Kinetic modeling of NOx storage and reduction using spatially resolved MS measurements
Graphical abstract
Introduction
The products formed during the burning process of internal combustion engines are major contributors to global air pollution, and CO, HC and NOX are the major components regulated in many countries. The three-way catalyst (TWC) can efficiently convert these compounds to benign species for gasoline stoichiometric engines. However, the emissions from diesel engines and lean burning gasoline engines have a large excess of oxygen. Due to the oxygen competing for a limited amount of reductants, TWCs cannot achieve reasonable NOX reduction in lean emissions. One solution to this problem is the use of NOX Storage and Reduction (NSR) catalysts, which was developed by Toyota in the 1990s [1]. In this concept, the catalyst is exposed to mixed lean-rich operation composed of relatively long lean (oxygen excess) pulses, followed by short rich (oxygen deficient) pulses. NOX is stored by the catalyst during the lean period and is released and reduced by short rich pulses, thereby regenerating the storage component. During the rich period, NOX is reduced with hydrocarbons (HC), H2 and CO, to produce CO2, H2O and N2. Furthermore, due to H2 and HC in the rich period [2], [3], [4], NH3 and N2O may also be produced. Ammonia slip from NSR catalysts has been observed [5], [6], [7], [8], [9], [10], [11], [12] during operation, and is a major problem for urban air quality [13]. NH3 forms secondary aerosols which are important contributors to urban fine particulate (PM2.5) pollution [14]. Although as yet an unregulated emission, N2O has 300-times greater greenhouse potential than CO2 on an equivalent-mass basis, and 5% of all U.S. greenhouse gas emissions from human activities [15]. Due to NH3 and N2O slip, strategies to minimize nitrogen-containing emissions other than N2 are desirable.
The NSR catalyst usually consists of a high surface area support, a storage component and a precious metal. The Pt/Ba/Al catalyst has been a well-studied system. Szailer et al. [16] showed that during the rich period, the main role of H2 is to keep the precious metal clean for dissociation of NOX by reacting with adsorbed oxygen to form H2O. Furthermore, it was suggested that the formation of NH3 occurs due to adsorbed nitrogen atoms reacting with H2. The selectivity of the formed NSR products was studied by Pihl et al. [17], who found that the formation of NH3 was favored when the ratio of reductant to NOX was high and the temperature low. Furthermore, knowing that NH3 is an effective reductant for both NO and NO2 [7], Pihl et al. observed that NH3 formed in the front parts of the catalyst at low temperatures was partially consumed further into the catalyst via regeneration reactions or the reduction of stored oxygen [17].
Studies to better understand ammonia formation from NSR catalysts have been driven by the need to avoid NH3 slip. Clayton et al. [12] showed that by progressively shortening the catalyst sample, H2 was reacting with stored NOX to form NH3 and then reduced downstream of the catalyst with stored NOX. Partridge et al. [18] further studied the intra-ammonia formation using Spatially Resolved Capillary Inlet Mass Spectrometry (SpaciMS). Transient NH3, NOX, N2 and H2 distributions within an operating NSR catalyst at different operating temperatures were studied. The results showed three distinct NH3 zones, a build-up zone in the front section of the catalyst, a balance zone, in addition to a zone at the back-end of the catalyst. More ammonia was generated than consumed in the front zone of the catalyst where most of the NOX was stored. However, downstream of the build-up zone, where remaining NOX was stored, there was a balance between NH3 production and reduction. Furthermore, the data showed that at the back-end of the catalyst, which did not show any significant NOX storage, a depletion zone existed where ammonia was consumed by surface oxygen. According to Partridge et al. [18], the SpaciMS results showed that there is a complex reaction network in the catalyst, mainly in the front, including NH3 generation, H2 and NH3 reduction, as well as parallel direct-H2 and intermediate-NH3 regeneration pathways.
There are multiple kinetic models describing the NSR process in the literature [12], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29]. Olsson et al. [19], [20], Tuttlies et al. [22] and Hepburn et al. [29] used global kinetic models to describe the NOX storage and reduction, where a shrinking-core model was used to describe the mass transport. Lindholm et al. [21] developed a detailed NSR kinetic model to describe the storage and regeneration using hydrogen, which also included ammonia formation over a Pt/Ba/Al catalyst. In this study, the reaction between stored NOX and NH3, according to SCR chemistry, was added.
However, to our knowledge, there are no kinetic models for NOX storage that have presented intra-catalyst concentrations and been verified by experiments using intra-catalyst measurements in the open literature. The objective of this study was to use both flow reactor experiments and SpaciMS to further investigate the mechanism of NOx storage and reduction over barium containing NSR catalysts using a global kinetic model focusing on NH3 and N2O breakthroughs. A Global Kinetic NSR Model was developed based on long experimental NSR cycles and validated against short experimental NSR conducted by Pihl et al. [17]. The simulated intra-catalyst concentration was thereafter verified with the help of SpaciMS measurements by Partridge et al. [18] to investigate the intra-catalyst concentrations during storage and reduction of NOX, as well the formation and reduction of ammonia.
Section snippets
Experimental
Two different catalysts were used in the present study. Catalyst 1 is a Commercial Gasoline Direct Injection (GDI) LNT Catalyst manufactured by Umicore (Auburn Hills, MI). The major washcoat components include alumina, ceria, zirconia, and barium. The washcoat also contains platinum, palladium, and rhodium at a ratio of 82:26:6 and a total loading of 3990 g/m3. The surface area of the material (including substrate) was determined by N2 adsorption resulting in 27.1 m2/g. More details about the
Reactor model
A commercial program, AVL BOOST [30], was used in combination with user defined files in FORTRAN to conduct kinetic modeling, as used previously in our group [31], [32]. The ideal gas law was applied and all gas properties were evaluated depending on the temperature, pressure and gas composition. The channel was discretized longitudinally into 15 grid-points, where all the equations including reactions were solved.
The assumptions made for the reactor model were:
- i.
Uniform radial flow distribution
Long cycles
Fig. 2, Fig. 3, Fig. 4, Fig. 5 show the experimental and simulated results from the long NSR cycles with a 900-s long lean period and a 600-s long rich period over the 200 to 500 °C temperature range. The catalyst (Cat. 1) was exposed to 300 ppm of NO, 10% O2, 5% H2O and 5% CO2 during the lean period and to 375 ppm H2, 625 ppm CO, 5% H2O and 5% CO2 during the rich period. Fig. 2 shows the experimental and simulated results conducted at 500 °C, where the upper panel shows the experimental and
Sensitivity analysis
The results from the sensitivity analysis are shown in Table 5, Table 6, Table 7. These show that the parameters for the diffusion rate are very important compared to other parameters during the lean period for the storage site S2 (r5–r6 in Table 5), especially during the low temperature NSR cycles where the storage site S2 is most active; the activation energy for the S2 sites is significant as well. This is also observed when the model is validated with SpaciMS measurements where the
Concluding remarks
The objective of the present study was to investigate the mechanisms of NH3 and N2O formation for NOX storage and reduction catalysts. The model was developed and fitted using long NSR cycles and was later validated against shorter and more realistic NSR cycles while keeping the parameters unchanged. To verify the intra-catalyst storage and reduction, the model was further validated against intra-catalyst measurements of a different catalyst focusing on the formation and consumption of ammonia.
Acknowledgments
This study was carried out at the Competence Centre for Catalysis, Chalmers University of Technology, Sweden and at the Oak Ridge National Laboratory (ORNL), USA. The funding from the Swedish Foundation for Strategic Research (F06-006) and Swedish Research Council (621-2011-4860) are gratefully acknowledged. The efforts at ORNL were sponsored by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Office of Vehicle Technologies, with Gurpreet Singh and Ken Howden as
References (52)
- et al.
Catal. Today
(1996) - et al.
Appl. Catal. B: Environ.
(2006) - et al.
J. Catal.
(2006) - et al.
Catal. Today
(2004) - et al.
J. Catal.
(2007) - et al.
Catal. Today
(2008) - et al.
Catal. Today
(2008) - et al.
Catal. Today
(2008) - et al.
J. Catal.
(2008) - et al.
Appl. Catal. B: Environ.
(2008)