Can the Observed Changes in the Size or Shape of a Colloidal Nanocatalyst Reveal the Nanocatalysis Mechanism Type: Homogeneous or Heterogeneous?

Synergistic Homogeneous Asymmetric Cu Catalysis with Pd Nanoparticle Catalysis in Stereoselective Coupling of Alkynes with Aldimine Esters

Understanding the nature of a transition-metal-catalyzed process, including catalyst evolution and the real active species, is rather challenging yet of great importance for the rational design and development of novel catalysts, and this is even more difficult for a bimetallic catalytic system. Pd(0)/carboxylic acid combined system-catalyzed allylic alkylation reaction of alkynes has been used as an atom-economical protocol for the synthesis of allylic products. However, the asymmetric version of this reaction is still rather limited, and the in-depth understanding of the nature of active Pd species is still elusive. Herein we report an enantioselective coupling between readily available aldimine esters and alkynes using a synergistic Cu/Pd catalyst system, affording a diverse set of α-quaternary allyl amino ester derivatives in good yields with excellent enantioselectivities. Mechanistic studies indicated that it is most likely a synergistic asymmetric molecular Cu catalysis with Pd nanoparticle catalysis. The Pd catalyst precursor is transformed to soluble Pd nanoparticles in situ, which are responsible for activating the alkyne to an electrophilic allylic Pd intermediate, while the chiral Cu complex of the aldimine ester enolate provides chiral induction and works in synergy with the Pd nanoparticles.

Fluorine Substitution of TCNQ Alters the Redox-Driven Catalytic Pathway for the Ferricyanide-Thiosulfate Reaction

Mechanistic variation in catalysis through substituent-based redox tuning is well established. Fluorination of TCNQ (TCNQ=tetracyanoquinodimethane) provides ~850 mV variation in the redox potentials of theTCNQF n 0 / 1 - ${{{\rm {TCNQF}}}_{{\rm {n}}}^{{\rm {0/1-}}}}$ andTCNQF n 1 - / 2 - ${{{\rm {TCNQF}}}_{{\rm {n}}}^{{\rm {1-/2-}}}}$ (n=0, 2, 4) processes. WithTCNQF 4 1 - ${{{\rm {TCNQF}}}_{{\rm {4}}}^{{\rm {1-}}}}$ , catalysis of the kinetically very slow ferrocyanide-thiosulfate redox reaction in aqueous solution occurs via a mechanism in which the catalystTCNQF 4 1 - ${{{\rm {TCNQF}}}_{{\rm {4}}}^{{\rm {1-}}}}$ is reduced toTCNQF 4 2 - ${{{\rm {TCNQF}}}_{{\rm {4}}}^{{\rm {2-}}}}$ when reacting withS 2 O 3 2 - ${{{\rm {S}}}_{{\rm {2}}}{{\rm {O}}}_{{\rm {3}}}^{{\rm {2-}}}}$ which is oxidised toS 4 O 6 2 - ${{{\rm {S}}}_{{\rm {4}}}{{\rm {O}}}_{{\rm {6}}}^{{\rm {2-}}}}$ . Subsequently,TCNQF 4 2 - ${{{\rm {TCNQF}}}_{{\rm {4}}}^{{\rm {2-}}}}$ reacts with[ Fe ( CN ) 6 ]​ 3 - ${{{\rm {[Fe(CN)}}}_{{\rm {6}}}{{\rm {]}}}^{{\rm {3-}}}}$ to form[ Fe ( CN ) 6 ]​ 4 - ${{{\rm {[Fe(CN)}}}_{{\rm {6}}}{{\rm {]}}}^{{\rm {4-}}}}$ and reform theTCNQF 4 1 - ${{{\rm {TCNQF}}}_{{\rm {4}}}^{{\rm {1-}}}}$ catalyst, in another thermodynamically favoured process. An analogous mechanism applies withTCNQF 2 1 - ${{{\rm {TCNQF}}}_{{\rm {2}}}^{{\rm {1-}}}}$ as a catalyst. In contrast, since the reaction ofS 2 O 3 2 - ${{{\rm {S}}}_{{\rm {2}}}{{\rm {O}}}_{{\rm {3}}}^{{\rm {2-}}}}$ withTCNQ 1 - ${{{\rm {TCNQ}}}^{{\rm {1-}}}}$ is thermodynamically unfavourable, an alternative mechanism is required to explain the catalytic activity observed in this non-fluorinated system. Here, upon addition ofTCNQ 1 - ${{{\rm {TCNQ}}}^{{\rm {1-}}}}$ , reduction of[ Fe ( CN ) 6 ]​ 3 - ${{{\rm {[Fe(CN)}}}_{{\rm {6}}}{{\rm {]}}}^{{\rm {3-}}}}$ to[ Fe ( CN ) 6 ]​ 4 - ${{{\rm {[Fe(CN)}}}_{{\rm {6}}}{{\rm {]}}}^{{\rm {4-}}}}$ occurs with concomitant oxidation ofTCNQ 1 - ${{{\rm {TCNQ}}}^{{\rm {1-}}}}$ toTCNQ 0 ${{{\rm {TCNQ}}}^{{\rm {0}}}}$ , which then acts as the catalyst forS 2 O 3 2 - ${{{\rm {S}}}_{{\rm {2}}}{{\rm {O}}}_{{\rm {3}}}^{{\rm {2-}}}}$ oxidation. Thermodynamic data explain the observed differences in the catalytic mechanisms.CuTCNQF n ${{{\rm {CuTCNQF}}}_{{\rm {n}}}}$ (n=0, 4) also act as catalysts for the ferricyanide-thiosulfate reaction in aqueous solution. The present study shows that homogeneous pathways are available following addition of these dissolved materials. Previously, theseCuTCNQF n ${{{\rm {CuTCNQF}}}_{{\rm {n}}}}$ (n=0, 4) coordination polymers have been regarded as insoluble in water and proposed as heterogeneous catalysts for the ferricyanide-thiosulfate reaction. Details and mechanistic differences were established using UV-visible spectrophotometry and cyclic voltammetry.

Enhancing catalytic activity of gold nanoparticles in a standard redox reaction by investigating the impact of AuNPs size, temperature and reductant concentrations

In this work, the catalytic activity of three different sizes of gold nano particles (AuNPs) (12, 30, and 45 nm) synthesized by the citrate reduction process studied in the conventional redox reaction of K₃Fe (CN₆)^-3 to K₄Fe (CN₆)^-4 using NaBH₄(reductant) at four different temperatures (5 °C, 10 °C, 15 °C, and 20 °C) and measured by UV-visible spectrophotometry. Comparative kinetic analysis of different sizes of AuNPs including rate constant, activation energy, Entropy values and Frequency of collisions are reported for the first time. Transmission electron microscopy analysis is employed to investigate morphology and particle size. Spherical nanoparticles of size 12, 30, and 45 nm were observed. The UV-visible spectra were recorded at regular intervals, and it was seen that the peak of K₃Fe (CN₆)^-3 decreased gradually with time, at the same time surface plasmon resonance of AuNPs remained constant. As reaction catalysts, AuNPs maintain their optical density which shows their stability during the course of reaction. The kinetic parameters i.e., rate constant, and activation energy (k, t_1/2, E_a) were determined for three distinct sizes of gold nanoparticles (AuNPs) using the reductant at various concentrations. The value of k increases by increasing reductant concentration. This rise was significant for the small AuNPs. Increasing gold nanoparticle size (12, 30, 45 nm) decreased rate constant. As the size of AuNPs decreased the E_a reduced as well, i.e. 17.325 k cal mol^-1 for 12 nm, 19 k cal mol^-1 for 30 nm and 21 k cal mol^-1 for 45 nm AuNPs. For 50 mM of NaBH₄, k for 45 nm AuNPs is 0.10728 s^-1, but for 12 nm AuNPs, the value of k is 0.145 s^-1, indicating that the 12 nm AuNPs have the greatest rate constant values. The rate of reaction rises with an increase in reductant concentration and temperature, but this increase is significant in the case of small-sized nanoparticles, i.e., 12 nm, which have a high surface area and low E_a. Besides being a model redox reaction, the reduction of K₃Fe (CN₆)^-3 to K₄Fe (CN₆)^-4 has industrial use in making fertilizers and paint industry, anti-coating agent in colour photography, in dot etching and in amperometric biosensors.

Harnessing immune response using reactive oxygen Species-Generating/Eliminating inorganic biomaterials for disease treatment

With the increasing understanding of various biological functions mediated by reactive oxygen species (ROS) in the immune system, a number of studies have been designed to develop ROS-generating/eliminating strategies to selectively modulate immunogenicity for disease treatment. These strategies potentially exploit ROS-modulating inorganic biomaterials to harness host immunity to maximize the therapeutic potency by eliciting a favorable immune response. Inorganic biomaterial-guided in vivo ROS scavenging can exhibit several effects to: i) reduce the secretion of pro-inflammatory factors, ii) induce the phenotypic transition of macrophages from inflammatory M1 to immunosuppressive M2 phase, iii) minimize the recruitment and infiltration of immune cells. and/or iv) suppress the activation of nuclear factor kappa-B (NF-κB) pathway. Inversely, ROS-generating inorganic biomaterials have been found to be capable of: i) inducing immunogenic cell death (ICD), ii) reprograming tumor-associated macrophages from M2 to M1 phenotypes, iii) activating inflammasomes to stimulate tumor immunogenicity, and/or iv) recruiting phagocytes for antimicrobial therapy. This review provides a systematic and up-to-date overview on the progress related to ROS-nanotechnology mediated immunomodulation. We highlight how the ROS-generating/eliminating inorganic biomaterials can converge with immunomodulation and ultimately elicit an effective immune response against inflammation, autoimmune diseases, and/or cancers. We expect that contents presented in this review will be beneficial for the future advancements of ROS-based nanotechnology and its potential applications in this evolving field.

Designing Sites in Heterogeneous Catalysis: Are We Reaching Selectivities Competitive With Those of Homogeneous Catalysts?

A critical review of different prominent nanotechnologies adapted to catalysis is provided, with focus on how they contribute to the improvement of selectivity in heterogeneous catalysis. Ways to modify catalytic sites range from the use of the reversible or irreversible adsorption of molecular modifiers to the immobilization or tethering of homogeneous catalysts and the development of well-defined catalytic sites on solid surfaces. The latter covers methods for the dispersion of single-atom sites within solid supports as well as the use of complex nanostructures, and it includes the post-modification of materials via processes such as silylation and atomic layer deposition. All these methodologies exhibit both advantages and limitations, but all offer new avenues for the design of catalysts for specific applications. Because of the high cost of most nanotechnologies and the fact that the resulting materials may exhibit limited thermal or chemical stability, they may be best aimed at improving the selective synthesis of high value-added chemicals, to be incorporated in organic synthesis schemes, but other applications are being explored as well to address problems in energy production, for instance, and to design greener chemical processes. The details of each of these approaches are discussed, and representative examples are provided. We conclude with some general remarks on the future of this field.

Sub nanometer clusters in catalysis

Sub nanometer transition metal clusters composed of a small number of atoms exhibit unexpected electronic, optical, magnetic and catalytic properties that often change substantially as a function of cluster atomicity. Several factors influence their unique catalytic behavior, including their discrete electronic structure of molecular-like orbitals and the accessibility of their low-coordinated atoms. In addition, these factors are strongly correlated so that changes in their morphology may provoke large modifications to their electronic structure and vice versa. The thermodynamic instability of clusters makes it necessary to stabilize them with protective ligands in solution or to support them on solid matrices for practical applications, which introduces non-negligible modifications into their properties. Understanding their cause and extent is the key point to potentially achieve a fine tuning of their catalytic behavior. Selected examples are discussed illustrating important points on this matter, such as the influence of cluster morphology on reactivity, the need of anchoring clusters to avoid sintering and deactivation, and the possible formation of clusters in solution or under reaction conditions, with the associated difficulty to identify them as the true active species.

Nano-kaolin/Ti4+/Fe3O4: a magnetic reusable nano-catalyst for the synthesis of pyrimido[2,1-b]benzothiazoles

Herein, nano-kaolin/Ti⁴⁺/Fe₃O₄ as a new magnetic nano-catalyst was synthesized, and its structural properties were characterized using various techniques such as Fourier transform infrared (FTIR) spectroscopy, field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), a vibrating sample magnetometer (VSM), thermogravimetric analysis (TGA) and energy-dispersive X-ray spectroscopy (EDX). This catalyst was used for the synthesis of pyrimido[2,1-b]benzothiazoles via the one-pot condensation of 2-aminobenzothiazole, an aldehyde and β-keto ester under solvent-free conditions at 100 °C. This simple protocol has many advantages such as easy workup, high product yields, short reaction times and reusability of the catalyst.

Herein, nano-kaolin/Ti⁴⁺/Fe₃O₄ as a new magnetic nano-catalyst was synthesized, and its structural properties were characterized using various techniques such as FTIR spectroscopy, FE-SEM, TEM, XRD, VSM, TGA and EDX.

Preparation and recyclable catalysis performance of functional macroporous polyHIPE immobilized with gold nanoparticles on its surface

High internal phase emulsion polymerization was adopted for preparing macroporous polymeric monoliths, polyHIPE–Br (PHIPE–Br). Macroporous PHIPE–Br was used as the initiator to initiate the atom transfer radical polymerization (ATRP) of glycidyl methacrylate (GMA), resulting in a dense coating of polymers on the PHIPE surface and PHIPE–PGMA was obtained. Through a ring-opening addition reaction with TETA, a surface amino-polymer modified functional macroporous PHIPE–PGMA–TETA, was prepared conveniently. Gold nanoparticles could be easily in situ prepared and immobilized on the surface of PHIPE–PGMA–TETA. Characterization by scanning electron microscopy (SEM), EDX-mapping and TGA showed that PHIPE–PGMA–TETA was immobilized by the gold nanoparticles, and presented good catalytic properties. Moreover, the macroporous catalytic material, PHIPE–PGMA–TETA/Au NPs, presented recyclable catalytic performance without any decrease in activity. The materials and methods to form the monoliths and immobilize metal nanoparticles were simple and efficient, thus, opening new possibilities for highly porous PHIPE in catalysis applications.

The macroporous material PHIPE–PGMA–TETA/Au NPs has an open-cell and porous structure, and can effectively catalyze the reduction of 4-nitrophenol.

Size effects in MgO cube dissolution

Stability parameters and dissolution behavior of engineered nanomaterials in aqueous systems are critical to assess their functionality and fate under environmental conditions. Using scanning electron microscopy, transmission electron microscopy, and X-ray diffraction, we investigated the stability of cubic MgO particles in water. MgO dissolution proceeding via water dissociation at the oxide surface, disintegration of Mg(2+)-O(2-) surface elements, and their subsequent solvation ultimately leads to precipitation of Mg(OH)2 nanosheets. At a pH ≥ 10, MgO nanocubes with a size distribution below 10 nm quantitatively dissolve within few minutes and convert into Mg(OH)2 nanosheets. This effect is different from MgO cubes originating from magnesium combustion in air. With a size distribution in the range 10 nm ≤ d ≤ 1000 nm they dissolve with a significantly smaller dissolution rate in water. On these particles water induced etching generates (110) faces which, above a certain face area, dissolve at a rate equal to that of (100) planes.1 The delayed solubility of microcrystalline MgO is attributed to surface hydroxide induced self-inhibition effects occurring at the (100) and (110) microplanes. The present work underlines the importance of morphology evolution and surface faceting of engineered nanomaterials particles during their dissolution.

A mechanistic study of cross-coupling reactions catalyzed by palladium nanoparticles supported on polyaniline nanofibers

Pd/PANI nanocomposites effect C–C coupling reactions mainly through a homogeneous mechanism.

Labile imidazolium salt protected palladium nanoparticles

An imidazolium (Im) salt with two long alkyl substituents at N atoms is employed to prepare cubelike palladium nanoparticles (PdNPs). The bilayer nature of the capped Im salts is characterized by thermogravimetric analysis and NMR studies. These capped Im salts are labile, as evidenced by their displacement reaction with dimethylaminopyridine, and the observation of fast exchange between those free and capped Im salts on the NMR time scale. NMR results also show that these capped Im salts exhibit different diffusion rates, and interesting spinning rate dependent chemical shifts. These cubelike PdNPs could catalyze the Suzuki coupling of aryl chlorides and boronic acids with high yields in 10 min, even at room temperature.

Single-particle spectroscopy reveals heterogeneity in electrochemical tuning of the localized surface plasmon

A hyperspectral imaging method was developed that allowed the identification of heterogeneous plasmon response from 50 nm diameter gold colloidal particles on a conducting substrate in a transparent three-electrode spectroelectrochemical cell under non-Faradaic conditions. At cathodic potentials, we identified three distinct behaviors from different nanoparticles within the same sample: irreversible chemical reactions, reversible chemical reactions, and reversible charge density tuning. The irreversible reactions in particular would be difficult to discern in alternate methodologies. Additional heterogeneity was observed when single nanoparticles demonstrating reversible charge density tuning in the cathodic regime were measured dynamically in anodic potential ranges. Some nanoparticles that showed charge density tuning in the cathodic range also showed signs of an additional chemical tuning mechanism in the anodic range. The expected changes in nanoparticle free-electron density were modeled using a charge density-modified Drude dielectric function and Mie theory, a commonly used model in colloidal spectroelectrochemistry. Inconsistencies between experimental results and predictions of this common physical model were identified and highlighted. The broad range of responses on even a simple sample highlights the rich experimental and theoretical playgrounds that hyperspectral single-particle electrochemistry opens.

Shape-controlled nanostructures in heterogeneous catalysis

Nanotechnologies have provided new methods for the preparation of nanomaterials with well-defined sizes and shapes, and many of those procedures have been recently implemented for applications in heterogeneous catalysis. The control of nanoparticle shape in particular offers the promise of a better definition of catalytic activity and selectivity through the optimization of the structure of the catalytic active site. This extension of new nanoparticle synthetic procedures to catalysis is in its early stages, but has shown some promising leads already. Here, we survey the major issues associated with this nanotechnology-catalysis synergy. First, we discuss new possibilities associated with distinguishing between the effects originating from nanoparticle size versus those originating from nanoparticle shape. Next, we survey the information available to date on the use of well-shaped metal and non-metal nanoparticles as active phases to control the surface atom ensembles that define the catalytic site in different catalytic applications. We follow with a brief review of the use of well-defined porous materials for the control of the shape of the space around that catalytic site. A specific example is provided to illustrate how new selective catalysts based on shape-defined nanoparticles can be designed from first principles by using fundamental mechanistic information on the reaction of interest obtained from surface-science experiments and quantum-mechanics calculations. Finally, we conclude with some thoughts on the state of the field in terms of the advances already made, the future potentials, and the possible limitations to be overcome.

Metastability and structural polymorphism in noble metals: the role of composition and metal atom coordination in mono- and bimetallic nanoclusters

This study examines structural variations found in the atomic ordering of different transition metal nanoparticles synthesized via a common, kinetically controlled protocol: reduction of an aqueous solution of metal precursor salt(s) with NaBH₄ at 273 K in the presence of a capping polymer ligand. These noble metal nanoparticles were characterized at the atomic scale using spherical aberration-corrected scanning transmission electron microscopy (C(s)-STEM). It was found for monometallic samples that the third row, face-centered-cubic (fcc), transition metal [(3M)-Ir, Pt, and Au] particles exhibited more coherently ordered geometries than their second row, fcc, transition metal [(2M)-Rh, Pd, and Ag] analogues. The former exhibit growth habits favoring crystalline phases with specific facet structures while the latter samples are dominated by more disordered atomic arrangements that include complex systems of facets and twinning. Atomic pair distribution function (PDF) measurements further confirmed these observations, establishing that the 3M clusters exhibit longer ranged ordering than their 2M counterparts. The assembly of intracolumn bimetallic nanoparticles (Au-Ag, Pt-Pd, and Ir-Rh) using the same experimental conditions showed a strong tendency for the 3M atoms to template long-ranged, crystalline growth of 2M metal atoms extending up to over 8 nm beyond the 3M core.

Morphology and lateral strain control of Pt nanoparticles via core-shell construction using alloy AgPd core toward oxygen reduction reaction

Controlling the morphology of Pt-based nanomaterials can be an effective way to improve the catalytic activity on a mass basis. Herein we demonstrate for the first time the synthesis of monodispersed core-shell AgPd@Pt nanoparticles with multiply twinned structures. These multiply twinned particles (MTPs), which possess the icosahedra structure, exhibit superior catalytic activity toward oxygen reduction reaction (ORR) in fuel cells. The Ag component of the alloy AgPd inner core is crucial for the construction of the multiply twinned structure of the core-shell nanoparticles, while the Pd component is used to reduce the tensile strain effect of the Ag on the deposited Pt layers, rendering the Pt binding energy in core-shell AgPd@Pt MTPs to be close to that of commercial Pt nanoparticles. The enhanced ORR activity of AgPd@Pt/C can be explained in terms of a much higher surface fraction of atoms on the (111) facets for icosahedral MTPs. This core-shell structure offers an interesting example to investigate the morphology and lateral strain effect of the substrate on the deposited layers, and their influence on the catalytic activity of metal catalysts.

Synthesis of supported metal nanoparticle catalysts using ligand assisted methods

The synthesis and characterization methods of metal nanoparticles (NPs) have advanced greatly in the last few decades, allowing an increasing understanding of structure-property-performance relationships. However, the role played by the ligands used as stabilizers for metal NPs synthesis or for NPs immobilization on solid supports has been underestimated. Here, we highlight some recent progress in the preparation of supported metal NPs with the assistance of ligands in solution or grafted on solid supports, a modified deposition-reduction method, with special attention to the effects on NPs size, metal-support interactions and, more importantly, catalytic activities. After presenting the general strategies in metal NP synthesis assisted by ligands grafted on solid supports, we highlight some recent progress in the deposition of pre-formed colloidal NPs on functionalized solids. Another important aspect that will be reviewed is related to the separation and recovery of NPs. Finally, we will outline our personal understanding and perspectives on the use of supported metal NPs prepared through ligand-assisted methods.