P$10-5{?}_A$ The rate law for the hydrogenation $(H)$ of ethylene $(E)$ to form ethane $(A)$ over a cobalt$-$molybdenum

catalyst $[$Collection Czech. Chem. Commun., $51,2760(1988)]$ is

$-r_E^{\text{'}}=\frac{k{P}_E{P}_H}{1+K_E{P}_E}$

$($a$)$ Suggest a mechanism and rate$-$limiting step consistent with the above rate law.

$($b$)$ What was the most difficult part in finding the mechanism?