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AC bimetal for gasification in supercritical water
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High performance noble-metal-free NiCo/AC bimetal for gasification in supercritical water Qingqing Guan a, Shanshuai Chen a, Yuan Chen b, Junjie Gu a, Bin Li a, Rongrong Miao a, Qiuling Chen a, Ping Ning a,* a
Collaborative Innovation Center of Western Typical Industry Environmental Pollution Control, Faculty of Environmental Science and Engineering, Kunming University of Science and Technology, Kunming 650500, China b Pacific Northwest National Laboratory, Richland, WA 99352, USA
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abstract
Article history:
We report high performance Noble-metal-free NiCo/AC bimetal for gasification in super-
Received 11 October 2016
critical water by the polyol reduction method with chloroplatinic acid as nucleation seed.
Received in revised form
At 475 C and 20 min, the carbon efficiency (CE) for gasification of phenol with NiCo/AC has
28 November 2016
reached about 65%, which represents a more than 20-fold enhancement of carbon effi-
Accepted 29 November 2016
ciency compared to the Co/AC and 6-fold compared to the Ni/AC. The CE of commercial
Available online xxx
noble Ru/C (5 wt%) catalyst is only about 15% higher than that of NiCo/AC catalyst. The catalyst was characterized by BET, XRD, XPS and TEM. The results indicate smaller and
Keywords:
higher disperse of NiCo alloy has been achieved. By using NiCo/AC catalyst, the CE can
Phenol
reach about 90% at 525 C and 60 min with a 5 wt% phenol, 0.098 kg/m3 density and catalyst
Gasification
loading of 0.5 g/g (the mass of NiCo/AC catalyst/the mass of phenol). The results of GCeMS
Supercritical water
indicate NiCo/AC can suppress tar successfully. A kinetic modeling was also proposed to
NiCo/AC
describe gaseous product and tar formation, which gives the activation energy (Ea) 123.56 ± 31 kJ/mol and the frequency factor 16.93 ± 0.48 for gaseous products. While for tar formation, the Ea is 112.95 ± 24 kJ/mol and the frequency factor is 13.99 ± 0.37. The successful use of noble-metal-free NiCo/AC shows the potential to replace noble Ru-catalysts in SCWG process for gases fuels by bimetal. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Gasification of biomass in supercritical water (SCW) has attracted increasing attention as an efficient technology for energy production which could help to reduce dependence on fossil fuels [1e3]. It has been proved that supercritical water (Tc 647 K, Pc 22.1 MPa) gasification (SCWG) is potential method for gasification [4e8]. In SCW condition reaction, water serves as both the solvent and reactant [9], since water
is miscible with the organic compounds to form a homogeneous fluid phase and takes part in steam reforming reactions. Comparing with other method such as direct gasification technologies and conventional pyrolysis, which usually require pretreatment to remove the extra water contained in the feedstock, SCW process can capture and use the energy invested in reaching the supercritical state [10]. From the thermodynamic perspective, liquefaction and gasification of wet feedstock such as algae and lignite in SCW is extraordinarily desirable.
* Corresponding author. E-mail address: [email protected] (P. Ning). http://dx.doi.org/10.1016/j.ijhydene.2016.11.191 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Guan Q, et al., High performance noble-metal-free NiCo/AC bimetal for gasification in supercritical water, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.191
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Uncatalyzed SCWG leads to a low gasification efficiency at lower temperatures (usually <600 C) and formation of tar [11]. The potential approach to achieve efficient gasification at a lower temperature and suppress the formation of tar is catalyzed gasification. There has been studied many catalysts including Co, Ni, Mo, Ru and so on [12]. Noble metal Ru is an efficient catalyst for SCWG [13e15]. Chen et al. [16] studied that lignite have been effectively gasified in supercritical water using Ru/CeO2eZrO2. Ni-catalyst can also clearly increase efficiency of SCWG [17]. However, the gasification efficiency is much lower than Ru. Additionally, the Ni can't serve as effective catalyst to suppress the formation of Tar. Meanwhile, the price of Ru-based catalyst usually is over 10 times than that of Ni-catalyst. Improvement of utilization of Ru on numerous carriers and efficiency of Ni-catalysts on different carriers are the common way to overcome the dilemma. However, no dramatic breakout was reported in recent years. An alternative concept is that using noble-metal-free bimetal for gasification. For instance, Damyanova et al. [18] reported NiCo/Al2O3 catalysts for oxidative steam reforming of ethanol that the second metal Co was added to monometallic Ni catalysts, which exhibited higher stability and activity than monometallic Ni catalyst. For gasification, Li et al. [19] also reported hydrogen production by glucose gasification in supercritical water with bimetallic Ni-M/Al2O3 catalysts (M ¼ Cu, Co and Sn), but only 20e30% increase of gasification efficiency can be found in an autoclave reactor at 400 C. However, Otomo et al. [20] have been studied that the complete gasification of glucose in SCW using ruthenium is accomplished at 400 C. To our best knowledge, no high performance noble-metal-free bimetal for gasification in supercritical water has been found until now. In those cases, NieCo alloy catalysts were prepared by the co-precipitation and impregnation methods [19,21]. It is very challenging to achieve homogeneous dispersion for effective alloy. To find efficient noble-metal-free bimetal for gasification in supercritical water, we screened Ni-M (M ¼ Fe, Zn, Cu, Co and Sn) by the polyol reduction method with chloroplatinic acid, which was used as the heterogeneous nucleation seed for bimetal. Herein, we reported a high performance NiCo/AC catalyst for gasification in supercritical water. We used phenol as a model of biomass, which is the basic unit of lignin and also typical aromatic pollutants in industrial wastewater [22,23]. In this study, we have tested NiCo/AC catalyst using for gasification of phenol in SCW at the temperature of 475, 500 and 525 C. The physico-chemical and structural properties of the catalyst are analysed by BET, XRD, TEM and XPS. Finally, to further study the SCWG of phenol, we also presented a kinetic model. The successful use of Noblemetal-free NiCo/AC bimetal for gasification shows a new approach of SCWG for gases fuels.
Experiments Pretreatment of activated carbon Activated Carbon (AC, Sinoplatinium and LXHG China) was pretreated in order to get rid of the metallic component. Briefly: a 100 ml 10% concentrated HNO3 was added into 20 g of
AC refluxed for 3 h at 80 C in a 150 ml beaker. After placing to room temperature, the mixture was washed and filtered with ultrapure water until the water filter out was neutral. Finally, we dried the pretreated AC at 100 C for 10 h and placed for further use.
Catalyst preparation NiCo/AC, Ni/AC, Co/AC catalysts were prepared by the polyol reduction method with chloroplatinic acid, which was used as the heterogeneous nucleation seed for bimetal. Ethylene glycol was used as solvent and reducing agent. The detail process is as follows: 200 ml of ethylene glycol and 1.36 g of PVP (Polyvinyl Pyrrolidone) were mixed in a 500 ml beaker. After the completely cleanout of the surfactant, 0.5 g of the pretreated AC, 2.4 g of sodium hydroxide, and the metal precursors i.e., Ni(NO3)2$6H2O and Co(NO3)2$6H2O with a given weight ratio were added into the mixture with magnetic stirring. After the complete dissolution of the precursors, we introduced 250 ml of 8 wt% chloroplatinic acid solution as the heterogeneous nucleation seed in the mixture above. The mixture was placed into an oil bath at 180 C and reduced for 4 h with continuous magnetic stirring. Finally, the mixture was washed and filtered with 4 l of water and 500 ml of ethanol. Then we dried catalyst at 100 C for 10 h. The catalyst composition is shown as “20 wt%Nie0.67Co/ AC”, where 20 represents the weight percentage of active metal (Ni þ Co) in the catalyst and 0.67 is the mass ratio of Co/Ni.
SCWG of phenol The experimental procedure and equipment operation were reported in the previous work [4,24e26]. In the test, the catalyst of 0.0098 g (50 wt% of phenol), phenol of 0.0196 g and ultrapure water of 392 ml were added into a 4 ml reactor made by swagelok. Then we eliminated air of the reactor by charging with helium gas and venting the mixed air three times. After that 200 kPa (gauge) helium gas was filled to the reactor (helium served as an internal standard for gas product analysis). Finally, the reactor was placed into A Techne fluidized sand bath (model SBL-2) and kept temperatures with a Techne TC8D temperature controller. After reaction, the reactor was took out of the sand bath and placed to room temperature.
Catalysts characterization BET surface area measurements were carried out on Quantachrome Instruments. The catalysts were degassed for 3 h at 300 C, and the N2 adsorption-isotherms of the catalysts were tested in liquid N2 (196 C). Then the pore size distribution was obtained from the description branch by the HorvatheKawazoe (HK) method. The X-ray diffraction (XRD) analysis of the catalyst was determined on a Bruker D8 Advance 19 diffractometer system with 40 kV an operating voltage and a 40 mA current, using a graphite monochromator and Cu Ka radiation (1.5406 Å). The catalyst was investigated at 6 min1 in a range of 5e90 . High Resolution Transmission Electron Microscopy (HRTEM) of the NiCo/AC, Ni/AC, Co/AC catalysts was carried out with an FEITecnai G2 F20 transmission electron microscope.
Please cite this article in press as: Guan Q, et al., High performance noble-metal-free NiCo/AC bimetal for gasification in supercritical water, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.191
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X-ray photoelectron spectrum (XPS) data were carried out with a ULVAC PHI 5000 Versa Probe-II instrument.
Sample analysis The gaseous products (e.g.CH4, H2, CO2, C2H4, C2H6 and CO) were analyzed with an Agilent Technologies model 7820 A gas chromatograph (GC) equipped with a thermal conductivity detector (GC-TCD) and a 15 ft 1/8 in. i.d. Stainless steel column, packed with 60 80 mesh Carboxen 1000 (Supelco). The carrier gas was Argon gas. The liquid of the product were wash out three times with dichloromethane and then analyzed by a GCeMS (PE SQ 8T-680) equipped with an Elite-5MS capillary column.
Evaluation of efficiency In this paper, carbon efficiency (%) and gas yield (mmol/g) are used to evaluate the gasification of phenol. The expressions are defined as follows: carbon efficiency ¼
the mass of carbon in gasous product the mass of carbon in phenol 100; %
gas yield ¼
the number of moles of produced gas the mass of phenol 100; mmol=g
Results and discussion Catalyst characterization The N2 absorptionedesorption isotherm of AC, Ni/AC, Co/AC and NiCo/AC catalysts and pore size distributions are given in Figs. 1 and 2. Fig. 1 displays the type of I which is belong to the characteristic curve for microporosity materials. Table 1 shows that AC support has a very high surface area (1045.31 m2/g), which was decreased by the loading of metallic Ni or Co. The BET surface area of NiCo/AC is 685.142 m2/g and
Fig. 1 e The N2 adsorptionedesorption isotherm of the catalyst (a) AC, (b) NiCo/AC, (c) Co/AC and (d) Ni/AC.
Fig. 2 e H.K. pore size distributions obtained from adsorption branches (a) AC, (b) NiCo/AC, (c) Co/AC and (d) Ni/AC.
Table 1 e BET surface area, pore volume and pore diameter. Sample
SBET (m2/g)
Pv (cm3/g)
Pd (nm)
AC Ni/AC Co/AC NiCo/AC
1045.31 576.093 579.652 685.142
0.47 0.23 0.23 0.21
0.9241 1.0608 1.2481 1.0793
the pore volume of NiCo/AC is 0.41 cm3/g. Compared with the AC catalyst, the BET surface area was reduced by ~34.45% and the pore volume obviously was reduced by ~34.14%. The results indicate that the pore volume and the BET surface area apparently decreased as bimetallic NiCo was introduced in AC. Because of the occupation of the pore space by bimetallic NiCo. The average pore diameter of Ni/AC, Co/AC and NiCo/AC is slightly changed than AC catalyst, as shown in Table 1. Fig. 3 shows the XRD patterns of the Ni/AC, NiCo/AC catalysts. The figure indicates that AC support is amorphous structure. The XRD patterns of the Ni/AC catalysts shows 2q values of diffraction peaks at 44.35, 51.67, 76.1, which match to
Fig. 3 e XRD patterns of (a) Ni/AC, (b) NiCo/AC catalyst.
Please cite this article in press as: Guan Q, et al., High performance noble-metal-free NiCo/AC bimetal for gasification in supercritical water, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.191
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metallic Ni planes (1 1 1), (2 0 0) and (2 2 0), respectively [27]. The addition of cobalt into metallic Ni causes that the intensity of XRD peaks is weaken, indicating that the crystallinity of Ni particles in NiCo/AC catalysts is also weaken [18] and it has prevented the growth of Ni particles [28]. Fig. 4 shows the XPS spectra of the element of O1s, Ni2p and Co2p in NiCo/AC catalyst. The peak convolution for O1s spectrum indicates two O-based diffraction species, including
CoO at 530.73 eV and NiO at 531.92 eV [29]. For the spectra of Ni2p3/2 at 855.08 eV can be corresponded to Ni and at 860.9 eV belong to shake-up peak of the Ni2þ [30]. The Co2p3/2 XPS spectra were also recorded, the peaks at 772.18 and 780.66 eV are corresponded to metallic Co and Co2þ [29,30]. The results confirm the CoO and Co do exist over the sample. Fig. 5 displays the morphology of the Ni/AC, NiCo/AC catalysts. In accordance with the XRD measurements, by using chloroplatinic acid, the Co was added into monometallic Ni catalysts successfully, leading clearly that the crystallinity NiCo catalysts become smaller and higher disperse in the AC supporter [19]. The results confirm that as chloroplatinic acid was used as the heterogeneous nucleation seed, homogeneous dispersion was enhanced to form homogeneous alloy catalyst.
Catalytic activity
Fig. 4 e XPS profiles of NiCo/AC.
In this section, we firstly compared NiCo/AC catalyst with Ni/ AC, Co/AC and commercial Ru/C (5 wt % Ru on carbon from SigmaeAldrich) catalysts. Fig. 6 displays the carbon efficiency and gas yields at the temperature of 475 C and 20 min. At the same conditions, the carbon efficiency of NiCo/AC has reached about 65%. Meanwhile, the carbon efficiencies of Co/ AC, Ni/AC are about 3% and 12% respectively. This represents a more than 20-fold enhancement of carbon efficiency compared to the Co/AC and 6-fold compared to the Ni/AC. It is worth noting that the carbon efficiency of commercial noble Ru/C catalyst is only about 15% higher than that of NiCo/AC catalyst. The gaseous yields of H2 of NiCo/AC and Ru/C are 30 and 11 mmol/g respectively, indicating that NiCo/AC shows high selectivity for hydrogen gas. In all, the noble-metal-free NiCo/AC bimetal shows the potential to replace noble Rucatalysts in SCWG process for gases fuels. Gasification of phenol in SCW was carried out at the temperature of 475e525 C with a 5 wt% phenol, 0.098 kg/m3 water density and 0.5 g NiCo/AC/g phenol catalyst. Fig. 7 displays the gaseous yields of H2, CH4 and CO2 only since other gaseous components, including C2H4, C2H6 and CO were difficultly detected. The gaseous yields of CH4 increased with the increase of time. CO2 was produced rapidly at initial 20 min and then kept stable. H2 reached the highest yield of about 29.9 mmol/g at 475 C and 20 min and 41 mmol/g at 500 C and 20 min and then reduced slowly, some indicating NiCo/AC can enhance methanation reaction. Huelsman et al. [31] reported non-catalytic gasification of phenol in SCW, their results indicate non-catalytic gasification leads dramatic formation of tar, which includes polycyclic aromatic hydrocarbons (PAHs) and dibenzofuran. To further understand the gasification of phenol with NiCo/AC, the reaction samples at 500 C and 30 min were extracted with dichloromethane and were detected by GCeMS. Fig. 8 shows the identities of the intermediate organic species. It should be noted that the GCeMS run about 3 h, but clear peaks can be found only between 10 and 22 min. Fig. 8 displays that 2methyl-phenol and 2-Vinylfuran are the main intermediates but the peaks are small. Thus, it is clear that the NiCo/AC catalyst can improve gasification of phenol in SCW and suppress the formation of PAHs and dimers.
Please cite this article in press as: Guan Q, et al., High performance noble-metal-free NiCo/AC bimetal for gasification in supercritical water, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.191
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Fig. 5 e TEM images of NiCo/AC (a, c) and Ni/AC (b, d).
Fig. 6 e Effect of different catalysts on carbon efficiency at 475 C and 20 min.
Kinetic model
Fig. 7 e The effect of batch holding time on yields of gaseous products (5 wt% phenol, 0.098 kg/m3 and 0.5 g/g NiCo/AC) at 475, 500 and 525 C.
As shown in Fig. 9, the NiCo/AC catalyst can significantly improve the carbon efficiency of phenol at 475, 500 and 525 C. The carbon efficiency can reach 79% at 475 C and 60 min. Rapid gasification of phenol happens within the initial 30 min. The carbon efficiency is close to 80% after 20 min at 525 C. For phenol SCWG reactions, the tar in the product is very difficult for gasification. Under the circumstances, linear
regression can't describe for phenol SCWG. There are two pathways for carbon concentration consumed, i.e., the formation of gaseous products from carbon gasification and coke formation to become tar. Such a power rate law for the carbon gasification of phenol would take the form of following Eq. (1):
Please cite this article in press as: Guan Q, et al., High performance noble-metal-free NiCo/AC bimetal for gasification in supercritical water, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.191
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Table 2 e Rate constants and Arrhenius parameters for the gasification of phenol in supercritical water over NiCo/AC catalyst. 475 C 500 C 525 C 1
K1 (min ) K2 (min1)
0.05364 0.01545
0.09928 0.02838
0.1864 0.04819
lnA
Ea (kJ/mol)
16.93 ± 0.48 13.99 ± 0.37
123.56 ± 31 112.95 ± 24
Fig. 8 e Chromatogram of liquid phase obtained from SCWG of phenol at 500 C for 30 min.
Fig. 10 e Arrhenius plot of reaction rate constants. Arrhenius plots for catalytic phenol SCWG over NiCo/AC catalyst. The k values increase with the temperature increases. Arrhenius equation as following: ln k ¼ ln A
Fig. 9 e The comparison between model and experimental results for carbon efficiency from SCWG of phenol with NiCo/AC.
dCphenol ¼ r1 þ r2 dt
Ea RT
where A is frequency factor and Ea is activation energy, respectively. As shown in Table 2. For k1, the Ea and the frequency factor are 123.56 ± 31 kJ/mol and 16.93 ± 0.48, respectively. While for k2, the Ea is 112.95 ± 24 kJ/mol and frequency factor is 13.99 ± 0.37. Fig. 11 displays the compared of the experimental carbon efficiency values and the model. The results show
(1)
r1 is the reaction rate for formation of gaseous products, and r2 is the reaction rate for formation of tar. The rate equation is supposed to be one-step reaction in phenol concentration (Cphenol). Formation of gaseous products is following Eq. (2): r1 ¼ k1½C ¼ A1exp
Ea1 jCj RTjKj
(2)
and formation of tar is following Eq. (3): r2 ¼ k2½C ¼ A2exp
Ea2 jCj RTjKj
(3)
where r, k, [C], A and Ea are the reaction rate, reaction rate constant, concentration of carbon, the frequency factor and activation energy, respectively. As also is shown in Fig. 9, we use Berkeley Madonna to fit between the experimental data to the model. Table 2 shows the k values at different temperature and Fig. 10 displays the
Fig. 11 e The comparison between model and experimental carbon efficiency values.
Please cite this article in press as: Guan Q, et al., High performance noble-metal-free NiCo/AC bimetal for gasification in supercritical water, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.191
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experiment data accord with the model, with an error value of about 0.05. The activation energy of gaseous produce formation is 123.56 ± 31 kJ/mol, compared with 112.95 ± 24 kJ/mol with tar formation, indicating that the decompose of phenol transform gaseous products is difficult than the formation of tar. Compared with that Huelsman et al. have been reported that the activation energy is 280 kJ/mol for phenol SCWG without catalyst [31], it is clear that the activation energy has been greatly reduced by using NiCo/AC catalyst and is even close to 84.24 ± 22 kJ/mol by Ru/CeO2 [4]. Thus, the NiCo/AC catalyst exhibits superior performance for gasification.
Conclusions A high performance Noble-metal-free NiCo/AC bimetal for gasification in supercritical water was prepared successfully by the polyol reduction method with chloroplatinic acid as nucleation seed. The catalyst was characterized by BET, XRD, XPS and TEM. The results indicate that the Co is added into monometallic Ni catalysts successfully, leading clearly that the crystallinity NiCo catalysts become smaller and higher disperse in the AC supporter. At 475 C and 20 min, the carbon efficiency for gasification of phenol with NiCo/AC has reached about 65%, which represents a more than 20-fold enhancement of carbon efficiency compared to the Co/AC and 6-fold compared to the Ni/AC. It is worth noting that the carbon efficiency of commercial noble Ru/C catalyst is only about 15% higher than that of NiCo/AC catalyst. The gaseous yields of H2 of NiCo/AC and Ru/C are 30 and 11 mmol/g respectively, indicating that NiCo/AC shows high selectivity for hydrogen gas. At 525 C and 60 min with a 5 wt% phenol, 0.098 kg/m3 density and 0.5 g NiCo/AC/g phenol catalyst, the carbon efficiency can reach about 90%. NiCo/AC can also suppress tar successfully. A kinetic modeling was proposed to describe gaseous product and tar formation, which gives the activation energy (Ea) 123.56 ± 31 kJ/mol and the frequency factor 16.93 ± 0.48 for gaseous products. While for tar formation, the Ea is 112.95 ± 24 kJ/mol and the frequency factor is 13.99 ± 0.37. In all, the noble-metal-free NiCo/AC bimetal shows the potential to replace noble Ru-catalysts in SCWG process for gases fuels.
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Acknowledgements This work is supported by the National Natural Science Foundation of China (21307049 and U1201234), the High Technology Talent Introduction Project of Yunnan in China (2010CI110) and Collaborative Innovation Center of Western Typical Industry Environmental Pollution Control.
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High performance noble-metal-free NiCo/AC bimetal for gasification in supercritical water Qingqing Guan a, Shanshuai Chen a, Yuan Chen b, Junjie Gu a, Bin Li a, Rongrong Miao a, Qiuling Chen a, Ping Ning a,* a
Collaborative Innovation Center of Western Typical Industry Environmental Pollution Control, Faculty of Environmental Science and Engineering, Kunming University of Science and Technology, Kunming 650500, China b Pacific Northwest National Laboratory, Richland, WA 99352, USA
article info
abstract
Article history:
We report high performance Noble-metal-free NiCo/AC bimetal for gasification in super-
Received 11 October 2016
critical water by the polyol reduction method with chloroplatinic acid as nucleation seed.
Received in revised form
At 475 C and 20 min, the carbon efficiency (CE) for gasification of phenol with NiCo/AC has
28 November 2016
reached about 65%, which represents a more than 20-fold enhancement of carbon effi-
Accepted 29 November 2016
ciency compared to the Co/AC and 6-fold compared to the Ni/AC. The CE of commercial
Available online xxx
noble Ru/C (5 wt%) catalyst is only about 15% higher than that of NiCo/AC catalyst. The catalyst was characterized by BET, XRD, XPS and TEM. The results indicate smaller and
Keywords:
higher disperse of NiCo alloy has been achieved. By using NiCo/AC catalyst, the CE can
Phenol
reach about 90% at 525 C and 60 min with a 5 wt% phenol, 0.098 kg/m3 density and catalyst
Gasification
loading of 0.5 g/g (the mass of NiCo/AC catalyst/the mass of phenol). The results of GCeMS
Supercritical water
indicate NiCo/AC can suppress tar successfully. A kinetic modeling was also proposed to
NiCo/AC
describe gaseous product and tar formation, which gives the activation energy (Ea) 123.56 ± 31 kJ/mol and the frequency factor 16.93 ± 0.48 for gaseous products. While for tar formation, the Ea is 112.95 ± 24 kJ/mol and the frequency factor is 13.99 ± 0.37. The successful use of noble-metal-free NiCo/AC shows the potential to replace noble Ru-catalysts in SCWG process for gases fuels by bimetal. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Gasification of biomass in supercritical water (SCW) has attracted increasing attention as an efficient technology for energy production which could help to reduce dependence on fossil fuels [1e3]. It has been proved that supercritical water (Tc 647 K, Pc 22.1 MPa) gasification (SCWG) is potential method for gasification [4e8]. In SCW condition reaction, water serves as both the solvent and reactant [9], since water
is miscible with the organic compounds to form a homogeneous fluid phase and takes part in steam reforming reactions. Comparing with other method such as direct gasification technologies and conventional pyrolysis, which usually require pretreatment to remove the extra water contained in the feedstock, SCW process can capture and use the energy invested in reaching the supercritical state [10]. From the thermodynamic perspective, liquefaction and gasification of wet feedstock such as algae and lignite in SCW is extraordinarily desirable.
* Corresponding author. E-mail address: [email protected] (P. Ning). http://dx.doi.org/10.1016/j.ijhydene.2016.11.191 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Guan Q, et al., High performance noble-metal-free NiCo/AC bimetal for gasification in supercritical water, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.191
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Uncatalyzed SCWG leads to a low gasification efficiency at lower temperatures (usually <600 C) and formation of tar [11]. The potential approach to achieve efficient gasification at a lower temperature and suppress the formation of tar is catalyzed gasification. There has been studied many catalysts including Co, Ni, Mo, Ru and so on [12]. Noble metal Ru is an efficient catalyst for SCWG [13e15]. Chen et al. [16] studied that lignite have been effectively gasified in supercritical water using Ru/CeO2eZrO2. Ni-catalyst can also clearly increase efficiency of SCWG [17]. However, the gasification efficiency is much lower than Ru. Additionally, the Ni can't serve as effective catalyst to suppress the formation of Tar. Meanwhile, the price of Ru-based catalyst usually is over 10 times than that of Ni-catalyst. Improvement of utilization of Ru on numerous carriers and efficiency of Ni-catalysts on different carriers are the common way to overcome the dilemma. However, no dramatic breakout was reported in recent years. An alternative concept is that using noble-metal-free bimetal for gasification. For instance, Damyanova et al. [18] reported NiCo/Al2O3 catalysts for oxidative steam reforming of ethanol that the second metal Co was added to monometallic Ni catalysts, which exhibited higher stability and activity than monometallic Ni catalyst. For gasification, Li et al. [19] also reported hydrogen production by glucose gasification in supercritical water with bimetallic Ni-M/Al2O3 catalysts (M ¼ Cu, Co and Sn), but only 20e30% increase of gasification efficiency can be found in an autoclave reactor at 400 C. However, Otomo et al. [20] have been studied that the complete gasification of glucose in SCW using ruthenium is accomplished at 400 C. To our best knowledge, no high performance noble-metal-free bimetal for gasification in supercritical water has been found until now. In those cases, NieCo alloy catalysts were prepared by the co-precipitation and impregnation methods [19,21]. It is very challenging to achieve homogeneous dispersion for effective alloy. To find efficient noble-metal-free bimetal for gasification in supercritical water, we screened Ni-M (M ¼ Fe, Zn, Cu, Co and Sn) by the polyol reduction method with chloroplatinic acid, which was used as the heterogeneous nucleation seed for bimetal. Herein, we reported a high performance NiCo/AC catalyst for gasification in supercritical water. We used phenol as a model of biomass, which is the basic unit of lignin and also typical aromatic pollutants in industrial wastewater [22,23]. In this study, we have tested NiCo/AC catalyst using for gasification of phenol in SCW at the temperature of 475, 500 and 525 C. The physico-chemical and structural properties of the catalyst are analysed by BET, XRD, TEM and XPS. Finally, to further study the SCWG of phenol, we also presented a kinetic model. The successful use of Noblemetal-free NiCo/AC bimetal for gasification shows a new approach of SCWG for gases fuels.
Experiments Pretreatment of activated carbon Activated Carbon (AC, Sinoplatinium and LXHG China) was pretreated in order to get rid of the metallic component. Briefly: a 100 ml 10% concentrated HNO3 was added into 20 g of
AC refluxed for 3 h at 80 C in a 150 ml beaker. After placing to room temperature, the mixture was washed and filtered with ultrapure water until the water filter out was neutral. Finally, we dried the pretreated AC at 100 C for 10 h and placed for further use.
Catalyst preparation NiCo/AC, Ni/AC, Co/AC catalysts were prepared by the polyol reduction method with chloroplatinic acid, which was used as the heterogeneous nucleation seed for bimetal. Ethylene glycol was used as solvent and reducing agent. The detail process is as follows: 200 ml of ethylene glycol and 1.36 g of PVP (Polyvinyl Pyrrolidone) were mixed in a 500 ml beaker. After the completely cleanout of the surfactant, 0.5 g of the pretreated AC, 2.4 g of sodium hydroxide, and the metal precursors i.e., Ni(NO3)2$6H2O and Co(NO3)2$6H2O with a given weight ratio were added into the mixture with magnetic stirring. After the complete dissolution of the precursors, we introduced 250 ml of 8 wt% chloroplatinic acid solution as the heterogeneous nucleation seed in the mixture above. The mixture was placed into an oil bath at 180 C and reduced for 4 h with continuous magnetic stirring. Finally, the mixture was washed and filtered with 4 l of water and 500 ml of ethanol. Then we dried catalyst at 100 C for 10 h. The catalyst composition is shown as “20 wt%Nie0.67Co/ AC”, where 20 represents the weight percentage of active metal (Ni þ Co) in the catalyst and 0.67 is the mass ratio of Co/Ni.
SCWG of phenol The experimental procedure and equipment operation were reported in the previous work [4,24e26]. In the test, the catalyst of 0.0098 g (50 wt% of phenol), phenol of 0.0196 g and ultrapure water of 392 ml were added into a 4 ml reactor made by swagelok. Then we eliminated air of the reactor by charging with helium gas and venting the mixed air three times. After that 200 kPa (gauge) helium gas was filled to the reactor (helium served as an internal standard for gas product analysis). Finally, the reactor was placed into A Techne fluidized sand bath (model SBL-2) and kept temperatures with a Techne TC8D temperature controller. After reaction, the reactor was took out of the sand bath and placed to room temperature.
Catalysts characterization BET surface area measurements were carried out on Quantachrome Instruments. The catalysts were degassed for 3 h at 300 C, and the N2 adsorption-isotherms of the catalysts were tested in liquid N2 (196 C). Then the pore size distribution was obtained from the description branch by the HorvatheKawazoe (HK) method. The X-ray diffraction (XRD) analysis of the catalyst was determined on a Bruker D8 Advance 19 diffractometer system with 40 kV an operating voltage and a 40 mA current, using a graphite monochromator and Cu Ka radiation (1.5406 Å). The catalyst was investigated at 6 min1 in a range of 5e90 . High Resolution Transmission Electron Microscopy (HRTEM) of the NiCo/AC, Ni/AC, Co/AC catalysts was carried out with an FEITecnai G2 F20 transmission electron microscope.
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X-ray photoelectron spectrum (XPS) data were carried out with a ULVAC PHI 5000 Versa Probe-II instrument.
Sample analysis The gaseous products (e.g.CH4, H2, CO2, C2H4, C2H6 and CO) were analyzed with an Agilent Technologies model 7820 A gas chromatograph (GC) equipped with a thermal conductivity detector (GC-TCD) and a 15 ft 1/8 in. i.d. Stainless steel column, packed with 60 80 mesh Carboxen 1000 (Supelco). The carrier gas was Argon gas. The liquid of the product were wash out three times with dichloromethane and then analyzed by a GCeMS (PE SQ 8T-680) equipped with an Elite-5MS capillary column.
Evaluation of efficiency In this paper, carbon efficiency (%) and gas yield (mmol/g) are used to evaluate the gasification of phenol. The expressions are defined as follows: carbon efficiency ¼
the mass of carbon in gasous product the mass of carbon in phenol 100; %
gas yield ¼
the number of moles of produced gas the mass of phenol 100; mmol=g
Results and discussion Catalyst characterization The N2 absorptionedesorption isotherm of AC, Ni/AC, Co/AC and NiCo/AC catalysts and pore size distributions are given in Figs. 1 and 2. Fig. 1 displays the type of I which is belong to the characteristic curve for microporosity materials. Table 1 shows that AC support has a very high surface area (1045.31 m2/g), which was decreased by the loading of metallic Ni or Co. The BET surface area of NiCo/AC is 685.142 m2/g and
Fig. 1 e The N2 adsorptionedesorption isotherm of the catalyst (a) AC, (b) NiCo/AC, (c) Co/AC and (d) Ni/AC.
Fig. 2 e H.K. pore size distributions obtained from adsorption branches (a) AC, (b) NiCo/AC, (c) Co/AC and (d) Ni/AC.
Table 1 e BET surface area, pore volume and pore diameter. Sample
SBET (m2/g)
Pv (cm3/g)
Pd (nm)
AC Ni/AC Co/AC NiCo/AC
1045.31 576.093 579.652 685.142
0.47 0.23 0.23 0.21
0.9241 1.0608 1.2481 1.0793
the pore volume of NiCo/AC is 0.41 cm3/g. Compared with the AC catalyst, the BET surface area was reduced by ~34.45% and the pore volume obviously was reduced by ~34.14%. The results indicate that the pore volume and the BET surface area apparently decreased as bimetallic NiCo was introduced in AC. Because of the occupation of the pore space by bimetallic NiCo. The average pore diameter of Ni/AC, Co/AC and NiCo/AC is slightly changed than AC catalyst, as shown in Table 1. Fig. 3 shows the XRD patterns of the Ni/AC, NiCo/AC catalysts. The figure indicates that AC support is amorphous structure. The XRD patterns of the Ni/AC catalysts shows 2q values of diffraction peaks at 44.35, 51.67, 76.1, which match to
Fig. 3 e XRD patterns of (a) Ni/AC, (b) NiCo/AC catalyst.
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metallic Ni planes (1 1 1), (2 0 0) and (2 2 0), respectively [27]. The addition of cobalt into metallic Ni causes that the intensity of XRD peaks is weaken, indicating that the crystallinity of Ni particles in NiCo/AC catalysts is also weaken [18] and it has prevented the growth of Ni particles [28]. Fig. 4 shows the XPS spectra of the element of O1s, Ni2p and Co2p in NiCo/AC catalyst. The peak convolution for O1s spectrum indicates two O-based diffraction species, including
CoO at 530.73 eV and NiO at 531.92 eV [29]. For the spectra of Ni2p3/2 at 855.08 eV can be corresponded to Ni and at 860.9 eV belong to shake-up peak of the Ni2þ [30]. The Co2p3/2 XPS spectra were also recorded, the peaks at 772.18 and 780.66 eV are corresponded to metallic Co and Co2þ [29,30]. The results confirm the CoO and Co do exist over the sample. Fig. 5 displays the morphology of the Ni/AC, NiCo/AC catalysts. In accordance with the XRD measurements, by using chloroplatinic acid, the Co was added into monometallic Ni catalysts successfully, leading clearly that the crystallinity NiCo catalysts become smaller and higher disperse in the AC supporter [19]. The results confirm that as chloroplatinic acid was used as the heterogeneous nucleation seed, homogeneous dispersion was enhanced to form homogeneous alloy catalyst.
Catalytic activity
Fig. 4 e XPS profiles of NiCo/AC.
In this section, we firstly compared NiCo/AC catalyst with Ni/ AC, Co/AC and commercial Ru/C (5 wt % Ru on carbon from SigmaeAldrich) catalysts. Fig. 6 displays the carbon efficiency and gas yields at the temperature of 475 C and 20 min. At the same conditions, the carbon efficiency of NiCo/AC has reached about 65%. Meanwhile, the carbon efficiencies of Co/ AC, Ni/AC are about 3% and 12% respectively. This represents a more than 20-fold enhancement of carbon efficiency compared to the Co/AC and 6-fold compared to the Ni/AC. It is worth noting that the carbon efficiency of commercial noble Ru/C catalyst is only about 15% higher than that of NiCo/AC catalyst. The gaseous yields of H2 of NiCo/AC and Ru/C are 30 and 11 mmol/g respectively, indicating that NiCo/AC shows high selectivity for hydrogen gas. In all, the noble-metal-free NiCo/AC bimetal shows the potential to replace noble Rucatalysts in SCWG process for gases fuels. Gasification of phenol in SCW was carried out at the temperature of 475e525 C with a 5 wt% phenol, 0.098 kg/m3 water density and 0.5 g NiCo/AC/g phenol catalyst. Fig. 7 displays the gaseous yields of H2, CH4 and CO2 only since other gaseous components, including C2H4, C2H6 and CO were difficultly detected. The gaseous yields of CH4 increased with the increase of time. CO2 was produced rapidly at initial 20 min and then kept stable. H2 reached the highest yield of about 29.9 mmol/g at 475 C and 20 min and 41 mmol/g at 500 C and 20 min and then reduced slowly, some indicating NiCo/AC can enhance methanation reaction. Huelsman et al. [31] reported non-catalytic gasification of phenol in SCW, their results indicate non-catalytic gasification leads dramatic formation of tar, which includes polycyclic aromatic hydrocarbons (PAHs) and dibenzofuran. To further understand the gasification of phenol with NiCo/AC, the reaction samples at 500 C and 30 min were extracted with dichloromethane and were detected by GCeMS. Fig. 8 shows the identities of the intermediate organic species. It should be noted that the GCeMS run about 3 h, but clear peaks can be found only between 10 and 22 min. Fig. 8 displays that 2methyl-phenol and 2-Vinylfuran are the main intermediates but the peaks are small. Thus, it is clear that the NiCo/AC catalyst can improve gasification of phenol in SCW and suppress the formation of PAHs and dimers.
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Fig. 5 e TEM images of NiCo/AC (a, c) and Ni/AC (b, d).
Fig. 6 e Effect of different catalysts on carbon efficiency at 475 C and 20 min.
Kinetic model
Fig. 7 e The effect of batch holding time on yields of gaseous products (5 wt% phenol, 0.098 kg/m3 and 0.5 g/g NiCo/AC) at 475, 500 and 525 C.
As shown in Fig. 9, the NiCo/AC catalyst can significantly improve the carbon efficiency of phenol at 475, 500 and 525 C. The carbon efficiency can reach 79% at 475 C and 60 min. Rapid gasification of phenol happens within the initial 30 min. The carbon efficiency is close to 80% after 20 min at 525 C. For phenol SCWG reactions, the tar in the product is very difficult for gasification. Under the circumstances, linear
regression can't describe for phenol SCWG. There are two pathways for carbon concentration consumed, i.e., the formation of gaseous products from carbon gasification and coke formation to become tar. Such a power rate law for the carbon gasification of phenol would take the form of following Eq. (1):
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Table 2 e Rate constants and Arrhenius parameters for the gasification of phenol in supercritical water over NiCo/AC catalyst. 475 C 500 C 525 C 1
K1 (min ) K2 (min1)
0.05364 0.01545
0.09928 0.02838
0.1864 0.04819
lnA
Ea (kJ/mol)
16.93 ± 0.48 13.99 ± 0.37
123.56 ± 31 112.95 ± 24
Fig. 8 e Chromatogram of liquid phase obtained from SCWG of phenol at 500 C for 30 min.
Fig. 10 e Arrhenius plot of reaction rate constants. Arrhenius plots for catalytic phenol SCWG over NiCo/AC catalyst. The k values increase with the temperature increases. Arrhenius equation as following: ln k ¼ ln A
Fig. 9 e The comparison between model and experimental results for carbon efficiency from SCWG of phenol with NiCo/AC.
dCphenol ¼ r1 þ r2 dt
Ea RT
where A is frequency factor and Ea is activation energy, respectively. As shown in Table 2. For k1, the Ea and the frequency factor are 123.56 ± 31 kJ/mol and 16.93 ± 0.48, respectively. While for k2, the Ea is 112.95 ± 24 kJ/mol and frequency factor is 13.99 ± 0.37. Fig. 11 displays the compared of the experimental carbon efficiency values and the model. The results show
(1)
r1 is the reaction rate for formation of gaseous products, and r2 is the reaction rate for formation of tar. The rate equation is supposed to be one-step reaction in phenol concentration (Cphenol). Formation of gaseous products is following Eq. (2): r1 ¼ k1½C ¼ A1exp
Ea1 jCj RTjKj
(2)
and formation of tar is following Eq. (3): r2 ¼ k2½C ¼ A2exp
Ea2 jCj RTjKj
(3)
where r, k, [C], A and Ea are the reaction rate, reaction rate constant, concentration of carbon, the frequency factor and activation energy, respectively. As also is shown in Fig. 9, we use Berkeley Madonna to fit between the experimental data to the model. Table 2 shows the k values at different temperature and Fig. 10 displays the
Fig. 11 e The comparison between model and experimental carbon efficiency values.
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experiment data accord with the model, with an error value of about 0.05. The activation energy of gaseous produce formation is 123.56 ± 31 kJ/mol, compared with 112.95 ± 24 kJ/mol with tar formation, indicating that the decompose of phenol transform gaseous products is difficult than the formation of tar. Compared with that Huelsman et al. have been reported that the activation energy is 280 kJ/mol for phenol SCWG without catalyst [31], it is clear that the activation energy has been greatly reduced by using NiCo/AC catalyst and is even close to 84.24 ± 22 kJ/mol by Ru/CeO2 [4]. Thus, the NiCo/AC catalyst exhibits superior performance for gasification.
Conclusions A high performance Noble-metal-free NiCo/AC bimetal for gasification in supercritical water was prepared successfully by the polyol reduction method with chloroplatinic acid as nucleation seed. The catalyst was characterized by BET, XRD, XPS and TEM. The results indicate that the Co is added into monometallic Ni catalysts successfully, leading clearly that the crystallinity NiCo catalysts become smaller and higher disperse in the AC supporter. At 475 C and 20 min, the carbon efficiency for gasification of phenol with NiCo/AC has reached about 65%, which represents a more than 20-fold enhancement of carbon efficiency compared to the Co/AC and 6-fold compared to the Ni/AC. It is worth noting that the carbon efficiency of commercial noble Ru/C catalyst is only about 15% higher than that of NiCo/AC catalyst. The gaseous yields of H2 of NiCo/AC and Ru/C are 30 and 11 mmol/g respectively, indicating that NiCo/AC shows high selectivity for hydrogen gas. At 525 C and 60 min with a 5 wt% phenol, 0.098 kg/m3 density and 0.5 g NiCo/AC/g phenol catalyst, the carbon efficiency can reach about 90%. NiCo/AC can also suppress tar successfully. A kinetic modeling was proposed to describe gaseous product and tar formation, which gives the activation energy (Ea) 123.56 ± 31 kJ/mol and the frequency factor 16.93 ± 0.48 for gaseous products. While for tar formation, the Ea is 112.95 ± 24 kJ/mol and the frequency factor is 13.99 ± 0.37. In all, the noble-metal-free NiCo/AC bimetal shows the potential to replace noble Ru-catalysts in SCWG process for gases fuels.
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Acknowledgements This work is supported by the National Natural Science Foundation of China (21307049 and U1201234), the High Technology Talent Introduction Project of Yunnan in China (2010CI110) and Collaborative Innovation Center of Western Typical Industry Environmental Pollution Control.
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Please cite this article in press as: Guan Q, et al., High performance noble-metal-free NiCo/AC bimetal for gasification in supercritical water, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.191