Effect of La promotion on Ni/Mg-Al hydrotalcite derived catalysts for glycerol steam reforming

https://doi.org/10.1016/j.jece.2020.104228Get rights and content

Highlights

  • La promoted Ni/Mg-Al hydrotalcite derived catalysts for glycerol steam reforming.

  • La influences reducibility and dispersion of Ni species.

  • La increases medium and strong basic site contribution.

  • Ni/Mg6Al1.6La0.4 catalyst showed superior activity.

  • La contributes to coke gasification.

Abstract

The influence of lanthanum promotion on the physico-chemical properties and catalytic activity of Ni/Mg-Al catalysts in the glycerol steam reforming (GSR) reaction was studied. An Mg-Al and different Mg-Al-La supports with various molar ratios were synthesized using a co-precipitation technique. The calcined supports were then impregnated with 5 wt% Ni. After calcination at 600 °C, physico-chemical properties of the Ni-based catalysts were investigated by XRD, BET, H2-TPR, and CO2-TPD techniques. H2-TPR analyses showed that the quantity of lanthanum in the support composition influenced the reducibility of Ni species as well as their dispersion. CO2-TPD analyses showed that lanthanum addition improved catalyst basicity. The activity of all catalysts in the GSR reaction was tested (T = 400−700 °C, water/glycerol feed ratio 9:1, flow rate of 0.025 mL/min). The composition of the Ni/MAL0.4 (Ni/Mg6Al1.6La0.4; subscripted numbers indicate the molar ratio of each metal) catalyst provided a good metal-support interaction and Ni dispersion which translated to an overall superior performance in terms of hydrogen yields, glycerol conversion to gaseous products, and total glycerol conversions. TG-DSC analyses of the spent catalysts showed that the lanthanum quantity also influenced the quantity of coke deposition. The formation of encapsulating coke over all catalysts was linked to high concentrations of dehydration products observed in liquid product analysis. A stability test conducted at 600 °C for Ni/MAL0.4 revealed its deactivation due to encapsulating coke formation which resulted in active metal site blockage. With a reduced flow rate (0.008 mL/min), a different coke formation mechanism allowed the catalyst to last for 24 h.

Introduction

With growing concerns regarding global warming and the depletion of existing fossil fuel reserves, the search for renewable energy alternatives has become a priority. The use of biodiesel as a substitute for conventional petroleum diesel is particularly promising given its non-toxic nature and biodegradability [1]. Biodiesel can be generated from the transesterification of vegetable oils with an alcohol [2]. Glycerol, also known as glycerin, is obtained as a by-product (∼10 wt% of produced biodiesel) of this transesterification reaction [3,4]. Commercially, purified glycerol is used in the food, oral care, cosmetics and tobacco industries [5]. Alternatively, the catalytic valorization of glycerol to hydrogen through the steam reforming process is a very attractive approach [6]. Hydrogen is a clean energy vector that can be used for the efficient generation of electricity via fuel cells [7]. The overall glycerol steam reforming (GSR) reaction can be depicted as follows:C3H8O3 + 3H2O ↔ 7H2 + 3CO2 ΔH298K = +128 kJ/mol

The GSR reaction (Eq. (1)) is in fact a resulting combination of the glycerol pyrolysis (Eq. (2)) and water gas shift (WGS) (Eq. (3)) reactions:C3H8O3 ↔ 4H2 + 3CO ΔH298K = +251 kJ/molCO + H2O ↔ CO2 + H2 ΔH298K = −41 kJ/mol

Various catalytic systems have been investigated in the GSR reaction. Noble metal based catalysts (Ru, Pt, Pd…) are known for their stability and high selectivity to Csingle bondC bond cleavage [8]. Nevertheless, compared to the limited availability of noble metals, transition metal based catalysts (Ni, Co, Cu…) are more attractive for industrial application due to their high catalytic activity and lower cost [9]. Nickel based systems in particular have received considerable attention given the capability of Ni for cleaving Csingle bondC bonds and for promoting the water gas shift reaction (WGS) which increases hydrogen production [10,11]. However, it has been found that nickel based catalysts tend to favor coke deposition [12]. Several studies have shown that increasing the basicity of the utilized support of the Ni-based catalyst can help inhibit carbon formation [[13], [14], [15]]. Dieuzeide et al. [16] studied the effect of Mg addition on the physico-chemical properties and activity of Ni/Al2O3 catalysts in GSR. The lower carbon formation observed over catalysts with a high Mg content was attributed to their higher basic character which favored carbon gasification.

Hydrotalcites, also known as layered double hydroxides, have the general formula: [M2+1-xM3+x(OH)2]x+(An−)x/n.yH2O where M2+ and M3+ stand for divalent and trivalent metal ions, and An- represents anions. Hydrotalcite compositions can be easily modified by the substitution of metal ions in the brucite layers with different M2+ and M3+ cations [17]. The thermal treatment of hydrotalcites results in the formation of metal oxides with high surface areas and basic properties, thus making them attractive for several catalytic applications. Mg-Al hydrotalcites and their obtained calcined mixed oxides (Mg(Al)O) are well established support materials in steam reforming reactions [18]. Moreover, the basic properties of Mg-Al hydrotalcites can be tuned by changing the nature of M2+ and M3+ ions [19]. The use of acidic supports for GSR is generally not preferred as acidic sites tend to dehydrate glycerol yielding undesired coke precursors. Accordingly, Mg-Al hydrotalcite derived supports were chosen for this particular study since their acido-basic properties can be tuned.

Promoters are added to reforming catalysts to improve active metal dispersion, carbon removal, and to minimize sintering [20,21]. It is already well established that the addition of lanthanum in particular as a promoter can modify the redox properties of a catalyst [22,23]. Lucredio et al. [24] showed that the addition of La to Ni-Mg-Al hydrotalcite derived catalysts increased hydrogen production in the steam reforming of ethanol. Iriondo et al. [25] studied the effect of La2O3 addition on the properties and activity of a Ni/Al2O3 catalyst in GSR. It was found that the acidic nature of alumina was reduced thus improving the catalyst ability to transform oxygenated hydrocarbons into gaseous products.

Several published papers focus on the use of La promoted Ni-Mg-Al hydrotalcite derived catalysts for reforming reactions [[26], [27], [28]]. However, in these studies the active phase (Ni) is incorporated during the co-precipitation step whereas in this work, the Ni is added via impregnation. This work aims to evaluate the influence of lanthanum addition on the physico-chemical properties and catalytic activity of Ni/Mg-Al catalysts in the GSR reaction.

Section snippets

Catalyst preparation

An Mg-Al and different Mg-Al-La hydrotalcite supports with a M2+/M3+ molar ratio of 3 were prepared by co-precipitation at pH = 9.5–10 and at 60 °C. An aqueous solution containing appropriate dissolved quantities of Mg(NO3)2.6H2O, Al(NO3)3.9H2O and La(NO3)3.6H2O was precipitated using a basic solution of NaOH (2 M) and Na2CO3 (1 M) under continuous stirring. The precipitate was aged for 1 h at 60 °C with stirring and dried at 60 °C for 18 h. This was followed by a filtration step and continuous

Characterization of the catalysts

X-ray diffractograms of the uncalcined supports [Fig. 1(a)] show the typical hydrotalcite structure (*, JCPDS 22−0700) for all the prepared supports. The introduction of La to the support composition had significant influences on its structure. Additional smaller peaks observed for MAL0.2 HT, MAL0.4 HT and MAL0.8 HT are assigned to the LaCO3OH (●, JCPDS 49−0981), La2(CO3)2(OH)2 (●, JCPDS 70–1774) and LaAl(OH)2(CO3)2 (●, JCPDS 52–1059) phases formed during co-precipitation as a result of the

Conclusion

An Mg-Al and different Mg-Al-La supports were prepared and impregnated with 5 wt% Ni followed by thermal treatment at 600 °C. H2-TPR profiles showed different interactions between the metal and support depending on the lanthanum content in the support composition. Stronger metal-support interactions were observed for small La quantities whereas weaker interactions were observed for larger La quantities. The differing metal-support interactions were linked to an influence on active metal

CRediT authorship contribution statement

Eliane Dahdah: Investigation, Writing - original draft, Writing - review & editing. Jane Estephane: Methodology, Supervision, Writing - review & editing. Cedric Gennequin: Supervision, Validation, Data curation. Antoine Aboukaïs: Conceptualization, Project administration. Samer Aouad: Project administration, Resources, Visualization, Supervision, Writing - review & editing. Edmond Abi-Aad: Supervision, Conceptualization, Resources, Funding acquisition.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors acknowledge the financial support given to this work by the Agence Universitaire de la Francophonie (AUF) Région du Moyen-Orient, Lebanon; the Lebanese CNRS; the University of Balamand (UOB), Lebanon; and the Université du Littoral Côte d'Opale (ULCO), France. This work was also supported by the ARCUS E2D2 project, the French Ministry of Foreign Affairs and the Région Nord-Pas de Calais, France.

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