Being a volcanic rock  pumice stone can withstand high temperature without being melted and is unreactive with concentrated nitric acid. Here is how these properties help during thermal decomposition of nitric acid.

When  concentrated nitric acid is made to fall dropwise on highly heated pumice stone it remains chemically unreacted with the stone but readily spreads over its highly heated  surface and thus the thin molecular layers of nitric acid easily  get the sufficient thermal kinetic energy required for their decomposition from the molecules of pumice stone vibrating with high thermal kinetic energy at high temperature and we see the following dismutation  reaction to occur ..

\(4HNO_3 \to 4NO_2 + 2NH_2O + O_2\)

Pumice stone is mainly used to separate out oxygen from Nitric acid.

Pumice stone is a volcanic rock and can be withstand high temperature. When Nitric acid is to be decomposed, it is made to fall dropwise on heated pumice stone, where nitric acid's thin layer gets sufficient kinetic energy for its decomposition.

\(4HNO_3 \to 4NO_2 + 2NH_2O + O_2\)