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

Practical catalysts that work well at a wide operation window for selective catalytic reduction of NO by NH (NH-SCR) are essential for the purification of non-isothermal emission such as vehicle exhaust. However, NH-SCR catalyst with high low-temperature performance has excellent NO activation and oxidation ability, leading inevitably to NH-intermediates over-oxidation and N selectivity deterioration at high operation temperatures. By far the best performance ceria-based catalyst with a super-wide temperature window of 175-400 C for 90% NO conversion in ideal environment and 225-475 C for 90% NO conversion by addition of 50 ppm SO and 5% HO is obtained via distributing phosphate over the outer of ceria. NH 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. NH was prone to be captured by the surface P-OH network, forming NH species, avoiding being oxidized and contributing to both low and high temperature activity. NO can also be readily absorbed and oxidized to the absorbed NO(ad) species over phosphate as reflected by in situ DRIFTS and DFT calculation, providing a facile pathway for 'fast SCR' by reacting with NH species to form N and HO. The reaction followed the L-H mechanism and contributed to catalytic activity under 300 C. This directional structure fabricate strategy helps to increases the NO conversion and N selectivity under a broaden temperature window. The enriched Brønsted acid sites over phosphate treated ceria were also demonstrated to have largely suppressed SO 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.