Rutile TiO2-Supported Pt Nanoparticle Catalysts for the Low-Temperature Oxidation of Ethane to Ethanol.

Structure-function relationships of supported metal nanoparticle catalysts in the CO-assisted oxidation of ethane to ethanol were investigated. A rutile TiO2-supported Pt nanoparticle catalyst exhibited the highest ethanol production rate and selectivity. During the reaction, sequential changes in the geometric/electronic states and the particle size of the Pt nanoparticles were observed. The comparison of the catalytic performances of model catalysts with controlled metal-support interactions revealed that Pt0 nanoparticles of 2-3 nm with a high fraction of the surface Ptδ+ species are highly active for the oxidation of ethane to ethanol. The coadded CO plays a pivotal role not only in tuning the oxidation state of the surface Pt but also in producing H2O2, which is the true oxidant for the reaction. The supported Pt nanoparticle uses in situ-generated H2O2 to activate ethane, where the C2H5OOH intermediate is formed through a nonradical mechanism and subsequently converted to C2H5OH. This reaction occurs even at 50 °C with an apparent activation energy of 32 kJ mol-1. The present study sheds light on the usefulness of surface-engineered Pt nanoparticles for the low-temperature oxidation of ethane to ethanol.

Theoretical Study on Carbon Monoxide Adsorption on Unsupported and γ-Al2O3-Supported Silver Nanoparticles: Size, Shape, and Support Effects.

Nanoparticles (NPs) supported on metal oxides exhibit high catalytic activities for various reactions. The shape and oxidation state of such NPs, which are related to the catalytic activity, are often determined by the support. Herein, we conducted a density functional theory study on isolated silver (Ag) NPs and two types of Ag-NPs supported on gamma-aluminum oxide (γ-Al2O3). First, carbon monoxide (CO) adsorption on the isolated Ag NPs was investigated for decahedra (D 5h ), icosahedra (I h ), and cuboctahedra (O h ) of various sizes. I h and O h NPs showed moderate size dependence, whereas D 5h NPs showed high size dependence when the height was below 1.4 nm. The enhancement of CO adsorption on D 5h NPs was attributed to the presence of superatomic states. Next, we performed geometrical optimization of Ag54/γ-Al2O3(110) with a decahedral shape. Two types of structures were obtained: amorphous Ag54(A) and locally fivefold symmetrical Ag54(B) structures. Both NPs on γ-Al2O3(110) were found to be positively charged, but electron transfer to the support occurred only from the Ag atoms at the two bottom layers, and the upper part of NPs was relatively neutral. The enhancement of CO adsorption on Ag54(B) disappeared due to loss of the high symmetry. In turn, the moderate size dependence of neutral isolated NPs can be applied.

How to Constrain Metal-Oxyl Bonds on a Solid Surface? Lesson from Isovalent Zn(II)-Oxyl and Ga(III)-Oxyl Bonds Isolated in Zeolite Matrix.

Isolation of the atomic O radical anion bound to a metal ion (metal-oxyl) on solid surfaces is highly desirable for an understanding of how we should design the surface structure for using oxyl as the reactive site. Owing to the analytical difficulty of oxyl, however, even identification of oxyl remains scarce. Herein, we report isovalent ZnII-oxyl and GaIII-oxyl bonds isolated in the zeolite matrix. Close similarities in reactivity, spectroscopic property, and bonding nature were observed between them, but their site requirements were entirely different; the former is generated at the monovalent ion-exchangeable site, whereas the latter at the divalent ion-exchangeable site. This study strongly suggests that tuning the polarization of the metal-oxygen bond using the charge-controlled lattice oxygens is a useful way to constrain surface metal-oxyl bonds.

Selective Hydrogenation of Cinnamaldehyde over the Stepped and Plane Surface of Pd Nanoparticles with Controlled Morphologies by CO Chemisorption.

Carbon monoxide (CO) molecules are attracting attention as capping agents that control the structure of metal nanoparticles. In this study, we aimed to control the shape and surface structure of Pd particles by reducing the supported Pd precursor with CO. The reduction of Pd nanoparticles with CO promoted the exposure of step sites and generated spherical and concave-tetrahedral Pd particles on carbon and SiO2 supports. On the other hand, conventional H2-reduced Pd particles show a flattened shape. The preferential exposure of the step sites by the adsorbed CO molecules was supported by the density functional theory-calculated surface energy and the Wulff construction. Morphology- and surface-controlled Pd nanoparticles were used to study the surface structure and morphology effects of Pd nanoparticles on cinnamaldehyde (CAL) hydrogenation. With an increase in the fraction of step sites on Pd nanoparticles, the hydrogenation activity and selectivity of hydrocinnamaldehyde (HCAL) increased. On step sites, the adsorption of the C═C bond of CAL proceeded preferentially, and HCAL was efficiently and selectively generated.

Enhancement of Alkaline Hydrogen Oxidation Reaction of Ru-Ir Alloy Nanoparticles through Bifunctional Mechanism on Ru-Ir Pair Site.

Anion exchange membrane fuel cells (AEMFCs) are being developed for practical use. However, it is necessary to improve the hydrogen oxidation reaction (HOR) under alkaline conditions to enhance the performance of AEMFCs. In this study, carbon-supported Ru-Ir alloy nanoparticle catalysts (Ru-Ir/C) were developed because they offer higher HOR activity compared with the Pt-based catalysts. The mechanism of HOR enhancement on Ru-Ir/C was studied to reveal the effect of the surface composition of Ru/Ir on the HOR activity. The results showed that the HOR activity is related to the surface pair probability of Ru-Ir, Ru-Ru, and Ir-Ir but not to the hydrogen binding energy (HBE). In addition, the Ru-Ir pair site was found to be highly active, which can promote the HOR through a bifunctional mechanism that involves Ru-Ir pairs providing reactive OH- and H species, respectively.

Structure-Activity Relationship of Iron Oxides for NO Reduction in the Presence of C3 H6 , CO, and O2.

Spinel-type NiFe2 O4 exhibited the highest NO reduction activity among base-metal oxides under simulated exhaust of a gasoline-powered vehicle. The structure-activity relationship of iron oxides has been investigated through both experimental and computational studies. Spinel iron oxide (γ-Fe2 O3 ) exhibited a much higher NO reduction activity than that of iron oxide with other structures (α-Fe2 O3 and LaFeO3 ). Operando IR measurements clarified that the spinel structure facilitated the reaction between NOx and adsorbed oxidized hydrocarbon or cyanide species. The high reactivity of the spinel structure was ascribed to the high adsorption energy of NO, as elucidated by DFT calculations. Furthermore, molecular orbital calculations demonstrated that the local coordination structure of the spinel iron oxide induced the involvement of not only σ but also π orbitals during NO adsorption on Fe atoms. This work clarified the origin of the structure-dependent activity of metal oxides, with a focus on their local coordination structures.

Identification of active sites in CO oxidation over a Pd/Al2O3 catalyst.

The active sites of Pd/Al2O3 catalysts for CO oxidations were identified by investigating the dependence of CO oxidation activities on the surface structure and morphology of Pd nanoparticles. The maximum catalytic activity was obtained for Pd particles approximately 2 nm in particle size. We performed structural analyses on the Pd surface through infrared (IR) spectroscopy of the adsorbed CO molecules. A positive correlation was obtained between catalytic activity and the fraction of linear CO adsorbed on Pd corner sites and Pd(111) facets, indicating that these sites are highly active for CO oxidation. X-ray absorption fine structure (XAFS) and spherical aberration-corrected scanning transmission electron microscopy (Cs-STEM) measurements demonstrated that Pd nanoparticles less than 2 nm in particle size with amorphous-like structures and Pd particles with large, well-ordered structures favor the formation of a high fraction of corner sites and Pd(111) facets, respectively.

Surface modification of Pt nanoparticles with other metals boosting the alkaline hydrogen oxidation reaction.

To enhance the hydrogen oxidation reaction (HOR) activity of Pt/C under alkaline conditions, the catalyst's surface was modified with fifteen metals. The surface modification enhanced the activity of Pt/C for the HOR beyond a change in hydrogen binding energy (HBE), and this can be attributed to a change in the reactivity of surface oxygen species to adsorbed hydrogen species.

Twinned/untwinned catalytic gold nanoparticles identified by applying a convolutional neural network to their Hough transformed Z-contrast images.

In this article, we demonstrate that a convolutional neural network (CNN) can be effectively used to determine the presence of twins in the atomic resolution scanning transmission electron microscopy (STEM) images of catalytic Au nanoparticles. In particular, the CNN screening of Hough transformed images resulted in significantly higher accuracy rates as compared to those obtained by applying this technique to the raw STEM images. The proposed method can be utilized for evaluating the statistical twining fraction of Au nanoparticles that strongly affects their catalytic activity.

Synthesis of Supported Bimetal Catalysts using Galvanic Deposition Method.

Supported bimetallic catalysts have been studied because of their enhanced catalytic properties due to metal-metal interactions compared with monometallic catalysts. We focused on galvanic deposition (GD) as a bimetallization method, which achieves well-defined metal-metal interfaces by exchanging heterogeneous metals with different ionisation tendencies. We have developed Ni@Ag/SiO2 catalysts for CO oxidation, Co@Ru/Al2 O3 catalysts for automotive three-way reactions and Pd-Co/Al2 O3 catalysts for methane combustion by using the GD method. In all cases, the catalysts prepared by the GD method showed higher catalytic activity than the corresponding monometallic and bimetallic catalysts prepared by the conventional co-impregnation method. The GD method provides contact between noble and base metals to improve the electronic state, surface structure and reducibility of noble metals.

Dynamic Behavior of Rh Species in Rh/Al2O3 Model Catalyst during Three-Way Catalytic Reaction: An Operando X-ray Absorption Spectroscopy Study.

The dynamic behavior of Rh species in 1 wt% Rh/Al2O3 catalyst during the three-way catalytic reaction was examined using a micro gas chromatograph, a NOx meter, a quadrupole mass spectrometer, and time-resolved quick X-ray absorption spectroscopy (XAS) measurements at a public beamline for XAS, BL01B1 at SPring-8, operando. The combined data suggest different surface rearrangement behavior, random reduction processes, and autocatalytic oxidation processes of Rh species when the gas is switched from a reductive to an oxidative atmosphere and vice versa. This study demonstrates an implementation of a powerful operando XAS system for heterogeneous catalytic reactions and its importance for understanding the dynamic behavior of active metal species of catalysts.

The Metal-Support Interaction Concerning the Particle Size Effect of Pd/Al2 O3 on Methane Combustion.

The particle size effect of Pd nanoparticles supported on alumina with various crystalline phases on methane combustion was investigated. Pd/θ, α-Al2 O3 with weak metal-support interaction showed a volcano-shaped dependence of the catalytic activity on the size of Pd particles, and the catalytic activity of the strongly interacted Pd/γ-Al2 O3 increased with the particle size. Based on a structural analysis of Pd nanoparticles using CO adsorption IR spectroscopy and spherical aberration-corrected scanning/transmission electron microscopy, the dependence of catalytic activity on Pd particle size and the alumina crystalline phase was due to the fraction of step sites on Pd particle surface. The difference in fraction of the step site is derived from the particle shape, which varies not only with Pd particle size but also with the strength of metal-support interaction. Therefore, this interaction perturbs the particle size effect of Pd/Al2 O3 for methane combustion.

Investigation of Reaction Mechanism of NO-C3H6-CO-O2 Reaction over NiFe2O4 Catalyst.

To elucidate the reaction mechanism of NO-C3H6-CO-O2 over NiFe2O4, we investigated the dynamics of the adsorbed and gaseous species during the reaction using operando Fourier transform infrared (FTIR). The NO reduction activity dependent on the C3H6 and CO concentrations suggested that NO is reduced by C3H6 under three-way catalytic conditions. From FTIR measurements and kinetic analysis, it was clarified that the acetate species reacted with NO-O2 to form N2 via NCO, and that the rate-limiting step of NO reduction was the reaction between CH3COO- and NO-O2. The NO reduction mechanism of the three-way catalyst on NiFe2O4 is different to that on platinum-group metal catalysts, on which NO reduction proceeds through N-O cleavage.

Preparation of TiO2-supported twinned gold nanoparticles by CO treatment and their CO oxidation activity.

We found that twinned gold nanoparticles (T-AuNPs) are generated at a high probability by CO treatment of TiO2-supported Au(iii) at room temperature. Because of the formation of T-AuNPs, the Au/TiO2 prepared by the CO treatment showed higher catalytic activity for CO oxidation than Au/TiO2 prepared by conventional H2 treatment.

Conversion of 5-hydroxymethylfurfural to a cyclopentanone derivative by ring rearrangement over supported Au nanoparticles.

Supported Au nanoparticles showed efficient catalytic performance for the ring rearrangement of 5-hydroxymethylfurfural (HMF) to a cyclopentanone derivative, 3-hydroxymethylcyclopentanone (HCPN), by taking advantage of the selective hydrogenation on Au nanoparticles and the Lewis acid catalysis of metal oxide supports. Among various metal oxide supported Au catalysts, the highest yield of HCPN was obtained by using Au/Nb2O5 (86% yield).

Statistical distribution of single atoms and clusters of supported Au catalyst analyzed by global high-resolution HAADF-STEM observation with morphological image-processing operation.

We have developed a quantitative particle size analytical method at the single atomic level employing electron microscopy and image processing for the investigation of supported metal catalysts. In the present study, a supported gold (Au) catalyst containing sub-nano clusters and individual atoms was globally observed by high-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) using spherical aberration (Cs)-corrected TEM. To fully extract structural information of the Au clusters and individual atoms from the HAADF-STEM images, a morphological image-processing operation was applied. The resulting mean particle size was in good agreement with particle sizes estimated from average information provided by X-ray absorption fine structure analysis. It is demonstrated that the present HAADF-STEM image analysis gives a quantitative particle size distribution measurement of supported Au clusters and individual atoms.

Size specifically high activity of Ru nanoparticles for hydrogen oxidation reaction in alkaline electrolyte.

The hydrogen oxidation reaction (HOR) in alkaline electrolyte was conducted on carbon-supported Ru nanoparticles (Ru/C) of which size was controlled in the range from approximately 2 to 7 nm. The HOR activity of Ru/C normalized by the metal surface area showed volcano shaped dependence on the particle size with a maximum activity at approximately 3 nm. The HOR activity of approximately 3 nm Ru/C was higher than commercially available Pt nanoparticles (ca. 2 nm) supported on carbon. The structural analysis of Ru/C using Cs-corrected scanning transmission electron microscopy with atomic resolution revealed the unique structural change of Ru/C different from Pt/C: Ru nanoparticle structure changed from amorphous-like structure below 3 nm to metal nanocrystallite with roughened surface at approximately 3 nm and then to that with well-defined facets above 3 nm, although Pt/C kept well-defined facets even at approximately 2 nm. It is proposed that the generation of unique structure observed on approximately 3 nm Ru nanoparticles, that is, long bridged coordinatively unsaturated Ru metal surface atoms on its nanocrystallite, is a key to achieve high HOR activity.

Surface oxygen-assisted Pd nanoparticle catalysis for selective oxidation of silanes to silanols.

Just add O(2): Based on the fact that an oxygen-adsorbed Pd metal surface shows higher reactivity for water dissociation than a clean Pd surface, carbon-supported Pd nanoparticles (NPs) with surface oxygen atoms were developed as a highly effective and reusable heterogeneous catalyst for selective oxidation of silanes to silanols with water as a green oxidant (see figure).