Low-temperature,dry,reforming,of,methane,tuned,by,chemical,speciations,of,active,sites,on,the,SiO2,and,γ-Al2O3,supported,Ni,and,Ni-Ce,catalysts

Yimin Zhang,Ruiming Zeng,Yun Zu,*,Linhua Zhu,Yi Mei,Yongming Luo,Dedong He,2,*

1 Faculty of Chemical Engineering,Kunming University of Science and Technology,Kunming 650500,China

2 National Engineering Laboratory for Flue Gas Pollutants Control Technology and Equipment,Tsinghua University,Beijing 100084,China

3 Faculty of Environmental Science and Engineering,Kunming University of Science and Technology,Kunming 650500,China

Keywords:CO2 dry reforming of methane Low-temperature Ni-based catalysts Chemical speciations Reforming reaction mechanisms

ABSTRACT The cognition of active sites in the Ni-based catalysts plays a vital role and remains a huge challenge in improving catalytic performance of low temperature CO2 dry reforming of methane (LTDRM).In this work,typical catalysts of SiO2 and γ-Al2O3 supported Ni and Ni-Ce were designed and prepared.Importantly,the difference in the chemical speciations of active sites on the Ni-based catalysts is revealed by advanced characterizations and further estimates respective catalytic performance for LTDRM.Results show that larger [Ni0n] particles mixed with [Ni-O-Sin]) species on the Ni/SiO2(R) make CH4 excessive decomposition,leading to poor activity and stability.Once the Ce species is doped,however,superior activity (59.0% CH4 and 59.8% CO2 conversions),stability and high H2/CO ratio (0.96) at 600 °C can be achieved on the Ni-Ce/SiO2(R),in comparison with other catalysts and even reported studies.The improved performance can be ascribed to the formation of integral ([Ni0n]-[CeIII-□-CeIII]) species on the Ni-Ce/SiO2(R) catalyst,containing highly dispersed [Ni0n] particles and rich oxygen vacancies,which can synergistically establish a new stable balance between gasification of carbon species and CO2 dissociation.With respect to Ni-Ce/γ-Al2O3(R),the Ni and Ce precursors are easily captured by extraframework Aln-OH groups and further form stable isolated ([Ni0n]-[Ni-O-Aln]) and [CeIII-O-Aln] species.In such a case,both of them preferentially accelerate CO2 adsorption and dissociation,causing more carbon deposition due to the disproportionation of superfluous CO product.This deep distinguishment of chemical speciations of active sites can guide us to further develop new efficient Ni-based catalysts for LTDRM in the future.

The catalytic conversion of C1 molecules (e.g.,CO,CO2,CH4and CH3OH,etc.) into fuels and value-added chemicals is a crucial process in the chemical industry,which is closely related to energy and environmental impact.In particular,the significance of comprehensive utilization of CO2and CH4,as the main greenhouse gases,has been widely noticed with the increasingly stringent environmental standards [1].To solve this difficulty,numerous processes have been adopted,such as steam reforming of methane (SRM) [2],CO2hydrogenation [3],and CO2dry reforming of methane (DRM) [4-12],etc.Among various alternatives,the DRM as an effective process plays an role in simultaneously converting them into syngas with a H2/CO molar ratio around 1 (cf.Eq.(1)),which can further serve to be transformed to value-added chemicals (e.g.,methanol and light olefins,etc.) through Fischer-Tropsch process [13].It is well known that the DRM process is an endothermic reaction.The high temperature above 700 °C is necessary.In such a case,it will usher in a huge challenge that the occurrence of several side reactions is direct impact on the DRM,such as CH4decomposition (cf.Eq.(2)),reverse Boudouard reaction (cf.Eq.(3)),reverse water gas shift reaction (RWGS,cf.Eq.(4)).

To lower the activation energy and decrease the side reactions,the development of efficient catalysts becomes crucial.Recent reports have given a fact that the Ni-based catalysts can exhibit comparable activity with noble metals (e.g.,Pt,Ru,Rh and Pd,etc.) at high temperatures and more promising application in industry,together with abundant resource and low cost [4,14-19].However,a universal phenomenon is that the catalysts suffer from deactivation rapidly due to coke formation derived from CH4excessive decomposition and CO disproportionation.Many literatures have generally considered that the rate of CH4decomposition is strongly dependent on Ni particle sizes and the formation of carbon species is favored on large Ni sizes[20-23].At such high temperature,therefore,the sintering of Ni particles should be limited.Besides,the H2/CO molar ratio (far below 1) also cannot meet for the industrial requirement of value-added syngas,on account of the RWGS.

To overcome abovementioned difficulties,many attempts for instance the introduction of promoters (i.e.,La,Ce,Mn,Zr and Mo,etc.) and the regulation of metal-support interactions have been employed to tune them into appropriate size and to embed(or confine)them into the support[23-32].In view of confinement effect,a Ni-yolk@Ni@SiO2catalyst designed by Liet al.[33] has showed stable and near equilibrium conversion with negligible carbon deposition.Besides,several reports also have shown that the nature and strength of the interaction of Ni particles with the supports (e.g.,TiO2,SiO2,Al2O3,CeO2,MgO and ZrO2,etc.) determine the catalytic activity for the DRM [19,24,26,29,34-36].For instance,Gong group [37] have found that both the reactivity and stability of La2O2CO3-modified Ni/Al2O3can be enhanced by moderate metal-support interactions.Although the designed Nibased catalysts have achieved some improvements for DRM,most studies are based on high temperatures (above 700 °C).Besides,high H2/CO molar ratio is difficult to be reached due to the occurrence of side reactions.

Thus,scholar’ gaze shifts to low temperature DRM (LTDRM)process because the coke deposition and Ni sintering can be thermodynamically limited and some side reactions can be prevented[21,26,27,38-40].For instance,Lemonidouet al.[39] have found that 5% (mass) Ni/CaO-Al2O3catalyst exhibited high stability,and importantly,the activity did not decrease even upon 50 h at 600 °C.Baudouinet al.[21] have investigated the Ni/SiO2catalyst that performed 7% and 13% for CH4and CO2conversions at 500 °C.They also have concluded that the activity for LTDRM increased with the decreasing of Ni particle sizes (from 7.3 to 1.6 nm).After doping Zr promoter,the conversions are 6.5% and 9.1%on the Ni-Zr/SiO2at 450°C,simultaneously accompanied with the enhancement of stability[25].It is worth noting that the DRM activity can be apparently promoted by exploiting Ni-Ce/SBA-15 catalyst at 600°C,especially for high H2/CO ratio(about 0.96)[40].

Given the current research,the coke-resistance,anti-sintering of Ni particles,and H2/CO ratio get an improvement at low temperature.However,the activities of catalysts are generally low,which is not consistent with detailed theoretical thermodynamic calculations by Nikooet al.[41] and Costaet al.[42].Through in-depth consideration,the phenomenon may be ascribed to the difference of DRM reaction mechanisms over specific active sites(e.g.,Ni particles and basic sites)of catalysts.At high temperatures,an acceptable mechanism is that the carbon species formed by the CH4decomposition on the Ni particle active sites can well consume the active oxygen species originated from the support (or promoter)that acted as the basic sites,accompanied with the generation of oxygen vacancies that facilitates the CO2dissociation.The stable balance between carbon gasification and CO2dissociation not only can improve the reactivity,but also can promote the carbon removal[4,20,43,44].However,other carbonate intermediates(e.g.,-CH2OH,,HCOO-and CO-2,etc.) on the Ni-based catalysts at low temperatures have been observed [23,27].The formation of these carbon species may be assigned to preferential CO2adsorption on those active sites,rather than CH4molecules.The unclear mechanisms may break previous DRM reaction pathways due to the complexity and ambiguousness of active sites,and a new one (or a new reaction balance) may be hidden.

In short,to improve catalytic activity for LTDRM,the chemical speciations of metal active sites on the catalysts and the elucidation of possible reforming mechanisms should be distinguished,which can give us a new idea to design excellent Ni-based catalysts and will be expected to build a new reaction balance for LTDRM.In general,silica(SiO2)and alumina(γ-Al2O3)as commercially applicable support materials due to high specific surface area and thermal stability have been employed to promote active metal dispersion on their surfaces,resulting in high catalytic activity[4,34,35,39].Meanwhile,the Ce species as a regular promoter also have been applied for DRM,owing to its unique redox properties(Ce4+-Ce3+) and high concentration of highly reactive oxygen species,which can act as local sources or reservoirs for oxygen species[26,27].

In consideration of above situation,in this work,four typical catalysts of SiO2and γ-Al2O3supported Ni and Ni-Ce are rationally designed and prepared by a incipient wetness impregnation method.Formation processes and chemical speciations of metal active sites on the Ni-based catalysts for LTDRM are systematacially revealed by employingin situFTIR,TG-DTG,XRD,H2-TPR,XPS,TEM and HAADF-STEM techniques.To deeply understand the formation and removal of carbon species,the reforming mechanisms of CH4and CO2on the active sites of catalysts are also investigated by using CH4-TPDe,CO2-TPD,Raman andin situFTIR.This paper aims to provide a theoretical foundation in the realization of developing efficient Ni-based catalysts for LTDRM in the future.

2.1.Chemicals and reagents

Ni(NO3)2·6H2O with a purity of 99.0%and Ce(NO3)3·6H2O with a purity of 99.5%were supplied by Sinopharm Chemical Reagent Ltd.,China.Pyridine with a purity of 99.8% was bought from the J&K Chemical Reagent Corporation.The commercial SiO2and γ-Al2O3were provided by Sigma-Aldrich.

2.2.Catalysts preparation

The catalyst samples that the Ni or Ni-Ce was supported on the SiO2or γ-Al2O3were prepared by a incipient wetness impregnation method.For the SiO2supported Ni and Ni-Ce catalysts,the industrial microsphere silica gel were mixed with the Ni(NO3)2-·6H2O with/without Ce(NO3)3·6H2O aqueous solution under constantly stirring to achieve a theoretical 10.0% (mass) Ni loading(or 10.0% (mass) Ni loading and 10.0% (mass) Ce loading).After stirring for 2 h,the resulting slurries were dried at 105°C overnight and then calcined in air at 700°C for 5 h with a heating rate of 1°-C·min-1.The obtained samples were donated as Ni/SiO2(C)and Ni-Ce/SiO2(C),respectively.The γ-Al2O3supported catalysts were prepared by the above same method.The obtained samples were named as Ni/γ-Al2O3(C) and Ni-Ce/γ-Al2O3(C),respectively.

To obtain metal active phase (metallic []),the calcined samples were reduced at 750°C for 1 h with a 10%H2/Ar(30 ml·min-1).The final reduced samples were presented as Ni/SiO2(R),Ni-Ce/SiO2(R),Ni/γ-Al2O3(R),and Ni-Ce/γ-Al2O3(R),respectively.

2.3.Catalysts characterization

2.3.1.X-ray diffraction (XRD)

The crystalline structures of all the catalyst samples before and after H2reduction at 750 °C were characterized on a Rigaku D/max-1200 diffractometer (Cu-Kα radiation,40 kV,30 mA).Data were collected with a diffraction angle (2θ) ranged from 10° to 80°.The scan slim and step size were 1° and 0.02°,respectively.

2.3.2.Hydrogen temperature programmed reduction (H2-TPR)

H2-TPR experiment was carried out on a Tianjin Golden eagle PX200 adsorption instrument,equipped with a thermal conductivity detector (TCD).Prior to experiment,the catalyst samples (ca.50 mg)were pre-treated under a Ar flow at 500°C for 1 h,cooling to room temperature,and then 10%H2/Ar(30 ml·min-1)was introduced.H2-TPR process was conducted from 40°C to 850 °C with a heating rate of 10 °C·min-1.

2.3.3.X-ray photoelectronic spectroscopy (XPS)

XPS was studied to analyze the atomic state of surface nickle,cerium,silicon,aluminum and oxygen conducted on a PHI 5000 Versa Probe II spectrometer,equipped with a monochromatic Al-Kα excitation.The typical base pressure was 5.0 × 10-7Pa.The correction of charge effect were performed by adjusting the characteristic binding energy (BE) peak of referencing carbon in the 1 s region of 284.6 eV.

2.3.4.Transmission electron microscopy (TEM)

Metal particle morphology(size,shape and lattice fringe)of catalysts was examined on a transmission electron microscopy(TEM,Tecnai G2 TF30).The scanning transmission electron microscopy(STEM) image was obtained by means of Z-contrast imaging,generated by a high angle annular dark field (HAADF).Bright field imaging and HAADF-STEM were both used to investigate the positioning of the Ni particles and Ce species on the supports.

2.3.5.Methane temperature programmed decomposition (CH4-TPDe)

The activation and decomposition of methane by the catalysts were investigated by CH4-TPDe performed using the Tianjin Golden eagle PX200 adsorption instrument.Prior to experiment,the samples (ca.20 mg) were pre-treated at 750 °C for 1 h with a 10% H2/Ar (30 ml·min-1).After pre-reduction,the system was purged under N2atmosphere for 1 h at 50 °C.Subsequently,the TPDe was carried out in a stream of 10% CH4/N2with a total flow rate of 30 ml·min-1under atmosphere pressure,with a heating rate of 10 °C·min-1.The corresponding CH4decomposition was on-line monitored using TCD detector equipped on the instrument.

2.3.6.CO2 temperature programmed desorption (CO2-TPD)

CO2-TPD experiment was also performed on the Tianjin Golden eagle PX200 adsorption instrument.Prior to adsorption experiments,the catalyst samples (ca.10 mg) were pre-treated at 750 °C for 1 h with a 10% H2/Ar (30 ml·min -1),and then cooled to room temperature in a He flow (30 ml·min-1) before the CO2adsorption.Subsequently,10% CO2/He (30 ml·min-1) was dosed into the system at 30 °C until the samples adsorbed to saturation.Then,the above samples were exposed to a He flow rate of 30 ml·min-1for 1 h at 30°C to remove free and physically adsorption CO2molecules.Desorption process was performed from 30°C to 1000 °C with a heating rate of 10 °C·min-1,in a He flow rate of 30 ml·min-1.The corresponding CO2desorption signal was measured using TCD detector.

2.3.7.Raman spectroscopy

Raman spectra were recorded on a Renishaw Raman spectrometer at room temperature with a 514 nm emission line from an Ar+laser.

2.3.8.Hydroxyls and CH4-CO2-dosing Fourier transform infrared spectroscopy (FTIR)

In situFTIR technique was applied to get more information about the formation and chemical speciations of active sites species on the Ni-based catalysts and to trace adsorbed species on the samples,in order to elucidate possible DRM reaction mechanisms at low temperatures.Firstly,the sample wafer (10-15 mg)was reduced in anin-situcell from 30 to 600 °C (10 °C·min-1) for 30 min with a flow of 10% H2/Ar (30 ml·min-1).At each H2reduction temperature,the sample was retained 5 min before the acquisition of IR spectrum.Afterwards,the sample was cooled to 100°C,followed by purging in Ar (30 ml·min-1) for 10 min.Background spectrum was recorded.Subsequently,the CH4-CO2-Ar (1:1:1)mixture was introduced into the cell for 10 min with a flow of 30 ml·min-1,and then was purged in Ar (30 ml·min-1) for 30 min.Final spectrum was collected at 100 °C.The spectrum of 200,300,400,and 500 °C were obtained abide by the above procedures.

2.4.Catalytic performance

The DRM was evaluated in a fixed bed reactor with the catalyst(ca.200 mg) sieved fraction between 0.250 to 0.425 nm.Prior to the reaction,all the catalysts were reduced at 750 °C for 1 h with a 10% H2balanced with Ar (30 ml·min-1).Subsequently,the reactant gases consisted of CH4(15 ml·min-1),CO2(15 ml·min-1) and Ar(15 ml·min-1)were passed into the reactor,the DRM were conducted at different temperatures (500 °C,600 °C and 700 °C).The reactants and products were on-line analyzed on a Fuli 9790II gas chromatography(GC)in combination with TCD and FID detectors,for the purpose of CH4,CO2,CO,and H2on-line analysis.The conversion of reactantsand H2/CO molar ratio were calculated as follows:

whereCrepresented the concentration of the reactants or products measured at the feed and effluent of the reactor.

The turnover frequencies(TOF)of reactants and particle sizes of[] on the catalysts were measured by H2-pluse chemisorption(cf.Table 1 and Table S1 in Supplementary Material).

Table 1 Textural properties(surface area,pore volume and pore diameter),the particle sizes of metallic[Ni0n],the ratio of[Ni0n]to([Ni2+]+[Ni0n]),and n(Ni)/n(Ce)of catalyst samples after reduction

3.1.Formation and chemical speciations of metal active sites on the catalysts

To elucidate the formation processes and chemical speciations of metal active sites on the Ni-based catalysts,hydroxyls FTIR spectra of catalysts during H2reduction are depicted in Fig.1.Two bands at 3738 cm-1and 3680 cm-1in the SiO2can be observed,assigned to the Si-OH and Si2-OH groups,respectively [45-47].After introducing Ni and Ni-Ce species,the intensities of two bands become weak on the Ni/SiO2(C).The result indicates that the Ni species can be captured by Sin-OH (n＞1) and possibly form the[Sin-O-Ni] species.However,the intensities of abovementionedbands are basically unchanged on the Ni-Ce/SiO2(C),meaning that the Ni species are easier to bind to the Ce species,rather than Sin-OH groups,possibly forming the [Ni-O-CeIV-O2-CeIV-O-Ni] speciesviathe hydroxyls condensation and dehydration processes.The conjecture also can be proved by thermal analysis.As shown in Fig.S1,one main stage of weight loss at about 279 °C can be observed on the uncalcined Ni/SiO2,which can be ascribed to the condensation between Ni(OH)+species and Sin-OH groups [47].While,for uncalcined Ni-Ce/SiO2,except for the abovementioned stage of weight loss,a new serious stage of weight loss appears,which can be exactly assigned to the dehydration condensation between the Ni(OH)+species and thespecies [47,48].

With the increasing of system temperature at H2atmosphere,the intensities of two bands on pure SiO2are gradually weakened,mainly ascribed to dehydration process of physically adsorbed water.However,it is worth noting that the intensities of bands on the Ni-Ce/SiO2(C),especially on the Ni/SiO2(C),are strengthened obviously.The phenomenon may be because the reduction of catalysts by hydrogen can also produce surface hydroxyls.For instance,if Me+(metal cations) are present on the surface,their reduction will proceed according to Eq.(8) and will cause the formation of surface hydroxyls.The results can further give the formation of supposed chemical speciations of Ni-Ce on the SiO2during calcination and reduction,as shown in Fig.2.For this situation,more evidences are provided in next sections.

However,the hydroxyls FTIR spectra show another situation after Ni and Ni-Ce supported on the γ-Al2O3.Three bands at 3723,3690 and 3550 cm-1in the γ-Al2O3can be observed,assigned to Al2-OH,Al3-OH and Aln-OH(n＞3)groups,respectively[45-47].For Ni/γ-Al2O3(C),the above bands (especially at 3723 cm-1) are weakened.After doping Ce species,trend in the intensities becomes more obvious.Meanwhile,a stage of weight loss at 241 °C for the Ni/γ-Al2O3and two stages of weight loss at 209 °C and 241 °C for the Ni-Ce/γ-Al2O3can be observed (cf.Fig.S1).These results imply that the Ni and Ce species are easier combined with those extra-framework Al-OH groups and can form more isolated[Ni-O-Aln]and[CeIII-O-Aln]species(n＞2)by hydroxyls condensation and dehydration processes[47].During H2reduction,the intensities of three bands decrease gradually,especially at 3723 cm-1,indicating that the chemical structures of Ni species and Ce species bonded to the Aln-OH (n＞2) groups are difficult to be reduced.Hence,the possible chemical speciations of Ni-Ce on the γ-Al2O3can be deduced preliminarily during calcination and reduction,seen in Fig.2,which can be further proved in next sections.

To further confirm chemical speciations of Ni and Ce species on the support,Fig.3 provides XRD patterns of Ni-based catalysts before and after reduction.For the Ni/SiO2(C),except for one broad peak shown at 22.1° assigned to amorphous SiO2,four diffraction peaks (empty black diamond,◇) also can be seen at around 2θ of 37.2°,43.3°,62.9° and 75.4°,which correspond to (1 1 1),(2 0 0),(2 2 0) and (3 1 1) planes for NiO,respectively.After reduction,the above peaks disappear,accompanied with the appearance of new peaks at 44.5°,51.8° and 76.5° (solid black diamond,◆),matching well with (1 1 1),(2 0 0),and (2 2 0) planes for metallic[],respectively.According to the H2-chemisorption,the particle size of [] is about 7.5 nm (cf.Table 1).When Ce species are doped onto the Ni/SiO2(C),the intensities of peaks associated with NiO phases on the Ni-Ce/SiO2(C) are weakened.In addition,four peaks at around 2θ of 28.5°,33.1°,47.3°,and 56.2° (black plum flower,♣),can be found,which correspond to (1 1 1),(2 0 0),(2 2 0),and (3 1 1) planes for CeO2,respectively.After reduction,the peaks associated with NiO phases disappear,simultaneously accompanied with the appearance of peaks ascribed to the []active phases.While,the intensities are weakened obviously.Besides,the peaks at 33.1°,47.3° and 56.2° associated with(2 0 0),(2 2 0),and (3 1 1) planes of CeO2phases disappear,and the intensity of (1 1 1) plane of CeO2phases is weakened apparently.The above results reveal that the [] particles perform a well dispersion on the Ni-Ce/SiO2(R)(cf.Table S1).Besides,the particle size of [] is about 4.3 nm.However,it is surprising that no diffraction peaks associated with the CeO2-xphases (0 ＜x≤0.5)can be observed [24,49,50],possibly mixed with [] particles.Hence,an interdependent relationship between [] active phase and CeO2-xspecies on the SiO2can be speculated,which is in line with hydroxyls FTIR results,exactly meaning the formation of integral ([]-[CeIII-□-CeIII]) species (cf.Fig.2).

Fig.1.Hydroxyls FTIR spectra acquired during H2 reduction from the calcined catalysts.The measurements are performed as a temperature series in 10 °C increments.

If the support is replaced by γ-Al2O3,XRD patterns perform another scene.As shown in Fig.3(b),the diffraction peaks associated with γ-Al2O3can be observed,indicting no phase transition after the samples treated at 750 °C.Besides,no diffraction peaks associated with NiO phases can be found on the Ni/γ-Al2O3(C),instead of the appearance of new peaks at 19.1°,37.3°,45.5°,and 66.7°,corresponding to (1 1 1),(2 0 0),(2 2 0),and (3 1 1) planes for nickel-aluminum spinel (NAS) phases.The result confirms that the Ni2+species are easily captured by extra-framework aluminum(EFAl) species on the γ-Al2O3(cf.Figs.1 and S2) [47,48],forming NAS phases.At present,the structure has been not clear,only the NiAl2O4phase generally accepted [35,51,52].Here,the NASs can be represented by basic chemical expression [Ni-O-Aln] (n＞2),in combination with the FTIR results.After reduction,the peaks associated with NAS phases are slightly weakened as well as the peaks assigned to [] phases.The particle size of [] on the Ni/γ-Al2O3(R) is about 4.6 nm (cf.Table 1).The results indicate that[]active phase also can be dispersed by the surface EFAl species of γ-Al2O3.However,some NAS phases cannot be reduced at 750 °C,indicating that the chemical speciations may be multifarious,which can be proved by the formation of different EFAl species(cf.Figs.1 and S2).When the Ce species are introduced onto the Ni/γ-Al2O3(C),the peaks associated with CeO2can be found and the intensities become stronger,implying that the CeO2species are difficult to be dispersed.After reduction,three new peaks around at 33.6°,47.8° and 55.8° can be observed,corresponding to ceriumaluminum spinel (CAS) phases [49],the basic chemical expression depicted by [CeIII-O-Aln] (n＞ 2).The formation may also be assigned to the combination between CeO2-xspecies and EFAl species.Differing from Ni-Ce/SiO2(R),only the peaks associated with CAS phases are weakened and the peaks associated with []active phases are weaker than that Ni/γ-Al2O3(R),implying that the N(C)AS phases are difficult to be reduced,leading to the difficult combination between Ni2+species and CeO2species,which confirms the FTIR results.Seen in Table 1,the particle size of[] is about 7.1 nm,which may be ascribed to some sites of EFAl species occupied by partial Ce species,accompanied with the decreasing of NAS phases and make the NiO particles increase.At this time,the interdependent between the [] and Ce species is weakened sharply,forming stable isolated ([]-[Ni-O-Aln]) and[CeIII-O-Aln] species (cf.Fig.2).

Fig.2.Possible formation processes of active sites species on the Ni-Ce/SiO2(R) and Ni-Ce/γ-Al2O3(R) catalysts.

Fig.3.XRD patterns of the Ni-based catalyst samples before and after reduction.

H2-TPR experiment is applied to further investigate the reducibility of active sites species on the Ni-based catalysts.As shown in Fig.4(a),two main H2consumption peaks at 405 °C and 471 °C can be observed on the Ni/SiO2(C),ascribed to the reduction of NiO particles [24].The H2consumption peak at low temperature (LT) may be associated with surface NiO particles.While,H2consumption peak at high temperature (HT) may be ascribed to NiO particles weakly bonded to SiO2.After doping Ce,the intensities of H2consumption peaks are weakened and the second H2consumption peak shifts to LT.Meanwhile,a new H2consumption peak appears,assigned to the reduction of CeO2particles.The results further confirm that more NiO particles can be combined with CeO2,which is consistent with the XRD and hydroxyls FTIR results.

In comparison with Ni/SiO2(C),the Ni/γ-Al2O3(C) shows more H2consumption peaks at 224,378,524,694,762 and 795 °C (cf.Fig.4(b)).The H2consumption peaks at 224 °C,378 °C and 524 °C can be ascribed to the reduction of surface NiO particles and NiO particles weakly bonded to γ-Al2O3,respectively [33,53].However,H2consumption peaks at HTs (694 °C,762 °C and 795°C)can be assigned to the difficult reducible NAS phases.Such interaction between NiO and γ-Al2O3is caused by the dissolution and incorporation of different EFAl species in NiO crystallites (cf.Figs.1 and S2),which makes the disruption of Ni-O bond difficult.Compared with Ni/γ-Al2O3(C),all the H2consumption peaks can be observed on the Ni-Ce/γ-Al2O3(C).However,H2consumption peaks associated with the Ce species cannot be found.This may be because the incorporation of EFAl species in CeO2crystallites needs HTs [51,52].Besides,all the H2consumption peaks associated with NAS phases drift to HTs.The intensity of reduction at 712 °C is obviously enhanced and the intensities of reduction at 773 °C and 802 °C are weakened.The results indicate that the Ce species are easier incorporated with EFAl species on the γ-Al2O3to form the CAS species than that nickel species,which make the NAS more difficult to be reduced.It gives a direct evidence for the mentioned FTIR results.

Fig.4.H2-TPR profiles of the Ni-based catalysts.

Fig.5.XPS spectra of Ni-based catalysts:(a),(b)Ni2p3/2 spectra;(c),(d)Ce3d spectra,Ce3+and Ce4+species content;(e),(f)Si2p and O1s spectra of Ni and Ni-Ce supported on the SiO2;(g),(h) Al2p and O1s spectra for Ni and Ni-Ce supported on the γ-Al2O3.

To get a deeper insights,XPS characterization is performed.Ni2p,Ce3d,Si2p,Al2p and O1s spectra of catalysts are collected(cf.Fig.5).The binding energy(BE)is analyzed to further rationalize chemical speciations of Ni species and Ce species on the support.Ni2p3/2XPS spectra of Ni/SiO2and Ni-Ce/SiO2catalysts are presented in Fig.5(a).It can be seen that both the Ni/SiO2(C) and Ni-Ce/SiO2(C) exhibit Ni2p3/2main peaks at 856.5 eV with satellites around 862.5 eV.The reported BE of Ni2p3/2on pure NiO was about 854.4 eV [54].After reduction,a new peak of metallic[] at 853.3 eV and 853.0 eV on the Ni/SiO2(R) and Ni-Ce/SiO2(R) can be observed,respectively.This indicates that parts of Ni2+species are reduced to metallic [],while the rest is remained,which may be ascribed to the reducibility of surface metallic [].Besides,special attention is that the introduction of Ce species causes the BE decreasing of surface [] content in the catalysts.Seen in Table 1,more Ni2+species are reduced to[] particles (about 61.0%) on the Ni-Ce/SiO2(R).Hence,more effective metallic [] can be applied to the DRM.

Meanwhile,Fig.5(b) gives Ni2p3/2XPS spectra of calcined and reduced Ni/γ-Al2O3and Ni-Ce/γ-Al2O3catalysts.The BE of Ni2p3/2associated with Ni2+species decreases,856.3 eV on the Ni/γ-Al2O3(C) and 855.9 eV on the Ni-Ce/γ-Al2O3(C),and both are lower than that on the SiO2.The differences may be ascribed to the chemical speciations of Ni species affected by Ce species and support.In combination with H2-TPR,the NAS phases strongly bonds to the γ-Al2O3.After reduction,a new contribution peak at 852.7 eV assigned to metallic [],can be observed,which is also lower than that on the Ni/SiO2(R)and Ni-Ce/SiO2(R).While,the percent can only reach about 17.8% on the Ni/γ-Al2O3(R) and 12.2% on the Ni-Ce/γ-Al2O3(R)(cf.Table 1).The results further verify that the resulting NAS phases are difficult to be reduced,which coincides with the FTIR and H2-TPR results.

To illustrate the role of Ce species,Ce3d XPS spectra of Ni-Ce/SiO2and Ni-Ce/γ-Al2O3are shown in Fig.5(c).Two groups of spin-orbital multiplets,corresponding to 3d3/2and 3d5/2,are denoted asuand v,respectively.With respect to Ni-Ce/SiO2(C),three main 3d5/2peaks at about 883.2 (v),889.2 (v’’),and 898.4(v’’’) eV and three main 3d3/2peaks at about 901.4 (u),907.4 (u’’),and 917.9 (u’’’) eV,can be assigned to the Ce4+species.The peaks near 886.2 eV(v’)and 904.2 eV(u’)can be ascribed to the Ce3+species.The content of Ce3+species can be estimated according the following Eq.(9):

By calculation,relative contents of Ce3+and Ce4+species for Ni-Ce/SiO2(C) are 21.3% and 78.7%,respectively (cf.Fig.5(d)).After reduction,the BE ofu’ andv’ on the Ni-Ce/SiO2(R) decreases slightly and the content of Ce3+species increases by 41.1%.The results indicate that more Ce3+(namely,[CeIII-□-CeIII]) species should strongly bond to[Ni0n]active phase,rather than the support,exactly echoing Ni2p2/3XPS,H2-TPR and FTIR results.Compared with Ni-Ce/SiO2(C),the BE of v on the Ni-Ce/γ-Al2O3(C) also decreases sharply,indicating that the Ce species also strongly bond to the γ-Al2O3,more stronger than that between Ni species and γ-Al2O3,in combination with H2-TPR results.At this time,chemical speciations of Ce species ought to be the CAS phases.Besides,the content of Ce3+and Ce4+species are about 18.9%and 81.1%,respectively.Based on H2-TPR,it can be deduced that some tetravalent Ce species in the mixtures are captured by EFAl species.After reduction,except for the BE decreasing of v,the BE ofu,v’’’,v’ and v decreases and the Ce3+species increase by 11.0%,indicating that only few Ce4+species in the abovementioned mixture can be reduced to further form [CeIII-O-Aln] species.

Meanwhile,the corresponding Si2p,Al2p and O1s spectra of Nibased catalysts are shown in Fig.5(e)-(h).The Si2p BE associated with the SiO2is about 103.9 eV.After reduction,a higher BE(103.3 eV) can be observed.However,there is no change in the BE of Si2p XPS spectra before and after the introduction of Ce species.The above results indicate that the Ni species are easier to combine the Ce species,rather than the support,which is consistent with the FTIR results.Meanwhile,no change in the O1s XPS spectra can be seen,corresponding to the SiO2and metal oxides(NiO and CeO2).The result further confirms the above conclusion.

For the γ-Al2O3,one contribution peak at about 74.2 eV ascribed to the NAS phases on the Ni/γ-Al2O3can be seen [51,54,55].After doping Ce species,the value of the BE peak decreases to 74.0 eV,further confirming the mutual interaction between Ni species (Ce species) and EFAls.Meanwhile,the chemical speciations of O species perform a similar phenomenon.As shown in Fig.5(h),one contribution peak (about 531.2 eV) in the O1s spectra on the Ni/γ-Al2O3(C) can be observed,which also can be assigned to the NAS phases (and/or CAS phases) [55,56].After reduction,the O1s BE in the spinel phases has no changeable,only a shift of low BE(like that Al2p).

The dispersion,structural,morphological properties of metal active sites on the catalysts are studied by TEM (cf.Figs.6 and S3).It can be intuitively observed that black particles (NiO-CeO2)are dispersed on the Ni-Ce/SiO2(C),compared with that of Ni/SiO2(-C)(cf.Fig.6(a)and S3(a)).The phenomenon just verifies the above XRD,H2-TPR and FTIR results.The distributions of Ni species and Ce species on the Ni-Ce/SiO2(C) are observed by linear scanning of HAADF-STEM.As shown in Fig.6(b) (insert picture),the Ni atomic percent at different position is apparently higher than that of Ce atomic percent,confirming that the Ce species well disperse NiO particles.The result is in line with the observation of high resolution TEM analyses in Fig.6(c).For Ni-Ce/SiO2(C),the lattice spacing,0.241 nm for NiO particles and 0.312 nm for CeO2particles,correspond well with the characteristic of NiO (1 0 1) and CeO2(1 1 1) planes,respectively [57-59] (cf.Fig.6(c)).It also can be clearly observed that the NiO particles and the CeO2particles locate on the SiO2separately and the NiO particles surround with the CeO2particles (the ratio about (2-3)/1,cf.Table 1).For Ni/SiO2(C),however,only aggregation of large NiO particles can be found(cf.Fig.S3(b)and S3(c)).The above results fully confirm that the Ce species can disperse the Ni species,consisting of the above mixtures in Fig.2.

For Ni-Ce/γ-Al2O3(C),no aggregation of larger black particles can be observed,same with Ni/γ-Al2O3(C) (cf.Fig.6(d) and S3(d)),indicating that the metal species can be both dispersed on the γ-Al2O3.Differing from Ni-Ce/SiO2(C),the Ce atomic percent before the position (＜100 nm) is apparently higher than that of Ni atomic percent,by the result of linear scanning of HAADFSTEM (cf.Fig.6(e)).Further lattice spacing can be observed,that is,0.243 nm for NAS particles,0.154 and 0.271 nm for CAS and CeO2particles,correspond to the(3 1 1),(2 1 1)and(2 0 0)planes[60,61],respectively (cf.Fig.6(f)).Besides,the CAS (2 1 1) plane is close to the NAS (3 1 1) plane.For Ni/γ-Al2O3(C),only the NAS(3 1 1) plane can be observed (cf.Fig.S3(e) and S3(f)).It can also be clearly found that the NAS particles and the CAS particles locate on the γ-Al2O3separately and the CAS particles surround with the NAS particles (the ratio about 1/(2-3),cf.Table 1).The results further confirm that the dispersion of metal active sites species is originated from the role of EFAl species.

3.2.Catalytic performance

Based on the distinguishment of chemical speciations on the Nibased catalysts,the catalytic performance are tested by CH4and CO2conversions together with the H2/CO molar ratio plotted against time on stream at 600 °C,as shown in Fig.7(a)-(c) and Table 2.For Ni/SiO2(C),the CH4and CO2conversions except for H2/CO ratio are very low as well as poor stability,which can be ascribed to carbon deposition(about 23.6%)on larger[]particles(cf.Table 1,Figs.S3 and S8).If the reaction temperature is at 500°C or 700 °C,the phenomenon seems worse (cf.Figs.S4 and S5).While,it is surprising that high CH4and CO2conversions (59.0%and 59.8%) as well as a high H2/CO ratio (0.96) can be achieved on the Ni-Ce/SiO2(R)catalysts at 600°C,compared with other studies(cf.Table 3).Besides,it also holds a certain steady after 360 min.The excellent DRM catalytic performance ought to be associated with the role of integral ([]-[CeIII-□-CeIII]) species.

With respect to Ni/γ-Al2O3(R),the CH4and CO2conversions as well as H2/CO ratio are about 55.0%,58.9%and 0.85%,respectively.However,the DRM catalytic performance occur a abrupt change after doping Ce species.Even if the Ni-Ce/γ-Al2O3(R) shows a certain steady,the CH4conversion decreases to 52.3%,especially for the CO2conversion(up to 62.0%),resulting in the H2/CO ratio down to 0.78.The results should have little relation with the pore structure of catalysts (cf.Figs.S6 and S7).Hence,the chemical speciations of active NAS-CAS species (namely,stable isolated ([]-[Ni-O-Aln]) and [CeIII-O-Aln] species) should go against LTDRM.In other word,the CO2adsorption and dissociation on the [Ni-OAln] and [CeIII-O-Aln] species active sites occur easily.While,CH4excessive decomposition takes place on the dispersed [] particles active sites,which can generate carbon precursors that are difficult to be consumed due to lack of synergy between the abovementioned active sites.Further evidences are provided in next section.The long-term stability of the catalyst was tested,and the results are shown in Fig.S10.Obviously,the addition of Ce improves the stability of the catalysts of the two samples,which maintains a stable reactivity within 20 h of reaction.

Fig.6.TEM and HAADF-STEM images of Ni-Ce/SiO2(C) (a)-(c) and Ni-Ce/γ-Al2O3(C) (d)-(f): (a),(c),(d),(f) TEM images;(b),(e) HAADF-STEM images,where bright spots correspond to Ni and Ce species.The inserts of (b) and (e) represent Ni and Ce atomic percent at different position.

Meanwhile,turnover frequencies (TOF) of DRM reaction on the catalysts are calculated(cf.Fig.7(d)and Table S1),which is considered as another important indicator.By contrast,the TOF value of Ni-Ce/γ-Al2O3(R) is higher than that of others,but the catalytic activity for CH4is relatively lower.This mainly because the amount of effective [] particles in the stable isolated ([]-[Ni-O-Aln])active sites species is very low (cf.Table 1).By contrast,superior activity can be achieved on the Ni-Ce/SiO2(R),but the TOF value is lower,mainly caused by the fabrication of rich effective[]particles with highly dispersion.To sum up,it can confirm that the improved performance should be ascribed to the formation of integral ([]-[CeIII-□-CeIII]) species on the Ni-Ce/SiO2(R) catalyst,which can synergistically establish a new stable balance between CH4decomposition(or gasification of carbon species)and CO2dissociation.However,the catalytic conversions of CH4and CO2occur separately on the stable isolated([]-[Ni-O-Aln])and[CeIII-O-Aln]species active sites of Ni-Ce/γ-Al2O3(R),causing the catalysts deactivation easily and excessive CO2conversion,which cannot build a new reforming reaction balance.As shown in Fig.S8,the amount of coke deposition on spent catalysts can be used to prove the above,which is as follows: Ni-Ce/SiO2(R)(about 7.3%) ＜Ni-Ce/γ-Al2O3(R)(about 20.5%).Besides,the main weight loss of spent Ni-Ce/SiO2(R)is at 625 °C,while is at 700 °C on spent Ni-Ce/γ-Al2O3(R).The results imply that the carbon species formed during reaction at 600 °C are easily consumed by CO2on the active sites of Ni-Ce/SiO2(R),which is in line with the abovementioned suppose.

Table 2 DRM catalytic activity over the Ni/SiO2(R),Ni-Ce/SiO2(R),Ni/γ-Al2O3(R) and Ni-Ce/γ-Al2O3(R) catalysts

Table 3 Catalytic properties of different catalysts for DRM reaction at low temperature

3.3.Reaction mechanisms

As analyzed above,the adsorption and transformation processes of CH4and CO2on the abovementioned active sites of catalysts should be very important in determining DRM catalytic performance and reforming reaction routes.Fig.8(a) and (b) provide CH4-TPDe profiles of catalysts.For Ni/SiO2(R) and Ni-Ce/SiO2(R),two broad CH4consumption peaks (ca.650 °C and 795 °C) can be observed and first one is obvious higher than that second,indicating that more CH4can be converted into H2and CHxspecies (0 ＜x＜4) on the Ni and Ni-Ce supported on the SiO2at 600 °C.With respect to Ni-Ce/SiO2(R),CH4consumption peaks drift towards LT(ca.620°C and 785°C),implying that the integral([]-[CeIII-□-CeIII]) species can be conducive to CH4decomposition,coinciding with the DRM catalytic performance.However,three CH4consumption peaks (ca.557 °C,687 °C and 799 °C) can be observed on the Ni/γ-Al2O3(R).After doping Ce species,the above peaks also shift to LT(ca.421°C,546°C and 753°C).Importantly,more CH4consumption occurs at 700-800 °C.The result implies that although majorities of CH4molecules are difficult to be decomposed over the active sites of Ni and Ni-Ce supported on the γ-Al2O3as DRM temperature at 600 °C,the efficiency of CH4LT decomposition may be higher,especially for Ni/γ-Al2O3(R).

In addition to CH4,another component,namely CO2conversion,is also important for CO formation.Fig.8(c) and (d) give CO2-TPD profiles to determine basic active sites of catalysts.As reported,it is generally considered that CO2adsorbed on weaker basic sites is desorbed at LT and that on strong basic sites is desorbed at HT[62,63].As shown in Fig.8(c),Ni/SiO2(R)shows a weak CO2desorption peak around 202 °C.It can be speculated that this desorption peak may be derived from both weakly chemisorbed CO2of the NiO,framework (SiO2) as well as the physically adsorbed CO2.Besides,another two desorption peaks centered around 739 °C and 868°C are also observed,which may be related to strong chemisorbed CO2.With respect to Ni-Ce/SiO2(R),however,the intensity of CO2desorption at HT becomes weak obviously,while that is no change at LT (only shift to HT,309 °C).The result gives an indication that the introduction of Ce species can only decrease the strong basic sites,preventing CO2excess dissociation at HT,which can exactly establish a new stable balance with CH4conversion,coinciding with the DRM results (cf.Fig.7).

Fig.7.DRM catalytic performance on the Ni-based catalysts at 600 °C: (a) CH4 conversion,(b) CO2 conversion,(c) H2/CO ratio and (d) TOF.Reaction conditions:mcatalyst=200 mg, Treduction=750 °C,CH4/CO2/Ar=1/1/1,GHSV=13500 ml·h-1·(g cat)-1.

For Ni/γ-Al2O3(R)and Ni-Ce/γ-Al2O3(R),three obvious CO2desorption peaks can be found,at about 150,200-300 and 500-600°C,and the desorption amount is far higher than that on the Ni/SiO2(R)and Ni-Ce/SiO2(R) catalysts (cf.Fig.8(c)).The result implies that the CO2molecules can easier convert into CO molecules at LT,further confirming the role of [Ni-O-Aln]) and [CeIII-O-Aln] species active sites.

From another viewpoint,oxygen vacancies in the catalysts are very important for CO2conversion.Thus,the catalysts arecharacterized by Raman spectroscopy to testify the formation of oxygen vacancies (cf.Fig.8(e) and (f)).Compared with Ni/SiO2(-R),except for the band assigned to NiO particles,a new Raman band near 454 cm-1can be observed for the Ni-Ce/SiO2(R),assigned to the vibrational mode of F2gsymmetry in a cubic fluorite lattice of CeO2-xparticles [54,62,64,65].Besides,another new Raman band at 601 cm-1is also found,ascribed to the oxygen vacancies.The results indicate that the [CeIII-□-CeIII]species with more oxygen vacancies in the Ni-Ce/SiO2(R) can be formed after reduction.For Ni-Ce/γ-Al2O3(R),the Raman band assigned to the F2gsymmetry occur a red shift,which may be ascribed to the structure change of Ce species.At this time,the intensity of band associated with oxygen vacancies is weakened obviously,indicating that the oxygen vacancies

content decreases as the Ni and Ce co-introduced onto the γ-Al2O3,forming the [CeIII-O-Aln] species.

Fig.8.CH4-TPDe profiles (a),(b),CO2-TPD profiles (c),(d) and Raman spectra (e),(f) on the Ni-based catalysts.

Fig.9. In situ FTIR spectra of CH4 and CO2 over the (a) Ni/SiO2(R),(b) Ni-Ce/SiO2(R),(c) Ni/γ-Al2O3(R) and (d) Ni-Ce/γ-Al2O3(R) catalysts.

To further elucidate reaction mechanisms,Fig.9 depictsin situFTIR spectra of reactant adsorbed on those active sties of catalysts.The bands at 1300-2000 cm-1can be assigned to the formation of carbon species.At 100°C,the bands at 1688,1625 and 1373 cm-1can be observed on the Ni/SiO2(R) and Ni-Ce/SiO2(R) (cf.Fig.9(a)and 9(b)),ascribed to C=O symmetrical stretching vibration and C-H bending vibration of CO2and CH4adsorbed on the active sites,respectively [23,27,43,66].As the temperature up to 200 °C,the above three bands disappear,while three new bands at 1947,1830 and 1589 cm-1can be found.The former two are associated with carbonate species and the later is assigned to the C=C symmetrical stretching vibration of carbon species [23,27,66],indicating that the intermediates of more carbon-based species are formed.With the increasing of system temperature,the intensities of mentioned bands weaken gradually,especially on the Ni-Ce/SiO2(R),implying that a new stable balance between gasification of carbon species and CO2dissociation is easier established on the integral([]-[CeIII-□-CeIII])species active sites with appropriate oxygen vacancies.The phenomenon can further explain why the Ni-Ce/SiO2(R) performs such high catalytic activity,while the poor catalytic performance appear on the Ni/SiO2(R)due to the formation of larger [] particles mixed with [Ni-O-Sin]) species.The possible reaction routes are depicted in Figs.10(a) and S9(a),respectively.

With respect to Ni/γ-Al2O3(R)and Ni-Ce/γ-Al2O3(R)(cf.Fig.9(c)and (d)),the bands at 1640 and 1580 cm-1can be observed,also assigned to C=O symmetrical stretching vibration of CO2and C=C symmetrical stretching vibration of carbon species on the active sites.Besides,the band at 1437 cm-1associated with other carbonate species can be found [27].The intensities of bands assigned to carbonate species are stronger on the Ni-Ce/γ-Al2O3(R) than that on the Ni/γ-Al2O3(R),indicating that the CO2molecules are easier adsorbed and dissociated on the isolated([]-[Ni-O-Aln]) and [CeIII-O-Aln] species active sites [43],which is consistent with CO2-TPD and DRM experimental results.At this time,more CO molecules are formed.At 500 °C,the band at 1640 cm-1is weakened obviously and the band at 1580 cm-1is strengthened,accompanied with the appearance of 1464 cm-1and 1373 cm-1.Besides,the intensities of bands at 1580 cm-1and 1464 cm-1are strengthened apparently.The results indicate that more species associated with carbon species and carbonate species are formed,especially on the Ni-Ce/γ-Al2O3(R).All the phenomena can illustrate the reason of high CO2conversion,low activity and low H2/CO ratio on Ni-Ce/γ-Al2O3(R) for LTDRM.In other words,the above isolated active sites species can preferentially accelerate CO2dissociation,and subsequently form more carbon deposition due to the CH4decomposition and the CO disproportionation.The possible reforming mechanisms of CO2and CH4on the Ni/γ-Al2O3(R) and Ni-Ce/γ-Al2O3(R) can be seen in Figs.S9(b)and Figs.10(b),respectively.

Fig.10.Possible reaction routes of CH4 and CO2 on the active sites of Ni-Ce/SiO2(R) (a) and Ni-Ce/γ-Al2O3(R) (b) catalyst.

In this work,Ni-based catalysts,namely Ni/SiO2(R),Ni-Ce/SiO2(R),Ni/γ-Al2O3(R) and Ni-Ce/γ-Al2O3(R),are rationally designed and prepared by a classical incipient wetness impregnation method.The difference in the catalytic performance is originated from the chemical speciations of active sites in the catalysts.Largerparticles mixed with [Ni-O-Sin]) on the Ni/SiO2(R)make excessive CH4decomposition,leading to poor activity and stability.After doping Ce species,superior activity (59.0% CH4and 59.8%CO2conversions),stability and high H2/CO ratio(0.96)at 600 °C can be achieved on the Ni-Ce/SiO2(R),in comparison with other catalysts and reported studies.The improved performance can be ascribed to the formation of integral ([]-[CeIII-□-CeIII])species on the Ni-Ce/SiO2(R) catalyst,containing highly dispersed[]particles and rich oxygen vacancies,which can synergistically establish a new stable balance between CH4decomposition and CO2dissociation.As for the Ni/γ-Al2O3(R) and Ni-Ce/γ-Al2O3(R),the Ni (and Ce precursors) are easily captured by the extraframework Aln-OH groups and further form stable isolated ([]-[Ni-O-Aln]) (and [CeIII-O-Aln]) species.In this case,the mentioned two chemical species preferentially accelerate CO2dissociation,simultaneously together with the formation of more carbon deposition due to the CH4decomposition and the CO disproportionation.This deep distinguishment of chemical speciations of active sites species can guide us to further develop new efficient Nibased catalysts for LTDRM in the future.

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 is financially supported by the National Natural Science Foundation of China(22006059,21968015),National Engineering Laboratory for Flue Gas Pollutants Control Technology and Equipment (NEL-KF-201905),Applied Basic Research Program of Yunnan Province,China (202101AU070154,2019FD034),Analysis and Testing Fund of Kunming University of Science and Technology(2020 T20200006).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.08.027.