Spatio-Temporal Behavior of NOx Storage and Reduction Monolith Catalysts

Michael  Harold, University of Houston

Robert Clayton
Ashok Kumar
Jin Xu
Michael P. Harold
Vemuri Balakotaiah

University of Houston
Department of Chemical and Biomolecular Engineering
Houston, TX 77204

Lean-burn gasoline and diesel-powered vehicles require significant reductions in emissions of nitrogen oxides (NOx) and particulate soot.  NOX Storage and Reduction (NSR) involves the sequential periodic reactive trapping of NOx and rapid reduction on multi-functional catalysts containing precious metal and storage components.  While NSR is effective in achieving high NOX conversion, elucidation of the selectivity of NOx reduction to desired product N2 and byproducts such as NH3 and N2O is paramount in optimizing the lean NOx trap (LNT) or hybrid LNT/SCR systems. A combination of Temporal Analysis of Products (TAP), and bench-scale reactor experiments, and microkinetic modeling is being carried out to elucidate the spatio-temporal features of NSR on monolithic Pt/Ba catalysts.
The regeneration of a model Pt/BaO/Al2O3 monolith catalyst was studied with hydrogen as the reductant to elucidate the reaction pathways.  Our results reveal that H2 is very effective in achieving a high time-averaged NOx conversion on Pt/Ba catalysts but the product distribution (N2, NH3, N2O) is a sensitive function of the operating conditions.  Storage and reduction cycles are identified that maximize the NOx conversion and minimize reductant requirements.  Experiments with series of monoliths of a range of lengths enabled the construction of spatio-temporal profiles of reactant and product concentrations.  The results show that there are two primary competing routes to the desired N2 product; a direct route from the reduction of stored NOx by H2 (H2 + NOx ?  N2) or by a sequential route through NH3 (H2 + NOx ?  NH3; NH3 + NOx ?  N2).  The results revealed H2 is the superior reductant, especially for temperatures below 230 oC.  At higher temperatures the reduction is feed-limited and the difference between the reductants H2 and NH3 is small.  The findings are pieced together to establish a phenomenological picture of the spatio-temporal features of the LNT and microkinetic-based description of the regeneration chemistry.