Temperature dependence of Cu(I) oxidation and Cu(II) reduction kinetics in the selective catalytic reduction of NOx with NH3 on Cu-chabazite zeolites

In this work, a kinetic model is developed for the reduction of CuII sites by NO + NH3 and the reoxidation of NH3-solvated CuI sites by O2 and NO in Cu-SSZ-13. Fourier transform infrared (FTIR) spectroscopy and spatially resolved capillary inlet mass spectrometry (SpaciMS) measurements during transient reactor experiments are utilized to identify the rate parameters associated with NO + NH3 RHC (reduction half-cycle), proposed to occur via two distinct pathways involving adsorbed NH3 and gas-phase NH3. The resulting NO + NH3 RHC model is validated using spatiotemporal N2 measurements covering a wide range of temperatures (200–450 °C) and space velocities (53 000–640 000 h–1). N2O formation is observed and modeled during NO + NH3 RHC, with quantitative validation under standard selective catalytic reduction (SCR) conditions. Experimentally measured enthalpic and entropic changes associated with O2 adsorption on NH3-solvated CuI (ZCu(NH3)2) complexes [KamasamudramK. Kamasamudram, K. Catal. Today2010151212222], along with activation energies estimated computationally for the intercage diffusion of ZCu(NH3)2 complexes [PaolucciC. Paolucci, C. Science2017357898903], are incorporated into a mean field kinetic model for the low-temperature oxidation half-cycle (OHC). Significant NH3 release is observed during the isothermal oxidation of CuI sites, attributed to desorption of NH3 ligands from NH3-solvated CuII dimers (Z2Cu2(NH3)4O2). Reduction of these dimeric complexes leads to the consumption of one NO/CuII, contradicting the expected reduction stoichiometry. Inclusion of a global Arrhenius rate for the NO titration of Z2Cu2(NH3)4O2 complexes provides accurate representations of standard SCR on reduced and oxidized catalysts, predicting transient NO and NH3 consumption between 150 and 250 °C as a function of hydrothermal aging. Deactivation of low-temperature standard SCR by NH3 is observed at high NH3 pressures, modeled via the formation of superoxo amino (ZCu(NH3)3OO*) complexes during NH3 titration of Z2Cu2(NH3)4O2 complexes [NegriC. Negri, C. J. Am. Chem. Soc.20201421588415896]. The redox kinetic model presented here provides a foundational description of active site redox during low-temperature standard SCR, combining the recent kinetic, spectroscopic, and computational findings on the mechanism of standard SCR over Cu-SSZ-13.

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“…Krishna et al . and Jones et al have recently reported on the Langmuirian dependence of standard SCR rates on O 2 pressure at low temperatures.…”

Section: Experimental and Kinetic Model Resultsmentioning

confidence: 98%

Kinetic Model for the Reduction of Cu^II Sites by NO + NH₃ and Reoxidation of NH₃-Solvated Cu^I Sites by O₂ and NO in Cu-SSZ-13

et al. 2022

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“…The activation energy (Ea) and pre-exponential factor (A) both increase with the increase in Cu loading, which was also observed by Gao et al 31 . Recently, Krisha et al reported that the Ea of Cu I oxidation increased monotonically with Cu density in a fixed kinetic regime due to the non-mean-field behavior of Cu-SSZ-13 in the NH 3 -SCR reaction and that the Ea of Cu II reduction was unchanged when the Cu load was higher than 0.69 wt.% 32 . On the other hand, the kinetic relevance of Cu II reduction increased with increasing Cu ion density, the Ea of which was higher than that of Cu I oxidation 30 , 32 .…”

Section: Resultsmentioning

confidence: 99%

“…Recently, Krisha et al reported that the Ea of Cu I oxidation increased monotonically with Cu density in a fixed kinetic regime due to the non-mean-field behavior of Cu-SSZ-13 in the NH 3 -SCR reaction and that the Ea of Cu II reduction was unchanged when the Cu load was higher than 0.69 wt.% 32 . On the other hand, the kinetic relevance of Cu II reduction increased with increasing Cu ion density, the Ea of which was higher than that of Cu I oxidation 30 , 32 . Therefore, the increase of the Ea in Cu I oxidation and kinetic relevance of Cu II reduction both contributed to the increase in the Ea of the SSCR reaction.…”

Section: Resultsmentioning

confidence: 99%

Strikingly distinctive NH3-SCR behavior over Cu-SSZ-13 in the presence of NO2

Shan

et al. 2022

Nat Commun

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Commercial Cu-exchanged small-pore SSZ-13 (Cu-SSZ-13) zeolite catalysts are highly active for the standard selective catalytic reduction (SCR) of NO with NH3. However, their activity is unexpectedly inhibited in the presence of NO2 at low temperatures. This is strikingly distinct from the NO2-accelerated NOx conversion over other typical SCR catalyst systems. Here, we combine kinetic experiments, in situ X-ray absorption spectroscopy, and density functional theory (DFT) calculations to obtain direct evidence that under reaction conditions, strong oxidation by NO2 forces Cu ions to exist mainly as CuII species (fw-Cu2+ and NH3-solvated CuII with high CNs), which impedes the mobility of Cu species. The SCR reaction occurring at these CuII sites with weak mobility shows a higher energy barrier than that of the standard SCR reaction on dynamic binuclear sites. Moreover, the NO2-involved SCR reaction tends to occur at the Brønsted acid sites (BASs) rather than the CuII sites. This work clearly explains the strikingly distinctive selective catalytic behavior in this zeolite system.

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“…In practice, the application of Cu-CHA catalysts for the NH 3 -SCR is restricted to ultralow-sulfur diesel fuels, due to the fact that a few ppm of SO 2 present in the exhaust gas drastically reduces the activity at low temperatures. ,, Multiple studies show that SO 2 affects the Cu mobility, the amount of Cu active sites, and the redox behavior of the Cu in the NH 3 -SCR cycle. , Most studies have focused on the overall effect of SO 2 on the performance of the catalysts, − while the chemistry behind SO 2 poisoning at the molecular level remains poorly understood. To determine a mechanism for SO 2 poisoning, one must identify the species in the Cu-CHA catalysts that interact with SO 2 .…”

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confidence: 99%

SO₂ Poisoning of Cu-CHA deNO_x Catalyst: The Most Vulnerable Cu Species Identified by X-ray Absorption Spectroscopy

Molokova

Borfecchia

Martini

et al. 2022

JACS Au

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Cu-exchanged chabazite zeolites (Cu-CHA) are effective catalysts for the NH 3 -assisted selective catalytic reduction of NO (NH 3 -SCR) for the abatement of NO x emission from diesel vehicles. However, the presence of a small amount of SO 2 in diesel exhaust gases leads to a severe reduction in the low-temperature activity of these catalysts. To shed light on the nature of such deactivation, we characterized a Cu-CHA catalyst under well-defined exposures to SO 2 using in situ X-ray absorption spectroscopy. By varying the pretreatment procedure prior to the SO 2 exposure, we have selectively prepared Cu I and Cu II species with different ligations, which are relevant for the NH 3 -SCR reaction. The highest reactivity toward SO 2 was observed for Cu II species coordinated to both NH 3 and extraframework oxygen, in particular for [Cu II 2 (NH 3 ) 4 O 2 ] 2+ complexes. Cu species without either ammonia or extraframework oxygen ligands were much less reactive, and the associated SO 2 uptake was significantly lower. These results explain why SO 2 mostly affects the low-temperature activity of Cu-CHA catalysts, since the dimeric complex [Cu II 2 (NH 3 ) 4 O 2 ] 2+ is a crucial intermediate in the low-temperature NH 3 -SCR catalytic cycle.

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Temperature dependence of Cu(I) oxidation and Cu(II) reduction kinetics in the selective catalytic reduction of NOx with NH3 on Cu-chabazite zeolites

Cited by 15 publications

References 46 publications

Kinetic Model for the Reduction of Cu^II Sites by NO + NH₃ and Reoxidation of NH₃-Solvated Cu^I Sites by O₂ and NO in Cu-SSZ-13

Kinetic Model for the Reduction of Cu^II Sites by NO + NH₃ and Reoxidation of NH₃-Solvated Cu^I Sites by O₂ and NO in Cu-SSZ-13

Strikingly distinctive NH3-SCR behavior over Cu-SSZ-13 in the presence of NO2

SO₂ Poisoning of Cu-CHA deNO_x Catalyst: The Most Vulnerable Cu Species Identified by X-ray Absorption Spectroscopy

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Temperature dependence of Cu(I) oxidation and Cu(II) reduction kinetics in the selective catalytic reduction of NOx with NH3 on Cu-chabazite zeolites

Cited by 15 publications

References 46 publications

Kinetic Model for the Reduction of CuII Sites by NO + NH3 and Reoxidation of NH3-Solvated CuI Sites by O2 and NO in Cu-SSZ-13

Kinetic Model for the Reduction of CuII Sites by NO + NH3 and Reoxidation of NH3-Solvated CuI Sites by O2 and NO in Cu-SSZ-13

Strikingly distinctive NH3-SCR behavior over Cu-SSZ-13 in the presence of NO2

SO2 Poisoning of Cu-CHA deNOx Catalyst: The Most Vulnerable Cu Species Identified by X-ray Absorption Spectroscopy

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Product

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Kinetic Model for the Reduction of Cu^II Sites by NO + NH₃ and Reoxidation of NH₃-Solvated Cu^I Sites by O₂ and NO in Cu-SSZ-13

Kinetic Model for the Reduction of Cu^II Sites by NO + NH₃ and Reoxidation of NH₃-Solvated Cu^I Sites by O₂ and NO in Cu-SSZ-13

SO₂ Poisoning of Cu-CHA deNO_x Catalyst: The Most Vulnerable Cu Species Identified by X-ray Absorption Spectroscopy