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

Abstract

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][...

Carbon Nanotube-Supported Metal Catalysts for NOx Reduction Using Hydrocarbon Reductants: Gas Switching and Adsorption Studies

“…Further, this switch evinces that the kinetics of the hydrocarbon combustion reaction are faster than those of NO oxidation and that both NO and C 3 H 6 are involved in the production of partially oxidized hydrocarbon species. These species are believed to play a crucial role in the occurrence of the HC-SCR reaction, as it has been proposed in previous reports. , In essence, all these observations are confirmed by the reverse (NO/C 3 H 6 /Ar → NO/C 3 H 6 /O 2 ) switch.…”

“…A 2 wt % Pt/fMWCNT catalyst was selected to study the surface state of a working HC-SCR catalyst by means of a transient technique. Details of the preparation, characterization, and evaluation of this catalyst as well as of the gas switching apparatus used in this study can be found elsewhere. Given that the feasibility of using fMWCNT-supported PGMs as HC-SCR catalysts is fully contingent on the ability of the carrier to withstand exposure to oxidizing conditions during operation, experiments were undertaken to validate the use of fMWCNTs as a support for HC-SCR catalysts.…”

“…This being said, the system employed in this study allowed for all the analytes of interest to be tracked quite satisfactorily, with a single exception: N 2 O could not be resolved due to interference by CO 2 . To simplify the analysis of the transient profiles, the semiquantitative data obtained for the analytes were normalized between 0 and 1 using previously reported equations …”

“…Further, these new types of mechanism have been proposed based on experimental evidence obtained through techniques alternative to infrared spectroscopy, addressing one of the main objections presented against schemes involving nitrogen-containing organic intermediates (Burch has claimed that the very detection of these species via IR spectroscopy proves that they are not real intermediates, since a true intermediate would react milliseconds after its formation). Most notably, García-Cortés et al have recently reported a mechanism involving partially oxidized intermediates (whose combustion by gaseous O 2 constitutes the clean off step) based on the study of reaction transients …”

Rate-determining Step of Selective Catalytic Reduction of NO by Acetylene Over HZSM-5

“…Since the first reports on selective catalytic reduction of NO by hydrocarbons (HC-SCR) [1,2], extensive research into the HC-SCR applications for the control of NO x in exhaust gases of diesel and lean-burn engines has been carried out [3][4][5][6][7][8][9]. For the practical application of the HC-SCR, an active catalyst capable of reducing NO in large space velocity at ca.…”