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Preparation and catalytic performance of Co3O4 catalysts for low-temperature CO oxidation

Preparation and catalytic performance of Co3O4 catalysts for low-temperature CO oxidation
Preparation and catalytic performance of Co3O4 catalysts for low-temperature CO oxidation

Preparation and catalytic performance of Co 3O 4catalysts

for low-temperature CO oxidation

Yong-Zhao Wang,Yong-Xiang Zhao,*Chun-Guang Gao,and Dian-Sheng Liu*

School of Chemistry and Chemical Engineering,Institute of Advanced Chemistry,Shanxi University,Taiyuan 030006,China

Received 25February 2007;accepted 17March 2007

Without use of any surfactant or oxidant,a series of Co 3O 4catalysts have been prepared from cobalt nitrate aqueous solution via a very simple liquid-precipitation method with ammonium acid carbonate followed by calcination at various temperatures.The catalytic performance of the Co 3O 4for CO oxidation has been studied with a continuous ?owing laboratory microreactor system.The results show that the CO conversion of all the samples can reach 100%at ambient temperature.The catalyst calcined at 300°C is able to maintain its activity for CO complete oxidation more than 500min at 25°C and about 240min even at )78°C.High reaction temperature results in a high catalytic stability,while the catalytic stability decreases with further increasing the reaction temperature.Characterizations with X-ray powder di?raction and transmission electron microscopy suggest that all the samples exist as a pure Co 3O 4phase with the spinel structure,the samples are apt to aggregate and the speci?c surface area gradually decreases with increasing the calcination temperature,which directly leads to the decrease of catalytic stability.Furthermore,the amount of active oxygen species measured by CO titration experiments appears to be critical for catalytic performance.

KEY WORDS:Co 3O 4;liquid-precipitation;low-temperature CO oxidation;active oxygen species.

1.Introduction

The low-temperature catalytic oxidation of CO has become an important research topic over the years due to its many potential areas of applications [1–9].Par-ticularly,these applications include air-puri?cation devices for respiratory protection,pollution control devices for reducing industrial and automotive emis-sions,closed-cycle carbon dioxide lasers,removing trace quantities of CO from the ambient air in sealed cabins such as submarines and space crafts on long duration missions and carbon monoxide gas sensors.Recently,the selective CO oxidation in the fuel cell at low tem-perature also becomes important.Precious metal cata-lysts,such as Pd/CeO 2–TiO 2,Pd/NaZSM-5,Pt/CeO 2and Sn–Pt/SiO 2[10–13],especially many gold-base cat-alysts supported on di?erent materials have been studied for CO oxidation and show high catalytic activities [14–20].However,the sensitivity of precious metals to sulfur poisoning and their high cost have long motivated the search for substitute catalysts including various transi-tion metal oxides [21–37].

As a promising candidate of precious metal catalysts,cobalt oxide catalysts have attracted considerable attention because of not only its high activity for CO

and hydrocarbon oxidation [26,38],but also its use in the Fischer–Tropsch synthesis [39],NO decomposition [40],and ammonia oxidation [41].Particularly,the low-temperature CO oxidation over Co 3O 4has been studied by several groups.Jia et al .[27]prepared nanometer Co 3O 4using dodecylbenzene sulfonic acid sodium salt (DBS)as the surfactant and found it was able to oxide completely CO at low temperature,but without using DBS the catalytic activity became very poor.Lin et al .[28–29]and Wang et al .[30]reported the synthesis of Co 3O 4by the precipitation-oxidation method with hydrogen peroxide as the oxidant combined with the temperature program reduction technique.Wang et al .found that the Co 3O 4prepared by precipitation-oxida-tion method with hydrogen peroxide exhibited higher activity than that prepared by impregnation or hydro-thermal due to the use of oxidant.Yu Yao [26]studied kinetic parameters and Jansson [32]studied the deacti-vation for the reaction over Co 3O 4obtained from the decomposition of Co 2O 3or commercial Co 3O 4,but the activity or stability was not good.From the results of the literatures it can be seen that the surfactant or oxi-dant is necessary to obtain a Co 3O 4catalyst with high catalytic activity and stability.However,about the above mentioned method there are many disadvantages such as the second pollution due to elimination of sur-factant,the complicated process of operation and the cost of surfactant and oxidant.

*To whom correspondence should be addressed.

E-mails:dsliu@https://www.sodocs.net/doc/278369061.html,;yxzhao@https://www.sodocs.net/doc/278369061.html, Catalysis Letters,Vol.116,Nos.3–4,August 2007(ó2007)136

DOI:10.1007/s10562-007-9099-4

1011-372X/07/0800–0136/0ó2007Springer Science +Business Media,LLC

In this paper,without use of any surfactant or oxi-dant,the Co3O4catalysts with high catalytic activity for low-temperature CO oxidation were obtained via better control of various preparation parameters in liquid-precipitation process.In?uences of activity test condi-tion on the catalytic performance were also discussed. The samples were characterized by means of X-ray powder di?raction(XRD),Transmission electron microscopy(TEM),N2adsorption-desorption,Infrared spectroscopy(FT-IR)and CO titration.The results of this physicochemical characterization were discussed in relation to the exhibited catalytic performance of the Co3O4catalysts.

2.Experimental

2.1.Preparation of catalysts

A Co3O4sample was prepared via a liquid-precipi-tation method in an aqueous solution.30ml of4.0M cobalt nitrate solution was added drop by drop to30ml of 2.0M ammonium acid carbonate solution under constant stirring at25°C.The temperature was then raised to30°C and maintained for3h.The?nal pH is about8.0.The resulting suspension was?ltered and the obtained precipitate was washed several times with deionized water,and then it was dried in air at110°C for12h.Finally,the samples were calcined at250,300, 400and600°C in?owing air for3h,respectively.The obtained samples were coded as LPC-T,where T refers to the calcination temperature.

2.2.Characterization of catalysts

XRD patterns of the samples were recorded on a Rigaku D/MAX-RB X-ray di?ractometer with a target of Cu K a operated at50kV and60mA with a scanning speed of0.5°/min and a scanning angle(2h)range of 15–75°.Average particle sizes of the Co3O4samples were measured from X-ray line broadening analysis (XLBA)using the well-known Scherrer equation[42]:

d?

0:89k Be2hTcos h

N2adsorption–desorption isotherms at)196°C were determined using CE SORPTOMATIC1990SERIES instrument.Prior to the sorption experiments,the samples were degassed under vacuum at150°C for 12h.The speci?c surface area was determined from the nitrogen adsorption isotherm.

The infrared spectra(FT-IR)of the samples were obtained on a1730Infrared Fourier Transform Spec-trometer(Perkin–Elmer)in the range of450–1500cm)1 with a resolution of4cm)1.One milligram of each powder sample was diluted with200mg of potassium bromide(KBr)powder.

Transmission electron microscopy(TEM)measure-ments were carried out by using Hitachi H-800TEM operated at80kv.Samples for TEM were dispersed by ultrasonic in acetone followed by deposition of the suspension onto a standard Cu grid covered with a holey carbon?lm.

The amount of active oxygen species on the catalysts was measured by CO titration at25°C.100mg of sample was loaded in a4mm i.d.quartz microreactor and heated in?owing air(30mL min)1)at200°C for 30min,then cooled to25°C in?owing N2(30mL min)1).1ml of3%CO/N2was injected with a syringe repeatedly until no CO2was observed in the e?uent gases.The pulses were performed at intervals of5min. The amount of active oxygen species was calculated from the amount of CO2obtained during the pulse.The analysis of the e?uent gases(CO and CO2)was per-formed with an on-line gas chromatograph equipped with a3m column packed with carbon molecular sieve, a methanator and a Flame Ionization Detector(FID). In order to enhance the sensitivity of the detection,CO and CO2were converted to CH4by the methanator at 360°C before entering into the FID.The minimum detection level was ca.10ppm.

2.3.Apparatus and method for activity test

The measurements of catalytic activity for low-tem-perature CO oxidation were carried out in a continuous ?ow laboratory microreactor under atmospheric pres-sure.The microreactor was a8mm i.d.quartz u-tube, and a thermocouple enveloped in glass was set into the catalyst bed to measure the temperature.The samples were sieved to40–60mesh so that pressure drop and concentration and temperature gradients over the cata-lyst bed were negligible.300mg of catalysts was used for each run.The feed gases adjusted by mass?ow con-trollers consisted of0.5vol%CO,14.4vol%O2and 85.1vol%N2passed through the catalyst bed with a total?ow rate of20mL min)1.Quantitative analysis of CO and CO2was the same as the method described in the CO titration experiment.

Prior to all catalytic experiments the catalysts were pretreated in?owing air at200°C for30min to yield clean surface and then cooled in the absence of?owing air.During the test,pretreatment and reaction gases were dried fully through the molecular sieve and silica gel.Control at low temperature()78,)20and0°C)was achieved by methanol–dry ice,ice–salt or ice–water mixture,respectively.For higher than room temperature a temperature controller was used.

The CO conversion was calculated based on the CO consumption as follows:

%of Conversion of CO?

[CO]inà[CO]out

[CO]in

?100

Y.-Z.Wang et al./Preparation and catalytic performance of Co3O4catalysts137

3.Results and discussion 3.1.Characterization of catalysts

The XRD patterns of a series of catalysts obtained at di?erent calcination temperatures are presented in ?g-ure 1.Powder XRD peaks of the cobalt oxide are well consistent with the data of the JCPDS ?le of Co 3O 4with cubic phase.It is clear that Co 3O 4is the only phase after the decomposition of the precipitate at 250,300,400and 600°C,and no di?raction peak other than those of Co 3O 4are present.All the samples are well crystallized and the degree of crystallinity increases gradually with increasing the calcination temperature.The average particle sizes,i.e.XLBA sizes,according to the (311)di?raction pattern of crystalline Co 3O 4,can be calculated using the Scherrer equation,which is in broad agreement with our TEM observation (will be presented shortly),listed in the third column of table 1.From the results it can be seen that the order of particle size is:LPC-600(62nm)>LPC-400(20nm)>LPC-300(10nm)>LPC-250(8nm).

Table 1shows the physicochemical properties of the samples obtained at calcination temperature ranging from 250to 600°C.The speci?c surface area (S BET ,m 2g )1)is listed in the second column of table 1.Calci-nation of the sample at 250and 300°C had a small e?ect on the speci?c surface area (61–54m 2g )1).How-ever,it was seen that the speci?c surface area was reduced dramatically to 30m 2g )1(LPC-400)and 9m 2g )1(LPC-600)when the calcination temperature was further increased to 400and 600°C,respectively.The decrease of the speci?c surface area is probably due to the growth and aggregation of the Co 3O 4nanopar-ticles with the increasing calcination temperature,which is also agreement with our TEM observation (will be presented shortly).

Figure 2shows the FT-IR spectra of the samples treated at di?erent temperatures.The appearance of two distinctive bands at ca.568(m 1)and 667cm )1(m 2),

originating from the stretching vibrations of the metal–oxygen bond,con?rms the formation of the Co 3O 4spinel oxide [43–45].The m 1band is associated with OB 3vibrations in the spinel lattice,where B denotes the Co 3+in an octahedral position,and the m 2band is attributed to the ABO 3vibrations,where A denotes the Co 2+in a tetrahedral position [46–47].With the decrease of the particle size,the two bands slightly shift to a lower wave number due to a large number of defects at the surface of the much smaller nanoparticles to weaken the Co–O bond strength.The redistribution of free electrons between the surface and the bulk results in a decrease of the bond force constant,and consequently absorption red shifts [30].The band near 1382cm )1of the LPC-250IR spectra suggests that the sample cal-cined at 250°C was contaminated with some nitrate radical residues [46],which disappear in other samples calcined at higher temperature.

It is clear that the TEM images of the Co 3O 4samples (?gure 3)calcined at 250and 300°C show monodis-perse and ultra?ne particles with uniform size about 5–13nm,which is generally consistent with the calculation from the broadening of powder di?raction peaks in X-ray patterns.The particles are apt to aggregate together randomly with increasing the calcination temperature.

It

Figure 1.XRD patterns of Co 3O 4calcined at di?erent temperatures:

(a)250°C;(b)300°C;(c)400°C and (d)600°C.

Table 1

S BET ,XLBA size,TEM size,Amount of active oxygen species and stability on CO oxidation of Co 3O 4calcined at di?erent temperatures Sample

S BET (m 2g )1)XLBA (nm)TEM (nm)Amount of active oxygen species (l molg )1)

Stability (nm)LPC-2506185–1099240LPC-30054108–13144500LPC-400302018–2291230LPC-600

9

62

44–80

21

50

Figure 2.FTIR spectra of Co 3O 4calcined at di?erent temperatures in

KBr:(a)250°C;(b)300°C;(c)400°C and (d)600°C.

Y.-Z.Wang et al./Preparation and catalytic performance of Co 3O 4catalysts

138

can be seen that there exists a certain extent of coagu-lation of the LPC-400,to a higher degree,of the LPC-600.The particle sizes estimated from TEM are pre-sented in the fourth column of table 1.

Figure 4shows the amount of active oxygen species,as obtained by CO titration and listed in the ?fth col-umn of table 1.The plot of active oxygen species shows a volcano curve with calcination temperature.The LPC-300possesses the most amounts of active oxygen spe-cies.The same as the speci?c surface area,the amounts of active oxygen species decrease with increasing the calcination temperature.However,the LPC-250pos-sesses fewer amount of active oxygen species than that of the LPC-300,though the speci?c surface area of the former is slightly larger.

3.2.Catalytic performance for low-temperature CO

oxidation 3.2.1.In?uence of calcination temperature on CO oxi-dation performance

The CO oxidation performance of LPC-250,LPC-300,LPC-400and LPC-600as a function of reaction time is compared and given at room temperature in ?gure 5.It is evident that all the catalysts exhibit very high activity for CO oxidation at room temperature and the calcination temperature has a strong in?uence on the catalytic performance.However,the results are di?erent from the reports presented by Jia [27],i.e.the high activity Co 3O 4catalyst was obtained only under using DBS as the surfactant during the preparation process and being calcined at 300°C.The results are also better than that of the literatures [28–30].Under the similar pretreatment and reaction conditions,Lin et al .[28–29]and Wang et al .[30]reported that the full conversion of CO is reached at about 130°C.It can also be observed that the reaction stability of CO oxidation increases gradually with the decrease of calcination temperature except 250°C.The time of CO complete conversion is 50,230,500and 240min,respectively,listed in the last column of table 1.

Clearly,all the samples possess high catalytic activity existing as a pure Co 3O 4phase with a spinel structure.However,the geometrical form and the particle size of Co 3O 4are quite di?erent for various catalysts.With increasing calcination temperature,the particles tended to aggregate and grow,which result in the decrease of the speci?c surface area,and the amount of active oxygen species on the samples.Hence,raising the calcination temperature results in a lower

stability

Figure 3.TEM images of Co 3O 4catalysts calcined at di?erent temperatures:(a)250°C;(b)300°C;(c)400°C and (d)600°C.

Y.-Z.Wang et al./Preparation and catalytic performance of Co 3O 4catalysts 139

(?gure 5).However,the stability of CO oxidation over the LPC-250is lower than that over the LPC-300,though the particle size of the former is smaller and the speci?c surface area is slightly larger.It should be noted that the amount of active oxygen species on the LPC-

250is fewer than that on the LPC-300,probably due to the presence of some nitrate radical residues on the surface of the LPC-250.

Jia et al .[27]reported that the particle size of Co 3O 4catalysts was crucial to the catalytic performance for low-temperature CO oxidation.Lin et al .[28]found that the catalytic activities toward the CO oxidation were decreased signi?cantly with the oxidation state of cobalt,i.e.CoO(+2)?Co 3O 4(+8/3))CoO(OH)(+3)?CoO x (>+3),and Wang et al .[30]reported that the T 50was decreased signi?cantly with increasing the S BET of the Co 3O 4.With the increase of calcination temper-ature,unlike the trend of particle size and speci?c sur-face area,the amount of active oxygen species showed a volcano curve.This indicates that the amount of active oxygen species is one of the main factors which a?ect the catalytic performance.

3.2.2.In?uence of reaction temperature on CO oxidation Figure 6shows the CO oxidation performance of the LPC-300catalyst as a function of reaction time at var-ious reaction temperatures,which are )78,)20,0,25and 50°C,respectively.Apparently,the catalyst showed very high activity for CO oxidation at low temperature and it was able to maintain its activity for CO

complete

Figure 4.E?ect of calcination temperature on the amount of active oxygen species on the Co 3O 4

catalysts.

Figure 5.Catalytic performance of Co 3O 4catalyst calcined at di?er-ent temperatures (reaction conditions:CO 0.5%,O 214.4%,GHSV 5000h )1,room temperature.).

Y.-Z.Wang et al./Preparation and catalytic performance of Co 3O 4catalysts

140

oxidation at )78°C about 240min followed by periodic oscillations of CO conversion,just as the result reported by Cunningham [31].The most probable explanation is the intermittent decomposition of carbonate intermedi-ates at the catalyst surface.It can be seen that the sta-bility of the LPC-300catalyst increases with increasing reaction temperature from )78to 25°C,though the di?erence of the stability at )20and 0°C is not obvious.The results are better than that of the literatures,and the T 50is )54,80,and )63°C [31–32,34],respectively.However,raising the reaction temperature to 50°C,the time of CO complete conversion decreased compared to that of 25°C.This is,perhaps,related to the strong thermal release originated from the CO oxidation,and the further investigation is still under discussion.

4.Conclusions

Without use of any surfactant or oxidant,a series of Co 3O 4catalysts with a high catalytic activity were pre-pared by a very simple liquid-precipitation method.It can be seen that all the samples calcined at di?erent temperatures exist as a pure Co 3O 4phase with the spinel structure.The samples calcined at 250and 300°C show monodisperse and ultra?ne particles with the particle size being 5–13nm.The particles tend to aggregate and the average size increases gradually with increasing the calcination temperature from 250to 600°C,and the speci?c surface area decreases from 61to 9m 2g )1.Particularly,the sample obtained at 250°C is contami-nated with some nitrate radical residues which disappear at other samples calcined at higher temperature.The present preparation method is facile,inexpensive and practical in scale up.

The samples are very e?ective for the low-tempera-ture CO oxidation.The catalytic stability of the sample

toward CO oxidation increases with decreasing calci-nation temperature from 600to 300°C due to the decrease of the average particle size and the increase of the speci?c surface area.CO titration experiments show that the amount of active oxygen species on the samples appears to be critical for catalytic performance.The LPC-300possesses the most amounts of active oxygen species and shows the highest catalytic stability com-pared to that of other samples.The LPC-300is able to maintain its activity for CO complete oxidation more than 500min at 25°C and about 240min even at )78°https://www.sodocs.net/doc/278369061.html,pared with Co 3O 4catalyst reported in the literatures,our catalysts exhibit better low-temperature catalytic performance.

Acknowledgments

The authors thank the Shanxi Natural Science Foundation (grants:20041017)and Shanxi Scienti?c &Technological Promoted Project of China (grants:031099)for the ?nancial support of this work.

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Y.-Z.Wang et al./Preparation and catalytic performance of Co3O4catalysts 142

雅思口语素材汇总之端午节

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expand/enlarge one’s scope of knowledge knowledge reserve/base/storage theoretical knowledge practical skills social experience broaden one’s knowledge base promote one’s overall/ comprehensive competence accumulate experiences learn lessons from past experiences Work and experience the scarcity of employment o p p o r t u n i t i lay the foundations for career p r o s p e r i t y

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一.雅思P2相关话题 一日假期 和外国朋友一起吃饺子的建议 1.The best way to eat a dumpling is in one bite. “Dumplings are designed to be consumed in one mouthful, as it’s the best way to enjoy the combination of the meat filling and the very thin and springy flour wrapper,” If you can’t use chopsticks, eat your dumplings with your fingers. Avoid using a fork at all costs, as piercing the dumpling will compromise the flavour. 3.Mix two parts vinegar with one part soy sauce for the perfect dumpling sauce. Add young ginger slices too, Chili oil is also a great addition when available. 4.Dumplings are just one element of dim sum. “Dim sum doesn’t just include dumplings. It’s also braised dishes like pork ribs, chicken feet, and beef balls. It’s actually small tapas-style dishes that are eaten in Cantonese restaurants at lunchtime,” 5.Xiao long bao dumplings are different from others as they contain broth. They originated in the Jiangnan region of China and are prepared in bamboo steaming baskets called xiao long, hence the name. 6.When eating xiao long bao or a dumpling with a ~soupy~ interior, opt for chopsticks and a spoon. “As soon as the dumplings arrive at your table, lift one from the steamer basket onto a soup spoon. Next, tear the skin of the dumpling by pressing the chopsticks from the side of the dumpling onto the spoon. The broth will ooze out onto the spoon. Sip the soup then enjoy the dumpling in one mouthful.” 7.You can tell whether your dumpling was cooked fresh or frozen by looking at the skin. “The skin of a freshly made dumpling is springy and light. “Frozen ones tend to be soggy.” 8.When makingdumplingsat home, try to keep your packages small.

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(完整word版)雅思口语素材整理汇总

雅思口语素材训练 by Tina Li Do you like music??A—肯定:Definitely yes, everyone enjoys music, and I am no exception! I love... 否定 :Well, honestly speaking, music is really not my cup of tea, simply because... ( 给出直接原因) What—pop, techno ( 电音音乐), hip-hop, rock, meditation ( 冥想乐) and especially light music.( 罗列名词) Where—Normally speaking, I would like to listen to music with my earphones when I take a ride on public transportation. ( 给出一个具体的场景) When—As long as I couldn’t go to sleep, I’d like to listen to some light music to calm myself down. ( 给出一个条件 :As long as I..., I would...)

Who—My most favourite singers include Adele, James Blunt, Avril Lavigne, and so forth. ( 喜欢的歌手) Why—I am fond of music mainly because it can cheer me up greatly when I feel down/low/ blue/bored/tired/depressed. ( 心情不好的时候让我高兴起来)?Besides, I also believe that music is an indispensable part of culture and tradition, through which I could have a better understanding of different cultures around the world, including cowboy culture, African-American street culture, the three main reli- gions and so on. ( 有助于理解不同的文化) Do you like watching movies?? A—Speaking of movies, yes, I am a big fan of all types of movies, such as...?What—comedy, action, romance, sci-fi, manga, vampire, zombie, animation...

英文翻译价格

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雅思口语part 2 素材分类整理

做有偿工作的人 Describe a person you know who is doing a paid job. You should say: Who this person is What job it is; How long the job lasted; And explain why you or this person chose to do this job. 让你笑的小孩 Describe a time that a child did something that made you laugh. You should say: When this happened Who the child was What the child did And explain why it was funny 特殊的旅行 Describe an educational trip you went on when you were in school. You should say: When and where you went; Who you went with; What you did; And explain what you learned on this trip.

Describe an electronic machine you want to buy. You should say: What it is When you know this machine What specific And explain why you want this machine 难忘的广告 Describe an unforgettable advertisement (that you saw or heard liked) You should say: Where you saw or heard it What kind of advertisement it was What the contents of the advertisement were (or, what product or service was advertised) And explain how you felt when you saw or heard this advertisement/why you like it

【2019-2020】雅思口语话题素材-精选word文档 (1页)

【2019-2020】雅思口语话题素材-精选word文档 本文部分内容来自网络整理,本司不为其真实性负责,如有异议或侵权请及时联系,本司将立即删除! == 本文为word格式,下载后可方便编辑和修改! == 雅思口语话题素材 雅思口语素材总结雅思口语要求学生平时多多的积累素材,在口语方面要不断的加强练习。烤鸭们要多模拟不同的场景,多用丰富的口语短语,这 样你的雅思口语水平才能越来越高。下面是常用的一些口语短语,供大家参考! 1、 a change of pace 改变步调;换口味 You cant do these chemistry experiments all day long . You certainly need a change of pace . 2、 a far cry from 相距甚远 The published book is a far cry from the early manuscript . 3、 and how 的确 A : Shes a good dancer . B : And how . 4、 a matter of time 时间问题 It is only a matter of time . 5、 a phone call away 一个电话之远,即愿意过来帮忙 If you need my help , do let me know . Just remember I am a phone call away . 6、 a while back 不久以前 Well , I listened to that CD you lent me a while back . 7、 all along 一直 I knew it all along . 8、 anything but 绝对不 I was anything but happy about going . 9、 account for 解释 How do you account for it ? 10、 after all 毕竟;终究 A : Ive just seen the X - rays and your teeth look just fine . B : I see . Then there is nothing to worry about after all . 11、allergic to 对过敏 Oh man ! Something in this room is making my eyes itch . I must be allergic to something . 12、 at sbs service 愿为某人服务 I am at your service at any time . 13、 around the clock 24小时不停 Martha studied around the clock for management exam . 14、 as far as I know 就我所知 But as far as I know , he once won the world champion at the Olympic Games . 15、 at home with 对很熟悉 She is at home with problems like this . 16、 back out 退出 A : Wasnt Bert supposed to sing tonight ? B : Yes , but he backed out at last minute . 17、be cut out for 适合于,有做某事物的天赋 She is cut out for a dancer . 18、 be absorbed in 全神贯注于某事物 She has been absorbed in a horrorfiction . I cant tear her away . 19、 be addicted to 对某事物上瘾 She has been addicted to drugs for years . 20、 be attached to 对某事物有感情 A : Im amazed that you are still driving that old car of yours . I thought you would have gotten rid of it years ago . B : It runs well and Ive actually been quite attached to it .

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