Ni supported on CaO-MgO-Al2O3 as a highly selective and stable catalyst for H2 production via the glycerol steam reforming reaction

doi:10.1016/j.ijhydene.2018.02.165

International Journal of Hydrogen Energy

Volume 44, Issue 1, 1 January 2019, Pages 256-273

https://doi.org/10.1016/j.ijhydene.2018.02.165 Get rights and content

Highlights

•
Ni on CaO-MgO-Al₂O₃ highly selective and stable for H₂ production via GSR reaction.
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Presence of CaO and MgO leads to significantly increased amount of Ni⁰ active phase.
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Higher conversion to gaseous products for Ni/modAl by favoring H₂ and CO₂ production.
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No liquid products produced by Ni/modAl over 550 °C, 100 °C lower than Ni/Al.
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Less amount and more defective structures of deposited coke for the modified sample.

Abstract

A comparative study of the GSR performance for Ni/CaO-MgO-Al₂O₃ and Ni/Al₂O₃ catalysts is reported. Catalysts were synthesized applying the wet impregnation method at a constant metal loading (8 wt %). Synthesized samples were characterized by N₂ adsorption/desorption, ICP, BET, XRD, NH₃-TPD, CO₂-TPD, H₂-TPR, XPS, TEM, STEM-HAADF and EDS. The carbon deposited on their surface under reaction conditions was characterized by TPO, Raman and TEM. It was proven that the use of CaO-MgO as alumina modifiers leads to smaller nickel species crystallite size, increased basicity and surface amount of Ni⁰ phase. Thus, it increases the conversion to gaseous products favoring H₂ and CO₂ production to the detriment of CO formation, by enhancing the water gas-shift (WGS) reaction. No liquid products were produced by the Ni/modAl catalyst over 550 °C, whereas time on stream results confirmed that deactivation can be prevented, as apart from decreasing the amount of coke deposition the nature of carbon was altered towards less graphitic and more defective structures.

Introduction

Biodiesel is currently being produced through the transesterification of vegetable oils and/or fats with alcohols [1], [2] and is thought of as a renewable, biodegradable, environmentally-friendly fuel [3]. The main by product of the process is glycerol, which accounts for 10% of the volume of oil undergoing the reaction [4], [5]. Although glycerol is a raw material with numerous uses in a multitude of industries, the glycerol produced by the transesterification process is ‘crude’, as it contains a number of impurities such as salts, methanol and fatty acid methyl esters. Its refining into high purity glycerol demands neutralization, stripping, filtration/centrifugation and vacuum distillation, which make the process highly costly [6], [7]. Thus, innovative options for the utilization of glycerol are needed that help minimize the environmental effects of biodiesel production and aid the industry to become more competitive by reducing its costs and/or add to its revenue streams.

Glycerol could also be used for the production of hydrogen, a fuel that is carbon-free and possesses the highest energy content compared to any known fuel [8], [9], [10], [11], via catalytic reactions, such as steam reforming (SR) [12], [13], [14], [15], [16], [17], aqueous phase reforming (APR) [18], [19] and auto-thermal reforming (ATR) [20], [21]. Glycerol steam reforming (GSR) presents two main advantages. Firstly, as deduced from the overall reaction (Eq. (1)), every mole of glycerol fed to the reactor can theoretically produce 7 mol of hydrogen. Secondly, SR is a mature industrial technology, which means that the shift from the commonly used feedstocks to glycerol will require relatively few adjustments [22], [23]. Note that the overall reaction is a combination of glycerol decomposition (Eq. (2)) and the water-gas shift reaction (Eq. (3)), but the process is complicated by a number of accompanying reactions that depend on the operating conditions [24], [25], [26].C₃H₈O₃ + 3H₂O → 3CO₂ + 7H₂ ΔH^o = 123 kJ/molC₃H₈O₃ → 3CO + 4H₂ ΔH^o = 245 kJ/molCO + H₂O ↔ CO₂ + H₂ ΔH^o = −41 kJ/mol

The thermodynamic studies that have been undertaken for the GSR show that hydrogen production is favored at atmospheric pressure, high temperatures and high water to glycerol feed ratios (WGFR > 9:1, molar) [27], [28], [29]. However, the reaction network followed during the decomposition of glycerol is quite complex as it involves a number of parallel and consecutive steps of reaction intermediates such as dehydrogenation, dehydration, polymerization and isomerization [30], [31]. Moreover, the catalyst that will be used should promote the cleavage of C-C, O-H, and C-H bonds in the glycerol mole, while the cleavage of the C-O bonds needs to be avoided as it leads to the production of alkanes, which in turn lead to carbon deposition. Additionally, the catalyst should also allow the WGS reaction to take place, so that absorbed CO from the surface can be removed as CO₂ [32], [33].

As in other reforming reactions, a number of researches have tested noble metal based catalysts, such as Platinum (Pt), Ruthenium (Ru), Rhodium (Rh), Iridium (Ir) and Palladium (Pd), and usually find them active and not easily susceptible to carbon formation [e.g. [34], [35], [36]]. The drawback is that their high cost makes their use on an industrial scale restrictive, which means that most research works found in the literature concern the development of catalysts that are based on transition metals. Although there are some works in the GSR that report on the performance of Cobalt (Co) and Copper (Cu) based catalysts [37], [38], [39], the majority focuses on Nickel (Ni) based systems as these are known to be active and cost effective [40], [41]. The drawback is that Ni catalysts are also known to deactivate due to carbon formation and metal particle sintering [42], [43].

As catalytic performance is influenced by the nature of the support [44], [45], [46], Ni based systems have been tested on different oxides, such as Al₂O₃ [48], [49], [50], [51], [52], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63], SiO₂ [49], [61], [64], [65], [66], [67], ZrO₂ [49], [52], [61], [63], [68], [69], CeO₂ [53], [60], [63], [70], La₂O₃ [61], [63], [67], MgO [60], [61], [63], [64], [70], [71] and TiO₂ [69], [70]. Clearly, alumina has been the most widely used catalyst support in the GSR. This is due to its high specific area (which improves metal dispersion) and its mechanical and chemical resistance under reaction conditions [72], [73]. However, as alumina is also known to induce deposition of carbon, transition (ZrO₂ [48], [52]), lanthanide (La₂O₃ [50], [74], [75], CeO₂ [53], [62], [74]), alkali (K₂O [58]) and alkaline earth (SrO [58], MgO [71], [76], [77], [78], [79]) metals have been used as additives in alumina supports, in an effort to induce support-mediated promotional effects on the catalytic system.

Dieuzeite et al. [71], [76], [77], [78] have published a series of works where they investigate the effect that the addition of MgO has on Ni/Al catalysts for the GSR and suggested that its addition can help improve Ni dispersion, as well as minimize carbon formation by enhancing the adsorption of H₂O, O₂, CO₂ and –OH fragments (thus facilitate carbon gasification). Wang et al. [79] have also studied the performance of a Ni-Mg-Al catalyst and reported high H₂ selectivity and high conversion of glycerol; however, they also observed significant carbon deposition at low temperatures.

The work presented herein forms part of a wide effort devoted to the design of appropriate catalysts that may be used for the steam reforming of glycerol. In some of our previous works, we studied the performance of different transition metals (Ni, Co, Cu) on alumina [37], of Ni catalysts based on alumina, zirconia, silica [49] and apatite-type lanthanum silicate [17] supports, the influence of the synthesis method on Ni/Al catalysts [42] and the effect of the addition of lanthana on Ni/Al catalysts [50]. In this study, we report on the catalytic performance of a Ni catalyst supported on CaO-MgO-Al₂O₃ and compare it with the performance of a Ni catalyst supported on pure alumina. An exhaustive literature search revealed that CaO and MgO have never been used together as alumina modifiers for the GSR reaction. The catalysts were synthesized applying the wet impregnation method at a constant metal loading (8 wt %). The synthesized samples, at their calcined and/or their reduced form, were characterized by N₂ adsorption/desorption, ICP, BET, XRD, NH₃-TPD, CO₂-TPD, H₂-TPR, XPS, TEM, STEM-HAADF and EDS. The carbon deposited on their surface under reaction conditions was characterized by TPO, Raman and TEM. Catalytic performance aimed at investigating the effect of the reaction temperature on: (i) Glycerol total conversion, (ii) Glycerol conversion to gaseous products, (iii) Hydrogen selectivity and yield, (iv) Selectivity of gaseous products, and (v) Selectivity of liquid products. The stability of the catalysts was also tested. Quantitative results are provided when reporting on the liquid products.

Section snippets

Catalyst preparation

The alumina support was purchased from Akzo and the modified-alumina was provided by Saint Gobain NorPro. Information on the properties of the untreated catalyst carriers is being presented in Table 1. Both supports had almost equal specific surface areas (SSA), but the modified alumina had a larger median pore diameter, and a larger pore volume. As the supports were in pellet form they were first crashed and then sieved to 350–500 μm. Calcination was undertaken at 800 °C for 4 h. The catalysts

Physicochemical, structural and textural properties of catalytic samples

The physicochemical properties of the calcined and reduced catalytic samples can be found in Table 2. For both samples, the drop of the specific surface area from its initial values (≈280 m²g⁻¹, Table 1) to those after the calcination of the support and the introduction of the Ni species (158 m²g⁻¹ for both calcined samples) and the values obtained after the activation procedure (136 and 123 m²g⁻¹ for the Ni/Al and Ni/modAl reduced samples, respectively) was substantial, but the catalysts did

Conclusions

A comparative investigation of the catalytic performance for H₂ production through the glycerol steam reforming reaction (GSR) of nickel catalysts supported on CaO-MgO-Al₂O₃ (Ni/modAl) and Al₂O₃ (Ni/Al) was performed. It was proven that use of CaO-MgO as alumina modifiers led to lower nickel species crystallite size (higher dispersion), increased catalyst's basicity and induced a redistribution of the acid sites in terms of strength and density, whereas a significantly increased amount of Ni⁰

Acknowledgements

Financial support by the program THALIS implemented within the framework of Education and Lifelong Learning Operational Programme, co-financed by the Hellenic Ministry of Education, Lifelong Learning and Religious Affairs and the European Social Fund, Project Title: ‘Production of Energy Carriers from Biomass by Products. Glycerol Reforming for the Production of Hydrogen, Hydrocarbons and Superior Alcohols’ is gratefully acknowledged by LAFEC's members. VS acknowledges financial support from

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