Hydrogen from Rice Husk Pyrolysis Volatiles via Non-Noble NiâFe

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Hydrogen from rice husk pyrolysis volatiles (RHPV) via non-noble Ni-Fe catalysts supported on 5 different treated rice husk pyrolysis carbon#RHPC# Xiwei Xu, Zhiyu Li, Ren Tu, Yan Sun, and EnChen Jiang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00343 • Publication Date (Web): 06 Jun 2018 Downloaded from http://pubs.acs.org on June 6, 2018

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ACS Sustainable Chemistry & Engineering

Corresponding author：Professor and Dr. Enchen Jiang

Professor Xiwei Xu

Email: [email protected] (Enchen Jiang) [email protected] (Xiwei Xu) Mailing address:

College of Materials and Energy, South China Agricultural University, No.483, Wushan Road, Tianhe

District, Guangzhou City, Guangdong Province, China, 510640

Hydrogen from rice husk pyrolysis volatiles (RHPV) via non-noble Ni-Fe catalysts supported on 5 different treated rice husk pyrolysis carbon（RHPC） Xu Xiwei*; Li Zhiyu; Tu Ren; Sun Yan; Jiang Enchen* College of Materials and Energy, South China Agricultural University, Guangzhou 510640

Abstract: A renewable H2 was obtained via catalytic reforming/cracking of RHPV with the catalysts employed RHPC as support. 5 different treated process such as pyrolysis-impregnation (P-I), impregnation-pyrolysis (I-P), activation, acid washing (A-W), and calcining in air was employed to improve the catalytic activity and stability of catalysts. The catalytic activity for bio-oil, gas and real pyrolysis volatiles was investigated. The role of Fe and Ni was also investigated. The H2 content reached 50% for bio-oil. The catalytic activity and stability were in the order: 0.1FeNi/RHPC(P-I) > 0.1FeNi/RHPC(A-W) ≈ 0.1FeNi/RHA > 0.1FeNi/RHPC-2(I-P) > 0.1FeNi/AC(Active carbon), which is consistent with the results of BET and NH3-TPD. It suggests that high active RHPC-support catalyst is prepared avoiding pretreatment, activation and calcination, which makes the preparation procedure of catalysts simple and energy saving. Bio-oil, gas, real pyrolysis volatile were employed to investigate the interaction and the catalytic reforming mechanism. Moreover, the characterization of catalysts showed that the active center is the FeNi

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nano-alloy and the RHPC supports was an intermediate reductant to keep the stability of catalyst via reducing the metal oxides.

Keywords: hydrogen; rice husk pyrolysis volatiles; different treated RHPC supports; 0.1FeNi/RHPC catalyst; Fe/Ni

Introduction Renewable energy was considered as an important ingredient in the transition to a more sustainable development [1]. Biomass energy was recognized as one of the most promising energy due to their carbon neutral and no environmental problems [2]. Moreover, H2 had a long history as an energy source or based chemical materials. H2 from biomass conversion via thermochemical including pyrolysis and gasification was considered to be the most promising process due to high conversion efficiency [3]. In China, the yield of rice husk reached 5.65*108 t in 2016 [4]. And the price of rice husk was only 60＄/t in China. However, the ratio of high quality utilization of rice husk was low and most of the rice husk was used as the fuel for directly combustion, or threw away as waste materials [5-6]. Recently, a large number of researchers had found that it was a promising method to obtain rich-H2 gas via catalytic reforming of pyrolysis volatiles of rice husk. For example, Li [7] obtained hydrogen-rich gas production by air-steam gasification of rice husk using supported nano-NiO/γ-Al2O3 catalyst. Kuo [8] investigated the hydrogen-rich syngas compositions from wet rice husk slurry steam reforming reactions using different catalysts. Khonde [9] obtained the hydrogen rich syngas by rice husk gasification in a two-stage fixed-bed gasifier. And the conversion was 91%. And the by-prodcuts pyrolysis carbon were widely used to prepare active


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carbon or directly used as fuel for combution. The problem was that the quality of RHPC was not so good for AC or combution due to the high silicon and ash content. Recently, carbon was used as catalysts supports. Zhou [10] used carboxylic multi-walled carbon nanotubes as the nano sheets supported of catalysts. Zhang [11] found that nitrogen-doped microporous carbon was an efficient oxygen reduction catalyst for Zn-air batteries. Varisli[12] found that iron incorporated mesoporous carbon catalysts played an important role in the microwave-assisted ammonia decomposition reaction. Moreover, rice husk carbon also was considered as the catalyst supports. For example, Lu [13] prepared cobalt catalyst which employed the porous carbon prepared from the rice husk as the support. Moreover, some researchers employed the carbon nanotube, mesoporous carbon or active carbon as the catalysts supports [14-15]. Moreover, it is widely accepted that the preparation process of carbon nanotube, mesoporous carbon is complexed and the cost is high [16-17]. It was well known that rice husk ashes was widely used as active silica production or a high SiO2-containing carbonaceous adsorbing material due to high heat conductivity and thermal stability [18]. Moreover, the active carbon is widely being used as the effective adsorbent materials for heavy metal or harmful pollutant like dibenzothiophenes in kerosene, phenol in the water due to high BET surface and good pore structure [19]. As usual, the pyrolysis carbon was washed with strong acid or base in order to remove alkali metal and the organic material in the pore or surface, improving the BET surface and enhancing the acid sites in the carbon. It was accepted that the influence is obvious different for the enhancing of BET surface or widening pore size of pyrolysis carbon with different pretreated methods. Products from rice husk pyrolysis contained condensable part, that is bio-oil (mainly



including acids, phenols, ketones, and alcohols) and incondensable parts (mainly including H2, CO, CO2, CH4, C2H4, and C2H6)[20]. In the manuscript, in order to investigate the pathway of biomass volatile catalytic conversion, we chose the mixture of acids, phenols, ketones, and alcohols as the model compounds of bio-oil and H2, CO, CO2, CH4, C2H4, and C2H6 as model compound of gas like many other papers [21-22]. Moreover, reforming reactions of bio-oil and gas were also relatively complicated. In the manuscript, we want to get rich-H2 gas products via catalytic reforming of pyrolysis volatiles of rice husk. Meanwhile, the by-products RHPC treated with 5 different methods was chosen as the supports of catalyst due to low price, easy prepared method and abundant resource. The high value utilization for all component of rice husk was achieved, which improved the utilization effectivity, reduced the environmental pollution, and increases the income of farmers in China as well as supplied one method for getting renewable H2. And the catalytic performance of 5 different treated carbon support catalysts was compared. The no-noble metal Fe and Ni was employed as the active parts.

Methods The preparation of catalyst Rice husk (RH), RHPC, RHPC washed with acid (RHPC A-W), and active carbon (AC) were selected as the catalyst supports. The physical and chemical properties were shown in the table 1. The preparation of catalysts supports RHPC was acquired through the biomass continuous pyrolysis system with screw conveyer. The screw conveyer is made of mild steel whose length and diameter were 1000 mm and 26 mm,


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respectively. Rice husk was continuously transported to the pyrolysis reactor from the hopper by the screw feeder. The feeding rate was controlled by the rotary speed of the screw. The rice husk feeding rate was 390-410 g/h. The pyrolysis temperature is 500℃. And the retained time is 10 min. After cooling, the RHPC was collected in the sealable bags. The support of RHPC (A-W) was obtained by washing RHPC with acid solution. The RHPC was dipping in the 10% HNO3 at room temperature for 4h. And then it was washed with deionized water till the pH is close to 7. The solid was dried at 80℃for 24h. Rice husk AC was purchased from Kailong Company in Henan. And particle size is 70mm. BET surface is 500m2/g. The pore size is 2-5nm. Table 1 the chemical and physical properties of rice husk and RHPC Samples

Moisture content（%）

Ash（%）

Volatiles（%）

Fixed carbon（%）

RH

10. 76

11. 55

63. 94

13. 75

RHPC

4. 17

30. 69

14. 30

50. 84

The preparation of catalysts The prepared method of different RHPC catalyst was shown in the scheme 1. The prepared method of 0.1FeNi-RHPC (pyrolysis-impregnation, P-I) was wet impregnated. The 10g RHPC was dipping in iron nitrate nonahydrate and nickel nitrate nonahydrate (Sigma) aqueous solution and stirred for 1h. Make sure that the content of Ni and Fe is 10%, respectively, based on the mass of RHPC. The mixture was dried at 70 °C for 24 h. And then it was calcined and reduced at 700 °C for 2 h in the 90%N2/10%H2. The prepared method of 0.1FeNi-RHPC (impregnation-pyrolysis, I-P) The difference between 0.1FeNi-RHPC (P-I) and 0.1FeNi-RHPC (I-P) is that directly



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dipping RH instead of RHPC into the metal nitrates solution for the 0.1FeNi-RHPC (I-P). And the mixture was dried at 70 °C for 24 h. And then the dried mixture was calcined and reduce at 700℃ in 10%H2/90N2 for 2h. The prepared method of 0.2Fe-RHPC (P-I) and 0.2Ni-RHPC (P-I) was the same with the 0.1FeNi-RHPC (P-I). Excepting for that the amount of iron or nickel was both controlled at 20 wt% based on the mass of RHPC. The prepared method of 0.1FeNi-RHA (rice husk ash): The 10g RHPC was dipping in iron nitrate nonahydrate and nickel nitrate nonahydrate (Sigma) aqueous solution and stirred for 1h. Make sure that the content of Ni and Fe is 10%, respectively, based on the mass of RHPC. The mixture was dried at 70 °C for 24 h. The dried mixture was calcined at 500℃ in the air for 2h. And then it was calcined and reduced at 700 °C for 2 h in the 90%N2/10%H2. The prepared method of 0.1FeNi-AC, 0.1FeNi-RHPC (acid washing, A-W): the prepared method of 0.1FeNi-AC, 0.1FeNi-RHPC (A-W) is the same with 0.1FeNi-RHPC (P-I), only replacing the RHPC with AC and RHPC (A-W). Ni(NO3)2

Fe(NO3)3

Active carbon

0.1FeNi/AC Activation

Rice husk RHPC Pyrolysis 500℃

Acid washing

Dip the carbon in Ni(NO3)2 and Fe(NO3)3 solution And then dry at 70℃ for 24h

Calcined in air at 500℃ for 2h

Reduced in 10%H2 /90N2 at 700℃ for 2h.

Rice husk ash

0.1FeNi/RH PC(P-I)

0.1FeNi/RHA 0.1FeNi/RH

PC (A-W) 0.1FeNi/RH PC(I-P)

Scheme 1. Preparation of Ni–Fe based on different RHPC supported catalysts

Catalysts characterization


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Specific surface areas and pore volumes were determined through N2 adsorption/desorption isotherms at 77 K and collected by a Gemini VII 2390 gas-adsorption analyzer. Before the test, all samples was heat at 300℃ for 6h in the vacuum. Chembet Pulsar TPR/TPD was used to analyze temperature-programmed ammonia desorption (NH3-TPD). Each sample was heated at 300 °C for 2 h and then cooled down to 50 °C. The samples (0.1 g) were then saturated with dried ammonia by replacing the N2-flow with NH3 for 2 h and treated with 50 ml/ min N2 flow for 1 h to remove remaining NH3. The temperature was slowly heated up to 800℃ at a rate of 10 ℃/min, and ammonia was flushed out with 50 ml/min N2. The Fourier transmission infrared (FT-IR) spectra of the surface morphology of the fresh catalysts and the carbon deposited on the spent catalysts were analyzed in the same way in our previous research [23]. The scanning electron microscopy (SEM) (Hitachi-S4800 FESEM) was used to investigate the surface morphology of the catalysts and the carbon deposition on the used catalysts. The acid-exchange capacity was determined by titration with NaOH to measure the acid contents for catalysts [24]. Catalytic reforming (CR) experiment and analysis In the paper, we chose the model bio-oil (alcohol: acetone: acetic acid: guaiacol: water= 1:1:3:1.5:3.5) and model pyrolysis gas (H2: CO: CO2: CH4: C2H4: C2H6 = 4: 37.8: 15.8: 38.6: 1.9: 1.9) as the raw material due to the complex of volatiles from rice husk pyrolysis. Catalytic tests of model bio-oil was conducted in a fixed bed with quartz tube reactor in scheme 2. The diameter and length of the quartz tube reactor were 3 cm and 120 cm, respectively.



And the thickness was 0.3 cm. The model bio-oil was continuously transported to quartz tube reactor from pipeline by the peristaltic pump. And the rate of feedstock was set as 0.29g/min by adjusting the peristaltic pump. And the carrier gas (100% N2) was 100ml/min. About 2.5g catalyst was placed in the catalyst bed in quartz tube reactor installed in the middle of the pyrolysis furnace, which was supported by insulation spacer. And catalyst was used in each test. The catalyst bed temperature was monitored by the central thermocouple in the catalytic cracking furnace. The experiments were carried out at 500 ℃. The bio-oil was heated by the furnace and was gasified before they contacted with the catalysts. The products was through the condensation reactor. And the gas products was collected with gas bag and analyzed with GC. And the liquid products was collected and analyzed with GC-MS. Catalytic tests of model pyrolysis gas was conducted in the same reactor in our previous reactor [20]. The gas rate is 100ml/min. And about 2.5g catalyst was used in each test. And the temperature was 500℃. The gas products was analyzed with GC. Catalytic tests of real pyrolysis volatiles was carried on a continue pyrolysis reactor which was used in our previous research [4] and it combined rice husk pyrolysis and catalytic reforming of pyrolysis volatiles. Both the pyrolysis and catalytic reforming temperature are 500℃. The rice husk feeding rate was 100 g/h. About 20g catalyst was fixed in the quartz tube. The gas gained by catalytic cracking was collected in gas bags. Then, it was analyzed separately by GC. GC analysis was performed in a Agilent 6820 gas chromatograph (for H2, CO, CO2, CH4, C2H4, and C2H6) with a thermal conductivity detector (TCD) with N2 carrier gas and HP-PLOT-Q column (30 m length, 30℃). The TCD temperature was 250℃ and the separation was done at the ratio of 1:10. The water content of liquid products is more than 98%. It indicated


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that most of the organic was converted into gas.

Analysis and test methods (1) Gas relative concentration was calculated as follows:

Yi =

Vi *100% E Vi

i= H2, CH4, CO, CO2, C2H4, C2H6 Results and discussion The characterization of catalysts N2-Physisorption Analysis

FeNi/RHPC(I-P) FeNi/RHPC(P-I)

FeNi/RHPC(A-W) FeNi/RHA FeNi/AC

160

Absorpted volume(cm3/g at STP)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


140 120 100 80 60 40 20 0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure(P/P0)

Figure 1. Nitrogen adsorption-desorption isotherms of catalysts with different carbon supports Figure 1 shows the N2 adsorption desorption isotherms of different RHPC supported catalysts. According to the Brunauer–Deming–Teller classification, it can be seen that there are obvious hysteresis loops in the curves of adsorption isotherms for all catalysts. And the adsorption isotherm for all catalyst obeys to type Ⅱ. Especially, when P/P0 is closed to 1, the isothermal line appears to rise sharply. Meanwhile, the slope increases again. It is possible that a large number of



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adsorbate was cooled down in the outer surface of the particles, or the mesoporous or macroporous existed in the catalysts [25]. And the hysteresis loop for FeNi/RHA is H3 type, H4 for the other catalyst. Table 2

Pore structures of different catalysts

Samples

BET Surface Area（m²/g）

Pore Volume （cm³/g）a

Pore Size （nm）b

Acid loading amount (mmol/g)

0.1FeNi/RHPC (I-P) 0.1FeNi/RHPC (P-I) 0.1FeNi/RHA 0.1FeNi/AC 0.1FeNi/RHPC (A-W)

186.49 210.85 15.47 419.21 258.86

0.131 0.117 0.042 0.225 0.151

6 6 25 3 6

0.13 0.07 0.18 0.10

a: single point adsorption total pore volume of pores

b: BJH Desorption average pore width

It can be seen from table 2 that the specific surface area is according to the following order: 0.1FeNi/AC> 0.1FeNi/RHPC(A-W)>0.1FeNi/RHPC (P-I)》0.1FeNi/RHPC (I-P)》0.1FeNi/RHA. The BET surface of 0.1FeNi/RHA catalyst is only 15.47 m2·g-1. The adsorption capacity Vm of monomolecular layer is only 4.12 cm3·g-1, and the total pore volume is 0.0188 cm3·g-1. The adsorption performance of 0.1FeNi/RHA catalyst is the lowest. It is possible that the organic materials consisted of frame structure of RHPC are destroyed and the pore structure collapse during the processing of calcination in the air. Meanwhile, the pore size focused on 25nm. It induced that the only macropore was left in the 0.1FeNi/RHA. It was also possible that the macropore was formed by the agglomerated alkali metal or silicide. The specific surface area of 0.1FeNi/AC is the highest, reaching 419.21 m2·g-1. It is possible that the particle size of AC is much smaller than the rice husk carbon. The total pore volume is 0.225 cm3·g-1. Compared the adsorption performance of 0.1FeNi /RHPC (P-I) and 0.1FeNi /RHPC (A-W), the BET surface is a bit high for 0.1FeNi /RHPC (A-W). It is possible that a part of


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organic materials (bio-oil) which was left in the pore or surface of RHPC was washed by acidic aqueous solution. Meanwhile, a part of small particle in the pore was washed away, therefore, the pore volume of 0.1FeNi/RHPC (A-W) is also higher than 0.1FeNi /RHPC (P-I). It is obviously that acid washing is convince for enhance the surface characterization of RHPC. Our observation is consistent with the other researchers’’ experimental results. Mathilakhath et al. found that the adsorbent of the rice husk carbon washed with acid is highly effective for the removal of metal ions[26]. Tzonghorng et al found that the ash content decreases significantly after the acid-leaching process, which indicates that the disappearance of silica simultaneously creates more surface area and new pore structures [27]. Moreover, Yeganeh et al. [28] reported that the surface area of activated carbon depends on the ash content: the higher the ash content, the lower the N2 surface area. Compared the 0.1FeNi /RHPC(P-I) and 0.1FeNi /RHPC(I-P) catalysts, we can know that the surface area of 0.1FeNi /RHPC(I-P) is a bit lower than 0.1FeNi /RHPC(P-I). It induced that order of catalyst preparation plays an important role in the structure of catalysts. It is possible that the pore volume and BET surface for rice husk is very small, which is not beneficial for the adsorbing of metal. At other side, the metal on the surface hinder the pyrolysis volatiles smoothly to get out of the rice husk, which is not advantage for the formation of microspore. It is also that possible that the active metal Fe or Ni on the surface or interior of the RHPC promotes organic compound in RHPC to take the catalytic pyrolysis reaction and makes the RHPC forming the new pore structure and morphology during the calcination and reduction at 700℃.

The analysis of NH3-TPD



0.1FeNi/RHA 0.1FeNi/ RHPC(P-I) 0.1FeNi/AC 0.1FeNi/ RHPC(A-W) 0.1FeNi/RHPC(I-P) 785℃

20

727℃

15

signal intensity/ a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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10 412℃ 445℃

5

0

-5 100

200

300

400

500

600

700

800

900

temperature/℃

Figure.2 NH3-TPD profiles of catalysts with different carbon supports The acidic intensity distribution of the catalyst surface can be analyzed from the spectrum of NH3-TPD in fig.2. It is accepted that the adsorption temperature of ammonia gas is lower than 300 ℃, corresponding to the weak acidic sites. When the temperature of NH3 adsorption peak is among 300 ℃ and 450 ℃, it stands for the medium acidic sites. And the temperature is higher than 450 ℃ , corresponding for the strong acidity [29]. It can be seen from Fig.2 that 0.1FeNi/RHA catalyst has no obvious desorption peak in the whole temperature range. It induces that there is almost no acidic sites in the surface of the catalyst. However, for the 0.1FeNi/RHPC (P-I) catalysts, the catalyst showed obvious acidity. There is a broad peak, ranging from 100-500℃, which is corresponding to the weak and medium intensity acidity. And the NH3 desorption temperature was at 785 ℃, corresponding to the strong acidic sites. Meanwhile, there are also 2 peaks in the NH3-TPD curve of 0.1FeNi /RHPC (A-W). Compared with 0.1FeNi /RHPC, the change is not obvious for the peak temperature of weak and medium strong acidity. But the peak area decreased slightly. It induced that not the acidic intensity but the amount of acidic sites decreased slightly. It is possible that the weak acidic site formed


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CO2 and H2O and disappeared during the acidic washing (it is observed that a number of gas goes out of the RHPC when put the HNO3 aqueous solution into the samples.). However, for strong acidic sites, the peak temperature decreased slightly from 785℃ to 727℃. Moreover, the peak area corresponding to strong acidic sites showed significant increase, indicating that the acid intensity decreased but the amount of strong acidic sites increases sharply after acid washing. It is possible that a part of the ionic acid H+ in the HNO3 aqueous solution was adsorbed in the surface or formed new bone with group existing in the carbon, causing the increase of strong acidic sites. For the NH3 desorption curve of 0.1FeNi /RHPC (I-P), the peak corresponding temperature is similar with 0.1FeNi /RHPC. However, the peak area for strong acidic sites is much bigger than that in the 0.1FeNi /RHPC. It induces that the method of impregnation-pyrolysis (I-P) is a favor for the formation of strong acidic sites. It is possible that the metal adsorbed in the rice husk formed the FeH+, NiH+ strong acidic sites with the abundant OH- group in RHPC. For the 0.1FeNi/AC catalyst, the NH3 desorption curve is similar with 0.1FeNi /RHPC (I-P). The area and peak temperature for strong acidic sites is the same. However, the peak temperature shifted from 445 to 412℃. It induced that the acidic intensity of 0.1FeNi/RHPC (I-P) is stronger than 0.1FeNi/AC. Moreover, the total acid amount was shown in table 2. The content for 0.1FeNi/RHPC (I-P) and 0.1FeNi/RHA samples was higher than others

The analysis of XRD



■

0.2Fe/RHPC(P-I)◆

●

▼ Ni ▼

▲

0.1FeNi/RHPC(P-I)

● Ni3Fe ●

▼

0.2Ni/RHPC(P-I)

●

0.1FeNi/RHPC(P-I)

◆Fe3O4 ■ Fe ◆◆ ◆■

◆

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0.1FeNi/RHPC(I-P)

▼

◆

●

◆

▲ Fe0.64Ni0.36 ▲

●

◆

● Ni3Fe ◆Fe3O4

◆ ◆

▲

0.1FeNi/AC

●

▲ 0.1FeNi/RHA

▲

0.1FeNi/RHPC(A-W)

▲

▲

● Ni3Fe ◆Fe3O4

◆

▲ Fe0.64Ni0.36

0.1FeNi/AC

▲ 0.1FeNi/RHPC(A-W)▲

▲ Fe0.64Ni0.36

●

▲ Fe0.64Ni0.36 ▲

▲ Fe0.64Ni0.36

▲ 0.1FeNi/RHA

●

▲

0.1FeNi/RHPC(I-P)

▲

●

20

30

40

50

●

▲ 10

10

● Ni3Fe

▲ Fe0.64Ni0.36

60

70

80

2θ (°) (a) :Fresh catalysts

20

30

40

50

60

2θ (°)

(b):Spent catalysts

Figure 3 XRD patterns of catalysts: (a) fresh samples and (b) spent samples. Fig. 3 shows the XRD analysis of fresh and spend catalysts. The crystallite size of the corresponding crystal phase in the catalysts (calculated by Scherrer Equation [30]) are shown in table 3. Comparing the diffraction peaks with reference patterns of Fe-Ni alloy (JCPDS: 47-1405, 2θ = 43.6, 50.8, 74.7, 90.6 and 95.9°) demonstrates the successful formation of Fe-Ni alloy Fe0.64Ni0.36 in all double metal FeNi/supports samples. Since the peaks related to reference patterns of Fe-Ni spine have not observed in any patterns. It can be mentioned that during the preparation, the alloy is the only crystal. Furthermore, the peaks of active phase (Ni, Fe, Fe3O4) corresponding to its standard patterns (JCPDS: 87-0712, 2θ = 44.496, 51.849 and 76.381°; JCPDS: 06-0696, 2θ = 44.67, 65.021, 82.33, 98.95, 116.38 and 137.14°; JCPDS: 88-0866, 2θ = 30.12, 35.48, 37.11, 43.12, 47.21, 53.50, 57.03, 62.62, 65.84, 66.90, 71.04, 74.09, 75.09 and 79.06°) were observed. Quantitatively, calculated the crystallite size based on the peak intensity and width


70

80

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of crystal. It is obvious that the crystal size of Fe0.64Ni0.36 in 0.1FeNi/RHPC (P-I) is smallest (21.1nm). Especially, the crystallite size of Ni (49nm) and Fe (38nm) in the single metal catalyst 0.2Fe/RHPC (P-I) and 0.2Ni/RHPC (P-I) is much bigger than double metal catalyst. It induced that the adding of Fe effectively enhanced the dispersion of Ni and formation the alloy, which reduced the particle size of active phase Fe0.64Ni0.36, eventually leading to better performance for catalytic reforming. It is noticeable that Ni3Fe (JCPDS: 88-1715, 2θ = 44.2, 25.1, 35.8, 51.5, 58.1, 64.3, 75.8, 81.4 and 86.8°), and Fe3O4 was found in the spent catalysts 0.1FeNi/RHPC (P-I), inducing that the alloy Fe0.64Ni0.36 was converted into Ni3Fe and Fe3O4 after catalytic cracking. It is possible that part of Fe separated out from alloy and formation Fe3O4. Table 3. Structural properties of RHPC supports nanocatalysts Catalysts Crystallite phase（fresh） a

0.1FeNi/RHPC(P-I)

0.1FeNi/AC

0.1FeNi/RHA

0.1FeNi/RHPC(A-W)

0.1FeNi/RHPC(I-P)

0.2Fe/RHPC(P-I)

0.2Ni/RHPC(P-I)

Fe0.64Ni0.36

Fe0.64Ni0.36

Fe0.64Ni0.36

Fe0.64Ni0.36

Fe0.64Ni0.36

Fe3O4/Fe

Ni

Crystallite size(nm) (fresh)

21.1

32.9

22.3

34.2

24.3

40/38.4

49

Crystallite phase（Spent）

Ni3Fe

Ni3Fe/Fe3O4

Ni3Fe

Fe0.64Ni0.36

Ni3Fe//Fe3O4

-

-

Crystal size(nm)（Spent）

18.5

36.4/49.2

29.2

21.2

32.2/66.7

-

-

a: Crystallite size estimated by Scherre's equation.

The analysis of FT-IR

I B G

Signal intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Spend 0.1Fe Ni/RHA

Fresh 0.1FeNi/RHA

Spend 0.1Fe Ni/RHC(A-W)

F

Fresh 0.1Fe Ni/RHC(A-W)

E

Spend 0.1Fe Ni/AC

D

Fresh 0.1Fe Ni/AC

C

Spend 0.1FeNi/RHPC(I-P)

H

Fresh 0.1FeNi/RHPC(P-I)

A

Spend 0.1FeNi/RHPC(P-I) 2928.5 3460.7

621.5 2354.7

792.5

1560.2 1090.6

4500 4000 3500 3000 2500 2000 1500 1000

465.3

500

0

-1

Wavennumber (cm )

Figure.4 FT-IR spectra of catalysts with different carbon supports



FT-IR spectrum for the fresh and spend catalysts shown in Figure 4. It is accepted that the wavenumber at 3480.7cm-1 is corresponding to the stretching vibration characteristic absorption peak of -OH group or intermolecular hydrogen bond. The band at 2928.5cm-1 is the -CH2 antisymmetric stretching vibration. The absorption peak of C-H bone is at 1560.2cm-1 and the C-O stretching vibration absorption peak appears at 1090.6cm-1. The characteristic absorption peaks for Ni appear at 792.5 cm-1 and 621.5 cm-1. The bending vibration of O-Si-O bone is at 470.8 cm-1. Compared with the fresh 0.1FeNi/RHPC (P-I), most of the groups keep the same, excepting for that the band at 2354.7 cm-1 corresponding to C≡C stretching vibration disappeared at spent 0.1FeNi/RHPC (P-I). For spend 0.1FeNi/RHPC (P-I), the stretching vibration peak of C≡C at 2354.7 cm-1 was more obvious than that of fresh one. And the peak became sharp for spent catalysts. For spend 0.1FeNi/RHA, the peak at 3480.7cm-1corresponding to -OH or intermolecular hydrogen bonds at 3480.7 cm-1 is obviously weakened. Moreover, for the fresh 0.1FeNi/AC, there is nearly no peaks. It is possible that the AC was prepared via steam activation at 900℃. And the group in the AC was degenerated at high temperature. However, for the spent 0.1FeNi/AC, there are new groups appearing due to the accumulation of intermediate products and reactants on the surface of catalysts and forms carbon deposition.

The analysis of SEM


Page 16 of 42

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a：RH

b：AC



c:0.1FeNi/RHPC（A-W）

d: 0.1FeNi/AC


Page 18 of 42

Page 19 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


e: 0.1FeNi/RHPC (I-P)

f: 0.1FeNi/RHPC (P-I)



g：0.1FeNi/RHA Figure 5 the SEM images of catalysts with different carbon support The SEM pattern of catalysts was shown in figure 5. For the support of RHPC, the shape of RHPC keeps the same with the RH. And there is no obvious pore in the surface of RHPC. On the contrary, the AC supports is a highly porous carbonaceous material. For 0.1FeNi/AC, there are obvious agglomerated particles with varied size on the surface. The EDS results show that the main composition of the particles is the compound of Fe-Ni. The amount of pores is significantly decreased. It is possible that the compound of Ni-Fe occupied a part of pores.

The effect of different supports on the catalytic reforming of model bio-oil


Page 20 of 42

Page 21 of 42

H2

CO

CH4

CO2

C2H4

C2H6

50

The concentration of gases (%)

50


40

30

20

10

H2

CO

CH4

CO2

C2H4

C2H6

40

30

20

10

0

0

10

20

30

40

50

60

Time (min)

10

20

30

40

50

60

Time (min)

a：RHA

b：RHPC H2

CO

CH4

CO2

C2H4

C2H6

40


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30

20

10

0 10

20

30

40

50

60

Time (min)

c：quartz sand Fig. 6 The effect of different supports on the catalytic reforming of model bio-oil （a：RHA，b：RHPC，c：quartz sand. The feed rate is 0.29g/min; the mass of catalysts was 2.5g; the reaction temperature is 500℃）

As shown in Fig. 6, the catalytic performance of different supports on bio-oil showed a significant difference as well as the quartz sand replacing the catalyst. In Fig. 6(a), it can be seen that the concentration of gas according to the order CH4> CO> H2> CO2> C2H4> C2H6 in the syngas by the catalytic reforming of simulated bio-oil via RHA support. The CH4 content is highest, reaching 44.5%. However, the H2 content is only 5%. In Fig. 6 (b), the concentration of CH4 at first is 45.3% and then decreases to 36%. Meanwhile, the H2 concentration decreases from 10.15% to 7.49%. When using SiO2 replacing the catalyst, it can be seen in fig. 6 (c), the



concentration of H2 shows a decreasing trend from 18.9% to 13.3%. It can be found that the concentration of H2 is the highest when using quartz sand as support. It is possible that the quartz sand is beneficial for strengthening of heat and mass transfer than RHPC and RHA, which promotes the reforming of bio-oil. The influence of catalyst with different supports on the catalytic reforming of model bio-oil H2

CO

CH4

CO2

C2H4

H2

CO

CH4

CO2

C2H4

C2H6

50



40

40 30 20 10

30

20

10

0 0

0

10

20

30

40

50

60

70

80

0

90

10

20

30

CO

CH4

CO2

50

60

70

80

C2H4

C2H6

H2

40

CO

CH4

CO2

C2H4

C2H6


50

30

20

10

0 10

20

30

40

50

90

b: 0.1FeNi/AC

a:0.1FeNi/RHPC（A-W） H2

40

Time (min)

Time (min)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 42

60

70

40 30 20 10 0 10

20

30

40

50

60

Time (min)

Time (min)

c: 0.1FeNi/RHPC (I-P)

d: 0.1FeNi/RHPC (P-I)


70

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


H2

CO

CH4

CO2

C2H4

20

30

40

50

C2H6

50

The concentration of gas (%)

Page 23 of 42

40

30

20

10

0

10

60

Time(%)

e：0.1FeNi/RHA

Fig.7 The influence of catalyst with different supports on the catalytic reforming of model bio-oil （a:0.1FeNi/RHPC（A-W）, b: 0.1FeNi/AC, c: 0.1FeNi/RHPC (I-P)，d:0.1FeNi/RHPC (P-I)，e：0.1FeNi/RHA The feed rate of model bio-oil is 0.29g/min; the mass of catalysts was 2.5g; the reaction temperature is 500℃）

In order to figure out the effect of different rice husk carbon support catalysts on the CR of bio-oil, the experiment with different catalysts was carried out. The reaction temperature is 500℃. Model bio-oil is the reactant. The feed rate is 0.29g/min and the catalyst usage is 2.5g. Figure 7(a) shows the effect of 0.1FeNi/RHPC(A-W) catalyst on the CR of bio-oil. The conversion of alcohol, acetone, and acetic acid is 100% for all samples. Only a part of guaiacol was left in the liquid products. Moreover, the water content is more than 98% for all liquid products, excepting for 0.1FeNi/AC (92%) and 0.1FeNi/RHPC (I-P) (93%). Therefore, we mainly analyzed the gas products to investigate the activity of catalysts with different supports. The concentration of gas is according to the order: H2> CH4> CO2> C2H4> C2H6. The concentration of H2 is highest, peaking at 49.8% at 40min. And then it goes down to 44.5% at 80min. CH4 content is stable at about 10%. CO and CO2 concentration slightly fluctuates among 23% and 20%, respectively. For 0.1FeNi/AC catalyst in figure 7(b), the gas concentration changed significantly during 80min. The concentration of H2 continued to decrease from 39.7% to 9.3%.



Page 24 of 42

On the contrary, the CH4 concentration increased significantly from 20.1% to 40.1%. Especially, the CO concentration decreased from 33.3% to 26.5%. Obviously, the concentration of H2 and CO is positively correlated. It is possible that model bio-oil takes the following reaction during steam catalytic reforming. C x H y + xH 2 O → xCO+(x+y/2)H 2 ( steam reforming )

（1）

C x H y + xCO 2 → (2x)CO+(y/2)H 2 (dry reforming)

（2）

CH4 + H2O → 3H2 + CO ( steam reforming methane reaction)

（3）

CO + H2 O → H2 + CO2 (water gas shift reaction)

（4）

C + H2 O → H2 + CO (water gas reaction)

（5）

However, the changed trend of CO and CO2 concentration is opposite. Especially, the CO2 concentration increased from 4.8% to 14.9%. It is possible that the competing reaction exists in the process of bio-oil catalytic reforming/cracking. Such as catalytic cracking or steam catalytic reforming of acetic acid and ethanol according to competing reaction. CH3COOH→CO2+CH4

(6)

CH3COOH→2CO+2H2

(7)

2CH3COOH→ (CH3)2CO+CO2+2H2O

(8)

C2H5OH+H2O→2H2+CO2+CH4

(9)

C2H5OH+H2O→4H2+2CO

(10)

2C2H5OH+3H2O→7H2+2CO2+CO+CH4

(11)

C2H5OH+H2O→6H2+2CO2

(12)

The C2H4 and C2H6 haven’t been detected. The change of the gases concentration induced that 0.1FeNi/AC is not stable during the whole process. When using 0.1FeNi/RHPC-2 (I-P), the


Page 25 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


change trend of gas concentration is similar with 0.1FeNi/AC. The H2 concentration decreases with time from 38.5% to 17.6%. And the CH4 concentration obviously competes with the H2. That is CH4 shows a steady upward trend, from 23.8% gradually increases to 34.3%. And CO content remained stable at about 20%. In fig.7 (d), the change of gas concentration is not obvious during 70min, inducing that the stability of 0.1FeNi/RHPC (P-I) is better than other catalysts. H2 concentration is stable at about 50%, CO content slightly fluctuated around 30%. And the concentration of CH4 is 10%. The H2 content decreased from 50.5% at 40min to 46% with the 0.1FeNi/RHA catalyst in fig. 7(e). A comparison of the catalytic reforming/cracking performance of five catalysts was investigated. It can be seen that the stability of 0.1FeNi/RHPC (P-I) catalyst is the best. And the H2 concentration is the highest, which is stable at 50% in 70 min. The activity of 0.1FeNi/RHPC (P-I) is highest in all catalyst, indicating that the active center is the alloy Ni0.64Fe0.36 and Ni3Fe combined with the results of XRD. Moreover, the crystallite size of 0.1FeNi/RHPC (P-I) is smaller than others in table 3, which promoted the catalytic performance. It is possible that the metal adsorbed in the RHPC promotes well-formed and stable channels and morphology to form during the process of calcination. It also enhances the formation and uniform distribution of acid sites which is beneficial for the RC of bio-oil. It was noticeable that the stability of 0.1FeNi/AC and 0.1FeNi/RHPC-2 (I-P) significantly decreased during the process of RC. It is possible that the alloy Fe0.64Ni0.36 was converted into Ni3Fe and Fe3O4 after catalytic cracking in XRD results. And the alloy Fe0.64Ni0.36 and Ni3Fe were the active center, excepting for Fe3O4, which causing the decrease of activity of 0.1FeNi/AC and 0.1FeNi/RHPC-2 (I-P). Moreover, for the 0.1FeNi/RHA catalysts, the H2 concentration is 50% in the first 40 minutes, and then began to decline. It is



accepted that the relative content of alkali metal such as K, Na, Ca, and Fe in the rice husk ash is much higher than other carbon. And the alkali metal improves the catalytic activity of catalytic reforming of bio-oil at first. However, there is no good structure and pore in the RHA (show in table 1). Therefore, intermediate products and reactant tends to accumulate on the surface of metal and forms carbon deposition, which covered some of the metal active sites. Moreover, there is no good support for the active metal, which is not advantage for active metal scatter, causing the agglomerate and sintering of alkali metal. Moreover, the melting of K and Na will cover the active metallic Fe and Ni, which causes the decreasing of activity. The results is consistent with the investigation of Chen who had found that alkali metals (K and Na) is easy to sinter due to low melting point (The melting point of Na and K is 97.81℃ and 63.65℃, respectively). And the alkali with SiO2 in the rice husk ash will produce the low melting-point silicates resulting in melting phenomenon occurring [31]. It is the reason for the reducing of catalytic activity of 0.1NiFe/RHA and hindering the catalytic reforming of bio-oil to produce H2. It is also induced that carbon in the 0.1FeNi/RHPC (P-I) plays a significant role in bio-oil catalytic conversion. On the one hand, the porous carbon can improve the surface area (17.93m2/g for 0.1NiFe/RHA vs 424.77m2/g for 0.1FeNi/RHPC (P-I) in table 2) which is beneficial for enhancing the sorption to bio-oil or gases. Moreover, we found that the some of the metallic had been converted into metal oxides, causing the activity decreasing during the catalytic reforming of bio-oil in our previous research [32]. Therefore, the carbon was considered as a medium as well as reducing gases in products for the transforming metal oxides (FexOy) into metallic states at high temperature by the following reductive reactions, which was contributed to the enhancement of bio-oil conversion [33-34]


Page 26 of 42

Page 27 of 42

Therefore, the carbon in 0.1FeNi/RHPC (P-I) was more useful for resisting the oxidation of metallic Ni and Fe during the bio-oil conversion due to the high oxygen amount in bio-oil. It was accepted that Fe and Ni in their metallic forms rather than oxide forms were considered as the main active sites for the tar reforming [35]. Compared with the catalyst (0.1FeNi/RHPC (P-I)), the catalytic activity of 0.1FeNi/RHPC (A-W) is similar. It induced that activation and pretreatment are not necessary for improving the activity of the 0.1FeNi/RHPC (P-I). Moreover, the stability of 0.1FeNi/AC and 0.1FeNi/RHPC-2 (I-P) is obviously poor. The selectivity of hydrogen is also lower than 0.1FeNi/RHPC (P-I) due to higher amount of strong acidic sites (shown in fig.2 NH3-TPD). It is accepted that strong acidic sites plays an important role in the carbon deposition [36-37]. Moreover, the BET surface of 0.1FeNi/RHPC-2 (I-P) is smaller (186.49 m2/g) than others and the pore size of 0.1FeNi/AC is smaller (3nm) than others catalysts. It is accepted that when BET surface is too small, the carbon deposition will occur. Because the pore size is so small that the reactants would be restricted to go through the catalysts, which causing the deactivation of catalysts [38].

The role of Fe and Ni on the activity of 0.1FeNi/RHPC (P-I) H2

CO

CH4

CO2

C2H4

C2H6

50


40


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30

20

10

0 10

20

30

40

50

60

H2

CO

CH4

CO2

C2H4

40

30

20

10

0 10

20

Time (min)

30

40

50

Time (min)

a：0.2Fe/RHRC (P-I)

C2H6

b：0.2Ni/RHPC (P-I)


60


H2

CO

CH4

CO2

C2H4

C2H6

50


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

40 30 20 10 0 10

20

30

40

50

60

70

Time (min)

c: 0.1FeNi/RHPC (P-I) Fig.8 The effect of single metal catalyst on the catalytic reforming of model bio-oil (a：0.2Fe/RHPC (P-I)，b：0.2Ni/RHPC (P-I)，c：0.1FeNi/RHPC (P-I) The feed rate of model bio-oil is 0.29g/min; the mass of catalysts was 2.5g; the reaction temperature is 500℃）

The role of Fe and Ni on the activity of 0.1FeNi/RHPC (P-I) was investigated. Fig. 8(a) shows the gas composition via catalytic reforming of bio-oil with 0.2Fe/RHRC. At the first 10min, the CH4 content increased rapidly from 13.2% to 35.3%, then remained stable at around 33%. The content of H2 also increased from 2.2% to about 16%. It is obvious that the gas concentration via 0.2Fe/RHRC is much lower than that with 0.1FeNi/RHPC (P-I) (fig.8c). It indicated that the catalytic activity of Fe and Fe3O4 is much low for RC combined with the results from XRD. It is possible that, the activation energy of C-H, H-H, H-O and C-O bond in the bio-oil molecules is high on the in Fe or Fe3O4 surface [33]. While the H2 concentration is about 48.9% with 0.2Ni/RHPC catalyst (fig.8b). It induces that the Ni-RHPC catalyst possesses much higher bio-oil catalytic cracking/reforming performances than Fe-based catalyst. It was reported that Nickel (Ni0) catalyst had much stronger ability for catalytic reforming/cracking of C–H and C–C bond in hydrocarbons than metallic iron (Fe0) catalyst [36]. Meanwhile, we found that the H2 concentration decreased from 48.9% to 39.6% at


Page 28 of 42

Page 29 of 42

50min, which indicated that the catalyst loss activity gradually. It is accepted that there are two reason for catalyst inactivity. One is carbon deposition [39-40]. And the other one is the sintering of active metal. Comparing the bimetallic catalysts of 0.1FeNi/RHPC (P-I) with monometallic catalysts 0.2Fe/RHRC and 0.2Ni/RHRC, 0.2Ni/RHRC has similar activity with 0.1FeNi/RHPC (P-I) (H2 concentration (50.5% vs.48.9%)) but poor stability. It induced that the Ni is also the active center 0.2Ni/RHRC catalysts combined with results in XRD. Moreover, it is obvious that the activity of 0.2Fe/RHRC catalysts is much lower than 0.2Ni/RHRC catalysts. However, the activity of 0.1FeNi/RHPC (P-I) is a bit higher than 0.2Ni/RHRC catalysts. It induced that the activity of alloy Ni-Fe is higher than Ni and the crystallite size of alloy is much smaller than Ni. Therefore, Fe was beneficial for the formation of Fe-Ni alloy and improved the activity of catalyst. Besides, when adding the Fe, the stability of 0.1FeNi/RHPC (P-I) was improved. It is consistent with the report that the Fe was conducive for resisting to carbon deposition and metal sintering [41]. It is possible the stability of Fe-Ni alloy is higher than Ni. The catalytic performance of Fe/Ni/RHPC on the model gas, model bio-oil and real RHPV H2

CO

CH4

CO2

C2H4

C2H6

H2

60

CO

CH4

CO2

C2H4

C2H6

50

50



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40 30 20 10

40 30 20 10 0

0 10

20

30

40

50

60

70

10

20

a：model gas

30

40

50

Time (min)

Time (min)

b：model bio-oil


60

70


H2

CO

CH4

CO2

C2H4

C2H6

30


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

20

10

0

0

10

20

30

40

50

60

70

80

90

100 110

Time (min)

c：Real pyrolysis volatiles Fig.9 The catalytic performance of Fe/Ni/RHPC on the model gas, bio-oil and real RHPV （a：model gas，b：model bio-oil，c：real pyrolysis volatiles; the feed rate of model bio-oil is 0.29g/min; the feed of model gas is 100ml/ml; the mass of catalysts was 2.5g for model gas and model bio-oil. The feed of rice husk is 100g/h, the mass of catalyst is 20g ; the reaction temperature is 500℃, the catalysts is Fe/Ni/RHPC(P-I).

Fig.9a shows the catalytic reforming of model gas via 0.1FeNi/RHPC (P-I). The influence of 0.1FeNi/RHPC (P-I) is not obvious. The concentration of gas is almost keeping the same with the mold gas. The real pyrolysis volatiles catalytic tests was carried on a continue pyrolysis reactor which was used in our previous research [4]. The results was shown in fig.9c. The concentration of H2 is stable at 23%. CH4 concentration is highest, slightly fluctuating around 30%. The performance of 0.1FeNi/RHPC (P-I) for catalytic reforming of real pyrolysis volatiles is not so good than model gas or bio-oil. It is possible that the composition of rice husk pyrolysis volatiles is very complexed [42-43], and the catalytic activity for catalytically reforming of all composition in the pyrolysis volatiles is not very high due to the selectivity of catalyst. Pyrolysis volatiles catalytic conversion scheme


Page 30 of 42

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Rice Husk

RHPV

RHPC

RHPV Gas

Tar

Fe/Ni/RHPC

Rich H2,CO,CO2

Tar : CH3COOH CH3 CH2 OH O

NiFe

OH O

CH3

C

Catalytic Reforming/Cracking

C-C

CH3

gas

CH3

H2O

NiFe

H2O

RHPC

CO+H2O+char==CO2+H2+CO

NiFe

NiFe

In situ oxidation or reduction Char,CO,H2+NimFem+Fe3O4== CO,CO2,H2O+FejNik

Fe NiFe

Gas: H2,CO,CO2,CH4, C2H4,C2H6 /H2O

Fe NiFe

NiFe

Catalytic Reforming

Rich H2 gas

Ni3Fe Ni0.64Fe0.36

The catalytic reforming of RHPV via Fe/Ni/RHPC

Fig.10 The rice husk pyrolysis volatiles conversion scheme via the 0.1FeNi/RHPC catalysts Fig.10 shows the rice husk pyrolysis volatiles conversion scheme via the 0.1FeNi/RHPC (P-I) catalysts. Rice husk was initially cracked into the pyrolysis volatile and char by thermo-chemical reactions. The pyrolysis volatiles including gas and tar can be catalytically cracking and reforming simultaneously over the 0.1FeNi/RHPC (P-I) under high temperature. And the main reactions contained rice husk pyrolysis, catalytic reforming/cracking and in-situ reduction. And all the reactions were shown as following: Rice husk pyrolysis: Rice husk → pyrolysis volatiles (bio-oil + gases) + rice husk pyrolysis char Pyrolysis volatiles catalytic reforming/cracking: Catalytic cracking: CnHmOy → ChHj +CxHyOz+gases+H2O

(1)

CnHm → gases + CxHy

(2)

Reforming: Dry reforming:



CnHmOy + nCO2 → 2nCO + (m/2)H2

(3)

C x H y + xCO 2 → (2x)CO+(y/2)H 2 (dry reforming)

(4)

Steam reforming: CnHmOy + nH2O → nCO + (n + m/2)H2

(5)

CnHmOy + 2nH2O → nCO2 + (m/2 + 2n)H2

(6)

C x H y + xH 2 O → xCO+(x+y/2)H 2 ( steam reforming )

(7)

CO + H2O → CO2 + H2

(8)

CH4 + H2O ↔ CO + 3H2

(9)

C + H2O → CO + H2

(10)

C + 2H2O → CO2 + 2H2

（11）

Stimulated Bio-oil and gases catalytic cracking/reforming: CH3COOH→CO2+CH4

(12)

CH3COOH→2CO+2H2

(13)

2CH3COOH→(CH3)2CO+CO2+2H2O

(14)

C2H5OH+H2O→2H2+CO2+CH4

(15)

C2H5OH+H2O→4H2+2CO

(16)

2C2H5OH+3H2O→7H2+2CO2+CO+CH4

(17)

C2H5OH+H2O→6H2+2CO2

(18)

CH3COCH3+5H2O→8H2+3CO2 [46]

(19)

C7H8O2 + 12H2O → 7 CO2 + 16H2 [47]

(20)

CH4 + H2O ↔ CO + 3H2

(21)

CO2 + 4H2↔ CH4 + 2H2O

(22)


Page 32 of 42

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Gases catalytic reforming: CH4+CO2 → CO + H2

(23)

C + CO2→ 2CO

(24)

In-situ oxidation: Ni0.64Fe0.36 → Fe3O4+Ni3Fe

(25)

In-situ reduction: nC + Fe3O4 → mFe + nCO

(26)

nH2 + Fe3O4→ mFe + nH2O

(27)

nCO + Fe4O3→ mFe + nCO2

(28)

nC + Fe3O4 + Ni3Fe→ nCO + Ni0.64Fe0.36

(29)

nH2 + Fe3O4 + Ni3Fe→ nH2O + Ni0.64Fe0.36

(30)

nCO + Fe4O3 + Ni3Fe→ nCO2 + Ni0.64Fe0.36

(31)

Fig.11 showed the mechanism of rice husk pyrolysis volatiles conversion via the 0.1FeNi/RHPC catalysts. Catalytic cracking reactions via cleaving the C-H and C-O bonds of the carbohydrate backbone produced a combination of gases such as CO, CO2, CH4, H2, and other small molecule hydrocarbon or oxygenated compounds (reaction (1)–(2)). The CnHmOy was aborted on the surface of RHPC, it was catalytic cracking via the combination of active center Ni0.64Fe0.36 alloy and acid sites. And the catalytic reforming reactions of pyrolysis volatiles contained dry reforming and steam catalytic reforming. When H2O from rice husk or products is present, which would be absorbed on the surface of RHPC and formed OH- groups. The main reaction was steam catalytic reforming (reactions (5)–(10)), including the hydrocarbon and gases steam reforming. For



example, CnHmOy, CxHy, CO and CH4 took the steam reformation reaction to produce more H2 and CO (CO2). It was accepted that compounds or gases in the pyrolysis volatiles can be adsorbed on the active sites of catalysts particles [44-45]. And if the adsorbed compound can’t convert into other products and leave the particle, it accumulated in the catalyst and formed the carbon deposition or cokes, which occupied the active site and reduced the activity of catalysts. It was noteworthy that the adsorbed tars and cokes can be converted into CO and H2 (reaction (5)-(7), (9)-(10)) by steam and dry thermochemical reactions, which was a good way for refreshing the active surface area of char at temperature above 800◦C as well as catalysts regeneration[46]. For the simulated gas or bio-oil, the main reaction is the steam catalytic reforming (reaction (12)-(24)). Moreover, the RHPC supports was an intermediate reductant to reduce the metal oxides as well as reduced gases CO and CH4 [34] (equation (25)-(31)), which improved the active sites of catalyst.


Page 34 of 42

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Rich H2 gas CO2,CO,H2,CH4 CnHmOy,Gas H2,CO

H2O

[Oy] Fe3O4

Ni3Fe

O-H

Ni0.64 Fe 0.36

Acid sites

Ni0.64 Fe 0.36

Rice husk pyrolysis carbon Carbon deposition Acid sites

Pores on the RHPC Carbon from the RHPC

Fig.11 The mechanism of rice husk pyrolysis volatiles conversion via the 0.1FeNi/RHPC catalysts

Conclusion (1) A renewable H2 was obtained via catalytic reforming/cracking of rice husk pyrolysis volatiles with the catalysts employed RHPC as support. Fe-Ni supports on different treated RHPC catalysts were prepared via wet impregnation methods. 5 different treated process such as pyrolysis-impregnation, impregnation-pyrolysis, activation, acid washing, and calcining in air was employed to improve the catalytic activity of catalysts. (2) Acid washing improves the BET surface. And FeNi/RHPC (P-I) possessed high activity due to active metallic Fe or Ni on the surface or interior of the RHPC promoting organic compound in RHPC to take the catalytic pyrolysis reaction and making the RHPC forming the new pore structure and morphology during the reduction at 700℃. The acidic intensity



Page 36 of 42

was higher due to the formation of strong acidic sites FeH+, NiH+. (3) The activity of 0.1FeNi/RHPC (P-I) is highest in all catalyst, indicating that the active center is the alloy Ni0.64Fe0.36 and Ni3Fe combined with the results of XRD. Moreover, the smaller crystallite size of 0.1FeNi/RHPC (P-I) promoted the catalytic performance. The carbon in RHPC was more useful for resisting the oxidation of alloy Ni3Fe in to Fe3O4 during the bio-oil conversion due to the high oxygen amount in bio-oil, which enhancing the catalytic performance. 0.1FeNi/RHA is not stable due to the sintering of alkali metal and the accumulation of intermediate products and reactant. (4) The activity of 0.2Fe/RHRC is much lower than the 0.1FeNi/RHPC (P-I) and 0.2Ni/RHRC. 0.2Ni/RHRC

has

similar

activity

with

0.1FeNi/RHPC

(P-I)

(H2

concentration

(50.5% vs.48.9%)) but poor stability. The adding of Fe promoted the formation of Fe-Ni alloy and reduced the crystallite sizes. (5) The catalytic activity and stability were in the order: 0.1FeNi/RHPC(P-I) > 0.1FeNi/RHPC(A-W)≈0.1FeNi/RHA > 0.1FeNi/RHPC-2 (I-P) > 0.1FeNi/AC, which was consistent with the results of NH3-TPD. It suggested that high active RHPC-support catalyst was prepared avoiding pretreatment, activation and calcination, which made the preparation procedure of catalysts become much simple and energy saving. (6) Bio-oil, gas, real pyrolysis volatile were employed to investigate the interaction and the catalytic reforming mechanism. Moreover, the RHPC supports was an intermediate reductant to reduce the metal oxides.

Acknowledge


Page 37 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Supported

by

Chinese National Natural Science Foundation

(Grant

No.51706075);

Chinese National Natural Science Foundation (Grant NO. 51576071); Cooperative exchange project between the National Natural Science Foundation of China (NSFC) and Royal Society (RS), China (Grant No.51711530230)；Science and Technology Planning Project of Guangdong Province, China (Grant No.2016A020210073), the Science and Technology Planning Project of Guangdong Province, China (Grant No.2015B020237010).

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Page 42 of 42

Rice Husk

RHPV

RHPV

RHPC

Rich H2 gas

Tar

Gas

NiFe RHPV

FeNi/RHPC

NiFe

Continue pyrolysis

Tar : CH3 COOH CH3 CH2 OH O OH

RHPC

O

Different treatment Activation Acid washing Calcination inair No treatment

Calcination and reduction

CH3

C

Catalytic Reforming/Cracking CH3

CO, H2 O+Char CO2 , H2 , CO

Fe / Ni/ RHPC impregnation

C-C

C-H

CH3

H2 O

Rich H2 gas

NiFe

NiFe

RHPC

NiFe

In- situ reduction Char, CO, H2 +Mem On CO, CO2 , H2 O+Me

NiFe Gas: CO, CO2 , H2 , CH4 , H2 O, C2 H4 , C2 H6

Catalytic Reforming

Rich H2 gas

Renewable H2 was obtained from the catalytic reforming of rice husk pyrolysis volatile and the pyrolysis carbon was used as the catalysts.


Hydrogen from Rice Husk Pyrolysis Volatiles via Non-Noble NiâFe

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