Cu oxide deposited on shape-controlled ceria nanocrystals for CO oxidation: influence of interface-driven oxidation states on catalytic activity

The design of a catalyst with a highly active and stable oxidation state is of great interest in heterogeneous catalysis. Herein, the relationship between catalytic activity and oxidation state on Cu deposited on CeO2 nanocrystals has been elucidated by varying the shape of the ceria (CeO2) support. Three types of CeO2 nanocrystals were prepared for supporting Cu oxide (CuOx): CeO2 nanocubes (NCs), nanorods (NRs) and nanospheres (NSs). The Cu oxide deposited on CeO2NC has shown higher CO oxidation activity at a lower temperature than that over the NR and NS surfaces. Furthermore, characterization of structure and oxidation states revealed that the stable Cu1+ oxidation state on the surface of CuOx/CeO2NC formed at a low loading of copper (similar to 1.5 wt%), which acts as an active site for the CO oxidation. In contrast to the high surface area and redox properties, a systematic catalytic activity trend was observed among the catalysts with the extent of the Cu1+ oxidation state. We demonstrate that the polar (100) surface facets of NCs contribute significantly to the formation of surface hydroxyl groups, which are required for the selective and stable Cu1+ state at a low loading.11Nsciescopu

The facet effect of ceria nanoparticles on platinum dispersion and catalytic activity of methanol partial oxidation

The effect of platinum-supported nano-shaped ceria catalysts on methanol partial oxidation and methyl formate product selectivity has been investigated. A Pt-supported CeO2 nanocube catalyst had a higher turnover frequency than nanosphere catalysts; however, nanosphere catalysts showed higher selectivity towards methyl formate. The observed ceria shape effect in catalysis was associated with the shape-dependent Pt dispersion and its oxidation states. Furthermore, in situ studies revealed that the reduced platinum and mono-dentate methoxy group were responsible for the higher turnover frequency.11Nsciescopu

Tuning CO2 Hydrogenation Selectivity through Reaction-Driven Restructuring on Cu-Ni Bimetal Catalysts

Tuning the selectivity of CO2 hydrogenation is of significant scientific interest, especially using nickel-based catalysts. Fundamental insights into CO2 hydrogenation on Ni-based catalysts demonstrate that CO is a primary intermediate, and product selectivity is strongly dependent on the oxidation state of Ni. Therefore, modifying the electronic structure of the nickel surface is a compelling strategy for tuning product selectivity. Herein, we synthesized well dispersed Cu-Ni bimetallic nanoparticles (NPs) using a simple hydrothermal method for CO selective CO2 hydrogenation. A detailed study on the monometallic (Ni and Cu) and bimetallic (CuxNi1-x) catalysts supported on gamma-Al2O3 was performed to increase CO selectivity while maintaining the high reaction rate. The Cu0.5Ni0.5/gamma-Al2O3 catalyst shows a high CO2 conversion and more CO product selectivity than its monometallic counterparts. The surface electronic and geometric structure of Cu0.5Ni0.5 bimetallic NPs was studied using ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and in situ diffuse reflectance infrared Fourier-transform spectroscopy under reaction conditions. The Cu core atoms migrate toward the surface, resulting in the restructuring of the Cu@Ni core-shell structure to a Cu-Ni alloy during the reaction and functioning as the active site by enhancing CO desorption. A systematic correlation is obtained between catalytic activity from a continuous fixed-bed flow reactor and the surface electronic structural details derived from AP-XPS results, establishing the structure-activity relationship. This investigation contributes to providing a strategy for controlling CO2 hydrogenation selectivity by modifying the surface structure of bimetallic NP catalysts.11Nsciescopu

Synergistic interactions between water and the metal/oxide interface in CO oxidation on Pt/CeO2 model catalysts

The significance of catalytic active interfacial sites in metal supported on metal oxide systems has been well established in prototypical CO oxidation reactions. Here, we investigate the role of water molecules in the CO oxidation reaction using Pt nanoparticles (Pt NPs) supported by a two-dimensional (2D) cerium oxide thin-film. The 2D model CeO2 thin films were fabricated using the e-beam evaporation technique and a monolayer of Pt nanoparticles deposited on CeO2 thin films by the Langmuir-Blodgett (LB) method. The well-defined reaction conditions, such as humid and dry conditions, were achieved using an ultra-high vacuum (UHV) batch chamber. Interestingly, we found a strong metal-support interaction (SMSI) in the Pt/CeO2 catalyst under dry reaction conditions, which was increased further by the presence of water molecules. However, water had a detrimental effect on a Pt thin-film model catalyst under humid reaction conditions. By altering the partial pressures of water molecules and CO, the reaction mechanism was established under humid conditions. The current results demonstrate that hydroxyl intermediates from water molecules provide an alternative pathway to the Pt/CeO2 catalyst, resulting in increased overall turnover rates. These findings shed light on the synergistic interplay of water and metal-oxide interfaces. © 2022 Elsevier B.V.11Nsciescopu

Copper Cobalt Sulfide Nanosheets Realizing a Promising Electrocatalytic Oxygen Evolution Reaction

Nanostructured CuCo₂S₄, a mixed metal thiospinel, is found to be a benchmark electrocatalyst for oxygen evolution reaction (OER) in this study with a low overpotential, a low Tafel slope, a high durability, and a high turnover frequency (TOF) at lower mass loadings. Nanosheets of CuCo₂S₄ are realized from a hydrothermal synthesis method in which the average thickness of the sheets is found to be in the range of 8–15 nm. Aggregated nanosheets form a highly open hierarchical structure. When used as an electrocatalyst, CuCo₂S₄ nanosheets offer an overpotential value of 310 mV at a 10 mA cm^–2 current density, which remains consistent for 10000 measured cycles in a 1 M KOH electrolyte. A chronoamperometric study reveals constant oxygen evolution for 12 h at a 10 mV s^–1 scan rate without any degradation of the activity. Furthermore, the calculated mass activity of the CuCo₂S₄ electrocatalyst is found to be 14.29 A/g and to afford a TOF value of 0.1431 s^–1 at 310 mV at a mass loading of 0.7 mg cm^–2. For comparison, nanostructures of Co₃S₄ and Cu_0.5Co_2.5S₄ have been synthesized using a similar method followed for CuCo₂S₄. When compared to the OER activities among these three thiospinels and standard IrO₂, CuCo₂S₄ nanosheets offered the highest OER activities at the same mass loading (0.7 mg/cm²). Extensive X-ray photoelectron spectroscopy and electron paramagnetic resonance analyses for a mechanistic study reveal that introduction of Cu into the Co₃S₄ lattice enhances the oxygen evolution and kinetics by offering Cu²⁺ sites for utilitarian adsorption of OH, O, and OOH reactive species and also by offering a highly active high-spin state of octahedral Co³⁺ for OER catalysis

Metallic Cobalt to Spinel Co₃O₄Electronic Structure Evolution by Near-Ambient Pressure Photoelectron Spectroscopy

In the present study, valence band (VB) and core level photoelectron spectroscopy was carried out to investigate the electronic structural changes from polycrystalline Co to spinel Co₃O₄, via CoO at near ambient pressures (NAP; ∼0.1 mbar). O₂–Co and H₂–CoO_<i>x</i> gas–solid oxidative and reductive interactions, respectively, have been explored with UV photons (He–I) or low kinetic energy electrons (≤16 eV) under NAP conditions. Typical VB features of Co metal, CoO, Co₃O₄, and a mixed phase between any two adjacent features were observed and well corroborated with core level changes. Very significant and characteristic changes were observed with Co 3d features in the VB for each stage from Co oxidation to Co₃O₄ as well as Co₃O₄ reduction to CoO. Co₃O₄ and CoO can be reversibly obtained by alternating the conditions between 0.1 mbar of H₂ at 650 K and 0.1 mbar of O₂ at 400 K, respectively. A meaningful correlation is observed between the changes in work function with cation oxidation state; small changes in the stoichiometry can strongly influence the shift in Fermi level and changes in work function under NAP conditions. Reversible work function changes are observed between oxidation and reduction conditions. While the O 2p derived feature for CoO_<i>x</i> was observed at a constant BE (∼5 eV) throughout the redox conditions, the Co 3d band and molecular oxygen or hydrogen vibration feature shifts significantly underscoring the physicochemical changes, such as charge transfer energy and hence changes in satellite intensity. The peak close to <i>E</i>_F originated from the 3d⁶<u>L</u> final state of the octahedral Co³⁺ 3d band of Co₃O₄

Efficient Organic Photovoltaics with Improved Charge Extraction and High Short-Circuit Current

Exciton generation, dissociation, free carrier transport, and charge extraction play an important role in the short-circuit current (<i>J</i>_sc) and power conversion efficiency of an organic bulk heterojunction (BHJ) solar cell (SC). Here we study the impact of band offset at the interfacial layer and the morphology of active layer on the extraction of free carriers. The effects are evaluated on an inverted BHJ SC using zinc oxide (ZnO) as a buffer layer, prepared via two different methods: ZnO nanoparticle dispersed in mixed solvents (ZnO A) and sol–gel method (ZnO B). The device with ZnO A buffer layer improves the charge extraction and <i>J</i>_sc. The improvement is due to the better band offset and morphology of the blend near the ZnO A/active layer interface. Further, the numerical analysis of current–voltage characteristics illustrates that the morphology at the ZnO A/active layer interface has a more dominant role in improving the performance of the organic photovoltaic than the band offset