Isopotential Titration for Quantifying Metal-Adsorbate Charge Transfer

Abstract

The extent of charge transfer between adsorbed reactants and a catalyst surface plays a key role in determining binding energy and catalytic activity. Here, we describe the technique of ‘isopotential titration’ (IPT) to quantify the magnitude and direction of charge transfer between adsorbates and catalytic surfaces. The method used a ‘catalytic condenser’ device (Pt/C/70 nm HfO2/p++-Si) to evaluate the adsorption of hydrogen on a platinum-on-carbon conductive surface on a HfO2 (70 nm) film on a silicon wafer, with a potentiostat applying fixed (zero) voltage between the conductive Pt/C and silicon wafer layers. Dissociative adsorption of hydrogen on Pt resulted in electron flow through the external circuit from the Pt electrode toward the Si substrate, indicating that adsorbed hydrogen atoms donated electron density to the Pt surface, which then equilibrated with electrons flowing through the potentiostat to the silicon substrate. Desorption of hydrogen from the Pt surface exhibited equal and opposite current flow. The magnitude of the measured charge transfer upon hydrogen adsorption increased with increasing temperature from 100 to 200 °C, consistent with a larger change in H surface coverage at higher temperatures for cycling gaseous H2 partial pressures between 0.5% and 99.999%. Charge transferred from H atoms to the Pt was estimated as 0.17% of an electron donated per adsorbed H atom. The extent of charge transfer was comparable with a computed Bader charge analysis, which calculated an average of 0.4% of an electron transferred from adsorbing H to Pt at high surface coverage. With hydrogen adsorption being an example, isopotential titrations provide a new tool to quantify charge transfer events in heterogeneous catalytic systems.