Perovskite oxides (ABO3) have been widely recognized as a class of promising noble-metal–free electrocatalysts due to their unique compositional flexibility and structural stability. Surprisingly, investigation into their size-dependent electrocatalytic properties, in particular barium titanate (BaTiO3), has been comparatively few and limited in scope. Herein, we report the scrutiny of size- and dopant-dependent oxygen reduction reaction (ORR) activities of an array of judiciously designed pristine BaTiO3and doped BaTiO3(i.e., La- and Co-doped) nanoparticles (NPs). Specifically, a robust nanoreactor strategy, based on amphiphilic star-like diblock copolymers, is employed to synthesize a set of hydrophobic polymer-ligated uniform BaTiO3NPs of different sizes (≤20 nm) and controlled compositions. Quite intriguingly, the ORR activities are found to progressively decrease with the increasing size of BaTiO3NPs. Notably, La- and Co-doped BaTiO3NPs display markedly improved ORR performance over the pristine counterpart. This can be attributed to the reduced limiting barrier imposed by the formation of -OOH species during ORR due to enhanced adsorption energy of intermediates and the possibly increased conductivity as a result of change in the electronic states as revealed by our density functional theory–based first-principles calculations. Going beyond BaTiO3NPs, a variety of other ABO3NPs with tunable sizes and compositions may be readily accessible by exploiting ourmore »
Accelerated Reaction Rates within Self-Assembled Polymer Nanoreactors with Tunable Hydrophobic Microenvironments
Performing reactions in the presence of self-assembled hierarchical structures of amphiphilic macromolecules can accelerate reactions while using water as the bulk solvent due to the hydrophobic effect. We leveraged non-covalent interactions to self-assemble filled-polymer micelle nanoreactors (NR) incorporating gold nanoparticle catalysts into various amphiphilic polymer nanostructures with comparable hydrodynamic nanoreactor size and gold concentration in the nanoreactor dispersion. We systematically studied the effect of the hydrophobic co-precipitant on self-assembly and catalytic performance. We observed that co-precipitants that interact with gold are beneficial for improving incorporation efficiency of the gold nanoparticles into the nanocomposite nanoreactor during self-assembly but decrease catalytic performance. Hierarchical assemblies with co-precipitants that leverage noncovalent interactions could enhance catalytic performance. For the co-precipitants that do not interact strongly with gold, the catalytic performance was strongly affected by the hydrophobic microenvironment of the co-precipitant. Specifically, the apparent reaction rate per surface area using castor oil (CO) was over 8-fold greater than polystyrene (750 g/mol, PS 750); the turnover frequency was higher than previously reported self-assembled polymer systems. The increase in apparent catalytic performance could be attributed to differences in reactant solubility rather than differences in mass transfer or intrinsic kinetics; higher reactant solubility enhances apparent reaction rates. Full conversion more »
- Award ID(s):
- 1651957
- Publication Date:
- NSF-PAR ID:
- 10215328
- Journal Name:
- Polymers
- Volume:
- 12
- Issue:
- 8
- Page Range or eLocation-ID:
- 1774
- ISSN:
- 2073-4360
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Self-assembled metal nanoparticle-polymer nanocomposite particles as nanoreactors are a promising approach for performing liquid phase reactions using water as a bulk solvent. In this work, we demonstrate rapid, scalable self-assembly of metal nanoparticle catalyst-polymer nanocomposite particles via Flash NanoPrecipitation. The catalyst loading and size of the nanocomposite particles can be tuned independently. Using nanocomposite particles as nanoreactors and the reduction of 4-nitrophenol as a model reaction, we study the fundamental interplay of reaction and diffusion. The induction time is affected by the sequence of reagent addition, time between additions, and reagent concentration. Combined, our experiments indicate the induction time is most influenced by diffusion of sodium borohydride. Following the induction time, scaling analysis and effective diffusivity measured using NMR indicate that the observed reaction rate are reaction- rather than diffusion-limited. Furthermore, the intrinsic kinetics are comparable to ligand-free gold nanoparticles. This result indicates that the polymer microenvironment does not de-activate or block the catalyst active sites.
-
Performing multiple reaction steps in “one pot” to avoid the need to isolate intermediates is a promising approach for reducing solvent waste associated with liquid phase chemical processing. In this work, we incorporated gold nanoparticle catalysts into polymer nanoreactors via amphiphilic block copolymer directed self-assembly. With the polymer nanoreactors dispersed in water as the bulk solvent, we demonstrated the ability to facilitate two reaction steps in one pot with spontaneous precipitation of the product from the reaction mixture. Specifically, we achieved imide synthesis from 4-nitrophenol and benzaldehyde as a model reaction. The reaction occured in water at ambient conditions; the desired 4-benzylideneaminophenol product spontaneously precipitated from the reaction mixture while the nanoreactors remained stable in dispersion. A 65% isolated yield was achieved. In contrast, PEGylated gold nanoparticles and citrate stabilized gold nanoparticles precipitated with the reaction product, which would complicate both the isolation of the product as well as reuse of the catalyst. Thus, amphiphilic nanoreactors dispersed in water are a promising approach for reducing solvent waste associated with liquid phase chemical processing by using water as the bulk solvent, eliminating the need to isolate intermediates, achieving spontaneous product separation to facilitate the recycling of the reaction mixture, and simplifyingmore »
-
Facile and large-scale synthesis of well-defined, thermally stable silver nanoparticles protected by polymer brushes for use in practical applications is still a challenge. Recent work has reported a nanoreactor approach that can be used to synthesize these silver nanoparticles. This approach uses amphiphilic star-block copolymers, which have a hydrophilic core surrounded by a hydrophobic exterior. These polymers thus can serve as the nanoreactors. In this study, we hypothesize that the local high concentration of silver ions in the inner hydrophilic cores of these star-block copolymers facilitates the nucleation and subsequent growth of silver nanoparticles. When all silver nanoparticles nucleate from the cores of the star-block copolymers in solution, the particle size can be controlled by the core size of the polymer. To test this hypothesis, a polyisoprene-b-poly(p-tert-butylstyrene) (PI-b-PtBS) star-block copolymer was functionalized with carboxylic acid groups using a high-efficiency, photo-initiated thiol-ene click reaction. We characterized this modified polymer using proton nuclear magnetic resonance spectroscopy, and the results indicated that ~60% of the double bonds in the polyisoprene block were successfully functionalized with carboxylic acid groups. When silver ions were added to a solution of these functionalized star-block copolymers, the negatively charged carboxylic acid groups would attract the positively charged silvermore »
-
Reducing the use of solvents is an important aim of green chemistry. Using micelles self-assembled from amphiphilic molecules dispersed in water (considered a green solvent) has facilitated reactions of organic compounds. When performing reactions in micelles, the hydrophobic effect can considerably accelerate apparent reaction rates, as well as enhance selectivity. Here, we review micellar reaction media and their potential role in sustainable chemical production. The focus of this review is applications of engineered amphiphilic systems for reactions (surface-active ionic liquids, designer surfactants, and block copolymers) as reaction media. Micelles are a versatile platform for performing a large array of organic chemistries using water as the bulk solvent. Building on this foundation, synthetic sequences combining several reaction steps in one pot have been developed. Telescoping multiple reactions can reduce solvent waste by limiting the volume of solvents, as well as eliminating purification processes. Thus, in particular, we review recent advances in “one-pot” multistep reactions achieved using micellar reaction media with potential applications in medicinal chemistry and agrochemistry. Photocatalyzed reactions in micellar reaction media are also discussed. In addition to the use of micelles, we emphasize the process (steps to isolate the product and reuse the catalyst).