CO INDUCED RECONSTRUCTION OF PD(110) CHARACTERIZED BY RAIRS

The IR spectra exhibited by the CO/Pd{110} system follow a complex and unique sequence at 300 K. Using RAIRS, LEED and TDS measurements we demonstrate that this behaviour arises from a novel phenomenon whereby CO induces a substantial reconstruction of the Pd{110} surface at intermediate coverages. Up to a coverage of 0.3, CO is adsorbed on the bulk truncation Pd{110}-(1x1) surface. At a critical coverage, however, the surface undergoes a first order transition to a (1x2) missing row structure, which is completed at 0.75 of a monolayer. Over the coverage range 0.3 < theta < 0.75 the reconstructed and unreconstructed surfaces coexist, giving rise to remarkably complex IR spectra with absorption features spanning a 300 cm-1 range! For coverages beyond 0.75, the reconstruction is lifted and a (2x1)p1g1 alternate-tilted CO overlayer forms. This unique behaviour arises because the small energy difference between the Pd{110}-(1x1) and the Pd{110}-(1x2) structures can be overcome, at intermediate coverages, by the higher heat of adsorption of CO on the Pd{110}-(1x2) missing row surface. This delicate balance between the heat of adsorption and the surface free energy leads to the surface switching from one structure to the other as the coverage range is traversed.