Mary Clare Sison Escaño

Electrocatalysis of borohydride oxidation: a review of density functional theory approach combined with experimental validation

The electrocatalysis of borohydride oxidation is a complex, up-to-eight-electron transfer process, which is essential for development of efficient direct borohydride fuel cells. Here we review the progress achieved by density functional theory (DFT) calculations in explaining the adsorption of BH4(-) on various catalyst surfaces, with implications for electrocatalyst screening and selection. Wherever possible, we correlate the theoretical predictions with experimental findings, in order to validate the proposed models and to identify potential directions for further advancements.

Crystal and electronic structure of Li15Si4

Silicon is one of the most promising anode materials for future rechargeable batteries because of its high theoretical capacity. New crystalline Li15Si4 was found as the fully electrochemical lithiated phase of crystalline Si or amorphous Si. Density functional theory was used to study the crystal and electronic structure of Li15Si4. Li15Si4 is formed by the unit figure in which six Li atoms surround a Si atom with two different Li–Si bond lengths. The Li atom has negative charge of 0.56–0.63 in Li15Si4. The average intercalation voltage for the lithium intercalation reaction from crystalline Si to Li15Si4 is 0.303 V, which is in good agreement with that predicted by the Coulometric titration experiment result for Li–Si alloys.

Nitric Oxide Adsorption Effects on Metal Phthalocyanines

The adsorption of nitric oxide (NO) on various metal phthalocyanines (MPc, M = Mn, Fe, Co) has been studied using first-principles calculations based on density functional theory (DFT). In this study, we investigated the fully optimized geometries and electronic structure of MPc. We found that the electronic structures of metal atoms are essential in shaping the ground-state electronic structure near the Fermi level. These states are defined mostly by the d orbitals of the transition-metal atoms and, to some degree, by the states of nitrogen and carbon atoms of the inner rings. The numerical calculations showed that NO strongly chemisorbs to the metal atom with an end-on configuration and results in a change in geometric and electronic structures of MPc. The N-O bond lengths are slightly longer than that of the isolated NO molecule. The orbital energy levels are shifted with respect to the Fermi level. The HOMO-LUMO gap widens as compared to bare MPc. These changes are attributed to the hybridization of the pi* orbital of NO and the d orbitals of the transition metal. Specifically, the interaction between dpi and the pi* orbital is significant for MnPc-NO, while the hybridization of d(z(2)) and the pi* orbital plays an important role in CoPc-NO.

First-principles calculations of the dissolution and coalescence properties of Pt nanoparticle ORR catalysts: The effect of nanoparticle shape

The degradation of Pt nanoparticles (NPs) in fuel cell cathodes leads to the loss of the precious metal catalyst. While the effect of NP size on Pt dissolution has been studied extensively, the influence of NP shape is largely unexplored. Because of the recent development of experimental methods to control the shape of metal NPs, rational guidelines/insights on the shape effects on NP stability are imperative. In this study, first-principles calculations based on density functional theory were conducted to determine the stability of 1–2 nm Pt NPs against Pt dissolution and coalescence with respect to NP shape. Toward dissolution, the stability of the Pt NPs increases in the following order: Hexagonal close-packed < icosahedral < cuboctahedral < truncated octahedral. This trend is attributed to the synergy of the oxygen adsorption strength and the local coordination of the Pt atoms. With respect to coalescence, the size of a NP is related to its propensity to coalesce or detach/migrate to form larger particles. The stability of the Pt NPs was found to increase in the following order: Hexagonal close-packed < truncated octahedral < cuboctahedral < icosahedral, and was correlated with the cohesive energies of the particles. By combining the characteristic stabilities of the shapes, new “metal-interfaced” Pt-based coreshell architectures were proposed that should be more stable than pure Pt nanoparticles with respect to both dissolution and coalescence.

The Role of Ferromagnetic Substrate in the Reactivity of Pt/Fe Overlayer: A Density Functional Theory Study

We investigated the energetics of O 2 reaction on a model bimetallic system, Pt bilayer on Fe(001), using six-dimensional potential-energy surface derived from spin-polarized density functional theory calculations. The model system renders same surface geometry and binding energy with the reference system, unresconstructed Pt(001), thus a systematic investigation on the role of Fe substrate was appropriately conducted. Results show that O 2 dissociation may proceed from a “no barrier” molecular adsorption on bridge with O–O axis lying parallel to the surface and spanning towards top sites ( t – b – t ) to translation towards four-fold hollow site ( b – h – b ) yielding dissociated O atoms on the bridge. In this reaction pathway, O 2 should overcome an activation barrier of ∼0.15 eV with respect to the initial state, which is comparable to that of the reference system (∼0.14 eV). However, the binding energy of O 2 with respect to its gas phase in all three states (initial, transition and final) is less as ...

Surface magnetism in O2 dissociation?from basics to application

First-principles calculations were performed to investigate magnetic phenomena in surface reactions involving O(2). We present two magnetized surface cases: (1) oxidation of paramagnetic Ag, and magnetic properties of the high coverage oxide phase, which correspond to a magnetic impurity superlattice on paramagnetic surfaces and (2) oxidation of ferromagnetic Pt, represented by the Pt layer on M (M = Fe and Co) relevant to the oxidation reduction reaction (ORR) on Pt, in relation to both fundamental and application interests. In the first case, we found that the dissociative adsorption of O(2), resulting in oxide phases in Ag(111), reveals interesting magnetic interactions. We note that the magnetic states are induced by the ferromagnetic superexchange interactions and Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions. Specifically, the superlattice structures with short O-O distances have an effective ferromagnetic superexchange and RKKY interaction. In the second case, we found that a magnetic moment is induced on the Pt layer by the M substrate. The spin polarization of Pt-d states is due to hybridization with M-d states. The d-band center (ε(d)) of Pt (on M), is shifted downwards with respect to pure Pt. However, because of the spin polarization, the otherwise filled spin-down d(zz) orbital in paramagnetic pure Pt is shifted towards the Fermi level. This promotes π(z↑)-d(zz↓) interactions, which influences the O(2)-Pt interaction at O(2) far from the surface. Details and mechanisms of these two magnetic phenomena are discussed.

Preferential sites for adsorption of methanol and methoxy on Pt and Pt-alloy surfaces

We studied the site preference of methanol (CH3OH) and methoxy (CH3O) on Pt, PtRu and PtRuMo surfaces using density functional theory. This work aimed to investigate the nature of methanol and methoxy adsorption in relation to energetic properties and charge transfers and to clarify the role of alloying metal Ru and Mo on adsorption properties. Similar adsorption geometry of methanol and methoxy on these three surfaces was observed. The largest charge transfer was observed on methanol and methoxy adsorption on PtRuMo-, followed by PtRu- and Pt-surfaces, resulting in the strength order of binding energy. We also found that Mo in PtRuMo donates the largest amount of charge, followed by Ru in PtRu and Pt in the pure Pt surface; we concluded that this is responsible for the preferential adsorption sites of methanol and methoxy.

The Adsorption of NO on Various Metal Tape-Porphyrins: A First-Principles Study

The adsorption of NO on various metal tape-porphyrins (Mn, Fe, and Co) is studied using first-principles calculations based on density functional theory (DFT). In this work we discuss the geometric structure and electronic properties of metal tape-porphyrins and the effect of the adsorption of NO on their properties. The results show that metal atoms protrude from the porphyrin plane toward the NO molecule. At the stable position, the variation of the metal–N–O angle is in the order: Co–N–O (123°) < Fe–N–O (148°) < Mn–N–O (180°). The N–O bond length in the metal tape-porphyrins is slightly longer than that of the isolated NO molecule. These results are consistent with other DFT calculations and experimental results. We also found that the binding energy of NO with metal porphyrins increases in the order CoTP–NO (1.718 eV) < FeTP–NO (1.719 eV) < MnTP–NO (1.736 eV). As regards the electronic properties, there is a metal–insulator transition in FeTP–NO. For CoTP–NO and MnTP–NO, we found that CoTP shows insul...

First-principles investigation on the atomic structure and stability of a Pt monolayer on Fe(001)

We investigated the atomic structure and stability of a Pt monolayer on Fe(001) using spin-polarized density functional theory (DFT)-based total energy calculations. The results show that addition of a Pt monolayer on Fe substrate completely removes Fe(001) surface relaxation and induces minimal disordering of Fe atoms in the interior region in accordance with experimental findings. The stable distance between the Pt monolayer and the Fe substrate is 1.630???and the binding energy of Pt on Fe (per surface atom) is 2.00?eV. Comparison of such a binding energy with Pt?Pt binding in the first adjacent fcc Pt(001) layers shows that Pt binds more on Fe than with its corresponding pure metal slab. Such strong binding results in stabilization of the Pt monolayer on Fe(001) as verified by an increase in charge density within the Pt?Fe interface. Interestingly, addition of a Pt monolayer also induces enhancement of Fe?Fe interlayer binding in the interior region.

Ru-Catalyzed Steam Methane Reforming:Mechanistic Study from First Principles Calculations

Elucidating the reaction mechanism of steam methane reforming (SMR) is imperative for the rational design of catalysts for efficient hydrogen production. In this paper, we provide mechanistic insights into SMR on Ru surface using first principles calculations based on dispersion-corrected density functional theory. Methane activation (i.e., C–H bond cleavage) was found to proceed via a thermodynamically exothermic dissociative adsorption process, resulting in (CHy + zH)* species (“*” denotes a surface-bound state, and y + z = 4), with C* and CH* being the most stable adsorbates. The calculation of activation barriers suggests that the conversion of C* into O-containing species via C–O bond formation is kinetically slow, indicating that the surface reaction of carbon intermediates with oxygen is a possible rate-determining step. The results suggest the importance of subsequent elementary reactions following methane activation in determining the formation of stable carbon structures on the surface that deactivates the catalyst or the conversion of carbon into O-containing species.