MgO-Al2O3 Catalyst for

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Synthesis of highly dispersed nano-sized NiO/MgOAl2O3 catalyst for the production of synthetic natural gas with enhanced activity and resistance to coke formation ahmad ebadi, Somayeh Tourani, and Farhad Khorasheh Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01878 • Publication Date (Web): 28 Aug 2018 Downloaded from http://pubs.acs.org on September 3, 2018

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Industrial & Engineering Chemistry Research

Synthesis of highly dispersed nano-sized NiO/MgO-Al2O3 catalyst for the production of synthetic natural gas with enhanced activity and resistance to coke formation

Ahmad Ebadi†, Somayeh Tourani*†, and Farhad Khorasheh‡

†

Department of Chemical Engineering, Mahshahr branch, Islamic Azad University, Mahshahr, Iran

‡

Department of Chemical and Petroleum Engineering, Sharif University of Technology, Tehran, Iran

Corresponding Author *E-mail: ([email protected])

1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT: Nickel nanoparticles supported on MgO-Al2O3 and Al2O3 were synthesized by an impregnation method using dinitrobisethylenediamine nickel and nickel nitrate hexahydrate as precursors and were used as catalysts for CO methanation. Different MgO contents (1.5-11.25 wt. %) were employed for the preparation of supports, and NiO loadings were in the range of 10 to 40 wt. %. The optimum catalyst prepared from proper amounts of MgO (≈2 wt. %) and NiO loading (20 wt. %.) with [Ni(en)2(H2O)2](NO3)2 as precursor and a mesoporous support with a wide range of mesopores, resulted in highly dispersed nickel nanoparticles that exhibited moderate metal-support interactions, lower acidic surface sites, and enhanced performance over the entire range of operating conditions. The optimum catalyst also showed a high activity and resistance to coke formation and sintering as compared with catalysts prepared with either MgO or [Ni(en)2(H2O)2](NO3)2. Complete CO conversion could be achieved at 300oC for this catalyst where other prepared catalysts had achieved about 60% CO conversion. KEYWORDS: CO Methanation, Ni-based catalyst, Ni(II)-ethylenediamine precursor, Carbon deposition resistance, MgO-Al2O3,

1. INTRODUCTION The CO methanation reaction was first investigated by Sabatier and Senderens in the early 20th century.1 Some of its industrial applications include elimination of CO from the feed for ammonia synthesis to prevent catalyst poisoning and in fuel cells to avoid poisoning of anodes,2-5and in production of synthetic natural gas (SNG) from syngas produced from coal,6-9 biomass,9,10 petroleum coke, and municipal solid wastes.9 There is currently a growing interest in the production of SNG from these feedstocks due to high energy consumption of many industries, increasing price of natural gas, lower CO2 emission, and higher energy savings.9-12 The methanation reactions 1 and 3 can occur simultaneously with other side reactions given below:13,14

CO + 3H2 ⇔ CH4 + H2O

∆HºR = - 206 kJ/mol

(1)

2CO + 2H2 ⇔ CH4 + CO2


(2)

CO2 + 4H2 ⇔ CH4 + 2H2O


(3)


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CO + H2O ⇔ CO2 + H2


(4)

2CO ⇔ CO2 + C

∆HºR = -172 kJ/mol

(5)

CH4 ⇔ C + 2H2

∆HºR = 73 kJ/mol

(6)

All of the above reactions, with the exception of reaction (6), are highly exothermic which could cause over-heating due to a high rate of heat release leading to catalyst deactivation. Methanation of

syngas is an important step of SNG processes and higher methane selectivity, lower carbon formation, and lower rate of catalyst sintering at high temperatures are important process objectives. Many investigations have therefore focused on the development of appropriate methanation catalysts. Different metals including Ru, Ir, Rh, Ni, Co, Pt, and Fe supported on Al2O3, SiO2, TiO2, ZrO2 or mixed oxides can catalyze the CO methanation reaction.15-23 Although both noble metals and nickel exhibit high methane selectivity, supported nickel catalysts are most attractive due to their low cost compared with noble metals.2,15,17,19,21,22 Loss of metal surface area by poisoning, coking, or sintering, however, remains a serious problem in a number of processes involving Ni catalysts.24,25 Several methods have been used for synthesis of alumina-supported nickel catalyst including co-precipitation,26 sol gel,27 and impregnation.28 Compared with other preparation methods, catalysts prepared by the conventional impregnation method often result in lower metal dispersion and active surface sites due to the agglomeration of nickel oxide particles on alumina.28 Furthermore, some of the nickel ions migrate to alumina leading to the formation of a nickel aluminate phase that is not reduced and remain inactive in the hydrogenation reaction.29-31 The use of ethylenediamine nickel complex and subsequent decomposition of the nickel precursor in an inert atmosphere during the preparation of nickel supported catalysts was reported to enhance the CO hydrogenation catalytic performance.31,32 However, there are no reports discussing the effects of catalyst preparation parameters such as the type of support, NiO loading, or using mixed oxides as support on the catalytic performance of catalysts prepared from this nickel



precursor for CO methanation. It is also well established that addition of an alkaline earth oxide such as MgO can promote the CO methanation reaction.33-36 This additive, however, has been reported to have no effects on the structural features of the support or on the active phase dispersion.34,37 In our current study, we focus on combining the special advantages of both of these modifications simultaneously to investigate their effects on the properties of the mesoporous support and on the characteristics and performance of the prepared catalysts for CO methanation that has not been reported in the open literature. The present work aims at preparing alumina supports with different MgO contents. The supports were then used to prepare supported NiO catalysts with different NiO loadings using Ni(II)-ethylenediamine as precursor. The performance of the synthesized catalysts in CO methanation was then compared with Ni/Al2O3 and Ni/MgO-Al2O3 catalysts prepared by the conventional methods. The performance of the optimum synthesized catalyst was also compared with the commercial catalyst for CO methanation under different operating conditions. This study reveals the challenges in simultaneously using MgO as a support additive, Ni(II)ethylenediamine as nickel precursor, and a mesoporous support for the synthesis of Ni/MgOAl2O3 catalyst and proposes a procedure to obtain a nano-sized catalyst with high nickel dispersion, enhanced activity and stability, and excellent resistance to coke formation.

2. EXPERIMENTAL 2.1. Catalysts preparation 2.1.1. Supports preparation method Mg-Al precursors with different Mg/Al atomic ratios were prepared by coprecipitation of an aqueous solution of Mg(NO3)2.6H2O and Al(NO3)3.9H2O containing the desired Mg/Al ratio at a constant pH of 9±0.5. A solution with a total [Mg+Al] cation concentration of 0.4 M was contacted with a basic solution of NaOH (2 M) and Na2CO3 (1 4 ACS Paragon Plus Environment

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M) by dropwise addition of both solutions into a stirred beaker containing 400 ml of deionized water held at 60oC. The precipitates were aged in their mother liquor for 3 h at 60oC and then filtered, washed with boiling deionized water and dried at 110oC overnight. These hydroxycarbonate precursors were decomposed at 560oC for 5 h with 5oC/min heating rate in order to obtain the corresponding Mg-Al mixed oxides supports (MgO contents 2.5, 6.25 and 12.5 wt. % of calcined support). Al2O3 support was prepared by similar precipitation with Al(NO3)3.9H2O aqueous solution except that pH of precipitation solution was constant at 8±0.5. The characteristics of different supports (S1 to S4) are presented in Table 1. 2.1.2. Preparation of [Ni(en)2(H2O)2](NO3)2 precursor salt The Ni(II) precursor salt was prepared by adding ethylenediamine (en) at a molar ratio en/Ni=2 to a hot methanol solution of nickel nitrate hexahydrate 2 M. After 10 min of magnetic stirring, the solvent was eliminated under vacuum at 70oC to obtain the crystals.38 2.1.3 Preparation of supported catalysts The catalysts were prepared by impregnation deposition method. Solutions of the nickel complex were prepared by adding the appropriate amounts of Ni(II)-ethylenediamine precursor salt to 50 ml of deionized water. The solution concentration was adjusted to obtain 10, 20 and 40 wt. % NiO for calcined catalysts. The calcined supports which were dried at 160oC for 90 min were slowly added to the solution under rapid stirring. After 30 minutes, the slurry was dried in a water bath to obtain the dried cakes and then, were dried in an air oven for 18 h at 120oC and subsequently calcined in a muffle furnace at 520oC for 4 h with 5oC/min heating rate. Commercial nickel nitrate hexahydrate solution was used for impregnation of S1 and S4 supports to obtain catalysts (S1N2 and S4N2). Properties of the prepared supports and catalysts and the corresponding sample codes are given in Table 1.



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2.2. Catalysts characterization Various analytical methods were carried out to investigate the physicochemical properties of the prepared catalysts and the details are presented in the supporting information. 2.3. Catalyst performance tests The performance of the prepared catalysts was evaluated for methanation of CO. Reactor tests were carried out in a fixed bed quartz reactor of 10 mm inner diameter. The amount of catalyst used was 0.1 g that was diluted with 0.1 g of quartz sand. The catalyst powder was crushed to obtain minus 200 mesh particles and reduced at 500oC for 1 h in a continuous flow of pure H2 (40 Nml/min). The mixed reactant gas consisted of H2/CO with a molar ratio of 4.5, 3, and 1. The inlet CO contained 2.2 mol% N2 which was used as an internal standard for analysis of the products. Gas hourly space velocity (GHSV, Nml(gas)/g(catalyst)·h) was selected to be 80000, 160000, and 320000 at 0.1 MPa and the reaction temperature was in the range of 200-500oC. The outlet gas stream was cooled to separate water by a cold trap before analyzing the product gases. Once the reactor had achieved steady-state operation, product gases were analyzed by a Varian Star 3400 gas chromatograph to determine CO conversion and CH4 selectivity. The contents of N2, CH4, and CO in the outlet gas were analyzed by a thermal conductivity detector (TCD). The carbon monoxide conversion (XCO), methane selectivity (SCH4), and methane yield (YCH4) are defined by equations (7), (8), and (9), respectively. For these calculations, the molar flow of each component, Fi, and the total output must be known. The outlet flow rates Fi,out of CH4 and CO were calculated using equations (10) and the known N2 inlet flow rate.

XCO =

F CO , in − F CO , out F CO , in


(7)

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SCH4 =

FCH 4 , out F CO , in − F CO , out

(8)

FCH 4, out FCO , in

(9)

FN 2 , in * xi , out xN 2 , out

(10)

YCH4 =

Fi,out =

Where Fi,in (Nml/min) and Fi,out (Nml/min) are the inlet and outlet volumetric flow rate of species i (i = CO, N2, and CH4), respectively, and xi, out is mole fraction of species i in reactor outlet gas obtained from GC analysis.

3. RESULTS AND DISCUSSION 3.1. Effect of MgO and [Ni(en)2(H2O)2](NO3)2 on catalyst properties Nitrogen adsorption-desorption isotherms and PSD curves of different supports are shown in Figure 1A. S1 was found to exhibit H3 type hysteresis loop between p/p0 = 0.4-0.9. This hysteresis is usually observed for solids consisting of aggregates or agglomerates of particles forming slit shaped pores. S4 was found to exhibit H2 hysteresis which is attributed to mesoporous solids consisting of particles crossed by nearly cylindrical channels or made by aggregates (consolidated) or agglomerates (unconsolidated) of spheroidal particles39 and also to pore-blocking.40,41 Both isotherms have type IV profile that can be ascribed to mesoporous structures and pore condensation. As indicated in Table 1, the surface area, pore volume, and pore diameter are greater in S1 support as compared with S4. XRD patterns of S1, S4, S4N2, S4N2en, S1N2, and S1N2en are presented in Figure 2A. The XRD patterns showed that both S1 and S4 exhibited similar diffraction patterns with main peaks at 2θ of 37.6o, 45.9o, and 67.0o corresponding to (311), (400), and (440) planes of γ-Al2O3 (JCPDS 00-010-0425), respectively. Although NiAl2O4, MgAl2O4 and γ-Al2O3



showed very similar diffraction peaks, a difference also existed. A simple method that has been suggested to determine whether or not the MgAl2O4 crystalline spinel is formed is based on the comparison of the relative intensities of the (400) and (311) peaks. A pure stoichiometric spinel should have X=I(400)/I(311) =0.65 (according to JCPDS Card No. 211152), while X>0.65 indicates a solid solution of γ-Al2O3 and MgAl2O4.42 For S1 with addition of MgO into the support, no additional diffraction peaks could be clearly observed indicating that all of MgO was highly dispersed as small particles. Pure crystalline spinel (MgAl2O4) was also not observed for S1 support maybe due to the low magnesia content43,44 and a low calcination temperature.45 As can be seen in Figure 2A, the supports impregnated with the nickel nitrate hexahydrate precursor, S1N2 and S4N2, showed small new peaks at 2θ = 37.3o, 43.3o, and 63o corresponding to NiO (111), (200) and (220) reflections (JCPDS 750197), respectively, and the peak at about 45.1o may be assigned to the (400) diffraction of spinel phase (JCPDS 10-0339). Surprisingly, XRD patterns of the S4N2en and S1N2en that were prepared with the [Ni(en)2(H2O)2](NO3)2 precursor showed different peaks intensity for NiO. The S4N2en catalyst showed three intense peaks which corresponded to the formation of larger NiO particles with a size of about 8.5 nm (determined by Scherrer's equation). In contrast, the S1N2en did not show the clear diffraction peaks of NiO at 37.3o and 43.3o implying that the NiO particles formed on the catalyst surface were small and more highly dispersed as compared with those for other catalysts. In addition, the position of NiAl2O4 and MgAl2O4 diffraction peaks were slightly lower than those for γ-Al2O3 and that the alumina peaks (45.9o and 67.0o) were shifted to slightly lower values (about 45.3o and 66o) for S1N2en and S1N2 catalysts possibly due to the presence of nonstoicheiometric Ni-Mg-(Al)-O solid solutions.33,46 Figure 2B shows the H2-TPR profiles for the calcined catalysts. H2-TPR was carried out to investigate the interaction between the NiO and the support. The reducible Ni2+ is


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classified into four types: α, β1, β2, and γ. Type α species with reduction temperature of 300530ºC were assigned to bulk or free NiO with no direct interaction with support (for the peaks about 300ºC)29,47 and NiO particles having a weak interaction with the support and reduced easily.48 Type β1 species with reduction temperature of 570-640oC corresponded to the NiO species with stronger interaction with the support than type α and were attributed to the NiO in Ni-rich mixed oxide phase. Type β2 species with reduction temperature of 697-737oC were related to NiO in Al-rich mixed oxide phase.49 Type γ species with reduction temperature above 750oC were assigned to nickel aluminate spinel in which the nickel species were strongly associated with the spinel structure.29,47,49-51 The S4N2 catalyst showed a small reduction peak at 380oC which could be ascribed to the reduction of α-type NiO, and a significant reduction peak at 660oC that corresponded to the NiO species between types β1 and β2. The main reduction peak of the S1N2 showed a significant decrease to 580oC as compared with S4N2, and a smaller broad peak at 460oC that can be attributed to the decreasing binding energy of Ni in the NiO/Al2O3 catalyst in presence of MgO as the Ni-Al interactions are significantly weakened in the NiO/MgO-Al2O3 catalysts.34,37 The main reduction peak of S4N2en was detected at 449oC which was assigned to αtype NiO and the second large intense peak at 315oC that could be ascribed to the reduction of bulk or free NiO which showed more reducibility as compared with the other two catalysts. This temperature decrease can be justified by scrutinizing the catalyst preparation method. The pH of impregnating solution was 8.2 for [Ni(en)2(H2O)2](NO3)2 that is close to alumina’s point of zero charge52,53 suggesting that electrostatic interactions between complexes and support can be negligible during impregnation. During catalyst preparation, capillary action draws the solution into the pores of the dry support and pore volume impregnation occurs. During deposition, a small quantity of mono-(diamine) complex (a minor species at equilibrium in the solution) is deposited onto the surface of the support. Due to the presence



of excess solution (more than pore volume of the support) used for impregnation and the gradual elimination of water during the impregnation step, the remaining salts are deposited on the surface of the support. During the drying stage, bis-(diamine) complexes deposit onto alumina.54 As indicated earlier, calcination is carried out in a muffle furnace in static air atmosphere with the catalyst placed in a lidded crucible to avoid air flow over the catalyst. Therefore ethylenediamine ligands are partially decomposed by nitrate ions.55 The nitrates that are also grafted on alumina have not chemically reacted and at higher temperatures, they react with the ligands and destroy them by oxidation.54 The results indicate that using [Ni(en)2(H2O)2](NO3)2 as catalysts precursor leads to lower reduction temperature. On the other hand, the main reduction peak of S1N2en was detected at 546oC that is between types α and β1 NiO and also a small peak that could be observed at 287oC would suggest the presence of small amounts of free NiO in catalyst. These results reveal that by using Ni(II)-ethylenediamine as precursor along with small amounts of MgO, while the reducibility does not increase, moderate interactions appear between metal and support. The large NiO particles in S4N2en with low interactions with the support (free NiO) also disappeared in S1N2en as shown in the XRD patterns and H2-TPR profile (Figures 2A and 2B) indicating higher uniformity for distribution of small size NiO nanoparticles on the support. These results can be explained in terms of the catalyst preparation method and supports properties. It is known that, the viscosity of [Ni(en)2(H2O)2](NO3)2 impregnation solution is higher than the nickel nitrate hexahydrate solution and the high viscosity could be due to the presence of colloids in the former solution.56 The high viscosity of the [Ni(en)2(H2O)2](NO3)2 impregnation solution would lead to the slow diffusion of the ions in the solution resulting in the formation of large crystals on the support surface during the deposition and subsequent crystal growth during the elimination of water. Therefore viscosity appears here to be a factor that hinders the transport of the precursor solution. After drying,


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crystallization would occur over the pore mouth for the larger crystals and at the pore mouth for the smaller crystals.56 As seen in Table 1, the lower surface area of 289 m2/g, pore volume of 0.44 m3/g, and especially average pore diameter of 6.05 nm for S4 (narrow pore size distribution for S4 and a wide pore size distribution for S1 as shown in Figure 1B) may lead to slower diffusion of the ions within the pores and thus a lower fraction of crystals would form inside the pores as compared with S1 having a surface area of 312 m2/g, pore volume of 0.7 m3/g, and average pore diameter of 8.98 nm. The finite surface area around the pore mouth would lead to the formation of some larger crystals with very weak interactions with the support surface. The higher fraction of pores with larger diameter in S1 can also be seen in Figure 1B which indicates maximum pore diameter of about 12 nm for S4 but close to 60 nm for S1. The larger crystals would form in the large pores close to the support surface and separated from other crystals. The pore size distributions also indicate a wider distribution for S1N2en catalyst compared with S4N2en catalyst similar to their corresponding support. Metal precursor distribution on S1 support surface was, therefore, more uniform due to the higher surface area of support and more accessible surface area within the pores. Furthermore, the splitting of the crystals during the exothermic combustion of the ligands by nitrates during the thermal treatment prior to reduction55,56 would also lead to the formation of highly dispersed NiO nanoparticles (in agreements with XRD results) with moderate interaction with the support for S1N2en catalyst. A question that remains is whether similar results could be achieved by using an alumina support with a wider pore size distribution containing larger diameter pores. To address this question, a commercial γ-alumina with nominal pore volume of 0.7 m3/g and pore diameter of 13 nm was impregnated by the same method. XRD patterns of γ-ALN2en (Figure 2A) shows smaller peaks for NiO corresponding to smaller NiO particles of 5.6 nm in size as compared with 8.5 nm for S4N2en catalyst. The pore size distribution for γ-ALN2en catalyst was also much wider as compared with S4N2en



catalyst as illustrated in Figure 1B. Furthermore, as shown in H2-TPR profiles (Figure 2B), the smaller peak at 298oC for γ-ALN2en indicates the presence of lower amounts of bulk NiO while the main reduction peak at 514oC corresponds to higher metal-support interactions as compared with S4N2en catalyst. These results support the above observations that lower surface area, pore volume, and pore diameter of the support could lead to the formation of free NiO and lower dispersion of NiO on the support when [Ni(en)2(H2O)2](NO3)2 is used as the nickel precursor in the impregnation deposition method. Metal support interactions were also higher in S1N2en catalyst as compared with [Ni(en)2(H2O)2](NO3)2 impregnated γAlumina due to the higher surface area and the presence of MgO in the former catalyst that led to higher NiO dispersion and the appearance of small amounts of non-stoichiometric NiMg-(Al)-O solid solutions for this catalyst as also supported from XRD results. The use of Ni(II)-ethylenediamine precursor also resulted in lower metal-support interactions as compared with the catalyst prepared from nickel nitrate hexahydrate impregnation on the MgO-Al2O3 support. These results point to the synergistic effect of MgO and dinitrobisethylenediamine nickel that led to the formation of highly dispersed NiO nanoparticles with moderate interactions with the MgO-Al2O3 support. The acidic properties of supported nickel catalysts were characterized by temperature programmed desorption (TPD) of NH3 adsorbed at room temperature in order to investigate the effect of MgO content and Ni salt precursors on catalyst properties. NH3-TPD curves are presented in Figure 3. Two types of acidic sites were observed in NH3-TPD curves of reduced NiO/MgO-Al2O3 catalysts.44,57 Bronsted acidity is attributed to the interaction of NH3 with surface hydroxyl groups, while Lewis acidity on the mixed oxides is attributed to nitrogen interaction with Al3+ cations in Al3+-O2--Mg2+ acid-base pair and for alumina, the Lewis acidity was more likely assigned to Al3+ cations in the abundant surface Al3+-O2− pairs. NH3TPD curves show a moderate (226-232oC) and a high temperature (333-356oC) peak that are


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attributed to Lewis acid sites. As can be seen in the Figure 3, the S1N2en catalyst showed a lower NH3 desorption peak compared with S1N2 catalyst which could be ascribed to the lower acidic surface. Both catalysts, however, were prepared from the same support having identical MgO content that is responsible for the basic sites on the catalyst surface. The lower acidic surface of the former may have resulted from the passivation of the acidic surface of alumina with highly dispersed nickel metal on the support surface and a high metallic surface area. The TEM images of the calcined catalysts are presented in Figure 4. The average NiO particle diameters were 4.8, 4.8, 5, 6.2, and 10.5 nm for S1N2en, S1N2, γ-ALN2en, S4N2, and S4N2en catalysts, respectively. NiO nanoparticles were found to be small and uniformly dispersed on the support for S1N2en (Figure 4A) as compared with other catalysts. The TEM images are in good agreement with the XRD results presented in Figure 2A. The results indicated that the NiO particles diameter, metal dispersion, and metal-support interactions were significantly influenced by the structural (especially pore diameter and the pore size distribution) and chemical properties of the support during the impregnation of [Ni(en)2(H2O)2](NO3)2 precursor. The use of an improper support may lead to non-uniform distribution of the NiO particles with large sizes without any or at best with low interactions with the support (formation of free NiO) that would render them unsuitable in catalytic reactions. Figure 5 shows CO conversion, methane selectivity, and methane yield for CO methanation over three different catalysts. Complete CO conversion was obtained at temperatures between 350 to 400oC for S4N2. Complete methane selectivity and methane yield from CO conversion was not achieved for any of the catalysts. This could be due to the low pressure (1 atmosphere) and relatively high GHSV and temperature (up to 500oC) conditions employed in this investigation under which the reverse methane CO2 reforming



reaction (reaction 2), water gas shift reaction (reaction 4), and the Boudouard reaction (reaction 5) compete with CO methanation reaction leading to CO2 and carbon formation.58 S4N2en, γ-ALN2en, and S1N2 show similar activity with a slightly higher methane selectivity and methane yield for S4N2en and γ-ALN2en compared with S1N2 at the same reaction temperatures. S1N2en catalyst was most active with complete CO conversion at about 300oC where others had achieved less than 60% CO conversion. These results are supported by the low reducibility of S4N2 as compared with other catalysts due to the presence of significant amounts of NiO species with strong NiO-Al2O3 interactions on the catalyst that were not reduced at the applied low reduction temperature of 500oC. The presence of small amounts of MgO could enhance the reducibility of NiO/Al2O3 leading to a higher concentration of active sites on the S1N2 catalyst. The highest reducibility when dinitrobisethylenediamine nickel was used as a precursor on Al2O3 support was obtained for S4N2en but the results were not significantly better than S1N2 possibly due to the appearance of free NiO with large particle sizes leading to lower NiO dispersion on the surface (as confirmed by TEM and XRD results) with lower catalytic activity.33 γ-ALN2en also had a similar catalytic performance compared with S4N2en albeit with a lower reducibility (Figure 2B). This is perhaps due to higher nickel dispersion with smaller particle sizes and lower amounts of free NiO for γ-ALN2en compared with S4N2en resulting in a similar content of accessible active sites on the surface of both catalysts. High catalytic activity for S1N2en can be attributed to higher surface area and the small size of highly dispersed nickel nanoparticles which was also confirmed by TEM images (Figure 4A) and XRD patterns (Figure 2A). Moreover, as indicated by the H2-TPR results, S1N2en had the moderate reducibility at the moderate temperature of 546oC (Figure 2B) leading to a high metallic surface area with lower acidic sites (as shown in Figure 3) exposure to reactants subsequent to a low reduction temperature of 500oC. Furthermore, the presence of rather basic and neutral sites on the


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surface of S1N2en led to higher methane selectivity by slowing down the methane cracking reaction (reaction 6)59 and Boudouard reaction (reaction 5) and by promoting the CO2 adsorption on the surface.37,43 These results indicate that the proper combination of the amounts of MgO and [Ni(en)2(H2O)2](NO3)2 could enhance the performance of NiO/Al2O3 catalysts to a greater extent than when each factor is applied individually. 3.2. Effect of MgO Loading The S1N1en catalyst was selected as a sample for chemical analysis by ICP to determine the agreement between actual and nominal chemical composition of the prepared catalysts. The content of each metal component was determined after the calcined sample was completely dissolved in mixed acids. Based on the ICP measurement, the contents of Ni, Mg, and Al were 8.01, 1.3, and 45.32 wt. %, respectively, that are very close to the nominal values of 7.86, 1.35, and 46.44 wt. %, respectively, in the precursor solution suggesting that Mg and Al salts precipitate to near completion during support synthesis with near complete subsequent deposition of Ni on the support. The BET surface area of calcined supports S1 to S3 decreased with increasing MgO content. It has been reported that the surface area of supports prepared by the co-precipitation method is almost independent on the compositional ratio of Mg/Al.44,60 However, it appears that the removal of CO2 and H2O during decomposition leads to the formation of significant porosity. As the Al content of the support precursor increased, the amount of CO2 formed during decomposition also increased leading to the observed trend in the surface area of S1 to S3 supports.61 The XRD patterns of the samples with different MgO content are shown in Figure 6A. The XRD patterns of the supports did not show any detectable diffraction peaks that could be assigned to free MgO species. With the addition of MgO, however, the ratio X=I(400)/I(311) decreased due to increasing solid solution of γ-Al2O3 and MgAl2O4 in S2 and S3 supports. As 15 ACS Paragon Plus Environment


evident in Figure 6A, new diffraction peaks appeared for calcined catalysts with the addition of 20 wt. % NiO on the supports. The appearance of diffraction peaks at 37.1o, 43.1o, and 62.6o for S2N2en and S3N2en may be attributed to MgNiO2 (JCPDS 24-0712). On the other hand, diffraction peaks at 45.1o, 65.7o would suggest the formation of Ni-Mg-(Al)-O spinel phase. These observations illustrated that NiO and MgO can form solid solutions when the MgO content is increased. Furthermore, as MgO content increased in the calcined catalyst, the NiO nanoparticles size as estimated by Scherrer's equation increased to 5 nm for S3N2en. Figure 6B shows the H2-TPR profiles of calcined catalysts with different MgO content indicating that with increasing MgO content, the reduction peaks shifted to the higher temperature; from 300oC and 546oC for S1N2en (MgO=2 wt. %) to 360oC and 738oC for S3N2en (MgO=10 wt. %). This could be attributed to the strong interaction between Ni and the support and the formation of Ni-Mg-(Al)-O solid solutions that was confirmed by XRD results.43,62 Reducibility of nickel ions on MgO-NiO solid solution depends on the position of Ni2+ on the MgO lattice. The peaks observed at temperatures between 500-750oC are usually assigned to the reduction of Ni2+ ions in the outermost layer and sub-surface layers of the MgO lattice.63 Addition of MgO in alumina leads to an inhomogeneous (MgO, Al2O3 and MgAl2O4) structure.44 On the other hand, MgO and NiO have similar rock salt cubic structure and lattice parameter (MgO=4.2212Ǻ and NiO=4.1684Ǻ) so they can theoretically form an “ideal” solid solution in any molar ratio.64,65 However, the formation of NiO-MgO solid solution is strongly influenced by the preparation conditions.66-68 Some of the NiO nanoparticles formed from the destruction of the ligand in the precursor can be incorporated into different species of the inhomogeneous support structure to form MgO-NiO and Ni-Mg(Al)-O solid solutions during the thermal treatment step. These results indicated that the use of Ni(II)-ethylenediamine instead of nickel nitrate hexahydrate as precursor did not inhibit the


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formation of NiO-MgO and Ni-Mg-(Al)-O solid solutions in high MgO loadings in NiO/MgO-Al2O3 catalysts. The TEM images of calcined S1N2en and S3N2en catalysts are presented in Figure 4. An increase in the MgO content from 2 wt. % (Figures 4A) to 10 wt. % of calcined catalysts (Figures 4F) led to a less uniform distribution of NiO and the average particle size increased from 4.8 to 5.6 nm that was in agreement with XRD results. It is evident from the results that although the use of Ni(II)-ethylenediamine as precursors and addition of small amounts of MgO led to superior properties of NiO/MgO-Al2O3 catalyst, addition of excess amounts of MgO would eventually lead to lower nickel particle dispersion, larger NiO particle size, and lower reducibility of catalyst. Figure 3 shows the NH3-TPD profiles for reduced SxN2en (x=1-3) catalysts. The broad desorption curves for all catalysts are indicative of an inhomogeneous surface containing different acidic site types and with different concentrations. The NH3-TPD profiles showed a moderate temperature peak (224-260oC) and a high temperature peak (319-356oC) attributed to Lewis acid sites. As given in Table 2, the concentration of acid sites decreased from 3.9 µmol/m2 for S1N2en to 3.1 µmol/m2 and 3 µmol/m2 for S2N2en and S3N2en, respectively, indicating a gradual decrease in the concentration of acid sites with an increase in the magnesium content from 2 to 10 wt. %. Figure S1 (the figure is presented in supporting information) shows the performance of catalysts with different MgO loadings. CO conversion slightly decreased with increasing MgO content for catalysts with similar Ni content. The lowest CO conversion, methane selectivity, and methane yield was obtained for catalysts with S3 support that contained 12.5 wt. % MgO. The low activity for these catalysts could be due to the formation of NiO-MgO and Ni-Mg-(Al)-O solid solutions (Figure 6A) that remain stable at low reduction temperatures as confirmed by H2-TPR profiles and would lead to the appearance of fewer



active sites on the surface of catalysts compared with the catalysts that were prepared from S2 and S1 supports. As illustrated in Figure S1, S3N1en had the lowest catalytic performance with maximum CO conversion (96.5%), methane selectivity (60%), and methane yield (58%) occurring at 400ºC (Figures S1A to S1C) while S1N4en had the highest activity (and only slightly better than S1N2en) with maximum CO conversion (100%), methane selectivity (91.1%), and yield occurring at 299ºC (Figures S1G to S1I). This observation indicated that decreasing NiO/MgO ratio and increasing MgO/Al2O3 ratio in NiO/MgO-Al2O3 catalysts would lead to a decrease in catalytic activity suggesting that smaller quantities of NiO in a high MgO content support favored the formation of NiO-MgO and Ni-Mg-(Al)-O solid solutions that were stable at low reduction temperatures and had a lower active metal surface area.62 3.3. Effect of NiO Loading The XRD patterns of calcined S1Nxen catalysts are shown in Figure 7A where x=1, 2 and 4 are assigned to 10, 20, and 40 wt. % NiO. There were no strong NiO diffraction peaks for S1N1en with 10 wt. % NiO loading indicating the formation of very small nanoparticles of NiO species on the catalyst or the presence of nickel ions in form of Ni-Al-O solid solutions.29,49,69,70 With increasing NiO content to 40 wt. % for S1N4en, strong and sharp diffraction peaks of NiO appeared at 2θ of 37.3o, 43.3o, 63o, 75.5o and 79.5o, corresponding to (111), (200), (220), (311), and (222) (JCPDS 75-0197). The size of NiO nanoparticles also increased with increasing Ni loading. In addition, it is difficult to confirm the formation of NiAl2O4 and MgAl2O4 from these diffraction patterns. H2-TPR profiles of calcined S1 catalysts with different Ni loadings are shown in Figure 7B. The main reduction peak of S1N1en catalyst was observed at 645oC attributed to type β1 NiO species assigned to NiO in Ni-rich mixed oxide phase. In addition, the peak at 780oC corresponded to type γ NiO species assigned to nickel aluminate spinel. The very small 18 ACS Paragon Plus Environment

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and broad peak at about 300oC suggested the lack of free NiO species on the catalyst surface. As the NiO loading increased, new reduction peaks started to emerge at lower temperatures (less than 500oC) suggesting that higher NiO contents would lead to the formation of NiO species with weaker interactions with the support.28,47,49,70 The main type of NiO species in S1N4en is type α assigned to NiO species having a weak interaction with the support and that are easily reduced. The appearance of a large intense peak at about 330oC for this catalyst indicates the presence of significant amounts of free NiO as compared with the catalysts with lower Ni contents. As illustrated above, the high viscosity of the impregnation solution led to the formation of large crystals on the support surface during the deposition and it is known that addition of nickel precursor in the impregnation solution could result in a higher viscosity. As the Ni loading increased, the amounts of [Ni(en)2(H2O)2](NO3)2 in the impregnation solution also increased and the excess amount of this species which was not deposited on the support may have formed larger crystals over the pore mouth by aggregating together during the final heat treatment step in catalyst preparation. This could also lead to some Ni species to be far from the support surface and deposit on the Ni species sub layers that are in direct interaction with the support.47,70 TEM images indicate that NiO particles for S1N4en (Figures 4G) were larger (8.7 nm) and less uniformly dispersed as compared with S1N2en (Figures 4A) indicating the aggregation of NiO particles with increasing NiO content in agreement with XRD (Figure 7A) and TPR results (Figure 7B). The diameter of NiO nanoparticles, metal particle dispersion on the catalyst surface, and the amount of free NiO formation from impregnation of [Ni(en)2(H2O)2](NO3)2 are, therefore, not only sensitive to the structural properties of the support, but also sensitive to the amounts of nickel complex and concentration of impregnation solution. These also affect the accessible surface area of the support to be exposed to the nickel complex during impregnation.



Figure S2 (the figure is presented in supporting information) shows the catalytic performance of all SxNyen catalysts (x=1, 2, and 3 and y= 1, 2, and 4) in CO methanation at 0.1 MPa. It was found that the conversion of CO as well as methane selectivity and yield increased with increasing Ni loading for all supports. The enhancement in catalyst performance was more pronounced when NiO loading was increased from 10 to 20 wt. % as compared with the enhancement resulting from an additional increase in Ni loading from 20 to 40 wt. %. These results are in agreement with XRD and H2-TPR results which indicated smaller quantities of reducible NiO species for the SxN1en catalysts. Increasing NiO loading led to the formation of free NiO particles with weaker interactions with the support that were reduced at low temperature thus enhancing the catalyst performance. But, care should be taken to prevent the formation of large amounts of free NiO. On the other hand, the appearance of free NiO with larger particle size in SxN4en catalysts (according to XRD and H2-TPR results of Figures 7A and 7B) led to lower Ni dispersion on the surface (TEM results of Figure 4G) and lower activity33 explaining the rather poor enhancement in catalyst performance with increase in NiO loading from 20 to 40 wt. %. It is also known that free NiO is susceptible to sintering. These results indicate that the activity of [Ni(en)2(H2O)2](NO3)2 impregnated catalysts was limited by the formation of free NiO on the support surface for high nickel loading catalysts. 3.4. Comparison of the performance of the optimum catalyst with a commercial catalyst Based on the catalytic performance and the properties of all prepared catalyst, we selected S1N2en catalyst as the optimum catalysts due to its high CO conversion, high methane yield and selectivity, and moderate NiO loading (20%). Performance of Meth134, a commercial methanation catalyst (Clariant, Süd Chemie, INDIA PVT. LTD) was also examined to provide an evaluation for the performance of the optimum catalyst under different operating conditions. The original commercial catalysts that were 3-6 mm spheres 20 ACS Paragon Plus Environment

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with a homogenous 25 wt. % NiO loading were crushed and sieved to obtain minus 200 mesh particles sizes. Prior to the reactor test, the catalysts were reduced at 500oC for 1 h in a continuous flow of pure H2 (40 Nml/min). The catalytic performance of the optimum and the commercial catalysts are presented in Figure S3 (the figure is presented in supporting information) indicating that the catalytic activity of the optimum catalyst was superior with maximum CO conversion, methane selectivity, and methane yield of 100%, 88.7%, and 88.7%, respectively, occurring at 300°C for S1N2en while maximum CO conversion, methane selectivity, and methane yield of 99.4%, 78.6%, and 78.2%, respectively, occurring at 400oC for the commercial catalyst. It should be noted that the NiO content of the commercial catalyst (25 wt. %) was even higher than S1N2en catalyst (20 wt. %). The effect of GHSV on the performance of the optimum and the commercial catalyst is presented in Figure 8 indicating that over the range of GHSV between 80000 and 320000 Nml(gas)/g(catalyst)·h employed in the performance tests, the activity of both catalysts decreased with increasing GHSV. Complete CO conversion for S1N2en catalyst was achieved at all GHSVs and the lowest methane selectivity of 82.3% was obtained for GHSV of 320000 Nml(gas)/g(catalyst)·h at the reaction temperature of about 400oC. For the commercial catalyst, however, there was a decrease in both CO conversion and methane selectivity from 99.4% and 78.6%, respectively, at GHSV of 80000 Nml(gas)/g(catalyst)·h to 93.1 and 68.3%, respectively, at GHSV of 320000 Nml(gas)/g(catalyst)·h for reaction temperature of about 400oC. These results illustrated that higher GHSVs led to higher amounts of by-products that are undesirable in the methanation process and that S1N2en demonstrated a better catalytic performance compared with the commercial catalyst over the entire range of GHSVs. Hydrogen to CO molar ratio in the industrial SNG production is about 3 in the feed gas71 and this ratio for the syngas produced from coal or biomass gasifiers is less than 3.6,72 It 21 ACS Paragon Plus Environment


is known that this ratio plays an important role on the catalyst performance on methanation reaction.73 Figure 9 illustrates the effects of different H2/CO ratio in the range of 1 to 4.5 on the performance of optimum and commercial catalysts. Both catalysts exhibited the same behavior with a slight decrease in catalytic activity with decreasing H2/CO ratio from 4.5 to 3 and a more pronounced decrease when H2/CO was further reduced to 1. Simultaneous reactions (reactions 2 to 6) that compete with the main methanation reaction become more significant at low H2/CO molar ratios. The Boudouard reaction (reaction 5) is the most undesired reaction leading to carbon deposition that is one of the main causes for methanation catalyst deactivation.25,73,74 As the H2/CO ratio was increased to 4.5 (above the stoichiometric ratio of 3), methane selectivity increased to a maximum possibly due to the slowing down of the water gas shift reaction (reaction 4). Carbon formation can also be inhibited by using excess hydrogen in the feed to drive reaction 6 forward.24,73,74 The maximum CO conversion (90.2%), methane selectivity (54.4%), and methane yield (48.7%) for H2/CO=1 that occurred at 340oC for the optimum catalyst were higher than those for the commercial catalyst that exhibited a maximum CO conversion of about 84%, methane selectivity of 49%, and methane yield of 41.8% at a higher temperature (400°C). The high methane selectivity and methane yield for the optimum catalyst at low H2/CO molar ratio was obtained due to the relatively basic properties of the catalyst as a result of the synergistic effects of MgO addition to the support and the use of [Ni(en)2(H2O)2](NO3)2 as precursor which would lead to rather basic and neutral sites on the catalyst surface (even as compared with S1N2 having the same support and MgO content as illustrated in Figure 3). It is known that the basic properties of the catalyst surface can slow down the methane cracking to carbon (reaction 6)59,75 and the Boudouard reaction (reaction 5) during CO methanation. The basic and neutral catalyst site could also promote CO2 adsorption on the catalyst surface and its subsequent reaction with the neighboring carbon deposits.43,37 Consequently, CO2 consumption could result in an


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increase in methane selectivity due to a shift in the reverse methane CO2 reforming reaction (reaction 2) towards methane production under low H2/CO molar ratio. Furthermore, highly dispersed NiO nanoparticles with moderate interactions with the mesoporous MgO-Al2O3 support would prevent the Ni crystallites from sintering during methanation reaction. Decreased carbon deposition due to slowing down of the methane cracking and Boudouard reactions is a significant consequence for industrial applications as it would lead to longer catalyst lifetime. These results illustrate that the optimum catalyst had enhanced performance under different operating conditions as compared with the commercial catalyst with even higher active metal loading. 3.5. Carbon deposition study To investigate the stability of the prepared catalysts in comparison with the commercial catalyst towards coke formation, the spent catalysts which were tested under the lowest H2/CO ratio where chosen for further analysis. The spent catalysts were subject to the severe reaction conditions suitable for coke formation (H2/CO=1, pressure of 1 atmosphere, temperatures from 200 to 500°C with time-on-stream of 40 minutes for each temperature stage, and GHSV of 80000 Nml(gas)/(g(catalyst)·h).76 Figure 10 shows the catalytic performance of S1N2en, S1N2, S4N2en and commercial catalysts in the CO methanation with above reaction condition. XRD patterns of spent S1N2en and commercial catalyst are presented in Figure 11 A and XRD patterns of spent S1N2 and S4N2en are shown in Figure 11B. The latter were obtained on a different diffractometer (X'pert PRO MPD PANanalytical instrument). The XRD patterns indicated three new peaks located at 44.5º, 51.8º and 76.4º assigned to (111), (200), and (220) planes of nickel (JCPDS 87-0712), respectively, and another strong peak at 26.3º corresponding to the (002) planes of carbon (JCPDS 02-0456) that were clearly



observed in the XRD patterns of the commercial and S4N2en catalysts. The intensity of nickel diffraction peaks for spent S1N2en were very weak in compared with other catalysts indicating that the Ni particles were still nano-sized even after being exposed to reduction and reaction conditions. As can be seen from the XRD patterns, the intensity of carbon diffraction peaks decreased in the order of S1N2en

MgO-Al2O3 Catalyst for

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