Explanation of the unusual peak of calorimetric heat in the adsorption of nitrogen, argon and methane on graphitized thermal carbon black

Abstract

Heats of adsorption and adsorption isotherms of argon, nitrogen and methane on a perfect graphitic surface and a defective graphitic surface are studied with a Grand Canonical Monte Carlo Simulation (GCMC). For the perfect surface, the isosteric heat versus loading shows a typical pattern of adsorption of simple fluids on graphite. Depending on adsorbate, degree of graphitization and temperature, a spike in the heat curve versus loading is observed when the first layer is mostly covered with adsorbate molecules. The heat spike is observed for argon and nitrogen at 77 K while for argon at 87.3 K it is no longer present. These simulation results are consistent with the experimental data of J. Rouquerol, S. Partyka and F. Rouquerol, J. Chem. Soc., Faraday Trans. 1, 1977, 73, 306. In the case of methane we observe heat spikes at low temperatures, 84.5, 92.5 and 104 K. The heat spike shifts to higher loading with temperature and it then disappears at high temperatures. These observations are in qualitative agreement with the experimental data of A. Inaba, Y. Koga and J. A. Morrison, J. Chem. Soc., Faraday Trans. 2, 1986, 82, 1635. In all cases where heat spikes are observed, the GCMC simulation results indicate that the heat spike is associated with the squeezing of molecules into the already dense first layer, and the rearrangement of molecules to form a highly structured fluid of this layer. While this squeezing into the first layer is happening, molecules continue to adsorb onto the relatively sparse second layer.

On the isosteric heat of adsorption of non-polar and polar fluids on highly graphitized carbon black

“…We observed this for both argon and nitrogen, and this is in agreement with the calorimetric measurements [4,25], and a number of our computer simulation studies [8,26,27]. The heat spike observed is lower than the calorimetric heat measured by Rouquerol and co-workers [4,25], and this is because our isosteric heat was calculated using the 77 K and 87 K isotherms, and therefore our heat curve corresponds to a temperature between these temperatures.…”

A novel application of kinetic Monte Carlo method in the description of N2 vapour–liquid equilibria and adsorption

“…The adsorption of N 2 on a graphite surface at 77 K was chosen for study with the kMC scheme, since the adsorption behaviour at this temperature is well known and in particular the existence of a spike in the plot of the experimental isosteric heat versus loading is welldocumented (Fan et al, 2012b;Grillet et al, 1979;Wongkoblap and Do, 2008). Therefore, it is a good test of the kMC scheme because the conventional GCMC fails to describe this.…”

On the anatomy of the adsorption heat versus loading as a function of temperature and adsorbate for a graphitic surface

“…The heat pattern shows a typical behaviour of type I, according to our classification. It is known that a heat spike may occur in the region of heat decrease from the first to second layer, which results from the squeezing of molecule into the dense first layer (giving rise to an increase in both the solid-fluid and fluid-fluid interactions [46]). We do not detect such a spike for xenon adsorption at 161 K because of the high thermal energy; however we do observe a small spike in the region of second layer (this occurs at about 15 μmol/m 2 , which is shown as filled symbol in Fig.…”