Mechanistic Cause of Hydrocarbon Specificity over Cu/ZSM-5 and Co/ZSM-5 Catalysts in the Selective Catalytic Reduction of NOx

Adelman, Brad; Beutel, T.; Lei, G.D.; Sachtler, W. M. H.

doi:10.1006/jcat.1996.0031

Cited by 218 publications

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“…On the other hand, Figure 9 reveals that |Cu x |[Si y Al]-MFI catalysts are not active in the reduction of NO in the presence of methane and also show low activity in the oxidation of this hydrocarbon. This behaviour was also observed by Li and Armor 27 and it is probably related to the high energy necessary for the C-H dissociation in methane, which is following Adelman et al 28 , the limiting step in the reduction of NO by alkanes over copper-containing ZSM-5 zeolites. For the |Cu x |[Si y Al]-MFI catalysts in the tests using propane, it can be seen that the conversion of NO increases more slightly at temperatures higher than 400 ºC.…”

Section: Catalytic Evaluationsupporting

confidence: 74%

Cu and Co exchanged ZSM-5 zeolites: activity towards no reduction and hydrocarbon oxidation

et al. 2006

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(y = 12, 25 and 45). The activity of the catalysts was evaluated in the reduction of NO to N 2 in an oxidative atmosphere using propane or methane as reducing agents. The Cu catalysts were only active with propane and they presented higher activity than the Co-based catalysts, the latter being active with both hydrocarbons. H 2 -TPR and DRS-UV/Vis data allowed correlation between the activity towards NO reduction and the presence of cationic charge-compensating species in the zeolite. It was also verified that the hydrocarbons are preferentially oxidised by O 2 , a reaction that occurs simultaneously with their oxidation with NO.

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Section: Catalytic Evaluationsupporting

confidence: 74%

Cu and Co exchanged ZSM-5 zeolites: activity towards no reduction and hydrocarbon oxidation

et al. 2006

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“…Others authors [12,[18][19][20][21][22][23][24] propose a path based on the formation of NO2 which reacts with propene to form different reaction intermediates. Most of them consider these intermediates to be organo-nitro and organo-nitrite species, although others [23] propose other partially oxidized intermediates such as γ-hydroxynitropropane.…”

Section: I) Reaction Intermediates From Partial Hydrocarbon Oxidationmentioning

confidence: 99%

“…Several authors claim that some deposits appearing on the catalyst during the NO/C 3 H 6 /O 2 reaction play an important role in the reaction, because they are the source of very reactive radical species able to take part as reaction intermediates in some reaction steps [6,7]. Pt phase might catalyze the formation of these carbonaceous deposits either by cracking or partial oxidation of the hydrocarbon, thus, generating a large amount of allyl groups [8,9], carbenium ions [10], groups such as -CN and -NCO [11-17]etc., that will then react with oxidizing species (NO, NO2, O2) yielding the reaction products (N2, N2O, CO2).Others authors [12,[18][19][20][21][22][23][24] propose a path based on the formation of NO2 which reacts with propene to form different reaction intermediates. Most of them consider these intermediates to be organo-nitro and organo-nitrite species, although others [23] propose other partially oxidized intermediates such as γ-hydroxynitropropane.…”

mentioning

confidence: 99%

The selective reduction of NO with propene on Pt-beta catalyst: A transient study

Cortés

Illán-Gómez

Lecea

2007

Applied Catalysis B: Environmental

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The mechanism of the NO/C3H6/O2 reaction has been studied on a Ptbeta catalyst using transient analysis techniques. This work has been designed to provide answers to the volcano-type activity behaviour of the catalytic system, for that reason, steady state transient switch (C 3 H 6 /NO/O 2 → C 3 H 6 /Ar/O 2 , C 3 H 6 /Ar/O 2 → C 3 H 6 /NO/O 2 ; C 3 H 6 /NO/O 2 → Ar/NO/O 2 , Ar/NO/O 2 → C 3 H 6 /NO/O 2 ; C 3 H 6 /NO/O 2 → C 3 H 6 /NO/Ar and C 3 H 6 /NO/Ar → C 3 H 6 /NO/O 2 ) and Thermal Programmed Desorption (TPD) experiments were conducted below and above the temperature of the maximum activity (Tmax). Below Tmax, at 200ºC, a high proportion of adsorbed hydrocarbon exists on the catalyst surface. There exists a direct competition between NO and O 2 for Pt free sites which is very much in favour of NO, and therefore NO reduction selectively takes place over hydrocarbon combustion. NO and C 3 H 6 are involved in the generation of partially oxidised hydrocarbon species. O 2 is essential for the oxidation of these intermediates closing the catalytic cycle. NO 2 is not observed in the gas phase. Above T max , at 230ºC, C 3 H 6ads coverage is negligible and the surface is mainly covered by O ads produced by the dissociative adsorption of O 2 . NO 2 is observed in gas phase and carbon deposits are formed at the catalyst surface. From these results, the state of Ptbeta catalyst at T max is inferred. The reaction proceeds through the formation of partially oxidised active intermediates (C x H y O z N w ) from C 3 H 6ads and NO ads . The combustion of the intermediates with O2(g) frees the Pt active sites so the reaction can continue.Temperature has a positive effect on the surface reaction producing active intermediates. On the contrary, formation of NO ads and C 3 H 6·ads are not favoured by an increase in temperature. Temperature has also a positive effect on the dissociation of O 2 to form O ads , consequently, the formation of NO 2 is favoured by temperature through the oxygen dissociation. NO2 is very reactive and produces the propene combustion without NO reduction. These facts will determine the maximum concentration of active intermediates and consequently the maximum of activity.Key words: NOx-SCR mechanism, propene, Pt-beta catalyst, transient experiments. Page 2 of 34A c c e p t e d M a n u s c r i p t 3 IntroductionNumerous papers deal with the study of the mechanism of NO x reduction by hydrocarbon in the presence of excess oxygen on platinum based catalysts. However there is no evidence that allows defining a single reaction path. Pt/Al 2 O 3 and Pt/SiO 2 catalysts have been the most studied catalytic systems and therefore, those about which more is known. One of the conclusions arising from a detailed study of the literature is the different reaction paths for different hydrocarbons, as it appears when comparing propane and propene [1]. The Pt oxidation state is a key issue in determining the reaction mechanism and this seems to be related to the different nature of the hydrocarbon-metal bond [2][...

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“…Adelman et al 21 Therefore the copper containing catalysts were tested only with propane and the cobalt containing catalysts only in the presence of methane. The specific activity in the reduction of NO (number of nitrogen molecules formed per mol of transition metal per hour) and in the alkane oxidation (number of alkane molecules consumed per mol of transition metal per hour) as a function of the reaction temperature is shown for the copper containing catalysts in Figure 7 and for the cobalt containing catalysts in Figure 8.…”

Section: Reduction Of No To Nmentioning

confidence: 99%

Selective catalytic reduction of NO to N2 with copper and cobalt exchanged ZSM-5 zeolites: the effect of calcium addition

Martins¹,

Peguin²,

Wallau³

et al. 2005

J. Braz. Chem. Soc.

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Zeólitas ZSM-5 contendo cobre ou cobalto, ou simultaneamente cobre e cálcio, ou cobalto e cálcio foram preparadas por troca-iônica e testadas como catalisadores na redução de NO a N 2 na presença de metano ou propano. . Nestes catalisadores a presença de cálcio não provocou diferenças significativas na atividade específica para a redução de NO ou para a oxidação do metano.ZSM-5 catalysts containing either Cu or Co cations individually or Cu/Ca or Co/Ca cation combinations were prepared by ion exchange. These materials were tested as catalysts for the reduction of NO to N 2 in the presence of propane or methane. Cu 2+ and [Cu-O-Cu] 2+ species were identified in ion exchangeable sites in the copper-containing catalysts. The amount of the latter cation increased with increasing copper content and in the presence of Ca 2+ and was the most active in the reduction of NO. However, at temperatures higher than 400 ºC its activity decreased markedly. For cobalt-exchanged zeolites, Co 2+ in charge-compensation sites or Co 3+ in extra-framework precipitates were present in the catalysts. It was verified that the amount of Co 3+ diminished with the presence of Ca 2+. For the catalysts prepared in this study no significant differences in specific activity for the reduction of NO or for methane oxidation were observed in the presence of calcium.

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Mechanistic Cause of Hydrocarbon Specificity over Cu/ZSM-5 and Co/ZSM-5 Catalysts in the Selective Catalytic Reduction of NOx

Cited by 218 publications

References 0 publications

Cu and Co exchanged ZSM-5 zeolites: activity towards no reduction and hydrocarbon oxidation

Cu and Co exchanged ZSM-5 zeolites: activity towards no reduction and hydrocarbon oxidation

The selective reduction of NO with propene on Pt-beta catalyst: A transient study

Selective catalytic reduction of NO to N2 with copper and cobalt exchanged ZSM-5 zeolites: the effect of calcium addition

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