Microemulsion droplets to catalytically active nanoparticles. How the application of colloidal tools in catalysis aims to well designed and efficient catalysts

Microemulsion Derived Titania Nanospheres: An Improved Pt Supported Catalyst for Glycerol Aqueous Phase Reforming

Glycerol aqueous phase reforming (APR) produces hydrogen and interesting compounds at relatively mild temperatures. Among APR catalysts investigated in literature, little attention has been given to Pt supported on TiO₂. Therefore, herein we propose an innovative titania support which can be obtained through an optimized microemulsion technique. This procedure provided high surface area titania nanospheres, with a peculiar high density of weak acidic sites. The material was tested in the catalytic glycerol APR after Pt deposition. A mechanism hypothesis was drawn, which evidenced the pathways giving the main products. When compared with a commercial TiO₂ support, the synthetized titania provided higher hydrogen selectivity and glycerol conversion thanks to improved catalytic activity and ability to prompt consecutive dehydrogenation reactions. This was correlated to an enhanced cooperation between Pt nanoparticles and the acid sites of the support.

Low-Temperature Miniemulsion-Based Routes for Synthesis of Metal Oxides

The use of miniemulsions containing chemical precursors in the disperse phase is a versatile method to produce nanoparticles and nanostructures of different chemical nature, including not only polymers, but also a variety of inorganic materials. This Minireview focuses on materials in which nanostructures of metal oxides are synthesized in processes that involve the miniemulsion technique in any of the steps. This includes in the first place those approaches in which the spaces provided by nanodroplets are directly used to confine precipitation reactions that lead eventually to oxides. On the other hand, miniemulsions can also be used to form functionalized polymer nanoparticles that can serve either as supports or as controlling agents for the synthesis of metal oxides. Herein, the description of essential aspects of the methods is combined with the most representative examples reported in the last years for each strategy.

Bicontinuous microemulsion as confined reaction media for the synthesis of plasmonic silver self-assembled hierarchical superstructures

Plasmonic superstructures may concentrate hot spots both on the external surface and within the inner gaps of the assembly. However, these materials are usually obtained by two-steps procedures from synthesis of plasmonic nanoparticles to their 3D assembly. The interconnected nano-network of water and oil channels in a bicontinuous microemulsion (BμE) may act as a preorganized reaction system giving reticulated materials. In this work, a silver hierarchical superstructure (HSS-AgCt) was obtained in the water channels of a BµE in a one-pot procedure. The characterization of the morphology and crystalline structure revealed that this superstructure is composed of silver nanoparticles embedded in polymeric silver citrate forming a 3D mesh of interconnected fibers with mean width of 30 nm. The aging of HSS-AgCt in the BμE allowed the degradation of the citrate fibers giving rise to interconnected spherical silver nanoparticles (HSS-Ag) of 8 nm as measured from TEM images. Rhodamine 6-G was detected by SERS up to 10-12 M with an analytical enhancement factor of 109 for both materials using a 633 nm laser operating at 0.85 mW (5% of the nominal power). These results introduce a novel route to obtain highly sensitive SERS substrates in one-pot procedures by using BμE as a nanoreactor and template.

Tailored surface composition of Au/Pt nanocatalysts synthesized in microemulsions: a simulation study

Au/Pt nanoparticles show an optimized catalytic activity when compared with Pt nanoparticles because Pt activity is improved by the presence of Au on the surface. It was checked whether a controllable surface composition can be achieved by the simple strategy of varying the Au : Pt ratio. We present an in-depth kinetic simulation study on the influence of Au : Pt ratio on the formation of Au/Pt nanoparticles synthesized in microemulsions. This study is able to explain the resulting nanoarrangement as a function of kinetic parameters such as Au : Pt ratio and intermicellar exchange rate. The role of the micelles as a dosing pump of the Au precursor explains that a higher Au amount results in a Au reduction which takes place over a longer period of time. It implies that Au is deposited until longer stages of the synthesis, so Au is present at the nanoparticle surface. Micelles as reaction media produce a minor impact on Pt due to its slower reduction. These different kinetic behaviours of Au and Pt give rise to a surface composition which can be tailored by tuning the Au : Pt ratio. Numerical results on surface composition successfully reproduce experimental data and further support the outcomes of the degree of atomic mixing under different Au : Pt ratios.

Pure Pt surface at low Au content and mixed surface at high Au content.

New trends in tailoring active sites in zeolite-based catalysts

This review discusses approaches for tailoring active sites in extra-large pore, nanocrystalline, and hierarchical zeolites and their performance in emerging catalytic applications.

Catalytically Active Pd-Ag Alloy Nanoparticles Synthesized in Microemulsion Template

This work investigates the possibility to form catalytically active bimetallic Pd-Ag nanoparticles synthesized in the water pools of a reversed microemulsion using methanol, a more environmental- and user-friendly reductant compared to hydrazine or sodium borohydride, which are commonly used for this type of synthesis. The nanoparticles were characterized with regards to crystallinity and size by X-ray diffraction and transmission electron microscopy. CO chemisorption and oxidation followed by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used for investigating the elemental composition of the surface and catalytic activity, respectively. Moreover, the structural composition of the bimetallic particles was determined by scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy. The particles were shown to be crystalline nanoalloys of around 5-12 nm. CO adsorption followed by in situ DRIFTS suggests that the particle surfaces are composed of the same Pd-Ag ratios as the entire particles, regardless of elemental ratio (i.e., no core-shell structures can be detected). This is also shown by numerical simulations using a Monte Carlo based model. Furthermore, CO oxidation confirms that the synthesized particles are catalytically active.