NO<sub>2</sub>–NH<sub>3</sub> SCR over Activated Carbon: A Combination of NH<sub>4</sub>NO<sub>3</sub> Formation and Consumption
Xiao Zhu, Liqiang Zhang, Yong Dong, Chunyuan Ma
Abstract
Selective catalytic reduction (SCR) in the presence of NH3 with different NO2/NOx ratios over activated carbon (AC) was investigated. NOx conversion at NO2/NOx = 1.0 was higher than that at NO2/NOx = 0 and 0.5 in the temperature range of 90–210 °C. NO2 was easily reduced to NO over AC, resulting in a steady-state NO2/NOx ratio in the NO2–NH3 SCR process, which is smaller than the feed NO2/NOx ratio set to 1.0. The results manifested that NH4NO3 was deposited on AC when the temperature was lower than 150 °C. When the reaction temperature was higher than or equal to 150 °C, no NH4NO3 deposition occurred, and NOx conversion eventually reached a steady state. Steady-state NOx conversion increased with the reaction temperature, reaching a maximum of 79.5% at 210 °C. Transient reaction analysis results showed that NH4NO3 could react with NO, and the reaction rate increased gradually with the increase in temperature. The results of in situ diffuse reflectance infrared Fourier-transform spectroscopy also indicated that NO could react with NH4+, confirming that NH4NO3 was an important immediate during the NO2–NH3 SCR process. The mechanism of NO2–NH3 SCR could be summarized as a combination of NH4NO3 formation from NH3 and NO2 and NH4NO3 consumption by NO and C(*). This study developed a systematic mechanism of NO2–NH3 SCR over AC and provided a basis for AC-based catalysts at low temperatures for NOx removal.