Sn(101) Derived from Metal-Organic Frameworks for Efficient Electrocatalytic Reduction of CO2.

The synthesis of a specific Sn plane as an efficient electrocatalyst for CO2 electrochemical reduction to generate fuels and chemicals is still a huge challenge. Density functional theory (DFT) calculations first reveal that the Sn(101) crystal plane is more advantageous for CO2 electroreduction. A metal-organic framework (MOF) precursor Sn-MOF has been carbonized and then etched to successfully fabricate Sn(101)/SnO2/C composites with good control of the carbonization time and the concentration of hydrochloric acid. The Sn(101) crystal plane of the catalyst could enhance the faradaic efficiency of formate to as high as 93.3% and catalytic stability up to 20 h. The promotion of the selectivity and activity by Sn(101) advances new possibilities for the rational design of high-activity Sn catalysts derived from MOFs.

Two-Dimensional Metal-Organic Framework Nanosheets with Cobalt-Porphyrins for High-Performance CO2 Electroreduction.

The electrochemical reduction of CO2 presents a promising strategy to mitigate the greenhouse effect and reduce excess carbon dioxide emission to realize a carbon-neutral energy cycle, but it suffers from the lack of high-performance electrocatalysts. In this work, catalytic active cobalt porphyrin [TCPP(Co)=(5,10,15,20)-tetrakis(4-carboxyphenyl)porphyrin-CoII ] was precisely anchored onto water-stable 2D metal-organic framework (MOF) nanosheets (Zr-BTB) to obtain ultrathin 2D MOF nanosheets [TCPP(Co)/Zr-BTB] with accessible catalytic sites for the CO2 reduction reaction. Compared with molecular cobalt porphyrin, the TCPP(Co)/Zr-BTB exhibits an ultrahigh turnover frequency (TOF=4768 h-1 at -0.919 V vs. reversible hydrogen electrode, RHE) owing to high active-site utilization. In addition, three post-modified 2D MOF nanosheets [TCPP(Co)/Zr-BTB-PABA, TCPP(Co)/Zr-BTB-PSBA, TCPP(Co)/Zr-BTB-PSABA] were obtained, with the modifiers of p-(aminomethyl)benzoic acid (PABA), p-sulfobenzoic acid potassium (PSBA), and p-sulfamidobenzoic acid (PSABA), to change the micro-environments around TCPP(Co) through the tuning of steric effects. Among them, the TCPP(Co)/Zr-BTB-PSABA exhibited the best performance with a faradaic efficiency (FECO ) of 85.1 %, TOF of 5315 h-1 , and jtotal of 6 mA cm-2 at -0.769 V (vs. RHE). In addition, the long-term durability of the electrocatalysts is evaluated and the role of pH buffer is revealed.

Indium-Based Metal-Organic Framework for High-Performance Electroreduction of CO2 to Formate.

It is urgent to find a catalyst with high selectivity and efficiency for the reduction of CO2 by renewable electric energy, which is the important means to reduce the greenhouse effect. In this work, we report that the metal-organic framework (MOF) indium-based 1,4-benzenedicarboxylate (In-BDC) catalyzes CO2 to formate with a Faradaic efficiency (FEHCOO-) of more than 80% in a wide voltage range between -0.419 and -0.769 V (vs. reversible hydrogen electrode, RHE). In-BDC performs at a maximum FEHCOO- of 88% at -0.669 V (vs. RHE) and a turnover frequency of up to 4798 h-1 at -1.069 V (vs. RHE). The long-term durability of 21 h and reusability of the electrocatalyst are clearly demonstrated. It opens up a new opportunity to utilize MOF with novel metal motifs for the efficient electroreduction of CO2.

Bioavailability and Speciation of Heavy Metals in Polluted Soil as Alleviated by Different Types of Biochars.

Biochar is an important material for remediation of contaminated soils, however, different biochars have variable effects on bioavailability of heavy metals. This experiment revealed that peanut shell biochar (PSB) has highest reduction of 78% concentration of Pb in plant roots. The maize straw biochar (MSB) has significantly decreased Zn and Cd concentration (mg/kg dry weight) in Chinese cabbage than other treatments of biochars. The plants of Chinese cabbage have exhibited an efficient transport capability for Zn and Cd. The biochars have reduced exchangeable form of Cd/Zn, enhanced residual heavy metals, and consequently diminished accumulation of heavy metals in plants. The straw block biochar (SBB), PSB and MSB have efficiently relieved the stresses of heavy metals in plants.

Bioavailability of heavy metals in contaminated soil as affected by different mass ratios of biochars.

The biochars are efficient adsorbent of heavy metals. However, the type of biochar species determines the effectiveness of biochars for immobilization of heavy metals. In this experiment, we mixed different biochars in variable mass ratios and evaluated their effects on plant growth and bioavailability of heavy metals. The results revealed that amendment of single and mixed biochars has increased pH, EC, available P and K of soil. The straw block biochar with maize straw biochar at a mass ratio of 1:2 (SDM) has significantly decreased Pb and Cu concentration in soil. The straw block biochar with maize straw biochar at a mass ratio of 2:1 (DSM) has highly increased the dry biomass of Chinese cabbage than single biochar or control, whereas plant physiological properties were mostly not affected. It is concluded that peanut shell biochar with maize straw biochar at a mass ratio of 2:1 (DPS) and DSM have significantly decreased Pb, Zn and Cu concentration in plants. The single peanut shell biochar has significantly decreased the plant Cd concentration. The ability of transport of heavy metals in Chinese cabbage was in the sequence of Cd > Zn > Cu > Pb. The mixing of different biochars has decreased the concentration of heavy metals in plants more effectively than single biochar.

Cathodized copper porphyrin metal-organic framework nanosheets for selective formate and acetate production from CO2 electroreduction.

An efficient and selective Cu catalyst for CO2 electroreduction is highly desirable since current catalysts suffer from poor selectivity towards a series of products, such as alkenes, alcohols, and carboxylic acids. Here, we used copper(ii) paddle wheel cluster-based porphyrinic metal-organic framework (MOF) nanosheets for electrocatalytic CO2 reduction and compared them with CuO, Cu2O, Cu, a porphyrin-Cu(ii) complex and a CuO/complex composite. Among them, the cathodized Cu-MOF nanosheets exhibit significant activity for formate production with a faradaic efficiency (FE) of 68.4% at a potential of -1.55 V versus Ag/Ag+. Moreover, the C-C coupling product acetate is generated from the same catalyst together with formate at a wide voltage range of -1.40 V to -1.65 V with the total liquid product FE from 38.8% to 85.2%. High selectivity and activity are closely related to the cathodized restructuring of Cu-MOF nanosheets. With the combination of X-ray diffraction, X-ray photoelectron spectroscopy, high resolution transmission electron microscopy and Fourier transform infrared spectroscopy, we find that Cu(ii) carboxylate nodes possibly change to CuO, Cu2O and Cu4O3, which significantly catalyze CO2 to formate and acetate with synergistic enhancement from the porphyrin-Cu(ii) complex. This intriguing phenomenon provides a new opportunity for the rational design of high-performance Cu catalysts from pre-designed MOFs.