Ryan Lacdao Arevalo

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.

NO dissociation on Cu(111) and Cu2O(111) surfaces: a density functional theory based study

NO dissociation on Cu(111) and Cu(2)O(111) surfaces is investigated using spin-polarized density functional theory. This is to verify the possibility of using Cu-based catalyst for NO dissociation which is the rate limiting step for the NO(x) reduction process. The dissociation of molecularly adsorbed NO on the surface is activated for both cases. However, from the reaction path of the NO-Cu(2)O(111) system, the calculated transition state lies below the reference energy which indicates the possibility of dissociation. For the NO-Cu(111) system, the reaction path shows that NO desorption is more likely to occur. The geometric and electronic structure of the Cu(2)O(111) surface indicates that the surface Cu atoms stabilize themselves with reference to the O atom in the subsurface. The interaction results in modification of the electronic structure of the surface Cu atoms of Cu(2)O(111) which greatly affects the adsorption and dissociation of NO. This phenomenon further explains the obtained differences in the dissociation pathways of NO on the surfaces.

Substrate dependence of Pt4 electronic properties

We show through first-principles calculations that the electronic properties of Pt4 clusters can be tuned by adsorption on substrates with different electronic valence characters. Pt clusters exhibit a metallic character on γ-Al2O3(111) and insulator properties on CaZrO3(001). The noted difference indicates the role of the electronic valence states of the substrate atoms that directly bond with Pt.

A theoretical study of the reactivity of Cu2O(111) surfaces: the case of NO dissociation

We compare the electronic properties of Cu(111) and Cu(2)O(111) surfaces in relation to the dissociation of NO using first principles calculations within density functional theory. We note a well-defined three-fold site on both O- and Cu-terminated Cu(2)O surfaces which is verified as the active site for the adsorption and dissociation of NO. The interaction of Cu with O atoms results in the forward shifting of the local density of states and formation of unoccupied states above the Fermi level, compared to the fully occupied d band of pure Cu. These results give valuable insights in the realization of a catalyst without precious metal for the dissociation of NO.

First Principles Calculations of Transition Metal Binary Alloys: Phase Stability and Surface Effects

The phase stability and surface effects on binary transition metal nano-alloy systems were investigated using density functional theory-based first principles calculations. In this study, we evaluated the cohesive and alloying energies of six binary metal alloy bulk systems that sample each type of alloys according to miscibility, i.e., Au-Ag and Pd-Ag for the solid solution-type alloys (SS), Pd-Ir and Pd-Rh for the high-temperature solid solution-type alloys (HTSS), and Au-Ir and Ag-Rh for the phase-separation (PS)-type alloys. Our results and analysis show consistency with experimental observations on the type of materials in the bulk phase. Varying the lattice parameter was also shown to have an effect on the stability of the bulk mixed alloy system. It was observed, particularly for the PS- and HTSS-type materials, that mixing gains energy from the increasing lattice constant. We furthermore evaluated the surface effects, which is an important factor to consider for nanoparticle-sized alloys, through analysis of the (001) and (111) surface facets. We found that the stability of the surface depends on the optimization of atomic positions and segregation of atoms near/at the surface, particularly for the HTSS and the PS types of metal alloys. Furthermore, the increase in energy for mixing atoms at the interface of the atomic boundaries of PS- and HTSS-type materials is low enough to overcome by the gain in energy through entropy. These, therefore, are the main proponents for the possibility of mixing alloys near the surface.

Oxidation of NO on Pt/M (M = Pt, Co, Fe, Mn): a first-principles density functional theory study

Platinum is commonly used as a catalyst for the oxidation of NO in exhaust gas purification systems. However, in addition to its high cost, the intrinsic NO + O → NO2 reaction is endothermic and the rate-limiting O2,gas dissociative adsorption is activated on Pt(111). In this paper, we show for the first time that the pseudomorphic Pt monolayer on 3d transition metals promotes a thermodynamically and kinetically favorable NO oxidation compared to pure Pt. Using density functional theory-based first principles calculations, we show that such results are attributed to the weaker binding of O and NO on the bimetallic surfaces and the change of the binding configuration of NO2 into a structure that promotes easier N–O bond formation. These results provide insights into the design of low-cost and efficient catalysts for NO oxidation.

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.

Tuning methane decomposition on stepped Ni surface: The role of subsurface atoms in catalyst design

The decomposition of methane (CH4) is a catalytically important reaction in the production of syngas that is used to make a wide spectrum of hydrocarbons and alcohols, and a principal carbon deposition pathway in methane reforming. Literatures suggest that stepped Ni surface is uniquely selective toward methane decomposition to atomic C, contrary to other catalysts that favor the CH fragment. In this paper, we used dispersion-corrected density functional theory-based first principles calculations to identify the electronic factors that govern this interesting property of stepped Ni surface. We found that the adsorption of atomic C on this surface is uniquely characterized by a 5–coordinated bonding of C with Ni atoms from both the surface and subsurface layers. Comparison with Ru surface indicates the importance of the subsurface atoms of stepped Ni surface on its selectivity toward methane decomposition to atomic C. Interestingly, we found that substituting these subsurface atoms with other elements can dramatically change the reaction mechanism of methane decomposition, suggesting a new approach to catalyst design for hydrocarbon reforming applications.

First principles study of methane decomposition on B5 step-edge type site of Ru surface

Many chemical reactions that produce a wide range of hydrocarbons and alcohols involve the breaking of C-H bonds in methane. In this paper, we analyzed the decomposition of this molecule on the B5 step-edge type site of Ru surface using first principles calculations based on dispersion-corrected density functional theory. Methane was found to be weakly adsorbed on the surface, characterized by the hybridization of its sp states with Ru-d xz,yz,zz states. Dissociative adsorption is energetically preferred over molecular methane adsorption, resulting in CH fragment. CH is strongly adsorbed on the surface due to the prevalence of low-energy sp-d bonding interaction over the electron-unoccupied anti-bonding states. This highly stable CH requires higher activation barrier for C-H bond cleavage than CH4.

Branching Reaction in Melanogenesis: The Effect of Intramolecular Cyclization on Thiol Binding

With the aid of density functional theory-based first principles calculations, we investigated energetics and electronic structure changes in reactions involving dopaquinone to give insights into the branching behaviors in melanogenesis. The reactions we investigated are the intramolecular cyclization and thiol binding, which are competing with each other. It was found that, in order to accomplish thiol binding, charge transfer of around one electron from thiol to dopaquinone occurs. Furthermore, intramolecular cyclization of dopaquinone increases the lowest unnoccupied molecular orbital level substantially. This result clearly shows prevention of the binding of thiol by intramolecular cyclization.