Abstract
Comparative studies over micro-/mesoporous Cu-containing zeolites ZSM-5 prepared by top-down treatment involving NaOH, TPAOH or mixture of NaOH/TPAOH (tetrapropylammonium hydroxide) were conducted. The results of the catalytic data revealed the highest activity of the Cu-ZSM-5 catalyst both in the absence and presence of water vapor. The physico-chemical characterization (diffuse reflectance UV-Vis (DR UV-Vis), Fourier transform infrared (FT-IR) spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, temperature-programmed desorption of NOx (TPD-NOx), and microkinetic modeling) results indicated that the microporous structure of ZSM-5 effectively stabilized isolated Cu ion monomers. Besides the attempts targeted to the modification of the textural properties of the parent ZSM-5, in the next approach, we studied the effect of the co-presence of sodium and copper cations in the microporous H-ZSM-5. The presence of co-cation promoted the evolution of [Cu–O–Cu]2+ dimers that bind NOx strongly with the desorption energy barrier of least 80 kJ mol−1. Water presence in the gas phase significantly decreases the rate of ammonia oxidation, while the reaction rates and activation energies of NH3-SCR remain unaffected.
Highlights
The selective catalytic reduction of NOx with ammonia (NH3-SCR) serves as one of the most efficient methods among the post-combustion technologies of NOx emission control
Besides the attempts targeted to the modification of the textural properties of the parent ZSM-5, in the approach, we studied the effect of the co-presence of sodium and copper cations in the microporous H-ZSM-5
Gao et al [6] investigated Cu-SSZ-13 (n(Si)/n(Al) = 6), and they showed that when co-cation, i.e., Na+, K+, Li+, is present the amount of introduced copper species increased from 0.87 wt % for Cu-H-SSZ-13 to 0.94–0.98 wt % for Cu(Li,Na,K)-SSZ-13
Summary
The selective catalytic reduction of NOx with ammonia (NH3-SCR) serves as one of the most efficient methods among the post-combustion technologies of NOx emission control. The higher content of reducible copper species of micro/mesoporous Cu-ZSM-5 resulted in enhanced activity and N2 selectivity during NH3-SCR. The authors revealed that the presence of certain amount of Na+ (up to 1.78 wt %) (or other alkali and alkaline earth cations) is beneficial to both low-temperature NOx conversion and hydrothermal stability of low-Cu loaded Cu-SSZ-13. These values (Na+ content up to 1.7 wt % among 0.1–3.4 wt % of Na) was confirmed in other studies over Na-Cu-SSZ-13 (n(Si)/n(Al) = 4, 2.4–2.7 wt %) applied in NH3-SCR [7]. The copper species (isolated Cu2+ and Cu+ as well as [Cu–O–Cu]2+) are reported to participate in NOx reduction; only limited evidence was present to suggest such participation of active copper species
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