Rafael Callejas-Tovar

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.

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.

Modeling oxidation of Pt-based alloy surfaces for fuel cell cathode electrocatalysts

Nanoparticles (especially metals and metal-oxides) have been used as catalysts much earlier than the beginning of the era of nanotechnology. This is because small particles having a large surface/volume ratio and a large proportion of low-coordinated sites may be much more reactive than flat surface...

Understanding Activity and Durability of Core/Shell Nanocatalysts for Fuel Cells

We review recent analyses of the various aspects related to the performance of core/shell nanocatalyst particles used as electrodes in proton exchange membrane fuel cells. These nanoparticles usually consist of a thin layer of pure Pt in the shell and a core alloy made of a combination of metal elements that are targeted to meet two main objectives: reducing the catalyst price and enhancing the activity of the surface layer with respect to an equivalent particle made of pure Pt. Even though both objectives have been shown to be met, a huge challenge remains that is related to the long-term durability of the particle. This is because the less noble components are prone to relatively easy dissolution in the harsh acid conditions in which low-temperature fuel cells operate. The catalytic behavior of the nanoparticle towards the oxygen reduction reaction (ORR) and the evolution of the catalytic particle under this complex environment require a combination of experimental modern surface science and electrochemical techniques but also the formulation of models that allow a better understanding and a rational catalyst design. In this chapter, we review the state-of-the-art modeling of core/shell catalysts for the ORR. This involves various aspects that are intrinsic to the core/shell structure: surface segregation, metal dissolution, and catalytic activity. A number of methods ranging from ab initio density functional theory to classical molecular dynamics and Kinetic Monte Carlo are included in our discussion.