Pussana Hirunsit

Evolution of Pt and Pt-Alloy Catalytic Surfaces Under Oxygen Reduction Reaction in Acid Medium

We review our recent work that employs a series of computational techniques including density functional theory, ab initio molecular dynamics, and classical molecular dynamics to investigate changes in the structure and electronic properties of Pt-based alloy catalysts under oxygen reduction reaction conditions in acid medium. We show density-functional theory-based correlations between surface segregation and the oxidation state of the subsurface atoms, and their effects on metal dissolution. Since the onset of Pt dissolution coincides with that of surface oxidation, surface reconstruction phenomena is evaluated using ab initio and classical molecular dynamics at increasing degrees of oxidation on extended surfaces and nanoparticles, including the effects of water and an acidic solution. Significant reconstruction and compositional changes are observed as the surface is modified by the presence of adsorbates and electrolyte components. Finally we discuss the consequences of dealloying and suggest an explanation for the enhanced activity observed experimentally in the resultant nanoporous structures.

Vibrational spectra of anhydrous and monohydrated caffeine and theophylline molecules and crystals.

Density functional theory and classical molecular dynamics simulations are used to investigate the vibrational spectra of caffeine and theophylline anhydrous and monohydrate molecules and those of their crystalline anhydrous and monohydrated states, with emphasis in the terahertz region of the spectra. To better understand the influence of water in the monohydrate crystal spectra, we analyze the vibrational spectra of water monomer, dimer, tetramer, and pentamer, and also those of liquid water at two different temperatures. In small water clusters, we observe the progressive addition of translational and librational modes to the terahertz region of the spectra. The water spectra predicted by rigid and flexible water models is examined with classical molecular dynamics, and the respective peaks, especially in the terahertz region, are compared with those found in the small clusters. Similar analysis done for caffeine and theophylline monohydrate molecules using density functional theory clearly shows the presence of water modes in the librational states and in the water stretching region. Molecular dynamics of caffeine and theophylline anhydrous and monohydrate crystals reveal the influence of vibrations from the molecule-molecule (caffeine or theophylline) crystal stacks and those from the water-molecule interactions found in the monohydrate molecules and new modes from molecule-molecule, water-molecule, and water-water hydrogen bonding interactions arising from collective effects in the crystal structure. Findings illustrate challenges of terahertz technology for the detection of specific substances in condensed phases.

Shell-anchor-core structures for enhanced stability and catalytic oxygen reduction activity

Density functional theory is used to evaluate activity and stability properties of shell-anchor-core structures. The structures consist of a Pt surface monolayer and a composite core having an anchor bilayer where C atoms in the interstitial sites lock 3d metals in their locations, thus avoiding their surface segregation and posterior dissolution. The modified subsurface geometry induces less strain on the top surface, thus exerting a favorable effect on the surface catalytic activity where the adsorption strength of the oxygenated species becomes more moderate: weaker than on pure Pt(111) but stronger than on a Pt monolayer having a 3d metal subsurface. Here we analyze the effect of changing the nature of the 3d metal in the subsurface anchor bilayer, and we also test the use of a Pd monolayer instead of Pt on the surface. It is found that a subsurface constituted by two layers with an approximate composition of M(2)C (M = Fe, Ni, and Co) provides a barrier for the migration of subsurface core metal atoms to the surface. Consequently, an enhanced resistance against dissolution in parallel to improved oxygen reduction activity is expected, as given by the values of adsorption energies of reaction intermediates, delayed onset of water oxidation, and/or low coverage of oxygenated species at surface oxidation potentials.

Challenges in the Design of Active and Durable Alloy Nanocatalysts for Fuel Cells

Nanoparticles –from a few Angstroms to tens of nanometers- have been used as catalysts well before the word nanotechnology became popular. It is not surprising to expect that very small particles, having a large surface/volume ratio and a large proportion of low-coordinated sites may be much more reactive than flat surfaces. However, obtaining a uniform catalyst material, with welldefined particle size and surface composition is still an extremely difficult task. If in addition, the catalyst needs to stand a harsh environment, where not only the target reaction, but also other undesired corrosion reactions may take place, the catalyst synthesis and the performance of the catalyst under reaction conditions becomes even much more complex.