Yoshiyasu Fujitani

Microstructural changes of diesel soot during oxidation

Abstract Changes in the microstructure of diesel soot during oxidation in air were examined by means of an analytical electron microscope. The soot was sampled using a stainless-steel mesh placed in an engine exhaust pipe. The samples at 25, 50, and 75 wt% burnoff were obtained by controlling the holding time in an electric furnace at 500°C. In the early stage of oxidation, an occluded “SOF (soluble organic fraction)” within the soot particles is released and the soot particles take on a porous structure. Also, layer planes of crystallites within the soot particles grow successively and are arranged, a turbostratic structure. In the last stage of oxidation, the size of the layer planes of the crystallites in the soot particles does not become smaller, but crystallites at the outermost shell of the soot particles flake off instead, reducing the size of the particles. That is, the oxidative disappearance of the soot particles is concluded to involve a disintegration and stripping of the outer layer, induced by strain energy in the layer plane that accompanies the growth of the crystallite layer under high temperature.

Nitric oxide reduction performance of automotive palladium catalysts

Reduction of nitric oxide over palladium catalysts under reducing conditions was examined by using cycled feeds and steady non-cycled feeds. The effects of lanthanum(III) oxide on the catalytic properties of α-alumina-supported palladium for the reduction of nitric oxide were studied by X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR) and chemisorption of propene. The reduction of nitric oxide on palladium catalysts is significantly inhibited by hydrocarbon. However, the reduction activity is improved by both the periodic operation effect and the addition lanthana to the catalyst. The mechanism of the periodic operation effect is interpreted in terms of the strong adsorption of hydrocarbons. The addition of lanthana to the catalyst substantially moderated this hydrocarbon poisoning. The conversion of nitric oxide on the Pd/La2O3 catalyst with cycled feeds was similar to that on a rhodium catalyst. The XPS, TPR and hydrocarbon chemisorption studies showed that the presence of lanthana provided palladium oxide in a hardly-reduced state and suppressed the chemisorption of hydrocarbons on the palladium oxide.

Periodic operation effects in propane and propylene oxidation over noble metal catalysts

Abstract It had been observed that under cycling conditions, the catalytic activities of three-way automotive catalysts are superior to those under static conditions, and that catalyst performance depends on the cycling period and feedstream conditions. The oxidation of propylene and propane over platinum, palladium and rhodium catalyst under static and periodic conditions was therefore investigated. The results were as follows; (i) the order of catalytic acitivities was Pd>Pt>Rh for propylene—oxygen and Pt>Rh>Pd for propane-oxygen; (ii) the periodic operation effect is dependent on both catalyst species and reaction temperature; (iii) the optimum period for the maximum conversion decreases with increasing temperature; and (iv) from results of the partial reaction order and evolution pattern analysis, either hydrocarbons or oxygen self-poison the reaction on the catalysts. The order of sensitivity to self-poisoning corresponds to the order of the periodic operation effect.

Effect of lanthanum on the no reduction over palladium catalysts

Abstract A palladium-lanthanum (Pd-La) catalyst was found to be more active for NO reduction than a palladium (Pd) catalyst under the reducing conditions found in engine exhaust gas. Both the Pd-La and Pd catalysts had same activities for the NO reduction with CO. However, in the presence of hydrocarbons, the NO reduction activity of a Pd-La catalyst was much better than that of a Pd catalyst. The NO reduction of the Pd catalyst was significantly inhibited by hydrocarbons. The behavior of the adsorbed hydrocarbons on the both catalysts were studied by means of the transient response method. The hydrocarbon was more strongly adsorbed on the Pd catalyst than on the Pd-La catalyst. Also, the partial oxidation activity of hydrocarbon for the Pd-La catalyst was much better than that for the Pd catalyst. The presence of lanthanum in the Pd-La catalyst perhaps weakens the adsorption strength of hydrocarbons.

Synthesis and Characterization of Si3N4 Powder Produced by Laser-Induced Chemical Reaction

In an attempt to prepare very fine, high-purity Si3N4 powder from an NH3/SiH4 system by CO2 laser induced reaction, the reaction parameters relating to the laser operation and the reaction conditions were examined systematically. Both pulsed and continuous high-power CO2 lasers were used, the Si3N4 powders obtained exhibited various colors from brown to white. Their particle size was 10~15 nm, and their nitrogen content was 37 wt%. The powders were composed of amorphous and partially-crystallized particles. The α-transformation in the heat treatment of two typical samples up to 1600°C was investigated by X-ray diffraction, and the surface area reduction produced by the heat treatment was also measured. The factors affecting the synthesis of the Si3N4 powder were found to be the NH3/SiH4 ratio, the reaction pressure, and the laser power density.

Effect of Severe Thermal Aging on Noble Metal Catalysts

Abstract Both sintering and activity behaviours over noble metal catalysts aged in oxidative atmospheres with various O 2 contents at 1100°C (or 1000° C) for 5 h were systematically characterized. With increasing O 2 contents, catalytic activities over aged Pt, Rh, and Pt/Rh catalysts decreased, and, in contrast, those over aged Pd and Pd/Rh catalysts increased. While, an order of sintering for the noble metal particles on aged catalysts agreed closely to that of the each percentage conversions as well as to that of vapour pressures of respective catalyst species, such as noble metals or their oxides. Selectivity profiles of the aged catalysts for converting NO and O 2 in a simulated exhaust gas were characteristic ones. It is confirmed through the above results that the performance of aged catalysts are tightly governed by the properties of noble metals and the selectivity data are also important for exploring the catalytic activities, in particular, over multi-functional catalysts such as automotive exhaust ones.

Rotational relaxation in free jet expansion for N2 from 300 to 1000 K

The object of this study is to obtain information on the rotation–translation transition probability during N2–N2 collisions. Rotational relaxation for N2 in the free expansion flow is studied by TOF (time‐of‐flight) measurement with a molecular beam sampling mass filter system. Source temperatures were heated up to 1300 K by an electric heater. The terminal rotational temperatures for various source temperatures and pressures were determined from an energy balance equation in conjunction with the TOF data. Recently, Pritchard et al. have proposed that the transition probability from the ith to the jth rotational states decreases in proportion to some power of the energy difference between the states. This power law was fitted to experimental results for rotational relaxation in the free jet expansion. The results show that the state‐to‐state rotational relaxation rates vary as the inverse 3/2 power of the energy difference between the initial and final states. The rotational distributions in the jet expa...

Periodic Operation Effects on Automotive Noble Metal Catalysts — Reaction Analysis of Binary Gas Systems

Catalytic activities and periodic operation effects in various binary gas systems (CO-O2, NO-CO, NO-H2, C3H6-O2, and C3H8-O2) over Pt, Pd, and Rh/α-Al2O3 were compared. In all reaction systems, periodic operation effects were found to some extent. That is, the conversion improved in the cycling feed compared to the static one. The periodic operation effects occurred most noticeably for catalysts having lower catalytic activity as a result of the difference of adsorption capability between the two reactants.

Improvement of Engine Emissions Tower Three-Way Catalyst by the Periodic Operations

The effects of periodic operation (variations of both AF modulation width and their frequency) on the activities of fresh and aged PtRh catalysts were systematically evaluated on engine dynamometer in terms of AF and cold-start performance. AF modulation width and its frequency were varied from +-2 to +-10% AF and from 0.5 to 5 Hz, respectively. The periodic operation effects for both catalysts were the most preferable for diminishing THC emission and a little effect was obtained for CO emission. A remarkable improvement of NO/sub x/ emission on the aged catalyst was confirmed, although a medium effect was appeared on the fresh catalyst.

Mass Separation during Molecular Beam Sampling Process

In a molecular beam sampling process, if the stagnation gas is a mixture, heavier species are concentrated onto the axis of the molecular beam, while lighter ones are diffused away from it. This phenomenon is generally called mass separation. Mass separation is caused mainly by three phenomena: (1) pressure diffusion, (2) Mach-number focusing and (3) skimmer interference. Pressure diffusion has already been analyzed by Sherman. In the present work, the mass separation for a binary mixture of Ar and He was experimentally measured by means of a molecular beam-mass filter system, and its kinetic analysis was carried out with regard to Mach-number focusing taking into consideration of the non-equilibrium of translational temperatures. The experimental results and the theoretical prediction agree well except in the cases of a shorter nozzle-skimmer distance, where skimmer interference is dominant, and in the cases of a higher Ar content.