CO induced reconstruction of Pd110 characterised by RAIRS

Abstract

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 Pd110 surface at intermediate coverages. Up to a coverage of 0.3, CO is adsorbed on the bulk truncation Pd110-(1×1) surface. At a critical coverage, however, the surface undergoes a first order transition to a (1×2) missing row structure, which is completed at 0.75 of a monolayer. Over the coverage range 0.3 < θ < 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 (2×l)plgl alternate-tilted CO overlayer forms. This unique behaviour arises because the small energy difference between the Pd110-(1×1) and the Pd110-(1×2) structures can be overcome, at intermediate coverages, by the higher heat of adsorption of CO on the Pd110-(1×2) 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.