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.     

Effects of potassium doping on CO hydrogenation over MoS2 catalysts: A first-principles investigation

The effects of potassium (K) doping on the reactivity of CO hydrogenation over MoS₂(100) catalysts are investigated using periodic density functional theory (DFT) calculations. The surface doped K species enhances the CO adsorption by providing both KO and KC bonding. DFT results show that K-doping promotes the CC coupling step forming the H₂CCO precursor that leads to the formation of mixed higher C_2 + oxygenates. Different reaction routes for CO hydrogenation on the Mo and the S edges over MoS₂(100) catalysts are identified.     

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.     

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.     

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...     

A comparative study of the selective catalytic reduction of NO by propylene over supported Pt and Rh catalysts

The catalytic performance of Pt and Rh catalysts for the selective catalytic reduction (SCR) of NO by propylene in the presence of excess oxygen has been investigated over catalysts supported on six different metal oxide carriers (CeO₂, Al₂O₃, TiO₂, YSZ, ZrO₂ and W⁶⁺-doped TiO₂). It has been found that the nature of the dispersed metal affects strongly the light-off temperature of propylene, the maximum NO conversion to reduction products and the selectivity towards nitrogen. For a given support, Pt catalysts are always more active for both NO reduction and propylene oxidation, but are much less selective towards N₂, compared to Rh catalysts. Rhodium catalysts are able to selectively reduce NO even in the absence of oxygen in the feed. However, their activity is suppressed with increasing oxygen feed concentration possibly due to the formation of less reactive rhodium oxides. In contrast, oxygen promotes the de-NO_x activity of platinum catalysts but decreases selectivity towards nitrogen. Results are explained by considering the effects of the nature of the metallic phase and the support on the elementary steps of the propylene-SCR reaction. It is concluded that the catalytic performance of both metals may be improved by proper selection of the support.     

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 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.     

Systematic study of H2 production from catalytic photoreforming of cellulose over Pt catalysts supported on TiO2

Hydrogen (H₂) production from photocatalytic reforming of cellulose is a promising way for sustainable H₂ to be generated. Herein, we report a systematic study of the photocatalytic reforming of cellulose over Pt/m-TiO₂ (i.e. mixed TiO₂, 80% of anatase and 20% of rutile) catalysts in water. The optimum operation condition was established by studying the effect of Pt loading, catalyst concentration, cellulose concentration and reaction temperature on the gas production rate of H₂ (r_H2) and CO₂ (r_CO2), suggesting an optimum operation condition at 40 ℃ with 1.0 g·L^-1 of cellulose and 0.75 g·L^-1 of 0.16-Pt/m-TiO₂ catalyst (with 0.16 wt% Pt loadting) to achieve a relatively sound photocatalytic performance with rH₂ = 9.95 μmol·h^-1. It is also shown that although the photoreforming of cellulose was operated at a relatively mild condition (i.e. with an UV-A lamp irradiation at 40 ℃ in the aqueous system), a low loading of Pt at ~0.16 wt% on m-TiO₂ could promote the H₂ production effectively. Additionally, by comparing the reaction order expressed from both r_H2 (a₁) and rCO₂ (a₂) with respect to cellulose and water, the possible mechanism of H₂ production was proposed.     

Systematic study of H_2 production from catalytic photoreforming of cellulose over Pt catalysts supported on TiO_2

Hydrogen(H_2) production from photocatalytic reforming of cellulose is a promising way for sustainable H_2 to be generated. Herein, we report a systematic study of the photocatalytic reforming of cellulose over Pt/m-TiO_2(i.e.mixed TiO_2, 80% of anatase and 20% of rutile) catalysts in water. The optimum operation condition was established by studying the effect of Pt loading, catalyst concentration, cellulose concentration and reaction temperature on the gas production rate of H_2(r_(H_2)) and CO_2(r_(CO_2)), suggesting an optimum operation condition at 40°C with 1.0 g·L~(-1) of cellulose and 0.75 g·L~(-1) of 0.16-Pt/m-TiO_2 catalyst(with 0.16 wt% Pt loadting) to achieve a relatively sound photocatalytic performance with rH_2= 9.95 μmol·h~(-1). It is also shown that although the photoreforming of cellulose was operated at a relatively mild condition(i.e. with an UV-A lamp irradiation at40 °C in the aqueous system), a low loading of Pt at ～0.16 wt% on m-TiO_2 could promote the H_2 production effectively. Additionally, by comparing the reaction order expressed from both r_(H_2)(a_1) and r_(CO_2)(a_2) with respect to cellulose and water, the possible mechanism of H_2 production was proposed.     

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.     

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.     

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.     

Oxidation of hydrocarbons by hydrogen peroxide over Ti catalysts: Kinetics and mechanistic studies

n-hexane conversion to hexanols and hexanones as well as the conversion of 2- and 3-hexanol to 2- and 3- hexanone, by reaction with aqueous hydrogen peroxide on titanium silicalite were the objects of kinetic investigations. The choice of these reactions is justified on the basis of an analysis of literature dealing with partial oxidation reactions performed on Ti sites in silica environments. The kinetic analysis based on equations derived using the mathematical theory of linear graphs allows several conclusions on the nature of surface intermediates and on the reaction mechanism. These conclusions are discussed in terms of the partial oxidation of organic materials by hydrogen peroxide over Ti catalysts. This review paper relies mostly on work by the authors and it intends to make an integrated presentation of the kinetics of some critically significant catalytic oxidation reactions. The content was the subject of a plenary lecture at the 5^th symposium of the Romanian Catalysis Society.     

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.