Denary High-Entropy Oxide Nanoparticles Synthesized by a Continuous Supercritical Hydrothermal Flow Process.

High-entropy oxide nanoparticles (HEO NPs) have been intensively studied because of their attractive properties, such as high stability and enhanced catalytic activity. In this work, for the first time, denary HEO NPs were successfully synthesized using a continuous supercritical hydrothermal flow process without calcination. Interestingly, this process allows the formation of HEO NPs on the order of seconds at a relatively lower temperature. The synthesized HEO NPs contained 10 metal elements, La, Ca, Sr, Ba, Fe, Mn, Co, Ru, Pd, and Ir, and had a perovskite-type structure. Atomic-resolution high-angle annular dark-field scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy measurements revealed homogeneous dispersion of the 10 metal elements. The obtained HEO NPs also exhibited a higher catalytic activity for the CO oxidation reaction than that of the LaFeO3 NPs.

Continuous-Flow Chemical Synthesis for Sub-2 nm Ultra-Multielement Alloy Nanoparticles Consisting of Group IV to XV Elements.

Multielement alloy nanoparticles have attracted much attention due to their attractive catalytic properties derived from the multiple interactions of adjacent multielement atoms. However, mixing multiple elements in ultrasmall nanoparticles from a wide range of elements on the periodic table is still challenging because the elements have different properties and miscibility. Herein, we developed a benchtop 4-way flow reactor for chemical synthesis of ultra-multielement alloy (UMEA) nanoparticles composed of d-block and p-block elements. BiCoCuFeGaInIrNiPdPtRhRuSbSnTi 15-element alloy nanoparticles composed of group IV to XV elements were synthesized by sequential injection of metal precursors using the reactor. This methodology realized the formation of UMEA nanoparticles at low temperature (66 °C), resulting in a 1.9 nm ultrasmall average particle size. The UMEA nanoparticles have high durability and activity for electrochemical alcohol oxidation reactions and high tolerance to CO poisoning. These results suggest that the multiple interactions of UMEA efficiently promote the multistep alcohol oxidation reaction.

Molybdenum-Ruthenium-Carbon Solid-Solution Alloy Nanoparticles: Can They Be Pseudo-Technetium Carbide?

Technetium (Tc), atomic number 43, is an element that humans cannot freely use even in the 21st century because Tc is radioactive and has no stable isotope. In this report, we present molybdenum-ruthenium-carbon solid-solution alloy (MoxRu1-xCy) nanoparticles (NPs) that are expected to have an electronic structure similar to that of technetium carbide (TcCy). MoxRu1-xCy NPs were synthesized by annealing under a helium/hydrogen atmosphere following thermal decomposition of metal precursors. The obtained NPs had a solid-solution structure in the whole composition range. MoxRu1-xCy with a cubic structure (down to 30 atom % Mo in the metal ratio) showed a superconducting state, and the transition temperature (Tc) increased with increasing Mo composition. The continuous change in Tc across that of TcCy indicates the continuous control of the electronic structure by solid-solution alloying, leading to pseudo-TcCy. Density functional theory calculations indicated that the synthesized Mo0.53Ru0.47C0.41 has a similar electronic structure to TcC0.41.

Crystal Structure Control of Binary and Ternary Solid-Solution Alloy Nanoparticles with a Face-Centered Cubic or Hexagonal Close-Packed Phase.

The crystal structure significantly affects the physical and chemical properties of solids. However, the crystal structure-dependent properties of alloys are rarely studied because controlling the crystal structure of an alloy at the same composition is extremely difficult. Here, for the first time, we successfully demonstrate the synthesis of binary Ru-Pt (Ru/Pt = 7:3) and Ru-Ir (Ru/Ir = 7:3) and ternary Ru-Ir-Pt (Ru/Ir/Pt = 7:1.5:1.5) solid-solution alloy nanoparticles (NPs) with well-controlled hexagonal close-packed (hcp) and face-centered cubic (fcc) phases, through the chemical reduction method. The crystal structure control is realized by precisely tunning the reduction speeds of the metal precursors. The effect of the crystal structure on the catalytic performance of solid-solution alloy NPs is systematically investigated. Impressively, all the hcp alloy NPs show superior electrocatalytic activities for the hydrogen evolution reaction in alkaline solution compared with the fcc alloy NPs. In particular, hcp-RuIrPt exhibits extremely high intrinsic (mass) activity, which is 3.1 (3.2) and 6.7 (6.9) times enhanced compared to that of fcc-RuIrPt and commercial Pt/C.

Noble-Metal High-Entropy-Alloy Nanoparticles: Atomic-Level Insight into the Electronic Structure.

The compositional space of high-entropy-alloy nanoparticles (HEA NPs) significantly expands the diversity of the materials library. Every atom in HEA NPs has a different elemental coordination environment, which requires knowledge of the local electronic structure at an atomic level. However, such structure has not been disclosed experimentally or theoretically. We synthesized HEA NPs composed of all eight noble-metal-group elements (NM-HEA) for the first time. Their electronic structure was revealed by hard X-ray photoelectron spectroscopy and density function theory calculations with NP models. The NM-HEA NPs have a lower degeneracy in energy level compared with the monometallic NPs, which is a common feature of HEA NPs. The local density of states (LDOS) of every surface atom was first revealed. Some atoms of the same constituent element in HEA NPs have different LDOS profiles, whereas atoms of other elements have similar LDOS profiles. In other words, one atom in HEA loses its elemental identity and it may be possible to create an ideal LDOS by adjusting the neighboring atoms. The tendency of the electronic structure change was shown by supervised learning. The NM-HEA NPs showed 10.8-times higher intrinsic activity for hydrogen evolution reaction than commercial Pt/C, which is one of the best catalysts.

Continuous-Flow Reactor Synthesis for Homogeneous 1 nm-Sized Extremely Small High-Entropy Alloy Nanoparticles.

High-entropy alloy nanoparticles (HEA NPs) emerged as catalysts with superior performances that are not shown in monometallic catalysts. Although many kinds of synthesis techniques of HEA NPs have been developed recently, synthesizing HEA NPs with ultrasmall particle size and narrow size distribution remains challenging because most of the reported synthesis methods require high temperatures that accelerate particle growth. This work provides a new methodology for the fabrication of ultrasmall and homogeneous HEA NPs using a continuous-flow reactor with a liquid-phase reduction method. We successfully synthesized ultrasmall IrPdPtRhRu HEA NPs (1.32 ± 0.41 nm), theoretically each consisting of approximately 50 atoms. This average size is the smallest ever reported for HEA NPs. All five elements are homogeneously mixed at the atomic level in each particle. The obtained HEA NPs marked a significantly high hydrogen evolution reaction (HER) activity with a very small 6 mV overpotential at 10 mA/cm-2 in acid, which is one-third of the overpotential of commercial Pt/C. In addition, although mass production of HEA NPs is still difficult, this flow synthesis can provide high productivity with high reproducibility, which is more energy efficient and suitable for mass production. Therefore, this study reports the 1 nm-sized HEA NPs with remarkably high HER activity and establishes a platform for the production of ultrasmall and homogeneous HEA NPs.

Fabrication of Integrated Copper-Based Nanoparticles/Amorphous Metal-Organic Framework by a Facile Spray-Drying Method: Highly Enhanced CO2 Hydrogenation Activity for Methanol Synthesis.

We report on Cu/amUiO-66, a composite made of Cu nanoparticles (NPs) and amorphous [Zr6 O4 (OH)4 (BDC)6 ] (amUiO-66, BDC=1,4-benzenedicarboxylate), and Cu-ZnO/amUiO-66 made of Cu-ZnO nanocomposites and amUiO-66. Both structures were obtained via a spray-drying method and characterized using high-resolution transmission electron microscopy, energy dispersive spectra, powder X-ray diffraction and extended X-ray absorption fine structure. The catalytic activity of Cu/amUiO-66 for CO2 hydrogenation to methanol was 3-fold that of Cu/crystalline UiO-66. Moreover, Cu-ZnO/amUiO-66 enhanced the methanol production rate by 1.5-fold compared with Cu/amUiO-66 and 2.5-fold compared with γ-Al2 O3 -supported Cu-ZnO nanocomposites (Cu-ZnO/γ-Al2 O3 ) as the representative hydrogenation catalyst. The high catalytic performance was investigated using in situ Fourier transform IR spectra. This is a first report of a catalyst comprising metal NPs and an amorphous metal-organic framework in a gas-phase reaction.

Platinum-Group-Metal High-Entropy-Alloy Nanoparticles.

The platinum-group metals (PGMs) are six neighboring elements in the periodic table of the elements. Each PGM can efficiently promote unique reactions, and therefore, alloying PGMs would create ideal catalysts for complex or multistep reactions that involve several reactants and intermediates. Thus, high-entropy-alloy (HEA) nanoparticles (NPs) of all six PGMs (denoted as PGM-HEA) having a great variety of adsorption sites on their surfaces could be ideal candidates to catalyze complex reactions. Here, we report for the first time PGM-HEA and demonstrate that PGM-HEA efficiently promotes the ethanol oxidation reaction (EOR) with complex 12-electron/12-proton transfer processes. PGM-HEA shows 2.5 (3.2), 6.1 (9.7), and 12.8 (3.4) times higher activity than the commercial Pd/C, Pd black and Pt/C catalysts in terms of intrinsic (mass) activity, respectively. Remarkably, it records more than 1.5 times higher mass activity than the most active catalyst to date. Our findings pave the way for promoting complex or multistep reactions that are seldom realized by mono- or bimetallic catalysts.

Significant Enhancement of Hydrogen Evolution Reaction Activity by Negatively Charged Pt through Light Doping of W.

We report novel PtW solid-solution nanoparticles (NPs) produced through electrochemical cleaning of core/shell PtW@WO3 NPs. The resulting PtW NPs achieved a record hydrogen evolution reaction (HER) performance as a class of Pt-based solid-solution alloys. A current density of 10 mA cm-2 was reached with an overpotential of 19.4 mV, which is significantly lower than that of a commercial Pt catalyst (26.3 mV). The PtW NPs also exhibited long-term stability. Theoretical calculations revealed that negatively charged Pt atoms adjacent to a W atom provide favorable hydrogen adsorption energies for the HER, realizing significantly enhanced HER activity.

Rational Synthesis for a Noble Metal Carbide.

Transition metal carbides have attractive physical and chemical properties that are much different from their parent metals. Particularly, noble metal carbides are expected to be promising materials for a variety of applications, particularly as efficient catalysts. However, noble metal carbides have rarely been obtained because carbide phases do not appear in noble metal-carbon phase diagrams and a reasonable synthesis method to make noble metal carbides has not yet been established. Here, we propose a new synthesis method for noble metal carbides and describe the first synthesis of rhodium carbide using tetracyanoethylene (TCNE). The rhodium carbide was synthesized without extreme conditions, such as the very high temperature and/or pressure typically required in conventional carbide syntheses. Moreover, we investigated the electronic structure and catalytic activity for the hydrogen evolution reaction (HER). We found that rhodium carbide has much higher catalytic activity for HER than pure Rh. Our study provides a feasible strategy to create new metal carbides to help advance the field of materials science.

A CO Adsorption Site Change Induced by Copper Substitution in a Ruthenium Catalyst for Enhanced CO Oxidation Activity.

Ru is an important catalyst in many types of reactions. Specifically, Ru is well known as the best monometallic catalyst for oxidation of carbon monoxide (CO) and has been practically used in residential fuel cell systems. However, Ru is a minor metal, and the supply risk often causes violent fluctuations in the price of Ru. Performance-improved and cost-reduced solid-solution alloy nanoparticles of the Cu-Ru system for CO oxidation are now presented. Over the whole composition range, all of the Cux Ru1-x nanoparticles exhibit significantly enhanced CO oxidation activities, even at 70 at % of inexpensive Cu, compared to Ru nanoparticles. Only 5 at % replacement of Ru with Cu provided much better CO oxidation activity, and the maximum activity was achieved by 20 at % replacement of Ru by Cu. The origin of the high catalytic performance was found as CO site change by Cu substitution, which was investigated using in situ Fourier transform infrared spectra and theoretical calculations.

Influence of the Crystal Structure of Titanium Oxide on the Catalytic Activity of Rh/TiO2 in Steam Reforming of Propane at Low Temperature.

Solid oxide fuel cells (SOFCs) with liquefied petroleum gas (LPG) reduce CO2 emissions due to their high-energy-conversion efficiency. Although SOFCs can convert LPG directly, coking occurs easily by decomposition of hydrocarbons, including C-C bonds on the electrode of fuel cell stacks. It is therefore necessary to develop an active steam pre-reforming catalyst that eliminates the hydrocarbons at low temperature, in which waste heat of SOFCs is used. Herein, we show that the crystal structure of the TiO2 that anchors Rh particles is crucial for catalytic activity of Rh/TiO2 catalysts for propane pre-reforming. Our experimental results revealed that strong metal support interaction (SMSI) induced during H2 pre-reduction were optimized over Rh/TiO2 with a rutile structure; this catalyst catalyzed the reaction much more effectively than conventional Rh/γ-Al2 O3 . In contrast, the SMSI was too strong for Rh/TiO2 with an anatase structure, and the surface of the Rh particles was therefore covered mostly with partially reduced TiO2 . The result was very low activity.

Solid-Solution Alloy Nanoparticles of the Immiscible Iridium-Copper System with a Wide Composition Range for Enhanced Electrocatalytic Applications.

For the first time, we synthesize solid-solution alloy nanoparticles of Ir and Cu with a size of ca. 2 nm, despite Ir and Cu being immiscible in the bulk up to their melting over the whole composition range. We performed a systematic characterization on the nature of the Irx Cu1-x solid-solution alloys using powder X-ray diffraction, scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. The results showed that the Irx Cu1-x alloys had a face-centered-cubic structure; charge transfer from Cu to Ir occurred in the alloy nanoparticles, as the core-level Ir 4f peaks shifted to lower energy region with the increase in Cu content. Furthermore, we observed that the alloying of Ir with Cu enhanced both the electrocatalytic oxygen evolution and oxygen reduction reactions. The enhanced activities could be attributed to the electronic interaction between Ir and Cu arising from the alloying effect at atomic-level.

Phase control of solid-solution RuIn nanoparticles and their catalytic properties.

The properties of solids could be largely affected by their crystal structures. We achieved, for the first time, the phase control of solid-solution RuIn nanoparticles (NPs) from face-centred cubic (fcc) to hexagonal close-packed (hcp) crystal structures by hydrogen heat treatment. The effect of the crystal structure of RuIn alloy NPs on the catalytic performance in the hydrogen evolution reaction (HER) was also investigated. In the hcp RuIn NPs, enhanced HER catalytic performance was observed compared to the fcc RuIn NPs and monometallic Ru NPs. The intrinsic electronic structures of the NPs were investigated by valence-band X-ray photoelectron spectroscopy (VB-XPS). The d-band centre of hcp RuIn NPs obtained from VB-XPS was deeper than that of fcc RuIn NPs and monometallic Ru NPs, which is considered to enable the hcp RuIn NPs to exhibit enhanced HER catalytic performance.

First synthesis of RuSn solid-solution alloy nanoparticles and their enhanced hydrogen evolution reaction activity.

Solid-solution alloys based on platinum group metals and p-block metals have attracted much attention due to their promising potential as materials with a continuously fine-tunable electronic structure. Here, we report on the first synthesis of novel solid-solution RuSn alloy nanoparticles (NPs) by electrochemical cyclic voltammetry sweeping of RuSn@SnOx NPs. High-angle annular dark-field scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy maps confirmed the random and homogeneous distribution of Ru and Sn elements in the alloy NPs. Compared with monometallic Ru NPs, the RuSn alloy NPs showed improved hydrogen evolution reaction (HER) performance. The overpotentials of Ru0.94Sn0.06 NPs/C and Ru0.87Sn0.13 NPs/C to achieve a current density of 10 mA cm-2 were 43.41 and 33.19 mV, respectively, which are lower than those of monometallic Ru NPs/C (53.53 mV) and commercial Pt NPs/C (55.77 mV). The valence-band structures of the NPs investigated by hard X-ray photoelectron spectroscopy demonstrated that the d-band centre of RuSn NPs shifted downward compared with that of Ru NPs. X-ray photoelectron spectroscopy and X-ray absorption near-edge structure analyses indicated that in the RuSn alloy NPs, charge transfer occurs from Sn to Ru, which was considered to result in a downward shift of the d-band centre in RuSn NPs and to regulate the adsorption energy of intermediate Hads effectively, and thus enable the RuSn solid-solution alloy NPs to exhibit excellent HER catalytic properties.

B2-structured indium-platinum group metal high-entropy intermetallic nanoparticles.

We first report the synthesis of B2-structured indium-platinum group metal high-entropy intermetallic nanoparticles (In-PGM HEI NPs). The synthesis was achieved by a wet-chemistry method and subsequent heat treatment. The crystal structure of these NPs is unique in the coexistence of completely orderly arranged indium and disorderly arranged PGMs.

Catalytic Behavior of K-doped Fe/MgO Catalysts for Ammonia Synthesis Under Mild Reaction Conditions.

An important part of realizing a carbon-neutral society using ammonia will be the development of an inexpensive yet efficient catalyst for ammonia synthesis under mild reaction conditions (<400 °C, <10 MPa). Here, we report Fe/K(3)/MgO, fabricated via an impregnation method, as a highly active catalyst for ammonia synthesis under mild reaction conditions (350 °C, 1.0 MPa). At the mentioned conditions, the activity of Fe/K(3)/MgO (17.5 mmol h-1 gcat -1 ) was greater than that of a commercial fused iron catalyst (8.6 mmol h-1 gcat -1 ) currently used in the Haber-Bosch process. K doping was found to increase the ratio of Fe0 on the surface and turnover frequency of Fe in our Fe/K(3)/MgO catalyst. In addition, increasing the pressure to 3.0 MPa at the same temperature led to a significant improvement of the ammonia synthesis rate to 29.6 mmol h-1 gcat -1 , which was higher than that of two more expensive, benchmark Ru-based catalysts, which are also potential alternative catalysts. A kinetics analysis revealed that the addition of K enhanced the ammonia synthesis activity at ≥300 °C by changing the main adsorbed species from NH to N which can accelerate dissociative adsorption of nitrogen as the rate limiting step in ammonia synthesis.

Co Nanoparticle Catalysts Encapsulated by BaO-La2O3 Nanofractions for Efficient Ammonia Synthesis Under Mild Reaction Conditions.

Ruthenium catalysts may allow for realization of renewable energy-based ammonia synthesis processes using mild reaction conditions (<400 °C, <10 MPa). However, ruthenium is relatively rare and therefore expensive. Here, we report a Co nanoparticle catalyst loaded on a basic Ba/La2O3 support and prereduced at 700 °C (Co/Ba/La2O3_700red) that showed higher ammonia synthesis activity at 350 °C and 1.0-3.0 MPa than two benchmark Ru catalysts, Cs+/Ru/MgO and Ru/CeO2. The synthesis rate of the catalyst at 350 °C and 1.0 MPa (19.3 mmol h-1 g-1) was 8.0 times that of Co/Ba/La2O3_500red and 6.9 times that of Co/La2O3_700red. The catalyst showed ammonia synthesis activity at temperatures down to 200 °C. Reduction at the high temperature induced the formation of BaO-La2O3 nanofractions around the Co nanoparticles by decomposition of BaCO3, which increased turnover frequency, inhibited the sintering of Co nanoparticles, and suppressed ammonia poisoning. These strategies may also be applicable to other non-noble metal catalysts, such as nickel.

Compositional dependence of structures and hydrogen evolution reaction activity of platinum-group-metal quinary RuRhPdIrPt alloy nanoparticles.

Platinum-group-metal quinary RuRhPdIrPt alloy nanoparticles were synthesised with compositions slightly away from equimolar, and their crystal and electronic structures were investigated. Their lattice constant changed linearly with composition, while the d-band centre changed nonlinearly. Their catalytic activities for the hydrogen evolution reaction were not correlated with their d-band centre.

Porous nanographene formation on γ-alumina nanoparticles via transition-metal-free methane activation.

γ-Al2O3 nanoparticles promote pyrolytic carbon deposition of CH4 at temperatures higher than 800 °C to give single-walled nanoporous graphene (NPG) materials without the need for transition metals as reaction centers. To accelerate the development of efficient reactions for NPG synthesis, we have investigated early-stage CH4 activation for NPG formation on γ-Al2O3 nanoparticles via reaction kinetics and surface analysis. The formation of NPG was promoted at oxygen vacancies on (100) surfaces of γ-Al2O3 nanoparticles following surface activation by CH4. The kinetic analysis was well corroborated by a computational study using density functional theory. Surface defects generated as a result of surface activation by CH4 make it kinetically feasible to obtain single-layered NPG, demonstrating the importance of precise control of oxygen vacancies for carbon growth.