The impact of ammonium nitrate species on low temperature NOx conversion over Cu/CHA SCR Catalyst

Paper #:
  • 2017-01-0953

Published:
  • 2017-03-28
Abstract:
Selective Catalytic Reduction of nitrogen oxides (NOx) with NH3 is a leading technology for lean-burn engines to meet the increasingly stringent environmental regulations worldwide. Among various SCR catalysts, Cu/CHA catalysts have been widely used in the industry, due to their desirable performance characteristics including their unmatched hydrothermal stability. While broadly recognized for their outstanding activity at or above 200oC, these catalysts may not show desired NOx conversion at lower temperatures. To achieve high NOx conversions it is desirable to have NO2/NOx close to 0.5 which is possible to achieve by DOC selection and optimization of its location. However even under such optimal gas feed conditions, sustained use of Cu/CHA below 200C leads to ammonium nitrate formation and accumulation leading to the inhibition of NOx conversion. In addition, the decomposition of accumulated NH4NO3 will lead to the formation of N2O, an undesired greenhouse gas. In this contribution, the formation and decomposition of NH4NO3 on a commercial Cu/CHA catalyst have been investigated systematically. First, the impact of NH4NO3 self-inhibition on SCR activity as a function of temperature and NO2/NOx ratios was investigate through reactor testing. Second, the formation of NH4NO3 was quantified through various adsorption and model reactions as well as its decomposition by the temperature-programed desorption (TPD). In addition the transient formation and accumulation of NH4NO under relevant process conditions were tracked by DRIFTS spectroscopy to identify its formation and accumulation mechanisms. This work provides insights into the limiting processes and SCR reactions at low temperatures that are important for addressing future low temperature NOx conversion needs.
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