Carbon Dioxide Conversion to Methanol over Size-selected Cu 4 Clusters at Low Pressures

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1 Carbon Dioxide Conversion to Methanol over Size-selected Cu 4 Clusters at Low Pressures Cong Liu a,, Bing Yang a,, Eric Tyo a, Soenke Seifert b, Janae DeBartolo b, Bernd von Issendorff c, Peter Zapol a, *, Stefan Vajda a,d-f, * and Larry A. Curtiss a, * a Materials Science Division, Argonne National Laboratory, 9700 S. Cass Ave. Lemont, IL 60439, USA; b X-ray Science Division, Argonne National Laboratory, 9700 S. Cass Ave. Lemont, IL 60439, USA; c Physikalisches Institut, Universität Freiburg, Stefan-Meier-Straße 21, Freiburg, Germany; d Nanoscience and Technology Division, Argonne National Laboratory, 9700 S. Cass Ave. Lemont, IL 60439, USA; e Department of Chemical and Environmental Engineering, School of Engineering & Applied Science, Yale University, 9 Hillhouse Avenue, New Haven, CT 06520, USA; f Institute for Molecular Engineering, University of Chicago, 5801 South Ellis Avenue Chicago, Illinois 60637, USA These authors contribute equally * Corresponding authors: zapol@anl.gov (PZ), vajda@anl.gov (SV) and curtiss@anl.gov (LAC) The supporting information includes experimental details (methods and reaction conditions), in-situ GIXANES measurement, multiple ion detection (MID) analysis of reaction products and computational details (methods and calculated energy data). 1. Experimental details Cu 4 clusters were synthesized using size-selected cluster source, and soft-landed onto three monolayer amorphous Al 2 O 3 thin film prepared by atomic layer deposition (ALD) on top of the native oxide of an N-doped silicon wafer (SiO 2 /Si(100)). The use of doped, i.e. conductive, Si wafer facilitates the on-line measurement of the flux of positively charged clusters landing on the surface and allows the charge of the clusters to be neutralized upon landing, using a picoampermeter (Keithley Model 6489). The surface coverage of the Si chip was set at 10% atomic Cu monolayer equivalent ~7 ng Cu loading to avoid aggregation of clusters on the surface. The Cu 4 sample was heated stepwise from 25 C up to 425 C under in-situ conditions with 20 sccm flow of 1% CO 2 and 3% H 2 gas mixture carried in helium at pressure of 1.25 atm. To enhance the surface sensitivity, X-ray beam was scattered off the sample surface at a grazing incident angle (α c = 0.18).The GIXANES data were collected at Cu K edge (8.9 KeV) by a fluorescence detector (Vortex) mounted parallel to the sample surface in order to minimize background from elastic scattering. 1 All reaction products were measured by a quadrupole mass spectrometer (Pfeiffer Prisma QMS 200) sampling with an electronic gas regulating valve (EVR 116, Pfeiffer) on the inlet of the mass spectrometer for assuring a constant pressure (5x10-6 mbar) in the mass spectrometer. The activity data were collected for 2 hr at each temperature. S1

2 2. In-situ GIXANES measurement As displayed in Fig. S1, the XANES spectra of Cu 4 clusters at 25 C resembles the Cu 2+ feature (reference spectra of CuO bulk standards in Fig. S1(b)). Upon heating to 75 C under CO 2 /H 2 gas mixture, down-shift of the absorption edge as well as the appearance of the second post-edge feature (~9 KeV) indicates the reduction of Cu clusters. Above 175 C, XANES feature thus presents characteristic metallic Cu 0 state which is stable up to 425 C. Figure S1. GIXANES measurements at Cu K edge (8.9 KeV) under in-situ reaction condition. a) XANES spectra of Al 2 O 3 supported Cu 4 clusters at five characteristic temperatures (25, 75, 175, 325 and 425 C). Gradual reduction of copper clusters can be identified with increasing temperature. b) Reference spectra of bulk Cu standards, including metallic Cu foil, Cu 2 O and CuO powder. 3. Multiple ion detection (MID) analysis of reaction products Figure S2 presents the background corrected MID data of reaction products, i.e. CH 3 OH (m/z 31) b); CH 4 (m/z 15) c); and H 2 O (m/z 18) d), under reaction conditions obtained by subtracting the signal measured under identical conditions on a blank support (i.e. without clusters) from the signal collected on the cluster sample. We note that the signal-noise ratio is low due to the very low partial pressure of CO 2 and H 2 in the gas feed and ultra-small catalyst loading. However, the excellent activity for methanol synthesis can be clearly identified between C. Figure 2 in the main text shows TOR calculated from the background corrected. MID data using mathematic averaging of 100 data points (100 points per 1hr) at each temperature and calibration of the mass spectrometer with certified gas mixtures of the products (Airgas). Besides, no obvious CH 4 (m/z 15) feature can be observed between C which supports the theoretical calculation that methanation of CO 2 is not favorable in this temperature range. We also observed higher m/z =15 (CH 4 ) and m/z=18 (H 2 O) signal at 375 C which indicates other reaction pathway(s) of CO 2 conversion, such as methanation and reverse water gas shift reaction. S2

3 Figure S2. Background corrected MID data of reaction products methanol, methane and water: a) Temperature profile of the applied temperature ramp. MID spectra of different reaction products: b) CH 3 OH (m/z 31);c) CH 4 (m/z 15); and d) H 2 O (m/z 18). The activity was measured for 2hr at each temperature from 25 C to 375 C. 4. Computational details Density functional theory (DFT) calculations were carried out to investigate the catalytic reaction mechanism of CO 2 hydrogenation on Al 2 O 3 supported Cu 4 clusters. The Al 2 O 3 supported Cu 4 cluster was constructed by binding the Cu 4 cluster onto a hydroxylated amorphous alumina surface. The initial structure of Cu 4 was taken from our previous DFT calculations in the gas phase 2. The initial structure of the hydroxylated Al 2 O 3 model was obtained from a molecular dynamics (MD) simulation using the same method in previous studies 3. The dimension of the MD box is a = b = 11 Å and c = Å with 8% H content and a total of 97 atoms. The structure from the MD simulation was then taken for DFT calculations. The unit cell of the amorphous Al 2 O 3 surface for periodic DFT calculation has a dimension of Å, with a vacuum space of 10 Å and a total of 97 atoms. On the optimized amorphous Al 2 O 3 surface, Al atoms have either four- or five- fold coordination. Oxygen atoms have either two- or three- fold coordination. Two types of hydroxyl groups are on the surface, the bridge-coordinated hydroxyl and single-coordinated hydroxyl groups. S3

4 Figure S3. Optimized structure of Al 2 O 3 supported Cu 4 cluster All the calculations were carried out using the PBE functional 4 with a plane wave basis set implemented in the Vienna Ab initio Simulation Package (VASP, version 5.3.3) 5-8. Spin-polarized calculations were performed for the systems that have Cu 4. The energy cutoff of 400 ev was used, and the Γ-point and a k-point mash were used to sample the Brillouin zones for the gas phase molecules and Al 2 O 3 supported systems, respectively. In the Al 2 O 3 supported systems, atoms in the top half of the Al 2 O 3 slab and adsorbed species were allowed to relax while the bottom half of the Al 2 O 3 slab were kept frozen. Transition states were searched via the climbingimage nudged elastic band method (CI-NEB) 9, and were confirmed with frequency calculations. The partial charges on Al 2 O 3 supported Cu 4 were calculated using Bader charge analysis 10. Table S1. Calculated partial charges ofal 2 O 3 supported Cu 4 using Bader charge analysis Atom number Partial charge ( e /atom) Cu Cu Cu Cu Total 0.63 Average 0.16 Previous DFT studies have shown that gas phase Cu 4 has rhombus geometry 2,11. However, a recent DFT study about Al 2 O 3 supported Cu 4 used the tetrahedral Cu 4 cluster 12. Thus in the present study, we considered both structures. Our preliminary calculations showed that Al 2 O 3 supported rhombus Cu 4 and tetrahedral Cu 4 are very close in energy (the former is only 0.02 ev lower than the latter), and the reaction pathways of CO 2 reduction to HCOOH on both clusters showed very similar energetic trends and barriers. This suggests that the geometry of Cu 4 clusters on the Al 2 O 3 support has little effect on the electronic and catalytic properties of Cu 4. Therefore, only the rhombus Cu 4 is presented in this work. The binding of Cu 4 onto the hydroxylated Al 2 O 3 surface was investigated in consideration of different binding sites. We considered three types of binding sites: three-fold oxygens, bridge-coordinated oxygens (with and without hydrogen) and single-coordinated hydroxyl groups. DFT calculations showed that bridge-coordinated oxygen tends to have the highest binding strength with Cu 4, while three-fold oxygen and single-coordinated hydroxyl groups both have weaker interactions with S4

5 Cu 4. This agrees with previous experimental observation 13. It is also interesting that Cu 4 is likely to replace the hydrogen on the bridge-coordinated hydroxyl group and binds to the oxygen. This can be explained by the strong acidity of these bridge-coordinated hydroxyl groups. The lowest energy structure of Al 2 O 3 supported Cu 4 is shown in Figure S3, in which Cu3 is bound to a bridge-coordinated O atom (O5), and Cu4 is bound to two single-coordinated hydroxyl groups (O6 and O7). The binding energy of Cu 4 onto the support is 1.96 ev. The calculated charges of the supported Cu 4 are shown in Table S1. Cu3 and Cu4 are partially oxidized by the oxygen atoms and thus have slight positive charges. The average partial charge on Cu 4 is 0.16 e /atom, agreed well with our experimental results (See main text). Table S2 shows the calculated relative energies of important states in the reduction of CO 2 to CH 3 OH, CO and CH 4 in Figure 3 in the main text. Table S2. Calculated relative energies of important states in the catalytic reaction pathways of CO 2 reduction to CH 3 OH, CO and CH 4 on Al 2 O 3 supported Cu 4 clusters CH 3 OH formation CO formation CH 4 formation State Relative energies (ev) State Relative energies (ev) State Relative energies (ev) * + CO 2 + 3H 2 0 * + CO 2 + H 2 0 * + CO 2 + 4H 2 0 H* + CO H H* + CO H H* + CO H TS-HCO 2 * + 2.5H TS-COOH* + 0.5H TS-HCO 2 * + 3.5H HCO 2 * + 2.5H COOH* + 0.5H HCO 2 * + 3.5H TS-HCO 2 H* + 2H 2 TS-(CO*+OH*) TS-HCO 2 H* + 3H H HCO 2 H* + 2H CO* + OH* + 0.5H HCO 2 H* + 3H TS-H 2 CO 2 H* + 1.5H CO + OH* + 0.5H TS-H 2 CO 2 H* + 2.5H H 2 CO 2 H* + 1.5H CO + TS-H 2 O* 0.69 H 2 CO 2 H* + 2.5H TS(H 2 CO* + OH*) + 1.5H CO + H 2 O* 0.11 TS(H 2 CO* + OH*) + 2.5H H 2 CO* + OH* + 1.5H CO + H 2 O + * 0.74 H 2 CO* + OH* + 2.5H TS-H 3 CO* +OH* + H TS-H 3 CO* +OH* + 2H H 3 CO* + OH* + H H 3 CO* + OH* + 2H TS(H 3 CO* + H 2 O*) + 0.5H TS(H 3 CO* + H 2 O*) + 1.5H H 3 CO* + H 2 O* + H 3 CO* + H 2 O* + 1.5H 2 0.5H H 3 CO* + H 2 O + 0.5H H 3 CO* + H 2 O + 1.5H TS-H 3 COH* + H 2 O TS(H 3 C* + O*) + 1.5H H 3 COH* + H 2 O H 3 C* + O* + 1.5H * + H 3 COH + H 2 O TS(CH 4 + O*) + H CH 4 + O* + H TS-OH* + CH H OH* + CH H TS-H 2 O* + CH H 2 O* + CH * + H 2 O + CH S5

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