Copper phenanthroline for selective electrochemical CO2 reduction on carbon paper.

We report a series of structurally relevant copper phenanthroline complexes as pre-catalysts for highly selective electrocatalytic reduction of CO2 to C2 products using inexpensive carbon paper electrodes. The Cu complexes with non-substituted phenanthroline promote the production of ethylene with a high faradaic efficiency of 71.2%, while the one with pyridinium-functionalized ligands is more selective for ethanol. The C2 selectivity can be effectively tuned by increasing the number of coordinated phenanthrolines and remains high at a wide range of potentials.

Molecular Nickel Thiolate Complexes for Electrochemical Reduction of CO2 to C1-3 Hydrocarbons.

We report the unprecedented electrocatalytic activity of a series of molecular nickel thiolate complexes (1-5) in reducing CO2 to C1-3 hydrocarbons on carbon paper in pH-neutral aqueous solutions. Ni(mpo)2 (3, mpo=2-mercaptopyridyl-N-oxide), Ni(pyS)3 - (4, pyS=2-mercaptopyridine), and Ni(mp)2 - (5, mp=2-mercaptophenolate) were found to generate C3 products from CO2 for the first time in molecular complex. Compound 5 exhibits Faradaic efficiencies (FEs) of 10.6 %, 7.2 %, 8.2 % for C1 , C2 , C3 hydrocarbons respectively at -1.0 V versus the reversible hydrogen electrode. Addition of CO to the system significantly promotes the FEC1-C3 to 41.1 %, suggesting that a key Ni-CO intermediate is associated with catalysis. A variety of spectroscopies have been performed to show that the structures of nickel complexes remain intact during CO2 reduction.

Improving Photocatalytic Hydrogen Production through Incorporating Copper to Organic Photosensitizers.

Organic dyes have been investigated extensively as promising photosensitizers in noble-metal-free photocatalytic systems for hydrogen production. However, other than functional group optimization, there are very few methods reported to be effective in improving their photocatalytic activity. Herein, we report the incorporation of Cu2+ into purpurin and gallein dyes for visible-light-driven hydrogen production. These Cu-dye chromophores significantly promote the photocatalytic activity of homogeneous systems when paired with a series of molecular Ni or Fe catalysts. Under optimal conditions, the Cu-purpurin and Cu-gallein photosensitizers exhibit more than 20-fold increases in turnover frequencies for hydrogen evolution when compared with purpurin and gallein. Catalytic systems with the Cu-purpurin chromophore show no decrease in activity over 120 h. Based on electrochemical and fluorescence quenching experiments, the enhancement of photocatalytic activity is likely due to the fact that Cu2+ can facilitate the transfer of electrons from the photosensitizers to the catalysts through creating highly reducing centers.

Selective CO2 Reduction to Ethylene Using Imidazolium-Functionalized Copper.

Electrochemical CO2 reduction is a promising approach to obtain sustainable chemicals in energy conversion. Improving the selectivity of CO2 reduction toward a particular C2 product such as ethylene remains a significant challenge. Herein, we report a series of imidazolium hexafluorophosphate compounds as surface modifiers for planar Cu foils to boost the Faradaic efficiency (FE) of ethylene from 5 to 73%, which is among the highest reported using polycrystalline Cu. The modified electrodes are convenient to prepare. The structure-function study demonstrates that varying the alkyl or aromatic substituents on the imidazolium nitrogen atoms has significant effects on the morphology of the deposited films and the product selectivity of CO2 reduction. Experimental FEC≥2, FEC2H4, ln(FEC≥2/FECH4), and ln(FEC2H4/FEC2H5OH) values show generally linear relationships with FEH2 while using different imidazolium modifiers, suggesting that factors governing proton reduction may also be directly related to both overall C≥2 generation and ethylene selectivity. This work presents an effective and practical way in tailoring the active sites of metallic surface for selective CO2 reduction.

Promoting photocatalytic CO2 reduction with a molecular copper purpurin chromophore.

CO2 reduction through artificial photosynthesis represents a prominent strategy toward the conversion of solar energy into fuels or useful chemical feedstocks. In such configuration, designing highly efficient chromophores comprising earth-abundant elements is essential for both light harvesting and electron transfer. Herein, we report that a copper purpurin complex bearing an additional redox-active center in natural organic chromophores is capable to shift the reduction potential 540 mV more negative than its organic dye component. When this copper photosensitizer is employed with an iron porphyrin as the catalyst and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole as the sacrificial reductant, the system achieves over 16100 turnover number of CO from CO2 with a 95% selectivity (CO vs H2) under visible-light irradiation, which is among the highest reported for a homogeneous noble metal-free system. This work may open up an effective approach for the rational design of highly efficient chromophores in artificial photosynthesis.

Preparation of Lignocellulose-Based Activated Carbon Paper as a Manganese Dioxide Carrier for Adsorption and in-situ Catalytic Degradation of Formaldehyde.

Formaldehyde is a colorless, highly toxic, and flammable gas that is harmful to human health. Recently, many efforts have been devoted to the application of activated carbon to absorb formaldehyde. In this work, lignocellulose-based activated carbon fiber paper (LACFP) loaded with manganese dioxide (MnO2) was fabricated for the adsorption and in-situ catalytic degradation of formaldehyde. LACFP was prepared by two-stage carbonization and activation of sisal hemp pulp-formed paper and was then impregnated with manganese sulfate (MnSO4) and potassium permanganate (KMnO4) solutions; MnO2 then formed by in situ growth on the LACFP base by calcination. The catalytic performance of MnO2-loaded LACFP for formaldehyde was then investigated. It was found that the suitable carbonization conditions were elevating the temperature first by raising it at 10°C/min from room temperature to 280°C, then at 2°C/min from 280 to 400°C, maintaining the temperature at 400°C for 1 h, and then increasing it quickly from 400 to 700°C at 15°C/min. The conditions used for activation were similar to those for carbonization, with the temperature additionally being held at 700°C for 2 h. The conditions mentioned above were optimized to maintain the fiber structure and shape integrity of the paper, being conducive to loading with catalytically active substances. Regarding the catalytic activity of MnO2-loaded LACFP, the concentration of formaldehyde decreased by 59 ± 6 ppm and the concentration of ΔCO2 increased by 75 ± 3 ppm when the reaction proceeded at room temperature for 10 h. The results indicated that MnO2-loaded LACFP could catalyze formaldehyde into non-toxic substances.

A new approach to recycle oxalic acid during lignocellulose pretreatment for xylose production.

BACKGROUND: Dilute oxalic acid pretreatment has drawn much attention because it could selectively hydrolyse the hemicellulose fraction during lignocellulose pretreatment. However, there are few studies focusing on the recovery of oxalic acid. Here, we reported a new approach to recycle oxalic acid used in pretreatment via ethanol extraction. RESULTS: The highest xylose content in hydrolysate was 266.70 mg xylose per 1 g corncob (85.0% yield), which was achieved using 150 mmol/L oxalic acid under the optimized treatment condition (140 °C, 2.5 h). These pretreatment conditions were employed to the subsequent pretreatment using recycled oxalic acid. Oxalic acid in the hydrolysate could be recycled according to the following steps: (1) water was removed via evaporation and vacuum drying, (2) ethanol was used to extract oxalic acid in the remaining mixture, and (3) oxalic acid and ethanol were separated by reduced pressure evaporation. The total xylose yields could be stabilized by intermittent adding oxalic acid, and the yields were in range of 46.7-64.3% in this experiment. CONCLUSIONS: This sustainable approach of recycling and reuse of oxalic acid has a significant potential application for replacing traditional dilute mineral acid pretreatment of lignocellulose, which could contribute to reduce CO2 emissions and the cost of the pretreatment.

New Understandings of the Relationship and Initial Formation Mechanism for Pseudo-lignin, Humins, and Acid-Induced Hydrothermal Carbon.

The generation of pseudo-lignin as byproduct during the lignocellulose acidic pretreatment has been proposed for many years. However, the detailed formation mechanism is still unclear. Moreover, there is a lack of understanding in the initial reaction during the formation of humins (byproducts in furfural production) and acid-induced hydrothermal carbon (carbon material). In this work, the initial formation of these three substances were investigated. We first found the common feature of their formation process was that carbohydrate-hydrolyzed compounds could form black polymers by condensing in acidic media, but the difference was dependent on the reaction degree. Furthermore, the results revealed that oxidation was an accelerator for condensations during producing black polymers because oxidized compounds could enhance the acidity of the reaction system. However, condensations of oxidized compounds were more difficult to proceed. Meanwhile, during the initial stage, the dominating pathway was that furfural condensed with itself and isomerized xylose via aldol-condensation.

Microwave-Assisted Oxalic Acid Pretreatment for the Enhancing of Enzyme Hydrolysis in the Production of Xylose and Arabinose from Bagasse.

In this study, highly-efficient hydrolysis of bagasse into xylose and arabinose sugars (C5 sugars) was developed by microwave-assisted oxalic acid pretreatment under mild reaction conditions. The effects of acid and hydrolysis conditions on the C5 sugar yields were discussed. The results showed that oxalic acid performed better than hydrochloric acid and maleic acid, and was a promising alternative to sulfuric acid for xylose production at the same acid concentration. The maximum yields of xylose (95.7%) and arabinose (91.5%) were achieved via the microwave-assisted oxalic acid pretreatment (120 °C, 10 min, 0.4 mol/L, solid-liquid ratio of 1:50 g/mL), indicating that almost all xylan-type hemicelluloses were released from the cell wall and hydrolyzed into C5 sugars. After pretreatment, more than 90% of the cellulose in the residual bagasse was converted to glucose (92.2%) by enzymatic hydrolysis. This approach could realize the highly-efficient hydrolysis of xylan from bagasse into C5 sugars, which would enhance the enzyme hydrolysis of treated bagasse into glucose.