO + keto-keto <==> H2 or H2O + keto-keto* <==> H2 or H2O + enol-keto* <==> H2 or H2O + 4,6-DCDF + HO is shown to be the preferred route. The other two routes proceed via closed shell processes (keto-keto <==> enol-keto <==> enol-enol <==> H2O + 4,6-DCDF) and (keto-keto <==> enol-keto <==> (H-,OH-) 4,6-DCDF <==> H2O + 4,6-DCDF). Results indicate that 1-MCDD should be the favored product in 2-chlorophenol pyrolysis in agreement with experimental findings. According to our results, tautomerization (inter-ring hydrogen transfer) and intra-annular displacement of HCl would not be competitive with paths deriving from H abstraction from the phenolic oxygen and the benzene ring followed by displacement of Cl in the formation of dibenzo-p-dioxin (DD) and 1-MCDD. The results presented here will assist in construction of detailed kinetic models to account for the formation of PCDD/PCDF from chlorophenols.     

Hydration of Cm3+ in aqueous solution from 20 to 200 degrees C. A time-resolved laser fluorescence spectroscopy study

Time-resolved laser fluorescence spectroscopy (TRLFS) is used to study the hydration of the Cm3+ ion in acidified (0.1 M perchloric acid) H2O and D2O from 20 to 200 degrees C. Strong temperature dependency is found for several of the spectroscopic quantities associated with the 6D'(7/2) --> 8S'(7/2) photoemission spectra, with similar relative changes in both solvents. The emission band shifts to lower energy with increasing temperature, which is attributed to an equilibrium between hydrated Cm3+ ions with different numbers of water molecules in the first coordination sphere, namely [Cm(H2O)9]3+ and [Cm(H2O)8]3+. Comparison with crystalline reference compounds and the analysis of hot bands corroborates the assignment of these species. The molar fraction of the octahydrated species increases from approximately 10% at room temperature to approximately 40% at 200 degrees C, indicating an entropy driven reaction. The corresponding thermodynamic parameters are obtained as Delta H degrees = + 13.1 +/- 0.4 kJ mol(-1), Delta S degrees = + 25.4 +/- 1.2 J mol(-1) K(-1), and Delta G298 = + 5.5 +/- 0.6 kJ mol(-1). Both the emission intensity and lifetime decrease with increasing temperature. The temperature dependency of the nonradiative decay rate of the emitting 6D'(7/2) level follows an Arrhenius equation with the activation energy 26.5 kJ mol(-1) (2250 cm(-1)) in both H2O and D2O, which is somewhat lower than the energy gap between 6D'(7/2) and 6P'(5/2) exited state levels.     

A simple FTIR technique for estimating the surface area of silica powders and films

A simple technique using FTIR spectroscopy to estimate the surface area of porous and non-porous silica powders is presented. The surface area is estimated by comparing the integrated area of the band due to isolated silanol groups on different silicas. We have found that by using a fumed silica as a calibrant, an accuracy of about 7% in the surface area of several silica materials is obtained when compared to the surface area computed by BET nitrogen adsorption techniques. The FTIR technique for computing surface area is simple and takes very little time to complete the analysis. The principle advantage of this method is that it enables surface area measurements of silica films on porous supports. To the best of our knowledge, there are no other methods that provide this information.     

Hydrolysis of a two-membered silica ring on the amorphous silica surface

We have combined density functional theory (DFT) with classical interatomic potential functions to model hydrolysis of amorphous silica surfaces. The water-silica interaction is described by DFT with incorporation of a long-range elastic field described by classical interatomic potentials. Both physisorption and chemisorption of water on a surface site, known as the two-membered silica ring, are studied in detail. The hybrid quantum-mechanical and classical mechanical method enables more realistic treatment of chemical processes on an extended surface than previous methods. We have studied cooperative events in the hydrolytic reactions and discovered a new reaction pathway that involves a double proton transfer process. In addition, the evaluation of the total energy in a hybrid quantum-mechanical and classical mechanical system is discussed.     

Theoretical study of the reactions of 2-chlorophenol over the dehydrated and hydroxylated silica clusters

Silica is the main component of combustion-generated fly ash and is expected to have an important impact on the formation of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) in municipal waste incinerators. In this work, we theoretically studied the reactions of 2-chlorinated phenol (2-CP) over the clusters (SiO(2))(3) and (SiO(2))(3)O(2)H(4), which mimic the dehydrated and hydroxylated silica structures, respectively. The dehydrated cluster is much more active toward the attack of 2-CP to form highly stable 2-chlorophenolate than the hydroxylated silica cluster. The further dissociation of chlorophenolates to form CP radicals (CPRs) is calculated to be very difficult. The calculated energy barrier of the reaction of 2-CP over the dehydrated (SiO(2))(3) cluster and IR data are in good agreement with early experimental observations. On the basis of the calculated results, we propose that the formation of PCDD/Fs from CPs over silica surfaces may not involve CPRs, but be relevant to the further conversion of chlorophenolates over silica surfaces. This mechanism is very different from the corresponding reactions mediated by transition metal oxides. The results presented here may be helpful to understand the chemisorption mechanism of CPs on silica surfaces in real waste combustion.     

Energy transfer on the MgO surface,monitored by UV-induced H2 chemisorption

Surface anions on edges (4-coordinated = 4C) and on corners (3-coordinated = 3C) of cubic MgO nanoparticles exhibit UV resonance absorptions around 5.5 and 4.6 eV, respectively. After monochromatic excitation of either site the electron paramagnetic resonance (EPR) spectrum exhibits exclusively signal components related to 3-coordinated O- radicals (O-(3C), electron hole centers), which are perfectly bleached by H(2) addition. The disappearance of the O-(3C) EPR signal components is paralleled by a depletion of the UV resonance absorption of the 3-coordinated O(2-) only and the appearance of one single band in the OH stretching region of the IR spectrum. Obviously the sites of UV excitation and subsequent UV induced surface reaction with H(2) are not the same. This may coherently be explained in terms of mobility of the exciton (O(2-)(4C)* or--after ionization--of the corresponding electron hole O-(4C) along the edge where it was created. Finally the mobile state is trapped at a corner site where the O(3C)H group is formed.     

The effect of hydrogen chemisorption on titanium surface bonding

The effect of hydrogen chemisorption on the strength of Ti-Ti bonds is studied byab initio configuration interaction techniques using an embedding theory to describe the electronic structure. A Ti adatom on Ti(0001) is modelled by a Ti₂₀H cluster with boundary potentials determined from the embedding treatment. Hydrogen atom chemisorption is highly exothermic for adsorption atop the adatom, a three-fold site formed by the adatom and in the interstitial site below the adatom. Compared to the planar Ti(0001) surface the adatom region binds hydrogen much more strongly. Removal of Ti from the surface is energetically much more favorable if H remains on the surface as opposed to the removal of TiH. The exchange reaction Ti₂₀+HTi₁₉H+Ti is endothermic by 0.3 eV. These results suggest high reactivity of the adatom region on Ti(0001) but not such that the surface is more easily fragmented by removal of Ti or TiH.Dedicated to Professor J. Koutecký on the occasion of his 65th birthday     

Single-step dispersion of functionalities on a silica surface

A new process for coating a mesoporous silica gel with a mixture of the grafting reagents para-aminophenyltrimethoxysilane and phenyltrimethoxysilane is thoroughly analyzed. The dilution of para-aminophenylsilane with phenylsilane at different ratios allows the density of the functional amino groups present on the silica surface to be controlled, while keeping constant the overall number of grafts. Furthermore, the choice of a rigid linker prevents undesirable interactions between the active function and the inorganic support that could alter the function reactivity. This simple and new method, which results in the improvement of the dispersion of a functionality in a one-pot synthesis, could be particularly interesting in the field of supported catalysis and molecular recognition. The dispersion of the functional groups of the synthesized hybrid solids is investigated using a pyrene derivative covalently linked to the free amino groups of the para-aminophenylsilanes by analyzing the excimer and monomer fluorescence properties of the probe.     

Inelastic scattering and chemisorption of CO on a Pt(111) surface

A semiclassical model is used to calculate energy transfer in collisions between CO and a Pt(111) surface. The sticking probability is found to be as large as 0.7–0.8 for small collision energies (≈0.14 eV). At higher energies (≈5 eV) it decreases to ≈0.3. Strong interaction between the adsorbed molecule and the phonons is observed.     

Dissociative chemisorption of D2 on a Ni13 cluster

Selected characteristic results of a quasiclassical trajectory study of dissociative adsorption of a D₂ molecule on a Ni₁₃ cluster are presented. These include detailed probabilities as functions of the impact parameter and of the relative translational energy of D₂, and cross sections as functions of this energy. The roles of the initial rovibrational state of the D₂ molecule and of the initial temperature of the cluster are examined. The effect of freezing the cluster into a rigid geometry is tested.