1.       Introduction

 Reported by the Energy White Paper of the Australian Department of Resource, Energy and Tourism, the world global energy demand in year 2035 will be higher than current level [1]. Nowadays, the majority of our energy derived from the consumption of fossil fuels, which will release pollutants to the atmosphere and lead to a series of environmental problems such as greenhouse effect and NOx emissions [2-4]. So, it urged to develop renewable clean energy [5].

 Biomass gasification is one of the most important technologies for the conversion of biomass to electricity, fuels, and chemicals [6-9]. However, this potential technology is limited by tar formation in the total process. In general, tar is complex and high-molecular weight compounds containing more than 10,000 species of aromatic which will impair the engine by deposition in the engine [10]. The reduction of tar content is the major point to utilize gasification technology successfully. Catalytic steam reforming is considered as the most effective method for reducing tar [11-15].

 Heterogeneous catalysts are widely used in steam reforming of tar because of their easy separation and recycle. Noble metals such as Pt [16], Rh [16,17], Ru [18], Pd [19] and Ir [20] manifest high activity in steam reforming. However, the noble metals are inappropriate for industrial application because of the high cost and rapid deactivation. Thus, it is of great significance to develop supported based-metal catalyst with excellent activity and stability.

 Supported Ni catalysts are drawing more and more attention because of the cheap cost and high activity [21-29]. In Zhang’s work [27], Ni-Ce-Mg/olivine was used as the catalyst for steam reforming of toluene. The highest conversion of 93% was reached with the reaction temperature of 1063K, catalyst weight to flow rate (W/F) of 11.4gžhžmol^-1, and steam to carbon ratio (S/C) of 3.5. Mukai and coworkers [28] reported that Ni/La_0.7Sr_0.3AlO_3-δ showed high activity for steam reforming of toluene. The conversion of 72.9% was reached under the reaction conditions (reaction temperature was 1073K, W/F was 3.4 gžhžmol^-1, S/C was 2). However, it is reported that Ni nanoparticles tended to sinter resulting in deactivation during the steam reforming.

 In order to solve the problem, great efforts have been paid to enhance the interaction between the supported metal [30-39]. According to Li and coworkers [36], alkaline-promoted Nial-Montmorillonite (NiAl-Mt) catalysts prepared by ion-exchange method showed high metal-support interaction and stability. However, it is difficult to obtain supported Ni nanoparticles with small mean particles size when the Ni loading amount is high through this approach. As reported by P. Burattin [37-39], the highly dispersed Ni nanoparticles supported on Phyllosilicate (PS) prepared by Deposition-Precipitation (DP) method, showed great stability. The excellent anti-sintering property of PS-derived catalysts could be ascribed to the confinement effect of the lamellar structure and the presence of Strong Metal-Support Interaction (SMSI). The Ni nanoparticles were highly dispersed with the average particle size of only 4.7 nm with the Ni content of 11.7 wt% However, nickel phyllosilicate (Ni/SiO_2P) is relatively less studied in catalytic reforming processes [33].

 In this paper, synthesis, characterization and activity test of Ni/SiO_2P are reported. As far as we know, application of Ni/SiO_2P to steam reforming of toluene has never been reported before. There may be three possible advantages by choosing PS as the support: first, because of the SMSI, it is easier to obtain highly dispersed Ni nanoparticles with high loading amount (for example 26.8 wt.%) which would be of great help for the promotion of the catalytic activity; second, the confinement effect of the lamellar structure and the presence of SMSI would lead to the excellent anti-sintering property and stability; third, PS is low cost and environmental friendly providing promising alternative for heterogeneous Ni-based catalysts. The Ni/SiO_2P was prepared by the DP method using silicone gel as the silicon source, and reduced with H₂. X-Ray Diffraction (XRD), Transmission Electron Microscope (TEM), H₂-Temperature Programmed Reduction (H₂-TPR) and Brunauer-Emmett-Teller (BET) analysis were used to characterize the structure and superficial properties of the Ni/SiO_2P.Catalytic activity and stability are investigated for the steam reforming of toluene. Effects of residence time, S/C ratio, and Ni content have been studied systematically in relation to the change in the conversion. And the stability of the as-prepared catalyst was investigated as well.

 2.       Experimental

 2.1.  Materials

 Ammonium Hydroxide [NH₃·H₂O, 25%] and SiO₂ were purchased from Sinopharm Chemical Reagent Co., LTD. Silica sol with 30 percent was purchased from Shandong peak-tech materials Co., LTD. Nickle (II) nitrate [Ni(NO₃)₂·6H₂O] was purchased from Fuchen Chemical Plant in Tianjin. The commercial ZSM-5 catalyst was purchased from the Catalyst Plant of Nankai University. Toluene was purchased from Beijing Chemical Plant.

 2.2.  Catalyst preparation and characterization

 The Ni/SiO_2P catalyst was prepared by DP method [33]. A certain amount of Ni(NO₃)₂·6H₂O and 8.4mL of NH₃·H₂O were dissolved in 80mL of deionized water, which were placed in a tank reactor and stirred at room temperature. 6 g of silica sol was added in the Ni-contain solution and stirred for 6 h at room temperature. Hereafter, the tank reactor was gradually heated to 353 K with stirred to evaporate ammonia. Nickel species were deposited on silica with the decrease of pH, and the tank reactor was cooled to room temperature when the pH value was below 7. Afterword the suspension was filtered by vacuum filtration and washed several times by deionized water. Finally, the solid was dried at 373 K for 12 h and reduced by hydrogen at 873 K.

 The Ni/SiO₂ catalyst and Ni/ZSM-5 catalyst was prepared by wetness impregnation method. A certain amount of Ni(NO₃)₂·6H₂O and SiO₂ were dissolved in 100mL of deionized water. After magnetic stirred for 2 h at room temperature, the Ni-containing solution was heated to 353 K to evaporate water. The resultant solid was dried at 373 K for 12 h, followed by reduction by hydrogen at 873 K.

 The calcined powders were studied by XRD using a Bruker D8-Advance X-ray diffractometer with Cu K α radiation (λ=0.154nm). The machine operated over a range of 2θ angles from 10^° to 90^°. The crystalline size was evaluated by the Scherrer’s equation: D=0.89λ/(β=cosθ). The specific surface area and pore structure were detected using Brunauer-Emmett-Teller (BET) analysis by nitrogen adsorption isotherms at 77K using a Micromeritics ASAP 2020 surface analyzer. H₂-TPR was performed by heating the catalysts to 1266 K with the heating rate of 5 K/min in 10% hydrogen and the total flow rate was 30 mL/min. TEM was used to characterize the size of particle.

 2.3.  Catalytic tests

 The steam reforming of toluene was performed in a fixed bed quartz reactor with internal diameter was 6mm. 0.3g of catalyst was fixed in the middle of quartz reactor. Water and toluene were fed into the reactor by injection pumps and vaporized before contact with the catalyst. N₂ with the flow rate of 30mL/min was used as the carrier gas. The products were flow through acetone and then collected by an airbag. Gas Chromatograph (GC) with a Flame Ionization Detector (FID) was used to analyze the organic species in acetone and GC with a Thermal Conductivity Detector (TCD) was used to analyze the gas in the airbag. The toluene conversion (X) and gas production selectivity were calculated by Equation (1) and (2),

                                                                                                                                    ( 1 )

                                                                                                              ( 2 )

Where X is the conversion of toluene, n_in is the mole of toluene and  is the mole of unreacted toluene S_i is the selectivity of i.

 3.       Results and Discussion

3.1.  Characteristics of Ni-based catalysts

 In order to makes comparisons, Ni nanoparticles were also supported on ZSM-5 zeolite and SiO₂ by the impregnation method. The structural properties and surface morphology of our catalysts are shown in (Table 1) and (Figure 1) respectively. The specific surface area of Ni/SiO_2P is 435.0 m²/g. And the pore volume and the average pore diameter are 0.92 mL/g and 8.4nm, respectively. Compared with Ni/SiO₂ and Ni/ZSM-5, the Ni/SiO_2P catalyst shows larger specific surface area and pore volume. (Figure 1) shows the N₂ adsorption-desorption isotherms of the catalysts. The hysteresis loops indicated the existence of mesoporous.

XRD patterns of Ni/SiO_2P, Ni/SiO₂ and Ni/ZSM-5catalysts are shown as (Figure 2).

Diffraction peaks at 21.6^°, 30.2^°, 35.6^°, 44.0^°, 60.4^°, and 71.3^° are contributed to the (111), (110), (220), (222), (121), (130) facets of PS support, respectively [40-41]. Due to the small size and good dispersion of Ni nanoparticles supported on SiO₂ by the DP method, the peak of Ni was difficult to discern. On the other hand, diffraction peaks at 44.5^°,51.8^°, and76.4^° appeared in the patterns of Ni/ZSM-5 and Ni/SiO₂ are contributed to (111), (200), and (220) facets of nickel nanoparticles. Diffraction peaks at 21.0^°, 26.7^°, 36.6^°, 39.7^° and 50.4^° of Ni/SiO₂ are contributed to the facets of SiO₂. The crystal Ni nanoparticle size of Ni/ZSM-5 and Ni/SiO₂ shown in (Table 1) were calculated to be 27 nm and 25 nm, respectively according to the Scherrer’s equation.

(Figure 3) shows the H₂-TPR results of Ni/SiO_2P, Ni/SiO₂ and Ni/ZSM-5. Ni/ZSM-5 showed two reduction peaks at around 559K and around 573 K, which are attributed to the reduction of NiO.

As for Ni/SiO₂, the main reduction peak around 583K can be ascribed to the reduction of NiO particles, the peak around 798 K could be assigned to the Ni²⁺ ions interacting with SiO₂. Two reduction peaks could also be observed in the result of Ni/SiO_2P, the small peak at 561 K could be ascribed to the reduction of NiO particles, while Ni²⁺ located in phyllosilicates were reduced at around 823K [42]. The highest main reduction peak of Ni/SiO_2P indicated the strongest metal-support interaction which would be benefit for the preparation of Ni nanoparticles with small size and narrow distribution. TEM images of the as-prepared catalysts are shown in (Figure 4).

 The perfectly formed structure of PS and mesoporous could be clearly observed. TEM images of Ni nanoparticles supported on PS with Ni content of 17.4 wt.%, 22.4 wt.%, 26.8 wt.%, 31.3 wt.% are shown as (Figure 4(a), 4(b), 4(c), and 4(d)), respectively. And TEM images of Ni/SiO₂ and Ni/ZSM-5 catalysts with the Ni content of 24 wt.% are shown as (Figure 4(e) and 4(f)) respectively. The corresponding distribution histograms of Ni nanoparticles are shown as (Figure 4(g), 4(h), 4(i), 4(j), 4(k), 4(l)) respectively. The histograms were derived from corresponding TEM images by more than 100 particles. The mean crystal nickel particle size is calculated using the equation ( where n_i is the number of corresponding nickel particles with a diameter of d_i.). The mean particles size of Ni nanoparticles are3.3nm, 3.6 nm, 4.2 nm and 6.0 nm for catalysts with Ni contents of 17.4 wt.%, 22.4 wt.%, 26.8 wt.%, and 31.3 wt.%, respectively. And the mean diameters of Ni particles supported on SiO₂ and ZSM-5 are 23.9 nm and 30.0 nm, respectively. Obviously, as expected, Ni nanoparticles supported on PS exhibited small mean particle size and narrow size distribution which might be of great help for improving the catalytic activity and stability.

 3.2.  Effect of residence time on conversion

 As shown in (Figure 5), the effect of residence time on conversion was investigated.

With the increase of residence time, the conversion of toluene increased gradually. The conversion was only 43.8% when the residence time was 0.1s, and the conversion rapidly increased to 99% when the residence time increased to 0.37s. Longer residence time means longer reaction duration, so the conversion would increase as well. TEM image of the used catalyst is shown as (Figure 5) in the supporting material. After the catalytic reaction, Ni nanoparticles still highly dispersed on the surface of PS.

 3.3.  Effect of S/C on conversion

 As shown in (Figure 6), catalytic performances under different S/C values were determined.

 When the S/C ratio was not higher than 3, the conversion of toluene increased with the increase of S/C ratio. And the conversion remained constant when the S/C ratio further increased. This could be explained as follows, the Equation (3) and (4) were promoted by the increase of water. So, the conversion of toluene will increase with the increase of S/C. But when the S/C was beyond 3, the conversion of toluene will not be improved because under these values of S/C, the main restrictive factor for steam reforming reaction was not the amount of steam, but the active sites of catalysts. Therefore, the S/C ratio was kept as 3 for subsequent research.

                                                                                                                               ( 3 )

                                                                                                                                         ( 4 )

3.4.  Effect of Ni content on conversion

Catalytic performances of Ni/SiO_2P catalysts with various Ni contents are shown in (Figure 7).

The conversion of toluene increased from 45% to 78% when the Ni content increased from 21.0 wt.% to 26.8 wt.%. However, further increase in Ni content resulted in reduction of reaction rate. Catalyst with the highest Ni content of 31.3 wt.% showed even worse performance than that of the catalyst with the Ni content of 26.8 wt.%. This may be driven by two factors: first, with the increase of the loading amount, the Ni particle size increased to almost 6.0 nm decreasing the catalytic activity (as shown in (Figure 4(d)); second, only the Ni nanoparticles loaded on the outer part played a role during the reaction.

3.5.  Catalysts stability study

The catalytic stability of our catalysts was studied. The performances of Ni/SiO_2P，Ni/SiO₂ and Ni/ZSM-5are shown in (Figure 8, 9, and 10), respectively.

As shown in (Figure 8), the initial toluene conversion reached 100%, and kept at 90% even after 660 min. The main gas production is H₂ with the selectivity of around 60%. Comparatively, Ni/SiO₂ and Ni/ZSM-5 showed much worse stability for the steam reforming of toluene. For Ni/ZSM-5, the initial conversion was58%, and the conversion is rapidly decreased. And the conversion of toluene was only 62%, and the conversion decreased to 31% after 660 min using Ni/SiO₂ as the catalyst. Above all, Ni/SiO_2P showed the highest catalytic activity and the best stability. Steam reforming of toluene has also been investigated in other works. According to Quitetea [43], the conversion of toluene decreased from 78% to 59% after 660 min. Comparisons in catalytic performances with other works are listed in (Table 2) [27,44,45]. Therefore, the PS supported with Ni nanoparticles showed high catalytic activity and stability.

4.       Conclusion

PS supported with Ni nanoparticles was successfully prepared and acted as a novel heterogeneous catalyst for steam reforming of toluene. Compared with Ni/SiO₂ and Ni/ZSM-5, the Ni/SiO_2P catalyst prepared in this work showed smaller Ni nanoparticles size and narrower size distribution. The mean Ni nanoparticle size of Ni/SiO_2P varied from 3.3 to 6.0nm with the loading amount increased from 17.4 wt.% to 31.3 wt.%. The pore volume, pore size and specific surface area of Ni/SiO_2P were 0.92 mL/g, 8.40 nm and 436.0 m²/g, respectively. Effects of residence time, S/C and Ni content on conversion were studied systematically. The conversion increased with the increase of residence time. The increase of S/C could also improve the conversion of toluene. The Ni content was optimized to be 26.8 wt.%. The as-prepared Ni/SiO_2P catalysts showed high catalytic activity and stability. The conversion of toluene reached 100% under the reaction conditions (reaction temperature was 923 K; 0.3 g catalyst; S/C was 3; Ni content of 26.8 wt.%; toluene feed rate was 2mL/h). And the conversion of toluene just decreased from 100% to 90% after 660 min.

5.       Acknowledgements

We gratefully acknowledge the support of the National Nature Science Foundation of China (21606008, 21436002), the National Basic Research Foundation of China (973 program) (2013CB733600), the Fundamental Research Funds for the Central Universities (ZY1630, JD1617), and the Fundamental Research Funds for the Central Universities (buctrc201616, buctrc201617).

Figure 1: N₂ Adsorption-Desorption Isotherms and BJH Mesopore Size Distribution (Inset) of Catalysts with Ni Content Of 26.8 W%; (A): Ni/PS, (B): Ni/SiO₂; (C): Ni/ZSM-5.

Figure 2: XRD Pattern of Catalysts with Ni Content Of 25 Wt.%; (A): Ni/PS; (B) Ni/SiO₂; (C) Ni/ZSM-5.

Figure 3: H₂-TPR Results of the As-Prepared Catalysts.

Figure 4(a-l): TEM Images of Catalysts; (a), (b), (c), and (d): Ni/SiO_2P with Ni Content of 17.4 wt.%, 22.4 wt.%, 26.8 wt.%, 31.3 wt.%; (e): Ni/SiO₂ with Ni Content of 26.8%; (f): Ni/ZSM-5 with Ni Content of 26.8%;(g), (h), (i), (j), (k), and (l): Histograms of Particle Size Distribution for Ni/SiO_2P, Ni/ SiO₂, and Ni/ ZSM-5, respectively.

Figure 5: Effect of Residence Time on Conversion; Reaction Temperature: 923K; 0.3 G Catalyst; Ni Content: 31.6 Wt.%; Toluene Feed Rate: 2ml/H; S/C: 1; The Flow Rate of N₂: 30ml/Min.

Figure 6: Effect of S/C on Conversion; Reaction Temperature: 923K; 0.3 G Catalyst; Ni Content: 31.6 Wt.%; Toluene Feed Rate: 2ml/H; the Flow Rate of N₂: 30ml/Min.

Figure 7: Effect of Ni Content on Conversion; Reaction Temperature: 923K; 0.3 G Catalyst; S/C: 3; Toluene Feed Rate: 4ml/H; The Flow Rate of N₂: 30ml/Min.

Figure 8: Catalysts Stability Study on Ni/SiO_2P; Reaction Temperature: 923K; 0.3 G Catalyst; S/C: 3; Ni Content: 26.8 Wt.%; Toluene Feed Rate: 2ml/H; The Flow Rate of N₂: 30ml/Min.

Figure 9: Catalysts Stability Study on Ni/SiO₂; Reaction Temperature: 923K; 0.3 G Catalyst; S/C: 3; Ni Content: 26.8 Wt.%; Toluene Feed Rate: 2ml/H; The Flow Rate of N₂: 30ml/Min.

Figure 10: Catalysts Stability Study on Ni/ZSM-5; Reaction Temperature: 923K; 0.3 G Catalyst; S/C: 3; Ni Content: 26.8 Wt.%; Toluene Feed Rate: 2ml/H; The Flow Rate of N₂: 30ml/Min.

Table 1: Structural Properties and Surface Morphology of Catalysts.

Table 2: Comparisons in Catalytic Performance with Other Works.

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