Kazuhiko Dohmae

In Situ Redispersion of Platinum Autoexhaust Catalysts: An On-Line Approach to Increasing Catalyst Lifetimes?†

Supported precious metals, such as platinum (Pt), rhodium (Rh), and palladium (Pd), are used to facilitate many industrial catalytic processes. Pt in particular is found at the core of catalysts used throughout the petrochemical industry: from bifunctional catalysts (isomerization/dehydrogenation) used for refining of hydrocarbon fuel stocks, to three-way (CO and hydrocarbon oxidation/NOx reduction) conversions within car exhausts. In this latter, ubiquitous application— commercialized in the USA and Japan in 1977—Pt has always been a pivotal component in the abatement of harmful gas emissions from gasolineor diesel-driven engines. The ever-increasing appreciation of the damage that noxious gas emissions are doing to our environment and the finite availability of noble metals provide strong drivers for the continued study and optimization of the behavior of Pt-based three-way catalysts (TWCs). Central to technological progress in this area is a fundamental understanding of how these materials behave, which may allow us to stop them degrading or deactivating during operation. A longstanding problem, affecting many applications that use highly dispersed metal nanoparticles, is loss of active surface area in the metal components as a result of “sintering”. This is a particularly pernicious problem in applications in which catalysts have to experience high temperatures—in excess of 800 8C in the case of modern car catalysts. This deleterious process causes the particle size of the metal to increase massively—through either particle diffusion or agglomeration or through “ripening” processes. The result is that a large fraction of the active metal is effectively “hidden away” within the bulk of these larger particles where it cannot be used to affect the desired chemical conversions that occur on the particle surface. This central issue of exhaust catalyst deactivation has long been recognized in the hydrocarbon reforming and emission abatement industries. In the former industry, “oxidative redispersion” has been utilized to reverse the effects of sintering and regenerate spent Pt-based reforming catalysts. However, whereas other noble metal particles such as Pd or Rh can be effectively redispersed by gaseous oxygen at certain temperatures, this method is efficient for Pt catalysts only when Cl is present either in the catalyst formulation or as an adjunct added during the redispersion process: in the absence of Cl, redispersion in Pt/Al2O3 by oxygen is limited both to a narrow temperature window (of around 500 8C) and a low level of redispersion. 6] Further, a continuous oxidative treatment over time is required for this redispersion process. Exhaust gases exiting from gasoline engines change quickly and dramatically during operation. Temperatures can rise transiently to around 1000 8C, and the exhaust gas composition itself fluctuates quickly between oxidative and reductive compositions. Clearly, the conventional approach to redispersion and reactivation is highly unsuitable on many counts for “on-board” redispersion and regeneration of TWCs. Other regeneration phenomena have recently been shown in some related cases. The “intelligent” catalyst system of Daihatsu shows in-built structural reversibility of the noble metal component. In this case, it is the structure of the perovskite support that provides the foundation for this extremely elegant piece of applied catalyst design. The possibility of forming very large particles is intrinsically reduced and, under some circumstances, this technology has been successfully commercialized. However, this approach is very much dependent upon the structure of a particular and low surface area support material and is limited in this sense. [*] Dr. Y. Nagai, K. Dohmae, T. Tanabe, Dr. H. Shinjoh TOYOTA Central R&D Labs., Inc. Nagakute, Aichi 480-1192 (Japan) Fax: (+ 81)561-63-6150 E-mail: e1062@mosk.tytlabs.co.jp

Regeneration of Palladium Subsequent to Solid Solution and Segregation in a Perovskite Catalyst: An Intelligent Catalyst

The object of this study was to provide a function for self-regeneration of precious metals in a usage ambience without auxiliary treatment. The strategy was to control the catalytic active site of those crystalline ceramics known as perovskite-type oxides at the atomic level in order to create the new, needed function. Three series of Pd-containing perovskite catalyst systems were prepared by coprecipitation of Pd with La, Fe, and Co using the alkoxide method. It was confirmed that Pd formed a solid solution of the perovskite-type oxide. And Pd in the perovskite crystal structure exhibited an abnormal oxidation number or higher binding energy than the normal bivalence, and it was presumed to be the reason for increasing the catalytic activity. The results of dissolution analysis for the aged Pd-perovskite catalyst suggested that Pd was not only dispersed on the surface of the perovskite crystal, but was present also in the solid solution of the perovskite crystal. The formation of a solid solution in this Pd-perovskite crystal was affected by the B site elements. And Pd in LaFe0.54Co0.36Pd0.10O3 system was more durable than in LaCo0.90Pd0.10O3 or in LaFe0.90Pd0.10O3. Furthermore, the formation of Pd solid solution into these perovskite crystals also depended on atmospheres and temperatures. It appeared that a high state of dispersion was maintained as Pd repeatedly forms solid solutions in the perovskite crystal or segregates out from the crystal depending on the fluctuation of redox conditions and temperatures in automotive catalyst ambience. We named such a catalyst, wherein a precious metal regenerates itself while in operation and remains highly active without requiring any auxiliary treatment, “an intelligent catalyst”.

Universal expressions for average projected range and average damage depth in ion-implanted substrates

Abstract Substrates of Al, Si, LiF, Al 2 O 3 , GaP, and GaAs were implanted with 45 to 420 keV N, Al, Ar, Mn, Ni, Zn, Te, and Xe ions. The reduced energies cover the range from 0.1 to 5. Depth distributions of implanted ions and displaced host atoms were determined by means of backscattering (including channeling) and nuclear resonance reaction measurements followed by computer-simulated spectrum analyses. The results are compared with other experimental data and theoretical predictions given by Gibbons et al. (GJM) and Winterbon et al. (WSS). For the latter theory, optimum WSS parameters are determined to give a good fit to the experimental data. It is concluded that reduced projected range and reduced damage depth are proportional to reduced energy but cannot be expressed by unified relations for all ion-substrate combinations. However, systematic investigation reveals that introduction of a new scaling coefficient gives two universal expressions for modified reduced projected range and modified reduced damage depth as a function of average reduced energy.

Real-Time Observation of Platinum Redispersion on Ceria-Based Oxide by In-situ Turbo-XAS in Fluorescence Mode

A real‐time observation of the redispersion behavior of sintered Pt on ceria‐based oxide was made possible by in‐situ time‐resolved Turbo‐XAS in fluorescence mode. 2 wt% Pt/Ce‐Zr‐Y mixed oxide samples were prepared, and then treated under an aging condition. The average Pt particle size measured by CO absorption method after aging was 7 nm. Redispersion treatments of the previously aged catalyst were carried out at 600°C within an in‐situ XAS cell in a cyclical flow of reducing/oxidizing gases. Pt L3‐edge XANES spectra were collected every 1.1 second under in‐situ conditions. From a change in the XANES spectra, we observed that the Pt particle size of the aged catalyst decreased from 7 to 5 nm after 60 seconds and then to 3 nm after 1000 seconds.

Ion-mixed Zr/ceramics interfaces — XPS study

Abstract Zr thin films (100 nm) were vacuum-deposited on SiC single crystals and silica glass, and these specimens were irradiated with 80 keV N + ions to a dose of 1 × 10 17 /cm 2 at about 100 K. RBS showed that an interdiffusion layer of about 7 nm thickness was formed around the interface between the Zr film and the SiC or the SiO 2 . XPS studies demonstrated that the N + implantations generate chemical bonds like ZrSi, ZrC, and ZrOSi in the intermixed layer.

The Effect of Oxygen Nonstoichiometry on the Structure of the Low Tc Phase in Bi-Sr-Ca-Cu-O Superconductors

The oxygen content of the low Tc phase was varied by annealing in N2 and O2 at 500–750 °C. The oxygen unloading increased the Tc up to 100 K and the length of c-axis by 0.01nm. The associated structural change was examined by X-ray diffraction, XPS, and HRTEM. XPS measurements were performed on the clean fracture surface. No major difference was detected on Bi and Cu spectra, indicating that the change of oxygen content was too small to cause detectable change in the valence of Bi and Cu ions. High resolution TEM showed that the average period of modulation in Bi-O layer was unvaried but the modulation was locally disordered by oxygen unloading. These results strongly indicate that oxygen unloading takes place from the Bi-O layer and the Tc of low Tc phase is mainly controlled by the oxygen in Bi-O layer. Preliminary EXAFS data were also consistent with this idea.