Study On Synergistic Effect Of Bimetal-promoted Zeolites Catalysts Applied In Diesel Engine Urea-SCR

The rapid growth of diesel engine production and ownership together with increasingly stringent emission regulation has triggered more awareness on NOx reduction technologies all around the world, among which selective catalytic reduction (SCR) is intended as one of the most promising techniques for the abatement of NOx from diesel engines. Note that R&D of environmently friendly and efficient catalyst is of critical importance to the automotive SCR technology. In this paper, we prepared bimetal-promoted zeolites catalysts: CuZr/ZSM-5 and MnZr/ZSM-5 via ion exchange method. We investigated the synergistic effect of Zr loading on bimetallic catalysts DeNOx performance as well as their physic-chemical characteristics. Main conclusions from this study are as follows:1. Results from catalytic activity tests over bimetallic catalysts indicated that doping of zirconium could enhance the low-temperature activity, because the temperature window has been shifted toward low temperatures. More specifically, T₅₀ and T₉₀ of bimetallic catalysts compared with monometal ones, have decreased ten degrees for Cu-based catalysts, while forty to fifty degrees for Mn-based catalysts.2. The micro-structure of bimetallic catalysts combined with their physic-chemical properties showed that the incorporation of zirconium promoted the dispersion of manganese oxides on catalysts surface and prevent its crystallization, thus ensuring active phases enrichment on ZSM-5 surface. What's more, the interactions between metal ions on catalysts surface were intensified due to the addition of zirconium, which also adjusted their reducibility demonstrated by more hydrogen consumption. These results as a whole could verify the promoting effect of zirconium doping on catalytic performance, especially in the low temperature range.3. The roles of bimetallic catalysts CuZr/ZSM-5 and MnZr/ZSM-5 played in SCR process could be depicted as:(ⅰ)Mn³⁺/Cu⁺ is preoxidized to Mn⁴⁺/Cu²⁺ by oxygen;(ⅱi)Oxygen in the atmosphere then reacts with NO to form intermediate in the form of nitrate and nitrite (Mn⁴⁺-N_xO_y/Cu²⁺-N_xO_y); (ⅲ) The resulting volatile products are able to react with ammonia to produce nitrogen and water, which reduces Mn⁴⁺/Cu²⁺ to Mn³⁺/Cu⁺. 4. The adverse effect of H₂O and SO₂ on catalytic performance is irreversible. Compired to monometal catalyst, bimetallic catalyst CuZr/ZSM-5 possesses better anti-sulfur property, which could be attributable to ZrO₂ on the catalyst surface. Because ZrO₂ could modify the catalysts surface Ph value, consequently preventing the adsorption and accumulation of SO₂ on the catalyst surface.