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Zr(Ce,Y)O2-δ catalysts
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Hydrogen production from glucose by supercritical water gasification with Ni/Zr(Ce,Y)O2-d catalysts Jianbing Huang a,*, Xiaoyan Lian a,b, Li Wang a, Chao Zhu a, Hui Jin a, Runyu Wang a a
State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi'an 710049, Shaanxi, China b College of Chemistry and Chemical Engineering, Baoji University of Arts and Science, Baoji 721013, Shaanxi, China
article info
abstract
Article history:
Catalytic supercritical water gasification (SCWG) of glucose over Ni/Zr(Ce,Y)O2-d catalysts
Received 17 June 2016
was conducted in a batch reactor at 500 C, 23e24 MPa with 10 wt.% feed concentration.
Received in revised form
The catalysts were prepared by carbonate co-precipitation method and the effects of
18 September 2016
nickel molar content and CeO2 loading on gasification performances were investigated.
Accepted 4 October 2016
The catalytic gasification experiments with Nix/Zr0.8Y0.2O2-d (x ¼ 0.1, 0.2, 0.3, 0.5, 0.7)
Available online xxx
(NZY) catalysts showed that carbon gasification efficiency increased gradually with the increase of nickel content, but hydrogen yield gradually increased then leveled off. The
Keywords:
maximum hydrogen yield of 22 mol kg1, about 10 times of that without catalyst, was
Hydrogen production
obtained with Ni0.5/Zr0.8Y0.2O2-d (NZY582) catalyst, and carbon gasification efficiency and
Supercritical water gasification
total gasification efficiency were 66% and 90%, respectively. In order to achieve higher
(SCWG)
gasification efficiency, the Zr(Y)O2-d support was modified by adding CeO2. The results
Glucose
showed that using Ni0.5/Zr0.4Ce0.4Y0.2O2-d (NZCY5442) catalyst, carbon gasification effi-
Catalysts
ciency reached 76%, about 15% higher than that of NZY582 catalyst, and methane yield increased by 1.9 times, while hydrogen yield decreased by 23%. Various characterization techniques were carried out on the reduced and used catalysts and it was found that the catalysts exhibited excellent hydrothermal stability and good anti-carbon deposition capability. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen is widely regarded as an ideal energy carrier since it is a high-quality, clean and efficient fuel for stationary and transportation applications by combustion in thermal engines or by electrochemical reaction in fuel cells, thus it has the potential to replace traditional fossil fuels in the future. However, current industrial-scale hydrogen
production relies on the fossil fuel feedstocks via the thermo-catalytic processes and gasification of methane, naptha, and coal [1], which ultimately increase CO2 concentration in the atmosphere. Hydrogen fuel derived from renewable biomass can realize carbon neutrality and nearzero pollution during utilization and it is crucial to the energy and environmental sustainability. Among various conversion processes from biomass to hydrogen, supercritical water gasification (SCWG) has
* Corresponding author. Fax: þ86 29 82669033. E-mail address: [email protected] (J. Huang). http://dx.doi.org/10.1016/j.ijhydene.2016.10.012 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Huang J, et al., Hydrogen production from glucose by supercritical water gasification with Ni/Zr(Ce,Y) O2-d catalysts, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.012
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attracted increasing attention in recent years [2]. Supercritical water (SCW, T > 374 C, P > 22.1 MPa) has properties entirely different from those of liquid water or steam, making it interesting as both solvent and reactant. As a homogenous phase with low viscosity and high diffusivity, mass transfer limitations can be overcome in SCW. Due to its greatly reduced dielectric constant, SCW behaves as an organic solvent, easily dissolving many organic species and gases while precipitating polar salts. SCW also participates in the steam reforming and water-gas shift reactions and contributes up to 50% of the H2 in biomass gasification process [3]. Compared with the traditional gasification technologies, SCWG has higher reaction efficiency and H2 selectivity, avoiding the drying of biomass and forming of char and tar. Therefore, SCWG is a very promising hydrogen production process with biomass resources. Much research has been reported that adopting appropriate catalysts in biomass SCWG process can significantly improve gasification efficiency and hydrogen selectivity at mild reaction temperature. Various kinds of homogeneous and heterogeneous catalysts have been investigated for SCWG of real biomass and model compounds [4]. Alkali such as NaOH, KOH, K2CO3, Na2CO3, etc. is a homogeneous catalyst which can be mixed with the feedstock uniformly and promote water-gas shift reaction thus to increase the hydrogen yield [5e11]. But the recovery of alkali from wastewater requires high consumption of energy. Compared with alkali catalysts, supported transition metals, a major category of heterogeneous catalysts, exhibit higher selectivity and are easier to be recycled. Ni and Ru perform quite well in SCWG processes but Ni catalyst is more frequently studied due to its low cost and efficient catalytic activity. However, both Ni catalyst and its support are unstable, sintering and deactivation occur in SCWG process in both batch and continuous-flow reactors. At present, most studies focus on the screening and designing of supported Ni catalysts with higher catalytic activities, better hydrothermal stabilities and anti-carbon deposition capabilities. Glucose, a typical model compound of biomass, is commonly used as the feedstock to evaluate the catalytic SCWG process. Ding et al. [11] studied the supercritical water catalytic gasification of glucose under the conditions of 12.5 wt.% feed concentration, 500 C, 30 min. Ni/activated carbon (AC) catalyst exhibited the best activity with the hydrogen yield increasing by 81%; Ni/MgO increasing by 62%; Ni/CeO2/Al2O3 increasing by 60%; Ni/Al2O3 increasing by 52%, compared with that without catalysts. Zhang et al. [12] screened several heterogeneous catalysts for hydrogen production from glucose via catalytic SCWG at 600 C and 24 MPa, and found that Ni supported on g-Al2O3 exhibited very high activity and H2 selectivity. As for the effects of catalyst support materials on activity, the following order of sequence was observed: g-Al2O3 > ZrO2 > AC. NieMgeAl catalysts with different Mg/Al molar ratios were synthesized by a co-precipitation method for hydrogen production by SCWG of glucose [13]. NiMg0.6Al1.9 showed the highest catalytic activity and the best hydrothermal stability probably due to the formation of MgAl2O4. It also found that Mg could efficiently improve the anti-carbon ability of NieAl catalyst
but MgO showed poor hydrothermal stability. A highly dispersed rutile-TiO2-supported Ni nanoparticle was synthesized by a solegel method and its catalytic performance for SCWG of glucose was studied [14]. It exhibited high activity and potential stability and anti-carbon ability for longterm SCWG. Lu et al. [15] compared the catalytic performances of Ni-based catalysts with different oxide supports for SCWG of glucose at 400 C and 23.5 MPa. H2 yield for different supports decreased in order: CeO2/Al2O3 > La2O3/ Al2O3 > MgO/Al2O3 > Al2O3 > ZrO2/Al2O3, and H2 selectivity decreased in order: CeO2/Al2O3 > La2O3/Al2O3 > ZrO2/ Al2O3 > Al2O3 > MgO/Al2O3. The catalyst deactivation was mainly due to sintering and carbon deposition and CeO2 could remove the surface carbon deposition. Lu et al. [16] further investigated the effect of Ce loading in Ni/CeO2/ Al2O3 catalysts on glucose SCWG, and the optimum Ce loading content of 8.46 wt.% was obtained. CeO2 plays an important role in inhibiting carbon deposition by increasing the Ni dispersion and reacting with deposited carbon. Azadi et al. [17] tested 44 supporting materials for Ni for SCWG of glucose under the conditions of 380 C, 23 MPa, 2 wt.% feed concentration. a-Al2O3, carbon nanotubes (CNTs), and MgO supports resulted in high carbon conversions, while SiO2, Y2O3, hydrotalcite, Y2O3-stabilized ZrO2 (YSZ), and TiO2 showed modest activities. Other supports such as zeolites, molecular sieves, CeO2 and ZrO2 showed no obvious activity under the testing conditions. Aside from the catalytic activity, the stable metal oxide supports were a-Al2O3, boehmite, YSZ, and TiO2. It indicated that the catalytic activity of Ni/ZrO2 catalyst can be improved by introducing Y2O3 into ZrO2 and forming a stable Y2O3eZrO2 solid solution. In the case of catalytic SCWG of real biomass or other model compounds, the screening of appropriate supports for Ni catalyst has also been carried out. Byrd et al. [18] studied hydrogen production from catalytic SCWG of the switchgrass biological diesel oil at 600 C and 250 bar. The supported Ni, Co and Ru catalysts with the supports of TiO2, ZrO2 and MgAl2O4 were prepared by impregnation method. It was found that the highest hydrogen yield of 0.98 mol H2$mol1 C was obtained over Ni/ZrO2 catalyst. Liu et al. [19] modified Ni/ZrO2 catalyst by MgO for glycerol reforming in supercritical water, and found that the MgO in the catalyst showed significant promotion effect for hydrogen production likely due to the formation of the alkaline active site. Therdthianwong et al. [20] investigated the hydrogen production from supercritical water reforming of bioethanol over Ni/Al2O3 and Ni/CeZrO2/ Al2O3 catalysts (500 C, 25 MPa). The results showed that hydrogen production increased 3e4 times of that without catalysts. After CeZrO2 modification, the catalytic activity increased significantly, reaching complete gasification with no tar formation. CeZrO2 could not only promote the watergas shift reaction and methanation reaction to obtain lower production of carbon monoxide and higher hydrogen yield, but also improve the hydrothermal stability of Al2O3. Although ZrO2 is considered as a stable support alternative to Al2O3 for Ni catalyst in SCWG of biomass, the catalytic activity of Ni/ZrO2 catalyst needs further improvement. From the aspect of support modification, doping ZrO2 with low valent metal oxide such as MgO,
Please cite this article in press as: Huang J, et al., Hydrogen production from glucose by supercritical water gasification with Ni/Zr(Ce,Y) O2-d catalysts, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.012
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Y2O3, etc. is an efficient way to improve the catalytic activity and maintain hydrothermal stability. Y2O3 stabilized ZrO2 (YSZ) or Zr(Y)O2-d is the most widely studied electrolyte material for solid oxide fuel cells. However, YSZ or Zr(Y)O2-d as catalyst for SCWG of biomass has not yet been intensively investigated. As mentioned above, CeO2 is a good promoter for Ni catalyst supported on both Al2O3 and ZrO2 for catalytic SCWG of biomass. Moreover, ZrO2eCeO2 solid solution is also a good promoter for Ni catalyst supported on Al2O3. Thus, it is expected that Ni/Zr(Ce,Y)O2d catalysts can balance the requirements of catalytic activity, hydrothermal stability and anti-carbon deposition capability. In this study, glucose was chosen as the representative model compound of real biomass and was catalytically gasified under SCW condition. Nix/Zr(Ce,Y)O2-d catalysts were prepared by carbonate co-precipitation method. The effect of nickel molar content and CeO2 loading on carbon gasification efficiency and hydrogen yield was investigated. In addition, XRD, physical and chemical adsorption-desorption, TG, SEM and XPS were performed to characterize the as-reduced and used catalysts to investigate the catalytic mechanism of the catalysts.
Experimental Catalyst preparation A series of Nix/Zr(Ce,Y)O2-d catalysts were prepared using carbonate co-precipitation method. All the chemical reagents (Ni(NO3)2$6H2O, Zr(NO3)4$4H2O, Ce(NO3)3$6H2O, Y(NO3)3$6H2O, Na2CO3, C6H12O6) were purchased from Sinopharm Chemical Reagent Co., Ltd. For each sample, a certain amount of 0.2 mol L1 Na2CO3 solution was added as the precipitant drop by drop through a constant flow pump into 0.15 mol L1 cations (Ni2þ, Zr4þ, Ce3þ, Y3þ) solution stirred at 40 C moderately with the final pH about 10, ensuring that the mole of CO2 3 was twice of that of the total cation. The obtained sediment was aged for 5 h, filtered by vacuum suction and washed by distilled water and ethanol for three times, respectively, and then dried in the vacuum at 105 C overnight. Next, the precipitates were grinded and sieved to more than 200 meshes. The ultrafine powders were calcined at 500 C for 180 min and then reduced by H2 with a flow rate of 50 mL min1 at 650 C for 150 min. Finally, the reduced catalysts were sealed to save for use.
The composition and abbreviation of the reduced Nix/ Zr(Ce,Y)O2-d catalysts were displayed in Table 1.
Catalyst characterization The crystalline phases of the calcined, reduced and used catalysts were identified by X-ray diffraction (XRD) using Nifiltered Cu Ka irradiation (wavelength 0.15406 nm) with the Bragg angles between 10 and 90. The analysis of N2 adsorption/desorption isotherms, specific surface area and pore size distributions of the catalysts was performed by the ASAP 2020 instrument. The samples were degassed at 300 C for 120 min before the testing. Surface area and pore size distributions of the catalysts were measured by the Brunauere-EmmetteTeller (BET) and Barrette-Joynere-Halenda (BJH) methods, respectively. The surface morphology of the catalysts was observed by the JEOL JSM-6700F field emission scanning electron microscopy (SEM). The catalysts were dispersed with ethanol and then dropped on silicon wafers before the scanning. All the used catalysts were analyzed by simultaneous DSC-TG instruments in air atmosphere with a heating rate of 10 C$min1 from 20 C to 1000 C. The DTG line was obtained by the analysis software. The surface chemical environment of some catalysts was detected by the X-ray photoelectron spectroscopy (XPS) instrument from Axis Ultra, Kratos (UK). Binding energy of each element was calibrated based on the C1s peak (284.8 eV) of the hydrocarbons adsorbed on the surface of the catalysts.
Catalytic testing The gasification of glucose was conducted through a highthroughput batch reactor made of Inconel 625 with the designed reaction condition of 750 C, 30 MPa. The schematic diagram of the reaction system is shown in Fig. 1 with the detailed introduction given in literature [21]. For each experiment, 1.5 g glucose solution of 10 wt.% was added into the reactor without or with 0.15 g catalyst and the residence time was 30 min at testing conditions. The testing conditions were 500 C and 23e24 MPa. The gaseous products collected by a gas bag were then analyzed by the gas chromatography (Agilent 7890A) equipped with a thermal conductivity detector (TCD). Argon was used as the carrier gas and a standard gas with certain concentrations of H2, CO, CH4 and CO2 was used for calibration. The catalytic gasification performance was evaluated by carbon gasification efficiency (CGE), total gasification
Table 1 e The composition and abbreviation of the reduced Nix/Zr(Ce,Y)O2-d catalysts. Catalyst composition Ni0.1/Zr0.8Y0.2O2-d Ni0.2/Zr0.8Y0.2O2-d Ni0.3/Zr0.8Y0.2O2-d Ni0.5/Zr0.8Y0.2O2-d Ni0.7/Zr0.8Y0.2O2-d Ni0.5/Zr0.6Ce0.2Y0.2O2-d Ni0.5/Zr0.5Ce0.3Y0.2O2-d Ni0.5/Zr0.4Ce0.4Y0.2O2-d
Nickel content/mol.%
Nickel content/wt.%
9.09 16.67 23.08 33.33 41.18 33.33 33.33 33.33
4.62 8.83 12.69 19.50 25.32 18.31 17.77 17.26
Zr/Ce
Abbreviation
6/2 5/3 4/4
NZY182 NZY282 NZY382 NZY582 NZY782 NZCY5622 NZCY5532 NZCY5442
Please cite this article in press as: Huang J, et al., Hydrogen production from glucose by supercritical water gasification with Ni/Zr(Ce,Y) O2-d catalysts, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.012
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Fig. 1 e The schematic diagram of the batch reactor system.
Catalytic activity of Nix/Zr(Ce,Y)O2-d catalysts
glucose with catalysts had a significant increase comparing to that of without any catalyst. CGE and TGE gradually increased with the increase of nickel molar content. However, H2 selectivity did not increase monotonically with increasing nickel content, and the highest H2 selectivity of 50% was obtained when the nickel content of the catalyst was 33.33 mol.%, which is 4-fold larger than that without catalyst addition. In this case, CGE and TGE was 66% and 90%, respectively. The distribution of gaseous products for the supercritical water gasification of glucose over NZY catalysts is presented in Fig. 2b and c. As shown in Fig. 2b and c, the main gaseous products of SCWG of glucose were H2, CO, CH4 and CO2, of which H2 and CO2 occupied the highest proportion, CO occupied the lowest proportion, indicating that nickel could promote the methanation reaction (CO þ 3H2 / CH4 þ H2O) and water-gas shift reaction (CO þ H2O / H2 þ CO2) of CO. Fig. 2b shows that, with the increase of nickel molar content of the catalysts, hydrogen yield gradually increased then leveled off. The maximum hydrogen yield of 22 mol kg1, about 10 times of that without catalyst, was obtained when the nickel content was 33.33 mol.%. Increasing the Ni content could increase the active sites of the catalyst, however, a high Ni content could also lead to reduced dispersion of the Ni in the catalyst, and decreased surface area and porosity of the catalyst, which would consequently reduce the catalytic activity [12]. According to Fig. 2, we could conclude that the best gasification performance of SCWG of glucose was achieved by the NYZ catalyst with the nickel content of 33.33 mol.%.
Effect of nickel molar content on catalytic activity
Effect of CeO2 loading on catalytic activity
Fig. 2 show the gasification results of SCWG of glucose without or with Nix/Zr0.8Y0.2O2-d (x ¼ 0.1, 0.2, 0.3, 0.5, 0.7) catalysts. As we can see from Fig. 2a, CGE and TGE of
In order to improve the gasification efficiency of SCWG of glucose, the Zr(Y)O2-d support was modified by adding CeO2 to partially replace ZrO2 and Ni0.5/Zr0.8-xCexY0.2O2-d (x ¼ 0.2, 0.3,
efficiency (TGE), H2 selectivity [20], gas yields and gas molar fraction defined as follows, respectively.
CGE (%) ¼ total carbon in the gaseous products/total carbon in the feedstock (dry basis) 100% (1)
TGE (%) ¼ total mass of gaseous products/total mass of the feedstock (dry basis) 100% (2)
H2 selectivity (%) ¼ H2 molecules in the gaseous products/total carbon atoms in the gaseous products (1/RR) 100% (3) Here, RR is the reforming ratio of H2/CO2 depending on the reactant compounds. The RR of glucose is 2 [20].
Gas yield (mol/kg) ¼ mole number of certain gaseous product/ total mass of the feedstock (dry basis) (4)
Gas molar fraction (%) ¼ mole number of a certain gaseous product/total mole numbers of the gaseous products (5)
Results and discussion
Please cite this article in press as: Huang J, et al., Hydrogen production from glucose by supercritical water gasification with Ni/Zr(Ce,Y) O2-d catalysts, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.012
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Fig. 2 e Gasification results over NZY catalysts for SCWG of glucose.
0.4) catalysts were prepared. The gasification results of SCWG of glucose after using Ni0.5/Zr0.8-xCexY0.2O2-d (x ¼ 0.2, 0.3, 0.4) catalysts are showed in Fig. 3. We can see that both CGE and TGE were improved by adding CeO2 into the Zr(Y)O2-d support, and they increased slightly with increasing CeO2 loading. The best gasification results were obtained over NZCY5442 catalyst, with CGE and TGE of 76% and 95%, respectively. However, H2 selectivity and H2 molar fraction over NZCY catalysts decreased comparing with that of NZY582 catalyst. Methane yield increased gradually from 3.3 mol kg1 with NZY582 catalyst to 6.2 mol kg1 with NZY5442 catalyst, but hydrogen yield decreased from 22 mol kg1 with NZY582 catalyst to 17 mol kg1 with NZY5442 catalyst, indicating that CeO2 could further promote methanation reaction [22]. It should be noted that H2 selectivity, H2 yield and H2 molar fraction for SCWG of glucose over NZCY catalysts were almost independent of CeO2 loading, due to the fact that the partial substitution of Zr4þ with Ce4þ in the lattice only increases the lattice constant and does not increase the concentration of oxygen vacancy significantly.
to the crystalline planes (1, 1, 1), (2, 0, 0) and (2, 2, 0) of the reduced and used catalysts, illustrating that NiO was well reduced to Ni0 after H2 reduction and further enhanced the reduced state after SCWG of glucose reaction due to its rich hydrogen environment. Moreover, there was no obvious carbon peaks in both figures indicating that no carbon or amorphous carbon was deposited during SCWG of glucose. It was further confirmed by thermogravimetric analysis that small amount of amorphous carbon was formed on the surface of the used catalysts. Besides the diffraction peaks of cubic ZrYO (No. 01-089-6687), the reduced NZY582 catalyst showed the characteristic peak of orthorhombic ZrO2 (No. 00-037-1413) at the 2q angles of 38.8 . In addition, the diffraction peaks of the support in the calcined, reduced and used NZCY5442 catalyst split into two sub-peaks of ZrCeO (No. 00-038-1436) and CeYO (No. 01-075-0176) when the molar fraction of CeO2 in the support increased to 40 mol.%, revealing that the solid solubility of CeO2 in the Zr(Y)O2-d support was not more than 40 mol.%.
Catalyst textures and structures
Pore structures and BET surface area analysis
Crystalline phase analysis XRD spectra of the calcined, reduced and used catalysts were presented in Fig. 4. It can be seen from that the diffraction peaks of NiO appeared at the 2q angles of about 37.2 , 43.3 and 62.8 of the calcined catalysts, and the diffraction peaks of Ni0 (2q ¼ 44.5 , 51.8 and 76.4 ) related
Fig. 5 presents the N2 adsorption/desorption isotherms and pore size distributions of the reduced and used catalysts. It can be seen from Fig. 5 that the N2 adsorption/desorption isotherms of the catalysts whether reduced or used were similar to Type IV with the hysteresis loop of Type H1 [23]. The N2 adsorption/desorption isotherms of Type IV with the hysteresis loop of Type H1 could probably be some
Please cite this article in press as: Huang J, et al., Hydrogen production from glucose by supercritical water gasification with Ni/Zr(Ce,Y) O2-d catalysts, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.012
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Fig. 3 e Gasification results over NZCY catalysts for SCWG of glucose.
Fig. 4 e XRD spectra of the calcined, reduced and used catalysts. Please cite this article in press as: Huang J, et al., Hydrogen production from glucose by supercritical water gasification with Ni/Zr(Ce,Y) O2-d catalysts, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.012
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Fig. 5 e N2 adsorption/desorption isotherms and pore size distributions of the reduced and used catalysts. Please cite this article in press as: Huang J, et al., Hydrogen production from glucose by supercritical water gasification with Ni/Zr(Ce,Y) O2-d catalysts, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.012
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Fig. 6 shows the morphology analysis of the catalysts by SEM. It can be seen from Fig. 6 that both the NZY and NZCY catalyst particles were regular spherical, but NZCY catalysts showed inconspicuous surface morphology variation after used compared with NZY catalysts, indicating that the NZCY catalysts had better hydrothermal stability than the NZY catalysts. The aggregated particles in the reduced NZY and NZCY catalysts became a little loose after used in accordance with the BET surface area results, and the reduced NZY catalyst particles became much smaller after used compared with the reduced NZCY catalysts as shown in Fig. 6. Nickel particles about 40e60 nm in diameter were highly dispersed in the reduced catalysts. Especially, the
TG analysis The simultaneous thermogravimetric method and differential scanning calorimeter method were used to further confirm the presence of the formation of carbon deposition. During the simultaneous TG-DSC testing in air atmosphere, the used catalysts may have these reactions as follows [24]: 4Ni þ 3O2 /2Ni2 O3
(6)
2Ni2 O3 /4NiO þ O2
(7)
2Ni þ O2 /2NiO
(8)
C þ O2 /CO2
(9)
1 1 =dCeO2 / =dCeO2d þ 2O2 ½24
(10)
1 1 =dZrO2 / =dZrO2d þ 2O2 ½24
(11)
1
1
=
Morphology analysis
NZCY catalysts maintained high porosity before and after use.
=
mesoporous materials with relatively narrow pore size distributions or some aggregates of spherical particles with relatively uniform geometry sizes, demonstrated by the pore size distribution graphs embedded in Fig. 5Ae(E) and the SEM images displayed below. The pore sizes of the NZY catalysts distributed between 20 and 50 nm. The pore sizes gradually increased then decreased with the increasing nickel content of the catalysts, and the pore sizes of NZY582 catalyst were the smallest. The main pore sizes of the catalysts changed little but its distribution became a bit widen maybe due to the growing up of several particles, revealing that the catalysts had a good hydrothermal stability. According to Fig. 5(D1)-(D3), the pore sizes of the NZCY catalysts distributed between 40 and 90 nm, which were about twice as those of the NZY catalysts. The BJH distribution shows that pore sizes of the catalysts gradually broadened with the increase of CeO2 molar fraction in the support, and the average pore size was increased as well, which can be attributed to the structural changes caused by the partial reduction of Ce4þ to Ce3þ. Table 2 shows the BET specific surface area of the catalysts. It can be seen from Table 2 that the BET surface area of most reduced catalysts increased after used, because the fairly weak hydrogen bonds will be broken when water reaches its supercritical state, thus weakening the agglomeration of catalyst particles and resulting in the increased BET surface area. With the increase of Ni molar content, the specific surface area of reduced NZY catalysts tended to increase first, then decrease, and then increase again. The reduced NZY282 catalyst had the maximum surface area of 24.87 m2 g1 and the reduced NZY582 catalyst had the minimum surface area of 10.31 m2 g1, as evidenced from the following morphology analysis. As for the reduced NZCY catalysts, the specific surface area decreased and then increased with increasing of CeO2 loading, which is also consistent with the morphology analysis.
The TG-DTG curves of the used catalysts are showed in Fig. 7 and the main information given from Fig. 7 are listed in Table 3. From Fig. 7 and Table 3 we can find that for the reduced catalysts, a significant weight increase appears between about 350 C and 400 C but then a slightly decrease appears at above 500 C, this may because Ni0 was oxidized to Ni2O3 (equation (6)), but Ni2O3 could not exist in the high temperature environment, then decomposed to NiO (equation (7)). Furthermore, the weight increase ratios increased with the increasing nickel content. The decreased weight increase ratios at 350e400 C for the used catalysts may be caused by the absorption loss of the nickel particles. In addition, it can be seen from Fig. 6 that the weight change trends of the used catalysts at above 500 C were different. For the used catalysts, the nickel particles of improved activity due to the higher dispersion degree were directly oxidized to the relatively stable NiO. The low active deposited carbon was oxidized, and with the oxidation of the deposited carbon, the gradually exposed nickel particles covered by the deposited carbon were oxidized immediately. We denoted the weight loss value resulted from carbon oxidation and CeO2 deoxygenation by Dm2 and the weight increase value resulted from the oxidation of the exposed nickel particles by Dm1. For the used NZY and NZCY5532 catalysts, Dm1 ¼ Dm2, the overall quality unchanged; for the used NZCY5622 catalyst, Dm1>Dm2, the overall quality increased; for the used NZCY5442 catalyst, Dm1
Table 2 e The BET surface area of the catalysts (m2·g¡1). Catalysts Reduced Used
NZY182
NZY282
NZY382
NZY582
NZY782
NZCY5622
NZCY5532
NZCY5442
19.38 35.12
24.87 33.07
24.29 23.00
10.31 23.06
14.35 23.64
18.67 27.33
13.95 18.56
15.68 21.81
Please cite this article in press as: Huang J, et al., Hydrogen production from glucose by supercritical water gasification with Ni/Zr(Ce,Y) O2-d catalysts, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.012
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Fig. 6 e SEM morphology analysis of the reduced and used catalysts: (A)e(E) reduced NZY182, NZY282, NZY382, NZY582 and NZY782; (a)e(e) used NZY182, NZY282, NZY382, NZY582 and NZY782; (D1)e(D3) reduced NZCY5622, NZCY5532, NZY5442; (d1)e(d3) used NZCY5622, NZCY5532, NZY5442.
XPS analysis Figs. 8 and 9 show the XPS patterns of Ni 2p and Ce 3d in order to analyze the surface chemistry environment of the reduced and used catalysts. Fig. 9 reveals that nickel is essentially in its reduction state of Ni0. The main peaks of Ni 2p3/2, Ni0 and Ni 2p1/2,Ni0 are observed at about 852.2e852.8 eV and 869.6e870.6 eV, respectively, with shake-up satellite peaks of Ni 2p3/2, Ni0 at about 858e959 eV (XPS database from LaSurface.com, NIST XPS database). It is clearly that the shake-up satellite peaks of Ni 2p1/2, Ni2þ appear at about 879e880 eV caused by the addition of CeO2. The peak intensity increased significantly when the nickel
content is lower, such as NZY182, NZY282 and NZY382 catalysts, because part of NiO doped into the gap positions of ZrYO crystal lattice was unable to separate out in atmospheric pressure, but it could spread from the gap positions to the surface along with the migration of oxygen vacancy in the high temperature and pressure environment. The NiO separated out then was reduced by hydrogen, leading to the increased surface nickel concentration and peak intensity. Furthermore, Ni 2p peaks of the used NZY catalysts shifted to the right comparing to the reduced catalysts, which may because the dispersion degree of nickel particles was improved due to the decreased agglomeration degree of the
Please cite this article in press as: Huang J, et al., Hydrogen production from glucose by supercritical water gasification with Ni/Zr(Ce,Y) O2-d catalysts, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.012
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Fig. 7 e TG-DTG analysis of the reduced and used catalysts.
Table 3a e Mass change for the reduced and used NZY catalysts. Catalysts
NZY182
NZY282
A Mass/% Temp/ C
0.87 375
a 0.31 744
1.77 388
B 1.71 354
NZY382 b
0.49 726
0.79 322 406
NZY582
C
c 0.66 737
2.72 354
1.82 383
NZY782
D 3.88 383
d 0.34 e
E
3.62 400
5.80 378
e 0.12 e
5.20 385
Table 3b e Mass change for the reduced and used catalysts. Catalysts
NZCY5622 d1
D1 Mass/% Temp/ C
3.39 345
NZCY5532
0.28 740
3.24 352
D2 3.73 342
used catalysts particles. Then the contacting area and acting force between Ni0 and ZrYO substrate increased, thus the binding energy increased and Ni 2p shifted to the right. But for the used NZCY catalysts, Ni 2p peaks shifted to the left
NZCY5442 d2
0.48 743
3.50 359
D3 2.70 334
d3 1.77 745
2.45 357
caused by the weakened acting force between Ni0 and ZrYO (ZrCeO and CeYO) substrates. It may because the binding force between Ce4þ (Ce3þ) and Zr4þ in the ceria-zirconia solid solution was the biggest. It can be seen from Fig. 9
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Fig. 8 e The Ni 2p XPS spectrums of the reduced and used catalysts. Please cite this article in press as: Huang J, et al., Hydrogen production from glucose by supercritical water gasification with Ni/Zr(Ce,Y) O2-d catalysts, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.012
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Fig. 9 e The Ce 3d XPS spectrums of the reduced and used catalysts.
that cerium was mainly in the state of Ce4þ but Ce3þ appeared after used, because part of Ce4þ was reduced to Ce3þ during the SCWG of glucose.
Conclusions The Nix/Zr(Ce,Y)O2-d were prepared by carbonate coprecipitation method and used in the hydrogen production from SCWG of glucose. CeO2 was added to improve the gasification efficiency. 1. The highest hydrogen yield of 22 mol kg1 and the maximum H2 selectivity of 50% were achieved by NZY582 catalyst, with CE of 66%. The optimum nickel content was 33.33 mol.%. 2. The highest CE of 76% was obtained by NZCY5442 catalyst, with methane yield increased by 1.9 times, but hydrogen yield decreased by 23%, because methanation reaction is a process of consuming hydrogen. 3. The SEM analysis showed the excellent hydrothermal stability of the catalysts and the TG analysis and the XRD spectra revealed the good anti-carbon deposition capability of the most catalysts. A relatively higher content of nickel could inhibit the carbon deposition and it was further enhanced by the synergism of CeO2.
Acknowledgment This work is currently supported by the National Key Project for Basic Research of China (973) through Contract No. 2012CB215303.
references
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[6] Yang F, Hanna MA, Marx DB, Sun R. Optimization of hydrogen production from supercritical water gasification of crude glycerol-byproduct of biodiesel production. Int J Energy Res 2013;37(13):1600e9. [7] Cao C, Guo L, Jin H, Guo S, Lu Y, Zhang X. The influence of alkali precipitation on supercritical water gasification of glucose and the alkali recovery in fluidized-bed reactor. Int J Hydrogen Energy 2013;38(30):13293e9. lu TG, Kurt S, Sag lam M, Yu¨ksel M, Go € kkaya D, [8] Madenog Ballice L. Hydrogen production from some agricultural residues by catalytic subcritical and supercritical water gasification. J Supercrit Fluids 2012;67:22e8. [9] Onwudili JA, Lea-Langton AR, Ross AB, Williams PT. Catalytic hydrothermal gasification of algae for hydrogen production: composition of reaction products and potential for nutrient recycling. Bioresour Technol 2013;127:72e80. [10] Ding N, Azargohar R, Dalai AK, Kozinski JA. Catalytic gasification of cellulose and pinewood to H2 in supercritical water. Fuel 2014;118:416e25. [11] Ding N, Azargohar R, Dalai AK, Kozinski JA. Catalytic gasification of glucose to H2 in supercritical water. Fuel Process Technol 2014;127:33e40. [12] Zhang L, Champagne P, Xu C. Screening of supported transition metal catalysts for hydrogen production from glucose via catalytic supercritical water gasification. Int J Hydrogen Energy 2011;36(16):9591e601. [13] Li S, Guo L, Zhu C, Lu Y. Co-precipitated NieMgeAl catalysts for hydrogen production by supercritical water gasification of glucose. Int J Hydrogen Energy 2013;38(23):9688e700. [14] Li S, Zhu C, Guo S, Guo L. A dispersed rutile-TiO2-supported Ni nanoparticle for enhanced gas production from catalytic hydrothermal gasification of glucose. RSC Adv 2015;5:81905e14. [15] Lu Y, Zhu Y, Li S, Zhang X, Guo L. Behavior of nickel catalysts in supercritical water gasification of glucose: influence of support. Biomass Bioenergy 2014;67:125e36.
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[16] Lu Y, Li S, Guo L. Hydrogen production by supercritical water gasification of glucose with Ni/CeO2/Al2O3: effect of Ce loading. Fuel 2013;103:193e9. [17] Azadi P, Afif E, Azadi F, Farnood R. Screening of nickel catalysts for selective hydrogen production using supercritical water gasification of glucose. Green Chem 2012;14:1766e77. [18] Byrd AJ, Kumar S, Kong L, Ramsurn H, Gupta RB. Hydrogen production from catalytic gasification of switchgrass biocrude in supercritical water. Int J Hydrogen Energy 2011;36(5):3426e33. [19] Liu Q, Liao L, Liu Z, Dong X. Hydrogen production by glycerol reforming in supercritical water over Ni/MgO-ZrO2 catalyst. J Energy Chem 2013;22(4):665e70. [20] Therdthianwong S, Srisiriwat N, Therdthianwong A, Croiset E. Reforming of bioethanol over Ni/Al2O3 and Ni/CeZrO2/Al2O3 catalysts in supercritical water for hydrogen production. Int J Hydrogen Energy 2011;36(4):2877e86. [21] Lan R, Jin H, Guo L, Ge Z, Guo S, Zhang X. Hydrogen production by catalytic gasification of coal in supercritical water. Energy Fuels 2014;28(11):6911e7. [22] Ding M, Tu J, Zhang Q, Wang M, Tsubaki N, Wang T, et al. Enhancement of methanation of bio-syngas over CeO2modified Ni/Al2O3 catalysts. Biomass Bioenergy 2016;85:12e7. [23] Sing KSW, Everett DH, Haul R, Moscou L, Pierotti RA, Rouquerol J, et al. Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity. Pure Appl Chem 1985;57(4):603e19. [24] Chueh WC, Falter C, Abbott M, Scipio D, Furler P, Haile SM, et al. High-flux solar-driven thermochemical dissociation of CO2 and H2O using nonstoichiometric ceria. Science 2010;330(6012):1797e801.
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Hydrogen production from glucose by supercritical water gasification with Ni/Zr(Ce,Y)O2-d catalysts Jianbing Huang a,*, Xiaoyan Lian a,b, Li Wang a, Chao Zhu a, Hui Jin a, Runyu Wang a a
State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi'an 710049, Shaanxi, China b College of Chemistry and Chemical Engineering, Baoji University of Arts and Science, Baoji 721013, Shaanxi, China
article info
abstract
Article history:
Catalytic supercritical water gasification (SCWG) of glucose over Ni/Zr(Ce,Y)O2-d catalysts
Received 17 June 2016
was conducted in a batch reactor at 500 C, 23e24 MPa with 10 wt.% feed concentration.
Received in revised form
The catalysts were prepared by carbonate co-precipitation method and the effects of
18 September 2016
nickel molar content and CeO2 loading on gasification performances were investigated.
Accepted 4 October 2016
The catalytic gasification experiments with Nix/Zr0.8Y0.2O2-d (x ¼ 0.1, 0.2, 0.3, 0.5, 0.7)
Available online xxx
(NZY) catalysts showed that carbon gasification efficiency increased gradually with the increase of nickel content, but hydrogen yield gradually increased then leveled off. The
Keywords:
maximum hydrogen yield of 22 mol kg1, about 10 times of that without catalyst, was
Hydrogen production
obtained with Ni0.5/Zr0.8Y0.2O2-d (NZY582) catalyst, and carbon gasification efficiency and
Supercritical water gasification
total gasification efficiency were 66% and 90%, respectively. In order to achieve higher
(SCWG)
gasification efficiency, the Zr(Y)O2-d support was modified by adding CeO2. The results
Glucose
showed that using Ni0.5/Zr0.4Ce0.4Y0.2O2-d (NZCY5442) catalyst, carbon gasification effi-
Catalysts
ciency reached 76%, about 15% higher than that of NZY582 catalyst, and methane yield increased by 1.9 times, while hydrogen yield decreased by 23%. Various characterization techniques were carried out on the reduced and used catalysts and it was found that the catalysts exhibited excellent hydrothermal stability and good anti-carbon deposition capability. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen is widely regarded as an ideal energy carrier since it is a high-quality, clean and efficient fuel for stationary and transportation applications by combustion in thermal engines or by electrochemical reaction in fuel cells, thus it has the potential to replace traditional fossil fuels in the future. However, current industrial-scale hydrogen
production relies on the fossil fuel feedstocks via the thermo-catalytic processes and gasification of methane, naptha, and coal [1], which ultimately increase CO2 concentration in the atmosphere. Hydrogen fuel derived from renewable biomass can realize carbon neutrality and nearzero pollution during utilization and it is crucial to the energy and environmental sustainability. Among various conversion processes from biomass to hydrogen, supercritical water gasification (SCWG) has
* Corresponding author. Fax: þ86 29 82669033. E-mail address: [email protected] (J. Huang). http://dx.doi.org/10.1016/j.ijhydene.2016.10.012 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Huang J, et al., Hydrogen production from glucose by supercritical water gasification with Ni/Zr(Ce,Y) O2-d catalysts, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.012
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attracted increasing attention in recent years [2]. Supercritical water (SCW, T > 374 C, P > 22.1 MPa) has properties entirely different from those of liquid water or steam, making it interesting as both solvent and reactant. As a homogenous phase with low viscosity and high diffusivity, mass transfer limitations can be overcome in SCW. Due to its greatly reduced dielectric constant, SCW behaves as an organic solvent, easily dissolving many organic species and gases while precipitating polar salts. SCW also participates in the steam reforming and water-gas shift reactions and contributes up to 50% of the H2 in biomass gasification process [3]. Compared with the traditional gasification technologies, SCWG has higher reaction efficiency and H2 selectivity, avoiding the drying of biomass and forming of char and tar. Therefore, SCWG is a very promising hydrogen production process with biomass resources. Much research has been reported that adopting appropriate catalysts in biomass SCWG process can significantly improve gasification efficiency and hydrogen selectivity at mild reaction temperature. Various kinds of homogeneous and heterogeneous catalysts have been investigated for SCWG of real biomass and model compounds [4]. Alkali such as NaOH, KOH, K2CO3, Na2CO3, etc. is a homogeneous catalyst which can be mixed with the feedstock uniformly and promote water-gas shift reaction thus to increase the hydrogen yield [5e11]. But the recovery of alkali from wastewater requires high consumption of energy. Compared with alkali catalysts, supported transition metals, a major category of heterogeneous catalysts, exhibit higher selectivity and are easier to be recycled. Ni and Ru perform quite well in SCWG processes but Ni catalyst is more frequently studied due to its low cost and efficient catalytic activity. However, both Ni catalyst and its support are unstable, sintering and deactivation occur in SCWG process in both batch and continuous-flow reactors. At present, most studies focus on the screening and designing of supported Ni catalysts with higher catalytic activities, better hydrothermal stabilities and anti-carbon deposition capabilities. Glucose, a typical model compound of biomass, is commonly used as the feedstock to evaluate the catalytic SCWG process. Ding et al. [11] studied the supercritical water catalytic gasification of glucose under the conditions of 12.5 wt.% feed concentration, 500 C, 30 min. Ni/activated carbon (AC) catalyst exhibited the best activity with the hydrogen yield increasing by 81%; Ni/MgO increasing by 62%; Ni/CeO2/Al2O3 increasing by 60%; Ni/Al2O3 increasing by 52%, compared with that without catalysts. Zhang et al. [12] screened several heterogeneous catalysts for hydrogen production from glucose via catalytic SCWG at 600 C and 24 MPa, and found that Ni supported on g-Al2O3 exhibited very high activity and H2 selectivity. As for the effects of catalyst support materials on activity, the following order of sequence was observed: g-Al2O3 > ZrO2 > AC. NieMgeAl catalysts with different Mg/Al molar ratios were synthesized by a co-precipitation method for hydrogen production by SCWG of glucose [13]. NiMg0.6Al1.9 showed the highest catalytic activity and the best hydrothermal stability probably due to the formation of MgAl2O4. It also found that Mg could efficiently improve the anti-carbon ability of NieAl catalyst
but MgO showed poor hydrothermal stability. A highly dispersed rutile-TiO2-supported Ni nanoparticle was synthesized by a solegel method and its catalytic performance for SCWG of glucose was studied [14]. It exhibited high activity and potential stability and anti-carbon ability for longterm SCWG. Lu et al. [15] compared the catalytic performances of Ni-based catalysts with different oxide supports for SCWG of glucose at 400 C and 23.5 MPa. H2 yield for different supports decreased in order: CeO2/Al2O3 > La2O3/ Al2O3 > MgO/Al2O3 > Al2O3 > ZrO2/Al2O3, and H2 selectivity decreased in order: CeO2/Al2O3 > La2O3/Al2O3 > ZrO2/ Al2O3 > Al2O3 > MgO/Al2O3. The catalyst deactivation was mainly due to sintering and carbon deposition and CeO2 could remove the surface carbon deposition. Lu et al. [16] further investigated the effect of Ce loading in Ni/CeO2/ Al2O3 catalysts on glucose SCWG, and the optimum Ce loading content of 8.46 wt.% was obtained. CeO2 plays an important role in inhibiting carbon deposition by increasing the Ni dispersion and reacting with deposited carbon. Azadi et al. [17] tested 44 supporting materials for Ni for SCWG of glucose under the conditions of 380 C, 23 MPa, 2 wt.% feed concentration. a-Al2O3, carbon nanotubes (CNTs), and MgO supports resulted in high carbon conversions, while SiO2, Y2O3, hydrotalcite, Y2O3-stabilized ZrO2 (YSZ), and TiO2 showed modest activities. Other supports such as zeolites, molecular sieves, CeO2 and ZrO2 showed no obvious activity under the testing conditions. Aside from the catalytic activity, the stable metal oxide supports were a-Al2O3, boehmite, YSZ, and TiO2. It indicated that the catalytic activity of Ni/ZrO2 catalyst can be improved by introducing Y2O3 into ZrO2 and forming a stable Y2O3eZrO2 solid solution. In the case of catalytic SCWG of real biomass or other model compounds, the screening of appropriate supports for Ni catalyst has also been carried out. Byrd et al. [18] studied hydrogen production from catalytic SCWG of the switchgrass biological diesel oil at 600 C and 250 bar. The supported Ni, Co and Ru catalysts with the supports of TiO2, ZrO2 and MgAl2O4 were prepared by impregnation method. It was found that the highest hydrogen yield of 0.98 mol H2$mol1 C was obtained over Ni/ZrO2 catalyst. Liu et al. [19] modified Ni/ZrO2 catalyst by MgO for glycerol reforming in supercritical water, and found that the MgO in the catalyst showed significant promotion effect for hydrogen production likely due to the formation of the alkaline active site. Therdthianwong et al. [20] investigated the hydrogen production from supercritical water reforming of bioethanol over Ni/Al2O3 and Ni/CeZrO2/ Al2O3 catalysts (500 C, 25 MPa). The results showed that hydrogen production increased 3e4 times of that without catalysts. After CeZrO2 modification, the catalytic activity increased significantly, reaching complete gasification with no tar formation. CeZrO2 could not only promote the watergas shift reaction and methanation reaction to obtain lower production of carbon monoxide and higher hydrogen yield, but also improve the hydrothermal stability of Al2O3. Although ZrO2 is considered as a stable support alternative to Al2O3 for Ni catalyst in SCWG of biomass, the catalytic activity of Ni/ZrO2 catalyst needs further improvement. From the aspect of support modification, doping ZrO2 with low valent metal oxide such as MgO,
Please cite this article in press as: Huang J, et al., Hydrogen production from glucose by supercritical water gasification with Ni/Zr(Ce,Y) O2-d catalysts, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.012
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Y2O3, etc. is an efficient way to improve the catalytic activity and maintain hydrothermal stability. Y2O3 stabilized ZrO2 (YSZ) or Zr(Y)O2-d is the most widely studied electrolyte material for solid oxide fuel cells. However, YSZ or Zr(Y)O2-d as catalyst for SCWG of biomass has not yet been intensively investigated. As mentioned above, CeO2 is a good promoter for Ni catalyst supported on both Al2O3 and ZrO2 for catalytic SCWG of biomass. Moreover, ZrO2eCeO2 solid solution is also a good promoter for Ni catalyst supported on Al2O3. Thus, it is expected that Ni/Zr(Ce,Y)O2d catalysts can balance the requirements of catalytic activity, hydrothermal stability and anti-carbon deposition capability. In this study, glucose was chosen as the representative model compound of real biomass and was catalytically gasified under SCW condition. Nix/Zr(Ce,Y)O2-d catalysts were prepared by carbonate co-precipitation method. The effect of nickel molar content and CeO2 loading on carbon gasification efficiency and hydrogen yield was investigated. In addition, XRD, physical and chemical adsorption-desorption, TG, SEM and XPS were performed to characterize the as-reduced and used catalysts to investigate the catalytic mechanism of the catalysts.
Experimental Catalyst preparation A series of Nix/Zr(Ce,Y)O2-d catalysts were prepared using carbonate co-precipitation method. All the chemical reagents (Ni(NO3)2$6H2O, Zr(NO3)4$4H2O, Ce(NO3)3$6H2O, Y(NO3)3$6H2O, Na2CO3, C6H12O6) were purchased from Sinopharm Chemical Reagent Co., Ltd. For each sample, a certain amount of 0.2 mol L1 Na2CO3 solution was added as the precipitant drop by drop through a constant flow pump into 0.15 mol L1 cations (Ni2þ, Zr4þ, Ce3þ, Y3þ) solution stirred at 40 C moderately with the final pH about 10, ensuring that the mole of CO2 3 was twice of that of the total cation. The obtained sediment was aged for 5 h, filtered by vacuum suction and washed by distilled water and ethanol for three times, respectively, and then dried in the vacuum at 105 C overnight. Next, the precipitates were grinded and sieved to more than 200 meshes. The ultrafine powders were calcined at 500 C for 180 min and then reduced by H2 with a flow rate of 50 mL min1 at 650 C for 150 min. Finally, the reduced catalysts were sealed to save for use.
The composition and abbreviation of the reduced Nix/ Zr(Ce,Y)O2-d catalysts were displayed in Table 1.
Catalyst characterization The crystalline phases of the calcined, reduced and used catalysts were identified by X-ray diffraction (XRD) using Nifiltered Cu Ka irradiation (wavelength 0.15406 nm) with the Bragg angles between 10 and 90. The analysis of N2 adsorption/desorption isotherms, specific surface area and pore size distributions of the catalysts was performed by the ASAP 2020 instrument. The samples were degassed at 300 C for 120 min before the testing. Surface area and pore size distributions of the catalysts were measured by the Brunauere-EmmetteTeller (BET) and Barrette-Joynere-Halenda (BJH) methods, respectively. The surface morphology of the catalysts was observed by the JEOL JSM-6700F field emission scanning electron microscopy (SEM). The catalysts were dispersed with ethanol and then dropped on silicon wafers before the scanning. All the used catalysts were analyzed by simultaneous DSC-TG instruments in air atmosphere with a heating rate of 10 C$min1 from 20 C to 1000 C. The DTG line was obtained by the analysis software. The surface chemical environment of some catalysts was detected by the X-ray photoelectron spectroscopy (XPS) instrument from Axis Ultra, Kratos (UK). Binding energy of each element was calibrated based on the C1s peak (284.8 eV) of the hydrocarbons adsorbed on the surface of the catalysts.
Catalytic testing The gasification of glucose was conducted through a highthroughput batch reactor made of Inconel 625 with the designed reaction condition of 750 C, 30 MPa. The schematic diagram of the reaction system is shown in Fig. 1 with the detailed introduction given in literature [21]. For each experiment, 1.5 g glucose solution of 10 wt.% was added into the reactor without or with 0.15 g catalyst and the residence time was 30 min at testing conditions. The testing conditions were 500 C and 23e24 MPa. The gaseous products collected by a gas bag were then analyzed by the gas chromatography (Agilent 7890A) equipped with a thermal conductivity detector (TCD). Argon was used as the carrier gas and a standard gas with certain concentrations of H2, CO, CH4 and CO2 was used for calibration. The catalytic gasification performance was evaluated by carbon gasification efficiency (CGE), total gasification
Table 1 e The composition and abbreviation of the reduced Nix/Zr(Ce,Y)O2-d catalysts. Catalyst composition Ni0.1/Zr0.8Y0.2O2-d Ni0.2/Zr0.8Y0.2O2-d Ni0.3/Zr0.8Y0.2O2-d Ni0.5/Zr0.8Y0.2O2-d Ni0.7/Zr0.8Y0.2O2-d Ni0.5/Zr0.6Ce0.2Y0.2O2-d Ni0.5/Zr0.5Ce0.3Y0.2O2-d Ni0.5/Zr0.4Ce0.4Y0.2O2-d
Nickel content/mol.%
Nickel content/wt.%
9.09 16.67 23.08 33.33 41.18 33.33 33.33 33.33
4.62 8.83 12.69 19.50 25.32 18.31 17.77 17.26
Zr/Ce
Abbreviation
6/2 5/3 4/4
NZY182 NZY282 NZY382 NZY582 NZY782 NZCY5622 NZCY5532 NZCY5442
Please cite this article in press as: Huang J, et al., Hydrogen production from glucose by supercritical water gasification with Ni/Zr(Ce,Y) O2-d catalysts, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.012
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Fig. 1 e The schematic diagram of the batch reactor system.
Catalytic activity of Nix/Zr(Ce,Y)O2-d catalysts
glucose with catalysts had a significant increase comparing to that of without any catalyst. CGE and TGE gradually increased with the increase of nickel molar content. However, H2 selectivity did not increase monotonically with increasing nickel content, and the highest H2 selectivity of 50% was obtained when the nickel content of the catalyst was 33.33 mol.%, which is 4-fold larger than that without catalyst addition. In this case, CGE and TGE was 66% and 90%, respectively. The distribution of gaseous products for the supercritical water gasification of glucose over NZY catalysts is presented in Fig. 2b and c. As shown in Fig. 2b and c, the main gaseous products of SCWG of glucose were H2, CO, CH4 and CO2, of which H2 and CO2 occupied the highest proportion, CO occupied the lowest proportion, indicating that nickel could promote the methanation reaction (CO þ 3H2 / CH4 þ H2O) and water-gas shift reaction (CO þ H2O / H2 þ CO2) of CO. Fig. 2b shows that, with the increase of nickel molar content of the catalysts, hydrogen yield gradually increased then leveled off. The maximum hydrogen yield of 22 mol kg1, about 10 times of that without catalyst, was obtained when the nickel content was 33.33 mol.%. Increasing the Ni content could increase the active sites of the catalyst, however, a high Ni content could also lead to reduced dispersion of the Ni in the catalyst, and decreased surface area and porosity of the catalyst, which would consequently reduce the catalytic activity [12]. According to Fig. 2, we could conclude that the best gasification performance of SCWG of glucose was achieved by the NYZ catalyst with the nickel content of 33.33 mol.%.
Effect of nickel molar content on catalytic activity
Effect of CeO2 loading on catalytic activity
Fig. 2 show the gasification results of SCWG of glucose without or with Nix/Zr0.8Y0.2O2-d (x ¼ 0.1, 0.2, 0.3, 0.5, 0.7) catalysts. As we can see from Fig. 2a, CGE and TGE of
In order to improve the gasification efficiency of SCWG of glucose, the Zr(Y)O2-d support was modified by adding CeO2 to partially replace ZrO2 and Ni0.5/Zr0.8-xCexY0.2O2-d (x ¼ 0.2, 0.3,
efficiency (TGE), H2 selectivity [20], gas yields and gas molar fraction defined as follows, respectively.
CGE (%) ¼ total carbon in the gaseous products/total carbon in the feedstock (dry basis) 100% (1)
TGE (%) ¼ total mass of gaseous products/total mass of the feedstock (dry basis) 100% (2)
H2 selectivity (%) ¼ H2 molecules in the gaseous products/total carbon atoms in the gaseous products (1/RR) 100% (3) Here, RR is the reforming ratio of H2/CO2 depending on the reactant compounds. The RR of glucose is 2 [20].
Gas yield (mol/kg) ¼ mole number of certain gaseous product/ total mass of the feedstock (dry basis) (4)
Gas molar fraction (%) ¼ mole number of a certain gaseous product/total mole numbers of the gaseous products (5)
Results and discussion
Please cite this article in press as: Huang J, et al., Hydrogen production from glucose by supercritical water gasification with Ni/Zr(Ce,Y) O2-d catalysts, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.012
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Fig. 2 e Gasification results over NZY catalysts for SCWG of glucose.
0.4) catalysts were prepared. The gasification results of SCWG of glucose after using Ni0.5/Zr0.8-xCexY0.2O2-d (x ¼ 0.2, 0.3, 0.4) catalysts are showed in Fig. 3. We can see that both CGE and TGE were improved by adding CeO2 into the Zr(Y)O2-d support, and they increased slightly with increasing CeO2 loading. The best gasification results were obtained over NZCY5442 catalyst, with CGE and TGE of 76% and 95%, respectively. However, H2 selectivity and H2 molar fraction over NZCY catalysts decreased comparing with that of NZY582 catalyst. Methane yield increased gradually from 3.3 mol kg1 with NZY582 catalyst to 6.2 mol kg1 with NZY5442 catalyst, but hydrogen yield decreased from 22 mol kg1 with NZY582 catalyst to 17 mol kg1 with NZY5442 catalyst, indicating that CeO2 could further promote methanation reaction [22]. It should be noted that H2 selectivity, H2 yield and H2 molar fraction for SCWG of glucose over NZCY catalysts were almost independent of CeO2 loading, due to the fact that the partial substitution of Zr4þ with Ce4þ in the lattice only increases the lattice constant and does not increase the concentration of oxygen vacancy significantly.
to the crystalline planes (1, 1, 1), (2, 0, 0) and (2, 2, 0) of the reduced and used catalysts, illustrating that NiO was well reduced to Ni0 after H2 reduction and further enhanced the reduced state after SCWG of glucose reaction due to its rich hydrogen environment. Moreover, there was no obvious carbon peaks in both figures indicating that no carbon or amorphous carbon was deposited during SCWG of glucose. It was further confirmed by thermogravimetric analysis that small amount of amorphous carbon was formed on the surface of the used catalysts. Besides the diffraction peaks of cubic ZrYO (No. 01-089-6687), the reduced NZY582 catalyst showed the characteristic peak of orthorhombic ZrO2 (No. 00-037-1413) at the 2q angles of 38.8 . In addition, the diffraction peaks of the support in the calcined, reduced and used NZCY5442 catalyst split into two sub-peaks of ZrCeO (No. 00-038-1436) and CeYO (No. 01-075-0176) when the molar fraction of CeO2 in the support increased to 40 mol.%, revealing that the solid solubility of CeO2 in the Zr(Y)O2-d support was not more than 40 mol.%.
Catalyst textures and structures
Pore structures and BET surface area analysis
Crystalline phase analysis XRD spectra of the calcined, reduced and used catalysts were presented in Fig. 4. It can be seen from that the diffraction peaks of NiO appeared at the 2q angles of about 37.2 , 43.3 and 62.8 of the calcined catalysts, and the diffraction peaks of Ni0 (2q ¼ 44.5 , 51.8 and 76.4 ) related
Fig. 5 presents the N2 adsorption/desorption isotherms and pore size distributions of the reduced and used catalysts. It can be seen from Fig. 5 that the N2 adsorption/desorption isotherms of the catalysts whether reduced or used were similar to Type IV with the hysteresis loop of Type H1 [23]. The N2 adsorption/desorption isotherms of Type IV with the hysteresis loop of Type H1 could probably be some
Please cite this article in press as: Huang J, et al., Hydrogen production from glucose by supercritical water gasification with Ni/Zr(Ce,Y) O2-d catalysts, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.012
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Fig. 3 e Gasification results over NZCY catalysts for SCWG of glucose.
Fig. 4 e XRD spectra of the calcined, reduced and used catalysts. Please cite this article in press as: Huang J, et al., Hydrogen production from glucose by supercritical water gasification with Ni/Zr(Ce,Y) O2-d catalysts, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.012
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Fig. 5 e N2 adsorption/desorption isotherms and pore size distributions of the reduced and used catalysts. Please cite this article in press as: Huang J, et al., Hydrogen production from glucose by supercritical water gasification with Ni/Zr(Ce,Y) O2-d catalysts, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.012
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Fig. 6 shows the morphology analysis of the catalysts by SEM. It can be seen from Fig. 6 that both the NZY and NZCY catalyst particles were regular spherical, but NZCY catalysts showed inconspicuous surface morphology variation after used compared with NZY catalysts, indicating that the NZCY catalysts had better hydrothermal stability than the NZY catalysts. The aggregated particles in the reduced NZY and NZCY catalysts became a little loose after used in accordance with the BET surface area results, and the reduced NZY catalyst particles became much smaller after used compared with the reduced NZCY catalysts as shown in Fig. 6. Nickel particles about 40e60 nm in diameter were highly dispersed in the reduced catalysts. Especially, the
TG analysis The simultaneous thermogravimetric method and differential scanning calorimeter method were used to further confirm the presence of the formation of carbon deposition. During the simultaneous TG-DSC testing in air atmosphere, the used catalysts may have these reactions as follows [24]: 4Ni þ 3O2 /2Ni2 O3
(6)
2Ni2 O3 /4NiO þ O2
(7)
2Ni þ O2 /2NiO
(8)
C þ O2 /CO2
(9)
1 1 =dCeO2 / =dCeO2d þ 2O2 ½24
(10)
1 1 =dZrO2 / =dZrO2d þ 2O2 ½24
(11)
1
1
=
Morphology analysis
NZCY catalysts maintained high porosity before and after use.
=
mesoporous materials with relatively narrow pore size distributions or some aggregates of spherical particles with relatively uniform geometry sizes, demonstrated by the pore size distribution graphs embedded in Fig. 5Ae(E) and the SEM images displayed below. The pore sizes of the NZY catalysts distributed between 20 and 50 nm. The pore sizes gradually increased then decreased with the increasing nickel content of the catalysts, and the pore sizes of NZY582 catalyst were the smallest. The main pore sizes of the catalysts changed little but its distribution became a bit widen maybe due to the growing up of several particles, revealing that the catalysts had a good hydrothermal stability. According to Fig. 5(D1)-(D3), the pore sizes of the NZCY catalysts distributed between 40 and 90 nm, which were about twice as those of the NZY catalysts. The BJH distribution shows that pore sizes of the catalysts gradually broadened with the increase of CeO2 molar fraction in the support, and the average pore size was increased as well, which can be attributed to the structural changes caused by the partial reduction of Ce4þ to Ce3þ. Table 2 shows the BET specific surface area of the catalysts. It can be seen from Table 2 that the BET surface area of most reduced catalysts increased after used, because the fairly weak hydrogen bonds will be broken when water reaches its supercritical state, thus weakening the agglomeration of catalyst particles and resulting in the increased BET surface area. With the increase of Ni molar content, the specific surface area of reduced NZY catalysts tended to increase first, then decrease, and then increase again. The reduced NZY282 catalyst had the maximum surface area of 24.87 m2 g1 and the reduced NZY582 catalyst had the minimum surface area of 10.31 m2 g1, as evidenced from the following morphology analysis. As for the reduced NZCY catalysts, the specific surface area decreased and then increased with increasing of CeO2 loading, which is also consistent with the morphology analysis.
The TG-DTG curves of the used catalysts are showed in Fig. 7 and the main information given from Fig. 7 are listed in Table 3. From Fig. 7 and Table 3 we can find that for the reduced catalysts, a significant weight increase appears between about 350 C and 400 C but then a slightly decrease appears at above 500 C, this may because Ni0 was oxidized to Ni2O3 (equation (6)), but Ni2O3 could not exist in the high temperature environment, then decomposed to NiO (equation (7)). Furthermore, the weight increase ratios increased with the increasing nickel content. The decreased weight increase ratios at 350e400 C for the used catalysts may be caused by the absorption loss of the nickel particles. In addition, it can be seen from Fig. 6 that the weight change trends of the used catalysts at above 500 C were different. For the used catalysts, the nickel particles of improved activity due to the higher dispersion degree were directly oxidized to the relatively stable NiO. The low active deposited carbon was oxidized, and with the oxidation of the deposited carbon, the gradually exposed nickel particles covered by the deposited carbon were oxidized immediately. We denoted the weight loss value resulted from carbon oxidation and CeO2 deoxygenation by Dm2 and the weight increase value resulted from the oxidation of the exposed nickel particles by Dm1. For the used NZY and NZCY5532 catalysts, Dm1 ¼ Dm2, the overall quality unchanged; for the used NZCY5622 catalyst, Dm1>Dm2, the overall quality increased; for the used NZCY5442 catalyst, Dm1
Table 2 e The BET surface area of the catalysts (m2·g¡1). Catalysts Reduced Used
NZY182
NZY282
NZY382
NZY582
NZY782
NZCY5622
NZCY5532
NZCY5442
19.38 35.12
24.87 33.07
24.29 23.00
10.31 23.06
14.35 23.64
18.67 27.33
13.95 18.56
15.68 21.81
Please cite this article in press as: Huang J, et al., Hydrogen production from glucose by supercritical water gasification with Ni/Zr(Ce,Y) O2-d catalysts, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.012
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Fig. 6 e SEM morphology analysis of the reduced and used catalysts: (A)e(E) reduced NZY182, NZY282, NZY382, NZY582 and NZY782; (a)e(e) used NZY182, NZY282, NZY382, NZY582 and NZY782; (D1)e(D3) reduced NZCY5622, NZCY5532, NZY5442; (d1)e(d3) used NZCY5622, NZCY5532, NZY5442.
XPS analysis Figs. 8 and 9 show the XPS patterns of Ni 2p and Ce 3d in order to analyze the surface chemistry environment of the reduced and used catalysts. Fig. 9 reveals that nickel is essentially in its reduction state of Ni0. The main peaks of Ni 2p3/2, Ni0 and Ni 2p1/2,Ni0 are observed at about 852.2e852.8 eV and 869.6e870.6 eV, respectively, with shake-up satellite peaks of Ni 2p3/2, Ni0 at about 858e959 eV (XPS database from LaSurface.com, NIST XPS database). It is clearly that the shake-up satellite peaks of Ni 2p1/2, Ni2þ appear at about 879e880 eV caused by the addition of CeO2. The peak intensity increased significantly when the nickel
content is lower, such as NZY182, NZY282 and NZY382 catalysts, because part of NiO doped into the gap positions of ZrYO crystal lattice was unable to separate out in atmospheric pressure, but it could spread from the gap positions to the surface along with the migration of oxygen vacancy in the high temperature and pressure environment. The NiO separated out then was reduced by hydrogen, leading to the increased surface nickel concentration and peak intensity. Furthermore, Ni 2p peaks of the used NZY catalysts shifted to the right comparing to the reduced catalysts, which may because the dispersion degree of nickel particles was improved due to the decreased agglomeration degree of the
Please cite this article in press as: Huang J, et al., Hydrogen production from glucose by supercritical water gasification with Ni/Zr(Ce,Y) O2-d catalysts, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.012
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Fig. 7 e TG-DTG analysis of the reduced and used catalysts.
Table 3a e Mass change for the reduced and used NZY catalysts. Catalysts
NZY182
NZY282
A Mass/% Temp/ C
0.87 375
a 0.31 744
1.77 388
B 1.71 354
NZY382 b
0.49 726
0.79 322 406
NZY582
C
c 0.66 737
2.72 354
1.82 383
NZY782
D 3.88 383
d 0.34 e
E
3.62 400
5.80 378
e 0.12 e
5.20 385
Table 3b e Mass change for the reduced and used catalysts. Catalysts
NZCY5622 d1
D1 Mass/% Temp/ C
3.39 345
NZCY5532
0.28 740
3.24 352
D2 3.73 342
used catalysts particles. Then the contacting area and acting force between Ni0 and ZrYO substrate increased, thus the binding energy increased and Ni 2p shifted to the right. But for the used NZCY catalysts, Ni 2p peaks shifted to the left
NZCY5442 d2
0.48 743
3.50 359
D3 2.70 334
d3 1.77 745
2.45 357
caused by the weakened acting force between Ni0 and ZrYO (ZrCeO and CeYO) substrates. It may because the binding force between Ce4þ (Ce3þ) and Zr4þ in the ceria-zirconia solid solution was the biggest. It can be seen from Fig. 9
Please cite this article in press as: Huang J, et al., Hydrogen production from glucose by supercritical water gasification with Ni/Zr(Ce,Y) O2-d catalysts, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.012
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Fig. 8 e The Ni 2p XPS spectrums of the reduced and used catalysts. Please cite this article in press as: Huang J, et al., Hydrogen production from glucose by supercritical water gasification with Ni/Zr(Ce,Y) O2-d catalysts, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.012
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Fig. 9 e The Ce 3d XPS spectrums of the reduced and used catalysts.
that cerium was mainly in the state of Ce4þ but Ce3þ appeared after used, because part of Ce4þ was reduced to Ce3þ during the SCWG of glucose.
Conclusions The Nix/Zr(Ce,Y)O2-d were prepared by carbonate coprecipitation method and used in the hydrogen production from SCWG of glucose. CeO2 was added to improve the gasification efficiency. 1. The highest hydrogen yield of 22 mol kg1 and the maximum H2 selectivity of 50% were achieved by NZY582 catalyst, with CE of 66%. The optimum nickel content was 33.33 mol.%. 2. The highest CE of 76% was obtained by NZCY5442 catalyst, with methane yield increased by 1.9 times, but hydrogen yield decreased by 23%, because methanation reaction is a process of consuming hydrogen. 3. The SEM analysis showed the excellent hydrothermal stability of the catalysts and the TG analysis and the XRD spectra revealed the good anti-carbon deposition capability of the most catalysts. A relatively higher content of nickel could inhibit the carbon deposition and it was further enhanced by the synergism of CeO2.
Acknowledgment This work is currently supported by the National Key Project for Basic Research of China (973) through Contract No. 2012CB215303.
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Please cite this article in press as: Huang J, et al., Hydrogen production from glucose by supercritical water gasification with Ni/Zr(Ce,Y) O2-d catalysts, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.012