Effect of Thermal Degradation on the CO, C3H6, and NO Oxidation

Mar 18, 2014 - (14) observed that C3H6 oxidation follows back-to-front light-off and that the ignition front of a thermally aged sample moved more slo...
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Effect of Thermal Degradation on the CO, C3H6, and NO Oxidation Performance of Pt/Al2O3 with a Zoned Distribution of Pt Ali Abedi and William Epling*,† Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario N2T 2S7, Canada S Supporting Information *

ABSTRACT: Temperature-programmed oxidation (TPO) experiments were used to evaluate the performance of uniformly (standard) and nonuniformly (zoned) distributed Pt/Al2O3 catalysts after homogeneous and heterogeneous thermal degradation. Both catalysts were homogeneously aged in an oven by exposing the entire catalyst to 750 °C in air for 4 h at atmospheric pressure. In heterogeneous aging, only the back of the catalysts was exposed at 675 °C, whereas the temperature at the inlet section did not exceed 550 °C. This was accomplished by introducing pulses of C3H6 and O2, with an inert gas between each pulse, for 50 cycles and using the heat generated by the exothermic oxidation reaction to thermally age zones of the samples. The zone-coated catalyst, with more Pt concentrated in the front of the monolith, showed better performance than the standard sample after heterogeneous aging. The reason is that most of the Pt in the zoned sample, which was located in the front half, was not affected by the more significant aging at the back of the monolith. On the other hand, at a higher total flow rate and higher temperature, the performance of the homogenously aged zoned catalyst was worse than that of the standard sample, because the effect of the Pt-rich upstream area of the zoned catalyst was lost through the formation of larger particles, widening the reaction zone into the less Pt-rich downstream region.

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INTRODUCTION Catalyst deactivation is common problem in diesel engine exhaust aftertreatment systems, where catalyst deactivation leads to a gradual decrease in performance, resulting in increased emissions from the vehicle. Catalysts can deactivate by a variety of mechanisms, including sulfur poisoning, mechanical deterioration, and thermal aging.1 With the new restrictions on sulfur content in diesel fuel, new-generation ultralow-sulfur fuels are expected to contribute less than 1 ppm to the total sulfur content of the exhaust gas,2 but this does not eliminate the issue because of accumulation over time. Thermal degradation occurs when the catalyst is exposed to high temperatures, from a hot inlet exhaust gas or from exothermic oxidation reactions occurring on the catalyst surface. The focus of this study is the latter degradation mechanism, thermal aging, of a diesel oxidation catalyst (DOC). Even though diesel engine exhaust temperatures are considered to be relatively low, the DOC can be exposed to higher temperatures as a result of heat generation from exothermic hydrocarbon oxidation and during the regeneration of the diesel particulate filter (DPF),3 which requires temperatures in excess of 550 °C for soot oxidation to commence. For example, during the regeneration of a DPF, the DOC bed temperature can reportedly reach 850 °C.4 In addition, during the lean-NOx-trap (LNT) desulfation process, the DOC can be exposed to temperatures of around 650 °C, as temperatures in excess of 600 °C are required to remove sulfur from an LNT.2,3 When the catalyst is exposed to such high temperatures, catalyst particles aggregate to form larger particles on the surface of the catalyst.5−8 This sintering effect results in a decrease in catalytic activity due to a loss of active surface area. With sintering, the DOC performance drops, typically expressed as an increase in reaction light-off © 2014 American Chemical Society

temperature, ultimately resulting in the reduced conversion of key species. In addition, DOC thermal deactivation can influence the performance of aftertreatment components downstream of the DOC, such as the selective-catalyticreduction (SCR) and LNT catalysts. For example, a decrease in NO-to-NO2 conversion over a DOC can reduce the SCR efficiency, as the fast SCR reaction requires a NO/NO2 ratio of 1:1,9 and reduce the LNT efficiency, as LNT catalysts trap NO2 more readily than NO.1 Bimetallic catalysts have been used to increase the thermal resistance of DOCs. For example, research has shown that adding Pd to Pt-based DOCs lowers the mobility of the particles on the surface, thereby reducing the sintering effect and increasing the thermal stability of the catalyst.10,11 Homogenous aging, by placing the monolith in a furnace so that the entire monolith is exposed to a constant temperature, is a common laboratory thermal degradation technique. However, several studies have shown that, in real applications, thermal degradation is nonuniform.8,12−14 Lambert et al.12 found that the outlet of the DOC deteriorated the most significantly, after aging a light-duty diesel truck aftertreatment system to the equivalent of 120000 miles. This is because the outlet of the DOC is exposed to the highest temperatures over time, being relatively adiabatic and catalyzing a variety of exothermic reactions. On the other hand, Winkler et al.,5 who also tested the catalytic activity of an engine-aged DOC, found that the degradation was nonuniform, with the front section being more affected than the rest of the catalyst. However, in Received: Revised: Accepted: Published: 5692

October 4, 2013 March 7, 2014 March 18, 2014 March 18, 2014 dx.doi.org/10.1021/ie403317w | Ind. Eng. Chem. Res. 2014, 53, 5692−5700

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Al2O3-washcoated cordierite monolith, with a 1.59 g/in.3 Al2O3 loading and a 325 cell/in.2 (cpsi) cell density, was provided by Johnson Matthey. Samples were cut to 1 in. in diameter and 3 in. in length. Five grams of tetraammine platinum(II) nitrate [Pt(NH3)4(NO3)2, Alfa Aesar] was first dissolved in 250 mL of water. To make a uniform catalyst with 1% Pt loading (1 wt % Pt/Al2O3, i.e., excluding the monolith material), the sample was dipped twice into the Pt solution, and each time, the loading provided 0.5 wt % Pt. The zoned catalyst with 1.5 wt % Pt loading in the front and 0.5 wt % Pt loading in the back was prepared by dipping the whole catalyst once and the front half two more times. All Pt loadings are reported as weight percentages relative to the Al2O3 loading. Between each exposure, the sample was dried in air overnight and heated in an oven to 300 °C for 1 h. Finally, both samples were calcined in air at 500 °C for 4 h. The amounts of Pt on the two catalysts were measured using the inductively coupled plasma (ICP) technique, with a Prodigy high-dispersion ICP spectrometer. Sample washcoat was scraped from the cordierite support and characterized. The homogeneous/standard sample contained 1.01 wt % Pt, and the front and back parts of the zoned sample contained 1.50 and 0.55 wt % Pt, respectively. The Pt/Al2O3 monolith samples were thermally aged using two different aging protocols, representing heterogeneous and homogeneous aging of a DOC. The same protocols were used for both the standard and zoned samples. In the first aging protocol, to heterogeneously age the back of the sample, the front of the monolith was exposed to 50 pulses of 7000 ppm C3H6, 10% O2, and 5% H2O, balanced with N2, for 10 s at 500 °C and a gas hourly space velocity (GHSV) of 25000 h−1. To avoid overheating the catalyst, an inert phase consisting of 5% H2O balanced with N2 was pulsed into the reactor for 150 s after each C3H6-containing pulse. As a result, the exothermic C3H6 oxidation reaction increased the temperature of the front section of the monolith to 680 °C after 50 cycles, whereas the temperature of the back of the catalyst did not exceed 550 °C. The reactor used was nonadiabatic; thus, we had to thermally damage the front and then switch the position of the sample in the reactor (flip it) to simulate a sample that is more heavily aged on the downstream section, as was noted by Lambert et al.12 In the second aging protocol, homogeneous aging, the samples were placed in an oven at atmospheric pressure and 750 °C for 4 h. For reaction performance testing, the sample was wrapped with 3M insulation and placed in a horizontal quartz tube with two thermocouples, one in the front and one in the back of the monolith, each about 1 mm inside the sample and radially centered. The performances of the heterogeneously and homogenously aged samples were tested using temperatureprogrammed oxidation (TPO) experiments. The inlet gas consisted of CO, C3H6, NO, or a mixture of C3H6 and NO, in the presence of 10% O2, 5% H2O, 5% CO2, and N2 as the balance gas. The inlet gas was fed into the reactor at 80 °C, and then the temperature was ramped at 5 °C/min to 500 °C. TPO experiments were run at two flow rates: 10 and 28 L/min. To ensure the consistency of the experimental conditions, the catalyst was pretreated before each experiment by flowing 10% O2, 5% CO2, and 5% H2O with a N2 balance at 500 °C for 20 min, after which the temperature was decreased to 80 °C with N2 only. The dispersions of the active sites over the surface of the catalysts were measured before and after aging by H 2

the analysis performed, the authors showed that the particle sizes at the back of the catalyst were larger than in the front, indicating that more thermal degradation had indeed occurred at the outlet section of the DOC. The poorer performance of the front of the catalyst was directly related to chemical poisoning by S, Zn, P, Mg, and Ca. Using IR thermography and spatially resolved capillary-inlet mass spectrometry (SpaciMS), Shakir el al.14 observed that C3H6 oxidation follows back-tofront light-off and that the ignition front of a thermally aged sample moved more slowly toward the inlet than that of a nonthermally degraded sample. Using the same technique and methodology, Russell et al.13 confirmed Shakir et al.’s results, observing an increase in time for the temperature and concentration waves to travel through the catalyst during back-to-front ignition. Furthermore, the reaction zones were spread farther into the catalyst relative to those observed before aging. In our previous study, unaged zoned and standard catalysts were compared.15 The zoned catalyst with more Pt located in the upstream portion and less located downstream showed better performance for CO and C3H6 oxidation at higher flow rates because of greater heat generation at the front by the exothermic reaction and a decreased self-poisoning effect downstream. In a NO/C3H6 mixture, the zoned catalyst showed higher NO conversion than the standard catalyst because most of the C3H6 was oxidized in a smaller volume in the front of the catalyst, leaving the rest of the catalyst available for NO oxidation. In contrast, the standard catalyst utilized a larger area to oxidize C3H6, thus leaving a smaller volume for NO oxidation. In this study, the influence of homogeneous and heterogeneous thermal degradation on the performances of a uniformly distributed (standard) catalyst and a nonuniformly distributed (zoned) catalyst was evaluated. Using the same amount of Pt for both samples, the Pt was homogeneously distributed on the standard sample, whereas the zoned sample had more Pt in the front section and less Pt in the back. To compare the performances of the two samples under different thermal aging situations, two aging protocols were used, namely, homogeneous and heterogeneous thermal aging, both representing different conditions ecountered in real applications. In homogeneous aging, the sample is heated in a furnace to a chosen temperature so that no temperature gradients exist in the catalyst. This method represents the case where the catalyst is exposed to constant and high temperatures, such as when the exhaust gas temperature is the only source of heat. In heterogeneous aging, the sample is thermally degraded by the heat of exothermic reactions. The heat released from the exothermic reaction causes an increase in the temperature of the zone where the reaction takes place, which causes a gradient in temperature in the catalyst. These temperature gradients lead to different degrees of thermal degradation along the axial length. This method represents the case when exothermic HC or CO oxidation occurs on the catalyst, causing an increase in temperature where the reaction occurs and, thus, in distinct catalyst zones. In real applications, degradation results from a combination of both exhaust-gas temperature and exothermic reactions.

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EXPERIMENTAL METHODS The uniformly (standard) and nonuniformly (zoned) distributed catalysts used in this study were prepared using the same method as described in previous work.15 In summary, an 5693

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chemisorption. For the fresh sample, two samples were prepared simultaneously, and we assumed that they had the same initial dispersions. For the measurements, different portions of the monolithic sample were crushed to fine powder. Then, the powder was loaded into a Hiden Catalab microreactor and was exposed to 26 pulses of 100 μL of 5% H2 with a He balance for 30 s, with 1 min of pure He between each pulse. Before each measurement, the catalyst was pretreated in 100 mL/min He at 25 °C for 25 min, after which 5% H2 was added and the temperature was ramped at 10 °C/min to 450 °C. The catalyst was exposed to 450 °C for 30 min, after which the H2 was turned off and the temperature was decreased to 25 °C before starting the H2 pulses. H2 chemisorption was measured based on irreversible H2 adsorption on a Pt site with a H2/Pt ratio assumed to be 1:2. Table 1 lists the Pt dispersions of the standard (STD) and zoned (Z) samples before and after thermal aging as determined by H2 chemisorption. Table 1. Pt Dispersions (%) of the Standard and Zoned Samples before and after Thermal Aging sample

fresh

heterogeneously aged

homogeneously aged

standard front standard back zoned front zoned back

11.7 11.7 8.5 39

11.0 2.9 6.0 4.9

2.4 2.4 1.1 3.8

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RESULTS AND DISCUSSION Heterogeneous Aging Protocol. Heterogeneous thermal degradation was simulated using a technique previously used to characterize C3H6 oxidation changes as a function of catalyst age.13 C3H6 and O2 are added to the feed gas in pulses, with an inert pulse between them. The catalyst temperature is initially high enough to guarantee that complete conversion will occur in a very narrow region at the upstream section of catalyst. The exothermic oxidation reaction causes a substantial amount of heat to be generated at and near the catalyst inlet face. Because the reactor is nonadiabatic, heat is lost to the surroundings, and the temperature decreases along the length of the catalyst. The temperature results obtained at four positions along each of the two samples during the heterogeneous thermal degradation protocol are shown in panels A and B of Figure 1 for the standard sample and the zoned sample, respectively. As shown, the temperature at the front of the catalyst was approximately 500 °C before the first pulse and returned to that value during the inert phase. During the reactive pulse, the temperature increased to 675 °C. At downstream positions, the temperature did not increase as much, with the back of the catalyst achieving only 548 °C. Thus, a temperature gradient was imposed, leading to different extents of thermal degradation throughout the catalyst. There were slight differences between the two samples in terms of highest temperature reached, but they were less than 5 °C. To simulate results obtained from on-engine tests, the samples were rotated after heterogeneous aging, so that the inlet side of the catalyst was the less thermally damaged side. Temperature-programmed oxidation (TPO) of CO, C3H6, and NO individually and of C3H6/NO as a mixture were analyzed. Throughout this article, the results obtained during the C3H6 oxidation experiments are shown, along with the data obtained for the mixture. CO and NO oxidation followed similar trends, so those data are not be presented in detail, but the results are available in the Supporting Information.

Figure 1. Temperature data obtained at different locations within the (A) standard and (B) zoned samples during heterogeneous aging.

C3H6 Oxidation. The results of C3H6 TPO experiments from the fresh and heterogeneously aged samples at low flow rate, 10 L/min, are shown in Figure 2. In all figures, solid and dashed lines represent data obtained from the samples before and after aging, respectively. A previous study comparing the

Figure 2. C3H6 conversion as a function of temperature before and after heterogeneous aging. The feed gas consisted of 1000 ppm C3H6, 10% O2, 5% CO2, and 5% H2O, balanced with N2 at 10 L/min. 5694

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performances of unaged standard and unaged zone-coated Pt/ Al2O3 catalysts demonstrated that the zoned catalyst, with a larger amount of Pt concentrated at the inlet, reached light-off at a lower temperature. With C3H6 oxidation in DOCs following a back-to-front ignition pattern under these conditions,8,13−18 the properties of the reaction as a function of temperature can be summarized as follows: at low temperature, C3H6 oxidation is inhibited by strong C3H6 adsorption on Pt, limiting oxygen access. As the catalyst temperature increases, some C3H6 desorbs, allowing oxygen to access the active sites and C3H6 to be oxidized. Consequently, exothermic heat is generated, thereby decreasing the effect of C3H6 self-poisoning and allowing the reaction to proceed more rapidly, which is sometimes called light-off. There is a significant change in the reaction rate as a function of temperature, and this inflection represents the time, or temperature, at which the reaction zone begins to propagate from the outlet of the catalyst sample toward the front. This shift in the reaction rate occurs at lower temperature for the zoned sample than for the standard sample. The standard (nonzoned) sample was more negatively affected after heterogeneous aging than the zoned sample. For example, as shown in Table 2, the difference in the inlet

Figure 3. C3H6 conversion as a function of temperature before and after heterogeneous aging. The feed gas consisted of 1000 ppm C3H6, 10% O2, 5% CO2, and 5% H2O, balanced with N2 at 28 L/min.

(Supporting Information) and for NO in Figures S3 and S4 (Supporting Information). Overall, for the single-reactant experiments, such as C3H6 oxidation, CO oxidation, and NO oxidation, the superior performance of the zoned sample over the standard sample was maintained at different flow rates after heterogeneous aging. Oxidation of C3H6 and NO as a Mixture. The performances of the standard and zoned samples were tested with a mixture of the reactants C3H6 and NO. The results obtained during TPO experiments, at the lower flow rate, after heterogeneous aging are shown in Figures 4 and 5. Table 3 lists the 50% C3H6 conversion temperatures, T50, and the 10% NO conversion temperatures, T10.

Table 2. T50 of C3H6 Oxidation for the Standard and Zoned Samples before and after Heterogeneous Aging T50 (°C) sample

FR (L/min)

fresh

aged

ΔT50 (°C)

standard zoned standard zoned

10 10 28 28

150 145 156 147

167 154 179 162

17 9 23 15

temperature corresponding to 50% conversion, ΔT50, between the heterogeneously aged sample and the fresh sample was 17 °C for the standard sample, whereas it was 9 °C for the zoned sample. The reason for the better performance of the zoned sample after heterogeneous aging is that heterogeneous aging affected the back of the catalyst. Thus, the zoned sample, which contained less catalyst in the back and more catalyst in the front, was less affected; that is, a proportionally higher amount of active sites in the downstream section were affected (sintering of the precious metal) on the sample with the uniform distribution. Therefore, the light-off temperature of the zoned sample was lower than that of the standard sample. It has been reported in the literature that a DOC is likely to suffer more extensive thermal degradation (sintering) at the outlet compared to the front;12 therefore, these results suggest that a zoned sample, with more catalyst concentrated in the front part, can enhance durability if heterogeneous thermal aging is the primary degradation mode, at least for the temperatures studied. At the higher flow rate, 28 L/min, similar trends were observed after heterogeneous aging, as shown in Figure 3. The difference in ΔT50 between the zoned sample and the standard sample was 8 °C, for both low and high flow rates. Similar results were observed with CO oxidation and NO oxidation; therefore, the results obtained during CO and NO TPO are not shown in this section for brevity. Oxidation data for the two samples before and after the heterogeneous thermal degradation protocol are plotted for CO in Figures S1 and S2

Figure 4. C3H6 conversion as a function of temperature before and after heterogeneous aging. The feed gas consisted of 1000 ppm C3H6, 200 ppm NO, 10% O2, 5% CO2, and 5% H2O, balanced with N2 at 10 L/min.

For both samples, C3H6 oxidation began at higher temperatures than in the absence of NO. NO is known to inhibit C3H6 oxidation,19 likely through competition for active sites. Therefore, although C3H6 oxidation can commence earlier in the absence of NO, with C3H6 desorption occurring and oxygen then being available for the reaction, in the presence of NO, once some C3H6 desorbs, NO then adsorbs, continuing to 5695

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At the higher flow rate, the effect of heterogeneous aging was more significant on both samples, yet the zoned sample maintained its superior performance over the standard sample, as seen in Figures 6 and 7. The change in T50, ΔT50, for the

Figure 5. NO conversion as a function of temperature before and after heterogeneous aging. The feed gas consisted of 1000 ppm C3H6, 200 ppm NO, 10% O2, 5% CO2, and 5% H2O, balanced with N2 at 10 L/ min.

Table 3. T50 of C3H6 Oxidation and T10 of NO Oxidation in the C3H6 + NO Mixture for the Standard and Zoned Samples before and after Heterogeneous Aging T50 (°C)

Figure 6. C3H6 conversion as a function of temperature before and after heterogeneous aging. The feed gas consisted of 1000 ppm C3H6, 200 ppm NO, 10% O2, 5% CO2, and 5% H2O, balanced with N2 at 28 L/min.

T10 (°C)

sample

FR (L/min)

fresh C3H6

aged C3H6

fresh NO

aged NO

standard zoned standard zoned

10 10 28 28

201 °C 197 206 201

218 202 241 217

207 202 214 203

221 203 250 223

block active sites. Once some NO can desorb, then oxygen can adsorb to the active sites, dissociate, and lead to oxidation. Also, when NO oxidation results were compared to those in the absence of C3H6 (Figure S3, Supporting Information), it was apparent that C3H6 also inhibited NO oxidation at low temperature. Previous research has shown that this effect is directly related not to active-site competition, but to product NO2 being consumed as an oxidant in C3H6 oxidation. Thus, when C3H6 is present, any NO2 formed readily reacts with the C3H6 and is reduced primarily back to NO.19,20 With the combined C3H6 and NO mixture, the zoned sample showed better performance than the standard sample at the lower flow rate, and the effect of heterogeneous aging was more significant on the standard sample than on the zoned sample. For example, in the case of C3H6 oxidation, the light-off temperature, T50, of the zoned sample increased by 5 °C, whereas that of the standard sample increased by 17 °C after heterogeneous aging. Similarly, in the case of NO oxidation, T10 for the zoned sample barely increased, by only 1 °C, whereas for the standard sample, it increased by 14 °C. Because the heterogeneous aging affected the back of the monolith, for the zoned sample, with less catalyst in the back and more concentrated in the front, most of the C3H6 was oxidized in a smaller volume of sample, leaving more volume for NO oxidationas was also observed when characterizing the reaction patterns of unaged samples.15 Once all or most of the C3H6 is oxidized, then none of the NO2 formed will be consumed; thus if a smaller amount of catalyst is used to completely oxidize the C3H6, more is available for NO oxidation.

Figure 7. NO conversion as a function of temperature before and after heterogeneous aging. The feed gas consisted of 1000 ppm C3H6, 200 ppm NO, 10% O2, 5% CO2, and 5% H2O, balanced with N2 at 28 L/ min.

standard sample was about twice that for the zoned sample after heterogeneous aging. For instance, in the case of the standard sample, ΔT50 was 35 °C for C3H6 oxidation, and ΔT10 was 36 °C for NO oxidation, whereas for the zoned sample, the values were 16 and 20 °C for C3H6 and NO oxidation, respectively. Therefore, the advantage of the zone-coated monolith is more pronounced under harsher conditions, such as higher flow rate and thermal aging, as has been observed in previous studies.15,21 Homogeneous Aging Protocol. The second method used for thermal deactivation was homogenously aging the samples by placing the monolith in a furnace so that the entire monolith was exposed to a constant temperature. This is a common laboratory thermal degradation technique. The 5696

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samples were placed in an oven at atmospheric pressure and 750 °C for 4 h. As for heterogeneous aging, the TPOs of CO, C3H6, NO, and a C3H6/NO mixture were performed; again however, data obtained during CO and NO oxidation are presented in the Supporting Information (Figures S5−S8), as they followed the same trends as the C3H6 data. C3H6 Oxidation. C3H6 TPO results are shown in Figure 8 for the fresh and homogeneously aged samples at the lower

Figure 9. C3H6 conversion as a function of temperature before and after homogeneous aging. The feed gas consisted of 1000 ppm C3H6, 10% O2, 5% CO2, and 5% H2O, balanced with N2 at 28 L/min.

the zoned and standard samples was greater at the higher flow rate than at the lower flow rate (Table 4). Unlike in the previous cases, at this higher flow rate and high conversions/ temperatures, the performance of the standard sample was better than that of the zoned sample. For example, as illustrated in Figure 9, at around 200 °C, corresponding to 60% C3H6 conversion, the rate of increase in C3H6 conversion with the zoned sample decreased, and the two samples resulted in similar conversions at 245 °C. As the temperature was increased beyond 245 °C, the performance of the standard sample was better than the performance of the zoned sample. The reason for this difference is that, as the temperature increased (T > 245 °C), the reaction zone moved toward the front of the monolith, where more of the active sites on the zoned sample were located. After homogeneous aging, the inlet half of the zoned sample had a Pt dispersion of 1.1%, compared to 2.4% for the standard sample. Moreover, because complete conversion was not attained, the back half of the monolith was obviously also catalyzing some of the reaction. If the performance loss was greater in the higher-density Pt front half of the zoned sample, then the reaction zone could not shift as far to the front, leaving more of the catalysis to be done by the back, which had a lower active-site density. Thus, with the significantly decreased dispersion in the front and the lower loading of Pt in the back, the reaction zone was spread through a wider portion of the zoned sample, and this sample therefore did not perform as well as the standard sample at high temperatures (high conversions). This result is also consistent with previous observations made in which thermally degrading the catalyst widened the reaction zone and slowed its propagation from back to front.13 To validate the reaction-zone position, spatially resolved capillary inlet mass spectrometry (SpaciMS) was used to evaluate the catalytic activity of the homogenously aged samples at different locations inside the monolith. C3H6 concentrations were measured at 270 °C under the higherflow-rate condition. The results are shown in Figure 10. The reaction zone of the standard sample reached the front section of the monolith, where more than 90% of C3H6 was converted in the first 3 cm of the monolith; however, the zoned sample converted only 10% of the C3H6 in the same location. These results further validate the discussion above: at high temperatures, when the reaction zone reached the front of the

Figure 8. C3H6 conversion as a function of temperature before and after homogeneous aging. The feed gas consisted of 1000 ppm C3H6, 10% O2, 5% CO2, and 5% H2O, balanced with N2 at 10 L/min.

Table 4. T50 of C3H6 Oxidation for the Standard and Zoned Samples before and after Homogeneous Aging T50 (°C) sample

FR (L/min)

fresh

homogeneously aged

ΔT50 (°C)

standard zoned standard zoned

10 10 28 28

150 °C 145 156 147

198 178 221 191

48 33 65 44

flow rate, 10 L/min. Table 4 lists the light-off temperatures, T50, before and after homogeneous aging. As for heterogeneous aging, the standard sample was more severely affected by homogeneous aging at the lower flow rate than the zoned sample. For example, the difference in light-off temperature, ΔT50, between the fresh and aged zoned sample was 15 °C lower than that of the standard sample. In addition, the performance after heterogeneous aging was better than that after homogeneous aging. The reasons for this behavior are as follows: (1) During homogeneous aging, the sample was exposed to higher temperature than during heterogeneous aging, and (2) during heterogeneous aging, only the back of the catalyst was affected by exposure to high temperature, whereas in homogeneous aging, the whole catalyst was exposed to high temperatures. Therefore, in all cases, the light-off temperature of the heterogeneously aged sample was lower than that of the homogeneously aged sample. The C3H6 TPO data obtained at the higher flow rate, 28 L/ min, are shown in Figure 9. At low temperature/conversion, the performance of the zoned sample was better than that of the standard sample. In fact, the difference in ΔT50 values between 5697

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Figure 10. C3H6 conversion as a function of position at 270 °C measured using SpaciMS after homogeneous aging. The feed gas consisted of 1000 ppm C3H6, 10% O2, and 5% H2O, balanced with N2 at 28 L/min.

Figure 12. NO conversion as a function of temperature before and after homogeneous aging. The feed gas consisted of 1000 ppm C3H6, 200 ppm NO, 10% O2, 5% CO2, and 5% H2O, balanced with N2 at 10 L/min.

monolith, the performance of the zoned sample, which contained larger particles in the front section of the monolith, became worse than that of the standard sample. Oxidation of C3H6 and NO as a Mixture. The performances of the homogeneously aged standard and zoned samples for the C3H6 and NO mixture were also evaluated. The TPO results are shown in Figures 11 and 12, and the T50 and

Table 5. T50 of C3H6 Oxidation and T10 of NO Oxidation in the C3H6 + NO Mixture for the Standard and Zoned Samples before and after Homogeneous Aging T50 (°C)

T10 (°C)

sample

FR (L/min)

fresh C3H6

aged C3H6

fresh NO

aged NO

standard zoned standard zoned

10 10 28 28

201 197 206 201

246 216 268 235

207 202 214 203

253 213 296 260

temperature, ΔT10, between the fresh and aged standard sample was more than four times that of the zoned sample after homogeneous aging. As mentioned above, this is due to the C3H6 being oxidized in a smaller volume of sample, leaving more volume for NO oxidation. Once all or most of the HC or CO has been oxidized, then none of the NO2 formed will be consumed, so if a smaller amount of catalyst is used to completely oxidize the HC or CO, then more is available for NO oxidation. At higher temperatures (T > 277 °C), however, the performance of the standard sample became better than that of the zoned sample, and the standard sample reached higher conversion, as shown in Figure 12. Just as with C3H6only oxidation, this indicates that, after homogenous aging of the samples, the effect of sintering became more noticeable in the front of the zoned sample, especially at high temperatures. C3H6 oxidation is known to follow back-to-front light-off;13,14 thus as the temperature increased, the reaction zone moved toward the front of the monolith. Because the homogeneous aging had a greater sintering effect on the front section of the zoned sample, at high temperature, when the reaction zone reached the front of the zoned sample, the reaction rate of the zoned sample dropped whereas that of the standard sample continued at the same rate of conversion. The C3H6 and NO conversions as functions of temperature for both the standard and zoned samples with the C3H6 + NO mixture at the higher flow rate are shown in Figures 13 and 14. Again, the performance of the standard sample was better than that of the zoned sample at high temperature/conversion. In the higher-conversion/higher-temperature regime, C3H6 oxidation was spread through more of the catalyst, as shown in

Figure 11. C3H6 conversion as a function of temperature before and after homogeneous aging. The feed gas consisted of 1000 ppm C3H6, 200 ppm NO, 10% O2, 5% CO2, and 5% H2O, balanced with N2 at 10 L/min.

T10 values listed in Table 5. As for the heterogeneously aged samples, the inhibition effects of C3H6 and NO were observed after homogeneous aging, as the light-off temperatures for C3H6 and NO oxidation were higher in the mixture than in the singlereactant experiments. The performance of the homogeneously aged zoned sample was better than that of the standard sample in the mixture at the lower flow rate. The difference in C3H6 light-off temperature, ΔT50, between the unaged and aged standard sample was more than twice that of the zoned sample after homogeneous aging. In terms of NO oxidation, at low temperature/conversion (T < 277 °C), better conversions were attained with the zoned sample at the lower flow rate. The difference in NO light-off 5698

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the higher level of Pt in the back half led to even more C3H6 oxidation, leaving some volume for observable NO oxidation because of the lower amount of C3H6 remaining.

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CONCLUSIONS TPO experiments were used to compare the performances of standard and zone-coated Pt/Al2O3 catalysts after heterogeneous and homogeneous aging. In most cases, the zone-coated catalyst showed better performance than the standard sample; however, after homogeneous aging, at the higher flow rate and at higher temperatures, the zone-coated catalyst was poorer because of a more significant loss in the inlet, Pt-rich zone. This inlet zone is key to high conversions, and the slower propagation of the reaction front toward the catalyst inlet under high-space-velocity conditions led to the decreased performance. The performance of the zoned sample after heterogeneous aging was similar to that of the fresh catalyst, because most of the Pt particles, concentrated in the front section, were not affected by the higher extent of thermal degradation at the back of the sample, which contained less Pt. In the case of a C3H6 and NO mixture, the difference between the standard and zoned catalysts was greater at higher flow rate because of a more significant effect of convective heat transfer, where the light-off temperature was higher, and thus, when it did occur, the hotter particles at the front of the zoned catalyst resulted in higher conversions more quickly. As for the singlereactant reactions, after heterogeneous aging, the performance of the zoned catalyst was better than that of the standard catalyst because the damage was less significant in the upstream volume where most of C3H6 was oxidized, thus leaving a larger volume available for NO oxidation.

Figure 13. C3H6 conversion as a function of temperature before and after homogeneous aging. The feed gas consisted of 1000 ppm C3H6, 200 ppm NO, 10% O2, 5% CO2, and 5% H2O, balanced with N2 at 28 L/min.

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ASSOCIATED CONTENT

S Supporting Information *

Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 14. NO conversion as a function of temperature before and after homogeneous aging. The feed gas consisted of 1000 ppm C3H6, 200 ppm NO, 10% O2, 5% CO2, and 5% H2O, balanced with N2 at 28 L/min.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address †

Department of Chemical and Biomolecular Engineering, University of Houston, Houston, TX 77204.

Figure 13, with C3H6 still observed in the effluent even at T > 300 °C, so that a significant portion of the NO2 being made could be consumed as an oxidant. Because the standard sample performed better in C3H6 oxidation (i.e., the C3H6 oxidation reaction zone was shifted more toward the front than that of the zoned sample), this resulted in more available catalyst for NO oxidation and thus better conversions. In summary, at the higher flow rate and after homogeneous thermal degradation, the width of the reaction zone led to better performance by the standard sample. The higher Pt loading in the front half of the zoned sample led to a higher level of sintering, and the larger particles were less effective in C3H6 oxidation. Thus, the reaction front did not propagate fully to the front because of the loss of exposed Pt surface area. The back half, having less Pt, was unable to oxidize a substantial amount of C3H6, such that any NO2 formed was then used to oxidize the remaining C3H6. In the standard sample, although sintering still occurred, the extent was not as dramatic, and enough of the Pt in the front half was active enough to oxidize more of the C3H6, and

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the Canadian Foundation for Innovation, the Natural Science and Engineering Research Council of Canada, and the Province of Ontario’s Early Research Award for financial support of the work presented herein.

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REFERENCES

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