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Hydrogenation of CO on ZnO/Cu(100) and ZnO/Cu(111) Catalysts: Role of Copper Structure and Metal-Oxide Interface in Methanol Synthesis Robert M. Palomino, Pedro J Ramirez, Zongyuan Liu, Rebecca Hamlyn, Iradwikanari Waluyo, Mausumi Mahapatra, Ivan Orozco, Adrian Hunt, Juan Pablo Simonovis, Sanjaya D. Senanayake, and Jose A. Rodriguez J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b06901 • Publication Date (Web): 21 Aug 2017 Downloaded from http://pubs.acs.org on August 25, 2017

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The Journal of Physical Chemistry

Hydrogenation of CO2 on ZnO/Cu(100) and ZnO/Cu(111) Catalysts: Role of Copper Structure and Metal-Oxide Interface in Methanol Synthesis

Robert M. Palomino,1 Pedro J. Ramírez,2,+ Zongyuan Liu,1 Rebecca Hamlyn,1 Iradwikanari Waluyo,3 Mausumi Mahapatra,1 Ivan Orozco,1 Adrian Hunt,3 Juan P. Simonovis,3 Sanjaya D. Senanayake,1 and José A. Rodriguez1*

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Chemistry Division, Brookhaven National Laboratory, Upton, NY 11973, USA

Facultad de Ciencias, Universidad Central de Venezuela, Caracas 1020-A, Venezuela

Photon Sciences Division, National Synchrotron Light Source II, Brookhaven National Laboratory, Upton, NY 11973, USA

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Current address: Zoneca-CENEX, R&D Laboratories, Alta Vista, 64770 Monterrey, Mexico. *Corresponding Author: [email protected]

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ABSTRACT The results of kinetic tests and ambient-pressure X-ray photoelectron spectroscopy (APXPS) show the important role played by a ZnO-copper interface in the generation of CO and the synthesis of methanol from CO2 hydrogenation. The deposition of nanoparticles of ZnO on Cu(100) and Cu(111), θoxi < 0.3 monolayer, produces highly active catalysts. The catalytic activity of these systems increases in the sequence: Cu(111) < Cu(100) < ZnO/Cu(111) < ZnO/Cu(100). The structure of the copper substrate influences the catalytic performance of a ZnO-copper interface. Furthermore, size and metal-oxide interactions affect the chemical and catalytic properties of the oxide making the supported nanoparticles different from bulk ZnO. The formation of a ZnO-copper interface favors the binding and conversion of CO2 into a formate intermediate that is stable on the catalyst surface up to temperatures above 500 K. Alloys of Zn with Cu(111) and Cu(100) were not stable at the elevated temperatures (500-600 K) used for the CO2 hydrogenation reaction. Reaction with CO2 oxidized the zinc enhancing its stability over the copper substrates.


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INTRODUCTION The conversion of CO2 to methanol or higher alcohols is an attractive approach for transforming a greenhouse gas present in the atmosphere into valuable chemicals. 1,2,3 The increasing levels of CO2 in the air and oceans are a serious concern for the future of humanity on our planet. 4 As a result there are major environmental and economic incentives to minimize the production of CO2 and finding methods to capture and use this molecule as chemical feedstock, rather than simply releasing it into the atmosphere.1-3 The most common catalyst used in the industry for the CO2 → CH3OH conversion consists of a mixture of copper and zinc oxide. 5 Recent studies utilizing high-resolution transmission electron microscopy (HRTEM) have observed the presence of ZnO aggregates on top of the copper particles typical of a Cu/ZnO catalyst active for methanol synthesis. 6,7,8 Furthermore, the deposition of ZnO nanostructures on Cu(111) produces an inverse oxide/metal catalyst with a catalytic activity 10-20 times larger than that of plain copper. 9 Synergistic effects between Cu and ZnO are thought to be responsible for the catalytic activity of Cu-ZnO.6-9 In addition, the ZnO aggregates dispersed on the copper could have special physical and chemical properties not seen for the bulk oxide.6,10 Copper is an essential element in methanol synthesis catalysts.5,9 The results of theoretical calculations indicate that CO2 interacts poorly with extended surfaces and nanoparticles of pure copper. 11,12 The Salmeron group has performed detailed studies for the interaction of CO2 with Cu(111) and Cu(100) surfaces using ambient-pressure X-ray photoelectron spectroscopy (APXPS) and high-pressure scanning tunneling miscroscopy (HP-STM). 13 It was found that the (100) face is more active for the dissociation of CO2 than the (111) face. On Cu(111), the results of APXPS showed a very small coverage of atomic oxygen (0.03 monolayer) and an adsorbed CO2δspecies (0.09 monolayer) for a CO2 pressure of 300 mTorr at 300K. Under similar conditions, a 3 ACS Paragon Plus Environment


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substantial coverage of atomic oxygen (0.34 monolayer) was detected on Cu(100) without any CO2δ- adsorbed. Images of HP-STM show that the (100) surface breaks up into nanoclusters in the presence of CO2 at 20 Torr and above, producing active kink and step sites.13 On the basis of these results, we decided to investigate the transformation of CO2 into methanol on Cu(100) and ZnO/Cu(100) surfaces. From previous works, it is known that the rate for the CO2 → CH3OH conversion varies when changing the structure or morphology of pure copper surfaces. 14,15,16 It is important to stablish the effect of the copper structure on the performance of inverse ZnO/Cu catalysts. Can the morphology of copper enhance oxide-metal interactions and affect the performance of the interface? A comparison of the behavior of ZnO/Cu(111) and ZnO/Cu(100) surfaces should clarify this point. In a previous study we reported the catalytic behavior of the ZnO/Cu(111) system.9 Here we focus on the ZnO/Cu(100) system showing its superior performance. This paper is organized as follows. In the next section we describe the experimental tools used and the methodology followed for the synthesis of the ZnO/Cu inverse catalysts. Then, we show kinetic data comparing the catalytic activities of ZnO/Cu(111) and ZnO/Cu(100) substrates. Finally, towards the end of the article, we describe experiments that use AP-XPS to study the surface chemistry for the transformation of CO2 into methanol and the reverse water-gas shift reaction (RWGS, CO2 + H2O → CO + H2O) on ZnO/Cu catalysts.

EXPERIMENTAL METHODS The catalyst systems were studied in a set-up that combines a ultra-high Vacuum (UHV) chamber for surface characterization (base pressure ~5×10-10 Torr) and a batch reactor for catalytic tests.9,11,17 The sample could be transferred between the reactor and the UHV chamber without 4 ACS Paragon Plus Environment

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exposure to air. The UHV chamber was equipped with instrumentation for XPS, ultraviolet photoelectron spectroscopy (UPS), low-energy electron diffraction (LEED), ion-scattering spectroscopy (ISS), and thermal-desorption mass spectroscopy (TDS).9,11,17 In the studies of CO2 hydrogenation, the sample was transferred to the reactor at ~ 300 K, then the reactant gases, 0.049 MPa (0.5 atm) of CO2 and 0.441 MPa (4.5 atm) of H2, were introduced and the sample was rapidly heated to the reaction temperature (500, 525, 550, 575 and 600 K).9,11,17 Product yields were analyzed by a mass spectrometer and/or a gas chromatograph.14 The amount of molecules (CO or CH3OH) produced in the catalytic tests was normalized by the active area exposed by the sample and the total reaction time. In the present experiments, data were collected at intervals of 15 min up to total reaction times of 270 min. The kinetic experiments were done in the limit of low conversion (< 5%). Most ambient AP-XPS measurements were conducted at the National Synchrotron Light Source II at Brookhaven National Laboratory (BNL) on beamline 23-ID-2 (CSX-2). 18 The sample preparation was performed in a preparation chamber connected to the analysis chamber. The analysis chamber is equipped with a SPECS Phoibos 150 NAP analyzer. A full description of the beamline, beam characteristics and end station configuration can be found elsewhere.18 The C 1s and O 1s XPS were collected with a photon energy of 750 eV, while the Cu and Zn 2p XPS and the Zn LMM Auger were probed with a photon energy of 1200 eV, and a resolution of 0.2–0.3 eV. Binding energy shifts are referenced to the Cu 3p of metallic Cu. The preparation of the ZnO/Cu(111) and ZnO/Cu(100) catalysts for the kinetic studies was performed by the deposition of zinc metal in an ambient of O2 (5x10-7Torr) onto the clean copper substrates at 600 K. To ensure full oxidation of the zinc, the samples were exposed to 1 Torr of O2 at 600 K in a reaction cell. This led to the formation of ZnO/CuOx/Cu(111) and ZnO/CuOx/Cu(100) 5 ACS Paragon Plus Environment


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surfaces which rapidly transformed into ZnO/Cu(111) and ZnO/Cu(100) after exposure to molecular hydrogen at 350-400 K. ISS and XPS were used to determine the coverage of ZnO on the copper substrates after reducing the ZnO/CuOx/Cu(111) and ZnO/CuOx/Cu(100) systems with hydrogen. Prior to Zn deposition, the Cu(111) and Cu(100) crystals were cleaned by repeated Ar+ sputter (1kV, 300 K) and anneal (900 K) cycles. RESULTS AND DISCUSSION A. Stability of Zn and ZnO overlayers on Cu(111) and Cu(100) Theoretical calculations and some experimental studies predict an enhancement in the chemical activity of copper upon the formation of Cu-Zn alloys. 19,20,21 We tried to generate CuZn alloys by vapor-depositing Zn metal on Cu(111) and Cu(100) surfaces at room temperature. Studies with XPS revealed that the Zn overlayers were unstable and diffused into the bulk of the samples upon heating to temperatures above 400 K (Figure 1A); a phenomenon also seen in previous studies. 22 At the temperatures typically used for the conversion of CO2 into methanol (450-600 K), we were not able to keep a significant amount of Zn on the Cu(111) and Cu(100) surfaces under ultra-high vacuum (UHV) conditions. Thus, it was impossible to test in a systematic way the catalytic activity of Zn/Cu(111) and Zn/Cu(100) surfaces. To keep the Zn near the surface, one must bind it to O, CO2 or HCOO oxidizing it. Salmeron and co-workers have reported the deposition of O atoms on Cu(100) as a consequence of CO2 dissociation.13 We found that these oxygen adatoms can be used to oxidize and keep Zn on top of Cu(100) and Cu(111) surfaces. Zn LMM Auger spectra collected under 5 mTorr of CO2 (Figure 1B) showed the transformation of Zn into ZnO.22,23 The larger the pressure of CO2 or the sample temperature, the faster the Zn → ZnO transformation. Figure 1A compares results of XPS obtained after annealing a plain


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Zn/Cu(100) surface and a surface exposed to 0.5 atm of CO2 at 300 K. In the case of the plain Zn/Cu(100) surface, there is a continuous decrease of the Zn 2p3/2 signal when the sample temperature is increased from 300 to 600 K. On the other hand, for the surface exposed to CO2, there is initially a decrease in the Zn 2p3/2 signal as a result of the formation of Zn-O bonds, but after that the signal remains essentially constant at temperatures between 300 and 600 K. In a set of experiments, we explored the formation of ZnO as an approach to stabilize zinc on the copper surface. Zinc was vapor deposited onto Cu(111) and Cu(100) under an atmosphere of O2. It is known that the pressure of O2 and temperature of the sample during the generation of the oxide film can have an important effect in the stability of ZnO overlayers on copper surfaces.22 Figure 2 shows Zn LMM Auger spectra collected after generating a ZnO/CuOx/Cu(111) surface by deposition of Zn metal at 600 K under an O2 pressure of 5 x 10-7 Torr. The line-shape of the Zn LMM Auger is typical of a ZnO compound and very different from that of metallic Zn. 23 The ZnO overlayer in Figure 2 (θZnO ~ 0.2 ML) did not reduce under a pressure of 50 mTorr of H2 at 600 K. In contrast, when the deposition of the zinc was done at 300 K plus subsequent heating in O2 to 600 K, the formed ZnO overlayer was not stable enough and reduced under a pressure of H2 with the zinc disappearing from the Cu(111) surface. The surfaces used in the kinetic tests described in the next section were prepared by depositing zinc metal in an ambient of O2 (5x107

Torr) onto the clean copper substrates at 600 K. To ensure full oxidation of the zinc, the samples

were exposed to 1 Torr of O2 at 600 K in a reaction cell. These samples, in general, were stable under an environment of CO2/H2 at elevated temperatures (500-600K). After reaction, XPS spectra indicated a small decrease in the intensity of the Zn 2p and Zn LMM features that could be a consequence of a minor ZnO→Zn reduction with Zn migrating deep into the bulk of Cu(111) and Cu(100). 7 ACS Paragon Plus Environment


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B. Catalytic activity of ZnO/Cu(111) and ZnO/Cu(100) for CO2 hydrogenation. In our studies, we found that plain Cu(111) and Cu(100) displayed a catalytic activity for CO2 hydrogenation smaller than that reported for Cu(110) under similar conditions.14 In all these copper surfaces, the rate for the production of CO through the RWGS reaction was 2-3 orders of magnitude larger than that seen for methanol synthesis. For the production of methanol at 550 K, we found turnover frequencies (TOFs) of 1.7 x 10-3 and 5.1 x 10-3 molecules/Cu atom sec over Cu(111) and Cu(100), respectively. A TOF of 8 x 10-3 molecules/Cu atom sec has been reported for Cu(110) at 530 K.14 Our estimated apparent activation energies for methanol synthesis changed from 25 kcal/mol on Cu(111) to 19 kcal/mol on Cu(100). Apparent activation energies in the range between 16 to 18 kcal/mol have been measured for methanol synthesis on Cu(100).15,16 All of these are larger than the value of 15 kcal/mol seen on Cu(110).14 In a previous article, we reported the positive effect of ZnO on the catalytic activity for CO2 hydrogenation of Cu(111).9 A much larger effect was found on Cu(100), see Figure 3. Thus, the structure of the copper substrate affects the performance of the oxide-metal interface. But on both surfaces, Cu(100) and Cu(111), the highest activity is seen at a relatively small coverage (0.15-0.2 ML) of ZnO. At these coverages, one probably has small mono– and bi-layer structures of ZnO on top of the copper substrates.22 The reactivity of ZnO in these small islands is enhanced due to their size and interactions with the metal underneath. Figure 4 displays Arrhenius plots for the synthesis of methanol on ZnO/Cu(111) and ZnO/Cu(100). In the range of temperatures investigated (500-600 K), ZnO/Cu(100) is the best catalyst for the conversion of CO2 to methanol. The apparent activation energy for the reaction decreases from 19 kcal/mol on Cu(100) to 11 kcal/mol on ZnO/Cu(100). On ZnO/Cu(100) the rate


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for the production of methanol is 15-45 times larger than on Cu(100). An enhancement of 18-80 times was observed when going from Cu(111) to ZnO/Cu(111) in our previous study for these two systems.9 As mentioned above, the main product for the hydrogenation of CO2 on the ZnO-Cu catalysts is CO. Figure 5 displays Arrhenius plots for the RWGS reaction on ZnO/Cu(111) and ZnO/Cu(100). Again ZnO/Cu(100) is the best catalyst in the range of temperatures investigated (500-600 K). The formation of a ZnO-Cu interface leads to a drop of 6-7 kcal/mol in the apparent activation energy for the RWGS. In Figures 4 and 5, the apparent activation energies for methanol synthesis and the RWGS on a given catalyst are similar. For example, on ZnO/Cu(100), both reactions exhibit an apparent activation energy of 11 kcal/mol but the rate for CO production is orders of magnitude larger. This phenomenon was first seen in studies for the hydrogenation of CO2 on Cu(110) and has also been observed on Cu/oxide catalysts.14,17 It suggests that both chemical processes share a key intermediate or reaction step,14 but one can have catalysts on which this is not the case.7,8,16 An important issue is the stability of the ZnO/Cu(111) and ZnO/Cu(100) catalysts under reaction conditions. The oxide overlayers used in the kinetic experiments of Figures 3-5 were prepared aiming for high temperature stability (see previous section). After evacuating the gases from the batch-cell at the studied reaction temperatures (500-600 K), we saw no indication for a clear reduction of ZnO in Zn 2p XPS and Zn LMM Auger spectra. A summary of these results is displayed in Figure 6. As shown in Figure 1A, metallic Zn is not stable on/in copper surfaces and migrates towards the bulk of the sample. We cannot rule out that part of the ZnO initially present in the catalysts was reduced with the Zn migrating deep into the copper substrates. In our XPS and



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Auger measurements we saw a small attenuation (10-15%) in the signal for ZnO that could be a consequence of this process or a result of the sintering of the oxide overlayer. A comparison of the catalytic activity reported in Figure 3 for ZnO/Cu(100) and ZnO/Cu(111) with that reported for the Cu/ZnO(000ī) system in the literature11 indicates that the oxide/metal configuration is by far a superior catalyst, Figure 7. Thus, it is not surprising that this is the active configuration in the industrial Cu/ZnO/Al2O3 powder catalyst.6 The deposition of nanoparticles of an oxide on a metal usually produces stronger metal-oxide interactions than observed after depositing nanoparticles of a metal on a bulk oxide due to the intrinsically low reactivity of bulk oxides where even wetting of many metals is problematic. 24,25 The strong bonding existing in oxide/metal configurations can also lead to perturbations or modifications in the electronic properties of the oxide giving novel chemical properties. 26 Thus, the ZnO clusters on top of copper are special entities. C. Surface chemistry for CO2 hydrogenation. Salmeron and co-workers have made major contributions to recent developments in APXPS. 27 The technique has evolved to become a very valuable tool to examine the surface chemistry associated with catalytic reactions. 28 The technique usually operates at pressures of 0.5-10 Torr.27 At these pressures, the production of CO through the RWGS reaction occurs in many metal and metal-oxide catalysts. We used AP-XPS to study the hydrogenation of CO2 on Cu(111) and ZnO/Cu(111) catalysts. In Figures 4 and 5, Cu(111) is the worst catalyst and undergoes the largest increase in activity upon the deposition of ZnO. Figure 8 shows O 1s and C 1s XPS collected while a CuOx/Cu(111) surface was exposed to 500 mTorr of H2 and 100 mTorr of CO2. The film of CuOx was produced by oxidizing Cu(111) in O2 and essentially consisted of layers of Cu2O with oxygen


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vacancies. 29 At 300 K, the O 1s region is dominated by the signal from the oxide film with a weak feature that can be attributed to adsorbed CO2δ-.17,30,31 A trace for this species is seen in the C 1s region and there is a small amount of surface carbon (< 0.15 ML) that in part could be coming from the dissociation of CO2 on oxygen vacancies of the copper oxide film. Heating to 400 K induces the complete reduction of the CuOx by the H2 in the background. At this temperature is clear the presence of an adsorbed species which probably corresponds to CO2δ-.17,30 This adsorbate essentially disappears from the copper surface upon heating to 500 and 575 K. At 575 K, a feature appears in the C 1s region at 291.5 eV that corresponds to CO gas produced by the RWGS (Figure 5). It is important to notice in Figure 8 the lack of peaks for adsorbed formate which is produced after exposing copper surfaces to high pressures (5-20 atm) of CO2/H2.16-20,32 The species can be a reaction intermediate or an spectator depending on its bonding configuration on the surface.17,32,33 AP-XPS showed clear differences in the surface chemistry for CO2 hydrogenation on Cu(111) and ZnO/Cu(111).

Figure 9 shows O 1s and C 1s XPS recorded while a

ZnO/CuOx/Cu(111) surface was exposed to 500 mTorr of H2 and 100 mTorr of CO2. In this case, we were working with a ZnO film produced by deposition of Zn in O2 at 300 K with subsequent heating to 600 K. The ZnO overlayer covered ~ 25% of the copper oxide substrate. Initially, the O 1s XPS region is dominated by the signal for ZnO/CuO. After introducing the reactant gases in the photoemission chamber at 300 K one sees features for carbon and a CO2δ- species.17,30 Heating to 400 K induces the reduction of CuOx to metallic Cu and a strong signal for formate appears.30 Some authors assign to this species a predominant role in the synthesis of methanol from CO2.11,12,19,21 Heating from 400 to 575 K induces a progressive reduction in the intensity of the formate peak. At 575 K, the peak has disappeared. The ZnO film used for the experiment in Figure 9 had a limited stability. Above 500 K it fell apart and no signal for ZnO or Zn was detected in the 11 ACS Paragon Plus Environment


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Zn 2p XPS and the Zn LMM Auger regions. We also did experiments with ZnO films that had a high stability (Figs. 2-5) and detected formate as a high coverage adsorbate. A comparison of the trends in Figures 8 and 9 show the important role played by the ZnO-Cu interface: It accelerates the binding and transformation of CO2. The CO2 → HCOO transformation is now seen at pressures in the mTorr range.

CONCLUSIONS In summary, the results of kinetic tests and AP-XPS show the important role played by a ZnO-copper interface in the synthesis of methanol from CO2 hydrogenation. Size and metal-oxide interactions affect the chemical and catalytic properties of the oxide making the supported nanoparticles different from bulk ZnO. The formation of a ZnO-copper interface favors the binding and conversion of CO2 into a formate intermediate that is stable on the catalyst surface up to temperatures above 500 K. When compared to ZnO/Cu(111), ZnO/Cu(100) is a much better catalyst for methanol synthesis and the reverse-water gas shift reaction. Thus, the structure of the copper substrate influences the catalytic performance of a ZnO-copper interface. Alloys of Zn with Cu(111) and Cu(100) were not stable at the elevated temperatures (500-600 K) used for the CO2 hydrogenation reaction. Reaction with CO2 oxidized the zinc enhancing its stability over the copper substrates.

AUTHOR INFORMATION Corresponding author *E-mail: [email protected] 12 ACS Paragon Plus Environment

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Notes The authors declare no competing financial interest

ACKNOWELDGEMENTS The research carried out in this manuscript performed at Brookhaven National Laboratory, was supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, and Catalysis Science Program under contract No. DE-SC0012704. These studies used resources of the 23-ID-2 beamline of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704.

Figure Captions Figure 1 (Part A) Black dots: Variation of the Zn 2p3/2 XPS signal for a plain Zn/Cu(111) surface, θZn= 0.21 monolayer (ML), heated from 300 to 600 K under UHV conditions. Red dots: Corresponding variation of the Zn 2p3/2 XPS signal of a Zn/Cu(111) surface, θZn= 0.23 ML, exposed to 0.5 atm of CO2 at 300 K in a reaction cell. (Part B) Zn LMM Auger spectra collected after depositing Zn on Cu(111) and under exposure to 5 mTorr of CO2 at 300 and 575 K. Figure 2 Zn LMM Auger spectra collected before and during exposure of a ZnO/CuOx/Cu(111) surface to 50 mTorr of H2 at 600 K. The ZnO overlayer was prepared by depositing Zn on Cu(111) at 600 K under an O2 pressure of 5 x 10-7 Torr. For comparison we also include a Zn LMM Auger spectra for pure metallic zinc. Figure 3 Rate for the conversion of CO2 to methanol on Cu(100) and Cu(111) as a function of the fraction of the metal surface covered by zinc oxide. Reaction conditions: PH2= 4.5 atm, PCO2= 0.5 atm, T= 550 K. Figure 4 Arrhenius plots for the conversion of CO2 to methanol on plain Cu(111) and Cu(100) and on metal surfaces covered ~ 20% by nanoparticles of ZnO. Reaction conditions: PH2= 4.5 atm, PCO2= 0.5 atm. Figure 5 Arrhenius plots for the production of CO through the reverse water-gas shift reaction on plain Cu(111) and Cu(100) and on metal surfaces covered ~ 20% by nanoparticles of ZnO. Reaction conditions: PH2= 4.5 atm, PCO2= 0.5 atm. 13 ACS Paragon Plus Environment


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Figure 6 Zn 2p3/2 XPS position measured after reaction on ZnO/Cu(111) and ZnO/Cu(100) surfaces. Reaction parameters: T= 550 K, PH2= 4.5 atm, PCO2= 0.5 atm. The gases were evacuated from the reactor at 550 K and then XPS spectra were collected. For comparison we also include the corresponding binding energies for metallic zinc and zinc oxide. Figure 7 Rates measured for the production of methanol on Cu(111),9 Cu(100), ZnO/Cu(111),9 ZnO/Cu(100) and Cu/ZnO(000ī).11 Reaction conditions: T= 550 K, PH2= 4.5 atm, PCO2= 0.5 atm. Figure 8 O 1s and C 1s XPS spectra of a CuOx/Cu(111) surface under 100 mTorr of CO2 and 500 mTorr of H2 at 300-575 K. Figure 9 O 1s and C 1s XPS spectra of a ZnO/CuOx/Cu(111) surface under 100 mTorr of CO2 and 500 mTorr of H2 at 300-575 K. The ZnO covered ~ 25% of the copper substrate.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1


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Page 19 of 27

Zn LMM

Fresh ZnO/Cu(111) After H2 pretreatment

Normalized Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Metallic Zn LMM reference

280

275

270

265

260

Binding Energy / eV

Figure 2


255

250


0.35

M ethanol molecules produced / 1015 molecules cm-2 s-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

On ZnO/Cu

0.30 0.25

Cu(100) 0.20 0.15 0.10 0.05

Cu(111)

0.00 0.0

0.2

0.4

0.6

0.8

1.0

Fraction of copper covered Figure 3


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CH3OH synthesis

0

s )}

ZnO/Cu(100)

-2 -1

Ln{rate/(1015 CH3OH molecules cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ea= 11 ± 2 kcal/mol -2

ZnO/Cu(111)

Ea= 16 ± 2 kcal/mol Cu(100) -4

Ea= 19 ± 2 kcal/mol

Cu(111)

-6

Ea= 25 ± 3 kcal/mol 1.65

1.70

1.75

1.80

1.85

1.90

1.95

2.00

1000 K/T

Figure 4



RWGS ZnO/Cu(100)

-2 -1

s )}

6

Ln{rate/(1015 CO molecules cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Ea= 11 ± 2 kcal/mol ZnO/Cu(111) 4

Ea= 15 ± 2 kcal/mol Cu(100) 2

Ea= 18 ± 2 kcal/mol Cu(111)

0

Ea= 22 ± 2 kcal/mol 1.65

1.70

1.75

1.80

1.85

1.90

1.95

2.00

1000 K/T

Figure 5


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Zn 2p3/2 peak position after reaction (eV)

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1021.8 2+

Zn , ZnO 1021.6

ZnO/Cu(100)

1021.4

ZnO/Cu(111)

1021.2

0

Zn , metal

1021.0 0.0

0.2

0.4

0.6

0.8

Fraction of copper surface covered Figure 6


1.0


0.35

550 K

ZnO-Cu

0.25

0.20

0.15

Cu/ ZnO

10

15

-2 -1 molecules cm s

0.30

CH3OH molecules produced/

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.10

0.05

0.00 Cu(111)

Cu(100)

ZnO/ ZnO/ Cu(111) Cu(100)

Cu/ ZnO(0001)

Figure 7


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 9


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC Graphic


Cu(111) Catalysts

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