Cauê A. Martins

Agglomeration and Cleaning of Carbon Supported Palladium Nanoparticles in Electrochemical Environment

AbstractHere we investigate the electrochemical behavior of Pd/C synthesized by reduction with ethylene glycol in the presence of polyvinylpyrrolidone (EG-PVP). EG-PVP produces nanoparticles (NPs) with a narrow size distribution, but some of them remain covered by impurities after the synthesis. After successive voltammetric cycles, NPs become cleaner, but some agglomeration and structural modification occur; these effects affect the electrochemical behavior of Pd/C in different ways, so we used CO as a probe to better understand the processes taking place. CO stripping shows that the general features of the multiple oxidation peaks change with the number of cycles. Possibly, CO and OH from different NPs react when the particles agglomerate, contributing to CO stripping changes. Finally, different active areas are found when the charges involved in CO oxidation and PdO reduction are compared. Such differences are rationalized in terms of a balance between the increase of sites which promote the oxidation of CO and the loss of area provoked by the growing of the particles. FigureAfter successive voltammetric cycles, Pd/C NPs become cleaner with slightly agglomeration, which lead to increase of the electrochemically active surface area. The value of area reaches a maximum, after this point the agglomeration is the main effect and contributes to the surface area decay. The agglomeration facilitates the CO electrooxidation reaction among NPs

A Pd nanocatalyst supported on multifaceted mesoporous silica with enhanced activity and stability for glycerol electrooxidation

Glycerol is massively produced as a byproduct of biodiesel manufacturing, which decreases its price and increases its inadequate disposal. The use of glycerol to feed anodes of alkaline fuel cells and electrolysers has emerged as potential alternatives for its use. However, the activity and stability of commercial Pd/C nanoparticles currently proposed do not reach the required performance. Here, we propose a new Pd catalyst deposited in mesoporous silica (SiO2) with enhanced activity and stability for glycerol electrooxidation in alkaline media. We synthesized well-ordered mesoporous silica with pores in the form of a fountain pen and lmm symmetry, in a cage-like arrangement. Such a structure was proved by experiments of microscopy, X-ray diffraction, N2 isotherms and small-angle X-ray scattering measured in synchrotron light. Next, we produced patterned Pd/SiO2 in a one-step synthesis and ordered Pd after template removal. The Pd catalysts were characterized by microscopy, dispersive X-ray spectrometry and X-ray diffraction. Pd/SiO2 and Pd showed improved current density and stability compared to Pd/C. The current density of Pd/SiO2 reached twice the values found for Pd/C. The enhancement is understood as a multiple role of the mesoporous silica support, which works as a seed mediator to order the Pd nanoparticles and mainly as a trap that confines the reactant inside the mesoporous structure, increasing the frequency of collision with active Pd sites. Furthermore, we showed that no carbonyl or intact glycerol was found on Pd/SiO2 after exhaustive cycles of use, except for adsorbed CO that was quickly removed from the surface, which is a probable reason for an improved pathway of glycerol electrooxidation via CO.

Alternative Uses for Biodiesel Byproduct: Glycerol as Source of Energy and High Valuable Chemicals

Glycerol was firstly faced as a residue, since it is massively produced from the transesterification of vegetable oils and animal fat, corresponding to roughly 10% of the total amount of biodiesel. The recent high availability of glycerol has decreased its price and increased the risks of environmentally inadequate disposal. Now, this small chain alcohol is faced as a powerful alternative for energy conversion and production of chemicals with commercial interest. Its three hydrated carbons make this organic a noticeable substrate to produce other carbonyl compounds and to be used to produce energy in electrochemical devices collectively known as direct alcohol fuel cells. The development in the electrocatalysis field opened up new strategies to convert glycerol into power and chemicals, by using fuel cells and electrolyzer reactors. In both systems, the efficiency of the process depends on several aspects, as the medium, applied potential, and mainly on the surface reaction taking place at the interface between solution and electrode. In this chapter, we discuss the use of glycerol in these electrochemical systems and illustrate some experimental results regarding fundamental and applied science. Namely, we describe some advances in the understanding of the glycerol electro-oxidation reaction interpreted by spectroscopy and chromatographic techniques. Novel nanomaterials currently applied to improve the catalysis of the reaction are also shown. Moreover, we comment some results regarding fuel cells, microfluidic fuel cells, and electrolyzers fed by glycerol and the perspectives and challenges of its use.

The Contribution of Carbon Supports on the Activity of Pt for Glycerol Electrooxidation: the Importance of Investigating the Derivative Voltammogram and Arrhenius Plots

Glycerol electrooxidation was evaluated on Pt electrodeposited over carbon Vulcan (CV), multi-walled carbon nanotubes (MWCNTs), graphene oxide nanoribbons (GONRs), and graphene nanoribbons (GNRs). Different masses of Pt were deposited under the same conditions, producing different surface areas of Pt. The presence of GNRs slightly enhanced the specific activity of the catalyst. By investigating the derivative voltammetry of glycerol, we found that the supports did not shift the onset potential towards lower values. Moreover, we found that the apparent activation energy did not vary by changing the carbon support. In this sense, we rationalized the slight improvement in specific activity of Pt deposited on GNRs as a consequence of the frequency of collision factor due to the availability of Pt over the longitudinal flat surface of nanoribbons, as shown by the high active surface area/mass of electrodeposited Pt ratio.

Estimating the Time-Dependent Performance of Nanocatalysts in Fuel Cells Based on a Cost-Normalization Approach

Researchers have developed new catalysts for fuel cells (FC), whose performances are compared after applying different normalization procedures. However, there is not a standard procedure. The current produced from CO electrooxidation was compared for Pt4Ru5Sn1/C and homemade Pt/C nanoparticles (NPs) normalized by different methods and the use of different methods renders different interpretations. Since the whole field aims to maximize the cost-effectiveness, a complementary method to normalize currents and power in terms of the total cost of the nanocatalyst, the Catalyst-based Cost method (CbC), was proposed. CbC considers the cost of all metals employed to build the catalyst, not only those ones with available surfaces. By applying a simple smoothing method on the prices in a time series, we were able to forecast the prices and consequently the power density of a FC. CbC provides tools for industrials forecast the designing of nanomaterials with improved efficiency and low cost.

New Electrochemical Flow-Cell Configuration Integrated into a Three-Dimensional Microfluidic Platform: Improving Analytical Application in the Presence of Air Bubbles

A newly configured electrochemical flow cell to be used for (end-channel) amperometric detection in a microfluidic device is presented. The design was assembled to place the reference electrode in a separated compartment, isolated from the flow in the microchannel, while the working and counter electrodes remain in direct contact with both compartments. Moreover, a three-dimensional coil-shaped microfluidic device was fabricated using a nonconventional protocol. Both devices working in association enabled us to solve the drawback caused by the discrete injection when the automatic micropipette was used. The high performance of the proposed electrochemical flow cell was demonstrated after in situ modifying the surface of the platinum working electrode with surfactant (e.g., using Tween 20 at 0.10%). As the reference electrode remained out of contact with the flowing solution, there was no trouble by air bubble formation (generated by accidental insertion or by presence of surfactants) throughout the measurements. This device was characterized regarding its analytical performance by evaluating the amperometric detection of acetaminophen, enabling determination from 6.60 to 66.0 μmol L-1. This issue is important since at high concentration (e.g., as assessed in clinical analysis) the acetaminophen is known to passivate the working electrode surfaces by electrogenerated products, impairing the accuracy of the electrochemical measurements.