We have carried out H2 adsorption and desorption measurements at 77 and 273 K using a low-pressure (0-1 bar) gas sorption analyzer (i.e. volumetric method) for a pure few-layer graphene powder (FLG) and a Pt-doped FLG analogue with 1 wt.% dopant (Pt-FLG). It should be pointed out that the pure FLG has a higher BET specific area than the Pt-FLG sample.
Both samples exhibited fully reversible H2 sorption isotherms at 77 K (the H2 uptake for the pure FLG sample was higher at 1 bar), however for the case of Pt-FLG at 273 K we have observed that desorption does not follow the same pathway with adsorption (even though there is a slightly higher H2 uptake for the Pt-FLG at 1 bar) and a small hysteresis is observed. To be more precise, the desorption isotherm is present above the adsorption isotherm and forms that hysteresis.
I would presume that this may attributed to weak chemisorption phenomena (e.g. spillover effect), which leads to a slightly higher H2 uptake at 273 K for the Pt-FLG relative to the pure FLG, as well as to a not fully-reversible H2 desorption.
Thank you in advance for your consideration and please let me know your thoughts.
Best regards,
Nikos
Dear Nikos,
In metals, such phenomena can be observed due to formation of hydrogen-induced vacancies. But I don't think it happen in carbon. Pt is known to be the high catalytic activities; so I guess that hydrocarbon compounds formed.
Does it make sense?
Hi Nikos,
This is interesting to me. We made similar observations when using Pd loaded on porous carbons. Palladium, like platinum, dissociates H2 into atoms which can enter a spillover process followed by chemisorption to exposed, unsaturated carbon atoms. Such effects have been documented for several Pt/carbon systems as well, and you can find extensive literature and reviews of spillover initiated by Pt on various carbon forms (porous, nanotuibes, nanohorns etc). I am not aware of reports for Pt/graphene, but I did not check recent literature. We worked on Pd/carbon (porous) and found three different effects: hydride formation (PdHx), Kubas bonding of molecular H2 to isolated Pd atoms Pd(H2)x, and chemisorption resulted from spillover. In your case, you may also have Kubas-type complexes to isolated Pt atoms, if you suspect that such Pt atoms are stable on graphene.
How irreversible is desorption ? Would H2 desorb at higher temperatures ? It would be interesting to do a TPD experiment on Pd/graphene after H2 uptake and isothermal desorption.
Best regards,
Cristian
What is your platinum loading ?
Would you expect your platinum to be in the form of metallic nanoparticles in your sample ? If yes (and if your platinum loading is significant), Irreversibility is likely related to dissociative chemisorption of H2 on platinum surface atoms. Actually, to determine the dispersion of platinum nanoparticles supported on a material, one commonly uses H2 chemisorption at room temperature. The process is the following: after a reduction step (under H2) H2 is desorbed from the sample at high temperature (in order to remove chemisorbed and physisorbed H2). After that a first H2 isotherm is measured and the sample is degassed at room temperature in order to remove physisorbed H2 (and leave chemisorbed H2 on the surface). A second isotherm is then measured, that will be characteristic of physisorption only. As we are interested in chemisorption, we substract the 2 isotherms in order to get the chemisorption isotherm.
Based on this protocole, I would suggest you to do something similar: after the first isotherm (chemisorption + physisorption) you could degass your sample at room temperarure and measure a second adsorption desorption isotherm. If your second isotherm does not show any irreversibility, that would be a good indication that the irreversibility in the first isotherm is due to chemisorption and give you a pure physisorption isotherm. Finally I would suggest you to use the difference between the two isotherms to evaluate the dispersion of the metallic phase (if this give you a dispersion that is consistent with your platinum loading, you will have a clear conclusion).
Dear Nikolaos,
I believe this paper may help you to understand the observed behaviour of your materials:
Understanding the mechanism of hydrogen uptake at low pressure in carbon/palladium nanostructured composites, J Mater Chem, 21 (2011) 17765-17775
Best regards
Dear Nikolaos,
Spill occurs hat elevated temperatures. The Hysteresis you see is more likely to be based on the pore structure of the material. Also the type of hyteresis you referre to is defined for physisorption only, hence, this nomenclature can not be used with respect to chemisorption-based phenomena.
Best,
Makus
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