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ZrO2 samples: ZrO2 crystalline phase & treatment impact
Journal of Energy Chemistry 25 (2016) 1070–1077
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CO2 selective hydrogenation to synthetic natural gas (SNG) over four nano-sized Ni/ZrO2 samples: ZrO2 crystalline phase & treatment impact Min Chen a,b, Zhanglong Guo a, Jian Zheng c, Fangli Jing a,b, Wei Chu a,b,∗ a b c
School of Chemical Engineering, Sichuan University, Chengdu 610065, Sichuan, China Institute of New Energy and Low-Carbon Technology, Sichuan University, Chengdu 610225, Sichuan, China Institute for Integrated Catalysis, Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352, United States
a r t i c l e
i n f o
Article history: Received 15 August 2016 Revised 4 October 2016 Accepted 18 October 2016 Available online 16 November 2016 Keywords: Monoclinic zirconia support Nano-sized nickel catalyst CO2 -TPD-MS TPSR-CH4 CO2 selective hydrogenation
a b s t r a c t Two type zirconia (monoclinic and tetragonal phase ZrO2 ) carriers were synthesized via hydrothermal route, and nano-sized zirconia supported nickel catalysts were prepared by incipient impregnation then followed thermal treatment at 30 0 °C to 50 0 °C, for the CO2 selective hydrogenation to synthetic natural gas (SNG). The catalysts were characterized by XRD, CO2 -TPD-MS, XPS, TPSR (CH4 , CO2 ) techniques. For comparison, the catalyst NZ-W-400 (monoclinic) synthesized in water solvent exhibited a better catalytic activity than the catalyst NZ-M-400 (tetragonal) prepared in methanol solvent. The catalyst NZ-W-400 displayed more H2 absorbed sites, more basic sites and a lower temperature of initial CO2 activation. Then, the thermal treatment of monoclinic ZrO2 supported nickel precursor was manufactured at three temperature of 350, 40 0, 50 0 °C. The TPSR experiments displayed that there were the lower temperature for CO2 activation and initial conversion (185 °C) as well as the lower peak temperature of CH4 generation (318 °C), for the catalyst calcined at 500 °C. This sample contained the more basic sites and the higher catalytic activity, evidenced byCO2 -TPD-MS and performance measurement. As for the NZ-W-350 sample, which exhibited the less basic sites and the lower catalytic activity, its initial temperature for CO2 activation and conversion was higher (214 °C) as well as the higher peak temperature of CH4 formation (382 °C). © 2016 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.
1. Introduction With more greenhouse gases (GHGs) effects in agriculture, water resources and human health, carbon dioxide with rising concentration is creating a critical risk for earth’s climate system and human’s daily life [1,2]. Large amount of reports have been illustrated about the global warming with surface temperature increasing, rising the sea level, melting in glaciers and so on [3]. The energy obtained from burning fossil fuels is vital for the developing of our civilization largely and still the dominant energy source. Therefore, the using of CO2 , efficiently, will be still a potential and needing-solving problem and arouse renewed interest [3–6]. The CO2 conversion to green fuels in an environment-friendly and sustainable way is still a challenge [7–9]. More efforts have been devoted to convert CO2 to valuable high energy density organic hy-
∗ Corresponding author at: School of Chemical Engineering, Sichuan University, Chengdu 610065, Sichuan, China. Fax: +86 2885461108. E-mail address: [email protected] (W. Chu).
drocarbons (e.g. CH3 OH [10]), rather than treating it as a waste in recent decades, which is also an effective way to mitigate the greenhouse effect [11–14]. The catalytic activity of supported catalysts is among other factors influenced by the support material used [14–16]. Various supports (e.g., Al2 O3 , SiO2 , ZrO2 , CNT) have been investigated for CO2 hydrogenation to methane [7,11,12,17–19]. Recently, zirconia (ZrO2 ) has received considerable attention and applied in a variety of reactions because of its amphoteric properties as well as high thermal and unique chemical stability [20–23]. Zirconia exists primarily in three different crystalline structures depending on the preparation method: cubic (c-ZrO2 ), monoclinic (m-ZrO2 ), and tetragonal (t-ZrO2 ) [24,25]. At ambient pressure, m-ZrO2 can exists under room temperature to 1175 °C, t-ZrO2 , from 1175 °C to 2370 °C, and c-ZrO2 , from 2370 °C to 2680 °C [26]. The morphology of zirconia was found to play a vital role in different catalytic reaction [27]. Pokrovski et al. [28] investigated the different zirconia phases showing different surface catalytically active sites (Bronsted acidic and basic hydroxyl groups). Samson et al. [29] explored that zirconia presence in tetragonal phase showed
http://dx.doi.org/10.1016/j.jechem.2016.11.008 2095-4956/© 2016 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.
M. Chen et al. / Journal of Energy Chemistry 25 (2016) 1070–1077
higher activity towards methanol synthesis from CO2 since oxygen vacancies stabilizes t-ZrO2 and then enhances catalytic activity. On the other way, Yamasaki et al. [27] found that CO2 methanation reaction showed better catalytic activity at low temperatures with increasing t-ZrO2 content, which was related to more CO2 adsorption sites. However, Rhodes et al. [10] drew an opposite conclusion for a fixed Cu surface area, Cu/m-ZrO2 was more active for methanol synthesis than Cu/t-ZrO2 from a feed of 3/1 H2 /CO at 3.0 MPa and temperatures between 200 and 250 °C. Pd supported on t-ZrO2 exhibited the highest selectivity towards benzene in hydrodeoxygenation of phenol, while significant formation of cyclohexanone was observed over Pd/m-ZrO2 [20]. Nevertheless, the mesoporous amorphous zirconia is beneficial to the dry reforming of methane reaction when zirconia was in amorphous, monoclinic, and tetragonal morphology and it was said that the amorphous zirconia enhanced the Ni dispersion [30]. In our recent researches, Chu and his workers have made a lot of efforts towards green chemistry catalyst through efficient carbon capture use and storage to valuable materials [5,6,13,31–34]. The chemical vapor deposition integrated process (CVD-IP) was developed in order to better use of the CO2 . In this work, the greenhouse gas was then converted to a new solid formed production [5,13]. On the previous research, we synthesized highly dispersed zirconia supported nano-sized nickel catalysts by a facile incipient wetness impregnation method under different thermal treatments in order to convert CO2 to synthetic natural gas (SNG). In this work, the phase influence and treatment impact on CO2 selective hydrogenation to SNG were studied over Ni/ZrO2 catalysts. A variety of characterization techniques including XRD, nitrogen adsorption–desorption, HRTEM, H2 -TPR, H2 -TPD, CO2 -TPD-MS, Raman, XPS, TPSR (CH4 , CO2 ) were used to explore the structureactivity relationship. 2. Experimental 2.1. Preparation of catalysts Zirconia supports were prepared via a solvothermal method which is reported elsewhere [35]. In a typical experiment, the reaction was performed at 140 °C for 20 h in a stainless-steel autoclave (50 mL) containing solutions of 4.805 g urea (CO(NH2 )2 ; > 99.9%), 2.138 g zirconyl nitrate (ZrO(NO3 )2 ·2H2 O; > 45.0% ZrO2 ) and 20 mL solvents. The deionized water or methanol (>99.9%) were intentionally selected as solvent and marked as W or M, respectively. The resulting precipitates were recovered by filtering with water and methanol and dried overnight at 110 °C. Zirconia supported nickel (15 wt%) catalysts were prepared by incipient wetness impregnation using Ni(NO3 )2 ·6H2 O as nickel source. The support was soaked in the precursor solution by ultrasonic treatment for 2 h at room temperature and then aged for 2 h. The solvent was flowing evaporated by heating. The obtained samples were calcined at the temperatures of 350 °C, 40 0 °C, and 50 0 °C for 4 h at a rate of 2 °C/min. The as-synthesized catalysts were finally denoted as NZ-W-x (water solvents) or NZ-M-x (methanol solvents), where the x noted as the calcining temperature. 2.2. Characterization of catalyst The X-ray diffraction (XRD) analysis was performed using a Philips X’pert PRO diffraction meter, with Cu Kα (40 kV, 30 mA) radiation. Scans were made in the 2θ range of 10°–80° with a step size of 0.03°. The specific surface areas, total pore volumes, and average pore diameters were determined by nitrogen adsorption/desorption at −196°C, which were measured using an automated surface area
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& pore size analyzer (Quadrasorb SI apparatus). Before each measurement, the samples were degassed in vacuum at 120°C for 3 h. Specific surface areas of samples were calculated by the Brunauer– Emmett–Teller (BET) method. The pore size distribution and average pore diameter were determined according to the Barrett– Joyner–Halenda (BJH) method applied to desorption isotherms. The transmission electron microscope (TEM) analysis of investigated catalysts was conducted using a Libra 200 FE microscope with a 200 kV voltage. The samples reduced at 450 °C in H2 for 1 h were dispersed in ethanol assisted by ultrasonic technique and then deposited on a carbon film covered copper grid for measurement. The reduced catalysts morphology were determined via scanning electron microscopy (SEM) using an Ultra 55 SEM. The temperature-programmed reduction with hydrogen (H2 TPR) was carried out in a fixed-bed reactor. 50 mg sample was loaded, and the reduction gas of 5% H2 in N2 was introduced at a flow rate of 30 mL/min. The temperature of the reactor was raised linearly from 100 °C to 800 °C at a rate of 10 °C/min by a temperature controller. The amount of hydrogen consumed was determined on-line using a SC-200 gas chromatograph with a thermal conductivity detector (TCD). The basicity of samples was determined by the temperatureprogrammed desorption of carbon dioxide technique (CO2 -TPDMS). 200 mg of catalyst was reduced at 450°C with H2 for 1 h, and then cooled down to 50 °C in Ar. CO2 was then introduced for 1 h at 50 °C. After purging with Ar for 2 h to purge the gas line and remove weakly adsorbed CO2 , the sample was heated from 50 °C to 800 °C with a ramp of 10 °C/min and the desorbed CO2 was detected on-line by a Hiden QIC-20 mass spectrometer. The mass signal at m/z = 44 was used to determine the amount of desorbed CO2 . Temperature-programmed desorption of hydrogen (H2 -TPD) experiments were carried out on a TP-5080 auto multi adsorption analyzer. 200 mg of sample was reduced with hydrogen at 450 °C for 1 h and then cooled to 50 °C, followed by H2 adsorption at 50 °C for 2 h. Then, the samples were flashed with nitrogen flow to remove the physically adsorbed hydrogen, followed by heating the sample at a rate of 10 °C/min in nitrogen from 50 °C to 800 °C, and the H2 -TPD spectra were recorded. The X-ray photoelectron spectroscopy (XPS) data were collected on a Kratos AXIS Ultra DLD spectrometer. The charging effects were corrected by adjusting the binding energy of C 1 s peak from carbon contamination to 284.6 eV. The samples were reduced at 450°C for 1 h under H2 atmosphere. In situ temperature-programmed surface reaction (TPSR) of CO2 and H2 was achieved with a fixed-bed reactor and then inspected on-line by a Hiden QIC-20 mass spectrometer. Before TPSR experiment, four catalysts were reduced under hydrogen flow at 450°C for 1 h, and then cooled to 150 °C and reactant gases mixed with argon were poured into the fix bed. The temperature of the reactor was raised linearly to 700 °C at a rate of 10 °C /min by a temperature controller. The mass signal at m/z = 44, 2, 28 and 16 were used to determine the amount of desorbed CO2 , H2 , CO and CH4 . 2.3. Catalytic activity test Catalytic tests were carried out using a quartz reactor (6 mm i.d.). In order to measure and control the temperature, 100 mg catalyst was placed in the center part of the catalyst bed where directly contact with a thermocouple. The catalysts were reduced in hydrogen (99.99%) at 450 °C for 1 h in a flow rate 30 mL/min. The H2 /CO2 ratio was 3.6:1 and GHSV was 30,0 0 0 mL/(h·gcat ) at 1 atm. The activity was tested in the range of 240–360°C every 20 °C with a heating rate 5 °C/min. The effluent gases from the reactor were analyzed on-line using a SC-200 model gas chromatograph equipped with a TDX-01 column and a thermal conductivity
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the catalysts NZ-W-350 and NZ-W-400 exhibited some characteristic peaks at 182, 339, 379, 479, 613 cm−1 , which attributed to the monoclinic zirconia. The Raman bands at 149, 195, and 608 cm−1 were assigned to the Raman-active modes for the tetragonal phase of ZrO2 [10,37–39]. With the calcining temperature raised from 350 °C to 500 °C, the intensities of peak at 182, 339, 379 and 479 cm−1 attributed to monoclinic zirconia decreased somewhat. It suggested that the portion of m-ZrO2 in zirconia was decreasing for the catalysts prepared in water solvent with raised calcining temperatures, while the catalyst prepared in methanol solvent exhibited tetragonal phase. This conclusion was well in agreement with the phenomenon of XRD representation. Ingeneral, the supports of catalysts NZ-W-350, NZ-W-400, NZW-500 and NZ-M-400 exhibited pure monoclinic, pure monoclinic, mainly monoclinic, tetragonal phase, respectively. In addition, the nickel oxide crystalline size was also influenced by the calcined temperature. These calculated data based on Scherrer Equation are summarized in Table S1, the crystalline sizes were between 11 and 14 nm. The size of NZ-W-400, NZ-W-50 0 and NZ-M-40 0 were roughly the same. 3.2. Morphology of the reduced catalysts using TEM, SEM and nickel particle size distribution
Fig. 1. XRD patterns of catalysts calcined at 350 °C to 500 °C.
detector (TCD). To determine conversion and selectivity, the products were collected after one hour of steady-state operation. CO2 conversion, CH4 selectivity and CH4 yield were defined as follows:
XC O2 ( % ) = 1 − SCH4 ( % ) =
CO2 CO2 +CH4 +CO
× 100
CH4 ×100 CO + CH4
YCH4 = XCO2 ×SCH4
(1) (2) (3)
where, X was the conversion, S was the selectivity and Y was the yield. 3. Results and discussion 3.1. Crystal phase analysis and textural properties of four catalysts XRD patterns of catalysts calcined at 350 °C to 500 °C (fresh catalysts) were shown in Fig. 1. Catalysts calcined at different temperatures showed different structures. The diffraction peak centered at 2θ = 30.1° was attributed to t-ZrO2 (111) (JCPDS card no. 491642). Four peaks at 2θ = 24, 28.2, 31.5, 34.6 were assigned to mZrO2 (JCPDS card no.37-1484). When the calcination temperature of catalysts with zirconia supports prepared in water solvent was raised from 350 °C to 400 °C, the diffraction peaks showed typically monoclinic crystalline and increased the peak intensity. It indicated an increase in the degree of crystallinity and in the particle sizes of NiO crystallites [36]. When the calcined temperature was up to 500 °C, the catalyst NZ-W-500 showed a partial transformation of the crystalline structure from monocline to tetragonal but still displayed mainly monoclinic phase. It could be concluded that the catalyst NZ-W-500 possessed both monoclinic to tetragonal phase, i.e., a mixture of these two structures. As for the catalyst NZ-M-400, showed an obvious peak at 2θ = 30.1° and no diffraction peaks were detected at 2θ = 28.2° and 31.5°, which were assigned to t-ZrO2 (111), m-ZrO2 (−111) and (111), respectively [21,28]. In conclusion, the zirconia support prepared by methanol showed only tetragonal phase after calcining at 400 °C. The characterization of Raman spectra in Fig. S2 also confirmed this conclusion. In the Raman spectra of catalysts calcined at 350–500 °C,
The TEM images of the reduced zirconia supported nickel catalysts were shown in Fig. S3. From the TEM images, reduced catalyst NZ-W-500 (R) showed better nickel nanoparticles dispersion than the other catalysts and some aggregation could be seen for the other catalysts. This phenomenon could also be seen from the SEM images in Fig. S4, the reduced sample NZ-W-500 (R) showed irregular sponge shape, different size particles exhibit a loose distribution and did not form aggregates. In comparison, the reduced sample NZ-M-400 (R) with a uniform spherical particle showed a large agglomerate. Besides, it could be seen that the surface of reduced sample NZ-M-400 (R) is highly compact while the reduced sample NZ-W-500 (R) show a smooth surface. Particle size distributions of the catalysts measured from TEM images were presented in Fig. 2. From the HRTEM images, the reduced catalysts NZ-W-350 (R), NZ-W-400 (R) exhibited d-spacing of 0.317 nm or 0.284 nm, which attributed to m-ZrO2 (−111) or m-ZrO2 (111) phase, respectively. On the other hand, the reduced catalyst NZ-W-500 contained both d-spacing of 0.317 nm and 0.297 nm assigned to m-ZrO2 (−111) and t-ZrO2 (111) phases. However, the reduced catalyst NZ-M-400 (R) showed t-ZrO2 (111) phase only. It could be seen from HRTEM that the catalysts NZW-350 (R), NZ-W-400 (R) and NZ-W-500 (R) existed monoclinic zirconia phase, and the catalysts NZ-W-500 (R) and NZ-M-400 (R) contained tetragonal zirconia phase. The reduced catalysts NZ-W350 (R) and NZ-M-400 (R) showed the higher portion of nickel particles with diameter larger than 20 nm, especially, the catalyst NZ-W-350 (R) exhibited a wider particle size distribution from 17 to 32 nm. On the other hand, the reduced catalysts NZ-W-500 (R) and NZ-W-400 (R) showed the higher distribution of nickel particles with diameter less than 16 nm. Interestingly, the catalysts NZW-500 (R) and NZ-W-400 (R) with smaller particle size displayed better catalytic activity than catalysts NZ-W-350 (R) and NZ-M-400 (R) with bigger nickel size. 3.3. The reducibility of four samples using temperature-programmed reduction H2 -TPR experiments for the four zirconia supported nickel catalysts were carried out to investigate the reduction behavior and interaction between nickel and zirconia support. From the results in Fig. 3, four catalysts exhibited three reduction peaks in the range of 20 0–60 0 °C, which could be attributed to the reduction of three
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nickel oxide containing species [40] and without ZrO2 reduction [37]. The first peak (below 300 °C) was assigned to the reduction of free nickel oxide species [22,41]. The second peak (300–470 °C) could be assigned to the reduction of weakly bound nickel oxide with the surface of ZrO2 [42]. The third peak (470–600 °C) could be attributed to the reduction of those nickel oxide species interacted strongly with support or the Ni2+ ions penetrating into zirconia lattice [37]. From the open literature, the second peak and the third one were assumed to be related to the reduction of NiOx species that had intimate interaction with the supports [22], while the first peak was assigned to free nickel oxide small particles that could be easily reduced and then it could be deactivated as a result of its high mobility and sintering, while bound-state nickel oxide species were formed due to the better metal dispersion and stronger metal-support interaction [43–46]. From Fig. 3, as for the two catalysts synthesized by water and methanol, respectively, when they were treated at 400 °C, the catalysts showed almost the same peak temperature between 30 0–60 0 °C except a 10 °C temperature difference at the third peak. This indicated that the two catalysts had almost the same metal-support interaction. With the calcining temperature increased from 350 to 500 °C, the peak temperature of the catalysts with zirconia prepared through water solvent had a positive shift to higher temperature, which indicated that the catalysts had an increasing stronger metal-support interaction. It was said that higher metal–support interaction presented in catalyst derived from higher active metal particle dispersion, which could be identified by H2 -TPD (Fig. S5) and XPS (Fig. 6) in our work. In the further characterization of H2 -TPD, the relatively much stronger intensity of NZ-W-500 and NZ-W-400 in comparison with that of NZ-W-350 and NZ-M-400, identified that the NZ-W-500 and NZ-W-400 samples have a higher nickel dispersion and more surface associated hydrogen could be generated on these samples [47]. 3.4. Catalytic performances and TPSR results analysis
Fig. 2. The HRTEM of (a) NZ-W-350 (R), (b) NZ-W-400 (R), (c) NZ-W-50 (R), and (d) NZ-M-400 (R); and the distribution of Ni nanoparticles of (e) NZ-W-350 (R), (f) NZ-W-400 (R), (g) NZ-W-500 (R) and (h) NZ-M-400 (R) of reduced catalysts.
Fig. 3. H2 -TPR profiles of four nickel catalysts.
The CO2 conversion and CH4 selectivity versus temperature under methanation conditions were studied and the results were shown in Fig. 4. It could be observed that NZ-W-500 exhibited much higher CO2 conversion than the other catalysts, especially at low temperature range. The CO2 conversion exceptionally underwent a fast leap from 26% at 240 °C to 80% at 280 °C for the NZ-W-500 catalyst. It increased slowly afterward when the temperature continued going up to 360 °C. As for catalysts based on pure monoclinic and tetragonal ZrO2 , the CO2 conversion was relatively low and the CO2 conversion only 57% and 68% for NZ-W-350 and NZ-M-400, respectively. It was worthy pointing out that the samples NZ-W-350 and NZ-W-500 showed as high as 100% selectivity of methane till the reaction temperature reach 340 °C, while there was significant difference for CO2 conversion. On the other side, NZ-M-400 showed a sharp drop of CH4 selectivity when the temperature was up to 300 °C. The differences in catalytic activity at low temperature were mainly due to crystalline phase effect, which leaded to different physical chemical properties. When the reaction temperature raised to 300 °C and above, the catalytic activity of catalysts NZ-W-500 and NZ-W-400 reached the thermodynamic equilibrium (calculated through Aspen Plus) and shifted to the maximum activity [48–50]. However, the catalytic activity of catalyst NZ-M-400 showed an upward trend since an increasing activated CO2 molecules converted to CO at higher temperature [51,52]. In order to further investigate the surface reaction of CO2 selective hydrogenation to synthetic natural gas (SNG), the CH4 and CO2 -TPSR were conducted as displayed in Fig. 5. The TPSR results provided more details of the start temperature points of CH4 formation and CO2 consumption. Since CO2 methanation was a
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100.0
(a)
CO 2 conversion (%)
80 60 40 NZ-W-350 NZ-W-400 NZ-W-500 NZ-M-400 Equilibrium
20 0 240
280
320
360
Selecticity of CH4 (%)
100
(b)
99.6
99.2
98.8
240
o
NZ-W-350 NZ-W-400 NZ-W-500 NZ-M-400
280
320
360
o
Temperature ( C)
Temperature ( C)
Fig. 4. (a) Conversion of CO2 versus temperature, (b) the selectivity of CH4 of four samples. Reaction condition: 101.3 kPa, GHSV = 30,0 0 0 mL/(h·gcat ), H2 /CO2 molar ratio = 3.6, catalyst 100 mg.
(a)
(b)
Fig. 5. (a) CH4 -TPSR, (b) CO2 -TPSR profiles of four catalysts.
Table 1. The TPSR results of CO2 activation and methane formation. Sample
The start point of CO2 consumption (°C)
NZ-W-350 NZ-W-400 NZ-W-500 NZ-M-400
214 195 185 201
The temperature of CH4 generation peak (°C) 382 338 318 365
strongly exothermal reaction, the highest temperature point of the reaction rate was also expected to be observed by TPSR. As displayed in Fig. 5 and Table 1, as for the catalysts synthesized under the same thermal treatment (400 °C) exhibited pure monoclinic or pure tetragonal phase zirconia support, the peak temperature of CH4 formation was 338 °C and 365 °C for the catalyst NZ-W-400 (monoclinic) and NZ-M-400 (tetragonal), respectively. Besides, the temperature of CO2 activation and initial conversion was 185 °C, for the catalyst NZ-W-400 (monoclinic), which turned out about 6 °C lower than that of the catalyst NZ-M-400 (tetragonal). On the other hand, among three catalysts prepared in water solvent but with thermal treatment from 350 °C to 500 °C, the TPSR displayed that the catalyst calcined at 500 °C exhibited the lower temperature of CO2 activation and initial conversion (185 °C) as well as the lower
temperature for CH4 generation (318 °C). For comparison, the temperature of CO2 activation and conversion (214 °C) as well as the peak temperature of CH4 formation (382 °C) were higher for the catalyst NZ-W-350. 3.5. X-ray photoelectron spectroscopy (XPS) measurement In order to determine the chemical state of Ni and Zr as well as a better understanding of the excellent catalytic performance, the XPS spectra was conducted to analyze the surface properties of reduced NZ-W-50 0 (R) and NZ-M-40 0 (R). The binding energy of each element was referenced by the C 1 s peak position at 284.6 eV. Fig. 6 depicted the Ni 2p and Zr 3d XPS spectra of Ni based ZrO2 catalysts. As shown in Fig. 6(a), two peaks emerged at binding energy of 852.1 eV and 854.9 eV, which were ascribed to characteristic Ni0 and Ni2+ , respectively [53–55]. Compared to NZW-500 (R), the two peaks shifted towards higher binding energy implied a stronger interaction between the Ni species and surface support. What’s more, the peak at 852.1 eV intensified, indicating that the Ni°content increased. The Ni0 /Ni2+ ratio for reduced NZM-40 0 (R) and NZ-W-50 0 (R) calculated from designated peak areas increased from 0.21 to 0.35, indicating the catalyst NZ-W-500 (R) displayed a higher reduction of Ni. The reducibility obtained from H2 -TPR also stated this phenomenon (Table S2). Two peaks at
M. Chen et al. / Journal of Energy Chemistry 25 (2016) 1070–1077
(a)
(b)
Fig. 6. XPS spectra of (a) Ni 2p level, (b) Zr 3d level for reduced catalysts NZ-W-500 (R) and NZ-M-400 (R).
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respectively [57]. As for the catalysts NZ-W-400 and NZ-M-400, the weak and strong sites were nearly the same in peak temperature and intensity, except that the NZ-W-400 catalyst showed more medium sites than the NZ-M-400 catalyst. As for the moderate and strong basic sites (β and γ peaks), the intensity of all catalysts were almost the same. For three catalysts whose supports prepared in water solvent, with the calcining temperature increasing from 350 °C to 500 °C, the weak basic site (α peak) slightly shifted to low temperature and the catalyst NZW-500 exhibited the higher intensity, while much low intensity were observed over the catalysts NZ-W-350, NZ-W-400. The total number basic sites could be arranged in the sequence of NZ-W50 0 >NZ-W-40 0 >NZ-W-350 >NZ-M-40 0. The increased CO2 desorption capacity demonstrated the more basic sites and stronger basic characteristics, which was beneficial to the activation of CO2 [58,59]. The connection between the zirconia phase and basicity was still suffered from controversy in the literature. Many authors had found a higher base sites for t-ZrO2 than m-ZrO2 [60,61]. However, the contrary phenomenon was detected for Pokrovski et al. [28,57]. Even so, little information was available for ZrO2 with mainly phases of monoclinic and little tetragonal phase. What’s more, the surface acidity and basicity for the zirconia supported catalyst may vary in spite of the general trend that t-ZrO2 exhibited a higher density of base sites than m-ZrO2 . Foraita et al. [62] reported that Ni supported mainly monoclinic phase of ZrO2 (83% monoclinic phase) exposed most base sites than pure mand t-ZrO2 (Ni/mix-ZrO2 : 0.69 μmol/m2 ; Ni/m-ZrO2 : 0.58 μmol/m2 ; Ni/t-ZrO2 :0.54 μmol/m2 ). The conclusions were consistent with our experiments, where NZ-W-500 shows mainly monoclinic phase of ZrO2 and contains the most basic sites. The catalysts NZ-W-400 and NZ-W-350 only showed pure monoclinic phase and emerged more basic sites than the catalyst NZ-M-400 which revealed only tetragonal monopoly of ZrO2 . Cai et al. [11] and Choudhary et al. [63] reported that the catalytic performance could be correlated with the basicity and strength of basic sites of the catalyst. In fact, the basic properties of the supports directly affect the activation of the CO2 and reacts toward CH4 by means of decomposing the CO2 into CO and then reacted with H2 .
4. Conclusions position 181.5 eV and 183.8 eV assigned to Zr4+ of Zr 3d5/2 and Zr 3d3/2 , respectively and no additional impurity peaks were detected over catalyst NZ-M-400 (R), as displayed in Fig. 6(b) [55,56]. Both catalysts NZ-W-500 (R) and NZ-M-400 (R) exhibited a spin orbit splitting of 2.4 eV. It could be obviously observed that the binding energy of NZ-W-500 (R) shifts towards a higher energy. This phenomenon agreed well with Fig. 6(a), which indicated a stronger interaction between nickel species and surface support. These findings were going along with H2 -TPR (Fig. 3) and H2 -TPD (Fig. S5) results. In the H2 -TPD characterization, the domain above 530 °C was excluded in the calculation of active nickel surface area and nickel dispersion since the reaction temperature below 400 °C. Moreover, it should be noted that NZ-W-500 shows high intensity, in comparison with that of NZ-M-400, demonstrating that the NZ-W-500 sample had a higher Ni dispersion and more surface associated hydrogen can be generated on these samples [47]. 3.6. Temperature-programmed desorption and chemisorption on four catalysts CO2 was taken as a probe gas to carry out the temperatureprogrammed desorption experiment in order to study the strength and basic sites presented in catalysts. As shown in Fig. 7, three peaks (α ,β ,γ ), corresponding to weak, medium, strong basic sites,
In summary, the morphology and structure of ZrO2 and the thermal treatment impact of the catalysts significantly affected the physicochemical properties of Ni/ZrO2 -T catalysts (T refers to the calcining temperature) and the subsequently catalytic activity of CO2 selective hydrogenation to synthetic natural gas. For comparison, the catalyst NZ-W-400 (monoclinic) synthesized in water solvent exhibited a better catalytic activity than the catalyst NZ-M400 (tetragonal) prepared in methanol solvent. The catalyst NZW-400 displayed more H2 absorbed sites, more basic sites and a lower temperature of CO2 activation and initial conversion as well as a lower peak temperature of CH4 generation. With the calcining temperature increased from 350 °C to 500 °C for the catalysts prepared in water solvent, the TPSR experiments displayed that there was a lower temperature for CO2 activation and initial conversion (185 °C) as well as a lower peak temperature of CH4 generation (318 °C), for the catalyst calcined at 500 °C. As for the catalyst NZ-W-350 which exhibited the less basic sites and lower catalytic activity, the temperature of CO2 activation and conversion was higher (214 °C) as well as the higher temperature of CH4 formation (382 °C). CO2 -TPD-MS revealed that the catalysts with pure or mainly monoclinic phase contained more basic sites than pure tetragonal phase. The total number of basic sites decreased in the sequence of
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(a)
(b)
Fig. 7. (a) CO2 -TPD-MS profiles, (b) peak area analysis of four catalysts.
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CO2 selective hydrogenation to synthetic natural gas (SNG) over four nano-sized Ni/ZrO2 samples: ZrO2 crystalline phase & treatment impact Min Chen a,b, Zhanglong Guo a, Jian Zheng c, Fangli Jing a,b, Wei Chu a,b,∗ a b c
School of Chemical Engineering, Sichuan University, Chengdu 610065, Sichuan, China Institute of New Energy and Low-Carbon Technology, Sichuan University, Chengdu 610225, Sichuan, China Institute for Integrated Catalysis, Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352, United States
a r t i c l e
i n f o
Article history: Received 15 August 2016 Revised 4 October 2016 Accepted 18 October 2016 Available online 16 November 2016 Keywords: Monoclinic zirconia support Nano-sized nickel catalyst CO2 -TPD-MS TPSR-CH4 CO2 selective hydrogenation
a b s t r a c t Two type zirconia (monoclinic and tetragonal phase ZrO2 ) carriers were synthesized via hydrothermal route, and nano-sized zirconia supported nickel catalysts were prepared by incipient impregnation then followed thermal treatment at 30 0 °C to 50 0 °C, for the CO2 selective hydrogenation to synthetic natural gas (SNG). The catalysts were characterized by XRD, CO2 -TPD-MS, XPS, TPSR (CH4 , CO2 ) techniques. For comparison, the catalyst NZ-W-400 (monoclinic) synthesized in water solvent exhibited a better catalytic activity than the catalyst NZ-M-400 (tetragonal) prepared in methanol solvent. The catalyst NZ-W-400 displayed more H2 absorbed sites, more basic sites and a lower temperature of initial CO2 activation. Then, the thermal treatment of monoclinic ZrO2 supported nickel precursor was manufactured at three temperature of 350, 40 0, 50 0 °C. The TPSR experiments displayed that there were the lower temperature for CO2 activation and initial conversion (185 °C) as well as the lower peak temperature of CH4 generation (318 °C), for the catalyst calcined at 500 °C. This sample contained the more basic sites and the higher catalytic activity, evidenced byCO2 -TPD-MS and performance measurement. As for the NZ-W-350 sample, which exhibited the less basic sites and the lower catalytic activity, its initial temperature for CO2 activation and conversion was higher (214 °C) as well as the higher peak temperature of CH4 formation (382 °C). © 2016 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.
1. Introduction With more greenhouse gases (GHGs) effects in agriculture, water resources and human health, carbon dioxide with rising concentration is creating a critical risk for earth’s climate system and human’s daily life [1,2]. Large amount of reports have been illustrated about the global warming with surface temperature increasing, rising the sea level, melting in glaciers and so on [3]. The energy obtained from burning fossil fuels is vital for the developing of our civilization largely and still the dominant energy source. Therefore, the using of CO2 , efficiently, will be still a potential and needing-solving problem and arouse renewed interest [3–6]. The CO2 conversion to green fuels in an environment-friendly and sustainable way is still a challenge [7–9]. More efforts have been devoted to convert CO2 to valuable high energy density organic hy-
∗ Corresponding author at: School of Chemical Engineering, Sichuan University, Chengdu 610065, Sichuan, China. Fax: +86 2885461108. E-mail address: [email protected] (W. Chu).
drocarbons (e.g. CH3 OH [10]), rather than treating it as a waste in recent decades, which is also an effective way to mitigate the greenhouse effect [11–14]. The catalytic activity of supported catalysts is among other factors influenced by the support material used [14–16]. Various supports (e.g., Al2 O3 , SiO2 , ZrO2 , CNT) have been investigated for CO2 hydrogenation to methane [7,11,12,17–19]. Recently, zirconia (ZrO2 ) has received considerable attention and applied in a variety of reactions because of its amphoteric properties as well as high thermal and unique chemical stability [20–23]. Zirconia exists primarily in three different crystalline structures depending on the preparation method: cubic (c-ZrO2 ), monoclinic (m-ZrO2 ), and tetragonal (t-ZrO2 ) [24,25]. At ambient pressure, m-ZrO2 can exists under room temperature to 1175 °C, t-ZrO2 , from 1175 °C to 2370 °C, and c-ZrO2 , from 2370 °C to 2680 °C [26]. The morphology of zirconia was found to play a vital role in different catalytic reaction [27]. Pokrovski et al. [28] investigated the different zirconia phases showing different surface catalytically active sites (Bronsted acidic and basic hydroxyl groups). Samson et al. [29] explored that zirconia presence in tetragonal phase showed
http://dx.doi.org/10.1016/j.jechem.2016.11.008 2095-4956/© 2016 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.
M. Chen et al. / Journal of Energy Chemistry 25 (2016) 1070–1077
higher activity towards methanol synthesis from CO2 since oxygen vacancies stabilizes t-ZrO2 and then enhances catalytic activity. On the other way, Yamasaki et al. [27] found that CO2 methanation reaction showed better catalytic activity at low temperatures with increasing t-ZrO2 content, which was related to more CO2 adsorption sites. However, Rhodes et al. [10] drew an opposite conclusion for a fixed Cu surface area, Cu/m-ZrO2 was more active for methanol synthesis than Cu/t-ZrO2 from a feed of 3/1 H2 /CO at 3.0 MPa and temperatures between 200 and 250 °C. Pd supported on t-ZrO2 exhibited the highest selectivity towards benzene in hydrodeoxygenation of phenol, while significant formation of cyclohexanone was observed over Pd/m-ZrO2 [20]. Nevertheless, the mesoporous amorphous zirconia is beneficial to the dry reforming of methane reaction when zirconia was in amorphous, monoclinic, and tetragonal morphology and it was said that the amorphous zirconia enhanced the Ni dispersion [30]. In our recent researches, Chu and his workers have made a lot of efforts towards green chemistry catalyst through efficient carbon capture use and storage to valuable materials [5,6,13,31–34]. The chemical vapor deposition integrated process (CVD-IP) was developed in order to better use of the CO2 . In this work, the greenhouse gas was then converted to a new solid formed production [5,13]. On the previous research, we synthesized highly dispersed zirconia supported nano-sized nickel catalysts by a facile incipient wetness impregnation method under different thermal treatments in order to convert CO2 to synthetic natural gas (SNG). In this work, the phase influence and treatment impact on CO2 selective hydrogenation to SNG were studied over Ni/ZrO2 catalysts. A variety of characterization techniques including XRD, nitrogen adsorption–desorption, HRTEM, H2 -TPR, H2 -TPD, CO2 -TPD-MS, Raman, XPS, TPSR (CH4 , CO2 ) were used to explore the structureactivity relationship. 2. Experimental 2.1. Preparation of catalysts Zirconia supports were prepared via a solvothermal method which is reported elsewhere [35]. In a typical experiment, the reaction was performed at 140 °C for 20 h in a stainless-steel autoclave (50 mL) containing solutions of 4.805 g urea (CO(NH2 )2 ; > 99.9%), 2.138 g zirconyl nitrate (ZrO(NO3 )2 ·2H2 O; > 45.0% ZrO2 ) and 20 mL solvents. The deionized water or methanol (>99.9%) were intentionally selected as solvent and marked as W or M, respectively. The resulting precipitates were recovered by filtering with water and methanol and dried overnight at 110 °C. Zirconia supported nickel (15 wt%) catalysts were prepared by incipient wetness impregnation using Ni(NO3 )2 ·6H2 O as nickel source. The support was soaked in the precursor solution by ultrasonic treatment for 2 h at room temperature and then aged for 2 h. The solvent was flowing evaporated by heating. The obtained samples were calcined at the temperatures of 350 °C, 40 0 °C, and 50 0 °C for 4 h at a rate of 2 °C/min. The as-synthesized catalysts were finally denoted as NZ-W-x (water solvents) or NZ-M-x (methanol solvents), where the x noted as the calcining temperature. 2.2. Characterization of catalyst The X-ray diffraction (XRD) analysis was performed using a Philips X’pert PRO diffraction meter, with Cu Kα (40 kV, 30 mA) radiation. Scans were made in the 2θ range of 10°–80° with a step size of 0.03°. The specific surface areas, total pore volumes, and average pore diameters were determined by nitrogen adsorption/desorption at −196°C, which were measured using an automated surface area
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& pore size analyzer (Quadrasorb SI apparatus). Before each measurement, the samples were degassed in vacuum at 120°C for 3 h. Specific surface areas of samples were calculated by the Brunauer– Emmett–Teller (BET) method. The pore size distribution and average pore diameter were determined according to the Barrett– Joyner–Halenda (BJH) method applied to desorption isotherms. The transmission electron microscope (TEM) analysis of investigated catalysts was conducted using a Libra 200 FE microscope with a 200 kV voltage. The samples reduced at 450 °C in H2 for 1 h were dispersed in ethanol assisted by ultrasonic technique and then deposited on a carbon film covered copper grid for measurement. The reduced catalysts morphology were determined via scanning electron microscopy (SEM) using an Ultra 55 SEM. The temperature-programmed reduction with hydrogen (H2 TPR) was carried out in a fixed-bed reactor. 50 mg sample was loaded, and the reduction gas of 5% H2 in N2 was introduced at a flow rate of 30 mL/min. The temperature of the reactor was raised linearly from 100 °C to 800 °C at a rate of 10 °C/min by a temperature controller. The amount of hydrogen consumed was determined on-line using a SC-200 gas chromatograph with a thermal conductivity detector (TCD). The basicity of samples was determined by the temperatureprogrammed desorption of carbon dioxide technique (CO2 -TPDMS). 200 mg of catalyst was reduced at 450°C with H2 for 1 h, and then cooled down to 50 °C in Ar. CO2 was then introduced for 1 h at 50 °C. After purging with Ar for 2 h to purge the gas line and remove weakly adsorbed CO2 , the sample was heated from 50 °C to 800 °C with a ramp of 10 °C/min and the desorbed CO2 was detected on-line by a Hiden QIC-20 mass spectrometer. The mass signal at m/z = 44 was used to determine the amount of desorbed CO2 . Temperature-programmed desorption of hydrogen (H2 -TPD) experiments were carried out on a TP-5080 auto multi adsorption analyzer. 200 mg of sample was reduced with hydrogen at 450 °C for 1 h and then cooled to 50 °C, followed by H2 adsorption at 50 °C for 2 h. Then, the samples were flashed with nitrogen flow to remove the physically adsorbed hydrogen, followed by heating the sample at a rate of 10 °C/min in nitrogen from 50 °C to 800 °C, and the H2 -TPD spectra were recorded. The X-ray photoelectron spectroscopy (XPS) data were collected on a Kratos AXIS Ultra DLD spectrometer. The charging effects were corrected by adjusting the binding energy of C 1 s peak from carbon contamination to 284.6 eV. The samples were reduced at 450°C for 1 h under H2 atmosphere. In situ temperature-programmed surface reaction (TPSR) of CO2 and H2 was achieved with a fixed-bed reactor and then inspected on-line by a Hiden QIC-20 mass spectrometer. Before TPSR experiment, four catalysts were reduced under hydrogen flow at 450°C for 1 h, and then cooled to 150 °C and reactant gases mixed with argon were poured into the fix bed. The temperature of the reactor was raised linearly to 700 °C at a rate of 10 °C /min by a temperature controller. The mass signal at m/z = 44, 2, 28 and 16 were used to determine the amount of desorbed CO2 , H2 , CO and CH4 . 2.3. Catalytic activity test Catalytic tests were carried out using a quartz reactor (6 mm i.d.). In order to measure and control the temperature, 100 mg catalyst was placed in the center part of the catalyst bed where directly contact with a thermocouple. The catalysts were reduced in hydrogen (99.99%) at 450 °C for 1 h in a flow rate 30 mL/min. The H2 /CO2 ratio was 3.6:1 and GHSV was 30,0 0 0 mL/(h·gcat ) at 1 atm. The activity was tested in the range of 240–360°C every 20 °C with a heating rate 5 °C/min. The effluent gases from the reactor were analyzed on-line using a SC-200 model gas chromatograph equipped with a TDX-01 column and a thermal conductivity
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the catalysts NZ-W-350 and NZ-W-400 exhibited some characteristic peaks at 182, 339, 379, 479, 613 cm−1 , which attributed to the monoclinic zirconia. The Raman bands at 149, 195, and 608 cm−1 were assigned to the Raman-active modes for the tetragonal phase of ZrO2 [10,37–39]. With the calcining temperature raised from 350 °C to 500 °C, the intensities of peak at 182, 339, 379 and 479 cm−1 attributed to monoclinic zirconia decreased somewhat. It suggested that the portion of m-ZrO2 in zirconia was decreasing for the catalysts prepared in water solvent with raised calcining temperatures, while the catalyst prepared in methanol solvent exhibited tetragonal phase. This conclusion was well in agreement with the phenomenon of XRD representation. Ingeneral, the supports of catalysts NZ-W-350, NZ-W-400, NZW-500 and NZ-M-400 exhibited pure monoclinic, pure monoclinic, mainly monoclinic, tetragonal phase, respectively. In addition, the nickel oxide crystalline size was also influenced by the calcined temperature. These calculated data based on Scherrer Equation are summarized in Table S1, the crystalline sizes were between 11 and 14 nm. The size of NZ-W-400, NZ-W-50 0 and NZ-M-40 0 were roughly the same. 3.2. Morphology of the reduced catalysts using TEM, SEM and nickel particle size distribution
Fig. 1. XRD patterns of catalysts calcined at 350 °C to 500 °C.
detector (TCD). To determine conversion and selectivity, the products were collected after one hour of steady-state operation. CO2 conversion, CH4 selectivity and CH4 yield were defined as follows:
XC O2 ( % ) = 1 − SCH4 ( % ) =
CO2 CO2 +CH4 +CO
× 100
CH4 ×100 CO + CH4
YCH4 = XCO2 ×SCH4
(1) (2) (3)
where, X was the conversion, S was the selectivity and Y was the yield. 3. Results and discussion 3.1. Crystal phase analysis and textural properties of four catalysts XRD patterns of catalysts calcined at 350 °C to 500 °C (fresh catalysts) were shown in Fig. 1. Catalysts calcined at different temperatures showed different structures. The diffraction peak centered at 2θ = 30.1° was attributed to t-ZrO2 (111) (JCPDS card no. 491642). Four peaks at 2θ = 24, 28.2, 31.5, 34.6 were assigned to mZrO2 (JCPDS card no.37-1484). When the calcination temperature of catalysts with zirconia supports prepared in water solvent was raised from 350 °C to 400 °C, the diffraction peaks showed typically monoclinic crystalline and increased the peak intensity. It indicated an increase in the degree of crystallinity and in the particle sizes of NiO crystallites [36]. When the calcined temperature was up to 500 °C, the catalyst NZ-W-500 showed a partial transformation of the crystalline structure from monocline to tetragonal but still displayed mainly monoclinic phase. It could be concluded that the catalyst NZ-W-500 possessed both monoclinic to tetragonal phase, i.e., a mixture of these two structures. As for the catalyst NZ-M-400, showed an obvious peak at 2θ = 30.1° and no diffraction peaks were detected at 2θ = 28.2° and 31.5°, which were assigned to t-ZrO2 (111), m-ZrO2 (−111) and (111), respectively [21,28]. In conclusion, the zirconia support prepared by methanol showed only tetragonal phase after calcining at 400 °C. The characterization of Raman spectra in Fig. S2 also confirmed this conclusion. In the Raman spectra of catalysts calcined at 350–500 °C,
The TEM images of the reduced zirconia supported nickel catalysts were shown in Fig. S3. From the TEM images, reduced catalyst NZ-W-500 (R) showed better nickel nanoparticles dispersion than the other catalysts and some aggregation could be seen for the other catalysts. This phenomenon could also be seen from the SEM images in Fig. S4, the reduced sample NZ-W-500 (R) showed irregular sponge shape, different size particles exhibit a loose distribution and did not form aggregates. In comparison, the reduced sample NZ-M-400 (R) with a uniform spherical particle showed a large agglomerate. Besides, it could be seen that the surface of reduced sample NZ-M-400 (R) is highly compact while the reduced sample NZ-W-500 (R) show a smooth surface. Particle size distributions of the catalysts measured from TEM images were presented in Fig. 2. From the HRTEM images, the reduced catalysts NZ-W-350 (R), NZ-W-400 (R) exhibited d-spacing of 0.317 nm or 0.284 nm, which attributed to m-ZrO2 (−111) or m-ZrO2 (111) phase, respectively. On the other hand, the reduced catalyst NZ-W-500 contained both d-spacing of 0.317 nm and 0.297 nm assigned to m-ZrO2 (−111) and t-ZrO2 (111) phases. However, the reduced catalyst NZ-M-400 (R) showed t-ZrO2 (111) phase only. It could be seen from HRTEM that the catalysts NZW-350 (R), NZ-W-400 (R) and NZ-W-500 (R) existed monoclinic zirconia phase, and the catalysts NZ-W-500 (R) and NZ-M-400 (R) contained tetragonal zirconia phase. The reduced catalysts NZ-W350 (R) and NZ-M-400 (R) showed the higher portion of nickel particles with diameter larger than 20 nm, especially, the catalyst NZ-W-350 (R) exhibited a wider particle size distribution from 17 to 32 nm. On the other hand, the reduced catalysts NZ-W-500 (R) and NZ-W-400 (R) showed the higher distribution of nickel particles with diameter less than 16 nm. Interestingly, the catalysts NZW-500 (R) and NZ-W-400 (R) with smaller particle size displayed better catalytic activity than catalysts NZ-W-350 (R) and NZ-M-400 (R) with bigger nickel size. 3.3. The reducibility of four samples using temperature-programmed reduction H2 -TPR experiments for the four zirconia supported nickel catalysts were carried out to investigate the reduction behavior and interaction between nickel and zirconia support. From the results in Fig. 3, four catalysts exhibited three reduction peaks in the range of 20 0–60 0 °C, which could be attributed to the reduction of three
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nickel oxide containing species [40] and without ZrO2 reduction [37]. The first peak (below 300 °C) was assigned to the reduction of free nickel oxide species [22,41]. The second peak (300–470 °C) could be assigned to the reduction of weakly bound nickel oxide with the surface of ZrO2 [42]. The third peak (470–600 °C) could be attributed to the reduction of those nickel oxide species interacted strongly with support or the Ni2+ ions penetrating into zirconia lattice [37]. From the open literature, the second peak and the third one were assumed to be related to the reduction of NiOx species that had intimate interaction with the supports [22], while the first peak was assigned to free nickel oxide small particles that could be easily reduced and then it could be deactivated as a result of its high mobility and sintering, while bound-state nickel oxide species were formed due to the better metal dispersion and stronger metal-support interaction [43–46]. From Fig. 3, as for the two catalysts synthesized by water and methanol, respectively, when they were treated at 400 °C, the catalysts showed almost the same peak temperature between 30 0–60 0 °C except a 10 °C temperature difference at the third peak. This indicated that the two catalysts had almost the same metal-support interaction. With the calcining temperature increased from 350 to 500 °C, the peak temperature of the catalysts with zirconia prepared through water solvent had a positive shift to higher temperature, which indicated that the catalysts had an increasing stronger metal-support interaction. It was said that higher metal–support interaction presented in catalyst derived from higher active metal particle dispersion, which could be identified by H2 -TPD (Fig. S5) and XPS (Fig. 6) in our work. In the further characterization of H2 -TPD, the relatively much stronger intensity of NZ-W-500 and NZ-W-400 in comparison with that of NZ-W-350 and NZ-M-400, identified that the NZ-W-500 and NZ-W-400 samples have a higher nickel dispersion and more surface associated hydrogen could be generated on these samples [47]. 3.4. Catalytic performances and TPSR results analysis
Fig. 2. The HRTEM of (a) NZ-W-350 (R), (b) NZ-W-400 (R), (c) NZ-W-50 (R), and (d) NZ-M-400 (R); and the distribution of Ni nanoparticles of (e) NZ-W-350 (R), (f) NZ-W-400 (R), (g) NZ-W-500 (R) and (h) NZ-M-400 (R) of reduced catalysts.
Fig. 3. H2 -TPR profiles of four nickel catalysts.
The CO2 conversion and CH4 selectivity versus temperature under methanation conditions were studied and the results were shown in Fig. 4. It could be observed that NZ-W-500 exhibited much higher CO2 conversion than the other catalysts, especially at low temperature range. The CO2 conversion exceptionally underwent a fast leap from 26% at 240 °C to 80% at 280 °C for the NZ-W-500 catalyst. It increased slowly afterward when the temperature continued going up to 360 °C. As for catalysts based on pure monoclinic and tetragonal ZrO2 , the CO2 conversion was relatively low and the CO2 conversion only 57% and 68% for NZ-W-350 and NZ-M-400, respectively. It was worthy pointing out that the samples NZ-W-350 and NZ-W-500 showed as high as 100% selectivity of methane till the reaction temperature reach 340 °C, while there was significant difference for CO2 conversion. On the other side, NZ-M-400 showed a sharp drop of CH4 selectivity when the temperature was up to 300 °C. The differences in catalytic activity at low temperature were mainly due to crystalline phase effect, which leaded to different physical chemical properties. When the reaction temperature raised to 300 °C and above, the catalytic activity of catalysts NZ-W-500 and NZ-W-400 reached the thermodynamic equilibrium (calculated through Aspen Plus) and shifted to the maximum activity [48–50]. However, the catalytic activity of catalyst NZ-M-400 showed an upward trend since an increasing activated CO2 molecules converted to CO at higher temperature [51,52]. In order to further investigate the surface reaction of CO2 selective hydrogenation to synthetic natural gas (SNG), the CH4 and CO2 -TPSR were conducted as displayed in Fig. 5. The TPSR results provided more details of the start temperature points of CH4 formation and CO2 consumption. Since CO2 methanation was a
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100.0
(a)
CO 2 conversion (%)
80 60 40 NZ-W-350 NZ-W-400 NZ-W-500 NZ-M-400 Equilibrium
20 0 240
280
320
360
Selecticity of CH4 (%)
100
(b)
99.6
99.2
98.8
240
o
NZ-W-350 NZ-W-400 NZ-W-500 NZ-M-400
280
320
360
o
Temperature ( C)
Temperature ( C)
Fig. 4. (a) Conversion of CO2 versus temperature, (b) the selectivity of CH4 of four samples. Reaction condition: 101.3 kPa, GHSV = 30,0 0 0 mL/(h·gcat ), H2 /CO2 molar ratio = 3.6, catalyst 100 mg.
(a)
(b)
Fig. 5. (a) CH4 -TPSR, (b) CO2 -TPSR profiles of four catalysts.
Table 1. The TPSR results of CO2 activation and methane formation. Sample
The start point of CO2 consumption (°C)
NZ-W-350 NZ-W-400 NZ-W-500 NZ-M-400
214 195 185 201
The temperature of CH4 generation peak (°C) 382 338 318 365
strongly exothermal reaction, the highest temperature point of the reaction rate was also expected to be observed by TPSR. As displayed in Fig. 5 and Table 1, as for the catalysts synthesized under the same thermal treatment (400 °C) exhibited pure monoclinic or pure tetragonal phase zirconia support, the peak temperature of CH4 formation was 338 °C and 365 °C for the catalyst NZ-W-400 (monoclinic) and NZ-M-400 (tetragonal), respectively. Besides, the temperature of CO2 activation and initial conversion was 185 °C, for the catalyst NZ-W-400 (monoclinic), which turned out about 6 °C lower than that of the catalyst NZ-M-400 (tetragonal). On the other hand, among three catalysts prepared in water solvent but with thermal treatment from 350 °C to 500 °C, the TPSR displayed that the catalyst calcined at 500 °C exhibited the lower temperature of CO2 activation and initial conversion (185 °C) as well as the lower
temperature for CH4 generation (318 °C). For comparison, the temperature of CO2 activation and conversion (214 °C) as well as the peak temperature of CH4 formation (382 °C) were higher for the catalyst NZ-W-350. 3.5. X-ray photoelectron spectroscopy (XPS) measurement In order to determine the chemical state of Ni and Zr as well as a better understanding of the excellent catalytic performance, the XPS spectra was conducted to analyze the surface properties of reduced NZ-W-50 0 (R) and NZ-M-40 0 (R). The binding energy of each element was referenced by the C 1 s peak position at 284.6 eV. Fig. 6 depicted the Ni 2p and Zr 3d XPS spectra of Ni based ZrO2 catalysts. As shown in Fig. 6(a), two peaks emerged at binding energy of 852.1 eV and 854.9 eV, which were ascribed to characteristic Ni0 and Ni2+ , respectively [53–55]. Compared to NZW-500 (R), the two peaks shifted towards higher binding energy implied a stronger interaction between the Ni species and surface support. What’s more, the peak at 852.1 eV intensified, indicating that the Ni°content increased. The Ni0 /Ni2+ ratio for reduced NZM-40 0 (R) and NZ-W-50 0 (R) calculated from designated peak areas increased from 0.21 to 0.35, indicating the catalyst NZ-W-500 (R) displayed a higher reduction of Ni. The reducibility obtained from H2 -TPR also stated this phenomenon (Table S2). Two peaks at
M. Chen et al. / Journal of Energy Chemistry 25 (2016) 1070–1077
(a)
(b)
Fig. 6. XPS spectra of (a) Ni 2p level, (b) Zr 3d level for reduced catalysts NZ-W-500 (R) and NZ-M-400 (R).
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respectively [57]. As for the catalysts NZ-W-400 and NZ-M-400, the weak and strong sites were nearly the same in peak temperature and intensity, except that the NZ-W-400 catalyst showed more medium sites than the NZ-M-400 catalyst. As for the moderate and strong basic sites (β and γ peaks), the intensity of all catalysts were almost the same. For three catalysts whose supports prepared in water solvent, with the calcining temperature increasing from 350 °C to 500 °C, the weak basic site (α peak) slightly shifted to low temperature and the catalyst NZW-500 exhibited the higher intensity, while much low intensity were observed over the catalysts NZ-W-350, NZ-W-400. The total number basic sites could be arranged in the sequence of NZ-W50 0 >NZ-W-40 0 >NZ-W-350 >NZ-M-40 0. The increased CO2 desorption capacity demonstrated the more basic sites and stronger basic characteristics, which was beneficial to the activation of CO2 [58,59]. The connection between the zirconia phase and basicity was still suffered from controversy in the literature. Many authors had found a higher base sites for t-ZrO2 than m-ZrO2 [60,61]. However, the contrary phenomenon was detected for Pokrovski et al. [28,57]. Even so, little information was available for ZrO2 with mainly phases of monoclinic and little tetragonal phase. What’s more, the surface acidity and basicity for the zirconia supported catalyst may vary in spite of the general trend that t-ZrO2 exhibited a higher density of base sites than m-ZrO2 . Foraita et al. [62] reported that Ni supported mainly monoclinic phase of ZrO2 (83% monoclinic phase) exposed most base sites than pure mand t-ZrO2 (Ni/mix-ZrO2 : 0.69 μmol/m2 ; Ni/m-ZrO2 : 0.58 μmol/m2 ; Ni/t-ZrO2 :0.54 μmol/m2 ). The conclusions were consistent with our experiments, where NZ-W-500 shows mainly monoclinic phase of ZrO2 and contains the most basic sites. The catalysts NZ-W-400 and NZ-W-350 only showed pure monoclinic phase and emerged more basic sites than the catalyst NZ-M-400 which revealed only tetragonal monopoly of ZrO2 . Cai et al. [11] and Choudhary et al. [63] reported that the catalytic performance could be correlated with the basicity and strength of basic sites of the catalyst. In fact, the basic properties of the supports directly affect the activation of the CO2 and reacts toward CH4 by means of decomposing the CO2 into CO and then reacted with H2 .
4. Conclusions position 181.5 eV and 183.8 eV assigned to Zr4+ of Zr 3d5/2 and Zr 3d3/2 , respectively and no additional impurity peaks were detected over catalyst NZ-M-400 (R), as displayed in Fig. 6(b) [55,56]. Both catalysts NZ-W-500 (R) and NZ-M-400 (R) exhibited a spin orbit splitting of 2.4 eV. It could be obviously observed that the binding energy of NZ-W-500 (R) shifts towards a higher energy. This phenomenon agreed well with Fig. 6(a), which indicated a stronger interaction between nickel species and surface support. These findings were going along with H2 -TPR (Fig. 3) and H2 -TPD (Fig. S5) results. In the H2 -TPD characterization, the domain above 530 °C was excluded in the calculation of active nickel surface area and nickel dispersion since the reaction temperature below 400 °C. Moreover, it should be noted that NZ-W-500 shows high intensity, in comparison with that of NZ-M-400, demonstrating that the NZ-W-500 sample had a higher Ni dispersion and more surface associated hydrogen can be generated on these samples [47]. 3.6. Temperature-programmed desorption and chemisorption on four catalysts CO2 was taken as a probe gas to carry out the temperatureprogrammed desorption experiment in order to study the strength and basic sites presented in catalysts. As shown in Fig. 7, three peaks (α ,β ,γ ), corresponding to weak, medium, strong basic sites,
In summary, the morphology and structure of ZrO2 and the thermal treatment impact of the catalysts significantly affected the physicochemical properties of Ni/ZrO2 -T catalysts (T refers to the calcining temperature) and the subsequently catalytic activity of CO2 selective hydrogenation to synthetic natural gas. For comparison, the catalyst NZ-W-400 (monoclinic) synthesized in water solvent exhibited a better catalytic activity than the catalyst NZ-M400 (tetragonal) prepared in methanol solvent. The catalyst NZW-400 displayed more H2 absorbed sites, more basic sites and a lower temperature of CO2 activation and initial conversion as well as a lower peak temperature of CH4 generation. With the calcining temperature increased from 350 °C to 500 °C for the catalysts prepared in water solvent, the TPSR experiments displayed that there was a lower temperature for CO2 activation and initial conversion (185 °C) as well as a lower peak temperature of CH4 generation (318 °C), for the catalyst calcined at 500 °C. As for the catalyst NZ-W-350 which exhibited the less basic sites and lower catalytic activity, the temperature of CO2 activation and conversion was higher (214 °C) as well as the higher temperature of CH4 formation (382 °C). CO2 -TPD-MS revealed that the catalysts with pure or mainly monoclinic phase contained more basic sites than pure tetragonal phase. The total number of basic sites decreased in the sequence of
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(a)
(b)
Fig. 7. (a) CO2 -TPD-MS profiles, (b) peak area analysis of four catalysts.
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