Assertion (A): Thermal decomposition of ${\mathrm{NH}}_3$in pressure of platinum at 1130 K and high pressure follows zero order kinetics.

Reason (R): At high pressure, the metal surface gets saturated with gas molecules.

Thermal decomposition of $N{H}_3$ on Pt surface follows zero order kinetics.  This is because at high pressure surface is totally covered with $N{H}_3$ molecules which makes reaction independent of it’s concentration