ZnO–ZnCr₂O₄ composite prepared by a glycine nitrate process method and applied for hydrogen production by steam reforming of methanol

Abstract

To address climate change, the energy crisis, and global warming, hydrogen (H₂) can be used as a potential energy carrier because it is clean, non-toxic and efficient. Today, the mainstream industrial method of H₂ generation is steam reforming of methanol (SRM). In this process, a zinc-based commercial catalyst is usually used. In this work, a ZnO–ZnCr₂O₄ catalyst was successfully synthesised by the glycine nitrate process (GNP) and developed for use in H₂ production by SRM. The specific surface area, porous structure and reaction sites of the zinc-based catalyst were effectively increased by the preparation method. The as-combusted ZnO–ZnCr₂O₄ composite catalyst had a highly porous structure due to the gas released during the GNP reaction process. Moreover, according to the ZnO distribution and different G/N ratios, the specific surface area (S_BET) of the as-combusted ZnO–ZnCr₂O₄ catalyst varied from 29 m² g⁻¹ to 46 m² g⁻¹. The ZnO–ZnCr₂O₄ composite catalyst (G/N 1.7) exhibited the highest hydrogen production, 4814 ml STP min⁻¹ g-cat⁻¹, at a reaction temperature of 450 °C without activation treatment. After activation, the ZnO–ZnCr₂O₄ composite catalyst achieved hydrogen production of 6299 ml STP min⁻¹ g-cat⁻¹ at a reaction temperature of 500 °C. The hydrogen production performance of the ZnO–ZnCr₂O₄ composite powder was improved by the uniform addition of ZnO to ZnCr₂O₄. Based on the performance, this ZnO–ZnCr₂O₄ composite catalyst has great potential to have industrial and economic impact due to its high efficiency in hydrogen production.

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1. Introduction

Because of environmental issues such as global warming, fossil energy use, and the energy crisis, hydrogen (H₂) energy has attracted the attention of industrial and scientific communities. Moreover, hydrogen energy is clean and harmless to the environment.^1–3 The nature and characteristics of hydrogen energy make it a possible replacement for fossil energy. Hence, hydrogen fuel cells are viewed as a possible energy source for the automotive field because of their stable operation, low cost, and carbon-free nature. Fuel cell-based chemical reactions to directly generate electricity also have the advantages of low pollution and high efficiency.¹

For the growth of a hydrogen energy economy, the challenges of the effective production, storage, and transport of hydrogen need to be met.⁴ Therefore, the progress of the popularization of fuel cell vehicles (FCV) in each country is directly affected by the hydrogen infrastructure, such as the cost and challenges of hydrogen storage, hydrogen transport, and leakage.⁵ The risk of flammability is high when the hydrogen concentration in air is between 4% and 75%. This range is wider than that of natural gas, which is between 5% and 15%. Given the risks, the safety of hydrogen storage techniques is important to society.^6–8 Most hydrogen storage media use metal hydrides, but such system storage capacity is insufficient.^9–11 To solve these challenges, hydrogen generation from a steam reforming system uses various conversion sources.^12–14

When methanol and water vapor react in the presence of a suitable catalyst, they can produce H₂ and CO. For H₂ conversion, methanol offers advantages over other resources such as methane, gasoline, and ethanol.¹⁵ Due to the lowest ratio of hydrogen to carbon from the methanol chemical formula, carbon emissions are lower than those of other liquid hydrogen conversion sources, leading to less carbide production.^16,17 Especially, because methanol has no C–C bond, the amount of coke fabricated in SRM is lower. The operation is safe, and the sulfur compound content is low (<5 ppm). For these reasons, SRM is the worthiest method for high-efficiency hydrogen production.¹⁸

The general reaction processes that produce hydrogen from methanol are mainly divided into three forms: (1) thermal decomposition, (2) partial oxidation, and (3) steam reforming.^19,20 However, they have important differences. The decomposition reaction has obvious and strong endothermic phenomena, and CO is produced as a byproduct. Hence, the decomposition reaction is not desirable in fuel cells.^21,22 Partial oxidation has a strong exothermic reaction, and if pure oxygen is replaced with air, the hydrogen production rate is 66%.²³ In contrast, the endothermic process of steam reforming allows efficient hydrogen production.²⁴ The by-product is CO, and the hydrogen production rate can reach 75%. Among the various methods, steam reforming of methanol has miscibility with water, a low reforming temperature (250 °C), and a low CO production rate, so it is preferred by industry for efficient hydrogen production.¹⁷

CH₃OH → CO + 2H₂, ΔH₀ = 128 kJ mol⁻¹ (1)

CH₃OH + 1/2O₂ → CO₂ + 2H₂, ΔH₀ = −155 kJ mol⁻¹ (2)

CH₃OH + H₂O → CO₂ + 3H₂, ΔH₀ = 131 kJ mol⁻¹ (3)

In the SRM process, hydrogen is generally produced from methanol in three ways: (4) water gas shift (WGS), (5) decomposition of methanol, and (6) methanol steam reforming, as shown below.²⁵

CH₃OH + H₂O → CO₂ + 3H₂ (4)

CH₃OH → CO + 2H₂ (5)

CO + H₂O → CO₂ + H₂ (6)

SRM is a simple and effective process to produce hydrogen due to the endothermic process during the reaction, so it is suitable for fuel cell applications. Moreover, the equipment of SRM requires no hydrogen storage station because of the gas existing. For the preparation of the metal-based composite catalysts used in SRM, elements with highly active elements, such as Zn, Cu, Ru, and Pd, are used. The various elements and reaction conditions have significant effects on performance. According to previous research, Cu-based catalysts produce outstanding results in various applications, especially in the hydrogen production reaction conditions, due to their lower reaction temperature and high sensitivity. Hence, Cu-based catalysts have good potential as materials and have received much interest in industry. However, when a Cu-based catalyst reacts at over 350 °C, the catalytic efficiency and life cycle decrease because the copper ions are easily over-reduced and crystallize into metallic copper particles, thereby losing the active sites. Furthermore, the deposition of carbon particles throughout the SRM process is the primary cause of decreased catalytic activity.^26–28 To extend the cycle-life and increase efficiency, different metal oxides, such as ZnO,^29–32 ZrO₂,^33,34 Al₂O₃,³⁵ Ga₂O₃,³⁶ Fe₂O₃,³⁷ and CeO₂,^38–40 can be incorporated to improve the function of the catalyst. Meanwhile, the perovskite oxides catalyst was expected as a potential material that was employed for thermochemical fuel production.^41–43 With proper methods for modifying the existing catalyst, the Cu-based catalyst has better the thermal stability, efficiency, and dispersion after the incorporation of the metal oxide in the Cu-based catalyst.

The glycine nitrate process (GNP), which is synthesis procedure, was proposed in 1967 by Professor A. G. Merzhanov.⁴⁴ This GNP synthesis technique uses the heat released by chemical reactions to promote synthesis, and it has been applied in several fields, such as flat plates and lithium batteries.⁴⁵ Meanwhile, the grain size can also affect the catalytic function. A special characteristic of GNP is that it can create a fluffy structure due to the gas produced in the reaction and restrain grain growth.^8,46 In the SRM process, Cu-based catalysts have high activity, but they also have the disadvantage of high sensitivity to the reaction environment.⁴⁷ Given the disadvantages of Cu-based catalysts, challenges still exist to their application. These challenges include long-term performance retention, stability under extreme reaction environments, and narrow reaction temperature range for optimal hydrogen production. Therefore, suitable alternatives need to be developed for the SRM process.²⁹

Zinc–chromium (Zn–Cr) oxide has long been used as a catalyst to generate methanol under high temperature and pressure. The good performance exhibited by Zn–Cr oxide is due to its crystallization during high-temperature steam reforming. Zn–Cr oxide can maintain high activity and high stability.⁴⁸ Zn–Cr based spinel oxide exhibits high function in applications such as CO and C_xH_y oxidation,⁴⁹ alcohol generation,⁵⁰ removals of organic contaminants,⁵¹ and isobutanol synthesis.⁴⁸ However, higher thermal treatments could deactivate the Zn–Cr catalyst and degrade its stability. According to Liu et al., the stable spinel structure can prevent a high drop in pressure, which otherwise could damage a vehicle.⁵² ZnCr₂O₄ has been used as a catalyst and demonstrated high thermal stability and mechanical stability.⁵³ Furthermore, ZnCr₂O₄ also has high activity and good photocatalytic performance.⁵⁴ Katte et al. revealed the synergistic effect of Cu and ZnO in promoting methanol production.⁵⁵ To avoid the drawbacks of higher sintering temperatures, the synergetic effect of contact between ZnO and ZnCr₂O₄ increases the catalytic function because of the strong attachment of the two phases.⁴⁸ In this project, the ZnO–ZnCr₂O₄ composite catalyst was made by the GNP method and applied to hydrogen generation by SRM. Moreover, the porous structure of the catalyst was increased by the GNP method, as compared to that resulting from the traditional solid-state reaction, and improved the function of the ZnO–ZnCr₂O₄ composite catalyst in hydrogen production.

2. Experiment procedure

2.1 Materials and methods

The starting reagents, namely, zinc nitrate hexahydrate [Zn(NO₃)₂·6H₂O], glycine [C₂H₅NO₂], and chromium nitrate nonahydrate [Cr(NO₃)₃·9H₂O] were purchased from SHOWA and Sigma-Aldrich. The as-combusted catalyst in this study was investigated by suitable instrumentation techniques. The crystalline structures of the ZnO–ZnCr₂O₄ composite catalyst and ZnCr₂O₄ catalyst were analyzed by X-ray diffractometric (D2 Phaser, Bruker) with a working voltage of 30 kV under Cu Kα radiation. The morphology and particle size of the as-combusted catalyst in this study were studied by field emission scanning electron microscopy (JEOL FE-SEM. JSM-7610F). The specific surface area (S_BET) was measured by Brunauer–Emmett–Teller (BET) method with a Gemini V Micromeritics, Surface Area, and Pore Size Analyzer. A suitable amount of prepared catalyst was degassed at 200 °C for 24 hours and the absorbed water was removed by passing high purity N₂ through the catalyst before BET measurement. N₂ adsorption isotherms were measured and investigated at various relative pressures (P/P₀) of 0 to 0.3 while the catalyst adsorbed N₂.

2.2 Preparation of ZnO–ZnCr₂O₄ composite and ZnCr₂O₄ catalysts

ZnO–ZnCr₂O₄ and ZnCr₂O₄ catalysts were prepared by as-combusted GNP method.⁵⁶ The molar ratios of zinc nitrate to glycine were 1 : 1.5 and 1 : 1.7, and the two catalysts were referred to as G/N-1.5 and G/N-1.7, respectively. Zinc nitrate to chromium nitrate molar ratios were 1 : 1 and 1 : 2 for the fabrication of the ZnO–ZnCr₂O₄ composite and ZnCr₂O₄. All starting reagents were dissolved in 80 ml DI water and stirred at 80 °C for 12 hours. After that, the precursor solution was dried in an oven at 100 °C for 48 hours to evaporate the remaining water until the precursor is gel-like. The obtained solution was heated on a hotplate and spontaneously combusted at approximately 300 °C. The reaction yielded gray powder and NO₂ gas.⁵⁷

2.3 Catalyst test

The as-combusted Zn-based catalysts were placed in a tubular reactor. The carrier gas used in the process was N₂ with a flow rate of 30 sccm and a quartz tube inner diameter of 1.2 cm. A flowchart of the SRM process is presented in Fig. 1. The gas product converted in the process was analyzed and measured several times at each reaction temperature with a GC-1000 gas chromatograph equipped with a thermal conductivity detector and the resulting values were averaged. Before the catalysts were testing, all of the specific samples with and without activation by flow reactor received pre-oxidation treatment by annealing in a mixture of gas of 10% H₂ and 90% N₂ at 650 °C for 1 hour. For every experiment, 0.02 g of catalyst was put into a quartz tube. A gas chromatograph (GC 1000 Chromatography with TCD) with one column (60/80 Carboxen® 1000) for H₂ (7 ft 1/16 in, stainless steel) was used to measure the hydrogen production. In the gas chromatograph, a thermal conductivity detector with a current of 50 mA was fitted. The H₂ production performances of the ZnO–ZnCr₂O₄ and ZnCr₂O₄ catalysts were investigated based on the H₂ production rate from the gas chromatograph.^58,59

Fig. 1 Flowchart diagram of the methanol steam reforming process.

3. Results and discussion

3.1 Characteristics of ZnO–ZnCr₂O₄ composite catalyst and ZnCr₂O₄ catalyst prepared by GNP

XRD studies of the as-combusted catalysts were performed with the database software Powder-XRD. Fig. 2(a–d) reveal the XRD patterns of as-combusted ZnCr₂O₄ (GNP: 1.5, 1.7) and ZnO–ZnCr₂O₄ (GNP: 1.5, 1.7). The XRD curves in Fig. 2(a) and (b) reveal the diffraction patterns of ZnCr₂O₄ prepared with G/N ratios of 1.5 and 1.7. The diffraction spectra of the cubic phase spinel ZnCr₂O₄ (PDF# 87-0028) showed diffractions at 30.3°, 35.7°, 43.4°, 53.8°, 57.4° and 63.1°, which corresponded to the (220), (311), (400), (422), (511) and (400) planes. On the other hand, the diffraction pattern of the ZnO phase in Fig. 2(c) was based on the increasing metal source ratio of zinc nitrate to chromium nitrate. Furthermore, Fig. 2(d) shows that the intensity of the ZnO diffraction peak decreased due to the decrease in the G/N ratio because the conditions were insufficient for the ZnO phase to crystallize. Meanwhile, the ZnO–ZnCr₂O₄ synthesized with a lower G/N ratio, the reaction temperature would decrease, and the diffraction peaks became broader which revealed the crystallite of the ZnO–ZnCr₂O₄ was smaller. The diffraction patterns of hexagonal phase ZnO (PDF# 79-0206) are shown in Fig. 2(c). The peaks at 31.7°, 34.4°, and 36.2° corresponded to the (100), (002) and (101) planes. Based on the XRD patterns of as-combusted ZnCr₂O₄ (GNP: 1.5, 1.7) and ZnO–ZnCr₂O₄ (GNP: 1.5, 1.7), immiscibility of ZnO and ZnCr₂O₄ was judged by the non-shift XRD peak from the ZnO–ZnCr₂O₄, and no other secondary phases such as Cr₂O₃ and ZnCrO₄ were observed in the pattern.

Fig. 2 XRD patterns of ZnCr₂O₄ catalysts prepared by the GNP method with G/N ratios of (a) 1.7 and (b) 1.5, and ZnO–ZnCr₂O₄ catalyst synthesis by the GNP method with G/N ratios of (c) 1.7 and (d) 1.5.

Moreover, the activated catalysts were respectively investigated for their characterization. Fig. 3 showed the XRD patterns of catalysts after activation. Fig. 3(a) ZnCr₂O₄ catalyst prepared by the GNP method with G/N ratios 1.7 revealed the phase remained ZnCr₂O₄ spinel phase and Fig. 3(b) ZnO–ZnCr₂O₄ catalyst prepared by the GNP method with G/N ratios 1.7 revealed the ZnCr₂O₄ spinel phase and slightly ZnO hexagonal phase, respectively.

Fig. 3 XRD patterns of activated (a) ZnCr₂O₄ and (b) ZnO–ZnCr₂O₄ catalyst prepared by the GNP method with G/N ratios 1.7.

The microscopic surface morphologies and the structures of the as-combusted catalysts were analyzed by FESEM. Fig. 4 presents FESEM images of (a) ZnCr₂O₄ prepared by the GNP method with a G/N ratio of 1.7, (b) ZnCr₂O₄ prepared by the GNP method with a G/N ratio of 1.5, (c) ZnO–ZnCr₂O₄ prepared by the GNP method with a G/N ratio of 1.7, and (d) ZnO–ZnCr₂O₄ prepared by the GNP method with a G/N ratio of 1.5 before the SRM treatment. The as-combusted ZnCr₂O₄ presented a coral-like porous structure due to the gas that was released during the reaction. However, the hexagonal flakes attached to the porous structure in Fig. 4(c) and (d) were ZnO, corresponding to the XRD result in Fig. 2. Based on the XRD and SEM analysis the of as-combusted catalysts, the ZnO was in the amorphous phase in the ZnO–ZnCr₂O₄ during the GNP when the G/N ratio was 1.5. Moreover, the as-combusted catalyst after activation still retains the porous structure which was shown in Fig. 5. However, compared to Fig. 4 and 5, the pore on the catalyst and the ZnO attached on the catalyst surface was shown to be smaller. Thus, it is exhibited that agglomeration occurs after activation.

Fig. 4 SEM images of ZnCr₂O₄ catalysts prepared by the GNP method with G/N ratios of (a) 1.7 and (b) 1.5, and ZnO–ZnCr₂O₄ catalysts prepared by the GNP method with G/N ratios of (c) 1.7 and (d) 1.5.

Fig. 5 SEM images of activated (a) ZnCr₂O₄ and (b) ZnO–ZnCr₂O₄ catalyst prepared by the GNP method with G/N ratios 1.7.

The specific surface areas of the as-combusted ZnO–ZnCr₂O₄ and ZnCr₂O₄ catalysts prepared by GNP (with G/N ratios of 1.5 and 1.7) are listed in Table 1. Before BET analysis, the remaining absorbed water was removed from the catalysts by high purity N₂ flow at 200 °C for 24 hours. N₂ adsorption isotherms were investigated at various relative pressures while the catalyst adsorbed N₂. A high specific surface area would facilitate the catalytic reaction.⁶⁰ Based on the BET results, the specific surface areas of the as-combusted ZnO–ZnCr₂O₄ and ZnCr₂O₄ catalysts ranged from 29 m² g⁻¹ to 46 m² g⁻¹, and the results of BET revealed a large S_BET. Furthermore, the surface areas of the ZnO–ZnCr₂O₄ and ZnCr₂O₄ catalysts decreased as the G/N ratio increased. Furthermore, the surface area of the activated ZnCr₂O₄ and ZnO–ZnCr₂O₄ (G/N = 1.7) catalyst was 27 m² g⁻¹ and 48 m² g⁻¹, respectively. Compared to the ZnO–ZnCr₂O₄ catalyst with G/N ratios of 1.7 before and after the activation, the surface area was increased by more than about 30% which could correspond to the catalyst test result shown in Table 3 and Fig. 9.

Table 1 The specific surface areas of ZnO–ZnCr₂O₄ and ZnCr₂O₄ catalysts prepared by GNP and the activated ZnO–ZnCr₂O₄ and ZnCr₂O₄ catalysts

Composition	Specific surface area (m^2 g^−1)
	Original catalyst	Activated catalyst
ZnCr2O4 G/N-1.7	29	27
ZnCr2O4 G/N-1.5	46	—
ZnO–ZnCr2O4 G/N-1.7	35	48
ZnO–ZnCr2O4 G/N-1.5	45	—

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3.2 Characteristics of ZnO–ZnCr₂O₄ composite catalyst and ZnCr₂O₄ catalyst after SRM treatment

Fig. 6 presents the X-ray diffraction patterns of ZnCr₂O₄ and ZnO–ZnCr₂O₄ prepared by the GNP method with G/N ratios of 1.5 and 1.7 after steam reforming treatment. As shown in the XRD patterns, the ZnCr₂O₄ and ZnO–ZnCr₂O₄ catalysts retained the cubic phase spinel structure of ZnCr₂O₄ (PDF# 87-0028) after the catalyst test, as revealed by Fig. 6(a–d). Moreover, after the SRM treatment, the crystallinity of the ZnO phase in the ZnO–ZnCr₂O₄ composite catalyst was significantly improved. This indicated that the ZnO particles attached to the surface of the catalyst experienced grain growth during the SRM process, as shown in Fig. 6(c) and (d).

Fig. 6 XRD patterns of ZnCr₂O₄ catalysts prepared by the GNP method with G/N ratios of (a) 1.7 and (b) 1.5, and those of ZnO–ZnCr₂O₄ catalysts prepared by the GNP method with G/N ratios of (c) 1.7 and (d) 1.5, after SRM treatment.

SEM images of the ZnCr₂O₄ and ZnO–ZnCr₂O₄ (G/N ratio 1.5 and 1.7) catalysts after steam reforming treatment are presented in Fig. 7. As can be seen in the figure, they still retained the porous structure produced in the GNP process. Because of the gas product generated during the reaction of SRM, the porous structure of ZnCr₂O₄ and ZnO–ZnCr₂O₄ catalyst was retained, as shown in Fig. 7(a–d). It can be seen from the SEM images in Fig. 7(c) and (d) that the addition of ZnO to the ZnCr₂O₄ catalyst increased the number of active sites and contributed to hydrogen production in the SRM process.

Fig. 7 SEM images of ZnCr₂O₄ catalysts prepared by the GNP method with G/N ratios of (a) 1.7 and (b) 1.5, and those of ZnO–ZnCr₂O₄ catalysts prepared by the GNP method with G/N ratios of (c) 1.7 and (d) 1.5, after SRM treatment.

For the catalyst test in this study, the ZnCr₂O₄ and ZnO–ZnCr₂O₄ catalysts prepared under different conditions were evaluated according to the hydrogen production rate (ml STP min⁻¹ g-cat⁻¹), and the efficiencies of the ZnCr₂O₄ and ZnO–ZnCr₂O₄ catalysts with and without activation were also compared. The H₂ production rate was estimated with a gas chromatograph equipped with TCD. The catalyst was placed in the reaction bed for the SRM process at temperatures of 350 °C to 500 °C, and then the reacted gas was assisted by the N₂ at a flow rate of 30 sccm. The catalyst was thermally treated at the reaction temperature prior to the reaction without contact with methanol vapor to maximize the hydrogen conversion capacity and the life of the catalyst.⁴⁷

The hydrogen production rates of the ZnCr₂O₄ and ZnO–ZnCr₂O₄ catalysts prepared with different G/N ratios at reaction temperatures of 350 °C to 500 °C are listed in Table 2. The H₂ production rates of the ZnCr₂O₄ and ZnO–ZnCr₂O₄ catalysts are illustrated in Fig. 8. It can be seen that, when the reaction temperature rose, the hydrogen production performance of the catalyst increased. The ZnO–ZnCr₂O₄ G/N-1.7 composite catalyst had the best hydrogen production efficiency when the gas product was carried by N₂, the N₂ flow rate was 30 sccm, and the temperature was 450 °C. The hydrogen production rate of the ZnO–ZnCr₂O₄ G/N-1.7 composite catalyst was estimated at 4814.25 ml STP min⁻¹ g-cat⁻¹. According to this result, the catalytic performance improved when the optimum catalyst reaction temperature decreased after the addition of ZnO. Zhang et al. reported that the incorporation of ZnO could affect the hydrogen production rate and that a suitable amount of ZnO content in the catalyst modified the active site on the surface and limited the agglomeration of the particles.⁶¹ Moreover, H₂ production enhancement by the ZnO incorporation into ZnCr₂O₄ which was binary crystal structure ZnO–ZnCr₂O₄ formed with a close connection of hexagonal ZnO and ZnCr₂O₄ phases and it was helpful to improve the interaction between ZnO and ZnCr₂O₄. Xiaofeng et al. reported the morphology and facet of ZnO played an important role in affecting its catalytic activity. Several studies have reported that the terminal polar (0001) facets were more active surfaces for catalysis than the nonpolar surfaces perpendicular to them.⁶²

Table 2 H₂ production rates of ZnO–ZnCr₂O₄ and ZnCr₂O₄ catalysts were prepared by the GNP method at different temperatures with a N₂ flow rate of 30 sccm

Rate of H2 production (ml STP min^−1 g-cat^−1)
Composition	350 °C	400 °C	450 °C	500 °C
ZnCr2O4 G/N-1.7	1714	2700	3549	2410
ZnCr2O4 G/N-1.5	2453	2753	4122	4402
ZnO–ZnCr2O4 G/N-1.7	1286	3046	4814	3954
ZnO–ZnCr2O4 G/N-1.5	1860	2573	4074	2663

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Fig. 8 The H₂ production rates of ZnO–ZnCr₂O₄ and ZnCr₂O₄ catalysts prepared by the GNP method at different temperatures with a N₂ flow rate of 30 sccm.

A high hydrogen production rate is essential for the realization of fuel cells for automotive and mobile applications. However, hydrogen production reactors are generally dangerous due to flammability and explosion hazards. However, the ZnO–ZnCr₂O₄ composite catalyst can be used directly without high-temperature activation, and it is extremely stable. Therefore, the catalyst in this study would be useful in vehicles with fuel cells because of its simple hydrogen production by SRM and its high efficiency.

Furthermore, the hydrogen production rates of the activated catalysts were also respectively investigated. The ZnO–ZnCr₂O₄ and ZnCr₂O₄ catalysts were activated in a mixture of 10% H₂ and 90% N₂ gas at 650 °C for 1 hour before the SRM process. The results of the catalyst tests are listed in Table 3 and illustrated in Fig. 9. According to the results, the optimal reaction temperature of ZnO–ZnCr₂O₄ composite increased to 500 °C. However, the optimal reaction temperature was increased because of the thermal treatment during the activation. Liyan et al. reported that ZnO–ZnCr₂O₄ composite treated with various thermal treatments revealed a reduction shift to the higher temperature in thermal treatment, indicating the synergetic effect shown by ZnO and ZnCr₂O_4.⁴⁸ The activated ZnO–ZnCr₂O₄ G/N-1.7 had the best hydrogen production rate, which reached 6299 ml STP min⁻¹ g-cat⁻¹. The activated ZnO–ZnCr₂O₄ composite catalyst also had higher activity and hydrogen production than those of the inactivated ZnO–ZnCr₂O₄ composite catalyst.

Table 3 H₂ production rates of activated ZnO–ZnCr₂O₄ and ZnCr₂O₄ catalysts (G/N-1.7) at different temperatures with a N₂ flow rate of 30 sccm

Rate of H2 production (ml STP min^−1 g-cat^−1)
Reactor temperature	ZnO–ZnCr2O4 G/N-1.7 (activated)	ZnCr2O4 G/N-1.7(activated)
350 °C	1681	1035
400 °C	2467	2272
450 °C	4185	3896
500 °C	6299	5865
550 °C	4503	3571

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Fig. 9 H₂ production rates of activated ZnO–ZnCr₂O₄ and ZnCr₂O₄ catalysts (G/N-1.7) at different temperatures with a N₂ flow rate of 30 sccm.

Fig. 10 compares the hydrogen production rates of ZnO–ZnCr₂O₄ and ZnCr₂O₄ powders prepared by GNP (with and without activation); a commercial catalyst;²⁴ CuCrO₂ bulk powder,²⁶ CuCrO₂, CuFeO_2, and CuFeO₂–CeO₂ nanopowders prepared by GNP.^8,14 The results show that the catalytic performance in hydrogen production reported in this study was higher than those of the aforementioned catalysts. Furthermore, the stability and the cycle-life were studied. Furthermore, the stability and the cycle-life were studied. In addition, to investigate the reusability of the ZnO–ZnCr₂O₄ composite catalyst, Fig. 11 was shown ZnO–ZnCr₂O₄ composite catalyst activity of changes with repeated cycle test during the SRM process which reacted at 450 °C with an N₂ flow rate of 30 sccm. ZnO–ZnCr₂O₄ composite after 3 times SRM treatment, the H₂ production rates of the ZnO–ZnCr₂O₄ composite G/N-1.7 decreased by about 90%.

Fig. 10 The H₂ production rates in SRM of the ZnO–ZnCr₂O₄ and ZnCr₂O₄ powders prepared by GNP with and without the activation inflow rate of 30 sccm, and the H₂ production rates of a commercial catalyst,²⁴ CuCrO₂ bulk powder,²⁶ and the CuCrO₂, CuFeO_2, and CuFeO₂–CeO₂ nanopowders prepared by GNP.^8,14

Fig. 11 The cycle test study of ZnO–ZnCr₂O₄ catalyst (G/N-1.7) during SRM at 450 °C with a N₂ flow rate of 30 sccm.

Based on the characteristics of the ZnO–ZnCr₂O₄ composite catalyst and ZnCr₂O₄ catalyst after SRM treatment, the ZnO–ZnCr₂O₄ and ZnCr₂O₄ catalysts prepared by the GNP method showed a reasonable degree of stability and exhibited better catalytic efficiency than those of the previous copper-based catalyst. The appropriate incorporation of ZnO in the ZnO–ZnCr₂O₄ catalyst increased the active sites and contributed to the hydrogen production function.

4. Conclusions

A zinc-based catalyst, namely, ZnO–ZnCr₂O₄ porous composite was prepared by GNP with G/N ratios of 1.7 and 1.5. The catalysts prepared in this study were applied to hydrogen production by SRM. The ZnO–ZnCr₂O₄ porous composite catalysts, before activation pretreatment, exhibited superior catalytic performance in the SRM process as compared with the previous copper-based catalyst and a commercial catalyst. Without activation pretreatment, the ZnO–ZnCr₂O₄ porous composite catalyst made with a G/N ratio of 1.7 revealed the best hydrogen production rate, 4814 ml STP min⁻¹ g-cat⁻¹, at a reaction temperature of 450 °C and N₂ flow rate of 30 sccm. After activation, the ZnO–ZnCr₂O₄ porous composite catalyst exhibited hydrogen production of 6299 ml STP min⁻¹ g-cat⁻¹ at 500 °C and a N₂ flow rate of 30 sccm. Furthermore, to investigate the characteristics of the catalyst prepared by GNP and the catalyst after the SRM process, the ZnO–ZnCr₂O₄ porous composite catalysts were analyzed by XRD, FE-SEM, and BET. The catalytic performance was determined by GC-TCD. Due to the GNP process, the catalyst had a porous structure, which effectively increased the specific surface area and thus the catalytic activity. The ZnO–ZnCr₂O₄ porous composite catalyst features a simple process with good performance for hydrogen production from SRM. The advantages of the ZnO–ZnCr₂O₄ composite catalyst make it a potential material for fuel cells in automobiles.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the Ministry of Science and Technology of Taiwan (MOST 108-2221-E-027-056, MOST 109-2221-E-027-068, MOST 109-2221-E-027-059, and MOST 109-2113-M-027-001-MY3). This work was supported by the National Science and Technology Council of Taiwan (NSTC 111-2221-E-027-104). This work was supported by University System of Taipei Joint Research Program (USTP-NTUT-TMU-111-01). The authors appreciate the Precision Research and Analysis Centre of National Taipei University of Technology (NTUT) for providing the measurement facilities.

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