Phosphorus modification to improve the hydrothermal stability of a Cu-SSZ-13 catalyst for selective reduction of NOx with NH3
Graphical abstract
Introduction
Various catalytic technologies have been developed to eliminate the emission of engines [[1], [2], [3]]. For diesel vehicles, the emission of hydrocarbons and carbon monoxide is controlled by a diesel oxidation catalyst, whereas the particulate matter is removed with a diesel particulate filter (DPF) [4]. Subsequently, nitrogen oxides (NOx) can be removed with either a lean NOx trap for light-duty vehicles, or a NH3 selective catalytic reduction (NH3-SCR) unit for heavy-duty ones [5,6]. Typically, the DPF requires regeneration in hot vapor at above 650 °C, inducing hydrothermal condition to the SCR catalysts in the downstream [[7], [8], [9]]. Therefore, a high hydrothermal durability is required for the SCR catalyst to achieve effective NOx emission control [[9], [10], [11], [12], [13]].
Cu−CHabazite (Cu−CHA) zeolites, including Cu-SSZ-13 and Cu-SAPO-34, have been successfully commercialized as catalysts for NH3-SCR reaction, to meet the stringent standards for diesel NOx emission in both North America and Europe, marking a significant breakthrough of catalytic technology in recent years [[14], [15], [16]]. Better hydrothermal stability has been shown with the Cu−CHA catalysts, as compared to the other zeolite-based catalysts, e.g., Cu-ZSM-5, Cu-Beta and Cu-Y [17]. One well-accepted explanation is that the unique topology and the small pore size in Cu−CHA zeolites prevent the detached Al(OH)3 moieties from escaping the CHA cavity [4,11].
Cu-SSZ-13 has been found inferior to Cu-SAPO-34 in the hydrothermal stability [18,19]. For the next generation Cu-SSZ-13 catalyst, stronger hydrothermal stability should be achieved to prolong the lifetime and lower the cost [17,[20], [21], [22], [23]]. One approach is to maintain the content of two adjacent framework Al sites (Al pairs), such as Al-O-Si-O-Al or Al-O-(Si-O)2-Al, stabilizing the isolated Cu2+ active sites in the six-membered rings (6MR) in the catalyst under hydrothermal conditions [[24], [25], [26], [27], [28]]. High content of the Al pairs requires a low Si/Al ratio, typically below 6 [22,29,30]. Nevertheless, the framework of the zeolite can be collapsed more easily with the low Si/Al ratio, caused the hydrothermal condition [31].
Phosphorus (P) is well known for improving the hydrothermal stability of H-ZSM-5 as well as the other zeolites with similar multidimensional 10 MR structures, such as ZSM-11, MCM-22, ITQ-13 and IM-5 [[32], [33], [34], [35], [36]]. Meanwhile, phosphorus in SAPO-34 has been proposed critical to stabilize the CHA type framework in hydrothermal condition [37]. It is therefore interesting to investigate the effect of P on the hydrothermal stability of the other CHA type catalyst, i.e. Cu-SSZ-13 with a low Si/Al ratio.
In this work, phosphorous is incorporated to modify the Cu-SSZ-13 catalyst with Si/Al ratio = 4. The effect of phosphorus on the framework structure, the cupric sites and the SCR performance will be discussed.
Section snippets
Chemicals
Copper(II) sulfate (CuSO4 < 99 wt.%), sodium aluminate (NaAlO2 < 98 wt.%) and phosphoric acid (80 wt.%) were purchased from Guangfu (Tianjin). Tetraethylenepentamine (TEPA < 98 wt.%,) and sodium hydroxide (NaOH < 98 wt.%) were purchased from Aladdin Industrial Corporation (Shanghai). Ammonium nitrate (NH4NO3 < 99 wt.%) was purchased from Yuanli (Tianjin). Colloidal silica (JN-30, SiO2 = 30 wt.%) was purchased from Haiyang (Qingdao). All the aqueous solutions were prepared using ultra-pure water
Effect of phosphorus on the SCR performance
The activity of the as-prepared and doped Cu-SSZ-13 was compared in Fig. 1a. The NOx conversion was around 20% with the as-prepared Cu-SSZ-13 at 150 °C, and increased rapidly to 78% at 200 °C. The conversion remained above 90% from 250 to 450 °C, then decreased with the further increase of the temperature, to approximately 77% NOx conversion at 550 °C. The P loading caused decrease of the activity at low temperatures, with the NOx conversion of only 67% and 56% at 200 °C for the P1Cu and P2Cu
Hydrothermal aging effect on the structure
The hydrothermal aging at 750 °C for 16 h destroyed the Cu-SSZ-13 structure completely for the sample without P loading (the sample Cu-H), according to the XRD pattern in Fig. 2a. The surface area and the pore volume of the sample lost almost completely as shown in Table 3.
It has been well documented that Brønsted acid sites (i.e., -Si-(OH)-Al-) in zeolites are the most vulnerable to H2O attack during hydrothermal aging, resulting in the collapse of the zeolite structure with dealumination [45
Conclusion
Doping with P as phosphate acid detach a part of TFAl to EFAl in the Cu-SSZ-13. Nevertheless, the formation of a framework silicoaluminophosphate interface retards the further dealumination of the Cu-SSZ-13 sample with phosphorus doping, retaining the CHA structure intact.
For the cupric sites, the isolated Cu2+ ions react partly with the phosphate, forming the Cu-phosphate species in the PxCu samples. The Cu-phosphate species may be inactive for NH3-SCR reaction due to the very high redox
Acknowledgments
This work was supported in part by the Program of Introducing Talents to the University Disciplines under file number B06006, and the Program for Changjiang Scholars and Innovative Research Teams in Universities under file number IRT 0641.
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