Flame spray pyrolysis makes highly loaded Cu nanoparticles on ZrO2 for CO2-to-methanol hydrogenation

Chemical Engineering Journal 381 (2020) 122750

Contents lists available at ScienceDirect

Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Flame spray pyrolysis makes highly loaded Cu nanoparticles on ZrO2 for CO2-to-methanol hydrogenation

T

Shohei Tadaa, , Kakeru Fujiwarab, Taihei Yamamuraa, Masahiko Nishijimac, Sayaka Uchidad, Ryuji Kikuchia ⁎

a

Department of Chemical System Engineering, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Department of Chemistry and Chemical Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan c The Electron Microscopy Center, Tohoku University, 2-1-1 Katahira, Aoba-ku, Miyagi 980-8577, 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 b

HIGHLIGHTS

GRAPHICAL ABSTRACT

catalysts were prepared by • CuO/ZrO a flame spray pyrolysis (FSP) tech2

nique.

FSP technique is promising as a • The simple and fast catalyst preparation process.

FSP technique can deposit small • The metal particles at high loading on metal oxide.

FSP catalysts hydrogenated CO • The to methanol. catalyst with 60 wt% of Cu was • The more active than a commercial CuO/ 2

ZnO/Al2O3.

ARTICLE INFO

ABSTRACT

Keywords: CO2 hydrogenation Methanol synthesis Copper Zirconia Flame spray pyrolysis

This paper deals with CuO/ZrO2 catalysts with extremely high Cu loading and their catalytic activity for CO2 hydrogenation to methanol. Because of aiming an industrial application, we chose a flame spray pyrolysis (FSP) technique as a simple and rapid catalyst preparation method. Thanks to the FSP, we succeeded to prepare 20–80 wt% CuO/ZrO2 catalysts. Interestingly, the catalyst structure changed with the Cu loading. In the case of Cu loading = 20 wt%, CuO nanoparticles (ca. 5 nm) were supported on tetragonal ZrO2 particles (5–10 nm), observed by high-angle annular dark-field scanning transmission electron microscopy. Of note, the catalyst with 60 wt% of Cu was ZrO2@CuO core-shell nanoparticles: ZrO2 aggregates were covered with many CuO nanoparticles (< 5 nm). When the Cu loading was 80 wt%, crystalline CuO particles (ca. 10 nm) as well as CuO nanoparticles (< 5 nm) were supported on the above ZrO2 aggregates. The catalysts reduced by H2 at 300 °C consisted of Cu nanoparticles (< 20 nm) and ZrO2 nanoparticles (5–10 nm). With decreasing the Cu loading, the interaction between the Cu and the ZrO2 became strong. The strong interaction caused high selectivity to methanol. In contrast to 20 wtCu% CuO/ZrO2, 80 wtCu% CuO/ZrO2 contained a large number of active sites for CO2 conversion, while the interaction between Cu and ZrO2 was weak. Therefore, the catalyst exhibited high yield and low selectivity to methanol. Among the prepared catalysts, at Cu loading = 60 wt%, the catalytic performance was better than that of a commercial CuO/ZnO/Al2O3. This is because the catalyst combined the ad-

⁎

Corresponding author. E-mail address: [email protected] (S. Tada).

https://doi.org/10.1016/j.cej.2019.122750 Received 5 July 2019; Received in revised form 2 September 2019; Accepted 6 September 2019 Available online 07 September 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.

Chemical Engineering Journal 381 (2020) 122750

S. Tada, et al.

vantages of both the 20 wt% CuO/ZrO2 (Cu-ZrO2 interaction) and the 80 wt% CuO/ZrO2 (a large number of active sites).

1. Introduction

relationship between the FSP condition and the catalytic performance, little is known about the relationship between the catalyst structure and the catalyst performance. In this study, we prepared several types of CuO/ZrO2 catalysts, with Cu loading = 20–80 wt%, using an FSP technique and studied the dependency of the catalyst structure on the Cu loading. In addition, we investigated the effect of the catalyst structure on methanol synthesis via CO2 hydrogenation using several characterization techniques. We succeeded to obtain high Cu loading catalysts (20–80 wt%). All the CuO/ZrO2 catalyst hydrogenated CO2 more selectively to methanol than did a commercial CuO/ZnO/Al2O3 catalyst. Of note, the as-prepared catalyst with Cu loading = 60 wt% had a unique core (ZrO2 particles)- shell (CuO layer) structure. The 60 wtCu% CuO/ZrO2 catalyst reduced by H2 at 300 °C consisted of Cu nanoparticles (10–20 nm) supported on ZrO2 particles (5–10 nm) which was active and selective to the CO2 hydrogenation. In addition, CO evolution was suppressed with decreasing Cu loading, probably because of strong Cu-ZrO2 interaction.

Methanol synthesis via CO2 hydrogenation is a key reaction to mitigate the increasing concentration of CO2 and then to achieve a sustainable methanol-based economy [1]. In the present industrial process, a Cu/ZnO/Al2O3 catalyst is used for methanol synthesis from syngas (CO/CO2/H2) to methanol at 220–300 °C and 5–10 MPa [2]. However, its catalytic activity is sensitive to the composition ratio of CO to CO2 in the reactant gas, and methanol production rate only via CO2 hydrogenation over Cu/ZnO/Al2O3 is less than that via hydrogenation of CO/ CO2 mixed gas [2]. Massive effort has been made to develop new catalysts for the CO2 hydrogenation such as Cu/ZrO2 [3–6], Cu/ZrO2/SiO2 [7], Cu/ZnO/ZrO2 [8,9], Cu/CeO2/TiO2 [10], Cu/TiO2/SiO2 [11], MnOx/m-CoOx [12], In2O3/ZrO2 [13], Ni-Ga/SiO2 [14], ZnO/ZrO2 [15], Pd/ZnO [16], and Pd-Cu/SiO2 [17], as well as Cu/ZnO/Al2O3 [18–20]. We have focused on Cu/ZrO2 which is active and selective in the CO2 hydrogenation [21–27], and reported that amorphous ZrO2 supported Cu catalysts (Cu/a-ZrO2) [24,25] was more selective to methanol than Cu/ZrO2 prepared by coprecipitation [23] and Cu/ZnO/ Al2O3 [23]. The CO2 hydrogenation proceeds at Cu-ZrO2 interfacial sites as follows: CO2 is hydrogenated to methanol (Eq. (1)) [21,22], and the thus-formed methanol is decomposed to CO (Eq. (2)) [23–25,27]. In parallel, on metallic Cu surface, CO2 is directly reduced to CO via reverse water gas shift reaction (RWGS reaction, Eq. (3)) [23–25,27]. Here, we have to design and prepare suitable catalysts to improve the methanol production and suppress the CO evolution.

CO2 + 3H2

CH3 OH + H2 O

r H(298K)

=

49.5kJ mol

1

2. Experimental procedures 2.1. Catalyst preparation An FSP reactor (Wegener consulting, LS-FSR 2.1) was used to prepare CuO/ZrO2 catalysts, as described in Scheme 1. The catalyst with × wt% of Cu was named xFSP (x = 0–80). Appropriate amount of copper acetate monohydrate (Wako, purity > 99.0%) and zirconyl 2ethylhexanoate in mineral spirits (Wako, 12% as Zr) were dissolved in a 1:1 volumetric mixture of 2-etylhexanolic acid (Sigma-Aldrich, purity > 99%) and methanol (Wako, purity > 99.5%) to be 0.2 M of the total metal concertation. Only for a Cu-free precursor, 1 vol% of acetic acid (Wako, purity > 99.7%) was mixed with the solvent before adding the Zr precursor. The precursor solution was fed to the FSP reactor at 2 mL min−1, dispersed to a fine spray by 8 L min−1 of oxygen and ignited/sustained by a CH4/O2 pilot-flame (CH4: 1.5 L min−1, O2: 3.2 L min−1) to produce particles. In this condition, the metal precursors were decomposed to metal oxides (see ESI and Fig. S1). The particles were collected on a glass-fiber filter (Albet LabScience, GF6,

(1)

[28]

CH3 OH

CO + 2H2

r H(298K)

= 90.6 kJ mol

(2)

1

[28]

CO2 + H2

CO + H2 O

r H(298 K)

= 41.2 kJ mol

1

(3)

[28] Since the active sites of the CO2 hydrogenation are along the CuZrO2 interface, methanol production over Cu/ZrO2 catalysts is expected to increase with increasing the number of the Cu-ZrO2 interfacial sites. Accordingly, the high-performance Cu/ZrO2 catalysts require the followings: high dispersion of Cu nanoparticles, high Cu loading, and high surface area of ZrO2 support. In addition, for industrial applications, simple and fast catalyst preparation processes are suitable. Here, we focus on flame spray pyrolysis (FSP) [29] as a continuous aerosol process that can be scaled up to kg h−1 [30] of the production rate. So far, FSP has been utilized to prepare various metal catalysts supported on metal oxides [31,32] including Cu catalysts for several reactions (e.g. CO hydrogenation to dimethyl ether [33], water gas shift reaction [34] and preferential CO oxidation [35]), proving their high catalytic activity. Previously, we reported 11–14 wt% Cu/ZrO2 catalysts for CO2to-methanol hydrogenation which had better performance than a commercial Cu/ZnO/Al2O3 catalyst [26]. Such FSP-made catalysts tend to have higher specific surface area and metal dispersion than the wetmade ones [31,32]. Furthermore, FSP can deposit small metal particles at extremely high metal loading on metal oxide particles which is hardly attained by wet-preparation methods, for example 50 wt% Ag/ SiO2 with the Ag size of 8.2 nm [36]. Thus, this capability could maximize the interfacial sites of Cu-ZrO2 by increasing Cu loading while keeping Cu particle small. Recently, we have investigated the effect of the flame condition on crystallinity and activity of 60 wt% CuO/ZrO2 catalysts and found the best operation condition of the preparation of the CuO/ZrO2 catalysts [37]. Though the study has revealed the

Scheme 1. Catalyst preparation by flame spray pyrolysis. 2

Chemical Engineering Journal 381 (2020) 122750

S. Tada, et al.

257 mm in diameter) located 62 cm above the FSP nozzle by a vacuum pump (Busch Seco SV1040C). In addition, a CuO/ZrO2 (Cu loading = 20 wt%) was also prepared by a wetness impregnation method. The FSP-made ZrO2 (0FSP) was impregnated with an aqueous solution of copper acetate. The obtained powders were dried at 110 °C for 12 h, and then calcined at 350 °C for 5 h. The obtained sample was named 20IMP. As a benchmark catalyst, we used a commercial catalyst for methanol synthesis CuO/ZnO/Al2O3 (MDC-3, Clariant Catalysts (Japan) K.K., prepared by a sol-gel based method [38]) and an amorphous ZrO2 supported Cu catalyst (9 wt% Cu/a-ZrO2) reported in our previous study [25].

content of the as-prepared catalysts, as listed in Table 1. The Cu content was almost the same to that estimated from the ratio of the Cu precursor to Zr precursor. Specific surface area (SSA) of Cu-based catalysts are shown in Table 1. The SSA of MDC-3 and 0FSP are 62 m2 g−1 and 235 m2 g−1, respectively. The SSA of FSP-made CuO-ZrO2 catalysts gradually decreases with increasing the Cu content from 0 to 80 wt%. Impregnation of Cu species into FSP-made ZrO2 also decreases the SSA from 235 to 123 m2 g−1. Fig. 1 shows the powder X-ray diffraction (PXRD) patterns of the asprepared catalysts with 20 wt% of NiO as an internal standard. In all of the patterns, tetragonal ZrO2 peaks are observed at 30 and 35°. These peaks are not shifted with Cu loading indicating that Cu species were present predominantly on the ZrO2 surface. In addition, their intensity becomes lower with the Cu loading. To identify CuO, we focused on the peak at 39° but not at 35° because it overlaps with the peak of tetragonal ZrO2 (2 0 0). Of note, the CuO peak does not appear for 20FSP, 40FSP, and 60FSP, while it does for 80FSP. Thus, for the FSP-made catalysts with Cu loading ≤ 60 wt%, the Cu species was mainly amorphous and/or the size of the Cu species was smaller than the detectable limit of PXRD (< 5 nm). The crystallite size of CuO (1 1 1) for 80FSP is 11 nm. Fig. S2 exhibits the PXRD patterns of 20IMP and MDC-3. For 20IMP, the peaks of tetragonal ZrO2 and CuO are observed. The crystallite size of CuO (1 1 1) for 20IMP is 18 nm. For MDC-3, the CuO peaks are overlapped with those of ZnO (at 32, 34, and 36°) and α-Al2O3 (at 26, 35, and 38°). Next, we carried out methanol vapor sorption on 0FSP to evaluate the surface property of the ZrO2. According to our previous work, methanol molecules more strongly adsorbs on a crystalline ZrO2 surface than do those on an amorphous ZrO2 surface, judging from the plots of methanol adsorption heat against methanol coverage [24]. Fig. 2 shows the adsorption heat of methanol on 0FSP against methanol coverage. The adsorption heat was calculated from methanol vapor sorption isotherms (Fig. S3) and the Clausius-Clapeyron equation (Eq. (S1)). When the methanol coverage is > 1, the adsorption heat is between 30 and 40 kJ mol−1. The adsorption heat dramatically increases with decreasing the coverage from 1 to 0.7. This behavior of 0FSP is similar to that of crystalline ZrO2, which we reported before [24]. Therefore, FSPmade ZrO2 was crystalline, in line with the above PXRD results (Fig. 1). We tried to obtain the morphology of the FSP-made catalysts. Figs. 3 and S4 show the HAADF-STEM images and EDX mapping of 20FSP, 60FSP, and 80FSP. For 20FSP, each ZrO2 particle (5–10 nm) was connected, leading to the aggregate formation. According to Cu EDX mapping, the size of Cu species is about 5 nm. Interestingly, for 60FSP and 80FSP, the location of Cu and Zr signals completely overlaps each other. By reducing 60FSP and 80FSP in 4%H2/Ar at 300 °C for 1 h, the Cu and Zr signals are segregated, which indicates that the resultant Cu particles of ca. 10–15 nm were sitting on the ZrO2 aggregates.

2.2. Reaction test Activity tests were carried out using a homemade fixed-bed tubular reactor. The shape of the catalyst bed was a column (radius = 6 mm, height = ca. 50 mm). We monitored a temperature at the center of the catalyst bed using a K-type thermocouple. The sample of 500 mg and quartz sand (Wako) of 1 g were placed into a catalyst bed. Next, 5% H2/ Ar gas (50 mL(STP) min−1) was introduced into the catalyst bed at 300 °C for 2 h at 1 bar. Then, the catalyst bed was cooled down to 230 °C. After that, we switched the 5% H2/Ar gas to a reaction gas (CO2/H2/N2 = 1/3/1, 50 mL(STP) min−1). While feeding the reaction gas, the pressure in the catalyst bed was fixed at 10 bar. We controlled the inlet gas flow rate from 50 to 20 mL(STP) min−1, to investigate the effect of the contact time. The composition of the product gas was analyzed using on-line gas chromatographs (Shimadzu, GC8A) equipped with a flame ionization detector (for methanol) and a thermal conductivity detector (for N2, CO, CO2, and CH4). The carbon balance was 100 ± 1% by taking into account the amount of CO2, CO, CH4 (negligible) and methanol at the outlet. The selectivity to A species (Eq. (4)), CO2 conversion (Eq. (5)), and the production rate of A species (rA, Eq. (6)) were determined as follows:

Selectivity to A species [ ] = FA,out (Fmethanol,out + FCO,out )

(4)

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

(5)

rA [mol A h

1g

cat

1]

(6)

= FA,out W , −1

where FA, in is the inlet flow rate of A species (mol h ), FA, out is the outlet flow rate of A species (mol h−1), and W is the amount of catalyst (gcat). 3. Results and discussion 3.1. Structure of FSP catalysts According to X-ray Fluorescence (XRF) results, we calculated the Cu

Table 1 Cu loading, specific surface area, Cu dispersion, Cu particle size, and Cu crystallite size. Sample

Cu loading/wt%

SSA /m2 g−1

Cu dispersionb/%

Cu particle size

0FSP 20FSP 40FSP 60FSP 80FSP 20IMP MDC-3

– 19 38 58 78 19 40a

235 184 176 136 102 123 62

– 2.1 1.7 1.7 1.1 1.2 4.9

– Ca. 5 d – 10–15b – – –

a b c d e f g

Provided from Clariant Catalysts (Japan) K.K. Catalyst reduced by H2 at 300 °C. Estimated from STEM. As-prepared catalyst. Spent catalyst. Estimated from PXRD. The corresponding peak is overlapped with the PXRD peak of α-Al2O3. 3

b,c

/nm

DCuO(1 1 1) d,f/nm

DCu(1 1 1) e,f/nm

– N. D. N. D. N. D. 11 18 –g

– N. D. 15 – 15 36 36

Chemical Engineering Journal 381 (2020) 122750

S. Tada, et al.

the interaction between Cu species and ZrO2. As shown in the HAADFSTEM images (Figs. 3 and S4), the size of catalyst powders was ca. 10 nm, which means that the XPS/AES signals contained not only the surface information but also the bulk information. In fact, the molar ratio of Cu to Zr estimated by XPS is identical to that estimated by XRF (Fig. S6). Fig. 5 shows Cu 2p3/2 XPS and Cu LMM AES spectra of the FSP-made catalysts and Table 2 summarizes their XPS/AES parameters. First, the as-prepared catalysts were examined. The XPS spectra have the main peak (933.5–933.8 eV) and large satellite peaks (940–945 eV), corresponding to Cu2+ [39–41]. The AES spectra have a strong peak (1G multiplet of the two-localized-hole d8 final state) at 916.2–917.3 eV, which is located at lower kinetic energy than that of CuO (918.0 eV) [39,42]. Here, we introduced the Auger parameter, α′, to determine the Cu oxidation state precisely. For 60FSP and 80 FSP, α′ values of the asprepared catalysts were similar to that of CuO (ca. 1851 eV) but not to Cu2O (ca. 1849 eV) reported in [39]. Interestingly, α′ gradually increased with the Cu loading from 20 wt% to 60 wt%. It is known that the peak position of Cu LMM Auger spectra as well as α′ is strongly affected by Cu nanoparticle shape and/or interaction between Cu and support [39,40,43]. Therefore, we expected that the Cu2+ species of 20FSP and 40FSP were well dispersed and/or interacted with ZrO2. Next, we studied the Cu oxidation state of the catalysts reduced by H2 at 300 °C using XPS/AES spectra (Fig. 6 and Table 2). In Cu 2p3/2 XPS spectra, an intense peak appears at around 932.4 eV, assigned to Cu0 or Cu+ [39,40]. In Cu LMM AES spectra of the reduced catalysts, the 1G peak at ca. 917 eV and the shoulder (the 3F multiplet of the localized d8 final state) at ca. 920 eV indicate the presence of metallic Cu [42]. However, the α′ value of the reduced catalysts (1849.4–1849.7 eV) is smaller than that of reported metallic Cu (1851.3 eV) and close to that of Cu2O (1849.3 eV). This might be due to the dispersion of metallic Cu species and the interaction between metallic Cu and ZrO2 [39,40,43–45]. Fig. 6 exhibits the Zr 3d XPS spectra of the as-prepared and H2reduced catalysts. We deconvoluted the spectra into four peaks, as summarized in Table 2. Two intense peaks at 182.0 eV and 184.4 eV are Zr 3d5/2 and Zr 3d3/2 peaks corresponding to Zr4+, respectively [46,47]. Most of the XPS spectra possess two additional peaks at ca. 183 and 186 eV, which are Zr 3d5/2 and Zr 3d3/2 peaks of Zr species bound to a more electron attractive species (Zrq+, q < 4) [46,47]. In Table 2, we listed the ratio of Zrq+ to Zr4+ for the FSP-made catalysts. The ratios for the as-prepared catalyst are similar to one another (0.05–0.09). The Zrq+/Zr4+ ratios of H2-reduced 20FSP and 40FSP are 0.09 and 0.30, respectively. The Zrq+ amounts of H2-reduced 20FSP and 40FSP are proportional to the amount of Cu species, while the Zrq+ amounts of H2-reduced 60FSP and 80FSP are almost zero. Fig. S8 shows the O 1s XPS spectra of the catalysts which were deconvoluted into three peaks. A peak at 530 eV (Table S1) corresponds to lattice oxygen (Olat) of ZrO2 [46,48] and CuO [49,50]. A peak at 531 eV (Table S1) is attributed to weakly-charged oxygen species (Oadd) on ZrO2 and CuO, like surface hydroxyl groups and defective oxygen [46,50–52]. A peak at 532 eV (Table S1) is attributed to organic compounds such as ester and carbonate [53] and/or adsorbed water [52]. For the H2-reduced catalysts, the sum of the amounts of Olat and Oadd is approximately two-fold more than the Zr amount (Table 2), indicating that the O-containing compounds in the H2-reduced catalysts were mostly ZrO2. We carried out N2O pulse titration of the Cu-based catalysts reduced at 300 °C to measure Cu dispersion (=the exposed Cu0 amount per unit Cu loading amount) on the catalysts. Accurate size determination for metallic Cu nanoparticles on ZrO2 is difficult because of a strong metalsupport interaction (SMSI) [24–26], while a higher dispersion can translate into a larger amount of the interfacial sites between Cu0 and ZrO2. We summarize the Cu dispersion of the catalysts in Table 1. For the FSP-made CuO-ZrO2 catalysts, the Cu dispersion gradually decreases with increasing the Cu loading. The Cu dispersion of 20IMP is

Fig. 1. PXRD patterns of as-prepared catalysts. The peak positions were corrected by NiO (1 1 1) peak (37°).

Fig. 2. Methanol adsorption heat of the ZrO2 estimated from methanol vapor sorption isotherms (Fig. S3) and the Clausius-Clapeyron equation (Eq. (S1)). A monolayer adsorption amount is 1.4 mmol g−1.

3.2. Reducibility of FSP-made catalysts We carried out H2-TPR measurements for the FSP-made catalysts, as shown in Fig. 4, to check the adequate reduction temperature and reducibility of their Cu species. Regardless of the Cu content, a broad peak appears at 300 °C. The peak grows with increasing the Cu content. We estimated the number of Cu species on ZrO2 surface, assuming that the peak corresponds to CuO reduction to Cu. Fig. S5 shows the correlation between Cu loadings determined by XRF and H2-TPR. The Cu content determined by XRF is almost identical to that determined by H2-TPR. Therefore, we concluded that the peak at 300 °C is attributed to Cu2+ reduction to Cu0, and that the Cu species in the FSP-made catalysts were completely reduced at 300 °C. We examined the oxidation states of Cu, O, and Zr species using XPS/AES to further investigate the oxidation state of Cu and Zr species before and after the reduction. The aim of the measurements is to reveal 4

Chemical Engineering Journal 381 (2020) 122750

S. Tada, et al.

Fig. 3. HAADF-STEM images of (a) 20FSP, (b) 60FSP, and (c) 60FSP reduced by 4% H2/Ar at 300 °C for 1 h. More images are shown in Fig. S4.

similar to that of 80FSP (1.1%). In addition, the Cu dispersion of MDC-3 is much larger than those of the other catalysts. 3.3. Catalytic activity Catalytic activity over Cu-based catalysts was examined using a high pressure fixed-bed tubular reactor. For all of the catalysts, the product of the CO2 conversion is CO and methanol. Fig. 7a summarizes methanol production rates over the Cu-based catalysts at the contact time = 600 gcat s L(STP)−1. The methanol production rate of the benchmark catalyst, MDC-3 (19 mL(STP) h−1 gcat−1), is higher than those of 20FSP and 40FSP, while it is lower than those of 60FSP and 80FSP. Interestingly, 20FSP is more active in methanol production than 20IMP. Fig. 7b shows CO production rates over the Cu-based catalysts. The CO production rate of MDC-3 (124 mL(STP) h−1 gcat−1) and 80FSP (58 mL(STP) h−1 gcat−1) are much higher than that of the other catalysts (< 15 mL(STP) h−1 gcat−1). We judged the quality of the active sites for each catalyst based on the relationship between CO2 conversion and methanol selectivity. Fig. 8 illustrates methanol selectivity against CO2 conversion for the Cubased catalysts at the different contact time (60–1500 gcat s L(STP)−1). Some data of 80FSP and MDC-3 are strongly affected by the chemical

Fig. 4. H2-TPR profiles for 20FSP, 40FSP, 60FSP, and 80FSP. 5

Chemical Engineering Journal 381 (2020) 122750

S. Tada, et al.

Fig. 5. (a) Cu LMM AES and (b) Cu 2p3/2 XPS spectra of FSP-made catalysts. An enlarged figure about Cu LMM AES spectrum of 20FSP is shown in Fig. S7.

equilibrium of CO2-to-methanol hydrogenation because the equilibrium conversion of CO2 under this condition is 14.9%. On the other hand, the equilibrium has a low impact on the data of 20FSP, 40FSP, 60FSP, and 20IWI because of their low CO2 conversion (< 5%). On each catalyst, CO2 conversion increases with increasing contact time. Simultaneously, methanol selectivity decreases. Thus, CO2 hydrogenation on the Cubased catalysts is step-wise from CO2-to-methanol hydrogenation (Eq. (1)) to methanol decomposition (a reverse reaction of CO-to-methanol hydrogenation, Eq. (2)). In Fig. 8, there are four interpolated lines indicating the different types of active sites of the catalysts [23–27]. For example, if the catalyst is highly selective in CO2-to-methanol hydrogenation, its interpolated line is located on the upper side of the figure. We observe the four types of the interpolated lines of CO2 conversion vs. methanol selectivity: (i) 20FSP, (ii) 40FSP and 60FSP, (iii) 80FSP and

20IMP, and (iv) MDC-3. It was obvious that FSP-made CuO/ZrO2 catalysts, especially 20FSP, 40FSP, and 60FSP were selective to methanol formation than MDC-3. For the four FSP-made catalysts, the position of the interpolated line moves up with decreasing Cu loading. According to the results of Fig. 7 and 8, 60FSP was the best catalyst with high activity and selectivity in CO2-to-methanol hydrogenation. 3.4. Crystal structure of spent catalysts We characterized the crystal structure of the spent catalysts using PXRD (Fig. 9). Prior to the measurements, the spent catalysts were taken out from the catalyst bed and stored at room temperature under air. In all patterns, the peaks at 31 and 36° are related to tetragonal ZrO2. The PXRD pattern of 20FSP has several peaks corresponding to

Table 2 XPS/AES parameters of FSP-made catalysts. Sample

α′a/eV

Position/eV Cu 2p3/2

Molar ratioc

Zr 3d5/2b

Cu

Cu LMM

Position/eV Zr4+

Zrq+

FWHM/eV

Zrq+/Zr4+

(Olat + Oad)/Zrd

As-prepared catalyst 20FSP 933.5 40FSP 933.8 60FSP 933.7 80FSP 933.8

916.2 916.9 917.2 917.3

1849.7 1850.4 1850.9 1851.1

182.0 182.0 182.0 182.0

183.2 183.6 183.2 183.2

1.5 1.5 1.4 1.4

0.08 0.05 0.09 0.07

3.0 3.9 4.7 9.6

H2-reduced catalyst 20FSP 932.3 40FSP 932.4 60FSP 932.3 80FSP 932.5

917.1 917.3 917.3 917.3

1849.4 1849.5 1849.6 1849.7

182.0 182.0 182.0 182.0

183.0 183.2 183.2 183.2

1.5 1.5 1.4 1.4

0.09 0.30 0.03 0.01

2.3 2.5 2.5 2.6

Reference Cue Cu2Oe CuOe

918.6 916.6 918.0

1851.3 1849.3 1851.3

a b c d e

932.7 932.5 933.6

Auger parameter = binding energy of Cu 2p3/2 + kinetic energy of Cu LMM. Binding energy of a Zr 3d5/2 peak is 2.4 eV lower than that of a Zr 3d3/2 peak [46,47]. The peak area ratio of Zr 3d5/2 to Zr 3d3/2 is 1.5. Estimated from XPS spectra in Figs. 5a, 6 and S8. Olat and Oad are characterized by the O 1s peaks at 529 and 531 eV, respectively. Cited from Ref. [39]. 6

Chemical Engineering Journal 381 (2020) 122750

S. Tada, et al.

Fig. 6. Zr 3d XPS peaks for (a-d) as-prepared and (e-h) H2-reduced catalysts. The catalysts reduced by H2 at 300 °C are named H2-reduced catalysts.

quartz that was probably contaminated because of insufficient separation of the quartz sand from the catalyst bed. Interestingly, for 20FSP, no peak of Cu species is observed, which means that the size of the Cu species was smaller than the PXRD detectable limit (< 5 nm). In the patterns of 40FSP, 80FSP, and 20IMP, a sharp peak is present at 43.3° related to Cu (1 1 1) phase. Their crystallite size of the Cu (1 1 1) phase (DCu(1 1 1)) was calculated from Scherrer′s equation (Eq. (S2)) to estimate the Cu particle size roughly (Table 1). Importantly, DCu(1 1 1) of 40FSP and 80FSP (ca. 15 nm) are smaller than that of 20IMP (36 nm). In addition, the pattern of 80FSP possesses two broad peaks at 37 and 42°, which are attributed to CuO. We expected that the storage of 80FSP under air proceeded the oxidation of metallic Cu to Cu2O and CuO. Of note, the crystallite size of Cu2O (1 1 1) phase, corresponding to the peak at 37°, is 8 nm, which is much smaller than DCu(1 1 1) of 20IMP. It

was concluded that the particle size of Cu species of the spent FSP-made catalysts was smaller than that of the catalyst prepared by an impregnation method. 3.5. Comparison with our benchmark catalyst We compared the catalytic performance of 60FSP and amorphous ZrO2 supported Cu (9 wt% Cu/a-ZrO2). Previously, we reported the latter catalyst as a high-performance catalyst [25]. Fig. 10 shows the methanol selectivity against CO2 conversion for 60FSP and Cu/a-ZrO2. It is noteworthy that the selectivity of 60FSP is higher than that of Cu/ a-ZrO2 at the same CO2 conversion. In addition, when the contact time is 600 gcat s L(STP)−1, the methanol formation rate of 60FSP is higher than that of Cu/a-ZrO2, while the CO formation rate of the catalysts is

Fig. 7. (a) Methanol and (b) CO production rate over Cu-based catalysts when the contact time was 600 gcat s L(STP)−1.

7

Chemical Engineering Journal 381 (2020) 122750

S. Tada, et al.

nucleate and grow faster than Cu species (e.g. Cu and CuO) resulting in the deposition of Cu species on ZrO2. Hence, we conclude that 60FSP consisted of ZrO2 aggregates surrounded by CuO nanoparticles (< 5 nm), such as ZrO2@CuO core-shell nanoparticles. With further increasing Cu loading from 60 wt% to 80 wt%, crystalline CuO particles with DCuO(1 1 1) = ca. 10 nm were observed by PXRD (Fig. 1). In the PXRD patterns, the CuO peak intensity of 80FSP was much weaker than that of our benchmark 20IMP (Figs. 1 and S2). Therefore, we expected that for 80FSP, crystalline CuO particles (ca. 10 nm) as well as CuO nanoparticles (< 5 nm) were supported on the above ZrO2 aggregates. We considered the structure of the activated catalysts by H2, as illustrated in Scheme 2. For all the catalysts, the crystalline structure of tetragonal ZrO2 remained unchanged during the catalysts testing (Fig. 9). For 20IMP, the particle of Cu species grew from 18 nm (=DCuO(1 1 1)) to 36 nm (=DCu(1 1 1), Table 1), which means that the large Cu particles (ca. 20–40 nm) were surrounded by the relativelysmall ZrO2 particles (5–10 nm). For 20FSP, the metallic Cu nanoparticles with the size < 5 nm were deposited on ZrO2. For 40FSP, 60FSP, and 80FSP, metallic Cu nanoparticles with the crystallite size of Cu species = ca. 15 nm were attached with tetragonal ZrO2 nanoparticles (5–10 nm), in line with XPS/AES (Fig. 5), PXRD (Fig. 9), and HAADF-STEM images (Figs. 3 and S4). Here, we reconsidered the interpolated line of CO2 conversion vs. methanol selectivity (Fig. 9). Interestingly, the position of the catalyst was related to Cu dispersion, DCu. We categorized the catalysts into three groups on DCu: (i) 20FSP (DCu = 2.1%), (ii) 40FSP and 60FSP (DCu = 1.7%), and (iii) 80FSP (DCu = 1.1%), as listed in Table 1. In other words, the position of the line moves up to the upper side of the figure with increasing DCu. Accordingly, 40FSP and 60FSP possessed similar active sites because (i) the plots of the above two catalysts are on the same interpolated line and (ii) DCu of the catalysts were identical to each other. The difference between the two catalysts was the number of specific active sites because the methanol production rate of 60FSP was faster than that of 40FSP (Fig. 7). This is in line with the fact that the Cu loading of 60FSP was larger than that of 40FSP in spite of the same DCu (1.7%). The active sites of 20FSP hydrogenated CO2 more selectively to methanol than those of 40FSP and 60FSP, while those of 80FSP did less selectively. The difference in Cu particle size between 20FSP and 40FSP may affect catalytic activity. Generally, with decreasing Cu particle size, the ratio of the number of “interfacial sites between Cu and ZrO2” to “Cu surface sites” increases [55]. Simultaneously, CO2 is catalyzed mainly at the interfacial sites which are active and selective in CO2-to-methanol hydrogenation. In fact, the size of Cu species on 20FSP was smaller than that on 40FSP, as discussed above. We considered why 60FSP showed higher selectivity to methanol than 80FSP. Based on simple geometry, with further increasing Cu loading, the volume of Cu particles overcomes that of tetragonal ZrO2 particles. According to the change of the volume ratio of Cu to ZrO2, the fraction of Cu-ZrO2 interfacial sites decreases and simultaneously that of metallic Cu sites increases. The loss of the interfacial sites leads to a decrease in methanol selectivity, while a large amount of the active metallic Cu sites results in high CO2 conversion. The former and latter cases can be attributed to 60FSP and 80FSP, respectively. It is noteworthily that metallic Cu on the H2-reduced catalysts decreased the electron density, as evidenced by a smaller α′ value (Table 2). Thus, the metallic Cu was strongly interacted with the surface of tetragonal ZrO2, forming the specific active sites between metallic Cu and the ZrO2 [21–26]. Even after the CO2 hydrogenation, interestingly, the Cu sintering for 20FSP was not observed, evidenced by PXRD results (Fig. 9). We have seen a similar phenomenon using 3.9 wt% Cu/ZrO2/ SiO2 prepared by grafting [22] and 11–14 wt% Cu/ZrO2 prepared by a two-nozzle flame spray pyrolysis [26]. The Cu sintering may be suppressed by the strong interaction between Cu and ZrO2. In this study, 60FSP was the best catalysts with high activity and selectivity in CO2-to-methanol hydrogenation, as described in Section

Fig. 8. Methanol selectivity against CO2 conversion over Cu-based catalysts. Contact time: 600–1500 gcat s L(STP)−1 for 20FSP, 40FSP, 60FSP, 80FSP, and 20IMP and 60–1500 gcat s L(STP)−1 for MDC-3.

Fig. 9. PXRD patterns of the spent catalysts. The number in the figure is the crystallite size of Cu (1 1 1) phase, estimated from the peak at 43.3°.

identical to one another. Thus, it was certain that the active sites of 60FSP were more suitable for methanol synthesis via CO2 hydrogenation than those of Cu/a-ZrO2. 3.6. Structure impact on catalyst performance We proposed the shape/size of Cu species of the as-prepared FSPmade catalysts, as illustrated in Scheme 2. For all the catalysts, the Cu species consisted of CuO, as evidenced by PXRD (Fig. 1) and XPS/AES (Fig. 5). HAADS-STEM images (Fig. 3a) reveal that, for 20FSP, CuO nanoparticles with the size of ca. 5 nm were supported on tetragonal ZrO2. Of note, in the HAADF-STEM images of 60FSP (Fig. 3b), Cu and Zr mappings were completely overlapping. However, we confirmed that no Cu species was inside the ZrO2 lattice and the Cu species can be amorphous and/or consist of CuO nanoparticles with the size of < 5 nm (Fig. 1). In FSP, the species having the higher boiling (and melting) temperature tend to nucleate and grow faster [54]: therefore, ZrO2 can 8

Chemical Engineering Journal 381 (2020) 122750

S. Tada, et al.

Fig. 10. (a) Methanol selectivity against CO2 conversion over 60FSP and 9 wt% Cu/a-ZrO2 prepared by an impregnation method. Contact time: 600–1500 gcat s L (STP)−1. (b) Methanol and CO production rate over 60FSP and 9 wt% Cu/a-ZrO2 when the contact time is 600 gcat s L(STP)−1.

Scheme 2. A schematic of (a) 20IMP and (b) FSP-made CuO/ZrO2 catalysts.

3.4. This is probably because 60FSP combined the advantages of both 20FSP (Cu-ZrO2 interaction) and 80FSP (a large number of active sites).

Second, the catalysts were tested in the CO2 hydrogenation at 230 °C and 10 bar. The catalysts with Cu loading < 60 wt% were highly selective to the CO2 hydrogenation, in other words, they suppressed undesirable CO evolution via methanol decomposition and/or RWGS reaction. In the cases of the Cu loading = 60 wt% and 80 wt%, their methanol yields were greater than those of a commercial CuO/ZnO/ Al2O3 catalyst and our benchmark catalyst 9 wt% Cu on amorphous ZrO2. We conclude from the activity and selectivity that the FSP-made CuO/ZrO2 with 60 wt% of Cu was the best catalyst for the CO2 hydrogenation. Finally, the structure and performance of the FSP-made catalysts were compared. We confirm a strong interaction between Cu and ZrO2 leading to electron transfer from Cu to ZrO2. The interaction formed specific active sites for CO2-to-methanol hydrogenation. With increasing Cu loading to 80 wt%, the volumetric ratio of Cu to ZrO2 was quite high, so that the fraction of Cu-ZrO2 interfacial sites decreased and simultaneously that of metallic Cu sites increased. The FSP-made catalyst with 60 wt% of Cu combined the advantages between 20FSP (Cu-ZrO2 interaction) and 80FSP (a large number of active sites).

4. Conclusions We prepared a series of FSP-made CuO/ZrO2 catalysts with high Cu loading (20–80 wt%). The effect of the structure and the Cu loading on methanol synthesis via CO2 hydrogenation to methanol was investigated. First, the catalysts were characterized by methanol vapor sorption, N2 adsorption, PXRD, XPS/AES, XRF, HAADF-STEM, H2-TPR, and N2O titration. Of note, the structure of the FSP-made CuO/ZrO2 was changed with Cu loading. At 20 wt% of Cu loading, CuO nanoparticles with the size of ca. 5 nm were supported on ZrO2 aggregates. At 60 wt% of Cu loading, ZrO2 aggregates were covered with CuO layer having a coreshell like structure. At 80 wt%, not only the CuO nanoparticles but also CuO particles with the size of ca. 10 nm were present. After H2 reduction, the FSP-made catalysts consisted of the mixture of metallic Cu nanoparticles (< 20 nm) and ZrO2 aggregates. 9

Chemical Engineering Journal 381 (2020) 122750

S. Tada, et al.

Declaration of Competing Interest [17]

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

[18]

Acknowledgements [19]

This work was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI, Japan (No. 18K04838), and Leading Initiative for Excellent Young Researchers (LEADER), the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan (No. 1039506). We appreciate Prof. Shigeo Satokawa at Seikei University, Japan, for kindly helping XRF, H2-TPR, N2 physisorption, and N2O titration. We thank Prof. Yuta Matsushima at Yamagata University, Japan for helping to analyze XRD data. We are grateful to Advanced Characterization Nanotechnology Platform of the University of Tokyo, supported by “Nanotechnology Platform” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, to conduct XPS measurements. Also, STEM measurements were supported by Tohoku University Advanced Characterization Nanotechnology platform in Nanotechnology platform project sponsored by MEXT, Japan.

[20] [21]

[22] [23] [24]

[25] [26]

Appendix A. Supplementary data

[27]

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.122750.

[28]

References

[29]

[1] G.A. Olah, Towards oil independence through renewable methanol chemistry, Angew. Chem. Int. Ed. 52 (2013) 104–107. [2] O. Martin, J. Perez-Ramirez, New and revisited insights into the promotion of methanol synthesis catalysts by CO2, Catal. Sci. Technol. 3 (2013) 3343–3352. [3] I.A. Fisher, A.T. Bell, In-Situ infrared study of methanol synthesis from H2/CO2 over Cu/SiO2 and Cu/ZrO2/SiO2, J. Catal. 172 (1997) 222–237. [4] K. Samson, M. Sliwa, R.P. Socha, K. Gora-Marek, D. Mucha, D. Rutkowska-Zbik, J.F. Paul, M. Ruggiero-Mikolajczyk, R. Grabowski, J. Sloczynski, Influence of ZrO2 structure and copper electronic state on activity of Cu/ZrO2 catalysts in methanol synthesis from CO2, ACS Catal. 4 (2014) 3730–3741. [5] S. Kattel, B.H. Yan, Y.X. Yang, J.G.G. Chen, P. Liu, Optimizing binding energies of key intermediates for CO2 hydrogenation to methanol over oxide-supported copper, J. Am. Chem. Soc. 138 (2016) 12440–12450. [6] T. Witoon, J. Chalorngtham, P. Dumrongbunditkul, M. Chareonpanich, J. Limtrakul, CO2 hydrogenation to methanol over Cu/ZrO2 catalysts: effects of zirconia phases, Chem. Eng. J. 293 (2016) 327–336. [7] I. Ro, Y.F. Liu, M.R. Ball, D.H.K. Jackson, J.P. Chada, C. Sener, T.F. Kuech, R.J. Madon, G.W. Huber, J.A. Dumesic, Role of the Cu-ZrO2 interfacial sites for conversion of ethanol to ethyl acetate and synthesis of methanol from CO2 and H2, ACS Catal. 6 (2016) 7040–7050. [8] T. Witoon, T. Numpilai, T. Phongamwong, W. Donphai, C. Boonyuen, C. Warakulwit, M. Chareonpanich, J. Limtrakul, Enhanced activity, selectivity and stability of a CuO-ZnO-ZrO2 catalyst by adding graphene oxide for CO2 hydrogenation to methanol, Chem. Eng. J. 334 (2018) 1781–1791. [9] F. Arena, G. Italiano, K. Barbera, S. Bordiga, G. Bonura, L. Spadaro, F. Frusteri, Solid-state interactions, adsorption sites and functionality of Cu-ZnO/ZrO2 catalysts in the CO2 hydrogenation to CH3OH, Appl. Catal. A: Gen. 350 (2008) 16–23. [10] J. Graciani, K. Mudiyanselage, F. Xu, A.E. Baber, J. Evans, S.D. Senanayake, D.J. Stacchiola, P. Liu, J. Hrbek, J.F. Sanz, J.A. Rodriguez, Highly active copperceria and copper-ceria-titania catalysts for methanol synthesis from CO2, Science 345 (2014) 546–550. [11] G. Noh, E. Lam, J.L. Alfke, K. Larmier, K. Searles, P. Wolf, C. Coperet, Selective hydrogenation of CO2 to CH3OH on supported cu nanoparticles promoted by isolated Ti-IV surface sites on SiO2, ChemSusChem 12 (2019) 968–972. [12] C.S. Li, G. Melaet, W.T. Ralston, K. An, C. Brooks, Y.F. Ye, Y.S. Liu, J.F. Zhu, J.H. Guo, S. Alayoglu, G.A. Somorjai, High-performance hybrid oxide catalyst of manganese and cobalt for low-pressure methanol synthesis, Nat. Commun. 6 (2015) 6538. [13] O. Martin, A.J. Martin, C. Mondelli, S. Mitchell, T.F. Segawa, R. Hauert, C. Drouilly, D. Curulla-Ferre, J. Perez-Ramirez, Indium oxide as a superior catalyst for methanol synthesis by CO2 hydrogenation, Angew. Chem. Int. Ed. 55 (2016) 6261–6265. [14] F. Studt, I. Sharafutdinov, F. Abild-Pedersen, C.F. Elkjaer, J.S. Hummelshoj, S. Dahl, I. Chorkendorff, J.K. Norskov, Discovery of a Ni-Ga catalyst for carbon dioxide reduction to methanol, Nat. Chem. 6 (2014) 320–324. [15] J. Wang, G. Li, Z. Li, C. Tang, Z. Feng, H. An, H. Liu, T. Liu, C. Li, A highly selective and stable ZnO-ZrO2 solid solution catalyst for CO2 hydrogenation to methanol, Sci. Adv. 3 (2017) e1701290. [16] H. Bahruji, M. Bowker, G. Hutchings, N. Dimitratos, P. Wells, E. Gibson, W. Jones,

[30] [31] [32] [33] [34] [35] [36] [37] [38] [39]

[40] [41] [42] [43] [44] [45] [46]

10

C. Brookes, D. Morgan, G. Lalev, Pd/ZnO catalysts for direct CO2 hydrogenation to methanol, J. Catal. 343 (2016) 133–146. X.W. Nie, X. Jiang, H.Z. Wang, W.J. Luo, M.J. Janik, Y.G. Chen, X.W. Guo, C.S. Song, Mechanistic understanding of alloy effect and water promotion for Pd-Cu bimetallic catalysts in CO2 hydrogenation to methanol, ACS Catal. 8 (2018) 4873–4892. M. Behrens, F. Studt, I. Kasatkin, S. Kuhl, M. Hävecker, F. Abild-Pedersen, S. Zander, F. Girgsdies, P. Kurr, B.L. Kniep, M. Tovar, R.W. Fischer, J.K. Nørskov, R. Schlögl, The active site of methanol synthesis over Cu/ZnO/Al2O3 industrial catalysts, Science 336 (2012) 893–897. S. Kuld, M. Thorhauge, H. Falsig, C.F. Elkjaer, S. Helveg, I. Chorkendorff, J. Sehested, Quantifying the promotion of Cu catalysts by ZnO for methanol synthesis, Science 352 (2016) 969–974. S. Kattel, P.J. Ramirez, J.G. Chen, J.A. Rodriguez, P. Liu, Active sites for CO2 hydrogenation to methanol on Cu/ZnO catalysts, Science 355 (2017) 1296–1299. K. Larmier, W.C. Liao, S. Tada, E. Lam, R. Verel, A. Bansode, A. Urakawa, A. ComasVives, C. Coperet, CO2-to-Methanol hydrogenation on zirconia-supported copper nanoparticles: reaction intermediates and the role of the metal-support interface, Angew. Chem. Int. Ed. 56 (2017) 2318–2323. E. Lam, K. Larmier, P. Wolf, S. Tada, O.V. Safonova, C. Coperet, Isolated Zr surface sites on silica promote hydrogenation of CO2 to CH3OH in supported cu catalysts, J. Am. Chem. Soc. 140 (2018) 10530–10535. S. Tada, F. Watanabe, K. Kiyota, N. Shimoda, R. Hayashi, M. Takahashi, A. Nariyuki, A. Igarashi, S. Satokawa, Ag Addition to CuO-ZrO2 catalysts promotes methanol synthesis via CO2 hydrogenation, J. Catal. 351 (2017) 107–118. S. Tada, A. Katagiri, K. Kiyota, T. Honma, H. Kamei, A. Nariyuki, S. Uchida, S. Satokawa, Cu species incorporated into amorphous ZrO2 with high activity and selectivity in CO2-to-methanol hydrogenation, J. Phys. Chem. C 122 (2018) 5430–5442. S. Tada, S. Kayamori, T. Honma, H. Kamei, A. Nariyuki, K. Kon, T. Toyao, K. Shimizu, S. Satokawa, Design of interfacial sites between cu and amorphous ZrO2 dedicated to CO2-to-methanol hydrogenation, ACS Catal. 8 (2018) 7809–7819. S. Tada, K. Larmier, R. Buchel, C. Copéret, Methanol synthesis via CO2 hydrogenation over CuO-ZrO2 prepared by two-nozzle flame spray pyrolysis, Catal. Sci. Technol. 8 (2018) 2056–2060. S. Tada, S. Satokawa, Effect of Ag loading on CO2-to-methanol hydrogenation over Ag/CuO/ZrO2, Catal. Commun. 113 (2018) 41–45. A. Alvarez, A. Bansode, A. Urakawa, A.V. Bavykina, T.A. Wezendonk, M. Makkee, J. Gascon, F. Kapteijn, Challenges in the greener production of formates/formic acid, methanol, and DME by heterogeneously catalyzed CO2 Hydrogenation processes, Chem. Rev. 117 (2017) 9804–9838. L. Madler, H.K. Kammler, R. Mueller, S.E. Pratsinis, Controlled synthesis of nanostructured particles by flame spray pyrolysis, J. Aerosol. Sci. 33 (2002) 369–389. K. Wegner, B. Schimmoeller, B. Thiebaut, C. Fernandez, T.N. Rao, Pilot plants for industrial nanoparticle production by flame spray pyrolysis, Kona Powder. Part. J. 29 (2011) 251–265. R. Koirala, S.E. Pratsinis, A. Baiker, Synthesis of catalytic materials in flames: opportunities and challenges, Chem. Soc. Rev. 45 (2016) 3053–3068. Y. Sheng, M. Kraft, R. Xu, Emerging applications of nanocatalysts synthesized by flame aerosol processes, Curr. Opin. Chem. Eng. 20 (2018) 39–49. R. Ahmad, M. Hellinger, M. Buchholz, H. Sezen, L. Gharnati, C. Woll, J. Sauer, M. Doring, J.D. Grunwaldt, U. Arnold, Flame-made Cu/ZnO/Al2O3 catalyst for dimethyl ether production, Catal. Commun. 43 (2014) 52–56. F. Huber, H. Meland, M. Ronning, H. Venvik, A. Holmen, Comparison of Cu-Ce-Zr and Cu-Zn-Al mixed oxide catalysts for water-gas shift, Top. Catal. 45 (2007) 101–104. R. Kydd, W.Y. Teoh, K. Wong, Y. Wang, J. Scott, Q.H. Zeng, A.B. Yu, J. Zou, R. Amal, Flame-synthesized ceria-supported copper dimers for preferential oxidation of CO, Adv. Funct. Mater. 19 (2009) 369–377. K. Fujiwara, Y. Deligiannakis, C.G. Skoutelis, S.E. Pratsinis, Visible-light active black TiO2-Ag/TiOx particles, Appl. Catal. B Environ. 154 (2014) 9–15. K. Fujiwara, S. Tada, T. Honma, H. Sasaki, M. NIshijima, R. KIkuchi, Influences of particle size and crystallinity of highly-loaded CuO/ZrO2 on CO2 hydrogenation to methanol, AIChE J. (2019) in press. T. Umegaki, Y. Kojima, K. Omata, Effect of silica coating on performance of copperzinc oxide based catalyst for methanol synthesis, Adv. Mater. Lett. 6 (2015) 1026–1030. J.P. Espinos, J. Morales, A. Barranco, A. Caballero, J.P. Holgado, A.R. GonzalezElipe, Interface effects for Cu, CuO, and Cu2O deposited on SiO2 and ZrO2. XPS determination of the valence state of copper in Cu/SiO2 and Cu/ZrO2 catalysts, J. Phys. Chem. B 106 (2002) 6921–6929. W. Grunert, N.W. Hayes, R.W. Joyner, E.S. Shpiro, M.R.H. Siddiqui, G.N. Baeva, Structure, chemistry, and activity of Cu-ZSM-5 catalysts for the selective reduction of NOx in the presence of oxygen, J. Phys. Chem. 98 (1994) 10832–10846. M.C. Biesinger, Advanced analysis of copper X-ray photoelectron spectra, Surf. Interface Anal. 49 (2017) 1325–1334. S.R. Barman, D.D. Sarma, Investigation of the L3–M45M45 auger-spectra of Cu, Cu2O and CuO, J. Phys. Condens. Matter 4 (1992) 7607–7616. S. Peters, S. Peredkov, M. Neeb, W. Eberhardt, M. Al-Hada, Size-dependent Auger spectra and two-hole coulomb interaction of small supported Cu-clusters, Phys. Chem. Chem. Phys. 15 (2013) 9575–9580. M. Decrescenzi, M. Diociaiuti, L. Lozzi, P. Picozzi, S. Santucci, C. Battistoni, G. Mattogno, Size effects on the linewidths of the auger-spectra of Cu clusters, Surf. Sci. 178 (1986) 282–289. T.A.D. Nguyen, Z.R. Jones, B.R. Goldsmith, W.R. Buratto, G. Wu, S.L. Scott, T.W. Hayton, A Cu25 nanocluster with partial Cu(0) character, J. Am. Chem. Soc. 137 (2015) 13319–13324. A.G. Sato, D.P. Volanti, D.M. Meira, S. Damyanova, E. Longo, J.M.C. Bueno, Effect of the ZrO2 phase on the structure and behavior of supported Cu catalysts for ethanol conversion, J. Catal. 307 (2013) 1–17.

Chemical Engineering Journal 381 (2020) 122750

S. Tada, et al. [47] S. Ardizzone, C.L. Bianchi, XPS characterization of sulphated zirconia catalysts: the role of iron, Surf. Interface Anal. 30 (2000) 77–80. [48] J.W. Liu, M.Y. Liao, M. Imura, A. Tanaka, H. Iwai, Y. Koide, Low on-resistance diamond field effect transistor with high-k ZrO2 as dielectric, Sci. Rep. 4 (2014) 6395. [49] D.A. Svintsitskiy, T.Y. Kardash, O.A. Stonkus, E.M. Slavinskaya, A.I. Stadnichenko, S.V. Koscheev, A.P. Chupakhin, A.I. Boronin, In situ XRD, XPS, TEM, and TPR study of highly active in CO oxidation CuO nanopowders, J. Phys. Chem. C 117 (2013) 14588–14599. [50] O. Akhavan, R. Azimirad, S. Safa, E. Hasani, CuO/Cu(OH)2 hierarchical nanostructures as bactericidal photocatalysts, J. Mater. Chem. 21 (2011) 9634–9640. [51] M.C. Biesinger, L.W.M. Lau, A.R. Gerson, R.S.C. Smart, Resolving surface chemical

[52] [53] [54] [55]

11

states in XPS analysis of first row transition metals, oxides and hydroxides: Sc, Ti, V, Cu and Zn, Appl. Surf. Sci. 257 (2010) 887–898. B. Barreca, G.A. Battiston, R. Gerbasi, E. Tondello, P. Zanella, Zirconium dioxide thin films characterized by XPS, Surf. Sci. Spectra 7 (2000) 303–309. G.P. Lopez, D.G. Castner, B.D. Ratner, XPS O 1s binding-energies for polymers containing hydroxyl, ether, ketone and ester groups surf, Interface Anal. 17 (1991) 267–272. R. Strobel, S.E. Pratsinis, Flame aerosol synthesis of smart nanostructured materials, J. Mater. Chem. 17 (2007) 4743–4756. S. Tada, R. Kikuchi, Mechanistic study and catalyst development for selective carbon monoxide methanation, Catal. Sci. Technol. 5 (2015) 3061–3070.