On the 2D-transition, hysteresis and thermodynamic equilibrium of Kr adsorption on a graphite surface

Abstract

The adsorption and desorption of Kr on graphite at temperatures in the range 60 K to 88 K, was systematically investigated using a combination of several simulation techniques including: grand canonical Monte Carlo (GCMC), canonical kinetic-Monte Carlo (C-kMC) and the Mid-Density scheme (MDS). Particular emphasis was placed on the gas-solid, gas-liquid and liquid-solid 2D phase transitions. For temperatures below the bulk triple point, the transition from a 2D-liquid-like monolayer to a 2D-solid-like state is manifested as a sub-step in the isotherm. A further increase in the chemical potential leads to another rearrangement of the 2D-solidlike state from a disordered structure to an ordered structure that is signalled by (1) another sub-step in the monolayer region and (2) a spike in the plot of the isosteric heat versus density at loadings close to the dense monolayer coverage concentration. Whenever a 2D transition occurs in a grand canonical isotherm it is always associated with a hysteresis, a feature that is not widely recognized in the literature. We studied in details this hysteresis with the analysis of the canonical isotherm, obtained with C-kMC, which exhibits a van der Waals (vdW) type loop with a vertical segment in the middle. We complemented the hysteresis loop and the vdW curve with the analysis of the equilibrium transition obtained with the MDS, and found that the equilibrium transition coincides exactly with the vertical segment of the C-kMC isotherm, indicating the coexistence of two phases at equilibrium. We also analysed adsorption at higher layers and found that the 2D-coexistence is also observed, provided that the temperature is well below the triple point. Finally the 2D-critical temperatures were obtained for the first three layers and they are in good agreement with the experimental data in the literature.

A GCMC simulation and experimental study of krypton adsorption/desorption hysteresis on a graphite surface

“…The simulations show two sub-steps at C and D in the first layer ( Fig. 2a), which have been widely reported experimentally [24][25][26] and are consistent with our previous work [1]. The gradual rise to the Point X in the experimental isotherm ( Fig.…”

“…In Fig. 6, the canonical isotherm exhibits a van der Waals-type loop with a vertical section that is associated with a two phase coexistence region, as noted in our earlier work at 73 K [1], which confirms the second layer hysteresis obtained with GCMC. The NVT-kMC simulation also substantiates the presence of a horizontal hysteresis loop, noting that the canonical isotherm fits in between the adsorption-desorption branch in the horizontal hysteresis loop region.…”

“…Our recent simulation study of Kr adsorption on a structureless graphite surface at very low temperatures [1] supports the interpretation of Inaba et al, showing that hysteresis occurs whenever a two-dimensional (2D) transition is observed in the grand canonical isotherm. This can be explained as a molecular rearrangement and progressive cohesiveness in the first adsorbed layer as adsorption in higher layers takes place.…”

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a b s t r a c tAdsorption isotherms and isosteric heats of krypton on a highly graphitized carbon black, Carbopack F, have been studied with a combination of Monte Carlo simulation and high-resolution experiments at 77 K and 87 K. Our investigation sheds light on the microscopic origin of the experimentally observed, horizontal hysteresis loop in the first layer, and the vertical hysteresis-loop in the second layer, and is found to be in agreement with our recent Monte Carlo simulation study (Diao et al, 2015). From detailed analysis of the adsorption isotherm, the latter is attributed to the compression of an imperfect solid-like state in the first layer, to form a hexagonally packed, solid-like state, immediately following the first order condensation of the second layer.

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“…Our recent study [1] suggests that the microscopic reason for the hysteresis lies in the restructuring and progressive cohesiveness of the adsorbate as loading increases. In this section we corroborate this argument by analysing the local density profiles at different stages in the adsorption (Fig.…”