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Molecules 2021, 26, 2144 24 of 37 Much attention has been focused on Au–Pd catalysts because of their excellent cat- alytic properties towards the hydrogenation of acetylene, the synthesis of acetate and the oxidation of alcohols. The presence of gold lowers the cost of the final catalytic material and its sensitivity towards poisoning with carbon oxide intermediates. This improvement is related to the synergic effect that takes place between gold and palladium in such materials. Gold induces an upshift of the palladium d-band center, which results in a stronger affinity for hydroxide ions. This effect enhances the adsorption of hydroxide ions and improves the kinetics of the EGOR [36,190]. Because of its relatively low price and very interesting properties, electrodes for the electrooxidation of EG based on gold, as the main metallic ingredient, have also been developed. This element shows catalytic properties not only towards the EGOR but also towards providing the hydroxide ions that are necessary for high-performance of the catalytic material and its immunity to poisoning [25,28,57,86,175,176,191] As with any of the other already-mentioned materials, gold-based catalysts show higher activity when their electroactive surfaces are increased. The easiest way to signifi- cantly increase the active area of the electrode is to increase its surface-to-volume ratio using electrode materials in the form of nanocompounds. Due to the sophisticated methods of their synthesis, gold-based nanomaterials are easy to shape and form, which allows simple alteration of ECSA. High electroactivity enhances the reactivity of the electrode material and improves the surface atom utilization, which allows the use of smaller doses of the catalyst for the same result, lowering the cost of the overall process [25,30,86,176,181,191]. Despite its catalytic properties, gold is rarely used as a catalytic material on its own because of its poisoning with reaction product and poor stability, both leading to a decrease of the reaction active centers [25,86,175]. Currently, gold catalysts are usually used with doping agents that enhance their performance [25] or with supporting materials that alter the catalyst properties [86]. One of the doping agents that can be used to improve the gold catalyst performance is silver. AuAg alloys show a bifunctional mechanism: on Au sites, alcohol molecules are adsorbed and oxidized, while on the surface of Ag, oxygenated species are promoted [25]. Metal oxides, such as CeO2, Fe2O3 and RuO2, used as embedding for gold catalysts, can also enhance the catalytic performance of gold nanoparticle catalysts [86]. Their highly oxophilic character provides conditions for the occurrence of a bifunctional mechanism that enhances the catalyst performance by improving the adsorption of hydroxide ions on the catalyst surface [86]. The use of metal oxides as support materials for golden nanoparticles simplifies the preparation of the catalytic system, which requires the formation of only one nanocompound while maintaining the benefits of these oxides as doping agents. However, this solution, as always, has flaws—too high an amount of iron and ruthenium oxide leads to a decrease in the electroactive catalyst, which is probably related to the formation of clusters from gold nanoparticles [86]. In addition, catalyst stability can decrease as a result of the presence of ruthenium oxide because it can over-provide the catalyst with oxygen species, which leads to surface poisoning with Au2O3–gold(III) oxide, which shows no catalytic properties towards EGOR and blocks bulk ethylene glycol molecules from adsorption on the electrode surface [86]. Additionally, the combination of platinum and gold has shown excellent results towards EG electrooxidation. Both of these metals have catalytic properties towards EGOR, and gold has an additional ability to prevent adsorption of the reaction interme- diate products on the platinum surface, which protects the active centers of the catalyst from poisoning [167,172]. Different kinds of palladium-based nanoparticles have been developed, from simple palladium nanoparticles [183] and PdNi nanocubes [22] to nanoflowers made of palla- dium and silver [192], to complicated ternary core–shell systems, such as FeCo@Fe@Pd particles [33]. Despite their differences related to different compositions, they are all char- acterized by high electroactive surface related to high volume to surface ratio. Nanoporous catalytic materials have also been examined as catalytic materials for the oxidation of smallPDF Image | Effect of Anode Material on Electrochemical Oxidation of Alcohols
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