CO2 reforming of methane on Ni catalysts: Effects of the support phase and preparation technique

The effects of the support phase and catalyst preparation methods on catalytic activity and carbon deposition were systematically investigated over nickel catalysts supported on Al2O3, SiO2 and MgO for the reforming reaction of methane with carbon dioxide. It is found that the pore structure of the support and metal-support interaction significantly affected the catalytic activity and coking resistance. Catalyst with well-developed porosity exhibited higher catalytic activity. Strong interaction between metal and the support made the catalyst more resistant to sintering and coking, thus resulting in a longer time of catalyst stability. (C) 1998 Elsevier Science B.V.

Reforming of methane with carbon dioxide over Ni/Al2O3 catalysts: Effect of nickel precursor

The catalytic activities of Ni/gamma-Al2O3 catalysts prepared using different nickel precursor compounds were studied for the reaction of methane reforming with CO2. It is found that the nickel precursor employed in the catalyst preparation plays an important role. The catalyst based on nickel nitrate exhibited higher catalytic activity and stability over a 24-h test period than the other two catalysts derived from nickel chloride and nickel acetylacetonate. A comprehensive characterisation of the catalysts showed that the weak interaction between Ni particles and gamma-Al2O3 resulted in more active sites on Ni nitrate-derived Ni/gamma-Al2O3 catalyst. Coking studies showed that carbon deposition on Ni catalysts derived from inorganic precursors (nitrate and chloride) were more severe than on the organic precursor-derived catalyst. However, the Ni nitrate-derived catalyst was found to have the highest stability (or lowest deactivation rate) mainly due to the active carbon species (-C-C-) of the resulting graphitic structure and their close contact with the metal particles. In contrast, the carbon formed on Ni-AA catalyst (from Ni acetylacetonate) is dominated by inactive -CO-C- species, thus leading to a rapid accumulation of carbon in this catalyst and more severe deactivation. (C) 1998 Elsevier Science B.V.

Effects of acidic treatments on the pore and surface properties of Ni catalyst supported on activated carbon

Activated carbon as catalyst support was treated with HCl, HNO3, and HF and the effects of acid treatments on the properties of the activated carbon support were studied by N-2 adsorption, mass titration, temperature-programmed desorption (TPD), and X-ray photoelectron spectrometry (XPS). Ni catalysts supported on untreated and treated activated carbons were prepared, characterized and tested for the reforming reaction of methane with carbon dioxide. It is found that acid treatment significantly changed the surface chemical properties and pore structure of the activated carbon. The surface area and pore volume of the carbon supports are generally enhanced upon acid treatment due to the removal of impurities present in the carbon. The adsorption capacity of Ni2+ on the carbon supports is also increased, and the increase can be closely correlated with the surface acidity. The impregnation of nickel salts decreases the surface area and pore volume of carbon supports both in micropores and mesopores. Acid treatment results in a more homogeneous distribution of the nickel salt in carbon. When the impregnated carbons are heated in inert atmosphere, there exists a redox reaction between nickel oxide and the carbon. Catalytic activity tests for methane reforming with carbon dioxide show that the activity of nickel catalysts based on the acid-treated carbon supports is closely related with the surface characteristics of catalysts. (C) 1998 Elsevier Science Ltd. All rights reserved.

Measurement of grain boundary composition for X52 pipeline steel

Analytical electron microscopy was used to measure the composition of grain boundaries (GBs) and interconstituent boundaries (IBs) of X52 pipeline steel using specimens about 40-60 nm in thickness. All elements of interest were examined with the exception of carbon. With this caveat; there was no segregation at proeutectoid ferrite GBs. This indicated that the commonly expected species S and P are not responsible for preferential corrosion of GBs during intergranular stress corrosion cracking of pipeline steels. Manganese was the only species measured to segregate at the IBs. Manganese segregated to the IBs between proeutectoid ferrite and pearlitic cementite, and desegregated from IBs between proeutectoid ferrite and pearlitic ferrite. The pearlitic cementite was Mn rich. There was no Mn segregation at the IBs between pearlitic ferrite and pearlitic cementite. The pattern of Mn segregation could be explained in terms of diffusion in the process zone ahead of the pearlite during the austenite to pearlite transformation and diffusion in the IBs between the proeutectoid ferrite and pearlite. (C) 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.

Thermogravimetric analysis of carbon deposition over Ni/gamma-Al2O3 catalysts in carbon dioxide reforming of methane

Carbon formation on Ni/gamma-Al2O3 catalysts and its kinetics during methane reforming with carbon dioxide was studied in the temperature range of 500-700 degrees C using a thermogravimetric analysis technique. The activation energies of methane cracking, carbon gasification in CO2, as well as carbon deposition in CO2-CH4 reforming were obtained. The results show that the activation energy for carbon gasification is larger than that of carbon formation in methane cracking and that the activation energy of coking in CO2-CH4 reforming is also larger than that of methane decomposition to carbon. The dependencies of coking rate on partial pressures of CH4 and CO2 indicate that methane decomposition is the main route for carbon deposition. A mechanism and kinetic model for carbon deposition is proposed.

Role of CeO2 in Ni/CeO2-Al2O3 catalysts for carbon dioxide reforming of methane

Ni catalysts supported on gamma-Al2O3, CeO2 and CeO2-A1(2)O(3) systems were tested for catalytic CO2 reforming of methane into synthesis gas. Ni/CeO2-Al2O3 catalysts showed much better catalytic performance than either CeO2- or gamma-Al2O3-supported Ni catalysts. CeO2 as a support for Ni catalysts produced a strong metal-support interaction (SMSI), which reduced the catalytic activity and carbon deposition. However, CeO2 had positive effect on catalytic activity, stability, and carbon suppression when used as a promoter in Ni/gamma-Al2O3 catalysts for this reaction. A weight loading of 1-5 wt% CeO2 was found to be the optimum. Ni catalysts with CeO2 promoters reduced the chemical interaction between nickel and support, resulting in an increase in reducibility and stronger dispersion of nickel. The stability and less coking on CeO2-promoted catalysts are attributed to the oxidative properties of CeO2. (C) 1998 Elsevier Science B.V. All rights reserved.

Factors controlling the origin of gas in Australian Bowen Basin coals

Open system pyrolysis (heating rate 10 degrees C/min) of coal maturity (vitrinite reflectance, VR) sequence (0.5%, 0.8% and 1.4% VR) demonstrates that there are two stages of thermogenic methane generation from Bowen Basin coals. The first and major stage shows a steady increase in methane generation maximising at 570 degrees C, corresponding to a VR of 2-2.5%. This is followed by a less intense methane generation which has not as yet maximised by 800 degrees C (equivalent to VR of 5%). Heavier (C2+) hydrocarbons are generated up to 570 degrees C after which only the C-1 (CH4, CO and CO2) gases are produced. The main phase of heavy hydrocarbon generation occurs between 420 and 510 degrees C. Over this temperature range,methane generation accounts for only a minor component, whereas the wet gases (C-2-C-5) are either in equal abundance or are more abundant by a factor of two than the liquid hydrocarbons. The yields of non-hydrocarbon gases CO2 and CO are greater then methane during the early stages of gas generation from an immature coal, subordinate to methane during the main phase of methane generation after which they are again dominant. Compositional data for desorbed and produced coal seam gases from the Bowen show that CO2 and wet gases are a minor component. This discrepancy between the proportion of wet gas components produced during open system pyrolysis and that observed in naturally matured coals may be the result of preferential migration of wet gas components, by dilution of methane generated during secondary cracking of bitumen, or kinetic effects associated with different activations for production of individual hydrocarbon gases. Extrapolation of results of artificial pyrolysis of the main organic components in coal to geological significant heating rates suggests that isotopically light methane to delta(13)C of -50 parts per thousand can be generated. Carbon isotope depletions in C-13 are further enhanced, however, as a result of trapping of gases over selected rank levels (instantaneous generation) which is a probable explanation for the range of delta(13)C values we have recorded in methane desorbed from Bowen Basin coals (-51 +/- 9 parts per thousand). Pervasive carbonate-rich veins in Bowen Basin coals are the product of magmatism-related hydrothermal activity. Furthermore, the pyrolysis results suggest an additional organic carbon source front CO2 released at any stage during the maturation history could mix in varying proportions with CO2 from the other sources. This interpretation is supported by C and O isotopic ratios, of carbonates that indicate mixing between magmatic and meteoric fluids. Also, the steep slope of the C and O isotope correlation trend suggests that the carbonates were deposited over a very narrow temperature interval basin-wide, or at relatively high temperatures (i.e., greater than 150 degrees C) where mineral-fluid oxygen isotope fractionations are small. These temperatures are high enough for catagenic production of methane and higher hydrocarbons from the coal and coal-derived bitumen. The results suggests that a combination of thermogenic generation of methane and thermodynamic processes associated with CH4/CO2 equilibria are the two most important factors that control the primary isotope and molecular composition of coal seam gases in the Bowen Basin. Biological process are regionally subordinate but may be locally significant. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.

Structural basis of autoregulation of phenylalanine hydroxylase

Phenylalanine hydroxylase converts phenylalanine to tyrosine, a rate-limiting step in phenylalanine catabolism and protein and neurotransmitter biosynthesis. It is tightly regulated by the substrates phenylalanine and tetrahydrobiopterin and by phosphorylation. We present the crystal structures of dephosphorylated and phosphorylated forms of a dimeric enzyme with catalytic and regulatory properties of the wild-type protein. The structures reveal a catalytic domain flexibly linked to a regulatory domain. The latter consists of an N-terminal autoregulatory sequence (containing Ser 16, which is the site of phosphorylation) that extends over the active site pocket, and an alpha-beta sandwich core that is, unexpectedly, structurally related to both pterin dehydratase and the regulatory domains of metabolic enzymes. Phosphorylation has no major structural effects in the absence of phenylalanine, suggesting that phenylalanine and phosphorylation act in concert to activate the enzyme through a combination of intrasteric and possibly allosteric mechanisms.

Turn up the HEAT

The recently determined crystal structure of the PR65/A subunit of protein phosphatase 2A reveals the architecture of proteins containing HEAT repeats, The structural properties of this solenoid protein explain many functional characteristics and account for the involvement of solenoids as scaffold, anchoring and adaptor proteins.

Molecular recognition of macrocyclic peptidomimetic inhibitors by HIV-1 protease

High-resolution crystal structures are described for seven macrocycles complexed with HIV-1 protease (HIVPR). The macrocycles possess two amides and an aromatic group within 15-17 membered rings designed to replace N- or C-terminal tripeptides from peptidic inhibitors of HIVPR. Appended to each macrocycle is a transition state isostere and either an acyclic peptide, nonpeptide, or another macrocycle. These cyclic analogues are potent inhibitors of HIVPR, and the crystal structures show them to be structural mimics of acyclic peptides, binding in the active site of HIVPR via the same interactions. Each macrocycle is restrained to adopt a P-strand conformation which is preorganized for protease binding. An unusual feature of the binding of C-terminal macrocyclic inhibitors is the interaction between a positively charged secondary amine and a catalytic aspartate of HIVPR. A bicyclic inhibitor binds similarly through its secondary amine that lies between its component N-terminal and C-terminal macrocycles. In contrast, the corresponding tertiary amine of the N-terminal macrocycles does not interact with the catalytic aspartates. The amine-aspartate interaction induces a 1.5 Angstrom N-terminal translation of the inhibitors in the active site and is accompanied by weakened interactions with a water molecule that bridges the ligand to the enzyme, as well as static disorder in enzyme flap residues. This flexibility may facilitate peptide cleavage and product dissociation during catalysis. Proteases [Aba(67,95)]HIVPR and [Lys(7),Ile(33),Aba(67,95)]- HIVPR used in this work were shown to have very similar crystal structures.

A comprehensive study on carbon dioxide reforming of methane over Ni/gamma-Al2O3 catalysts

Carbon dioxide reforming of methane into syngas over Ni/gamma-Al2O3 catalysts was systematically studied. Effects of reaction parameters on catalytic activity and carbon deposition over Ni/gamma-Al2O3 catalysts were investigated. It is found that reduced NiA1204, metal nickel, and active species of carbon deposited were the active sites for this reaction. Carbon deposition on Ni/gamma Al2O3 varied depending on the nickel loading and reaction temperature and is the major cause of catalyst deactivation. Higher nickel loading produced more coke on the catalysts, resulting in rapid deactivation and plugging of the reactor. At 5 wt % Ni/gamma-Al2O3 catalyst exhibited high activity and much lesser magnitude of deactivation in 140 h. Characterization of carbon deposits on the catalyst surface revealed that there are two kinds of carbon species (oxidized and -C-C-) formed during the reaction and they showed different reactivities toward hydrogenation and oxidation. Kinetic studies showed that the activation energy for CO production in this reaction amounted to 80 kJ/mol and the rate of CO production could be described by a Langmuir-Hinshelwood model.

Crystal structure of mammalian purple acid phosphatase

Background: Mammalian purple acid phosphatases are highly conserved binuclear metal-containing enzymes produced by osteoclasts, the cells that resorb bone. The enzyme is a target for drug design because there is strong evidence that it is involved in bone resorption. Results: The 1.55 Angstrom resolution structure of pig purple acid phosphatase has been solved by multiple isomorphous replacement. The enzyme comprises two sandwiched beta sheets flanked by or-helical segments. The molecule shows internal symmetry, with the metal ions bound at the interface between the two halves. Conclusions: Despite less than 15% sequence identity, the protein fold resembles that of the catalytic domain of plant purple acid phosphatase and some serine/threonine protein phosphatases. The active-site regions of the mammalian and plant purple acid phosphatases differ significantly, however. The internal symmetry suggests that the binuclear centre evolved as a result of the combination of mononuclear ancestors. The structure of the mammalian enzyme provides a basis for antiosteoporotic drug design.

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.

The crack tip strain field of AISI 4340 - Part I - Measurement technique

This is the first paper in a study on the influence of the environment on the crack tip strain field for AISI 4340. A stressing stage for the environmental scanning electron microscope (ESEM) was constructed which was capable of applying loads up to 60 kN to fracture-mechanics samples. The measurement of the crack tip strain field required preparation (by electron lithography or chemical etching) of a system of reference points spaced at similar to 5 mu m intervals on the sample surface, loading the sample inside an electron microscope, image processing procedures to measure the displacement at each reference point and calculation of the strain field. Two algorithms to calculate strain were evaluated. Possible sources of errors were calculation errors due to the algorithm, errors inherent in the image processing procedure and errors due to the limited precision of the displacement measurements. Estimation of the contribution of each source of error was performed. The technique allows measurement of the crack tip strain field over an area of 50 x 40 mu m with a strain precision better than +/- 0.02 at distances larger than 5 mu m from the crack tip. (C) 1999 Kluwer Academic Publishers.

The crack tip strain field of AISI 4340 - Part II - Experimental measurements

Crack tip strain maps have been measured for AISI 4340 high strength steel. No significant creep was observed. The measured values of CTOD were greater than expected from the HRR model. Crack tip branching was observed in every experiment. The direction of crack branching was in the same direction as a major ridge'' of epsilon(yy) strain, which in turn was in the same direction as predicted by the HRR model. Furthermore, the measured magnitudes of the epsilon(y)y strain in this same direction were in general greater than the values predicted by the HRR model. This indicates more plasticity in the crack tip region than expected from the HRR model. This greater plasticity could be related to the larger than expected CTOD values. The following discrepancies between the measured strain fields for AISI 4340 and the HRR predictions are noteworthy: (1) The crack branching. (2) Values of CTOD significantly higher than predicted by HRR. (3) The major ridge'' of epsilon(yy) strain an angle of about 60 degrees with the direction of overall propagation of the fatigue precrack, in which the measured magnitudes of the epsilon(yy) strain were greater than the values predicted by the HRR model. (4) Asymmetric shape of the plastic zone as measured by the epsilon(yy) strain. (5) Values of shear strain gamma(xy) significantly higher than predicted by the HRR model. (C) 1999 Kluwer Academic Publishers.