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Abstract: Hydrotalcite-derived Ni and Fe-promoted hydrotalcite-derived Ni catalysts were found to outperform industrial catalysts in the CO2 methanation reaction, however the origin of the improved activity and selectivity of these catalysts is not clear. Here, we report a study of these systems by means of in situ X-ray photoelectron spectroscopy and near-edge X-ray absorption spectroscopy elucidating the chemical nature of the catalysts` surface under reaction conditions and revealing the mechanism by which Fe promotes activity and selectivity towards methane. We show that the increase of the conversion leads to hydroxylation of the Ni surface following the formation of water during the reaction. This excessive Ni surface hydroxylation has however a detrimental effect as shown by a controlled study. A dominant metallic Ni surface exists in conditions of higher selectivity towards methane whereas if an increase of the Ni surface hydroxylation occurs, a higher selectivity towards carbon monoxide is observed. The electronic structure analysis of the Fe species under reaction conditions reveals the existence of predominantly Fe(III) species at the surface, whereas a mixture of Fe(II)/Fe(III) species is present underneath the surface. Our results highlight that Fe(II) exerts a beneficial effect on maintaining Ni in a metallic state, whereas the extension of the Fe oxidation front from the surface towards the bulk is accompanied by a more extended Ni surface hydroxylation with a negative impact on the selectivity towards methane.

Open Access

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Abstract: The carbon–carbon coupling via electrochemical reduction of carbon dioxide represents the biggest challenge for using this route as platform for chemicals synthesis. Here we show that nanostructured iron (III) oxyhydroxide on nitrogen-doped carbon enables high Faraday efficiency (97.4%) and selectivity to acetic acid (61%) at very-low potential (−0.5 V vs silver/silver chloride). Using a combination of electron microscopy, operando X-ray spectroscopy techniques and density functional theory simulations, we correlate the activity to acetic acid at this potential to the formation of nitrogen-coordinated iron (II) sites as single atoms or polyatomic species at the interface between iron oxyhydroxide and the nitrogen-doped carbon. The evolution of hydrogen is correlated to the formation of metallic iron and observed as dominant reaction path over iron oxyhydroxide on oxygen-doped carbon in the overall range of negative potential investigated, whereas over iron oxyhydroxide on nitrogen-doped carbon it becomes important only at more negative potentials.

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Abstract: Nanostructured carbons with different pore geometries are prepared with a liquid-free nanocasting method. The method uses gases instead of liquid to disperse carbon precursor, leach templates and remove impurities, minimizing synthetic procedures and the use of chemicals. The method is universal and demonstrated by the synthesis of 12 different porous carbons with various template sources. The effects of pore geometries in catalysis can be isolated and investigated. Two of the resulted materials with different pore geometries are studied as supports for Ru clusters in the hydrogenolysis of 5-hydroxymethylfurfural (HMF) and electrochemical hydrogen evolution (HER). The porous carbon supported Ru catalysts outperform commercial ones in both reactions. It was found that Ru on bottle-neck pore carbon shows highest yield in hydrogenolysis of HMF to 2,5-dimethylfuran (DMF) due to a better confinement effect. A wide temperature operation window from 110 °C to 140 °C, with over 75% yield and 98% selectivity of DMF has been achieved. Tubular pores enable fast charge transfer in electrochemical HER, requiring only 16 mV overpotential to reach current density of 10 mA cm.

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Abstract: The general and cost-effective synthesis of single atom electrocatalysts (SAECs) still remains a great challenge. Herein, we report a general synthetic protocol for the synthesis of SAECs via a simple condensation–carbonization process, in which furfural and cyanamide were condensation polymerized in the presence of polystyrene nanospheres and metal ions, followed by a pyrolysis to N-doped carbon nanosheet (NCNS) supported SAECs. Six types of SAECs containing platinum, palladium, gold, nickel, cobalt and iron were synthesized to demonstrate the generality of the synthesis protocol. This methodology affords a facile solution to the trade-off between support conductivity and metal loading of SAECs by optimizing the ratio of carbon/nitrogen precursors, i.e., furfural and cyanamide. The presence of single metal atoms was confirmed by high-angle annular dark field scanning transmission electron microscopy and X-ray absorption fine structure measurements. The three-dimensional distribution of single platinum atoms was vividly revealed by depth profile analysis using a scanning transmission electron microscope. The resulting SAECs showed excellent performance for glycerol electro-oxidation and water splitting in alkaline solutions. Notably, Pt/NCNSs possessed an unprecedent mass-normalized current density of 5.3 A per milligram of platinum, which is 32 times that of the commercial Pt/C catalyst. Density functional theory calculations were conducted to reveal the adsorption behavior of glycerol over the SAECs. Using Ni/NCNSs and Co/NCNSs as anodic and cathodic electrocatalysts, we constructed a solar panel powered electrolytic cell for overall water splitting, leading to an overall energy efficiency of 8.8%, which is among the largest solar-to-hydrogen conversion efficiencies reported in the literature.

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Abstract: Single-site catalysts can demonstrate high activity and selectivity in many catalytic reactions. The synthesis of these materials by impregnation from strongly oxidizing aqueous solutions or pH-controlled deposition often leads to low metal loadings or a range of metal species. Here, we demonstrate that simple impregnation of the metal precursors onto activated carbon from a low-boiling-point, low-polarity solvent, such as acetone, results in catalysts with an atomic dispersion of cationic metal species. We show the generality of this method by producing single-site Au, Pd, Ru and Pt catalysts supported on carbon in a facile manner. Single-site Au/C catalysts have previously been validated commercially to produce vinyl chloride, and here we show that this facile synthesis method can produce effective catalysts for acetylene hydrochlorination in the absence of the highly oxidizing acidic solvents previously used.