PYRIDINE ON RU(001) - THERMAL EVOLUTION

The thermal evolution of the various states of chemisorbed pyridine on Ru(001) has been studied in detail using vibrational spectroscopy and thermal desorption. At low coverages a parallel π-bonded pyridine state is formed, but at higher coverages the configuration changes to an inclined, mainly N-bonded state. This disparity of adsorption is reflected in the formation of different intermediate states and dissociation products upon heating as well as in the evolution and nature of the desorption products. It is demonstrated that the low coverage state maintains a parallel ring orientation as dehydrogenation occurs; no inclined ring can be found, and in this respect the decomposition is analogous to that of benzene o on the same surface. The onset of ring cleavage can be correlated with the appearance of adsorbed N atoms around 450 K. In contrast, the decomposition of inclined pyridine leads to the formation of α-pyridyl with a strongly inclined ring; the formation and decomposition of this and other species have been followed by vibrational spectroscopy. Contrary to the thermal evolution of low coverage layers the vibrational features of Nads are observed above 600 K for the saturated layer. This is interpreted as hindered decomposition of the (originally standing) pyridine species. As a consequence a HCN species is formed which desorbs around 660 K and which also shows up in the vibrational spectra by an increase of the vC-H loss intensity above 600 K. © 1990 Elsevier Science Publishers B.V. (North-Holland).