Hydrogen adsorption on platinum catalysts. Quantitative determination of the various species population

Abstract

Hydrogen adsorption isotherms and thermodesorption curves have been established for a large variety of unsupported and supported catalysts. Upon adsorption at 300 K, 2 species, β₁ and β₂, were detected during desorption. The formation of β₂ is activated but not that of β₁. On Raney platinum a determination of the population of these two species was possible and we found (0.85 ± 0.1)× 10¹⁵ and (1.25 ± 0.1)× 10¹⁵ H atoms cm^–2 for β₁ and β₂, respectively. The total volume adsorbed at 300 K corresponds to 1.4–1.8 H per superficial Pt atom, depending on the assumed surface structure of the metal. At 900 K, this volume is increased by 10%. On supported catalysts the adsorbed amount depends on the interaction between metal and support. For a weak interaction, as on Pt—SiO₂ Eurocat, the total adsorbed amount corresponds to 1.8 hydrogen per surface metal atom under the two forms β₁ and β₂. For a strong interaction between metal and support, as on Pt-zeolite, the volume of hydrogen adsorbed at 300 K corresponds to only 1.1 H per surface atom. A new species, β₃, is formed upon adsorption at 900 K which produces an increase of 30% in the total adsorbed volume. β₃ is attributed to H atoms in a bridged position between platinum and oxygen. When oxygen is admitted at 300 K, the surface coverage is 0.8–0.9. Subsequent hydrogen adsorption on the oxygen covered surface is not disturbed by the formation of water.

Excellent agreement was generally found with recent results obtained on single crystals.