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,
β
1
and
β
2
, were detected during desorption. The formation of
β
2
is activated but not that of
β
1
. On Raney platinum a determination of the population of these two species was possible and we found (0.85 ± 0.1)× 10
15
and (1.25 ± 0.1)× 10
15
H atoms cm
-2
for
β
1
and
β
2
, 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-SiO
2
Eurocat, the total adsorbed amount corresponds to 1.8 hydrogen per surface metal atom under the two forms
β
1
and
β
2
. 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,
β
3
, is formed upon adsorption at 900 K which produces an increase of 30% in the total adsorbed volume.
β
3
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.