Surface Structure and Catalytic Performance of Ni-Fe Catalyst for Low-Temperature CO Hydrogenation

Catalysts 16NixFe/Al 2 O 3 (x is 0, 1, 2, 4, 6, 8) were prepared by incipient wetness impregnationmethod and the catalytic performance for the production of synthetic natural gas (SNG) from CO hydrogenation in slurry-bed reactor were studied. The catalysts were characterized by BET, XRD, UV-Vis DRS, H 2 -TPR, CO-TPD, and XPS, and the results showed that the introduction of iron improved the dispersion of Ni species, weakened the interaction between Ni species and support and decreased the reduction temperature and that catalyst formed Ni-Fe alloy when the content of iron exceeded 2%. Experimental results revealed that the addition of iron to the catalyst can effectively improve the catalytic performance of low-temperature CO methanation. Catalyst 16Ni4Fe/Al 2 O 3 with the iron content of 4% exhibited the best catalytic performance, the conversion of CO and the yield of CH 4


Introduction
The production of synthetic natural gas (SNG) from coal or biomass has attracted much attention in recent years, especially in China, because of the increasing demands for natural gas and the wish for enhancing domestic energy security [1,2].The gasification of coal results in the production of syngas containing mainly H 2 and CO; after the gas cleaning, the H 2 /CO ratio of the syngas can be adjusted to suitable ratio by water-gas shift reaction (WGSR) and finally the syngas can be converted to SNG through CO methanation.Among the SNG production processes, the methanation of syngas is one of the most critical steps, and the methanation reaction is a strong exothermic reaction; for every 1% of CO completely converted into CH 4 , gas adiabatic temperature rise is approximately 70 ∘ C [3].Thus, the major challenge is to remove in time the highly exothermic heat effectively.Recently, many efforts have been made to develop a number of methanation reactors for SNG production, including fixed-bed [1,4], fluidized-bed [5][6][7], and slurrybed reactors [8,9].Among them, the slurry-bed reactor can remove the reaction heat efficiently and keep the reaction at isotherm low temperature so as to minimize the catalyst deactivation and avoid the limitation on CH 4 yield resulting from thermodynamic equilibrium [9,10].But to develop an active and stable low-temperature methanation catalyst for a slurry-bed reactor is more difficult, and now, only a few studies have been done in the laboratory [11,12].
Numerous catalysts have been used for fixed-bed methanation reaction [9].Among these catalysts, Ni-based catalysts have been widely employed due to the relatively low cost, high activity, and high selectivity to methane [13][14][15][16].It is known that conventional Ni-based catalysts showed high catalytic activity at the high reaction temperature; however, the catalytic activity of CO conversion and CH 4 selectivity was low at the low reaction temperature.To overcome this problem, addition of second metal such as Sm, Ce, and Fe has been attempted to enhance the catalytic activity and stability of Ni-based catalysts for CO methanation reaction, because the second metal can improve the dispersion of active metal and adjust the interaction between Ni and the support [17][18][19][20].Hwang et al. [18,19] prepared nickel-M-Al xerogel (NiMAX) catalysts doped with different second metal (M = Fe, Ni, Co, Ce, and La) by sol-gel method, and they found that NiFeAX catalyst exhibited the best catalytic performance for CO or CO 2 methanation at low reaction temperature (220 ∘ C and 230 ∘ C).Therefore, the investigations over the Ni-Fe catalysts for the low-temperature CO methanation in slurry-bed would be worthwhile.
Our previous work [21,22] has optimized the nickel precursor and Al 2 O 3 support of Ni-Fe/Al 2 O 3 catalysts for CO methanation.In this work, a series of Ni-Fe/Al 2 O 3 catalysts with different Fe content were prepared by incipient wetness impregnation method and characterized by BET, XRD, UV-Vis DRS, H 2 -TPR, CO-TPD, and XPS.The effects of Fe content on surface structure and catalytic performance of the catalysts for low-temperature CO methanation in slurry-bed reactor were studied.The BET specific surface area, pore volume, and the average pore diameter of samples were measured by the multipoint BET analysis method using N 2 adsorption isotherms at 77 K.The measurement was performed on Micrometrics ASAP 2100 automatic device.

Experiment
X-ray photoelectron spectroscopy (XPS) data were collected on Thermo Fisher ESCALAB 250Xi with Al  radiation.The binding energies were calibrated by the carbon (C1s = 284.8eV).
Ultraviolet-visible diffuse reflectance spectra (UV-VIS DRS) were performed at room temperature in the range of 250∼800 nm by a CARY 300 spectrophotometer, using MgO as the reference material.
Temperature-programmed reduction of H 2 (H 2 -TPR) and temperature-programmed desorption of CO (CO-TPD) experiments were carried out with Micrometrics Autochem II 2920 model multifunctional adsorption instrument.Prior to the TPR measurements, 20 mg of the samples was pretreated at 350 ∘ C for 0.5 h in flowing He (50 mL/min), after cooling the reactor to room temperature, a 10% H 2 /90% Ar (50 mL/min) gas mixture was introduced, and the sample was heated to 800 ∘ C. Prior to the TPD measurements, 40 mg of the samples was reduced at 550 ∘ C and then cooled to 50 ∘ C. 10% CO diluted in He was passed though the sample until saturation, and the TPD profile was obtained by heating the sample from 50 ∘ C to 800 ∘ C.

Catalytic Activity Evaluation.
Catalytic performance evaluation was carried out in a 250 mL slurry-bed agitated autoclave.In each experiment, 2.0 g of catalyst (100-140 mesh particle) and 120 mL of paraffin (boiling point higher than 350 ∘ C) were added into the reactor vessel and rotated at 750 r/min.N 2 (99.995%) was introduced into the reactor at a flow rate of 50 mL/min to remove the air and the reactor was heated at a rate of 2 ∘ C/min from room temperature to 280 ∘ C.Then, the syngas of H 2 (99.995%) and CO (99.9%) was switched as feed gas at 75 mL/min and 25 mL/min, respectively, to synthesize methane at 1.0 MPa with the space velocity of 3000 mL/g cat ⋅ h.The products from the reactor were condensed by ethanol at 2 ∘ C, the cooling liquid was removed by gas-liquid separator, and the outlet gas was quantitatively analyzed by on-line gas chromatography (Agilent 7890A), which was equipped with flame ionization detector (FID) and thermal conductivity detector (TCD), using He as carrier gas.The conversion of CO, the selectivity of CH 4 , CO 2 , and C 2∼4 , and the yield of CH 4 were calculated using the following expressions: where The introduction of iron in 16NiFe/Al 2 O 3 catalyst greatly improved the catalytic performance.A rise in iron content greatly enhanced CO conversion, reaching the maximum for the catalyst containing 4% iron content with outstanding conversion of CO and the yield of CH 4 at 97.2% and 84.9%, respectively.Further rise in iron content resulted in the decrease of catalytic performance.It is well known that Fe-based catalysts show high water gas shift activity, and it can be seen that the increase of iron content aggravated the water gas shifted reaction [23], resulting in the more production of CO 2 .In addition, the selectivity of C 2∼4 changed slightly with the increase of iron content except the catalyst with 1% iron content.

BET.
Textural properties of -Al 2 O 3 support and 16NiFe/Al 2 O 3 ( = 0, 1, 4, 8) catalysts are summarized in Table 2.The -Al 2 O 3 support got a high BET surface area of 191 m 2 /g, and the pore volume and the average pore diameter were 0.38 cm 3 /g and 5.81 nm, respectively.After the loading of 16% Ni, part of nickel species began to aggregate and formed crystalline nickel oxide clusters, which may block the pores of -Al 2 O 3 and thus make the BET surface area of catalyst 16Ni/Al 2 O 3 decreases to 148 m 2 /g [4,24]; in addition, both the pore volume and average pore diameter of catalyst 16Ni/Al 2 O 3 decreased.However, the catalysts 16NiFe/Al 2 O 3 ( = 1, 4, 8) modified with iron showed a slightly higher BET surface area compared with 16Ni/Al 2 O 3 catalyst, implying that the addition of iron improved the dispersion of Ni species and decreased the agglomeration; similar results were also reported in other literatures [4,24,25].( ≥ 2) exhibited lower angles by increasing the iron content; although this change was not obvious, it is an effect of a significant interaction between iron and Ni, and the reason was due to the formation of Ni-Fe alloy, which appeared at 44.3 ∘ , 51.5 ∘ , and 75.9 ∘ [20,26], and the intensity of metal Ni decreased, indicating that the crystallite size of Ni decreased.Combined with the catalytic performance listed in Table 1, the results suggested that the formation of Ni-Fe alloy must be an important factor for CO methanation reaction over the Ni species that existed in the catalysts.600 nm.Compared with catalyst 16Ni/Al 2 O 3 (329 nm), the absorption bands of 16NiFe/Al 2 O 3 catalysts in the spectra were all red-shifted with the increase of iron content, and the maximum absorption band at 353 nm was found on catalyst 16Ni4Fe/Al 2 O 3 , indicating that there was a strongly interaction between Ni species and iron species [28].While for the catalyst 8Fe/Al 2 O 3 , the absorption band was exhibited at 534 nm, which can be ascribed to the existence of Fe 2 O 3 species [29].Catalysts 16NiFe/Al 2 O 3 with the iron content above 4% also exhibited the same absorption band position at 534 nm, except for the absorption band in the range of 300∼400 nm.The results indicated that part of iron existed as Fe 2 O 3 , although it was not detected by XRD patterns in Figure 1(a).(peak II) and the reduction peak of -NiO, which had a strong interaction state with the support; additionally, the reduction peak in the range of 550∼750 ∘ C was mainly attributed to the reduction of NiAl 2 O 4 spinel [31,32].

UV-Vis DRS.
After the introduction of iron, the reduction peaks of 16NiFe/Al 2 O 3 shifted to lower temperature, and the total reduction peak areas increased gradually with the increase of iron content, indicating that the addition of iron facilitated the reduction of catalysts.It is believed that the addition of iron weakened the interaction between Ni and Al 2 O 3 support, changed the microchemical environment of Ni and iron species on the surface of the catalyst, and thus increased the amount of reduction species [33][34][35].
In order to clarify the effect of iron content on the reduction of Ni species, Gaussian fitting analysis was conducted, and the reduction peak area of Ni species was obtained by subtracting the reduction peak area of corresponding Fe content from the total hydrogen consumption of each catalyst; the results are shown in Figure 3 and Table 3.According to the iron content of 16NiFe/Al 2 O 3 catalysts and taking the hydrogen consumption of 8Fe/Al 2 O 3 catalyst as a benchmark, it can be seen that the total hydrogen consumption increased with the increase of iron content, which was due to the reduction of Fe species and the Ni species.Catalyst 16Ni/Al 2 O 3 got the smallest amount of  reduction peak, and the amount of  peak increased gradually with the increase of iron content, while the amount of  reduction peak decreased, indicating that the increase of iron content improved the reduction species of NiO.Among the catalysts examined, when the iron content reached 4%, the amount of  reduction peak reached the maximum (45.3%); further increase of iron content resulted in the decrease of  peak amount.The results were consistent with the catalytic performance of catalysts 16NiFe/Al 2 O 3 ; thus, it can be concluded that the -NiO species was the main reason for enhancing the catalytic activities of CO methanation.Similar results have been reported in Hu et al. 's study [15].In addition, it can be seen that the reduction peak area of peak I of catalyst 8Fe/Al 2 O 3 was larger than peak II, and the shape of peak I was sharp, while peak II was broad, indicating that the reduction degree of Fe 2 O 3 to Fe 3 O 4 was high and the reduction rate was fast, while the reduction It is reported that Fe 3 O 4 species was the main active component of water gas shift reaction [23], indicating that the catalyst with larger content of Fe 3 O 4 favored more CO 2 production, and the results were in agreement with the catalytic performance listed in Table 1.

CO-TPD.
Figure 4 shows the CO-TPD profiles of 16NiFe/Al 2 O 3 catalysts.All the catalysts exhibited a lowtemperature peak centered at ∼85 ∘ C, which was attributed to the desorption of weak chemisorption or physical adsorption of CO.The weak peaks centered at around 265 ∘ C could be due to desorption of the middle intensive CO adsorption, and the highest desorption temperature shoulder spanning from 400 to ∼500 ∘ C was probably attributed to the strong adsorption of CO [36].It also can be seen that catalyst 8Fe/Al 2 O 3 exhibited almost no desorption peaks, which means that the adsorption of CO of catalysts 16NiFe/Al 2 O 3 was mostly attributed to the active component Ni and that the increase of CO adsorption amount was attributed to the increase of Fe content and the interaction between Fe and Ni.The former results of XRD, BET, and H 2 -TPR showed that the introduction of Fe improved the dispersion of Ni and increased the BET specific surface area and the amount of active component Ni which facilitated the adsorption of more CO.were 856.1 eV and 855.9 eV, respectively, which were higher than that pure NiO (854.4 eV) and NiAl 2 O 4 (856.2eV) [37], indicating that there might be two kinds of Ni species (NiO and NiAl 2 O 4 ) on the surface of catalysts.The large amount of NiO could be formed by reoxidation of metallic Ni through contacting oxygen in air during the XPS experiments [38].It could be noted that the binding energy of the Ni 2p 3/2 peak of 16Ni/Al 2 O 3 sample was slightly higher than that of 16Ni4Fe/Al 2 O 3 , which could be attributed to the stronger interaction between Ni and support in 16Ni/Al 2 O 3 [39].

Conclusions
The catalytic performance of catalyst 16NiFe/Al 2 O 3 for CO methanation in slurry-bed reactor was greatly influenced by the content of iron.The introduction of iron improved the dispersion of Ni species and decreased the reduction temperature, and Ni-Fe alloy was formed when the content of iron exceeded 2%, which was favorable for the methanation of CO.With the increase of iron content, the catalytic performance of CO methanation increased at first and then decreased.Excessive iron content covered the active component Ni and aggravated the water gas shift reaction.
Figure 1(a) shows the XRD patterns of calcined catalysts 16NiFe/Al 2 O 3 .The diffraction peaks of -Al 2 O 3

Figure 2
depicts the UV-Vis DRS of calcined 16NiFe/Al 2 O 3 catalysts.The absorption band position was determined by the first-derivative of absorption bands[27].It can be seen that all catalysts showed the absorption bands (optical absorption threshold) in the range of 300∼

3. 5 .
H 2 -TPR.The H 2 -TPR profiles of 16NiFe/Al 2 O 3 catalysts are shown in Figure 3.For comparison, the TPR profile of catalyst 8Fe/Al 2 O 3 is also presented in Figure 3.The

Figure 5
shows the Ni 2p 3/2 core level spectra of reduced catalysts 16Ni/Al 2 O 3 and 16Ni4Fe/Al 2 O 3 .The binding energies of Ni 2p 3/2 for 16Ni/Al 2 O 3 and 16Ni4Fe/Al 2 O 3 2.1.Catalyst Preparation.The commercial -Al 2 O 3 (100-140 mesh, supported by Shandong Alumina Company) precalcined at 550 ∘ C for 6 h was used in this study.The Ni-based catalysts were prepared by incipient wetness impregnation method, described as follows: nickel nitrate hexahydrate (Sinopharm Chemical Reagent Co., Ltd) and ferric nitrate nonahydrate (Sinopharm Chemical Reagent Co., Ltd) were dissolved in deionized water, followed by the addition of -Al 2 O 3 : the slurry was continuously stirred at room temperature for 24 h and then dried at 120 ∘ C for 12 h and calcined at 450 ∘ C for 4 h in muffle oven; and the obtained sample was reduced at 550 ∘ C for 6 h in a flow of 25% H 2

Table 1 :
Catalytic performance of 16NixFe/Al 2 O 3 catalysts for CO methanation in slurry-bed reactor.Catalytic Performance of 16NiFe/Al 2 O 3 Catalysts for CO Methanation.The catalytic performance of 16NiFe/Al 2 O 3 catalysts with different iron content for low-temperature CO methanation in slurry-bed reactor is listed in Table1.The reference catalyst 16Ni/Al 2 O 3 showed the conversion of CO and the yield of CH 4 at 82.5% and 72.1%, respectively, which were much higher than that of catalyst 8Fe/Al 2 O 3 .However, the catalytic performance of the above two catalysts were lower than the commercial catalyst J108-2Q.
CO ,  CH 4 ,  C ,  CO 2 , and  CH 4 refer to the conversion of CO; the selectivity of CH 4 , C 2∼4 , and CO 2 ; and the yield of CH 4 , respectively.C 2∼4 represents C 2 H 4 , C 2 H 6 , C 3 H 6 , C 3 H 8 , C 4 H 8 , and C 4 H 10 . and  cat represent the flow of CO, C 2∼4 , CH 4, and CO 2 (mL/min, STP) and the weight of catalyst, respectively.

Table 2 :
Textural properties of -Al 2 O 3 support and 16NiFe/Al 2 O 3 catalysts.Al 2 O 3 .It can be seen that all the reduced catalysts exhibited the characteristic peaks of Ni at 2 angle of 44.3 ∘ , 51.8 ∘ and 76.0 ∘ , indicating that NiO was reduced to metallic Ni.The diffraction peaks due to iron species were not detected.It is noted that, compared with reference catalyst 16Ni/Al 2 O 3 , the diffraction peak of catalysts 16NiFe/Al 2 O 3 ∘ , 45.8 ∘ , and 66.9 ∘ , and characteristic diffraction peaks due to NiO were observed at 2 of 37.4 ∘ , 43.6 ∘ , and 63.1 ∘ , and no diffraction peaks of nickel nitrate hexahydrate were observed, indicating that Ni species in 16NiFe/Al 2 O 3 catalyst were mainly presented as NiO.It is interesting to note that the characteristic peaks of NiO gradually decreased with the increase of iron content.In addition, no diffraction peaks of iron species were observed in XRD patterns even if the content of iron reached 8%.Figure1(b) displays the XRD patterns of reduced catalysts 16NiFe/

Table 3 :
Gaussian fitting analysis of H 2 -TPR patterns of 16NixFe/Al 2 O 3 catalysts.The value calculated by different Fe content based on the H 2 -consumpution of 8Fe/Al 2 O 3 catalyst.As expected, catalyst 16Ni4Fe/Al 2 O 3 possessed slightly higher Ni/Al atomic ratio (0.085) than that of 16Ni/Al 2 O 3 (0.082), indicating that the introduction of Fe improved the dispersion of Ni species on the surface of catalyst 16Ni4Fe/Al 2 O 3 , which was in accordance with the results of XRD, H 2 -TPR, and BET. a