Jin-li Zhng,Shung Wu,Hui Hu∗,Go-ming Wu,Zho-wei Zeng

a.School of Environmental Science and Engineering,Huazhong University of Science and Technology, Wuhan 430074,China

b.Wuhan Iron and Steel(Group)Corp.,Wuhan 430083,China

(Dated:Received on October 29,2013;Accepted on December 26,2013)

N2O Decomposition over K-Ce Promoted Co-M-Al Mixed Oxide Catalysts Prepared from Hydrotalcite-like Precursors

Jin-li Zhanga,Shuang Wua,Hui Hua∗,Gao-ming Wub∗,Zhao-wei Zenga

a.School of Environmental Science and Engineering,Huazhong University of Science and Technology, Wuhan 430074,China

b.Wuhan Iron and Steel(Group)Corp.,Wuhan 430083,China

(Dated:Received on October 29,2013;Accepted on December 26,2013)

A series of mixed oxide catalysts with dif f erent composition of Co-M-Al and Co-M-Ce-Al(M=Zn,Ni,Cu)were prepared by co-precipitation method from hydrotalcite-like compounds.The experimental results revealed the catalytic activity of Co-Ni-Al is slightly higher than that of Co-Zn-Al and much higher than that of Co-Cu-Al for direct decomposition of N2O.Moreover,addition of small amounts of CeO2improved the catalytic activity significantly and made the decomposition temperatures at which the N2O conversion was 50% and 90%(T50and T90)both decreased 80◦C than those of Co-M-Al catalysts without CeO2added.Further,potassium-load also promoted the catalytic activity,and the decomposition temperatures of T50and T90both decreased approximately 50◦C.It is signif i cant for decomposing N2O from industries and reducing carbon emission from atmosphere.

N2O,Catalytic decomposition,Hydrotalcite-like compound,CeO2,Alkali metal

I.INTRODUCTION

Nitrous oxide(N2O),as one of the major greenhouse gases in Kyoto Protocol and also as the gas depleting the ozone layer,has a global warming potential (GWP)of 310 and longer atmospheric residence time (120 years).Therefore,N2O abatement has received more and more attentions recently[1].However,N2O concentration has risen continuously by 0.2%-0.3%per year[2],which emits from both natural and anthropogenic sources such as coal combustion,nitric acid plants,adipic acid plants and f l uidized bed combustors [3-6].Direct catalytic decomposition is considered as a perfect method because it can decompose directly N2O to N2and O2without accretion and the process is without secondary pollution.Various types of catalysts have been reported to decompose N2O[7-11].

Inrecentyears,mixedoxidesderivedfrom hydrotalcite-like compounds(HTLCs)as new catalysts exhibited high catalytic activity in decomposing N2O to N2and O2[12],and transition metals such as Co[13],Ni [14],and Zn[15]were used respectively to replace Mg of hydrotalcite-like compounds and showed better decomposition activity.Kar´askov´a et al.revealed alkali metal potassium in Co-Mn-Al mixed oxide as a promoter to improve catalytic activity for N2O catalytic decomposition[16].Xue et al.found the addition of CeO2to the cobalt spinel led to an enhanced catalytic activity for the direct decomposition of N2O[17].Hu et al.investigated the ef f ect of SO2reduction over CeO2-La2O3/γ-Al2O3and also showed the rare earth metal Ce could accelerate the transfer of surface O and eventually improve catalytic activity[18].However,the ef f ect of rare earth metal Ce on hydrotalcite-like compounds for N2O decomposition was rarely investigated,and there were no reports about the synergistic ef f ect of three aspects of transition metal,rare-earth metal,and alkali metal on hydrotalcite-like compounds for direct decomposition of N2O.

In this work,catalysts Co-M-Al and Co-M-Ce-Al (M=Zn,Ni,Cu)with dif f erent compositions were prepared by co-precipitation method,and then Co-M-Ce-Al(M=Zn,Ni)mixed oxide catalysts as the carriers of the catalysts to be loaded potassium oxide by impreasgnation method,f i nally were tested for the catalytic decomposition of N2O.X-ray powder dif f raction(XRD), Fourier-transform infrared absorption spectra(FTIR) and X-ray photoelectron spectroscopy(XPS)methods were used to characterize the catalysts.

II.EXPERIMENTS

A.Catalyst preparation and characterization

The catalysts of Co-M-Al and Co-M-Ce-Al(M=Zn, Ni,Cu)were prepared by the co-precipitation method. The precursor salts were metal nitrates includingCo(NO3)2·6H2O,Zn(NO3)2·6H2O,Ni(NO3)2·6H2O, Cu(NO3)2·2H2O,and(NO3)3·9H2O(Sinopharm Chemical Reagent Co.,Ltd.,A.C.S.grade).CoxM3-xAl-HTLC(x=0,0.5,1,1.5,2,2.5,or 3,where x is the Co/M molar ratio)was added to the NaOH-Na2CO3solution drop wise under stirring at 40◦C and the pH was controlled to 10.The precipitate slurry was stirred for 1 h and heated at 65◦C for 24 h.The resulting products were separated and washed with distilled water until the pH reached 7,and then being dried at 100◦C for 18 h. These CoxM3-x-Cey-Al-HTLCs samples with dif f erent composition were also prepared using the above method and designated as CoxM3-x-Cey-Al-HTLC,where y is expressed as the molar ratio of Ce/(Co+M).Co-M-Al-HTLC and Co-M-Ce-Al-HTLC samples were calcined at 500◦C to form Co-M-Al and Co-M-Ce-Al mixed oxide catalysts.

Supported catalyst Co-M-Ce-Al(M=Zn or Ni)was prepared by co-precipitation method as above and then impregnated into K2CO3(Sinopharm Chemical Reagent Co.,Ltd,A.C.S.grade)solution.The mixed liquid was treated for 1 h in ultrasonic generator and then dried in a vacuum oven at 100◦C for 18 h,f inally crushed and calcined at 500◦C for 1 h in a muffle furnace.The prepared catalysts were thus referred as zK/CoxM3-x-Cey(z indicates the loading in mass ratio of K2O,and z=0%,0.5%,1%,1.5%,2%).

The samples were characterized by X-ray dif f raction (XRD,PANalytical,Netherlands,Cu Kα radiation,and 1.5418 nm)to check the crystal structure of catalysts with a 2θ range of 5◦-70◦in 0.01◦/step.FTIR were recorded using the KBr pellet technique on the spectrometer FTIR(VERTEX 70,Germany)in the range 4000-400 cm-1and the resolution of 4 cm-1.The catalysts were analyzed using XPS(Vacuum Generators, UK,Al Kα radiation,1486.6 eV)to identify the surface nature and concentration of the active species with a constant pass energy of 50 eV and charging ef f ects were corrected by referencing C1s measurements at 284.6 eV.

B.Catalytic activity test

The catalytic reaction was carried out in a f i xedbed quartz f l ow reactor(ϕ=8 mm),containing approximately 1.0 g of catalyst in all the experiments.The reactor was heated by a temperature controlled furnace with a thermocouple.The testing system consisted of of N2,f l owmeter,mixed gas chamber,reactor and gas chromatograph.Before the experiment,all samples were pretreated for 30 min in air at 500◦C, and then decreased to the reaction temperature.Finally reaction mixture of N2O(4000 ppm)in N2was introduced into the reactor at a f l ow rate of 400 mL and the space velocity(W/F)was approximately 24000 h-1. For reliable N2O conversion rate,the reaction system was kept for 1 h at each reaction temperature to reach steady state and the reaction products were analyzed in of f-gases with a gas chromatographs(Agilent 6890A)equipped with HP-PLOT/Q capillary columns (30 m×0.32 mm×20µm)and ECD detector,respectively.In all tests,N2and O2were the only gaseous products we observed.The temperature range was from 200◦C to 650◦C,and the ef f ect of catalytic activity was expressed by the conversion rate of N2O.The conversion of reactant X was calculated using Eq.(1).

FIG.1 XRD patterns of(a)Co-Ni-Al-HTLCs and(b)Co-Ni-Al mixed oxides.

where X(N2O)is conversion rate of N2O,Cin(N2O)and Cout(N2O)are concentrations of N2O(ppm)in inlet and outlet,respectively.

III.RESULTS AND DISCUSSION

A.Structural characteristics of Co-M-Al-HTLCs and catalytic activity of derived Co-M-Al mixed oxides

FIG.2 FTIR spectra of(a)Co-Zn-Al-HTLCs,(b)Co-Ni-Al-HTLCs,and(c)Co-Cu-Al-HTLCs.

In this work,all Co-M-Al-HTLCs and Co-M-Al mixed oxides were characterized by XRD.The XRD spectra of Co-Ni-Al-HTLCs and Co-Ni-Al mixed oxides are shown in Fig.1.From Fig.1(a),the crystallographic indices in XRD patterns of fresh Co-M-Al-HTLCs such as(003),(006),(110),and(113)dif f raction lines exhibited the crystal characteristics of hydrotalcite-like materials,it indicated all Co-Ni-Al-HTLCs samples formed the hydrotalcite characteristic dif f raction peak [19].XRD patterns of the calcined Co-Ni-Al-HTLCs are shown in Fig.1(b).The main crystal phase of Co0Ni3and Co0.5Ni2.5was NiO,but the main crystal phase of Co2.5Ni0.5and Co3Ni0was spinel phase.It was seen that the characteristic dif f raction peaks of HTLCs disappeared,indicating the complete collapse of double layered structure after being calcined at 500◦C.Compared with Co2.5Ni0.5and Co0.5Ni2.5,we found that spinel type dif f raction peak became increasingly sharp with the Co content increasing,and the peak intensity increased,f i nally the crystallinity became better.Moreover,the segregation phenomenon of NiO decreased gradually and the NiO dif f raction peak became more dispersed.

It can be seen from Fig.2 the FTIR spectra of all the Co-M-Al-HTLCs samples also exhibited some characteristics of hydrotalcite-like materials.The broad band around 3450 cm-1was ascribed to the stretching mode of hydroxyl groups.The intense band at 1380 cm-1was due to the asymmetric stretching mode of C=O in CO32-and shifted to lower frequency in contrast to that in CaCO3,indicating the strong hydrogen bonding of water molecules with interlayer carbonate anions. The band below 800 cm-1was the skeletal vibration of metal-O bonds and hydrotalcite-like[20].

FIG.3 Conversion of N2O as a function of reaction temperature over(a)Co-Zn-Al,(b)Co-Ni-Al,and(c)Co-Cu-Al mixed oxides.Reaction conditions:4000 ppm N2O,N2balance,GHSV=24000 h-1.

The N2O conversions over Co-M-Al mixed oxides with dif f erent compositions are shown in Fig.3.Compared with Co0M3Al,it could be seen that the partial replacement of M2+(M=Zn,Ni,Cu)by Co2+with different degree led to a signif i cant improvement in the catalytic activity for the N2O decomposition.With the increasing of Co content(0＜x＜3),the catalytic activity was improved gradually.Especially among Co-M-Al mixed oxide catalysts,the group with Co2.5M0.5exhibited the best catalytic activity.A further increase in Co2+content caused a decrease in the N2O decomposition activity.

B.Impact of Ce on catalytic activity of Co-M-Al samples

Figure 4 revealed the catalytic activity of N2O over Co-M-Ce-Al mixed oxides.It can be seen that the mixed Ce in catalyst Co-M-Al gave rise to an increase of the catalytic activity and the activity decreased with increasing of the Ce amount.When the molar ratio ofmixed Ce was 0.05,the catalyst Co2.5M0.5Ce0.05mixed oxide represented the best catalytic activity for N2O decomposition and the temperatures of 50%and 90% conversion of N2O(T50and T90)were the lowest.

FIG.4 Conversion of N2O as a function of reaction temperature over Co-M-Ce-Al mixed oxides.Reaction conditions: 4000 ppm N2O,N2balance,GHSV=24000 h-1.

Table I shows the temperatures of 50%and 90%conversion of N2O(T50and T90)over Co-M-Ce-Al mixed oxide catalysts.The temperatures of 50%and 90% conversion of N2O over Co3M0were higher than those of the other catalysts which were partially replaced by M2+,indicating that the replacement of M2+improved the catalytic activity.Moreover,compared with Co2.5Zn0.5,T50and T90over Co2.5Zn0.5Ce0.05decreased 78 and 91◦C,respectively(shown in Table I),and T50and T90over Co2.5Ni0.5Ce0.05also decreased 107 and 89◦C than those of Co2.5Ni0.5,respectively,which was signif i cant in practice.However,Co2.5Cu0.5Ce0.05had the relatively poor catalytic activity,and T50and T90over Co2.5Cu0.5Ce0.05than those of Co2.5Cu0.5decreased 83 and 42◦C,respectively.

FIG.5 XRD patterns of(a)Co-Ni-Ce-Al-HTLCs and(b) Co-Ni-Ce-Al mixed oxides.

TABLE I Temperatures of 50%and 90%conversion of N2O (T50and T90)over Co-M-Ce-Al mixed oxide catalysts Catalyst.

In order to inspect the crystal forms and structures of Co-M-Ce-Al hydrotalcite-like catalysts before and after calcining,we chose Co-Ni-Ce-Al-HTLCs and Co-Ni-Ce-Al to be characterized by XRD.From Fig.5(a),we found that the crystallographic indices in XRD patterns of fresh Co-Ni-Ce-Al-HTLCs such as(003),(006),(110), and(113)dif f raction lines exhibited the crystal characteristics of hydrotalcite-like materials.With the Ce content increasing,the shape of dif f raction peaks became more dispersive and the peak intensity became weaker.

FIG.6 Conversion of N2O as a function of reaction temperature over(a)K/Co-Zn-Ce-Al and(b)K/Co-Ni-Ce-Al mixed oxides.Reaction conditions:4000 ppm N2O,N2balance,GHSV=24000 h-1.

The XRD patterns of calcined Co-Ni-Ce-Al mixed oxides are shown in Fig.5(b).From Fig.5(b),we found the main crystalline phase of Co2.5Ni0.5was spinel phase,while the crystalline phase of Co2.5Ni0.5Ce0.05and Co2.5Ni0.5Ce0.2included spinel phase and CeO2phase,however,the characteristic dif f raction peaks of HTLCs disappeared in these samples,indicating the complete collapse of double layered structure after calcination at 500◦C.Compared with Co2.5Ni0.5Ce0.05and Co2.5Ni0.5Ce0.2,we found that the dif f raction peak of CeO2phase became increasingly sharp with the Co content increasing,the peak intensity increased,and the crystalline became better,however,the dif f raction peak of spinel phase was weaken gradually.

Widmann et al.considered CeO2,as one of rare earth materials,was very easy to transform from one to the other because of two kinds of valence states and could be stored and released at the same time to possess special oxidation-reduction properties with excellent reversibility[21].Hu et al.also conf i rmed the transformation from Ce4+to Ce3+in catalytic desulfurization over rare oxide catalyst[18].Therefore,we suggested the addition of CeO2in catalysts led to support more chances for O transference,and it was related to the contribution of oxygen from CeO2to improve the catalytic activity.

C.Ef f ect of potassium on catalytic activity of K-Ce promoted Co-M-Al mixed oxides

Figure 6 illustrates the N2O conversions over Co-Zn/Ni-Ce-Al mixed oxide catalysts modif i ed by loadingvarious amount of potassium.From Fig.6,we found the K2CO3-doped catalysts were more active than bare Co-Zn/Ni-Ce-Al oxides.With the content increase of potassium loaded,the catalytic activities of the Co-Zn/Ni-Ce-Al mixed oxide catalysts were enhanced signif i cantly until the optimal value of potassium-loaded at 1.5%.

TABLE II Temperatures of 50%and 90%conversion of N2O (T50and T90)over K-Co-Ni-Ce-Al mixed oxide catalysts.

In particular,as shown in Table II,when the mass ratio of K2O on Co-Zn-Ce-Al mixed oxide was equal to 1.5%,the promotional ef f ect of K2O was so remarkable that T50and T90were lowered by 49 and 73◦C in comparison with Co-Zn-Ce-Al mixed oxides,and lowered by 127 and 164◦C in comparison with Co3O4,respectively. While T50and T90over 1.5%K/Co2.5Ni0.5Ce0.05Al were 261 and 346◦C,respectively.T50and T90decreased by 49 and 34◦C in comparison with Co-Ni-Ce-Al mixed oxide,and decreased by 156 and 123◦C,in comparison with Co3O4,respectively.Such a pronounced enhancement was signif i cant for industrialization to decompose N2O and reduce carbon emission from atmosphere.

The dominant state of surface nickel element was NiO,andfromFig.7(a)wefoundthepeaksof Ni2p1/2and Ni2p3/2of 1.5%K/Co2.5Ni0.5Ce0.05catalyst moved to lower banding energy in comparison with un-promoted catalyst,indicating the weakness of surface Ni-O bond due to the electron donation of K.In Fig.7(b),the peaks of corresponding binding energy(BE)of Co2p3/2and Co2p1/2for 1.5%K/Co2.5Ni0.5Ce0.05were 779.6 and 794.7 eV,respectively.Moreover,the∆E(Co2p1/2-Co2p3/2)was 15.1 eV.BE of Co2p3/2for 1.5%K/Co2.5Ni0.5Ce0.05decreased 0.3 eV,which was same as that of Co2p1/2,in comparison with Co2.5Ni0.5Ce0.05,resulting in the further weakness of surface Co-O bond and the desorption of adsorbed oxygen species becoming easier.Eventually the catalytic activity was improved.When potassium was loaded,the banding energy decreased,which indicated the electronic state of Co moved to the lower state.Asano et al.suggested that small amount of potassium ions could be adsorbed on the particles tocompensate the negative charge of the particles[22]. Alkali cations were Lewis acids and had no electron donation ef f ects.However,oxygen anions in the coordination sphere of the cations were highly basic and therefore electron donation from highly basic oxygen anions surrounding the alkali cation toward cobalt ion increased the electron density of cobalt ions and played an important role in N2O decomposition.

FIG.7 XPS of(a)Ni2p,(b)Co2p,and(c)Ce3d level for 1.5%K/Co2.5Ni0.5Ce0.05Al mixed oxide.

Figure 7(c)revealed the XPS of Ce3d level for Co2.5Ni0.5Ce0.05and 1.5%K/Co2.5Ni0.5Ce0.05catalysts, including three patterns for Ce3d5/2(UI,UII,UIII)and three patterns for Ce3d3/2(VI,VII,VIII)[17]to correspond to Ce4+,which indicated the loading of alkali metal oxide on Co2.5Ni0.5Ce0.05mixed oxide did not signif i cantly inf l uence the valence state distribution of Ce.From Fig.7(c),we found that the XPS ofCe3d level for 1.5%K/Co2.5Ni0.5Ce0.05was more intensive than Co2.5Ni0.5Ce0.05and BE of Ce3d5/2and Ce 3d3/2were slightly dif f erent.It might be one reason for the dif f erence of catalytic activity.

Figure 8 shows the O1s spectra of the catalysts.The peak at 529.0-529.5 eV was attributed to the O2-anions of the crystalline network[23].The O1s peak slightly shifted toward the lower BE side by the addition of K,indicating that the electronic density of oxygen increased,i.e.,the basicity of oxygen anion became high.

IV.CONCLUSION

Among the Co-M-Al(M=Zn,Ni,Cu)mixed oxide catalysts prepared from hydrotalcite-like precursors with dif f erent compositions,the series of Co2.5M0.5mixed oxides showed better catalytic activities.Moreover,the catalytic activity of Co-Ni-Al was slightly higher than that of Co-Zn-Al and much higher than that of Co-Cu-Al in decomposing N2O.

The experiment of Co-M-Ce-Al catalyst for N2O decomposition indicated CeO2displayed a key role in improving the catalytic activity and made the decomposition temperatures of both T50and T90decreased 80◦C compared with Co-M-Al catalysts without CeO2added. Furthermore,the series of Co2.5M0.5Ce0.05mixed oxides had higher catalytic activities.

Potassium-load also had some ef f ect on promoting catalytic activity for direct decomposition of N2O. When the optimal value of potassium-load was 1.5%, the decomposition temperatures T50and T90both decreased approximately 50◦C.It is signif i cant for decomposing N2O from anthropogenic source such as the nitric acid plant and reducing carbon emission from atmosphere.

V.ACKNOWLEDGMENTS

This work was supported by the National High-tech R&D Program of China(No.2012AA062501)and the Postgraduates Innovation Foundation of Huazhong University of Science and Technology of China(No.HF-08-11-2011-261).We also thank the analytical support from Analytical and Testing Center of Huazhong University of Science&Technology.

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∗Authors to whom correspondence should be addressed.E-mail：zjlzjl@hust.edu.cn,gm-wu1031@163.com,Tel.：+86-27-87542224,FAX：+86-27-87792101