NO x uptake mechanism on Pt/BaO/Al 2 O 3 catalysts
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Catalysis Letters Vol. 111, Nos. 3–4, November 2006 (Ó 2006) DOI: 10.1007/s10562-006-0153-4
NOx uptake mechanism on Pt/BaO/Al2 O3 catalysts Ja Hun Kwak, Do Heui Kim,* Tama´s Szailer, Charles H. F. Peden, and Ja´nos Szanyi Institute for Interfacial Catalysis, Pacific Northwest National Laboratory, Richland, WA 99352, USA
Received 30 May 2006; accepted 8 August 2006
The NOx adsorption mechanism on Pt/BaO/Al2 O3 catalysts was investigated by performing NOx storage/reduction cycles, NO2 adsorption and NO + O2 adsorption on 2%Pt=ðxÞBaO/Al2 O3 (x = 2, 8, and 20 wt%) catalysts. NOx uptake profiles on 2%Pt=20%BaO/Al2 O3 at 523 K show complete uptake behavior for almost 5 min, and then the NOx level starts gradually increasing with time and it reaches 75% of the inlet NOx concentration after 30 min time-on-stream. Although this catalyst shows fairly high NOx conversion at 523 K, only 2.4 wt% out of 20 wt% BaO is converted toBa(NO3 Þ2 . Adsorption studies by using NO2 and NO + O2 suggest two different NOx adsorption mechanisms. The NO2 uptake profile on 2%Pt=20%BaO/Al2 O3 shows the absence of a complete NOx uptake period at the beginning of adsorption and the overall NOx uptake is controlled by the gas– solid equilibrium between NO2 and BaO/BaðNO3 Þ2 phase. When we use NO + O2, complete initial NOx uptake occurs and the time it takes to convert 4% of BaO to BaðNO3 Þ2 is independent of the NO concentration. These NOx uptake characteristics suggest that the NO + O2 reaction on the surface of Pt particles produces NO2 that is subsequently transferred to the neighboring BaO phase by spill over. At the beginning of the NOx uptake, this spill-over process is very fast and so it is able to provide complete NOx storage. However, the NOx uptake by this mechanism slows down as BaO in the vicinity of Pt particles are converted to BaðNO3 Þ2 . The formation of BaðNO3 Þ2 around the Pt particles results in the development of a diffusion barrier for NO2, and increases the probability of NO2 desorption and consequently, the beginning of NOx slip. As NOx uptake by NO2 spill-over mechanism slows down due to the diffusion barrier formation, the rate and extent of NO2 uptake are determined by the diffusion rate of nitrate ions into the BaO bulk, which, in turn, is determined by the gas phase NO2 concentration. KEY WORDS: NOx storage/reduction; Pt/BaO/Al2 O3 ; NO2 adsorption; NO + O2 adsorption; spill-over; bulk diffusion; kinetic control.
1. Introduction The control of NOx, (NO and NO2) emission from combustion processes, including vehicle engines, remains a challenge particularly for systems operating at high air-to-fuel ratios (so-called ‘lean’ combustion). The current ‘‘3-way’’, precious metal-based catalytic converters are unable to selectively reduce NOx with reductants (e.g., CO, H2, and residual unburned hydrocarbon) in the presence of excess O2. In the last few years, worldwide environmental regulations regarding NOx emissions from diesel engines (inherently operated ‘lean’) have become significantly more stringent resulting in considerable
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