Cu Species Incorporated into Amorphous ZrO2 with High Activity and

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Article 2

Cu Species Incorporated into Amorphous ZrO with High Activity and Selectivity in CO-to-Methanol Hydrogenation 2

Shohei Tada, Ayaka Katagiri, Keiko Kiyota, Tetsuo Honma, Hiromu Kamei, Akane Nariyuki, Sayaka Uchida, and Shigeo Satokawa J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11284 • Publication Date (Web): 13 Feb 2018 Downloaded from http://pubs.acs.org on February 18, 2018

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The Journal of Physical Chemistry

Cu Species Incorporated into Amorphous ZrO2 with High Activity and Selectivity in CO2-to-methanol Hydrogenation Shohei Tada,a* Ayaka Katagiri,a Keiko Kiyota,a Tetsuo Honma,b Hiromu Kamei,c Akane Nariyuki,c Sayaka Uchida,d Shigeo Satokawa a* a

Department of Materials and Life Science, Faculty of Science and Technology, Seikei

University, 3-3-1 Kichijoji-kitamachi, Musashino-shi, Tokyo 180-8633, Japan. b

Japan Synchrotron Radiation Research Institute, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan

c

Nikki-Universal Co., Ltd., 7-14-1 Hiratsuka-shi, Kanagawa 254-0014, Japan.

d

Department of Basic Sciences, School of Arts and Sciences, The University of Tokyo, 3-8-1

Komaba, Meguro-ku, Tokyo 153-8902, Japan

ACS Paragon Plus Environment

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Abstract

We prepared Cu/a-ZrO2 (a-ZrO2: amorphous ZrO2), Cu/m-ZrO2 (m-ZrO2: monoclinic ZrO2), Cu/a-ZrO2/KIT-6, and Cu/t-ZrO2/KIT-6 (t-ZrO2: tetragonal ZrO2) by a simple impregnation method and examined the effect of ZrO2 phase on CO2-to-methanol hydrogenation. We discovered a-ZrO2-containing catalysts with high activity and selectivity in CO2-to-methanol hydrogenation. Next, we focused on Cu species formation on the above-described catalysts. While pure CuO was observed on Cu/m-ZrO2 and Cu/t-ZrO2/KIT-6, copper-zirconium mixed oxide (CuxZryOz), not pure CuO, was formed on Cu/a-ZrO2 and Cu/a-ZrO2/KIT-6, as evidenced by X-ray absorption spectroscopy (XAS) and the powder color. After reducing a-ZrO2containing catalysts with H2 at 300 °C, we observed highly dispersed Cu nanoparticles in close contact with a-ZrO2 (or CuxZryOz). In addition, methanol vapor sorption revealed that methanol adsorbed more weakly on a-ZrO2 than on m-ZrO2. Therefore, the high dispersion of Cu species and weak adsorption of methanol led to high activity and selectivity in CO2-to-methanol hydrogenation.

1. Introduction Methanol has been produced by the hydrogenation of CO (Eq. 1) since Imperial Chemical Industries developed a CuO/ZnO/Al2O3 catalyst in the sixties. Subsequently, methanol synthesis became a key process in petrochemical industries. The direct hydrogenation of CO2 (in place of CO) to methanol has become a very active field of research since CO2 may be actively recycled, allowing mitigation of the greenhouse effect of CO2

1-5

. Because of the reactivity difference

between CO and CO2, new catalysts specific to CO2-to-methanol hydrogenation should be


2

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developed. Methanol synthesis by CO2 hydrogenation (Eq. 2) is exothermic, and the number of product moles is lower than that of the reactants. At a high temperature, the endothermic reverse water gas shift reaction (RWGS reaction, Eq. 3) becomes dominant, converting CO2 into CO. Hence, for methanol production via CO2 hydrogenation, low temperatures and high pressures are typically required.

CO + 2H2 ⇄ CH3OH

∆rH (298K) = - 91 kJ mol-1

(Eq. 1)

CO2 + 3H2 ⇄ CH3OH+ H2O ∆rH (298K) = - 49 kJ mol-1

(Eq. 2)

CO2 + H2 ⇄ CO + H2O

(Eq. 3)

∆rH (298K) = 42 kJ mol-1

Numerous experimental and theoretical studies have been performed to evaluate Cu-based catalysts for the hydrogenation of CO2 into methanol. Various supports and promoters have been reported, e.g., ZnO,6-15 ZrO2,6, 13-24 CeO2 25 and MgO 26. For instance, addition of ZnO to a CuOZrO2 catalyst enhanced methanol production rate via CO2 hydrogenation selectivity to methanol

6, 14

13

, but decreased

. In addition, according to our previous work, CuO-ZrO2 catalysts

showed higher selectivity in CO2-to-methanol hydrogenation than a commercial CuO/ZnO/Al2O3 catalyst 21. Therefore, we consider that CuO-ZrO2 catalysts will be a specific catalyst of CO2-tomethanol hydrogenation. A plausible reaction mechanism of CO2-to-methanol hydrogenation over Cu/ZrO2 was proposed by Bell et al.

18

First, CO2 adsorbs on ZrO2 surface, and the CO2 is hydrogenated to

formate species on ZrO2. Afterwards, H2 adsorbs dissociatively on metallic Cu surface and the


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thus-formed H species spills over to the Cu-ZrO2 interface, allowing the conversion of the formate species to methoxy species at the Cu-ZrO2 interface. In the end, the methoxy species is hydrogenated to methanol. According to the previous work of one of the authors (Tada), the above reaction mechanism was elucidated using spectroscopic experiments and theoretical calculations 22. In addition, an argument persists concerning the ZrO2 phase effect on CO2-to-methanol hydrogenation over Cu/ZrO2 catalysts. Jung and Bell concluded that Cu on monoclinic-ZrO2 (mZrO2) was much more active in CO2-to-methanol hydrogenation than Cu on tetragonal-ZrO2 (tZrO2).

23

Grabowski et al. investigated Cu nanoparticles on polymorphic ZrO2, which was

between t-ZrO2 and m-ZrO2. With increasing t-ZrO2 content on the Cu/ZrO2 catalysts, the methanol production rate increased. The presence of oxygen vacancies on the ZrO2 stabilized both Cu+ cations and thermodynamically unstable t-ZrO2. The sites of Cu+ next to the oxygen vacancies played an important role as acid centers for methanol production.

17

Witoon et al.

prepared three types of Cu/ZrO2 catalysts by incipient wetness impregnation: Cu/t-ZrO2, Cu/mZrO2 and Cu/a-ZrO2 (a-ZrO2: amorphous ZrO2). The turnover frequency of the methanol production over Cu/t-ZrO2 was higher than those over Cu/a-ZrO2 and Cu/m-ZrO2. 16 One of the central issues for Cu/ZrO2 catalysts is the incorporation of Cu species into ZrO2. Although this incorporation has been reported by numerous researchers, most of them used a coprecipitation technique for the catalyst preparation. 6, 17, 27-30 During the coprecipitation process using NaOH and NH3 solution, however, Cu precursors are converted to Cu complexes (e.g., Cu hydroxy complex). The Cu complexes are easily removed by washing the precipitate with water.21 Accordingly, the formation of Cu complexes makes it difficult to control the physical properties of the Cu species on ZrO2, which is a disadvantage of the coprecipitation technique.


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As an alternative to the coprecipitation method, Chen et al. 31, 32 and Grabowski et al. 17 used an incipient wetness impregnation method to prepare CuO/t-ZrO2. The impregnation method is known as a simpler and more convenient preparation technique compared with the coprecipitation method. In fact, the researchers succeeded in observing Cu incorporation into tZrO2 on the prepared catalyst. As a drawback, however, it is very difficult to synthesize pure tZrO2 without a stabilizer, such as Y2O3, MgO, CaO, or CeO2, because the tetragonal phase is metastable and readily converted to the stable monoclinic phase at < 1170 °C.

33-38

Here, we

wondered whether Cu species is incorporated into a-ZrO2 and m-ZrO2 when CuO/a-ZrO2 and CuO/m-ZrO2 are prepared using an impregnation method. Witoon et al. 16, Shimokawabe et al. 39

, and Bueno et al. 40 provided telling clues about this problem: a strong interaction between Cu

species and a-ZrO2 was observed for Cu/a-ZrO2 prepared using an impregnation method. In the present study, we investigated the effect of ZrO2 phase on Cu species formation of CuZrO2-based catalysts and on CO2-to-methanol hydrogenation. To verify the effect, we used three types of ZrO2: ZrO2 deposited onto mesoporous silica KIT-6, commercial m-ZrO2 and commercial a-ZrO2. According to Satokawa’s work,41 the ZrO2 phase of ZrO2/KIT-6 was controlled by calcining ZrO(NO3)2/KIT-6: we obtained a-ZrO2/KIT-6 by calcining ZrO(NO3)2/KIT-6 at 400 °C, while we obtained t-ZrO2/KIT-6 by calcining ZrO(NO3)2/KIT-6 at 800 °C. As will be shown, a-ZrO2-containing Cu catalysts exhibited high activity and selectivity in CO2-to-methanol hydrogenation. When a-ZrO2 and a-ZrO2/KIT-6 were impregnated with Cu nitrate solution and then calcined at 350 °C, Cu species was incorporated into a-ZrO2, resulting in the formation of green CuxZryOz. After the green powders were reduced by H2, Cu nanoparticles with a crystallite size < 10 nm were formed on the catalysts. Hence, we concluded that H2 reduction of the CuxZryOz is a key feature of the formation of highly dispersed Cu


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nanoparticles. In addition, we found that methanol more weakly adsorbed on a-ZrO2 than on mZrO2, resulting in suppression of the undesirable methanol decomposition and enhancement of the selectivity towards methanol.

2. Experimental Procedures 2.1 Catalyst preparation. Mesoporous silica KIT-6 was prepared according to a previously reported procedure (see ESI) 42

. The Catalysis Society of Japan provided a-ZrO2 (aZ, JRC-ZRO-5) and m-ZrO2 (mZ, JRC-

ZRO-3). A series of ZrO2/KIT-6 were prepared using a wetness impregnation method.

41

The

KIT-6 was impregnated with an aqueous solution of ZrO(NO3)2･xH2O (Wako Pure Chemical Industries, Ltd.) and dried at 70 °C under a vacuum (110 hPa). The crude material was heated at 110 °C overnight under air and then calcined at T °C (T = 400, 600, and 800) for 4 h. The obtained ZrO2/KIT-6 calcined at T °C was named Z/K(T). Supported Cu catalysts were also prepared using an incipient wetness impregnation method. The abovementioned Z/K(T), aZ, or mZ was impregnated with an aqueous solution of Cu(NO3)2･3H2O (Wako Pure Chemical Industries, Ltd.), dried at 110 °C overnight, and calcined at 350 °C for 2 h. The catalysts containing Z/K(T), aZ, and mZ were named C/Z/K(T), C/aZ, and C/mZ, respectively. The loadings of Cu and ZrO2 for the prepared catalysts are summarized in Table 1.

2.2 Characterization


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Powder X-ray diffraction (PXRD). The crystalline phases of the catalysts were determined by powder X-ray diffraction (Rigaku, Ultima IV) with a Cu Kα radiation source at a voltage of 40 kV and a current of 40 mA. The crystallite size (D) was estimated from the diffraction peak using the Scherrer’s equation (Eq. 4),

D=

Kλ β cosθ

(Eq. 4)

where K is the shape factor (0.89), λ is the X-ray wavelength (0.154 nm), β is the line broadening at half the maximum intensity in radians, and θ is the Bragg angle.

Adsorption. The specific surface area and pore width for each catalyst were estimated by N2 adsorption using a MicrotracBEL BELSORP-mini II. Prior to the measurements, the sample (ca. 100 mg) was loaded into a sample cell and dried under vacuum at 150 °C overnight. CO2 adsorption isotherms of aZ and mZ were measured at 5, 15, 35 and 50 °C using an automatic volumetric adsorption apparatus (MicrotracBEL, BELSORP-max). Approximately 100 mg of the samples were evacuated at 150 °C for 2 h to form the guest-free phases. According to Figure S1, CO2 is removed by the evacuation at 150 °C if CO2 adsorbed on the samples at room temperature. Sorption equilibrium was judged by the following criteria: ± 0.3% of pressure change in 5 min. The amount of adsorbed CO2 was measured in the pressure range from 0 to 91 kPa.


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Methanol sorption isotherms of aZ and mZ were measured at 15, 20, 25 and 30 °C using an automatic volumetric vapor sorption apparatus (MicrotracBEL, BELSORP-max). As a pretreatment, approximately 100 mg of the samples were evacuated at 150 °C for 2 h. Sorption equilibrium was judged by the following criteria: ± 0.3% of pressure change in 2.5 min. The vapor saturation pressure (P0) of methanol at 15, 20, 25 and 30 °C was 9.8, 13.0, 16.9 and 21.8 kPa, respectively. The amount of adsorbed methanol was measured in the range of relative pressure from 0 to 0.8. The adsorption heats of CO2 and methanol on aZ and mZ (q, kJ mol-1) were estimated using the Clausius-Clapeyron equation (Eq. 5),

∂ Ln(Px ) ∂T

= θx

q

(Eq. 5)

RT2

where P is the equilibrium pressure of x species (Pa), T is the adsorption temperature (K), R is the gas constant (8.31 J mol-1 K-1), and θx is the coverage of x species (-). In addition, methanol vapor sorption on aZ and mZ was examined by in-situ Fourier transform infrared spectroscopy (in-situ FTIR) measurements. The FTIR cell equipped with high purity CaF2 windows and capability for heating and cooling was placed in a JASCO FT/IR 4600 instrument with a mercury cadmium telluride detector. The powder samples (ca. 20 mg cm2

) were pressed into a thin self-supporting disk and set in the cell, heated at 280 °C for 2 h under

vacuum, and cooled down to 205 °C under vacuum. Then background spectra were collected at 205 °C. The samples were exposed to methanol vapor (3 kPa) for 20 min at 205 °C. Afterwards,


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the methanol vapor was removed by vacuum treatment at 205 °C. Typically, 20 scans were collected for 1 spectrum, and the results were presented as absorbance spectra.

Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES). The Cu loading was measured using an ICP-AES (Shimadzu, ICPS-7500). Approximately 30 mg of the samples were dissolved in a mixture of 0.5 mL of HNO3 (69-70%, Wako Pure Chemical Industries) and 1 mL of HF (46-48%, Morita Chemical Industries) prior to the measurements.

N2O titration. An N2O titration was carried out as previously described. 43, 44 Approximately 200 mg of catalyst was placed in a quartz tube connected to a flow system (MicrotracBEL, BELCAT-A) and treated at 300 °C for 30 min with 4% H2/Ar. He was used as a carrier gas at 30 mL(STP) min-1, and successive doses of 10% N2O/He gas were subsequently introduced into the He stream using a calibrated injection valve (27 µLN2O(STP) pulse-1) at 90 °C. A thermal conductivity detector analyzed the amount of outlet N2O and N2. The number of accessible Cu surface atoms was estimated according to Eq. 6.

2Cu + N2O → Cu2O + N2

(Eq. 6)


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Using N2O consumption, the average particle size of Cu0, dCu (m), was calculated by Eq. 7, which assumes that the Cu particles on the supports are semispherical and have a uniform volume and density.

dCu =

6 WCu nN2O A

(Eq. 7)

YN2O Nav ρCu

In these equations, YN2O is the amount of N2O uptake (molN2O gcat-1), WCu is the Cu loading (gCu gcat-1), Nav is Avogadro’s constant (6.02 × 1023 atom mol-1), nN2O is the stoichiometry of N2O to Cu (0.5 molN2O molCu-1), A is the number of Cu surface atoms per unit area (1.46 × 1019 atomCu m-2), and ρCu is the density of Cu (8.94 gCu cm-3).

Temperature programmed reduction by H2 (H2-TPR). The reducibility of the Cu species in the prepared catalysts was investigated by H2-TPR in a flow system (MicrotracBEL, BELCAT-A). Approximately 30 mg of the samples were placed in a quartz tube and heated at 300 °C for 1 h under an Ar flow. Next, the samples were cooled to 50 °C under an Ar flow, after which the cell was purged with 4% H2/Ar. The temperature was then raised from 50 °C to 500 °C at a heating rate of 5 °C min-1 under 4% H2/Ar flow (30 mL(STP) min-1).

Transmission Electron Microscopy. The morphologies of the catalysts were observed by highangle annular dark field scanning transmission electron microscopy (HAADF-STEM) performed using a Hitachi HDCS2700CS microscope.


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X-ray absorption spectroscopy (XAS). Using XAS at the BL14B2 beamline at SPring-8, the Cu K-edge and Zr K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) were measured. XAS data were collected in quick mode. The Si (3 1 1) monochromator was continuously moved from 25.415° to 22.05° in 313 sec for the Cu K-edge, while it was moved from 12.27° to 11.505° in 152 sec for the Zr K-edge. Spectra were collected in transmission mode using ion chambers filled with an Ar/N2 mixture on pressed pellets of the samples. The samples and BN were mixed using a mortar and a pestle and pressed into a thin disk using a 10-mm die set. In addition, we prepared pellets of the samples reduced by H2 using a 7-mm die set in a glove box. The as-prepared samples were placed into a quartz tube of a fixed-bed quartz tubular reactor (MicrotracBEL, BELCAT-A), reduced at 300 °C under 20% H2/He for 15 min, and then cooled down to room temperature under a He flow. Spectra were corrected and normalized using the Athena and Artemins

45

. Fourier-transformed data were

analyzed by a curve-fitting method using theoretical phase-shift and amplitude functions derived with the FEFF8 program.

2.3 Reaction test CO2 hydrogenation was conducted in a fixed-bed tubular reactor at 10 bars (PID Eng&Tech, Microactivity Effi reactor). After loading 500 mg of catalyst powder into the reactor, the catalyst was treated under a flow of 16% H2/N2 (72 mL(STP) min-1) at 300 °C for 2 h under ambient pressure. After cooling to 270 °C, a flow of CO2/H2/N2 (1/3/1, 14 mL(STP) min-1) was passed through the catalyst bed for 12 h at 10 bars, during which steady state was reached. Subsequently,


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the catalyst bed was cooled to 230 °C under the reaction conditions, and the products were analyzed by an online gas chromatograph (Shimadzu, GC8A) equipped with FID (for methanol) and TCD (for N2, CO2, CO, and CH4). In all experiments, the CH4 concentration in the outlet gas was nearly zero. Thus, the carbon balance was estimated from the concentrations of CO2, CO, and methanol using N2 as an internal standard. The reaction gas flow was changed from 14 to 70 mL(STP) min-1, and the effect of space velocity on CO2 hydrogenation to methanol was also examined. The contact time, selectivity of x species, and production rate of x species (rx) were determined by Eqs. 8, 9, 10, and 11: 21

Contact time [s] = Vcat / Ftotal

(Eq. 8)

Selectivity of x species [-] = Fx,out / (Fmethanol, out + FCO,out)

(Eq. 9)

CO2 conversion [-] = (Fmethanol, out + FCO,out) / FCO2,in

(Eq. 10)

rx [molx h-1 gcat-1] = Fx, out / W

(Eq. 11)

where Vcat is the catalyst bed volume (mL), Ftotal is the total gas inlet flow rate (mL(STP) s-1), Fx, in

is the total gas inlet flow rate of x species (mol h-1), Fx, out is the total gas outlet flow rate of x

species (mol h-1) and W is the amount of catalyst (gcat).

3. Results and Discussion 3.1 Properties of the support materials


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Figure S2 shows N2 adsorption/desorption isotherms for KIT-6, Z/K(T), aZ and mZ. For all the samples except for aZ, the profiles exhibited the typical type IV(a) isotherms with an H1 hysteresis loop, indicating the mesostructure of the samples. For aZ, the profile showed the typical type II isotherm, which indicated the microporous structure of aZ. The specific surface area of KIT-6 was 987 m2 g-1. Impregnation of ZrO2 into KIT-6 decreased the specific surface area from 987 to 298-435 m2 g-1, as shown in Table 1, probably because of some insertion of the ZrO2 additive into the pores of KIT-6. The ZrO2 loadings of Z/K(T) were estimated by ICP-AES to be 43-47 wt %. Next, PXRD measurements on the samples were performed (Figure 1). For the three catalysts and KIT-6 (Figure 1a), a broad peak at 22° was observed, which was attributed to an amorphous SiO2 phase. Additionally, ZrO2 in Z/K(400) and Z/K(600) was mainly amorphous, whereas ZrO2 in Z/K(800) was tetragonal.

41

These results were also

supported by XAS (Figures S3 and S4). It is noteworthy that a part of ZrO2 in Z/K(600) was tetragonal because two tiny peaks of t-ZrO2 were observed in the XRD pattern. Additionally, the ZrO2 crystal phase in aZ was amorphous, whereas the phase in mZ was mainly monoclinic with a minor amount of tetragonal phase.

3.2 Structures of as-prepared Cu catalyst Figure S5 shows the N2 adsorption/desorption for as-prepared C/Z/K(T), C/aZ, and C/mZ. The typical type IV(a) isotherms with an H1 hysteresis loop show the mesostructure of C/Z/K(T) and C/mZ, whereas the type II isotherm exhibits the microstructure of C/aZ. As shown in Table 1, the specific surface area of C/Z/K(T) was 211-303 m2 gcat-1, that of C/aZ was 177 m2 gcat-1 and


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that of C/mZ was 107 m2 gcat-1. According to the ICP results, the Cu loading of C/Z/K(T) was 12-16 wt %, and those of C/aZ and C/mZ were ca. 9 wt %. Figure 1b shows the PXRD patterns of the as-prepared Cu catalysts. First, we discuss the ZrO2 phase for the catalysts. The ZrO2 phase for Z/K(T), aZ, and mZ were almost the same to those for C/Z/K(T), C/aZ, and C/mZ, respectively: ZrO2 in C/Z/K(400), C/Z/K(600) and C/aZ was mainly amorphous, ZrO2 in C/Z/K(800) was tetragonal, and ZrO2 in C/mZ was monoclinic. In addition, the peak position of t-ZrO2 (111) phase was observed at 30.3 ° for Z/K(800) (Figure 1a) and as prepared C/Z/K(800) (Figure 1b), and the peak position of m-ZrO2 (-111) phase was observed at 28.2 ° for mZ (Figure 1a) and as prepared C/mZ (Figure 1b). Furthermore, the Zr K-edge XANES spectra (Figure S6) and radial structure functions (Figure S7) of the Cu catalysts supported the PXRD results. Next, we analyzed Cu species on the as-prepared C/Z/K(T), C/aZ and C/mZ using PXRD, as shown in Figure 1b. The Cu species on C/Z/K(600), C/Z/K(800) and C/mZ was CuO. The Cu species on C/Z/K(400) and C/aZ were not identified by PXRD. By comparison, copper oxides deposited on KIT-6 (Cu loading: 13wt %) were prepared using an incipient wetness impregnation, and the PXRD patterns showed strong peaks of CuO (Figure S8). When Cu species were deposited on a-ZrO2 by an incipient wetness impregnation, the nature of a-ZrO2 was considered to affect the particle formation of the Cu species. We attempted to obtain detailed information regarding the local structure of the Cu species using XAS. Figure 2a shows the Cu K-edge XANES spectra for the as-prepared Cu catalysts. We also used Cu foil, Cu2O powder (Wako Pure Chemical Industries, Ltd., 99.5%) and CuO powder (Kanto Chemical, 99.9%) as standard reference samples. For C/Z/K(600), C/Z/K(800)


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and C/mZ, the XANES spectra were similar to that of CuO and the pre-edge peak at 8985 eV appeared, which suggested that the Cu species in the catalysts was present as CuO. For C/Z/K(400) and C/aZ, while the XANES spectra were also similar to that of CuO, the slope of the edge absorption was relatively reduced. The loss of the pre-edge peak was reported by CuOZrO2 containing catalyst prepared by thermal decomposition

46

, co-precipitation

30

, and

impregnation 40. The intensity and position of the pre-edge peak were derived from coordination structure of Cu species. According to Ressler et al. 30, the distorted CuO species was the main Cu phase in C/Z/K(400) and C/aZ. The first derivatives of the XANES spectra for the as-prepared catalysts (Figure 2b) also revealed the difference between CuO and the CuO-like species. The energy position of the first inflection points for C/Z/K(600), C/Z/K(800) and C/mZ was identical to that of CuO (8984 eV). The absorption edge positions of C/Z/K(400) and C/aZ (8985 eV) were higher than that of CuO (8984 eV). Furthermore, the shapes of the peak at ca. 8990 eV for C/Z/K(600), C/Z/K(800) and C/mZ were similar to that for CuO and different from those for C/Z/K(400) and C/aZ. The differences in the Cu K-edge radial structure functions (RSFs) are clearly visible in Figure 3. The average coordination numbers for Cu-O and Cu-Cu were estimated by EXAFS curve-fitting for the prepared catalysts, where we assumed the presence of bulk CuO (Table 2). For CuO, the RSF exhibited a strong backscattering peak at 1.5 Å, which could be assigned to the contribution of the nearest-neighbor Cu-O at 1.96 Å. Furthermore, the RSF possessed the other strong peak at 2.5 Å, which was related to the second-nearest Cu-O (2.78 Å), the nearestneighbor Cu-Cu (2.91 Å) and the second-nearest Cu-Cu (3.08 Å) in CuO (Table S1). Because the RSFs of C/Z/K(600), C/Z/K(800) and C/mZ consisted of bulk CuO, the Cu species in the three samples were CuO. Of note, a peak at 2.5 Å in the RSFs of C/Z/K(400) and C/aZ was


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much weaker than those in C/Z/K(600), C/Z/K(800), C/mZ and CuO, while a peak of the nearest-neighbor Cu-O was observed at 1.5 Å. Hence, the coordination structure of Cu species in C/Z/K(400) and C/aZ differed from CuO and was distorted, as summarized in Table 2. Figure 4 shows the photographs of the as-prepared catalysts. The powder color of C/Z/K(800) and C/mZ was gray, and that of C/Z/K(600) was pale gray. Therefore, we can expect the presence of CuO in the three catalysts based on the color (Scheme 1). The color of C/Z/K(400) and C/aZ was green, resulting from the presence of green CuxZryOz

47

. Because of the green

color of C/Z/K(400), the Cu species on it was not CuO but CuxZryOz. Thus, the Cu species was selectively deposited on a-ZrO2 by a simple impregnation technique, but not on SiO2 (in this case KIT-6) (Scheme 1). By recognizing the presence of CuxZryOz, C/Z/K(600) exhibited a color between gray (CuO) and green (CuxZryOz).

3.3 Structures of the reduced catalysts Figure 5 shows the TPR profiles of the Cu catalysts. For C/Z/K(400), C/aZ and C/mZ, reduction peaks were observed at less than 200 °C. According to previous works, the peaks are attributed to the reduction of highly dispersed CuO

16, 31, 39, 48-50

. For C/Z/K(600) and

C/Z/K(800), we observed peaks at less than 200 °C as well as at greater than 200 °C. The peaks at higher temperature can be assigned to the reduction of crystalline CuO

16, 31, 39, 48, 49

. At >

300 °C, there was no peak in the TPR profile, indicating that the reduction of Cu catalysts was completed at less than 300 °C. Next, we estimated the amount of H2 consumption for the catalysts based on the area of the corresponding H2 consumption peaks. Table 1 shows the ratio of the H2 consumption amount to the loaded Cu2+ amount (RH2 cons.) for each catalyst. The RH2 cons.


16

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of C/Z/K(400) was only 44%, whereas those for the other catalysts were ca. 100%. Therefore, Cu2+ cations in C/Z/K(400) were partially reduced by H2. Figure 6a shows that the Cu K-edge XANES spectra for the Cu catalysts which were reduced by 20% H2/N2 at 300 °C for 15 min. For all the catalysts except C/Z/K(400), the XANES spectra were identical to that for Cu foil, which means that the Cu species on the catalysts were completely reduced to metallic Cu. In contrast, according to the XANES spectrum shape of C/Z/K(400), the reduced C/Z/K(400) consisted of Cu0 (12%), Cu2O (31%) and unreduced C/Z/K(400) (57%), as shown in Figure 6b. This XAS study, as well as H2-TPR, clarified that part of the Cu species in C/Z/K(400) was reduced by H2, resulting in the formation of Cu/Cu+ species (Scheme 1). The first derivatives of the XANES spectra (Figure S9) and RSFs (Figure S10) for the reduced catalysts also revealed differences in Cu species between C/Z/K(400) and the others. In addition, the reduction treatment did not change the ZrO2 phase of the catalysts, as shown in the Zr K-edge XANES spectra (Figure S6) and RSFs (Figure S7): the ZrO2 of reduced C/Z/K(400), C/Z/K(600) and C/aZ was amorphous, that of C/Z/K(800) was tetragonal and that of C/mZ was monoclinic.

3.4 Activity test For the as-prepared catalysts, catalytic activity tests were conducted using a high-pressure fixed-bed flow reactor (10 bars, 230 °C). Figure 7 shows CO2 conversion, methanol selectivity and CO selectivity against the contact time over the prepared catalysts. For all catalysts, CO2 conversion rose as the contact time increased. Under the reaction condition, the equilibrium conversion of CO2 was 15% if only the reactions of Eqs. 1-3 were assumed to take place. The


17


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order of CO2 conversion at the same contact time was C/Z/K(400) > C/mZ > C/aZ = C/Z/K(600) > C/Z/K(800), indicating that C/Z/K(400) was highly active in CO2 hydrogenation. We note that CO selectivity rose with an increase in contact time, while methanol selectivity decreased. Based on the shape of the selectivity curve,

51, 52

methanol is a primary product and

CO is a secondary product. Thus, this reaction is considered to be a step-wise reaction of CO2-tomethanol hydrogenation (Eq. 2) and methanol decomposition to CO (the reverse reaction of COto-methanol hydrogenation, Eq. 1). It is important to discuss the methanol selectivity of the prepared catalysts at the same CO2 conversion. Figure 8a shows the methanol selectivity against CO2 conversion for C/Z/K(T), C/mZ and C/aZ. The CO2 conversion was varied by changing the W/Ftotal from 430 to 2100 mgcat s mL(STP)-1. We note that the trend lines in Figure 8a moved to a region of higher methanol selectivity and higher CO2 conversion with decreasing calcination temperatures of the Z/K(T) samples. In particular, C/Z/K(400) and C/aZ showed excellent selectivity in CO2-tomethanol hydrogenation. Because the types of active sites should be reflected in the trend curves of methanol selectivity against CO2 conversion, the active sites on C/Z/K(400) and C/aZ were considered to be suitable for the hydrogenation of CO2 into methanol. Figure 8b summarizes the methanol and CO production rates for the prepared catalysts when W/Ftotal was 430 mgcat s mL(STP)-1. The order of methanol production rates was C/Z/K(400) (0.88 mmol h-1 gcat-1) > C/aZ (0.70 mmol h-1 gcat-1) > C/mZ (0.64 mmol h-1 gcat-1) > C/Z/K(600) (0.53 mmol h-1 gcat-1) > C/Z/K(800) (0.18 mmol h-1 gcat-1). While C/mZ was also active in CO2-to-methanol hydrogenation, the production rate of CO was highest (1.3 mmol h-1 gcat-1) among the prepared catalysts. Therefore, C/Z/K(400) and C/aZ are promising catalysts with high activity and selectivity in CO2-to-methanol hydrogenation. As described later, the low activity of C/Z/K(800)


18

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can be explained by the deposition of Cu species on KIT-6. It was reported that Cu/SiO2 was much less active in CO2 hydrogenation than Cu/ZrO2 22.

3.5 Characterization of spent catalysts PXRD patterns for the spent catalysts are displayed in Figure 1c. The ZrO2 phase in the samples, excluding C/aZ, was not changed by the reaction tests. The ZrO2 in spent C/aZ was mainly amorphous, with a minor amount of tetragonal and monoclinic phases. Because of the appearance of ZrO2-related peaks for spent C/Z/K(800) and C/mZ, the size of ZrO2 in the catalysts was larger than the detection limit for PXRD (> 10 nm). Two strong peaks of metallic Cu were observed at 43 and 51° for C/Z/K(600), C/Z/K(800) and C/mZ, indicating metallic Cu formation with a crystallite size of 40-50 nm by reduction of the catalysts. We can conclude that a large amount of Cu species on C/Z/K(600) and C/Z/K(800) were deposited on KIT-6 because the crystallite size of Cu (40-50 nm) for the catalysts was much larger than the pore size of KIT6 (ca. 8 nm). Importantly, weak and broad peaks of metallic Cu in Figure 1c were observed for C/Z/K(400) and C/aZ, indicating the formation of highly dispersed Cu nanoparticles (< 10 nm) on the catalysts. Figures 9 and S11-S13 show HAADF-STEM images of spent C/Z/K(T). In Figure 9, a well-ordered array of mesopores (ca. 8 nm) can be observed and was attributed to the mesoporous structure of KIT-6. Many brighter dots were observed in the images and attributed to Cu species or ZrO2. Because the Cu and ZrO2 appeared to be dispersed by the presence of KIT-6, KIT-6 acted as a spacer. According to the EDX analyses (Figures S11-S14), Cu and Zr species were detected in most areas, as indicated by boxes. This result probably indicates that the Cu species were located in close contact with ZrO2 (or CuxZryOz). However, it is difficult to


19


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conclude whether the Cu and ZrO2 particles were deposited into KIT-6 pores due to the larger size of several brighter dots than the KIT-6 pore size (ca. 8 nm). As described above, the PXRD results also indicated the formation of Cu and ZrO2 particles with a size greater than 10 nm. N2O titrations were conducted using the spent catalysts, as listed in Table 1. On the spent C/Z/K(T) catalysts, N2O consumption decreased from 29 to 0 µmol gcat-1 with increasing T (the calcination temperature of Z/K(T)). For the spent C/aZ and C/mZ, N2O consumption was 32 µmol gcat-1. Based on the N2O consumption, we estimated the average particle sizes of Cu. The sizes of C/Z/K(400), C/Z/K(600), C/aZ and C/mZ were 36, 77, 23 and 22 nm, respectively. Finally, for C/Z/K(400) and C/aZ, the Cu size which was calculated from N2O titration (36 or 22 nm) was much larger than the crystallite size of Cu (111) (< 10 nm, as described above). As reported in our previous work

21

, determination of an accurate Cu particle size for Cu-ZrO2

catalysts using N2O titration is difficult, probably due to a strong metal-support interaction (SMSI) effect 44.

3.6 Cu incorporation into a-ZrO2 An important aspect of this work is the formation of CuxZryOz only on a-ZrO2 by a simple impregnation technique. As shown in Figure 4, the powder color of C/aZ was green, providing evidence of CuxZryOz formation. If CuO particles were on the catalyst, the powder color of the sample would be gray, like C/Z/K(800) and C/mZ. However, we did not observe the incorporation of Cu species into the t-ZrO2 of Z/K(800) because the peak position of the t-ZrO2 (111) phase for Z/K(800) (Figure 1a) was identical to that for C/Z/K(800) (Figure 1b). In


20

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addition, we conclude that no Cu species were incorporated into m-ZrO2 based on the peak position of the m-ZrO2 (-111) phase for mZ (28.2 °, Figure 1a) and C/mZ (28.2 °, Figure 1b). Another key result is that small particles of CuxZryOz formed on KIT-6 of C/Z/K(400) (Scheme 1). The powder color of C/Z/K(400) was green, similar to that of C/aZ (Figure 4), providing simple and direct evidence of CuxZryOz formation. We observed a distortion of the coordination structure of copper oxides in C/Z/K(400) using XAS (Figure 3). According to the work reported by Shimokawabe et al., Cu2+ ions in Cu nitrate aqueous solution can adsorb on the a-ZrO2 surface during the impregnation process. 39

3.7 Role of a-ZrO2 for Cu/a-ZrO2 An important aspect of this work is the easy preparation of highly dispersed Cu nanoparticles on a-ZrO2, which is one of the advantages of the system of Cu and a-ZrO2. Here, we tried to interpret the mechanism of metallic Cu formation for the prepared catalysts by comparing C/aZ, C/mZ, C/Z/K(600) and C/Z/K(800). The CuO particles on C/Z/K(800), as well as C/Z/K(600) and C/mZ, were reduced by H2, resulting in metallic Cu formation with a crystallite size of 4050 nm (Scheme 1). In contrast, we must consider the presence of CuxZryOz on C/aZ. The CuxZryOz changed to Cu nanoparticles on a-ZrO2 (or on remaining copper zirconate) after being reduced by H2 at 300 °C and then treated with the reaction gas mixture. In fact, after the reaction test, the formed Cu nanoparticles on C/aZ were much smaller than those on C/Z/K(600), C/Z/K(800) and C/mZ, as evidenced by PXRD (Figure 1c).


21


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The Cu species on a-ZrO2 exhibited high selectivity in CO2-to-methanol hydrogenation (Figure 8a). As described in Section 3.4, CO2 hydrogenation over Cu/ZrO2 catalysts is a stepwise reaction of CO2-to-methanol hydrogenation and methanol decomposition. When the former reaction is improved and the latter reaction is suppressed, the selectivity to methanol should be high. First, we investigated the adsorption properties of CO2 on ZrO2, which should be related to the CO2-to-methanol hydrogenation activity

6, 18, 19, 22, 53

. Figure S15 shows the CO2 adsorption

isotherms for aZ and mZ with changing adsorption temperature. With increasing temperature, the amount of adsorbed CO2 decreased. Based on the Clausius-Clapeyron equation, the adsorption heats of CO2 on aZ and mZ were estimated to be 30-35 kJ mol-1. In this work, little difference was observed in the CO2 adsorption properties between aZ and mZ, which can mean that CO2 adsorption is not the rate-determining step for CO2-to-methanol hydrogenation. Next, we examined the adsorption properties of methanol on ZrO2 because we hypothesized that the produced methanol molecules could easily detach from the surface of a-ZrO2, resulting in the suppression of methanol decomposition (Scheme 2). One of the authors (Satokawa) reported methanol oxidative decomposition over ZrO2-supported Ag

54

and Z/K(T)

41

. It was

concluded that methanol was barely oxidized over the a-ZrO2-containing catalyst below 200 °C, while it was converted over the m-ZrO2-containing catalyst at ca. 200 °C. Therefore, when aZrO2 is used as a support, methanol decomposition can be suppressed. Figure 10a shows the methanol vapor sorption isotherms of aZ and mZ. For both ZrO2, the shapes of the methanol adsorption isotherms were similar to those of the N2 adsorption isotherms (Figure S2). For aZ, the amount of adsorbed methanol rapidly increased as PMeOH rose to 1 kPa. After that, the amount gradually increased as PMeOH, indicating the formation of multilayer physisorbed


22

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methanol on aZ. For mZ, the amount of adsorbed methanol increased to approximately 0.8 mmol g-1 with increasing PMeOH and then was steady, indicating the formation of a methanol monolayer on mZ. The amount subsequently increased again, which implied the formation of multilayer physisorbed methanol on mZ. Next, we estimated the adsorption heat of methanol on aZ and mZ using the ClausiusClapeyron equation (Eq. 5). For aZ (Figure 10b), regardless of the methanol coverage, the adsorption heat was 40-50 kJ mol-1. For mZ (Figure 10c), with increasing coverage from 1.0 to 1.5, the adsorption heat rapidly decreased from 66 to 38 kJ mol-1. When the methanol coverage on mZ was > 1.5, the adsorption heat was 38 kJ mol-1. Thus, it was clarified that methanol molecules more weakly adsorbed on the aZ surface than on the mZ surface when the methanol coverage was low. Finally, we adsorbed methanol on aZ and mZ and measured IR spectra, as shown in Figure 11. When 3 kPa of methanol was passed over aZ at 205 °C, a broad peak at 3100-2800 cm-1 was observed (Figure 11a) and attributed to gas phase methanol

55

. Of note, after evacuation of the

gaseous methanol at 205 °C, the band immediately disappeared (Figure 11b), which means that species containing C-H bonds did not exist on aZ surface. Next, we examined methanol adsorption on mZ using FTIR measurements. After 3 kPa of methanol was passed over mZ at 205 °C, gas phase methanol was observed as a characteristic band between 3100-2800 cm-1 (Figure 11c). In addition, two sharp peaks at 2930 cm-1 and 2817 cm-1 overlapped with the gas phase methanol peak. After the methanol was degassed at 205 °C, the two sharp peaks at 2930 and 2817 cm-1 were observed due to the methoxy species on ZrO2 (Figure 11d)

18, 22

. Through

these studies, we elucidated that at 205 °C methanol molecules did not adsorb on the aZ surface


23


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and strongly adsorbed on the mZ surface. This conclusion was also supported by the methanol vapor sorption experiments (Figure 10).

3.8 Metallic Cu surface on C/aZ The metallic Cu surface is well known as the active sites for RWGS. According to Copéret et al., CO2 conversion for Cu/SiO2 was much lower than that for Cu/ZrO2

22

. Thus, main active

sites on Cu/ZrO2 was the Cu-ZrO2 interface, not metallic Cu surface. However, the CO2 conversion on the metallic Cu surface was not negligible. When the trend line was extrapolated in Figure 8a, we estimated methanol selectivity at zero conversion of CO2. The methanol selectivity for C/aZ was ca. 80%. On the other hand, the CO selectivity at zero conversion of CO2 was ca. 20%, which can be derived from RWGS reaction over metallic Cu surface (Scheme 2).

3.9 Role of SiO2 in C/Z/K(400) Finally, we must consider why C/Z/K(400) produced methanol much faster than C/aZ. The difference between C/Z/K(400) and the others was the reducibility of Cu species by hydrogen, as shown in Section 3.3. After hydrogen treatment at 300 °C, Cu species were basically reduced to metallic Cu. In contrast, Cu species on C/Z/K(400) were incompletely reduced, leading to the formation of Cu/Cu+ species with crystallite sizes < 10 nm. We also verified the reproducibility of the Cu/Cu+ species formation, as shown in Figure S16. This phenomenon has also been reported by Dumesic et al. using Cu/ZrO2/SiO2 catalysts

20

. There is a possibility that the


24

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presence of SiO2 in Cu/ZrO2/SiO2 catalysts stabilized Cu+, which is well known to be an active species in CO2-to-methanol hydrogenation

6, 17, 56

. In addition, the ZrO2 phase of C/Z/K(400)

was amorphous, which could enhance the selectivity in CO2-to-methanol hydrogenation, as described in Section 3.7. At the present stage, however, it is difficult to precisely determine the role of SiO2 in C/Z/K(400).

4. Conclusions The effect of the ZrO2 phase on CO2-to-methanol hydrogenation was studied using Cu/a-ZrO2, Cu/m-ZrO2, Cu/a-ZrO2/KIT-6, and Cu/t-ZrO2/KIT-6. All of the catalysts were prepared by an incipient wetness impregnation, which is known as a simple preparation technique. Of note, methanol production for Cu/a-ZrO2 and Cu/a-ZrO2/KIT-6 was higher than those for the other catalysts. When a-ZrO2 or a-ZrO2/KIT-6 were used as a support, Cu species were highly dispersed on the ZrO2 because Cu nanoparticles were formed by hydrogen reduction of green CuxZryOz, which was observed only on a-ZrO2 containing Cu catalysts. In addition, methanol molecules more weakly adsorbed on a-ZrO2 than on m-ZrO. According to the activity tests, we clarified that CO2 hydrogenation over ZrO2-supported Cu catalysts is a step-wise reaction of CO2-to-methanol hydrogenation and subsequent methanol decomposition. Therefore, the weak adsorption of methanol on a-ZrO2 can suppress undesirable methanol decomposition, leading to a high selectivity towards methanol.

Corresponding Authors *

Shohei Tada ([email protected])

*

Shigeo Satokawa ([email protected])


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Supporting information Preparation procedure of KIT and additional figures including N2 and CO2 adsorption, XAS, and STEM-EDX.

Acknowledgements We are grateful to Dr. Büchel Robert and Dr. Frank Krumeich, ETH Zürich, for their kind help with STEM. The synchrotron radiation experiments were performed at the BL14B2 of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI, Proposal No. 2017A1557). This work was supported by the Japan Society for the Promotion of Science (JSPS, NO. 15J10157 and 16K18293).

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Table 1 Summary of the characteristics of the prepared catalysts. / N2O cons. a

R H2 cons. b

SSA

Vm

Loading wt%

/ m2 gcat-1

/ mLstp gcat-1

Cu

ZrO2 / µmol gcat-1

/%

KIT-6

987

1.33

-

-

-

-

Z/K(400)

435

0.53

-

43

-

-

Z/K(600)

298

0.39

-

45

-

-

Sample


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Z/K(800)

309

0.44

-

47

-

-

aZ

248

0.32

-

100 c -

-

mZ

92

0.28

-

100 c -

-

C/Z/K(400) 303

0.41

13

37

29

44

C/Z/K(600) 272

0.36

16

38

17

90

C/Z/K(800) 211

0.31

12

37

C/aZ

177

0.22

C/mZ

107

0.39

9.1 8.5

0d

97

76

32

105

82

32

91

a

Spent catalyst. b Ratio of H2 consumption to Cu2+ loadings for the prepared catalysts. The amount was estimated using the area of the corresponding H2 consumption peaks in TPR profiles (Figure 5). c ZrO2 + HfO2. d N. D.

Table 2 Parameters calculated by fitting the EXAFS signals of CuO, C/Z/K(800), C/Z/K(600), C/Z/K(400), C/aZ and C/mZ shown in Figure 3. Sample

Shell

R/Å

CN

σ2 / Å 2

∆E0 / eV

R factor

CuO

Cu-O

1.96

4 (fixed)

0.004

3.955

0.003

Cu-O

2.78

2 (fixed)

0.005

Cu-Cu

2.91

4 (fixed)

0.003


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Cu-Cu

3.08

4 (fixed)

0.005

C/Z/K(800) Cu-O

1.95

4.1

0.004

Cu-O

2.79

2.0

0.009

Cu-Cu

2.90

2.9

0.004

Cu-Cu

3.08

2.9

0.004

C/Z/K(600) Cu-O

1.95

4.0

0.004

Cu-O

2.79

2.0

0.014

Cu-Cu

2.90

2.2

0.003

Cu-Cu

3.07

2.2

0.004

1.94

4.3

0.006

Cu-Cu

2.92

2.3

0.016

C/aZ

Cu-O

1.94

4.0

C/mZ

Cu-O

1.95

Cu-O

3.535

0.002

3.392

0.003

-2.572

0.007

0.006

2.451

0.019

4.3

0.004

3.679

0.002

2.80

2.1

0.020

Cu-Cu

2.89

2.4

0.004

Cu-Cu

3.06

2.4

0.005

C/Z/K(400) Cu-O

Notations: R, distance; CN, coordination number; σ, Debye-Waller factor; ∆E0, increase of the threshold energy. Intrinsic loss factor S02 = 0.816 (nearest-neighbor Cu-O from CuO) for Cu-O and S02 = 0.748 (nearest-neighbor Cu-Cu from CuO) for Cu-Cu.


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Figure 1 PXRD patterns of the prepared samples. (a) Support, (b) as-prepared Cu catalyst, and (c) spent Cu catalysts.


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Figure 2 (a) Normalized Cu K-edge XANES spectra and (b) derivative XANES spectra of C/Z/K(T), C/aZ, C/mZ, Cu foil, Cu2O and CuO.


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Figure 3 Fourier transforms of k3-weighted EXAFS oscillation measured at room temperature near the Cu K-edge of C/Z/K(T), C/aZ, C/mZ, Cu foil, Cu2O and CuO. k range: 30-110 nm-1. R range: 1.0-3.2 Å. Solid line-experiment; dashed red line-EXAFS model.


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Figure 4 Photographs of as-prepared C/Z/K(T), C/aZ and C/mZ.


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Figure 5 TPR profiles for C/Z/K(T), C/aZ and C/mZ.


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Figure 6 (a) Normalized Cu K-edge XANES spectra of reduced C/Z/K(T), C/aZ and C/mZ. Cu foil, Cu2O and CuO were used as standard references. The catalysts were reduced at 300 °C for 15 min with 20% H2/He. (b) Linear combination fitting analysis of a XANES spectrum for reduced C/Z/K(400). The blue line represents the experimental data, the dotted black line is the fit and the red lines are the standards. The contributions of the reference Cu compounds are as follows: 0.573 (0.018) for as-prepared C/Z/K(400), 0.115 (0.012) for Cu foil and 0.312 (0.012) for Cu2O.


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Figure 7 CO2 conversion, methanol selectivity and CO selectivity against the contact time over (a) C/Z/K(400), (b) C/Z/K(600), (c) C/Z/K(800), (d) C/aZ and (e) C/mZ. Reaction conditions: CO2/H2/N2 = 1/3/1, catalyst loading = 500 mg, reaction temperature = 230 °C, pressure = 10 bar. Before the reaction test, all the catalysts were reduced at 300 °C by 16% H2/N2 (72 mL(STP) min-1) for 2 h under ambient pressure.


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Figure 8 (a) Methanol selectivity at different CO2 conversions for C/Z/K(400), C/Z/K(600), C/Z/K(800), C/aZ and C/mZ. The CO2 conversion was varied by changing the contact time from 430 to 2100 mgcat s mL(STP)-1. (b) Methanol and CO production rates for the catalysts when W/Ftotal was 430 mgcat s mL(STP)-1. Reaction conditions: CO2/H2/N2 = 1/3/1, catalyst loading = 500 mg, reaction temperature = 230 °C, pressure = 10 bar. Before the reaction test, all catalysts were reduced at 300 °C by 16% H2/N2 (72 mL(STP) min-1) for 2 h under ambient pressure.


43


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Figure 9 HAADF STEM images of spent C/Z/K(T). The scale bar in the images is 20 nm. EDX analyses were carried out in the area indicated by yellow boxes. The EDX results are summarized in Figure S11. The other HAADF-STEM images are summarized in Figures S1214.


44

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Figure 10 (a) Methanol vapor sorption isotherms of (filled symbols) aZ and (open symbols) mZ at different temperatures: (circle) 15 °C, (square) 20 °C, (diamond) 25 °C and (triangle) 30 °C. Adsorption heat of methanol for (b) aZ and (c) mZ estimated using the Clausius-Clapeyron equation (Eq. 5). Monolayer adsorption amounts of aZ and mZ were 1.6 and 0.67 mmol g-1, respectively.


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Figure 11 FTIR spectra of methanol and methoxy species on (a, b) aZ and (c, d) mZ at 205 °C. (a, c) The spectra were recorded after admission of methanol (3 kPa) for 20 min at 205 °C. (b, d) The spectra were recorded after admission of methanol (3 kPa) for 20 min at 205 °C and then evacuation of methanol for 10 min at 205 °C.


46

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Scheme 1 Schematic image of the catalyst preparation of C/aZ, C/Z/K(400), C/Z/K(800) and C/mZ (from the top).


47


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Scheme 2 Proposed mechanism of CO2-to-methanol hydrogenation of Cu nanoparticles on ZrO2. (i) CO2 is hydrogenated to methanol at the interface between Cu and ZrO2. (ii) When methanol strongly adsorbs on the ZrO2 surface, methanol is decomposed to CO. (iii) When methanol weakly adsorbs on the ZrO2 surface, the methanol easily desorbs from the surface, resulting in the suppression of methanol decomposition. (iv) CO2 is converted to CO via RWGS reaction on the metallic Cu surface.


48

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TOC 133x69mm (96 x 96 DPI)


Cu Species Incorporated into Amorphous ZrO2 with High Activity and

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