Synthesis of Heterogeneous Catalysts in Catalyst Informatics to Bridge Experiment and High-Throughput Calculation.

The coupling of high-throughput calculations with catalyst informatics is proposed as an alternative way to design heterogeneous catalysts. High-throughput first-principles calculations for the oxidative coupling of methane (OCM) reaction are designed and performed where 1972 catalyst surface planes for the CH4 to CH3 reaction are calculated. Several catalysts for the OCM reaction are designed based on key elements that are unveiled via data visualization and network analysis. Among the designed catalysts, several active catalysts such as CoAg/TiO2, Mg/BaO, and Ti/BaO are found to result in high C2 yield. Results illustrate that designing catalysts using high-throughput calculations is achievable in principle if appropriate trends and patterns within the data generated via high-throughput calculations are identified. Thus, high-throughput calculations in combination with catalyst informatics offer a potential alternative method for catalyst design.

Infusion of Variable Chemical Structure to Tune Stacking among Metal-Organic Layers in 2D Nano MOF.

Thickness of two-dimensional (2D) metal-organic frameworks (MOFs) govern their intriguing functionalities. Primarily this thickness is controlled by the stacking between the metal-organic layers (MOL). It is observed that until now such modulating factors for stacking efficiency of MOL are not well studied. Here, we report a fundamental hypothesis to comprehend regulation of stacking efficiency among MOLs as a function of chemical structure of organic ligands (dicarboxylic acids and pillar linkers). This basically involves a series of isostructural three-dimensional (3D) MOFs which contain linkers of variable chemical nature that could be depillared to generate 2D stacked MOFs of different thickness. Depending on the linkers, we encountered the formation of single MOL to stacked multiple MOLs as evidenced from atomic force microscopic and other experimental analysis. The present study gives a concrete correlation between the stacking within 2D MOFs (from monolayer to multilayers), and their 3D counter parts, which may provide a thickness tuning pathway for 2D MOFs.

Infusion of Variable Chemical Structure to Tune Stacking among Metal-Organic Layers in 2D Nano MOFs.

Invited for the cover of this issue is the group of Biplab Manna at the University of Kumamoto. The image depicts various kinds of stacking between the metal-organic layers of MOFs. Read the full text of the article at 10.1002/chem.202201665.

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.

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.

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.

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.

Catalytic NH3 oxidation affected by the nanometric roughness of the platinum overlayer.

Pulsed cathodic arc-plasma deposition was employed to create a few nanometre-thick Pt overlayer on a 50 µm-thick Fe-Cr-Al metal (SUS) foil, resulting in an effective NH3 oxidation catalyst fabrication. This catalyst exhibited a turnover frequency (TOF) exceeding 100 times that of Pt nanoparticles. In this study, Pt overlayer catalysts with varying degrees of surface roughness were fabricated using different metal foil substrates: mirror-polished (Pt/p-SUS), unpolished (Pt/SUS) and roughened by the formation of a surface oxide layer (Pt/Al2O3/SUS). The nanoscale roughness was comprehensively analysed using electron microscopy, laser scanning confocal microscopy and chemisorption techniques. NH3 oxidation activity, measured at 200 °C, followed an increasing trend in the order of Pt/Al2O3/SUS < Pt/SUS < Pt/p-SUS, despite a decrease in the apparent Pt surface area in the same order. Consequently, the calculated TOF was markedly higher for Pt/p-SUS (267 min-1) compared to Pt/SUS (107 min-1) and Pt/Al2O3/SUS (≤22 min-1). The smooth Pt overlayer surface also favoured N2 yield over N2O at this temperature. This discovery enhances our fundamental understanding of high-TOF NH3 oxidation over Pt overlayer catalysts, which holds significance for the advancement and industrial implementation of selective NH3 oxidation processes.

One-pot gram-scale rapid synthesis of MN4 complexes with 14-membered ring macrocyclic ligand as a precursor for carbon-based ORR and CO2RR catalysts.

Herein, CoN4, CuN4, and NiN4 complexes with a 14-membered ring hexaazamacrocycle ligand H2HAM were synthesised as precursors for ORR and CO2RR catalysts via a one-pot, gram-scale synthesis procedure, which involved microwave heating for only 10 min. Detailed structures of the obtained 14MR-MN4 complex were revealed by single-crystal X-ray diffraction measurements.

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.

Catalysts informatics: paradigm shift towards data-driven catalyst design.

Designing catalysts is a challenging matter as catalysts are involved with various factors that impact synthesis, catalysts, reactor and reaction. In order to overcome these difficulties, catalysts informatics is proposed as an alternative way to design and understand catalysts. The underlying concept of catalysts informatics is to design the catalysts from trends and patterns found in catalysts data. Here, three key concepts are introduced: experimental catalysts database, knowledge extraction from catalyst data via data science, and a catalysts informatics platform. Methane oxidation is chosen as a prototype reaction for demonstrating various aspects of catalysts informatics. This work summarizes how catalysts informatics plays a role in catalyst design. The work covers big data generation via high throughput experiments, machine learning, catalysts network method, catalyst design from small data, catalysts informatics platform, and the future of catalysts informatics via ontology. Thus, the proposed catalysts informatics would help innovate how catalysts can be designed and understood.

Atomic-Scale 3D Structure of a Supported Pd Nanoparticle Revealed by Electron Tomography with Convolution Neural Network-Based Image Inpainting.

Electron tomography based on scanning transmission electron microscopy (STEM) is used to analyze 3D structures of metal nanoparticles on the atomic scale. However, in the case of supported metal nanoparticle catalysts, the supporting material may interfere with the 3D reconstruction of metal nanoparticles. In this study, a deep learning-based image inpainting method is applied to high-angle annular dark field (HAADF)-STEM images of a supported metal nanoparticle to predict and remove the background image of the support. The inpainting method can separate an 11 nm Pd nanoparticle from the θ-Al2 O3 support in HAADF-STEM images of the θ-Al2 O3 -supported Pd catalyst. 3D reconstruction of the extracted images of the Pd nanoparticle reveals that the Pd nanoparticle adopts a deformed structure of the cuboctahedron model particle, resulting in high index surfaces, which account for the high catalytic activity for methane combustion. Using the xyz coordinate of each Pd atom, the local Pd-Pd bond distance and its variance in a real supported Pd nanoparticle are visualized, showing large strain and disorder at the Pd-Al2 O3 interface. The results demonstrate that 3D atomic-scale analysis enables atomic structure-based understanding and design of supported metal catalysts.

Cr-Fe-Ni-Cu Quaternary Nanostructure as a Substitute for Precious Metals in Automotive Three-Way Catalysts.

The replacement of precious metals (Rh, Pd, and Pt) in three-way catalysts with inexpensive and earth-abundant metal alternatives is an ongoing challenge. In this research, we examined various quaternary metal catalysts by selecting from six 3d transition metals, i.e., Cr, Mn, Fe, Co, Ni, and Cu, equimolar amounts (0.1 mol each), which were prepared on the Al2O3 support (1 mol Al) using H2 reduction treatment at 900 °C. Among 15 combinations, the best catalytic performance was achieved by the CrFeNiCu system. Light-off of NO-CO-C3H6-O2-H2O mixtures proceeded at the lowest temperature of ≤200 °C for CO, ≤300 °C for C3H6, and ≤400 °C for NO when the molar fraction of Cr in Cr x Fe0.1Ni0.1Cu0.1 was around x = 0.1. The activity for CO/C3H6 oxidation was superior to that of reference Pt/Al2O3 catalysts but was less active for NO reduction. The structural analysis using scanning transmission electron microscopy and X-ray absorption spectroscopy showed that the as-prepared catalyst consisted of FeNiCu alloy nanoparticles dispersed on the Cr2O3-Al2O3 support. However, the structural change occurred under a catalytic reaction atmosphere, i.e., producing NiCu alloy nanoparticles dispersed on a NiFe2O4 moiety and Cr2O3-Al2O3 support. The oxidation of CO/C3H6 can be significantly enhanced in the presence of Cr oxide, resulting in a faster decrease in O2 concentration and thus regenerating the NiCu metallic surface, which is active for NO reduction to N2.

High Proton Conductivity of 3D Graphene Oxide Intercalated with Aromatic Sulfonic Acids.

The development of efficient proton conductors that are capable of high power density, sufficient mechanical strength, and reduced gas permeability is challenging. Herein, we report the development of a series of aromatic sulfonic acid/graphene oxide hybrid membranes incorporating benzene sulfonic acid (BS), naphthalene sulfonic acid (NS), naphthalene disulfonic acid (DS) or pyrene sulfonic acid (PS) using a facile freeze dried method. For out-of-plane proton conductivity, the 3DGO-BS and 3DGO-NS yielded proton conductivities of 4.4×10-2  S cm-1 and 3.1×10-2  S cm-1 , respectively; this represents a two-times higher value than that which occurs for three dimensional graphene oxide (3DGO). Additionally, the respective prepared films as membranes in a proton exchange membrane fuel cell (PEMFC) show maximum power density of 98.76 mW cm-2 for 3DGO-NS while it is 92.75 mW cm-2 for 3DGO-BS which are close to double that obtained for 3DGO (50 mW cm-2 ).

Synergistic Strengthening in Graphene Oxide and Oxidized Single-walled Carbon Nanotube Hybrid Material for use as Electrolytes in Proton Exchange Membrane Fuel Cells.

Herein, we report an efficient proton exchange membrane formed from a synergistic combination of graphene oxide (GO) and oxidized single-walled carbon nanotube (CNTOX) by the freeze-drying route that gives rise to enhanced fuel cell power density. At 25 °C and 100% relative humidity (RH), the 3DGO-CNTOX hybrid shows remarkably high out-of-plane and in-plane proton conductivities of 6.64×10-2 and 5.08 S cm-1 , respectively. Additionally, the measured performance using prepared films as proton conduction membranes in a proton exchange membrane fuel cell (PEMFC) exhibited a peak power density of 117.21 mW cm-2 . The high performance of these films can be ascribed to the freeze-dried-driven structural morphology of 3DGO-CNTOX that facilitates higher water retention capacity as well as the synergistic strengthening effect between GO and CNTOX with a highly interconnected proton conduction network. The current results imply that the new 3DGO-CNTOX hybrid material has potential for wide application as a proton exchange membrane.

An in situ quick XAFS spectroscopy study on the formation mechanism of small gold nanoparticles supported by porphyrin-cored tetradentate passivants.

We previously reported that a porphyrin-cored tetradentate passivant, which has two disulfide straps over one face of the porphyrin plane, can produce monolayer-protected gold nanoparticles, 2-4 nm in size, by the one-pot reduction of HAuCl(4) in DMF. The resulting nanoparticles are smaller than those prepared using the same S/Au molar ratio of a monodentate passivant. To examine the formation mechanism of small gold nanoparticles, the formation of gold nanoparticles in the presence of porphyrin-cored tetradentate passivants or a structurally related monodentate passivant was studied by time-resolved quick X-ray absorption fine structure spectroscopy. The results demonstrated that all of Au ions in solution are reduced to compose small Au clusters, i.e. nuclei, just after the NaBH(4) reduction of HAuCl(4) in both cases, but their size varied with the initial S/Au molar ratios and structure of the passivants. Thus, the size of Au nuclei was kinetically controlled by the passivants. Interestingly, the porphyrin-cored tetradentate passivant could stabilize smaller gold nanoparticles, 2-4 nm in size, but it was less efficient in trapping the Au nuclei formed at a very early stage, in comparison to the monodentate passivant.

Representing the Methane Oxidation Reaction via Linking First-Principles Calculations and Experiment with Graph Theory.

Representing the chemical reaction is a challenging matter faced in chemistry due to the complex molecular interactions and difficulties faced when determining intermediate reactions that may occur throughout the reaction. Graph theory and network analysis are used with first-principles calculations and experiments to investigate possible intermediate reactions that may occur during a reaction; in this case, catalyst-free methane oxidation is chosen as the prototype reaction. Network analysis is used to help illuminate several key intermediate compounds that potentially appear throughout the course of the prototype reaction and the detailed mechanisms of methane oxidation while showing good agreement with experimental data. Presenting the chemical reaction as a network, therefore, makes it possible to link experimental and computational data in a space that accounts for the impact of intermediate reactions upon the outcome of the overall reaction, thereby making network analysis an alternative method for representing chemical reactions.

High Durability of a 14-Membered Hexaaza Macrocyclic Fe Complex for an Acidic Oxygen Reduction Reaction Revealed by In Situ XAS Analysis.

Nonplatinum metal (NPM) catalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs) have been developed; however, NPM catalysts still need to be improved in terms of both their catalytic activity and durability. To overcome these problems, an Fe active site contained within a more compact ligand than conventional, porphyrinic, 16-membered ring ligands, or more specifically, a hexaaza macrocyclic ligand with a 14-membered ring (14MR), was developed. In this study, the durability of the Fe-14MR complex was compared to that of Fe phthalocyanine (FePc), which has a 16-membered ring ligand, using in situ X-ray absorption spectroscopy; demetalation of the Fe complexes was directly observed during electrochemical experiments performed under acidic ORR conditions. It was found that Fe-14MR is significantly more resistant to demetalation than FePc during the ORR.

Data science assisted investigation of catalytically active copper hydrate in zeolites for direct oxidation of methane to methanol using H2O2.

Dozens of Cu zeolites with MOR, FAU, BEA, FER, CHA and MFI frameworks are tested for direct oxidation of CH4 to CH3OH using H2O2 as oxidant. To investigate the active structures of the Cu zeolites, 15 structural variables, which describe the features of the zeolite framework and reflect the composition, the surface area and the local structure of the Cu zeolite active site, are collected from the Database of Zeolite Structures of the International Zeolite Association (IZA). Also analytical studies based on inductively coupled plasma-optical emission spectrometry (ICP-OES), X-ray fluorescence (XRF), N2 adsorption specific surface area measurement and X-ray absorption fine structure (XAFS) spectral measurement are performed. The relationships between catalytic activity and the structural variables are subsequently revealed by data science techniques, specifically, classification using unsupervised and supervised machine learning and data visualization using pairwise correlation. Based on the unveiled relationships and a detailed analysis of the XAFS spectra, the local structures of the Cu zeolites with high activity are proposed.