Global warming, resulting from the emission of greenhouse gases, has been a concern for several years. CO₂ represents more than 60% of these gases, with an emission rate above 33 Gt/year. In the 1990s, the growth in CO₂ emissions from fossil energy sources was around 0.9%/year. In the 2000s, the growth rate jumped more than three times to around 3%/year, decreasing again to 0.9%/year from 2010 onward (A (org.) (2020). Global, 2020). Although emission reductions in the United States and EU have been observed, emerging countries drive the growth of this rate. Investment in low-carbon technologies must go hand in hand, with the implementation of public policies aimed at the progressive reduction until the elimination of the use of fossil fuels (Peters et al., 2020).

Graphene is a carbon material; in fact, it is a single layer of graphite. Despite being studied for many years, interest in graphene has been growing in recent years, as it has very interesting physical, chemical, and mechanical properties. In 1940, a series of theoretical analyses suggested that a single layer of graphite, if isolated, could have particularly useful electrical characteristics, such as high conductivity (Dai et al., 2012). It is possible to obtain graphene by two different routes: top-down, the method to designate the preparation of graphene from graphite; bottom-up, which refers to obtaining graphene from the synthesis, starting from varied carbon sources (Huang et al., 2012). Pure graphene can be obtained by mechanical exfoliation of graphite (Novoselov et al., 2005). However, the most used method is the oxidation of graphite followed by its exfoliation (Huang et al., 2011). This work will focus on a specific bottom-up method: magnesiothermic reaction.

The ability of magnesium to reduce CO₂ by forming graphitic structures (Luo et al., 2013) is known in the literature. There are some problems when the reaction is considered, the most important of which is the usual low yield and quality of graphene. In an attempt to eliminate these two drawbacks, an auxiliary metal could be a solution. By its physicochemical properties, zinc seemed to be adequate to improve yield and could also improve graphene quality (Luo et al., 2013).

The most used method for CO₂ separation in post-combustion processes is the use of solvents, mainly amine-based (Ben-Mansour et al., 2016). The advantages are the high efficiency for CO₂ capture (almost 98%), requirement of low gas pressure, and the possibility of recovery after use. There are disadvantages such as equipment corrosion, high cost of solvent regeneration, and the toxicity of amine-based solvents (Sevilla and Fuertes, 2011). To eliminate the corrosion problem and to reduce costs of regeneration, solid materials were studied, such as zeolites, activated carbons, alumina, and hollow fibers (Lee and Park, 2015; Bakhtyari et al., 2020), to name just a few.

Porous carbon materials are quite promising in CO₂ capture and could be an option to amine solutions (Lee and Park, 2015; Bakhtyari et al., 2020). Porous carbons have some characteristics that could be of interest in the area of CO₂ adsorption, such as thermal and chemical stabilities and large surface areas (Wang et al., 2012). Studies show that the adsorption of CO₂ in porous materials is more efficient when subjected to an environment with high pressure when the material’s pores are larger than 1 nm. Materials with smaller pores favor the adsorption of carbon dioxide at 1 atm (Lee and Park, 2015). Therefore, the graphene produced here, in the absence and in the presence of Zn, was tested in CO₂ capture at 1 atm pressure and compared with commercial graphene under the same reaction conditions.

Figure 1 shows the graphene x-ray diffractograms of samples prepared in this work, in the absence of Zn (Figure 1A) and with increasing concentrations of the second metal (Figure 1B–H). Two characteristic peaks can be observed at 26.0 and 43.5° 2θ, related to the 002 and 100 diffractions (Cai et al., 2014), respectively. The weak halo between 20 and 30° 2θ is the result of a certain extension of material disorganization.

Raman spectroscopy was used to determine the graphenization index. This is the intensity ratio between the two characteristic bands of graphitic materials, the D band and the G band, that is the I_D/I_G index, presented in Figure 2. The D band, at 1,346 cm⁻¹, corresponds to the bonds between sp (Dai et al., 2012) carbon atoms in the material, indicating the number of defects present in the structure of the material obtained, while the G band, at 1,579 cm⁻¹, represents the characteristic bonds of graphene, between sp (Peters et al., 2020) carbon atoms. There is also a third band, called G’ or 2D (since it is approximately twice the frequency of the D band), which indicates the number of layers in the material, when it is not aggregated (Tian et al., 2016). Table 1 shows the values of I_D/I_G indexes and the standard deviation which provides the homogeneity of the solid: the lower the standard deviation, the more homogeneous the solid is. It is important to note that the measured graphenization index is close to one, or, in other words, the solids display a close concentration of C–C sp (Peters et al., 2020) bonds and C–C sp (Dai et al., 2012) defect. Only the samples prepared with Zn/Mg M ratios 3 and 4 present different graphenization indexes, being considered the best samples.

Another important graphene characteristic is the porosity. Through the analysis of the N₂ adsorption isotherm, as shown in Figure 3 (and Supplementary Material), pore volumes were obtained and are presented in Table 1. It is possible to see a large increase in the pore volume when zinc was added to the reaction: from 1.27 cm³ g⁻¹ in Zn/Mg = 0 to 1.64 cm³ g⁻¹ at Zn/Mg = 4, and decreasing again when Zn/Mg > 4.

The adsorption isotherm of the sample Zn/Mg = 0, as shown in Figure 3, is a mixture of two types of isotherms: Type I(a) and Type IV(a).

Type I(a) isotherm presents adsorption of N₂ at very low P/Po; it is related to the formation of a single layer of adsorbate on the solid surface. This is due to the adsorption on micropores. Type IV(a) isotherms are characteristics of mesoporous adsorbents where capillary condensation is accompanied by hysteresis. This is because the pore size exceeds the critical limit, which for adsorption of N₂ at cryogenic temperatures is approximately 4 nm (Dreyer et al., 2010).

The desorption hysteresis of this material is Type H2(b) and is associated with cylindrical pores with necks whose diameter is smaller than the diameter of the cylinders, with a wide distribution of the pore neck width, through which N₂ is released into the atmosphere (Dreyer et al., 2010).

The reaction yield, also presented in Table 1, shows us a trend of increased yields up to Zn/Mg = 3, decreasing after that. When only Mg was used to obtain graphene, the reaction yield was about 40%, increasing as the Zn/Mg ratios increase and reaching a maximum value of about 86% when the Zn/Mg M ratio was 3, at Zn/Mg > 3, decreased yields are observed. It is possible to conclude that the presence of zinc increases yield, more than duplicating it, but is deleterious at larger Zn/Mg M ratios.

Figure 4 shows the results of CO₂ adsorption in the materials prepared by the method discussed here as compared with the performance of commercial graphene. It is clear that commercial graphene, as shown in Figure 4A, adsorbs very small amounts of CO₂ under 1 atm: 0.02 mmol g⁻¹ at 50°C. When graphene is prepared by the metallothermal method, using only magnesium, the performance changes considerably. The solid adsorbs CO₂ in the three temperatures tested, being more efficient at the lowest temperature tested, 25°C. The values of adsorbed capacities for each temperature and material are shown in Table 2. When Zn is also present during graphene synthesis, an even better performance is observed, still at low temperatures. In this case, the material behaves very differently in the three temperatures. Another point to call attention is the fact that under certain conditions, not all CO₂ adsorbed is released.

The sample prepared with Zn/Mg M ratio 3 was characterized by AFM, as shown in Figure 5. Figure 5A shows the potential difference at various points in the sample: the green area of the image is the potential applied by the AFM tip when in contact with the sample holder; the small red dots correspond to positions where less conductive materials are deposited: the darker blue areas are those where the potential drop is greater, resulting in regions with larger conductivities in the sample. This potential drop is not uniform due to the large porosity presented by the material. The topography collected at the AFM shows the particle size range as 1.2–0.3 µm, as shown in Figura 5B, but there are also smaller particles in the range from 4 to 6 nm, as shown in Figure 5C.

CO₂ can be reduced with Mg to graphene under certain reaction conditions. The solid obtained is more porous than commercial graphene and displays a graphenization index that suggests that it contains defects in the form of sp₃ C atoms. The addition of Zn during graphene synthesis increases porosity even more, as well as the yield in graphene; if its concentration remains within a certain range, larger or smaller concentrations of Zn do not improve the porosity or yield. Metallothermal graphene can adsorb CO₂ at temperatures as low as ambient; the desorption of the gas indicates that there may be two adsorption processes, one of them is physical and can be reverted at room temperature and the other bears characteristics of chemisorption, demanding higher temperatures for desorption.