Adsorption of mixtures of methane and methyl merkaptan in nanoporous carbon

ethane is the main constituent of natural gas (NG). As fuel for vehicular appli- cations NG requires sorbents that allow e±cient, reversible and safe storage at room temperature and moderate pressure. To enable easy human detection of gas leaks the fuel gas should be added with compounds having low odor threshold, such as thi- ols (mercaptans). Thus a full understanding of the behavior of methane-mercaptan mixtures is necessary for the development of safe storage systems. In this talk we present results of molecular dynamics simulations in the temperature range 150{350 K and for a large range of methane partial pressures, up to the saturation pressure of methane. We observe the presence of 2D (and to a lesser degree 3D) diÆusion of the thiols indicating that though thiols adsorb preferentially relative to methane, the adsorption is still reversible. We estimate that only a small increase in mercaptan concentration is necessary for the desorbed phase to be above the threshold for human detection.

In recent years, a great emphasis has been placed on replacing fossil fuels with clean, renewable energy for use in vehicles. One potential solution is the use of hydrogen gas as a fuel source to power a fuel cell. For vehicular use, the US Department of Energy (DOE) has identified three major challenges to implementing a hydrogen-powered solution: (i) hydrogen production costs must be substantially lowered, (ii) a substantial reduction in fuel-cell costs, (iii) and hydrogen storage systems capable of delivering a driving range of hundreds of kilometers, without major detrimental effects to vehicle cost, safety, or cargo capacity must be developed. Mechanism of hydrogen adsorption in carbon porous structures is a fundamental problem for these applications. There exist many carbon porous structures which exhibit different capacities of adsorbed hydrogen, practically, all of them too low for practical applications.It can be shown that it is not possible to increase hydrogen storage capacity only by modification of slit geometry without simultaneous increase of the spe-cific surface. So, we have introduced structures with higher surfaces and ana-lyzed their adsorption properties. These new models of hypothetical structures represent ordered carbon structure with low density architecture required for effective application of porous carbons for mobile storage. We call them Open Carbon Frameworks (OCF). Theoretically they may have the specific surfaces exceeding 6000 m2/g.

Early results on optimization of gravim. excess adsorption for undoped carbons, with KOH activation: 1.Excess adsorption and gravimetric storage capacity both increase with increasing KOH:C ratio and activation temperature. Excess adsorption dominated by KOH:C ratio. 2.Volumetric storage capacity decreases with increasing activation temperature. 3. 2.5K activated at 700 oC has record volumetric storage capacity of 132 g CH4/liter carbon at 35 bar and 22 oC (200 V/V, 110% of DOE target of 118 g/liter). Has only micropores (< 2nm). Large surface area and low porosity is key to volumetric storage capacity. 4. 4K activated at 790 oC has record gravimetric storage capacity of 256 g CH4/kg carbon at 35 bar and 22 oC. Presence of mesopores raises gravimetric storage capacity. 5. Langmuir and Ono-Kondo analysis of CH4 excess isotherms at 295 K gives surface area (2400-2700 m2/g, agree with BET area), film thickness (~0.4 nm), saturated film density (320-440 g/cm3), and average binding energy (8-9 kJ/mol). 6.Briquetting can increase volumetric storage capacity by factor of 2 if surface area remains constant.

Adsorption mechanism depends on adsorbent geometry. The most important factors defined by geometry are: the surface accessible for adsorption and the distribution of energy of adsorption. In a particular type of pores with walls built from graphene layer fragments, we observe a competition between these two factors: an increase of adsorption due to the additional adsorption surface introduced by the walls' edges and a decrease of adsorption uptake due to smaller adsorption energy near the edges. We present the results of Monte Carlo simulations of N2 and H2 adsorption in model, finite size carbon slit pores. This model mimics real carbon samples better than usually used infinite pore models, whatever is the assumed pore shape (slit-like, tubular, curved etc). In particular, it allows us to account for the adsorption at the pore edges, totally neglected in infinite models. We show that the contribution of edges to the total adsorption is not negligible: in smallest pores, with the diameters comparable with the pore width, the edge contribu-tion can be even equally or more important than that of the pore inside. We discuss and compare Monte Carlo simulations and DFT approach in similar pore geometries.

In recent years, a great emphasis has been placed on replacing fossil fuels with clean, renewable energy for use in vehicles. One potential solution is the use of hydrogen gas as a fuel source to power a fuel cell. For vehicular use, the US Department of Energy (DOE) has identified three major challenges to implementing a hydrogen-powered solution: (i) hydrogen production costs must be substantially lowered, (ii) a substantial reduction in fuel-cell costs, (iii) and hydrogen storage systems capable of delivering a driving range of hundreds of kilo-meters, without major detrimental effects to vehicle cost, safety, or cargo capacity must be developed. Mechanism of hydrogen adsorption in carbon porous structures is a fundamental problem for these applications. There exist many carbon porous structures which exhibit different capacities of adsorbed hydrogen, most of them too low for practical applications.It has shown that it is not possible to increase hydrogen storage capacity only by modification of slit geometry without simultaneous increase of the specific surface. So, we have introduced structures with higher surfaces and analyzed their adsorption properties. These new models of hypothetical structures represent activated carbons with low density architecture required for effective application of porous carbons for mobile storage. We call them Open Carbon Frameworks (OCF). Theoretically they may have the specific surfaces exceeding 6000 m2/g. From such perspective it is interesting to discuss the recent exceptional predictions of enhanced storage capacity of Covalent Organic Frameworks (COF) materials and Porous Aromatic Frameworks (PAF). The question arises: Is it possible to propose a porous carbon structure that will have properties required for hydrogen adsorption? Our discussion is focused on hypothetical model materials that have not been yet practically synthesized. It shows a vision and one possible way of searching for new porous carbon-based structures with properties required for storage applications.

We report here the Molecular Dynamic (MD) simulations of a series of self-assembled mo-lecular monolayers adsorbed on graphite. The building bloc of the layers is a molecule of 1,3,5 - trisyrylbenzene substituted in 3,5 positions by alkoxy chains CnH2n+1O- (n= 6,8,10 and 12). Recent experimental studies [1-4] shows that at room temperature the the molecules aggregate on HOPG (highly oriented pyrolitic graphite) and form porous monolayers with the size of pore proportional to the length of alkoxy chain. The structure of the adlayer remains stable and commensurate with underlying graphite, even if orientation of molecules fluctuates within the layer. We will show that such behavior is only possible if the gauche defects in the adjacent molecular clips are formed in a cooperative way. In parallel, we investigate the stability of the film in the configurations that are - or are not - commensurate with graphite.