Kinds Of Active Sites Research Articles

Investigation on Surface Metallic Active Structure of Supported Ruthenium Catalyst

The metallic surface structure of supported Ru catalysts, activated at different pretreatment conditions, was investigated by following the development of IR bands due to adsorbed CO, as well as by following the methane formation in temperature-programmed surface reaction (TPSR-MS) of adsorbed CO in a H-2 flow. It revealed that there existed three kinds of active sites, designated as S-1, S-2 and S-3 sites,respectively. The S-1 sites, characterized by low frequency (LF) bands of 1980 similar to 2060cm(-1) for linearly adsorbed CO, were attributed to metallic Ru particles with almost no interaction with the support. The TPSR-MS results showed that, when reaction with H-2, the adsorbed CO on these sites changed into CH4 at a lower temperature of ca. 450K, varying with activated conditions. On the other hand, the S-2 sites were visualized to be interfacial Ru particles in direct contact with supporting materials, and exhibited stronger interaction with support. As a result of this MSI effect, the S-2 sites would result in the formation of electron-deficient Ru delta+ sites, and maintain high dispersive states of metal catalysts, and also adsorb CO with configuation of Ru delta+-CO at 2075 +/- 5cm(-1) medium frequency (MF2). Upon hydrogenation, the adsorbed CO formed methane at a much higher temperature of ca. 650K. Obviously, the S-1 and S-2 sites are static sites of the conventional nature.

Heterogeneous nucleation of aspartame from aqueous solutions

Waiting times, the time from the instant of quenching needed for a first nucleus to appear, were measured at constant supercoolings for primary nucleation of aspartame (α-L-aspartyl-L-phenylalanine methylester) from aqueous solutions, which were sealed into glass ampoules (solution volume = 3.16 cm 3). Since the waiting time became shorter by filtering the solution prior to quenching, the nucleation was concluded to be heterogeneously induced. The measured waiting time consisted of two parts: time needed for the nucleus to grow to a detactable size (growth time) and stochastic time needed for nucleation (true waiting time). The distribution of the true waiting time, is well explained by a stochastic model, in which nucleation is regarded to occur heterogeneously and in a stochastic manner by two kinds of active sites. The active sites are estimated to be located on foreign particles in which such elements as Si, Al and Mg were contained. The amount of each element is very small in the order of magnitude of ppb (mass basis) of the whole solution. The growth time was correlated with the degree of supercooling.

Support effect on the catalytic activity and properties of sulfided molybdenum catalysts

The effect of the support on the catalytic activity and properties of molybdenum sulfide catalysts was investigated in order to obtain the fundamental information necessary for designing better hydrotreating catalysts. The catalytic activity for each catalyst was evaluated using model test reactions under high hydrogen pressure. The order of supports for hydrogenation (HYD) of 1-methylnaphthalene was A1 2O 3 > TiO 2 > MgO > SiO 2, while that for hydrocracking (HYC) of diphenylmethane and hydrodesulfurization (HDS) of dibenzothiophene was TiO 2 > SiO 2 > A1 2O 3 > MgO. Characterization of catalysts clarified that the catalytic activities depend on the structure and properties of the metal oxide which is the precursor of the active sulfide species. Formation of highly dispersed two-dimensional polymolybdate structures, which were observed on the A1 2O 3 and the TiO 2 supports, were the most favorable for high HYD activity. In contrast, the data indicate that the enhancement of HYC activity requires an electronegative form of the molybdenum species on which Brønsted acid sites can be formed under the reaction condition. HDS of DBT can be catalyzed on both kinds of active sites, although the product distribution is different.

Local reaction environments and their properties for ethene deuterogenation on the surfaces of SMSI catalysts

Ethene deuterogenation and H2–D2 exchange reaction over Nb2O5-supported Rh and Ir catalysts have been investigated in relation to strong metal–support interaction (SMSI) phenomena. The activation energies for these reactions were considerably changed by high-temperature reduction of the catalyst in the case of Ir/Nb2O5, but were not modified in the case of Rh/Nb2O5. The change is ascribed to a reduction in the energy barrier for deuterium dissociation. The deuterium distribution in ethane formed during ethene deuterogenation was also investigated at various reaction temperatures and as a function of the reduction time of the catalyst. By studying the catalysts in their working state instead of by static adsorption measurements two kinds of active sites in different environments are suggested to exist on the surface of these catalysts in the SMSI states. One of these sites (site I) is on the bare metal surface and the other (site II) is on the perimeter of a migrating NbOx island. The surface isotopic ratio of hydrogen during ethene deuterogenation is different at sites I and II. Site I, on which D2 dissociates, acts as a deuterium supply for site II. A model for the deuterogenation of ethene on the SMSI catalysts is proposed.

Magnesium chloride supported high mileage catalysts for olefin (decene‐l) polymerization XIV. Propagation and chain transfer

AbstractDecene‐l was polymerized with the MgCl2/ethylebenzoate/p‐cresol/AIEt3/TiCl4‐AlEt3/methyl‐p‐toluate catalyst at 50° using an A/T ratio of 167 and a range of monomer concentration. The concentration of the two kinds of active sites are [Ti] = 12% and [Ti] = 4% of the total titanium. The rate constants of propagation are 24 M−1 s−1. Chain transfers to AIEt3, monomer, and by β‐hydride elimination have rate constant values of 1.7 × 10−3 M−1 s−1, 1.34 × 10−2 M−1 s−1, and 1.7 × 10−2 s−1, respectively. Poly(decene‐l) have relatively narrow MW which are unchanged during the course of a polymerization. Therefore, the active site concentrations in the CW catalyst for propylene and decene polymerization are identical and their rate constant values agree within a factor of 2. However, the rate of decene polymerization depends on fractional order of monomer concentration and decreases with the increase of activator concentration. Furthermore, the formation of metal polymer bonds has a rate independent of these concentrations. These kinetic behaviors are a manifestation of absorption processes of these species which are not seen in propylene polymerizations.

Linear potential scan voltammetry for irreversible co-adsorption of electroactive species

Irreversible electrochemical co-adsorption of cations on two different kinds of active sites on the electrode surface has been studied theoretically for the conditions of the linear potential scan voltammetry (LPSV). The analytical expressions for the LPSV peak currents and peak potentials for the partial co-adsorption processes have been derived. The normalized, dimensionless LPSV characteristics have been computed using an implicit digital simulation procedure. The effect of the lateral self-interactions and exchange interactions, as well as effects of the kinetic parameters on the LPSV curves have been examined. An interesting phenomenon of splitting of the partial-current peak (for the adsorption reaction on one kind of active site) caused by coadsorption has been found. The potential distributions of the partial coverages have also been analyzed.

Characterization of the ATPase active site in myosin subfragment-1 with the use of vanadate plus ADP as a reversible "affinity-labeling" reagent: evidence for heterogeneity in the active sites.

Our previous work showed that the active site heterogeneity in heavy meromyosin (HMM) becomes evident when highly reactive SH-groups in HMM are modified by thimerosal (Kawamura, Higuchi, Emoto, & Tawada (1985) J. Biochem. 97, 1583-1593). The heterogeneity was revealed by "affinity-labeling" analysis with vanadate plus ADP, which was developed in the previous paper. To see whether this heterogeneity is due to the head-head interaction or two different alkali light chains present in HMM, we carried out similar studies with myosin subfragment-1 (S1) and one of the isozymes, S1(A1), which contains only the alkali light chain 1, and obtained essentially the same results as those previously obtained with HMM. The S1 results are easily explained by the same hypothesis previously used for explaining the HMM results: SH-modified S1 or S1(A1) contains two kinds of active site in a 1:1 ratio with almost the same ATPase activity: one hydrolyzes ATP by a mechanism giving a protein Trp fluorescence enhancement, whereas the other hydrolyzes ATP by another mechanism giving no fluorescence enhancement.

On the active site of β-hexosaminidase from latex of Ficus glabrata

N-Acetyl-β- d-hexosaminidase (EC 3.2.1.52), active against both p-nitrophenyl- N-acetyl-β- d-glucosaminide and p-nitrophenyl- N-acetyl-β- d-galactosaminide, is present in latex of Ficus glabrata. The final specific activity was increased 150-fold from crude extract after ammonium sulphate fractionation and affinity chromatography on Concanavalin A-Sepharose. The activity ratio β- N-acetylglucosaminidase-β- N-acetylgalactosaminidase remained constant. Substrate competition, competitive inhibition studies and Arrhenius plots confirm that, in the hexosamidase, only one kind of active site is responsible for both activities. Acetate and acetamide are more effective competitive inhibitors than iodoacetamide, N-acetylglucosamine and N-acetylgalactosamine more than glucosamine and galactosamine, and α-methylmannoside more than mannose, suggesting that the active site binds the N-acetyl moiety of the substrate and a hydrophobic interaction of the methyl group is involved. The difference between the strength of the inhibition by mannosamine with respect to glucosamine and galactosamine, that do not inhibit, seems to be due to the position at C-2 of the amino group in the pyranose ring.

炭化水素の気相接触酸化

The background and main points of recent works on vapor phase oxidation of C3, C4 olefins and ethylbenzene with multi-component oxide catalysts are described.Fe-As and Mo-Ce (or Al, Ba, Sr) -As oxides were found to have high catalytic activity for producing acrolein by vapor phase oxidation of propylene with air. The activity of Fe-As oxide decreased with reaction time because of the escape of As component, especially in the reaction at high temperature, such as oxidation of i-butylene to methacrolein.This defect was eliminated and initial activity was kept by adding Li component to Fe-As system. With Fe-Li-As oxide of atomic ratio= 1 : 2 : 2.5 1 : 3 : 3, high yield of methacrolein was obtained from i-butylene at the temperature of 470°C.By means of searching direct oxidation of the olefins with Fe-As5+ oxide systems and formation of As5+ in As3+ oxide systems and studying the structure of catalyst, following conclusions were obtained.The catalytic oxidation of α-CH bond to aldehyde is carried out by a cycle consisting of two reactions : 2 As5+ oxide + CH2 = CR-CH3 → 2 As3+oxide CH2= CR-CHO and2 As 3+oxide + O2 → 2 As5+oxide (R : H or -CH 3).The former reaction, remarkably promoted by the Fe component in the catalyst, is always accompanied with formation of CO2, but proceeds more dominantly when As5+ concentration is high. Two structures of -Fe-O-As- and -Li -O-As- were observed in Fe-Li-As oxide catalyst; the former is more easily reduced with olefin than the latter.-Fe-O-As- structure has at least two kinds of active site. Unsaturated aldehyde forms on one site and CO2 on another site. Li-O-As structure is stable and instrumental in preventing the escape of As component into vapor phase.The activity of Mo-X-As (X : alkaline earth metal) oxide catalyst for oxidative dehydrogenation of 1-butene was studied in connection of its preparation and physical property.A Si-Ce-Sb oxide catalyst of new type showed relatively high activity for styrene formation by oxidative dehydrogenation of ethylbenzene.

Adsorption characteristics of nitric oxide on various adsorbents determined by gas—solid chromatography

The adsorption of nitric oxide on 13X and 5A molecular sieves, alumina (boehmite) and nickel—kieselguhr and copper oxide—zinc oxide catalysts was studied by using a chromatographic technique. Moment analysis was used for determining the adsorption equilibrium constant K A. The nitric oxide is purely physically adsorbed on 13X molecular sieve and alumina in the ranges of temperature between 50 and 100 °C and between −8 and 50 °C respectively. The adsorption capacities of 13X molecular sieve and alumina are 0.5 cm 3 g −1 and 1.0 cm 3 g −1 respectively at 50 °C. The nitric oxide reacts with 5A molecular sieve and changes its adsorption capacity with time from 3.4 cm 3 g −1 for fresh adsorbent at 100 °C. For the nickel—kieselguhr catalyst the adsorption of nitric oxide at 50 °C and 110 °C takes place on two kinds of active sites with slow and fast reversible adsorption respectively. The total equilibrium constant, for fast and slow adsorption, is 0.90 cm 3 g −1 at 50 °C and 0.32 cm 3 g −1 at 110 °C. The nitric oxide reacts with the copper oxide—zinc oxide catalyst between 50 and 200 °C and changes the physical properties of the bed probably as a result of shrinkage of the particles. Therefore, quantitative data could not be obtained for this system.

The effect of sulphidation on the nature of active sites on nickel-molybdenum-alumina hydrorefining catalysts

A strong influence of sulphidation on the hydrogenation and hydrogenolytic activity of Mo-Al2O3 and Ni-Mo-Al2O3 hydrotreating catalysts has been found. Two kinds of active sites are distinguished, the first consisting of surface molybdenum atoms and serving mostly for hydrogenation, the second type formed by interaction of Mo with Ni atoms and being active in sulphided form for direct hydrogenolysis of the C-S bonds.

Conversions in catalytic deamination calculated by stopped-flow gas chromatography

By calculating absolute values for the intercepts of the rate equation that describes the deamination of aminocyclohexane on aluminium oxide, the components of a weighted mean rate constant can be found. From these the conversion of the reactant amine to product(s) can be computed, and this is in good agreement with the conversion determined directly by separate experiments. This is an indication of the internal consistency of the theoretical rate equation, derived on the basis of a particular reaction model with many kinds of active sites. The conversions are much less than 100%, showing that a considerable fraction of the reactant is adsorbed on sites which transform it to irreversibly adsorbed products. The conversion to gaseous cyclohexene increases with increasing flow-rate of the carrier gas. Finally, it was found that the effective duration of the stops is very different from the real duration.

Catalytic deamination on solid surfaces. Part 1.—Kinetic studies by stopped-flow gas chromatography

Stopped-flow gas chromatography was used to study the detailed kinetics of formation of unsaturated hydrocarbons by deamination on aluminium oxide and on porous glass of aminocyclohexane, 2-aminopropane, 1-aminobutane, 2-aminobutane, 1-amino-2-methylpropane and 2-amino-2-methylpropane. The results are consistent with a proposed mechanism in which the reacting amine is rapidly adsorbed on two or three kinds of active sites of aluminium oxide or porous glass, respectively, leading to unsaturated hydrocarbons through the formation of an intermediate adsorbed product on each site. Based on this mechanism, theoretical equations are derived which adequately describe the experimental reaction rate as a function of time. Analysis of the data by means of these equations permits the calculation of the various rate constants for the surface processes. True activation energies and frequency factors for the deamination of aminocyclohexane on aluminium oxide have also been determined.

Ideal stopped-flow gas chromatography with chemical reactions in the stationary phase

By assuming a general mechanism for surface reactions on a chromatographic column, equations are derived from which can be calculated the elution curves of the reactant, the products and the peaks produced by stopping the carrier-gas flow for a short time. Particularly, the area under the curve of these stop-peaks can be calculated as a function of the time of cessation of the gas flow, and this can be used to determine, from experimental values, the rate constants of the surface reactions. Application of the method requires specification of the mechanism assumed in each case. As an example, the equations derived are applied for a simple first-order reaction on one kind of active sites, with no intermediate. In this limiting case, two of the final equations coincide with the results of earlier static derivations.

Hydrogenation of ethylene on a MoS 2 catalyst

The evacuated MoS 2 surface has two kinds of active sites with different degrees of coordinative unsaturation named B-sites and C-sites. The hydrogenation of ethylene, the exchange between D 2 and C 2H 4 and the H 2D 2 equilibration proceed on the CH 2 and CH-sites, however, the BH-sites promotes the hydrogen exchange between C 2H 4 and C 2D 4. The hydrogen addition on the CH 2-sites should maintain the hydrogen molecular identity, but facile reverse process of the ethyl intermediate on the CH 2-sites lowers the conservation of the hydrogen molecularity in the ethane compared with the other olefins. By treating with H 2S, the C-sites on the surface are diminished, but the specific sites remained in the layer of the MoS 2 promote the H 2D 2 equilibration.

Evidence for two kinds of active sites on a molybdenum sulfide

Evidence for two kinds of active sites on a molybdenum sulfide

The nature of active sites on aluminum phosphate for the isomerization of butenes

The nature of active sites on aluminum phosphate for the isomerization of n-butenes was investigated by means of a tracer study which consisted of isomerization over deuterated catalyst and co-isomerization of light and perdeuterated butene. There exist three kinds of active sites, Brönsted acid sites, Lewis acid sites where intermolecular hydrogen transfer occurs (L-inter sites), and Lewis acid sites where intramolecular hydrogen transfer occurs (L-intra sites). Their contributions to n-butene isomerization, which were quite dependent on the thermal treatment of the catalyst, were calculated for different evacuation temperatures. At low evacuation temperature, Brönsted acid sites and L-inter sites are the main active sites, while L-intra sites become prominent at high evacuation temperature. L-intra sites are characteristically active for cis-trans rotation. The adsorption state of butene molecules was also examined. From the isotopic distributions of butene isomers, it was concluded that secondary butyl carbenium ion intermediates on Brönsted acid sites and L-inter sites were adsorbed weakly, and all bonds of the intermediates rotated freely. Over L-intra sites, the isomerization may be initiated by the abstraction of H − at the vinyl hydrogen of butene.

Temperature-dependent change in rate-limiting step of the magnesium-stimulated ITPase of myosin.

The effects of temperature on Mg-ITPase activity of heavy meromyosin and myosin subfragment 1 were measured in 0.1 M KC1. The initial burst of Pi liberation was one mol per mol of heavy meromyosin or two mol of myosin subfragment 1, i.e. one mol per two mol of myosin active sites, at 20 degrees C. However, it was almost zero mol below 8degrees C. Effects of KC1 concentration and pH on ITPase activity of heavy meromyosin at 20 degrees C were different from those below 8 degrees C, suggesting that the rate-limiting step in the Mg-ITP hydrolysis of myosin depends on temperature. The effect of temperature on the actin activation of heavy meromyosin Mg-ITPase was analyzed by measuring the temperature dependence of double-reciprocal plots of ITPase activity against actin concentration. The extent of actin activation was larger at low temperture. The results presented in this paper might be explained by assuming the existence of two kinds of active sites on a myosin molecule.

Kinetic study of slow processes in gas chromatography

A method is developed for determining the rate constant of slow diffusion and/or slow adsorption-desorption processes in gas chromatography. Rate-constant measurements have been performed by applying the method to the desorption of n-butane and trans-butene from an aluminium oxide surface modified with cerium trichloride. The kinetic feature of every desorption experiment in the systems studied is two distinct rate constants, which might be attributed to two kinds of active sites on the surface. The activation parameters of desorption corresponding to the two kinds of sites have been determined from Arrhenius plots.

CATALYST FOULING IN THE DEHYDROGENATION OF n-BUTANE OVER CHROMIA-ALUMINA CATALYSTS

The fouling mechanism of the dehydrogenation of n-butane over chromia-alumina catalysts was studied both theoretically and experimentally. The reaction experiments were carried out using three kinds of chromia-alumina catalysts of different compositions and it was clarified that dehydrogenation of n-butane was effective only on chromia, whereas formation occurred on alumina as well as on chromia. The experimental results could be well explained by the dual active site model in which two kinds of active sites on the catalytic surface and a one-to-one correspondence between the number of molecules of coke and the number of active sites were assumed.