CONDITIONS AND KINETICS OF FACETING OF A PT(110) SURFACE DURING CATALYTIC COOXIDATION

During the catalytic oxidation of CO the Pt(110) surface undergoes a facetting process that is associated with an increase in catalytic activity. The kinetics of the reaction-induced facetting can therefore be followed by measuring the increase of the reaction rate with the P(o2), P(Co), and T parameters kept constant. The temperature range between 380 and 530 K for a fixed P(o2) = 1.5 X 10(-4) Torr and varying P(Co) values was investigated. The conditions and the kinetics of the facetting process can qualitatively be well described by assuming that both the CO-induced 1 X 1 <----> 1 X 2 phase transition and the surface reaction have to proceed simultaneously in order to initiate facet growth. In a broad temperature region between approximately 400 and approximately 500 K the rate of facetting is only weakly dependent on the temperature. Beyond 480 K the rate of facetting decreases with increasing temperature until a (pressure-dependent) high-temperature limit at 530 K is reached, beyond which the surface remains essentially flat. The negative influence of higher temperatures on the facetting process is due to the operation of a thermal reordering process which restores the flat (110) surface and which becomes dominant towards higher temperatures. For T > 500 K facetting may lead to the development of kinetic oscillations either as transient behavior or as stable dynamical state.