The decomposition of phosphine (PH3)on tungsten at low pressure is a first-order reaction. It is because the:
Rate is proportional to the surface coverage
This is an example of surface catalyzed unimolecular decomposition.
For the above reaction, rate is given as
Where, p =partial pressure of absorbing substrate.
At low pressure, ∝p << 1 or Rate =k∝p
So, (∝p+1) can be neglected.
Thus, the decomposition is predicted to be first order.