Janaina F. Gomes

Straightforward Synthesis of Carbon-Supported Ag Nanoparticles and Their Application for the Oxygen Reduction Reaction

In this paper we report a simple and environmentally friendly synthesis of silver nanoparticles (AgNps) and their activities towards the oxygen reduction reaction (ORR). Ultraviolet spectroscopy (UV–vis) and transmission electron microscopy confirmed the formation of poly(vinyl pyrrolidone)-protected colloidal AgNps through direct reduction of Ag+ by glycerol in alkaline medium at room temperature. For the ORR tests, the AgNps were directly produced onto carbon to yield the Ag/C catalyst. Levich plots revealed the process to occur via 2.7 electrons, suggesting that the carbon support contributes to the ORR. We discuss here possibilities of improving the catalytic properties of the Ag/C for ORR by optimizing the parameters of the synthesis.

MORPHOLOGICAL STUDY OF Sr 2 CeO 4 BLUE PHOSPHOR WITH FINE PARTICLES

Morphological and spectroscopic studies of Sr2CeO4 blue phosphor in the form of fine particles prepared from a powdered multi-component precursor, via a combustion method, are reported. Samples were also prepared through a solid-state reaction and from a polymeric precursor for comparison. Citric acid or glycine as fuels in the combustion method lead to a mixture which is heated at 950 oC for 4 h, resulting in spheroidal particles with a diameter between 250-550 nm. Samples from the polymeric precursor result in spheroidal particles (350-550 nm) and from the solid-state reaction in irregular particles (~ 5 mm). Therefore, the combustion method is adequate for preparation of Sr2CeO4 in the form of spherical fine particles.

Eco-friendly synthesis of bimetallic AuAg nanoparticles

In this study we report a friendly route to produce colloidal AuAg bimetallic nanoparticles (NPs) using glycerol as a reducing chemical in alkaline medium. Ultraviolet spectroscopy (UV-Vis) and transmission electron microscopy (TEM) confirmed the formation of colloidal NPs. The nanoparticles were also directly produced on carbon to yield Au/C, Ag/C and AuAg/C. Extended X-ray absorption fine-structure (EXAFS) spectroscopy at the Au L3 edge evidenced surface segregation of silver in the AuAg/C material. Also the X-ray absorption near edge structure (XANES) region indicated a slight reduction in the Au 5d band occupancy due to the presence of silver. Since a fuel cell is a potent source of energy for the future, the catalytic properties of the NPs were investigated for anodic glycerol electro-oxidation reaction and results for AuAg NPs are compared with those for monometallic Au and Ag NPs. When compared with Au/C, the AuAg/C material displayed lower potential for glycerol electro-oxidation, reducing by 120 mV the onset potential for glycerol electro-oxidation. This means that the electro-oxidation of glycerol on the bimetallic catalyst is mainly affected by the ligand effect.

Hollow AgPt/SiO2 nanomaterials with controlled surface morphologies: is the number of Pt surface atoms imperative to optimize catalytic performances?

We describe herein an investigation on how the number of Pt surface atoms and nature of exposed surface facets affect the catalytic performances of AgPt nanomaterials displaying controlled surface morphologies (smooth or rough surfaces), shapes (spherical or one-dimensional), and hollow interiors towards CO oxidation. More specifically, we focused on AgPt nanoshells (smooth surfaces), assembled nanoparticles (rough surfaces), nanotubes with smooth surfaces, and nanotubes with rough surfaces. We found that their catalytic performances followed the order: nanotubes with smooth surfaces > nanoshells, nanotubes with rough surfaces > assembled nanoparticles. The better catalytic activity observed for the nanoshells relative to the assembled nanoparticles can be associated with their higher number of Pt surface atoms. Even though the nanotubes with rough surfaces had a higher number of Pt surface atoms relative to the nanotubes with smooth surfaces, the latter displayed higher catalytic activities as a result of the preferential exposure of {100} facets, which are the most active towards CO oxidation relative to {111} and {110}. Interestingly, the nanotubes with smooth surfaces also displayed higher catalytic activities when compared to the nanoshells, showing that the preferential exposure of {100} side facets compensated the decrease in their number of Pt surface atoms relative to the nanoshells. Our data showed that the catalytic performances were strongly dependent on the surface morphologies, in which the preferential exposure of more active surface facets may play a significant role in the optimization of performances relative to the number of Pt surface atoms.

Reactivity of Alcohols with Three-Carbon Atom Chain on Pt in Acidic Medium

Abstractn-Propanol, 2-propanol, 1,2-propanediol, 1,3-propanediol, and glycerol are all saturated alcohols with a three-carbon atom chain. The difference between them is the number of hydroxyl groups and their positions in these molecules. The interaction between alcohols with a three-carbon atom chain and model surfaces and the further reaction have been widely investigated in the last three decades. In this short review, we will concentrate our attention on the reactivity of n-propanol, 2-propanol, 1,2-propanediol, 1,3-propanediol, and glycerol on smooth Pt in acidic medium. We will present the most important results reported in the literature in this field, highlighting the main particularities related to their reactions, addressing key points such as the impact of the position of the OH group in the molecule, number of OH groups in the molecule, and presence of vicinal OH groups in the molecule on the reactivity of these alcohols over Pt. Finally, we will present some perspectives concerning the application of alcohols with a three-carbon atom chain, mainly glycerol, in fuel cells.

Influence of Poly(Vinyl Alcohol) (PVA) on the Cyclic-Voltammetry Behavior of Single-Crystal Pt Surfaces in Aqueous H2SO4

The study of the interaction of polymers with metallic surfaces is of pivotal importance not only from the fundamental point of view (e.g.: hydrogen UPD on single crystals in the presence of surface blocking species), 1 but also from the applied one since polymers are widely employed as stabilizing agents in the production of nanoparticles. 2 Poly(vinyl alcohol) (PVA) is a water-soluble hydrophilic polymer and has been employed in different applications. 3 It reacts with metal salts in aqueous solution leading to a metal chelate, being then usually selected as an ion and cluster capping agent to prevent excessive nanoparticle growth. 4–8 The mechanism of protection involves donation of the –OH lone electron pairs to the metals and prevention of aggregation through steric effect. Although this mechanism is generally accepted, the literature completely lacks information on the interaction between PVA and metal surfaces. It is well known that adsorption is not only affected by the electronic properties of the surface but also by its structure (crystallographic orientation). As an example, Morin and Conway 9 found the chemisorption and surface reactivity behavior of acetonitrile at distinct Pt single crystal planes to be very specific to the geometry of the surface. The understanding of molecular adsorption at well-ordered single crystal surfaces may enable, in principle, the production of preferentially oriented nanoparticles. 10 In this work we studied, for the first time, the electrochemical behavior of PVA at well-ordered single crystal as well as polycrystalline Pt surfaces in H2SO4 solution. We show that PVA not only behaves distinctly on different oriented surfaces, but also suffer oxidation to some extent.

Luminóforo azul preparado a partir do método de combustão

This work reports on the preparation of Sr2CeO4 blue phosphor with fine particles by using a powder multi-component precursor, via combustion method, varying the fuel. Citric acid or glycine as fuels lead to the formation of different phase mixture. After heating under 950oC both different phase mixtures result on a material that the mainly phase is the Sr2CeO4. White blue luminescence is only observed for these samples after thermal treatment and both of them present similar emission and excitation spectra that are characteristics of Sr2CeO4 phase. Therefore, the combustion method is adequate for blue phosphor preparation.

Electro-oxidation of 3-Carbon Alcohols and Its Viability for Fuel Cell Application

N-propanol, 2-propanol, 1,2-propanediol, 1,3-propanediol, glycerol, allyl and propargyl are all aliphatic alcohols containing three carbon atoms.

How reactive are metal surfaces in solution? A comparison of the electrochemical adsorption of organic molecules on Pt at low potentials

IntroductionTransition metal surfaces are widely employed as catalysts ofchemical reactions, both at the metal/gas [1] and at the metal/solution [2] interfaces. Well-known examples are ceramic-supportedcatalystsusedingasexhaustsofvehiclestooxidizepartially burnt fuels [3] and metal-based catalysts used in fuelcell devices to promote electrochemical oxidation of organicmolecules for direct conversion of chemical energy into elec-trical energy with high efficiency [4].In this context, the electrochemical oxidation of alcoholshasbeenextensivelystudiedinthelastdecades[5]duetotheirpotentialfor practical applicationsinfuelcells.Directalcoholfuelcell(DAFC),especiallydirectethanolfuelcell(DEFC),isbecominga veryattractivealternative energysourcefor fossilfuel combustion engines due to several advantages, includingits high theoretical efficiency, high energy density of alcohols(comparable to gasoline) [6], and the possibility of energyproduction from renewable sources [7]. However, for theDEFC to become commercially attractive, a few issues mustbeovercome.Oneofthedifficultiesisthatthecurrentstate-of-the-art anodes are not capable of fully oxidizing ethanol toCO

Combining active phase and support optimization in MnO2-Au nanoflowers: Enabling high activities towards green oxidations

Among the several classes of chemical reactions, the green oxidation of organic compounds has emerged as an important topic in nanocatalysis. Nonetheless, examples of truly green oxidations remain scarce due to the low activity and selectivity of reported catalysts. In this paper, we present an approach based on the optimization of both the support material and the active phase to achieve superior catalytic performances towards green oxidations. Specifically, our catalysts consisted of ultrasmall Au NPs deposited onto MnO2 nanoflowers. They displayed hierarchical morphology, large specific surface areas, ultrasmall and uniform Au NPs sizes, no agglomeration, strong metal-support interactions, oxygen vacancies, and Auδ+ species at their surface. These features led to improved performances towards the green oxidations of CO, benzene, toluene, o-xylene, glucose, and fructose relative to the pristine MnO2 nanoflowers, commercial MnO2 decorated with Au NPs, and other reported catalysts. We believe that the catalytic activities, stabilities, and mild/green reaction conditions described herein for both gas and liquid phase oxidations due to the optimization of both the support and active phase may inspire the development of novel catalytic systems for a wealth of sustainable transformations.