Abstract
As a contribution to the “surface thermodynamics” of supported platinum catalysts, the adsorption and desorption of oxygen on platinum-alumina and platinum-zeolite (Type Y) has been studied at 475 and 525 °C. H 2 chemisorption at 200 °C was used as a measure of Pt dispersion. The adsorption isotherms for O 2 on Pt-zeolite showed a semistep at partial pressures near the estimated dissociation pressures of bulk PtO; the limiting O:Pt atom ratio at higher pressures was about 0.86:1. No such steps were observed in the isotherms for O 2 on PtAl 2O 3, and the atom ratio of net O:total Pt reached the surprisingly high value of 3.7:1 at 525 °. This is attributed to a partial reduction of the Al 2O 3 surface, catalyzed by the Pt, during the catalyst prereduction with H 2 at 550 °C. Measurements of Pt dispersion by H 2 chemisorption showed that: (a) a limiting atom ratio of net H:total Pt of 1:1 was observed for PtAl 2O 3, but 2:1 for PtHY; (b) PtHY was more susceptible to sintering than was PtAl 2O 3; (c) “redispersion” of Pt in PtAl 2O 3 on cyclic exposure to O 2 and H 2 occurred during the reduction step; and (d) the “redispersed” Pt in PtAl 2O 3 was quite resistant to subsequent sintering in cyclic O 2H 2 treatment.
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