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Supercritical water synthesis of bimetallic catalyst and its application in hydrogen production by furfural gasification in supercritical water
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Supercritical water synthesis of bimetallic catalyst and its application in hydrogen production by furfural gasification in supercritical water Hui Jin*, Xiao Zhao, Xiaohui Su, Chao Zhu, Changqing Cao, Liejin Guo State Key Laboratory of Multiphase Flow in Power Engineering (SKLMF), Xi'an Jiaotong University, 28 Xianning West Road, Xi'an 710049, Shaanxi, China
article info
abstract
Article history:
Supercritical water not only provides good solvent of biomass gasification but also causes
Received 11 September 2016
catalyst agglomeration. Therefore, the supercritical water synthesis method was utilized to
Received in revised form
guarantee the stability of the catalyst. In order to take into account the advantages of
17 October 2016
different metal catalysts, bimetallic catalysts were prepared based on Ni, Co, Zn and Cu,
Accepted 19 October 2016
with TiO2 as carrier. Hydrogen production by furfural gasification in supercritical water
Available online xxx
was conducted with different catalysts. The experimental results turned out that all
Keywords:
Cu þ Zn showed better selectivity on hydrogen, and Co þ Ni showed better carbon gasi-
Biomass
fication efficiency. Spherical coke particles were obtained after the reaction with bimetallic
Hydrogen
catalysts, while the coke particles were amorphous after the reaction with K2CO3.
combinations showed remarkable stability except for the combination Co þ Ni. Zn þ Ni and
Supercritical water
© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Bimetal catalysts Nano particles
Introduction While the consumption of traditional fossil fuels becomes more and more fast, it is notoriously necessary to lay the foundation for the future renewable energy. Due to the vast amount and its renewable property, biomass has been a hot issue for researchers over the recent years [1e5]. However, there are many disadvantages of biomass, such as low energy density, high moisture content and low energy conversion efficiency [6e10]. Depolymerisation is a favourable method for the high efficiency and clean utilization of biomass for liquid fuel. Meanwhile, the waste depolymerisation liquid was produced as by products with furfurals as one of the main components. Moreover, the liquid fuel can be upgraded by
subsequent hydrogenation [11e13]. Therefore, hydrogen production by the waste depolymerisation liquid makes the whole conversion system more complete and independent, which also consumes the waste liquid. Nevertheless, the concentration of the organic content is low, and the cost for conversion is extremely high due to the expensive drying process during the traditional thermochemical conversion [14e17]. Supercritical water (SCW) gasification is a promising method to produce hydrogen from biomass in water phase [18e22] and the system economy will be greatly improved owing to the omitting of drying process. The key to a more efficient system is a proper catalyst with great performance in SCW environment. Recent years, many researchers [1,23e29] have paid great attention to catalysts in SCW to improve the
* Corresponding author. E-mail address: [email protected] (H. Jin). http://dx.doi.org/10.1016/j.ijhydene.2016.10.096 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Jin H, et al., Supercritical water synthesis of bimetallic catalyst and its application in hydrogen production by furfural gasification in supercritical water, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.10.096
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Nomenclature CE
(total carbon in the gaseous products)/(total carbon in furfural) 100% GE (mass of the gaseous products)/(mass of furfural) 100% HE (total hydrogen in the gaseous products)/(total hydrogen in furfural) 100% YH2 (yield of H2) molar amount of H2 after reaction/ mass of furfural, mol/kg whole system. Kruse [30] studied the effect of KOH on the gasification of biomass in SCW. It turned out that the yield of hydrogen increased threefold. Ott [31] used zinc sulfate for acrolein production in sub- and supercritical water. Heterogeneous catalysts in SCW tend to sintering and carbonize, so that catalysts are easily inactivated [25,32]. As a result, a proper method to prepare catalyst is of great importance. Masaru used ZrO2 for catalytic hydrogen generation from glucose and cellulose [33]. Byrd [34]conducted hydrogen production from glycerol by reforming in supercritical water over Ru/Al2O3 catalyst. In their experiment, feed concentration of glycerol was up to 40 wt%. “Many researchers synthesized catalyst in supercritical water environment because supercritical water has high performance in micro- or nanoparticle formation” [35]. Sue [36] used micro reactor to prepare ZnO nano crystals and conducted rapid hydrothermal synthesis without organics [37]. Kojima [38] used sodium borohydride for hydrogen generation accelerated by applying metal metal oxide catalysts such as Pt/TiO2, Pt/CoO and Pt/ LiCoO2. Kawasaki [39] synthesized NiO nano-particles using a T-shaped mixer with a continuous supercritical hydrothermal method. The relation between particle diameter and the heating rate was investigated. The aim of this paper is firstly to enlarge the supercritical water synthesis method for the preparation for bimetallic catalyst to ensure the hydrothermal stability of the heterogeneous catalyst. The combination effect of inexpensive Zn, Ni, Cu and Co was test to evaluation the synergetic catalytic effect. Multiple analysis methods such as SEM, XRD and EDS were conducted to reveal the catalytic mechanism. Furfural was selected to be the model reactant for the waste depolymerisation liquid, in order that a novel catalyst preparation was obtained for biomass advanced utilization.
Experimental section
the reactor using an inserted thermocouple. The operating condition for the gasification is within the following range: residence time 20 min, pressure 23e25 MPa, temperature 200e400 C. To acquire the combination of different metal elements, metal salt solution of two different elements were mixed with enough time to be distributed uniformly. Afterwards, measured TiO2 particles were added into the reactor and deionized water was injected into the reactor with a syringe. Then, a purging process was conducted to replace the air to prevent the undesired effect of air. As for metal catalysts, after being dried in an oven, they were analysed by multiple methods to obtain the properties. Gasification characteristics were used to analyse the catalytic effects and make a comparison. The operating condition for the synthesis process is within the following range: residence time 20 min, pressure 23e25 MPa, temperature 400 C.
Materials Reagents are commercial products bought from special companies and their content information is listed in Table 1. It can be seen that acetate was applied in the work, and suppose metal M is positive 1, the reaction pathway is showed as Eqs. (1)e(3): Acetate was used because it can be gasified in supercritical to omit the unwanted effect of anion. Moreover, it can prevent the severe acid cession caused by chlorate or sulphate. MCH2COOH þ H2O / MOH þ CH3COOH
(1)
CH3COOH þ H2O / CO2 þ H2
(2)
2MOH / M2O þ H2O
(3)
Characterization and analytical methods All the characterizations were obtained on freeze-dried powders. The samples were characterized by X-ray diffraction (XRD) using a PANalytical X'pert MPD Pro X automatic diffractometer. The volt and current is 40 kV and 40 mA, respectively. Powders were all observed by scanning electron microscopy (SEM). These experiments were conducted on a JEOL JSM-7800F electron microscope coupled with an energy dispersive spectroscope. The composition of the gaseous products was analyzed by gas chromatography (Agilent
Apparatus and procedure Table 1 e Reagents involved in this paper. A high-throughput batch reactor system with 6 subsystems was used for the synthesis and gasification processes. The reactor was fabricated from Inconel 625 and designed with a maximum temperature and pressure of 750 C and 30 MPa, respectively. A subsystem consists of batch reactor, electric furnace, and data acquisition station of temperature and pressure. The electric furnace operates at a certain temperature for experiment with a temperature controller. The DAS can monitor and record the temperature and pressure inside
Chemicals Furfural Zinc acetate Nickel acetate Copper acetate Cobalt acetate
Company
Content no less than
SigmaeAldrich Tianjin Shengao Reagents Tianjin Yongsheng Fine Chemicals Tianjin Shengmiao Fine Chemicals Tianjin Fuchen Chemicals
98.0% 99.0% 98.0% 99.0% 99.5%
Please cite this article in press as: Jin H, et al., Supercritical water synthesis of bimetallic catalyst and its application in hydrogen production by furfural gasification in supercritical water, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.10.096
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e8
7890A) with thermal conductivity detectors (TCDs). The highpurity argon was used as the carrier gas. CE, GE and HE are used to describe the gasification process with the equation shown in nomenclature and from the point of view of carbon, mass, hydrogen conversion progress respectively.
Results and discussion Based on our previous work, different metal elements performed certain effects. In particular, Ni showed great stability in crystalline phase, and Zn showed better performance in the selectivity of hydrogen. Since each element had its own advantage, the combination of the bimetallic catalyst may hold the advantages together. In order to identify this hypothesis, different acetic salts are uniformly mixed to be loaded on TiO2 particles, which was famous for its hydrothermal stability in supercritical water [40,41]. Through three steps, metal elements can be dispersed on TiO2 particles in the form of oxides. For quantification, mass of metal in acetic salt was 20 wt% of TiO2. Then we conducted XRD analysis to figure out whether the two metal elements were well dispersed on TiO2 particles. Generally, four combinations were selected, Cu þ Zn, Cu þ Ni, Co þ Ni, and Zn þ Ni. Each combination at least has one of these two elements considering the advantages of Zn and Ni. Fig. 1 showed the XRD analysis of the catalyst after preparation. Generally, anion was always titan acid. Cation consists of metal elements such as NiZnTiO4 and CuZnTiO4. However, as for combination Co þ Ni, Co and Ni were found in two forms of titanates, Co2TiO4 and NiTiO3. There was even NiO in the combination Cu þ Ni, along with CuNi.5Ti.5O2, which meant that a single uniform crystalline phase cannot be generated in this combination. After catalyst preparation, we conducted gasification experiments with each kind of catalyst. Mass of furfural and deionized water were 0.5 g and 5 g, respectively. Mass of TiO2 was 0.25 g and residence time is 20 min. After filtration and drying, residues were investigated by multiple characterization methods. As shown in Fig. 2, for combination Co þ Ni, Co and Ni were still in two different crystalline phases: Co2TiO4, Ni3TiO5. It can be seen that compared with Fig. 1, the crystalline phase for the experimental situation of Ni þ Co, NiTiO3 transmitted Ni3TiO5. Probably because in the supercritical water process, NiTiO3 decomposed to Ni3TiO5 and TiO2 under the catalytic effect of Co. Moreover, as the experimental situation of Cu þ Ni, the products were also in two different crystalline phases: NiO, CuNi.5Ti.5O2. The crystalline phase for Zn þ Ni and Cu þ Zn was NiZnTiO4 and ZuCuTiO4 respectively. Table 2 showed all the phase transitions in every reaction condition. All other combinations containing element Ni show obvious stability except for the combination Co þ Ni, which verified our previous work that Ni had better stability, and it can be successfully reserved in two metal elements combination. Surprisingly, for combination Cu þ Zn which did not contain elements Ni, stability in crystalline phase was also obtained. As a result, the combination of Cu þ Zn provides a new direction that the combination of two different elements can show a new capability which these two elements do not have before.
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For the microstructure of synthesized catalyst, SEM and EDS analysis was used to reveal the surface condition from the micro perspective. Fig. 3 showed the observation area under the magnification of 100,000 times. It can be seen that polyhedral catalyst particles by supercritical water synthesis performed an excellent dispersity. The particle diameters in Cu þ Ni experimental situation were about 30 nm. The diameters for the rest of the situation were about 20 nm. Probably the crystal growth ability of NiO or CuNi.5Ti.5O2 was stronger than that in the other situation. Fig. 3 illustrates that there was a combination of two metal elements, and a uniform nano-structure can be also formed in spite of different crystalline phases. Take Fig. 3 (a) for example, energy spectrum analysis is listed in Table 3. As can be seen in Table 3, since TiO2 was the carrier, element Ti and O took the most positions on the surface of the catalyst.
Fig. 1 e XRD plot of two metal elements loaded on TiO2 particles in SCW.
Fig. 2 e XRD plot of different metal elements loaded on TiO2 particles after reaction in SCW.
Please cite this article in press as: Jin H, et al., Supercritical water synthesis of bimetallic catalyst and its application in hydrogen production by furfural gasification in supercritical water, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.10.096
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Table 2 e Differences of metal elements between before and after reaction.
Table 3 e EDS analysis results of catalyst surface (Zn þ Ni loaded on TiO2, 20 wt%, 400 C).
Combination
Before reaction
After reaction
Element
Zn þ Ni Co þ Ni Cu þ Ni Cu þ Zn
NiZnTiO4 Co2TiO4, NiTiO3 NiO, CuNi.5Ti.5O2 ZuCuTiO4
NiZnTiO4 Co2TiO4, Ni3TiO5 NiO, CuNi.5Ti.5O2 ZuCuTiO4
O Ti Ni Zn
Meanwhile, element Ni and Zn were successfully loaded on TiO2 particles uniformly. It showed that hydrothermal method using TiO2 particles and mixed metal salt water solutions is a promising catalyst preparation method and turned out to be a very convincing result with a combination of two different metal elements to integrate their separate properties. Catalyst particles were collected for SEM analysis in order to understand the surface topography of the catalysts after reaction in SCW. Fig. 4 showed SEM images of these particles for each combination, and the magnification was 5000 times. Overall, a number of spherical particles were generated on the surface of catalysts. EDS analysis was carried out to find out that these particles were mainly coke. The reasons for the formation of the coke particles was analysed as following. The solubility of the precursor of char was relative high, therefore the precursor was uniformly distributed in the supercritical water. After reaction, supercritical water was cooled to be in liquid phase and the solubility of the precursor of char was vastly dropped so that the char precursor might grow up its
Mass percentage (%)
Atom percentage (%)
47.05 42.69 3.57 6.69
73.61 22.31 1.52 2.56
size around a nano-bimetallic catalyst. Due to the surface tension of the char precursor, spherical particles were formed [42,43]. This phenomenon was not found when using K2CO3 as a homogeneous catalyst in supercritical water. Fig. 5 showed the gasification results of furfural with and without different metal element combination catalysts. In Fig. 5(a), the fraction of H2 in gaseous products was only 6.28% without any catalyst. Totally, by comparison, every combination increases the fraction of H2. When combinations were Cu þ Zn, Co þ Ni, Cu þ Ni and Zn þ Ni, the fraction of H2 were 19.56%, 13.80%, 12.33% and 18.63% respectively. It seemed that the combinations contain Zn element maintained the advantage of single Zn because combinations of Cu þ Zn and Zn þ Ni had a relatively higher fraction of H2. This result again proved the rationality of the combination of two different metal elements. When there was no catalyst added, the gas products appeared to contain no CH4. All catalysts can help produce CH4 even though the ratio is only approximately 5%, which does not have obvious difference with single element catalysts [44]. At the same time, all combinations reduced the fraction of CO2
Fig. 3 e SEM images of different combinations loaded on TiO2 particles in SCW. (a) Zn þ Ni, 20 wt%; (b) Co þ Ni, 20 wt%; (c) Cu þ Ni, 20 wt%; (d) Cu þ Zn, 20 wt%. Please cite this article in press as: Jin H, et al., Supercritical water synthesis of bimetallic catalyst and its application in hydrogen production by furfural gasification in supercritical water, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.10.096
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Fig. 4 e SEM images of metal element combination loaded on TiO2 particles after reaction in SCW: (a) Zn þ Ni, 20 wt%; (b) Co þ Ni, 20 wt%; (c) Cu þ Ni, 20 wt%; (d) Cu þ Zn, 20 wt%.
Fig. 5 e Gasification results of furfural with different metal element combination catalysts (furfural 10 wt%, residence time 20 min).
from 82.29% to around 70% when there was no catalyst. It can be observed from Fig. 5(b) that HE was 4.26% with catalyst, and the combination of Cu þ Zn and Zn þ Ni bimetallic catalyst increased HE to 15.64% and 14.06% respectively. As for YH2, Cu þ Ni had the most appealing result which was 18.92 mol/kg, which was better that the result with only Zn in our previous work. In general, combinations of catalyst performed advantages of two elements and obtained a synergy effect. To make a comparison with conventional homogeneous catalyst, K2CO3 was used to catalyze gasification of furfural. Fig. 6 showed that when the temperature increased from 200 C to 400 C, the ratio of H2 increased from 1.98% to 17.98%.
And when the temperature was above the critical point, the ratio of CO and CO2 decreased from 1.31% and 83.12% to 0.61% and 72.85% respectively. As a result, the parameter above the critical point was proved to be necessary for good gasification results. Meanwhile, K2CO3 can increase the fraction of H2 from 6.34% to 17.98% and decrease the CO fraction from 23.34% to 0.61%. It can be explained by Eq. (1) that K2CO3 accelerated the subsequent reaction. However, as many other homogeneous catalysts, K2CO3 was difficult to be recycled, because when the operating temperature and pressure deceased below the critical point of water, water can dissolve K2CO3 with other complicated solutes.
Please cite this article in press as: Jin H, et al., Supercritical water synthesis of bimetallic catalyst and its application in hydrogen production by furfural gasification in supercritical water, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.10.096
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Fig. 6 e Gasification results of furfural under different temperature with K2CO3 catalyst (furfural 10 wt%, residence time 20 min, ratio of K2CO3 and furfural is 0.5:1).
Fig. 7 e SEM images of solid gasification products after reaction (furfural 10 wt%, residence time 20 min) (a)200 C; (b)250 C; (c)300 C; (d)350 C; (e)400 C. Please cite this article in press as: Jin H, et al., Supercritical water synthesis of bimetallic catalyst and its application in hydrogen production by furfural gasification in supercritical water, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.10.096
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CO þ H2O / CO2 þ H2
(4)
It can be seen from Fig. 7 that solid coke was generated and the coke had no fixed structure instead of spherical structure. Generally the diameter of coke was a few microns as Fig. 7 (e) showed, and the sized of the carbon was larger than that of bimetallic catalyst. Because K2CO3 was easily to grow up to larger particles compared with the well dispersed nano bimetallic particles.
Conclusions The catalytic effect of bimetallic catalyst prepared by supercritical water synthesis was investigated in the process of supercritical water gasification of furfural, and different metal element combinations were investigated as Cu þ Zn, Co þ Ni, Cu þ Ni and Zn þ Ni. Based on the results of XRD analysis, Zn þ Ni and Cu þ Zn had better crystal stability, and two elements were combined as cation with titanate as anion. SEM images revealed that two different elements can be loaded on TiO2 particles uniformly and the diameter was around 30 nm. From gasification results, Cu þ Zn and Ni þ Zn showed yield gas with higher hydrogen fraction, which was 19.56% and 18.63%, respectively. Co þ Ni had the highest CE, which was 18.92 mol/kg. Generally speaking, combinations of two different metal elements certainly integrated the advantages of each element, and combinations showed superiority over single metal catalysts for gasification of biomass in SCW.
Acknowledgement This work was financially supported by the National Natural Science Foundation of China (Contract nos. 51306145 and 51236007) and the China National Key Research and Development Plan Project (Contract no. 2016YFB0600100).
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Please cite this article in press as: Jin H, et al., Supercritical water synthesis of bimetallic catalyst and its application in hydrogen production by furfural gasification in supercritical water, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.10.096
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Supercritical water synthesis of bimetallic catalyst and its application in hydrogen production by furfural gasification in supercritical water Hui Jin*, Xiao Zhao, Xiaohui Su, Chao Zhu, Changqing Cao, Liejin Guo State Key Laboratory of Multiphase Flow in Power Engineering (SKLMF), Xi'an Jiaotong University, 28 Xianning West Road, Xi'an 710049, Shaanxi, China
article info
abstract
Article history:
Supercritical water not only provides good solvent of biomass gasification but also causes
Received 11 September 2016
catalyst agglomeration. Therefore, the supercritical water synthesis method was utilized to
Received in revised form
guarantee the stability of the catalyst. In order to take into account the advantages of
17 October 2016
different metal catalysts, bimetallic catalysts were prepared based on Ni, Co, Zn and Cu,
Accepted 19 October 2016
with TiO2 as carrier. Hydrogen production by furfural gasification in supercritical water
Available online xxx
was conducted with different catalysts. The experimental results turned out that all
Keywords:
Cu þ Zn showed better selectivity on hydrogen, and Co þ Ni showed better carbon gasi-
Biomass
fication efficiency. Spherical coke particles were obtained after the reaction with bimetallic
Hydrogen
catalysts, while the coke particles were amorphous after the reaction with K2CO3.
combinations showed remarkable stability except for the combination Co þ Ni. Zn þ Ni and
Supercritical water
© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Bimetal catalysts Nano particles
Introduction While the consumption of traditional fossil fuels becomes more and more fast, it is notoriously necessary to lay the foundation for the future renewable energy. Due to the vast amount and its renewable property, biomass has been a hot issue for researchers over the recent years [1e5]. However, there are many disadvantages of biomass, such as low energy density, high moisture content and low energy conversion efficiency [6e10]. Depolymerisation is a favourable method for the high efficiency and clean utilization of biomass for liquid fuel. Meanwhile, the waste depolymerisation liquid was produced as by products with furfurals as one of the main components. Moreover, the liquid fuel can be upgraded by
subsequent hydrogenation [11e13]. Therefore, hydrogen production by the waste depolymerisation liquid makes the whole conversion system more complete and independent, which also consumes the waste liquid. Nevertheless, the concentration of the organic content is low, and the cost for conversion is extremely high due to the expensive drying process during the traditional thermochemical conversion [14e17]. Supercritical water (SCW) gasification is a promising method to produce hydrogen from biomass in water phase [18e22] and the system economy will be greatly improved owing to the omitting of drying process. The key to a more efficient system is a proper catalyst with great performance in SCW environment. Recent years, many researchers [1,23e29] have paid great attention to catalysts in SCW to improve the
* Corresponding author. E-mail address: [email protected] (H. Jin). http://dx.doi.org/10.1016/j.ijhydene.2016.10.096 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Jin H, et al., Supercritical water synthesis of bimetallic catalyst and its application in hydrogen production by furfural gasification in supercritical water, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.10.096
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Nomenclature CE
(total carbon in the gaseous products)/(total carbon in furfural) 100% GE (mass of the gaseous products)/(mass of furfural) 100% HE (total hydrogen in the gaseous products)/(total hydrogen in furfural) 100% YH2 (yield of H2) molar amount of H2 after reaction/ mass of furfural, mol/kg whole system. Kruse [30] studied the effect of KOH on the gasification of biomass in SCW. It turned out that the yield of hydrogen increased threefold. Ott [31] used zinc sulfate for acrolein production in sub- and supercritical water. Heterogeneous catalysts in SCW tend to sintering and carbonize, so that catalysts are easily inactivated [25,32]. As a result, a proper method to prepare catalyst is of great importance. Masaru used ZrO2 for catalytic hydrogen generation from glucose and cellulose [33]. Byrd [34]conducted hydrogen production from glycerol by reforming in supercritical water over Ru/Al2O3 catalyst. In their experiment, feed concentration of glycerol was up to 40 wt%. “Many researchers synthesized catalyst in supercritical water environment because supercritical water has high performance in micro- or nanoparticle formation” [35]. Sue [36] used micro reactor to prepare ZnO nano crystals and conducted rapid hydrothermal synthesis without organics [37]. Kojima [38] used sodium borohydride for hydrogen generation accelerated by applying metal metal oxide catalysts such as Pt/TiO2, Pt/CoO and Pt/ LiCoO2. Kawasaki [39] synthesized NiO nano-particles using a T-shaped mixer with a continuous supercritical hydrothermal method. The relation between particle diameter and the heating rate was investigated. The aim of this paper is firstly to enlarge the supercritical water synthesis method for the preparation for bimetallic catalyst to ensure the hydrothermal stability of the heterogeneous catalyst. The combination effect of inexpensive Zn, Ni, Cu and Co was test to evaluation the synergetic catalytic effect. Multiple analysis methods such as SEM, XRD and EDS were conducted to reveal the catalytic mechanism. Furfural was selected to be the model reactant for the waste depolymerisation liquid, in order that a novel catalyst preparation was obtained for biomass advanced utilization.
Experimental section
the reactor using an inserted thermocouple. The operating condition for the gasification is within the following range: residence time 20 min, pressure 23e25 MPa, temperature 200e400 C. To acquire the combination of different metal elements, metal salt solution of two different elements were mixed with enough time to be distributed uniformly. Afterwards, measured TiO2 particles were added into the reactor and deionized water was injected into the reactor with a syringe. Then, a purging process was conducted to replace the air to prevent the undesired effect of air. As for metal catalysts, after being dried in an oven, they were analysed by multiple methods to obtain the properties. Gasification characteristics were used to analyse the catalytic effects and make a comparison. The operating condition for the synthesis process is within the following range: residence time 20 min, pressure 23e25 MPa, temperature 400 C.
Materials Reagents are commercial products bought from special companies and their content information is listed in Table 1. It can be seen that acetate was applied in the work, and suppose metal M is positive 1, the reaction pathway is showed as Eqs. (1)e(3): Acetate was used because it can be gasified in supercritical to omit the unwanted effect of anion. Moreover, it can prevent the severe acid cession caused by chlorate or sulphate. MCH2COOH þ H2O / MOH þ CH3COOH
(1)
CH3COOH þ H2O / CO2 þ H2
(2)
2MOH / M2O þ H2O
(3)
Characterization and analytical methods All the characterizations were obtained on freeze-dried powders. The samples were characterized by X-ray diffraction (XRD) using a PANalytical X'pert MPD Pro X automatic diffractometer. The volt and current is 40 kV and 40 mA, respectively. Powders were all observed by scanning electron microscopy (SEM). These experiments were conducted on a JEOL JSM-7800F electron microscope coupled with an energy dispersive spectroscope. The composition of the gaseous products was analyzed by gas chromatography (Agilent
Apparatus and procedure Table 1 e Reagents involved in this paper. A high-throughput batch reactor system with 6 subsystems was used for the synthesis and gasification processes. The reactor was fabricated from Inconel 625 and designed with a maximum temperature and pressure of 750 C and 30 MPa, respectively. A subsystem consists of batch reactor, electric furnace, and data acquisition station of temperature and pressure. The electric furnace operates at a certain temperature for experiment with a temperature controller. The DAS can monitor and record the temperature and pressure inside
Chemicals Furfural Zinc acetate Nickel acetate Copper acetate Cobalt acetate
Company
Content no less than
SigmaeAldrich Tianjin Shengao Reagents Tianjin Yongsheng Fine Chemicals Tianjin Shengmiao Fine Chemicals Tianjin Fuchen Chemicals
98.0% 99.0% 98.0% 99.0% 99.5%
Please cite this article in press as: Jin H, et al., Supercritical water synthesis of bimetallic catalyst and its application in hydrogen production by furfural gasification in supercritical water, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.10.096
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e8
7890A) with thermal conductivity detectors (TCDs). The highpurity argon was used as the carrier gas. CE, GE and HE are used to describe the gasification process with the equation shown in nomenclature and from the point of view of carbon, mass, hydrogen conversion progress respectively.
Results and discussion Based on our previous work, different metal elements performed certain effects. In particular, Ni showed great stability in crystalline phase, and Zn showed better performance in the selectivity of hydrogen. Since each element had its own advantage, the combination of the bimetallic catalyst may hold the advantages together. In order to identify this hypothesis, different acetic salts are uniformly mixed to be loaded on TiO2 particles, which was famous for its hydrothermal stability in supercritical water [40,41]. Through three steps, metal elements can be dispersed on TiO2 particles in the form of oxides. For quantification, mass of metal in acetic salt was 20 wt% of TiO2. Then we conducted XRD analysis to figure out whether the two metal elements were well dispersed on TiO2 particles. Generally, four combinations were selected, Cu þ Zn, Cu þ Ni, Co þ Ni, and Zn þ Ni. Each combination at least has one of these two elements considering the advantages of Zn and Ni. Fig. 1 showed the XRD analysis of the catalyst after preparation. Generally, anion was always titan acid. Cation consists of metal elements such as NiZnTiO4 and CuZnTiO4. However, as for combination Co þ Ni, Co and Ni were found in two forms of titanates, Co2TiO4 and NiTiO3. There was even NiO in the combination Cu þ Ni, along with CuNi.5Ti.5O2, which meant that a single uniform crystalline phase cannot be generated in this combination. After catalyst preparation, we conducted gasification experiments with each kind of catalyst. Mass of furfural and deionized water were 0.5 g and 5 g, respectively. Mass of TiO2 was 0.25 g and residence time is 20 min. After filtration and drying, residues were investigated by multiple characterization methods. As shown in Fig. 2, for combination Co þ Ni, Co and Ni were still in two different crystalline phases: Co2TiO4, Ni3TiO5. It can be seen that compared with Fig. 1, the crystalline phase for the experimental situation of Ni þ Co, NiTiO3 transmitted Ni3TiO5. Probably because in the supercritical water process, NiTiO3 decomposed to Ni3TiO5 and TiO2 under the catalytic effect of Co. Moreover, as the experimental situation of Cu þ Ni, the products were also in two different crystalline phases: NiO, CuNi.5Ti.5O2. The crystalline phase for Zn þ Ni and Cu þ Zn was NiZnTiO4 and ZuCuTiO4 respectively. Table 2 showed all the phase transitions in every reaction condition. All other combinations containing element Ni show obvious stability except for the combination Co þ Ni, which verified our previous work that Ni had better stability, and it can be successfully reserved in two metal elements combination. Surprisingly, for combination Cu þ Zn which did not contain elements Ni, stability in crystalline phase was also obtained. As a result, the combination of Cu þ Zn provides a new direction that the combination of two different elements can show a new capability which these two elements do not have before.
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For the microstructure of synthesized catalyst, SEM and EDS analysis was used to reveal the surface condition from the micro perspective. Fig. 3 showed the observation area under the magnification of 100,000 times. It can be seen that polyhedral catalyst particles by supercritical water synthesis performed an excellent dispersity. The particle diameters in Cu þ Ni experimental situation were about 30 nm. The diameters for the rest of the situation were about 20 nm. Probably the crystal growth ability of NiO or CuNi.5Ti.5O2 was stronger than that in the other situation. Fig. 3 illustrates that there was a combination of two metal elements, and a uniform nano-structure can be also formed in spite of different crystalline phases. Take Fig. 3 (a) for example, energy spectrum analysis is listed in Table 3. As can be seen in Table 3, since TiO2 was the carrier, element Ti and O took the most positions on the surface of the catalyst.
Fig. 1 e XRD plot of two metal elements loaded on TiO2 particles in SCW.
Fig. 2 e XRD plot of different metal elements loaded on TiO2 particles after reaction in SCW.
Please cite this article in press as: Jin H, et al., Supercritical water synthesis of bimetallic catalyst and its application in hydrogen production by furfural gasification in supercritical water, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.10.096
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Table 2 e Differences of metal elements between before and after reaction.
Table 3 e EDS analysis results of catalyst surface (Zn þ Ni loaded on TiO2, 20 wt%, 400 C).
Combination
Before reaction
After reaction
Element
Zn þ Ni Co þ Ni Cu þ Ni Cu þ Zn
NiZnTiO4 Co2TiO4, NiTiO3 NiO, CuNi.5Ti.5O2 ZuCuTiO4
NiZnTiO4 Co2TiO4, Ni3TiO5 NiO, CuNi.5Ti.5O2 ZuCuTiO4
O Ti Ni Zn
Meanwhile, element Ni and Zn were successfully loaded on TiO2 particles uniformly. It showed that hydrothermal method using TiO2 particles and mixed metal salt water solutions is a promising catalyst preparation method and turned out to be a very convincing result with a combination of two different metal elements to integrate their separate properties. Catalyst particles were collected for SEM analysis in order to understand the surface topography of the catalysts after reaction in SCW. Fig. 4 showed SEM images of these particles for each combination, and the magnification was 5000 times. Overall, a number of spherical particles were generated on the surface of catalysts. EDS analysis was carried out to find out that these particles were mainly coke. The reasons for the formation of the coke particles was analysed as following. The solubility of the precursor of char was relative high, therefore the precursor was uniformly distributed in the supercritical water. After reaction, supercritical water was cooled to be in liquid phase and the solubility of the precursor of char was vastly dropped so that the char precursor might grow up its
Mass percentage (%)
Atom percentage (%)
47.05 42.69 3.57 6.69
73.61 22.31 1.52 2.56
size around a nano-bimetallic catalyst. Due to the surface tension of the char precursor, spherical particles were formed [42,43]. This phenomenon was not found when using K2CO3 as a homogeneous catalyst in supercritical water. Fig. 5 showed the gasification results of furfural with and without different metal element combination catalysts. In Fig. 5(a), the fraction of H2 in gaseous products was only 6.28% without any catalyst. Totally, by comparison, every combination increases the fraction of H2. When combinations were Cu þ Zn, Co þ Ni, Cu þ Ni and Zn þ Ni, the fraction of H2 were 19.56%, 13.80%, 12.33% and 18.63% respectively. It seemed that the combinations contain Zn element maintained the advantage of single Zn because combinations of Cu þ Zn and Zn þ Ni had a relatively higher fraction of H2. This result again proved the rationality of the combination of two different metal elements. When there was no catalyst added, the gas products appeared to contain no CH4. All catalysts can help produce CH4 even though the ratio is only approximately 5%, which does not have obvious difference with single element catalysts [44]. At the same time, all combinations reduced the fraction of CO2
Fig. 3 e SEM images of different combinations loaded on TiO2 particles in SCW. (a) Zn þ Ni, 20 wt%; (b) Co þ Ni, 20 wt%; (c) Cu þ Ni, 20 wt%; (d) Cu þ Zn, 20 wt%. Please cite this article in press as: Jin H, et al., Supercritical water synthesis of bimetallic catalyst and its application in hydrogen production by furfural gasification in supercritical water, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.10.096
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Fig. 4 e SEM images of metal element combination loaded on TiO2 particles after reaction in SCW: (a) Zn þ Ni, 20 wt%; (b) Co þ Ni, 20 wt%; (c) Cu þ Ni, 20 wt%; (d) Cu þ Zn, 20 wt%.
Fig. 5 e Gasification results of furfural with different metal element combination catalysts (furfural 10 wt%, residence time 20 min).
from 82.29% to around 70% when there was no catalyst. It can be observed from Fig. 5(b) that HE was 4.26% with catalyst, and the combination of Cu þ Zn and Zn þ Ni bimetallic catalyst increased HE to 15.64% and 14.06% respectively. As for YH2, Cu þ Ni had the most appealing result which was 18.92 mol/kg, which was better that the result with only Zn in our previous work. In general, combinations of catalyst performed advantages of two elements and obtained a synergy effect. To make a comparison with conventional homogeneous catalyst, K2CO3 was used to catalyze gasification of furfural. Fig. 6 showed that when the temperature increased from 200 C to 400 C, the ratio of H2 increased from 1.98% to 17.98%.
And when the temperature was above the critical point, the ratio of CO and CO2 decreased from 1.31% and 83.12% to 0.61% and 72.85% respectively. As a result, the parameter above the critical point was proved to be necessary for good gasification results. Meanwhile, K2CO3 can increase the fraction of H2 from 6.34% to 17.98% and decrease the CO fraction from 23.34% to 0.61%. It can be explained by Eq. (1) that K2CO3 accelerated the subsequent reaction. However, as many other homogeneous catalysts, K2CO3 was difficult to be recycled, because when the operating temperature and pressure deceased below the critical point of water, water can dissolve K2CO3 with other complicated solutes.
Please cite this article in press as: Jin H, et al., Supercritical water synthesis of bimetallic catalyst and its application in hydrogen production by furfural gasification in supercritical water, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.10.096
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Fig. 6 e Gasification results of furfural under different temperature with K2CO3 catalyst (furfural 10 wt%, residence time 20 min, ratio of K2CO3 and furfural is 0.5:1).
Fig. 7 e SEM images of solid gasification products after reaction (furfural 10 wt%, residence time 20 min) (a)200 C; (b)250 C; (c)300 C; (d)350 C; (e)400 C. Please cite this article in press as: Jin H, et al., Supercritical water synthesis of bimetallic catalyst and its application in hydrogen production by furfural gasification in supercritical water, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.10.096
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CO þ H2O / CO2 þ H2
(4)
It can be seen from Fig. 7 that solid coke was generated and the coke had no fixed structure instead of spherical structure. Generally the diameter of coke was a few microns as Fig. 7 (e) showed, and the sized of the carbon was larger than that of bimetallic catalyst. Because K2CO3 was easily to grow up to larger particles compared with the well dispersed nano bimetallic particles.
Conclusions The catalytic effect of bimetallic catalyst prepared by supercritical water synthesis was investigated in the process of supercritical water gasification of furfural, and different metal element combinations were investigated as Cu þ Zn, Co þ Ni, Cu þ Ni and Zn þ Ni. Based on the results of XRD analysis, Zn þ Ni and Cu þ Zn had better crystal stability, and two elements were combined as cation with titanate as anion. SEM images revealed that two different elements can be loaded on TiO2 particles uniformly and the diameter was around 30 nm. From gasification results, Cu þ Zn and Ni þ Zn showed yield gas with higher hydrogen fraction, which was 19.56% and 18.63%, respectively. Co þ Ni had the highest CE, which was 18.92 mol/kg. Generally speaking, combinations of two different metal elements certainly integrated the advantages of each element, and combinations showed superiority over single metal catalysts for gasification of biomass in SCW.
Acknowledgement This work was financially supported by the National Natural Science Foundation of China (Contract nos. 51306145 and 51236007) and the China National Key Research and Development Plan Project (Contract no. 2016YFB0600100).
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Please cite this article in press as: Jin H, et al., Supercritical water synthesis of bimetallic catalyst and its application in hydrogen production by furfural gasification in supercritical water, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.10.096