A theoretical and experimental approach for correlating nanoparticle structure and electrocatalytic activity

Advanced Functionalized Nanoclusters (Cu, Ag, and Au) as Effective Catalyst for Organic Transformation Reactions

A considerable amount of research has been carried out in recent years on synthesizing metal nanoclusters (NCs), which have wide applications in the field of optical materials with non-linear properties, bio-sensing, and catalysis. Aside from being structurally accurate, the atomically precise NCs possess well-defined compositions due to significant tailoring, both at the surface and the core, for certain functionalities. To illustrate the importance of atomically precise metal NCs for catalytic processes, this review emphasizes 1) the recent work on Cu, Ag, and Au NCs with their synthesis, 2) the parameters affecting the activity and selectivity of NCs catalysis, and 3) the discussion on the catalytic potential of these metal NCs. Additionally, metal NCs will facilitate the design of extremely active and selective catalysts for significant reactions by elucidating catalytic mechanisms at the atomic and molecular levels. Future advancements in the science of catalysis are expected to come from the potential to design NCs catalysts at the atomic level.

Bifunctional Electrocatalysts for Overall and Hybrid Water Splitting

Electrocatalytic water splitting driven by renewable electricity has been recognized as a promising approach for green hydrogen production. Different from conventional strategies in developing electrocatalysts for the two half-reactions of water splitting (e.g., the hydrogen and oxygen evolution reactions, HER and OER) separately, there has been a growing interest in designing and developing bifunctional electrocatalysts, which are able to catalyze both the HER and OER. In addition, considering the high overpotentials required for OER while limited value of the produced oxygen, there is another rapidly growing interest in exploring alternative oxidation reactions to replace OER for hybrid water splitting toward energy-efficient hydrogen generation. This Review begins with an introduction on the fundamental aspects of water splitting, followed by a thorough discussion on various physicochemical characterization techniques that are frequently employed in probing the active sites, with an emphasis on the reconstruction of bifunctional electrocatalysts during redox electrolysis. The design, synthesis, and performance of diverse bifunctional electrocatalysts based on noble metals, nonprecious metals, and metal-free nanocarbons, for overall water splitting in acidic and alkaline electrolytes, are thoroughly summarized and compared. Next, their application toward hybrid water splitting is also presented, wherein the alternative anodic reactions include sacrificing agents oxidation, pollutants oxidative degradation, and organics oxidative upgrading. Finally, a concise statement on the current challenges and future opportunities of bifunctional electrocatalysts for both overall and hybrid water splitting is presented in the hope of guiding future endeavors in the quest for energy-efficient and sustainable green hydrogen production.

Evolutionary Monte Carlo of QM Properties in Chemical Space: Electrolyte Design

Optimizing a target function over the space of organic molecules is an important problem appearing in many fields of applied science but also a very difficult one due to the vast number of possible molecular systems. We propose an evolutionary Monte Carlo algorithm for solving such problems which is capable of straightforwardly tuning both exploration and exploitation characteristics of an optimization procedure while retaining favorable properties of genetic algorithms. The method, dubbed MOSAiCS (Metropolis Optimization by Sampling Adaptively in Chemical Space), is tested on problems related to optimizing components of battery electrolytes, namely, minimizing solvation energy in water or maximizing dipole moment while enforcing a lower bound on the HOMO-LUMO gap; optimization was carried out over sets of molecular graphs inspired by QM9 and Electrolyte Genome Project (EGP) data sets. MOSAiCS reliably generated molecular candidates with good target quantity values, which were in most cases better than the ones found in QM9 or EGP. While the optimization results presented in this work sometimes required up to 106 QM calculations and were thus feasible only thanks to computationally efficient ab initio approximations of properties of interest, we discuss possible strategies for accelerating MOSAiCS using machine learning approaches.

Equivariant Graph-Representation-Based Actor-Critic Reinforcement Learning for Nanoparticle Design

We have developed an actor-critic-type policy-based reinforcement learning (RL) method to find low-energy nanoparticle structures and compared its effectiveness to classical basin-hopping. We took a molecule building approach where nanoalloy particles can be regarded as metallic molecules, albeit with much higher flexibility in structure. We explore the strengths of our approach by tasking an agent with the construction of stable mono- and bimetallic clusters. Following physics, an appropriate reward function and an equivariant molecular graph representation framework is used to learn the policy. The agent succeeds in finding well-known stable configuration for small clusters in both single and multicluster experiments. However, for certain use cases the agent lacks generalization to avoid overfitting. We relate this to the pitfalls of actor-critic methods for molecular design and discuss what learning properties an agent will require to achieve universality.

One-Step Phase Separation for Core-Shell Carbon@Indium Oxide@Bismuth Microspheres with Enhanced Activity for CO2 Electroreduction to Formate

It is a substantial challenge to construct electrocatalysts with high activity, good selectivity, and long-term stability for electrocatalytic reduction of carbon dioxide to formic acid. Herein, bismuth and indium species are innovatively integrated into a uniform heterogeneous spherical structure by a neoteric quasi-microemulsion method, and a novel C@In₂ O₃ @Bi₅₀ core-shell structure is constructed through a subsequent one-step phase separation strategy due to melting point difference and Kirkendall effect with the nano-limiting effect of the carbon structure. This core-shell C@In₂ O₃ @Bi₅₀ catalyst can selectively reduce CO₂ to formate with high selectivity (≈90% faradaic efficiency), large partial current density (24.53 mA cm^-2 at -1.36 V), and long-term stability (up to 14.5 h), superior to most of the Bi-based catalysts. The hybrid Bi/In₂ O₃ interfaces of core-shell C@In₂ O₃ @Bi will stabilize the key intermediate HCOO* and suppress CO poisoning, benefiting the CO₂ RR selectivity and stability, while the internal cavity of core-shell structure will improve the reaction kinetics because of the large specific surface area and the enhancement of ion shuttle and electron transfer. Furthermore, the nano-limited domain effect of outmost carbon prevent active components from oxidation and agglomeration, helpful for stabilizing the catalyst. This work offers valuable insights into core-shell structure engineering to promote practical CO₂ conversion technology.

Enhanced oxygen reduction activity of size-selected platinum subnanocluster catalysts: Ptn (n = 3–9)

Ptn subnanoclusters (n = 3–9) on a carbon substrate exhibit 1.6–2.2 times higher activity than the standard Pt/C catalysts. EXAFS experiments and DFT calculations show plausible structures and energetics for reaction intermediates in the processes.

Multicompartment dendrimicelles with binary, ternary and quaternary core composition

Hierarchically built-up multicompartment nanoaggregate systems are of interest for, e.g., novel materials and medicine. Here we present a versatile strategy to generate and unambiguously characterize complex coacervate-core micelles by exploiting four different dendrimeric subcomponents as core-units. The resulting mesoscale structures have a hydrodynamic diameter of 50 nm and a core size of 33 nm, and host about thirty 6th generation polyamidoamine (PAMAM) dendrimers. We have used FRET (efficiency of ∼0.2) between fluorescein and rhodamine moieties immobilized on separate PAMAM dendrimers (G6-F and G6-R, respectively) to prove synchronous encapsulation in the micelle core. Tuning the proximity of the FRET pair molecules either by varying the G6-F : G6-R ratio, or by co-assembling non-functionalized dendrimer (G6-E) in the core, reveals the optimal FRET efficiency to occur at a minimum of 70% loading with G6-F and G6-R. Additional co-encapsulation of 6th generation gold dendrimer-encapsulated nanoparticles (G6-Au) in the micelle core shows a dramatic reduction of the FRET efficiency, which can be restored by chemical etching of the gold nanoparticles from within the micellar core with thiols, leaving the micelle itself intact. This study reveals the controlled co-assembly of up to four different types of subcomponents in one single micellar core and concomitantly shows the wide variety of structures that can be made with a well-defined basic set of subcomponents. It is straightforward to design related strategies, to incorporate inside one micellar core, e.g., even more than 4 different dendrimers, or other classes of (macro)molecules, with different functional groups, other FRET pairs or different encapsulated metal nanoparticles.

In Situ/Operando Electrocatalyst Characterization by X-ray Absorption Spectroscopy

During the last decades, X-ray absorption spectroscopy (XAS) has become an indispensable method for probing the structure and composition of heterogeneous catalysts, revealing the nature of the active sites and establishing links between structural motifs in a catalyst, local electronic structure, and catalytic properties. Here we discuss the fundamental principles of the XAS method and describe the progress in the instrumentation and data analysis approaches undertaken for deciphering X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra. Recent usages of XAS in the field of heterogeneous catalysis, with emphasis on examples concerning electrocatalysis, will be presented. The latter is a rapidly developing field with immense industrial applications but also unique challenges in terms of the experimental characterization restrictions and advanced modeling approaches required. This review will highlight the new insight that can be gained with XAS on complex real-world electrocatalysts including their working mechanisms and the dynamic processes taking place in the course of a chemical reaction. More specifically, we will discuss applications of in situ and operando XAS to probe the catalyst's interactions with the environment (support, electrolyte, ligands, adsorbates, reaction products, and intermediates) and its structural, chemical, and electronic transformations as it adapts to the reaction conditions.

Three-Dimensional TEM Study of Dendrimer-Encapsulated Pt Nanoparticles for Visualizing Structural Characteristics of the Whole Organic-Inorganic Hybrid Nanostructure

Here, we report three-dimensional (3-D) visualization of dendrimer-encapsulated Pt nanoparticles (Pt DENs) by using 3-D electron tomography to reveal intricate structural characteristics of their whole organic-inorganic hybrid nanostructure. We reconstructed the 3-D spatial volume of Pt DENs by back-projecting a tilt series of two-dimensional (2-D) projections of Pt nanoparticles encapsulated inside dendrimers negatively stained with uranyl acetate. The direct 3-D visualization of Pt DENs elucidated their encapsulation characteristics with the spatial imaging of Pt nanoparticles embraced inside dendrimers in three dimensions. The encapsulation characteristics of Pt DENs were further verified with selective electrochemical poisoning experiments. In addition, quantitative 3-D structural characterization of Pt DENs provided more accurate and precise size distributions of nanoparticles than those obtained from conventional 2-D transmission electron microscopy analysis relying only on a 3-D structure projected on a 2-D plane.

Propene oxidation catalysis and electronic structure of M55 particles (M = Pd or Rh): differences and similarities between Pd55 and Rh55

Propene oxidation is one of the important reactions that occurs in the presence of a three-way catalyst but its reaction mechanism is unclear. The reaction mechanisms and differences in catalysis between Pd and Rh particles were investigated by DFT calculations employing Pd₅₅ and Rh₅₅ as the model catalysts. The O-attack mechanism, in which the O atom adsorbed on the Pd₅₅ and Rh₅₅ surfaces attacks the C[double bond, length as m-dash]C double bond of propene, needs to overcome a large activation barrier (E_a). On the other hand, C-H bond cleavage of the methyl group of propene easily occurs with moderate E_a; the mechanism initiated by this C-H activation is named H-transfer mechanism. In this mechanism, the next step is allyl alcohol formation, followed by the second C-H bond activation of the CH₂OH species of allyl alcohol, and the final step is proton transfer from OH-substituted π-allyl species to the OH group on the metal surface to yield acrolein and water molecules with the regeneration of M₅₅. The rate-determining step is the second C-H bond activation. Its E_a is 17.4 kcal mol^-1 for the reaction on Pd₅₅ and 34.4 kcal mol^-1 for the reaction on Rh₅₅. These results indicate that Pd particles are more active than Rh particles in propene oxidation, which agrees with the experimental findings. The larger E_a for Rh₅₅ than that for Pd₅₅ arises from the stronger Rh-OH bond than the Pd-OH bond. The higher energy d-valence band-top of Rh₅₅ than that of Pd₅₅ is the origin of the stronger Rh-OH bond than the Pd-OH bond. Thus, the d-valence band-top energy is an important property for understanding and designing catalysts for alkene oxidation.

Multilayer electrodeposition of Pt onto 1-2 nm Au nanoparticles using a hydride-termination approach

Here we report on hydride-terminated (HT) electrodeposition of Pt multilayers onto ∼1.6 nm Au nanoparticles (NPs). The results build on our earlier findings regarding electrodeposition of a single monolayer of Pt onto Au NPs and reports relating to HT Pt electrodeposition onto bulk Au. In the latter case, it was found that electrodeposition of Pt from a solution containing PtCl42- can be limited to a single monolayer of Pt atoms if it is immediately followed by adsorption of a monolayer of H atoms. The H-atom capping layer prevents deposition of Pt multilayers. In the present report we are interested in comparing the structure of NPs after multiple HT Pt electrodeposition cycles to the bulk analog. The results indicate that a greater number of HT Pt cycles are required to electrodeposit both a single Pt monolayer and Pt multilayers onto these Au NPs compared to bulk Au. Additionally, detailed structural analysis shows that there are fundamental differences in the structures of the AuPt materials depending on whether they are prepared on Au NPs or bulk Au. The resulting structures have a profound impact on formic acid oxidation electrocatalysis.

Strategy used to synthesize high activity and low Pd catalyst for Suzuki coupling reaction: an experimental and theoretical investigation

Compared with Pd₁₄₇ DEN, Pd₇₄Cu₇₃ DEN shows comparable catalytic performance, with TOFs of 8519 and 8728 h⁻¹, and provides a valuable strategy for designing a low Pd dosage and high-activity catalyst for Suzuki coupling.

Combined Experimental and Theoretical Study of the Structure of AuPt Nanoparticles Prepared by Galvanic Exchange

In this article, experiment and theory are combined to analyze Pb and Cu underpotential deposition (UPD) on ∼1.7 nm Au nanoparticles (NPs) and the AuPt structures that result after galvanic exchange (GE) of the UPD layer for Pt. Experimental Pb (0.49 ML) and Pt (0.50 ML) coverages are close to values predicted by density functional theory-molecular dynamics (DFT-MD, 0.59 ML). DFT-MD reveals that the AuNPs spontaneously reconstruct from cuboctahedral to a (111)-like structure prior to UPD. In the case of Pb, this results in the random electrodeposition of Pb onto the Au surface. This mechanism is a consequence of opposing trends in Pb-Pb and Pb-Au coordination numbers as a function of Pb coverage. Cu UPD is more complex, and agreement between theory and experiment takes into account ligand effects (e.g., SO₄^2- present as the electrolyte) and the electric double layer. Importantly, AuPt structures formed upon Pt GE are found to differ markedly depending on the UPD metal. Specifically, cyclic voltammetry indicates that the Pt coverage is ∼0.20 ML greater for Cu UPD/Pt GE (0.70 ML) than for Pb UPD/Pt GE (0.50 ML). This difference is corroborated by DFT-MD theoretical predictions. Finally, DFT-MD calculations predict the formation of surface alloy and core@shell structures for Pb UPD/Pt GE and Cu UPD/Pt GE, respectively.

Enhanced oxygen reduction activity of platinum subnanocluster catalysts through charge redistribution

Single-size platinum Pt6 subnanoclusters exhibit superior mass-specific and surface-specific activities for the oxygen reduction reaction. The enhanced activity is attributed to polarized electron distributions based on rigorous structure characterization by X-ray absorption fine structure spectroscopy and density functional theory.

Computational screening of M/Cu core/shell nanoparticles and their applications for the electro-chemical reduction of CO2 and CO

To improve the catalytic activity of copper nanoparticles (Cu NPs) in the electro-chemical catalysis of the CO2 reduction reactions (CO2RRs), the formation and use of core/shell nanoparticles (CSNPs) with Cu as the shell composite may be an effective way. Using Cu79 NP as the representative, we performed computational screening and confirmed four Mx@Cu79-x CSNPs that can stably exist. Then, the catalytic performance of the screened CSNPs was tested through first-principles calculations. The free energy profiles indicate that Fe19@Cu60 is more desirable for CO2RR catalysis due to its high selectivity for CO rather than HCOOH at a low potential. Moreover, when it electro-catalyzes CO2 into CH4, the Fe19@Cu60 CSNP exhibits much lower limiting potential (-0.58 V) compared with pure Cu79 NP (-0.86 V) or the Cu (211) surface (-0.70 V). Taking the cost into consideration, the Fe19@Cu60 CSNP is highly recommended as a promising electro-catalyst for CO2RRs. In addition, when CO is taken as the initial reactant to be reduced, all the screened CSNPs exhibit lower limiting potentials than Cu79 NP. From the view of material design, the significant weakening of CO binding originating from the change in the d-band center could be the reason why the formation of a core/shell structure will enhance the catalytic performance of Cu NPs in CO reduction.

Solving the Structure and Dynamics of Metal Nanoparticles by Combining X-Ray Absorption Fine Structure Spectroscopy and Atomistic Structure Simulations

Extended X-ray absorption fine structure (EXAFS) spectroscopy is a premiere method for analysis of the structure and structural transformation of nanoparticles. Extraction of analytical information about the three-dimensional structure and dynamics of metal-metal bonds from EXAFS spectra requires special care due to their markedly non-bulk-like character. In recent decades, significant progress has been made in the first-principles modeling of structure and properties of nanoparticles. In this review, we summarize new approaches for EXAFS data analysis that incorporate particle structure modeling into the process of structural refinement.

Effects of ensembles, ligand, and strain on adsorbate binding to alloy surfaces

Alloying elements with strong and weak adsorption properties can produce a catalyst with optimally tuned adsorbate binding. A full understanding of this alloying effect, however, is not well-established. Here, we use density functional theory to study the ensemble, ligand, and strain effects of close-packed surfaces alloyed by transition metals with a combination of strong and weak adsorption of H and O. Specifically, we consider PdAu, RhAu, and PtAu bimetallics as ordered and randomly alloyed (111) surfaces, as well as randomly alloyed 140-atom clusters. In these alloys, Au is the weak-binding component and Pd, Rh, and Pt are characteristic strong-binding metals. In order to separate the different effects of alloying on binding, we calculate the tunability of H- and O-binding energies as a function of lattice constant (strain effect), number of alloy-substituted sublayers (ligand effect), and randomly alloyed geometries (ensemble effect). We find that on these alloyed surfaces, the ensemble effect more significantly tunes the adsorbate binding as compared to the ligand and strain effects, with the binding energies predominantly determined by the local adsorption environment provided by the specific triatomic ensemble on the (111) surface. However, we also find that tuning of adsorbate binding from the ligand and strain effects cannot be neglected in a quantitative description. Extending our studies to other bimetallics (PdAg, RhAg, PtAg, PdCu, RhCu, and PtCu), we find similar conclusions that the tunability of adsorbate binding on random alloys is predominately described by the ensemble effect.

Origin of the catalytic activity of phosphorus doped MoS2 for oxygen reduction reaction (ORR) in alkaline solution: a theoretical study

Phosphorus doped MoS₂ nanosheets (P-doped MoS₂) have been reported as excellent oxygen reduction reaction (ORR) catalysts with four-electron selectivity in alkaline solution. By performing density functional theory (DFT) calculations, we revealed the detailed reaction mechanism and the key reaction sites on surface of P-doped MoS₂ for ORR catalysis. The double P-doped MoS₂ (2P-MoS₂) is calculated to be more stable than the single P-doped MoS₂ (P-MoS₂), and the configuration with two P atoms in neighboring sites exhibits the highest stability. The surface of P-doped MoS₂ is found highly active for dissociation of O₂. Comparative calculations reveal that P-MoS₂ is unsuitable as ORR catalyst due to the high dissociation barrier of H₂O (1.19 and 2.06 eV for the first and second adsorbed H₂O), while the 2P-MoS₂ shows good ORR catalytic activity with much lower dissociation barrier of H₂O (0.62 eV). Furthermore, we elucidated that the ORR catalytic activity in 2P-MoS₂ originates from the activated S2 atom, which provides an extra adsorption site for the first H₂O and the following OH group benefited from the enhanced hydrogen bond interaction. Our results illustrate the mechanisms of doped MoS₂ based catalysts and provide rational way for designing ORR catalysts with high activity.

Catalysis by Metallic Nanoparticles in Solution: Thermosensitive Microgels as Nanoreactors

Metallic nanoparticles have been used as catalysts for various reactions, and the huge literature on the subject is hard to overlook. In many applications, the nanoparticles must be affixed to a colloidal carrier for easy handling during catalysis. These “passive carriers” (e.g. dendrimers) serve for a controlled synthesis of the nanoparticles and prevent coagulation during catalysis. Recently, hybrids from nanoparticles and polymers have been developed that allow us to change the catalytic activity of the nanoparticles by external triggers. In particular, single nanoparticles embedded in a thermosensitive network made from poly(N-isopropylacrylamide) (PNIPAM) have become the most-studied examples of such hybrids: immersed in cold water, the PNIPAM network is hydrophilic and fully swollen. In this state, hydrophilic substrates can diffuse easily through the network, and react at the surface of the nanoparticles. Above the volume transition located at 32°C, the network becomes hydrophobic and shrinks. Now hydrophobic substrates will preferably diffuse through the network and react with other substrates in the reaction catalyzed by the enclosed nanoparticle. Such “active carriers”, may thus be viewed as true nanoreactors that open new ways for the use of nanoparticles in catalysis. In this review, we give a survey on recent work done on these hybrids and their application in catalysis. The aim of this review is threefold: we first review hybrid systems composed of nanoparticles and thermosensitive networks and compare these “active carriers” to other colloidal and polymeric carriers (e.g. dendrimers). In a second step we discuss the model reactions used to obtain precise kinetic data on the catalytic activity of nanoparticles in various carriers and environments. These kinetic data allow us to present a fully quantitative comparison of different nanoreactors. In a final section we shall present the salient points of recent efforts in the theoretical modeling of these nanoreactors. By accounting for the presence of a free-energy landscape for the reactants’ diffusive approach towards the catalytic nanoparticle, arising from solvent-reactant and polymeric shell-reactant interactions, these models are capable of explaining the emergence of all the important features observed so far in studies of nanoreactors. The present survey also suggests that such models may be used for the design of future carrier systems adapted to a given reaction and solvent.

Electrochemistry at a single nanoparticle: from bipolar regime to tunnelling

This paper is concerned with long-distance interactions between an unbiased metal nanoparticle (NP) and a nanoelectrode employed as a tip in the scanning electrochemical microscope (SECM).

Liquid-phase catalysis by single-size palladium nanoclusters supported on strontium titanate: size-specific catalysts for Suzuki–Miyaura coupling

Size-specific catalysis by single-size palladium nanoclusters.

Structural Characterization of Rh and RhAu Dendrimer-Encapsulated Nanoparticles

We report the structural characterization of 1-2 nm Rh and RhAu alloy dendrimer-encapsulated nanoparticles (DENs) prepared by chemical reduction with NaBH₄. In contrast to previously reported results, in situ and ex situ X-ray absorption spectroscopic experiments indicate that only a fraction of the Rh³⁺ present in the precursors are reduced by NaBH₄. Additional structural analysis of RhAu alloy DENs using extended X-ray absorption fine structure spectroscopy leads to a model in which there is significant segregation of Rh and Au within the nanoparticles. In Rh-rich alloy DENs, Au atoms are segregated on the nanoparticle surface.

Understanding the atomic-level process of CO-adsorption-driven surface segregation of Pd in (AuPd)147 bimetallic nanoparticles

When the elements that compose bimetallic catalysts interact asymmetrically with reaction feedstock, the surface concentration of the bimetallic catalysts and the morphology of the reaction center evolve dynamically as a function of environmental factors such as the partial pressure of the triggering molecule. Relevant experimental and theoretical findings of the dynamic structural evolution of bimetallic catalysts under the reaction conditions are emerging, thus enabling the design of more consistent, reliable, and efficient bimetallic catalysts. In an initial attempt to provide an atomic-level understanding of the adsorption-induced structural evolution of bimetallic nanoparticles (NPs) under CO oxidation conditions, we used density functional theory to study the details of CO-adsorption-driven Pd surface segregation in (AuPd)₁₄₇ bimetallic NPs. The strong CO affinity of Pd provides a driving force for Pd surface segregation. We found that the vertex site of the NP becomes a gateway for the initial Pd-Au swapping and the subsequent formation of an internal vacancy. This self-generated internal vacancy easily diffuses inside the NP and activates Pd-Au swapping pathways in the (100) NP facet. Our results reveal how the surface and internal concentrations of bimetallic NPs respond immediately to changes in the reaction conditions. Our findings should aid in the rational design of highly active and versatile bimetallic catalysts by considering the environmental factors that systematically affect the structure of bimetallic catalysts under the reaction conditions.

Precise localization of metal nanoparticles in dendrimer nanosnakes or inner periphery and consequences in catalysis

Understanding the relationship between the location of nanoparticles (NPs) in an organic matrix and their catalytic performances is essential for catalyst design. Here we show that catalytic activities of Au, Ag and CuNPs stabilized by dendrimers using coordination to intradendritic triazoles, galvanic replacement or stabilization outside dendrimers strongly depends on their location. AgNPs are found at the inner click dendrimer periphery, whereas CuNPs and AuNPs are encapsulated in click dendrimer nanosnakes. AuNPs and AgNPs formed by galvanic replacement are larger than precursors and only partly encapsulated. AuNPs are all the better 4-nitrophenol reduction catalysts as they are less sterically inhibited by the dendrimer interior, whereas on the contrary CuNPs are all the better alkyne azide cycloaddition catalysts as they are better protected from aerobic oxidation inside dendrimers. This work highlights the role of the location in macromolecules on the catalytic efficiency of metal nanoparticles and rationalizes optimization in catalyst engineering.

Electrocatalytic Reduction of Oxygen on Platinum Nanoparticles in the Presence and Absence of Interactions with the Electrode Surface

We report that ultraviolet/ozone (UV/O3) treatment can be used to remove sixth-generation, hydroxyl-terminated poly(amidoamine) (PAMAM) dendrimers from dendrimer-encapsulated Pt nanoparticles (Pt DENs) previously immobilized onto a pyrolyzed photoresist film (PPF) electrode. Results from X-ray photoelectron spectroscopy, scanning transmission electron microscopy, and electrochemical experiments indicate that removal of the dendrimer proceeds without changes to the size, shape, or electrocatalytic properties of the encapsulated nanoparticles. The UV/O3 treatment did not damage the PPF electrode. The electrocatalytic properties of the DENs before and after removal of the dendrimer were nearly identical.