Give examples of heterogeneous catalysis (i) in gas phase and (ii) in liquid phase.
Now, I came to know that heterogeneous catalysis involves different phases between the catalyst used and the reactants. That way, either the catalyst can be in solid phase where the reactants can be in gas/liquid phase. But in the above question, what does "in gas phase" mean? Is it about the gas phase of the catalyst or the gas phase of the reactants?
Also, it would be of great help if you could provide me with examples of the above question.