Alternately, would subjecting the GAC to vacuum, and replacing the gas with CO2 effectively remove oxygen. Seeking a way to degas (not necessarily all gas, but oxygen) GAC without a furnace. Not necessarily every last molecule, just most of the oxygen. And if one of the processes I describe would work, would some other gas such as nitrogen or argon also work? Any other suggestions to remove the oxygen? Thanks.
Yes, sure you will deplace oxygen until it is almost completely replaced by CO2. It'll work through continuous flushing, leading to a well-known breakthrough adsorption curve, but you'll save time and gas by using successive vacuum - CO2 pressure cycles. Not as efficient, however, as outgassing with the help of temperature, as heating will allow desorbing some (not all) oxigenated surface functions, i.e., not only adsorbed O2.
Alain
The oxygen is present on the reactive surface of activated carbon as carboxylic acid and carbonyl groups. The oxygen containing groups on the surface bring about that the activated carbon is hydrophilic. Usually activated carbon exposed to atmospheric air is hydrophikuc, To remove the oxygen containing groups keeping the carbon at a temperature above about 200oC in a gas flow not containing molecular oxygen will remove the oxygen-containing surface groups. However, exposure to atmospheric air for about a week will re-establish the oxygen-containing groups on the surface.
Activated carbon exhibits a large surface area and very narrow pores. If you expose bodies of activated carbon of a size of some mm's to a flow of carbon dioxide, it will take you unmeasurable periods of time to replace the gas atmosphere within the narrow pores with carbon dioxide and remove the oxygen. Consequently keeping the activated carbon at an elevated temperature is the only possibility to remove the oxygen by flow; molecular diffusion is much too slow with narrow pores.
Thank you Dr. Geus. An experiment I performed yesterday confirmed that the oxygen could not be removed with brief vacuum and CO2 treatment. Your explanation is enlightening, thanks for taking the time to answer.
I confirm that if you had been talking about surface functions, I would have given a different reply, but you only talked about adsorbed molecules (assumed to be physisorbed in the absence of additional detail). For your information, each oxigenated function has its own range of "desorption" temperature, and the last ones (hydroxyl groups) evolve only above 700°C.
Alain
Thank you Dr. Celzard. I was naïve and didn't understand that most of the oxygen was present as functional groups, so I was indeed just talking about adsorbed molecular oxygen. I imagine my experiment actually did remove some of that, so I don't think your initial reply was incorrect. Thank you also for taking time to teach!
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