The high through-put synthesis & screening of electrocatalysts for the reduction of nitrate in groundwater and waste streams

Abstract

The pollution of groundwater and waste streams by nitrate is an increasing problem. The electrochemical reduction of nitrate is a viable technology to convert nitrate to a less harmful species such as N2. However, Rh, a rare precious metal is the most active catalyst for the electrochemical reduction of nitrate. The discovery of new electrocatalysts is required for this technology to be commercially viable. As nitrate is a complex reaction with up to eight different reaction products it is vital to be able to determine the product distribution. From the viewpoint of environmental remediation it is undesirable to form species such as NH3, which are more toxic than nitrate itself.
Differential electrochemical mass spectrometry (DEMS) was highlighted as a useful technique to determine the gaseous reaction products. A DEMS system was designed and implemented to be used in conjunction with a high through-put electrochemical screening array. Samples were deposited on this electrochemical array featuring 100 electrodes by using a high through-put physical vapour deposition methodology [A] to create libraries of electrocatalyst materials. This method is highly versatile and was used to produce binary alloys over a wide composition range, as well as nanoparticles of Pt and alloy particles of PdCu. The advantage of high through-put methods is the fast synthesis and screening of potential electrocatalysts to determine active compositional ranges or particle size. The electrocatalysts prepared in this manner were characterised using energy dispersive X-ray spectroscopy, transmission electron microscopy and X-ray diffraction prior to the electrochemical screening. The development of DEMS has allowed the activity of the electrocatalysts to be screened whilst monitoring the reaction products.
Using the methods described above the activity and product selectivity of PdCu and PdSn alloy electrocatalysts are discussed in detail for the electrochemical reduction of nitrate in both acid and alkaline electrolyte. The PdCu and PdSn alloy systems which are frequently discussed in the literature were used to demonstrate the capabilities of the DEMS technique. These electrocatalysts still utilise a relatively expensive metal, Pd. Therefore further systems presented in this work include Pt nanoparticles, PdCu alloy nanoparticles (which reduce the precious metal loading) in addition to alloys of WCu and WC.
References:
[A] Guerin, S., Hayden, B., Physical Vapour Deposition Method for the High Through-put Synthesis of Solid-State Material Libraries, J. Combi. Chem. 2006, 8, (1), 66-73

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ORCID for Brian E. Hayden: orcid.org/0000-0002-7762-1812

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