Abstract
The definitions of absolute, excess and net adsorption in microporous materials are used to identify the correct limits at zero and infinite pressure. Absolute adsorption is shown to be the fundamental thermodynamic property and methods to determine the solid density that includes the micropore volume are discussed. A simple means to define when it is necessary to distinguish between the three definitions at low pressure is presented. To highlight the practical implications of the analysis the case of adsorption of helium is considered in detail and a combination of experiments and molecular simulations is used to clarify how to interpret adsorption measurements for weakly adsorbed components.
Highlights
Despite several decades of research, there is still some confusion about what definition of adsorption to use for microporous materials and how to convert consistently between these properties
A simple means to define when it is necessary to distinguish between the three definitions at low pressure is presented
These issues have been recently brought in a particular spotlight due to the current ongoing interest in high pressure adsorption of weakly adsorbing gases, such as hydrogen, where the ambiguity in the definition of properties and procedures of conversion between them can lead to appreciable differences
Summary
Despite several decades of research, there is still some confusion about what definition of adsorption (absolute, excess, net) to use for microporous materials and how to convert consistently between these properties. The system is defined as a rigid microporous solid, shown schematically, as assumed by Myers and Monson (2014) This is the obvious definition of a system for absolute adsorption and it becomes effectively the system in net and excess adsorption even when a volume external to the solid is considered since the effect of the external volume cancels out in the ‘‘net’’ and ‘‘excess’’ frameworks. The terms on the RHS are the moles in the gas phase; the moles in the macropores; and the moles adsorbed in the microporous material in terms of the adsorption isotherm This shows clearly that to formulate the mass balance, the volume of the particles which includes the micropores is needed. A discussion of this point is included in Appendix 3
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
