Modification of Ammonia Decomposition Activity of Ruthenium Nanoparticles by N-Doping of CNT Supports

Abstract

The use of ammonia as a hydrogen vector has the potential to unlock the hydrogen economy. In this context, this paper presents novel insights into improving the ammonia decomposition activity of ruthenium nanoparticles supported on carbon nanotubes (CNT) by nitrogen doping. Our results can be applied to develop more active systems capable of delivering hydrogen on demand, with a view to move towards the low temperature target of less than 150 °C. Herein we demonstrate that nitrogen doping of the CNT support enhances the activity of ruthenium nanoparticles for the low temperature ammonia decomposition with turnover frequency numbers at 400 °C of 6200 LH₂ mol_Ru ^-1 h^-1, higher than the corresponding value of unmodified CNT supports under the same conditions (4400 LH₂ mol_Ru ^-1 h^{- 1}), despite presenting similar ruthenium particle sizes. However, when the nitrogen doping process is carried out with cetyltrimethylammonium bromide (CTAB) to enhance the dispersion of CNTs, the catalyst becomes virtually inactive despite the small ruthenium particle size, likely due to interference of CTAB, weakening the metal-support interaction. Our results demonstrate that the low temperature ammonia decomposition activity of ruthenium can be enhanced by nitrogen doping of the CNT support due to simultaneously increasing the support's conductivity and basicity, electronically modifying the ruthenium active sites and promoting a strong metal-support interaction.

Keywords: Ammonia decomposition; In situ H2 production; N-CNT; Nitrogen; Nitrogen-doped CNT; Ruthenium.

Fig. 1

Representative TEM micrographs of CNT supports a unmodified CNT, b N-CNT1-CTAB, c N-CNT2-CTAB, d N-CNT1 and e N-CNT2

Fig. 2

Ammonia decomposition (GHSV = 6000 cm³ g⁻¹ h⁻¹, pre-reduction temperature = 230 °C) a conversion versus temperature and b Arrhenius plot for 7 wt.% Ru catalysts supported on CNT (filled circle), N-CNT1-CTAB (x) and N-CNT2-CTAB (+)

Fig. 3

Ammonia decomposition activity (GHSV = 6000 cm³ g⁻¹ h⁻¹, pre-reduction temperature = 230 °C) a conversion versus temperature and b Arrhenius plot for 7 wt.% Ru catalysts supported on CNT (filled circle), N-CNT1 (filled triangle) and N-CNT2 (filled square)

Fig. 4

TEM micrographs and Ru particle diameter distributions for reduced catalyst a 7Ru/CNT, b 7Ru/N-CNT1 and c 7Ru/N-CNT1-CTAB

Fig. 5

High resolution TEM micrographs for a 7Ru/CNT, b 7Ru/N-CNT1 and c 7Ru/N-CNT1-CTAB

Fig. 6

Thermogravimetric analysis of CNT supports: unmodified CNT (solid grey line), N-CNT1 (dashed blue line) and N-CNT1-CTAB (black dots)

Fig. 7

FTIR spectra of CNT supports: unmodified CNT (solid black line), N-CNT1 (solid dark grey line) and N-CNT1-CTAB (solid pale grey line)

Fig. 8

Temperature programmed reduction profile for catalysts supported on a CNT, b N-CNT1 and c N-CNT1-CTAB in which support only is shown by dashed line and support with 7 wt.% Ru catalyst is represented by a solid line

Fig. 9

XPS spectra in the N 1 s region for a CNT, b N-CNT1 and c N-CNT1-CTAB