Controlled Synthesis and Enhanced Catalytic Activity of Well-Defined Close-Contact Pd-ZnO Nanostructures

In this study, PdZn-ordered intermetallic nanoparticles (NPs) were prepared in liquid phase by butyllithium co-reduction of their precursors at 240 °C. Through calcination and subsequent reduction with H₂, the synthesized PdZn NPs were then in situ transformed into Pd-ZnO heteroaggregate nanocatalysts on alumina supports. Various characterization techniques, such as diffuse reflectance Fourier transform infrared with CO probes, transmission electron microscopy, X-ray diffraction, H₂ temperature-programmed reduction, and X-ray photoelectron spectra, reveal that PdZn NPs are ordered intermetallic compounds, and in situ transformation of PdZn alloy NPs results in close-contact Pd-ZnO heteroaggregates, where the interfaces are highly active and the interaction between Pd and ZnO prevents the active particles from agglomeration. The catalytic hydrogenations of nitrophenols over Pd/Al₂O₃ and Pd-ZnO/Al₂O₃ were investigated. The results show that Pd-ZnO/Al₂O₃ illustrates an enhanced catalytic activity relative to Pd/Al₂O₃, and no obvious activity degradation was observed in the recycle catalytic experiments over such nanostructures. It is concluded that the Pd-ZnO interaction not only enhances the catalytic hydrogenation activity but also promotes the thermal and catalytic stability.

Promotion of Pt/CeO2 catalyst by hydrogen treatment for low-temperature CO oxidation

Low temperature CO oxidation reaction is known to be facilitated over platinum supported on a reducible cerium oxide. Pt species act as binding sites for reactant CO molecules, and oxygen vacancies on surface of cerium oxide atomically activate the reactant O₂ molecules. However, the impacts of size of Pt species and concentration of oxygen vacancy at the surface of cerium oxide on the CO oxidation reaction have not been clearly distinguished, thereby various diverse approaches have been suggested to date. Here using the co-precipitation method we have prepared pure ceria support and infiltrated it with Pt solution to obtain 0.5 atomic% Pt supported on cerium oxide catalyst, and then systematically varied the size of Pt from single atom to ∼1.7 nm sized nanoparticles and oxygen vacancy concentration at surface of cerium oxide by controlling the heat-treatment conditions, which are temperature and oxygen partial pressure. It is found that Pt nanoparticles in range of 1–1.7 nm achieve 100% of CO oxidation reaction at ∼100 °C lower temperature compared to Pt single atom owing to the facile adsorption of CO but weaker binding strength between Pt and CO molecules, and the oxygen vacancy in the vicinity of Pt accelerates CO oxidation below 150 °C. Based on this understanding, we show that a simple hydrogen reduction at 550 °C for the single atom Pt supported on CeO₂ catalyst induces the formation of highly dispersed Pt nanoparticles with size of 1.7 ± 0.2 nm and the higher concentration of surface oxygen vacancies simultaneously, enabling 100% conversion from CO to CO₂ at 200 °C as well as 16% conversion even at 150 °C owing to the synergistic effects of Pt nanoparticles and oxygen vacancies.

Understanding on effects of Pt size and oxygen vacancy at CeO₂ surface in Pt/CeO₂ catalyst for CO oxidation reaction enables to boost catalytic activity.

Palygorskite Supported AuPd Alloy Nanoparticles as Efficient Nano-Catalysts for the Reduction of Nitroarenes and Dyes at Room Temperature

In this work, AuPd alloy palygorskite based Pal-NH₂@AuPd nano-catalysts were prepared and used as catalysts for the reduction of nitroarenes and dyes at room temperature. The surface of palygorskite (Pal) was first modified with 3-aminpropyltriethoxysilane, and then covered with AuPd alloy nanoparticles through co-reduction of HAuCl₄ and K₂PdCl₄. The morphology and structures of the Pal-NH₂@AuPd nano-catalysts were characterized by X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM). The as-synthesized Pal-NH₂@AuPd nano-catalysts displayed excellent catalytic performance in reducing 4-nitrophenol (4-NP) and various other nitroaromatic compounds. Moreover, the catalytic activities of the Pal-NH₂@AuPd nano-catalysts were adjustable via changing the atomic ratio of AuPd alloy nanoparticles, leading to the Pal-NH₂@Au₄₈Pd₅₂ component as having the best atomic ratio. The Pal-NH₂@Au₄₈Pd₅₂ continued to display good catalytic stability after being reused for several cycles and there were no obvious changes, either of the morphology or the particle size distribution of the nano-catalysts. Furthermore, these Pal-NH₂@Au₄₈Pd₅₂ nano-catalysts also provided a convenient and accessible way for the degradation of dyes in artificial industrial wastewater.

Highly Reducible Nanostructured CeO2 for CO Oxidation

Ceria in nanoscale with different morphologies, rod, tube and cube, were prepared through a hydrothermal process. The structure, morphology and textural properties were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and isothermal N2 adsorption-desorption. Ceria with different morphologies were evaluated as catalysts for CO oxidation. CeO2 nanorods showed superior activity to the others. When space velocity was 12,000 mL·gcat−1·h−1, the reaction temperature for 90% CO conversion (T90) was 228 °C. The main reason for the high activity was the existence of large amounts of easily reducible oxygen species, with a reduction temperature of 217 °C on the surface of CeO2 nanorods. Another cause was their relatively large surface area.

Versatile Synthesis of Pd and Cu Co-Doped Porous Carbon Nitride Nanowires for Catalytic CO Oxidation Reaction

Developing efficient catalyst for CO oxidation at low-temperature is crucial in various industrial and environmental remediation applications. Herein, we present a versatile approach for controlled synthesis of carbon nitride nanowires (CN NWs) doped with palladium and copper (Pd/Cu/CN NWs) for CO oxidation reactions. This is based on the polymerization of melamine by nitric acid in the presence of metal-precursors followed by annealing under nitrogen. This intriguingly drove the formation of well-defined, one-dimensional nanowires architecture with a high surface area (120 m2 g−1) and doped atomically with Pd and Cu. The newly-designed Pd/Cu/CN NWs fully converted CO to CO2 at 149 °C, that was substantially more active than that of Pd/CN NWs (283 °C) and Cu/CN NWs (329 °C). Moreover, Pd/Cu/CN NWs fully reserved their initial CO oxidation activity after 20 h. This is mainly attributed to the combination between the unique catalytic properties of Pd/Cu and outstanding physicochemical properties of CN NWs, which tune the adsorption energies of CO reactant and reaction product during the CO oxidation reaction. The as-developed method may open new frontiers on using CN NWs supported various noble metals for CO oxidation reaction.

In Situ Embedded Pseudo Pd-Sn Solid Solution in Micropores Silica with Remarkable Catalytic Performance for CO and Propane Oxidation

Most of the industrial and environmental catalytic reactions are operated at high temperature for a long time, and the sintering of the active centers is the main factor leading to catalysts deactivation, especially for noble metal catalysts. Herein we develop a dual confinement (enhanced metal-oxide interaction and the porous shell confinement) strategy to prepare Pd-Sn pseudo solid solution and in situ embedded in microporous silica for the first time and showed superior catalytic performance for CO and propane total oxidation (two main vehicle emission gases), even stored more than 640 days.

TiO2-nanosheet-assembled microspheres as Pd-catalyst support for highly-stable low-temperature CO oxidation

The Pd-embedded-in-TiO₂ structure could improve the activity and stability of the Pd/TiO₂ catalyst.

Identification of a flattened Pd–Ce oxide cluster as a highly efficient catalyst for low-temperature CO oxidation

An improved deposition method was employed to prepare a Pd/CeO₂ catalyst, which exhibited highly efficient activity in low-temperature CO oxidation (LTO).

Synthesis of K–Pd doped NiCo2O4−δ by reactive calcination route for oxidation of CO–CH4 emissions from CNG vehicles

The present study is devoted to formulating a doped spinel catalyst by a novel route of calcination for the oxidation of the CO–CH₄ mixture emitted from compressed natural gas (CNG) vehicles.

Ligand-Assisted, One-Pot Synthesis of Rh-on-Cu Nanoscale Sea Urchins with High-Density Interfaces for Boosting CO Oxidation

Predictable synthesis of bimetallic nanocrystals with spatially controlled metal distributions offers a versatile route to the development of highly efficient nanocatalysts. Here we report a one-pot synthesis of super branched Rh-on-Cu nanoscale sea urchins (Rh-Cu NSUrs) with a high density of Cu-Rh interfaces by manipulating the ligand coordination chemistry. Structural analysis and UV-vis spectra reveal that ascorbic acid can serve as a Rh-selective coordination ligand in the nonaqueous synthesis to reverse the reduction potentials of Rh³⁺ and Cu²⁺ cations. The sequential reduction of Cu²⁺ and then Rh³⁺ cations, as well as the island epitaxial growth of Rh atoms on Cu cores, leads to the formation of Rh-on-Cu nanostructures mimicking sea urchin. The size of the Cu cores and the density of Rh branches can both be facilely regulated by tuning the mole ratio of Cu to Rh. The Cu-Rh NSUrs show enhanced activity and stability in catalyzing CO oxidation, as the intrinsic Cu-Rh interfaces can act as catalytic hot spots through a bifunctional mechanism. The Cu-Rh two-component system can separate the adsorption and activation of CO and O₂ on the Rh and Cu surfaces, respectively, accelerating the generation of CO₂ at the interfaces.

Nanoparticle/Metal-Organic Framework Composites for Catalytic Applications: Current Status and Perspective

Nanoparticle/metal-organic frameworks (MOF) based composites have recently attracted significant attention as a new class of catalysts. Such composites possess the unique features of MOFs (including clearly defined crystal structure, high surface area, single site catalyst, special confined nanopore, tunable, and uniform pore structure), but avoid some intrinsic weaknesses (like limited electrical conductivity and lack in the "conventional" catalytically active sites). This review summarizes the developed strategies for the fabrication of nanoparticle/MOF composites for catalyst uses, including the strategy using MOFs as host materials to hold and stabilize the guest nanoparticles, the strategy with subsequent MOF growth/assembly around pre-synthesized nanoparticles and the strategy mixing the precursors of NPs and MOFs together, followed by self-assembly process or post-treatment or post-modification. The applications of nanoparticle/MOF composites for CO oxidation, CO₂ conversion, hydrogen production, organic transformations, and degradation of pollutants have been discussed. Superior catalytic performances in these reactions have been demonstrated. Challenges and future developments are finally addressed.

Atomically dispersed Pd catalysts in graphyne nanopore: formation and reactivity

The formation of single-atom noble metal catalysts on carbon materials remains a challenge due to the weak interaction between metals and pristine carbon. By means of density functional theory (DFT) calculations, it is found that the atomically dispersed Pd in graphyne nanopore is much more stable than that of relative Pd clusters. The large diffusion barrier of Pd from the most stable hollow site to the bridge site confirms the kinetic stability of such structures. While CO adsorption causes the pulling of Pd from graphyne nanopore due to the low diffusion barrier, based on DFT calculations, which can be further confirmed by ab initio molecular dynamic simulations. Finally, CO oxidation on the reconstruction of Pd@graphyne exhibits an energy barrier of 0.62 eV in the rate-limiting step through the Langmuir-Hinshelwood mechanism. After the reaction, the catalyst can be restored to the original atomically dispersed state again. This study shows graphyne is an excellent support for an atomically dispersed or single-metal catalyst.

An In situ TEM study of the surface oxidation of palladium nanocrystals assisted by electron irradiation

The surface oxidation of palladium nanocrystals plays an important role in changing the active sites and subsequently influencing the catalytic reactivity. Such a microscopy study on surface oxidation, down to the atomic scale, is essential for understanding the structure-property correlations of palladium nanocrystal based catalysts. Herein, we present an in situ atomic scale study on the surface oxidation behavior of palladium nanocrystals, which is induced by electron beam irradiation under low oxygen partial pressure and at room temperature inside an environmental transmission electron microscope. We found that: (i) surface oxidation initially started at the edge sites with atomic steps or vertex sites, which served as active sites for oxidation; (ii) the oxidation reaction proceeded with a much faster rate on the {111} surface, indicating a certain crystallography preference; (iii) nanometer-sized palladium monoxide islands were formed on the surfaces eventually. The results from our in situ studies provide insightful knowledge, and will be of certain importance for the design of improved functional catalysts in future.

Structural transformation of Pd-α-Fe2O3 and Pd-γ-Fe2O3 catalysts and application in the CO oxidation reaction

Pd-α-Fe₂O₃ and Pd-γ-Fe₂O₃ catalysts can be obtained by redox pretreatment and exhibit different reactive performances during CO oxidation.

A promising single atom catalyst for CO oxidation: Ag on boron vacancies of h-BN sheets

Due to “CO-Promoted O₂ Activation”, the termolecular Eley–Rideal (TER) mechanism is the most relevant one for CO oxidation over the SAC, Ag₁/BN.

Enhanced CO oxidation on CeO2/Co3O4 nanojunctions derived from annealing of metal organic frameworks

The interface of nanojunctions plays an important role in the performance of heterogeneous catalysts. However, it is highly challenging to construct nanojunctions which are usually prepared by complex multistep processes. Metal-organic frameworks (MOFs), with designable metal centers and tunable organic ligands, are promising precursors for the one-step synthesis of nanojunctions. Herein, we prepared porous CeO₂/Co₃O₄ nanojunctions by direct annealing of MOFs in air. These unique nanojunctions exhibit remarkable catalytic activity for CO oxidation, which can achieve complete oxidization of CO to CO₂ at 110 °C. In contrast, the temperature required for 100% CO oxidation is 190 °C for pure Co₃O₄. Moreover, the nanojunctions can maintain complete CO conversion after 16 h at 110 °C. Density functional theory calculations revealed that the enhancement in the catalytic activity of CeO₂/Co₃O₄ nanojunctions can be attributed to the charge transfer through the interfaces of the nanojunctions.

Immobilization of Highly Dispersed Ag Nanoparticles on Carbon Nanotubes Using Electron-Assisted Reduction for Antibacterial Performance

Silver nanoparticles (Ag NPs) supported on certain materials have been widely used as disinfectants. Yet, to date, the antibacterial activity of the supported Ag NPs is still far below optimum. This is mainly associated with the easy aggregation of Ag NPs on the supporting materials. Herein, an electron-assisted reduction (EAR) method, which is operated at temperatures as low as room temperature and without using any reduction reagent, was employed for immobilizing highly dispersed Ag NPs on aminated-CNTs (Ag/A-CNTs). The average Ag NPs size on the EAR-prepared Ag/A-CNTs is only 3.8 nm, which is much smaller than that on the Ag/A-CNTs fabricated from the traditional thermal calcination (25.5 nm). Compared with Ag/A-CNTs fabricated from traditional thermal calcination, EAR-prepared Ag/A-CNTs shows a much better antibacterial activity to E. coli/S. aureus and antifouling performance to P. subcordiformis/T. lepidoptera. This is mainly originated from the significantly enhanced Ag(+) ion releasing rate and highly dispersed Ag NPs with small size on the EAR-prepared Ag/A-CNTs. The findings from the present work are helpful for fabricating supported Ag NPs with small size and high dispersion for efficient antibacterial process.

Theoretical investigations of non-noble metal single-atom catalysis: Ni1/FeOx for CO oxidation

The single-atom catalyst Ni₁/FeO_x has a high activity for CO oxidation and the oxygen vacancy on the surface of this catalyst can be created at room temperature.

Low-temperature catalytic oxidation of CO over highly active mesoporous Pd/CeO2–ZrO2–Al2O3catalyst

Addition of zirconium and aluminum into Pd/CeO₂led to an increase of surface atomic ratios of Ce³⁺/Ce⁴⁺andO_adsorbed/O_lattice, pore volume, ratios and size of mesopores, catalytic activity, as well as decrease of particle size,E_aandT₁₀₀(60 °C).

Palladium-Based Nanomaterials: A Platform to Produce Reactive Oxygen Species for Catalyzing Oxidation Reactions

Oxidation reactions by molecular oxygen (O2 ) over palladium (Pd)-based nanomaterials are a series of processes crucial to the synthesis of fine chemicals. In the past decades, investigations of related catalytic materials have mainly been focused on the synthesis of Pd-based nanomaterials from the angle of tailoring their surface structures, compositions and supporting materials, in efforts to improve their activities in organic reactions. From the perspective of rational materials design, it is imperative to address the fundamental issues associated with catalyst performance, one of which should be oxygen activation by Pd-based nanomaterials. Here, the fundamentals that account for the transformation from O2 to reactive oxygen species over Pd, with a focus on singlet O2 and its analogue, are introduced. Methods for detecting and differentiating species are also presented to facilitate future fundamental research. Key factors for tuning the oxygen activation efficiencies of catalytic materials are then outlined, and recent developments in Pd-catalyzed oxygen-related organic reactions are summarized in alignment with each key factor. To close, we discuss the challenges and opportunities for photocatalysis research at this unique intersection as well as the potential impact on other research fields.

Pd1/BN as a promising single atom catalyst of CO oxidation: a dispersion-corrected density functional theory study

Single metal atom catalysts exhibit extraordinary activity in a large number of reactions, and some two-dimensional materials (such as graphene and h-BN) are found to be prominent supports to stabilize single metal atoms.

Synthesis of alumina supported Pd–Cu alloy nanoparticles for CO oxidation via a fast and facile method

A fast, facile and environmentally friendly method for synthesizing supported Pd–Cu alloy nanoparticles with enhanced CO oxidation activity is reported.