Phosphate on ceria with controlled active sites distribution for wide temperature NH₃-SCR

Practical catalysts that work well at a wide operation window for selective catalytic reduction of NO_x by NH₃ (NH₃-SCR) are essential for the purification of non-isothermal emission such as vehicle exhaust. However, NH₃-SCR catalyst with high low-temperature performance has excellent NO activation and oxidation ability, leading inevitably to NH₃-intermediates over-oxidation and N₂ selectivity deterioration at high operation temperatures. By far the best performance ceria-based catalyst with a super-wide temperature window of 175-400 ^oC for 90% NO_x conversion in ideal environment and 225-475 ^oC for 90% NO_x conversion by addition of 50 ppm SO₂ and 5% H₂O is obtained via distributing phosphate over the outer of ceria. NH₃ protection strategy is the key for keeping high-temperature activity. Brønsted acidity surged as the formation of P-OH network via a charge compensatory mechanism of phosphate. NH₃ was prone to be captured by the surface P-OH network, forming NH₄⁺ species, avoiding being oxidized and contributing to both low and high temperature activity. NO can also be readily absorbed and oxidized to the absorbed NO₂(ad) species over phosphate as reflected by in situ DRIFTS and DFT calculation, providing a facile pathway for 'fast SCR' by reacting with NH₄⁺ species to form N₂ and H₂O. The reaction followed the L-H mechanism and contributed to catalytic activity under 300 ^oC. This directional structure fabricate strategy helps to increases the NO_x conversion and N₂ selectivity under a broaden temperature window. The enriched Brønsted acid sites over phosphate treated ceria were also demonstrated to have largely suppressed SO₂ adsorption, which significantly slowed down the catalyst poisoning. A dynamic equilibrium between the poisoning and regeneration process can be achieved according to the shrinking-core model for each nanosphere, leading to the excellent resistance.