Mechanism of Autocatalytic Reduction of CO2 over MgCO3 to High Value-Added Chemicals: A DFT & AIMD Study.

Calcination of MgCO3 is an important industrial reaction, but it causes significant and unfavorable CO2 production. Calcination in a reducing green hydrogen atmosphere can substantially reduce CO2 release and produce high value-added products such as CO or hydrocarbons, but the mechanism is still unclear. Here, the in situ transformation process of MgCO3 interacting with hydrogen and the specific formation mechanism of the high value-added products are thoroughly investigated based on reaction thermodynamic, ab initio molecular dynamics (AIMD) simulations, and density functional theory (DFT) calculations. The reaction thermodynamic parameters of MgCO3 coupled with hydrogen to produce CO or methane are calculated, revealing that increasing and decreasing the thermal reductive decomposition temperature favors the production of CO and methane, respectively. Kinetically, the energy barriers of each possible production pathway for the dominant products CO and methane are further calculated in conjunction with the AIMD simulation results of the transformation process. The results suggest that CO is produced via the MgO catalytic-carboxyl pathway (CO2*→ COOH*trans→ COOH*cis→ CO*→ CO), which is autocatalyzed by MgO derived from the thermal reductive decomposition of MgCO3. For the mechanism of methane formation, it prefers to be produced by the stepwise interaction of carbonates in the MgCO3 laminates with hydrogen adsorbed on their surfaces (direct conversion pathway: sur-O-CO → sur-O-HCO → sur-O-HCOH → sur-O-HC → sur-O-CH2 → sur-O-CH3 → sur-O + CH4*).

Theoretical study on electrocatalytic carbon dioxide reduction over copper with copper-based layered double hydroxides.

The conversion of CO2 into valuable fuels and multi-carbon chemical substances by electrical energy is an effective strategy to solve environmental problems by using renewable energy sources. In this work, the density functional theory (DFT) method is used to reveal the electrocatalytic mechanism of CO2 reduction reaction (CO2RR) over the surface of CuAl-Cl-layered double hydroxides (LDHs) with Cu monoatoms (Cu@CuAl-Cl-LDH), Cu2 diatoms (Cu2@CuAl-Cl-LDH), orthotetrahedral Cu4 clusters (Td-Cu4@CuAl-Cl-LDH) and planar Cu4 clusters (Pl-Cu4@CuAl-Cl-LDH). The active sites, density of states, adsorption energy, charge density difference and free energy are calculated. The results show that CO2RR over all the above five catalysts can generate C2 products. Pl-Cu4@CuAl-Cl-LDH tends to generate C2H5OH, while the remaining four structures all tend to produce C2H4. Cuδ+ favors CO2RR, and Td-Cu4@CuAl-Cl-LDH with a larger positively charged area at the active site has the better electrocatalytic performance among the calculated systems with a maximum step height of 0.78 eV. The selectivity of the products C2H4 and C2H5OH depends on the dehydration of the intermediate *C2H2O to *C2H3O or *CCH; if the dehydration produces *CCH intermediate, the final product is C2H4, and if no dehydration occurs, C2H5OH is produced. This work provides theoretical information and guidance for further rational design of efficient CO2RR catalysts for energy saving and emission reduction.

Three undescribed compounds from Diaporthe biguttusis T-24, an endophytic fungus isolated from Ligularia fischeri.

Diaporthpyran A (1), diaporthester E (2) and diaporthester F (3), three new compounds along with four known compounds (4-7) were isolated from the crude extract of Diaporthe biguttusis T-24, an endophytic fungus isolated from Ligularia fischeri. The planar structures of compounds 1-3 including the relative and absolute configurations were elucidated on the basis of HRMS, NMR, J-based coupling constant analysis, CD, and calculated ECD analysis. In addition, compounds 1 and 3 were evaluated for their cytotoxic activities against four human cancer cell lines.