Effect of Textural Properties and Presence of Co-Cation on NH3-SCR Activity of Cu-Exchanged ZSM-5

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.

Influence of Framework n(Si)/n(Al) Ratio on the Nature of Cu Species in Cu‐ZSM‐5 for NH3‐SCR‐DeNOx

“…We identified three isolated Cu 2 + complexes (A, B and C in Figure 8) from the spectra of dehydrated Cu-ZSM-5-com, Cu-ZSM-5-100 and Cu-ZSM-5-120 samples with spin Hamiltonian parameters very similar to the ones reported in previous studies (e. g., [32,45] ). A and B species have been assigned to isolated copper complexes in square-pyramidal configuration (likely…”

“…According to the literature indications, the adsorption bands at about 200–260 nm can be assigned to oxygen‐to‐metal charge transfer related to Cu + or Cu 2+ cations stabilized by the zeolite framework [30,31] . The bands in the range of 260–550 nm proved the presence of the CuO species and [Cu‐O‐Cu] 2+ species [32,33] . The absorption in the range 550–900 nm is related to the d‐d transitions of Cu 2+ ions in pseudo‐octahedral coordination (e. g., Cu(H 2 O) 6 2+ ) [30,31] .…”

“…We identified three isolated Cu 2+ complexes ( A , B and C in Figure 8) from the spectra of dehydrated Cu‐ZSM‐5‐com, Cu‐ZSM‐5‐100 and Cu‐ZSM‐5‐120 samples with spin Hamiltonian parameters very similar to the ones reported in previous studies (e. g., [32,45] ). A and B species have been assigned to isolated copper complexes in square‐pyramidal configuration (likely differing in the crystal‐field symmetry) located in α six‐membered rings, whereas species C is assigned to isolated Cu 2+ with a square‐planar configuration [45,46] .…”

“…[30,31] The bands in the range of 260-550 nm proved the presence of the CuO species and [Cu-O-Cu] 2 + species. [32,33] The absorption in the range 550-900 nm is related to the d-d transitions of Cu 2 + ions in pseudooctahedral coordination (e. g., Cu(H 2 O) 6 2 + ). [30,31] Clearly, for the materials with ZSM-5 synthesized at higher temperatures, the two maxima appear centered at ca.…”

Review of the application of Cu-containing SSZ-13 in NH3-SCR-DeNOx and NH3-SCO

“…The ion-exchange properties can also be varied for the micro-/mesoporous materials. [55][56][57] In the case of SSZ-13, 58 mesopores with diameters of 2-10 nm were introduced into the zeolite H-SSZ-13 aer treatment with NaOH. Cu-containing SSZ-13 with the support treated with an aqueous solution of 0.1 M NaOH showed enhanced activity in NH 3 -SCR-DeNO x .…”