Development of Low Temperature Selective Catalytic Reduction (SCR) Catalysts for Future Emissions Regulations

Abstract

<div class="section abstract"><div class="htmlview paragraph">A series of novel metal-oxide (TiO<sub>2</sub>, TiO<sub>2</sub>-SiO<sub>2</sub>)-supported Mn, Fe, Co, V, Cu and Ce catalysts were prepared by incipient wetness technique and investigated for the low-temperature selective catalytic reduction (SCR) of NO<sub>x</sub> with ammonia at industrial relevantly conditions. Among all the prepared catalysts, Cu/TiO<sub>2</sub> showed superior de-NO<sub>x</sub> performance in the temperature range of 150-200 °C followed by Mn/TiO<sub>2</sub> in the temperature range of 200-250 °C. The Ce/TiO<sub>2</sub> catalyst exhibited a broad temperature window with notable de-NO<sub>x</sub> performance in the temperature regime of 250-350 °C. The phyico-chemical characterization results revealed that the activity enhancement was correlated with the properties of the support material. All the anatasetitania-supported catalysts (M/TiO<sub>2</sub> (Hombikat)) demonstrated significantly high de-NO<sub>x</sub> performance above 150 °C. Rutile-phase TiO<sub>2</sub> leads to smaller surface areas and decreased reactivity in gas-solid reactions, whereas anatase-phase TiO<sub>2</sub> (Hombikat) is a highly active carrier that can help the active element highly disperse onto the catalyst surface. This behavior was attributed to the markedly stronger support interaction of crystalline titania compared with titaniasilica support. As a result of the weak support interaction, the surface metal oxide species supported on silica-titania tended to agglomerate when exposed to higher temperatures under SCR conditions. This agglomeration suppressed when titania rich support used as the carrier. Our H<sub>2</sub>-TPR studies illustrate that the decreasing amount of SiO<sub>2</sub> in the TiO<sub>2</sub>−SiO<sub>2</sub> support promotes the formation of surface MnO<sub>2</sub> (Mn<sup>4+</sup>) species. Conversely, the decreasing amount of SiO<sub>2</sub> in the TiO<sub>2</sub>−SiO<sub>2</sub> support inhibits the formation of surface Mn<sub>2</sub>O<sub>3</sub> (Mn<sup>4+</sup>) and MnO (Mn<sup>2+</sup>) species, which is evident from the reduction peaks (T<sub>2</sub> and T<sub>3</sub>) shift to higher temperatures. Our NH<sub>3</sub>-temperature programmed desorption (NH<sub>3</sub>-TPD) studies illustrate that the number of surface Lewis acid sites slightly increase and the surface Brönsted acid sites monotonically increase as the SiO<sub>2</sub> content increase in the TiO<sub>2</sub>−SiO<sub>2</sub> support. The evolution of a new ammonia desorption peak at high temperature indicates the formation of isolated Brönsted acid sites in Si rich M/TiO<sub>2</sub>-SiO<sub>2</sub> samples. The introduction of SiO<sub>4</sub> tetrahedral network into the TiO<sub>2</sub> crystallites may lead to the reduction of elemental number in crystal grain, network strain disintegration and deviation of adjacent oxygen atoms.Interestingly, Ce/TiO<sub>2</sub> and Ce/TiO<sub>2</sub>−SiO<sub>2</sub> samples exhibited strong inhibition for the unselective ammonia oxidation reaction. Future low emissions (LD Tier-3 regulations) standards for light duty diesel and gasoline engine vehicles are forcing automobile and catalyst manufacturers to focus on reducing cold start and low load (California 2020HD regulations) NO<sub>x</sub> emissions. These legislations implement very stringent limits on NO<sub>x</sub> emissions and trigger a renewed attention in NO<sub>x</sub> mitigation at low temperatures. Light duty diesel and gasoline vehicles will require >95% NO<sub>x</sub> conversion over the Federal Test Procedure (FTP) to meet future Tier-3 standards. This requirement is especially demanding for the low temperature exhaust applications that mostly operate in the 100-350°C temperature regime (cold start). Low temperature SCR catalyst systems would be a good alternative solution to avoid all the problems associated with the existing commercial catalytic systems during the cold start and low idle conditions. In the present work, we have developed several low temperature catalysts and investigated for potential catalytic reduction of NO<sub>x</sub> in the temperature range between 100 and 500 °C. The novel, highly active nano structured metal oxide formulations are able to represent the innovative and most attractive solution for the reduction of NO<sub>x</sub> emissions during cold start low idle conditions.</div></div>