Dopant-induced particle size control governs carbon yield in methane pyrolysis

doi:10.1016/S1872-2067(26)65277-1

Abstract

Abstract:

Methane pyrolysis is a promising route for low-emission H₂ production with valuable nanostructured carbon as coproduct. Tuning catalyst performance through the addition of dopants has transformed many catalytic reactions, yet their role in methane pyrolysis remains largely unexplored. In this study, the role of three alkali oxide dopants (MgO, Na₂O and K₂O) is investigated in Ni-catalysed methane pyrolysis. The dopants greatly affected the lifetime and carbon yield of Ni/C in a thermogravimetric reactor: Mg-doping resulted in a threefold increase in carbon yield and more than double the catalyst lifetime, while K-doping decreased the carbon yield by around 50% at a concentration of 5 at%. This unexpectedly large effect of the dopants was elucidated with high-resolution electron microscopy. We revealed that the dopants remained well-dispersed on the support during catalysis and that they control particle growth at the start of the reaction there. Mg- and Na-doping enhanced Ni particle growth from 5‒6 nm to a more optimal size of 10‒12 nm, whereas K-doping stabilizes smaller Ni particles (7 nm). The final carbon yield was found to be critically dependent on the active Ni particle size, suggesting that the particle growth mediated by the dopants defines for a large part the overall performance of the catalyst in methane pyrolysis.

Key words: Methane pyrolysis, Heterogeneous catalysis, Nickel, Alkali, Particle size, Carbon nanofibers

Daniël P. C. van Eck, Suzan E. Schoemaker, Stefan Bismeijer, Erik Betz-Güttner, Tom A. J. Welling, Petra E. de Jongh. Dopant-induced particle size control governs carbon yield in methane pyrolysis[J]. Chinese Journal of Catalysis, 2027, 92: 397-409.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(26)65277-1

https://www.cjcatal.com/EN/Y2027/V92/I1/397

Figures/Tables 8

Table 1 Metal contents and structural properties of the reduced-passivated catalysts.

Dopant	Catalyst	Ni content^a (wt%)	Dopant content^a (wt%)	Dopant/Ni^a (at%)	BET surface area^b (m²/g)	Total pore volume^b (cm³/g)	d_n ± σ_n^c (nm)	d_s ± σ_s^c (nm)	Ni crystallite size^d (nm)
	Ni/C	14.1	—	—	302	0.49	6.5 ± 1.8	7.6 ± 2.1	4.0
Na	4Na-Ni/C	14.0	0.29	5.3	301	0.48	—	—	—
	5Na-Ni/C	13.9	0.37	6.8	299	0.49	5.4 ± 1.2	5.9 ± 1.2	4.8
	10Na-Ni/C	13.8	0.63	12	290	0.52	6.7 ± 1.7	7.6 ± 1.9	—
	20Na-Ni/C	13.7	1.2	22	245	0.49	—	—	—
Mg	4Mg-Ni/C	14.0	0.22	3.8	344	0.60	—	—	—
	5Mg-Ni/C	13.9	0.32	5.5	315	0.49	5.6 ± 1.4	6.3 ± 1.4	4.6
	10Mg-Ni/C	13.6	0.61	11	284	0.45	6.5 ± 1.7	7.4 ± 2.0	—
	20Mg-Ni/C	13.3	1.2	21	290	0.45	—	—	—
K	1K-Ni/C	13.8	0.13	1.4	—	—	—	—	—
	2K-Ni/C	13.6	0.23	2.4	—	—	—	—	—
	4K-Ni/C	13.6	0.27	3.0	292	0.48	—	—	—
	5K-Ni/C	13.7	0.45	5.0	295	0.48	5.2 ± 1.7	6.4 ± 2.4	4.4

Fig. 1. TEM images and corresponding particle size distributions of the reduced-passivated Ni/C and 5X-Ni/C (X = Na, K, Mg) catalysts.

Fig. 2. Influence of 5 at% doping of Ni/C on carbon yield and lifetime in methane pyrolysis using TGA. (a) Nickel weight-normalized carbon evolution of a Ni/C catalyst and 5 at% doped X-Ni/C (X = Mg, Na, K) catalysts. (b) Corresponding growth rate curves. Reaction conditions: 500 °C in 34% CH4/ Ar (50 mL/min).

Table 2 Key parameters from catalytic evaluation of undoped and 5 at% doped X-Ni/C (X = Mg, Na, K) catalysts. The deactivation order is assumed to be 0.5. The parameter rd reflects the rate of deactivation but is dependent on temperature and partial pressures of H2 and CH4. It is reported here using the standard conditions: 1 mg catalyst, 34% CH4/Ar (50 mL/ min), T = 500 °C.

Catalyst	Yield (g_c/g_Ni)	Lifetime (min)	Deactivation rate constant (r_d)
5Mg-Ni/C	13.1 ± 1.0	33.6	0.053 ± 1.9E-5
5Na-Ni/C	7.83 ± 0.46	18.8	0.094 ± 1.4E-4
Ni/C	4.84 ± 1.26	14.0	0.13 ± 5.7E-4
5K-Ni/C	2.35 ± 0.55	8.7	0.29 ± 4.1E-3

Fig. 3. Effect of dopant concentration on the total carbon yield of Ni/C catalysts. Reaction conditions: 500 °C in 34% CH4/Ar (50 mL/min). Carbon yield measured after deactivation to <1% of initial growth rate.

Fig. 4. Distribution of metals in the spent Ni/C catalysts doped with Mg (a‒c), K (d‒f) and Na (g‒i) species. For each sample the elemental map with Ni and the dopant, and the HAADF image are shown. EDX spectra from active NP and support aggregates were extracted from the highlighted areas in the HAADF images. The EDX spectra are normalized to the Ni-Lα signal.

Fig. 5. Particle size analysis on a spent catalyst. (a) Representative TEM image of a spent 5Mg-Ni/C catalyst showing inactive Ni particles and grown carbon nanofibers. (b) Corresponding size distribution, distinguishing inactive particles and carbon nanofibers. (c) Equivalence of active Ni particle size and carbon nanofiber diameter on three nanofibers of different sizes: 6.5, 12.3 and 17.6 nm. (d) Close-up of carbon structures grown using Ni/C, showing a hollow fishbone-type carbon nanofiber morphology and carbon-encapsulated Ni particles.

Fig. 6. Particle size evolution during catalysis. (a) Mean Ni particle size of undoped and 5 at% doped Ni/C catalysts at different stages of the reaction. Reduced: After reduction-passivation. During: After 5 min under reaction conditions. Spent: After complete deactivation under reaction conditions. Active fibers of each catalyst are plotted separately from the inactive Ni particles of all catalysts combined (grey trace). (b) Correlation of active Ni particle sizes during catalysis and the deactivation rate constant rd, assuming half-order deactivation and under the standard reaction conditions. (c) Correlation of active Ni particle sizes during catalysis and the final carbon yield.

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