I want to know that if we perform 2 hrs degassing and 12 hours degassing separately of mesoporous metal oxide (silica) does it will effect of BET surface area. Temperature is 150 oC.
I have experienced short hours (4hr) degassing which produced low surface area of mesoporous silica compared to long hours (12 to 16hours). However, depending on degassing condition, you should get appreciable results.
it depended on your preparation method for your samples, for example if you synthesis your sample with solvothermal in 100 C you must degassing in temperature lower than 100 for several time.
Thanks for your Reply. I prepared my sample by sol gel and calcined it temperature at 400-600 oC. It is mesoporous silica nanoparticle. How much degassing time is sufficient ?
I am not so sure on the lenght of time so much as the outgassing temperatura. I had a colleague who worked with a mesoporous alumina that was being outgassed at 140°C overnight, whose results were all over the place and not reproducible. Sincé his samples had been heat treated at 500°C I decided to outgas at 300°C (a temperatura that is usually advised if you are dealing with a microporous material). With that outgassing temperatura his results were consistant and reproducible. His problema was a very strong adsorption of wáter molecules. When I had a good vacuum at 140°C and then raised the temperatura to 200°C I noticed immediately how the vacuum became worse and then recovered. So although many people seem to use 140°C as a standard, since your material has already been treated at 400°C or higher you will not harm it by increasing your outgassing temperatura to 300°C
Thank you very much Mozaffar Abdollahifar and M. Yates for reply ,,,
M. Yates thanks for a very good explanation and advise.
Kindly take TGA first before start any degassing, Then identify degassing temp. , During degassing observe the pressure of the sample
To find out the correct degassing time. Sample must maintain the pressure less than 20umHg pressure, So it will be near to vacuum. So you can confirm the degassing is completed. Thus you will get better result as well as you can reduce the run time of degassing.
@Behzad Shiroud Heidari : What about the rare earth doped metal oxides? How can we decide degassing conditions without TGA?
Thanks. I referred few works. IN one of them, they have used degassing at 400 degree for 4 hours for Ce-ZnO. And few they have taken 200 degrees for 2 hours
Depends on the material under investigation. 150C is probably low for silica. You want to make sure you remove all residual moisture from pores to have an accurate BET surface area. So going with higher temperature with longer degassing time is safer as long as your material is not damaged/change structure under those conditions.
An empirical relation given by Orr and dallevalle suggests the safe temperature limit for ordinary degassing of solids at pressure lower than 5*10-6 mm Hg can be given as,
Θ = 14.4*104t-1.77 [so, if t=300C, Θ is appx. 5.9hrs, You could use this condition for silica]
Where θ is in hours and t is in °C (applicable between 100°C and 400°C). However, this is just a generalized formula.
[Ref. Particle Size Measurement: Volume 2: Surface Area and Pore Size Determination. Section 2.18.3, Page 79]
I had a friend who used mesoporous silica as the support for his catalysts that were outgassed at 150°C and the results were all over the place. When I outgassed his samples although I achieved a good vacuum at this temperature when I raised it to 300°C the vacuum got worse before getting better, revealing that 150°C was not enough as water molecules were strongly adsorbed onto the catalytic sites. On outgassing all samples at 300°C prior to measurement we obtained sensible and reproducible results. If a sample has already been treted at higher temperatures in air then outgassing at 300°C should do no harm and lead to reproducible results.
Degassing time and temperature are very important issues for measuring and reporting of N2 adsorption and SBET surface area. Unfortunately, we see many reported isotherms in literatures carried out by inexpert, which were measured without sufficient degassing, as can be inferred from the isotherm itself. I urge all the interested researchers to refer to the IUPAC regulation for reporting of gas adsorption data (see the reference below). It should be noted that variable degasing temperatures (150 – 300°C) and periods (2.0 – 12 h) may be required depending on the sample nature and previous thermal treatment history.
K. Sing et al, Reporting physisorption data for gas/solid systems with special referenceto the determination of surface area and porosity, Pure Appl. Chem. 54 (1982) 2201–2218, https://doi.org/10.1351/pac198254112201.
It depends on a type of the material. Normally, I put for min. 20 h to feel safe for the reliability of the results. I mean, you can set the processing overnight and by the noon time the degassed sample is ok for the BET measurements. More critical is the temperature of the processing.
Degassing time during the BET analysis depends upon the following fators:
1. Nature of material
2. Particle Size of material
3. Temperature resist by material
4. Morphology of the material
5. Porosity
And so on.
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