Interaction of O 2, CO 2, CO, C 2H 4 and C 2H 4O with Ag(110)

Abstract

The interaction of O 2, CO 2, CO, C 2H 4 AND C 2H 4O with Ag(110) has been studied by low energy electron diffraction (LEED), temperature programmed desorption (TPD) and electron energy loss spectroscopy (EELS). For adsorbed oxygen the EELS and TPD signals are measured as a function of coverage (θ). Up to θ = 0.25 the EELS signal is proportional to coverage; above 0.25 evidence is found for dipole-dipole interaction as the EELS signal is no longer proportional to coverage. The TPD signal is not directly proportional to the oxygen coverage, which is explained by diffusion of part of the adsorbed oxygen into the bulk. Oxygen has been adsorbed both at pressures of less than 10 -4 Pa in an ultrahigh vacuum chamber and at pressures up to 10 3 Pa in a preparation chamber. After desorption at 10 3 Pa a new type of weakly bound subsurface oxygen is identified, which can be transferred to the surface by heating the crystal to 470 K. CO 2 is not adsorbed as such on clean silver at 300 K. However, it is adsorbed in the form of a carbonate ion if the surface is first exposed to oxygen. If the crystal is heated this complex decomposes into O ad and CO 2 with an activation energy of 27 kcal/mol(1 kcal = 4.187 kJ). Up to an oxygen coverage of 0.25 one CO 2 molecule is adsorbed per two oxygen atoms on the surface. At higher oxygen coverages the amount of CO 2 adsorbed becomes smaller. CO readily reacts with O ad at room temperature to form CO 2. This reaction has been used to measure the number of O atoms present on the surface at 300 K relative to the amount of CO 2 that is adsorbed at 300 K by the formation of a carbonate ion. Weakly bound subsurface oxygen does not react with CO at 300 K. Adsorption of C 2H 4O at 110 K is promoted by the presence of atomic oxygen. The activation energy for desorption of C 2H 4O from clean silver is ∼ 9 kcal/mol, whereas on the oxygen-precovered surface two states are found with activation energies of 8.5 and 12.5 kcal/mol. The results are discussed in terms of the mechanism of ethylene epoxidation over unpromoted and unmoderated silver.