The modification of Pt(110) by ruthenium: CO adsorption and electro-oxidation

The adsorption and incorporation of ruthenium on Pt(110) have been investigated using the techniques of low energy ion scattering spectroscopy (LEISS), X-ray photoelectron spectroscopy (ARXPS) and low energy electron diffraction (LEED). Ruthenium lifts the Pt(110)-(1 x 2) reconstruction by a coverage of 0.5, with evidence of only a small amount of clustering following deposition at 300 K. LEISS and XPS measurements indicate that ruthenium can be excluded from the top layer by flash annealing, but incorporated into the second and third layers. After annealing to over 1000 K, bulk dissolution of ruthenium is observed. Temperature programmed desorption (TPD) of CO on the adsorbed phases of ruthenium indicates that CO coverages remain relatively high, and CO is adsorbed on both the platinum and ruthenium. TPD of CO on the alloyed phases has demonstrated that second layer ruthenium has a significant effect on the binding of CO on the top platinum atoms. The maximum saturation coverage of CO at 325 K observed on these alloyed phases is significantly reduced. Surfaces in which ruthenium is incorporated in the top surface layer provide a phase which reduces the overpotential of CO electro-oxidation to 0.6 V RHE. No promotion is observed when ruthenium is excluded from the top layer, but substantial concentrations are present in the second and third layers (the conditions that gave rise to the strongest perturbation in CO adsorption behaviour). This suggests that the effect of ruthenium is primarily to reduce the overpotential in the activation of water to produce the surface oxidising species in the electro-oxidation process, and that it is water activation that is rate limiting in the overall reaction. The incorporated ruthenium phase in the top layer is significantly more stable with respect to oxidative dissolution than the adsorbed phase. Oxidative cycling of surfaces with ruthenium in the second and third layers results in the segregation of ruthenium to the top layer, and such modified surfaces result in overpotentials for electro-oxidation as low as 0.52 V RHE. (C) 2000 Elsevier Science B.V. All rights reserved.