Dr. Sergey Stolbov, Research Associate Professor, Physics Department, UCF

First principles computational studies of adsorption, diffusion and reactions of atoms and molecules on solid surfaces and nanostructures

My research interests focus on computational studies of the microscopic mechanisms of adsorption, diffusion and reaction of atoms and molecules on transition metal, metal oxide surfaces, and nanostructures. These are the elementary processes underlying important phenomena such as heterogeneous catalysis, growth and stability of thin films and nanostructures, coating, and corrosion. Ability to reveal the relevant elementary processes and to control their energetics by rational modification of composition and/or geometry of the system is critical for tailoring novel materials by design.

Since these processes involve creation, breaking or modification of chemical bonds, their computational studies require quantum mechanical approaches. Among such approaches are the methods based on the density functional theory (DFT), which are widely used in the area.

Real catalysts work at finite temperature and in contact with a gas phase environment at ambient pressure, which put them beyond the DFT conditions. Exchange of atoms with the environment may cause dramatic modification of the surface structure (composition and geometry) that in turn totally changes its properties (for instance the mechanism of catalytic reactions). To handle such cases, we use the ab initio thermodynamics approach that considers the surface in equilibrium with the gas phase environment and requires calculation of the free energy.

Here are some our current and future projects:

1. Effects of co-adsorbed C and alkalis on CO oxidation on Pd(111). Carbon may atomically adsorb on catalyst surface as an intermediate during CO oxidation and there are indications that it poisons surface reactivity. In contrast, alkali metals, in particular potassium, are used as promoters for many reactions and are intentionally co-adsorbed on catalyst surfaces. To reveal the mechanisms of both promotion and poison effects, we study the reaction energetics and pathways versus coverage of the co-adsorbates and distance between reactants and co-adsorbates. We find K to reduce the activation barrier for the oxidation, while C to increase it even at low coverage. Further analysis of the effects of the co-adsorbates on the local densities of electronic states and valence electron density is expected to provide an insight into the mechanisms of the effects.

2. Adsorption and oxidation of carbon monoxide on Cu₂O surface. Experimental observations suggest that Cu₂O is an efficient catalyst for CO oxidation. Our computational studies show that CO adsorbed on Cu₂O surface spontaneously oxidizes to CO₂ by consuming surface oxygen:

3. CO adsorption and diffusion on Pt nano-islands deposited on Ru particles. Proton exchange fuel cells are promising tools for hydrogen economy. However, CO, present in hydrogen, blocks active Pt sites of anode that poisons its reactivity. As reported [1], small coverage of Pt on Ru nanoparticles is much less sensitive to CO than commercial catalysts. To understand this effect, we study stability of Pt nano-island on Ru surface, as well as adsorption and diffusion of CO on the islands and substrate. [1]. S. R. Brankovic, et al., Electrochem. Sol.-St. Lett. 4 (12) A217-A220 (2001).

4. Systematic studies of reactivity of gold nano-clusters on oxide surfaces. Everyone knows that gold is very inert metal. Surprisingly, small Au nano-particles deposited on some oxide surface are found to be very efficient catalysts for CO oxidation and other reactions. We are planning extensive study of this phenomenon in collaboration with experimental group of Dr. Beatriz Roldan Cuenya, Physics Department, UCF.