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FAU zeolite catalysts modified by Cu for reverse water gas shift reaction
Applied Catalysis A, General 592 (2020) 117415
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Catalytic performance of MoO3/FAU zeolite catalysts modified by Cu for reverse water gas shift reaction
T
Atsushi Okemoto*, Makoto R. Harada, Takayuki Ishizaka, Norihito Hiyoshi, Koichi Sato National Institute of Advanced Science and Technology, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Reverse water gas shift reaction Zeolite Molybdenum catalysis Metal–metal interaction Reduction reaction
The reverse water gas shift (rWGS) reaction using a supported zeolite catalyst containing molybdenum (Mo) was investigated. The Mo-based catalyst is expected to be effective for selective conversion of CO2 into CO. The focus of this work was on the performance of the Mo-based catalyst and the effect of adding copper (Cu) to the catalyst. The Mo catalyst exhibited high CO selectivity (99 %) with CO2 conversion of 14.3 % at 773 K. The activity of the catalyst containing Cu as an additive depended on the Mo/Cu ratio. For a series of Mo(x)Cu(1-x)/Faujasite (FAU) catalysts (x: metal content, mmol/g), the Mo(0.8)Cu(0.2)/FAU catalyst exhibited the best performance. X-ray diffraction, X-ray photoelectron spectroscopy and temperature-programmed reduction with hydrogen analysis of the Mo(x)Cu(1-x)/FAU catalysts revealed that the presence of Cu as an additive influenced the reduction step of MoO3 to MoO2, with MoO3 being the active species. Transmission electron microscopy verified morphology of MoO3 and Cu particles on the support. The excellent performance of the prepared Mo(0.8)Cu(0.2)/FAU catalyst was due to facilitation of the reduction process involving the Mo species.
1. Introduction The recent increase in carbon dioxide (CO2) emissions has resulted in climate change and has had a major impact on human society and the global environment [1–3]. Therefore, the development of sustainable technology that can recover CO2 emitted by industry and reconvert the CO2 into a useful resource is considered critical for alleviating this trans-national environmental issue [4–6]. Most of the CO2 is generated by consumption of fossil fuels, such that the increase in the CO2 emissions directly impacts the global environment. This cannot continue in societies that consume large amounts of limited fossil resources if the world is to achieve sustainable development over the long term. There is a need to create a carbonneutral society that introduces renewable energy and uses it effectively [7–9]. At present, one of the approaches for solving the CO2 problem is to exploit chemical reaction technology whereby CO2 is converted into a chemical product or hydrocarbon fuel through reaction with hydrogen (H2). In this regard, many gas-phase catalytic reactions using solid catalysts have been studied. For example, there is a rich body of catalysis research involving methanation, methanol synthesis, and the reverse water gas shift (rWGS) reaction [10–14]. The rWGS is a reaction that reduces CO2 to CO, and has been regarded as a useful process for
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arbitrarily controlling the H2:CO composition ratio in synthesis gas, which is in high demand as a raw material for C1 chemistry [15–18]. This report concerns the development of a supported metal catalyst that exhibits high selectivity for CO in the rWGS reaction. Inorganic materials such as zeolites have been studied extensively as catalyst supports, zeolites are particularly useful for supports due to polymorphous porous structures, specific surface area and thermal stability [19,20]. A Y-type (Faujasite-type) zeolite, which is one of 12-memered ring zeolites, was used as a support for highly dispersed metal particles [21,22]. When the rWGS reaction was evaluated for activity using several metals, it was found that Mo exhibited high selectivity for CO. However, the Mo oxide supported catalyst has the problem of low CO2 conversion [23,24]. Some researchers have reported that the CO2 conversion in the rWGS reaction was related to the reducibility of Pt or Cu supported catalysts [25,26]. In contrast, Mo oxide is not readily reduced [27,28]. Therefore, in this work, a second active metal was added for the purpose of enhancing the activity of the Mo oxide supported catalyst. It was found that Cu promoted the reduction of Mo and high activity was realized. The support state and the reducibility of the Mo catalyst were evaluated, and the catalyst was characterized by temperature-programmed reduction with H2 (H2-TPR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Transmission electron microscopy (TEM) to gain insights into the effect of adding Cu,
Corresponding author. E-mail address: [email protected] (A. Okemoto).
https://doi.org/10.1016/j.apcata.2020.117415 Received 26 July 2019; Received in revised form 7 January 2020; Accepted 11 January 2020 Available online 13 January 2020 0926-860X/ © 2020 Published by Elsevier B.V.
Applied Catalysis A, General 592 (2020) 117415
A. Okemoto, et al.
and the composition of the catalyst that gave the highest activity was found.
CO yield (%) =
CH 4 yield (%) =
2. Experimental 2.1. Materials
CO outlet (mol) × 100 CO2 inlet (mol)
CH4 outlet (mol) × 100 CO2 inlet (mol)
CO selectivity (%) =
All materials were used as received without further purification. The Faujasite-type zeolite (FAU, HSZ-330HUA) was purchased from Tosoh Corporation. Hexammonium heptamolybdate tetrahydrate ((NH4)6Mo7O24·4H2O, Guaranteed Reagent [GR]), copper(II) chloride (CuCl2, GR), nickel(II) chloride hexahydrate (NiCl2(II)·6H2O, GR), and hydrochloric acid (HCl, GR) were purchased from Wako Pure Chemical Corporation. Anhydrous iron(III) chloride (FeCl3, Trace Metal Basis) was purchased from Sigma-Aldrich.
CO outlet (mol) × 100 CO2 inlet − CO2 outlet (mol)
2.4. XRD XRD studies were carried out using a SmartLab diffractometer (Rigaku). The XRD patterns were recorded using monochromatic Cu Kα radiation (λ =1.5418 Å) at room temperature. The 2θ range from 20° to 50° was scanned using a step size of 0.04° and a counting time of 4 s per step. 2.5. XPS
2.2. Catalyst preparation
The oxidization states of the metals on the surfaces of the catalysts were measured by X-ray photoelectron spectroscopy using a PHI 5000 VersaProbe II (ULVAC-PHI) spectrometer with a monochromatic Al Kα source (hν =1486.6 eV, 24.92 W). The oxidation states of the elements measured were Mo 3d and Cu 2p. MultiPak software (ULVAC PHI) was used for all data processing. The charge shift was corrected by using the peak of adventitious carbon (284.5 eV). The software used Gaussian–Lorentzian functions to generate the fitting curves for the recorded XPS data. The multiple peak for Mo 3d was deconvoluted by a Gaussian–Lorentzian function with restricted conditions for the peak area (Mo 3d 3/2 : Mo 3d 5/2 = 2:3) and the peak distance between Mo 3d 3/2 and Mo 3d 5/2 (3.1 eV).
Mo and Cu supported catalysts were prepared by incipient wetness impregnation using FAU as a support. The metal loading was fixed at 1.0 mmol/g-cat for each catalyst. Prior to impregnation, 1.0 g of FAU placed under vacuum in a side-arm flask for 30 min at room temperature to remove adsorbed water molecules from the surface. Hexammonium heptamolybdate tetrahydrate (0.206 g), copper(II) chloride (0.146 g), anhydrous iron(III) chloride (0.176 g), or nickel(II) chloride hexahydrate (0.257 g) was dissolved into 10 mL of HCl solution (0.01 mol/L). The impregnation solution was put into the flask with the FAU and left for 30 min at room temperature. The solution was evaporated in a 300 mL round-bottom flask for 1 h, followed by drying overnight in an electric oven at 328 K. Then, the sample was calcined in air at 773 K for 10 h. The obtained catalysts were named M(x)/FAU (M and x refer to element and its metal content per gram of the catalyst, 0 ≤ x ≤ 1 mmol/g). The metal content was determined by inductively coupled plasma atomic emission spectroscopy (SII SPS-7800, Hitachi). The specific surface areas of the catalysts were measured using a BELSORPmini system (Microtrac BEL Corporation). A glass cell charged with the sample (0.050 g) was heated at 523 K for 2 h in a flow of N2 before measurement. The N2 adsorption/desorption isotherms were obtained at liquid N2 temperature (77 K). The surface areas of the samples were evaluated using the Brunauer–Emmett–Teller (BET) method.
2.6. TEM TEM images were recorded using a FEI TecnaiG2 20 electron microscope operating at 200 kV. The samples were grinding in a mortar, and then mounted on a microgrid carbon polymer supported on a copper grid by placing a droplet of a sample dispersion in ethanol on the grid. 2.7. H2-TPR The H2-TPR was recorded using a BEL-CAT system (Microtrac BEL Corporation) equipped with a TCD. The sample (80 mg) was charged in a quartz cell and pretreated in a flow of He at 573 K for 120 min. The sample was then cooled to room temperature, and heated to 1073 K at a rate of 10 K/min in 10 % H2/Ar. The H2 consumption was recorded by the TCD.
2.3. Reactivity test for the rWGS reaction The rWGS reaction was carried out in a fixed-bed continuous-flow reactor. An α-alumina tube (12 mm inner diameter) was used as the reaction tube. Catalyst (200 mg) diluted with 1.0 g of inert quartz sand was placed in the tube, which contained 1.0 g of pure quartz sand as an inner liner. The catalyst was pretreated with a 55 % H2:N2 mixture at a flow rate of 22.5 mL/min at 873 K for 1 h before performing the reaction. After pretreatment, the catalyst was set at the reaction temperature in N2 gas, and the reaction gas (H2:CO2:N2 = 12.5:12.5:10 mL/min, total flow: 35 mL/min) was fed into the reactor with a space velocity of 7500 mL·h−1 g−1. The products of the reaction were analyzed by two separate on-line GC systems equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID), respectively. The gas lines from the reaction tube to the sampling valves were heated by electric heaters to avoid condensation of products. The reaction products were measured as follows: H2, CO, and CH4 were measured by GC-TCD with a Molecular Sieve 13X column; CO2 and H2O were measured by GC-TCD with a Gaskuropack 54 column (GL Science); and CH3OH was measured by GC-FID with a Molecular Sieve 13X column. The conversion, yield, and selectivity for each product were calculated using the following equations:
3. Results and discussion 3.1. Catalytic test for the rWGS reaction The metal contents and BET surface areas for the prepared catalysts are listed in Table 1. The Si/Al ratio and the BET surface area of the FAU (raw) used as the catalyst support were 2.9 and 593 m2/g, respectively. The Si/Al ratio values for all materials were between 2.7–2.9. The catalyst preparation procedure had little influence on the Si/Al ratio. On the other hand, the BET surface areas for all metal supported catalysts decreased by 10–14 % in comparison with the FAU (raw) because the metal clusters partially covered the pores of the zeolite or the weight of the catalysts increased as a result of adding the supporting metal cluster. In the rWGS reaction, the catalytic performance of a single-metal supported zeolite catalyst and Cu/ZnO/Al2O3, which is generally known as a catalyst for the water gas shift reaction, was evaluated and 2
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Table 1 List of catalysts. Catalyst
FAU (raw) Mo(1)/FAU Cu(1)/FAU Fe(1)/FAU Ni(1)/FAU Mo(0.9)Cu(0.1)/ FAU Mo(0.8)Cu(0.2)/ FAU Mo(0.7)Cu(0.3)/ FAU Mo(0.5)Cu(0.5)/ FAU Mo(0.2)Cu(0.8)/ FAU
Si/Al
a)
Metal content
a)
(mmol/g)
Physical property b) BET area (m2/ g)
Mo
Cu
Fe
Ni
2.9 2.9 2.8 2.8 2.7 2.9
– 1.02 – – – 0.91
– – 0.98 – – 0.11
– – – 0.96 – –
– – – – 0.96 –
593 512 545 530 531 524
2.9
0.80
0.18
–
–
533
2.8
0.66
0.28
–
–
529
2.9
0.48
0.46
–
–
536
2.7
0.77
0.18
–
–
530
Fig. 1. CO2 conversion over Mo(1)/FAU for the reverse water gas shift reaction at 773 K. Reaction conditions. Reactor: alumina tube φ12 mm; catalyst: 200 mg; GHSV: 7500 mL·h−1 g−1; feed gas: H2:CO2:N2 = 12.5:12.5:10 (mL/min); pre-treatment gas: H2:N2 = 12.5:10 (mL/min); pre-treatment temperature: 873 K; reaction temperature: 773 K.
a) Measured by inductively coupled plasma atomic emission spectroscopy (SII SPS-7800, Hitachi). b) Measured by N2 adsorption (BEL-mini, Microtrac Bell).
results are listed in Table 2. Chen et al. reported that a Cu-Zn-Al catalyst gave a 17.4 % yield and 100 % selectivity for CO at 773 K [26]. The catalyst was activated in a reduced gas flow (H2:N2 = 12.5:10 mL/min) at 873 K before reaction. The activity was measured at 773 K 3 h after the start of the reaction. Mo(1)/FAU exhibited relatively high activity with a 14.3 % yield and 99 % selectivity for CO 3 h after the start of the reaction. Little CH4 was formed as a by-product (< 1 %) and the carbon balance was more than 95 %. Although the activity was less than that of the Cu-Zn-Al catalyst, it was found that the single-metal MoO3 supported catalyst exhibited acceptable performance in the rWGS reaction. Fe(1)/FAU and Cu(1)/FAU gave yields of 5.9 % and 6.8 %, respectively, and selectivity of 98 % for CO. The yield for Ni(1)/FAU (17.2 %) was higher than that for Mo(1)/FAU, but the CO selectivity for Ni(1)/FAU was relatively low at 45 % due to high conversion of CO2 into CH4. The CO yields for the time-on-stream for these catalysts are shown in Fig. 1. Ni(1)/FAU and Mo(1)/FAU showed high initial activity but there was a decrease in activity after 5 h, probably due to migration of metal species and deposition of carbon on the catalyst surface [29,30]. A significant decrease in conversion of CO2 relative to Mo(1)/FAU, Cu(1)/ FAU, and Fe(1)/FAU was not observed. After reaction for 20 h, the conversion became higher for Mo(1)/FAU than for Ni(1)/FAU. That is, Mo(1)/FAU was very effective in the rWGS reaction because of the high CO2 conversion, the high CO selectivity, and a relative absence of catalytic deactivation. To improve the activity of the supported Mo catalyst, Cu was added to the catalyst to facilitate the reduction of the active Mo species. The activities of the Mo-Cu co-supported catalysts with different Mo/Cu ratios for the rWGS reaction are presented in Fig. 2. Although there were small differences in the activities of the catalysts at a reaction temperature of 673 K, the differences became more pronounced at 723
Fig. 2. CO yield over Mo(x)Cu(1-x)/FAU catalysts (0 ≤ x ≤ 1 mmol/g-cat.) for the reverse water gas shift reaction at different temperatures. Reaction conditions. Reactor: alumina tube φ12 mm; catalyst: 200 mg; GHSV: 7500 mL·h−1 g−1; feed gas: H2:CO2:N2 = 12.5:12.5:10 (mL/min); pre-treatment gas: H2:N2 = 12.5:10 (mL/min); pre-treatment temperature: 873 K; reaction time: 3 h.
K. The order of catalytic activity was Mo(01)/FAU > Mo(0.8)Cu(0.2)/ FAU > Mo(0.9)Cu(0.1)/FAU > Mo(0.7)Cu(0.3)/FAU > Mo(0.5)Cu (0.5)/FAU > Mo(0.2)Cu(0.8)/FAU > Cu(1)/FAU. In other words, Cu (1)/FAU exhibited the lowest activity in the series. Note that the order was not dependent on the loading of Cu. In particular, Mo(0.8)Cu(0.2)/ FAU showed high activity, with a CO yield of 18.5 % at 773 K. The
Table 2 Comparison of CO2 conversion and CO selectivity for the single-metal supported catalysts and values reported for Cu-Zn-Al catalyst. Reaction conditions: 0.1 MPa, 773 K, H2:CO2 = 12.5:12.5 (mL/min), GHSV =7500 mL·h−1 g−1. [[In expressions below the table, please use subscripts for the chemical symbols.]]. Catalyst
H2:CO ratio
CO2 conversion (%)
CO selectivity (%)
CH4 yield (%)
Ref.
Cu-Zn-Al (36 wt%) Mo(1)/FAU Cu(1)/FAU Fe(1)/FAU Ni(1)/FAU
2:1 1:1 1:1 1:1 1:1
17.4 14.3 6.8 5.9 17.2
100 99 98 98 45
0 1 2 2 55
[1] This This This This
[1] Chen, C.-S.; Cheng, W.-H.; Lin, S.-S. Catal. Lett. 2000, 68 (1), 45−48. GSHV: gas space hourly velocity. 3
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Fig. 3. CO yield over Mo(x)Cu(1-x)/FAU catalysts (0 ≤ x ≤ 1 mmol/g-cat.) for the reverse water gas shift reaction with a reaction time of 3 h. Reaction conditions. Reactor: alumina tube φ12 mm; catalyst: 200 mg; GHSV: 7500 mL·h−1 g−1; feed gas: H2:CO2:N2 = 12.5:12.5:10 (mL/min); pre-treatment gas: H2:N2 = 12.5:10 (mL/min); pre-treatment temperature: 873 K; reaction temperature: 773 K; reaction time: 3 h.
effect of Cu addition on the rWGS reaction at 733 K is summarized in Fig. 3. The yields of CO for Mo-Cu co-supported catalysts with different Mo/Cu ratios at 773 K are illustrated in Fig. 3. A volcano curve having a peak top at Mo(0.8)Cu(0.2)/FAU was obtained for the plot of the CO yield. Addition of a small amount of Cu to the Mo catalyst at a Mo/Cu ratio of 4/1 led to an increase in the catalytic performance in the rWGS reaction. Mo(0.5)Cu(0.5)/FAU, Mo(0.2)Cu(0.8)/FAU, and Cu(1)/FAU showed less activity than Mo(1)/FAU because of a decrease in the Mo content of the catalysts. This result confirmed that a Mo species is a key active species and the addition of Cu influences catalytic performance in the rWGS reaction. 3.2. Relationship between activity and structure of catalysts 3.2.1. Specific surface area and structure The crystal structures of all metal supported catalysts were characterized by XRD and the results are shown in Fig. 4. Fig. 4(a) displays the patterns for the raw FAU zeolite as a support and Mo(1)/FAU, Cu (1)/FAU, Fe(1)/FAU, and Ni(1)/FAU after calcination at 773 K. The peak at 23.5° in the patterns for Mo(1)/FAU and FAU zeolite is ascribed to α-MoO3, [31–33]. Other forms of Mo species were not observed. In the pattern for Cu(1)/FAU, the peak at 38° was assigned to CuO(111) [34]. The pattern for Fe(1)/FAU showed peaks at 35.5° and 41° and these were assigned to the Fe3O4 (311) and the Fe3O4 (400) phases, respectively, signifying that Fe species are supported as Fe3O4 after calcination at 773 K [35,36]. The pattern for Ni(1)/FAU showed two prominent peaks corresponding to NiO(111) and NiO(200) at 37.1° and 42.2°, respectively [37]. Therefore, Ni species were present as NiO in Ni (1)/FAU. In the case of the single-metal supported catalyst, the load metal existed as a single-phase metal oxide. The XRD patterns for Mo-Cu co-supported catalysts after calcination at 773 K are presented in Fig. 4(b). Although there is a peak corresponding to α-MoO3 at 23.5° in the patterns for Mo(1)/FAU and Mo (0.8)Cu(0.2)/FAU, there is no peak at 23.5° in the XRD patterns for Mo (0.5)Cu(0.5)/FAU and Mo(0.2)Cu(0.8)/FAU. In contrast, the CuO peak at 38° is observed for Cu(1)/FAU and Mo(0.2)Cu(.8)/FAU. When the loading amount of Cu was under 50 %, peaks for Cu compounds were not observed. That is, the Cu oxide peak did not appear in cases when
Fig. 4. X-ray diffraction patterns for the single-metal supported catalysts: (a) single-metal supported catalysts after calcination at 773 K; (b) Mo and Cu supported catalysts after calcination at 773 K; (c) Mo-Cu co-supported catalysts after H2 reduction at 873 K.
4
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Fig. 5. X-ray photoelectron spectroscopy spectra of Mo(1)/FAU and Mo(0.8)Cu(0.2)/FAU in the region of Mo 3d: (a) Mo(1)/FAU calcined at 773 K; (b) Mo(0.8)Cu (0.2)/FAU calcined at 773 K; (c) Mo(1)/FAU reduced by hydrogen at 873 K; (d) Mo(0.8)Cu(0.2)/FAU reduced by hydrogen at 773 K; (e) Mo(0.8)Cu(0.2)/FAU after the reverse water gas shift reaction at 773 K for 6 h.
the peaks were changed and the peak positions were moved to lower binding energies, appearing at 228.0 eV and 231.5 eV, and these can be assigned to Mo(IV) 3d 5/2 and Mo(IV) 3d 3/2, respectively [39–41]. This indicates that some of the MoO3 was reduced to MoO2 after the samples were reduced with H2 at 873 K for 1 h. In comparison with the peak intensities of the reduced Mo(1)/FAU and the reduced Mo(0.8)Cu (0.2)/FAU, the peak for Mo(IV) in Mo(0.8)Cu(0.2)/FAU was larger than that for Mo(1)/FAU. The ratio of the peak area of Mo(VI) and Mo(IV) represents the degree of reduction of Mo. The ratios for Mo(1)/FAU and Mo(0.8)Cu(0.2)/FAU after reduction by H2 were 0.25 and 0.45, respectively. These results suggest that MoO3 in Mo(0.8)Cu(0.2)/FAU underwent more reduction than that in Mo(1)/FAU. Further, in the spectrum of Mo(0.8)Cu(0.2)/FAU after the rWGS reaction, the peak assigned to MoO2 had intensity as strong as that in the reduced Mo (0.8)Cu(0.2)/FAU. This finding suggested that MoO3 was reduced as the rWGS reaction proceeded. The XPS spectra for Mo(1)/FAU and Mo(0.8)Cu(0.2)/FAU before and after H2 reduction are shown in Fig. 6. In Fig. 6 (a), two peaks appeared at 934.0 eV and 953.5 eV in the Cu 2p region. The peaks were assigned to Cu(II) 2p 3/2 and Cu(II) 2p 1/2 [41–44]. These results indicate that Cu was present as CuO in the Cu(1)/FAU catalyst after calcination. The spectrum of Mo(0.8)Cu(0.2)/FAU after calcination also
the metal loading was less than 0.5 mmol/g, owing to the small size of the oxide crystals (< 4 nm), thus precluding detection by XRD. It was noted that the complex oxide of Mo and Cu did not form after calcination of the catalyst. The XRD patterns for the Mo-Cu co-supported catalysts reduced by H2 at 873 K are shown in Fig. 4(c) In the spectra for all reduced samples, the peak at 23.5°, which is ascribed to α-MoO3, was not observed. On the other hand, a small peak was detected at 37.1° and was assigned to MoO2 (211). Although the main peak for MoO2 (110) at 26.1° was observed despite the overlapping by the peak due to FAU, it is suggested that MoO2 was formed by H2 reduction. 3.2.2. XPS The XPS studies were conducted to determine the surface oxidization states of the Mo(1)/FAU and Mo(0.8)Cu(0.2)/FAU catalysts before and after treatment with H2. The XPS spectra for Mo(1)/FAU and Mo (0.8)Cu(0.2)/FAU are shown in Fig. 5. In Fig. 5(a) and (b), Mo(1)/FAU and Mo(0.8)Cu(0.2)/FAU after calcination each gave two peaks at 231.8 eV and 250.0 eV in the Mo 3d region. These peaks were attributed to the Mo(VI) 3d 5/2 and the Mo(VI) 3d 3/2 spin–orbit components, respectively [38]. In the spectra for the reduced Mo(1)/FAU and Mo(0.8)Cu(0.2)/FAU (Fig. 5(b) and 5(c), respectively), the shapes of 5
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Fig. 6. X-ray photoelectron spectroscopy spectra of Cu(1)/FAU and Mo(0.8)Cu(0.2)/FAU in the region of Cu 2p. (a) Cu(1)/FAU calcined at 773 K; (b) Mo(0.8)Cu(0.2)/FAU calcined at 773 K; (c) Cu(1)/FAU reduced by H2 at 873 K; (d) Mo(0.8)Cu(0.2)/FAU reduced by H2 at 873 K; (e) Mo(0.8)Cu(0.2)/FAU after the reverse water gas shift reaction at 773 K for 6 h.
lattice interval of 0.38-0.40 nm was found in Fig. 7(b), and was assigned to MoO3 with the lattice parameter =0.39 nm, in good agreement with the literature values (JCPDS file no. 05-0508) [45]. There was a light-gray particle in contiguity with the MoO3 particle, which was assigned to Cu with lattice parameter =0.21 nm, in good agreement with the literature values (JCPDS file no. 04-0836). [46–48]. The results verified that Cu particles was contiguous to MoO3 particles (ca. 15 nm diameter)
showed two peaks at 934.2 eV and 954.0 eV, indicating the presence of CuO as well as Cu(1)/FAU. The spectrum for the reduced Cu(1)/Y in Fig. 6(c) showed two peaks at 933.0 eV and 952.0 eV, indicating that CuO was reduced to Cu or Cu2O as a result of reduction by H2. The spectra for Mo(0.8)Cu(0.2)/FAU after H2 reduction and Mo(0.8)Cu (0.2)/FAU after the rWGS reaction also show two peaks around 933.0 eV and 952.2 eV, indicating reduction of CuO. Although it is difficult to decide from the XPS spectra whether the peaks were attributable to Cu or Cu2O, it is speculated that CuO was reduced to Cu based on the XPS and XRD data. The XPS analysis did clarify that Mo and Cu were reduced to MoO2 and Cu/Cu2O by pretreatment with H2 at 873 K and that the oxidized state was maintained under the rWGS reaction conditions at 773 K.
3.2.4. H2-TPR For the Mo-Cu co-supported catalysts, the XPS and XRD results suggested that the reduction behavior depended on the contents of Mo and Cu. To examine the reduction process for the Mo-Cu co-supported catalysts, the H2-TPR profiles for the series of catalysts were measured and results are shown in Fig. 8. Five peaks (labeled (a)–(e)) are shown in the figure. In the TPR profile for Mo(1)/FAU, two peaks were present at 750 K (peak (d)) and 800 K (peak (e)), which corresponded to the reduction of Mo. Peak (e) was suppressed in the case of Mo(0.8)Cu (0.2)/FAU. The species corresponding to peak (a) at 570 K and peak (b) at 630 K were responsible for the reduction of CuO. Peak (c) at 680 K was observed in the profile for Mo(0.2)Cu(0.8)/FAU and Mo(0.5)Cu (0.2)/FAU. Two-step reactions caused by the H2 reduction of the MoO3.
3.2.3. TEM The morphology of immobilized MoOx and Cu was observed by the TEM measurement. Fig. 7 represents TEM micrographs (scale bars: 20 and 5 nm, respectively) of Mo(0.8)Cu(0.2)/FAU after H2 pretreatment. In Fig. 7(a), it was clearly seen that the catalyst maintained its zeolite structure with lattice diameter of 1.4 nm, whereas dark nanoparticles supported on the edge of the zeolite. Fig. 7(b) is the enlarged figure of the nanoparticles in Fig. 7(a). Large darker particles (up to 15 nm) with 6
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Fig. 7. Representative TEM images of Mo(0.8)Cu(0.2)/FAU.
Cu. This result is consistent with the XPS data of Figs. 5 and 6. Thus, it is suggested that the effect of adding Cu to Mo catalysts in the rWGS reaction was to accelerate the reduction of the Mo species. To verify the influence of the reduction of MoO3 on the catalytic activity, the reaction was carried out for Mo(1)/FAU without H2 reduction and for Mo(1)/FAU with H2 reduction at 673 K and 873 K. The results are shown in Fig. 9. Below a reaction temperature of 673 K, Mo (1)/FAU after calcination showed the lowest CO yield compared with the results for the catalysts with H2 reduction. This was because it was difficult for the reduction of MoO3 to proceed at 673 K given the slow rate of reaction. On the other hand, the sample with pre-reduction at 873 K showed high activity, indicating the reduced Mo species was important in the rWGS reaction.
4. Conclusion In this study, the characteristics and mechanisms for the rWGS reaction catalyzed by Mo supported zeolite and Mo-Cu co-supported zeolite catalysts were examined. As shown by XRD, XPS, TEM, and H2TPR data, MoO2 species were formed from reduction of MoO3 by the Mo-based catalysts. Moreover, the addition of Cu to the catalyst resulted in an improvement in the reducibility of MoO3. The Mo(0.8)Cu (0.2)/FAU catalyst, which contains co-supported Mo-Cu at an atomic
Fig. 8. Temperature-programmed reduction with H2 spectra of Mo(x)Cu(1-x)/ FAU catalysts (x: metal content, mmol/g). Measurement conditions. Sample: 80 mg; flow: H2(10 %)/Ar 40 mL/min; 323–1073 K, 10 K/min; pre-treatment: He flow, 573 K, 2 h.
In the profile for Mo(1)/FAU, the two step reaction that occurred in the range 700–773 K was assigned to the reduction process MoO3→MoO2 [49,50]. Kim et al. reported that the first step corresponded to the reduction process MoO3→MoO2. This reaction occurs over a temperature range of 723–923 K [51]. Below 723 K, the reduction proceeded very slowly, yielding intermediate oxides of Mo4O11 between MoO3 and MoO2. The second step of the reduction process occurred in the range 773–823 K and proceeded very slowly, resulting in the formation of MoO2, which originated from the intermediate oxides Mo4O11. The TPR profile for Cu(1)/FAU indicated a two-step reduction, corresponding to CuO→Cu2O→Cu in the range 473–673 K, but there was no peak above 673 K [52,53]. The TPR profiles for Mo-Cu co-supported catalysts showed new peaks in the range 573–823 K. It is noteworthy that the peaks in Fig. 8 were shifted to lower temperatures by the co-supporting Mo-Cu catalyst. Of note, the second reduction step for MoO3 and MoO2 to Mo4O11 was absent for the Mo-Cu co-supported catalysts [54]. The shift in the reduction temperature for MoO3 indicated that the addition of Cu led to an easier reduction for Mo oxide, which for Mo(1)/FAU was reduced at 823 K. It is reasonable to conclude, therefore, that the reduction behavior of MoO3 was strongly influenced by the addition of
Fig. 9. Catalytic activity of Mo(1)/FAU for different pretreatment conditions: △: after calcination at 773 K; ◇: after H2 reduction at 673 K; ○: after H2 reduction at 873 K. Reaction conditions. Catalyst: 200 mg; GHSV: 7500 mL·h−1 g−1; H2:CO2:N2 = 12.5:12.5:10 (mL/min). 7
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A. Okemoto, et al.
ratio of 4:1, exhibited a higher CO yield compared with the supported Mo catalyst. The Mo(0.8)Cu(0.2)/FAU catalyst exhibited CO yield of 18.5 % and selectivity of 99 % for the rWGS reaction with the feed gas (H2:CO2 = 1:1) at atmosphere pressure and 773 K. The Mo-based catalyst is expected to be effective for selective conversion of CO2 to CO because of its high selectivity and high deactivation resistance. In future studies, optimization of the preparation method for the Mo catalyst together with better understanding of the reaction mechanisms for the Mo species may lead to activity enhancements for the CO2 conversion process by H2.
[20] [21] [22] [23] [24] [25] [26] [27] [28] [29]
Credit author statement
[30]
Authors’ contributions Atsushi Okemoto designed the study, and wrote the initial draft of the manuscript. Koichi Sato, Norihito Hiyoshi, Takayuki Ishizaka and Makoto R. Harada contributed to analysis and interpretation of data, and assisted in the preparation of the manuscript. All other authors have contributed to data collection and interpretation, and critically reviewed the manuscript. All authors approved the final version of the manuscript, and agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved
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Applied Catalysis A, General journal homepage: www.elsevier.com/locate/apcata
Catalytic performance of MoO3/FAU zeolite catalysts modified by Cu for reverse water gas shift reaction
T
Atsushi Okemoto*, Makoto R. Harada, Takayuki Ishizaka, Norihito Hiyoshi, Koichi Sato National Institute of Advanced Science and Technology, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Reverse water gas shift reaction Zeolite Molybdenum catalysis Metal–metal interaction Reduction reaction
The reverse water gas shift (rWGS) reaction using a supported zeolite catalyst containing molybdenum (Mo) was investigated. The Mo-based catalyst is expected to be effective for selective conversion of CO2 into CO. The focus of this work was on the performance of the Mo-based catalyst and the effect of adding copper (Cu) to the catalyst. The Mo catalyst exhibited high CO selectivity (99 %) with CO2 conversion of 14.3 % at 773 K. The activity of the catalyst containing Cu as an additive depended on the Mo/Cu ratio. For a series of Mo(x)Cu(1-x)/Faujasite (FAU) catalysts (x: metal content, mmol/g), the Mo(0.8)Cu(0.2)/FAU catalyst exhibited the best performance. X-ray diffraction, X-ray photoelectron spectroscopy and temperature-programmed reduction with hydrogen analysis of the Mo(x)Cu(1-x)/FAU catalysts revealed that the presence of Cu as an additive influenced the reduction step of MoO3 to MoO2, with MoO3 being the active species. Transmission electron microscopy verified morphology of MoO3 and Cu particles on the support. The excellent performance of the prepared Mo(0.8)Cu(0.2)/FAU catalyst was due to facilitation of the reduction process involving the Mo species.
1. Introduction The recent increase in carbon dioxide (CO2) emissions has resulted in climate change and has had a major impact on human society and the global environment [1–3]. Therefore, the development of sustainable technology that can recover CO2 emitted by industry and reconvert the CO2 into a useful resource is considered critical for alleviating this trans-national environmental issue [4–6]. Most of the CO2 is generated by consumption of fossil fuels, such that the increase in the CO2 emissions directly impacts the global environment. This cannot continue in societies that consume large amounts of limited fossil resources if the world is to achieve sustainable development over the long term. There is a need to create a carbonneutral society that introduces renewable energy and uses it effectively [7–9]. At present, one of the approaches for solving the CO2 problem is to exploit chemical reaction technology whereby CO2 is converted into a chemical product or hydrocarbon fuel through reaction with hydrogen (H2). In this regard, many gas-phase catalytic reactions using solid catalysts have been studied. For example, there is a rich body of catalysis research involving methanation, methanol synthesis, and the reverse water gas shift (rWGS) reaction [10–14]. The rWGS is a reaction that reduces CO2 to CO, and has been regarded as a useful process for
⁎
arbitrarily controlling the H2:CO composition ratio in synthesis gas, which is in high demand as a raw material for C1 chemistry [15–18]. This report concerns the development of a supported metal catalyst that exhibits high selectivity for CO in the rWGS reaction. Inorganic materials such as zeolites have been studied extensively as catalyst supports, zeolites are particularly useful for supports due to polymorphous porous structures, specific surface area and thermal stability [19,20]. A Y-type (Faujasite-type) zeolite, which is one of 12-memered ring zeolites, was used as a support for highly dispersed metal particles [21,22]. When the rWGS reaction was evaluated for activity using several metals, it was found that Mo exhibited high selectivity for CO. However, the Mo oxide supported catalyst has the problem of low CO2 conversion [23,24]. Some researchers have reported that the CO2 conversion in the rWGS reaction was related to the reducibility of Pt or Cu supported catalysts [25,26]. In contrast, Mo oxide is not readily reduced [27,28]. Therefore, in this work, a second active metal was added for the purpose of enhancing the activity of the Mo oxide supported catalyst. It was found that Cu promoted the reduction of Mo and high activity was realized. The support state and the reducibility of the Mo catalyst were evaluated, and the catalyst was characterized by temperature-programmed reduction with H2 (H2-TPR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Transmission electron microscopy (TEM) to gain insights into the effect of adding Cu,
Corresponding author. E-mail address: [email protected] (A. Okemoto).
https://doi.org/10.1016/j.apcata.2020.117415 Received 26 July 2019; Received in revised form 7 January 2020; Accepted 11 January 2020 Available online 13 January 2020 0926-860X/ © 2020 Published by Elsevier B.V.
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and the composition of the catalyst that gave the highest activity was found.
CO yield (%) =
CH 4 yield (%) =
2. Experimental 2.1. Materials
CO outlet (mol) × 100 CO2 inlet (mol)
CH4 outlet (mol) × 100 CO2 inlet (mol)
CO selectivity (%) =
All materials were used as received without further purification. The Faujasite-type zeolite (FAU, HSZ-330HUA) was purchased from Tosoh Corporation. Hexammonium heptamolybdate tetrahydrate ((NH4)6Mo7O24·4H2O, Guaranteed Reagent [GR]), copper(II) chloride (CuCl2, GR), nickel(II) chloride hexahydrate (NiCl2(II)·6H2O, GR), and hydrochloric acid (HCl, GR) were purchased from Wako Pure Chemical Corporation. Anhydrous iron(III) chloride (FeCl3, Trace Metal Basis) was purchased from Sigma-Aldrich.
CO outlet (mol) × 100 CO2 inlet − CO2 outlet (mol)
2.4. XRD XRD studies were carried out using a SmartLab diffractometer (Rigaku). The XRD patterns were recorded using monochromatic Cu Kα radiation (λ =1.5418 Å) at room temperature. The 2θ range from 20° to 50° was scanned using a step size of 0.04° and a counting time of 4 s per step. 2.5. XPS
2.2. Catalyst preparation
The oxidization states of the metals on the surfaces of the catalysts were measured by X-ray photoelectron spectroscopy using a PHI 5000 VersaProbe II (ULVAC-PHI) spectrometer with a monochromatic Al Kα source (hν =1486.6 eV, 24.92 W). The oxidation states of the elements measured were Mo 3d and Cu 2p. MultiPak software (ULVAC PHI) was used for all data processing. The charge shift was corrected by using the peak of adventitious carbon (284.5 eV). The software used Gaussian–Lorentzian functions to generate the fitting curves for the recorded XPS data. The multiple peak for Mo 3d was deconvoluted by a Gaussian–Lorentzian function with restricted conditions for the peak area (Mo 3d 3/2 : Mo 3d 5/2 = 2:3) and the peak distance between Mo 3d 3/2 and Mo 3d 5/2 (3.1 eV).
Mo and Cu supported catalysts were prepared by incipient wetness impregnation using FAU as a support. The metal loading was fixed at 1.0 mmol/g-cat for each catalyst. Prior to impregnation, 1.0 g of FAU placed under vacuum in a side-arm flask for 30 min at room temperature to remove adsorbed water molecules from the surface. Hexammonium heptamolybdate tetrahydrate (0.206 g), copper(II) chloride (0.146 g), anhydrous iron(III) chloride (0.176 g), or nickel(II) chloride hexahydrate (0.257 g) was dissolved into 10 mL of HCl solution (0.01 mol/L). The impregnation solution was put into the flask with the FAU and left for 30 min at room temperature. The solution was evaporated in a 300 mL round-bottom flask for 1 h, followed by drying overnight in an electric oven at 328 K. Then, the sample was calcined in air at 773 K for 10 h. The obtained catalysts were named M(x)/FAU (M and x refer to element and its metal content per gram of the catalyst, 0 ≤ x ≤ 1 mmol/g). The metal content was determined by inductively coupled plasma atomic emission spectroscopy (SII SPS-7800, Hitachi). The specific surface areas of the catalysts were measured using a BELSORPmini system (Microtrac BEL Corporation). A glass cell charged with the sample (0.050 g) was heated at 523 K for 2 h in a flow of N2 before measurement. The N2 adsorption/desorption isotherms were obtained at liquid N2 temperature (77 K). The surface areas of the samples were evaluated using the Brunauer–Emmett–Teller (BET) method.
2.6. TEM TEM images were recorded using a FEI TecnaiG2 20 electron microscope operating at 200 kV. The samples were grinding in a mortar, and then mounted on a microgrid carbon polymer supported on a copper grid by placing a droplet of a sample dispersion in ethanol on the grid. 2.7. H2-TPR The H2-TPR was recorded using a BEL-CAT system (Microtrac BEL Corporation) equipped with a TCD. The sample (80 mg) was charged in a quartz cell and pretreated in a flow of He at 573 K for 120 min. The sample was then cooled to room temperature, and heated to 1073 K at a rate of 10 K/min in 10 % H2/Ar. The H2 consumption was recorded by the TCD.
2.3. Reactivity test for the rWGS reaction The rWGS reaction was carried out in a fixed-bed continuous-flow reactor. An α-alumina tube (12 mm inner diameter) was used as the reaction tube. Catalyst (200 mg) diluted with 1.0 g of inert quartz sand was placed in the tube, which contained 1.0 g of pure quartz sand as an inner liner. The catalyst was pretreated with a 55 % H2:N2 mixture at a flow rate of 22.5 mL/min at 873 K for 1 h before performing the reaction. After pretreatment, the catalyst was set at the reaction temperature in N2 gas, and the reaction gas (H2:CO2:N2 = 12.5:12.5:10 mL/min, total flow: 35 mL/min) was fed into the reactor with a space velocity of 7500 mL·h−1 g−1. The products of the reaction were analyzed by two separate on-line GC systems equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID), respectively. The gas lines from the reaction tube to the sampling valves were heated by electric heaters to avoid condensation of products. The reaction products were measured as follows: H2, CO, and CH4 were measured by GC-TCD with a Molecular Sieve 13X column; CO2 and H2O were measured by GC-TCD with a Gaskuropack 54 column (GL Science); and CH3OH was measured by GC-FID with a Molecular Sieve 13X column. The conversion, yield, and selectivity for each product were calculated using the following equations:
3. Results and discussion 3.1. Catalytic test for the rWGS reaction The metal contents and BET surface areas for the prepared catalysts are listed in Table 1. The Si/Al ratio and the BET surface area of the FAU (raw) used as the catalyst support were 2.9 and 593 m2/g, respectively. The Si/Al ratio values for all materials were between 2.7–2.9. The catalyst preparation procedure had little influence on the Si/Al ratio. On the other hand, the BET surface areas for all metal supported catalysts decreased by 10–14 % in comparison with the FAU (raw) because the metal clusters partially covered the pores of the zeolite or the weight of the catalysts increased as a result of adding the supporting metal cluster. In the rWGS reaction, the catalytic performance of a single-metal supported zeolite catalyst and Cu/ZnO/Al2O3, which is generally known as a catalyst for the water gas shift reaction, was evaluated and 2
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Table 1 List of catalysts. Catalyst
FAU (raw) Mo(1)/FAU Cu(1)/FAU Fe(1)/FAU Ni(1)/FAU Mo(0.9)Cu(0.1)/ FAU Mo(0.8)Cu(0.2)/ FAU Mo(0.7)Cu(0.3)/ FAU Mo(0.5)Cu(0.5)/ FAU Mo(0.2)Cu(0.8)/ FAU
Si/Al
a)
Metal content
a)
(mmol/g)
Physical property b) BET area (m2/ g)
Mo
Cu
Fe
Ni
2.9 2.9 2.8 2.8 2.7 2.9
– 1.02 – – – 0.91
– – 0.98 – – 0.11
– – – 0.96 – –
– – – – 0.96 –
593 512 545 530 531 524
2.9
0.80
0.18
–
–
533
2.8
0.66
0.28
–
–
529
2.9
0.48
0.46
–
–
536
2.7
0.77
0.18
–
–
530
Fig. 1. CO2 conversion over Mo(1)/FAU for the reverse water gas shift reaction at 773 K. Reaction conditions. Reactor: alumina tube φ12 mm; catalyst: 200 mg; GHSV: 7500 mL·h−1 g−1; feed gas: H2:CO2:N2 = 12.5:12.5:10 (mL/min); pre-treatment gas: H2:N2 = 12.5:10 (mL/min); pre-treatment temperature: 873 K; reaction temperature: 773 K.
a) Measured by inductively coupled plasma atomic emission spectroscopy (SII SPS-7800, Hitachi). b) Measured by N2 adsorption (BEL-mini, Microtrac Bell).
results are listed in Table 2. Chen et al. reported that a Cu-Zn-Al catalyst gave a 17.4 % yield and 100 % selectivity for CO at 773 K [26]. The catalyst was activated in a reduced gas flow (H2:N2 = 12.5:10 mL/min) at 873 K before reaction. The activity was measured at 773 K 3 h after the start of the reaction. Mo(1)/FAU exhibited relatively high activity with a 14.3 % yield and 99 % selectivity for CO 3 h after the start of the reaction. Little CH4 was formed as a by-product (< 1 %) and the carbon balance was more than 95 %. Although the activity was less than that of the Cu-Zn-Al catalyst, it was found that the single-metal MoO3 supported catalyst exhibited acceptable performance in the rWGS reaction. Fe(1)/FAU and Cu(1)/FAU gave yields of 5.9 % and 6.8 %, respectively, and selectivity of 98 % for CO. The yield for Ni(1)/FAU (17.2 %) was higher than that for Mo(1)/FAU, but the CO selectivity for Ni(1)/FAU was relatively low at 45 % due to high conversion of CO2 into CH4. The CO yields for the time-on-stream for these catalysts are shown in Fig. 1. Ni(1)/FAU and Mo(1)/FAU showed high initial activity but there was a decrease in activity after 5 h, probably due to migration of metal species and deposition of carbon on the catalyst surface [29,30]. A significant decrease in conversion of CO2 relative to Mo(1)/FAU, Cu(1)/ FAU, and Fe(1)/FAU was not observed. After reaction for 20 h, the conversion became higher for Mo(1)/FAU than for Ni(1)/FAU. That is, Mo(1)/FAU was very effective in the rWGS reaction because of the high CO2 conversion, the high CO selectivity, and a relative absence of catalytic deactivation. To improve the activity of the supported Mo catalyst, Cu was added to the catalyst to facilitate the reduction of the active Mo species. The activities of the Mo-Cu co-supported catalysts with different Mo/Cu ratios for the rWGS reaction are presented in Fig. 2. Although there were small differences in the activities of the catalysts at a reaction temperature of 673 K, the differences became more pronounced at 723
Fig. 2. CO yield over Mo(x)Cu(1-x)/FAU catalysts (0 ≤ x ≤ 1 mmol/g-cat.) for the reverse water gas shift reaction at different temperatures. Reaction conditions. Reactor: alumina tube φ12 mm; catalyst: 200 mg; GHSV: 7500 mL·h−1 g−1; feed gas: H2:CO2:N2 = 12.5:12.5:10 (mL/min); pre-treatment gas: H2:N2 = 12.5:10 (mL/min); pre-treatment temperature: 873 K; reaction time: 3 h.
K. The order of catalytic activity was Mo(01)/FAU > Mo(0.8)Cu(0.2)/ FAU > Mo(0.9)Cu(0.1)/FAU > Mo(0.7)Cu(0.3)/FAU > Mo(0.5)Cu (0.5)/FAU > Mo(0.2)Cu(0.8)/FAU > Cu(1)/FAU. In other words, Cu (1)/FAU exhibited the lowest activity in the series. Note that the order was not dependent on the loading of Cu. In particular, Mo(0.8)Cu(0.2)/ FAU showed high activity, with a CO yield of 18.5 % at 773 K. The
Table 2 Comparison of CO2 conversion and CO selectivity for the single-metal supported catalysts and values reported for Cu-Zn-Al catalyst. Reaction conditions: 0.1 MPa, 773 K, H2:CO2 = 12.5:12.5 (mL/min), GHSV =7500 mL·h−1 g−1. [[In expressions below the table, please use subscripts for the chemical symbols.]]. Catalyst
H2:CO ratio
CO2 conversion (%)
CO selectivity (%)
CH4 yield (%)
Ref.
Cu-Zn-Al (36 wt%) Mo(1)/FAU Cu(1)/FAU Fe(1)/FAU Ni(1)/FAU
2:1 1:1 1:1 1:1 1:1
17.4 14.3 6.8 5.9 17.2
100 99 98 98 45
0 1 2 2 55
[1] This This This This
[1] Chen, C.-S.; Cheng, W.-H.; Lin, S.-S. Catal. Lett. 2000, 68 (1), 45−48. GSHV: gas space hourly velocity. 3
work work work work
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Fig. 3. CO yield over Mo(x)Cu(1-x)/FAU catalysts (0 ≤ x ≤ 1 mmol/g-cat.) for the reverse water gas shift reaction with a reaction time of 3 h. Reaction conditions. Reactor: alumina tube φ12 mm; catalyst: 200 mg; GHSV: 7500 mL·h−1 g−1; feed gas: H2:CO2:N2 = 12.5:12.5:10 (mL/min); pre-treatment gas: H2:N2 = 12.5:10 (mL/min); pre-treatment temperature: 873 K; reaction temperature: 773 K; reaction time: 3 h.
effect of Cu addition on the rWGS reaction at 733 K is summarized in Fig. 3. The yields of CO for Mo-Cu co-supported catalysts with different Mo/Cu ratios at 773 K are illustrated in Fig. 3. A volcano curve having a peak top at Mo(0.8)Cu(0.2)/FAU was obtained for the plot of the CO yield. Addition of a small amount of Cu to the Mo catalyst at a Mo/Cu ratio of 4/1 led to an increase in the catalytic performance in the rWGS reaction. Mo(0.5)Cu(0.5)/FAU, Mo(0.2)Cu(0.8)/FAU, and Cu(1)/FAU showed less activity than Mo(1)/FAU because of a decrease in the Mo content of the catalysts. This result confirmed that a Mo species is a key active species and the addition of Cu influences catalytic performance in the rWGS reaction. 3.2. Relationship between activity and structure of catalysts 3.2.1. Specific surface area and structure The crystal structures of all metal supported catalysts were characterized by XRD and the results are shown in Fig. 4. Fig. 4(a) displays the patterns for the raw FAU zeolite as a support and Mo(1)/FAU, Cu (1)/FAU, Fe(1)/FAU, and Ni(1)/FAU after calcination at 773 K. The peak at 23.5° in the patterns for Mo(1)/FAU and FAU zeolite is ascribed to α-MoO3, [31–33]. Other forms of Mo species were not observed. In the pattern for Cu(1)/FAU, the peak at 38° was assigned to CuO(111) [34]. The pattern for Fe(1)/FAU showed peaks at 35.5° and 41° and these were assigned to the Fe3O4 (311) and the Fe3O4 (400) phases, respectively, signifying that Fe species are supported as Fe3O4 after calcination at 773 K [35,36]. The pattern for Ni(1)/FAU showed two prominent peaks corresponding to NiO(111) and NiO(200) at 37.1° and 42.2°, respectively [37]. Therefore, Ni species were present as NiO in Ni (1)/FAU. In the case of the single-metal supported catalyst, the load metal existed as a single-phase metal oxide. The XRD patterns for Mo-Cu co-supported catalysts after calcination at 773 K are presented in Fig. 4(b). Although there is a peak corresponding to α-MoO3 at 23.5° in the patterns for Mo(1)/FAU and Mo (0.8)Cu(0.2)/FAU, there is no peak at 23.5° in the XRD patterns for Mo (0.5)Cu(0.5)/FAU and Mo(0.2)Cu(0.8)/FAU. In contrast, the CuO peak at 38° is observed for Cu(1)/FAU and Mo(0.2)Cu(.8)/FAU. When the loading amount of Cu was under 50 %, peaks for Cu compounds were not observed. That is, the Cu oxide peak did not appear in cases when
Fig. 4. X-ray diffraction patterns for the single-metal supported catalysts: (a) single-metal supported catalysts after calcination at 773 K; (b) Mo and Cu supported catalysts after calcination at 773 K; (c) Mo-Cu co-supported catalysts after H2 reduction at 873 K.
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Fig. 5. X-ray photoelectron spectroscopy spectra of Mo(1)/FAU and Mo(0.8)Cu(0.2)/FAU in the region of Mo 3d: (a) Mo(1)/FAU calcined at 773 K; (b) Mo(0.8)Cu (0.2)/FAU calcined at 773 K; (c) Mo(1)/FAU reduced by hydrogen at 873 K; (d) Mo(0.8)Cu(0.2)/FAU reduced by hydrogen at 773 K; (e) Mo(0.8)Cu(0.2)/FAU after the reverse water gas shift reaction at 773 K for 6 h.
the peaks were changed and the peak positions were moved to lower binding energies, appearing at 228.0 eV and 231.5 eV, and these can be assigned to Mo(IV) 3d 5/2 and Mo(IV) 3d 3/2, respectively [39–41]. This indicates that some of the MoO3 was reduced to MoO2 after the samples were reduced with H2 at 873 K for 1 h. In comparison with the peak intensities of the reduced Mo(1)/FAU and the reduced Mo(0.8)Cu (0.2)/FAU, the peak for Mo(IV) in Mo(0.8)Cu(0.2)/FAU was larger than that for Mo(1)/FAU. The ratio of the peak area of Mo(VI) and Mo(IV) represents the degree of reduction of Mo. The ratios for Mo(1)/FAU and Mo(0.8)Cu(0.2)/FAU after reduction by H2 were 0.25 and 0.45, respectively. These results suggest that MoO3 in Mo(0.8)Cu(0.2)/FAU underwent more reduction than that in Mo(1)/FAU. Further, in the spectrum of Mo(0.8)Cu(0.2)/FAU after the rWGS reaction, the peak assigned to MoO2 had intensity as strong as that in the reduced Mo (0.8)Cu(0.2)/FAU. This finding suggested that MoO3 was reduced as the rWGS reaction proceeded. The XPS spectra for Mo(1)/FAU and Mo(0.8)Cu(0.2)/FAU before and after H2 reduction are shown in Fig. 6. In Fig. 6 (a), two peaks appeared at 934.0 eV and 953.5 eV in the Cu 2p region. The peaks were assigned to Cu(II) 2p 3/2 and Cu(II) 2p 1/2 [41–44]. These results indicate that Cu was present as CuO in the Cu(1)/FAU catalyst after calcination. The spectrum of Mo(0.8)Cu(0.2)/FAU after calcination also
the metal loading was less than 0.5 mmol/g, owing to the small size of the oxide crystals (< 4 nm), thus precluding detection by XRD. It was noted that the complex oxide of Mo and Cu did not form after calcination of the catalyst. The XRD patterns for the Mo-Cu co-supported catalysts reduced by H2 at 873 K are shown in Fig. 4(c) In the spectra for all reduced samples, the peak at 23.5°, which is ascribed to α-MoO3, was not observed. On the other hand, a small peak was detected at 37.1° and was assigned to MoO2 (211). Although the main peak for MoO2 (110) at 26.1° was observed despite the overlapping by the peak due to FAU, it is suggested that MoO2 was formed by H2 reduction. 3.2.2. XPS The XPS studies were conducted to determine the surface oxidization states of the Mo(1)/FAU and Mo(0.8)Cu(0.2)/FAU catalysts before and after treatment with H2. The XPS spectra for Mo(1)/FAU and Mo (0.8)Cu(0.2)/FAU are shown in Fig. 5. In Fig. 5(a) and (b), Mo(1)/FAU and Mo(0.8)Cu(0.2)/FAU after calcination each gave two peaks at 231.8 eV and 250.0 eV in the Mo 3d region. These peaks were attributed to the Mo(VI) 3d 5/2 and the Mo(VI) 3d 3/2 spin–orbit components, respectively [38]. In the spectra for the reduced Mo(1)/FAU and Mo(0.8)Cu(0.2)/FAU (Fig. 5(b) and 5(c), respectively), the shapes of 5
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Fig. 6. X-ray photoelectron spectroscopy spectra of Cu(1)/FAU and Mo(0.8)Cu(0.2)/FAU in the region of Cu 2p. (a) Cu(1)/FAU calcined at 773 K; (b) Mo(0.8)Cu(0.2)/FAU calcined at 773 K; (c) Cu(1)/FAU reduced by H2 at 873 K; (d) Mo(0.8)Cu(0.2)/FAU reduced by H2 at 873 K; (e) Mo(0.8)Cu(0.2)/FAU after the reverse water gas shift reaction at 773 K for 6 h.
lattice interval of 0.38-0.40 nm was found in Fig. 7(b), and was assigned to MoO3 with the lattice parameter =0.39 nm, in good agreement with the literature values (JCPDS file no. 05-0508) [45]. There was a light-gray particle in contiguity with the MoO3 particle, which was assigned to Cu with lattice parameter =0.21 nm, in good agreement with the literature values (JCPDS file no. 04-0836). [46–48]. The results verified that Cu particles was contiguous to MoO3 particles (ca. 15 nm diameter)
showed two peaks at 934.2 eV and 954.0 eV, indicating the presence of CuO as well as Cu(1)/FAU. The spectrum for the reduced Cu(1)/Y in Fig. 6(c) showed two peaks at 933.0 eV and 952.0 eV, indicating that CuO was reduced to Cu or Cu2O as a result of reduction by H2. The spectra for Mo(0.8)Cu(0.2)/FAU after H2 reduction and Mo(0.8)Cu (0.2)/FAU after the rWGS reaction also show two peaks around 933.0 eV and 952.2 eV, indicating reduction of CuO. Although it is difficult to decide from the XPS spectra whether the peaks were attributable to Cu or Cu2O, it is speculated that CuO was reduced to Cu based on the XPS and XRD data. The XPS analysis did clarify that Mo and Cu were reduced to MoO2 and Cu/Cu2O by pretreatment with H2 at 873 K and that the oxidized state was maintained under the rWGS reaction conditions at 773 K.
3.2.4. H2-TPR For the Mo-Cu co-supported catalysts, the XPS and XRD results suggested that the reduction behavior depended on the contents of Mo and Cu. To examine the reduction process for the Mo-Cu co-supported catalysts, the H2-TPR profiles for the series of catalysts were measured and results are shown in Fig. 8. Five peaks (labeled (a)–(e)) are shown in the figure. In the TPR profile for Mo(1)/FAU, two peaks were present at 750 K (peak (d)) and 800 K (peak (e)), which corresponded to the reduction of Mo. Peak (e) was suppressed in the case of Mo(0.8)Cu (0.2)/FAU. The species corresponding to peak (a) at 570 K and peak (b) at 630 K were responsible for the reduction of CuO. Peak (c) at 680 K was observed in the profile for Mo(0.2)Cu(0.8)/FAU and Mo(0.5)Cu (0.2)/FAU. Two-step reactions caused by the H2 reduction of the MoO3.
3.2.3. TEM The morphology of immobilized MoOx and Cu was observed by the TEM measurement. Fig. 7 represents TEM micrographs (scale bars: 20 and 5 nm, respectively) of Mo(0.8)Cu(0.2)/FAU after H2 pretreatment. In Fig. 7(a), it was clearly seen that the catalyst maintained its zeolite structure with lattice diameter of 1.4 nm, whereas dark nanoparticles supported on the edge of the zeolite. Fig. 7(b) is the enlarged figure of the nanoparticles in Fig. 7(a). Large darker particles (up to 15 nm) with 6
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Fig. 7. Representative TEM images of Mo(0.8)Cu(0.2)/FAU.
Cu. This result is consistent with the XPS data of Figs. 5 and 6. Thus, it is suggested that the effect of adding Cu to Mo catalysts in the rWGS reaction was to accelerate the reduction of the Mo species. To verify the influence of the reduction of MoO3 on the catalytic activity, the reaction was carried out for Mo(1)/FAU without H2 reduction and for Mo(1)/FAU with H2 reduction at 673 K and 873 K. The results are shown in Fig. 9. Below a reaction temperature of 673 K, Mo (1)/FAU after calcination showed the lowest CO yield compared with the results for the catalysts with H2 reduction. This was because it was difficult for the reduction of MoO3 to proceed at 673 K given the slow rate of reaction. On the other hand, the sample with pre-reduction at 873 K showed high activity, indicating the reduced Mo species was important in the rWGS reaction.
4. Conclusion In this study, the characteristics and mechanisms for the rWGS reaction catalyzed by Mo supported zeolite and Mo-Cu co-supported zeolite catalysts were examined. As shown by XRD, XPS, TEM, and H2TPR data, MoO2 species were formed from reduction of MoO3 by the Mo-based catalysts. Moreover, the addition of Cu to the catalyst resulted in an improvement in the reducibility of MoO3. The Mo(0.8)Cu (0.2)/FAU catalyst, which contains co-supported Mo-Cu at an atomic
Fig. 8. Temperature-programmed reduction with H2 spectra of Mo(x)Cu(1-x)/ FAU catalysts (x: metal content, mmol/g). Measurement conditions. Sample: 80 mg; flow: H2(10 %)/Ar 40 mL/min; 323–1073 K, 10 K/min; pre-treatment: He flow, 573 K, 2 h.
In the profile for Mo(1)/FAU, the two step reaction that occurred in the range 700–773 K was assigned to the reduction process MoO3→MoO2 [49,50]. Kim et al. reported that the first step corresponded to the reduction process MoO3→MoO2. This reaction occurs over a temperature range of 723–923 K [51]. Below 723 K, the reduction proceeded very slowly, yielding intermediate oxides of Mo4O11 between MoO3 and MoO2. The second step of the reduction process occurred in the range 773–823 K and proceeded very slowly, resulting in the formation of MoO2, which originated from the intermediate oxides Mo4O11. The TPR profile for Cu(1)/FAU indicated a two-step reduction, corresponding to CuO→Cu2O→Cu in the range 473–673 K, but there was no peak above 673 K [52,53]. The TPR profiles for Mo-Cu co-supported catalysts showed new peaks in the range 573–823 K. It is noteworthy that the peaks in Fig. 8 were shifted to lower temperatures by the co-supporting Mo-Cu catalyst. Of note, the second reduction step for MoO3 and MoO2 to Mo4O11 was absent for the Mo-Cu co-supported catalysts [54]. The shift in the reduction temperature for MoO3 indicated that the addition of Cu led to an easier reduction for Mo oxide, which for Mo(1)/FAU was reduced at 823 K. It is reasonable to conclude, therefore, that the reduction behavior of MoO3 was strongly influenced by the addition of
Fig. 9. Catalytic activity of Mo(1)/FAU for different pretreatment conditions: △: after calcination at 773 K; ◇: after H2 reduction at 673 K; ○: after H2 reduction at 873 K. Reaction conditions. Catalyst: 200 mg; GHSV: 7500 mL·h−1 g−1; H2:CO2:N2 = 12.5:12.5:10 (mL/min). 7
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ratio of 4:1, exhibited a higher CO yield compared with the supported Mo catalyst. The Mo(0.8)Cu(0.2)/FAU catalyst exhibited CO yield of 18.5 % and selectivity of 99 % for the rWGS reaction with the feed gas (H2:CO2 = 1:1) at atmosphere pressure and 773 K. The Mo-based catalyst is expected to be effective for selective conversion of CO2 to CO because of its high selectivity and high deactivation resistance. In future studies, optimization of the preparation method for the Mo catalyst together with better understanding of the reaction mechanisms for the Mo species may lead to activity enhancements for the CO2 conversion process by H2.
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Credit author statement
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Authors’ contributions Atsushi Okemoto designed the study, and wrote the initial draft of the manuscript. Koichi Sato, Norihito Hiyoshi, Takayuki Ishizaka and Makoto R. Harada contributed to analysis and interpretation of data, and assisted in the preparation of the manuscript. All other authors have contributed to data collection and interpretation, and critically reviewed the manuscript. All authors approved the final version of the manuscript, and agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved
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