Characterization of Supported Silver Catalysts II. Adsorption Studies of Well-Dispersed Ag on η-Al 2 O 3

Abstract

Chemisorption of H₂, O₂, and N₂O at 443 K as well as XRD and TEM techniques were used to characterize small (3–4 nm) silver crystallites supported on η-Al₂O₃. The effects of different pretreatment procedures were also examined. The amounts of chemisorbed H₂ on reduced Ag surfaces were insignificant (<10% of the O-monolayer). Adsorption-decomposition of N₂O according to the following reaction:

${\mathrm{N}}_2{\mathrm{O}}_{(\mathrm{g})}+{\text{Ag}}_{\mathrm{s}}\to {\text{Ag}}_{\mathrm{s}}-{\mathrm{O}}_{\mathrm{a}}+{\mathrm{N}}_{2(\mathrm{g})}$

provides oxygen coverages similar to those obtained using O₂ as the adsorbate. In either case, the adsorbed oxygen layer can be stoichiometrically titrated with H₂ at 443 K by the reaction:

${\text{Ag}}_{\mathrm{s}}-{\mathrm{O}}_{\mathrm{a}}+{\mathrm{H}}_{2(\mathrm{g})}\to {\text{Ag}}_{\mathrm{s}}+{\mathrm{H}}_2{\mathrm{O}}_{(\text{ads})}$

after correction for the small amount of hydrogen adsorption which occurs on reduced silver surfaces. This titration reaction provides significant advantages for characterizing silver catalysts, which are discussed.