Influence of dispersion on the energies of adsorption: H2, CO, propylene and propyne on supported Pd or Pt
The aim of this work is to contribute to an explanation of the lowering of the specific activity of Pd/Al2O3 catalysts, towards hydrogenation of acetylenic molecules, as the dispersion increases. For this purpose, the interaction energy between these catalysts and H2, CO, propylene and propyne was directly measured by adsorption microcalorimetry at 323 K. In the case of propyne, the enthalpy of adsorption was found to be more than 50% higher for the catalysts with 97% dispersion than for the one with 26% dispersion, for coverages ranging from 0.2 to 0.7. This high enthalpy of adsorption (up to 290 kJ.mol−1) is attributed to a different proportion of edges and crystalline planes in the smallest Pd crystallites and is indeed consistent with a “blocking” chemisorption or with a hindered desorption of the products of a catalytic hydrogenation, i.e. with a lowering of the specific activity.
PACS36.40 82.65.Jv 82.60.Cx
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