Study of hydrogen adsorption-desorption kinetics on ordered Cu3Pt(111) anddisordered Pt/Cu(111) surface alloys

The adsorption-desorption kinetics of hydrogen on an ordered Cu3Pt(111) bul k alloy surface and on a 0.25-ML Pt-covered Cu(111) surface alloy have been studied by Auger electron spectroscopy (AES), thermal desorption spectrosc opy (TDS), and work-function change measurements (Delta Phi). Adsorption an d dissociation of hydrogen proceed via platinum sites. H-2-TDS data suggest that these processes display second-order kinetics with respect to the num ber of Pt-sites available. Desorption spectra obtained experimentally have been successfully simulated by a lattice gas model. From the simulations, t he desorption and the interaction energies could be deduced and showed weak hydrogen-hydrogen repulsion. H-induced work-function changes on both surfa ces obtained during hydrogen adsorption are always positive, in contrast to Pt(111) and Cu(111). This indicates the specific property of the alloy sur face that alters the electronic and the structural properties of bimetallic surfaces. The dipole moment that is associated with an adsorbed hydrogen a tom remains constant throughout the active coverage range, and the actual v alue of the dipole moment can be calculated from the saturation coverage an d the work-function change by using the well-known Helmholtz equation.