Understanding the Water Effect for Selective Catalytic Reduction of NOx with NH3 over Cu-SSZ-13 Catalysts

Abstract

The catalytic activity of commercial Cu-SSZ-13 monolith catalysts was measured in the presence and absence of water, which demonstrated an inhibiting role below 300 °C and a promoting role above 300 °C. Furthermore, temperature-programed desorption illustrated that hydrolysis of Z2Cu would take place to generate ZCuIIOH and [Cu(H2O)6]2+ species, which inhibited ammonia adsorption over Lewis acid sites but promoted nitrate formation and gaseous NO2 desorption. The temperature-programed surface reaction and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) were further performed to elucidate the influence of water on reaction mechanisms. Basically, the NH3-SCR reaction mainly took place at Cu centers acting as Lewis acid sites of Cu-SSZ-13 catalysts in the low-temperature regime. Water inhibited the reaction by occupying and blocking Lewis acid sites to prevent [Cu(NH3)2]2+ formation and NO oxidation from occurring, limiting the formation of NH4NO3 intermediates. In the high-temperature regime, gaseous NO2 could directly interact with NH4+ molecules adsorbed onto Brønsted acid sites. The addition of water could facilitate the production of gaseous NO2 decomposed by surface nitrates and further promote the NH3-SCR reaction rate at high temperatures.