NO x uptake mechanism on Pt/BaO/Al2O3 catalysts

Abstract

The NO x adsorption mechanism on Pt/BaO/Al 2 O 3 catalysts was investigated by performing NO x storage/reduction cycles, NO 2 adsorption and NO + O 2 adsorption on 2%Pt=ðxÞBaO/Al 2 O 3 (x = 2, 8, and 20 wt%) catalysts. NO x uptake profiles on 2%Pt=20%BaO/Al 2 O 3 at 523 K show complete uptake behavior for almost 5 min, and then the NO x level starts gradually increasing with time and it reaches 75% of the inlet NO x concentration after 30 min time-on-stream. Although this catalyst shows fairly high NO x conversion at 523 K, only $2.4 wt% out of 20 wt% BaO is converted toBa(NO 3 Þ 2 . Adsorption studies by using NO 2 and NO + O 2 suggest two different NO x adsorption mechanisms. The NO 2 uptake profile on 2%Pt=20%BaO/Al 2 O 3 shows the absence of a complete NO x uptake period at the beginning of adsorption and the overall NO x uptake is controlled by the gassolid equilibrium between NO 2 and BaO/BaðNO 3 Þ 2 phase. When we use NO + O 2 , complete initial NO x uptake occurs and the time it takes to convert $4% of BaO to BaðNO 3 Þ 2 is independent of the NO concentration. These NO x uptake characteristics suggest that the NO + O 2 reaction on the surface of Pt particles produces NO 2 that is subsequently transferred to the neighboring BaO phase by spill over. At the beginning of the NO x uptake, this spill-over process is very fast and so it is able to provide complete NO x storage. However, the NO x uptake by this mechanism slows down as BaO in the vicinity of Pt particles are converted to BaðNO 3 Þ 2 . The formation of BaðNO 3 Þ 2 around the Pt particles results in the development of a diffusion barrier for NO 2 , and increases the probability of NO 2 desorption and consequently, the beginning of NO x slip. As NO x uptake by NO 2 spill-over mechanism slows down due to the diffusion barrier formation, the rate and extent of NO 2 uptake are determined by the diffusion rate of nitrate ions into the BaO bulk, which, in turn, is determined by the gas phase NO 2 concentration.

Investigating the Effect of NO Versus NO2 on the Performance of a Model NO X Storage/Reduction Catalyst

“…The thermal stability of nitrate species is higher than nitrite species [4,7], and therefore they are less likely to decompose during the lean phase (via an adsorption/ desorption like equilibrium) leading to more NO X trapped. Most evidence, however, suggests that at temperatures below 300°C, a mixture of nitrites and nitrates form when NO is used, but at higher temperatures, although a mixture may initially form, the nitrites are quickly oxidized to nitrates and have therefore not been observed [6,7]. This is therefore unlikely to significantly influence higher temperature performance and therefore cannot explain the differences observed at 400 and 500°C.…”

“…Along the same concept, nitrates are more thermally stable than nitrites. More nitrates form with NO 2 introduced as compared to NO, which at low temperatures is trapped as a nitrite or nitrite/ nitrate pair [5,6,15,44]. Therefore at low temperature, nitrite decomposition could lead to a higher amount of unreduced NO X , while the nitrates formed with NO 2 as the inlet NO X source would lead to less.…”

“…This technology operates in two modes: lean and rich. In the lean mode, which is where the engine normally operates, NO is oxidized to NO 2 over precious metal sites and the NO and/or NO 2 is then stored on the trapping material as a nitrate and/or nitrite [4][5][6][7][8]. When some portion of these trapping materials becomes saturated, the engine exhaust is switched to a reductant-rich mode where reductants such as H 2 , CO, and HC are introduced to induce the reduction of the stored NO X to N 2 [9][10][11].…”

“…First, if trapping is a function of NO 2 partial pressure [6], then when NO 2 is used, the very inlet of the catalyst can participate in trapping whereas with NO, oxidation to NO 2 must occur before efficient trapping is realized. A second possibility involves the presence of multiple types of trapping sites [28][29][30].…”

Promotional Effects of H2O Treatment on NO x Storage Over Fresh and Thermally Aged Pt–BaO/Al2O3 Lean NO x Trap Catalysts

“…Consistent with this conclusion, TEM results demonstrated a morphological change in the Ba and alumina catalyst components resulting from the water treatment, which suggests a redistribution of barium species via the carbonation reaction. Such explanations are reasonable because an intimate Pt-Ba interaction [18,19] has been regarded as a primary route to NO x storage since NO 2 , formed on Pt, may then readily spill over to the neighboring barium sites. Therefore, it can be summarized that the H 2 O treatment at room temperature enhances the NO x uptake for the fresh and thermally aged samples by promoting the Pt-BaO interaction through a redistribution of barium species on the LNT catalysts.…”