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Electrochemical CO2reduction to produce value-added chemicals and fuels is one of the research hotspots in the field of energy conversion. The development of efficient catalysts with high conductivity and readily accessible active sites for CO2electroreduction remains challenging yet indispensable. In this work, a reliable poly(ethyleneimine) (PEI)-assisted strategy is developed to prepare a hollow carbon nanocomposite comprising a single-site Ni-modified carbon shell and confined Ni nanoparticles (NPs) (denoted as Ni@NHCS), where PEI not only functions as a mediator to induce the highly dispersed growth of Ni NPs within hollow carbon spheres, but also as a nitrogen precursor to construct highly active atomically-dispersed Ni-Nxsites. Benefiting from the unique structural properties of Ni@NHCS, the aggregation and exposure of Ni NPs can be effectively prevented, while the accessibility of abundant catalytically active Ni-Nxsites can be ensured. As a result, Ni@NHCS exhibits a high CO partial current density of 26.9 mA cm–2and a Faradaic efficiency of 93.0% at −1.0 V vs.RHE, outperforming those of its PEI-free analog. Apart from the excellent activity and selectivity, the shell confinement effect of the hollow carbon sphere endows this catalyst with long-term stability. The findings here are anticipated to help understand the structure-activity relationship in Ni-based carbon catalyst systems for electrocatalytic CO2reduction. Furthermore, the PEI-assisted synthetic concept is potentially applicable to the preparation of high-performance metal-based nanoconfined materials tailored for diverse energy conversion applications and beyond.

Among insulating transition-metal oxides, ZrO2has the potential to be a photocatalyst; however, its wide bandgap (∼5 eV) interferes with its ability to absorb sufficient light. Defect engineering—that is, the creation of oxygen vacancies (VOs) and/or Zr3+ions, is a promising strategy for adjusting the bandgap of ZrO2for photocatalytic applications. This study demonstrates that the included VOs play an important role in boosting photocatalytic activity. In addition, the crystal phase of ZrO2(e.g., monoclinic or tetragonal) strongly influences both VOformation and photocatalytic activity. After a Pt cocatalyst was deposited onto ZrO2with a single crystal phase (i.e., monoclinic (m-) or tetragonal (t-)), the specimens were reduced at 200, 400, and 600 °C to obtain photocatalysts with different amounts of VOs. As-synthesized m-ZrO2possessed Zr3+ions in the absence of VOs, and the reduction treatment promoted VOformation accompanied by the disappearance of Zr3+. The as-synthesized t-ZrO2originally included VOs, and their amount in Pt/t-ZrO2−xincreased with increasing reduction temperature. Pt/t-ZrO2−xpossessed a greater amount of VOs than Pt/m-ZrO2−xreduced at the same temperature, suggesting that VOformation occurs more easily in t-ZrO2than in m-ZrO2. Photocatalytic activity increased with increasing reduction temperature, which is the same tendency observed for the VOamount. In a comparison of the crystal phases, Pt/m-ZrO2−xexhibited relatively higher activity than Pt/t-ZrO2−xdespite its fewer VOs, suggesting that the VOformed in the monoclinic ZrO2contributes to the enhancement of the photocatalytic activity more efficiently than in the tetragonal one.

Palladium nanoparticles (Pd NPs) were synthesized by the reduction-by-solvent method using polyvinylpirrolidone (PVP) as capping agent. The nonstatic interaction between PVP and the metallic surface may change the properties of the NPs due to the different possible interactions, through either the O or N atoms of the PVP. In order to analyze these effects and their repercussions in their catalytic performance, Pd NPs with various PVP/Pd molar ratios (1, 10, and 20) were prepared, deposited on silica, and tested in the formic acid decomposition reaction. The catalytic tests were conducted using catalysts prepared by loading NPs with three different time lapses between their purification and their deposition on the silica support (1 day, 1 month, and 6 months). CO adsorption, FTIR spectroscopy, XPS, and TEM characterization were used to determine the accessibility of the Pd NPs surface sites, the electronic state of Pd, and the average NPs size, respectively. The H2production from the formic acid decomposition reaction has a strong dependence on the Pd surface features, which in turn are related to the NPs aging time due to the progressive removal of the PVP.

Au–CeO2catalysts are prepared from amorphous Ce–Al alloys by the dealloying method, and their catalytic performances are examined in the oxidation of benzyl alcohol to benzaldehyde using O2as an oxidant. The catalytic activity of the Au–CeO2catalysts is improved using an amorphous alloy as a precursor, and further improved activity is attained by directly alloying Au with Ce–Al rather than by depositing Au on preformed CeO2. Consequently, Au–CeO2prepared directly from an amorphous Au–Ce–Al alloy exhibits the highest activity among the prepared samples, despite its low surface area. Detailed characterization by means of XPS analysis indicates that most of the Au species in Au–CeO2prepared from the amorphous Au–Ce–Al alloy exist in a +1 oxidation state (Au+). The presence of Au+facilitates the migration of oxygen atoms through the lattice, which likely contributes to the high catalytic activity of the Au–CeO2catalyst.