Direct synthesis of highly loaded and well-dispersed NiO/SBA-15 for producer gas conversion

Abstract

To design a new Ni based catalyst for producer gas (CO₂ + H₂) conversion with high conversion and selectivity, highly loaded and well-dispersed NiO/SBA-15 was obtained for the first time by the direct synthesis method. The NiO particles were dispersed into the SiO₂ structure of SBA-15, unlike when prepared by the post synthesis method. The NiO/SBA-15 exhibited excellent efficiency and selectivity for producer gas conversion, comparable to that obtained by the post synthesis method. The synthesis method affected the CO selectivity. The temperature and H₂/CO₂ ratio played an important role in CO₂ conversion, indicating that a high temperature and high H₂/CO₂ ratio favored CO₂ conversion. The NiO loading did not affect the CO₂ conversion. Although there was no difference in the CO selectivity when the NiO loading was increased at high temperature, it was influenced greatly by NiO loading at low temperature as a result of CH₄ formation. In NiO/SBA-15 with low NiO loading, it can be considered that the NiO particles were separated into single NiO particles, which only catalyzed the reverse water gas shift (RWGS) reaction, regardless of the temperature, resulting in a CO selectivity of 100%. However, in NiO/SBA-15 with high NiO loading, the NiO particles aggregated to result in one or more NiO particles existing near each other. In this case only the RWGS reaction could occur at high temperature, and both methanation and the RWGS reaction were catalyzed at low temperature, resulting in a CO selectivity of less than 100%.

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1 Introduction

Biomass use has attracted great interest as an important renewable energy resource because bioenergy does not damage the environment.¹ Gasification is an important process with regards to the utilization of biomass in energy recycling, namely, converting biomass into a producer gas mixture consisting of carbon monoxide (CO), hydrogen (H₂), carbon dioxide (CO₂) and other trace species.^2,3 As the gas mixture has a low energy density, its utilization value is very low. To increase the energy density or calorific value, the producer gas is often reformed to produce useable H₂,^4–10 CO^11–13 and synthetic natural gas (SNG),^14–17 CO₂ is the most difficult of these producer gases to convert. Although CO₂ can be converted to CO by reforming methane (CH₄) with the addition of extra CH₄,^11–13 there has been no report on CO₂ conversion without an external supply of chemical materials. Therefore, the conversion of CO₂ without an external H₂ supply remains an important topic.

The conversion of CO₂ to valuable chemical materials has been proposed as a possible way of utilizing non-combustible CO₂.^18–23 In terms of CO₂ conversion, since CO is a valuable material in many chemical processes, the conversion of CO₂ to CO by catalytic hydrogenation is recognized as the most promising process using a producer gas (CO₂ + H₂) without a supply of extra H₂. The reverse water gas shift (RWGS) reaction is a highly desirable reaction for CO₂ to CO conversion using a producer gas. Because the RWGS reaction is endothermic, a high temperature will facilitate the conversion of CO₂ to CO. Copper and cerium-based catalysts are effective for the RWGS reaction.^24–33 However, these catalysts are not suitable for use at high temperature because of their poor thermal stability. Therefore, to increase the CO₂ equilibrium conversion and selectivity (the production of methane (CH₄) as a by-product affects CO selectivity), there is still a need to develop improved catalysts that can be employed at high temperature.

Nickel (Ni)-based catalysts remain the most extensively studied well-dispersed materials, and Ni based ceria has been investigated for the RWGS reaction.³⁴ The discovery of ordered mesostructured silica SBA-15 with an adjustable pore size and a high specific surface area^35,36 has provided the opportunity to produce well-dispersed Ni catalysts. Although NiO/SBA-15 has been realized with a post synthesis method,^37,38 there has been no report on an RWGS reaction using Ni based mesoporous SBA-15 as catalyst. In addition, because SBA-15 is synthesized in strong acid media, it is difficult to obtain NiO/SBA-15 by the direct synthesis method due to the high solubility of Ni salt in acid media.

On the basis of the above options, we must develop a new Ni based catalyst with good CO₂ conversion and CO selectivity for producer gas (CO₂ + H₂) conversion, to convert non-combustible CO₂ to combustible CO and hence increase the energy density of the producer gas. Therefore, we have designed and synthesized highly loaded and well-dispersed NiO/SBA-15 by the direct synthesis method for the first time. The RWGS reaction using a producer gas (CO₂ + H₂) without the addition of extra H₂ was further investigated using the obtained NiO/SBA-15 as a catalyst in a temperature range of 400 to 900 °C, and the result compared with that obtained by the post synthesis method.

2 Experimental

2.1 Direct synthesis of NiO/SBA-15

NiO/SBA-15 was obtained by the direct synthesis method. A block copolymer Pluronic P123 (Aldrich, EO₂₀PO₇₀EO₂₀, M_av = 5800) was dissolved in 105 g of deionized water, and then 19.83 g of sulfuric acid (H₂SO₄, Wako Chemical, Japan, 95.0%) and an appropriate amount of nickel(II) nitrate hexahydrate (Ni(NO₃)₂·6H₂O, Wako Chemical, Japan, 99.5%) were added at room temperature. The mixture was stirred for 30 min, and then 8.52 g of tetraethylorthosilicate (Si(OC₂H₅)₄, TEOS, Wako Chemical, Japan, 95.0%) was added. The obtained starting mixture was aged at 60 °C until a white precipitate appeared, and then immediately evaporated at 100 °C overnight (≈ 14 h). The resulting solid product was dried at 150 °C for 5 h, and then calcined at 500 °C for 10 h and at 800 °C for 2 h.

2.2 Characterization

The X-ray diffraction (XRD) patterns of the solid products were collected by a powder X-ray diffractometer (Bruker AXS D8 Advance) with graphite monochromatized Cu-Kα radiation at 40 kV and 40 mA. Nitrogen adsorption isotherms at −196 °C were performed using a conventional volumetric apparatus (Quadrasorb USA). Prior to adsorption measurements, the powders (≈ 0.1 g) were treated at 300 °C for 5 h in vacuum. The specific surface area was calculated by the BET method. The pore size was calculated from the desorption branch of nitrogen adsorption–desorption isotherms by the Barrett–Joyner–Halenda (BJH) method. The pore volume was taken at a single point P/P₀ = 0.95. The morphology was observed with a scanning electron microscope (SEM, OXFORD instruments). The TEM observation of the solid product was carried out using a transmission electron microscope (TEM, Hitachi High-Technologies, HF-2000) at an electron acceleration of 200 kV.

2.3 Reverse water gas shift (RWGS) reaction

Before the RWGS reaction, the catalysts were pre-reduced in situ in an 80% H₂/N₂ stream for 2 h with a total gas flow of 50 ml min⁻¹ at 900 °C with a heating ramp rate of 100 °C h⁻¹. The RWGS reaction was performed at atmospheric pressure in a fixed bed quartz reactor with an inside diameter of 20 mm in an initial temperature range of 400 to 900 °C. A 30 mm long catalyst between two layers of quartz wool was loaded into the reactor. A thermocouple was directly inserted into the center of the catalyst bed to measure the actual pretreatment and reaction temperature in situ. The reactor was heated in a furnace (KTF-035N, Koyo Thermo Systems, CO., Ltd.) equipped with a temperature controller. All reactant gases were monitored with a mass flow meter (E-40) and a controller (PE-D20) (HORIBA STEC, Co., Ltd.). The flow of the product was measured with a film flow meter (VP-3, HORIBA STEC, Co., Ltd.) and analyzed with a GC-TCD after the RWGS reaction had become stable.

3 Results and discussion

3.1 Direct synthesis and characterization of NiO/SBA-15

In our previous research,⁴⁰ we employed H₂SO₄ to investigate a novel approach for rapidly synthesizing ordered mesoporous silica SBA-15, and we were able to obtain the SBA-15 overnight. In this study, to obtain the NiO/SAB-15 by the direct synthesis method, we modified the previous method⁴⁰ by adding Ni(NO₃)₂·6H₂O to the starting synthesis mixture. We found, using powder X-ray diffraction (XRD) analysis, that the solid product we obtained was NiSO₄/SBA-15, and so we performed the calcination at a different temperature. We found that from 650 °C part of the NiSO₄ began to decompose to NiO, until 800 °C when the decomposition was completed and we could obtain NiO/SBA-15. Fig. 1 shows the small-angle (A) and wide-angle (B) (XRD) patterns of the product (catalyst no. 1) with a NiO loading of 10 wt% calcined at 800 °C. The small-angle XRD pattern (Fig. 1 (A) had three well-resolved peaks that could be indexed as (100), (110) and (200) diffraction peaks, which indicated a typical two-dimensional hexagonally ordered mesostructure (p6mm) of SBA-15. The wide-angle XRD peaks (Fig. 1 (B) can be indexed to a face-centered cubic crystalline NiO structure, indicating that the NiO particles were occluded in the SBA-15. In addition, a broad peak at around 2θ = 23° caused by the amorphous SiO₂ structure of SBA-15 was also clearly observed in the wide-angle XRD pattern, indicating that the ordered mesoporous SBA-15 structure was not disturbed by the included NiO. As the NiO loading increased, the peaks of the small-angle XRD pattern (not shown here) shifted to slightly higher angles, and the peak intensities decreased. This result suggests that the structural degradation of SBA-15 was caused by NiO loading. In addition, the peak intensities of the wide-angle XRD pattern (not shown here) increased as the NiO loading increased, which may indicate that large NiO particles were formed due to the aggregates that resulted from the high loading. Therefore, well-dispersed NiO/SBA-15 with high loading could be obtained by the direct synthesis method.

Fig. 1 XRD patterns of NiO/SBA-15 (Catalyst no.1).

Fig. 2 shows the nitrogen adsorption–desorption isotherm of NiO/SBA-15 with a NiO loading of 10 wt% calcined at 800 °C. The isotherm was similar to the type IV IUPAC classification,⁴¹ clearly indicating that these materials possessed mesoporous structures, and a type-H1 hysteresis loop was observed. The isotherm was typical for SBA-15, indicating the formation of a p6mm structure with open cylindrical mesopores. As the NiO loading increased, the nitrogen adsorption–desorption isotherms (not shown here) remained unchanged, and there was no blocking effect, unlike that obtained with the post synthesis method.^37,38 This result suggests that NiO particles were dispersed into the SiO₂ structure of SBA-15, not in the SBA-15 pores, unlike that prepared by the post synthesis method.

Fig. 2 Nitrogen adsorption–desorption isotherm of NiO/SBA-15 (Catalyst no.1).

The NiO/SBA-15 characteristics are summarized in Table 1. Although there was no difference in pore size with increased NiO loading, the surface area and pore volume decreased, indicating that the SBA-15 structure was degraded by NiO loading.

Table 1 Characteristics of mesoporous NiO-SBA-15^a

Catalyst no.	NiO-SBA-15
	NiO wt%	Surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)	Pore size (nm)
a Synthesis conditions: H2SO4/TEOS = 4.7; P123/TEOS = 1.65 × 10^−2; H2O/TEOS = 140.
1	10	319.71	1.05	9.12
2	20	298.99	0.96	9.12
3	30	288.26	0.82	9.11
4	40	203.17	0.54	9.15

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The SEM image in Fig. 3 shows that the SBA-15 morphology consisted of rod-like aggregates, and this is the same as that of typical SBA-15, as in the previous report.⁴⁰ The TEM image is shown in Fig. 4. It had an ordered mesoporous structure for typical SBA-15. No aggregates of NiO particles were observed, indicating that the NiO particles were highly dispersed into the SBA-15 structure. Therefore, highly loaded and well-dispersed NiO/SBA-15 could be obtained by the direct synthesis method.

Fig. 3 SEM image of NiO/SBA-15 (Catalyst no. 1).

Fig. 4 TEM image of NiO/SBA-15 (Catalyst no. 1).

3.2 RWGS reaction

To investigate the influence of catalysts, we performed the RWGS reaction using producer gas from biomass gasification with and without catalysts by fixing the CO₂/H₂ ratio at 1, and the result is shown in Fig. 5 (A). In Fig. 5 (A), we can see that the RWGS reaction occurred even without a catalyst. Although there was almost no CO₂ conversion at temperatures below 600 °C, the CO₂ conversion increased with increasing temperature. When catalyst no. 1 with a NiO loading of 10 wt% was employed, the CO₂ conversion was enhanced compared with that obtained without catalysts, and it increased with increasing temperature. Therefore, the catalysts played the important role of increasing the CO₂ conversion in the RWGS reaction. Because the CO₂/H₂ ratio in producer gas is often not 1, the influence of the H₂/CO₂ ratio on CO₂ conversion at 800 °C was also investigated and is shown in Fig. 5 (B). CO₂ conversion using catalyst no. 1 with a NiO loading of 10 wt% increased as the H₂/CO₂ ratio increased, indicating that the complete consumption of CO₂ was possible at a high H₂ concentration (the excess H₂ can be recycled after the reaction). These results suggest that high equilibrium conversion can be achieved at a high temperature and a high H₂/CO₂ ratio, indicating that a high temperature and H₂/CO₂ ratio can shift the equilibrium in the RWGS reaction to the right. The CO₂ conversion at 900 °C was very high compared with that in other reports.^24–33 Also, to confirm this conclusion, we used 1 wt% NiO/SBA-15 and CuO/SBA-15 as a catalyst to investigate the RWGS reaction, and the result is shown in Fig. 6. The turnover frequency (TOF) using NiO/SBA-15 was greater than that using CuO/SBA-15. Therefore, the catalytic activity of NiO/SBA-15 was higher than that using the commonly employed CuO/SBA-15.

Fig. 5 Using NiO/SBA-15 with different NiO loading, (A) CO₂ conversion at different temperature (CO₂ : H₂ = 60 : 60 ml min⁻¹) and (B) with H₂/CO₂ ratio (CO₂ : H₂ = 60 : X ml min⁻¹) at 800 °C.

Fig. 6 TOF obtained using 1 wt% oxides/SAB-15 at different temperature. CO₂ : H₂ = 120 : 120 ml min⁻¹.

Although there was no difference in the CO₂ conversion when the NiO loading of 10 wt% was increased to 40 wt% at a high temperature of over 600 °C, suggesting that the RWGS reaction achieved equilibrium, at a low temperature of 500 °C, the CO₂ conversion seemed to increase as the NiO loading increased. The CO selectivity when using NiO/SBA-15 with different NiO loadings at different temperatures is shown in Fig. 7. With an NiO loading of 10 wt%, the CO selectivity was 100%, regardless of temperature. When the NiO loading exceeded 20 wt%, the CO selectivity was 100% at a temperature of over 600 °C, regardless of NiO loading. In contrast, the CO selectivity was not 100% due to the formation of CH₄ at temperatures below 500 °C, indicating that the NiO loading affects the CO selectivity at low reaction temperatures. The CO selectivity seemed to decrease as the NiO loading increased at low temperature. Therefore, to obtain CO with a selectivity of 100%, the RWGS reaction should be carried out at a suitable temperature and using NiO/SBA-15 with an appropriate NiO loading as a catalyst. In other words, the temperature and NiO loading were important factors influencing CO selectivity.

Fig. 7 CO selectivity at different temperature using NiO/SBA-15 with different NiO loading. CO₂ : H₂ = 60 : 60 ml min⁻¹.

A TEM image shows that the NiO particles were well dispersed into SBA-15. When the NiO loading was low (10 wt%), the NiO particles may have been completely dispersed into individual NiO particles by the SiO₂ of SBA-15. Namely, one NiO particle does not exist near another NiO particle in NiO/SBA-15. However, as the NiO loading increased, the environment surrounding the NiO particles began to change, and one or more NiO particles appeared in close proximity to other NiO particles. In accordance with a previous report_,³⁹ the mechanism of CO₂ methanation over Ni/SBA-15 was considered and is shown in Fig. 8. In the CO₂ adsorption step on the Ni surface, two active Ni particles (paired Ni–Ni) are required, and then CO₂ methanation can occur to form CH₄ and CO as a by-product. In other words, when only one active metallic Ni (single active Ni particle) was present, CO₂ methanation cannot occur. Therefore, we can explain why the NiO loading affects the CO selectivity at a low reaction temperature. As shown in Fig. 9, with a low NiO loading of 10 wt%, because only a single (isolated) Ni particle was present, only the RWGS reaction occurred. Therefore, CO₂ methanation cannot be catalyzed, so no CH₄ is formed. When the NiO loading is increased above 20 wt%, paired Ni–Ni particles began to appear, so the CO₂ methanation occurred in addition to the RWGS reaction, resulting in CO + CH₄ formation. Namely, CO was formed by two reactions (CO as the main product from the RWGS reaction, and as a by-product of CO₂ methanation). Therefore, the CO₂ conversion increased with increasing NiO loading at a low temperature of 500 °C, due to the occurrence of CO₂ methanation.

Fig. 8 Mechanism of CO₂ methanation over Ni/SBA-15.

Fig. 9 Coordination of Ni particle and its catalyzed reaction.

To validate our above explanation, we employed 10 wt% NiO supported SBA-15 prepared by the post synthesis method as a reference catalyst to investigate the RWGS reaction. SBA-15 was synthesized according to the reported method.⁴² The starting mixture was prepared as follows: 4 g of Pluronic P123 (P123, EO₂₀PO₇₀EO₂₀, Aldrich, USA) was dissolved in 105 g of deionized water at room temperature, followed by 8.52 g of tetraethylorthosilicate (Si(OC₂H₅)₄, TEOS, Wako Chemical, Japan, 95.0%). The mixture was then stirred for 30 min and hydrochloric acid (HCl, Wako Chemical, Japan, 35.0–37.0%) was added to obtain the starting synthesis mixture. The obtained starting mixture was aged immediately at 60 °C for 24 h under reflux conditions. The chemical composition of the prepared starting mixture was as follows: 1.00 TEOS : 1.65 × 10⁻² P123 : 5.00–6.95·HCl : 140·H₂O. The resulting white solid was filtered, washed thoroughly with deionized water, dried overnight in an oven at 80 °C, and calcined at 550 °C for 10 h. NiO/SBA-15 was prepared by the conventional solvent impregnation (SI) method using ethanol (EtOH, Wako Chemical, Japan, 99.5%) as a solvent: After Ni(NO₃)₂·6H₂O had been completely dissolved in EtOH under ultrasonication, SBA-15 was added and dispersed for 6 h also under ultrasonication, then dried at room temperature and calcined at 400 °C for 5 h.

In this NiO/SBA-15 reference catalyst obtained by the post synthesis method, it can be considered that the NiO particles were occluded in the SBA-15 pores, so quantities of paired NiO particles were present. Namely, the dispersion degree of the reference catalyst was lower than that of NiO/SBA-15 obtained by the direct synthesis method. As shown in Fig. 10 (A), when the reference catalyst was employed, at high temperatures of over 600 °C, the CO₂ conversion increased with the increase in temperature, and was lower than that obtained using NiO/SBA-15 synthesized by the direct synthesis method. However, at low temperatures of below 500 °C, because the CO₂ methanation (catalyzed by the paired Ni–Ni) occurred, the CO₂ conversion was higher than that obtained using NiO/SBA-15 synthesized by the direct synthesis method. The CO selectivity is shown in Fig. 10 (B). Although when using the reference catalyst, the CO selectivity was 100% at temperatures above 700 °C, it was not 100% at temperatures below 600 °C due to the formation of CH₄. This differs from the result obtained using NiO/SBA-15 with a NiO loading of over 20 wt% synthesized by the direct synthesis method. When the reference catalyst was used at low temperatures, the CO selectivity (about 60%) was far lower than that obtained using NiO/SBA-15 synthesized by the direct synthesis method due to the difference in the quantity of paired Ni–Ni particles, and in this case, the CH₄ selectivity exceeded 40%. Therefore, the CO selectivity was also affected by the catalyst synthesis method. This result further indicates that the paired Ni–Ni catalyzed the CO₂ methanation and confirms our above explanation.

Fig. 10 (A) CO₂ conversion and (B) CO selectivity using catalyst no. 1 (•) and reference catalyst (▴) at different temperatures. CO₂ : H₂ = 60 : 60 ml min⁻¹.

4 Conclusions

In summary, well-dispersed NiO/SBA-15 with a high loading could be obtained by the direct synthesis method. The obtained NiO/SBA-15 catalysts exhibited excellent efficiency, and selectivity with regards to reducing CO₂ in a producer gas to CO by the RWGS reaction. The CO₂ conversion increased as the temperature and H₂/CO₂ ratio increased, indicating that a high temperature and a high H₂/CO₂ ratio favors CO₂ conversion. The CO selectivity was affected by the NiO loading and synthesis method. This can be explained by the fact that a single (isolated) Ni particle catalyzes the RWGS reaction, giving a high selectivity of 100% for reducing CO₂ to CO. The paired Ni–Ni particles catalyze CO₂ methanation, giving a low selectivity of less than 100% for reducing CO₂ to CO due to CH₄ formation.

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