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Effect of the pressure on the catalytic oxidation of volatile organic compounds over AgAl2O3

Effect of the pressure on the catalytic oxidation of volatile organic compounds over Ag/Al2O3catalyst

Yi Li a,Xiuli Zhang a,Hong He a,*,Yunbo Yu a,Tao Yuan b,Zhenyu Tian b,Jing Wang b,Yuyang Li b

a Research Center for Eco-Environmental Sciences,Chinese Academy of Sciences,Beijing100085,PR China

b National Synchrotron Radiation Laboratory,University of Science and Technology of China,Hefei,Anhui230029,PR China

1.Introduction

Increasing interest is being shown in catalytic combustion

processes,which are convenient ways for the prevention of

emission(including the control of nitrogen oxides(NOx)and

unburned hydrocarbons in heat and power generation plants)as

well as clean-up processes(VOC removal and automobile exhaust

converters)[1].Compared with?ame or thermal combustion,

catalytic combustion exhibits advantages such as high ef?ciency,

ultra low pollutant emissions,stable combustion,and so on.

Fundamental knowledge of catalytic oxidation mechanisms is

valuable for the development of effective methods for using

catalysts to control air pollution.

The catalytic oxidation of VOCs over various catalysts,as a

means to control air pollution,has been investigated[2].Although

silver is not the best oxidation catalyst,it is a well-known catalyst

for the conversion of alcohols to aldehydes and ketones,and the

only metal that selectively catalyzes ethylene epoxidation to

ethylene oxide[3,4].Some researchers have studied the catalytic

oxidation of alcohols over silver based catalysts[5–11].Yao[5]

studied the catalytic oxidation of ethanol at low concentrations,

and found that nearly100%ethanol could be oxidized to

acetaldehyde over Ag/Al2O3catalyst,accompanied by a small

amount of CO,C2H4and CH3COOH.Cordi and Falconer[6]studied

the oxidation of?ve VOCs including methanol,ethanol,acetalde-

hyde,formic acid and acetic acid over Ag/Al2O3catalyst using

temperature-programmed desorption(TPD)and oxidation(TPO).

They reported that acetaldehyde and acetic acid were the partial

oxidation products of ethanol,while formic acid was the partial

oxidation product of methanol.

The selective catalytic reduction(SCR)of NOx by hydrocarbons

is a potential method for removing NOx from oxygen rich exhausts

of automobiles and stationary combustion facilities.Ag/Al2O3is

known to be one of the most effective catalysts for the SCR of NOx

by hydrocarbons.He et al.[9–12]found that an enolic surface

species could be formed on Ag/Al2O3during the partial oxidation of

CH3CHO,C2H5OH,C3H6,1-propanol and2-propanol over Ag/Al2O3

catalyst using in situ diffuse re?ectance infrared Fourier transform

spectroscopy(DRIFTS),and found that this highly active species

was the key intermediate in the process of NOx reduction by

hydrocarbons.Recently,Taatjes et al.[13]studied24different

?ames of14prototypical single fuels and low-pressure?ames of

commercial gasoline using synchrotron VUV photoionization mass

spectrometry,and found that enols are common intermediates in

hydrocarbon oxidation.It is well known that there is less collision

between molecules under low pressure,so that one could expect to

obtain rarer partial oxidation intermediates of VOC catalytic

oxidation under such reaction conditions.More recently,enols

with two to four carbon atoms have been found in propanols and

butanols fueled low-pressure?ames[14,15].It is very interesting

to note that enols can be formed from low-pressure plasma

discharges of alcohols as well[16].In addition,Ferguson et al.[17]

Applied Catalysis B:Environmental89(2009)659–664

A R T I C L E I N F O

Article history:

Received23September2008

Received in revised form5December2008

Accepted28January2009

Available online12February2009

Keywords:

Silver/alumina

Volatile organic compounds

Catalytic partial oxidation

Synchrotron VUV photoionization mass

spectrometry

A B S T R A C T

The catalytic oxidation of volatile organic compounds(VOCs:ethanol,1-propanol and2-propanol)over

Ag/Al2O3catalyst under low and normal atmospheric pressure conditions has been studied with

synchrotron vacuum ultraviolet(VUV)photoionization mass spectrometry(PIMS).The partial oxidation

intermediates of the VOCs under different pressures were identi?ed by PIMS and their photoionization

ef?ciency(PIE)spectra.Alkene is preferentially formed under the low pressure conditions,while

aldehyde and acid are favored under the normal atmospheric pressure conditions.In addition,the low

pressure conditions are more suitable for observing the active intermediates,such as ethenol,ketene and

propenal.The results indicate that the pressure has a signi?cant effect on the oxidation pathway of VOCs

over Ag/Al2O3catalyst.

*Corresponding author.Tel.:+861062849123;fax:+861062849123.

E-mail address:honghe@https://www.wendangku.net/doc/e96316044.html,(H.He).

Contents lists available at ScienceDirect

Applied Catalysis B:Environmental

j o u r n a l h o m e p a g e:w w w.e l se v i e r.c om/l oc a t e/a p c a t b

doi:10.1016/j.apcatb.2009.01.029

reported that elevated pressure could accelerate the oxidation reactions showing that the effect of pressure was important for the investigation of the reaction products.

In this article,the reaction mechanism of catalytic partial oxidation of VOCs(such as ethanol,1-propanol,and2-propanol) over Ag/Al2O3catalyst was studied under low and normal atmo-spheric pressure conditions using tunable synchrotron VUV photo-ionization mass spectrometry(PIMS)combining with molecular-beam sampling technique.The intermediates of partial oxidation were identi?ed by the measurements of PIMS and photoionization ef?ciency(PIE)spectra.The results could provide new insights into the mechanisms for the SCR of NOx by alcohols and provide signi?cant contributions to the practical use of Ag/Al2O3catalysts.

2.Experimental

2.1.Catalyst preparation

2.1.1.Preparation of Ag/Al2O3powders

The4%Ag/Al2O3catalysts were prepared by an impregnation method using an aqueous solution of silver nitrate.After impregnation,the excess water was removed in a rotary evaporator at333K.The samples were?rst dried at393K for 12h,followed by calcination at873K for3h.

2.1.2.Preparation of washcoated honeycomb catalysts

Washcoated honeycomb catalysts were prepared using the4% Ag/Al2O3powder.Washcoat slurries were then prepared by mixing 250g Ag/Al2O3powder and750g water in a ball mill.Cordierite honeycombs with400cells per square inch(90mm in diame-ter?110mm in length)were dipped into the washcoat slips,and excess slurry was blown out with an air knife.The samples were then dried and calcined at873K for3h.The washcoat loading was 110g là1after calcination.

2.2.Experimental setup

The experiments were carried out at National Synchrotron Radiation Laboratory in Hefei,China.A molecular-beam mass spectrometer(MBMS)using synchrotron VUV light as the ionization source provided a novel and selective method for the analysis of individual intermediates and products of the reaction. Synchrotron radiation from a bend magnet of the800MeV electron storage ring was monochromized with a1-m Seya-Namioka monochromator equipped with two gratings(2400and 1200grooves/mm),which provided single-photon ionization (SPI)and minimized fragmentation due to the near-threshold ionization.The wavelength of the monochromator was calibrated with the known ionization energies of the inert gases.The energy resolving power(E/D E)was about500–1000,depending on the slit widths.A LiF window with1.0mm thickness was used to eliminate higher order radiation of the dispersed light in the wavelength region longer than105nm.

With the tunability of the light source,a series of mass spectra from8.63to11.15eV were measured.The measurement time for each mass spectrum was120s.The baseline was subtracted from the integrated ion intensity.Each mass peak was integrated to yield the PIE spectra,a plot of ion intensity vs.photon energy.The value of IE could be directly obtained from the PIE spectra. Considering the energy resolution of the monochromator and the cooling effect of molecular beam[18],the experimental error of the measured IEs was less than0.05eV.Thus,all components could be identi?ed by their molecular weights from the photoionization mass spectrometry near threshold ionization and the ionization energies from photoionization ef?ciency spectra.The experimen-tal IEs were compared with known literature values.According to our previous studies under the same conditions,the?nal products of VOCs oxidation over Ag/Al2O3catalyst are CO2and small amount of CO[10].In this study,we focused on the intermediates of catalytic partial oxidation of VOCs,which are not in the same magnitude with the?nal products of CO2and CO,thereby we did not measure the photoionization ef?ciency(PIE)spectra of?nal products.

The details of MBMS have been described elsewhere[18].It was originally designed for the study of combustion.Some modi?ca-tions were made to detect products of the catalytic oxidation in this study.In brief,a schematic diagram of the experimental instrument is shown in Fig.1.A syringe pump(ISCO Inc.,USA)was used to control the?ow rate of the VOC aqueous solution(ethanol, 1-propanol,and2-propanol)at0.500ml/min.The

aqueous

Fig.1.Schematic diagram of the instrument under low pressure(A)and normal atmospheric pressure(B),including the sample inlet system(I),the catalyst bed(II),the differential pumped system(III),and the photoionization chamber with the re?ectron TOF MS(IV).The other components include:(1)the syringe pump;(2)the carrier helium inlet;(3)the temperature controller;(4)the vaporizer;(5)the catalyst bed;(6)the quartz nozzle with an ori?ce of500m m at the tip;(7)the nickel skimmer;(8)to a turbo molecular pump with the pump speed of3500l/s;(9)the ionization region;(10)to a turbo molecular pump with the pump speed of1600l/s;(11)the ion trajectory;

(12)the ion optics of the re?ectron;(13)the MCP detector;(14)the quartz sampling tube.

Y.Li et al./Applied Catalysis B:Environmental89(2009)659–664

660

solution of VOCs was vaporized with the carrier gas(Ar)and passed into the catalyst bed(II).The temperature of the vaporizer was maintained at423K to ensure that the aqueous solution was vaporized completely.As shown in Fig.1A,the catalyst bed was put in a low-pressure chamber($1.07KPa)for the low pressure experiment and just outside the chamber for the experiment under normal atmospheric pressure conditions(Fig.1B).The tempera-ture of the catalyst bed was controlled by a temperature controller. The appropriate temperatures were adjusted to keep the conver-sion of VOCs at about50%.The?ow rates of Ar and O2were1.500 and0.500standard liters per minute(SLM),respectively, controlled by a mass?ow controller(MKS Instruments Inc.).For the low pressure experiments,the gas-phase mixture of unburned VOCs,partial oxidation products of VOCs and water from the catalyst bed outlet was sampled through a quartz cone-like nozzle with an ori?ce of$500m m in diameter.A nickel skimmer with a2-mm diameter aperture was located at18mm downstream from the sampling nozzle.For the experiments under normal atmo-spheric pressure conditions,a quartz tube with2.0mm in diameter was used to sample the gases,as shown in Fig.1B.The sample gases formed a molecular beam,which passed horizontally through the 10mm gap between the repeller and extractor plates of the re?ectron TOF MS[19].The molecular beam intersected perpen-dicularly with the synchrotron VUV light beam.The ion signal was detected with the re?ectron TOF MS,which was installed vertically in the photoionization chamber(see Fig.1).A pulsed voltage of 200V was used to propel ions into the?ight tube,and?nally to a microchannel plate(MCP)detector.Detailed information on the RTOF MS can be found in previous reports[18,19].For the low pressure experiments,the pressures in the sample inlet chamber,

the differentially pumped chamber and the ionization chamber were800,10à3and10à3Pa,respectively.For the experiments under normal atmospheric pressure conditions,the pressures in the sample inlet chamber,the differentially pumped chamber and the ionization chamber were800,10à2and10à3Pa,respectively.

3.Results and discussion

3.1.Catalytic partial oxidation of ethanol over Ag/Al2O3catalyst under low and normal atmospheric pressure conditions

The partial catalytic oxidation of ethanol on the Ag/Al2O3 catalyst was conducted at603K under low pressure and at563K under normal atmospheric pressure.

Fig.2shows four VUV photoionation mass spectra of the partial oxidation of ethanol on Ag/Al2O3catalyst under low pressure and at different photon energies.Several peaks in the mass range of20–70 are observed at the photon energy of10.78eV,which correspond to hydrocarbons ranging from C2to C4.However,some peaks do not originate from ethanol oxidation.The peaks with odd m/z are attributed to13C isotope contribution.Furthermore,the peaks with odd m/z can also be produced from the fragmentation of large hydrocarbons,as the photon energy is high enough to produce fragmentation.In this study,the measurement of the PIE spectra of mass45is attributed to the fragmentation of ethanol.

Comparing the experimental IEs with the known values from the literature[20],several kinds of catalytic partial oxidation intermediates are identi?ed.The measured ionization thresholds and ion intensity for all the observed peaks are listed in Table1. Under low pressure conditions,ethylene,ketene,acetaldehyde, ethenol,propenal,acetone and2-butenal can be identi?ed;while under normal atmospheric pressure conditions,acetaldehyde, acetone,acetic acid,2-butenal and ethyl ether are identi?ed.Based on these results,it is obvious that the pressure has a signi?cant effect on the partial catalytic oxidation pathway of ethanol over Ag/Al2O3catalyst.

As can be seen in Table1,acetaldehyde,acetone,and2-butenal are all present during the partial oxidation of C2H5OH over Ag/ Al2O3catalyst under both low and normal atmospheric pressure conditions,and acetaldehyde is the main product with the highest intensity among them.It is considered that acetaldehyde is produced from the dehydrogenation of ethanol over Ag/Al2O3 catalyst;acetone is formed by the oxidation of ethanol with molecular oxygen;and2-butenal is formed from the self-condensation of acetaldehyde.Based on a previous study[6], acetaldehyde was the main partial oxidation product of ethanol during the TPO experiment and oxygen adsorbed on the silver site activates neighboring silver sites for dehydrogenation,forming acetaldehyde and H2.Yao[5]found that Ag is a well-known catalyst for the conversion of alcohols to aldehydes and ketones, and nearly100%ethanol was oxidized to acetaldehyde over Ag/ Al2O3catalysts under appropriate experimental conditions.They also found that acetone was formed by ethanol oxidation with molecular oxygen during the catalytic oxidation of ethanol over Pd/Al2O3catalysts.Furthermore,the aldol condensation reaction is an important method for the synthesis of unsaturated carbonyl compounds in organic chemistry.

The ethylene,ketene,ethenol and propenal in Table1are only identi?ed under low pressure conditions,whereas only acetic acid and ethyl ether are identi?ed under normal atmospheric pressure conditions.Ethenol,as the isomer of acetaldehyde,has the same molecular weight as acetaldehyde and is only observed under low pressure conditions.Here the ethenol and acetaldehyde can be differentiated by comparing their different IEs from the PIE spectra with literature values.The PIE spectra of m/z=44(C2H4O)(Fig.3) illustrate how to identify the isomers.Under low pressure,two onsets are clearly observed at9.33and10.21eV,which correspond to the IEs of ethenol and acetaldehyde according to the literature values of9.33?0.01,and10.21?0.01eV[20].Ethenol(CH 2HC–OH),bearing OH groups adjacent to carbon–carbon double bonds,is thermodynamically unstable relative to acetaldehyde(CH3–HC O). Until now,only a few experimental data are available on neutral

enol Fig.2.VUV photoionization mass spectra of partial oxidation of ethanol over Ag/ Al2O3catalyst under low pressure at the photon energies of10.78,10.60,10.42,and 10.25eV,respectively.

Y.Li et al./Applied Catalysis B:Environmental89(2009)659–664661

chemistry.In the low-pressure?ames of hydrocarbon oxidation, Taatjes et al.[13]found that enols are the common intermediates in hydrocarbon oxidation.Qi et al.[14,15]found that enols with two to four carbon atoms were in propanols and butanols fueled?ames. Also,it is very interesting that enols can also be formed from plasma discharges of alcohols[16].Under normal pressure,He et al.[9–12] observed the formation of the enolic species during the partial oxidation of alcohols on Ag/Al2O3catalyst using in situ DRIFTS measurements,where the Ag/Al2O3could provide a suitable surface to stabilize the ethenol.In the case of catalytic oxidation of ethanol over Ag/Al2O3,we deduce that C2H5OH is?rst adsorbed on the surface of Ag/Al2O3,an enolic anion(CH 2CH–O)àM+is then formed on the surface via the dehydrogenation of C2H5OH.A part of the enolic anion subsequently desorbs from the Ag/Al2O3catalyst,becoming gaseous ethenol.Owing to its thermodynamic instability under normal pressure,ethenol subsequently transforms rapidly into acetaldehyde. Therefore,we consider that ethenol is the key intermediate during the catalytic partial oxidation of C2H5OH on the Ag/Al2O3catalyst,and the formation of ethenol is signi?cant for the SCR of NOx by C2H5OH over the Ag/Al2O3catalyst.In contrast,under normal atmospheric pressure conditions,ethenol cannot be observed in gas phase.However,in our previous study[9–12],we found that an enolic surface species was formed on Ag/Al2O3under normal pressure during the partial oxidation of CH3CHO,C2H5OH,C3H6,1-propanol and2-propanol over Ag/Al2O3catalyst by in situ DRIFTS measurement.Therefore,we conclude that ethenol is formed on the surface of Ag/Al2O3catalyst, but desorbed ethenol subsequently transforms rapidly into acet-aldehyde in gas phase so that it cannot be detected even using synchrotron VUV photoionization mass spectrometry under normal pressure conditions.

Besides ethenol,the ethylene,ketene and propenal can only be identi?ed under low pressure conditions.It is deduced that ethylene is formed by the dehydration of ethanol over Ag/Al2O3 catalyst.Because of its high intensity,the formation of ethylene is another main reaction pathway for the partial oxidation of ethanol over Ag/Al2O3catalyst under low pressure conditions.It is also supposed that ketene is formed by the further dehydrogenation of acetaldehyde and the formation of propenal is a result of the rearrangement and further dehydrogenation of acetone.According to the literature[6],ethylene may be formed by dehydration of ethanol on Al2O3.

On the other hand,acetic acid and ethoxy ethane are only observed under normal atmospheric pressure,as shown in Table1. It is supposed that acetic acid is formed by the further oxidation of the generated acetaldehyde and ethyl ether is formed by the intermolecular dehydration of ethanol.In organic chemistry,the catalytic oxidation of aldehydes to the corresponding carboxylic acids is an important method for the preparation of acetic acid. Furthermore,in industrial applications,ethanol can be dehydrated to form ethyl ether on Al2O3with more than95%yield under suitable reaction conditions.

On the basis of the above results,we can conclude that pressure signi?cantly affects the formation pathway of the partial oxidation intermediates on Ag/Al2O3catalyst,as shown in Scheme1.The low pressure conditions favor the dehydration and dehydrogenation of ethanol over the Ag/Al2O3catalyst.However,normal atmospheric pressure conditions favor the partial catalytic oxidation of ethanol. In addition,low pressure conditions favor the observation of active intermediates,such as ethenol,ketene and propenal,and the

Table1

List of the partial oxidation species of ethanol identi?ed by their IEs in the?ow of ethanol over Ag/Al2O3catalyst under low pressure at603K and normal atmospheric pressure at563K.

m/z Formula Low pressure Normal atmospheric pressure Species Ref.[20]

Measured(?0.05eV)Ion intensity(arb.unit)Measured(?0.05eV)Ion intensity(arb.unit)

28C2H410.5222.46––Ethylene10.50?0.02 42C2H2O9.627.49––Ketene9.614?0.008 44C2H4O10.2167.3710.2187.72Acetaldehyde10.21?0.01 C2H4O9.330.90––Ethenol9.33?0.01 56C3H4O10.110.45––Propenal10.10?0.01 58C3H6O9.70.459.70.88Acetone9.71?0.01 60C2H4O2––10.65 6.58Acetic acid10.66?0.05 70C4H6O9.740.909.74 2.192-Butenal9.73?0.01

74C4H10O––9.53 2.63Ethyl ether9.53?

0.02

Fig.3.The PIE spectra of m/z=44(C2H4O)measured in the?ow of ethanol over Ag/ Al2O3catalyst under low pressure at603K and normal atmospheric pressure at 563

Scheme1.Possible formation pathways of ethanol partial oxidation intermediates

over Ag/Al2O3catalyst under low pressure at603K(solid frame)and normal

atmospheric pressure at563K(dotted frame).

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662

formation of ethenol is a key step during the catalytic oxidation of C2H5OH over the Ag/Al2O3catalyst.

3.2.Catalytic partial oxidation of1-propanol and2-propanol over Ag/ Al2O3catalyst under low and normal atmospheric pressure conditions To further investigate the effect of the pressure on the formation pathway of the partial oxidation products on Ag/ Al2O3catalyst,we also studied the catalytic partial oxidation of1-propanol and2-propanol over the catalyst.The catalytic partial oxidation of1-propanol was carried out at603K under low pressure and523K under normal atmospheric pressure.The measured ionization thresholds and ion intensity for all observed peaks are listed in Table2,along with the IEs from the literature [20].

Under low pressure conditions,ethylene,propene,propenal, propanal and benzene are identi?ed,whereas acetaldehyde, propanal,and propanoic acid are identi?ed under normal atmo-spheric pressure conditions.The formation pathways of the1-propanol partial oxidation intermediates over Ag/Al2O3catalyst are shown in Scheme2.

Comparing the results under low pressure with those under normal atmospheric pressure in Table2,only propanal is the common partial oxidation intermediate formed by the dehy-drogenation of1-propanol.The PIE spectra for propanal under both low and normal atmospheric pressure conditions are shown in Fig.4.Under normal atmospheric pressure,propanal shows the highest signal intensity,indicating that it is the main intermediate of the partial oxidation of1-propanol over Ag/ Al2O3catalyst.As shown in Table2,ethylene,propene,propenal and benzene are only observed under low pressure.It is deduced that propenal is formed by the further dehydrogenation of propanal.Propene is formed by the dehydration of1-propanol and shows the highest intensity,indicating that propene is the main intermediate of1-propanol partial oxidation over Ag/Al2O3 catalyst.Subsequently,a part of propene decomposes to ethylene.Trace amounts of benzene are formed by further dehydrogenation and condensation of ethylene or propene or both.Under normal atmospheric pressure,only acetaldehyde and propanoic acid are observed.The formation of acetaldehyde can be explained by the decomposition of propanal over Ag/ Al2O3catalyst,and propanoic acid is formed by the further oxidation of propanal.

A similar experiment was performed for the catalytic partial oxidation of2-proponal over Ag/Al2O3catalyst at603K under low pressure and at473K under normal atmospheric pressure.The measured ionization thresholds and ion intensities for all observed peaks are listed in Table3,along with the IEs from the literature [20].

Under low pressure conditions,propene,acetaldehyde,acetone, methylethyl ether and benzene are identi?ed,whereas only acetone appears under normal atmospheric pressure conditions. The formation pathways of the partial oxidation intermediates of 2-propanol over Ag/Al2O3catalyst are shown in Scheme3.

The results in Table3show that acetone is the common partial oxidation intermediate,formed by the dehydrogenation of2-propanol.The PIE spectra for acetone are shown in Fig.5,its high intensity indicating that the formation of acetone is the main reaction pathway in the partial oxidation of2-propanol over Ag/ Al2O3catalyst under low and normal atmospheric pressure conditions.

Under low pressure conditions,the other four products are observed,that is,propene,acetaldehyde,methylethyl ether and benzene.The formation of propene with high intensity can be

Table2

List of the partial oxidation species of1-propanol identi?ed by their IEs in the?ow of1-propanol over Ag/Al2O3catalyst under low pressure at603K and normal atmospheric pressure at523K.

m/z Formula Low pressure Normal atmospheric pressure Species Ref.[20]

Measured(?0.05eV)Ion intensity(arb.unit)Measured(?0.05eV)Ion intensity(arb.unit)

28C2H410.5 5.85––Ethylene10.50?0.02 42C3H69.7370.26––Propene9.73?0.01 44C2H4O––10.2 4.62Acetaldehyde10.21?0.01 56C3H4O8.95 3.75––Propenal8.95

58C3H6O9.9616.399.9686.71Propanal9.96

74C3H6O2––10.218.67Propanoic acid10.24?0.03

78C6H69.21 3.75––Benzene9.2?

0.1

Scheme 2.Possible formation pathways of1-propanol partial oxidation intermediates over Ag/Al2O3catalyst under low pressure at603K(solid frame) and normal atmospheric pressure at523K(dotted

frame).Fig.4.The PIE spectra for m/z=58(C3H6O)measured in the?ow of1-propanol over Ag/Al2O3catalyst under low pressure at603K and normal atmospheric pressure at 523K.

Y.Li et al./Applied Catalysis B:Environmental89(2009)659–664663

explained by the dehydration of the adsorbed 2-propanol over Ag/Al 2O 3catalyst,which is another main reaction pathway of the partial oxidation of 2-propanol on Ag/Al 2O 3catalyst under low pressure.Subsequently,trace amounts of benzene are formed by further dehydrogenation and condensation of propene.It is also supposed that methylethyl ether is formed by the intermolecular dehydration and decomposition of 2-propanol.Further decom-position may produce acetaldehyde.

Based on these results,it is noteworthy that the pressure has a signi?cant effect on the formation pathway of the partial oxidation products of 1-propanol and 2-propanol over Ag/Al 2O 3catalyst.Low pressure conditions favor the dehydration and dehydrogenation of 1-propanol and 2-propanol over the Ag/Al 2O 3catalyst.In contrast,normal atmospheric pressure conditions favor the partial oxida-tion of 1-propanol and 2-propanol.In addition,low pressure conditions favor observation of active intermediates such as propenal.

4.Conclusions

Synchrotron VUV photoionization mass spectrometry combined with a molecular-beam sampling technique has been used to study the partial catalytic oxidation of volatile organic compounds (ethanol,1-propanol and 2-propanol)over Ag/Al 2O 3catalyst under low pressure and normal atmospheric pressure conditions.It was found that the pressure has a signi?cant effect on the reaction pathways for the partial oxidation of the VOCs.Low pressure conditions favor the dehydration and dehydrogenation of VOCs over the Ag/Al 2O 3catalyst,and normal atmospheric pressure conditions favor the further partial oxidation of the VOCs.In addition,ethenol,ketene and propenal can be unambiguously identi?ed as active reaction intermediates under low pressure conditions.For the oxidation of ethanol,the formation of ethenol is a key step during the catalytic oxidation of C 2H 5OH on the Ag/Al 2O 3catalyst under both low pressure and normal atmospheric pressure conditions.Acknowledgements

Authors are grateful to Prof.Fei Qi for experimental design and modi?cation.This work was ?nancially supported by the National Natural Science Foundation of China (20773158)and the Ministry of Science and Technology of China (2007AA061402and 2007DFA61310).References

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[18]F.Qi,R.Yang,B.Yang,C.Q.Huang,L.X.Wei,J.Wang,L.S.Sheng,Y.W.Zhang,Rev.

Sci.Instrum.77(2006)084101.

[19]C.Q.Huang,B.Yang,R.Yang,J.Wang,L.X.Wei,X.B.Shan,L.S.Sheng,Y.W.Zhang,F.

Qi,Rev.Sci.Instrum.76(2005)126108.

[20]P.J.Linstrom,W.G.Mallard,NIST Chemistry Webbook,NIST Standard Reference

Database Number 69,National Institute of Standards and Technology,Gaithers-burg,MD,2003,https://www.wendangku.net/doc/e96316044.html, .

Table 3

List of the partial oxidation species of 2-propanol identi?ed by their IEs in the ?ow of 2-propanol over Ag/Al 2O 3catalyst under low pressure at 603K and normal atmospheric pressure at 473K.m /z

Formula

Low pressure Normal atmospheric pressure Species

Ref.[20]

Measured (?0.05eV)

Ion intensity (arb.unit)Measured (?0.05eV)Ion intensity (arb.unit)42C 3H 69.7321.34––Propene

9.73?0.0144C 2H 4O 10.21 2.78––Acetaldehyde 10.21?0.0158C 3H 6O 9.6974.219.70100.00Acetone

9.69?0.0160C 3H 8O 9.70.93––Methylethyl ether 9.6?0.178

C 6H 6

9.21

0.74

–

Benzene

9.2?

0.1

Scheme 3.Possible formation pathways of the 2-propanol partial oxidation intermediates over Ag/Al 2O 3catalyst under low pressure at 603K (solid frame)and normal atmospheric pressure at 473K (dotted

frame).

Fig.5.The PIE spectra for m /z =58(C 3H 6O)measured in the ?ow of 2-propanol over Ag/Al 2O 3catalyst under low pressure at 603K and normal atmospheric pressure at 473K.

Y.Li et al./Applied Catalysis B:Environmental 89(2009)659–664

664

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On the contrary Onthecontrary, I have not yet begun. 正好相反，我还没有开始。 https://www.wendangku.net/doc/e96316044.html, Onthecontrary, the instructions have been damaged. 反之，则说明已经损坏。 https://www.wendangku.net/doc/e96316044.html, Onthecontrary, I understand all too well. 恰恰相反，我很清楚 https://www.wendangku.net/doc/e96316044.html, Onthecontrary, I think this is good. ⑴我反而觉得这是好事。 https://www.wendangku.net/doc/e96316044.html, Onthecontrary, I have tons of things to do 正相反，我有一大堆事要做 Provided by jukuu Is likely onthecontrary I in works for you 反倒像是我在为你们工作 https://www.wendangku.net/doc/e96316044.html, Onthecontrary, or to buy the first good. 反之还是先买的好。 https://www.wendangku.net/doc/e96316044.html, Onthecontrary, it is typically american. 相反，这正是典型的美国风格。 222.35.143.196 Onthecontrary, very exciting.

恰恰相反，非常刺激。 https://www.wendangku.net/doc/e96316044.html, But onthecontrary, lazy. 却恰恰相反，懒洋洋的。 https://www.wendangku.net/doc/e96316044.html, Onthecontrary, I hate it! 恰恰相反，我不喜欢！ https://www.wendangku.net/doc/e96316044.html, Onthecontrary, the club gathers every month. 相反，俱乐部每个月都聚会。 https://www.wendangku.net/doc/e96316044.html, Onthecontrary, I'm going to work harder. 我反而将更努力工作。 https://www.wendangku.net/doc/e96316044.html, Onthecontrary, his demeanor is easy and nonchalant. 相反，他的举止轻松而无动于衷。 https://www.wendangku.net/doc/e96316044.html, Too much nutrition onthecontrary can not be absorbed through skin. 太过营养了反而皮肤吸收不了. https://www.wendangku.net/doc/e96316044.html, Onthecontrary, I would wish for it no other way. 正相反，我正希望这样 Provided by jukuu Onthecontrary most likely pathological. 反之很有可能是病理性的。 https://www.wendangku.net/doc/e96316044.html, Onthecontrary, it will appear clumsy. 反之，就会显得粗笨。 https://www.wendangku.net/doc/e96316044.html,

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M A: Has the case been closed yet? B: No, the magistrate still needs to decide the outcome. magistrate n.地方行政官，地方法官，治安官 A: I am unable to read the small print in the book. B: It seems you need to magnify it. magnify vt.１．放大，扩大；２．夸大，夸张 A: That was a terrible storm. B: Indeed, but it is too early to determine the magnitude of the damage. magnitude n.１．重要性，重大；２．巨大，广大 A: A young fair maiden like you shouldn’t be single. B: That is because I am a young fair independent maiden. maiden n.少女，年轻姑娘，未婚女子 a.首次的，初次的 A: You look majestic sitting on that high chair. B: Yes, I am pretending to be the king! majestic a.雄伟的，壮丽的，庄严的，高贵的 A: Please cook me dinner now. B: Yes, your majesty, I’m at your service. majesty n.１．［Ｍ－］陛下（对帝王，王后的尊称）；２．雄伟，壮丽，庄严 A: Doctor, I traveled to Africa and I think I caught malaria. B: Did you take any medicine as a precaution? malaria n.疟疾 A: I hate you! B: Why are you so full of malice? malice n.恶意，怨恨 A: I’m afraid that the test results have come back and your lump is malignant. B: That means it’s serious, doesn’t it, doctor? malignant a.１．恶性的，致命的；２．恶意的，恶毒的 A: I’m going shopping in the mall this afternoon, want to join me? B: No, thanks, I have plans already. mall n.（由许多商店组成的）购物中心 A: That child looks very unhealthy. B: Yes, he does not have enough to eat. He is suffering from malnutrition.

base on的例句

意见应以事实为根据. 3 来自辞典例句 192. The bombers swooped ( down ) onthe air base. 轰炸机突袭空军基地. 来自辞典例句 193. He mounted their engines on a rubber base. 他把他们的发动机装在一个橡胶垫座上. 14 来自辞典例句 194. The column stands on a narrow base. 柱子竖立在狭窄的地基上. 14 来自辞典例句 195. When one stretched it, it looked like grey flakes on the carvas base. 你要是把它摊直, 看上去就象好一些灰色的粉片落在帆布底子上. 18 来自辞典例句 196. Economic growth and human well - being depend on the natural resource base that supports all living systems. 经济增长和人类的福利依赖于支持所有生命系统的自然资源. 12 1 来自辞典例句 197. The base was just a smudge onthe untouched hundred - mile coast of Manila Bay. 那基地只是马尼拉湾一百英里长安然无恙的海岸线上一个硝烟滚滚的污点. 6 来自辞典例句 198. You can't base an operation on the presumption that miracles are going to happen. 你不能把行动计划建筑在可能出现奇迹的假想基础上.

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(完整版)主谓造句

主语+谓语 1. 理解主谓结构 1) The students arrived. The students arrived at the park. 2) They are listening. They are listening to the music. 3) The disaster happened. 2．体会状语的位置 1) Tom always works hard. 2) Sometimes I go to the park at weekends.. 3) The girl cries very often. 4) We seldom come here. The disaster happened to the poor family. 3. 多个状语的排列次序 1) He works. 2) He works hard. 3) He always works hard. 4) He always works hard in the company. 5) He always works hard in the company recently. 6) He always works hard in the company recently because he wants to get promoted. 4. 写作常用不及物动词 1. ache My head aches. I’m aching all over. 2. agree agree with sb. about sth. agree to do sth. 3. apologize to sb. for sth. 4. appear (at the meeting, on the screen) 5. arrive at / in 6. belong to 7. chat with sb. about sth. 8. come (to …) 9. cry 10. dance 11. depend on /upon 12. die 13. fall 14. go to … 15. graduate from 16. … happen 17. laugh 18. listen to... 19. live 20. rise 21. sit 22. smile 23. swim 24. stay (at home / in a hotel) 25. work 26. wait for 汉译英： 1.昨天我去了电影院。 2.我能用英语跟外国人自由交谈。 3.晚上7点我们到达了机场。 4.暑假就要到了。 5.现在很多老人独自居住。 6.老师同意了。 7.刚才发生了一场车祸。 8.课上我们应该认真听讲。9. 我们的态度很重要。 10. 能否成功取决于你的态度。 11. 能取得多大进步取决于你付出多少努力。 12. 这个木桶能盛多少水取决于最短的一块板子的长度。

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【it's time to和it's time for】 ——————这其实是一个句型,只不过后面要跟不同的东西. ——————It's time to跟的是不定式（to do）.也就是说,要跟一个动词,意思是“到做某事的时候了”.如： It's time to go home. It's time to tell him the truth. ——————It's time for 跟的是名词.也就是说,不能跟动词.如： It's time for lunch.(没必要说It's time to have lunch) It's time for class.(没必要说It's time to begin the class.) They can't wait to see you Please ask liming to study tonight. Please ask liming not to play computer games tonight. Don’t make/let me to smoke I can hear/see you dance at the stage You had better go to bed early. You had better not watch tv It’s better to go to bed early It’s best to run in the morning I am enjoy running with music. With 表伴随听音乐 I already finish studying You should keep working. You should keep on studying English Keep calm and carry on 保持冷静继续前行二战开始前英国皇家政府制造的海报名字 I have to go on studying I feel like I am flying I have to stop playing computer games and stop to go home now I forget/remember to finish my homework. I forget/remember cleaning the classroom We keep/percent/stop him from eating more chips I prefer orange to apple I prefer to walk rather than run I used to sing when I was young What’s wrong with you There have nothing to do with you I am so busy studying You are too young to na?ve I am so tired that I have to go to bed early

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《The Kite Runner》追风筝的人--------------------------------美句摘抄 1.I can still see Hassan up on that tree, sunlight flickering through the leaves on his almost perfectly round face, a face like a Chinese doll chiseled from hardwood: his flat, broad nose and slanting, narrow eyes like bamboo leaves, eyes that looked, depending on the light, gold, green even sapphire 翻译：我依然能记得哈桑坐在树上的样子，阳光穿过叶子，照着他那浑圆的脸庞。他的脸很像木头刻成的中国娃娃，鼻子大而扁平，双眼眯斜如同竹叶，在不同光线下会显现出金色、绿色，甚至是宝石蓝。 E.g.: A shadow of disquiet flickering over his face. 2.Never told that the mirror, like shooting walnuts at the neighbor's dog, was always my idea. 翻译：从来不提镜子、用胡桃射狗其实都是我的鬼主意。E.g.:His secret died with him, for he never told anyone. 3.We would sit across from each other on a pair of high

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The effective sentences:(improve the sentences!) 1.She hopes to spend this holiday either in Shanghai or in Suzhou. 2.Showing/to show sincerity and to keep/keeping promises are the basic requirements of a real friend. 3.I want to know the space of this house and when it was built. I want to know how big this house is and when it was built. I want to know the space of this house and the building time of the house. 4.In the past ten years,Mr.Smith has been a waiter,a tour guide,and taught English. In the past ten years,Mr.Smith has been a waiter,a tour guide,and an English teacher. 5.They are sweeping the floor wearing masks. They are sweeping the floor by wearing masks. wearing masks,They are sweeping the floor. 6.the drivers are told to drive carefully on the radio. the drivers are told on the radio to drive carefully 7.I almost spent two hours on this exercises. I spent almost two hours on this exercises. 8.Checking carefully,a serious mistake was found in the design. Checking carefully,I found a serious mistake in the design.

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M1 U1 一. 把下列短语填入每个句子的空白处（注意所填短语的形式变化）： add up (to) be concerned about go through set down a series of on purpose in order to according to get along with fall in love (with) join in have got to hide away face to face 1 We’ve chatted online for some time but we have never met ___________. 2 It is nearly 11 o’clock yet he is not back. His mother ____________ him. 3 The Lius ___________ hard times before liberation. 4 ____________ get a good mark I worked very hard before the exam. 5 I think the window was broken ___________ by someone. 6 You should ___________ the language points on the blackboard. They are useful. 7 They met at Tom’s party and later on ____________ with each other. 8 You can find ____________ English reading materials in the school library. 9 I am easy to be with and _____________my classmates pretty well. 10 They __________ in a small village so that they might not be found. 11 Which of the following statements is not right ____________ the above passage? 12 It’s getting dark. I ___________ be off now. 13 More than 1,000 workers ___________ the general strike last week. 14 All her earnings _____________ about 3,000 yuan per month. 二.用以下短语造句： 1.go through 2. no longer/ not… any longer 3. on purpose 4. calm… down 5. happen to 6. set down 7. wonder if 三. 翻译： 1.曾经有段时间，我对学习丧失了兴趣。（there was a time when…） 2. 这是我第一次和她交流。（It is/was the first time that …注意时态） 3.他昨天公园里遇到的是他的一个老朋友。（强调句） 4. 他是在知道真相之后才意识到错怪女儿了。（强调句） M 1 U 2 一. 把下列短语填入每个句子的空白处（注意所填短语的形式变化）： play a …role (in) because of come up such as even if play a …part (in) 1 Dujiangyan(都江堰) is still ___________in irrigation(灌溉) today. 2 That question ___________ at yesterday’s meeting. 3 Karl Marx could speak a few foreign languages, _________Russian and English. 4 You must ask for leave first __________ you have something very important. 5 The media _________ major ________ in influencing people’s opinion s. 6 _________ years of hard work she looked like a woman in her fifties. 二.用以下短语造句： 1.make (good/full) use of 2. play a(n) important role in 3. even if 4. believe it or not 5. such as 6. because of

英语造句

English sentence 1、(1)、able adj. 能句子：We are able to live under the sea in the future. (2)、ability n. 能力句子：Most school care for children of different abilities. (3)、enable v. 使。。。能句子：This pass enables me to travel half-price on trains. 2、(1)、accurate adj. 精确的句子：We must have the accurate calculation. (2)、accurately adv. 精确地句子：His calculation is accurately. 3、(1)、act v. 扮演句子：He act the interesting character.（2）、actor n. 演员句子：He was a famous actor. (3)、actress n. 女演员句子：She was a famous actress. (4)、active adj. 积极的句子：He is an active boy. 4、add v. 加句子：He adds a little sugar in the milk. 5、advantage n. 优势句子：His advantage is fight. 6、age 年龄n. 句子：His age is 15. 7、amusing 娱人的adj. 句子：This story is amusing. 8、angry 生气的adj. 句子：He is angry. 9、America 美国n. 句子：He is in America. 10、appear 出现v. He appears in this place. 11. artist 艺术家n. He is an artist. 12. attract 吸引 He attracts the dog. 13. Australia 澳大利亚 He is in Australia. 14.base 基地 She is in the base now. 15.basket 篮子 His basket is nice. 16.beautiful 美丽的 She is very beautiful. 17.begin 开始 He begins writing. 18.black 黑色的 He is black. 19.bright 明亮的 His eyes are bright. 20.good 好的 He is good at basketball. 21.British 英国人 He is British. 22.building 建造物 The building is highest in this city 23.busy 忙的 He is busy now. 24.calculate 计算 He calculates this test well. 25.Canada 加拿大 He borns in Canada. 26.care 照顾 He cared she yesterday. 27.certain 无疑的 They are certain to succeed. 28.change 改变 He changes the system. 29.chemical 化学药品