Yincong Liu,Lingjun Zhu,Xiaoliu Wang,Shi Yin,Furong Leng,Fan Zhang,Haizhou Lin,Shurong Wang*

State Key Laboratory of Clean Energy Utilization,Zhejiang University,Hangzhou 310027,China

1.Introduction

As an effective technology to synthesize the substitute of natural gas,methanation has aroused extensive attention of researchers[1–3].At present,researches of syngas methanation mainly focus on the methanation catalyst including active components,auxiliaries and supports.Millset al.[4]found that the methanation catalytic activities of the metals were in accordance with the following order:Ru＞Ir＞Rh＞Ni＞Co＞Os＞Pt＞Fe＞Mo＞Pd＞Ag.Precious metals such as Rh,Ir and Ru have better catalytic activities,however the cost is pretty high[5–8];Co has better catalytic activity at low temperature and can avoid coking deactivation[9],while the selectivity of CH4is poor.Fe-based catalyst has extensive sources with lower price but it is easy to cause carbon deposition[10].Compared with the catalysts mentioned above,Ni-based catalyst for CO methanation has a broad research prospect.

The activity of Ni-based catalyst can be affected by the supports,preparation methods and auxiliaries.The common supports of Nibased catalysts include Al2O3,SiO2and ZrO2etc.,and these supports can affect the catalytic activities of Ni-based catalyst through changing the particle size of active components[11].Al2O3as support[12]can interact with the super ficial NiO species to form ionic bonds promoting the dispersion of NiO species over the surface,which exhibited higher catalytic activity during the research of syngas methanation[3,13].ZrO2has been studied widely in recent years[11,14].ZrO2with the properties of N type semiconductor can generate strong interaction with the metal loaded on its surface,and occurs negative charge absorption with oxygen easily[15].Under the condition of less oxygen,the existence of redox pair Ce3+/Ce4+makes Ce move fast between CeO2and Ce2O3thus forming unstable oxygen vacancy and generate strong interaction with oxygen atoms of CO molecules,which makes C–Obonds easily break to form activated carbon over the surface of support.CeO2,as an important rare earth oxide with special property,shows more potential in the study of support[16].

As a consequence,Al2O3,ZrO2and CeO2were employed to be catalyst supports for the research of methanation.Catalytic activity of catalysts for methanation is also affected by preparation method.Wanget al.[17]found that the particle size of catalyst was smaller prepared by co-precipitation,and the active components evenly dispersed over the support,which led to better catalytic performance.Therefore this paper adopted co-precipitation method to prepare Ni/Al2O3,Ni/ZrO2and Ni/CeO2.The catalytic activities of these catalysts were compared in order to find out the most optimal catalyst which could display higher CO conversion and better selectivity to CH4.Meanwhile the effect of different supports on Ni-based catalysts was investigated.In addition,the catalyst with the best result of methanation was taken to life test in aim to study the stability of the catalyst at high temperature.

2.Experimental

2.1.Catalyst preparation

Ni/Al2O3,Ni/ZrO2and Ni/CeO2catalysts were prepared by coprecipitation method with the nominal Ni loading of 15 wt%.The reagents were purchased from Sinopharm Chemical Reagent Co.

Ni/Al2O3catalyst was prepared through the following steps:3.7 g Ni(NO3)2·6H2O and 15.6 g Al(NO3)3·9H2O were added to 125 mL deionized water to get the uniform aqueous solution after continuous stirring.Then 1 moL·L−1K2CO3solution was dropwise added as a precipitant to the Ni(NO3)2and Al(NO3)3solution until the pH value reached 9–10,followed by stirring for 2 h at 70 °C.After that,the suspension was filtered and washed using deionized water to neutral state.The obtained precipitate was dried at 120°C overnight and calcined at 500°C in air for 4 h.

For the preparation of Ni/ZrO2and Ni/CeO2catalysts,15.6 g Al(NO3)3·9H2O was replaced by 14.8 g Zr(NO3)3.5H2O and 10.7 g Ce(NO3)3.6H2O respectively,while other procedures were the same as that for the preparation of Ni/Al2O3catalyst.

2.2.Catalyst characterization

The surface area(SBET)and pore structures of the catalysts were measured by N2physical adsorption(Quadrasorb SI).Before this measurement,the sample was degassed at 300°C for 3 h.N2adsorption–desorption isotherms were recorded at −196 °C.The surface area was obtainedviaBrunauer–Emmett–Teller(BET)method while the pore size and volume were calculated from Barrett–Joynerr–Halenda(BJH)model.The real content of nickel in the catalyst was analyzed by inductively coupled plasma atomic emission spectrometry(ICP-AES),using a Thermo iCAP 6000 device.

Powder X-ray diffraction(XRD)patterns were characterized on a PANalytical X'Pert PRO X-ray diffractometer with a CuKαradiation source at 40 kV and 30 mA.The diffraction angle 2θ ranged from 10°to 90°and the scan speed was 5(°)·min−1.The crystallite size was calculated according to the Scherrer equation.

Reduction feature of the catalysts was analyzed through hydrogen temperature programmed reduction(H2-TPR)on an AutoChem II 2920 instrument(Micromeritics Instrument Corp).The samples(20 mg)were purged by argon at 200 °C for 90 min and then cooled to 50 °C.Then,the reduction was carried out with a mixture of 10%H2/Ar at a heating rate of 10 °C·min−1up to 900 °C.During the reduction,the consumption amount of H2was recorded with a thermal conductivity detector.

The amount of active sites on the reduced catalysts was measured according to the hydrogen adsorption capacity and adsorption strength by H2-TPD(temperature programmed desorption)experiments using an AutoChem II 2920 instrument(Micromeritics Instrument Corp).Prior to the H2-TPD measurement,50 mg catalyst sample was initially reduced with a mixed stream of 10%H2/Ar.After cooling the sample to 50°C,purging for 90 min under H2/Ar atmosphere and then purging under argon for another 90 min to remove the physicsorbed hydrogen,the temperature was increased from 50 °C to 800 °C at a heating rate of 10 °C·min−1.The desorbed hydrogen was detected by using a thermal conductivity detector(TCD).

2.3.Test of catalytic activity

The catalytic test was conducted in a fixed-bed reactor.In the experiment,1 g catalyst was packed in a stainless steel tube reactor and sandwiched by silica wool on the top and at the bottom.A thermocouple was centered in the catalyst layer to measure reaction temperatures.Before reactions,the catalysts were reduced by hydrogen(50 ml·min−1)at 470 °C for 2 h with a heating rate of 5 °C·min−1.After reduction,the catalyst bed temperature was decreased to the reaction temperature(200–440 °C)and the pressure was maintained at 1 MPa.The syngas had a composition ofn(H2)/n(CO)=3 and the gas hourly space velocity(GHSV)was at 20000 ml·h−1·g−1controlled by the mass flowmeter(Brooks,5850E).The production gas flow was measuredviasoap film flowmeter(GILIBRATOR-2).The product gases were analyzed with an online gas chromatograph(Agilent 7890A).The hydrocarbon was separated by HP-PLOT Qcapillary column and detected by a hydrogen flame ionization detector(FID).H2,CO,and CO2in the gas were separated by Porapak Q and carbon sieve packed columns and then measured by TCD.Under each reaction condition,several groups of data were obtained to get an average value.The molecular CO conversion(XCO)and product selectivity were presented in Eqs.(1)and(2)as follows:

wherencoinandncooutrepresented the CO molecular numbers in feed gas and product gas,respectively,andmstood for the carbon number of product Cm.

3.Results and Discussion

3.1.Catalyst characterization

N2physical adsorption of the three prepared catalysts was carried out to characterize their BET surface area and pore structure.As shown in Table 1,the BET surface area of Ni/Al2O3catalyst is 266.3 m2·g−1,which is much larger than the other catalysts,13.4 and 32.9 m2·g−1for Ni/ZrO2and Ni/CeO2catalysts respectively.Meanwhile,the largest pore volume of 0.27 ml·g−1was found in Ni/Al2O3catalyst.For catalyst of Ni/ZrO2,its pore volume is only 0.02 ml·g−1,indicating the less developed pore structure.Ni/Al2O3and Ni/CeO2catalysts show the isotherms of type IV(Fig.1(a))and hysteresis loops of type H1,indicating the characteristic of particulate adsorbents with mesopores.However,the hysteresis loop in Ni/Al2O3catalyst is not quite obvious due to its less porous structure.The BJH plots(Fig.1(b))reveal that compared with the other two Ni-based catalysts,the Ni/Al2O3is a porosity catalyst with the most probable pore size of 3.8 nm.

Fig.2 depicts the XRDpatterns of reduced Ni-based catalysts.For the Ni/Al2O3catalyst,the broad peaks at2θof19.6°,31.9°,37.6°,45.8°,60.5°,66.8°,84.5°assignable to Al2O3(JCPDS 29-0063)were found.The diffraction peak of Ni at 2θ=44.5°(JCPDS 04-0850)was overlapped by the peak at2θ=45.8°assigned to Al2O3.Itis indicated that Niwashighly dispersed on Al2O3and was too small to be detected[19],which is in accordance with the result of TEM.For the catalyst of Ni/CeO2,two weak diffraction peaks at 2θ of 44.5°and 51.8°which were characteristics of metallic Ni(JCPDS 04-0850)were detected,suggesting the increased crystalline size of Ni.The Ni crystallite size is 3.4,6.3 and 12.1 nm for Ni/Al2O3,Ni/ZrO2and Ni/CeO2catalysts respectively.The results revealed that the Ni species could be highly distributed on the supports of Al2O3and ZrO2though the Ni/ZrO2catalyst with low BET surface area and pore volume.

The characterization technology of TEM was applied to obtain the morphology of Ni catalysts.As shown in Fig.3,the particles of active components are uniformly dispersed on the support with sphericalshape.Al2O3and ZrO2supported Ni catalysts have smaller particle size(corresponding to 3.4 nm and 6.8 nm,respectively)and better dispersion than Ni/CeO2catalyst,with an average particle size of 12.4 nm.The presence of metallic state Ni was verified by their interplanar crystal spacing on the HRTEM images of the reduced catalysts.In addition,more well-de fined lattice fringe was found for Ni/CeO2catalyst and the lattice fringe for part of Ni particle was ambiguous,indicating the presence of amorphous Ni in the catalyst of Ni/Al2O3and the increasing of crystallization degree for Ni/CeO2.

Table 1Textural properties of the Ni-based catalysts after reduction

The particle size of active species and their interactions with supports are the two key factors for the reduction of metal oxide.The TPR patternsofNi/Al2O3,Ni/ZrO2and Ni/CeO2are shown in Fig.4.Awide reduction peak at the temperature of 300–800 °C was found for Ni/Al2O3catalyst.The reduction peak from 300 to 500°C corresponds to the week interaction between NiO species and Al2O3on the surface[20]while the reduction peak at 500–800 °C is related to the strong interaction[21].It is observed that Ni/CeO2catalyst exhibited two reduction peaks,the one at the temperature around 325°C represent the reduction of the NiO species[22],and the other one at 725°C was for the reduction of CeO2species[23].For Ni/ZrO2catalyst,three peaks at 400 °C,480 °C and 520 °C respectively were found and parts of them were overlapped,indicating that there were several kinds of NiO species on ZrO2support[22].Alternatively,it may be caused by the heterogeneous distribution of Ni species.The reduction temperature of the three catalysts is in the order as follows:Ni/Al2O3＞Ni/ZrO2＞Ni/CeO2,which is on the contrary of the Ni crystalline size calculated by XRD.It can be speculated that strong interaction exists between Ni and Al2O3,ZrO2supports,leading to high reduction temperature of NiO species,which makes metals difficult to sinter and reunite during the high temperature reduction as well as the reaction process.

The amount of active sites in the catalyst is an important parameter for catalyst activity.So the H2-TPD experiments were performed over the three catalysts in order to determine the active nickel surface area of catalysts and the results were shown in Fig.5.All the three catalysts presented the desorbed peak at low temperature(＜300°C)corresponding to weak active site,and the temperature of the desorbed peak for Ni/CeO2and Ni/ZrO2is relatively low(100–150 °C)while Ni/Al2O3has a higher one(about 250°C).In addition,Ni/Al2O3and Ni/ZrO2also showed desorbed peak at high temperature(300–600 °C)relevant to strong active site.While the desorbed peak appearing above 600°C attributed to hydrogen migration to subsurface layer or spillover,which was excluded in the calculation of nickel surface area.The nickel surface area was then calculated by the areas of H2-TPD pro files from 50 °C to 600 °C[24].It was observed that Ni/Al2O3catalyst retained larger amount of hydrogen desorption and higher nickel surface area than the other two catalysts.The larger desorbed peak area representing more active site amounts on the surface derived from the strong interaction between nickel and Al2O3,which contributed to the high dispersion of the metallic Ni.

Fig.1.N2 adsorption–desorption isotherms(a)and corresponding BJH pore size distributions(b)for the Ni-based catalysts after reduction.

Fig.2.XRD patterns of the Ni-based catalysts after reduction.

3.2.Catalytic activities

Fig.3.TEM images of the Ni-based catalysts after reduction:(A1)and(A2)for Ni/Al2O3,(B1)and(B2)for Ni/ZrO2,(C1)and(C2)for Ni/CeO2.

Fig.4.H2-TPR pro files of the Ni-based catalysts.

Fig.5.H2-TPD pro files of the Ni-based catalysts.

Syngas methanation was carried out on Ni/Al2O3,Ni/ZrO2and Ni/CeO2catalysts respectively,with the reaction conditions of 1 MPa,n(H2)/n(CO)=3 and a GHSV of 20000 ml·h−1·g−1.The main purpose of the research is to study the influence of temperature on the catalytic activity.As shown in Fig.6,with the increase of temperature,three catalysts presented the similar change trend of CO conversion.They all showed low catalytic activity at low temperature,but the CO conversion increased as temperature rose and reached about 100% finally,attributing to the hydrogen liquidity on the surface of catalysts improving with the temperature increasing,which promoted the conversion of CO[25].Besides,when three catalysts reached their respective transition temperature,the CO conversion increased significantly just because the Ni-based catalysts have a light-off temperature.When the reaction temperature achieved light-off temperature,the catalysts were activated and the CO conversion reached above 99%in a short time.The catalytic activities were analyzed as followed.It was found that Ni/Al2O3catalyst showed the best activity at low temperature and the CO conversion reached 99.5%at only 240°C.Meanwhile,the maximum CO conversion(99.9%)was obtained at 400°C.However,Ni/CeO2presented the worst activity,not showing its catalytic performance until 300°C.When the temperature reached 320°C,the CO conversion reached the peak value of 99.8%.The activity of Ni/ZrO2was just a little lower than that of Ni/Al2O3and the CO conversion of 99.7%was obtained at 260°C.The catalytic performance of the three catalysts ranked from high to low is Ni/Al2O3＞Ni/ZrO2＞Ni/CeO2,while the order is on the contrary to the size of Ni particles.Smaller Ni particle size led to higher nickel dispersion on the support and better catalytic activity,which was consistent with the results of XRD and TEM.All the catalysts were prepared by co-precipitation method,with the solution PH of 9–10.Al3+and Zr3+were mainly precipitated in a colloidal form.Both Ni/Al2O3and Ni/ZrO2exhibited stronger interaction between the support and the active component Ni,which resulted in smaller particle size and higher dispersion of Ni.Meanwhile,Ni/Al2O3and Ni/ZrO2have a closer distance between metal sites and acid sites.Then the transport properties of products increased between the two sites,which improves the activity of catalysts[26].Nematollahiet al.[16]found that CeO2could have an obvious interaction with Ni in methanation.However,H2-TPR analysis showed that the interactions of the other two supports(Al2O3,ZrO2)with Ni are stronger than that of CeO2.Besides that,H2-TPD results also indicated that more active sites are distributed in Ni/Al2O3and Ni/ZrO2.Therefore,Ni/Al2O3and Ni/ZrO2have better catalytic activities in methanation due to their abundant active sites and stronger interaction between the support and the active component.

The product selectivities of the three catalysts were shown in Fig.7.With the increase of temperature,the CH4selectivity of the three catalysts showed the same trend,increasing during the temperaturerise period and arriving above 90%ultimately.The products of methanation at low temperature were complex including methane,carbon dioxide,alkanes,and olefin.But when the temperature reached their respective transition temperature,the main product was only methane,the selectivities of CO2and other hydrocarbon became lower simultaneously.

Making a comparison among the three catalysts,it was found that Ni/Al2O3showed the best catalytic activity and the selectivity of CH4reached 81.8%at only 220°C.Meanwhile,the methane selectivity of the catalyst increased as temperature rose and the highest value reached to 94.5%at 380°C,but then decreased slightly as the temperature continued increasing.For the limitation of thermodynamic and meanwhile the CO methanation is a strong exothermic reaction,elevated temperature is adverse to CO conversion,which results in slight reducing of CH4selectivity at high temperature[27].However,Ni/CeO2presented the worst methane selectivity with the maximum value of only 92.3%at 360°C.The methane selectivity of Ni/ZrO2was between that of Ni/Al2O3and Ni/CeO2and the maximum selectivity reached 93.6%at 320°C.In summary,the methane selectivity of the three catalysts was in the following order:Ni/Al2O3＞Ni/ZrO2＞Ni/CeO2.

Fig.6.The influence of temperature on the CO conversion of the Ni-based catalysts.

Fig.7.The product selectivity in methanation on the Ni-based catalysts after reduction:(a)Ni/Al2O3,(b)Ni/ZrO2,(c)Ni/CeO2.

Fig.8.XRD patterns of the Ni-based catalysts after life test.

3.3.Catalyst stability

For practical industrial applications of methanation,long-term stability is an important factor for a promising catalyst besides activity and selectivity.The catalytic stability test of Al2O3catalyst with the best performance was carried out at 450°C,1 MPa,n(H2)/n(CO)=3 with a GHSV of 20000 ml·h−1·g−1.In the reaction of 30 h,the Ni/Al2O3catalyst presented stable reactivity,the CO conversion of the catalyst decreased slightly and maintained at about 100%.

The crystalline structures of the catalyst after reaction and the size of the Ni particles on the catalyst were measured using XRD,the results are shown in Fig.8.For Ni/Al2O3,the Ni exhibited three obvious characteristic diffraction peaks at 2θ =44.5°,51.8°and 76.4°(JCPDS 04-0850).Meanwhile,the Ni crystallite size increased from 3.4 nm before reaction to 7.3 nm(calculated by Scherrer equation)after test.It is indicated that slight sintering of Ni crystal occurred during the life test.

In order to further explore the dispersion of Ni particles on the support afterlife test,TEM characterization was adopted and the results were shown in Fig.9.Compared with the TEM images of Ni/Al2O3catalyst before reaction(Fig.3(A1)and(A2)),it could be found that the size of Ni particles was increased and the dispersion of Ni particles was reduced.The Ni particle size grew from 3.4 nm to 6.7 nm,which is consistent with the result of XRD.Presumably,the size of Ni particles increased leading to the decrease of its dispersion on the support during the life test.But it did not affect the activity of catalyst a lot during the test up to 30 h.

4.Conclusions

In this study,three high activity Ni-based catalysts(Ni/Al2O3,Ni/ZrO2and Ni/CeO2)with different supports were prepared by co-precipitation method.Their catalytic activity in methanation was analyzed by a series of characterization and experiments.

The low-temperature activity of the three catalysts were in the order of Ni/Al2O3＞Ni/ZrO2＞Ni/CeO2,and Ni/Al2O3showed the best performance in methanation.At the condition ofP=1 MPa,n(H2)/n(CO)=3 and GHSV=20000 ml·h−1·g−1,a high selectivity of 94.5%to CH4could be achieved over Ni/Al2O3.And Ni/Al2O3still maintained a high catalytic activity after the reaction of 30 h.

Results indicated that different supports have their own effects during the reaction.Al3+and Zr3+were mainly precipitated in a colloidal form.Both Ni/Al2O3and Ni/ZrO2exhibited stronger interaction between the support and the active component Ni.In addition,more active sites are distributed in Ni/Al2O3and Ni/ZrO2.Therefore,Ni/Al2O3and Ni/ZrO2have better catalytic activities in methanation due to their abundant active sites and stronger interaction between the support and the active component.

Fig.9.TEM images of the Ni-based catalysts after life test.

Acknowledgments

The authors are grateful for the financial support from the National Science and Technology Supporting Plan through contract(2015BAD15B06)and the National Natural Science Foundation of China(51661145011).

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