Hydrotalcites derived catalysts for syngas production from biogas reforming: Effect of nickel and cerium load

doi:10.1016/j.cattod.2016.08.022

Catalysis Today

Volume 289, 1 July 2017, Pages 78-88

https://doi.org/10.1016/j.cattod.2016.08.022 Get rights and content

Highlights

•
NiMgAlCe hydrotalcites catalysts are suitable to produce syngas to FT synthesis.
•
Carbon produced in Ni10%/MgAlCe was less stable than deposits over Ni10%/MgAl.
•
XANES suggest that nickel was more dispersed in Ni10%/MgAlCe than in Ni10%/MgAl.

Abstract

Mixed oxide catalysts derived from hydrotalcites were applied to model biogas reforming (CH₄:CO₄ = 1.5:1). Ni_x/MgAl and Ni_x/MgAlCe (x = 10% or 25% w/w nominal nickel content) were prepared by the coprecipitation method. Both Ni10%/MgAl and Ni10%/MgAlCe catalysts showed potential for syngas production due to their stability. CO₂-TPD measurements and XANES results helped understand the Ni10%/MgAl and Ni10%/MgAlCe catalysts performance be similar. XANES spectra suggested that the majority of Ni⁺² would be present inside the solid solution in Ni10%/MgAlCe, making the activation process more difficult, which probably compromised the conversions enhancement in relation to Ni10%/MgAl. The H₂/CO ratio lower than unity makes the obtained syngas suitable for the Fischer-Tropsch process. The carbon deposits on Ni10%/MgAlCe catalyst were less stable, which means that it could be gasified more easily.

Graphical abstract

Introduction

The biogas produced through anaerobic digestion of organic wastes from landfills and vinasse, with an average volumetric composition of 55% methane (CH₄) and over 30% carbon dioxide (CO₂), both greenhouse gases, could be used as raw material to obtain syngas (H₂ + CO) from methane dry reforming [1], represented by Eq. (1).CH₄ + CO₂ → 2H₂ + 2CO ΔH⁰_298K = +247 kJ/mol

In GTL plants (Fischer-Tropsch Synthesis), syngas is converted into liquid fuels, an alternative to gasoline and diesel [2].

The main problem with the reforming reaction is the undesired coke formation by CH₄ decomposition, by the Boudoard reaction and by the reaction between H₂ and CO (Eqs. (2), (3) and (4), respectively) [3]:CH₄ → C + 2H₂ ΔH⁰_298K = +74.9 kJ/mol2CO ↔ C + CO₂ ΔH⁰_298K = −171.0 kJ/molH₂ + CO ↔ C + H₂O ΔH⁰_298K = −175.3 kJ/mol

Coke formation is intensified by nickel-based catalysts, the most used in industrial plants. The literature reports several studies of nickel based catalysts applied to methane and biogas reforming, whose precursors are hydrotalcite mixed oxides [4], [5] with cerium as a promoter. These studies have been seeking improvement in catalyst basicity and better stability during the CH₄ reforming reaction [6].

The aim of this work is to apply hydrotalcite derived catalysts − composed of nickel, magnesium, aluminum and cerium − in biogas reforming to produce syngas and investigate the effect of a cerium addition in two catalysts with distinct nickel contents on reducing carbon deposits.

Section snippets

Precursors syntheses

Hydrotalcites were synthesized by coprecipitation: a nitrate solution (1 mol.L⁻¹) containing Ni⁺², Mg⁺² and Al⁺³ (M⁺²/M⁺³ = 3) was added to an alkaline solution (1 molL⁻¹; OH^‐1/CO^‐2₃ = 11.5), heated to 60 °C. The initial molar ratio CO^‐2₃/Al⁺³ was 0.8. To obtain 10% and 25% (w/w) of nickel content in the mixed oxide, Ni⁺²/Mg⁺² molar ratios of 0.12 (Al⁺³/Mg⁺² = 0.37) and 0.39 (Al⁺³/Mg⁺² = 0.46) were used, respectively.

5% w/w of cerium was added to hydrotalcites by coprecipitation, using a Al/Ce molar

XRD

The X-ray patterns (Fig. 1a) show sharp and symmetric peaks at 2θ° = 11°, 23°, 61° and 62° which confirms the hydrotalcite phase formation [7]. According to Daza and coworkers [6], the presence of additional peaks related to ceria phase (CeO₂) indicates that Ce⁺³ was not included in hydrotalcite layers. Ce⁺³ ions are quickly oxidized to Ce⁺⁴.

Fig. 1b shows the X-ray diffractograms of calcined samples. The hydrotalcite characteristic peaks disappeared due to the collapse of the lamellar structure

Conclusions

The Ni10%/MgAl and Ni10%/MgAlCe catalysts were active and stable in biogas reforming to produce syngas. CH₄ and CO₂ conversions were practically the same for both catalysts, probably due to the Ni10%/MgAlCe total basicity not being substantially enhanced with Ce promotion. Also, from XANES results, it was inferred that the non-promoted catalyst was reduced to a larger extent than Ce-promoted and the majority of Ni⁺² would be present inside the solid solution in Ni10%/MgAlCe, making the

Acknowledgments

The authors would like to thank ANP-PRH44/MCTI/FINEP and CAPES for the studentship and would like to acknowledge the sponsorship of FAPESP and BG Brasil through the Research Centre for Gas Innovation, FAPESP Grant Proc. 2014/50279-4, and to the Brazilian National Synchrotron Light Laboratory (LNLS) for the XANES experiments.

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Hydrotalcites derived catalysts for syngas production from biogas reforming: Effect of nickel and cerium load

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Precursors syntheses

XRD

Conclusions

Acknowledgments

Fuel Process. Technol.

Appl. Surf. Sci.

Biomass Bioenergy

Int. J. Hydrogen Energy

Appl. Catal. A: Gen.

J. Catal.

Appll Catall A:Gen.

Int. J. Hydrogen Energy

Catal. Today

Int. J. Hydrogen Energy

Int. J. Hydrogen Energy

Appl. Clay Sci.

Appl. Catal. A: Gen.

Catal. Today

Fuel Process. Technol.

Catal. Today

Solid States Ionic

Catal. Today

Chem. Eng. J.

Appl. Catal. B: Environ.

Appl. Catal. A: Gen.

J. Catal.

Chem. Eng. J.