In-situ scanning tunneling microscopy of carbon monoxide adsorbed on Au(111) electrode

CO electroxidation on gold in alkaline media: a combined electrochemical, spectroscopic, and DFT study

The aim of the present work is to provide a deeper understanding of gold catalysis for CO electrooxidation in alkaline media, through a combined electrochemical, spectroscopic, and DFT study. Voltammetric and spectroscopic measurements evidence that the amount of CO irreversibly adsorbed on gold increases as the adsorption potential becomes more negative (vs SHE). This explains why higher CO coverages can be achieved in more alkaline solutions, since the value of adsorption potential vs RHE becomes more negative vs SHE with increasing pH. On the other hand, the combination of FTIRRAS experiments and DFT calculations shows that the adsorption site of irreversibly adsorbed CO on Au(111) depends on the value of the adsorption potential. It is concluded that CO adsorption on top sites takes place at all studied potentials, and hollow and bridge sites also become occupied for adsorption potentials lower and higher than 0 V vs RHE, respectively. However, it should be noted that our DFT calculations give values of the CO binding energies that are not strong enough to explain CO irreversible adsorption. This may be partly attributed to the fact that OH coadsorption is not included in the calculations. Indeed, this work presents two experimental facts that suggest that CO adsorption on gold promotes the coadsorption of OH species: (i) CO irreversibly adsorbed on Au(111) and Au(100) leads to an unusual voltammetric feature, whose charge indicates the stabilization of one OH species per adsorbed CO species; (ii) the apparent transfer coefficient of this unusual state is close to unity, suggesting that it is due to a presumed structural transformation coupled to OH adsorption. Finally, the effect of the adsorption potential on the bulk CO electrooxidation is also studied. It is found that, on Au(111), an increased occupation of CO on multifold (hollow) sites seems to result in a less efficient catalysis. However, on Au(110), an increased coverage of CO on top sites does not produce any significant change in catalysis.

Self-promotion mechanism for CO electrooxidation on gold

CO electrooxidation on Au(111), Au(100) and Au(110) electrodes in 0.1 M HClO(4) and 0.1 M NaOH solutions has been studied by means of voltammetric measurements with hanging meniscus rotating bead-type electrodes. It is found that the reaction order in CO concentration is close to unity at potentials close to the onset of CO oxidation, and then it becomes higher than unity at higher potentials for all studied surfaces except for Au(111) in 0.1 M HClO(4). This behavior indicates that CO oxidation on gold takes place through a self-promotion mechanism, in which the presence of CO on the gold surface enhances the adsorption of its own oxidant. Specifically, this mechanism offers a plausible explanation for the higher catalytic activity in alkaline solutions, since CO adsorption is stronger under these conditions, and thus it can lead to a higher enhancement of the adsorption of the oxidant species.