Low-temperature steam reforming of toluene and biomass tar over

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Low-temperature steam reforming of toluene and biomass tar over biochar-supported Ni nanoparticles Zhen-Yi Du, Zhi-Hua Zhang, Chen Xu, Xingbao Wang, and Wen Ying Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04872 • Publication Date (Web): 15 Nov 2018 Downloaded from http://pubs.acs.org on November 15, 2018

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ACS Sustainable Chemistry & Engineering

Low-temperature steam reforming of toluene and biomass tar over biochar-supported Ni nanoparticles Zhen-Yi Du, Zhi-Hua Zhang, Chen Xu, Xing-Bao Wang*, Wen-Ying Li* Training Base of State Key Laboratory of Coal Science and Technology Jointly Constructed by Shanxi Province and Ministry of Science and Technology, Taiyuan University of Technology, Taiyuan 030024, China *Corresponding

authors.

Tel.: +86 351 6018957; fax: +86 351 6018453. E-mail addresses: [email protected] (XB Wang). Tel.: +86 351 6018957; fax: +86 351 6018453. E-mail addresses: [email protected] (WY Li). Full mailing address of all the authors: 79 Yingze West Street, Taiyuan, Shanxi Province 030024, China.

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Abstract Developing efficient, inexpensive in situ tar reforming technologies under mild conditions is an important practical aspect of biomass gasification. In this study, a series of biochar-supported Ni catalysts (Ni/BC) were prepared via a simple one-step pyrolytic approach, and firstly explored for steam reforming of toluene as a tar model compound at a relatively low temperature of 600 °C. The as-prepared catalysts can be directly used without further reduction process. The abundant surface oxygen-containing groups of the starting biomass and the high porosity of Ni/BC assisted with the dispersion of the Ni nanoparticles. The in situ generating process of metallic Ni nanoparticles via carbothermal reduction was manipulated to modulate the Ni particle size. A size-dependent behavior was observed, wherein 5Ni-600/BC (pyrolyzed at 600 °C, 5% Ni loading) with the smallest Ni particle size (4.2 nm) showed superior catalytic performance in terms of the initial intrinsic activity (turnover frequency value of 1.64 s–1) and stability over others, indicating the positive role of small particles with more corner and step sites, which was further proved by DFT calculations. Besides, 5Ni-600/BC was found to be effective in steam reforming of real biomass tar by reducing both the tar amount and the molecular weight. Keywords: Low-temperature steam reforming; Biomass tar; Toluene; Ni/biochar; Ni particle size.


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INTRODUCTION Biomass gasification is one of the most promising technologies for the production of renewable fuels and chemicals. Although significant technological advances have been made in recent decades, tar remains the most challenging problem to date. Consisting largely of aromatic compounds, tar not only reduces the overall energy efficiency of the entire gasification system, but also causes pipeline clogging problems.1-4 Catalytic steam reforming can transform tar into usable gaseous products,1 and the key issue in tar steam reforming is catalyst development. Considering the complexity of real tar, low-cost and disposable catalysts are more likely to be applied in practical gasification plants. Therefore, biochar and biochar-supported catalysts have recently emerged as novel tar reforming catalysts.5 Biochar-derived catalysts can be prepared in situ from biomass itself at low cost, and, more importantly, the spent catalysts can be gasified or combusted to recover energy and recycle the loaded metal species.5 Furthermore, biochar has abundant porosity with good diffusion properties, and therefore it has been used as a novel catalyst support in a number of reactions, such as catalytic pyrolysis of biomass, lignin conversion, biomass reforming and etc.612

Ni as a non-precious metal has been widely used for tar reforming. Shen et al.9 prepared a rice husk char-supported Ni catalyst, which showed a better performance in tar reforming than the blank char alone (500–900 °C). Qian et al.10 prepared biochar-supported Ni catalysts and studied the effects of nickel precursors and hydrazine reduction on the performance of toluene and naphthalene reforming (700–900 °C). Wang et al.13 developed wood-char-supported Ni monolith catalysts, in which the wood channels greatly attenuated coke formation at 700 °C. However, as revealed by Al-Rahbi et al.14, char can be consumed quickly by steam at high temperatures above 800 °C, which is typically used for traditional oxide-supported Ni catalysts. In other words, char-supported Ni catalysts must be supplemented continuously to compensate for their consumption, making the unit operation complicated and expensive in industrial applications. In addition, high temperature operation makes the reforming process energy-intensive. Whether charsupported metal catalysts can be applied under relatively low reforming temperature (for example, 600 °C 3 ACS Paragon Plus Environment


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in this study) should be of great significance for their practical utilization in industry. However, low temperature operation could result in low activity from the kinetics perspective, and therefore it is important to identify the key factors affecting catalytic performance and develop highperformance catalysts. Gai et al.15-16 synthesized hydrochar supported Fe catalysts for phenol decomposition, and they found that the catalysts prepared via a one-pot hydrothermal carbonization method with smaller particle size performed better than those prepared by the impregnation method with larger particle size. Wu et al.17 also found that Ni/Al2O3 with small nickel particle size possessed the highest hydrogen yield during steam reforming of glycerol. As such, the metal particle size of reforming catalysts plays a significant role in catalytic performance. However, biochar as a support has significantly different characteristics from tranditional metal oxides supports. The formation of metallic Ni particles, the evolution of Ni particles during reaction, and the structure-performance of Ni/biochar (denoted Ni/BC hereafter) catalysts are not clearly understood. Therefore, in this study, a series of Ni/BC catalysts were prepared via a simple one-step pyrolytic approach, and explored for steam reforming of tar and its model compound toluene at a relatively low temperature of 600 °C for what is, to the best of our knowledge, the first time. Toluene is always selected as the model compound as it is one of the most abundant compounds in tar and bares the stable aromatic characteristics of tar

18-20.

The evolution of the catalyst structure during carbothermal reduction was

revealed. The in situ generating process of metallic Ni nanoparticles was manipulated by varying the pyrolysis temperature and Ni loading amount to modulate the Ni particle size. Density functional theory (DFT) calculations were performed to elucidate the toluene decomposition pathways and the effects of particle size on the catalytic activity. Catalysts before and after the reaction were characterized in detail to understand their catalytic behaviors. EXPERIMENTAL SECTION Catalyst preparation and characterization


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Corncobs collected from a local farm were used as the biomass feedstock. The proximate and ultimate analyses of the starting materials are listed in Table S1. The Ni/BC catalysts were prepared via a simple one-step pyrolytic approach. The samples were denoted xNi-y/BC (x = 5, 10, 20, and 30; y = 200, 300, 400, 500, 600, 700, 750, and 800), with x and y indicating the nominal Ni loading percentage in the final char catalysts and pyrolysis temperature during preparation, respectively. The detailed preparation procedures and catalyst characterization are provided in the Supporting Information. Catalyst evaluation and theoretical DFT calculations Toluene was selected as the model tar compound to evaluate the performance of Ni/BC catalysts. Catalytic tests were carried out in a fixed-bed quartz reactor. DFT calculations were performed to clarify the toluene decomposition pathways. The catalysts were also tested for the reforming of real biomass tar on a two-stage reactor used in our previous studies21-22. The experimental process and the DFT calculation details are included in the Supporting Information. The schematic of the experimental apparatus is shown in Figure S1. RESULTS AND DISCUSSION Formation process of Ni nanoparticles Thermogravimetric analysis-mass spectrometry (TG-MS) measurements in N2 gas was used to study the decomposition characteristics of Ni/BC catalyst precursors. Figure 1(a–c) shows the TG and DTG profiles of corncob biomass without Ni impregnation, and with 5% Ni and 30% Ni impregnation amounts. The corresponding releasing profiles of H2, CO, CO2, and CH4 are shown in Figure 1(d–f). The TG curves displayed continuous weight loss with increasing temperature up to 1000 °C for all three samples. The major weight loss takes place between 200 °C and 500 °C for all three samples, together with a significant release of CO and CO2. Therefore, the largest weight loss rate centered at approximately 330 °C is due to the decomposition of oxygen-containing functional groups in biomass. Compared with the unloaded samples, those impregnated with Ni salts exhibit additional DTG peaks centered at approximately 171 °C



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and 420 °C, respectively, and these peaks become more intense with the increasing Ni amount. A closer examination reveals that these two DTG peaks coincide with the additional CO and CO2 release compared with the unloaded biomass. As reported in the literature,23-24 between 130 °C and 262 °C, nickel nitrate decomposes into Ni(NO3)(OH)·2H2O with the release of HNO3 and NOx, which can oxidize the biomass to yield CO and CO2. As shown in Figure S2, the sharp peak of m/z (30) at 171 °C is found for the 5% and 30% Ni-loaded samples, while this peak does not exist for the unloaded one. This proves that this peak is related to NO release. Regarding the m/z (30) peak at 341 °C for the unloaded sample, it should be related to the molecular fragments with an m/z value of 30 from biomass volatiles during pyrolysis instead of NO release, since minor amounts of nitrogen exist in the starting biomass. The CO release at 414 °C is related to the reduction of NiO by carbon in the biomass, i.e., carbothermal reduction. H2 and CO release were significantly enhanced for Ni-impregnated samples, especially above 500 °C, when the major biomass devolatilization already ended. This indicates that Ni species enhanced the decomposition and aromatization process of the biochar support.

Figure 1. (a–c) TG and DTG profiles of corncob biomass with 0%, 5%, and 30% Ni loadings; (d–f) 6 ACS Paragon Plus Environment

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corresponding gas-release profiles determined with MS. Figure 2(a) presents the X-ray diffraction (XRD) patterns of the Ni/BC samples of 5% Ni loading at different final pyrolysis temperatures from 200 °C to 800 °C. As expected, the crystalline phases of Ni (JCPDS 70-1849) appeared when the pyrolysis temperature was above 400 °C, which is consistent with the TG-MS data. Figure 2(b) shows the XRD patterns of the Ni/BC samples of different Ni loadings at the same pyrolysis temperature of 600 °C, and only obvious peaks of Ni were observed. Taken together the TG-MS and XRD data, it is found that Ni precursors can be carbothermally reduced to metallic Ni in the studied pyrolysis preparation temperature range (600–800 °C). Therefore additional hydrogen reduction was not required for the Ni/BC catalysts before the steam reforming reactions.

Figure 2. XRD patterns of as-prepared Ni/BC catalysts. Structural and surface chemical properties All of the Ni/BC samples display type I isotherms (Figure S3(a)), except for 5Ni-800/BC, which



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shows a hysteresis loop. The considerable uptake of N2 at low relative pressures is indicative of the microporous characteristic of the Ni/BC samples. Table 1 lists the surface areas and pore volumes of the samples. Compared with BC with no metal loading, most Ni/BC samples exhibited higher surface areas and pore volumes, but the surface areas decreased with the increasing Ni loading amount. This indicates that the presence of Ni-promoted pore formation, but an excessive amount of Ni may block the pore channels. Furthermore, increasing temperature to 800 °C induced the formation of mesopores and resulted in a significant decrease in surface area. Typical SEM photographs of Ni/BC samples are shown in Figure S4. All of the fresh Ni/BC samples displayed a similar honeycomb-type structure with many large holes measuring tens of micrometers. Secondary holes were also observed to be distributed on the walls of large pores. This kind of structure, together with a large specific surface area, should be beneficial for the distribution of Ni nanoparticles and the mass transfer of reactants. Table 1 Main physico-chemical properties of as-prepared catalysts and TOF values in steam reforming of toluene. Sample 0Ni600/BC 5Ni600/BC 10Ni600/BC 20Ni600/BC 30Ni600/BC 5Ni700/BC 5Ni750/BC 5Ni800/BC a The

SBET (m2/g)a Fresh Spentc

Vp (cm3/g)a Fresh Spentc

dXRD (nm)b Ni dTEM Fresh Spentc (wt.%)d (nm)e

437.7

107.9

0.20

0.35

-

-

-

-

-

-

-

524.6

233.9

0.23

0.46

4.2

27.6

5.2

4.8

61.6

1.64

1.64

461.9

172.8

0.21

0.45

4.8

31.1

9.3

5.0

77.8

1.30

1.50

417.1

117.8

0.21

0.26

4.9

32.6

16.3

5.1

83.6

1.21

1.35

380.5

88.5

0.15

0.18

5.0

34.7

25.7

5.1

91.7

1.10

1.11

431.4

207.2

0.20

0.54

4.4

36.2

5.7

4.9

61.6

1.56

1.41

409.1

162.1

0.19

0.38

28.0

44.1

5.5

-

65.0

0.69

0.77

351.9

83.2

0.20

0.24

37.2

38.1

5.5

-

145.9

-

-

dH DNi (nm)f (%)f

TOF (s-1)

BET surface area, pore volume were determined by nitrogen physical adsorption. b Crystallite size of

Ni calculated from XRD analysis. c Spent catalysts after 10-h TOS for toluene steam reforming. d Ni weight


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percentage determined with Inductively coupled plasma mass spectrometry (ICP-MS). e Average particle size calculated based on statistical analysis on TEM results. f Particle size and Ni dispersion determined by H2 pulse chemisorption. The X-ray photoelectron spectroscopy (XPS) profiles of C 1s and Ni 2p are shown in Figure S5. The C 1s spectra were deconvoluted into three peaks, and that corresponding to C–C was set at 284.6 eV to calibrate the binding energies (BEs) of other elements. The C–C peak is accompanied by a broad and less intense tail, composed of two peaks centered around 286 and 290 eV, which are indicative of C–O and C═O bonds. The Ni 2p spectra are characterized by two spin-orbit components, Ni 2p3/2 and Ni 2p1/2. The Ni 2p3/2 spectra of all of the catalysts are well resolved, with BEs between 853.0 and 854.0 eV (Ni0), between 856.1 and 856.8 eV (Ni2+), and between 861.0 and 861.8 eV (Ni2+ satellite).25 Increasing the pyrolysis temperature from 600 °C to 800 °C, the BEs of Ni 2p shifted slightly to higher values. The positive shift in BE indicates an electron transfer from the metal to the carbon support. This phenomenon was also observed on carbon-supported platinum catalysts.26 The surface atomic concentrations of catalysts are compiled in Table S2. As expected, the C ratio increased and the O ratio decreased with temperature due to a more severe extent of graphitization of the char supports. The percentage of surface Ni also decreased, which can be due to the increased particle size on one hand, and carbon encapsulation on the other. Graphitic carbon layers were found around the Ni particles as indicated in the HRTEM image of 5Ni800/BC (Figure S6), which coincides with the results of Yu’s group7,

27

and Wang et al.13 at high

temperatures above 900 °C. Modulation of Ni particle size The Ni particle size was modulated by varying the Ni loading amount and pyrolysis preparation temperature. Based on the XRD patterns in Figure 2, the crystalline sizes were calculated with the Scherrer equation from the Ni(111) plane at 44.5° (Table 1). Below 700 °C, the crystalline sizes were less than 4.4 nm. A sudden increase to 28.0 nm was observed when the pyrolysis temperature was increased to 750 °C, indicating severe agglomeration of Ni particles under high temperatures. In addition, the peak 9 ACS Paragon Plus Environment


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corresponding to graphitic carbon (JCPDS 41-1487) at 2θ of 26.1° appeared for the sample pyrolyzed at 800 °C, suggesting that the biochar support was partially graphitized at this temperature. Surprisingly, under the same pyrolysis temperature of 600 °C, with the Ni amount increasing from 5% to 30%, the average Ni crystalline size increased only from 4.2 nm to 5.0 nm (Table 1), demonstrating that very high dispersion can be achieved on biochar support even at a high loading of 30%. The size increase degree is quite small compared with metal oxide-supported Ni catalysts,28-31 wherein the size increased by several fold with increasing metal loading in the same range as in this study. It is inferred that biomass is rich in oxygencontaining functional groups, and Ni2+ exchange with the protons of those functional groups occurs during impregnation. Thus, high abundance of these functional groups leads to the high dispersion of Ni metal particles even at a high loading of 30%. Transmission electron microscopy (TEM) images of the as-prepared catalysts are displayed in Figure 3. The high-resolution TEM (HRTEM) image (Figure 3(e)) of the representative particles showed dspacings of 0.20 and 0.18 nm, corresponding to the (111) and (200) planes of metallic Ni, which is consistent with the selected-area electron diffraction (SAED) pattern. This again proved that Ni precursors were fully reduced to metallic Ni during the pyrolysis process. For catalysts prepared at 600 °C, Ni nanoparticles were uniformly dispersed over the char matrix (Figure 3(a-d)). In agreement with the sizevariation trend obtained from XRD measurements, the average particle size measured from TEM pictures increased only from 4.8 nm for 5Ni-600/BC to 5.2 nm for 30Ni-600/BC as well. However, the temperature exhibited a more significant influence on the dispersion of Ni nanoparticles. At relatively low temperatures of 600 °C and 700 °C (Figure 3f), Ni particles distributed uniformly. When increasing the pyrolysis temperature to 750 °C and 800 °C, although small well-dispersed particles (smaller than 7.5 nm) still existed, large coagulated particles (larger than 20 nm) were clearly observed, as indicated in the red circles (Figure 3(g-h)). H2-pulse chemisorption was carried out to measure the Ni dispersion and the results are shown in Table 1. The Ni dispersions showed a downward trend with increasing Ni loading amount and increasing 10 ACS Paragon Plus Environment

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pyrolysis temperature. At 800 °C, almost no difference was observed between the H2-pulse peaks, and therefore this data is unavailable. In fact, the active particle diameters determined by H2-pulse chemisorption were much larger than those obtained from TEM and XRD analysis, again indicating that the Ni nanoparticles were partially embedded or covered by the char matrix.

Figure 3. TEM images of as-prepared catalysts (a–d and f–h); inset at the lower right-hand corner of (a) is the corresponding SAED pattern; (e) HRTEM image. Catalytic performance in steam reforming of toluene Toluene steam reforming was carried out in a temperature range 400–650 °C with an Steam/carbon (S/C) ratio of 3 and a space time defined as the ratio between the mass of catalyst and the mass flow rate of toluene (W/F) of 0.5 h. Catalyst 5Ni-600/BC was used and the performance data at a time on steam (TOS) = 2 h are reported in Table S3. The products are composed of benzene, H2, CO, CO2, and a negligible amount of CH4. During toluene reforming, several reactions (Eqs. (1) – (5)) take place in parallel, among which the main reactions include the following. Steam reforming reaction C7H8+7H2O → 7CO+11H2

(1)



C7H8+14H2O → 7CO2+18H2

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(2)

Water gas shift (WGS) reaction CO+H2O ↔ CO2+H2

(3)

Cracking reaction C7H8 → 7C+4H2

(4)

Boudouard reaction 2CO ↔ CO2+C

(5)

In general, toluene conversion and hydrogen yield increased with temperature, except for 650 °C. In fact, at 650 °C, toluene conversion was 100% at TOS = 0.5 h and it decreased quickly to 78.3% at TOS of 2 h, indicating the instability of this catalyst. Typically, metal nanoparticles could coagulate at high temperatures, leading to catalyst deactivation. In addition, as stated in the Introduction, biochar as a carbon material could be gasified more quickly by steam at higher temperatures. A similar phenomenon was also observed by Cao et al.32 namely that Ni-loaded on lignite char was consumed quickly at 650 °C in steam atmosphere. Carbon balances were low on this catalyst in the studied temperature range, with the highest value of 81.4% achieved at 600 °C. The unaccountable carbon can be mainly assigned to carbon deposition on catalysts formed from side reactions such as Eqs. (4) and (5). From 400 °C to 600 °C, the increased CO and CO2 selectivity together with better C balance proved the enhanced reforming ability of the catalyst with the increasing temperature. Further increasing the temperature to 650 °C resulted in higher CO selectivity and lower CO2 selectivity, which is probably due to the improved reverse WGS activity. In addition, CO2 can react with the char catalysts more easily at high temperatures to form CO. As introduced by Heo et al.,33 an index R is defined as the ratio between the calculated H2 amount only from reforming reactions (Eqs. (1) and (2)) and the actual H2 amount, i.e.,


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R

Calculated hydrogen amount from reforming reactions Actual total hydrogen amount

(6)

Higher R values mean that hydrogen was mainly produced via reforming, and those side reactions that also yield hydrogen, such as Eq. (4), were suppressed. Therefore, as shown in Table S3, R values display the same trend as carbon balance values with temperature. Based on the above analysis, the operating temperatures of Ni/BC need to be carefully selected, as Ni/BC displayed low activity at lower temperatures, but they were quickly consumed by steam at higher temperatures. Therefore, an intermediate temperature of 600 °C was selected as the reaction temperature for catalytic performance evaluation in the following sections. Toluene conversion as a function of TOS for Ni/BC catalyst with varying Ni loading amount under the reaction conditions of 600 °C, W/F = 0.5 h, and S/C = 3 are plotted in Figure 4(a). Biochar itself with no Ni loading (0Ni-600/BC) exhibited very limited initial catalytic activity, which quickly diminished to zero after 2 h TOS. In sharp contrast, Ni/BC catalysts all displayed an initial toluene conversion of 100%, demonstrating that Ni nanoparticles were the major catalytic sites for toluene conversion. According to the studies of Feng et al.34, O-containing functional groups in biochar can catalyze tar destruction. However, compared with metallic Ni sites, their catalytic ability and stability were found to be very low, especially at a relatively low temperature of 600 °C. Toluene conversions over 5Ni-600/BC, 10Ni-600/BC and 20Ni600/BC catalysts all showed a slight decrease in the 10-h tests. When Ni loading was increased to 30%, toluene conversion dropped monotonically to 52 % at TOS = 10 h, suggesting that significant deactivation took place. The significant deactivation at high metal loadings can be assigned to more severe coke formation, which is evidenced by the decreasing carbon balance data and R values with increasing Ni loadings, as shown in Table S4. Hydrogen yield decreased with increasing Ni loadings, consistent with the trend of decreased activity. Benzene is a by-product due to the incomplete decomposition of toluene, and therefore its selectivity increased with Ni loadings. According to the characterization results, Ni/BC catalysts with different Ni loadings all exhibited good Ni metal dispersion, and higher Ni loadings should



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provide more catalytic sites. However, higher Ni loadings could lead to a greater tendency of metal-particle aggregation. This is perhaps why all of these catalysts converted toluene completely at the beginning of the reaction and those with higher loadings deactivated more quickly. The effects of pyrolysis temperature during the preparation of Ni/BC catalysts were further explored under the reaction condition of 600 °C, W/F = 0.5 h, and S/C = 3. As shown in Figure 4(b), a decreasing trend can be observed for toluene conversion with increasing pyrolysis temperature. In addition, as shown in Table S4, the hydrogen yield also suffered significant reduction, accompanied with the increase of CO selectivity and decrease of CO2 selectivity, suggesting the hampered activity of the WGS reaction. These results are consistent with the decreasing trend of Ni metal dispersion with increasing pyrolysis temperature, thereby leading to less available catalytic sites. The carbon balance and R values also decreased with increasing pyrolysis temperature. As discussed earlier, catalysts prepared at relatively high pyrolysis temperatures of 750 °C and 800 °C had larger Ni particle sizes, which could result in more severe coke deposition. It has been reported that coke deposition during reforming is structure-sensitive, and larger metal particles with more terrace sites favor coke formation and propagation, thereby leading to catalyst deactivation.35-37


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Figure 4. Catalytic activities of Ni/BC catalysts for steam reforming of toluene. Reaction conditions: 600 °C, W/F = 0.5 h, S/C = 3, 1 vol.% of toluene in N2 gas, 1 atm. Reaction mechanism and size-dependent performance 1 1 To obtain the intrinsic activity data, turnover frequencies (TOFs; molToluene molSurface )were Ni s

calculated according to Eq. (7) using the initial conversion data taken at TOS = 20 min:

TOF  s 1  

FToluene,in  X Toluene 100 Wcat  xNi D M Ni

(7)

where Wcat is the amount of catalyst used, xNi the actual Ni loading percentage determined with ICP-MS, MNi the molecular weight of Ni (58.69 g/mol), and D the Ni dispersion in Table 1. Reaction conditions were determined to exclude the external and internal mass-transfer limitations. Toluene steam reforming on 5Ni-600/BC at 600 °C under three reaction conditions were carried out as shown in Figure S7, indicating that mass transfer-effects were eliminated with the total feeding flow of 15 ACS Paragon Plus Environment


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125.6 mL/min and catalyst size of 0.25–0.42 mm. For different catalysts, toluene conversions were kept below 20% at low W/F values by adjusting the catalyst amount between 4 and 9 mg. As shown in Table 1, the TOFs were largely in a narrow variation range, and a gradual decreasing trend was observed with the Ni crystalline size determined from XRD for Ni/BC catalysts. The decrease was more evident for 5Ni750/BC, which had a significantly larger crystalline size than others. In theory, the fraction of corner and step sites increases, and the terrace sites decrease when the metal size becomes smaller.35, 38 Therefore, the correlation between Ni crystalline size and TOF values demonstrated that toluene steam reforming is favored on the corner and step sites, i.e., toluene reforming on Ni nanoparticles is a structure-sensitive reaction. To gain mechanistic insight for rationalizing the structure sensitivity of the reaction, toluene decomposition pathways were determined on the flat and stepped Ni(111) surfaces using DFT. To reduce the calculation workload, our calculations were mainly based on a previous study,39 wherein all of the possible reactions were taken into account on pure and boron-doped Ni(111). The models of flat and stepped Ni(111), adsorption configurations of toluene and transition state structures involved in the most favorable pathways are shown in Figure 5. The most favorable decomposition pathways on both surfaces can be described as follows: toluene adsorbs in a parallel configuration to the surfaces, and then the stepwise dehydrogenation of the methyl group takes place. After that, the aromatic C–H bond at the ortho position breaks and the aromatic ring opening occurs via the C1–C2 bond cleavage. The ring-opened aliphatic C7 hydrocarbon is further dissociated into C3 and C4 fragments. However, toluene binds more strongly on the stepped Ni(111) than on the flat Ni(111). Besides, as shown in the potential energy diagram (Figure S8), all of the decomposition steps are promoted to various extents (activation energies lowered by 0.03 to 0.41 eV) on stepped Ni(111), except for the ring-opening reaction, in which the energy barrier is higher by 0.04 eV than on flat Ni(111). The reduction of the activation barriers is related to the coordination unsaturation of the step sites, which is in good agreement with the experimental results that smaller particles displayed higher TOF values.


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Figure 5. Adsorption configurations of toluene and transition state structures involved in the most favorable pathways on flat and stepped Ni(111) surfaces. (a and aʹ) Structure of flat and stepped Ni(111) surfaces; (b and bʹ) top view of the most stable adsorption configurations of toluene on flat and stepped Ni(111) surfaces; top view of transition states along most favorable decomposition pathway of toluene on flat and stepped Ni(111) surfaces: (c and cʹ) first aliphatic C–H activation; (d and dʹ) second aliphatic C–H activation; (e and eʹ) third aliphatic C–H activation; (f and fʹ) aromatic C–H activation at ortho position. (g and gʹ) Ring opening at C1–C2 position; (h and hʹ) C–C cleavage of hydrocarbon chain. Adsorption energies, activation barriers (eV), and distances (Å) are also shown. Small gray, small white, and large blue and purple spheres represent C, H, and Ni atoms, respectively. Deactivation analysis The N2 adsorption-desorption isotherms of the spent Ni/BC catalysts after the 10-h TOS tests are shown in Figure S3(b). In contrast to the fresh catalysts, the spent ones displayed hysteresis loops, indicating the formation of widened mesopores. N2 adsorption at relatively low pressure was significantly reduced, indicating the decrement of micropores. These results suggested that the structure of Ni/BC 17 ACS Paragon Plus Environment


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catalysts strongly deteriorated during reactions, mainly due to coke deposition. Moreover, as shown in Table 1, Ni/BC catalysts experienced different degrees of reduction in BET surface area due to coke deposition, while the pore volume increased due to mesopore formation. It is obvious that the decreasing extent was more severe for 5Ni-750/BC and 5Ni-800/BC, which had much larger Ni particles than others, again indicating that coke formation was enhanced with increasing Ni particle size. Coke formation was also confirmed by XRD analysis in Figure S9, which shows that graphitic carbon was produced on all of the spent catalysts. Furthermore, the Ni crystalline size increased for all of the Ni/BC samples, indicative of the sintering of Ni particles (Table 1). The surface morphologies of the spent catalysts were further examined with SEM as shown in Figure S10(a-d). The spent catalyst surface was covered with coke deposition, masking the original honeycomb-type structure. Thus, Ni/BC catalysts deactivated because of Ni sintering and coke-deposition problems, which need to be addressed with methods such as surface modification to enhance the metal-support interaction in future work. In situ steam reforming of biomass tar using Ni/BC catalysts To test the effectiveness of Ni/BC catalysts in real tar reforming, the 5Ni-600/BC catalyst was subjected to the in situ steam reforming of corncob-derived tar, with unloaded BC and quartz sand for reference. Figure 6(a) shows the product distribution and the H2/CO ratio based on the average performance of the catalysts for 1 h of reaction time. Char yield in the top stage remained almost constant. Compared with inert quartz sand, BC showed somewhat catalytic activity in tar-breakdown and gas-production promotion. When 5Ni-600/BC catalyst was applied, drastic effects were observed in terms of significantly reduced tar yield of 6.0% and improved gas yield of 44.3%. Furthermore, the carbon loss can be largely attributed to coke formation. If the reforming ability of a catalyst was low, then the decomposition or degradation of the feedstocks dominated, leading to a large amount of coke formation. Thus, the best mass enclosure was obtained on the 5Ni-600/BC catalyst as well. In particular, H2/CO ratio is considered as an indicator of WGS activity. The gas product distribution is shown in Figure S11. The much larger H2/CO value of 5Ni-600/BC over quartz sand and BC again demonstrated the capability of Ni in facilitating the 18 ACS Paragon Plus Environment

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WGS reaction. In addition to the amount, the composition of tar is also important in the evaluation of catalyst performance. Gel permeation chromatography (GPC) was employed to measure the molecular-weight distribution of tar at the UV detection length of 254 nm. The GPC profiles are shown in Figure 6(b) and the number average molecular weight (Mn), weight average molecular weight (Mw), and polydispersity index ( d 

Mw ) are given in Table S5. Obviously, Ni/BC performed best in decreasing the overall Mn

molecular weight of tar compounds, due to the cracking ability of Ni, while BC itself only displayed a slight effect on decreasing the molecular weight of tar compared with inert quartz sand. The lower d value indicates a narrower molecular-weight distribution of tar on Ni/BC catalyst. These results demonstrate that Ni loaded on a biochar support not only reduced the overall amount of tar, but also changed the composition of tar by cracking large molecules into smaller ones.

Figure 6. (a) Catalytic performance for steam reforming of real biomass tar: comparison between quartz 19 ACS Paragon Plus Environment


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sand, BC, and 5Ni-600/BC; (b) GPC profiles of tars obtained after reforming on quartz sand, BC, and 5Ni-600/BC. CONCLUSION Ni/BC catalysts were prepared for steam reforming of toluene as a tar model compound. The steam reforming of toluene was found to be a structure-sensitive reaction by a combined experimental and theoretical effort. Metallic Ni nanoparticles were formed by carbothermal reduction above 400 °C. Compared with Ni loading, pyrolysis temperature exhibited a more significant effect on Ni particle size. Large metal clusters were formed when the pyrolysis temperature was above 750 °C. 5Ni-600/BC showed the highest intrinsic activity and best stability among the Ni/BC catalysts, and this can be ascribed to the small Ni particle size on this catalyst. In addition, steam reforming of real biomass tar over 5Ni-600/BC demonstrated its ability to transform heavy fractions into lighter ones and usable gases (CO and H2). ASSOCIATED CONTENT Supporting Information Details of catalyst preparation and characterization methods, catalyst evaluation and theoretical DFT calculations and related references. Tables of the proximate and ultimate analyses of starting samples, XPS atomic concentrations, product distribution, and average molecular weight of tars. Figures of experimental apparatus, TG-MS profiles, BET isotherms, XPS spectra, XRD patterns, SEM images, reaction energy profiles, and mass-transfer limitation tests. ACKNOWLEDGEMENTS The work was supported by the National Natural Science Foundation of China (Grant Nos. 51776134 and 51406129), the Technology Foundation for Selected Overseas Chinese Scholars, Ministry of Human Resources and Social Security of China (2016), the Program for the Top Young Academic Leaders of Higher Learning Institutions of Shanxi, and the Research Project Supported by Shanxi Scholarship Council of China (2015-046).


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Table of Contents

Synopsis Biochar-supported Ni nanoparticles were prepared via a simple one-step pyrolytic approach to catalyze low-temperature steam reforming of toluene and tar.


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Low-temperature steam reforming of toluene and biomass tar over

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