Potassium Atom can act as reducing agent, yet ion cannot?
The ability of K to act as a reducing agent is mainly due to its electronic structure. 
$K = [Ar] 4s^1$ . In this case, such atom tends to donate his $4s^1$ electron (acting as a reducing agent). This is due to the gain in energy that produces to have its octet complete and stay in the cationic form $K^{+}=[Ar]$ which is quite more stable that the pure element one.

This is common to all elements in the first column of the periodic table, just because all of them have a quite similar electronic structure, just with more electrons on their inner orbitals.