Influence of o-fluorophenylbiguanidine on the kinetics of hydrogen evolution reaction on iron, the nature of rate-determining step and hydrogen diffusion through a steel membrane

It is shown, hydrogen evolution reaction (HER) on iron occurs according to Volmer–Tafel mechanism in acidic aqueous chloride solutions with constant ionic strength. Relation between o-fluorophenylbiguanidine (o-FPBG) concentration and the kinetics and the nature of rate-determining step (rds) of hydrogen evolution reaction (HER) on iron in acidic chloride media with constant ionic strength has been established. Kinetic parameters of HER – dE/dlog ic, dE/dlog CH+, (dlog ic/dlog CH+)E, dη/dlog ic, dη/dlog CH+ and (∂log ic/∂log CH+)η – have been obtained at different concentrations of o-FPBG and compared with theoretical values appropriate to Volmer–Tafel mechanism. An increase in the o-FPBG concentration provokes sequential change of the rds in the hydrogen evolution reaction according to scheme: slow discharge – slow recombination – slow surface diffusion – slow discharge. It is proved that in the background solutions the slow discharge is followed by chemical recombination, but hydrogen diffusion rate through a steel membrane practically does not depend on the rds nature. Observed phenomena are interpreted from a position of existence of two adsorbed hydrogen forms – subsurface and raised above the surface – and difference in surface coverage by either of them.