(498c) Dynamic Behavior of Fe-Zeolite Urea-SCR Catalysts

Selective Catalytic Reduction (SCR) of NOx with urea is one of the primary candidate technologies for meeting the increasingly stringent diesel exhaust emission regulations worldwide. Several types of metal-exchanged zeolite catalysts have been shown to have excellent hydrothermal stability and steady-state NOx conversion over a broad range of diesel exhaust conditions, with Fe- and Cu-zeolites being the two leading candidate groups [1,2]. However, realizing these high conversion efficiencies in the practical applications requires detailed understanding of the key individual functions contributing to the overall NOx reduction performance of these urea-SCR catalysts, including their capability to catalyze in-situ oxidation of NO and NH3, as well as their NH3 storage and release characteristics [2]. The latter factor is often responsible for large differences between the commonly reported steady-state conversion values, and real conversion efficiencies observed in practice, due to rapidly varying temperature, gas flow rate, and exhaust gas composition. In this study, dynamic responses of several key functions of a commercial Fe-zeolite SCR catalyst were individually characterized over a broad range of experimental conditions relevant to automotive applications (temperatures, flow rate, NO2/NOx and NH3/NOx ratio) and related to the NOx conversion efficiency.

Materials and Methods

A 4-step test experimental protocol, developed at Cummins and used in this study, provides independent measurements of a variety of SCR catalyst functions. These include dependence of the instantaneous NOx conversion on NH3 coverage, as well as a number of steady-state characteristics, for example NO and NH3 oxidation. A commercial Fe-zeolite catalyst coated on cordierite honeycomb monolith, with cell density of 300 cells/in2, supplied by Johnson Matthey, was cut in to 2.5 cm diameter and 7.6 cm long cylindrical samples and is used in this experimental work. Its performance was investigated in the temperature range of 150-600°C at a fixed space velocity (GHSV) of 40,000 h-1. A typical reaction mixture contained 10 vol% O2, 8 vol% CO2, 7 vol% H2O, 200 ppm NOx (NO2/NOx=0 to 1, when used), 200ppm NH3 and balance N2. The concentrations of various exhaust species were measured by MKS FT-IR 2030 MultiGas Analyzer.

Results and Discussion

Several steady state NOx conversion studies reported that the catalyst NH3 storage capacity and NOx oxidation ability are two critical catalyst functions that will affect the rate of NOx reduction. The studied catalyst, Fe-zeolite, was found to have a large NH3 storage capacity, such that it took tens of minutes from the beginning of NH3 dosing to reach stead state surface coverage under SCR reaction conditions. During that period, instantaneous or dynamic NOx conversion efficiency was changing substantially as a function of NH3 coverage. Using NO only for NOx reduction, at relatively low catalyst surface coverage by NH3, NOx conversion increased with the coverage as expected. However, at higher NH3 surface coverage under SCR conditions, realized at low temperatures, the NOx conversion declined when approaching saturation. Independent measurements of NO oxidation showed that the studied catalyst has appreciable ability to generate NO2 in-situ under the reaction conditions, which is beneficial when the NO2/NOx ratio in the inlet gas composition is below the optimal value of 0.5. The decrease of NOx conversion with increase in NH3 concentration and the increase in NOx conversion with increase in NO2 concentration suggest that the in-situ NO oxidation is inhibited at higher levels of NH3 coverage, resulting in the decline of SCR efficiency. The employed experimental protocol also allowed us to distinguish between the ?dynamic? NH3 storage capacity reached at steady-state SCR conversion conditions, and ?total? NH3 storage capacity, achievable at the same conditions in the absence of the SCR reaction. The difference between the dynamic and total NH3 capacity can be very substantial, depending on the rate of the SCR and NH3 oxidation reactions. In this work we have also shown that pre-saturating the catalyst to an optimal level of NH3 storage at a given set of conditions allows us to reach the best NOx reduction efficiency more rapidly.

References

1. Grossale. A, Nova. I, Tronconi. E, Chatterjee. D, Weibel. M, J. Catal., 256, 312 (2008)

2. Olsson. L, Sjovall, H, Blint. R. J, Appl. Catal. B 81, 203 (2008)