Mechanistic and microkinetic studies of Nitrogen Oxide Storage and Reduction (NSR) on Platinum/Barium Oxide/AIuminum Trioxide Catalysts using Temporal Analysis of Products (TAP)

Engines that operate under lean conditions have significantly better fuel economy but higher NOx emissions compared to stoichiometric engines. NOx storage and reduction (NSR) is a promising technology for meeting upcoming stringent NOx regulations for such engines. We have carried out a systematic study over Pt/Al2O3 and Pt/BaO/Al2O3 powder catalysts using Temporal Analysis of Products (TAP) to elucidate the reaction mechanisms and transient kinetics for the NOx/H2 system. NO pulsing and NO-H2 pump-probe experiments demonstrate the effect of catalyst temperature, NO-H 2 delay time and H2/NO ratio on N2, N2O and NH3 selectivity. A detailed model is derived which explains the key features of the reaction system and used to estimate key kinetic parameter for NO -- H2 reaction. The TAP is utilized to study the relative competition of N2 and NH3 formation with respect to various parameters like H2/NO ratio, temperature and NO -- H2 delay time.;The sequential pre-nitration of Pt/BaO/Al2O3 using NO and 15NO followed by reduction with H2 results in preferential evolution of 15N-containing species during initial H2 pulses. The evolution shifts toward unlabeled N-containing species in later H2 pulses. The data suggests that NOx storage proceeds radially outward from the Pt crystallites and some mobility of stored NOx exists. Further, it confirms the involvement of spillover process at the Pt/BaO interface and its importance for NOx storage and reduction.;The NOx transport parameter, i.e. the diffusivity of NO x in the BaO phase, is also an important parameter affecting the NO x storage and reduction. A methodology is developed to operate in the NOx transport limited regime in the barium phase of Pt/BaO/Al 2O3. A moment analysis is used to calculate the effective distance, effective time and eventually effective diffusivity and diffusional activation energy of stored NOx in the barium phase.;This study is an effort towards understanding the chemistry, kinetics and physics of transient NW system using non-traditional means like isotopes, vacuum experiments and multi-pulse transient modeling. The estimated rate parameters for NO -- H2 system are important for optimum design of NSR or combined NSR -- SCR system. The results suggest that local gradients in the stored NOx at the Pt/BaO interface are important and that should be taken into account in NSR catalyst design and modeling.