Studies of hydrogen spillover. Part 5.—Electrochemical measurement of hydrogen spillover from platinum black to various carbons

Abstract

The spillover of H atoms from Pt blacks to Vulcan 3-G carbon black has been measured galvanostatically in a cell in which a mixture could first be treated with wet or dry gaseous H₂ for various times before immersion in the electrolyte. With O₂-free H₂, the spillover process is complete within ca. 3 h at room temperature: the quantity of H on the carbon after 20 h increased with Pt content of the mixture to a limiting value of 2.2 cm³ g^–1 at 30 w/w% Pt. This represents a surface coverage of ca. 7%. The spillover process is retarded by traces of O₂ in the H₂ and by acid or alkali pretreatment of the carbon. The quantity of H spillover increased with decreasing particle size of the Pt black; Sterling FT-G carbon black, which has a larger particle size than the Vulcan carbon black, acquired a proportionately smaller amount of H. A supported 3% Pt on Vulcan carbon behaved similarly to a mixture of the same composition. A simple model is described to account for the results; it appears that spillover can only occur at room temperature from a Pt particle to a carbon particle with which it is in immediate contact.