CHEMISORPTION AND THERMAL-DECOMPOSITION OF BENZENE ON PD(110) - HIGH-RESOLUTION ELECTRON-ENERGY LOSS SPECTROSCOPY, LOW-ENERGY ELECTRON-DIFFRACTION, AND THERMAL-DESORPTION STUDIES

The adsorbed state of benzene on Pd(110) at 300 K and its thermal decomposition process in the temperature region up to 700 K have been investigated by using high-resolution electron energy loss spectroscopy, low-energy electron diffraction, and multiplexed thermal desorption spectroscopy. Vibrational spectra show the existence of two adsorbed states of benzene on Pd(110) at 300 K. In one state, benzene is adsorbed with its C ring nearly parallel to the surface ("flat" benzene), and in the other state, at some angle ("tilted" benzene). The tilted benzene is located in the c(4 x 2) domains. For a small exposure (less-than-or-similar-to 0.3 langmuir; fractional coverage theta-C6H6 less-than-or-similar-to 0.07), the flat benzene predominates. With increasing exposure, conversion from flat to tilted benzene occurs, and the amount of tilted benzene is increased relative to that of flat benzene. For the saturation exposure (3 langmuirs; theta-C6H6 approximately 0.27), the tilted benzene predominates; the flat benzene also exists. Thermal decomposition of benzene on Pd(110) has been studied in detail. For a small exposure (0.3 langmuir), heating to 380-600 K forms the C(x)H(y) (x greater-than-or-equal-to 1, y = 0, 1) species as the decomposition products. By heating the sample to 600 K, we find only C adatoms exist on Pd(110). For the saturation exposure (3 langmuirs), by heating the sample to 400 K, we find the c(4 x 2) structure is well developed and the Pd(110) surface is mostly covered by tilted benzene. Heating to 400-650 K forms the C(x)H(y) (x = 1 or greater-than-or-equal-to 3, y = 0, 1) species. The decomposition temperature is shifted toward higher temperatures by the site-blocking effect of benzene admolecules themselves. The decomposition is accompanied by H2 desorption.