Unsupported cobalt nanoparticles as catalysts: Effect of preparation method on catalytic activity in CO2 methanation and ethanol steam reforming

doi:10.1016/j.ijhydene.2019.08.228

International Journal of Hydrogen Energy

Volume 44, Issue 50, 18 October 2019, Pages 27319-27328

https://doi.org/10.1016/j.ijhydene.2019.08.228 Get rights and content

Highlights

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B-containing samples are very active in ethanol steam reforming at 673–773 K.
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B-containing samples are inactive in CO₂ methanation.
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B-free NPs have high activity in CO₂ methanation but deactivate at 773 K.
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B-free sample in ESR is slightly less selective to H₂, due to higher activity in ethanol cracking to CH₄.
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B impurities tend to poison the ethanol cracking by-reaction improving catalyst activity for ESR.

Abstract

Cobalt nanoparticles (10–50 nm) have been prepared by different procedures. Materials produced by reduction of cobalt chloride and nitrate by NaBH₄ contain B impurities as borates or borides. They are very active in ethanol steam reforming at 673–773 K with up to 85% hydrogen yield at 773 K. B-free samples obtained by thermal decomposition of Co₂(CO)₈ is slightly less selective to hydrogen, due to its activity in ethanol cracking to methane which is probably poisoned by boron impurities on the other catalysts. B-containing samples are inactive in CO₂ methanation and have weak activity in the reverse water gas shift (RWGS) reaction to CO. B-free nanoparticles have high activity in both CO₂ methanation and RWGS. However, methanation activity is reduced fast by growth of encapsulating carbon species. These particles however also show quite stable activity in RWGS to CO, attributed to CoO impurities.

Graphical abstract

Introduction

Supported Cobalt catalysts, in particular Co/Al₂O₃ and/or Co/ZrO₂–SiO₂, are used industrially for the low temperature Fischer Trøpsch process producing Diesel fuel and waxes from syngases [1], [2], where methane is the main product on molar basis. Some studies report on a remarkable activity of cobalt catalysts also for methanation of CO and CO₂ [3] while others [4] ranked low cobalt catalyst for these reactions for low activity, selectivity and stability. Cobalt catalysts are also of interest for Ethanol Steam Reforming (ESR) [5], [6], a potentially useful process for producing renewable hydrogen from bioethanol [7], [8]. We previously reported that also unsupported cobalt nanoparticles [9], [10], or nanoparticles mechanically supported on a low surface area support as α-Al₂O₃ [11], have interesting activity in ESR. This may indicate that the role of support in catalytic activity is not relevant or even negligible for catalytic activity in this reaction [12], being probably mostly associated to improving catalyst stability.

To go deeper in our studies, we prepared unsupported nanoparticles starting from different precursors and with different procedures, and we tested them in both CO₂ methanation and ESR. The aim was to reveal the role of preparation method on activity and short-term stability in these two reactions also in relation to the role of precursor on the behavior of cobalt catalysts.

Section snippets

Samples preparation

The Co nanoparticles were prepared via reduction method in aqueous solution, starting from CoCl₂·6H₂O or Co(NO₃)₂·6H₂O, as precursor salts and using NaBH₄ as reducing agent. The reaction was carried out using a 1.5*10⁻² molL⁻¹ final concentration of the corresponding aqueous solution, and a proper ratio Me^2+:BH₄⁻, depending on the precursor salt; it is equal to 1:4 for CoCl₂·6H₂O and 1:10 for Co(NO₃)₂·6H₂O. The solution is maintained under vigorous mechanical stirring for 15 min. The appearance

Synthesis and characterization of cobalt nanoparticles

In Table 1, the experimental conditions adopted to synthetize the Co-based catalysts are summarized.

The synthetic procedure of nanoparticles strongly affects their morphological and compositional properties and has a strong effect on their catalytic activity. In this case, the effect of using different cobalt precursor salts for NaBH₄ route allows to investigate the role of different anions since the already deep knowledge on the effect of preparation parameters [10]. Thus, sample A has been

Discussion

The paper reports on the successful preparation of cobalt nanoparticles using four different procedures. For the first time it has been shown that the route implying reduction of Co²⁺ with NaBH₄ can be applied not only to cobalt chloride but also to cobalt nitrate, although in this case a higher B/Co ratio has been used. The as cast materials are all amorphous with particle sizes in the range 10–30 nm, but crystallize producing cubic and hexagonal metallic cobalt by annealing at 773 K and also

Conclusions

The main conclusions from our work are the following:

1.
The adopted procedures allow to produce amorphous cobalt nanoparticles (10–30 nm size) which crystallize as a mixture of hexagonal and cubic cobalt during reaction or upon annealing. Samples arising from the borohydride reducing route, applied to both cobalt chloride and nitrate, contain borate and boride impurities, while all may contain some cobalt oxide (Co₃O₄ or CoO).
2.
The B-containing sample are inactive in CO₂ methanation, and have low

Acknowledgements

The collaboration of Elena Reghitto during the preparation of her master theses is gratefully acknowledged.

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Unsupported cobalt nanoparticles as catalysts: Effect of preparation method on catalytic activity in CO2 methanation and ethanol steam reforming

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Samples preparation

Synthesis and characterization of cobalt nanoparticles

Discussion

Conclusions

Acknowledgements

Catal Today

Energy Procedia

Int J Hydrogen Energy

Fuel

Int J Hydrogen Energy

Appl Catal Gen

J Mol Catal A Chem

Appl Catal Gen

Thermochim Acta

Mater Chem

Chem Rev

Top Catal

ChemSusChem

Unsupported cobalt nanoparticles as catalysts: Effect of preparation method on catalytic activity in CO₂ methanation and ethanol steam reforming