Coadsorption of bismuth with electrocatalytic molecules: A study of formic acid oxidation on Pt(100)

Abstract

Bismuth is an effective promoter in formic acid electrocatalysis on Pt(100), yet this system is problematic and the reaction mechanisms not well understood. We investigate this from a surface science perspective by studying Bi adsorption and coadsorption with CO, O2, H2O, and HCOOH on Pt(100) with low-energy electron diffraction (LEED) and thermal desorption spectroscopy (TDS). The ‘‘hex’’ reconstruction of clean Pt(100) is gradually and completely lifted by increasing Bi coverage up to 0.25 monolayer (ML); saturation coverage is 0.5 ML with a c(2×2) LEED pattern. Low coverages of Bi (below 0.25 ML) exhibit moderate attractive interactions, whereas repulsive interactions occur for higher coverages. The heat of adsorption for Bi in the limit of zero coverage is 380 kJ/mol. Bismuth is an effective site blocker for CO and O2 with each adatom blocking two CO molecules and two O atoms. Strangely, Bi is not hydrated by coadsorbed H2O and a Pt(100) surface with only 0.2 ML Bi is hydrophobic. Both clean and Bi-covered Pt(100) are inert to HCOOH; only molecular adsorption occurs below 220 K. Preadsorbed oxygen dramatically enhances HCOOH reaction, however, with complete oxidation to CO2 and H2O occurring at 300 K via the formate intermediate. The implications of these results towards electrocatalysis are discussed.