Toshitaka Tanabe

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

ESR study of the deactivation of Cu-ZSM-5 in a net oxidizing atmosphere

A microscopic study of the deactivation of Cu-ZSM-5 was carried out. The activity of Cu-ZSM-5 for NOx reduction with hydrocarbons decreased when heated above 600°C in a simulated net oxidixing exhaust gas.27Al and29Si nuclear magnetic resonance results revealed that a dealumination of the zeolite occurs in the deactivated Cu-ZSM-5. Electron spin reonance (ESR) results indicated that aggregation of copper ions did not occur and copper remained as isolated ions in Cu-ZSM-5 even after the deactivation. The ESR spectra of dehydrated Cu-ZSM-5 suggested the migration of copper ions in the zeolite after the deactivation. To investigate the interactions between copper ions and NO/C3H6, the ESR spectra were measured after exposure to nitric oxide and propene. The ESR results indicated that the oxidation state and coordination structure of the copper ions were changed by the adsorption of nitric oxide or propene in active Cu-ZSM-5. On the other hand, no such reaction was observed in the deactivated Cu-ZSM-5. In the deactivated Cu-ZSM-5, copper ions remain atomically dispersed but cannot interact with nitric oxide or propene. In conclusion, the deactivation of Cu-ZSM-5 is suggested to occur through the migration of copper ions to sites where gas molecules like nitric oxide and propene cannot reach them. This migration is triggered by the dealumination of the zeolite.

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.

Comparative NOx reduction behavior of Pt, Pd, and Rh supported catalysts in simulated exhaust gases as a function of oxygen concentration

NOx reduction activity on Pt and Pd catalysts had a maximum for S value as stoichiometry number at a fixed temperature, and the S value at the maximum NOx conversion increased with decreasing temperature. NOx conversion on Rh catalyst increased with decreasing S value, but independent of temperature. As for the effect of HC on NOx reduction behavior, it was concluded that, for Pt and Pd catalysts, HC adsorbs strongly on the catalysts surface to cause the self-inhibition. Increasing O2 concentration lead to oxidation of HC, but decreased the value of NO/O2 ratio. The balance point of the two factors generated a maximum NOx conversion. For Rh catalyst, the strongly adsorbed oxygen is more reactive with decreasing S value, and thus NOx conversion is increased.

Effect of Ba Addition on Catalytic Activity of Pt and Rh Catalysts Loaded on γ-Alumina

The catalytic activity of Pt catalyst loaded on γ-alumina was improved by Ba addition in simulated automotive exhaust gases. On the other hand, the result of Rh catalyst was the opposite. From the results of the partial reaction orders in C3H6–O2 reaction and TPR, it was concluded that the Ba addition to Pt catalyst suppressed the hydrocarbon chemisorption on the Pt catalyst and therefore allowed the catalytic reaction to proceed smoothly. On the other hand, Ba addition to Rh catalyst caused such a strong oxygen adsorption on Rh that rejected the hydrocarbon adsorption and suppressed the reaction.

Enhanced oxygen storage capacity of cation-ordered cerium–zirconium oxide induced by titanium substitution

Herein, we report on the synthesis of Ce0.5Zr0.5-xTixO2 oxygen storage materials prepared via a solution combustion method. Ce0.5Zr0.4Ti0.1O2 showed an outstanding oxygen storage capacity (1310 μmol-O per g) at 200 °C compared to conventional κ-Ce2Zr2O8 (650 μmol-O per g) due to its cation ordering and the formation of weakly bound oxygen atoms induced by Ti substitution.

102 NOx selective catalytic reduction over Pt supported catalyst promoted by zeolite and CeO2-ZrO2

Publisher Summary This chapter proposes a new concept of NOx selective reduction catalyst with high durability. The NOx reduction activities on the catalysts were investigated using a simulated diesel engine exhaust gas. X-ray photoelectron spectroscopy (XPS) measurements were performed to characterize the oxidation state of supported Pt. The chapter reveals NOx reduction activity on the catalyst containing mordenite in light-off test with increasing temperature at 20K/min from 423K to 773K after hydrocarbon (HC) adsorption pre-treatment at 423K. Maximum of NOx conversion increased with increasing HC adsorption amount. This result suggests that the adsorbed HC on catalyst at low temperature should promote NOx reduction at elevating temperature. However, HC adsorption treatment degraded catalytic activity at low temperature because of HC poisoning. To suppress this poisoning some oxides that have redox activity were added. The addition of CeO 2 –ZrO 2 improved the NOx reduction activity below 473K.