Al2O3改性CuO/Fe2O3催化劑水煤氣變換反應(yīng)性能

林性貽 殷 玲 范言語(yǔ) 陳崇啟(福州大學(xué)化肥催化劑國(guó)家工程研究中心,福州350002)

Al2O3改性CuO/Fe2O3催化劑水煤氣變換反應(yīng)性能

林性貽*殷 玲 范言語(yǔ) 陳崇啟
(福州大學(xué)化肥催化劑國(guó)家工程研究中心,福州350002)

采用分步共沉淀法制備了不同Al2O3含量(0%-15%(w))的CuO/Fe2O3催化劑,并進(jìn)行水煤氣變換反應(yīng)(WGSR)評(píng)價(jià)測(cè)試.制得的催化劑中含有復(fù)合物CuFe2O4,其晶粒尺寸,氧化還原性質(zhì)和表面Cu分散通過(guò)相應(yīng)表征手段加以研究.X射線粉末衍射(XRD),拉曼(Raman)光譜,N2物理吸附,N2O分解和CO2程序升溫脫附(CO2-TPD)等表征技術(shù)說(shuō)明適量Al2O3的加入可以促進(jìn)尖晶石CuFe2O4發(fā)生由四方相向立方相的轉(zhuǎn)變,阻止催化劑中Cu燒結(jié),增大表面Cu分散,增加弱堿性位點(diǎn)的數(shù)量.此外,采用H2程序升溫還原(H2-TPR)技術(shù)探究改性的CuO/Fe2O3催化劑的還原性能.關(guān)聯(lián)結(jié)果發(fā)現(xiàn),Al2O3摻雜在增大銅物種的耗氫量,降低其還原溫度方面起著重要的作用.即Al2O3的添加促進(jìn)CuO/Fe2O3催化劑中銅鐵物種之間的協(xié)同作用.結(jié)合活性測(cè)試和表征結(jié)果,適量的Al2O3(10%(w))改性的催化劑具有較小的Cu顆粒尺寸、較大的Cu分散、較強(qiáng)的還原性能、較多數(shù)量的弱堿性位點(diǎn),因此具有更好的初始活性和熱穩(wěn)定性.

CuFe2O4;水煤氣變換反應(yīng);Al2O3改性;Cu分散;弱堿性位點(diǎn);N2O分解

?Editorial office ofActa Physico-Chimica Sinica

1 Introduction

Hydrogen is a potential clean fuel for satisfying many of our energy demands in the future.1At present,most of the hydrogen is supplied by the reforming of traditional fossil energy,resulting in CO with a content of～10%(volume fraction)in the reforming gas.However,the slight content of CO degrades the performance of the Pt electrode in fuel cell systems.It is reported that watergas shift reaction(WGSR)and CO oxidation process are crucial steps to reduce the CO concentration and get clean hydrogen for fuel cell.2Herein,scientists pay their attentions to water-gas shift reaction again.It is well known that the water-gas shift reaction is usually carried out via two stages:a high-temperature stage at 350-450°C using Fe-Cr oxide catalyst and a low-temperature stage at 200-250°C using Cu-based catalysts,as well as supported precious metal catalysts,3-5such as Pt/TiO2,6Au/CeO2,7Pt/ CeO28and so on.Considering the high cost of the precious metal, great efforts have been done on non-precious metal catalysts. Budiman et al.9reported that highly dispersed Cu/ZnO/Al2O3catalysts were prepared by modifying a conventional co-precipitation method.The improved activity during WGSR was attributed to the high Cu active surface.Wang et al.10found that highly dispersed copper nanoparticles contributed to the high activity and selectivity over the Cu/SiO2catalysts.Lin et al.11prepared copper ferrite-based catalysts by co-precipitation method with KOH as the precipitants,and showed high catalytic performance for WGSR.Kameoka et al.12found that spinel CuFe2O4was an effective precursor for a high performance copper catalyst.Lin et al.13studied the Cu/Fe2O3catalyst with different Cu loadings, and found that CuFe2O4with spinel structure played an important role in improving catalytic activity.

Many metal oxides have been used to promote catalytic activity of catalysts in WGRS.Du et al.14reported a highly effective ZrO2-promoted Cu-Mn spinel catalyst in WGSR.Ayastuy et al.15used Co as a promoter to modify Cu/CeO2catalyst,finding that Co-CuCe catalyst was highly selective and active for preferential oxidation of CO(CO-PROX)reaction,but low activity in WGSR and oxygen-enhanced water-gas shift reaction(OWGSR).Li et al.16reported that Cu/CeO2catalyst doped with aluminum prepared by co-precipitation method showed the high catalytic activity and thermal stability for WGSR.Whereas,aluminum promoted CuO/Fe2O3catalyst synthesized by co-precipitation method for WGSR has not been reported yet.

In the present work,a series of Al2O3-modified CuO/Fe2O3catalysts were prepared by stepwise co-precipitation method.The catalytic activities of CuO/Fe2O3-Al2O3catalysts under WGSR condition were investigated in detail.The results show that the sample CCA-10(CuO/Fe2O3catalyst modified by 10%(w)Al2O3) exhibits higher initial activity and thermal stability.The correlation of catalyst structure and catalytic properties were measured by X-ray diffraction(XRD),Raman,N2physisorption,N2O decomposition,temperature-programmed reduction of H2(H2-TPR), and temperature-programmed desorption of CO2(CO2-TPD) techniques.

2 Experimental

2.1 Preparation of catalysts

A series of aluminum doped CuO/Fe2O3catalysts with copper content fixed at 20%(w)(calculated as CuO)and a given aluminum content(e.g.,0%,3%,5%,10%,15%(w),calculated as Al2O3)were synthesized by stepwise co-precipitation method. Stoichiometric amounts of copper(II)nitrate[Cu(NO3)2·3H2O], iron(Ⅲ)nitrate[Fe(NO3)3·9H2O]were dissolved in deionized water to form the salt solution for 400 mL,and various contents of aluminum nitrate nonahydrate[Al(NO3)3·9H2O]were also dissolved for 25 mL.Then the salt solution was co-precipitated with an aqueous solution of potassium hydroxide(KOH)with vigorous stirring at T=80°C and pH=10±1.Materials Cu(NO3)2· 3H2O,Fe(NO3)3·9H2O,Al(NO3)3·9H2O,KOH are all analytical grades and were purchased from Shanghai Chemical Reagent Ltd. without further purification.After the copper and iron nitrate salt solution dripped off,the auxiliary Al2O3was dripped into the precipitate under the same condition.The final precipitate was aged with continuous stirring at T=80°C for 4 h.The obtained precipitate finally was centrifuged and washed by deionized water for 7 times,and then dried at 110°C for 12 h and calcined at 650°C for 4 h(heating rate was 5°C·min-1)subsequently.The x%(w) Al2O3modified catalysts were denoted as CCA-x(x=0,3,5,10, 15).

2.2 Characterizations of catalysts

XRD patterns of the samples were recorded by a PANalytical X'pertPro diffractometer(Netherlands)equipped with Co Kα(λ= 0.1789 nm)radiation.The samples were operated at 40 kV and 40 mA for 2θ angles ranging from 10°to 80°.Step size was kept at 0.0167°and each catalyst was detected in 6 min.The diffraction pattern was identified by comparing with Joint Committee of Powder Diffraction Standards(JCPDS)cards.

Raman spectrum was recorded using the Lab-Ram HR800 spectrometer(JobinYvon-Horiba)(British Renishawn Company) with a 514.2 nm AreKr 2018RM laser(Spectrum Physics)as the excitation source.

Cu dispersion was measured by the N2O decomposition with 3.35%(volume fraction,φ)N2O/He gas.50 mg of the sample was pretreated with highly pure helium(flow rate=30 mL·min-1)at 250°C for 1 h.After cooled down to room temperature,the sample was reduced by 10%(φ)H2/Ar from room temperature to 350°C.Once the sample was cooled down to 50°C,N2O/He gas was introduced to flow about 0.5 h.After that,the sample was flowed with highly pure helium to remove the remaining gas. Then continue one more TPR measurement.The hydrogen consumption was monitored by thermal conductivity detector(TCD) in the AutoChem 2910 apparatus(American Micrometric Company).The difference in the hydrogen consumption between two TPR measurements was used to calculate the Cu dispersion.The size of Cu metal particles(d/nm)was calculated by the equation: d=1.1/dispersion.17

The BET surface area and pore volumes were measured by American Micrometrics ASAP 2020 instrument using nitrogenadsorption-desorption at 77 K.

H2-TPR was measured on anAutoChem 2910 apparatus equipped with a TCD.50 mg of the sample was pretreated in high purity argon gas at 250°C for 1 h.After being cooled to room temperature,the sample was reduced by 10%H2/Ar(flow rate=30 mL· min-1)from 50 to 800°C at a heating rate of 10°C·min-1.

CO2-TPD was performed with AutoChem 2920 instrument (American Micrometric Company).50 mg of the sample was firstly pre-reduced at 250°C for 1 h by 10%H2/Ar(flow rate=50 mL·min-1).Then CO2was introduced for 1 h after the sample was cooled to 50°C.Subsequently,the sample was purged by high purity helium gas for 1 h to remove physically adsorbed CO2. After that,it was heated from room temperature to 450°C at a heating rate of 10°C·min-1,the amount of desorbed CO2was monitored by TCD.

2.3 Evaluation of catalytic performance

The catalytic activity and thermal stability of the catalysts for WGSR were tested in a fixed-bed reactor at atmospheric pressure. The catalytic activity of all the samples was measured from 200 to 400°C.In order to investigate the thermal stability of the catalysts,all the samples were kept at 400°C for 10 h and then decreased to 200°C before continuing the second evaluating cycle.Besides,the sample CCA-10 was maintained at 250°C for 120 h to further investigate the thermal stability of the catalyst.2 g of fresh catalysts(20-40 mesh)after calcination were placed between two layers of quartz granules inside a stainless tube(i.d.= 12 mm).The reaction temperature was controlled by a thermocouple located near the center of the catalyst bed.The experiment was performed with a feed gas(10%CO,60%H2,12%CO2,and balance N2)flowing at 78.2 mL·min-1.The volume ratio of vapor to feed gas was maintained at 0.6:1 and gas hourly space velocity (GHSV)was kept at 7500 h-1.The residual water of the outlet was removed by a condenser before entering a gas chromatograph which was equipped with a TCD.The activity was expressed by the conversion of CO,defined as:XCO(%)=(1-φ'CO/φCO)/(1+φ'CO)× 100%,where φCOand φ'COare the inlet and outlet volume fractions of CO(dry base),respectively.

3 Results and discussion

3.1 Structural and textural studies of the asprepared CuO/Fe2O3-Al2O3catalysts

The XRD patterns of CuO/Fe2O3-Al2O3catalysts,as well as that of non-modified CuO/Fe2O3sample,are presented in Fig.1.Two sets of diffraction patterns are found for all the CuO/Fe2O3samples:those marked with“#”are indexed to α-Fe2O3(JCPDS file No.01-089-0596),while those labeled with“*”and“■”are ascribed to cubic(JCPDS No.01-077-0010)and tetragonal (JCPDS No.00-034-0425)CuFe2O4,respectively.As depicted in Fig.1,there are no obvious diffraction peaks of crystalline CuO or Al2O3for all the samples,suggesting that the CuO orAl2O3species might be highly dispersed or present in an amorphous state on the surface.Besides,the diffraction peaks of tetragonal CuFe2O4are presented from CCA-0 to CCA-5 at 21.3°,34.9°,42.1°,43.3°, 51.4°,63.6°,while for CCA-10 and CCA-15,the diffraction peaks at 21.3°,35.1°,43.4°,74.2°are indexed to cubic CuFe2O4.It is worth noting that the peaks of CuFe2O4shift slightly for CCA-10 and CCA-15,comparing with the others.It may indicate that the crystal structure of the CuFe2O4is changed by the appropriate introduction of Al2O3.The peak intensities of the CuFe2O4for CCA-10 and CCA-15 are weaker than those of the other samples, judging from the fact that the diffraction peak at 42.1°is too weak to see.It may illustrate that more copper may highly disperse on the surface and result in decreasing of CuFe2O4.Furthermore,the crystalline sizes of CuFe2O4and Fe2O3are calculated by Scherrer equation and the results are listed in Table 1.It is clear that the crystalline sizes of CuFe2O4for CCA-10 and CCA-15,which present cubic spinel structure,are smaller than the others.Cubic spine structure was reported to be in favor of refraining crystallite growth,18exactly explained the smaller crystalline sizes.It indicates that the crystalline sizes of catalysts are greatly affected by dopingAl2O3.

Fig.1 XRD patterns of(A)the as-prepared aluminum doped CuO/Fe2O3catalysts and(B)the samples after reduction with the reactant gas at 250°C

Furthermore,the structural studies of the as-prepared CuO/ Fe2O3-Al2O3catalysts are complemented by Raman spectra,as presented in Fig.2.All the spectra exhibit α-Fe2O3characteristic bands located at 220,410,and 608 cm-1which can be attributed to A1gand Egvibrational spectral modes.19The peak located at 1320 cm-1is assigned to a two-magnon scattering which arises from the interaction of two magnons created on antiparallel closespin sites.20295,496,and 663 cm-1are reported to be the characteristic peaks of CuFe2O4,representing two modes.One at 663 cm-1is related to T-site mode that reflects the local lattice effect in the tetrahedral sublattice.The other two at 295 and 496 cm-1are corresponding to O-site mode,which reflects the local lattice effect in the octahedral sublattice.21Reddy et al.22reported that octahedral cations in ferrospinel played an important role in affecting catalytic activity in WGSR.The peaks at 295 and 496 cm-1for CCA-10 are stronger than the others.For nanocrystal CuO, peak at 280 cm-1is assigned to theAgmode and peaks at 332,618 cm-1are corresponding to the Bgmodes,reported by Xu et al.23As seen in Fig.2,there are no obvious peaks of nanocrystal CuO or Al2O3for the samples.This is well consistent with XRD result, indicating that most of copper species might be highly dispersed or present in an amorphous state on the surface.

Table 1 Texture properties of CuO/Fe2O3catalysts doped with aluminium

Fig.2 Raman spectra of the as-prepared CuO/Fe2O3-Al2O3catalysts

As depicted in Table 1,the total pole volume and BET surface area increase with the increasing the content of doped Al2O3,and the average pore diameter is larger than the non-doped one.From Fig.3A,it is seen that the pore distribution for CCA-10 is relatively more concentrated.The adsorption-desorption curves of the samples are type IV gas adsorption isothermal classified by International Union of Pure and Applied Chemistry(IUPAC),24presented in Fig.3B,and all the samples are with conspicuous hysteresis loops.H1 hysteresis loops that exhibit parallel and nearly vertical branches are observed for CCA-3,CCA-5,and CCA-10.Type H1 loops are reported to be comprised of agglomerates or compacted of approximately spherical particles arranged in a fairly uniform way.The appearance of type H1 loop indicates its relatively high pore size uniformity and facile pore connectivity.25While CCA-0 and CCA-15 are typical H2 hysteresis loops,which has a triangular shape and steep desorption branch.H2 hysteresis loops are considered to be with relatively uniform channel-like pores for materials.25

Fig.3 (A)Effect of dopingAl2O3on pore size distribution of CuO/Fe2O3catalysts and(B)adsorption-desorption isothermal curves of the samples

Fig.1B shows the XRD patterns of the as-prepared CuO/Fe2O3-Al2O3catalysts after reduction with the reactant gas at 250°C.As shown in Fig.1B,the main phases are Cu and Fe3O4,meaning that CuO is reduced to Cu,α-Fe2O3reduced to Fe3O4,and CuFe2O4reduced to Cu and Fe3O4.The characteristic peak of Cu(JCPDS 00-004-0836)is observed at 50.5°of 2θ.The intensity of diffraction peak of characteristic Cu at 59.2°is too weak to be found. The weak intensity of peaks is because of the high dispersion and little particle size of Cu metal.

Nitrous oxide is reported to be used for the estimation of thesurface area of metallic copper in catalysts,since the surface atoms of Cu-crystals can be oxidized selectively using N2O in a temperature interval from 20 to 120°C by the reaction:N2O+2Cu= Cu2O+N2.26By assuming the metallic copper entirely oxidized,the Cu dispersion can be calculated.As listed in Table 1,the addition of Al2O3does favor for the copper dispersion.The Cu dispersion values of the samples CCA-10 and CCA-15 are 67.6%and 68.8%, respectively,much higher than that of the CCA-0,which is only 43.7%.For the two doped catalysts,the Cu particle size is as small as 1.6 nm.That is why the peak of Cu is too weak to see.It is reported that the better the dispersion of Cu is,the higher catalytic performance of supported Cu catalyst will be.10,27-29Hence,little particle size and high dispersion of Cu may result in high catalytic activity and stability.

3.2 Redox properties of the as-synthesized catalystsThe redox properties of the as-synthesized catalysts are investigated by H2-TPR profiles.H2-TPR profiles are presented in Fig.4.In our previous research,11the overlapping peak β below 200°C and one small peak γ at 236°C are assigned to the reduction of CuFe2O4to Cu and Fe3O4,and α-Fe2O3to Fe3O4,respectively.One small shoulder peak α is attributed to the reduction of highly dispersed CuO species,as seen in Fig.4.The temperature of the reduction of highly dispersed CuO for the sample doped with aluminium is lower than that of the non-doped one. The H2consumption of copper species for all the catalysts is listed inTable 2.The sum of H2consumption of copper species for CCA-10 and CCA-15 is larger than the others,meaning that more copper species can be reduced.Besides,the H2consumption of highly dispersed CuO increases while that of CuFe2O4decreases with increasing the content of Al2O3,illustrating that more copper are highly dispersed on the surface.This is well consistent with the results of XRD and N2O decomposition.Still,the temperature of the reduction of CuFe2O4for the sample doped with aluminium is lower than that of the CCA-0,indicating that the synergistic interaction between copper and iron oxides is enhanced by doping Al2O3,especially for CCA-10.Clearly,reducibility of the doped catalysts is improved by the introduction ofAl2O3.

Fig.4 H2-TPR profiles of the as-synthesized catalysts

Table 2 H2consumption of copper species for all the catalysts

3.3 Surface basic properties of the as-synthesized catalysts

Basic sites are reported to play an important role in affecting catalytic performance of WGSR.30Hence,CO2-TPD is used to investigate the basicity of catalysts.Fig.5 presents the CO2-TPD analysis results of the CuO/Fe2O3-Al2O3catalysts.Each CO2-TPD profile shows a main low-temperature desorption peak around 76°C and high-temperature desorption peak at 200-450°C,which was assigned to weak and strong basic sites,respectively.Clearly,The samples CCA-5 and CCA-10 show an obvious peak at 100-200°C, which can be attributed to medium basic sites,according to previous work by our group.11,31,32The low and medium temperature peaks were assigned to the CO2desorption of monodentate carbonate formed on weak and medium basic sites,while the hightemperature peak was assigned to the CO2desorption of bidentate carbonate formed on strong basic sites,which was much more difficult to desorb due to the bond between carbonate and oxide surface,reported by Li et al.32

It is evident that the basic properties of CuO/Fe2O3catalysts are influenced by the introduction of Al2O3,as depicted in Fig.5.The sample CCA-10 shows large peak area in low temperature and small peak area in high temperature,illustrating that there are more weak basic sites,and less strong basic sites,which are in favor of the WGSR.Also,the medium basic sites do favor for the reaction.But the sample CCA-10 has less medium basic sites than CCA-5,meaning that the weak basic sites are the main factor in affecting catalytic activity.

Fig.5 CO2-TPD profiles of the CuO/Fe2O3-Al2O3catalysts

Fig.6 WGSR activity of CuO/Fe2O3-Al2O3catalysts

Fig.7 Catalytic activity of sample CCA-10 maintained at 250°C as a function of time-on-stream for the WGSR

3.4 Catalytic performances of the as-synthesized catalysts in WGSR

The catalytic performances of the CuO/Fe2O3-Al2O3catalysts vs temperature are presented in Fig.6.It is evident that the performances of CuO/Fe2O3catalyst are improved by adding Al2O3.For all the samples,CO conversion increases with the increasing reaction temperature from 200 to 250°C.The samples CCA-10 andCCA-15 are with high initial activity of 52%and 59%,while the CCA-0 is 39%at 200°C,as seen in Fig.6A.The CO conversions of catalysts CCA-3,CCA-5,CCA-10,and CCA-15 reach the maximum values of 92.2%,91.4%,92.5%,and 90.0%at 250°C, respectively.Meanwhile,the conversion of sample CCA-0 is 88.0%at 250°C and reaches its maximum(90.6%)at 300°C. These point out that the introduction of Al2O3not only improves the catalytic performance but also shifts the optimal reaction temperature to lower point.It is well known that Fe-based catalysts in WGSR are used in high temperature(300-450°C),while Cu-based catalysts are used in low temperature(200-250°C).33-35It is also reported that the metal Cu is the active site of the WGSR.16,34Based on the XRD results,there are only Cu and Fe3O4left after the catalysts reduced by the reactant gas at 250°C.We can draw a conclusion that metallic Cu can be responsible for the high catalytic activity of catalysts.In combination with the result of N2O chemisorption,little particle size and high Cu dispersion of the sample CCA-10 and CCA-15 are able to provide more active Cu metal,well explaining the high initial performance.

After the first cycle,the catalysts were exposed to the reactant gas at the given temperature procedure(kept at 400°C for 10 h and then cooled down to 200°C),then started the second evaluation cycle to investigate the thermal stability.It is observed from Fig.6B that all the catalysts deactivate at some extent.But the introduction of Al2O3enhances the thermal stability of CuO/Fe2O3catalysts.The CO conversions for CCA-0,CCA-10,CCA-15 are 16.2%,26.6%,23.2%at 200°Cand71.1%,89.4%,86.7%at 250°C, respectively.Combined with CO2-TPD results,the weak basic sites of the sample CCA-10 are larger than the others.In order to further investigate the thermal stability of CCA-10,the catalyst was tested at 250°C for 120 h.As shown in Fig.7,the CO conversion almost remains unchanged.It illustrates that the catalyst is with well thermal stability and has a long lifetime.Above all, the sample CCA-10 shows high activity and well thermal stability in WGSR.

4 Conclusions

A series of CuO/Fe2O3catalysts doped with Al2O3prepared by stepwise co-precipitation method were evaluated for WGSR.The catalytic activity and thermal stability were improved by introducing Al2O3.Considered the initial activity and thermal stability, the catalysts follow the sequence:CCA-10＞CCA-15＞CCA-5＞CCA-3＞CCA-0.The characterization results indicate that the sample CCA-10 is with relatively smaller crystallite,higher BET surface area,and larger pore volume.Its high initial activity and well thermal stability are contributed to its smaller particle size, higher copper dispersion,greater reducibility,and more weak basic sites.

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Performance of Al2O3-Modified CuO/Fe2O3Catalysts in the Water-Gas Shift Reaction

LIN Xing-Yi*YIN Ling FAN Yan-Yu CHEN Chong-Qi
(National Engineering Research Center of Chemical Fertilizer Catalysts,Fuzhou University,Fuzhou 350002,P.R.China)

The water-gas shift reaction(WGSR)has been carried out over CuO/Fe2O3catalysts modified by different loadings of Al2O3(0%-15%(w)),prepared by a stepwise co-precipitation method.Composite mixture CuFe2O4was produced,and the crystalline size,redox property,and surface metallic Cu dispersion were manipulated.The appropriate introduction of Al2O3can promote the phase transition of spinel CuFe2O4from tetragonal to cubic,inhibit aggregation of Cu-crystallite,improve Cu dispersion,and increase the amount of weak basic sites,as confirmed using powder X-ray diffraction(XRD),Raman spectroscopy,N2physisorption,N2O decomposition,and temperature-programmed desorption of carbon dioxide(CO2-TPD)techniques.In addition, a temperature-programmed reduction of hydrogen(H2-TPR)technique was used to investigate the reducibility of the modified CuO/Fe2O3catalysts.It was found that the Al2O3-doping plays an important role in increasing the hydrogen consumption of the copper species,and decreasing reduction temperature.This means that the Al2O3can promote a synergistic interaction between the copper and iron species in the CuO/Fe2O3catalysts. Overall,the Al2O3-modified catalyst(10%(w))has a smaller Cu particle size,better Cu dispersion,greater reducibility,and larger amount of weak basic sites,resulting in a much higher initial catalytic activity and better thermal stability.

CuFe2O4;Water-gas shift reaction;Al2O3-modification;Cu dispersion;Weak basic site; N2O decomposition

The project was supported by the National Natural Science Foundation of China(21346007).

國(guó)家自然科學(xué)基金(21346007)資助項(xiàng)目

O643.3

10.3866/PKU.WHXB201501091www.whxb.pku.edu.cn

Received:November 13,2014;Revised:January 5,2015;Published on Web:January 9,2015.?