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 NOx 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% NOx conversion in ideal environment and 225-475 oC for 90% NOx 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 NOx 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.