Ind. Eng. Chem. Res. 2002, 41, 6589-6592
6589
SEPARATIONS Temperature-Programmed Adsorption and Characteristics of Honeycomb Hydrocarbon Adsorbers Dae Jung Kim,* Jin Won Kim, and Jae Eui Yie School of Chemical Engineering and Biotechnology, Ajou University, Suwon 442-749, Korea
Hee Moon Faculty of Applied Chemistry, Chonnam National University, Gwangju 500-757, Korea
Hydrocarbon emissions of a vehicle during cold start must be reduced to meet stringent emission regulations. In this paper, three honeycomb hydrocarbon adsorbers were prepared by using H-ZSM-5 and materials of conventional three-way catalysts. A new method was proposed, namely, temperature-programmed adsorption, to evaluate simultaneously the performance of adsorption and conversion of decane on the adsorber. The adsorption amount and conversion of decane on the adsorbers were varied with the Si/Al ratio of H-ZSM-5 and the loading amount of precious metal (PM) and hydrothermal aging. There was a trade-off between the adsorption amount and the conversion. The coexistent gas effect on the adsorption of decane on the twocycle-aged adsorber seemed to increase in the order of CO, NO, O2, and H2O in the conditions simulated to the emissions of a vehicle. The pore structure at fresh and hydrothermal aged states of the adsorbers was observed by using nitrogen. Introduction A large portion (above 70%) of hydrocarbon emissions for a typical vehicle occurs mainly during cold start. To meet stringent regulations, the hydrocarbons must be reduced. Several technologies such as an electrically heated catalyst,1 close-coupled catalyst,2 exhaust gas burner,3 and hydrocarbon adsorber4,5 have been developed. Generally, a hydrocarbon adsorber system is composed of two bricks. The first brick of a hydrocarbon adsorber is followed by the second brick of a light-off catalyst. A hydrocarbon adsorber would first trap any hydrocarbons on a zeolite or similar adsorbent material during cold start before it has lit off and then release the hydrocarbons once a light-off catalyst is hot enough to allow conversion of the hydrocarbons. The adsorption capacity of hydrocarbons on a hydrocarbon adsorber using zeolite as the adsorbing material can be affected by the Si/Al ratio of the zeolite.5 In this paper, three honeycomb hydrocarbon adsorbers were employed based on the Si/Al ratio of H-ZSM-5 and precious metal (PM) loading. A new method was proposed, namely, temperature-programmed adsorption (TPA), to evaluate simultaneously the adsorption, desorption, and conversion of hydrocarbons on a hydrocarbon adsorber. To select the best adsorber, TPA experiments were carried out both at the fresh state and after the hydrothermal aging state of the three adsorbers. Isothermal adsorption and temperature-programmed desorption (TPD) experiments were carried out to check * To whom correspondence should be addressed. E-mail:
[email protected]. Tel.: +82-31-219-2518. Fax: +82-31214-8918.
the effect of coexistent gases such as O2, NO, CO, and H2O. The pore structure at fresh and hydrothermal aged states of the adsorbers was observed by using nitrogen adsorption. Experimental Section Hydorcarbon Adsorber and Adsorbate. Three adsorber samples used in this study were obtained by coating washcoat onto a cordierite honeycomb ceramic substrate [cell density of 62 cells/cm2 (400 cells/in.2), wall thickness of 0.165 mm, 19 mm (D) × 30 mm (L)]. The washcoat consisted of H-ZSM-5, γ-Al2O3, and base metals (Ba, Ce, and Zr) used in a conventional threeway catalyst. In this study, to evaluate of Si/Al ratio effect of H-ZSM-5 on TPA behavior and hydrothermal stability, two types of Si/Al ratios, 40/1 or 150/1, were employed. Also, to study the effect of the presence of PMs on the adsorber, Pd and Rh were used. The first adsorber had the 150/1 Si/Al ratio adsorbent with impregnated Pd/Rh in a 14/1 ratio, and it was named as ADS #1. The second one had the same Si/Al ratio adsorbent without PMs, and it was named as ADS #2. The rest had the 40/1 Si/Al ratio adsorbent with impregnated Pd/Rh in a 10/1 ratio, and it was named as ADS #3. For three adsorbers, washcoat and PM loading amounts were identical for 140 and 4.5 g/L. The washcoats of ADS #1 and ADS #2 were named as washcoat A, and the washcoat of ADS #3 was named as washcoat B. When PMs were impregnated, PdCl2 and RhCl3‚3H2O were prepared as the precursors of Pd and Rh. All samples were dried at 150 °C for 5 h and calcined at 550 °C for 4 h. The samples were denoted as “fresh”.
10.1021/ie020165b CCC: $22.00 © 2002 American Chemical Society Published on Web 11/13/2002
6590
Ind. Eng. Chem. Res., Vol. 41, No. 25, 2002
Figure 2. Relative surface area of fresh and two-cycle-aged adsorbers. Figure 1. Apparatus for adsorption, desorption, and conversion.
To check the adsorption ability of a hydrocarbon on hydrocarbon adsorbers, decane (Aldrich, 99%) was used as an adsorbate. O2 (1.47%), NO (1000 ppm), CO (1%), and H2O (10%) were used to evaluate the coexistence effect on the adsorption of decane on hydrocarbon adsorbers. Rapid Aging of Hydrocarbon Adsorbers. Three fresh adsorbers were aged for two cycles with a rapid aging mode.6 One cycle duration of the rapid aging mode was 9 h. The maximum surface temperature of a sample in the rapid aging mode was set to 900 °C. Air and H2O were supplied to the front of a sample. These samples were denoted as “two-cycle-aged”. The two-cycle-aged state by the rapid aging mode was proven already to be the correspondent to the 50 000-mile-aged state of a vehicle. Nitrogen Adsorption and Desorption. Nitrogen adsorption and desorption experiments were carried out using the AS-1 apparatus (Quantachrom) to measure the surface area and pore volume of a hydrocarbon adsorber. Before the experiments were done, all samples were evacuated to 1 × 10-6 Torr and kept at 300 °C for 8 h. The experiments were conducted at 77 K. TPA, Isothermal Adsorption, and TPD. TPA, isothermal adsorption, and TPD experiments were carried out in the apparatus shown in Figure 1. For all experiments the dimensions of a sample [19 mm (D) × 30 mm (L)] were identical. Before TPA experiment was done, the sample was oxidized at 400 °C for 1 h in air (1 L/min), and then it was purged with N2 (1 L/min) at 400 °C for 1 h and cooled to 30 °C in N2 (1 L/min). After that the sample was heated in a 1000 ppm decane/N2 mixture (1 L/min) at 30-300 °C with the ramping rate of 1 °C/min. An isothermal adsorption experiment was conducted in a 1000 ppm decane/N2 mixture (1 L/min) at 30 °C. After that the sample was heated in N2 gas (1 L/min) at 30-300 °C with the ramping rate of 1 °C/min. In addition, to evaluate a conversion of decane, O2 was supplied with the concentration of 1.47%. The concentrations of decane at the inlet and outlet of the reactor were measured using a gas chromatograph (HP5890) with a flame ionization detector and a DB1 column. The effect of coexistent gases such as O2 (1.47%), NO (1000 ppm), CO (1%), and H2O (10%) on the adsorption of decane was observed by the isothermal adsorption and TPD experiments as mentioned. Decane and H2O in each bubbler, which was controlled by each water bath, were vaporized and supplied to the reactor by using the loading gas of N2. The vapor pressure of decane and H2O was calculated by using the Reid equation.7
Figure 3. Relative micropore volume of fresh and two-cycle-aged adsorbers.
Figure 4. TPA results of fresh adsorbers in the absence of O2.
Results and Discussion Surface Area and Pore Volume. The relative surface areas of the fresh and two-cycle-aged adsorbers are shown in Figure 2. The relative surface area is based on the surface area of a fresh sample. At a two-cycleaged state, ADS #2 showed a low loss of surface area in comparison to ADS #1 and ADS #3. This result suggests that a PM coated on an adsober can promote the reduction of the surface area by aging. This may be due to the interaction between PM and washcoat, blocking pores by the sintered PM. Figure 3 shows a relative micropore volume under 20 Å for three adsorbers according to aging. The reduction of the micropore volume of adsorbers by aging was similar to that of the surface area. According to the results of Figures 2 and 3, the hydrothermal resistance of ADS #1 appeared to be superior to that of ADS #3. This result implies that the higher the Si/Al ratio of zeolite, the better the hydrothermal resistance.8 TPA. The TPA results of fresh ADS #1, ADS #2, and ADS #3 are shown in Figure 4. The supplied gas in this figure was a 1000 ppm decane/N2 mixture. This figure represents relative concentration versus temperature.
Ind. Eng. Chem. Res., Vol. 41, No. 25, 2002 6591
Figure 5. TPA results of fresh adsorbers in the presence of O2.
The relative concentration means the ratio of outlet concentration (Co) to inlet concentration (Ci). The value of 1 stands for the full saturation of decane on an adsorber. The value over 1 represents the sum of supplied decane and desorbed decane from an adsorber. Decane was emitted more rapidly due to the loading of PM. Under condition of the absence of H2O in supplied gases, hydrocarbon is more adsorbed on an adsorber at lower Si/Al ratio.5 In comparison to ADS #1 and ADS #3, the higher adsorption amount of ADS #3 can be attributed to the lower Si/Al ratio. At the temperature of over 200 °C, decane appeared to be under the value of 1 in the cases of ADS #1 and ADS #3. This result may be due to the reduction of decane by the oxidation reaction of decane and surface oxygen adsorbed on the adsorber. According to additional gas of O2, TPA results of fresh ADS #1, ADS #2, and ADS #3 are shown in Figure 5. The supplied gases were 1000 ppm decane, 1.47% O2, and N2, and the total flow was 1 L/min. In the case of PM-loaded adsorbers, ADS #1 and ADS #3, TPA curves of decane on adsorbers were a little affected by the O2 supply gas under 100 °C. However, over 100 °C TPA curves changed from positive to negative. That may be attributed to the reduction of decane by the oxidation reaction of decane and supplied O2 gas over 100 °C. This behavior was advanced in ADS #3. This result may be attributed to the increase of the Rh loading amount. In the case of the absence of PM in washcoat, TPA curves of ADS #2 showed the value under 1 over 230 °C. This means that, even though PM is not loaded, the oxidation reaction of decane occurs over 230 °C by the interaction of washcoat and supplied O2. From the TPA results of Figures 4 and 5, a clue can be obtained that PM makes the decreasing of the saturation temperature and the adsorption amount of decane. However, PM has an advantage for the conversion of decane by the oxidation reaction under the presence of O2 supply gas. It is recommended that when an adsorber is designed, a trade-off of adsorption and reduction of hydrocarbons is always considered. Figure 6 shows the TPA results of two-cycle-aged ADS #1, ADS #2, and ADS #3. The experimental conditions were identical to those of Figure 4. According to twocycle aging, the saturation temperature and adsorption amount were reduced. In contrast to the TPA results of fresh ADS #1 and ADS #3, at a two-cycle-aged state, ADS #1 was higher than ADS #3 for the adsorption amount of decane under the value of 1. This result can be attributed mainly to the difference of hydrothermal resistance according to the Si/Al ratio. At higher Si/Al ratio, zeolite shows good hydrothermal resistance.5 This result agrees well with the surface area and micropore
Figure 6. TPA results of two-cycle-aged adsorbers in the absence of O2.
Figure 7. TPA results of two-cycle-aged adsorbers in the presence of O2.
Figure 8. Adsorption results of decane onto two-cycle-aged ADS #1.
volume according to aging. In contrast to the TPA results of fresh ADS #1 and ADS #3, at the temperature of over 200 °C, the conversion of decane by the oxidation reaction did not appear. It is mainly due to the sintering of PM and washcoat by hydrothermal aging. Figure 7 shows the TPA results of two-cycle-aged ADS #1, ADS #2, and ADS #3 under the conditions of an additional O2 supply. The experiment conditions were identical to those of Figure 5. According to hydrothermal aging, the activation temperature required for conversion of decane in ADS #1 and ADS #3 was higher, and the conversion of decane in ADS #2 did not occur. From the results of Figures 6 and 7, it can be seen that the adsorbing materal of washcoat A is better than that of washcoat B in the point of the adsorption amount according to hydrothermal aging. This means that the Si/Al ratio of ADS #1 is superior to ADS #3 in view of hydrothermal resistance. Isothermal Adsorption and TPD. Figure 8 shows breakthrough curves of decane on two-cycle-aged ADS #1 at 30 °C. It was presented according to a relative concentration of decane and time. The inlet concentra-
6592
Ind. Eng. Chem. Res., Vol. 41, No. 25, 2002
relative desorption amount of decane is shown in Figure 10. The order of the relative desorption amount was found to be decane + H2O, decane + O2, decane + NO, decane + CO, and decane. From the results of Figures 8-10, the coexistent gas effect on the adsorption of decane seemed to increase in the order of CO, NO, O2, and H2O. Conclusions
Figure 9. TPD results of decane from two-cycle-aged ADS #1.
Figure 10. Relative desorption amount of decane from two-cycleaged ADS #1.
tion of decane was 1000 ppm. In this figure, O2, NO, and CO mean coexistent gases with decane. The concentrations of O2, NO, and CO were 1.47%, 1000 ppm, and 1%. The concentrations of coexistent gases were simulated to the concentrations of emissions from a real vehicle. For all cases the total flow rate of supplied gases was 2 L/min. Although the concentration of CO was higher than that of NO, in the case of decane + NO, decane was saturated more rapidly. The saturation order of decane was found to be decane + O2, decane + NO, decane + CO, and decane. Figure 9 shows TPD curves of decane under N2 gas. Adsorption of decane was carried out at 30 °C. The concentration of coexistent gases such as CO, NO, and O2 was the same as that in Figure 8. In addition, 10% H2O was employed to evaluate the H2O effect. The concentrations of coexistent gases were simulated to the concentrations of emissions from a real vehicle. TPD was carried out in N2 gas (1 L/min) at 30-300 °C (1 °C/min). In the case of decane, two peaks appeared at 75 and 125 °C. Decane was fully desorbed at over 230 °C. According to the presence of coexistent gas, the peaks were shifted to low temperature. It may be attributed to the competition adsorption of decane and coexistent gas. Adsorption sites of decane were more lowered by the adsorption of coexistent gas, and decane was easily emitted at the lower temperature. The
In the fresh state, the surface area, pore volume, and adsorbed amount of decane were reduced with the increase of the Si/Al ratio of H-ZSM-5 and PM loading. They were also reduced by the hydrothermal aging treatment. The proposed TPA method explains well the difference of adsorption, desorption, and conversion of decane on three adsorbers (ADS #1, ADS #2, and ADS #3). By the TPA results, washcoat A was better than washcoat B in the point of the adsorption amount according to the hydrothermal aging. It is well matched with the results of the surface area and pore volume experiments. The coexistent gas effect on the adsorption of decane on the two-cycle-aged adsorebr seemed to increase in the order of CO, NO, O2, and H2O in the conditions simulated to the emissions of a vehicle. Literature Cited (1) Socha, L. S.; Thompson, D. F.; Weber, P. A. Optimization of Extruded Electrically Heated Catalysts. SAE 1994, Paper No. 940468. (2) Kim, D. J.; Son, G. S.; Lee, K. Y.; Choi, B. C.; Kang, S. R. Developmemt of Close-Coupled Catalyst(CCC) System to Meet EC Stage 2. J. Korean Soc. Autom. Eng. 1996, 4, 140-146. (3) Ma, T.; Collings, N.; Hands, T. Exhaust Gas IgnitionsA New Concept For Rapid Light off of Automotive Exhaust Catalysts. SAE 1992, Paper No. 920400. (4) Williams, J. L.; Patil, M. D.; Hertl, W. By-Pass Hydrocarbon Adsorber System for ULEV. SAE 1996, Paper No. 960343. (5) Engler, B. H.; Lindner, D.; Lox, E. S.; Ostgathe, K.; SchaferSindlinger, A.; Muller, W. Reduction of Exhaust Gas Emissions by Using Hydrocarbon Adsorber Systems. SAE 1993, Paper No. 930738. (6) Son, G. S.; Lee, G. Y.; Lee, K. Y.; Choi, B. C. Study I of Catalyst Aging. J. Korean Soc. Autom. Eng. 1997, 5, 86-94. (7) Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The Properties of Gases & Liquids, 4th ed.; McGraw-Hill: New York, 1988. (8) Descorme, C.; Gelin, P.; Lecuyer, C.; Primet, M. Palladiumexchanged MFI-type zeolites in the catalytic reduction of nitrogen monoxide by methanesInfluence of the Si/Al ration an the activity and the hydrothermal stability. Appl. Catal. B 1997, 13, 185195. (9) Lee, C. H.; Chen, Y. W. Effect of additives on Pd/Al2O3 for CO and propylene oxidation at oxygen-deficient conditions. Appl. Catal. B 1998, 17, 279-291.
Received for review February 27, 2002 Revised manuscript received September 11, 2002 Accepted September 13, 2002 IE020165B
