Copper Cluster Size Effect in Methanol Synthesis from CO 2

May 8, 2017 - Size-selected Cun catalysts (n = 3, 4, 20) were synthesized on Al2O3 thin films using mass-selected cluster deposition. A systematic stu...
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Copper Cluster Size Effect in Methanol Synthesis from CO2 Bing Yang,†,# Cong Liu,‡,# Avik Halder,† Eric C. Tyo,† Alex B. F. Martinson,† Sönke Seifert,§ Peter Zapol,*,† Larry A. Curtiss,*,† and Stefan Vajda*,† †

Materials Science Division, ‡Chemical Sciences and Engineering Division, and §X-ray Science Division, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, Illinois 60439, United States S Supporting Information *

ABSTRACT: Size-selected Cun catalysts (n = 3, 4, 20) were synthesized on Al2O3 thin films using mass-selected cluster deposition. A systematic study of size and support effects was carried out for CO2 hydrogenation at atmospheric pressure using a combination of in situ grazing incidence X-ray absorption spectroscopy, catalytic activity measurement, and firstprinciples calculations. The catalytic activity for methanol synthesis is found to strongly vary as a function of the cluster size; the Cu4/Al2O3 catalyst shows the highest turnover rate for CH3OH production. With only one atom less than Cu4, Cu3 showed less than 50% activity. Density functional theory calculations predict that the activities of the gas-phase Cu clusters increase as the cluster size decreases; however, the stronger charge transfer interaction with Al2O3 support for Cu3 than for Cu4 leads to remarkably reduced binding strength between the adsorbed intermediates and supported Cu3, which subsequently results in a less favorable energetic pathway to transform carbon dioxide to methanol. measured Cu0 surface area. In addition, support composition, e.g., ZnO,9 ZrO2,20 and CeO2,6 is another key factor that influences the activity of methanol synthesis by stabilizing copper catalyst with different cluster size and oxidation state. Due to the structural complexity of a working copper catalyst, the clear assignment of the active Cu species is not yet achieved and is still under debate. Recently, size-selected subnanometer clusters have received considerable attention in catalysis because of their unique electronic and catalytic properties.10,21−35 Our previous study on a subnanometer Cu4 catalyst has revealed that ultrasmall copper clusters can exhibit extraordinary catalytic activity for methanol synthesis at near atmospheric pressure compared with other, larger size catalysts.10 In this article, we report a more systematic study on atomically precise Cun catalysts (n = 3, 4, 20) supported on Al2O3 using both experimental and computational approaches. We have chosen 3, 4, and 20 atoms to represent odd/even numbers of atoms in the clusters and different shapes of the clusters, because these clusters can have different reactivities.36 Three-atom clusters are always planar, four-atom clusters are planar in the gas phase but can be tetrahedral on the surface or under reaction conditions, and 20atom clusters are 3D in the gas phase and remain 3D on the surface. In situ grazing incidence X-ray absorption near-edge spectroscopy (GIXANES) and catalytic activity measurements were carried out under reaction conditions to study the sizedependent activity of Cun clusters for hydrogenation of CO2 at

1. INTRODUCTION The development of a sustainable, long-term solution to meeting the world’s energy needs is one of the defining issues of our time. The recent development of new technologies in carbon dioxide capture1−3 make the utilization of CO2 a focus of interest for sustainable energy solutions. CO2 can be recycled from power plant flue-gas emissions, recovered from automotive exhaust gas, or derived from carbonic acid in seawater, and further converted to syngas, methanol, or directly to liquid fuels.3−6 Catalytic conversion of the chemically stable CO2 thus plays a key role in sustainable carbon recycling. Copper catalysts have been extensively studied for methanol (CH3OH) synthesis from syngas (CO, CO2, and H2).7−10 Unlike Fe/Co catalysts that are used in Fischer−Tropsch synthesis to produce hydrocarbons,11 Cu catalysts exhibit extraordinary activity and selectivity in low-temperature methanol synthesis by CO2 hydrogenation (CO2 + 3H2 → CH3OH + H2O).7,9,12,13 Many efforts have been devoted to discovering the reaction mechanism and the nature of the active sites in Cu catalysts, including cluster size, oxidation state, and support effects. It has been shown that for Cu/Zn/Zr/Ga/Y catalysts with Cu particle sizes from 10 to 27 nm, smaller particles enhanced the rate of methanol formation,14 whereas another study using Cu/ZnO catalysts (Cu particle sizes between 8.5 and 37.3 nm) proposed that the size of copper nanoparticles does not influence the intrinsic rate of methanol formation.15 The oxidation state of active copper species in methanol synthesis catalysts has also been debated. It has been suggested that the ionic form of Cu+ was the active center for methanol synthesis,16,17 but other investigations18,19 report a direct correlation between methanol synthesis activity and the © XXXX American Chemical Society

Received: February 24, 2017 Revised: April 21, 2017

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Source in transmission mode, as reference spectra for linear combination fit (LCF) analysis. We observed no change in the XANES features between spectra recorded with very different X-ray fluxes nor when moving from one spot illuminated for extended time to a “fresh” spot, thus excluding beam damage. During the in situ experiments, the possible sintering of the clusters was monitored with grazing incidence small-angle X-ray scattering (GISAXS).42 No change in the GISAXS patterns was observed which would indicate the formation of aggregates (see Figure S1 in the Supporting Information), thus confirming sintering resistant clusters. 2.3. Catalytic Activity Measurement. The catalytic activity of Cun catalysts was measured separately in the laboratory using an identical setup and under reaction conditions (25−425 °C, with 20 sccm flow of 1% CO2 and 3% H2 gas mixture carried in helium at pressure of 1.25 atm) identical to those used for in situ GIXAS measurements at synchrotron. 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 (5 × 10−6 mbar) in the mass spectrometer. Before each reaction, the reaction gas mixture was flowed continuously over 12 h to minimize and stabilize the background signal of the mass spectrometer. The catalytic data were collected for 2 h at each temperature. The mass spectrometer was calibrated using gases diluted in helium purchased from AirGas; three concentrations were used (including 100% He) by further mixing with pure helium in a gas mixer manifold equipped with precision mass flow controllers (Brooks), to ensure linearity. The turnover rate (TOR) was calculated by dividing the production rate of methanol in molecules per second over the total number of deposited copper atoms. The estimated error in the reported TORs is about 35% or better. 2.4. Computational Details. All calculations were carried out using the PBE functional44 with a plane wave basis set implemented in the Vienna Ab Initio Simulation Package (VASP, version 5.3.5).45−48 Spin-polarized calculations were performed for all the systems that have Cu. An energy cutoff of 400 eV was used, and the Γ-point and a 2 × 2 × 1 k-point mesh were used to sample the Brillouin zones for the gas-phase molecules (including all gas-phase clusters and isolated molecules) and Al2O3-supported systems, respectively. A model of hydroxylated amorphous alumina was chosen for the support based on the following considerations. In this study, the amorphous Al2O3 thin film was grown by ALD, by alternating trimethylaluminum and water vapor doses. The mechanism for alumina growth using trimethylaluminum and water is well-known from a number of surface chemistry studies. The widely accepted mechanism is a complete removal of the methyl termination from a recently trimethylaluminumexposed surface upon exposure to water vapor to produce a surface with ∼8 hydroxyls per nm2 when saturating dose is utilized. (See, for example, ref 49 and references therein.) The substantial density of hydroxyl groups was also indicated by previous Raman studies of ALD alumina films.50 Therefore, the surface structure of alumina support was modeled according to our previous studies of hydroxylated amorphous alumina.51 In the Al2O3-supported systems, atoms in the top half of the Al2O3 slab and adsorbed species were allowed to relax while atoms in the bottom half of the Al2O3 slab were kept frozen. Transition states were calculated using the climbing-image nudged elastic

near atmospheric pressure. A strong size dependence was observed in the activity of Cun clusters with similar oxidation states at elevated temperatures. Density functional theory (DFT) calculations provided insights into the cluster size effect and the support effect (charge transfer) of Cu clusters for CO2 hydrogenation to CH3OH.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Size-selected Cun clusters (n = 3, 4, 20) were synthesized using a mass-selected cluster deposition method described previously.10,37 In brief, Cu clusters/ions were produced in the gas phase using magnetron sputtering and mass-selected by quadrupole mass filter according to their mass to charge ratio. Subsequently, the atomically precise, mass-selected, positively charged Cun clusters (Cu3+, Cu4+, and Cu20+) were directed toward the substrate by a quadrupole deflector and soft landed (with a kinetic energy of less than 5 eV per cluster) onto an ultrathin amorphous Al2O3 film as support material. The amorphous Al2O3 thin film of ∼3 ML thickness was prepared by six cycles of atomic layer deposition (ALD) by alternating trimethylaluminum and water doses at 150 °C on top of the native silicon dioxide of an n-type (phosphorus-doped) silicon wafer (SiO2/ Si(100)). The charge of the clusters was neutralized upon landing and counted by a picoampermeter (Keithley Model 6489). The loading (surface coverage) of all Cun catalysts was set at 10% atomic Cu monolayer equivalent (∼14 ng), within two deposition spots of 9 mm in diameter, to avoid aggregation of clusters on the surface. (1 monolayer equivalent = ∼1.4 × 1014 atoms/cm2.) Previous studies have also shown that such a film can keep a variety of clusters from sintering under reaction conditions.22,38−42 All samples were exposed to air after deposition, which led to the oxidization of the copper clusters as shown by GIXANES measurements. Identical pairs of samples were synthesized for each cluster size: one sample for catalytic testing in the lab and the other for in situ characterization. 2.2. In Situ Grazing Incidence X-ray Absorption Near Edge Spectroscopy (GIXANES). In situ GIXANES measurements were performed at beamline 12-ID-C of the Advanced Photon Source at the Argonne National Laboratory. The experimental setup has previously been reported elsewhere.10,23,38,41−43 The Cun clusters were placed in a homebuilt cell reactor10,25,43 equipped with Kapton windows that allow X-ray transmission. The reaction gas was mixed in a manifold gas mixer to maintain a 20 sccm flow of 1% CO2 and 3% H2 gas mixture carried in helium. After an initial 30 min purge of the reactor with the gas mixture, the samples were heated stepwise from 25 °C up to 425 °C under in situ reaction conditions at a constant pressure of 1.25 atm. To maximize the sensitivity of the experiment to the particles sitting on the surface of the support, the X-ray beam was scattered off the sample surface close to the angle of total reflection, i.e., the critical grazing incident angle (αc = 0.18). An additional advantage of the low angle is that a long stripe of the sample surface is illuminated, which significantly increases measured signal levels in samples with very low surface coverage. The Xray absorption near edge structure (XANES) 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. The spectra of the Cu metal foil, Cu2O, CuO, and Cu(OH)2 bulk standards were collected at the 12-BM beamline of the Advanced Photon B

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Figure 1. In situ GIXAS measurements at Cu K edge of single-size copper catalysts supported on Al2O3 thin film: (a) Cu3/Al2O3, (b) Cu4/Al2O3, and (c) Cu20/Al2O3. For each sample, XANES spectra are plotted at four temperatures: 25, 75, 175, and 325 °C. (d) Reference XANES spectra of copper bulk standards: Cu metal, Cu2O, CuO, and Cu(OH)2 compounds.

band method (CI-NEB)52 and were confirmed with frequency calculations. The charges on Al2O3-supported Cu clusters were calculated using Bader charge analysis.53 The Al2O3-supported Cu clusters were constructed using the same approach as our previous studies.10

3. RESULTS AND DISCUSSION 3.1. Size Dependence in Cu Oxidation State. In situ GIXANES measurements were conducted to track the evolution of the oxidation state of atomically precise Cun/ Al2O3 catalysts (n = 3, 4, 20) at elevated temperatures under reaction conditions. The X-ray absorption near edge structure (XANES) spectra of the clusters collected at the Cu K edge are presented in Figure 1a−c. Figure 1d displays the XANES spectra of bulk Cu standards: Cu metal foil, Cu2O, CuO, and Cu(OH)2. The as-deposited metallic Cun clusters clearly undergo oxidation upon exposure of the samples to air. In a control experiment, no changes in GIXANES and GISAXS patterns were observed at room temperature when switching from a pure CO2 environment to an undiluted 1:3 CO2:H2 mixture and over 3 h long exposure to the reaction mixture, thus indicating that the clusters are stable at room temperature. Comparison of the GIXANES spectra collected on the cluster samples (Figure 1a−c) with the GIXANES spectra of bulk standards (Figure 1d) shows a close resemblance with the spectrum of CuO, indicating that the Cu in the clusters exhibit an average oxidation state of ∼+2. With increasing temperature under the reaction environment, a size-dependent reduction of copper with temperature was observed in the Cun/Al2O3 catalysts, identified by the emerging double-peak feature typical of the metallic state (Cu0). To demonstrate the evolution of the oxidation states of the copper catalysts quantitatively, linear combination fit (LCF) analysis was performed using Cu metal (Cu0), Cu2O (Cu+), CuO (Cu2+), and Cu(OH)2 (Cu2+) bulk standards. The analysis yields the relative fractions of the Cu standards for each Cun/Al2O3 catalyst as a function of reaction temperatures (See Figure S2 and typical fits in Figure S3), and the average Cu charge state can thus be calculated as shown in Figure 2. According to LCF analysis, at room temperature the investigated Cu clusters were present in an average charge state between +1.6e and +1.8e per Cu atom. The error in the average charge state was about 15%. (Please see Table S1 in the Supporting Information for more details.) Among all three sizes, Cu20 possesses the lowest Cu charge state of 1.6 at 25 °C. Heating to 75 °C induces a sharp drop in the charge state of copper in the Cu20/Al2O3 sample, and a

Figure 2. Average Cu charge state of single-size copper catalysts with increasing reaction temperatures: (a) Cu3/Al2O3, (b) Cu4/Al2O3, and (c) Cu20/Al2O3. The average charge state is calculated based on results of the linear combination fit (LCF) using Cu metal (charge state 0), Cu2O (charge state +1), and CuO (charge state +2) bulk standards. Curve b was adapted from the Cu4 XANES data in ref 10.

stable charge state of about +0.1 is achieved above 125 °C. Cu4 possesses a slightly higher oxidation state of 1.7 at 25 °C. A stable charge state of copper was achieved in the Cu4/Al2O3 sample above 125 °C, with an average charge of around +0.1, similar to the Cu20/Al2O3 sample. Among the studied cluster sizes, Cu3 possesses the highest charge state of ∼1.8 at 25 °C prior to reduction. Reduction under the reaction atmosphere above 175 °C results in a stable charge state of ∼+ 0.1 in the Cu3/Al2O3 sample. Here, we also note the presence of copper hydroxide at room temperature, particularly in the case of Cu3 cluster (Figure S2) which is practically absent in Cu20. The presence of Cu(OH)2 is consistent with our previous observations in ultrasmall Cu clusters after exposure to air.54 A clear size dependence for the reduction of Cun clusters under reaction atmosphere can thus be identified, as shown in Figure 2 with the reduction temperature decreasing with increasing cluster size: Cu3/Al2O3 > Cu4/Al2O3 > Cu20/Al2O3. The Cu clusters, reduced under reaction conditions, possess a low average charge state of about +0.1, according to LCF analysis. This finding is in good agreement with other available in situ XAS data from literature19,55,56 that metallic Cu is the most abundant copper species under reducing and reaction environment. The small positive charge is attributed to the cluster−support interaction. A similar metal−support interaction of copper on silica has been reported by Xu et al.57 indicating that a small fraction of oxidized copper species can C

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which may indicate higher activation energy for the trimer, stronger binding of reaction intermediates and/or products, or the combination thereof. In order to visualize trends, estimated activation energies were calculated based on the TORs shown in Figure 3, while taking into consideration the lowest two or three temperature points in order to minimize the possible contribution of rWGS at higher temperatures and yielded 0.51, 0.35, and 0.35 eV for the Cu3, Cu4, and Cu20 clusters, respectively. While size effects in catalysis14,15 have been well documented for Cu nanoparticles, size effects in supported ultrasmall Cu clusters with only a few atoms have not yet been investigated. Particularly for atmospheric-pressure CO2 hydrogenation, our previous experimental and computational results have demonstrated that the atomic-sized Cu4 clusters exhibit lower activation barrier for methanol synthesis in contrast to nonselected Cu nanoparticle and bulk Cu(111) surface.10,60 The undercoordinated sites in ultrasmall clusters can enhance CO2 and hydrogen adsorption at low pressures and thus facilitate CO2 activation to form CH3OH. However, in the case of Cu3/Al2O3, the trend seems to be reversed. This stark difference in activity may be a result of a slightly higher oxidation state of Cu3 under reaction conditions from among the three investigated cluster sizes, likely due to charge transfer from the support, which will be further discussed in the next section. We also note that with further increase in temperature (>275 °C), methanol activity of all three Cun catalysts starts to decline. This is because of the exothermic nature of methanol synthesis from CO2, which is not thermodynamically favorable in the higher temperature regime. Instead, undesired CO and H2O products may occur via endothermic reverse water−gas shift reaction (rWGS, CO2 + H2→ CO + H2O), consistent with the literature12,13 indicating that at higher temperatures copper promotes rWGS instead of methanol synthesis. With our current experimental setup, a clear assignment of CO (m/z = 28) in mass spectrometry is not straightforward due to its overlap with fragment ions of CO 2 present at high concentrations in the feedstock. However, the increasing water signal (m/z 18) observed at the highest temperatures when methanol production declines, as reported previously,10 can reflect the onset of another reaction, such as rWGS. 3.3. Cluster Size vs Support Effect. Density functional theory calculations were carried out for both gas-phase and supported Cu clusters to investigate the effect of cluster size and support in CO2 hydrogenation to CH3OH (Figure 4 and Figures S5, S6, and S7 in the Supporting Information). Our results for the gas-phase clusters showed that the apparent barriers and the hydrogen adsorption energies follow the same trend as a function of cluster size: Cu3 < Cu4 < Cu20, suggesting that in gas phase the activity of the Cu systems for CO2 → CH3OH is expected to increase as the cluster size goes down (see Figure S5 in the Supporting Information). This is due to the fact that the binding strengths between the intermediate adsorbates and the Cu cluster/surface decrease considerably as the cluster size increases, leading to less stable intermediate/ transition states, and thus the energetic pathways become higher lying (see Figure 4a for comparison between Cu3 and Cu4, and Figure S6a for comparison among Cu3, Cu4, and Cu20). We can see the same trend for the hydrogen adsorption reaction energies (Figure S5). Because the apparent barriers are closely correlated with the experimental activity for the CO2 hydrogenation to CH3OH, the catalyst activity is essentially

be stabilized by nonbridging oxygens on the support. Other Xray photoelectron spectroscopy studies have also revealed that surface hydroxyls on heavily hydroxylated α-Al2O3 (0001) can strongly promote the Cu+ formation.58 3.2. Size Dependence in Methanol Activity. The activity test was conducted under the same conditions as the in situ GIXANES measurements. The catalytic activity of methanol formation was monitored by mass spectrometer at m/z 31 (see Figure S3 in the Supporting Information). The calculated per Cu atom TORs are presented in a bar plot shown in Figure 3

Figure 3. Methanol activity on Cu3/Al2O3 (red) Cu4/Al2O3 (green), and Cu20/Al2O3 (blue) catalyst. The activity test was performed in a home-built flow reactor with 20 sccm flow of 1% CO2 and 3% H2 gas mixture carried in helium at pressure of 1.25 atm. The Cun catalysts were heated stepwise from 25 °C up to 425 °C under reaction conditions. The production rate of methanol (CH3OH) was monitored at m/z 31 using a mass spectrometer. The turnover rate (TOR) was calculated by dividing the production rate of methanol molecules per second over total number of copper atoms in the catalyst. Cu4/Al2O3 data were adapted from the previous activity data in ref 10.

within the temperature range from 125 to 325 °C. The Cu3/ Al2O3, Cu4/Al2O3, and Cu20/Al2O3 are represented as red, green, and blue bars, respectively. The onset of methanol activity occurs at 125 °C for Cu4 and Cu20. In the case of Cu3, it starts at much higher temperature around 225 °C. Recalling the reduction profile of all three copper clusters shown in Figure 2, we thus find that the onset activity for CH3OH synthesis correlates with the reduction temperature of Cun clusters. This suggests that methanol synthesis from CO2 is mainly catalyzed by the reduced phase of the copper catalyst. There is a debate in literature whether the metallic Cu0 or ionic form of Cu+ is the active site in methanol synthesis catalysts.59 Our results thus demonstrate a strong correlation between the onset activity and reduced Cu0 phase. The TOR of methanol also strongly varies as a function of cluster size. Cu4/Al2O3 shows the highest TOR of CH3OH among all three sizes, of 4.1 × 10−4 per second per total Cu atom at 225 °C. At the same temperature, the TOR for Cu20 and Cu3 is 1.7 × 10−4 and 0.5 × 10−4 per second per total Cu atom, respectively. Even if considering the maximal TOR for Cu3/Al2O3 at 275 °C, the methanol synthesis activity is still lower by more than 50% compared to catalysis with Cu3 which differs by a single atom. This indicates that methanol synthesis over ultrasmall Cu clusters is very sensitive to the cluster size and that the catalytic activity can be greatly modified by adding/removing only one metal atom to/from the clusters. Moreover, methanol production on Cu3 clusters sets off at higher temperatures in comparison with Cu4 and Cu20 clusters, D

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in charges between Cu3/Al2O3 and Cu4/Al2O3 helps to explain why Cu4/Al2O3 is more active than Cu3/Al2O3 for CO2 hydrogenation to CH3OH; the higher charge state of Cu3 on Al2O3 compared with Cu4 leads to weaker binding of the intermediates and thus a less favorable energetic pathway for CO2 hydrogenation to CH3OH.

4. CONCLUSIONS Methanol synthesis from CO2 over size-selected Cun catalysts supported on hydroxylated amorphous Al2O3 was investigated by in situ GIXANES and catalytic activity measurements under near-atmospheric pressure and density functional theory calculations. A very strong cluster size dependence of the catalytic activity was observed. The temperature at which the initially oxidized clusters reduce decreases with increasing cluster size: Cu3/Al2O3 > Cu4/Al2O3 > Cu20/Al2O3. The onset of methanol production correlates with the reduction of Cun clusters, suggesting that methanol synthesis from CO2 is catalyzed by the reduced phase of the copper catalyst. Per copper atom methanol activity varies with the cluster size in the order of Cu4/Al2O3 > Cu20/Al2O3 > Cu3/Al2O3. By removing a single atom from Cu4 to Cu3, the rate of CH3OH production drops dramatically, in excess of 50%. DFT calculations performed for both gas-phase and supported Cu clusters reveal that the activity of Cun clusters is determined by the combination of the cluster size and its charge state. The higher charge state of Cu in Cu3/Al2O3 compared with Cu4/Al2O3 leads to a weaker binding with intermediates, which is ultimately responsible for its lower per atom activity.

Figure 4. Calculated reaction pathways (including all the intermediate states) of gas-phase Cu3 and Cu4 clusters (a) vs hydroxylated Al2O3 supported Cu3 and Cu4 clusters (b).

correlated with the binding strengths between the intermediate adsorbates and the Cu cluster/surface.9,10 The decreased binding strengths with the increased cluster size are attributed to the decreased fraction of undercoordinated Cu atoms in the Cu system.10 It is worth noting that for Cu3, Cu4, and Cu20, the apparent barrier is from the initial binding of CO2, H* + CO2 → HCOO* (Figure 4a), suggesting that the binding of CO2 is a critical step for CO2 hydrogenation on small Cu clusters. Although the cluster size effect showed that in the gas phase the smaller the Cu cluster the more active it is expected to be for CO2 hydrogenation to CH3OH, the charge transfer from the support plays a significant role in the catalyst activity of the supported small Cu clusters (i.e., Cu3/Al2O3 and Cu4/Al2O3). Figure 4b shows the support effect of hydroxylated Al2O3 on Cu3 and Cu4 clusters for CO2 hydrogenation to CH3OH. As explained in the previous paragraph, Figure 4a shows that the gas-phase Cu4 has a higher-lying energetic pathway than Cu3 (gas-phase) due to the reduced binding strengths with the intermediate adsorbates; all the intermediate states of the Cu4 reaction pathway are relatively less stable relative to the initial state compared with the Cu3 (gas-phase) pathway. This suggests that the Cu3 pathway is energetically more favorable than Cu4 for the gas phase. However, Al2O3-supported Cu3 and Cu4 clusters have a strikingly opposite order (Figure 4b); the energetic pathway of Cu3/Al2O3 lies higher than that of Cu4/ Al2O3, indicating that Cu3/Al2O3 is less active than Cu4/Al2O3, which agrees with the experimental results. This difference between gas-phase and supported Cu clusters is due to the charge-transfer effect from the hydroxylated Al2O3 support and is confirmed by the changes observed for the estimated activation energies. Our Bader charge analysis showed that the average charge on the Cu atom of Cu3/Al2O3 is +0.20 |e|, slightly higher than that of Cu4/Al2O3 (+0.16 |e|). This charge is due to the interaction between the Cu cluster and the hydroxyl and the oxygen groups on the Al2O3.10 This difference

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b01835. Linear combination fits of XANES spectra; catalytic activity measurement by mass spectrometry of reaction products; DFT barriers, energies, and Bader charges for gas-phase and supported clusters (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (P.Z.). *E-mail: [email protected] (L.A.C.). *E-mail: [email protected] (S.V.). ORCID

Alex B. F. Martinson: 0000-0003-3916-1672 Peter Zapol: 0000-0003-0570-9169 Larry A. Curtiss: 0000-0001-8855-8006 Stefan Vajda: 0000-0002-1879-2099 Author Contributions #

B.Y. and C.L. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the U.S. Department of Energy, Office of Science, BES-Materials Science and Engineering, under Contract DE-AC-02-06CH11357, with UChicago Argonne, LLC, the operator of Argonne National Laboratory. C.L. acknowledges support from the U.S. Department of Energy, Office of Science, BES-Chemical Sciences, GeoE

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The Journal of Physical Chemistry C

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sciences, and Biosciences, under Contract DE-AC-0206CH11357. The use of the 12-ID-C and 12-BM beamlines of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC0206CH11357. The authors thank Dr. Sungsik Lee for the measurement of the XANES of the Cu bulk standards in transmission mode at the 12-BM beamline.

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DOI: 10.1021/acs.jpcc.7b01835 J. Phys. Chem. C XXXX, XXX, XXX−XXX