Reaction pathways and poisons—II: The rate controlling step for electrochemical oxidation of hydrogen on Pt in acid and poisoning of the reaction by CO

The reaction mechanism for hydrogen molecule oxidation on platinum electrocatalysts in acid solutions is deduced by comparing kinetic rate parameters obtained electrochemically, to those rate parameters obtained in the gas phase from H₂—D₂ exchange. The electrochemical rate parameters were obtained from potentiodynamic scanning of smooth platinum and from polarisation curves of porous platinum black flooded electrode structures. The rate controlling step for hydrogen molecule oxidation on Pt is the dual site dissociative chemisorption of the hydrogen molecule H₂ → 2H (known as the Tafel reaction). Specific poisons for this reaction are chemisorbed carbon monoxide and adsorbed hydrogen, producing a simple site elimination for dissociative chemisorption of the hydrogen molecule and confirm the dual site mechanism. The electrochemical reaction rate parameters are the same on smooth platinum, unsupported platinum black and platinum crystallites supported on graphitised carbon, correlating with the H₂-H₂ exchange in the gas-phase, both with and without chemisorbed carbon monoxide and are dependent only upon the surface areas of platinum catalysts.