Article

Adsorption of simple fluid on silica surface and nanopore: effect of surface chemistry and pore shape.

Institut Charles Gerhardt Montpellier, CNRS (UMR 5253) and Université Montpellier 2, Montpellier, France.
Langmuir (Impact Factor: 4.38). 07/2008; 24(14):7285-93. DOI: 10.1021/la800567g
Source: PubMed

ABSTRACT This paper reports a molecular simulation study on the adsorption of simple fluids (argon at 77 K) on hydroxylated silica surfaces and nanopores. The effect of surface chemistry is addressed by considering substrates with either partially or fully hydroxylated surfaces. We also investigate the effect of pore shape on adsorption and capillary condensation by comparing the results for cylindrical and hexagonal nanopores having equivalent sections (i.e., equal section areas). Due to the increase in the polarity of the surface with the density of OH groups, the adsorbed amounts for fully hydroxylated surfaces are found to be larger than those for partially hydroxylated surfaces. Both the adsorption isotherms for the cylindrical and hexagonal pores conform to the typical behavior observed in the experiments for adsorption/condensation in cylindrical nanopores MCM-41. Capillary condensation occurs through an irreversible discontinuous transition between the partially filled and the completely filled configurations, while evaporation occurs through the displacement at equilibrium of a hemispherical meniscus along the pore axis. Our data are also used to discuss the effect of surface chemistry and pore shape on the BET method. The BET surface for fully hydroxylated surfaces is much larger (by 10-20%) than the true geometrical surface. In contrast, the BET surface significantly underestimates the true surface when partially hydroxylated surfaces are considered. These results suggest that the surface chemistry and the choice of the system adsorbate/adsorbent is crucial in determining the surface area of solids using the BET method.

0 Bookmarks
 · 
233 Views
  • [Show abstract] [Hide abstract]
    ABSTRACT: We have analyzed various phenomena that occur in nanopores, focusing on elucidating their key mechanisms, to advance the effective engineering use of nanoporous materials. As ideal experimental systems, molecular simulations can effectively provide information at the molecular level that leads to mechanistic insight. In this short review, several of our recent results are presented. The first topic is the critical point depression of Lennard-Jones fluid in silica slit pores due to finite size effects, studied by our original Monte Carlo (MC) technique. We demonstrate that the first layers of adsorbed molecules in contact with the pore walls act as a “fluid wall” and impose extra finite size effects on the fluid confined in the central portion of the pore. We next present a new kernel for pore size distribution (PSD) analysis, based entirely on molecular simulation, which consists of local isotherms for nitrogen adsorption in carbon slit pores at 77 K. The kernel is obtained by combining grand canonical Monte Carlo (GCMC) method and open pore cell MC method that was developed in the previous study. We show that overall trends of the PSDs of activated carbons calculated with our new kernel and with conventional kernel from non-local density functional theory are nearly the same; however, apparent difference can be seen between them. As the third topic, we apply a free energy analysis method with the aid of GCMC simulations to investigate the gating behavior observed in a porous coordination polymer, and propose a mechanism for the adsorption-induced structural transition based on both the theory of equilibrium and kinetics. Finally, we construct an atomistic silica pore model that mimics MCM-41, which has atomic-level surface roughness, and perform molecular simulations to understand the mechanism of capillary condensation with hysteresis. We calculate the work required for the gas–liquid transition from the simulation data, and show that the adsorption branch with hysteresis for MCM-41 arise from spontaneous capillary condensation from a metastable state.
    Adsorption 01/2014; 20(2-3). · 1.74 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We construct an atomistic silica pore model mimicking templated mesoporous silica MCM-41, which has molecular-level surface roughness, with the aid of the electron density profile (EDP) of MCM-41 obtained from X-ray diffraction data. Then, we present the GCMC simulations of argon adsorption on our atomistic silica pore models for two different MCM-41 samples at 75, 80, and 87 K, and the results are compared with the experimental adsorption data. We demonstrate that accurate molecular modeling of the pore structure of MCM-41 by using the experimental EDP allows the prediction of experimental capillary evaporation pressures at all investigated temperatures. The experimental desorption branches of the two MCM-41 samples are in good agreement with equilibrium vapor–liquid transition pressures from the simulations, which suggests that the experimental desorption branch for the open-ended cylindrical pores is in thermodynamic equilibrium.
    Adsorption 04/2013; 19(2-4). · 1.74 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Amine grafted mesoporous materials such as MCM-41, etc. have been approved as a type of good candidates for CO2 capture by many experimental results. However, the theoretical or molecular simulation studies about the effects of the grafted amine groups on the CO2 adsorption are far from sufficient to deeply understand the adsorption mechanism. The difficulty lies in the fact that it is hard to formulate a precise interaction between CO2 and amine group. In this work, CO2 adsorption in the amine grafted mesoporous material was simulated by grand canonical Monte Carlo (GCMC) method. Enhanced physical interaction was used to describe the weak chemical interaction between CO2 and amine group. The effects of the density of grafted amine groups on properties of the mesoporous materials, the adsorption quantity, as well as the adsorption selectivity were investigated. The simulation results were quite comparable to the real experimental ones.
    Catalysis Today 10/2012; 194(1):53–59. · 3.31 Impact Factor

Full-text (2 Sources)

Download
80 Downloads
Available from
May 19, 2014