New Findings in Hydrothermal Deactivation Research on the Vanadia-Selective Catalytic Reduction Catalyst

Abstract

Considering the risks of hydrothermal deterioration in vehicles, power plants, and oceangoing vessels, V 2 O 5 -WO 3 /TiO 2 catalysts were subject to hydrothermal and thermal aging at 600, 625, 635, and 650 °C for 4–48 h. The different ratio and significant loss of active sites are main reasons for catalyst deactivation. Both Lewis and Brønsted acid sites are involved in the selective catalytic reduction reaction. Brønsted acid sites are more susceptible. High temperature plays a major role in the aging. It causes sintering, particle growth, and the anatase phase transition. Phase transformation turns out to be less important than sintering. Sintering leads to the reduction of the BET surface area, which in turn causes decrease of NH 3 adsorption amount and changes of active sites. Aging time can accelerate the degree of deactivation. It also helps to change the proportion of active sites. Water vapor has no significant effect on NO X conversion rates.

Hydrothermal Aging Mechanism and Modeling for SCR Catalysts

“…The change function of S1 in the hydrothermal aging process is given as follows f 1 ( T , t ) = prefix− 3.38 · ln nobreak0em.25em⁡ false( T − 200 false) − 0.300 · ln nobreak0em.25em⁡ t + 22.8 The change function of S2 in the hydrothermal aging process is given as follows f 2 ( T , t ) = prefix− 4.02 · ln ( T − 200 ) − 0.366 · ln nobreak0em.25em⁡ t + 27.3 The weighting coefficient of temperature m 1 is greater than the weighting coefficient of aging time m 2 , so in the process of hydrothermal aging, the influence of temperature is higher than the influence of aging time, which is consistent with our previous study . The different functions between the two active sites reflect their different sensitivities to hydrothermal aging.…”

“…Regarding the V 2 O 5 /TiO 2 catalyst, the Brønsted active site is a weak acidic site, while the Lewis acid site is a strong acidic site . Our previous study reported that in the process of hydrothermal aging, the deactivation rules below 350 °C and above 350 °C are obviously different . Therefore, the peak area below 350 °C is regarded as active site 1 (S1), and S1 is a combination of weak acidic sites and medium-strong acidic sites. , The peak area above 350 °C is regarded as active site 2 (S2), and S2 is the Lewis acid site.…”

“…In recent years, the models of V 2 O 5 /TiO 2 catalysts established by Yun et al, Lothongkum et al, Yu et al, Usberti et al, Beretta et al, Shin et al, Xiong et al, and Soleimanzadeh et al according to different research purposes and application scenarios are all single-active-site models. In our previous study, a hydrothermal aging mechanism based on dual active sites was proposed, so it is reasonable and necessary to establish a dual-active-site model. In addition, there are not many studies on the hydrothermal aging model of SCR catalysts.…”

Development of a Nonequilibrium Multisite Kinetic Model for NH3-SCR of NOx on CHA Cu-SAPO-34: Impact of Active Site Configurations and Locations

“…Thus, concerning NH 3 oxidation transient experiment, Figure 2, it is interesting to notice that the ramp of 10 °C min −1 makes the catalyst desorb a large amount of weakly stored NH 3 between 200 and 400 °C, which is usually observed for Cu-CHA during similar transient experiments. 37,97 Indeed, these steep temperature elevations avoid NH 3 oxidation (eqs 16,17) at low temperatures on corresponding active redox sites, W \which means that for each desorption peak, the N 2 concentration consequently drops until that NH 3 readsorbs on free active sites at the temperature step after 100 and 150 min. Here, the model does not completely represent this behavior, and only a small increase in NH 3 concentration is calculated at the beginning of each temperature ramp up to 350 °C.…”

“…Among all the NH 3 -SCR catalysts developed, copper (Cu) and iron (Fe)-exchanged microporous zeolites, and, in particular, small pore-sized chabazite (CHA)-based zeolites, have received much attention because of their high hydrothermal stability and SCR performance. , These zeolites are considered as a competitive alternative to the previous vanadia-based catalysts, − displaying good hydrothermal and sulfur resistance without employing rare metals. , Small-pore ( Ø = 0.38 nm) eight-membered ring (MR) CHA-based catalysts are constituted of a succession of tetrahedral SiO 4 , AlO 4 (SSZ-13 zeolite), and PO 4 (SAPO-34 zeolite) groups according to the zeolite chemical composition. This construction consequently forms a unique organization of cages between 8MR, 6MR, and 4MR interconnections, giving rise to different redox active site configurations within the zeolite lattice. − The active site configuration greatly depends on the catalyst synthesis route and has an important impact on overall SCR performance. , Typically, CHA catalysts are composed of two active sites of different nature: Brønsted and Lewis acid sites.…”