Characteristics of adsorbed intermediates in the hydrogenative ring opening of methylcyclobutane and methylcyclopentane over silica-supported Pt and Pd catalysts

The hydrogenative ring opening of methylcyclobutane and methylcyclopentane was studied over silica-supported Pt and Pd catalysts by kinetic and infrared (IR) spectroscopic measurements. First, the temperature (473-673 K) and hydrogen pressure (3.3-73.2 kPa) dependences of the ring-opening reactions were determined. The reaction rates were determined over the initial and the working catalysts for both compounds. According to the type of product formation vs. hydrogen pressure dependence curves either dissociative or associative adsorption was suggested for methylcyclobutane and methylcyclopentane. The geometry of adsorbed intermediates was proposed on the basis of the regioselectivity of ring opening. The regioselectivity data show close to statistical ring opening for both compounds, which indicate flat-lying adsorbed intermediates. The structure of adsorbed intermediates was also studied by transmission IR spectroscopy. The H-D exchange reactions between the surface hydroxyl groups and adsorbed species in most cases confirmed the type of adsorption suggested by kinetic measurements. This revised version was published online in July 2006 with corrections to the Cover Date.     

The selectivity of porous catalysts: parallel reactions

The influence of intraparticle mass diffusion on the overall and relative rates of the parallel reactions A → B and A → C is examined for the case of an isothermal catalyst particle with no external concentration or temperature gradients. Calculations for three pairs of reaction orders: (0,1), (0,2) and (1,2), show that internal concentration gradients can cause the yield of the product formed in the higher-order reaction to decrease by as much as 80 percent. Extension of the analysis to other reaction orders is discussed. Criteria for close approach to various asymptotic limits are presented, together with an asymptotic expression which allows the maximum effect of pore diffusion on selectivity to be estimated for any pair of reaction orders.     

Tuning the selectivity of MoOx supported catalysts for cyclohexane photo oxidehydrogenation

The photocatalytic properties of sulphated MoO_x/γ-Al₂O₃ catalysts in cyclohexane oxidative dehydrogenation have been determined in a two-dimensional fluidized bed photoreactor and compared to those of sulphated MoO_x/TiO₂ catalysts. Photocatalytic tests on MoO_x/γ-Al₂O₃ at 8 wt% MoO₃ and various sulphate contents showed the selective (100%) formation of cyclohexene, without production of benzene, as instead found with MoO_x/TiO₂. These results show that the selectivity of photocatalytic cyclohexane oxydehydrogenation is dramatically influenced by the catalyst support.

Maximum cyclohexane conversion and cyclohexene yield of 11% were obtained for SO₄ content of 2.6 wt% at 120 °C. Physico-chemical characterisation of catalysts indicates the presence of both octahedral polymolybdate and sulphate species on alumina surface, as previously found for titania. Increasing sulphate load, thermogravimetry evidenced the presence of up to three sulphate species at different thermal stability. The lower activity observed at high sulphate content is likely due to polymolybdate decoration by sulphates.     

NO x adsorption study over Pt–Ba/alumina catalysts: FT-IR and reactivity study

Topics in Catalysis - The NO x storage process over Ba/Al2O3 and Pt–Ba/Al2O3 NSR catalysts has been analyzed in this study by performing experiments at 350&;nbsp;°C with NO2 and NO/O2...     

The isothermal selectivity of porous catalysts: competitive—consecutive reactions

The influence of mass transfer on the selectivity of three reaction types has been studied for an isothermal catalyst particle: Type 1 is a consecutive reaction, Types 2 and 3 are competitive—consecutive reactions. Type 1 and Type 2 are treated analytically for the case that the diffusivities are not equal. Equations and diagrams are presented that show the importance of different parameters to maximize the concentration of an intermediate in a plug-flow reactor. Type 3 has been dealt with on an analog computer. It can be proved that the selectivity loss for Type 3 is less than for Type 1.     

A kinetic and DRIFTS study of supported Pt catalysts for NO oxidation

NO oxidation was studied over Pt/CeO₂ and Pt/SiO₂ catalysts. Apparent activation energies (E _a) of 31.4 and 40.6 kJ/mole were determined for Pt/CeO₂ and Pt/SiO₂, respectively, while reaction orders for NO and O₂ were fractional and positive for both catalysts. Pre-treatment of the catalysts with SO₂ caused a decrease in the E _a values, while the reaction orders were only slightly changed. In situ DRIFTS measurements indicated that high concentrations of nitrate species were formed on the surface of Pt/CeO₂ during NO oxidation, while almost no surface species could be detected on Pt/SiO₂. The addition of SO₂ resulted in the formation of a highly stable sulfate at the expense of nitrate species and caused an irreversible loss of catalytic activity for Pt/CeO₂.     

A mechanistic study into the reactions of ammonium nitrate with pyrite

The reaction of ammonium nitrate with pyrite was studied using a simultaneous differential scanning calorimetry and thermogravimetric analyser (TGA/DSC). When a mixture of pyrite and ammonium nitrate is heated at a constant heating rate of from room temperature to , two exothermic reactions occur at about 200 and , respectively. The first exothermic reaction is considered to take place between ammonium nitrate and pyrite where NO, NH₃, SO₂ and N₂O gases are produced. The second exothermic reaction is due to the oxidation of the remaining pyrite by atmospheric oxygen. Based on the quantitative analysis of the gaseous and solid products of the reaction, a new overall reaction is proposed at the first exothermic peak of interest, which is thermodynamically favourable. The results have significant implication in the understanding of stability of ammonium nitrate-based industrial explosives in reactive mining grounds containing pyritic minerals.     

A kinetic study of NOx reduction over Pt/SiO2 model catalysts with hydrogen as the reducing agent

Flow reactor experiments and kinetic modeling have been performed in order to study the mechanism and kinetics of NO_x reduction over Pt/SiO₂ catalysts with hydrogen as the reducing agent. The experimental results from NO oxidation and reduction cycles showed that N₂O and NH₃ are formed when NO_x is reduced with H₂. The NH₃ formation depends on the H₂ concentration and the selectivity to NH₃ and N₂O is temperature dependent. A previous model has been used to simulate NO oxidation and a mechanism for NO_x reduction is proposed, which describes the formation/consumption of N₂, H₂O, NO, NO₂, N₂O, NH₃, O₂ and H₂. A good agreement was found between the performed experiments and the model.     

Oxygen effect on the selectivity of ketone hydrogenation reaction on Pt and Pd catalysts

An oxygen deactivated metal catalyst that exhibits less catalytic activity can catalyze an extensive ketone hydrogenation reaction which results in the formation of alkanes, but a fully reduced metal catalyst that exhibits stronger catalytic activity only catalyzes a mild hydrogenation reaction which results in the formation of alcohols.     

On the mechanisms and the selectivity determining steps in syngas conversion over supported metal catalysts: An IR study

An IR study of syngas and methanol conversion has been performed over Cu–ZnO–Al₂O₃ methanol synthesis catalyst, Ni–Al₂O₃ methanation catalyst and Co–Al₂O₃ Fischer Tropsch catalyst. The data, obtained at low pressure, provide unequivocal evidence of the existence of a way via oxygenated intermediates (formates, possibly dioxymethylene, methoxy groups) in the three cases. In the selectivity determining step, methoxy groups desorb associatively as methanol on Cu–ZnO–Al₂O₃. Methoxy groups are selectively hydrogenolyzed to methane over Ni–Al₂O₃. Over Co–Al₂O₃ oxygenated surface species may be involved in the chain growth to give C₂₊ compounds. It is possible that this mechanism coexists with the via-carbide all-metallic catalysis reported for methanation and FT synthesis, on the basis of studies performed on pure metals.     

Butane dehydrogenation over Pt/alumina: activation, deactivation and the generation of selectivity

The dehydrogenation of butane over a Pt/alumina catalyst has been studied using pulse-flow techniques. The selectivity to butenes is generated as carbonaceous material is deposited. The final catalyst is sensitive to whether reduction has taken place in hydrogen or butane, with the more effective dehydrogenation catalyst being generated from a hydrogen reduction.     

Hydroisomerization-cracking of gasoline distillate from Fischer-Tropsch synthesis over bifunctional catalysts containing Pt and heteropolyacids

A gasoline distillate from the Fischer-Tropsch synthesis (so-called as F-T gasoline) was collected in a cold trap set in a gas-flowed slurry reaction system. The F-T gasoline contained 92.8 wt.% n-alkanes (ranged from n-C₄H₁₀ to n-C₁₄H₃₀), 2.4 wt.% 1-alkenes, and 4.8 wt.% iso-alkanes. By the hydroisomerization-cracking in an atmospheric flowed fixed-bed reactor over the catalysts containing Pt and heteropoly compound Cs_2.5H_0.5PW₁₂O₄₀ (abbreviated as Cs2.5), the F-T gasoline was converted to gasoline distillated mixed alkanes with a high iso/n ratio. A mechanical mixture of Pt/Al₂O₃ and Cs2.5 (noted as Pt/Al₂O₃ + Cs2.5) showed the highest catalytic performance among various catalysts. The product over Pt/Al₂O₃ + Cs2.5 after 5 h on-steam at 523 K contained 94.4% C₅-C₉ (gasoline components) with a high iso/n ratio of 8.45. The co-grinding time of Pt/Al₂O₃ and Cs2.5 influenced the catalytic performance when the time was shorter than 10 min but gave little influence on the catalytic performance when the time was longer than 10 min. Because the iso/n ratio of products over Pt/Al₂O₃ + Cs2.5 increased by adding Pt in Cs2.5 and decreased with increasing H₂/feedstock, the reaction proceeded through a bifunctional mechanism in which Pt sites achieved a hydrogenation/dehydrogenation function and acid sites achieved an isomerization/cracking function. The balance of Pt and solid acids was important to obtain a high catalytic performance in the hydroisomerization-cracking of F-T gasoline. Because Cs2.5 possessed moderate acidic strength and uniformly distributed acidic sites, Pt/Al₂O₃ + Cs2.5 showed a higher catalytic stability than that over Pt/Al₂O₃ + SO₄/ZO₂ and showed a higher catalytic activity than that over Pt/Al₂O₃ + H-ZSM-5.     

Pt catalysts supported on zeolitized-pumice for the selective hydrogenation of campholenic aldehyde: A characterization and kinetic study

A detailed characterization study of Pt catalysts supported on a novel zeolitized-pumice (Z-PM) support is reported. Two catalysts, named Pt(Cl)/Z-PM and Pt(Ac)/Z-PM, prepared by impregnation from H₂PtCl₆ and Pt(acac)₂ as precursors and subsequent reduction under H₂ at 623 K, were investigated in terms of microstructure, chemical and surface properties by using X-ray diffraction (XRD), transmission electron microscopy (TEM), temperature programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS).Characterization data showed that the microstructural stability of the support was preserved by using Pt(acac)₂ as precursor. It was verified instead that, by using H₂PtCl₆, the microstructure of the support changed during the thermal activation step under hydrogen, favouring a strong interaction between Pt and support as well as the formation of bimetallic Pt–Fe alloys. Moreover, a moderate increase of Pt d-band vacancies with respect to metallic Pt was observed in this catalyst by XPS.The above catalysts were tested in the selective hydrogenation of campholenic aldehydes to the corresponding unsaturated alcohol, naturanol. In comparison to the Pt(Ac)/Z-PM sample, the ex-chloride Pt(Cl)/Z-PM catalyst showed a higher selectivity to naturanol. This behaviour was interpreted on the basis of the different microstructural and electronic properties as evinced by the characterization data.     

Novel catalysts for the ring-opening metathesis polymerization of norbornene-type monomers

A novel family of metallate compounds has been developed for use in catalyst systems for the ring-opening polymerization of dicyclopentadiene (DCPD) and other norbornene-type monomers. Examples of these novel catalyst compounds are the organoammonium isopoly-and heteropolymetallates. Applications of these metallates in combination with alkylaluminum-type reducing cocatalysts are found in the production of both solution polymers, and cross-linked resins from reaction injection molding (RIM). These catalyst systems have significant advantages over prior art catalysts. They are highly soluble in DCPD and other norbornene-type monomers, thus eliminating the need for a reaction solvent. They also are insensitive to air and moisture, have unlimited shelf-life in solution in norbornene-type monomers, and do not function as Lewis acids. The chemistry of these catalyst systems is discussed, along with the benefits they provide to the properties of the final polymer products. © 1993 John Wiley & Sons, Inc.     

Rational determination of exchange current density for hydrogen electrode reactions at carbon-supported Pt catalysts

The rotating disk electrode (RDE) is a useful technique for precise determination of exchange current density (j₀) in electrochemistry. For the study of powder catalysts, a common practice is to apply the powder onto an inert disk substrate (such as glassy carbon). However, this approach in its usual version will lead to wrong results for the exchange current density of hydrogen electrode reactions at carbon-supported Pt nanoparticles (Pt/C) because of the poor utilization of the loaded Pt nanoparticles. Our new approach is to dilute the Pt/C powder with a large amount of pristine carbon support to make the catalyst layer. In this way, all the catalyst particles in the catalyst layer have nearly the same and much enhanced mass transport so that rational exchange current density can be obtained. Using the new approach, the current density for hydrogen electrode reactions at Pt/C in 0.1 M perchloric acid at 25 °C is found to be 27.2 ± 3.5 mA/cm² with an apparent activation energy 43 kJ/mol. These results are in agreement with the j₀ estimation based on real fuel cell experiments.     

Equations for the kinetics of liquid-phase reactions over solid catalysts and for activity coefficients

Equations are proposed for the kinetics of heterogeneous catalytic liquid-phase reactions that allow for the Langmuir-Hinshelwood sorption concept and the concept of local concentrations and equations for activity coefficients in homogeneous and separating systems based on the combination of the Wilson and Margules concepts.     

Electrochemical dissolution of surface alloys in acids: Thermodynamic trends from first-principles calculations

A simple procedure is introduced to use periodic Density Functional Theory calculations to estimate trends in the thermodynamics of surface alloy dissolution in acidic media. With this approach, the dissolution potentials for solute metal atoms embedded in the surface layer of various host metals (referenced to the dissolution potential of the solute in its pure, metallic form) are calculated. Periodic trends in the calculated potentials are found to be related to trends in surface segregation energies of the various solute/host pairs. The effects of water splitting and concomitant hydroxyl adsorption on the dissolution potentials are also considered; these effects do not change the potentials for highly oxophilic solutes embedded in less active hosts, but they do decrease the dissolution potential for more inert solutes on oxophilic hosts. Finally, the dissolution of Pt “skin” layers from Pt₃X (X = Fe, Co, and Ni) bulk alloys is analyzed; the Pt skins are found to be stabilized compared to pure Pt.     

A revisitation of the polymorphism of poly(butylene-2,6-naphthalate) from periodic first-principles calculations

Periodic density functional theory calculations are here used to investigate polymorphism in poly(butylene-2,6-naphthalate) (PBN) and to understand the intra- and inter-molecular effects which are responsible for its behavior. Despite its similarity to poly(butylene terephthalate), the larger π-electron conjugation promoted by the presence of naphtyl rings generates peculiar intramolecular effects and stronger interchain packing interactions which cause some differences between the two polymers. This is particularly evident by comparing the structural and spectroscopic data predicted for the α and β crystals with respect to the respective one dimensional infinite chain models. Two different interpretations have been proposed in the previous literature to describe the structural transitions from the α to the β polymorph of PBN upon mechanical deformation or thermal treatments: from one hand, the setting on of a transplanar conformation on the methylene chains has been proposed while, on the other hand, a larger coplanarity of the ester groups and the naphtyl rings has been suggested. Our calculations reveal that both these effects are present in β-PBN and should be both taken into account to give an interpretation of the trends observed by IR spectroscopy and structural characterization.     

Effect of Pt dispersion on the reduction of NO by propene over alumina-supported Pt catalysts under lean-burn conditions

The hydrogen exchange in the propane‐d ₈ loaded zeolite H‐ZSM‐5 was monitored by in situ ¹H MAS NMR spectroscopy within the temperature range 457–543 K. Measurements of the H/D exchange between the acidic hydroxyl groups of the zeolite and the adsorbed deuterated propane molecules show that both methyl and methylene groups of alkane are involved in the exchange. The comparison of the experimentally obtained apparent activation energies for the exchange in methyl groups (108 ± 7 kJ mol^-1) and methylene groups (117 ± 7 kJ mol^-1) with theoretical values for methane and ethane supports the assumption that the H/D exchange for methyl and methylene groups takes place via a pentavalent transition state. This revised version was published online in July 2006 with corrections to the Cover Date.