Many reactions involving heterogeneous catalysts are zeroth...

Many reactions involving heterogeneous catalysts are zeroth order; that is, rate $=k .$ An example is the decomposition of phosphine $\left(\mathrm{PH}_{3}\right)$ over tungsten $(\mathrm{W}):$ $$4 \mathrm{PH}_{3}(g) \longrightarrow \mathrm{P}_{4}(g)+6 \mathrm{H}_{2}(g)$$ It is found that the reaction is independent of $\left[\mathrm{PH}_{3}\right]$ as long as phosphine's pressure is sufficiently high $(\geq 1$ atm). Explain.

Many reactions involving heterogeneous catalysts are zeroth order; that is, rate $=k .$ An example is the decomposition of phosphine $\left(\mathrm{PH}_{3}\right)$ over tungsten $(\mathrm{W}):$
$$4 \mathrm{PH}_{3}(g) \longrightarrow \mathrm{P}_{4}(g)+6 \mathrm{H}_{2}(g)$$
It is found that the reaction is independent of $\left[\mathrm{PH}_{3}\right]$ as long as phosphine's pressure is sufficiently high $(\geq 1$ atm). Explain.

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Key Concepts

Surface Saturation

Surface saturation in the context of catalysis occurs when all the catalytic sites on a solid are occupied by reactant molecules. Once saturation is achieved, additional increase in reactant concentration or pressure does not lead to an increase in the number of molecules interacting with the catalyst surface, keeping the reaction rate constant. Thus, despite changes in bulk reactant concentration, the rate remains determined by the number of active sites on the catalyst.

Zeroth Order Kinetics

Zeroth order kinetics describes reactions whose rate is independent of the concentration of one or more reactants. This occurs when the reaction rate is limited by another factor rather than by the reactant's concentration, such as the number of available catalytic sites on a solid surface. Under these conditions, even if the concentration changes, the reaction rate remains unchanged.

Heterogeneous Catalysis

Heterogeneous catalysis involves catalysts that are in a different phase (usually solid) than the reactants (often gases or liquids). The reaction typically occurs on the surface of the catalyst, where active sites provide a platform for reactant molecules to adsorb, react, and then desorb as products. The process is highly dependent on the surface properties and the availability of these active sites.

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