Lean-rich Cycling Conditions Research Articles

Oxidation of C3H8, iso-C5H12 and C3H6 under near-stoichiometric and fuel-lean conditions over aged Pt–Pd/Al2O3 catalysts with different Pt:Pd ratios

Effective control of hydrocarbon (HC) emissions at low temperatures is critical for emission compliance, since a large fraction (typically 70–80%) of tailpipe HC emissions occurs during the first couple of minutes after an engine cold-start. The oxidation of propane, iso-pentane and propylene was investigated over Pd-only, Pt-only and Pt–Pd catalysts aged under lean-rich cycling conditions. Various characterizations such as XRD, XANES, STEM, CO chemisorption and H2-TPR were performed over the initial catalysts. Moreover, before and after the reaction, the states of the catalysts were characterized by XRD, XANES and STEM. The oxidation activity tests were carried out under both near-stoichiometric and fully lean conditions in the feedstream containing HC, O2, CO, H2, NO and H2O. Under the near-stoichiometric condition, the Pt/Al2O3 catalyst showed much higher activity for propane oxidation than the Pd/Al2O3 and Pt–Pd/Al2O3 catalysts. For propylene oxidation, on the other hand, the Pd/Al2O3 catalyst was the most active, and the Pt/Al2O3 catalyst exhibited the lowest activity due to the fact that the co-presence of CO and propylene on the Pt surface leads to their competition for a limited amount of adsorbed oxygen. Under the fully lean condition, metallic Pd in the Pd/Al2O3 catalyst was gradually oxidized to PdO and consequently poisoned by H2O, whereas there was little change in the state of the Pt/Al2O3 catalyst, indicating higher stability of metallic Pt than Pd. However, a drastic decrease in the propane oxidation activity was observed over Pt/Al2O3 in the presence of excess oxygen in the fully lean feed since preferential adsorption of oxygen on the Pt surface inhibited propane adsorption and thus the oxidation reaction. In addition, the propylene oxidation activity under the fully lean condition was greatly enhanced over all three catalysts compared to that observed under the near-stoichiometric condition, with Pd/Al2O3 still being more active than Pt/Al2O3. The catalyst activity for iso-pentane oxidation was affected only to a relatively small extent by the high oxygen concentration, so that the same activity order among the catalysts (Pt > Pt–Pd > Pd) was observed under both the slightly lean and fully lean conditions. These results indicate that both hydrocarbon type and oxygen concentration level are important factors determining the relative oxidation activities of the various noble metal catalysts. Relatively small beneficial effects of Pt–Pd alloying were observed during iso-pentane oxidation at low temperatures under the near-stoichiometric conditions and during propane oxidation at high temperatures under the fully lean condition. However, alloy formation was found to be detrimental for the propane oxidation under the slightly lean condition.

Effects of four-mode hydrothermal aging on three-way catalysts for passive selective catalytic reduction to control emissions from lean-burn gasoline engine

Passive selective catalytic reduction (SCR) is a promising approach for the control of NOX emissions in lean burn gasoline exhausts. It requires ammonia (NH3) to be produced over a three-way catalyst (TWC) during periods of fuel-rich operation for the reduction of NOX during periods of fuel-lean operation. Previous research has shown the viability of this system but has not examined the effects of hydrothermal degradation. This work is focused on evaluating the effects of hydrothermal aging on the TWC in a passive SCR system. Two catalysts were studied: a Pd-TWC, and a NOX storage and reduction (NSR) TWC. Samples were aged at 920 °C for 100 h using a four-mode hydrothermal aging procedure. This causes the catalyst to be oxidized and reduced, as it would in a real system. The effects of aging were evaluated using simulated exhaust under both steady state and lean-rich cycling conditions. Hydrothermal aging caused significant changes in catalyst activity, leading to a decrease in low temperature conversion of carbon monoxide (CO) and propane (C3H8) on both catalysts, and degradation of oxygen storage and NOX storage components. However, the catalysts maintained their activity for NOX conversion and NH3 production, showing sufficient activity for the operation of a passive SCR with an optimum projected fuel consumption of 92–98% compared to stoichiometric operation.

Pt/Ba/Co1Mg2Al1Ox with dual adsorption sites: A novel NOx storage and reduction catalyst

A novel NOx storage and reduction catalyst, namely 1Pt/20Ba/Co1Mg2Al1Ox with dual NOx adsorption sites was developed. The layered double hydroxide derived mixed metal oxide Co1Mg2Al1Ox not only serves as catalyst support but also provides additional NOx adsorption catalytic sites. The influence of Pt and BaO loadings, adsorption and calcination temperatures, H2O resistance and lean-rich cycling conditions on the performance of the stated catalyst composition were systematically investigated. We demonstrate that this new 1Pt/20Ba/Co1Mg2Al1Ox catalyst possesses much higher NOx storage capacity (1.38mmol/g) than the conventional Pt/Ba/γ-Al2O3 (0.56mmol/g). It also exhibits good H2O resistance, stability in lean-rich cycling conditions and a high NOx removal efficiency (ca. 82.2%), thus suggesting a great potential for practical lean-burn deNOx applications.

NOx storage and reduction properties of model manganese-based lean NOx trap catalysts

In order to study the role of manganese in LNT catalysis, model Pd/Mn/Ba/Al, Pd/Mn/Al and Pd/Ba/Al catalysts were prepared and characterized. Both Mn-containing catalysts exhibited higher activity for NO oxidation to NO2 than the Pd/Ba/Al reference catalyst, although NOx-TPD experiments showed that adsorbed NOx species were weakly bound on Pd/Mn/Al. Consequently, the presence of Ba was essential for high storage capacity. Compared to the Pd/Ba/Al reference, Pd/Mn/Ba/Al showed significantly improved NOx conversion under lean-rich cycling conditions as a consequence of its increased activity for NO oxidation and hence, superior NOx storage efficiency. In addition, the presence of Mn greatly lessened the inhibiting effects of H2O and CO2 on cycle-averaged NOx conversion, this being due to the facile decomposition of manganese carbonate at low temperatures as evidenced by DRIFTS. Significantly, the Pd/Mn/Ba/Al catalyst displayed comparable activity to a traditional LNT catalyst of the Pt/Ba/Al type, showing the promising prospect of such new type of LNT catalysts.

Pt/CexPr1−xO2 (x = 1 or 0.9) NOx storage–reduction (NSR) catalysts

Model Pt/Ce0.9Pr0.1O2 and Pt/CeO2 NOx storage–reduction catalysts were prepared via nitrate calcination, co-precipitation and carbon-templating routes. Raman spectroscopic data obtained on the catalysts indicated that the introduction of praseodymium into the ceria lattice increased the concentration of defect sites (vacancies), arising from the higher reducibility of the Pr4+ cation compared to Ce4+. For the Pr-promoted samples, H2-TPR profiles contained high temperature bulk reduction peaks which were less pronounced compared with their ceria analogs, indicating that the presence of praseodymium enhances oxygen mobility due to the creation of lattice defects. Under lean-rich cycling conditions, the cycle-averaged NOx conversion of the Pt/Ce0.9Pr0.1O2 samples was in each case substantially higher than that of the Pt/CeO2 analog, amounting to a difference of 10–15% in the absolute NOx conversion in some cases. According to DRIFTS data, a double role can be assigned to Pr doping; on the one hand, Pr accelerates the oxidation of adsorbed NOx species during the lean periods. On the other hand, Pr doping destabilizes the adsorbed NOx species during the rich periods, and the kinetics of nitrate decomposition are faster on Pt/Ce0.9Pr0.1O2, leading to improved catalyst regeneration. These results suggest that ceria-based mixed oxides incorporating Pr are promising materials for NOx storage–reduction catalysts intended for low temperature operation.

Passive-ammonia selective catalytic reduction (SCR): Understanding NH3 formation over close-coupled three way catalysts (TWC)

NH3 formation was examined under steady-state and lean/rich cycling conditions over four commercial catalysts including: (1) a Pd-only, high precious metal loading (HPGM) three-way catalyst, (2) a Pd/Rh+CeO2, low precious metal loading (LPGM) three-way catalyst, (3) a combination of a HPGM and a LPGM (Dual-Zone) catalyst and (4) a lean NOX trap (LNT) catalyst. The goal of this work was to evaluate these catalysts for their potential use as the upstream component in a passive-NH3 SCR configuration. NH3 formation during steady-state operation was found to be dependent on the air-to-fuel ratio (AFR), temperature and catalytic formulation used. While all of the formulations produced significant amounts of NH3 when operated under sufficiently rich conditions, in general the steady-state NH3 yield decreased in the following order: HPGM≥Dual-Zone>>LPGM≈LNT. Under lean-rich cycling conditions that would be required for this mode of operation, lower air-to-fuel ratios were required to generate the same amount of NH3 as under steady-state conditions. Results obtained with the LNT catalyst demonstrated that at moderate temperatures (i.e., 275–500°C) NOX storage capacity significantly increased the amount of NH3 produced in relation to the amount of NOX slipped. Consequently, the addition of an “optimum” amount of NOX storage capacity in addition to well-controlled lean-rich timing, could significantly improve the performance of the three-way catalyst used as the upstream component in a passive-NH3 SCR configuration. When the CO, C3H6 and N2O concentrations in the effluent were considered in addition to the NH3 formation, an optimum temperature of 400–450°C was determined for the operation of these catalysts.

Isocyanate formation and reactivity on a Ba-based LNT catalyst studied by DRIFTS

Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and mass spectrometry (MS), coupled with the use of isotopically-labeled reactants (15N18O and 13CO), were employed to study the formation of isocyanate species during NOx reduction with CO, as well as isocyanate reactivity toward typical exhaust gas components. DRIFTS demonstrated that both Ba–NCO and Al–NCO were simultaneously formed during NOx reduction by CO under dry lean-rich cycling conditions. The Ba–NCO band was more intense than that of Al–NCO, and became comparatively stronger at high temperatures. During rich purging at 300 and 400°C, a near linear relationship was found between the increase in Ba–NCO band intensity and the decrease in Ba–NO3 band intensity, suggesting that Ba–NCO is directly derived from the reaction of Ba nitrate with CO. Both temperature-programmed surface reaction (TPSR) and isothermal reaction modes (ISR) were utilized to study the reactivity of isocyanate species under lean conditions. Simultaneous DRIFTS and mass spectrometric measurements during TPSR indicated that isocyanate reaction with H2O, O2, NO and NO/O2 took place almost immediately the temperature was raised above 100°C, and that all NCO species were removed below 300°C. The evolution of the NCO IR bands during ISR at 350°C demonstrated that the kinetics of NCO hydrolysis are fast, although a delay in N2 formation indicated that N2 is not the initial product of the reaction. In contrast, immediate N2 evolution was observed during NCO reaction with O2 and with NO+O2. Overall, it can be inferred that under dry cycling conditions with CO as the sole reductant, N2 is mainly generated via NCO reaction with NO/O2 after the switch to lean conditions, rather than being evolved during the rich phase. However, in the presence of water, isocyanate undergoes rapid hydrolysis in the rich phase, N2 generation proceeding via NH3.

N2O Mitigation in a Coupled LNT–SCR System

N2O formation and consumption were investigated over a coupled LNT–SCR system consisting of a low-precious metal loaded Pt/Rh LNT catalyst and a commercial Cu–zeolite SCR catalyst. Under lean–rich cycling conditions, N2O emissions from the LNT were found to be partially mitigated by the downstream SCR catalyst. N2O decomposition over the SCR catalyst was observed in the absence of reductant immediately after the switch to rich conditions, while N2O reduction occurred after subsequent breakthrough of the reductant from the LNT. Steady-state data indicate that the former process is weakly promoted by NO which breaks through the LNT at the same time as the N2O. Steady-state experiments revealed the order H2 > NH3 > CO > C3H6 for the efficacy of N2O reduction with different reductants. These findings suggest that coupled LNT–SCR systems can not only improve overall NO x conversion levels but can also mitigate N2O emissions from the LNT catalyst under actual driving conditions.

NOx storage and reduction properties of model ceria-based lean NOx trap catalysts

Three kinds of model ceria-containing LNT catalysts, corresponding to Pt/Ba/CeO2, Pt/CeO2/Al2O3 and Pt/BaO/CeO2/Al2O3, were prepared for comparison with a standard LNT catalyst of the Pt/BaO/Al2O3 type. In these catalysts ceria functioned as a NOx storage component and/or a support material. The influence of ceria on the microstructure of the catalysts was investigated, in addition to the effect on NOx storage capacity, regeneration behavior and catalyst performance during lean/rich cycling. The Pt/Ba/CeO2 and Pt/BaO/CeO2/Al2O3 catalysts exhibited higher NOx storage capacity at 200 and 300°C relative to the Pt/BaO/Al2O3 catalyst, although the latter displayed better storage capacity at 400°C. Catalyst regeneration behavior at low temperature was also improved by the presence of ceria, as reflected by TPR measurements. These factors contributed to the superior NOx storage-reduction performance exhibited by the Pt/Ba/CeO2 and Pt/BaO/CeO2/Al2O3 catalysts under cycling conditions in the temperature range 200–300°C. Overall, Pt/BaO/CeO2/Al2O3 (which displayed well balanced NOx storage and regeneration behavior), showed the best performance, affording consistently high NOx conversion levels in the temperature range 200–400°C under lean-rich cycling conditions.

A non-NH3 pathway for NOx conversion in coupled LNT-SCR systems

NOx storage-reduction experiments were performed using a coupled LNT-SCR system consisting of a low-precious metal loaded Pt/Rh LNT catalyst and a commercial Cu–zeolite SCR catalyst. Cycling experiments revealed that when a CO+H2+C3H6 mixture or C3H6 by itself was used as the reductant, the NOx conversion over the SCR catalyst exceeded the conversion of NH3 over the same catalyst. This is explained by the presence of propene, which slipped through the LNT catalyst and reacted with the LNT NOx slip. Separate experiments, conducted under continuous flow and lean-rich cycling conditions, confirmed the ability of propene, as well as ethene, to function as a NOx reductant over the SCR catalyst. Cycling experiments also revealed that the SCR catalyst was able to store propene, such that NOx reduction by stored propene continued into the lean phase (after the switch from rich conditions). According to adsorption experiments, significant co-adsorption of NH3 and propene occured in the SCR catalyst, while under lean-rich cycling conditions the contributions of NH3 and C3H6 to NOx conversion were found to be essentially additive. These findings suggest that under actual driving conditions, NOx reduction by non-NH3 reductants (olefins and possibly other hydrocarbons) in the SCR catalyst can contribute to the mitigation of lean and rich phase NOx.

NOx storage and reduction properties of Pt/CexZr1−xO2 mixed oxides: Sulfur resistance and regeneration, and ammonia formation

The influence of the ceria-zirconia mixed oxide composition in Pt/CexZr1−xO2 catalysts was studied toward NOx storage capacity (NSC), including sulfur poisoning and sulfur regeneration, and NOx reduction efficiency in lean–rich cycling conditions. The results are compared with a Pt/Ba/Al model catalyst. The samples were characterized by N2 adsorption, XRD and H2-TPR. The behaviors of the ceria-zirconia supported catalysts are quite similar whatever their composition. They are sensitive to a reducing pretreatment which leads to an increase of (i) the cerium reducibility/oxygen mobility, (ii) the NO oxidation rate and (iii) the NOx storage capacity at 300 and 400°C. The sulfating treatment leads to a dramatic decrease of the NOx storage capacity for all catalysts, the decrease being more pronounced for the Zr-rich samples. H2-TPR experiments show that the sulfates amount and their stability tend to increase with the Zr content, but these sulfates are significantly less stable compared with Pt/Ba/Al. The sulfur elimination rates in rich mixture at 550°C are higher than 90% with the ceria-zirconia supported catalysts versus 56% with Pt/Ba/Al.The ceria-zirconia supported catalysts are able to convert NOx in lean–rich cycling condition. Compared with Pt/Ba/Al, the NOx conversions are a little lowered but the ammonia selectivity is significantly decreased with the Ce–Zr mixed oxides, with a beneficial influence of the cerium content.

Effect of Ceria on the Storage and Regeneration Behavior of a Model Lean NO x Trap Catalyst

In this study the effect of ceria addition on the performance of a model Ba-based lean NOx trap (LNT) catalyst was examined. The presence of ceria improved NOx storage capacity in the temperature range 200–400 °C under both continuous lean and lean-rich cycling conditions. Temperature-programmed experiments showed that NOx stored in the ceria-containing catalyst was thermally less stable and more reactive to reduction with both H2 and CO as reductants, albeit at the expense of additional reductant consumed by reduction of the ceria. These findings demonstrate that the incorporation of ceria in LNTs not only improves NOx storage efficiency but also positively impacts LNT regeneration behavior.

SOx Storage Materials under Lean−Rich Cycling Conditions. Part I: Identification of Transient Species

The SO(x) uptake of second generation sulfur trapping materials was studied by in situ IR spectroscopy under lean-rich cycling conditions. The combination of advanced chemometric methods including generalized 2D correlation analysis, 2D sample-sample correlation analysis, and multivariate curve resolution with alternating least squares allowed the detection of the species involved in the storage process. The formation of the bulk sulfate species was always accompanied by the consumption of carbonates. The reduction of a transient surface sulfate species was identified as the key parameter in the storage process under dynamic conditions. Three distinct reaction regimes were differentiated on the industrial materials under SO(x) trapping conditions being imperceptible from conventional spectra.