Electrocatalytic methane oxidation to formate on magnesium based metal-organic frameworks.

The electrochemical methane oxidation reaction is a potential approach for upgrading the nature-abundant methane (CH4) into value-added chemicals, while the activity and selectivity have remained substantially low due to the extremely inert chemical property of CH4. Inspired by the methane mono-oxygenase in nature, we demonstrated Mg-substituted metal-organic frameworks (Mg-MOF-74) containing a uniform distribution of Mg-oxo-Mg nodes as efficient catalytic sites. Compared to MgNi-MOF-74 and Mg(OH)2 without the Mg-oxo-Mg nodes, the Mg-MOF-74 presented a much enhanced CH4 electrooxidation performance, with a unique selectivity of producing formate. The maximum Faradaic efficiency of all liquid products reached 10.9% at 1.60 V versus reversible hydrogen electrode (RHE), corresponding to the peak production rate of 126.6 µmol·h-1·g-1.

Lithium Vacancy-Tuned [CuO4 ] Sites for Selective CO2 Electroreduction to C2+ Products.

Electrochemical CO2 reduction to valuable multi-carbon (C2+ ) products is attractive but with poor selectivity and activity due to the low-efficient CC coupling. Herein, a lithium vacancy-tuned Li2 CuO2 with square-planar [CuO4 ] layers is developed via an electrochemical delithiation strategy. Density functional theory calculations reveal that the lithium vacancies (VLi ) lead to a shorter distance between adjacent [CuO4 ] layers and reduce the coordination number of Li+ around each Cu, featuring with a lower energy barrier for COCO coupling than pristine Li2 CuO2 without VLi . With the VLi percentage of ≈1.6%, the Li2- x CuO2 catalyst exhibits a high Faradaic efficiency of 90.6 ± 7.6% for C2+ at -0.85 V versus reversible hydrogen electrode without iR correction, and an outstanding partial current density of -706 ± 32 mA cm-2 . This work suggests an attractive approach to create controllable alkali metal vacancy-tuned Cu catalytic sites toward C2+ products in electrochemical CO2 reduction.

Lithiation-Enabled High-Density Nitrogen Vacancies Electrocatalyze CO2 to C2 Products.

Electrochemical CO2 reduction to produce valuable C2 products is attractive but still suffers with relatively poor selectivity and stability at high current densities, mainly due to the low efficiency in the coupling of two *CO intermediates. Herein, it is demonstrated that high-density nitrogen vacancies formed on cubic copper nitrite (Cu3 Nx ) feature as efficient electrocatalytic centers for CO-CO coupling to form the key OCCO* intermediate toward C2 products. Cu3 Nx with different nitrogen densities are fabricated by an electrochemical lithium tuning strategy, and density functional theory calculations indicate that the adsorption energies of CO* and the energy barriers of forming key C2 intermediates are strongly correlated with nitrogen vacancy density. The Cu3 Nx catalyst with abundant nitrogen vacancies presents one of the highest Faradaic efficiencies toward C2 products of 81.7 ± 2.3% at -1.15 V versus reversible hydrogen electrode (without ohmic correction), corresponding to the partial current density for C2 production as -307 ± 9 mA cm-2 . An outstanding electrochemical stability is also demonstrated at high current densities, substantially exceeding CuOx catalysts with oxygen vacancies. The work suggests an attractive approach to create stable anion vacancies as catalytic centers toward multicarbon products in electrochemical CO2 reduction.

Electrocatalytic Methane Oxidation Greatly Promoted by Chlorine Intermediates.

Renewable energy-powered methane (CH4 ) conversion at ambient conditions is an attractive but highly challenging field. Due to the highly inert character of CH4 , the selective cleavage of its first C-H bond without over-oxidation is essential for transforming CH4 into value-added products. In this work, we developed an efficient and selective CH4 conversion approach at room temperature using intermediate chlorine species (*Cl), which were electrochemically generated and stabilized on mixed cobalt-nickel spinels with different Co/Ni ratios. The lower overpotentials for *Cl formation enabled an effective activation and conversion of CH4 to CH3 Cl without over-oxidation to CO2 , and Ni3+ at the octahedral sites in the mixed cobalt-nickel spinels allowed to stabilize surface-bound *Cl species. The CoNi2 Ox electrocatalyst exhibited an outstanding yield of CH3 Cl (364 mmol g-1 h-1 ) and a high CH3 Cl/CO2 selectivity of over 400 at room temperature, with demonstrated capability of direct CH4 conversion under seawater working conditions.

Double sulfur vacancies by lithium tuning enhance CO2 electroreduction to n-propanol.

Electrochemical CO2 reduction can produce valuable products with high energy densities but the process is plagued by poor selectivities and low yields. Propanol represents a challenging product to obtain due to the complicated C3 forming mechanism that requires both stabilization of *C2 intermediates and subsequent C1-C2 coupling. Herein, density function theory calculations revealed that double sulfur vacancies formed on hexagonal copper sulfide can feature as efficient electrocatalytic centers for stabilizing both CO* and OCCO* dimer, and further CO-OCCO coupling to form C3 species, which cannot be realized on CuS with single or no sulfur vacancies. The double sulfur vacancies were then experimentally synthesized by an electrochemical lithium tuning strategy, during which the density of sulfur vacancies was well-tuned by the charge/discharge cycle number. The double sulfur vacancy-rich CuS catalyst exhibited a Faradaic efficiency toward n-propanol of 15.4 ± 1% at -1.05 V versus reversible hydrogen electrode in H-cells, and a high partial current density of 9.9 mA cm-2 at -0.85 V in flow-cells, comparable to the best reported electrochemical CO2 reduction toward n-propanol. Our work suggests an attractive approach to create anion vacancy pairs as catalytic centers for multi-carbon-products.

Amine-functionalized MIL-53(Al) with embedded ruthenium nanoparticles as a highly efficient catalyst for the hydrolytic dehydrogenation of ammonia borane.

Well-dispersed ruthenium nanoparticles (Ru NPs) are immobilized within the pores of amine-functionalized MIL-53 via an in situ impregnation-reduction method. The resulting Ru/MIL-53(Al)-NH2 catalyst exhibits superior catalytic performance for the dehydrogenation of ammonia borane (AB) at ambient temperature relative to the Ru/MIL-53(Al) catalyst; it has a turnover frequency (TOF) of 287 mol H2 min-1 (mol Ru)-1 and an activation energy (E a) of 30.5 kJ mol-1. The amine groups present in the MIL-53(Al)-NH2 framework facilitate the formation and stabilization of ultra-small Ru NPs by preventing their aggregation. Additionally, the Ru/MIL-53(Al)-NH2 catalyst exhibits satisfactory durability and reusability: 72.4% and 86.3% of the initial catalytic activity was maintained after the fifth successive cycle of the hydrolytic dehydrogenation of AB in the two respective tests.