nh3 storage on a zeolite scr catalyst measured using a microwave based method reduced sensitivity for more strongly held nh3

Available online at www.sciencedirect.com ScienceDirect Procedia Engineering 168 (2016) 11 – 14 30th Eurosensors Conference, EUROSENSORS 2016 NH3 storage on a zeolite SCR catalyst measured using a microwavebased method: Reduced sensitivity for more strongly held NH3 D Kubinskia*, A Bognerb a Ford Research and Innovation Center, 2101 Village Rd., Dearborn, Michigan, 48121, USA b Department of Functional Materials, University of Bayrueth, Bayreuth 95440, Germany Abstract One method for reduction of nitrogen oxides from the exhaust of modern diesel vehicles is the selective catalytic reduction (SCR) of NOx by NH3 injected onto a zeolite SCR catalyst Knowledge of the amount of NH3 stored on these catalysts is useful for optimal conversion Here, measurements of the NH3 storage on a Cu-chabazite zeolite SCR catalyst were done using the microwave-frequency resonator cavity method Shown are results for NH loading on a previously empty catalyst and for the subsequent desorption of the more weakly held NH Temperature was held constant at 250 °C We demonstrate that a comparison of the frequency changes of the resonant modes can help determine variations in the spatial profile of the NH3 storage along the SCR axis Analysis of desorption data shows that the sensitivity of the resonant frequency change per mass of NH3 stored on the catalyst is lower for that more strongly held 2016The TheAuthors Authors Published by Elsevier © 2016 Published by Elsevier Ltd Ltd This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference Keywords: selective catalytic reduction (SCR); Cu-chabazite; ammonia storage; resonant cavity; microwave Introduction One of the more common after treatment methods used for NOx reduction in today’s diesel vehicles is the selective catalytic reduction (SCR) of NOx by NH3 In these systems, urea containing solution injected into the exhaust gas stream is converted to NH3 which selectively reacts with the NOx on the SCR catalyst to produce N2 and * Corresponding author Tel.: +1-313-322-3810 E-mail address: dkubinsk@ford.com 1877-7058 © 2016 The Authors Published by Elsevier Ltd This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference doi:10.1016/j.proeng.2016.11.114 12 D Kubinski and A Bogner / Procedia Engineering 168 (2016) 11 – 14 water The SCR catalyst is often a zeolite material and conversion of NOx is optimized when excess NH3 is stored on its acidic sites Information about the amount of NH3 stored on the catalyst is desired, as too little can result in poor NOx conversion and too much in NH3 emissions In a series of papers, it has been demonstrated that it is possible to measure this quantity directly and in-situ using the microwave resonator cavity method [1-3] Resonant frequency changes were demonstrated to correlate with the mass of NH stored on the catalyst A temperature dependency was noted Here we demonstrate an additional factor affecting the sensitivity of this technique We report on the changes in sensitivity measured as the more weakly held NH3 desorbs from the catalyst at constant temperature Experimental In this study the SCR catalyst was a commercial Cu-chabazite zeolite used in serial application Figure shows the configuration of the cylindrical microwave resonant cavity used to measure the NH3 storage on the catalyst The stainless-steel cavity was 17.1 cm long and ~4.4 cm in diameter with metal stainless screens welded on each end The Cu-chabazite SCR catalyst, cored from a larger piece, was 7.6 cm long and ~4.2 mm in diameter and was mounted approximately in the center of the cavity A single antenna positioned between the catalyst and back screen enabled measurement of the S11 reflection parameter, with microwave signals generated and detected by an Agilent E5071C network analyzer A 60 liter/min gas flow was heated via an in-line heater upstream of the cavity, with temperature probes placed near the center of the metal screens at the ends of the cavity The composition of the outlet gas was monitored by a FTIR analyzer (MIDAC) Additionally, automotive NOx sensors, which are also sensitive to NH3, were placed both upstream and downstream of the cavity All measurements were made in 5% accompanying O2 and 4% water The gas temperature measured a few mm upstream of the front screen was kept at 250 °C Temperature just downstream the cavity was ~238 °C The mass of NH3 stored on the catalyst was calculated from differences in the gas concentrations following the procedure described in Ref Fig Schematic of the cylindrical resonant cavity and Cu-chabazite SCR catalyst placed within Overlaid are plots of the square of the electric field variation along the cavity axis for the TE111 and TE112 resonant modes In general, the resonant frequency, fo, of the cavity of volume Vc will alter due to a change in dielectric permittivity, 'H, of the material within For the TE resonant modes this is given by [4]: & ³ 'H (V ) |E | 'f V | C fo & ³ |E | 2 dV (1) dV VC Overlaid in Fig are the squares of the calculated electric field strengths along the SCR axis for the TE 111 and TE112 resonant modes For the TE112, the transverse electric field magnitude is zero midway along the axis of the catalyst, and Eq predicts that this mode will not respond to changes in the dielectric permittivity occurring there Comparatively, the electric field for the TE111 mode is non-zero in that same location and will register a response These differences will be used to determine information about variations in the NH storage along the SCR catalyst axis D Kubinski and A Bogner / Procedia Engineering 168 (2016) 11 – 14 Results Figure shows the measured correlation between the estimated mass of NH3 stored on the SCR and the corresponding resonant frequency changes for the TE 111 and TE112 modes at 250 °C Identified in the figure are the intervals corresponding to the loading of the 400 ppm feed-gas NH3 on the previously empty catalyst (between and 3500 sec) and the desorption of the more weakly held NH3 beginning after the feed-gas was set back to ppm Note that for the loading interval when NH3 was in the feed-gas, both the resonant frequencies and mass of stored NH saturate after ~1000 sec The most notable difference between the responses for the two resonant modes during this period is the reduced sensitivity observed for the TE112 mode near ~500 sec The magnitude of 'f/fo decreased for both modes during the desorption interval Very little NH3 was measured downstream of the SCR catalyst at the end of desorption at 11,000 sec At that time the SCR still contained NH3, about 50% full, which was held on average more strongly [1] This was reacted off with NO added to the gas stream, bringing the frequencies back to their original values This final step is not shown here, but the data are similar to that previously reported [1-3] Fig Top plot shows the feed-gas and downstream NH3 concentrations Middle plot shows the mass of NH3 estimated on the catalyst with the loading and desorption intervals identified Lower plot shows the corresponding change in resonant frequencies for the TE111 and TE112 modes Data at 250 °C with O2 = 5% and water = 4% Space velocity = 30,000 hr-1 Fig A plot of the fractional frequency change of the TE 112 mode versus that for the TE111 during the NH3 loading (blue) and NH3 desorption (red) intervals Point A denotes the empty catalyst, point B the fully loaded by the 400 ppm NH3, and point C the end of the desorption interval where only the more strongly held NH3 remain These same points are noted in Fig To better elucidate differences in the NH3 adsorption and desorption behaviors, the frequency change of the TE112 mode is plotted relative to the corresponding change for the TE111 in Fig Data are shown for both the NH3 adsorption and desorption intervals Differences in the responses for these modes during the loading interval (blue curve from A o B) are a consequence of the NH3 storage moving through the catalyst with time, filling it from front to back, and encountering different electric field profiles as the NH3 storage “front” moves along the SCR axis The region with reduced slope in the middle of the adsorption curve is a consequence of the loss of sensitivity of the TE112 mode at the cavity center where its electric field is zero For the desorption interval we observed a very different behavior, with Fig showing a straight line response (red line from B o C) Since there are obvious differences in the electric field distribution across the SCR axis for the two modes, this indicates little spatial variation along the SCR’s axis in the relative rate of NH3 desorption during this interval Discussion 13 14 D Kubinski and A Bogner / Procedia Engineering 168 (2016) 11 – 14 We define the sensitivity, S, at each moment as the magnitude of the percent change in resonant frequency (relative to the unloaded state) per gram of NH3 stored on the catalyst Careful analysis of the data for both modes in Fig shows a reduced sensitivity at the end desorption interval compared to that at its start when the catalyst was full (points C and B in Fig 2, respectively) Figure shows this in detail, plotting the fractional change in sensitivity for the two modes during the desorption interval The sensitivity decreased for both resonant modes as the NH3 desorbed, dropping at the end to less than 0.6 of its starting value The nearly equal values for the fractional change in S observed for the two modes further demonstrate the relative desorption of NH3 during this interval being independent of axial position along the catalyst Figure 4: Comparison of the relative RF sensitivity (S = 'freq/massNH3) for the TE111 and TE112 modes during the NH3 desorption interval Data are plotted as a function of the mass of NH3 stored on the catalyst The path B o C is as identified in Fig The values have been normalized relative to the sensitivity a point B, SB, the value for the fully loaded catalyst The constant temperature desorption removes the more weakly held NH3, with only the more strongly held remaining on the SCR at point C [1] The observed sensitivity decrease for the more strongly held NH may be related to a corresponding reduction in the proton conductivity on the zeolite surface [5] Following the argument given elsewhere, we expect the more weakly held NH3 on the zeolite surface, possibly a (NH3)nNH4+ complex, to have a lower activation energy for mobility and interact with the microwave electric fields more easily (i.e., higher sensitivity) than is the case for that more strongly held, most likely as NH4+ [5, 6] Conclusions Using the microwave resonant method we found that although loading of NH3 on the SCR Cu-chabazite catalyst proceeded in a front to back direction with time, its subsequent desorption after it was fully loaded occurred uniformly along the catalyst axis We also demonstrated that the sensitivity of the microwave resonant cavity method (defined as frequency change per gram of stored NH3) decreased as the more weakly held NH3 desorbed at constant temperature The more strongly held NH remaining on the catalyst at the end of the desorption interval showed only ~60% the sensitivity value measured when it was fully loaded References [1] D Rauch, D Kubinski, G Cavataio, D Upadhyay, R Moos, Ammonia loading detection of zeolite SCR catalysts using a radio frequency based method, SAE Int J Eng 8(3) (2015) 1126-1135 [2] D Rauch, D Kubinski, U Simon, R Moos, Detection of the ammonia loading of a Cu Chabazite SCR catalyst by a radio frequency-based method, Sens and Act B: Chemical 205 (2014) 88-93 [3] R Moos, D Rauch1, M Votsmeier, D Kubinski, Review on radio frequency based monitoring of SCR and three way catalysts, Top Catal 59 (2016) 961–969 [4] L.F Chen C.K Ong, C.P Neo, V.V Varadan, V.K Varadan, Microwave Electronics, first ed., John Wiley and Sons, West Sussex, 2004 [5] U Simon, U Flesch, W Maunz, R Muller, C Plog, The effect of NH3 on the ionic conductivity of dehydrated zeolites Na beta and H beta, Microporous and Mesoporous Materials 21 (1998) 111–116 [6] L Rodríguez-González, E Rodríguez-Castellón, A Jiménez-López, U Simon, Correlation of TPD and impedance measurements on the desorption of NH3 from zeolite H-ZSM-5, Solid State Ionics 179 (2008) 1968-1973  ... as the more weakly held NH3 desorbs from the catalyst at constant temperature Experimental In this study the SCR catalyst was a commercial Cu-chabazite zeolite used in serial application Figure... sensitivity) than is the case for that more strongly held, most likely as NH4+ [5, 6] Conclusions Using the microwave resonant method we found that although loading of NH3 on the SCR Cu-chabazite... in that same location and will register a response These differences will be used to determine information about variations in the NH storage along the SCR catalyst axis D Kubinski and A Bogner