Neal W. Currier

Overview of the Fundamental Reactions and Degradation Mechanisms of NOx Storage/Reduction Catalysts

Abstract Over the last several years, nitrogen oxide(s) (NOx) storage/reduction (NSR) catalysts, also referred to as NOx adsorbers or lean NOx traps, have been developed as an aftertreatment technology to reduce NOx emissions from lean‐burn power sources. NSR operation is cyclic: during the lean part of the cycle, NOx are trapped on the catalyst; intermittent rich excursions are used to reduce the NOx to N2 and restore the original catalyst surface; and lean operation then resumes. This review will describe the work carried out in characterizing, developing, and understanding this catalyst technology for application in mobile exhaust‐gas aftertreatment. The discussion will first encompass the reaction process fundamentals, which include five general steps involved in NOx reduction to N2 on NSR catalysts; NO oxidation, NO2 and NO sorption leading to nitrite and nitrate species, reductant evolution, NOx release, and finally NOx reduction to N2. Major unresolved issues and questions are listed at the end of each of the reaction process fundamental sections. Degradation mechanisms and their effects on NSR catalyst performance are also described in relation to these generalized reactions. Since at this stage it does not appear possible to arrive at a complete and consistent mechanistic model describing the broad, existing experimental phenomenology for these processes, this review is primarily focused on summarizing and evaluating literature data and reconciling the many differences presented.

Experimental determination of the kinetics of Diesel soot oxidation by O2: Modeling consequences

Several complementary experimental techniques were applied to investigate kinetics of diesel soot oxidation by O 2 in an attempt to provide accurate data for modeling of the Diesel Particulate Filters regeneration process. For two diesel soot samples with measurably different properties, it was shown that the complexity of their overall kinetic behavior was due to an initial period of rapidly changing reactivity. This initial high reactivity was understood not to be related to the SOF, and was quantitatively correlated to the extent of soot pre-oxidation. This initial reactivity can affect the averaged apparent kinetic parameters, for example resulting in the lower apparent activation energy values. After the initial soot pre-oxidation, which consumed ∼10-25% of carbon, the remaining soot was behaving very uniformly, producing linear Arrhenius plots in a remarkably broad range of temperatures (330-610°C) and integral conversions (up to 90%). The two samples of diesel soot were found to behave differently even after the SOF desorption and pre-oxidation, with the measurable difference in the light-off characteristics (-20-30°C) and activation energies (126′3 and 143′3 kJ/mol). Good mathematical description of the results was obtained using the hypothesis that reaction order with respect to soot mass is close to unity. Implications of the first order are discussed, including its effect on the model structure.

Time-Resolved Measurements of Emission Transients By Mass Spectrometry

Transient emissions occur throughout normal engine operation and can significantly contribute to overall system emissions. Such transient emissions may originate from various sources including cold start, varying load and exhaust-gas recirculation (EGR) rates; all of which are dynamic processes in the majority of engine operation applications (1). Alternatively, there are systems which are inherently dynamic even at steady-state engine-operation conditions. Such systems include catalytic exhaust-emissions treatment devices with self-initiated and sustained oscillations (2) and NOX adsorber systems (3,4,5). High-speed diagnostics, capable of temporally resolving such emissions transients, are required to characterize the process, verify calculated system inputs, and optimize the system.

Deactivation of Cu-SSZ-13 by SO2 exposure under SCR conditions

A deactivation study of Cu-SSZ-13 has been conducted using SO2 exposure under SCR conditions and examining its effect on different reactions involving NH3-SCR. Several reactions, including NH3 storage/TPD, NO/NH3 oxidation, standard SCR, fast SCR and SCR with 75% NO2, as well as NH3–NO2 storage/TPD, were investigated at a temperature range of 100–400 °C after exposing the catalyst to 30 ppm SO2 under SCR conditions at 300 °C for 90 min. The catalyst was characterized using XRD, BET, ICP-SFMS and H2-TPR. The BET surface area and pore volume decreased after the sulfur treatment presumably due to blocking by sulfur and/or ammonium–sulfur species. It was found that sulfur was not uniformly deposited along the monolith channel. The deposition occurred from the inlet towards the outlet, as evident from ICP-SFMS measurements. Part of the sulfur was removed after an SCR experiment up to 400 °C. However, this removal was observed only in the inlet half of the sample and not in the outlet. Ammonia TPD experiments revealed that the sulfur poisoning resulted in additional sites that were capable of adsorbing ammonia, resulting in increased ammonia storage. Moreover, standard SCR was significantly deactivated by SO2 poisoning under SCR conditions. Due to the site-blocking effect of the ammonium–sulfur species, fewer copper sites are likely available for the redox SCR cycle. Furthermore, the effect of sulfur poisoning on NH3 oxidation and NO2-SCR as well as N2O production in various SCR reactions were observed. Finally, it was found that the conditions for the sulfur poisoning were critical in which SO2 deactivation under SCR conditions (NH3 + NO + O2 + H2O) was more severe compared to SO2 poisoning in O2 + H2O alone.

The Effect of NO2/NOx Feed Ratio on the NH3-SCR System Over Cu–Zeolites with Varying Copper Loading

In this study we examine the NO/NO2–NH3-SCR system over Cu–BEA with varying Cu loading. Significantly higher selective catalytic reduction (SCR) activity is observed at low temperature on highly loaded copper samples, whereas the reverse trend is noticed at high temperature. The N2O formation is substantially increased over “over-exchanged” Cu sites, where Cu co-ordinate to one Al and charge-balanced with one OH-group. This is also the case for NO2 reaction with NH3 to produce NO. Using transient experiments the formation/decomposition of ammonium nitrate species are examined. The decomposition depends on the temperature, the sequence of the feed as well as the type of copper species present.Graphical Abstract

Final Report of a CRADA Between Pacific Northwest National Laboratory and Cummins, Incorporated (CRADA No.PNNL/283): “Enhanced High and Low Temperature Performance of NOx Reduction Catalyst Materials”

In this annual CRADA program report, we will briefly highlight results from our recent studies of the stability of candidate K-based high temperature NSR materials, and comparative studies of low temperature performance of SSZ-13 and SAPO-34 CHA catalysts; in particular, recent results comparing Fe- and Cu-based CHA materials.

CRADA Final Report: Mechanisms of Sulfur Poisoning of NOx Adsorber Materials

This annual report will review progress of the initial 4 months of a three-year effort between Cummins Engine Company and Pacific Northwest National Laboratory to understand and improve the performance and sulfur tolerance of the materials used in the NOx adsorber after-treatment technology in order to meet both performance and reliability standards required for diesel engines. The goal of this project is to enable NOx after-treatment technologies that will meet both EPA 2007 emission standards and customer cost, reliability and durability requirements. The project will consist of three phases. First, the efforts will focus on understanding the current limitation of capture, regeneration and durability of existing NOx adsorber materials, especially with respect to their sulfur tolerance. With this developing understanding, efforts will also be focused on the optimization of the NOx absorber chemical and material properties to increase performance and durability over many regeneration cycles. We anticipate that improved materials will be tested and evaluated, in partnership with Cummins, on diesel vehicle engines over expected operating conditions.