Research Group - Prof. Gunther Wittstock

Nanoporous gold (npAu) has already been demonstrated or can be expected to have a number of unique features as electrode material for electrocatalysis or direct electron transfer to enzymes. Like in any porous electrode material limitation by external mass transport in the liquid phase is complemented by internal mass transfer problems. In addition, selective transport phenomena for ions can be expected similar to the case of nanotubular membranes with charged walls. In this sub-project we want to investigate how structure and composition of nanoporous Au electrodes with minor bulk elements and surface modifications influence electrocatalytic reactions and internal mass transport properties. The simple surface modification by self-assembled monolayer or by UPD layers offers ways to alter the surface without changing the macroporous structure of the material.

In this sub-project we will investigate how morphology and surface composition of nanoporous gold influences electrocatalytic reactions and internal mass transport conditions. During the execution of potential programme or during storage at elevated temperatures the ligaments of nanoporous gold coarsen. This changes mass transport condition and available surface area during the use. The time course of the coarsening process can be influenced by adsorbates at the surface of the nanoporous material. Such adlayers have an influence on the catalytic activity of the material.

We focus on the electrochemical methods to determine the internal surface area and the effective diffusion coefficient inside the nanoporous gold. These methods are applicable without removal of the sample from the electrochemical cell. The results will be correlated to selected measurements using scanning electron microscopy. Furthermore, we investigate how the addition of specifically adsorbing anions and the formation of monolayers of foreign metals by under-potential deposition will change the kinetics of the coarsening process.

In a second approach we study the amount of surface oxides and surface-bound reaction intermediates using the surface interrogation mode of scanning electrochemical microscopy and complementary ex-situ spectroscopic techniques, mainly x-ray photoelectron spectroscopy (XPS). We will then study, how monolayer of foreign metal obtained by under-potential deposition will change the amount of surface oxides and reaction intermediates as a function of applied potential.

From the thus obtained insights we would like to devise ways for effective modification of nanoporous gold as electrocatalyst for methanol oxidation reaction that has been selected as the model reaction. The results of the electrocatalytic experiments will be compared to the oxidation of methanol by molecular oxygen in liquid phase and in gas phase.