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CeO2 catalysts
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The evaluation of autothermal methane reforming for hydrogen production over Ni/CeO2 catalysts Soodeh Sepehri a,b, Mehran Rezaei b,c,**, Yuan Wang d, Aryan Younesi e, Hamidreza Arandiyan f,* a
Young Researchers and Elite Club, Kashan Branch, Islamic Azad University, Kashan, Iran Catalyst and Advanced Materials Research Laboratory, Chemical Engineering Department, Faculty of Engineering, University of Kashan, Kashan, Iran c Institute of Nanoscience and Nanotechnology, University of Kashan, Kashan, Iran d School of Chemistry, The University of New South Wales, Sydney, New South Wales 2052, Australia e Chemical Engineering Faculty, Tafresh University, Iran f Laboratory of Advanced Catalysis for Sustainability, School of Chemistry, The University of Sydney, Sydney 2006, Australia b
article info
abstract
Article history:
Nanocrystalline Ni/CeO2 catalysts with various loadings of Ni (10, 15, 20, and 25%) were
Received 8 August 2018
synthesised by a facile solvent deficient precipitation method for methane autothermal
Received in revised form
reforming process. The characterisation techniques such as XRD, BET, TPH, H2-TPR were
30 September 2018
carried out on fresh and spent samples to investigate the catalytic properties of the Ni/
Accepted 2 October 2018
CeO2. On the basis of characterisation results, the 20% Ni/CeO2 performs the best activity
Available online xxx
among the catalysts with different Ni contents. The optimal reaction conditions for autothermal methane reforming has been investigated by evaluating the effect of reaction
Keywords:
parameters including the reactivity temperature, the gas hourly space velocity (GHSV) and
Autothermal reforming
H2O/CH4 (S/C) and O2/CH4 (O/C) molar ratios. The stability of 20 wt% Ni/CeO2 catalyst at
Methane conversion
700 C is examined for 20 h on-stream reaction. It reveals that the methane conversion
Synthesis gas production
starts a graduate decrease trend from the second 10 h, which is found to be because of the
Ni supported catalyst
sintering of Ni nanoparticles by TPH and BET analysis.
Sintering
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Autothermal methane reforming (ATR) is an important process to generate synthesis gas (syngas), a combination gas of CO and H2, through the combination of adiabatic steam
reforming and non-catalytic partial oxidation. Syngas is a valuable chemical raw material for methanol synthesis. Both precious and non-precious metal and metal oxide catalysts have been developed to be efficient catalysts for catalysing ATR reaction. A precious metal catalyst such as Rh supported catalyst has been proved to possess high performance for the
* Corresponding author. ** Corresponding author. Catalyst and Advanced Materials Research Laboratory, Chemical Engineering Department, Faculty of Engineering, University of Kashan, Kashan, Iran. E-mail addresses: [email protected] (M. Rezaei), [email protected] (H. Arandiyan). https://doi.org/10.1016/j.ijhydene.2018.10.016 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Sepehri S, et al., The evaluation of autothermal methane reforming for hydrogen production over Ni/ CeO2 catalysts, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.10.016
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reforming process however it has limited source storage on the earth and high price which prohibits the widely usage in practical applications [1,2]. Ni catalysts, on the other hand, are better fit for industrial applications in large scale production because of the low cost. However, Ni catalysts encounter the activity loss by sintering and coke formation and deposition [3e6]. To solve these problems, proper support is attempted to be selected on the well dispersion of Ni catalysts. Variety kinds of metal oxide catalysts are well studied as support for reforming process catalysts, which can be divided into two groups: reducible oxides such as ZrO2, CeO2, TiO2, Ta2O5, Nb2O5, GdO2 and irreducible oxides such as La2O3, MgO, Al2O3 [7]. Supports can have a significant influence on the catalytic performance of catalysts when a support has been applied to ensure a homogeneous dispersion of active metals. Additionally, the support can improve the catalyst's stability by interacting with the active metal. Although the primitive activity of the supported catalyst has been proposed as independent from their supports [8], and it only depends on the dispersion [9], some researchers have argued that the turnover frequency is affected by the support nature, and catalyst deactivation is related to the support as well [7,10]. It is not easy to make direct association between the results regarding the function of the substrate due to nature of a substance, the strategy of preparation, pretreatment and the experimental environment in different studies vary. To choose a support material, a comparison is made between material cost and activity. Since irreducible oxides have a lower cost, they have been broadly applied in industrial applications. In spite of the economical concerns of reducing oxides, some of them are desirable since the lattice oxygen existed in their structure is capable of reducing carbon accumulation by participating in the reaction. Since precious metals like Rh, Pt, Ru and Pd are not appropriate candidates due to their high price and limited sources, the development of Ni-based catalyst for ATR has been extensively researched [5,11,12]. The reducible substrate materials are specified by involving their lattice oxygen in the catalytic reaction, which subsequently produce localised oxygen on the surface. The localised oxygen accelerates the gasification rate of adsorbed carbon which leads to confine carbon accumulation over the active metals. It is known that during this process the stable oxygen vacancies formed, and feed oxygen or oxygen from steam is likely to fill into these vacancies. Numerous studies have led CeO2 as support for reducing carbon formation by its redox properties and oxygen store/release ability [13,14]. However, this ability is limited to low temperatures around 350e400 C for bare CeO2. Above this temperature, the reducing atmosphere eliminates the CeO2 particles morphology which causes pore blockage and subsequently led to the reduction of surface area [13]. Our previous works were assigned to the studies on Nibased catalysts and its promoted catalysts supported on Al2O3 for ATR which was synthesised via a sol-gel and solventdeficient precipitation (SDP) method [6,11,15,16]. In this work, we investigated the efficient ATR on a Ni supported on CeO2 catalysts synthesised by one-step SDP method. The SDP method is a robust, fast and one-pot synthesise strategy. Moreover, to evaluate the optimal conditions of this process, the effects of the temperature, the gas hourly space velocity
(GHSV) and the molar ratios of H2O/CH4 (S/C) and O2/CH4 (O/C) were investigated on the Ni-based catalysts.
Catalyst preparation and characterizations Catalyst fabrication The Ni/CeO2 catalysts are fabricated by solvent deficient precipitation method. To be specific, the Ce(NO3)3$6H2O (Merck) is utilised as precursor salt to mix with proper amount of Ni(NO3)2$6H2O (Merck) to achieve the desired Ni loadings of 10, 15, 20, 25 wt%. The desired amount of the (NH4)2CO3 was subsequently added to the above mixture. After a uniform precursor was obtained a mortar and pestle was used to grand the mix for 20 min until the pasty-like materials formed which is due to the contain of hydrate water in the precursors. After being dried at ambient temperature for 24 h, the obtained materials have been calcined at 600 C for 2 h. To prevent the catalyst from fast dehumidification and generate a uniform pore structure in the final product, the rate has been set as 2 C/min [17]. The obtained samples were denoted as 10Ni/ CeO2, 15Ni/CeO2, 20Ni/CeO2, 25Ni/CeO2.
Evaluation of catalytic activity Catalytic performance evaluation was conducted in a fixed bed quartz reactor at a continuous flow regime, and ambient pressure toward the ATR from 550 C to 700 C. The prepared samples were treated into the tablet and selected the particle size in the range of 0.25e0.5 mm to perform the reactivity test in order to exclude the pressure drop and mass heat transfer effects. Each sample was initially reduced in a H2flow at 700 C for 3h. The gas mixture was composite of O2, CH4 and H2O with a volume ratio of O2/CH4/H2O ¼ 0.25/1/0.5. A programmable syringe pump (IVAC P1000, UK) was applied to control the flow rate of water while the flow rates were managed by the mass flow controllers (Brooks 5850). The outlet gases from the microreactor were detected online by an on-line gas chromatograph (Varian 3400) equipped with a Carboxen-1000 PLOT column and a thermal conductivity detector (TCD). For the purpose to protect the column, the outlet gas was dehydrated by passing through the cold trap before introducing to GC.
Characterization N2 adsorption-desorption isotherms, pore parameters, and surface areas of the samples have been carried out through N2 adsorption experiments at 196 C on a Tristar 3020 Micromeritics adsorption analyser. The specific surface areas and pore size distributions were estimated by the BrunauerEmmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) strategies. Prior to analysis a VacPrep 061 degas system (Micromeritics) was applied at 250 C for 2 h to purge samples in nitrogen gas. The X-ray diffraction (XRD) experiment was carried out in the PANalytical X'Pert-Pro with a Cu-Ka monochromatized radiation over the range of 2q ¼ 10e80 at 0.06 per step to study the crystalline structure of samples. The reducibility of the samples was discovered using hydrogen
Please cite this article in press as: Sepehri S, et al., The evaluation of autothermal methane reforming for hydrogen production over Ni/ CeO2 catalysts, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.10.016
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temperature programmed reduction (H2-TPR) analysis by a Micromeritics chemisorb 2750. The samples were purged with inert gas (Ar, 20 mL/min) at 200 C for 2 h to clean the water and impurities on the catalyst. Then, the reducing gas (10% H2 in Ar, 20 mL/min) was brought in at a ramp rate of 10 C/min from room temperature to 900 C. Temperature-programmed hydrogenation (TPH) was conducted on the similar temperature program and apparatus as H2-TPR experiments.
Results and discussion Calcined catalysts specifications XRD diffraction peak profile of the Ni/CeO2 with Ni content of 10e25 wt% is displayed in Fig. 1. Comparing with the standard card in the XRD database, it has been confirmed that the asobtained Ni/CeO2 samples have a pure crystalline phase which is consist of NiO and ceria fluorite phase according to the JCPDS card No. 01-075-0076. It shows that the intensity of the diffraction patterns referred to NiO at 2q ¼ 37.4, 43.3, 63.1, 75.6 have increased in respect of that of peaks corresponded to CeO2 at 2q ¼ 28.7, 33.2, 47.7, 56.6, 59.4, 69.7, 77.1 . The Scherrer equation has been used to estimate NiO crystalline sizes, and the resulted values are reported in Table 1. The minimum crystalline size value is found to be 21 nm for 20Ni/ CeO2 sample among the Ni/CeO2 catalysts (20.7e33.7 nm).
Fig. 1 e XRD analysis of the Ni/CeO2 catalysts calcined at 700 C.
The structural properties of catalysts have been obtained using N2 adsorption-desorption analysis for both calcined samples and spent samples after activity test as displayed in Table 1. The Ni/CeO2 samples after calcination possess the surface area, pore volume and pore diameter in the range of 11.6e26.3 m2/g, 0.06e0.09 cm3/g and 11.4e23.2 nm, respectively. It is found that the BET surface area of Ni/CeO2 samples declines with the growing of Ni contents. The N2 adsorption-desorption isotherms and pore size distribution for Ni/CeO2 catalysts have been presented in Fig. 2. Fig. 2a illustrated pore size distribution of Ni/CeO2 that 10Ni/CeO2 has a narrow size distribution centred in 20 nm whereas 15Ni/CeO2, 20Ni/CeO2 and 25Ni/CeO2 samples possess a broad size distribution in the larger range from 30 to 80 nm. This result is consistent with the trend of the variation of BET of the Ni loading samples. Therefore, it can be concluded that loading with Ni on the CeO2 could affect the porosity of CeO2 support by blocking the pores with large particles. The isotherms of Ni/CeO2 exhibit IV type with an H3 hysteresis loop, demonstrating the existance of mesopores and macropores in the structure, which is formed from the collapsing of layered structure or pore damaging during the thermal treatment. The average particle sizes of the samples were estimated by BET results and summarized in Table 1. The 10-25Ni/CeO2 catalysts have particle sizes in the range of 30e70 nm, confirming the nanostructured morphology of the prepared Ni/CeO2. H2-TPR analysis of calcined samples is illustrated in Fig. 3. The CeO2 reduction curve has been presented separately in Fig. 3b in a proper y-axis scale for better recognition of the reduction peaks. It shows that CeO2 support shows two broad peaks which suggest two different reduction stages. The first reduction peak at 520 C is related to the reduction of surface capping oxygen onions connected to the Ce4þ surface ions in octagon coordination [18] and the peak at higher temperature around 875 C is assigned to the reduction of lattice oxygen connected to two Ce4þ in bulk ceria which is driven by the phase evolution from CeO2 to Ce2O3 [19,20]. In the reduction curve of all Ni/CeO2 catalysts, three main peaks can be seen. They are one small peak at temperature range between 196 C and 215 C, a larger domain peak at temperature range between 379 C and 399 C with should peak at range between 447 C and 537 C, and a wide peak with low intensity at 870 C. It has been reported in the previous study that the reduction of NiO is around 250e300 C [21]. In Fig. 3a, the reduction peaks located at 196e215 and 379e399 C are due to the hydrogen consumption by surface NiO species and NiO reduction in the bulk. The shoulder peak around 447e537 C and broad peak at 870 C with low intensity are corresponding to the reduction of
Table 1 e Textural properties of Ni/CeO2 catalysts with different Ni contents. Sample
10Ni/CeO2 15Ni/CeO2 20Ni/CeO2 25Ni/CeO2
Ni content (wt%)
10 15 20 25
Surface areas (m2/g)
Pore volume (cm3/g)
Calcined
Spent
Calcined
Spent
26.3 22.1 13.8 11.6
7.2 6.4 0.5 0.1
0.09 0.08 0.07 0.06
0.06 0.06 0.03 0.03
Pore size (nm) Calcined Spent 11.4 11.8 20.6 23.2
19.7 25.6 47.2 48.1
NiO crystalline Particle size size (nm) (nm) 25.7 23.1 20.7 33.7
30.3 36.3 58.6 70.3
Please cite this article in press as: Sepehri S, et al., The evaluation of autothermal methane reforming for hydrogen production over Ni/ CeO2 catalysts, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.10.016
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Fig. 2 e (a) Pore-size distributions and (b) N2 adsorption-desorption isotherms of Ni/CeO2 calcined samples.
Fig. 3 e H2-TPR profiles of (a) calcined CeO2 and Ni/CeO2 samples and (b) calcined CeO2.
surface CeO2 [22] and CeO2 to Ce2O3 which is a coincidence with the result of CeO2 in Fig. 3b. Along with the growth of Ni contents the surface CeO2 reduction peak move to a higher temperature, suggesting the interaction effect between metal and support.
Catalytic performance in autothermal reforming Temperature function of activity and stability of the Ni/CeO2 samples were examined towards ATR to produce syngas. The results shown in Fig. 4a indicate that the activity increase along with the increase of Ni content from 10% to 20% and the reactivity decrease for the higher loading content of 25% of Ni. The catalysts containing 15% and 20% Ni demonstrated the highest activities among the samples. H2/CO ratio of the products for the catalyst with different Ni loading is used as an indicator for the H2 selectivity as shown in Fig. 4b. It is interesting to show that H2/CO ratio decreases when temperature increases from 550 to 700 C, which is found out to be correlated with the thermodynamic trend of water-gas shift reaction. Moreover, the H2 selectivity is also affected significantly by the Ni content, and 25Ni/CeO2 with the highest Ni loading exhibits the highest H2/CO ratio at a whole range of the
reaction temperatures. The stability of Ni/CeO2 samples has been tested at 700 C for 240 min. All as-obtained samples show excellent stability without activity loss as shown in Fig. 4c. Performance evaluation of Ni/CeO2 sample, the effect of water and O2 content, as well as the effect of GHSV on methane conversion, were investigated as shown in Fig. 5. The result shows that water has moderate effect on the activity of 20Ni/CeO2 since the CH4 conversion grow slowly along with the increase of S/C ratio. Oxygen content, on the other hand, has a significant influence on the reactivity. Fig. 5a shows that the CH4 conversion is enhanced from about 60% for O/C ¼ 0.2 to almost 100% when O/C ¼ 1.0. Recently King et al. [23] reported that methane conversion decrease in the existent of excess steam which is due to the sintering of Ni nanoparticles (NPs). These Ni NPs tend to solve the catalyst support and compose a solid solution during thermal treatment. They can be extracted from the solid solution to form larger NPs than that of the fresh catalyst during the following reduction step in the rich steam environment. After a certain period, observed an agglomeration of these small particles, and the particle size stabilises.
Please cite this article in press as: Sepehri S, et al., The evaluation of autothermal methane reforming for hydrogen production over Ni/ CeO2 catalysts, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.10.016
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Fig. 4 e (a) Methane conversion, (b) H2/CO ratios of Ni/CeO2 catalysts, and (c) stability test at 700 C for 240 min. All the reactions follow the same conditions in CH4/H2O/O2 ¼ 1/ 0.5/0.25 and GHSV ¼ 2.2 £ 104 (mL g¡1 h¡1).
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Fig. 5 e Methane conversion of 20Ni/CeO2 at (a) different S/ C and O/C ratios (GHSV ¼ 2.2 £ 104 mL g¡1 h¡1, T ¼ 650 C), (b) effect of GHSV (CH4/H2O/O2 ¼ 1/0.5/0.25, T ¼ 650 C) and (c) stability test for 20 h (CH4/H2O/O2 ¼ 1/0.5/0.25, T ¼ 700 C).
Please cite this article in press as: Sepehri S, et al., The evaluation of autothermal methane reforming for hydrogen production over Ni/ CeO2 catalysts, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.10.016
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Fig. 5b has shown the effect of GHSV on the CH4 conversion over 20Ni/CeO2. It shows that the larger GHSV results in the poor activity, which can be due to the insufficient contact time between the catalyst and reactants for the larger GHSV. Apart from the stability test of all samples for 240 min, the long-term stability test for 20Ni/CeO2 was performed for 20 h at 700 C as shown in Fig. 5c. The catalytic activity remains unchanged until 10 h on-stream reaction, showing slightly drop of CH4 conversion (about 8% decreases). Typically, the loss of activity of Ni-based catalysts during reforming process is due to one or more of the following reasons: nickel re-oxidation, nickel sintering and carbon formation [23e26]. Nickel re-oxidation cannot be the cause of activity loss because each test was repeated three times in our experiments. To investigate the main reasons for the loss of activity, it is necessary to
Fig. 6 e TPH analysis of spent Ni/CeO2 catalysts.
investigate the carbon deposition on the surface of catalyst. For this purpose, the TPH analysis of the spent catalysts was conducted.
Specifications of spent catalysts The TPH analysis results for the aged catalysts after activity are demonstrated in Fig. 6. There is an apparent sharp peak centred around 200e250 C and a small and broad peak located at 900 C. As described by Mirodatos et al. [27], the peak at a lower temperature (250 C) is related to highly reactive carbon species, which can be converted to CH4 by reacting with hydrogen. A broad peak with low intensity observed in temperature ranges from 400 to 680 C only for 25Ni/CeO2 sample is related to the inactive carbon sitting on the nickel surface, which is the primary cause of catalyst deactivation [27]. This type of carbon is found in the sample with high Ni loading (25%). The peak located at high temperature around 900 C is due to the reduction of the CeO2 support, which is corresponding to the result of the H2-TPR analysis. It can be summarised from the TPH results that the carbon formation is not the domain cause for activity decrease over the 20% Ni catalyst. Moreover, to investigate the catalyst sintering N2 adsorption-desorption test of the spent samples was performed and the results are shown in Fig. 7. Additionally, the BET surface area and pore size information are displayed in Table 1. According to Fig. 7, the pore size distribution and pore size of spent catalysts were larger and wider, respectively. According to Table 1, it is also determined that the BET of the used catalysts have encountered reduction drastically. The isotherms of the spent catalysts exhibit type IV with the H3 hysteresis loop. Regarding the presence of steam and reduced BET surface area of spent catalysts, perhaps the most important reason for reducing activity of 20% Ni catalyst can be nickel sintering.
Fig. 7 e (a) Pore-size distributions and (b) N2 adsorption/desorption isotherms of Ni/CeO2 spent catalysts.
Please cite this article in press as: Sepehri S, et al., The evaluation of autothermal methane reforming for hydrogen production over Ni/ CeO2 catalysts, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.10.016
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Conclusion The Ni loading effect on the catalytic activity in ATR over Ni/ CeO2 catalysts was studied. The Ni/CeO2 catalysts were successfully synthesised through a facile SDP method. XRD analysis approved the formation of nanocrystallinity of the samples. The results show that 20Ni/CeO2 shows the highest CH4 conversion while 25Ni/CeO2 exhibits the highest H2 selectivity among the catalysts with different Ni contents. The stability test for 20 h at 700 C demonstrated the excellent thermal stability of the resulted Ni/CeO2 catalyst. The good performance of the 20Ni/CeO2 catalyst is due to the various factors, i.e. high Ni dispersion, large surface area of CeO2 support and good reducibility. With the confirmation of TPH results, no noticeable carbon deposition formed on the 20Ni/ CeO2 with the highest catalytic performance for ATR reaction.
Acknowledgements The authors gratefully acknowledge the support from University of Kashan for supporting this work by Grant No. 158426/377.
references
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Please cite this article in press as: Sepehri S, et al., The evaluation of autothermal methane reforming for hydrogen production over Ni/ CeO2 catalysts, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.10.016
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ScienceDirect journal homepage: www.elsevier.com/locate/he
The evaluation of autothermal methane reforming for hydrogen production over Ni/CeO2 catalysts Soodeh Sepehri a,b, Mehran Rezaei b,c,**, Yuan Wang d, Aryan Younesi e, Hamidreza Arandiyan f,* a
Young Researchers and Elite Club, Kashan Branch, Islamic Azad University, Kashan, Iran Catalyst and Advanced Materials Research Laboratory, Chemical Engineering Department, Faculty of Engineering, University of Kashan, Kashan, Iran c Institute of Nanoscience and Nanotechnology, University of Kashan, Kashan, Iran d School of Chemistry, The University of New South Wales, Sydney, New South Wales 2052, Australia e Chemical Engineering Faculty, Tafresh University, Iran f Laboratory of Advanced Catalysis for Sustainability, School of Chemistry, The University of Sydney, Sydney 2006, Australia b
article info
abstract
Article history:
Nanocrystalline Ni/CeO2 catalysts with various loadings of Ni (10, 15, 20, and 25%) were
Received 8 August 2018
synthesised by a facile solvent deficient precipitation method for methane autothermal
Received in revised form
reforming process. The characterisation techniques such as XRD, BET, TPH, H2-TPR were
30 September 2018
carried out on fresh and spent samples to investigate the catalytic properties of the Ni/
Accepted 2 October 2018
CeO2. On the basis of characterisation results, the 20% Ni/CeO2 performs the best activity
Available online xxx
among the catalysts with different Ni contents. The optimal reaction conditions for autothermal methane reforming has been investigated by evaluating the effect of reaction
Keywords:
parameters including the reactivity temperature, the gas hourly space velocity (GHSV) and
Autothermal reforming
H2O/CH4 (S/C) and O2/CH4 (O/C) molar ratios. The stability of 20 wt% Ni/CeO2 catalyst at
Methane conversion
700 C is examined for 20 h on-stream reaction. It reveals that the methane conversion
Synthesis gas production
starts a graduate decrease trend from the second 10 h, which is found to be because of the
Ni supported catalyst
sintering of Ni nanoparticles by TPH and BET analysis.
Sintering
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Autothermal methane reforming (ATR) is an important process to generate synthesis gas (syngas), a combination gas of CO and H2, through the combination of adiabatic steam
reforming and non-catalytic partial oxidation. Syngas is a valuable chemical raw material for methanol synthesis. Both precious and non-precious metal and metal oxide catalysts have been developed to be efficient catalysts for catalysing ATR reaction. A precious metal catalyst such as Rh supported catalyst has been proved to possess high performance for the
* Corresponding author. ** Corresponding author. Catalyst and Advanced Materials Research Laboratory, Chemical Engineering Department, Faculty of Engineering, University of Kashan, Kashan, Iran. E-mail addresses: [email protected] (M. Rezaei), [email protected] (H. Arandiyan). https://doi.org/10.1016/j.ijhydene.2018.10.016 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Sepehri S, et al., The evaluation of autothermal methane reforming for hydrogen production over Ni/ CeO2 catalysts, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.10.016
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reforming process however it has limited source storage on the earth and high price which prohibits the widely usage in practical applications [1,2]. Ni catalysts, on the other hand, are better fit for industrial applications in large scale production because of the low cost. However, Ni catalysts encounter the activity loss by sintering and coke formation and deposition [3e6]. To solve these problems, proper support is attempted to be selected on the well dispersion of Ni catalysts. Variety kinds of metal oxide catalysts are well studied as support for reforming process catalysts, which can be divided into two groups: reducible oxides such as ZrO2, CeO2, TiO2, Ta2O5, Nb2O5, GdO2 and irreducible oxides such as La2O3, MgO, Al2O3 [7]. Supports can have a significant influence on the catalytic performance of catalysts when a support has been applied to ensure a homogeneous dispersion of active metals. Additionally, the support can improve the catalyst's stability by interacting with the active metal. Although the primitive activity of the supported catalyst has been proposed as independent from their supports [8], and it only depends on the dispersion [9], some researchers have argued that the turnover frequency is affected by the support nature, and catalyst deactivation is related to the support as well [7,10]. It is not easy to make direct association between the results regarding the function of the substrate due to nature of a substance, the strategy of preparation, pretreatment and the experimental environment in different studies vary. To choose a support material, a comparison is made between material cost and activity. Since irreducible oxides have a lower cost, they have been broadly applied in industrial applications. In spite of the economical concerns of reducing oxides, some of them are desirable since the lattice oxygen existed in their structure is capable of reducing carbon accumulation by participating in the reaction. Since precious metals like Rh, Pt, Ru and Pd are not appropriate candidates due to their high price and limited sources, the development of Ni-based catalyst for ATR has been extensively researched [5,11,12]. The reducible substrate materials are specified by involving their lattice oxygen in the catalytic reaction, which subsequently produce localised oxygen on the surface. The localised oxygen accelerates the gasification rate of adsorbed carbon which leads to confine carbon accumulation over the active metals. It is known that during this process the stable oxygen vacancies formed, and feed oxygen or oxygen from steam is likely to fill into these vacancies. Numerous studies have led CeO2 as support for reducing carbon formation by its redox properties and oxygen store/release ability [13,14]. However, this ability is limited to low temperatures around 350e400 C for bare CeO2. Above this temperature, the reducing atmosphere eliminates the CeO2 particles morphology which causes pore blockage and subsequently led to the reduction of surface area [13]. Our previous works were assigned to the studies on Nibased catalysts and its promoted catalysts supported on Al2O3 for ATR which was synthesised via a sol-gel and solventdeficient precipitation (SDP) method [6,11,15,16]. In this work, we investigated the efficient ATR on a Ni supported on CeO2 catalysts synthesised by one-step SDP method. The SDP method is a robust, fast and one-pot synthesise strategy. Moreover, to evaluate the optimal conditions of this process, the effects of the temperature, the gas hourly space velocity
(GHSV) and the molar ratios of H2O/CH4 (S/C) and O2/CH4 (O/C) were investigated on the Ni-based catalysts.
Catalyst preparation and characterizations Catalyst fabrication The Ni/CeO2 catalysts are fabricated by solvent deficient precipitation method. To be specific, the Ce(NO3)3$6H2O (Merck) is utilised as precursor salt to mix with proper amount of Ni(NO3)2$6H2O (Merck) to achieve the desired Ni loadings of 10, 15, 20, 25 wt%. The desired amount of the (NH4)2CO3 was subsequently added to the above mixture. After a uniform precursor was obtained a mortar and pestle was used to grand the mix for 20 min until the pasty-like materials formed which is due to the contain of hydrate water in the precursors. After being dried at ambient temperature for 24 h, the obtained materials have been calcined at 600 C for 2 h. To prevent the catalyst from fast dehumidification and generate a uniform pore structure in the final product, the rate has been set as 2 C/min [17]. The obtained samples were denoted as 10Ni/ CeO2, 15Ni/CeO2, 20Ni/CeO2, 25Ni/CeO2.
Evaluation of catalytic activity Catalytic performance evaluation was conducted in a fixed bed quartz reactor at a continuous flow regime, and ambient pressure toward the ATR from 550 C to 700 C. The prepared samples were treated into the tablet and selected the particle size in the range of 0.25e0.5 mm to perform the reactivity test in order to exclude the pressure drop and mass heat transfer effects. Each sample was initially reduced in a H2flow at 700 C for 3h. The gas mixture was composite of O2, CH4 and H2O with a volume ratio of O2/CH4/H2O ¼ 0.25/1/0.5. A programmable syringe pump (IVAC P1000, UK) was applied to control the flow rate of water while the flow rates were managed by the mass flow controllers (Brooks 5850). The outlet gases from the microreactor were detected online by an on-line gas chromatograph (Varian 3400) equipped with a Carboxen-1000 PLOT column and a thermal conductivity detector (TCD). For the purpose to protect the column, the outlet gas was dehydrated by passing through the cold trap before introducing to GC.
Characterization N2 adsorption-desorption isotherms, pore parameters, and surface areas of the samples have been carried out through N2 adsorption experiments at 196 C on a Tristar 3020 Micromeritics adsorption analyser. The specific surface areas and pore size distributions were estimated by the BrunauerEmmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) strategies. Prior to analysis a VacPrep 061 degas system (Micromeritics) was applied at 250 C for 2 h to purge samples in nitrogen gas. The X-ray diffraction (XRD) experiment was carried out in the PANalytical X'Pert-Pro with a Cu-Ka monochromatized radiation over the range of 2q ¼ 10e80 at 0.06 per step to study the crystalline structure of samples. The reducibility of the samples was discovered using hydrogen
Please cite this article in press as: Sepehri S, et al., The evaluation of autothermal methane reforming for hydrogen production over Ni/ CeO2 catalysts, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.10.016
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temperature programmed reduction (H2-TPR) analysis by a Micromeritics chemisorb 2750. The samples were purged with inert gas (Ar, 20 mL/min) at 200 C for 2 h to clean the water and impurities on the catalyst. Then, the reducing gas (10% H2 in Ar, 20 mL/min) was brought in at a ramp rate of 10 C/min from room temperature to 900 C. Temperature-programmed hydrogenation (TPH) was conducted on the similar temperature program and apparatus as H2-TPR experiments.
Results and discussion Calcined catalysts specifications XRD diffraction peak profile of the Ni/CeO2 with Ni content of 10e25 wt% is displayed in Fig. 1. Comparing with the standard card in the XRD database, it has been confirmed that the asobtained Ni/CeO2 samples have a pure crystalline phase which is consist of NiO and ceria fluorite phase according to the JCPDS card No. 01-075-0076. It shows that the intensity of the diffraction patterns referred to NiO at 2q ¼ 37.4, 43.3, 63.1, 75.6 have increased in respect of that of peaks corresponded to CeO2 at 2q ¼ 28.7, 33.2, 47.7, 56.6, 59.4, 69.7, 77.1 . The Scherrer equation has been used to estimate NiO crystalline sizes, and the resulted values are reported in Table 1. The minimum crystalline size value is found to be 21 nm for 20Ni/ CeO2 sample among the Ni/CeO2 catalysts (20.7e33.7 nm).
Fig. 1 e XRD analysis of the Ni/CeO2 catalysts calcined at 700 C.
The structural properties of catalysts have been obtained using N2 adsorption-desorption analysis for both calcined samples and spent samples after activity test as displayed in Table 1. The Ni/CeO2 samples after calcination possess the surface area, pore volume and pore diameter in the range of 11.6e26.3 m2/g, 0.06e0.09 cm3/g and 11.4e23.2 nm, respectively. It is found that the BET surface area of Ni/CeO2 samples declines with the growing of Ni contents. The N2 adsorption-desorption isotherms and pore size distribution for Ni/CeO2 catalysts have been presented in Fig. 2. Fig. 2a illustrated pore size distribution of Ni/CeO2 that 10Ni/CeO2 has a narrow size distribution centred in 20 nm whereas 15Ni/CeO2, 20Ni/CeO2 and 25Ni/CeO2 samples possess a broad size distribution in the larger range from 30 to 80 nm. This result is consistent with the trend of the variation of BET of the Ni loading samples. Therefore, it can be concluded that loading with Ni on the CeO2 could affect the porosity of CeO2 support by blocking the pores with large particles. The isotherms of Ni/CeO2 exhibit IV type with an H3 hysteresis loop, demonstrating the existance of mesopores and macropores in the structure, which is formed from the collapsing of layered structure or pore damaging during the thermal treatment. The average particle sizes of the samples were estimated by BET results and summarized in Table 1. The 10-25Ni/CeO2 catalysts have particle sizes in the range of 30e70 nm, confirming the nanostructured morphology of the prepared Ni/CeO2. H2-TPR analysis of calcined samples is illustrated in Fig. 3. The CeO2 reduction curve has been presented separately in Fig. 3b in a proper y-axis scale for better recognition of the reduction peaks. It shows that CeO2 support shows two broad peaks which suggest two different reduction stages. The first reduction peak at 520 C is related to the reduction of surface capping oxygen onions connected to the Ce4þ surface ions in octagon coordination [18] and the peak at higher temperature around 875 C is assigned to the reduction of lattice oxygen connected to two Ce4þ in bulk ceria which is driven by the phase evolution from CeO2 to Ce2O3 [19,20]. In the reduction curve of all Ni/CeO2 catalysts, three main peaks can be seen. They are one small peak at temperature range between 196 C and 215 C, a larger domain peak at temperature range between 379 C and 399 C with should peak at range between 447 C and 537 C, and a wide peak with low intensity at 870 C. It has been reported in the previous study that the reduction of NiO is around 250e300 C [21]. In Fig. 3a, the reduction peaks located at 196e215 and 379e399 C are due to the hydrogen consumption by surface NiO species and NiO reduction in the bulk. The shoulder peak around 447e537 C and broad peak at 870 C with low intensity are corresponding to the reduction of
Table 1 e Textural properties of Ni/CeO2 catalysts with different Ni contents. Sample
10Ni/CeO2 15Ni/CeO2 20Ni/CeO2 25Ni/CeO2
Ni content (wt%)
10 15 20 25
Surface areas (m2/g)
Pore volume (cm3/g)
Calcined
Spent
Calcined
Spent
26.3 22.1 13.8 11.6
7.2 6.4 0.5 0.1
0.09 0.08 0.07 0.06
0.06 0.06 0.03 0.03
Pore size (nm) Calcined Spent 11.4 11.8 20.6 23.2
19.7 25.6 47.2 48.1
NiO crystalline Particle size size (nm) (nm) 25.7 23.1 20.7 33.7
30.3 36.3 58.6 70.3
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Fig. 2 e (a) Pore-size distributions and (b) N2 adsorption-desorption isotherms of Ni/CeO2 calcined samples.
Fig. 3 e H2-TPR profiles of (a) calcined CeO2 and Ni/CeO2 samples and (b) calcined CeO2.
surface CeO2 [22] and CeO2 to Ce2O3 which is a coincidence with the result of CeO2 in Fig. 3b. Along with the growth of Ni contents the surface CeO2 reduction peak move to a higher temperature, suggesting the interaction effect between metal and support.
Catalytic performance in autothermal reforming Temperature function of activity and stability of the Ni/CeO2 samples were examined towards ATR to produce syngas. The results shown in Fig. 4a indicate that the activity increase along with the increase of Ni content from 10% to 20% and the reactivity decrease for the higher loading content of 25% of Ni. The catalysts containing 15% and 20% Ni demonstrated the highest activities among the samples. H2/CO ratio of the products for the catalyst with different Ni loading is used as an indicator for the H2 selectivity as shown in Fig. 4b. It is interesting to show that H2/CO ratio decreases when temperature increases from 550 to 700 C, which is found out to be correlated with the thermodynamic trend of water-gas shift reaction. Moreover, the H2 selectivity is also affected significantly by the Ni content, and 25Ni/CeO2 with the highest Ni loading exhibits the highest H2/CO ratio at a whole range of the
reaction temperatures. The stability of Ni/CeO2 samples has been tested at 700 C for 240 min. All as-obtained samples show excellent stability without activity loss as shown in Fig. 4c. Performance evaluation of Ni/CeO2 sample, the effect of water and O2 content, as well as the effect of GHSV on methane conversion, were investigated as shown in Fig. 5. The result shows that water has moderate effect on the activity of 20Ni/CeO2 since the CH4 conversion grow slowly along with the increase of S/C ratio. Oxygen content, on the other hand, has a significant influence on the reactivity. Fig. 5a shows that the CH4 conversion is enhanced from about 60% for O/C ¼ 0.2 to almost 100% when O/C ¼ 1.0. Recently King et al. [23] reported that methane conversion decrease in the existent of excess steam which is due to the sintering of Ni nanoparticles (NPs). These Ni NPs tend to solve the catalyst support and compose a solid solution during thermal treatment. They can be extracted from the solid solution to form larger NPs than that of the fresh catalyst during the following reduction step in the rich steam environment. After a certain period, observed an agglomeration of these small particles, and the particle size stabilises.
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Fig. 4 e (a) Methane conversion, (b) H2/CO ratios of Ni/CeO2 catalysts, and (c) stability test at 700 C for 240 min. All the reactions follow the same conditions in CH4/H2O/O2 ¼ 1/ 0.5/0.25 and GHSV ¼ 2.2 £ 104 (mL g¡1 h¡1).
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Fig. 5 e Methane conversion of 20Ni/CeO2 at (a) different S/ C and O/C ratios (GHSV ¼ 2.2 £ 104 mL g¡1 h¡1, T ¼ 650 C), (b) effect of GHSV (CH4/H2O/O2 ¼ 1/0.5/0.25, T ¼ 650 C) and (c) stability test for 20 h (CH4/H2O/O2 ¼ 1/0.5/0.25, T ¼ 700 C).
Please cite this article in press as: Sepehri S, et al., The evaluation of autothermal methane reforming for hydrogen production over Ni/ CeO2 catalysts, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.10.016
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Fig. 5b has shown the effect of GHSV on the CH4 conversion over 20Ni/CeO2. It shows that the larger GHSV results in the poor activity, which can be due to the insufficient contact time between the catalyst and reactants for the larger GHSV. Apart from the stability test of all samples for 240 min, the long-term stability test for 20Ni/CeO2 was performed for 20 h at 700 C as shown in Fig. 5c. The catalytic activity remains unchanged until 10 h on-stream reaction, showing slightly drop of CH4 conversion (about 8% decreases). Typically, the loss of activity of Ni-based catalysts during reforming process is due to one or more of the following reasons: nickel re-oxidation, nickel sintering and carbon formation [23e26]. Nickel re-oxidation cannot be the cause of activity loss because each test was repeated three times in our experiments. To investigate the main reasons for the loss of activity, it is necessary to
Fig. 6 e TPH analysis of spent Ni/CeO2 catalysts.
investigate the carbon deposition on the surface of catalyst. For this purpose, the TPH analysis of the spent catalysts was conducted.
Specifications of spent catalysts The TPH analysis results for the aged catalysts after activity are demonstrated in Fig. 6. There is an apparent sharp peak centred around 200e250 C and a small and broad peak located at 900 C. As described by Mirodatos et al. [27], the peak at a lower temperature (250 C) is related to highly reactive carbon species, which can be converted to CH4 by reacting with hydrogen. A broad peak with low intensity observed in temperature ranges from 400 to 680 C only for 25Ni/CeO2 sample is related to the inactive carbon sitting on the nickel surface, which is the primary cause of catalyst deactivation [27]. This type of carbon is found in the sample with high Ni loading (25%). The peak located at high temperature around 900 C is due to the reduction of the CeO2 support, which is corresponding to the result of the H2-TPR analysis. It can be summarised from the TPH results that the carbon formation is not the domain cause for activity decrease over the 20% Ni catalyst. Moreover, to investigate the catalyst sintering N2 adsorption-desorption test of the spent samples was performed and the results are shown in Fig. 7. Additionally, the BET surface area and pore size information are displayed in Table 1. According to Fig. 7, the pore size distribution and pore size of spent catalysts were larger and wider, respectively. According to Table 1, it is also determined that the BET of the used catalysts have encountered reduction drastically. The isotherms of the spent catalysts exhibit type IV with the H3 hysteresis loop. Regarding the presence of steam and reduced BET surface area of spent catalysts, perhaps the most important reason for reducing activity of 20% Ni catalyst can be nickel sintering.
Fig. 7 e (a) Pore-size distributions and (b) N2 adsorption/desorption isotherms of Ni/CeO2 spent catalysts.
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Conclusion The Ni loading effect on the catalytic activity in ATR over Ni/ CeO2 catalysts was studied. The Ni/CeO2 catalysts were successfully synthesised through a facile SDP method. XRD analysis approved the formation of nanocrystallinity of the samples. The results show that 20Ni/CeO2 shows the highest CH4 conversion while 25Ni/CeO2 exhibits the highest H2 selectivity among the catalysts with different Ni contents. The stability test for 20 h at 700 C demonstrated the excellent thermal stability of the resulted Ni/CeO2 catalyst. The good performance of the 20Ni/CeO2 catalyst is due to the various factors, i.e. high Ni dispersion, large surface area of CeO2 support and good reducibility. With the confirmation of TPH results, no noticeable carbon deposition formed on the 20Ni/ CeO2 with the highest catalytic performance for ATR reaction.
Acknowledgements The authors gratefully acknowledge the support from University of Kashan for supporting this work by Grant No. 158426/377.
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Please cite this article in press as: Sepehri S, et al., The evaluation of autothermal methane reforming for hydrogen production over Ni/ CeO2 catalysts, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.10.016