Next Article in Journal
A Comprehensive Overview on Biochar-Based Materials for Catalytic Applications
Next Article in Special Issue
Lanthanide Oxides in Ammonia Synthesis Catalysts: A Comprehensive Review
Previous Article in Journal
Enhancing the Production of Syngas from Spent Green Tea Waste through Dual-Stage Pyrolysis and Catalytic Cracking
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:

Hydrogen Production via Methanol Steam Reforming over CuO/ZnO/Al2O3 Catalysts Prepared via Oxalate-Precursor Synthesis

School of Mechanical-Electronic and Vehicle Engineering, Beijing University of Civil Engineering and Architecture, Beijing 100044, China
Author to whom correspondence should be addressed.
Catalysts 2023, 13(10), 1335;
Submission received: 4 September 2023 / Revised: 25 September 2023 / Accepted: 28 September 2023 / Published: 30 September 2023


CuO/ZnO/Al2O3 catalysts are commonly used for the methanol steam reforming reaction. The oxalate precursor of CuO/ZnO/Al2O3 catalysts were prepared via the co-precipitation method using oxalic acid as the precipitator, deionized water and ethanol as the solvent, and microwave radiation and water baths as aging heating methods, respectively. This suggests that ethanol selects the crystalline phase composition of oxalate precursors and limits their growth. Microwave irradiation prompted the isomorphous substitution between Cu2+ of CuC2O4 and Zn2+ of ZnC2O4 in the mother liquid; Zn2+ in ZnC2O4·xH2O was substituted with Cu2+ in CuC2O4, forming the master phase (Cu,Zn)C2O4 in the precursor. Moreover, the solid solution Cu-O-Zn formed after calcination, which exhibited nano-fibriform morphology. It has the characteristics of small CuO grains, a large surface area, and strong synergistic effects between CuO and ZnO, which is conducive to improving the catalytic performance of methanol steam reforming. The conversion rate of methanol reached 91.2%, the space time yield of H2 reached 516.7 mL·g−1·h−1, and the selectivity of CO was only 0.29%.

Graphical Abstract

1. Introduction

With exponential growth in global energy consumption and disorderly emissions of greenhouse gases, the production of sustainable and ecofriendly renewable energy is highly urgent [1]. Hydrogen energy is called “the ultimate energy of the 21st century”, and hydrogen energy has a huge space for development. Among various hydrogen production methods, hydrogen production via methanol is increasingly favored by researchers, and methanol is easy to store and transport as a raw material [2]. Currently, there are four main ways to produce hydrogen from methanol: methanol decomposition, (MD, Equation (1)), partial oxidative reforming of methanol (POM, Equation (2)) methanol steam reforming (MSR, Equation (3)), and oxidized methanol steam reforming (OSRM, Equation (4)), respectively [3].
CH3OH → 2H2 + CO ΔH = +128.0 kJ·mol−1
CH3OH + 0.5O2 → 2H2 + CO2 ΔH = −192.2 kJ·mol−1
CH3OH + H2O → 3H2 + CO2 ΔH = +49.4 kJ·mol−1
CH3OH + (1-n)H2O + 0.5nO2 → (3-n)H2 + CO2 ΔH = +49(1-n) − 192.2n kJ·mol−1
Among the four methods of hydrogen production, MSR is the most widely and deeply studied method. MSR produces the highest amount of hydrogen per mole of methanol, and has the advantages of high hydrogen purity and low CO content [4]. Moreover, MSR technology has a low reaction temperature, low energy consumption, and low investment [5].
Therefore, various transition metal oxides are widely used in MSR. There are two types of catalysts used in MSR for hydrogen production: one type is precious metal catalysts (such as Pd/ZnO, etc.); the other type is non-precious metal catalysts, including non-copper-based catalysts (such as Zn-Cr, etc.) and copper-based catalysts (such as CuO/ZnO/Al2O3, etc.). CuO/ZnO, as a promising catalytic material, is widely used and reported on due to its low cost, abundant availability, easy synthesis, low toxicity, and chemical stability [6,7,8].
Cu-based catalysts can produce H2 with high selectively at low temperatures and have low CO selectivity; as a result, Cu-based catalysts are widely used. The Cu-based catalysts for MSR have been widely studied, and it was found that the effects of the synergistic effect between CuO and ZnO and the surface structure of CuO/ZnO/Al2O3 catalysts on the catalytic activities are critical [9,10].
Co-precipitation is a common method for preparing CuO/ZnO/Al2O3 catalysts, and the precipitation process has a profound influence on the structure and properties of the prepared catalysts. Inui et al. [11] studied the effects of pH and temperature on catalyst precursors in the precipitation process. It indicated that the formation of Cu2(NO3)(OH)3 is advantageous when pH ≤ 6, whereas (Cu,Zn)2CO3(OH)2 is dominant when pH ≥ 7. The effect of temperature on the precursor is mainly to change the reaction rate; it has almost no effect on its phase composition. Spencer et al. [12] studied the phase transition process in the mother liquid; it was indicated that amorphous Cu2CO3(OH)2 was first generated, which gradually transformed into (Cu,Zn)2CO3(OH)2 during the aging process. Fang et al. [13] studied the effects of different feeding methods. Cu2(NO3)(OH)3 mainly formed as a result of the forward addition method, whereas amorphous Cu2CO3(OH)2 mainly formed as a result of the concurrent flow method, which interacts with Zn5(CO3)2(OH)6 and transforms into (Cu,Zn)2CO3(OH)2 and (Cu,Zn)5(CO3)2(OH)6, respectively. A CuO-ZnO solid solution formed after decomposition, which is the active phase of the MSR reaction.
The solvents and heating methods are the main factors in the precipitation process. Ma et al. [14] prepared a CuO/ZnO/Al2O3 catalyst using ethanol and diethylene glycol as solvents, which possessed a larger superficial area and exhibited higher catalytic performance. Zhang et al. [15,16] synthesized the smaller particle CuO/ZnO/Al2O3 catalyst via oxalate co-precipitation using ethanol as a solvent; the catalyst showed better catalytic performance for MSR. Dai et al. [17] investigated the surface property of CuO/ZnO/Al2O3 catalysts prepared via oxalate co-precipitation, and explained that isomorphous substitution promoted synergistic effects between CuO and ZnO and increased the superficial content of CuO.
Microwave irradiation heating was rapid, and even in the preparation of catalysts, the active components were well-distributed on the support. Moreover, microwave irradiation could control the micro structure of materials and enhance the selectivity of the target product [18]. It is reported that microwave irradiation had obvious effects on the preparation of ZnO and Al2O3 nanoparticles [19]. Zhang et al. [20] treated CuO/ZnO/Al2O3 catalysts with microwave irritation (200 W) for 3–10 min; the results showed that the catalyst micro structure was significantly improved, and the catalytic activity of MSR increased by 7%. Fernández et al. [21] synthesized CuO/ZnO precursor mainly containing aurichalcite and CuO/ZnO/Al2O3 precursor only containing hydrotalcite, respectively, under microwave irradiation. The aurichalcite was burned into Cu-O-Zn solid solution, which exhibited strong synergistic effects and excellent activity and stability in MSR reactions.
At present, researchers generally believe that the strong synergy between Cu and Zn is conducive to hydrogen production in MSR, providing a lot of direct evidence for the evolution of the structure, morphology, and coordination state of Cu-O-Zn solid solutions, and laying a foundation for the identification of active sites in the catalytic process and the study of interface effects [22,23,24].
In present work, the synergistic effect between CuO and ZnO was further enhanced using oxalate precursor instead of carbonate precursor, ethanol instead of water as the solvent, and a microwave instead of the conventional heating method. The effects of solvent and heating method on the composition of oxalate precursor, the structure and properties of calcined catalyst, and the final MSR reaction performance were studied from atomic scale to nano scale.

2. Results and Discussion

2.1. XRD Characterization of Precursors

Figure 1 shows the XRD patterns of all catalyst precursors. It is seen that the phases of CuC2O4·xH2O (2θ = 18.5°, 23.1°, 31.2°, 36°, 38.5°, JCPDS card No. 48-1054), α-ZnC2O4·2H2O (2θ = 18.9°, 35.1°, JCPDS card No. 25-1029), and β-ZnC2O4 (2θ = 24°, 25.1°, 29.1°, 34.1°, 36.9°, JCPDS card No. 37-0718) were observed in the precursor of WWP prepared via water solvent and water bath heating. However, only diffraction peaks of CuC2O4·xH2O (2θ = 23.1°, 36°, 38.5°, JCPDS card No. 48-1054) and weak peaks of β-ZnC2O4 (2θ = 24°, 36.9°, JCPDS card No. 37-0718) existed in the precursor of EWP prepared using ethanol solvent and water bath heating; no α-ZnC2O4·2H2O phase was observed. It indicated that ethanol solvent restrained the formation of α-ZnC2O4·2H2O, and enhanced the phase selectivity of the product. Microwave irradiation promoted the isomorphous substitution between Cu2+ of CuC2O4·xH2O and Zn2+ of ZnC2O4 in mother liquid, thus, WMP and EMP mainly contained (Cu,Zn)C2O4 and partial CuC2O4 failing to be substituted during aging; thus, the diffraction peaks of CuC2O4·xH2O (2θ = 23.1°, 36°, JCPDS card No. 48-1054) and β-ZnC2O4 (2θ = 24°, 36.9°, JCPDS card No. 37-0718) overlapped each other and deviated from their original positions. Compared with WWP and EWP, WMP and EMP did not contain diffraction peaks of α-ZnC2O4·2H2O and β-ZnC2O4, and the crystal degree failed to be detected due to isomorphous substitution; this indicated that microwave irradiation has strong selectivity on the formation of the crystal phase [25,26].
CuC2O4·xH2O, α-ZnC2O4·2H2O, and β-ZnC2O4 generated in the mother liquid in co-precipitation, as shown in Equations (5)–(7). The isomorphous substitution mainly occurred in the aging process, it means that the Cu2+ of CuC2O4 entered into ZnC2O4 and the Zn2+ of ZnC2O4 entered into CuC2O4-formed (Cu,Zn)C2O4, as shown in Equations (8) and (9). However, the concentration of Cu2+ was higher than that of Zn2+; it primarily produced CuC2O4 as Equation (5) in the mother liquid, and the amount of ZnC2O4 was small. In other words, Reactions (8) and (9) were promoted simultaneously via microwave irradiation, whereas CuC2O4 was not substituted completely.
Cu2+ + C2O42− + xH2O → CuC2O4·xH2O
Zn2+ + C2O42− + 2H2O → α-ZnC2O4·2H2O
Zn2+ + C2O42− → β-ZnC2O4
CuC2O4 + xZn2+ → (Cu1−x,Znx)C2O4
ZnC2O4 + xCu2+ → (Cux,Zn1−x)C2O4

2.2. DTG Characterization of Precursors

DTG curves of different catalyst precursors are shown in Figure 2. Three weight loss peaks for WWP were observed at 118 °C, 308 °C, and 343 °C, respectively, as shown in Equations (10)–(13); the peak at 118 °C was due to the desorption of physically absorbed water of α-ZnC2O4·2H2O, whereas the peak around 308 °C was ascribed to the decomposition of CuC2O4·xH2O or (Cu,Zn)C2O4, and the peak at about 343 °C was attributed to decomposition of β-ZnC2O4 and further decomposition of α-ZnC2O4 [15]. The weight loss peak for CuC2O4·xH2O at 307 °C and weak peak for β-ZnC2O4 at 343 °C were observed from curve of EWP, which verified the XRD analysis of its precursor.
α-ZnC2O4·2H2O → α-ZnC2O4 + 2H2O  t ≈ 118 °C
CuC2O4·xH2O → CuO + CO2 + H2O t ≈ 308 °C
α-ZnC2O4 → ZnO + CO2  t ≈ 343 °C
β-ZnC2O4 → ZnO + CO2  t ≈ 343 °C
(Cu,Zn)C2O4 → (Cu,Zn)O + CO2  t ≈ 296 °C
WMP and EMP prepared under microwave irradiation only included the weight loss peak for (Cu,Zn)C2O4 or CuC2O4·xH2O at about 300 °C; it can be speculated that microwave irradiation accelerated the isomorphous substitution between Cu2+ and Zn2+; the lesser Zn2+ was incorporated into CuC2O4, so there is little content of α-ZnC2O4·2H2O or β-ZnC2O4, and there were no corresponding decomposition peaks. In particular, EMP had a large quantity of substitution and produced more (Cu,Zn)C2O4; the crystal phase trended to uniformity. Therefore, the decomposition peak of EMP became narrower and the decomposition temperature decreased by 12 °C [25]; the peak was mainly ascribed to (Cu,Zn)C2O4, and partial CuC2O4 was unable to be substituted as Equations (11) and (14).

2.3. SEM Images of Precursors and Catalysts

SEM images of different precursors are shown in Figure 3, and SEM images of different catalysts after calcining the corresponding precursors are shown in Figure 4. The appearance of catalysts kept coherence with precursors to some extent by comparing Figure 3 and Figure 4. WWP is a spherical particle with a diameter of about 400 nm. After calcination, it appears as spherical particles and gathers together, and its particle size is reduced to about 300 nm. EWC consists of irregular bars and blocks with a size of 400 nm, indicating that ethanol solvent has an effect on the forming process of precursors in the mother liquor. Compared with EWP, EWC has not changed much in appearance and size; its size is about 300–400 nm, but there are many tiny porous channels in its surface structure.
After microwave irradiation, the morphology of WMP can be observed to be an irregular single wafer, the size of which is between 300 and 500 nm. WMC is an acicular flake, not completely decomposed, with a cross-section diameter of 50–100 nm. When ethanol was further selected as the solvent, EMP mainly contained unidirectional ordered fiber nanoparticles with a cross-section diameter of about 50nm, which were fine, fibrous, or flocculent, and dispersed uniformly after calcination [27].
After combining the SEM photos of the precursor and catalyst, it can be concluded that the precursor forms towards the lower size under the action of bulk heating via microwave. The precursor prepared via water bath was spherical and massive, whereas the precursor prepared via microwave irradiation was single-fiber, and its particle size was smaller than the former. The particle size of EMC is only 50nm, with good dispersion and uniform distribution of surface active sites, which is conducive to improving the catalytic performance of MSR.

2.4. XRD Characterization of Catalysts

XRD patterns of different catalysts can be seen in Figure 5. The average grain size calculated via the Scherrer formula at 2θ ≈ 35.5°, 38.7° and the texture parameters of different catalysts are listed in Table 1. There were no peaks assigned to Al2O3 in the four patterns, indicating that Al2O3 existed as amorphous or that the content of Al2O3 was low. Diffraction peaks of CuO appeared in all the four catalysts at 2θ of 38.7°; CuO peaks of WWC were very sharp, and the grain size of CuO was comparatively large, up to 18.8 nm as shown in Table 1. The CuO peaks of EWC became smoother when using ethanol as a solvent, in which the ZnO peaks at 2θ = 34.4°, and 36.3° overlapped with the CuO peak at 2θ = 35.5°. The CuO grain size of EWC reduced to 12 nm, which indicated that viscous ethanol restricted the growth of the precursor in the mother liquid, resulting in a decrease in CuO grain size after calcination.
From patterns of WMC and EMC in Figure 5, it can be observed that the CuO peak becomes smoother, after introducing microwaves. Weak peaks of ZnO could still be seen in WMC; however, no peaks assigned to ZnO were detected in EMC. It indicated that isomorphous substitution took place in mother liquid when microwave irradiation was introduced, partial Cu2+ of CuC2O4 was incorporated into ZnC2O4 and almost all Zn2+ of ZnC2O4 was incorporated into CuC2O4, thereby, the original crystal structure were destroyed and the crystal degree declined after substitution. Cu-O-Zn solid solutions were obtained after calcining WMP and EMP, in which CuO and ZnO coordinate and interact closely with each other in the catalysts. This is because microwave irradiation promotes the substitution of Zn2+ in ZnC2O4·xH2O compound with Cu2+, and increases the content of the (Cu,Zn)C2O4 phase in precursors during the catalyst preparation process. In the calcined catalysts, Cu and Zn atoms are in close contact, dispersed, and perfectly connected, which, accordingly, enhances the synergistic effect of Cu/ZnO/Al2O3 catalysts. The CuO crystal was surrounded by the ZnO crystal, which restricted the growth of CuO crystallites; thus, the grain sizes of CuO in WMC and EMC were merely 8.9 nm and 8.4 nm, respectively [26].
As shown in Table 1, catalysts prepared under microwave irradiation possessed larger surface area and bigger pore volume than that prepared in the water bath. The surface area and pore volume of EMC were 77.2 m2/g and 0.36 cm3/g, respectively. This is because the (Cu,Zn)C2O4 content was higher in precursors prepared under microwave irradiation; the precursor were converted into (Cu,Zn)O (or Cu-O-Zn) solid solution after calcination, in which ZnO and CuO arrayed homogeneously and closely, restricting the agglomeration of CuO. Therefore, the catalysts were very fine and possessed a large surface area. The mother liquid was sol-like after aging when ethanol was selected as a solvent, and the catalyst possessed a large surface area after calcining the corresponding precursor [28]; therefore, the pore volume of EWC and EMC was comparatively large, as shown in Table 1.

2.5. H2-TPR Characterization of Catalysts

Figure 6 shows H2-TPR curves of different catalysts. Ethanol restricted the growth of the phase in the precursor and favored the formation of fine catalysts, which made the reduction process easy. It manifested that the reduction temperature of catalysts prepared using ethanol as a solvent was lower than that using water. The catalysts prepared via water bath heating were difficult to reduce; the reduction temperature of WWC and EWC was approximately 240 °C, whereas the reduction temperature of catalysts prepared via microwave irradiation was lower (around 220 °C). It proved again that microwave irradiation selected the phase in the generation process of the precursor; the catalyst was arranged orderly in the micro structure, which decreased the difficulty of reducing CuO with H2.
As shown in Figure 6, one low temperature reduction peak and one high emerged in curves of WWC, EWC, and WMC; however, three peaks emerged in the curve of EMC. The first two are low temperature reduction peaks, the third is high temperature reduction peak. Low temperature reduction peaks were assigned to the reduction in scattered phase CuO (Peak I), whereas the other was assigned to the reduction in bulk phase CuO (Peak II). The amount of isomorphous substitution was less when aging mother liquids heated via water bath; thus, its synergistic effect between CuO and ZnO was weak, and bulk phase CuO was mainly found in the corresponding catalyst; thus, Peak II was larger. The amount of substitution between Cu2+ and Zn2+ was great when aging under microwave irradiation, they mainly generated (Cu,Zn)C2O4 in precursors which were calcined into Cu-O-Zn solid solutions. CuO and ZnO coordinate and interact closely with each other in the catalysts; Cu and Zn atoms are in close contact, dispersed and perfectly connected, which accordingly enhances the synergistic effect of Cu/ZnO/Al2O3 catalysts. The catalysts were homogeneous and dispersive, and the synergistic effect was strong; thus, Peak I was larger. In particular, the amount of substitution for EMC was great, and the reduction peak assigned to high temperature was much smaller than others [29,30].

2.6. XPS and AES Characterization of Catalysts

XPS spectra of catalysts are shown in Figure 7. As shown in Figure 7a, all catalysts showed a distinctive BE (binding energy) of Cu 2p3/2 around 932–933 eV, accompanying a characteristic satellite peak between 940 and 945 eV due to the electron shakeup process, which indicated that Cu species were present as CuO. The BE of Cu 2p3/2 around 932–933 eV in the spectra of WWC, EWC, WMC, and EMC were 932.0 eV, 932.05 eV, 932.10 eV, and 932.95 eV, respectively, increasing gradually. The BE of Zn2p3/2 (Figure 7b) in the corresponding spectra were 1021.45 eV, 1020.6 eV, 1020.35 eV, and 1020.15 eV, decreasing gradually, which were lower than the BE (1022.2 eV) of pure ZnO [31].
The results show that homogeneous substitution occurs when microwave is introduced in the aging process. The chemical environment and energy state of copper and zinc in Cu-O-Zn solid solution were changed. Since the electronegativity of zinc is higher than that of copper, the outermost electrons of copper move towards zinc, and the electron density of copper decreased and the binding energy increased, while the electron density of zinc increased and the binding energy decreased [25].
Table 2 lists the data analysis based on XPS and AES spectra and the superficial elements concentration of different catalysts. EMC possessed higher superficial copper content and lower superficial zinc content, its ratio of XCu/XZn was up to 8.98, which exceeded all others, it can be verified by the intensity of spectra in Figure 7 and Figure 8. In Table 2, it also could be observed that ethanol solvent help to improve the content of superficial copper atoms. Moreover, microwave irradiation raised the yield of (Cu,Zn)C2O4 in precursor, leading to lower KE (kinetic energy) of Cu LMM and higher KE of Zn LMM in final catalyst as shown in Figure 8. This was coincident with the results reported in the literature [17].

2.7. Catalytic Performance Test for MSR Reaction

The catalytic performance of different catalysts for MSR is listed in Table 3. When comparing WWC and EWC, it can be concluded that ethanol solvent is conducive to reducing the CO selectivity of the catalyst. As can be seen from the XRD data, ethanol optimized the precursor crystal phase composition, thereby reducing the CO selectivity. When comparing WWC and WMC, it can be concluded that microwave heating can improve the methanol conversion of the catalyst. It can be seen from the BET data and SEM images that microwave radiation increases the specific surface area of the catalyst, improves the dispersion of copper, and increases more active sites, thus increasing the methanol conversion rate. It can be seen from the XRD data that microwave radiation promotes equimolar substitution between copper and zinc oxalate precursors; moreover, Cu-O-Zn solid solution formed after calcination, which possessed strong synergistic effects and benefited the MSR reaction.
In the aging process of catalyst precipitation mother liquid, microwave irradiation can promote the generation of the (Cu,Zn)C2O4 phase, and the synergistic effect between CuO and ZnO after the phase roasting is stronger; moreover, the CuO and ZnO grains are smaller, and the surface Cu content is higher, so the active sites are more uniform. The chemical environment and energy state of Cu and Zn components in the catalyst have changed. As the electronegativity of Zn is higher than that of Cu, the outermost electrons of Cu shift to Zn, thus reducing the electron cloud density of Cu and increasing the electron-binding energy, while the electron cloud density of Zn increases and decreasing the electron-binding energy. Thus, it illustrated that both ethanol and microwave irradiation benefited to improve the catalytic performance of MSR, the catalyst EMC prepared via ethanol solvents and microwave heating exhibited optimal catalytic performance; the conversion of methanol was 91.2%, the space time yield (STY) of H2 reached 516.7 mL·g−1·h−1, and its selectivity of CO was only 0.29%. In stark contrast, the catalyst WWC prepared via water solvent and water bath heating showed the worst catalytic performance. Its conversion of methanol was only 53.6%, and the STY of H2 was 300.0 mL·g−1·h−1; however, its selectivity of CO was as high as 1.53%.

3. Experimental

3.1. Catalyst Preparation

The Cu-O-Zn/Al2O3 catalyst precursors were prepared by dropping 1 mol/L Cu(NO3)2-Zn(NO3)2-Al(NO3)3 (Cu2+/Zn2+/Al3+ = 16/8/1 (molar ratio)) solution and 1 mol/L H2C2O4 solution simultaneously into a beaker while stirring constantly and keeping them in a water bath at 70 °C. Then, the suspension was aged in a microwave oven or a water bath with circulating cooling equipment after co-precipitation; the aging process was conducted at 80 °C for 1 h. The precipitate was filtered and washed with distilled water or ethanol, then the precursor was obtained after drying at 110 °C for 12 h, and the catalyst was obtained after calcining the corresponding precursor at 350 °C for 4 h in the muffle furnace. The precursor was designated as XYP, and the catalyst was designated as XYC; X is W for water, or E for ethanol; Y is W for water bath, or M for microwave irradiation; P is for precursor; C is for calcined catalyst). Table 4 shows the summary of the preparation conditions of the catalysts.

3.2. Catalyst Characterization

X-ray diffraction (XRD) patterns of solid samples were recorded using a Rigaku D/max 2500 power diffractometer with Cu Kα radiation at 40 kV and 100 mA with a scanning rate of 8°/min in the 2θ ranges from 10° to 40°.
Temperature-programmed reduction (H2-TPR) was performed in an Autochem II 2920. About 20 mg catalyst sample was set in a U-mode quartz tube, pretreated in helium at 50 °C for 30 min, then heated to 300 °C at a rate of 10 °C /min, under a mixture of 10 vol% H2/Ar (50 mL/min), the sample was then heated to 600 °C at a rate of 10 °C /min, The consumption of hydrogen was monitored using a thermal conductivity detector.
Differential thermal gravity (DTG) measurement was executed in a STA409C thermal analyzer. 30 mg sample was heated to 600 °C at a rate of 8 °C/min in a gas mixture of 20 vol% O2/N2 (50 mL/min).
JSM-6700F cold-field scanning electron microscope (SEM) was used to characterize the size and morphology of the samples.
BET specific surface areas and pore distribution of catalyst was measured with a SORPTMATIC 1990 automatic adsorption instrument employing N2 as the adsorbent. BET specific surface areas were calculated by applying the Brunauer–Emmett–Teller (BET) method.
X-ray photoelectron spectroscopy (XPS) spectra of the samples were collected on an ESCAL-ab 220i-XL electron spectrometer using Al Kα radiation at 300 W. The samples were compressed into the pellets of 2 mm thickness and then mounted on a sample holder. The chamber was maintained at a pressure lower than 10−10 Torr. The binding energies were calibrated using C1s as the reference energy (C1s = 284.6 eV).

3.3. Catalytic Performance of MSR

The performance evaluation of MSR was conducted on a continuous flow fixed bed device with a catalyst loading of 2 g, using industrial refined methanol as raw material. Evaluation conditions: raw material methanol aqueous solution (the molar ratio of water to methanol is 1.5), reaction temperature 260 °C, pressure 0.5 MPa, WHSV = 1.0 h−1. After cooling, gas and liquid samples were analyzed using chromatographs equipped with PorpakT columns and TDX-01 columns, respectively, and thermal conductivity cell detectors. The methanol conversion rate and product distribution were calculated.

4. Conclusions

In the process of preparing CuO/ZnO/Al2O3 catalyst precursors, ethanol solvent selected the crystal phase composition in precursor and restricted its growth. Microwave irradiation promoted the homogeneous substitution of Cu2+ of CuC2O4 and Zn2+ of ZnC2O4 in the mother liquid, Zn2+ in ZnC2O4·xH2O was replaced with Cu2+ in CuC2O4, and the main phase (Cu, Zn) C2O4 was formed in the precursor; the solid solution Cu-O-Zn was formed after calcination, which showed a nanofiber shape. It has the characteristics of small CuO grains, large surface area, and a strong synergistic effect between CuO and ZnO, which is helpful for improving the catalytic performance of methanol steam reforming. The methanol conversion rate reached 91.2%, the H2 spacetime yield reached 516.7mL·g−1·h−1, while the CO selectivity was only 0.29%.

Author Contributions

Conceptualization, H.W. and Y.L.; methodology, J.Z.; investigation, J.Z. and Y.L.; resources, J.Z.; data curation, H.W. and Y.L.; writing—original draft preparation, H.W.; writing—review and editing, H.W. and Y.L. supervision, Y.L. and J.Z.; funding acquisition, Y.L. All authors have read and agreed to the published version of the manuscript.


Project Supported by National Natural Science Foundation of China (51976007).

Data Availability Statement

Data available on request from the authors.


This study was supported by Open Fund Project of State Key Laboratory of Automotive Safety and Energy (KFY2218).

Conflicts of Interest

The authors declare no conflict of interest.


  1. Poudel, M.B.; Kim, H.J. Confinement of Zn-Mg-Al-layered double hydroxide and α-Fe2O3 nanorods on hollow porous carbon nanofibers: A free-standing electrode for solid-state symmetric supercapacitors. Chem. Eng. J. 2022, 429, 132345. [Google Scholar] [CrossRef]
  2. Kawamura, Y.; Ogura, N.; Igarashi, A. Hydrogen production by methanol steam reforming using microreactor. J. Jpn. Petrol. Inst. 2013, 56, 288–297. [Google Scholar] [CrossRef]
  3. Yong, S.T.; Ooi, C.W.; Chai, S.P.; Wu, X.S. Review of methanol reforming-Cu-based catalysts, surface reaction mechanisms, and reaction schemes. Int. J. Hydrogen Energy 2013, 38, 9541–9552. [Google Scholar] [CrossRef]
  4. Xu, X.; Shuai, K.; Xu, B. Review on copper and palladium based catalysts for methanol steam reforming to produce hydrogen. Catalysts 2017, 7, 183. [Google Scholar] [CrossRef]
  5. Iulianelli, A.; Ribeirinha, P.; Mendes, A.; Basile, A. Methanol steam reforming for hydrogen generation via conventional and membrane reactors, a review. Renew. Sustain. Energy Rev. 2014, 29, 355–368. [Google Scholar] [CrossRef]
  6. Poudel, M.B.; Lohani, P.C.; Acharya, D.; Kandel, D.; Kim, A.A.; Yoo, D.J. MOF derived hierarchical ZnNiCo-LDH on vapor solid phase grown CuxO nanowire array as high energy density asymmetric supercapacitors. J. Energy Storage 2023, 72, 108220. [Google Scholar] [CrossRef]
  7. Iwasa, N.; Masuda, S.; Ogawa, N.; Takezawa, N. Steam reforming of methanol over Pd/ZnO, effect of the formation of PdZn alloys upon the reaction. Appl. Catal. A 1995, 125, 145–157. [Google Scholar] [CrossRef]
  8. Shokrani, R.; Haghighi, M.; Jodeiri, N.; Ajamein, H.; Abdollahifar, M. Fuel cell grade hydrogen production via methanol steam reforming over CuO/ZnO/Al2O3 nanocatalyst with various oxide ratios synthesized via urea-nitrates combustion method. Int. J. Hydrogen Energy 2014, 39, 13141–13155. [Google Scholar] [CrossRef]
  9. Shishido, T.; Yamamoto, Y.; Morioka, H.; Takehira, K. Production of hydrogen from methanol over Cu/ZnO and Cu/ZnO/Al2O3 catalysts prepared by homogeneous precipitation: Steam reforming and oxidative steam reforming. J. Mol. Catal. A 2007, 268, 185–194. [Google Scholar] [CrossRef]
  10. Agrell, J.; Birgersson, H.; Boutonnet, M.; Melián-Cabrera, I.; Navarro, R.M.; Fierro, J.L.G. Production of hydrogen from methanol over Cu/ZnO catalysts promoted by ZrO2 and Al2O3. J. Catal. 2003, 219, 389–403. [Google Scholar] [CrossRef]
  11. Spencer, M.S. Precursors of copper/zinc oxide catalysts. Catal. Lett. 2000, 66, 255–257. [Google Scholar] [CrossRef]
  12. Li, J.; Inui, T. Characterization of precursors of methanol synthesis catalysts copper/zinc/aluminum oxides precipitated at different pH and temperatures. Appl. Catal. A 1996, 137, 105–117. [Google Scholar] [CrossRef]
  13. Fang, D.; Ren, W.; Liu, Z.; Xu, X.; Xu, L.; Lv, H.; Liao, W.; Zhang, H. Synthesis and applications of mesoporous Cu-Zn-Al2O3 catalyst for dehydrogenation of 2-butanol. J. Nat. Gas. Chem. 2009, 18, 179–182. [Google Scholar] [CrossRef]
  14. Ma, Y.; Sun, Q.; Wu, D.; Fan, W.; Zhang, Y.; Deng, J. A practical approach for the preparation of high activity Cu/ZnO/ZrO2 catalyst for methanol synthesis from CO2 hydrogenation. Appl. Catal. A 1998, 171, 45–55. [Google Scholar] [CrossRef]
  15. Zhang, Y.; Sun, Q.; Deng, J.; Wu, D.; Chen, S. A high activity Cu/ZnO/Al2O3 catalyst for methanol synthesis: Preparation and catalytic properties. Appl. Catal. A 1997, 158, 105–120. [Google Scholar] [CrossRef]
  16. Zhang, X.; Wang, L.; Yao, C.; Cao, Y.; Dai, W.; He, H.; Fan, K. A highly efficient Cu/ZnO/Al2O3 catalyst via gel-coprecipitation of oxalate precursors for low-temperature steam reforming of methanol. Catal. Lett. 2005, 102, 183–190. [Google Scholar] [CrossRef]
  17. Dai, W.; Sun, Q.; Deng, J.; Wu, D.; Sun, Y. XPS studies of Cu/ZnO/Al2O3 ultra-fine catalysts derived by a novel gel oxalate co-precipitation for methanol synthesis by CO2+H2. Appl. Surf. Sci. 2001, 177, 172–179. [Google Scholar] [CrossRef]
  18. Wu, Z.; Ge, S.; Zhang, M.; Li, W.; Tao, K. Synthesis of a supported nickel boride catalyst under microwave irradiation. Catal. Commun. 2008, 9, 1432–1438. [Google Scholar] [CrossRef]
  19. Sule, E.E.; Sadik, C.; Siddik, I. Conventional and microwave-assisted synthesis of ZnO nanorods and effects of PEG400 as a surfactant on the morphology. Inorg. Chim. Acta 2009, 362, 1855–1858. [Google Scholar]
  20. Zhang, X.; Wang, L.; Cao, Y.; Dai, W.; He, H.; Fan, K. A unique microwave effect on the microstructural modification of Cu/ZnO/Al2O3 catalysts for steam reforming of methanol. Chem. Commun. 2005, 102, 4104–4106. [Google Scholar] [CrossRef]
  21. Fernández, Y.; Menéndez, J.; Arenillas, A.; Fuente, E.; Peng, J.; Zhang, Z.; Li, W.; Zhang, Z. Microwave-assisted synthesis of CuO/ZnO and CuO/ZnO/Al2O3 precursors using urea hydrolysis. Solid State Ion. 2009, 180, 1372–1378. [Google Scholar] [CrossRef]
  22. Nakamura, J.; Choi, Y.; Fujitani, T. On the issue of the active site and the role of ZnO in Cu/ZnO methanol synthesis catalysts. Top. Catal. 2003, 22, 277–285. [Google Scholar] [CrossRef]
  23. Kasatkin, I.; Kurr, P.; Kniep, B.; Trunschke, A.; Schlögl, R. Role of lattice strain and defects in copper particles on the activity of Cu/ZnO/Al2O3 catalysts for methanol synthesis. Angew. Chem. Int. Ed. 2007, 46, 7324–7327. [Google Scholar] [CrossRef] [PubMed]
  24. Behrens, M.; Studt, F.; Kasatkin, I.; Kühl, S.; Hävecker, M.; Abild-Pedersen, F. The active site of methanol synthesis over Cu/ZnO/Al2O3 industrial catalysts. Science 2012, 336, 893–897. [Google Scholar] [CrossRef] [PubMed]
  25. Wang, L.; Liu, Y.; Chen, M.; Cao, Y.; He, H.; Wu, G.; Dai, W.; Fan, K. Production of hydrogen by steam reforming of methanol over Cu/ZnO catalysts prepared via a practical soft reactive grinding route based on dry oxalate-precursor synthesis. J. Catal. 2007, 246, 193–204. [Google Scholar] [CrossRef]
  26. Ning, W.; Shen, H.; Liu, H. Study of the effect of preparation method on CuO-ZnO-Al2O3 Catalyst. Appl. Catal. A 2001, 211, 153–157. [Google Scholar] [CrossRef]
  27. An, X.; Li, J.; Zuo, Y.; Zhang, Q.; Wang, D.; Wang, J. A Cu/Zn/Al/Zr fibrous catalyst that is an improved CO2 hydrogenation to methanol catalyst. Catal. Lett. 2007, 118, 264–269. [Google Scholar] [CrossRef]
  28. Bao, J.; Liu, Z.; Zhang, Y.; Tsubaki, N. Preparation of mesoporous Cu/ZnO catalyst and its application in low-temperature methanol synthesis. Catal. Commun. 2008, 9, 913–918. [Google Scholar] [CrossRef]
  29. Yang, R.; Yu, X.; Zhang, Y.; Li, W.; Tsubaki, N. A new method of low-temperature methanol synthesis on Cu/ZnO/Al2O3 catalysts from CO/CO2/H2. Fuel 2008, 87, 443–450. [Google Scholar] [CrossRef]
  30. Bae, J.; Kang, S.; Lee, Y.; Jun, K. Synthesis of DME from syngas on the bifunctional Cu–ZnO–Al2O3/Zr-modified ferrierite: Effect of Zr content. Appl. Catal. B 2009, 90, 426–435. [Google Scholar] [CrossRef]
  31. Baltes, C.; Vukojević, S.; Schüth, F. Correlations between synthesis; precursor; and catalyst structure and activity of a large set of CuO/ZnO/Al2O3 catalysts for methanol synthesis. J. Catal. 2008, 258, 334–344. [Google Scholar] [CrossRef]
Figure 1. XRD patterns of the catalyst precursors.
Figure 1. XRD patterns of the catalyst precursors.
Catalysts 13 01335 g001
Figure 2. DTG curves of catalyst precursors.
Figure 2. DTG curves of catalyst precursors.
Catalysts 13 01335 g002
Figure 3. SEM images of catalyst precursors. (a) WWP; (b) EWP; (c) WMP; (d) EMP.
Figure 3. SEM images of catalyst precursors. (a) WWP; (b) EWP; (c) WMP; (d) EMP.
Catalysts 13 01335 g003
Figure 4. SEM images of catalysts. (a) WWC; (b) EWC; (c) WMC; (d) EMC.
Figure 4. SEM images of catalysts. (a) WWC; (b) EWC; (c) WMC; (d) EMC.
Catalysts 13 01335 g004aCatalysts 13 01335 g004b
Figure 5. XRD patterns of catalysts.
Figure 5. XRD patterns of catalysts.
Catalysts 13 01335 g005
Figure 6. H2-TPR curves of catalysts.
Figure 6. H2-TPR curves of catalysts.
Catalysts 13 01335 g006
Figure 7. XPS spectra of catalysts. (a) Cu 2p1/2 and Cu 2p3/2; (b) Zn 2p3/2.
Figure 7. XPS spectra of catalysts. (a) Cu 2p1/2 and Cu 2p3/2; (b) Zn 2p3/2.
Catalysts 13 01335 g007
Figure 8. AES patterns of catalysts. (a) Cu LMM; (b) Zn LMM.
Figure 8. AES patterns of catalysts. (a) Cu LMM; (b) Zn LMM.
Catalysts 13 01335 g008
Table 1. Grain size values and textural properties of catalysts.
Table 1. Grain size values and textural properties of catalysts.
CatalystGrain Size/nmTextural Properties
2θ ≈ 35.5°2θ ≈ 38.7°Surface Area (m2/g)Pore Volume (cm3/g)
Table 2. XPS and AES data of different catalysts.
Table 2. XPS and AES data of different catalysts.
CatalystSurface Atom/%BE(Cu 2p3/2)
BE(Zn 2p3/2)
Table 3. Catalytic performance of different catalysts for MSR reaction.
Table 3. Catalytic performance of different catalysts for MSR reaction.
Manner a
/% b
a Evaluation conditions: the molar ratio of water to methanol is 1.5 in the methanol aqueous solution, T = 260 °C, P = 0.5 MPa, WHSV = 1.0 h−1. b WB represents water bath, MI represents microwave irradiation. SCO: CO selectivity.
Table 4. Summary of the preparation condition of catalysts.
Table 4. Summary of the preparation condition of catalysts.
CatalystPrecursorSolventHeating Manner
WWCWWPWaterWater bath (WB)
EWCEWPEthanolWater bath (WB)
WMCWMPWaterMicrowave irradiation (MI)
EMCEMPEthanolMicrowave irradiation (MI)
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Wang, H.; Liu, Y.; Zhang, J. Hydrogen Production via Methanol Steam Reforming over CuO/ZnO/Al2O3 Catalysts Prepared via Oxalate-Precursor Synthesis. Catalysts 2023, 13, 1335.

AMA Style

Wang H, Liu Y, Zhang J. Hydrogen Production via Methanol Steam Reforming over CuO/ZnO/Al2O3 Catalysts Prepared via Oxalate-Precursor Synthesis. Catalysts. 2023; 13(10):1335.

Chicago/Turabian Style

Wang, Haiguang, Yongfeng Liu, and Jun Zhang. 2023. "Hydrogen Production via Methanol Steam Reforming over CuO/ZnO/Al2O3 Catalysts Prepared via Oxalate-Precursor Synthesis" Catalysts 13, no. 10: 1335.

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop