Yasuo Ikeda

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

NOx Storage-Reduction Three-Way Catalyst with Improved Sulfur Tolerance

The NOx storage-reduction catalyst (NSR catalyst) is poisoned by SO2 caused by fuel sulfur, thus its activity is reduced. In order to improve the NSR catalyst, the sulfur poisoning phenomenon has been analyzed. Based on this result, we developed TiO2 and Rh/ZrO2 to promote the sulfur desorption. The developed catalyst has made remarkable progress in its sulfur tolerance, about 50% improvement in NOx purification performance compared with the conventional one.

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.