Short communicationCatalysts Cu/ZSM-5 for N2O decomposition obtained with copper complexes of various structures
Graphical abstract
Introduction
Nitrous oxide (N2O) represents one of the potential contributors to the environmental problems. This harmful gas contributes to both, stratospheric ozone destruction and greenhouse warming. One of the promising ways to reduce N2O emission is the direct catalytic decomposition of N2O over the Fe/ZSM-5 and Cu/ZSM-5 zeolite catalysts [[1], [2], [3], [4], [5], [6]].
The synthesis and identification of the nature of active copper species of Cu/ZSM-5 catalysts have already been carried out for a long time and are still being discussed [4,6]. In catalysts synthesized by the ion-exchange method, the single Cu2+ ions located in structural cationic sites and some types of extra framework binuclear or dispersed oligonuclear CuO-species [3,4,[7], [8], [9], [10]] were established. In [11] was shown that multinuclear copper sites as [Cu2(μ-O)2]2+ are the most active for the N2O decomposition. The high activity is explained by facile oxygen migration over the Cu sites. In accordance with [12] the active species may be presented as follows: bis(μ-oxo)dicopper cores > other EPR-silent Cu sites > isolated, EPR-active Cu sites. In Cu/ZSM-5, copper species on the external surface of zeolite crystals may also form [13,14]. But in the literature, there is no consensus about activity of CuO-like species for the DeNOx reactions [3,4,[13], [14], [15]].
In this study, we used the copper complexes of different structure to prepare Cu/ZSM-5 catalysts by ion exchange method and investigated the effect of copper precursors on their catalytic properties in N2O decomposition.
Section snippets
Experimental
For preparation of Cu/ZSM-5 catalysts HZSM-5 zeolite (Plant of Chemical concentrates, Novosibirsk, Russia) with Si/Al = 17, the crystalline degree of 95% and contents of Fe3+ and Na+ impurities 0.09 and 0.05 wt% was used as a powder. The catalysts were prepared by ion-exchange. The starting copper solutions were prepared by dissolving of Cu(NO3)2.3H2O (Aldrich, USA) in aqueous ammonia with a preset ammonia concentration. The copper concentration in all cases was 10 g/L, the ratios of NH4OH/Cu
Results and discussion
Stabilization of Cu2+ ions in concentrated water-ammonia copper nitrate solutions in a wide range of ammonium ion concentrations has been studied in [17]. It was found by ESR method that the ammonia concentration in a solution has a decisive effect on the type of orbital ordering of Cu2+ ions in their associated species. In all cases, Cu2+ ordering in associated copper species is caused by the existence of bridging OH groups in the axial and equatorial positions of [Cu(NH3)n(H2O)6 – n]2+ (n
Conclusions
It can be concluded that the copper precursor species of different structure CC-I and CC-III used for the preparation of Cu/ZSM-5 catalysts by ion-exchanged method is a key parameter determining the nature, the concentration of the active copper species, and their performance in decomposition of N2O. In Cu/ZSM-5 catalysts prepared by using of CC-III and CC-I at the similar copper concentration, the catalysts prepared by using CC-III are more active.
The results of these studies might be helpful
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was conducted within the framework of the budget project for Boreskov Institute of Catalysis Siberian Branch Russian Academy of Sciences.
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