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摘要: 利用密度泛函理论研究了CO2在Fe3O4（111）表面Fetet1和Feoct2两种终结的吸附行为。在Fetet1终结表面，当覆盖度为1/5 ML时，CO2倾向于线性吸附；而在高覆盖度下，弯曲的CO2与表面O作用形成CO32-结构。在Feoct2终结表面，CO2倾向于弯曲吸附，在1/6 ML和1/3 ML覆盖度时都可以形成CO32-和-COO结构。覆盖度对Fetet1终结的表面影响很弱，但是对Feoct2终结的表面影响很大。从热力学上来说，CO2在Feoct2终结表面的吸附要比Fetet1终结表面更有利。Abstract: Density functional theory calculations were used to investigate CO2 adsorption behaviors on Fetet1-and Feoct2-terminated surface of Fe3O4 (111). The results indicated that on the Fetet1-terminated surface, the linear CO2 is favored at 1/5 monolayer (ML), whereas the bent CO2 bonded to surface O, i.e. carbonate structure, becomes possible at higher coverage. On the Feoct2-terminated surface, the bent CO2 is favored; both carbonate and carboxylate structure are formed at both 1/6 and 1/3 ML. Meanwhile, the Fetet1-terminated Fe3O4(111) surface has weak coverage effects, whereas the Feoct2-terminated Fe3O4(111) surface has strong coverage effects; the Feoct2-terminated surface is thermodynamically more favorable than the Fetet1-terminated surface for CO2 adsorption.
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Key words:
- Fe3O4 /
- density functional theory /
- CO2 /
- adsorption
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Figure 1 Front (F) and top (T) views of the Fetet1 (a)- and Feoct2 (b)-terminated surfaces of Fe3O4(111) in a p(1×1) unit cell
(purple, Fe atom; red, oxygen atom)
Figure 2 CO2 adsorption on Fetet1-terminated Fe3O4(111) surface at 1/5 and 2/5 ML
(purple, Fe atom; red, oxygen atom; grey, carbon atom)
Figure 3 CO2 adsorption on the Feoct2-terminated Fe3O4(111) surface at 1/6 ML
(purple, Fe atom; red, oxygen atom; grey, carbon atom)
Figure 4 CO2 adsorption on the Feoct2-terminated Fe3O4(111) surface at 1/3 ML
(purple, Fe atom; red, oxygen atom; grey, carbon atom)
Figure 5 Local density of states of adsorbed CO2 on Fe3O4(111) surfaces (a)/(b): Figure 2(b)/2(c) on Fetet1-terminated surface; (c)/(d): Figure 3(c)/ Figure 4(a) on Feoct2-terminated surface
(solid lines, after adsorption; dotted lines, before adsorption; red and blue lines, the front and back of two co-chemisorbed CO2 molecules on the surface)
Figure 6 Schematic diagram of molecular orbital for the adsorbed CO2 on Fetet1-(a) and Feoct2-(b) terminated Fe3O4(111) surface
(a): 1 at 1/5 ML; 3 at 2/5 ML; (b): 4, 5, and 6 at 1/6 ML; 11 at 1/3 ML
Table 1 Computed net charges (q) of CO2 on the Fetet1- and Feoct2-terminated Fe3O4(111) surface
Fe3O4 surface Adsorption model ML OCO qC qO(1) qO(2) qCO2 None Free CO2 180.0 1.02 -0.51 -0.51 0 Fetet1-terminated surface 2(a) 1/5 180.0 0.95 -0.51 -0.41 0.03 2(b) 1/5 177.8 0.95 -0.50 -0.44 0.01 2(c) 2/5a 178.1 0.98 -0.51 -0.42 0.05 2/5b 131.4 0.73 -0.56 -0.55 -0.38 Feoct2-terminated surface 3(a) 1/6 179.7 0.94 -0.53 -0.46 -0.05 3(b) 1/6 146.4 0.46 -0.50 -0.47 -0.51 3(c) 1/6 126.3 0.72 -0.58 -0.58 -0.44 3(d) 1/6 127.5 0.71 -0.59 -0.56 -0.44 3(e) 1/6 141.9 0.39 -0.52 -0.47 -0.60 3(f) 1/6 126.1 0.66 -0.57 -0.56 -0.47 4(1) 1/3a 146.0 0.50 -0.49 -0.42 -0.41 1/3b 128.1 0.70 -0.59 -0.55 -0.44 4(b) 1/3a 147.6 0.51 -0.48 -0.43 -0.40 1/3b 127.7 0.70 -0.56 -0.59 -0.45 note: a and b represent the front and back of two co-chemisorbed CO2 molecules on the surface model, respectively 
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