Catalytic Water Production from First-Principles Calculations

Doctoral thesis, 2005

The main subject of this thesis is the catalytic water production reaction on metal surfaces. This reaction is of great current interest due to its importance in fuel cells, a clean and efficient way to convert chemical energy into electrical. Fuel cells constitute a vital part in a remedy to the pressing environmental issue of the increasing green house effect, and may furthermore be used as a convenient and powerful replacement to conventional batteries in mobile applications such as cellular phones and computers. A strong connection also exists between the water production reaction and processes occurring in corrosion and electrochemistry. In particular, similar mixtures of water and hydroxyl are expected in all these cases. This thesis studies the catalytic water production reaction on metal surfaces using first-principles based calculations at the quantum-mechanical level within the framework of density functional theory. Extension to both equilibrium and kinetic properties of larger length scale adsorbate structures are provided by Monte Carlo calculations. In particular, the work focuses on the following parts: (i) A quantum treatment of hydrogen adsorbed on Pt(111). (ii) The nature and properties of the low temperature intermediate in the catalytic water production reaction on Pt(111). This intermediate turns out to consist of a mixture of water and hydroxyl adsorbed on the surface. An extensive model to simulate large length scale thermodynamics and kinetics for mixed water and hydroxyl overlayers is developed. (iii) Adsorption trends for the water, hydroxyl, oxygen and hydrogen on transition metal surfaces are studied. Taken together the results yield a fundamental understanding of the water production reaction and water related adsorbate-adsorbate interaction on metal surfaces. In particular, the strength and directional character of the hydrogen bonds in these structures are analyzed. Of all hydrogen bonds in this study the ones occurring between hydroxyl and water are found to be the strongest. A directional asymmetry in the hydrogen bond strength is found for adsorbed hydroxyl molecules. Processes such as proton transport and hydroxyl decomposition are studied for the water-hydroxyl mixed overlayers. Foundations for generalizing the results are provided by adsorption trends on metals close to platinum in the periodic table. From the trend study it is among others found that the topmost metal layer in the substrate dominates the adsorption characteristics.