Catalytic Implications Research Articles

On the formation of type I and type II NiMoS phases in NiMo/Al 2O 3 hydrotreating catalysts and its catalytic implications

Two active phases can exist in sulfided NiMo/Al 2O 3 catalysts. Impregnation of an ammoniacal NiMo solution leads eventually to NiMoS phase I, which is strongly bonded to the alumina, while the use of nitrilotriacetic acid gives rise to the totally sulfided NiMoS phase II. Phase I is less active in gas-phase thiophene hydrodesulfurization (HDS) and trickle-flow quinoline hydrodenitrogenation (HDN), but more active in trickle-flow dibenzthiophene HDS. The presence of phosphoric acid in the impregnation solution results in partial formation of NiMoS II upon sulfidation; P itself does not have a direct effect in HDS, but does influence HDN performance, mainly through increasing the catalyst's affinity for quinoline. Extended X-ray absorption fine structure analysis data are consistent with this picture. In-situ Raman spectroscopy and NO chemisorption/infra-red spectroscopy cannot distinguish between NiMo, NiMoP, and NiMo(nitrilotriacetic acid) catalysts. In-situ fluoridation of the sulfided NiMo/Al 2O 3 catalyst leads to a fairly complete phase I å phase II transformation. Again, however, in-situ Raman spectroscopy does not detect the difference. The implications of these findings for catalyst design are discussed.

Molecular orbital calculation of the soft-hard acidity of zeolites and its catalytic implications

The relative hardness of different compositions of model clusters of acid zeolites and the same clusters containing a metoxy group as alkylating agent were evaluated using the energy of the lowest unoccupied molecular orbital as the index. Different basis sets and pseudopotentials were used in ab initio calculations. Semiempirical MNDO-PM3 calculations were also performed. The results show that the hardness of the zeolite increases when the Si/Al ratio decreases. On the basis of Pearson's HSAB principle, the selectivity obtained during the alkylation of toluene with methanol catalyzed by acid zeolites was interpreted.

Neutral imidazole is the electrophile in the reaction catalyzed by triosephosphate isomerase: structural origins and catalytic implications

To illuminate the role of histidine-95 in the catalytic reaction mediated by triosephosphate isomerase, 13C and 15N NMR titration studies have been carried out both on the wild-type enzyme and on a mutant isomerase in which the single remaining histidine (that at the active site) has been isotopically enriched in the imidazole ring. 15N NMR has proved especially useful in the unambiguous demonstration that the imidazole ring of histidine-95 is uncharged over the entire pH range of isomerase activity, between pH 5 and pH 9.9. The results require that the first pKa of histidine-95 is below 4.5. This abnormally low pKa rules out the traditional view that the positively charged imidazolium cation of histidine-95 donates a proton to the developing charge on the substrate's carbonyl oxygen. 15N NMR experiments on the enzyme in the presence of the reaction intermediate analogue phosphoglycolohydroxamate show the presence of a strong hydrogen bond between N epsilon 2 of histidine-95 and the bound inhibitor. These findings indicate that, in the catalyzed reaction, proton abstraction from C-1 of dihydroxyacetone phosphate first yields an enediolate intermediate that is strongly hydrogen bonded to the neutral imidazole side chain of histidine-95. The imidazole proton involved in this hydrogen bond then protonates the enediolate, with the transient formation of the enediol-imidazolate ion pair. Abstraction of the hydroxyl proton on O-1 now produces the other enediolate intermediate, which collapses to give the product glyceraldehyde 3-phosphate.(ABSTRACT TRUNCATED AT 250 WORDS)

The influence of water on the chemisorption of CO 2 onto Pd(110)

A chemisorbed species of CO 2 has been observed on Pd(110) in the presence of coadsorbed water. It has been identified using vibrational spectroscopy as a bent, negatively charged CO δ− 2 species which has previously been observed on a number of clean and alkali metal predosed surfaces. The donation of negative charge from water to the palladium surface is identified as the key factor in the formation of CO δ− 2. The catalytic implications of this observation are discussed.

Structure and Catalytic Implications of Taka-Amylase A.

Taka-amylase has a (αβ)8 barrel structure and the active site is located at the C-terminal end of a β-strand as reported earlier. In this paper, we describe about the direction of substrate amylose binding with respect to the barrel structure. A possible mechanism of hydrolysis is also proposed, in which Glu 230, Asp 297 and Asp 206 located near the active site are essentially involved.

Stability of bulk and surface carbide layers and their relation to the Fischer-Tropsch hydrocarbon synthesis

Using a combination of the recently developed selfconsistent layer Koringa-Kohn-Rostoker LKKR) scheme and a non-selfconsistent cluster method, the local densities of states for bulk and surface carbides of iron, nickel and copper have been determined. Ia the bulk carbides, a decrease in carbon-metal bonding is observed, due both to decreased splitting of the carbon p orbitals, and an increase in filling of the antibonding p orbital. This exactly mirrors the observed trend in stability of the carbides, decreasing from iron to nickel, with copper carbide unknown. Similar trends are observed from the LDOS of the surface carbides, and it is concluded that the carbidic carbon species is thermodynamically unstable at copper surfaces. The catalytic implication of these results and those of Feibelman for ruthenium and rhodium carbides is discussed, with particular reference to their implications for the mechanism and selectivity of these metals in Fischer-Tropsch synthesis.

ChemInform Abstract: Effect of Cavity Size on the Charge Distribution in Carbido‐Metal Carbonyl Clusters and Its Possible Catalytic Implications.

AbstractMO calculations on carbido‐transition metal carbonyl cluster compounds with 4‐12 metal atoms where the C atom lies in a large cavity indicate a substantial buildup of negative charge on the C atom.

Effect of cavity size on the charge distribution in carbido-metal carbonyl clusters and its possible catalytic implications

Molecular orbital calculations on carbido-transition-metal carbonyl cluster compounds where the carbon atom lies in a large cavity indicate a substantial buildup of negative charge on the carbon atom. Besides making the carbon more susceptible to protonic attack the large cavity assists the stabilization of interstitial C-H fragments.

Catalytic implications of local electronic interactions between carbon monoxide and coadsorbed promoters on nickel surfaces

Recent experimental and theoretical work has highlighted the electronic factors that influence the interactions between CO and coadsorbed species on transition-metal surfaces. Although the Blyholder model and modifications thereof provide a useful framework for interpreting many of these results, the microscopic mechanisms by which coadsorbates either poison or promote catalytic reactions are still not well understood. The development of theoretical approaches that can account for established experimental data, while having the flexibility to explore possible reaction paths is therefore an important step in being able to design more efficient catalysts. We present multiple scattering-Xα and Green function density of states calculations for Ni(100) and Ni(111) with CO and coadsorbed poisons (e.g. sulphur) and promoters (e.g. lithium potassium) for the methanation reaction. For the promoted Ni(100) surface we find that the polarisation of the alkali-metal charge towards the surface induces a decrease in the work function and a large electrostatic shift of the CO levels to greater binding energies, which increases the occupancy of the 2π*orbital. There is furthermore evidence of direct CO–promoter interaction in the 1π and 2π* levels, although the 5σ appears to be largely unaffected by the alkali metal. The implications of these calculations for the promotion of the methanation reaction by nickel catalysts are discussed. For sulphur on Ni(111) we find an oscillatory interaction, with some sites within the range of poisoning showing an enhanced local density of states. This is not observed on the (100) surface and is attributed to the absence of a CO adsorption site at the appropriate distance from the sulphur atom.

Surface charge measurements on Micrococcus lysodeikticus and the catalytic implications for lysozyme

Electrophoresis measurements on Micrococcus lysodeikticus have shown that the net surface charge density on the cell wall is constant at around −1.5 μC/cm 2 for the pH range 4–8. This result has enabled a quantitative analysis to be made of how the electrostatic field associated with the negatively charged cell wall influences the ionic strength and pH dependency of the lytic activity of lysozyme towards M. lysodeikticus. A dominant effect is the creation of a local pH gradient at the cell wall, and at high ionic strengths the lytic activity is found to be controlled by an electrostatic force of attraction between the lysozyme molecule and the cell wall. As the ionic strength of the supporting electrolyte is decreased, however, an electrostatic force of repulsion becomes dominant and is associated with a negative charge carried by the lysozyme molecule, which could possibly be the ionized Asp-52 residue at the active site. This is considered to arise from the fact that at low ionic strengths the fine details of the heterogeneous charge distribution on the cell wall and lysozyme molecule are only partially screened by counter ions.

ChemInform Abstract: Isomorphic: Substitution in Zeolites: Its Potential Catalytic Implications

ChemInform Abstract: Isomorphic: Substitution in Zeolites: Its Potential Catalytic Implications

Catalytic implications of the unstable lattice oxygen of praseodymium oxide

The structural conversion of Pr 6O 11 during reduction was investigated by means of temperature-programmed reduction (TPR), measurement of the kinetics of reduction by hydrogen in the temperature range 518–681 K and X-ray photoelectron spectroscopy (XPS). The TPR profile showed that praseodymium oxide is quantitatively reduced to stoichiometric Pr 2O 3 at temperatures below 700 K. Both the gravimetric and the XPS results indicated that the unstable lattice oxygen which evolved during the reduction was produced from both surface and subsurface layers of the oxide. The kinetics of reduction was satisfactorily described by the Erofeev-Avrami equation. Catalysts which had been prereduced at 723–753 K were tested for the conversion of 2-butanol; dehydration and dehydrogenation reactions were found. Product distributions and the possible influence of the unstable lattice oxygen of the oxide are discussed.

Electronic structure of carbidic and graphitic carbon on Ni(111)

The electronic structure of carbon overlayers on Ni(111) has been investigated by using photoemission, energy-loss, and ionization-loss spectroscopy techniques. The combined use of these spectroscopies gives a detailed picture of these surfaces which have important catalytic implications. The carbidic carbon overlayer shows states at \ensuremath{\sim}1-, 4.2-, and 13-eV binding energy. Of particular significance is the detection (both in photoemission and in ionization-loss spectroscopy) of a carbon-derived state very near the Fermi level, in excellent agreement with recent theoretical predictions. The graphitic overlayer has an electronic structure very close to the electronic structure of pure graphite. We detect, however, an almost rigid shift of both valence and conduction bands which points to a charge transfer of about 0.02 electrons per carbon atom between the nickel substrate and the graphitic overlayer.

Characterization of the structures and active sites in sulfided [formula omitted] and [formula omitted] catalysts by NO chemisorption

Infrared and volumetric studies of NO adsorption have been used to elucidate the type of surface structures present in sulfided Co Al 2 O 3 , Ni Al 2 O 3 , Mo Al 2 O 3 , CoMo Al 2 O 3 , and NiMo Al 2 O 3 catalysts. Catalytic implications were obtained from measurements of thiophene hydrodesulfurization (HDS) activities. The content of active material in the catalysts and calcination temperature were varied in these studies. The ir bands were observed to be very different for NO adsorbed on Co, Ni, and Mo atoms and are sensitive to the surface concentration of the element, the nature of the surface phase and the extent of sulfiding or reduction. For Mo Al 2 O 3 catalysts, the NO most probably adsorbs on edge or corner sites of MoS 2-like structures and the adsorption therefore reflects the edge dispersion of these structures. In the case of the promoted CoMo Al 2 O 3 and NiMo Al 2 O 3 catalysts, the ir studies give simultaneous information on the NO adsorption occurring on the Co or Ni promoter atoms and that occurring on the Mo atoms. It is seen that the addition of promoter atoms results in a decreased adsorption on the Mo atoms. This indicates that the promoter atoms occupy edge positions of the MoS 2 “support”. The Co atoms located in such positions are found to be related to those present in the so-called CoMoS structure identified previously by Mössbauer emission spectroscopy. Evidence for similar Ni-Mo-S type structures is found. The HDS activity correlated neither with the total amount of chemisorbed NO nor with the amount of NO adsorbed on the Mo atoms. However, for all the catalysts a good correlation was observed between the HDS activity and the amount of NO adsorbed on the Co or Ni promoter atoms. This further supports the “CoMoS model” in which the primary role of the promoter atoms is to create new sites (associated with the promoter atoms) with higher intrinsic HDS activity than that of the unpromoted sites. Pyridine, which is known to be a partial HDS poison, was observed to block a large fraction of the NO adsorption sites.

Catalytic activity of metals in petroleum and their removal

Experiments are described to elucidate the transformations of metals and metalloporphyrins during hydrosulphurization and to probe the catalytic implications of these transformations. It is shown that at least two reactions involving metal compounds are important, the catalytic hydrogenation reaction and a direct metal extraction by H 2S, both of which result in deposition of metal as sulphide on the catalyst. The former reaction predominates with metalloporphyrins but there is evidence that the latter may be significant with non-porphyrinic metals. The catalytic activity of the resultant sulphide deposits, in particular VS x, although measurable, is perhaps an order of magnitude lower than that of CoMo.

The surface chemistry of some perovskite oxides

The surface hydroxyl concentrations of BaTiO 3, SrTiO 3, and LaCoO 3 were determined by exchange with D 2 as a function of the dehydroxylation temperature. The results suggest that the surface chemistry of these materials resembles that of certain other oxide systems such as alumina and titania. Values approaching the number of hydroxyls required to terminate the lattice (1.4 × 10 15/cm 2) were obtained for samples evacuated at room temperature, but these values fell by about an order of magnitude as the pretreatment temperature was increased to 450 °C and by another factor of about 2 at 600 °C. In this way coordinatively unsaturated centers were produced. The experimental procedure was complicated somewhat by the fact that all three oxides were reduced to a greater or lesser extent when contacted with D 2 at elevated temperatures. This amount was negligibly small at 450 °C (the temperature required for equilibration) with BaTiO 3 and SrTiO 3 but it amounted to about 1 e/mole for LaCoO 3 ( e = electrons). The former two oxides could be reduced by about 0.4 e/mole at 1200 °C. In all cases an amount of H 2O equivalent to that of the H 2 consumed was produced in the reduction step and the oxygen removed could be readily replaced on contact with O 2 at the same temperature. LaCoO 3 could be reduced by up to 3 e/mole at 500 °C and the oxygen again replaced stoichiometrically. X-Ray patterns of the parent perovskites were in good agreement with literature values. The pattern for LaCoO 3 changed slightly when it was reduced by 1 e/mole, but returned to that of the parent sample on reoxidation at 450 °C. The pattern of La 2O 3 appeared when the sample was reduced by 3 e/mole, but it again became nearly identical with that of the parent perovskite upon reoxidation. Lines for metallic Co did not appear at 500 °C but became evident when the reduced sample was sintered in He at 800 °C. In the latter case the perovskite lines returned only very slowly as Co 3O 4 reacted with La 2O 3 at this temperature. The catalytic implications of these observations are briefly discussed.

A Monte Carlo simulation of two-point adsorption on binary metal alloy surfaces

The Monte Carlo method has been applied to the chemisorption of a two-point immobile species on binary alloys consisting of active and inert component metals. The results of the simulations over the complete alloy composition range highlight an important and essentially geometric feature: the variation of the statistically isolated lone sites at saturation. On a polycrystalline fcc sample, this shows a maximum coverage of 12.1% at 42 at% of active alloy component. The results of the simulation also show that a plot relating saturation uptake of a two-point species with alloy composition will deviate from a linear relationship, as has been observed experimentally. When one-point and two-point adsorption occur simultaneously, the plot curves in the other direction. Attention is drawn to the catalytic implications arising from the lone site statistics.

Catalytic implications of electrostatic potentials: The lytic activity of lysozyme as a model

A detailed kinetic study (kcat, Km(app)) of the lytic activity of lysozyme towards Micrococcus luteus cell walls, has been carried out as a function of both pH and ionic strength. Whereas the hydrolytic activity of lysozyme on oligosaccharides (small substrates) has a pH optimum of 5·2 independent of ionic strength, the following observations were made with the bacterial substrate: (1) at low ionic strength (I=0·01) lytic activity was detected at very alkaline pH values; (2) as ionic strength was increased, the kcat-pH profiles were shifted towards more acidic values and approached, for I=0·16, that value obtained for the hydrolysis of oligosaccharides; (3) at a given pH value, kcat was first increased, then decreased by increasing ionic strength; (4) the Michaelis constant Km (app) was strongly dependent on ionic strength, and increased with it.

Electron paramagnetic resonance studies of some supported organometallic catalysts

E.p.r. spectroscopy has been used to study catalysts obtained by supporting the following complexes on high surface area silica: Mo2(CH3CO2)4, M2[CH2Si(CH3)3]6(M = Mo or W), W(CO)6, W(CH3)6, Cr(CO)6, Cr[CH2C(CH3)3]4. The supported molybdenum complexes activated at sufficiently high temperatures give rise to e.p.r. signals identical to those observed previously from silica supported Mo(CO)6 and MoO3 catalysts, due to MoV in square pyramidal co-ordination, and O–2 formed by interaction of O2 with tetrahedrally coordinated MoV. The supported tungsten complexes give only an O–2 signal on exposure to O2. The spectrum of adsorbed CH3 radicals is detected during decomposition of supported W(CH3)6. The supported chromium complexes activated in vacuo and exposed to O2 give CrV signals identical to those observed from silica supported CrO3. The catalytic implications of these observations are discussed.

A spectroscopic study of the surface of Y zeolite: III. Reexamination of the temperature dependence of the infrared spectra of hydrogen Y zeolite

The infrared spectra of several ammonium Y zeolites of different exchange levels were studied, after deammination at 470 °C, as a function of temperature. In the temperature range of 200–470 °C, the 3640-cm −1 band changes in integrated intensity by about 35% of its 200 °C value or 55% of its room temperature value. This change in intensity is too large to be due to simple thermal effects and is probably due to proton delocalization. No integrated intensity change was observed for the hydroxyl groups on Cab-O-Sil silica. The catalytic implications of these observations are discussed.