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Highly Efficient and Selective Cu/MnOx Catalysts for Carbon Dioxide Reduction Haiwei Du, Yuan Wang, Tao Wan, Hamidreza Arandiyan, and Dewei Chu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00548 • Publication Date (Web): 21 Jun 2018 Downloaded from http://pubs.acs.org on June 24, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Applied Energy Materials

Highly Efficient and Selective Cu/MnOx Catalysts for Carbon Dioxide Reduction Haiwei Du,a,‡ Yuan Wang,b,‡ Tao Wan,a Hamidreza Arandiyanb, c,* and Dewei Chua,* a

School of Materials Science and Engineering, UNSW Sydney, NSW 2052, Australia

b

Particles and Catalysis Research Group, School of Chemical Engineering, UNSW Sydney, NSW

2052, Australia c

Laboratory of Advanced Catalysis for Sustainability, School of Chemistry, The University of Sydney,

Sydney 2006, Australia Corresponding author email: [email protected] (D. Chu) and [email protected] (H. Arandiyan) ‡

These authors contributed equally to this work.

Abstract Catalytic reduction of carbon dioxide (CO2) to chemical and energy feedstocks is of great importance to both environmental improvement and energy regeneration. Herein, a Cu decorated MnOx nanowires based heterogeneous catalyst was rationally designed via a facile hydrothermal method towards the optimization of their performance in CO2 reduction. The synthesised Cu/MnOx combines well dispersed Cu nanoparticles on MnOx nanowires, exhibiting nearly complete conversion efficiency and 100% selectivity towards reverse water gas shift to produce CO under mild reaction conditions (425 °C). It is found that Cu/MnOx with Cu/Mn molar ratio of 1:1 with a strong interaction between metal and support possessed rich surface oxygen vacancies and lattice strain, resulting in the optimal activity with CO2 conversion rate of 13.8 µmol gcat-1 s-1 and TOF of 4.32 mol molcat-1 h-1. The strategy to optimize the metal-support interaction in this work has opened a new avenue for the design of advanced noble-metal-free catalysts for more heterogeneous catalytic applications. Keywords: Heterogeneous catalyst; MnO2 nanowire; Copper; CO2 conversion; Reverse water-gas shift; CO selectivity

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The excessive carbon dioxide (CO2) emissions due to the reliance on fossil fuels have triggered severe climate changes and have been considered as a global environmental issue. Currently, a worldwide demand to maintain the sustainability of the earth is the conversion of CO2 to chemical feedstocks or fuels. There are three main types of critical technologies including thermochemical, electrochemical and photocatalytic reduction of CO2 which are developed as direct solutions for the capture and recycling of CO2.1 Especially, the commonly known reverse water–gas shift reaction (RWGS) (eq. 1) for catalytic reduction of CO2 to CO, is a very promising and desired route for industrial application. Generally, the direct route for converting CO2 into hydrocarbons is a two-step reaction: (i) the reduction of CO2 to CO via the RWGS reaction and (ii) conversion of CO to hydrocarbons via Fischer– Tropsch synthesis (FTS).2

CO + H ⇋ CO + H O

∆H298 K = 41.2 kJ/mol

(1)

Exploring highly efficient metal-based heterogeneous catalysts is implemented with the rapid progress of nanotechnology over the past decades. Among the candidates, Cu-based catalysts3 have been popularly employed on an industrial scale and the combination of Cu metal particles with metal oxide supports always plays a dominant role in developing novel alternatives. First, decorating metal nanoparticles on the metal oxide supports not only combines the respective physicochemical properties of the metal nanoparticles and supports but also facilitates the complementation between metal and support, which is the recognized strong metal-support interaction (SMSI).4 Second, the metal oxides in this strategy often act as the promoter to increase the surface area by avoiding the copper nanoparticle agglomeration and provide a better dispersion of copper species. Consequently, a higher catalytic activity is achieved as the hydrogen molecules can be dissociated easily at the highly

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dispersed metal sites to promote atomic hydrogen spillover and then hydrogenate CO2 on the supports.5 Third, abundant metal/support interfaces are also able to (i) provide more active sites, (ii) generate oxygen defects and (iii) induce lattice microstrain by the epitaxial bonding or electronic modification by the charge and/or mass transfer among metal and supports owing to the SMSI effect. Given that the catalytic activity is strongly influenced by the metal nanoparticles, searching catalyst supports for more uniform dispersion of metal nanoparticles and better contact between metal and support should be taken into consideration. Based on this viewpoint, onedimensional (1D) nanomaterials with tunable aspect ratios show a promoting effect on metal dispersion.6 Transition metal oxides with high electrical and thermal conductivity are considered as promising catalyst supports, however, sometimes the lower surface area limits the sufficient contact between metal and support, deferring the SMSI effects. Typically, as a common member of 1D nanomaterials, MnO2 nanowire is favourable for metal nanoparticle dispersion7 when using as the catalyst support. Manganese oxide itself as either loaded nanoparticles or supports also shows a promising activity for CO2 hydrogenation.8 In this work, Cu/MnOx nanowire heterogeneous catalysts were prepared via a two-step hydrothermal method. After a compositional optimization, CO2 conversion efficiency was enhanced significantly and all catalysts were highly selective to CO. The improvement of catalytic activity was attributed to the presence of metallic Cu nanoparticles, the increased amount of oxygen vacancies as well as a local structural strain induced by the strong metalsupport interaction.

Scheme 1 Schematic of synthesis of Cu/MnOx catalysts.

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Fig. 1 TEM images of the as-synthesized CM nanostructures: (a, b) CM-0, (c) CM-0.5, (d, e) CM-1 and (f) CM-2. HAADF-STEM-EDS mapping of CM-1 specimen: (g) SEM imaging, (h) STEM imaging, elemental mapping of (i) Mn, (j) O, (k) Cu, and (l) overlay mapping of Cu and Mn.

The synthesis process of CuO/MnO2 (CM) catalysts is depicted in Scheme 1. MnO2 nanowires were hydrothermally prepared as the template for the subsequent decorating CuO nanoparticles with a well dispersion. The designed molar ratio of Cu/Mn was 0, 0.5, 1 and 2, and the corresponding samples were named as CM-0, CM-0.5, CM-1 and CM-2, respectively. A further annealing in reducing atmosphere is necessary before evaluating the CO2 catalytic activity. The microstructures of the CM samples are observed by TEM. From Fig. 1a, the as4


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synthesized MnO2 with smooth surface shows uniform one-dimensional nanowire morphology. The diameter is 20~50 nm while the length of ultra-long MnO2 nanowires can be reached to more than 50 µm (Fig. S3a). A typical interplanar spacing of 0.498 nm corresponding to the (200) plane, is determined by the high resolution TEM (HRTEM) image (Fig. 1b). After the second hydrothermal step, CuO nanoparticles were successfully grown on the MnO2 nanowire supports. It is noted that the morphological evolution is strongly dependent on the concentration of copper precursor. As shown in Fig. 1c, CuO nanoparticles were randomly dispersed on the nanowire surface when the amount of copper acetate is relatively lower, resulting in a thin layer of CuO in homogeneously decorated on the MnO2 nanowires. However, after increasing the Cu/Mn ratio the CuO nanoparticles are almost fully decorated on MnO2 nanowires, forming a “core-shell” like heterogeneous nanostructure (Fig. 1d) and even a thicker shell layer in CM-2 sample (Fig. 1f). Fig. 1e reveals the CuO/MnO2 interface of CM-1 nanostructure and indicates the (111) plane of CuO. An additional evidence for the heterogeneous nanostructure is the elemental mapping. As shown in Fig. 1gl, MnO2 is confined in the core while CuO is homogeneously dispersed on the nanowire as the shell, indicating a uniform distribution. The average grain size of CuO nanoparticles obtained by the TEM images is around 6 nm (Fig. S2). Moreover, the morphology of assynthesised CM nanostructures is also observed by SEM (Fig. S3). The nanowire surface becomes coarse after CuO loading while the shortening in nanowire length is probably due to the “etching effect” by ethanol and acetate in the second hydrothermal treatment.

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Fig. 2 (a) XRD patterns, (b) H2-TPR profiles, (c) N2 adsorption-desorption isotherm and (d) the pore size distribution of as-synthesized CM nanostructures.

Fig. 2a shows the XRD patterns of as-synthesized CM nanostructures. It is seen that the assynthesized MnO2 is well indexed to tetragonal α-MnO2 (JCPDS 44-0141). The diffraction peaks corresponding to CuO (JCPDS 80-1916) start to appear and the peak intensity increases gradually with the increasing amount of copper precursor, indicating more CuO nanoparticles are formed. The as-grown CuO nanoparticles may have a smaller grain size, which is reflected by the broad diffraction peaks and matched with the TEM observation. The H2-TPR profiles were conducted to investigate the redox properties (Fig. 2b), and the quantitative analysis is shown in Table S1. MnO2 exhibits two reduction peaks at around 330 °C and 372 °C, respectively. The reduction process should take place at a temperature range and cannot be determined very accurately since the reduction of MnO2 to MnO may involve Mn2O3 and Mn3O4 as the intermediate.9 After decorating CuO nanoparticles, the reduction peaks shift towards lower temperature below 300 °C, suggesting that the MnO2 6


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reduction is promoted by the presence of Cu in the catalysts. This is because the metallic Cu from Cu2+ reduction acts as the H2 activation site to facilitate the reduction of Mn oxides with H2.10 It is reported that the former TPR peak corresponds to the highly dispersed CuO species.11 With the increasing Cu loading, the reduction peaks shifts towards low-temperature region, and the CM-1 with reduction peaks at 193 °C and 238 °C may have the optimal initial CuO dispersion and a better interaction with the metal oxide support.12 However with further increasing CuO, the reduction peaks of CM-2 shift a little back to higher temperature, implying a poor dispersion due to the loading saturation as the CuO nanoparticles have fully covered the MnO2 nanowire surface (Fig. 1f). The specific surface area and pore size distribution were obtained by N2 adsorption-desorption tests (Fig. 2c and 2d) as summarized in Table S2. From Table S2, the surface area for CM nanostructures has been improved by decorating CuO nanoparticles, increasing from 47.2 m2/g for the CM-0 to 98.9 m2/g for the CM-2 sample. The improvement in surface area is attributed to (i) the dispersed CuO nanoparticles on the nanowire surface and (ii) the “etching effect” during the second reaction step (Fig. S2). In contrast to the variation in surface area, the pore size (Table S2) decreases slightly first and then the change becomes significantly, due to the reduction in the nanoparticle gap by the overloading as the pores primarily derive from the spaces between CuO nanoparticles. The pore volume increases after decorating CuO nanoparticles first and keeps almost unchanged with further loading.

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Fig. 3 TEM images of the reduced CM catalysts: (a) rCM-0, (b) rCM-0.5, (c) rCM-1 and (d) rCM-2. The copper nanoparticles are marked with yellow dashed circles. HAADF-STEM-EDS mapping of the reduced CM-1 specimen: (e) SEM imaging, (f) STEM imaging, elemental mapping of (g) O, (h) Mn, (i) Cu and (j) overlay mapping of Cu and Mn. (k) The XRD patterns, (l) Mn 2p and (m) O 1s spectra from the XPS spectra of the reduced catalysts: (i) rCM-0, (ii) rCM-0.5, (iii) rCM-1 and (iv) rCM-2.

Before catalytic reaction, the CM catalysts were annealed in the reducing atmosphere to reduce the CuO to metallic Cu nanoparticles. Fig. S4 shows the obvious colour change for the Cu containing catalysts from dark to reddish after the H2 reduction. Fig. 3a-d shows the TEM images of reduced catalysts, where the microstructural change can be found after the annealing treatment. It is seen that the reduced MnOx support still shows nanowire 8


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morphology while an obvious deformation takes place. This phenomenon is probably attributed to two aspects: (i) sintering by thermal treatment and (ii) the phase transformation from tetragonal MnO2 to cubic MnO. Another distinct feature is that after reduction the Cu grains show a nanoparticle agglomeration and coalescence (Fig. 3b-d) in comparison to the well dispersed CuO nanoparticles in the as-synthesized samples. As shown in Fig. 3e-j, most of copper oxide nanoparticles have been transformed to copper clusters on the MnOx support. The particle agglomeration and coalescence which may be the co-results of thermal induced copper sintering and diffusion,13 is more visible in Fig. S5. Compared with the as-synthesized nanostructures, the grain size of aggregated Cu nanoparticles increases after the post annealing, ranging from 20 to 80 nm, and this increase in grain size is consistent with the XRD analysis (Table 1) regardless of the slight difference/error in characterisations. In addition, metallic Cu nanoparticles can be observed in an HRTEM image (Fig. S6) as two different fast Fourier transforms (FFT) indicate the co-existence of metallic Cu and CuO nanoparticles. The presence of CuO is due to the rapid oxidation of surface atoms. In spite of a small amount of residual KCl formed during the synthesis process of MnO2 nanowires, there are two crystal structure transformations after annealing, as shown in the XRD patterns of the reduced catalysts (Fig. 3k). On the one hand, the diffraction peaks for α-MnO2 disappear while the diffraction peaks for MnO (JCPDS 07-0230) can be seen. Since reduction from oxide to metallic phase is highly unlikely for manganese14 and no reduction peak was detected after 450 °C in the H2-TPR profiles (Fig. 2b), α-MnO2 has been probably transformed to MnO. On the other hand, two diffractions peaks with around 43° and 50° corresponding to Cu (111) and (200) (JCPDS 04-0836) are detected, and the peak intensity increases with the increasing copper concentration, as a result of reduction from CuO to metallic Cu0 phase. Moreover, compared with the broad peaks for CuO (Fig. 2a), the sharp Cu peaks with high intensity indicate the increase in grain size. Similar with other Cu/metal

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oxide catalysts,15 the larger grain size of Cu nanoparticles (Table 1) in the reduced CM nanostructures shows a grain growth during the further crystallization accompanied by the reduction process. The chemical bonding and oxidation states of the reduced CM nanostructure were examined by XPS spectra. Fig. 3l shows the comparison of Mn 2p signal in all reduced CM nanostructures. It is seen that Mn 2p3/2 and 2p1/2 for rCM-0 centered at 641.7 and 653.3 eV respectively, indicating the main oxidation state of Mn is +2.16 The separation of peak binding energies of Mn 3s (∆E3s) for rCM-0 is ~5.8 eV (Fig. S7), which is also consistent with the previous reported MnO nanoparticles.17 As shown in Fig. 3l, a Cu Auger peak starts to appear and becomes more obvious with the increasing amount of Cu. Moreover, after increasing Cu amount the binding energy of Mn 2p peaks slightly shift towards lower energies. This phenomenon is a result of an electronic structure modification due to the electron transfer18 from Cu to MnOx supports, indirectly indicating the SMSI effect. The O 1s spectra are shown in Fig. 3m in order to investigate the surface active oxygen species. The primary peaks can be split to three sub peaks: intrinsic lattice oxygen (Olatt, centred at ~529.7 eV), oxygen vacancies (Ov, centred at ~531.2 eV) and other species (centred at ~532.6 eV) which are derived from hydroxyl species, water or carbonates.19 Generally, the area ratio of Ov/Olatt is calculated to evaluate the proportion of oxygen defects as a higher ratio of Ov/Olatt indicates more active vacant oxygen species.20 As shown in Table 1, the area ratio of Ov/Olatt increases after loading Cu nanoparticles, reaching the maximum value of 1.4 when the nominal molar ratio of Cu/Mn is 1. The oxygen vacancies reflected by XPS analysis are consistent with previous H2-TPR results (Fig. 2b).

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Fig. 4 (a) CO2 conversion with the increasing temperature, (b) the conversion at 350 °C, (c) CO2 conversion rate and TOF, and (d) the CO selectivity of the reduced CM catalysts.

Scheme 2 Schematic of the promoted RWGS on Cu/MnOx catalysts.

Table 1 Summary of XRD and XPS analysis. Lattice parameter of the Samples

reduced catalysts (Å)

MnO

b

Cu

c

Strain (%) a

Grain size (nm)

Area ratio of Ov/Olatt in

MnO

Cu

dCuO

11


d

dCu

e

XPS results


a

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rCM-0

4.4456

—

0.013

—

—

—

0.626

rCM-0.5

4.4525

3.6239

0.168

0.246

12.6

49.1

1.167

rCM-1

4.4545

3.6248

0.213

0.271

11.0

54.8

1.400

rCM-2

4.4535

3.6233

0.191

0.229

12.5

55.2

0.904

Strain = (a-a0)/a0 × 100%, the theoretical lattice parameters (a0) of MnO (JCPDS 07-0230) and Cu (JCPDS 04-

0836) are 4.445 Å and 3.615 Å respectively. b

Determined by Rietveld refinement for MnO peaks.

c

Determined by Rietveld refinement for Cu peaks.

d

Calculated by CuO (111) peak in the XRD data of the as-synthesized CM catalysts.

e

Calculated by Cu (111) peak in the XRD data of the reduced CM catalysts.

It should be noted that the metallic Cu is easily to be oxidized under ambient condition (Fig. S6). Thus the reduction procedure for catalysts was just conducted in situ in the reactor before starting the activity test. Fig. 4a shows the catalytic activity in terms of CO2 conversion over the reduced CM catalysts as a function of temperature from 200 to 550 °C. The catalytic results indicate that decorating Cu on MnOx is conducive to improving the CO2 reduction as the temperature for nearly complete CO2 conversion has been significantly lowered by around 100 °C. Meanwhile, the conversion efficiency at 350 °C is selected to evaluate the activity (Fig. 4b) and the conversion efficiency follows the sequence: rCM-1 > rCM-0.5 > rCM-2 > rCM-0, suggesting that an optimal Cu loading is 50 mol% (Cu:Mn = 1:1). Fig. 4c shows the CO2 conversion rate and turnover frequency (TOF) of Cu-Mn samples at 350 °C. It is obvious that the addition of Cu on MnOx has enhanced the reactivity and the rCM-1 exhibits the highest values with CO2 conversion rate of 13.8 µmol gcat-1 s-1 and TOF of 4.32 mol molcat-1 h-1, which is about 7 times higher than that of MnOx (1.96 µmol gcat-1 s-1 and 0.61 mol molcat-1 h-1). For a comparison, a sample consisting of Cu decorated on commercial MnO2 powder was synthesised (Cu:Mn = 1:1) and the inferior activity (Fig. S8) indicates that MnO2 nanowire support is beneficial for higher catalytic efficiency. For the Cu-based catalysts, given that the CuO has been reduced to Cu before the catalytic reaction, the surface

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redox is a regenerative process since copper is successively oxidised and reduced by CO2 and H2. Thus the RWGS reaction takes place according to the following equation:21

CO g + 2Cu s ⇋ COg + Cu Os

H g + Cu Os ⇋ H Og + 2Cu s

The improvement of CO2 reduction activity in this work can be attributed to three aspects. First, the metallic Cu nanoparticles serve as the active sites for the H2 dissociation and CO2 activation and hydrogenation. Second, it is known that oxygen vacancies often play a critical role in modulating the CO2 hydrogenation efficiency since the advantages of oxygen vacancies such as being active sites for CO2 activation, facilitating a stronger CO2 adsorption strength and resulting in more oxygen can be dissociatively adsorbed, have been reported in other catalysts.22 Based on the XPS results, the amount of oxygen vacancies significantly increases after adding Cu nanoparticles on the MnOx support. The formed oxygen vacancies are preferentially located at the Cu/MnOx interfaces to participate in the catalytic reaction (Scheme 2). Third, the strong interplay between Cu and MnOx supports may result in the presence of microstrain. As another lattice defect, microstrain often induces the local structural disorder and the catalytic activity of Cu-based catalysts is found to positively correlate with the microstrain.23 A larger strain means a higher degree of structural disorder in the Cu nanoparticles, and the lattice defects tend to terminate at the surface and consequently become effective for catalysis.23 Also, strain is an indicative of an increased deviation from the “ideal” structure by the increased interface contact area between metal and support.24 In our catalysts, the strain in the metal particles and support is caused by (i) the atom diffusion into the lattice structure during the reduction process and (ii) a large lattice mismatch due to an epitaxial bonding of Cu on the support at the interface. By Rietveld

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refinement for the XRD results (Fig. S9 and Table S3), the MnO and Cu phases of rCM-1 catalyst show the maximum lattice parameters and consequently the strain originating from the lattice mismatch at the Cu/MnO interfaces is the largest as well. The defective catalysts are more active during the catalytic reaction and the catalytic activity is enhanced. Moreover, the CO2 reduction is dependent on a compositional effect as the catalytic activity decreases when the Cu loading is more than 50 mol%. On the one hand, excessive Cu loading leads to a bigger grain size (55.2 nm) in rCM-2 the aggregation of Cu domain25 and poor CO2 dissociative chemisorption energy on metallic Cu.26 On the other hand, the amount of oxygen vacancies also decreased since the formation energy of oxygen vacancy is related to the amount of Cu.26 This compositional effect is also reported in Cu/ZnO catalyst, in which a better CO formation rate and lower activation energy are achieved when Cu/Zn ratio is similar.27 Therefore, the aggregation of Cu domain by overloading and the reduced amount of oxygen vacancies should be responsible for the decreased catalytic activity. Moreover, after catalysis there is no significant change in the TEM images and XPS results (Fig. S10 and S11), indicating a good stability of the catalysts. It is known that the CO selectivity plays a key role in controlling the reaction as a partial consumption of hydrogen by the competition of CO2 methanation with RWGS reaction always limits the CO production. As shown in Fig. 4d, all the catalysts show a 100% CO selectivity in the test regardless of the amount of Cu loading, indicating that designing Cu/MnOx can not only facilitate the CO2 reduction but also maintain the high selectivity towards CO. The 100% CO selectivity is also reported in other Cu-based catalysts such as Cu/ZnO/Al2O3,25 Cu-ZnGaZrO28 and CeCu composite.29 Previous theoretical study also revealed that the desorption of CO from Cu(100) surface is expected to be less endergonic, due to a weaker interaction between Cu and CO,30 which suppresses the further hydrogenation of CO to CH4 on the catalysts. Thus a high selectivity towards CO is

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maintained. The high CO2 conversion efficiency and selectivity are better or comparable to the previous reported catalysts (Table S4). In conclusion, Cu/MnOx heterostructures were hydrothermally constructed by decorating CuO nanoparticles on MnO2 nanowires. Afterwards, the redox properties and surface area were improved significantly. The highly dispersed CuO with a small grain size grown on the MnO2 supports was further reduced to metallic Cu nanoparticles, accompanied by a phase transformation from MnO2 to MnO and a grain growth/agglomeration in Cu nanoparticles simultaneously. The Cu/MnOx heterogeneous catalysts were very active for the RWGS reaction and especially highly selective towards CO. Our findings also revealed that the catalytic activity can be optimized by compositional modulation, and the highest activity was achieved when the molar fraction of Cu and Mn was equal. Detailed structural characterizations demonstrated that the strong interaction between Cu and MnOx support provided more active oxygen species and induced lattice strain in Cu nanoparticles, which accounts for the improved activity for the RWGS reaction.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. XRD pattern and SEM image of MnO2 nanowire synthesised at 200 °C for 12 h, higher magnification TEM images of the fresh and reduced CM nanostructures, quantitative analysis of H2-TPR results, BET surface area, pore size, pore volume and SEM images of the assynthesized CM nanostructures, photographs of the CM catalysts before and after reducing annealing, HRTEM image of the reduced CM-1 catalyst, Mn 3s spectra of the reduced CM nanostructures, comparison of CO2 conversion for Cu/commercial MnO2 and rCM-1 catalysts,

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TEM images and O 1s spectra of used catalysts, Rietveld refinement analysis for XRD patterns and comparison of Cu/MnOx and previously reported catalysts.

Acknowledgments This work is funded by the Australian Research Council Project (Grant No. FT140100032). The authors thank Dr. Yin Yao, Ms. Katie Levick and Dr. Bill Gong from the UNSW Mark Wainwright Analytical Centre and Dr. David Mitchell from the Electron Microscopy Centre, University of Wollongong for their assistance with SEM, TEM, XPS and HAADF-STEM analyses.

References 1.

Xu, X.; Moulijn, J., Mitigation of CO2 by Chemical Conversion: Plausible Chemical

Reactions and Promising Products. Energy & Fuels 1996, 10 (2), 305-325. 2.

Yang, H.; Zhang, C.; Gao, P.; Wang, H.; Li, X.; Zhong, L.; Wei, W.; Sun, Y., A

Review of the Catalytic Hydrogenation of Carbon Dioxide into Value-Added Hydrocarbons.

Catal. Sci. Technol. 2017, 7 (20), 4580-4598. 3.

Wang, W.; Wang, S.; Ma, X.; Gong, J., Recent Advances in Catalytic Hydrogenation

of Carbon Dioxide. Chem. Soc. Rev. 2011, 40 (7), 3703-3727. 4.

Sá, S.; Silva, H.; Brandão, L.; Sousa, J. M.; Mendes, A., Catalysts for Methanol

Steam Reforming—A Review. Appl. Catal. B 2010, 99 (1), 43-57. 5.

Porosoff, M. D.; Yan, B.; Chen, J. G., Catalytic Reduction of CO2 by H2 for Synthesis

of CO, Methanol and Hydrocarbons: Challenges and Opportunities. Energy Environ. Sci. 2016, 9 (1), 62-73. 6.

Yang, H.-M.; Liao, P.-H., Preparation and Activity of Cu/ZnO-CNTs Nano-Catalyst

on Steam Reforming of Methanol. Appl. Catal. A 2007, 317 (2), 226-233.

16


Page 16 of 20

Page 17 of 20 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


7.

Xu, R.; Wang, X.; Wang, D.; Zhou, K.; Li, Y., Surface Structure Effects in

Nanocrystal MnO2 and Ag/MnO2 Catalytic Oxidation of CO. J. Catal. 2006, 237 (2), 426430. 8.

Li, C.-S.; Melaet, G.; Ralston, W. T.; An, K.; Brooks, C.; Ye, Y.; Liu, Y.-S.; Zhu, J.;

Guo, J.; Alayoglu, S., High-Performance Hybrid Oxide Catalyst of Manganese and Cobalt for Low-Pressure Methanol Synthesis. Nat. Commun. 2015, 6, 6538. 9.

Liang, S.; Teng, F.; Bulgan, G.; Zong, R.; Zhu, Y., Effect of Phase Structure of MnO2

Nanorod Catalyst on the Activity for CO Oxidation. J. Phys. Chem. C 2008, 112 (14), 53075315. 10.

Einaga, H.; Kiya, A.; Yoshioka, S.; Teraoka, Y., Catalytic Properties of Copper-

Manganese Mixed Oxides Prepared by Coprecipitation Using Tetramethylammonium Hydroxide. Catal. Sci. Technol. 2014, 4 (10), 3713-3722. 11.

Chary, K. V.; Seela, K. K.; Sagar, G. V.; Sreedhar, B., Characterization and

Reactivity of Niobia Supported Copper Oxide Catalysts. J. Phys. Chem. B 2004, 108 (2), 658-663. 12.

Avgouropoulos, G.; Ioannides, T., Effect of Synthesis Parameters on Catalytic

Properties of CuO-CeO2. Appl. Catal. B 2006, 67 (1), 1-11. 13.

Deng, D.; Jin, Y.; Cheng, Y.; Qi, T.; Xiao, F., Copper Nanoparticles: Aqueous Phase

Synthesis and Conductive Films Fabrication at Low Sintering Temperature. ACS Appl. Mater.

Interfaces 2013, 5 (9), 3839-3846. 14.

Stobbe, E.; De Boer, B.; Geus, J., The Reduction and Oxidation Behaviour of

Manganese Oxides. Catal. Today 1999, 47 (1), 161-167. 15.

Yao, C.-Z.; Wang, L.-C.; Liu, Y.-M.; Wu, G.-S.; Cao, Y.; Dai, W.-L.; He, H.-Y.; Fan,

K.-N., Effect of Preparation Method on the Hydrogen Production from Methanol Steam Reforming over Binary Cu/ZrO2 Catalysts. Appl. Catal. A 2006, 297 (2), 151-158.

17



16.

Strohmeier, B. R.; Hercules, D. M., Surface Spectroscopic Characterization of

Mn/Al2O3 Catalysts. J. Phys. Chem. 1984, 88 (21), 4922-4929. 17.

Lee, J. H.; Sa, Y. J.; Kim, T. K.; Moon, H. R.; Joo, S. H., A Transformative Route to

Nanoporous Manganese Oxides of Controlled Oxidation States with Identical Textural Properties. J. Mater. Chem. A 2014, 2 (27), 10435-10443. 18.

Wang, Y.; Arandiyan, H.; Scott, J.; Akia, M.; Dai, H.; Deng, J.; Aguey-Zinsou, K.-F.;

Amal, R., High Performance Au-Pd Supported on 3d Hybrid Strontium-Substituted Lanthanum Manganite Perovskite Catalyst for Methane Combustion. ACS Catal. 2016, 6 (10), 6935-6947. 19.

Wang, Y.; Arandiyan, H.; Tahini, H. A.; Scott, J.; Tan, X.; Dai, H.; Gale, J. D.; Rohl,

A. L.; Smith, S. C.; Amal, R., The Controlled Disassembly of Mesostructured Perovskites as an Avenue to Fabricating High Performance Nanohybrid Catalysts. Nat. Commun. 2017, 8, 15553. 20.

Li, J.; Liang, X.; Xu, S.; Hao, J., Catalytic Performance of Manganese Cobalt Oxides

on Methane Combustion at Low Temperature. Appl. Catal. B 2009, 90 (1), 307-312. 21.

Ginés, M.; Marchi, A.; Apesteguia, C., Kinetic Study of the Reverse Water-Gas Shift

Reaction over CuO/ZnO/Al2O3 Catalysts. Appl. Catal. A 1997, 154 (1-2), 155-171. 22.

Wang, F.; Li, C.; Zhang, X.; Wei, M.; Evans, D. G.; Duan, X., Catalytic Behavior of

Supported Ru Nanoparticles on the {100}, {110}, and {111} Facet of CeO2. J. Catal. 2015,

329, 177-186. 23.

Kasatkin, I.; Kurr, P.; Kniep, B.; Trunschke, A.; Schlögl, R., Role of Lattice Strain

and Defects in Copper Particles on the Activity of Cu/ZnO/Al2O3 Catalysts for Methanol Synthesis. Angew. Chem. 2007, 119 (38), 7465-7468.

18


Page 18 of 20

Page 19 of 20 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


24.

Ressler, T.; Kniep, B. L.; Kasatkin, I.; Schlögl, R., The Microstructure of Copper Zinc

Oxide Catalysts: Bridging the Materials Gap. Angew. Chem. Int. Ed. 2005, 44 (30), 47044707. 25.

Zhang, X.; Zhu, X.; Lin, L.; Yao, S.; Zhang, M.; Liu, X.; Wang, X.; Li, Y.-W.; Shi, C.;

Ma, D., Highly Dispersed Copper over β-Mo2C as an Efficient and Stable Catalyst for the Reverse Water Gas Shift (RWGS) Reaction. ACS Catal. 2016, 7 (1), 912-918. 26.

Daza, Y. A.; Maiti, D.; Hare, B. J.; Bhethanabotla, V. R.; Kuhn, J. N., More Cu, More

Problems: Decreased CO2 Conversion Ability by Cu-Doped La0.75Sr0.25FeO3 Perovskite Oxides. Surf. Sci. 2016, 648, 92-99. 27.

Galván, C. Á.; Schumann, J.; Behrens, M.; Fierro, J. L. G.; Schlögl, R.; Frei, E.,

Reverse Water-Gas Shift Reaction at the Cu/ZnO Interface: Influence of the Cu/Zn Ratio on Structure-Activity Correlations. Appl. Catal. B 2016, 195, 104-111. 28.

Liu, X.; de la Piscina, P. R.; Toyir, J.; Homs, N., CO2 Reduction over Cu-ZnGaMO

(M= Al, Zr) Catalysts Prepared by a Sol-Gel Method: Unique Performance for the RWGS Reaction. Catal. Today 2017, 296 (1), 181-186. 29.

Zhou, G.; Dai, B.; Xie, H.; Zhang, G.; Xiong, K.; Zheng, X., CeCu Composite

Catalyst for CO Synthesis by Reverse Water-Gas Shift Reaction: Effect of Ce/Cu Mole Ratio.

J. CO2 Util. 2017, 21, 292-301. 30.

Liu, C.; Cundari, T. R.; Wilson, A. K., CO2 Reduction on Transition Metal (Fe, Co,

Ni, and Cu) Surfaces: In Comparison with Homogeneous Catalysis. J. Phys. Chem. C 2012,

116 (9), 5681-5688.

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