Mengjuan Zhang,Panpan Li,Mingyuan Zhu,Zhiqun Tian,Jianming Dan,3,4,Jiangbing Li,Bin Dai,Feng Yu,*

1Key Laboratory for Green Processing of Chemical Engineering of Xinjiang Bingtuan,School of Chemistry and Chemical Engineering,Shihezi University,Shihezi 832003,China

2Collaborative Innovation Center of Renewable Energy Materials,Guangxi University,Nanning 530004,China

3Engineering Research Center of Materials-Oriented Chemical Engineering of Xinjiang Production and Construction Corps,Shihezi 832003,China

4Key Laboratory of Materials-Oriented Chemical Engineering of Xinjiang Uygur Autonomous Region,Shihezi 832003,China

Keywords:Nickel-based catalysts Synthetic gas Synthetic natural gas Two-dimensional vermiculite Carbon monoxide methanation Plasma irradiation method

A B S T R A C T Nickel-based catalysts represent the most commonly used systems for CO methanation.We have successfully prepared a Ni catalyst system supported on two-dimensional plasma-treated vermiculite(2D-PVMT)with a very low Ni loading(0.5 wt%).The catalyst precursor was subjected to heat treatment via either conventional heat treatment(CHT)or the plasma irradiation method(PIM).The as-obtained CHT-Ni/PVMT and PIM-Ni/PVMT catalysts were characterized with scanning electron microscopy(SEM),energy dispersive X-ray(EDX),X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS),inductively coupled plasma-atomic emission spectroscopy(ICP-AES)and high-angle annular dark field scanning transmission electron microscopy(HAADF-STEM).Additionally,CHT-NiO/PVMT and PIM-NiO/PVMT catalysts were characterized with hydrogen temperature programmed reduction(H2-TPR).Compared with CHT-Ni/PVMT,PIM-Ni/PVMT exhibited superior catalytic performance.The plasma treated catalyst PIM-Ni/PVMT achieved a CO conversion of 93.5%and a turnover frequency(TOF)of 0.8537 s-1,at a temperature of 450°C,a gas hourly space velocity of 6000 ml·g-1·h-1,a synthesis gas flow rate of 65 ml·min-1,and a pressure of 1.5 MPa.Plasma irradiation may provide a successful strategy for the preparation of catalysts with very low metal loadings which exhibit excellent properties.

1.Introduction

Natural gas,consisting mostly of methane,is one of the most important fossil fuels in the low-carbon life.Compared with coal,natural gas represents a cleaner fossil fuel,is 73%more energy efficient and can reduce CO2emissions by 84%[1–3].Synthetic natural gas(SNG)has been synthesized via the methanation reaction of CO from coal-derived synthetic gas(SG),which represents the most efficient way to convert SG into a useful energy source,and attempts to further optimize the reaction process have attracted recent attention[4].Generally,methanation of SG represents an efficient way of using abundant coal resources,especially those of lower quality.Moreover,CO methanation has the potential to satisfy the increasing demand of SNG in world markets[5,6],and represents a cleaner energy source than the coal raw material from which SG is derived[7].

Ni-has been widely used in CO methanation catalysts due to the low cost and wide availability of this metal[8].SiO2has been a commonly used support in such catalytic systems,and corresponding Ni/SiO2catalysts have been reported with Ni metal loadings ranging from 10 wt%to 55 wt%,such as 55 wt%Ni@SiO2[9],30 wt%Ni/SiO2[10],20 wt%Ni/SiO2[11],13 wt%Ni/SiO2[12],and 10 wt%Ni/SiO2[13].However,in such catalytic systems the amount of catalytically active Ni species is limited,due to the presence of blocked and ineffectual active sites.Recently,catalysts with much lower metal loadings(particularly single-atom catalysts),have attracted considerable attention and have opened a new frontier in heterogeneous catalysis.As the size of the metal nanostructures decreases from the nanometer to the subnanometer scale and ultimately to single atoms,the catalytic performance may change significantly.Thus far,single-atom heterogeneous catalysts have been developed with noble metals such as Pt,Pd,Au,Ir,Ag,and Rh,and have been reported to have high activity and selectivity.For non-noble metal catalysts,the metal nanoparticles have a tendency to undergo agglomeration,and the nanoparticle dispersion is usually low[14].Interestingly,a catalyst consisting of single-atom nickel dopants supported on nanoporous graphene has been used as a stable catalyst for hydrogen production,which shows the possibility of using a non-noble metal catalyst such as Ni as single-atom catalysts[15].

Herein,plasma-treated vermiculite(PVMT)was used as the support for the Ni/PVMT catalyst precursor,which underwent successful heat treatment via the plasma irradiation method(PIM)to form a catalyst PIM-Ni/PVMT with an ultralow Ni loading(i.e.,0.5 wt%).VMT,as a two-dimensional(2D)layer-structured clay mineral,mainly consists of SiO2,MgO and Al2O3[19–21].Active components can be evenly dispersed on 2D VMT,such as HgCl2/VMT for acetylene hydrochlorination[16]and 10 wt%Ni/VMT for CO methanation[17].Excellent catalytic performance can be obtained due to the highly exposed active centers present on 2D nanolayers[18].Compared with conventional heat treatment(CHT)of the catalyst precursor via calcination,plasma irradiation can activate molecules,reduce metal nanoparticle size and improve the dispersion of catalysts[19,20].The asobtained PIM-Ni/PVMT catalyst exhibits good catalytic performance superior to CHT-Ni/PVMT.

2.Experimental

2.1.Catalyst preparation

Raw vermiculite(VMT,30g)and H2O2(25wt%,300ml)were placed in a 500 ml beaker,which was immersed in a temperature controlled water bath set at 80°C for 4 h.Then,the mixture in the beaker was allowed to cool and was dried in an oven at 100°C.The dried contents were then crushed and passed over a 100 mesh screen.To obtain the plasma-treated VMT(PVMT),the vermiculite was subjected to plasma treatment for 30 min with air by applying an electric current of 2 A and voltage of 60 V to the discharging electrode.The catalyst precursor was prepared by incipient wetness impregnation:the as-obtained PVMT was impregnated with an aqueous solution of Ni(NO3)2to achieve a 0.5 wt%Ni loading,and then vacuum freeze-dried with a vacuum freeze-drying machine LGJ-10D.The as-prepared precursor was divided into two parts for the preparation of CHT-and PIM-treated catalysts.One part was calcined at 550°C for 4 h to obtain CHT-NiO/PVMT;another part was subjected to plasma treatment for 40 min by applying an electric current of 2 A and voltage of 80 V to the discharging electrode to create non-thermal plasma,and was named PIM-NiO/PVMT.The CHT-NiO/PVMT and PIM-NiO/PVMT were respectively reduced with flowing H260 ml·min-1for 2 h in a stainless steel tubular microreactor to obtain CHT-Ni/PVMT and PIM-Ni/PVMT,respectively.

2.2.Characterization of the catalysts

The chemical composition of CHT-Ni/PVMT and PIM-Ni/PVMT was identified by inductively coupled plasma-atomic emission spectroscopy(ICP-AES)on an Iris Advantage equipment from Thermo Jarrell Ash Corporation.The samples were dissolved with 40%HF and then diluted with water.The crystallographic properties of the materials were determined through XRD analysis.The X-ray diffraction(XRD)patterns were obtained with a BrukerD8 Advance X-ray diffractometer with Cu Kαradiation in the 2 θ range of 0–90°.The morphology of the catalyst was characterized by scanning electron microscopy(SEM,Hitachi S-4300 microscope).High-angle annular dark field scanning transmission electron microscopy(HAADF-STEM)images were performed on a Titan ETEM G2 80-300 with an X-FEG 300 kV electron gun.X-ray photoelectron spectroscopy(XPS)experiments were carried out with an AMICUS/ESCA 3400 electron spectrometer from Kratos Analytical using Mg Kα(20 mA,12 kV)radiation.Catalyst reducibility was ascertained by performing hydrogen temperature programmed reduction(H2-TPR)with Micromeritics TPx System equipment.The catalyst sample(100 mg)was reduced under a Ar/H2atmosphere(40 ml·min-1),which was heated up to 900 °C at a rate of 10 °C min-1.

2.3.Catalytic performance tests

Experiments to determine the catalytic performance of the catalysts were conducted in a fixed bed microreactor.A 0.65 g sample of the catalyst was placed in a stainless steel tubular microreactor.The CHT-or PIM-prepared NiO/PVMT catalyst was purged with N2at 60 ml·min-1as the sample was heated up to 500°C.When this temperature was reached,the sample was reduced with flowing H260 ml·min-1for 2 h before the catalytic reaction was conducted.The temperature was then lowered to 250°C,and syngas was introduced into the microreactor with a H2:CO ratio of 3:1,a weight hourly space velocity(WHSV)of 6000 ml·g-1·h-1,and pressure of 1.5 MPa.The outlet gases were separated and analyzed online by gas chromatography(GC-2014C,SHIMADZU).

3.Results and Discussion

The actually Ni containing of catalysts CHT-Ni/PVMT and PIM-Ni/PVMT characterized by ICP-AES.The experimental Ni loading was 0.5 wt%,and the Ni loadings of catalysts CHT-Ni/PVMT and PIM-Ni/PVMT were 0.45 wt%and 0.49 wt%,respectively.So in fact,the experimental Ni loadings on the supports agree well with the expected values[21].The XRD spectra of the CHT-Ni/PVMT and PIM-Ni/PVMT catalysts do not exhibit any clear diffraction peaks corresponding to the Ni phase(three of the main peaks would be observed at 37.2°,43.2°,and 62.8°),as shown in Fig.1.Hence,under conditions of very low Ni loadings(0.5 wt%in our case),and reduced nanoparticle sizes,XRD spectra of such catalysts fail to exhibit the main peaks of the Ni phase[22].SEM and the elemental mapping images of the Ni/PVMT precursor,and the heat treated samples(CHT-Ni/PVMT and PIM-Ni/PVMT)are shown in Fig.2.The images illustrate that Ni particles of PIM-Ni/PVMT are dispersed evenly on the PVMT because of plasma irradiation[23].These uniformly dispersed Ni particles could be responsible for excellent catalytic performance.The corresponding HAADF-STEM images of CHT-Ni/PVMT and PIM-Ni/PVMT are shown in Fig.3.Compared with agglomerated Ni nanoparticles of CHT-Ni/PVMT,Ni nanoparticles of PIM-Ni/PVMT were dispersed more uniformly consistent with Fig.2.

Fig.1.XRD images of PVMT,PIM-Ni/PVMT,and CHT-Ni/PVMT.

Fig.2.SEM images and elemental mapping images of Ni/PVMT precursor(a and b),CHT-Ni/PVMT(c and d),and PIM-Ni/PVMT(e and f).

From the XPS spectra of the CHT-Ni/PVMT and PIM-Ni/PVMT samples(Fig.4),it is clear that the Ni 2p spectra can be divided into separate 2p3/2 and 2p1/2 peaks[24].In the Ni 2p XPS spectrum of the CHT-Ni/PVMT catalyst,the observed peaks with a binding energy of 852.9 eV and 856.3 eV were assigned to Ni0and Ni 2p3/2,respectively.The distinct peak at 862.2 eV could be ascribed to satellite.Similarly,in the same spectrum the two peaks with binding energies of 873.8 eV and 879.2 eV can be assigned to Ni 2p1/2 and its satellite peak,respectively.With regard to the PIM-Ni/PVMT catalyst,the peak at 853.1 eV can be assigned to Ni0species.The peaks at 856.3 eV and 862.3 eV correspond to Ni 2p3/2 and a satellite peak,respectively,while the two peaks with binding energies of 873.9 eV and 879.8 eV are ascribed to Ni 2p1/2 and the satellite peak,respectively[25].Hence,as expected both CHT-and PIM-treated catalysts contain Ni0and Ni(II),demonstrating that both treatment processes successfully decomposed nickel nitrate to NiO.Interestingly,the PIM-treated sample shows a slightly higher Ni 2p1/2 binding energy,indicating that Ni and NiO species more strongly interact with PVMT support[10].

The H2-TPR profiles of the CHT-NiO/PVMT and PIM-NiO/PVMT catalysts(Fig.5)show two peaks at 273.5 °C and 416.8 °C,respectively,which may correspond to the reduction of Ni(II)in the outermost region of the catalyst's surface,while the large peaks at 634.3 °C and 603.2 °C respectively could be assigned to the reduction of Ni(II)nanoparticles located deeper from the catalyst surface.The lower temperature for the reduction of NiO nanoparticles on the subsurface of the PIM-NiO/PVMT catalyst(603.2°C)could be due to the smaller,more uniformly dispersed nature of the NiO nanoparticles[13,26].However,the outermost NiO nanoparticles are reduced at a higher temperature in the PIM-treated catalyst(416.8°C)because of a stronger interaction between NiO and PVMT as a result of plasma treatment,which can provide rapid nickel nitrate decomposition and avoid NiO nanoparticle grain growth and aggregation[10,27].

A schematic illustration is proposed in Fig.6 for the formation mechanism of CHT-Ni/PVMT and PIM-Ni/PVMT catalysts.First,the PVMT support was impregnated in Ni(NO3)2solution to form the Ni/PVMT precursor,which was then subjected to freeze drying.Secondly,the Ni/PVMT precursor was subjected to either of two different heat treatments.The precursor was either calcined in a muffle furnace to obtain CHT-NiO/PVMT,or heated through plasma(avoiding the high temperature required for calcination),affording PIM-NiO/PVMT.Plasma irradiation provides a unique environment which is favorable for the formation of NiO nanoparticles and avoids particle aggregation.After reduction at Ar/H2atmosphere,the Ni particles of PIM-Ni/PVMT distributed evenly on the support PVMT.

Fig.3.HAADF-STEM image of(a,b)CHT-Ni/PVMT and(c,d)PIM-Ni/PVMT.

The efficiency of the CHT-and PIM-prepared catalysts was investigated by measuring three dependent variables:CO conversion,CH4selectivity,and TOF(turn-over frequency)(Fig.7a–c,respectively).In the temperature range of 250 °C–550 °C,a maximum CO conversion of 93.5%at 450°C was observed for PIM-Ni/PVMT,while the maximum conversion for the CHT-prepared catalyst in this temperature range was 64.7%at 550°C,representing an increase of nearly 30%for the PIM-prepared catalyst under milder conditions.The higher conversion for the PIM-prepared catalyst may be due to the plasma treatment of the nickel catalyst precursor which causes rapid decomposition of nickel nitrate,and results in the Ni nanoparticles being dispersed more evenly,making the Ni active sites more easily accessible and therefore increasing the catalytic activity[28,29].At temperatures greater than 250°C,the TOF of PIM-Ni/PVMT was considerably greater than that of the CHT-prepared catalyst[30].In addition,the highest TOF value of the CHT-Ni/PVMT catalyst(0.3882 s-1),is much lower than that of the PIM-prepared catalyst(0.8537s-1).Overall,interms of CO conversion,selectivity at temperatures which yield significant conversions,and TOF,the catalytic performance of the PIM-Ni/PVMT catalyst is clearly superior to that of CHT-Ni/PVMT.The results for the methanation of CO reported in the literature with different catalyst systems are shown in Table 1,along with those using the optimized conditions for the CHT-and PIM-prepared catalysts from the present study[31].

Fig.4.XPS spectra of Ni 2p3/2 and Ni 2p1/2 for(a)CHT-Ni/PVMT and(b)PIM-Ni/PVMT.

Fig.5.H2-TPR patterns of CHT-NiO/PVMT and PIM-NiO/PVMT.

4.Conclusions

The catalyst precursor(Ni/PVMT)was formed via incipient wetness impregnation with a very low Ni loading(0.5 wt.%),and subsequently heat treated with either conventional air calcination(CHT-)or plasma treatment(PIM-).The plasma treated catalyst(PIM-Ni/PVMT)exhibits excellent and superior performance compared to the CHT-prepared catalyst for the methanation of CO.The CO conversion,CH4selectivity and TOF values with the plasma treated catalyst were significantly higher than in the calcined catalyst,with the CO conversion and TOF reaching 93%and 0.8537 s-1,respectively,at 450°C and a gas hourly space velocity of 6000 ml·g-1·h-1.We are confident that plasma treatment can lead in the future to the development of novel catalyst systems with very low metal loadings that exhibit excellent catalytic performance.

Fig.6.Schematic illustration of processing steps to prepare CHT-NiO/PVMT and PIM-NiO/PVMT.

Fig.7.(a)CO conversion,(b)CH4selectivity,and(c)TOF(turn-over frequency)values of the as-obtained CHT-Ni/PVMT and PIM-Ni/PVMT.

Table 1Comparison of the CO conversion,CH4selectivity,and TOF values for CO methanation with different catalysts under various conditions