Untersuchung Von Oberflächenreaktionen An Kupfer Mittels Beugung Langsamer Elektronen (LEED). II

Abstract

The interaction of C 2H 4, CO, and H 2O with clean (110)-, (100)-, and (111)-surfaces of copper, the subsequent adsorption of oxygen on preadsorbed layers of another surface species, and the kinetics of some surface reactions has been studied by low energy electron diffraction. Adsorption of ethylene on the (110)-surface causes the appearance of half-order streaks, corresponding to the formation of a stable chain-like adsorption layer which is of onedimensional order. A structure model is proposed which is based on geometrical considerations. Additional oxygen partially displaces the ordered C 2H 4 and leads to the formation of a partly disordered adsorption layer and small O-covered domains. With the (100)-face diffuse spots of an unstable 2 × 2-structure are visible; on the (111)-face no adsorption occurs. Adsorption of water occurs on regular lattice points without formation of new extra spots. Interaction with carbon monoxide: On the (110)-face sometimes formation of weak streaks similar to the adsorption of C 2H 4 occurs, but normally with all planes no extra spots appear at low temperatures. At higher temperatures catalytic decomposition of CO and formation of carbon takes place. This causes the appearance of a 2 × 3-structure on the (110)-face. Additional oxygen adsorbs on separate non-covered surface regions. On the (100)-face upon interaction with CO at 500 °C formation of two structures, with 2 × 2- and a more complicated symmetry, occurs. On a surface covered with carbon in the 2 × 2- structure oxygen adsorption takes place only above 200°C with formation of an “alloy”- phase, which decomposes on annealing at 450°C into separate C- or O-covered domains. The carbon is completely oxidized at 500 °C. Kinetic studies: The intensity change of an extra spot with time and temperature is a measure of the velocity of a surface reaction. The activation energy for the formation of the O 2-II-structure on the (110)-face is found to be 3.5 kcal mole , the activation energy for desorption 53 kcal mole , and therefore the heat of adsorption about 50 kcal mole . The initial activation energies for the reaction CO+O ad→CO 2 on the (110)- and (100)-face are 5 and 30 kcal mole respectively. Longer interaction with oxygen leads to a partial poisoning of the catalyst. There is some other evidence on the participation of non-periodic structure elements (structure defects, impurities) at surface reactions on single crystal planes which can play the role of “active centers”.