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Ammonium Nitrate Formation and Decomposition on an Extruded Vanadium-Based SCR Catalyst

Cummins Emission Solutions-Nathan Ottinger, Yuanzhou Xi, Christopher Keturakis, Z. Gerald Liu
  • Technical Paper
  • 2020-01-1320
To be published on 2020-04-14 by SAE International in United States
In this study, the formation and decomposition of ammonium nitrate (AN) on a state-of-the-art extruded vanadium-based SCR catalyst (V-SCR) under simulated exhaust conditions has been evaluated. Results show that AN readily forms and accumulates at temperatures below 200°C when exposed to NH3 and NO2. The rate of AN accumulation increases with decreasing temperature. A new low temperature NH3 release peak (not present following NH3 storage conditions with NH3 only) becomes apparent after AN accumulation at 100 and 125°C. This new NH3 release, with a peak release temperature of approximately 180°C, is evaluated in detail to better determine its origin. BET surface area, and thermal gravimetric analysis/differential scanning calorimetry (TGA/DSC), and reactor-based experiments are all used to characterize AN formed on the V-SCR catalyst in comparison to pure AN. Experiments on pure AN crystals reveal that the new low temperature NH3 peak is attributable to the release of NH3 from AN that is not related to AN volatilization, since this does not occur until above T > 200°C.This study expands the present understanding of AN formation…
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A Case Study of a Cu-SSZ-13 SCR Catalyst Poisoned by Real-World High Sulfur Diesel Fuel

Cummins Emission Solutions-Yuanzhou Xi, Nathan Ottinger, Christopher Keturakis, Z. Gerald Liu
  • Technical Paper
  • 2020-01-1319
To be published on 2020-04-14 by SAE International in United States
To meet increasingly stringent diesel engine emission regulations, diesel engines are required to use ultra-low sulfur diesel (ULSD) and are equipped with advanced aftertreatment systems. Cu-SSZ-13 zeolite catalysts are widely used as selective catalytic reduction (SCR) catalysts due to their high NOx reduction and excellent hydrothermal stability. However, active Cu sites of Cu-SSZ-13 catalysts can be poisoned by exposure to engine exhaust sulfur species. This poison effect can be mitigated with the use of ULSD and high temperature exposure from engine operation. On the other hand, ULSD is still not universally available where regulations require it, and vehicles may inadvertently operate with high sulfur diesel fuel (HSD) in some locations. The high concentration of exhaust sulfur species resulting from HSD combustion may rapidly poison the Cu-SSZ-13 SCR catalyst. In this study, the catalytic performance of a sulfur poisoned Cu-SSZ-13 SCR catalyst is analyzed. Results show that the as received SCR catalyst displays substantially low NOx conversion below 350 °C. A thermal treatment at 550 °C can recover most of its lost performance. Temperature programmed desorption…
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Low-Temperature NH3 Storage, Isothermal Desorption, Reactive Consumption, and Thermal Release from Cu-SSZ-13 and V2O5-WO3/TiO2 Selective Catalytic Reduction Catalysts

Cummins Emission Solutions-Nathan Ottinger, Yuanzhou Xi, Christopher Keturakis, Z. Gerald Liu
Published 2019-04-02 by SAE International in United States
Worldwide, regulations continue to drive reductions in brake-specific emissions of nitric oxide (NO) and nitrogen dioxide (NO2) from on-highway and nonroad diesel engines. NOx, formed as a byproduct of the combustion of fossil fuels (e.g., natural gas, gasoline, diesel, etc.), can be converted to dinitrogen (N2) through ammonia (NH3) selective catalytic reduction (SCR). In this study, we closely examine the low-temperature storage, isothermal desorption, reactive consumption, and thermal release of NH3 on commercial Cu-SSZ-13 and V2O5-WO3/TiO2 SCR catalysts. Catalyst core-reactor, N2 adsorption (BET) surface area, and in-situ diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) experiments are utilized to investigate the fundamental chemical processes relevant to low-temperature (T < 250°C) NH3 SCR. Results show that NH3 stored at low-temperature continuously, yet slowly releases from the SCR catalysts, and that nearly all of the weakly bound NH3 stored on Cu2+ sites of the Cu-SSZ-13 catalyst will isothermally desorb from the catalyst in the absence of NOx. However, in the presence of NOx, a large fraction of this weakly bound NH3 will react with NOx, contributing to the…
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NO2 Formation and Mitigation in an Advanced Diesel Aftertreatment System

Cummins Emission Solutions-Nathan Ottinger, Yuanzhou Xi, Niklas Schmidt, Z. Gerald Liu
Published 2018-04-03 by SAE International in United States
Nitrogen dioxide (NO2) is known to pose a risk to human health and contributes to the formation of ground level ozone. In recognition of its human health implications, the American Conference of Governmental Industrial Hygienists (ACGIH) set a Threshold Limit Value (TLV) of 0.2 ppmv NO2 in 2012. For mobile sources, NO2 is regulated as a component of NOx (NO + NO2). In addition, the European Commission has indicated it is considering separate Euro 6 light-duty diesel and Euro VI heavy-duty diesel NO2 emissions limits likely to mitigate the formation of ground level ozone in urban areas. In this study, we conduct component-level reactor-based experiments to understand the effects that various aftertreatment catalyst technologies including diesel oxidation catalyst (DOC), diesel particulate filter (DPF), selective catalytic reduction (SCR) catalyst and ammonia oxidation (AMOX) catalyst have on the formation and mitigation of NO2 emissions. Finally, emissions from a nonroad Tier 4 Final/Stage IV engine equipped with an advanced aftertreatment system are analyzed to understand the effect of real-world engine operating conditions on NO2 emissions. Experiments were conducted…
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The Dynamics of Methane and NOx Removal by a Three-Way Catalyst: A Transient Response Study

SAE International Journal of Engines

Cummins Emission Solutions-Yuanzhou Xi, Nathan Ottinger, Z. Gerald Liu
  • Journal Article
  • 2018-01-1270
Published 2018-04-03 by SAE International in United States
Natural gas-powered engines are widely used due to their low fuel cost and in general their lower emissions than conventional diesel engines. In order to comply with emissions regulations, an aftertreatment system is utilized to treat exhaust from natural gas engines. Stoichiometric burn natural gas engines use three-way catalyst (TWC) technology to simultaneously remove NOx, CO, and hydrocarbon (HC). Removal of methane, one of the major HC emissions from natural gas engines, is difficult due to its high stability, posing a challenge for existing TWC technologies. In this work, degreened (DG), standard bench cycle (SBC)-aged TWC catalysts and a DG Pd-based oxidation catalyst (OC) were evaluated and compared under a variety of lean/rich gas cycling conditions, simulating stoichiometric natural gas engine emissions. Transient response techniques were applied to reveal the effect of the oxygen storage component on the performance of DG TWC as well as upon SBC aging in comparison to DG OC. It is illustrated that the oxygen storage component of TWC can extend the rich phase’s high conversion efficiency of both CH4 and…
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Formation and Decomposition of Ammonium Nitrate on an Ammonia Oxidation Catalyst

Cummins Emission Solutions-Nathan Ottinger, Yuanzhou Xi, Z. Gerald Liu
Published 2018-04-03 by SAE International in United States
Achieving high NOx conversion at low-temperature (T ≤ 200 °C) is a topic of active research due to potential reductions in regulated NOx emissions from diesel engines. At these temperatures, ammonium nitrate may form as a result of interactions between NH3 and NO2. Ammonium nitrate formation can reduce the availability of NH3 for NOx conversion and block active catalyst sites. The thermal decomposition of ammonium nitrate may result in the formation of N2O, a regulated Greenhouse Gas (GHG). In this study, we investigate the formation and thermal and chemical decomposition of ammonium nitrate on a state-of-the-art dual-layer ammonia oxidation (AMOX) catalyst. Reactor-based constant-temperature ammonium nitrate formation, temperature programmed desorption (TPD), and NO titration experiments are used to characterize formation and decomposition. N2 adsorption and diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) experiments are also conducted to elucidate the physical and chemical impacts of ammonium nitrate formation on the AMOX catalyst. The insights provided herein support the diesel aftertreatment communities’ ongoing efforts to understand low-temperature chemical processes such as ammonium salt formation and their impact on emissions.
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Impact of Hydrothermal Aging on the Formation and Decomposition of Ammonium Nitrate on a Cu/zeolite SCR Catalyst

SAE International Journal of Engines

Cummins Emission Solutions-Nathan Ottinger, Yuanzhou Xi, Christopher Keturakis, Z. Gerald Liu
  • Journal Article
  • 2017-01-0946
Published 2017-03-28 by SAE International in United States
Low-temperature (T ≤ 200°C) NOx conversion is receiving increasing research attention due to continued potential reductions in regulated NOx emissions from diesel engines. At these temperatures, ammonium salts (e.g., ammonium nitrate, ammonium (bi)sulfate, etc.) can form as a result of interactions between NH3 and NOx or SOx, respectively. The formation of these salts can reduce the availability of NH3 for NOx conversion, block active catalyst sites, and result in the formation of N2O, a regulated Greenhouse Gas (GHG). In this study, we investigate the effect of hydrothermal aging on the formation and decomposition of ammonium nitrate on a state-of-the-art Cu/zeolite selective catalytic reduction (SCR) catalyst. Reactor-based constant-temperature ammonium nitrate formation, temperature programmed oxidation (TPO), and NO titration experiments are used to characterize the effect of hydrothermal aging from 600 to 950°C. N2 adsorption (BET) surface area and diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) experiments are also conducted in order to correlate the morphological effects of hydrothermal aging with concomitant changes in ammonium nitrate chemistry. The insights provided herein support the diesel aftertreatment communities’ ongoing…
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Lab Study of Urea Deposit Formation and Chemical Transformation Process of Diesel Aftertreatment System

Cummins Emission Solutions-Yuanzhou Xi, Changsheng Su, Z. Gerald Liu
Massachusetts Institute of Technology-Haomiao Zhang
Published 2017-03-28 by SAE International in United States
Diesel exhaust fluid, DEF, (32.5 wt.% urea aqueous solution) is widely used as the NH3 source for selective catalytic reduction (SCR) of NOx in diesel aftertreatment systems. The transformation of sprayed liquid phase DEF droplets to gas phase NH3 is a complex physical and chemical process. Briefly, it experiences water vaporization, urea thermolysis/decomposition and hydrolysis. Depending on the DEF doser, decomposition reaction tube (DRT) design and operating conditions, incomplete decomposition of injected urea could lead to solid urea deposit formation in the diesel aftertreatment system. The formed deposits could lead to engine back pressure increase and DeNOx performance deterioration etc. The formed urea deposits could be further transformed to chemically more stable substances upon exposure to hot exhaust gas, therefore it is critical to understand this transformation process. In this work, lab experiments were designed to simulate urea deposit formation process by treatment of pure urea at 100-250 °C for various durations up to 100 h. The effect of water vapor was also investigated. The lab formed urea deposits were subsequently characterized by thermogravimetric analysis…
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Development of a Lab Reactor System for the Evaluation of Aftertreatment Catalysts for Stoichiometric Natural Gas Engines

Cummins Emission Solutions-Yuanzhou Xi, Nathan Ottinger, Z. Gerald Liu
Published 2017-03-28 by SAE International in United States
Natural gas powered vehicles are attractive in certain applications due to their lower emissions in general than conventional diesel engines and the low cost of natural gas. For stoichiometric natural gas engines, the aftertreatment system typically consists only of a three-way catalyst (TWC). However, increasingly stringent NOx and methane regulations challenge current TWC technologies. In this work, a catalyst reactor system with variable lean/rich switching capability was developed for evaluating TWCs for stoichiometric natural gas engines. The effect of varying frequency and duty-cycle during lean/rich gas switching experiments was measured with a hot-wire anemometer (HWA) due to its high sensitivity to gas thermal properties. A theoretical reactor gas dispersion model was then developed and validated with the HWA measurements. The model is capable of predicting the actual lean/rich gas exposure to the TWC under different testing conditions. Finally, three platinum group metal (PGM) based aftertreatment catalysts with significant differences in Pt/Pd ratio, and oxygen storage component (OSC) were evaluated under simulated stoichiometric natural gas engine exhaust conditions. The results for the three catalysts are compared…
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Effect of Reductive Regeneration Conditions on Reactivity and Stability of a Pd-Based Oxidation Catalyst for Lean-Burn Natural Gas Applications

Cummins Emission Solutions-Yuanzhou Xi, Nathan Ottinger, Z. Gerald Liu
Published 2016-04-05 by SAE International in United States
Regulations on methane emissions from lean-burn natural gas (NG) and lean-burn dual fuel (natural gas and diesel) engines are becoming more stringent due to methane’s strong greenhouse effect. Palladium-based oxidation catalysts are typically used for methane reduction due to their relative high reactivity under lean conditions. However, the catalytic activity of these catalysts is inhibited by the water vapor in exhaust and decreases over time from exposure to trace amounts of sulfur. The reduction of deactivated catalysts in a net rich environment is known to be able to regenerate the catalyst. In this work, a multicycle methane light-off & extinction test protocol was first developed to probe the catalyst reactivity and stability under simulated exhaust conditions. Then, the effect of two different regeneration gas compositions, denoted as regen-A and regen-B, was evaluated on a degreened catalyst and a catalyst previously tested on a natural gas engine. The results from light-off & extinction test cycles reveal that the reactivity and stability of the Pd-based catalysts change upon reaction and the change becomes more significant upon regeneration.…
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