Properties and Roles of Adsorbed NH3 and NOx over Cu/SAPO-34 Zeolite Catalyst in NH3-SCR Process

To investigate the adsorption properties and roles of different feed gases in the selective catalytic reduction by ammonia (NH3-SCR), the adsorption sites, strength, and amount as well as reaction rates of NH3 and NOx (a mixture of NO and NO2) on exchanged Cu/SAPO-34 (chabazite zeolite) catalyst were studied. Transient response, temperature programmed desorption (TPD), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments were performed to characterize the catalyst. Transient response experiments showed that NH3 was adsorbed by the catalyst. TPD and DRIFTS indicated that NH3 can be adsorbed at both Bronsted and Lewis acid sites to form various NH3 adsorption species that show different SCR activities. The adsorption rate of NH3 by Cu2+ cations was the fastest and the adsorption bond strength of NH3-Cu2+ was the strongest between NH3 and the Cu/SAPO-34 catalyst. NOx can be oxidized and stored as nitrates and nitrites on the Cu catalyst. The intermediate species formed at Cu active sites during the NH3-SCR reaction are discussed, allowing SCR reaction mechanisms to be inferred.