Methanol Reforming over Cu-Ce-Al Catalysts Prepared by Solution Combustion Synthesis Method

Abstract

The demand for environmentally friendly types of energy is growing all over the world, which naturally increases the intensity of studies on fuel mixtures that have high contents of hydrogen. In this case, methanol steam reforming is a leading effective research area, as it is a process with low energy consumption. The results of the steam reforming of methanol on synthesized catalysts by the solution combustion synthesis (SCS), self-propagating high-temperature synthesis (SHS), and moisture impregnation capacity methods are presented. A study was conducted to evaluate the activity of Cu-Ce-Al catalysts with varying ratios of components for hydrogen production, comparing the SCS method with the other mentioned methods. The methanol conversion reached 99% and the selectivity of H₂ was 88% at 500 °C. The study showed that the replacement of Al³⁺ ions with Cu²⁺ and Ce³⁺ cations leads to the formation of spinels, such as CuAl₂O₄ and CeAlO₃. As a consequence, the CuAl₂O₄ and CeAlO₃ lattice parameters increase because of the difference in the ionic radii of Al³⁺ (0.53 Å), Cu²⁺ (0.73 Å), and Ce³⁺ (1.07 Å). Advantages of SCS catalysts in the process of the steam reforming of methanol have been demonstrated. The goal of this research is to create a new catalyst for methanol’s conversion into hydrogen-containing fuel mixtures, the production of which, in the future, will be a huge step in the transition to more energy-efficient and environmentally friendly methods of their synthesis.

1. Introduction

As we all know, population increases and economic growth have naturally provoked increases in energy consumption [1]. Today, oil, natural gas, and coal are the main sources of the energy produced worldwide and account for almost 70–80% of it [2], and, at the same time, the hydrogen synthesis cost remains very expensive. As fossil fuel reserves decline, dependence on renewable energy has become crucial, while the continued use of fossil fuels leads to considerable environmental and economic costs. The increasing costs of our current energy systems highlight the potential of hydrogen proton-exchange-membrane fuel cells as a viable alternative to internal combustion engines, though they require a reliable H₂ supply [3]. Consequently, having a liquid hydrogen source that can provide H₂ on demand is beneficial, with hydrogen production from methanol via steam reforming over copper-based catalysts emerging as a notable method.

Currently, as an energy carrier, hydrogen remains invaluable due to its high efficiency and cleanliness [4]. The most common process in production organizations is steam reforming, which, in turn, involves the decomposition of molecules of hydrocarbons and alcohols, in particular methanol and ethanol, using superheated steam, the result of which is hydrogen and carbon oxides. But these processes are energy-consuming, since they take place at high temperatures and, in addition to the target products, produce a large number of undesirable accompanying by-products. Alternatively, hydrogen can be synthesized using the process of water electrolysis [5] (which breaks down into O₂ and H₂)—a less cost-effective method due to the amount of electricity required for the electrolysis. The approach we propose can completely or partially solve all these problems. This method requires only an inexpensive catalyst and a mixture of methanol and water.

Under much milder conditions, catalytic processes of the steam reforming of aliphatic alcohols take place, which, moreover, can be obtained from renewable raw materials (biomass (bioalcohols)). Methanol, as a low-carbon alcohol, which contains a high amount of H₂, is an available liquid hydrogen carrier and can easily react with water to release the main product under relatively mild conditions for this type of process compared to gaseous hydrogen carriers. This can be easily explained by the fact that, compared to other multi-carbon hydrocarbon feedstocks, methanol lacks robust C-C bonds. [6]. The steam reforming of methanol (SRM) seems attractive because the process is characterized by low energy costs and the raw material itself is relatively cheap [7], and, in addition to traditional methods, it is combined with a wide range of alternative sources for methanol synthesis [8]:

• Biomass (agricultural waste, timber);
• Household waste with high organic contents;
• Waste gases from enterprises.

Their gasification will make it possible to obtain a gas mixture for the subsequent synthesis of methanol along the “green” route, without using organic fossil fuel. Most likely, in the near future, alternative sources of methanol synthesis in the form of biogas will become one of the few solutions to the energy supply problems in the world. It is no secret that reserves of natural hydrocarbons are being depleted every year, and this direction may well become the only variable method of producing both motor fuel and petrochemical products.

A promising direction for the resulting methanol is its use as a motor fuel for internal combustion engines and fuel cells. Compared to traditional fuels, methanol has a number of advantages, such as renewability, a smaller carbon footprint, and high calorific value [9]. All of them are the main drivers of the demand for methanol for use as a raw material and energy carrier in hydrogen production. It is expected that by 2030, the methanol production in the world will exceed 130 million tons per year.

The SRM process involves the following chemical reactions:

CH₃OH + H₂O → CO₂ + 3H₂, ∆H° = 49.7 kJ/mol,

(1)

CH₃OH → CO + 2H₂, ∆H° = 90.8 kJ/mol,

(2)

CO + H₂O → CO₂ + H₂, ∆H° = −41.1 kJ/mol.

(3)

The first (1) and second (2) chemical reactions are endothermic and reversible, the courses of which always lead to an increase in volume, while the last (3) reaction is exothermic, occurs without an increase in volume, and is also known as a water–gas-shift reaction [10]. As a result of these reactions, a mixture of gases from carbon and hydrogen oxides is formed. The conditions of the methanol-reforming process, such as the temperature, pressure, and volumetric velocity, as well as the characteristics of the catalyst, directly affect the ratio between the products formed.

The development of a new effective catalyst for the process of the steam reforming of methanol into hydrogen-containing gas is the aim of this work, which, in the future, will be a significant advancement in transitioning to more energy-efficient and environmentally friendly synthesis methods.

Bulk metals [11,12] and oxides [13,14], along with those backed on diverse kinds of carriers [15], could play a role as catalysts for hydrocarbon conversion. Nevertheless, in the past few years, new kinds of catalysts with the very-high-activity characteristics produced through the combustion process have emerged. Self-propagating high-temperature synthesis (SHS) [16], and specifically its relatively new and contemporary version, solution combustion synthesis (SCS) [17,18,19], is a novel approach for acquiring a contemporary range of unique catalysts derived from oxides, metals, spinels, alloys, and similar materials. In the SHS process, an intense exothermic reaction (combustion reaction) occurs, where the heat release is concentrated within a layer and is transferred from one layer to another through heat transfer. Catalytic process conditions, such as high rates of combustion reactions and the instantaneous cooling of the sample after the combustion process, lead to the formation of catalysts with high densities of defective structures on their surfaces, which directly affect the increase in the activity of SCS catalysts. In the previous studies of the authors, the results of comprehensive studies on the mechanical and physicochemical characteristics of synthesized SHS and SCS catalysts with various contents were examined and discussed in detail [20,21,22,23]. During these investigations, materials produced via the SHS process were synthesized, possessing characteristics typical of highly active catalysts. It is anticipated that, in the future, they will be promising for diverse industrial processes, including partial oxidation, reduction, and hydrocarbon and organic alcohol conversion, among others [24,25]. This study involved synthesizing a set of catalysts utilizing Cu, Ce, and Al through both SCS/SHS and conventional impregnation techniques, followed by characterization using various physicochemical analyses. The catalysts underwent examination in a continuous fixed-bed reactor during methanol steam reforming.

2. Results and Discussion

2.1. Characterization of Catalysts

This study presents the results of catalysts based on the Cu-Ce-Al system, obtained through the solution combustion synthesis method. Post-combustion, the catalysts were analyzed by X-ray diffraction (XRD) to identify and determine the phases. Brunauer–Emmett–Teller (BET) and scanning electron microscopy (SEM) analyses were employed to ascertain the specific surface areas and content of the catalysts.

Table 1. Al(NO₃)₃ + Ce(NO₃)₃ + Cu(NO₃)₂ + H₂O + glycine system’s solution combustion reactions.

Chemical Reactions	Remarks
Cu(NO3)2 + Ce(NO3)3 + 2Al(NO3)3 + 5C2H5NO2 → Cu + CuO + CuxAly + CuCeOx + CeO2 + CuxAl2−xO4 + CeAlO3 + 5CO2 + 8N2 + 10H2O	The total reaction of the SCS process at 500 °C
2Al2O3 + 3C → 4Al + 3CO2, 2CuO + C → 2Cu + CO2	Carbon formed after combustion of glycine subsequently participates in reduction of oxides into their metals
4Al + 3O2 → 2Al2O3	Exothermic reaction with ΔH°278 equal to −3352 kJ/mol. The metals that have been reduced may undergo partial oxidation or interact with another metal
2Cu + O2 → 2CuO	Exothermic reaction where ΔH°278 = −475.8 kJ/mol
Al2O3 + CuO → CuAl2O4, Al2O3 + Ce2O3 → 2CeAlO3	Endothermic reactions followed by obtainment of spinels
Cu + Al → CuxAly	In this stage, intermetallic compounds are synthesized

presents the most possible chemical reactions of the solution combustion synthesis process.

Table 2. The starting mixtures of the materials with different ratios and synthesized-catalyst contents (the preheating temperature in the furnace was 500 °C).

Initial Materials	Catalyst Contents
50% Cu(NO3)2 + 30% Al(NO3)3 + 20% Ce(NO3)3 + 50% glycine + 10 mL H2O	CuAl2O4, CeAlO3, CeO2, CuO, Al, AlCu
40% Cu(NO3)2 + 30% Al(NO3)3 + 30% Ce(NO3)3 + 50% glycine + 10 mL H2O	CuAl2O4, CeAlO3, CeO2, CuO, Al, AlCu
30% Cu(NO3)2 + 30% Al(NO3)3 + 40% Ce(NO3)3 + 50% glycine + 10 mL H2O	CuAl2O4, CeAlO3, CeO2, CuO, Al, AlCu
20% Cu(NO3)2 + 30% Al(NO3)3 + 50% Ce(NO3)3 + 50% glycine + 10 mL H2O	CuAl2O4, CeAlO3, CeO2, CuO, Al, AlCu

shows the initial compositions of the nitrates and glycine solution mixtures at the respective concentrations (wt.), as well as the qualitative contents of the produced catalysts in the form of individual compounds.

2.1.1. XRD and BET Analysis

The final catalysts had comparable contents by quality but their phase ratios were different. As can be seen from Figure 1, the relational intensities of the X-ray diffraction peaks were mounted for each individual phase, and then the estimated phase ratio was revealed.

Figure 1 presents the XRD diffraction patterns of the synthesized catalysts. After obtaining the catalysts via solution combustion synthesis, the Cu(NO₃)₂ species were identified as CuO, Al-Cu (intermetallic), and CuAl₂O₄ (spinels). As for the aluminum nitrate, it mainly passed through an intermediate product in the form of aluminum oxide into cerium and copper spinels, as well as into intermetallites. The clear overlap among these diffraction peaks can be explained by the small amount of Al₂O₃ in the final compounds.

Research on methanol’s decomposition and adsorption onto copper-based surfaces revealed that the methanol exposes dissociative adsorption, resulting in the formation of methoxy species (CH₃O) [26]. It is assumed that absorbed O is the initiator of the formation of methoxy forms. Studies by some authors [27] have shown the following important information: oxygen may originate from the deficient reduction of the catalysts, like the cerium’s lattice oxygen, or from possible residual wetness in the methanol feed.

The species of cerium in the prepared catalyst system of Cu(NO₃)₂ + Ce(NO₃)₃ + Al(NO₃)₃ + glycine in each option were recognized as cerium oxide (CeO₂) and spinels (CeAlO₃). As is shown in the XRD patterns of the catalysts, the wide diffraction peaks of CuO indicate that the cerium enhanced the level of dispersion and reduced the crystalline size of the copper at the same time, which has also been mentioned in previous studies [28].

Based on the above and taking into account the research of previous authors, it can be assumed that the cerium in this catalyst acts as a promoter in the methanol-reforming process, with CeO₂ playing this role. This can be explained by the fact that cerium has a cubic structure, and its ions are easily transferred at low temperatures. Additionally, cerium ions can easily switch between oxidation states. Considering the catalyst preparation process by the combustion method with a high combustion rate, it can be said that the part of the catalyst where cerium oxide or spinels (CeAlO₃) have formed possesses a defective structure and has greater potential for oxygen accumulation. This will influence the activity of the catalyst, as confirmed by the analyses presented below.

The graphs of the temperatures recorded during the solution combustion synthesis exhibited subsequent peaks following the SCS, as shown in Figure 2. Only metal oxides and carbon reaction can account for this. In this case, the reaction Al₂O₃ + C → Al + CO₂ may give an explanation for the existence of aluminum in the reaction products, due to hydrogen, that come out under the reaction. The aluminum oxide cannot be reduced under the process conditions. This has already been found in earlier studies [29].

At first, a muffle furnace was heated up to 500 °C, and then the series of needed catalysts were synthesized in it. The temperatures were measured by three different thermocouples, which were placed in a glass with a solution of nitrates and glycine. Each of them measured the temperature in the lower, middle, and upper parts of the glass during the entire combustion process, the results of which are demonstrated in Figure 2. During the synthesis process, two combustion modes were carried out: (1) volumetric explosion and (2) self-propagating modes. During the volumetric-explosion mode, the solution heats up, causing water to evaporate. When the H₂O evaporates, a gel forms. The furnace temperature steadily rises until it reaches a critical point, and then the exothermic reaction occurs across the entirety of the catalyst volume.

As shown in Figure 2, during the synthesis of the catalyst, the solution evaporates when it reaches T = 100 °C, and when it reaches 172 °C, a gel is formed. The maximum temperature peak is reached at 445 s, and the main synthesis reactions take place in this zone at over 1000 °C.

The unspent catalysts were examined by the BET method, and the adsorption and desorption isotherms by nitrogen were plotted for these catalysts (Figure 3). According to the IUPAC classification, all the samples were identified and classified as type IV isotherms, suggesting the presence of mesoporous structures.

All catalysts prepared by any kind of synthesis exhibited hydrogen hysteresis rings, indicating that the sizes of their pores were wide and diverse. These included different pore types, such as “ink bottle” pores, tubular pores with uneven sizes, and tightly packed spherical particle gap pores [30].

All the acquired catalyst’s surface areas were assessed using the BET method. The obtained results are shown in Figure 4.

The concentration of cerium nitrate directly affects the surface areas of these catalysts. As can be seen from Figure 4, an increase in the Ce(NO₃)₃ content leads to an increase in the surface area, which further affects the activity of the catalyst. As is clear, the 20% Cu(NO₃)₂ + 50% Ce(NO₃)₃ + 30% Al(NO₃)₃ + 50% glycine catalyst had the largest specific surface area among them. It should be noted that the catalyst’s specific surface areas in this system are relatively low. This can be explained by the high combustion temperatures while the synthesis of the catalyst is taking place. However, the prepared final catalysts are very active, which allows them to be on par with expensive catalysts.

It is also worth noting that a change in the concentration of cerium in the initial composition affects the pore size, which consequently leads to a change in the surface area of the entire catalyst. Thus, in the sample with a high content of cerium nitrate, the pore sizes were decreased, but, due to its defective structures, more micropores appeared, which contributed to an increase in the surface area. Reducing the pore size increases the hydrogen selectivity without decreasing the methanol conversion. The direct relationship between the surface area and pore size with the catalyst activity is described in the section on the catalyst performance in methanol steam reforming.

2.1.2. SEM Analysis

The surface structure and morphology of the synthesized catalysts were analyzed using scanning electron microscopy. The results of the SEM analysis for catalysts containing 50% cerium nitrate and 20% copper nitrate are presented in Figure 5, Figure 6, Figure 7 and Figure 8. The chemical analysis confirmed that the phase composition (CuAl₂O₄, Al, Al-Cu, CuO, CeO₂, CeAlO₃) aligns with the XRD data.

The cerium, copper, aluminum, and oxygen compositions were located and identified in various areas (Figure 6). The very high contents of these components match the abovementioned spinel phases.

Chemical analysis (Figure 8) was carried out for the catalyst shown in Figure 7.

In sum, Cu(NO₃)₂-Ce(NO₃)₃-Al(NO₃)₃ catalysts with different element ratios were synthesized, and their combustion characteristics, compositions, structures, specific surface area, and pores were analyzed.

2.2. Catalytic Activity Results

For the determination of the activity of the synthesized series of catalysts, we studied the process of the steam reforming of methanol in a temperature range between 250 and 650 °C. According to the data [31], the catalytic activity of the catalysts was directly affected by the methanol-steam-reforming temperature. Figure 9 shows the results of these catalyst-series activities on the conversion of methanol depending on the process temperature.

As is demonstrated in Figure 9, the highest CH₃OH conversion (99.3%) is observed for the catalyst containing 20% Cu(NO₃)₂ + 20% Ce(NO₃)₃ + 30% Al(NO₃)₃ at the 600 °C temperature, and then it starts to decrease. Actually, good results are shown between 500 and 650 °C for all the samples, but the amount of cerium nitrate plays a significant role in this system of catalysts. Decreasing the content of Ce(NO₃)₃ naturally leads to a decrease in the catalyst activity, which confirms the previously discussed results of the BET analysis. The results of the studies on the influence of the temperature and component contents of the catalysts on the yield of the target product are shown in Figure 10.

As displayed in Figure 10, the H₂ yields of the four different catalysts demonstrated initial increases and then reductions after 600 °C with the increase in temperature. The catalyst containing 50% Cu(NO₃)₂ + 20% Ce(NO₃)₃ + 30% Al(NO₃)₃ + 50% exhibited a greater variance with the temperature changes. This could be because of the sintering of the active components of copper at high temperatures, resulting in a reduction in the H₂ yield. In this case, the content of Ce showed the same effect as in the conversion of methanol. The sample, which had 50% cerium nitrate, had a higher catalytic activity than the 20% sample, which might be explained by the improvement in the dispersion of Cu by increasing the content of cerium. The yields of carbon oxide are shown in Figure 11.

In Figure 11, it can be seen that an increase in temperature promoted carbon oxide generation. Also, it can be recognized that an increase in the concentration of cerium nitrate in the content of the initial catalyst unexpectedly resulted in a decline in the ratio of hydrogen to carbon monoxide (H₂/CO) in this series of catalysts almost three times. This is because the Ce could inhibit the CH₃OH decomposition and reverse the reactions of the water–gas shift, ultimately resulting in a product stream with a low CO content but that is rich in hydrogen.

Due to the fact that the ratio of hydrogen to carbon in the methanol atom is very high, it is a good feedstock for the reforming process. Based on the results of this research, it can be understood that methanol, including only one carbon atom, already easily begins to transform into a hydrogen-containing gas with a low content of carbon oxides at a temperature of 300–350 °C. Chemical reactions (1)–(3) are basic for the reforming process, but the following reactions are also possible:

CH₃OH → 2H₂ + CH₄ + CO₂, ∆H° = −67 kJ/mol,

(4)

2CH₃OH → CH₃OCH₃ + H₂O, ∆H° = −23 kJ/mol,

(5)

CH₃OH → CH₂O + H₂, ∆H° = 85 kJ/mol,

(6)

2CH₃OH → HCO₂CH₃ + 2H₂, ∆H° = 64 kJ/mol,

(7)

These reactions are mainly methanol decomposition reactions. But the probability of their occurrence is not so high, especially since this is influenced by the content of the catalyst and its microstructure. In addition, if you reduce the ratio of raw materials in the reforming process (methanol:water), then the likelihood of these reactions occurring increases.

Based on the results of the above-described analyses, the most active catalyst (the 20% Cu(NO₃)₂ + 50% Ce(NO₃)₃ + 30% Al(NO₃)₃ + 50% glycine sample) among those synthesized by the SCS method was identified, and an additional analysis was carried out for it in the form of the temperature-programmed desorption of oxygen. The results are shown in Figure 12.

The high intensity (over 5000) of the oxygen desorption at 550–570 °C was noticed, but, in the spent catalyst sample, the intensity decreased. The overall quantity of the emitted oxygen steadily changes within this range of heat treatment temperatures.

The catalysts maintained a constant weight following the catalytic activity studies, indicating no coke formation. The stability of the catalysts was tested over a span of 70 h. During this period, the conversion rates of the CH₃OH exhibited minor fluctuations. Further and more comprehensive investigations into coke formation are currently being conducted.

In addition, the analysis of the spent catalysts showed that there was a greater concentration of copper in the starting material, and that more changes occurred in the entire mass. This may be due to the fact that copper particles are more resistant to sintering and coking. There is also the possibility that a small amount of the product formed could react with the copper oxide, displacing oxygen, or include only pure copper in the catalyst. In this regard, the role of cerium may be opposed, acting as an inhibitor of the coking process in this catalyst.

The best SCS catalyst was evaluated against samples of identical compositions that were prepared using the impregnation and self-propagating high-temperature methods. The methanol steam reforming was conducted in the reaction mixture of it with H₂O at a ratio 2.5:1 under an Ar flow with a 50 mL/min rate.

Table 3. Comparison of Cu-Ce-Al catalysts prepared using the SCS, SHS, and impregnation methods.

Catalyst Preparation Method	Conversion of CH3OH (%)	Selectivity of H2 (%)	Specific Surface Area (m2/g)
SCS	99.3	88.8	25.8
Impregnation	75.6	74.7	102.9
SHS	83.8	63.2	18.5

shows the results of the work.

The methanol conversion values are the best for the SCS catalysts, followed by the samples prepared by the SHS and impregnation methods. The H₂ selectivity is also higher for the SCS catalysts at 88%, compared to 63–74% for the other catalysts. The largest surface (Langmuir) area had a catalyst synthesized by the impregnation method and was over 100 m²/g, followed by the SCS catalyst with almost 26 m²/g, and then the SHS sample with 18 m²/g. Therefore, new composite materials produced through the SCS method possess a notable advantage. The generated hydrogen and CO mixture is pure and does not need further purification.

It should be noted that when comparing the labor and energy costs during the preparation of catalysts synthesized by different methods, the catalysts obtained by combustion methods rightfully take the lead. This is because they have several advantages over traditional methods. For instance, in the preparation of catalysts by the impregnation method, it is necessary to separately pre-dry the carrier in the form of aluminum oxide at 110 °C for 2–8 h in a muffle furnace, whereas this step is not required in combustion methods. Even this single difference implies a significantly lower consumption of electricity, thereby reducing the final cost of the ready catalyst. Additionally, the catalyst synthesis process by the impregnation method involves calcining the initial solutions applied to the aluminum oxide at 500 °C for 4–6 h continuously, while, for the synthesis by the combustion method, it takes only 1–2 min to complete the process, and the samples cool relatively quickly, significantly shortening the entire catalyst preparation path. These factors significantly influence the final cost of the catalysts in favor of combustion methods.

In early studies by the authors of [29], a comparative analysis of the activity of catalysts prepared by the SHS and impregnation methods was also carried out, proving the advantage of the former. Thus, in the process of reforming methanol with carbon dioxide, SHS catalysts demonstrate greater activity in the yield and a greater selectivity toward hydrogen when the process is carried out in the presence of water vapor. This and other studies further confirm the effectiveness of this method of preparing catalysts for reforming processes.

3. Materials and Methods

Catalysts consisting of Cu, Ce, and Al were synthesized using the solution combustion synthesis method. In the process of the catalyst preparation, pre-determined quantities of nitrate salts were employed: Cu(NO₃)₂ × 6H₂O (98–99%, Sigma, Aldrich, St. Louis, MI, USA), Ce(NO₃)₃ × 6H₂O (98–99%, Sigma, Aldrich), Al(NO₃)₃ × 9H₂O (98–99%, Sigma, Aldrich), and glycine (98%, Oxford lab fine Chem LLP, Vasai East, India). The salts were initially ground in an agate mortar before being combined in a porcelain cup. Then, the nitrate mixture was diluted with 10 mL of water, and the resulting solution was mixed at room temperature in the open air. It took only a few minutes until the salts were completely dissolved in water during the process. During the preparation for the synthesis of the catalyst, the muffle furnace was heated up to 500 °C beforehand, which was selected as optimal for the process. The ready-made blend was removed from a porcelain cup to a glass beaker capable of withstanding high temperatures and with a 200 mL volume, and it then was put inside the preheated muffle furnace. Approximately 2–3 min later, while the door of the muffle furnace was not completely open, the combustion process occurring within the solution was observable and resulted in the mixture boiling over the edges of the glass, where it could be seen and caught. In order to enhance the characteristics of the combustion process itself, glycine (50 wt.%) was thrown onto the initial composition of the synthesized catalyst. The inclusion of glycine in the catalyst caused the solution to change to a dark-green/brown color in the middle of the combustion. Then, the glass with a burned mixture was cooled at room temperature in air, and the ready-made catalyst was put into a glass jug.

Using the traditional impregnation-by-moisture method, the other series of catalysts were prepared. Analytically pure copper nitrate hexahydrate (Cu(NO₃)₂ × 6H₂O) (98–99%, Sigma, Aldrich) and cerium nitrate hexahydrate (Ce(NO₃)₃ × 6H₂O) (98–99%, Sigma, Aldrich) were used for preparing the catalysts. These samples of catalysts were obtained by using the same quantity of this technique. The Cu(NO₃)₂ × 6H₂O and Ce(NO₃)₃ × 6H₂O samples were weighed on an electronic balance, and a needed precursor solution of the active metal element component was prepared, with the concentration controlled by adjusting the molar ratio of Cu to Ce. In addition, the activated alumina was pre-treated and then put into a conical flask, and then the previously prepared nitrate solution was also poured in until it was completely soaked into the activated alumina. The conical flask was placed in a water bath shaker and shaken for 2 h, with the water bath temperature maintained at 25 °C. The support material was dried at 110 °C for 8 h. The catalyst was roasted in a muffle furnace at a controlled roasting temperature of 500 °C for 6 h.

The SHS catalysts were from the same material composition. Cylindrical specimens, 10 mm in diameter and 20 mm long, were obtained using uniaxial pressing under a pressure of approximately 10 MPa. The starting reaction blend was preheated in the muffle furnace at a temperature of 500–700 °C for 3–5 min while initiating the SHS reaction.

The temperature profiles were recorded under the combustion synthesis of the mentioned catalysts in a muffle furnace preheated up to 500 °C. We set three thermocouples at the top of the muffle furnace, and all of them were put into the glass with the solution. These thermocouples monitored the lower, middle, and upper layers of the glass. During the catalyst preparation by the SCS method, two combustion types were carried out: (1) volumetric explosion, and (2) self-propagating options. During the volumetric-explosion mode, the solution heats up, causing water to evaporate. When the H₂O evaporates, a gel forms. The furnace temperature steadily rises until it reaches a critical point, and then the exothermic reaction occurs across the entirety of the catalyst volume.

The atomic structures of the catalysts were analyzed using X-ray diffraction measurements conducted on a Siemens Spellman DF3 spectrometer (Munich, Germany) with Cu-Kα radiation. A 10% KCl solution was introduced to the samples as an internal standard to facilitate the semi-quantitative XRD analysis. Brunauer–Emmett–Teller (BET) specific surface area analysis was carried out on a GAPP V-Sorb 2800 Analyzer (Gold APP Instruments, Xi’an, China) using nitrogen as the carrier gas. During the solution combustion synthesis process, nanopowders were produced and their porosities were subsequently determined. The materials’ microstructure was analyzed following spatter coating with gold (with a coating thickness of 5–10 nm) using a scanning electron microscope (Quanta Inspect from FEI, Hillsboro, OR, USA), along with point EDX elemental analysis.

Investigations into the steam reforming of methanol to hydrogen and carbon oxides were conducted on a flow-type installation at atmospheric pressure in a tubular quartz reactor with a fixed catalyst bed without any pre-reduction. Catalysts were put into the middle part of the reactor, and quartz wool was put above and below the catalyst bed. The catalytic reaction was conducted in a temperature range between 250 and 650 °C using a mixture of CH₃OH:H₂O in a ratio of 2.5:1 as the feed. It should be noted that by-products were detected in negligible quantities for most catalysts, indicating a very high level of selectivity towards H₂.

The analysis of the starting blend and products of reaction was conducted by using a chromatograph “Chromos GC-1000” with “Unichrom v. 5.1.15.265” software (Moscow, Russia), as well as on a chromatograph “Agilent Technologies 6890N” (Santa Clara, CA, USA) with “OpenLab ChemStation LTS 01.11” software. Chromatograph “Chromos GC-1000” has both packed and capillary columns in it. The packed column was utilized to analyze hydrogen, oxygen, nitrogen, methane, ethane, ethylene, hydrocarbons with a carbon chain (C₃–C₄), carbon monoxide, and carbon dioxide, whereas the capillary column was employed for hydrocarbon analysis. The temperature of the TC detector was set to 200 °C, the temperature of the evaporator to 280 °C, and the temperature of the columns to 40 °C. The carrier gas (Ar) flow rate was set to 10 mL/min. A HP-PLOT Q capillary column (Agilent) with a length of 30 m and a diameter of 0.53 mm filled with polystyrene–divinylbenzene was employed for analysis on an “Agilent Technologies 6890N” chromatograph.

The chromatographic peaks were determined using calibration curves plotted for the respective products with “Chromos” software. For this, accurately measured quantities of pure substances or mixtures with known concentrations were injected into the chromatograph using a microsyringe. Based on the measured areas of the peaks, corresponding to the amounts of the introduced substance, a calibration curve (V = f(S)) was constructed, where V is the amount of substance in milliliters and S is the peak area in square centimeters. The concentrations of the resulting products were defined from these calibration curves. The balance of the reference substances and products was maintained within ±3.0%.

4. Conclusions

The catalysts obtained by using the solution combustion synthesis method based on Cu(NO₃)₂-Ce(NO₃)_3-Al(NO₃)₃ systems in different ratios of components were examined in the methanol-steam-reforming (MSR) process, which flows by its conversion reactions. Analyzing the catalysts through the XRD, SEM, TPD, and BET methods yielded valuable insights into their catalytic activity. The impact of the content of the starting compounds on the formation of oxides and spinels, which demonstrated high activity in the methanol-steam-reforming process, was fixed. The catalyst with the 50% content of Ce(NO₃)₃ at T—450–550 °C and P—0.27 bar showed the best results. These conditions are very suitable for the industry. Catalysts prepared by the SCS method offer advantages compared to catalysts prepared using the traditional impregnation and SHS methods in methanol steam reforming. SCS is a rapid synthesis method, completing the catalyst production within minutes. It is cost-effective, utilizing exothermic reactions to create spinels at lower temperatures. The process allows for the synthesis of spinels by forming oxides from nitrates with a high defect structure, enabling reactions at significantly lower temperatures compared to oxides with well-formed crystal lattices. Due to the exothermic nature of SCS, the temperatures exceed 1000 °C, resulting in a fast reaction rate and the formation of a defective crystal lattice, which is highly active in catalysis. The abovementioned conditions are likewise ideal for adjusting the crystalline structure of spinels and enhancing the catalysts’ activity.