Tianyu Guo ,Jianping Du ,Jinting Wu ,Jinping Li,*

1 Research Institute of Special Chemicals,Taiyuan University of Technology,Taiyuan 030024,China

2 College of Chemistry and Chemical Engineering,Taiyuan University of Technology,Taiyuan 030024,China

1.Introduction

With the depletion of fossil energy resources,the demand for new energy resources is growing.At present,it is significant to develop and utilize new energy.Methane is considered as one of the main gas energies and its applications have been attracted great attention.Coal bed methane(CBM)possesses a large amount of methane source[1].However,with extremely low concentration methane(0.1 vol%–1.0 vol%),called “ventilation air methane(VAM)”[2],is usually emitted into atmosphere.Due to the large flow rate of VAM and the low content of methane[3],it is a great challenge to capture methane and avoid its emission into atmosphere.

As we know,the greenhouse effect can easily bring about serious problems of ecological environment such as global warming,sea level rise and abnormal climate.The warming effect of carbon dioxide as a greenhouse gas has been well known,however,the warming potential of methane is 21–23 times higher than that of carbon dioxide[4–6].Therefore,it is significant to reduce the emission of methane into the atmosphere to mitigate the global warming in the short term[5].

Now,many works focus on the utilizing of low-concentration methane by the oxidation method,which includes conventional combustion and catalytic oxidation in terms of the combustion kinetic mechanisms[2].Compared to the conventional combustion,catalytic oxidation is more helpful to remove low-concentration methane from the VAM[7,8].

Generally,catalysts used for methane oxidation include noble metal supported catalysts[9,10],perovskite[11],hexaaluminate[12]and solid solution catalysts[13].Among them,supported-Pd catalysts exhibit high activity in the complete oxidation of methane[14].However,high cost of noble metal limits its use[15].We expectnovel materials to substitute for noble metal.Recently,some works focused on research about solid solution oxide as catalyst in some reactions due to its good mechanical and electrical properties[16],which include methanereforming[17],photocatalysis[18],energy transfer[19],CO2methanation[20]and methane oxidation[21,22].It is con firmed that ceria is a potential catalytic material because of the redox ability involving the Ce4+/Ce3+couple and high oxygen storage capacity[23,24].Moreover,Zr cation doped in the CeO2lattice can promote the oxygen mobility,improve its thermal stability and enhance its redox property[25].CeO2–ZrO2mixed oxide possesses oxygen storage–release capacity[26],which may endow it some advantages in low-concentration methane oxidation.To the best of our knowledge,the study on catalytic performances of CeO2–ZrO2solid solution in low-concentration methane oxidation has not been addressed.

Herein,mesoporous solid solution(CeZr)O2was synthesized by sol–gel method and exhibited excellent catalytic properties,particularly,its stability,in methane oxidation.More importantly,the performances of as-synthesized CeO2–ZrO2solid solution were further enhanced by doping of nickel,implying that it will be a potential catalyst substitution for noble metal in decreasing emission of methane.

Fig.1.XRD patterns of as-prepared solid solution catalysts(a)CZS,(b)MgO–CZS and(c)NiO–CZS.

2.Experimental

2.1.Catalyst preparation

All the reagents were obtained from commercial sources and used without further purification.CeO2–ZrO2solid solution(CZS)was prepared by urea hydrolysis-assisted method.In the typical process,9.77 g cerium(III)nitrate hexahydrate and 1.73 g zirconium(IV)oxynitrate hydrate were dissolved in distilled water(100 ml)and the mixture was stirred for 1 h.Molar ratio of Ce/Zr is 3:1.Then urea solution(0.3 mol·L?1)was added into the above solution and continuously stirred for 3 h.The powder was maintained at 100°C for 32 h.Finally,the product was obtained by being calcined at500°C for4 h.The obtained solid solution powder was modified with metal oxides by wet incipient impregnation method.Nickel nitrate and magnesium nitrate were used as NiO and MgO precursors,respectively.The weight percent of Ni and Mg was 1–20 wt%and 10 wt%,respectively.The obtained samples were dried for 12 h at 120°C,followed by being calcined for 4 h at 500°C.Doped-metal samples were denoted as MO/CZS(M:Ni and Mg).

2.2.Characterization

The structures of the as-prepared materials were measured by X-ray powder diffraction(XRD)ata scan rate with 2θ values ranging from 20°to 80°.Transmission electron microscopy(TEM,JEOL)images were obtained by a JEM-2100F electron microscope.The specific surface areas were measured by adsorption–desorption of N2at 77 K by using full automatic adsorption instrument ASAP2010M.The pore size distribution was obtained by using the Barrett–Joyner–Halenda(BJH)method.Before the tests,the samples were degassed in a nitrogen atmosphere at 200°C for 2 h.The H2-temperature programmed reduction experiments(H2-TPR)were carried out on a Micromeritics Auto Chem II 2920 instrument.Prior to H2-TPR experiment,50 mg samples were pretreated under Ar with a total flow rate of 30 ml·min?1at 400 °C for 60 min and then cooled to ambient temperature.H2/Ar with a flow rate of 30 ml·min?1was used,and the temperature was increased from 50 °C to 1000 °C at a heating rate of 10 °C·min?1.

2.3.Catalytic property

The activities of catalysts were measured in a fixed bed quartz microreactor(i.d.=6 mm)at atmospheric pressure.The feed gas consisted of 1 vol%CH4,20 vol%O2and 79 vol%N2.The total flowrate of gas was setto 40 ml·min?1,corresponding to the gas hourly space velocity(GHSV)of 110000 h?1.For each run,150 mg catalyst(250–380 μm)was packed into the center of the reactor using quartz wool as plugs.The temperature of the catalyst bed was carried out by a thermocouple.The product gases were analyzed by using a GC-6890A equipped with a TDX-01 column and a thermal conductivity detector(TCD).The minimum CH4concentration measured by the GC is 100–200 mg·L?1.Methane catalytic oxidation was performed in the temperature range of 250–700 °C in 50°C increments.Prior to each run,the catalysts were pretreated for 1 h at200°C in a stream of nitrogen to clean their surface.In all measurements,when methane reacted with oxygen completely,CO2and H2O were the only reaction products.The produced water vapor was collected in a condenser connected to the outlet of the reactor.The activity is evaluated byT10%andT90%,representing the methane conversions are 10%and 90%respectively at this temperature.

3.Results and Discussion

3.1.XRD analysis

Fig.2.N2 adsorption–desorption isotherms(a)and pore-size distributions(b)of the fresh catalysts.

X-ray diffraction patterns of the as-prepared samples were presented in Fig.1.The characteristic peaks of the sample are ascribed to the structure of Ce0.75Zr0.25O2,which is consistent with reported results[27].All peaks appearing at 29°,33°,48°and 57°are indexed to the diffraction of(111),(200),(220)and(311)planes respectively(JCPDS No.28-0271),which are attributed to cubic structure.No other peaks corresponding to either ZrO2or CeO2phase can be observed,which suggests that Zr4+is doped into the CeO2lattice to for ma solid solution with the fluorite structure[28].The characteristic diffraction peaks of metal oxides can also be observed.The two peaks at near 42.9°and 62.3°(2θ),which are related to the diffraction peaks of MgO crystal.The diffraction peaks of NiO are observed at 2θ =37.2°and 43.2°.All peak intensities are very low,suggesting that they have small particle size or high dispersion.

Table 1Physical properties of fresh catalysts

3.2.Textural properties

From Fig.2,it can be seen that all samples show type IV isotherms with a H3 hysteresis loop occurring atP/P0between 0.2 and 1.0,indicating the presence of the mesoporous.The H3 hysteresis loop is usually observed in slit-shaped pores formed by the aggregates of plate-like particles[29].Moreover,modification with oxides affects the BET surface area and pore volume of catalysts.As shown in Table 1,the pore volume decreases as the order of CZS＜NiO–CZS＜MgO–CZS,which is consistent with the change of specific surface areas.The result shows that introduction of oxides decreases the BET surface area and pore volume of the CZS,indicating that the oxides may cause some blockage of CZS pores[30].Especially,the pore volume of MgO–CZS reduced by 57.7%compared to that of CZS.The BJH pore size distributions of the as-prepared catalysts are shown in Fig.2b.The CZS catalyst has a wide pore-size distribution from 2 to 45 nm.Additionally,the sample exhibits a sharp peak atca.8 nm,showing the presence of mesoporous structure.After CZS was modified by the oxides,the pore size distributions have almost no change,implying that there is no effect of metal oxides on the structure of the CZS sample.

Fig.3 displays the TEM images and EDS spectrum of NiO–CZS.The large CZS particles consist of small ones and NiO nanoparticles are dispersed uniformly on the surface of solid solution Ce0.75Zr0.25O2(Fig.3a).The size distribution is about 2–3 nm and CZS particles are observed clearly(Fig.3b).The lattice fringes of single NiO nanoparticles are clear and the lattice distance is about 0.205 nm(Fig.3c).From EDS spectrum,cerium,zirconium,nickel and oxygen elements are measured,which indicates the composition of NiO doped solid solution and their high purity(Fig.3d).

3.3.Catalytic performance

3.3.1.The effect of doped metal

For all catalysts,besides carbon dioxide and water,no other products can be detected in the final products,implying that methane can converse completely.The properties of catalysts(MO–CZS)in lean methane oxidation are shown in Fig.4.Fig.4a shows the light-off temperature for methane oxidation over CZS and metal oxide-modified CZS catalysts.The initial high activities are indicated by the lower temperature(T10%),which means that there is 10%conversion of methane at this temperature.For different catalysts,the initial activity order is NiO–CZS＞CZS＞MgO–CZS.When methane conversion is up to 90%,the reaction temperatures(T90%)are about 500 °C,600 °C and 650 °C respectively.This results show that NiO doped CZS catalyst exhibits higher activity at lower temperature compared with MgO–CZS and undoped solid solution catalysts,which is superior to MnOx–CeO2,La2CoMnO6/CeO2and Ce(5)MgAlO catalysts[31–33].The possible reason is that NiO promotes the reduction of surface oxygen.Hence,it provides more activity oxygen species for the methane oxidation and thereby makes more methane molecules activated.

Fig.3.(a)TEM and(b,c)HRTEM images of CZS modified with NiO catalysts(d)EDS spectrum of selected areas in(b).

The further evidences are provided by H2-TPR(Fig.5).For CZS,two reduction peaks were observed.One is related to reduction of the surface oxygen of CeO2at low temperature(500°C)and the other is attributed to reduction of the bulk oxygen of CeO2at high temperature(730 °C)[34].NiO–CZS catalyst shows two H2consumption peaks at 244 °C and 331 °C.The first peak is attributed to the reduction of free NiO particles at 244 °C,while the other peak(331 °C)is assigned to the overlap peaks of the reduction of NiO strongly interacting with CZS support and surface oxygen of CeO2[13,35].Apparently,the reduction peak of the surface oxygen moves toward lower temperature,suggesting that the addition of NiO promotes the reduction of surface oxygen and thereby enhances oxygen mobility,and hence high catalytic activity as a consequence.For MgO–CZS,one shoulder peak can be observed at 532°C,which is ascribed to the reduction of surface oxygen of CeO2and the peak shifts toward higher temperature.The possible reason is that MgO suppress the reduction of surface oxygen leading to low oxygen mobility,thereby suppress the catalytic activity.The reduction peak of MgO can't be detected due to highly stability of Mg2+ions in MgO structure[36].

In order to further evaluate the stability,all catalysts were tested for 20 h at 550 °C respectively.Fig.4b shows the stabilities of the MO–CZS catalysts over 20 h.At the temperature of 550°C,the conversion of methane has been closed to 100%over NiO–CZS.Only a slight decrease can be observed after 8 h and then it maintains relative stability.Other catalysts also exhibit high stability although methane conversion over them is lower than that over NiO–CZS.The results indicate that the as prepared solid solution(CZS)is effective catalyst for oxidation of low concentration methane.Its catalytic performance can be further improved by modifying with NiO.Because porous channels favor the diffusion of reactants and products molecules[37,38],high stability of solid solution catalysts may be ascribed to their pristine mesoporous structures partly,which are con firmed by analysis of pore characteristic(Fig.2b).

In order to obtain further information,XRD analysis was conducted.From the results of XRD patterns of the used catalysts,the obvious sintering of MO–CZS particle is not found when experiencing a high temperature(Fig.6a)or a long time(Fig.6b),which can be con firmed by the peak intensities of XRD patterns compared to that of the fresh samples.Only weak diffraction peaks of metal oxides(NiO and MgO)are observed at corresponding diffraction angles(denoted with black arrow)compared to that before reaction(Fig.1),suggesting that metal oxides maintain initial dispersion and size distribution without obvious sintering.The solid solution phases still have perfect crystal structure after reaction at different temperatures from 250 to 700°C(Fig.6a)and running for 20 h(Fig.6b)respectively.Therefore,the catalytic stability is not only attributed to the less sintering of oxide particles,but is related to the CZS structure which may play a spatial confinement effect role in particle dispersion.

Fig.4.(a)Light-off curves of solid solution catalysts at different temperatures.(b)Catalyst stability of solid solution catalysts at 550°C for 20 h.

Fig.5.H2-TPR pro files of different solid solution catalysts.

Fig.6.XRD patterns of the used solid solution catalysts(a)after reaction at various temperatures(250–700 °C)and(b)after reaction at 550 °C for 20 h.

3.3.2.The effect of NiO-doped contents

According to the above result,doping NiO into CZS material improves the property of CZS catalyst.Here,the amount of doping Ni has been optimized.Catalytic properties of NiO–CZS(NiO content:1,2,5,10,15 and 20 wt%)have also been performed.From XRD results of the catalysts,the diffraction peaks of NiO for NiO–CZS catalysts with low contents(1,2 wt%)aren't found(Fig.7).The possible reason is due to their low content and high dispersion on the surface of CZS.

The activities of NiO–CZS catalysts with various NiO loadings are shown in Fig.8.It can be seen that NiO contents have an obvious effect on the catalytic properties of NiO–CZS.That is,a maximum methane conversion over NiO–CZS catalysts can be obtained by adjusting guest-NiO contents.Obviously,methane conversion displays a trend offastincrease when NiO contents range from 1 to 5 wt%,and 15 wt%NiO–CZS catalyst exhibits excellent catalytic activity for methane oxidation.When the NiO content rises to 20 wt%,methane conversion does not increase continuously.To further analyze the catalytic properties,H2-TPR experiments were performed.Fig.9 shows the H2-TPR pro files of the NiO–CZS catalysts with different NiO contents.It is found that the reduction temperature of NiO strongly interacting with CZS support increases as NiO contents rising from 1 to 10 wt%,indicating stronger interaction between NiO and CZS support.The strong interaction can lead to high dispersion of particles and hence high activity as a consequence.Excess NiO weakens the interaction between metal oxide and support,which is evidenced by lower reduction temperature.Moreover,a small peak atca.240°C can be observed for the high NiO loadings,revealing the occurrence of free NiO particles caused by high NiO contents.Therefore,it is important to optimize the guest metal oxide contents for improving property of catalyst.

Fig.7.XRD patterns of the NiO–CZS catalysts with various NiO contents:(a)1 wt%,(b)2 wt%,(c)5 wt%,(d)10 wt%,(e)15 wt%,and(f)20 wt%.

Fig.8.Methane conversion over the NiO–CZS catalysts with various NiO contents.

Fig.9.H2-TPR pro files of NiO–CZS catalysts with various NiO contents:(a)1 wt%,(b)2 wt%,(c)5 wt%,(d)10 wt%,(e)15 wt%,and(f)20 wt%.

4.Conclusions

Mesoporous Ce0.75Zr0.25O2solid solution was synthesized and modified by metal oxides.The catalytic properties and thermal stabilities of different catalysts modified with metal oxides were also investigated by low-concentration methane oxidation.Nickel oxides improved the activity of solid solution effectively,and the optimal loading is 15 wt%.High activities are ascribed to high oxygen mobility compared to magnesium oxide and CZS.The stability of solid solution catalysts con firms that the mesoporous structure of solid solution is helpful to promote fast diffusion of reactants and products,and suppresses oxide particle sintering,which implies that the metal-doped Ce0.75Zr0.25O2solid solution has a potential application in removal of methane from coal bed methane and related fields.

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