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A kinetic study of the reduction of NOx stored on Pt-Ba/Al2O3 catalyst

Paper ID Volume ID Publish Year Pages File Format Full-Text
47221 46464 2010 11 PDF Available
Title
A kinetic study of the reduction of NOx stored on Pt-Ba/Al2O3 catalyst
Abstract

A kinetic study for the reduction of NOx stored onto a Pt-Ba/Al2O3 LNT catalyst is developed, based on previous experimental work where regeneration of stored NOx was accomplished under isothermal wet conditions using ammonia and hydrogen as reducing agents [L. Lietti, I. Nova, P. Forzatti, J. Catal. 257 (2008) 270–282]. The kinetic model developed in the present paper differs from those available in the literature because it considers the most relevant physico-chemical features of regeneration of LNT catalysts under nearly isothermal conditions. It is based exclusively on a consecutive reaction scheme with ammonia as intermediate, as proposed in the previous study, and on a mechanism which considers only surface reactions of stored NOx. The model includes the elementary-like steps of adsorption–desorption of hydrogen, ammonia and water at Pt sites, and the lumped steps of reduction of stored nitrates by hydrogen to give ammonia and of reduction of residual stored nitrates by ammonia to give nitrogen. A surface dependent energy of activation for the reduction of nitrates with ammonia is required to describe the data in the T interval 150–300 °C. This is due to the presence of nitrates with high and low reactivity that are envisaged as close and far from the noble metal crystallites, respectively. A low energy of activation (22.94 kJ/mol) is estimated for the reduction of nitrates by hydrogen to give ammonia. This applies above a consumption of 1.5–2 × 10−4 mol of stored NOx/gcat, where the residual NOx species are expectedly stored far from the Pt crystallites and the rate of their consumption is limited by surface diffusion. Before this consumption level the reaction is limited by the concentration of hydrogen at any temperature in the T interval 150–350 °C. The model is shown to capture the major features of the isothermal experiments performed under wet conditions over Pt-BaO/Al2O3 LNT catalyst with hydrogen and ammonia as reducing agents in the T interval 150–350 °C and is able to describe nicely on a purely predictive basis independent NH3-TPSR experiments. However the model underestimates H2 consumption and net NH3 production during independent H2-TPSR experiments below 100 °C, where the consumption of stored NOx is less than 1 × 10−4 mol/gcat and is associated with fast regeneration of Ba sites in close proximity to the Pt crystallites. The model is also used to simulate the axial concentration profiles in the reactor for gas and surface species as function of time during regeneration with ammonia.

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Keywords
NOx reduction; Pt-Ba/Al2O3; NOx Trap; Kinetics; Hydrogen; Ammonia; Spatiotemporal resolution
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Publisher
Database: Elsevier - ScienceDirect
Journal: Applied Catalysis B: Environmental - Volume 99, Issues 1–2, 31 August 2010, Pages 145–155
Authors
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Subjects
Physical Sciences and Engineering Chemical Engineering Catalysis
Get Full-Text Now
Don't Miss Today's Special Offer
Price was $35.95
You save - $31
Price after discount Only $4.95
100% Money Back Guarantee
Full-text PDF Download
Online Support
Any Questions? feel free to contact us