The decomposition of phosphine $$\left( {P{H_3}} \right)$$  on tungsten at low pressure is a first-order reaction. It is because the

Solution :
\[P{{H}_{3}}\xrightarrow{W}P+\frac{3}{2}{{H}_{2}}\]
This is an example of surface catalysed unimolecular decomposition.
For the above reaction, rate is given as
$${\text{Rate}} = \frac{{k\alpha p}}{{1 + \alpha p}}$$
where, $$p =$$  partial pressure of absorbing substrate.
At low pressure, $$\alpha p < < 1\,\,{\text{or}}\,\,{\text{Rate = }}k\alpha p$$
So, $$\left( {\alpha p + 1} \right)$$  can be neglected.
Thus, the decomposition is predicted to be first order.