The Role of Impregnated Sodium Ions in Cu/SSZ-13 NH3-SCR Catalysts

Abstract

To reveal the role of impregnated sodium (Na) ions in Cu/SSZ-13 catalysts, Cu/SSZ-13 catalysts with four Na-loading contents were prepared using an incipient wetness impregnation method and hydrothermally treated at 600 • C for 16 h. The physicochemical property and selective catalytic reduction (SCR) activity of these catalysts were studied to probe the deactivation mechanism. The impregnated Na exists as Na + on catalysts and results in the loss of both Brönsted acid sites and Cu 2+ ions. Moreover, the high loading of Na ions destroy the framework structure of Cu/SSZ-13 and forms new phases (SiO 2 /NaSiO 3 and amorphous species) when Na loading was higher than 1.0 mmol/g. The decreased Cu 2+ ions finally transformed into Cu x O, CuO, and CuAlO x species. The inferior SCR activity of Na impregnated catalysts was mainly due to the reduced contents of Cu 2+ ions at kinetic temperature region. The reduction in the amount of acid sites and Cu 2+ ions, as well as copper oxide species (Cu x O and CuO) formation, led to low SCR performance at high temperature. Our study also revealed that the existing problem of the Na ions' effect should be well-considered, especially at high hydrothermal aging when diesel particulate filter (DPF) is applied in upstream of the SCR applications.Catalysts 2018, 8, 593 2 of 15 (>0.5%) mainly due to the decreased amount of isolated Cu 2+ formed CuO x clusters. Wang et al. [10] further found that alkali decreased the number of Brönsted acid sites and NH 3 coverage, and would decrease the SCR reaction rate. In our previous study [11], the different contents of Na impact on Cu/SAPO-34 were also studied. Except for the decrease of active sites and acidity, the results also showed the framework of Cu/SAPO-34 damaged and CuAlO x species formed at a high content of sodium (>0.8%), and all these factors hindered the SCR activity.In recent years, some studies have also considered the Na ions' effect on Cu/SSZ-13 catalysts, but most of them focused on co-cation Na ions. Gao et al. [12] investigated the effects of co-cation Na on Cu/SSZ-13 catalyst. They found ≈1.78% Na promoted SCR performance at low temperatures and helped to improve the hydrothermal stability of Cu/SSZ-13 because Na ions modified the redox of active sites and protected the framework of CHA structure. Zhao et al. [13] found a high amount of co-cation Na ions decreased the hydrothermal stability of Al-rich Cu/SSZ-13 at 750 • C for 5 h because the excess amount of Na ions weakened the interaction between Cu ions and the zeolitic framework and formed Cu x O species. Xie et al. [14] found one-pot-synthesized Cu/SSZ-13 with higher co-cation Na contents showed poorer hydrothermal stability at 750 • C for 16 h, which was attributed to Cu species with poor stability and CHA structure deterioration. Even though some achievements have been made on co-cation Na, the conclusion could not be applied in real-world applications because co-cation Na ions have already existed before Cu exchange and could not represent the deposition of Na in the ...

The Role of SO3 Poisoning in CU/SSZ-13 NH3-SCR Catalysts

“…NMR measurements were used to gain insight into the SO3 poisoning effect on the CHA framework, and hence, the chemical environment of 27 Al and 29 Si on F-Cu, S-Cu-0 and S-Cu-22 were measured. For he 27 Al NMR spectra (Figure 3a), one dominating peak at ~55 ppm is attributed to Al feature framework, while two peaks in the 29 Si NMR spectra ( Figure 3b) at −105 and −111 ppm are assigned to Si(3Si, 1Al) and Si(4Si, 0Al), respectively [5,15,34]. It is worth noting that sulfation at SO2 or SO3 resulted in almost no changes in the 27 Al NMR spectra, and as a result, identical 29 Si NMR spectra are shown in Figure 3b.…”

“…Considering the intact topological structure of SOx sulfated Cu/SSZ-13, Cu/SSZ-13 catalysts show better acid resistance than Cu/SAPO-34. In fact, as SSZ-13 have a desorption peak of NH3 at higher temperature compared with that of SAPO-34 (~420 °C for SSZ-13 vs. ~350 °C for SAPO-34) [15,39], SSZ-13 shows stronger acidity than SAPO-34. Similarly, with the mechanism of SO2 effect on zeolites, the stronger acidity of SSZ-13 help withstand SO3 with more acidity.…”

“…Actually, a number of essential conclusions and contributions have been made to properly understand the nature of copper ions and their impact on NH 3 -SCR mechanisms over Cu/SSZ-13 catalysts [6][7][8]. When facing real applications; however, the Cu/SSZ-13 catalysts are invariably affected by impurities, like sulfur, phosphorus, or alkali metal ions, and hence, a deep comprehension of their impacts on catalysts is very useful for their application [9][10][11][12][13][14][15]. 2 of 13 Among these impurities, sulfur poisoning is a common issue in SCR catalysts [16][17][18], and even when the amount of sulfur is quite low, sulfur exposure could result in serious deactivation of SCR activity [19].…”

Catalyst Deactivation, Poisoning and Regeneration

“…The complexity of catalyst poisoning obviously increases along with the increasing use of biomass/waste-derived/residual feedstocks [2,3] and with requirements for cleaner and novel sustainable processes, such as those implementing a catalytic assisted chemical looping approach [4,5].This Special Issue provides insight for several specific scientific and technical aspects of catalyst poisoning and deactivation, proposing more tolerant catalyst formulations and exploring possible regeneration strategies. In particular, 14 research articles focus on heterogeneous catalysts by investigating thermal [6-8], physical [9,10] and chemical [11][12][13][14][15][16][17][18][19] deactivation phenomena, and also exploring less conventional poisons related to the increasing use of bio-fuels [17]. Some regeneration strategies [11,16], together with solutions to prevent or limit deactivation phenomena [7,9,11,16], are also discussed.…”

“…In addition, alkali and alkali earth metals are often responsible for the severe deactivation of SCR catalysts, especially when treating exhaust from the combustion of renewable fuels. Indeed, those elements can potentially come from bio-fuels or urea solutions in diesel engines [17] or even from fossil fuels in thermal power plants burning carbon [18]. Accordingly, in [18] the authors investigate the impact of the deposition of sulphur containing sodium salts onto a V 2 O 5 -WO 3 /TiO 2 SCR catalyst with special regards to the NO removal rate as well as to the oxidation of SO 2 to SO 3 .…”