What is your reaction? Normally, no one want to passivate catalyst, the objective is to enhance the stability of catalyst

BR,

It's hydrogenation. Then, in which case to we passivate the catalyst? Just out of curiosity

For example, when people want to take the catalyst out of the reactor but the catalyst could react with oxygen in air and cause some bad effect due to the exothermic reaction, they have to passivate the catalyst before open the reactor.

For the hydrogenation, you can reduce your catalyst in-situ and perform your reaction without the passivation step.

BR, 

For Hydrogenation , Catalyst passivation might be required for transition metal phosphide catalyst to prevent drastic oxidation when exposed to oxygen.