In-Situ Drifts Studies of Selective Catalytic Reduction of NO With C3H6 On Cu-Chabazite Catalyst

In-situ DRIFTS Studies of Selective Catalytic Reduction of NO with C₃H₆ on Cu-chabazite Catalyst

Richa Raj, Michael P. Harold^[1], and Vemuri Balakotaiah^[2]

Department of Chemical & Biomolecular Engineering, University of Houston, Houston, TX, United States.

Abstract

In-situ DRIFTS studies were carried out to elucidate the underlying mechanism of NO_x reduction with C₃H₆. In-situ DRIFTS was performed using a Nicolet Nexus 470 spectrometer, equipped with a small collector with a dual sample environmental chamber (Thermo Fisher Scientific). The sample of catalyst powder was prepared by scraping the washcoat off the monolith channels. The feed gas, at a total flow rate of 50 cm³/min at STP, was mixed using Bronkhorst mass flow controllers; and its composition, with all gases present, was 500 ppm C₃H₆, 500 ppm NO, 1% or 5% O₂, 0 or 2.5% H₂O balanced by He. Before each experiment, the catalyst sample of 60 mg was treated with 5% O₂ in He for 1 hour at 600^oC.

            Fig. 1 shows the evolution of surface species with time for C₃H₆ + O₂ reaction. The growth of the bands around 1550 cm ^-1 and 2900 cm^-1 correspond to stretching of C-C and C-H bonds of adsorbed hydrocarbon species, respectively. The broad band near 3060 cm^-1 corresponds to vibration of C-H bond of condensed aromatic rings. The growth of these bands with time suggest the formation and accumulation of coke like species on the catalyst surface formed via oligomerization of adsorbed C₃H₆ derived hydrocarbon species. The appearance of band at 1660 cm^-1 corresponds to C=O stretching of acrolein like species. Similarly, the peak at 1450 cm^-1 corresponds to adsorbed carboxylate species. Hence, oxygenates are the intermediates in C₃H₆ oxidation.  Fig. 2 shows the spectra obtained for different times on introduction of NO in feed after exposure to C₃H₆ + O₂. On exposure to NO a new peak at 2250 cm^-1 is observed which is attributed to –NCO species. There is increase in the peak intensity corresponding to –NCO species with time suggesting its accumulation on the catalyst surface. It clearly indicates that N-containing hydrocarbon species act as intermediate in NO_x reduction with C₃H₆.

 Figure 1 DRIFTS spectra obtained at different times upon Cu-SSZ13 exposure to 500 ppm C₃H₆ at 380 ^oC (500 ppm C₃H₆, 1% O₂, balanced by He)

Figure 2 Emergence of new peak upon exposure to 500 ppm NO after 45 minutes of exposure to 500 ppm C₃H₆ at 380 ^oC (500 ppm C₃H₆, 500 ppm NO, 1% O₂, balanced by He)

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[1]Corresponding author: Tel.: +1 713 743 4322; Fax: +1 713 743 4323; Email: mharold@uh.edu (M. P. Harold)

[2]Corresponding author: Tel.: +1 713 743 4318; Fax: +1 713 743 4323; Email: bala@uh.edu (V. Balakotaiah)