A comparative study of N2O conversion to N-2 over Co/AC and Cu/AC catalysts

Catalytic conversion of N2O to N-2 over Cu- and Co-impregnated activated carbon catalysts (Cu/AC and Co/AC) was investigated. Catalytic activity measurements were carried out in a fixed-bed flow reactor at atmospheric pressure. The catalysts were characterized by N-2 adsorption, X-ray diffraction (XRD) and thermogravimetric analysis (TGA). This study aimed to provide insights into the following aspects: the metal dispersion, changes in pore structure, influence of catalyst loading on reaction, and reaction mechanism. Increasing loading of Co or Cu led to decreasing dispersion, but 20 wt % loading was an upper limit for optimal activities in both cases, with too high loading causing sintering of metal. Co exhibited a relatively better dispersion than Cu. Impregnation of metal led to a large decrease in surface area and pore volume, especially for 30 wt % of loading. 20 wt % of loading has proved to be the optimum for both Cu and Co, which shows the highest activity. Both N2O-Co/AC and -Cu/AC reactions are based upon a redox mechanism, but the former is limited by the oxygen transfer from catalysts to carbon, while N2O chemisorption on the surface of Cu catalyst controls the latter. The removal of oxygen from cobalt promotes the activity of Co/AC, but it is beneficial for Cu/AC to keep plenty of oxygen to maintain the intermediate oxidation of copper-Cu1+. The different nature of the two catalysts and their catalytic reaction mechanisms are closely related to their different electronegativities.

Carbon structure and porosity of carbonaceous adsorbents in relation to their adsorption properties

A number of carbonaceous adsorbents were prepared by carbonisation at 600 degrees C following acidic oxidation under various conditions. Effects of the chemical nature of the precursor, such as the ratio of aromatic to aliphatic carbons and oxygen content, on the chemical and structural characteristics of the resultant chars were investigated using C-13 NMR and Raman spectroscopy, respectively. The C-13 NMR spectral parameters of the coal samples show that as the severity of oxidation conditions increased, the ratio of aromatic to aliphatic carbons increased. Furthermore, it was also found that the amount of disorganised carbon affects both the pore structure and the adsorption properties of carbonaceous adsorbents. It is demonstrated that higher amount of the disorganised carbon indicates smaller micropore size. (C) 1999 Elsevier Science Ltd. All rights reserved.

Nickel complexes with tris(2-aminoethyl)amine (tren): [Ni-3(tren)(4)(H2O)(2)][Cr(ox)(3)](2)center dot 6H(2)O (ox = oxalate), {[Ni-2(tren)(3)][ClO4](4)center dot H2O}(n), and [Ni-2(tren)(2)(aepd)][ClO4](4)center dot 2H(2)O (aepd = N-(2-aminoethyl)pyrrolidi

Reaction of K-3[Cr(ox)(3)] (ox = oxalate) with nickel(II) and tris(2-aminoethyl)amine (tren) in aqueous solution resulted in isolation of the bimetallic assembly [Ni-3(tren)(4)(H2O)(2)][Cr(ox)(3)](2). 6H(2)O. The polymeric complex {[Ni-2(tren)(3)][ClO4](4). H2O}(n) has been prepared by reaction of nickel(II) perchlorate and tren in aqueous solution. From the same reaction mixture the complex [Ni-2(tren)(2)(aepd)][ClO4](4). 2H(2)O (aepd = N-(2-aminoethyl)pyrrolidine-3,4-diamine), in which a bridging tren ligand contains a carbon-carbon bond between two arms forming a substituted pyrrolidine, has been isolated. The complexes have been characterized by X-ray crystallography. The magnetic susceptibility (300-4.2 K) and magnetization data (2, 4 K, H = 0-5 T) for {[Ni-2(tren)(3)][ClO4](4). H2O}(n) (300 K , 4.23 mu(B)) exhibit evidence of weak antiferromagnetic coupling and zero field splitting (2J = -1.8 cm(-1); \ D\ = 2 cm(-1)) at low temperature. For [Ni-3(tren)(4)(H2O)(2)][Cr(ox)(3)](2). 6H(2)O the susceptibility data at 300 K are indicative of uncoupled nickel(II) and chromium(III) sites with zero-field splitting and intramolecular antiferromagnetic coupling predicted at low temperature.

Metal-centred versus ligand-centred luminescence quenching of a macrocyclic copper(II) complex

The new macrocyclic ligand trans-6-(9-anthracenylmethylamino)-6,13-dimethyl-1,4,8,11-tetraazacyclotetradecan-13-amine has been synthesized and characterised as its copper(II) complex and the crystal structure of this complex has been determined. Fluorescence of the anthracenyl group of the macrocycle is quenched in its free base form and when complexed with Cu-II. Fluorescence returns when Lewis acids such as H+ and Zn-II are added to solutions of the ligand, indicating that photoinduced electron transfer from the amine lone pairs is responsible for fluorescence quenching in the free base form. By contrast, fluorescence of the complex is quenched by intramolecular electronic energy transfer.

Catalytic conversion to N2O to N2 over potassium catalyst supported on activated carbon

Catalytic conversion of N2O to N-2 With potassium catalysts supported on activated carbon (K/AC) was investigated. Potassium proves to be much more active and stable than either copper or cobalt because potassium possesses strong abilities both for N2O chemisorption and oxygen transfer. Potassium redispersion is found to play a critical role in influencing the catalyst stability. A detailed study of the reaction mechanism was conducted based upon three different catalyst loadings. It was found that during temperature-programmed reaction (TPR), the negative oxygen balance at low temperatures (< 50 degrees C) is due to the oxidation of the external surface of potassium oxide particles, while the bulk oxidation accounts for the oxygen accumulation at higher temperatures (below ca. 270 degrees C). N2O is beneficial for the removal of carbon-oxygen complexes because of the formation of CO2 instead of CO and because of its role in making the chemisorption of produced CO2 on potassium oxide particles less stable. A conceptual three-zone model was proposed to clarify the reaction mechanism over K/AC catalysts. CO2 chemisorption at 250 degrees C proves to be an effective measurement of potassium dispersion. (C) 1999 Academic Press.

Electrochemistry of macrocyclic cobalt(III/II) hexaamines: Electrocatalytic hydrogen evolution in aqueous solution

The macrocyclic cobalt hexaamines [Co(trans-diammac)](3+) and [Co(cis-diammac)](3+) (diammac = 6,13-dimethyl-1,4,8,11-tetraazacyclotetradecane-6,13-diamine) are capable of reducing the overpotential for hydrogen evolution on a mercury cathode in aqueous solution. Protons are reduced in a catalytic process involving reoxidation of the Co-II species to its parent Co-III complex. The cycle is robust at neutral pH with no decomposition of catalyst. The stability of the [Co(trans-diammac)](2+) and [Co(cis-diammac)](2+) complexes depends on the pH of the solution and the coordinating properties of the supporting electrolyte. Electrochemical studies indicate that the adsorbed Co-II complex on the surface of mercury is the active catalyst for the reduction of protons to dihydrogen.

Effects of acid treatments of carbon on N2O and NO reduction by carbon-supported copper catalysts

The influences of HCl, HNO3 and HF treatments of carbon on N2O and NO reduction with 20 wt% Cu-loaded activated carbon were studied. The order of activity in both N2O and NO is as follows: Cu20/AC-HNO3>Cu20/AC>Cu20/AC-HF>Cu20/AC-HCl. The same sequence was also observed for the amount of CO2 evolved during TPD experiments of supports acid for the catalyst dispersion. On the other hand, N2O exhibited a higher reaction rate than NO and a higher sensitivity to acid treatments, and the presence of gas-phase O-2 had opposite effects in N2O and NO reduction. The key role of carbon surface chemistry is examined to rationalize these findings and the relevant mechanistic and practical implications are discussed. The effects of oxygen surface groups on the pore structure of supports and catalysts are also analyzed, (C) 2000 Elsevier Science Ltd. All rights reserved.

Role of the crystallite phase of TiO2 in heterogeneous photocatalysis for phenol oxidation in water

A series of TiO2 samples with different anatase-to-rutile ratios was prepared by calcination, and the roles of the two crystallite phases of titanium(IV) oxide (TiO2) on the photocatalytic activity in oxidation of phenol in aqueous solution were studied. High dispersion of nanometer-sized anatase in the silica matrix and the possible bonding of Si-O-Ti in SiO2/TiO2 interface were found to stabilize the crystallite transformation from anatase to rutile. The temperature for this transformation was 1200 degrees C for the silica-titania (ST) sample, much higher than 700 degrees C for Degussa P25, a benchmarking photocatalyst. It is shown that samples with higher anatase-to-rutile ratios have higher activities for phenol degradation. However, the activity did not totally disappear after a complete crystallite transformation for P25 samples, indicating some activity of the rutile phase. Furthermore, the activity for the ST samples after calcination decreased significantly, even though the amount of anatase did not change much. The activity of the same samples with different anatase-to-rutile ratios is more related to the amount of the surface-adsorbed water and hydroxyl groups and surface area. The formation of rutile by calcination would reduce the surface-adsorbed water and hydroxyl groups and surface area, leading to the decrease in activity.

Controlled doping of transition metal cations in alumina pillared clays

A two-step method of loading controlled amounts of transition metal cations into alumina pillared clays (Al-PILCs) is proposed. First, calcined Al-PILC was dispersed into an aqueous solution of sodium or ammonium ions. Increasing the pH of the dispersion resulted in an increase in the amount of cations loaded into the clay. The ion-doped Al-PILC was then exchanged with an aqueous solution of transition metal salt at a pH of similar to 4.5 to replace Na+ or NH4+ ions by transition metal cations. Analytical techniques such as atomic absorption spectroscopy, X-ray diffraction, diffuse reflectance-ultraviolet-visible spectroscopy, as well as N-2 adsorption were used to characterize the PILC products with and without the loading of metal ions. The introduced transition metal species exist in the forms of hydrated ions in the PILC hosts. The content of transition metal ions in the final product increased with the amount of Na+ or NH4+ loaded in the first step so that by controlling the pH of the dispersion in the first step, one can control the doping amounts of transition metal cations into Al-PILCs. A sample containing 0.125 mmol/g of nickel was thus obtained, which is similar to 3 times of that obtained by directly exchanging Al-PILC with Ni(NO3)(2) solution, while the pillared layered structures of the Al-PILC remained. The porosity analysis using N-2 adsorption data indicated that most of the doped transition metal ions dispersed homogeneously in the micropores of the Al-PILC, significantly affecting the micropore structure.

Mesoionic pyridopyrimidinylium and pyridooxazinylium olates and non-mesoionic pyridopyrimidinones. Structures in the solid state, solution, and matrices

X-Ray crystal structures, C-13 NMR spectra and theoretical calculations (B3LYP/6-31G*) are reported for the mesoionic (zwitterionic) pyridopyrimidinylium- and pyridooxazinyliumolates 2a, 3a and 5a,b as well as the enol ether 11b and the enamine 11c. The 1-NH compounds like 1a, 2a and 3a exist in the mesoionic form in the crystal and in solution, but the OH tautomers such as 1b and 2b dominate in the gas phase as revealed by the Ar matrix IR spectra in conjunction with DFT calculations. All data indicate that the mesoionic compounds can be regarded as intramolecular pyridine-ketene zwitterions (cf. 16 --> 17) with a high degree of positive charge on the pyridinium nitrogen, a long pyridinium N-CO bond (ca. 1.44-1.49 Angstrom), and normal C=O double bonds (ca. 1.22 Angstrom). All mesoionic compounds exhibit a pronounced tilting of the olate C=O groups (the C=O groups formally derived from a ketene) towards the pyridinium nitrogen, giving NCO angles of 110-118 degrees. Calculations reveal a hydrogen bond with 6-CH, analogous to what is found in ketene-pyridine zwitterions and the C3O2-pyridine complex. The 2-OH tautomers of type 1b, 2b, and 11 also show a high degree of zwitterionic character as indicated by the canonical structures 11 12.

Reply to comments on "Oxidation mechanism of Si3N4-bonded SiC ceramics by CO, CO2 and steam," by Balat et al.

First of all, we would like to clarify that the passive to active transition was determined not by using Solgasmix [1], but by combining thermodynamic equilibrium and mass balance for the oxidation of SiC under pure CO2 and pure CO. The model used in our paper [2]was an extension ofWagner’s model [3], in a similar way as Balat et al. [4] did for the oxidation of SiC in oxygen.

Mechanisms of substitution reactions on cyclometallated platinum(IV) complexes: "Quasi-labile" systems

The substitution reactions of SMe2 by phosphines (PMePh2, PEtPh2, PPh3, P(4-MeC6H4)(3), P(3-MeC6H4)(3), PCy3) on Pt-IV complexes having a cyclometalated imine ligand, two methyl groups in a cis-geometrical arrangement, a halogen, and a dimethyl sulfide as ligands, [Pt(CN)(CH3)(2)(X)(SMe2)], have been studied as a function of temperature, solvent, and electronic and steric characteristics of the phosphines and the X and CN ligands. In all cases, a limiting dissociative mechanism has been found, where the dissociation of the SMe2 ligand corresponds to the rate-determining step. The pentacoordinated species formed behaves as a true pentacoordinated Pt-IV compound in a steady-state concentration, given the solvent independence of the rate constant. The X-ray crystal structures of two of the dimethyl sulfide complexes and a derivative of the pentacoordinate intermediate have been determined. Differences in the individual rate constants for the entrance of the phosphine ligand can only be estimated as reactivity ratios. In all cases an effect of the phosphine size is detected, indicating that an associative step takes place from the pentacoordinated intermediate. The nature of the (CN) imine and X ligands produces differences in the dimethyl sulfide dissociation reactions rates, which can be quantified by the corresponding DeltaS double dagger values (72, 64, 48, 31, and 78 J K-1 mol(-1) for CN/X being C6H4CHNCH2C6H5/Br, C6H4CHNCH2-(2,4,6-(CH3)(3))C6H2/Br, C6H4CHNCH2C6H5/Cl, C6Cl4CHNCH2C6H5/Cl, and C6W4CH2NCHC6H5/ Pr, respectively). As a whole, the donor character of the coordinated C-aromatic and X atoms have the greatest influence on the dissociativeness of the rate-determining step.

Characterization of the structural and surface properties of chemically modified MCM-41 material

Mesoporous Mobil catalytic materials of number 41 (MCM-41) silica was chemically modified using both inorganic and organic precursors and characterized using the techniques, XRD, XPS, MAS NMR, FTIR, W-Vis, and physical adsorption of nitrogen, hydrocarbons (hexane, benzene, acetone, and methanol) and water vapor. Modification using organic reagents was found to result in a significant loss in porosity and a shape change of surface properties (increased hydrophobicity and decreased acidity). With inorganic modifying reagents, the decrease in porosity was also observed while the surface properties were not significantly altered as reflected by the adsorption isotherms of organics and water vapors. Chemical modifications can greatly improve the hydrothermal stability of MCM-41 material because of the enhanced surface hydrophobicity (with organic modifiers) or increased pore wall thickness (with inorganic modifiers). (C) 2000 Elsevier Science B.V. All rights reserved.

Porous clays and pillared clays-based catalysts. Part 2: A review of the catalytic and molecular sieve applications

Metal oxide pillared clay (PILC) possesses several interesting properties, such as large surface area, high pore volume and tunable pore size (from micropore to mesopore), high thermal stability, strong surface acidity and catalytic active substrates/metal oxide pillars. These unique characteristics make PILC an attractive material in catalytic reactions. It can be made either as catalyst support or directly used as catalyst. This paper is a continuous work from Kloprogge's review (J.T. Kloprogge, J. Porous Mater. 5, 5 1998) on the synthesis and properties of smectites and related PILCs and will focus on the diverse applications of clay pillared with different types of metal oxides in the heterogeneous catalysis area and adsorption area. The relation between the performance of the PILC and its physico-chemical features will be addressed.

An insight into the thermostability of a pair of xylanases: the role of hydrogen bonds

Xylanases are enzymes that are very tolerant to temperature. Their potential use in several biotechnological applications, such as animal food manufacture and pulp bleaching, is due to their intrinsic thermostability. The present report deals with two xylanases, the mesophilic xylanase from Bacillus circulans, BCX, and the thermophilic xylanase from Thermomyces lanuginosus,TLX. These enzymes belong to family 11, and they are the most structurally similar mesophilic-thermophilic pair. Molecular dynamics simulations were employed to investigate the factors responsible for the different thermostabilities exhibited by these structurally similar enzymes. Their active site is their most rigid region, and it is equally rigid at all temperatures. Inter and intramolecular interactions, hydrogen bonds in particular, are the key to the main differences between BCX and TLX. The intramolecular hydrogen bonds and salt bridges are important for maintenance of the backbone rigidity even at high temperature, and the highly solvated surface is a clear optimization in TLX compared with BCX. The main differences between these two enzymes can be found on the fingers domain, which indicates that this domain must be the target for the site-directed mutagenesis responsible for improving the temperature tolerance of this family of enzymes.