Intra-channel evolution of carbon monoxide and its implication on the regeneration of a monolithic Pt/K/Al2O3 NOx storage-reduction catalyst

Understanding how a reductant evolves and is utilized during regeneration of NOx storage-reduction catalyst can lead to predictive kinetic models, improved catalysts and energy-efficient engine-catalyst systems. We performed practically relevant NOx storage/regeneration cycling (56 s/4 s) experiments over a monolithic Pt/K/Al2O3 catalyst in a bench-flow reactor and resolved multiple transient reactions and exotherms. Carbon monoxide was the reductant and intra-channel speciation and temperature measurements were instrumental in resolving CO chemistry. Gas-phase O2 reacted with CO very fast over the entire regeneration time, and was depleted at the catalyst front. The resulting exotherm was significant and dissipated slowly over time raising the subsequent storage temperature considerably. NOx release/reduction by CO was also vigorous and primary NOx removal occurred at early regeneration times. The NOx-attributable exotherm was smaller than that of the O2–CO reaction, but extended deeper into the front portion of catalyst due to axially distributed NOx storage. Secondary NOx release/reduction occurred after the primary and produced NH3 as the main product. Hydrogen appeared when and where both the O2 consumption and major NOx release/reduction were near complete. We proposed that H2 produced via water–gas shift (WGS) reaction had little impact on O2 depletion and primary NOx release/reduction under the conditions studied. Further study is necessary to assess the impact of WGS reaction on secondary NOx release/reduction.