Assume the gas is some inert gas and the substrate is V2O5 nanostructure.
If you know the gas(es), one alternative is to perform a temperature programmed desorption (TPD) experiment analyzing the exit gases in a mass spectrometer
I believe you meant to say Gas Adsorption Analysis instead of BET method. BET method does not tell you if the molecule is chemisorbed or physisorbed. The BET method is an equation, nothing else, which a Gas Adsorption data is used to obtain the specific surface area. The BET method considers the multi-layer adsorption of an inert gas onto a flat and energetically homogeneous surface. These assumption are rare, and therefore the C constant of the BET equation must be carefully discussed. Additionally, the relative pressure range used to calculate surface area must provide a linear fit. IUPAC recommends P/Po between 0.05 to 0.35. I particularly use 0.05 to 0.2 when there is no micropores (with at least 6 data points).
Regards to the analysis in question there is no universal method, you need to think about what kind of adsorption sites you want to analyse. If an inert gas is used, there will be no chemisorption, mainly at temperatures above the condensation temperature. If you desire to obtain information about the adsorption strength of a chemisorbed molecule, the Temperature-Programmed Desorption (TPD) is a good start. For instance, you can use NH3-TPD to evaluate acid sites and CO2-TPD to evaluate basic sites. Also, your sample must be stable at the temperature you are using for analysis. More discussions about this method can be found in this paper (DOI: 10.1021/acsami.6b15507), and references therein [Ref 55-58]. Some times a second source of characterization is needed, such as TPD coupled with a FTIR detector. Moreover, this review (DOI: 10.1021/cm0101069) has being sited more thand 1500 times and discuss the adsorption on nanostructured materials. You may find it useful.
As Leon and Alexandre comment, temperature-programmed desorption mass spectrometer (TPD/MS)is most widely used to identify and analyze the concentration changes of the surface chemical species. The position of the peak ( peak temperature) in TPD graph is related to the adsorption enthalpy , i.e. to the strength of binding to the surface
You can use Cvetanovich equation ( or other) in TPD for calculating the activation energy of desorptin which is best parameter for understanding of process nature
Dear All, i am really thankful to you for giving me a valuable suggestion. I will try to follow the TPD method. Thanks again for your help.
Another approach to distinguish between physisorption and chemisorption is analyzing the heat of adsorption. Is the heat of adsorption below approx. 50 kJ/mol than you have very likely physisorption, is it above then chemisorption is likely. To analyze the heat of adsorption two to three adsorption isotherms at different temperatures are sufficient. You can either use a graphical method to estimate the heat of adsorption or a analytical method.
For the graphical method you have to draw isosteres, means lines with the same loading of your respective gas e.g. 0.5 mmol/g in a log(p)-1/T plot. The slope of the linear fit to your data points in this plot gives you the heat of adsorption if you multiply it by the ideal gas constant.
log(p)/(1/T) = Hads /R
For the analytical method you need also two to three adsorption isotherms and should fit them with a model e.g. Langmuir, Freundlich, Virial etc. The chosen model should fit your isotherms well. From the model you can calculated all adsorbed amounts you want to have. Then plot them in the same manner as for the graphical method to estimate the heat of adsorption.
The heat of adsorption can be or is usually depended on the amount adsorbed. So it may be slightly different depending on the adsorbed amount you are looking at.
I hope you are not working with completely unknown substrates where you may not be able to predict if they react chemically. If you know the adsorbent and adsorbate, try to see if any chemical reaction has been reported, what are the conditions and end products. Then you can run TPD/ TPDMS to analyse the products to match with physisorption/chemisorption phenomena.
You can also measure two successive adsorption isotherms on the test sample with in between degassing at the adsorption temperature. If the second adsorption isotherm coincides with the first then you have physisorption, but if it determines lower adsorption values (i.e. it lies below the first isotherm) then you have chemisorption.
Regards,
Mohamed
TG/DSC a very simple method can be used to check the adsorption behavior of a gas As adsorption is exothermic process and desorption is an endothermic process heat of reaction could be measured from DSC.
Heat of adsorption less than 50 KJ/mole physisorption. From 50-200 KJ/mole weak chemisorption and above 200 fairly good chemisorption.
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