Ensemble size estimation in formic acid oxidation on Bi-modified Pt(111)

The physical dimensions of ensemble structures formed by adsorbed Bi on Pt(111) were estimated and correlations were established between ensemble size and oxidation activity. Measurements were made by examining electrochemical scanning tunneling microscopy (EC-STM) images of Bi irreversibly adsorbed on Pt(111). Percentage coverages of Bi, CO, poison, and ensemble were determined by both EC-STM and cyclic voltammetry. As the fractional Bi coverage increased, from 0.03 to 0.21, the ensemble size decreased, from approximately 9 to 1 nm. Poison formation was inhibited at ensemble sizes less than 2 nm and the turnover frequency of formic acid was enhanced by a factor of 100–200. Bi is hypothesized to not only physically produce the ensembles, but may also chemically alter their electronic properties.