Surface area of material has correlation with adsorption uptake (H2 or CO2 uptake) in a material (Chahine's Rule) but in case of my synthesized material I am not finding the same correlation even after repeating experiment.
In adsorption process, it is not only the surface area that react there is also functional groups present on the surface of area. To confirm this you have to de characterisation of your samples and also the modelling of equilibrium isotherm to better understand the mechanism of adsorption involved
Hi Anant
I think it depends on the conditions under which you are measuring these quantities and the magnitude of the difference between both the uptakes and surface areas.
The Chahine rule applies to high pressure hydrogen uptake. I've not seen it applied to carbon dioxide, although there is likely to be some correlation between determined surface area and the amount of CO2 adsorbed. However, as Mouhamadou says, it's not all about surface area. If you are measuring the amount of CO2 adsorbed at (or near) saturation, then that may correlate more with the total pore volume (or perhaps total micropore volume) than surface area. It depends, to some extent, on the shape of the isotherm. If, on the other hand, you are measuring CO2 uptake at relatively low pressures, well away from saturation, then surface area might not be the main factor.
The Chahine rule is also only a crude rule of thumb - just an empirical observation - so there is usually scatter in the data (the plot of uptake vs surface area for different materials). That's why I mention the magnitude of differences between the values. If you are measuring high pressure CO2 uptake and, for instance, you are seeing twice as much uptake in a material with half the surface area, then something is probably wrong (e.g. there may be a problem with the isotherm data). If, however, you are seeing relatively small differences - just a small increase for a slightly lower surface area - then it could be genuine and due to some other property of the material; and just be part of the natural scatter you might expect. Or it could be due to a relatively small error in the surface area value.
Best regards,
Darren
Sometimes adsorption can be influenced by the pore volume, not only surface area.
Low temperature (for ex: 77K) & high pressure => adsorbed amount is correlated to total pore volume and specific area.
Mild (Room T°) to high temperature & low pressure (ie: Henry's law domain) => adsorbed amount is correlated to microporous (or even ultramicroporous) volume and, possibly, to heteroatom content (specific chemical functionalities). The latter correlation can be due to specific interactions between CO2 and sorbent.
These trends depend, of course, on gas you use.
Three physical quantities should always be taken into account: specific surface area, particle porosity, and average pore size. You should also remember about the possibility of capillary condensation.