CONDITIONS AND KINETICS OF THE FACETING OF PT(110) AND PT(210) IN CO+O2

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 PO2, PCO, T-parameters kept constant. The temperature range between 380 and 530 K for a fixed PO2 = 1.5 x 10(-4) Torr and varying PCO 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 reversible 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 500 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 temperature 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. Experiments with a Pt(210) single crystal demonstrated that this surface also undergoes facetting during catalytic CO oxidation, but different from Pt(110) the facetting is associated with a decrease in catalytic activity.