Study of CO, NO, and H2 adsorption on model Pdα-Al2O3(0001) catalysts

We have examined the adsorption of CO, NO, and H2 on α-Al2O3(0001)-supported, Pd particles using temperature-programmed desorption (TPD) in order to determine the effect of particle size on the desorption rates. For vapor deposition of Pd at 295 K, one obtains layer-by-layer growth of the Pd film. The film coalesces into a relatively uniform distribution of particles above ∼ 600K, as shown by TEM, with the particle sizes depending on the metal coverage. TPD curves for CO from all particle sizes are very similar to curves reported for bulk metals, with a main desorption feature at ∼ 490K and a second peak at ∼ 375K; however, the relative amount of CO desorbing from the low-temperature feature, assigned to linear CO, increases significantly for very small particles (∼ 1.6nm). For NO, more than half of the molecules desorb intact from large particles (5nm), with the remainder forming N2O and N2 at ∼ 550K and leaving adsorbed oxygen. The fraction of molecules which dissociate during TPD increases significantly for small particles and only N2 is observed as a product above 600 K. For H2 at 120 K, desorption occurs in two regions on large particles, between 250 and 400 K for surface hydrogen and at ∼ 180K for hydrogen from the bulk. The only significant difference on small particles is the absence of the 180 K state. These results are discussed in terms of their implications for Pd particle-size effects in catalysis.