Hydrogen production via ammonia decomposition on molybdenum carbide catalysts: Exploring the Mo/C ratio and phase transition dynamics

doi:10.1016/S1872-2067(25)64726-7

Abstract

Abstract:

The deployment of non-precious metal catalysts for the production of CO_x-free hydrogen via the ammonia decomposition reaction (ADR) presents a promising yet great challenge. In the present study, two crystal structures of α-MoC and β-Mo₂C catalysts with different Mo/C ratios were synthesized, and their ammonia decomposition performance as well as structural evolution in ADR was investigated. The β-Mo₂C catalyst, characterized by a higher Mo/C ratio, demonstrated a remarkable turnover frequency of 1.3 s^-1, which is over tenfold higher than that of α-MoC (0.1 s^-1). An increase in the Mo/C ratio of molybdenum carbide revealed a direct correlation between the surface Mo/C ratio and the hydrogen yield. The transient response surface reaction indicated that the combination of N^* and N^* derived from NH₃ dissociation represents the rate-determining step in the ADR, and β-Mo₂C exhibited exceptional proficiency in facilitating this pivotal step. Concurrently, the accumulation of N^* species on the carbide surface could induce the phase transition of molybdenum carbide to nitride, which follows a topological transformation. It is discovered that such phase evolution was affected by the Mo-C surface and reaction temperature simultaneously. When the kinetics of combination of N^* was accelerated by rising temperatures and its accumulation on the carbide surface was mitigated, β-Mo₂C maintained its carbide phase, preventing nitridation during the ADR at 810 °C. Our results contribute to an in-depth understanding of the molybdenum carbides’ catalytic properties in ADR and highlight the nature of the carbide-nitride phase transition in the reaction.

Key words: Molybdenum carbides, Phase transition, Nitridation, Recombination, Ammonia decomposition reaction

Bowen Sun, Siyun Mu, Bingbing Chen, Guojun Hu, Rui Gao, Chuan Shi. Hydrogen production via ammonia decomposition on molybdenum carbide catalysts: Exploring the Mo/C ratio and phase transition dynamics[J]. Chinese Journal of Catalysis, 2025, 74: 365-376.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(25)64726-7

https://www.cjcatal.com/EN/Y2025/V74/I7/365

Figures/Tables 6

Fig. 1. Characterization of the prepared α-MoC and β-Mo2C catalysts. (a) Schematic illustration of α-MoC and β-Mo2C catalysts prepared from MoO3 by different carburization pathway. XRD pattern (b), C 1s and Mo 3d (c) XPS spectra of α-MoC and β-Mo2C. (d) Surface Mo/C atomic ratios of Mo carbides calculated from Mo 3d and C 1s spectra. (e,f) HRTEM images of α-MoC and β-Mo2C samples.

Fig. 2. (a) Temperature-dependent NH3 conversion of α-MoC and β-Mo2C in pure NH3 at a WHSV of 30,000 mL/(g·h). H2 yield normalized by the specific surface area of catalysts (b) and TOFs comparison (c) at 450 °C over α-MoC and β-Mo2C catalysts at a WHSV of 30000 mL/(g·h). (d) XRD patterns of α-MoC-t (t = 30, 60, 90, and 120 min) treated with H2/Ar flow. (e) NH3 conversion for the α-MoC-t catalysts at a WHSV of 30000 mL/(g·h). (f) Dependence of H2 production rate at 550 °C (WHSV of 30000 mL/(g·h)) on surface Mo/C atomic ratio. (g) Stability tests of α-MoC and β-Mo2C catalysts at 600 °C with a space velocity of 60000 mL/(g·h).

Fig. 3. NH3-TPD (a) and H2-TPD (b) profiles over α-MoC and β-Mo2C catalysts. (c) NH3-pulse reaction at 400 °C for the α-MoC and β-Mo2C. (d) Subsequent to the NH3-pulse reaction, an N2-TPD test was then carried out.

Fig. 4. (a) XRD pattern of fresh and spent Mo carbide catalysts. (b) XRD pattern of α-MoC and β-Mo2C after NH3 stability testing at different temperatures. Reaction conditions: 50 mg catalyst, WHSV of 30000 mL/(g·h), pure NH3, total flow rate of 25 mL/min. (c) NH3-TPSR and corresponding CH4 signal for the α-MoC and β-Mo2C. The CH4 signal was detected by an infrared absorption spectrometer (SICK-MAIHAK-S710). (d,e) HRTEM images and EDS elemental maps of spent α-MoC and β-Mo2C. Reaction conditions for the catalytic test for (a), (d), and (e): 50 mg catalyst at 600 °C, WHSV of 60000 mL/(g·h), pure NH3, total flow rate of 50 mL/min.

Table 1 Physical properties of the fresh and spent catalysts.

Sample	S_BET ^a (m²/g)	C^b (wt%)	N^b (wt%)	Phase structure ^c
α-MoC-fresh	126	8.94	—	α-MoC
α-MoC-spent-4 h	98	1.13	9.08	C doped Mo₂N
α-MoC-spent-120 h	79	0.58	7.02	Mo₂N
β-Mo₂C-fresh	13	5.78	—	β-Mo₂C
β-Mo₂C-spent-4 h	9	2.67	8.02	β-Mo₂C and MoN
β-Mo₂C-spent-120 h	7	0.70	9.96	MoN

Fig. 5. The energy profiles of NH3 dissociation on the catalysts of α-MoC and β-Mo2C. (a) the most favorable route of ADR. (b,c) The possible NHx coupling reactions, in which the corresponding structures are inserted and the Mo, C and N atom are marked in cyan, grey and blue, respectively.

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