Nickel-based catalysts for hydrogen production by

Nickel-based catalysts for hydrogen production by steam reforming of glycerol

O. Parlar Karakoc, M. E. Kibar, A. N. Akin & M. Yildiz

International Journal of Environmental Science and Technology ISSN 1735-1472 Int. J. Environ. Sci. Technol. DOI 10.1007/s13762-018-1875-8

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Author's personal copy International Journal of Environmental Science and Technology https://doi.org/10.1007/s13762-018-1875-8

ORIGINAL PAPER

Nickel‑based catalysts for hydrogen production by steam reforming of glycerol O. Parlar Karakoc1 · M. E. Kibar1 · A. N. Akin1 · M. Yildiz1 Received: 2 January 2018 / Revised: 1 June 2018 / Accepted: 13 June 2018 © Islamic Azad University (IAU) 2018

Abstract As the renewable energy technologies are making progress every day, hydrogen energy technologies are taking huge part of this development. Nowadays, there are several industries using hydrogen. Also, the increase at biodiesel production amounts has increased the glycerol production as by-product. One of the main objectives of this study is to compare the effects Al2O3-, SiO2- and CeO2-supported nickel catalysts at steam reforming of glycerol in order to produce hydrogen. All catalysts were prepared by incipient to wetness method. The obtained steam reforming results showed that nickel catalysts supported on ceria showed the highest activity. Also the effect of nickel amount on C eO2 support material has been studied. The highest hydrogen amounts were obtained with 15 wt% nickel loading. Also the increase at water/glycerol ratio increased the hydrogen production amount. The highest hydrogen yield was obtained as 4.82 mol per glycerol mole, whereas the theoretical hydrogen yield is 7 mol, by using 15 wt% Ni/CeO2 catalyst, water/glycerol ratio of 15 and at 650 °C reaction temperature. Keywords Hydrogen · Glycerol · Steam reforming · Catalysts

Introduction The decline of fossil oil reserves and having serious environmental problems have led researchers to study on clean and sustainable resources (Zamsuri et al. 2017; Demsash and Mohan 2016). Nowadays, hydrogen has become a very important and promising energy carrier. Because it is a very important alternative to fossil fuels and is an environmentally friend fuel. Gasification, electrolysis and biological methods provide hydrogen production, but these methods have disadvantages due to excessive chemical consumption, non-renewable sources or the presence of C O2 in the final product (Celik and Yildiz 2017). Hydrogen is commercially obtained with natural gas, but unfortunately this production method cannot be a recipe to the reduction in greenhouse gases (Wu et al. 2013; Carrero et al. 2017; Dang et al. 2017). In the literature, there are several studies about hydrogen production by using biomass-based compounds. Ethylene Editorial responsibility: Iskender Akkurt. * M. Yildiz [email protected] 1

Department of Chemical Engineering, Kocaeli University, 41380 Kocaeli, Turkey

glycol, ethanol, methanol and glycerol are some of these compounds (Kim et al. 2012; Davda et al. 2005; Tuza et al. 2013). Having higher hydrogen content, non-toxicity, storage and transport properties present primacy of glycerol compared to the other compounds (Papageridis et al. 2016). The most important main product of the biodiesel production process is glycerol, and since it is formed in large quantities, hydrogen production from glycerol can reduce the disadvantages of biodiesel production. (Tamosiunas et al. 2017). Steam reforming is the most preferred method for the conversion of glycerol to hydrogen according to its high reaction efficiency [Silva]. The reaction of glycerol with water vapor consists of glycerol pyrolysis (Eq. 1) and the water–gas-shift reaction (Eq. 2). The overall reaction can be described by Eq. 3 as x value can differ from 0 to 3. In this overall reaction, it is important to note that: when x is equal to 0, the glycerol pyrolysis reaction occurs, whereas when x is equal to 3, the hydrogen formation reaction takes place. Each mole of glycerol consumed can provide theoretically 7 mol of hydrogen, but in the literature, less hydrogen was produced than in the theoretical quantities (Lin 2013; Koc and Avci 2017). The reaction stoichiometry tells us that carbon monoxide produced by pyrolysis is completely converted by water–gasshift (WGS) reaction. But depending on the performance

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International Journal of Environmental Science and Technology

of the WGS reaction, CO may be present in the product mixture. Also, several side reactions occur beside gas steam reforming: methanation and dry reforming of methane, and also carbon formation can be observed because of the several side reactions (Koc and Avci 2017).

C3 H8 O3 → 3CO + 4H2 CO + H2 O → CO2 + H2

0

ΔH = 251 kJ mol

−1

ΔH0 = −41 kJ mol−1

C3 H8 O3 + xH2 O → (3 − x) CO + xCO2 + (4 + x) H2

(1) (2)

(3) ΔH0 = 251 − 41 ∗ x kJ mol−1 In recent years, several papers have been published that aimed to investigate and to compare the activities of the catalysts as Pt, Ir, Ni, Co, Ru and Rh. (Carrero et al. 2017; Dang et al. 2017; Kim et al. 2012; Tuza et al. 2013; Papageridis et al. 2016; Koc and Avci 2017; Suffredini et al. 2017; Vaidya 2009; Silva et al. 2015; Subramanian et al. 2016). It has become an important research area to design redox catalysts that can highly be reducible and with high oxygen mobility properties. Among these catalysts, it is more economical to develop nickel-based non-noble metal catalysts because of their high performance and their high resistance to carbon deposition (Jun et al. 2007). Nickel can easily break C–C, O–H and C–H bonds and catalyze the water–gas-shift reaction (Adhikari et al. 2009). Surface oxidation and sintering of the Ni species are the main drawbacks of the mentioned catalysts by comparing with noble ones. Nickel/ɣ-alumina catalyst has been extensively used for the production of hydrogen steam reforming. However, at high temperatures (> 700 °C), thermal degradation of the support takes place and α-alumina phase forms. This transformation causes a decrease in catalytic stability of this catalyst (Suffredini et al. 2017). CeO2-supported nickel catalyst has been used for mentioned reaction. CeO2 support has increased the activity of the catalyst because of its distinctive oxygen storage capacity (Silva et al. 2015). Pant et al. (Pant et al. 2011) also observed that the A l2O3 support caused higher sintering than the CeO2-supported Ni-based catalysts. ZrO2 support has gained nickel-based catalyst higher surface area and higher stability at lower temperatures (Nichele et al. 2012). The study which was performed in Kocaeli University— Catalyst Research and Development Laboratory (2017)— aimed to investigate the performance of C eO2-, Al2O3- and SiO2-supported nickel catalysts that are approximated to have high glycerol conversion and hydrogen yield for steam reforming. For the purpose of achieving high glycerol conversion and hydrogen selectivity, the types of the catalyst support, the amount of nickel and water/glycerol ratios at the reaction medium were studied. All catalysts prepared have been characterized by Brunauer–Emmett–Teller method (BET) and X-ray diffraction (XRD).

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Materials and methods Catalyst preparation The nickel–ceria catalysts were prepared by using incipient to wetness method at 5, 10, 15 and 20% of nickel loading by weight to determine the loading effect. Beside the ceriasupported nickel catalysts, nickel–alumina and nickel–silica catalysts having 15 wt% nickel were prepared by incipient to wetness impregnation method using aqueous solutions of nickel nitrate (Ni(NO3)2.6H2O, Merck). However, cerium (ΙΙΙ) nitrate hexahydrate (Ce(NO3)3. 6 H2O, Merck) was used to produce CeO2 support. Catalyst support material, CeO2, was obtained by calcination of cerium (III) nitrate hexahydrate at 650 °C for 4 h. Commercial ɣ-Al2O3 and SiO2 powders were obtained from Sigma-Aldrich. The metal salt solution, prepared in the appropriate amount, was added dropwise onto the support material with the aid of the peristaltic pump under vacuum. After impregnation, the catalysts were dried for 24 h. At 100 °C and calcined at 500 °C for 5 h, the catalysts were crushed and sieved to less than 45 mesh size.

Catalyst characterization BET method was used to determine the surface areas of the calcined catalysts. Analysis was carried out on ASAP 2020 Micromeritics analyzer. It was determined by multipoint technique and adsorption with nitrogen from nitrogen–helium mixtures at liquid nitrogen temperature. X-Ray diffraction (XRD) analysis were carried out on Rigaku, Miniflex 2 (Japan) analyser to examine crystal structure of materials. 2θ values were scanned from 10° to 80° with a step size of 0.01°/min using CuKα radiation (λ = 0.15418 nm) at 45kV/40mA.

Catalytic experiments The reactions were performed in a fixed bed reactor capable of reaching high temperatures as illustrated in Fig. 1. One gram of catalysts was placed in the middle of the reactor and was reduced at 700 °C for an hour before the reactions. The reactor was made of stainless steel with 1.27 cm outer diameter, 0.21 cm wall thickness and 1-m length. The aqueous solution of glycerol–water was mixed in a separate container at preselected molar ratios prior to the experiments. A high-pressure liquid chromatography (HPLC) pump was used to introduce the mixture into the tubular reactor with 1 mL/dk volumetric flow rate. Pure Argon was used as a carrier gas and the reactor was heated to 650 °C. The reactions were carried out at atmospheric pressure for 4 h. A cold trap, consisting of an ice bath, was used to cool the liquid products. The product gases were analyzed using gas chromatography (GC6890-Agilent Technologies) equipped with

Author's personal copy International Journal of Environmental Science and Technology

Fig. 1 Experimental setup

molecular sieve-5A capillary column and thermal conductivity detector (TCD). A high-pressure liquid chromatography (HPLC) was used to determine the glycerol amount at the liquid product. A refractive index detector (RID) was equipped with HPLC for glycerol determination. A 5 mmol L−1 H2SO4 aqueous solution was a mobile phase with a 0.6 mL min−1 flow rate. The glycerol conversion and H2 yield were calculated according to the following definitions (Eqs. 4, 5): Hydrogen yield =

H2 produced experimentally H2 calculated according to theoretical quantity

(4)

Glycerol conversion (%) =

mole of glycerol in feed − mole of glycerol in product mole of glycerol in feed × 100

(5)

Results and discussion Catalyst characterization The XRD diffraction patterns of calcined 15 wt% Ni/Al2O3, Ni/SiO2, Ni/CeO2 and the Ni/CeO2 catalyst after the reaction are presented in Fig. 2. The characteristic peaks for NiO were determined at 37.4, 43.5 and 62.8 (COD # 1010095) for 2θ values. The peaks at 2θ values 26.2 and 43.4 are attributed to carbon formation (COD # 1101021) after the reaction. The crystallite sizes of the oxide catalysts (NiO) were calculated according to Scherrer equation with the full width half maximum (fwhm) values are 0.38, 0.51 and 0.61 for Ni/CeO2, Ni/SiO2 and Ni/ Al2O3, respectively, and were given in Table 1. Table 1 also illustrates the textural properties of the 15 wt% Ni-loaded catalysts.

Fig. 2 XRD patterns of the 15 wt% Ni-loaded catalyst

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As seen in Table 1, total surface area of C eO2-supported catalyst has increased after reaction. This is probably due to carbon formation. In Fig. 3, pore size distribution of Ni/ CeO2 catalysts is given. Although Ni/CeO 2 catalysts were mesoporous (2 nm 5 wt%) is due to agglomeration of nickel, which inhibits the catalyst activity. Zamsuri et al. (2017) have confirmed these results for ˃7.5 nickel loading values. In this study, we observed that the glycerol conversion due to agglomeration of nickel was stable for 10, 15 and 20 wt% of nickel. However, when

Fig. 3 Pore size distribution of Ni/CeO2 catalysts a calcined catalyst, b after reaction

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Author's personal copy International Journal of Environmental Science and Technology Fig. 4 Effect of support type with 15 wt% Ni metal loading on glycerol conversion (650 °C reaction temp., water/glycerol ratio = 6, catalyst amount = 1 g, 4 h reaction time)

Fig. 5 Effect of support type with 15 wt% Ni metal loading on hydrogen yield (650 °C reaction temp., water/glycerol ratio = 6, catalyst amount = 1 g, 4 h reaction time)

Fig. 6 Effect of Ni amount on glycerol conversion (650 °C reaction temp., water/glycerol ratio = 6, catalyst amount = 1 g, 4 h reaction time)

Fig. 7 is examined, it is seen that the hydrogen yield for the catalyst containing 5 and 10 wt% Ni is too much lower compared with 15 and 20 wt% Ni/CeO2 catalysts. This can be interpreted as the fact that the converted glycerol is not in favor of hydrogen, but in favor of undesired substances or by-products as previously mentioned (Koc and Avci 2017).

In the literature, various nickel percentages were used for steam reforming reactions. It is known that for the catalytic reactions, the reaction conversion increases by increasing the percentage of nickel in the catalyst. But more than 10–15% by weight of the nickel metal content causes deactivation of the catalyst (Wu et al. 2013; Tuza et al. 2013). In this

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Fig. 7 Effect of Ni amount on hydrogen yield (650 °C reaction temp., water/glycerol ratio = 6, catalyst amount = 1 g, 4 h reaction time)

Fig. 8 Effect of water/glycerol ratio with 15 wt% Ni catalyst on glycerol conversion (650 °C reaction temp., catalyst amount = 1 g, 4 h reaction time)

study, 20 wt% nickel loading caused a decrease at hydrogen production likely because of deactivation of the catalyst. Therefore, 15 wt% nickel–ceria catalyst was selected for the rest of the study.

Effect water/glycerol ratio The effect of liquid medium (water/glycerol ratio) on the glycerol conversion and hydrogen yield by using 15 wt% Ni/CeO2 catalyst for steam reforming reaction at 650 °C and feed flow rate 1 ml/min has been presented in Figs. 8 and 9. The highest glycerol conversion was observed at water/ glycerol ratio 6 as 87.7% as shown at Fig. 7. Then, conversion values of 82.1, 85.8 and 83.3% were obtained for water/ glycerol ratio of 9, 12 and 15, respectively. The hydrogen yield has increased with water/glycerol increasing. The production of H 2 increased with increasing steam/ glycerol ratio due to decreasing of the glycerol in feed. When the excess water was utilized for the glycerol reforming it favors the water–gas-shift reaction as given in literature (Mitran et al. 2016). Also in the literature it is given that the maximum hydrogen production amount is 7 for stoichiometric ratio. In this

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study, we obtained maximum H 2 production amount as 4.82 mol per mole of glycerol which means 69% hydrogen selectivity that it is a promising result compared with the literature. As the reactions were carried out at water/glycerol ratios of 6, 9, 12 and 15, the highest hydrogen gain was obtained when this ratio was 12 and 15.

Conclusion This study aimed to investigate the performance of C eO2, Al2O3 and S iO2 for steam reforming of glycerol reactions. For the purpose of achieving high glycerol conversion and hydrogen selectivity, the types of the catalyst support, the amount of nickel and water/glycerol ratios at the reaction medium were studied. In this study, we have investigated the effect of Ni-based catalysts for the steam reforming of glycerol to form hydrogen. It was concluded that catalyst activities were greatly dependent on the type of support material, nickel amount and water/gas ratio of the reaction medium. Using nickelbased catalysts, the most favorable support is found as C eO2 with 15 wt% nickel loading. 15 wt% Ni/CeO2 catalyst caused

Author's personal copy International Journal of Environmental Science and Technology Fig. 9 Effect of water/glycerol ratio with 15 wt% Ni catalyst on hydrogen yield (650 °C reaction temp., catalyst amount = 1 g, 4 h reaction time)

higher hydrogen production than S iO2- and A l2O3-supported catalysts. Also, amount of nickel was investigated. The maximum glycerol conversion was obtained with 5 wt% Ni/CeO2, whereas the highest hydrogen amount was achieved with 15 wt% Ni/CeO2. And also, water/glycerol ratio at the reaction medium was investigated and it was deduced that hydrogen production amount was increased by increasing water/ glycerol ratio. The highest hydrogen yield was obtained as 4.82 mol per mole of glycerol by using 15 wt% Ni/CeO2 catalyst and water/glycerol ratio of 15 at 650 °C reaction temperature. Acknowledgements Financial support is provided by Kocaeli University through Project 2013/10.

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