Estimating the temperatures of the precious metal sites on a lean NOx trap during oxidation reactions

Applied Catalysis B: Environmental 75 (2007) 39–51 www.elsevier.com/locate/apcatb

Estimating the temperatures of the precious metal sites on a lean NOx trap during oxidation reactions Joseph R. Theis a,*, Erdogan Gulari b b

a Chemical Engineering Department, Ford Motor Company, 2101 Village Road, Dearborn, MI 48124, USA Department of Chemical Engineering, 3302 GG Brown, University of Michigan, Ann Arbor, MI 48109, USA

Received 15 November 2006; received in revised form 16 March 2007; accepted 26 March 2007 Available online 30 March 2007

Abstract The temperatures of the platinum sites on a 0.64 cm long monolithic platinum/potassium/alumina lean NOx trap have been estimated during CO oxidation from the conversion of a trace amount of hydrocarbon (HC) and a HC conversion versus temperature calibration curve. The base exhaust contained 5% O2, 10% H2O, 10% CO2, and 200 ppm of the HC in N2. For CO levels of 2 to 5%, CO injection times of 5 to 180 s, and base temperatures of 300 to 500 8C, the inferred platinum site temperatures were in good agreement with tracer gases of propane, butane, hexane, and methane (when used), even though the levels of HC conversion were significantly different. At a base temperature of 300 8C with 2% CO, the steady-state precious metal temperature was approximately 130 8C higher than the temperature of the exhaust gas exiting the catalyst. The difference in PGM temperature and gas temperature decreased with increasing base temperature and increasing CO level, suggesting that radiation is an important mechanism of heat transfer within a catalyst during exothermic reactions. # 2007 Elsevier B.V. All rights reserved. Keywords: Catalyst; Precious metal; Temperature; CO oxidation; Radiation

1. Introduction The three-way catalyst (TWC) used to treat the exhaust emissions on gasoline-powered vehicles typically consists of a cordierite monolith (e.g., 62 cells per square cm, 0.17 mm wall thickness) that supports a washcoat of gamma-alumina (gAl2O3). The thickness of this washcoat depends on the washcoat loading and formulation and is usually thickest in the corners of a cell and considerably thinner along the walls between the corners [1]. The Al2O3 is highly porous and has a surface area of approximately 100 m2/g. Throughout the pores, a small quantity of platinum (Pt), palladium (Pd), and/or rhodium (Rh) (collectively known as platinum group metal or PGM) is dispersed as tiny particles with initial diameters on the ˚ as measured by X-ray diffraction [2]. Other order of 50 A components of the TWC include ceria, zirconia, H2S inhibitors, and thermal stabilizers. During closed-loop operation on a vehicle, the average air/fuel (A/F) ratio of the exhaust is * Corresponding author. Tel.: +1 313 3376941; fax: +1 313 594 2963. E-mail addresses: [email protected] (J.R. Theis), [email protected] (E. Gulari). 0926-3373/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2007.03.007

controlled at the stoichiometric A/F ratio of approximately 14.6 using feedback from a heated exhaust gas oxygen (HEGO) sensor that is placed in the exhaust system. Under these conditions, the TWC is very effective at converting the hydrocarbons (HC), carbon monoxide (CO), oxides of nitrogen (NOx), and hydrogen (H2) in the exhaust into non-toxic water (H2O), carbon dioxide (CO2), and nitrogen (N2). The NOx conversion of three-way catalysts drops to very low levels under lean conditions. To provide effective NOx control at lean A/F ratios, a lean NOx trap (LNT) may be used [3]. A LNT is essentially a TWC that contains high levels of alkalineearth metal (e.g., barium (Ba)) and/or alkali metal (e.g., potassium (K)), which serve as NOx storage materials. Under lean conditions, the NOx storage materials adsorb and store the NOx by forming adsorbed NOx species, such as nitrites (e.g., Ba(NO2)2) or nitrates (e.g., Ba(NO3)2). After 30 to 60 s of lean operation, the NOx storage efficiency will start to drop as the NOx storage sites begin to saturate. At that point, the A/F ratio can be driven to a rich condition (i.e., A/F < 14.6) for 2 to 5 s. Under these conditions, the reductants in the rich exhaust (i.e., H2, CO, and HC) react with the stored NOx to produce N2. Then the lean/rich cycle repeats.

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Nomenclature A Cp HEGO LNT PGM Q TWC

area of heat transfer heat capacity at constant pressure heated exhaust gas oxygen sensor lean NOx trap platinum group metal heat three-way catalyst

For both three-way catalysts and lean NOx traps, the reactions of HC, CO, H2, and NOx occur primarily on the PGM sites of the catalyst. These reactions are all exothermic, releasing heat which can generate high temperatures on the PGM sites. Some of this heat may be transferred directly to the exhaust gas by convection and radiation, while some of the heat is conducted to the catalyst washcoat in the local vicinity of the PGM sites. Heat in the washcoat is also transferred to the exhaust gas passing over it, again by convection and radiation. Under steady-state exothermic conditions, where the temperatures of the PGM sites, the washcoat, and the exhaust gas have stabilized, the total amount of exothermic heat generated at the PGM sites is equivalent to the total amount of heat transferred to the exhaust gas. Several researchers have attempted to quantify the temperatures generated on the PGM sites of a supported catalyst during chemical reactions. Some of these studies were intended to explain the PGM sintering that could occur during catalyst deactivation. Prater [4] used traditional transport equations to model a catalyst pellet as a continuum. Under steady-state conditions, he estimated a maximum difference of 1 to 2 8C between the interior and exterior of a pellet during the regeneration of beads fouled with carbonaceous material. Wei [5] allowed for the existence of hot spots on a porous catalyst during transient conditions and estimated a maximum ˚ hot spot and the pellet difference of 34 8C between a 2 A surface during catalyst regeneration. Mills et al. [6] suggested that the temperatures generated on a platinum cluster during the oxidation of CO to CO2 could exceed the bulk temperature by 200 8C or higher. He concluded that conduction from the platinum sites to the alumina washcoat was the dominant mode of heat transfer within a catalyst. Luss [7] modeled the temperature rise on a supported catalyst by assuming that the reactions occurred during a period of time that was less than the hopping time of a molecule, which was on the order of 1012 s. This reaction time was several orders of magnitude shorter than the duration between reaction events (e.g., 103 s). For a reaction time of 1013 s, his onedimensional model indicated that the maximum temperature rise on the surface of the metal crystallites ranged from 285 to 511 8C, depending on the thickness of the crystal. Luss’ model assumed that the temperature of the support remained constant under reaction conditions. Due to rapid heat transfer at the metal particle/support interface, the metal temperature cooled back to the original temperature during a relaxation time that

was similar to the reaction time of 1013 s. Therefore, the temperature of the metal crystals was higher than the washcoat temperature only during the short reaction periods and was effectively equal to the washcoat temperature the majority of the time. In contrast to Mills’ conclusions, Ruckenstein [8] suggested that radiation and convection were the dominant modes of heat transfer within a catalyst, due to poor acoustic matching between a catalyst particle and the support along with the low conductivity of the support. Also, he suggested that Luss’ temperature estimations could not account for the observed sintering of metal sites during catalyst deactivation, which required that the metal crystals remain at an elevated temperature for a period of time orders of magnitude longer than the reaction time of 1013 s. Chan et al. [9] modified Luss’ model to account for the finite propagation speed of heat transfer but still assumed that the temperature of the support remained constant under reaction conditions. He predicted significantly higher increases in metal temperature for a reaction time of 1013 s when the heat propagation speed was approximated by the speed of sound in crystals. Steinbruchel and Schmidt [10] argued that the reaction time should be on the order of 1012 s, as it would take several vibrational periods for an excited reaction product to release its energy. They modified Luss’ model to account for thermal boundary resistance at the interface between the metal sites and the support, still assuming that the support temperature remained constant. Depending on the particle size, their model predicted comparable or higher temperature increases on the metal sites as well as significantly slower cooling rates (e.g., 1010 s) relative to Luss’ model. However, this cooling time was still orders of magnitude shorter than the time between reaction events, suggesting that there would be no sustained temperature increase on the metal crystals, even for high turnover frequencies. Other researchers have attempted to measure the temperatures of the metal sites and the support through experimentation. Sharma et al. [11] performed experiments with a thin film of platinum on a non-porous quartz crystal. During the oxidation of CO with O2, the temperature of the support was estimated by measuring its temperature-dependent vibrational frequencies, while the temperature of the platinum film was estimated from its resistivity. At a gas temperature of 200 8C, the steady-state temperatures of the support and the platinum were determined to be 210 8C and 235 8C, respectively, resulting in a steady-state temperature gradient of 25 8C between the active metal and the support. Kember and Sheppard [12] recorded the infrared emissions from pressed discs of a Pd/SiO2 catalyst during the oxidation of CO with O2. They found that the blackbody emission was significantly higher than that which would be expected from the temperature measured with a thermocouple in contact with the silica. They suggested that this higher emission resulted from the Pd particles and estimated that the temperature of the particles was at least 190 8C higher than that of the silica. Kaul and Wolf [13] measured the surface temperatures on a 5%Pt/ SiO2 wafer while studying self-sustaining temperature oscillations during CO oxidation. They measured fluctuations in the

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surface temperature of 150 8C and speculated that the metal crystal temperatures would be significantly higher. These investigations required an experimental setup where the catalyst was physically exposed to an infrared camera or spectrometer. As discussed before, the precious metals in an automotive catalyst are dispersed as tiny particles throughout the pores of the washcoat, making access to them virtually impossible. It was desired to estimate the temperatures of these precious metal sites during oxidation reactions under realistic test conditions that reflected the intended use of the catalyst on a vehicle (i.e., high levels of reductant, conversion levels approaching 100%, reasonable space velocities (e.g., 37 000– 100 000 h1), atmospheric pressure, and a full complement of exhaust components including H2O and CO2). These temperature estimations could be used to refine current computer models that simulate the operation of automotive catalysts [14]. Also, information about the precious metal temperatures could improve the understanding of heat transfer within a catalyst under reaction conditions. Finally, the temperatures of the precious metal sites would be useful in improving the understanding of catalyst deactivation. Due to the extreme difficulty of measuring the PGM site temperatures in a monolithic catalyst, a technique was developed to infer or estimate the temperatures from chemical reactions [15]. A large exotherm was generated on a lean NOx trap containing potassium by oxidizing high concentrations of CO under net lean conditions for various durations of time. A trace concentration of a hydrocarbon was injected into the exhaust gas upstream of the LNT before, during, and after the CO injection periods. The PGM temperatures were then inferred from the conversion of the hydrocarbon using a calibration curve that related the HC conversion to temperature. Tests were performed with different hydrocarbons to provide multiple estimations of the site temperatures and thereby increase the confidence in the inferred temperatures. Since the majority of the exothermic reactions occurred on the front portion of the monolith, it was expected that the PGM sites in that zone would be hotter than the PGM sites in the rear portion of the monolith. Therefore, to characterize the PGM temperatures at various positions along the axis of the LNT during CO oxidation, experiments were performed on catalyst samples of different length. This paper summarizes the results of these investigations. An advantage of using a LNT for this work is that the potassium degraded the HC conversion of the catalyst, so that higher temperatures were required to achieve high conversion levels. As a result, the HC conversion could be used to infer the PGM temperatures during tests with high base temperatures and high levels of CO. Also, another method was developed to infer the temperature of the NOx storage sites on the LNT during oxidation reactions. The NOx site temperature was used as an estimate of the temperature of the catalyst washcoat, since the NOx storage sites are part of the washcoat. This allowed a comparison of the temperatures of the precious metal sites and the NOx storage sites (i.e., the washcoat) during exothermic reactions. Similar to the precious metal sites, access to the NOx storage sites in a monolithic catalyst would be extremely

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difficult. Therefore, another technique involving a chemical reaction was developed and used to estimate the temperature of these NOx storage sites during exothermic reactions. That work is discussed in a companion paper [16]. 2. Experimental 2.1. Catalyst preparation These experiments were performed on monolithic samples of a model lean NOx trap containing potassium, platinum, and alumina. To prepare the catalysts, 15.24 cm long cores with a diameter of 1.75 cm were drilled from a 1.2 L cordierite substrate (61 cells/sq cm, 0.17 mm wall thickness) that had been pre-coated with 0.18 g/cc of gamma-alumina (g-Al2O3). The Al2O3 washcoat was approximately 250–260 m thick in the corners of the cells and approximately 40–60 m thick along the walls between the corners of the cells. From BET analysis, ˚ with an average the pore diameters ranged from 20 to 2000 A ˚ diameter of approximately 110 A. The 15.24 cm cores were cut with a diamond-blade saw to produce several samples 2.54 cm in length. The 1.75 cm diameter  2.54 cm long samples were loaded with platinum by dipping them in an aqueous solution of dihydrogen hexachloroplatinate (IV) [H2PtCl66H2O]. After drying, the samples were calcined in air at 350 8C for approximately 2 h and then reduced in 1% H2 + 10% H2O in N2 at 500 8C for approximately 2.5 h. The samples were then dipped in an aqueous solution of potassium nitrate [KNO3], dried, calcined in air at 550 8C for approximately 3 h, and then reduced in 1% H2 + 10% H2O in N2 at 500 8C for 2 h. The final catalysts contained 3.5 g/L of platinum (approximatly 2% of the washcoat weight) and 5.7% potassium (also as a percentage of the washcoat weight). Two of the 2.54 cm long samples were cut axially into four 0.64 cm long pieces so that experiments could be performed on samples that were 0.64, 1.28, and 1.92 cm in length. Fresh catalysts samples were used for all of the tests in this paper. 2.2. Laboratory reactor The performance tests were performed on a laboratory reactor, shown in Fig. 1. Mass flow controllers were used to inject CO2, O2, and N2 into a mixing manifold. A peristaltic pump injected liquid water onto a wick in a heated tube, where the water evaporated and blended with the gas mixture. A trace amount of hydrocarbon was injected into the gas mixture so that the conversion of the hydrocarbon could be used to infer the temperatures of the precious metal sites. The hydrocarbon was either methane (CH4), propane (C3H8), butane (C4H10), or hexane (C6H14). These species were chosen because their conversion levels increased from 0 to 100% over a relatively broad range of temperature, allowing the PGM temperature to be estimated for a variety of test conditions. To minimize the exotherm from the hydrocarbon oxidation, the HC concentration was limited to 200 ppm (on a C3 basis), which would produce a maximum exotherm of approximately

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analyzers for measuring NO and total NOx, a paramagnetic analyzer for measuring O2, and a flame ionization detector (FID) for measuring HC. 2.3. Calibration curves

Fig. 1. Experimental apparatus.

13 8C. A mass flow controller was used to inject either 2% methane in N2, 5% propane in N2, 2% butane in N2, or 1000 ppm hexane in N2 into the gas mixture. For the different hydrocarbons, the flow rate of the bulk N2 was adjusted to produce a total gas flow rate of 2.5 L/min, resulting in a space velocity of approximately 100 000 h1 for a 0.64 cm long sample. The gas concentrations were typically 10% CO2, 10% H2O, 5% O2, 200 ppm HC (C3 basis), and the balance N2. The gas mixture was directed into a furnace, where a 0.64 cm long LNT sample was placed inside of a quartz tube. A 1.6 mm type K thermocouple was used to measure the temperature of the exhaust gas entering the LNT sample. The tip of a second 1.6 mm type K thermocouple was placed against the rear face of the LNT sample to measure the temperature of the exhaust gas exiting the LNT sample. To generate an exotherm on the LNT sample, high concentrations of CO were injected for various durations of time. A mass flow controller continuously injected a 70% CO/ 30% N2 mixture into a solenoid valve, and an external timer was used to direct the CO/N2 mix either into the gas mixture or into a ventilation line. The CO was injected into the exhaust for periods of 5, 10, 20, 30, 60, 120, or 180 s, and the flow rate of the injected CO/N2 mixture was varied to produce final CO concentrations of 2%, 3%, or 5%. To insulate the catalyst and minimize the heat losses from the reactor, 1.28 cm long pieces of uncoated cordierite (62 cells per sq cm, 0.17 mm wall thickness) were placed before and after the 0.64 cm LNT sample. A small hole was drilled through the downstream piece of cordierite so that the rear 1.6 mm thermocouple could still be placed against the rear face of the LNT sample. In addition, a large hole was drilled through a 7.5 cm diameter  10.2 cm long catalyst brick, and this cordierite brick was placed around the outside of the quartz sample holder, as shown in Fig. 1. The exhaust exiting the LNT reactor was diluted with 22.5 L/min N2 to provide enough flow for all of the analyzers and to reduce the H2O concentration from 10 to 1%, in order to prevent condensation in unheated sections of tubing or the analyzers. The emission benches included NDIR analyzers for measuring CO2, CO, and N2O, two chemiluminescence

This technique required that accurate conversion versus temperature curves (referred to as ‘‘lightoff curves’’) be developed for each hydrocarbon species. For multiple samples of the same catalyst formulation, there could be differences in hydrocarbon conversion due to slight differences in the sample length, the platinum concentration, the potassium concentration, and/or the potassium/platinum interaction. As a result, the lightoff curves for a particular sample could only be used to infer the precious metal temperatures for that specific sample. Therefore, for each sample, slow temperature ramps were performed with each of the four hydrocarbons. During these slow ramps, the exhaust contained 5% O2, 200 ppm of the hydrocarbon, 10% CO2, 10% H2O, and the balance N2. Since the lightoff curves were developed without NO or NO2, the experiments with CO injection were performed without NOx, in order to prevent the hydrocarbon from reacting with the feedgas NOx or with NOx that was released from the LNT during the CO injections. 2.4. Tests on samples in series To quantify the precious metal temperatures in a second 0.64 cm sample when two catalyst samples were placed in series, two different techniques were used. In the first technique, the samples were butted together, and the CO and the hydrocarbon were injected before the first sample. The HC conversion of the first sample and the HC conversion of the two samples in series were used to calculate the HC conversion of the second sample. Then the lightoff curves for the second sample were used to estimate the precious metal temperatures in that sample. The rear thermocouple was used to measure the temperature of the exhaust gas exiting the second sample. Similar work was performed on three 0.64 cm samples in series, where the HC conversions of the two samples in series and the three samples in series were used to calculate the HC conversion of the third sample. In the second technique, the two samples were separated by a gap. The inlet thermocouple was removed and replaced with a 1.6 mm tube, which was inserted through a hole in the middle of the first 0.64 cm sample. The HC was injected through the tube into the gap between the first LNT sample and the second LNT sample, while the CO was injected before the first LNT sample. The HC conversion of the second sample was used with its calibration curve to infer the PGM temperatures on that sample. The second sample was placed far enough from the HC injection point to allow the injected HC to mix with the exhaust. Again, the rear thermocouple was used to measure the temperature of the exhaust gas exiting the second sample. It was expected that there would be heat losses between the first and second samples with this technique. Therefore, the inferred PGM temperatures in the second sample were compared to the

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temperature of the gas exiting that sample. Any difference between these two temperatures would be added to (or subtracted from) the temperature of the gas exiting the second sample when the two samples were butted together (i.e., from the first technique). 3. Results and discussion 3.1. Inferring PGM temperatures from HC conversion To justify the use of the HC conversion as a means of inferring the precious metal temperatures, it needed to be shown that the HC oxidation occurred primarily on the precious metal sites and not on other components of the catalyst, such as the cordierite, the alumina, or the NOx storage components, as the temperatures of these components were expected to be different from the temperature of the precious metal sites under exothermic reaction conditions. Therefore, the conversion of 200 ppm C3H8 was determined during slow temperature ramps for the following cases: - Empty quartz reactor tube. - With two 1.27 cm long pieces of uncoated cordierite. - With a 0.64 cm piece of cordierite coated with only Al2O3 between the two 1.27 cm pieces of cordierite. - With a 0.64 cm piece of cordierite coated with Al2O3 and 5.7% potassium (K) between the two 1.27 cm pieces of cordierite. - With a 0.64 cm piece of cordierite coated with Al2O3, 5.7% K, and 3.5 g/L of platinum between the two 1.27 cm pieces of cordierite. - With a 0.64 cm piece of cordierite coated with Al2O3 and 3.5 g/L of platinum between the two 1.27 cm pieces of cordierite. The last two tests demonstrated the effect of the potassium on the HC conversion. Fig. 2 shows the C3H8 conversion as a function of the measured gas temperature for these different cases. Very little C3H8 conversion was observed up to 600 8C for any of the cases without precious metal. Potassium is known to be an oxidation catalyst, but at this high space velocity (100 000 h1) only a small amount of conversion was observed up to 700 8C for the case with potassium but no PGM. This indicated that essentially all of the HC conversion occurred on the precious metal sites, particularly when the temperatures were below 600 8C. Since the HC conversion continued to increase over a broad range of temperature, it was concluded that the HC conversion could be used to infer the temperature of the PGM sites. While the HC conversion occurred primarily on the PGM sites and could be used to infer the PGM temperatures, this did not by itself prove that the PGM sites were at a higher temperature than the washcoat. This issue will be addressed in a later section, where the inferred PGM temperatures are compared directly to the inferred temperatures of the NOx storage sites on the LNT under exothermic reaction conditions. Since the NOx storage sites are part of the washcoat, this

Fig. 2. Propane lightoff curves for fresh catalysts with different components. Test conditions: 5% O2, 10% H2O, 10% CO2, 200 ppm C3H8 (C3 basis), balance N2, 100 000 h1 space velocity.

effectively compares the PGM temperature with the washcoat temperature. The last two tests indicated that the presence of potassium increased the temperature required for 50% conversion of C3H8 by approximately 150 8C. This allowed PGM temperatures as high as 700 8C to be inferred with this technique. One potential issue with this technique is that the presence of CO could affect the conversion of the hydrocarbon. If CO was adsorbed on the surface of the platinum, this could affect the conversion of the hydrocarbon and therefore the inferred PGM temperatures. However, Wolf and Kaul [13] used FTIR to study CO coverage on a Pt/SiO2 catalyst at different temperatures and showed clearly that the CO coverage on Pt dropped to extremely low levels above 300 8C. Similar results were obtained by Me´norval et al. [17], whose results showed that, even in the absence of oxygen, the surface coverage of CO adsorbed on alumina-supported Pt crystallites decreased to essentially zero at 250 8C. Since the experiments in this present work were always performed above 300 8C, the CO coverage on the platinum could be neglected. Another potential issue with this technique was that the O2 concentration over the catalyst dropped during the CO injections, as no attempt was made to maintain a constant O2 level by injecting additional O2 during the CO injections. As a result, the O2 concentration dropped from 5% to 4.0%, 3.5%, and 2.5%, respectively, when 2%, 3%, and 5% CO was injected. However, it was confirmed experimentally that the HC conversion was constant over this range of O2 concentrations. Therefore, it was concluded that the HC conversion reflected the thermal environment of the PGM sites and was unaffected by these potential chemical effects. 3.2. Lightoff curves The technique required that accurate hydrocarbon lightoff curves be determined for each catalyst sample. Therefore, slow temperature ramps were performed from approximately 300 8C to approximately 800 8C to determine the conversion of each hydrocarbon species as a function of the gas temperature that was measured with the thermocouple placed against the rear

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face of the sample. The low HC concentration of 200 ppm limited the exotherm on the catalyst to a maximum of 13 8C during the ramps, and the use of slow ramps insured that the temperature of the 0.64 cm long catalyst was fairly uniform. Therefore, as an approximation, it was assumed that the PGM temperature was equal to the temperature of the exhaust gas exiting the catalyst during the ramps. To infer the PGM temperatures from the measured conversions, the measured gas temperature (i.e., PGM temperature) was plotted as a function of the HC conversion for each hydrocarbon. The curves were fit to polynomial expressions to allow the PGM temperatures to be calculated from the measured HC conversion.

temperature dropped to the same level as the exit gas temperature in a few seconds. The PGM and gas temperatures then continued to drop at approximately the same rate over the next 40–50 s. Since the source of exothermic heat (i.e., the CO) was turned off, it was reasonable that the PGM temperature would match the exit gas temperature during this cooling period. 3.4. Tests with different CO levels and different injection times

To demonstrate the procedure, a test was performed on a 0.64 cm long LNT while injecting 2% CO for 180 s at a base temperature of approximately 390 8C, where 200 ppm C4H10 was used for the tracer gas. The results of the test are shown in Fig. 3. Before the CO was injected at approximately 125 s, the inferred PGM temperatures were approximately the same as the measured gas temperature. This was expected, since the only exotherm resulted from the low conversion (<10%) of the C4H10. When the CO was initiated, the C4H10 concentration dropped rapidly, producing a rapid increase in the HC conversion and the inferred PGM temperature. In contrast, the measured gas temperature increased slowly over time as the heat generated at the precious metal sites was transferred to the exhaust gas. Immediately after the CO injection was initiated, the inferred PGM temperature was approximately 140 8C hotter than the measured gas temperature as a result of the rapid increase in PGM temperature and the slow increase in gas temperature. At the end of the 180 s injection, the steady-state PGM temperature was still about 80 8C above the gas temperature (i.e., approximately 560 8C versus 480 8C). When the CO was turned off, the HC conversion and therefore the inferred PGM temperatures began to decrease. The drop in PGM temperature was slower than the increase in PGM temperature when the CO was initiated. The PGM

Using butane for the tracer gas, tests were performed on the 0.64 cm LNT where 5% CO was injected for different durations between 10 and 180 s. The tests were repeated with hexane, propane, and methane as the tracer gas. Tests with propane, butane, and hexane were also performed with CO concentrations of 3% and 2% (low conversion levels made it difficult to infer the PGM temperatures with methane on these tests). Fig. 4 shows the measured increase in the gas temperature and the inferred increase in the PGM temperature (relative to the base temperature of 390 8C) for the three CO levels and the different HC tracer gases as a function of the CO injection time. Fig. 5 shows the maximum hydrocarbon conversion of the four tracer gases as a function of the injection time with 5% CO. For each CO level, Fig. 4 shows that the inferred PGM temperatures were in very good agreement for the different tracer gases, even though Fig. 5 shows that the levels of hydrocarbon conversion were significantly different for the four tracer gases. With 5% CO, the inferred PGM temperatures increased from the base value by about 200 8C during the first 10 s, and the increase in the PGM temperature stabilized at approximately 290 8C after about 120 s. The increase in the gas temperature was only about 60 8C after 10 s, but it stabilized at about 240 8C after the 180 s injection. Due to the difference in response times, the average difference between the PGM temperature and the gas temperature was approximately 140 8C after 10 s and was still about 50 8C for injection times of 120 and 180 s. The difference in the PGM temperature and gas temperature at the higher injection times was approximately 70 8C with 3% CO and approximately 80 8C with 2% CO.

Fig. 3. Inferred PGM temperature and exit gas temperature for fresh 0.64 cm LNT with 3.5 g/L Pt and 5.7% K. Base exhaust: 390 8C, 100 000 h1 space velocity, 5% O2, 10% H2O, 10% CO2, 200 ppm C4H10 (C3 basis), balance N2. 2% CO injected for 180 s.

Fig. 4. Inferred PGM exotherms and exit gas exotherms for fresh 0.64 cm LNT with 3.5 g/L Pt and 5.7% K. Base exhaust: 390 8C, 100 000 h1 space velocity, 5% O2, 10% H2O, 10% CO2, 200 ppm CH4, C3H8, C4H10, or C6H14 (C3 basis), balance N2. 2%, 3%, or 5% CO injected for various durations of time.

3.3. Demonstration of concept

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Fig. 5. Maximum HC conversions for fresh 0.64 cm LNT with 3.5 g/L Pt and 5.7% K. Base exhaust: 390 8C, 100 000 h1 space velocity, 5% O2, 10% H2O, 10% CO2, 200 ppm CH4, C3H8, C4H10, or C6H14 (C3 basis), balance N2. 5% CO injected for various durations of time.

Fig. 7. Inferred PGM exotherms and exit gas exotherms for fresh 0.64 cm LNT with 3.5 g/L Pt and 5.7% K. Base exhaust: 300–500 8C, 100 000 h1 space velocity, 5% O2, 10% H2O, 10% CO2, 200 ppm C3H8, C4H10, or C6H14 (C3 basis), balance N2. 2%, 3%, or 5% CO injected for 180 s.

For each CO level, the CO concentration exiting the 0.64 cm LNT sample was very low, indicating that essentially all of the CO was being oxidized by this first sample. This information will be useful later when inferring the PGM temperatures of second and third LNT samples placed downstream of the first sample.

for CO levels of 5%, 3%, and 2%, respectively, but the gas temperatures increased by only 30 8C, 17 8C, and 8 8C. The standard deviations of the maximum PGM temperatures were 1.4 8C, 1.1 8C, and 1.5 8C, respectively. These low standard deviations indicate that the technique resulted in reproducible PGM temperatures.

3.5. Reproducibility of inferred PGM temperatures

3.6. Tests at different temperatures

To evaluate the reproducibility of the estimated PGM temperatures, replicate tests were performed on a fresh 0.64 cm LNT with 5 s injections of 2%, 3%, and 5% CO. The CO injections were separated by 120 second periods without CO. Hexane was the tracer gas, and the base temperature was approximately 390 8C. Fig. 6 shows the gas temperatures before the CO injections, the maximum gas temperatures with the CO injections, and the maximum PGM temperatures for multiple injections with the different CO levels. For these relatively short injection periods, the inferred PGM temperatures increased by approximately 210 8C, 190 8C, and 155 8C

Using butane, propane, or hexane as the tracer gas, tests were performed at base temperatures ranging from 300 to 500 8C with 180 s injections of 2%, 3%, or 5% CO. For the three CO levels, Fig. 7 shows the inferred increase in PGM temperature and the measured increase in gas temperature (relative to the base temperature) as a function of the base temperature. For each CO level, the increase in the gas temperature was relatively constant at the different base temperatures (approximately 250 8C, 150 8C, and 100 8C for CO levels of 5%, 3%, and 2%, respectively). The slight decrease in the gas exotherms with increasing temperature can be attributed to the fact that the heat capacity of the exhaust gas increased by approximately 7% between 300 and 500 8C. The inferred PGM temperatures were higher than the measured gas temperatures in all cases, although the difference decreased with increasing temperature. For 5% CO, the maximum PGM temperature was approximately 53 8C higher than the maximum gas temperature at a base temperature of 300 8C but only about 17 8C higher at a base temperature of 500 8C. For the injections with 3% CO, the maximum PGM temperature was approximately 109 8C higher than the maximum gas temperature at a base temperature of 300 8C but only about 35 8C higher at a base temperature of 500 8C. For the injections with 2% CO, the corresponding differences were 129 8C and 44 8C. Potential reasons for the decrease in this temperature difference with increasing base temperature and increasing CO level will be addressed later. Fig. 8 shows the stabilized difference between the inferred PGM temperature and the measured gas temperature for tests performed with 5% CO at a base temperature of 300 8C, 3% CO at a base temperature of 430 8C, and 2% CO at a base

Fig. 6. Reproducibility of inferred PGM temperatures and exit gas temperatures for fresh 0.64 cm LNT with 3.5 g/L Pt and 5.7% K. Base exhaust: 390 8C, 100 000 h1 space velocity, 5% O2, 10% H2O, 10% CO2, 200 ppm C6H14 (C3 basis), balance N2. 2%, 3%, or 5% CO injected for 5 s periods multiple times, with 120 s between injections.

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Fig. 8. Difference in inferred PGM temperature and exit gas temperature for fresh 0.64 cm LNT with 3.5 g/L Pt and 5.7% K. Base exhaust: 100 000 h1 space velocity, 5% O2, 10% H2O, 10% CO2, 200 ppm C4H10 or C6H14 (C3 basis), balance N2. 180 s injections with 5% CO at base of 300 8C, 3% CO at base of 430 8C, or 2% CO at base of 480 8C. Exit gas temperature 560–570 8C.

temperature of 480 8C; butane and hexane were used as the tracer gases. All of these conditions produced exit exhaust gas temperatures of approximately 560–570 8C. The difference between the PGM temperature and the exit gas temperature was reasonably consistent with the different CO concentrations, which supports the earlier assumption that the presence of CO does not have a significant effect on the HC conversion under the conditions of these experiments. 3.7. Inferred PGM temperatures in a second catalyst sample It was desired to characterize the PGM temperatures along the axis of the catalyst during the oxidation reactions. Therefore, experiments were performed to determine the PGM temperatures in a second 0.64 cm LNT placed downstream of the first 0.64 cm LNT. Two different techniques were used for this assessment. One technique involved injecting the CO before the first sample and injecting the HC between the two samples. The HC conversion of the second sample was used to infer the PGM temperature in that sample. In the other technique, the two samples were butted together, and the CO and the HC were both injected before the first sample. The HC conversions for the first sample and for the two samples in series were used to determine the HC conversion of the second sample, which was then used to infer the PGM temperature for that sample. Each of these techniques had advantages and disadvantages. Injecting the HC before the two samples in series had the advantage that there would be no heat losses between the samples. However, a disadvantage was that the HC conversion of the second sample was dependent on the HC conversion of the first sample. It had to be assumed that the HC conversion of the first sample remained exactly the same when tested by itself and when tested in series with the second sample. A small change in the HC conversion of the first sample due to small differences in test conditions and/or catalyst deactivation could have affected the calculated HC conversion of the second

sample and thereby affected the inferred PGM temperature for that sample. Another issue with the series orientation was that it placed limitations on the test conditions that could be used, specifically the type of hydrocarbon and the temperature. If the HC conversion of the first sample was very high, the HC concentration entering the second sample was low, and this limited the accuracy of the inferred PGM temperature in the second sample. This prohibited the use of easy-to-oxidize hydrocarbons (e.g., hexane) at high base temperatures when testing two samples in series. The injection of the HC between the two samples had the advantage that the HC conversion over the second sample was not dependent on the HC conversion of the first sample. Also, this expanded the range of test conditions that could be used. The disadvantage of this technique was that a large gap had to be placed between the two samples to allow the injected hydrocarbon to mix with the exhaust gas. As a result of this large gap, there were substantial heat losses between the two catalyst samples. Therefore, the inferred PGM temperature in the second sample was lower that it would have been if the samples had been placed in series. However, this problem was overcome by comparing the inferred PGM temperature in the second sample to the measured gas temperature exiting that sample, as the heat losses would affect both temperatures similarly. Any difference between these two temperatures could be added to (or subtracted from) the temperature of the gas exiting the second sample when the two samples were butted together (i.e., from the first technique). 3.7.1. Injecting HC between catalyst samples The technique of injecting the HC between the two catalyst samples will be discussed first. To determine the gap necessary between the catalyst samples to allow good mixing of the hydrocarbon with the exhaust, a series of slow temperature ramps were performed with methane while varying the gap between the two samples. Another test was performed on the second LNT sample by itself where the methane was completely mixed with the exhaust by injecting it upstream of the reactor. Gaps of 5.08 and 10.16 cm provided similar lightoff performance as the premixed case, but a gap of 2.54 cm resulted in lower conversion performance. This suggested that a minimum gap of 5.08 cm was required to allow good mixing of the methane with the exhaust gas. Similar results were obtained with propane and butane. However, a 5.08 cm gap was not sufficient for hexane, as the conversion was significantly lower with a 5.08 cm gap relative to the premixed case, particularly at high temperatures. Tests were run with the different hydrocarbons with a 5.08 cm gap between the two samples. The initial temperature was 470 8C, and 5% CO was injected before the first LNT for 180 s. The hydrocarbon was injected between the two samples continuously, and the PGM temperature was inferred from the HC conversion of the second sample. Fig. 9 compares the inferred PGM temperatures from all four hydrocarbons to the gas temperature measured with a thermocouple placed against the rear face of the sample. The figure also shows the HC conversion for the four hydrocarbons. Even though the conversions of the

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Fig. 9. Inferred PGM temperatures and HC conversions for second 0.64 cm LNT with 3.5 g/L Pt and 5.7% K. 5.08 cm gap between LNT samples. Base exhaust: 470 8C, 5% O2, 10% H2O, 10% CO2, balance N2. 5% CO injected before first 0.64 cm LNT for 180 s. 200 ppm CH4, C3H8, C4H10, or C6H14 (C3 basis) injected continually between LNT samples to infer PGM temperatures of second sample. Gas temperature measured behind second sample.

Fig. 10. Inferred PGM temperatures, exit gas temperatures, and HC conversions for both 0.64 cm LNT samples when samples tested in series. Base exhaust: 400 8C, 5% O2, 10% H2O, 10% CO2, 200 ppm C6H14 (C3 basis), balance N2. 3% CO injected for 5 min. C6H14 conversion of first LNT sample assumed constant between 180 and 300 s.

four hydrocarbons were significantly different, the PGM temperatures inferred from the HC conversions were essentially equivalent. This was true even for hexane, although a 5.08 cm gap was not sufficient to achieve complete mixing for this hydrocarbon species. This is indicated by the fact that the hexane conversion was lower than that of propane and butane in Fig. 9. However, the incomplete mixing of hexane was integrated into the lightoff curve, so the inferred PGM temperatures with hexane were similar to the temperatures inferred with the other HC species. It is noteworthy that, unlike the first 0.64 cm sample, the inferred PGM temperatures in the second 0.64 cm sample were similar to the temperature of the gas exiting the sample. As mentioned earlier, there was essentially no CO exiting the first 0.64 cm sample. Therefore, it was reasonable that there would be no difference between the inferred PGM temperature in the second sample and the gas temperature exiting that sample under steady-state conditions. The excellent agreement between the measured gas temperature and the inferred PGM temperatures for the second sample (Fig. 9) along with the excellent agreement in the inferred PGM temperatures from the different HC species for the first sample (Figs. 4 and 7) support the validity of using the HC conversion to diagnose the temperature of the precious metal sites in a catalyst.

performed for methane with concentrations of 200 ppm and 50 ppm and for propane and hexane with concentrations of 200 ppm, 100 ppm, and 50 ppm. For each hydrocarbon, there was very little difference in the lightoff curves for the different feedgas concentrations. This confirmed that the HC conversion was indeed a first-order reaction. Therefore, after time-aligning the test data for the first sample and for the two samples in series, the HC conversion of the second sample was calculated continuously from the HC concentrations exiting the first sample and the second sample. CO injection tests were run at different temperatures for the different hydrocarbons. As an example, Fig. 10 shows the results of a test performed at a base temperature of 400 8C while injecting 3% CO before the samples. Hexane was used as the tracer hydrocarbon for this test. The CO injection periods were extended to 5 min to allow ample time for the temperatures in the second sample to stabilize. Due to slight differences in the feedgas HC concentration between the two tests, the secondby-second HC conversion for the first sample and the feedgas HC concentration for the test on the two samples in series were used to calculate the second-by-second HC concentration exiting the first sample during the test on the two samples in series. Since the maximum CO injection time for the first sample was 180 s, it was assumed that the HC conversion of the first sample remained constant between 180 and 300 s. The figure shows the HC conversion of the first sample, the second sample, and the two samples in series. The figure also shows the exit gas temperature and the PGM temperature for the first sample and the second sample. Finally, the figure shows the difference between the inferred PGM temperature and the exit gas temperature for the first sample and the second sample. At the end of the 300 s injection, this difference was approximately 70 8C for the first sample but only about 4 8C for the second sample. Similar to the test where the HC was injected between the two samples, the test on the two samples in series suggests that the PGM temperatures in the second sample were essentially equivalent to the temperature of the gas exiting that sample.

3.7.2. Series testing with two catalyst samples The second technique of inferring the PGM temperatures in a second sample is discussed now. When tested in series, the second LNT sample would be converting the hydrocarbons emitted by the first sample, and this concentration would vary during the test as the first sample was heating up from the oxidation of the injected CO. So this technique required that the HC conversion of the second sample follow first order kinetics, so that the HC conversion of the second sample did not depend on the HC concentration to which it was exposed. Therefore, a series of slow temperature ramps were performed on the second sample with different HC concentrations. Lightoff tests were

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Fig. 11. Inferred PGM temperatures and exit gas temperatures along the axis of a fresh 1.92 cm LNT with 3.5 g/L Pt and 5.7% K. Base exhaust: 470 8C, 5% O2, 10% H2O, 10% CO2, 200 ppm C4H10 (C3 basis), balance N2. 2% CO injected for 3 min (first 0.64 cm sample) or 5 min (two or three 0.64 cm samples in series). C4H10 conversion of first 0.64 cm sample assumed constant between 180 and 300 s.

Similar testing was performed with three samples in series. Similar to the results for the second sample in Fig. 10, the inferred PGM temperatures in the third sample were essentially the same as the gas temperature measured at the rear face of that sample under steady-state conditions. 3.8. PGM temperature along axis of catalyst For samples tested in series, Fig. 11 summarizes the steadystate PGM temperatures in the first, second, and third 0.64 cm catalyst samples and the measured gas temperature exiting those samples after 3 min injections (first sample) or 5 min injections (two or three samples in series) with 2% CO at a base temperature of 470 8C. Butane was used as the tracer gas for these tests. Again, it was assumed that the HC conversion of the first sample remained constant between 180 and 300 s. The PGM temperature was highest in the front sample and was approximately 50 8C higher than the gas temperature exiting that sample. The PGM temperatures in the second and third samples were essentially the same as the gas temperatures exiting those samples. The data in Fig. 11 indicate that the axial profile of gas temperatures measured with thermocouples can be significantly different from the profile of PGM temperatures when exothermic reactions are occurring on the catalyst. 3.9. Comparison of PGM temperature and NOx storage site temperature In reference [16], a procedure is described for inferring the temperatures of the NOx storage sites of a LNT under exothermic reaction conditions. Briefly, the LNT is saturated with NO2, and then a large concentration of propylene (C3H6) is injected for various durations of time. The resulting exotherm causes some of the stored NOx to be released, and the amount of NOx remaining on the sample is calculated. This remaining NOx is used with a NOx capacity calibration curve to estimate the final temperature of the NOx storage sites. The temperature

of the NOx storage sites can be used as an estimate for the temperature of the washcoat, since the NOx storage sites are part of the washcoat. It was desired to compare the temperatures of the PGM sites and the NOx storage sites (i.e., washcoat) under exothermic reaction conditions. However, since the PGM temperatures were inferred during CO oxidation and the NOx site temperatures were inferred during C3H6 oxidation, the temperatures could not be compared directly. On the other hand, these temperatures could be compared under conditions where the increase in the exit gas temperature was similar, as this implied equivalent heat release at the PGM sites. The inferred PGM temperature and the exit gas temperatures were determined during 180 s injections with 2% CO at base temperatures ranging from 300 to 500 8C, using propane, butane, and hexane as tracer gases. The exit gas temperature increased by approximately 100 8C at the different base temperatures. Reference [16] explains how the NOx site temperatures were estimated for a fresh 0.64 cm LNT with 2.6 g/L Pt and 5.7% K under conditions that would produce this same increase in gas temperature at base temperatures of 318, 372, and 463 8C. Fig. 12 compares the inferred increase in PGM temperature, the inferred increase in NOx site temperature, and the measured increase in gas temperature for the different base temperatures. Under steady-state conditions, the PGM temperatures were significantly higher than the NOx site temperatures, which were slightly higher than the exit exhaust gas. Since the NOx site temperature can represent the washcoat temperature, this demonstrates that there is a sustained difference in temperature between the PGM sites and the washcoat during exothermic reactions, in agreement with the results of Sharma et al. [11].

Fig. 12. Steady-state increase in PGM temperature and NOx storage site temperature for fresh 0.64 cm LNT samples under conditions producing similar increases in exit gas temperature. PGM temperatures: LNT with 3.5 g/L Pt and 5.7% K, base temperatures of 300–500 8C, 100 000 h1 space velocity, 5% O2, 10% H2O, 10% CO2, 200 ppm C3H8, C4H10, or C6H14 (C3 basis), balance N2. 2% CO injected for 180 s. NOx storage site temperatures: LNT with 2.6 g/L Pt and 5.7% K, base temperatures of 318 8C, 372 8C, or 463 8C, 100 000 h1 space velocity, 5% O2, 10% H2O, 10% CO2, balance N2. NOx site temperatures inferred from NOx release from saturated LNT during 180 s injections of different C3H6 concentrations under net lean conditions. At base temperatures of 318, 372, and 463 8C, experimental data interpolated to generate NOx site temperature corresponding to same increase in exhaust gas temperature indicated in the figure (i.e., 100, 96, and 91 8C).

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To summarize the results thus far, the time-averaged and spatially-averaged temperatures of the PGM sites on a fresh 0.64 cm monolithic lean NOx trap during lean CO oxidation have been inferred from the conversion of a trace amount of hydrocarbon. The PGM temperatures inferred with tracer gases of methane, propane, butane, or hexane were in excellent agreement, although the levels of HC conversion were significantly different. When two or three catalyst samples were tested in series, the inferred PGM temperatures in the second and third samples agreed with the temperatures of the exhaust gas exiting those samples. This agreement was expected, since essentially all of the CO was oxidized on the first sample. The results of this work suggest that there is a significant and sustained increase in the time-averaged temperature of the PGM sites on a polycrystalline catalyst during oxidation reactions. This is in contrast to the modeling results of previous researchers [7,9,10], who suggested that the temperature of the PGM sites would be elevated only during reaction times and cooling times that were on the order of 1010 to 1013 s. These reaction and cooling times were orders of magnitude shorter than the time between reaction events. As a result, those models would predict that the time-averaged PGM temperatures would be essentially equivalent to the base temperature, even for high turnover frequencies. The results shown in this paper suggest otherwise. It should be mentioned that the steady-state PGM temperatures inferred from the HC conversion in this work were the time-averaged or sustained temperatures of the PGM sites. It is possible that the large and rapid temperature spikes predicted by Luss, Chan, and Steinbruchel were superimposed on the stabilized temperatures inferred from the HC oxidation. Temperature spikes occurring for a period of time on the order of 1013 to 1010 s would not be detected with the technique used here or by any other known technique. Also, the inferred PGM temperature was the spatially-averaged temperature for all of the PGM sites on the 0.64 cm long sample. While a short monolith sample was used in an attempt to minimize axial temperature gradients, it is probable that the PGM sites on the front of the sample were exposed to a higher concentration of CO than the PGM sites on the rear of the sample. Thus, the sites on the front may have oxidized more CO and therefore may have generated higher PGM temperatures. So the PGM temperature inferred from the HC conversion is the average temperature for the entire 0.64 cm sample. The steady-state increases in gas temperature with 5%, 3%, and 2% CO were approximately 250 8C, 150 8C, and 100 8C, respectively, which were lower than the calculated adiabatic temperature increases of 410 8C, 250 8C, and 170 8C. This was attributed to conductive and radiative heat losses from the reactor during the CO injection experiments, even though efforts were taken to minimize these losses. To investigate this, CO oxidation experiments were performed with different exhaust flow rates on a fresh 2.54 cm long catalyst sample with 3.5 g/L Pt but no potassium. The steady-state increases in exit gas temperature with 3% CO were approximately 155 8C, 207 8C, and 228 8C for base exhaust flow rates of 2.5, 3.5, and

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4.5 L/min, respectively. So the measured gas exotherm approached the adiabatic exotherm of 250 8C as the flow rate increased, presumably because the axial heat transfer by convection began dominating the radial heat losses due to radiation and/or conduction. It was assumed that these heat losses, which decreased the measured gas exotherm during the CO injection, also decreased the HC conversion of the sample, which resulted in a lower PGM temperature relative to that which would have been obtained under adiabatic conditions. Therefore, the inferred PGM temperature was appropriate for the measured exit gas temperature. This is supported by the very good agreement between the inferred PGM temperature and the exit gas temperature for the second and third LNT samples when injecting CO before two or three samples in series. Fig. 7 indicated that the steady-state increase in the PGM temperature was significantly higher than the measured increase in the temperature of the exhaust gas exiting the sample, although the total amount of heat transferred to the exhaust gas through convection and radiation was equal to the total amount of heat generated at the PGM sites. This was largely attributed to the higher heat capacity of the exhaust relative to that of platinum (Cp of N2 is roughly eight times larger than that of Pt). In addition, it was reasonable that the PGM temperature were higher than the exhaust gas temperature under steady-state conditions, as this provided the driving force for heat transfer from the PGM sites to the exhaust gas. While the steady-state PGM temperatures were always higher than the exit gas temperatures in Fig. 7, this temperature difference (referred to as the ‘‘delta’’ temperature for brevity) decreased as the base temperature increased and as the amount of CO increased. For example, at a base temperature of 300 8C, the delta temperatures were approximately 130 8C, 110 8C, and 50 8C for injections of 2%, 3%, and 5% CO, respectively. One potential explanation for this observation was that the higher concentrations of gas-phase CO resulted in higher degrees of CO coverage on the PGM sites, which could impede the conversion of the hydrocarbon and result in lower inferred PGM temperatures. However, Wolf and Kaul demonstrated clearly that the surface coverage of CO on a Pt/SiO2 catalyst fell to very low levels at temperatures above 300 8C [13]. Since the PGM temperatures were far above 300 8C during these experiments, a surface coverage mechanism would not account for the observed drop in the delta temperature with increasing CO level. Some insight into the potential reasons for the drop in delta temperature was obtained by plotting the delta temperature as a function of the exit gas temperature for the different base temperatures and CO levels, as shown in Fig. 13. Here the PGM temperatures were inferred from tracer gases of propane, butane, and hexane. Whether the higher temperatures were generated by increasing the base temperature or by increasing the level of CO, the delta temperature continued to decrease with increasing gas temperature. Again, this suggested that the drop in delta temperature was due to thermal effects and not due to chemical effects (i.e., differences in the amount of CO). For modeling purposes, Fig. 13 suggests that the PGM temperature

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in the front section of the catalyst (where the majority of the exothermic reactions occur) can be calculated from the temperature of the exhaust gas exiting that section of the catalyst, regardless of the composition of the exhaust entering the catalyst. The large drop in the delta temperature with increasing base temperature implied that radiation was contributing to the heat transfer within the catalyst. Heat transfer due to radiation is governed by the equation [18]:

power of the temperature, Eq. (1) suggests that, for a constant level of CO (i.e., constant Q), the delta temperature must decrease as the temperatures increase in order to maintain a constant level of radiative heat transfer. It is difficult to calculate the absolute level of radiation from the PGM sites to the exhaust due to uncertainties in the area for heat transfer. However, Eq. (1) can be used to estimate the change in the delta temperature with increasing temperature. The inferred PGM temperature and measured gas temperature at 400 8C were used to predict the PGM temperatures for several higher gas temperatures, assuming constant radiative heat transfer. The predicted change in the delta temperature was calculated as a function of the gas temperature, neglecting the change in the emissivity of platinum with temperature. Next, to account for the increase in platinum emissivity with temperature, a linear equation was used to estimate the emissivity of platinum as a function of temperature [19]. Since the value of e1 in Eq. (1) depends on the final temperature T1, an iterative solver was used to calculate the PGM temperature T1 for several values of the gas temperature T2. Fig. 14 compares the experimental delta temperatures from Fig. 13 to the predicted delta temperatures with and without allowing for the increase in Pt emissivity with temperature. The predicted change in delta temperature while accounting for the change in Pt emissivity agrees reasonably well with the experimental data. This supports the conjecture that heat transfer by radiation from the PGM sites is an important mechanism of heat transfer within a catalyst during exothermic reactions.

Q ¼ sAðe1 T14  a1 T24 Þ

4. Conclusions

Fig. 13. Difference in inferred PGM temperature and exit gas temperature as a function of exit gas temperature for fresh 0.64 cm LNT with 3.5 g/L Pt and 5.7% K. Base exhaust: 300–500 8C, 100 000 h1 space velocity, 5% O2, 10% H2O, 10% CO2, 200 ppm C3H8, C4H10, or C6H14 (C3 basis), balance N2. 2%, 3%, or 5% CO injected for 180 s.

(1)

where s is the Stefan–Boltzmann constant, A is the area of heat transfer, T1 and T2 are the temperatures of component 1 and component 2, e1 is the emissivity of component 1 at T1, and a1 is the absorptivity of component 1 at T2. The absorptivity a1 can be estimated from the emissivity of component 1 at T2. Applying this equation to a catalyst, T1 can refer to the temperature of the PGM sites, and T2 can refer to the temperature of the exhaust gas. Since heat transfer by radiation follows the 4th

Fig. 14. Projected difference between inferred PGM temperature and exit gas temperature as a function of exit gas temperature assuming constant heat transfer by radiation, compared to experimental data from Fig. 13. Projections performed for constant Pt emissivity and for Pt emissivity increasing with temperature.

The temperature of the precious metal sites on a 0.64 cm long monolithic platinum/potassium/Al2O3 lean NOx trap has been inferred during CO oxidation from the conversion of a trace amount of hydrocarbon and a HC conversion versus temperature calibration curve. For an exhaust stream containing 5% O2, 10% H2O, 10% CO2, and 200 ppm of the HC in N2, CO was injected to generate concentrations of 2%, 3%, or 5% for periods of time ranging from 5 to 180 s. The inferred temperatures of the precious metal sites on the LNT were in good agreement with tracer gases of propane, butane, hexane, and methane (when experimental conditions permitted its use). The steady-state precious metal temperature exceeded the gas temperature by as much as 130 8C, attributable to the much higher heat capacity of the exhaust relative to that of platinum. Also, this temperature difference provided the driving force for heat transfer from the platinum sites to the exhaust gas. The difference between the precious metal temperature and the gas temperature decreased with increasing gas temperature. This suggested that radiation plays an important role in transferring heat from the precious metal sites during exothermic reactions. When the technique was applied to two or three short monolithic samples in series, the precious metal temperatures in the second and third samples were similar to the temperatures of the exhaust gas exiting those samples. This result supported the validity of the technique, since essentially all of the CO was oxidized on the first catalyst sample.

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