Analyse of System Factors Affecting Performance in Lean NOx Catalysis. 2. The Deleterious Role of Parasitic Homogeneous Hydrocarbon Oxidation on the Performance of High Temperature Lean NOx Catalysts

Kharas, K. C. C.; Miller, Michael; Yan, Ji-Yang

doi:10.4271/982604

Cited by 3 publications

(1 citation statement)

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“…The complex chemical environments typical of an automotive exhaust stream have generally only been evaluated in more applied screening or durability studies [6,11,12,32,[36][37][38][39][40]. Virtually all mechanistic studies to date related to active lean-NOx catalysis have involved simple, high purity feed-gas mixtures (often including only a single hydrocarbon reductant).…”

Section: Low Na Pt/hto:si Catalystsmentioning

confidence: 99%

Preparation Effects on the Performance of Silica-Doped Hydrous Titanium Oxide (HTO:Si)-Supported Pt Catalysts for Lean-Burn NOx Reduction by Hydrocarbons

Gardner¹,

McLaughlin²,

Mowery³

et al. 2002

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This report describes the development of bulk hydrous titanium oxide (HTO)-and silicadoped hydrous titanium oxide (HTO:Si)-supported Pt catalysts for lean-burn NOx catalyst applications. The effects of various preparation methods, including both anion and cation exchange, and specifically the effect of Na content on the performance of Pt/HTO:Si catalysts, were evaluated. Pt/HTO:Si catalysts with low Na content (< 0.5 wt.%) were found to be very active for NOx reduction in simulated lean-burn exhaust environments utilizing propylene as the major reductant species. The activity and performance of these low Na Pt/HTO:Si catalysts were comparable to supported Pt catalysts prepared using conventional oxide or zeolite supports. In ramp down temperature profile test conditions, Pt/HTO:Si catalysts with Na contents in the range of 3-5 wt.% showed a wide temperature window of appreciable NOx conversion relative to low Na Pt/HTO:Si catalysts. Full reactant species analysis using both ramp up and isothermal test conditions with the high Na Pt/HTO:Si catalysts, as well as diffuse reflectance FTIR studies, showed that this phenomenon was related to transient NOx storage effects associated with NaNO 2 /NaNO 3 formation. These nitrite/nitrate species were found to decompose and release NOx at temperatures above 300°C in the reaction environment (ramp up profile). A separate NOx uptake experiment at 275°C in NO/N 2 /O 2 showed that the Na phase was inefficiently utilized for NOx storage. Steady state tests showed that the effect of increased Na content was to delay NOx light-off and to decrease the maximum NOx conversion. Similar results were observed for high K Pt/HTO:Si catalysts, and the effects of high alkali content were found to be independent of the sample preparation technique. Catalyst characterization (BET surface area, H 2 chemisorption, and transmission electron microscopy) was performed to elucidate differences between the HTO-and HTO:Sisupported Pt catalysts and conventional oxide-or zeolite-supported Pt catalysts.4

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