Hydrogen gas bubbles up through the liquid, dissolves, and diffuses
to the metal catalyst bed at the bottom of the reactor. Substrate
molecules with a C=C double bond (orange, kinked) adsorb onto the
catalyst surface; the metal splits adsorbed H₂ into atomic hydrogen and
delivers it to the bond, straightening the molecule into its saturated
product (blue). This is a heterogeneous, gas-liquid-solid reaction, so
its rate depends on how fast H₂ can reach the catalyst, not just on
chemistry.
rate ∝ k(T)·[catalyst active sites]·P(H2)·(stirring ↑ mass transfer)
k(T) = A·exp(−Ea/RT) (Arrhenius)
- Catalyst — Pd/C is mild and fast for simple alkenes; PtO₂ (Adams' catalyst) tolerates more functional groups; Raney Ni is cheap but less selective and needs higher pressure.
- H₂ pressure — more dissolved hydrogen at the catalyst surface speeds the reaction and pushes it further before H₂ becomes limiting.
- Temperature — speeds the surface reaction (Arrhenius) but past a point starts also reducing other groups, hurting selectivity.
- Stirring rate — poor stirring makes H₂ mass transfer, not surface chemistry, the bottleneck; the rate plateaus once stirring is fast enough.