Abstract
We demonstrate copper-based (Cu,M)(O,S) oxysulfide catalysts with M = Ni, Sn, and Co for the abiotic chemical synthesis of ethanol (EtOH) with the C-C bond formation by passing carbon dioxide (CO2) through an aqueous dispersion bath at ambient environment. (Cu,Ni)(O,S) with 12.1% anion vacancies had the best EtOH yield, followed by (Cu,Sn)(O,S) and (Cu,Co)(O,S). The ethanol yield with 0.2 g (Cu,Ni)(O,S) catalyst over a span of 20 h achieved 5.2 mg. The ethanol yield is inversely proportional to the amount of anion vacancy. The kinetic mechanism for converting the dissolved CO2 into the C2 oxygenate is proposed. Molecular interaction, pinning, and bond weakening with anion vacancy of highly strained catalyst, the electron hopping at Cu+/Cu2+ sites, and the reaction orientation of hydrocarbon intermediates are the three critical issues in order to make the ambient chemical conversion of inorganic CO2 to organic EtOH with the C-C bond formation in water realized. On the other hand, Cu(O,S) with the highest amount of 22.7% anion vacancies did not produce ethanol due to its strain energy relaxation opposing to the pinning and weakening of O-H and C-O bonds.
Highlights
Ethanol from the CO2 conversion is much more difficult than MeOH due to the C-C chain formation
The understanding on the C-C bond formation by such a thermodynamically difficult reaction to mimic photosynthesis can help in the catalyst design for converting inorganic into organic species
The particle size and morphology of the catalysts were examined by high resolution transmission electron microscopy (HR-TEM, H-7000, Hitachi)
Summary
Distorted lattice structure randomize the chemical bonding of catalysts with no characteristic peaks to show up. It is apparent that the ambient conversion of CO2 in water is strongly related to the stored strain energy in catalyst. Cu(O,S) releases its strain energy due to the 22.7% anion vacancies and cannot be used for chemical conversion. Based upon the anion vacancy and the distorted and strained lattice in catalysts, two kinetic mechanisms are proposed for the ambient conversion of CO2 to EtOH with the C-C bond formation in water: the oxygen-exchangeable mechanism and the ethanol-forming kinetic mechanism, as shown in Fig. 4a,b, respectively. The anion vacancies of catalyst surrounding with cations prefer to trap the highly electronegative oxygen of H2O, simultaneously with the strain energy release from (Cu,Ni)(O,S) “nanobomb”. H2O molecules are pinned at the anion vacancies of (Cu,Ni)(O,S) surfaces with the outward-pointed O-H bond weakened This pinning-to-bond weakening step of H2O can be viewed as the activation step of reaction initiation. The H2O-pinning equation by anion vacancy of catalyst, Vanion(catal.), is shown in Eq (1) to form the pinned and active H2O (H2O*): Catalyst
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