Catalytic steam reforming of complex gasified biomass tar model toward hydrogen over dolomite promoted nickel catalysts

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 1 3 0 3 e2 1 3 1 4

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Catalytic steam reforming of complex gasified biomass tar model toward hydrogen over dolomite promoted nickel catalysts Ru Shien Tan a,b, Tuan Amran Tuan Abdullah a,b,*, Saiful Azam Mahmud a,c, Rohaya Md Zin c, Khairuddin Md Isa d a

Centre of Hydrogen Energy, Institute of Future Energy, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia b School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia c Industrial Centre of Innovation in Energy Management, SIRIM Berhad, 40700 Shah Alam, Selangor, Malaysia d School of Environmental Engineering, Universiti Malaysia Perlis, Kompleks Pusat Pengajian Jejawi 3, 02600 Arau, Perlis, Malaysia

highlights

graphical abstract


Dolomite promoted Ni catalysts studied for steam reforming of gasified biomass tar.
Dolomite promoted metal-support interaction and basicity of the catalyst.
Low amounts of CO2 gas was released from steam reforming of tar.
Ni/dolomite/La2O3 catalyst showed superior catalytic performance.
Small amount of filamentous coke deposited on spent Ni/dolomite/ La2O3.

article info

abstract

Article history:

The complex mixture of gasified tar model (phenol, toluene, naphthalene, and pyrene) was

Received 21 March 2019

steam reformed for hydrogen production over 10 wt% nickel based catalysts. The catalysts

Received in revised form

were prepared by co-impregnation method with dolomite promoter and various oxide

13 June 2019

supports (Al2O3, La2O3, CeO2, and ZrO2). Steam reforming was carried out at 700  C at at-

Accepted 21 June 2019

mospheric pressure with steam to carbon molar ratio of 1 and gas hourly space velocity of

Available online 13 July 2019

20 L/h$gcat. The catalysts were characterised for reducibility, basicity, crystalline, and total surface area properties. Dolomite promoter strengthened the metal-support interaction

* Corresponding author. Centre of Hydrogen Energy, Institute of Future Energy, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia. E-mail address: [email protected] (T.A. Tuan Abdullah). https://doi.org/10.1016/j.ijhydene.2019.06.125 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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Keywords: Nickel Dolomite Catalyst Hydrogen Steam reforming

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and basicity of catalyst. The Ni/dolomite/La2O3 (NiDLa) catalyst with mesoporous structure (26.42mn), high reducibility (104.42%), and strong basicity (5.56 mmol/g) showed superior catalytic performance in terms of carbon conversion to gas (77.7%), H2 yield (66.2%) and H2/ CO molar ratio (1.6). In addition, the lowest amount of filamentous coke was deposited on the spent NiDLa after 5 h. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Tar

Introduction Recently, the depletion of fossil fuel is increasingly becoming a global concern. Among the possible alternatives, hydrogen (H2) gas produced from biomass gasification could potentially reduce the dependence on fossil fuels. However, the formation of unacceptable levels of tar in the syngas is the primary challenge and urgently facing the gasification process [1]. Tar is a complex mixture composed of various condensable hydrocarbons (HCs), varying from monocyclic to polycyclic aromatic HCs and also primary oxygenated to heavier deoxygenated HCs [2]. The presence of tar in syngas contributes to end-use problems such as blockages and corrosion in downstream filters, fuel line, engine nozzles and turbines [3]. Therefore, removal of tar by steam reforming is an attractive technology for converting HCs into H2 rich gas [4]. To date, many research studies deal with the steam reforming based on an individual tar model compound, typically phenol, benzene, toluene or naphthalene over a variety of supported metal catalysts [5e8]. Artetxe et al. [3], examined the efficient of Ni/Al2O3 catalyst for steam reforming of phenol at 750  C. It was found that 56% of carbon conversion and 11% of H2 yield were obtained. Quitete et al. [9], studied the steam reforming of toluene over Ni/dolomite and mixed oxides (i.e. Ce0.4Ni0.6AlO3 and La0.2Ni08AlO3) catalysts. The authors observed that Ni/dolomite presented lower toluene conversion than mixed oxides due to hydration and carbonation of dolomite support. Josuinkas et al. [10], investigated the steam reforming of different composition of tar model (benzene, toluene, naphthalene) over Ni/MgO/Al2O3. The authors reported that naphthalene has higher thermal stability. It inhibited toluene conversion and increased the favorable temperature for toluene complete conversion from 700  C to 900  C during steam reforming of naphthalene/toluene. In this study, a multi-compound tar model is used. The selected tar model compounds are phenol for phenolic and heterocyclic HCs, toluene for one-ring aromatic HCs, naphthalene for tworing aromatic HCs and pyrene for four-ring and higher HCs. Among the various existing catalysts, Ni-based catalysts have been extensively employed for steam reforming. This is due to the economic viability of Ni-based catalysts (e.g. 100e150 times cheaper than noble metals) [11,12]. Besides, Ni has high activity for tar destruction (e.g. CeC, CeO, and CeH bonds cleavage) along with the additional activity for water gas shift (WGS) reaction [13,14]. According to Wang et al. [15], and Furusawa et al. [16], Ni showed a more promising performance for steam reforming compared to Co and Pt.

However, Ni-based catalysts are prone to deactivation of its active sites by coke formation [17,18]. The oxide supports used in this study include alumina (Al2O3), lanthana (La2O3), ceria (CeO2) and zirconia (ZrO2). Notably, Al2O3 is often used as catalyst support because it is inexpensive with high thermal stability and large specific surface area [19]. Nonetheless, its high acidity promotes coke deposition on catalysts [20,21]. The alkalinity and high hygroscopic nature of La2O3 are reportedly beneficial to the neutralisation of the acid sites responsible for oligomerisation and the enhancement of water adsorption for coke gasification [22]. Recent research has demonstrated that CeO2 and ZrO2 facilitate coke gasification and the water gas shift (WGS) reaction. This ability is due to its redox and high storage properties, which enhance the mobility of oxygen [23,24]. However, the small surface area of CeO2 and unactive CeC bond cleavage of ZrO2 offer a low catalytic activity during steam reforming [16]. Recently, it has been reported that addition of alkaline earth metal oxides (MgO and CaO) could neutralise the acidity of the catalyst and improve steam-carbon reaction, which in turn increases the coke suppression rate and catalytic stability. Ashok and Kawi [25] reported that CaO-doped catalyst could activate water molecules to produce OH groups on the catalyst which promotes the CeC bond cleaving, enhances WGS reaction, and accelerates the oxidation of coke. Moreover, Udomsirichakorn et al. [26], claimed that the spatially diffuse electron clouds of CaO play a role in tar reforming. This disrupts the stability of the p-electron cloud of tar compounds at 650  C, consequently leading to aromatic ring breaking. Zhang et al. [27], found that MgO promoter improved the Ni dispersion and enhanced the Ni-support interaction by the formation of NiOeMgO solid solutions, subsequently resulting in the high conversion of toluene via steam reforming reaction. As reported by Koike et al. [28], the MgO-based solid solution could eliminate the catalyst deactivation by suppressing the aggregation and sintering of metal. Low cost and abundance naturally occurring minerals such as dolomite that contains both CaO and MgO. Recently, dolomite (CaMg(CO3)2) has been used as the catalyst support [29] or CO2 sorbent [30] in the steam reforming process. Wang et al. [31], examined the steam reforming of naphthalene over Nidolomite catalyst. The authors observed that coke was mainly distributed on the surface of dolomite compared to Ni [31]. Sisinni et al. [30], reported that dolomite is considered as CO2 sorbent which adsorbs the CO2 produced from steam reforming and lowered its partial pressure, thereby shifts the thermodynamic equilibrium of WGS reaction towards H2

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production. However, to the best of the authors’ knowledge, there is no study reported on the utilisation of dolomite as a promoter. In this study, a combination of oxide support and dolomite promoter was suggested to address the weaknesses of Nibased catalysts. Therefore, this study aims to investigate the catalytic activity of the oxide supported Ni-based catalysts promoted with dolomite through steam reforming of selected tar model compounds based on catalytic activity and coke formation.

Experimental methods Catalyst preparation The 10 wt% Ni-based catalysts were synthesised by a coimpregnation method based on the composition listed in Table 1. Nickel nitrate hexahydrate (Ni(NO3)2ˑ6H2O) (99%, Sigma-Aldrich) was used as the Ni precursor. The catalysts were formulated with the dolomite promoter (comprised of 57.3 wt% CaCO3 and 41.8 wt% MgCO3) and various oxide supports including g-Al2O3 (99.9%, Merck), La2O3 (99.9%, SigmaAldrich), CeO2 (90%, Sigma-Aldrich), and ZrO2 (99%, SigmaAldrich). Firstly, the active metal, oxide support, and dolomite were added into deionised water and continuously stirred at 90  C until the mixture turned into a viscous paste. Next, the paste was dried entirely at 110  C for 12 h. After drying, the dried catalyst was calcined in the muffle furnace at 750  C for 3 h. Lastly, the catalyst was pelletized, ground, and sieved into a particles size between 34 and 35 mesh.

Catalyst characterisation Thermogravimetric analysis (TGA) was performed on the prior-calcined and spent catalysts using a thermogravimetric analyser (Shimadzu TG-50, Japan). Each catalyst was heated in-situ with continuous airflow at the heating rate of 10  C/ min. The weight loss of the catalyst was recorded over a temperature range of 30e900  C. Temperature-programmed reduction (TPR) analysis was performed using a chemisorption analyser (Micromeritics Chemisorb 2720, USA) equipped with a thermal conductivity detector (TCD). Before the reduction process, the calcined catalyst was pretreated with helium (He) flow at 300  C for 30 min to remove moisture and contaminants from the catalyst surface. The TPR profile was obtained by heating the calcined catalyst over a temperature range of 25e1000  C at a

linearly programmed rate of 20  C/min with a 20 mL/min of 10 vol% H2/Ar flow. The crystalline structure of the reduced catalyst was analysed by X-ray diffraction (XRD) using the high-resolution Xray diffractometer (Shimadzu XRD 600, Japan) with a Cu target Ka radiation at 30 kV and 30 mA. The X-ray diffractogram was developed in the scanning angle (2q) range of 10e80 at a scanning speed of 1 per minute. The textural properties of the reduced catalyst were determined using the Beckman Coulter SA3100 surface area analyser with N2 as the adsorptive gas. Before each analysis, the reduced catalyst was degassed at 300  C with He flow for 1 h to remove contaminants from the catalyst surface. The N2 adsorption was conducted at the temperature of liquid N2 (77 K) over a relative pressure (0e1). Micromeritics Chemisorb 2720 apparatus was used for temperature-programmed desorption of carbon dioxide (CO2TPD) analysis. The reduced catalyst was purged at 300  C in a He flow for 30 min to remove the adsorbed impurities. The reduced catalyst was saturated with 20 mL/min pure CO2 at 50  C for 30 min and purged with He for another 30 min to remove the excess CO2. Next, the CO2 desorption was conducted from saturation temperature to 900  C at 20  C/min linear heating rate with a 20 mL/min of He flow. The nature of the coke deposited on the spent catalyst was physically observed through the JEOL JSM-IT300LV variablepressure scanning electron microscope (VP-SEM) at an accelerating voltage of 10 kV.

Catalytic activity tests The catalytic performance was investigated by steam reforming of tar model compounds from biomass gasification. The gasified biomass tar model was comprised of 15 wt% of phenol, 50 wt% of toluene, 30 wt% of naphthalene, and 5 wt% of pyrene. The selected components are the major chemicals contained in the gasified biomass tar as reported by Singh et al., [32]. Experiments were performed at atmospheric pressure in a fixed bed stainless steel tubular reactor (27 cm length  1.27 cm inner diameter). Fig. 1 illustrates the schematic diagram of the catalytic steam reforming experimental setup. For each test, about 0.8 g of catalyst was diluted with silicon carbide (1:2.5 wt ratio) to avoid the formation of local hot spots within the catalyst bed during reaction [33]. The catalyst bed was loaded on the stainless steel mesh located at the middle of the reactor. Next, a stream of 10 vol% H2/N2 with a flow rate of 50 mL/min was passed through the reactor to reduce the

Table 1 e Composition (wt.%) of catalysts. Catalyst Ni/dolomite Ni/dolomite/Al2O3 Ni/dolomite/La2O3 Ni/dolomite/CeO2 Ni/dolomite/ZrO2 Ni/La2O3

Symbol

Ni (wt.%)

Dolomite (wt.%)

Al2O3 (wt.%)

La2O3 (wt.%)

CeO2 (wt.%)

ZrO2 (wt.%)

NiD NiDAl NiDLa NiDCe NiDZr NiLa

10 10 10 10 10 10

90 10 10 10 10 e

e 80 e e e e

e e 80 e e 90

e e e 80 e e

e e e e 80 e

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Fig. 1 e Schematic diagram of the steam reforming experimental setup: (1) Syringe pump; (2) High pressure liquid pump; (3) 10% H2/N2 cylinder; (4) N2 cylinder; (5) MKS mass flow controller; (6) K-type thermocouple; (7) Preheater; (8) Reactor; (9) Catalyst; (10) Furnace; (11) Condenser; (12) Temperature controller; (13) GC.

fresh catalyst at the selected temperature (900  C for Al2O3 supported catalyst and 800  C for others) for 60 min. The selected gasified biomass tar model compound was subsequently steam reformed at 700  C in the reactor. Next, about 0.02 mL/min of gasified biomass tar model was directly fed into the reactor using a syringe pump (KD Scientific Series 100, USA). About 1.66 mL/h of water was fed into the preheater using a high-pressure liquid pump (Lab Alliance Series II).

Carbon conversion to gas ð%Þ ¼

free basis as expressed in Eq. (2). For catalytic activity evaluation, carbon conversion to gas (Eq. (3)) [13,35] and H2 yield (Eq. (4)) were also considered in this research. C6 H6 O þ C7 H8 þ C10 H8 þ C16 H10 þ 38 H2 O/39 CO þ 54 H2 SX ðmol %Þ ¼

mole of X in the product gas  100 total mole of gaseous product

mole of carbon in the product gas  100 mole of carbon in the tar fed

Steam to carbon (S/C) molar ratio of 1 was selected for the feed stream. This is because the stoichiometric value of S/C molar ratio is 0.97 corresponding to steam reforming of gasified biomass tar (Eq. (1)). Preheater was ramped to 250  C to vaporise the water before entry to the reactor. In addition, 50 mL/min of N2 carrier gas was continuously introduced into the preheater to sweep the vaporised steam into the reactor. The reaction products produced were cooled after passing through a condenser filled with an ice and ethanol mixture. The gas product was analysed on-line by gas chromatography (GC) (Agilent 6890N) fitted with Carboxen 1010 PLOT capillary GC column (30 m L  0.53 mm ID, average thickness 30 mm), and equipped with TCD. Many parallel reactions are known to occur during catalytic steam reforming [34]. The completion of these reactions determines the product selectivity. In this study, the product selectivity (SX) was evaluated by calculating the composition of each gaseous product (X ¼ H2, CO, CO2, CH4) on a dry and N2

H2 yield ð%Þ ¼

(1)

(2)

(3)

mole of H2 in the product gas mole of H2 in tar fed þ mole of H2 in steam

 100 (4)

Results and discussion Catalyst characterisation The thermal stability of the prior-calcined catalysts was evaluated by TGA. As observed in Fig. 2, the initial weight loss of the catalysts occurred below 200  C. This could be related to the thermal dehydration of physically absorbed water and

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Fig. 2 e Thermogravimetric curves of the prior-calcined catalysts.

volatile impurities during synthesis route. The weight loss of catalysts above 200  C could be attributed to the transformation of amorphous to crystalline phase (decomposition of Ni(NO3)2ˑ6H2O to NiO) and thermal decomposition of dolomite (CaMg(CO3)2) to MgO and CaO. From Fig. 2, it can be seen that the zero weight changes are observed at above 750  C, indicating that the decomposition of catalysts is completed. Therefore, the catalysts must be calcined at an optimum calcination temperature of 750  C to ensure the complete nickel oxide phase formation and stabilise the mechanical properties (structure and textual) of catalysts. The reducibility of the calcined catalysts was subsequently characterised by H2-TPR. The results showed that the catalysts exhibited multiple-stage reduction peaks between 300 and 1000  C, due to the different degrees of reducible metal oxide-support interaction. It is interesting to note that the addition of dolomite promoter shifts the reduction peak toward higher temperature region, compared to those without dolomite promoter as reported by other researchers [36e40]. The TPR profiles are divided into 4 phases in a specific temperature region, as portrayed in Fig. 3. Based on the ease of

Fig. 3 e H2-TPR profiles of calcined catalysts.

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reduction, the free-state NiO particle (phase I; < 400  C) is the easiest to reduce [41] followed by the bulk NiO particle (phase II; 400e500  C) [29] and support interacted NiO (phase III; 500e650  C) [41]. Lastly, spinel type NiO (i.e. NiAl2O4 and NiLa2O4 spinels) [42,43] and solid solution (i.e. NiOeCaO, NiOeMgO, and NiOeCeO2 solid solutions) [5,41,44] is the hardest to reduce (phase IV; > 650  C). Among the catalysts, NiDAl catalyst had the highest reduction temperature with the main peak observed at 857  C. These findings along with XRD analysis, confirm the reduction peak is due to the reduction of non-stoichiometric spinel type metals (NiAl2O4). For NiD, a main peak at 700  C is ascribed to the reduction of NiO arising from solid solution which is distributed within the CaO lattice [45] and MgO lattice [44]. Besides, a reduction of MgO also occurred around 700  C [46]. Thus, the higher peak at 700  C compared to other catalysts is due to the higher composition of dolomite. For the NiD, NiDLa and NiDZr catalysts, the H2 consumption increased sharply at the low-temperature region (phase II), revealing that the existence of bulk NiO as the prevailing metal oxide to a significant extent. Free NiO was also observed in the NiD, NiDLa, and NiDZr catalysts. Typically, at lower reduction temperature results in the incomplete reduction of NiOx to metallic Ni, while at higher temperatures the catalyst tends to deactivate due to metallic Ni agglomeration. Hence, in this research, 800  C was selected as the reduction temperature for NiD, NiDLa, NiDCe and NiDZr catalysts. Due to the presence of less reducible species in the NiDAl catalyst, the primary reduction peak shifted to a higher temperature region. This finding suggests that a higher reduction temperature (900  C) is required for the NiDAl catalyst. The reduction degree of each calcined catalysts is listed in Table 2. The reduction degree was calculated by assuming NiO is sufficiently reduced to metallic Ni0 and the contribution of partial reduction of support is negligible [25]. It is found that the degree of reduction ranged from 72% to 238%. However, the reduction degree for NiD, NiDLa and NiDZr were above 100%, indicating that the occurrence of partial reduction of the support. Similar observations were reported by other researchers, indicating the reduction of CeO2 occurs at 927  C [47], ZrO2 at 580e650  C [48], MgO at 706  C [46] and CaO at 610  C [49]. As shown in Fig. 4, the crystallographic nature of the reduced catalysts was characterised by XRD. All the reduced catalysts exhibited low intensity or zero diffraction peaks attributed to NiO (2q ¼ 37.3 , 43.3 , 62.2 ), whereas the strong, intense diffraction peaks corresponding to metallic Ni (2q ¼ 44.5 , 51.8 , 53.8 , 76.3 ) were observed. This indicates that all the catalysts are activated by reducing NiO to metallic Ni, which is deemed as the principal active phase in the steam reforming of tar. The size of Ni crystalline and Ni dispersion were calculated to be 29.1e37.3 nm and 2.7e3.5%, respectively (see Table 2). As revealed in XRD profiles, the appearance of NiAl2O4 on the NiDAl catalyst at 2q ¼ 38.3 and 45.5 suggests that the NiO combined with Al2O3 to form the spinel during calcination and it cannot be reduced entirely at 900  C. The diffraction peaks of NiO were not detected on the NiDLa catalyst, and a small amount of NiLa2O4 was detected at 31.5 , which revealed that

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Table 2 e Reducibility, textural characteristic, crystallite properties and basicity of catalysts. Catalyst

NiD NiDAl NiDLa NiDCe NiDZr a b c d e f g

Reduction degree (%)a

Ni crystallite size (nm)b

238.4 72.9 104.4 81.9 118.4

37.31 32.65 37.33 37.32 29.05

Ni BET surface area Pore volume dispersion (m2/g)d (cm3/g)e c (%) 2.7 3.1 2.7 2.7 3.5

9.71 62.86 12.07 7.44 7.43

0.12 0.24 0.08 0.04 0.05

Average pore size (nm)f 47.21 15.11 26.42 20.64 26.82

Basicity (mmol/g)g Weak Strong Total e 0.47 e 0.002 e

15.81 0.14 5.56 0.003 0.78

15.81 0.62 5.56 0.005 0.78

Reduction degree ¼ (H2 consumption by TPR/theoretical H2 consumption)  100, assuming Ni2þ þ H2 / Ni0 þ 2H2þ. Calculated from Ni (111) at 2q ¼ 44.5 by the Scherrer equation, crystallite size ¼ 0.89l/bcosq where l ¼ 0.15406 nm. Dispersion ¼ 101/crystallite size, assuming that Ni particles exhibit a spherical geometry. Determined at p/po of 0.05e0.30 using BET equation by assuming the N2 molecule cross sectional area of 0.162 nm2. Determined at the highest p/po of 0.99. Pore size ¼ (4000  pore volume)/BET surface area [78]. Determined by CO2-TPD.

Fig. 5 e BJH pore size distribution of reduced catalysts. Fig. 4 e XRD patterns of the reduced catalysts. Crystalline phases: Ni (-); NiO (*); MgO (C); CaO (&); Ca(OH)2 ()); Al2O3 (D); NiAl2O4 (£); La2O3 (#); La(OH)3 (,); NiLa2O4 (þ); CeO2 (B); ZrO2 (▽). all the Ni oxide species were almost completely reduced during the reduction process. These observations conform to the TPR results presented in Table 2. However, the existence of NiO species on NiD are not consistent with the TPR result which shows a complete reduction of the Ni oxides. There is a high chance that the NiO and MgO phases have similar 2q values. Therefore, it is difficult to differentiate between the two phases by XRD technique [50]. Besides, there were no characteristic peaks for CaO in the NiDLa and NiDZr catalysts, suggesting that CaO is either well dispersed or presented as amorphous structure in such catalysts [35,51]. For the NiD and NiDZr catalysts, the portlandite phase, namely Ca(OH)2 had been detected as a result of the hydroxylation process of water humidity during handling and storage [52,53]. However, the appearance of lanthanum hydroxide (La(OH)3) in NiDLa is expected due to the hygroscopic nature of La2O3 support [54,55]. The textural characteristics of the reduced catalysts were analysed by N2 physisorption isotherms, as presented in Table 2. As observed, the NiDAl catalyst showed the highest BET

surface area (62.86 m2/g) and pore volume (0.24 cm3/g) with the smallest pore size (15.11 nm) compared to other reduced catalysts. Furthermore, the BET surface area of reduced catalysts increased in the order: NiDZr < NiDCe < NiD < NiDLa < NiDAl. Fig. 5 illustrates the Brunauer-Joyner-Halenda (BJH) pore size

Fig. 6 e CO2-TPD profiles of reduced catalysts.

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distribution of all reduced catalysts. As depicted in Fig. 5, all of the reduced catalysts were dominated by mesopores (2e50 nm), where small mesopores (<10 nm) constituted a large percentage of the total pore volume. It can be confirmed that mesoporous structures are successfully developed on all catalysts. As reported by other researches, the mesoporous structure of any catalyst is beneficial to reactant and heat diffusion during steam reforming reaction along with good endurance to coke formation and Ni sintering [56,57]. The CO2-TPD patterns of reduced catalysts are presented in Fig. 6. The strength of basic sites is related to the peak position, which indicates the interaction between the CO2 and the basic sites. Typically, strong basic sites are characterized by high desorption peak temperature [58]. As shown in Fig. 6, the weak and strong basic strengths correspond to the desorption peaks at temperature below 250  C and above 400  C, respectively. The total number of basic sites on the catalysts were in the order NiD > NiDLa > NiDZr > NiDAl > NiDCe. The results indicate that the NiD catalyst had the highest number of basic sites and desorbed CO2 at 448 and 635  C. On the other hand, the basicity of NiDAl was mainly attributed to weak basic sites, which constituted 75.6% of the total number of basic sites. The negligible of basic sites on NiDCe is due to the weak interaction between CO2 and CeO2 support [59]. The addition of the dolomite promoter increased the basicity of the catalysts compared to others without the promoter as also reported by other researchers [37,60,61].

Catalytic activity Fig. 7 reflects the average value of total gas production, carbon conversion to gas and H2 yield for various catalysts during 5 h’ steam reforming of tar model compounds from biomass gasification. The catalytic performance in carbon conversion to gas is ranked in the order of NiDLa > NiDCe > NiDZr > NiDAl > NiD > NiLa. However, the H2 yield for each catalyst follows the order NiDLa > NiDAl > NiDZr > NiD > NiDCe > NiLa. Despite its low

Fig. 7 e The average value of total gas production, carbon conversion to gas and H2 yield in the 5 h steam reforming of gasified biomass tar model over various catalysts. Reaction conditions: temperature ¼ 700  C; S/C molar ratio ¼ 1; catalyst mass ¼ 0.8 g; GHSV ¼ 20,453 mL/h·gcat.

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surface area (12.07 m2/g) and pore volume (0.08 cm3/g), NiDLa catalyst demonstrated the most significant amount of total gas production (134.9 mmol/mLtar), the highest carbon conversion to gas (77.7%), and the highest H2 yield (66.2%) among the catalysts studied. It is shown that the textural properties scarcely affects catalytic activity on steam reforming of gasified biomass tar model whereas the selection of the appropriate support material was more important [62,63]. To compare the activity between dolomite promoted and non-promoted catalysts, NiLa catalyst was prepared using the same preparation method for NiDLa. As can be seen in Fig. 7, NiLa catalyst was the least efficient catalyst for steam reforming, offering 64.47 mmol/mLtar of total gas production, 28.5% of carbon conversion to gas, and 37.5% of H2 yield. The catalytic performance of dolomite promoted NiLa catalyst showed better performance over NiLa catalyst. This is because CaO could catalytically facilitates the tar cracking and tar-steam reaction [64]. Fig. 8 depicts the average selectivity towards the gaseous product and H2/CO molar ratio of reformate that resulted from the 5 h steam reforming of gasified biomass tar model over various catalysts. As observed, the main gaseous products are H2 (58.0e50.8 mol %) and CO (47.2e34.8 mol %) followed by CO2 (1.9e8.5 mol %) and CH4 (0.0e1.0 mol %). The results indicate that the main reaction during the process is steam reforming followed by WGS reaction. However, CH4 was undetectable along the steam reforming reaction over NiD and NiDAl catalysts. Two reasons can explain the absence of CH4: (i) the parallel reaction such as hydroalkylation does not take place in a significant extent throughout the catalytic activity test or (ii) the CH4 produced has been further converted into H2 and CO through the methane steam reforming [65]. Furthermore, NiD, NiDLA, and NiLa catalysts exhibited an H2/CO molar ratio of 1.5, 1.6, and 3.9, respectively. Their H2/CO molar ratio is higher than the expected value from stoichiometry (1.38) of steam reforming reaction of tar model (Eq. (1)). Compared to other researchers, the CO2 constituent of reformate (1.5e8.5 mol %) produced in this work is much lower than typically reported in the literature ranging from 10% to

Fig. 8 e The average gaseous product selectivity and H2/CO molar ratio during 5 h' steam reforming of gasified biomass tar model over various catalysts. Reaction conditions: temperature ¼ 700  C; S/C molar ratio ¼ 1; catalyst mass ¼ 0.8 g; GHSV ¼ 20,453 mL/h·gcat.

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25% [22,65,66]. From this work, it is also observed that the CO2 production of NiDLa catalyst is approximately twice less than NiLa. This is because of the CO2 sorption by CaO contained in calcined dolomite (MgO/CaO) via the carbonation reaction (Eq. (5)). Hlaing et al. studied the effect of carbonation temperature on CO2 adsorption capacity of CaO [67]. The authors observed that at similar reaction temperatures of this research (700  C), CaO reached the highest capacity of adsorption which is 0.682 g-CO2/g-CaO. CaOðsÞ þ CO2ðgÞ /CaCO3ðsÞ

(5)

The variation in carbon conversion to gas and H2 yield with time on stream is displayed in Fig. 9. It is noted that all catalysts exhibited different trends in carbon conversion to gas and H2 yield over 300 min of reaction time: (i) relative consistency over NiDLa and NiLa catalysts, (ii) fluctuated trend over NiDCe and NiDZr catalysts, (iii) slightly decreased trend over NiDAl catalyst, and (iv) marked decreased trend over NiD catalyst. The activity of NiD catalyst is in accordance with the finding of Shuai et al. [8] that the catalytic activity of CaO in the steam reforming decreased dramatically over the reaction time due to the loss of its catalytic activity in the carbonated form. Furthermore, the NiD catalyst also became soft and frangible after 300 min of catalytic activity test. This phenomenon was also observed by Nordgreen et al. [68], who showed that calcined dolomite is soft and has weak attrition resistance. The dolomite promoted catalysts possess better catalytic performance than the dolomite supported catalyst. Therefore, in this case, dolomite is more suitable act as a promoter rather than support.

Fig. 10 e TG curves of spent catalysts after catalytic activity test.

Table 3 e Weight loss of spent catalysts during TG analysis. Catalyst

Amorphous region

Filamentous region

Weight loss Weight gain Weight loss Weight gain (wt.%) (wt.%) (wt.%) (wt.%) NiD NiDAl NiDLa NiLa NiDCe NiDZr

8.1 7.3 0.0 3.8 0.0 0.0

3.2 0.0 2.7 1.8 0.9 1.2

6.4 10.6 11.0 12.2 13.3 13.5

0.0 0.0 0.0 0.0 0.0 0.0

Characterisation of spent catalyst The coking resistance and properties of the coke deposited on each catalyst after 5 h of catalytic activity test was investigated using TGA (in air), as presented in Fig. 10. The catalyst weight loss during the TG analysis is associated with the oxidation of deposited coke. Table 3 summarized the weight loss and weight gain during TG analysis. The coke deposited over the catalyst surface is classified into two groups, namely; amorphous (oxidised below 550  C) and filamentous carbons (oxidised at 550e750  C) [69]. In literature, amorphous carbon deactivates the catalyst, whereas filamentous carbon does not significantly contribute to deactivation but leads to reactor

blockage and pressure depression [18]. Furthermore, an additional weight gained was observed for all the spent catalysts at the region of amorphous coke, except the spent NiDAl catalyst (see Fig. 10). This might be due to (i) the formation of NiO species through oxidation of metallic Ni particles, (ii) formation of La2O2CO3 through the reaction between La2O3 and CO2 (e.g. produced from the oxidation of deposited coke), and (iii) formation of CaCO3 through carbonation of CaO [70,71]. Based on Fig. 10, the weight gained by the formation of CaCO3 in the region of amorphous carbon is favourable.

Fig. 9 e - (a) Carbon conversion to gas and (b) H2 yield as a function of time on stream during steam reforming of gasified biomass tar model over various catalysts. Reaction conditions: temperature ¼ 700  C; S/C molar ¼ 1; catalyst mass ¼ 0.8 g; GHSV ¼ 20,453 mL/h·gcat.

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Fig. 11 e VP-SEM micrograph of spent (a) NiDAl and (b) NiDLa catalysts with 1000 £ magnification. As observe in Fig. 10, the addition of dolomite eliminates the formation of amorphous coke on NiDLa catalyst. The weight loss of spent NiDLa catalyst only occurs at the region of filamentous carbon. Besides, spent NiDLa catalyst shows the smallest amount of weight loss compared to other spent catalysts. Thus, it is suggested that the spent NiDLa catalyst had the smallest amount of coke formation. This is because CaO and MgO adsorb CO2 to formed active carbonate species, which provide oxygen atoms to gasify coke that deposited on the catalyst [72,73]. Besides, NiDLa catalyst with higher total number of basic sites also restricts the coke formation by enhancing steam-coke reaction and preventing oligomerisation reaction [74]. The coke suppression is also attributed to the reaction of La2O2CO3 with deposited carbon on the adjacent Ni sites [75]. As a result, the NiDLa catalyst is more active and stable during steam reforming, as observed in Fig. 9. The mechanism of the coke suppression feature of La2O3 support is expressed in Eqs. (6) and (7) [74]. La2 O3ðsÞ þ CO2ðgÞ #La2 O2 CO3ðsÞ

(6)

La2 O2 CO3ðsÞ þ CðsÞ #La2 O3ðsÞ þ 2 COðgÞ

(7)

thereby producing an oxygen lattice. Next, the oxygen lattices formed diffuse into the Ni sites to facilitate coke suppression [77]. The reversible reaction (Eq. (9)) and coke removal reaction (Eq. (10)) are presented below. CaCO3ðsÞ / CaOðSÞ þ CO2ðgÞ

(8)

MO2ðsÞ #MO2xðsÞ 475 þ OxðgÞ

(9)

CðsÞ þ OxðgÞ /Ox1ðgÞ þ COðgÞ

(10)

where M is Ce or Zr; Ox is lattice oxygen on the support surface; C(s) is carbon deposited on the catalyst; and Ox-1 is reduced site of support. Based on characterisation, activity test, and coke formation analysis results, NiDLa catalyst is the best catalyst for steam reforming of gasified biomass tar among the catalyst examined in this study.

Conclusions

In contrast, the spent NiDAl catalyst shows a relatively high amount of total weight loss. This is due to the high acidity of Al2O3 support promotes the dehydrogenation of hydrocarbon into coke over metal phases [76]. Thus, the decreasing of catalytic activity shown in Fig. 9 is related to the deposition of amorphous coke on the NiDAl catalyst. In order to confirm the coke formation on spent NiDAl and NiDLa catalysts, VP-SEM analysis was carried out to observe the surface morphology, as shown in Fig. 11. Only the filamentous carbon was visible on the spent NiDLa catalyst but both amorphous and filamentous carbon were observed on spent NiDAl catalyst, matching the finding of TGA. As observed in Fig. 10, the spent NiD catalyst shows a significant weight loss between 400  C and 700  C, which may be ascribed to not only coke oxidation but also the calcination of CaCO3 (Eq. (8)) [70]. The finding in Fig. 2 is evidence of the occurrence of CaCO3 calcination. The lower amount of weight loss associated with filamentous coke deposited on the NiDCe and NiDZr spent catalysts is attributed to the redox capability of CeO2 and ZrO2 [19,20]. During the steam reforming process, water is dissociated on the oxygen vacancies on the support,

The steam reforming of the complex gasified biomass tar model compounds was carried out over NiD, NiDAl, NiDLa, NiDCe, NiDZr and NiLa catalysts. The findings revealed that the addition of dolomite promoter reduced the undesired CO2 gas emission. Moreover, the addition of dolomite promoter also improved the metal-support interaction and anti-deactivation capacity. Among the catalysts, NiDLa is considered the most promising for H2 production through steam reforming of the tar model compounds due to its excellent catalytic performance for carbon conversion to gas, H2 yield, and the outstanding anticoking ability. Furthermore, the type of coke deposited on the spent NiDLa catalyst is filamentous carbon which does not diminish the activity of catalyst. Therefore, the capability of NiDLa to retain the catalytic activity during the steam reforming of gasified biomass tar is established.

Acknowledgement This work was supported by the Universiti Teknologi Malaysia through the Research University Grant (GUP Tier 1: 20H52).

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