With the development of human society, massive emissions of CO
2 from fossil fuel combustion cause serious pollution on the environment and pose a threat to human health [
1]. A post-combustion capture (PCC) technology including chemical and physical absorption methods can capture effectively CO
2 from flue gas [
2], but a chemical absorption method using organic amine (such as alkanolamine and ethanolamine) now faces the dilemma of equipment corrosion, high capital cost, and solvent consumption [
3]; thus, a physical absorption method using carbon-based porous materials, such as activated carbons (ACs), has been extensively explored, to remove CO
2 from flue gas, due to its advantages of being low in cost, highly efficient, and environmentally sustainable [
4]. In addition, the coal as raw materials, instead of traditional biomasses and some wastes, can meet the industrial requirements.
At present, the preparation of coal-based ACs with a desirable physicochemical structure is the key factor to realizing high efficiency of CO
2 adsorption. First, an excellent CO
2 capture is related to the micropores quantity of ACs. According to the theory of micropore filling [
5], the gas molecules within the micropore presented an adsorbed state at lower pressure. When pore size was small, the adsorption potential energy within the pores was enhanced by the superposition of van der Waals force on the pore wall, resulting in the increase of CO
2 adsorption. Yi et al. [
6] studied CO
2 adsorption performance of carbon materials with three kinds of pore structures (hierarchical pore, ultra-micropore, and ordered mesoporous) at low pressure. Correlation analysis indicated that the adsorption properties of materials depended entirely on the number of micropores. Zhao et al. [
7] proved that the diffusion rate of CO
2 molecules in the ultra-micropore with a pore diameter of about 0.5 nm was the highest by simulation and calculation method. Wickramaratne [
8] and Presser et al. [
9] found that the super-micropore with a pore diameter of between 0.5 and 0.8 nm had a significant effect on increasing CO
2 adsorption. The contents of nitrogen-containing functional groups also have an important impact on CO
2 adsorption of ACs. Hohm et al. [
10] believed that the interaction between CO
2 molecules with strong quadrupole moments and polar nitrogen-containing functional groups promoted CO
2 adsorption of ACs. Khoerunnisa [
11] and Gong et al. [
12] found that the electronegativity of nitrogen atoms was stronger than that of carbon atoms; thus, the charge density surrounding carbon atoms was increased by the incorporation of nitrogen atoms, which strengthened CO
2 adsorption capacity. Wu et al. [
13] prepared nitrogen-doped mesoporous carbon in situ by introducing diazonitrile amine. The nitrogen content of mesoporous carbon was as high as 13.1 wt.%, and N mainly existed in the form of pyrrole and Ph-NH
2; this nitrogen-doped mesoporous carbon presented excellent CO
2 adsorption (3.2 mmol/g) and selectivity at room temperature and atmospheric pressure. Wilke et al. [
14] found that N-doped carbon materials with a graded sponge shape introducing SiO
2 nanoparticles into melamine resin presented high CO
2 selectivity in CO
2/CH
4 mixture. Finally, in terms of the preparation of AC, traditional physical activation using H
2O, O
2, and CO
2, or their mixtures, displays the characteristics of simplicity, environmental friendliness, and economy. Some researchers found that ammonia as an activation agent at a high temperature could realize simultaneously the incorporation of nitrogen atoms and pores development. Zhai et al. [
15] used cellulose as raw material to prepared porous carbon materials with a nitrogen content of 10.43%, under ammonia activation conditions. Jin et al. [
16] used resorcinol formaldehyde (RF) xerogel as raw material to prepare the different nitrogen-doped porous carbon materials under NH
3 and NH
3/N
2 mixtures at a high temperature. Kim et al. [
17] have prepared the nitrogen-doped mesoporous carbon with 8% nitrogen content in ammonia/nitrogen (volume ratio 1:1) mixture at 80 °C. However, the pore development follows a branched model during physical activation [
18,
19,
20,
21]; more concretely, the micropore is formed on the surface of particles in the initial stage of activation, and then the successive diffusion of the activated agent from the surface to the core helps the formation of a new micropore; meanwhile, the formation of mesopores and macropores originates from the enlargement of the former micropore. This development process inevitably leads to a low specific surface area (S
BET), even in various activation conditions (such as activation temperature, activation time, activated gas species, and so forth). After physical activation, the pore size of carbon materials can be adjusted further by chemical vapor deposition (CVD), using organic carbon source (such as benzene, toluene, methane, acetylene, ethylene, or ethanol) [
22,
23,
24]. Cansado et al. [
25] used the benzene as carbon source to adjust the micropore size of asphalt-based activated carbon fibers by CVD method. The obtained carbon molecular sieves (CMS) with 93% microporosity had a good screening ability for CO
2/CH
4. Kang et al. [
26] found that benzene vapor as a carbon source was the most favorable to adjust pore size at 700~800 °C. However, the application of benzene as an organic carbon source is limited because of its high toxicity, volatility, and relatively high price [
27]. Therefore, methane, as a clean gas, with the advantages of nontoxicity, low cost, and abundant sources, has gradually attracted people’s attention. Villar-Rodil et al. [
28] found that the preparation of CMS by using methane as a carbon source could accurately adjust the pore size to promote CO
2 adsorption and separation.
In this paper, the effects of ammonia activation and subsequent CVD on the physicochemical structure of ACs in the whole preparation process were investigated. First, coal was activated under different volume ratios of ammonia and nitrogen, at 900 °C, to introduce the nitrogen-containing functional groups and form the original pores. After that, the further adjustment of pore size could be performed by CVD, using methane as a carbon source, at 900 °C, for different times. In addition, the results of physicochemical structure of all samples were measured by a D/max-rb X-ray diffractometer (XRD), Raman spectroscopy, nitrogen adsorption, X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscope (HRTEM), and scanning electron microscope (SEM). Finally, in order to verify the application potentials of ACs samples with ideal physicochemical structure, a CO2 adsorption test was carried out by gas adsorption instrument.