Federico Calle-Vallejo

Universality in Oxygen Evolution Electrocatalysis on Oxide Surfaces

Trends in electrocatalytic activity of the oxygen evolution reaction (OER) are investigated on the basis of a large database of HO* and HOO* adsorption energies on oxide surfaces. The theoretical overpotential was calculated by applying standard density functional theory in combination with the computational standard hydrogen electrode (SHE) model. We showed that by the discovery of a universal scaling relation between the adsorption energies of HOO* vs HO*, it is possible to analyze the reaction free energy diagrams of all the oxides in a general way. This gave rise to an activity volcano that was the same for a wide variety of oxide catalyst materials and a universal descriptor for the oxygen evolution activity, which suggests a fundamental limitation on the maximum oxygen evolution activity of planar oxide catalysts.

Fast Prediction of Adsorption Properties for Platinum Nanocatalysts with Generalized Coordination Numbers

Platinum is a prominent catalyst for a multiplicity of reactions because of its high activity and stability. As Pt nanoparticles are normally used to maximize catalyst utilization and to minimize catalyst loading, it is important to rationalize and predict catalytic activity trends in nanoparticles in simple terms, while being able to compare these trends with those of extended surfaces. The trends in the adsorption energies of small oxygen- and hydrogen-containing adsorbates on Pt nanoparticles of various sizes and on extended surfaces were analyzed through DFT calculations by making use of the generalized coordination numbers of the surface sites. This simple and predictive descriptor links the geometric arrangement of a surface to its adsorption properties. It generates linear adsorption-energy trends, captures finite-size effects, and provides more accurate descriptions than d-band centers and usual coordination numbers. Unlike electronic-structure descriptors, which require knowledge of the densities of states, it is calculated manually. Finally, it was shown that an approximate equivalence exists between generalized coordination numbers and d-band centers.

Design of an Active Site towards Optimal Electrocatalysis: Overlayers, Surface Alloys and Near-Surface Alloys of Cu/Pt(111)**

Therefore,thedesign of the appropriate active site is crucial to obtain highcatalytic activity, especially where multi-functionality isneeded. However, the control of a given surface on anatom-by-atom basis is particularly challenging.The electrochemical oxidation of CO is the prototypicalbifunctional reaction.

On the behavior of Brønsted-Evans-Polanyi relations for transition metal oxides

Versatile Brønsted-Evans-Polanyi (BEP) relations are found from density functional theory for a wide range of transition metal oxides including rutiles and perovskites. For oxides, the relation depends on the type of oxide, the active site, and the dissociating molecule. The slope of the BEP relation is strongly coupled to the adsorbate geometry in the transition state. If it is final state-like the dissociative chemisorption energy can be considered as a descriptor for the dissociation. If it is initial state-like, on the other hand, the dissociative chemisorption energy is not suitable as descriptor for the dissociation. Dissociation of molecules with strong intramolecular bonds belong to the former and molecules with weak intramolecular bonds to the latter group. We show, for the prototype system La-perovskites, that there is a cyclic behavior in the transition state characteristics upon change of the active transition metal of the oxide.

Density functional theory based screening of ternary alkali-transition metal borohydrides: A computational material design project

We present a computational screening study of ternary metal borohydrides for reversible hydrogen storage based on density functional theory. We investigate the stability and decomposition of alloys containing 1 alkali metal atom, Li, Na, or K (M(1)); and 1 alkali, alkaline earth or 3d/4d transition metal atom (M(2)) plus two to five (BH(4))(-) groups, i.e., M(1)M(2)(BH(4))(2-5), using a number of model structures with trigonal, tetrahedral, octahedral, and free coordination of the metal borohydride complexes. Of the over 700 investigated structures, about 20 were predicted to form potentially stable alloys with promising decomposition energies. The M(1)(Al/Mn/Fe)(BH(4))(4), (Li/Na)Zn(BH(4))(3), and (Na/K)(Ni/Co)(BH(4))(3) alloys are found to be the most promising, followed by selected M(1)(Nb/Rh)(BH(4))(4) alloys.

Trends in Stability of Perovskite Oxides

Perovskite oxides with general formula AMO3 have a large variety of applications as dielectrics and piezoelectrics, ferroelectrics and/or ferromagnetic materials, among others. Rare earth and alkaline earth metal perovskites are useful as catalysts for hydrogen generation, as oxidation catalysts for hydrocarbons, and as effective and inexpensive electrocatalysts for state-of-the-art fuel cells, mainly due to the possibility of tuning their mixed ionic–electronic conductivity through substitution of A and M and subsequent formation of oxygen vacancies. Despite the general interest in perovskites, so far there have been no ab initio studies devoted to their formation energies, and the trends in stability are unknown. Among the available theoretical techniques to investigate perovskites, DFT is an appealing candidate, since it has proved useful for understanding metals and alloys at the atomic scale. Nevertheless, the well-known shortcoming of DFT in describing strongly correlated systems has prevented its use for the estimation of properties such as band gaps and electron localization–delocalization of oxides, and there are numerous corrections. Despite these limitations, Figure 1a shows the experimental formation energies from elements and O2 of 20 perovskites at 298 K and the corresponding standard DFT energies using the RPBE-GGA exchange-correlation functional. The simulations are able to reproduce trends in the formation energies, and the calculated energies are shifted by about 0.75 eV compared to experiments. The A component is Y, La, Ca, Sr, or Ba, while M is a 3d metal from Ti to Cu. However, it is possible to combine the formation energies of these compounds with those of their sesquioxides (A2O3 and M2O3), rutile dioxides (MO2), monoxides (AO and MO), and O2 to reproduce the energetics of several reactions (Figure 1b–d). The reactions are shown in the Supporting Information. The excellent correspondence between experiments and theory shows that DFT very accurately captures the mixing energies between oxides. The chemical reaction depicted in Figure 1a and the way of representing its Gibbs energy, are given by Equations (1) and (2).

Trends in Metal Oxide Stability for Nanorods, Nanotubes, and Surfaces

The formation energies of nanostructures play an important role in determining their properties, including their catalytic activity. For the case of 15 different rutile and 8 different perovskite metal oxides, we used density functional theory (DFT) to calculate the formation energies of (2,2) nanorods, (3,3) nanotubes, and the (110) and (100) surfaces. These formation energies can be described semiquantitatively (mean absolute error ≈ 0.12 eV) by the fraction of metal−oxygen bonds broken and the metal d-band and p-band centers in the bulk metal oxide.

Electro-Catalysis of Oxygen Reduction Reaction

This paper is a short review of the recent developments in understanding trends in electro-catalysis of the oxygen reduction reaction (ORR). Our focus is on atomic scale simulations at the density functional theory level. First, we investigate the models and the approximations that have been assumed, and thence we reach the conclusion that trends in electrocatalysis are well captured by only considering binding energies of reaction intermediates to the catalyst surface. We show, assuming a simple and very likely series of intermediates, existence- of a universal scaling relation common to all ORR catalysts, which defines the overpotential.

Enabling Generalized Coordination Numbers to Describe Strain Effects

The worlds growing energetic demand calls for efficient generation and interconversion of different types of energy. Heterogeneous catalysis can help cope with such demand, provided that rational, accurate and affordable design methods lead to the discovery of cost-effective and efficient catalysts. Here we derive a simple descriptor to simultaneously capture two parameters commonly used in catalytic materials design: strain and coordination. We test the descriptor with four different adsorbates on four active sites of two metals, and applying strain in the range of ±3u2009%, usually observed experimentally at catalytic metal surfaces. Furthermore, we use the descriptor to illustrate catalyst design availing strain and nearest-neighbor effects simultaneously for the oxygen reduction reaction, a reaction of high importance in fuel cells. The connection between coordination and strain helps in the search for robust yet rapid catalyst design methodologies.

How Au Outperforms Pt in the Catalytic Reduction of Methane Towards Ethane and Molecular Hydrogen

Within the context of a “hydrogen economy”, it is paramount to guarantee a stable supply of molecular hydrogen to devices such as fuel cells. Besides, catalytic conversion of the environmentally harmful methane into ethane, which has a significantly lower Global Warming Potential, is an important endeavour. Herein we propose a novel proof-of-concept mechanism to accomplish both tasks simultaneously. We provide transition-state barriers and reaction Helmholtz free energies obtained from first-principles Density Functional Theory by taking account vibrations for