SEPARATIONS

Temperature-Programmed Adsorption and Characteristics ofHoneycomb 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 emissionregulations. In this paper, three honeycomb hydrocarbon adsorbers were prepared by usingH-ZSM-5 and materials of conventional three-way catalysts. A new method was proposed,namely, temperature-programmed adsorption, to evaluate simultaneously the performance ofadsorption and conversion of decane on the adsorber. The adsorption amount and conversion ofdecane on the adsorbers were varied with the Si/Al ratio of H-ZSM-5 and the loading amountof precious metal (PM) and hydrothermal aging. There was a trade-off between the adsorptionamount and the conversion. The coexistent gas effect on the adsorption of decane on the two-cycle-aged adsorber seemed to increase in the order of CO, NO, O2, and H2O in the conditionssimulated to the emissions of a vehicle. The pore structure at fresh and hydrothermal agedstates of the adsorbers was observed by using nitrogen.

Introduction

A large portion (above 70%) of hydrocarbon emissionsfor a typical vehicle occurs mainly during cold start. Tomeet stringent regulations, the hydrocarbons must bereduced. Several technologies such as an electricallyheated catalyst,1 close-coupled catalyst,2 exhaust gasburner,3 and hydrocarbon adsorber4,5 have been devel-oped. Generally, a hydrocarbon adsorber system iscomposed of two bricks. The first brick of a hydrocarbonadsorber is followed by the second brick of a light-offcatalyst. A hydrocarbon adsorber would first trap anyhydrocarbons on a zeolite or similar adsorbent materialduring cold start before it has lit off and then releasethe hydrocarbons once a light-off catalyst is hot enoughto allow conversion of the hydrocarbons. The adsorptioncapacity of hydrocarbons on a hydrocarbon adsorberusing zeolite as the adsorbing material can be affectedby the Si/Al ratio of the zeolite.5

In this paper, three honeycomb hydrocarbon adsorb-ers were employed based on the Si/Al ratio of H-ZSM-5and precious metal (PM) loading. A new method wasproposed, namely, temperature-programmed adsorption(TPA), to evaluate simultaneously the adsorption, de-sorption, and conversion of hydrocarbons on a hydro-carbon adsorber. To select the best adsorber, TPAexperiments were carried out both at the fresh state andafter the hydrothermal aging state of the three adsorb-ers. Isothermal adsorption and temperature-programmeddesorption (TPD) experiments were carried out to check

the effect of coexistent gases such as O2, NO, CO, andH2O. The pore structure at fresh and hydrothermal agedstates of the adsorbers was observed by using nitrogenadsorption.

Experimental Section

Hydorcarbon Adsorber and Adsorbate. Threeadsorber samples used in this study were obtained bycoating washcoat onto a cordierite honeycomb ceramicsubstrate [cell density of 62 cells/cm2 (400 cells/in.2), wallthickness of 0.165 mm, 19 mm (D) × 30 mm (L)]. Thewashcoat consisted of H-ZSM-5, γ-Al2O3, and basemetals (Ba, Ce, and Zr) used in a conventional three-way catalyst. In this study, to evaluate of Si/Al ratioeffect of H-ZSM-5 on TPA behavior and hydrothermalstability, two types of Si/Al ratios, 40/1 or 150/1, wereemployed. Also, to study the effect of the presence ofPMs on the adsorber, Pd and Rh were used. The firstadsorber had the 150/1 Si/Al ratio adsorbent withimpregnated Pd/Rh in a 14/1 ratio, and it was namedas ADS #1. The second one had the same Si/Al ratioadsorbent without PMs, and it was named as ADS #2.The rest had the 40/1 Si/Al ratio adsorbent withimpregnated Pd/Rh in a 10/1 ratio, and it was namedas ADS #3. For three adsorbers, washcoat and PMloading amounts were identical for 140 and 4.5 g/L. Thewashcoats of ADS #1 and ADS #2 were named aswashcoat A, and the washcoat of ADS #3 was namedas washcoat B. When PMs were impregnated, PdCl2 andRhCl3‚3H2O were prepared as the precursors of Pd andRh. All samples were dried at 150 °C for 5 h andcalcined at 550 °C for 4 h. The samples were denotedas “fresh”.

* To whom correspondence should be addressed. E-mail:kim8682@empal.com. Tel.: +82-31-219-2518. Fax: +82-31-214-8918.

6589Ind. Eng. Chem. Res. 2002, 41, 6589-6592

10.1021/ie020165b CCC: $22.00 © 2002 American Chemical SocietyPublished on Web 11/13/2002

To check the adsorption ability of a hydrocarbon onhydrocarbon adsorbers, decane (Aldrich, 99%) was usedas an adsorbate. O2 (1.47%), NO (1000 ppm), CO (1%),and H2O (10%) were used to evaluate the coexistenceeffect on the adsorption of decane on hydrocarbonadsorbers.

Rapid Aging of Hydrocarbon Adsorbers. Threefresh adsorbers were aged for two cycles with a rapidaging mode.6 One cycle duration of the rapid aging modewas 9 h. The maximum surface temperature of a samplein the rapid aging mode was set to 900 °C. Air and H2Owere supplied to the front of a sample. These sampleswere denoted as “two-cycle-aged”. The two-cycle-agedstate by the rapid aging mode was proven already to bethe correspondent to the 50 000-mile-aged state of avehicle.

Nitrogen Adsorption and Desorption. Nitrogenadsorption and desorption experiments were carried outusing the AS-1 apparatus (Quantachrom) to measurethe surface area and pore volume of a hydrocarbonadsorber. Before the experiments were done, all sampleswere evacuated to 1 × 10-6 Torr and kept at 300 °C for8 h. The experiments were conducted at 77 K.

TPA, Isothermal Adsorption, and TPD. TPA,isothermal adsorption, and TPD experiments werecarried out in the apparatus shown in Figure 1. For allexperiments the dimensions of a sample [19 mm (D) ×30 mm (L)] were identical. Before TPA experiment wasdone, 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) at400 °C for 1 h and cooled to 30 °C in N2 (1 L/min). Afterthat the sample was heated in a 1000 ppm decane/N2mixture (1 L/min) at 30-300 °C with the ramping rateof 1 °C/min. An isothermal adsorption experiment wasconducted in a 1000 ppm decane/N2 mixture (1 L/min)at 30 °C. After that the sample was heated in N2 gas (1L/min) at 30-300 °C with the ramping rate of 1 °C/min.In addition, to evaluate a conversion of decane, O2 wassupplied with the concentration of 1.47%. The concen-trations of decane at the inlet and outlet of the reactorwere 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 adsorptionof decane was observed by the isothermal adsorptionand TPD experiments as mentioned. Decane and H2Oin each bubbler, which was controlled by each waterbath, were vaporized and supplied to the reactor byusing the loading gas of N2. The vapor pressure ofdecane and H2O was calculated by using the Reidequation.7

Results and Discussion

Surface Area and Pore Volume. The relativesurface areas of the fresh and two-cycle-aged adsorbersare shown in Figure 2. The relative surface area is basedon the surface area of a fresh sample. At a two-cycle-aged state, ADS #2 showed a low loss of surface area incomparison to ADS #1 and ADS #3. This result suggeststhat a PM coated on an adsober can promote thereduction of the surface area by aging. This may be dueto the interaction between PM and washcoat, blockingpores by the sintered PM.

Figure 3 shows a relative micropore volume under 20Å for three adsorbers according to aging. The reductionof the micropore volume of adsorbers by aging wassimilar to that of the surface area. According to theresults of Figures 2 and 3, the hydrothermal resistanceof ADS #1 appeared to be superior to that of ADS #3.This result implies that the higher the Si/Al ratio ofzeolite, the better the hydrothermal resistance.8

TPA. The TPA results of fresh ADS #1, ADS #2, andADS #3 are shown in Figure 4. The supplied gas in thisfigure was a 1000 ppm decane/N2 mixture. This figurerepresents relative concentration versus temperature.

Figure 1. Apparatus for adsorption, desorption, and conversion.

Figure 2. Relative surface area of fresh and two-cycle-agedadsorbers.

Figure 3. Relative micropore volume of fresh and two-cycle-agedadsorbers.

Figure 4. TPA results of fresh adsorbers in the absence of O2.

6590 Ind. Eng. Chem. Res., Vol. 41, No. 25, 2002

The relative concentration means the ratio of outletconcentration (Co) to inlet concentration (Ci). The valueof 1 stands for the full saturation of decane on anadsorber. The value over 1 represents the sum ofsupplied decane and desorbed decane from an adsorber.Decane was emitted more rapidly due to the loading ofPM. Under condition of the absence of H2O in suppliedgases, hydrocarbon is more adsorbed on an adsorber atlower Si/Al ratio.5 In comparison to ADS #1 and ADS#3, the higher adsorption amount of ADS #3 can beattributed to the lower Si/Al ratio. At the temperatureof over 200 °C, decane appeared to be under the valueof 1 in the cases of ADS #1 and ADS #3. This resultmay be due to the reduction of decane by the oxidationreaction of decane and surface oxygen adsorbed on theadsorber.

According to additional gas of O2, TPA results of freshADS #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 ofPM-loaded adsorbers, ADS #1 and ADS #3, TPA curvesof decane on adsorbers were a little affected by the O2supply gas under 100 °C. However, over 100 °C TPAcurves changed from positive to negative. That may beattributed to the reduction of decane by the oxidationreaction of decane and supplied O2 gas over 100 °C. Thisbehavior was advanced in ADS #3. This result may beattributed to the increase of the Rh loading amount. Inthe case of the absence of PM in washcoat, TPA curvesof ADS #2 showed the value under 1 over 230 °C. Thismeans that, even though PM is not loaded, the oxidationreaction of decane occurs over 230 °C by the interactionof washcoat and supplied O2. From the TPA results ofFigures 4 and 5, a clue can be obtained that PM makesthe decreasing of the saturation temperature and theadsorption amount of decane. However, PM has anadvantage for the conversion of decane by the oxidationreaction under the presence of O2 supply gas. It isrecommended that when an adsorber is designed, atrade-off of adsorption and reduction of hydrocarbonsis always considered.

Figure 6 shows the TPA results of two-cycle-aged ADS#1, ADS #2, and ADS #3. The experimental conditionswere identical to those of Figure 4. According to two-cycle aging, the saturation temperature and adsorptionamount were reduced. In contrast to the TPA results offresh ADS #1 and ADS #3, at a two-cycle-aged state,ADS #1 was higher than ADS #3 for the adsorptionamount of decane under the value of 1. This result canbe attributed mainly to the difference of hydrothermalresistance according to the Si/Al ratio. At higher Si/Alratio, zeolite shows good hydrothermal resistance.5 Thisresult agrees well with the surface area and micropore

volume according to aging. In contrast to the TPAresults of fresh ADS #1 and ADS #3, at the temperatureof over 200 °C, the conversion of decane by the oxidationreaction did not appear. It is mainly due to the sinteringof 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 anadditional O2 supply. The experiment conditions wereidentical to those of Figure 5. According to hydrothermalaging, the activation temperature required for conver-sion of decane in ADS #1 and ADS #3 was higher, andthe conversion of decane in ADS #2 did not occur.

From the results of Figures 6 and 7, it can be seenthat the adsorbing materal of washcoat A is better thanthat of washcoat B in the point of the adsorption amountaccording to hydrothermal aging. This means that theSi/Al ratio of ADS #1 is superior to ADS #3 in view ofhydrothermal resistance.

Isothermal Adsorption and TPD. Figure 8 showsbreakthrough curves of decane on two-cycle-aged ADS#1 at 30 °C. It was presented according to a relativeconcentration of decane and time. The inlet concentra-

Figure 5. TPA results of fresh adsorbers in the presenceof O2.

Figure 6. TPA results of two-cycle-aged adsorbers in theabsence of O2.

Figure 7. TPA results of two-cycle-aged adsorbers in thepresence of O2.

Figure 8. Adsorption results of decane onto two-cycle-agedADS #1.

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tion of decane was 1000 ppm. In this figure, O2, NO,and CO mean coexistent gases with decane. The con-centrations of O2, NO, and CO were 1.47%, 1000 ppm,and 1%. The concentrations of coexistent gases weresimulated to the concentrations of emissions from a realvehicle. For all cases the total flow rate of supplied gaseswas 2 L/min. Although the concentration of CO washigher than that of NO, in the case of decane + NO,decane was saturated more rapidly. The saturationorder 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. Theconcentration of coexistent gases such as CO, NO, andO2 was the same as that in Figure 8. In addition, 10%H2O was employed to evaluate the H2O effect. Theconcentrations of coexistent gases were simulated to theconcentrations of emissions from a real vehicle. TPDwas carried out in N2 gas (1 L/min) at 30-300 °C (1°C/min). In the case of decane, two peaks appeared at75 and 125 °C. Decane was fully desorbed at over 230°C. According to the presence of coexistent gas, thepeaks were shifted to low temperature. It may beattributed to the competition adsorption of decane andcoexistent gas. Adsorption sites of decane were morelowered by the adsorption of coexistent gas, and decanewas easily emitted at the lower temperature. The

relative desorption amount of decane is shown in Figure10. The order of the relative desorption amount wasfound to be decane + H2O, decane + O2, decane + NO,decane + CO, and decane. From the results of Figures8-10, the coexistent gas effect on the adsorption ofdecane seemed to increase in the order of CO, NO, O2,and H2O.

Conclusions

In the fresh state, the surface area, pore volume, andadsorbed amount of decane were reduced with theincrease of the Si/Al ratio of H-ZSM-5 and PM loading.They were also reduced by the hydrothermal agingtreatment. The proposed TPA method explains well thedifference of adsorption, desorption, and conversion ofdecane on three adsorbers (ADS #1, ADS #2, and ADS#3). By the TPA results, washcoat A was better thanwashcoat B in the point of the adsorption amountaccording to the hydrothermal aging. It is well matchedwith the results of the surface area and pore volumeexperiments. The coexistent gas effect on the adsorptionof decane on the two-cycle-aged adsorebr seemed toincrease in the order of CO, NO, O2, and H2O in theconditions simulated to the emissions of a vehicle.

Literature Cited

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(4) Williams, J. L.; Patil, M. D.; Hertl, W. By-Pass HydrocarbonAdsorber System for ULEV. SAE 1996, Paper No. 960343.

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(6) Son, G. S.; Lee, G. Y.; Lee, K. Y.; Choi, B. C. Study I ofCatalyst Aging. J. Korean Soc. Autom. Eng. 1997, 5, 86-94.

(7) Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The Properties ofGases & Liquids, 4th ed.; McGraw-Hill: New York, 1988.

(8) Descorme, C.; Gelin, P.; Lecuyer, C.; Primet, M. Palladium-exchanged MFI-type zeolites in the catalytic reduction of nitrogenmonoxide by methanesInfluence of the Si/Al ration an the activityand the hydrothermal stability. Appl. Catal. B 1997, 13, 185-195.

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Received for review February 27, 2002Revised manuscript received September 11, 2002

Accepted September 13, 2002

IE020165B

Figure 9. TPD results of decane from two-cycle-aged ADS #1.

Figure 10. Relative desorption amount of decane from two-cycle-aged ADS #1.

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