Favorable electronic interactions between active sites and substrates or transition states are essential to promoting a catalytic reaction. However, tuning the electronic state is often coupled with compositional or structural changes of active sites, leading to uncertainties in the catalyst design. Herein, we isolate the impact of the Fermi level of Pt, Pd, and Rh nanoparticles on thermocatalytic (hemi)hydrogenation of ethylene and acetylene with electric polarization. Through a combination of kinetic, spectroscopic, isotopic labeling, and computational investigations, we show that the electric polarization by applying a potential bias tunes the rate and product distribution of ethylene hydrogenation by altering the coverages of key intermediates, e.g., hydrogen, acetonitrile, and ethylene. The proposed mechanistic framework rationalizes the simultaneous increase in the rate and selectivity of acetylene hydrogenation on Pt as the Fermi level is increased by applying a negative potential bias. This work highlights the promise of leveraging electric polarization as a strategy to advance the mechanistic understanding and optimize performance in heterogeneous catalysis.