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Research Article Cite This: ACS Catal. 2018, 8, 6495−6506

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Low-Temperature Catalytic CO2 Dry Reforming of Methane on Ni-Si/ ZrO2 Catalyst Ye Wang,†,‡ Lu Yao,§ Yannan Wang,‡ Shenghong Wang,†,‡ Qing Zhao,‡ Dehua Mao,‡ and Changwei Hu*,†,‡ †

College of Chemical Engineering, Sichuan University, Chengdu, Sichuan 610065, People’s Republic of China Key Laboratory of Green Chemistry and Technology, Ministry of Education, College of Chemistry, Sichuan University, Chengdu, Sichuan 610064, People’s Republic of China § College of Architecture and Environment, Sichuan University, Chengdu, Sichuan 610065, People’s Republic of China Downloaded via UNIV OF WYOMING on June 18, 2018 at 21:50:28 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: The activity of a ZrO2-supported nickel catalyst promoted by silica (Ni-Si/ZrO2) in CO2 dry reforming of methane was carried out at 400 and 450 °C. The catalysts were prepared by an impregnation method and characterized by H2-TPR, XRD, TEM, TG-MS, Raman, XPS, and in situ XPS and DRIFTS. It was discovered that Ni-Si/ ZrO2 showed higher initial conversion of CH4 (0.50 s−1) and CO2 (0.44 s−1), and stability for low temperature (400 °C) DRM reaction in comparison to an SiO2-supported nickel catalyst promoted by zirconia (Ni-Zr/SiO2) (0.32 s−1 for both CO2 and CH4). The Ni-Si/ZrO2 catalyst featured the formation of active nickel particles with a small size of 6−9 nm and with slightly strong electronic donor ability, stabilization of the initial metal nickel state under the reaction conditions, and the formation of easily removed C1 coke. However, for the 450 °C DRM reaction, the coke that formed on the Ni-Si/ZrO2 catalyst was mainly C2 coke that was difficult to remove, because the CO2 preferred to combine with H species rather than react with the coke. For the Ni-Zr/SiO2 catalyst, the Ni0 species was oxidized to a NiO species under the reaction conditions at 400 °C and could not be restored, leading to its deactivation. KEYWORDS: Ni-based catalysts, low temperature, zirconium, methane, CO2 2CO → CO2 + C

1. INTRODUCTION Carbon dioxide reforming of methane (dry reforming of methane, DRM) has received much attention in the last few decades as an effective way of using both abundant carbon dioxide and methane to reduce the greenhouse effect.1−3 However, the main issue about DRM is its high endothermicity, which usually results in high reaction temperature (above 650 °C).4,5 However, high-temperature DRM usually causes serious sintering of metals and further deactivation of the catalysts.6−8 Also, the formation of a large amount of coke at high temperature also deactivates the catalyst.8−10 In addition, other reactions influence the performance of the catalysts for the DRM reaction (eq 1), among which are methane cracking (eq 2), the reverse water-gas shift (RWGS) reaction (eq 3), and the disproportionation of CO (eq 4).11,12 CH4 + CO2 → 2CO + 2H 2

(1)

CH4 → C + 2H 2

(2)

H 2 + CO2 → H 2O + CO

(3)

© XXXX American Chemical Society

(4)

Because the low-temperature DRM reaction is thermodynamically practical (the equilibrium conversions of CH4 and CO2 at 400 °C for the DRM reaction would be about 47.6% and 35.0%, respectively),13,14 more and more investigations have focused on the low-temperature DRM reaction, which could not only decrease the energy consumption and cost but also restrain the sintering of metals.7,15 Thus, the synthesis of a high-activity catalyst capable of being used at low temperature for the DRM reaction has attracted much attention. Ni-based catalysts have been extensively researched for the dry reforming of methane because of their high activity and low cost.16−24 Bradford et al.16 studied MgO, TiO2, SiO2, and activated carbon supported Ni catalysts and found that the Ni/ MgO catalyst showed the best stability. They pointed out that the activation of the C−H bond for its cleavage needed electron donation and that the support could alter the electronReceived: February 10, 2018 Revised: May 5, 2018

6495

DOI: 10.1021/acscatal.8b00584 ACS Catal. 2018, 8, 6495−6506

Research Article

ACS Catalysis

by a total immersion method using a commercial SiO2 support.15 2.2. Catalytic Activity Tests. Activity tests were carried out in a fixed-bed tubular quartz microreactor under atmospheric pressure. In this experiment, 0.25 g of the catalyst was reduced at 450 °C for 1 h with F(H2) = F(Ar) = 30 mL min−1 and then cooled to 400 °C before introduction of the reactant gases (F(CO2) = F(CH4) = 30 mL min−1) for the DRM reaction. The feed and the products were analyzed by an online gas chromatograph (Plot-C2000 capillary column). For the sake of comparison, the previously reported Ni-Zr/SiO2 catalyst was also retested. The CH4 (or CO2) turnover frequencies were calculated by the number of CH4 (or CO2) groups converted per second per exposed Ni atom. The number of CH4 (or CO2) groups converted per second was estimated by the CH4 (or CO2) conversion at the initial 1 h, while the number of exposed Ni atoms was determined by H2 chemisorption experiments. The conversion and turnover frequencies of the CO2 and CH4 and the selectivity and yield of the products were calculated as follows: nCH4,in − nCH4,out XCH4 = × 100% nCH4,in

donating ability to improve the activity of the catalyst. Li et al.25 also found that the moderate electronic donor ability of the catalyst could promote the performance of the catalysts. Alkaline-earth oxides, such as MgO, BaO, CaO, and SrO, and supports with strong Lewis basicity could also promote the performance of the catalyst.18,26 Sokolov et al.17 prepared a Nibased catalyst with various supports for the DRM reaction at 400 °C. It was found that Ni/La2O3−ZrO2 catalyst showed the highest stability without deactivation for 180 h on stream. Further, they noted that the deactivation of the catalyst was due to the formation of graphene-like coke covering on the catalyst and the NiO shell covering on the Ni particle rather than the sintering of Ni species. Gonzalez et al.19 discussed the nickel metallic particle size on a Ni/ZrO2 catalyst modified by H2 or CO treatment. They found that the interaction of nickel and zirconia prevented the Ni species from being lost. Recently, some improvements have been made in characterizing the catalysts, using such as in situ XPS and in situ DRIFTS techniques.27−33 Liu et al.28 found that methane could dissociate to form CHx and COx species at rather low temperature on Ni/CeO2 catalyst as revealed by in situ XPS, due to the strong interaction between metal and support. Pakharukov et al.29 carried out in situ XPS studies of methane oxidation and found that the platinum oxide was covered on the active phase Pt surface, thereby leading to the deactivation of the catalyst for methane oxidation. In addition, Jodłowski et al.,34 Yao et al.,15 and Xu et al.32 found possible intermediates (formate) using in situ DRIFTS experiments. Jiang et al.35 found by in situ DRIFTS experiments that the decomposition of CH4 could promote the dissociation of CO2 to form CO. In our previous work,15 it was found that the Ni-Zr/SiO2 catalyst showed DRM reaction activity at low temperature of (400 °C) for the conversion of both methane and carbon dioxide. By introduction of a Zr promoter, the nature of the active site changed and the formation of a low-temperature active site for the DRM reaction was promoted by ZrO2. In order to further investigate the low-temperature DRM reaction, we prepared a Ni-Si/ZrO2 catalyst, which showed a higher catalytic activity for the DRM reaction in comparison to the Ni-Zr/SiO2 catalyst under the same conditions, and the structure of the Ni-Si/ZrO2 catalyst is discussed in detail.

XCO2 =

SH 2 = SCO =

nCO2,in − nCO2,out nCO2,in

nH2 2nCH4,in − 2nCH4,out (nCH4,in

× 100%

× 100%

nCO × 100% + nCO2,in) − (nCH4,out + nCO2,out)

Yi = X i × Si × 100% CO/H 2 = YCO/YH2

TOF(CH4) = N (CH4)/N (Ni)

TOF(CO2 ) = N (CO2 )/N (Ni)

where Xi, Si, Yi, and TOFi are the conversion, the selectivity, the yield, and the turnover frequencies of i species (%), respectively, ni is the number of moles of i species, N(CH4) (or N(CO2)) is the number of CH4 (or CO2) groups converted per second, and N(Ni) is the number of exposed Ni atoms per gram of catalyst. 2.3. Catalyst Characterization. The ex situ and in situ XPS experiments were carried out on an AXIS Ultra DLD (KRATOS) spectrometer with a catalytic chamber, and the data were acquired using monochromated Al radiation. All of the data were referenced using the Si 2p peak at 103.5 eV. Prior to experiment, the sample was reduced at 450 °C for 1 h in the Catalytic Chamber. After reduction, the sample was cooled down to room temperature under an H2 atmosphere. For another treatment, after reduction, the feed gases (F(CH4) = F(CO2) = 30 mL min−1) were introduced, and the reaction was carried out for 20 or 60 min at 400 °C. After treatment, the sample was directly introduced into the XPS vacuum chamber to avoid exposure in air, and the spectra were collected at room temperature. The hydrogen temperature-programmed reduction (H2TPR) measurements were carried out with a Micromeritics

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The ZrO2 support was prepared by a precipitation method. A certain amount of Zr(NO3)4 was dissolved in deionized water with constant stirring. After that, aqueous ammonia was added to the Zr(NO3)4 solution until pH 9, and then the mixture was filtered for 2 h at room temperature and washed three times with deionized water. The obtained sample was dried at 110 °C for 4 h and then calcined at 600 °C for 5 h to obtain the ZrO2 support. The Ni-Si/ZrO2 catalyst (zirconia was used as the support and silica was used as the promoter) was prepared by a coimpregnation method. Ni(NO3)2 and (C2H5O)4Si were dissolved in alcohol (95%). The ZrO 2 support was impregnated with the aforementioned ethanol solution for 24 h at room temperature. After that, the sample was dried at 80 °C for 2 h and then at 110 °C for 4 h. Finally, the obtained material was calcined at 500 °C for 5 h to obtain the Ni-Si/ ZrO2 catalyst. The Ni-Zr/SiO2 catalyst (silica was used as the support and zirconia was used as the promoter) was prepared 6496


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Figure 1. Catalytic performance of Ni-Si/ZrO2 and Ni-Zr/SiO2 at (a) 400 °C and (b) 450 °C. Isothermal DRM reactions at 400 °C with a feed rate (CO2/CH4 = 1/1) of 60 mL min−1 for Ni-Si/ZrO2 and Ni-Zr/SiO2 catalyst: (c) CH4 and CO2 conversion; (d) yield of H2 and CO. Isothermal DRM reactions at 450 °C with a feed rate (CO2/CH4 = 1/1) of 60 mL min−1 for Ni-Si/ZrO2 and Ni-Zr/SiO2 catalyst: (e) CH4 and CO2 conversion; (f) yield of H2 and CO.

AutoChem II Chemisorption Analyzer. The temperature was raised from 25 to 800 °C at a heating rate of 8 °C min−1. Chemisorption experiments were performed also on a Micromeritics AutoChem II Chemisorption Analyzer. About 100 mg of the catalyst was reduced in situ under a 60 mL min−1 flow of an H2/Ar gas mixture at 450 °C for 1 h. The chemisorption measurements were carried out at 0 °C. The dispersion of Ni atoms was calculated by the exposed Ni atoms measured by the number of hydrogens desorbed. X-ray diffraction (XRD) measurements of the samples were carried out with Cu Kα radiation at 40 kV and 25 mA. The diffraction patterns were recorded in the range 5° < 2 θ < 80° with a scan speed of 0.3 s step−1 and a step size of 0.03°. Transmission electron microscopy (TEM) experiments were carried out on an FEI Tecnai G2 20 Twin instrument at an acceleration voltage of 200 kV. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) analysis combined with mass spectrometry

(MS) was also used to characterize the used catalysts. The temperature was increased from 30 to 800 °C with a heating rate of 5 °C min−1, and the air flow rate was 30 mL min−1. Raman spectroscopy measurements were carried out on a HORIBA R-XploRA Plus instrument with a 638 nm excitation wavelength. The wavenumber values were measured over the range 1000−3500 cm−1 with an average of three scans. The in situ diffuse reflectance infrared Fourier transform spectra (in situ DRIFTS) experiments were carried out on a Bruker infrared spectrometer (FT-IR V70). The sample was reduced at 450 °C in situ in H2/Ar for 1 h and then cooled to room temperature. The in situ DRIFTS experiments were started after introducing the dilute feed gas for 1 h at room temperature, and the system was heated from room temperature to 450 °C at a rate of 10 °C min−1. In situ DRIFTS CO2 and CH4 temperature-programmed reactions (TPRes) were carried out in a gas mixture with F(CH4) = F(CO2) = 4 mL min−1 and F(Ar) = 12 mL min−1. The in situ DRIFTS CH4 6497


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for 15 h. However, the conversion of CO2 decreased slightly with time on stream, to 2.8% after reaction for 7 h and to 2.3% for 15 h, which might be caused by coke formation. In parallel, the rates of CO and H2 formation followed a similar downward trend of methane, and the ratio of CO/H2 stabilized to about 1.5. A small amount of water was detected simultaneously, due to the reverse water-gas shift (RWGS) reaction (eq 3). As is shown in Figure 1e,f, at 450 °C the DRM reaction only proceeded for 2 h on stream on Ni-Si/ZrO2 catalyst, due to the blocking of the fixed-bed reactor by coke formation, thereby stopping the reaction. However, the activity on Ni-Zr/SiO2 catalyst decreased gradually. 3.2. Reducibility of Ni Species. The reducibility of the catalysts was characterized by H2-TPR (Figure 2). There were

temperature-programmed decompositions (TPDes) were performed under a flow with F(CH4) = 4 mL min−1 and F(Ar) = 16 mL min−1. CO2 temperature-programmed activation (TPA) was conducted with the following flow rates: F(CO2) = 4 mL min−1 and F(Ar) = 16 mL min−1.

3. ACTIVITY AND STRUCTURE OF NI-SI/ZRO2 AND NI-ZR/SIO2 CATALYSTS 3.1. Catalytic Activities. The catalytic activity results of the Ni-Si/ZrO2 and Ni-Zr/SiO2 catalysts are shown in Figure 1. The activity results of the previously reported Ni-Zr/SiO2 catalyst were the same: that is, the activation of both methane and carbon dioxide at 400 °C was achieved over Ni-Zr/SiO2 catalyst. The initial (at 1 h) conversion of both CH4 and CO2 was 2.0%, and the yields of H2 and CO were 0.8% and 1.2%, respectively.15 It was interesting to note that the Ni-Si/ZrO2 catalyst with similar Ni, Zr, and Si components exhibited higher catalytic activity in comparison to that of Ni-Zr/SiO2 catalyst at both 400 and 450 °C. It was especially noticeable that the Ni-Si/ZrO2 catalyst exhibited remarkably enhanced activity at 400 °C. As shown in Table 1, the metal dispersion Table 1. Ni Dispersion As Determined by H2 Chemisorption, Ni Loadings As Determined by ICP, and TOF for 400 and 450 °C DRM Reactions on Ni-Si/ZrO2 and Ni-Zr/SiO2 Catalysts (Reduced at 450 °C) TOF (s−1)c T = 400 °C

T = 450 °C

sample

Ni dispersion (%)a

Ni (wt %)b

CH4

CO2

CH4

CO2

Ni-Zr/SiO2 Ni-Si/ZrO2

0.39 0.58

8.3 7.8

0.32 0.50

0.32 0.44

1.06 1.38

1.48 1.30

Figure 2. H2-TPR profiles of Ni-Si/ZrO2 catalysts.

a

From H2 chemisorption. bDetermined by ICP. cEvaluated by Ni dispersion and conversion.

three peaks at about 447, 510, and 550 °C on the Ni-Zr/SiO2 catalyst.36 The peaks at 510 and 550 °C implied that it was very difficult to reduce this NiOx species, which had a strong interaction with SiO2.37,38 The Ni-Si/ZrO2 catalyst showed two distinct reduction peaks at about 343 and 410 °C and a broad shoulder at around 500 °C. The peak at about 343 °C could be attributed to the reduction of bulk NiOx species, and the second peak at about 410 °C could be assigned to the reduction of NiOx species weakly interacting with the support, while the additional shoulder at about 500 °C corresponded to a NiOx species having stronger interaction with the support or silica on the Ni-Si/ZrO2 catalyst. In brief, the reduction peaks on Ni-Si/ZrO2 catalyst shifted to low temperature in comparison to those on Ni-Zr/SiO2 catalyst, due to the weaker interaction between the NiOx species and the support. Thus, taking the reduction temperature (450 °C) into consideration, Ni species could be reduced more completely on Ni-Si/ZrO2 catalyst than on Ni-Zr/SiO2 catalyst. In order to further study the metal dispersion, H2-chemisorption experiments were carried out. As shown in Table 1, the controlled nickel content was the same, while the percentages of Ni species evaluated by ICP were 7.8% and 8.3% for Ni-Si/ ZrO2 and Ni-Zr/SiO2 catalysts, respectively. In spite of the slightly higher nickel content on Ni-Zr/SiO2 catalyst, the metal dispersion estimated by H2-chemisorption experiments on NiSi/ZrO2 catalyst (0.58%) was much higher than that on Ni-Zr/ SiO2 catalyst (0.39%). This indicated that the content of exposed Ni metal on Ni-Si/ZrO2 catalyst was much higher than that on Ni-Zr/SiO2 catalyst after reduction at 450 °C.

estimated by chemisorption experiments on Ni-Si/ZrO2 catalyst (0.58%) was much higher than that on Ni-Zr/SiO2 catalyst (0.39%). The CH4 and CO2 conversions on Ni-Si/ ZrO2 catalysts were 4.3% and 3.8% with turnover frequencies of 0.50 and 0.44 s−1, respectively, which were about 1.5 times that on Ni-Zr/SiO2 (0.32 s−1), as shown in Figure 1a and Table 1. In addition, higher yields of H2 (1.5%) and CO (2.0%) were also obtained over Ni-Si/ZrO2 catalyst. However, at 450 °C, the initial (at 1 h) conversions of CH4 and CO2 on Ni-Si/ZrO2 catalyst were 11.9% and 11.2% (Figure 1b), respectively, and the yields of CO and H2 were 5.7% and 3.5%, respectively. The CH4 turnover frequency on Ni-Si/ZrO2 (1.38 s−1) was just slightly higher than that on Ni-Zr/SiO2 (1.06 s−1). However, the CO2 turnover frequency on Ni-Zr/ SiO2 (1.48 s−1) was slightly higher than that on Ni-Si/ZrO2 (1.30 s−1). In addition, in comparison to the reaction at 400 °C, more water formed during the 450 °C reaction on Ni-Si/ ZrO2 catalyst, which indicated that Ni-Si/ZrO2 catalyst showed higher activity for the reverse water-gas shift (RWGS) reaction (eq 3) at 450 °C. In addition, the stabilities of catalysts determined at 400 °C for 15 h on stream showed also similar differences. It can be seen from Figure 1c,d that the Ni-Zr/SiO2 catalyst was almost deactivated within 5 h, whereas the conversions of CH4 and CO2 on Ni-Si/ZrO2 catalyst at 5 h were 3.3% and 3.5%. The conversion of CH4 almost stabilized to 3.2% even after reaction 6498


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Figure 3. (A) XRD patterns of Ni-Zr/SiO2 catalyst on (a) reduction, (b) reaction at 400 °C for 1 h, and (c) reaction at 400 °C for 5 h and Ni-Si/ ZrO2 catalyst on (d) reduction, (e) reaction at 400 °C for 1 h and (f) reaction at 400 °C for 5 h. (B) XRD patterns of Ni-Zr/SiO2 catalysts on (a) reaction at 450 °C for 1 h and (b) reaction at 450 °C for 5 h and Ni-Si/ZrO2 catalyst on (c) reaction at 450 °C for 1 h and (d) reaction at 450 °C for 5 h.

3.3. XRD Results. Figure 3A shows the XRD patterns of Ni-Si/ZrO2 and Ni-Zr/SiO2 catalysts after reduction and after reaction for 1 h at 400 °C. The intensity of the peak for metallic Ni on the reduced Ni-Si/ZrO2 catalyst was more distinct than that on reduced Ni-Zr/SiO2 catalyst, which is in good agreement with the results of H2-TPR. After reaction for 1 h, the intensity of the metallic Ni diffraction peak decreased and the diffraction peak of NiO appeared over Ni-Zr/SiO2 catalyst. After reaction for 5 h, the Ni0 diffraction peak faded away, while the NiO diffraction peak increased with a particle size of 13 nm on the Ni-Zr/SiO2 catalyst (see Table 2).

observed, which indicated that nickel species preferred aggregating on the catalyst, as shown in Figure 4(c) and (d). The highly dispersed Ni species on Ni-Si/ZrO2 increased the activation of CH4 and CO2.

4. CARBON DEPOSITION Figure 5 shows the TG-MS profiles of the Ni-Si/ZrO2 and NiZr/SiO2 catalysts after DRM reactions at 400 and 450 °C. The amount of the deposited carbon on used Ni-Si/ZrO2 catalyst was much higher than that on Ni-Zr/SiO2 (Figure 5a,b) catalyst at both reaction temperatures. This illustrated that the coke formation reaction proceeded seriously on the Ni-Si/ ZrO2 catalyst. In addition, in comparison to the reaction at 400 °C, the content of carbon deposited on Ni-Si/ZrO2 catalyst after reaction at 450 °C increased from 5.7% to 13.6% (Figure 5c,d). In addition, after reaction, the DSC curve showed two exothermic peaks for the used catalyst (Figure 5), which corresponded to the temperature zone of coke removal; at the same time, only CO2 (m/e 44) was detected by the mass spectrometer. According to the different temperature ranges of carbon removal, carbon deposition was defined as two types, C1 (removable at 360−500 °C) and C2 (removable at 500− 600 °C), and the contents of the two kinds of carbon deposited on the used catalysts are shown in Table 3. On NiZr/SiO2 catalyst after the 400 °C reaction, the content of C1 coke was 11 mg gcat−1, which was slightly higher than that of C2 coke (7 mg gcat−1). However, after reaction at 450 °C, the C1 coke content remained at about 11 mg gcat−1, while the C2 coke content increased to 11 mg gcat−1. Nevertheless, on Ni-Si/ZrO2 catalyst, almost the same contents of C1 and C2 were observed after reaction at 400 °C (29 and 28 mg gcat−1 for C1 and C2, respectively); however, after reaction at 450 °C, the C1 content decreased to 16 mg gcat−1 while that of C2 increased remarkably to 120 mg gcat−1. This phenomenon showed that a much greater amount of C2 coke formed on Ni-Si/ZrO2 catalyst after reaction at 450 °C, which was the main reason for the deactivation of the catalyst, because the C2 coke is very difficult to remove. Except for the type and content, the initial temperature for removing coke could evaluate how high a temperature was needed to remove the coke. The further identification of carbon was carried out using Raman and XPS experiments (see Figures S1 and S2). The results also showed

Table 2. Sizes of the Ni and NiO Species in Reactions at 400 and 450 °C Ni-Zr/SiO2 (nm) reaction temp (°C) 400 450

Ni NiO Ni NiO

0h

1h

11

9 11 9 12

11

2h

Ni-Si/ZrO2 (nm)

5h

0h

1h

12

10

12

11

2h

5h 9

13 11

12

However, for the Ni-Si/ZrO2 catalyst, the intensity of the Ni peaks decreased slightly after reaction for 5 h. The crystal size of metallic Ni decreased slightly, from 12 to 9 nm, which is in accordance with the results of Gonzalez et al.19 The XRD results of both catalysts after reaction at 450 °C were similar to the results after reaction at 400 °C (see in Figure 3B). These phenomena indicated that the Ni metal might be oxidized by CO2 on Ni-Zr/SiO2 catalysts, causing the enlargement of NiO particles and leading to the deactivation of the catalyst, which was similar to what was reported by Sokolov et al.17 However, the Ni metal could maintain its original state or be restored in situ on the Ni-Si/ZrO2 catalyst, thereby showing higher stability in the 400 °C DRM reaction. 3.4. TEM Results. Figure 4 shows the morphology of the reduced catalyst measured by TEM. It can be observed from Figure 4a,b that the Ni species on Ni-Si/ZrO2 was more evenly distributed on the support with particle sizes of 6−9 nm. On the Ni-Zr/SiO2 catalyst, the particle sizes of Ni species were widely distributed, and the majority were concentrated in the range of 10−12 nm with big nickel particles (above 20 nm) 6499


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Figure 4. TEM images of catalysts (a) Ni-Zr/SiO2 and (b) Ni-Si/ZrO2 and particle sizes of catalysts (c) Ni-Zr/SiO2 and (d) Ni-Si/ZrO2.

Si/ZrO2 catalyst, the intensity of Ni species decreased after reaction at 400 °C, due to the formation of a carbon covering on Ni species. It was noted that most Ni0 species could be observed obviously on Ni-Si/ZrO2 catalyst within 20 min of reaction. Even after reaction for 60 min, the Ni0 species still existed clearly on the Ni-Si/ZrO2 catalyst. This phenomenon showed that part of the Ni0 species could maintain the initial state or be restored in situ on a ZrO2 support because of the interaction between Ni species and the support and/or promoter, which was in agreement with the results of XRD. However, for the Ni-Zr/SiO2 catalyst, the Ni 2p signal shifted to higher BE values under the reaction conditions and the Ni0 species decreased after reaction for 20 min. When the reaction was carried out for 60 min, the Ni0 species almost faded away and NiO species formed on the Ni-Zr/SiO2 catalyst, which indicated that most of the Ni0 species was oxidized to NiO species with time on stream; as a consequence, the activity of Ni-Zr/SiO2 catalyst was almost lost within 5 h. Interestingly, the position of P1 (Ni2+ BE at approximately 854.5 eV) species and P2 (Ni2+ BE at approximately 857.0 eV)39−41 peaks on NiZr/SiO2 catalyst shifted slightly to high binding energy in comparison to those on Ni-Si/ZrO2 catalyst under the reaction conditions, which suggested that the ability of nickel species to donate electrons to ZrO2 modified by silica was stronger than that to SiO2 modified by ZrO2. Figure 6c.d gives the XPS spectra of Zr 3d on Ni-Si/ZrO2 and Ni-Zr/SiO2 catalysts, respectively. The peak about at 182.6 eV corresponded to the ZrO2.43−45 In addition to the decrease in intensity of ZrO2

that two kinds of carbon formed, which was in agreement with the TG-MS results. On combination of the results of XPS, TGMS, and Raman, the C1 coke might be defective carbon materials (C−O and CO species), while C2 coke might be graphitic carbon (C−C species). After 400 °C DRM, the initial coke removal temperatures were 360 and 382 °C for Ni-Si/ ZrO2 and Ni-Zr/SiO2 catalysts, respectively. However, after 450 °C DRM, these temperatures were 432 and 383 °C for NiSi/ZrO2 and Ni-Zr/SiO2 catalysts, respectively. Ni-Si/ZrO2 catalyst after reaction at 400 °C showed the lowest initial temperature (360 °C) for removing deposited carbon, which indicated that the coke was the easiest to remove. After reaction at 450 °C, the initial temperature for removing coke increased to 432 °C, which was much higher in comparison to the others: that is to say, the coke was very difficult to remove. Therefore, in 400 and 450 °C DRM reactions, the coke species formed on Ni-Si/ZrO2 catalyst might be different.

5. IN SITU XPS AND IN SITU DRIFTS 5.1. In Situ XPS Results. To investigate the state of Ni, Si, and Zr under the reaction conditions, in situ XPS was carried out, and the results are shown in Figure 6. The XPS spectra of Ni 2p are displayed in Figure 6a,b. On reduction at 450 °C, the Ni0 (BE at approximately 852.3 eV) species39−41 were clearly observed on both catalysts, which indicated that most NiO species were reduced to form Ni0 species. It is well-known that the activation of C−H bonds takes place on metallic Ni sites, which is the limiting step for DRM reactions.13,42 For the Ni6500


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Figure 5. TG-DSC-MS profiles of Ni-Zr/SiO2 catalyst after reaction at 400 °C (a) and 450 °C (b) and of Ni-Si/ZrO2 catalysts after reaction at 400 °C (c) and 450 °C (d).

Table 3. Amount of Carbon Deposition on Used Catalysts, Characterized by TG-MS carbon deposition C1 (peak 1)

C2 (peak 2)

catalyst

reaction temp (°C)

initial temp (°C)a

positionb (m/z)

weight(mg gcat.−1

position (m/z)

weight(mg gcat.−1)

Ni-Zr/SiO2

400 450 400 450

382 383 360 432

440 441 457 460

11 11 29 16

529 527 515 541

7 11 28 120

Ni-Si/ZrO2 a

The initial temperature for eliminating carbon deposition. bThe center position of the peak on CO2 MS single curve.

original state under the reaction conditions (see Figure 6g,h).49,50 5.2. In Situ DRIFTS Results. In order to get mechanistic information about the DRM reaction on Ni-Si/ZrO2 catalyst, in situ diffuse reflectance infrared spectroscopy (DRIFT) was carried out. Figure 7a shows the results of CO2 and CH4 temperature-programmed reactions (TPRes) on Ni-Si/ZrO2 catalyst. The IR bands at 1410, 1512, and 1665 cm−1 were characteristic of monodentate and bidentate carbonates and were more distinct than those on Ni-Zr/SiO2 catalyst (see ref 15),15,31,34,51,52 and the bands at 1270 and 1300 cm−1 corresponded to gaseous methane. With an increase in temperature, the intensity of gaseous methane and carbonates decreased slowly, due to the conversion or desorption of CO2 and adsorption of CH4. At 150 °C, new bands at about 1590, 1968, and 1865 cm−1 were detected, which corresponded to formate and bridge adsorbed CO.31,34 The intensity of formate increased with temperature, while it decreased rapidly when the temperature was higher than 300 °C. Moreover, linearly adsorbed CO (2040 cm−1) sprang up at 400 °C with the

after the reaction, the peak of ZrO2 shifted from 182.6 eV to a lower BE value (182.3 eV) on Ni-Si/ZrO2 catalyst with time on stream, indicating that a great number of charges transferred to the surface of ZrO2. This transfer might be attributed to the strong electron-donating ability of nickel species on ZrO2 modified by silica, which could contribute to the activity of the catalyst. However for the Ni-Zr/SiO2 catalyst, the peak shifted from 182.4 to 182.9 eV after a reaction of 60 min. These phenomena illuminated that no charge accumulated on the promoter Zr surface. Figure 6e,f gives the O 1s XPS spectra of Ni-Si/ZrO2 and Ni-Zr/SiO2 catalysts, respectively. The BE at about 528−530 eV corresponded to the oxygen ions in the crystal lattice (O2−).46−48 This mobile lattice oxygen species could promote the activation of C−H bonds.47 The relative content of O2− species on Ni-Si/ZrO2 catalyst was higher than that on Ni-Zr/ SiO2 catalyst, which improved the ability of C−H bond cleavage, thereby enhancing the activity of Ni-Si/ZrO2 catalyst. According to the Si 2p spectra results, the Si species belonged to the SiO2 on both catalysts, and the SiO2 could maintain its 6501


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Figure 6. In situ XPS spectra of Ni-Si/ZrO2 and Ni-Zr/SiO2 catalysts after reduction at 450 °C with a mixed flow (F(H2) = F(Ar) = 30 mL min−1) and reaction at 400 °C with a mixed flow (F(CH4) = F(CO2) = 30 mL min−1): (a) Ni 2p, (c) Zr 3d, (e) O 1s, and (g) Si 2p for Ni-Si/ZrO2 catalyst and (b) Ni 2p, (d) Zr 3d, (f) O 1s, and (h) Si 2p for Ni-Zr/SiO2 catalyst. 6502


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Figure 7. DRIFTS spectra of (a) CO2 and CH4 TPRe, (b) CH4 TPDe, and (c) CO2 TPA on Ni-Si/ZrO2 catalyst.

apperance of gaseous CO.31,32,35,53 The above phenomenon indicated that some carbonates decomposed to formate and further decomposed to CO. The formate that formed might be the intermediate species for the DRM reaction on Ni-Si/ZrO2 catalyst. According to the literature,54,55 it was considered that carboxyl species (HOCO) should be involved in the mechanism of the RWGS reaction. Solis et al.56 thought that no carboxyl species could be observed during the CO2 methanation reaction on Ni/ZrO2 catalyst, and so carboxyl species could not be involved on Ni/ZrO2 catalyst. Under our reaction conditions, the carboxyl species was not observed. However, we could not rule out the possibility of the formation of carboxyl species, because they were unstable at high temperature.57 In addition, the IR bands at 1490 and 1358 cm−1 could be attributed to the symmetric and asymmetric deformation vibrations of adsorbed CH3.15,58,59 It seemed that the adsorbed CH3 species could be detected on Ni-Si/ZrO2 catalyst, which was similar to the literature result that CH4 could be dissociated on IrO2 surface to form adsorbed CH3 and adsorbed H species at low temperature (lower than 240 °C).60 Figure 7b shows the results of CH4 temperatureprogrammed decomposition (TPDe) on Ni-Si/ZrO2 catalyst. The adsorbed CH3 species were very distinct on the Ni-Si/ ZrO2 catalyst at 50 °C, and the intensity decreased with temperature, indicating the loss of adsorbed CH3 species, due to the recombination of adsorbed CH3 species and H species or the decomposition of adsorbed CH3 species.60 Figure 7c shows the results of CO2 temperature-programmed activation (TPA) on the Ni-Si/ZrO2 catalyst. The linearly adsorbed CO (2020 cm−1) developed starting from 250 °C, while the bridge adsorbed CO (1850 cm−1) developed starting at 300 °C, and

the intensity of both peaks increased with the temperature. Moreover, no gaseous CO signal could be observed in the TPA reaction. It is worth pointing out that, in comparison to the TPDe reaction, the frequency of CO adsorbed species over the TPA reaction decreased from 2040 and 1865 cm−1 to 2020 and 1850 cm−1 for linear and bridged CO adsorption (red shift), respectively. This red shift indicated a slight increase of electron density on Ni, because of the adsorption of CO2 species on the Ni-Si/ZrO2 catalyst. As a consequence, the oxygen concentration at the interface of metal and support decreased.31,33 This phenomenon indicated that CO2 could be dissociated on Ni-Si/ZrO2 catalyst: that is to say, the Ni-Si/ ZrO2 could promote not only the adsorption of CO2 but also the dissociation of CO2. According to the in situ DRIFTS results and refs 31, 33, 35, and 53, CH4 could be dissociated and adsorbed as CH3 and H species on the Ni-Si/ZrO2 catalyst, whereas CO2 could be adsorbed and dissociated to form adsorbed CO and O species. Part of the adsorbed H species could combine with adsorbed CO2 to form formate, while part of the adsorbed O species could react with adsorbed CH3 species, thereby promoting the dissociation of methane, which might occur as follows: CH4 ↔ CH3,ads + Hads I:

CO2,ads ↔ COads + Oads 2Hads ↔ H 2 COads ↔ COg

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II:

higher elimination temperature of C2 coke might be caused by the larger amount of deposited carbon and structural variation caused by temperature. The different electronic effects on the two catalysts might mainly affect the selectivity of different reactions in the system, further leading to the promotion of the RWGS reaction on the Ni-Si/ZrO2 catalyst. Hence, the role of electron transfer on the performance of Ni-Si/ZrO2 catalysts should be further investigated through adjusting the interaction of nickel and support. In particular, keeping the high activity of CH4 cracking, increasing the activity of coke removal at low temperature, and decreasing the activity of reverse water gas shift reaction would be promising for a low-temperature DRM catalyst.

CO2,ads + Hads ↔ COOHads ↔ Coads + OHads (like RWGS) 2OHads ↔ H 2 + H 2O COads ↔ COg

6. DISCUSSION The Ni-Si/ZrO2 catalyst exhibited higher catalytic activity and stability than the Ni-Zr/SiO2 catalyst in low-temperature DRM reactions. At 400 °C, the CH4 and CO2 turnover frequencies on Ni-Si/ZrO2 catalyst were about 1.5 times of those on NiZr/SiO2. At 450 °C, the CH4 turnover frequency on Ni-Si/ ZrO2 became just slightly higher than that on Ni-Zr/SiO2. However, the CO2 turnover frequency on Ni-Zr/SiO2 was slightly higher than that on Ni-Si/ZrO2. The interaction between nickel and support and/or promoters could affect the environment of Ni. Because of the interaction of nickel with silica, many NiO species that were difficult to reduce formed on Ni-Zr/SiO2 catalyst,36 resulting in the fact that the Ni0 species reduced at 450 °C on the catalyst were oxidized by CO2 to NiO species, which could not be restored in the reaction process at 400 °C, causing the enlargement of NiO particles. However, most NiO species were easily reduced on Ni-Si/ZrO2 catalyst, due to the small amount of silica added; furthermore, the NiO formed under the reaction conditions could be restored to Ni0 species, which contributed to maintaining the original state of nickel. The interaction between nickel and zirconia promoted the dispersion of nickel.21,40,61 Therefore, 10−12 nm particles formed on the NiZr/SiO2 catalyst, due to the small amount of zirconia added, while active nickel particles a small size of 6−9 nm formed on the Ni-Si/ZrO2 catalyst. Meanwhile, these appropriate interactions were characterized by partial charge transfer from nickel particles to zirconia, which might prevent the sintering of nickel species. ZrO2 could promote the adsorption of CO2, since ZrO2 could increase the number of basic sites,21,62 thereby decreasing of the adsorption of CO2 on Ni0 species and thus relieving the oxidation of Ni0 species and maintaining the original metal Ni state on Ni-Si/ZrO2 catalyst during the process of DRM reaction. In addition, Ni-Si/ZrO2 catalyst could dissociate CO2, forming COads and Oads, while Oads could promote the cleavage of C−H bonds.29,35 Guo et al.63 discovered that the performance of catalysts for the DRM reaction could affect the amount of carbon deposition, as well as the type of coke. The results in the present work are in agreement with their report. According to the initial temperature of eliminating deposited carbon on both catalysts, we speculated that the carbon depositions on both catalysts were similar and the above difference might mainly be caused by the amount of carbon deposition and the effects of temperature on the final carbon formation. With increasing reaction temperature, the ability to remove the coke diminished, leading to the accumulation of coke, as well as a gradual transformation to inactive coke. In addition, for the Ni-Si/ZrO2 catalyst, on the one hand the decomposition of CH4 was very fast and on the other hand CO2 preferred to react with H2 at 450 °C, thereby promoting the reverse water-gas shift (RWGS) reaction, thus a fair amount of CO2 reacted with H2 rather than coke. This was the reason for the large amount of carbon decomposition. The

7. CONCLUSION Ni-Si/ZrO2 catalyst showed higher initial conversion of both CH4 (4.3%) and CO2 (3.8%) and was relatively stable in a 400 °C DRM reaction in comparison to Ni-Zr/SiO2 catalyst. Even after reaction for 15 h, CH4 and CO2 conversions on Ni-Si/ ZrO2 catalyst were 3.2% and 2.3%, respectively, while the NiZr/SiO2 catalyst was almost completely deactivated. Ni0 was oxidized to NiO on the Ni-Zr/SiO2 catalyst, resulting in its deactivation. The proper interaction of ZrO2 and Ni on Ni-Si/ ZrO2 catalyst promoted the formation of small active particles (6−9 nm) with slightly strong electron donor ability and maintenance of the state of Ni metal under the reaction conditions. In addition, mostly easily removed C1 coke formed on the Ni-Si/ZrO2 catalyst after reaction at 400 °C. Because CO2 reacted with H species more easily than with coke, mostly C2 coke that was very difficult be remove deposited on Ni-Si/ ZrO2 catalyst after reaction at 450 °C. The results on the novel Ni-Si/ZrO2 catalyst provided new clues toward the development of a low-temperature DRM reaction catalyst.

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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/acscatal.8b00584.

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Raman results of carbon deposited on catalysts after reaction and XPS results of carbon deposited on catalysts after reaction (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail for C.H.: [email protected]. ORCID

Changwei Hu: 0000-0002-4094-6605 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (No. 20976109 and 111 project (B17030)). We thank Yunfei Tian of analytical & testing center of Sichuan University for XPS experiments.

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