Ryosuke Jinnouchi

What Makes the Photocatalytic CO2 Reduction on N-Doped Ta2O5 Efficient: Insights from Nonadiabatic Molecular Dynamics

Recent experimental studies demonstrated that photocatalytic CO2 reduction by Ru catalysts assembled on N-doped Ta2O5 surface is strongly dependent on the nature of the anchor group with which the Ru complexes are attached to the substrate. We report a comprehensive atomistic analysis of electron transfer dynamics in electroneutral Ru(di-X-bpy) (CO)2Cl2 complexes with X = COOH and PO3H2 attached to the N-Ta2O5 substrate. Nonadiabatic molecular dynamics simulations indicate that the electron transfer is faster in complexes with COOH anchors than in complexes with PO3H2 groups, due to larger nonadiabatic coupling. Quantum coherence counteracts this effect, however, to a small extent. The COOH anchor promotes the transfer with significantly higher frequency modes than PO3H2, due to both lighter atoms (C vs P) and stronger bonds (double vs single). The acceptor state delocalizes onto COOH, but not PO3H2, further favoring electron transfer in the COOH system. At the same time, the COOH anchor is prone to decomposition, in contrast to PO3H2, making the former show smaller turnover numbers in some cases. These theoretical predictions are consistent with recent experimental results, legitimating the proposed mechanism of the electron transfer. We emphasize the role of anchor stability, nonadiabatic coupling, and quantum coherence in determining the overall efficiency of artificial photocatalytic systems.

First principles study of sulfuric acid anion adsorption on a Pt(111) electrode

A first principles theory combined with a continuum electrolyte theory is applied to adsorption of sulfuric acid anions on Pt(111) in 0.1 M H(2)SO(4) solution. The theoretical free energy diagram indicates that sulfuric acid anions adsorb as bisulfate in the potential range of 0.41 < U ≤ 0.48 V (RHE) and as sulfate in 0.48 V (RHE) < U. This diagram also indicates that sulfate inhibits formations of surface oxide and hydroxide. Charge analysis shows that the total charge transferred for the formation of the full coverage sulfate adlayer is 90 μC cm(-2), and that the electrosorption valency value is -0.45 to -0.95 in 0.41 < U ≤ 0.48 V (RHE) and -1.75 to -1.85 in U > 0.48 V (RHE) in good agreement with experiments reported in the literature. Vibration analysis indicates that the vibration frequencies observed experimentally at 1250 and 950 cm(-1) can be assigned, respectively, to the S-O (uncoordinated) and symmetric S-O stretching modes for sulfate, and that the higher frequency mode has a larger potential-dependence (58 cm(-1) V(-1)) than the lower one.

Activities and Stabilities of Au-Modified Stepped-Pt Single-Crystal Electrodes as Model Cathode Catalysts in Polymer Electrolyte Fuel Cells

The purpose of this study is to test the concept of protecting vulnerable sites on cathode catalysts in polymer electrolyte fuel cells. Pt single-crystal surfaces were modified by depositing Au atoms selectively on (100) step sites and their electrocatalytic activities for oxygen reduction reaction (ORR) and stabilities against potential cycles were examined. The ORR activities were raised by 70% by the Au modifications, and this rise in the activity was ascribed to enhanced local ORR activities on Pt(111) terraces by the surface Au atoms. The Au modifications also stabilized the Pt surfaces against potential cycles by protecting the low-coordinated (100) step sites from surface reorganizations. Thus, the surface modification by selective Au depositions on vulnerable sites is a promising method to enhance both the ORR activity and durability of the catalysts.

Catalytic Activity of Pt/TaB2(0001) for the Oxygen Reduction Reaction

Proton-exchange-membrane fuel cells (PEMFCs) are a promising power source for automobiles. For their wide application, however, there still remain several problems. 2] One problem is the limited mass activity (reaction rate per mass) of cathode electrocatalysts for the oxygen reduction reaction (ORR). Bulk Pt has a high specific activity (reaction rate per surface area), and the specific activity can be further increased by alloying the subsurfaces with several nonprecious metals, such as Fe, Co, Ni, Cu, Sc, or Y, or by replacing subsurfaces with Pd. However, the specific areas (surface area per mass of the precious metal) of bulk materials are small, and therefore, the mass activities (specific activity multiplied by specific area) are also small. To increase the mass activity, the specific surface area should be increased by decreasing the catalyst size to the nanometer scale. Although Pt nanoparticles supported on carbon (Pt/C) are used practically in PEMFCs, the mass activity is not sufficiently high because the decrease in the size of the catalyst leads to a decrease in the specific activity as a result of the so-called particle-size effect. 9, 10] To avoid the particlesize effect, the specific surface area must be increased while maintaining the extended bulklike surface morphology. This new approach was employed by the company 3M in the development of nanostructured thin-film (NSTF) catalysts, in which Pt films with a thickness of a few tens of nanometers are deposited on organic nanostructured whiskerlike supports. The discovery of these new electrocatalysts inspired a number of studies on the fabrication of electrocatalysts with an extended Pt surface and high specific surface area with the aim of further increasing the mass activity. Herein, we show that a high mass activity of 1890 Ag , which is six times as high as that of Pt/C (299 Ag ), can be attained by the use of an epitaxial Pt thin film with a thickness of 1.5 nm on a TaB2(0001) single-crystal substrate. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of the Pt/TaB2 structure at the 10 10 TaB2 incidence and the 2 1 10 TaB2 incidence are shown in Figure 1. TaB2(0001) was selected because of its strong bonding with Pt, as shown by our DFT calculations, which indicated a cohesive energy of the Pt monolayer on TaB2(0001) terminated with Ta of 6.47 eV, which is much larger than that of a Pt monolayer on a graphene sheet (3.76 eV) or on Pt(111) (5.06 eV). We deposited Pt on the cleaned TaB2(0001) substrate and carried out CO annealing to obtain a flat and uniform Pt surface. Figure 2a shows the cyclic voltammogram (CV) recorded during CO annealing. In the anodic scan of the first cycle, the oxidation current appeared at approximately 0.5 V and then gradually increased (preignition potential region). This oxidation current disappeared in following cycles. The oxidation current in the preignition region is due to CO oxidation at the sites of Pt adatoms and adislands; thus, the disappearance of this current suggests the elimination of the Pt adatoms and adislands. Figure 2b shows the voltammogram for CO stripping in an argon-purged solution. The electrochemical surface area (ECSA) was estimated from the charge of 420 mC cm 2 required for CO oxidation to be 0.21 cm, which corresponds to a roughness factor (ECSA/ geometrical surface area) of 1.06. The ORR activity of the Pt/ TaB2(0001) alloy was evaluated by linear sweep voltammetry with a rotating disk electrode (measurement at 1600 rpm) under oxygen-saturated conditions (Figure 2c). The current on Pt/TaB2(0001) was corrected to compensate for the geometrical conditions of the working electrode (see Figure S1 in the Supporting Information for details). The specific activity of Pt/TaB2(0001) (kinetic current at 0.9 V) was 4961 mAcm , which is more than twice that observed for polycrystalline Pt (1400 mAcm ) and Pt(111) (1867 mAcm ). The mass activity of Pt/TaB2(0001) was 1890 Ag , which is almost six times that of Pt/C (299 Ag ). The CVs recorded under argon-saturated conditions are shown in Figure 2d. The shape of the CV of Pt/ TaB2(0001) is more similar to that of polycrystalline Pt than to Figure 1. HAADF-STEM images of the epitaxial Pt thin film on the TaB2(0001) single-crystal substrate as observed for: a) 10 10 1⁄2 TaB2 incidence; b) 2 1 10 1⁄2 TaB2 incidence.

Predicting Catalytic Activity of Nanoparticles by a DFT-Aided Machine-Learning Algorithm

Catalytic activities are often dominated by a few specific surface sites, and designing active sites is the key to realize high-performance heterogeneous catalysts. The great triumphs of modern surface science lead to reproduce catalytic reaction rates by modeling the arrangement of surface atoms with well-defined single-crystal surfaces. However, this method has limitations in the case for highly inhomogeneous atomic configurations such as on alloy nanoparticles with atomic-scale defects, where the arrangement cannot be decomposed into single crystals. Here, we propose a universal machine-learning scheme using a local similarity kernel, which allows interrogation of catalytic activities based on local atomic configurations. We then apply it to direct NO decomposition on RhAu alloy nanoparticles. The proposed method can efficiently predict energetics of catalytic reactions on nanoparticles using DFT data on single crystals, and its combination with kinetic analysis can provide detailed information on structures of active sites and size- and composition-dependent catalytic activities.

Theoretical Insights into the Impact of Ru Catalyst Anchors on the Efficiency of Photocatalytic CO2 Reduction on Ta2O5

We present a computational study of the dynamical and electronic structure origins of the impact of anchoring groups, PO3H2, COOH, and OH, on the efficiency of photochemical CO2 reduction in Ru(di-X-bpy)(CO)2Cl2/Ta2O5 systems. Recent experimental studies indicate that the efficiency may not directly correlate with the driving force for electron transfer (ET) in these systems, prompting the need for further investigation of the role of anchor groups. Our analysis shows that there are at least two key roles of the anchor in determining the efficiency of CO2 reduction by the Ru complex. First, depending on local steric interactions, different tilting angles and their fluctuations may emerge for different anchors, affecting the magnitude of the donor-acceptor coupling. Second, depending on localization of acceptor states on the anchor, determined by the anchors tendency to form conjugate subsystems, the yields of ET to the catalytic center may vary, directly affecting the photocatalytic efficiency. Finally, our calculations indicate that surface modeling with N-doping and many-body effects are needed to describe the ET process in the systems properly. N-doping imparts the Ta2O5 surface with a dipole moment, while Coulomb and exchange contributions to the electron-hole interaction can produce excitons that should be taken into account.

Kinetically induced irreversibility in electro-oxidation and reduction of Pt surface

A mean field kinetic model was developed for electrochemical oxidations and reductions of Pt(111) on the basis of density functional theory calculations, and the reaction mechanisms were analyzed. The model reasonably describes asymmetric shapes of cyclic voltammograms and small Tafel slopes of relevant redox reactions observed in experiments without assuming any unphysical forms of rate equations. Simulations using the model indicate that the oxidation of Pt(111) proceeds via an electrochemical oxidation from Pt to PtOH and a disproportionation reaction from PtOH to PtO and Pt, while its reduction proceeds via two electrochemical reductions from PtO to PtOH and from PtOH to Pt.

A Universal 3D Voxel Descriptor for Solid-State Material Informatics with Deep Convolutional Neural Networks

Material informatics (MI) is a promising approach to liberate us from the time-consuming Edisonian (trial and error) process for material discoveries, driven by machine-learning algorithms. Several descriptors, which are encoded material features to feed computers, were proposed in the last few decades. Especially to solid systems, however, their insufficient representations of three dimensionality of field quantities such as electron distributions and local potentials have critically hindered broad and practical successes of the solid-state MI. We develop a simple, generic 3D voxel descriptor that compacts any field quantities, in such a suitable way to implement convolutional neural networks (CNNs). We examine the 3D voxel descriptor encoded from the electron distribution by a regression test with 680 oxides data. The present scheme outperforms other existing descriptors in the prediction of Hartree energies that are significantly relevant to the long-wavelength distribution of the valence electrons. The results indicate that this scheme can forecast any functionals of field quantities just by learning sufficient amount of data, if there is an explicit correlation between the target properties and field quantities. This 3D descriptor opens a way to import prominent CNNs-based algorithms of supervised, semi-supervised and reinforcement learnings into the solid-state MI.

Analysis of Electronic State of Pt Small Particles on Carbon Substrate by sXPS

INTRODUCTION For wide commercialization of fuel cell technologies, reduction in the required amount of platinum is one of the top-prioritized issues. The catalytic activity toward oxygen reduction reaction of platinum, which is the element having the highest activity as a single metal, is known to be dependent on kinds of neighboring elements: i.e. alloy etc., and particle size. This dependency and catalytic activities of modified platinum should stem from the electronic state of platinum particles because catalytic reactions proceed by the electronic interaction among catalysts and reactants. Nevertheless, actual measurement of realistic platinum particles has not been carried out to clarify the relation between the catalytic activity and electronic state of Pt. In this communication, the electronic state of surface platinum atoms on carbon substrate was measured by sXPS (soft X-ray Photoelectron Spectroscopy) and calculated by DFT as the first steps of the discussion on the electronic state and catalytic activity.