- Email: [email protected]
Comparative study between supported and doped MgO catalysts in supercritical water gasification for hydrogen production
international journal of hydrogen energy xxx (xxxx) xxx
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Comparative study between supported and doped MgO catalysts in supercritical water gasification for hydrogen production Mohd Sufri Mastuli a,b,c,d,*, Norlida Kamarulzaman c, Muhd Firdaus Kasim c, Zulkarnain Zainal b, Yukihiko Matsumura e, Yun Hin Taufiq-Yap a,b,** a
Catalysis Science and Technology Research Centre, Faculty of Science, Universiti Putra Malaysia, UPM Serdang, 43400 Selangor, Malaysia b Department of Chemistry, Faculty of Science, Universiti Putra Malaysia, UPM Serdang 43400, Selangor, Malaysia c Centre for Nanomaterials Research, Institute of Science, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia d School of Chemistry and Environment, Faculty of Applied Sciences, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia e Department of Mechanical Science and Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527, Japan
article info
abstract
Article history:
Different catalyst structures may influence the catalytic performance of catalysts in su-
Received 24 August 2018
percritical water gasification (SCWG). This study reports the catalytic activity of supported
Received in revised form
(SP) and doped (DP) MgO catalysts in catalyzing the gasification of oil palm frond (OPF)
11 December 2018
biomass in supercritical water to produce hydrogen. Two types of supported catalysts,
Accepted 14 December 2018
labelled as Ni-SP (nickel supported MgO) and Zn-SP (zinc supported MgO), were synthe-
Available online xxx
sized via impregnation method. Another two types of doped catalysts, labelled as Ni-DP (nickel doped MgO) and Zn-DP (zinc doped MgO), were synthesized by using the self-
Keywords:
propagating combustion method. All the synthesized catalysts were found to be pure
Catalyst
with the doped catalysts exhibited small crystallites, in comparison to that produced by the
Hydrogen
supported catalysts. The specific surface area increased in the order of Ni-DP
SCWG
(67.9 m2 g1) > Zn-DP (36.3 m2 g1) > Ni-SP (30.1 m2 g1) > Zn-SP (13.1 m2 g1). Regardless
Supercritical water
of supported or doped, the Ni-based catalysts always had larger specific surface area than
Gasification
that in the Zn-based catalysts. Unexpectedly, the Zn-based catalysts with smaller surface area for SCWG produced higher hydrogen (H2) yield from the OPF biomass. When compared to the non-catalytic reaction, the H2 yield increased by 187.2% for Ni-SP, 269.0% for Zn-SP, 361.7% for Ni-DP, and 438.1% for Zn-DP. Among the studied catalysts, the Zn-DP displayed the highest H2 yield because it had the highest number of basic sites; approximately twenty-fold higher than that of the Zn-SP catalyst. The Zn-DP also proved to be the most stable catalyst, as verified from the X-Ray photoelectron spectroscopy (XPS) results. As such, this study concludes that the catalytic performances of the synthesized catalysts
* Corresponding author. School of Chemistry and Environment, Faculty of Applied Sciences, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia. ** Corresponding author. Catalysis Science and Technology Research Centre, Faculty of Science, Universiti Putra Malaysia, UPM Serdang, 43400 Selangor, Malaysia. E-mail address: [email protected] (Y.H. Taufiq-Yap). https://doi.org/10.1016/j.ijhydene.2018.12.102 0360-3199/© 2018 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. Please cite this article as: Mastuli MS et al., Comparative study between supported and doped MgO catalysts in supercritical water gasification for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.102
2
international journal of hydrogen energy xxx (xxxx) xxx
do not only depend on the specific surface area, but they are also influenced by the number of basic sites and the catalyst stability. It is trustworthy to note that this is the initial study that associated SCWG with doped catalysts. The doped catalysts, hence, may serve as a new catalyst system to generate SCWG reactions. © 2018 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.
Introduction Without any doubt, hydrogen is the future energy carrier for the transportation sector upon success of hydrogen fuel cell technology [1]. The production of hydrogen from biomass makes it a renewable and sustainable resource [2]. The combustion of hydrogen in fuel cell produces only water as its byproduct, unlike fossil fuels that emit carbon dioxide (CO2) that promotes global warming. The potential of hydrogen as an alternative fuel minimizes the dependency on conventional fuels and can cater to the escalating energy demands. As such, numerous advanced technologies have been developed for the conversion of biomass into combustible hydrogen [3e5]. The SCWG refers to a promising technology that generates hydrogen from biomass. Biomass, which usually contains high moisture content, can be employed directly in SCWG without any costly drying pre-treatment because supercritical water (Tc > 374 C, Pc > 22.1 MPa) is applied as the reaction medium to produce hydrogen [6]. Additionally, hydrogen, which is produced at high pressure, requires less energy to pressurize the hydrogen in storage tank [7]. Tar and char issues can be suppressed during the biomass SCWG [8]. Nevertheless, the SCWG product is not limited to H2 alone as it generates carbon monoxide (CO), carbon dioxide (CO2), and methane (CH4) gases with varied ratios. This gas mixture is also called syngas (synthesis gas). Therefore, the addition of catalyst in the SCWG process is vital to alleviate H2 yield and to suppress the formation of other gases. Several review papers have published the role of catalysts in SCWG reactions [9e11]. The gas product can be either H2 rich syngas or CH4 rich syngas, depending on the type of catalysts used. Mostly, Ni-based catalysts enhance hydrogen production. For example, nickel metal and Raney nickel have been widely used in SCWG due to comparable catalytic performances to that of noble metal catalysts [12,13]. These catalysts are also relatively lower in cost. Nickel catalysts promote the hydrogenation reaction to form H2 and CO from the biomass, which in turn, can generate CH4 and CO2 via methanation reaction. A. Sinag et al. [14] catalyzed the SCWG of glucose using Raney nickel and K2CO3. They discovered that the Raney nickel increased CH4 yield, but not H2 yield. The H2 yield was only increased when K2CO3 was used as the catalyst. A significant increment of H2 yield was also observed when NaOH and KOH were used as the catalysts for SCWG of sugarcane bagasse [15]. As explained in Ref. [9], the Raney nickel catalyst strongly promotes the CeO cleavage and reacts with some the produced H2; resulting in CH4 rich syngas. Alkali catalysts are favorable to produce hydrogen due to their basicity properties that are crucial in increasing H2 yield via water-gas shift reaction.
Therefore, catalyst modification is required when nickel is considered as the catalyst for SCWG of biomass. One common way to modify the catalyst is by supporting the nickel onto metal oxide. Most nickel-supported catalysts were synthesized via impregnation method. Among them, the nickel supported onto MgO has received greater attention for SCWG due to its highly basic active sites [16e22]. The catalytic activity of the nickel supported MgO catalyst can be further increased through the addition of other non-noble metals, such as copper and zinc [23e25]. In fact, our previous work compared the catalytic performances of Ni, Cu, and Zn oxides that were supported onto MgO in SCWG of OPF biomass for hydrogen production [26]. The H2 yield increased in the order of Zn-supported MgO > Ni-supported MgO > Cu-supported MgO. The catalytic performances of these supported catalysts were mainly controlled by the basicity properties and the catalyst stability. It is also believed that the stability of the catalyst can be improved by doping (instead of supported) the active metal into the crystal structure of MgO. The doped catalyst can be synthesized by using the self-propagating combustion method [27,28]. Varied catalyst structures (supported and doped) may influence their catalytic performances in SCWG reactions. In this study, two groups of catalysts; supported and doped Ni and Zn onto/into MgO, were synthesized and characterized. The catalyst properties and catalytic performances of the synthesized catalysts in SCWG of OPF biomass for H2 rich syngas are discussed in detail.
Materials and methods Materials All the chemicals used were analytical grade and directly used as received without further purification. High purity of nickel (II) nitrate hexahydrate (Ni(NO3)2$6H2O, 98% purity), zinc (II) nitrate hexahydrate (Zn(NO3)2$6H2O, 98% purity), magnesium (II) nitrate hexahydrate (Mg(NO3)2$6H2O, 98% purity), magnesium oxide (MgO, > 99.5% purity), and citric acid (C6H8O7, >99.5% purity) were used for the synthesis of catalysts. All these chemicals were procured from SigmaAldrich Co., St. Louis, MO, USA. The ultra-pure deionized water (EDI: 15e10 MU cm) used in this study was obtained from TKA Labtower. The OPF biomass was used as feedstock for SCWG to produce hydrogen. The preparation and the properties of the feedstock are given in Ref. [26].
Catalysts synthesis Two groups of catalysts were synthesized and the related details are summarized in Table 1. The first group refers to
Please cite this article as: Mastuli MS et al., Comparative study between supported and doped MgO catalysts in supercritical water gasification for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.102
international journal of hydrogen energy xxx (xxxx) xxx
the supported catalysts (Ni-SP and Zn-SP) that were synthesized via impregnation method and the details are as described in our previous work [26]. Meanwhile, the second group is the doped catalysts that were synthesized by using the self-propagating combustion method. Stoichiometric amount of metal salts (20 wt% of Ni or Zn, and 80 wt% of Mg) were dissolved in minimal amount of deionized water. Citric acid as an oxidizing agent (also known as a combustion agent) was added slowly into the aqueous metal solution under vigorous stirring to form a homogeneous mixture. The ratio between metal salt and oxidizing agent was 1:1. After that, the solution mixture was heated at 350 C without stirring until it combusted and produced dried powders. Note that the combustion process took place rapidly within a few minutes. The obtained powders were grounded before calcined at 600 C for 6 h, which was similar to that performed on the supported catalysts, thus forming the doped catalysts (Ni-DP and Zn-DP).
Catalysts characterization Various characterization techniques were used to study the properties of both supported and doped catalysts. The formation of these catalysts was confirmed by using X-Ray diffraction (XRD; PANalytical X'Pert Pro MPD) that was operated at 45 kV and 40 mA using Bragg-Brentano configuration for data collection. Their morphology and elemental composition were investigated by using field emission scanning electron microscope integrated with energy dispersive X-Ray (FESEM-EDX; JEOL JSM-7600F). The morphology of the catalysts was also examined under high resolution of transmission electron microscope (HRTEM; JEOL JEM-2100 F). The nitrogen (N2) adsorption-desorption isotherms of the catalysts were measured by using BELSORP-mini II instrument from BEL Japan Inc. Additionally, Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods were employed to calculate the specific surface area and the distribution of pore size. Temperature programmed desorption of carbon dioxide (TPD-CO2: Thermo Finnigan TPDRO 1100) that was equipped with a thermal conductivity detector had been applied to measure the strength and the amount of basicity of the catalysts. The chemical environment, such as binding energy (BE) and oxidation state of the synthesized catalysts, were studied via XPS (JEOL JPS-9200).
3
batch reactor system, as schematically illustrated in Fig. 1. The reactor was made of stainless steel tubing (SS316, OD ¼ 3/ 8 in., ID ¼ 7.44 mm, length ¼ 13 cm) with an internal volume of 6.2 ml. A ball valve (SS-4SKPS4) was connected to one end of the reactor tube through another stainless steel tubing (SS316, OD ¼ 1/16 in., ID ¼ 1 mm, length ¼ 35.5 cm) to collect the generated gas. As for the non-catalytic SCWG, 0.0402 g of OPF biomass was added into the reactor tube together with 1.3 ml of deionized water to make 3 wt% suspension of biomass in water. On the other hand, for the catalytic SCWG, 0.0020 g of the synthesized catalysts, which is 5% from the weight of OPF biomass used, was added into the reactor tube together with the biomass suspension, and the reactor tube was closed tightly. The batch reactor was immersed into molten salt bath at 400 C to raise its pressure to 25 MPa and this temperature was retained for 30 min. After that, the reactor was withdrawn from the molten salt bath and cooled down immediately to room temperature using cold water. The collected gas was transferred into a gas bottle and its volume was measured by using the water displacement method.
Gas products analysis The gas products were analyzed via gas chromatography (GC; Shimadzu GC-14B) that was equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID). The yield and the composition of the gas products, such as H2, CO, CO2, CH4, C2H6, and C2H4, were quantified. GC-TCD was used to detect H2, CO, and CO2 gases with helium (He) as the carrier gas for H2 analysis, while N2 as the carrier gas for CO and CO2 analyses. Meanwhile, CH4, C2H6, and C2H4 gases were detected by using GC-FID with He as the carrier gas. A standard gas mixture of H2, CO, CO2, CH4, C2H6, and C2H4 was used for GC calibration and determination of gas composition.
Results and discussion Physicochemical properties of supported and doped MgO catalysts In this study, two types of catalyst structures; supported and doped, were synthesized and characterized. Additionally, their catalytic performances in SCWG were compared for the first time. Fig. 2(a) shows the schematic illustration of crystal
Catalytic SCWG The catalytic performances of the supported and doped catalysts were investigated in SCWG of OPF biomass to generate hydrogen. The SCWG experiments were conducted by using a
Table 1 e Details of synthesized catalysts. Catalyst Composition Catalyst group Synthesis method Ni-SP Zn-SP Ni-DP Zn-DP
20NiO/MgO 20ZnO/MgO Ni0$2Mg0$8O Zn0$2Mg0$8O
Supported catalyst Supported catalyst Doped catalyst Doped catalyst
SPC: self-propagating combustion.
Impregnation Impregnation SPC SPC
Fig. 1 e Experimental apparatus for batch SCWG reactor.
Please cite this article as: Mastuli MS et al., Comparative study between supported and doped MgO catalysts in supercritical water gasification for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.102
4
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 2 e (a) Schematic illustration for crystal structure and (bec) XRD patterns of supported and doped catalysts.
structure for both types of catalysts. One can observe that the active metal (Ni or Zn) is formed either outside (for supported catalyst) or inside (for doped catalyst) the cubic structure of MgO, depending on the synthesis method. The XRD was used to confirm the formation of the synthesized catalysts, as displayed in Fig. 2(b) and (c). All the catalysts showed the XRD peaks of MgO that well-matched our previous work [29]. As for the Ni-SP catalyst, the NiO and MgO diffraction patterns seemed to coincide with each other due to their similar crystal structures of cubic. Nonetheless, no XRD peak coincidence was noted for Zn-SP catalyst because ZnO has a different crystal structure, which is
hexagonal [30]. As reported in the literature, the supported catalysts with mixed phase of XRD peaks verified the active metal formed outside the crystal structure of the support [31e33]. Kamarulzaman et al. synthesized various types of doped metal oxides for electrical and electrochemical applications, such as Hf-doped Al2O3 [28], Cr-doped Al2O3 [27], Cudoped ZnO [34], Mn-doped ZnO [35], and Li-doped MgO [36]. All these doped materials portrayed single phase of XRD peaks that revealed the dopant was actually located inside the crystal structure. This means; Ni and Zn substituted the Mg position in the cubic structure for Ni-DP and Zn-DP catalysts. The XRD results suggest that all the synthesized supported and doped MgO catalysts are indeed pure without any impurity. Fig. 3 illustrates the FESEM micrographs for the synthesized catalysts. The doped catalysts are smaller than the supported catalysts. As exhibited in Fig. 3(a) and (b), the supported catalysts have irregular particles shapes that are flaky for Ni-SP and spherical for Zn-SP. Meanwhile, the morphology for the doped catalysts is not clearly discernible due to the minute crystallites, as shown in Fig. 3(c) and (d). This required observation under higher resolution of TEM. The HRTEM micrographs (Fig. 3(e) and (f)) clearly display the shapes of Ni-DP and Zn-DP catalysts. Both have spherical shape with crystallite sizes below 50 nm. In addition, the dispersion of the doped catalysts is better than that of the supported catalysts, which is an important factor to be highly active in catalytic reaction [37,38]. The Ni and Zn elemental analysis was carried out by using FESEM-EDX measurement. The metal loading for Ni-SP, Zn-SP, Ni-DP, and Zn-DP catalysts is 13.0 wt%, 17.8 wt%, 20 wt%, and 16.9 wt%, respectively. This means; the doped catalysts formed the highest metal loading, when compared to the supported catalysts. During the synthesis process, the metal salt of Ni nitrate or Zn nitrate was combusted simultaneously with Mg nitrate to form Ni-DP and Zn-DP catalysts, thus resulting smaller crystallite sizes with higher metal loading. Nonetheless, as for the supported catalysts, a bulk MgO was directly used during the impregnation method to limit the formation of Ni or Zn onto the surface of MgO. The bulk MgO is large in size and thermally stable. Other researchers reported that the crystallite size of the supported catalysts derived from the impregnation method are usually larger than those synthesized using other synthesis methods, such as sol-gel [39], precipitation [40], and template [41]. Fig. 4 shows the N2 adsorption-desorption isotherms and the pore size distribution of the synthesized catalysts. Based on the classification standard of International Union of Pure and Applied Chemistry (IUPAC), the adsorption isotherms can be classified as Type II (Fig. 4(a) and (b)) for the supported catalysts, while Type IV (insert Fig. 4(a) and (b)) for the doped catalysts. This means; both Ni-SP and Zn-SP catalysts are macroporous, whereas Ni-DP and Zn-DP are mesoporous. The pore size distribution of the doped catalysts was determined by employing the Barrett-Joyner-Halenda (BJH) method. The pore size distribution for the Ni-DP catalyst appeared to be narrower than that of the Zn-SP catalyst with centered at 4.8 nm and 11.0 nm, respectively. The Ni-DP catalyst displayed smaller pore size, when compared to the Zn-DP catalyst, and agreed to the average pore diameter tabulated in Table 2.
Please cite this article as: Mastuli MS et al., Comparative study between supported and doped MgO catalysts in supercritical water gasification for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.102
international journal of hydrogen energy xxx (xxxx) xxx
5
Fig. 3 e FESEM ((a) Ni-SP, (b) Zn-SP, (c) Ni-DP, (d) Zn-DP) and HRTEM ((e) Ni-DP, (f) Zn-DP) micrographs of the synthesized catalysts.
Please cite this article as: Mastuli MS et al., Comparative study between supported and doped MgO catalysts in supercritical water gasification for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.102
6
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 4 e (aeb) N2 adsorption-desorption isotherms, (ced) pore size distributions and (eef) illustration of pore trapping and pore blockages by oxide clusters. The specific surface area, the total pore volume, and the average pore diameter for all the synthesized catalysts calculated using the BET method are presented in Table 2. The doped catalysts have larger specific surface area than the supported catalysts in the following order: Ni-DP > ZnDP > Ni-SP > Zn-SP. These findings are in agreement with the outcomes derived from FESEM and HRTEM. The smallest crystallite gives the largest specific surface area. The BET analysis also revealed that the catalyst structure (supported or doped) affected the aspect of porosity. It is believed that for
Table 2 e Textural properties of supported and doped catalysts. Catalyst Ni-SP Zn-SP Ni-DP Zn-DP
SBET (m2 g1)
Total pore volume (cm3 g1)
Average pore diameter (nm)
30.1 13.1 67.9 36.3
0.4541 0.1372 0.1412 0.3320
50.3 41.9 10.8 36.6
Please cite this article as: Mastuli MS et al., Comparative study between supported and doped MgO catalysts in supercritical water gasification for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.102
7
international journal of hydrogen energy xxx (xxxx) xxx
the supported catalysts, some Ni and Zn oxides possess a greater tendency to enter the pores and block the pore entrances, which may reduce the total pore volume and the average pore diameter. These drawbacks, however, cannot be avoided for the supported catalysts [42e44]. It is noteworthy to highlight that the trapping and blocking of pores by the oxide clusters may not occur for the doped catalysts, mainly because the Ni and Zn elements are positioned within the cubic MgO crystals. Fig. 4(e) and (f) show the pore illustration for all supported and doped catalysts. It appears that the doped catalysts have larger specific surface area, higher total pore volume, and smaller average pore diameter to benefit the catalytic reaction. The TPD-CO2 was applied to determine the strength and the amount of basicity of the synthesized catalysts. The basicity strength can be expressed in terms of CO2 desorption temperature, as recorded by the TPD-CO2 profiles and presented in Fig. 5. The CO2 desorption peaks for the catalysts ranged between 300 and 600 C, which can be said to have medium basic strength, with the doped catalysts slightly higher than the supported catalysts. Table 3 shows that the doped catalysts have greater amount of basicity than that of the supported catalysts. The amount of basicity increased in the following order: Zn-SP < Ni-SP < NiDP < Zn-DP. The Zn-DP catalyst displayed the highest amount of active basic sites with more than twenty-fold increment than the Zn-SP catalyst. This shows that the catalyst structure has greatly influenced both the strength and the amount of the basicity properties. Hence, it is more interesting to examine the catalytic performances of both supported and doped catalysts.
Table 3 e Basicity of supported and doped catalysts. Catalyst Ni-SP Zn-SP Ni-DP Zn-DP
CO2 desorption temperature ( C)
Amount of CO2 desorbed (mmol g1)
344 386 469 437
794.4 345.9 5740.9 7252.6
OPF biomass produced a gas yield of 142.6 ml g1, which further increased by 28.5%, 32.0%, 35.7%, and 55.0% with the addition of Ni-SP, Zn-SP, Ni-DP, and Zn-DP catalysts, respectively, during the SCWG reactions. The outcomes suggest that the lignocellulosic waste, such as OPF biomass, can be gasified at a lower temperature by using catalytic SCWG. Similar findings are reported in the literature with various nickel supported catalysts [45e47]. Herein, for the first time, the catalytic activity of doped catalysts is studied in SCWG. Surprisingly, the doped catalysts (Ni-DP and Zn-DP) gave higher gas yields than those of the supported catalysts (Ni-SP and ZnSP). All the synthesized catalysts accelerated the steam reforming reactions of Eqs. (1) and (2), apart from generating syngas with various gas compositions [48]. The x and y refer to the elemental molar ratios of H/C and O/C in the biomass, respectively. CHxOy þ (1 y)H2O / (1 y þ x/2)H2 þ CO
(1)
CHxOy þ (2 y)H2O / (2 y þ x/2)H2 þ CO2
(2)
The catalytic performances between supported (Ni-SP and ZnSP) and doped (Ni-DP and Zn-DP) MgO catalysts were compared by catalyzing the SCWG of OPF biomass to produce syngas with high H2 yield. For each SCWG experimental run, the reaction was performed at 400 C and 25 MPa for 30 min with 5 wt% of the synthesized catalyst. Without catalyst, the
Fig. 6 illustrates the effects of supported and doped MgO catalysts on the composition of gas products. For noncatalytic SCWG, the produced gas contained 41.1 vol% of CO, 29.2 vol% of CO2, 14.6 vol% of CH4, 10.6 vol% of H2, 3.8 vol% of C2H6, and 0.7 vol% of C2H4. CO emerged as the main gas product, followed by CO2, CH4, and H2 gases. Other hydrocarbon gases, such as C2H6 and C2H4, were also detected at very minimal amount. Nevertheless, the CO yield decreased with the addition of synthesized catalysts during SCWG of OPF biomass. It is believed that the added catalysts promoted the water-gas shift reaction, which promoted H2 yield in each gas product. The findings also showed that the CH4 yield remained constant although with the presence of catalysts. This means; both supported and doped catalysts were only
Fig. 5 e TPD-CO2 profiles of (a) Ni-SP, (b) Zn-SP, (c) Ni-DP and (d) Zn-DP catalysts.
Fig. 6 e Effects of supported and doped catalysts on gas composition for catalytic SCWG of OPF biomass.
Catalytic SCWG of OPF biomass
Please cite this article as: Mastuli MS et al., Comparative study between supported and doped MgO catalysts in supercritical water gasification for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.102
8
international journal of hydrogen energy xxx (xxxx) xxx
active for the water-gas shift reaction (Eq. (3)), but appeared to be inactive for the methanation reaction (Eqs. (4) and (5)). CO þ H2O / H2 þ CO2
(3)
CO þ 3H2 / CH4 þ H2O
(4)
CO2 þ 4H2 / CH4 þ 2H2O
(5)
Fig. 7 displays the impacts of each catalyst on the individual gas yield. It seemed that H2 yield increased tremendously with the decreasing yield of other gases. When compared to those without any catalyst, the supported catalysts; Ni-SP and Zn-SP increased the H2 yield by 187.2% and 269.0%, respectively. These H2 yields momentously increased when the doped catalysts were applied, wherein increment up to 361.7% and 438.1% had been recorded for Ni-DP and Zn-DP. In fact, two important findings need to be highlighted. First, the doped catalysts performed better than the supported catalysts. This suggests that the position of active element (Ni or Zn), regardless of outside or inside the cubic structure of the MgO, influenced the catalytic performances. Second, the Znbased catalysts always gave higher H2 yields than that of the Ni-based catalysts. The catalytic activity of the synthesized catalysts is in the order of Zn-DP > Zn-SP > Ni-DP > Ni-SP. The promotional effects of all the catalysts toward watergas shift reaction had been verified by the reduction of CO yield in each gas product. This means; the CO yield decreased as the H2 yield increased. Among the catalysts, the Zn-DP displayed the highest reduction of CO yield with 82.4% decrement as it produced the highest H2 yield by more than four times over Ni-SP, Zn-SP, and Ni-DP catalysts. Unfortunately, the CO2 yield did not adhere to the expected trend. During the water-gas shift reaction, as the produced CO reacted with water, both H2 and CO2 yields should be increased simultaneously. However, the CO2 yield in this study decreased in the range of 24.8%e41.9% although the added catalysts promoted the water-gas shift reaction, hence indicating that some CO2 could have been absorbed on the catalyst surfaces, as explained in our previous works [49,50]. The ability of the synthesized catalysts to absorb the produced CO2 is an advantage for the catalytic SCWG of OPF biomass to
Fig. 7 e Increment or decrement of gas composition after the addition of supported and doped catalysts for catalytic SCWG of OPF biomass.
produce H2 rich syngas. Another advantage is that all the synthesized catalysts did not catalyze the methanation reaction as the undesired CH4 gas was formed with constant gas yield. Hence, one can conclude that the supported and doped Ni and Zn on the MgO-based catalysts are active for water-gas shift reaction, but inactive for methanation reaction. The doped catalyst of Zn-DP caused the SCWG of OPF biomass to produce syngas with the highest H2 yield and the lowest CH4 yield, in comparison to other synthesized catalysts.
Relationship between catalyst properties and catalytic performances in SCWG reactions As depicted, varied catalyst structures (supported and doped) can significantly influence the catalytic performances of the synthesized catalysts. As reported previously [26], for the supported catalysts, the Zn-based catalyst exhibited the highest H2 yield from the OPF biomass even though it has the smallest specific surface area. The main reason for the Znbased catalyst to perform better during the SCWG reaction is due to the higher catalyst stability, as confirmed in the XPS outcomes. The ZnO that was supported on the MgO surface increased the strength of OeMg bond, which further increased the stability of the catalyst. Therefore, the stability of the catalyst is an important factor to be considered while designing a new catalyst system for the SCWG reaction. It is believed that the catalyst stability can be further improved by inserting (or doped substitutionally) the active metal into the crystal structure of the catalyst. Next, comparisons were made for the catalyst properties and the catalytic performances between supported and doped Ni and Zn onto/into MgO catalysts. The catalyst properties of both supported (Ni-SP and ZnSP) and doped (Ni-DP and Zn-DP) catalysts differed from each other. All the synthesized catalysts were highly pure, as indicated by the XRD patterns with mixed phase for the supported catalysts and the single-phase for the doped catalysts. The doped catalysts have very small crystallites, as compared to the supported catalysts, as observed vividly from the FESEM and HRTEM micrographs. These results are further confirmed using the BET analyses with the Ni-DP and Zn-DP catalysts showing larger specific surface area than the Ni-SP and Zn-SP catalysts. The smaller crystallite size provides larger specific surface area. Furthermore, the doped catalysts attained higher metal loading with higher active basic sites at medium basic strength. Interestingly, the doped catalysts had managed to catalyze the SCWG of OPF biomass effectively by increasing the H2 yield up to 361.7% and 438.1% for Ni-DP and Zn-DP catalysts, respectively. Although the Zn-DP catalyst has slightly smaller surface area than the Ni-DP catalyst, it has the highest number of active basic sites that are crucial to promote catalytic reaction. The basic catalysts possess greater tendency in promoting the water-gas shift reaction to increase the production of hydrogen [51,52]. The effectiveness of the doped catalysts for SCWG reaction was further influenced by catalyst stability, which was elucidated via XPS measurements. To better understand the chemical environment of both supported and doped catalysts, XPS investigation was performed. The BE values for all the elements are listed in Table 4.
Please cite this article as: Mastuli MS et al., Comparative study between supported and doped MgO catalysts in supercritical water gasification for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.102
international journal of hydrogen energy xxx (xxxx) xxx
9
Table 4 e Binding energy (BE) for Ni-SP, Ni-DP, Zn-SP and Zn-DP catalysts. Catalyst Ni-SP
Component Mg 2p Ni 2p3/2 Ni 2p1/2 O 1s
Ni-DP
Mg 2p Ni 2p3/2 Ni 2p1/2 O 1s
Zn-SP
Zn-DP
Mg 2p Zn 2p3/2 Zn 2p1/2 O 1s Mg 2p Zn 2p3/2 Zn 2p1/2 O 1s
Ni2þ Ni3þ OeMg OeNi Ni2þ Ni3þ OeMg OeNi Zn2þ OeMg OeZn Zn2þ OeMg OeZn
BE (eV) 48.86 852.16 853.37 869.62 530.06 527.98 48.98 852.85 854.07 870.30 530.02 528.33 49.14 1017.13 1040.14 530.27 528.29 49.23 1018.05 1040.98 530.31 528.35
% 75.5 24.5
77.2 22.8
100
100
All the BEs are corrected based on the C 1s peak (BE ¼ 284.2 eV) as depicted in Ref. [53], while Shirley background was applied for peak fitting. The BE values are also in agreement with the NIST database. Fig. 8 illustrates the XPS narrow scans of Mg 2p and O 1s for both supported and doped catalysts. As shown in Fig. 8(a), all the synthesized catalysts have symmetrical Mg 2p peaks; indicating only one oxidation state of þ2 present for Mg in each catalyst. Besides, the BE values of the Mg 2p retrieved for doped catalysts appeared to be higher than those obtained for supported catalysts (refer to Table 4). Regardless of supported or doped structures, the Zn-based catalysts shifted the Mg 2p peaks to higher BE values, as compared to the Ni-based catalysts. This phenomenon is believed to occur due to the bonding character of ionic bonds in the MgO. It is well known that in ionic bonding, the positions of cation (Mg2þ) and anion (O2) are locked by a strong electrostatic attraction force beA) is tween the ions. When larger ionic radius of Zn2þ (0.74 supported or doped onto/into the MgO crystal lattice, a greater electron repulsion is felt by the electron clouds surrounding the ions (Mg2þ and O2) that pushes the electrons into each Mg and O orbital closer to their nucleus. Note that the ionic radius A. This explains why the BEs of Mg 2p peak for of Mg2þ is 0.72 both Zn-SP and Zn-DP catalysts are higher, as compared to those of Ni-SP and Ni-DP catalysts, which have smaller ionic A). The ionic radii for all the cations adhere radius of Ni2þ (0.69 to the coordination number 6 of the cubic structure [54]. Between the Zn-based catalysts, the Zn-DP displayed the highest BE value for the Mg 2p peak than the Zn-SP. This can be further explained by examining the crystal structure of the catalysts, as illustrated in Fig. 2 (a). As for the supported catalyst, the active metal of Zn was only formed on the MgO surface. In contrast, for the doped catalyst, the Zn2þ took place in the Mg2þ site at the MgO lattice structure. The incorporation of Zn2þ into the MgO host lattice can cause the surrounding atoms (Mg and O) to experience more electron repulsion from
Fig. 8 e XPS narrow scans for (a) Mg 2p and (b) O 1s of the supported and doped catalysts. the electron cloud of Zn2þ than being only supported on the MgO surface. The greater electron repulsion leads the electrons in each orbital of Mg and O ions to move closer to their nucleus, which can result in higher BE values. This is well supported with the BE value of O 1s for the Zn-DP catalyst (see Table 4). In Fig. 8(b), the BE of O 1s for Zn-DP shifted to a higher value; indicating that its OeMg bond is stronger (shorter distance) than the other catalysts. The shorter distance between OeMg bond implies that more energy is needed to break this bond. Hence, it is can be concluded that the Zn-DP catalyst has higher stability of catalyst structure, when compared to the rest. Fig. 9 illustrates the XPS narrow scans of the active metals (Ni and Zn) for both supported and doped catalysts. The doped catalysts recorded higher BE values than the supported catalysts. This is mainly because the Ni and Zn ions took place in the Mg2þ sites in the MgO lattice. In Fig. 9(a), both the Nisupported and doped catalysts possess asymmetry shape of Ni 2p3/2 peaks, which reflect the existence of more than one oxidation state for Ni cations. The deconvolution on Ni 2p3/2 peak was performed for both catalyst structures. An example of deconvolution of Ni 2p3/2 peak for Ni-DP catalyst is shown in Fig. 10. The analysis revealed that the Ni-based catalysts have mixture of Ni2þ and Ni3þ, as tabulated in Table 4. On the contrary, Fig. 9(b) presents that both the Zn-supported and doped catalysts have symmetrical Zn 2p3/2 peaks. This implies
Please cite this article as: Mastuli MS et al., Comparative study between supported and doped MgO catalysts in supercritical water gasification for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.102
10
international journal of hydrogen energy xxx (xxxx) xxx
synthesized catalysts. The doped catalysts are more stable than the supported catalysts with the Zn-based catalysts always being more reactive than the Ni-based catalysts during the SCWG of OPF biomass. The catalytic performances of the synthesized catalysts are given in the following order: ZnDP > Zn-SP > Ni-DP > Ni-SP. This means; the doped catalysts can be considered as new promising catalysts for SCWG reactions.
Conclusion Supported (Ni-SP and Zn-SP) and doped (Ni-DP and Zn-DP) catalysts were synthesized successfully and catalyzed the SCWG of OPF biomass for hydrogen production. The study outcomes revealed that the doped catalysts have larger specific surface area than the supported catalysts with Zn-DP catalyst, which led to the highest H2 yield up to 438.1% of increment, when compared to that without catalyst. The catalytic activity of the doped catalysts seemed to be affected by the active basic sites and the catalyst stability. The Znbased catalysts generated the highest H2 yield, when compared to the Ni-based catalysts for each supported and doped catalysts. In future work, the compositions of Ni-DP and Zn-DP will be varied and used as new promising catalysts to catalyze the SCWG reactions.
Acknowledgement Fig. 9 e XPS narrow scans for active metal of (a) Ni and (b) Zn of the supported and doped catalysts.
that only one oxidation state exists for the Zn cation, which is þ2. This is further confirmed by the energy splitting of approximately 23 eV between Zn 2p1/2 and Zn 2p3/2 peaks. This finding is in line with the NIST database. Based on these results, the active element (Ni or Zn) and the catalyst structure (supported or doped) are greatly affected by the chemical environments, such as BE and oxidation state of the
Fig. 10 e The deconvolution for Ni 2p3/2 peak of Ni-DP catalyst.
This work was supported by the Malaysian Ministry of Higher Education (MOHE) through Fundamental Research Grant Scheme (600-RMI/FRGS5/3(13/2013)). M. S. Mastuli would like to thank to MOHE for the PhD scholarship.
references
[1] Dodds PE, Staffell I, Hawkes AD, Li F, Gru¨newald P, McDowall W, et al. Hydrogen and fuel cell technologies for heating: a review. Int J Hydrogen Energy 2015;40:2065e83. [2] Ehsan S, Wahid MA. Hydrogen production from renewable and sustainable energy resources : promising green energy Carrier for clean development. Renew Sustain Energy Rev 2016;57:850e66. [3] Sansaniwal SK, Pal K, Rosen MA, Tyagi SK. Recent advances in the development of biomass gasification technology: a comprehensive review. Renew Sustain Energy Rev 2017;72:363e84. [4] Pereira EG, Da Silva JN, De Oliveira JL, MacHado CS. Sustainable energy: a review of gasification technologies. Renew Sustain Energy Rev 2012;16:4753e62. [5] Puig-Arnavat M, Bruno JC, Coronas A. Review and analysis of biomass gasification models. Renew Sustain Energy Rev 2010;14:2841e51. [6] Guo Y, Wang SZ, Xu DH, Gong YM, Ma HH, Tang XY. Review of catalytic supercritical water gasification for hydrogen production from biomass. Renew Sustain Energy Rev 2010;14:334e43. [7] Abdoulmoumine N, Adhikari S, Kulkarni A, Chattanathan S. A review on biomass gasification syngas cleanup. Appl Energy 2015;155:294e307.
Please cite this article as: Mastuli MS et al., Comparative study between supported and doped MgO catalysts in supercritical water gasification for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.102
international journal of hydrogen energy xxx (xxxx) xxx
[8] Shen Y, Yoshikawa K. Recent progresses in catalytic tar elimination during biomass gasification or pyrolysis - a review. Renew Sustain Energy Rev 2013;21:371e92. [9] Azadi P, Farnood R. Review of heterogeneous catalysts for sub- and supercritical water gasification of biomass and wastes. Int J Hydrogen Energy 2011;36:9529e41. [10] Reddy SN, Nanda S, Dalai AK, Kozinski JA. Supercritical water gasification of biomass for hydrogen production. Int J Hydrogen Energy 2014;39:6912e26. [11] Chan FL, Tanksale A. Review of recent developments in Nibased catalysts for biomass gasi fi cation. Renew Sustain Energy Rev 2014;38:428e38. https://doi.org/10.1016/ j.rser.2014.06.011. [12] Yoshida T, Oshima Y, Matsumura Y. Gasification of biomass model compounds and real biomass in supercritical water. Biomass Bioenergy 2004;26:71e8. [13] Nguyen HT, Yoda E, Komiyama M. Catalytic supercritical water gasi fication of proteinaceous biomass: catalyst performances in gasification of ethanol fermentation stillage with batch and flow reactors. Chem Eng Sci 2014;109:197e203. [14] Sinag A, Kruse A, Rathert J. Influence of the heating rate and the type of catalyst on the formation of key intermediates and on the generation of gases during hydropyrolysis of glucose in supercritical water in a batch reactor. Ind Eng Chem Res 2004;43:502e8. [15] Ebrahimi-Nik M, Almassi M, Sheikhdavoodi MJ, Kruse A, Bahrami H. Gasification of sugarcane bagasse in supercritical water; evaluation of alkali catalysts for maximum hydrogen production. J Energy Inst 2015;88:450e8. [16] Furusawa T, Sato T, Saito M, Ishiyama Y, Sato M, Itoh N, et al. The evaluation of the stability of Ni/MgO catalysts for the gasification of lignin in supercritical water. Appl Catal Gen 2007;327:300e10. [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;36:3426e33. [19] Ding N, Azargohar R, Dalai AK, Kozinski JA. Catalytic gasification of glucose to H2 in supercritical water. Fuel Process Technol 2014;127:33e40. [20] Kang K, Azargohar R, Dalai AK, Wang H. Systematic screening and modification of Ni based catalysts for hydrogen generation from supercritical water gasification of lignin. Chem Eng J 2016;283:1019e32. [21] Sato T, Furusawa T, Ishiyama Y, Sugito H, Miura Y, Sato M, et al. Effect of water density on the gasification of lignin with magnesium oxide supported nickel catalysts in supercritical water. Ind Eng Chem Res 2006;45:615e22. [22] Furusawa T, Sato T, Sugito H, Miura Y, Ishiyama Y, Sato M, et al. Hydrogen production from the gasification of lignin with nickel catalysts in supercritical water. Int J Hydrogen Energy 2007;32:699e704. [23] Li S, Lu Y, Guo L, Zhang X. Hydrogen production by biomass gasification in supercritical water with bimetallic Ni-M/ gAl2O3 catalysts (M ¼ Cu, Co and Sn ). Int J Hydrogen Energy 2011;36:14391e400. [24] Tavasoli A, Barati M, Karimi A. Sugarcane bagasse supercritical water gasification in presence of potassium promoted copper nano-catalysts supported on g-Al2O3. Int J Hydrogen Energy 2016;41:174e80. [25] Barati M, Babatabar M, Tavasoli A, Dalai AK, Das U. Hydrogen production via supercritical water gasification of bagasse
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42]
11
using unpromoted and zinc promoted Ru/g-Al2O3 nanocatalysts. Fuel Process Technol 2014;123:140e8. Mastuli MS, Kamarulzaman N, Kasim MF, Sivasangar S, Saiman MI, Taufiq-Yap YH. Catalytic gasification of oil palm frond biomass in supercritical water using MgO supported Ni, Cu and Zn oxides as catalysts for hydrogen production. Int J Hydrogen Energy 2017;42:11215e28. Badar N, Kamarulzaman N, Rusdi R, Abdul Aziz ND, Kun Fun H. Increased conductivities of Cr doped Al2-xCrxO3 powders due to band gap narrowing. Phys B Condens Matter 2014;437:32e5. Kamarulzaman N, Mahat AM, Badar N, Zhao CZ. Structural and band gaps studies of novel Al2-xHfxO3 materials toward MOS applications. Mater Chem Phys 2018;216:237e42. Mastuli MS. Growth mechanisms of MgO nanocrystals via a sol-gel synthesis using different complexing agents. Nanoscale Res Lett 2014;9:134. Rusdi R, Rahman AA, Mohamed NS, Kamarudin N, Kamarulzaman N. Preparation and band gap energies of ZnO nanotubes, nanorods and spherical nanostructures. Powder Technol 2011;210:18e22. Tushar MSHK, Dutta A, (Charles) Xu C. Catalytic supercritical gasification of biocrude from hydrothermal liquefaction of cattle manure. Appl Catal B Environ Environ 2016;189:119e32. Guan Q, Chen S, Chen Y, Gu J, Li B, Miao R, et al. High performance noble-metal-free NiCo/AC bimetal for gasification in supercritical water. Int J Hydrogen Energy 2016;42:6511e8. Kang K, Azargohar R, Dalai AK, Wang H. Hydrogen production from lignin, cellulose and waste biomass via supercritical water gasification: catalyst activity and process optimization study. Energy Convers Manag 2016;117:528e37. Kamarulzaman N, Kasim MF, Chayed NF. Elucidation of the highest valence band and lowest conduction band shifts using XPS for ZnO and Zn0.99Cu0.01O band gap changes. Results Phys 2016;6:217e30. Kamarulzaman N, Kasim MF, Rusdi R. Band gap narrowing and widening of ZnO nanostructures and doped materials. Nanoscale Res Lett 2015;10:1e12. Chayed NF, Badar N, Rusdi R, Azahidi A, Kamaru. Band gap energies of Li2xMg(1-x)O materials synthesized by the sol-gel method. J Cryst Growth 2013;362:268e70. Huang J, Lian X, Wang L, Zhu C, Jin H, Wang R. Hydrogen production from glucose by supercritical water gasification with Ni/Zr(Ce , Y)O2 catalysts. Int J Hydrogen Energy 2016;42:4613e25. Hossain MZ, Chowdhury MBI, Jhawar AK, Charpentier PA. Supercritical water gasification of glucose using bimetallic aerogel Ru-Ni-Al2O3 catalyst for H2 production. Biomass Bioenergy 2017;107:39e51. Kou J, Xu J, Jin H, Guo L, Zhang D, Cao W. Evaluation of modified Ni/ZrO2 catalysts for hydrogen production by supercritical water gasification of oil-containing wastewater. Int J Hydrogen Energy 2018;43:13896e903. Li S, Guo L, Zhu C, Lu Y. Co-precipitated Ni-Mg-Al catalysts for hydrogen production by supercritical water gasification of glucose. Int J Hydrogen Energy 2013;38:9688e700. Hossain MZ, Chowdhury MBI, Alsharari Q, Jhawar AK, Charpentier PA. Effect of mesoporosity of bimetallic Ni-RuAl2O3 catalysts for hydrogen production during supercritical water gasification of glucose. Fuel Process Technol 2017;159:55e66. Lee I-G. Effect of metal addition to Ni/activated charcoal catalyst on gasification of glucose in supercritical water. Int J Hydrogen Energy 2011;36:8869e77.
Please cite this article as: Mastuli MS et al., Comparative study between supported and doped MgO catalysts in supercritical water gasification for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.102
12
international journal of hydrogen energy xxx (xxxx) xxx
[43] Li S, Savage PE, Guo L. Stability and activity maintenance of Al2O3- and carbon nanotube-supported Ni catalysts during continuous gasification of glycerol in supercritical water. J Supercrit Fluids 2018;135:188e97. [44] Zhang L, Champagne P, Xu C, Chunbao. Supercritical water gasification of an aqueous by-product from biomass hydrothermal liquefaction with novel Ru modified Ni catalysts. Bioresour Technol 2011;102:8279e87. [45] Behnia I, Yuan Z, Charpentier P, Xu C, Charles. Production of methane and hydrogen via supercritical water gasification of renewable glucose at a relatively low temperature: effects of metal catalysts and supports. Fuel Process Technol 2016;143:27e34. [46] Guo Y, Wang S, Yeh T, Savage PE. Catalytic gasification of indole in supercritical water. Appl Catal B Environ 2015;166e167:202e10. [47] Louw J, Schwarz CE, Burger AJ. Catalytic supercritical water gasification of primary paper sludge using a homogeneous and heterogeneous catalyst: experimental vs thermodynamic equilibrium results. Bioresour Technol 2016;201:111e20. [48] Guo LJ, Lu YJ, Zhang XM, Ji CM, Guan Y, Pei AX. Hydrogen production by biomass gasification in supercritical water: a
[49]
[50]
[51]
[52]
[53]
[54]
systematic experimental and analytical study. Catal Today 2007;129:275e86. Sivasangar S, Mastuli MS, Islam A, Taufiq-Yap YH. Screening of modified CaO-based catalysts with a series of dopants for the supercritical water gasification of empty palm fruit bunches to produce hydrogen. RSC Adv 2015;5:36798e808. Ismail K, Yarmo MA, Taufiq-Yap YH, Ahmad A. The effect of particle size of CaO and MgO as catalysts for gasification of oil palm empty fruit bunch to produce hydrogen. Int J Hydrogen Energy 2012;37:3639e44. Youssef EA, Chowdhury MBI, Nakhla G, Charpentier P. Effect of nickel loading on hydrogen production and chemical oxygen demand (COD) destruction from glucose oxidation and gasification in supercritical water. Int J Hydrogen Energy 2010;35:5034e42. Yanik J, Ebale S, Kruse A, Saglam M, Yuksel M. Biomass gasification in supercritical water: Part 1. Effect of the nature of biomass. Fuel 2007;86:2410e5. Ikeo N, Iijima Y, Niimura N, Sigematsu M, Tazawa T, Matsumoto S. Handbook of X-Ray photoelectron spectroscopy. 1991. Lide DR. Handbook of Chemistry and physics. 1995.
Please cite this article as: Mastuli MS et al., Comparative study between supported and doped MgO catalysts in supercritical water gasification for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.102
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Comparative study between supported and doped MgO catalysts in supercritical water gasification for hydrogen production Mohd Sufri Mastuli a,b,c,d,*, Norlida Kamarulzaman c, Muhd Firdaus Kasim c, Zulkarnain Zainal b, Yukihiko Matsumura e, Yun Hin Taufiq-Yap a,b,** a
Catalysis Science and Technology Research Centre, Faculty of Science, Universiti Putra Malaysia, UPM Serdang, 43400 Selangor, Malaysia b Department of Chemistry, Faculty of Science, Universiti Putra Malaysia, UPM Serdang 43400, Selangor, Malaysia c Centre for Nanomaterials Research, Institute of Science, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia d School of Chemistry and Environment, Faculty of Applied Sciences, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia e Department of Mechanical Science and Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527, Japan
article info
abstract
Article history:
Different catalyst structures may influence the catalytic performance of catalysts in su-
Received 24 August 2018
percritical water gasification (SCWG). This study reports the catalytic activity of supported
Received in revised form
(SP) and doped (DP) MgO catalysts in catalyzing the gasification of oil palm frond (OPF)
11 December 2018
biomass in supercritical water to produce hydrogen. Two types of supported catalysts,
Accepted 14 December 2018
labelled as Ni-SP (nickel supported MgO) and Zn-SP (zinc supported MgO), were synthe-
Available online xxx
sized via impregnation method. Another two types of doped catalysts, labelled as Ni-DP (nickel doped MgO) and Zn-DP (zinc doped MgO), were synthesized by using the self-
Keywords:
propagating combustion method. All the synthesized catalysts were found to be pure
Catalyst
with the doped catalysts exhibited small crystallites, in comparison to that produced by the
Hydrogen
supported catalysts. The specific surface area increased in the order of Ni-DP
SCWG
(67.9 m2 g1) > Zn-DP (36.3 m2 g1) > Ni-SP (30.1 m2 g1) > Zn-SP (13.1 m2 g1). Regardless
Supercritical water
of supported or doped, the Ni-based catalysts always had larger specific surface area than
Gasification
that in the Zn-based catalysts. Unexpectedly, the Zn-based catalysts with smaller surface area for SCWG produced higher hydrogen (H2) yield from the OPF biomass. When compared to the non-catalytic reaction, the H2 yield increased by 187.2% for Ni-SP, 269.0% for Zn-SP, 361.7% for Ni-DP, and 438.1% for Zn-DP. Among the studied catalysts, the Zn-DP displayed the highest H2 yield because it had the highest number of basic sites; approximately twenty-fold higher than that of the Zn-SP catalyst. The Zn-DP also proved to be the most stable catalyst, as verified from the X-Ray photoelectron spectroscopy (XPS) results. As such, this study concludes that the catalytic performances of the synthesized catalysts
* Corresponding author. School of Chemistry and Environment, Faculty of Applied Sciences, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia. ** Corresponding author. Catalysis Science and Technology Research Centre, Faculty of Science, Universiti Putra Malaysia, UPM Serdang, 43400 Selangor, Malaysia. E-mail address: [email protected] (Y.H. Taufiq-Yap). https://doi.org/10.1016/j.ijhydene.2018.12.102 0360-3199/© 2018 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. Please cite this article as: Mastuli MS et al., Comparative study between supported and doped MgO catalysts in supercritical water gasification for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.102
2
international journal of hydrogen energy xxx (xxxx) xxx
do not only depend on the specific surface area, but they are also influenced by the number of basic sites and the catalyst stability. It is trustworthy to note that this is the initial study that associated SCWG with doped catalysts. The doped catalysts, hence, may serve as a new catalyst system to generate SCWG reactions. © 2018 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.
Introduction Without any doubt, hydrogen is the future energy carrier for the transportation sector upon success of hydrogen fuel cell technology [1]. The production of hydrogen from biomass makes it a renewable and sustainable resource [2]. The combustion of hydrogen in fuel cell produces only water as its byproduct, unlike fossil fuels that emit carbon dioxide (CO2) that promotes global warming. The potential of hydrogen as an alternative fuel minimizes the dependency on conventional fuels and can cater to the escalating energy demands. As such, numerous advanced technologies have been developed for the conversion of biomass into combustible hydrogen [3e5]. The SCWG refers to a promising technology that generates hydrogen from biomass. Biomass, which usually contains high moisture content, can be employed directly in SCWG without any costly drying pre-treatment because supercritical water (Tc > 374 C, Pc > 22.1 MPa) is applied as the reaction medium to produce hydrogen [6]. Additionally, hydrogen, which is produced at high pressure, requires less energy to pressurize the hydrogen in storage tank [7]. Tar and char issues can be suppressed during the biomass SCWG [8]. Nevertheless, the SCWG product is not limited to H2 alone as it generates carbon monoxide (CO), carbon dioxide (CO2), and methane (CH4) gases with varied ratios. This gas mixture is also called syngas (synthesis gas). Therefore, the addition of catalyst in the SCWG process is vital to alleviate H2 yield and to suppress the formation of other gases. Several review papers have published the role of catalysts in SCWG reactions [9e11]. The gas product can be either H2 rich syngas or CH4 rich syngas, depending on the type of catalysts used. Mostly, Ni-based catalysts enhance hydrogen production. For example, nickel metal and Raney nickel have been widely used in SCWG due to comparable catalytic performances to that of noble metal catalysts [12,13]. These catalysts are also relatively lower in cost. Nickel catalysts promote the hydrogenation reaction to form H2 and CO from the biomass, which in turn, can generate CH4 and CO2 via methanation reaction. A. Sinag et al. [14] catalyzed the SCWG of glucose using Raney nickel and K2CO3. They discovered that the Raney nickel increased CH4 yield, but not H2 yield. The H2 yield was only increased when K2CO3 was used as the catalyst. A significant increment of H2 yield was also observed when NaOH and KOH were used as the catalysts for SCWG of sugarcane bagasse [15]. As explained in Ref. [9], the Raney nickel catalyst strongly promotes the CeO cleavage and reacts with some the produced H2; resulting in CH4 rich syngas. Alkali catalysts are favorable to produce hydrogen due to their basicity properties that are crucial in increasing H2 yield via water-gas shift reaction.
Therefore, catalyst modification is required when nickel is considered as the catalyst for SCWG of biomass. One common way to modify the catalyst is by supporting the nickel onto metal oxide. Most nickel-supported catalysts were synthesized via impregnation method. Among them, the nickel supported onto MgO has received greater attention for SCWG due to its highly basic active sites [16e22]. The catalytic activity of the nickel supported MgO catalyst can be further increased through the addition of other non-noble metals, such as copper and zinc [23e25]. In fact, our previous work compared the catalytic performances of Ni, Cu, and Zn oxides that were supported onto MgO in SCWG of OPF biomass for hydrogen production [26]. The H2 yield increased in the order of Zn-supported MgO > Ni-supported MgO > Cu-supported MgO. The catalytic performances of these supported catalysts were mainly controlled by the basicity properties and the catalyst stability. It is also believed that the stability of the catalyst can be improved by doping (instead of supported) the active metal into the crystal structure of MgO. The doped catalyst can be synthesized by using the self-propagating combustion method [27,28]. Varied catalyst structures (supported and doped) may influence their catalytic performances in SCWG reactions. In this study, two groups of catalysts; supported and doped Ni and Zn onto/into MgO, were synthesized and characterized. The catalyst properties and catalytic performances of the synthesized catalysts in SCWG of OPF biomass for H2 rich syngas are discussed in detail.
Materials and methods Materials All the chemicals used were analytical grade and directly used as received without further purification. High purity of nickel (II) nitrate hexahydrate (Ni(NO3)2$6H2O, 98% purity), zinc (II) nitrate hexahydrate (Zn(NO3)2$6H2O, 98% purity), magnesium (II) nitrate hexahydrate (Mg(NO3)2$6H2O, 98% purity), magnesium oxide (MgO, > 99.5% purity), and citric acid (C6H8O7, >99.5% purity) were used for the synthesis of catalysts. All these chemicals were procured from SigmaAldrich Co., St. Louis, MO, USA. The ultra-pure deionized water (EDI: 15e10 MU cm) used in this study was obtained from TKA Labtower. The OPF biomass was used as feedstock for SCWG to produce hydrogen. The preparation and the properties of the feedstock are given in Ref. [26].
Catalysts synthesis Two groups of catalysts were synthesized and the related details are summarized in Table 1. The first group refers to
Please cite this article as: Mastuli MS et al., Comparative study between supported and doped MgO catalysts in supercritical water gasification for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.102
international journal of hydrogen energy xxx (xxxx) xxx
the supported catalysts (Ni-SP and Zn-SP) that were synthesized via impregnation method and the details are as described in our previous work [26]. Meanwhile, the second group is the doped catalysts that were synthesized by using the self-propagating combustion method. Stoichiometric amount of metal salts (20 wt% of Ni or Zn, and 80 wt% of Mg) were dissolved in minimal amount of deionized water. Citric acid as an oxidizing agent (also known as a combustion agent) was added slowly into the aqueous metal solution under vigorous stirring to form a homogeneous mixture. The ratio between metal salt and oxidizing agent was 1:1. After that, the solution mixture was heated at 350 C without stirring until it combusted and produced dried powders. Note that the combustion process took place rapidly within a few minutes. The obtained powders were grounded before calcined at 600 C for 6 h, which was similar to that performed on the supported catalysts, thus forming the doped catalysts (Ni-DP and Zn-DP).
Catalysts characterization Various characterization techniques were used to study the properties of both supported and doped catalysts. The formation of these catalysts was confirmed by using X-Ray diffraction (XRD; PANalytical X'Pert Pro MPD) that was operated at 45 kV and 40 mA using Bragg-Brentano configuration for data collection. Their morphology and elemental composition were investigated by using field emission scanning electron microscope integrated with energy dispersive X-Ray (FESEM-EDX; JEOL JSM-7600F). The morphology of the catalysts was also examined under high resolution of transmission electron microscope (HRTEM; JEOL JEM-2100 F). The nitrogen (N2) adsorption-desorption isotherms of the catalysts were measured by using BELSORP-mini II instrument from BEL Japan Inc. Additionally, Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods were employed to calculate the specific surface area and the distribution of pore size. Temperature programmed desorption of carbon dioxide (TPD-CO2: Thermo Finnigan TPDRO 1100) that was equipped with a thermal conductivity detector had been applied to measure the strength and the amount of basicity of the catalysts. The chemical environment, such as binding energy (BE) and oxidation state of the synthesized catalysts, were studied via XPS (JEOL JPS-9200).
3
batch reactor system, as schematically illustrated in Fig. 1. The reactor was made of stainless steel tubing (SS316, OD ¼ 3/ 8 in., ID ¼ 7.44 mm, length ¼ 13 cm) with an internal volume of 6.2 ml. A ball valve (SS-4SKPS4) was connected to one end of the reactor tube through another stainless steel tubing (SS316, OD ¼ 1/16 in., ID ¼ 1 mm, length ¼ 35.5 cm) to collect the generated gas. As for the non-catalytic SCWG, 0.0402 g of OPF biomass was added into the reactor tube together with 1.3 ml of deionized water to make 3 wt% suspension of biomass in water. On the other hand, for the catalytic SCWG, 0.0020 g of the synthesized catalysts, which is 5% from the weight of OPF biomass used, was added into the reactor tube together with the biomass suspension, and the reactor tube was closed tightly. The batch reactor was immersed into molten salt bath at 400 C to raise its pressure to 25 MPa and this temperature was retained for 30 min. After that, the reactor was withdrawn from the molten salt bath and cooled down immediately to room temperature using cold water. The collected gas was transferred into a gas bottle and its volume was measured by using the water displacement method.
Gas products analysis The gas products were analyzed via gas chromatography (GC; Shimadzu GC-14B) that was equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID). The yield and the composition of the gas products, such as H2, CO, CO2, CH4, C2H6, and C2H4, were quantified. GC-TCD was used to detect H2, CO, and CO2 gases with helium (He) as the carrier gas for H2 analysis, while N2 as the carrier gas for CO and CO2 analyses. Meanwhile, CH4, C2H6, and C2H4 gases were detected by using GC-FID with He as the carrier gas. A standard gas mixture of H2, CO, CO2, CH4, C2H6, and C2H4 was used for GC calibration and determination of gas composition.
Results and discussion Physicochemical properties of supported and doped MgO catalysts In this study, two types of catalyst structures; supported and doped, were synthesized and characterized. Additionally, their catalytic performances in SCWG were compared for the first time. Fig. 2(a) shows the schematic illustration of crystal
Catalytic SCWG The catalytic performances of the supported and doped catalysts were investigated in SCWG of OPF biomass to generate hydrogen. The SCWG experiments were conducted by using a
Table 1 e Details of synthesized catalysts. Catalyst Composition Catalyst group Synthesis method Ni-SP Zn-SP Ni-DP Zn-DP
20NiO/MgO 20ZnO/MgO Ni0$2Mg0$8O Zn0$2Mg0$8O
Supported catalyst Supported catalyst Doped catalyst Doped catalyst
SPC: self-propagating combustion.
Impregnation Impregnation SPC SPC
Fig. 1 e Experimental apparatus for batch SCWG reactor.
Please cite this article as: Mastuli MS et al., Comparative study between supported and doped MgO catalysts in supercritical water gasification for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.102
4
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 2 e (a) Schematic illustration for crystal structure and (bec) XRD patterns of supported and doped catalysts.
structure for both types of catalysts. One can observe that the active metal (Ni or Zn) is formed either outside (for supported catalyst) or inside (for doped catalyst) the cubic structure of MgO, depending on the synthesis method. The XRD was used to confirm the formation of the synthesized catalysts, as displayed in Fig. 2(b) and (c). All the catalysts showed the XRD peaks of MgO that well-matched our previous work [29]. As for the Ni-SP catalyst, the NiO and MgO diffraction patterns seemed to coincide with each other due to their similar crystal structures of cubic. Nonetheless, no XRD peak coincidence was noted for Zn-SP catalyst because ZnO has a different crystal structure, which is
hexagonal [30]. As reported in the literature, the supported catalysts with mixed phase of XRD peaks verified the active metal formed outside the crystal structure of the support [31e33]. Kamarulzaman et al. synthesized various types of doped metal oxides for electrical and electrochemical applications, such as Hf-doped Al2O3 [28], Cr-doped Al2O3 [27], Cudoped ZnO [34], Mn-doped ZnO [35], and Li-doped MgO [36]. All these doped materials portrayed single phase of XRD peaks that revealed the dopant was actually located inside the crystal structure. This means; Ni and Zn substituted the Mg position in the cubic structure for Ni-DP and Zn-DP catalysts. The XRD results suggest that all the synthesized supported and doped MgO catalysts are indeed pure without any impurity. Fig. 3 illustrates the FESEM micrographs for the synthesized catalysts. The doped catalysts are smaller than the supported catalysts. As exhibited in Fig. 3(a) and (b), the supported catalysts have irregular particles shapes that are flaky for Ni-SP and spherical for Zn-SP. Meanwhile, the morphology for the doped catalysts is not clearly discernible due to the minute crystallites, as shown in Fig. 3(c) and (d). This required observation under higher resolution of TEM. The HRTEM micrographs (Fig. 3(e) and (f)) clearly display the shapes of Ni-DP and Zn-DP catalysts. Both have spherical shape with crystallite sizes below 50 nm. In addition, the dispersion of the doped catalysts is better than that of the supported catalysts, which is an important factor to be highly active in catalytic reaction [37,38]. The Ni and Zn elemental analysis was carried out by using FESEM-EDX measurement. The metal loading for Ni-SP, Zn-SP, Ni-DP, and Zn-DP catalysts is 13.0 wt%, 17.8 wt%, 20 wt%, and 16.9 wt%, respectively. This means; the doped catalysts formed the highest metal loading, when compared to the supported catalysts. During the synthesis process, the metal salt of Ni nitrate or Zn nitrate was combusted simultaneously with Mg nitrate to form Ni-DP and Zn-DP catalysts, thus resulting smaller crystallite sizes with higher metal loading. Nonetheless, as for the supported catalysts, a bulk MgO was directly used during the impregnation method to limit the formation of Ni or Zn onto the surface of MgO. The bulk MgO is large in size and thermally stable. Other researchers reported that the crystallite size of the supported catalysts derived from the impregnation method are usually larger than those synthesized using other synthesis methods, such as sol-gel [39], precipitation [40], and template [41]. Fig. 4 shows the N2 adsorption-desorption isotherms and the pore size distribution of the synthesized catalysts. Based on the classification standard of International Union of Pure and Applied Chemistry (IUPAC), the adsorption isotherms can be classified as Type II (Fig. 4(a) and (b)) for the supported catalysts, while Type IV (insert Fig. 4(a) and (b)) for the doped catalysts. This means; both Ni-SP and Zn-SP catalysts are macroporous, whereas Ni-DP and Zn-DP are mesoporous. The pore size distribution of the doped catalysts was determined by employing the Barrett-Joyner-Halenda (BJH) method. The pore size distribution for the Ni-DP catalyst appeared to be narrower than that of the Zn-SP catalyst with centered at 4.8 nm and 11.0 nm, respectively. The Ni-DP catalyst displayed smaller pore size, when compared to the Zn-DP catalyst, and agreed to the average pore diameter tabulated in Table 2.
Please cite this article as: Mastuli MS et al., Comparative study between supported and doped MgO catalysts in supercritical water gasification for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.102
international journal of hydrogen energy xxx (xxxx) xxx
5
Fig. 3 e FESEM ((a) Ni-SP, (b) Zn-SP, (c) Ni-DP, (d) Zn-DP) and HRTEM ((e) Ni-DP, (f) Zn-DP) micrographs of the synthesized catalysts.
Please cite this article as: Mastuli MS et al., Comparative study between supported and doped MgO catalysts in supercritical water gasification for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.102
6
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 4 e (aeb) N2 adsorption-desorption isotherms, (ced) pore size distributions and (eef) illustration of pore trapping and pore blockages by oxide clusters. The specific surface area, the total pore volume, and the average pore diameter for all the synthesized catalysts calculated using the BET method are presented in Table 2. The doped catalysts have larger specific surface area than the supported catalysts in the following order: Ni-DP > ZnDP > Ni-SP > Zn-SP. These findings are in agreement with the outcomes derived from FESEM and HRTEM. The smallest crystallite gives the largest specific surface area. The BET analysis also revealed that the catalyst structure (supported or doped) affected the aspect of porosity. It is believed that for
Table 2 e Textural properties of supported and doped catalysts. Catalyst Ni-SP Zn-SP Ni-DP Zn-DP
SBET (m2 g1)
Total pore volume (cm3 g1)
Average pore diameter (nm)
30.1 13.1 67.9 36.3
0.4541 0.1372 0.1412 0.3320
50.3 41.9 10.8 36.6
Please cite this article as: Mastuli MS et al., Comparative study between supported and doped MgO catalysts in supercritical water gasification for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.102
7
international journal of hydrogen energy xxx (xxxx) xxx
the supported catalysts, some Ni and Zn oxides possess a greater tendency to enter the pores and block the pore entrances, which may reduce the total pore volume and the average pore diameter. These drawbacks, however, cannot be avoided for the supported catalysts [42e44]. It is noteworthy to highlight that the trapping and blocking of pores by the oxide clusters may not occur for the doped catalysts, mainly because the Ni and Zn elements are positioned within the cubic MgO crystals. Fig. 4(e) and (f) show the pore illustration for all supported and doped catalysts. It appears that the doped catalysts have larger specific surface area, higher total pore volume, and smaller average pore diameter to benefit the catalytic reaction. The TPD-CO2 was applied to determine the strength and the amount of basicity of the synthesized catalysts. The basicity strength can be expressed in terms of CO2 desorption temperature, as recorded by the TPD-CO2 profiles and presented in Fig. 5. The CO2 desorption peaks for the catalysts ranged between 300 and 600 C, which can be said to have medium basic strength, with the doped catalysts slightly higher than the supported catalysts. Table 3 shows that the doped catalysts have greater amount of basicity than that of the supported catalysts. The amount of basicity increased in the following order: Zn-SP < Ni-SP < NiDP < Zn-DP. The Zn-DP catalyst displayed the highest amount of active basic sites with more than twenty-fold increment than the Zn-SP catalyst. This shows that the catalyst structure has greatly influenced both the strength and the amount of the basicity properties. Hence, it is more interesting to examine the catalytic performances of both supported and doped catalysts.
Table 3 e Basicity of supported and doped catalysts. Catalyst Ni-SP Zn-SP Ni-DP Zn-DP
CO2 desorption temperature ( C)
Amount of CO2 desorbed (mmol g1)
344 386 469 437
794.4 345.9 5740.9 7252.6
OPF biomass produced a gas yield of 142.6 ml g1, which further increased by 28.5%, 32.0%, 35.7%, and 55.0% with the addition of Ni-SP, Zn-SP, Ni-DP, and Zn-DP catalysts, respectively, during the SCWG reactions. The outcomes suggest that the lignocellulosic waste, such as OPF biomass, can be gasified at a lower temperature by using catalytic SCWG. Similar findings are reported in the literature with various nickel supported catalysts [45e47]. Herein, for the first time, the catalytic activity of doped catalysts is studied in SCWG. Surprisingly, the doped catalysts (Ni-DP and Zn-DP) gave higher gas yields than those of the supported catalysts (Ni-SP and ZnSP). All the synthesized catalysts accelerated the steam reforming reactions of Eqs. (1) and (2), apart from generating syngas with various gas compositions [48]. The x and y refer to the elemental molar ratios of H/C and O/C in the biomass, respectively. CHxOy þ (1 y)H2O / (1 y þ x/2)H2 þ CO
(1)
CHxOy þ (2 y)H2O / (2 y þ x/2)H2 þ CO2
(2)
The catalytic performances between supported (Ni-SP and ZnSP) and doped (Ni-DP and Zn-DP) MgO catalysts were compared by catalyzing the SCWG of OPF biomass to produce syngas with high H2 yield. For each SCWG experimental run, the reaction was performed at 400 C and 25 MPa for 30 min with 5 wt% of the synthesized catalyst. Without catalyst, the
Fig. 6 illustrates the effects of supported and doped MgO catalysts on the composition of gas products. For noncatalytic SCWG, the produced gas contained 41.1 vol% of CO, 29.2 vol% of CO2, 14.6 vol% of CH4, 10.6 vol% of H2, 3.8 vol% of C2H6, and 0.7 vol% of C2H4. CO emerged as the main gas product, followed by CO2, CH4, and H2 gases. Other hydrocarbon gases, such as C2H6 and C2H4, were also detected at very minimal amount. Nevertheless, the CO yield decreased with the addition of synthesized catalysts during SCWG of OPF biomass. It is believed that the added catalysts promoted the water-gas shift reaction, which promoted H2 yield in each gas product. The findings also showed that the CH4 yield remained constant although with the presence of catalysts. This means; both supported and doped catalysts were only
Fig. 5 e TPD-CO2 profiles of (a) Ni-SP, (b) Zn-SP, (c) Ni-DP and (d) Zn-DP catalysts.
Fig. 6 e Effects of supported and doped catalysts on gas composition for catalytic SCWG of OPF biomass.
Catalytic SCWG of OPF biomass
Please cite this article as: Mastuli MS et al., Comparative study between supported and doped MgO catalysts in supercritical water gasification for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.102
8
international journal of hydrogen energy xxx (xxxx) xxx
active for the water-gas shift reaction (Eq. (3)), but appeared to be inactive for the methanation reaction (Eqs. (4) and (5)). CO þ H2O / H2 þ CO2
(3)
CO þ 3H2 / CH4 þ H2O
(4)
CO2 þ 4H2 / CH4 þ 2H2O
(5)
Fig. 7 displays the impacts of each catalyst on the individual gas yield. It seemed that H2 yield increased tremendously with the decreasing yield of other gases. When compared to those without any catalyst, the supported catalysts; Ni-SP and Zn-SP increased the H2 yield by 187.2% and 269.0%, respectively. These H2 yields momentously increased when the doped catalysts were applied, wherein increment up to 361.7% and 438.1% had been recorded for Ni-DP and Zn-DP. In fact, two important findings need to be highlighted. First, the doped catalysts performed better than the supported catalysts. This suggests that the position of active element (Ni or Zn), regardless of outside or inside the cubic structure of the MgO, influenced the catalytic performances. Second, the Znbased catalysts always gave higher H2 yields than that of the Ni-based catalysts. The catalytic activity of the synthesized catalysts is in the order of Zn-DP > Zn-SP > Ni-DP > Ni-SP. The promotional effects of all the catalysts toward watergas shift reaction had been verified by the reduction of CO yield in each gas product. This means; the CO yield decreased as the H2 yield increased. Among the catalysts, the Zn-DP displayed the highest reduction of CO yield with 82.4% decrement as it produced the highest H2 yield by more than four times over Ni-SP, Zn-SP, and Ni-DP catalysts. Unfortunately, the CO2 yield did not adhere to the expected trend. During the water-gas shift reaction, as the produced CO reacted with water, both H2 and CO2 yields should be increased simultaneously. However, the CO2 yield in this study decreased in the range of 24.8%e41.9% although the added catalysts promoted the water-gas shift reaction, hence indicating that some CO2 could have been absorbed on the catalyst surfaces, as explained in our previous works [49,50]. The ability of the synthesized catalysts to absorb the produced CO2 is an advantage for the catalytic SCWG of OPF biomass to
Fig. 7 e Increment or decrement of gas composition after the addition of supported and doped catalysts for catalytic SCWG of OPF biomass.
produce H2 rich syngas. Another advantage is that all the synthesized catalysts did not catalyze the methanation reaction as the undesired CH4 gas was formed with constant gas yield. Hence, one can conclude that the supported and doped Ni and Zn on the MgO-based catalysts are active for water-gas shift reaction, but inactive for methanation reaction. The doped catalyst of Zn-DP caused the SCWG of OPF biomass to produce syngas with the highest H2 yield and the lowest CH4 yield, in comparison to other synthesized catalysts.
Relationship between catalyst properties and catalytic performances in SCWG reactions As depicted, varied catalyst structures (supported and doped) can significantly influence the catalytic performances of the synthesized catalysts. As reported previously [26], for the supported catalysts, the Zn-based catalyst exhibited the highest H2 yield from the OPF biomass even though it has the smallest specific surface area. The main reason for the Znbased catalyst to perform better during the SCWG reaction is due to the higher catalyst stability, as confirmed in the XPS outcomes. The ZnO that was supported on the MgO surface increased the strength of OeMg bond, which further increased the stability of the catalyst. Therefore, the stability of the catalyst is an important factor to be considered while designing a new catalyst system for the SCWG reaction. It is believed that the catalyst stability can be further improved by inserting (or doped substitutionally) the active metal into the crystal structure of the catalyst. Next, comparisons were made for the catalyst properties and the catalytic performances between supported and doped Ni and Zn onto/into MgO catalysts. The catalyst properties of both supported (Ni-SP and ZnSP) and doped (Ni-DP and Zn-DP) catalysts differed from each other. All the synthesized catalysts were highly pure, as indicated by the XRD patterns with mixed phase for the supported catalysts and the single-phase for the doped catalysts. The doped catalysts have very small crystallites, as compared to the supported catalysts, as observed vividly from the FESEM and HRTEM micrographs. These results are further confirmed using the BET analyses with the Ni-DP and Zn-DP catalysts showing larger specific surface area than the Ni-SP and Zn-SP catalysts. The smaller crystallite size provides larger specific surface area. Furthermore, the doped catalysts attained higher metal loading with higher active basic sites at medium basic strength. Interestingly, the doped catalysts had managed to catalyze the SCWG of OPF biomass effectively by increasing the H2 yield up to 361.7% and 438.1% for Ni-DP and Zn-DP catalysts, respectively. Although the Zn-DP catalyst has slightly smaller surface area than the Ni-DP catalyst, it has the highest number of active basic sites that are crucial to promote catalytic reaction. The basic catalysts possess greater tendency in promoting the water-gas shift reaction to increase the production of hydrogen [51,52]. The effectiveness of the doped catalysts for SCWG reaction was further influenced by catalyst stability, which was elucidated via XPS measurements. To better understand the chemical environment of both supported and doped catalysts, XPS investigation was performed. The BE values for all the elements are listed in Table 4.
Please cite this article as: Mastuli MS et al., Comparative study between supported and doped MgO catalysts in supercritical water gasification for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.102
international journal of hydrogen energy xxx (xxxx) xxx
9
Table 4 e Binding energy (BE) for Ni-SP, Ni-DP, Zn-SP and Zn-DP catalysts. Catalyst Ni-SP
Component Mg 2p Ni 2p3/2 Ni 2p1/2 O 1s
Ni-DP
Mg 2p Ni 2p3/2 Ni 2p1/2 O 1s
Zn-SP
Zn-DP
Mg 2p Zn 2p3/2 Zn 2p1/2 O 1s Mg 2p Zn 2p3/2 Zn 2p1/2 O 1s
Ni2þ Ni3þ OeMg OeNi Ni2þ Ni3þ OeMg OeNi Zn2þ OeMg OeZn Zn2þ OeMg OeZn
BE (eV) 48.86 852.16 853.37 869.62 530.06 527.98 48.98 852.85 854.07 870.30 530.02 528.33 49.14 1017.13 1040.14 530.27 528.29 49.23 1018.05 1040.98 530.31 528.35
% 75.5 24.5
77.2 22.8
100
100
All the BEs are corrected based on the C 1s peak (BE ¼ 284.2 eV) as depicted in Ref. [53], while Shirley background was applied for peak fitting. The BE values are also in agreement with the NIST database. Fig. 8 illustrates the XPS narrow scans of Mg 2p and O 1s for both supported and doped catalysts. As shown in Fig. 8(a), all the synthesized catalysts have symmetrical Mg 2p peaks; indicating only one oxidation state of þ2 present for Mg in each catalyst. Besides, the BE values of the Mg 2p retrieved for doped catalysts appeared to be higher than those obtained for supported catalysts (refer to Table 4). Regardless of supported or doped structures, the Zn-based catalysts shifted the Mg 2p peaks to higher BE values, as compared to the Ni-based catalysts. This phenomenon is believed to occur due to the bonding character of ionic bonds in the MgO. It is well known that in ionic bonding, the positions of cation (Mg2þ) and anion (O2) are locked by a strong electrostatic attraction force beA) is tween the ions. When larger ionic radius of Zn2þ (0.74 supported or doped onto/into the MgO crystal lattice, a greater electron repulsion is felt by the electron clouds surrounding the ions (Mg2þ and O2) that pushes the electrons into each Mg and O orbital closer to their nucleus. Note that the ionic radius A. This explains why the BEs of Mg 2p peak for of Mg2þ is 0.72 both Zn-SP and Zn-DP catalysts are higher, as compared to those of Ni-SP and Ni-DP catalysts, which have smaller ionic A). The ionic radii for all the cations adhere radius of Ni2þ (0.69 to the coordination number 6 of the cubic structure [54]. Between the Zn-based catalysts, the Zn-DP displayed the highest BE value for the Mg 2p peak than the Zn-SP. This can be further explained by examining the crystal structure of the catalysts, as illustrated in Fig. 2 (a). As for the supported catalyst, the active metal of Zn was only formed on the MgO surface. In contrast, for the doped catalyst, the Zn2þ took place in the Mg2þ site at the MgO lattice structure. The incorporation of Zn2þ into the MgO host lattice can cause the surrounding atoms (Mg and O) to experience more electron repulsion from
Fig. 8 e XPS narrow scans for (a) Mg 2p and (b) O 1s of the supported and doped catalysts. the electron cloud of Zn2þ than being only supported on the MgO surface. The greater electron repulsion leads the electrons in each orbital of Mg and O ions to move closer to their nucleus, which can result in higher BE values. This is well supported with the BE value of O 1s for the Zn-DP catalyst (see Table 4). In Fig. 8(b), the BE of O 1s for Zn-DP shifted to a higher value; indicating that its OeMg bond is stronger (shorter distance) than the other catalysts. The shorter distance between OeMg bond implies that more energy is needed to break this bond. Hence, it is can be concluded that the Zn-DP catalyst has higher stability of catalyst structure, when compared to the rest. Fig. 9 illustrates the XPS narrow scans of the active metals (Ni and Zn) for both supported and doped catalysts. The doped catalysts recorded higher BE values than the supported catalysts. This is mainly because the Ni and Zn ions took place in the Mg2þ sites in the MgO lattice. In Fig. 9(a), both the Nisupported and doped catalysts possess asymmetry shape of Ni 2p3/2 peaks, which reflect the existence of more than one oxidation state for Ni cations. The deconvolution on Ni 2p3/2 peak was performed for both catalyst structures. An example of deconvolution of Ni 2p3/2 peak for Ni-DP catalyst is shown in Fig. 10. The analysis revealed that the Ni-based catalysts have mixture of Ni2þ and Ni3þ, as tabulated in Table 4. On the contrary, Fig. 9(b) presents that both the Zn-supported and doped catalysts have symmetrical Zn 2p3/2 peaks. This implies
Please cite this article as: Mastuli MS et al., Comparative study between supported and doped MgO catalysts in supercritical water gasification for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.102
10
international journal of hydrogen energy xxx (xxxx) xxx
synthesized catalysts. The doped catalysts are more stable than the supported catalysts with the Zn-based catalysts always being more reactive than the Ni-based catalysts during the SCWG of OPF biomass. The catalytic performances of the synthesized catalysts are given in the following order: ZnDP > Zn-SP > Ni-DP > Ni-SP. This means; the doped catalysts can be considered as new promising catalysts for SCWG reactions.
Conclusion Supported (Ni-SP and Zn-SP) and doped (Ni-DP and Zn-DP) catalysts were synthesized successfully and catalyzed the SCWG of OPF biomass for hydrogen production. The study outcomes revealed that the doped catalysts have larger specific surface area than the supported catalysts with Zn-DP catalyst, which led to the highest H2 yield up to 438.1% of increment, when compared to that without catalyst. The catalytic activity of the doped catalysts seemed to be affected by the active basic sites and the catalyst stability. The Znbased catalysts generated the highest H2 yield, when compared to the Ni-based catalysts for each supported and doped catalysts. In future work, the compositions of Ni-DP and Zn-DP will be varied and used as new promising catalysts to catalyze the SCWG reactions.
Acknowledgement Fig. 9 e XPS narrow scans for active metal of (a) Ni and (b) Zn of the supported and doped catalysts.
that only one oxidation state exists for the Zn cation, which is þ2. This is further confirmed by the energy splitting of approximately 23 eV between Zn 2p1/2 and Zn 2p3/2 peaks. This finding is in line with the NIST database. Based on these results, the active element (Ni or Zn) and the catalyst structure (supported or doped) are greatly affected by the chemical environments, such as BE and oxidation state of the
Fig. 10 e The deconvolution for Ni 2p3/2 peak of Ni-DP catalyst.
This work was supported by the Malaysian Ministry of Higher Education (MOHE) through Fundamental Research Grant Scheme (600-RMI/FRGS5/3(13/2013)). M. S. Mastuli would like to thank to MOHE for the PhD scholarship.
references
[1] Dodds PE, Staffell I, Hawkes AD, Li F, Gru¨newald P, McDowall W, et al. Hydrogen and fuel cell technologies for heating: a review. Int J Hydrogen Energy 2015;40:2065e83. [2] Ehsan S, Wahid MA. Hydrogen production from renewable and sustainable energy resources : promising green energy Carrier for clean development. Renew Sustain Energy Rev 2016;57:850e66. [3] Sansaniwal SK, Pal K, Rosen MA, Tyagi SK. Recent advances in the development of biomass gasification technology: a comprehensive review. Renew Sustain Energy Rev 2017;72:363e84. [4] Pereira EG, Da Silva JN, De Oliveira JL, MacHado CS. Sustainable energy: a review of gasification technologies. Renew Sustain Energy Rev 2012;16:4753e62. [5] Puig-Arnavat M, Bruno JC, Coronas A. Review and analysis of biomass gasification models. Renew Sustain Energy Rev 2010;14:2841e51. [6] Guo Y, Wang SZ, Xu DH, Gong YM, Ma HH, Tang XY. Review of catalytic supercritical water gasification for hydrogen production from biomass. Renew Sustain Energy Rev 2010;14:334e43. [7] Abdoulmoumine N, Adhikari S, Kulkarni A, Chattanathan S. A review on biomass gasification syngas cleanup. Appl Energy 2015;155:294e307.
Please cite this article as: Mastuli MS et al., Comparative study between supported and doped MgO catalysts in supercritical water gasification for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.102
international journal of hydrogen energy xxx (xxxx) xxx
[8] Shen Y, Yoshikawa K. Recent progresses in catalytic tar elimination during biomass gasification or pyrolysis - a review. Renew Sustain Energy Rev 2013;21:371e92. [9] Azadi P, Farnood R. Review of heterogeneous catalysts for sub- and supercritical water gasification of biomass and wastes. Int J Hydrogen Energy 2011;36:9529e41. [10] Reddy SN, Nanda S, Dalai AK, Kozinski JA. Supercritical water gasification of biomass for hydrogen production. Int J Hydrogen Energy 2014;39:6912e26. [11] Chan FL, Tanksale A. Review of recent developments in Nibased catalysts for biomass gasi fi cation. Renew Sustain Energy Rev 2014;38:428e38. https://doi.org/10.1016/ j.rser.2014.06.011. [12] Yoshida T, Oshima Y, Matsumura Y. Gasification of biomass model compounds and real biomass in supercritical water. Biomass Bioenergy 2004;26:71e8. [13] Nguyen HT, Yoda E, Komiyama M. Catalytic supercritical water gasi fication of proteinaceous biomass: catalyst performances in gasification of ethanol fermentation stillage with batch and flow reactors. Chem Eng Sci 2014;109:197e203. [14] Sinag A, Kruse A, Rathert J. Influence of the heating rate and the type of catalyst on the formation of key intermediates and on the generation of gases during hydropyrolysis of glucose in supercritical water in a batch reactor. Ind Eng Chem Res 2004;43:502e8. [15] Ebrahimi-Nik M, Almassi M, Sheikhdavoodi MJ, Kruse A, Bahrami H. Gasification of sugarcane bagasse in supercritical water; evaluation of alkali catalysts for maximum hydrogen production. J Energy Inst 2015;88:450e8. [16] Furusawa T, Sato T, Saito M, Ishiyama Y, Sato M, Itoh N, et al. The evaluation of the stability of Ni/MgO catalysts for the gasification of lignin in supercritical water. Appl Catal Gen 2007;327:300e10. [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;36:3426e33. [19] Ding N, Azargohar R, Dalai AK, Kozinski JA. Catalytic gasification of glucose to H2 in supercritical water. Fuel Process Technol 2014;127:33e40. [20] Kang K, Azargohar R, Dalai AK, Wang H. Systematic screening and modification of Ni based catalysts for hydrogen generation from supercritical water gasification of lignin. Chem Eng J 2016;283:1019e32. [21] Sato T, Furusawa T, Ishiyama Y, Sugito H, Miura Y, Sato M, et al. Effect of water density on the gasification of lignin with magnesium oxide supported nickel catalysts in supercritical water. Ind Eng Chem Res 2006;45:615e22. [22] Furusawa T, Sato T, Sugito H, Miura Y, Ishiyama Y, Sato M, et al. Hydrogen production from the gasification of lignin with nickel catalysts in supercritical water. Int J Hydrogen Energy 2007;32:699e704. [23] Li S, Lu Y, Guo L, Zhang X. Hydrogen production by biomass gasification in supercritical water with bimetallic Ni-M/ gAl2O3 catalysts (M ¼ Cu, Co and Sn ). Int J Hydrogen Energy 2011;36:14391e400. [24] Tavasoli A, Barati M, Karimi A. Sugarcane bagasse supercritical water gasification in presence of potassium promoted copper nano-catalysts supported on g-Al2O3. Int J Hydrogen Energy 2016;41:174e80. [25] Barati M, Babatabar M, Tavasoli A, Dalai AK, Das U. Hydrogen production via supercritical water gasification of bagasse
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42]
11
using unpromoted and zinc promoted Ru/g-Al2O3 nanocatalysts. Fuel Process Technol 2014;123:140e8. Mastuli MS, Kamarulzaman N, Kasim MF, Sivasangar S, Saiman MI, Taufiq-Yap YH. Catalytic gasification of oil palm frond biomass in supercritical water using MgO supported Ni, Cu and Zn oxides as catalysts for hydrogen production. Int J Hydrogen Energy 2017;42:11215e28. Badar N, Kamarulzaman N, Rusdi R, Abdul Aziz ND, Kun Fun H. Increased conductivities of Cr doped Al2-xCrxO3 powders due to band gap narrowing. Phys B Condens Matter 2014;437:32e5. Kamarulzaman N, Mahat AM, Badar N, Zhao CZ. Structural and band gaps studies of novel Al2-xHfxO3 materials toward MOS applications. Mater Chem Phys 2018;216:237e42. Mastuli MS. Growth mechanisms of MgO nanocrystals via a sol-gel synthesis using different complexing agents. Nanoscale Res Lett 2014;9:134. Rusdi R, Rahman AA, Mohamed NS, Kamarudin N, Kamarulzaman N. Preparation and band gap energies of ZnO nanotubes, nanorods and spherical nanostructures. Powder Technol 2011;210:18e22. Tushar MSHK, Dutta A, (Charles) Xu C. Catalytic supercritical gasification of biocrude from hydrothermal liquefaction of cattle manure. Appl Catal B Environ Environ 2016;189:119e32. Guan Q, Chen S, Chen Y, Gu J, Li B, Miao R, et al. High performance noble-metal-free NiCo/AC bimetal for gasification in supercritical water. Int J Hydrogen Energy 2016;42:6511e8. Kang K, Azargohar R, Dalai AK, Wang H. Hydrogen production from lignin, cellulose and waste biomass via supercritical water gasification: catalyst activity and process optimization study. Energy Convers Manag 2016;117:528e37. Kamarulzaman N, Kasim MF, Chayed NF. Elucidation of the highest valence band and lowest conduction band shifts using XPS for ZnO and Zn0.99Cu0.01O band gap changes. Results Phys 2016;6:217e30. Kamarulzaman N, Kasim MF, Rusdi R. Band gap narrowing and widening of ZnO nanostructures and doped materials. Nanoscale Res Lett 2015;10:1e12. Chayed NF, Badar N, Rusdi R, Azahidi A, Kamaru. Band gap energies of Li2xMg(1-x)O materials synthesized by the sol-gel method. J Cryst Growth 2013;362:268e70. Huang J, Lian X, Wang L, Zhu C, Jin H, Wang R. Hydrogen production from glucose by supercritical water gasification with Ni/Zr(Ce , Y)O2 catalysts. Int J Hydrogen Energy 2016;42:4613e25. Hossain MZ, Chowdhury MBI, Jhawar AK, Charpentier PA. Supercritical water gasification of glucose using bimetallic aerogel Ru-Ni-Al2O3 catalyst for H2 production. Biomass Bioenergy 2017;107:39e51. Kou J, Xu J, Jin H, Guo L, Zhang D, Cao W. Evaluation of modified Ni/ZrO2 catalysts for hydrogen production by supercritical water gasification of oil-containing wastewater. Int J Hydrogen Energy 2018;43:13896e903. Li S, Guo L, Zhu C, Lu Y. Co-precipitated Ni-Mg-Al catalysts for hydrogen production by supercritical water gasification of glucose. Int J Hydrogen Energy 2013;38:9688e700. Hossain MZ, Chowdhury MBI, Alsharari Q, Jhawar AK, Charpentier PA. Effect of mesoporosity of bimetallic Ni-RuAl2O3 catalysts for hydrogen production during supercritical water gasification of glucose. Fuel Process Technol 2017;159:55e66. Lee I-G. Effect of metal addition to Ni/activated charcoal catalyst on gasification of glucose in supercritical water. Int J Hydrogen Energy 2011;36:8869e77.
Please cite this article as: Mastuli MS et al., Comparative study between supported and doped MgO catalysts in supercritical water gasification for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.102
12
international journal of hydrogen energy xxx (xxxx) xxx
[43] Li S, Savage PE, Guo L. Stability and activity maintenance of Al2O3- and carbon nanotube-supported Ni catalysts during continuous gasification of glycerol in supercritical water. J Supercrit Fluids 2018;135:188e97. [44] Zhang L, Champagne P, Xu C, Chunbao. Supercritical water gasification of an aqueous by-product from biomass hydrothermal liquefaction with novel Ru modified Ni catalysts. Bioresour Technol 2011;102:8279e87. [45] Behnia I, Yuan Z, Charpentier P, Xu C, Charles. Production of methane and hydrogen via supercritical water gasification of renewable glucose at a relatively low temperature: effects of metal catalysts and supports. Fuel Process Technol 2016;143:27e34. [46] Guo Y, Wang S, Yeh T, Savage PE. Catalytic gasification of indole in supercritical water. Appl Catal B Environ 2015;166e167:202e10. [47] Louw J, Schwarz CE, Burger AJ. Catalytic supercritical water gasification of primary paper sludge using a homogeneous and heterogeneous catalyst: experimental vs thermodynamic equilibrium results. Bioresour Technol 2016;201:111e20. [48] Guo LJ, Lu YJ, Zhang XM, Ji CM, Guan Y, Pei AX. Hydrogen production by biomass gasification in supercritical water: a
[49]
[50]
[51]
[52]
[53]
[54]
systematic experimental and analytical study. Catal Today 2007;129:275e86. Sivasangar S, Mastuli MS, Islam A, Taufiq-Yap YH. Screening of modified CaO-based catalysts with a series of dopants for the supercritical water gasification of empty palm fruit bunches to produce hydrogen. RSC Adv 2015;5:36798e808. Ismail K, Yarmo MA, Taufiq-Yap YH, Ahmad A. The effect of particle size of CaO and MgO as catalysts for gasification of oil palm empty fruit bunch to produce hydrogen. Int J Hydrogen Energy 2012;37:3639e44. Youssef EA, Chowdhury MBI, Nakhla G, Charpentier P. Effect of nickel loading on hydrogen production and chemical oxygen demand (COD) destruction from glucose oxidation and gasification in supercritical water. Int J Hydrogen Energy 2010;35:5034e42. Yanik J, Ebale S, Kruse A, Saglam M, Yuksel M. Biomass gasification in supercritical water: Part 1. Effect of the nature of biomass. Fuel 2007;86:2410e5. Ikeo N, Iijima Y, Niimura N, Sigematsu M, Tazawa T, Matsumoto S. Handbook of X-Ray photoelectron spectroscopy. 1991. Lide DR. Handbook of Chemistry and physics. 1995.
Please cite this article as: Mastuli MS et al., Comparative study between supported and doped MgO catalysts in supercritical water gasification for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.102