Active role of carbon surface chemistry in the electrooxidation of cinnamyl alcohol over AuPd-modified carbon nanofibers

The selective electrooxidation of biomass-derived compounds is gaining increasing attention to simultaneously produce valuable chemical feedstock and store renewable electricity. In this scenario, the selective electrooxidation of cinnamyl alcohol can be considered a reference reaction. In this work, three carbon nanofibers (CNFs) prepared with different post-treatment temperatures (700, 1500 and 3000 °C) were investigated supporting on them Au, Pd and bimetallic AuPd nanoparticles (NPs), for cinnamyl alcohol electrooxidation. The characterization of CNFs by BET, Raman and XPS showed an increase of the graphitisation degree increasing the post-treatment temperature, while the oxygen content decrease. After NPs deposition, HAADF-STEM/EDX showed smaller particles when the graphitization degree increased. The electrochemical study allowed to predict the reactivity and selectivity of the materials in cinnamyl alcohol electrooxidation. Remarkably, the initial investigation of the pristine CNFs showed that not only they can detect both cinnamyl alcohol and aldehyde, but form further products, whose distribution was different depending on the presence of O-functionalities. Precisely, huge number of O-functionalities increased the formation of the acid. It is thus possible to suppose that the metal phase cooperates with the carbon support in directing the reaction pathway: while Au favored the oxidation reaction of alcohol to aldehyde and to the acid, Pd notably favored the hydrogenation path to 3-Phenyl-1-propanol. However, the hydrogenation path ruled by Pd was blunted when it was supported on highly O-functionalized CNF, as well as the acid formation ruled by Au can be slowed by AuPd alloy, enhancing the selectivity to the aldehyde. Overall, the tuning of the O-functionalities of the carbon support coupled with the presence of specific metal species can rule the selectivity of the reaction.