Coalbed Methane Adsorption on Al-, Si-, P- and S-containing Coal Models Predicted by Density Functional Theory†

doi:10.7503/cjcu20150134

Abstract

Abstract:

Al-, Si-, P- and S-doped graphenes(X-Gr, X=Al, Si, P and S) were used to represent the surface models of coal with the structural heterogeneity. Based on the spin-polarized density-functional theory, coalbed methane(CBM, the main components are CH₄, CO₂ and H₂O) adsorption on X-Gr was investigated and the corresponding adsorption energy, adsorption equilibrium distance, Mulliken charge, charge density differences and density of states were well discussed. The result show that Al- and Si-Gr are sensitive to CBM, whereas CBM adsorptions on P- and S-Gr are found to be physical adsorption. In brief, the interactions between CBM and X-Gr are slightly depend on the orientations of CBM gas molecules and the metallicity of the dopants in Gr. The higher adsorption energy, the higher metallicity of the dopants becomes, namely Al-Gr>Si-Gr>P-Gr>S-Gr. Through systematical and theoretical analysis, we found that CO₂ and H₂O were easy to adsorb on surface models of coal than CH₄. Hence, injection of CO₂ or H₂O in coal seams could enhance CH₄ recovery. In addition, we suggest that Al- and Si-Gr could be a good gas sensors for CBM because of the higher interactions between Al-/Si-Gr and CBM.

Key words: Coalbed methane, Coal model, Doping, Adsorption, Density functional theory

CLC Number:

O641

TrendMD:

HE Xu, LIU Xiaoqiang, HE Bing, WANG Hanguang, LIU Xudong, TIAN Zhiyue, CHU Wei, XUE Ying. Coalbed Methane Adsorption on Al-, Si-, P- and S-containing Coal Models Predicted by Density Functional Theory^†[J]. Chem. J. Chinese Universities, 2015, 36(9): 1786.

Figures/Tables 7

Fig.1 5×5×1 Graphene and X-Gr in calculation(A) Three adsorption sites for Gr, namely, B(the midpoint of a carbon-carbon bond), C(the center of a hexagon) and T(above a carbon atom); (B) the structure of X-Gr, the colorized atom(Al, Si, P, S), h: the distance between the doped atom and Gr plane, (B1) top view, (B2) side view; (C) the adsorption configuration of CH4, CO2, and H2O; (D) the adsorption sites of CH4, CO2, and H2O on X-Gr, d: the distance between the doped atom and Gs.

Fig.2 Optimized partial geometrical structures of X-Gr (A) Al-Gr; (B) Si-Gr; (C) P-Gr; (D) S-Gr.

Table 1 Geometrical parameters of the Gr/X-Gr, including the calculated lattice constants(a), the embossment height(h) of doped atoms, dX—C(X=C, Al, Si, P, S ), formation and cohesive energies of Gr/X-Gr

System	a/nm	h/nm	d_X—C/nm	ΔE_f/(kJ·mol^-1)	ΔE_c/(kJ·mol^-1)
Gr	0.245	0	0.141		834.6
Ref.	0.244^[36]	0^[36]	0.141^[36]		849.1^[39]
Expt.	0.246^[36]	0^[36]	0.142^[36]	-	762.2^[40]
Al-Gr	1.226	0.169	0.184	1026.6	814.3
Ref.		0.176^[37]	0.185^[36,37]	970.6^[38]	-
Si-Gr	1.226	0.142	0.174	722.7	820.10
Ref.		0.146^[32,37]	0.176^[37]	715.0^[32]	-
P-Gr	1.224	0.137	0.175	688.9	821.1
Ref.		0.146^[38]	0.177^[38]	683.1^[38]	-
S-Gr	1.224	0.124	0.174	863.5	818.2
Ref.		0.110^[36]	0.174^[36]	807.6^[32]	-

Fig.3 Most favored adsorption configuration and electronic density difference images of CH4 on X-Gr(A) Al-Gr; (B) Si-Gr; (C) P-Gr; (D) S-Gr.

Fig.4 Most favored adsorption energies of Gs (CH4, CO2, H2O) on X-Gr

Fig.5 Partial density of states(PDOS) of X-Gr(A) Al-Gr; (B) Si-Gr; (C) P-Gr; (D) S-Gr. a. s; b. p; c. Cs; d. Cp.

Fig.6 Charge density differences of X-Gr(A) Al-Gr; (B) Si-Gr; (C) P-Gr; (D) S-Gr.

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