Steam reforming of toluene as a tar model compound with modified nickel-based catalyst

Abstract

Catalytic steam reforming is a promising route for tar conversion to high energy syngas in the process of biomass gasification. However, the catalyst deactivation caused by the deposition of residual carbon is still a major challenge. In this paper, a modified Ni-based Ni-Co/Al₂O₃-CaO (Ni-Co/AC) catalyst and a conventional Ni/Al₂O₃ (Ni/A) catalyst were prepared and tested for tar catalytic removal in which toluene was selected as the model component. Experiments were conducted to reveal the influences of the reaction temperature and the ratio between steam to carbon on the toluene conversion and the hydrogen yield. The physicochemical properties of the modified Ni-based catalyst were determined by a series of characterization methods. The results indicated that the Ni-Co alloy was determined over the Ni-Co/AC catalyst. The doping of CaO and the presence of Ni-Co alloy promoted the performance of toluene catalytic dissociation over Ni-Co/AC catalyst compared with that over Ni/A catalyst. After testing in steam for 40 h, the carbon conversion over Ni-Co/AC maintained above 86% and its resistance to carbon deposition was superior to Ni/A catalyst.

Graphical abstract

Keywords

catalytic steam reforming / tar model compound / Ni-based catalyst / carbon resistance

1 Introduction

The environmental problems caused by the emission of greenhouse gases that are produced from the process of fossil fuel combustion have necessitated the studies of renewable energy utilization [1–3]. Among the various sources of renewable energies, biomass is easily available and can be converted to solid, gaseous and liquid fuels, so as to partly replace the fossil fuels [4–7]. Gasification offers a promising method to produce bio-syngas from solid biomass, which can be widely applied in many industrial chemical processes, i.e., the production of electric power and heat, the synthesis of liquid biofuels, and high-grade chemicals [8–10]. Bio-syngas consists of hydrogen (H₂), carbon monoxide (CO), methane (CH₄), and other undesirable contaminants such as tar, soot, sulfur compounds, and chloride compounds [11]. Tar is a major impediment that hinders the commercialization of biomass gasification, as it is a complex mixture of various organic compounds mainly including monocyclic and polycyclic aromatic hydrocarbons as well as oxygenated compounds, which is easy to be condensed at ambient temperature to cause the blockage of pipelines and valves [12,13].

Catalytic steam reforming is an effective and renewable route for tar elimination from biomass gasification products, through which most of the tar can be mainly converted into hydrogen, in favor of elevating the syngas heating value and adjusting the ratio of H₂/CO which significantly affects the synthesis of liquid fuels by the Fischer-Tropsch method [14–16]. At present, numerous catalysts have been proposed for catalytic steam reforming of tar, and nickel-based (Ni-based) catalysts have been widely used due to their excellent activity, abundance, and lower cost [17]. However, the fast losing of their activity induced by metal sintering and carbon deposition on the active sites hinders the further utilization of Ni-base catalysts [18,19]. Alumina is commonly considered to be a suitable support for Ni-based catalysts, which possesses many interesting features, i.e., large surface area, superb thermal, and mechanical stability [19]. However, the acidity of alumina support causes undesirable carbon accumulation over the surface of the catalyst, resulting in further deactivation [20].

To promote the resistance to carbon deposition on Ni-based catalysts, numerous approaches have been explored, such as support modification, alloying Ni with various transition metals (Co, Cu, Fe, etc.), the addition of alkaline earth metals, the selection of proper synthesis route, etc [21]. Among these methods, the presence of alkaline earth metals is confirmed to be effective for enhancing the activity of Ni-based catalysts. Elias et al. investigated the influence of CaO addition on the performance of Ni-Ca/Al₂O₃ catalysts. The results indicated that the addition of CaO significantly decreased the acidity of the support, which suppressed the carbon formation and improved the catalyst activity [19]. The modification of alumina support by MgO was found to significantly improve the catalyst activity and stability [22]. Ashok et al. explored the effect of CaO doping on the performance of tar catalytic removal on Ni-based catalysts. The results revealed that all catalysts containing CaO exhibited a good resistance to carbon deposition [23].

In addition, alloying nickel with transition metals (Co, Fe, Cu, etc.) was confirmed to be essential for promoting the catalytic activity and coking hindrance. Li et al. synthesized Ni-Cu alloy catalysts and determined that the alloying Ni with Cu promoted the steam adsorption of the catalysts during the catalytic removal of 1-methylnaphthalene [24]. You et al. reported that the addition of Co to Ni/γ-Al₂O₃ improved the stability and carbon hindrance of the catalyst while methane conversion was relatively compromised [25]. Koh et al. explored the performance of Co-Ni catalysts supported with CaAl₂O₄/Al₂O₃ on the catalytic oxidation of methane and established that the optimal Ni/Co ratio was 2 [26].

In the present paper, two modifications for the conventional Ni-based alumina-supported catalyst were adopted to promote its capacity of carbon resistance in the process of tar catalytic steam reforming. Toluene was chosen as a model compound of biomass tar since it is a main biomass tar component [27]. Both the conventional Ni/Al₂O₃ (Ni/A) and the modified Ni-based catalyst Ni-Co/Al₂O₃-CaO (Ni-Co/AC) were prepared and tested. In addition, the catalysts were systematically characterized by a series of characterization methods, aiming to determine the influences of support modification and Co presence on the catalyst properties and performance during the catalytic steam reforming of toluene. The results could provide some fundamental instructions for the synthesis and development of novel Ni-based catalysts for tar catalytic steam reforming in the process of biomass gasification.

2 Experimental

2.1 Catalyst preparation

The Al₂O₃-CaO (AC) support was prepared by the incipient wetness impregnation method with a molar ratio Al³⁺/Ca²⁺ of 3. 113.0 g of Ca(NO₃)₂·4H₂O was dissolved in deionized water to obtain a homogeneous solution at room temperature. Afterward, 73.2 g of γ-Al₂O₃ was added to the solution and stirred for 2 h. Then, the slurry was dried at 110°C for 48 h. The solid was calcined in air at 800°C for 6 h and then crushed to fine particles.

To load the active metals on the AC support, 29.7 g of Ni(NO₃)₂·6H₂O as a main active metal precursor and 14.8 g of Co(NO₃)₂·6H₂O as a promoter precursor were dissolved in deionized water and stirred until a clear solution was formed. Subsequently, 51.0 g of AC support was added to the solution and stirred for 2 h. The slurry obtained was then dried in an electric oven at 1100°C overnight and calcined at 6500°C for 4 h. The solid was further crushed and sieved to the sizes of 0.30–0.45 mm. The Ni and Co metal loadings were 10% (wt) and 5% (wt), respectively. For comparison, Ni/A catalyst was also prepared by the incipient wetness impregnation method. 29.7 g of Ni(NO₃)₂·6H₂O was dissolved in deionized water, and then 54.0 g of γ-Al₂O₃ was added to the solution and stirred for 2 h. Afterward, the obtained slurry was dried in an electric oven at 1100°C overnight and calcined at 6500°C for 4 h to obtain the Ni/A catalyst with a Ni content of 10% (wt).

2.2 Catalyst performance evaluation

Figure 1 depicts the schematic representation of the experimental setup for catalytic steam reforming of toluene. The experimental setup mainly consists of a reduction gas mixture cylinder (6% H₂ balanced by Ar), a carrier gas cylinder (high purity N₂), two mass flow meters (Alicat Scientific), a peristaltic pump (Leadfluid) for deionized water feeding, a syringe pump (LongerPump) for toluene feeding, a quartz tube reactor (with an internal diameter of 32 mm) placed vertically in an electric furnace, a temperature controller connected to a thermocouple (with an uncertainty of ±1°C) which was placed at the outer side of the quartz reactor, an ice condenser to condense unconverted water and toluene, a calcium chloride column gas dryer, and an online micro gas chromatograph (GC, INFICON 3000) equipped by four thermal conductivity detectors capable of detecting H₂, N₂, CO₂, CO, CH₄ and lighter hydrocarbons (C2–C3). The GC was calibrated by a mixture of standard calibration gas (H₂, CH₄, CO, CO₂, and N₂), purchased from Beijing Lian You Fa Commercial and Trading Co., Ltd. The calibration factor was calculated based on the peak area value from the GC, which was then used to determine the volume fraction of each gas in the product. The standard errors associated with the reproducibility of the gas components, i.e., H₂, CH₄, CO, CO₂, and N₂, were ±0.1%, ±0.2%, ±0.2%, ±0.2%, and ±0.2%, respectively.

In prior to the experiments, 1.2 mL of the targeted catalyst was located inside the quartz reactor, sandwiched between two quartz wool pieces. The catalyst was first reduced under a mixture of 6% H₂/Ar (100 mL/min). Meanwhile, the reactor was heated from ambient temperature to 750°C with a heating rate of 10°C/min, and it was maintained at 750°C for 1.5 h. Following the catalyst reduction, the flow of the reducing gas mixture was stopped and the carrier gas was purged into the reactor to sweep the reducing mixture. At the same time, the temperature was set to the desired reaction value for toluene reforming. When the desired temperature was reached, the toluene and water were fed into the reactor by the syringe and peristaltic pump, respectively. In these experiments, the gas hourly space velocity (GHSV) was set to 20000 h⁻¹ (calculated at the standard condition), which was defined as the total flow rates of nitrogen, water, and toluene to the volume of the catalyst bed. The flue gas was cooled in the ice condenser to collect the unconverted water and toluene, followed by undergoing further drying by the column of calcium chloride. Then, part of the effluent gas was injected into the online gas chromatograph and the residuals were emitted into the atmosphere. The period of full gas analysis for the online gas chromatograph was 2.8 min and each test lasted for 2 h, in which 42 circles of full gas analysis were performed.

The main reactions that occurred during the catalytic steam reforming of tar were as follows.

(a) Steam reforming:

(1){{\displaystyle \mathrm{C}}}_{\mathrm{7}}{{\displaystyle \mathrm{H}}}_{\mathrm{8}}+\mathrm{7}{{\displaystyle \mathrm{H}}}_{\mathrm{2}}\mathrm{O}\to \mathrm{7}\mathrm{C}\mathrm{O}+\mathrm{11}{{\displaystyle \mathrm{H}}}_{\mathrm{2}}

(2){{\displaystyle \mathrm{C}}}_{\mathrm{7}}{{\displaystyle \mathrm{H}}}_{\mathrm{8}}+\mathrm{14}{{\displaystyle \mathrm{H}}}_{\mathrm{2}}\mathrm{O}\to \mathrm{7}{{\displaystyle \mathrm{C}\mathrm{O}}}_{\mathrm{2}}+\mathrm{18}{{\displaystyle \mathrm{H}}}_{\mathrm{2}}

(b) Water-gas shift:

(3)\mathrm{C}\mathrm{O}+{{\displaystyle \mathrm{H}}}_{\mathrm{2}}\mathrm{O}\rightleftharpoons {{\displaystyle \mathrm{C}\mathrm{O}}}_{\mathrm{2}}+{{\displaystyle \mathrm{H}}}_{\mathrm{2}}

Since the flow rate of nitrogen was determined by the mass flowmeter, the flow rates of the other effluent gas components were calculated according to the method of nitrogen balance.

(4){{\displaystyle \varphi }}_{\mathrm{i}}={\displaystyle \frac{{{\displaystyle \varphi }}_{{\mathrm{N}}_{\mathrm{2}}}\times V_{\mathrm{i}}}{{{\displaystyle V}}_{{\mathrm{N}}_{\mathrm{2}}}}},

where ϕ_i is the flow rate of gas component (mL/min), V_i is the volume fraction of gas component, {\mathit{\varphi }}_{{\mathrm{N}}_2} is the nitrogen flow rate (mL/min), and V_{{\mathrm{N}}_2}is the nitrogen volume fraction in flue gas.

The conversion of toluene was determined as a ratio of the total carbon molar flow rate in flue gas to the carbon molar flow rate in feeding toluene, which is given in Eq. (5).

(5){{\displaystyle X}}_{\mathrm{C}}={\displaystyle \frac{{{\displaystyle n}}_{{\mathrm{C}\mathrm{O}}_{\mathrm{2}}}+{{\displaystyle n}}_{\mathrm{C}\mathrm{O}}+{{\displaystyle n}}_{{\mathrm{C}\mathrm{H}}_{\mathrm{4}}}}{\mathrm{7}\times {{\displaystyle n}}_{{\mathrm{C}}_{\mathrm{7}}{\mathrm{H}}_{\mathrm{8}}}}}\times \mathrm{100},

where X_C is carbon conversion, {{\displaystyle n}}_{{\mathrm{C}\mathrm{O}}_{\mathrm{2}}} is carbon dioxide molar flow rate (mL/min), n_CO is carbon monoxide molar flow rate (mL/min), {{\displaystyle n}}_{{\mathrm{C}}_{\mathrm{7}}{\mathrm{H}}_{\mathrm{8}}} is toluene molar flow rate (mL/min), and {{\displaystyle n}}_{{\mathrm{C}\mathrm{H}}_{\mathrm{4}}} is methane molar flow rate (mL/min).

Hydrogen yield was defined in terms of the hydrogen molar flow rate in the effluent gas to the molar flow rate of maximum hydrogen that could theoretically be formed during the catalytic steam reforming of toluene, which was as follows.

(6){{\displaystyle Y}}_{{\mathrm{H}}_{\mathrm{2}}}={\displaystyle \frac{{{\displaystyle \varphi }}_{{\mathrm{H}}_{\mathrm{2}}}}{\mathrm{18}\times {{\displaystyle \varphi }}_{{\mathrm{C}}_{\mathrm{7}}{\mathrm{H}}_{\mathrm{8}}}}}\times \mathrm{100},

where {{\displaystyle Y}}_{{\mathrm{H}}_{\mathrm{2}}} is hydrogen yield, {{\displaystyle \varphi }}_{{\mathrm{H}}_{\mathrm{2}}} is hydrogen flow rate (mL/min), and {{\displaystyle \varphi }}_{{\mathrm{C}}_{\mathrm{7}}{\mathrm{H}}_{\mathrm{8}}} is toluene flow rate (mL/min).

2.3 Catalysts characterizations

The physical and textural properties of the fresh and aged catalysts that were recovered after the stability tests for 40 h were characterized by the nitrogen adsorption/desorption method (Autosorb-iQ-MP, Quantachrome). In addition, the specific surface areas were determined by the Brunauer-Emmett-Teller (BET) method and the features of pores were obtained by the Barrett-Joyner-Halenda (BJH) method.

The reducibility of the catalysts was examined by means of temperature-programmed reduction by hydrogen (H₂-TPR), which was conducted on a chemisorption analyzer with a thermal conductivity detector (AutoChem 2920). During the H₂-TPR tests, the specimens were heated from 20°C to 900°C at a heating rate of 10°C/min in the flow of H₂ balanced by Ar of 94%.

X-ray diffraction (XRD) was applied to determine the structures of crystallites, which was performed on a Bruker D8 Advance X-ray powder diffractometer equipped with a Co Kα radiation, operating at 40 kV and 40 mA from 10° to 90° with a ramp of 6°/min.

X-ray photoelectron spectroscopy (XPS) was conducted by utilizing an X-ray photoelectron spectrometer (Thermo Escalab 250) equipped with a radiation source of Al Kα (hn = 1486.6 eV). The binding energies were corrected by C1s (284.4 eV).

In addition, the formed carbon on the aged catalysts was also determined by utilizing a carbon/sulfur analyzer (Vario MACRO cube).

It is notable that the fresh catalyst samples were reduced under the same reduction conditions as that in Section 2.2 before being characterized by nitrogen adsorption/desorption, XRD, and XPS.

3 Results and discussion

3.1 Activity testing

3.1.1 Effect of temperature on toluene conversion and hydrogen yield

The influences of reacting temperature on toluene conversion and H₂ yield were investigated at a steam to carbon ratio (S/C) of 2.5 and a GHSV of 20000 h⁻¹. Figure 2(a) illustrates the tendencies of toluene conversion at different temperatures. Along with increasing temperature, the conversion of toluene was remarkably enhanced over both Ni/A and Ni-Co/AC catalysts. As the reacting temperature was raised from 550°C to 700°C, the efficiency of toluene conversion increased from 37% to 81% over Ni/A, and from 45% to 93% over Ni-Co/AC, respectively. The Ni-Co/AC catalyst exhibited a better conversion efficiency of toluene at all temperatures. This result demonstrated that the modifications significantly enhanced the catalytic efficiency of the conventional Ni-based catalyst.

According to Fig. 2(b), the yield of hydrogen also increased with increasing temperatures for both catalysts, indicating that the toluene reforming reaction (1) was enhanced at high temperatures. The effect of increasing temperatures on hydrogen yield was more significant at lower reaction temperatures than that at higher temperatures. With increasing reaction temperatures, the reverse reaction (3) was promoted, in which more hydrogen and CO₂ were consumed. Thus, the hydrogen yield increment was lower at higher temperatures [28,29]. In addition to the effects of reactions (1) and (3) on hydrogen yield, the methanation reactions (7) and (8) could lower the hydrogen yield by consuming partial H₂, CO, and CO₂ that were produced from reactions (1) and (3). Figure 3 presents the effects of reaction temperatures on gas compositions in flue gas. The content of CH₄ was higher with the Ni-Co/AC catalyst. According to Refs. [30,31], Co could promote the methanation reaction from H₂ by enhancing reactions (7) and (8) respectively, especially at lower temperatures. Therefore, the concentration of hydrogen decreased and that of CH₄ increased over Ni-Co/AC catalysts at 550°C.

(7)\mathrm{C}\mathrm{O}+\mathrm{3}{{\displaystyle \mathrm{H}}}_{\mathrm{2}}\rightleftarrows {{\displaystyle \mathrm{C}\mathrm{H}}}_{\mathrm{4}}+{{\displaystyle \mathrm{H}}}_{\mathrm{2}}\mathrm{O}

(8){{\displaystyle \mathrm{C}\mathrm{O}}}_{\mathrm{2}}+\mathrm{4}{{\displaystyle \mathrm{H}}}_{\mathrm{2}}\rightleftarrows {{\displaystyle \mathrm{C}\mathrm{H}}}_{\mathrm{4}}+\mathrm{2}{{\displaystyle \mathrm{H}}}_{\mathrm{2}}\mathrm{O}

3.1.2 Effect of S/C ratios on toluene conversion and hydrogen yield

The influences of S/C ratios on toluene conversion and H₂ yield at 700°C and a GHSV of 20000 h⁻¹ were investigated at S/C ratios ranging from 1.0 to 2.5, as shown in Fig. 4, in which Fig. 4(a) indicated that the toluene conversion increased remarkably with increasing S/C ratios. Generally, the partial pressure of the steam could be enhanced in the condition of high S/C ratios, which was one of the determining factors for reforming reactions [32]. This resembled the results reported in Ref. [33]. Furthermore, the increase in steam partial pressure promoted the production of hydrogen by enhancing reforming reactions, as displayed in Fig. 4(b). Besides, based on the Le Chatelier’s principle, the increase in S/C ratios accelerated the shifting of reaction (3) to the production of hydrogen, which also facilitated the yield of hydrogen.

3.1.3 Performance of stability test

The stability tests for Ni/A and Ni-Co/AC catalysts were conducted at 700°C at an S/C ratio of 2.5 and a GHSV of 20000 h⁻¹. The test lasted for 40 h in steam. As depicted in Fig. 5, the conversion efficiency of toluene decreased from 81% to 68% and from 93% to 86% for Ni/A and Ni-Co/AC respectively after 40 h. This result demonstrated that the Ni-Co/AC catalyst retained a better resistance for carbon deposition compared with the Ni/A catalyst. According to the results revealed by Elias et al. [19], the addition of alkaline metal to alumina improved the stability of catalysts by enhancing the resistance of carbon formation over the surface of catalysts. Besides, the doping of Co could also promote the carbon resistance over Ni-based catalysts, which was similar to the findings of You et al. [25] and Chen et al. [33].

3.2 Catalyst characterization

3.2.1 Structural analysis

In the present paper, both the fresh and aged catalysts were subjected to a series of physical and chemical characterizations. Table 1 gives the results of the microstructures of the fresh and the aged catalysts. The Ni/A catalyst had a higher specific surface area and pore volume compared with the Ni-Co/AC catalyst, owing to the properties of alumina support. The small surface areas of the Ni-Co/AC catalyst were caused by the modification of Al-support, the Co loading and the relatively high temperature of modified support calcination [34]. Besides, the Ni-Co/AC catalyst possessed a higher average pore diameter in contrast to the Ni/A catalyst. The possible reason for this is that the collapse of the small pores results in the formation of large ones. Nevertheless, despite the fact that the Ni-Co/AC catalyst had a lower specific surface area, it exhibited a stronger activity for toluene conversion. After testing for 40 h in steam, the specific surface area and the pore volume of the Ni/A catalyst decreased significantly, whereas the Ni-Co/AC catalyst suffered a less decrement in surface area. Moreover, both the pore volume and the average pore diameter of the Ni-Co/AC remained almost unchanged.

Figure 6 illustrates the XRD patterns of the fresh and the aged catalysts. Two intense peaks at 52.18° and 61.02° ascribing to Ni species [35] were detected in the Ni/A catalyst. In the Ni-Co/AC catalyst, these peaks shifted slightly to 52.05° and 60.89° caused by the formation of the nickel-cobalt alloy. Three peaks of Al₂O₃ at 43.96°, 53.80°, and 79.60° were also observed. The Ni-Co/AC catalyst exhibited weaker peaks of Al₂O₃ compared with that of the Ni/A catalyst. The reason could be attributed to the perfect dispersion of CaO into alumina which decreased the crystallinity of Al-support. The peak at 43.25° ascribing to NiAl₂O₄ spinal was detected over the fresh Ni-Co/AC, while it was not observed over the aged one. Although carbon peaks were detected in both aged catalysts, the intensity of carbon peak over the aged Ni-Co/AC was less than that over the aged Ni/A. No peaks of CaO were observed due to its low concentration in the support.

Additionally, the metal particle size was estimated according to the Scherrer formula as follows.

(9){{\displaystyle D}}_{\mathrm{p}}=\left(\mathrm{0.94}\lambda \right)/\left(\beta \cos \theta \right),

where l is the wavelength of the X-ray used (1.7902 nm), b is the full width at half maximum intensity (FWHM) (rad), and q is the Bragg’s angle (rad). The detailed calculation results were listed in Table 1. The results obtained suggested that the Ni-Co/AC catalyst had relatively smaller particle sizes compared to Ni/A.

3.2.2 Reducibility analysis

To reveal the influence of support modification and cobalt addition on the reducibility of catalyst, a series of H₂-TPR tests over fresh Ni/A and Ni-Co/AC were conducted, whose results are given in Fig. 7. The peak at 440°C in the reduction curve of fresh Ni/A was assigned to the hydrogen depletion of weakly bound NiO. Besides, the intense peak at 765°C was ascribed to the hydrogen consumption of strongly bond NiAl₂O₄ [36,37]. Four different peaks were observed in the reduction profile of Ni-Co/AC. The first peak from 240°C to 350°C was ascribed to the hydrogen depletion of Co₃O₄ to CoO [38,39]. The second peak ranging from 400°C to 580°C was attributed to the hydrogen depletion of CoO and the bond of NiO species [40]. This indicated that the Co-Ni alloy was formed in the presence of Co species, which facilitated the simultaneous reduction of the NiO catalyst. The result was consistent with a previous suggestion made by Wang et al. [41]. The third intense peak at 710°C was ascribed to the hydrogen reduction of strongly bound NiAl₂O₄. The fourth peak around 850°C was assigned to the hydrogen reduction of the CoAl₂O₄ spinel [42]. For the Ni-Co/AC catalyst, the peak of strongly bond NiAl₂O₄ shifted to a lower temperature. This trend could be attributed to the preference of calcium aluminate formation, which suppressed the formation of NiAl₂O₄ spinel. Therefore, the NiAl₂O₄ stayed in the form of agglomerated oxide, which could be reduced at lower temperatures [43]. The Ni-Co/AC catalyst showed a better reduction at the low temperature than Ni/A, indicating that the addition of CaO to the Al-support increased the weak bond of NiO species on the catalyst surface in which the interaction of NiO with Al₂O₃ was restrained [44]. This confirmed that the addition of CaO could promote the reducibility of the NiO catalyst by increasing the amounts of active Ni species over the surface of the catalyst, which further enhanced the catalytic conversion of toluene over Ni-Co/AC.

3.2.3 Chemical states analysis

The chemical states of Ni species in the fresh and the aged catalysts were determined by XPS. Figure 8 indicated that Ni mainly existed in two forms in catalysts, which were metallic nickel (Ni⁰) and oxidized nickel (Ni²⁺). Generally, the peaks at 852.8 eV and 870.8 eV were related to the Ni⁰ in the spectra of Ni 2p_3/2 and Ni 2p_1/2 separately [45,46]. The Ni²⁺ species in the Ni 2p_3/2 and Ni 2p_1/2 spectra were assigned to the peaks of 856.2 eV and 875.2 eV respectively. Ni⁰ was the active center of catalytic steam reforming, which enhanced the activity of the catalysts. Table 2 shows the results of the ratio of Ni⁰/(Ni²⁺ + Ni⁰). The Ni-Co/AC catalyst contained a higher Ni⁰ and the decrease in active centers of Ni-Co/AC was less than that of the Ni/A catalyst. This result indicated that the CaO addition promoted the reducibility of the catalyst. Besides, the presence of Co also enhanced the reducibility of the nickel compounds [41], which agreed with the results of catalytic tests and H₂-TPR characterization.

3.2.4 Analysis of deposited carbon contents

Table 3 presents the amount of carbon deposited and the decrement of conversion efficiency over the aged catalysts, which was calculated as follows.

(10)\mathrm{D}\mathrm{e}\mathrm{c}\mathrm{r}\mathrm{e}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{o}\mathrm{f}\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{e}\mathrm{f}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{y}={\displaystyle \frac{{{\displaystyle X}}_{{\mathrm{C}}_{\mathrm{0}}}-{{\displaystyle X}}_{{\mathrm{C}}_{\mathrm{1}}}}{{{\displaystyle X}}_{{\mathrm{C}}_{\mathrm{0}}}}}\times \mathrm{100}\mathrm{\%},

where {{\displaystyle X}}_{{\mathrm{C}}_{\mathrm{0}}} is the conversion efficiency of toluene at the beginning of the stability test, and {{\displaystyle X}}_{{\mathrm{C}}_1} was the conversion efficiency of toluene after 40 h.

Depending on the results in Table 3, the Ni-Co/AC catalyst exhibited a perfect residence of coke deposition and a less decrement of conversion efficiency compared with Ni/A. The contribution of the modifications to the improvement of carbon resistance could be attributed to the increase in the basicity and reducibility of the catalyst, due to the addition of CaO and the formation of Ni-Co alloy in the presence of Co species. Therefore, the Ni-Co/AC catalyst was superior in resisting carbon accumulation. In addition, compared with the conventional Ni/A, the conversion efficiency of toluene was relatively substantial over the aged Ni-Co/AC catalyst.

4 Conclusions

The catalytic performance of toluene steam reforming over conventional Ni/Al₂O₃ (Ni/A) catalyst and modified Ni-based catalyst Ni-Co/Al₂O₃-CaO (Ni-Co/AC) was systematically investigated in this paper. The results revealed that the Ni-Co/AC catalyst exhibited a better performance in terms of activity and hindrance to carbon deposition in the process of toluene catalytic conversion. The support modification by the addition of CaO improved the basicity of the catalyst, which promoted the nickel reducibility and enhanced the resistance to carbon deposition. Even though the Ni/A had a larger surface area compared to Ni-Co/AC, Ni-Co/AC retained a relatively smaller metal particle size, owing to the metal dispersion improvement of the CaO addition. Furthermore, it was confirmed that the presence of Ni-Co alloy promoted the activity of the catalysts. The ratio of Ni⁰/ (Ni⁰ + Ni²⁺) was higher for the Ni-Co/AC catalyst. In addition, the Ni-Co/AC catalyst exhibited less conversion decrement after aging in steam for 40 h, which was ascribed to the lower coking and the less decrease of active sites over the catalyst.

References

[1] Pinton N, Vidal M V, Signoretto M, Ethanol steam reforming on nanostructured catalysts of Ni, Co and CeO₂: influence of synthesis method on activity, deactivation and regenerability. Catalysis Today, 2017, 296: 135–143

[2] Huber G W, Iborra S, Corma A. Synthesis of transportation fuels from biomass: chemistry, catalysts, and engineering. Chemical Reviews, 2006, 106(9): 4044–4098

[3] Zhang L, Yang K, Zhao R, Nanostructure and reactivity of soot from biofuel 2,5-dimethylfuran pyrolysis with CO₂ additions. Frontiers in Energy, 2020, online, doi: 10.1007/s11708-020-0658-3

[4] Shen Y, Fu Y. Advances in: in situ and ex situ tar reforming with biochar catalysts for clean energy production. Sustainable Energy and Fuels, 2018, 2(2): 326–344

[5] Nabgan W, Tuan Abdullah T A, Mat R, Hydrogen production from catalytic steam reforming of phenol with bimetallic nickel-cobalt catalyst on various supports. Applied Catalysis A, General, 2016, 527: 161–170

[6] Chaudhari S T, Dalai A K, Bakhshi N N. Production of hydrogen and/or syngas (H₂+CO) via steam gasification of biomass-derived chars. Energy & Fuels, 2003, 17(4): 1062–1067

[7] Rupesh S, Muraleedharan C, Arun P. Energy and exergy analysis of syngas production from different biomasses through air-steam gasification. Frontiers in Energy, 2020, 14(3): 607–619

[8] Teo S H, Yap D K Y, Mansir N, Facile recoverable and reusable macroscopic alumina supported Ni-based catalyst for efficient hydrogen production. Scientific Reports, 2019, 9(1): 16358

[9] Ersöz A, DurakÇetin Y, Sarıoğlan A, Investigation of a novel and integrated simulation model for hydrogen production from lignocellulosic biomass. International Journal of Hydrogen Energy, 2018, 43(2): 1081–1093

[10] Nakamura K, Miyazawa T, Sakurai T, Promoting effect of MgO addition to Pt/Ni/CeO₂/Al₂O₃ in the steam gasification of biomass. Applied Catalysis B: Environmental, 2009, 86(1–2): 36–44

[11] Zamboni I, Courson C, Kiennemann A. Fe-Ca interactions in Fe-based/CaO catalyst/sorbent for CO₂ sorption and hydrogen production from toluene steam reforming. Applied Catalysis B: Environmental, 2017, 203: 154–165

[12] Xiao X, Liu J, Gao A, The performance of nickel-loaded lignite residue for steam reforming of toluene as the model compound of biomass gasification tar. Journal of the Energy Institute, 2018, 91(6): 867–876

[13] Buchireddy P R, Bricka R M, Rodriguez J, Biomass gasification: catalytic removal of tars over zeolites and nickel supported zeolites. Energy & Fuels, 2010, 24(4): 2707–2715

[14] Yang J, Kaewpanha M, Karnjanakom S, Steam reforming of biomass tar over calcined egg shell supported catalysts for hydrogen production. International Journal of Hydrogen Energy, 2016, 41(16): 6699–6705

[15] Alipour Z, Rezaei M, Meshkani F. Effect of alkaline earth promoters (MgO, CaO, and BaO) on the activity and coke formation of Ni catalysts supported on nanocrystalline Al₂O₃ in dry reforming of methane. Journal of Industrial and Engineering Chemistry, 2014, 20(5): 2858–2863

[16] Michel R, Łamacz A, Krzton A, Steam reforming of α-methylnaphthalene as a model tar compound over olivine and olivine supported nickel. Fuel, 2013, 109: 653–660

[17] Kong M, Fei J, Wang S, Influence of supports on catalytic behavior of nickel catalysts in carbon dioxide reforming of toluene as a model compound of tar from biomass gasification. Bioresource Technology, 2011, 102(2): 2004–2008

[18] Chen S L, Zhang H L, Hu J, Effect of alumina supports on the properties of supported nickel catalysts. Applied Catalysis, 1991, 73(2): 289–312

[19] Elias K F M, Lucrédio A F, Assaf E M. Effect of CaO addition on acid properties of Ni-Ca/Al₂O₃ catalysts applied to ethanol steam reforming. International Journal of Hydrogen Energy, 2013, 38(11): 4407–4417

[20] Furusawa T, Saito K, Kori Y, Steam reforming of naphthalene/benzene with various types of Pt- and Ni-based catalysts for hydrogen production. Fuel, 2013, 103: 111–121

[21] Baidya T, Cattolica R J, Seiser R. High performance Ni-Fe-Mg catalyst for tar removal in producer gas. Applied Catalysis A, General, 2018, 558: 131–139

[22] Lu Q, Hou Y, Laraib S R, Electro-catalytic steam reforming of methane over Ni-CeO₂/g-Al₂O₃-MgO catalyst. Fuel Processing Technology, 2019, 192: 57–64

[23] Ashok J, Kawi S. Steam reforming of biomass tar model compound at relatively low steam-to-carbon condition over CaO-doped nickel-iron alloy supported over iron-alumina catalysts. Applied Catalysis A, General, 2015, 490: 24–35

[24] Li D, Lu M, Aragaki K, Characterization and catalytic performance of hydrotalcite-derived Ni-Cu alloy nanoparticles catalysts for steam reforming of 1-methylnaphthalene. Applied Catalysis B: Environmental, 2016, 192: 171–181

[25] You X, Wang X, Ma Y, Ni-Co/Al₂O₃ bimetallic catalysts for CH₄ steam reforming: elucidating the role of Co for improving coke resistance. ChemCatChem, 2014, 6(12): 3377–3386

[26] Koh A C W, Chen L, Kee Leong W, Hydrogen or synthesis gas production via the partial oxidation of methane over supported nickel-cobalt catalysts. International Journal of Hydrogen Energy, 2007, 32(6): 725–730

[27] Tao J, Lu Q, Dong C, Effects of electric current upon catalytic steam reforming of biomass gasification tar model compounds to syngas. Energy Conversion and Management, 2015, 100: 56–63

[28] Tao J, Zhao L, Dong C, Catalytic steam reforming of toluene as a model compound of biomass gasification tar using Ni-CeO₂/SBA-15 catalysts. Energies, 2013, 6(7): 3284–3296

[29] Chen W H, Chen C Y. Water gas shift reaction for hydrogen production and carbon dioxide capture: a review. Applied Energy, 2020, 258: 114078

[30] He L, Parra J M S, Blekkan E A, Towards efficient hydrogen production from glycerol by sorption enhanced steam reforming. Energy & Environmental Science, 2010, 3(8): 1046–1056

[31] Vannice M A. The catalytic synthesis of hydrocarbons from H₂ CO mixtures over the group VIII metals. III. Metal-support effects with Pt and Pd catalysts. Journal of Catalysis, 1975, 40(1): 129–134

[32] Shen C, Zhou W, Yu H, Ni nanoparticles supported on carbon as efficient catalysts for steam reforming of toluene (model tar). Chinese Journal of Chemical Engineering, 2018, 26(2): 322–329

[33] Chen L, Zhu Q, Wu R. Effect of Co-Ni ratio on the activity and stability of Co-Ni bimetallic aerogel catalyst for methane Oxy-CO₂ reforming. International Journal of Hydrogen Energy, 2011, 36(3): 2128–2136

[34] Feng H Z, Lan P Q, Wu S F. A study on the stability of a NiO-CaO/Al₂O₃ complex catalyst by La₂O₃ modification for hydrogen production. International Journal of Hydrogen Energy, 2012, 37(19): 14161–14166

[35] Rashid M H, Raula M, Mandal T K. Polymer assisted synthesis of chain-like cobalt-nickel alloy nanostructures: magnetically recoverable and reusable catalysts with high activities. Journal of Materials Chemistry, 2011, 21(13): 4904–4917

[36] Zhang L, Wang X, Tan B, Effect of preparation method on structural characteristics and propane steam reforming performance of Ni-Al₂O₃ catalysts. Journal of Molecular Catalysis A Chemical, 2009, 297(1): 26–34

[37] Denis A, Grzegorczyk W, Gac W, Steam reforming of ethanol over Ni/support catalysts for generation of hydrogen for fuel cell applications. Catalysis Today, 2008, 137(2–4): 453–459

[38] Karim A M, Su Y, Engelhard M H, Catalytic roles of Co⁰ and Co²⁺ during steam reforming of ethanol on Co/MgO catalysts. ACS Catalysis, 2011, 1(4): 279–286

[39] Furusawa T, Tsutsumi A. Development of cobalt catalysts for the steam reforming of naphthalene as a model compound of tar derived from biomass gasification. Applied Catalysis A, General, 2005, 278(2): 195–205

[40] Horlyck J, Lawrey C, Lovell E C, Elucidating the impact of Ni and Co loading on the selectivity of bimetallic NiCo catalysts for dry reforming of methane. Chemical Engineering Journal, 2018, 352: 572–580

[41] Wang L, Li D, Koike M, Catalytic performance and characterization of Ni-Co catalysts for the steam reforming of biomass tar to synthesis gas. Fuel, 2013, 112: 654–661

[42] Xu J, Zhou W, Li Z, Biogas reforming for hydrogen production over nickel and cobalt bimetallic catalysts. International Journal of Hydrogen Energy, 2009, 34(16): 6646–6654

[43] Dias J A C, Assaf J M. Influence of calcium content in Ni/CaO/g-Al₂O₃ catalysts for CO₂ reforming of methane. Catalysis Today, 2003, 85(1): 59–68

[44] Choong C K S, Zhong Z, Huang L, Effect of calcium addition on catalytic ethanol steam reforming of Ni/Al₂O₃: I. Catalytic stability, electronic properties and coking mechanism. Applied Catalysis A, General, 2011, 407(1–2): 145–154

[45] Kavalerskaya N E, Lokteva E S, Rostovshchikova T N, Hydrodechlorination of chlorobenzene in the presence of Ni/Al₂O₃ prepared by laser electrodispersion and from a colloidal dispersion. Kinetics and Catalysis, 2013, 54(5): 597–606

[46] Lei H, Song Z, Bao X, XRD and XPS studies on the ultra-uniform Raney-Ni catalyst prepared from the melt-quenching alloy. Surface and Interface Analysis, 2001, 32(1): 210–213

RIGHTS & PERMISSIONS

Higher Education Press