Dynamics and selectivity of NOx reduction in NOx storage catalytic monolith

Abstract Several nitrogen compounds can be produced during the regeneration phase in periodically operated NOx storage and reduction catalyst (NSRC) for conversion of automobile exhaust gases. Besides the main product N2, also NO, N2O, and NH3 can be formed, depending on the regeneration phase length, temperature, and gas composition. This contribution focuses on experimental evaluation of the NOx reduction dynamics and selectivity towards the main products (NO, N2 and NH3) within the short rich phase, and consequent development of the corresponding global reaction-kinetic model. An industrial NSRC monolith sample of PtRh/Ba/CeO2/ γ -Al2O3 type is employed in nearly isothermal laboratory micro-reactor. The oxygen and NOx storage/reduction experiments are performed in the temperature range 100–500 °C in the presence of CO2 and H2O, using H2, CO and C3H6 as the reducing agents. The spatially distributed NSRC model developed earlier is extended by the following reactions: NH3 is formed by the reaction of H2 with NOx and it can further react with oxygen and NOx deposited on the catalyst surface, producing N2. Considering this scheme with ammonia as an active intermediate of the NOx reduction, a good agreement with experiments is obtained in terms of the NOx reduction dynamics and selectivity. A reduction front travelling in the flow direction along the reactor is predicted, with the NH3 maximum on the moving boundary. When the front reaches the reactor outlet, the NH3 peak is observed in the exhaust gas. It is assumed that the ammonia formation during the NOx reduction by CO and HCs at higher temperatures proceed via the water gas shift and steam reforming reactions producing hydrogen. It is further demonstrated that oxygen storage effects influence the dynamics of the stored NOx reduction. The temperature dependences of the outlet ammonia peak delay and the selectivity towards NH3 are correlated with the effective oxygen and NOx storage capacity.

[1]  Michael P. Harold,et al.  Mechanistic and kinetic studies of NOx storage and reduction on Pt/BaO/Al2O3 , 2004 .

[2]  Koji Yokota,et al.  The new concept 3-way catalyst for automotive lean-burn engine: NOx storage and reduction catalyst , 1996 .

[3]  W. Epling,et al.  Overview of the Fundamental Reactions and Degradation Mechanisms of NOx Storage/Reduction Catalysts , 2004 .

[4]  Jae-Soon Choi,et al.  Intra-channel evolution of carbon monoxide and its implication on the regeneration of a monolithic Pt/K/Al2O3 NOx storage-reduction catalyst , 2006 .

[5]  W. Epling,et al.  The effects of regeneration conditions on NOX and NH3 release from NOX storage/reduction catalysts , 2007 .

[6]  E. Tronconi,et al.  New insights in the NOx reduction mechanism with H2 over Pt–Ba/γ-Al2O3 lean NOx trap catalysts under near-isothermal conditions , 2006 .

[7]  Yu‐Shu Su,et al.  In situ FTIR studies of the mechanism of NOx storage and reduction on Pt/Ba/Al2O3 catalysts , 2004 .

[8]  M. Kubicek,et al.  Modelling of catalytic monolith converters with low- and high-temperature NOx storage compounds and differentiated washcoat , 2004 .

[9]  E. Fridell,et al.  A combined transient in situ FTIR and flow reactor study of NOX storage and reduction over M/BaCO3/Al2O3 (M=Pt, Pd or Rh) catalysts , 2006 .

[10]  E. Tronconi,et al.  NOx Storage Reduction over PtBa/γ-Al2O3 Catalyst , 2001 .

[11]  Erik Fridell,et al.  Mean field modelling of NOx storage on Pt/BaO/Al2O3 , 2002 .

[12]  Petr Kočí,et al.  Meso-scale modelling of CO oxidation in digitally reconstructed porous Pt/γ-Al2O3 catalyst , 2006 .

[13]  Tomáš Gregor,et al.  Transient behaviour of catalytic monolith with NOx storage capacity , 2007 .

[14]  S. E. Voltz,et al.  Kinetic Study of Carbon Monoxide and Propylene Oxidation on Platinum Catalysts , 1973 .

[15]  Gerhart Eigenberger,et al.  A mechanistic simulation model for NOx storage catalyst dynamics , 2004 .

[16]  Petr Kočí,et al.  Catalytic Converters for Automobile Diesel Engines with Adsorption of Hydrocarbons on Zeolites , 2005 .

[17]  Pore-scale modeling of non-isothermal reaction phenomena in digitally reconstructed porous catalyst , 2007 .

[18]  Neal W. Currier,et al.  Ammonia is a hydrogen carrier in the regeneration of Pt/BaO/Al2O3 NOx traps with H2 , 2007 .

[19]  F. Basile,et al.  Reactivity of Mg-Ba catalyst in NOx storage/reduction , 2007 .

[20]  James E. Parks,et al.  Further evidence of multiple NOx sorption sites on NOx storage/reduction catalysts , 2004 .

[21]  Mathematical modelling of catalytic monolithic reactors with storage of reaction components on the catalyst surface , 1999 .

[22]  Pio Forzatti,et al.  NOx adsorption study over Pt-Ba/alumina catalysts: FT-IR and pulse experiments , 2004 .

[23]  R. T. Yang,et al.  A highly sulfur resistant Pt-Rh/TiO2/Al2O3 storage catalyst for NOx reduction under lean-rich cycles , 2001 .

[24]  Petr Kočí,et al.  Modeling of Three-Way-Catalyst Monolith Converters with Microkinetics and Diffusion in the Washcoat , 2004 .