Adsorption of some small molecules on a Pd field emitter Original Research Article

The adsorption of CO, H2 and NO on Pd field emitters has been studied using Field Emission Microscopy (FEM). The results are compared with earlier studies on Rh and Pt tips. The isoteric heat of adsorption of CO was found to be ∼154 kJ mol−1 in the limit of zero coverage, decreasing to ∼133 kJ mol−1 at a coverage of 0.5. At 300 K, a saturated layer of CO caused a work function increase of 0.98 eV above that of the clean surface. The (110) and (311) areas of the tip experience greater increases in work function than the other planes present on the tip. The initial heat of adsorption of H2 was found to be ∼127 kJ mol−1, decreasing to ∼104 kJ mol−1 at a coverage of 0.5. The work function increased by adsorption of H2 by a maximum of 0.21 eV. At 300 K, NO while increasing the work function by 0.46 eV retained the same pattern as the clean tip. Heating the tip above 400 K, with NO in the gas phase produced an increase in emission around the (311) and (310) planes, with the possible formation of subsurface oxygen. On introducing NO to Pd at 550 K, almost no change was seen in the work function. When H2 was in the gas phase, a similar phenomenon already occurred at 450 K. Subsequent removal of the H2led to an increase in emission, which could be restored by simply adding H2 again.