Article pubs.acs.org/est
Low Temperature Activation of CO Removal by O3‑Assisted Catalysis Sungkwon Jo, Kwan-Tae Kim, Dae Hoon Lee,* Young-Hoon Song, and Jae-Ok Lee Korea Institute of Machinery and Materials, 156 Gajeongbuk-ro, Yuseong-gu, Daejeon 305-343, Republic of Korea
Taewoo Lee and Hyun-Sik Han Heesung Catalysts Corporation, 507-1 Da, 1251-6 Jungwang-Dong, Shiheung City, Kyungki-Do 429-450, Republic of Korea ABSTRACT: Catalytic CO oxidation was activated at low temperature by injecting O3 as an additive. It was empirically confirmed that CO removal rate was dramatically enhanced by supplying a small amount of O3, and the reaction temperature was almost half that required for CO oxidation when using a catalyst only. By optimizing the concentration of O3, catalytic CO oxidation could be achieved within 1 min at low operational temperature. The removal rate of CO was sensitive to the concentration of O3, and a deduced reaction mechanism is discussed to explain how catalytic CO oxidation is activated but subsequently deactivated at higher O3 concentration. Moreover, the presence of C3H8 and C3H6 were considered to evaluate the effects of each gas on the enhancement of CO removal rate by O3. Finally, the rate of CO removal was evaluated with increasing O3 concentration for practical applications such as the cold-start problem in automobile engines.
1. INTRODUCTION In the automobile industry, many researchers have been interested in the treatment of exhaust emissions from gasoline and diesel engines.1−4 The trends in the development of both gasoline and diesel engines are toward highly fuel-efficient engines that produce cooler exhaust gas. As a result of a reduction in the temperature of exhaust gas, the development of treatment technologies should focus on the removal of hydrocarbons (HCs) and carbon monoxide (CO). Even though this advancement leads to efficient utilization of fuels, the lower exhaust gas temperature results in different compositions of emissions. At low operational temperature, emissions of HCs and CO become severe issues, whereas the previous problem of NOx emission becomes less important.5 For automobile exhaust treatment, three-way catalysts (TWCs) or diesel-oxidation catalysts (DOCs) that were already widely used can activate only at the light-off temperature above. Lower temperature exhaust gas therefore unfavorably affects the catalytic reaction of emission gases, thereby exacerbating emissions. In order to overcome these issues, various trials have sought to activate TWCs or DOCs at low temperatures.6−8 Recently, the plasma−catalyst combination has been investigated using the advantages of both high selectivity induced by the catalyst and low-temperature activation driven by plasma. Recent studies have reported the synergistic effect of using this combination in removal of volatile organic compounds (VOCs),9−11 methane reforming,12−14 methanol reforming,15 and carbon dioxide reduction.16,17 For the treatment of automobile exhaust gas, various configurations have been © XXXX American Chemical Society
proposed using the combination of nonthermal plasma and industrially proven honeycomb catalysts, and those studies demonstrated the synergistic effects of combining a catalyst and plasma generation.18−21 However, there remain limitations in application to real engines due to the excessive energy consumption involved in plasma generation and the difficulty in realizing the proposed systems in vehicles. In this work, we focus on the removal of CO emitted from automobile engines at low temperature. HCs emitted prior to light-off could be captured by an adsorbent until normal operation of the catalyst, and this can give several options to eliminate HCs by operational processes of engines. However, it is difficult to adsorb CO, and the emission must be continuously removed by catalytic oxidation during the coldstart period. As one promising method, we investigated O3 injection for enhancing catalytic CO oxidation at relatively low temperature, in comparison to the performance when using a catalyst only. Ozone (O3) can be easily produced by a dielectric barrier discharge (DBD), and the device for generating O3 can be fabricated in a compact form suitable for real automobile applications.22 Prior to this work, the injection of O3 was proposed by Kirkpatrick et al., who reported engine bench-test results from coupling nonthermal plasma with diesel oxidation catalysis.20 However, the study focused the work on the realization of the system in engine bench but did not provided Received: July 10, 2014 Revised: November 3, 2014 Accepted: November 14, 2014
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was evaluated by gradually increasing the temperature of the reactant (see Figure 2). The initial concentration of supplied
detail mechanism on O3 assisted oxidation process. As a further investigation of the role of O3 in catalytic CO oxidation, we test CO removal rate by varying the concentration of O3 and the HC species. HC species that possibly exist in the exhaust gas are modeled by C3H8 and C3H6. Moreover, the mechanisms for the effect of O3 in catalytic CO oxidation are suggested based on the empirical results. Finally, we examine the maximum achievable rate of CO removal by adjusting the concentration of O3 at low temperature similar to a cold-start scenario, during which a large amount of CO is released.
2. EXPERIMENTAL SECTION The experimental setup mainly consists of an air blower, an electrical heater, an O3 generator, and a catalytic reactor. The schematic for the experimental apparatus is shown in Figure 1.
Figure 2. Variation of CO concentration with increasing reactant temperature, and temperature profiles at the locations before and after passing through the used catalyst.
CO was approximately 252 ppm, plotted as the dotted line in Figure 2. The reactant temperature was controlled based on the temperature measured at the location before the catalyst, and was increased from about 50 °C to about 120 °C. As a result of heat loss, the temperature measured at the location after the catalyst was much lower than that before the catalyst. It can be expected that the real temperature of the catalyst would be between the upstream and downstream temperatures measured here. For convenience, the upstream temperature, which is higher than that of the real catalyst, is used as a representative temperature when describing some phenomena related to catalyst temperature in this work. The amount of CO was reduced at the outlet side mainly because of oxidation, wherein CO was converted to CO2. However, at the start of the reaction, just adsorbed amount of CO on active sites without oxidation can possibly occupy certain portion of the reduced amount of CO. Considering this, all of the test conditions were applied after sufficient time with monitoring saturation state of CO to remove possible misleading of the CO conversion. Initial CO concentration was about 230 ppm at about 50 °C, and the concentration decreased with increasing reactant temperature. At approximately 90 °C, CO concentration dramatically decreased, and at temperatures of more than 120 °C the CO removal rate exceeded 90%. This result confirms that the tested catalyst requires an operational temperature of at least 90 °C for effective CO catalytic oxidation in the absence of O3 injection. The role of O3 in CO catalytic oxidation was investigated by measuring the variation in CO concentration with O3 injection into the catalyst reactor. O3 was injected at identical reactant temperature of about 55 °C, at which the removal rate of CO by catalytic reaction alone was approximately 10%. The experimental result using a fresh catalyst is shown in Figure 3. O 3 injection can be confirmed by measuring the concentration of O3, which showed that the concentration of CO dramatically decreased when O3 was supplied into the catalyst. When O3 was supplied at about 55 ppm, the CO concentration was additionally reduced by 150 ppm and the removal rate of CO was approximately 72% at approximately 55 °C. This removal rate corresponds to the performance at
Figure 1. Schematic diagram of experimental setup.
The air blower can supply a large volume of atmospheric air into the catalytic reactor, and the air flow rate can be controlled and monitored by a mass flow meter. The CO, C3H6, and C3H8 test gases were prepared as mixtures with air balance using gas cylinders. After mixing air and test gases, the mixture was heated by the electrical heater (LE-10000S, Leister). The heated mixture passed through a mixer with several swirling blades and was then supplied into the catalytic reactor. The O3 generator (OZC-62-2, Ozonetech) creates O3 by using O2, and the amount of O3 is linearly dependent on the power applied to a plasma reactor inside the O3 generator. The generated O3 was injected before the heated mixture was passed through the mixer. The O3 concentrations ranged from 55 to 260 ppm, corresponding to the applied power of 70 to 210 W. During experiments, temperature and the concentrations of CO and O3 were measured using a nondispersive infrared absorption analyzer (VA-3000 series, Horiba) for CO and a Lambert− Beer law O3 analyzer (OZM-5000G, Horiba) at measurement locations before and after the catalytic reactor. We used a flame ionization analyzer (FIA-510, Horiba) to measure total hydrocarbons. All measured data were collected by a data logger (34970A, Agilent) and synchronized data were used for analysis. Pt was used as a catalyst for CO oxidation, and a cordierite monolith was used as a catalytic support. After coating Al2O3 on the cordierite monolith, a coating up to 0.57 g/m3 of Pt was applied to the Al2O3. The cordierite monolith was of a commercially available shape (diameter 110 mm, length 94.5 mm) with a cell density of 400 cells per inch (cpi). In all experiments, total flow rate was fixed to about 750 LPM and the value of space velocity was about 50 000 h−1 considering the volume of the used catalyst.
3. RESULTS AND DISCUSSION 3.1. O3-Enhanced Reaction. Before supplying O3 into the catalyst reactor, the CO oxidation performance of the catalyst B
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site of the catalyst. Case (1) was evaluated by using only cordierite monolith without the alumina and catalyst. No meaningful conversion of CO via gas phase reaction below 100 °C was observed. Case (2) was evaluated by locating aluminacoated cordierite monolith without a catalyst on it. It was confirmed that the concentration of CO does not decrease by only Al2O3-coated monolith. Only under a high-temperature condition, (125 °C) a small amount of reduction was observed in CO concentration (18 ppm of conversion from 250 ppm). Based on these results, it is believed that the phenomenon that enhanced catalytic CO oxidation by supplying O3 was induced by the interaction between the Pt catalyst and O3. It is well-known that Pt catalyst can be easily deactivated by CO. Therefore, the experiment above was repeated with the catalyst deactivated. To simulate the deactivation of the catalyst, 250 ppm of CO was supplied for several hours. After that, the CO removal rate was evaluated as shown in Figure 4. The
Figure 3. Variation of CO concentration containing a fresh catalyst with increasing O3 concentration from 55 to 90 ppm at a constant temperature. O3 slip refers to O3 measured at the location after the catalyst.
approximately 100 °C without O3 injection. In other words, supplying a small amount of O3 into the catalyst achieves the same CO removal at approximately half the temperature. The reaction described by eq 1 on active sites of the catalyst could be easily accepted as explaining the reaction of CO in the presence of increased O3 supply. It is interesting that the reaction in eq 2 must be considered to analyze the empirical result, because only 55 ppm of O3 was required to convert CO of 150 ppm to CO2. In other words, an O3 molecule can convert more than one CO molecule. O3(g) + CO(g) → O2 (s) + O(s) + CO(s) → O2 (g) + CO2 (g)
O3(g) + 3CO(g) → 3O(s) + 3CO(s) → 3CO2 (g)
(1)
Figure 4. Variation of CO concentration containing the old catalyst with increasing O3 concentration from 55 to 90 ppm at a constant temperature. O3 slip refers to O3 measured at the location after the catalyst.
(2)
In eqs 1 and 2, (g) denotes the species in gas and (s) denotes the species on surface. In order to confirm the generation of CO2 via eqs 1 and 2, gas samples were analyzed through gas chromatography (GC) to evaluate carbon balance by comparing the quantities of the removed CO and generated CO2. The initial gas consisted of CO and CO2 concentrations of 250.1 and 429 ppm, respectively. The high concentration of CO2 was a result of atmospheric air entrained by the use of blower for a high flow rate. After supplying 55 ppm of O3, the removed CO and generated CO2 concentrations were measured as 215 and 230 ppm, respectively. The carbon balance could be calculated to be 107%, which is acceptable considering the level of uncertainty in the concentration of CO2. Following the injection of 55 ppm of O3, CO concentration was measured at increasing O3 concentrations of 75 and 90 ppm. When increasing O3 concentration to 75 ppm, the concentration of CO initially declined but then began to increase with time. At O3 concentration of 90 ppm, this phenomenon was noticeably increased. To verify the role of the Pt catalyst, it is necessary to study (1) the homogeneous reaction of CO and O3, (2) the heterogeneous reaction of CO and O3 without the catalyst (Pt) but with a catalyst support (alumina), and (3) the heterogeneous reaction of CO and O3 on the catalyst. Here, case (2) was intended to investigate the possible contribution of O3 decomposition on the alumina not present on the active
experiment using the deactivated catalyst still shows identical behavior, in which CO concentration is reduced by supplying O3. However, in this case there was considerably less enhancement of CO removal rate by O3 injection. In addition, unlike the result using the fresh catalyst, additional injection of O3 made the catalyst lose catalytic reactivity, with remarkable decline in CO oxidation. Moreover, O3 slip, which refers to the amount of O3 detected at the location after the catalyst, increased noticeably. From the two experiments above, it is inferred that the enhanced removal of CO by O3 takes place on the active sites of the catalyst, and that supplying a small amount of O3 can considerably affect the reactivity between CO and O3. 3.2. The Effect of Preoccupying Species. In order to establish why CO removal rate decreased at O3 concentrations of more than 75 ppm, the experiment was reconfigured to change the order of the supply between CO and O3. The reactant temperature was set to a constant 60 °C, and 55 ppm of O3 was supplied prior to the introduction of CO. As shown in Figure 5, the concentration of CO increased dramatically and stabilized at about 210 ppm. After a few minutes, the injection of O3 was removed and the concentration of CO was monitored. Unlike the previous result, the CO concentration was not affected by whether O3 was supplied or not. After another few minutes, the concentration of CO decreased C
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Figure 7. Proposed mechanism for the interaction of O3 with a catalyst in CO oxidation: The case of active sites preoccupied by CO.
blocked by CO molecules preoccupying active sites of the catalyst, whereas CO was blocked by O3 preoccupying molecules. It seems likely that Pt catalyst has a much stronger interaction with O3 than CO. The reaction mechanisms for each condition were deduced based on all experimental results. Although the estimation is valuable in providing insights on the role of O3 in catalytic CO oxidation, a full understanding of the proposed reaction mechanism requires further investigation into the effect of various conditions such as catalyst, concentrations of each gas, and so on. 3.3. The Effect of Hydrocarbon Species. Considering potential application to automotive systems, the effect of other species on the above reaction mechanism should be investigated because exhaust gases generally include not only CO, but also H2O, CO2, unburned hydrocarbons, and so on. Since unburned hydrocarbons can affect the role of O3 in catalytic CO oxidation, additional experiments were performed using C3H8 and C3H6 to represent unburned hydrocarbon species. The effects of the hydrocarbon species were evaluated with partly deactivated catalyst considering the real application environment. Catalytic function was evaluated with variation of CO concentration by supplying O3 at 55 ppm without hydrocarbons. The results or the reference data for further experiment are shown in Figure 8. Although the enhancement
Figure 5. Effect of initial gas supply (CO or O3) on the interaction between O3 and catalytic CO oxidation; O3 is supplied first up to about 16 min, and CO is supplied first from about 26−36 min.
sharply when O3 was injected again. The results show that catalyst performance is highly dependent on the species preoccupying the active sites of the catalyst. When O3 preoccupied the active sites of the catalyst, the supplied O3 could be decomposed into atomic O and oxygen on the catalyst. In this condition, there is less probability for the CO to be adsorbed, and it instead follows an Eley−Rideal type reaction in which the atomic O can directly oxidize CO into CO2. This reaction mechanism, which is described in Figure 6,
Figure 6. Proposed mechanism for the interaction of O3 with a catalyst in CO oxidation: the case of active sites preoccupied by O3.
would be dominant rather than the reaction of direct CO oxidation by the catalyst (Langmuir−Hinshelwood mechanism). This explains why the supply of O 3 at high concentration is associated with a large drop in CO removal rate. As a large amount of O3 is supplied to the catalyst, the catalyst can be easily blocked by the O3, and the atomic O decomposed from O3 by the catalyst would have a greater probability to react with another atomic O into O2, not with CO. On the other hand, when the active sites of the catalyst are already occupied by CO, the supply of a small amount of O3 resulted in dramatically increased catalytic oxidation of CO. Compared to the reaction between CO and O2 on the catalyst, the reaction rate would be considerably enhanced between CO and O3 because using O3, it is much easier to generate atomic O, which reacts preferably with CO rather than O2 even at low temperatures. In other words, it is believed that the injection of O3 into the catalytic CO oxidation reaction lowers the activation energy of CO oxidation sufficiently to react at low temperatures compared to that of catalytic reaction without O3. Moreover, O3 concentration of approximately 55 ppm is more effective than higher O3 concentration in removing CO. Based on these findings, the mechanism of CO oxidation with O3 is described in Figure 7. An interesting point is that O3 cannot be
Figure 8. Variation of CO concentration with 55 ppm of O3 injection at a steady temperature.
of CO removal rate by O3 injection declined slightly in the follow-up experiments, the role of O3 in catalytic CO oxidation was still evident. At reaction temperature of about 60 °C, CO removal rates were approximately 10% and 32% without and with injection of 55 ppm of O3, respectively. Initially, the mixture of CO and C3H8 was investigated to test whether the C3H8 affects the role of O3 in catalytic CO oxidation. The reference experiment above was repeated under identical conditions with C3H8, and the result is shown in D
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3.4. The Effect of High O3 Concentration. The effect of excessive supply of O3 was investigated to determine the optimal strategy for O3 supply. Three different (high) concentrations of O3 were compared at identical temperature of about 60 °C, and the result is plotted in Figure 11. At 55
Figure 9 for the initial concentration (red dotted line) and the variation (red solid line) of C3H8. There is no difference
Figure 9. Variation of CO and C3H8 concentrations with 55 ppm of O3 injection at a steady temperature.
between the two lines, and the trend of CO concentration was almost the same as that in the reference result, thereby indicating that C3H8 did not affect the enhancement of CO removal rate observed when adding O3. The experiment was repeated using a mixture of CO and C3H6; Figure 10 shows the initial concentration (red dotted
Figure 11. Variation of CO concentration after injecting different concentrations of O3.
ppm of O3, the decrease in CO was maintained after about 2 min. With increasing concentration of O3, the catalytic CO oxidation is not maintained by deactivation from the O3 accumulated on the active sites of used catalyst, whereas the removal of CO occurs much faster than that in the case of 55 ppm of O3. The degree of CO removal with 160 ppm of O3 was almost same at that with 55 ppm of O3, but the highest removal rate was achieved very rapidly, within approximately 1 min. When applying 260 ppm of O3, there was no noticeable difference in the gradient of CO removal rate, whereas the degree of CO removal declined. These findings confirm the possibility of rapid CO removal in a cold-start engine, but there are still many issues to address, such as deactivation of the catalyst, control of O3 concentration, and so on. In summary, the emission of CO from automobile engines has been identified as an environmental problem, especially in cold-start conditions during which the catalyst for CO removal is not fully heated to light-off temperature by the exhaust gases from the engine. Therefore, the rate of CO removal at low exhaust temperatures is an important issue. We investigated the role of O3 in catalytic CO oxidation performed with a Pt catalyst. The results confirm that the removal rate of CO is dramatically enhanced by supplying a small amount of O3. In the presence of O3, the same CO removal rate was achieved at half the operational temperature required when using the catalyst only. The enhancement of CO oxidation by the addition of O3 was largely dependent on the availability of active sites on the catalyst. Further test results indicated that the reaction mechanism between CO and O3 on the catalyst differs according to whether the active sites on the catalyst are preoccupied by CO or O3, and that high concentration of O3 can block the active sites of the catalyst from reaction with CO. Therefore, the optimum concentration of O3 is that which maintains the enhanced reaction of catalytic CO oxidation. The effects of hydrocarbons on the role of O3 in catalytic CO oxidation and the variation in CO removal rate at high O3
Figure 10. Variation of CO and C3H6 concentrations with 55 ppm of O3 injection at a steady temperature.
line) and the variation (red solid line) of C3H6. Unlike the result for C3H8, after supplying O3 into the catalyst, variations were observed in the concentrations of both CO and C3H6. Following O3 injection, the concentration of C3H6 declined from 210 ppm to about 185 ppm. Only about 20 ppm of CO was removed by adding O3 in the presence of C3H6, whereas 80 ppm of CO was removed in the scenario without C3H6. When supplying C3H8, the activation energy of C3H8 is insufficient to react with O3 at low temperatures, whereas C3H6 reacts with O3 as catalytic CO oxidation occurs. The results confirm that both C3H6 and CO have a competitive relationship with O3. From the two experiments above with C3H8 and C3H6, it could be expected that, compared to the alkane group with relatively high activation energy, the alkene group should be considered as competitive species when catalytic CO oxidation is enhanced by O3 injection. E
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concentrations were additionally investigated to determine the performance during cold-start applications with real exhaust gas. It was found that the alkene group such as C3H6 competed with CO to react with O3, whereas the alkane group such as C3H8 did not. From the latter experiment, we observed variation in the rate of CO concentration. By optimizing the concentration of O3, catalytic CO oxidation could be achieved within 1 min. The findings of the present study suggest controlled O3 injection as a promising solution for CO removal during the cold-start period, but many associated factors require further investigation.
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AUTHOR INFORMATION
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[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the financial support from the Ministry of Knowledge Economy (MKE) and the Korea Research Council for Industrial Science and Technology (ISTK) of the Republic of Korea, grant number B551179-1103-00.
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REFERENCES
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