Hydrogen Production via Methanol Steam Reforming over CuO/ZnO/Al 2 O 3 Catalysts Prepared via Oxalate-Precursor Synthesis

: CuO/ZnO/Al 2 O 3 catalysts are commonly used for the methanol steam reforming reaction. The oxalate precursor of CuO/ZnO/Al 2 O 3 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 Cu 2+ of CuC 2 O 4 and Zn 2+ of ZnC 2 O 4 in the mother liquid; Zn 2+ in ZnC 2 O 4 · xH 2 O was substituted with Cu 2+ in CuC 2 O 4 , forming the master phase (Cu,Zn)C 2 O 4 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 H 2 reached 516.7 mL · g − 1 · h − 1 , and the selectivity of CO was only 0.29%.


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].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-copperbased catalysts (such as Zn-Cr, etc.) and copper-based catalysts (such as CuO/ZnO/Al 2 O 3 , 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 H 2 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/Al 2 O 3 catalysts on the catalytic activities are critical [9,10].
Co-precipitation is a common method for preparing CuO/ZnO/Al 2 O 3 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 Cu 2 (NO 3 )(OH) 3 is advantageous when pH ≤ 6, whereas (Cu,Zn) 2 CO 3 (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 Cu 2 CO 3 (OH) 2 was first generated, which gradually transformed into (Cu,Zn) 2 CO 3 (OH) 2 during the aging process.Fang et al. [13] studied the effects of different feeding methods.Cu 2 (NO 3 )(OH) 3 mainly formed as a result of the forward addition method, whereas amorphous Cu 2 CO 3 (OH) 2 mainly formed as a result of the concurrent flow method, which interacts with Zn 5 (CO 3 ) 2 (OH) 6 and transforms into (Cu,Zn) 2 CO 3 (OH) 2 and (Cu,Zn) 5 (CO 3 ) 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/Al 2 O 3 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/Al 2 O 3 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/Al 2 O 3 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 Al 2 O 3 nanoparticles [19].Zhang et al. [20] treated CuO/ZnO/Al 2 O 3 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/Al 2 O 3 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., 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-ZnC 2 O 4 •2H 2 O and β-ZnC 2 O 4 , 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].
In present work, the synergistic effect between CuO and ZnO was further en using oxalate precursor instead of carbonate precursor, ethanol instead of wate solvent, and a microwave instead of the conventional heating method.The effect vent and heating method on the composition of oxalate precursor, the structure an erties of calcined catalyst, and the final MSR reaction performance were studie atomic scale to nano scale.8) and (9).However, the concentration of Cu 2+ was higher than that of Zn 2+ ; it primarily produced CuC 2 O 4 as Equation ( 5) in the mother liquid, and the amount of ZnC 2 O 4 was small.In other words, Reactions (8) and (9) were promoted simultaneously via microwave irradiation, whereas CuC 2 O 4 was not substituted completely.

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 Catalysts 2023, 13, x FOR PEER REVIEW 4 of 13 CuC2O4•xH2O, α-ZnC2O4•2H2O, and β-ZnC2O4 generated in the mother liquid in coprecipitation, as shown in Equations ( 5)-( 7).The isomorphous substitution mainly occurred in the aging process, it means that the Cu 2+ of CuC2O4 entered into ZnC2O4 and the Zn 2+ of ZnC2O4 entered into CuC2O4-formed (Cu,Zn)C2O4, as shown in Equations ( 8) and ( 9).However, the concentration of Cu 2+ was higher than that of Zn 2+ ; 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. (5)

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.WMP and EMP prepared under microwave irradiation only included the weight loss peak for (Cu,Zn)C 2 O 4 or CuC 2 O 4 •xH 2 O at about 300 • C; it can be speculated that microwave irradiation accelerated the isomorphous substitution between Cu 2+ and Zn 2+ ; the lesser Zn 2+ was incorporated into CuC 2 O 4 , so there is little content of α-ZnC 2 O 4 •2H 2 O or β-ZnC 2 O 4 , and there were no corresponding decomposition peaks.In particular, EMP had a large quantity of substitution and produced more (Cu,Zn)C 2 O 4 ; 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)C 2 O 4 , and partial CuC 2 O 4 was unable to be substituted as Equations ( 11) and ( 14).

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 Figures 3  and 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.
(Cu,Zn)C2O4 → (Cu,Zn)O + CO2 t ≈ 296 °C WMP and EMP prepared under microwave irradiation only included the weigh peak for (Cu,Zn)C2O4 or CuC2O4•xH2O at about 300 °C; it can be speculated microwave irradiation accelerated the isomorphous substitution between Cu 2+ and the lesser Zn 2+ was incorporated into CuC2O4, so there is little content of α-ZnC2O4• or β-ZnC2O4, and there were no corresponding decomposition peaks.In particular, had a large quantity of substitution and produced more (Cu,Zn)C2O4; the crystal trended to uniformity.Therefore, the decomposition peak of EMP became narrowe the decomposition temperature decreased by 12 °C [25]; the peak was mainly ascrib (Cu,Zn)C2O4, and partial CuC2O4 was unable to be substituted as Equations (11) and

SEM Images of Precursors and Catalysts
SEM images of different precursors are shown in Figure 3, and SEM imag different catalysts after calcining the corresponding precursors are shown in Figure appearance of catalysts kept coherence with precursors to some extent by comp Figures 3 and 4. WWP is a spherical particle with a diameter of about 400 nm.calcination, it appears as spherical particles and gathers together, and its particle s reduced to about 300 nm.EWC consists of irregular bars and blocks with a size of 40 indicating that ethanol solvent has an effect on the forming process of precursors mother liquor.Compared with EWP, EWC has not changed much in appearance and its size is about 300-400 nm, but there are many tiny porous channels in its su 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.After microwave irradiation, the morphology of WMP can be observed to irregular single wafer, the size of which is between 300 and 500nm.WMC is an ac flake, not completely decomposed, with a cross-section diameter of 50-100nm.ethanol was further selected as the solvent, EMP mainly contained unidirectional or fiber nanoparticles with a cross-section diameter of about 50nm, which were fine, fib or flocculent, and dispersed uniformly after calcination [27].
After combining the SEM photos of the precursor and catalyst, it can be conc that the precursor forms towards the lower size under the action of bulk heatin microwave.The precursor prepared via water bath was spherical and massive, wh the precursor prepared via microwave irradiation was single-fiber, and its particl was smaller than the former.The particle size of EMC is only 50nm, with good dispe and uniform distribution of surface active sites, which is conducive to improvin catalytic performance of MSR.

XRD Characterization of Catalysts
XRD patterns of different catalysts can be seen in Figure 5.The average grai calculated via the Scherrer formula at 2θ ≈ 35.5°, 38.7° and the texture paramet different catalysts are listed in Table 1.There were no peaks assigned to Al2O3 in th patterns, indicating that Al2O3 existed as amorphous or that the content of Al2O3 wa Diffraction peaks of CuO appeared in all the four catalysts at 2θ of 38.7°; CuO pe WWC were very sharp, and the grain size of CuO was comparatively large, up to 18 as shown in Table 1.The CuO peaks of EWC became smoother when using ethano solvent, in which the ZnO peaks at 2θ = 34.4°,and 36.3°overlapped with the CuO p 2θ = 35.5°.The CuO grain size of EWC reduced to 12 nm, which indicated that vi ethanol restricted the growth of the precursor in the mother liquid, resulting in a dec in CuO grain size after calcination.After microwave irradiation, the morphology of WMP can be observed t irregular single wafer, the size of which is between 300 and 500nm.WMC is an flake, not completely decomposed, with a cross-section diameter of 50-100nm ethanol was further selected as the solvent, EMP mainly contained unidirectional fiber nanoparticles with a cross-section diameter of about 50nm, which were fine, or flocculent, and dispersed uniformly after calcination [27].
After combining the SEM photos of the precursor and catalyst, it can be con that the precursor forms towards the lower size under the action of bulk hea microwave.The precursor prepared via water bath was spherical and massive, w the precursor prepared via microwave irradiation was single-fiber, and its parti was smaller than the former.The particle size of EMC is only 50nm, with good dis and uniform distribution of surface active sites, which is conducive to improv catalytic performance of MSR.

XRD Characterization of Catalysts
XRD patterns of different catalysts can be seen in Figure 5.The average gr calculated via the Scherrer formula at 2θ ≈ 35.5°, 38.7° and the texture param different catalysts are listed in Table 1.There were no peaks assigned to Al2O3 in patterns, indicating that Al2O3 existed as amorphous or that the content of Al2O3 w Diffraction peaks of CuO appeared in all the four catalysts at 2θ of 38.7°; CuO p WWC were very sharp, and the grain size of CuO was comparatively large, up to as shown in Table 1.The CuO peaks of EWC became smoother when using etha solvent, in which the ZnO peaks at 2θ = 34.4°,and 36.3°overlapped with the CuO 2θ = 35.5°.The CuO grain size of EWC reduced to 12 nm, which indicated that ethanol restricted the growth of the precursor in the mother liquid, resulting in a d in CuO grain size after calcination.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/Al 2 O 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 m 2 /g and 0.36 cm 3 /g, respectively.This is because the (Cu,Zn)C 2 O 4 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.

H 2 -TPR Characterization of Catalysts
Figure 6 shows H 2 -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 H 2 .
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 Cu 2+ and Zn 2+ was great when aging under microwave irradiation, they mainly generated (Cu,Zn)C 2 O 4 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/Al 2 O 3 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].As shown in Figure 6, one low temperature reduction peak and one high em curves of WWC, EWC, and WMC; however, three peaks emerged in the curve The first two are low temperature reduction peaks, the third is high temperature re peak.Low temperature reduction peaks were assigned to the reduction in scattere CuO (Peak I), whereas the other was assigned to the reduction in bulk phase Cu II).The amount of isomorphous substitution was less when aging mother liquid via water bath; thus, its synergistic effect between CuO and ZnO was weak, a phase CuO was mainly found in the corresponding catalyst; thus, Peak II was lar amount of substitution between Cu 2+ and Zn 2+ was great when aging under mic irradiation, they mainly generated (Cu,Zn)C2O4 in precursors which were calcin Cu-O-Zn solid solutions.CuO and ZnO coordinate and interact closely with each the catalysts; Cu and Zn atoms are in close contact, dispersed and perfectly con which accordingly enhances the synergistic effect of Cu/ZnO/Al2O3 catalysts.The c were homogeneous and dispersive, and the synergistic effect was strong; thus, Pe larger.In particular, the amount of substitution for EMC was great, and the re peak assigned to high temperature was much smaller than others [29,30].

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 2p 3/2 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 2p 3/2 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 Zn2p 3/2 (Figure 7b) in the corresponding spectra were 1021.45eV, 1020.6 eV, 1020.35eV, and 1020.15eV, decreasing gradually, which were lower than the BE (1022.2eV) of pure ZnO [31].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 Cu 2+ and Zn 2+ 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].

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 1021.45eV, 1020.6 eV, 1020.35eV, and 1020.15eV, decreasing gradually, which were lower than the BE (1022.2eV) 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 X Cu /X Zn was up to 8.98, which exceeded all others, it can be verified by the intensity of spectra in Figures 7 and 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)C 2 O 4 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].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 Figures 7 and 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].

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.

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)C 2 O 4 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 electronbinding 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 H 2 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 H 2 was 300.0 mL•g −1 •h −1 ; however, its selectivity of CO was as high as 1.53%.

Catalyst Preparation
The Cu-O-Zn/Al 2 O 3 catalyst precursors were prepared by dropping 1 mol/L Cu(NO 3 ) 2 -Zn(NO 3 ) 2 -Al(NO 3 ) 3 (Cu 2+ /Zn 2+ /Al 3+ = 16/8/1 (molar ratio)) solution and 1 mol/L H 2 C 2 O 4 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.

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 (H 2 -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% H 2 /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% O 2 /N 2 (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 N 2 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).

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.

Conclusions
In the process of preparing CuO/ZnO/Al 2 O 3 catalyst precursors, ethanol solvent selected the crystal phase composition in precursor and restricted its growth.Microwave irradiation promoted the homogeneous substitution of Cu 2+ of CuC 2 O 4 and Zn 2+ of ZnC 2 O 4 in the mother liquid, Zn 2+ in ZnC 2 O 4 •xH 2 O was replaced with Cu 2+ in CuC 2 O 4 , and the main phase (Cu, Zn) C 2 O 4 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 H 2 spacetime yield reached 516.7mL•g −1 •h −1 , while the CO selectivity was only 0.29%.

Figure 1 .
Figure 1.XRD patterns of the catalyst precursors.

CuC 2 O 4 •
xH 2 O, α-ZnC 2 O 4 •2H 2 O, and β-ZnC 2 O 4 generated in the mother liquid in coprecipitation, as shown in Equations (5)-(7).The isomorphous substitution mainly occurred in the aging process, it means that the Cu 2+ of CuC 2 O 4 entered into ZnC 2 O 4 and the Zn 2+ of ZnC 2 O 4 entered into CuC 2 O 4 -formed (Cu,Zn)C 2 O 4 , as shown in Equations ( • C was due to the desorption of physically absorbed water of α-ZnC 2 O 4 •2H 2 O, whereas the peak around 308 • C was ascribed to the decomposition of CuC 2 O 4 •xH 2 O or (Cu,Zn)C 2 O 4 , and the peak at about 343 • C was attributed to decomposition of β-ZnC 2 O 4 and further decomposition of α-ZnC 2 O 4 [15].The weight loss peak for CuC 2 O 4 •xH 2 O at 307 • C and weak peak for β-ZnC 2 O 4 at 343 • C were observed from curve of EWP, which verified the XRD analysis of its precursor.

Figure 1 .
Figure 1.XRD patterns of the catalyst precursors.

2. 4 .
XRD Characterization of CatalystsXRD patterns of different catalysts can be seen in Figure5.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 Table1.There were no peaks assigned to Al 2 O 3 in the four patterns, indicating that Al 2 O 3 existed as amorphous or that the content of Al 2 O 3 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 Table1.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.
XPS spectra of catalysts are shown in Figure7.As shown in Figure7a, all c showed a distinctive BE (binding energy) of Cu 2p3/2 around 932-933 eV, accompa characteristic satellite peak between 940 and 945 eV due to the electron shakeup which indicated that Cu species were present as CuO.The BE of Cu 2p3/2 around eV in the spectra of WWC, EWC, WMC, and EMC were 932.0 eV, 932.05 eV, 93

Table 1 .
Grain size values and textural properties of catalysts.From patterns of WMC and EMC in Figure5, 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 CuC 2 O 4 was incorporated into ZnC 2 O 4 and almost all Zn 2+ of ZnC 2 O 4 was incorporated into CuC 2 O 4 , 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 Zn 2+ in ZnC 2 O 4 •xH 2 O compound with Cu 2+ , and increases the content of the (Cu,Zn)C 2 O 4 phase in precursors during the catalyst preparation process.

Table 2 .
XPS and AES data of different catalysts.

Table 2 .
XPS and AES data of different catalysts.

Table 3 .
Catalytic performance of different catalysts for MSR reaction.

Table 4 .
Summary of the preparation condition of catalysts.