Display omitted •Catalytic steam reforming of methane in an electric field was investigated over Pd/CeO2.•Operando–DRIFTS using isotope has been used to investigate the reaction mechanism.•Inverse kinetic isotope effect based on CHH vibration was observed.•Proton collision promoted methane activation irreversibly at low temperatures in an electric field. To elucidate the reaction mechanism of catalytic steam reforming of methane in an electric field, operando–diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) has been used to investigate methane dissociative adsorption at low temperatures using CH4, H2O, and their isotopes. Operando-analyses demonstrated that methane was adsorbed and converted into CO and CO2 more when CD4 and/or D2O was used instead of CH4 and/or H2O. This observed “inverse” kinetic isotope effect is based on the difference of CHH vibrational energy level at the transition state. Furthermore, CO2-methanation, which is the reverse reaction of steam reforming of methane, was irreversible with H2O in an electric field. These results demonstrate that the proton collision promoted methane activation irreversibly, even at low temperatures in an electric field.