Ni-Al mixed metal oxide with rich oxygen vacancies:CO methanation performance an

Zhouxin Chang ,Feng Yu,2,*,Zhisong Liu ,Zijun Wang ,Jiangbing Li ,Bin Dai ,Jinli Zhang

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

2 Bingtuan Industrial Technology Research Institute,Shihezi University,Shihezi 832003,China

Keywords: Natural gas Carbon monoxide methanation High shear mixer Oxygen vacancy Homogeneous catalysis

ABSTRACT Ni-Al mixed metal oxides have been successfully prepared by high shear mixer (HSM) and coprecipitation(CP)methods for low temperature CO methanation.In this work,Ni-Al(HSM-CP)catalyst presented small Ni crystallite size and high surface area,which all contribute to the methanation reaction at low temperature conditions.The obtained Ni-Al (HSM-CP) sample exhibited a mass of defective oxygen,thereby accelerating the dissociation of CO and ultimately increasing the activity of the catalyst.Ni-Al(HSM-CP) catalyst offered the best activity with CO conversion=100% and CH4 selectivity=93% at 300°C,and the CH4 selectivity can reach 81.8% at 200°C.In situ Fourier transform infrared spectroscopy and density functional theory show that CHO and COH intermediates with lower activation energy barriers are produced during the reaction,and hydrogen-assisted carbon–oxygen bond scission is more favorable.

1.Introduction

Removal of toxic and combustible CO from coke oven gas(COG)is essential to save energy and protect the environmental.The removal of CO through CO methanation is of great significance for H2purification in water–gas shift reaction (WGSR) [1–4].At the same time,the CO methanation reaction is the core technology of the coal-to-natural gas project.Coal-to-natural gas technology can play a vital role in the alleviation of energy crisis,solving the problem of natural gas demand and reduction of environmental pollution[5–7].CH4is a kind of green gas with high calorific value,clean and efficient,and it is also a raw material for the production of chemical products [8–11].Methanation technology is regarded as a strategic plan to meet energy demand,improve energy structure,and protect the environment[12].Therefore,CO methanation has a bright future [13,14].

The catalyst will be deactivated due to carbon deposition and sintering at high temperature[15–18].Considering the thermodynamic and kinetic balance,low temperature is beneficial to the methanation reaction.Thus,it is necessary to develop low temperature methanation catalyst.Generally,noble metal catalysts have good low temperature catalytic performance [19–21].However,noble metals are relatively expensive and resources are scarce so that they cannot be industrially produced.Consequently,nonnoble metal nickel-based catalysts have received extensive research due to their low price and high activity.At present,the widely used nickel-based catalysts include Ni/CeO2[22,23],Ni/Al2O3[24],Ni/MgAl [3],Ni/MgO [5],Ni/SiO2[25],etc.

Different preparation methods of catalyst will have different effects on the catalytic reaction [26].

Traditional coprecipitation and impregnation method can cause active metals to block the pores of the carrier,resulting in a reduction in specific surface area and active sites,thereby reducing the catalyst active.In this study,high-shear-mixer (HSM) assisted coprecipitation(CP)method is used to replace the traditional impregnation method.HSM has the advantages of low energy consumption,high shear force and short preparation time.In addition,HSM is widely used in controlling crystal structure,fine dispersion and material mixing [27–30].

In this paper,Ni-Al mixed metal oxide was prepared by HSM and CP method for CO methanation.The methanation activity of the two catalysts was tested within a certain temperature range.The Ni-Al (HSM-CP) catalyst showed best activity at 200 °C.Besides,the formation of methanation intermediates and reaction mechanism were studied byin situFourier transform infrared spectroscopy (FTIR) and density functional theory (DFT),which provided help for the design of high activity methanation catalyst.This work will contribute to the study of low temperature CO methanation,which provides an effective method for energy development and environmental protection.

2.Experimental

2.1.Samples synthesis

0.043 mol Ni(NO3)2∙6H2O (Fuchen Chemical,AR,99%) and 0.075 mol Al(NO3)3∙9H2O (Macklin,AR,98%) were dissolved in 250 ml deionized water.Subsequently,the pH value of solution was adjusted to 9.36 by ammonia solution,and stirred with HSM(FLUKO,Shanghai,Model FA25)at 1.0×104r∙min-1for 1 h,which was washed with a mixed solution of distilled water and ethanol until pH=7.Next,the samples were dried for 12 h in a blast drying oven at 80°C.The sample was ground into powder and calcined in a muffle furnace with a heating rate of 2°C∙min-1at 400°C for 2 h.The catalysts were marked as Ni-Al(HSM-CP).Under the condition of keeping the rotation speed and the stirring time consistent,HSM was changed to mechanical stirring.The catalyst prepared by mechanical stirring was named as Ni-Al (CP).And other preparation processes remained unchanged.

2.2.Catalyst characterization

X-ray diffraction (XRD) was committed to measure crystalline structure of the catalyst.The instrument is Bruker D8 with source of CuKα,scanning range from 10° to 90°.The nitrogen adsorption and desorption of the sample was performed on the ASAP 2460,USA.The Brunauer-Emmett-Teller (BET) was used to measure the surface area of the catalyst.The pore structure of the sample was characterized by the Barrett-Joyner-Halenda (BJH) method.The chemical composition of catalyst surface was done by X-ray photoelectron spectroscopy (XPS,Thermo Escalab 250XI) with AlKα sources (1486.6 eV).The electron paramagnetic resonance (EPR)was measured with A300-10/12 instrument (Bruker,Germany) at room temperature.High-resolution transmission electron microscope (TEM) image was obtained on F200X.The scanning electron microscopy (SEM) was taken on Hitachi S-4800 instrument with current of 10 uA and voltage of 15 kV.The element mapping image was observed by JEM-2100F(Japan)with operating at 200 kV.The CO temperature-programmed desorption (CO-TPD) was test by Auto Chem II 2920 (US).Firstly,50 mg samples were reduced by H2(30 ml∙min-1) at 500 °C for 2 h.Then,sample was cooled to 50 °C under argon protection.It was adsorbed in 10% CO/He mixture flow (30 ml∙min-1) at 50 °C for 1 h.To remove the adsorbed CO,the catalyst was purged for 1 h in an argon flow with a flow rate of 30 ml∙min-1at 50 °C.Finally,in the temperature range of 50 °C to 900 °C,the desorption experiment was carried out in an argon flow with a flow rate of 30 ml∙min-1.H2temperature programmed reduction (H2-TPR) was detected through Auto Chem II 2920 (US).At 300 °C,use argon with a flow rate of 30 ml∙min-1to purge 50 mg sample for 0.5 h,and then cool to 50 °C.Then the catalyst was heated to 900 °C at a heating rate of 10 °C∙min-1in a 10% (volume) H2/Ar mixed gas flow with a flow rate of 30 ml∙min-1.In-situFTIR was tested by Bruker 80 v,the acquisition resolution was 4 cm-1,and the number of scans was 32.Thein-situFTIR test conditions are the same as the methanation performance test process.

2.3.Performance measurement

The methanation activity test was carried out using 150 mg of catalyst on an atmospheric pressure fixed bed microreactor.The reactor consists of a tube furnace and a stainless-steel reaction tube with a length of 75 cm and an inner diameter of 10 mm.Insert a thermocouple on the catalyst bed to detect the reaction temperature.Initially,the sample was heated to 500°C at a heating rate of 5°C∙min-1in a nitrogen flow with a flow rate of 65 ml∙min-1.The catalyst was reduced in hydrogen at a flow rate of 65 ml∙min-1for 2 hours at 500°C.After reduction,the temperature was lowered to 150 °C in a nitrogen atmosphere.Secondly,the methanation performance test was carried out in the temperature range of 150–500 °C,at 0.1 MPa.The ratio of H2to CO in the synthesis gas was 3:1,and the weight hourly space velocity (WHSV) is 26,000 ml∙g-1∙h-1.The carrier gas is helium with a flow rate of 30 ml∙min-1.The produced water was discharged from the bottom of the reactor.Use the GC-2014C gas chromatograph to detect the outlet gas online.Gas chromatograph has TDX-01 column with a column temperature of 80 °C and thermal conductivity detector is DTCD1.

The value of CO conversion and the value of CH4selectivity are calculated by the following formula:

where,nCO,inis the molar fraction of CO in the inlet;nCO,outandare the molar fraction of CO and CH4in the outlet.

2.4.Computational method

The energy calculation,structural optimization,and transition state search were executed in the framework of generalized gradient approximation (GGA) [31,32] represented by Perdew-Burke-Ernzerhof (PBE) methods [33],as implemented in QuantumATK code.PseudoDojo pseudopotentials and high-size of linear combination of atomic orbitals (LCAO) basis set was used throughout the calculations.Use the projector augmented-wave(PAW) method to describe the core electrons [34].In terms of methanation,relevant studies have shown that the crystal surface of Ni (111) is good for methanation reaction [32,35].The active substance was simulated by a three-layer (3×3) Ni(111) supercell.The top two layers of the models and the surface adsorbates were completely relaxed.In the geometric optimization converges,the maximal residual force below the values of 0.2 eV∙nm-1.A vacuum spacing of at least 1.5 nm along the normal direction (z) was set to avoid spurious interactions.The Brillouin zone was sampled by a 2×2×1 array of k-points in the Monkhorst–Pack grid [36].By removing an oxygen atom from the surface of the catalyst,a surface with oxygen vacancies was constructed.Use the method of climbing image nudged elastic band (CI-NEB) to search transition state (TSs) of the basic reaction [37–39].Use six images between the initial state (IS)and the final state (FS) to discrete the minimum energy pathway of each basic response.The maximum amplitude image along the minimum energy path identifies TS.The reaction energy(ΔE) and activation energy (Ea) were calculated from the energy difference of Born-Oppenheimer between the IS,TS and FS:

Among them,EISon behalf of the initial state of energy,ETSrepresents the transition state of energy,andEFSrepresents the final state of energy,calculation formula of the adsorption energy(Eads):

In the formula,Etotalis the total energy of the substrate and the adsorbate,Esubstrateis the energy of the substrate,andEadsorbateis the energy of the adsorbate,respectively.

3.Results and Discussion

3.1.Characterization

Fig.1 revealed SEM images and N2adsorption–desorption results.In Fig.1(a) and (b),the nanoparticles on the Ni-Al (HSMCP) catalyst surface are loosely arranged.In Table 1,the pore size and pore volume of the Ni-Al(CP)catalyst are significantly smaller than that of the Ni-Al(HSM-CP).The morphology analysis result is consistent with the structural properties,indicating that HSM can change the pore structure of the material.There is IV type H2 shaped hysteresis loops,indicating that the catalyst presented mesoporous structure [3].Besides,BET surface area of Ni-Al(HSM-CP)catalyst (271.0 m2∙g-1) is larger than Ni-Al (CP) catalyst(191.0 m2∙g-1).These advantages promise their potential applications as catalyst.

The XRD was employed to characterize the crystal structure of catalyst.In Fig.2(a),all the catalysts display obvious NiO (JCPDS no.47-1049) diffraction peaks at 37.2° (111),43.3° (200),62.9°(220) and 75.4° (311).The diffraction peaks at 37.6°,45.9°,and 67.0° corresponds to Al2O3(JCPDS no.10-0425).However,the diffraction peak of NiO in Ni-Al (HSM-CP) is not obvious,which indicates that the dispersion of NiO is high [40–42].It can be seen from Fig.2(b) that both samples have Ni (JCPDS no.04-0850)diffraction peaks.We calculated the crystallite size of the Ni(111) crystal plane (2θ=44.5°) through the Debye-Scherrer formula.In Table 1,the crystallite size of Ni-Al (HSM-CP) catalyst was 19.18 nm,while Ni-Al(HSM-CP)catalyst was 24.19 nm.Therefore,HSM can reduce the crystallite size of the active material Ni.

Fig.1.SEM images (a) and (b),BET surface area (c) and pore size (d).

Table 1 N2 adsorption desorption analysis and crystallite size of catalysts

Fig.2.The XRD patterns of the catalyst (a) and the XRD patterns after H2 reduction (b).

Fig.3.HRTEM images (a-d) and element mapping images of the Ni-Al (HSM-CP) catalysts (e).

Fig.3 described HRTEM and element mapping images.The HRTEM results clearly show the crystal structure of the two samples.In Fig.3(a),there is obvious agglomeration for Ni-Al(CP)catalyst.The dispersion of metal particles in the Ni-Al(HSM-CP)is relatively higher than that of Ni-Al(CP)catalyst.The NiO(200)crystal is exposed on the catalyst surface.Fig.3(e) is the mapping images of the Ni-Al (HSM-CP),which indicates that Al,Ni and O elements are evenly distributed.This reveals that Ni-Al (HSM-CP) sample has good dispersion.

HRTEM images of catalyst reduced by hydrogen were presented in Fig.4.The Ni-Al (CP) catalyst exhibits obvious agglomeration.However,the Ni nanoparticles of Ni-Al(HSM-CP)catalyst have better dispersibility.In addition,both catalysts expose the Ni (111)crystal plane and the lattice spacing is 0.203 nm.Therefore,the HSM process can effectively suppress the agglomeration of Ni nanoparticles.

As shown in Fig.5(a),the two peaks with binding energy values of 856.0 eV and 873.4 eV are Ni 2p3/2and Ni 2p1/2,their satellite peak located at 862.1 eV and 880.6 eV [43].Spectral peaks of Ni3+(Ni2O3)at around 857.4 eV and 875.4 eV[44].The Ni2+spectral peaks on the catalyst surface located near 855.5 eV and 873.4 eV.In Fig.5(b),there is oxygen (Olatt),surface oxygen (Osurf) and defect oxygen (Odef).The Odefis located at 531.6 eV,which has low oxygen coordination [45].The Osurfrepresent O—H that comes from surface adsorbed water [46].In Table 2,Ni-Al (HSM-CP) catalyst contains more Olatt,which helps to enhance the stability [47].

Fig.4.HRTEM images of the catalyst after hydrogen reduction.

In addition,the Odefof Ni-Al (HSM-CP) catalyst is 55.2% in Table 2,which is higher than that of Ni-Al (CP).This suggests that HSM can improve more oxygen vacancies[48].The oxygen vacancy was studied through the EPR,as shown in Fig.5(d).The value of g equal to 2.002 corresponds to unpaired electrons captured by oxygen vacancies.The spectra of the Ni-Al(HSM-CP)catalyst are both broad and strong,indicating that it contains more oxygen vacancies.Oxygen vacancies can interact strongly with the oxygen in CO,thereby accelerating the dissociation of CO and ultimately increasing the activity of the catalyst [49–52].In addition,oxygen vacancies can also effectively prevent the generation of CO2,thereby improving CH4selectivity [53,54].

Table 2 Surface quantitative analysis by XPS

The effect of oxygen vacancies (OV) on catalytic activity was studied by optimizing the geometric structure of oxygen vacancies,generating energy and density of states (DOS) in Fig.5(e) and (f).According to XPS and EPR data,HSM increases the oxygen vacancy content of Ni-Al (HSM-CP).In Fig.5(e),formation energy of OVis 4.13 eV.And in Fig.5(f),before the formation of OV,Nidorbital electrons are mainly concentrated on the left side of the Fermi level.After the OVis formed,the DOS blue shifts and approaches the Fermi level,indicating that the electrons have undergone a transition.The electronic transition is beneficial to charge transfer,marking that Ni-Al (HSM-CP) catalyst is relatively more active in the methanation reaction [55–57].

The reducibility of catalysts was investigated by H2-TPR analyse.As observed in Fig.6,according to literature research,the reduction peaks can be divided into three classes (β1,β2,and γ)[58].Generally,α-NiO comes from free NiO on the catalyst surface and is generally reduced in 200–360 °C,which has weak interaction with carrier.β-NiO has a strong interaction with the carrier,and is generally reduced in the middle temperature region (450–700 °C) [59].Furthermore,β-NiO can be divided into β1 and β2.β1-NiO is easier to reduced and β2-NiO is difficult to be reduced[59,60].γ-NiO is assigned to the nickel aluminum spinel,and its characteristic is that it may be reduced in a high temperature region above 700 °C [61].It can be seen from Fig.6 that reduction temperature corresponding to Ni-Al(HSM-CP)catalyst moves negatively compared to Ni-Al(CP)catalyst.Therefore,under the same conditions,Ni-Al (HSM-CP) catalyst is more easily reduced by H2.In summary,HSM helps to improve the reducibility of the catalyst.These are also essential for good low temperature CO methanation activity.

CO-TPD was used to explore the effect of HSM on CO adsorption.In Fig.7,CO desorption peak in the CO-TPD profiles at around 150 °C.Because of single-site chemisorption of CO,the peak at 110 °C is correlated with the emission of CO [52,62].The peaks in 140–400°C corresponds to the emission of CO2.CO2may be produced by the reaction of dissociated CO with oxygen[63].The high temperature peak of the Ni-Al(CP)catalyst(446°C)represents the dissociation of CO2.This CO2may come from the disproportionation reaction of CO[64,65].The funny thing is that there is no high temperature peak in Ni-Al (HSM-CP) catalyst.Therefore,the adsorbed CO by the Ni-Al (HSM-CP) catalyst may not participate in the side reaction.Furthermore,the CO molar desorption of Ni-Al (HSM-CP) catalyst is 156.6 μmol∙g-1and Ni-Al (CP) catalyst is 147.3 μmol∙g-1.This is because the surface of the Ni-Al (HSMCP) catalyst contains a large amount of oxygen vacancies,which accelerate the adsorption and dissociation of CO [52].This result is consistent with the analysis in Fig.5.Consequently,Ni-Al(HSM-CP) has excellent catalytic performance.

3.2.Catalytic activity

To evaluate the catalytic activity of catalysts,CO methanation data were obtained (Fig.8).It is obvious that the CO methanation activity after HSM treatment was significantly improved at 200 °C in Fig.8(a) and (b).For Ni-Al (HSM-CP) catalyst,CO conversion reached equilibrium conversion at 250 °C,and CO conversion and CH4selectivity was 100% and 93% at 300°C.Stability of the catalyst at 200 °C and 300°C were shown in Fig.8(c) and (d),respectively.As for the stability,the activity of the Ni-Al (HSM-CP) catalyst has remained stable without attenuation,indicating that Ni-Al (HSMCP)catalyst has good stability.In conclusion,Ni-Al(HSM-CP)catalyst has the excellent catalytic activity.

Fig.5.XPS spectra of(a)Ni 2p,(b)O 1s and(c)Al 2p,(d)EPR spectra.Optimized geometry and generation energy of oxygen vacancy(e),density of states(DOS)before and after the formation of the oxygen vacancy (f).Zero eV is the Fermi energy.

3.3.Catalytic mechanism

Thein-situFTIR of the two catalysts at different temperatures was further tested in Fig.9.Notably,there is an obvious adsorption vibration peak in the range of 1300–1600 cm-1,which belong to formyl species (COH,CHO) [66].The spectra at 1340 cm-1belongs to monodentate formyl species and the spectra at 1590 cm-1was ascribed to bidentate formyl species[67,68].Compared with Fig.9(b),the adsorption vibration peaks at 1590 cm-1of the Ni-Al(HSM-CP) catalyst at different temperatures are overall clearer and the intensity in the low temperature region is greater.In addition,Ni-Al (HSM-CP) catalyst has a strong low-temperature adsorption vibration peak of the monodentate formyl species.According to related research reports,the monodentate formyl species is more helpful to the hydrogenation reaction[69].The formyl species can participate in the formation of methane with active hydrogen on the nickel surface.The band around 2117 cm-1can be assigned to Ni(CO)n(n=2 or 3),which is easy to form in steps,corners and kinks.Besides,the band around 2179 cm-1is interpreted as CO[68].Therefore,based on the better methanation activity of the Ni-Al (HSM-CP),it can be speculated that the abundant formyl species can effectively improve the low-temperature catalytic activity.

Fig.6.H2-TPR profiles of catalysts.

Fig.7.CO-TPD profiles of catalysts.

Fig.8.Catalytic performance of CO methanation:conversion of CO (a),selectivity of CH4 (b) and stability data at 200 °C (c) and 300 °C (d).

Fig.9. In-situ FTIR spectra of the catalyst at different temperature.

Fig.10.Transition states and potential reaction pathways of CO+H2 on Ni(111)surface.The red,green,gray and white spheres represent O,Ni,C and H atoms,respectively.

According to the CO methanation reaction and relevant literature,three potential reaction pathways are proposed.Fig.10(a)and (b) show that the breaking of the C-O bond occurs with the aid of hydrogen [70].Hydrogen is adsorbed by active sites and decomposed to produce active hydrogen (*H).The adsorption of active hydrogen on the CO surface can be divided into C-endonadsorption and O-endonadsorption.In Fig.10(a),intermediates such as CHO and CHOH are formed in the reaction path.Then the C-O bond breaks to generate H2O,and the CH*intermediate undergoes continuous hydrogenation reactions to generate CH4.Fig.10(b),as the adsorption reaction progresses,O-endonadsorption path generates water and activated carbon (*C)when the C-O bond is broken.*C reacts with *H to form CH4.The formyl species signal was observed by thein-situFTIR.Therefore,pathway 1 and pathway 2 are more in line with the experimental observations in this work.In Fig.10(c),the direct dissociation of CO is considered to be a possible reaction pathway for CO methanation[68].Adsorbed CO firstly undergo the cleavage of the C-O bond,and then *C generates CH4through the hydrogenation reaction.

Fig.11 shows energy and reaction path of CO methanation.The adsorption energy of H2and CO in the initial state is-0.35 eV.CH4formation from CO hydrogenation proceeds through the pathway 1 CO* →CHO* →HCOH*→CH* →CH2* →CH3* →CH4*.The dissociation of H2into H*shows a lower activation energy barrier(Ea)of 0.17 eV,and an exotherm of 0.16 eV.The adsorption of H*occurs at theend,and the generated H*is adsorbed on the active sites on the Ni surface(see H*structure).This process is accompanied by anEaof 0.12 eV and an exotherm of 0.30 eV.The endothermic reaction of the formation of H2O has anEaof 0.18 eV.Next,the hydrogenation reaction produces methane.In pathway 2,CO hydrogenation produces CH4viachannel:CO* → CHO* → C* → CH* →CH2*→CH3*→CH4*.The difference of this path is that the adsorption of H*occurs at theend.TheEaof this process is 0.28 eV and an exotherm of 0.44 eV.The pathway 3 CO* →C* →CH* →CH2*→CH3*→CH4*.After the transition state TS1,the C-O bond breaks and H2dissociation.TheEaexhibited by this process is 0.58 eV,and the heat release is 0.22 eV.Then C* continuously undergoes hydrogenation reaction to produce methane.

Fig.11.Energy distribution of the reaction path.

Besides,theEafor the formation of CHO is 0.12 eV in pathway 1.In pathway 2,Eafor the formation of COH is 0.28 eV.In pathway 3,Eafor the formation of CH* intermediate is 0.15 eV.José L.C.Fajínet al.[71] found through DFT calculations that theEaof CHO in CO methanation is 1.25 eV,andEaof COH is 1.42 eV.Wanget al.[72] calculated by DFT thatEais 1.70 eV for CHO andEais 1.93 eV for COH.TheEaof CHO species is 0.12 eV,which is favorable than pathway 2.Therefore,the Ni-Al (HSMCP) in this work have the best catalytic activity may because of the formation of CHO or COH intermediate species with lower activation energy barriers.Combined with the formyl species observed inin-situFTIR,the breaking of the C-O bond occurs with the aid of hydrogen is more favorable than direct CO dissociation.

4.Conclusions

Ni-Al mixed metal oxide catalyst was prepared by HSM assisted CP method for low temperature CO methanation.The CO conversion rate of Ni-Al(HSM-CP)at 300°C is 100% and the CH4selectivity is 93% .Moreover,the Ni-Al (HSM-CP) sample offered 89.7% CO conversion and 81.8% CH4selectivity at 200 °C.Besides,the Ni-Al(HSM-CP) catalyst maintained good stability.The catalyst has improved catalytic performance may come from the high BET surface area,good dispersion and improved reducibility.In addition,Ni-Al (HSM-CP) catalyst contains more oxygen vacancy,which can accelerate the dissociation of CO and prevent the generation of CO2,thereby improving the activity.Based onin-situFTIR and DFT analysis,formyl species with lower activation energy barriers are formed and the results indicate that hydrogen-assisted CO dissociation is the preferred route.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by National Natural Science Foundation of China(No.22068034)and Science and Technology Innovation Talents Program of Bingtuan (No.2019CB025).