SIMS, AES, work function and flash desorption measurements of the CO adsorption on NiCu alloy surfaces

Abstract

The chemisorption of CO on Cu, Ni and CuNi alloy surfaces was examined by SIMS, work function measurements and desorption spectroscopy. Using a dynamic SIMS technique the M +, M + 2, MCO + and M 2CO + emission at different temperatures (100–400 K) was measured as a function of CO exposure. In agreement with the work function and desorption experiments an increase of M + and MCO + emission due to the CO adsorption on Cu was found only at low temperatures (100–190 K). On the Ni surface an increase of Ni +, NiCO + and Ni 2CO + was measured up to 400 K. The adsorption of CO on CuNi alloy surfaces — as derived from the work function measurements — can be described by the assumption of two different states of adsorbed carbon monoxide. They can be characterized by different binding energies and from sign and magnitude different work function changes. These states were interpreted as adsorption at Ni or Cu sites of the alloy surfaces, respectively. To a certain extent the SIMS results from the alloy surfaces are incompatible with the work function measurements and desorption spectroscopy and the SIMS studies on the pure metals. A Cu + emission with comparable intensity to the Ni + emission was found for alloys with bulk concentrations of 60 and 40 at% Cu at 300 K. The ratio Ni + Cu + was nearly independent of CO pressure and temperature. The measured ratios of Cu + 2 ( Cu + + Ni +) , Ni + 2 ( Cu + + Ni +) and CuNi + ( Cu + + Ni +) with values about 10 −2 can be explained the basis of a statistical arrangement of Cu and Ni atoms in the alloy surface. The intensities of the MCO + emissions are 10 2 times smaller than the corresponding values of the pure metals. No emission of M 2CO + was found on CuNi during CO adsorption.