Supported Ni Catalysts from Ni-Mg-Al Hydrotalcite-like Compounds：Preparation and Catalytic Performance for Ammonia Decomposition

doi:10.15541/jim20240140

Abstract

Abstract:

Ammonia decomposition is a promising approach for on-site hydrogen generation for fuel cells, and the development of a cost-effective and efficient catalyst is highly desired. In this study, a series of Ni_xMg_75-_xAl₂₅ hydrotalcite-like compounds (HTlc) with different Ni contents were synthesized by co-precipitation, followed by calcination and reduction treatments. Influences of Ni content and ammonia reduction on the catalytic performance for ammonia decomposition were investigated. The characterization results of the as-prepared samples showed that HTlc was decomposed into Mg(Ni, Al)O solid solution by calcination, which displayed a strong interaction between Ni species and support, while upon reduction with ammonia at 750 ℃, well-dispersed Ni metal nanoparticles with an average crystallite size range of 5.9-7.7 nm were formed. No nitrogen oxides (NO_x) were produced during the NH₃ reduction process as indicated by mass spectrometry analysis, and the catalyst reduced with ammonia showed comparable activity with that reduced with hydrogen, suggesting that ammonia can be used as a reductant gas. The catalyst activity increased with the increase of Ni content and reduction temperature. Among the catalysts, the Ni₂₀Mg₅₅Al₂₅ catalyst reduced with ammonia at 750 ℃ showed the best activity, which afforded 98% ammonia conversion at 600 ℃ at a space velocity of 30000 mL·g_cat^-1·h^-1, and no evident deactivation was observed during a 100 h test, demonstrating good activity, stability, and sintering resistance.

Key words: hydrogen production, ammonia decomposition, nickel catalyst, hydrotalcite-like compound

CLC Number:

O643

LIAN Minli, SU Jiaxin, HUANG Hongyang, JI Yuyin, DENG Haifan, ZHANG Tong, CHEN Chongqi, LI Dalin. Supported Ni Catalysts from Ni-Mg-Al Hydrotalcite-like Compounds：Preparation and Catalytic Performance for Ammonia Decomposition[J]. Journal of Inorganic Materials, 2025, 40(1): 53-60.

Figures/Tables 10

Fig. 1 (A) XRD patterns and (B) SEM images of the as-synthesized NixMg75-xAl25 HTlc precursors (a) Ni5Mg70Al25-HTlc; (b) Ni10Mg65Al25-HTlc; (c) Ni15Mg60Al25-HTlc; (d) Ni20Mg55Al25-HTlc; (e) Ni25Mg50Al25-HTlc

Table 1 Metal contents and lattice parameters of the NixMg75-xAl25 HTlc precursors

Sample	Metal content/(%, in atom)			Lattice parameter/nm
Sample	Ni	Mg	Al	a	c
Ni₅Mg₇₀Al₂₅-HTlc	5.5	68.1	26.4	0.306	2.344
Ni₁₀Mg₆₅Al₂₅-HTlc	10.4	63.1	26.5	0.305	2.344
Ni₁₅Mg₆₀Al₂₅-HTlc	15.9	58.4	25.7	0.306	2.344
Ni₂₀Mg₅₅Al₂₅-HTlc	20.5	53.5	26.0	0.305	2.344
Ni₂₅Mg₅₀Al₂₅-HTlc	25.6	48.9	25.5	0.305	2.347

Fig. 2 (A) XRD patterns and (B) H2-TPR profiles of calcined NixMg75-xAl25 (a) Ni5Mg70Al25; (b) Ni10Mg65Al25; (c) Ni15Mg60Al25; (d) Ni20Mg55Al25; (e) Ni25Mg50Al25

Table 2 Textural properties of the calcined samples and crystallite size of Ni metal in the reduced catalysts

Sample	Surface area/ (m²·g_cat^-1)	Pore volume/ (cm³·g_cat^-1)		Ni crystal size/ nm
Ni₅Mg₇₀Al₂₅	197.3	0.45	7.5	7.7
Ni₁₀Mg₆₅Al₂₅	173.0	0.31	5.6	7.6
Ni₁₅Mg₆₀Al₂₅	217.4	0.36	5.6	5.9
Ni₂₀Mg₅₅Al₂₅	218.6	0.38	5.5	5.9
Ni₂₅Mg₅₀Al₂₅	204.4	0.43	7.0	6.7

Fig. 3 Analysis of NH3-TPR mass spectrum for Ni20Mg55Al25 Colorful figure is available on website

Fig. 4 XRD patterns of NixMg75-xAl25 catalysts reduced at 750 ℃ with NH3 (a) Ni5Mg70Al25; (b) Ni10Mg65Al25; (c) Ni15Mg60Al25; (d) Ni20Mg55Al25; (e) Ni25Mg50Al25

Fig. 5 (a) HAADF-STEM image, (b-f) EDX elemental mappings, (g) TEM image, (h) HRTEM image, and (i) Ni particle size distribution for the Ni20Mg55Al25 catalyst reduced at 750 ℃ with NH3

Fig. 6 Catalytic ammonia decomposition over the NixMg75-xAl25 catalysts Influence of (A) Ni contents, (B) reduction temperature, and (C) reduction atmosphere

Fig. 7 (A) Catalytic activities at different space velocities and (B) long-term stability of Ni20Mg55Al25 (pre-reduced with NH3 at 750 ℃), and (C) XRD patterns of the fresh and used Ni20Mg55Al25

Table 3 Catalytic performance of Ni catalysts for ammonia decomposition

Catalyst	Feed gas	Space velocity/ (mL·g_cat^-1·h^-1)	Temperature/ ℃	NH₃ conversion/ %	NH₃ reaction rate/ (mmol· g_cat^-1·min^-1)	Ref.
Ni₂₀Mg₅₅Al₂₅	Pure NH₃	30000	550	68.5	15.3	This work
NiCe_0.85Zr_0.15O	Pure NH₃	30000	550	63	14.1	[33]
Ni/cCeO₂(IMP)	Pure NH₃	30000	550	51	11.4	[34]
Ni_MgAl(6:1)	Pure NH₃	30000	550	48	10.7	[26]
Ni/MCM-41(TIE)	Pure NH₃	30000	550	47.6	10.6	[31]
Ni/BaZrO₃	Pure NH₃	20000	550	62	9.2	[29]
Ni_1.2Ce_0.1Al	Pure NH₃	45000	550	26	8.7	[32]
Ni/La₂O₃	Pure NH₃	6000	550	78.9	3.5	[30]

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