In 2013, a team from NREL, Oak Ridge National Laboratory, Ford, MECA (Manufacturers of Emission Controls Association) and the University of Tennessee - Knoxville published an SAE paper investigating the impact of alkali (e.g., Na and K) and alkaline earth metals impurities in a light-duty application.

They found that alkali metals are volatile at temperatures typically seen in diesel exhaust, allowing them to migrate into the catalyst washcoat leading to catalyst deactivation. However, they also found that there was sufficient unaltered catalyst volume to maintain mandated NO_x emission levels on the dynamometer after the simulated equivalent of 150,000 miles of exposure. I.e., by loading enough catalyst into the system, the deactivation problem can be managed.

In the new work out of China, the researchers note that:

Although catalysts simultaneously suffer severe deactivation by the two poisons under practical conditions, efforts have typically focused on one or the other individually, because it is extremely difficult to protect catalysts simultaneously against poisoning from “basic” alkali metals and “acidic” SO₂ molecules.

—Huang et al.

• Alkali poisoning occurs when catalytically active sites are blocked by the alkalis. One solution for this has been found by designing catalysts with rich alkali-trapping sites independent from catalytically active sites.

• Sulfur-poisoning predominantly originates from the preferential adsorption of SO₂ on catalysts’ surfaces and subsequent oxidation. One possible strategy is to develop an “acidic” catalyst with a weak adsorption ability to acidic SO₂ molecules.

Catalysts with acidic V₂O₅ as an active component are proven to have an excellent resistance against SO₂ poisoning than other “basic” metal oxides such as M_nO_x, CuO, CeO₂, and so on, although the vanadium-based catalysts are prone to poisoning by alkalis due largely to their strong acidity.

… In this work, we develop a V₂O₅/HWO catalyst with strong resistance simultaneously to alkalis and SO₂ poisoning. Two acidic oxides with strong resistance to SO₂ poisoning, V₂O₅ and HWO, are designed as an active component and a support, respectively. The HWO have rich size-suitable alkali-trapping sites with high specificity, and can professionally trap alkalis in the presence of high-concentration SO₂, even though alkalis are initially accumulated on the V₂O₅ surfaces under normal SCR conditions.

—Huang et al.

Testing showed that the V₂O₅/HWO catalyst exhibited strong resistance to alkali poisoning, and the catalysts with high K⁺ loading of 350 μmol g⁻¹ did not decrease their high SCR activity even in the presence of the high-concentration SO₂, whereas conventional V₂O₅/WO₃−TiO₂ catalysts almost completely lost SCR activity under the same conditions.

The experimental results coupled with theoretical calculations demonstrated that the strong resistance of the V2O5/HWO catalysts to alkali and sulfur poisoning mainly originated from the specific alkali- trapping sites of the HWO. Alkalis accumulated on the catalytically active surface sites of the V₂O₅/HWO catalysts could spontaneously migrate into the HWO tunnels during the SCR reactions, and arrived at a separate state from the catalytically active sites, thus leading to simultaneous resistance to alkalis and SO₂ poisoning. Therefore, the V₂O₅/HWO catalysts with a hexagonal structure of WO₃ are promising candidates for controlling NO_x emissions from the stationary source and the mobile source against alkali and sulfur poisoning.

—Huang et al.

Resources

• Zhiwei Huang, Hao Li, Jiayi Gao, Xiao Gu, Li Zheng, Pingping Hu, Ying Xin, Junxiao Chen, Yaxin Chen, Zhaoliang Zhang, Jianmin Chen, and Xingfu Tang (2015) “Alkali- and Sulfur-Resistant Tungsten-Based Catalysts for NOx Emissions Control” Environmental Science & Technology doi: 10.1021/acs.est.5b03972

• Yasser Jangjou and William Epling (2015) “Sulfur Poisoning of the NH₃-SCR Reaction over Cu-SAPO-34” 2015 AIChE Annual Meeting

• Williams, A., Burton, J., McCormick, R., Toops, T. et al. (2013) “Impact of Fuel Metal Impurities on the Durability of a Light-Duty Diesel Aftertreatment System,” SAE Technical Paper 2013-01-0513 doi: 10.4271/2013-01-0513