Fabrication, Modification And Electrocatalytic Activity Of Gold Nanofilm Electrodes And Nanoporous Palladium Electrodes

Nanomaterials, especially noble metals like platinum and palladium, have showed promising applications in many fields such as optics, electrics, magnetics, biology, and chemistry. Nowadays novel methods for the fabrication of nanomaterials are various, among which electrochemical method has many advantages like good controllability, mild reaction conditions, environmental benignancy, and wide range of applications, therefore it is an excellent method for preparing nanostructured materials. Meanwhile, nanostructured platinum, palladium and gold play very important roles in catalysis, what’s more, the bimetallic and multimetallic catalysts show excellent catalytic performance that has attracted many researches in the related fields. In this paper, we aim at utilizing simple and novel electrochemical methods to fabricate nanostructured noble metals, such as nanoparticles and porous materials, and at studying their applications in electrooxidation of methanol and formic acid for fuel cells or electrochemical degradation of chlorinated organic compounds. The main contents of this paper include:(1) Using a simple, wet chemical method to fabricate gold nanoprisms in aqueous solutions with PVP as stabilizer and shape controller, ethylene glycol as a weak reducing agent. The as-prepared gold nanoprisms grow on the treated indium tin oxide (ITO) glass, and the coverage of the gold nanoprisms on the ITO substrate is about 80%, which may act as gold prism film electrodes. The coverage of gold nanoprisms of this electrode is higher than other electrodes prepared by similar methods. Moreover, this kind of film electrodes can be used as the substrate to construct bimetallic or multimetallic electrodes by electrochemical underpotential deposition (UPD) of copper and replacement of noble metals on the electrodes and used for electrooxidation of methanol in acid medium. In the study we found that the Au nanoprism films (Au-PF) electrode behaved poor electrochemical catalytic activity to methanol in acid solutions, while played good performance in the alkaline electrolyte, especially strong resistance to CO poisoning. When Au-PF electrodes were modified with tiny Pt, Pd or Au, the as-prepared bimetallic or multimetallic electrodes played excellent electrocatalytic oxidation activity of methanol in acid electrolyte. Compared with commercial platinum catalyst, these electrodes had wonderful electrocatalytic activity and good resistance of CO poisoning. The reason of this phenomenon is that the synergistic effect of noble metals could improve the catalytic activity of the electrodes. One of the most obvious advantages of the Au-PF electrodes is very low consumption of noble metals, and the other is that the electrodes display high electrochemical catalytic activity and strong resistance against CO poisoning.(2) Using electrochemical dealloying method to fabricate the nanoporous palladium (np-Pd) in ionic liquid or sulfuric acid solutions by selective removal Cu in Pd-Cu alloy. Ionic liquid has a wider electrochemical window than water, in which we able to select an appropriate anodic potential to induce fast dissolution of Cu while assuring appropriate surface diffusion kinetics of un-attacked Pd atoms to self-organize into a uniform porous network structure. In H2SO4 solutions, OH- ions and other anions usually participate in the selective anodic dissolution of Cu, forming deposits or salt films on the surface of a dissolving alloy, which will bring an adverse effect to the surface diffusion of Pd atoms. The nanoporous Pd fabricated in ionic liquid or in H2SO4 still contains residual Cu. To remove the residual Cu, the np-Pd sample was further immersed in Pd(Ⅱ) solution via galvanic replacement. This simple method not only can remove Cu, but also can refresh the porous structures due to formation of some fresh and tiny nanoparticles, which make the np-Pd electrode possess higher electrocatalytic activity. Because Pd nanostructures display very high catalytic activity towards the electrochemical reductive dechlorination of chlorinated organic compounds, we chose carbon tetrachloride (CT) and chlorobenzene (CB) as the target pollutants, and use the nanoporous Pd as the work electrodes to carry out electrochemical reductive dechlorination of CT or CB at different potentials. Based on the electrocatalytic dechlorination results, we found that the four kinds of electrode materials followed this order in electrochemical dechlorination activity:t-np-Pdn, np-Pdn, np-Pdsuifuric acid and bulk-Pd. This further confirms that the stability and the high specific surface area of the t-np-Pdn electrode makes its catalytic activity greatly enhance. Besides, it is very interesting that the dechlorination efficiency of three nanostructure palladium electrodes has the same variation trends, namely the dechlorination efficiency at-0.22 V is the highest, followed by-0.06 V and the worst at-0.15 V. However this is not the same on the bulk Pd electrode. Electrochemical dealloying method has many advantages; fabricating nanoporous Pd electrodes using this method don’t need the substrate electrode and don’t cause obvious loss of precious metals. These kinds of nanoporous Pd materials display excellent dechlorination performance and therefore have potential applications in environment protection such as waste water treatment project.(3) Using electrochemical dealloying method to fabricate the nanoporous palladium (np-Pd) and nanoporous Pd-Au alloy. The fabrication method of nanoporous Pd-Au is similar as that of the nanoporous Pd. Pd-Au-Cu alloy was selectively corroded at two appropriate potentials to prepare nanoporous Pd-Au alloy electrodes. The nanoporous Pd-Au alloy fabricated at the potential of 1.1 V has smaller pore size, and the atom ratio of Pd and Au is close to 1:1. While the nanoporous treated at 1.4 V has bigger pore size and with the Pd/Au atomic ratio of 1:2.7. Besides the diffraction peak of the XRD of PdiAu2.7 shifts to right side more than that of PdjAui sample. This means that dealloying at 1.4 V would remove a part of Pd atoms except Cu atoms. Recently the common catalyst used in fuel cells is still Pt-based catalyst; however, Pt is very precious and expensive. To find an alternative to Pt catalyst is needed. The chemical properties of Pd is very similar to that of Pt, therefore, nanoporous Pd and Pd-Au alloy can be acted as the catalysts in fuel cells instead of Pt. In this paper, nanoporous Pd or Pd-Au alloy was used as anode catalyst and formic acid as fuel for electrochemical oxidation. The electrochemical activities of different electrodes were discussed. The nanoporous Pd-Au alloy fabricated by electrochemical dealloying has high surface area and excellent electrocatalytic activity, which displays promising applications in direct formic acid fuel cell (DFAFC).