Replacing PVP by macrocycle cucurbit[6]uril to cap sub-5 nm Pd nanocubes as highly active and durable catalyst for ethanol electrooxidation

Pd nanocubes (NCs) enclosed by six {100} facets are fascinating model materials for both fundamental studies and practical applications. However, the only available method to prepare well-defined sub-10 nm Pd NCs was developed by Xia et al. more than 10 years ago, unavoidably using polyvinylpyrrolidone (PVP) polymer to prevent particle aggregation. The strongly adsorbed PVP extremely deteriorates the catalysts' efficiency because of the high coverage of accessible surface-active sites. Numerous efforts have been devoted to replacing PVP with weaker capping agents but with limited progress predominately due to the difficulties in tuning the growth kinetics of Pd NCs. For the first time, we report that macrocycle cucurbit[6]uril (CB[6]) can replace PVP in the synthesis of Pd NCs by dedicatedly controlling the growth parameters. CB[6] capped Pd NCs showed 1.1-1.5 times increased specific surface area compared to the surfactant-free commercial Pd catalysts. Moreover, X-ray photoelectron spectroscopy demonstrated the modified electronic structure of Pd NCs through the carbonyl group of CB[6]. Consequently, compared to the commercial catalysts, the obtained Pd NCs exhibited 7 times higher current density towards ethanol oxidation reaction. Remarkably, after 17 h of continuous work, it reduced deactivation by up to 1-4 orders of magnitude.