Gustavo E. Ramírez-Caballero

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.

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.

Effect of Temperature and Catalyst Concentration on Polyglycerol during Synthesis

Morphology, molecular weight, polydispersity, functionality, and thermal properties are important characteristics when using polyglycerol as a building block in the development of materials for industrial applications such as hydrogels, surfactants, asphalts additives, cosmetics, pharmaceutical, biomedical, and drug delivery systems. In this study several experimental techniques are used to understand the effect of process variables during synthesis in the catalyzed etherification of glycerol, a coproduct of biodiesel industry. Biobased polyglycerol is a high-valued product, which is useful as building block material because of its remarkable features, for instance, multiple hydrophilic groups, excellent biocompatibility, and highly flexible aliphatic polyether backbone. A connection between polyglycerol characteristics and process variables during synthesis allows the control of glycerol polymerization through reaction conditions. We show that temperature and catalyst concentration can be tuned with the aim of tailoring fundamental polyglycerol parameters including molecular weight, polydispersity, morphology, and functionality.

p–n Junction at the Interface between Metallic Systems

Density functional theory is used to evaluate the electronic properties in a composite metallic material consisting of two subsystems made of interacting metallic thin films separated by a subnanometer gap. One of the subsystems, M/Pt-M/Pt, has a monolayer of metal M over a core of Pt atoms, and the other is Pt-Pt, where the interacting surfaces are made of pure Pt. At equilibrium, this composite material exhibits a potential barrier at the interface, resembling a semiconductor p-n junction. In the gap region of M/Pt-M/Pt, the amount of electrons correlates with the surface layer degree of polarization, which depends on electronegativity and number of unpaired electrons in the external shells. The electron density in the gap, the system work function, and the built-in potential at the interface of the composite system calculated for various metal skins correlate with the degree of reduction of the Pt atoms located at the junction area.

Enhanced acidity of defective MOF-808: effects of the activation process and missing linker defects

Among the zirconium-based metal–organic frameworks, MOF-808 has been reported as a promising heterogeneous catalyst due to its inherent missing linker defects. In this work, we perform state-of-the-art density functional theory calculations along with ab initio molecular dynamics and classical reactive molecular dynamics studies of the activation processes of the MOF-808 material and correlate them to the type and strength of the proposed active sites. By starting with a defect-free structure, we added formic acid as a modulator and evaluated the effect of its removal after the activation process using calculations of adsorption of ammonia molecules. Results show that the activation process induces proton mobility on the zirconium node leading to its rearrangement and the release of water molecules. We evaluate the strength of the acid sites accompanied by Bader charge analysis, vibrational frequency data, and density of states examinations. The dehydroxylation and modulator removal that occur during the activation process affect the acidity of MOF-808. This theoretical rationalization of the activation process can serve as a basis for engineering defects in MOF-808 materials.

Stimuli-Responsive Hydrogels Based on Polyglycerol Crosslinked with Citric and Fatty Acids

Polyglycerol-based hydrogels from biodegradable raw materials were synthesized by crosslinking reactions of polyglycerol with citric and fatty acids. Three hydrogels were studied varying molar ratios of crosslinking agent. It was found that crosslink amount, type, and size play a crucial role in swelling, thermal, mechanical, and stimuli-responsive properties. The hydrogels absorption capacity changed in response to temperature and pH external stimuli. The hydrogel with the highest swelling capacity absorbed more than 7 times its own weight at room temperature and pH 5. This material increased 14 times its own weight at pH 10. Creep-recovery tests were performed to study the effect of crosslinking agent on mechanical properties. Deformation and percentage of recovery of synthesized hydrogels were obtained. Formation of hydrogels was confirmed using FTIR, and physicochemical properties were analyzed by Scanning Electron Microscopy (SEM), Differential Scanning Calorimetric (DSC), and Dynamic Mechanical Analysis (DMA). This paper aims to give a contribution to biobased hydrogel knowledge from chemical, physicochemical, and mechanical point of view.

A quantum chemical study for exploring the inhibitory effect of nitrogen containing species on the adsorption of polynuclear aromatic hydrocarbons over a Bronsted acid site

The analysis of the inhibitory effect of nitrogenated compounds on the hydroprocessing and hydropurification of oil derived fuels is important to produce cleaner fuels. In this work, density functional theory calculations were performed to investigate the effect of the nitrogen containing molecules on the adsorption of Polynuclear Aromatic Hydrocarbons (PAHs). Mordenite was chosen as a zeolitic structure for simulating a Bronsted acid site. The character of the acid site was confirmed by both a vibrational frequency calculation and a Bader charge analysis. From the adsorption calculations, it was found that the adsorption energy of PAHs increases with the number of aromatic rings in the structure. Also, the nitrogen containing species possibly inhibit more extensively two and three rings PAHs because of their lower adsorption energies. Finally, it was observed that the nitrogen species tend to drag the proton from the mordenite acid site. This explains the inhibitory effect in the adsorption of PAHs and contributes to understanding the dynamics of hydrocarbon hydroprocessing in refineries.