All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger.
If you are able to prostrate on the ground, then it is not sufficient for you to perform the prostration of recitation while sitting on the chair. However, if you are unable to prostrate on the ground, then it is sufficient for you to nod.
The Permanent Committee for Iftaa was asked, “I always recite the Quran while sitting at my desk, and when I reach a verse that requires a prostration of recitation, I prostrate on the desk; is this valid, or should I prostrate on the ground?”
"The prostration is performed by putting the forehead on the ground or on anything that is placed over it, such as a carpet or the like, whether it be for the prostration of recitation or any other prostration. It does not suffice to perform the prostration by nodding except for the person who is unable to prostrate on the ground."
As for the voluntary prayer, it is valid to perform it while sitting, even if the person is able to stand; however, he earns half the reward earned for performing it while standing. Shaykh Ibn ‘Uthaymeen wrote, ‘As for the voluntary prayer, it is valid to perform it while sitting even if he can stand, but he receives half of the reward of the one who stands.’” [Majmoo’ Al-Fataawa]
Allah knows best.