Introduction:
Catalysts are substances that speed up a chemical reaction without being consumed in the process. Different catalysts are used for different chemical reactions. In this question, we will discuss which catalyst can be used for a specific conversion.

Conversion:
We are given a conversion to be carried out. Let's look at the reaction:
C6H6 + H2 → C6H12
This reaction is the hydrogenation of benzene to cyclohexane.

Catalysts:
Two catalysts are given as options for the conversion of benzene to cyclohexane. Let's evaluate each catalyst in detail.

H2/Ni:
H2/Ni is hydrogen gas with nickel as the catalyst. This catalyst is widely used in the hydrogenation of organic compounds. However, it is not suitable for the hydrogenation of benzene to cyclohexane. The reason behind this is that benzene is an aromatic compound that is very stable due to its delocalized pi electrons. It requires more energy to break the pi bond and hydrogenate the ring. The H2/Ni catalyst is not strong enough to break this pi bond, and therefore, it is not suitable for this reaction.

H2/Pd:
H2/Pd is hydrogen gas with palladium as the catalyst. This catalyst is suitable for the hydrogenation of benzene to cyclohexane. Palladium is a stronger catalyst than nickel and can break the pi bond in the aromatic ring. The hydrogenation of benzene with H2/Pd catalyst is an important industrial process, and it is widely used to produce cyclohexane.

Conclusion:
In conclusion, H2/Pd catalyst is suitable for the hydrogenation of benzene to cyclohexane while H2/Ni catalyst is not suitable for this reaction. The choice of catalyst is critical in determining the efficiency and selectivity of a chemical reaction.