First-Principles Calculation, Synthesis, and Catalytic Properties of Rh-Cu Alloy Nanoparticles

Atomically precise rhodium-indium carbonyl nanoclusters: synthesis, characterization, crystal structure and electron-sponge features

In this paper we present the investigation of the reactivity of [Rh7(CO)16]3- with InCl3, with the aim of expanding the more general study that allowed us to obtain, among other species, the icosahedral [Rh12E(CO)27]n- (n = 4 when E = Ge or Sn; n = 3 when E = Sb or Bi) family of clusters. Indeed, the study resulted in the isolation and characterization of the analogous In-centred icosahedral [Rh12In(CO)28]3- nanocluster (1), which is isoelectronic and isostructural with the [Rh12E(CO)27]n- congeners. During the course of the reaction two more new species, namely the octahedral [Rh6(CO)15InCl3]2- (2) and the dimeric [{Rh6(CO)15InCl2}2]2- (3) have also been identified. The reaction between [Rh7(CO)16]3- and InCl3 proved to be poorly selective; nevertheless, by fine tuning some reaction parameters it was possible to drive the reaction more towards one product or the other. Alternatively, [Rh6(CO)15InCl3]2- can be more selectively prepared by reacting either [Rh5(CO)15]- or, less efficiently, [Rh6(CO)15]2- with InCl3. As for the dimeric [{Rh6(CO)15InCl2}2]2- species, this was only isolated by carrying out the reaction with [Rh7(CO)16]3- under inert atmosphere, as opposed to under CO. All clusters were characterized by IR spectroscopy and ESI-MS, and their molecular structures were fully established by single-crystal X-ray diffraction studies. The [Rh12In(CO)28]3- species was also analysed by EDS via SEM, and further investigated through in situ infrared spectroelectrochemistry and CV experiments to check its multivalence nature. Indeed, [Rh12In(CO)28]3- can reversibly undergo two monoelectronic oxidation and one bi-electronic reduction processes, behaving like an electron sponge and, thus, giving rise to the further [Rh12In(CO)28]n- derivatives (n = 1, 2 and 5). These results parallel the findings for the [Rh12E(CO)27]n- series. The geometry variations of the metal framework associated with the changes in the cluster negative charge were investigated by means of DFT calculations.

Slow Synthesis Methodology-Directed Immiscible Octahedral Pdx Rh1-x Dual-Atom-Site Catalysts for Superior Three-Way Catalytic Activities over Rh

This study provided an effective strategy to construct dual-atom sites by solid-solution alloying. A slow synthesis methodology was established for the solid-solution preparations as dual-atom-site catalysts. The atomic-level homogeneous Pd_x Rh_1-x dual-atom-site catalysts were successfully synthesized over the whole composition range, as evidenced by X-ray powder diffraction and scanning transmission electron microscope energy-dispersive X-ray spectroscopy mapping measurements. The challenging morphology formation in the immiscible alloys was achieved by an energy-controlling process as the octahedral Rh-rich alloys. The Pd_0.3 Rh_0.7 dual-atom-site catalyst had unique surface states to activate the key reactants of CO and NO in the complex three-way catalytic reactions, and it performed significantly better than pure Rh.

A highly efficient three-solvent methodology for separating colloidal nanoparticles

This study has established a three-solvent methodology for separating nanomaterials, such as monometallic nanoparticles, miscible and immiscible nanoalloys. After systematically investigating the separation methods in two-solvent and three-solvent systems, a three-solvent theoretical model has been proposed to thoroughly reveal the centrifugation mechanism of colloidal particles. PVP plays an important role in the formation of emulsion droplets as the key factor for separation. Based on the three-solvent model, a novel solvent system has been discovered, with low-toxicity solvents and high separation efficiency. This study can open a new path for the separation of colloidal particles in both research and industrial fields and further promote the development of functional nanomaterials in greener pathways.

Phase Control of Noble Monometallic and Alloy Nanomaterials by Chemical Reduction Methods

In recent years, the phase control of monometallic and alloy nanomaterials has attracted great attention because of the potential to tune the physical and chemical properties of these species. In this Review, an overview of the latest research progress in phase-controlled monometallic and alloy nanomaterials is first given. Then, the phase-controlled synthesis using a chemical reduction method are discussed, and the formation mechanisms of these nanomaterials are specifically highlighted. Lastly, the challenges and future perspectives in this new research field are discussed.

Synthesis of a binary alloy nanoparticle catalyst with an immiscible combination of Rh and Cu assisted by hydrogen spillover on a TiO2 support

This work demonstrated the use of TiO2 as a promising platform for the synthesis of non-equilibrium RhCu binary alloy nanoparticles (NPs). These metals are regarded as immiscible based on their phase diagram but form NPs with the aid of the significant hydrogen spillover on TiO2 with concurrent proton-electron transfer. The resulting RhCu/TiO2 exhibited 2.6 times higher catalytic activity than Rh/TiO2 during hydrogen production from the hydrolysis of ammonia borane (AB), due to a synergistic effect. Theoretical simulations showed a higher energy value for the adsorption of AB on the RhCu alloy and a lower activation energy for the rate determining N-B bond dissociation by the attack of H2O during AB hydrolysis compared to monometallic Rh. High-angle annular dark-field scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy confirmed the formation of RhCu alloy NPs with a mean diameter of 2.0 nm on the TiO2. H2-temperature programmed reduction and in situ X-ray absorption fine structure analyses at elevated temperature under H2 demonstrated that Rh3+ and Cu2+ precursors were simultaneously reduced only on the TiO2 support. This effect resulted from the improved and limited reducibility of Cu2+ and Rh3+, respectively. The rate of hydrogen spillover of TiO2 is faster as compared to γ-Al2O3 and MgO as evidenced by sequential H2/D2 exchanges during in situ Fourier transform infrared analyses. Density functional theory calculations also showed that the migration of H atoms on TiO2 proceeds with a lower energy barrier than that on Al2O3, and the reduction of Cu2+ species is facilitated by H spillover on the support rather than by direct reduction by H2. These results confirm the vital role of TiO2 in the formation of the alloy and may represent a new strategy for the synthesis of different non-equilibrium solid solution alloys.

Ultrathin Pd-based nanosheets: syntheses, properties and applications

Two-dimensional (2D) noble metal-based nanosheets (NSs) have received considerable interest in recent years due to their unique properties and widespread applications. Pd-based NSs, as a typical member of 2D noble metal-based NSs, have been most extensively studied. In this review, we first summarize the research progress on the synthesis of Pd-based NSs, including pure Pd NSs, Pd-based alloy NSs, Pd-based core-shell NSs and Pd-based hybrid NSs. The synthetic strategy and growth mechanism are systematically discussed. Then their properties and applications in catalysis, biotherapy, gas sensing and so on are introduced in detail. Finally, the challenges and opportunities towards the rational design and controlled synthesis of Pd-based NSs are proposed.

A Cu-Pd single-atom alloy catalyst for highly efficient NO reduction

A series of Cu-Pd alloy nanoparticles supported on Al2O3 were prepared and tested as catalysts for deNO x reactions. XRD, HAADF-STEM, XAFS, and FT-IR analyses revealed that a single-atom alloy structure was formed when the Cu/Pd ratio was 5, where Pd atoms were well isolated by Cu atoms. Compared with Pd/Al2O3, Cu5Pd/Al2O3 exhibited outstanding catalytic activity and N2 selectivity in the reduction of NO by CO: for the first time, the complete conversion of NO to N2 was achieved even at 175 °C, with long-term stability for at least 30 h. High catalytic performance was also obtained in the presence of O2 and C3H6 (model exhaust gas), where a 90% decrease in Pd use was achieved with minimum evolution of N2O. Kinetic and DFT studies demonstrated that N-O bond breaking of the (NO)2 dimer was the rate-determining step and was kinetically promoted by the isolated Pd.

Hydrogen in Palladium and Storage Properties of Related Nanomaterials: Size, Shape, Alloying, and Metal-Organic Framework Coating Effects

One of the key issues for an upcoming hydrogen energy-based society is to develop highly efficient hydrogen-storage materials. Among the many hydrogen-storage materials reported, transition-metal hydrides can reversibly absorb and desorb hydrogen, and have thus attracted much interest from fundamental science to applications. In particular, the Pd-H system is a simple and classical metal-hydrogen system, providing a platform suitable for a thorough understanding of ways of controlling the hydrogen-storage properties of materials. By contrast, metal nanoparticles have been recently studied for hydrogen storage because of their unique properties and the degrees of freedom which cannot be observed in bulk, i. e., the size, shape, alloying, and surface coating. In this review, we overview the effects of such degrees of freedom on the hydrogen-storage properties of Pd-related nanomaterials, based on the fundamental science of bulk Pd-H. We shall show that sufficiently understanding the nature of the interaction between hydrogen and host materials enables us to control the hydrogen-storage properties though the electronic-structure control of materials.

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 Cu_x Ru_1-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.

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 Ir_x Cu_1-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 Ir_x Cu_1-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.

Selective control of fcc and hcp crystal structures in Au-Ru solid-solution alloy nanoparticles

Binary solid-solution alloys generally adopt one of three principal crystal lattices—body-centred cubic (bcc), hexagonal close-packed (hcp) or face-centred cubic (fcc) structures—in which the structure is dominated by constituent elements and compositions. Therefore, it is a significant challenge to selectively control the crystal structure in alloys with a certain composition. Here, we propose an approach for the selective control of the crystal structure in solid-solution alloys by using a chemical reduction method. By precisely tuning the reduction speed of the metal precursors, we selectively control the crystal structure of alloy nanoparticles, and are able to selectively synthesize fcc and hcp AuRu₃ alloy nanoparticles at ambient conditions. This approach enables us to design alloy nanomaterials with the desired crystal structures to create innovative chemical and physical properties.

The crystal structure of a solid-solution alloy is generally determined by its elemental composition, limiting synthetic control over the alloy’s properties. Here, the authors are able to selectively control the crystal structure of Au–Ru alloy nanoparticles by rationally tuning the reduction speed of the metal precursors.

Solid-Solution Alloying of Immiscible Ru and Cu with Enhanced CO Oxidation Activity

We report on novel solid-solution alloy nanoparticles (NPs) of Ru and Cu that are completely immiscible even above melting point in bulk phase. Powder X-ray diffraction, scanning transmission electron microscopy, and energy-dispersive X-ray measurements demonstrated that Ru and Cu atoms were homogeneously distributed in the alloy NPs. Ru_0.5Cu_0.5 NPs demonstrated higher CO oxidation activity than fcc-Ru NPs, which are known as one of the best monometallic CO oxidation catalysts.

Electronic Structure Evolution with Composition Alteration of RhxCuy Alloy Nanoparticles

The change in electronic structure of extremely small RhxCuy alloy nanoparticles (NPs) with composition variation was investigated by core-level (CL) and valence-band (VB) hard X-ray photoelectron spectroscopy. A combination of CL and VB spectra analyses confirmed that intermetallic charge transfer occurs between Rh and Cu. This is an important compensation mechanism that helps to explain the relationship between the catalytic activity and composition of RhxCuy alloy NPs. For monometallic Rh and Rh-rich alloy (Rh0.77Cu0.23) NPs, the formation of Rh surface oxide with a non-integer oxidation state (Rh(3-δ)+) resulted in high catalytic activity. Conversely, for alloy NPs with comparable Rh:Cu ratio (Rh0.53Cu0.47 and Rh0.50Cu0.50), the decreased fraction of catalytically active Rh(3-δ)+ oxide is compensated by charge transfer from Cu to Rh. As a result, ensuring negligible change in the catalytic activities of the NPs with comparable Rh:Cu ratio to those of Rh-rich and monometallic Rh NPs.

First-Principles Calculation, Synthesis, and Catalytic Properties of Rh-Cu Alloy Nanoparticles

The first synthesis of pure Rh_1−xCu_x solid‐solution nanoparticles is reported. In contrast to the bulk state, the solid‐solution phase was stable up to 750 °C. Based on facile density‐functional calculations, we made a prediction that the catalytic activity of Rh_1−xCu_x can be maintained even with 50 at % replacement of Rh with Cu. The prediction was confirmed for the catalytic activities on CO and NO_x conversions.