Commercial Lean NOx Trap Catalyst Research Articles

Optimization of the Washcoat Slurry for Hydrotalcite-Based LNT Catalyst

This work aimed to optimize the washcoat slurry for hydrotalcite-based lean NOx trap (LNT) catalyst. The effects of the slurry properties including pH, solid content, binder and additive on the hydrotalcite-based slurry viscosity were investigated. The particle size distribution of the optimal hydrotalcite-based slurry was measured. A cordierite material was used to coat the optimal slurry, and the washcoat was characterized by X-ray diffraction, scanning electron microscopy and N2 adsorption. The optimal slurry containing Pt and Ba was coated on the cordierite for the preparation of hydrotalcite-based LNT catalyst, and the performances of this catalyst were evaluated by NOx storage test, temperature programmed desorption and NOx reduction. For comparison, the performance of the commercial LNT catalyst with Pt/BaO/Al2O3 was analyzed. After coating, the hydrotalcite-based washcoat was closely contacted with the support, being the main phase MgO and presenting a specific surface area of 86.3 m2/g. The hydrotalcite-based LNT catalyst had better NOx storage and desorption ability, selectivity to N2 and LNT efficiency than the Pt/BaO/Al2O3 catalyst.

Deactivation reactions on a commercial lean nox-trap - Effect of hydrocarbon nature, concentration and operation temperature

Engine-out emissions during rich phase operation of a lean NOX trap (LNT) catalyst contain a mixture of different hydrocarbons (HCs) depending on engine operation and fuel type. The impact of temperature, as well as type and concentration of HC on reactions and deactivation potential on a commercial LNT catalyst was analyzed by lean-rich cycle measurements and subsequent temperature programmed oxidation. Deactivation during the rich phase is omitted initially by O species adsorbed or stored by CeO2, but formation of amorphous carbon and significant deactivation of noble metal sites occur in the presence of C2H2. Carbon formation and deactivation of Pt sites correlate with the reforming reaction of C2H2, thus emphasizing the significance of this reaction in the rapid build-up of carbonaceous deposits. Coking and deactivation decreases with decreasing C2H2 concentration and reaction temperature that determine the extent of reforming reaction. DFT calculations of adsorption energies on sites of Pt (111) or Pt10/CeO2 verify that abundance of HCs on the Pt surface, relative to other feed components, determines which reactions occur and thus the coking degree. This aligns with the finding that HCs with lower adsorption energy, such as C2H4 or C3H6, are less reactive and significant catalyst deactivation is absent during their presence.

New insights on N2O formation pathways during lean/rich cycling of a commercial lean NOx trap catalyst

Pathways for N2O formation during lean/rich cycling of a commercial LNT catalyst (containing Pt, Pd, Rh as well as Ba, CeZr, MgAl and Al oxides) were investigated based on bench-reactor experiments with spatiotemporally resolved gas measurements (SpaciMS) and in situ surface analysis (DRIFTS). The inlet gas temperature, composition of the rich gas mixture and regeneration length were varied in order to reveal the underlying mechanisms. Particular attention was given to low temperatures where the regeneration was kinetically limited and N2O was emitted in two peaks. The primary N2O peak appeared immediately after rich-phase inception when platinum-group-metal (PGM) sites over which NOx reduction took place were only partially reduced, and tailed off with the breakthrough of the reductant front. The secondary N2O peak appeared at the rich-to-lean transition as a result of reactions between surface-deposited reductive species (NH3, CO and/or isocyanates) and residual stored NOx. Therefore, more thorough regeneration (longer rich phase, more efficient reductant, or higher temperature) led to a reduced secondary N2O peak. In contrast, CO either produced from reverse water gas shift or injected as a feed could poison PGM sites resulting in self-inhibition of the regeneration and a subsequent significant secondary N2O peak.

Effect of sulfur loading on the desulfation chemistry of a commercial lean NOx trap catalyst

We investigate the effects of initial sulfur loadings on the desulfation chemistry and subsequent final activity of a commercial LNT catalyst. Identical total amounts of SO2 are applied to the samples, albeit with the frequency of desulfation varied. The results indicate that performance is better with less frequent desulfations. The greater the amount of sulfur deposited before desulfation, the more amount of SO2 evolution before H2S is observed during desulfation, which can be explained by two sequential reactions; initial conversion of sulfate to SO2, followed by the reduction of SO2 to H2S. After completing all sulfation/desulfation steps, the sample with only a single desulfation results in a fairly uniform sulfur distribution along the z-axis inside of the monolith. We expect that the results obtained in this study will provide useful information for optimizing regeneration strategies in vehicles that utilize the LNT technology.

Spatially-Resolved Thermal Degradation Induced Temperature Pattern Changes along a Commercial Lean NOX Trap Catalyst

Spatially-Resolved Thermal Degradation Induced Temperature Pattern Changes along a Commercial Lean NOX Trap Catalyst

Study of Ammonia Formation During the Purge of a Lean NO x Trap

The aim of this work was to study the formation of ammonia during the regeneration step of a commercial lean NOxTrap catalyst in real conditions. Experiments were carried out on an engine test bench equipped with 2.2 L common rail diesel engine. The after-treatment system was composed of a commercial NSR monolith catalyst with a volume of 2.2 L. This material was composed of platinum, palladium and rhodium, as active components, with barium oxide and alumina as storage component and support, respectively. Catalytic measurements were carried out at 280 °C. Particular attention has been paid to formation of ammonia during the purge using hydrogen and CO as reducing agents. The time dependency of the extent of ammonia formation and the influence of hydrogen concentration were carefully examined. It was observed that the temperature rose in the first step of the regeneration process, after the switch in rich conditions, due to the occurrence of exothermic oxidation (H2/O2, CO/O2) accompanied with the releasing of available vacant noble metal sites which may provide a route for H2 and NO dissociation and/or subsequent surface reactions leading to the ultimate formation of ammonia. It was found that ammonia formation strongly depends on the purge duration and on the concentration of the reducing agent available during the purge.