- Email: [email protected]
γ-Al2O3 catalysts prepared by solution combustion method for CO methanation
Fuel 137 (2014) 155–163
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Highly efficient NiAl2O4-free Ni/c-Al2O3 catalysts prepared by solution combustion method for CO methanation Yan Zeng, Hongfang Ma ⇑, Haitao Zhang, Weiyong Ying, Dingye Fang Engineering Research Center of Large Scale Reactor Engineering and Technology, Ministry of Education, State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Ti-doped Ni/c-Al2O3 catalysts were
prepared by solution combustion method for CO methanation. Ti species suppressed forming NiAl2O4 and thus enhanced utilization of Ni species. More exposed active surfaces of Tidoped catalyst increased CO adsorption capacity. Electron transfer from TiOx improved the catalytic performance of Ti-doped catalysts.
a r t i c l e
i n f o
Article history: Received 26 May 2014 Received in revised form 31 July 2014 Accepted 2 August 2014 Available online 13 August 2014 Keywords: NiAl2O4 spinel Ni/c-Al2O3 CO methanation Titanium Solution combustion method
a b s t r a c t Ti-doped Ni/c-Al2O3 catalysts with Ti loadings varying from 0 wt% to 5 wt% were prepared in glycol by solution combustion method for CO methanation. Compared to Ni/c-Al2O3 catalyst, Ti-doped Ni/c-Al2O3 catalysts exhibited higher activity. In particular, an optimal Ti-doped Ni/c-Al2O3 catalyst with Ti content of 3 wt% (G-3Ti) achieved almost 100% CO conversion and 98.7% CH4 selectivity at 300 °C, 0.1 MPa, and a WHSV of 12,000 mL g 1 h 1, and showed good stability at 600 °C. On analysis of characterization results, Ti species was found effectively restrict the formation of NiAl2O4 spinel phase, leading to a higher utilization of Ni species and thus more exposed active surfaces, which enhanced the CO adsorption capacity. In addition, the electron cloud density of Ni was increased by electron transfer from TiOx, which could facilitate the dissociation of CO on the catalyst surfaces. Moreover, solvents (i.e., ethanol, n-propanol and glycol) also significantly affected the physical–chemical properties of the catalysts, and catalyst prepared in glycol exhibited better catalytic performance for CO methanation. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Natural gas has been recognized as a promising energy source, but reserves poorly in some regions of the world. CO methanation as a useful technology to synthesize natural gas from syngas, has ⇑ Corresponding author. Tel./fax: +86 21 6425 2192. E-mail address: [email protected] (H. Ma). http://dx.doi.org/10.1016/j.fuel.2014.08.003 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved.
attracted much attention from both academia and industry in the last decade [1,2]. Since Sabatier and Senderens’ pioneering work in 1902 on discovering methane formation by the reaction of carbon monoxide and hydrogen over a nickel catalyst [3], great efforts have been made to produce high-efficiency catalysts retaining high activity at low-temperature (ca. 300 °C) and good stability at hightemperature (ca. 600 °C). Nickel supported on alumina is the most commonly used catalyst for CO methanation due to its good
156
Y. Zeng et al. / Fuel 137 (2014) 155–163
catalytic performance and relatively low price [4,5]. However, Ni/ c-Al2O3 catalyst prepared by conventional methods, such as coprecipitation, impregnation, sol–gel and mechanical mixing, showed low activity at low-temperature, and deactivated easily at hightemperature [6]. Moreover, nickel and alumina easily form NiAl2O4 spinel phase which is difficult to reduce even at high-temperature [7], leading to relatively low catalytic performance [8]. In our previous work [9], Solution combustion method (SCM) was employed to prepare Ni-based catalysts for CO methanation, which enhanced the low-temperature activity and high-temperature stability. With the advantage of economically viable, large-scale production, fast and simple technique, SCM has been widely used for the preparation of porous materials generally used as catalysts [10–12]. Whereas, Ni/c-Al2O3 catalysts prepared by SCM also easily form NiAl2O4 spinel because of the rapid heating and high-temperature during the combustion process. If the formation of NiAl2O4 could be suppressed, the catalytic activity of Ni/c-Al2O3 catalysts would be improved a lot. The formation of NiAl2O4 spinel could be attributed to the strong interaction between NiO and c-Al2O3, and easily happens at relatively low-temperature (400–450 °C). It was reported that reducing calcination temperature was a useful method to decrease the amount of NiAl2O4 spinel [13,14]. However, since CO methanation is an intensely exothermic reaction and commonly operated in the temperature range of 260–600 °C [15], catalysts prepared at low-temperature would deactivate easily at high-temperature. Interestingly, it was also reported that the addition of a second metal could decrease the strong metal-support interaction (SMSI) [16–22]. For instance, by grafting Ti on the surface of c-Al2O3, the linkage of the titanium oxide sublayer to the alumina support stabilized the catalyst against the strong interaction between Pt and c-Al2O3 [23,24]. Thus, adding a second metal to weaken the interaction between NiO and c-Al2O3 would be another efficient
Fig. 1. XRD patterns of (a) calcined samples, and (b) pure c-Al2O3 and reduced samples.
method to lower down the formation of NiAl2O4 spinel. To the best of our knowledge, few researches related to the developing of NiAl2O4-free Ni/c-Al2O3 catalyst in a facile and efficient way has been reported. In this work, we incorporated Ti species in Ni/c-Al2O3 catalysts and aim to inhibit the formation of NiAl2O4 spinel phase. A series of Ni/c-Al2O3 catalysts with Ti loadings varying form 0 wt% to 5 wt% were synthesized via SCM for CO methanation. Ti-doped Ni/cAl2O3 catalyst was more active than the conventional Ni/c-Al2O3 catalyst. The fresh as well as the used catalysts were characterized by XRD, H2-TPR, HRTEM, CO-TPD, XPS, and N2 adsorption–desorption. The possible mechanism for promotional effect of Ti species was proposed. Moreover, the effect of solvents (i.e., ethanol, n-propanol and glycol) on the physical–chemical properties and catalytic performance of the Ti-doped Ni/c-Al2O3 catalysts was also studied.
2. Experimental 2.1. Catalyst preparation Ti-doped Ni/c-Al2O3 catalysts, with constant Ni loading of 30 wt% and Ti content form 0 wt% to 5 wt%, were prepared by SCM in different combustible solution (ethanol, n-propanol and glycol). In a typical experiment, certain amounts of Ni(NO3)26H2O and Al(NO3)39H2O were dissolved in 200 mL combustible solution.
Fig. 2. (a) H2-TPR profiles for calcined G-0Ti and G-1Ti, (b) CO-TPD patterns for reduced G-0Ti and G-1Ti, and (c) XPS spectra of Ni 2p3/2 for calcined G-0Ti and G1Ti.
Y. Zeng et al. / Fuel 137 (2014) 155–163
Then, Ti(OC4H9)4 (TBOT), in accordance with the final catalyst composition requirements, was added dropwise to the mixture solution under continuously vigorous stirring, and 2 mol L 1 HNO3 was simultaneously introduced into the above slurry to maintain it transparent. After the mixture placed in drying oven with constant temperature of 70 °C for 6 h, the concentrated viscous solution transferred to an open china crucible was heated to 700 °C under static air in a muffle furnace with the heating rate of 2 °C min 1 (combustion occurred spontaneously during the heating process), and then kept at 700 °C for 7 h. The as-obtained catalysts were denoted as M–xTi, in which M represented the solvents (M = E, P, G, and E = ethanol, P = n-propanol, G = glycol) and x indicated the Ti weight content (x = 0, 1, 3, 5). For comparison, pure c-Al2O3 was also prepared by SCM in glycol. 2.2. Characterization of catalysts Nitrogen adsorption–desorption isotherms were conducted on by ASAP 2020 (Micromeritics, USA). Specific surface areas were measured by Brunauer Emmet Teller (BET) method. Total pore volume and average pore diameter were evaluated using the standard Barrett–Joyner–Halenda (BJH) treatment. Powder X-ray diffraction (XRD) patterns were carried out on a Rigaku D/Max2550VB/PC Xray diffractometer (Rigaku, Japan) with Cu Ka radiation
157
(c = 1.54056 Å). Hydrogen temperature programmed reduction (H2-TPR) and carbon monoxide temperature programmed desorption (CO-TPD) were conducted on AutoChemII2920 (Micromeritics, USA). High-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and elemental mapping were conducted on transmission electron microscope apparatus (JEOL JEM-2100, Japan) with an accelerating voltage of 200 kV, fitted with an energy dispersive X-ray detector (EDX, Oxford-INCA). The binding energy of Ni was analyzed by the X-ray photo-electron spectra (XPS) measurement by an ESCALAB 250Xi system (Thermo Fisher Scientific, UK) with Al Ka radiation (hm = 1486.6 eV), and calibrated by referencing to the C 1s peak form the contaminated carbon that was assumed to have a binding energy of 284.6 eV. Thermogravimetric analysis (TG) was conducted on a Q600 analyzer (TA Instruments, USA) in a temperature range of room temperature to 800 °C at a heating rate of 10 °C min 1 in air flow of 100 mL min 1. 2.3. Catalytic evaluation Syngas (carbon oxides and hydrogen) methanation reaction was carried out in a fixed-bed reactor (U14 2 500 mm). Inert Al2O3 was added in both ends of the uniform temperature zone to keep the catalyst in the thermostatic area. Corresponding to a weight
Fig. 3. (a) HRTEM of reduced G-1Ti with d (1 1 1) fringes at 0.203 nm, and (b) SAED patterns of reduced G-1Ti.
158
Y. Zeng et al. / Fuel 137 (2014) 155–163
hourly space velocity (WHSV) of 12,000 mL g 1 h 1, 500 mg catalyst was used with the total gas flow rate of 100 mL min 1. The catalyst was heated from ambient temperature to 700 °C in nitrogen flow within 300 min and reduced by pure hydrogen for 3 h. After the catalyst was cooled to reaction temperature under nitrogen flow, the syngas (17.3 vol.% CO, 9.5 vol.% CO2, 67.7 vol.% H2, 5.5 vol.% N2) was introduced to investigate the low-temperature catalytic activity of Ni-based catalysts at 260–340 °C, 0.1 MPa and 12,000 mL g 1 h 1. High-temperature stability test was conducted over G-0Ti and G-3Ti at 600 °C, 2 MPa, 12,000 mL g 1 h 1 and with 30 mol% water.
3. Results and discussion 3.1. Catalyst characterization 3.1.1. XRD XRD patterns of calcined and reduced samples are shown in Fig. 1a and b, respectively. For the calcined samples, the diffraction peaks at 19.1°, 31.4°, 37.2°, 45.2°, 59.7°and 65.8° are attributed to NiAl2O4 (JCPDS 10-0339), and the diffraction peaks at 37.3°, 43.3°, 62.9°, 75.5° and 79.4° derive from NiO (JCPDS 47-1049). For the
reduced samples, the diffraction peaks at 37.3°, 45.6° and 67.0° are attributed to c-Al2O3 (JCPDS 10-0425), and the diffraction peaks at 44.5°, 51.8° and 76.3° are attributed to distinct peaks of Ni (JCPDS 04-0850). There is, however, no diffraction peaks attributed to Ti species appearing in both Fig. 1a and b, indicating that Ti species may highly disperse in the catalysts. As shown in Fig. 1a, the intensity of NiAl2O4 diffraction peaks in G-0Ti is obviously higher than that in G-1Ti, implying that the addition of Ti can inhibit forming NiAl2O4 to some extent. For the two reduced catalysts in Fig. 1b, distinct peaks for NiAl2O4 spinel still existed in G-0Ti, while no obvious NiAl2O4 diffraction peaks assigned to NiAl2O4 can be found in G-1Ti, which may be due to the fully reduction of Ni oxides in G-1Ti. To further understand this, we characterized the samples by TPR and HRTEM. 3.1.2. H2-TPR H2-TPR profiles of the calcined catalysts are shown in Fig. 2a. The low-temperature peaks (<500 °C) are attributed to the reduction of free Ni oxides species (a-type), which have weak interaction with the support, whereas the mid-temperature peaks (500–800 °C) are assigned to the reduction of NiO retaining stronger interaction with the support (b-type) [9,25]. The high-temperature peaks (>800 °C) are ascribed to NiAl2O4 with a spinel
Fig. 4. Typical elemental maps of the reduced G-1Ti.
Y. Zeng et al. / Fuel 137 (2014) 155–163
159
structure (c-type), which is the most difficult to reduce [26–28]. Obviously, as compared with G-0Ti, the reduction peaks of G-1Ti shifted to low-temperature side. This result suggested that the introduction of Ti can effectively weaken the interaction between Ni and c-Al2O3, and thus improved the reducibility of Ni oxides species. Moreover, the c-type NiO species was the dominating part in G-0Ti, while only a small shoulder peak possibly attributed to the c-type NiO species could be observed in G-1Ti. It indicated that the addition of Ti effectively resisted forming NiAl2O4 spinel. Clearly, the conclusion agreed with XRD results well.
the reactants. CO-TPD technology was therefore employed to obtain more insights. As shown in Fig. 2b, the amount of desorbed CO of G-1Ti was much more than that of G-0Ti, indicating that G1Ti had more accessible active surfaces. Moreover, peaks observed at low-temperature (<300 °C) represent desorption at the low activation energy sites, which contribute little toward methanation, while the relatively high-temperature desorption peaks (300– 600 °C) significantly contribute toward methanation [29]. Obviously, the amount of desorbed CO at high-temperature of G-1Ti was more than that of G-0Ti, suggesting a higher activity for G-1Ti.
3.1.3. HRTEM HRTEM image of reduced G-1Ti is shown in Fig. 3a. A planar lattice of 0.203 nm can be observed, which could be assigned to the Ni (1 1 1) face. The results of SAED (Fig. 3b) further confirm this, in which the four rings corresponded to Ni (1 1 1), (2 0 0), (2 2 0) and (3 1 1), respectively. Notably, NiAl2O4 spinel could not be identified in both characterizations, which indicated the absence of NiAl2O4 spinel in G-1Ti, in accordance with the results of both XRD and H2-TPR. The effect of Ti species on resisting the formation of NiAl2O4 spinel may be attributed to that Ni and Al particles were scattered by Ti species, which obstructed the immediate strong interaction between Ni and Al particles. This hypothesis was studied in more detail by mapping. Typical elemental maps of reduced G-1Ti are presented in Fig. 4a–e. As can be clearly deduced from the mapping, Ni and Al particles were spatially isolated by Ti species.
3.1.5. XPS XPS analysis was employed to further study the oxidative state of Ni species, and the XPS spectra of Ni 2p3/2 for calcined G-0Ti and G-1Ti are shown in Fig. 2c. The binding energy of Ni 2p3/2 was 856.9 eV for G-0Ti, and shifted to 856.3 eV for G-1Ti. The result indicated that an interaction may occur between Ni and Ti species in combustion process, and the decrease binding energy may be due to the increase electron cloud density of Ni atoms (Ni0 ? Nid ), which could be explained by electron transfer from Ti4+/Ti3+ redox couples in the catalyst [21]. Previous results [30,31] indicated that the decrease binding energy of Ni 2p3/2 can promote CO dissociation because of the enhanced Ni–C bond and subdued C–O bond. Combining the XPS result with previous results, the addition of Ti species could improve CO dissociation because of electron transfer from TiOx to Ni atoms. 3.2. Evaluation of the catalytic behavior for G-0Ti and G-1Ti
3.1.4. CO-TPD The fully reduction of Ni oxides in G-1Ti improved the utilization of Ni species, which may expose more accessible surfaces to
CO conversion versus temperature profiles over G-0Ti and G-1Ti are shown in Fig. 5a. The initial CO conversions at 260 °C of G-0Ti
Fig. 5. (a) CO conversion and (b) CH4 selectivity of G-0Ti and G-1Ti at 0.1 MPa and 12,000 mL g 1 h 1 in the temperature range of 260–340 °C.
Fig. 6. Effect of solvents on (a) CO conversion and (b) CH4 selectivity at 0.1 MPa and 12,000 mL g 1 h 1 in the temperature range of 260–340 °C.
160
Y. Zeng et al. / Fuel 137 (2014) 155–163
and G-1Ti were 38.5% and 53.5%, respectively. A similar CH4 selectivity for both G-0Ti and G-1Ti in the measured temperature range is shown in Fig. 5b, although Ti-doped G-1Ti showed slightly higher selectivity. Moreover, CO was completely converted over G-1Ti below 310 °C, while 100% CO conversion can be achieved over G-0Ti at the temperature higher than 320 °C. The higher activity of G-1Ti could be attributed to two reasons: (1) the addition of Ti species improved the utilization of Ni species by resisting the formation of NiAl2O4 spinel, which promoted CO adsorption capacity, and (2) the increased electron cloud density of Ni facilitated the dissociation of CO on the catalyst surfaces. Since CO dissociation is the rate-determining step in CO methanation [32], the enhanced
CO dissociation ability of G-1Ti can explain its relatively high catalytic activity. To optimize the Ti-doped Ni/c-Al2O3 catalyst, we also studied the influence of solvents and Ti loadings on catalytic performance, as discussed below. 3.3. Effect of solvents CO conversions versus temperature profiles over E-1Ti, P-1Ti, and G-1Ti are shown in Fig. 6a. It can be observed that G-1Ti prepared in glycol retained much higher CO conversion and CH4 selectivity (see Fig. 6b) than the others. The higher activity of G-1Ti was
Fig. 7. TEM images of (a) E-1Ti, (b) P-1Ti, and (c) G-1Ti, (d) particle size distribution, and (e) CO-TPD of the three catalysts.
Y. Zeng et al. / Fuel 137 (2014) 155–163
possibly because of its better distribution of Ni particles (see Fig. 7a–d) and higher CO adsorption ability (see Fig. 7e). In addition, solvents also affected the structure of the catalysts (see Table 1). The various porous properties may be attributed to that the different exothermicity of solvents resulted in a diverse gases evolution rate and volume during combustion process. Bhaduri et al. [33] also found that the evolution of gases affected the structure of porous catalysts. 3.4. Effect of Ti loadings CO conversions of Ni–Al2O3 catalysts with various Ti loadings are presented in Fig. 8a. CO conversions increased with the Ti loadings Table 1 Surface area, pore volume, average pore size, and Ni particle size for samples. Catalyst d
E-1Ti P-1Tid G-1Tid G-0Tie G-3Tie G-0Tif G-3Tif a b c d e f
SBET (m2 g 184.8 201.4 124.7 139.5 116.8 143.9 118.2
1
)
Vpa (cm3 g 0.08 0.27 0.48 0.30 0.51 0.28 0.47
1
)
Dpb (nm)
DNic (nm)
3.1 3.9 13.4 9.0 13.9 8.3 13.5
– – – 21.7 21.6 21.9 21.6
BJH desorption pore volume. BJH desorption average pore size. Calculated from Ni (2 0 0) plane by Scherrer’s equation. Fresh calcined samples. Catalysts before 150 h lifetime test. Catalysts after 150 h lifetime test.
Fig. 8. Effect of Ti loadings on (a) CO conversion and (b) CH4 selectivity at 0.1 MPa and 12,000 mL g 1 h 1 in the temperature range of 260–340 °C.
161
rising from 0 wt% to 3 wt%, and then decreased significantly with further increase to 5 wt%. CO conversion of G-3Ti achieved 100% at 300 °C, while that of G-0Ti, G-1Ti and G-5Ti were 87.7%, 92.1% and 84.8%, respectively. However, all the four catalysts retained a similar CH4 selectivity in the measured temperature range as shown in Fig. 8b. XPS and CO-TPD were conducted to obtain more insights. As shown in Fig. 9a, the binding energy of Ni 2p3/2 only slightly shifted to lower side with the increase of Ti content, indicating that further increase of Ti content did not significantly impact on CO dissociation. However, it can be seen from Fig. 9b that Ti content observably affected the CO adsorption ability of the Ti-doped catalysts. The CO adsorption ability first increased as Ti content varied from 1% to 3%, and then decreased dramatically with further increase of Ti content to 5%. The improved CO adsorption ability of G-3Ti implied more accessible active surfaces, which may be the main reason for its optimal catalytic activity. Moreover, the lower CO adsorption ability of G-5Ti may be due to the partial coverage of the active metal by excessive TiOx, possibly explaining its lower catalytic activity.
3.5. Stability test The catalytic stability is an important indicator of the catalyst performance, and it is related to the industrial application. The catalytic stability test was conducted on the optimal G-3Ti at 600 °C, 2 MPa and 12,000 mL g 1 h 1. For comparison, that of G-0Ti was also investigated at the same condition. Fig. 10a presents the catalytic activity in terms of CO conversion over G-3Ti and G-0Ti. CO conversions of G-3Ti and G-0Ti were 93.3% and 92.7%, respectively, and almost remained unchanged during the 150 h stability test. The good stability of both G-0Ti and G-3Ti could be attributed
Fig. 9. (a) XPS spectra of Ni 2p3/2 for calcined samples, and (b) CO-TPD patterns for reduced samples.
162
Y. Zeng et al. / Fuel 137 (2014) 155–163
more exposed accessible surfaces enhanced the CO adsorption capacity. Electron transfer from TiOx increased electron cloud density of Ni atoms, which can promote CO dissociation on the catalyst surfaces, leading to a relatively high catalytic performance. Catalytic activity increased with the Ti loadings rising from 0 wt% to 3 wt%, and then decreased significantly with further increase to 5 wt%, which could be due to the partial coverage of the active metal by excessive TiOx. G-3Ti with Ti content of 3 wt%, as the most potential catalyst, achieved almost 100% CO conversion and 98.7% CH4 selectivity at 300 °C, 0.1 MPa, and a WHSV of 12,000 mL g 1 h 1. Moreover, both Ti-free G-0Ti and Ti-doped G3Ti showed good stability during the life-time tests. In addition, solvents significantly affected the physical–chemical properties of the catalysts, and catalyst prepared in glycol exhibited better catalytic performance for CO methanation. It is expected that this strategy can be extended for the fabrication of other metal heterogeneous catalyst with significantly improved behavior.
Acknowledgment This work is financially supported by the National Science and Technology Supporting Plan (2012AA050102).
References
Fig. 10. (a) Stability test over G-0Ti and G-3Ti at 600 °C, 2 MPa, and 12,000 mL g 1 h 1, (b) XRD patterns, and (c) TG curves of G-0Ti and G-3Ti before and after 150 h stability test.
to sintering- and coking-resistant. Nitrogen adsorption–desorption and XRD were conducted to obtain more insight into the physicochemical properties of the samples before and after lifetime test. It can be seen from Table 1 and Fig. 10b that no obviously change of textural properties and Ni particle size can be found in both G-0Ti and G-3Ti after lifetime test. The results indicated that the catalysts prepared by SCM retained good resistance to catalysts sintering, which may be attributed to that Ni particles were scattered and spatially isolated by Al2O3, thus preventing the sintering of Ni particles [9]. Moreover, TG profiles of G-0Ti and G-3Ti before and after lifetime test are shown in Fig. 10c. Weight losses attributed to carbon deposition (the difference before and after lifetime test) over G-0Ti and G-3Ti were 0.5% and 0.8%, respectively, suggesting that both G-0Ti and G-3Ti showed good ability to resist carbon deposition. In addition, the weight increment in the temperature range of 300–500 °C was ascribed to the oxidation of metallic nickel species [34]. 4. Conclusions Ni/c-Al2O3 catalysts with Ti contents ranging from 0 wt% to 5 wt% were prepared in different solvents by SCM for CO methanation. The addition of titanium improved the utilization of Ni species by inhibiting the formation of NiAl2O4 spinel phase, and the
[1] Xavier KO, Sreekala R, Rashid KKA, Yusuff KKM, Sen B. Doping effects of cerium oxide on Ni/Al2O3 catalysts for methanation. Catal Today 1999;49:17–21. [2] Derekaya FB, Yasßar G. The CO methanation over NaY-zeolite supported Ni/ Co3O4, Ni/ZrO2, Co3O4/ZrO2 and Ni/Co3O4/ZrO2 catalysts. Catal Commun 2011;13:73–7. [3] Sabatier P, Senderens JB. Hydrogenation of CO over nickel to produce methane. J Soc Chim Ind 1902;21:504–6. [4] Pan ZY, Dong MH, Meng XK, Zhang XX, Mu XH, Zong BN. Integration of magnetically stabilized bed and amorphous nickel alloy catalyst for CO methanation. Chem Eng Sci 2007;62:2712–7. [5] Zhang WD, Liu BS, Zhu C, Tian YL. Preparation of La2NiO4/ZSM-5 catalyst and catalytic performance in CO2/CH4 reforming to syngas. Appl Catal A Gen 2005;292:138–43. [6] Inui T, Hagiwara T, Takegami Y. Prevention of catalyst deactivation caused by coke formation in the methanation of carbon oxides. Fuel 1982;61:537–41. [7] Cai MD, Wen J, Chu W, Cheng XQ, Li ZJ. Methanation of carbon dioxide on Ni/ ZrO2–Al2O3 catalysts: effects of ZrO2 promoter and preparation method of novel ZrO2–Al2O3 carrier. J Nat Gas Chem 2011;20:318–24. [8] Patil KC, Aruna ST, Mimani T. Combustion synthesis: an update. Curr Opin Solid State Mater Sci 2002;6:507–12. [9] Zhao AM, Ying WY, Zhang HT, Ma HF, Fang DY. Ni–Al2O3 catalysts prepared by solution combustion method for syngas methanation. Catal Commun 2012;17:34–8. [10] Sharma S, Hu Z, Zhang P, McFarland EW, Metiu H. CO2 methanation on Rudoped ceria. J Catal 2011;278:297–309. [11] Lathika Devi SK, Sudarsana Kumar K, Balakrishnan A. Rapid synthesis of pure and narrowly distributed Eu doped ZnO nanoparticles by solution combustion method. Mater Lett 2011;65:35–7. [12] Prakash AS, Shivakumara C, Hegde MS. Single step preparation of CeO2/ CeAlO3/c-Al2O3 by solution combustion method: phase evolution, thermal stability and surface modification. Mater Sci Eng B 2007;139:55–61. [13] Cesteros Y, Salagre P, Medina F, Sueiras JE. Preparation and characterization of several high-area NiAl2O4 spinels. Study of their reducibility. Chem Mater 2000;12:331–5. [14] Rynkowski JM, Paryjczak T, Lenik M. On the nature of oxidic nickel phases in NiO/c-Al2O3 catalysts. Appl Catal A Gen 1993;106:73–82. [15] Kopyscinski J, Schildhauer TJ, Biollaz SMA. Production of synthetic natural gas (SNG) from coal and dry biomass – a technology review from 1950 to 2009. Fuel 2010;89:1763–83. [16] Yang XZ, Wang X, Gao GJ, Wendurima, Liu EM, Shi QQ, et al. Zhang JN, Han CH, Wang J, Lu HL, Liu J, Tong M. Nickel on a macro-mesoporous Al2O3@ZrO2 core/ shell nanocomposite as a novel catalyst for CO methanation. Int J Hydrogen Energy 2013;38:13926–37. [17] Macleod N, Cropley R, Keel JM, Lambert RM. Exploiting the synergy of titania and alumina in lean NOx reduction: in situ ammonia generation during the Pd/ TiO2/Al2O3-catalysed H2/CO/NO/O2 reaction. J Catal 2004;221:20–31. [18] Macleod N, Lambert RM. Low-temperature NOx reduction with H2+CO under oxygen-rich conditions over a Pd/TiO2/Al2O3 catalyst. Catal Commun 2002;3:61–5. [19] Surisetty VR, Eswaramoorthi I, Dalai A. Comparative study of higher alcohols synthesis over alumina and activated carbon-supported alkali-modified MoS2 catalysts promoted with group VIII metals. Fuel 2012;96:77–84.
Y. Zeng et al. / Fuel 137 (2014) 155–163 _ Interaction between nickel and molybdenum [20] Aksoylu AE, Mısırlı Z, Önsan ZI. in Ni–Mo/Al2O3 catalysts: I CO2 methanation and SEM-TEM studies. Appl Catal A Gen 1998;168:385–97. [21] Hegde MS, Madras G, Patil KC. Noble metal ionic catalysts. Acc Chem Res 2009;42:704–12. [22] Zhang JY, Xin Z, Meng X, Lv YH, Tao M. Effect of MoO3 on the heat resistant performances of nickel based MCM-41 methanation catalysts. Fuel 2014;116:25–33. [23] de Resende NS, Eon JG, Schmal M. Pt–TiO2–c Al2O3 catalyst: I. Dispersion of platinum on alumina-grafted titanium oxide. J Catal 1999;183:6–13. [24] Mendoza-Nieto JA, Puente-Lee I, Salcedo-Luna C, Klimova T. Effect of titania grafting on behavior of NiMo hydrodesulfurization catalysts supported on different types of silica. Fuel 2012;100:100–9. [25] Wang K, Li XJ, Ji SF, Shi XJ, Tang JJ. Effect of CexZr1 xO2 promoter on Ni-Based SBA-15 catalyst for steam reforming of methane. Energy Fuels 2009;23:25–31. [26] Zou XJ, Wang XG, Li L, Shen K, Lu XG, Ding WZ. Development of highly effective supported nickel catalysts for pre-reforming of liquefied petroleum gas under low steam to carbon molar ratios. Int J Hydrogen Energy 2010;35:12191–200. [27] Koo KY, Roh HS, Seo YT, Seo DJ, Yoon WL, Bin Park S. A highly effective and stable nano-sized Ni/MgO–Al2O3 catalyst for gas to liquids (GTL) process. Int J Hydrogen Energy 2008;33:2036–43.
163
[28] Yang J, Wang XG, Li L, Shen K, Lu XG, Ding WZ. Catalytic conversion of tar from hot coke oven gas using 1-methylnaphthalene as a tar model compound. Appl Catal B Environ 2010;96:232–7. [29] Razzaq R, Zhu HW, Jiang L, Muhammad U, Li CS, Zhang SJ. Catalytic methanation of CO and CO2 in coke oven gas over Ni–Co/ZrO2–CeO2. Ind Eng Chem Res 2013;52:2247–56. [30] Luo LT, Wang MW, Li FY, Wang JJ. Effect of La2O3 on the methanation of CO and CO2 over Ni–Mo/c-Al2O3 catalyst. J Chin Rare Earth Soc 1999;17:120–4. [31] Zhang JY, Xin Z, Meng X, Lv YH, Tao M. Effect of MoO3 on structures and properties of Ni–SiO2 methanation catalysts prepared by the hydrothermal synthesis method. Ind Eng Chem Res 2013;52:14533–44. [32] Panagiotopoulou P, Kondarides DI, Verykios XE. Selective methanation of CO over supported noble metal catalysts: effects of the nature of the metallic phase on catalytic performance. Appl Catal A Gen 2008;344:45–54. [33] Bhaduri S, Bhaduri S, Zhou E. Auto ignition synthesis and consolidation of Al2O3–ZrO2 nano/nano composite powders. J Mater Res 1998;13:156–65. [34] Zhang ZL, Verykios XE. Carbon dioxide reforming of methane to synthesis gas over supported Ni catalysts. Catal Today 1994;21:589–95.
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Highly efficient NiAl2O4-free Ni/c-Al2O3 catalysts prepared by solution combustion method for CO methanation Yan Zeng, Hongfang Ma ⇑, Haitao Zhang, Weiyong Ying, Dingye Fang Engineering Research Center of Large Scale Reactor Engineering and Technology, Ministry of Education, State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Ti-doped Ni/c-Al2O3 catalysts were
prepared by solution combustion method for CO methanation. Ti species suppressed forming NiAl2O4 and thus enhanced utilization of Ni species. More exposed active surfaces of Tidoped catalyst increased CO adsorption capacity. Electron transfer from TiOx improved the catalytic performance of Ti-doped catalysts.
a r t i c l e
i n f o
Article history: Received 26 May 2014 Received in revised form 31 July 2014 Accepted 2 August 2014 Available online 13 August 2014 Keywords: NiAl2O4 spinel Ni/c-Al2O3 CO methanation Titanium Solution combustion method
a b s t r a c t Ti-doped Ni/c-Al2O3 catalysts with Ti loadings varying from 0 wt% to 5 wt% were prepared in glycol by solution combustion method for CO methanation. Compared to Ni/c-Al2O3 catalyst, Ti-doped Ni/c-Al2O3 catalysts exhibited higher activity. In particular, an optimal Ti-doped Ni/c-Al2O3 catalyst with Ti content of 3 wt% (G-3Ti) achieved almost 100% CO conversion and 98.7% CH4 selectivity at 300 °C, 0.1 MPa, and a WHSV of 12,000 mL g 1 h 1, and showed good stability at 600 °C. On analysis of characterization results, Ti species was found effectively restrict the formation of NiAl2O4 spinel phase, leading to a higher utilization of Ni species and thus more exposed active surfaces, which enhanced the CO adsorption capacity. In addition, the electron cloud density of Ni was increased by electron transfer from TiOx, which could facilitate the dissociation of CO on the catalyst surfaces. Moreover, solvents (i.e., ethanol, n-propanol and glycol) also significantly affected the physical–chemical properties of the catalysts, and catalyst prepared in glycol exhibited better catalytic performance for CO methanation. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Natural gas has been recognized as a promising energy source, but reserves poorly in some regions of the world. CO methanation as a useful technology to synthesize natural gas from syngas, has ⇑ Corresponding author. Tel./fax: +86 21 6425 2192. E-mail address: [email protected] (H. Ma). http://dx.doi.org/10.1016/j.fuel.2014.08.003 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved.
attracted much attention from both academia and industry in the last decade [1,2]. Since Sabatier and Senderens’ pioneering work in 1902 on discovering methane formation by the reaction of carbon monoxide and hydrogen over a nickel catalyst [3], great efforts have been made to produce high-efficiency catalysts retaining high activity at low-temperature (ca. 300 °C) and good stability at hightemperature (ca. 600 °C). Nickel supported on alumina is the most commonly used catalyst for CO methanation due to its good
156
Y. Zeng et al. / Fuel 137 (2014) 155–163
catalytic performance and relatively low price [4,5]. However, Ni/ c-Al2O3 catalyst prepared by conventional methods, such as coprecipitation, impregnation, sol–gel and mechanical mixing, showed low activity at low-temperature, and deactivated easily at hightemperature [6]. Moreover, nickel and alumina easily form NiAl2O4 spinel phase which is difficult to reduce even at high-temperature [7], leading to relatively low catalytic performance [8]. In our previous work [9], Solution combustion method (SCM) was employed to prepare Ni-based catalysts for CO methanation, which enhanced the low-temperature activity and high-temperature stability. With the advantage of economically viable, large-scale production, fast and simple technique, SCM has been widely used for the preparation of porous materials generally used as catalysts [10–12]. Whereas, Ni/c-Al2O3 catalysts prepared by SCM also easily form NiAl2O4 spinel because of the rapid heating and high-temperature during the combustion process. If the formation of NiAl2O4 could be suppressed, the catalytic activity of Ni/c-Al2O3 catalysts would be improved a lot. The formation of NiAl2O4 spinel could be attributed to the strong interaction between NiO and c-Al2O3, and easily happens at relatively low-temperature (400–450 °C). It was reported that reducing calcination temperature was a useful method to decrease the amount of NiAl2O4 spinel [13,14]. However, since CO methanation is an intensely exothermic reaction and commonly operated in the temperature range of 260–600 °C [15], catalysts prepared at low-temperature would deactivate easily at high-temperature. Interestingly, it was also reported that the addition of a second metal could decrease the strong metal-support interaction (SMSI) [16–22]. For instance, by grafting Ti on the surface of c-Al2O3, the linkage of the titanium oxide sublayer to the alumina support stabilized the catalyst against the strong interaction between Pt and c-Al2O3 [23,24]. Thus, adding a second metal to weaken the interaction between NiO and c-Al2O3 would be another efficient
Fig. 1. XRD patterns of (a) calcined samples, and (b) pure c-Al2O3 and reduced samples.
method to lower down the formation of NiAl2O4 spinel. To the best of our knowledge, few researches related to the developing of NiAl2O4-free Ni/c-Al2O3 catalyst in a facile and efficient way has been reported. In this work, we incorporated Ti species in Ni/c-Al2O3 catalysts and aim to inhibit the formation of NiAl2O4 spinel phase. A series of Ni/c-Al2O3 catalysts with Ti loadings varying form 0 wt% to 5 wt% were synthesized via SCM for CO methanation. Ti-doped Ni/cAl2O3 catalyst was more active than the conventional Ni/c-Al2O3 catalyst. The fresh as well as the used catalysts were characterized by XRD, H2-TPR, HRTEM, CO-TPD, XPS, and N2 adsorption–desorption. The possible mechanism for promotional effect of Ti species was proposed. Moreover, the effect of solvents (i.e., ethanol, n-propanol and glycol) on the physical–chemical properties and catalytic performance of the Ti-doped Ni/c-Al2O3 catalysts was also studied.
2. Experimental 2.1. Catalyst preparation Ti-doped Ni/c-Al2O3 catalysts, with constant Ni loading of 30 wt% and Ti content form 0 wt% to 5 wt%, were prepared by SCM in different combustible solution (ethanol, n-propanol and glycol). In a typical experiment, certain amounts of Ni(NO3)26H2O and Al(NO3)39H2O were dissolved in 200 mL combustible solution.
Fig. 2. (a) H2-TPR profiles for calcined G-0Ti and G-1Ti, (b) CO-TPD patterns for reduced G-0Ti and G-1Ti, and (c) XPS spectra of Ni 2p3/2 for calcined G-0Ti and G1Ti.
Y. Zeng et al. / Fuel 137 (2014) 155–163
Then, Ti(OC4H9)4 (TBOT), in accordance with the final catalyst composition requirements, was added dropwise to the mixture solution under continuously vigorous stirring, and 2 mol L 1 HNO3 was simultaneously introduced into the above slurry to maintain it transparent. After the mixture placed in drying oven with constant temperature of 70 °C for 6 h, the concentrated viscous solution transferred to an open china crucible was heated to 700 °C under static air in a muffle furnace with the heating rate of 2 °C min 1 (combustion occurred spontaneously during the heating process), and then kept at 700 °C for 7 h. The as-obtained catalysts were denoted as M–xTi, in which M represented the solvents (M = E, P, G, and E = ethanol, P = n-propanol, G = glycol) and x indicated the Ti weight content (x = 0, 1, 3, 5). For comparison, pure c-Al2O3 was also prepared by SCM in glycol. 2.2. Characterization of catalysts Nitrogen adsorption–desorption isotherms were conducted on by ASAP 2020 (Micromeritics, USA). Specific surface areas were measured by Brunauer Emmet Teller (BET) method. Total pore volume and average pore diameter were evaluated using the standard Barrett–Joyner–Halenda (BJH) treatment. Powder X-ray diffraction (XRD) patterns were carried out on a Rigaku D/Max2550VB/PC Xray diffractometer (Rigaku, Japan) with Cu Ka radiation
157
(c = 1.54056 Å). Hydrogen temperature programmed reduction (H2-TPR) and carbon monoxide temperature programmed desorption (CO-TPD) were conducted on AutoChemII2920 (Micromeritics, USA). High-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and elemental mapping were conducted on transmission electron microscope apparatus (JEOL JEM-2100, Japan) with an accelerating voltage of 200 kV, fitted with an energy dispersive X-ray detector (EDX, Oxford-INCA). The binding energy of Ni was analyzed by the X-ray photo-electron spectra (XPS) measurement by an ESCALAB 250Xi system (Thermo Fisher Scientific, UK) with Al Ka radiation (hm = 1486.6 eV), and calibrated by referencing to the C 1s peak form the contaminated carbon that was assumed to have a binding energy of 284.6 eV. Thermogravimetric analysis (TG) was conducted on a Q600 analyzer (TA Instruments, USA) in a temperature range of room temperature to 800 °C at a heating rate of 10 °C min 1 in air flow of 100 mL min 1. 2.3. Catalytic evaluation Syngas (carbon oxides and hydrogen) methanation reaction was carried out in a fixed-bed reactor (U14 2 500 mm). Inert Al2O3 was added in both ends of the uniform temperature zone to keep the catalyst in the thermostatic area. Corresponding to a weight
Fig. 3. (a) HRTEM of reduced G-1Ti with d (1 1 1) fringes at 0.203 nm, and (b) SAED patterns of reduced G-1Ti.
158
Y. Zeng et al. / Fuel 137 (2014) 155–163
hourly space velocity (WHSV) of 12,000 mL g 1 h 1, 500 mg catalyst was used with the total gas flow rate of 100 mL min 1. The catalyst was heated from ambient temperature to 700 °C in nitrogen flow within 300 min and reduced by pure hydrogen for 3 h. After the catalyst was cooled to reaction temperature under nitrogen flow, the syngas (17.3 vol.% CO, 9.5 vol.% CO2, 67.7 vol.% H2, 5.5 vol.% N2) was introduced to investigate the low-temperature catalytic activity of Ni-based catalysts at 260–340 °C, 0.1 MPa and 12,000 mL g 1 h 1. High-temperature stability test was conducted over G-0Ti and G-3Ti at 600 °C, 2 MPa, 12,000 mL g 1 h 1 and with 30 mol% water.
3. Results and discussion 3.1. Catalyst characterization 3.1.1. XRD XRD patterns of calcined and reduced samples are shown in Fig. 1a and b, respectively. For the calcined samples, the diffraction peaks at 19.1°, 31.4°, 37.2°, 45.2°, 59.7°and 65.8° are attributed to NiAl2O4 (JCPDS 10-0339), and the diffraction peaks at 37.3°, 43.3°, 62.9°, 75.5° and 79.4° derive from NiO (JCPDS 47-1049). For the
reduced samples, the diffraction peaks at 37.3°, 45.6° and 67.0° are attributed to c-Al2O3 (JCPDS 10-0425), and the diffraction peaks at 44.5°, 51.8° and 76.3° are attributed to distinct peaks of Ni (JCPDS 04-0850). There is, however, no diffraction peaks attributed to Ti species appearing in both Fig. 1a and b, indicating that Ti species may highly disperse in the catalysts. As shown in Fig. 1a, the intensity of NiAl2O4 diffraction peaks in G-0Ti is obviously higher than that in G-1Ti, implying that the addition of Ti can inhibit forming NiAl2O4 to some extent. For the two reduced catalysts in Fig. 1b, distinct peaks for NiAl2O4 spinel still existed in G-0Ti, while no obvious NiAl2O4 diffraction peaks assigned to NiAl2O4 can be found in G-1Ti, which may be due to the fully reduction of Ni oxides in G-1Ti. To further understand this, we characterized the samples by TPR and HRTEM. 3.1.2. H2-TPR H2-TPR profiles of the calcined catalysts are shown in Fig. 2a. The low-temperature peaks (<500 °C) are attributed to the reduction of free Ni oxides species (a-type), which have weak interaction with the support, whereas the mid-temperature peaks (500–800 °C) are assigned to the reduction of NiO retaining stronger interaction with the support (b-type) [9,25]. The high-temperature peaks (>800 °C) are ascribed to NiAl2O4 with a spinel
Fig. 4. Typical elemental maps of the reduced G-1Ti.
Y. Zeng et al. / Fuel 137 (2014) 155–163
159
structure (c-type), which is the most difficult to reduce [26–28]. Obviously, as compared with G-0Ti, the reduction peaks of G-1Ti shifted to low-temperature side. This result suggested that the introduction of Ti can effectively weaken the interaction between Ni and c-Al2O3, and thus improved the reducibility of Ni oxides species. Moreover, the c-type NiO species was the dominating part in G-0Ti, while only a small shoulder peak possibly attributed to the c-type NiO species could be observed in G-1Ti. It indicated that the addition of Ti effectively resisted forming NiAl2O4 spinel. Clearly, the conclusion agreed with XRD results well.
the reactants. CO-TPD technology was therefore employed to obtain more insights. As shown in Fig. 2b, the amount of desorbed CO of G-1Ti was much more than that of G-0Ti, indicating that G1Ti had more accessible active surfaces. Moreover, peaks observed at low-temperature (<300 °C) represent desorption at the low activation energy sites, which contribute little toward methanation, while the relatively high-temperature desorption peaks (300– 600 °C) significantly contribute toward methanation [29]. Obviously, the amount of desorbed CO at high-temperature of G-1Ti was more than that of G-0Ti, suggesting a higher activity for G-1Ti.
3.1.3. HRTEM HRTEM image of reduced G-1Ti is shown in Fig. 3a. A planar lattice of 0.203 nm can be observed, which could be assigned to the Ni (1 1 1) face. The results of SAED (Fig. 3b) further confirm this, in which the four rings corresponded to Ni (1 1 1), (2 0 0), (2 2 0) and (3 1 1), respectively. Notably, NiAl2O4 spinel could not be identified in both characterizations, which indicated the absence of NiAl2O4 spinel in G-1Ti, in accordance with the results of both XRD and H2-TPR. The effect of Ti species on resisting the formation of NiAl2O4 spinel may be attributed to that Ni and Al particles were scattered by Ti species, which obstructed the immediate strong interaction between Ni and Al particles. This hypothesis was studied in more detail by mapping. Typical elemental maps of reduced G-1Ti are presented in Fig. 4a–e. As can be clearly deduced from the mapping, Ni and Al particles were spatially isolated by Ti species.
3.1.5. XPS XPS analysis was employed to further study the oxidative state of Ni species, and the XPS spectra of Ni 2p3/2 for calcined G-0Ti and G-1Ti are shown in Fig. 2c. The binding energy of Ni 2p3/2 was 856.9 eV for G-0Ti, and shifted to 856.3 eV for G-1Ti. The result indicated that an interaction may occur between Ni and Ti species in combustion process, and the decrease binding energy may be due to the increase electron cloud density of Ni atoms (Ni0 ? Nid ), which could be explained by electron transfer from Ti4+/Ti3+ redox couples in the catalyst [21]. Previous results [30,31] indicated that the decrease binding energy of Ni 2p3/2 can promote CO dissociation because of the enhanced Ni–C bond and subdued C–O bond. Combining the XPS result with previous results, the addition of Ti species could improve CO dissociation because of electron transfer from TiOx to Ni atoms. 3.2. Evaluation of the catalytic behavior for G-0Ti and G-1Ti
3.1.4. CO-TPD The fully reduction of Ni oxides in G-1Ti improved the utilization of Ni species, which may expose more accessible surfaces to
CO conversion versus temperature profiles over G-0Ti and G-1Ti are shown in Fig. 5a. The initial CO conversions at 260 °C of G-0Ti
Fig. 5. (a) CO conversion and (b) CH4 selectivity of G-0Ti and G-1Ti at 0.1 MPa and 12,000 mL g 1 h 1 in the temperature range of 260–340 °C.
Fig. 6. Effect of solvents on (a) CO conversion and (b) CH4 selectivity at 0.1 MPa and 12,000 mL g 1 h 1 in the temperature range of 260–340 °C.
160
Y. Zeng et al. / Fuel 137 (2014) 155–163
and G-1Ti were 38.5% and 53.5%, respectively. A similar CH4 selectivity for both G-0Ti and G-1Ti in the measured temperature range is shown in Fig. 5b, although Ti-doped G-1Ti showed slightly higher selectivity. Moreover, CO was completely converted over G-1Ti below 310 °C, while 100% CO conversion can be achieved over G-0Ti at the temperature higher than 320 °C. The higher activity of G-1Ti could be attributed to two reasons: (1) the addition of Ti species improved the utilization of Ni species by resisting the formation of NiAl2O4 spinel, which promoted CO adsorption capacity, and (2) the increased electron cloud density of Ni facilitated the dissociation of CO on the catalyst surfaces. Since CO dissociation is the rate-determining step in CO methanation [32], the enhanced
CO dissociation ability of G-1Ti can explain its relatively high catalytic activity. To optimize the Ti-doped Ni/c-Al2O3 catalyst, we also studied the influence of solvents and Ti loadings on catalytic performance, as discussed below. 3.3. Effect of solvents CO conversions versus temperature profiles over E-1Ti, P-1Ti, and G-1Ti are shown in Fig. 6a. It can be observed that G-1Ti prepared in glycol retained much higher CO conversion and CH4 selectivity (see Fig. 6b) than the others. The higher activity of G-1Ti was
Fig. 7. TEM images of (a) E-1Ti, (b) P-1Ti, and (c) G-1Ti, (d) particle size distribution, and (e) CO-TPD of the three catalysts.
Y. Zeng et al. / Fuel 137 (2014) 155–163
possibly because of its better distribution of Ni particles (see Fig. 7a–d) and higher CO adsorption ability (see Fig. 7e). In addition, solvents also affected the structure of the catalysts (see Table 1). The various porous properties may be attributed to that the different exothermicity of solvents resulted in a diverse gases evolution rate and volume during combustion process. Bhaduri et al. [33] also found that the evolution of gases affected the structure of porous catalysts. 3.4. Effect of Ti loadings CO conversions of Ni–Al2O3 catalysts with various Ti loadings are presented in Fig. 8a. CO conversions increased with the Ti loadings Table 1 Surface area, pore volume, average pore size, and Ni particle size for samples. Catalyst d
E-1Ti P-1Tid G-1Tid G-0Tie G-3Tie G-0Tif G-3Tif a b c d e f
SBET (m2 g 184.8 201.4 124.7 139.5 116.8 143.9 118.2
1
)
Vpa (cm3 g 0.08 0.27 0.48 0.30 0.51 0.28 0.47
1
)
Dpb (nm)
DNic (nm)
3.1 3.9 13.4 9.0 13.9 8.3 13.5
– – – 21.7 21.6 21.9 21.6
BJH desorption pore volume. BJH desorption average pore size. Calculated from Ni (2 0 0) plane by Scherrer’s equation. Fresh calcined samples. Catalysts before 150 h lifetime test. Catalysts after 150 h lifetime test.
Fig. 8. Effect of Ti loadings on (a) CO conversion and (b) CH4 selectivity at 0.1 MPa and 12,000 mL g 1 h 1 in the temperature range of 260–340 °C.
161
rising from 0 wt% to 3 wt%, and then decreased significantly with further increase to 5 wt%. CO conversion of G-3Ti achieved 100% at 300 °C, while that of G-0Ti, G-1Ti and G-5Ti were 87.7%, 92.1% and 84.8%, respectively. However, all the four catalysts retained a similar CH4 selectivity in the measured temperature range as shown in Fig. 8b. XPS and CO-TPD were conducted to obtain more insights. As shown in Fig. 9a, the binding energy of Ni 2p3/2 only slightly shifted to lower side with the increase of Ti content, indicating that further increase of Ti content did not significantly impact on CO dissociation. However, it can be seen from Fig. 9b that Ti content observably affected the CO adsorption ability of the Ti-doped catalysts. The CO adsorption ability first increased as Ti content varied from 1% to 3%, and then decreased dramatically with further increase of Ti content to 5%. The improved CO adsorption ability of G-3Ti implied more accessible active surfaces, which may be the main reason for its optimal catalytic activity. Moreover, the lower CO adsorption ability of G-5Ti may be due to the partial coverage of the active metal by excessive TiOx, possibly explaining its lower catalytic activity.
3.5. Stability test The catalytic stability is an important indicator of the catalyst performance, and it is related to the industrial application. The catalytic stability test was conducted on the optimal G-3Ti at 600 °C, 2 MPa and 12,000 mL g 1 h 1. For comparison, that of G-0Ti was also investigated at the same condition. Fig. 10a presents the catalytic activity in terms of CO conversion over G-3Ti and G-0Ti. CO conversions of G-3Ti and G-0Ti were 93.3% and 92.7%, respectively, and almost remained unchanged during the 150 h stability test. The good stability of both G-0Ti and G-3Ti could be attributed
Fig. 9. (a) XPS spectra of Ni 2p3/2 for calcined samples, and (b) CO-TPD patterns for reduced samples.
162
Y. Zeng et al. / Fuel 137 (2014) 155–163
more exposed accessible surfaces enhanced the CO adsorption capacity. Electron transfer from TiOx increased electron cloud density of Ni atoms, which can promote CO dissociation on the catalyst surfaces, leading to a relatively high catalytic performance. Catalytic activity increased with the Ti loadings rising from 0 wt% to 3 wt%, and then decreased significantly with further increase to 5 wt%, which could be due to the partial coverage of the active metal by excessive TiOx. G-3Ti with Ti content of 3 wt%, as the most potential catalyst, achieved almost 100% CO conversion and 98.7% CH4 selectivity at 300 °C, 0.1 MPa, and a WHSV of 12,000 mL g 1 h 1. Moreover, both Ti-free G-0Ti and Ti-doped G3Ti showed good stability during the life-time tests. In addition, solvents significantly affected the physical–chemical properties of the catalysts, and catalyst prepared in glycol exhibited better catalytic performance for CO methanation. It is expected that this strategy can be extended for the fabrication of other metal heterogeneous catalyst with significantly improved behavior.
Acknowledgment This work is financially supported by the National Science and Technology Supporting Plan (2012AA050102).
References
Fig. 10. (a) Stability test over G-0Ti and G-3Ti at 600 °C, 2 MPa, and 12,000 mL g 1 h 1, (b) XRD patterns, and (c) TG curves of G-0Ti and G-3Ti before and after 150 h stability test.
to sintering- and coking-resistant. Nitrogen adsorption–desorption and XRD were conducted to obtain more insight into the physicochemical properties of the samples before and after lifetime test. It can be seen from Table 1 and Fig. 10b that no obviously change of textural properties and Ni particle size can be found in both G-0Ti and G-3Ti after lifetime test. The results indicated that the catalysts prepared by SCM retained good resistance to catalysts sintering, which may be attributed to that Ni particles were scattered and spatially isolated by Al2O3, thus preventing the sintering of Ni particles [9]. Moreover, TG profiles of G-0Ti and G-3Ti before and after lifetime test are shown in Fig. 10c. Weight losses attributed to carbon deposition (the difference before and after lifetime test) over G-0Ti and G-3Ti were 0.5% and 0.8%, respectively, suggesting that both G-0Ti and G-3Ti showed good ability to resist carbon deposition. In addition, the weight increment in the temperature range of 300–500 °C was ascribed to the oxidation of metallic nickel species [34]. 4. Conclusions Ni/c-Al2O3 catalysts with Ti contents ranging from 0 wt% to 5 wt% were prepared in different solvents by SCM for CO methanation. The addition of titanium improved the utilization of Ni species by inhibiting the formation of NiAl2O4 spinel phase, and the
[1] Xavier KO, Sreekala R, Rashid KKA, Yusuff KKM, Sen B. Doping effects of cerium oxide on Ni/Al2O3 catalysts for methanation. Catal Today 1999;49:17–21. [2] Derekaya FB, Yasßar G. The CO methanation over NaY-zeolite supported Ni/ Co3O4, Ni/ZrO2, Co3O4/ZrO2 and Ni/Co3O4/ZrO2 catalysts. Catal Commun 2011;13:73–7. [3] Sabatier P, Senderens JB. Hydrogenation of CO over nickel to produce methane. J Soc Chim Ind 1902;21:504–6. [4] Pan ZY, Dong MH, Meng XK, Zhang XX, Mu XH, Zong BN. Integration of magnetically stabilized bed and amorphous nickel alloy catalyst for CO methanation. Chem Eng Sci 2007;62:2712–7. [5] Zhang WD, Liu BS, Zhu C, Tian YL. Preparation of La2NiO4/ZSM-5 catalyst and catalytic performance in CO2/CH4 reforming to syngas. Appl Catal A Gen 2005;292:138–43. [6] Inui T, Hagiwara T, Takegami Y. Prevention of catalyst deactivation caused by coke formation in the methanation of carbon oxides. Fuel 1982;61:537–41. [7] Cai MD, Wen J, Chu W, Cheng XQ, Li ZJ. Methanation of carbon dioxide on Ni/ ZrO2–Al2O3 catalysts: effects of ZrO2 promoter and preparation method of novel ZrO2–Al2O3 carrier. J Nat Gas Chem 2011;20:318–24. [8] Patil KC, Aruna ST, Mimani T. Combustion synthesis: an update. Curr Opin Solid State Mater Sci 2002;6:507–12. [9] Zhao AM, Ying WY, Zhang HT, Ma HF, Fang DY. Ni–Al2O3 catalysts prepared by solution combustion method for syngas methanation. Catal Commun 2012;17:34–8. [10] Sharma S, Hu Z, Zhang P, McFarland EW, Metiu H. CO2 methanation on Rudoped ceria. J Catal 2011;278:297–309. [11] Lathika Devi SK, Sudarsana Kumar K, Balakrishnan A. Rapid synthesis of pure and narrowly distributed Eu doped ZnO nanoparticles by solution combustion method. Mater Lett 2011;65:35–7. [12] Prakash AS, Shivakumara C, Hegde MS. Single step preparation of CeO2/ CeAlO3/c-Al2O3 by solution combustion method: phase evolution, thermal stability and surface modification. Mater Sci Eng B 2007;139:55–61. [13] Cesteros Y, Salagre P, Medina F, Sueiras JE. Preparation and characterization of several high-area NiAl2O4 spinels. Study of their reducibility. Chem Mater 2000;12:331–5. [14] Rynkowski JM, Paryjczak T, Lenik M. On the nature of oxidic nickel phases in NiO/c-Al2O3 catalysts. Appl Catal A Gen 1993;106:73–82. [15] Kopyscinski J, Schildhauer TJ, Biollaz SMA. Production of synthetic natural gas (SNG) from coal and dry biomass – a technology review from 1950 to 2009. Fuel 2010;89:1763–83. [16] Yang XZ, Wang X, Gao GJ, Wendurima, Liu EM, Shi QQ, et al. Zhang JN, Han CH, Wang J, Lu HL, Liu J, Tong M. Nickel on a macro-mesoporous Al2O3@ZrO2 core/ shell nanocomposite as a novel catalyst for CO methanation. Int J Hydrogen Energy 2013;38:13926–37. [17] Macleod N, Cropley R, Keel JM, Lambert RM. Exploiting the synergy of titania and alumina in lean NOx reduction: in situ ammonia generation during the Pd/ TiO2/Al2O3-catalysed H2/CO/NO/O2 reaction. J Catal 2004;221:20–31. [18] Macleod N, Lambert RM. Low-temperature NOx reduction with H2+CO under oxygen-rich conditions over a Pd/TiO2/Al2O3 catalyst. Catal Commun 2002;3:61–5. [19] Surisetty VR, Eswaramoorthi I, Dalai A. Comparative study of higher alcohols synthesis over alumina and activated carbon-supported alkali-modified MoS2 catalysts promoted with group VIII metals. Fuel 2012;96:77–84.
Y. Zeng et al. / Fuel 137 (2014) 155–163 _ Interaction between nickel and molybdenum [20] Aksoylu AE, Mısırlı Z, Önsan ZI. in Ni–Mo/Al2O3 catalysts: I CO2 methanation and SEM-TEM studies. Appl Catal A Gen 1998;168:385–97. [21] Hegde MS, Madras G, Patil KC. Noble metal ionic catalysts. Acc Chem Res 2009;42:704–12. [22] Zhang JY, Xin Z, Meng X, Lv YH, Tao M. Effect of MoO3 on the heat resistant performances of nickel based MCM-41 methanation catalysts. Fuel 2014;116:25–33. [23] de Resende NS, Eon JG, Schmal M. Pt–TiO2–c Al2O3 catalyst: I. Dispersion of platinum on alumina-grafted titanium oxide. J Catal 1999;183:6–13. [24] Mendoza-Nieto JA, Puente-Lee I, Salcedo-Luna C, Klimova T. Effect of titania grafting on behavior of NiMo hydrodesulfurization catalysts supported on different types of silica. Fuel 2012;100:100–9. [25] Wang K, Li XJ, Ji SF, Shi XJ, Tang JJ. Effect of CexZr1 xO2 promoter on Ni-Based SBA-15 catalyst for steam reforming of methane. Energy Fuels 2009;23:25–31. [26] Zou XJ, Wang XG, Li L, Shen K, Lu XG, Ding WZ. Development of highly effective supported nickel catalysts for pre-reforming of liquefied petroleum gas under low steam to carbon molar ratios. Int J Hydrogen Energy 2010;35:12191–200. [27] Koo KY, Roh HS, Seo YT, Seo DJ, Yoon WL, Bin Park S. A highly effective and stable nano-sized Ni/MgO–Al2O3 catalyst for gas to liquids (GTL) process. Int J Hydrogen Energy 2008;33:2036–43.
163
[28] Yang J, Wang XG, Li L, Shen K, Lu XG, Ding WZ. Catalytic conversion of tar from hot coke oven gas using 1-methylnaphthalene as a tar model compound. Appl Catal B Environ 2010;96:232–7. [29] Razzaq R, Zhu HW, Jiang L, Muhammad U, Li CS, Zhang SJ. Catalytic methanation of CO and CO2 in coke oven gas over Ni–Co/ZrO2–CeO2. Ind Eng Chem Res 2013;52:2247–56. [30] Luo LT, Wang MW, Li FY, Wang JJ. Effect of La2O3 on the methanation of CO and CO2 over Ni–Mo/c-Al2O3 catalyst. J Chin Rare Earth Soc 1999;17:120–4. [31] Zhang JY, Xin Z, Meng X, Lv YH, Tao M. Effect of MoO3 on structures and properties of Ni–SiO2 methanation catalysts prepared by the hydrothermal synthesis method. Ind Eng Chem Res 2013;52:14533–44. [32] Panagiotopoulou P, Kondarides DI, Verykios XE. Selective methanation of CO over supported noble metal catalysts: effects of the nature of the metallic phase on catalytic performance. Appl Catal A Gen 2008;344:45–54. [33] Bhaduri S, Bhaduri S, Zhou E. Auto ignition synthesis and consolidation of Al2O3–ZrO2 nano/nano composite powders. J Mater Res 1998;13:156–65. [34] Zhang ZL, Verykios XE. Carbon dioxide reforming of methane to synthesis gas over supported Ni catalysts. Catal Today 1994;21:589–95.