Ca- and P-Induced Local Microenvironment Modulation of Redox and Acidic Sites for NO_<i>x</i> Catalytic Reduction

The presence of alkali and alkaline earth metals, as well as the increased levels of phosphorus, in biodiesel exhausts poses a great challenge to the durability of emission control catalysts. Thus, a mechanistic understanding of these poisoning effects individually and in combination is crucial for addressing these problems. Herein, we report calcium (Ca)- and phosphorus (P)-induced local microenvironment modulation of the redox and acidic sites on a V₂O₅-WO₃/TiO₂ catalyst for NO_<i>x</i> reduction. Phosphorus and calcium poison the catalyst and tune the local microenvironments of redox V-O and acidic W-O sites in different ways. Phosphorus poisons the catalyst primarily via impairing the functions of the V-O sites (e.g., redox property and polymerization state) by binding P to the VO_<i>x</i> species, forming surface phosphate and crystalline VO_<i>x</i>. Calcium, on the other hand, decreases the activity mainly through direct bonding with the acidic W-O sites, which significantly alters the initial W⁶⁺-O-V⁴⁺ local electronic structure and dramatically decreases its ability to adsorb and activate NH₃ due to the diminished active VO_<i>x</i> species and weakened redox functionality. In the presence of both Ca and P, the crystalline vanadia species are depolymerized, and the Ca-bonded tungsten species are released, restoring the initial local microenvironment of the acidic WO₃ and redox V₂O₅ sites for SCR catalysis. These findings are supported by our DFT calculations, where Ca-P copoisoning results in a more favorable thermodynamics due to calcium phosphate formation. This work provides new insights about the deactivation mechanism of vanadia-based SCR catalysts in exhausts containing multiple poison components, and these insights would benefit the design of more durable NO_<i>x</i> reduction catalysts for biodiesel applications.