Hongyan Pan ,Zhiyan He ,Qian Lin *,Fei Liu Zhong Li

1 School of Chemical Engineering,Guizhou University,Guiyang 550025,China

2 Key Laboratory of Guizhou Province for Green Chemical Industry and Clean Energy Technology,Guiyang 550025,China

3 Research Institute of Chemical Engineering,South China University of Technology,Guangzhou 510640,China

1.Introduction

Styrene is an important aromatic compound and widely used as a raw materialfor the production of plastics,rubbers,insulation,pharmaceuticals,dyes,and pesticides[1].Due to its high volatility at ambient condition,styrene can easily emit into atmosphere from manufacturing units.And itcan be absorbed through the human's respiratory tract,skin and digestive tractfora long exposure to styrene[2],which isharmfulto the respiratory system and central nervous system even at low concentrations.Thus it is regarded as one kind of the volatile organic compounds(VOCs).Therefore it is necessary to remove styrene for legislation and human health.

In comparison with conventional treatment methods such as thermal destruction and adsorption[3],catalytic combustion is an important method to remove VOCs due to its final disposal and energy saving process[4,5].The catalystplays an important role in the catalytic combustion technology.Noble metal based catalysts and transitional metal based catalysts are usually used for VOC combustion.Noble metal based catalysts show higher catalytic activity and stability compared with transitional metal based catalysts,whereas,their application in industry is limited due to its rarity and valuableness.Therefore,many researchers devoted to designing transitional metal based catalysts with higher catalytic activity.Among them,copper based catalysts were reported to show higher activity for VOC combustion[6].However,its catalytic activity was affected by water vapor of the feed stream in practical applications.The water vapor mainly came from many off-gases such as previous combustions and watercontact operations.For example,Pan et al.[7]reported that water vapor had a negative effect on the activity of the copper based catalysts for styrene combustion,and the styrene conversion of the catalyst CuO/γ-Al2O3decreased from 90%to 70%when the water vapor volume concentration increased from 0.21%to 2.1%at 300°C,which was due to the competitive adsorption of styrene and water on the active sites of the catalysts.Wang et al.[8]reported that the conversion of toluene on the catalyst CuO/γ-Al2O3decreased from 85 to 62%when the water vapor concentration increased from 0 to 10 vol.%at 300°C,which was also due to the competitive adsorption of toluene and water vapor.Similar results were reported by Li[9],who prepared copper-manganese catalysts for toluene combustion in the presence of water vapor.Thus,in order to ensure the copper based catalysts with high activity and durability to water vapor,it is necessary to suppress the adsorption of water vapor and promote the adsorption of styrene on the active sites of the catalysts.

In ourprevious researches,itwas reported that when Ag+was loaded on AC,it could enhance the adsorption ability to dibenzothiophene because Ag+was a soft acid and dibenzothiophene was a soft base[10],whereas,it could weaken the adsorption ability to dichloromethane because dichloromethane was a hard base[11].The above presentation was supported by Hard and soft acids and bases(HSAB)principle,which shows that soft acids like to bond to soft bases and hard acids like to bond to hard bases.According to the classification of Pearson,Cu+belongs to the soft acid,styrene and water belong to the base[12,13].Thus,if the more content of soft acid Cu+on the copper based catalysts is,the stronger the adsorption ability to styrene will be because styrene is a kind ofsoftbase,instead,the weakerthe adsorption ability to water will be because water is a kind of hard base.Therefore,raising the Cu+content can improve its catalytic activity of the copper based catalysts and inhibit the negative in fluence of water vapor.

Previous studies[14]found that the metal dispersion degree of the palladium and platinum-based catalysts can be increased when they were calcined and then reduced.Wang et al.[15]reported that the particle size of Co metal of the Co/Mg/Al(10/40/50)is about 14 nm when the catalyst was reduced by H2.It can be speculated that,the use of H2to reduce the copper oxide based catalysts can increase the dispersion degree of copper,and CuO or Cu2O can be produced when the reduced copper based catalysts were then oxidized under the crystallization temperature.

Based on this perception,in order to raise the content of Cu+ion of the copper based catalysts,we used the reduction-oxidation method to prepare the catalyst CuOx/γ-Al2O3-IH with different copper loading amounts.By comparison,the catalyst CuOx/γ-Al2O3-IM was also prepared by ultrasonic impregnation methods.The copper valence and content of the catalysts were characterized by XRD,TPR,XPS and N2 adsorption.The catalytic activity of the catalysts for styrene combustion was tested in the presence of water vapor whose volume concentration was in the range of 0.21%-3.75%.Moreover,the effect of copper valence and contentof the catalysts on the catalytic activity for styrene combustion in the absence and presence of water vapor was also analyzed and discussed here with the help of HSAB theory.

2.Materials and Methods

2.1.Catalysts preparation

Synthesis ofCuOx/γ-Al2O3-IM: first,theγ-Al2O3[(60-80)mesh],obtained from Alfa Aesar company(America),was separately added to aqueous solution of copper nitrate(Cu(NO3)2,99%)with different concentrations.The chemical reagent was supplied from Guangzhou Chemical reagent factory(China),and no additional purification was done.Second,the slurry was stirred for 24 h at 30°C,and then put in an ultrasonic field stirred for 1 h.In this experiment,the ultrasonic sound of frequency was 45 kHz and its power was 185 W.After that the slurry was filtered and then dried at 120°C for 12 h.Finally,the samples were calcined at 550°C for 5 h.The obtained catalysts were denoted as CuOx/γ-Al2O3-IM with 8.2%-12.3%(by mass)CuO loading.

Synthesis of CuOx/γ-Al2O3-IH:About 500 mg catalyst CuOx/γ-Al2O3-IM with 8.2%-12.3%(by mass)CuO loading was placed in a stainless steel pipe;both sides of the catalyst bed were sealed with quartz wool.The catalyst was treated by 30 ml·min-1gas flow of H2at 350°C for 2 h,followed by calcination at the same temperature in air for 2 h.Quantitative uptakes of the copper loading on the catalysts were verified by atomic adsorption spectroscopy.

2.2.Characterization of catalysts

X-ray diffraction(XRD)was conducted in a Bruker D8 Advance diffractometer using Cu Karadiation(λ=0.15406 nm).The X-ray tube was operated at 40 kV and 40 mA.

Temperature-programmed reduction of hydrogen(H2-TPR)was conducted using a Micromeritics Autochem 2920 analyzer.About 100 mg catalyst was placed on a U shape quartz tube and dried at 300 °C for 2 h in a helium flow rate of 40 ml· min-1,and then cooled to room temperature.After that,the catalyst was reduced by 30 ml·min-1gas mixture composed of 10%H2/Ar,and then the catalyst bed temperature was heated to 750°C at a rate of 10°C ·min-1.A thermal conductivity detector(TCD)was used to measure the amount of H2consumption.

X-ray photoelectron spectra(XPS)was used to analyze the photoelectronic signals of Cu 2p by a Physical Electronics PHI 300 spectrometer with nonmonochromatic AlKa(1486.6 eV).Collected data were corrected for charge shifting using standard C1s binding energy of 284.6 eV.Data analysis with background subtraction and curve fitting was done on XPS Peak 4.1.

The surface area and pore structures of catalysts were measured by nitrogen adsorption at 196°C using a Micromeritics ASAP-2020 analyzer.The BET surface area was calculated using standard Brunauer-Emmett-Teller equation,and the total pore volume was estimated on the basis of the volume adsorbed when p/p0=0.99.Average pore diameter was calculated through 4 V·A-1.

2.3.Catalytic activity of catalysts

Catalytic activated tests for styrene combustion in the presence of different water vapor concentrations were conducted in a fixed bed reactor with an internal catalyst bed at atmospheric pressure.

Before the catalytic experiment,about 100 mg catalyst sample was placed in the center of the stainless steel reactor tube,and both sides of the catalyst bed were filled with quartz wool.The reactant gas mixture of styrene vapor,water vapor and pure air gas(Air,0.9999)was continuously delivered to the catalyst bed.Reactant gas flow-rate through the catalyst bed was controlled by mass flow controllers(MFC)(BJQXHC Electron Company,China).Temperature of the catalyst bed was controlled by a temperature controller(XMYD Electron Company,China),and a K-type thermocouple was used to measure the upstream side temperature of the catalyst bed.The effluent of styrene was analyzed by on line gas chromatography(Hua'ai GC 9160 chromatograph equipped with a FIDdetector).In this work,the reactant gas flow rate was 180 ml·min-1,and the concentration of styrene was 5.17 g·m-3at the gas hourly space velocity(GHSV)of 54000 h-1.The controlled volume concentration of water vapor in reactant gas stream was 0.21%,2.1%,2.94%and 3.78%separately,which was measured by hygrometers.The relative standard uncertainty of the MFC controllers and temperature controller was to be 1%and±0.2°C respectively in this work.

3.Results and Discussion

3.1.Characterization of catalysts

3.1.1.X-ray diffraction(XRD)

Fig.1 presents the XRD patterns of the two catalysts CuOx/γ-Al2O3-IM,CuOx/γ-Al2O3-IH and the support γ-Al2O3respectively.Both γ-Al2O3and the two copper based catalysts show three main diffraction peaks at 2θ values of 36,46 and 66°,which represent typical characteristic diffraction peaks ofγ-Al2O3.When 10.1%ofcopperoxide(by mass)is loaded on γ-Al2O3,the two catalysts display some new diffraction peaks besides the same structure of γ-Al2O3.It is can be seen easily that these additional new diffraction peaks appear at 2θ value of 35.3,38.8 and 48.7°(form CuO(JCPDS45-0937)),and 36.2,42.2 and 61.2°(form Cu2O(JCPDS78-2076))respectively,which indicates that both CuO and Cu2O coexist on the surface of the two catalysts.The mean diameter of CuO and Cu2O crystallites is calculated by the Scherrer equation,and the data are shown in Table 1.The data in Table 1 show that the crystallite sizes of CuO and Cu2O are 33.66 and 20.23 nm on the catalyst CuOx/γ-Al2O3-IM,and 20.23 and 22.69 nm on the catalyst CuOx/γ-Al2O3-IH,separately.It can be seen that the crystallite size of CuO on the catalyst CuOx/γ-Al2O3-IM is much higher than that on the catalyst CuOx/γ-Al2O3-IH,whereas,there is a small difference in the crystallite size ofCu2Obetween the two catalysts.The largerthe crystallite sizes of CuO of the catalyst CuOx/γ-Al2O3-IM is,the lower the dispersion is,which would decrease the number of active sites,and therefore decrease its catalytic activity.

Fig.1.XRD patterns of γ-Al2O3 and two catalysts with 10.1%copper species.

Table 1 Area percentage of each reduction peak and its peak temperature over the two catalysts

3.1.2.TEM measurement

Fig.2 shows the TEM images of the two catalysts CuOx/γ-Al2O3-IH and CuOx/γ-Al2O3-IM.It can be seen that CuxO is not easy to be distinguished from the support γ-Al2O3under low magnification(Fig.2(a)and(c),because the particle size of γ-Al2O3is small.However,CuxO is easy observed under high magnification because CuxO and γ-Al2O3show different crystal lattice(Fig.2(b)and(d)).It indicated that CuxO is present on the carrier γ-Al2O3and its particle size is nanometer,which is in agreement with the XRD analysis.

3.1.3.TPR analysis of the catalysts

Fig.3 presents the TPR profiles of the two catalysts CuOx/γ-Al2O3-IH and CuOx/γ-Al2O3-IM.Two reduction peaks are observed between 100 and 300°C in both ofthem.For the two catalysts,the lower temperature reduction peak is located at 173 and 196°C and ascribed to the reduction of Cu+,which follows the reaction,Cu2O+H2→2Cu+H2O;and the higher temperature reduction peak is located at 219 and 254°C respectively and ascribed to the reduction ofCu2+,which follows another reaction,CuO+H2→Cu+H2O.It is consistent with the previous report[16].

It is noticed that the reduction temperature of CuO and Cu2O on the catalyst CuOx/γ-Al2O3-IH is 23 and 35 °C respectively,lower than that on the catalyst CuOx/γ-Al2O3-IM.Pecchi and Morales[17]indicated that the lower the reduction temperature of copper oxides on the catalyst was,the easier it could be to be reduced and show higher catalytic activity.Dow et al.[18]pointed out that the shift of the reduction temperature of CuO was related to the reducibility of CuO,which was associated with the activity of catalysts.Therefore,it implies that the activity of the catalyst CuOx/γ-Al2O3-IH is higher than that of the catalyst CuOx/γ-Al2O3-IM,which will be proved in Section 3.2.

Table 1 summarizes the reduction peak area percentage and temperature of CuO and Cu2O on the two catalysts.It shows that the reduction peak area percentages of Cu2O and CuO on the catalyst CuOx/γ-Al2O3-IM are 44%and 56%respectively;while for the catalyst CuOx/γ-Al2O3-IH,that of Cu2O and CuO are 64%and 36%respectively.The larger the reduction peak area of copper oxide is,the more the hydrogen consumption is,which means there are more content of copper oxides on the catalyst surface.Therefore,it can be speculated that the content of Cu2O on the catalyst CuO/γ-Al2O3-IH is larger than that on the catalyst CuOx/γ-Al2O3-IM,which will be proved by XPS results in Section 3.1.4.

3.1.4.XPS studies of the catalysts

Fig.4 shows XPS spectra of Cu 2p region of the catalysts CuOx/γ-Al2O3-IH and CuOx/γ-Al2O3-IM.It shows that Cu 2p peak is composed by Cu 2p1/2and Cu 2p3/2,and between the two peaks there is a carryon satellite peak(shake-up),which stands for the existing of Cu2+.The core level spectrum of Cu 2p3/2on the catalyst CuOx/γ-Al2O3-IM is centered at 933.65 eV,which can be decomposed into two peaks,a lower intensity at 932.45 eV,and another one,more intense,at 933.97 eV(as shown in Fig.4(a)).For the catalyst CuOx/γ-Al2O3-IH as shown in Fig.4(b),the Cu 2p3/2peak is centered at 933.78 eV,and can also be decomposed into two peaks,corresponding to the binding energy at 933.98 eV and 931.69 eV respectively.The higher binding energy peaks occur at 933.97 eV and 933.98 eV,which can be attributed to CuO;the lower binding energy peaks occur at 932.45 eV and 931.69 eV,which can be ascribed to Cu2O[16,19].These data indicate that Cu2+and Cu+coexist on the two catalysts and these results are agreement with the XRD and H2-TPR results.

Table 2 summarizes the binding energy values and peak area of the CuII2p3/2and CuI2p3/2for the two catalysts.It shows that the percentage of Cu2+and Cu+on the catalyst CuOx/γ-Al2O3-IM surface are 89.68%and 10.32%respectively,while forthe catalyst CuOx/γ-Al2O3-IH,Cu2+and Cu+are 64.21%and 35.79%respectively,which indicates that the Cu+content of the former catalyst is significantly higher than that of the latter.

3.1.5.Textural properties of the catalysts

Table 3 shows textural properties of the catalysts CuOx/γ-Al2O3-IH and CuOx/γ-Al2O3-IM and support γ-Al2O3.It indicates that with the loading of CuOxon γ-Al2O3,the BET surface area,pore diameter and pore volume of the copper based catalysts are decreased,which is mainly attributed to the introduction of copper species on the catalysts surfaces.However,both the two copper based catalysts show similar textural properties.Therefore it can be suggested that there is no linear relationship between the catalytic activity and pore structure parameters.

3.2.Effect of water vapor on the catalytic activity for styrene combustion

Fig.5 shows the effect of water vapor on the catalytic activity of the catalysts CuOx/γ-Al2O3-IM and CuOx/γ-Al2O3-IH for styrene combustion.The water vapor is added to the feed stream lasting for about 355 min.It can be seen that styrene conversion decrease to a low level of the two catalysts when 2.1%(by volume)water vapor is added to the feed stream,and then stable throughout the same reaction condition.However,the catalytic activity can return to its original state when water vapor is removed from the reactant stream.

It indicates that the effect of water vapor on the catalytic activity of the catalysts is reversible due to the competitive adsorption between water molecular and styrene on active sites of the catalysts surface.That is,once water vapor is added to the feed stream,it can occupy some active sites of the catalysts surface,which can reduce the adsorption of styrene,and thus decrease the catalytic activity of catalysts.Once water vapor is removed from the feed stream,the active sites that are occupied by water vapor are then exposed to provide the blank for styrene to absorb and react,leading to the activity of catalysts recovered to its original state.

Fig.2.TEM images of the two catalysts with 10.1%copper species.(a,b)CuO x/γ-Al2O3-IM;(c,d)CuO x/γ-Al2O3-IH.

Fig.3.TPR profiles for two catalysts with 10.1%copper species.

Furthermore,it can be seen that styrene conversion decrease from 95%to 80%for the catalyst CuOx/γ-Al2O3-IM when 2.1%(by volume)water vapor is added to the feed stream,and the conversion drops 15%,while for the catalyst CuOx/γ-Al2O3-IH,styrene conversion decrease from 97 to 90%at the same reaction condition,and the conversion drops 7%merely.The above results indicate that,compared with the CuOx/γ-Al2O3-IM,the catalyst CuOx/γ-Al2O3-IH has better durability to the negative in fluence of water vapor,which is due to its higher amount of soft acid Cu+.

Fig.6 shows the effect of water vapor concentration on the catalytic activity of the two catalysts for styrene combustion.It can be seen that styrene conversion over the two catalysts decreases with the increasing of water vapor concentration.This is because when higher water vapor concentrations are added to the feed stream,the more active sites on the catalysts surface would be occupied by watermolecules,which suppress the adsorption and reaction of styrene on the active sites and reduce the catalytic activity of the catalysts for styrene combustion.

It also indicates that the degradation degrees of catalytic activity of the two catalysts affected by water vapor concentration are different.The T90(reaction temperature required for 90%conversion of styrene)is used to evaluate the catalytic activity of catalysts at different water vapor concentrations,and the results are shown in Table 4.

The data in Table 4 show that the T90values of the catalyst CuOx/γ-Al2O3-IH and CuOx/γ-Al2O3-IM are 270 and 325 °C respectively when the water vapor volume concentration is 0.21%.The lower T90values of the catalystCuOx/γ-Al2O3-IH means it shows higher catalytic activity.Both CuO and Cu2O can catalyze combust VOCs,especially Cu2O which shows higher catalytic activity because it belongs to p-type semiconductor catalyst,has hole-conduction behavior,and shows better adsorption to O2.The adsorbed O2is activated by a series of processes such as synergy,electron transition,dissociative adsorption and entering the lattice of oxide catalyst[20],and then produce higher active oxygen species,such as O2-,O-,and O2-.These species are easily to oxide VOCs to from CO2,which make the catalyst CuOx/γ-Al2O3-IH with more content of Cu2O showing higher catalytic activity.

Fig.4.XPS spectra of the two catalysts with 10.1%copper species.

Table 2 XPS fitting data of Cu2p region on surface of catalysts

In addition,it can be seen that the T90values increase from 270 to 300°C when water vapor concentration increases from 0.21%to 3.78%for the catalysts CuOx/γ-Al2O3-IH,and the T90value increases by 11.1%.However,the T90values increase from 329 to 382°C for the catalyst CuOx/γ-Al2O3-IM in the same reaction condition,and its value increases by 17.5%.The results indicate that the former catalyst shows higher durability to water vapor,and its causes will be discussed in detail in Section 3.3.

3.3.Effect of the local hardness on the catalytic activity of the catalysts for styrene combustion

These data above indicate that the two catalysts have different catalytic activities for styrene combustion under various water vapor concentrations,which can be attributed to the changes in the surface chemical properties of the two catalysts pretreated with and without H2.In this work,HSAB principle is used to explain the effect of surfaces chemical properties on the catalytic activity for styrene combustion in the presence of water vapor.

HSAB theory is proposed by Pearson in 1963,it has become one of the foundations of modern chemistry,and its basic statement is“the hard acids prefer to be bonded to hard bases,and the soft acids prefer to be bonded to soft bases”[11,13].According to the classification of the Pearson HSAB theory,the base property of a species can be determined by the electronegativity(χ).A species belongs to hard bases if its χ is above 3,a species belongs to borderline bases if its χ is between 2.8 and 3,and a species belongs to softbases ifitsχis below 2.8.Theχof styrene is 2.567,being below 2.8,so it belongs to soft bases.Whereas,the χ of H2O is 3.84,being above 2.8,so it belongs to hard bases.Theχ of styrene and H2O was calculated by molecular simulation software Hyperchem 8.0 on the basis of density functional theory(DFT),the calculation method was stated in detail in Ref.[16].According to the classification of Pearson's HSAB theory,various transitional metal ions are denoted as Lewis acid,among them,Cu2+ions belong to borderline acid and Cu+ions belong to soft acids[12].

Table 3 Textural properties of the catalysts with 10.1%copper species

The higher the contentofsoftacid Cu+on the catalysts surface is,the stronger the local acid softness of catalysts will be.According to the HSAB principle“the hard acids prefer to be bonded to hard bases,and the soft acids prefer to be bonded to soft bases”,the stronger the local acid softness of catalysts surface is,the higher the adsorption ability of soft base and the lower the adsorption ability of hard base will be.In comparison with the catalyst CuOx/γ-Al2O3-IM,the catalyst CuOx/γ-Al2O3-IH with higher content of soft acid Cu+exhibits stronger local acid softness,which can enhance the adsorption toward styrene and reduce the adsorption toward water vapor because styrene is a kind of soft base and water is a kind of hard base.These make the catalyst CuOx/γ-Al2O3-IH show higher catalytic activity for styrene combustion and resistance to water vapor.That is consistent with the experimental results.

3.4.The catalytic performance of catalysts with different CuO loadings

Fig.7 shows the effect of loading amount of CuO over the catalyst CuOx/γ-Al2O3-IH on its catalytic activity for styrene combustion.It shows that styrene conversion increases from 61 up to 63.7%when CuOloadings increase from 8.2%up to 10.1%at240°C.However,styrene conversion decreases to 55.3%when the loading amount of CuO continuously increases to 12.3%at the same temperature.That is to say the catalytic activity of catalyst for styrene combustion firstly rises and then drops as the increasing of CuO loading.The optimal CuO loading is 10.1%among the as-prepared catalysts in this work.

Fig.5.Effect of water vapor on the activity of the copper based catalysts for styrene combustion.2.1%(by volume)of steam was added to the feed for 355 min,T=330°C.

Fig.6.Styrene conversion on the two catalysts depending on the H2O concentration.

Table 4 Effect of the amount of water vapor added on the temperature of T90

4.Conclusions

XRD,H2-TPR and XPS analyses indicated that both Cu+and Cu2+coexisted on the surface of the two catalysts CuOx/γ-Al2O3-IH and CuOx/γ-Al2O3-IM.Meanwhile the content of Cu+on the former catalyst is higher than that on the latter.Water vapor can inhibit the catalytic activity of the two catalysts in styrene combustion and its effect is reversible.Catalytic activity of the catalysts gradually decreases with the increasing of water vapor concentration in the feed stream.The catalyst CuOx/γ-Al2O3-IH shows higher catalytic activity and inhibitory effect of water vapor in styrene combustion than the catalyst CuOx/γ-Al2O3-IM.This is because the catalyst has higher content of the soft acid ions Cu+,which can lead to an increase in the partial local acid hardness of catalytic surface,improve the adsorption ability of soft base styrene,and prevent the adsorption for hard base water.

Fig.7.Effect of CuO x loading on the catalytic activity of the catalyst CuO x/γ-Al2O3-IH.

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