Adsorbate-induced "Breathing" in Carbonaceous Adsorbents

Matthew J. Connolly (University of Missouri)

Adsorbent materials such as activated carbon and metal organic frameworks (MOFs) have received significant attention for their potential for storage of hydrogen and natural gas. Typically the adsorbent is assumed to consist of rigid slit- or cylindrical-shaped pores. Recent work, however, probed the importance of the mechanical response of the adsorbent in the presence of an adsorbate: when a gas enters a micropore the gas-gas and gas-substrate interactions give rise to large excess pressures on the pore walls. For a slit-shaped pore, this effect is due to the adsorbed film is dependent on pore width and the bulk gas pressure upon loading. For very narrow pores, in which the pore wall interactions overlap to produce very deep potential wells and adsorbate strong binding, the adsorbed film puts a large pressure on the pore walls and the pore tends to expand. For larger pores, the attractive potential of the adsorbed film coupled with weaker substrate interactions creates a tension, and the pore tends to contract. These effects are amplified at higher bulk gas pressures.

In this work we present a combination of theoretical, computational and experimental evidence demonstrating that graphene-like adsorbents experience significant conformational changes ("breathing") due to adsorption.

Molecular dynamics simulations show the potential for supercritical adsorbed hydrogen to open new pores in a carbonaceous material. Monte Carlo calculations of the free energy landscape over a range of adsorbate coverage are presented. This conformability of the pore structure has significant consequences for high-pressure adsorption, resulting in modified adsorption isotherms observed in activated carbon samples. Neutron diffraction measurements of the solid structure and quasi-elastic neutron scattering measurements of the adsorbed film, as functions of loading pressure, will be presented.

Additionally, extensions of the models and techniques used, along with residual stress neutron diffraction measurements, are proposed for the study of hydrogen-assisted fatigue crack growth in pipeline steel.

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