Kinds Of Active Sites Research Articles

Surface Active Sites: An Important Factor Affecting the Sensitivity of Carbon Anode Material toward Humidity

We report that various kinds of active sites on graphite surface including active hydrophilic sites markedly affect the electrochemical performance of graphite anodes for lithium-ion batteries under different humidity conditions. After depositing metals such as Ag and Cu by immersion and heat-treating, these active sites on the graphite surface were removed or covered and its electrochemical performance under the high humidity conditions was markedly improved. This suggests that lithium-ion batteries can be assembled under less strict conditions and provides a valuable direction toward lowering the manufacturing cost for lithium-ion batteries.

Methanol steam reforming over Cu/ZnO/Al 2O 3 catalyst: kinetics and effectiveness factor

Steam reforming of methanol over a commercial Cu/ZnO/Al 2O 3 catalyst was studied at atmospheric pressure and in a temperature range between 160 and 260 °C. The reaction rate depended upon methanol and hydrogen partial pressures, and was independent of the partial pressures of carbon oxides and water, which was in excess of the methanol partial pressure. Small amounts of carbon monoxide, less than 1% in the product gas, were formed at high temperatures; the amounts were well below the equilibrium amounts of reverse water–gas-shift reaction (RWGS). This was in support of the reaction sequence of methanol steam reforming followed by the RWGS. A power-law and a Langmuir–Hinselwood rate expression were developed for the reforming reaction by fitting the expressions to the experimental data. As the data were found to be affected by internal diffusion at high temperatures, the effectiveness factor of the catalyst particle was estimated in the fitting in order to obtain the intrinsic kinetics. Details of the estimation of the factor are elucidated. In order to predict a non-zero, finite rate in the absence of hydrogen, the hydrogen partial pressure term in the power-law expression was corrected by a fitted constant to avoid an infinite reaction rate, since the exponent of the hydrogen partial pressure was a negative number due to the hydrogen inhibition effect in the reforming; in the reaction mechanism for the Langmuir–Hinselwood expression, it was necessary to assume two different kinds of active sites on the catalyst: one for adsorbed methoxy and the other for adsorbed hydrogen. In addition, an excellent fitting of the data by the Langmuir–Hinselwood expression indicates that dehydrogenation of the adsorbed methoxy to the adsorbed oxymethylene is the rate-determining step (RDS), and that adsorption of all the species other than methoxy and hydrogen on the active sites is negligible.

Gas phase polymerization of 1,3‐butadiene with supported neodymium‐based catalyst: Investigation of molecular weight

AbstractThe gas phase polymerization of 1,3‐butadiene (Bd), with supported catalyst Nd(naph)3/Al2Et3Cl3/Al(i‐Bu)3 or/and Al(i‐Bu)2H, was investigated. The polymerization of Bd with neodymium‐based catalysts yielded cis‐1,4 (97.2–98.9%) polybutadiene with controllable molecular weight (MW varying from 40 to 80 × 104 g mol−1). The effects of reaction temperature, reaction time, Nd(naph)3/Al(i‐Bu)3 molar ratio, and cocatalyst component on the catalytic activity and molecular weight of polymers were examined. It was found that there are two kinds of active sites in the catalyst system, which mainly influenced the MW and molecular weight distribution of polybutadiene. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1945–1949, 2004

906 Nutritional counselling vs commercial supplements vs ad lib: a prospective randomised controlled trial in head-neck cancer patients undergoing radiotherapy

To better understand the nature of carbon nanotubes supported Co-Mo catalysts (Co-Mo/CNTs) for selective hydrodesulfurization (HDS) of fluid catalytic cracking (FCC) gasoline, studies are carried out using in situ Fourier transform infrared spectroscopy (FT-IR). The catalytic performances of Co-Mo/CNTs catalysts were evaluated with a mixture of cyclohexane, diisobutylene, cyclohexene, 1-octene (60:30:5:5, volume ratio) and thiophene (0.5%, ratio of total weight) as model compounds to simulate FCC gasoline. The HDS experimental results suggested that the HDS activity and selectivity of Co-Mo/CNTs catalysts were affected by Co/Mo ratio; the optimal Co/Mo atomic ratio is about 0.4, and the optimum reaction temperature is 260° C in situ FT-IR studies revealed that 1-octene can be completely saturated at 200° C. In the FT-IR spectra of diisobutylene, the characteristic absorption peak around 3081 cm−1 for the stretching vibration peak of =C-H bond was still clear at 320° C, indicating that diisobutylene is difficult to be hydrogenated. As for the thiophene, no characteristic absorption peak could be found around 3092 cm−1 and 835 cm−1 when the reaction temperature was raised to 280° C, indicating that thiophene had been completely hydrodesulfurized. On the basis of FT-IR results, it can be deduced that thiophene HDS reaction occurred mainly through direct hydrogenolysis route, whereas thiophene HDS and diisobutylene hydrogenation reaction over Co-Mo/CNTs catalysts might occur on two different kinds of active sites.

Kinetic evaluation of various isospecific active sites on MgCl2‐supported Ziegler catalysts

AbstractThe formation, variation and conversion of isospecific active sites were investigated, based on the isotacticity distribution of the polypropenes analyzed by the temperature rising elution fractionation (TREF) method. Stopped‐flow polymerization of propene was carried out with a MgCl2‐supported Ziegler catalyst in the absence or presence of an internal or external electron donor so that the effects and roles of the electron donors could be clarified. The results showed that various kinds of active sites with different isospecificities, including the highest isospecific active sites responsible for producing the highest isotactic fraction (elution temperature: > 112°C) existed, even in the electron donor‐free catalyst system. The isospecificity of the active sites in the donor‐free catalyst might have originated from a surface monolayer multinuclear titanium species, namely an “island” of titanium species. The addition of the external electron donor converted a part of the aspecific and/or low isospecific active sites into the second highest isospecific active sites, but showed no effect on the formation of the highest isospecific active sites, whereas the addition of an internal donor played an important role in creating the highest isospecific active sites as well as suppressing the formation of the aspecific active sites.

Steam Reforming and Graphite Formation on Ni Catalysts

Based on density functional theory calculations, kinetic measurements, microkinetic and Monte Carlo simulations, thermogravimetric analysis (TGA) experiments, extended X-ray absorption spectroscopy (EXAFS) measurements, and experimental results from the literature, we present a detailed and comprehensive mechanistic picture of the steam reforming process on a Ni catalyst. The complete potential energy diagram for the full reaction is presented and compared to experiments. Structures, energies of all reaction intermediates, and activation barriers are identified. We conclude that there are at least two kinds of active sites with different reactivities for steam reforming: a more active one associated with defect (step) sites, and a less active one associated with close-packed facets. It is further suggested that the nucleation of graphite is initiated at the step sites. We show that additives like potassium, sulfur, and gold preferentially bind to the step sites of Ni, and on this basis it is suggested that the promotion by these additives in terms of a higher tolerance toward graphite formation involves the same basic mechanism: blocking of the step sites.

Methane reforming over Ni/Ce-ZrO 2 catalysts: effect of nickel content

The effect of Ni content on the Ni/Ce-ZrO 2 catalyst has been investigated in the methane conversion reactions to syngas, such as oxy-reforming, steam reforming and oxy-steam reforming. Among the catalysts examined, Ni/Ce-ZrO 2 catalyst with 15% Ni loading exhibits not only the highest catalytic activity and selectivity but also remarkable stability. The TPR results reveal that strong interaction between support and metal exists and that some part of NiO incorporates into the surface of the Ce-ZrO 2 support. Combined with H 2 chemisorption results, one may deduce that Ni surface area and the chemical environment of nickel, as well as the properties of the Ce-ZrO 2 support, play very important roles in the catalytic activity and stability of Ni/Ce-ZrO 2 catalysts. It seems that two kinds of active sites, i.e. one for methane and the other for steam or oxygen, are well-balanced on 15% Ni/Ce-ZrO 2 catalyst.

Copolymerization of butadiene and styrene with neodymium naphthenate based catalyst

Copolymerization of butadiene (Bd) and styrene (St) was carried out in toluene at 50 °C by a conventional rare earth catalytic system, Nd(naph) 3–Al(i-Bu) 3–Al(i-Bu) 2Cl. It exhibited a high catalytic activity and high stereospecificity in the copolymerization. The influences of the conditions in polymerization on the yield, composition, microstructure and molecular weight of copolymer were thoroughly studied. According to the 13 C-NMR spectrum, the resultant copolymer containing 18% St units, and the diad fraction of St– trans Bd or St–vinyl Bd can hardly be found in its 13 C-NMR. The cis-1,4 content of Bd unit of the copolymer decreased little with the increase of St content. The GPC curves indicate the presence of two kinds of active sites in the polymerization.

Characterization of active sites working on FSM-16 during the vapor-phase Beckmann rearrangement of cyclohexanone oxime

The behaviors of ε-caprolactam, cyclohexanone oxime and Beckmann rearrangement by-products on FSM-16 and Al 2O 3/FSM-16 have been investigated in detail by means of TPD and reactions in the presence of organic acids and pyridine. The TPD spectra on FSM-16 of ε-caprolactam, cyclohexanone oxime, Beckmann rearrangement by-products and mixtures of by-products can be explained by postulating that there are four kinds of active sites, i.e. two kinds of nitrile formation sites (Sites A and R), a cyclohexanone formation site (Site B) and a nitrile adsorption site (Site α). The TPDs of cyclohexanone oxime adsorbed on Al 2O 3/FSM-16 in the presence and absence of water vapor suggest that the presence of water accelerates the formation of cyclohexanone on Site B. Addition of acetic acid suppresses conversion of ε-caprolactam to nitriles, and that addition of pyridine has no effect on the TPD spectrum. Addition of acetic acid to the reactant during the Beckmann rearrangement on Al 2O 3/FSM-16 improved the ε-caprolactam yield and catalyst’s life.

A general model for both three-way and deNO x catalysis: dissociative or associative nitric oxide adsorption, and its assisted decomposition in the presence of a reductant : Part I. Nitric oxide decomposition assisted by CO over reduced or oxidized rhodium species supported on ceria

A selective overview of recent studies on both three-way and deNO x catalysis (we shall note ‘deNO x ’, the removal of NO in the presence of an excess of oxygen) leads to an unique and general model of these reactions, based on kinetic concepts. Two kinds of active sites have to be first defined: cationic and zero-valent metal ones. The first type can form either through the reduction of the support ( V)-Ce 3+-( V) or (ii) a strong metal-support interaction ( V)-Rh +-( V)/CeO 2, both linked to adjacent oxygen vacancies ( V) of the reducible support, or (iii) a surface transition metal (TM) complex (TM in zeolite for instance). The second kind of sites are accessible supported-zero-valent noble metal atoms. These two kinds of sites are involved in three-way Catalysts (TWC), whereas only the cationic ones concern deNO x reactions. Therefore, nitric oxide chemisorption can be either ‘associative’ on the first kind of sites — leading to dinitrosyl or hyponitrite species — or ‘dissociative’, on the second ones, leading to oxygen and nitrogen atoms adsorbed on the sites. Two different catalytic sequences of elementary steps can then be defined. On cationic sites, successive N–O bond scissions of dinitrosyl or hyponitrite species occur, potentially able to produce intermediate N 2O, and in all cases leaving oxygen atoms adsorbed on the active site, and inhibiting a further adsorption of NO. A reductant is then necessary to remove these oxygen atoms and permit the reaction to proceed further. On zero-valent metallic sites, at the temperature of reaction, NO suffers a dissociative chemisorption leading again to surface oxygen atoms. Again, a reductant is necessary to remove these oxygen species and permit the reaction to proceed again. In this first paper, TWC are considered and the reductant is CO. Two catalytic cycles are considered based on our results on temperature-programmed desorption and surface reactions of NO in a stoichiometric CO/NO/O 2 mixture.

Functional conservation of the active sites of human and Drosophila angiotensin I-converting enzyme.

Human somatic angiotensin I-converting enzyme (sACE) has two active sites present in two homologous protein domains, resulting from a tandem gene duplication. It has been proposed that the N- and C-terminal active sites can have specific in vivo roles. In Drosophila melanogaster, Ance and Acercode for two ACE-like single-domain proteins, also predicted to have distinct physiological roles. We have investigated the relationship of Ance and Acer to the N- and C-domains of human sACE by genomic sequence analysis and by using domain-selective inhibitors, including RXP 407, a selective inhibitor of the human N-domain. These phosphinic peptides were potent inhibitors of Acer, but not of Ance. We conclude that the active sites of the N-domain and of Acer share structural features that permit the binding of the unusual RXP407 inhibitor and the hydrolysis of a broader range of peptide structures. In comparison, Ance, like the human C-domain of ACE, displays greater inhibitor selectivity. From the analysis of the published sequence of the Adh region of Drosophila chromosome 2, which carries Ance, Acer, and four additional ACE-like genes, we also suggest that this functional conservation is reflected in an ancestral gene structure identifiable in both protostome and deuterostome lineages and that the duplication seen in vertebrate genomes predates the divergence of these lineages. The conservation of ACE enzymes with distinct active sites in the evolution of both vertebrate and invertebrate species provides further evidence that these two kinds of active sites have different physiological functions.

Effect of Water Vapor on Reaction Rates of Limestone-Catalyzed NH3 Oxidation and Reduction of N2O under Fluidized Bed Combustion Conditions

The effect of water vapor concentration on the reaction rates of NH3 oxidation and N2O decomposition catalyzed by calcined limestone was investigated by use of a fixed bed reactor under atmospheric fluidized bed combustion conditions. Both reactions were inhibited by the presence of water vapor. Inhibition of NH3 oxidation by H2O could be expressed as a decrease in the number of available active sites according to Langmuir-type adsorption of H2O on one kind of active site. Inhibition of N2O decomposition by H2O could be approximated by assuming two kinds of active sites, both of which adsorb H2O according to Langmuir-type adsorption equilibrium, but the adsorption equilibrium constant was different between two types of active sites. The equilibrium constant of H2O adsorption on active sites for NH3 oxidation was found to be different from those of H2O adsorption on active sites for N2O decomposition.

Studies of the cerium-metal–oxygen–hydrogen system (metal=Cu, Ni)

In situ X-ray diffraction (XRD), in situ X-ray photoelectron spectroscopy (XPS), electron microscopy and thermogravimetric techniques were used to characterize CeM x O y mixed oxides in the oxidized and H 2 reduced state. The processes occurring during the hydrogen treatment, and the amounts of hydrogen occluded in the solids when treated at different temperatures under flowing hydrogen have also been determined. The CeM x O y oxides can be described as a mixture of copper oxide or nickel oxide and of ceria modified by the insertion of a part of copper or nickel in its lattice. The size of the copper oxide or nickel oxide varies considerably from clusters to a crystallized material, depending on the x value and on the experimental conditions. In the partially reduced state anionic vacancies able to receive hydrogen, probably in a hydridic form, are created in the bulk and at the surface of the solid and a reduction mechanism partly based on heterolytic dissociation of H 2 is proposed (O 2−+□+H 2→H −+OH −). Moreover, it appears that different kinds of active sites which differ from each other in terms of the environment of nickel or copper can exist.

Time Distribution of Adsorption Energies, Local Monolayer Capacities, and Local Isotherms on Heterogeneous Surfaces by Inverse Gas Chromatography

A new inverse gas chromatographic methodology is introduced to measure local (homogeneous) adsorption energies, e, local monolayer capacities, , and adsorption isotherms, θi(p,T,e) for probe gases on heterogeneous solid surfaces in the presence of nitrogen as carrier gas. The method does not depend on analytical solutions of the classical integral equation comprising the adsorption energy distribution function f(e) as unknown, nor on numerical solutions and estimations from this equation, using powerful computers. It simply uses a time function of the chromatographic peaks obtained by short flow-reversals of the carrier gas, combined with the local isotherm model of Jovanovic. All three adsorption parameters e, , and θi(p,T,e) mentioned above can be calculated as a function of the experimental time and refer to instantaneous equilibration of the probe gases with the heterogeneous surface, with different kinds of active sites i being involved at different times. The kinetic physicochemical parameters for t...

Stereoregular polymerization of glycidyl phenyl ether catalyzed by dibutyltin oxide-tributyl phosphate condensates

Polymerization of glycidyl phenyl ether was carried out with the dibutyltin oxide-tributyl phosphate condensate as catalyst. The product was fractionated. The yield of the acetone-insoluble fraction was in the range of 85% to 96%. The tacticity of the polymer fractions was studied using 13 C NMR spectroscopy. The results indicated that the microstructure of PGPE was dependent on the catalyst structure and the reaction conditions. By using the condensate of appropriate molecular weight as catalyst, the isotacticity of the acetone-insoluble fractions can reach as high as 87.4%. In some cases, however, the fractions rich in syndiotactic structure were obtained in addition to the isotactic fraction. It was suggested that more than one kind of active sites were present in the catalyst.

Modelling study of transient behaviour of elimination reactions of alcohols and amines on oxide catalysts

Transient responses of the proposed mechanism of dehydration of alcohols and deamination of amines are investigated theoretically by means of computer modelling. In the mechanism two different kinds of active sites and one blocking (self-inhibiting) step are involved. The nature of the nonstationary production rate increase (the so-called ‘stop-effect’), which was observed experimentally, is analyzed on the molecular level. Relationships between the shapes of transient responses and kinetics of the elementary steps are presented. An unusual illustration of the rate-determining step in the nonsteady state is presented.

Characterizations and catalytic behaviors of vanadium-antimony oxide catalysts in propane ammoxidation

The ammoxidation of propane to acrylonitrile was investigated over a series of V/Sb mixed oxide catalysts. Characterizations of catalysts were carried out by X-ray diffraction, temperature-programmed desorption of ammonia and infrared spectroscopy. The rutile-type of VSbO 4 is found to be highly selective for the formation of acrylonitrile due to its enhancing effect of reoxidation. Correlation of the XRD patterns and NH 3-TPD profiles indicates that the ammonia desorption peak appearing at about 90°C is caused by the ammonia adsorption on the VSbO 4 rutile-type surface. Ammonia inhibits the formation of carbon oxides and acrylonitrile, and therefore the competition between ammonia and oxygen takes place on the same kind of active site. Propane is found to be adsorbed on another site. With regards to the reaction mechanism, it is proposed that the adsorption of propane on the surface occurs first through a formation of propylene-like adsorption intermediate, which appears to react with oxygen to form an oxidized site. The further reaction with ammonia forms a second oxidized site and finally, acrylonitrile is produced from the second oxidized site. A Langmuir-Hinshelwood model fits fairly the experimental data.

Reduction of Lower Concentration NO with Ammonia at Room Temperature over Pitch Based Active Carbon Fibers Calcined and Further Activated with Sulfuric Acid.

Reduction of lower-concentration NO (10ppm) with NH3 (20ppm) over pitch based active carbon fibers (p-ACF) at room temperature was examined to find higher activity in humid air, by further activation with H2SO4, after the calcination. The activation with H2SO4 increased the NO conversion from 50% over the as-received ACF by W/F=5×10-3g•min•ml-1 to 63% in the dry air. The increase of the activity in wet air (80% relative humidity), however, remained in the comparatively lower range of 5 to 12%. The calcination at 800°C is effective to increase the conversions to 63% in dry air, and 23% in wet air. The successive activation with H2SO4 was very effective to enhance the activity in wet air, giving a conversion of 40%, although the conversion in the dry air increased conservatively to 68%. The activation with H2SO4 increased the surface oxygen groups, which can enhance the reduction of NO in dry air. Their excess presence, however, does not lead to higher activity in the humid air because such low concentrations of NO and NH3 may require a small number of highly active sites, while inhibition of H2O may be more important. Very sensitive control of functional groups of surface oxygen and extent of graphitization, thus, appear highly influential on the reduction of NO with NH3 at their present lower level of concentrations, in the humid air at room temperature. The calcination prior to the activation by H2SO4 may remove excess oxygen functional groups introduced at activation by steam to enhance hydrophobicity and to control surface reactivity through partial graphitization. Successive activation by H2SO4 may introduce the optimum number and kinds of active sites containing oxygen functional groups.

CO hydrogenation on alumina pre-treated with sodium acetate aqueous solution

CO hydrogenation was studied over various alumina catalysts, untreated, pre-treated with sodium acetate (NaOAC) and calcined at 773 K after treatment. The catalysts were characterized by means of XRD and IR spectroscopy after pyridine adsorption. An apparent increase of ethylene selectivity over NaOAC pre-treated catalyst was observed. Both CO conversion and ethylene selectivity depended on the temperature of CO hydrogenation and a new γ-Al2O3 layer formed by surface boehmite transformation at elevated temperature on the original alumina surface. Several probe molecules, including HC1, NH3 and pyridine, were used to detect the active sites for CO hydrogenation. There are two kinds of active sites (e.g. Lewis acidic and basic sites) which seem to be involved in the heterolytic dissociation of hydrogen to form a hydroxyl. Thus, the hydroxyl possibly plays a very important role in the formation of formyl, which may be the intermediate of the methoxide, and the methoxide seems to be the intermediate for hydrocarbons.

Hydrogenation catalysts based on nickel and rare earth oxides

In a previous article we selected a preparation process of cerium and nickel oxides which permits catalysts to be obtained which contain as much hydrogen as the intermetallic compounds with the same composition, and which are more active in benzene hydrogenation. Moreover the catalysts' behaviour varies with the Ni/Ce ratio (x) and at least two zones can be distinguished. In the present paper we have found correlations between catalytic activity, hydrogen content and some physical and chemical characteristics of the solids, both in the oxidized and reduced states. The techniques used were electron microscopy (TEM, SEM, EPMA), X-ray diffraction and X-ray photoelectron spectroscopy. Whatever the method, the catalysts are classified into two families. Forx⩽ 0.5 nickel is inserted in the ceria lattice to form a solid solution. Above 0.5, both crystallized nickel oxide and solid solution coexist. In the reduced state anionic vacancies able to receive hydrogen, probably in an hydridic form, are created in the bulk and at the surface of the solid solution. The catalytic results can then be explained by assuming the existence of three kinds of active sites which differ from each other in terms of the environment of nickel, and an explanation for the higher efficiency of the catalyst withx =5 is advanced. Finally the situation is shown to be almost identical in ceria-supported nickel catalysts.