The decomposition of phosphine (${\text{PH}}_3$) on tungsten at low pressure is a first-order reaction. It is because the:

For heterogeneous reactions on surfaces:At high pressure, the surface is completely covered by reactant molecules (saturation), making the rate independent of concentration (zero-order).At low pressure, the surface coverage ($\theta$) is proportional to the partial pressure of the gas ($\text{Langmuir Adsorption Isotherm}$). Since the rate is proportional to the number of molecules on the surface (coverage), at low pressure, $\text{Rate}\propto \text{Pressure}$, which makes it a first-order reaction.