Shielding Protection by Mesoporous Catalysts for Improving Plasma-Catalytic Ambient Ammonia Synthesis

Abstract

Plasma catalysis is a promising technology for decentralized small-scale ammonia (NH₃) synthesis under mild conditions using renewable energy, and it shows great potential as an alternative to the conventional Haber-Bosch process. To date, this emerging process still suffers from a low NH₃ yield due to a lack of knowledge in the design of highly efficient catalysts and the in situ plasma-induced reverse reaction (i.e., NH₃ decomposition). Here, we demonstrate that a bespoke design of supported Ni catalysts using mesoporous MCM-41 could enable efficient plasma-catalytic NH₃ production at 35 °C and 1 bar with >5% NH₃ yield at 60 kJ/L. Specifically, the Ni active sites were deliberately deposited on the external surface of MCM-41 to enhance plasma-catalyst interactions and thus NH₃ production. The desorbed NH₃ could then diffuse into the ordered mesopores of MCM-41 to be shielded from decomposition due to the absence of plasma discharge in the mesopores of MCM-41, that is, "shielding protection", thus driving the reaction forward effectively. This promising strategy sheds light on the importance of a rational design of catalysts specifically for improving plasma-catalytic processes.

Figure 1

HRTEM image and PSD of (a) Ni/MCM-out, (b) Ni/MCM-both, and (c) Ni/MCM-in (all in the reduced state).

Figure 2

(a) H₂-TPR of the as-prepared Ni/MCM-out, Ni/MCM-both, and Ni/MCM-in, and the corresponding TEM images for out (α), window-like (β), and in (γ) Ni sites; (b) H₂ consumption (by TPR) for the as-prepared catalysts (-a) and the H₂ plasma-treated catalysts (-p) (at a specific energy input, SEI, of 24 kJ/L, SEI = plasma discharge power over gas flow rate); and (c) schematics of the supported Ni catalysts on MCM-41 (top) and shielding effect of mesoporous MCM-41 on Ni species within its mesopores in an H₂ plasma discharge.

Figure 3

(a) R_NH3 and NH₃ yield of plasma alone and plasma-catalytic systems (with SiO₂, MCM-41 and catalysts based on them, SEI = 36 kJ/L, Q_gas = 40 mL/min, at 35 °C and 1 bar. Each experiment lasted 3 h). (Errors were derived from three tests under the same condition). (b) Turnover frequency (TOF, of plasma-catalytic systems) as a function of SEI. (c) Logarithmic reaction rate (ln(R_NH3)) vs 1/discharge power_DBD plots for the plasma alone and hybrid systems (based on the MCM-41 support and Ni/MCM-41 catalysts). (d) R_NH3 over Ni/MCM-out as a function of ToS for 150 h (The continuous sampling interval is 20 s). (e) Reported energy yield vs NH₃ concentration in the plasma-catalytic NH₃ synthesis over different metallic catalysts using DBD. Plotted experimental data are reproduced from the works of Bai et al., Barboun et al., Gómez-Ramírez et al., Herrera et al., Iwamoto et al.,^, Kim et al., Li et al., Mizushima et al.,^, Patil et al., Peng et al.,^,, Shah et al.,^, Wang et al., Xie et al., and Zhu et al.

Figure 4

In situ FTIR spectra of (a) N₂ activation and adsorption (in N₂ plasma, Q_gas = 40 mL/min) and (b) hydrogenation of the adsorbed N₂ (in an H₂ plasma, Q_gas = 40 mL/min) at 5 min of discharge time. (c) IR spectra of plasma-assisted ammonia synthesis (discharge time = 15 min, Q_gas = 40 mL/min, N₂/H₂ = 1:3, full scale as shown in Figure S17) on MCM-41, Ni/MCM-in, and Ni/MCM-out, respectively. (d) N 1s XPS profiles of the spent catalysts and MCM-41 after the reaction. (e) Normalized relative intensities of N, H_α, and NH peaks as measured by optical emission spectrometry (SEI = 24 kJ/L, 35 °C, 1 bar, full scale as shown in Figure S20a) (Errors were obtained from three tests under the same conditions).

Figure 5

(a) NH₃ synthesis rate vs the external Ni site densities of different supported Ni catalysts on SiO₂ and MCM-41 (at 36 kJ/L, 35 °C and 1 bar). (b) Desorbed NH₃ concentration and (c) corresponding accumulated amount vs purging time in the plasma alone and plasma-catalyst systems (catalyst amount: ∼0.5 g, mixed N₂/H₂ as the purge gas). (d) Plasma-driven decomposition of NH₃ in the plasma alone and plasma-SiO₂/-MCM-41 systems (init. adsorption: initial adsorption stage; re-adsorption: NH₃ re-adsorption stage; NH₃ dec. rate: NH₃ decomposition rate; discharge power = 5 W, with ∼0.1 g catalysts).

Figure 6

Schematic of the proposed mechanism for the plasma-assisted surface reaction and the “shielding protection” effect of mesoporous MCM-41.