Abstract
Two copper-exchanged zeolites, Cu/SSZ-13 and Cu/BEA, were studied as catalysts for the selective reduction of NOx by NH3 (NH3-SCR). Their activities for standard SCR (NOx = NO) and fast SCR (NOx = 50% NO + 50% NO2) were measured before and after sulfur poisoning at 250 °C. The effect of 30 ppm SO2 and a mixture of 24 ppm SO3 + 6 ppm SO2 was evaluated. The repetition of subsequent activity measurements served as regeneration method in SCR conditions. SO2 deactivated Cu/SSZ-13 whereas Cu/BEA was only moderately affected. SO3 led to stronger deactivation of both catalysts than SO2. However, also for this case, the Cu/BEA was significantly less affected than Cu/SSZ-13, even though Cu/BEA contained larger amount of stored sulfur. One possible reason for this could be the large pores of Cu/BEA, where the sulfur species possibly resulted in less sterical hindrance than in the small pore SSZ-13 structure. NH3 temperature-programmed desorption (NH3-TPD) showed no loss of storage sites upon sulfur treatment and subsequent regeneration. Partial activity recovery was observed after a period in SCR conditions at 400 °C and 500 °C. Temperature at 300 °C was insufficient to regenerate the catalysts. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of NO adsorption suggested that SO2 interacts with the ZCuOH sites on Cu/SSZ-13, causing the strong poisoning.
Highlights
Diesel and lean-burn gasoline engines present the advantage to be more fuel-efficient and to emit less CO2 than gasoline engines
In order to aid the understanding of the poisoning and regeneration ability of Cu/SSZ-13, we compare it with another Cu/zeolite, namely Cu/BEA due to the high activity of Cu/BEA for SCR with NH3
It should be noted that Cu/BEA is not commercially applicable due to thermal degradation [22] and hydrocarbon poisoning [23] issues and was chosen in this work for mechanistic reasons together with Cu/SSZ-13
Summary
Diesel and lean-burn gasoline engines present the advantage to be more fuel-efficient and to emit less CO2 than gasoline engines. These combustion systems produce nitrogen oxides (NOx). NH3-SCR is an efficient process to reduce N Ox by N H3 over a metal-exchanged zeolite or V2O5-based catalyst. The exhaust gas contains other chemical compounds that can interfere with the NH3-SCR process and deactivate the widely used, efficient, Cu-exchanged zeolite catalyst. The oxidation catalyst is a noble metal-based catalyst which oxidizes CO and hydrocarbons into CO2 and water vapor. It oxidizes a large fraction of NO to NO2 and SO2 to SO3 [13,14,15,16].
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