- Email: [email protected]
Y2O3 and its catalytic performance in methane conversion to syngas
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 4 4 4 7 e1 4 4 5 4
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Properties of Ni/Y2O3 and its catalytic performance in methane conversion to syngas Huimin Liu, Dehua He* Innovative Catalysis Program, Key Laboratory of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, PR China
article info
abstract
Article history:
Ni/Y2O3, with Y2O3 support prepared by the conventional precipitation method, was
Received 9 June 2011
prepared by an impregnation method. The physicochemical properties of Y2O3 and Ni/Y2O3
Received in revised form
were characterized by BET, CO2-TPD, NH3-TPD, TPR, XRF and TGA, and compared with
28 July 2011
those of g-Al2O3 and Ni/g-Al2O3, respectively. The catalytic performance of Ni/Y2O3 in the
Accepted 7 August 2011
reaction of partial oxidation of methane (POM) to syngas was evaluated and compared with
Available online 13 September 2011
that of Ni/g-Al2O3 catalyst, too. The results showed that, Y2O3 was a basic support with few
Keywords:
more easily to be reduced than those supported on g-Al2O3. In the partial oxidation of
acidic sites while g-Al2O3 was an acidic support. NiO particles supported on Y2O3 were Ni/Y2O3 catalyst
methane, Ni/Y2O3 catalyst showed high catalytic activity and exhibited better catalytic
Partial oxidation of methane
stability than Ni/g-Al2O3. After POM reaction at 700 C for 550 h, methane conversion decreased little and only 2.2 wt% carbon was deposited on Ni/Y2O3 catalyst. Ni/Y2O3 was
Syngas
stable in POM even after a series of reaction temperature variations within the temperature range of 400 w 800 C. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Catalytic conversion of methane to syngas (and also to hydrogen gas) is an important route for the effective utilization of natural gas [1e4]. Generally, there are three ways for the conversion of methane to syngas, including the steam reforming of methane (SRM), the carbon dioxide reforming of methane (CRM) and the partial oxidation of methane (POM). SRM is a traditional industrial route, whereas CRM and POM are potential technical routes and have attracted more and more attentions in the recent 20 w 30 years. Particularly, POM has overwhelmed SRM due to its obvious advantages, such as high energy efficiency [5], suitable H2/CO ratio for methanol synthesis and FischereTropsch processes. In the partial oxidation of methane, compared with noble metal-based catalysts, such as Rh, Ru, Ir, Pd, Pt [6e10],
Ni-based catalysts have been widely studied because of their good catalytic performances as well as the low costs. However, Ni-based catalysts are suffered from deactivation due to carbon deposition in POM [11,12]. Lots of methods have been employed to improve their abilities to resist carbon deposition. Reports showed that the acidic-basic properties of Ni-based catalysts could affect the amount of carbon deposited. Choudhary et al. [13,14] discovered that, no obvious carbon was deposited on Ni/CaO after 15 h reaction in POM. Miao et al. [15] modified Ni/Al2O3 catalyst with Li2O and La2O3, and the obtained LiNiLaOx/Al2O3 exhibited improved ability to resist carbon deposition during the 50 h life-test in POM. Similar results were also obtained on ZrO2, MgO and La2O3, which were also used as the supports of Ni-based catalysts in POM [16e18]. Except oxide supports, non-oxide supports, such as SiC and Si3N4, were also employed in POM. Shang et al. [19]
* Corresponding author. Tel/Fax: þ86 10 62773346. E-mail address: [email protected] (D. He). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.08.025
14448
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 4 4 4 7 e1 4 4 5 4
studied the catalytic performance of nitrified Ni/SiC in the partial oxidation of methane, and the results showed that, compared with Ni/SiC, less carbon was deposited on the nitrified Ni/SiC. Ni/Si3N4 also exhibited good catalytic performance in POM [20], and almost no carbon was deposited. However, it is not the truth that, the more basic of catalysts, the more able to resist carbon deposition, because methane coupling reaction might occur on catalysts with the supports of strong basicity [21]. Therefore, searching suitable supports for POM is still an important issue. Y2O3, an important metal oxide, has been applied in a wide range of areas because of its optical, thermal and chemical stabilities [22e24]. In the recent years, Y2O3 has also been applied in some catalytic reactions. Yao et al. [25] showed that the addition of a small quantity of Y2O3 and K2O to Ag/Al2O3 could adjust the electronic density of adjacent silver atoms and produce proper adsorbed oxygen species, and the conversion of propylene and the selectivity to propylene oxide were high on the obtained AgeY2O3eK2O/Al2O3 catalyst in the reaction of epoxidation of propylene. Costa et al. [26] studied Pd/CeO2 and Pd/Y2O3 in the partial oxidation of ethanol, and it was found that the selectivity to CO was higher over Pd/Y2O3 catalyst, since Pd/Y2O3 catalyst favored the transformation of ethoxy species to acetate whereas Pd/CeO2 catalyst facilitated the further oxidization of CO to CO2. Wu et al. [27] employed different Rh supported catalysts in the steam reforming of ethanol (SRE), and the order of catalytic activity was: Rh/Y2O3 > Rh/CeO2 > Rh/La2O3 > Rh/Al2O3. The activity of Rh/Y2O3 in SRE was promoted due to the surface oxygen vacancies of Y2O3. Yttria stabilized zirconia (YSZ) was also used as supports in the steam reforming of ethanol [28], the selective reduction of NO [29], the carbon dioxide reforming of methane [30] and the partial oxidation of methane [31]. Santos et al. added a small amount of Y2O3 to Al2O3, and the obtained Ni/Al2O3eY2O3 catalyst presented higher catalytic activity than Ni/Al2O3 in the methane autothermal reforming reaction (ATR) [32]. Fu et al. [33] used different oxides, including Y2O3, to modify Al2O3 support, and found that the ability to resist carbon deposition of the prepared Nibased catalysts (measured after 1 h TOS of ATR) decreased in the following order: Ni/CaOeAl2O3 > Ni/MgOeAl2O3 > Ni/ TiO2eAl2O3 > Ni/CeO2eAl2O3 > Ni/La2O3eAl2O3 > Ni/ Y2O3eAl2O3 > Ni/Fe2O3eAl2O3 > Ni/Al2O3. Wang et al. [34] employed Y2O3 promoted metallic Ni catalyst in POM. The catalyst was acid treated nickel sponge and modified with Y2O3 by an impregnation method. The results showed that the conversion of CH4 and the selectivities to H2 and CO were increased on the Y2O3 modified Ni sponge. However, the amount of carbon deposited was not analyzed and long time life-test was not reported by Wang et al. [34]. What’s more, in this case, Y2O3 was not used as catalyst support, instead, only a small amount of Y2O3 was used (the content of Y2O3 was in the range of 1.38 w 10.1%). On the other hand, Y2O3 was also used as a support of Ru catalyst in the partial oxidation of methane [35], and Nishimoto et al. reported that Ru/Y2O3 was an active catalyst in POM and no carbon was deposited after 10 h reaction. However, the physicochemical properties of Y2O3 were not revealed or longer time life-test was not reported either. In the previous studies of Y2O3 as supports, no matter Rh/Y2O3 in SRE, Ni/Al2O3eY2O3 in ATR, or Ru/Y2O3 in
POM, the relationship between the properties of Y2O3 and the catalytic performance of the relevant catalysts was not investigated intensively. However, the properties of Y2O3 might exert an influence on the performance of Ni/Y2O3 catalyst in POM. Therefore, in this paper, the physicochemical properties of Y2O3 and Ni/Y2O3, as well as the catalytic performance of Ni/Y2O3 catalyst in POM, were studied and compared with those of Al2O3 and Ni/Al2O3, respectively.
2.
Experimental section
2.1.
Catalyst preparation
Y2O3 was prepared by the conventional precipitation method. 200 mL Y(NO3)3$6H2O aqueous solution (0.25 M) containing 0.05 mol Y(NO3)3 was added dropwise into 145 mL NH3 aqueous solution (2.5 wt%) under the conditions of pH 10 w 11 and vigorous stirring. After 0.5 h of stirring, the white precipitate of Y(OH)3 was aged at room temperature for 12 h. Then, the precipitate was filtered and washed thoroughly with deionized water till the pH of the filtrated mother liquor was 7. Afterward, the Y(OH)3 hydrogel was dried at 110 C for 12 h in air and then calcined at 500 C for 5 h in air. Ni/Y2O3 catalyst was prepared by impregnating Ni(NO3)2∙6H2O aqueous solution (analytical grade reagent, provided by Shantou Xilong chemical factory) with the above obtained Y2O3 support at room temperature for 10 h. After water being removed by vaporization, the precursor of Ni/Y2O3 was then dried at 110 C in air for 12 h and calcined at 650 C in air for 5 h. For comparison, Ni/g-Al2O3 was also prepared with the impregnation method, and the detailed information was available in our previous paper [36].
2.2.
Catalyst characterization
The specific surface areas of the catalysts and the supports were measured by N2 adsorption-desorption with the BET method on a Micromeritics ASAP 2010 C analyzer. The crystalline phases of the Ni particles and the supports were investigated by X-ray diffraction (XRD). The reduction behaviors of the catalysts were characterized by Temperature programmed reduction (TPR). The amounts of carbon deposited on the catalysts were evaluated by thermogravimetric analysis (TGA). The acidic properties of the catalysts were measured by NH3-temperature programmed desorption (NH3TPD). The actual contents of Ni on the catalysts were measured by X-ray fluorescence spectrometer (XRF). The detailed experimental procedures of BET, XRD, TPR, TGA, XRF and NH3-TPD were consistent with those in our previous paper [36]. The basic properties of the catalysts were measured by CO2-temperature programmed desorption (CO2-TPD). The CO2-TPD profiles were measured by Quantachrome adsorption instrument (Chembet-3000 TPR/TPD). The catalysts (0.1 g) were firstly treated in highly pure He (99.999%, 110 mL/min) at 500 C for 0.5 h. After that, the catalysts were saturated with flowing highly pure CO2 at 100 C, and then flushed with highly pure He (110 mL/min) to remove the physically adsorbed CO2. Finally, the desorption of CO2 was carried out in
14449
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 4 4 4 7 e1 4 4 5 4
*
Y2O3
*
+ Ni
*
Intensity (a.u.)
*
Ni/Y2O3 (550h POM)
* * * *+* * * ****
*
Ni/Y2O3 (125h POM)
+ Ni/Y2O3 (10 min POM)
NiO/Y2O3 Y2O3
10
20
30
40
50
60
70
80
2 Theta degree Fig. 1 e XRD patterns of Y2O3 support and Ni/Y2O3 catalyst.
flowing highly pure He (110 mL/min) from 100 C to 800 C with a heating ramp of 15 C/min. Nevertheless, carbonate (Y2O2CO3) may be formed on Y2O3 and Ni/Y2O3 due to the preservation of the samples in air [37]. In order to eliminate the influence of Y2O2CO3 on the CO2-TPD profiles, CO2-TPD profiles obtained by pretreating catalysts at 700 C for 0.5 h before CO2 adsorption was used as reference.
with the inlet and outlet sides of the catalyst bed, were used to measure the temperatures of the inlet and outlet sides of the bed. Before reaction, the catalyst was reduced with 20 mol% H2/Ar at 600 C for 2 h, then the temperature was raised to the reaction temperature (700 C), and then the mixture gas of CH4 and O2 with a molar ratio of 2/1 was introduced into the reactor at a total flow rate of 165 mL/min (The concentration of CH4 is 66.7 mol% and the concentration of O2 is 33.3 mol%, and no inert gas was introduced). The effluent gas was firstly cooled down in a cool trap (about 0 C) to remove water in the products and the gaseous products were analyzed by two online gas chromatographs (GC), both equipped with TDX-01 columns and connected with TCD detectors. The GC-A with Ar as the carrier gas was used to analyze the volume ratio of H2/CO in the products, and the GC-B with H2 as the carrier gas was used to analyze the components of CH4, CO, O2 and CO2 in the products. The relative amount of the gases in the products was calculated by the normalization method, and the data obtained by these two GC were linked by CO amount. The equations were shown as follows: Conversion of CH4 ¼ FinCH4 FoutCH4
Selectivity of CO ¼ FoutCO = FinCH4 FoutCH4 100% Selectivity of H2 ¼ FoutH2 =2= FinCH4 FoutCH4 100% F volume flow rate; mL=min
3. 2.3.
FinCH4 100%
Results and discussion
Catalytic performance
The apparatus used to evaluate the catalytic performance of Ni/Y2O3 in POM was the same as that in our previous paper [36]. It was conducted in a fixed-bed quartz reactor with an inner diameter of 5 mm under atmosphere. A sample (0.1 g) of the catalysts was packed in the center of the quartz reactor. A furnace was used to heat the quartz tube reactor. A thermocouple, placed in the center of the furnace, was used to control the temperature of the furnace. Another two thermocouples, installed in the quartz tube reactor and contacted
3.1. Crystal structures and textural properties of supports and catalysts The XRD pattern of Y2O3 support is shown in Fig. 1. It could be seen that the Y2O3 prepared by the conventional precipitation method was cubic Y2O3 [01-082-2415]. The result was consistent with those reported by Lin et al. [22] and Wang et al. [38], who found that cubic Y2O3 could be obtained by calcining Y(OH)3 hydrogel at temperatures higher than 500 C. The crystalline phase of g-Al2O3 was also confirmed by XRD, and
Table 1 e Textural properties of supports and amount of carbon deposition. Specific surface areas (m2/g)a
Pore size distributions (nm)b
Actual content of Ni (wt %)c
TOS(h)
Amount of carbon deposition (%)d
Y 2O 3 g-Al2O3e Ni/Y2O3
50.8 142.5 43.4
2e8 2e8 5e20
e e 5.28
Ni/g-Al2O3e
116.9
2e8
5.45
e e 125 550 50
e e 1.9 2.2 3.1
Catalyst/support
a b c d e
Measured by N2 adsorption-desorption with the BET method. Measured by BJH. Measured by XRF. Measured by TGA. Data was cited from Ref. [36].
14450
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 4 4 4 7 e1 4 4 5 4
3.2. Acidic and basic properties of supports and catalysts
Volume adsorbed, cm 3 /g-STP
The acidic properties of Y2O3, g-Al2O3 and the relevant supported Ni catalysts were measured by NH3-TPD, and the profiles are shown in Fig. 3. For Y2O3 support, there was a broad and weak NH3 desorption peak at the temperature of 125 w 300 C, whereas for g-Al2O3 support, broad NH3 desorption peaks with high intensity were observed within the temperature range of 125 w 500 C. Besides, the area of NH3 desorption peaks on g-Al2O3 was much larger than that on Y2O3. That is to say, there were more acidic sites on g-Al2O3 than on Y2O3. For NiO/g-Al2O3, the area and the intensity of the desorption peaks of NH3 decreased a little compared with g-Al2O3 support. For NiO/Y2O3, the area and the intensity of
Al2O3 Ni/Al2O3
Intensity (a.u.)
the result was shown in our previous paper [36]. The textural properties of Y2O3 and g-Al2O3, measured by N2 adsorptiondesorption isotherm method, are listed in Table 1. The specific surface area of Y2O3 was 50.8 m2/g, lower than that of g-Al2O3 (142.5 m2/g). However, both Y2O3 and g-Al2O3 exhibited IV-type isotherms with pore size distributions within the range of 2 w 8 nm (Fig. 2). After loading of NiO on Y2O3 (the actual content of Ni was measured by XRF and listed in Table 1), the XRD pattern of NiO/Y2O3 (before reduction) is shown in Fig. 1. It could be seen that no peak attributed to NiO phase was observed, implying that NiO particles were well dispersed on Y2O3. The phase of cubic Y2O3 could be clearly observed in the XRD pattern of NiO/Y2O3. The textural properties of Ni/Y2O3 and Ni/g-Al2O3 are listed in Table 1. Both catalysts exhibited IV-type isotherms (Fig. 2). However, for both catalysts, the specific surface areas decreased somewhat compared with the relevant supports. The specific surface area of Ni/Y2O3 was 43.4 m2/g and that of Ni/g-Al2O3 was 116.9 m2/g. Besides, for Ni/g-Al2O3, the average pore size didn’t change compared with g-Al2O3, while for Ni/Y2O3, the pore size increased to the range of 5 w 20 nm.
Ni/Y2O3
Y2O3 100
200
300
400
500
600
o
Temperature ( C) Fig. 3 e NH3-TPD profiles of Y2O3, g-Al2O3 and relevant supported Ni catalysts. Data of Al2O3 was cited from Ref. [36].
the desorption peak of NH3 changed little after the impregnation of NiO on Y2O3. The basic properties of Y2O3, g-Al2O3 and the relevant supported Ni catalysts were measured by CO2-TPD, and the profiles are shown in Fig. 4. It could be seen that, for Y2O3 support, there was a broad CO2 desorption peak with high intensity at 110 w 550 C, on the contrary, a small and weak CO2 desorption peak was observed on g-Al2O3 support. Nevertheless, the CO2 desorption peak for Y2O3 was centered at about 250 C while that for g-Al2O3 was centered at about 180 C. What’s more, the area of CO2 desorption peak on Y2O3 was much larger than that on g-Al2O3. That is to say, in comparison to g-Al2O3, there were more basic sites on Y2O3 with stronger basic strength. For NiO/Y2O3 and NiO/Al2O3 catalysts, the number of basic sites and the strength of basicity changed little compared with the relevant supports. However, in the present research, wide and broad CO2 desorption peaks were also observed at 550 w 800 C on the CO2-TPD profiles of Y2O3 and NiO/Y2O3. In order to investigate
Y2O3
Ni/Y2O3
Ni/Y2O3
Intensity (a.u.)
Y2O3
Ni/Al2O3
Al2O3
Al2O3
Ni/Al2O3
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P0 ) Fig. 2 e Adsorption-desorption isotherms of Y2O3, g-Al2O3 and relevant supported Ni catalysts Data of Al2O3 was cited from Ref. [36].
100
200
300
400
500
600
700
800
o
Temperature ( C) Fig. 4 e CO2-TPD profiles of Y2O3, g-Al2O3 and relevant supported Ni catalysts.
14451
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 4 4 4 7 e1 4 4 5 4
o
Intensity (a.u.)
Treated at 500 C before CO2 adsorption
o
Treated at 700 C before CO2 adsorption
100
200
300
400
500
600
700
800
900
three reduction peaks, centered at 280 C, 400 C and 500 C respectively, were observed. Generally speaking, Ni-based catalysts would exhibit different reduction peaks, which depend on the nature of the supports [42,43]. Bulk reduction peaks of free NiO particles that don’t interact with supports generally appear at about 400 C, and the interactions between NiO and supports decrease the tendency of Ni cation to be reduced to metallic Ni [44]. Therefore, it could be speculated that, the two reduction peaks on NiO/Y2O3 centered at 400 C and 500 C could be attributed to the reduction of bulk NiO and the reduction of NiO that weakly interacted with Y2O3. As for the reduction peak on NiO/Y2O3 at about 280 C in TPR, it could be ascribed to the reduction of surface NiO particles promoted by the surface oxygen vacancies of Y2O3 [45].
o
Fig. 5 e CO2-TPD profiles of Y2O3 at different pretreatment conditions.
the attribution of CO2 desorption peak on Y2O3 and NiO/Y2O3 at 550 w 800 C, a comparative CO2-TPD experiment was carried out. In the comparative CO2-TPD experiment, Y2O3 was treated at 700 C for 0.5 h in the flowing He under atmospheric pressure before CO2 adsorption. The CO2-TPD profile obtained is shown in Fig. 5. Obviously, the CO2 desorption peak at 550 w 800 C disappeared on the profile obtained by pretreating Y2O3 at 700 C before CO2 adsorption. This implied that the CO2 desorption peak of Y2O3 at 550 w 800 C could not be ascribed to the basic properties of Y2O3. The result was in accordance with that of Sato et al.’s [37]. Sato et al. [37] reported that, on the CO2-TPD profile of Y2O3, no CO2 desorption peaks could be observed at temperatures higher than 500 C. Then, what’s the reason for the desorption of CO2 on Y2O3 and NiO/Y2O3 at 550 w 800 C in the present experiment? Sato et al. [37] also reported that, CO2 desorbed at temperatures above 500 C could be attributed to the decomposition of carbonate. In the present research, since Y2O3 and NiO/Y2O3 were preserved in air before the CO2-TPD experiment, Y2O2CO3 might be formed on Y2O3 due to the adsorption of CO2 in the air. Miao et al. [39] had proposed that Y2O2CO3 would decompose at about 640 C. Then, it’s speculated that the desorbed CO2 peak at 550 w 800 C on the CO2-TPD profiles of Y2O3 and NiO/Y2O3 could be attributed to the decomposition of Y2O2CO3 on the surface of Y2O3 and NiO/Y2O3.
3.3.
Reduction behaviors of supports and catalysts
The reduction behaviors of Y2O3, g-Al2O3 and the relevant supported Ni catalysts (NiO/Y2O3 and NiO/g-Al2O3) were measured by TPR, and the results are shown in Fig. 6. Clearly, g-Al2O3 was irreducible under the conditions employed. As for Y2O3, a weak reduction peak centered at about 600 C was observed. It has been reported that, Y2O3 could be partially reduced due to the lattice oxygen on its surface [40]. For NiO/gAl2O3, two reduction peaks were observed at 400 w 650 C (the peak was weak) and 850 C, respectively. The peak at 400 w 650 C was attributed to the reduction of NiO that weakly interacted with g-Al2O3 and the peak at about 850 C was ascribed to the reduction of NiAl2O4 [41]. For NiO/Y2O3,
3.4.
Catalytic performance of Ni/Y2O3
The catalytic performances of Ni/Y2O3 and Ni/g-Al2O3 in POM are shown in Fig. 7. It could be seen that the conversion of CH4 on Ni/Y2O3 catalyst at the initial stage of reaction was about 82%, with CO selectivity and H2 selectivity higher than 90%. On Ni/g-Al2O3, the initial conversion of CH4 was about 82%, too, and CO selectivity and H2 selectivity were also higher than 90%. However, CH4 conversion on Ni/g-Al2O3 decreased from 82% to 79% after 50 h reaction in POM. By comparison, on Ni/Y2O3 catalyst, CH4 conversion decreased little even after 125 h reaction in POM. Further prolonging the reaction time over Ni/Y2O3 to 550 h, still no obvious deactivation occurred (Fig. 8). That is to say, Ni/Y2O3 showed high stability in POM. In order to further examine the stability of Ni/Y2O3 catalyst over a wide range of reaction temperatures, the reaction temperatures were varied periodically during POM. Firstly, the initial reaction temperature was 700 C and kept for 24 h. Then it was increased to 800 C and kept at 800 C for 24 h, and then decreased to 700 C, 600 C, 500 C and 400 C successively, and at each temperature it was kept for 24 h. Finally it was raised again to 700 C and kept for 24 h. The results are shown in Fig. 9. The data for the first 24 h were similar to those in Figs. 7 and 8, and the catalytic performance of Ni/Y2O3 catalyst changed little at 700 C after a series of reaction temperature variations. This indicates that Ni/Y2O3 was rather stable in
Ni/Al2O3 Al2O3
Intensity (a.u.)
Temperature ( C)
o
950 C
Ni/Y2O3
Y2O3
100 200 300 400 500 600 700 800 900 950 950 950 o
Temperature ( C) Fig. 6 e TPR profiles of Ni/Y2O3 and Ni/g-Al2O3 Data of Al2O3 was cited from Ref. [36].
14452
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 4 4 4 7 e1 4 4 5 4
90
H 2 selectivity
CH4 conversion (%)
85
Conversion or selectivity (%)
100 Ni/Y2O3
80
Ni/Al2O3 75 70 65
CO selectivity
80 CH 4 conversion 60
o
700 C
o
0
H2 selectivity (%)
o
500 C
400 C
o
700 C
40
20
40
60
80
100
0
120
20
40
60
80
100 120 140 160
Time on stream (h)
Time on stream (h)
Fig. 9 e Stability test of Ni/Y2O3 in POM (Reaction temperature varied).
100
CO selectivity (%)
o
o
600 C
20
60
80 60
Ni/Y2O3
Ni/Al2O3
reason was probably that water-gas shift reaction was favored at low temperatures [46,47].
40 20 0
3.5. Effects of GHSV (gas hourly space velocity) on the catalytic performance of Ni/Y2O3 catalyst
100 80 60
Ni/Y2O3
Ni/Al2O3
40 20 0
0
20
40
60
80
100
120
Time on stream (h) Fig. 7 e Comparison of catalytic performance of Ni/Y2O3 and Ni/g-Al2O3 in POM Data of Ni/g-Al2O3 was cited from Ref. [36].
H2 selectivity
100
Fig. 10 shows the effects of GHSV (in the range of 9 104 h1 to 24 104 h1) on the catalytic performance of Ni/Y2O3 in POM. It could be discovered that, CH4 conversion increased slightly with the increase of GHSV, whereas CO selectivity and H2 selectivity increased apparently with the increase of GHSV. The results were similar with that reported by Wang et al. [31]. POM is an exothermic reaction, that is, it could be influenced by heat transfer [48]. The increase in GHSV could result in more heat generation per unit time due to the release of reaction heat, and meanwhile, it could also improve the heat transfer in the catalyst bed. There was a balance between heat generation and heat transportation out of the reaction system. At higher GHSV, more heat energy released, which could lead to an increase in reaction temperature and finally higher
CO selectivity 80
CH4 conversion 60
40
CO selectivity
100
Conversion or selectivity (%)
POM within the temperature range of 400 w 800 C. Besides, it could be seen that, higher catalytic activities were observed at higher reaction temperatures. H2 selectivity was higher at lower reaction temperatures (400 w 700 C), while CO selectivity was higher at higher reaction temperature (800 C). The
Conversion or selectivity (%)
o
800 C 700 C
90 H2 selectivity
80 CH4 conversion
70
60 50 8
10
12
14
16
18 4
20
22
24
26
-1
GHSV (×10 h )
20 0
100
200
300
400
500
Time on stream (h) Fig. 8 e Stability test of Ni/Y2O3 in POM (550 h TOS).
Fig. 10 e Effects of GHSV on the catalytic performance of Ni/ Y2O3 in POM Reaction conditions: 700 C, atmospheric pressure, CH4/O2 [ 2.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 4 4 4 7 e1 4 4 5 4
conversion of methane [49]. The reason for the selectivities to H2 and CO increase with GHSV was probably due to the reduction of secondary reactions, which resulted in higher selectivities to the target products, H2 and CO.
3.6. Crystalline structures of Ni/Y2O3 and anti-carbon deposition properties of Ni/Y2O3 The crystalline structures of Ni/Y2O3 catalyst after POM reaction with different time on streams are shown in Fig. 1. It can be seen that after 10 min reaction in POM, the diffraction peaks attributed to the phases of Ni or NiO could not be observed. This implies that Ni particles were still well dispersed on the Ni/Y2O3 catalyst after the reduction and 10 min reaction in POM. However, after 125 h and 550 h reaction in POM, the diffraction peaks of Ni phases were detected on Ni/Y2O3, indicating Ni particles aggregated somewhat during POM reactions. From the XRD patterns in Fig. 1, no diffraction peak attributed to graphic carbon was observed on the three used Ni/Y2O3 catalysts after different reaction time in POM. This means that no obvious carbon deposition occurred on these catalysts. TGA was carried out to evaluate the amount of carbon deposited on Ni/Y2O3 and Ni/g-Al2O3 after different reaction time in POM, and the results are listed in Table 1. It could be seen that the amount of carbon deposited on Ni/gAl2O3 was 3.1% after 50 h reaction in POM, whereas that on Ni/ Y2O3 was only 1.9% after 125 h reaction and 2.2% after 550 h reaction in POM. The amount of carbon deposited on Ni/Y2O3 was much less than that on Ni/g-Al2O3 catalyst. Namely, Ni/ Y2O3 possessed stronger ability to resist carbon deposition. Guo et al. [50] reported that the acidic-basic properties of catalysts could affect the ability of Ni-based catalysts to resist carbon deposition. The more basic of the catalysts, the more chances for the reaction between deposited carbon and adsorbed CO2, and the less chances for Boudouard reaction (CO / CO2 þ C) [51,52]. Then it could be inferred that the basicity of Y2O3 contributed to the less amount of carbon deposited on it in POM reaction.
4.
Conclusions
Y2O3, prepared by the conventional precipitation method, was a basic support with more basic sites and almost no acidic sites. After loading of Ni, the prepared Ni/Y2O3 catalyst showed relatively high catalytic activity and high selectivities to CO and H2 in POM reaction. Ni/Y2O3 was a stable and potential catalyst in POM, and it showed strong capacity to resist carbon deposition, after 550 h POM reaction, negligible amount of carbon deposited.
Acknowledgement We acknowledge the financial support of this work from National Basic Research Program of China (973 Program, 2011CB201405), NSFC (21073104, 20921001) and Analytical fund of Tsinghua University.
14453
references
[1] Choudhary VR, Rane VH. Acidity basicity of rare-earth oxides and their catalytic activity in oxidative coupling of methane to C2-hydrocarbons. J Catal 1991;130:411e22. [2] Jones CJ, Taube D, Ziatdinov VR, Periana RA. Selective oxidation of methane to methanol catalyzed, with C-H activation, by homogeneous, cationic gold. Angew Chem Int Edit 2004;43:4626e9. [3] Cheekatamarla PK, Finnerty CM. Synthesis gas production via catalytic partial oxidation reforming of liquid fuels. Int J Hydrogen Energy 2008;33:5012e9. [4] Hickman DA, Haupfear EA, Schmidt LD. Synthesis Gaformation by direct oxidation of methane over Rh monoliths. Catal Lett 1993;17:223e37. [5] Freni S, Calogero G, Cavallaro S. Hydrogen production from methane through catalytic partial oxidation reactions. J Power Sources 2000;87:28e38. [6] Tanaka H, Kaino R, Okumura K, Kizuka T, Yoshinao N, Tomishige K. Comparative study of Rh/MgO modified with Fe, Co or Ni for the catalytic partial oxidation of methane at short contact time. Part I: characterization of catalysts. Appl Catal A 2010;378:175e86. [7] Perkasa N, Zhongb Z, Chenb L, Bessonc M, Gedanken A. Sonochemically prepared high dispersed Ru/TiO2 mesoporous catalyst for partial oxidation of methane to syngas. Catal Lett 2005;103:9e14. [8] Basini L, Aragno A. Molecular mechanisms in partial oxidation of methane on Ir/alpha-Al2O3: reactivity dependence on catalyst properties and transport phenomena limitations. Catal Lett 1996;39:49e55. [9] Feio LSF, Hori CE, Mattos LV, Zanchet D, Noronha FB, Bueno JMC. Partial oxidation and autothermal reforming of methane on Pd/CeO2eAl2O3 catalysts. Appl Catal A 2008;348: 183e92. [10] Corbo P, Fortunato M. Hydrogen production by catalytic partial oxidation of methane and propane on Ni and Pt catalysts. Int J Hydrogen Energy 2007;32:55e66. [11] Ji YY, Li WZ, Xu HY, Chen YX. A study of carbon deposition on catalysts during the catalytic partial oxidation of methane to syngas in a fluidized bed. React Kinet Catal Lett 2001;73: 27e32. [12] Claridge JB, Green MLH, Tsang SC, York APE, Ashcroft AT, Battle PD. A study of carbon deposition on catalysts during the partial oxidation of methane to synthesis gas. Catal Lett 1993;22:299e305. [13] Choudhary VR, Rajput AM, Prabhakar B. Low-temperature oxidative conversion of methane to syngas over NiOeCaO catalyst. Catal Lett 1992;15:363e70. [14] Choudhary VR, Rane VH, Rajput AM. Beneficial effects of cobalt addition to Ni-catalysts for oxidative conversion of methane to syngas. Appl Catal A 1997;162:235e8. [15] Miao Q, Xiong GX, Sheng SS, Cui W, Xu L, Guo XX. Partial oxidation of methane to syngas over nickel-based catalysts modified by alkali metal oxide and rare earth metal oxide. Appl Catal A 1997;157:17e27. [16] Song YQ, He DH, Xu BQ. Effects of preparation methods of ZrO2 support on catalytic performances of Ni/ZrO2 catalysts in methane partial oxidation to syngas. Appl Catal A 2008; 337:19e28. [17] Requies J, Cabrero MA, Barrio VL, Guemez MB, Cambra JF, Arias PL, et al. Partial oxidation of methane to syngas over Ni/MgO and Ni/La2O3 catalysts. Appl Catal A 2005;289:214e23. [18] Weng WZ, Luo CR, Huang JJ, Liao YY, Wan HL. Comparative study on the mechanism of partial oxidation of methane to syngas over Rhodium supported on SiO2 and gama-Al2O3. Top Catal 2003;22:87e93.
14454
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 4 4 4 7 e1 4 4 5 4
[19] Shang RJ, Wang YY, Jin GQ, Guo XY. Partial oxidation of methane over nickel catalysts supported on nitrogen-doped SiC. Catal Comm 2009;10:1502e5. [20] Shang RJ, Sun WZ, Wang YY, Jin GQ, Guo XY. Silicon nitride supported nickel catalyst for partial oxidation of methane to syngas. Catal Comm 2008;9:2103e6. [21] Miao Q, Xiong GX, Li XS, Sheng SS, Guo XX. Acid-base properties and the directions of oxidative transformation of methane over nickel-based catalysts. Catal Lett 1996;41: 165e9. [22] Lin CK, Zhang CM, Lin J. Sol-gel derived Y2O3 as an efficient bluish-white phosphor without metal activator ions. J Lumin 2009;129:1469e74. [23] Wang ZX, Xu HL, Zhang ZY, Wang S, Ding L, Zeng QS, et al. Growth and performance of yttrium oxide as an ideal highkappa gate dielectric for carbon-based electronics. Nano Lett 2010;10:2024e30. [24] Kodo M, Soga K, Yoshida H, Yamamoto T. Doping effect of divalent cations on sintering of polycrystalline yttria. J Eur Ceram Soc 2010;30:2741e7. [25] Yao W, Lu GZ, Guo YL, Guo Y, Wang YQ, Zhang ZG. Promotional effect of Y2O3 on the performance of Ag/aeAl2O3 catalyst for epoxidation of propylene with molecular oxygen. J Mol Catal A Chem 2007;276:162e7. [26] Costa LOO, Silva AM, Borges LEP, Mattos LV, Noronha FB. Partial oxidation of ethanol over Pd/CeO2 and Pd/Y2O3 catalysts. Catal Today 2008;138:147e51. [27] Wu XS, Kawi S. Steam reforming of ethanol to H2 over Rh/ Y2O3: crucial roles of Y2O3 oxidizing ability, space velocity, and H2/C. Energy Environ Sci 2010;3:334e42. [28] Resini C, Delgado MHC, Presto S, Alemany LJ, Riani P, Marazza R, et al. Yttria-stabilized zirconia (YSZ) supported NieCo alloys (precursor of SOFC anodes) as catalysts for the steam reforming of ethanol. Int J Hydrogen Energy 2008;33: 3728e35. [29] Lintanf A, Djurado E, Vernoux P. Pt/YSZ electrochemical catalysts prepared by electrostatic spray deposition for selective catalytic reduction of NO by C3H6. Solid State Ionics 2008;178:1998e2008. [30] Blanchard J, Nsungui AJ, Abatzoglou N, Gitzhofer F. Dry reforming of methane with a Ni/Al2O3eYSZ catalyst: the role of the catalyst preparation protocol. Can J Chem Eng 2007;85: 889e99. [31] Wang YQ, Wang WJ, Hong XB, Li YH, Zhang ZG. Yttriumstabilized zirconia-promoted metallic nickel catalysts for the partial oxidation of methane to hydrogen. Int J Hydrogen Energy 2009;34:2252e9. [32] Santos DC, Madeira L, Passos FB. The effect of the addition of Y2O3 to Ni/alpha-Al2O3 catalysts on the autothermal reforming of methane. Catal Today 2010;149:401e6. [33] Fu LY, Xie WG, Lv SJ, Qiu FL. Influence of support on resistance to carbon deposition of catalyst for CH4, CO2 with O2 to synthesis gas. Sci China Ser B Chem 2000;2:154e61. [34] Wang YQ, Hong XB, Li BB, Wang WJ, Wang DL. Yttria promoted metallic nickel catalysts for the partial oxidation of methane to synthesis gas. J Nat Gas Chem 2008;17:344e50.
[35] Nishimoto H, Nakagawa K, Ikenaga N, Suzuki T. Partial oxidation of methane to synthesis gas over Ru-loaded Y2O3 catalyst. Catal Lett 2002;82:161e7. [36] Liu HM, He DH. Physicochemical properties of Ni/ gammaeAl2O3eAlN and effects of AlN on catalytic performance of Ni/gammaeAl2O3eAlN in partial oxidation of methane. J Phys Chem C 2010;114:13716e21. [37] Sato S, Takahashi R, Kobune M, Gotoh H. Basic properties of rare earth oxides. Appl Catal A 2009;356:57e63. [38] Wang SJ, Zhong SL, Ou-Yang XJ, Hu N, Chen XS, Wang SP, et al. Y(OH)3 and Y2O3 with novel structures: formation and mechanism. Mater Sci Eng B 2009;162:200e4. [39] Miao F, Yu L, Sun M, Li CS, Zhang L. Preparation and characterization of nanometer yttrium oxide. Fine Chem 2009;4:313e7. [40] Sun GB, Hidajat K, Wu XS, Kawi S. A crucial role of surface oxygen mobility on nanocrystalline Y2O3 support for oxidative steam reforming of ethanol to hydrogen over Ni/Y2O3 catalysts. Appl Catal B 2008;81:303e12. [41] Hou ZY, Osamu Y, Takumi T, Tatsuaki Y. Investigation of CH4 reforming with CO2 on meso-porous Al2O3-supported Ni catalyst. Catal Lett 2003;89:121e7. [42] Dewaele O, Froment GF. Tap study of the mechanism and kinetics of the adsorption and combustion of methane on Ni/Al2O3 and NiO/Al2O3. J Catal 1999;184:499e513. [43] Hoffer BW, Langeveld AD, Janssens JP, Bonne RLC, Lok CM, Moulijin JA. Stability of highly dispersed Ni/Al2O3 catalysts: effects of pretreatment. J Catal 2000;192:432e40. [44] Komandur VRC, Pendyala VRR, Vattikonda VR. Catalytic functionalities of nickel supported on different polymorphs of alumina. Catal Comm 2008;9:886e93. [45] Bellido JDA, Assaf EM. Effect of the Y2O3eZrO2 support composition on nickel catalyst evaluated in dry reforming of methane. Appl Catal A 2009;352:179e87. [46] Zhu JJ, Ommen JG, Knoester A, Lefferts L. Effect of surface composition of yttrium-stabilized zirconia on partial oxidation of methane to synthesis gas. J Catal 2005;230: 291e300. [47] Liu TF, Snyder C, Veser G. Catalytic partial oxidation of methane: is a distinction between direct and indirect pathways meaningful? Ind Eng Chem Res 2007;46:9045e52. [48] Rabe S, Truong TB, Vogel F. Low temperature catalytic partial oxidation of methane for gas-to-liquids applications. Appl Catal A 2007;318:54e62. [49] Bodke AS, Bharadwaj SS, Schmidt LD. The effect of ceramic supports on partial oxidation of hydrocarbons over noble metal coated monoliths. J Catal 1998;179:138e49. [50] Guo CL, Zhang JL, Zhang XL. Comparative study of LaNiO3 and La2NiO4 catalysts for partial oxidation of methane. React Kinet Catal Lett 2008;95:89e97. [51] Yamazaki O, Nozaki T, Omatak YK, Fujimoto K. Reduction of carbon-dioxide by methane with Ni-on-MgOeCaO containing catalysts. Chem Lett 1992;10:1953e4. [52] Geon JK, Dong SC, Kwang HK, Jong HK. The reaction of CO2 with CH4 to synthesize H2 and CO over nickel-loaded Yzeolites. Catal Lett 1994;28:41e52.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Properties of Ni/Y2O3 and its catalytic performance in methane conversion to syngas Huimin Liu, Dehua He* Innovative Catalysis Program, Key Laboratory of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, PR China
article info
abstract
Article history:
Ni/Y2O3, with Y2O3 support prepared by the conventional precipitation method, was
Received 9 June 2011
prepared by an impregnation method. The physicochemical properties of Y2O3 and Ni/Y2O3
Received in revised form
were characterized by BET, CO2-TPD, NH3-TPD, TPR, XRF and TGA, and compared with
28 July 2011
those of g-Al2O3 and Ni/g-Al2O3, respectively. The catalytic performance of Ni/Y2O3 in the
Accepted 7 August 2011
reaction of partial oxidation of methane (POM) to syngas was evaluated and compared with
Available online 13 September 2011
that of Ni/g-Al2O3 catalyst, too. The results showed that, Y2O3 was a basic support with few
Keywords:
more easily to be reduced than those supported on g-Al2O3. In the partial oxidation of
acidic sites while g-Al2O3 was an acidic support. NiO particles supported on Y2O3 were Ni/Y2O3 catalyst
methane, Ni/Y2O3 catalyst showed high catalytic activity and exhibited better catalytic
Partial oxidation of methane
stability than Ni/g-Al2O3. After POM reaction at 700 C for 550 h, methane conversion decreased little and only 2.2 wt% carbon was deposited on Ni/Y2O3 catalyst. Ni/Y2O3 was
Syngas
stable in POM even after a series of reaction temperature variations within the temperature range of 400 w 800 C. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Catalytic conversion of methane to syngas (and also to hydrogen gas) is an important route for the effective utilization of natural gas [1e4]. Generally, there are three ways for the conversion of methane to syngas, including the steam reforming of methane (SRM), the carbon dioxide reforming of methane (CRM) and the partial oxidation of methane (POM). SRM is a traditional industrial route, whereas CRM and POM are potential technical routes and have attracted more and more attentions in the recent 20 w 30 years. Particularly, POM has overwhelmed SRM due to its obvious advantages, such as high energy efficiency [5], suitable H2/CO ratio for methanol synthesis and FischereTropsch processes. In the partial oxidation of methane, compared with noble metal-based catalysts, such as Rh, Ru, Ir, Pd, Pt [6e10],
Ni-based catalysts have been widely studied because of their good catalytic performances as well as the low costs. However, Ni-based catalysts are suffered from deactivation due to carbon deposition in POM [11,12]. Lots of methods have been employed to improve their abilities to resist carbon deposition. Reports showed that the acidic-basic properties of Ni-based catalysts could affect the amount of carbon deposited. Choudhary et al. [13,14] discovered that, no obvious carbon was deposited on Ni/CaO after 15 h reaction in POM. Miao et al. [15] modified Ni/Al2O3 catalyst with Li2O and La2O3, and the obtained LiNiLaOx/Al2O3 exhibited improved ability to resist carbon deposition during the 50 h life-test in POM. Similar results were also obtained on ZrO2, MgO and La2O3, which were also used as the supports of Ni-based catalysts in POM [16e18]. Except oxide supports, non-oxide supports, such as SiC and Si3N4, were also employed in POM. Shang et al. [19]
* Corresponding author. Tel/Fax: þ86 10 62773346. E-mail address: [email protected] (D. He). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.08.025
14448
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 4 4 4 7 e1 4 4 5 4
studied the catalytic performance of nitrified Ni/SiC in the partial oxidation of methane, and the results showed that, compared with Ni/SiC, less carbon was deposited on the nitrified Ni/SiC. Ni/Si3N4 also exhibited good catalytic performance in POM [20], and almost no carbon was deposited. However, it is not the truth that, the more basic of catalysts, the more able to resist carbon deposition, because methane coupling reaction might occur on catalysts with the supports of strong basicity [21]. Therefore, searching suitable supports for POM is still an important issue. Y2O3, an important metal oxide, has been applied in a wide range of areas because of its optical, thermal and chemical stabilities [22e24]. In the recent years, Y2O3 has also been applied in some catalytic reactions. Yao et al. [25] showed that the addition of a small quantity of Y2O3 and K2O to Ag/Al2O3 could adjust the electronic density of adjacent silver atoms and produce proper adsorbed oxygen species, and the conversion of propylene and the selectivity to propylene oxide were high on the obtained AgeY2O3eK2O/Al2O3 catalyst in the reaction of epoxidation of propylene. Costa et al. [26] studied Pd/CeO2 and Pd/Y2O3 in the partial oxidation of ethanol, and it was found that the selectivity to CO was higher over Pd/Y2O3 catalyst, since Pd/Y2O3 catalyst favored the transformation of ethoxy species to acetate whereas Pd/CeO2 catalyst facilitated the further oxidization of CO to CO2. Wu et al. [27] employed different Rh supported catalysts in the steam reforming of ethanol (SRE), and the order of catalytic activity was: Rh/Y2O3 > Rh/CeO2 > Rh/La2O3 > Rh/Al2O3. The activity of Rh/Y2O3 in SRE was promoted due to the surface oxygen vacancies of Y2O3. Yttria stabilized zirconia (YSZ) was also used as supports in the steam reforming of ethanol [28], the selective reduction of NO [29], the carbon dioxide reforming of methane [30] and the partial oxidation of methane [31]. Santos et al. added a small amount of Y2O3 to Al2O3, and the obtained Ni/Al2O3eY2O3 catalyst presented higher catalytic activity than Ni/Al2O3 in the methane autothermal reforming reaction (ATR) [32]. Fu et al. [33] used different oxides, including Y2O3, to modify Al2O3 support, and found that the ability to resist carbon deposition of the prepared Nibased catalysts (measured after 1 h TOS of ATR) decreased in the following order: Ni/CaOeAl2O3 > Ni/MgOeAl2O3 > Ni/ TiO2eAl2O3 > Ni/CeO2eAl2O3 > Ni/La2O3eAl2O3 > Ni/ Y2O3eAl2O3 > Ni/Fe2O3eAl2O3 > Ni/Al2O3. Wang et al. [34] employed Y2O3 promoted metallic Ni catalyst in POM. The catalyst was acid treated nickel sponge and modified with Y2O3 by an impregnation method. The results showed that the conversion of CH4 and the selectivities to H2 and CO were increased on the Y2O3 modified Ni sponge. However, the amount of carbon deposited was not analyzed and long time life-test was not reported by Wang et al. [34]. What’s more, in this case, Y2O3 was not used as catalyst support, instead, only a small amount of Y2O3 was used (the content of Y2O3 was in the range of 1.38 w 10.1%). On the other hand, Y2O3 was also used as a support of Ru catalyst in the partial oxidation of methane [35], and Nishimoto et al. reported that Ru/Y2O3 was an active catalyst in POM and no carbon was deposited after 10 h reaction. However, the physicochemical properties of Y2O3 were not revealed or longer time life-test was not reported either. In the previous studies of Y2O3 as supports, no matter Rh/Y2O3 in SRE, Ni/Al2O3eY2O3 in ATR, or Ru/Y2O3 in
POM, the relationship between the properties of Y2O3 and the catalytic performance of the relevant catalysts was not investigated intensively. However, the properties of Y2O3 might exert an influence on the performance of Ni/Y2O3 catalyst in POM. Therefore, in this paper, the physicochemical properties of Y2O3 and Ni/Y2O3, as well as the catalytic performance of Ni/Y2O3 catalyst in POM, were studied and compared with those of Al2O3 and Ni/Al2O3, respectively.
2.
Experimental section
2.1.
Catalyst preparation
Y2O3 was prepared by the conventional precipitation method. 200 mL Y(NO3)3$6H2O aqueous solution (0.25 M) containing 0.05 mol Y(NO3)3 was added dropwise into 145 mL NH3 aqueous solution (2.5 wt%) under the conditions of pH 10 w 11 and vigorous stirring. After 0.5 h of stirring, the white precipitate of Y(OH)3 was aged at room temperature for 12 h. Then, the precipitate was filtered and washed thoroughly with deionized water till the pH of the filtrated mother liquor was 7. Afterward, the Y(OH)3 hydrogel was dried at 110 C for 12 h in air and then calcined at 500 C for 5 h in air. Ni/Y2O3 catalyst was prepared by impregnating Ni(NO3)2∙6H2O aqueous solution (analytical grade reagent, provided by Shantou Xilong chemical factory) with the above obtained Y2O3 support at room temperature for 10 h. After water being removed by vaporization, the precursor of Ni/Y2O3 was then dried at 110 C in air for 12 h and calcined at 650 C in air for 5 h. For comparison, Ni/g-Al2O3 was also prepared with the impregnation method, and the detailed information was available in our previous paper [36].
2.2.
Catalyst characterization
The specific surface areas of the catalysts and the supports were measured by N2 adsorption-desorption with the BET method on a Micromeritics ASAP 2010 C analyzer. The crystalline phases of the Ni particles and the supports were investigated by X-ray diffraction (XRD). The reduction behaviors of the catalysts were characterized by Temperature programmed reduction (TPR). The amounts of carbon deposited on the catalysts were evaluated by thermogravimetric analysis (TGA). The acidic properties of the catalysts were measured by NH3-temperature programmed desorption (NH3TPD). The actual contents of Ni on the catalysts were measured by X-ray fluorescence spectrometer (XRF). The detailed experimental procedures of BET, XRD, TPR, TGA, XRF and NH3-TPD were consistent with those in our previous paper [36]. The basic properties of the catalysts were measured by CO2-temperature programmed desorption (CO2-TPD). The CO2-TPD profiles were measured by Quantachrome adsorption instrument (Chembet-3000 TPR/TPD). The catalysts (0.1 g) were firstly treated in highly pure He (99.999%, 110 mL/min) at 500 C for 0.5 h. After that, the catalysts were saturated with flowing highly pure CO2 at 100 C, and then flushed with highly pure He (110 mL/min) to remove the physically adsorbed CO2. Finally, the desorption of CO2 was carried out in
14449
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 4 4 4 7 e1 4 4 5 4
*
Y2O3
*
+ Ni
*
Intensity (a.u.)
*
Ni/Y2O3 (550h POM)
* * * *+* * * ****
*
Ni/Y2O3 (125h POM)
+ Ni/Y2O3 (10 min POM)
NiO/Y2O3 Y2O3
10
20
30
40
50
60
70
80
2 Theta degree Fig. 1 e XRD patterns of Y2O3 support and Ni/Y2O3 catalyst.
flowing highly pure He (110 mL/min) from 100 C to 800 C with a heating ramp of 15 C/min. Nevertheless, carbonate (Y2O2CO3) may be formed on Y2O3 and Ni/Y2O3 due to the preservation of the samples in air [37]. In order to eliminate the influence of Y2O2CO3 on the CO2-TPD profiles, CO2-TPD profiles obtained by pretreating catalysts at 700 C for 0.5 h before CO2 adsorption was used as reference.
with the inlet and outlet sides of the catalyst bed, were used to measure the temperatures of the inlet and outlet sides of the bed. Before reaction, the catalyst was reduced with 20 mol% H2/Ar at 600 C for 2 h, then the temperature was raised to the reaction temperature (700 C), and then the mixture gas of CH4 and O2 with a molar ratio of 2/1 was introduced into the reactor at a total flow rate of 165 mL/min (The concentration of CH4 is 66.7 mol% and the concentration of O2 is 33.3 mol%, and no inert gas was introduced). The effluent gas was firstly cooled down in a cool trap (about 0 C) to remove water in the products and the gaseous products were analyzed by two online gas chromatographs (GC), both equipped with TDX-01 columns and connected with TCD detectors. The GC-A with Ar as the carrier gas was used to analyze the volume ratio of H2/CO in the products, and the GC-B with H2 as the carrier gas was used to analyze the components of CH4, CO, O2 and CO2 in the products. The relative amount of the gases in the products was calculated by the normalization method, and the data obtained by these two GC were linked by CO amount. The equations were shown as follows: Conversion of CH4 ¼ FinCH4 FoutCH4
Selectivity of CO ¼ FoutCO = FinCH4 FoutCH4 100% Selectivity of H2 ¼ FoutH2 =2= FinCH4 FoutCH4 100% F volume flow rate; mL=min
3. 2.3.
FinCH4 100%
Results and discussion
Catalytic performance
The apparatus used to evaluate the catalytic performance of Ni/Y2O3 in POM was the same as that in our previous paper [36]. It was conducted in a fixed-bed quartz reactor with an inner diameter of 5 mm under atmosphere. A sample (0.1 g) of the catalysts was packed in the center of the quartz reactor. A furnace was used to heat the quartz tube reactor. A thermocouple, placed in the center of the furnace, was used to control the temperature of the furnace. Another two thermocouples, installed in the quartz tube reactor and contacted
3.1. Crystal structures and textural properties of supports and catalysts The XRD pattern of Y2O3 support is shown in Fig. 1. It could be seen that the Y2O3 prepared by the conventional precipitation method was cubic Y2O3 [01-082-2415]. The result was consistent with those reported by Lin et al. [22] and Wang et al. [38], who found that cubic Y2O3 could be obtained by calcining Y(OH)3 hydrogel at temperatures higher than 500 C. The crystalline phase of g-Al2O3 was also confirmed by XRD, and
Table 1 e Textural properties of supports and amount of carbon deposition. Specific surface areas (m2/g)a
Pore size distributions (nm)b
Actual content of Ni (wt %)c
TOS(h)
Amount of carbon deposition (%)d
Y 2O 3 g-Al2O3e Ni/Y2O3
50.8 142.5 43.4
2e8 2e8 5e20
e e 5.28
Ni/g-Al2O3e
116.9
2e8
5.45
e e 125 550 50
e e 1.9 2.2 3.1
Catalyst/support
a b c d e
Measured by N2 adsorption-desorption with the BET method. Measured by BJH. Measured by XRF. Measured by TGA. Data was cited from Ref. [36].
14450
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 4 4 4 7 e1 4 4 5 4
3.2. Acidic and basic properties of supports and catalysts
Volume adsorbed, cm 3 /g-STP
The acidic properties of Y2O3, g-Al2O3 and the relevant supported Ni catalysts were measured by NH3-TPD, and the profiles are shown in Fig. 3. For Y2O3 support, there was a broad and weak NH3 desorption peak at the temperature of 125 w 300 C, whereas for g-Al2O3 support, broad NH3 desorption peaks with high intensity were observed within the temperature range of 125 w 500 C. Besides, the area of NH3 desorption peaks on g-Al2O3 was much larger than that on Y2O3. That is to say, there were more acidic sites on g-Al2O3 than on Y2O3. For NiO/g-Al2O3, the area and the intensity of the desorption peaks of NH3 decreased a little compared with g-Al2O3 support. For NiO/Y2O3, the area and the intensity of
Al2O3 Ni/Al2O3
Intensity (a.u.)
the result was shown in our previous paper [36]. The textural properties of Y2O3 and g-Al2O3, measured by N2 adsorptiondesorption isotherm method, are listed in Table 1. The specific surface area of Y2O3 was 50.8 m2/g, lower than that of g-Al2O3 (142.5 m2/g). However, both Y2O3 and g-Al2O3 exhibited IV-type isotherms with pore size distributions within the range of 2 w 8 nm (Fig. 2). After loading of NiO on Y2O3 (the actual content of Ni was measured by XRF and listed in Table 1), the XRD pattern of NiO/Y2O3 (before reduction) is shown in Fig. 1. It could be seen that no peak attributed to NiO phase was observed, implying that NiO particles were well dispersed on Y2O3. The phase of cubic Y2O3 could be clearly observed in the XRD pattern of NiO/Y2O3. The textural properties of Ni/Y2O3 and Ni/g-Al2O3 are listed in Table 1. Both catalysts exhibited IV-type isotherms (Fig. 2). However, for both catalysts, the specific surface areas decreased somewhat compared with the relevant supports. The specific surface area of Ni/Y2O3 was 43.4 m2/g and that of Ni/g-Al2O3 was 116.9 m2/g. Besides, for Ni/g-Al2O3, the average pore size didn’t change compared with g-Al2O3, while for Ni/Y2O3, the pore size increased to the range of 5 w 20 nm.
Ni/Y2O3
Y2O3 100
200
300
400
500
600
o
Temperature ( C) Fig. 3 e NH3-TPD profiles of Y2O3, g-Al2O3 and relevant supported Ni catalysts. Data of Al2O3 was cited from Ref. [36].
the desorption peak of NH3 changed little after the impregnation of NiO on Y2O3. The basic properties of Y2O3, g-Al2O3 and the relevant supported Ni catalysts were measured by CO2-TPD, and the profiles are shown in Fig. 4. It could be seen that, for Y2O3 support, there was a broad CO2 desorption peak with high intensity at 110 w 550 C, on the contrary, a small and weak CO2 desorption peak was observed on g-Al2O3 support. Nevertheless, the CO2 desorption peak for Y2O3 was centered at about 250 C while that for g-Al2O3 was centered at about 180 C. What’s more, the area of CO2 desorption peak on Y2O3 was much larger than that on g-Al2O3. That is to say, in comparison to g-Al2O3, there were more basic sites on Y2O3 with stronger basic strength. For NiO/Y2O3 and NiO/Al2O3 catalysts, the number of basic sites and the strength of basicity changed little compared with the relevant supports. However, in the present research, wide and broad CO2 desorption peaks were also observed at 550 w 800 C on the CO2-TPD profiles of Y2O3 and NiO/Y2O3. In order to investigate
Y2O3
Ni/Y2O3
Ni/Y2O3
Intensity (a.u.)
Y2O3
Ni/Al2O3
Al2O3
Al2O3
Ni/Al2O3
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P0 ) Fig. 2 e Adsorption-desorption isotherms of Y2O3, g-Al2O3 and relevant supported Ni catalysts Data of Al2O3 was cited from Ref. [36].
100
200
300
400
500
600
700
800
o
Temperature ( C) Fig. 4 e CO2-TPD profiles of Y2O3, g-Al2O3 and relevant supported Ni catalysts.
14451
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 4 4 4 7 e1 4 4 5 4
o
Intensity (a.u.)
Treated at 500 C before CO2 adsorption
o
Treated at 700 C before CO2 adsorption
100
200
300
400
500
600
700
800
900
three reduction peaks, centered at 280 C, 400 C and 500 C respectively, were observed. Generally speaking, Ni-based catalysts would exhibit different reduction peaks, which depend on the nature of the supports [42,43]. Bulk reduction peaks of free NiO particles that don’t interact with supports generally appear at about 400 C, and the interactions between NiO and supports decrease the tendency of Ni cation to be reduced to metallic Ni [44]. Therefore, it could be speculated that, the two reduction peaks on NiO/Y2O3 centered at 400 C and 500 C could be attributed to the reduction of bulk NiO and the reduction of NiO that weakly interacted with Y2O3. As for the reduction peak on NiO/Y2O3 at about 280 C in TPR, it could be ascribed to the reduction of surface NiO particles promoted by the surface oxygen vacancies of Y2O3 [45].
o
Fig. 5 e CO2-TPD profiles of Y2O3 at different pretreatment conditions.
the attribution of CO2 desorption peak on Y2O3 and NiO/Y2O3 at 550 w 800 C, a comparative CO2-TPD experiment was carried out. In the comparative CO2-TPD experiment, Y2O3 was treated at 700 C for 0.5 h in the flowing He under atmospheric pressure before CO2 adsorption. The CO2-TPD profile obtained is shown in Fig. 5. Obviously, the CO2 desorption peak at 550 w 800 C disappeared on the profile obtained by pretreating Y2O3 at 700 C before CO2 adsorption. This implied that the CO2 desorption peak of Y2O3 at 550 w 800 C could not be ascribed to the basic properties of Y2O3. The result was in accordance with that of Sato et al.’s [37]. Sato et al. [37] reported that, on the CO2-TPD profile of Y2O3, no CO2 desorption peaks could be observed at temperatures higher than 500 C. Then, what’s the reason for the desorption of CO2 on Y2O3 and NiO/Y2O3 at 550 w 800 C in the present experiment? Sato et al. [37] also reported that, CO2 desorbed at temperatures above 500 C could be attributed to the decomposition of carbonate. In the present research, since Y2O3 and NiO/Y2O3 were preserved in air before the CO2-TPD experiment, Y2O2CO3 might be formed on Y2O3 due to the adsorption of CO2 in the air. Miao et al. [39] had proposed that Y2O2CO3 would decompose at about 640 C. Then, it’s speculated that the desorbed CO2 peak at 550 w 800 C on the CO2-TPD profiles of Y2O3 and NiO/Y2O3 could be attributed to the decomposition of Y2O2CO3 on the surface of Y2O3 and NiO/Y2O3.
3.3.
Reduction behaviors of supports and catalysts
The reduction behaviors of Y2O3, g-Al2O3 and the relevant supported Ni catalysts (NiO/Y2O3 and NiO/g-Al2O3) were measured by TPR, and the results are shown in Fig. 6. Clearly, g-Al2O3 was irreducible under the conditions employed. As for Y2O3, a weak reduction peak centered at about 600 C was observed. It has been reported that, Y2O3 could be partially reduced due to the lattice oxygen on its surface [40]. For NiO/gAl2O3, two reduction peaks were observed at 400 w 650 C (the peak was weak) and 850 C, respectively. The peak at 400 w 650 C was attributed to the reduction of NiO that weakly interacted with g-Al2O3 and the peak at about 850 C was ascribed to the reduction of NiAl2O4 [41]. For NiO/Y2O3,
3.4.
Catalytic performance of Ni/Y2O3
The catalytic performances of Ni/Y2O3 and Ni/g-Al2O3 in POM are shown in Fig. 7. It could be seen that the conversion of CH4 on Ni/Y2O3 catalyst at the initial stage of reaction was about 82%, with CO selectivity and H2 selectivity higher than 90%. On Ni/g-Al2O3, the initial conversion of CH4 was about 82%, too, and CO selectivity and H2 selectivity were also higher than 90%. However, CH4 conversion on Ni/g-Al2O3 decreased from 82% to 79% after 50 h reaction in POM. By comparison, on Ni/Y2O3 catalyst, CH4 conversion decreased little even after 125 h reaction in POM. Further prolonging the reaction time over Ni/Y2O3 to 550 h, still no obvious deactivation occurred (Fig. 8). That is to say, Ni/Y2O3 showed high stability in POM. In order to further examine the stability of Ni/Y2O3 catalyst over a wide range of reaction temperatures, the reaction temperatures were varied periodically during POM. Firstly, the initial reaction temperature was 700 C and kept for 24 h. Then it was increased to 800 C and kept at 800 C for 24 h, and then decreased to 700 C, 600 C, 500 C and 400 C successively, and at each temperature it was kept for 24 h. Finally it was raised again to 700 C and kept for 24 h. The results are shown in Fig. 9. The data for the first 24 h were similar to those in Figs. 7 and 8, and the catalytic performance of Ni/Y2O3 catalyst changed little at 700 C after a series of reaction temperature variations. This indicates that Ni/Y2O3 was rather stable in
Ni/Al2O3 Al2O3
Intensity (a.u.)
Temperature ( C)
o
950 C
Ni/Y2O3
Y2O3
100 200 300 400 500 600 700 800 900 950 950 950 o
Temperature ( C) Fig. 6 e TPR profiles of Ni/Y2O3 and Ni/g-Al2O3 Data of Al2O3 was cited from Ref. [36].
14452
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 4 4 4 7 e1 4 4 5 4
90
H 2 selectivity
CH4 conversion (%)
85
Conversion or selectivity (%)
100 Ni/Y2O3
80
Ni/Al2O3 75 70 65
CO selectivity
80 CH 4 conversion 60
o
700 C
o
0
H2 selectivity (%)
o
500 C
400 C
o
700 C
40
20
40
60
80
100
0
120
20
40
60
80
100 120 140 160
Time on stream (h)
Time on stream (h)
Fig. 9 e Stability test of Ni/Y2O3 in POM (Reaction temperature varied).
100
CO selectivity (%)
o
o
600 C
20
60
80 60
Ni/Y2O3
Ni/Al2O3
reason was probably that water-gas shift reaction was favored at low temperatures [46,47].
40 20 0
3.5. Effects of GHSV (gas hourly space velocity) on the catalytic performance of Ni/Y2O3 catalyst
100 80 60
Ni/Y2O3
Ni/Al2O3
40 20 0
0
20
40
60
80
100
120
Time on stream (h) Fig. 7 e Comparison of catalytic performance of Ni/Y2O3 and Ni/g-Al2O3 in POM Data of Ni/g-Al2O3 was cited from Ref. [36].
H2 selectivity
100
Fig. 10 shows the effects of GHSV (in the range of 9 104 h1 to 24 104 h1) on the catalytic performance of Ni/Y2O3 in POM. It could be discovered that, CH4 conversion increased slightly with the increase of GHSV, whereas CO selectivity and H2 selectivity increased apparently with the increase of GHSV. The results were similar with that reported by Wang et al. [31]. POM is an exothermic reaction, that is, it could be influenced by heat transfer [48]. The increase in GHSV could result in more heat generation per unit time due to the release of reaction heat, and meanwhile, it could also improve the heat transfer in the catalyst bed. There was a balance between heat generation and heat transportation out of the reaction system. At higher GHSV, more heat energy released, which could lead to an increase in reaction temperature and finally higher
CO selectivity 80
CH4 conversion 60
40
CO selectivity
100
Conversion or selectivity (%)
POM within the temperature range of 400 w 800 C. Besides, it could be seen that, higher catalytic activities were observed at higher reaction temperatures. H2 selectivity was higher at lower reaction temperatures (400 w 700 C), while CO selectivity was higher at higher reaction temperature (800 C). The
Conversion or selectivity (%)
o
800 C 700 C
90 H2 selectivity
80 CH4 conversion
70
60 50 8
10
12
14
16
18 4
20
22
24
26
-1
GHSV (×10 h )
20 0
100
200
300
400
500
Time on stream (h) Fig. 8 e Stability test of Ni/Y2O3 in POM (550 h TOS).
Fig. 10 e Effects of GHSV on the catalytic performance of Ni/ Y2O3 in POM Reaction conditions: 700 C, atmospheric pressure, CH4/O2 [ 2.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 4 4 4 7 e1 4 4 5 4
conversion of methane [49]. The reason for the selectivities to H2 and CO increase with GHSV was probably due to the reduction of secondary reactions, which resulted in higher selectivities to the target products, H2 and CO.
3.6. Crystalline structures of Ni/Y2O3 and anti-carbon deposition properties of Ni/Y2O3 The crystalline structures of Ni/Y2O3 catalyst after POM reaction with different time on streams are shown in Fig. 1. It can be seen that after 10 min reaction in POM, the diffraction peaks attributed to the phases of Ni or NiO could not be observed. This implies that Ni particles were still well dispersed on the Ni/Y2O3 catalyst after the reduction and 10 min reaction in POM. However, after 125 h and 550 h reaction in POM, the diffraction peaks of Ni phases were detected on Ni/Y2O3, indicating Ni particles aggregated somewhat during POM reactions. From the XRD patterns in Fig. 1, no diffraction peak attributed to graphic carbon was observed on the three used Ni/Y2O3 catalysts after different reaction time in POM. This means that no obvious carbon deposition occurred on these catalysts. TGA was carried out to evaluate the amount of carbon deposited on Ni/Y2O3 and Ni/g-Al2O3 after different reaction time in POM, and the results are listed in Table 1. It could be seen that the amount of carbon deposited on Ni/gAl2O3 was 3.1% after 50 h reaction in POM, whereas that on Ni/ Y2O3 was only 1.9% after 125 h reaction and 2.2% after 550 h reaction in POM. The amount of carbon deposited on Ni/Y2O3 was much less than that on Ni/g-Al2O3 catalyst. Namely, Ni/ Y2O3 possessed stronger ability to resist carbon deposition. Guo et al. [50] reported that the acidic-basic properties of catalysts could affect the ability of Ni-based catalysts to resist carbon deposition. The more basic of the catalysts, the more chances for the reaction between deposited carbon and adsorbed CO2, and the less chances for Boudouard reaction (CO / CO2 þ C) [51,52]. Then it could be inferred that the basicity of Y2O3 contributed to the less amount of carbon deposited on it in POM reaction.
4.
Conclusions
Y2O3, prepared by the conventional precipitation method, was a basic support with more basic sites and almost no acidic sites. After loading of Ni, the prepared Ni/Y2O3 catalyst showed relatively high catalytic activity and high selectivities to CO and H2 in POM reaction. Ni/Y2O3 was a stable and potential catalyst in POM, and it showed strong capacity to resist carbon deposition, after 550 h POM reaction, negligible amount of carbon deposited.
Acknowledgement We acknowledge the financial support of this work from National Basic Research Program of China (973 Program, 2011CB201405), NSFC (21073104, 20921001) and Analytical fund of Tsinghua University.
14453
references
[1] Choudhary VR, Rane VH. Acidity basicity of rare-earth oxides and their catalytic activity in oxidative coupling of methane to C2-hydrocarbons. J Catal 1991;130:411e22. [2] Jones CJ, Taube D, Ziatdinov VR, Periana RA. Selective oxidation of methane to methanol catalyzed, with C-H activation, by homogeneous, cationic gold. Angew Chem Int Edit 2004;43:4626e9. [3] Cheekatamarla PK, Finnerty CM. Synthesis gas production via catalytic partial oxidation reforming of liquid fuels. Int J Hydrogen Energy 2008;33:5012e9. [4] Hickman DA, Haupfear EA, Schmidt LD. Synthesis Gaformation by direct oxidation of methane over Rh monoliths. Catal Lett 1993;17:223e37. [5] Freni S, Calogero G, Cavallaro S. Hydrogen production from methane through catalytic partial oxidation reactions. J Power Sources 2000;87:28e38. [6] Tanaka H, Kaino R, Okumura K, Kizuka T, Yoshinao N, Tomishige K. Comparative study of Rh/MgO modified with Fe, Co or Ni for the catalytic partial oxidation of methane at short contact time. Part I: characterization of catalysts. Appl Catal A 2010;378:175e86. [7] Perkasa N, Zhongb Z, Chenb L, Bessonc M, Gedanken A. Sonochemically prepared high dispersed Ru/TiO2 mesoporous catalyst for partial oxidation of methane to syngas. Catal Lett 2005;103:9e14. [8] Basini L, Aragno A. Molecular mechanisms in partial oxidation of methane on Ir/alpha-Al2O3: reactivity dependence on catalyst properties and transport phenomena limitations. Catal Lett 1996;39:49e55. [9] Feio LSF, Hori CE, Mattos LV, Zanchet D, Noronha FB, Bueno JMC. Partial oxidation and autothermal reforming of methane on Pd/CeO2eAl2O3 catalysts. Appl Catal A 2008;348: 183e92. [10] Corbo P, Fortunato M. Hydrogen production by catalytic partial oxidation of methane and propane on Ni and Pt catalysts. Int J Hydrogen Energy 2007;32:55e66. [11] Ji YY, Li WZ, Xu HY, Chen YX. A study of carbon deposition on catalysts during the catalytic partial oxidation of methane to syngas in a fluidized bed. React Kinet Catal Lett 2001;73: 27e32. [12] Claridge JB, Green MLH, Tsang SC, York APE, Ashcroft AT, Battle PD. A study of carbon deposition on catalysts during the partial oxidation of methane to synthesis gas. Catal Lett 1993;22:299e305. [13] Choudhary VR, Rajput AM, Prabhakar B. Low-temperature oxidative conversion of methane to syngas over NiOeCaO catalyst. Catal Lett 1992;15:363e70. [14] Choudhary VR, Rane VH, Rajput AM. Beneficial effects of cobalt addition to Ni-catalysts for oxidative conversion of methane to syngas. Appl Catal A 1997;162:235e8. [15] Miao Q, Xiong GX, Sheng SS, Cui W, Xu L, Guo XX. Partial oxidation of methane to syngas over nickel-based catalysts modified by alkali metal oxide and rare earth metal oxide. Appl Catal A 1997;157:17e27. [16] Song YQ, He DH, Xu BQ. Effects of preparation methods of ZrO2 support on catalytic performances of Ni/ZrO2 catalysts in methane partial oxidation to syngas. Appl Catal A 2008; 337:19e28. [17] Requies J, Cabrero MA, Barrio VL, Guemez MB, Cambra JF, Arias PL, et al. Partial oxidation of methane to syngas over Ni/MgO and Ni/La2O3 catalysts. Appl Catal A 2005;289:214e23. [18] Weng WZ, Luo CR, Huang JJ, Liao YY, Wan HL. Comparative study on the mechanism of partial oxidation of methane to syngas over Rhodium supported on SiO2 and gama-Al2O3. Top Catal 2003;22:87e93.
14454
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 4 4 4 7 e1 4 4 5 4
[19] Shang RJ, Wang YY, Jin GQ, Guo XY. Partial oxidation of methane over nickel catalysts supported on nitrogen-doped SiC. Catal Comm 2009;10:1502e5. [20] Shang RJ, Sun WZ, Wang YY, Jin GQ, Guo XY. Silicon nitride supported nickel catalyst for partial oxidation of methane to syngas. Catal Comm 2008;9:2103e6. [21] Miao Q, Xiong GX, Li XS, Sheng SS, Guo XX. Acid-base properties and the directions of oxidative transformation of methane over nickel-based catalysts. Catal Lett 1996;41: 165e9. [22] Lin CK, Zhang CM, Lin J. Sol-gel derived Y2O3 as an efficient bluish-white phosphor without metal activator ions. J Lumin 2009;129:1469e74. [23] Wang ZX, Xu HL, Zhang ZY, Wang S, Ding L, Zeng QS, et al. Growth and performance of yttrium oxide as an ideal highkappa gate dielectric for carbon-based electronics. Nano Lett 2010;10:2024e30. [24] Kodo M, Soga K, Yoshida H, Yamamoto T. Doping effect of divalent cations on sintering of polycrystalline yttria. J Eur Ceram Soc 2010;30:2741e7. [25] Yao W, Lu GZ, Guo YL, Guo Y, Wang YQ, Zhang ZG. Promotional effect of Y2O3 on the performance of Ag/aeAl2O3 catalyst for epoxidation of propylene with molecular oxygen. J Mol Catal A Chem 2007;276:162e7. [26] Costa LOO, Silva AM, Borges LEP, Mattos LV, Noronha FB. Partial oxidation of ethanol over Pd/CeO2 and Pd/Y2O3 catalysts. Catal Today 2008;138:147e51. [27] Wu XS, Kawi S. Steam reforming of ethanol to H2 over Rh/ Y2O3: crucial roles of Y2O3 oxidizing ability, space velocity, and H2/C. Energy Environ Sci 2010;3:334e42. [28] Resini C, Delgado MHC, Presto S, Alemany LJ, Riani P, Marazza R, et al. Yttria-stabilized zirconia (YSZ) supported NieCo alloys (precursor of SOFC anodes) as catalysts for the steam reforming of ethanol. Int J Hydrogen Energy 2008;33: 3728e35. [29] Lintanf A, Djurado E, Vernoux P. Pt/YSZ electrochemical catalysts prepared by electrostatic spray deposition for selective catalytic reduction of NO by C3H6. Solid State Ionics 2008;178:1998e2008. [30] Blanchard J, Nsungui AJ, Abatzoglou N, Gitzhofer F. Dry reforming of methane with a Ni/Al2O3eYSZ catalyst: the role of the catalyst preparation protocol. Can J Chem Eng 2007;85: 889e99. [31] Wang YQ, Wang WJ, Hong XB, Li YH, Zhang ZG. Yttriumstabilized zirconia-promoted metallic nickel catalysts for the partial oxidation of methane to hydrogen. Int J Hydrogen Energy 2009;34:2252e9. [32] Santos DC, Madeira L, Passos FB. The effect of the addition of Y2O3 to Ni/alpha-Al2O3 catalysts on the autothermal reforming of methane. Catal Today 2010;149:401e6. [33] Fu LY, Xie WG, Lv SJ, Qiu FL. Influence of support on resistance to carbon deposition of catalyst for CH4, CO2 with O2 to synthesis gas. Sci China Ser B Chem 2000;2:154e61. [34] Wang YQ, Hong XB, Li BB, Wang WJ, Wang DL. Yttria promoted metallic nickel catalysts for the partial oxidation of methane to synthesis gas. J Nat Gas Chem 2008;17:344e50.
[35] Nishimoto H, Nakagawa K, Ikenaga N, Suzuki T. Partial oxidation of methane to synthesis gas over Ru-loaded Y2O3 catalyst. Catal Lett 2002;82:161e7. [36] Liu HM, He DH. Physicochemical properties of Ni/ gammaeAl2O3eAlN and effects of AlN on catalytic performance of Ni/gammaeAl2O3eAlN in partial oxidation of methane. J Phys Chem C 2010;114:13716e21. [37] Sato S, Takahashi R, Kobune M, Gotoh H. Basic properties of rare earth oxides. Appl Catal A 2009;356:57e63. [38] Wang SJ, Zhong SL, Ou-Yang XJ, Hu N, Chen XS, Wang SP, et al. Y(OH)3 and Y2O3 with novel structures: formation and mechanism. Mater Sci Eng B 2009;162:200e4. [39] Miao F, Yu L, Sun M, Li CS, Zhang L. Preparation and characterization of nanometer yttrium oxide. Fine Chem 2009;4:313e7. [40] Sun GB, Hidajat K, Wu XS, Kawi S. A crucial role of surface oxygen mobility on nanocrystalline Y2O3 support for oxidative steam reforming of ethanol to hydrogen over Ni/Y2O3 catalysts. Appl Catal B 2008;81:303e12. [41] Hou ZY, Osamu Y, Takumi T, Tatsuaki Y. Investigation of CH4 reforming with CO2 on meso-porous Al2O3-supported Ni catalyst. Catal Lett 2003;89:121e7. [42] Dewaele O, Froment GF. Tap study of the mechanism and kinetics of the adsorption and combustion of methane on Ni/Al2O3 and NiO/Al2O3. J Catal 1999;184:499e513. [43] Hoffer BW, Langeveld AD, Janssens JP, Bonne RLC, Lok CM, Moulijin JA. Stability of highly dispersed Ni/Al2O3 catalysts: effects of pretreatment. J Catal 2000;192:432e40. [44] Komandur VRC, Pendyala VRR, Vattikonda VR. Catalytic functionalities of nickel supported on different polymorphs of alumina. Catal Comm 2008;9:886e93. [45] Bellido JDA, Assaf EM. Effect of the Y2O3eZrO2 support composition on nickel catalyst evaluated in dry reforming of methane. Appl Catal A 2009;352:179e87. [46] Zhu JJ, Ommen JG, Knoester A, Lefferts L. Effect of surface composition of yttrium-stabilized zirconia on partial oxidation of methane to synthesis gas. J Catal 2005;230: 291e300. [47] Liu TF, Snyder C, Veser G. Catalytic partial oxidation of methane: is a distinction between direct and indirect pathways meaningful? Ind Eng Chem Res 2007;46:9045e52. [48] Rabe S, Truong TB, Vogel F. Low temperature catalytic partial oxidation of methane for gas-to-liquids applications. Appl Catal A 2007;318:54e62. [49] Bodke AS, Bharadwaj SS, Schmidt LD. The effect of ceramic supports on partial oxidation of hydrocarbons over noble metal coated monoliths. J Catal 1998;179:138e49. [50] Guo CL, Zhang JL, Zhang XL. Comparative study of LaNiO3 and La2NiO4 catalysts for partial oxidation of methane. React Kinet Catal Lett 2008;95:89e97. [51] Yamazaki O, Nozaki T, Omatak YK, Fujimoto K. Reduction of carbon-dioxide by methane with Ni-on-MgOeCaO containing catalysts. Chem Lett 1992;10:1953e4. [52] Geon JK, Dong SC, Kwang HK, Jong HK. The reaction of CO2 with CH4 to synthesize H2 and CO over nickel-loaded Yzeolites. Catal Lett 1994;28:41e52.