Effect of Length on the Performance of Lean NO\u3csub\u3ex\u3c/sub\u3e Traps

The effect of monolith length on the NOx performance of two different Lean NOx Traps has been investigated using a bench flow reactor (BFR). The washcoat composition of one of the catalysts consists of Pt/K/γ-Al2O3; the major components of the other catalyst include Pt, Pd, Rh, barium, ceria and zirconia supported on γ-alumina washcoat. Samples of 2.22-cm in diameter and length of 2.54, 5.08 and 7.62-cm within each LNT were evaluated with long and short-cycle experiments at a fixed gas hourly space velocity, and results were compared between samples of different lengths. No significant difference in performance was observed in long and short-cycle experiments with full regeneration. On a contrary, significant difference was observed in short-cycle experiments with partial regeneration: the longer the sample the better the performance. The intra-catalyst concentration of H2 measured at different axial locations in short-cycle experiments indicated higher H2 consumption in shorter samples. A series of experiments was carried out to ascertain different mechanisms of H2 consumption, and results indicated different degrees of lean and rich front back-mixing for samples of different sizes. Higher back-mixing in shorter samples resulted in a higher H2 loss via its oxidation by O2 and lesser H2 availability for reducing stored NOx, which in turn affected catalyst’s performance when regeneration was limited by amount of H2 available

The Effects of Ceria Loading on Three-Way Catalysts for Passive SCR Operation

Passive SCR systems, which employ both a three-way catalyst and SCR catalyst, are effective for the reduction of nitrogen oxide (NOx) emissions from lean burn gasoline engines. However, questions remain regarding the effect of three-way catalyst formulations on their performance in these systems. Here, Pd/CeOx/Al2O3 catalysts with variable CeOx loading were synthesized, characterized, and evaluated to determine the effects of CeOx on catalyst performance. While a small amount of ceria was beneficial for promoting essential reactions, excess ceria was detrimental due to the increase in oxygen storage capacity. Additionally, insights into potential reaction pathways for NH3 production were determined

The Effects of Ceria Loading on Three-Way Catalysts for Passive SCR Operation

Passive SCR systems, which employ both a three-way catalyst and SCR catalyst, are effective for the reduction of nitrogen oxide (NOx) emissions from lean burn gasoline engines. However, questions remain regarding the effect of three-way catalyst formulations on their performance in these systems. Here, Pd/CeOx/Al2O3 catalysts with variable CeOx loading were synthesized, characterized, and evaluated to determine the effects of CeOx on catalyst performance. While a small amount of ceria was beneficial for promoting essential reactions, excess ceria was detrimental due to the increase in oxygen storage capacity. Additionally, insights into potential reaction pathways for NH3 production were determined

Passive Ammonia-SCR Catalyst System for NO\u3csub\u3eX\u3c/sub\u3e Abatement from Lean-Burn Gasoline Engines: NH\u3csub\u3e3\u3c/sub\u3e formation over TWC

This dissertation summarizes experimental and computational observations from investigations of a selective catalytic reduction (SCR) system for reducing nitrogen oxides (NOX) in lean gasoline engine exhaust based on utilizing ammonia (NH3) generated by a three-way catalyst (TWC) during brief periods of fuel-rich engine operation. NH3 released from the TWC is stored and available to reduce NOX on a downstream SCR catalyst during subsequent periods of lean engine operation. The experimental results include high-speed measurements of transient NH3 formation on the TWC monolith catalysts, as the catalysts were exposed to lean gasoline engine exhaust from a commercial engine. In addition to the experimental investigations, dynamic computational simulations of NH3 generation on the TWC catalyst were implemented to provide more detailed information about NH3 generation on TWCs based on available reaction kinetic mechanisms. Based on the experimental and computational results, estimates of the potential fuel efficiency gains and emissions relevant to simulated drive cycles indicate that passive SCR can potentially achieve significant fuel efficiency benefits while still meeting regulated NOX emissions limits for vehicles powered by lean gasoline engines. However, optimal performance of the system will most likely require development of emission control methods that include accurate models for SCR catalyst NH3 storage and reaction under realistic drive-cycle transients

Effects of including a NOx storage component on a TWC when using a lean spark ignition gasoline engine combined with a passive SCR system✰

A three-way catalyst (TWC) and a TWC with a NOx storage component (NS-TWC) were evaluated on a lean spark ignition (SI) engine platform to reduce the fuel consumption and emissions of a passive selective catalytic reduction (pSCR) emission control system. The pSCR system is an approach for controlling NOx emissions from lean SI engines. It relies on onboard NH3 generation over a TWC during brief periods of fuel-rich operation. The NH3 is then stored on a downstream SCR catalyst and is available for NOx reduction during subsequent periods of lean engine operation. The NS-TWC addition enabled longer lean operation and more efficient NH3 use, which lowered fuel penalty of the pSCR system. Over a pseudo-transient drive cycle, the lean SI engine with pSCR that included NS-TWC demonstrated a 8.3% reduction in gasoline consumption over stoichiometric-only engine operation, and the NOx and non-CH4 organic gas emissions were consistent with Environmental Protection Agency (EPA) Tier 3 levels. The CO emissions, primarily from rich operation, exceeded the EPA Tier 3 levels. A cleanup catalyst (CUC) with high oxygen storage capacity was used to oxidize tailpipe CO during rich excursions by using the stored oxygen from the preceding lean operation. Although the CUC decreased CO emissions and reduced NH3 slip, some of the NH3 was converted back to NOx. Furthermore, rich CO control remains challenging. The results of this work demonstrate significant improvement in fuel consumption and emissions with a modified pSCR system architecture