Effect of Carbon Deposits on Reactivity of Supported Pd Model Catalysts

Alumina-supported Pd model catalysts were prepared by Pd evaporation onto a thin alumina film grown on a NiAl(110) substrate. Adsorption and co-adsorption of ethene, CO and hydrogen on Pd/Al₂O₃/NiAl(110) covered by carbon species, formed by ethene dehydrogenation at 550 K, was studied by temperature programmed desorption (TPD). TPD results show that carbon deposits do not prevent adsorption but inhibit dehydrogenation of di- bonded ethene. Carbon species suppress CO adsorption in the highly coordinated sites and also suppress the formation of hydrogen ad-atoms on the surface. The ethene hydrogenation reaction performed by co-adsorption of hydrogen and ethene is inhibited by the presence of carbon deposits. The inhibition is independent of particle size studied (1-3 nm). The effects are rationalized in terms of a site-blocking behavior of carbon species occupying highly coordinated sites on the Pd surface.     

Stoichiometric hydrogenation of ethene on Rh(111); mechanism,importance of weakly adsorbed ethene,and relationship to homogeneous catalysis

The hydrogenation of ethene is an important reaction in heterogeneous catalysis and, despite its apparent simplicity, many aspects of the reaction mechanism remain unclear. By contrast, the mechanism using homogeneous catalysts such as Wilkinson's catalyst [(RhCl(PPh₃)₃] is thought to be well understood. To allow a comparison between the homogeneous and heterogeneous reactions we have studied ethene/hydrogen interactions on the (111) plane of rhodium in the temperature range 160–500 K. Under UHV conditions no catalytic reaction was detected. However, we have been able to observe stoichiometric hydrogenation and exchange in the chemisorbed layer. A mixed adlayer of either ethene/deuterium (or perdeuteroethene and hydrogen) was formed at ca. 160 K, and allowed to warm up. From previous spectroscopic studies, ethene is adsorbed at 165 K as partially rehybridised, bonded species with a C-C bond order of ca. 1.5, similar to ethene in Zeise's salt. At 190–210 K we observe coincident desorption of undeuterated ethene — the major species — together with much smaller quantities of deuterated ethane and partially deuterated ethenes. The influence of both hydrogen and ethene pre-coverage has been studied as has the relative extent of hydrogenation and exchange. The ethane formation results parallel those reported by other authors on Pd(110) and Pt(111) and Pt(110). We propose that on all three metals both hydrogenation and exchange follow the same pathway, with a common intermediate for exchange and hydrogenation. This isa weakly held, bonded species formed during the desorption process, which can be convertedreversibly into an adsorbed ethyl species. A detailed comparison indicates that the mechanism of heterogeneous hydrogenation closely parallels that in the homogeneous phase.     

The pyrolysis of benzonitrile

The pyrolysis of benzonitrile under nitrogen at atmospheric pressure has been studied at temperatures of 823–873 K in a flow reactor. The results demonstrate that conversion to hydrogen cyanide occurs directly by a free radical mechanism. The dominant products detected are hydrogen cyanide, monocyanodiphenyl, benzene, dicyanodiphenyl and dicyanobenzene. Reaction orders and activation energies have been determined for product formation. A reaction scheme involving three competing chain reactions in the gas phase with chain carriers H^., C₆H and ^.C₆H₄CN is proposed to explain the observed kinetics. A mechanism is advanced for the formation of significant quantities of polymer, consistent with infra-red spectra and elemental analysis.     

Harmful by-products in selective catalytic reduction of nitrogen oxides by olefins over alumina

The selective reduction of nitrogen dioxide and nitrogen monoxide by olefins (ethene, propene) has been studied over two different -aluminium oxides in the temperature range 473–873 K. Nitrogen dioxide was reduced more effectively than nitrogen monoxide with both, ethene and propene, as a reductant. At temperatures exceeding 700 K, ammonia was formed as a by-product over one type of alumina. Concentrations in the range 30–40 ppm were determined for propene in combination with both, NO and NO₂, while no ammonia was produced with ethene as a reductant. In addition, significant formation of hydrogen cyanide up to 70 ppm was observed with propene over both aluminium oxides starting from either NO or NO₂. In contrast, hydrogen cyanide formation remained below 10 ppm with ethene as a reductant. Nitrous oxide formation did not exceed 10 ppm for all investigations. The results show that for alumina catalysts ethene is a more suitable reductant than propene due to its lower tendency to form undesired by-products.     

Hreels study and catalytic significance of low-temperature interaction of isolated carbon atoms with hydrogen on Pt(111)

Isolated atoms of carbon evaporated on to Pt(111) react with hydrogen atT170 K to form methine species, characterized with vibrational modesv(CH) at 2960 and (CH) at 800 cm^–1. The high reactivity ofC _ads is in line with their ability to take part as intermediates in the metanation reaction. CH_ads species are stable up toT 500 K; further heating leads to their dissociation accompanied by H₂ desorption and formation of unreactive graphite-like islands.     

The activity and selectivity of oxygen atoms adsorbed on a Ag/α-Al2O3 catalyst in ethene epoxidation

The bonding of the oxygen species held on a Ag/-Al₂O₃ catalyst has been studied by temperature programmed desorption and their reactivity in ethene epoxidation by temperature programmed reduction using ethene as the reductant. The Ag/-Al₂O₃ catalyst was produced by the thermal decomposition of a Ag oxalate/-Al₂O₃ precursor. Oxygen desorbs from this Ag/-Al₂O₃ catalyst in two states, one (peak maximum temperature 520 K) having a desorption activation energy of 140 kJ mol^–1 – oxygen desorbing from Ag(111), and one (peak maximum temperature 573 K) having a desorption activation energy of 155 kJ mol^–1 – oxygen desorbing from a highly stepped or defected Ag surface. Temperature programmed reduction of the two oxygen states existing on the surface of the Ag/-Al₂O₃ catalyst using ethene as the reductant produced two peaks at 373 and 473 K in which ethene epoxide and CO₂ evolved coincidently. The peak at 373 K derives from the reduction of oxygen atoms adsorbed on Ag(111). The higher temperature peak (473 K) corresponds to the reduction of oxygen atoms adsorbed on highly stepped or defected Ag surface. The selectivity to ethene epoxide for the 373 K peak is ~ 57%, while that of the 473 K peak is 34%. The coincident evolution of ethene epoxide and CO₂ shows that the selective and unselective reaction pathways have a common surface intermediate – probably an oxametallacycle. The higher selectivity of the oxametallacycle formed by the bonding of ethene to the weaker Ag-O bond is considered to result from its having a lower activation energy to cyclisation than that produced by ethene bonding to the higher Ag-O bond.     

1H- and 13C-n.m.r. studies on naphtha and light distillate saturate hydrocarbon fractions obtained from in-situ shale oil

Shale oil, obtained from an in-situ oil shale experiment, from the Green River formation in Southwestern Wyoming was thermally fractionated into naphtha, light distillate, heavy distillate and residue fractions. The naphtha and light distillate fractions were further separated into saturates, olefins and aromatic subfractions. ¹H- and ¹³C-n.m.r. spectra and mass spectral data were obtained for the saturated hydrocarbons of the naphtha and light distillate fractions. Resonances in the n.m.r. spectra were assigned to normal alkanes and to methyl- and dimethyl-branched alkanes. The composition as determined by n.m.r. of the naphtha saturates was found to be ≈82% straight-chain alkanes with an average carbon-chain-length of ≈C₁₁ and ≈18% branched/cyclo-alkanes. The composition of the light distillate saturates was found to be ≈ 69% straight-chain alkanes with an average carbon-chain-length of ≈C₁₅and ≈31% branched/cyclo-alkanes. The dimethyl-branched alkanes in both saturate fractions were proposed to have the molecular substructure of saturated isoprenoids. In the naphtha saturates the isoprenoid substructure

is evident. However, in the light distillate saturates, both isoprenoid substructures,

and

, are evident. ¹³C spin-lattice re-laxation times also were determined for the dominant resonances observed in the spectra of the naphtha and light distillate saturates. Relaxation times for the molecular species in the naphtha saturate fraction were observed to be longer than those observed for the light distillate saturate fraction. It was found that the ratio of the overall average relaxation times for the naphtha and light distillate saturate fractions corresponded to the inverse ratio of the average molecular weights of each fraction as determined from average alkane carbon-chain-length determination. Intermolecular (segmental) motion of the carbon chain of the straight-chain alkanes in both saturate fractions is also evident from the relaxation time measurements.     

Catalytic reactions of dioxygen with ethane and methane on platinum clusters: Mechanistic connections,site requirements,and consequences of chemisorbed oxygen

C₂H₆ reactions with O₂ only form CO₂ and H₂O on dispersed Pt clusters at 0.2–28 O₂/C₂H₆ reactant ratios and 723–913 K without detectable formation of partial oxidation products. Kinetic and isotopic data, measured under conditions of strict kinetic control, show that CH₄ and C₂H₆ reactions involve similar elementary steps and kinetic regimes. These kinetic regimes exhibit different rate equations, kinetic isotope effects and structure sensitivity, and transitions among regimes are dictated by the prevalent coverages of chemisorbed oxygen (O^*). At O₂/C₂H₆ ratios that lead to O^*-saturated surfaces, kinetically-relevant CH bond activation steps involve O^*O^* pairs and transition states with radical-like alkyls. As oxygen vacancies (¹) emerge with decreasing O₂/alkane ratios, alkyl groups at transition states are effectively stabilized by vacancy sites and CH bond activation occurs preferentially at O^** site pairs. Measured kinetic isotope effects and the catalytic consequences of Pt cluster size are consistent with a monotonic transition in the kinetically-relevant step from CH bond activation on O^*O^* site pairs, to CH bond activation on O^** site pairs, to O₂ dissociation on ^** site pairs as O^* coverage decrease for both C₂H₆ and CH₄ reactants. When CH bond activation limits rates, turnover rates increase with increasing Pt cluster size for both alkanes because coordinatively unsaturated corner and edge atoms prevalent in small clusters lead to more strongly-bound and less-reactive O^* species and lower densities of vacancy sites at nearly saturated cluster surfaces. In contrast, the highly exothermic and barrierless nature of O₂ activation steps on uncovered clusters leads to similar turnover rates on Pt clusters with 1.8–8.5 nm diameter when this step becomes kinetically-relevant at low O₂/alkane ratios. Turnover rates and the O₂/alkane ratios required for transitions among kinetic regimes differ significantly between CH₄ and C₂H₆ reactants, because of the different CH bond energies, strength of alkylO^* interactions, and O₂ consumption stoichiometries for these two molecules. Vacancies emerge at higher O₂/alkane ratios for C₂H₆ than for CH₄ reactants, because their weaker CH bonds lead to faster scavenging of O^* and to lower O^* coverages, which are set by the kinetic coupling between CH and OO activation steps. The elementary steps, kinetic regimes, and mechanistic analogies reported here for C₂H₆ and CH₄ reactions with O₂ are consistent with all rate and isotopic data, with their differences in CH bond energies and in alkyl binding, and with the catalytic consequences of surface coordination and cluster size. The rigorous mechanistic interpretation of these seemingly complex kinetic data and cluster size effects provides useful kinetic guidance for larger alkanes and other catalytic surfaces based on the thermodynamic properties of these molecules and on the effects of metal identity and surface coordination on oxygen binding and reactivity.     

Characterization of pyrolysis products of harbolite and Avgamasya asphaltites: comparison with solvent extracts

The products of pyrolysis at 525 and 840 °C of two asphaltites from South-Eastern Turkey have been analysed and compared with the bitumen obtained by solvent extraction. The yield of oil product is reasonably similar for all three treatments, with gas (hydrogen, ethene, C₁C₄ alkanes and hydrogen sulphide) being liberated during pyrolysis. Greater percentages of alkanes with shorter chain lengths (along with some alkenes), and of pentane-soluble aromatic oils with reduced molecular masses, are generated during pyrolysis, at the expense of asphaltenes. The extra alkanes are generated partly by the cracking of aromatic side-chains and also from kerogen. Pyrolysis reduces the number of sulphur linkages in the oil, but nitrogen- and oxygen-containing structures are liberated from kerogen during heating.     

The activation of O2 and the oxidative dehydrogenation of C2H6 over SmOF catalyst

The activation of O₂ over SmOF was studied by in situ laser Raman spectrometry and temperature programmed desorption (TPD). When the hydrogen- and helium-treated (1 h for each gas at 973 K) SmOF sample was cooled to 303 K in oxygen, Raman bands which correspond to the existence of O ₂ ^2– , O ₂ ^n– (2 >n > 1), O ₂ ^– and O ₂ ^- (1 > > 0) species were observed. From 303 to 973 K, there was no O₂ desorption but the Raman bands observed at 303 K reduced in intensity and vanished completely at 973 K, even though the sample was under an atmosphere of oxygen. We suggest that as the sample temperature increased, dioxygen species were converted to mono-oxygen species such as O^– which were undetectable by Raman spectrometry. O₂ desorption occurred above 973 K, giving a TPD-peak at 1095 K. When C₂ H₆ was pulsed over the sample pretreated with oxygen and helium at 973 K, C₂H₄ selectivity was 91.8%. We conclude that the mono-oxygen species is responsible for the oxidative dehydrogenation of ethane to ethene.     

Metallocene‐Catalyzed Solution Polymerization of Ethene at Elevated Pressure

The kinetic of ethene polymerization catalyzed by (ⁿBuCp)₂ZrCl₂/AlⁱBu₃/[Me₂PhNH]⁺[B(C₆F₅)₄]^? has been investigated at 100 and 140 °C at pressures from 2 to 7 MPa. The initial polymerization rate R_p0 increases linearly with increasing catalyst concentration, whereas a second‐order dependence of R_p0 on the ethene concentration is found the number‐average molecular weight M _n increases with increasing ethene pressure (ethene concentration). The relation between M _n and ethene concentration can be explained by an equation based on a kinetic model involving a single center, two‐state catalyst system.

Influence of the Zr concentration on the initial polymerization rate R_p0. p_Ethene = 7 MPa, solvent (toluene/ethene) = 275 mL.     

Thermodynamics of polymer systems

Summary Gas chromatography has been employed to determine partial molar free energies of sorption, C₁ ^s, as well as partial molar free energies of mixing, C₁ , of atactic poly(styrene) with linear and branched alkanes (C₆-C₉), alkenes (C₈), cyclohexane and alkylbenzenes (Ph-H to PhC₆H₁₃) within the temperature range from 403 to 463 K. The influence of nature and constitution of the solute molecule on sorption and mixing properties in poly(styrene) are discussed in terms of the competing group contributions of the components. Knowledge of this influence may be transduced to understand polymer-polymer compatibility. The cohesive energy density concept has been used to calculate the infinite dilution solubility parameter for the polymer, with the aid of G ₁ . From the high temperature range the HILDEBRAND parameter ₂ was extrapolated to 298 K. The value obtained, 9.14, indicates that gas chromatography is an promising alternative to the conventional methods for determination of the solubility parameter for polymers.Herrn Prof. Dr. R. C. Schulz zu seinem 65. Geburtstag herzlichst gewidmet     

Highly Stable Performance of Methane Dehydroaromatization on Mo/HZSM-5 Catalyst with a Small Amount of H2 Addition into Methane Feed

With a small amount of H₂ (3 6%) addition into methane feed, coke formation on 6 wt% Mo catalyst during the methane dehydroaromatization reaction was effectively suppressed and the catalyst stability was increased evidently under the reaction conditions of 1023K, 0.3MPa and 2520 mL g-MFI^-1 h^-1 of methane space velocity.     

Investigation of bifurcated hydrogen bonds within the thermotropic liquid crystalline polyurethanes

The polyurethanes are synthesized from the biphenyl-4,4′-diol (mesogenic biphenol) and 1,3-Bis(isocyanatomethyl) cyclohexane, using (CH₂) of 2, 6 and 11 units as flexible alkylene spacer, respectively. FTIR detects the hydrogen bond in the thermotropic liquid crystalline polyurethane. FTIR spectra show a new CO absorption with lower wavenumber at around 1658 cm^?1 is assigned to “bifurcated” hydrogen bonded CO group—a CO with higher strength hydrogen bonds. The distributions of “bifurcated” hydrogen bonded CO are increased substantially along with increasing the flexible spacer length in polymer backbone. The “bifurcated” hydrogen bond existed not only at the temperature below T_g, but also existed at the temperature far higher than T_m and T_i. It almost is independent of temperature and exhibits a stable interaction (or strucuture) throughout a wide temperature range, differences from the normal liquid crystalline polyurethanes. It is worthy of predicting the thermotropic liquid crystalline polyurethane with “bifurcated” hydrogen bond would enhance its performances.     

Untersuchungen zur Kinetik der Diffusion von Ethen in 4A-Zeolithen

Investigation on the Kinetics of Diffusion of Ethene on 4A Zeolites The kinetics of diffusion of ethene on 4A zeolites was examined. Conclusions on the transport mechanism were obtained by the loading dependence on the diffusion coefficients. Contrary to propene and the n-butene isomers, ethene can cross the windows to the cavities blocked up by Na⁺ ions in the NaA and MeNa_0,8A zeolites (Me = Mg, Ca, Zn, Ba) by a translatory jump motion. In NaKA zeolites – by reason of additional blocking up of windows with K⁺ ions – the ethene molecules overcome the barrier only through the ethene/Na⁺(1) addition complexes.     

Infrared spectroscopy of some carbon-based materials relevant in combustion: Qualitative and quantitative analysis of hydrogen

A detailed procedure for the quantitative analysis of aromatic and aliphatic hydrogen based on infrared spectroscopy was set up and implemented on some carbon-based materials produced from organic precursors (naphthalene pitch) and/or relevant in combustion field (asphaltenes, carbon particulate matter, carbon black), spanning in the H/C atomic ratio range from 0.1 to 1. The quantitative FT-IR analysis involved the spectral deconvolution in the CH vibrations regions and the calibration factors of diverse standard species having spectral characteristics suitable for the detailed peak-to-peak analysis of the CH stretching (3100–2800 cm⁻¹) and aromatic CH bending (900–700 cm⁻¹) regions. The good agreement between the H/C atomic ratio obtained by quantitative FT-IR analysis and elemental analysis showed a reasonable reliability of the procedure. The major value of the developed FT-IR quantitative technique relies also on the capacity of discriminating between the different kinds of aliphatic and aromatic hydrogen. The quantitative and detailed analysis of hydrogen in form of CH₃, CH₂ and CH groups and in form of solo, duo and trio/quatro aromatic hydrogens showed to be useful also for inferring the structure of the aromatic moieties constituting the CC backbone of carbon materials.     

Entropy considerations in monomolecular cracking of alkanes on acidic zeolites

Compensation between adsorption entropies and enthalpies results in less than a two-fold variation in adsorption equilibrium constants for C₃–C₆ alkanes at temperatures relevant for monomolecular cracking; the size-independent activation energy for CC bond activation in C₃–C₆ alkanes indicates that the marked increase in monomolecular cracking turnover rates observed with alkane chain size reflects a concurrent increase in activation entropies. Thermodynamic treatments for non-ideal systems rigorously describe confinement effects within zeolite channels and show that pre-exponential factors depend on solvation effects of the zeolite-host environment through variations in the thermodynamic activity of the zeolitic proton. Observed differences in rates and selectivities of monomolecular alkane activation with zeolite structure, after normalization to intrazeolitic concentrations, reflect differences in intrinsic rate constants.     

Fischer-Tropsch reactor selection

A comparison of slurry versus fixed-bed reactor design principles for methanol and Fischer-Tropsch distillate production.Notation a gas-liquid interfacial area, m^–1 - CCat catalyst concentration, kg mole/m³ - C _HG hydrogen concentration in gas phase, kg mole/m³ - C ^* _HL hydrogen concentration, liquid, in equilibrium with gas, kg mole/m³ - C _HL hydrogen concentration in the liquid phase, kg mole/m³ - D I.D. of reactor, m - GHSV Gas hourly space velocity, Nm³ (H₂ + CO)/[h · m³ reactor volume], (reactor volume is expanded slurry height times cross section area) - H solubility coefficient of hydrogen =C _HG/C ^* C _HL - l Inlet ratio of CO/H₂ - k _L liquid side mass transfer coefficient, m/s - k _H effective reaction rate constant for hydrogen consumption, s^–1 (note that to agree with space velocity in Nm³/[s · kgCat],k _H =k _H ·CCat wherek _H is in m³/[kg · s] - L Length of expanded slurry bed, m - P pressure, kPa - r rate of hydrogen consumption,r =k _H ·C _HL, kg moles/[m³ · s] - SV Space velocity in actual m³ inlet gas/[s · m³] - T temperature, K - U Usage ratio of CO/H₂ - X _H hydrogen fractional conversion per pass (IfU=l,X _H =X _CO) - contraction factor, = [m³/s(X _{H 2+CO} = 1)-m³/s(inlet)]/[m³/s(inlet)] - * contraction factor modified for H₂ conversion, * = · (1 +U)/(1 +l) - _L fractional liquid hold-up