Dynamics of N2O decomposition over HZSM-5 with low Fe content

doi:10.1016/j.jcat.2003.11.012

Journal of Catalysis

Volume 222, Issue 2, 10 March 2004, Pages 389-396

https://doi.org/10.1016/j.jcat.2003.11.012 Get rights and content

Abstract

The dynamics of N₂O decomposition to gaseous nitrogen and oxygen over HZSM-5 catalysts with a low iron content (200 and 1000 ppm) was studied by the transient response method in the temperature range 523–653 K. The active catalysts were prepared from HZSM-5 with Fe in the framework on its steaming at 823 K followed by thermal activation in He at 1323 K. Two main steps were distinguished in the dynamics of N₂O decomposition. The first step represents N₂O decomposition forming gaseous nitrogen and surface atomic oxygen. The second step is associated with surface oxygen recombination and desorption. At 523–553 K only the first step is observed. Above 573 K the decomposition of N₂O to O₂ and N₂ in stoichiometric amounts starts at a rate increasing with time until a steady-state value is reached. This increase was assigned to the catalysis by adsorbed NO formed slowly on the catalyst surface from N₂O, as indicated by temperature-programmed desorption. The catalytic effect of the adsorbed NO was also confirmed by transient experiments with forced addition of NO in the stream of N₂O during its decomposition. A simplified kinetic model is proposed to explain the autocatalytic reaction. Catalyst pretreatment in O₂ did not affect N₂O decomposition, but irreversible water vapor adsorption at 603 K resulted in a twofold decrease in surface oxygen loading from N₂O and complete inhibition of the oxygen desorption.

Introduction

N₂O is a greenhouse gas with a warming potential 310 times higher than that of CO₂, and its concentration in the atmosphere is still on the rise [1]. Metal- and metal oxide-doped zeolites have been reported as promising catalysts for N₂O decomposition [2]. HZSM-5 with a low content of iron (hereafter H(Fe)ZSM-5) was shown to be active in nitrous oxide decomposition, forming surface atomic oxygen (O)_ad and gaseous N₂ and O₂ [3]. The sites in H(Fe)ZSM-5 active in the above-mentioned processes have been assigned to iron cations [3], [4], [5], [6]. But until now very little quantitative information has been available on the total loading of (O)_ad and its role during N₂O decomposition to O₂ and N₂. Moreover, H(Fe)ZSM-5 is able to hydroxylate benzene to phenol and oxidize methane to methoxy species by N₂O at low temperatures [7], [8]. For these reactions, N₂O decomposition to gaseous oxygen and nitrogen is a side reaction decreasing the concentration of the reactant [9], which should be avoided.

The kinetics of N₂O decomposition was shown to be of first order in N₂O concentration [4]. The activity of zeolites in N₂O decomposition is known to depend on the presence of NO_x, O₂, H₂O, SO₂, CO, and hydrocarbons in streams. El-Malki et al. [4], [10] and Zhu et al. [5] did not find any effect of oxygen on this reaction, but in the presence of water vapor, isothermal oscillations were observed during N₂O decomposition with Fe/MFI [4], [10]. Perez-Ramirez et al. [6], [11] studied the effect of NO on N₂O decomposition and showed that oxygen desorption is the rate-limiting step, which could be accelerated by NO. The objective of the present study was to determine the effect of O₂, NO, and H₂O on the dynamics of N₂O decomposition over H(Fe)ZSM-5. The transient response method gives valuable information concerning reaction dynamics and the effect of different gases. This method was applied earlier to study N₂O decomposition over Cu-ZSM-5 catalysts [12] and may shed additional light on the mechanism of the reaction over H(Fe)ZSM-5. In our previous study [13] it was shown that the transient response method allows determination of the concentration of Fe sites active in the formation of surface atomic oxygen via N₂O decomposition to gaseous N₂ and (O)_ad.

Section snippets

Experimental

In our previous study [13] it was found that thermal pretreatment of steamed zeolites in He at 1323 K increases the concentration of sites active in N₂O decomposition, which were assigned to surface Fe-containing species. Therefore, this treatment was used during this study. Zeolites with very small concentrations of iron ( $⩽ 1000$ ppm) were employed, aiming at the formation of uniform Fe species. Low temperatures were used to study N₂O decomposition to discriminate between the different reaction

Dynamics of N₂O decomposition

The transient response experiment with the HZSM-5_1000Fe catalyst at 603 K is presented in Fig. 1 and illustrates the formation of N₂ and O₂. N₂ concentration first passes a sharp maximum and then slowly increases, while oxygen concentration is seen to continuously increase. After 45 min of reaction a steady state is reached with a conversion of around 50%. Nitrous oxide evolution is delayed with respect to Ar (inert tracer) due to product formation and reversible adsorption on the catalyst

Discussion

A reaction scheme is proposed to explain the experimental data: $N_{2} O ↔ N_{2} O_{ad} (reversible adsorption),$ $N_{2} O +()→ N_{2} +(O)_{ad} fast,$ $N_{2} O +[]→[NO]+0.5 N_{2} slow,$ $[NO]+2(O)_{ad} →[NO]+ O_{2} +2() relatively fast,$ $2(O)_{ad} → O_{2} +2() slow .$

At 523 K only the first two steps seem to occur. The first step corresponds to surface atomic oxygen formation. Extraframework Fe²⁺ sites created by autoreduction of Fe³⁺ to Fe²⁺ during thermal treatment of the zeolite in He or vacuum [13], [21], [22], [23], [24], [25] are suggested as the active sites

Conclusions

1.
N₂O decomposition over isomorphously substituted H(Fe)ZSM-5 was studied by the transient response method. The catalysts were activated by steaming at 823 K followed by thermal treatment in He at 1323 K.
2.
Two steps were distinguished from the reaction dynamics: (1) a fast surface atomic oxygen deposition from N₂O with gaseous nitrogen release, and (2) a slow atomic oxygen recombination/desorption (the rate-limiting step).
3.
Oxygen recombination/desorption was observed to be accelerated by small

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N₂O decomposition is investigated on Cu, Co and Fe-exchanged SSZ-13 zeolite catalysts at relatively low metal loadings. The catalysts are synthesized by solution ion exchange, and subjected to X-ray diffraction (XRD), temperature-programed-reduction by H₂ (H₂-TPR), temperature-programed-reaction of N₂O (N₂O-TPR) coupled with in-situ transmission FTIR, and finally steady-state flow reaction tests. At low N₂O pressures (<0.05 kPa), all catalysts display pseudo first-order kinetics. From Arrhenius analysis, Cu and Fe-SSZ-13 display very different apparent activation energies but similar pre-exponential factors, suggesting their similar reaction mechanisms. N₂O decomposition follows a dual-site mechanism, occurring on dimeric M-O-M sites in these catalysts, and O₂ is formed by the combination of two O ad-atoms from two vicinal metal sites. Under low N₂O pressure (0.05 kPa) and first-order kinetic regime, the reaction is limited by N-O cleavage on bare metal active sites. In comparison to Cu-SSZ-13, the much higher N₂O decomposition rate over Fe-SSZ-13 is attributed to the much lower activation barriers for the N-O cleavage step. N₂O decomposition occurs on isolated Co²⁺ ions in Co-SSZ-13. The rate-limiting step is N-O cleavage on an O-occupied Co site in the low-pressure first order kinetic regime. This single-site mechanism leads to much higher pre-exponential factors as compared to the dual-site mechanism. This beneficial factor for reaction rate enhancement, however, is compromised by the much higher activation barriers over this catalyst.
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Citation Excerpt :
The rate of reaction is dependent on oxidant concentrations, total feedflow rate, temperature, pressure, and catalyst [51]. Literature suggests a fairly complex mechanism for N2O decomposition reaction [52–54]. The overall quantification of the gaseous products was performed for a diluted N2O (about 7% in feed balanced with helium) feed over the same Co-ZSM-5 catalyst in a continuous fixed bed flow reactor (total feed rate = 230 mL/min) in a temperature range of 300 °C–500 °C.
The aim of the present study is to explore the coherence of thermodynamic equilibrium predictions with the actual catalytic reaction of CH₄ with N₂O, particularly at higher CH₄ conversions. For this purpose, key process variables, such as temperature (300 °C–550 °C) and a molar feed ratio (N₂O/CH₄ = 1, 3, and 5), were altered to establish the conditions for maximized H₂ yield. The experimental study was conducted over the Co-ZSM-5 catalyst in a fixed bed tubular reactor and then compared with the thermodynamic equilibrium compositions, where the equilibrium composition was calculated via total Gibbs free energy minimization method.
The results suggest that molar feed ratio plays an important role in the overall reaction products distribution. Generally for N₂O conversions, and irrespective of N₂O/CH₄ feed ratio, the thermodynamic predictions coincide with experimental data obtained at approximately 475 °C–550 °C, indicating that the reactions are kinetically limited at lower range of temperatures. For example, theoretical calculations show that the H₂ yield is zero in presence of excess N₂O (N₂O/CH₄ = 5). However over a Co-ZSM-5 catalyst, and with a same molar feed ratio (N₂O/CH₄) of 5, the H₂ yield is initially 10% at 425 °C, while above 450 °C it drops to zero. Furthermore, H₂ yield steadily increases with temperature and with the level of CH₄ conversion for reactions limited by N₂O concentration in a reactant feed. The maximum attainable (from thermodynamic calculations and at a feed ratio of N₂O/CH₄ = 3) H₂ yield at 550 °C is 38%, whereas at same temperature and over Co-ZSM-5, the experimentally observed yield is about 19%.
Carbon deposition on Co-ZSM-5 at lower temperatures and CH₄ conversion (less than 50%) was also observed. At higher temperatures and levels of CH₄ conversion (above 90%), the deposited carbon is suggested to react with N₂O to form CO₂.
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Promotional effects of Ce⁴⁺, La³⁺ and Nd³⁺ incorporations on FeCu₂O_3.5/ATS for decomposing N₂O were investigated. Results showed that all the ions especially La³⁺ can dramatically improve the catalytic performance. FeCu₂La_1.5O_x/ATS, the optimal catalyst, can decompose 100% N₂O at 550 °C. The formed perovskite-type LaFeO₃ and spinel-type CuLa₂O₄ largely improved catalytic performance. The ion modification increased N₂O adsorption capacity, and La³⁺-doped catalyst enhanced desorption ability of surface oxygen. Both effects can accelerate the rate-determine steps of N₂O decomposition reaction, promote catalytic performance. Moreover, the supported catalysts studied were economic efficiency for application.
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Reactivity of oxygen species generated by N₂O decomposition over Fe-ZSM-5 and Fe-Beta zeolites was investigated using oxygen isotopic exchange as test reaction. The generated species are very stable up to 300 °C in the absence of the organic traces or residual N₂O in the gas phase. The reactivity of the oxygen species towards organics depends on the size of the organic molecules and their ability to penetrate into the zeolite pores. For example, in case of Fe-ZSM-5 zeolite, the oxygen species react readily with toluene, but stay intact with more bulky 1,3,5-trimethylbenzene.
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Journal of Catalysis

Abstract

Introduction

Section snippets

Experimental

Dynamics of N₂O decomposition

Discussion

Conclusions

References (28)

Coord. Chem. Rev.

Appl. Catal. B

J. Catal.

J. Catal.

J. Catal.

J. Mol. Catal. A

Catal. Today

Micropor. Mesopor. Mater.

J. Catal.

J. Catal.

Colloid Surf. A

J. Catal.

J. Catal.

Catal. Today

Cited by (56)

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Promotional effects of Ce<sup>4+</sup>, La<sup>3+</sup> and Nd<sup>3+</sup> incorporations on catalytic performance of Cu-Fe-O<inf>x</inf> for decomposition of N<inf>2</inf>O

Direct Catalytic Decomposition of N<inf>2</inf>O over Cu- and Fe-Zeolites

Location, stability, and reactivity of oxygen species generated by N <inf>2</inf>O decomposition over Fe-ZSM-5 and Fe-Beta zeolites

Direct nitrous oxide decomposition with a cobalt oxide catalyst