Thermal desorption studies of CO and H 2 from the CuNi alloy

Abstract

The desorption order and activation energies of CO and H 2 from a (110) oriented CuNi single crystal have been studied by thermal desorption spectroscopy. The surface composition of the alloy can be varied by annealing (after sputtering treatment which reduces the surface Cu concentration) in a temperature range where the surface composition is controlled by bulk diffusion. The surface composition is determined by the relative line intensities of the Auger M 1M 4,5M 4,5 Cu and Ni transitions at ∼-100 eV electron energy, and therefore represents an average concentration measurement within the electron escape length (approximately the first couple of atomic layers). The CO desorption spectra are mainly interpreted in terms of the ensemble effect, which relates the heat of chemisorption to the number and the species of the surface atoms in contact with the CO molecule. The CO desorption kinetics is first order. Pour desorption states are resolved, two of which are readily identified as desorption from a pure Cu or pure Ni ensemble (i.e., pure Cu or pure Ni binding site). The pure Ni binding state activation energy decreases linearly with increasing surface Cu concentration, indicating that a ligand (long range) effect is also present in the bonding. The H 2 desorption kinetics is second order. Two binding states are resolved, and both of them are shown to have a pure Ni origin. The activation energy of the high temperature bonding state remains constant with changing surface composition.