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ZnO catalyst promoted by Sc2O3 for hydrogen production from methanol reforming
Fuel 241 (2019) 607–615
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Full Length Article
An improved Cu/ZnO catalyst promoted by Sc2O3 for hydrogen production from methanol reforming
T
Yun-Chuan Pu, Shui-Rong Li , Shuai Yan, Xiao Huang, Duo Wang, Yue-Yuan Ye, Yun-Quan Liu ⁎
⁎
College of Energy, iChEM, Xiamen University, Xiamen 361102, China
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Methanol reforming Cu/Sc2O3-ZnO catalyst Doping Hydrogen production Autothermal
Copper sintering at temperatures above 200 °C has been a big challenge for the application of copper-based catalysts in methanol reforming for hydrogen production. In the present work, a Cu/ZnO catalyst promoted by Sc2O3 was developed, which is capable of converting methanol into hydrogen-rich gas with good activity and decent stability in the temperature range of 220–600 °C. The role of Sc in Cu/Sc2O3-ZnO catalysts was explored through the XRD, H2-TPR, N2O chemisorption, TEM, Raman, and XPS analyses. The results indicated that ZnO lattice was doped with the Sc3+ ion, which led to increased metal dispersion, strengthened interaction between Cu metal and support oxide, and subsequently enhanced catalytic activity and stability of the Cu/Sc2O3-ZnO catalyst in methanol reforming for hydrogen production.
1. Introduction Particulate matters (PM) and nitrogen oxides (NOx) from automobile exhaust are generally considered the main culprits of the notorious smog in cities. PM is usually generated by the incomplete
⁎
combustion of fuel while NOx is formed when free radicals react due to local high temperatures in the combustion chamber of engines. Thus, it is critical to reduce engine emissions by improving the quality and efficiency of combustion. Among many endeavors so far, the introduction of a second fuel into diesel to form a mixed fuel has been one of the
Corresponding authors. E-mail addresses: [email protected] (S.-R. Li), [email protected] (Y.-Q. Liu).
https://doi.org/10.1016/j.fuel.2018.12.067 Received 2 June 2018; Received in revised form 21 October 2018; Accepted 13 December 2018 0016-2361/ © 2018 Elsevier Ltd. All rights reserved.
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most important methods for reducing engine emissions [1]. These mixed fuels could be natural gas-diesel [2,3], reformed gas-diesel [4], hydrogen-diesel [3,5,6], biodiesel-diesel [7], kerosene-diesel [8] and butanol-gasoline-diesel [9], etc. Compared to other fuels, hydrogen is the lightest, requires minimum ignition energy, has the fastest flame propagation speed, wide explosive limit, and a short quenching distance. These properties significantly expand the range of the combustible fuel ratio, reduce the ignition temperature, promote homogeneous combustion, improve the combustion efficiency, and reduce PM while inhibiting the formation of NOx. Thus, the co-combustion of hydrogen with diesel can help achieve “zero emissions” in engine exhaust [3,6]. There are many compounds that are rich in hydrogen and can be considered as hydrogen carriers, among which methanol is obviously the most suitable one due to its low cost and easy storage as a liquid. Methanol is a primary alcohol with a high hydrogen to carbon ratio but no carbon-carbon bonds, thus allowing it to be easily converted into a hydrogen-rich gas at relatively low temperatures (200–300 °C) [10]. Compared to other fuels, such as methane, with a reforming temperature of above 600 °C [11], and ethanol, which is usually reformed at around 500 °C [12], methanol has a lower energy requirement for conversion into hydrogen-rich gas via reforming. In addition, methanol can be produced from renewable resources such as biomass, which is usually sulfur-free, so there will be less or no sulfur dioxide emitted during the conversion process [13]. In general, there are three major routes for hydrogen generation from methanol via reforming [13]: steam reforming (SR), partial oxidation (POX), and autothermal reforming (ATR). Both methanol and water contribute to the production of hydrogen in SR, which guarantees a high hydrogen yield, yet the reaction is strongly endothermic, which requires extra energy to sustain. POX, on the contrary, is an exothermic reaction, which means it gives off heat by consuming part of the fuel. While methanol is partially oxidized by O2 to produce CO2 and H2 in POX, the consumption of hydrogen by combustion is inevitable, and this greatly reduces the hydrogen yield. For ATR, however, it is a combination of SR and POX, and thus has the advantages of both SR and POX. The ATR reaction is described below:
CH3 OH + (1
2p)H2 O + pO2
CO2 + (3
2p)H2
(0
p
high temperature stability. At high temperatures, one of the biggest challenges is the sintering of Cu particles. Although the structure-activity relationship for Cu/ZnO/Al2O3 catalysts remains unclear, ZnO has been identified as a textural promoter in segregating the active Cu component, and thus a lot efforts have been made to maximize Cu-ZnO contact [25]. Various promoters, such as Ce, Y, and Zr, had been introduced into Cu-based catalysts to improve their catalytic performance [10,26–30]; however, it was found that only the addition of Zr to Cu/ ZnO or Cu/ZnO/Al2O3 would promote Cu dispersion and the formation of Cu-Zn alloy, which could enhance the catalytic performance with a higher methanol conversion and lower CO selectivity [26,29]. It is believed that the formation of surface lattice defect in the oxide support would benefit the metal-support interactions [31]. For ZnO, it was found that Sc could be readily embedded in ZnO crystal lattice in the form of Sc3+ through ion-doping due to its comparable ionic radius with Zn2+, and Sc could stabilize the support metal through the accommodation of metal ions at the surface vacant cation-sites [18,32]. Therefore, in the present work, a series of Sc promoted Cu/ZnO catalysts were prepared with the reverse precipitation method and were tested in methanol reforming at the wider temperature range of 220–600 °C. Furthermore, the structure-activity relationship of the Cubased catalysts was studied with a focus on the activity and stability, followed by the study of the possible roles of Sc in the catalysts. 2. Experimental 2.1. Chemicals Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, ≥99.0%), Copper (II) nitrate trihydrate (Cu(NO3)2·3H2O, ≥99.0%), and Scandium nitrate (Sc (NO3)3, ≥99.999%) were purchased from Shanghai Macklin Biochemical Co., Ltd., Aladdin Reagent (Shanghai) Co., Ltd., and Shandong Xiya Chemical Industry Co., Ltd., respectively. Sodium hydroxide (NaOH, ≥96.0%) and Methanol (≥99.5%, 0.791–0.793 g/mL) were received from Xilong Science Co., Ltd., and Sinopharm Chemical Reagent Co., Ltd., respectively. Hydrogen, oxygen, and nitrogen (all ≥99.999%) were supplied by Fujian Nan'an Success Gas Co., Ltd. Deionized water was produced in the lab with a laboratory water purification system (Manufacturer: Hitech Instruments Co., Ltd, China). All chemicals were used as received without further treatments.
0.5)
This reaction ensures both relatively high hydrogen yield and high overall efficiency compared to POX. Also, by varying the feed composition, ATR can be operated at a wide range of operating conditions, from endothermic to exothermic, but most frequently, at a condition of heat self-sustaining. In addition, ATR has a more rapid start than SR, which makes it more suitable for portable applications. The commonly used methanol reforming catalysts for hydrogen production can be divided into three categories: low temperature copper-based catalysts, high temperature Zn-Cr catalysts, and all-round precious metal catalysts. Although the precious metal catalysts, such as Pd and Pt-based [14–16], showed the best catalytic activity and stability for methanol reforming at a wide temperature range, the copperbased catalysts, particularly Cu/ZnO/Al2O3, are still most widely used in industry because of their low cost and good catalytic performance at low temperatures (150–300 °C) [17]. The Zn-Cr catalysts also exhibited good performance in terms of catalytic activity and hydrogen selectivity, but they operated at relatively high temperatures (> 450 °C) [18–20]. As for the application of methanol reforming in hydrogen enhanced combustion for internal combustion engines, the reformer is usually installed in the tailpipe to make full use of the waste heat carried by the exhaust [21]. Since the temperature of diesel engine exhaust varies in the temperature range of 200–600 °C depending on the diesel type and operating conditions [22–24], a cost effective catalyst with high catalytic activity and stability at a wide temperature range would be ideal for practical applications. To achieve this goal, copper-based catalysts must have the improved
2.2. Catalysts preparation The Cu/Sc2O3-ZnO catalysts were prepared through the reverse precipitation method. In a typical process, nitrate salts of Cu(NO3)2, Zn (NO3)2 and Sc(NO3)3 were dissolved in deionized water. The total metal concentration in the aqueous solution was 1 M while the concentration of NaOH in another solution (serving as the precipitant) was 3 M. The nitrate salt solution was then added dropwise to the precipitating agent with vigorous stirring at 75 °C, which resulted in precipitation. The stirring was continued for another 2 h while the temperature was maintained at 75 °C. Then, the precipitate was cooled to room temperature, and the suspension was filtered, and washed with deionized water repeatedly until the filtrate became neutral in pH. Drying was performed at 110 °C for 12 h followed by the calcination in a muffle furnace with stagnant air at 350 °C for 5 h. The obtained composites were denoted as CZ, CZS-3, CZS-5 and CZS-7, which were corresponding to the Cu/Sc2O3-ZnO catalysts with different Sc/Zn molar ratio of 0, 0.03, 0.05 and 0.07, respectively. Cu metal loading was set at 15 wt% for all the catalysts prepared. The catalyst samples were crushed and sieved before use, and powders of size 20–40 mesh were selected for the subsequent catalytic performance tests and characterization.
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2.3. Catalytic performance tests
nitrous oxide at each step, as listed below, were recorded by a TCD [33].
The ATR activity of methanol was performed in a fixed-bed reactor at atmospheric pressure. The catalysts were put in a quartz tube of 6 mm ID to form a packed bed, in which a coaxially centered thermocouple was installed to measure the bed temperature. All the catalysts samples were reduced in a 10% H2/90% N2 flow (80 mL/min) at 300 °C for 2 h before the testing. During the test, an aqueous solution of methanol at a rate of 0.05 mL/min was fed into the vaporizer through a high-pressure liquid pump (MP5, Elite, China) for vaporization, and then was introduced into the reactor. A mixed gas flow of N2 (fixed at 50 mL/min) and O2 was also introduced into the vaporizer to help carry the feed vapor into the reactor. The inert N2 also acted as the reference gas for quantitative calculation. The effluent of the reactor was analyzed by a gas chromatography equipped with a Porapack Q column and a flame ionization detector (FID) for organic species (e.g. CH3OH and CH4), a TDX-01 column and a thermal conductivity detector (TCD) for the gaseous products (e.g. H2, CO2, CO, and CH4). For the stability tests, the reaction was kept at 500 °C for 15 h and then cooled to 300 °C for another 4 h.
CuO + H2
(1)
Cu + H2 O
Hydrogen consumption = A1
2Cu + N2 O
(2)
Cu2 O+ N2
Nitrous oxide consumption = B
Cu2 O+ H2
(3)
2Cu + H2 O
Hydrogen consumption = A2 In order to quantify accurately the amount of N2O consumed, a blank switch over the oxidation catalyst was required as a reference. Dispersion of Cu (dCu) was calculated with the following formula:
dCu =
2A2 × 100% A1
Specific area of metallic copper was calculated from the amount of N2O consumption (B) with 1.46 × 1019 copper atoms per m2 [33]. Temperature-programmed reduction and oxidation (TPR/TPO) experiments were carried out on a Micromeritics AutoChem II 2920 apparatus. The sample (100 mg) was pretreated in an Ar flow (30 mL/ min) at 300 °C for 30 min and cooled to 50 °C in the same atmosphere. The reactor temperature was then raised to 500 °C at 10 °C/min in a 10% H2/90% Ar flow of 30 mL/min. After being cooled to 50 °C in an Ar flow (30 mL/min), the sample was heated up again to 450 °C at 10 °C/ min in a 3% O2/He flow (30 mL/min) and held there for 1 h. Then, the sample was cooled to 50 °C in Ar (30 mL/min), and a second run of H2TPR was performed by raising the temperature to 500 °C (10 °C/min) in a 10% H2/90% Ar flow (30 mL/min). The H2 consumption during each step was monitored by a TCD. A thermos flask of gel formed by liquid N2 and isopropanol was used as a cryogenic trap to keep water from entering the detector. Raman spectra data were collected on Renishaw inVia Raman microscope spectrometer with lines of laser at 532 nm. X-ray photoelectron spectroscopy (XPS) experiments were carried out on a Thermo Fischer ESCALAB 250Xi spectrometer with Al Kα radiation (hv = 1486.6 eV) at 8 × l0−10 Pa, 12.5 kV, 16 mA, and 10 cycles of signal accumulation. For the experiments, the O 1s, Cu 2p, Zn 2p and Sc 2p core-level spectra were recorded, the Passing-Energy was 40 eV in step of 0.1 eV, and the corresponding binding energies were referred to the C 1s line at 284.60 eV.
2.4. Catalysts characterization Multi-element analysis was carried out by X-ray fluorescence spectroscopy (XRF, Bruker S8 Tiger). The samples were ground, tableted, and placed in a ceramic crucible for analysis at 40 kV and 10 mA. N2 adsorption-desorption isotherms were obtained at −196 °C using a TriStar II 3020 Micromeritics apparatus. Prior to N2 adsorption, the samples were degassed at 200 °C for 2 h. The specific surface areas (SBET) were determined using the Brunauer-Emmett-Teller (BET) equation. The pore volumes (Vp), average pore diameters (Dp) were determined by the Barrett-Joyner-Halenda (BJH) method from the desorption branches of the isotherms. Powder X-ray diffraction (XRD) was performed by a Rigaku Ultima IV X-ray diffractometer employing Cu-Kα (40 kV, 30 mA) radiation. The morphological information of the catalysts was characterized by field emission scanning electron microscopy (FE-SEM, Zeiss Supra 55) and by high-resolution transmission electron microscopy (HR-TEM, JEM 2100). N2O chemisorption experiments were performed on an AutoChem II 2920 Micromeritics apparatus. The sample (100 mg) was purged at 200 °C for 30 min with Ar gas (30 mL/min) and cooled to 50 °C before heated up again in a 10% H2/90% Ar atmosphere (30 mL/min) to 300 °C (10 °C/min), where the temperature was held for another 2 h before cooled to 60 °C again in an Ar flow (30 mL/min). Then, the gas flow was switched to 10% N2O/90% Ar (30 mL/min) and held for 30 min before heated up again to 500 °C (10 °C/min) in a 10% H2/90% Ar atmosphere (30 mL/min). The consumptions of hydrogen and
3. Results and discussion 3.1. Characterizations of the catalysts Table 1 summarizes the physicochemical properties of the copper catalysts. The real compositions of the catalysts were examined by XRF
Table 1 Physico-chemical properties of Cu/ZnO/Sc2O3 catalysts. Catalyst
CZ CZS-3 CZS-5 CZS-7
Compositiona (wt%)
Sc/Zna (mol/mol)
Cu
Zn
Sc
15.4 15.1 14.8 14.8
64.6 63.6 62.8 62.6
– 1.2 1.8 2.5
0 0.03 0.05 0.07
SBETb (m2/g)
14.3 20 25.6 34.7
Vpc (cm3/g)
0.27 0.3 0.36 0.3
a
Dpc (nm)
70.1 46.4 42.9 28.2
DCud (nm) Fresh
Used
26.5 12.1 11.9 6.2
65.9 56.6 45.5 52.0
SCue (m2/g)
dCuf (%)
35.2 46.6 52.4 49.2
5.2 6.9 7.7 7.3
Chemical analysis from XRF. Specific surface area (SBET) of the catalysts calculated by BET equation from N2 adsorption-desorption isotherms. c Pore volume (Vp) and average pore size (Dp) of the catalysts calculated by BJH equation from N2 adsorption-desorption isotherms. d Crystal size of Cu estimated by the Debye-Scherrer equation from the (1 1 1) plane of Cu in XRD patterns. e Specific surface area of Cu metal (SCu) calculated by the amount of N2O consumption from N2O chemisorption with 1.46 × 1019 copper atoms per m2. f Dispersion of Cu metal (dCu) calculated by the ratio of two times of the amount of hydrogen consumption in the second reduction run to the amount of hydrogen consumption in the first reduction run from N2O chemisorption. b
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With the addition of scandium, the intensities of characteristic peaks for both Cu and ZnO decreased gradually. The estimation of crystal size by the Debye-Scherer formula from the (1 1 1) plane of Cu, as displayed in Table 1, shows a continuous decrease of crystal size for Cu metal for all the samples. This is in agreement with the calculated results based on the N2O chemisorption experiments, which showed an increase in specific surface area of the exposed Cu metal (SCu) with the addition of Sc. After reaching the maximum of 52.4 m2/g for CZS-5, a decrease in SCu was observed again for the CZS-7, which is the catalyst with higher content of Sc (see Table 1). Correspondingly, the CZS-5 showed the highest dispersion of 7.7% for Cu metal among all the samples. The observation of nanoparticles with an average lattice spacing of 0.21 nm in the HR-TEM images of CZ and CZS-5 catalysts (Fig. 3e, f) were assigned to the (1 1 1) plane of Cu metal with a nominal spacing of 0.209 nm. The measured sizes of 26.9 nm and 10.2 nm for CZ and CZS-5 respectively, were very close to those calculated from the corresponding XRD patterns. The H2-TPR profiles of the as-prepared and re-oxidized Cu/Sc2O3ZnO catalysts obtained from the H2-TPR/O2-TPO experiments are shown in Fig. 4. H2-TPR profile of the as-prepared CZ catalysts indicated a peak at 172 °C, which could be ascribed to the reduction of CuO nanoparticles [34,35]. As for the Sc promoted copper catalysts, a shift of the reduction peak to higher temperature was observed for all the samples, which indicated an enhanced metal-support interaction in these catalysts [36]. It is reported that the doping of Sc3+ in ZnO lattice induces the formation of oxygen vacancy, which could contribute to the strong metal-support interactions in the Sc promoted catalysts [18]. In addition, the small reduction peaks at about 150 °C were also found on the Sc promoted samples, which implied the presence of small CuO nanoparticles in these samples as well. In comparison, CZS-5 showed the biggest peak around this temperature, which might be the reason for its high metal dispersion as revealed by the N2O chemisorption. After a re-oxidation by O2-TPO, the profile obtained from the second time H2-TPR experiment showed a shift of the reduction peak to higher temperature for the Cu/ZnO catalyst, which might be due to the aggregation of Cu or CuO particles during the redox cycles. In comparison, little change was found on the Sc promoted catalysts as shown in Fig. 4, implying a good structure stability for these samples. Fig. 5 showed the Raman spectra of the as-prepared Cu/Sc2O3-ZnO catalysts. Four Raman modes at 328, 375, 432 and 575 cm−1 were observed for the CZ sample. The phonon mode at 328 cm−1 is a secondorder phonon, which is originated from the zone-boundary phonons of 2E2 (low) [37,38]. The latter three phonon modes correspond to A1 (TO), E2 (high) and A1 (LO) of ZnO, respectively [39]. With the addition of scandium, a shift of the above four modes to lower energies and the appearance of new Raman peaks labeled as ZnO* in Fig. 5 at 268–284 cm−1, were observed on the CZS-3, CZS-5 and CZS-7 samples. Similar phenomena were also reported in the relevant studies, which were considered as the characteristics of dopant vibrations [40,41], and confirmed the successful doping of the extrinsical ions in the ZnO lattice. In addition, the intensity of the LO mode, which was related with the defects of O-vacancy in ZnO [38], became stronger with the addition of Sc, inferring an increased amount of oxygen defects in the Sc promoted samples. Fig. 6 shows the XPS core level spectra of the Cu 2p and Zn 2p for the reduced CZ and CZS-5 catalysts. Generally, Cu metal, Cu+ and Cu2+ ions show intensive peaks of split spin-orbit components of Cu 2p at about 933 eV and 952.75 eV, which makes chemical state differentiation of Cu by XPS only difficult. However, the strong peaks at about 943 eV and 962.75 eV are usually ascribed to the collection of satellite features of Cu2+. As shown in Fig. 6a, satellite features at 943 eV and 962.75 eV were observed for both samples, while strengthened intensities of the satellite peaks were observed on the CZS-5 sample, indicating a higher content of Cu2+ species in this sample. Considering
Fig. 1. X-ray diffractograms of (a) freshly reduced and (b) used catalysts ((♦) Cu; (•) ZnO).
analysis, and a slightly lower concentration of Sc was detected in CZS-5 and CZS-7, which could be due to the partial segregating of Sc in the samples. The specific surface area (SBET) of the catalysts increased with the addition of Sc while the average pore size decreased with the increased Sc, indicating an improved sintering resistance of the Sc promoted Cu catalysts. The XRD patterns of the pre-reduced Cu/Sc2O3-ZnO catalysts are shown in Fig. 1a. The characteristic peaks of Cu (JCPDS: 04-0836) and ZnO (JCPDS: 36-1451) are indicated in the diffractogram. Although the diffraction peak at 2θ ≈ 31.5°, referring to ZnO (1 0 0), is very close to the characteristic peak of Sc2O3 (4 0 1) (JCPDS: 42-1463), it failed to find the characteristic peak of Sc2O3 (3 1 3) at 2θ ≈ 52.5°. The missing characteristic XRD peaks of Sc2O3 were also reported on a Sc2O3 doped Co-ZnO catalyst prepared by co-precipitation method [32]. Since scandium was present in small quantities, it might be present in Sc2O3 that was too small in crystal size to be identified in the XRD due to line-broadening or highly dispersed state in ZnO lattice as dopant. Considering that the ionic radius of Sc3+ (0.745 nm) is very close to that of Zn2+ (0.74 nm), the latter is preferred, which is also reported in the literature [18,32]. The embedded graph of Fig. 1a shows a slight shift of the enlarged (1 0 1) diffraction peak of ZnO to higher Brag angle with the addition of Sc3+, indicating a good doping of Sc3+ cation in the wurtzite structure of ZnO. With the addition of Sc, a continuous decrease of the relative intensity of (0 0 2) plane of ZnO (2θ ≈ 34.4°) was also observed. This change might be a possible microscopic reason for the observed crushing of rod-shaped zinc oxides after the addition of scandium in their SEM and TEM images, as shown in Figs. 2a, b and 3a, b.
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Fig. 2. FE-SEM images of (a) freshly reduced CZ, (b) freshly reduced CZS-5, (c) CZ after stability test, and (d) CZS-5 after stability test.
that a higher specific surface area of Cu metal was obtained on the Sc promoted samples as revealed by the N2O chemisorption (Table 1), the presence of abundant Cu2+ in the reduced CZ and CZS-5 samples could be attributed to a high dispersion of Cu and the rapid oxidation of Cu metal into CuO, when exposed to air before the experiments. Upon careful deconvolution, two peaks at 932.6 eV and 934.1 eV were obtained from the Cu 2p3/2 signal, which were attributed to Cu0 and Cu+ that showed a complete superposition of respective Cu 2p3/2 signals, and Cu2+, respectively. Calculation of Cu2+/(Cu0 + Cu+) from the corresponding peak areas showed high relative intensities of 1.14 and 1.40 for CZ and CZS-5, respectively. It could be contributed by a strong interaction between Cu and ZnO that strips the electron of Cu metal to the oxide and/or the doping of Cu2+ in the ZnO lattice, considering that Cu2+ also shows a comparative ionic radius (0.73 nm) with that of Zn2+. This was confirmed by the significant broadening of Zn 2p peak width (Fig. 6b), 1.68 eV for CZ and 1.7 eV for CZS comparing with 1.60 eV for pure ZnO, which implied the presence of more than one Zn species in the sample, indicating the possible formation of Cu–Zn–O and/or Sc–Zn–O bonds [41].
of the catalysts. Among all the catalysts studied, the CZS-5 showed the best catalytic activity at temperature below 400 °C in terms of methanol conversion, which was consistent with the fact that the highest metal dispersion was achieved on this sample. The hydrogen production yields of different copper catalysts are shown in Fig. 8. A continuous increase of the hydrogen production yield was observed for all catalysts at temperature below 400 °C, which was similar to the trend observed for the methanol conversion. Particularly, a hydrogen production yield of 140 μmol g−1 s−1 was obtained for CZS5 at 300 °C, which is the highest at a relatively high temperatures, although the complete conversion of methanol was not yet achieved for any of the catalysts tested. Also, this yield is higher than the previously reported hydrogen yield in literature [42] for the catalysts with the similar Cu/Zn ratio in methanol steam reforming. With further increase of temperature, a slight decrease in hydrogen production yield was observed for each of the catalysts individually after the production yield reached a maximum. For all the catalysts, only CO2, CO and trace amount of CH4 were detected as the carbonaceous products. Fig. 9 showed the selectivity of CO for the different catalysts, which was detectable at temperatures above 300 °C and increased with the temperature thereafter. The CO may be produced from the decomposition of methanol with co-production of CH4 or the reversed water gas shift reaction (R-WGS) with the consumption of H2. Since only trace amount of CH4 was detected and a decrease in hydrogen production rate was exhibited at elevated temperatures for all catalysts, the increase in catalytic activity of R-WGS should take responsibility for the increase of CO selectivity and also the decrease of hydrogen production rate at high temperatures [37]. Therefore, the fact that hydrogen production yield was surpassed by CZS-3 at temperatures above 350 °C for CZS-5 could be attributed to an enhanced R-WGS activity, which was usually related to the enriched surface oxygen species on the CZS-5 catalyst. Fig. 10 shows the results of stability tests for the copper catalysts developed. As seen, all samples started with a full conversion of methanol at 500 °C, then the conversion was decreased with the time-on-
3.2. Catalytic performance The catalytic activities of the different Cu/Sc2O3-ZnO catalysts at 220–600 °C were shown in Fig. 7. A control group with packed quartz sand of the same amount as the copper catalysts showed that while methanol could not be reformed at temperature below 450 °C, its conversion rose to a low level of 21% at 600 °C under the same reaction conditions. With the introduction of copper catalysts, however, the methanol conversion increased with the temperature, passed a boom at 260–400 °C, and finally exceeded 95% for all catalysts. It can be concluded that the activity of hydrogen production from methanol reforming was almost contributed by the catalyst. In comparison, the enhanced catalytic activities at low temperatures were achieved on the Sc promoted catalysts, indicating a positive impact of Sc on the activity
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Fig. 3. TEM images of (a) freshly reduced CZ, (b) freshly reduced CZS-5, (c) CZ after stability test, (d) CZS-5 after stability test, and HR-TEM images of (e) freshly reduced CZ, (f) freshly reduced CZS-5.
stream for 15 h, and finally dropped significantly after the temperature was cooled to 300 °C (Fig. 10a). Specifically, the activity of the CZ catalyst declined in a relatively higher rate at 500 °C, and then dropped sharply from 80% to 10% when the temperature was reduced to 300 °C. As for the Sc promoted copper catalysts, a slower decline in catalytic activities was observed at 500 °C during the first 15 h, but significant difference of the activities was observed for the rest of time when the
temperature was reduced to 300 °C. For all the catalysts studied, it was found that CZS-5 kept its activity (i.e. methanol conversion) at about 55%, which is obviously better than the other catalysts, demonstrating that it has the best catalytic stability for methanol reforming. In general, hydrogen production yield showed a similar declining trend to that of the methanol conversion for all the catalysts (Fig. 10b). Although the Sc promoted catalysts showed slightly lower hydrogen
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Fig. 4. H2-TPR profiles of Cu/ZnO/Sc2O3 catalysts. The fine and the coarse lines indicates the first and the second runs of the temperature programmed reduction experiments, respectively.
Fig. 6. XPS core level spectra of CZ and CZS-5, (a) Cu 2p and (b) Zn 2p.
Fig. 5. Raman spectra of Cu/ZnO/Sc2O3 catalysts.
yields than that of the CZ sample due to the consumption of hydrogen by a higher R-WGS activity, much higher hydrogen yields were maintained on these samples when the temperature was reduced to 300 °C. In contrast, a highest hydrogen yield of 81 μmol g−1 s−1 was obtained on the CZS-5 catalyst while only 10 μmol g−1 s−1 was realized on the CZ sample. Copper sintering is generally more pronounced in the ATR process due to the formation of hot spots resulted from the combustion reactions [43]. XRD patterns of all catalysts after stability tests were shown in Fig. 1b. Similar to the patterns of fresh catalysts, no diffraction peaks corresponding to Sc2O3 (JCPDS: 42-1463) were found, but Cu (JCPDS: 04-0836) and ZnO (JCPDS: 36-1451) were detected in the used samples, confirming that good doping of Sc in the ZnO lattice achieved. Significant increases in particle sizes of Cu, which was listed in Table 1, were found for all the samples when comparing with those of the fresh catalysts, which may be a main reason for causing the decrease of catalytic activity observed on the catalysts [25]. Evidently, a little
Fig. 7. Methanol conversions vs. reaction temperatures for the Cu/ZnO/Sc2O3 catalysts. (S/C = 1.5, O2/CH3OH = 0.2, 0.2 g catalyst, WHSV = 13.4 h−1.)
increase in particle size of Cu metal was observed in the Sc promoted copper catalysts. This was confirmed by the observation of smaller Cu metal particles in the TEM images for CZS-5 when comparing with the CZ sample (Fig. 3c, d). The finding is in agreement with the result of the TPR/TPO experiments, which implied that the presence of scandium favored the stabilization of Cu metal particles at high temperature. Additionally, no sign of carbon deposition was observed on the TEM
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Fig. 8. H2 production yields vs. reaction temperature for the Cu/ZnO/Sc2O3 catalysts. (S/C = 1.5, O2/CH3OH = 0.2, 0.2 g catalyst, WHSV = 13.4 h−1.)
Fig. 10. Stability tests of the Cu/ZnO/Sc2O3 catalysts, (a) Methanol conversion vs. time, (b) H2 production yields vs. time. (S/C = 1.5, O2/CH3OH = 0.2, 0.2 g catalyst, WHSV = 13.4 h−1.)
Fig. 9. CO selectivity vs. reaction temperature for the Cu/ZnO/Sc2O3 catalysts. (S/C = 1.5, O2/CH3OH = 0.2, 0.2 g catalyst, WHSV = 13.4 h−1.)
the produced hydrogen is used as the secondary fuel for internal combustion engines or any other combustors. Finally, an optimized amount of Sc (Sc/ZnO = 0.05 mol/mol) was proposed in this work, which we believe would effectively increase the catalytic activity and stability at the temperature range of 220–600 °C when the catalysts are used for hydrogen production from methanol via autothermal reforming.
images for the used CZ and CZS-5 catalysts, which indicated that carbon deposition should not be a main reason for the decrease of catalytic activity during the stability tests. On the other hand, SEM image of the used CZ (Fig. 2c) showed a severe agglomeration of the ZnO rods after the stability test. The agglomeration of ZnO would wrap the Cu metal nanoparticles on it, which could also cause the catalytic activity to decrease. As for the CZS-5 sample, a relatively good dispersion of ZnO particles was maintained (Fig. 2d), implying a positive role of Sc in the stabilization of ZnO oxide besides serving as the support metal, thus it contributed to a well improved catalytic stability.
Acknowledgements This work was supported by Fujian Department of Science and Technology (2016H6024), Natural Science Foundation of China (NSFC21276214), Natural Science Foundation of Fujian Province of China (2015J05033), and the Fundamental Research Funds for the Central Universities (20720170043).
4. Conclusions A series of Cu/Sc2O3-ZnO catalysts were prepared and studied for the production of H2 from methanol reforming at a wide range temperature of 220–600 °C. Characterizations of XRD, SEM, TEM and N2O chemisorption showed that a reduced particle size and an increased metal dispersion of Cu were obtained on the Sc promoted catalysts. Raman and XPS spectra in combination with the analysis of XRD patterns, together with the TPR/TPO experiments indicated that the successful doping of Sc in ZnO lattice would enhance the interaction between the metal and support, improve the metal dispersion and sintering resistance of the catalysts. The addition of Sc into the catalysts not only increased the catalytic activity at the temperatures below 400 °C, but also led to the higher selectivity towards CO at elevated temperatures due to the increased R-WGS activity. While the generation of CO is adverse to the hydrogen production, it is less detrimental when
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Full Length Article
An improved Cu/ZnO catalyst promoted by Sc2O3 for hydrogen production from methanol reforming
T
Yun-Chuan Pu, Shui-Rong Li , Shuai Yan, Xiao Huang, Duo Wang, Yue-Yuan Ye, Yun-Quan Liu ⁎
⁎
College of Energy, iChEM, Xiamen University, Xiamen 361102, China
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Methanol reforming Cu/Sc2O3-ZnO catalyst Doping Hydrogen production Autothermal
Copper sintering at temperatures above 200 °C has been a big challenge for the application of copper-based catalysts in methanol reforming for hydrogen production. In the present work, a Cu/ZnO catalyst promoted by Sc2O3 was developed, which is capable of converting methanol into hydrogen-rich gas with good activity and decent stability in the temperature range of 220–600 °C. The role of Sc in Cu/Sc2O3-ZnO catalysts was explored through the XRD, H2-TPR, N2O chemisorption, TEM, Raman, and XPS analyses. The results indicated that ZnO lattice was doped with the Sc3+ ion, which led to increased metal dispersion, strengthened interaction between Cu metal and support oxide, and subsequently enhanced catalytic activity and stability of the Cu/Sc2O3-ZnO catalyst in methanol reforming for hydrogen production.
1. Introduction Particulate matters (PM) and nitrogen oxides (NOx) from automobile exhaust are generally considered the main culprits of the notorious smog in cities. PM is usually generated by the incomplete
⁎
combustion of fuel while NOx is formed when free radicals react due to local high temperatures in the combustion chamber of engines. Thus, it is critical to reduce engine emissions by improving the quality and efficiency of combustion. Among many endeavors so far, the introduction of a second fuel into diesel to form a mixed fuel has been one of the
Corresponding authors. E-mail addresses: [email protected] (S.-R. Li), [email protected] (Y.-Q. Liu).
https://doi.org/10.1016/j.fuel.2018.12.067 Received 2 June 2018; Received in revised form 21 October 2018; Accepted 13 December 2018 0016-2361/ © 2018 Elsevier Ltd. All rights reserved.
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most important methods for reducing engine emissions [1]. These mixed fuels could be natural gas-diesel [2,3], reformed gas-diesel [4], hydrogen-diesel [3,5,6], biodiesel-diesel [7], kerosene-diesel [8] and butanol-gasoline-diesel [9], etc. Compared to other fuels, hydrogen is the lightest, requires minimum ignition energy, has the fastest flame propagation speed, wide explosive limit, and a short quenching distance. These properties significantly expand the range of the combustible fuel ratio, reduce the ignition temperature, promote homogeneous combustion, improve the combustion efficiency, and reduce PM while inhibiting the formation of NOx. Thus, the co-combustion of hydrogen with diesel can help achieve “zero emissions” in engine exhaust [3,6]. There are many compounds that are rich in hydrogen and can be considered as hydrogen carriers, among which methanol is obviously the most suitable one due to its low cost and easy storage as a liquid. Methanol is a primary alcohol with a high hydrogen to carbon ratio but no carbon-carbon bonds, thus allowing it to be easily converted into a hydrogen-rich gas at relatively low temperatures (200–300 °C) [10]. Compared to other fuels, such as methane, with a reforming temperature of above 600 °C [11], and ethanol, which is usually reformed at around 500 °C [12], methanol has a lower energy requirement for conversion into hydrogen-rich gas via reforming. In addition, methanol can be produced from renewable resources such as biomass, which is usually sulfur-free, so there will be less or no sulfur dioxide emitted during the conversion process [13]. In general, there are three major routes for hydrogen generation from methanol via reforming [13]: steam reforming (SR), partial oxidation (POX), and autothermal reforming (ATR). Both methanol and water contribute to the production of hydrogen in SR, which guarantees a high hydrogen yield, yet the reaction is strongly endothermic, which requires extra energy to sustain. POX, on the contrary, is an exothermic reaction, which means it gives off heat by consuming part of the fuel. While methanol is partially oxidized by O2 to produce CO2 and H2 in POX, the consumption of hydrogen by combustion is inevitable, and this greatly reduces the hydrogen yield. For ATR, however, it is a combination of SR and POX, and thus has the advantages of both SR and POX. The ATR reaction is described below:
CH3 OH + (1
2p)H2 O + pO2
CO2 + (3
2p)H2
(0
p
high temperature stability. At high temperatures, one of the biggest challenges is the sintering of Cu particles. Although the structure-activity relationship for Cu/ZnO/Al2O3 catalysts remains unclear, ZnO has been identified as a textural promoter in segregating the active Cu component, and thus a lot efforts have been made to maximize Cu-ZnO contact [25]. Various promoters, such as Ce, Y, and Zr, had been introduced into Cu-based catalysts to improve their catalytic performance [10,26–30]; however, it was found that only the addition of Zr to Cu/ ZnO or Cu/ZnO/Al2O3 would promote Cu dispersion and the formation of Cu-Zn alloy, which could enhance the catalytic performance with a higher methanol conversion and lower CO selectivity [26,29]. It is believed that the formation of surface lattice defect in the oxide support would benefit the metal-support interactions [31]. For ZnO, it was found that Sc could be readily embedded in ZnO crystal lattice in the form of Sc3+ through ion-doping due to its comparable ionic radius with Zn2+, and Sc could stabilize the support metal through the accommodation of metal ions at the surface vacant cation-sites [18,32]. Therefore, in the present work, a series of Sc promoted Cu/ZnO catalysts were prepared with the reverse precipitation method and were tested in methanol reforming at the wider temperature range of 220–600 °C. Furthermore, the structure-activity relationship of the Cubased catalysts was studied with a focus on the activity and stability, followed by the study of the possible roles of Sc in the catalysts. 2. Experimental 2.1. Chemicals Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, ≥99.0%), Copper (II) nitrate trihydrate (Cu(NO3)2·3H2O, ≥99.0%), and Scandium nitrate (Sc (NO3)3, ≥99.999%) were purchased from Shanghai Macklin Biochemical Co., Ltd., Aladdin Reagent (Shanghai) Co., Ltd., and Shandong Xiya Chemical Industry Co., Ltd., respectively. Sodium hydroxide (NaOH, ≥96.0%) and Methanol (≥99.5%, 0.791–0.793 g/mL) were received from Xilong Science Co., Ltd., and Sinopharm Chemical Reagent Co., Ltd., respectively. Hydrogen, oxygen, and nitrogen (all ≥99.999%) were supplied by Fujian Nan'an Success Gas Co., Ltd. Deionized water was produced in the lab with a laboratory water purification system (Manufacturer: Hitech Instruments Co., Ltd, China). All chemicals were used as received without further treatments.
0.5)
This reaction ensures both relatively high hydrogen yield and high overall efficiency compared to POX. Also, by varying the feed composition, ATR can be operated at a wide range of operating conditions, from endothermic to exothermic, but most frequently, at a condition of heat self-sustaining. In addition, ATR has a more rapid start than SR, which makes it more suitable for portable applications. The commonly used methanol reforming catalysts for hydrogen production can be divided into three categories: low temperature copper-based catalysts, high temperature Zn-Cr catalysts, and all-round precious metal catalysts. Although the precious metal catalysts, such as Pd and Pt-based [14–16], showed the best catalytic activity and stability for methanol reforming at a wide temperature range, the copperbased catalysts, particularly Cu/ZnO/Al2O3, are still most widely used in industry because of their low cost and good catalytic performance at low temperatures (150–300 °C) [17]. The Zn-Cr catalysts also exhibited good performance in terms of catalytic activity and hydrogen selectivity, but they operated at relatively high temperatures (> 450 °C) [18–20]. As for the application of methanol reforming in hydrogen enhanced combustion for internal combustion engines, the reformer is usually installed in the tailpipe to make full use of the waste heat carried by the exhaust [21]. Since the temperature of diesel engine exhaust varies in the temperature range of 200–600 °C depending on the diesel type and operating conditions [22–24], a cost effective catalyst with high catalytic activity and stability at a wide temperature range would be ideal for practical applications. To achieve this goal, copper-based catalysts must have the improved
2.2. Catalysts preparation The Cu/Sc2O3-ZnO catalysts were prepared through the reverse precipitation method. In a typical process, nitrate salts of Cu(NO3)2, Zn (NO3)2 and Sc(NO3)3 were dissolved in deionized water. The total metal concentration in the aqueous solution was 1 M while the concentration of NaOH in another solution (serving as the precipitant) was 3 M. The nitrate salt solution was then added dropwise to the precipitating agent with vigorous stirring at 75 °C, which resulted in precipitation. The stirring was continued for another 2 h while the temperature was maintained at 75 °C. Then, the precipitate was cooled to room temperature, and the suspension was filtered, and washed with deionized water repeatedly until the filtrate became neutral in pH. Drying was performed at 110 °C for 12 h followed by the calcination in a muffle furnace with stagnant air at 350 °C for 5 h. The obtained composites were denoted as CZ, CZS-3, CZS-5 and CZS-7, which were corresponding to the Cu/Sc2O3-ZnO catalysts with different Sc/Zn molar ratio of 0, 0.03, 0.05 and 0.07, respectively. Cu metal loading was set at 15 wt% for all the catalysts prepared. The catalyst samples were crushed and sieved before use, and powders of size 20–40 mesh were selected for the subsequent catalytic performance tests and characterization.
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2.3. Catalytic performance tests
nitrous oxide at each step, as listed below, were recorded by a TCD [33].
The ATR activity of methanol was performed in a fixed-bed reactor at atmospheric pressure. The catalysts were put in a quartz tube of 6 mm ID to form a packed bed, in which a coaxially centered thermocouple was installed to measure the bed temperature. All the catalysts samples were reduced in a 10% H2/90% N2 flow (80 mL/min) at 300 °C for 2 h before the testing. During the test, an aqueous solution of methanol at a rate of 0.05 mL/min was fed into the vaporizer through a high-pressure liquid pump (MP5, Elite, China) for vaporization, and then was introduced into the reactor. A mixed gas flow of N2 (fixed at 50 mL/min) and O2 was also introduced into the vaporizer to help carry the feed vapor into the reactor. The inert N2 also acted as the reference gas for quantitative calculation. The effluent of the reactor was analyzed by a gas chromatography equipped with a Porapack Q column and a flame ionization detector (FID) for organic species (e.g. CH3OH and CH4), a TDX-01 column and a thermal conductivity detector (TCD) for the gaseous products (e.g. H2, CO2, CO, and CH4). For the stability tests, the reaction was kept at 500 °C for 15 h and then cooled to 300 °C for another 4 h.
CuO + H2
(1)
Cu + H2 O
Hydrogen consumption = A1
2Cu + N2 O
(2)
Cu2 O+ N2
Nitrous oxide consumption = B
Cu2 O+ H2
(3)
2Cu + H2 O
Hydrogen consumption = A2 In order to quantify accurately the amount of N2O consumed, a blank switch over the oxidation catalyst was required as a reference. Dispersion of Cu (dCu) was calculated with the following formula:
dCu =
2A2 × 100% A1
Specific area of metallic copper was calculated from the amount of N2O consumption (B) with 1.46 × 1019 copper atoms per m2 [33]. Temperature-programmed reduction and oxidation (TPR/TPO) experiments were carried out on a Micromeritics AutoChem II 2920 apparatus. The sample (100 mg) was pretreated in an Ar flow (30 mL/ min) at 300 °C for 30 min and cooled to 50 °C in the same atmosphere. The reactor temperature was then raised to 500 °C at 10 °C/min in a 10% H2/90% Ar flow of 30 mL/min. After being cooled to 50 °C in an Ar flow (30 mL/min), the sample was heated up again to 450 °C at 10 °C/ min in a 3% O2/He flow (30 mL/min) and held there for 1 h. Then, the sample was cooled to 50 °C in Ar (30 mL/min), and a second run of H2TPR was performed by raising the temperature to 500 °C (10 °C/min) in a 10% H2/90% Ar flow (30 mL/min). The H2 consumption during each step was monitored by a TCD. A thermos flask of gel formed by liquid N2 and isopropanol was used as a cryogenic trap to keep water from entering the detector. Raman spectra data were collected on Renishaw inVia Raman microscope spectrometer with lines of laser at 532 nm. X-ray photoelectron spectroscopy (XPS) experiments were carried out on a Thermo Fischer ESCALAB 250Xi spectrometer with Al Kα radiation (hv = 1486.6 eV) at 8 × l0−10 Pa, 12.5 kV, 16 mA, and 10 cycles of signal accumulation. For the experiments, the O 1s, Cu 2p, Zn 2p and Sc 2p core-level spectra were recorded, the Passing-Energy was 40 eV in step of 0.1 eV, and the corresponding binding energies were referred to the C 1s line at 284.60 eV.
2.4. Catalysts characterization Multi-element analysis was carried out by X-ray fluorescence spectroscopy (XRF, Bruker S8 Tiger). The samples were ground, tableted, and placed in a ceramic crucible for analysis at 40 kV and 10 mA. N2 adsorption-desorption isotherms were obtained at −196 °C using a TriStar II 3020 Micromeritics apparatus. Prior to N2 adsorption, the samples were degassed at 200 °C for 2 h. The specific surface areas (SBET) were determined using the Brunauer-Emmett-Teller (BET) equation. The pore volumes (Vp), average pore diameters (Dp) were determined by the Barrett-Joyner-Halenda (BJH) method from the desorption branches of the isotherms. Powder X-ray diffraction (XRD) was performed by a Rigaku Ultima IV X-ray diffractometer employing Cu-Kα (40 kV, 30 mA) radiation. The morphological information of the catalysts was characterized by field emission scanning electron microscopy (FE-SEM, Zeiss Supra 55) and by high-resolution transmission electron microscopy (HR-TEM, JEM 2100). N2O chemisorption experiments were performed on an AutoChem II 2920 Micromeritics apparatus. The sample (100 mg) was purged at 200 °C for 30 min with Ar gas (30 mL/min) and cooled to 50 °C before heated up again in a 10% H2/90% Ar atmosphere (30 mL/min) to 300 °C (10 °C/min), where the temperature was held for another 2 h before cooled to 60 °C again in an Ar flow (30 mL/min). Then, the gas flow was switched to 10% N2O/90% Ar (30 mL/min) and held for 30 min before heated up again to 500 °C (10 °C/min) in a 10% H2/90% Ar atmosphere (30 mL/min). The consumptions of hydrogen and
3. Results and discussion 3.1. Characterizations of the catalysts Table 1 summarizes the physicochemical properties of the copper catalysts. The real compositions of the catalysts were examined by XRF
Table 1 Physico-chemical properties of Cu/ZnO/Sc2O3 catalysts. Catalyst
CZ CZS-3 CZS-5 CZS-7
Compositiona (wt%)
Sc/Zna (mol/mol)
Cu
Zn
Sc
15.4 15.1 14.8 14.8
64.6 63.6 62.8 62.6
– 1.2 1.8 2.5
0 0.03 0.05 0.07
SBETb (m2/g)
14.3 20 25.6 34.7
Vpc (cm3/g)
0.27 0.3 0.36 0.3
a
Dpc (nm)
70.1 46.4 42.9 28.2
DCud (nm) Fresh
Used
26.5 12.1 11.9 6.2
65.9 56.6 45.5 52.0
SCue (m2/g)
dCuf (%)
35.2 46.6 52.4 49.2
5.2 6.9 7.7 7.3
Chemical analysis from XRF. Specific surface area (SBET) of the catalysts calculated by BET equation from N2 adsorption-desorption isotherms. c Pore volume (Vp) and average pore size (Dp) of the catalysts calculated by BJH equation from N2 adsorption-desorption isotherms. d Crystal size of Cu estimated by the Debye-Scherrer equation from the (1 1 1) plane of Cu in XRD patterns. e Specific surface area of Cu metal (SCu) calculated by the amount of N2O consumption from N2O chemisorption with 1.46 × 1019 copper atoms per m2. f Dispersion of Cu metal (dCu) calculated by the ratio of two times of the amount of hydrogen consumption in the second reduction run to the amount of hydrogen consumption in the first reduction run from N2O chemisorption. b
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With the addition of scandium, the intensities of characteristic peaks for both Cu and ZnO decreased gradually. The estimation of crystal size by the Debye-Scherer formula from the (1 1 1) plane of Cu, as displayed in Table 1, shows a continuous decrease of crystal size for Cu metal for all the samples. This is in agreement with the calculated results based on the N2O chemisorption experiments, which showed an increase in specific surface area of the exposed Cu metal (SCu) with the addition of Sc. After reaching the maximum of 52.4 m2/g for CZS-5, a decrease in SCu was observed again for the CZS-7, which is the catalyst with higher content of Sc (see Table 1). Correspondingly, the CZS-5 showed the highest dispersion of 7.7% for Cu metal among all the samples. The observation of nanoparticles with an average lattice spacing of 0.21 nm in the HR-TEM images of CZ and CZS-5 catalysts (Fig. 3e, f) were assigned to the (1 1 1) plane of Cu metal with a nominal spacing of 0.209 nm. The measured sizes of 26.9 nm and 10.2 nm for CZ and CZS-5 respectively, were very close to those calculated from the corresponding XRD patterns. The H2-TPR profiles of the as-prepared and re-oxidized Cu/Sc2O3ZnO catalysts obtained from the H2-TPR/O2-TPO experiments are shown in Fig. 4. H2-TPR profile of the as-prepared CZ catalysts indicated a peak at 172 °C, which could be ascribed to the reduction of CuO nanoparticles [34,35]. As for the Sc promoted copper catalysts, a shift of the reduction peak to higher temperature was observed for all the samples, which indicated an enhanced metal-support interaction in these catalysts [36]. It is reported that the doping of Sc3+ in ZnO lattice induces the formation of oxygen vacancy, which could contribute to the strong metal-support interactions in the Sc promoted catalysts [18]. In addition, the small reduction peaks at about 150 °C were also found on the Sc promoted samples, which implied the presence of small CuO nanoparticles in these samples as well. In comparison, CZS-5 showed the biggest peak around this temperature, which might be the reason for its high metal dispersion as revealed by the N2O chemisorption. After a re-oxidation by O2-TPO, the profile obtained from the second time H2-TPR experiment showed a shift of the reduction peak to higher temperature for the Cu/ZnO catalyst, which might be due to the aggregation of Cu or CuO particles during the redox cycles. In comparison, little change was found on the Sc promoted catalysts as shown in Fig. 4, implying a good structure stability for these samples. Fig. 5 showed the Raman spectra of the as-prepared Cu/Sc2O3-ZnO catalysts. Four Raman modes at 328, 375, 432 and 575 cm−1 were observed for the CZ sample. The phonon mode at 328 cm−1 is a secondorder phonon, which is originated from the zone-boundary phonons of 2E2 (low) [37,38]. The latter three phonon modes correspond to A1 (TO), E2 (high) and A1 (LO) of ZnO, respectively [39]. With the addition of scandium, a shift of the above four modes to lower energies and the appearance of new Raman peaks labeled as ZnO* in Fig. 5 at 268–284 cm−1, were observed on the CZS-3, CZS-5 and CZS-7 samples. Similar phenomena were also reported in the relevant studies, which were considered as the characteristics of dopant vibrations [40,41], and confirmed the successful doping of the extrinsical ions in the ZnO lattice. In addition, the intensity of the LO mode, which was related with the defects of O-vacancy in ZnO [38], became stronger with the addition of Sc, inferring an increased amount of oxygen defects in the Sc promoted samples. Fig. 6 shows the XPS core level spectra of the Cu 2p and Zn 2p for the reduced CZ and CZS-5 catalysts. Generally, Cu metal, Cu+ and Cu2+ ions show intensive peaks of split spin-orbit components of Cu 2p at about 933 eV and 952.75 eV, which makes chemical state differentiation of Cu by XPS only difficult. However, the strong peaks at about 943 eV and 962.75 eV are usually ascribed to the collection of satellite features of Cu2+. As shown in Fig. 6a, satellite features at 943 eV and 962.75 eV were observed for both samples, while strengthened intensities of the satellite peaks were observed on the CZS-5 sample, indicating a higher content of Cu2+ species in this sample. Considering
Fig. 1. X-ray diffractograms of (a) freshly reduced and (b) used catalysts ((♦) Cu; (•) ZnO).
analysis, and a slightly lower concentration of Sc was detected in CZS-5 and CZS-7, which could be due to the partial segregating of Sc in the samples. The specific surface area (SBET) of the catalysts increased with the addition of Sc while the average pore size decreased with the increased Sc, indicating an improved sintering resistance of the Sc promoted Cu catalysts. The XRD patterns of the pre-reduced Cu/Sc2O3-ZnO catalysts are shown in Fig. 1a. The characteristic peaks of Cu (JCPDS: 04-0836) and ZnO (JCPDS: 36-1451) are indicated in the diffractogram. Although the diffraction peak at 2θ ≈ 31.5°, referring to ZnO (1 0 0), is very close to the characteristic peak of Sc2O3 (4 0 1) (JCPDS: 42-1463), it failed to find the characteristic peak of Sc2O3 (3 1 3) at 2θ ≈ 52.5°. The missing characteristic XRD peaks of Sc2O3 were also reported on a Sc2O3 doped Co-ZnO catalyst prepared by co-precipitation method [32]. Since scandium was present in small quantities, it might be present in Sc2O3 that was too small in crystal size to be identified in the XRD due to line-broadening or highly dispersed state in ZnO lattice as dopant. Considering that the ionic radius of Sc3+ (0.745 nm) is very close to that of Zn2+ (0.74 nm), the latter is preferred, which is also reported in the literature [18,32]. The embedded graph of Fig. 1a shows a slight shift of the enlarged (1 0 1) diffraction peak of ZnO to higher Brag angle with the addition of Sc3+, indicating a good doping of Sc3+ cation in the wurtzite structure of ZnO. With the addition of Sc, a continuous decrease of the relative intensity of (0 0 2) plane of ZnO (2θ ≈ 34.4°) was also observed. This change might be a possible microscopic reason for the observed crushing of rod-shaped zinc oxides after the addition of scandium in their SEM and TEM images, as shown in Figs. 2a, b and 3a, b.
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Fig. 2. FE-SEM images of (a) freshly reduced CZ, (b) freshly reduced CZS-5, (c) CZ after stability test, and (d) CZS-5 after stability test.
that a higher specific surface area of Cu metal was obtained on the Sc promoted samples as revealed by the N2O chemisorption (Table 1), the presence of abundant Cu2+ in the reduced CZ and CZS-5 samples could be attributed to a high dispersion of Cu and the rapid oxidation of Cu metal into CuO, when exposed to air before the experiments. Upon careful deconvolution, two peaks at 932.6 eV and 934.1 eV were obtained from the Cu 2p3/2 signal, which were attributed to Cu0 and Cu+ that showed a complete superposition of respective Cu 2p3/2 signals, and Cu2+, respectively. Calculation of Cu2+/(Cu0 + Cu+) from the corresponding peak areas showed high relative intensities of 1.14 and 1.40 for CZ and CZS-5, respectively. It could be contributed by a strong interaction between Cu and ZnO that strips the electron of Cu metal to the oxide and/or the doping of Cu2+ in the ZnO lattice, considering that Cu2+ also shows a comparative ionic radius (0.73 nm) with that of Zn2+. This was confirmed by the significant broadening of Zn 2p peak width (Fig. 6b), 1.68 eV for CZ and 1.7 eV for CZS comparing with 1.60 eV for pure ZnO, which implied the presence of more than one Zn species in the sample, indicating the possible formation of Cu–Zn–O and/or Sc–Zn–O bonds [41].
of the catalysts. Among all the catalysts studied, the CZS-5 showed the best catalytic activity at temperature below 400 °C in terms of methanol conversion, which was consistent with the fact that the highest metal dispersion was achieved on this sample. The hydrogen production yields of different copper catalysts are shown in Fig. 8. A continuous increase of the hydrogen production yield was observed for all catalysts at temperature below 400 °C, which was similar to the trend observed for the methanol conversion. Particularly, a hydrogen production yield of 140 μmol g−1 s−1 was obtained for CZS5 at 300 °C, which is the highest at a relatively high temperatures, although the complete conversion of methanol was not yet achieved for any of the catalysts tested. Also, this yield is higher than the previously reported hydrogen yield in literature [42] for the catalysts with the similar Cu/Zn ratio in methanol steam reforming. With further increase of temperature, a slight decrease in hydrogen production yield was observed for each of the catalysts individually after the production yield reached a maximum. For all the catalysts, only CO2, CO and trace amount of CH4 were detected as the carbonaceous products. Fig. 9 showed the selectivity of CO for the different catalysts, which was detectable at temperatures above 300 °C and increased with the temperature thereafter. The CO may be produced from the decomposition of methanol with co-production of CH4 or the reversed water gas shift reaction (R-WGS) with the consumption of H2. Since only trace amount of CH4 was detected and a decrease in hydrogen production rate was exhibited at elevated temperatures for all catalysts, the increase in catalytic activity of R-WGS should take responsibility for the increase of CO selectivity and also the decrease of hydrogen production rate at high temperatures [37]. Therefore, the fact that hydrogen production yield was surpassed by CZS-3 at temperatures above 350 °C for CZS-5 could be attributed to an enhanced R-WGS activity, which was usually related to the enriched surface oxygen species on the CZS-5 catalyst. Fig. 10 shows the results of stability tests for the copper catalysts developed. As seen, all samples started with a full conversion of methanol at 500 °C, then the conversion was decreased with the time-on-
3.2. Catalytic performance The catalytic activities of the different Cu/Sc2O3-ZnO catalysts at 220–600 °C were shown in Fig. 7. A control group with packed quartz sand of the same amount as the copper catalysts showed that while methanol could not be reformed at temperature below 450 °C, its conversion rose to a low level of 21% at 600 °C under the same reaction conditions. With the introduction of copper catalysts, however, the methanol conversion increased with the temperature, passed a boom at 260–400 °C, and finally exceeded 95% for all catalysts. It can be concluded that the activity of hydrogen production from methanol reforming was almost contributed by the catalyst. In comparison, the enhanced catalytic activities at low temperatures were achieved on the Sc promoted catalysts, indicating a positive impact of Sc on the activity
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Fig. 3. TEM images of (a) freshly reduced CZ, (b) freshly reduced CZS-5, (c) CZ after stability test, (d) CZS-5 after stability test, and HR-TEM images of (e) freshly reduced CZ, (f) freshly reduced CZS-5.
stream for 15 h, and finally dropped significantly after the temperature was cooled to 300 °C (Fig. 10a). Specifically, the activity of the CZ catalyst declined in a relatively higher rate at 500 °C, and then dropped sharply from 80% to 10% when the temperature was reduced to 300 °C. As for the Sc promoted copper catalysts, a slower decline in catalytic activities was observed at 500 °C during the first 15 h, but significant difference of the activities was observed for the rest of time when the
temperature was reduced to 300 °C. For all the catalysts studied, it was found that CZS-5 kept its activity (i.e. methanol conversion) at about 55%, which is obviously better than the other catalysts, demonstrating that it has the best catalytic stability for methanol reforming. In general, hydrogen production yield showed a similar declining trend to that of the methanol conversion for all the catalysts (Fig. 10b). Although the Sc promoted catalysts showed slightly lower hydrogen
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Fig. 4. H2-TPR profiles of Cu/ZnO/Sc2O3 catalysts. The fine and the coarse lines indicates the first and the second runs of the temperature programmed reduction experiments, respectively.
Fig. 6. XPS core level spectra of CZ and CZS-5, (a) Cu 2p and (b) Zn 2p.
Fig. 5. Raman spectra of Cu/ZnO/Sc2O3 catalysts.
yields than that of the CZ sample due to the consumption of hydrogen by a higher R-WGS activity, much higher hydrogen yields were maintained on these samples when the temperature was reduced to 300 °C. In contrast, a highest hydrogen yield of 81 μmol g−1 s−1 was obtained on the CZS-5 catalyst while only 10 μmol g−1 s−1 was realized on the CZ sample. Copper sintering is generally more pronounced in the ATR process due to the formation of hot spots resulted from the combustion reactions [43]. XRD patterns of all catalysts after stability tests were shown in Fig. 1b. Similar to the patterns of fresh catalysts, no diffraction peaks corresponding to Sc2O3 (JCPDS: 42-1463) were found, but Cu (JCPDS: 04-0836) and ZnO (JCPDS: 36-1451) were detected in the used samples, confirming that good doping of Sc in the ZnO lattice achieved. Significant increases in particle sizes of Cu, which was listed in Table 1, were found for all the samples when comparing with those of the fresh catalysts, which may be a main reason for causing the decrease of catalytic activity observed on the catalysts [25]. Evidently, a little
Fig. 7. Methanol conversions vs. reaction temperatures for the Cu/ZnO/Sc2O3 catalysts. (S/C = 1.5, O2/CH3OH = 0.2, 0.2 g catalyst, WHSV = 13.4 h−1.)
increase in particle size of Cu metal was observed in the Sc promoted copper catalysts. This was confirmed by the observation of smaller Cu metal particles in the TEM images for CZS-5 when comparing with the CZ sample (Fig. 3c, d). The finding is in agreement with the result of the TPR/TPO experiments, which implied that the presence of scandium favored the stabilization of Cu metal particles at high temperature. Additionally, no sign of carbon deposition was observed on the TEM
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Fig. 8. H2 production yields vs. reaction temperature for the Cu/ZnO/Sc2O3 catalysts. (S/C = 1.5, O2/CH3OH = 0.2, 0.2 g catalyst, WHSV = 13.4 h−1.)
Fig. 10. Stability tests of the Cu/ZnO/Sc2O3 catalysts, (a) Methanol conversion vs. time, (b) H2 production yields vs. time. (S/C = 1.5, O2/CH3OH = 0.2, 0.2 g catalyst, WHSV = 13.4 h−1.)
Fig. 9. CO selectivity vs. reaction temperature for the Cu/ZnO/Sc2O3 catalysts. (S/C = 1.5, O2/CH3OH = 0.2, 0.2 g catalyst, WHSV = 13.4 h−1.)
the produced hydrogen is used as the secondary fuel for internal combustion engines or any other combustors. Finally, an optimized amount of Sc (Sc/ZnO = 0.05 mol/mol) was proposed in this work, which we believe would effectively increase the catalytic activity and stability at the temperature range of 220–600 °C when the catalysts are used for hydrogen production from methanol via autothermal reforming.
images for the used CZ and CZS-5 catalysts, which indicated that carbon deposition should not be a main reason for the decrease of catalytic activity during the stability tests. On the other hand, SEM image of the used CZ (Fig. 2c) showed a severe agglomeration of the ZnO rods after the stability test. The agglomeration of ZnO would wrap the Cu metal nanoparticles on it, which could also cause the catalytic activity to decrease. As for the CZS-5 sample, a relatively good dispersion of ZnO particles was maintained (Fig. 2d), implying a positive role of Sc in the stabilization of ZnO oxide besides serving as the support metal, thus it contributed to a well improved catalytic stability.
Acknowledgements This work was supported by Fujian Department of Science and Technology (2016H6024), Natural Science Foundation of China (NSFC21276214), Natural Science Foundation of Fujian Province of China (2015J05033), and the Fundamental Research Funds for the Central Universities (20720170043).
4. Conclusions A series of Cu/Sc2O3-ZnO catalysts were prepared and studied for the production of H2 from methanol reforming at a wide range temperature of 220–600 °C. Characterizations of XRD, SEM, TEM and N2O chemisorption showed that a reduced particle size and an increased metal dispersion of Cu were obtained on the Sc promoted catalysts. Raman and XPS spectra in combination with the analysis of XRD patterns, together with the TPR/TPO experiments indicated that the successful doping of Sc in ZnO lattice would enhance the interaction between the metal and support, improve the metal dispersion and sintering resistance of the catalysts. The addition of Sc into the catalysts not only increased the catalytic activity at the temperatures below 400 °C, but also led to the higher selectivity towards CO at elevated temperatures due to the increased R-WGS activity. While the generation of CO is adverse to the hydrogen production, it is less detrimental when
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