Abiotic Synthesis with the C-C Bond Formation in Ethanol from CO2 over (Cu,M)(O,S) Catalysts with M = Ni, Sn, and Co

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.

nanoflowers had been presented 28 . Here with lattice strain energy to replace sunlight energy as the driving force to weaken chemical bonds, our aqueous (Cu,M)(O,S) catalyst system with M = Ni, Sn, and Co is demonstrated to produce EtOH from the dissolved CO 2 and water with the C-C bond formation at normal temperature and pressure. 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.
After stirring for 30 min, 1.5 g thioacetamide (CH 3 CSNH 2 ) was added into the mixed solution for another 30  Surface composition and chemical state of the (Cu,M)(O,S) catalyst were investigated with XPS (VG Scientific ESCALAB 250) photoelectron spectrometry under the Al Kα X-rays (hv = 1486.6 eV) radiation with carbon C1s (Ea = 284.62 eV) for calibration. The particle size and morphology of the catalysts were examined by high resolution transmission electron microscopy (HR-TEM, H-7000, Hitachi). The crystal structure of samples was characterized by using X-ray diffraction analysis (Bruker D2 phaser, Japan) using Cu Kα radiation.
The conversion of CO 2 by (Cu,M)(O,S) (M = Ni, Sn, and Co) was carried out in a home-made and jacketed quartz reactor. In the experiment, 0.2 g catalyst was added into the reactor with 70 mL distilled water, then CO 2 gas was passed into the reactor by adding droplets of the HNO 3 aqueous solution into the NaHCO 3 -dispersed solution. The amount of HNO 3 added to NaHCO 3 -dispersed solution was controlled by automatic infusion machine with the flow rate of 0.5 mL/h. Prior to adding HNO 3 to the solution, argon gas at 100 mL/min was flowed into the reactor to purge out all the atmospheric gases in reactor. The experiments were executed in a period of 20 h to collect sufficient amount of product to minimize the system errors including alcohol vaporization. The collected and centrifuged reaction solutions were analyzed by HP 6890 series gas chromatography equipped with HP 5973 mass selective detector, i.e. by GC-MS. During the experiment, the unreacted gas was collected in a sample bag for further GC analysis. Our experimental setup is schematically shown in Fig. S1 in Supporting Information. Figure 1a shows the XRD diffraction pattern of (Cu,Ni)(O,S  Figure 1b shows the FE-SEM image of (Cu,Ni)(O,S) with particle size in the range of 300~500 nm and the shape of petal-gathered flowers. Figure 1c shows the TEM image of (Cu,Ni)(O,S), which further verifies the (Cu,Ni)(O,S) microstructure. Figure 1d Figure 2a shows the high resolution (HR) XPS spectrum of Cu2p. The asymmetric shape of the Cu2p peak represented the different chemical states of Cu in the (Cu,Ni)(O,S) catalyst. The two peak positions of 2p3/2 and 2p1/2 at 933.2 and 953.2 eV, respectively, with a peak separation of 20.0 eV due to the spin-orbit splitting indicated that copper was in the Cu + state 30 . On the other hand, the two peak positions at 935.1 and 955.5 eV were to identify the monovalent Cu 2+ 31 . The Cu + and Cu 2+ molar contents were calculated to be 76.7% and 23.3%, respectively. The ratio between Cu + and Cu 2+ is about 3.29. Figure 2b shows HR-XPS spectra of Ni2p for (Cu,Ni)(O,S) catalyst. The two peak positions of 2p3/2 and 2p1/2 were observed at 853.7 and 871.7 eV, respectively, and were contributed from Ni 2+ 31 . Figure 2c shows the HR-XPS spectra of O1s for (Cu,Ni)(O,S) catalyst. The asymmetric shape of the O1s peak was attributed to the hydroxyl oxygen O-H (531.8 eV) and the lattice oxygen (530.3 eV) [32][33][34][35] . Figure 2d shows the HR-XPS spectra of S2p for (Cu,Ni)(O,S) catalyst. The peaks at 161.7 and 163.5 eV were attributed to S −2 and the peaks at 168.4 and 169.8 eV to S 6+ 36, 37 . The S 6+ and S 2− molar contents were calculated to be 19.05% and 80.95%, respectively. Such a large amount of S 6+ in ionically bonded materials is rare to see. Similar analyses were shown for (Cu,Sn)(O,S) in . For covellite structure of CuS, the cation: anion ratio remains 1:1. Based upon the total cation site of 1, the above molecular formula can be re-written as  Figure 3b shows the mass spectra for the species in the retention time from 2.332 to 2.415 min, as compared with the standard mass spectra of ethanol in Fig. 3c. The first peak at the retention time of ~2.1 min was originated from the CO 2 contribution. The mass spectra of our reaction solution were contributed from nitrogen, oxygen, and CO 2 with the m/z ratios at 28, 32, and 44, respectively. Nitrogen and oxygen peaks were contributed from the air during the GC measurement. The CO 2 peak indicates the existences of dissolved CO 2 in solution. It is obvious that ethanol peaks in (Cu,Ni)(O,S) with a good ethanol yield for the CO 2 hydrogenation by flowing CO 2 into the catalyst-dispersed aqueous solution shows several unique characteristics: (1) Cu + is dominant over Cu 2+ in the Cu 2+ S covellite structure with a ratio of Cu + /Cu of ~0.77, (2) there are 10.9% S 6+ in cation and 12.1% anion vacancies, (3) the cation site contains four kinds of ions in Cu + , Cu 2+ , Ni 2+ , and S 6+ and three kinds of valence charges in 1 + , 2 + , and 6 + , (4) the anion site is occupied with vacancy, O 2− , and S 2− , and (5) (Cu,Ni)(O,S) can generate EtOH, but it is not for Cu(O,S). As there are much more Cu + cations than Cu 2+ in the Cu 2+ S structure (Fig. 2a), anion vacancy and S 6+ cation have to form in order to keep charge neutrality and to hold the covellite structure. The multiple cations in Cu + of 0.77 Å in radius, Cu 2+ of 0.73 Å, Ni 2+ of 0.69 Å, and S 6+ of only 0.29 Å and the multiple anions in O 2− of 1.40 Å and S 2− of 1.84 Å 38 are the characters of (Cu,Ni)(O,S). Together with the high configuration entropy due to the complex composition, the highly distorted and heavily strained (Cu,Ni)(O,S) catalyst is at a high strain energy state and is much active. Figure S7 displays the high-resolution TEM image of (Cu,Sn)(O,S). In that figure, an enlarged image is to demonstrate the bent and distorted lattice structure of (Cu,Sn)(O,S). Figure S8 shows Raman spectra of (Cu,M)(O,S), Cu(O,S), and commercially available CuS. The flatten Raman spectra for self-made oxysulfides did not show any characteristic chemical bonding. The multiple cations and anions and the . It is apparent that the ambient conversion of CO 2 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. The best metal M in catalyst is the one to build inside the highest strain energy, which involves the optimization among the ionic size effect, the type and content of M, the equilibrium defect configuration, the charge state of cation etc.
Based upon the anion vacancy and the distorted and strained lattice in catalysts, two kinetic mechanisms are proposed for the ambient conversion of CO 2 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. For the formation of EtOH from CO 2 , it needs the hydrogenation and the C-C bond formation. The addition of hydrogen, which solely comes from water, for CO 2 reduction has to happen. Water needs to involve with the CO 2 -to-EtOH conversion. As our (Cu,Ni)(O,S) has its lattice highly distorted, each of its nanoparticle can be  When CO 2 starts to flow through the reactor, it always undergoes adsorption, migration, and reaction with the trapped and bond-weaken H 2 O*. Therefore, the next step after water trapping is the reaction between the in-coming and adsorbed CO 2 and the trapped and weakened H 2 O* on catalyst surface, as shown in the step (1) in Fig. 4a,b. As the adsorbed CO 2 reacts with the dissociated protons from H 2 O * , oxygen remains trapped at vacancy. For the charge neutrality consideration, the formation of each trapped oxygen needs to accompany with the transition of 2Cu + → 2Cu 2+ + 2e' with the release of 2 electrons. The released electrons are required for the CO 2 reduction reaction with protons. After CO 2 reaction, the anion vacancy of catalyst covers with oxygen ion and the Cu 2+ content increases, as shown in step (2)   Cu 2+ and its reverse one from Cu 2+ to Cu + (Fig. 4a) can re-install the Cu + /Cu 2+ ratio back to its original charge state or the catalyst is refreshed back to its highly strain energy state. The refreshed and re-appeared anion vacancy will trap H 2 O again or to wait for the arrival of reaction intermediates, as discussed next.
On the CO 2 reaction part, when it approaches the trapped and bond weakened H 2 O*, their molecules interact together to weaken and break two HO−H bonds in H 2 O* and to form CHO(OH) with the simultaneous assistance of 2e − from the oxygen-exchangeable mechanism in Fig. 4a. CHO(OH) further converts to CH 2 (OH) 2 with additional 2 H + from the HO−H bond weakening and 2e' from the Cu + /Cu 2+ transition, and then to form the adsorbed formaldehyde (CH 2 O (ad.) ) by de-hydrolysis with the liberation of one H 2 O molecule, as shown in step (2) of Fig. 4b. At this stage, the catalyst surface is covered with the "refreshed and empty" and the "H 2 O * -trapped" anion vacancies together with the adsorbed CH 2 O (ad.) , which is evaluated as the basic unit for natural saccharides with the general formula of (CH 2 O) n from natural photosynthesis. The acetyl group of CH 3 CO − was proposed as the intermediate for the formation of EtOH from the gas phase reaction between CO 2 and H 2 20 , while CH x O was recommended from the thermo-chemical reaction between CO and H 2 21 . Some surface-diffusive CH 2 O (ad.) molecules on catalyst surface in this work have their negatively charged ends of the C=O functional group trapped or pinned at the refreshed anion vacancies and then form active CH 3 OH* after hydrogenation. The trapped CH 3 OH* shows the weakened C-O bond due to the molecule pinning on catalyst. Some surface-diffusive CH 2 O (ad.) molecules react with the pinned and weakened CH 3 OH* to form the desorbed C 2 H 5 OH, as shown in step (3) in Fig. 4b. Here the ethanol formation is proposed by the reaction between the pinned and aligned and the adsorbed and movable species. The formed and adsorbed ethanol loses its strong interaction with the anion vacancy of catalyst and is released into solvent. Therefore, there are no C n species with n > 2.
Based upon the ability of catalyst in dynamically supplying protons and electrons, as shown in Fig. 4a, the formation reaction of ethanol is similar to the natural photosynthesis with the following reaction: The driving force to overcome this reaction barrier is the "released strain energy" from catalyst instead of sunlight. Our net reaction equation is shown below: With the aids of (a) the strain energy release, (b) the trapped and active H 2 O* with weakened O-H bonds to elevate its chemical potential above the standard Gibbs formation energy, and (c) the heat liberation during the interaction between the dissolved CO 2 and the activated H 2 O * , the conversion of CO 2 to EtOH in water is proceeded. The CO 2 -to-EtOH conversion cannot proceed without electron transport. Therefore, we make the first proposal: Catalyst with the feasibility to reversibly form the multiple charged cation states can benefit the electron hopping transport required for CO 2 hydrogenation. Saito et al. referred that the high activity in methanol synthesis over the Cu/ZnO-based catalyst by the thermo-conversion was related to the ratio of Cu + /Cu at ~0.7 39 , which is consistent with our ratio of 0.755-0.789. Lin and Frei observed the oxidation of Cu(I) at the CO 2 photoreduction 40 .
Water being trapped and weakened over catalyst by anion vacancy are crucial to initiate the CO 2 conversion reaction. The function of water in catalysis with inorganic oxides has been identified [41][42][43] . Therefore, we make the second proposal: Catalyst with the adequate amount of anion vacancies and the higher oxygen-exchangeable ability with water due to the strain energy consideration can weakened the HO−H bonds in the trapped and active water for generating protons required in Eq. (2). As anion vacancies are needed for ambient conversion, however, Cu(O,S) with a large amount of anion vacancies (22.7%) has a too loose structure instead of strained one to establish the sufficient strain energy and to initiate the HO−H bond trapping/weakening in active H 2 O*. J.C. Frost 44 mentioned that the productivity of methanol from H 2 /CO/CO 2 depends on the junction between metal and oxide and is related to oxygen or anion vacancies. Recently, Tisseraud et al. explained the Cu-ZnO synergy in methanol thermo-synthesis in terms of the formation of the Cu x Zn (1−x) O y interlayered phase between Cu and ZnO at 350 °C 45 . The role of oxygen vacancies in interacting with gaseous CO 2 has been widely investigated for the gas-phase thermo-chemical reaction 6,20 . Our (Cu,Ni)(O,S) closely related to the formed defective interlayered phase has successfully demonstrated the conversion of CO 2 to EtOH even at the ambient environment.
The artificial synthesis of ethanol with the C-C bond formation is rare at the natural environment. The third proposal is given as: The ethanol formation is related to the reaction between the pinned and the adsorbed oxygenates in considering the stereochemistry. If all molecules on catalyst surface are held straight through the C-O bond pinning, they are all fixed and do not have the flexibility to form the C-C bonds.
This study demonstrates the conversion of dissolved CO 2 to ethanol with the C-C bond formation over covellite-based (Cu,M)(O,S) oxysulfide catalysts with M = Ni, Sn, and Co in water. Several factors need to simultaneously operate together in order for this ambient conversion to occur, which are (1) the highly strained catalyst with anion vacancies to trap, pin, and weaken water and oxygenates, (2) the charge transport ability in semiconductor-type catalyst, and (3) the reaction between the pinned and the adsorbed C 1 oxygenates to form the C 2 ones. The amounts of ethanol derived from CO 2 over a period of 20 h were 5.2, 1.5, 0.30, and 0 mg for (Cu,Ni) (O,S), (Cu,Sn)(O,S), (Cu,Co)(O,S), and Cu(O,S), respectively. Here the C 2 oxygenate of ethanol with the C-C bond formation is formed from adsorbed CO 2 and the pinned and active H 2 O at natural environment with inorganic catalysts. Therefore, CO 2 , water, and the suitable inorganic mineral are sufficient to execute the abiotic chemical synthesis of C 2 oxygenate of ethanol.