Proton shuttling flattens the energy landscape of nitrite catalytic reduction

•Kinetic isotope experiments for hydrogen indicate that H-insertion is part of the rate determining step.•Hydrogen reaction orders on Pd vary from 1.5 to 0.5 indicating a shift in the rate determining step.•Negative reaction orders for nitrite suggest competitive adsorption with hydrogen.•A smoother energy landscape of the reaction leads to co-limiting hydrogen insertion on NO* and HNO*.•Degree of rate control calculations show that the rate determining steps shift earlier in the mechanism with increasing pressure.•DFT calculations show that explicit water molecules can capture the proton-shuttling mechanism.