Carbon Deposition Characteristics in Thermal Conversion of Methane for Sustainable Fuel

Abstract

Low-carbon powertrains and sustainable fuels are closely linked as they both aim to reduce carbon emissions and transition away from reliance on fossil fuels. The methane from biogas, biomass, and organic waste can serve as an alternative energy source to traditional fossil fuels. The process of obtaining sustainable fuel (e.g., hydrogen and syngas) from methane is commonly confronted with the problems of carbon deposition on metal oxide. The study of carbon deposition characteristics during methane thermal conversion processes is particularly crucial for low-carbon powertrains. Herein, the carbon deposition on CoAl₂O₄ and strongly alkali-etched CoAl₂O₄ (CoAl^vO₄) spinel oxides from the CH₄ stage was investigated. We demonstrate that reaction time, calcination temperature, and reaction temperature have no effect on the compositions of carbon deposition, and the material itself plays a crucial role in carbon deposition. The graphitization degree for CoAl^vO₄ is lower than that for CoAl₂O₄. The strong alkali etching in CoAl₂O₄ only affects contents in different composition carbon deposition. This is mainly attributed to the introduction of Al³⁺ vacancies by alkali etching, which efficiently tunes the surface electronic structure in CoAl₂O₄. These findings guide designing efficient and clean low-carbon powertrains, especially in the development of removal carbon deposition technologies and catalysts.

1. Introduction

Low-carbon powertrains are essential for sustainable development and minimizing carbon emissions [1], which play a pivotal role in shaping a more environmentally conscious future. The utilization of sustainable fuels represents a significant step towards enhancing eco-friendliness and operational efficiency across various sectors [2]. The transition to sustainable fuels not only supports the shift towards low-carbon powertrains but also contributes to global endeavors aimed at achieving climate goals through the substantial reduction in greenhouse gas emissions originating from conventional fossil fuels [3]. The energy demand is expected to be met largely by sustainable energy and natural gas [4,5].

Methane derived from biogas, biomass, and organic waste [6], along with sustainable fuels such as hydrogen and syngas extracted from methane, are increasingly being adopted across diverse industries, signaling a paradigm shift towards a more sustainable and environmentally responsible energy landscape. Hydrogen is widely recognized as a sustainable and clean energy carrier, prized for its high energy density and emission of only water upon utilization [7,8]. Meanwhile, syngas serves as a crucial chemical intermediate, playing an indispensable role across various industries. It can be transformed into high-value-added products such as alcohols and aldehydes, as well as fuels including olefins, waxes, and diesel, among others [9,10].

The methane thermal conversion process is commonly confronted with the problems of carbon deposition on metal oxide due to the reaction of methane decomposition (1). Carbon formation over solid catalysts is mainly due to the metal-catalyzed decomposition of these feedstocks at high temperatures [11] and involves hydrogen transfer, dehydrogenation, and polycondensation steps [12]. As a non-desorbed product, carbon is located in the pores and on the external surface, thus covering active sites and blocking pores, resulting in catalyst deactivation [12,13,14]. Noble metal catalysts show excellent activities with minimal carbon deposition. However, they have limited application value of sustainable practices due to their high cost and low abundance [15]. As an alternative, spinel oxides containing Co element are known for high methane conversion ability due to the high oxygen equilibrium partial pressure of Co [16,17]; however, the deactivation catalysts due to carbon deposition still pose challenges for industrialization [18].

CH₄ → C + 2H₂

(R1)

The effective regeneration of catalysts is critical for sustainable development, thus achieving the stability of catalysts after repeated reaction–regeneration cycles. Therefore, the elimination of carbon deposition is necessary to restore catalyst activity and to ensure continuous operation in industrial processes. Currently, the commonly used methods for eliminating carbon deposits include using oxidation in Air/O₂, gasification in CO₂/H₂O, and so on [12,14,19,20]. The elimination of carbon with air combustion is often the preferred and most practical route. However, this process produces a large amount of carbon dioxide (CO₂) emissions [12,21], and high exothermicity can easily cause overheating, leading to metal sintering and catalyst deactivation.

To meet the concept of “green carbon science” and make full use of carbon deposition, carbon can be significantly eliminated by gasification using H₂O and CO₂. Synthesis gas, instead of CO₂, is the main product in these processes [22]. Compared to steam gasification, CO₂ gasification has several advantages. Unlike steam, CO₂ is a gas and thus requires no vaporization before gasification. CO₂, as an important greenhouse gas [23], can be utilized to produce additional CO by reacting with carbon (C + CO₂ = 2CO) [20], thus realizing the resource utilization of CO₂, and the generated CO can be used to adjust the H₂/CO ratio of syngas or be utilized independently for other processes [24]. In addition, steam can attack the Al-O bond of the catalyst at high temperatures, which leads to the collapse of the catalyst framework [12]. This does not occur in CO₂ gasification. The regeneration of catalysts via gasification in CO₂ is an ideal process for the development of a sustainable economy. There must be a relationship between the effectiveness of the regeneration of catalysts and the catalyst’s nature, types of carbon deposition, and so on [18]. Usually, the main forms of carbon are amorphous carbon, graphitic carbon, and filamentous carbon [25]. Thus, the carbon deposition characteristics of Co-based spinel oxides are worthy of consideration.

In this work, the carbon deposition on CoAl₂O₄ and strongly alkali-etched CoAl₂O₄ (CoAl^vO₄) spinel oxides from the CH₄ stage were investigated. We examine the impact of various factors (reaction time, calcination temperature of metal oxide, and reaction temperature) on the composition of carbon deposition. The compositions of carbon deposition are differentiated via fixed-bed in situ oxidation of CO₂ and air as well as CO₂/O₂-TG methods for eliminating carbon deposition. We demonstrate that reaction time, calcination temperature, and reaction temperature have no effect on the compositions of carbon deposition, and the material itself plays a crucial role in carbon deposition. This is mainly attributed to the introduction of Al³⁺ vacancies by alkali etching, which efficiently tunes the surface electronic structure in CoAl₂O₄.

2. Materials and Methods

2.1. Hydrothermal Synthesis of CoAl Spinel Oxides

The Co-Al spinel oxides were synthesized via a typical hydrothermal method followed by a thermal annealing process. The solution was formulated by mixing 0.01 mol Co(NO₃)₂·6H₂O, Al(NO₃)₂·9H₂O (99.5%, Sinopharm, Shanghai, China) (Co/Al = 1/2 for CoAl₂O₄), and 0.05 mol urea into 60 mL of deionized water and stirring. The obtained solution was poured into a stainless-steel Teflonlined autoclave (100 mL) followed by reacting in a preheated oven at 120 °C for 12 h. The precipitate was washed with deionized water three times and then dried overnight at 60 °C.

Synthesis of CoAl₂O₄: The resulting solids powder was annealed in air at 750 °C for 2 h at a heating rate of 5 °C min⁻¹.

Synthesis of CoAl^vO₄ (v refers to cationic vacancies): An amount of 1 g of as-synthesized powder was soaked in 40 mL of 6.0 M NaOH at 60 °C for 60 min. The etched products were washed with deionized water several times and dried overnight at 60 °C. The resulting solids powder was annealed in air at 750 °C for 2 h at a heating rate of 5 °C min⁻¹.

The effect of different factors on the production of carbon deposition was examined, with materials designated according to the metal oxide–calcination temperature–reaction temperature. For instance, CoAl₂O₄−750−700 refers to the metal oxide CoAl₂O₄, which was calcined in air at 750 °C and then underwent reduction reactions at 700 °C. The parameters varied include the following:

(1)

Reaction time: Reaction durations of 20 and 4 min were selected, with the corresponding samples labeled as CoAl₂O₄−20 and CoAl₂O₄−4, respectively.

(2)

Calcination temperature of metal oxide: Temperatures of 700 and 800 °C were selected, and the corresponding samples were denoted as CoAl₂O₄−700−700, CoAl^vO₄−700−700 and CoAl₂O₄−800−700, CoAl^vO₄−800−700, respectively.

(3)

Reaction temperature: Temperatures of 700 and 800 °C were selected, and the corresponding samples were denoted as CoAl₂O₄−750−700, CoAl^vO₄−750−700 and CoAl₂O₄−750−800, CoAl^vO₄−750−800, respectively.

2.2. Physical and Chemical Characterization

XRD (X-ray diffraction) patterns were acquired over a 2θ angular range of 20 to 90 degrees, employing a SmartLab 9 KW diffractometer (Rigaku, Tokyo, Japan) that utilized Cu Kα radiation source with a wavelength of λ = 1.5418 Å. Raman spectroscopy was collected on a HORIBA EVOLUTION Raman spectrometer (HORIBA, Kyoto, Japan) with a 532 nm laser excitation source. The morphologies of the samples were characterized by JEM-2100F transmission electron microscopy (TEM), Japan Electron Optics Laboratory Co., Ltd., Akishima, Tokyo, Japan. The carbon deposition on metal oxide was analyzed using thermogravimetric analysis (TGA). The sample (10 mg) was placed in an alumina crucible. The temperature was then raised from room temperature to 800 °C at a rate of 10 °C min⁻¹ under air/CO₂ flow (50 mL min⁻¹). The sample remained heated at 800 °C for 10 min until no weight change was detected.

2.3. Reactivity Tests

The formation and elimination of carbon on metal oxide was carried out in a lab-scale fixed bed reactor. The 0.2 g samples (particle size range of 20–40 mesh) were placed in a quartz tube and heated to 700 °C with a heating rate of 10 °C min⁻¹ under an N₂ atmosphere flowing at a rate of 160 mL min⁻¹. During the experiments, a changeover between 10 ± 0.5% CH₄/N₂, 10 ± 0.5% CO₂/N_2, and synthetic air gas streams was executed, maintaining the same flow rate of 160 mL min⁻¹. The product gases were analyzed by an online gas analyzer (MRU Vario Plus, Norwegian Subsea, Hovfaret 8, Oslo, Norway).

3. Results and Discussion

3.1. Effect of Different Factors on the Production of Carbon Deposition

3.1.1. Reaction Time

The compositions of carbon deposition formed during the methane conversion process are differentiated via subsequent in situ reaction of CO₂–air oxidation for eliminating carbon deposition.

At various methane reaction times, the gas product distribution during the reduction–oxidation reaction of CoAl₂O₄ with CH₄−CO₂−O₂ is shown in Figure 1. In the CO₂ oxidation stage, the trend of the CO₂ conversion into CO was observed on both CoAl₂O₄−20 and CoAl₂O₄−4. During the air oxidation process, a certain concentration of CO₂ was detected on CoAl₂O₄−20 and CoAl₂O₄−4 (Figure 1a,b), which suggests that some carbon deposition on CoAl₂O₄ is resistant to CO₂ oxidation and necessitate further oxidation in air.

The CO yield decreased from 54.78 mmol g⁻¹ in CoAl₂O₄−20 to 6.41 mmol g⁻¹ in CoAl₂O₄−4 during the CO₂ oxidation processes (Figure 1c), and similar declining trends were observed on CoAl₂O₄−20 and CoAl₂O₄−4 in air (Figure 1d), revealing that reaction time has a minimal effect on the compositions of carbon deposition on CoAl₂O₄, and has an effect on the amount of carbon.

3.1.2. Calcination Temperature of Metal Oxide

The crystal structures of CoAl₂O₄ and CoAl^vO₄ metal oxides calcined at different temperatures (700, 750, and 800 °C) were characterized and analyzed via XRD patterns, as illustrated in Figure 2.

For CoAl₂O₄, the diffraction peaks at 31.2, 36.7, 44.7, 55.5, 59.2, and 65.0° are assigned to (220), (311), (400), (422), (511), and (440) planes with d values of 0.29, 0.24, 0.20, 0.17, 0.160, and 0.14 nm, respectively. The diffraction peaks for CoAl₂O₄−700, CoAl₂O₄−750, and CoAl₂O₄−800 consistently maintain the same crystal structure after different calcination temperatures (Figure 2a). Similarly, the diffraction peaks in CoAl^vO₄−700, CoAl^vO₄−750, and CoAl^vO₄−800 agreed well with CoAl₂O₄, and no additional crystal phases were observed (Figure 2b). Obviously, the XRD patterns of all the CoAl^vO₄ agreed well with CoAl₂O₄ (PDF #44-0160). These results demonstrate that calcination temperature does not affect the crystal structures of CoAl₂O₄ and CoAl^vO₄.

At various calcination temperatures (700 and 800 °C) of metal oxides, the gas product distribution during the reduction–oxidation reaction of CoAl₂O₄ and CoAl^vO₄ with CH₄−CO₂–air is shown in Figure 3.

The elimination process of carbon deposition on CoAl₂O₄ and CoAl^vO₄ metal oxides with calcination temperatures at 700 °C is shown in Figure 3a,b. In the CO₂ oxidation stage, similar trends were observed on both CoAl₂O₄−700−700 and CoAl^vO₄−700−700 metal oxides for the CO₂ conversion into CO. However, notable differences emerged in the air oxidation process. A certain concentration of CO₂ was detected on CoAl₂O₄−700−700, whereas CoAl^vO₄−700−700 exhibits an extremely low CO₂ concentration, suggesting that carbon deposition on CoAl^vO₄−700−700 is almost completely removed during the CO₂ oxidation process. The trends observed for CoAl₂O₄−800−700 and CoAl^vO₄−800−700 agree with those recorded on CoAl₂O₄−700−700 and CoAl^vO₄−700−700 (Figure 3c,d).

The elimination process of carbon deposition on CoAl₂O₄ metal oxides with calcination temperatures at 700 and 800 °C is shown in Figure 3a,c. In the CO₂ oxidation stage, the phenomenon of the CO₂ conversion into CO was observed on both CoAl₂O₄−700−700 and CoAl₂O₄−800−700 metal oxides, and a certain concentration of CO₂ was detected on CoAl₂O₄−700−700 and CoAl₂O₄−800−700, showing that calcination temperature of metal oxide has a minor effect on carbon deposition. These results show that the calcination temperature of metal oxides has no effect on the compositions of carbon deposition, and the material itself plays a crucial role in influencing carbon deposition.

The CO and CO₂ yield during CO₂ and air oxidation processes are illustrated in Figure 4. During the CO₂ oxidation process, the CO yield increased from 12.46 mmol g⁻¹ in CoAl₂O₄−700−700 to 14.25 mmol g⁻¹ in CoAl^vO₄−700−700 (Figure 4a), which attributed to the superior methane conversion on CoAl^vO₄−700−700. In contrast to CoAl₂O₄−700−700, both CO and CO₂ yield on CoAl^vO₄−700−700 metal oxide decreases in the air oxidation process. The CO yield decreases from 0.02 mmol g⁻¹ to 0.01 mmol g⁻¹, while the CO₂ yield decreases from 0.41 mmol g⁻¹ to 0.14 mmol g⁻¹ (Figure 4b). The trends in CO and CO₂ yield observed for CoAl₂O₄−800−700 and CoAl^vO₄−800−700 agree with those recorded on CoAl₂O₄−700−700 and CoAl^vO₄−700−700 (Figure 4c,d).

For CO yield obtained at the different calcination temperatures (Figure 4a,c), as the calcination temperature increases from 700 to 800 °C, the CO yield decreased from 12.46 mmol g⁻¹ in CoAl₂O₄−700−700 to 7.98 mmol g⁻¹ in CoAl₂O₄−800−700. CoAl^vO₄−700−700 and CoAl^vO₄−800−700 showed the same trend. The possible reason is that the increase in calcination temperature leads to an increase in catalytic particle size, resulting in decreased activity of methane decomposition.

3.1.3. Reaction Temperature

At various reaction temperatures (700 and 800 °C), the gas product distribution during the reduction–oxidation reaction of CoAl₂O₄ and CoAl^vO₄ with CH₄−CO₂–air is shown in Figure 5.

The elimination process of carbon deposition on CoAl₂O₄ and CoAl^vO₄ metal oxides with reaction temperatures at 700 °C is shown in Figure 5a,b. In the CO₂ oxidation stage, similar trends were observed on both CoAl₂O₄−750−700 and CoAl^vO₄−750−700 for the C→CO. However, in the air oxidation process, a certain concentration of CO₂ was detected on CoAl₂O₄−750−700, whereas a low CO₂ concentration was exhibited on CoAl^vO₄−750−700, suggesting that carbon deposition on CoAl^vO₄−750−700 is almost completely removed during the CO₂ oxidation process. The trends observed for CoAl₂O₄−750−800 and CoAl^vO₄−750−800 agree with those recorded on CoAl₂O₄−750−700 and CoAl^vO₄−750−700 (Figure 5c,d), suggesting that the composition of carbon deposition remains unchanged despite variations in reaction temperature.

The CO and CO₂ yield during CO₂ and air oxidation processes are illustrated in Figure 6. During the CO₂ oxidation process, the CO yield increased from 6.4 mmol g⁻¹ in CoAl₂O₄−750−700 to 8.92 mmol g⁻¹ in CoAl^vO₄−750−700 (Figure 6a). In the air oxidation process, the CO yield decreases from 0.02 mmol g⁻¹ in CoAl₂O₄−750−700 to 0.01 mmol g⁻¹ in CoAl^vO₄−750−700, while the CO₂ yield decreases from 0.41 mmol g⁻¹ to 0.1 mmol g⁻¹ (Figure 6b). The trends in CO and CO₂ yield observed for CoAl₂O₄−750−800 and CoAl^vO₄−750−800 agree with those recorded on CoAl₂O₄−750−700 and CoAl^vO₄−750−700 (Figure 6c,d). However, CoAl₂O₄−750−800 and CoAl^vO₄−750−800, during the CO₂ oxidation process, exhibit CO yield than that of CoAl₂O₄−750−700 and CoAl^vO₄−750−700, indicating that an increase in reaction temperature leads to an increase carbon deposition from CH₄ conversion, resulting in higher CO yields during CO₂ conversion.

3.2. Characterization of Carbon Deposition

3.2.1. Raman Analysis

To further confirm the elimination of carbon deposition in the oxidation process, Raman spectroscopy analyses were performed on CoAl₂O₄−750−700 and CoAl^vO₄−750−700 metal oxides under reduction–oxidation reaction with CH₄−CO₂–air, as shown in Figure 7. The Raman characteristic peaks appeared at ~1350 cm⁻¹ corresponded to the D-band, and ~1580 cm⁻¹ corresponded to the G-band, which represent lattice defects or amorphous carbon and highly ordered graphitic carbon structure, respectively [26].

The Raman spectra of CoAl₂O₄−750−700 and CoAl^vO₄−750−700 metal oxides show obvious differences during the oxidation process of CO₂ and air. For CoAl₂O₄−750−700, D and G peaks appear in the Raman spectrum during CH₄ conversion, but these peaks’ intensity is significantly reduced during the CO₂ oxidation process. After O₂ oxidation, D and G peaks disappear (Figure 7a), indicating that carbon deposition cannot be completely removed in CO₂, which can be completely removed through oxidation in air. For CoAl^vO₄−750−700, it is worth noting that D and G peaks disappear in the Raman spectrum during the CO₂ oxidation process (Figure 7b).

The structure and composition of carbon species on CoAl₂O₄ and CoAl^vO₄ metal oxides were characterized using Raman spectroscopy, as shown in Figure 8. Raman spectroscopy is used to characterize the graphitization degree of carbon materials. The ID/IG ratio is used to assess the degree of graphitization of the carbon deposition on materials [27,28,29]. A lower ID/IG value indicates a higher degree of graphitization of the carbon species [26,27].

Raman spectroscopy showed ID/IG increased from 1.42 in CoAl₂O₄−750−700 to 1.45 in CoAl^vO₄−750−700 (Figure 8a), and similar trends were observed in CoAl₂O₄−700−700 and CoAl^vO₄−700−700 (Figure 8b), which indicates that the graphitization degree for CoAl^vO₄ is lower than that for CoAl₂O₄. The research indicates that a more disordered carbon structure exists in the carbon deposition with ID/IG values greater than 1 [27]. Consequently, the degree of graphitization of the carbon deposition decreases following treatment with strong alkali etching.

3.2.2. Microstructure Characterization

Some carbon nanotubes and carbon nano-onions have been observed on the surface of the CoAl₂O₄−750−700 and CoAl^VO₄−750−700 catalysts after CH₄ conversion at 700 °C, as shown in Figure 9.

The carbon nanotubes are straight and bamboo-like, with a hollow channel and metal particles on the end side (Figure 9a); the metal particles are observed to be trapped in a carbon nanotube cage (Figure 9b). The carbon nano onions (thick layer of graphite encapsulating metal particles) were observed on the CoAl₂O₄−750−700 surface (Figure 9b). In addition to carbon nanotubes and carbon nano-onions (Figure 9d), spiral carbon nanofibers were observed on the CoAl^vO₄−750−700 surface (Figure 9c). Therefore, the difference in carbon deposition behavior may be correlated to the difference in the material itself.

3.2.3. Thermogravimetric (TG) Analysis

To investigate the compositions of carbon deposition on CoAl₂O₄ and CoAl^vO₄ metal oxides, carbon deposition from the methane stage was analyzed using thermogravimetric analysis (TG) in O₂ and CO₂ [30].

The O₂ and CO₂-TGA analyses of CoAl₂O₄ and CoAl^vO₄ metal oxide are illustrated in Figure 10. The TGA curve could be divided into three stages of mass changes: (1) an initial slight weight loss(Yellow area) before 200 °C due to the elimination of absorbed water; (2) an increase in weight (White area) between approximately 200 and 300 °C, attributing to the oxidation of reduced CoAl₂O₄ and CoAl^vO₄ in the air; (3) a sharp decline (Purple area) in weight between approximately 300 and 450 °C, resulting from the removal of carbon deposition.

The weight-loss onset temperature of CoAl^vO₄−750−700 is especially higher than that of CoAl₂O₄−750−700 in O₂ oxidation (Figure 10a); however, this phenomenon is not obvious in CO₂ oxidation (Figure 10b), attributing to the prior restoration reaction of oxygen species in CoAl^vO₄ metal oxide before the carbon removal reaction.

The O₂ and CO₂−DTG analyses of CoAl₂O₄ and CoAl^vO₄ metal oxide are illustrated in Figure 11. In O₂−DTG curves, both CoAl₂O₄ and CoAl^vO₄ metal oxides exhibit a sharp derivative thermogravimetric (DTG) peak (Figure 11a). The maximum weight loss corresponding temperature of CoAl^vO₄−750−700 (447 °C) is higher than that of CoAl₂O₄−750−700 (399 °C).

According to the CO₂−DTG curve (Figure 11b), there were two obvious weightlessness stages of CoAl₂O₄ and CoAl^vO₄, which were located at ~500 and 570 °C, respectively. These results indicated the presence of two carbon compositions in CoAl₂O₄ and CoAl^vO₄. Normally, weight loss below 500 °C is caused by the oxidation of amorphous carbon and/or graphitic carbon fibers, whereas weight loss above 500 °C stems from the combustion of highly ordered graphitic carbon [28,31,32]. The research suggests that the carbon located close to the metal site is more easily removed, while carbon situated on the support requires higher temperatures to be eliminated [9]. For CoAl₂O₄−750−700, the temperature of the maximum rate of weight loss is 567 °C, while the temperature in CoAl^vO₄−750−700 is 523 °C, which indicates a higher content of graphitic carbon fibers in CoAl^vO₄−750−700. Therefore, CoAl₂O₄ by strong alkali etching only affects contents in different compositions of carbon deposition but does not affect compositions of carbon deposition. It is mainly attributed to the cation vacancies produced by alkali etching, which efficiently modulates the surface electronic structure of CoAl₂O₄ [33].

4. Conclusions

In this study, the carbon deposition on CoAl₂O₄ and strongly alkali-etched CoAl₂O₄ (CoAl^vO₄) from the CH₄ stage was investigated. We examine the impact of various factors (reaction time, calcination temperature of metal oxide, and reaction temperature) on the composition of carbon deposition. The compositions of carbon deposition are differentiated via fixed-bed in situ oxidation of CO₂ and air and CO₂/O₂−TG methods for eliminating carbon deposition. The main findings are presented below.

(1)

The composition of carbon deposition over CoAl₂O₄ and CoAl^vO₄ depends on the material itself and reaction time; calcination temperature and reaction temperature do not affect the compositions of carbon deposition.

(2)

In the CO₂ oxidation, similar trends were observed on both CoAl₂O₄ and CoAl^vO₄ for C→CO. However, a certain concentration of CO₂ was detected on CoAl₂O₄ in the air oxidation process, whereas CoAl^vO₄ exhibits an extremely low CO₂ concentration, suggesting that carbon deposition on CoAl^vO₄ is almost completely removed during the CO₂ oxidation process. This trend is also confirmed by Raman spectra.

(3)

Raman spectroscopy showed that ID/IG in CoAl^vO₄ is higher than that of CoAl₂O₄, indicating the decrease in the degree of graphitization of the carbon deposition on CoAl^vO₄. Thermogravimetric analysis showed that the strong alkali etching in CoAl₂O₄ only affects contents in different types of carbon deposition. It is mainly attributed to the cation vacancies produced by alkali etching, which efficiently modulates the surface electronic structure of CoAl₂O₄, thus resulting in differences in carbon deposition on CoAl^vO₄ surface compared to CoAl₂O₄.

Notably, CO₂ can be utilized to produce CO by eliminating carbon deposition on the CoAl spinel oxides, which realizes the utilization of CO₂, and the generated CO can be used to adjust the H₂/CO ratio of syngas or be utilized independently for other processes. Meanwhile, carbon deposition on CoAl^vO₄ can be removed by oxidation with CO₂, avoiding hot spots caused by the oxidation with air, thereby reducing damage to the catalyst.