Impact of preparation methods on the performance of Cu/Ni/Zr catalysts for methanol decomposition

Utilizing waste heat from engine exhausts to decompose methanol into a hydrogen (H2) and carbon monoxide (CO) mixture, subsequently reintroduced into the engine, offers a significant potential to enhance engine efficiency and reduce emissions. The efficacy of the catalyst is crucial, as it directly influences the composition of the decomposition gases, thereby impacting energy conservation and emissions reduction. This study investigates the impact of various preparation methods for the self-developed Cu/Ni/Zr catalyst for methanol hydrogenation decomposition. These techniques include the co-precipitation method, co-impregnation method, and citrate complexation method, evaluated within a temperature spectrum of 220 °C–320 °C. Employing analytical methods such as x-ray Photoelectron Spectroscopy (XPS), x-ray Diffraction (XRD), Thermogravimetric Analysis-Differential Scanning Calorimetry (TGA-DSC), Brunauer–Emmett–Teller (BET), Temperature-Programmed Reduction (TPR), and Scanning Electron Microscopy (SEM) analysis, the study elucidates the mechanism of methanol decomposition catalyzed by Cu/Ni/Zr. The findings indicate that the catalyst’s activity, in terms of decomposition rate and hydrogen content, ranks in descending order from the co-impregnation method, followed by the citrate complexation method, to the co-precipitation method.


Introduction
In the era of rapid global industrialization, the escalation of environmental pollution has led to an increased demand for energy.Within this framework, the cultivation of low-carbon, clean, and renewable energy sources emerges as a critical necessity [1].Methanol, a hydrocarbon compound with high energy density, can be produced by reacting carbon dioxide with hydrogen.This production process poses lower environmental hazards and offers a relatively low production cost, marking methanol as an ideal low-carbon fuel [2].As an effective energy carrier, methanol can serve as a power source in transportation vehicles, either through direct use in fuel cells or by generating power via combustion [3,4].
Our research group has developed a technique that leverages the residual heat from engine exhaust to decompose methanol, generating a hydrogen-rich gas mixture subsequently introduced into the engine's combustion chamber.This approach offers significant benefits, notably enhancing engine thermal efficiency and reducing carbon monoxide and hydrocarbon emissions [5].Utilizing the residual heat from engine exhaust to heat the decomposer catalyzes the conversion of liquid methanol into H 2 and CO.This integration into the engine's combustion process augments thermal efficiency and curtails pollutant emissions [6].The primary reaction in methanol decomposition, identified as Reaction 1, is endothermic.Theoretically, complete decomposition of 1mol of methanol yields 1 mol of CO and 2 mol of H 2 [7].

kJ mol 1
For methanol decomposition without catalysts, temperatures exceeding 800 °C are required, often surpassing the residual heat capacity of internal combustion engines.However, with catalysts, this decomposition is attainable at substantially lower temperatures, under ambient or low-pressure conditions, yielding H 2 and CO.These catalysts primarily encompass copper-based [8,9], nickel-based [10,11], and precious metal catalysts [12].
Copper-based catalysts, the earliest and most extensively researched for methanol decomposition, primarily consist of an active copper component supported by SiO 2 , Al 2 O 3 , or activated carbon.Typical additives include La, Al, Ti, Cr, Mn, Ce, Zn, Fe, Si, and Ni [13][14][15], with the reaction usually occurring between 200 °C and 300 °C.At temperatures above 450 °C, these catalysts become susceptible to sintering.Furthermore, this catalytic decomposition reaction is known to generate by-products, including formaldehyde, methane, methyl formate, and dimethyl ether.To overcome these challenges, extensive research has been conducted on the development of fabrication techniques and the optimization of elemental ratios.Li et al [16] investigated the catalytic properties of CuAl 2 O 4 spinel catalysts synthesized using co-impregnation and citrate complexation methods, discovering that the catalysts fabricated through citrate complexation exhibited enhanced methanol decomposition capabilities compared to those prepared via co-impregnation.However, the inclusion of potassium in CuAl 2 O 4 spinel catalysts through co-impregnation improved H 2 +CO selectivity.Velinov et al [17] employed both thermochemical and mechanochemical treatments on bicarbonate precursors to produce two variants of Cu 0.5 Co 0.5 Fe 2 O 4 ferrites.Their study highlighted the importance of iron dispersion and crystallinity in methanol decomposition, pinpointing the optimal conditions as a thermochemical treatment temperature of 300 °C and a mechanochemical grinding duration of 1 h.Tsoncheva et al [18] compared four copper-based activated carbon catalysts, each prepared through distinct modified impregnation methods.Their research revealed that impregnation and initial wet impregnation methods were more effective for creating copper-based activated carbon catalysts, promoting a favorable distribution of precursors within the micro and mesopores of the carrier, and facilitating significant formation of Cu 1+ ions on the carrier surface.Moreover, these catalysts maintained high decomposition rates, stability, and H 2 +CO selectivity even at elevated temperatures.
Nickel-based catalysts, known for their stability, broad application range, and resistance to poisoning, however, are prone to sintering at high temperatures, which can lead to deactivation [19].Their catalytic activity primarily relies on the surface's zero-valent nickel, with the size of the surface metal particles being a crucial determinant of the catalyst's performance [20].Extensive research in methanol decomposition reactions has been conducted on Ni/SiO 2 , Ni-Cu/SiO 2 , Ni 3 Al metal compounds, and nickel alloys.Matsumura et al [21] examined Ni/SiO 2 catalysts prepared through various methodologies and found that those with larger Ni particles displayed superior catalytic activity.For Ni content below 5%, the activity of impregnated catalysts surpassed that of sol-gel catalysts, while for Ni content between 5% and 40%, sol-gel prepared catalysts showed higher activity.Chun et al [22] conducted methanol catalytic decomposition experiments on Ni 3 Al foils prepared via single-crystal cold rolling.They found that although the Ni 3 Al foil had a small specific surface area, it underwent spontaneous activation as the reaction progressed.After 29 h at 520 °C, the methanol decomposition ratio achieved 98% without notable activity loss, suggesting that Ni 3 Al foil could serve as both a catalyst precursor for methanol decomposition and as a structural material for micro hydrogen reactors.
While precious metal catalysts exhibit outstanding catalytic performance surpassing that of non-precious metal catalysts [23,24], their high precious metal content leads to significantly elevated preparation costs, rendering them less viable for widespread industrial application.In contrast, nickel-based catalysts, though stable, show limited activity at low temperatures, thus proving less effective for harnessing engine waste heat.
Copper-based catalysts, conversely, demonstrate higher activity at lower temperatures.The incorporation of specific additives, such as Ni and Zr, can markedly boost the catalyst's activity, substantially improving its lowtemperature selectivity, all while being more cost-effective than precious metal alternatives.
Our research group has previously concentrated on copper-based catalysts, employing the impregnation method to synthesize a variety of single-metal catalysts including Cu, Zn, and Ni, as well as multi-metallic combinations like CuZn, CuNi, ZnNi, and CuZnNi, further augmented by modifiers such as Mg, Mn, La, and Zr.These catalysts were evaluated within a temperature range of 200 °C-400 °C, at a weight hourly space velocity of 1.1 h −1 , and under a pressure of 0.1 MPa for their activity and selectivity.Our findings indicated that the Cu/Ni catalyst, particularly with an optimal elemental weight ratio of 3:1, exhibited the most effective catalytic performance.The introduction of Zr notably amplified the methanol decomposition rate and the H 2 +CO selectivity of the Cu/Ni catalyst.The composition of the catalyst was ultimately established as Cu/Ni/ Zr with an elemental mass ratio of 3/1/1.Some experimental results are depicted in figures 1 and 2.
Building upon the preliminary research of our group, this paper further investigates the fabrication methods for the Cu/Ni/Zr methanol decomposition catalyst.We utilized co-impregnation, co-precipitation, and citrate complexation as preparation methods.The synthesized catalysts' microstructures were characterized using advanced techniques such as x-ray Photoelectron Spectroscopy (XPS), x-ray Diffraction (XRD), Thermogravimetric Analysis-Differential Scanning Calorimetry (TGA-DSC), Brunauer-Emmett-Teller (BET), Temperature-Programmed Reduction (TPR), and Scanning Electron Microscopy (SEM).Their performance was tested within a temperature range of 220 °C-320 °C, at a weight hourly space velocity of 1.1 h −1 and a pressure of 0.1 MPa.
The objective of this study is to prepare a methanol decomposition catalyst that meets the requirements of engines.The criteria for the catalyst include low cost, high methanol decomposition rate at low temperatures, high selectivity for H 2 , CO, and CH 4 , as well as excellent thermal and time stability.The superiority of the research lies in its focused approach towards specific industrial requirements, with outcomes that can guide the development of industrial products.

Methodology
This section elaborates on the reagents and instruments employed in the synthesis of the catalyst for methanol decomposition.The preparation methods included co-precipitation, co-impregnation, and citrate complexation.The synthesized catalysts were tested on a custom-made fixed-bed reactor.

Catalyst preparation 2.1.1. Co-precipitation method
Adhering to the Cu/Ni/Zr mass fraction ratio of 3/1/1, corresponding quantities of Cu(NO 3 and Zr(NO 3 ) 4 •5H 2 O, cumulatively weighing 5g, were weighed and poured into a beaker.The mixture was thoroughly stirred in a predetermined volume of deionized water until fully dissolved.The beaker was then placed on a magnetic stirrer, with the speed adjusted for intense stirring.Subsequently, a solution containing 5 g of Na 2 CO 3 was gradually added dropwise to the nitrate solution.After complete addition, the mixture was allowed to age for an additional 2 h.The mass of a filter paper was recorded, and the paper was placed inside a vacuum filter, pre-wetted with deionized water for airtightness, before starting the vacuum  pump.The aged mixture was then cautiously transferred from the beaker into the filter.Post initial filtration, the residue was rinsed two to three times with deionized water to remove any soluble matter on the surface.The filter cake was then placed in an oven, set at 110 °C, and dried for 16 h.Afterwards, the dried residue was transferred to a crucible, which, using crucible tongs, was placed in a muffle furnace set at 450 °C for 3 h of calcination.After calcination, the catalyst is obtained.

Co-impregnation method
Activated alumina balls, ground to a 20-40 mesh size, were utilized as the carrier.They underwent triple washing with both deionized water and anhydrous ethanol, followed by drying in an oven at 110 °C for 16 h, and subsequent activation in a muffle furnace at 500 °C for 4 h.After cooling to room temperature, they were reserved for later use.Adhering to the Cu/Ni/Zr mass fraction ratio of 3/1/1, corresponding quantities of Cu(NO 3 ) 2 •3H 2 O, Ni(NO 3 ) 2 •6H 2 O, and Zr(NO 3 ) 4 •5H 2 O, cumulatively weighing 5 g, were weighed and poured into a beaker.The mixture was thoroughly stirred in a predetermined volume of deionized water until fully dissolved.Next, 5 g of the carrier was weighed and incorporated into the solution.Following 24 h immersion at room temperature, the blend was dried in an oven at 110 °C for 16 h before being placed in a muffle furnace.In the furnace, it underwent calcination at 450 °C for 3 h, culminating in the catalyst's synthesis.

Citrate complexation method
Adhering to the Cu/Ni/Zr mass fraction ratio of 3/1/1, corresponding quantities of Cu(NO 3 and Zr(NO 3 ) 4 •5H 2 O, cumulatively weighing 5 g, were weighed and poured into a beaker.The mixture was thoroughly stirred in a predetermined volume of deionized water until fully dissolved.Slowly add 500 ml of a 0.1 mol/L sodium citrate solution to the nitrate solution.The blue aqueous solution is then placed in a rotary evaporator for vacuum distillation, yielding a blue flocculent material.This is placed in an oven set at 110 °C and dried for 16 h.It is then transferred to a muffle furnace, with the temperature set to 450 °C, and calcinated for 3 h.After calcination, the catalyst is obtained.
Figure 3 presents a schematic representation of the preparation methods for Cu/Ni/Zr catalysts using three different techniques.

Catalyst evaluation
The schematic representation for evaluating the catalyst activity is illustrated in figure 4 and the actual setup of the catalyst evaluation apparatus is shown in figure 5. Before use, 2.5 g of Catalyst 2B is packed into a tubular furnace.Hydrogen, air, and nitrogen pass through the first gas purifier, 3C, to serve as carrier gases for the hydrogen flame detector, 3A.Nitrogen and air, after passing through the second gas purifier, 3D, serve as carrier gases for the thermal conductivity pool detector, 3B, completing the start-up and self-check procedures for detectors 3A and 3B.Catalyst 2B is placed in the center of quartz tube 2F.Turn on the mass flow meter 2E and the first double valve 2D, then close the second double valve 3G and the third double valve 3H.
During operation, liquid methanol is drawn from storage tank 1A and gently introduced into the water bath 1C via the peristaltic pump 1B.The water bath's temperature, being higher than methanol's boiling point, ensures methanol enters the reaction system 2 in gaseous form.Subsequently, it enters the quartz tube 2F, undergoing catalytic decomposition on the active surface of Catalyst 2B, and is then vented outside through the first double valve 2D.When the temperature of the tubular furnace 2A reaches the designated level, the second double valve 3G is opened, and the first double valve 2D is closed.The reactants, in gaseous form, traverse the heating belt 2C, entering the flame ionization detector (FID) 3A, where substances like methanol, alkanes, alkenes, and carbon monoxide are tested.The carrier gas within the FID 3A is high-purity nitrogen, and the chromatographic column is a white silanized carrier.Column temperature is set at 60 °C, detector at 230 °C, injector at 200 °C, and auxiliary furnace at 180 °C.
The third double valve 3H is then opened, closing the second double valve 3G.Reactants pass through the filter 3E, filtering out methanol and other water-soluble byproducts.They enter the thermal conductivity pool detector 3B, completing the hydrogen content testing in the reactants.This detector, 3B, is a dual Thermal Conductivity Detector (TCD).Specifically, TCD1 analyzes CO and CO 2 with the Porapak-Q chromatographic column, using high-purity hydrogen as its carrier gas at a flow rate of 20 mL/min.TCD2 analyzes H 2 , using both the Porapak-Q and 5A columns, with high-purity nitrogen as its carrier gas at the same flow rate.The column temperature is set at 80 °C, and the detector at 125 °C.
Finally, the data is relayed to computer 3F, which is connected to detectors 3A/3B.Through external standard methods, calibration for each component of the methanol decomposition reactants is achieved, yielding the results.
Throughout the process, the temperature of the reaction system is controlled by various temperature controllers and heaters.The tubular furnace 2A and heating belt 2C both provide effective temperature The decomposition ratio of methanol (X MeOH ), as well as the selectivity of H 2 (S H 2 ) and CO (S CO ), are calculated using carbon balance and hydrogen balance.The equations are as follows:   ( ) Where: C total is the total carbon content in the product.H total is the total hydrogen content in the product.C MeOH is the CH 3 OH content in the product.C CH 4 is the CH 4 content in the product.C CO is the CO content in the product.In this study, the reagents for the catalyst were primarily sourced from the China National Medicines Corporation Ltd. and Shanghai Macklin Biochemical Technology Co., Ltd.Key instruments, such as the chromatograph and air generator, were provided by Zhejiang Fuli Analytical Instruments Co., Ltd.Three preparation methods were employed: co-precipitation method, co-impregnation method, and citrate complexation method.The catalysts were characterized using techniques such as XPS, XRD, TGA-DSC, BET, TPR, and SEM.Evaluation was carried out using a custom-built fixed-bed reactor, and the decomposition ratio and selectivity were determined using an elemental balance method.

Results and discussion
In this study, Cu/Ni/Zr catalysts were prepared using three methods: co-precipitation, co-impregnation, and citrate complexation.Their microstructures were characterized using techniques such as XPS, XRD, TGA-DSC, TPR, SEM, and BET.The activity and selectivity of the catalysts were investigated under specific conditions: temperature range of 220 °C-320 °C, weight hourly space velocity of 1.1 h −1 , and a pressure of 0.1 MPa.

XPS analysis results
Figure 6 displays the XPS analysis results for catalysts synthesized using different methods.Each method resulted in catalysts composed of Cu, Ni, and Zr.It is noteworthy that the catalyst produced via the co-impregnation method also contained Al, attributed to the use of Al 2 O 3 in the impregnation process.Table 1 summarizes the peak areas of each element across the various preparation methods.The proportion of Cu in the coimpregnation method was approximately one-fourth of that in the co-precipitation and citrate complexation methods.This discrepancy arises from the inherent nature of the impregnation method utilizing Al 2 O 3 as a support, where the Cu content is reduced to about one-fourth compared to the other methods, for an equivalent total mass.In the catalyst developed through the citrate complexation method, there was a noticeable issue with the mixing of Ni, leading to a diminished synergistic effect with Cu.This could be attributed to two factors: (1) the analysis considered only the 2p orbital of Ni, omitting other orbitals; (2) XPS provides only a semiquantitative analysis, focusing on surface components rather than the entire catalyst composition.

XRD analysis results
Figure 7 displays the XRD analysis results for the different preparation methods.The primary diffraction angles for CuO are at 29.4°, 35.7°, and 38.7°.In the sample prepared using the co-impregnation method, no distinct peaks for CuO are evident, suggesting that CuO has poor crystallinity or is amorphous at this stage.For the sample prepared by the co-precipitation method, there are no pronounced peaks for NiO, indicating that NiO has a lower crystallinity and is uniformly dispersed.The catalyst prepared using the citrate complexation method showed the most distinct peaks for CuO, indicating superior crystallinity.The ZrO 2 diffraction peaks were primarily observed at 30.2°, with broader peaks in both the co-precipitation and citrate complexation results, indicating lower crystallinity of ZrO 2 .In the co-impregnation results, the absence of ZrO 2 diffraction peaks suggests a smaller crystallite size, allowing for a more uniform dispersion on the carrier's surface.

TGA-DSC analysis results
Figure 8 presents the TGA-DSC results for catalysts prepared using different methods.Each catalyst was heated from ambient temperature to 800 °C in an oxygen-rich environment.The analysis demonstrated that the catalyst synthesized via the citrate complexation method exhibited minimal weight loss, predominantly due to water evaporation below 100 °C, with a total weight reduction of about 0.93%, indicating commendable thermal stability.In contrast, the catalyst produced by the co-precipitation method experienced two significant weight loss events: first, water evaporation below 100 °C, and subsequently, between 610 °C and 685 °C, attributed to the decomposition of carbonates and a dehydroxylation reaction.The total weight loss for the coimpregnation method was noted to be 10.23%.When considering thermal stability, the catalysts rank as follows: citrate complexation method > co-impregnation method > co-precipitation method.

BET characterization results
The size of the specific surface area and pore volume significantly influences the catalytic activity.Table 2 displays the BET test data for different preparation methods.As evident from the table, the catalyst prepared using the co-impregnation method has the largest specific surface area and a pore volume of 0.397 cm 3 g −1 , due to its support on Al 2 O 3 , which inherently has a larger pore diameter.The catalyst made through the citrate complexation method has the smallest specific surface area and pore volume.

TPR characterization results
Figure 9 presents the TPR properties for different preparation methods.Each TPR peak mainly corresponds to the reduction of CuO to metallic Cu.As shown in table 3, the primary peak temperatures for the highest reduction are concentrated between 550 °C-580 °C.The variance in the onset of the reduction temperature and the maximum reduction temperature mainly reflects the different influences of the preparation methods on the interaction of CuO.The baseline drift is primarily related to the decreased promoting effect of NiO on the  reduction result.Notably, the catalyst from the citrate complexation method has a higher peak concentration, suggesting a more uniform distribution and higher content of its active components.

SEM characterization results
Figure 10 shows the SEM images of the catalysts prepared by different methods.From the images, it is evident that the catalyst prepared using the co-precipitation method primarily consists of particle shapes with smaller grains.As a result, it has a smaller specific surface area and pore volume, which is consistent with the BET test results.For the catalyst prepared by the co-impregnation method, CuO adheres to the surface of Al 2 O 3 and is distributed uniformly.The catalyst made through the citrate complexation method displays more lamellar structures, with an increase in pore depth.Its average pore diameter is the largest, and its crystallinity is the best, aligning with the XRD and BET characterization results.Figure 11 shows the element mapping patterns of the Cu/Ni/Zr catalyst, which demonstrate the presence of Cu, Ni, and Zr elements in the catalyst.

Catalytic performance analysis
Figure 12 illustrates the influence of different preparation methods on catalyst performance within the temperature range of 220 °C-320 °C.As depicted in figure 12(a), the catalyst prepared using the coimpregnation method has the highest decomposition rate.In contrast, the catalysts made by the co-precipitation and citrate complexation methods have subpar low-temperature decomposition performance.Based on the BET and SEM characterization results, the primary reason for this disparity is that the catalyst prepared using the co-impregnation method has the largest specific surface area and pore volume.This allows for the introduction of more methanol to react with the catalyst per unit surface area.As depicted in figure 12(b), in terms of selectivity for H 2 and CO, the catalyst prepared using the citrate complexation method remains above 90% and approaches 100% with increasing temperature, making its overall selectivity the highest.The primary reason for this is its highest crystallinity of CuO and NiO and superior thermal stability.On the other hand, the catalyst prepared using the co-precipitation method shows decreasing selectivity with increasing temperature.As evidenced by SEM and BET characterizations, it has a smaller pore volume.According to XRD characterization, its CuO has lower crystallinity, and its Ni content is minimal.As the temperature rises, CuO agglomerates,  The catalyst made by the co-impregnation method shows gradually increasing selectivity with rising temperature.Specifically, the H 2 selectivity approaches 90% at 320 °C.As seen in figures 12(c) and (d), the contents of CH 4 and CO 2 first increase and then decrease with temperature, peaking at 260 °C and then decreasing gradually.As indicated by XPS and XRD characterizations, the main reason is that this catalyst has the lowest Cu content and the least crystallinity of CuO.At lower temperatures, the activity of CuO is inadequate, leading to more side reactions.As the temperature increases, the activity of CuO intensifies.The presence of elements like Ni and Zr also enhances the activity and selectivity of CuO, promoting more decomposition reactions, indicating a strong synergistic effect between Ni, Zr, and Cu.
As depicted in figure 12(f), concerning the H 2 content, 280 °C stands as a critical temperature threshold.Below this temperature, the catalyst prepared by the co-impregnation method exhibits the highest content.Above it, the catalyst produced via the citrate complexation method dominates.Though the citrate complexation method nudges the selectivity for H 2 close to 100%, at lower temperatures, as illustrated in table 4, the H 2 content and decomposition ratio for the co-impregnation method significantly surpass those of the citrate complexation and co-precipitation methods.Considering the working conditions of engine operations, where H 2 , CO, and CH 4 all serve as fuels and can be combusted in the engine, the methanol decomposition catalyst demands the highest decomposition ratio.Thus, the co-impregnation method is selected as the optimal preparation technique for this methanol decomposition catalyst.

Conclusion
This study investigated the impact of different preparation methods on the performance of Cu/Ni/Zr methanol decomposition catalysts.BET and SEM characterization results revealed that catalysts prepared using the coimpregnation method have significantly higher surface area and pore volume compared to those prepared by coprecipitation and citrate complexation methods.The active components in the co-impregnation method are more uniformly dispersed, leading to a greater number of methanol molecules reacting per unit surface area, resulting in a higher decomposition rate.However, characterizations using XPS, TGA-DSC, XRD, and TPR revealed that catalysts prepared by the co-impregnation method have the lowest crystallinity of active components and contain the least amount of reducible components.In contrast, the citrate complexation method produces catalysts with the highest crystallinity and content of active components.Consequently, when considering H 2 and CO selectivity, catalysts prepared by the citrate complexation method outperform, maintaining over 90% selectivity across all temperature ranges.Thus, surface area and pore volume are the primary factors influencing the decomposition rate of Cu3Ni1Zr1 catalysts, while the crystallinity and content of active components primarily determine H 2 and CO selectivity.
Within the temperature range of 220 °C-320 °C, the effectiveness of the catalysts, as determined by their decomposition rates and hydrogen content, varies according to the preparation method.The activity of the catalysts, in descending order of performance, is as follows: co-impregnation method, citrate complexation method, and co-precipitation method.Catalysts produced by the co-precipitation method undergo more side reactions as the temperature rises, leading to an increase in the production of CH 4 and CO 2 .Catalysts prepared by the citrate complexation method have the highest selectivity for H 2 and CO and possess the highest crystallinity and content of active components.Catalysts made using the co-impregnation method have the highest decomposition rates and H 2 content at lower temperatures.Evaluating the ease of catalyst preparation, the efficiency of the citrate complexation method is relatively low, due to the necessity of a rotary evaporator for vacuum distillation, making it unsuitable for large-scale production.In contrast, the co-impregnation method is straightforward and suitable for mass production.Therefore, considering the actual working conditions of engines, the co-impregnation method is ultimately chosen as the primary method for producing vehicular methanol decomposition catalysts.

Figure 1 .
Figure 1.Effects of varying Cu/Ni mass ratios on (a) methanol decomposition ratio, and (b) hydrogen selectivity.

Figure 2 .
Figure 2. Effects of adding different elements on (a) methanol decomposition ratio, and (b) hydrogen selectivity.

Figure 4 .
Figure 4. Schematic of the catalyst evaluation device.

Figure 5 .
Figure 5. Actual image of the catalyst evaluation apparatus.

Figure 6 .
Figure 6.XPS analysis results for different preparation methods.

Figure 7 .
Figure 7. XRD analysis results for different preparation methods.

Figure 9 .
Figure 9. TPR analysis results for different preparation methods.

Figure 10 .
Figure 10.SEM images of the catalysts prepared by different methods.(a) Co-precipitation method, (b) Co-impregnation method, (c) Citrate complexation method.

Figure 12 .
Figure 12.Impact of different preparation methods on catalyst performance.(a) Catalyst activity, (b) H 2 selectivity, (c) CO selectivity, (d) CH 4 content, (e) CO 2 content, (f) H 2 content. 2 CO 2 is the CO 2 content in the product.C MF is the Methyl Formate (MF) content in the product.C DME is the Dimethyl Ether (DME) content in the product.H H 2 is the H 2 content in the product.X MeOH is the decomposition ratio of CH 3 OH.S CO is the selectivity of CO. S H 2 is the selectivity of H 2 .

Table 1 .
Peak areas of elements for different preparation methods.

Table 2 .
BET results for different preparation methods.

Table 3 .
TPR data for different preparation methods.

Table 4 .
Catalyst performance at 260 °C for different preparation methods.