Applied Surface Science 331 (2015) 225–233 Contents lists available at ScienceDirect Applied Surface Science jou rn al h om ep age: www.elsevier.com/locate/apsusc Analyzing adsorption characteristics of CO 2 , N 2 and H 2 O in MCM-41 silica by molecular simulation Shing-Cheng Chang a , Shih-Yao Chien a , Chieh-Li Chen b , Cha’o-Kuang Chen a,∗ a Department of Mechanical Engineering, National Cheng Kung University, Tainan, Taiwan b Department of Aeronautics and Astronautics, National Cheng-Kung University, Tainan, Taiwan a r t i c l e i n f o Article history: Received 4 December 2014 Received in revised form 12 January 2015 Accepted 13 January 2015 Available online 19 January 2015 Keywords: MCM-41 Water adsorption Molecular simulation a b s t r a c t The adsorption characteristics of carbon dioxide, nitrogen and water molecules in MCM-41 meso- porous molecular sieve have been investigated by the molecular simulation. We evaluate the pressure–adsorption isotherms and adsorption density profiles under variant gas pressure, operating temperature and mesopore radius of MCM-41 by the grand canonical Monte Carlo simulation. Accord- ing to the calculated adsorption energy distributions, the adsorption mechanisms of gas in MCM-41 are mainly divided into three types, namely “surface adsorption” on the pore wall, “multilayer adsorption” on the adsorbed gas molecules and “molecular self-aggregation” near the pore center. In addition, the adsorption characteristics of water molecules in MCM-41 are found to be quite different from those of carbon dioxide and nitrogen due to the hydrogen bonds effect. The results indicate that the MCM-41 is practicable in engineering application for the capture, storage, and re-use of water molecules, since it is temperature-sensitive and can achieve significant adsorption loadings within a small range of pressure values via the capillary condensation phenomena. © 2015 Elsevier B.V. All rights reserved. 1. Introduction The capture of gas adsorbates has emerged an important means of dealing with the huge amount of useless, harmful, or even noxious gases emitted by daily human activities. For instance, industrial processes produce large volumes of flue gas, a com- bustion exhaust gas consisting of nitrogen, carbon dioxide and water vapor. Many studies have proposed schemes to capture, sep- arate and regenerate flue gas. Other studies have addressed the potential re-utilization of material and energy from flue gas. Sev- eral effective technologies have been developed for the solvent absorption, cryogenic distillation and biotransformation of carbon dioxide [1–3]. Solid sorbents are widely used to remove water vapor from compressed air, including silica gel, zeolite, and carbon-based adsorbents such as activated carbon, carbon nanotubes, and carbon fibers. A saturated adsorbent can be easily regenerated by inducing a hot airflow, which transports enough heat energy to allow the water molecules to escape from the surface. Desorption tempera- ture and efficiency depend on the adsorbent’s structural properties and heat transfer characteristics. ∗ Corresponding author. Tel.: +886 62757575x62140. E-mail address: ckchen@mail.ncku.edu.tw (C.-K. Chen). Silica gel, SiO 2 ·xH 2 O, is an amorphous material that can eas- ily adsorb water vapor, ammonia and methanol because of its surface silanol groups (Si–O–H) [4]. It is non-toxic, odorless, non- flammable and non-corrosive to metal and exhibits good chemical and physical stability. In recent years, the adsorption of gas through porous silica materials has shown considerable promise and cost- effectiveness. Porous materials with a high surface area and high porosity are often used as adsorbents or catalysts in engineering applications. For example, zeolite materials generally have a pore size of less than 1 nm and hence can be used to select molecules of specific sizes for reaction in configurations referred to as molecular sieves. Composed of amorphous silica, MCM-41 (Mobile Composite of Matter 41) is a mesoporous molecular sieve containing a hexag- onal array of nearly uniform pores with diameters varying from 15 ˚ A to 100 ˚ A [5–7]. Unlike zeolite materials, MCM-41 allows larger molecules to participate in the reaction. Many studies have focused on the adsorption/desorption and catalytic characteristics of MCM-41 [8–10], but the molecular model is still insufficiently detailed. Given the difficulty and cost of observing microscopic transport phenomena through experi- ments, calculation models of molecular scale materials are needed to further develop porous silica materials for adsorbate capture. In molecular simulations, MCM-41 is usually simplified into a smooth and homogeneous cylindrical pore. The interactions of the skeleton and the fluid molecules are modeled by suitable potential fields. http://dx.doi.org/10.1016/j.apsusc.2015.01.084 0169-4332/© 2015 Elsevier B.V. All rights reserved.