Applied Catalysis B:Environmental24(2000)
169–173
Enhanced activity of an In–Fe2O3/H-ZSM-5catalyst for NO
reduction with methane
Xiaodong Wang,Tao Zhang?,Xiaoying Sun,Wen Guan,Dongbai Liang,Liwu Lin
State Key Laboratory of Catalysis,Dalian Institute of Chemical Physics,Chinese Academy of Sciences,Dalian116023,China Received28May1999;received in revised form18August1999;accepted18August1999
Abstract
Selective reduction of NO by CH4on an In–Fe2O3/H-ZSM-5catalyst was investigated in the presence of excess oxygen. Compared with In/H-ZSM-5,the In–Fe2O3/H-ZSM-5catalyst with high Fe2O3contents showed higher activity in a wide range of reaction temperatures.It was found that the addition of Fe2O3yielded a promotion effect on CH4activation.The in?uence of water vapor on NO conversion was also investigated.The activity of the In/H-ZSM-5catalyst has been found to be strongly inhibited by water vapor,while the In–Fe2O3/H-ZSM-5catalyst remained fairly active in the presence of3.3% steam.?2000Elsevier Science B.V.All rights reserved.
Keywords:Indium;Iron oxide;H-ZSM-5;Methane;Nitrogen oxide reduction
1.Introduction
Selective catalytic reduction(SCR)of NO by hy-drocarbons is believed to be one of the promising routes to eliminate nitrogen oxides(NO x),because this process takes advantage of using a gas mixture very similar to that found in the automobile exhaust as reductants.Reductants such as propane,propene and ethylene have been extensively studied on zeolite-and alumina-supported catalysts[1–7].The CH4-SCR process has recently attracted great attention,since methane is almost present in all combustion ex-hausts,particularly in the gas-cogeneration systems. Several studies were reported on various types of catalysts for CH4-SCR and indicated that Co-ZSM-5,?Corresponding author.Tel.:+86-411-4678-404;fax: +86-411-4691-570.
E-mail address:taozhang@https://www.doczj.com/doc/9f1745600.html,(T.Zhang).Ga–H-ZSM-5and In–H-ZSM-5exhibited high cat-alytic activities for NO reduction when CH4was used as a reductant[8–12].
Kikuchi et al.?rst found that NH4-ZSM-5,when ion-exchanged with indium ion(In-ZSM-5)was ac-tive for the SCR of NO with CH4[12]and we reported the high activity of indium impregnated H-ZSM-5 (In/H-ZSM-5)[13,14].Modi?cations of In-zeolite cat-alysts with various metals have also been investigated and precious metals of Ir,Pt,Rh,as well as Co(II), were reported to be effective promoters[15–18].Re-cently we have found that the direct incorporation of iron oxide into the In/H-ZSM-5catalyst caused the catalyst to exhibit higher activity and durability.In this paper,we report on the investigation results of this In–Fe2O3/H-ZSM-5catalyst,which was prepared by a special procedure,showed enhanced activity for NO reduction by CH4in the presence of excess oxygen and water vapor.
0926-3373/00/$–see front matter?2000Elsevier Science B.V.All rights reserved. PII:S0926-3373(99)00101-0
170X.Wang et al./Applied Catalysis B:Environmental24(2000)169–173
2.Experimental
Mixtures of commercial grade Fe2O3and H-ZSM-5 powder(SiO2/Al2O3=25,supplied by the Nankai University,China)were impregnated with aqueous solutions prepared by dissolving In(NO3)3·4.5H2O (supplied by the Tianjin No.3Agents Plant,China). The samples were stirred vigorously for 1.5h at 80?C,dried at120?C for6h and calcined at700?C for6h.The catalysts were denoted by the weight ratios of indium,iron oxide and H-ZSM-5zeolite, for example,In–Fe2O3/H-ZSM-5(1:4:20)means In:Fe2O3:H-ZSM-5=1:4:20(weight ratio). Steady-state activities of the catalysts were evalu-ated using a single-pass?ow micro-reactor made of quartz,with an internal diameter of8mm.The cat-alysts were pelletized,crushed and sieved to40–60 mesh before use.0.5g of the catalyst sample was used for each activity evaluation.The?ow-rate of the feed was60cm3/min(GHSV=3600h?1).The feeding mixture was prepared by mixing four?ow-ing gases,namely NO,CH4,O2and He,at the inlet of the reactor,each of them was independently con-trolled with a mass?ow regulator.The feed-gas was controlled to consist of2500ppm NO,2000ppm CH4, 4.0%O2,with He as the balance.The reaction ef-?uent was analyzed with a NO x monitor and a gas chromatograph equipped with a thermal conductivity detector(TCD),a13X Zeolite column and a Porapak QS column.The yield of N2was used to calculate the NO conversion.
3.Results and discussion
The conversions of NO reduction to N2and methane oxidation to CO2over the In–Fe2O3/H-ZSM-5 (1:4:20)and the In/H-ZSM-5(1:20)catalysts re-spectively are shown in Fig.1as a function of tem-perature.In these experiments,the conversion points were obtained by increasing the reaction temperature stepwise from low to high temperature.All opera-tions were identical with respect to data collection at ascending or descending temperature.We can see that the temperature dependence trend of the conversion was similar for the two catalysts.The conversions in-creased with temperature before a certain temperature. However,up to a certain temperature,the
methane Fig.1.Conversions of NO(solid symbols)and CH4(open symbols) as a function of reaction temperature over In-Fe2O3/H-ZSM-5 (1:4:20squares)and In/H-ZSM-5(1:20circles),2500ppm NO, 2000ppm CH4,4.0%O2,GHSV=3600h?1.
oxidation rate became so rapid that all the methane in the feed gas was used up,and NO conversion reached a maximum.Then,further increase in reaction tem-perature caused the NO conversion to decrease with temperature.On the other hand,the temperature pro-?les of the CH4conversion were also almost identical for both of the catalysts.It is worth to point out that in spite of the trend of temperature dependence was sim-ilar for the two catalysts,the NO conversion was dis-tinctly higher over the In–Fe2O3/H-ZSM-5(1:4:20) catalyst.In this work,SCR selectivity is de?ned as the ratio of CH4consumption for NO reduction to total CH4consumption[13],and this implied that the In–Fe2O3/H-ZSM-5catalyst exhibited a higher SCR selectivity of NO,or in other words,a lower selec-tivity for the combustion reaction of CH4with O2. These results indicated that,with the addition of iron oxide to the In/H-ZSM-5catalyst,higher activity and selectivity of the catalyst in excess oxygen for a wide range of reaction temperatures were developed. Fig.2shows the effect of Fe2O3content on the activity of the catalyst.With the increasing of the Fe2O3content of the catalyst,NO conversion was enhanced.However,excess addition of Fe2O3sup-pressed the SCR activity.It was very interesting to?nd that even for a high iron oxide content of In:Fe2O3:H-ZSM-5=1:20:20,the SCR activ-ity was not greatly decreased and NO could still
X.Wang et al./Applied Catalysis B:Environmental24(2000)169–173
171
Fig. 2.Effect of Fe2O3contents on the conversions of NO and CH4over In–Fe2O3/H-ZSM-5with2500ppm NO,2000ppm CH4, 4.0%O2,GHSV=3600h?1.In/H-ZSM-5(1:20?), In–Fe2O3/H-ZSM-5(1:1:20?),In–Fe2O3/H-ZSM-5(1:8:20 ),In–Fe2O3/H-ZSM-5(1:20:20?).
be completely reduced into N2between500and 550?https://www.doczj.com/doc/9f1745600.html,pared with the catalyst with lower con-tent of Fe2O3(i.e.In:Fe2O3:H-ZSM-5=1:1:20), the In–Fe2O3/H-ZSM-5(1:20:20)catalyst showed lower CH4conversion at low temperatures(below 450?C)and higher CH4conversion at high tempera-tures(above500?C),which,however,was not due to the reaction with NO but due to the competitive reac-tion with O2,as considering the activity suppression. These results show that addition of a suitable amount of Fe2O3to the In/H-ZSM-5catalyst can promote CH4reduction of NO on one hand,and inhibit CH4 combustion with O2on the other hand.Effective Fe2O3ratios(In:Fe2O3:H-ZSM-5)in the
catalyst Fig.3.Conversions of NO(solid symbols)and CH4(open sym-
bols)as a function of O2concentration over In–Fe2O3/H-ZSM-5 (1:8:20squares)and In/H-ZSM-5(1:20circles)at450?C, 2500ppm NO,2000ppm CH4,4.0%O2,GHSV=3600h?1
. Fig.4.Conversions of NO(solid symbols)and CH4(open symbols) as a function of space velocity over In–Fe2O3/H-ZSM-5(1:8:20 squares)and In/H-ZSM-5(1:20circles)at600?C,2500ppm NO,
2000ppm CH4,4.0%O2.
ranged from1:1:20to1:8:20,as revealed by the results of this investigation.
Figs.3and4show the changes in NO and CH4con-versions with respect to O2concentration and space velocity at450and600?C over an In/H-ZSM-5(1:20) and an In–Fe2O3/H-ZSM-5(1:8:20)catalyst,respec-tively.For the In–Fe2O3/H-ZSM-5(1:8:20)catalyst, the increase in O2concentration from2to8%en-hanced the NO conversion,then followed by a slight decrease with higher concentrations of O2.For the In/H-ZSM-5(1:20)catalyst,however,NO conver-
172X.Wang et al./Applied Catalysis B:Environmental 24(2000)169–173
sion was enhanced only up to 4%O 2,then showed a decrease with higher O 2concentrations.Moreover,the difference in CH 4conversion between these two catalysts was remarkable,CH 4conversion over the In–Fe 2O 3/H-ZSM-5(1:8:20)was higher than that over the In/H-ZSM-5(1:20),although both conver-sions slightly increased with the increase in O 2con-centration.This indicates that iron oxide addition is favorable for CH 4activation.As for the effect of space velocity variation,an increase in space velocity en-hanced NO conversion over the In-Fe 2O 3/H-ZSM-5(1:8:20)catalyst,whilst an opposite trend was ex-hibited over the In/H-ZSM-5(1:20).The difference in the effect of space velocity on CH 4conversion for the two catalysts was more interesting.On the In/H-ZSM-5(1:20),CH 4conversion was lower than that on the In–Fe 2O 3/H-ZSM-5(1:8:20)catalyst,and was strongly dependent on space velocity.In contrast,CH 4conversion on the In–Fe 2O 3/H-ZSM-5(1:8:20)leveled-off with space velocity,and reached 100%.This again illustrates that the addition of iron oxide played an accelerative role in CH 4activation.
Fig.5compares the in?uence of water vapor on the activities of the In–Fe 2O 3/H-ZSM-5(1:4:20)and the In/H-ZSM-5(1:20)catalysts.Although both catalysts were sensitive to water vapor,however,the retard-ing effect of water vapor on the In–Fe 2O 3/H-ZSM-5(1:4:20)catalyst was relatively small,
especially
Fig.5.Effect of water vapor on the conversions of NO (solid sym-bols)and CH 4(open symbols)over In–Fe 2O 3/H-ZSM-5(1:4:20squares)and In/H-ZSM-5(1:20circles)with 2500ppm NO,2000ppm CH 4,4.0%O 2,3.3%H 2O,GHSV =3600h ?1.
when the temperature was above 450?C,and the NO conversion reached 91%at 500?C in the presence of 3.3%H 2O.Furthermore,CH 4conversion over this catalyst was higher than that over the In/H-ZSM-5(1:20)at this temperature region.Interestingly,the presence of water vapor greatly promoted the CH 4conversion over the In–Fe 2O 3/H-ZSM-5(1:4:20)at temperatures above 450?C.Obviously the addition of Fe 2O 3was responsible for this result.It is worth to point out that,manganese oxide,when mixed physi-cally with an active de-NO x catalyst,has been reported to enhance the activity [19–23].However,the effect of adding iron oxide to de-NO x catalysts has not been reported so far.Further studies are in progress in our laboratory to elucidate the effect of iron oxide and the in?uence of the preparation method of the catalyst.4.Conclusions
The addition of iron oxide has a promotion effect on the catalytic activity of In/H-ZSM-5catalyst for the SCR of NO with methane.In–Fe 2O 3/H-ZSM-5cata-lysts with Fe 2O 3ratios (In :Fe 2O 3:H-ZSM-5)rang-ing from 1:1:20to 1:8:20showed high activities for the SCR of NO to N 2by methane in the presence of excess oxygen with high space velocity,as well as in the presence of water vapor.Fe 2O 3addition also promoted CH 4conversions.References
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