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Molecules 2021, 26, 2144 16 of 37 For C1 products, a reaction between adsorbed (CH3CO)ads and OHads look differently [148]: Pd-(CH3CO)ads +Pd-OHads +6OH−→2CO2 +6H2O+4Pd+6e− (22) Comparing reaction (20) to reaction (22), the importance of the hydroxide ions becomes clear. It not only provide conditions that enable ethanol oxidation towards C1 products but also allows usage of less expansive, non-noble metals as electrode materials [10]. Palladium-based catalysts can be used in acidic media. For example, Co-Pd/Sn anodes have been proven to have significant electrocatalytic activity for the EOR [10], but because of advantages that come with the usage of alkaline media that have been mentioned above, most of the researchers have focused on developing electrocatalytic materials for alkaline media [10]. Different metals have been investigated as palladium cocatalysts for ethanol electroox- idation in alkaline media. Copper is an inexpensive metal showing catalytic properties towards ethanol ox- idation, making it a promising cocatalyst for palladium [13,32,148,149]. The presence of copper in the catalyst is leading to the electronic (ligand) effect, which enhances the performance of the anodic material [13]. The use of core–shell nanoparticles, where a PdCu alloy mixed with pure palladium is the shell material, and copper is the core, has shifted the performance, even more, thanks to a higher active area related to a higher surface-to-volume ratio [32]. The addition of copper has not only increased the oxidation peak current density but also increased durability in alkaline media and the immunity towards CO poisoning [32]. Platinum also has been tested as a cocatalyst material for Pd-based anodic materials for ethanol oxidation because they combine the advantages of both materials—the high activity of both platinum and palladium towards alcohol oxidation and enhanced immunity towards CO poisoning thanks to the presence of Pd [11,137,150–152]. According to the literature, the rate-determining step in ethanol electrooxidation is the reaction between the adsorbed CH3CO(ads) and adsorbed OH(ads) (reaction (20)) [11,137] or breaking of the C–H bond to obtain C1 products [151]. The catalytic effect observed for the PdPt catalyst is probably the result of an electronic effect where the d-band palladium center is shifted in the presence of platinum. Shifted Pd centers promote OH adsorption, which increases the rate of adsorbed species reaction and thus improves the overall reaction kinetics [11]. Doping with silver leads to electrocatalytic materials that are more active and more stable towards the EOR in alkaline environments [26,136,153,154]. The addition of silver to a palladium catalyst leads to a lower potential of the reaction peak than pure Pd under the same conditions, which may contribute to the acceleration of the reaction rate and the smaller size of the obtained catalytic particles, leading to a larger electroactive surface on the working electrode [26,136,153,154]. To obtain a larger electroactive surface on such electrodes, different core–shell structures, such as PdAg@Pd core–shell worm-like structures, which are shown in Figure 2, have been developed [26]. Because of its structural properties, this type of nanomaterials has shown better stability and higher residual activity and better noble-metal utilization than other catalysts [26,136,153,154]. Nickel as a metal shows catalytic properties towards the oxidation of alcohols and other low molecular weight organic compounds [12,13,136,155–158]. When nickel is added to alkaline media at the reaction potential, it oxidizes into nickel hydroxide, which increases the surface coverage of OHads species, leading to an increasing overall reaction rate since the reaction between the adsorbed species on the palladium surface is the rate-determining step [12,157]. The addition of nickel not only increases the amount of adsorbed hydrogen ions but also changes the electronic structure of palladium thanks to the electronic (ligand) effect, which improves the catalyst reactivity and its immunity towards CO poisoning [12]. The dissociative adsorption of ethanol proceeds quickly, so the rate-determining step of this process is the reaction between the adsorbed hydroxyl groups and adsorbed acyl groups, which leads to the removal of the adsorbed species [12]. The use of PdNi nanoparticles has shifted the catalytic properties of the material thanks to a higher volume to surfacePDF Image | Effect of Anode Material on Electrochemical Oxidation of Alcohols
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