Low Temperature NOx Adsorbers Research Articles

Achieving High Dispersion of Pd in Small-Pore Zeolite SSZ-13: A High-Efficiency Low-Temperature NOx Adsorber

Achieving High Dispersion of Pd in Small-Pore Zeolite SSZ-13: A High-Efficiency Low-Temperature NOx Adsorber

A Study of CeSnOx and Pd/CeSnOx as Low-Temperature NOx Adsorbers with Excellent Hydrothermal Stability.

In the present work, we report on two passive NOx adsorber (PNA) material candidates: the novel support CeSnOx with and without Pd loading. The NOx adsorption and storage capacities of fresh and hydrothermally aged CeSnOx and Pd/CeSnOx were investigated. The results show that CeSnOx exhibits a rather large NOx uptake and storage capacity (28.9 μmol/g), while the loading of Pd on CeSnOx can further increase the storage capacity to 37.6 μmol/g and affect the desorption temperature of NOx. It was found that the NOx desorption temperature of Pd/CeSnOx was compatible with the efficient operating window of selective catalytic reduction (SCR) catalysts. After a hydrothermal aging treatment at 800 °C for 12 h, the NOx adsorption and storage capacities of CeSnOx and Pd/CeSnOx increased, indicating excellent hydrothermal stability. The interaction of Pd with CeSnOx, the state of Pd species, and the structure of CeSnOx and Pd/CeSnOx are studied by combination of the characterization results.

An Assessment of Zeolite Framework Effect for Low-Temperature NOX Adsorbers

Pd-promoted zeolites (Y, ZSM-5, FER, SSZ-13) were prepared and characterized to analyze their properties as low-temperature NOx adsorbers. The samples were investigated by BET and XRD and by in situ FT-IR spectroscopy of CO and NO adsorption to probe the Pd sites and the nature of the adsorbed NOx species. The NOx adsorption/desorption performances at low temperatures were examined by microreactor measurements upon NO/O2 adsorption followed by TPD in the presence of water and carbon dioxide. It was enlightened that: (i) the zeolite framework influences the Pd dispersion: the smaller the zeolite cage, the higher the Pd dispersion, irrespective of the Si/Al ratio. Accordingly, the following Pd dispersion order has been observed, inversely to the zeolite cage size: Pd/SSZ-13 > Pd/ZSM-5 ~ Pd/FER >> Pd/Y; (ii) Pd is present as isolated Pdn+ species and in PdOx particles; (iii) the Pd dispersion governs the NOx storage capacity: the smaller the zeolite cage, the higher the Pd dispersion and the storage capacity; (iv) NO adsorbs mainly in the form of Pd nitrosyls and nitrates; (v) NO desorption occurs both at a temperature below 200 °C and in a high-temperature range (near 350 °C).

NO adsorption/desorption pathways over Pd/SSZ-13 low temperature NOx adsorbers investigated by operando FT-IR spectroscopy and microreactor study

In this study a Pd-doped SSZ-13 zeolite sample is synthesized, characterized and evaluated for potential use in the low-temperature NOx adsorption. TEM/HR-TEM and in-situ CO/NO adsorptions followed by FT-IR spectroscopy are used to gain information on the nature and accessibility of the Pd sites. It is found that over the calcined sample Pd is present as isolated Pd+ and Pd2+ cations formed by ion exchange with the Brønsted acid sites of the zeolite, and as Pd+ and Pd2+ species belonging to PdOx particles on the external surface of the zeolite. Isolated Pd2+ ions prevail on the calcined fresh sample. The NOx adsorption/desorption performances, investigated by microreactor studies and operando FT-IR under relevant operating conditions, indicate that significant amounts of NO are stored on the catalyst (i.e. ca. 80–95 μmol/gcat of NOx in the temperature range 50–150 °C) which suggests the participation in the NOx uptake of both isolated Pdn+ sites and PdOx particles. NO is adsorbed as nitrosyls (anhydrous and hydrated) over both Pd2+ and Pd+; at low temperatures the formation of nitrates is also observed. The NO uptake as nitrosyls occurs with evolution of NO2 according with a NO storage mechanism which implies (i) the initial adsorption of NO on Pd2+, followed by (ii) the reduction of Pd2+ to Pd+ by NO accompanied by NO2 release and eventually (iii) the adsorption of NO on Pd+. The suggested interconversion of Pd2+ into Pd+ during NO adsorption is herein discussed based on FT-IR evidences. The stored NOx species are decomposed upon heating; nitrates are responsible for low temperature NOx desorption (below 200 °C), whereas nitrosyls show much higher stability and lead to a high temperature NO desorption (above 200 °C). The presence of NO2 in the feed stream increases the NOx adsorption capacity and results in the formation of mainly surface/bulk nitrates and nitro-compounds at the expense of nitrosyls.

Assessment of zeolite-based Low temperature NOx adsorbers: Effect of reductants during multiple sequential cold starts

Low temperature NOx adsorbers (LTNA) are designed to adsorb NOx from a diesel engine during a cold start and then release the stored NOx when the downstream SCR catalyst is above 200 °C and able to reduce the NOx with NH3. Monolithic catalysts with washcoats containing beta (BEA), chabazite (CHA), or ZSM5 zeolites with Pt and Pd (4.2 g/L at 1:4) were prepared, degreened, and evaluated for NOx storage performance with different reductants (C2H4, a CO/H2 mixture, both, or none). The reductants improved the NOx storage efficiency of all three zeolites. CHA provided the best NOx storage and release performance, as BEA released all of the stored NOx below 200 °C with or without reductants, and ZSM5 released most of the stored NOx below 200 °C with most feedgas compositions. After aging 80 h at high temperatures (650–800 °C) under lean conditions, BEA was the most thermally durable of the three zeolites, while ZSM5 was the least durable. The NOx storage performances of the three degreened zeolites were stable during multiple NOx storage tests without reductant, with C2H4 alone, and with H2 alone, but the performance degraded from test to test when the feedgas contained CO. This performance degradation was attributed to reduction of the Pd by the CO. Lean CH4 lightoff tests performed after the NOx storage tests confirmed that the Pd in all three zeolites was not reduced after multiple tests with C2H4, H2, or no reductant, but the Pd was significantly reduced after multiple tests with CO. The decrease in NOx storage capacity was less for ZSM5 than for BEA and CHA during multiple tests with 900 ppm CO/300 ppm H2, and the Pd in ZSM5 was also less reduced. However, tests with 3600 ppm CO/1200 ppm H2 reduced the Pd in ZSM5 much more rapidly and consequently caused a more rapid deterioration of the NOx storage performance.

Effect of Reductants on the NOx Storage Performance of a Pd/CZO Low Temperature NOx Adsorber

Low temperature NOx adsorbers (LTNA) adsorb the NOx emissions from diesel engines during the cold start period and then thermally release the NOx when the downstream urea SCR system is operational. A degreened monolithic core sample of a LTNA with palladium on a ceria-zirconia washcoat (Pd/CZO) was evaluated for NOx storage performance under lean conditions using a reactor-based transient temperature test with and without different reductants [i.e., ethylene (C2H4), carbon monoxide (CO), hydrogen (H2), or a 3:1 CO:H2 mix]. Relative to tests without a reductant, all four reductants increased the amount of NOx stored on the LTNA. The NOx storage performance decreased gradually during consecutive tests. Lean methane (CH4) oxidation was used to probe the average oxidation state of the PdOx in the LTNA after the transient tests. The PdOx was reduced similarly on tests with NO alone and with NO and each reductant but significantly less with the reductant alone. NO alone reduced the PdOx by reacting with the PdOx to form NO2 between 200 and 210 °C. CH4 oxidation tests were also performed after the bed temperature exceeded 185 °C on transient tests with and without the reductants (i.e., after the NOx storage period). There was little PdOx reduction with NO alone or reductant alone, but there was significant and similar PdOx reduction with NO and each reductant. This supported a mechanism where NO and the reductant interacted to form a chemical intermediate on a PdOx site, reducing the PdOx in the process. The oxidation state of the PdOx was also determined as a function of time during transient tests with and without the reductants. CO and CO/H2 reduced the PdOx more rapidly than C2H4 or H2, which correlated with the superior NOx storage performance with CO and CO/H2. It is proposed that the PdOx reduction was responsible for the gradual drop in NOx storage efficiency on consecutive tests. Consistent with that, the maximum performance could be recovered by oxidizing the catalyst between 700 and 750 °C. Future work includes a similar assessment of zeolite-based LTNAs which are more sulfur tolerant than ceria-based LTNAs.

Mn-based mixed oxides for low temperature NOx adsorber applications

Pd-promoted ternary oxides of the type Mn-Ce-Zr and Mn-Pr-Zr were characterized and evaluated for low temperature NOx storage applications such as diesel vehicle cold starts. While X-ray diffraction data were in all cases consistent with the formation of solid solutions, Mn/Zr and Mn/Ce(Pr) ratios found by XPS were consistently higher than the bulk values, indicative of an enrichment of Mn at the surface of the solids. Both sets of Pd-promoted mixed oxides showed remarkably high NOx storage efficiency in the range 80–160 °C, while a 1.8% Pd/Mn(27)-Ce(7)-Zr catalyst showed excellent NOx storage in simulated cold start experiments. Moreover, ramping the temperature to 370 °C in these experiments, simulating higher speed operation, resulted in near complete purging of stored NOx from the catalyst. NOx storage efficiency in isothermal storage experiments was found to improve with increasing Mn content for the 1%Pd/Mn(x)-Ce(7)-Zr series (x = 9, 18, 27 wt.%), DRIFTS measurements showing that relative to Ce-Zr mixed oxides, Mn incorporation favored NOx storage as nitrate. During temperature programmed desorption (TPD) two main desorption events were observed, corresponding to decomposition of nitrites (up to 200 °C), followed by loss of nitrates (⁓200–400 °C). Nitrates stored on Pd/Mn-Ce(Pr)-Zr mixed oxides desorbed during TPD at lower temperatures than for CeO2-ZrO2 mixed oxides, a finding attributed to the lower basicity of Mn compared to Ce. Hydrothermal aging of 1.8%Pd/Mn(27)-Ce(7)-Zr at 700 °C reduced NOx storage efficiency, although the catalyst was still able to store significant amounts of NOx. However, catalyst sulfation led to a large decrease in NOx storage efficiency and the efficiency could not be completely recovered with lean or rich desulfations at high temperatures.

Mechanistic assessment of low temperature NOx adsorbers for cold start NOx control on diesel engines

Low temperature NOx adsorbers (LTNA) adsorb NOx from diesel engines during cold starts and release the NOx when the SCR system is operational. H2O inhibited NOx storage on Pd/CZO LTNAs below 100 °C, but above 100 °C C2H4 and H2 increased the NOx storage. CO improved the NOx storage below 100 °C even with H2O. Two potential mechanisms were identified for these improvements: (1) the reaction of NO and reductant to form intermediates on the LTNA; (2) partial reduction of some of the PdOx. The different effects of C2H4, CO, and H2 on the NOx storage supported the first mechanism, as did the very rapid increase in NOx slip when the reductant was turned off during a test. DRIFTS analysis confirmed NCO formation when NO and CO were fed to the sample at 100 °C. Based on CH4 oxidation, NO alone or reductant alone did not reduce the PdOx significantly during the first 5 min of the test, but significant reduction occurred with both NO and a reductant. This suggests that the NO and reductant interacted on the PdOx, reducing the PdOx in the process. Thus, the primary benefit of the reductants is to react with NO to form intermediates on the Pd/CZO LTNA.

The Effects of CO, C2H4, and H2O on the NOx Storage Performance of Low Temperature NOx Adsorbers for Diesel Applications

The Effects of CO, C₂H₄, and H₂O on the NO_x Storage Performance of Low Temperature NO_x Adsorbers for Diesel Applications

An assessment of Pt and Pd model catalysts for low temperature NOx adsorption

Low temperature NOx adsorbers (LTNA) are being developed to adsorb the cold start NOx emissions from diesel engines and release the NOx when the urea/SCR system is operational. To assess the contributions of the precious metal and washcoat, model catalysts with 0.43g/L (26 gpcf) of platinum (Pt) or palladium (Pd) on washcoats of alumina (Al2O3) or ceria/zirconia (CZO) were evaluated for lean NO storage and release with and without C2H4 and H2O. The degreened samples were evaluated on a reactor-based transient test that partially simulated the exhaust temperatures during the first two phases of the light-duty Federal Test Procedure (FTP) followed by a Supplemental FTP (US06) test. After oxidation, the Pd/CZO catalyst stored NO at the lowest temperatures of the test (∼90°C) without C2H4 and H2O, and the NO storage was enhanced by the presence of C2H4 but inhibited by H2O at bed temperature under 100°C. Most of the NOx emissions during the FTP simulation were in the form of NO, and the catalyst produced low N2O levels on tests with C2H4. The oxidized Pd/Al2O3 sample exhibited poor NO storage with or without C2H4 and H2O, suggesting that cerium is important for NOx storage when the catalyst is oxidized. Both Pd technologies stored more NO when the samples were reduced, indicating that metallic Pd is effective for storing NO. The Pt/Al2O3 sample exhibited high NO storage efficiency between 90 & 120°C without H2O & C2H4 but lower efficiency with these species. Above 120°C, the catalyst produced high NO2 levels and high N2O levels on the tests with C2H4. The Pt/CZO sample performed similarly to the Pd/CZO sample except for higher NO2 and N2O levels. The Pd/CZO catalyst provided the best combination of NO storage with C2H4 and H2O above 100°C, essentially complete NOx release at the maximum bed temperature of the US06 simulation (400°C), low NO2 and N2O formation, and cost effectiveness.

An assessment of low temperature NOx adsorbers for cold-start NOx control on diesel engines

Fresh and thermally aged laboratory samples of several low temperature NOx adsorber (LTNA) formulations were evaluated for low temperature NOx storage and release under lean conditions to assess their potential for treating the cold-start NOx emissions on diesel engines before the SCR system becomes operational. Steady-state tests were used to determine the NOx storage capacity as a function of temperature and to determine the minimum temperature for purging the LTNA of stored NOx. The samples were also evaluated on a transient test performed on the reactor that simulated the thermal environment during the first two phases of the light-duty Federal Test Procedure (FTP-75) followed by a Supplemental FTP (US06) test. The Pt-rich LTNAs exhibited low NOx storage performance during the initial 200s of this transient test due to low storage efficiency of NO and weak NO oxidation capability. A Pd-rich formulation provided efficient storage of NO and C2H4 on the transient test and released most of the stored NO by 350°C. The presence of C2H4 improved its NOx storage performance after thermal aging. SO2 poisoned the storage of NO2 as nitrates at 170°C, but the storage of NO as nitrites at lower temperatures (e.g., 90–140°C) was more robust to the SO2. A lean desulfation at 750°C recovered some of the storage performance. The Pd-rich LTNA performed best when highly oxidized. This catalyst demonstrated a gradual loss of NOx storage performance when evaluated on consecutive lean transient tests. This was attributed to the gradual reduction of the palladium oxide from reacting with the feedgas NO and C2H4 to form NO2 and CO at temperatures near 90°C. Methane oxidation was used to probe the oxidation state of the palladium oxide and confirmed that the palladium oxide was partially reduced after only two lean transient tests.

Investigation of Ag-based low temperature NOx adsorbers

Future emission standards are becoming increasingly stringent as California will phase in LEV III regulations starting in 2015 and EPA will implement Tier 3 regulations starting in 2017. Current vehicles emit greater than 50% of targeted tailpipe emissions during the first minute or two following “cold-start” mainly due to the ineffectiveness of existing catalytic converters at low temperatures (<180°C). Thus, developing low temperature emission control technologies has become a top priority of automobile manufacturers and their suppliers. In this article, Ag-based catalysts (Ag/Al2O3) were evaluated as a low temperature NOx adsorber using a laboratory fixed-bed flow reactor system. For the first time superior low temperature NO to NO2 conversion efficiencies and NOx storage capacity in the presence of hydrogen were demonstrated. Sulfation and desulfation experiments were conducted to examine the impact of sulfur on the low temperature NOx storage. Several supplier catalysts, including two diesel oxidation catalysts (DOC) and a passive NOx adsorber (PNA), were also tested to provide comparisons to the Ag-based catalysts.