- Email: [email protected]
Natural clay-based Ni-catalysts for dry reforming of methane at moderate temperatures
G Model
ARTICLE IN PRESS
CATTOD-10507; No. of Pages 7
Catalysis Today xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Natural clay-based Ni-catalysts for dry reforming of methane at moderate temperatures Hongrui Liu a , Lu Yao a,b , Haithem Bel Hadj Taief a,c , Mourad Benzina a,c , Patrick Da Costa a,∗ , Maria Elena Gálvez a a Sorbonne Universités, UPMC, Univ. Paris 6, CNRS, UMR 7190, Institut Jean Le Rond d’Alembert, 2 Place de la gare de ceinture, 78210 Saint-Cyr-L’Ecole, France b Department Key Laboratory of Green Chemistry and Technology, Ministry of Education, College of Chemistry, Sichuan University, China c Laboratoire Eau, Energie et Environnement (LR3E), Code: AD-10-02, Ecole Nationale d’Ingénieurs de Sfax, Université de Sfax, B.P1173.W.3038 Sfax, Tunisie
a r t i c l e
i n f o
Article history: Received 23 September 2016 Received in revised form 16 November 2016 Accepted 8 December 2016 Available online xxx Keywords: Dry reforming of methane Syngas Hydrogen production Clays Nickel
a b s t r a c t Natural clay based Ni-containing catalysts for dry methane reforming (DRM) were prepared using a Fe and Cu-modified Tunisian clay as support. The catalysts were characterized by means of X-ray diffraction (XRD). H2 -temperature programmed reduction (H2 -TPR) and CO2 -temperature programmed desorption (TPD). The catalysts were either reduced at 800 or at 900 ◦ C prior to the DRM tests. Reduction temperature has a determinant influence in Ni crystal size and basicity. The catalysts reduced at 800 ◦ C showed better catalytic performance than those reduced at 900 ◦ C. The catalysts prepared using the Cu-modified clay yielded the highest CO2 and methane conversions all the time. At 850 ◦ C, 72% CO2 conversion was measured, corresponding to a H2 /CO ratio near 1, close to the thermodynamically forecasted value. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Dry reforming of methane (DRM) has received considerable attention in the last years, as one of the most promising routes for CO2 valorization [1,2]. It involves consumption of two important greenhouse gases methane and CO2 , resulting in the formation of a syngas (CO + H2 gas mixture) having a H2 /CO ratio equal to 1, ideal for liquid fuel synthesis through Fischer-Tropsch or for chemical synthesis through hydroformylation [3]. DRM is nevertheless an endothermic reaction (H0 298K = +247 kJ mol−1 ) that is only thermodynamically favorable at moderate-high temperatures (>750 ◦ C). At these moderate-high temperatures the reaction is still kinetically slow and only takes place in the presence of a catalyst, i.e. Ni-containing catalyst, preferred to noble metal-based ones due to lower cost and higher availability. In the whole range of reaction temperatures but especially at low-moderate reaction temperatures (<750 ◦ C) carbon forming reactions are bound to take place, among other side reactions, resulting in the formation of deposition of carbon on the catalyst surface. This problem of catalyst deactivation needs to be assessed in order to increase the stability of the catalytic system in view of its practical utilization. Different
∗ Corresponding author. E-mail address: patrick.da [email protected] (P. Da Costa).
approaches, in terms of the utilization of nano-structured supports [4,5], and in terms of increasing the dispersion and controlling the crystal size of the Ni particles deposited on the catalysts surface [6,7], proof the possibility of tailoring the selectivity of the DRM reaction with the final aim of minimizing carbon deposition. In general, the support itself is not catalytically active, however it may strongly influence the catalytic performance of the overall catalytic system. Dependent on the type of support used, the interaction active phase-support may be favored, it can also provide acid or basic sites that may promote the adsorption of reaction species, and furthermore, it offers adequate surface area and may improve the mechanical properties of the catalytic system. Clays have been widely used as supports for the preparation of diverse catalysts, including Ni-containing catalyst for dry methane reforming. Among them, synthetic clays, i.e. hydrotalcites, have shown to yield high activity, selectivity and catalytic stability in DRM, since they provide adequate surface area, mostly in terms of mesopores and intrinsic surface basicity, resulting in enhanced Ni dispersion and reducibility [6–9]. Natural clays stand as a most interesting option, since they are naturally available, green and low-cost materials. They have been recently used as supports in several catalytic applications [10–15], including DRM [15–20]. Either raw [15] or Alpillared [16] smectites were used for in the preparation of Ni-based catalysts for DRM. Daza et al. [17], Wang et al. [18], and Barama et al. [19] considered the utilization of raw, LaAl and Al-pillared
http://dx.doi.org/10.1016/j.cattod.2016.12.017 0920-5861/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: H. Liu, et al., Natural clay-based Ni-catalysts for dry reforming of methane at moderate temperatures, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.12.017
G Model CATTOD-10507; No. of Pages 7
ARTICLE IN PRESS H. Liu et al. / Catalysis Today xxx (2016) xxx–xxx
2
montmorillonites. Hao et al. [20] employed a Zr-pillared laponites. All these works point out the benefits of using these natural clays as supports in the preparation of DRM catalysts. Moreover Wang et al. reported a direct influence of the mesoporous surface area of LaAl-pillared clays on their activity, in terms of methane conversion [18]. The use of natural Fe or Cu-pillared clays as supports in the preparation of Ni-based catalysts for DRM has never been described in the existing literature. The presence of Fe and Cu pillars may influence not only the type of porosity of the support, and therefore Ni distribution and catalyst stability, but also the activity and selectivity of these materials, due to the inherent presence of Fe and Cu together with Ni. The present paper describes the DRM behavior of Ni-catalysts prepared using a raw, Fe-pillared and Cupillared natural clay. The so-prepared catalysts were characterized from a physico-chemical point of view with the aim of assessing the influence of important properties such as porosity, Ni distribution and crystal size, as well as basicity, for two different reduction temperatures, i.e. 800 and 900 ◦ C. 2. Experimental 2.1. Preparation of the clay-based catalysts A natural clay from the deposit of Jebal Cherahil (Kairouan, Central-West of Tunisia) was chosen as the raw material. The procedure for its purification and Na-ion exchange of its surface have been previously described elsewhere [14,21]. The Fe-pillared clay, Fe-Clay, was synthesized as described in detail in our previously published work [21]. The pillaring solution was prepared by slow addition of Na2 CO3 powder (97%, MERCK) into a 0.2 M solution of Fe(NO3 )3 (Fe (NO3 )3 ·9H2 O 97%, MERCK) while stirring at 100 rpm for 2 h at room temperature until the molar ratio Fe/Na2 CO3 reached 1:5. The solution was then aged during 4 days at 60 ◦ C. Finally, the resulting oligomeric Fe(III) solution was added into a 2% wt. aqueous dispersion of the purified Na-exchanged clay, at a ratio of 10−3 mol of Fe3+ per gram of clay. The dispersion was agitated at 100 rpm for 24 h, then filtered, washed by with deionized water several times, and finally centrifuged at 4000 rpm for 10 min. The resulting solid material was calcined at 300 ◦ C for 24 h, and subsequently ground to 100-mesh. For the preparation of the Cu-pillared clay, Cu-Clay, 1 g of Na-exchanged raw clay and 100 mL of 0.02 M copper acetate (Cu (CH3 COO)2 (98%) MERCK) solution were stirred at a pH of 5.2 and 40 ◦ C for 24 h. The resulting suspension was filtered and the precipitate was washed several times with deionized water. The catalyst was then dried at 120 ◦ C for 12 h and subsequently calcined at 400 ◦ C for 5 h. The composition of raw, Fe and Cu-pillared clays are shown in Table 1. Nickel was introduced into each clay support by means of a conventional impregnation method, using an aqueous solution of nickel nitrate hexahydrate (Ni(NO3 )2 ·6H2 O, Aldrich) as metal precursor. The Ni loading for all catalysts was fixed as 15 wt%. Upon impregnation, the catalysts were dried overnight at 100 ◦ C and subsequently calcined at 550 ◦ C for 4 h. 2.2. Physicochemical characterization The chemical composition of the natural and modified clays was analyzed by means of X-ray fluorescence (XRF, ARL1 9800 XP spectrometer). The textural properties of both the clay supports and the catalysts were studied by means of N2 adsorption at −196 ◦ C (ASAP 2020, Micromeritics). The samples were previously degased at 250 ◦ C. X-ray diffraction (XRD) patterns were obtained in a PANalytical-Empyrean diffractometer, equipped with CuK␣ ( = 1.5406 Å) radiation source and 2 range between 3 and
90◦ , with a step size of 0.02◦ /s. The Scherrer equation was used for calculating the crystal size of nickel on the reduced clay based catalyts. The profiles of temperature-programmed reduction (H2 -TPR) were acquired in a BELCAT-M (BEL Japan) device, equipped with a thermal conductivity detector (TCD). The calcined clay-based catalysts were first outgassed at 100 ◦ C for 2 h, then reduced in 5% (vol.) H2 /Ar, while the temperature was increased from 100 ◦ C to 900 ◦ C at a heating rate of 7.5 ◦ C/min. The same apparatus was used for the acquisition of the CO2 -TPD profiles. The materials were first degassed for 2 h at 500 ◦ C at a heating rate of 10 ◦ C/min, cooled to 80 ◦ C in pure helium, and subsequently exposed to a gaseous mixture of CO2 (10 vol.%)/He for 1 h. Helium was then flown for 15 min in order to desorb the physically adsorbed CO2 . Temperature programmed desorption (TPD) of CO2 was carried out heating the samples at 10 ◦ C/min up to 950 ◦ C under He flow. The desorbed CO2 was measured with the aid of the TCD detector. 2.3. Catalytic DRM experiments The DRM experiments were carried out in a tubular quartz reactor (8 mm internal diameter) at temperatures from 850 ◦ C to 600 ◦ C, under 100 mL/min reactant gas flow (GHSV = 20,000 h− 1), composed of CH4 /CO2 /Ar = 1/1/8. The catalyst was submitted to each reaction temperature during at least 30 min reaction time, and until steady state conversions were reached. An overall time-on-stream of 5 h can be considered for all the catalysts tested. Prior to DRM experiment the calcined catalysts were reduced in situ either at 800 ◦ C and 900 ◦ C, for 1 h in a stream of 5% (vol.) H2 /Ar. These reduction temperatures were chosen at the sight of the H2 -TPR profiles obtained for the catalysts. Though, in principle, 800 ◦ C reduction temperature would be enough, the catalysts were as well reduced at 900 ◦ C, due to the slight tailing of the last peak observed in the TPR profile acquired for the Ni/Clay catalyst. The compositions of the products were analyzed in a gas chromatograph (Varian GC4900), equipped with a thermal conductivity detector. The conversions of CO2 and CH4 , as well as the ratio H2 /CO were calculated as follows: out in XCO2 = (nin CO − nCO )/nCO × 100
(1)
out in XCH4 = (nin CH − nCH )/nCH × 100
(2)
out H2 /CO = nout H /nCO
(3)
2
4
2
4
2
4
2
and nout denote the concentration for each species enterWhere nin i i ing and exiting the reactor. 3. Results and discussion 3.1. Textural, structural and chemical properties of the natural clay based Ni-catalysts Table 2 contains the results of the textural characterization of the clays and natural clay based Ni-catalysts, performed by means of N2 adsorption. The adsorption isotherms obtained (Fig. 1) correspond to type IV isotherms, according to IUPAC classification, presenting H3 hysteresis loops, typical of laminar-structured materials containing slit-shaped pores. They all show a strong increase of adsorption at high relative pressured, linked to the presence of macropores. The desorption branch of the isotherms closes at low relative pressures (below 0.4), pointing to impeded diffusion and trapping of N2 molecules inside the partially collapsed and far too complex laminar structure, maybe as a consequence of an excess of Fe and Cu pillars. The excess of Fe pillars is clear at the sight of the lower surface area of this material, vis-à-vis the non-modified clay: some pores remain blocked, maybe also due to the effect of a partial collapse of the pillars.
Please cite this article in press as: H. Liu, et al., Natural clay-based Ni-catalysts for dry reforming of methane at moderate temperatures, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.12.017
G Model
ARTICLE IN PRESS
CATTOD-10507; No. of Pages 7
H. Liu et al. / Catalysis Today xxx (2016) xxx–xxx
3
Table 1 Composition of raw, Fe and Cu-pillared clays (XRF analysis). Composition (% wt.)
Raw clay Fe-Clay Cu-Clay
SiO2
Al2 O3
Fe2 O3
MgO
K2 O
Na2 O
CaO
CuO
43.4 38.4 37.01
16.8 15.4 14.32
7.7 32.5 6.9
5.5 1.1 1.3
0.8 0.6 1.1
2.5 1.0 2.61
7.9 1.13 0.61
– – 11.1
Table 2 BET surface area, pore volumes and average pore size for the raw and modified clays, as well as for the natural clay based Ni-catalysts, together with Ni crystal sized calculated from XRD patterns and carbon balance from DRM tests. Material
Raw clay Fe-Clay Cu-Clay Ni/Clay Ni/Fe-Clay Ni/Cu-Clay a
SBET (m2 /g)
Vp a (cm3 /g)
73.6 60.9 102.8 55.4 44.3 84.3
0.148 0.111 0.195 0.112 0.108 0.159
VBJH b (cm3 /g)
0.146 0.112 0.193 0.111 0.108 0.156
Vmicro c (cm3 /g)
0.012 0.011 0.016 0.007 0.006 0.003
dp d (cm3 /g)
8.1 6.6 7.7 8.9 8.6 8.2
Ni crystal size (nm)
C balance (mg)
800 ◦ C
900 ◦ C
800 ◦ C
900 ◦ C
– – – 18 16.4 18.7
– – – 13.1 12.4 11.7
– – – 129.6 76.8 146.6
– – – 82.2 109.4 119.6
Single point desorption total pore volume. T-plot micropore volume. BJH desorption cumulative pore volume. BJH desorption average pore width.
b c d
H2 Consumption (a.u.)
Raw Clay Fe-Clay Cu-Clay Ni/Fe-Clay
3
Adsorbed volume (cm /g)
150
100
50
0 0.0
Ni/Cu-Clay
Ni/Fe-Clay
Ni/Clay 0.2
0.4
0.6
0.8
1.0
0
Relative pressure (p/p) Fig. 1. Adsorption isotherms for the raw and modified clays, as well as for the calcined Ni/Fe-Clay catalysts.
BET surface area decreases upon the addition of Fe and increases through the formation of Cu pillars. Ni loading generally causes a decrease of surface area, due to partial blockage of micropores and narrow mesopores. Average pore sizes around 8–9 nm were found for the three natural clay based Ni-catalysts. The results of the H2 -TPR experiments are plotted in Fig. 2, for the three natural clay based Ni-catalysts. CuO reduction to Cu0 should occur at relatively low temperatures, i.e. between 270 and 310 ◦ C [22]. No peaks are clearly observed at these temperatures for the catalyst prepared using the Cu-modified clay. However, the shoulder appearing at about 400 ◦ C can be ascribed to the presence of mixed Cu-Ni oxides [23,24]. In the case of the catalysts, Fe3 O4 oxidation to Fe2 O3 is expected to take place at temperatures between 450 and 480 ◦ C, whereas Fe2 O3 oxidation to metallic Fe0 occurs at higher temperatures, in a wide range, i.e. from 480 to 600 ◦ C [25]. The reduction of bulk and weakly bonded Ni-oxide species will take place almost simultaneously. However, the presence of a second peak of H2 consumption at about 650–700 ◦ C points as well to the presence of finely dispersed NiOx , having a stronger interaction with the clay support, or, in the case of the Fe-Clay based
200
400
600
800
Temperature [°C] Fig. 2. Temperature programmed reduction (H2 -TPR) profiles acquired for the natural clay based Ni-catalysts.
catalysts, even with iron [26–28]. The area of this peak decreases in the catalysts prepared either the Fe or the Cu-modified clay, pointing to the presence of Ni-species of increased reducibility, maybe due to impeded interaction of Ni and the support in the presence of the Fe or Cu-pillars. In this sense, we can conclude that Fe and Cu presence slightly weakens the interaction between the Ni phase and the clay support. At the sight of the H2 -TPR profiles acquired for this series of catalysts, two reduction temperatures were chosen, 800 and 900 ◦ C, in order to pre-treat the catalysts prior to the DRM catalytic tests. Fig. 3a and b shows the XRD patterns acquired for the catalysts reduced either at 800 ◦ C (Fig. 3a) or at 900 ◦ C (Fig. 3b), under 5% (vol.) H2 /Ar for 1 h. Independently of the reduction temperature, the XRD patterns for the natural clay based Ni-catalysts evidence the typical reflections from quartz and silica, corresponding to the clay support matrix, together with the diffraction peaks at 2 = 44.3◦ , 52.7◦ and 76.3◦ , due to the presence of metallic Ni. Ni crystal sizes were calculated applying Scherrer equation to the diffraction peak
Please cite this article in press as: H. Liu, et al., Natural clay-based Ni-catalysts for dry reforming of methane at moderate temperatures, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.12.017
G Model
ARTICLE IN PRESS
CATTOD-10507; No. of Pages 7
H. Liu et al. / Catalysis Today xxx (2016) xxx–xxx
4
a)
b)
Fig. 3. XRD patterns acquired for the catalysts reduced at a) 800 ◦ C and b) 900 ◦ C.
Table 3 Total basicity and deconvolution of the CO2 -TPD profiles. Material
Raw clay Fe-Clay Cu-Clay Ni/Clay (800 ◦ C) Ni/Fe-Clay (800 ◦ C) Ni/Cu-Clay (800 ◦ C) Ni/Clay (900 ◦ C) Ni/Fe-Clay (900 ◦ C) Ni/Cu-Clay (900 ◦ C)
Deconvolution (mol CO2 /g) Weak
Medium
Strong
4.5 4.7 0.4 5.9 7.0 9.3 9.9 4.8 0.5
11.3 5.8 3.7 19.5 6.7 8.4 7.7 19.6 2.2
5.5 2.0 2.7 30.8 8.9 1.1 6.0 6.0 1.4
Total basicity (mol CO2 /g)
21.3 12.5 6.8 56.2 22.6 18.8 23.6 30.4 4.1
appearing at 76.3◦ . The values are shown in Table 2, together with the results of the textural characterization. Increasing the reduction temperature from 800 to 900 ◦ C results in a decrease in Ni crystal size, i.e. values around 11.7–13.1 nm were calculated from the catalysts reduced at 900 ◦ C, vis-à-vis the crystal sizes around 16.4–18.7 nm determined for the catalysts reduced at 800 ◦ C. Ni redispersion through the enhancement of the interaction between Ni-species, clay support and maybe Fe or Cu pillars is favored when increasing the temperature of this reduction pre-treatment. In fact, the reduction pretreatment of the catalysts will result in the reduction of Fe and Cu pillaring species. However, the structure of the clay seems to be preserved at the sight of the XRD patterns obtained for the reduced catalysts. Unfortunately, XRD does not provide further information about the structural changes induced as a consequence of the reduction of Fe and Cu, being very difficult to distinguish segregated Cu0 , FeO or Fe2 O3 phases in these patterns. Fig. 4 presents the CO2 -TPD profiles acquired for the catalysts reduced either at 800 ◦ C (Fig. 3a) or at 900 ◦ C (Fig. 3b). Such CO2 desorption profiles typical exhibit three contributions, corresponding to different types of basic groups: weak Brønsted basic sites such as surface OH groups (low temperature, 100–250 ◦ C), medium-strength Lewis acid-base sites (intermediate temperature, 250–400 ◦ C), and low-coordination oxygen anions acting as strong basic sites (high temperature, 400–600 ◦ C) [29]. The deconvolution of the TPD profiles was performed taking into account these criteria. The results are shown in Table 3. Even at simple sight and independently of the reduction temperature, the peak corresponding to strong basic sites almost disappears for the catalysts prepared using either the Fe or the Cu-modified clays. For these catalysts, basicity is mostly of weak or moderate strength.
The results presented in Table 3 evidence a decrease in total basicity upon the introduction of Fe and Cu, especially in the case of the Cu-modified clay. This trend is maintained in the Ni-catalysts prepared using these materials. A decrease in basicity is further observed after reduction at 900 ◦ C, probably due to Ni re-dispersion (smaller Ni crystal sizes). The lower presence of basic oxides in the modified clays, such as MgO, may explain this general decrease in basicity. Note however that this corresponds to a decrease in strong basic sites, and that this strong basic sites, probably MgO sites, are not the ones preferred in reactions involving CO2 . 3.2. Catalytic activity and selectivity of the natural clay based Ni-catalyst in DRM Fig. 5 presents the CH4 and CO2 conversions, as well as the H2 /CO ratio obtained during the DRM experiments in the presence of the natural clay based Ni-catalysts reduced at 800 ◦ C. Methane and CO2 conversions increase with increasing reaction temperature. However, and especially in the case of methane conversion, the values reached are quite far from those predicted by the thermodynamic equilibrium calculations. The CO2 conversion measured is very similar but always a little bit higher than methane conversion. Direct methane decomposition occurs but remains relatively slow, even in the presence of the Ni-containing catalysts. However, the hydrogen generated in this reaction, favored at low temperatures, may react with CO2 through the reverse water gas shift reaction, which yields CO and water and further explain the low H2 /CO values obtained. As temperature increases, the DRM reaction becomes favorable from a thermodynamic point of view. At 800 ◦ C it may contribute to an important extent to the generation of both H2 and CO, even though the parallel reactions described, among others, such as Boudouard, steam reforming, etc., will continue taking place. In fact, at 800 ◦ C, the value of the H2 /CO ratio measured approached the thermodynamically forecasted value of 1. Among the catalysts reduced at 800 ◦ C, the one prepared using the Fe-modified clay shows the lowest activity. Note here that this catalyst shows the lowest Ni crystal sizes of this series, i.e. 16.4 nm vs. 18.7 nm calculated for Ni/Cu-Clay. Moreover, the presence of Cu seems to be positive, yielding increased CH4 and CO2 conversions. In fact, Cu-promoted catalysts are well known for its activity in reverse water gas shift and methanol synthesis reactions involving CO2 [23,30,31], what may explain its better performance in spite of its lower total basicity. Fig. 6 shows the results obtained in the DRM experiments in the presence of the catalysts reduced at 900 ◦ C. Such catalysts showed decreased Ni crystal sizes, probably due to favored support-Ni
Please cite this article in press as: H. Liu, et al., Natural clay-based Ni-catalysts for dry reforming of methane at moderate temperatures, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.12.017
G Model
ARTICLE IN PRESS
CATTOD-10507; No. of Pages 7
H. Liu et al. / Catalysis Today xxx (2016) xxx–xxx
a)
5
b)
Ni/Cu-Clay
CO2 Desorption (a.u.)
Ni/Cu-Clay
CO2 Desorption (a.u.)
Ni/Fe-Clay
Ni/Clay
100
Ni/Fe-Clay
Ni/Clay
200
300
400
500
100
600
200
Temperature [°C]
300
400
500
600
Temperature [°C]
Fig. 4. CO2 -TPD profiles obtained for the natural clay based Ni-catalysts reduced at a) 800 ◦ C and b) 900 ◦ C.
b)
100 Ni/Clay red. @ 800°C Ni/Fe-Clay red. @ 800°C Ni/Cu=Clay red. @ 800°C thermodynamic equilibrium
CH4 conversion [%]
80
60
100 Ni/Clay red. @ 800°C Ni/Fe-Clay red. @ 800°C Ni/Cu=Clay red. @ 800°C thermodynamic equilibrium
80
CO2 Conversion [%]
a)
40
20
0
60
40
20
0
600
650
700
750
800
600
650
Temperature [°C]
700
750
800
Temperature [°C]
c)
Ni/Clay red. @ 800°C Ni/Fe-Clay red. @ 800°C Ni/Cu=Clay red. @ 800°C thermodynamic equilibrium
2.0
H2/CO
1.5
1.0
0.5
600
650
700
750
800
Temperature [°C] Fig. 5. Results of the DRM catalytic tests in the presence of the natural clay based Ni-catalysts reduced at 800 ◦ C, a) CH4 conversion, b) CO2 conversion, and c) H2 /CO ratio.
phase interaction and to a re-dispersion of Ni-species during reduction at higher temperatures. They yield however lower CH4 and CO2 conversion than the ones measured in the presence of the catalysts reduced at 800 ◦ C. The general decrease in basicity may explain this fact, together with the lower Ni crystal sizes obtained [7]. Again, the Cu-containing catalyst shows the highest activity. Table 2 contains as well the results of the carbon balance performed over the whole dynamic DRM tests for each catalyst tested. The extent of carbon deposition is higher for the catalysts reduced
at 800 ◦ C, corresponding to higher activity and bigger Ni0 crystal size, as could be expected. The Ni/Cu-Clay reduced at 800 ◦ C shows the highest total amount of C deposited, corresponding to its higher activity, above all in terms of CO2 conversion. Since direct methane decomposition results in C formation, together with H2 production, the H2 generated can further react with CO2 by means of the reverse water gas shift reaction, resulting in higher CO2 conversion. Slightly lower stability can be thus expected when using this Cu-clay based catalyst.
Please cite this article in press as: H. Liu, et al., Natural clay-based Ni-catalysts for dry reforming of methane at moderate temperatures, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.12.017
G Model
ARTICLE IN PRESS
CATTOD-10507; No. of Pages 7
H. Liu et al. / Catalysis Today xxx (2016) xxx–xxx
6
b)
100 Ni/Clay red. @ 900°C Ni/Fe-Clay red. @ 900°C Ni/Cu=Clay red. @ 900°C thermodynamic equilibrium
CH4 Conversion [%]
80
60
40
20
0 600
650
700
750
100 Ni/Clay red. @ 900°C Ni/Fe-Clay red. @ 900°C Ni/Cu=Clay red. @ 900°C thermodynamic equilibrium
80
CO2 Conversion [%]
a)
800
850
60
40
20
0 600
650
Temperature [°C]
700
750
800
850
Temperature [°C]
c)
Ni/Clay red. @ 900°C Ni/Fe-Clay red. @ 900°C Ni/Cu=Clay red. @ 900°C thermodynamic equilibrium
2.0
H2/CO
1.5
1.0
0.5
0.0 600
650
700
750
800
850
Temperature [°C] Fig. 6. Results of the DRM catalytic tests in the presence of the natural clay based Ni-catalysts reduced at 900 ◦ C, a) CH4 conversion, b) CO2 conversion, and c) H2 /CO ratio.
4. Conclusions
References
Natural clay based Ni-containing catalysts were prepared using a Tunisian clay that was either modified using Fe or Cu. Their activity was tested in the dry reforming of methane (DRM) at temperatures between 600 and 800 ◦ C. Prior to these catalytic experiments, the catalysts were reduced in 5% (vol.) H2 -Ar at two different temperatures: 800 and 900 ◦ C. Reduction temperature had a big influence on Ni crystal size. Smaller Ni crystal sizes were found in the catalysts reduced at 900 ◦ C. Basicity, i.e. evaluated by means of CO2 -TPD, was found to be lower in the catalysts prepared using the Cu-pillared clay. Moreover, increasing reduction temperature from 800 to 900 ◦ C resulted in a considerable decrease of basicity. In fact, as a consequence of this generally decreased basicity and smaller Ni crystal size, the catalysts reduced at 900 ◦ C showed lower activity in DRM than those reduced at 800 ◦ C. In spite of their lower basicity, however, the Cu-containing catalysts yielded all the time highest CH4 and CO2 conversions. This is maybe linked to the well known activity of Cu-catalysts towards reverse water gas shift reaction, which effectively takes place in parallel to reforming reactions in the presence of the natural clay based Ni-containing catalysts.
[1] N. Homs, J. Toyir, P.R. Piscina, Catalytic processes for activation of CO2 , in: S.L. Suib (Ed.), New and Future Developments in Catalysis. Activation of Carbon Dioxide, Elsevier, 2013, pp. 1–26. [2] G. Centi, S. Perathoner, Opportunities and prospects in the chemical recycling of carbon dioxide to fuels, Catal. Today 148 (2009) 191. [3] J.H. Edwards, A.M. Maitra, The chemistry of methane reforming with carbon dioxide and its current and potential applications, Fuel Process. Technol. 42 (1995) 269. [4] A. Albarazi, M.E. Galvez, P. Da Costa, Synthesis strategies of ceria-zirconia doped Ni/SBA-15 catalysts for methane dry reforming, Catal. Commun. 59 (2015) 108–112. [5] P. Frontera, A. Macario, A. Aloise, F. Crea, P.L. Antonucci, J.B. Nagy, F. Frusteri, G. Giordano, Catalytic dry-reforming on Ni-zeolite supported catalyst, Catal. Today 179 (2012) 52–60. ˛ [6] R. Debek, M. Radlik, M. Motak, M.E. Galvez, W. Turek, P. Da Costa, T. Grzybek, Ni-containing Ce-promoted hydrotalcite derived materials as catalysts for methane reforming with carbon dioxide at low temperature—on the effect of basicity, Catal. Today 257 (2014) 59–65. ˛ [7] R. Debek, M. Motak, D. Duraczyska, F. Launay, M.E. Galvez, T. Grzybek, P. Da Costa, Methane dry reforming over hydrotalcite-derived Ni-Mg-Al mixed oxides: the influence of Ni content on catalytic activity, selectivity and stability, Catal. Sci. Technol. 6 (2016) 6705–6715. [8] C.E. Daza, J. Gallego, F. Mondragón, S. Moreno, R. Molina, High stability of Ce-promoted Ni/Mg-Al catalysts derived from hydrotalcites in dry reforming of methane, Fuel 89 (2010) 592–603. [9] A.R. Gonzalez, Y.J.O. Asencios, E.M. Assaf, J.M. Assaf, Dry reforming of methane on Ni-Mg-Al nano-spheroid oxide catalysts prepared by the sol-gel method from hydrotalcite-like precursors, Appl. Surf. Sci. 280 (2013) 876–887. [10] J. Herney Ramirez, C.A. Costa, L.M. Madeiraa, G. Matab, M.A. Vicenteb, M.L. Rojas-Cervantesc, A.J. López-Peinadoc, R.M. Martín-Arandac, Fenton-like oxidation of Orange II solutions using heterogeneous catalysts based on saponite clay, Appl. Catal. B: Environ. E (2007) 44–56. [11] A. Gil, S.A. Korili, M.A. Vicente, Recent advances in the control and characterization of the porous structure of pillared clay catalysts, Catal. Rev. 50 (2008) 153–221.
Acknowledgement H. Liu and L. Yao thank China Scholarship Council for their doctoral scholarship at UPMC Sorbonne Universités.
Please cite this article in press as: H. Liu, et al., Natural clay-based Ni-catalysts for dry reforming of methane at moderate temperatures, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.12.017
G Model CATTOD-10507; No. of Pages 7
ARTICLE IN PRESS H. Liu et al. / Catalysis Today xxx (2016) xxx–xxx
[12] C.H. Zhou, An overview on strategies towards clay-based designer catalysts for green and sustainable catalysis, Appl. Clay Sci. 53 (2011) 87–96. [13] J.L. Cao, G.S. Shao, Y. Wang, Y. Liu, Z.Y. Yuan, CuO catalysts supported on attapulgite clay for low-temperature CO oxidation, Catal. Commun. 9 (2008) 2555–2559. [14] H.B. Hadjltaief, P. Da Costa, P. Beaunier, M.E. Gálvez, M. Ben Zina, Fe-clay-plate as a heterogeneous catalyst in photo-Fenton oxidation of phenol as probe molecule for water treatment, Appl. Clay Sci. 91–92 (2014) 46–54. [15] N. Iwasa, M. Takizawa, M. Arai, Preparation and application of nickel-containing smectite-type clay materials for methane reforming with carbon dioxide, Appl. Catal. A: Gen. 314 (2006) 32–39. [16] O. Gamba, S. Moreno, R. Molina, Catalytic performance of Ni-Pr supported on delaminated clay in the dry reforming of methane, Int. J. Hydrogen Energy 36 (2011) 1540–1550. [17] C.E. Daza, A. Kiennemann, S. Moreno, R. Molina, Dry reforming of methane using Ni-Ce catalysts supported on a modified mineral clay, Appl. Catal. A: Gen. 364 (2009) 65–74. [18] S. Wang, H.Y. Zhu, G.Q. Lu, Preparation characterization, and catalytic properties of clay-based nickel catalysts for methane reforming, J. Colloid Interface Sci. 204 (1998) 128–134. [19] S. Baramaa, C. Dupeyrat-Batiot, M. Capron, E. Bordes-Richard, O. Bakhti-Mohammedi, Catalytic properties of Rh, Ni, Pd and Ce supported on Al-pillared montmorillonites in dry reforming of methane, Catal. Today 141 (2009) 385–392. [20] Z. Hao, H.Y. Zhu, G.Q. Lu, Zr-laponite pillared clay-based nickel catalysts for methane reforming with carbon dioxide, Appl. Catal. A: Gen. 242 (2003) 275–286. [21] H. Bel Hadjltaief, P. Da Costa, M.E. Galvez, M. Ben Zina, Influence of operational parameters in the heterogeneous photo-fenton discoloration of wastewaters in the presence of an iron-pillared clay, Ind. Eng. Chem. Res. 52 (2013) 16656–16665.
7
[22] M.F. Luo, P. Fang, M. He, Y.L. Xie, In situ XRD Raman, and TPR studies of CuO/Al2 O3 catalysts for CO oxidation, J. Mol. Catal. A: Chem. 239 (2005) 243–248. [23] Y. Li, Q. Fu, M. Flytzani-Stephanopoulos, Appl. Catal. B: Environ. 27 (2000) 179–191. ˜ G. Baronetti, M. Jobbagy, M. Laborde, Cu-Ni-K/␥-Al2 O3 supported [24] F. Marino, catalysts for ethanol steam reforming: formation of hydrotalcite-type compounds as a result of metal-support interaction, Appl. Catal. A: Gen. 238 (2003) 41–54. [25] H.Y. Chen, W.M.H. Sachtler, Activity and durability of Fe/ZSM-5 catalysts for lean burn NOx reduction in the presence of water vapor, Catal. Today 42 (1998) 73–83. [26] L. Wang, D. Li, M. Koike, S. Koso, Y. Nakagawa, Y. Xu, K. Tomishige, Catalytic performance and characterization of Ni-Fe catalysts for the steam reforming of tar from biomass pyrolysis to synthesis gas, Appl. Catal. A: Gen. 392 (2011) 248–255. [27] A. Djaidja, H. Messaoudi, D. Kaddeche, A. Barama, Study of Ni-M/MgO and Ni-M-Mg/Al (M = Fe or Cu) catalysts in the CH4 -CO2 and CH4 O reforming, Int. J. Hydrogen Energy 40 (2015) 4989–4995. [28] J. Ashok, S. Kawi, Nickel-iron alloy supported over iron-alumina catalysts for steam reforming of biomass tar model compound, ACS Catal. 4 (2014) 289–301. [29] H. Liu, D. Wierzbicki, R. Debek, M. Motak, T. Grzybek, P. Da Costa, M.E. Gálvez, La-promoted Ni-hydrotalcite-derived catalysts for dry reforming of methane at low temperatures, Fuel 182 (2016) 8–16. [30] J. Qi, Y. Sun, Z. Xie, M. Collins, H. Du, T. Xiong, Development of Cu foam-based Ni catalyst for solar thermal reforming of methane with carbon dioxide, J. Energy Chem. 24 (2015) 786–793. [31] T.-J. Huang, S.-Y. Jhao, Ni-Cu/samaria-doped ceria catalysts for steam reforming of methane in the presence of carbon dioxide, Appl. Catal. A: Gen. 302 (2006) 325–332.
Please cite this article in press as: H. Liu, et al., Natural clay-based Ni-catalysts for dry reforming of methane at moderate temperatures, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.12.017
ARTICLE IN PRESS
CATTOD-10507; No. of Pages 7
Catalysis Today xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Natural clay-based Ni-catalysts for dry reforming of methane at moderate temperatures Hongrui Liu a , Lu Yao a,b , Haithem Bel Hadj Taief a,c , Mourad Benzina a,c , Patrick Da Costa a,∗ , Maria Elena Gálvez a a Sorbonne Universités, UPMC, Univ. Paris 6, CNRS, UMR 7190, Institut Jean Le Rond d’Alembert, 2 Place de la gare de ceinture, 78210 Saint-Cyr-L’Ecole, France b Department Key Laboratory of Green Chemistry and Technology, Ministry of Education, College of Chemistry, Sichuan University, China c Laboratoire Eau, Energie et Environnement (LR3E), Code: AD-10-02, Ecole Nationale d’Ingénieurs de Sfax, Université de Sfax, B.P1173.W.3038 Sfax, Tunisie
a r t i c l e
i n f o
Article history: Received 23 September 2016 Received in revised form 16 November 2016 Accepted 8 December 2016 Available online xxx Keywords: Dry reforming of methane Syngas Hydrogen production Clays Nickel
a b s t r a c t Natural clay based Ni-containing catalysts for dry methane reforming (DRM) were prepared using a Fe and Cu-modified Tunisian clay as support. The catalysts were characterized by means of X-ray diffraction (XRD). H2 -temperature programmed reduction (H2 -TPR) and CO2 -temperature programmed desorption (TPD). The catalysts were either reduced at 800 or at 900 ◦ C prior to the DRM tests. Reduction temperature has a determinant influence in Ni crystal size and basicity. The catalysts reduced at 800 ◦ C showed better catalytic performance than those reduced at 900 ◦ C. The catalysts prepared using the Cu-modified clay yielded the highest CO2 and methane conversions all the time. At 850 ◦ C, 72% CO2 conversion was measured, corresponding to a H2 /CO ratio near 1, close to the thermodynamically forecasted value. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Dry reforming of methane (DRM) has received considerable attention in the last years, as one of the most promising routes for CO2 valorization [1,2]. It involves consumption of two important greenhouse gases methane and CO2 , resulting in the formation of a syngas (CO + H2 gas mixture) having a H2 /CO ratio equal to 1, ideal for liquid fuel synthesis through Fischer-Tropsch or for chemical synthesis through hydroformylation [3]. DRM is nevertheless an endothermic reaction (H0 298K = +247 kJ mol−1 ) that is only thermodynamically favorable at moderate-high temperatures (>750 ◦ C). At these moderate-high temperatures the reaction is still kinetically slow and only takes place in the presence of a catalyst, i.e. Ni-containing catalyst, preferred to noble metal-based ones due to lower cost and higher availability. In the whole range of reaction temperatures but especially at low-moderate reaction temperatures (<750 ◦ C) carbon forming reactions are bound to take place, among other side reactions, resulting in the formation of deposition of carbon on the catalyst surface. This problem of catalyst deactivation needs to be assessed in order to increase the stability of the catalytic system in view of its practical utilization. Different
∗ Corresponding author. E-mail address: patrick.da [email protected] (P. Da Costa).
approaches, in terms of the utilization of nano-structured supports [4,5], and in terms of increasing the dispersion and controlling the crystal size of the Ni particles deposited on the catalysts surface [6,7], proof the possibility of tailoring the selectivity of the DRM reaction with the final aim of minimizing carbon deposition. In general, the support itself is not catalytically active, however it may strongly influence the catalytic performance of the overall catalytic system. Dependent on the type of support used, the interaction active phase-support may be favored, it can also provide acid or basic sites that may promote the adsorption of reaction species, and furthermore, it offers adequate surface area and may improve the mechanical properties of the catalytic system. Clays have been widely used as supports for the preparation of diverse catalysts, including Ni-containing catalyst for dry methane reforming. Among them, synthetic clays, i.e. hydrotalcites, have shown to yield high activity, selectivity and catalytic stability in DRM, since they provide adequate surface area, mostly in terms of mesopores and intrinsic surface basicity, resulting in enhanced Ni dispersion and reducibility [6–9]. Natural clays stand as a most interesting option, since they are naturally available, green and low-cost materials. They have been recently used as supports in several catalytic applications [10–15], including DRM [15–20]. Either raw [15] or Alpillared [16] smectites were used for in the preparation of Ni-based catalysts for DRM. Daza et al. [17], Wang et al. [18], and Barama et al. [19] considered the utilization of raw, LaAl and Al-pillared
http://dx.doi.org/10.1016/j.cattod.2016.12.017 0920-5861/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: H. Liu, et al., Natural clay-based Ni-catalysts for dry reforming of methane at moderate temperatures, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.12.017
G Model CATTOD-10507; No. of Pages 7
ARTICLE IN PRESS H. Liu et al. / Catalysis Today xxx (2016) xxx–xxx
2
montmorillonites. Hao et al. [20] employed a Zr-pillared laponites. All these works point out the benefits of using these natural clays as supports in the preparation of DRM catalysts. Moreover Wang et al. reported a direct influence of the mesoporous surface area of LaAl-pillared clays on their activity, in terms of methane conversion [18]. The use of natural Fe or Cu-pillared clays as supports in the preparation of Ni-based catalysts for DRM has never been described in the existing literature. The presence of Fe and Cu pillars may influence not only the type of porosity of the support, and therefore Ni distribution and catalyst stability, but also the activity and selectivity of these materials, due to the inherent presence of Fe and Cu together with Ni. The present paper describes the DRM behavior of Ni-catalysts prepared using a raw, Fe-pillared and Cupillared natural clay. The so-prepared catalysts were characterized from a physico-chemical point of view with the aim of assessing the influence of important properties such as porosity, Ni distribution and crystal size, as well as basicity, for two different reduction temperatures, i.e. 800 and 900 ◦ C. 2. Experimental 2.1. Preparation of the clay-based catalysts A natural clay from the deposit of Jebal Cherahil (Kairouan, Central-West of Tunisia) was chosen as the raw material. The procedure for its purification and Na-ion exchange of its surface have been previously described elsewhere [14,21]. The Fe-pillared clay, Fe-Clay, was synthesized as described in detail in our previously published work [21]. The pillaring solution was prepared by slow addition of Na2 CO3 powder (97%, MERCK) into a 0.2 M solution of Fe(NO3 )3 (Fe (NO3 )3 ·9H2 O 97%, MERCK) while stirring at 100 rpm for 2 h at room temperature until the molar ratio Fe/Na2 CO3 reached 1:5. The solution was then aged during 4 days at 60 ◦ C. Finally, the resulting oligomeric Fe(III) solution was added into a 2% wt. aqueous dispersion of the purified Na-exchanged clay, at a ratio of 10−3 mol of Fe3+ per gram of clay. The dispersion was agitated at 100 rpm for 24 h, then filtered, washed by with deionized water several times, and finally centrifuged at 4000 rpm for 10 min. The resulting solid material was calcined at 300 ◦ C for 24 h, and subsequently ground to 100-mesh. For the preparation of the Cu-pillared clay, Cu-Clay, 1 g of Na-exchanged raw clay and 100 mL of 0.02 M copper acetate (Cu (CH3 COO)2 (98%) MERCK) solution were stirred at a pH of 5.2 and 40 ◦ C for 24 h. The resulting suspension was filtered and the precipitate was washed several times with deionized water. The catalyst was then dried at 120 ◦ C for 12 h and subsequently calcined at 400 ◦ C for 5 h. The composition of raw, Fe and Cu-pillared clays are shown in Table 1. Nickel was introduced into each clay support by means of a conventional impregnation method, using an aqueous solution of nickel nitrate hexahydrate (Ni(NO3 )2 ·6H2 O, Aldrich) as metal precursor. The Ni loading for all catalysts was fixed as 15 wt%. Upon impregnation, the catalysts were dried overnight at 100 ◦ C and subsequently calcined at 550 ◦ C for 4 h. 2.2. Physicochemical characterization The chemical composition of the natural and modified clays was analyzed by means of X-ray fluorescence (XRF, ARL1 9800 XP spectrometer). The textural properties of both the clay supports and the catalysts were studied by means of N2 adsorption at −196 ◦ C (ASAP 2020, Micromeritics). The samples were previously degased at 250 ◦ C. X-ray diffraction (XRD) patterns were obtained in a PANalytical-Empyrean diffractometer, equipped with CuK␣ ( = 1.5406 Å) radiation source and 2 range between 3 and
90◦ , with a step size of 0.02◦ /s. The Scherrer equation was used for calculating the crystal size of nickel on the reduced clay based catalyts. The profiles of temperature-programmed reduction (H2 -TPR) were acquired in a BELCAT-M (BEL Japan) device, equipped with a thermal conductivity detector (TCD). The calcined clay-based catalysts were first outgassed at 100 ◦ C for 2 h, then reduced in 5% (vol.) H2 /Ar, while the temperature was increased from 100 ◦ C to 900 ◦ C at a heating rate of 7.5 ◦ C/min. The same apparatus was used for the acquisition of the CO2 -TPD profiles. The materials were first degassed for 2 h at 500 ◦ C at a heating rate of 10 ◦ C/min, cooled to 80 ◦ C in pure helium, and subsequently exposed to a gaseous mixture of CO2 (10 vol.%)/He for 1 h. Helium was then flown for 15 min in order to desorb the physically adsorbed CO2 . Temperature programmed desorption (TPD) of CO2 was carried out heating the samples at 10 ◦ C/min up to 950 ◦ C under He flow. The desorbed CO2 was measured with the aid of the TCD detector. 2.3. Catalytic DRM experiments The DRM experiments were carried out in a tubular quartz reactor (8 mm internal diameter) at temperatures from 850 ◦ C to 600 ◦ C, under 100 mL/min reactant gas flow (GHSV = 20,000 h− 1), composed of CH4 /CO2 /Ar = 1/1/8. The catalyst was submitted to each reaction temperature during at least 30 min reaction time, and until steady state conversions were reached. An overall time-on-stream of 5 h can be considered for all the catalysts tested. Prior to DRM experiment the calcined catalysts were reduced in situ either at 800 ◦ C and 900 ◦ C, for 1 h in a stream of 5% (vol.) H2 /Ar. These reduction temperatures were chosen at the sight of the H2 -TPR profiles obtained for the catalysts. Though, in principle, 800 ◦ C reduction temperature would be enough, the catalysts were as well reduced at 900 ◦ C, due to the slight tailing of the last peak observed in the TPR profile acquired for the Ni/Clay catalyst. The compositions of the products were analyzed in a gas chromatograph (Varian GC4900), equipped with a thermal conductivity detector. The conversions of CO2 and CH4 , as well as the ratio H2 /CO were calculated as follows: out in XCO2 = (nin CO − nCO )/nCO × 100
(1)
out in XCH4 = (nin CH − nCH )/nCH × 100
(2)
out H2 /CO = nout H /nCO
(3)
2
4
2
4
2
4
2
and nout denote the concentration for each species enterWhere nin i i ing and exiting the reactor. 3. Results and discussion 3.1. Textural, structural and chemical properties of the natural clay based Ni-catalysts Table 2 contains the results of the textural characterization of the clays and natural clay based Ni-catalysts, performed by means of N2 adsorption. The adsorption isotherms obtained (Fig. 1) correspond to type IV isotherms, according to IUPAC classification, presenting H3 hysteresis loops, typical of laminar-structured materials containing slit-shaped pores. They all show a strong increase of adsorption at high relative pressured, linked to the presence of macropores. The desorption branch of the isotherms closes at low relative pressures (below 0.4), pointing to impeded diffusion and trapping of N2 molecules inside the partially collapsed and far too complex laminar structure, maybe as a consequence of an excess of Fe and Cu pillars. The excess of Fe pillars is clear at the sight of the lower surface area of this material, vis-à-vis the non-modified clay: some pores remain blocked, maybe also due to the effect of a partial collapse of the pillars.
Please cite this article in press as: H. Liu, et al., Natural clay-based Ni-catalysts for dry reforming of methane at moderate temperatures, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.12.017
G Model
ARTICLE IN PRESS
CATTOD-10507; No. of Pages 7
H. Liu et al. / Catalysis Today xxx (2016) xxx–xxx
3
Table 1 Composition of raw, Fe and Cu-pillared clays (XRF analysis). Composition (% wt.)
Raw clay Fe-Clay Cu-Clay
SiO2
Al2 O3
Fe2 O3
MgO
K2 O
Na2 O
CaO
CuO
43.4 38.4 37.01
16.8 15.4 14.32
7.7 32.5 6.9
5.5 1.1 1.3
0.8 0.6 1.1
2.5 1.0 2.61
7.9 1.13 0.61
– – 11.1
Table 2 BET surface area, pore volumes and average pore size for the raw and modified clays, as well as for the natural clay based Ni-catalysts, together with Ni crystal sized calculated from XRD patterns and carbon balance from DRM tests. Material
Raw clay Fe-Clay Cu-Clay Ni/Clay Ni/Fe-Clay Ni/Cu-Clay a
SBET (m2 /g)
Vp a (cm3 /g)
73.6 60.9 102.8 55.4 44.3 84.3
0.148 0.111 0.195 0.112 0.108 0.159
VBJH b (cm3 /g)
0.146 0.112 0.193 0.111 0.108 0.156
Vmicro c (cm3 /g)
0.012 0.011 0.016 0.007 0.006 0.003
dp d (cm3 /g)
8.1 6.6 7.7 8.9 8.6 8.2
Ni crystal size (nm)
C balance (mg)
800 ◦ C
900 ◦ C
800 ◦ C
900 ◦ C
– – – 18 16.4 18.7
– – – 13.1 12.4 11.7
– – – 129.6 76.8 146.6
– – – 82.2 109.4 119.6
Single point desorption total pore volume. T-plot micropore volume. BJH desorption cumulative pore volume. BJH desorption average pore width.
b c d
H2 Consumption (a.u.)
Raw Clay Fe-Clay Cu-Clay Ni/Fe-Clay
3
Adsorbed volume (cm /g)
150
100
50
0 0.0
Ni/Cu-Clay
Ni/Fe-Clay
Ni/Clay 0.2
0.4
0.6
0.8
1.0
0
Relative pressure (p/p) Fig. 1. Adsorption isotherms for the raw and modified clays, as well as for the calcined Ni/Fe-Clay catalysts.
BET surface area decreases upon the addition of Fe and increases through the formation of Cu pillars. Ni loading generally causes a decrease of surface area, due to partial blockage of micropores and narrow mesopores. Average pore sizes around 8–9 nm were found for the three natural clay based Ni-catalysts. The results of the H2 -TPR experiments are plotted in Fig. 2, for the three natural clay based Ni-catalysts. CuO reduction to Cu0 should occur at relatively low temperatures, i.e. between 270 and 310 ◦ C [22]. No peaks are clearly observed at these temperatures for the catalyst prepared using the Cu-modified clay. However, the shoulder appearing at about 400 ◦ C can be ascribed to the presence of mixed Cu-Ni oxides [23,24]. In the case of the catalysts, Fe3 O4 oxidation to Fe2 O3 is expected to take place at temperatures between 450 and 480 ◦ C, whereas Fe2 O3 oxidation to metallic Fe0 occurs at higher temperatures, in a wide range, i.e. from 480 to 600 ◦ C [25]. The reduction of bulk and weakly bonded Ni-oxide species will take place almost simultaneously. However, the presence of a second peak of H2 consumption at about 650–700 ◦ C points as well to the presence of finely dispersed NiOx , having a stronger interaction with the clay support, or, in the case of the Fe-Clay based
200
400
600
800
Temperature [°C] Fig. 2. Temperature programmed reduction (H2 -TPR) profiles acquired for the natural clay based Ni-catalysts.
catalysts, even with iron [26–28]. The area of this peak decreases in the catalysts prepared either the Fe or the Cu-modified clay, pointing to the presence of Ni-species of increased reducibility, maybe due to impeded interaction of Ni and the support in the presence of the Fe or Cu-pillars. In this sense, we can conclude that Fe and Cu presence slightly weakens the interaction between the Ni phase and the clay support. At the sight of the H2 -TPR profiles acquired for this series of catalysts, two reduction temperatures were chosen, 800 and 900 ◦ C, in order to pre-treat the catalysts prior to the DRM catalytic tests. Fig. 3a and b shows the XRD patterns acquired for the catalysts reduced either at 800 ◦ C (Fig. 3a) or at 900 ◦ C (Fig. 3b), under 5% (vol.) H2 /Ar for 1 h. Independently of the reduction temperature, the XRD patterns for the natural clay based Ni-catalysts evidence the typical reflections from quartz and silica, corresponding to the clay support matrix, together with the diffraction peaks at 2 = 44.3◦ , 52.7◦ and 76.3◦ , due to the presence of metallic Ni. Ni crystal sizes were calculated applying Scherrer equation to the diffraction peak
Please cite this article in press as: H. Liu, et al., Natural clay-based Ni-catalysts for dry reforming of methane at moderate temperatures, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.12.017
G Model
ARTICLE IN PRESS
CATTOD-10507; No. of Pages 7
H. Liu et al. / Catalysis Today xxx (2016) xxx–xxx
4
a)
b)
Fig. 3. XRD patterns acquired for the catalysts reduced at a) 800 ◦ C and b) 900 ◦ C.
Table 3 Total basicity and deconvolution of the CO2 -TPD profiles. Material
Raw clay Fe-Clay Cu-Clay Ni/Clay (800 ◦ C) Ni/Fe-Clay (800 ◦ C) Ni/Cu-Clay (800 ◦ C) Ni/Clay (900 ◦ C) Ni/Fe-Clay (900 ◦ C) Ni/Cu-Clay (900 ◦ C)
Deconvolution (mol CO2 /g) Weak
Medium
Strong
4.5 4.7 0.4 5.9 7.0 9.3 9.9 4.8 0.5
11.3 5.8 3.7 19.5 6.7 8.4 7.7 19.6 2.2
5.5 2.0 2.7 30.8 8.9 1.1 6.0 6.0 1.4
Total basicity (mol CO2 /g)
21.3 12.5 6.8 56.2 22.6 18.8 23.6 30.4 4.1
appearing at 76.3◦ . The values are shown in Table 2, together with the results of the textural characterization. Increasing the reduction temperature from 800 to 900 ◦ C results in a decrease in Ni crystal size, i.e. values around 11.7–13.1 nm were calculated from the catalysts reduced at 900 ◦ C, vis-à-vis the crystal sizes around 16.4–18.7 nm determined for the catalysts reduced at 800 ◦ C. Ni redispersion through the enhancement of the interaction between Ni-species, clay support and maybe Fe or Cu pillars is favored when increasing the temperature of this reduction pre-treatment. In fact, the reduction pretreatment of the catalysts will result in the reduction of Fe and Cu pillaring species. However, the structure of the clay seems to be preserved at the sight of the XRD patterns obtained for the reduced catalysts. Unfortunately, XRD does not provide further information about the structural changes induced as a consequence of the reduction of Fe and Cu, being very difficult to distinguish segregated Cu0 , FeO or Fe2 O3 phases in these patterns. Fig. 4 presents the CO2 -TPD profiles acquired for the catalysts reduced either at 800 ◦ C (Fig. 3a) or at 900 ◦ C (Fig. 3b). Such CO2 desorption profiles typical exhibit three contributions, corresponding to different types of basic groups: weak Brønsted basic sites such as surface OH groups (low temperature, 100–250 ◦ C), medium-strength Lewis acid-base sites (intermediate temperature, 250–400 ◦ C), and low-coordination oxygen anions acting as strong basic sites (high temperature, 400–600 ◦ C) [29]. The deconvolution of the TPD profiles was performed taking into account these criteria. The results are shown in Table 3. Even at simple sight and independently of the reduction temperature, the peak corresponding to strong basic sites almost disappears for the catalysts prepared using either the Fe or the Cu-modified clays. For these catalysts, basicity is mostly of weak or moderate strength.
The results presented in Table 3 evidence a decrease in total basicity upon the introduction of Fe and Cu, especially in the case of the Cu-modified clay. This trend is maintained in the Ni-catalysts prepared using these materials. A decrease in basicity is further observed after reduction at 900 ◦ C, probably due to Ni re-dispersion (smaller Ni crystal sizes). The lower presence of basic oxides in the modified clays, such as MgO, may explain this general decrease in basicity. Note however that this corresponds to a decrease in strong basic sites, and that this strong basic sites, probably MgO sites, are not the ones preferred in reactions involving CO2 . 3.2. Catalytic activity and selectivity of the natural clay based Ni-catalyst in DRM Fig. 5 presents the CH4 and CO2 conversions, as well as the H2 /CO ratio obtained during the DRM experiments in the presence of the natural clay based Ni-catalysts reduced at 800 ◦ C. Methane and CO2 conversions increase with increasing reaction temperature. However, and especially in the case of methane conversion, the values reached are quite far from those predicted by the thermodynamic equilibrium calculations. The CO2 conversion measured is very similar but always a little bit higher than methane conversion. Direct methane decomposition occurs but remains relatively slow, even in the presence of the Ni-containing catalysts. However, the hydrogen generated in this reaction, favored at low temperatures, may react with CO2 through the reverse water gas shift reaction, which yields CO and water and further explain the low H2 /CO values obtained. As temperature increases, the DRM reaction becomes favorable from a thermodynamic point of view. At 800 ◦ C it may contribute to an important extent to the generation of both H2 and CO, even though the parallel reactions described, among others, such as Boudouard, steam reforming, etc., will continue taking place. In fact, at 800 ◦ C, the value of the H2 /CO ratio measured approached the thermodynamically forecasted value of 1. Among the catalysts reduced at 800 ◦ C, the one prepared using the Fe-modified clay shows the lowest activity. Note here that this catalyst shows the lowest Ni crystal sizes of this series, i.e. 16.4 nm vs. 18.7 nm calculated for Ni/Cu-Clay. Moreover, the presence of Cu seems to be positive, yielding increased CH4 and CO2 conversions. In fact, Cu-promoted catalysts are well known for its activity in reverse water gas shift and methanol synthesis reactions involving CO2 [23,30,31], what may explain its better performance in spite of its lower total basicity. Fig. 6 shows the results obtained in the DRM experiments in the presence of the catalysts reduced at 900 ◦ C. Such catalysts showed decreased Ni crystal sizes, probably due to favored support-Ni
Please cite this article in press as: H. Liu, et al., Natural clay-based Ni-catalysts for dry reforming of methane at moderate temperatures, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.12.017
G Model
ARTICLE IN PRESS
CATTOD-10507; No. of Pages 7
H. Liu et al. / Catalysis Today xxx (2016) xxx–xxx
a)
5
b)
Ni/Cu-Clay
CO2 Desorption (a.u.)
Ni/Cu-Clay
CO2 Desorption (a.u.)
Ni/Fe-Clay
Ni/Clay
100
Ni/Fe-Clay
Ni/Clay
200
300
400
500
100
600
200
Temperature [°C]
300
400
500
600
Temperature [°C]
Fig. 4. CO2 -TPD profiles obtained for the natural clay based Ni-catalysts reduced at a) 800 ◦ C and b) 900 ◦ C.
b)
100 Ni/Clay red. @ 800°C Ni/Fe-Clay red. @ 800°C Ni/Cu=Clay red. @ 800°C thermodynamic equilibrium
CH4 conversion [%]
80
60
100 Ni/Clay red. @ 800°C Ni/Fe-Clay red. @ 800°C Ni/Cu=Clay red. @ 800°C thermodynamic equilibrium
80
CO2 Conversion [%]
a)
40
20
0
60
40
20
0
600
650
700
750
800
600
650
Temperature [°C]
700
750
800
Temperature [°C]
c)
Ni/Clay red. @ 800°C Ni/Fe-Clay red. @ 800°C Ni/Cu=Clay red. @ 800°C thermodynamic equilibrium
2.0
H2/CO
1.5
1.0
0.5
600
650
700
750
800
Temperature [°C] Fig. 5. Results of the DRM catalytic tests in the presence of the natural clay based Ni-catalysts reduced at 800 ◦ C, a) CH4 conversion, b) CO2 conversion, and c) H2 /CO ratio.
phase interaction and to a re-dispersion of Ni-species during reduction at higher temperatures. They yield however lower CH4 and CO2 conversion than the ones measured in the presence of the catalysts reduced at 800 ◦ C. The general decrease in basicity may explain this fact, together with the lower Ni crystal sizes obtained [7]. Again, the Cu-containing catalyst shows the highest activity. Table 2 contains as well the results of the carbon balance performed over the whole dynamic DRM tests for each catalyst tested. The extent of carbon deposition is higher for the catalysts reduced
at 800 ◦ C, corresponding to higher activity and bigger Ni0 crystal size, as could be expected. The Ni/Cu-Clay reduced at 800 ◦ C shows the highest total amount of C deposited, corresponding to its higher activity, above all in terms of CO2 conversion. Since direct methane decomposition results in C formation, together with H2 production, the H2 generated can further react with CO2 by means of the reverse water gas shift reaction, resulting in higher CO2 conversion. Slightly lower stability can be thus expected when using this Cu-clay based catalyst.
Please cite this article in press as: H. Liu, et al., Natural clay-based Ni-catalysts for dry reforming of methane at moderate temperatures, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.12.017
G Model
ARTICLE IN PRESS
CATTOD-10507; No. of Pages 7
H. Liu et al. / Catalysis Today xxx (2016) xxx–xxx
6
b)
100 Ni/Clay red. @ 900°C Ni/Fe-Clay red. @ 900°C Ni/Cu=Clay red. @ 900°C thermodynamic equilibrium
CH4 Conversion [%]
80
60
40
20
0 600
650
700
750
100 Ni/Clay red. @ 900°C Ni/Fe-Clay red. @ 900°C Ni/Cu=Clay red. @ 900°C thermodynamic equilibrium
80
CO2 Conversion [%]
a)
800
850
60
40
20
0 600
650
Temperature [°C]
700
750
800
850
Temperature [°C]
c)
Ni/Clay red. @ 900°C Ni/Fe-Clay red. @ 900°C Ni/Cu=Clay red. @ 900°C thermodynamic equilibrium
2.0
H2/CO
1.5
1.0
0.5
0.0 600
650
700
750
800
850
Temperature [°C] Fig. 6. Results of the DRM catalytic tests in the presence of the natural clay based Ni-catalysts reduced at 900 ◦ C, a) CH4 conversion, b) CO2 conversion, and c) H2 /CO ratio.
4. Conclusions
References
Natural clay based Ni-containing catalysts were prepared using a Tunisian clay that was either modified using Fe or Cu. Their activity was tested in the dry reforming of methane (DRM) at temperatures between 600 and 800 ◦ C. Prior to these catalytic experiments, the catalysts were reduced in 5% (vol.) H2 -Ar at two different temperatures: 800 and 900 ◦ C. Reduction temperature had a big influence on Ni crystal size. Smaller Ni crystal sizes were found in the catalysts reduced at 900 ◦ C. Basicity, i.e. evaluated by means of CO2 -TPD, was found to be lower in the catalysts prepared using the Cu-pillared clay. Moreover, increasing reduction temperature from 800 to 900 ◦ C resulted in a considerable decrease of basicity. In fact, as a consequence of this generally decreased basicity and smaller Ni crystal size, the catalysts reduced at 900 ◦ C showed lower activity in DRM than those reduced at 800 ◦ C. In spite of their lower basicity, however, the Cu-containing catalysts yielded all the time highest CH4 and CO2 conversions. This is maybe linked to the well known activity of Cu-catalysts towards reverse water gas shift reaction, which effectively takes place in parallel to reforming reactions in the presence of the natural clay based Ni-containing catalysts.
[1] N. Homs, J. Toyir, P.R. Piscina, Catalytic processes for activation of CO2 , in: S.L. Suib (Ed.), New and Future Developments in Catalysis. Activation of Carbon Dioxide, Elsevier, 2013, pp. 1–26. [2] G. Centi, S. Perathoner, Opportunities and prospects in the chemical recycling of carbon dioxide to fuels, Catal. Today 148 (2009) 191. [3] J.H. Edwards, A.M. Maitra, The chemistry of methane reforming with carbon dioxide and its current and potential applications, Fuel Process. Technol. 42 (1995) 269. [4] A. Albarazi, M.E. Galvez, P. Da Costa, Synthesis strategies of ceria-zirconia doped Ni/SBA-15 catalysts for methane dry reforming, Catal. Commun. 59 (2015) 108–112. [5] P. Frontera, A. Macario, A. Aloise, F. Crea, P.L. Antonucci, J.B. Nagy, F. Frusteri, G. Giordano, Catalytic dry-reforming on Ni-zeolite supported catalyst, Catal. Today 179 (2012) 52–60. ˛ [6] R. Debek, M. Radlik, M. Motak, M.E. Galvez, W. Turek, P. Da Costa, T. Grzybek, Ni-containing Ce-promoted hydrotalcite derived materials as catalysts for methane reforming with carbon dioxide at low temperature—on the effect of basicity, Catal. Today 257 (2014) 59–65. ˛ [7] R. Debek, M. Motak, D. Duraczyska, F. Launay, M.E. Galvez, T. Grzybek, P. Da Costa, Methane dry reforming over hydrotalcite-derived Ni-Mg-Al mixed oxides: the influence of Ni content on catalytic activity, selectivity and stability, Catal. Sci. Technol. 6 (2016) 6705–6715. [8] C.E. Daza, J. Gallego, F. Mondragón, S. Moreno, R. Molina, High stability of Ce-promoted Ni/Mg-Al catalysts derived from hydrotalcites in dry reforming of methane, Fuel 89 (2010) 592–603. [9] A.R. Gonzalez, Y.J.O. Asencios, E.M. Assaf, J.M. Assaf, Dry reforming of methane on Ni-Mg-Al nano-spheroid oxide catalysts prepared by the sol-gel method from hydrotalcite-like precursors, Appl. Surf. Sci. 280 (2013) 876–887. [10] J. Herney Ramirez, C.A. Costa, L.M. Madeiraa, G. Matab, M.A. Vicenteb, M.L. Rojas-Cervantesc, A.J. López-Peinadoc, R.M. Martín-Arandac, Fenton-like oxidation of Orange II solutions using heterogeneous catalysts based on saponite clay, Appl. Catal. B: Environ. E (2007) 44–56. [11] A. Gil, S.A. Korili, M.A. Vicente, Recent advances in the control and characterization of the porous structure of pillared clay catalysts, Catal. Rev. 50 (2008) 153–221.
Acknowledgement H. Liu and L. Yao thank China Scholarship Council for their doctoral scholarship at UPMC Sorbonne Universités.
Please cite this article in press as: H. Liu, et al., Natural clay-based Ni-catalysts for dry reforming of methane at moderate temperatures, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.12.017
G Model CATTOD-10507; No. of Pages 7
ARTICLE IN PRESS H. Liu et al. / Catalysis Today xxx (2016) xxx–xxx
[12] C.H. Zhou, An overview on strategies towards clay-based designer catalysts for green and sustainable catalysis, Appl. Clay Sci. 53 (2011) 87–96. [13] J.L. Cao, G.S. Shao, Y. Wang, Y. Liu, Z.Y. Yuan, CuO catalysts supported on attapulgite clay for low-temperature CO oxidation, Catal. Commun. 9 (2008) 2555–2559. [14] H.B. Hadjltaief, P. Da Costa, P. Beaunier, M.E. Gálvez, M. Ben Zina, Fe-clay-plate as a heterogeneous catalyst in photo-Fenton oxidation of phenol as probe molecule for water treatment, Appl. Clay Sci. 91–92 (2014) 46–54. [15] N. Iwasa, M. Takizawa, M. Arai, Preparation and application of nickel-containing smectite-type clay materials for methane reforming with carbon dioxide, Appl. Catal. A: Gen. 314 (2006) 32–39. [16] O. Gamba, S. Moreno, R. Molina, Catalytic performance of Ni-Pr supported on delaminated clay in the dry reforming of methane, Int. J. Hydrogen Energy 36 (2011) 1540–1550. [17] C.E. Daza, A. Kiennemann, S. Moreno, R. Molina, Dry reforming of methane using Ni-Ce catalysts supported on a modified mineral clay, Appl. Catal. A: Gen. 364 (2009) 65–74. [18] S. Wang, H.Y. Zhu, G.Q. Lu, Preparation characterization, and catalytic properties of clay-based nickel catalysts for methane reforming, J. Colloid Interface Sci. 204 (1998) 128–134. [19] S. Baramaa, C. Dupeyrat-Batiot, M. Capron, E. Bordes-Richard, O. Bakhti-Mohammedi, Catalytic properties of Rh, Ni, Pd and Ce supported on Al-pillared montmorillonites in dry reforming of methane, Catal. Today 141 (2009) 385–392. [20] Z. Hao, H.Y. Zhu, G.Q. Lu, Zr-laponite pillared clay-based nickel catalysts for methane reforming with carbon dioxide, Appl. Catal. A: Gen. 242 (2003) 275–286. [21] H. Bel Hadjltaief, P. Da Costa, M.E. Galvez, M. Ben Zina, Influence of operational parameters in the heterogeneous photo-fenton discoloration of wastewaters in the presence of an iron-pillared clay, Ind. Eng. Chem. Res. 52 (2013) 16656–16665.
7
[22] M.F. Luo, P. Fang, M. He, Y.L. Xie, In situ XRD Raman, and TPR studies of CuO/Al2 O3 catalysts for CO oxidation, J. Mol. Catal. A: Chem. 239 (2005) 243–248. [23] Y. Li, Q. Fu, M. Flytzani-Stephanopoulos, Appl. Catal. B: Environ. 27 (2000) 179–191. ˜ G. Baronetti, M. Jobbagy, M. Laborde, Cu-Ni-K/␥-Al2 O3 supported [24] F. Marino, catalysts for ethanol steam reforming: formation of hydrotalcite-type compounds as a result of metal-support interaction, Appl. Catal. A: Gen. 238 (2003) 41–54. [25] H.Y. Chen, W.M.H. Sachtler, Activity and durability of Fe/ZSM-5 catalysts for lean burn NOx reduction in the presence of water vapor, Catal. Today 42 (1998) 73–83. [26] L. Wang, D. Li, M. Koike, S. Koso, Y. Nakagawa, Y. Xu, K. Tomishige, Catalytic performance and characterization of Ni-Fe catalysts for the steam reforming of tar from biomass pyrolysis to synthesis gas, Appl. Catal. A: Gen. 392 (2011) 248–255. [27] A. Djaidja, H. Messaoudi, D. Kaddeche, A. Barama, Study of Ni-M/MgO and Ni-M-Mg/Al (M = Fe or Cu) catalysts in the CH4 -CO2 and CH4 O reforming, Int. J. Hydrogen Energy 40 (2015) 4989–4995. [28] J. Ashok, S. Kawi, Nickel-iron alloy supported over iron-alumina catalysts for steam reforming of biomass tar model compound, ACS Catal. 4 (2014) 289–301. [29] H. Liu, D. Wierzbicki, R. Debek, M. Motak, T. Grzybek, P. Da Costa, M.E. Gálvez, La-promoted Ni-hydrotalcite-derived catalysts for dry reforming of methane at low temperatures, Fuel 182 (2016) 8–16. [30] J. Qi, Y. Sun, Z. Xie, M. Collins, H. Du, T. Xiong, Development of Cu foam-based Ni catalyst for solar thermal reforming of methane with carbon dioxide, J. Energy Chem. 24 (2015) 786–793. [31] T.-J. Huang, S.-Y. Jhao, Ni-Cu/samaria-doped ceria catalysts for steam reforming of methane in the presence of carbon dioxide, Appl. Catal. A: Gen. 302 (2006) 325–332.
Please cite this article in press as: H. Liu, et al., Natural clay-based Ni-catalysts for dry reforming of methane at moderate temperatures, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.12.017