Al2O3: Effect of Ce loading

Fuel 103 (2013) 193–199

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Hydrogen production by supercritical water gasification of glucose with Ni/CeO2/Al2O3: Effect of Ce loading Youjun Lu ⇑, Sha Li, Liejin Guo ⇑ State Key Laboratory of Multiphase Flow in Power Engineering (SKLMF), Xi’an Jiaotong University, Xi’an 710049, Shaanxi, China

h i g h l i g h t s " Ni/CeO2/Al2O3 were prepared successfully by co-impregnation method. " The effects of Ce loading in catalysts on glucose gasification were studied. " The oxidant kinetic data of deposited carbon in catalysts with air was obtained. " The role of Ce in the glucose gasification was discussed.

a r t i c l e

i n f o

Article history: Received 23 February 2012 Received in revised form 20 April 2012 Accepted 24 April 2012 Available online 10 May 2012 Keywords: Hydrogen production Supercritical water gasification Ni catalyst Ce promoter

a b s t r a c t Ni-based SCWG catalysts attract more attention for its high activity and relatively low cost. However, carbon deposition, which is one of the serious problems, reduces the activity of Ni-based catalysts. Ni/ CeO2/Al2O3 catalysts with different Ce loading were prepared by a wet impregnation method. The performance of catalyst in supercritical water was tested in an autoclave reactor at 673 K, 24.5 MPa. The catalysts before and after reaction were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), BET specific surface area measurements and thermo-gravimetric analyses (TGA). The effects of Ce loading in catalysts on glucose gasification were studied. The results showed that hydrogen yield and hydrogen selectivity increased sharply with addition of Ni/CeO2/Al2O3 catalysts. When the Ce loading content was 8.46 wt.%, the maximum H2 yield and H2 selectivity were obtained. The carbon deposition and coking will lead to the deactivation of the catalysts. Based on the thermo-gravimetric analyses, the oxidant kinetic data of carbon deposited on the used catalysts with air was obtained. CeO2 in the Ni/CeO2/Al2O3 catalyst plays an important role in inhibiting carbon deposition by increasing the Ni dispersion and reacting with deposited carbon. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Hydrogen is widely regarded as a promising energy carrier in the future because of its high energy density by weight, storable, clean and efficient properties. In China, hydrogen production from biomass cannot only promote the large-scale exploitation of renewable energy, but also improve the energy consumption structure and reduce the environmental pollution. Moreover, it should be one of the most feasible ways for hydrogen production from renewable energy in the near future. One of the methods for producing hydrogen from biomass is supercritical water gasification (SCWG) that can avoid high drying costs in conventional thermo-chemical gasification process, especially for wet biomass [1]. In last two decades, SCWG was concerned widely by many scholars of USA [2,3], Europe [1,4], ⇑ Corresponding authors. Tel.: +86 29 8266 4345; fax: +86 29 8266 9033. E-mail addresses: [email protected] (Y. Lu), [email protected] (L. Guo). 0016-2361/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2012.04.038

Japan [5,6], China [7,8] and other countries [9–11]. The reaction temperature required for non-catalytic complete gasification of biomass in SCW is about 873 K that is much lower than the reaction temperature of the conventional thermo-chemical gasification for hydrogen production [12]. However, the operation pressure of SCWG is generally higher than the critical pressure of water (22.1 MPa). High pressure and temperature makes the gasification apparatus more demand for containment structure and use of costly alloys [13], which increases the investment costs of this hydrogen production technology. Therefore, reducing the reaction temperature is crucial for reducing the hydrogen production cost of SCWG at high operation pressure. The catalysts provide an opportunity to gasify biomass at mild reaction temperature. In all available SCWG catalysts, Ni-based catalysts attract more attention for its high activity and relatively low cost [14]. Elliott from Pacific Northwest Laboratory used several different forms of nickel as catalysts for p-cresol gasification in sub-critical water (623 K, 17–23 MPa), and CH4-rich

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gaseous products were obtained [15]. Minowa studied cellulose decomposition in hot-compressed water using a reduced nickel catalyst on different carrier materials [16]. DiLeo et al. used Ni wire as a catalyst in the SCWG of phenol and glycine, and found that the reaction rates are greatly increased by adding Ni wire [17]. The results from Azadia’s study showed that Raney-nickel was a more effective catalyst for glucose hydrothermal gasification compared to homogeneous Ni(acac)2, Co(acac)2, and Fe(acac)3 catalysts [18].Yoshida used a commercial nickel catalyst (Ni-5312P, Engealhard) in gasification of cellulose and lignin at 673 K, 25 MPa [19]. It was found that nickel catalysts could suppress tar productivity. Yan et al [8] investigated the performance of Ni/ZrO2, Co/ZrO2 and W/ZrO2 catalysts in SCWG (663 K, 24 MPa) of polyethylene glycol, and Ni/ZrO2 showed the highest catalytic activity. Taylor and coworkers reported that carbon nanotube/fiber and aluminosilicate supported Ni catalysts were active in the gasification of biomass in SCW [20]. Lee gasified glucose in supercritical water using Ni/activated charcoal. Ni/activated charcoal showed a good yield of hydrogen [21]. In our previous work, the alumina-supported nickel catalysts modified with Cu, Co and Sn were used for hydrogen production in SCWG of glucose [22]. The results showed that Cu could improve the catalytic activity of Ni in reforming methane to produce hydrogen and Co was found to be an excellent promoter of Ni-based catalyst in terms of hydrogen selectivity. In most cases, carbon deposition, which is one of the serious problems, reduces the activity of Ni-based SCWG catalysts. Adding promoter into the Ni-based catalysts is a commonly way to prevent carbon deposition. CeO2 is an effective promoter and/or supporter for Ni catalysts since it can enhance the activity, stability, and carbon resistance of Ni catalysts. Therdthianwong et al. reformed bioethanol in SCW (773 K and 25 MPa) with Ni/Al2O3 and Ni/CeZrO2/Al2O3 catalysts for high-pressure hydrogen production. They found that the CeZrO2-promoted catalyst had higher activity than the un-promoted catalyst because of higher metal dispersion and smaller active site particles of the promoted catalyst [9]. Hao et al. gasified cellulose in SCW with CeO2 particle, nano-CeO2, nano-(CeZr)xO2 as catalysts [23]. More H2 and CO2 but less CO was produced compared to that without any catalyst, and the reaction of water–gas shift was enhanced by these catalysts. In our previous work, Ni/CeO2/Al2O3 catalyst was synthesized successfully by wet impregnation method. Compared with Ni/Al2O3, the catalyst showed higher activity and carbon resistance for SCWG of glucose [14]. In this paper, the influences of Ce loading on the activity of Ni/CeO2/Al2O3 in SCWG of glucose were studied by gasification reaction tests and characterization of fresh and used catalysts. The effects of Ce loading on H2 yield, H2 selectivity and carbon deposition were studied. The role of Ce in the glucose gasification and its mechanism of carbon resistance were also discussed.

the solid residue was grounded into powders. Subsequently, it was calcinated into catalyst precursors in air with a 7 K min1 ramp from 300 K to 973 K, holding 5 h at 973 K, and then cooled naturally into ambient temperature. Before used, the precursors were reduced by H2 (50 ml/min) with a 9 K min1 ramp from 300 K to 923 K, holding at 923 K for 2.5 h. The catalysts are referred to here after as Ni/xCe–Al, where x refers to the theoretical mass percentage of CeO2 in the catalysts. In this study, x equals to 1.22, 3.66, 6.07, 8.46 and 10.83, respectively.

2.2. Characterization of catalysts X-ray diffraction (XRD) patterns of the synthesized catalysts were obtained from a PANalytical X’pert MPD Pro diffractometer using Ni-filtered Cu Ka irradiation (wavelength 1.5406 Å). N2 adsorption-desorption isotherms of catalysts were conducted at 77 K in the Beckman Coulter SA3100 plus instrument. Samples were degassed at 573 K for 60 min, prior to measurements. Surface area was determined using the Brunauer–Emmett–Teller (BET) methods. The sample morphology was observed (before and after the catalytic tests) by a JEOL JSM-6700F field emission scanning electron microscopy (SEM). Thermo-gravimetric analyses (TGA) were carried out using a DSC/DTA-TG (NETZSCH STA449C) with air carrier to determine the amount of carbon deposited on catalysts and the kinetics data of deposited carbon oxidant.

2.3. Evaluation of catalytic activity Catalytic activity test were carried out in a 140 mL, 316L stainless steal, high-pressure autoclave. Detailed description of the experimental apparatus can be referred to the Ref. [14]. Firstly, 11 g mixed solution of 9.09 wt.% glucose and 0.2 g catalyst was added into the reactor for these experiments. The reactor was heated from ambient temperature to reaction temperature with a heating rate of about 9 K min1. After gasification reaction, the reactor was cooled to below 473 K in 1 min and below 373 K in 2.5 min. The sampled gases were analyzed using a Hewlett-Packard model 6890 gas chromatograph (GC) with a thermal conductivity detector. A Carbon-2000 capillary column was used (£0.53 mm  25 m), operating at 333 K for 7 min. N2 was used as the carrier gas and its flow rate is 10 ml min1. A standard gas mixture with 6 kinds of contents used for calibration is bought from and compounded by Beijing AP Beifen Gases Industry Limited Company.

16 H2 CH4

2. Apparatus and experimental procedures 2.1. Prepararion of Ni/CeO2/Al2O3 Ni/CeO2/Al2O3 were prepared by co-impregnation of a commercial c-Al2O3 (surface area, 140 m2/g) with aqueous solutions of Ni(NO3)26H2O and Ce(NO3)36H2O. To begin with, a mixture of certain amount of nickel nitrate, cerium nitrate and c-Al2O3 was prepared. Different Ce loading was achieved by varying Ce(NO3)3 solution concentration. Ni loading in different catalysts was selected in order to achieve theoretical mass fraction of Ni to 20%. Secondly, the solution mixture was heated under stirring for about 3 h until the water evaporation was finished and then

Gas Yield (mol/kg)

12

CO2

8

4

0 No catalyst

1.22

3.66

6.07

8.46

10.83

x (%) Fig. 1. Effects of Ce loading on gas yield of glucose gasification in SCW.

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Y. Lu et al. / Fuel 103 (2013) 193–199 60

a *

#

x=1.22

*

+ CeO2

#

*

#

40

Intensity (A.U.)

H2 selectivity (%)

* γ−Al2O3

# NiO #

50

30

20

+

x=3.66

+

+

+

x=6.07

x=8.46

10

x=10.83 0 No Catalyst

1.22

3.66

6.07

8.46

10.83 10

x (%)

20

30

40

50

60

70

80

2θ/ (degree) Fig. 2. Effects of Ce loading on H2 selectivity of glucose gasification in SCW.

b

3. Results and discussion

3.2. XRD patterns of fresh and used catalysts with different Ce loading The XRD patterns of calcinated catalysts are shown in Fig. 3a. All calcinated catalysts exhibit the characteristic peaks of crystalline c-Al2O3 (at 2h angle of 37.5°, 46° and 66.7°), NiO (at 2h angle of 37°, 43°, 62.5° and 75.5°) and CeO2 (at 2h angle of 28.6°, 33.1°, 47.4° and 56.3°). No other impurity crystalline phases are detected. With the increase of Ce loading amount, the intensity of the peaks corresponding to CeO2 increases and these of Al2O3 and NiO have a tendency of decrease. Fig. 3b presents the XRD patterns of H2 reduced catalysts. The characteristic peaks of CeO2 and c-Al2O3 are also shown up in Fig. 3b. The diffraction peaks corresponding to Ni0 (at 2h angle of 44.6°, 51.9° and 76.6°) are detected instead of

Intensity (A.U.)

The catalytic activity of the synthesized Ni/Al2O3 catalysts with different Ce loading was tested at 673 ± 3.0 K and 24.5 ± 0.5 MPa with the reaction time of 20 min. We regard the gas yield and H2 selectivity [24] to be indicative of the SCWG efficiency. Fig. 1 displays the effects of Ce loading on gas yield of glucose gasification. It can be seen from Fig. 1 that with the catalysts added, the H2 yield of glucose gasification increases as 404.7–899.2% than that without catalyst. The coke and black tar-like substance were observed in the products of glucose gasification at 673 K although the catalyses were used. With the increasing Ce loading, the H2 yield increases firstly and but decreases afterwards. When x is equal to 8.46, the maximum H2 yield of 12.99 mol/kg is obtained with no coke and a small amount of tar in the reaction residues. The CH4 yield varies with different catalysts revealed the same trend as that of H2 yield, and it reaches to a maximum of 2.63 mol/kg when x equals 6.07. Fig. 2 displays the effects of Ce loading on H2 selectivity. As shown in Fig. 2, the H2 selectivity of glucose gasification increases sharply with the catalysts addition. Similarly, with increasing Ce loading, the H2 selectivity increases firstly and then decreases. When x equals 8.46, we obtain the maximum H2 selectivity of 56.18% which is about 2-fold larger than that without catalyst addition. The addition of CeO2 can inhibit carbon deposition and coking of the catalyst surface, which could improve the activity of the catalyst (This will be explained in detail below). However, the surface area of the catalysts decreases as the Ce loading increases (see Section 3.3), which could reduce the activity of the catalyst. As a result, the H2, CH4, CO2 yields and H2 selectivity increase and then decrease as Ce loading increases from 1.22 to 10.83, and there is an optimum Ce loading for hydrogen yield and selectivity.

*

γ−Al O

2 3

+ CeO2

Ni

*

x=3.66

x=6.07 x=8.46 + x=10.83

20

+

+

30

40

+

50

60

70

80

2θ/ (degree)

c

* x=1.22

Intensity (A.U.)

3.1. Effects of Ce loading on catalytic activity

*

x=1.22

*

γ−Al O

2 3

*

*

+ CeO2

Ni

*

x=3.66 x=6.07

x=8.46 x=10.83 +

20

30

40

50

60

70

80

2θ/ (degree) Fig. 3. The XRD patterns of Ni/CeO2/Al2O3 catalysts: (a) calcined catalysts (b) H2 reduced catalysts and (c) used catalysts.

NiO, which indicates that Ni2+ is reduced to Ni0. Similarly, the intensity of the peaks corresponding to Ni0 decreases as the Ce loading increases. Lower peak intensity means the better dispersion of Ni0 crystallite in CeO2/Al2O3 supports which favors hydrogen production. The peak positions of c-Al2O3,CeO2 (2h = 28.6°) and Ni0 (2h = 44.6°, 51.9° and 76.6°) as shown in Fig. 3c are also detected in all used samples, indicating that the crystal structures of used catalysts is not changed obviously. 3.3. Effects of Ce loading on N2 adsorption/desorption of reduced and used catalysts Fig. 4 shows the N2 adsorption and desorption isothermal curves of fresh catalysts. As shown in the figure, all catalyst give

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Y. Lu et al. / Fuel 103 (2013) 193–199 80

a

b

70

Adsorption Desorption

250

Volume (cc/g(STP))

Volume (cc/g(STP))

60 50 40 30 20

Adsorption Desorption

200 150 100 50

10 0 0.0

300

0 0.2

0.4

0.6

0.8

0.0

1.0

0.2

0.4

c

0.8

1.0

0.8

1.0

d 80

120

70

Adsorption Desorption

Volume (cc/g(STP))

100

Volume (cc/g(STP))

0.6

P/P0

P/P0

80 60 40 20

Adsorption Desorption

60 50 40 30 20 10

0 0.0

0.2

0.4

0.6

0.8

0 0.0

1.0

0.2

0.4

P/P0

e

0.6

P/P0

70

Volume (cc/g(STP))

60

Adsorption Desorption

50 40 30 20 10 0 0.0

0.2

0.4

0.6

0.8

1.0

P/P0 Fig. 4. The N2 adsorption and desorption isothermal curves of fresh catalysts: (a) x = 1.22; (b) x = 3.66; (c) x = 6.07; (d) x = 8.46; and (e) x = 10.83.

the carbon deposition on the catalyst surface and catalyst sintering [14].

Table 1 BET surface area of fresh and used catalysts. Ce loading (x, wt.%) 2

Surface area of fresh catalysts (m /g) Surface area of used catalysts (m2/g)

1.22

3.66

6.07

8.46

10.83

107.2 24.9

109.3 59.3

104.1 69.9

97.1 46.2

84.9 23.8

the typical type II adsorption/desorption isotherms which is characteristic for macroporous (d > 50 nm) and non-porous adsorbents. The BET specific surface areas of fresh and used Ni/Al2O3 catalysts with different loading amount of CeO2 are listed in Table 1. The surface areas of Ni/Al2O3 catalysts decreases with the increasing of CeO2 amount, which may due to the decrease of c-Al2O3 support amount or the blocking of porous structures by the bulk CeO2. Compared to that of the fresh catalysts, the surface area of used ones decreases to various degrees, which may due to

3.4. Crystallite morphologic of fresh and used catalysts Fig. 5 shows the typical SEM images of fresh and used Ni/CeO2/ Al2O3 catalysts. Nickel particles about 20–30 nm in diameter are highly dispersed on the reticular c-Al2O3 support in the fresh catalysts. Ni/8.46Ce–Al has smaller nickel particles than Ni/6.07Ce–Al, which increase active sites of nickel on the surface of the catalysts. Carbon deposition on the catalysts surface can be observed from the SEM images as shown in Fig. 5b and d. In addition, the used Ni/6.07Ce–Al agglomerated into bulks, indicating that the sintering of catalysts happens during the reaction. However, Ni/8.46Ce–Al shows no agglomeration, but a little amount of carbon deposited on Ni/8.46Ce–Al.

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Fig. 5. Typical SEM images of reduced and used catalysts: (a) x = 6.07, reduced catalyst; (b) x = 6.07, used catalyst; (c) x = 8.46, reduced catalyst; and (d) x = 8.46, used catalyst.

Table 2 Carbon deposition determined by TGA on used catalysts. Ce loading (x, wt.%)

1.22

3.66

6.07

8.46

10.83

Amount of deposited carbon (g C/g Catalyst)

0.655

0.503

0.433

0.375

0.447

3.5. TGA analysis of used catalysts In order to obtain the amount of deposited carbon on the used catalysts, thermo-gravimetric analyses (TGA) were carried out with air as the carrier gas. The results are shown in Table 2. The amount of deposited carbon on Ni/Al2O3 catalysts with various Ce loading decreases first while increased afterwards, and reaches the minimum with the CeO2 loading percentage of 8.46 wt.%. The trend of hydrogen yields of Ni/xCe–Al catalysts with different Ce addition is in opposite direction with that of deposited carbon on the used catalysts, which indicates that the amount of H2 yield is correlated with the amount of deposited carbon. In fact, the active sites on the catalyst surface decreases accordingly with the increasing amount of deposited carbon, which reduces the catalytic activity. As a result, the H2 yield and hydrogen selectivity decreases. Also, thermo-gravimetric analyzer is widely used to determine the oxidation kinetic data of deposited carbon on the used catalysts [25], which can be used to determine the type of carbon on the used catalysts. For the oxidation of deposited carbon on the used catalysts with air, the simplified gas-solid chemical kinetics is usually given by:

da ¼ kð1  aÞn dt

ð1Þ

where a is the dimensionless degree of reaction or conversion which is calculated as follow equation:

a¼

m0  m m0  m1

ð2Þ

where m0 is the starting mass, m is the mass of the sample at time and m1 is the mass of the sample at the end of reaction. In Eq. (1), n is the reaction order of carbon oxidant. It is assumed that the reaction order is one (n = 1). If the Arrhenius temperature dependence is assumed, the reaction rate constant k is given by

k ¼ k0 exp



EA RT

 ð3Þ

Where k0 is Arrhenius pre-exponential, EA is activation energy, R is gas constant and T is temperature. The heating rate U (dT/dt) is the constant of 10 K min1 during the carbon oxidation in this paper. Therefore, the Eq. (1) can be expressed as follows:

da k ¼ ð1  aÞ dT /

ð4Þ

Based on the Eq. (4) and the a–T function, the functions of ln k  1/T can be obtained, which can be used to calculate the kinetics dates including EA and k0. Table 3 displays the kinetic data for deposited carbon oxidation. The activation energy of the oxidation reaction of deposited carbon on all used catalysts is from 64.06 to 67.44 kJ/mol, which shows no distinguished difference. It suggests that the carbon on all of the catalysts is the same type (graphitic or amorphous). In fact, the XRD patterns of used catalysts did not show the diffraction peaks of graphite (for example, 2h angle of 26.47° which correspond to the characteristic peak of graphite (0 0 2) [26]). The typical value of EA recommended in literatures is 170 kJ/mol for graphite oxidant [27], which is much larger than that of deposited carbon oxidant in this work. Therefore, the deposited

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Table 3 Oxidation kinetic data of deposited carbon on the catalyst with air. Ce loading (x, wt.%)

1.22

3.66

6.07

8.46

10.83

EA (kJ/mol) k0 (s1)

64.06 1.58  102

67.43 5.32  102

66.19 3.41  102

64.17 3.44  102

67.44 5.70  102

carbon on the Ni/CeO2/Al2O3 catalysts must be the amorphous carbon, which is also confirmed in our previous wok by carbon oxidant with the peak occurring at different temperature [14]. Bolova et al. studied on the kinetics of the catalyzed oxidant of amorphous carbon (carbon black) in presence of Ce–Al oxides [28]. The results showed that the reaction is first order reaction and the activation energy is about 90 kJ/mol, which is still higher than that obtained in this work. The possible reason is that the Ni also catalyzed the carbon black oxidant and reduces the activation energy.

During the SCWG of glucose, the carbon could be formed by two pathways. One is by intermediate liquid products decomposition and the other is by product gas. Sinag et al. proposed the mechanism of solid carbon formation in SCWG of glucose [29]. Glucose is decomposed into furfurals and organic acids firstly. The furfurals can be decomposed in SCW with the formation of solid carbon, phenols and organic acids. The phenolic compounds can be converted further into solid carbon and organic acids. The product gas that consists of H2, CH4, CO, and CO2, is formed by organic acids decomposition. Ni catalysts can promote the decomposition of glucose into organic acid, but inhibit the formation of the phenolic compounds from the furfurals. Therefore, Ni catalysts addition will reduce the amount of solid carbon. The carbon formation could also occur due to the side reactions of product gas. Laosiripojana et al. proposed the most probable reactions that could lead to the carbon formation, in steam reforming of CH4 [30].The reactions are as follows:

CH4 $ 2H2 þ C

ð5Þ

2CO $ CO2 þ C

ð6Þ

CO þ H2 $ H2 O þ C

ð7Þ

CO2 þ 2H2 $ 2H2 O þ C

ð8Þ

In SCWG of glucose, the carbon is most likely formed by reaction 7, 8 but not by reaction 5, 6, because at low reaction temperature (673 K), Eqs. (7) and (8) are favorable, while Eqs. 5, 6 are thermodynamically unfavorable [30]. At the same time, the equilibrium of water-gas shift reaction moves forward and produces more CO2 rather than CO with the increase of water to glucose ratio. Therefore, high water feed in SCWG can also inhibit carbon deposition via Eq. (6). In comparison, at low temperature solid carbon is more likely formed by the first pathway rather than the second. High catalytic activity and resistance toward carbon formation of Ni/CeO2/Al2O3 could be mainly due to the redox property of ceria [30]. CeO2-based materials have high oxygen storage capacity and oxygen mobility. These characteristics are related to their rapid reduction/oxidation capability by releasing and uptaking oxygen owing to the reversible reaction of [31],

ð9Þ

where Ox is the lattice oxygen at CeO2 surface. Carbon monoxide can adsorb and react with the lattice oxygen on the surface of ceria to form carbon dioxide,

CO þ Ox ! CO2 þ Ox1

CðsÞ þ Ox ! Ox1 þ CO

ð10Þ

ð11Þ

Similarly, in presence of lattice oxygen, Ni oxidation may also take place according to the following reaction,

Ni þ Ox ! Ox1 þ NiO

3.6. Carbon deposition mechanism and the role of Ce

CeO2 $ CeO2x þ Ox

where Ox1 is the reduced site of ceria. More CO2 are produced rather than CO, which can inhibit carbon deposition via the Eq. (6) and (7). As a lattice oxygen provider, CeO2 may oxidize the solid carbon in the following reaction [31],

ð12Þ

It is unlikely that CeO2 oxidizes Ni to NiO, which has less catalyst activity than Ni species, whereas NiO possibly reacts with reduced site of Ce, Ox1, and is reduced to Ni, maintaining Ni activity for hydrogen production [31]. Also, H2O can react with the reduced site of ceria, Ox1. The steady state reforming rate is mainly due to the continuous supply of the oxygen source by H2O [30].

Ox1 þ H2 O ! H2 þ Ox

ð13Þ

Because Ce as promoter can help to remove the deposited carbon, and Ni catalysts have more active sites, which will inhibit the carbon formation further. At the same time, Ni interacts with CeO2 strongly over the catalysts prepared by the co-impregnation method and this interaction can lead to high catalytic performance [32], which also benefits the inhibition of carbon formation. 4. Conclusion Ni/CeO2/Al2O3 catalysts with different Ce loading were synthesized successfully by a co-impregnation method. With addition of the Ni/CeO2/Al2O3 catalysts, the H2 yield of glucose gasification in SCW increases by 404.7–899.2% than that without catalyst. Ce as a promoter can inhibit the solid carbon formation. When the Ce loading percentage is 8.46 wt.%, the catalyst behaves the highest catalytic activity and the amount of deposited carbon is the smallest. Activity energy of deposited carbon oxidant varies from 64.06 to 67.44 kJ/mol, which has no distinguished difference. The carbon on all of the used catalysts is the same type (amorphous). The carbon could be formed by intermediate liquid products and product gas. In comparison, solid carbon is more likely formed by liquid products rather than gas in low temperature SCWG of glucose. The resistance toward carbon formation of Ni/CeO2/Al2O3 could be mainly due to the lattice oxygen on the surface of ceria. Acknowledgements This work is currently supported by the National Key Project for Basic Research of China (973) through Contract Nos. 2009CB220000, 2012CB215303 and National Nature Science Foundation of China through Contract No. 50906069. References [1] Kruse A, Gawlik A. Biomass conversion in water at 330–410 °C and 30–50 MPa: identification of key compounds for indication different chemical reaction pathways. Ind Eng Chem Res 2003;42(2):267–79. [2] Antal JM, Allen SG, Schulman D, Xu X, Divilio RJ. Biomass gasification in supercritical water. Ind Eng Chem Res 2000;39(11):4044–53.

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