An ultrathin oxide film supported on a metal substrate (Figure 1) sometimes exhibits extraordinary catalytic activities. To explore the potential of such structures as automotive exhaust catalysts, we have examined NO-CO reaction over oxide/metal slab models by plane-wave density-functional theory calculations. This article summarizes our computational results for TiO2/Ag and ZrO2/Cu slabs (Figure 2). For both systems, we found that NO can be adsorbed and activated on the cationic site of the oxide by receiving electronic charge from the metal (Figure 4). The activated NO species then dimerizes to form ONNO (Figure 5). This ONNO decomposes into N2O and O (Figure 5), and N2O then decomposes into N2 and O (Figure 6). The O adatoms left on the surface react with CO to form CO2, thus completing the catalytic cycle (Figure 7). The calculated energy barriers and adsorption energies indicate the superiority of ZrO2/Cu compared to TiO2/Ag. This difference can be attributed to a difference in charge on the cations and a difference in inter-cation distances.
Various important reactions proceed at electrode-electrolyte interface, where we can find specific solvent structures quite different from those in bulk solutions. Classical molecular dynamics simulation is one of useful approaches to obtain microscopic understanding of the interfaces. In this article, we will describe three points that require special attention in such simulations: interaction potential functions, polarization of metal electrodes and constant electrode potential condition, and long-range electrostatic interactions in slab systems.
Solid electrolyte interphase (SEI) is formed under the first charging at the interface between anode and electrolyte solution in Lithium ion battery (LIB), and governs the performance and the safety. However, the difficulty in in-situ measurements as well as operando measurements has prevented substantial understanding of the interfacial phenomena and processes on the electronic and atomic scale. To resolve such issues, theoretical calculations with sufficient accuracy play an important role. In this paper, I provide a comprehensive survey of density-functional-theory-based first-principles calculations studies on the SEI-related microscopic mechanisms, including our recent works.
Recent theoretical studies of mixed-metal clusters/particles are reviewed here. Mixed-metal clusters/particles, which consist of more than two kinds of metal element, are attracting a lot of interests to find a new heterogeneous catalyst using abundant cheap metals instead of precise metals. One good example is core-shell structure consisting of active and precise metal on the shell and abundant metals in the core. In this review, we report theoretical studies of Cu/M core-shell cluster Cu32M6 (M = Ru to Ag and Os to Au) with Cu element on the shell and Pt/M core-shell cluster Pt42M13 (M = Ru, Rh, Os, and Ir) with Pt element on the shell. Their stabilities are explored on the basis of the electronic structure. As important factors for stability and electronic structure, the charge-transfer from the shell to the core is discussed here. Such charge-transfer provides significant influence to the band structure, in particular, density of state (DOS) around the Fermi level, and also reactivity of catalysis.
In the supported metal catalyst, where the metal nanoparticle is highly dispersed on the support surface, the metal–surface interaction plays an important role in determining catalytic activity by suppressing particle aggregation and influencing frontier orbitals via interaction between metal particle and surface. First principles calculation is useful in understanding and predicting the nature of the metal-surface interaction. However, in the plane-wave DFT method of the slab model, which is widely used for theoretical study of surface systems, powerful analysis methods of electronic structure have not been proposed. Also, the use of the hybrid DFT and post Hartree-Fock methods is still difficult for a realistic slab model. To solve these problems, we developed the analysis method based on molecular orbital–band interaction, and the embedded cluster model incorporating periodic electrostatic potential. In this review, we describe an overview of these methods and their application examples to Rh/AlPO4 and Rh/Al2O3 systems.
Atomic clusters generally possess many structural isomers due to the degrees of freedom. Conventional theoretical studies begin by searching stable structures for clusters followed by analyzing the electronic properties for the lowest energy isomer. Recent development in structural and chemical reaction pathway search technique allows us to investigate isomerization among low-energy isomers, as well as catalytic properties of a cluster considering the structural isomers in addition to the lowest energy one. In this study, we focus on small Au, Ag, and Cu clusters to study the stable structures, isomerization between these isomers, and NO dissociation reaction catalyzed by these clusters. We find that isomerization in Au and Ag is relatively easier than Cu, corresponding to the Mohs hardness scale of their bulk counterparts. As for the NO dissociation reaction, Au and Ag have low activity, while Cu has higher activity, suggesting the potential applications toward low-cost abundant catalysts for three-way catalysis.
NOx reduction reactions, occurring in the three-way catalyst, are highly sensitive to the reaction condition: The reaction is inactive in both too low and too high temperature conditions, as well as high O2 partial pressure condition. In the present theoretical study, we performed kinetic analyses of the NO-CO-O2 model reaction system, taking place on Rh(111) surface, on the basis of density functional theory calculations. The temperature/pressure dependences were incorporated into account by coverage dependent reaction and activation free energies. The kinetic analyses revealed the existence of peak temperature for NO reduction reaction as well as the switching of N + NO and N + N reaction mechanisms with respect to the temperature. The analyses for the surface coverages of intermediate species revealed that the coverages of N ad-atom and NO ad-molecule species are the crucial factor for the activity and selectivity of NO reduction reactions.
We investigate the Na-ion battery characteristics of SnS as a negative electrode for Na ion battery by first-principles calculations. Using energy analyses, we show a phase diagram of Na-Sn-S ternary systems by convex-hull curves, and clarify a possible reaction route considering intermediate products in discharge reactions. We calculate the voltage-capacity curves based on the Na / SnS reaction path obtained from the ternary phase diagram, and compare with the experimental result. It is found that the conversion reactions and subsequently the alloying reactions proceed in the SnS electrode, contributing to its high capacity compared with the metallic Sn electrode, in which only the alloying reactions progresses stepwise. To verify our calculation result, x-ray absorption spectra (XAS) are computed and compared with experimental XAS at S K-edge. The Na2S reaction product can precipitate in the SnS electrodes during the discharge process, and it is expected that the electrode is recovered to be SnS again after charging.
This article presents a density functional theory (DFT) study that explores the chemical interactions and mechanisms in Li/Na-MXene systems with the aim of improving the performance of rechargeable batteries. Experimental studies indicate the presence of chemical and physical adsorption mechanisms in these systems. To understand the interaction mechanisms in the charging/discharging process, we investigated the ion intercalation/adsorption process and the induced chemical shielding. Different possible surface terminations have been investigated to determine which type of interaction is more likely to exist at the interlayer surfaces. The DFT results obtained in this study suggested the use of various methods, such as surface modification and expansion of the interlayer distance, to enhance the energy storage performance; nuclear magnetic resonance measurements can be used to check whether the ideal surface modifications have been experimentally achieved.
Both rhodium and copper show a catalytic activity for nitric oxide (NO) reduction; however, the reaction mechanisms can be different. Herein, we elucidate the difference in the NO reduction mechanisms between Rh and Cu clusters regarding the electronic structures using DFT computations and small cluster models involving four metal atoms. The computational results show that the dissociative adsorption proceeds on the Rh cluster with the reaction barrier of 33 kcal mol−1. The calculated heat of the reaction is almost zero. On the Cu cluster, the calculated reaction barrier reaches to 78 kcal mol−1 indicating that the dissociative adsorption hardly occurs. Instead of the dissociative adsorption, dimerization of NO initiates the catalytic NO reduction on Cu cluster. The calculated energy barrier for the dimerization is 8 kcal mol−1. The adsorbed NO dimer has a similar stability to co-adsorbed two NO molecules. In contrast, the dimerization hardly occurs on the Rh cluster; the reaction pathway is remarkably endothermic, and a stable adsorbed product is not found. The adsorption structures of NO can explain such differences. On Cu cluster, NO takes bent-nitrosyl conformation that acts as an electron acceptor. On Rh cluster, NO acts as an electron donor having linear-nitrosyl conformation.