ELECTROCHEMICAL-BEHAVIOR OF PT(100) IN VARIOUS ACIDIC MEDIA .1. ON A NEW VOLTAMMETRIC PROFILE OF PT(100) IN PERCHLORIC-ACID AND EFFECTS OF SURFACE-DEFECTS

A new voltammetric profile is obtained for Pt(100) electrodes in perchloric acid solutions when the amount of oxygen adsorbed during the flame cleaning procedure is reduced to a minimum by cooling the sample in a H2 + Ar atmosphere. A reduction in the amount of hydrogen adsorbed below 0.2 V as well as the positive shift of the onset of electrochemical oxygen adsorption suggest that these cooling conditions give rise to a surface with a very low content of surface defects. On the other hand, the increase in the population of the adsorption states between 0.35 and 0.70 V indicates an increase of the two-dimensional long range order of the (100) surface domains. This is concluded from the effect on these states of repeated cycles of electrochemical oxygen adsorption and desorption, which are known to disrupt the two-dimensional long range order. These conclusions parallel those reached for the same orientation in sulphuric acid. A difference between the two media is a well marked peak for oxygen adsorption at 1.025 V appearing in perchloric acid for the H2 + Ar cooled electrode. An excess of charge measured in perchloric medium between 0.15 and 0.73 V is tentatively assigned to an increase of the density of hydrogen adsorption sites since SHG results indicate that hydrogen is the only species giving rise to the adsorption states in this potential range. According to quantitative data for hydrogen adsorption, the new voltammetric profile can be ascribed provisionally to the Pt(100) (5 x 20) closely packed surface structure. The effect of electrochemical oxygen adsorption is discussed and electrochemical criteria for the absence of surface defects on the voltammograms obtained for Pt(100) and Pt(111) in perchloric solutions are given.