Copper Nanoparticles Installed in Metal–Organic Framework Thin Films are Electrocatalytically Competent for CO₂ Reduction

Copper nanoparticles are embedded into a solvothermally grown thin film of a zirconium metal–organic framework (MOF), NU-1000, by installing single-site Cu­(II) into the NU-1000 thin film via solvothermal deposition in MOFs (SIM) followed by electrochemical reduction of Cu­(II) to metallic Cu. The obtained Cu nanoparticles are electrochemically addressable and exhibit promising electrocatalytic activity for CO₂ reduction in an aqueous electrolyte

Effect of Redox “Non-Innocent” Linker on the Catalytic Activity of Copper-Catecholate-Decorated Metal–Organic Frameworks

Two new UiO-68 type of Zr-MOFs featuring redox non-innocent catechol-based linkers of different redox activities have been synthesized through a de novo mixed-linker strategy. Metalation of the MOFs with Cu­(II) precursors triggers the reduction of Cu­(II) by the phenyl-catechol groups to Cu­(I) with the concomitant formation of semiquinone radicals as evidenced by EPR and XPS characterization. The MOF-supported catalysts are selective toward the allylic oxidation of cyclohexene and it is found that the presence of in situ-generated Cu­(I) species exhibits enhanced catalytic activity as compared to a similar MOF with Cu­(II) metalated naphthalenyl-dihydroxy groups. This work unveils the importance of metal–support redox interactions in the catalytic activity of MOF-supported catalysts which are not easily accessible in traditional metal oxide supports

Photoelectrochemical Proton-Coupled Electron Transfer of TiO₂ Thin Films on Silicon

TiO2 thin films are often used as protective layers on semiconductors for applications in photovoltaics, molecule–semiconductor hybrid photoelectrodes, and more. Experiments reported here show that TiO2 thin films on silicon are electrochemically and photoelectrochemically reduced in buffered acetonitrile at potentials relevant to photoelectrocatalysis of CO2 reduction, N2 reduction, and H2 evolution. On both n-type Si and irradiated p-type Si, TiO2 reduction is proton-coupled with a 1e–:1H+ stoichiometry, as demonstrated by the Nernstian dependence of the Ti4+/3+ E1/2 on the buffer pKa. Experiments were conducted with and without illumination, and a photovoltage of ∼0.6 V was observed across 20 orders of magnitude in proton activity. The 4 nm films are almost stoichiometrically reduced under mild conditions. The reduced films catalytically transfer protons and electrons to hydrogen atom acceptors, based on cyclic voltammogram, bulk electrolysis, and other mechanistic evidence. TiO2/Si thus has the potential to photoelectrochemically generate high-energy H atom carriers. Characterization of the TiO2 films after reduction reveals restructuring with the formation of islands, rendering TiO2 films as a potentially poor choice as protecting films or catalyst supports under reducing and protic conditions. Overall, this work demonstrates that atomic layer deposition TiO2 films on silicon photoelectrodes undergo both chemical and morphological changes upon application of potentials only modestly negative of RHE in these media. While the results should serve as a cautionary tale for researchers aiming to immobilize molecular monolayers on “protective” metal oxides, the robust proton-coupled electron transfer reactivity of the films introduces opportunities for the photoelectrochemical generation of reactive charge-carrying mediators

An Exceptionally Stable Metal–Organic Framework Supported Molybdenum(VI) Oxide Catalyst for Cyclohexene Epoxidation

Molybdenum­(VI) oxide was deposited on the Zr₆ node of the mesoporous metal–organic framework NU-1000 via condensed-phase deposition where the MOF is simply submerged in the precursor solution, a process named solvothermal deposition in MOFs (SIM). Exposure to oxygen leads to a monodisperse, porous heterogeneous catalyst, named <b>Mo-SIM</b>, and its structure on the node was elucidated both computationally and spectroscopically. The catalytic activity of <b>Mo-SIM</b> was tested for the epoxidation of cyclohexene. Near-quantitative yields of cyclohexene oxide and the ring-opened 1,2-cyclohexanediol were observed, indicating activity significantly higher than that of molybdenum­(VI) oxide powder and comparable to that of a zirconia-supported analogue (Mo-ZrO₂) prepared in a similar fashion. Despite the well-known leaching problem of supported molybdenum catalysts (i.e., loss of Mo species thus causes deactivation), <b>Mo-SIM</b> demonstrated no loss in the metal loading before and after catalysis, and no molybdenum was detected in the reaction mixture. In contrast, Mo-ZrO₂ led to significant leaching and close to 80 wt % loss of the active species. The stability of <b>Mo-SIM</b> was further confirmed computationally, with density functional theory calculations indicating that the dissociation of the molybdenum­(VI) species from the node of NU-1000 is endergonic, corroborating the experimental data for the <b>Mo-SIM</b> material

Effect of Redox “Non-Innocent” Linker on the Catalytic Activity of Copper-Catecholate-Decorated Metal–Organic Frameworks

Two new UiO-68 type of Zr-MOFs featuring redox non-innocent catechol-based linkers of different redox activities have been synthesized through a de novo mixed-linker strategy. Metalation of the MOFs with Cu­(II) precursors triggers the reduction of Cu­(II) by the phenyl-catechol groups to Cu­(I) with the concomitant formation of semiquinone radicals as evidenced by EPR and XPS characterization. The MOF-supported catalysts are selective toward the allylic oxidation of cyclohexene and it is found that the presence of in situ-generated Cu­(I) species exhibits enhanced catalytic activity as compared to a similar MOF with Cu­(II) metalated naphthalenyl-dihydroxy groups. This work unveils the importance of metal–support redox interactions in the catalytic activity of MOF-supported catalysts which are not easily accessible in traditional metal oxide supports

Room Temperature Synthesis of an 8‑Connected Zr-Based Metal–Organic Framework for Top-Down Nanoparticle Encapsulation

Room Temperature Synthesis of an 8‑Connected Zr-Based Metal–Organic Framework for Top-Down Nanoparticle Encapsulatio

Fine-Tuning the Activity of Metal–Organic Framework-Supported Cobalt Catalysts for the Oxidative Dehydrogenation of Propane

Few-atom cobalt-oxide clusters, when dispersed on a Zr-based metal–organic framework (MOF) NU-1000, have been shown to be active for the oxidative dehydrogenation (ODH) of propane at low temperatures (<230 °C), affording a selective and stable propene production catalyst. In our current work, a series of promoter ions with varying Lewis acidity, including Ni­(II), Zn­(II), Al­(III), Ti­(IV) and Mo­(VI), are anchored as metal-oxide,hydroxide clusters to NU-1000 followed by Co­(II) ion deposition, yielding a series of NU-1000-supported bimetallic-oxo,hydroxo,aqua clusters. Using difference envelope density (DED) analyses, the spatial locations of the promoter ions and catalytic cobalt ions are determined. For all samples, the promoter ions are sited between pairs of Zr₆ nodes along the MOF <i>c</i>-axis, whereas the location of the cobalt ions varies with the promoter ions. These NU-1000-supported bimetallic-oxide clusters are active for propane ODH after thermal activation under O₂ to open a cobalt coordination site and to oxidize Co­(II) to Co­(III), as evidenced by operando X-ray absorption spectroscopy at the Co K-edge. In accord with the decreasing Lewis acidity of the promoter ion, catalytic activity increases in the following order: Mo­(VI) < Ti­(IV) < Al­(III) < Zn­(II) < Ni­(II). The finding is attributed to increasing ease of formation of Co­(III)–O^• species and stabilization of a cobalt­(III)-oxyl/propane transition state as the Lewis acidity of the promoter ions decreases. The results point to an increasing ability to fine-tune the structure-dependent activity of MOF-supported heterogeneous catalysts. Coupled with mechanistic studiescomputational or experimentalthis ability may translate into informed prediction of improved catalysts for propane ODH and other chemical reactions