Synthesis of Pd/Fe₃O₄ Hybrid Nanocatalysts with Controllable Interface and Enhanced Catalytic Activities for CO Oxidation

Palladium is an important catalyst for many industrial processes and chemical reactions. The conjunction of Pd and a metal oxide is of particular interest for improving catalytic performance in heterogeneous catalysis. Here we report the synthesis of Pd/Fe₃O₄ hybrid nanoparticles with controllable interface and the evaluation of their catalytic activities for CO oxidation. The synthesis involves a seed-mediated process in which Pd nanoparticles serve as seeds, followed by the deposition of the Fe₃O₄ layer in the solution phase. The adhesion of the oxide layer to the metal surface is through the reduced form of Fe. Upon thermal annealing, the Fe₃O₄ layer evolved from complete to partial coverage on the Pd core surface. This process is accompanied by increased crystallinity of Fe₃O₄. The resultant Pd–Fe₃O₄ nanoparticles with a partial Fe₃O₄ shell significantly lower the light-off temperature of CO oxidation

Selective Cation Exchange Enabled Growth of Lanthanide Core/Shell Nanoparticles with Dissimilar Structure

Core/shell nanostructure is versatile for improving or integrating diverse functions, yet it is still limited to homeomorphism with isomorphic core and shell structure. Here, we delineate a selective cation exchange strategy to construct lanthanide core/shell nanoparticles with dissimilar structure. Hexagonal NaLnF₄, a typical photon conversion material, was selected to grow cubic CaF₂ shell to protect surface exposed Ln³⁺. Preferential cation exchange between Ca²⁺ and Na⁺ triggered the surface hexagonal-to-cubic structure evolution, which remediated the large barrier for heteroepitaxy of monocrystalline CaF₂ shell. The heterostructured CaF₂ shell leads to greatly enhanced upconversion emission with increased absolute quantum yield from 0.2% to 3.7%. Moreover, it is advantageous in suppressing the interfacial diffusion of Ln³⁺, as well as the leakage of Ln³⁺ from nanoparticle to aqueous system. These findings open up a new avenue for fabricating heterostructured core/shell nanoparticles, and are instructive for modulating various properties

Synthesis and Demonstration of Subnanometric Iridium Oxide as Highly Efficient and Robust Water Oxidation Catalyst

Development of a highly efficient and robust water oxidation catalyst (WOC) with reduced usage of noble metals is extremely crucial for water splitting and CO₂ reduction by photocatalysis or electrolysis. Herein, we synthesized subnanometric iridium dioxide clusters supported on multiwalled carbon nanotubes (MWCNTs) by a chemical vapor deposition method (nominated as IrO₂/CNT). Benefiting from a mild oxidation process in air at 303 K, the deposited iridium clusters can be controlled to have a narrow size distribution from several atoms to 2 nm, having an average size of ca. 1.1 nm. The subnanometric iridium-containing sample is demonstrated to be highly efficient and robust for water oxidation. The optimal turnover frequency (TOF) of chemical water oxidation on the as-obtained sample can reach 11.2 s^–1, and the overpotential of electrochemical water oxidation is 249, and 293 mV at 10 mA cm^–2 in 1.0 M KOH (pH: 13.6), and 0.5 M H₂SO₄ (pH: 0), respectively. On the basis of the structural characterizations and theory simulation, the extraordinary performances of the ultrasmall iridium dioxide are proposed to mainly originate from enhanced number of unsaturated surface Ir atoms and change of local coordination environment. Our work highlights the importance of subnanometric size of iridium dioxide in water oxidation

Highly Efficient K_0.15MnO₂ Birnessite Nanosheets for Stable Pseudocapacitive Cathodes

In this paper, we reported a facile synthesis of Birnessite K_0.15MnO₂·0.43H₂O nanosheets in a solution phase. The structural and electrochemical properties of the K_0.15MnO₂ nanosheets for supercapacitor (SC) reactions were studied, and a gravimetric capacitance of 303 F/g was obtained at a charge/discharge current of 0.2 A/g. Electrochemical kinetics showed that a non-Faradaic (electrical double layer) current existed throughout the charging potential range, while a dominant Faradaic (pseudocapacitive) current was observed at high and low potentials during anodic and cathodic scans, respectively. Asymmetric pseudocapacitive full-cells were constructed with both anodic and cathodic K_0.15MnO₂ composite materials and subjected to long-term galvanostatic charge/discharge analyses. A specific capacitance of 67.8 F/g was obtained for the cathodic K_0.15MnO₂ full-cells after 1000 cycles, with a capacitive retention of 87.8% and Coulombic and energy efficiencies of ∼100 and ∼90%, respectively. <i>In situ</i> X-ray absorption near edge spectroscopy further corroborated the potential-dependent Faradaic reactions, suggesting a predominant change in valence state of K_0.15MnO₂ to occur between 0.3 and 0.6 V (vs Ag/AgCl). The present study not only underscores the structure–function relationship of MnO₂-based electrode materials for SC reactions but also provides a new approach in fabricating advanced pseudocapacitors by utilizing cost-effective transition metal oxide materials

Effects of Multiple Platinum Species on Catalytic Reactivity Distinguished by Electron Microscopy and X‑ray Absorption Spectroscopy Techniques

Supported platinum species in the forms of single atoms, ultrafine clusters, and metallic particles have been widely investigated because of their unique catalytic properties in diverse redox reactions. In this work, we used thermally stable ceria–zirconia–lanthana (Ce_0.5Zr_0.42La_0.08O_<i>x</i>) as an active oxide support to deposit platinum with different loading amounts from 0.5 to 2 at. % via an incipient wetness impregnation. The as-obtained samples were measured under the methane oxidation reaction conditions with high space velocities up to 100,000 mL·g^–1·h^–1. Here, 1 at. % Pt sample showed the best catalytic performance with a total reaction rate of 1.93 μmol_CH4·g_cat^–1·s^–1 and exclusive platinum rate of 24.4 mmol_CH4·mol_Pt^–1·s^–1 at 450 °C. Multiple characterization means, especially aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and X-ray absorption fine structure (XAFS) with the related profile fittings, were carried out to determine the electronic and local coordination structures of platinum. On the basis of these experimental evidence, we have distinguished the effects of different components and found that platinum oxide clusters (Pt_<i>x</i>O_<i>y</i>) with averaged sizes from subnanometer to 2–3 nm play an essential role for the oxidation of methane. Metallic Pt particles are probably active species, but their large-size characteristics impair the reactivity. However, ionic platinum single atoms may not be appropriate for this catalytic process

Shaping Single-Crystalline Trimetallic Pt–Pd–Rh Nanocrystals toward High-Efficiency C–C Splitting of Ethanol in Conversion to CO₂

Atomic-scale construction and high-throughput screening of robust multicomponent nanocatalysts with tunable well-defined surface structures and associated active sites for the ethanol electro-oxidation reaction (EOR) in high activity and selectivity, referring to C–C bond cleavage and full oxidation of ethanol as a clean and sustainable energy source, has remained a great challenge. Herein, we demonstrate a powerful conceptual approach to design, synthesize, and optimize single-crystalline Pt–Pd–Rh nanocrystals of altered shapes and compositions for enhanced EOR performance, based on combined density functional theory (DFT) calculations and experiment study. We prepared (111)-terminated Pt–Pd–Rh nanotruncated-octahedrons (NTOs) and (100)-terminated Pt–Pd–Rh nanocubes (NCs) with varied-compositions by regulating the reduction tendency of metal precursors in a facile hydrothermal method. Aided by DFT calculations, Pt₃PdRh NTOs, PtPdRh NTO, and 8.8 nm PtPdRh NCs-200 were screened to be the best performing catalysts with the highest EOR activity (five times as much as that of commercial Pt black) at 0.5 V vs NHE. Among these catalysts, PtPdRh NTOs exhibited the highest selectivity to CO₂ at 0.5 V and the noteworthy capability to fully oxidize ethanol at extremely low potential (0.35 V); 8.8 nm PtPdRh NCs-200 possessed the best durability. Morphology and surface composition correlated to the synergistic effect of three metals were verified to affect the EOR performance of well-shaped Pt–Pd–Rh nanocrystals. Combined with in situ FTIR, it was deduced that appropriate surface composition and exposed facets were the key factors to the promoted capability in the cleavage of C–C bond down to low potential. Through adjusting surface composition and morphology of Pt–Pd–Rh nanocrystals with homogeneous element distribution, enhanced EOR performance was achieved in light of DFT simulations of the two elementary reactions (i.e., breakdown of C–C bond and oxidation of CO_ad). This work has offered an effective and useful strategy to promote the reactivity of multicomponent heterogeneous nanocatalysts with optimized compositions and surface structures for many industrial catalytic processes

Dopant-Induced Modification of Active Site Structure and Surface Bonding Mode for High-Performance Nanocatalysts: CO Oxidation on Capping-free (110)-oriented CeO₂:Ln (Ln = La–Lu) Nanowires

Active center engineering at atomic level is a grand challenge for catalyst design and optimization in many industrial catalytic processes. Exploring new strategies to delicately tailor the structures of active centers and bonding modes of surface reactive intermediates for nanocatalysts is crucial to high-efficiency nanocatalysis that bridges heterogeneous and homogeneous catalysis. Here we demonstrate a robust approach to tune the CO oxidation activity over CeO₂ nanowires (NWs) through the modulation of the local structure and surface state around Ln_Ce′ defect centers by doping other lanthanides (Ln), based on the continuous variation of the ionic radius of lanthanide dopants caused by the lanthanide contraction. Homogeneously doped (110)-oriented CeO₂:Ln NWs with no residual capping agents were synthesized by controlling the redox chemistry of Ce­(III)/Ce­(IV) in a mild hydrothermal process. The CO oxidation reactivity over CeO₂:Ln NWs was dependent on the Ln dopants, and the reactivity reached the maximum in turnover rates over Nd-doped samples. On the basis of the results obtained from combined experimentations and density functional theory simulations, the decisive factors of the modulation effect along the lanthanide dopant series were deduced as surface oxygen release capability and the bonding configuration of the surface adsorbed species (i.e., carbonates and bicarbonates) formed during catalytic process, which resulted in the existence of an optimal doping effect from the lanthanide with moderate ionic radius

Structural Determination of Catalytically Active Subnanometer Iron Oxide Clusters

Supported subnanometer clusters exhibit superiority in catalytic performance compared to common nanoparticles, due to their higher fraction of exposed surfaces and larger number of active species at the metal–support interface, responding to the size effect and the support effect in heterogeneous catalysis. Here, we report the synthesis of subnanometer iron oxide clusters anchored to the surfaces of two types of ceria nanoshapes (nanorods and nanopolyhedra), as well as the structure–activity relation investigation for Fischer–Tropsch synthesis. On the basis of the comprehensive structural characterizations including aberration-corrected scanning transmission electron microscopy (STEM) and X-ray absorption fine structure (XAFS), we demonstrated that the subnanometer clusters of iron oxide are stable and catalytically active for the Fischer–Tropsch synthesis reaction. Furthermore, it is identified that finely dispersed iron oxide clusters (Fe–O_<i>x</i>–Fe_<i>y</i>) consisted of partially reduced Fe^δ+ (δ = 2.6–2.9) species in ceria nanorods are active for Fischer–Tropsch synthesis; however, another type of iron oxide cluster (Fe–O_<i>x</i>–Ce_<i>y</i>) composed of fully oxidized Fe³⁺ ions strongly interacted with the ceria nanopolyhedra support but exhibits relatively poorer activity for the reaction. These results have broad implications on the fundamental understanding of active site of supported metal catalysts at the atomic level

Efficient Tailoring of Upconversion Selectivity by Engineering Local Structure of Lanthanides in Na_<i>x</i>REF_3+<i>x</i> Nanocrystals

Efficient tailoring of upconversion emissions in lanthanide-doped nanocrystals is of great significance for extended optical applications. Here, we present a facile and highly effective method to tailor the upconversion selectivity by engineering the local structure of lanthanides in Na_<i>x</i>REF_3+<i>x</i> nanocrystals. The local structure engineering was achieved through precisely tuning the composition of nanocrystals, with different [Na]/[RE] ([F]/[RE]) ratio. It was found that the lattice parameter as well as the coordination number and local symmetry of lanthanides changed with the composition. A significant difference in the red to green emission ratio, which varied from 1.9 to 71 and 1.6 to 116, was observed for Na_<i>x</i>YF_3+<i>x</i>:Yb,Er and Na_<i>x</i>GdF_3+<i>x</i>:Yb,Er nanocrystals, respectively. Moreover, the local structure-dependent upconversion selectivity has been verified for Na_<i>x</i>YF_3+<i>x</i>:Yb,Tm nanocrystals. In addition, the local structure induced upconversion emission from Er³⁺ enhanced 9 times, and the CaF₂ shell grown epitaxially over the nanocrystals further promoted the red emission by 450 times, which makes it superior as biomarkers for <i>in vivo</i> bioimaging. These exciting findings in the local structure-dependent upconversion selectivity not only offer a general approach to tailoring lanthanide related upconversion emissions but also benefit multicolor displays and imaging

Chemical Insights into the Design and Development of Face-Centered Cubic Ruthenium Catalysts for Fischer–Tropsch Synthesis

Ruthenium is a promising low-temperature catalyst for Fischer–Tropsch synthesis (FTS). However, its scarcity and modest specific activity limit its widespread industrialization. We demonstrate here a strategy for tuning the crystal phase of catalysts to expose denser and active sites for a higher mass-specific activity. Density functional theory calculations show that upon CO dissociation there are a number of open facets with modest barrier available on the face-centered cubic (fcc) Ru but only a few step edges with a lower barrier on conventional hexagonal-closest packed (hcp) Ru. Guided by theoretical calculations, water-dispersible fcc Ru catalysts containing abundant open facets were synthesized and showed an unprecedented mass-specific activity in the aqueous-phase FTS, 37.8 mol_CO·mol_Ru^–1·h^–1 at 433 K. The mass-specific activity of the fcc Ru catalysts with an average size of 6.8 nm is about three times larger than the previous best hcp catalyst with a smaller size of 1.9 nm and a higher specific surface area. The origin of the higher mass-specific activity of the fcc Ru catalysts is identified experimentally from the 2 orders of magnitude higher density of the active sites, despite its slightly higher apparent barrier. Experimental results are in excellent agreement with prediction of theory. The great influence of the crystal phases on site distribution and their intrinsic activities revealed here provides a rationale design of catalysts for higher mass-specific activity without decrease of the particle size