Adsorption of some small molecules on a Pd field emitter

The adsorption of CO, H-2 and NO on Pd field emitters has been studied usin g Field Emission Microscopy (FEM). The results are compared with earlier st udies on Rh and Pt tips. The isoteric heat of adsorption of CO was found to be similar to 154 kJ mol(-1) in the limit of zero coverage, decreasing to similar 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 s urface. The (110) and (311) areas of the tip experience greater increases i n work function than the other planes present on the tip. The initial heat of adsorption of H-2 was found to be similar to 127 kJ mol(-1) decreasing t o similar to 104 kJ mol(-1) at a coverage of 0.5. The work function increas ed by adsorption of H-2 by a maximum of 0.21 eV. At 300 K, NO while increas ing 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 increas e in emission around the (311) and (310) planes, with the possible formatio n of subsurface oxygen. On introducing NO to Pd at 550 K, almost no change was seen in the work function. When H-2 was in the gas phase, a similar phe nomenon already occurred at 450 K. Subsequent removal of the H-2 led to an increase in emission, which could be restored by simply adding H-2 again. ( C) 1999 Elsevier Science B.V. All rights reserved.