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Ni supported on the CaO modified attapulgite as catalysts for hydrogen production from glycerol steam reforming
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Ni supported on the CaO modified attapulgite as catalysts for hydrogen production from glycerol steam reforming P. Feng a, K. Huang b, Q. Xu a,b,*, W. Qi c,**, S. Xin d, T. Wei a, L. Liao a, Y. Yan a a
Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, Research Center for Biomass Energy, East China University of Science and Technology, Shanghai, China b National Engineering Research Center for Non-Food Biorefinery, Guangxi Key Laboratory of Biorefinery, Guangxi Academy of Sciences, Nanning, China c Guangzhou Institute of Energy Conversion, Chinese Academy of Science, Guangzhou, China d Hubei Key Laboratory of Industrial Fume and Dust Pollution Control, Jianghan University, Wuhan, China
highlights NieCaO-ATP exhibits the highest glycerol conversion and hydrogen yield. Attapulgite (ATP) has good carbon deposition resistance. CaO increases the activity of catalyst and the inhibition of carbon deposition.
article info
abstract
Article history:
The Ni catalyst supported on CaO-modified attapulgite (CaO-ATP) were synthesized by wet
Received 28 October 2019
impregnation method at a constant Ni metal loading (10 wt%). The catalyst was tested by
Received in revised form
carrying out a glycerin steam reforming reaction under the following conditions: 400
20 December 2019
e800 C, W/G is 3, GHSV is 1 h1. NieCaO-ATP exhibited the highest hydrogen yield (85.30%)
Accepted 2 January 2020
and glycerol conversion (93.71%) at 600
Available online xxx
adsorption/desorption, BET, XRD, H2-TPR, TG and SEM. The results show that ATP has good
C. The catalysts were characterized by N2
resistance to carbon deposition. As an attapulgite modifier of NieCaO-ATP, CaO promotes Keywords:
the dispersion of the active component nickel species, which would promote the water gas
Hydrogen
shift reaction, leading to the improving of hydrogen yield. In addition, the addition of Ca
Steam reforming
would further enhance the inhibition of carbon deposition and prolong the life of the Ni
Attapulgite
eCaO-ATP catalyst.
Ni catalysts
© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
CaO
* Corresponding author. Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, Research Center for Biomass Energy, East China University of Science and Technology, Shanghai, China. ** Corresponding author. E-mail addresses: [email protected] (Q. Xu), [email protected] (W. Qi). https://doi.org/10.1016/j.ijhydene.2020.01.013 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Feng P et al., Ni supported on the CaO modified attapulgite as catalysts for hydrogen production from glycerol steam reforming, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.013
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Introduction In the past few decades, the problems caused by the large use of non-renewable resources have been extensively studied. Because biomass is a renewable, environmentally friendly, abandant and widely distributed, it has received worldwide attention. Biomass energy is an ideal alternative energy source considering traditional energy sources are exhausted [1]. In the utilization of biomass energy, biodiesel is widely studied and applied, and it’s currently produced by transesterification [2,3]. In the process of producing biodiesel by a transesterification reaction, about 10% of by-product glycerol is inevitably formed [4]. Although glycerin is a widely used raw material, the reuse of glycerol is limited because it contains many impurities (such as methanol, methyl esters, inorganic salts, unreacted reactants and large amounts of impurities) [5]. Therefore, glycerin has low commercial value and high toxicity, and it is expensive to further purification, but a pity to waste it [6]. According to the previous research [7e9], glycerin can be converted to hydrogen. Glycerin is a good substitute for hydrogen production compared to methanol and ethanol, mainly because it doesn’t swell Nafion membranes in proton exchange membrane fuel cell (PEMFC) [10]. At present, the hydrogen production technology of glycerol is mainly through catalytic reactions, such as aqueous phase reforming [11,12], autothermal reforming [13,14], chemical looping reforming [15] and steam reforming [16e19]. Among them, glycerin steam reforming has great appeal because it can theoretically produce 7 mol of hydrogen per mole of glycerol involved in the reaction according to the equation (Eq. (1)). The glycerin steam reforming reaction is carried out at a high temperature, and thus the glycerin undergoes a decomposition reaction (Eq. (2)). The carbon monoxide generated by the decomposition of glycerol is further subjected to a water gas shift reaction with steam (Eq. (3)), which is believed to promote hydrogen production [20]. In addition, glycerol steam reforming has other side reactions which complicate the entire reforming reaction. C3 H8 O3 þ 3H2 O/3CO2 þ 7H2 DH ¼ 123kJ=mol
(1)
C3 H8 O3 / 3CO þ 4H2 DH ¼ 245kJ=mol
(2)
CO þ H2 O/CO2 þ H2 DH ¼ 41kJ=mol
(3)
Current research on glycerin steam reforming (GSR) has focused on catalysts that promote the cleavage of CeC, CeH and OeH bonds in glycerin and maintain C]O bonds. The cleavage of the C]O bond results in the formation of alkanes and further carbon deposition, resulting in higher H2 selectivity [21]. Due to their high selectivity and excellent stability, many studies have researched noble metal-based catalysts such as ruthenium (Ru) [22], iridium (Ir) [23], rhodium (Rh) [24], palladium (Pd) [25] and platinum (Pt) [8]. Senseni et al. [26] performed GSR experiments on MgOeAl2O3 catalysts, which were promoted by different noble metals (Rh, Ru, Pt, Ir). The catalytic results show that the addition of precious metal improves H2 yield and glycerol conversion in the prepared catalyst. Among them, the Pt-based and Rh-based catalysts have the highest activity, and the Rh catalyst has the highest stability with time in the production process. Despite many
advantages of precious metal GSR reactions, the high cost has limited their use on an industrial scale. To realize the economics of the catalysts used in the GSR reaction, the researchers turned to the development of transition metal-based catalysts. Although there are some GSR researches of the cobalt (Co) and copper (Cu) based catalysts [20], most are concentrated on nickel (Ni) based system due to its high ability to break CeC bonds and lower costs [27,28]. The disadvantages of nickel-based catalysts are that they have a high carbon deposition rate and the nickel material is s easily sintered, which results in a large amount of deactivation of the catalyst in the GSR process [25]. Wu et al. [9] studied the synthesis of La1-XCaXNiO3 perovskite oxide and its application in glycerin steam reforming. The results show that the perovskite can promote the uniform distribution of Ni, La and Ca elements during the reduction process, thereby increasing the interfacial area between oxide and Ni by limiting action. The catalyst of La0.5Ca0.5NiO3 is the most optimized composition for glycerol steam reforming, but it is inactivated after 18 h, with a total carbon deposit of about 200 mg/gcat [29]. In order to solve the problem of deactivation of the nickelbased catalyst due to carbon deposition, the researchers further studied Ni-based catalysts based on other supports. Some carriers having a larger specific surface area, pore volume, and a unique intermediate structure are used for the preparation of catalysts such as SBA-15 [30] and ZrO2 [31]. These unique properties are highly dispersible in the active phase (Nio) and form a strong interaction with the nickel species, which is believed to be important for the catalytic performance and stability of the catalyst [32]. Calles et al. [30] studied Ca and Mg modified Ni-SBA-15 catalysts and found that the NieCa-SBA-15 catalyst achieved the highest glycerol conversion (98.4%) at 600 C. The disadvantage is that the hydrogen selectivity is low. Attapulgite (ATP) is a natural crystalline hydrated magnesium aluminosilicate mineral with a unique layered chain structure. Its structure is a 2:1 type clay mineral. In each 2:1 unit structure layer, the tetrahedral wafer corners are reversed at a certain distance to form a layer chain [33]. Due to its unique structure, ATP has many excellent properties such as cation exchangeability, thermal stability and high specific surface area [34]. Therefore, ATP is widely used as a carrier for catalysts in catalytic reactions, such as ethanol steam reforming [35e37], low temperature oxidation of CO [38], and polymerization of olefins [39,40]. Chen M et al. [41] studied a transition metal-loaded ATP catalyst and found that the NiATP catalyst has a higher H2 selectivity, while the Co-ATP catalyst has a higher glycerol conversion at 600 C. Further, as a catalyst carrier, attapulgite has a high specific surface area and good thermal stability, which is advantageous for glycerol steam reforming. In this paper, ATP was chosen as the carrier to prepare and Ca-modified Ni/ATP catalysts by wet impregnation method. The synthesized catalyst was characterized by N2 adsorption/ desorption, BET, XRD, H2-TPR and NH3-TPD. The catalyst was tested by carrying out a glycerol steam reforming reaction under the following conditions: 400e800 C, W/G is 3, N2 flow rate is 0.15 L/min, GHSV is 1 h1. In addition, carbon deposition on the catalyst surface was characterized by TG and SEM, and the cause of catalyst deactivation was analyzed.
Please cite this article as: Feng P et al., Ni supported on the CaO modified attapulgite as catalysts for hydrogen production from glycerol steam reforming, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.013
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Experimental Catalyst preparation Natural attapulgite mineral clay was produced by Zhongke (Huai’an) New Energy Technology Development Co., Ltd. Jiangsu Province, China. The alumina carrier was purchased from Shanghai Titan Technology Co., Ltd., Ni(NO3)2$6H2O was purchased from Shanghai Titan Technology Co., Ltd., and Ca(NO3)2$4H2O was purchased from Hubei Guangao Biotechnology Co., Ltd. First, Al2O3 and ATP were separately ground and crushed, then sieved to 350e500 mm, and calcined at 700 C for 4 h. Ni-Al2O3 and Ni-ATP catalysts containing 10 wt% Ni were prepared by wet impregnation method using Ni(NO3)2$6H2O aqueous solution with a concentration of 0.2 mol/L. The catalysts were then transferred to an oven and dried at 110 C for 12 h. These were then placed in a muffle furnace and calcined at 700 C for 4 h. After the calcination was completed, the temperature was lowered to room temperature, and placed the catalsyt in a desiccator for use. Preparation of Ni-Ca-ATP. A modified ATP carrier containing 10% by weight of CaO was obtained by an incipient wetness impregnation method using an appropriate concentration of Ca(NO3)2$4H2O aqueous solution. Using a 0.1 mol/L aqueous solution of Ni(NO3)2$6H2O, a catalyst having a 10 wt% Ni content was obtained by impregnation. The catalyst was then transferred to an oven and dried at 110 C for 12 h. It was then placed in a muffle furnace and calcined at 700 C for 4 h. After the calcination was completed, the temperature was lowered to room temperature, and placed the catalsyt in a desiccator for use. The catalyst was named Ni-Ca-ATP.
Catalyst characterization The N2 adsorption/desorption isotherm was measured and recorded using a JW-BK200 high performance surface area and pore size analyzer. The catalyst surface area was calculated by the BET method, and the cumulative volume (Vpore) and average pore diameter (Dpore) of the pores were calculated by the BJH method. The crystal structure of the support and the catalyst was measured by a TD-3700 X-ray diffractometer. Based on the characteristic peaks, the average crystal diameter of the metal particles was calculated according to the Scherrer formula. In the FineSorb 3010 temperature-programmed reduction apparatus, H2 temperature-programmed reduction (H2-TPR) was performed to analyze the reduction behavior of the catalyst. Measure the surface acidity of the catalyst by the NH3 programmed temperature desorption method (NH3-TPD). Scanning electron microscopy (SEM) analysis was measured on a Phenom LE electron microscope to measure the surface morphology of the catalyst. Thermogravimetric analysis (TG) was used to study carbon deposition on used catalyst by thermogravimetric analyzer TGA 8000.
Experimental methods and data analysis The glycerol steam reforming reaction is carried out in a cylindrical fixed bed reactor. The experimental instrument is shown in our previous research [42]. The catalyst was reduced at 600 C for 2 h under a flow of H2/N2 at 0.1 L/min. Thereafter, the reactor was purged with N2 (0.15 L/min) for 20 min before the experiment and then heated to the reaction temperature. The mixture of glycerol (analytical purity) and water was fed into the reactor using an HPLC pump. N2 at a rate of 0.15 L/min was introduced into the reactor through a mass flow controller. The discharged gas was collected in a balloon through a condenser tube and analyzed by gas chromatography equipped with TCD. The gas chromatography analytical condition is shown in our previous research [43]. Give the following definition %Glycerol conversionðglobal conversionÞ ¼
Glycerolin Glycerolout Glycerolin
100 %Glycerol conversionðgaseous productsÞ ¼
C atoms in gas products total C atoms in feedstock
100 H2 yield ¼
H2 moles producted 100% moles of glycerol in the feedstock 7
H2 selectivity ¼
H2 moles produced 1 100% C atoms produced in the gas phase RR
RR is the reforming ratio (7/3), defined as the ratio of moles of H2 to CO2 formed. W=G ¼
moles of H2 O in the feedstock moles of glycerol in the feedstock
WHSV ¼
Mass flow rate of liquid feed Catalyst quality
Results and discussion Catalyst characterization The N2 adsorption-desorption isotherm and pore size distribution are shown in Fig. 1. It can be seen from Fig. 1 that the Ni-Al2O3 catalyst exhibits a type Ⅳ isotherm with H2-type hysteresis loop, and the Ni-ATP and Ni-CaO-ATP exhibit a type III isotherm with H3 hysteresis loop. It can be observed that when the relative pressure is greater than 0.4, the amount of nitrogen adsorption is significantly increased, which is characterized by the presence of a large number of mesopores. The structural characteristics of all the catalysts are shown in Table 1. Compared with the Ni-Al2O3 catalyst, the specific surface area and pore volume of Ni-ATP catalyst are significantly smaller, but the pore diameter is similar. For the Ni–CaATP catalyst, the addition of CaO results in a significant increase in the specific surface area of the catalyst as well as a decrease in pore volume and pore size.
Please cite this article as: Feng P et al., Ni supported on the CaO modified attapulgite as catalysts for hydrogen production from glycerol steam reforming, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.013
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Fig. 1 e N2 adsorption-desorption isotherms. Fig. 2 (a) indicates the XRD patterns of the catalyst of NiAl2O3, Ni-ATP and Ni-CaO-ATP after calcination. For Ni-Al2O3 catalyst, g-Al2O3 is observed at 2q ¼ 37.2 , 47.2 and 67.6 without corresponding transitional oxidation. The spinel nickel aluminate (NiAl2O4) is observed at 2q ¼ 19.0 , 32.0 , 37.0 , 45.0 , 60.2 and 65.9 . It is well known that NiAl2O4 is formed by the reaction of NiO and Al2O3 at high temperature and is difficult to be reduced at low temperature. For Ni-ATP and Ni-CaO-ATP catalyst, the peaks observed at 8.4 and 35.5 are both characteristic peaks of ATP. The characteristic peaks of NiO are at 37.3 , 43.3 and 62.6 . Since ATP is a silicate, some silicon oxides are produced during calcination and reduction. Therefore, the peaks of 20.7 and 26.6 are
attributed to SiO2 of different crystallinity, respectively. The XRD pattern of the catalyst after reduction is shown in Fig. 2 (b). Compared with the calcined catalyst, there is no significant change in the structure of Al2O3 and ATP, indicating that Al2O3 and ATP have higher thermal stability. For the three kinds of catalysts, the peaks at 44.5 and 51.6 indicate the formation of a nickel metal phase characteristic, which are related to the NiO crystal faces of (111) and (200), respectively. For the Ni-CaO-ATP catalyst, the characteristic peaks of CaO are shown at 31.3 and 52.5 . In addition, the NiO crystallite sizes of Ni-Al2O3, Ni-ATP, and Ni-CaO-ATP were calculated using the Scherrer formula, and the values are shown in Table 1: Ni-CaO-ATP (10.2 nm) < Ni-ATP (12.6 nm) < Ni-Al2O3 (16.3 nm). The addition of a small amount of alkaline cocatalyst such as CaO or MgO results in a higher metal dispersion, which is consistent with the literature [44]. The H2-TPR experiment was carried out to study the reducibility of all calcined samples, and the results are shown in Fig. 3. For the Ni-Al2O3 catalyst, three characteristic peaks of low (200e300 C), medium (400e600 C), and high (600e800 C) are observed. The small peak observed at low temperature can be attributed to the decrease of the volume nickel oxide phase, and the peak at the intermediate temperature can be attributed to the interaction of the NiO substance with the surface of the carrier to form a NiO-AlOX monolayer, and the peak at high temperature indicates there is a large amount of NiAl2O4. Ni-ATP catalyst exhibits two H2 peaks at 330 C and 590 C during the reduction process. The first broad peak around 330 C is caused by NiO crystallites on the outer surface of the support, and can be easily reduced at low temperature. The broad peak around 590 C is attributed to the reduction of NiO in the portion interacting with the ATP carrier, which requires high temperatures to reduce [45,46]. Compared with Ni-AL2O3 catalyst, Ni-ATP catalyst has a lower reduction temperature, indicating that it is easier to be reduced. The peak of the NiCaO-ATP catalyst is similar to that of Ni-ATP, and the high temperature peak may also be due to an increase in the strength of the NieO bond near Ca2þ. The NH3-TPD experiment was carried out to study the surface acidity of the catalyst, and the results are shown in Fig. 4. The adsorption force of NH3 on the sample is proportional to the acidity of the adsorption site. Generally, the stronger the acidity, the greater the adsorption power, and the higher the temperature required for desorption. It is generally believed that the desorption peak below 200 C corresponds to a weak acid site, the desorption peak at 200e400 C corresponds to a medium strong acid site, and the desorption peak above 400 C is a strong acid site. It can be seen from the figure that among the three catalysts, NieAl2O3 is the most acidic and Ni-ATP is weaker. This is because the density of Al3þ on the surface of the NieAl2O3 catalyst is high, which results in the support being acidic. At the same time, we can see that for
Table 1 e The physical properties of different catalysts. Samples NieAl2O3 Ni-ATP Ni-CaO-ATP
SBET(m2/g)
Vpore(cm3/g)
Dpore(nm)
DXRD(nm)
D’XRD(nm)
101.71 76.92 109.54
0.34 0.22 0.18
9.62 8.33 6.27
16.3 12.6 10.2
22.8 16.4 13.9
Please cite this article as: Feng P et al., Ni supported on the CaO modified attapulgite as catalysts for hydrogen production from glycerol steam reforming, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.013
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Fig. 2 e XRD profiles of calcined (a) and reduced (b) catalysts. Ni-CaO-ATP, the addition of CaO reduces the acidity of the catalyst, because the presence of Ca2þ provides more basic sites.
The catalytic activity was investigated in the fixed bed reactor at 400e800 C, W/G is 3, N2 flow rate is 0.15 L/min, and WHSV is1h1. The effects of reaction temperature on the conversion of glycerol (XC3H8O3, %) and the conversion of glycerol to gaseous product (XC3H8O3 gaseous into production, %) are shown in Fig. 5. It can be seen from Fig. 5 that the conversion of glycerol under the action of the three catalysts is very close, and after 600 C, the conversion of glycerol exceeds 90%, and the conversion of glycerol under the action of Ni-CaO-ATP catalyst is slightly higher. Since the overall GSR process is endothermic, the glycerol conversion increases rapidly with the temperature increasing. Compared with the Al2O3 support, the ATP support catalyst achieves a higher gaseous product conversion at lower temperature. In particular, the activity of the NiCaO-ATP catalyst is significantly improved after the addition
of the CaO modifier, and the maximum gaseous product conversion is substantially achieved at 600 C (Fig. 5). It can be seen from Fig. 5 that the conversion of gaseous products is higher than that of NieAl2O3 at 400e600 C, although the glycerol conversion is lower under the action of Ni-ATP. This would be due to the special spatial structure of ATP that is more conducive to the glycerol steam reforming reactions, which also improves the carbon deposition resistance of the catalyst [38]. It has been reported that the decomposition of glycerol includes a thermal cracking reaction before entering the catalyst bed and an acid-base catalytic reaction in the acid-basic part of the catalyst carrier [47]. It can be seen from the Fig. 5 that the Ni-CaO-ATP catalyst is more active and produces more gaseous products. These results indicate that the use of a small amount of a basic accelerator results in a higher metal dispersion, thereby promoting the conversion of glycerol to gaseous products. This is consistent with the study by Dieuzeide et al. [48]. Effects of temperature on hydrogen yield and hydrogen selectivity are shown in Fig. 6. It can be seen from the Fig. 6 that the catalytic effect of Ni-ATP is similar to that of NieAl2O3. Compared with the catalytic effect of NieAl2O3 at
Fig. 3 e H2-TPR profiles of catalyst.
Fig. 4 e NH3-TPD curves for all reduced catalysts.
Catalyst performance
Please cite this article as: Feng P et al., Ni supported on the CaO modified attapulgite as catalysts for hydrogen production from glycerol steam reforming, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.013
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Fig. 5 e Effects of temperature on glycerol conversion and gaseous product conversion. 400e800 C, the hydrogen yield and hydrogen selectivity of NiATP is slightly better. ATP is a natural carrier that is less expensive than Al2O3. Therefore, ATP has a better industrial application value, and it was chosen as the catalyst carrier in this paper. Compared with Ni-ATP, Ni-CaO-ATP shows a significantly higher hydrogen yield and selectivity. It would be due to the fact the surface properties of ATP are changed by the addition of CaO, leading to promote the steam-carbon reaction (C þ H2O]CO þ H2), water gas shift reaction (CO þ H2O]CO2þH2) and the inhibition of cracking and polymerization reactions [49,50]. Effects of reaction temperature on the molar ratio of H2/CO and CO/CO2 are shown in Fig. 7. For the Ni/Al2O3 catalyst, the ratio of H2/CO and CO/CO2 is between 2 and 3, and it is relatively stable over the entire temperature range. For Ni-ATP catalyst, the ratio of H2/CO is between 2 and 4 and CO/CO2 value is between 1 and 2. Both of them are relatively stable and similar to Ni/Al2O3. For Ni-CaO-ATP catalyst, the CO/CO2 molar ratio is lower than that of Ni/Al2O3 and Ni-ATP
catalysts, but the ratio of H2/CO is more than that of that of Ni/Al2O3 and Ni-ATP catalysts over the entire temperature range. The H2/CO molar ratio of Ni-CaO-ATP catalyst increases with temperature and reaches its maximum (about 6.5) at about 600 C and begins to decrease to about 4 at higher temperature. This is due to the fact that the CaO modifier has an important influence on the distribution of gaseous products. The main effect of CaO is to increase the selectivity of hydrogen, which is advantageous for the water gas shift reaction [50]. It can be seen from Figs. 6 and 7 that Ni-CaO-ATP exhibits high activity at 600 C, the glycerol conversion and hydrogen yield are relatively stable at 600e800 C. In addition, H2/CO is a maximum value (about 6.5) and CO/CO2 was low at 600 C. Therefore, 600 C was chosen as the optimum temperature for the GSR reaction of Ni-CaO-ATP. The distribution of three-phase products of Ni-CaO-ATP catalyzed glycerol reforming at 600 C is shown in Fig. 8. It can be seen from the Fig. 8 that up to 85.48% of glycerol is
Fig. 6 e Effect of temperature on hydrogen yield and hydrogen selective. Please cite this article as: Feng P et al., Ni supported on the CaO modified attapulgite as catalysts for hydrogen production from glycerol steam reforming, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.013
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converted to gaseous products and only 2.63% of glycerol is converted to liquid products, which indicates that the Ni-CaOATP catalyst has a good catalytic effect and is conducive to the production of hydrogen-rich gas. However, the carbon content in the solid product was higher, reaching 11.14%. Higher carbon deposits will lead to catalyst deactivation, which is adverse to the hydrogen production of glycerin steam reforming. Therefore, we should study how to further reduce the carbon production in the future. In addition, 0.75% of the carbon was lost, which was caused by a small amount of carbon products contaminating the reactor wall and glassware wall.
The test of catalytic The stability of NieAl2O3, Ni-ATP and Ni-CaO-ATP catalysts was studied under the following conditions: temperature was 600 C, W/G was 3, N2 flow rate was 0.15 L/min, and WHSV was 1 h1. The result is shown in Fig. 9. It can be seen from Fig. 9
Fig. 7 e Effect of temperature on the molar ratio of H2/CO and CO/CO2.
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that the glycerol conversion of three kinds of catalyst is decreased gradually, indicating that its ability to destroy the CeC bond of the glycerol molecule is decreasing slowly with time on stream. For the NieAl catalyst, the hydrogen yield and selectivity are reduced rapidly. The result indicates that the nickel catalyst is gradually deactivating over time on stream due to the carbon formation and sintering of catalyst (see Figs. 10e12). It is worth noting that the hydrogen selectivity of NiATP catalyst increases with time on stream, which would be due to the good carbon deposition resistance of ATP. The glycerol conversion and hydrogen yield of NieCaO-ATP are maintained at a higher level, indicating that the catalyst has the best activity, and it is optimal in suppressing carbon deposition and sinter [49,50].
Characterization of used catalysts The catalyst used in the above stability test was characterized by XRD, and the results are shown in Fig. 10. As can be seen from the Fig. 10, on the XRD patterns of Ni-ATP and NieCaO-ATP catalysts, the characteristic diffraction peaks of the ATP supporter is shown at 8.9 and 35.6 , indicating that the crystal structure of ATP remains unchanged after the reaction, which indicated that ATP has a good hydrothermal durability as a catalyst carrier. Compared with the reduction catalyst, the XRD pattern of all of catalysts after the reaction is widened at 26.0 and the strength is obviously weakened, which is due to the deposition of carbon on the catalyst after the reaction (see Figs. 11 and 12). After calculation, the crystal size of the catalyst after the reaction was as shown in Table 1: Ni-CaO-ATP (13.9 nm) < Ni-ATP (16.4 nm) < NieAl2O3 (22.8 nm). Compared with the crystal size of the reduction catalyst, NieAl2O3 increased most, Ni-ATP followed, and NiCaO-ATP increased the least. This indicates that Ni-ATP catalyst has better resistance to sintering than NieAl2O3 catalyst, and Ni-CaO-ATP catalyst has better resistance to sintering than Ni-ATP catalyst, which is attributed to the inhibition of cracking and polymerization reaction of basic cocatalyst [49,50]. The carbon deposit on the catalyst after the reaction was characterized by TG, and the results are shown in Fig. 11. As can be seen from the Fig. 11, the amount of carbon deposited
Fig. 8 e Distribution of three-phase products.
Please cite this article as: Feng P et al., Ni supported on the CaO modified attapulgite as catalysts for hydrogen production from glycerol steam reforming, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.013
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Fig. 9 e Stability of the catalysts with time on stream.
Fig. 10 e XRD patterns of used catalysts.
follows the following sequence: Ni-CaO-ATP (11.6%) < Ni-ATP (13.9%) < NieAl2O3 (17.9%). It can be seen from the TG and DTG curves that for the NieAl2O3 and Ni-ATP catalysts, the curve has a decrease within 300 C, which may be due to the
removal of easily oxidizable carbonaceous materials and volatile materials [51]. On the contrary, the TG and DTD curves of Ni-CaO-ATP catalyst hardly decreased at 300 C, which indicates that Ca has a good inhibitory effect on the easily oxidizable carbonaceous substance and the volatile substance [49]. TG graph of the NieAl2O3 catalyst shows that the curve starts to decrease dramatically from about 400 C, while the Ni-ATP and Ni-CaO-ATP curves decrease rapidly from 500 C or even higher temperature. It can also be seen from the DTG graph that for the NieAl2O3 catalyst, the curve starts to decrease dramatically at around 400 C, and the rate of weight loss further increases at about 500 C. It has been reported that peak below 500 C is designated as oxidation of amorphous carbon, while the peak of high temperature is attributed to oxidation of highly ordered graphitic carbon [52,53]. Compared with NieAl2O3, Ni-ATP avoided the formation of the former peak, and the amorphous carbon that forms this peak is considered to be the main reason for the catalyst deactivation [54,55]. There is only one weight loss peak on the TG and DTG of Ni-CaO-ATP catalyst, indicating that the addition of calcium would inhibit the carbon deposition and prolong the life of the catalyst [52]. The used catalysts were analyzed by scanning electron microscopy (SEM), and the results are shown in Fig. 12. It can be seen from Fig. 12 that the surface of NieAl2O3 is covered by
Fig. 11 e TG and DTG profiles for all used catalysts. Please cite this article as: Feng P et al., Ni supported on the CaO modified attapulgite as catalysts for hydrogen production from glycerol steam reforming, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.013
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Fig. 12 e SEM image of the used catalyst: (a) NieAl2O3, (b) Ni-ATP, (c) Ni-CaO-ATP. a large amount of carbon filaments, the surface carbon deposition of Ni-ATP is less, and the carbon deposition of NiCaO-ATP is the least. It can be concluded that as a carrier ATP exhibited superior carbon deposition resistance than that o f Al2O3, which is consistent with the results of TG and DTG. And as a modifier, CaO further reduced the formation of carbon on the surface of the catalyst, indicating that CaO has good properties for inhibiting carbon deposition. This is consistent with the documents of Wang et al. [49] and Charisiou N D et al. [50].
Conclusion The catalytic performance of NieAl2O3, Ni-ATP and Ni-CaOATP was compared by glycerol steam reforming reaction (GSR). For all catalysts, as the reaction temperature increased from 500 C to 700 C, the total conversion of glycerol and its catalytic activity for catalytic conversion to gaseous products increased. As a carrier, ATP is superior to Al2O3 because of its unique intermediate structure. When CaO is used as an ATP modifier, which would promote the water gas shift reaction, leading to improve hydrogen yield, hydrogen selectivity and decrease the formation of CO. The stability results show that ATP has better carbon deposition resistance than Al2O3, and the addition of CaO further reduces carbon production. Therefore, a Ni-based catalyst supported on a CaO-modified attapulgite can be used as a candidate material for good glycerin steam reforming to produce H2.
Acknowledgements This project was financially supported by the National Natural Science Foundation of China (No.21376084), Guangxi Key Laboratory of Bio-refinery (GXKLB19-03) and Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development (Y909kn1001). The authors thank Research Center of Analysis and Test of East China University of Science and Technology for the help on the characterization.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2020.01.013.
references
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Ni supported on the CaO modified attapulgite as catalysts for hydrogen production from glycerol steam reforming P. Feng a, K. Huang b, Q. Xu a,b,*, W. Qi c,**, S. Xin d, T. Wei a, L. Liao a, Y. Yan a a
Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, Research Center for Biomass Energy, East China University of Science and Technology, Shanghai, China b National Engineering Research Center for Non-Food Biorefinery, Guangxi Key Laboratory of Biorefinery, Guangxi Academy of Sciences, Nanning, China c Guangzhou Institute of Energy Conversion, Chinese Academy of Science, Guangzhou, China d Hubei Key Laboratory of Industrial Fume and Dust Pollution Control, Jianghan University, Wuhan, China
highlights NieCaO-ATP exhibits the highest glycerol conversion and hydrogen yield. Attapulgite (ATP) has good carbon deposition resistance. CaO increases the activity of catalyst and the inhibition of carbon deposition.
article info
abstract
Article history:
The Ni catalyst supported on CaO-modified attapulgite (CaO-ATP) were synthesized by wet
Received 28 October 2019
impregnation method at a constant Ni metal loading (10 wt%). The catalyst was tested by
Received in revised form
carrying out a glycerin steam reforming reaction under the following conditions: 400
20 December 2019
e800 C, W/G is 3, GHSV is 1 h1. NieCaO-ATP exhibited the highest hydrogen yield (85.30%)
Accepted 2 January 2020
and glycerol conversion (93.71%) at 600
Available online xxx
adsorption/desorption, BET, XRD, H2-TPR, TG and SEM. The results show that ATP has good
C. The catalysts were characterized by N2
resistance to carbon deposition. As an attapulgite modifier of NieCaO-ATP, CaO promotes Keywords:
the dispersion of the active component nickel species, which would promote the water gas
Hydrogen
shift reaction, leading to the improving of hydrogen yield. In addition, the addition of Ca
Steam reforming
would further enhance the inhibition of carbon deposition and prolong the life of the Ni
Attapulgite
eCaO-ATP catalyst.
Ni catalysts
© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
CaO
* Corresponding author. Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, Research Center for Biomass Energy, East China University of Science and Technology, Shanghai, China. ** Corresponding author. E-mail addresses: [email protected] (Q. Xu), [email protected] (W. Qi). https://doi.org/10.1016/j.ijhydene.2020.01.013 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Feng P et al., Ni supported on the CaO modified attapulgite as catalysts for hydrogen production from glycerol steam reforming, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.013
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Introduction In the past few decades, the problems caused by the large use of non-renewable resources have been extensively studied. Because biomass is a renewable, environmentally friendly, abandant and widely distributed, it has received worldwide attention. Biomass energy is an ideal alternative energy source considering traditional energy sources are exhausted [1]. In the utilization of biomass energy, biodiesel is widely studied and applied, and it’s currently produced by transesterification [2,3]. In the process of producing biodiesel by a transesterification reaction, about 10% of by-product glycerol is inevitably formed [4]. Although glycerin is a widely used raw material, the reuse of glycerol is limited because it contains many impurities (such as methanol, methyl esters, inorganic salts, unreacted reactants and large amounts of impurities) [5]. Therefore, glycerin has low commercial value and high toxicity, and it is expensive to further purification, but a pity to waste it [6]. According to the previous research [7e9], glycerin can be converted to hydrogen. Glycerin is a good substitute for hydrogen production compared to methanol and ethanol, mainly because it doesn’t swell Nafion membranes in proton exchange membrane fuel cell (PEMFC) [10]. At present, the hydrogen production technology of glycerol is mainly through catalytic reactions, such as aqueous phase reforming [11,12], autothermal reforming [13,14], chemical looping reforming [15] and steam reforming [16e19]. Among them, glycerin steam reforming has great appeal because it can theoretically produce 7 mol of hydrogen per mole of glycerol involved in the reaction according to the equation (Eq. (1)). The glycerin steam reforming reaction is carried out at a high temperature, and thus the glycerin undergoes a decomposition reaction (Eq. (2)). The carbon monoxide generated by the decomposition of glycerol is further subjected to a water gas shift reaction with steam (Eq. (3)), which is believed to promote hydrogen production [20]. In addition, glycerol steam reforming has other side reactions which complicate the entire reforming reaction. C3 H8 O3 þ 3H2 O/3CO2 þ 7H2 DH ¼ 123kJ=mol
(1)
C3 H8 O3 / 3CO þ 4H2 DH ¼ 245kJ=mol
(2)
CO þ H2 O/CO2 þ H2 DH ¼ 41kJ=mol
(3)
Current research on glycerin steam reforming (GSR) has focused on catalysts that promote the cleavage of CeC, CeH and OeH bonds in glycerin and maintain C]O bonds. The cleavage of the C]O bond results in the formation of alkanes and further carbon deposition, resulting in higher H2 selectivity [21]. Due to their high selectivity and excellent stability, many studies have researched noble metal-based catalysts such as ruthenium (Ru) [22], iridium (Ir) [23], rhodium (Rh) [24], palladium (Pd) [25] and platinum (Pt) [8]. Senseni et al. [26] performed GSR experiments on MgOeAl2O3 catalysts, which were promoted by different noble metals (Rh, Ru, Pt, Ir). The catalytic results show that the addition of precious metal improves H2 yield and glycerol conversion in the prepared catalyst. Among them, the Pt-based and Rh-based catalysts have the highest activity, and the Rh catalyst has the highest stability with time in the production process. Despite many
advantages of precious metal GSR reactions, the high cost has limited their use on an industrial scale. To realize the economics of the catalysts used in the GSR reaction, the researchers turned to the development of transition metal-based catalysts. Although there are some GSR researches of the cobalt (Co) and copper (Cu) based catalysts [20], most are concentrated on nickel (Ni) based system due to its high ability to break CeC bonds and lower costs [27,28]. The disadvantages of nickel-based catalysts are that they have a high carbon deposition rate and the nickel material is s easily sintered, which results in a large amount of deactivation of the catalyst in the GSR process [25]. Wu et al. [9] studied the synthesis of La1-XCaXNiO3 perovskite oxide and its application in glycerin steam reforming. The results show that the perovskite can promote the uniform distribution of Ni, La and Ca elements during the reduction process, thereby increasing the interfacial area between oxide and Ni by limiting action. The catalyst of La0.5Ca0.5NiO3 is the most optimized composition for glycerol steam reforming, but it is inactivated after 18 h, with a total carbon deposit of about 200 mg/gcat [29]. In order to solve the problem of deactivation of the nickelbased catalyst due to carbon deposition, the researchers further studied Ni-based catalysts based on other supports. Some carriers having a larger specific surface area, pore volume, and a unique intermediate structure are used for the preparation of catalysts such as SBA-15 [30] and ZrO2 [31]. These unique properties are highly dispersible in the active phase (Nio) and form a strong interaction with the nickel species, which is believed to be important for the catalytic performance and stability of the catalyst [32]. Calles et al. [30] studied Ca and Mg modified Ni-SBA-15 catalysts and found that the NieCa-SBA-15 catalyst achieved the highest glycerol conversion (98.4%) at 600 C. The disadvantage is that the hydrogen selectivity is low. Attapulgite (ATP) is a natural crystalline hydrated magnesium aluminosilicate mineral with a unique layered chain structure. Its structure is a 2:1 type clay mineral. In each 2:1 unit structure layer, the tetrahedral wafer corners are reversed at a certain distance to form a layer chain [33]. Due to its unique structure, ATP has many excellent properties such as cation exchangeability, thermal stability and high specific surface area [34]. Therefore, ATP is widely used as a carrier for catalysts in catalytic reactions, such as ethanol steam reforming [35e37], low temperature oxidation of CO [38], and polymerization of olefins [39,40]. Chen M et al. [41] studied a transition metal-loaded ATP catalyst and found that the NiATP catalyst has a higher H2 selectivity, while the Co-ATP catalyst has a higher glycerol conversion at 600 C. Further, as a catalyst carrier, attapulgite has a high specific surface area and good thermal stability, which is advantageous for glycerol steam reforming. In this paper, ATP was chosen as the carrier to prepare and Ca-modified Ni/ATP catalysts by wet impregnation method. The synthesized catalyst was characterized by N2 adsorption/ desorption, BET, XRD, H2-TPR and NH3-TPD. The catalyst was tested by carrying out a glycerol steam reforming reaction under the following conditions: 400e800 C, W/G is 3, N2 flow rate is 0.15 L/min, GHSV is 1 h1. In addition, carbon deposition on the catalyst surface was characterized by TG and SEM, and the cause of catalyst deactivation was analyzed.
Please cite this article as: Feng P et al., Ni supported on the CaO modified attapulgite as catalysts for hydrogen production from glycerol steam reforming, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.013
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Experimental Catalyst preparation Natural attapulgite mineral clay was produced by Zhongke (Huai’an) New Energy Technology Development Co., Ltd. Jiangsu Province, China. The alumina carrier was purchased from Shanghai Titan Technology Co., Ltd., Ni(NO3)2$6H2O was purchased from Shanghai Titan Technology Co., Ltd., and Ca(NO3)2$4H2O was purchased from Hubei Guangao Biotechnology Co., Ltd. First, Al2O3 and ATP were separately ground and crushed, then sieved to 350e500 mm, and calcined at 700 C for 4 h. Ni-Al2O3 and Ni-ATP catalysts containing 10 wt% Ni were prepared by wet impregnation method using Ni(NO3)2$6H2O aqueous solution with a concentration of 0.2 mol/L. The catalysts were then transferred to an oven and dried at 110 C for 12 h. These were then placed in a muffle furnace and calcined at 700 C for 4 h. After the calcination was completed, the temperature was lowered to room temperature, and placed the catalsyt in a desiccator for use. Preparation of Ni-Ca-ATP. A modified ATP carrier containing 10% by weight of CaO was obtained by an incipient wetness impregnation method using an appropriate concentration of Ca(NO3)2$4H2O aqueous solution. Using a 0.1 mol/L aqueous solution of Ni(NO3)2$6H2O, a catalyst having a 10 wt% Ni content was obtained by impregnation. The catalyst was then transferred to an oven and dried at 110 C for 12 h. It was then placed in a muffle furnace and calcined at 700 C for 4 h. After the calcination was completed, the temperature was lowered to room temperature, and placed the catalsyt in a desiccator for use. The catalyst was named Ni-Ca-ATP.
Catalyst characterization The N2 adsorption/desorption isotherm was measured and recorded using a JW-BK200 high performance surface area and pore size analyzer. The catalyst surface area was calculated by the BET method, and the cumulative volume (Vpore) and average pore diameter (Dpore) of the pores were calculated by the BJH method. The crystal structure of the support and the catalyst was measured by a TD-3700 X-ray diffractometer. Based on the characteristic peaks, the average crystal diameter of the metal particles was calculated according to the Scherrer formula. In the FineSorb 3010 temperature-programmed reduction apparatus, H2 temperature-programmed reduction (H2-TPR) was performed to analyze the reduction behavior of the catalyst. Measure the surface acidity of the catalyst by the NH3 programmed temperature desorption method (NH3-TPD). Scanning electron microscopy (SEM) analysis was measured on a Phenom LE electron microscope to measure the surface morphology of the catalyst. Thermogravimetric analysis (TG) was used to study carbon deposition on used catalyst by thermogravimetric analyzer TGA 8000.
Experimental methods and data analysis The glycerol steam reforming reaction is carried out in a cylindrical fixed bed reactor. The experimental instrument is shown in our previous research [42]. The catalyst was reduced at 600 C for 2 h under a flow of H2/N2 at 0.1 L/min. Thereafter, the reactor was purged with N2 (0.15 L/min) for 20 min before the experiment and then heated to the reaction temperature. The mixture of glycerol (analytical purity) and water was fed into the reactor using an HPLC pump. N2 at a rate of 0.15 L/min was introduced into the reactor through a mass flow controller. The discharged gas was collected in a balloon through a condenser tube and analyzed by gas chromatography equipped with TCD. The gas chromatography analytical condition is shown in our previous research [43]. Give the following definition %Glycerol conversionðglobal conversionÞ ¼
Glycerolin Glycerolout Glycerolin
100 %Glycerol conversionðgaseous productsÞ ¼
C atoms in gas products total C atoms in feedstock
100 H2 yield ¼
H2 moles producted 100% moles of glycerol in the feedstock 7
H2 selectivity ¼
H2 moles produced 1 100% C atoms produced in the gas phase RR
RR is the reforming ratio (7/3), defined as the ratio of moles of H2 to CO2 formed. W=G ¼
moles of H2 O in the feedstock moles of glycerol in the feedstock
WHSV ¼
Mass flow rate of liquid feed Catalyst quality
Results and discussion Catalyst characterization The N2 adsorption-desorption isotherm and pore size distribution are shown in Fig. 1. It can be seen from Fig. 1 that the Ni-Al2O3 catalyst exhibits a type Ⅳ isotherm with H2-type hysteresis loop, and the Ni-ATP and Ni-CaO-ATP exhibit a type III isotherm with H3 hysteresis loop. It can be observed that when the relative pressure is greater than 0.4, the amount of nitrogen adsorption is significantly increased, which is characterized by the presence of a large number of mesopores. The structural characteristics of all the catalysts are shown in Table 1. Compared with the Ni-Al2O3 catalyst, the specific surface area and pore volume of Ni-ATP catalyst are significantly smaller, but the pore diameter is similar. For the Ni–CaATP catalyst, the addition of CaO results in a significant increase in the specific surface area of the catalyst as well as a decrease in pore volume and pore size.
Please cite this article as: Feng P et al., Ni supported on the CaO modified attapulgite as catalysts for hydrogen production from glycerol steam reforming, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.013
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Fig. 1 e N2 adsorption-desorption isotherms. Fig. 2 (a) indicates the XRD patterns of the catalyst of NiAl2O3, Ni-ATP and Ni-CaO-ATP after calcination. For Ni-Al2O3 catalyst, g-Al2O3 is observed at 2q ¼ 37.2 , 47.2 and 67.6 without corresponding transitional oxidation. The spinel nickel aluminate (NiAl2O4) is observed at 2q ¼ 19.0 , 32.0 , 37.0 , 45.0 , 60.2 and 65.9 . It is well known that NiAl2O4 is formed by the reaction of NiO and Al2O3 at high temperature and is difficult to be reduced at low temperature. For Ni-ATP and Ni-CaO-ATP catalyst, the peaks observed at 8.4 and 35.5 are both characteristic peaks of ATP. The characteristic peaks of NiO are at 37.3 , 43.3 and 62.6 . Since ATP is a silicate, some silicon oxides are produced during calcination and reduction. Therefore, the peaks of 20.7 and 26.6 are
attributed to SiO2 of different crystallinity, respectively. The XRD pattern of the catalyst after reduction is shown in Fig. 2 (b). Compared with the calcined catalyst, there is no significant change in the structure of Al2O3 and ATP, indicating that Al2O3 and ATP have higher thermal stability. For the three kinds of catalysts, the peaks at 44.5 and 51.6 indicate the formation of a nickel metal phase characteristic, which are related to the NiO crystal faces of (111) and (200), respectively. For the Ni-CaO-ATP catalyst, the characteristic peaks of CaO are shown at 31.3 and 52.5 . In addition, the NiO crystallite sizes of Ni-Al2O3, Ni-ATP, and Ni-CaO-ATP were calculated using the Scherrer formula, and the values are shown in Table 1: Ni-CaO-ATP (10.2 nm) < Ni-ATP (12.6 nm) < Ni-Al2O3 (16.3 nm). The addition of a small amount of alkaline cocatalyst such as CaO or MgO results in a higher metal dispersion, which is consistent with the literature [44]. The H2-TPR experiment was carried out to study the reducibility of all calcined samples, and the results are shown in Fig. 3. For the Ni-Al2O3 catalyst, three characteristic peaks of low (200e300 C), medium (400e600 C), and high (600e800 C) are observed. The small peak observed at low temperature can be attributed to the decrease of the volume nickel oxide phase, and the peak at the intermediate temperature can be attributed to the interaction of the NiO substance with the surface of the carrier to form a NiO-AlOX monolayer, and the peak at high temperature indicates there is a large amount of NiAl2O4. Ni-ATP catalyst exhibits two H2 peaks at 330 C and 590 C during the reduction process. The first broad peak around 330 C is caused by NiO crystallites on the outer surface of the support, and can be easily reduced at low temperature. The broad peak around 590 C is attributed to the reduction of NiO in the portion interacting with the ATP carrier, which requires high temperatures to reduce [45,46]. Compared with Ni-AL2O3 catalyst, Ni-ATP catalyst has a lower reduction temperature, indicating that it is easier to be reduced. The peak of the NiCaO-ATP catalyst is similar to that of Ni-ATP, and the high temperature peak may also be due to an increase in the strength of the NieO bond near Ca2þ. The NH3-TPD experiment was carried out to study the surface acidity of the catalyst, and the results are shown in Fig. 4. The adsorption force of NH3 on the sample is proportional to the acidity of the adsorption site. Generally, the stronger the acidity, the greater the adsorption power, and the higher the temperature required for desorption. It is generally believed that the desorption peak below 200 C corresponds to a weak acid site, the desorption peak at 200e400 C corresponds to a medium strong acid site, and the desorption peak above 400 C is a strong acid site. It can be seen from the figure that among the three catalysts, NieAl2O3 is the most acidic and Ni-ATP is weaker. This is because the density of Al3þ on the surface of the NieAl2O3 catalyst is high, which results in the support being acidic. At the same time, we can see that for
Table 1 e The physical properties of different catalysts. Samples NieAl2O3 Ni-ATP Ni-CaO-ATP
SBET(m2/g)
Vpore(cm3/g)
Dpore(nm)
DXRD(nm)
D’XRD(nm)
101.71 76.92 109.54
0.34 0.22 0.18
9.62 8.33 6.27
16.3 12.6 10.2
22.8 16.4 13.9
Please cite this article as: Feng P et al., Ni supported on the CaO modified attapulgite as catalysts for hydrogen production from glycerol steam reforming, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.013
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Fig. 2 e XRD profiles of calcined (a) and reduced (b) catalysts. Ni-CaO-ATP, the addition of CaO reduces the acidity of the catalyst, because the presence of Ca2þ provides more basic sites.
The catalytic activity was investigated in the fixed bed reactor at 400e800 C, W/G is 3, N2 flow rate is 0.15 L/min, and WHSV is1h1. The effects of reaction temperature on the conversion of glycerol (XC3H8O3, %) and the conversion of glycerol to gaseous product (XC3H8O3 gaseous into production, %) are shown in Fig. 5. It can be seen from Fig. 5 that the conversion of glycerol under the action of the three catalysts is very close, and after 600 C, the conversion of glycerol exceeds 90%, and the conversion of glycerol under the action of Ni-CaO-ATP catalyst is slightly higher. Since the overall GSR process is endothermic, the glycerol conversion increases rapidly with the temperature increasing. Compared with the Al2O3 support, the ATP support catalyst achieves a higher gaseous product conversion at lower temperature. In particular, the activity of the NiCaO-ATP catalyst is significantly improved after the addition
of the CaO modifier, and the maximum gaseous product conversion is substantially achieved at 600 C (Fig. 5). It can be seen from Fig. 5 that the conversion of gaseous products is higher than that of NieAl2O3 at 400e600 C, although the glycerol conversion is lower under the action of Ni-ATP. This would be due to the special spatial structure of ATP that is more conducive to the glycerol steam reforming reactions, which also improves the carbon deposition resistance of the catalyst [38]. It has been reported that the decomposition of glycerol includes a thermal cracking reaction before entering the catalyst bed and an acid-base catalytic reaction in the acid-basic part of the catalyst carrier [47]. It can be seen from the Fig. 5 that the Ni-CaO-ATP catalyst is more active and produces more gaseous products. These results indicate that the use of a small amount of a basic accelerator results in a higher metal dispersion, thereby promoting the conversion of glycerol to gaseous products. This is consistent with the study by Dieuzeide et al. [48]. Effects of temperature on hydrogen yield and hydrogen selectivity are shown in Fig. 6. It can be seen from the Fig. 6 that the catalytic effect of Ni-ATP is similar to that of NieAl2O3. Compared with the catalytic effect of NieAl2O3 at
Fig. 3 e H2-TPR profiles of catalyst.
Fig. 4 e NH3-TPD curves for all reduced catalysts.
Catalyst performance
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Fig. 5 e Effects of temperature on glycerol conversion and gaseous product conversion. 400e800 C, the hydrogen yield and hydrogen selectivity of NiATP is slightly better. ATP is a natural carrier that is less expensive than Al2O3. Therefore, ATP has a better industrial application value, and it was chosen as the catalyst carrier in this paper. Compared with Ni-ATP, Ni-CaO-ATP shows a significantly higher hydrogen yield and selectivity. It would be due to the fact the surface properties of ATP are changed by the addition of CaO, leading to promote the steam-carbon reaction (C þ H2O]CO þ H2), water gas shift reaction (CO þ H2O]CO2þH2) and the inhibition of cracking and polymerization reactions [49,50]. Effects of reaction temperature on the molar ratio of H2/CO and CO/CO2 are shown in Fig. 7. For the Ni/Al2O3 catalyst, the ratio of H2/CO and CO/CO2 is between 2 and 3, and it is relatively stable over the entire temperature range. For Ni-ATP catalyst, the ratio of H2/CO is between 2 and 4 and CO/CO2 value is between 1 and 2. Both of them are relatively stable and similar to Ni/Al2O3. For Ni-CaO-ATP catalyst, the CO/CO2 molar ratio is lower than that of Ni/Al2O3 and Ni-ATP
catalysts, but the ratio of H2/CO is more than that of that of Ni/Al2O3 and Ni-ATP catalysts over the entire temperature range. The H2/CO molar ratio of Ni-CaO-ATP catalyst increases with temperature and reaches its maximum (about 6.5) at about 600 C and begins to decrease to about 4 at higher temperature. This is due to the fact that the CaO modifier has an important influence on the distribution of gaseous products. The main effect of CaO is to increase the selectivity of hydrogen, which is advantageous for the water gas shift reaction [50]. It can be seen from Figs. 6 and 7 that Ni-CaO-ATP exhibits high activity at 600 C, the glycerol conversion and hydrogen yield are relatively stable at 600e800 C. In addition, H2/CO is a maximum value (about 6.5) and CO/CO2 was low at 600 C. Therefore, 600 C was chosen as the optimum temperature for the GSR reaction of Ni-CaO-ATP. The distribution of three-phase products of Ni-CaO-ATP catalyzed glycerol reforming at 600 C is shown in Fig. 8. It can be seen from the Fig. 8 that up to 85.48% of glycerol is
Fig. 6 e Effect of temperature on hydrogen yield and hydrogen selective. Please cite this article as: Feng P et al., Ni supported on the CaO modified attapulgite as catalysts for hydrogen production from glycerol steam reforming, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.013
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converted to gaseous products and only 2.63% of glycerol is converted to liquid products, which indicates that the Ni-CaOATP catalyst has a good catalytic effect and is conducive to the production of hydrogen-rich gas. However, the carbon content in the solid product was higher, reaching 11.14%. Higher carbon deposits will lead to catalyst deactivation, which is adverse to the hydrogen production of glycerin steam reforming. Therefore, we should study how to further reduce the carbon production in the future. In addition, 0.75% of the carbon was lost, which was caused by a small amount of carbon products contaminating the reactor wall and glassware wall.
The test of catalytic The stability of NieAl2O3, Ni-ATP and Ni-CaO-ATP catalysts was studied under the following conditions: temperature was 600 C, W/G was 3, N2 flow rate was 0.15 L/min, and WHSV was 1 h1. The result is shown in Fig. 9. It can be seen from Fig. 9
Fig. 7 e Effect of temperature on the molar ratio of H2/CO and CO/CO2.
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that the glycerol conversion of three kinds of catalyst is decreased gradually, indicating that its ability to destroy the CeC bond of the glycerol molecule is decreasing slowly with time on stream. For the NieAl catalyst, the hydrogen yield and selectivity are reduced rapidly. The result indicates that the nickel catalyst is gradually deactivating over time on stream due to the carbon formation and sintering of catalyst (see Figs. 10e12). It is worth noting that the hydrogen selectivity of NiATP catalyst increases with time on stream, which would be due to the good carbon deposition resistance of ATP. The glycerol conversion and hydrogen yield of NieCaO-ATP are maintained at a higher level, indicating that the catalyst has the best activity, and it is optimal in suppressing carbon deposition and sinter [49,50].
Characterization of used catalysts The catalyst used in the above stability test was characterized by XRD, and the results are shown in Fig. 10. As can be seen from the Fig. 10, on the XRD patterns of Ni-ATP and NieCaO-ATP catalysts, the characteristic diffraction peaks of the ATP supporter is shown at 8.9 and 35.6 , indicating that the crystal structure of ATP remains unchanged after the reaction, which indicated that ATP has a good hydrothermal durability as a catalyst carrier. Compared with the reduction catalyst, the XRD pattern of all of catalysts after the reaction is widened at 26.0 and the strength is obviously weakened, which is due to the deposition of carbon on the catalyst after the reaction (see Figs. 11 and 12). After calculation, the crystal size of the catalyst after the reaction was as shown in Table 1: Ni-CaO-ATP (13.9 nm) < Ni-ATP (16.4 nm) < NieAl2O3 (22.8 nm). Compared with the crystal size of the reduction catalyst, NieAl2O3 increased most, Ni-ATP followed, and NiCaO-ATP increased the least. This indicates that Ni-ATP catalyst has better resistance to sintering than NieAl2O3 catalyst, and Ni-CaO-ATP catalyst has better resistance to sintering than Ni-ATP catalyst, which is attributed to the inhibition of cracking and polymerization reaction of basic cocatalyst [49,50]. The carbon deposit on the catalyst after the reaction was characterized by TG, and the results are shown in Fig. 11. As can be seen from the Fig. 11, the amount of carbon deposited
Fig. 8 e Distribution of three-phase products.
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Fig. 9 e Stability of the catalysts with time on stream.
Fig. 10 e XRD patterns of used catalysts.
follows the following sequence: Ni-CaO-ATP (11.6%) < Ni-ATP (13.9%) < NieAl2O3 (17.9%). It can be seen from the TG and DTG curves that for the NieAl2O3 and Ni-ATP catalysts, the curve has a decrease within 300 C, which may be due to the
removal of easily oxidizable carbonaceous materials and volatile materials [51]. On the contrary, the TG and DTD curves of Ni-CaO-ATP catalyst hardly decreased at 300 C, which indicates that Ca has a good inhibitory effect on the easily oxidizable carbonaceous substance and the volatile substance [49]. TG graph of the NieAl2O3 catalyst shows that the curve starts to decrease dramatically from about 400 C, while the Ni-ATP and Ni-CaO-ATP curves decrease rapidly from 500 C or even higher temperature. It can also be seen from the DTG graph that for the NieAl2O3 catalyst, the curve starts to decrease dramatically at around 400 C, and the rate of weight loss further increases at about 500 C. It has been reported that peak below 500 C is designated as oxidation of amorphous carbon, while the peak of high temperature is attributed to oxidation of highly ordered graphitic carbon [52,53]. Compared with NieAl2O3, Ni-ATP avoided the formation of the former peak, and the amorphous carbon that forms this peak is considered to be the main reason for the catalyst deactivation [54,55]. There is only one weight loss peak on the TG and DTG of Ni-CaO-ATP catalyst, indicating that the addition of calcium would inhibit the carbon deposition and prolong the life of the catalyst [52]. The used catalysts were analyzed by scanning electron microscopy (SEM), and the results are shown in Fig. 12. It can be seen from Fig. 12 that the surface of NieAl2O3 is covered by
Fig. 11 e TG and DTG profiles for all used catalysts. Please cite this article as: Feng P et al., Ni supported on the CaO modified attapulgite as catalysts for hydrogen production from glycerol steam reforming, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.013
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Fig. 12 e SEM image of the used catalyst: (a) NieAl2O3, (b) Ni-ATP, (c) Ni-CaO-ATP. a large amount of carbon filaments, the surface carbon deposition of Ni-ATP is less, and the carbon deposition of NiCaO-ATP is the least. It can be concluded that as a carrier ATP exhibited superior carbon deposition resistance than that o f Al2O3, which is consistent with the results of TG and DTG. And as a modifier, CaO further reduced the formation of carbon on the surface of the catalyst, indicating that CaO has good properties for inhibiting carbon deposition. This is consistent with the documents of Wang et al. [49] and Charisiou N D et al. [50].
Conclusion The catalytic performance of NieAl2O3, Ni-ATP and Ni-CaOATP was compared by glycerol steam reforming reaction (GSR). For all catalysts, as the reaction temperature increased from 500 C to 700 C, the total conversion of glycerol and its catalytic activity for catalytic conversion to gaseous products increased. As a carrier, ATP is superior to Al2O3 because of its unique intermediate structure. When CaO is used as an ATP modifier, which would promote the water gas shift reaction, leading to improve hydrogen yield, hydrogen selectivity and decrease the formation of CO. The stability results show that ATP has better carbon deposition resistance than Al2O3, and the addition of CaO further reduces carbon production. Therefore, a Ni-based catalyst supported on a CaO-modified attapulgite can be used as a candidate material for good glycerin steam reforming to produce H2.
Acknowledgements This project was financially supported by the National Natural Science Foundation of China (No.21376084), Guangxi Key Laboratory of Bio-refinery (GXKLB19-03) and Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development (Y909kn1001). The authors thank Research Center of Analysis and Test of East China University of Science and Technology for the help on the characterization.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2020.01.013.
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Please cite this article as: Feng P et al., Ni supported on the CaO modified attapulgite as catalysts for hydrogen production from glycerol steam reforming, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.013