Ni–MnOx Catalysts Supported on Al2O3-Modified Si Waste with

Article pubs.acs.org/IECR

Ni−MnOx Catalysts Supported on Al2O3‑Modified Si Waste with Outstanding CO Methanation Catalytic Performance Xiaopeng Lu,†,‡ Fangna Gu,*,† Qing Liu,† Jiajian Gao,† Lihua Jia,*,‡ Guangwen Xu,† Ziyi Zhong,§ and Fabing Su*,† †

State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China ‡ College of Chemistry and Chemical Engineering, Qiqihaer University, Qiqihaer, Heilongjiang Province 161006, China § School of Chemical & Biomedical Engineering, Nanyang Technological University (NTU), 62 Nanyang Drive, Singapore 637459 S Supporting Information *

ABSTRACT: A series of MnOx-promoted Ni catalysts supported on Si waste contact mass (W) modified by Al2O3 were successfully prepared by the deposition−precipitation method for CO methanation. Compared with the Ni catalysts directly supported on W, Ni/Al2O3−Si and MnOx-promoted Ni−MnOx/Al2O3−Si catalysts exhibited much better catalytic performance for CO methanation, particularly the latter, which exhibited the least decrease in CO conversion (1%) and lowest carbon deposit (0.3 wt %) in a 110 h lifetime test. The structural characterizations showed the highest Ni dispersion and highest oxygen vacancy concentration in the Ni−MnOx/Al2O3−Si catalyst. The added Al2O3 improves the dispersion of Ni, and the MnOx promoter can restrain the sintering and aggregation of Ni particles during the process of reduction and reaction at high temperatures and provides more oxygen vacancies, which is conducive to the removal of carbonaceous species on the catalyst surface for anticoking.

1. INTRODUCTION In coal-rich regions around the world, gasification of coal to syngas followed with methanation reaction to synthetic natural gas (SNG) is believed to be one of the possible approaches to meeting the increasing demand for the clean energy.1−4 In recent years, great efforts have been made to develop highly efficient catalysts for syngas methanation (CO + 3H2 → CH4 + H2O, ΔH298 K= −206.1 kJ mol−1), and the ideal methanation catalysts should possess both high activity at low temperature and high stability at high temperature. Among various methanation catalysts, Ni/Al2O3 catalysts have been studied extensively because of their relatively high activity and low cost.5,6 At low temperature (ca. 300 °C), they exhibit poor activity for methanation, and in the absence of promoter, they often suffer from sintering of the Ni particles as well as coke formation.7 In recent years, there have been great efforts made to improve their catalytic performance through enhancing their antisintering and anticoke properties, e.g., by use of various promoters, such as MgO, 4,8 CeO 2 ,9 ZrO 2 , 10 MoO 3 ,11 V2O 3,12−14and MnOx,15,16 by developing Ni bimetallic catalysts,3,17,18 and by application of composite support.19,20 It is reported that several support materials, such as hexaaluminate barium,21,22 calcium titanate,7 and SiC, with both excellent thermostability and high thermal conductivity also have been developed as support in CO methanation.23,24 However, the complexity in catalyst preparation and high cost of these materials limit their industrial application. Theoretically, Si has high thermal conductivity and is often used to improve thermal stability of some materials25 and can form silicon−nickel intermetallic compounds to prevent the adsorption of resilient carbon species in CO methanation.26 As © 2015 American Chemical Society

we know, there is large amount of Si waste contact mass (composed of 70−90 wt % of Si particles and 10−30 wt % of Cu-based catalyst) generated in organosilane industry every year. Generally, these solid wastes are treated by acid dissolution followed by precipitation with alkaline solution to recover the copper component, but the residual Si particles are not found to have the high-value utilization. Considering the high thermal conductivity of Si and the efficient utilization of waste Si particles, particularly for improving the antisintering property of the porous Al2O3 at high temperature in the strongly exothermic methanation reaction, in this work we first designed and prepared the Ni catalysts supported on pretreated Si waste contact mass modified with Al2O3. This unique structure of the synthesized catalysts can not only transfer the exothermal heat timely but also prevent the Ni metal particles from migrating easily over Si surface, thus improving the activity in antisintering of the catalysts. Additionally, MnOx was added as the promoter to improve the low-temperature activity15 and prevent carbon deposition by promoting the surface carbon gasification and/or the water gas shift (WGS) reaction in the CO methanation reaction.27 As demonstrated, the developed Ni catalysts supported on Al2O3-modified Si waste and promoted by MnOx showed outstanding catalytic performance in CO methanation. Combined with its simple preparation and low cost, this type of catalyst should be promising for industrial application. Received: Revised: Accepted: Published: 12516

September 7, 2015 November 16, 2015 November 27, 2015 November 27, 2015 DOI: 10.1021/acs.iecr.5b03327 Ind. Eng. Chem. Res. 2015, 54, 12516−12524

Article

Industrial & Engineering Chemistry Research

2. EXPERIMENTAL SECTION 2.1. Materials. Chemicals of analytical grade including aluminum nitrate nonahydrate (Al(NO3)3·9H2O), nickel(II) nitrate hexahydrate (Ni(NO3)2·6H2O), 50 wt % Mn(NO3)2 solution, and potassium hydroxide (KOH) were purchased from Sinopharm Chemical Reagent Co., Ltd., China, and used without further treatment. 2.2. Catalysts Preparation. The Si waste contact mass (WCM, for short) (Jiangsu Hongda New Material Co., Ltd.) was first pretreated by excessive concentrated nitric acid and then dried at 120 °C in air for 10 h, and the obtained sample was denoted as W. The purity of W can reach 96.5%, and the main impurities are O (2.3%), Al (0.2%), Ca (0.2%), and Fe (0.7%);28 among them, Fe is in the form of FeSi2, confirmed by XRD results. The control test reveals that W shows no activity in CO methanation; thus, W was used without other treatment. The Ni/Al2O3−Si catalysts were prepared by the deposition− precipitation (DP) method. Typically, 2.40 g of the W support was added to a round-bottomed flask with 50 mL of deionized water to form a slurry; the obtained slurry was then slowly heated to 60 °C in a water bath under vigorous stirring. Subsequently, 2.44 g of Ni(NO3)2·6H2O and a certain amount of Al(NO3)3·9H2O were dissolved in 50 mL of deionized water, respectively, followed by dropwise addition of the above two solutions to the slurry of W simultaneously; meanwhile, 0.1 M KOH solution was added to keep the pH value of the mixture at around 9.0. The resulting mixture was stirred vigorously for 8 h, then cooled down to room temperature, further filtered under vacuum, and washed several times with deionized water. The product was dried at 120 °C in air and then calcined at 400 °C for 2 h in air at the heating rate of 2 °C min−1. The collected catalysts were denoted as 20N/20A-W, in which 20 represents the loadings of NiO and Al2O3 in weight percentage. To optimize the catalyst composition, the Ni−Al−Si catalysts with Al2O3 concentrations of 10 and 30 wt % were also prepared, which were denoted as 20N/10A-W and 20N/30A-W. Similarly, the catalysts with different loadings of MnOx (2, 5, and 8 wt %) were synthesized by the DP method and denoted as 20NxM/20A-W (x = 2, 5 and 8). In addition, three catalysts were synthesized using the similar experimental conditions to that of 20N/20A-W but calcined at other temperatures of 600, 800, 1000 °C for 2 h, and the obtained samples were denoted as 20N/20A-W (Cal-600, Cal-800 and Cal-1000 °C), respectively. The 20N5M/20A-W catalysts were respectively reduced at the temperature of 500, 650, and 700 °C for 2 h, which were denoted as 20N5M/20A-W (Re-500, Re-650 and Re-700 °C). For the detailed analysis, 20N15M/20A-W, 20M/ 20A-W, and pure MnO x were also prepared by the aforementioned method, calcined at 400 °C for 2 h, and reduced at 650 °C for 2 h. 2.3. Catalysts Characterization. N2 adsorption at −196 °C was analyzed using a Quantachrome surface area and pore size analyzer (NOVA 3200e). Before the analysis, the sample was degassed at 200 °C for 3 h under vacuum. The specific surface area was determined according to the Brunauer− Emmett−Teller (BET) method under the relative pressure range of 0.05−0.2. The pore size distribution (PSD) was calculated by the Barett−Joyner−Halenda (BJH) method by the adsorption isotherm branch. Powder X-ray diffraction (XRD) patterns were recorded by a PANanalytica X’Pert PRO MPD with Cu Kα radiation (λ = 0.1541 nm) at 40 kV and 40 mA. The crystallite sizes of Ni particles were calculated by the

Debye−Scherrer equation. The microscopic feature of the samples was observed by field-emission scanning electron microscope (SEM) (JSM-6700F, JEOL, Tokyo, Japan) and transmission electron microscopy (TEM) (JEM-2010F, JEOL, Tokyo, Japan). Before the TEM measurement, the Ni catalysts were reduced in a fixed bed at 650 °C for 2 h under H2 flow. Thermogravimetric analysis (TGA) was conducted (EXSTAR TG/DTA 6300, Seiko Instruments) in air at a flow rate of 100 mL min−1 and a temperature ramp of 10 °C min−1. The surface chemical composition of the samples was determined by X-ray photoelectron spectroscopy (XPS) on a VG ESCALAB 250 spectrometer (Thermo Electron, U.K.), using a nonmonochromatized Al Kα X-ray source (1486 eV). Zeta potential analysis was carried out on a Beckman Coulter DelsaNano C particle size and zeta potential analyzer. Typically, certain amounts of W, Ni(NO3)2·6H2O, Al(NO3)3·9H2O, and 50 wt % Mn(NO3)2 were separately added into a round-bottomed flask with 50 mL of deionized water to form a slurry; then, 0.1 M KOH solution was added to keep the pH value of the slurry at around 9.0. After that, the above four kinds of turbid liquids were analyzed by the zeta potential. Temperature-programmed reduction with H2 (H2-TPR) was carried out on an automated chemisorption analyzer (chemBET pulsar TPR/TPD, Quantachrome), and the operations are similar to our previous work.7 For the H2-TPR test, 0.07 g of the sample was loaded into a quartz U-tube and heated from room temperature to 300 °C at 10 °C min−1 and maintained for 1 h under Ar flow. Then, the sample was cooled down to room temperature and then heated to 1000 °C at 10 °C min−1 under the flow of a binary gas (10.0 vol % H2/Ar) at 30 mL min−1. For H2-TPD, 0.2 g of the catalyst was used and reduced in situ by H2 flow previously. Then, the sample was cooled down to room temperature and saturated with H2. After removing the physically adsorbed H2 by flushing with Ar for 2 h, the sample was heated to a given temperature at a ramping rate of 10 °C min−1 in Ar flow (30 mL min−1). The released H2 was detected continuously as a function of increasing temperature using a thermal conductivity detector (TCD). The dispersion of Ni was calculated on the basis of the volume of chemisorbed H2 using the simplified equation:9 D (%) =

2 × Vad × M × SF × 100 m × P × Vm × dr

(1)

where Vad (mL) is the volume of chemisorbed H2 in the TPD measurement at standard temperature and pressure (STP) conditions; m is the mass of sample (g); M is the molecular weight of Ni (58.69 g mol−1); P is weight fraction of Ni in the sample determined by ICP; SF is the stoichiometric factor (the Ni/H molar ratio in the chemisorption), which is taken as 1; Vm is molar volume of H2 (22414 mL mol−1) at STP; and dr is the reduction degree of nickel calculated on the basis of H2TPR. 2.4. Catalytic Measurement. Typically, the CO methanation reaction evaluations were carried out in a fixed-bed quartz tube reactor with an inner diameter of 8 mm at 0.1 MPa at the temperature range of 260−550 °C. First, 0.2 g of catalyst (20−40 mesh) mixed with 5.0 g of quartz sand (20−40 mesh) was loaded in the reactor. The catalyst was reduced at the given temperature under pure H2 (100 mL min−1) and then cooled down to the starting reaction temperature under pure H2. The mixed gases of H2 and CO reactants as well as N2 (as an internal standard) were introduced into the reactor at a 12517

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from their N2 adsorption isotherms (Figure S1). Compared with the W support, the 20N/W catalyst has larger specific surface area, probably because the NiO nanoparticles make the surface of the W support coarser. Not surprisingly, the Al2O3modified 20N/W sample exhibits larger specific surface area than 20N/W, which can be attributed to the partial substitution of high-density W by the low-density Al2O3 with more mesopores (Figure S1B). After the addition of MnOx, the specific surface area of the catalysts is further increased slightly because of the formation of more meso- and macropores stacked by the MnOx particles. The isotherms of W and all the catalysts belong to type IV with hysteresis loops of type H3 (Figure S1A), suggesting the presence of mesopores and macropores in the support and the catalysts. 3.2. XRD, H2-TPR and H2-TPD Analysis. Figure 1A and the enlarged view in Figure 1B show the XRD patterns of the

stoichiometric molar ratio of H2/CO/N2 = 3/1/1 with the total flow rate of 100 mL min−1 corresponding to a WHSV of 30 000 mL (gas) g−1 (catalyst) h−1. The outlet gas stream was cooled using a cold-water trap. The inlet and outlet gases were analyzed on line using a Micro GC (3000A; Agilent Technologies) after 1 h of steady-state operation at each temperature. The concentrations of H2, N2, CH4, and CO in the outlet gas were detected by a TCD with a Molecular Sieve column, and the concentrations of CO2, C2H4, C2H6, C3H6, and C3H8 were analyzed by using another TCD with a Plot Q column. A stability test at 0.1 MPa was carried out using the reduced catalysts. After reduction at the given temperature under pure H2 (100 mL min−1) for 1 h, the H2 flow was changed to the reaction mixture gas to perform the stability test. For the CO methanation lifetime test at 0.1 MPa and 550 °C, 0.15 g of catalyst was mixed with 5.0 g of quartz sands, and the initial gas flow was 150 mL min−1 (WHSV = 60 000 mL g−1 h−1), which was subsequently set to 300 mL min−1 (WHSV = 120 000 mL g−1 h−1) and 375 mL min−1 (WHSV is 150 000 mL g−1 h−1) to distinguish the difference of the catalysts. The catalytic results are calculated according to the following formulas: CO conversion: XCO (%) =

FCO,in − FCO,out FCO,in

× 100 (2)

CH4 selectivity: SCH4 (%) =

FCH4,out FCO,in − FCO,out

× 100 (3)

CH4 yield: YCH4 (%) =

XCO × SCH4 100

=

FCH4,out FCO,in

× 100 (4)

Here, XCO is the conversion of CO, SCH4 is the selectivity of CH4, YCH4 is the yield of CH4, and Fi,in and Fi,out are the volume flow rate of species i (i = CO or CH4) at the inlet and outlet, respectively.

Figure 1. (A)XRD, (B) enlarged view of XRD, (C) H2-TPR, and (D) H2-TPD curves of the support and reduced catalysts. (a) W, (b) 20N/ W, (c) 20N/20A-W, (d) 20N2M/20A-W, (e) 20N5M/20A-W, (f) 20N8M/20A-W, and (g) 20M/20A-W.

3. RESULTS AND DISCUSSION 3.1. Pore Structure Analysis. Table 1 compiles the surface areas and pore volumes of the supports and catalysts derived

support and the reduced Ni catalysts. All the samples display the diffraction peaks at 28.5, 47.4, 56.2, 69.2, and 76.4° belonging to Si (Powder Diffraction File (PDF) 00-027-1402, International Centre for Diffraction Data (ICDD), [1976]), and the weak diffraction peaks at 37.5 and 48.7° belonging to FeSi2 (PDF 00-001-1285, ICDD, [1938]), which may be introduced during the process of industrial production, suggesting that the treated W is not very pure. The control test reveals that W shows no activity in CO methanation (figure not shown); thus, W was used without other treatment. For all the catalysts, only one characteristic diffraction peak of Ni (PDF 01-070-1849, ICDD, [1956]) can be seen at 44.5°, and its intensity is very weak, even in the enlarged view (Figure 1B). The calculated Ni particle sizes are all very small (3−5 nm), suggesting that the Ni species are well-dispersed in the catalysts prepared by the DP method. To know the real state of MnOx in the catalysts, we further investigated the as-calcined and reduced 20N15M/20A-W, 20M/20A-W, and pure MnOx (Figure S2). The pure oxidized MnOx consists of Mn3O4 (PDF 00-001-1127, ICDD, [1938]) with diffraction peaks at

Table 1. Physical and Chemical Properties of the Supports and Catalysts samples

SBET (m2 g−1)a

V (cm g−1)b

Ni size (nm)c

H2 uptake (μmol gcat−1)

D (%)d

W 20N/W 20N/20A-W 20N2M/20A-W 20N5M/20A-W 20N8M/20A-W

12.7 43.0 113.5 115.8 127.5 140.0

0.02 0.15 0.23 0.24 0.26 0.29

4.8 4.7 4.2 3.4 4.8

117.3 171.3 193.7 244.9 191.5

8.8 12.8 14.5 18.3 14.3

3

a Surface area calculated using BET equation. bPore volume, obtained from the volume of nitrogen adsorbed at the relative pressure of 0.97. c Crystal size of Ni, derived from XRD by Debye−Scherrer equation. d Ni dispersion, calculated on the basis of the H2-TPR and H2-TPD results.

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the increase of the H2 adsorption ability of the Ni nanoparticles in the 20NxM/20A-W (x = 2, 5, and 8) catalysts. The H2 uptakes and Ni dispersion in the catalysts are listed in Table 1. The maximum H2 uptake is 244.9 μmol gcat−1 for 20N5M/20AW, which has the highest Ni dispersion of 18.3%, indicating that the addition of proper amount of MnOx can highly increase the dispersion of Ni nanoparticles whereas an excess of MnOx loading may cover some of the Ni active sites and decrease the H2 chemisorption ability which are closely related to the catalytic activity of the Ni catalysts. 3.3. TEM and SEM Analysis. Figure 2a shows the TEM image of W support. The surface of the W support is rough and thus favorable for supporting Ni species or coating Al2O3 on it. Figure 2b indicates that the metallic Ni particles are highly dispersed in 20N/W prepared by the DP method. After adding Al2O3 into the catalysts, the surface of the W support becomes rougher, the Al2O3 disperses on the surface of W support, and the Ni particles in 20N/20A-W are also uniform with the Ni particle sizes varying from 3 to 6 nm (Figure 2c). Figure 2d shows that the Ni particles are uniform and small in size (about 4 nm), and no agglomeration can be observed in the MnOxdoped 20N5M/20A-W. Figure 2e shows the HRTEM image of 20N5M/20A-W, in which the Al2O3 decorated around Si can be seen clearly, and both Ni and MnOx are dispersed on the Al2O3−Si composite support uniformly. The lattice spacing of ca. 0.31 nm in the further enlarged view of area 1 corresponds to the Si (111) plane (Figure 2f). The lattice spacing at ca. 0.21 and 0.25 nm in the further enlarged view of area 3 in Figure 2g,h correspond to the Mn2O3 (420) plane and the Mn3O4 (420) plane, respectively, and the lattice spacing at ca. 0.20 nm corresponds to the Ni (111) plane. Figure 3 gives the element mappings of the reduced 20N5M/ 20A-W catalyst. It confirms that Al2O3 can increase the specific surface area (Table 1) by modifying the surface of the catalysts; thus, higher dispersion of Ni and MnOx species on the support (Figure 3c,d) and higher catalytic activity can be obtained. 3.4. Surface Analysis of the Reduced Catalysts. Figure 4a shows the Ni 2p3/2 XPS spectra of the reduced 20N/20A-W and 20N5M/20A-W catalysts. From Ni 2p3/2 spectra of the two catalysts it can be seen that they mainly consist of large portion of Ni2+ (ca. 856.0 eV) and a small portion of Ni0 (ca. 853.0 eV) which may be because the surface Ni was easily oxidized after exposure to air during the sample transfer.21 After doping with MnOx, the binding energy of Ni 2p3/2 for 20N5M/20A-W (856.4 eV) shifts to a higher value as compared with that of 20N/20A-W (856.0 eV). The shift toward higher binding energy should be attributed to the enhanced interaction between Ni and MnOx,30 which is in agreement with the H2TPR result. The Mn 2p3/2 peaks (Figure 4b) of reduced 20N5M/20A-W can be fitted to three subpeaks:34 Mn2+ (640.6 eV), Mn3+ (641.5 eV), and Mn4+ (643.1 eV), consistent with the above XRD and TEM results that show there is coexistence of various states of Mn species (Mn2+, Mn3+, and Mn4+) in 20N5M/20A-W. Figure 4c shows the spectra of O 1s, which can be fitted into two subpeaks: OI (lattice oxygen) at 531.2 eV and OII (adsorbed oxygen) at 532.1 eV. The integrated area ratio (Table S2) of OII/(OII + OI) of 20N5M/20A-W (47.8 atom %) is higher than that of 20N/20A-W (22.7 atom %), suggesting an increase of oxygen vacancies.35 The generation of more oxygen vacancies can improve the oxygen mobility, which is crucial in the catalytic oxidation reaction and in removal of carbonaceous species.36,37

29.2, 31.2, 32.5, 36.2, 44.6, 51.1, 54.0, 58.6, and 60.0° and Mn2O3 (PDF 00-002-0902, ICDD, [1930]) with diffraction peaks at 33.0, 60.9, 64.6, and 74.1°, whereas the reduced MnOx is made up of Mn2O3 (PDF 00-002-0902, ICDD, [1930]) with diffraction peaks at 35.2 and 70.5° and MnO (PDF 00-0031145, ICDD, [1925]) with peaks at 41.0, 59.2, and 74.1°. Likewise, as-calcined 20M/20A-W shows the typical diffraction peaks assigned to both Mn3O4 and Mn2O3, and reduced 20M/ 20A-W exhibits the diffraction peaks of both Mn2O3 and MnO. In contrast, there is no diffraction peak of MnOx observed in either the as-calcined or the reduced catalysts with low Mn species loading, but a weak diffraction peak at 36.2° is observed in the as-calcined 20N15M/20A-W sample, indicating the high dispersion of Mn species in all the samples. Figure 1C presents the H2-TPR profiles of the support, catalysts, and 20M/20A-W. It can be seen that there is no obvious reduction peak for the support of W, implying that W is unreducible under this condition. 20M/20A-W shows two weak peaks at ca. 318 and 513 °C, respectively, corresponding to the two-step reduction of MnO2/Mn2O3 → Mn3O4 → MnO, which is in good accordance with the previous reports that the reduction of bulk MnO2 and/or Mn2O3 takes place through a distinct two-step process.29,30 Considering the much lower loading of Mn species in the catalysts than that of NiO, we will mainly focus on the reduction of NiO. Compared with 20N/W, which has only a broad reducible peak centered at ca. 660 °C, 20N/20A-W, which possesses higher specific surface area and higher Ni dispersion, shows different reduction peaks: The one at ca. 340 °C can be assigned to the free nickel oxide species with a weak interaction with the Al2O3−Si composite support, which is conductive to the activity for methanation.31 The ones at ca. 570 and 660 °C can be assigned to the reduction of nickel oxide species with the middle and strong interaction with Al2O3−Si support, respectively, whereas the small one appeared in the high-temperature range (above 750 °C) to the reduction of nickel aluminate phase with the spinel structure. After addition of proper MnOx promoter, the lowtemperature peak at ca. 340 °C becomes larger than that of the catalyst without MnOx, accompanied by a shift of the starting reduction peak of NiO to lower temperature, which is consistent with the results that MnOx can promote the reducibility of NiO.16 However, the reduction of 20N8M/ 20A-W seems to be more difficult than that of other 20NxM/ 20A-W catalysts with less MnOx addition, especially the much increased reduction peak at ca. 660 °C for the former, suggesting that the doping of excessive MnOx is deleterious to the reduction of nickel oxide species, which may be due to the enhanced interaction between Ni and MnOx or the support. Figure 1D presents the H2-TPD profiles of the support, catalysts, and 20M/20A-W. For 20N/W, there is only one H2 desorption peak at ca. 145 °C. In contrast, 20NxM/20A-W (x = 2, 5, and 8) show two main H2 desorption peaks at ca. 203 and 411 °C, suggesting the Ni dispersion in the Al2O3−Si composite support is different from that in the single support of W. The former peak is attributed to the chemisorbed hydrogen on the highly dispersed Ni nanoparticles with a large density of surface defects, which often serve as capture traps for surface hydrogen diffusion, thus reducing the activation energy of hydrogen dissociation,32 whereas the latter peak can be derived from the H2 adsorbed in the subsurface layers of Ni atoms and/ or to the spillover of H2.5,33 After addition of the Mn promoter, the intensity of the low-temperature peak becomes stronger accompanying weakening of the peak at ca. 350 °C, suggesting 12519

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Figure 3. (a) SEM image of the reduced 20N5M/20A-W; elemental mapping images of (b) Al, (c) Ni, and (d) Mn.

support, the catalytic activity of 20N/20A-W is obviously enhanced, and the maximum of CO conversion reaches 97.6% at 320 °C, which should be attributed to the enhanced H2 adsorption ability (Figure 1D). Compared with that of 20N/ 20A-W, the catalytic activity of 20N5M/20A-W is further improved, with the maximum CO conversion of 100% at 300 °C, which should be related to the MnOx promoter that can increase the electron cloud density of Ni atoms and promote the cleavage of the CO molecules thus leading to the enhanced performance of the catalyst.9 In addition, after molding the 20N5M/20A-W catalyst into 1.2 × 5 mm tablets with high radial crushing strength (Figure S9 and Table S3), the catalytic test demonstrates that this catalyst even has better performance than the commercial HT-1 catalyst (LiaoNing HaiTai SCITECH Development CO., Ltd., China) for CO methanation at a high WHSV of 120 000 mL g−1 h−1 (Figure S10), thus this catalyst is promising in the industrial application for methanation. 3.6. Lifetime Test of the Catalysts. Figure 6 presents the lifetime test results of the catalysts carried out at 550 °C (the temperature of furnace) and 0.1 MPa for 110 h with high WHSVs of 60 000, 120 000 and 150 000 mL g−1 h−1, respectively, of which the real reaction temperature may reach 580−670 °C, on the basis of our previous experience (Table S5). It can be seen that the catalytic activity of the 20N/ W catalyst decreases at each stage with different WHSVs, e.g., the CO conversion is decreased from 74 to 70% in 30 h, then to 39% at the end of the 80 h, and finally to 23% at the end of 110 h. In contrast, the stability of the 20N/20A-W catalyst is obviously improved, and its catalytic performance is not changed in the first and second steps. The CO conversion declines only 3% by the end of the third step even with the very high WHSV of 150 000 mL g−1 h−1. Regarding the 20N5M/ 20A-W catalyst with MnOx promoter, it is also very stable in each step, and the CO conversion only declines by 1% when the WHSV is increased to 150 000 mL g−1 h−1, which is obviously superior to the performance of the catalyst reported previously by us with the same amount of Ni under harsher reaction conditions during the long lifetime test.26 3.7. Characterization of the Catalysts After the Lifetime Test. The above three catalysts after 110 h lifetime tests were named as 20N/W-ST, 20N/20A-W-ST, and

Figure 2. TEM images of the support and reduced catalysts: (a) W, (b) 20N/W, (c) 20N/20A-W, and (d) 20N5M/20A-W; HRTEM image of (e) selected areas of d and (f−h) further enlarged views, corresponding to the areas 1−3 in e, respectively.

3.5. Catalytic Performances of the Catalysts. The details regarding the catalyst optimization can be found in section S-4, and the selected catalysts are discussed below. The CO methanation reaction was carried out in the temperature range of 260−550 °C at 0.1 MPa and a WHSV of 30 000 mL g−1 h−1, and the results are shown in Figure 5a−c. It can be seen that the catalytic activity of 20N/W is very poor and that the maximum CO conversion and CH4 yield at 450 °C is only 91.1 and 65.1%, respectively, mainly because of its poor H2 adsorption ability. After addition of Al2O3 into the catalyst 12520

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Figure 4. XPS spectra of the reduced catalysts: (a) Ni 2p, (b) Mn 2p, and (c) O 1s.

Figure 5. Catalytic performances of the reduced catalysts for CO methanation at 0.1 MPa and WHSV of 30 000 mL g−1 h−1: (a) CO conversion, (b) CH4 selectivity, and (c) CH4 yield.

Figure 6. Catalytic performances of the reduced catalysts for CO methanation at 0.1 MPa, 550 °C, and different WHSVs: (a) CO conversion, (b) CH4 selectivity, and (c) CH4 yield.

the mean size of Ni particles in 20N5M/20A-W is almost not changed as compared with that before the lifetime test. It should be noted that the mean Ni particles size calculated by Debye−Scherrer equation are much larger than that observed from TEM images because of the aggregation of crystals.22 The amount of the deposited carbon on the used catalysts is determined by TGA, as presented in Figure 7h. The weight increase in the TGA curves from 250 to 500 °C can be attributed to the oxidation of metallic Ni into NiO,22 and that after 700 °C is due to the oxidation of the pure Si (Figure S11b). The deposited carbon amount in the 20N/W-ST, 20N/ 20A-W-ST, and 20N5M/20A-W-ST catalysts is estimated to be 2.6, 0.9, and 0.3%, respectively, which is in agreement with the results of the TEM and SEM characterizations. It is noted that the oxidation of metallic Ni in 20N/W-ST is obviously delayed, which is due to the fact that Ni was covered in part by the

20N5M/20A-W-ST and characterized by TEM, SEM, XRD, and TGA. Figure 7a−c,e−g show the TEM and SEM images of them. It can be seen that some large deposited carbon filaments with the diameter of ca. 40−200 nm are formed on 20N/W-ST (Figures 7a, e and S11c), and the Ni particles are seriously aggregated. In contrast, there is very little carbon deposit on 20N/20A-W-ST, and the carbon filament diameter is small (7− 8 nm) (Figure 7b,f). As anticipated, almost no visible carbon deposition can be seen in the TEM and SEM images of the used 20N5M/20A-W. The mean crystal size of Ni in the 20N/ W-ST is calculated to be 7.5 nm (Table S4) on the basis of the XRD characterization, which is 1.5 times that of fresh 20N/W, indicating the poor thermal stability of this catalyst. Meanwhile, there is only a slight increase of the Ni particle size in 20N/ 20A-W-ST as compared with that of fresh 20N/20A-W (from 4.7 to 5.7 nm). After further addition of the MnOx promoter, 12521

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deposited carbon in the catalyst,38 and the presence of more bigger Ni particles in it compared to those in 20N/20A-W-ST and 20N5M/20A-W-ST catalysts. Figure S11a shows that the H2 adsorption ability of the 20N/W-ST catalyst after the 110 h lifetime test becomes significantly lower because of the sintering of Ni particles and large amount of carbon deposition. From the above results, we can conclude that both the smaller specific surface area of the W support and the lower Ni dispersion are not conducive to the heat transfer during the reaction at high temperatures, thus causing serious Ni sintering and the further increased carbon deposition. These poor antisintering and the low anticoking properties jointly lead to the fast deactivation of 20N/W-ST catalyst. In contrast, 20N/ 20A-W has both higher specific surface area and higher Ni dispersion, and the heat can be transferred quickly during the reaction because of the high thermal conductivity of Si. As evidenced, the Ni particle size (5.7 nm) in 20N/20A-W-ST after the 110 h lifetime test was increased only a little, and the amount of the carbon deposition was only 0.9%. For 20N5M/ 20A-W, besides the merits of 20N/20A-W, the added MnOx promoter also can act as the physical barrier to restrain the increase of Ni particles during the reaction at high temperatures. Thus, in this catalyst the Ni size after the 110 h lifetime test was almost the same as that in the fresh 20N5M/20A-W, and the amount of the carbon deposition was only 0.3%. 3.8. The Relationship between the Structure and the Performance of Catalysts. The zeta potentials of the stabilized W support, Al, Ni, and Mn precursor are −62.3, −1.5, +4.8, and +1.7 mV, respectively, indicating that the W support and Al precursor bear negative surface charges, whereas Ni and Mn precursors show positive surface charge. According to the principle of electrostatic interaction, in the preparation process of the 20NxM/20A-W catalysts (Figure 8c), part of Ni and Mn precursor will be preferentially adsorbed on the surface of the W support. After the surface charge is weakened, the Al precursor will deposit on the subsurface of the W support sequentially, followed by deposition of Ni and Mn precursor on the Al precursor. On the basis of the above zeta potentials results, a preparation process of the catalysts is outlined in Figure 8, and the structure−property relationship is revealed as below. As reported by Happel, the slowest step in the methanation reaction is the hydrogenation of CHx species,39 and the limiting factor is the concentration of adsorbed hydrogen atoms on the surface of the catalyst. From our H2-TPD results, it is clear that the addition of the Al2O3 and MnOx to the catalyst can increase H2 uptakes, thus improving the activity for CO methanation, which is consistent with the previous research results.12

Figure 7. TEM images of the used catalysts: (a) 20N/W-ST, (b) 20N/ 20A-W-ST, and (c) 20N5M/20A-W-ST, XRD patterns of the used catalysts (d), SEM images of the used catalysts: (e) 20N/W-ST, (f) 20N/20A-W-ST and (g) 20N5M/20A-W-ST, and TGA curves of the used catalysts in air (h).

Figure 8. Schematic of the formation process of (a) 20N/W, (b) 20N/20A-W, and (c) 20NxM/20A-W catalysts. 12522

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In 20N/W and 20N/20A-W, it seems that the calculated Ni particle sizes from XRD patterns by Debye−Scherrer equation are quite similar. However, we should point out that there is a limitation for this estimation method, which can just reflect the size of the primary particle in the sample. 20N/W (Figure 8a) shows a poor catalytic performance due to its H2 uptakes and a continuous decline in activity in the 110 h lifetime test caused by the severe Ni sintering and the carbon deposit. After modification by porous Al2O3, the obtained 20N/20A-W exhibits much enhanced activity and stability in the 110 h lifetime test because of its higher specific surface area and higher Ni dispersion and the faster transfer of the heat generated in the methanation reaction through the Si support. For the 20NxM/20A-W catalysts (Figure 8c), the added MnOx can act as the physical barrier to restrain Ni particles from sintering and agglomeration at high reaction temperatures and also improve the low-temperature performance through the electronic effect. The MnOx promoter can generate more oxygen vacancies, as proven by the observation of the higher integrated area ratio (Table S2) of OII/(OII + OI) in the 20N5M/20A-W catalyst (47.8 atom %) relative to that in the 20N/20A-W (22.7 atom %) catalyst. These oxygen vacancies can promote the dissociation of CO2 and generate surface oxygen intermediates that can react with carbon deposit, thus reducing the carbon deposition on the Ni particles in CO methanation.12,35,40

Present Address

Z.Z.: Institute of Chemical Engineering, Singapore 159088, and School of Chemical & Biomedical Engineering, Nanyang Technological University (NTU), Singapore 637459. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b03327. N2 adsorption isotherms, XRD patterns, XRD and H2TPR comparisons, catalyst property tables, TEM images, catalytic performance data, photos of molded catalysts, and catalyst bed temperature data. (PDF)

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ACKNOWLEDGMENTS

We gratefully acknowledge support from the National Natural Science Foundation of China (nos. 21476238 and 21206173), the National Basic Research Program (no. 2014CB744306), National High Technology Research and Development Program of China (863) (no. SS2012AA021402), and “Strategic Priority Research Program” of the Chinese Academy of Sciences (nos. XDA07010100 and XDA07010200).

4. CONCLUSIONS A series of Ni catalysts supported on the Al2O3-modified Si waste contact mass (W) are successfully prepared by the deposition−precipitation (DP) method, and the MnO x promoter is introduced to improve the catalytic properties further. Compared to the 20N/W catalyst, the 20N/20A-W catalyst supported on the Al2O3−Si composite exhibits better catalytic performance and higher anticoking and antisintering abilities in the lifetime test at extremely high WHSVs. After adding the MnOx promoter, the performance of the 20NxM/ 20A-W catalysts is further improved, as evidenced by the observation of excellent catalytic activity and stability in the harsh condition at high temperatures and WHSVs. 20NxM/ 20A-W contains small Ni particles, porous Al2O3, and the highly thermal conductive Si. The combination of these can facilitate the fast transfer of the heat generated in the reaction, the enhanced interaction between Ni and MnOx that prevents the Ni agglomeration, and the increased oxygen vacancies provided by the MnOx promoter that can improve the anticoking property of the catalyst.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +86-10-82544851; Fax: +86-1082544851. *E-mail: [email protected]. 12523

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