Towards an efficient CoMo/γ-Al2O3 catalyst using metal amine metallate as an active phase precursor: Enhanced hydrogen production by ammonia decomposition

doi:10.1016/j.ijhydene.2014.06.081

International Journal of Hydrogen Energy

Volume 39, Issue 24, 13 August 2014, Pages 12490-12498

https://doi.org/10.1016/j.ijhydene.2014.06.081 Get rights and content

Highlights

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Co–Mo bimetallic catalyst shows the synergistic effect for ammonia decomposition.
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Metal amine metallate is a good active phase precursor of bimetallic catalyst.
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Co–Mo nanoparticles are highly dispersed on γ-Al₂O₃ support.
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The catalyst with monocomponent Co–Mo precursor shows higher activity and stability.

Abstract

Two kinds of Co-Mo bimetallic catalysts (i.e., CoMo-I/γ-Al₂O₃ and CoMo-II/γ-Al₂O₃) prepared by using different active phase precursors as well as Co and Mo monometallic catalysts were used to catalyze ammonia decomposition. The Co-Mo bimetallic catalysts show higher activity than the Co and Mo monometallic catalysts, indicating the synergistic effect between Co and Mo. More interestingly, the CoMo-I/γ-Al₂O₃ catalyst using monocomponent metal amine metallate (i.e., Co(en)₃MoO₄) as the active phase precursor exhibits higher activity and stability than the CoMo-II/γ-Al₂O₃ using bicomponent Co(NO₃)₂ and (NH₄)₆Mo₇O₂₄ as the active phase precursor, which could be linked to the higher content of active species Co₃Mo₃N for the CoMo-I/γ-Al₂O₃ catalyst.

Graphical abstract

Introduction

The efficient and low-cost catalysts for ammonia decomposition have long been actively pursued in energy and environmental industries [1], [2], [3]. Previous studies have shown that the binding energy of N on transition metal surfaces is a good descriptor to search for highly efficient catalysts for ammonia decomposition [4], [5]. There exists a volcano-type relationship between the N binding energy and the catalytic activity [4], [5], [6]. Although Ru shows the highest activity for catalyzing this process among single metals because of the optimal N binding energy, its expense and scarcity are prohibitive to the large-scale commercialization. Therefore, developing efficient and low-cost alternative catalysts are highly desirable.

In view of the low-cost and abundant resources, non-noble metal-based catalysts in unsupported as well as supported forms are more attractive for ammonia decomposition despite showing lower activity than noble Ru catalysts [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. Specifically, for the non-noble metal catalysts, adding other metals to manipulate the electronic properties and thus tune the N binding energy is found to be an effective way to further enhance the activity [5], [10], [11], [12], [16]. For example, Simonsen et al. changed Ni-to-Fe ratio to tune the catalytic activity, and found that for the gas atmosphere of 1000 ppm NH₃ balanced by Ar and H₂ (vol:vol = 1:1) mixture, the catalytic activity of Ni–Fe/Al₂O₃ catalysts with 20 wt% Ni in the active phase is comparable to or even better than that of Ru catalyst [11]. Notably, when catalyzing decomposition of pure ammonia, Co–Mo bimetallic catalyst with Co/Mo atomic ratio of ca. 7/3 showed superior activity compared with the reported Fe–Co, Ni–Mo, Co–Mo and Ni–Pt bimetallic catalysts [16]. Taking into account these bimetallic catalysts usually prepared by using bicomponent as the active phase precursor, employing monocomponent as the active phase precursor is thus expected to show a better synergistic effect.

In this work, we performed a comparative study of two kinds of Co–Mo bimetallic catalysts (i.e., CoMo-I/γ-Al₂O₃ and CoMo-II/γ-Al₂O₃) as well as Co and Mo monometallic catalysts to catalyze ammonia decomposition, in which the CoMo-I/γ-Al₂O₃ catalyst was prepared by using monocomponent metal amine metallate Co(en)₃MoO₄ (en = ethylenediamine) as the active phase precursor, while the CoMo-II/γ-Al₂O₃ catalyst was prepared by using bicomponent Co(NO₃)₂ and (NH₄)₆Mo₇O₂₄ as the active phase precursor. Activity and stability of both bimetallic and monometallic catalysts were tested. The Co–Mo bimetallic catalysts were found to be higher than the monometallic Co and Mo catalysts, indicating the synergistic effect between Co and Mo. The fresh and/or used catalysts were characterized by Multisizer, ICP-AES, N₂ physisorption, H₂-TPR, XRD, TEM, Raman and XPS. Finally, the relationship between the type of active phase precursor of Co–Mo bimetallic catalysts and the activity and stability was correlated.

Section snippets

Catalyst preparation

Metal amine metallate Co(en)₃MoO₄ was prepared according to the method described earlier [23]. A mixture of (NH₄)₆Mo₇O₂₄∙4H₂O, H₂O and ethylenediamine with molar ratio of 0.007:2.06:0.15 was stirred for 30 min in N₂ atmosphere in an ice bath. Subsequently, CoCl₂∙6H₂O mixed with methanol with molar ratio of (0.05:2.85) was gradually added into the above mixture at a rate of 10 mL min⁻¹ using a pump. After precipitation, the mixture was filtered in a N₂ atmosphere box. The filter cake was fully

XRD and TGA of Co(en)₃MoO₄

Fig. 1a shows the photograph and XRD pattern of the as-obtained Co(en)₃MoO₄ sample. The color peach and XRD pattern of the sample are observed to be in good agreement with the previous results [25], indicating that the sample is the form of metal amine metallate Co(en)₃MoO₄. In order to understand the thermal stability of the sample, the TGA analysis of Co(en)₃MoO₄ sample was carried out, and the results are shown in Fig. 1b. It can be seen that the TGA curve shows a continuous weight loss of

Conclusions

Monocomponent metal amine metallate Co(en)₃MoO₄ has been successfully synthesized. When it was employed as the active phase precursor to prepare Co–Mo bimetallic catalyst, the resulting CoMo-I/γ-Al₂O₃ catalyst shows higher activity and stability for ammonia decomposition than the CoMo-II/γ-Al₂O₃ catalyst prepared using Co(NO₃)₂ and (NH₄)₆Mo₇O₂₄ as the active phase precursor. This exceptional performance is a consequence of the monocomponent Co(en)₃MoO₄ as the active phase precursor (e.g., the

Acknowledgments

This work is financially supported by the Natural Science Foundation of China (21306046 and 21374062), the Shanghai Natural Science Foundation (12ZR1407300), the China Postdoctoral Science Foundation (2012M520041 and 2013T60428), and the Fundamental Research Funds for the Central Universities (WA1214020 and WG1213011).

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Towards an efficient CoMo/γ-Al2O3 catalyst using metal amine metallate as an active phase precursor: Enhanced hydrogen production by ammonia decomposition

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Catalyst preparation

XRD and TGA of Co(en)3MoO4

Conclusions

Acknowledgments

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Appl Catal A Gen

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Mater Chem Phys

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Appl Surf Sci

Surf Coat Technol

J Catal

Appl Catal A Gen

Appl Catal A Gen

Towards an efficient CoMo/γ-Al₂O₃ catalyst using metal amine metallate as an active phase precursor: Enhanced hydrogen production by ammonia decomposition

XRD and TGA of Co(en)₃MoO₄