SiO2 Prepared by the Complexed

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Kinetics, Catalysis, and Reaction Engineering

A high performance Ni/SiO2 prepared by complexed- impregnation method with citric acid for carbon dioxide reforming of methane Hua-Ping Ren, Qing-Qing Hao, siyi ding, yuzhen zhao, min zhu, shaopeng tian, qiang ma, wenqi song, zongcheng miao, and Zhao-Tie Liu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03897 • Publication Date (Web): 09 Nov 2018 Downloaded from http://pubs.acs.org on November 12, 2018

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Industrial & Engineering Chemistry Research

A high performance Ni/SiO2 prepared by complexedimpregnation method with citric acid for carbon dioxide reforming of methane Hua-Ping Rena, Qing-Qing Haob, Si-Yi Dinga, Yu-Zhen Zhaoa, Min Zhua, Shao-Peng Tiana, Qiang Maa, Wen-Qi Songa, Zongcheng Miaoa,*, and Zhao-Tie Liuc,d,* a

School of Science, Xijing University, Xi’an, Shaanxi 710123, China

b

School of Chemical Engineering, Northwest University, Xi’an, Shaanxi 710069, China Key Laboratory of Applied Surface and Colloid Chemistry (Ministry of Education), School of Chemistry & Chemical Engineering, Shaanxi Normal University, Xi’an, Shaanxi 710119, China d College of Chemistry and Chemical Engineering, Shaanxi University of Science & Technology, Xi’an, 710021, China c

*Corresponding authors: Zongcheng Miao School of Science Xijing University, Xi’an 710123, China Tel: +86-29-8562-8154 E-mail: [email protected]

Zhao-Tie Liu School of Chemistry & Chemical Engineering Shaanxi Normal University, Xi’an 710119, China Tel: +86-29-8153-0802 E-mail: [email protected]

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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

Abstract: A group of Ni/SiO2 was prepared via complexed-impregnation method with different amounts of citric acid (Ni-C/SiO2). The Ni-C/SiO2 and Ni/SiO2 prepared by conventional impregnation method were relatively tested for carbon dioxide reforming of methane (CDR) at CO2/CH4 = 1.0, gas hourly space velocity (GHSV) of 60000 mL·g-1·h-1, and reaction temperature of 750 oC. The characterization results indicated that Ni-C/SiO2 showed higher Ni dispersion and stronger Ni-support interaction than Ni/SiO2. As a result that the superior anti-coke and anti-sintering of Ni was obtained over the Ni-C/SiO2 during CDR operated period. Importantly, a highly active and stable catalyst was obtained by optimizing the amount of citric acid added, and an unobservable deactivation was exhibited over Ni-C/SiO2 with the molar ratio of citric acid to Ni of 2.0 for the CDR testing within the TOS = 100 h. Key words: Complexed-impregnation; Reforming of methane; Carbon dioxide; Nickel; Stability

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1. Introduction With the advancement of science and technology, modern industry has developed rapidly, resulting in the ongoing consumption of fossil fuels; this consumption has led to the energy source shortage and environmental issues, especially global warming, becoming increasingly serious and of global concern.1,2 The utilization and conversion of the potent CO2 and CH4 greenhouse gases have attracted intense attention.3,4 Considerable attention has been devoted to the carbon dioxide reforming of methane (CDR), which produces syngas with low molar ratio of H2 to CO. Moreover, the syngas from the CDR process is a crucial feedstock for synthesizing high-value chemicals such as liquid fuel from F-T synthesis, methanol and higher oxygenates hydrocarbons production.5-7 Thus, many catalysts, such as precious metal-based catalyst (Rh, Ru, and Pt),8-11 have been investigated for the CDR because of their superior catalytic activity and coke resistance. But considering the limited resources and economic cost, numerous studies have been directed to the transition metal catalyst, particularly the Ni-based catalyst.12,13 It is well-known that the deactivation of catalyst caused by coke deposition and nickel sintering at high temperature is the biggest challenge for the industrial application of Ni-based catalyst for CDR.14-16 Hence, many efforts have been made to promote resistant coke and anti-sintering properties of nickel over Ni-based catalyst for CDR. Thus, continuous research has been carried out for the design of Ni-based catalyst with high activity and stability for the CDR. It has been reported that the reasons affecting the CDR performance of Ni-based catalyst may be summarized as follows: (1) the Ni particle size and the Ni-support interaction. Smaller Ni particle and suitable Ni-support interaction lead to good performance for CDR because of excellent anti-coke and anti-sintering of Ni.14 (2) the surface structure of the Ni. It has been reported that the Ni (111) surface showed better anti-coke property than that over Ni (100) and (110) planes because the deposited coke could be quickly gasified by CO2 over the Ni (111) surface.14,17,18 Thus, different measures have been taken for this purpose. The first is to synthesize core-shell structure nanocatalysts, such as Ni-yolk@Ni@SiO2,19 Ni@SiO2,20 and [email protected] The second is to obtain the confinement of Ni-based catalyst, for example, by using ordered mesoporous Ni-Ce-Al oxide material,22 nickel phyllosilicate (PS) 3



and nanotubes.23 The third is to adjust the structure characteristics and surface feature of Ni-based catalyst via plasma treatment.16,24,25 However, these methods require greater amounts of materials and energy than the conventional methods, which is unfavorable for their commercialization. For the supported Ni catalyst, a lower dispersion of Ni is the obstacle for CDR. Liu et al.26,27 observed that Ni dispersion was significantly promoted by complexation of the Ni2+ with various complexants. Moreover, many organic ligands, such as acetic acid, ethylenediamine, or L-arginine, were added as a complexant in the process of Ni impregnation.28-31 Because it is cheap and less toxic, citric acid was also used as a complexant for synthesis of the high-dispersion metal catalysts, which were mainly used as hydrodesulfurization (HDS) catalysts.32,33 Recently, citric acid was as complexant to improve Ni dispersion for CO methanation and reforming of CH4, but the effect of the amount of citric acid and origin on the high dispersion of Ni were not reported.29,30,34 In this work, commercial SiO2 with high specific surface area was used as the support, Ni(NO3)2·6H2O was used as Ni source, and citric acid was used as the complexant, the complexed-impregnation method was used to adjust the Ni particle size and Ni-support interaction, and the obtained catalyst was evaluated for CDR at 750 °C, gas hourly space velocity (GHSV) of 60000 mL·g-1·h-1, and CO2/CH4 = 1.0. Compared to Ni/SiO2 prepared with the conventional impregnation method, all Ni-C/SiO2 showed superior CDR performance, and in particular, near thermodynamic equilibrium of CH4 conversion was obtained over the optimized Ni-C/SiO2, no observable deactivation was observed for 100 h. This approach is likely for the synthesis of an excellently active and stable CDR Ni-based catalyst, which is promisingly applied on industrial scale because of its facile synthesis. 2 Experimental section 2.1 Catalyst preparation Ni-based catalyst was synthesized by the citric acid complexed-impregnation method. The load of nickel on catalyst was 10 wt.%, and different amounts of citric acid were added. Prior to the impregnation, commercial SiO2, which was purchased from Fujisilicia (Q-50), was treated at 750 °C for 4 h. After impregnation, the sample was stored in the air overnight, and evaporated at 80 °C for 12 h. Then, the solid was transferred to a muffle furnace and calcined 4


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at 500 °C for 4 h, obtained sample was listed as Ni-C-x/SiO2, where x represents the molar ratio of citric acid to Ni. For comparison, the Ni/SiO2 was synthesized by the same process but without addition of citric acid. 2.2 Characterization technique of the catalyst The crystal structure and phase analysis of the sample were obtained using Bruker powder X-ray diffractometer (D8 advanced). The sample was tested at the conditions as follows: Cu Kα, λ = 1.5418 Å, 40 KV, and 40 mA. The grain size of NiO was calculated according to the (200) diffraction peak of the fresh sample, while the grain size of Ni was calculated based on the (111) diffraction peak of the reduced and spent catalysts based on formula of Scherrer. The textural properties of the sample, such as BET surface areas, pore volumes, and pore sizes distribution, were characterized by a BelSorp-Max physisorption analyzer at -196 oC.

Before the testing, all samples were pre-treated at 300 oC for 12 h to remove adsorbed gas

and moisture. The BET surface areas were acquired by BET method, while pore volumes were tested from the nitrogen adsorption data at the relative pressure (P/P0) = 0.99. The reduction behaviors of the sample were characterized by H2-temperature programmed reduction (H2-TPR) on a Micromeritics Autochem 2920 apparatus with a thermal conductivity detector (TCD). Prior to the testing, approximately 0.0500 g sample was loaded and treated at 300 °C for 60 min with Ar. Then, the sample was cooled to 100 oC. H2-TPR was recorded from 100 oC to 1000 °C at the heating rate of 10 °C min-1 in 10 vol% H2/Ar gas flow. TEM image of the reduced sample was acquired using a JEOL transmission electron microscope (JEM-2100). The sample was dispersed in ethanol using ultrasonic for 30 min. The suspension was dripped onto a carbon-enhanced copper grid and dried in air. XPS was used for analysis of the surface of the sample using a Kratos X-ray photoelectron spectrometer with monochromatic Al Kα radiation. Sample was tested at about 5 × 10-9 Torr, and the peak shift was calibrated by C1s binding energy of 284.8 eV. The coke deposited on spent catalyst was measured using a thermoanalyzer systems of TA instruments (Q1000DSC+LNCS+FACS Q600SDT) at an air flow with a heating rate of 10 °C/min from 100 to 1000 oC. 2.3 Activity evaluation of the catalysts 5



CDR testing was performed on a fixed bed quartz tubular reactor (i.d. = 8 mm). 0.10 g catalyst, which was diluted with 5 times the weight of quartz sand, was loaded and sandwiched by two quartz layers. Before testing, the catalyst was reduced in flow of 20% H2/N2 (50 mL/min) at 700 °C for 150 min and then heated to 750 °C in N2 atmosphere. Then, the CO2 and CH4 with the molar ratio of 1.0 were injected into the reactor. The reaction was performed at 750 °C, and GHSV of 60000 mL·g-1·h-1. The product gases were analyzed online by gas chromatography (GC9720 II, Zhejiang Fuli chromatographic analysis Co., Ltd) after cooling to room temperature and condensing the water. A thermal conductivity detector (TCD) was used to analyze the amounts of the effluent products with the Molecular Sieve 5A and Porapak Q capillary columns. 3 Results and discussion 3.1 Structural properties of Ni-based catalyst X-ray diffraction patterns of Ni-C/SiO2 by the complexed-impregnation method with various citric acid contents are shown in Figure 1. For comparison, the data for Ni/SiO2 prepared with the conventional incipient impregnation method are also presented in Figure 1. As shown in Figure 1, all samples exhibited the similar diffraction peaks, and in particular, the diffraction peak at 2θ ≈ 23° was attributed to amorphous silica species.35 Moreover, the diffraction peaks at 2θ = 37, 43, 63, 75, and 79° can be ascribed to (111), (200), (220), (311), and (222) planes of cubic NiO,26,27 suggesting that the crystal morphology of the sample did not change for preparing with or without citric acid. However, for the intensity of diffraction peaks, obvious differences were showed on Ni/SiO2 and Ni-C/SiO2. Compared with Ni/SiO2, the intensity of NiO diffraction peaks of Ni-C/SiO2 was significantly weaker, and full width at half maximum (FWHM) was obvious broader, implying that smaller NiO particle was obtained after the addition of citric acid, and with the increase of the molar ratio of citric acid to Ni from 0.2 to 4.0, the FWHM of the NiO diffraction peaks broader slightly. The grain size of NiO was calculated by the diffraction peak of the (200) plane based on the formula of Scherrer, which is listed in Table 1. A significantly effect of sample preparation method on Ni grain size was obtained. Ni/SiO2 exhibited the biggest NiO grain size (~21 nm), but Ni-C/SiO2 showed a smaller value (~10-11 nm). Moreover, the NiO grain size was slightly decreased from 10.9 nm to 9.6 nm with the increase of the molar ratio of citric acid to Ni from 0.2 to 4.0. This can 6


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be explained as due to the complexes formed by citric acid and the precursor of Ni2+ and the Ni(II)-CA species being determined by the pH of solution, i.e., the amount of citric acid in our experiment.32 The pH of catalyst solution was roughly measured by pH meter, i.e., 5.23, 3.71, 3.39, 3.21, 3.08, and 2.76 for Ni-C-0.2/SiO2, Ni-C-0.5/SiO2, Ni-C-1.0/SiO2, Ni-C-1.3/SiO2, Ni-C-2.0/SiO2, and Ni-C-4.0/SiO2, respectively. 3.2 Structural properties of reduced Ni-based catalyst The XRD patterns of reduced catalyst, which was reduction at 700 °C for 150 min, were obtained and are presented in Figure 2. All samples also showed very similar diffraction peaks. In addition to the peak at 2θ ≈ 23°, which was attributed to amorphous silica species, three clear diffraction peaks at 2θ = 44, 52, and 76° are observed, these are ascribed to the (111), (200), and (220) planes of Ni.[26,27,29] The diffraction peak of the (111) plane was also used to estimate the Ni grain size based on the formula of Scherrer, and the data is listed in Table 1. The Ni grain size was really affected by sample preparation method. The Ni grain size on the reduced Ni/SiO2 was approximately 30 nm, and ~10-14 nm on the Ni-C/SiO2. When these results were used to indicate the dispersion of Ni,36 the opposite trend of the changes with the Ni grain size was obtained, and presented in Table 1. Moreover, when the grain size of Ni and NiO was carefully compared, it was unexpectedly found that the NiO grain size of fresh samples was smaller than Ni grain size of corresponding reduced samples. This may be interpreted metal Ni aggregated under high temperature reduction conditions. Except for the diffraction peaks of Ni, NiO and/or NiSiOx were not observable over the reduced catalyst from Figure 2, suggesting a very suitable for the reduction of the supported Ni catalyst at 700 oC for 150 min. For an in-depth comparison of the particle size and the Ni distribution on the reduced catalyst, the typical samples were characterized using TEM, and the TEM images presented in Figure 3. It was clear that larger Ni particle size was obtained on reduced Ni/SiO2 while smaller Ni particle size and a narrower distribution of Ni particle were observed on reduced Ni-C-2.0/SiO2. These results are consistent with the XRD results (Table 1), suggesting that the high dispersion of Ni particle was obtained by complexing with citric acid. 3.3 Textural features of catalyst The textural features of catalyst calculated from the N2 adsorption-desorption isotherms are 7



also listed in Table 1. Compared to SiO2, the BET surface areas, pore volumes, and pore diameter of Ni/SiO2 and Ni-C/SiO2 catalysts were clearly decreased. This can be easily assigned to the loaded NiO on the surface and tunnel structure of SiO2 leading to the decreased available surface areas. Moreover, the BET surface areas were slightly increased from 65 to 77 m2·g-1 with the increase in the molar ratio of citric acid to nickel from 0 to 1.3 while it was stable (approximately 77 m2·g-1) with the molar ratio of citric acid to nickel further increasing from 1.3 to 4.0, indicating that the amount of the added citric acid played a crucial impact in determining the BET surface areas of Ni supported on SiO2. This may be interpreted as due to appearance of new pores formed by the decomposition of the complex between Ni2+ and citric acid. However, when the molar ratio of citric acid to Ni is greater than 1.3, a complete complex of Ni was formed,33,34 which caused the BET surface areas to remain stable. Moreover, the pore volumes and pore diameter were slightly varied by approximately 1.00 cm3·g-1 and 50 nm. According to the mechanism of incipient impregnation, the pore volumes and pore diameter were only affected by the amount of Ni loading since the citric acid was removed by calcination.34 3.4 Reduction properties of catalyst The reducibility of the catalyst was tested by H2-TPR, and the results are presented in Figure 4. An obvious reduction peak centered at 362 °C was found over the Ni/SiO2. However, in addition to the peak centered at about 360 °C, an obvious peak were also observed at approximately 500-600 °C on the Ni-C/SiO2, and the area of the reduction peak was increased with the increase of the molar ratio of citric acid to nickel from 0.2 to 1.3, while it remained constant with the further increase of the ratio to 4.0. Generally, the reduction peak of the larger NiO particle was at lower temperatures (below 400 °C) while the reduction peak of the smaller NiO particle and the NiO with stronger interaction with SiO2 was at higher temperatures.27,37 Based on above analysis, only NiO species was presented on all of the samples, and the peak at the lower temperatures approximately 360 °C can be reasonably ascribed to the reduction of NiO with larger particle, while the peak at 500-600 °C was attributed to the reduction of NiO with smaller particle or stronger interaction and support. Therefore, the reducibility of catalyst was obviously affected on the addition of the various amounts of citric acid. This may be explained by the fact that higher NiO dispersion and 8


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stronger NiO and SiO2 interaction due to the formation of Ni-O(H)-SiO was observed over the Ni-C/SiO2.29,38 It was reported that the larger Ni particle and the weak interaction between the Ni species and support led to severe coke deposits and sintering of Ni for CDR, leading to a weaker CDR performance. To confirm our speculation that the stronger Ni and SiO2 interaction was formed over the catalyst with the addition of citric acid, the typical reduced Ni/SiO2 and Ni-C-2.0/SiO2 were characterized by XPS analysis, and the Ni2p3/2 spectra are shown in Figure 5. Based on the previous reports, about 852 eV of peak was reasonably assigned to metallic Ni,39 and about 858 eV of peak was also attributed to NiO species formed by oxidation at air. Moreover, no peak was assigned to the NiSiOx species based on Ref.40 Thus, no NiSiOx phase was formed while a small amount of NiO on the surface of reduced catalyst was formed by oxidation in air, which could not be detected by XRD. As expected, reduced Ni/SiO2 showed an obviously lower binding energies of metallic Ni (~852.15 eV) and Ni2+ (~857.86 eV) than reduced Ni-C-2.0/SiO2, i.e., 852.75 for metallic Ni and 858.59 eV for Ni2+, respectively. These results reveal that stronger Ni and SiO2 interaction was obtained on Ni-C-2.0/SiO2 compared to Ni/SiO2, which is in agreement with the TPR results (Figure 4). 3.5 Catalytic performance All the catalysts were tested for CDR, and CH4 conversion over reaction time is presented in Figure 6. Near thermodynamic equilibrium of CH4 conversion (approximately 83%) was obtained over all catalysts,27 indicating that Ni has a high CDR activity. Moreover, the CH4 conversion on Ni/SiO2 was sharply declined to approximately 10% after TOS = 20 h, which could be explained as due to the bigger Ni particle size shown in Figure 2 causing a severe coke deposition leading to the deactivation of Ni/SiO2. But, Ni-C/SiO2 showed higher stability than the Ni/SiO2, indicating that the stability was increased by the addition of citric acid. With the mole ratio of citric acid to Ni increasing, the stability of catalyst was increased follow a sequence of Ni/SiO2

SiO2 Prepared by the Complexed

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