금속 산화물과 메조포러스 물질에 흡착된 가스 분자의 열역학적 특성 연구

Abstract

Magnesium oxide (MgO) and calcium oxide (CaO) powders were synthesized by using a radio-frequency (RF)-induction heating method, and Santa Barbara Amorphous No. 15 (SBA-15) and Ti-doped SBA-15 (Ti-SBA-15) samples were prepared by using a hydrothermal reaction method. X-ray diffraction, transmission electron microscopy, Fourier-transform infrared analysis, and absorption isotherms apparatus were used to analyze the structures and thermodynamic properties of samples. It was confirmed that atomic layers of methane form on MgO powder surface in a layer-by-layer fashion at 77.3 K. In the porous-structured material, filling of gas molecules in the pore was observed by the existence of a hysteresis loop from nitrogen adsorption–desorption isotherms at 77.3 K. Accordingly, in the case of CaO, the layering properties of methane were also measured through an adsorption isotherm experiment at 77.3–90.2 K in which it was found that at least three layers were formed. Values of the two-dimensional compressibility (K2D) reveal that a phase transition from solid to liquid occurred at 80.0 K. An analysis of the isosteric heat of adsorption (Qst) indicated that interaction between the methane molecules and CaO powder is 124 meV. Adsorption isotherm experiments with nitrogen and argon were conducted on pure SBA-15 and Ti-SBA-15 with uniform pore structures to analyze their thermodynamic properties near the triple point. As results, two isotherm steps were formed. Particularly, calculating of the full width at half maximum values of K2D demonstrates that a phase transition occurred while the gas molecules being adsorbed into the pores as the temperature increased. For pure SBA-15 samples, the change of intensity of K2D showed significant differences as ~6.0 × 105, ~2.0 × 105 mm/N for N2, and Ar. The intensity change for Ti-SBA-15 was approximately 10 times lower than that of SBA-15 as of N2: ~3.2 × 104 mm/N and Ar: ~5.1 × 104 mm/N. This suggests that the film thickness of the adsorption layer varies by both the temperature and the level of Ti doping.