Oxygen vacancy rich Cu2O based composite material with nitrogen doped carbon as matrix for photocatalytic H2 production and organic pollutant removal

One stone, two birds: A Cu-S cluster as a laccase-mimicking nanozyme and sulfite activator for phenol remediation in marine environments

Phenols are infamous pollutants in marine environments and present a grave danger to human health, which makes their efficient detection and removal serious issues. Colorimetry is a simple method for detecting phenols in water because phenols can be oxidized by natural laccase and generate a brown product. However, high cost and poor stability impede the wide-spread implementation of natural laccase in phenol detection. To reverse this adverse situation, a nanoscale Cu-S cluster, Cu₄(MPPM)₄ (Cu₄S₄, MPPM = 2-mercapto-5-n-propylpyrimidine), is synthesized. As a stable and inexpensive nanozyme, Cu₄S₄ shows excellent laccase-mimicking activity and prompts the oxidation of phenols. This characteristic makes Cu₄S₄ a perfect option for phenol detection with colorimetry. In addition, Cu₄S₄ also exhibits sulfite activation properties. It can degrade phenols and other pollutants with advanced oxidation processes (AOPs). Theoretical calculations show good laccase-mimicking and sulfite activation properties originating from appropriate interactions between Cu₄S₄ and substrates. We anticipate that the phenol detection and degradation characteristics of Cu₄S₄ make it a promising material to be used for practical phenol remediation in water environments.

Role of Efficient Charge Transfer at the Interface between Mixed-Phase Copper-Cuprous Oxide and Conducting Polymer Nanostructures for Photocatalytic Water Splitting

Photocatalytic hydrogen generation from water splitting is regarded as a sustainable technology capable of producing green solar fuels. However, the low charge separation efficiencies and the requirement of lowering redox potentials are unresolved challenges. Herein, a multiphase copper-cuprous oxide/polypyrrole (PPy) heterostructure has been designed to identify the role of multiple oxidation states of metal oxides in water reduction and oxidation. The presence of a mixed phase in PPy heterostructures enabled an exceptionally high photocatalytic H₂ generation rate of 41 mmol h^-1 with an apparent quantum efficiency of 7.2% under visible light irradiation, which is a 7-fold augmentation in contrast to the pure polymer. Interestingly, the copper-cuprous oxide/PPy heterostructures exhibited higher charge carrier density, low resistivity, and 6 times higher photocurrent density compared to Cu₂O/PPy. Formation of a p-p-n junction between polymer and mixed-phase metal oxide interfaces induce a built-in electric field which influences directional charge transfer that improves the catalytic activity. Notably, photoexcited charge separation and transfer have been significantly improved between copper-cuprous oxide nanocubes and PPy nanofibers, as revealed by femtosecond transient absorption spectroscopy. Additionally, the photocatalyst demonstrates excellent stability without loss of catalytic activity during cycling tests. The present study highlights a superior strategy to boost photocatalytic redox reactions using a mixed-phase metal oxide in the heterostructure to achieve enhanced light absorption, longer charge carrier lifetimes, and highly efficient photocatalytic H₂ and O₂ generation.

Designing Highly Efficient Cu2O-CuO Heterojunction CO Oxidation Catalysts: The Roles of the Support Type and Cu2O-CuO Interface Effect

In this work, a series of Cu2O/S (S = α-MnO2, CeO2, ZSM-5, and Fe2O3) supported catalysts with a Cu2O loading amount of 15% were prepared by the facile liquid-phase reduction deposition-precipitation strategy and investigated as CO oxidation catalysts. It was found that the Cu2O/α-MnO2 catalyst exhibits the best catalytic activity for CO oxidation. Additionally, a series of Cu2O-CuO/α-MnO2 heterojunctions with varied proportion of Cu+/Cu2+ were synthesized by further calcining the pristine Cu2O/α-MnO2 catalyst. The ratio of the Cu+/Cu2+ could be facilely regulated by controlling the calcination temperature. It is worth noting that the Cu2O-CuO/α-MnO2-260 catalyst displays the best catalytic performance. Moreover, the kinetic studies manifest that the apparent activation energy could be greatly reduced owing to the excellent redox property and the Cu2O-CuO interface effect. Therefore, the Cu2O-CuO heterojunction catalysts supported on α-MnO2 nanotubes are believed to be the potential catalyst candidates for CO oxidation with advanced performance.

Reheat treatment under vacuum induces pre-calcined α-MnO2 with oxygen vacancy as efficient catalysts for toluene oxidation

Engineering α-MnO₂ with abundant oxygen vacancies is efficient to enhance its catalytic activity towards toluene oxidation. A simple and facile method was introduced to fabricate oxygen vacancies on α-MnO₂ surface by reheating the pre-calcined samples under vacuum condition. The reheat treatment especially at 180 °C is beneficial for the formation of oxygen vacancies on α-MnO₂ surface, enhancing the oxidation of toluene. The toluene conversion is up to 100% at 270 °C, which is 30 °C lower than that of α-MnO₂ without reheat treatment. The apparent activation energy (16.8 kJ mol^-1) of MnO₂-180 catalyst is lowest among these catalysts, which is essential for accelerating the oxidation of toluene. In-situ DRIFTS results indicate that the MnO₂-180 sample promotes the formation of benzaldehyde and the occurrence of ring-opening reaction, thus effectively improving the catalytic performance for toluene oxidation. A possible catalytic oxidation mechanism of toluene over α-MnO₂ catalysts after reheat treatment was proposed.

Boosting electrochemical nitrite-ammonia conversion properties by a Cu foam@Cu2O catalyst

Electrocatalytic reduction of nitrite (NO₂^-) to ammonia (NH₃) can simultaneously achieve wastewater treatment and ammonia production, but it needs efficient catalysts. Herein, Cu₂O particles self-supported on Cu foam with enriched oxygen vacancies are developed to enable selective NO₂^- reduction to NH₃, exhibiting a maximum NH₃ yield rate of 7510.73 μg h^-1 cm^-2 and high faradaic efficiency of 94.21% at -0.6 V in 0.1 M PBS containing 0.1 M NaNO₂. Density functional theory calculations reveal the vital role of oxygen vacancies during the nitrite reduction process, as well as the reaction mechanisms and the potential limiting step involved. This work provides a new avenue to the rational design of Cu-based catalysts for NH₃ electrosynthesis.

Photoenhanced Water Electrolysis in Separate O2 and H2 Cells Using Pseudocapacitive Electrodes

Water electrolysis has received much attention in recent years as a means of sustainable H₂ production. However, many challenges remain in obtaining high-purity H₂ and making large-scale production cost-effective. This study provides a strategy for integrating a two-cell water electrolysis system with solar energy storage. In our proposed system, CuO-Cu(OH)₂/Cu₂O was used as a redox mediator between oxygen and hydrogen evolution components. The system not only overcame the gas-mixing issue but also showed high gas generation performance. The redox reaction (charge/discharge) of CuO-Cu(OH)₂/Cu₂O led to a significant increase (51%) in the initial rate of H₂ production from 111.7 μmol h^-1 cm^-2 in the dark to 168.9 μmol h^-1 cm^-2 under solar irradiation. The effects of light on the redox reaction of CuO-Cu(OH)₂/Cu₂O during water electrolysis were investigated by in situ X-ray absorption and photoemission spectroscopy. These results suggest that surface oxygen vacancies are created under irradiation and play an important role in increased capacitance and gas generation. These findings provide a new path to direct storage of abundant solar energy and low-cost sustainable hydrogen production.

Magnetic Fe3O4-encapsulated VAN@MIL-101(Fe) with mixed-valence sites and mesoporous structures as efficient bifunctional water splitting photocatalysts

Fe3O4/VAN@MIL-101(Fe) with both mesoporous and mixed-valence Fe3+/Fe2+ structures was controllably synthesized in the synthesis of MIL-101(Fe), and it was used as a bifunctional photocatalyst in both oxygen evolution reactions (OERs) and hydrogen evolution reactions (HERs) of photocatalytic water splitting. By the reduction of auxiliary ligand vanillin (VAN) and the introduction of Fe3O4, the mixed-valence Fe3+/Fe2+ structure in Fe3O4/VAN@MIL-101(Fe) was obtained, which improves the band gap of the Fe3+ reactive active center and increases the separation efficiency of photogenerated carriers. Owing to the partial difference in the structure between VAN and ligand terephthalic acid (H2BDC), hierarchical porous and vacant structures were effectively improved in Fe3O4/VAN@MIL-101(Fe), which can induce more active sites to adsorb more water molecules and shorten the electron-hole migration distance to improve the transfer efficiency of photogenerated carriers. Therefore, Fe3O4/VAN@MIL-101(Fe) presents excellent photocatalytic activities for improving the O2 and H2 production rate up to 360 000 μmol g-1 h-1 and 584 μmol g-1 h-1, respectively. Meanwhile, Fe3O4/VAN@MIL-101(Fe) maintains the excellent catalytic activity in OERs and HERs after recycling for 5 times. Moreover, the introduction of magnetic Fe3O4 nanoplates into Fe3O4/VAN@MIL-101(Fe) can make it easily recyclable by magnetic separation, which can maximize its performance.

Solar-Driven Water-Gas Shift Reaction over CuOx /Al2 O3 with 1.1 % of Light-to-Energy Storage

Hydrogen production from coal gasification provides a cleaning approach to convert coal resource into chemical energy, but the key procedures of coal gasification and thermal catalytic water-gas shift (WGS) reaction in this energy technology still suffer from high energy cost. We herein propose adopting a solar-driven WGS process instead of traditional thermal catalysis, with the aim of greatly decreasing the energy consumption. Under light irradiation, the CuO_x /Al₂ O₃ delivers excellent catalytic activity (122 μmol g_cat ^-1  s^-1 of H₂ evolution and >95 % of CO conversion) which is even more efficient than noble-metal-based catalysts (Au/Al₂ O₃ and Pt/Al₂ O₃ ). Importantly, this solar-driven WGS process costs no electric/thermal power but attains 1.1 % of light-to-energy storage. The attractive performance of the solar-driven WGS reaction over CuO_x /Al₂ O₃ can be attributed to the combined photothermocatalysis and photocatalysis.