Effect of Additives on Cracking Characteristics of Dust-Containing Tar

May 24, 2016 - Juan YuDechang MengHuawei ZhangJunqiang GaoYaqing ZhangTiantain JiaoPeng Liang. Industrial & Engineering Chemistry Research ...
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Effect of Additives on Cracking Characteristics of Dust-Containing Tar over Nickel-Based Catalysts Peng Liang,* Xiaohang Wang, Yaqing Zhang, Juan Yu, and Xiwang Zhang College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao, Shandong 266590, People’s Republic of China ABSTRACT: To develop tar-cracking catalysts with an industrial application value and to study the effect of dust species on the cracking characteristics of dust-containing tar, four kinds of modified nickel (Ni)-based catalysts were prepared by the incipient wetness impregnation method. Alkali and alkaline earth metal oxides K2O and MgO and transition metal oxides CeO2 and ZrO2 were used as additives. The activities and stability performances of these catalysts were investigated in a fixed-bed apparatus. The additions of K2O and MgO neutralized the acidity of the catalysts and reduced carbon deposition on the catalyst surface, and the additions of CeO2 and ZrO2 improved active ingredient dispersion. X-ray diffraction studies revealed that the (Zr0.85Ce0.15)O2 solid solution has good stability. The 2% NiO/1% CeO2/2% ZrO2−Al2O3 catalyst shows the highest activity and stability among the four modified Ni-based catalysts. Depositions of different species of dust on the catalyst surface have different influences on the catalyst performance; MgO, CaO, and Fe2O3 increase the catalytic activity, while SiO2 and Al2O3 decrease the catalytic activity.

1. INTRODUCTION As a traditional coal chemical process, coking provides large amounts of raw material for the steel and chemical industries. Coke-oven crude gas contains ∼100 g m −3 benzene, naphthalene, and other heavy tar components and a small amount of dust that forms a tar residue. Heavy tar and dust not only clog pipes and corrode equipment but also are waste resources and pollute the environment.1 Currently, heavy tar is usually disposed by catalytic cracking with nickel (Ni)-based catalysts.2−4 Most studies focus on the effect of additives that are able to delay the deactivation as a result of carbon deposition and sintering of the active ingredients.5 Lee et al.6 showed that the Ni/CeO2−Al2O3 catalyst was deactivated by coke deposition and active metal sintering, but it maintained stable activity for steam propane reforming in cyclic oxidation operation for 500 h. The studies by Yu et al.7 on Ni/Ce−ZrO2/Al2O3 catalysts with different Ce−ZrO2 contents and Ni loadings have revealed that the addition of Ce−ZrO2 limited the sintering of Ni particles and enhanced catalytic activity. The catalyst is promising for the direct removal of tar compounds from hot coke-oven gas with low S/C ratios. Yang et al.8 have reported that the addition of MgO to Ni/MgO/Al2O3 catalysts could significantly enhance the resistance to carbon deposition. Liang et al.9 reported that adding K reduces the surface acidity and improves the catalytic activity of Ni-based catalysts; however, K is easily lost during catalyst calcination. Juan-Juan et al.10 revealed that the presence of potassium in Ni/Al2O3 catalysts hinders the accumulation of coke on the catalyst surface during the dry reforming of methane. In this study, four Ni-based catalysts were prepared by adding alkali and alkaline earth metal oxides K2O and MgO and transition metal oxides CeO2 and ZrO2 as additives. The cracking characteristics and anti-dust ability of the catalysts are investigated and compared in detail. The present study will provide a general understanding of the effect of additives under © XXXX American Chemical Society

the same experimental conditions and elucidate the cracking characteristics of dust-containing tar over Ni-based catalysts.

2. EXPERIMENTAL SECTION 2.1. Preparation of Catalysts. To compare the effect of four different additives in the same standard, an incipient wetness impregnation method was chosen to prepare four kinds of modified Ni-based catalysts. γ-Al2O3 particles (1−2 mm) were impregnated with an aqueous solution of M1 for 12 h. After impregnation, the particles were evaporated to dryness at 80 °C and dried overnight at 110 °C. Next, the particles were calcined at temperature T1 for time t1. Then, the particles were impregnated with an aqueous solution of M2 for 12 h, evaporated to dryness at 80 °C, dried overnight at 110 °C, and calcined at temperature T2 for time t2. The steps are shown in Figure 1. The reagents used and the optimized calcination conditions are given in Table 1. The reagents are all of analytical grade. 2.2. Catalyst Characterization. The specific surface areas and pore sizes were measured by the Brunauer−Emmett−Teller (BET) method in an ASAP2020 adsorption instrument using N2 as the adsorbent. The surface morphologies of the catalysts were observed using scanning electron microscopy (SEM, Zeiss EVO MA 10/LS 10) under an accelerated voltage of 10 kV. The surface phase composition was studied by X-ray diffraction (XRD, D/max-2550) using Cu Kα radiation at 200 mA and 40 kV. The samples were scanned over 2θ = 20−80° at a ramp rate of 2°/min. Elemental analysis of the catalyst surface was carried out using a JEDL JXA-8230 election probe microanalyzer equipped with an energy-dispersive spectrometer (EDS 350). The chemical states of the active ingredient in used catalysts were observed by X-ray photoelectron spectroscopy (XPS, ESCALAB 250). Ammonia temperature-programmed desorption (NH3-TPD) was tested in a FINETEC FINESORB-3010 chemisorption analyzer. Approximately 50 mg of each tested catalyst sample was placed in the reaction tube. After pretreatment at 400 °C for 1 h under a He flow of 30 mL min−1, the sample was cooled to 50 °C and treated with Received: March 8, 2016 Revised: May 6, 2016

A

DOI: 10.1021/acs.energyfuels.6b00557 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 1. Flowchart for catalyst preparation.

Table 1. Reagents and Optimized Calcination Conditions sample 2% 2% 1% 3%

NiO/1% CeO2−Al2O3 NiO/1% CeO2/2% ZrO2−Al2O3 NiO/0.1% MgO−Al2O3 NiO/5% K2O−Al2O3

M1

T1 (°C)

t1 (h)

M2

T2 (°C)

t2 (h)

Ce(NO3)3·6H2O Ce(NO3)3·6H2O and Zr(NO3)4·5H2O Mg(NO3)2·6H2O Ni(NO3)2·6H2O

500 500 550 550

3 3 10 10

Ni(NO3)2·6H2O Ni(NO3)2·6H2O Ni(NO3)2·6H2O KCl

800 800 800 450

4 4 10 5

Figure 2. Schematic diagram of the fixed-bed reaction equipment: (1) nitrogen, (2) hydrogen, (3) air, (4) water, (5) model tar compounds, (6) mixture of dust and water, (7) preheater furnace, (8) preheater, (9) reactor furnace, (10) catalyst, (11) reactor, (12) condenser, (13) gas−liquid separator, (14) condensate, and (15) gas chromatograph. anhydrous NH3 gas (0.5% NH3 in He, with a flow rate of 25 mL min−1). After NH3 adsorption, the sample was purged with He at 100 °C for 1 h. NH3-TPD was then performed with a ramp of 8 °C min−1 from 100 to 700 °C under a He atmosphere. 2.3. Catalytic Tests. The catalytic tar reforming experiment was performed on a fixed-bed reaction equipment, the schematic of which is shown in Figure 2. A total of 2 g of catalyst was loaded in the middle of the fixed-bed reactor with a 20 mm diameter. Reduction gas of composition 15 vol % H2/85 vol % N2 with a flow rate of 120 mL min−1 was introduced into the reactor, maintaining the temperature at 800 °C for 3 h. The model tar compounds (57 wt % toluene, 14 wt % methylnaphthalene, 14 wt % cyclohexane, and 15 wt % dodecane) and water (preheated at 300 °C) were injected into the rector using two peristaltic pumps. At the same time, carrier gas of composition 50 vol % N2/50 vol % H2 was introduced, at a flow rate determined according to space velocity. In each run, the cracking reaction of tar was conducted under predetermined conditions for 5 h. To simulate real dust-containing tar, typical metal oxides (analytical reagents) were added to the feedwater and injected into the reactor. The particle size of these materials did not exceed 74 μm. The total amount of dust did not exceed 10 wt % of the catalyst mass.

Small-molecular gases, such as H2, N2, CO, and CH4, were tested online by a thermal conductivity detector (TCD) on a SP-6800A gas chromatograph. Hydrocarbon gases, such as CH4, C2H6, C2H4, C3H8, C3H6, and C4, were tested online by a flame ionization detector (FID) on a SP-6890 gas chromatograph. The catalytic activity for cracking of the model tar compounds was evaluated by calculating the yield of the gaseous products H2, CO, CH4, C2+, and Ct, using eqs 1−5 described below

YH2 =

YCO =

4FCin7H8

7FC7H8 +

YCnHm = B

+

FHout2 − FHin2 in 5FC11H10 + 6FCin6H12

11FCin11H10

+ 13FCin12H26

× 100

out FCO × 100 + 6FCin6H12 + 12FCin12H26

nFCout nH m 7FC7H8 +

(1)

11FCin11H10

+ 6FCin6H12 + 12FCin12H26

(2)

× 100 (3)

DOI: 10.1021/acs.energyfuels.6b00557 Energy Fuels XXXX, XXX, XXX−XXX

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YC2+ =

∑ YCnHm n=2

(4)

4

YCt = YCO +

∑ YCnHm n=1

(5)

where F and F are the molar flow rates of the inlet and outlet components, respectively. in

out

3. CATALYTIC PERFORMANCE OF THE NI-BASED CATALYSTS 3.1. Determination of the Optimized Process Conditions. To compare the catalytic performances of the four Nibased catalysts, at first, the influence of process conditions was investigated. The effects of the reaction temperature, space velocity (determined by the total volume of feeding H2, H2O, and tar model compounds that pass through the catalyst per hour), and H2O/ingredient ratio on the catalyst activities were studied. Figure 3 shows that the catalytic activity increases with

Figure 4. Effect of the space velocity on the catalyst activity: (a) 2% NiO/1% CeO2−Al2O3, (b) 2% NiO/1% CeO2/2% ZrO2−Al2O3, (c) 1% NiO/0.1% MgO−Al2O3, and (d) 3% NiO/5% K2O−Al2O3.

Figure 3. Effect of the reaction temperature on the catalyst activity: (a) 2% NiO/1% CeO2−Al2O3, (b) 2% NiO/1% CeO2/2% ZrO2−Al2O3, (c) 1% NiO/0.1% MgO−Al2O3, and (d) 3% NiO/5% K2O−Al2O3.

Figure 5. Effect of the H2O/ingredient ratio on the catalyst activity: (a) 2% NiO/1% CeO2−Al2O3, (b) 2% NiO/1% CeO2/2% ZrO2− Al2O3, (c) 1% NiO/0.1% MgO−Al2O3, and (d) 3% NiO/5% K2O− Al2O3.

an increasing reaction temperature. The chosen reaction temperature is 800 °C because the temperature of the cokeoven crude gas is generally ∼800 °C and higher temperatures result in deep cracking of the small molecules. The influence of space velocity on the catalyst activity is shown in Figure 4. The concentration of H2, H2O, and tar model compounds increases with an increasing space velocity, which is beneficial for the tar cracking reaction. On the other hand, the residence time decreases with an increasing space velocity, which is not conducive to the tar cracking reaction. For 2% NiO/1% CeO2− Al2O3 and 3% NiO/5%K2O-Al2O3, increasing the concentration has a greater influence on the reaction at lower space velocities, whereas at higher space velocities, the residence time is too short to complete the adsorption and reaction. However, the activities of 2% NiO/1% CeO2/2% ZrO2−Al2O3 and 1% NiO/0.1% MgO−Al2 O3 increase continuously with an increasing space velocity because the adsorption and reaction on their surfaces are faster. These results indicate that, within the range of experimental space velocities examined, the reaction is controlled by the concentration. Figure 5 shows that the yields of H2 and CO increase with an increasing H2O/ ingredient ratio, because the −OH groups probably decrease the rate of coke deposition on the active sites of the

catalysts.11,12 The reaction C + H2O = CO + H2 has been confirmed for elimination of carbon deposition.13 The excess steam is attributed to the formation of a Ni oxide phase, which leads to hydrothermal deactivation.14,15 However, the catalytic activity of 2% NiO/1% CeO2/2% ZrO2−Al2O3 shows a continuous increase, indicating better hydrothermal tolerance within the range of the experimental study. The activities of the other three catalysts first increased and then decreased. The optimal operation conditions of each Ni-based catalyst are shown in Table 2. 3.2. Comparison of the Effect of These Four Kinds of Additives. The BET surface area (SBET) and pore structure properties of the catalysts before and after the tar cracking reaction are shown in Table 3. Among the four catalysts tested, a SBET of 3% NiO/5% K2O−Al2O3 after 5 h of reaction shows the largest decrease compared to fresh catalyst. SEM images of 3% NiO/5% K2O−Al2O3 before and after reaction are compared in Figure 6. The image of used 3% NiO/5% K2O− Al2O3 shows the formation of many filaments not firmly attached to the surface of the catalyst. Figure 7d shows the XRD pattern of 3% NiO/5% K2O−Al2O3 after reaction for 5 h, indicating that a variety of K-containing crystalline substances appears. Moreover, the EDS results show that the content of K C

DOI: 10.1021/acs.energyfuels.6b00557 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 2. Optimized Process Conditions of the Four Kinds of Ni-Based Catalysts optimized process conditions sample 2% 2% 1% 3%

temperature (°C)

space velocity (h−1)

H2O/ingredient ratio

800 800 800 800

11000 16000 16000 12000

3:1 5:1 4:1 4:1

NiO/1% CeO2−Al2O3 NiO/1% CeO2/2% ZrO2−Al2O3 NiO/0.1% MgO−Al2O3 NiO/5% K2O−Al2O3

Table 3. SBET and Pore Structure Properties of the Catalysts sample 2% NiO/1% CeO2−Al2O3 before reaction 2% NiO/1% CeO2−Al2O3 after reaction 2% NiO/1% CeO2/2% ZrO2−Al2O3 before reaction 2% NiO/1% CeO2/2% ZrO2−Al2O3 after reaction 1% NiO/0.1% MgO−Al2O3 before reaction 1% NiO/0.1% MgO−Al2O3 after reaction 3% NiO/5% K2O−Al2O3 before reaction 3% NiO/5% K2O−Al2O3 after reaction

SBET (m2 g−1)

Vp (cm3 g−1)

dp (nm)

106 73 135

0.36 0.26 0.37

14.1 9.0 11.4

79

0.31

7.8

126

0.39

12.9

66 158 75

0.22 0.32 0.14

8.8 8.1 7.6

Figure 7. XRD patterns of catalysts before and after reaction. Before reaction: (a) 1% NiO/0.1% MgO−Al2O3 and (c) 3% NiO/5% K2O− Al2O3. After reaction: (b) 1% NiO/0.1% MgO−Al2O3 and (d) 3% NiO/5% K2O−Al2O3.

in the filaments is 13.04 wt %, whereas that in the body of the catalyst is 0.35 wt %. It is suggested that the filaments observed in the SEM images are the K-containing crystalline substances indicated by the XRD analysis. The pore volume change between fresh and used 2% NiO/1% CeO2/2% ZrO2−Al2O3 is small, from 0.37 to 0.31 cm3 g−1. After the reaction, pore volumes of the catalysts are larger than those of others, because the additive of ZrO2 increases the stability of the catalyst structure. For used catalysts, the average pore sizes of 2% NiO/ 1% CeO2−Al2O3 and 1% NiO/0.1% MgO−Al2O3 are larger than the other two kinds of catalysts. The XRD patterns of the catalysts before and after reaction are depicted in Figures 7 and 8. The formation of a spinel compound NiAl2O4 is indicated with all four types of additives. The solid solution (Ce0.85Zr0.15)O2 peak at 2θ = 29.7° appears in the XRD pattern of the used 2% NiO/1% CeO2/2% ZrO2− Al2O3 catalyst, shown in Figure 8d. For both fresh and used 1% NiO/0.1% MgO−Al2O3 catalyst, NiAl2O4−MgAl2O4 solid solution peaks appear at 2θ = 37.7°, 45.7°, and 67.2°. The two above-mentioned solid solutions are stable and contribute to the dispersion of Ni. The XRD pattern of the used 3% NiO/ 5% K2O−Al2O3 catalyst shows KO2 and KAl5O8 peaks, which are in agreement with the corresponding SEM image.

Figure 8. XRD patterns of catalysts before and after reaction. Before reaction: (a) 2% NiO/1% CeO2−Al2O3 and (b) 2% NiO/1% CeO2/ 2% ZrO2−Al2O3. After reaction: (c) 2% NiO/1% CeO2−Al2O3 and (d) 2% NiO/1% CeO2/2% ZrO2−Al2O3.

NH3-TPD profiles of the Ni−Al2O3 catalysts are shown in Figure 9. The NH3 desorption profiles obtained were deconvoluted into two peaks for the combined Ni−Al2O3

Figure 6. SEM images of 3% NiO/5% K2O−Al2O3 (left panel) before and (right panel) after reaction. D

DOI: 10.1021/acs.energyfuels.6b00557 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 9. NH3-TPD profiles of the Ni−Al2O3 catalysts.

catalyst solid acids over the temperature range of 180−400 °C. The first peak, at ∼190 °C, is related to weakly adsorbed NH3 or NH3 adsorbed on separated Al2O3. The second peak at 280 °C corresponds to NH3 adsorbed on extra-framework aluminum species.16 The peak between 400 and 600 °C is not a NH3 desorption peak17 but is probably a H2O desorption peak. In comparison to the NH3-TPD profiles of the Ni−Al2O3 carrier with no additives, the peaks as a result of Ni−Al2O3 with MgO and K2O additives have narrow temperature ranges and a significantly reduced area. The result shows that MgO and K2O do not change the types of solid acid; however, they reduce the amount of acids and, hence, the acidity of the catalysts. Previous studies18 have reported that coke is preferentially deposited at the sites with the highest acidity; hence, MgO and K2O are beneficial to prevent coke deposition. Figure 10 shows the comparison chart of the catalytic activities of the catalysts under their individual optimal process

Figure 11. Stability test of the catalysts under their individual optimized process conditions: (a) 2% NiO/1% CeO2−Al2O3, (b) 2% NiO/1% CeO2/2% ZrO2−Al2O3, (c) 1% NiO/0.1% MgO−Al2O3, and (d) 3% NiO/5% K2O−Al2O3.

the highest activity and stability among the four Ni-based catalysts. 3.3. Cracking Characteristics of Dust-Containing Tar over 2% NiO/1% CeO2/2% ZrO2−Al2O3. Figure 12 shows

Figure 12. Ni 2p XPS profiles for 2% NiO/1% CeO2−Al2O3, 2% NiO/1% CeO2/2% ZrO2−Al2O3, and 2% NiO/1% CeO2/2% ZrO2− Al2O3 with MaOb: (a) 2% NiO/1% CeO2−Al2O3, (b) 2% NiO/1% CeO2/2% ZrO2−Al2O3, (c) 2% NiO/1% CeO2/2% ZrO2−Al2O3 with SiO2, (d) 2% NiO/1% CeO2/2% ZrO2−Al2O3 with MgO, (e) 2% NiO/1% CeO2/2% ZrO2−Al2O3 with Fe2O3, (f) 2% NiO/1% CeO2/ 2% ZrO2−Al2O3 with CaO, and (g) 2% NiO/1% CeO2/2% ZrO2− Al2O3 with Al2O3.

Figure 10. Comparison of gas yields of the catalysts under their individual optimized process conditions.

the Ni 2p XPS profiles for the tested catalyst samples. The Ni 2p XPS profiles comprise Ni 2p1/2 and Ni 2p3/2 peaks.19 Except for 2% NiO/1% CeO2/2% ZrO2−Al2O3 with Al2O3, the binding energy of Ni 2p3/2 on the other samples is larger than that on pure Ni, indicating that the additives of CeO2 and ZrO2 decrease the electron density on the Ni atom surface. Although it does not contribute to the CH4 reforming reaction,20 it contributes to the adsorption of aromatic substances with larger electron densities. After introduction of MgO, SiO2, and Fe2O3, the Ni 2p1/2 peak appears. The binding energies of Ni 2p3/2 on the three catalysts are lower than that of 2% NiO/1% CeO2/2% ZrO2−Al2O3 with no dust, although they are still larger than that of pure Ni. Among the three kinds of dust species, SiO2 has the most significant influence on the binding energy of Ni 2p3/2.

conditions. The comparison curve of the stability test of the catalysts performed under the optimized process conditions is shown in Figure 11. The combination of these two figures shows that the yield of H2 is the lowest over the 2% NiO/1% CeO2−Al2O3 catalyst but the yields of CH4, C2+, and Ct are the highest, indicating that catalytic cracking is incomplete for this catalyst and its stability is worst among the four catalysts tested. The 1% NiO/0.1% MgO−Al2O3 catalyst has the highest H2 yield in the initial stage; however, the yield of H2 decreases significantly after 6 h. In contrast, the catalytic activity of the 3% NiO/5% K2O−Al2O3 catalyst begins to decrease after 28 h. Overall, the 2% NiO/1% CeO2/2% ZrO2−Al2O3 catalyst has E

DOI: 10.1021/acs.energyfuels.6b00557 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Hence, the effect on catalytic activity is obvious. After introduction of CaO, the Ni 2p3/2 peak area reduces. However, CaO also has some catalytic activity;21 thus, the activity of 2% NiO/1% CeO2/2% ZrO2−Al2O3 with CaO did not decrease. This is consistent with the experimental results shown in Figure 14. Figure 13 shows the Ce 3d XPS profiles for the samples. The Ce 3d XPS profiles comprise Ce 3d3/2 and Ce 3d5/2 peaks. The

Figure 14. Effect of dust species on catalytic activity of 2% NiO/1% CeO2/2% ZrO2−Al2O3.

increase upon adding MgO and Fe2O3 dust, because MgO reduces the acidity of the catalyst and Fe is a second active ingredient for Ni-based catalysts.22 When CaO is introduced, the yields of H2, CH4, and Ct increase but that of CO decreases, probably because CaO not only has some catalytic activity but can also neutralize the acidity of the catalyst surface, thus preventing carbon deposition.23 In comparison to the blank experiment, the yields of all gas products decrease upon introducing SiO2, which has no catalytic activity and covers the active sites of the Ni-based catalysts. The yields of H2, CO, and Ct decrease upon adding Al2O3, which promotes carbon deposition.

Figure 13. Ce 3d XPS profiles for 2% NiO/1% CeO2−Al2O3, 2% NiO/1% CeO2/2% ZrO2−Al2O3, and 2% NiO/1% CeO2/2% ZrO2− Al2O3 with MaOb: (a) 2% NiO/1% CeO2−Al2O3, (b) 2% NiO/1% CeO2/2% ZrO2−Al2O3, (c) 2% NiO/1% CeO2/2% ZrO2−Al2O3 with SiO2, (d) 2% NiO/1% CeO2/2% ZrO2−Al2O3 with MgO, (e) 2% NiO/1% CeO2/2% ZrO2−Al2O3 with Fe2O3, (f) 2% NiO/1% CeO2/ 2% ZrO2−Al2O3 with CaO, and (g) 2% NiO/1% CeO2/2% ZrO2− Al2O3 with Al2O3.

binding energies of Ce 3d3/2 on 2% NiO/1% CeO2/2% ZrO2− Al2O3 and 2% NiO/1% CeO2−Al2O3 catalysts are larger than that of pure Ce, indicating that the addition of CeO2 enhances the stability of the catalyst structure. The binding energy of Ce 3d decreases upon adding SiO2 dust. Relative contents of the surface elements of the used samples, as measured by XPS, are shown in Table 4. The C content on the surface of 2% NiO/1% CeO2/2% ZrO2−Al2O3 is lower than that on the surface of 2% NiO/1% CeO2−Al2O3, implying that the simultaneous addition of CeO2 and ZrO2 reduces carbon deposition. Introduction of MgO, CaO, SiO2, and Fe2O3 further reduces the C content on the 2% NiO/1% CeO2/2% ZrO2−Al2O3 catalyst surface. However, the C content increases from 47.20 to 61.77% upon adding Al2O3 dust, implying that Al2O3 promotes carbon deposition. The effect of the dust species on the catalytic activity of 2% NiO/1% CeO2/2% ZrO2−Al2O3 is shown in Figure 14. Five kinds of oxides, SiO2, MgO, Fe2O3, Al2O3, and CaO, were introduced as typical dusts, with the mass of each dust sample being 10% of the catalyst mass. The yields of H2, CO, and Ct

4. CONCLUSION In this study, K2O, MgO, CeO2, and ZrO2 were introduced as additives in Ni−Al2O3 catalysts to study their effects on catalytic activity and stability. The influence of dust species on tar cracking characteristics was also investigated. The addition of K2O and MgO neutralized the catalyst acidity and reduced carbon deposition on the catalyst surface, while the addition of CeO2 and ZrO2 improved the stability of catalysts. XRD analysis showed that the CeO2 and ZrO2 additives formed stable (Zr0.85Ce0.15)O2 solid solution. The 2% NiO/1% CeO2/ 2% ZrO2−Al2O3 catalyst shows the highest activity and stability among the four Ni-based catalysts tested. MgO, CaO, and Fe2O3 increase the catalytic activity when added as typical dust particles, while SiO2 and Al2O3 decrease the catalytic activity.

■

AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-13678890728. Fax: +86-532-86057718. Email: [email protected].

Table 4. Relative Content of Surface Elements of the Used Samples relative content of surface elements of the used samples (atom ratio %) 2% 2% 2% 2% 2% 2% 2%

NiO/1% NiO/1% NiO/1% NiO/1% NiO/1% NiO/1% NiO/1%

used sample

C

O

Ni

Al

Ce

Zr

CeO2−Al2O3 CeO2/2% ZrO2−Al2O3 CeO2/2% ZrO2−Al2O3 CeO2/2% ZrO2−Al2O3 CeO2/2% ZrO2−Al2O3 CeO2/2% ZrO2−Al2O3 CeO2/2% ZrO2−Al2O3

49.62 47.20 43.36 41.15 61.77 41.28 44.23

28.41 30.54 32.67 34.66 21.89 33.44 36.45

0.32 0.67 0.42 0.61 0.36 0.56 0.40

21.52 20.85 22.54 22.2 15.63 23.54 17.83

0.13 0.22 0.19 0.24 0.15 0.21 0.29

0.51 0.40 0.55 0.20 0.44 0.52

with with with with with

CaO MgO Al2O3 SiO2 Fe2O3

F

Ca

Mg

Si

Fe

0.30 0.59 0.53 0.28 DOI: 10.1021/acs.energyfuels.6b00557 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Notes

(18) Schuurman, Y.; Delattre, C.; Pitault; Reymond, J. P.; Forissier, M. Effect of coke deposition on transport and sorption in FCC catalysts studied by temporal analysis of products. Chem. Eng. Sci. 2005, 60, 1007−1017. (19) Liu, F.; Zhao, Z. J.; Qiu, L. M.; Zhao, L. Z. Tables of peak positions for XPS photoelectron and auger electron peaks. Anal. Test. Technol. Instrum. 2009, 15, 151−17 (in Chinese). (20) Shi, K. Y.; Xu, H. Y.; Zhang, G. L.; Wang, Y. Z.; Xu, G. L.; Wei, Y. D. Study on reforming natural gas−carbon dioxide−steam−oxygen to syngas: The role of rare earth additives. Chin. J. Catal. 2002, 23, 15−18. (21) Ashok, J.; Kathiraser, Y.; Ang, M. L.; Kawi, S. Bi-functional hydrotalcite-derived NiO-CaO-Al2O3 catalysts for steam reforming of biomass and/or tar model compound at low steam-to-carbon conditions. Appl. Catal., B 2015, 172−173, 116−128. (22) Sun, Y. J.; Jiang, J. C.; Kantarelis, E.; Xu, J. M.; Li, L. N.; Zhao, S. H.; Yang, W. H. Development of a bimetallic dolomite based tar cracking catalyst. Catal. Commun. 2012, 20, 36−40. (23) Horiuchi, T.; Sakuma, K.; Fukui, T.; Kubo, Y.; Osaki, T.; Mori, T. Suppression of carbon deposition in the CO2-reforming of CH4 by adding basic metal oxides to a Ni/Al2O3 catalyst. Appl. Catal., A 1996, 144, 111−120.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the National Natural Science Foundation of China (Grants 21006059 and 21376142). REFERENCES

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DOI: 10.1021/acs.energyfuels.6b00557 Energy Fuels XXXX, XXX, XXX−XXX