Aluminum, known for its strong reducing properties, faces a challenge due to the presence of an aluminum oxide film on its surface. However, by inducing pitting corrosion, this barrier can be overcome, allowing aluminum to be used effectively as a reducing agent for the synthesis of metals and alloys. During the pitting corrosion process of aluminum in certain solvents, hydrogen is produced and acts as the reducing agent.
Mechanism of Al as a Reducing Agent
The aluminum oxide film on aluminum foil hinders its use as a reducing agent, but pitting corrosion can help bypass this limitation. By destroying aluminum alloys, pitting corrosion allows aluminum to be an efficient reducing agent in wet-chemical synthesis.
In addition to its role as a reducing agent, aluminum also offers the advantage of being a cost-effective and readily available material for use in wet-chemical synthesis processes. Its ability to efficiently reduce metals and alloys makes it a valuable tool in the laboratory for producing desired chemical compounds.
Activation of Palladium Nanoparticles
UV-Vis Spectra and Mechanism
Results from UV-Vis spectra demonstrate that Na2PdCl4 can be reduced by aluminum foil at room temperature in just a few minutes. This reduction mechanism involves pitting corrosion to break through the oxide film on the aluminum foil.
Corrosion and Cu Particle Formation
Pitting corrosion plays a crucial role in enhancing aluminum’s capabilities as a reducing agent, facilitating the formation of copper particles in solutions containing CuF2, CuBr2, and CuCl2. Notably, anions like SO4 2− and NO3 − do not trigger pitting corrosion of aluminum foil.
Moreover, water molecules can penetrate the oxide film and react with the metallic aluminum underneath, leading to the production of H2 gas, with hydrogen acting as an intermediate in the process. While H2 gas, hydrogen, and metallic aluminum are potential reducing agents in this system, experiments indicate that H2 gas alone cannot reduce an aqueous solution of Na2PdCl4. Conversely, hydrogen is more active and capable of reducing metal oxides like CuO at ambient temperature. The evolution of Pd particles on the aluminum foil during the early stages of pitting corrosion, prior to breaking the oxide film, signifies a unique electro-reduction process. This intricate pathway involves the participation of hydrogen in the reduction of [PdClx(OH)y] 2−, showcasing the interplay between aluminum and various reactive species.
By employing Al as a reducing agent, metallic nanostructures can be synthesized efficiently through a controlled pitting corrosion process. The resulting products demonstrate the diverse applications of this innovative approach.
Applications: synthesis of metallic nanomaterials
An investigation into the utilization of aluminum as a reducing agent for wet-chemical synthesis involved the preparation of two carbon-supported metallic nanomaterials: 40 wt.% Pd on carbon support (Pd/C) for formic acid electro-oxidation in fuel cells and intermetallic Cu2Sb/C as an anode material for lithium-ion batteries. The choice of aluminum foil over powder was influenced by the higher reduction rate, ease of product separation, and improved handling in stirring reactions.
Discussion
The experimental findings confirm that aluminum, activated through pitting corrosion, effectively operates as a reducing agent in wet-chemical synthesis. This novel mechanism generates atomic hydrogen, which serves as the primary reducing agent. Different from reactive metals like Zn and Fe, aluminum’s reduction process hinges on the rapid formation of an oxide film and the unique characteristics of this film, enabling selective reduction of metal compounds.
This innovative approach to synthesis utilizing aluminum as a reducing agent presents a valuable method for producing metallic nanoparticles with diverse applications.
Synthesis
Throughout the experimental process, various precursors, including aluminum foil and powder, different metal compounds, and stabilizers, were employed. Solvents such as deionized water and ethylene glycol were used in conjunction with these materials.
Optimizing reaction conditions and harnessing aluminum’s potential as a reducing agent offer a versatile and sustainable avenue for tailoring metallic nanoparticles and compounds to suit a range of applications.
The Cu2Sb intermetallic catalyst supported on carbon black (acetylene black, compressed, Alfa Aaser) was synthesized following a procedure similar to the one used for Pd/C-EG. Carbon was dispersed in ethylene glycol (EG) to form a 20% composite with Cu2Sb. CuCl2 and SbCl3 were dissolved in a 25:1 molar ratio of aluminum to copper + antimony under a nitrogen atmosphere. After a 4-hour reaction, any unreacted aluminum was removed, and the mixture was washed with isopropanol before being dried in a vacuum oven.
The pH of the solutions was determined using a Corning 313 pH/Temperature meter. The concentration of Na2PdCl4 solutions was identified by d–d spin-forbidden transition peaks in the UV-Vis spectra between 420 – 450 nm. X-ray diffraction (XRD) patterns were obtained and analyzed, scanning electron microscopy (SEM) images were captured, and compositions were determined using INCA software and energy-dispersive X-ray spectroscopy (EDS). Transmission electron microscopy (TEM) images were also obtained to study the particle size distributions.
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Acknowledgments:
This research was funded by the Office of Naval Research MURI grant No. N00014-07-1-0758 and Welch Foundation grant F-1254. We would like to extend our special thanks to Dr. Xinsheng Zhao and Kristen Bateman for their valuable contributions to the study.
Affiliation:
Electrochemical Energy Laboratory, Materials Science and Engineering Program, The University of Texas at Austin, Austin, TX, 78712, USA.