How to determine the correct sample volume by gravimetric sorption measurements

Net, excess and absolute adsorption and adsorption of helium

“…As representative examples see: the comparison of adsorption of carbon dioxide on coal above 100 bar studied by different European laboratories (Gensterblum et al 2010); the adsorption of carbon dioxide on semi-crystalline polymers (Lorenz and Wessling 2013); the adsorption of light gases on HKUST-1 (Moellmer et al 2011) where two approaches for determining the absolute amount adsorbed are suggested. The discussion in the existing literature on this topic is extensive and an attempt to summarise it is not within the scope of this contribution.…”

Gravimetric adsorption measurements of helium on natural clinoptilolite and synthetic molecular sieves at pressures up to 3500 kPa

“…To measure adsorption of gases on porous solids with a high degree of accuracy requires the adsorbent's impenetrable solid volume (Vs) to be determined precisely. The most common method used to determine Vs is by volumetric helium pycnometry [2][3][4] because helium is small enough to access narrow pores (kinetic diameter of 2.60 Å [5]) and helium has only weak interactions with solid surfaces. Typically, helium pycnometry is performed either in-situ within an adsorption measurement apparatus or in a separate pycnometer, and this procedure is based on the assumption that helium adsorption is negligible (i.e.…”

“…This assumption is reasonable for many adsorbents and conditions of relevance to most industrial processes. However, there is evidence in the literature [2,7] that the uptake of helium on certain adsorbents may significantly affect the measurement of Vs at low temperatures and high pressures. Thus, at high pressure conditions the standard helium expansion methods may lead to significant errors in the determination of Vs [1,8], and these errors propagate as uncertainties in the estimation of the Gibbs dividing surface excess (GSE) [8,9] and consequently as uncertainties in equilibrium capacities of other species such as CO2, CH4 and N2 measured at high pressures.…”

“…Although helium pycnometry is ubiquitous in the characterisation of porous materials, there are few reports of high pressure helium adsorption measurement techniques [1,2,7,8,[10][11][12][13][14].…”

Surface excess amounts in high-pressure gas adsorption: Issues and benefits

“…The usual way for assessing the dead volume is based on expansion of a gas assumed not to adsorb in the conditions chosen for the measurement. Helium is commonly used, though it is well known that helium is prone to adsorption in micropores, then leading to a larger apparent dead volume than expected [12][13][14][15][16][17][18][19], an effect which can be minimized by increasing the measurement temperature [20,21] and which can be taken into account by appropriate procedures [22,17,23]. Nevertheless, even then, and even in the absence of any adsorption, the small size of the helium molecule may allow assessing a larger volume than that available to the adsorptive molecule, as represented in Figure 3-a.…”