Kinetic behaviour of HC-SCR over Ag/alumina catalyst using a model paraffinic second generation biodiesel compound

The effect of longer paraffins on the mechanism of the HC-SCR reaction over a 1.91 wt.% Ag/alumina catalyst was investigated by kinetic studies. Hexadecane (C₁₆H₃₄) was chosen as a model compound as it is also a representative molecule for a second generation biodiesel consisting of only long-chain paraffins. The kinetic behaviour of the catalytic reduction of NO_x was examined at steady-state conditions in the temperature range 250–550 °C (50 °C ramping) and by using the following gas concentrations: P_NO = 100, 250, 500, 750 or 1000 ppm, P_hexadecane = 31, 94, 188, 281 or 375 ppm and P_O2=1.5, 3, 4.5, 6 or 9 vol.%. Results showed that in the temperature range 250–425 °C high hexadecane concentration had an inhibiting effect on the NO reduction. At temperatures above 350 °C the apparent reaction orders for hexadecane with respect to hexadecane increased to close or above 1. Reaction orders towards NO were close to 0.55 indicating that NO adsorption on the catalyst surface is stronger than hexadecane adsorption. Based on the experimental data it is proposed that small clusters alone cannot be the active sites for HC-SCR over Ag/Al₂O₃ but the important requirement for high activity over the catalyst is the local concentrations of hydrocarbon and NO on the interface of silver and the support.     

Reactivity of surface isocyanate species with NO, O2 and NO+O2 in selective reduction of NOχ over Ag/Al2O3 and Al2O3 catalysts

Reactivity of surface isocyanate (NCO(a)) species with NO, O₂ and NO+O₂ in selective reduction of NOχ over Ag/Al₂O₃ and Al₂O₃ catalysts was studied by a pulse reaction technique and an in situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. The NCO(a) species on Ag/Al₂O₃ reacted with O₂ or NO+O₂ mixture gas to produce N₂ effectively above 200°C, while the reaction of NCO(a) with NO hardly produced N₂ even at 350°C. In the case of Al₂O₃ alone, less N₂ was detected in the reaction of NCO(a) with NO+O₂, indicating that silver plays an important role in the N₂ formation from NCO(a). These behaviors of the reactivity of NCO(a) species with reactant gases were in good agreement with the changes in NCO(a) bands shown by in situ DRIFT measurements. Based on these findings, the role of NCO(a) species in the selective reduction of NOχ on Ag/Al₂O₃ and Al₂O₃ catalysts is discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

The effect of alumina contamination from the ball-milling of fused silica on the high temperature properties of injection moulded silica ceramic components

The amount of alumina contamination present in ball-milled silica powders has been shown to increase with increased mill time for materials manufactured during the same time period. This alumina contamination level has also been observed to vary depending on the date, and possibly the state of repair, of the ball mill itself. The associated alumina level has been shown to significantly influence the high temperature properties (at 1475 °C) of the materials, with high contamination levels not only resulting in increased flexural strength and creep resistance, but also increasing the thermal contraction of the materials when dilatometer measurements were performed to 1600 °C.     

Properties and rapid consolidation of nanostructured Al2O3-Al2SiO5 composites by high frequency induction heated sintering

The rapid sintering of nanostructured Al₂O₃ and Al₂O₃ to Al₂SiO₅ composites was investigated by a high-frequency induction heating sintering process. The advantage of this process is that it allows very quick densification to near theoretical density and inhibition of grain growth. Highly dense nanostructured Al₂O₃ and Al₂O₃ to Al₂SiO₅ composites were produced with simultaneous application of a 80 MPa pressure and induced output current of a total power capacity (15 kW) within 3 min. The sintering behavior, grain size and mechanical properties of Al₂O₃ and Al₂O₃ to Al₂SiO₅ composites were investigated.     

Crystallization kinetics of amorphous alumina–zirconia–silica ceramics

Crystallization kinetics of amorphous alumina–zirconia–silica ceramics was studied by nonisothermal differential scanning calorimetry (DSC). Different amorphous materials were produced by plasma spraying of near-eutectic Al₂O₃–ZrO₂–SiO₂ mixtures. Phase composition and microstructure of the amorphous materials and nanocrystalline products were analyzed. All of the investigated materials show an exothermic peak between 940 and 990 °C in the DSC experiments. The activation energies calculated from DSC traces decrease with increasing SiO₂ concentration. Values of the Avrami coefficients together with results of the microstructural observations indicate that tetragonal zirconia crystallization from materials containing more than 10 wt.% SiO₂ proceeds by a diffusion-controlled mechanism with nucleation occurring predominantly at the beginning of the process. In contrast, material with almost no SiO₂ exhibited a value of the Avrami exponent consistent with the crystal growth governed by processes at the phase boundary.     

Reduced strength degradation of alumina–aluminium titanate composite subjected to low-velocity impact loading

The impact behaviour of monophase alumina and alumina–aluminium titanate monolithic composite ceramics that present flaw tolerant behaviour was studied. Low-velocity impact loading tests were performed on bending bars and the residual strength after the impact was evaluated by four-point bending tests. The impact tests were monitored using an instrumented drop-weight machine. During impact, the composites absorbed higher energy than the monophase material. The strength retention, in percentage, after the impact was significantly higher for the composite that presented damage tolerance for impact energy levels higher than monophase alumina. These results are discussed and fractographic analysis was used to identify the mechanisms responsible for the lower strength degradation of the composite.     

Performance of a slurry bubble column reactor for Fischer-Tropsch synthesis: Determination of optimum condition

The CO conversion and selectivity to C₁+ and C₁₁+ wax products over Co/Al₂O₃ as well as Ru/Co/Al₂O₃ Fischer-Tropsch (F-T)catalysts were investigated by varying reaction temperature (210-250 °C), system pressure (1.0-3.0 MPa), GHSV (1000-6000 L/kg/h), superficial gas velocity (1.7-13.6 cm/s) and slurry concentration (9.09-26.67 wt.%) in a slurry bubble column reactor (0.05 m diameter × 1.5 m height) to determine the optimum operating conditions. Squalane or paraffin wax was used as initial liquid media. The overall CO conversion increased with increasing reaction temperature, system pressure and catalyst concentration. However, the local maximum CO conversion was exhibited at GHSV of 1500-2000 L/kg/h and superficial gas velocity of 3.4-5.0 cm/s. The CO conversion in the case of Ru/Co/Al₂O₃ was much higher and stable than that in the case of Co/Al₂O₃. The selectivity to C₁₁+ wax products increased slightly with increasing GHSV; on the other hand, it decreased with increasing reaction temperature, system pressure, and solid concentration in a slurry bubble column reactor. It could be concluded that the optimum operating conditions based on the yield of hydrocarbons and wax products were; U_G = 6.8-10 cm/s, Cs = 15 wt.%, T = 220-230 °C, P = 2.0 MPa in a slurry bubble column reactor for F-T synthesis.     

Improved wear behaviour of alumina-aluminium titanate laminates with low residual stresses and large grained interfaces

The wear behaviour of a monolithic alumina and an alumina-aluminium titanate laminated structure was studied. The laminate, containing surface fine grained alumina layers and internal composite layers with 10 vol.% of aluminium titanate, showed relatively low (≅20 MPa) compressive residual stresses at the surface. Interfaces between layers were constituted by large alumina grains (up to ≅50 μm) that promoted toughening due to crack deflection and branching. Wear tests were performed on square specimens (30 mm × 30 mm × 6 mm) using the pin-on-disc method. The laminates showed higher wear resistance than the monolithic alumina. The analysis of the results together with SEM-EDX observations was performed to identify possible wear mechanisms. The wear resistance improvements are discussed in terms of the residual stresses in the laminate and the properties provided by the special microstructure of the interfaces.     

Decomposition of ozone on Ag/SiO2 catalyst for abatement of waste gases emissions

A silica-supported Ag system made by the incipient wetness impregnation method was investigated in the reaction of heterogeneous catalytic decomposition of ozone. It was established that the catalytic ozone decomposition on Ag/SiO₂ proceeded in the temperature interval −40 °C to 25 °C as a first order reaction with activation energy of 65 kJ/mol (pre-exponential factor 5.0 × 10¹⁴ s⁻¹). Based on the results from the instrumental methods (SEM, XRD, XPS, EPR, TPD) it can be concluded that in presence of ozone the silver is oxidized to a complicated mixture of Ag₂O₃ and AgO. Due to the high activity and stability of the Ag/SiO₂ catalyst, it is promising for neutralization of waste gases containing ozone.     

The Temperature-Programmed Desorption of Oxygen from an Alumina-Supported Silver Catalyst

The associative desorption kinetics of O₂ from a 15 wt% Ag/-Al₂O₃ atalyst were studied under atmospheric pressure in a microreactor set-up by performing temperature-programmed desorption (TPD) experiments. Saturation with chemisorbed atomic oxygen (O*) was achieved by dosing O₂ for 1 h at 523 K and at atmospheric pressure followed by cooling in O₂ to room temperature. The TPD spectra showed almost symmetric O₂ peaks centred above 500 K, indicating associative desorption of O₂ from Ag metal surface sites. By varying the heating rates from 2 to 20 K min^-1, the O₂ TPD peak maxima were found to shift from 508 to 542 K, respectively. A microkinetic analysis of these TPD traces yielded an activation energy for desorption of 149±2 kJ mol^-1 and a corresponding pre-exponential factor of 2×10¹²±1×10¹² s^-1 in good agreement with the kinetic parameters reported for O₂ desorption under UHV conditions from Ag(111) and Ag(110) single crystal surfaces.     

Preparation of ceramic nanospheres by CO2 laser vaporization (LAVA)

In order to prepare ceramic nanoparticles by CO₂ laser vaporization (LAVA) coarse ceramic powders (e.g. ZrO₂, Al₂O₃, and TiO₂) were used as raw materials. The raw powder was vaporized into a flowing process gas under normal pressure. Expanding into the gas, the vapor instantly cools down. Gas-phase condensation leads to the formation of nanoscale particles with a composition that corresponds to that of the raw powder. LAVA nanoparticles are chemically pure, spherical, crystalline, exhibit a narrow size distribution, and form merely soft agglomerates. The co-laser vaporization of mixtures of ceramic raw powders allows for the preparation of nanoparticles of multi-phase (e.g. Fe₂O₃-SiO₂) or single-phase (e.g. CaTiO₃) mixed-oxides and dispersion ceramics (e.g. ZrO₂-Al₂O₃). In order to modify the surface of the nanoparticles they can be coated in-process with an organic additive. Thus, the LAVA method allows for the targeted development of a wide range of ceramic nanopowders with tailored properties.     

Selective catalytic reduction of NOx using propene and ethanol over catalysts of Ag/Al2O3 prepared by microemulsion and promotional effect of hydrogen

The present study explores the possibilities of catalysts of Ag/Al₂O₃, in which silver has been deposited using reverse microemulsions with the aim of getting maximum dispersion and homogeneity in the active superficial species, for the selective catalytic reduction of NO_x in excess of oxygen, using both propene and ethanol as reductants and in the scope of the control of the emissions produced by vehicles that operate in conditions of lean mixture like the diesel engine or those of gasoline direct injection. The promotional effect of the hydrogen presence in the reactive mixture has also been analyzed. For both reductants, when in presence of hydrogen, an important enhancement in NO_x conversion is produced, in particular for a catalyst with 3 wt.% silver. The production of acetaldehyde during the reaction employing ethanol is also analyzed and its role on the NO_x reduction process has been examined. The interpretation of catalytic properties has been complemented by means of in-situ DRIFTS.     

Reduction of lean NOx by ethanol over Ag/Al2O3 catalysts in the presence of H2O and SO2

The reduction of lean NOx using ethanol in simulated diesel engine exhaust was carried out over Ag/Al₂O₃ catalysts in the presence of H₂O and SO₂. The Ag/Al₂O₃ catalysts are highly active for the reduction of lean NOx by ethanol but the reaction is accompanied by side reactions to form CH₃CHO, CO along with small amounts of hydrocarbons (C₃H₆, C₂H₄, C₂H₂ and CH₄) and nitrogen compounds such as NH₃ and N₂O. The presence of H₂O enhances the NOx reduction while SO₂ suppresses the reduction. The presence of SO₂ along with H₂O suppresses the formation of acetaldehyde and NH₃. By infrared spectroscopy, it was revealed that the reactivity of NCO species formed in the course of the reaction was greatly enhanced in the presence of H₂O. The NCO species readily reacts with NO in the presence of O₂ and H₂O at room temperature, being converted to N₂ and CO₂ (CO). Addition of SO₂ suppresses the formation of NCO species and lowers the reactivity of the NCO species. However, the reduction of NOx is still kept at high conversion levels in the presence of H₂O and SO₂ over the present catalysts. About 80% of NOx in the simulated diesel engine exhaust was removed at 743 K. This revised version was published online in July 2006 with corrections to the Cover Date.     

Comparison of product selectivity during hydroprocessing of bitumen derived gas oil in the presence of NiMo/Al2O3 catalyst containing boron and phosphorus

A detailed experimental study was performed in a trickle-bed reactor using bitumen derived gas oil. The objective of this work was to compare the activity of NiMo/Al₂O₃ catalyst containing boron or phosphorus for the hydrotreating and mild hydrocracking of bitumen derived gas oil. Experiments were performed at the temperature and LHSV of 340-420 °C and 0.5-2 h⁻¹, respectively, using NiMo/Al₂O₃ catalysts containing 1.7 wt% boron or 2.7 wt% phosphorus. In the temperature range of 340-390 °C, higher nitrogen conversion was observed from boron containing catalyst than that from phosphorus containing catalyst whereas in the same temperature range, phosphorus containing catalyst gave higher relative removal of sulfur than boron containing catalyst. Phosphorus containing catalyst showed excellent hydrocracking and mild hydrocracking activities at all operating conditions. Higher naphtha yield and selectivity were obtained using phosphorus containing catalyst at all operating conditions. Maximum gasoline selectivity of ∼45 wt% was obtained at the temperature, pressure, and LHSV of 400 °C, 9.4 MPa and 0.5 h⁻¹, respectively, using catalyst containing 2.7 wt% phosphorus.     

Preparation of Al2O3–SiC composite powder from kyanite tailings via carbothermal reduction process

ABSTRACT

Al₂O₃–SiC composite powders were prepared from kyanite tailings mixed with 20% excess carbon coke via carbothermal reduction (CR) reaction. The optimised synthesis condition for synthesising Al₂O₃–SiC composite powders was at 1600°C for 6?h. The equilibrium relationship curves of the condensed phases were presented and the temperature dependence of the phase composition was also studied. The results show that irregular Al₂O₃ and SiC grains first formed at 1500°C, and the elements C, O, Al, and Si randomly distributed in the each crystal particles. The amount of Al₂O₃–SiC composites increased with the increasing synthesis temperature and reaction time. Finally, Al₂O₃–SiC composite bulk materials were further prepared by pressureless sintering using the synthesised Al₂O₃–SiC composite powders as raw materials, and their mechanical properties were investigated in detail. All these results indicate that the CR method can offer a niche application for the development of kyanite tailings.     

Non-destructive characterisation of alumina/aluminium titanate composites using a micromechanical model and ultrasonic determinations: Part II. Evaluation of microcracking

A method for non-destructive detection of microcracks in ceramic composites is described. The method involves the combination of ultrasound characterisation with the application of a three-phase micromechanical model, which considers cracks and pores as void constituents. Four alumina-aluminium titanate materials with different levels of microcracking, from no cracks (monophase alumina) to severely cracked (alumina + 40 vol.% of aluminium titanate) including an alumina + 10 vol.% aluminium titanate material with incipient microcracking have been developed to test the validity of the method. Specimens have been fabricated by colloidal processing and the longitudinal and transverse ultrasound velocities have been determined by the ultrasonic pulse-echo and transmission ultrasound-immersion techniques, employing a digital signal processing. It has been demonstrated that it is possible to differentiate between pores and microcracks, both modelled as void constituents, in terms of the aspect ratio.     

Selectivity enhancement of Co-Mo/Al2O3 FCC gasoline hydrodesulfurization catalysts via incorporation of mesoporous Si-SBA-15

Mesoporous Si-SBA-15 was applied to enhance the FCC gasoline selective hydrodesulfurization (HDS) performance of conventional Co-Mo/Al₂O₃ catalysts and the physicochemical properties of the resulting catalyst were compared with those of Co-Mo/Al₂O₃ catalysts incorporated with macroporous kaolin, mesoporous Si-MCM-41 and microporous Si-ZSM-5. The selective HDS performances of all the catalysts were assessed with different FCC gasolines as feedstocks. The results showed that the HDS selectivity of the catalysts was closely related to the Mo sulfidation that depends on catalyst surface area and metal-support interaction. With the superior Mo sulfidation, the Co-Mo/Si-SBA-15-Al₂O₃ catalyst had the optimal HDS selectivity for not only the full-range FCC gasolines but also the heavy fractions thereof. The present article demonstrates the significance of enhancing Mo sulfidation in improving HDS selectivity and thus sheds a light on the development of highly selective HDS catalysts.