HREELS AND TDS STUDIES OF NO ADSORPTION AND NO+H-2 REACTION ON PT(100) SURFACES

The adsorption of NO and the reaction of a saturated NOads layer with hydrogen on Pt(100)-(hex) and Pt(100)-(1 X 1) surfaces have been studied at 300 K with high resolution electron energy loss spectroscopy (HREELS), thermal desorption spectroscopy (TDS) and low energy electron diffraction (LEED) techniques. It has been shown by HREELS that two Pt phases are distinct in their capacity to dissociate NO: for the 1 x 1 phase the process occurs already at 300 K, while for the hex surface it requires some activation and starts at 375 K. On the hex phase the dissociation seems to involve first NOads adsorbed on defective sites formed during the back-reconstructive NO adsorption. The saturated NOads prepared on both the hex and 1 X 1 surfaces cannot react with hydrogen unless a previous heating in vacuum at similar to 375 K and higher is performed in order to provide room for NOads dissociation and H-2 adsorption. The further 300 K reaction with H-2 proceeds on the 1 X 1 phase only. For the original hex surface, the active phase is presented by 1 X 1 patches covered by NOads. When the reaction occurs on the original hex surface, the reaction reveals an autocatalytic behavior. A pronounced induction period is correlated with the titration of the NOads molecules located on the defects near the boundaries of the NOads/1 X 1 patches. At this early stage of the NOads + H-2 reaction, the appearance and disappearance of an NHads particle is observed which is an intermediate tentatively identified by HREELS. A possible mechanism of the NOads + H-2 reaction is discussed.