Calorimetric Heat of Adsorption—Nitrogen on Tungsten

The heat of adsorption of nitrogen on tungsten ribbon has been measured calorimetrically. Up to a coverage of 2.5×1014 molecules/cm2 the heat is sensibly constant at 4 ev/molecule. This is 20% less than the change in enthalpy of nitrogen adsorbed on smilar ribbons determined from quasi-equilibrium adsorption isotherms. The coverage at which the heat departs from its constant value in the present room temperature experiments is twice that at which the change in enthalpy decreases sharply in the high-temperature equilibrium experiments. This is explained in terms of two types of adsorption site for nitrogen atoms on the exposed tungsten crystal planes. A pair consisting of one site of each type is occupied when a molecule is adsorbed at room temperature, but when the temperature is high enough to permit surface migration of nitrogen atoms, the more tightly binding sites are preferentially occupied. At high coverage, where the sticking probability falls to a low value, the calorimetric heat of adsorption depends critically on the microscopic details of the surface structure.