THERMAL-DISSOCIATION OF NO ON PD SURFACES - THE INFLUENCE OF STEP SITES

The thermal behavior of NO on (flat) Pd(111) and (stepped) Pd(112) has been investigated by temperature programmed desorption (TPD), high re solution electron energy loss spectroscopy (HREELS), and Auger electro n spectroscopy (AES) techniques. NO is shown to adsorb molecularly on both Pd(111) and Pd(112) in the temperature range 300-373 K. NO desorb s molecularly from Pd(111) near 500 K with evidence for slight NO diss ociation. In contrast, on Pd(112), in addition to NO, relatively large amounts of N-2(7x) and N2O(15x) are observed to desorb near 500 K, co mpared to Pd(111). This result indicates that the influence of the ste p sites on Pd(112) is to catalyze the decomposition of NO upon heating . This is a surprising result in light of the fact that NO molecules p referentially bind to terrace sites, instead of step sites, on Pd(112) . HREELS measurements indicate the presence of small amounts of surfac e-bound O (resulting from NO thermal decomposition) coadsorbed with NO on both Pd(111) and Pd(112) after NO adsorption at 373 K, followed by 490 K annealing. In addition, HREELS provides evidence for the existe nce of subsurface O formed only on Pd(112) during this procedure. The presence of steps on Pd(112) presumably offers an efficient pathway fo r O incorporation within the outermost Pd layers, as no spectroscopic evidence for subsurface O exists for Pd(111). Annealing both surfaces to 550 K induces the diffusion of both surface O and subsurface O into the Pd bulk.