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Ce–Zr–O catalyst for high CO2 conversion during reverse water gas shift reaction (RWGS)
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Ni/CeeZreO catalyst for high CO2 conversion during reverse water gas shift reaction (RWGS) Feng-man Sun a,b, Chang-feng Yan a,b,*, Zhi-da Wang a, Chang-qing Guo a, Shi-lin Huang a a b
Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangdong, 510640, China University of Chinese Academy of Sciences, Beijing, 100039, China
article info
abstract
Article history:
A series of Ni-based catalysts supported on CeeZreO were synthesized via impregnation
Received 14 July 2015
and applied in the reverse water gas shift reaction (RWGS). BET, XRD, TPR, SEM, TEM and
Received in revised form
XPS were employed as the characterization of catalysts. Catalytic activities of Ni/CeeZreO
14 September 2015
with different Ni contents were evaluated in RWGS under atmospheric pressure and at the
Accepted 2 October 2015
temperature range of 550e750 C in a fixed-bed quartz reactor. CO2 conversion reaches
Available online 11 November 2015
49.66% and maintains stable after 80 h of reaction over 3 wt% Ni/CeeZreO. CO selectivity
Keywords:
vides high activity, stability and selectivity in the conversion of CO2 to CO at high tem-
Ni/CeeZreO
perature. CeeZreO solid solution is formed by co-precipitation and Ni is able to be
Catalyst
incorporated into the CeeZreO lattice via impregnation. The Ni species on catalyst surface
RWGS
can be considered as the active site for RWGS.
CO2 utilization
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
can reach 99.65% for the catalyst with 10 wt% Ni at 750 C. The Ni/CeeZreO catalyst pro-
reserved.
H2 utilization
Introduction Conventional energy consumption has caused depleting of the non-renewable fossil fuels gradually and led to increasing anthropogenic emissions of CO2. As a result, carbon capture is of great interest not only for the storage of energy but also for the limitation of greenhouse effect. Using CO2 rather than fossil fuel as feedstock to produce a series of precursors relevant with industrial reaction and organic materials presents a sustainable and long-term blueprint [1e3]. The reverse water gas shift reaction (RWGS) is a promising candidate, in which the carbon capture, storage and utilization (CCSU) and effective utilization of hydrogen can be realized. And hydrogen
obtained from electrolysis of water by consuming wind or solar power which are abandoned due to causing instability of the grid makes this process feasible and sustainable [4]. A lot of chemicals such as methanol can be produced by employing synthesis gas, which traditionally is generated from natural gas reforming and coal gasification [5] with the non-renewable fossil fuel as carbon source. It has been proposed that the atmospheric CO2 and H2 could be converted to renewable energy sources through RWGS according to Eqns. (1) and (2) [6e8]: CO2 þH2 %CO þ H2 O DH ¼ 41:2 kJ=mol
(1)
CO þ 3H2 %CH4 þH2 O DH ¼ 206:1 kJ=mol
(2)
* Corresponding author. Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangdong, 510640, China. Tel./fax: þ86 2087057729. E-mail address: [email protected] (C.-f. Yan). http://dx.doi.org/10.1016/j.ijhydene.2015.10.004 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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Generally, the conversion of CO2 in RWGS is limited by thermodynamic equilibrium due to the reversibility. High temperature is beneficial to improve CO selectivity because the primary reaction (Eqn. (1)) is endothermic and the side reaction (Eqn. (2)) is exothermic [9]. Many researchers regard CO2 as the direct source of methanol [10e12], but the yield of methanol from CO2 is lower than from CO. It can be attributed to that CO2 is a highly stable molecule. RWGS is the first step of the synthesis of hydrocarbons from CO2 [13]. The Cu-based catalyst is used for RWGS in the earlier time [14,15]. However, it is not the proper catalyst for RWGS at high temperature due to its poor thermal stability. ZnO-based catalyst shows high deactivation rate at high temperature, which is related to the vaporization of Zn during the reaction [6]. Noble metals, such as Pt and Au [16,17], are also employed but they are very expensive. Ni is a kind of cheaper metal with high activity in most of the catalytic reactions and is more economical than noble metals. However, coke deposition and deactivation are easy to occur on the Nibased catalyst during the catalytic process [18]. As alkaline earth metal oxide, ZrO2 is a promising support which has a unique interaction with the active component by changing the electronic distribution. Researchers have proved that ZrO2 can prevent carbon deposition and facilitate the gasification of coke [19,20] for its low concentration of Lewis sites and noted stability [21]. On the other hand, there have been many studies about different metals supported on CeO2 for RWGS reaction [22,23]. The redox properties of CeO2 could be enhanced if additional Ni was introduced into CeO2 lattice by forming solid solutions [24,25]. The rare earth metal oxide CeO2 as promoter or support plays an important role in the generation of surface oxygen species and anionic vacancies during catalytic reaction. Therefore, CeeZreO supported catalyst has potential not only for its outstanding performance but also for the mitigation of carbon deposition [26,27]. The stable activities have been attributed to the ability to facilitate the carbon gasification [28]. At the same time, the oxygen storage capability and thermal stability can be improved on CeeZreO solid solution compared with pure oxides [29e32]. On the basis of the character of RWGS, the catalyst should be thermally stable and have high oxygen storage capacity. We aimed to develop an efficient catalyst for RWGS. Based on the performance of the materials mentioned above, Ni/ CeeZreO may be a promising choice in designing a catalyst for RWGS. In this work, a series of Ni/CeeZreO with different Ni contents were synthesized and evaluated in the RWGS reaction and the catalyst performed high activity, selectivity and stability in the goal reaction.
Experiments Catalyst preparation The CeeZreO support was prepared by the co-precipitation method. The aqueous solution of Ce(NO3)3$6H2O and ZrOCl2$8H2O with a tailored composition was added into a precursor solution of CH3CH2OH and NH4OH, and then kept for several hours under vigorous stirring at 60 C and pH of 9. The precipitate was filtered and washed with deionized water
followed by CH3CH2OH, then dried at 110 C for 12 h. Finally, the CeeZreO support (7.8 wt% of Ce oxide) was naturally cooled to room temperature after calcination at 600 C for 6 h. ZrO2 support was prepared as described above without addition of Ce(NO3)3$6H2O. A series of Ni catalysts supported on CeeZreO were prepared via the impregnation method [33]. The solid residues were dried at 110 C for 12 h, and then heated to 800 C at a rate of 5 C min1 and calcined for 6 h. Finally, Ni/CeeZreO catalysts with Ni loading of 1, 3, 5, 7.8, 10 wt% were prepared.
Catalyst characterization The nitrogen adsorptionedesorption isotherms of catalyst at 196 C were obtained on a Micromeritics ASAP-2010 apparatus. Multi-plot Braunauer-Emmett-Teller (BET) method was used to calculate the specific surface area. Pore volume and pore size distribution were obtained by BarretteJoynereHalenda (BJH) method. X-ray diffraction (XRD) analysis was conducted in the PANalytical X'Pert diffractometer (X'Pert PRO MPD, PW3040/60) with Cu-Ka (l ¼ 0.154060 nm) radiation (40 kV, 40 mA) and Scherrer equation was used for the crystallite size. The catalyst was characterized by the Scanning electron microscope (SEM) with a S-4800 instrument at 2.0 kV for the surface morphology followed by the Energy dispersive spectrometer (EDS) analysis for the elemental composition. The morphology of catalyst was further characterized by Transmission electron microscopy (TEM) on a JEM-2100 microscope. Temperatureprogrammed reduction (TPR) was performed on a Micromeritics AutoChem 2920 apparatus using a reducing gas of 10 vol% H2/Ar in a quartz tube from 30 C to 1000 C. The X-ray photo electron spectroscopy (XPS) measurements were performed on an ESCALAB 250Xi spectrometer with Mg Ka source (hv ¼ 1253.6 eV) under vacuum. The binding energies were calibrated using the contaminant carbon (C1s ¼ 284.8 eV) when analyzed by Thermo Avantage software.
Catalytic activity test The RWGS reaction was conducted in a fixed-bed quartz tubular reactor (4 mm inner diameter and 300 mm length) under atmospheric pressure in a temperature range of 550e750 C, and a total flow of 50 ml/min with H2:CO2 ¼ 1:1. Before the RWGS reaction, 0.3 g Ni/CeeZreO catalyst (40e60 mesh) and 1.2 g quartz sand (40e60 mesh) supported on quartz wool was pre-reduced in situ in a 5 vol% H2/Ar stream for 5 h at a temperature of 600 C and a heating ramp rate of 8 C min1 with a total gas flow of 150 ml/min. A thermocouple was inserted directly into the center of the catalyst bed to measure the actual temperatures of the pretreatment and reaction. All the reactant gases were monitored with mass flow meter and the flow of the product was measured by a soap bubble flow meter. When the reaction became stable, the output gas flow was analyzed online in the gas chromatographs (GC 7890Ⅱ) equipped with a thermal conductivity detector (TCD) and a TDX-01 column for H2, CO2, CO, CH4 after condensation and drying. The stability of 3 wt% Ni/CeeZreO catalyst was tested for 80 h, at 750 C, 1 atm, and a total flow of 50 ml/min with H2:CO2 ¼ 1:1.
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Data analysis The RWGS reaction performance over Ni/CeeZreO was evaluated by conversion of the CO2 and selectivity of CO, CH4. The representative equations are given as follows:
XCO2 (%) ¼ ([CO2]ine[CO2]out)/[CO2]in 100
SCO (%) ¼ [CO]out /([CO2]ine[CO2]out) 100
SCH4 (%) ¼ [CH4]out /([CO2]ine[CO2]out) 100 Where XCO2, SCO, SCH4 are CO2 conversion, CO selectivity and CH4 selectivity, respectively.
Fig. 1 e Pore size distribution curves of the reduced Ni/ CeeZreO catalysts with different Ni contents.
Results and discussion Catalytic characterization Compared with CeeZreO support, all the specific surface areas of Ni/CeeZreO catalyst significantly decrease as listed in Table 1. The pore volumes decrease with the increasing Ni loading except 10 wt% Ni/CeeZreO. However, the surface area and pore size of Ni/CeeZreO catalyst are independent on the Ni content, which indicates that the CeeZreO structure is degraded slightly because of the incorporated Ni [34]. These materials have mesoporous structures from the pore size data. Fig. 1 shows the pore size distribution curves from the desorption branch of isotherm. Each curve has one narrow peak, suggesting that mesoporous Ni/CeeZreO has uniform pore size. The peaks of ZrO2 are found at about 2q of 28.175 , 31.468 , 34.483 , 35.309 , 50.116 , 50.559 , as shown in Fig. 2, which represent the monoclinic structure of (111), (111), (020), (002), (022), (221) planes, respectively. For CeeZreO, the characteristic diffraction peaks are corresponding to the tetragonal structure. Ni/CeeZreO consists of ZrO2 monoclinic phase, Ce0.1Zr0.9O2 tetragonal phase and Ni cubic phase. This suggests that Ce4þ is incorporated into the ZrO2 lattice and a solid solution is formed by co-precipitation. Among them, the dominant Ce0.1Zr0.9O2 phase shows peaks at 2q of 29.974 , 34.379 , 34.931 , 59.607 , which correspond to the planes of (101), (002), (110), (211). The introduction of Ce species into the Zr4þ lattice generates active oxygen and increases oxygen
storage capability because the local oxygen environment is modified around Ce and Zr [35]. From this point, Ce0.1Zr0.9O2 has an advantage over ZrO2 monoclinic structure for the RWGS because of its good oxygen storage space [36]. The weak and broad diffraction peak at 44.497 agrees with the typical (111) plane of Ni cubic phase and implies that Ni is probably well dispersed on the mesoporous CeeZreO with low Ni content (1 wt%). Then the Ni peak becomes sharp and narrow with the increase of Ni loading, indicating that Ni begins to aggregate on the surface of the support. This is similar to the result in literature [37]. The crystal plane spacing slightly decreases when Ni is loaded on the CeeZreO support as shown in Table 2 (P represents plane of support and S represents spacing). On the one hand, this suggests that Ni2þ ions are incorporated into the support lattice and replace some Ce4þ or Zr4þ because the ionic radius of Ni2þ (0.072 nm) is smaller than those of Zr4þ
Table 1 e Specific surface area and pore structure of the reduced Ni/CeeZreO catalysts. Catalyst Ni (wt%) 0 1 3 5 7.8 10
BET surface area (m2g1)
Pore volume (cm3g1)
Pore size (nm)
62.895 32.261 35.809 29.923 35.465 31.320
0.471 0.314 0.249 0.215 0.196 0.227
29.947 38.955 27.818 29.066 22.075 29.031
Fig. 2 e XRD patterns for the reduced Ni/CeeZreO catalysts with different Ni contents.
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Table 2 e The crystal plane spacing of each plane and Ni crystallite size of the reduced Ni/CeeZreO catalysts with different Ni contents. S(nm) Ni (wt%)
0 1 3 5 7.8 10
Ni crystallite size(nm)
p (111)
(101)
(111)
(002)
(110)
(022)
(221)
(211)
0.31782 0.31664 0.31741 0.31761 0.31722 0.31635
0.29736 0.29653 0.29641 0.29726 0.29693 0.29686
0.28519 0.28510 0.28512 0.28510 0.28589 0.28565
0.26064 0.26044 0.26053 0.26046 0.26405 0.26375
0.25593 0.25514 0.25541 0.25615 0.25505 0.25545
e 0.18540 0.18530 0.18560 0.18556 0.18532
e 0.18192 0.18247 0.18235 0.18256 0.18206
0.15477 0.15399 0.15442 0.15443 0.15461 0.15643
(0.084 nm) and Ce4þ (0.097 nm) [38]. The imbalance of charge and the lattice distortion occurring within the catalyst result in the generation of oxygen vacancies [24,39,40]. On the other hand, the defect formed during the process of preparation leads to shrinkage of the lattice. However, the crystal plane spacing increases for (111), (002) planes of 7.8 wt% Ni/ CeeZreO and for (111), (002), (211) planes of 10 wt% Ni/ CeeZreO. The reason is conjectured to be that the excess Ni leads to the structure change of CeeZreO. The Ni crystallite size of 3 wt% Ni/CeeZreO calculated from Scherrer equation was the smallest in Table 2, which may mean more active sites. Indeed, this is in consistent with the BET result that 3 wt % Ni/CeeZreO has the largest surface area. The Ni crystallite size becomes larger with the increasing Ni content. Since the surface Ni species is considered as the active site in the conversion of CO2 to CO, the aggregation of Ni is one of the reasons for the decreasing activity. Fig. 3 gives SEM images of the surface morphology of the reduced 3 wt% Ni/CeeZreO. The particle size is uniform and there is little change before (a) and after (b) the stability test. Besides, neither obvious carbon deposition nor sintering is seen after the stability test in Fig. 3(b). The stable structure of catalyst determines the stability of catalytic performance. Elemental composition of the catalyst was analyzed by EDS. There is either no significant change in the component content for each element before and after the stability test in Table 3. For example, the C wt% changes from 1.64 to 1.66 when Ni wt% changes from 2.56 to 2.53 after reaction. In addition, 3 wt% Ni/CeeZreO catalyst maintains high activity after 80 h of reaction, indicating that the Ni/CeeZreO catalyst has remarkable stability. In order to explore the structure features, 3 wt% Ni/CeeZreO catalyst was further investigated
e e 13.00 14.85 17.33 20.78
Table 3 e Elemental composition of the reduced 3 wt% Ni/ CeeZreO before and after the stability test. Element
Content Before
After
Weight (%) Atomic (%) Weight (%) Atomic (%) O Ni Ce Zr C
32.63 2.56 5.50 57.67 1.64
70.57 1.51 1.37 21.82 4.73
32.82 2.53 5.11 57.88 1.66
70.65 1.49 1.26 21.86 4.76
by TEM. From the TEM image presented in Fig. 4(a), the particle sizes of 3 wt% Ni/CeeZreO catalyst are 20e40 nm. The lattice spacing of 0.298 nm is assigned to (101) plane of Ce0.1Zr0.9O2 and Ni particle size is about 14 nm in Fig. 4(b), which are in consistent with the calculated result from XRD. As shown in Fig. 5, the pure CeeZreO support is not reduced under 1000 C. Only when NiO is loaded, the reduction peak appears. The main peaks can be attributed to the reduction of superficial NiO species and the shoulder (at higher temperature) is assigned to the reduction of the bulk NiO species. With the increase of Ni content, the shoulder becomes more and more obvious, indicating that the crystallinity increases, which is in agreement with the XRD analysis. In addition, the main peak has a shift to the low temperature with the increase of hydrogen consumption. Distortion of O2 sublattice in the support results in a higher mobility of the lattice oxygen, so that the reduction is no longer confined to the surface and penetrates into the bulk [41]. From the structural point of view, a quite significant mismatch happens between the ionic radius of Ce4þ (0.097 nm) and Zr4þ
Fig. 3 e SEM images of the reduced 3 wt% Ni/CeeZreO before (a) and after (b) the stability test.
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Fig. 4 e TEM images of the reduced 3 wt% Ni/CeeZreO catalyst.
(0.084 nm) and is responsible for the modifications of the ZrO2 lattice upon insertion of CeO2 [42]. Element species on the surface were measured by XPS. The Zr3d spectrum of 3 wt% Ni/CeeZreO catalyst is shown in Fig. 6(a). The binding energy of Zr3d5/2 and Zr3d3/2 are 181.83 eV and 184.19 eV, respectively, which could be attributed to the characteristics of Zr4þ [43]. The Ce3d spectrum in Fig. 6(b) indicates the presence of two different Ce species, Ce3þ and Ce4þ, on the catalyst surface [44]. The states and binding energy of different Ce species are listed in Table 4. The existence of Ce species in ZrO2 leads to more defects of the lattice, which eventually enhances the mobility of oxygen in bulk of the support. As shown in Fig. 6(c), peak 1 of 853.05 eV (with the satellite peak 3 of 857.01 eV) is the binding energy of Ni combined with Ni and peak 2 of 855.01 eV (with the satellite peak 4 of 862.14 eV) means Ni is bonded to CeeZreO. The peaks at 870.16 eV and 873.42 eV are the corresponding Ni2p1/2 peaks of the main peak 1 and peak 2 [21,45], respectively. Fig. 6(d) shows the spectrum of O1s of 3 wt% Ni/CeeZreO. The lower binding energy of 529e530 eV can be ascribed to the
Fig. 5 e H2-TPR profiles of Ni/CeeZreO catalysts with different Ni contents.
lattice oxygen ions bonded with metal cations, while the higher of 531e532 eV might be assigned to the surface oxygen ions bonded to metal cations [46].
Catalytic performance The reaction temperature range from 550 C to 750 C was investigated for Ni/CeeZreO at 1 atm and a total flow of 50 ml/min with H2:CO2 ¼ 1:1. Over all the catalysts, CO2 conversion increases with the increase of temperature, indicating that high temperature is favorable for the CO2 conversion in Fig. 7(a). In XRD result, Ni crystallite size calculated from Scherrer equation increases with the increase of Ni wt% due to the Ni aggregation. In TPR profiles, the shoulder peak assigned to the bulk NiO reduction becomes more clear with the increase of Ni loading. This is attributed to the increasing crystallinity and in accordance with the XRD result. Ni crystallite size is an important factor for the CO2 conversion since that the Ni species on surface play an important role in RWGS [34]. In general, small crystallite size means larger specific surface area and more active sites. As a result, CO2 conversion decreases with the increasing Ni loading except 1 wt% Ni/CeeZreO. The highest conversion obtained over the catalyst with 3 wt% Ni can be attributed to its largest specific surface area and suitable active sites based on the characterization results. The Ni crystallite size of 3 wt % Ni/CeeZreO is smallest, indicating that more active sites can be provided. Although Ni is also well dispersed for 1 wt% Ni/CeeZreO, it does not perform the best activity due to the very low content of Ni. Meanwhile, RWGS is a reverse reaction and the conversion is limited by the thermodynamic equilibrium. Maybe the promoted adsorption of CO2 by coupling CeeZreO with Ni leads to the enhanced CO2 conversion and the phenomenon can be attributed to the high interface between Ni and support [47]. CO selectivity increases with the increase of temperature over Ni/CeeZreO with different Ni contents in Fig. 7(b), which is determined by the nature of the endothermic reaction. When the temperature is higher than 700 C, the CO selectivity is over 93%, stable and independent on Ni amount. The best
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Fig. 6 e XPS spectra of Zr3d (a), Ce3d (b), Ni2p (c), O1s (d) of the reduced 3 wt% Ni/CeeZreO.
CO selectivity can reach 99.65% at 750 C for 10 wt% Ni/ CeeZreO. Fig. 7(c) shows that the CH4 selectivity decreases with the increase of temperature and no CH4 is formed above 700 C, indicating that the generation of CH4 can be inhibited at high temperature.
Stability test The stability of 3 wt% Ni/CeeZreO catalyst was tested for 80 h at 750 C. It can be seen that the catalyst maintained stable and high activity after the long time reaction from Fig. 8. The stable CO2 conversion can be ascribed to the good thermal stability and resistance to carbon deposition of Ni/CeeZreO catalyst. In terms of this aspect, Ni/CeeZreO catalyst may be one promising candidate for the industrial application in the future.
Conclusion Mesoporous Ni-based catalysts with different Ni contents give outstanding performance in reverse water gas shift reaction (RWGS) at different temperatures. The study shows that CO2 conversion reaches 49.66% and maintains stable after 80 h of reaction over 3 wt% Ni/CeeZreO. CO selectivity can reach 99.65% on 10 wt% Ni/CeeZreO catalyst at 750 C. Ni/CeeZreO catalyst performs high activity, stability and selectivity in the conversion of CO2 to CO at high temperature. The solid solution CeeZreO is formed by co-precipitation method and Ni is able to be incorporated into the CeeZreO lattice via impregnation. There is rare carbon deposition after long time reaction. The Ni species on catalyst surface can be considered as the active site for RWGS. According to redox mechanism of
Table 4 e The states and binding energy of different Ce species in Fig. 6(b). Ce4þ
Species State Peak Binding energy (eV)
3d94f0 O2p6 10 5 917.49 898.86
3d94f1 O2p5 9 4 907.97 889.07
Ce3þ 3d94f2 O2p4 7 2 902.22 883.80
3d94f1 O2p6 8 3 905.08 886.46
3d94f2 O2p5 6 1 900.44 881.80
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Fig. 7 e The catalytic activity over Ni/CeeZreO with different Ni contents as a function of temperature were obtained: (a) CO2 conversion, (b) CO selectivity, (c) CH4 selectivity.
RWGS, the enhanced oxygen storage capability of CeeZreO by impregnated Ni can promote the goal reaction.
Acknowledgments The authors are grateful for the financial support of CAS Renewable Energy Key Lab., Natural Science Foundation of China (51576201), National Natural Science Foundation of Guangdong province (2015A030312007), Guangdong Science and Technology Project (2013B050800007), Guangzhou Science and Technology Project (2013J4500027).
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Fig. 8 e The CO2 conversion over 3 wt% Ni/CeeZreO catalyst for 80 h at 750 C was obtained.
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[23] Wang L, Liu H, Chen Y, Zhang R, Yang S. K-promoted CoCeO2 catalyst for the reverse water-gas shift reaction. Chem Lett 2013;42:682e3. [24] Shan W. Reduction property and catalytic activity of Ce1XNiXO2 mixed oxide catalysts for CH4 oxidation. Appl Catal A General 2003;246:1e9. [25] Li Yue, Fu Qi, Flytzani-Stephanopoulos Maria. Lowtemperature water-gas shift reaction over Cu- and Niloaded cerium oxide catalysts. Appl Catal B Environ 2000;27:179e91. [26] Passos FB, de Oliveira ER, Mattos LV, Noronha FB. Partial oxidation of methane to synthesis on the stability of the catalysts. Catal Today 2005;101:23e30. [27] Laosiripojana N, Assabumrungrat S. Methane steam reforming over Ni/CeeZrO2 catalyst: Influences of CeeZrO2 support on reactivity, resistance toward carbon formation, and intrinsic reaction kinetics. Appl Catal A General 2005;290:200e11. [28] Chen XY, Yik E, Butler J, Schwank JW. Gasification characteristics of carbon species derived from model reforming compound over Ni/Ce-Zr-O catalysts. Catal Today 2014;233:14e20. [29] Hyun-Seog Roh K-WJ, Dong Wen-Sheng, Park Sang-Eon, Baek Young-Soon. Highly stable Ni catalyst supported on CeeZrO2 for oxy-steam reforming of methane. Catal Lett 2001;74:31e6. [30] Chen J, Wu Q, Zhang J, Zhang J. Effect of preparation methods on structure and performance of Ni/Ce0.75Zr0.25O2 catalysts for CH4eCO2 reforming. Fuel 2008;87:2901e7. n A. Methane [31] Montoya JA, R-P E, Gimon C, Del Angel P, Monzo reforming with CO2 over Ni/ZrO2ecatalysts prepared by solegel. Catal Today 2000;63:71e85. [32] de Leitenburg Carla, Trovarelli Alessandro, Llorca Jordi, Cavani Fabrizio, Bini Gianluca. The effect of doping CeO2 with zirconium in the oxidation of isobutane. Appl Catal A General 1996;139:161e73. [33] Basagiannis AC, X V. Reforming reactions of acetic acid on nickel catalysts over a wide temperature range. Appl Catal A 2006;308:182e93. [34] Lu B, Kawamoto K. Preparation of mesoporous CeO2 and monodispersed NiO particles in CeO2, and enhanced selectivity of NiO/CeO2 for reverse water gas shift reaction. Mater Res Bull 2014;53:70e8. [35] Quan ZA, Qin YN, Liu C. Promoting effect of cerium oxide in supported nickel-catalyst for hydrocarbon steam-reforming. Appl Catal 1991;70:1e8. [36] Zheng X-x, Yan C-f, Hu R-r, Li J, Hai H, Luo W-m, et al. Hydrogen from acetic acid as the model compound of biomass fast-pyralysis oil over Ni catalyst supported on ceriaezirconia. Int J Hydrogen Energ 2012;37:12987e93. [37] Tao W, Cheng GW, Yao WL, Lu XG, Zhu QH, Li GS, et al. Syngas production by CO2 reforming of coke oven gas over Ni/La2O3-ZrO2 catalysts. Int J Hydrogen Energ 2014;39:18650e8. [38] Roh HS, Eum IH, Jeong DW. Low temperature steam reforming of methane over Ni-Ce(1-x)Zr(x)O2 catalysts under severe conditions. Renew Energ 2012;42:212e6. [39] Yisup N, Cao Y, Feng W-L, Dai W-L, Fan K-N. Catalytic oxidation of methane over novel Ce-Ni-O mixed oxide catalysts prepared by oxalate gel-coprecipitation. Catal Lett 2005;99:207e13. [40] Li Y, Zhang B, Tang X, Xu Y, Shen W. Hydrogen production from methane decomposition over Ni/CeO2 catalysts. Catal Commun 2006;7:380e6. [41] Benjaram M, Reddy AK, Lakshmanan Pandian. Structural characterization of Nanosized CeO2-SiO2, CeO2-TiO2, and CeO2-ZrO2 catalysts by XRD, Raman, and HREM techniques. J Phys Chem B 2005;109:3355e63.
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Ni/CeeZreO catalyst for high CO2 conversion during reverse water gas shift reaction (RWGS) Feng-man Sun a,b, Chang-feng Yan a,b,*, Zhi-da Wang a, Chang-qing Guo a, Shi-lin Huang a a b
Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangdong, 510640, China University of Chinese Academy of Sciences, Beijing, 100039, China
article info
abstract
Article history:
A series of Ni-based catalysts supported on CeeZreO were synthesized via impregnation
Received 14 July 2015
and applied in the reverse water gas shift reaction (RWGS). BET, XRD, TPR, SEM, TEM and
Received in revised form
XPS were employed as the characterization of catalysts. Catalytic activities of Ni/CeeZreO
14 September 2015
with different Ni contents were evaluated in RWGS under atmospheric pressure and at the
Accepted 2 October 2015
temperature range of 550e750 C in a fixed-bed quartz reactor. CO2 conversion reaches
Available online 11 November 2015
49.66% and maintains stable after 80 h of reaction over 3 wt% Ni/CeeZreO. CO selectivity
Keywords:
vides high activity, stability and selectivity in the conversion of CO2 to CO at high tem-
Ni/CeeZreO
perature. CeeZreO solid solution is formed by co-precipitation and Ni is able to be
Catalyst
incorporated into the CeeZreO lattice via impregnation. The Ni species on catalyst surface
RWGS
can be considered as the active site for RWGS.
CO2 utilization
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
can reach 99.65% for the catalyst with 10 wt% Ni at 750 C. The Ni/CeeZreO catalyst pro-
reserved.
H2 utilization
Introduction Conventional energy consumption has caused depleting of the non-renewable fossil fuels gradually and led to increasing anthropogenic emissions of CO2. As a result, carbon capture is of great interest not only for the storage of energy but also for the limitation of greenhouse effect. Using CO2 rather than fossil fuel as feedstock to produce a series of precursors relevant with industrial reaction and organic materials presents a sustainable and long-term blueprint [1e3]. The reverse water gas shift reaction (RWGS) is a promising candidate, in which the carbon capture, storage and utilization (CCSU) and effective utilization of hydrogen can be realized. And hydrogen
obtained from electrolysis of water by consuming wind or solar power which are abandoned due to causing instability of the grid makes this process feasible and sustainable [4]. A lot of chemicals such as methanol can be produced by employing synthesis gas, which traditionally is generated from natural gas reforming and coal gasification [5] with the non-renewable fossil fuel as carbon source. It has been proposed that the atmospheric CO2 and H2 could be converted to renewable energy sources through RWGS according to Eqns. (1) and (2) [6e8]: CO2 þH2 %CO þ H2 O DH ¼ 41:2 kJ=mol
(1)
CO þ 3H2 %CH4 þH2 O DH ¼ 206:1 kJ=mol
(2)
* Corresponding author. Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangdong, 510640, China. Tel./fax: þ86 2087057729. E-mail address: [email protected] (C.-f. Yan). http://dx.doi.org/10.1016/j.ijhydene.2015.10.004 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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Generally, the conversion of CO2 in RWGS is limited by thermodynamic equilibrium due to the reversibility. High temperature is beneficial to improve CO selectivity because the primary reaction (Eqn. (1)) is endothermic and the side reaction (Eqn. (2)) is exothermic [9]. Many researchers regard CO2 as the direct source of methanol [10e12], but the yield of methanol from CO2 is lower than from CO. It can be attributed to that CO2 is a highly stable molecule. RWGS is the first step of the synthesis of hydrocarbons from CO2 [13]. The Cu-based catalyst is used for RWGS in the earlier time [14,15]. However, it is not the proper catalyst for RWGS at high temperature due to its poor thermal stability. ZnO-based catalyst shows high deactivation rate at high temperature, which is related to the vaporization of Zn during the reaction [6]. Noble metals, such as Pt and Au [16,17], are also employed but they are very expensive. Ni is a kind of cheaper metal with high activity in most of the catalytic reactions and is more economical than noble metals. However, coke deposition and deactivation are easy to occur on the Nibased catalyst during the catalytic process [18]. As alkaline earth metal oxide, ZrO2 is a promising support which has a unique interaction with the active component by changing the electronic distribution. Researchers have proved that ZrO2 can prevent carbon deposition and facilitate the gasification of coke [19,20] for its low concentration of Lewis sites and noted stability [21]. On the other hand, there have been many studies about different metals supported on CeO2 for RWGS reaction [22,23]. The redox properties of CeO2 could be enhanced if additional Ni was introduced into CeO2 lattice by forming solid solutions [24,25]. The rare earth metal oxide CeO2 as promoter or support plays an important role in the generation of surface oxygen species and anionic vacancies during catalytic reaction. Therefore, CeeZreO supported catalyst has potential not only for its outstanding performance but also for the mitigation of carbon deposition [26,27]. The stable activities have been attributed to the ability to facilitate the carbon gasification [28]. At the same time, the oxygen storage capability and thermal stability can be improved on CeeZreO solid solution compared with pure oxides [29e32]. On the basis of the character of RWGS, the catalyst should be thermally stable and have high oxygen storage capacity. We aimed to develop an efficient catalyst for RWGS. Based on the performance of the materials mentioned above, Ni/ CeeZreO may be a promising choice in designing a catalyst for RWGS. In this work, a series of Ni/CeeZreO with different Ni contents were synthesized and evaluated in the RWGS reaction and the catalyst performed high activity, selectivity and stability in the goal reaction.
Experiments Catalyst preparation The CeeZreO support was prepared by the co-precipitation method. The aqueous solution of Ce(NO3)3$6H2O and ZrOCl2$8H2O with a tailored composition was added into a precursor solution of CH3CH2OH and NH4OH, and then kept for several hours under vigorous stirring at 60 C and pH of 9. The precipitate was filtered and washed with deionized water
followed by CH3CH2OH, then dried at 110 C for 12 h. Finally, the CeeZreO support (7.8 wt% of Ce oxide) was naturally cooled to room temperature after calcination at 600 C for 6 h. ZrO2 support was prepared as described above without addition of Ce(NO3)3$6H2O. A series of Ni catalysts supported on CeeZreO were prepared via the impregnation method [33]. The solid residues were dried at 110 C for 12 h, and then heated to 800 C at a rate of 5 C min1 and calcined for 6 h. Finally, Ni/CeeZreO catalysts with Ni loading of 1, 3, 5, 7.8, 10 wt% were prepared.
Catalyst characterization The nitrogen adsorptionedesorption isotherms of catalyst at 196 C were obtained on a Micromeritics ASAP-2010 apparatus. Multi-plot Braunauer-Emmett-Teller (BET) method was used to calculate the specific surface area. Pore volume and pore size distribution were obtained by BarretteJoynereHalenda (BJH) method. X-ray diffraction (XRD) analysis was conducted in the PANalytical X'Pert diffractometer (X'Pert PRO MPD, PW3040/60) with Cu-Ka (l ¼ 0.154060 nm) radiation (40 kV, 40 mA) and Scherrer equation was used for the crystallite size. The catalyst was characterized by the Scanning electron microscope (SEM) with a S-4800 instrument at 2.0 kV for the surface morphology followed by the Energy dispersive spectrometer (EDS) analysis for the elemental composition. The morphology of catalyst was further characterized by Transmission electron microscopy (TEM) on a JEM-2100 microscope. Temperatureprogrammed reduction (TPR) was performed on a Micromeritics AutoChem 2920 apparatus using a reducing gas of 10 vol% H2/Ar in a quartz tube from 30 C to 1000 C. The X-ray photo electron spectroscopy (XPS) measurements were performed on an ESCALAB 250Xi spectrometer with Mg Ka source (hv ¼ 1253.6 eV) under vacuum. The binding energies were calibrated using the contaminant carbon (C1s ¼ 284.8 eV) when analyzed by Thermo Avantage software.
Catalytic activity test The RWGS reaction was conducted in a fixed-bed quartz tubular reactor (4 mm inner diameter and 300 mm length) under atmospheric pressure in a temperature range of 550e750 C, and a total flow of 50 ml/min with H2:CO2 ¼ 1:1. Before the RWGS reaction, 0.3 g Ni/CeeZreO catalyst (40e60 mesh) and 1.2 g quartz sand (40e60 mesh) supported on quartz wool was pre-reduced in situ in a 5 vol% H2/Ar stream for 5 h at a temperature of 600 C and a heating ramp rate of 8 C min1 with a total gas flow of 150 ml/min. A thermocouple was inserted directly into the center of the catalyst bed to measure the actual temperatures of the pretreatment and reaction. All the reactant gases were monitored with mass flow meter and the flow of the product was measured by a soap bubble flow meter. When the reaction became stable, the output gas flow was analyzed online in the gas chromatographs (GC 7890Ⅱ) equipped with a thermal conductivity detector (TCD) and a TDX-01 column for H2, CO2, CO, CH4 after condensation and drying. The stability of 3 wt% Ni/CeeZreO catalyst was tested for 80 h, at 750 C, 1 atm, and a total flow of 50 ml/min with H2:CO2 ¼ 1:1.
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Data analysis The RWGS reaction performance over Ni/CeeZreO was evaluated by conversion of the CO2 and selectivity of CO, CH4. The representative equations are given as follows:
XCO2 (%) ¼ ([CO2]ine[CO2]out)/[CO2]in 100
SCO (%) ¼ [CO]out /([CO2]ine[CO2]out) 100
SCH4 (%) ¼ [CH4]out /([CO2]ine[CO2]out) 100 Where XCO2, SCO, SCH4 are CO2 conversion, CO selectivity and CH4 selectivity, respectively.
Fig. 1 e Pore size distribution curves of the reduced Ni/ CeeZreO catalysts with different Ni contents.
Results and discussion Catalytic characterization Compared with CeeZreO support, all the specific surface areas of Ni/CeeZreO catalyst significantly decrease as listed in Table 1. The pore volumes decrease with the increasing Ni loading except 10 wt% Ni/CeeZreO. However, the surface area and pore size of Ni/CeeZreO catalyst are independent on the Ni content, which indicates that the CeeZreO structure is degraded slightly because of the incorporated Ni [34]. These materials have mesoporous structures from the pore size data. Fig. 1 shows the pore size distribution curves from the desorption branch of isotherm. Each curve has one narrow peak, suggesting that mesoporous Ni/CeeZreO has uniform pore size. The peaks of ZrO2 are found at about 2q of 28.175 , 31.468 , 34.483 , 35.309 , 50.116 , 50.559 , as shown in Fig. 2, which represent the monoclinic structure of (111), (111), (020), (002), (022), (221) planes, respectively. For CeeZreO, the characteristic diffraction peaks are corresponding to the tetragonal structure. Ni/CeeZreO consists of ZrO2 monoclinic phase, Ce0.1Zr0.9O2 tetragonal phase and Ni cubic phase. This suggests that Ce4þ is incorporated into the ZrO2 lattice and a solid solution is formed by co-precipitation. Among them, the dominant Ce0.1Zr0.9O2 phase shows peaks at 2q of 29.974 , 34.379 , 34.931 , 59.607 , which correspond to the planes of (101), (002), (110), (211). The introduction of Ce species into the Zr4þ lattice generates active oxygen and increases oxygen
storage capability because the local oxygen environment is modified around Ce and Zr [35]. From this point, Ce0.1Zr0.9O2 has an advantage over ZrO2 monoclinic structure for the RWGS because of its good oxygen storage space [36]. The weak and broad diffraction peak at 44.497 agrees with the typical (111) plane of Ni cubic phase and implies that Ni is probably well dispersed on the mesoporous CeeZreO with low Ni content (1 wt%). Then the Ni peak becomes sharp and narrow with the increase of Ni loading, indicating that Ni begins to aggregate on the surface of the support. This is similar to the result in literature [37]. The crystal plane spacing slightly decreases when Ni is loaded on the CeeZreO support as shown in Table 2 (P represents plane of support and S represents spacing). On the one hand, this suggests that Ni2þ ions are incorporated into the support lattice and replace some Ce4þ or Zr4þ because the ionic radius of Ni2þ (0.072 nm) is smaller than those of Zr4þ
Table 1 e Specific surface area and pore structure of the reduced Ni/CeeZreO catalysts. Catalyst Ni (wt%) 0 1 3 5 7.8 10
BET surface area (m2g1)
Pore volume (cm3g1)
Pore size (nm)
62.895 32.261 35.809 29.923 35.465 31.320
0.471 0.314 0.249 0.215 0.196 0.227
29.947 38.955 27.818 29.066 22.075 29.031
Fig. 2 e XRD patterns for the reduced Ni/CeeZreO catalysts with different Ni contents.
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Table 2 e The crystal plane spacing of each plane and Ni crystallite size of the reduced Ni/CeeZreO catalysts with different Ni contents. S(nm) Ni (wt%)
0 1 3 5 7.8 10
Ni crystallite size(nm)
p (111)
(101)
(111)
(002)
(110)
(022)
(221)
(211)
0.31782 0.31664 0.31741 0.31761 0.31722 0.31635
0.29736 0.29653 0.29641 0.29726 0.29693 0.29686
0.28519 0.28510 0.28512 0.28510 0.28589 0.28565
0.26064 0.26044 0.26053 0.26046 0.26405 0.26375
0.25593 0.25514 0.25541 0.25615 0.25505 0.25545
e 0.18540 0.18530 0.18560 0.18556 0.18532
e 0.18192 0.18247 0.18235 0.18256 0.18206
0.15477 0.15399 0.15442 0.15443 0.15461 0.15643
(0.084 nm) and Ce4þ (0.097 nm) [38]. The imbalance of charge and the lattice distortion occurring within the catalyst result in the generation of oxygen vacancies [24,39,40]. On the other hand, the defect formed during the process of preparation leads to shrinkage of the lattice. However, the crystal plane spacing increases for (111), (002) planes of 7.8 wt% Ni/ CeeZreO and for (111), (002), (211) planes of 10 wt% Ni/ CeeZreO. The reason is conjectured to be that the excess Ni leads to the structure change of CeeZreO. The Ni crystallite size of 3 wt% Ni/CeeZreO calculated from Scherrer equation was the smallest in Table 2, which may mean more active sites. Indeed, this is in consistent with the BET result that 3 wt % Ni/CeeZreO has the largest surface area. The Ni crystallite size becomes larger with the increasing Ni content. Since the surface Ni species is considered as the active site in the conversion of CO2 to CO, the aggregation of Ni is one of the reasons for the decreasing activity. Fig. 3 gives SEM images of the surface morphology of the reduced 3 wt% Ni/CeeZreO. The particle size is uniform and there is little change before (a) and after (b) the stability test. Besides, neither obvious carbon deposition nor sintering is seen after the stability test in Fig. 3(b). The stable structure of catalyst determines the stability of catalytic performance. Elemental composition of the catalyst was analyzed by EDS. There is either no significant change in the component content for each element before and after the stability test in Table 3. For example, the C wt% changes from 1.64 to 1.66 when Ni wt% changes from 2.56 to 2.53 after reaction. In addition, 3 wt% Ni/CeeZreO catalyst maintains high activity after 80 h of reaction, indicating that the Ni/CeeZreO catalyst has remarkable stability. In order to explore the structure features, 3 wt% Ni/CeeZreO catalyst was further investigated
e e 13.00 14.85 17.33 20.78
Table 3 e Elemental composition of the reduced 3 wt% Ni/ CeeZreO before and after the stability test. Element
Content Before
After
Weight (%) Atomic (%) Weight (%) Atomic (%) O Ni Ce Zr C
32.63 2.56 5.50 57.67 1.64
70.57 1.51 1.37 21.82 4.73
32.82 2.53 5.11 57.88 1.66
70.65 1.49 1.26 21.86 4.76
by TEM. From the TEM image presented in Fig. 4(a), the particle sizes of 3 wt% Ni/CeeZreO catalyst are 20e40 nm. The lattice spacing of 0.298 nm is assigned to (101) plane of Ce0.1Zr0.9O2 and Ni particle size is about 14 nm in Fig. 4(b), which are in consistent with the calculated result from XRD. As shown in Fig. 5, the pure CeeZreO support is not reduced under 1000 C. Only when NiO is loaded, the reduction peak appears. The main peaks can be attributed to the reduction of superficial NiO species and the shoulder (at higher temperature) is assigned to the reduction of the bulk NiO species. With the increase of Ni content, the shoulder becomes more and more obvious, indicating that the crystallinity increases, which is in agreement with the XRD analysis. In addition, the main peak has a shift to the low temperature with the increase of hydrogen consumption. Distortion of O2 sublattice in the support results in a higher mobility of the lattice oxygen, so that the reduction is no longer confined to the surface and penetrates into the bulk [41]. From the structural point of view, a quite significant mismatch happens between the ionic radius of Ce4þ (0.097 nm) and Zr4þ
Fig. 3 e SEM images of the reduced 3 wt% Ni/CeeZreO before (a) and after (b) the stability test.
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Fig. 4 e TEM images of the reduced 3 wt% Ni/CeeZreO catalyst.
(0.084 nm) and is responsible for the modifications of the ZrO2 lattice upon insertion of CeO2 [42]. Element species on the surface were measured by XPS. The Zr3d spectrum of 3 wt% Ni/CeeZreO catalyst is shown in Fig. 6(a). The binding energy of Zr3d5/2 and Zr3d3/2 are 181.83 eV and 184.19 eV, respectively, which could be attributed to the characteristics of Zr4þ [43]. The Ce3d spectrum in Fig. 6(b) indicates the presence of two different Ce species, Ce3þ and Ce4þ, on the catalyst surface [44]. The states and binding energy of different Ce species are listed in Table 4. The existence of Ce species in ZrO2 leads to more defects of the lattice, which eventually enhances the mobility of oxygen in bulk of the support. As shown in Fig. 6(c), peak 1 of 853.05 eV (with the satellite peak 3 of 857.01 eV) is the binding energy of Ni combined with Ni and peak 2 of 855.01 eV (with the satellite peak 4 of 862.14 eV) means Ni is bonded to CeeZreO. The peaks at 870.16 eV and 873.42 eV are the corresponding Ni2p1/2 peaks of the main peak 1 and peak 2 [21,45], respectively. Fig. 6(d) shows the spectrum of O1s of 3 wt% Ni/CeeZreO. The lower binding energy of 529e530 eV can be ascribed to the
Fig. 5 e H2-TPR profiles of Ni/CeeZreO catalysts with different Ni contents.
lattice oxygen ions bonded with metal cations, while the higher of 531e532 eV might be assigned to the surface oxygen ions bonded to metal cations [46].
Catalytic performance The reaction temperature range from 550 C to 750 C was investigated for Ni/CeeZreO at 1 atm and a total flow of 50 ml/min with H2:CO2 ¼ 1:1. Over all the catalysts, CO2 conversion increases with the increase of temperature, indicating that high temperature is favorable for the CO2 conversion in Fig. 7(a). In XRD result, Ni crystallite size calculated from Scherrer equation increases with the increase of Ni wt% due to the Ni aggregation. In TPR profiles, the shoulder peak assigned to the bulk NiO reduction becomes more clear with the increase of Ni loading. This is attributed to the increasing crystallinity and in accordance with the XRD result. Ni crystallite size is an important factor for the CO2 conversion since that the Ni species on surface play an important role in RWGS [34]. In general, small crystallite size means larger specific surface area and more active sites. As a result, CO2 conversion decreases with the increasing Ni loading except 1 wt% Ni/CeeZreO. The highest conversion obtained over the catalyst with 3 wt% Ni can be attributed to its largest specific surface area and suitable active sites based on the characterization results. The Ni crystallite size of 3 wt % Ni/CeeZreO is smallest, indicating that more active sites can be provided. Although Ni is also well dispersed for 1 wt% Ni/CeeZreO, it does not perform the best activity due to the very low content of Ni. Meanwhile, RWGS is a reverse reaction and the conversion is limited by the thermodynamic equilibrium. Maybe the promoted adsorption of CO2 by coupling CeeZreO with Ni leads to the enhanced CO2 conversion and the phenomenon can be attributed to the high interface between Ni and support [47]. CO selectivity increases with the increase of temperature over Ni/CeeZreO with different Ni contents in Fig. 7(b), which is determined by the nature of the endothermic reaction. When the temperature is higher than 700 C, the CO selectivity is over 93%, stable and independent on Ni amount. The best
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Fig. 6 e XPS spectra of Zr3d (a), Ce3d (b), Ni2p (c), O1s (d) of the reduced 3 wt% Ni/CeeZreO.
CO selectivity can reach 99.65% at 750 C for 10 wt% Ni/ CeeZreO. Fig. 7(c) shows that the CH4 selectivity decreases with the increase of temperature and no CH4 is formed above 700 C, indicating that the generation of CH4 can be inhibited at high temperature.
Stability test The stability of 3 wt% Ni/CeeZreO catalyst was tested for 80 h at 750 C. It can be seen that the catalyst maintained stable and high activity after the long time reaction from Fig. 8. The stable CO2 conversion can be ascribed to the good thermal stability and resistance to carbon deposition of Ni/CeeZreO catalyst. In terms of this aspect, Ni/CeeZreO catalyst may be one promising candidate for the industrial application in the future.
Conclusion Mesoporous Ni-based catalysts with different Ni contents give outstanding performance in reverse water gas shift reaction (RWGS) at different temperatures. The study shows that CO2 conversion reaches 49.66% and maintains stable after 80 h of reaction over 3 wt% Ni/CeeZreO. CO selectivity can reach 99.65% on 10 wt% Ni/CeeZreO catalyst at 750 C. Ni/CeeZreO catalyst performs high activity, stability and selectivity in the conversion of CO2 to CO at high temperature. The solid solution CeeZreO is formed by co-precipitation method and Ni is able to be incorporated into the CeeZreO lattice via impregnation. There is rare carbon deposition after long time reaction. The Ni species on catalyst surface can be considered as the active site for RWGS. According to redox mechanism of
Table 4 e The states and binding energy of different Ce species in Fig. 6(b). Ce4þ
Species State Peak Binding energy (eV)
3d94f0 O2p6 10 5 917.49 898.86
3d94f1 O2p5 9 4 907.97 889.07
Ce3þ 3d94f2 O2p4 7 2 902.22 883.80
3d94f1 O2p6 8 3 905.08 886.46
3d94f2 O2p5 6 1 900.44 881.80
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Fig. 7 e The catalytic activity over Ni/CeeZreO with different Ni contents as a function of temperature were obtained: (a) CO2 conversion, (b) CO selectivity, (c) CH4 selectivity.
RWGS, the enhanced oxygen storage capability of CeeZreO by impregnated Ni can promote the goal reaction.
Acknowledgments The authors are grateful for the financial support of CAS Renewable Energy Key Lab., Natural Science Foundation of China (51576201), National Natural Science Foundation of Guangdong province (2015A030312007), Guangdong Science and Technology Project (2013B050800007), Guangzhou Science and Technology Project (2013J4500027).
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Fig. 8 e The CO2 conversion over 3 wt% Ni/CeeZreO catalyst for 80 h at 750 C was obtained.
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