Adsorption and adhesion energies of n-Decane on the Pt(111) surface by calorimetry

Abstract

The heat of adsorption of n-decane molecularly adsorbed on Pt(111) at 150 K was measured by single-crystal adsorption calorimetry (SCAC). The heat of adsorption was initially ∼152 kJ/mol on terrace sites up to a coverage of Θ = 0.045 ML . Previous TPD measurements of the desorption energies of shorter n-alkanes (C1 to C6) on Pt(111) terraces showed a linear increase with chain length which predicts a desorption energy for n-decane in very good agreement with this new result. A heat of 169 kJ/mol was measured below 0.004 ML and attributed to defect sites, probably step edges. At coverages above Θ = 0.27 ML (∼3 molecular layers), the multilayer heat of adsorption was constant at ∼85 kJ/mol. Using the heat of adsorption versus coverage at 150 K up through this bulk-like multilayer, the adhesion energy per unit area of liquid n-decane on Pt(111) terraces at room temperature was estimated to be 0.148 J/m2. This is very similar to the value estimated for liquid n-hexane on Pt(111) from TPD energies, which shows that while adsorption energy increases nearly proportional to the number of CH2 plus CH3 groups, as the adhesion energy is a per unit area value, it remains nearly constant (0.15 J/m2) between hexane and decane.