NO ADSORPTION ON PD(111) IN THE TEMPERATURE-RANGE BETWEEN 20-K AND 300-K

The adsorption of NO on Pd(111) was studied by means of HREELS, LEED, UPS and Δø measurements in the temperature range between 20 and 300 K. TDS results show that after NO exposure at temperatures between 100 and 200 K, desorption takes place from three distinct desorption states (α, β and γ). Saturation of the γ state is characterized by two losses at 41 and 197 meV, which can be explained with non-dissociative adsorption on bridge sites. Only shifts to 39 and 200 meV respectively are observed in the β state. The a state is characterized by two new losses at 60 and 217 meV in addition to the old ones. These observations can be explained with a model in which two thirds of the molecules are adsorbed at on-top sites and one third at bridge sites. At 20 K a first layer of chemisorbed NO is formed followed by a multilayer in which NO dimers are formed as is concluded from HREELS and UPS. Models for the arrangement of NO in the α, β and γ states are given. An idea has been developed to explain why no ordered structure is observed for the γ state. © 1990.