Physicochemical properties and ammonia selective catalytic reduction reaction performance of copper-based molecular sieve catalysts under hydrothermal aging at different temperatures and atmospheres
摘要
To illuminate the hydrothermal aging mechanism of Cu-SSZ-13 catalysts, catalytic performances and physicochemical properties of selective catalytic reduction (SCR) samples at different aging temperatures (600 ~ 900 °C) and water vapor contents (WVC, 5 ~ 20 V%) were investigated via catalyst sample testing device and micro characterization tests. The impacting laws of inlet conditions on SCR reaction progress and axial reactor performance were revealed by one-dimensional reactor models. The results show that both the increasing aging temperature and the decreasing WVC induced the collapse of molecular sieve structure and the decrease of CHA diffraction peak values. The Cu2+ inside molecular sieve diffused to catalyst surface and aggregated to form CuxO species that blocked the pore channels. The decreases in specific surface area and pore volume, as well as the increase in pore size, weakened the catalytic and adsorption capabilities. The active site amount and storage capacity of NH3 also decreased, which increased the transient catalytic response speed and the corresponding side reaction rate. Thus the NO conversion rate was significantly reduced. Overall, the effects of aging temperature on catalytic performance and physicochemical property were significant than that of aging atmosphere. Additionally, with the increasing axial position, the increase slope of conversion rates of NOx and NH3 reduced at both fast and standard SCR progress because the lowered NH3 concentration in the second half decreased the adsorption rate in catalyst surface. The increasing ammonia/nitrogen ratio (ANR) and the decreasing space velocity (SV) also strengthened the SCR catalytic efficiency. The NO2/NOx range during 40 ~ 60% further promoted the catalytic progress due to the dominant role of fast SCR reactions on the catalyst surface. Interestingly, the aging sample B exhibited stronger catalytic and adsorption activities than fresh sample (at the temperature of 600 °C and WVC of 10 V%). It meant that moderate degree of hydrothermal aging could unblock partial molecular sieve pores and optimize physicochemical parameters. The research results can provide theoretical guidance for developing high-efficiency and long-life cycle catalysts with strong resistance to hydrothermal aging.