I've been using VASP for DFT calculations and came across with an adsorption energy value over $-8\,eV$.
Is this a reasonable value for S-H bond?
From my own experience of calculating adsorption energy of small molecules on the surface of TMDs, an adsorption energy of -8 eV for an S-H bond in DFT calculation using VASP sounds unusually high and potentially unrealistic.
Such a large negative value of -8 eV suggests an extremely strong interaction between the adsorbate and the surface, which is unlikely for VOCs on the surface of MoS2, where most of the interatcion is coming from vdW forces.
In general, adsorption energies for weakly bound species like molecular adsorbates on surfaces are often in the range of a few tenths to a few eV. There could be several reasons for such a large and negative adsorption energy.
It is possible that there are errors in the calculation, such as convergence issues, inadequate basis set, or improper treatment of the system. It is also possible that the interaction between the S-H bond and the surface is being overestimated due to the limitations of the exchange-correlation functional used in the DFT calculation.
To confirm the validity of the result, I would recommend you to share your input files so people can help you find out more about the problem you are facing.