Preparation Of Wire-mesh Honeycomb Catalysts And Selective Catalytic Reduction Of NO_x With Propene Under Lean-burn Conditions

Metallic monoliths offer plenty of advantages over ceramic monoliths, such as stronger mechanical strength, higher thermal conductibility and lower pressure drop, and now have become most attractive supports in automotive emissions controls. However, the metallic monoliths could not be applied in practice because of the low specific surface area. It is necessary to deposit the ceramic oxide washcoat with high surface area over metallic monoliths. Since thermal expansion coefficient of the metallic support is different from the ceramic oxide washcoat, the adhesion of washcoat is poor which leads to the washcoat chap and flake away. Otherwise, metallic monoliths are expensive due to complicated manufacturing and the high price of metal materials. Recently, lean-bum engines have become the main option for the vehicles due to lower consumption of fuel and lower emission. However, the exhaust from the lean-burn engines contains a large amount of oxygen, the three-way catalysts have been proved to be not efficient on removing NO_x. The selective catalytic reduction of NO_x by hydrocarbons (HC-SCR) has attracted considerable interest as a method to control emissions from engines operated under lean-burn conditions. Nevertheless, the rather high reaction temperature and narrow window are the main drawbacks of these catalysts to be used under lean-burn conditions. Specially, supported precious metal catalysts exhibit low selectivity of N₂, N₂O being formed in substantial amounts.In this study,γ-Al₂O₃ washcoat on wire mesh was prepared by electrophoretic deposition and selective catalytic reduction of NO_x by propene over the wire-mesh honeycomb catalysts was performed under lean burn conditions. The main works are as following:(1) Washcoat deposited on metallic wire mesh was prepared usingγ-Al₂O₃ powders by electrophoretic deposition. The preparation parameters of washcoat were investigated, such as the deposited solution, the deposited voltage, the deposited time, Zeta potential and the amount of polyscylic acid. The alumina powders were dispersed in ethanol and pH was about 8.8. Then, polyscylic acid was added as additives and its concentration was 2.035 mg·L^-1. Finally, the suspension was mixed in an ultrasonic bath for 40 min. The suspension was stable and suitable for the electrophoretic deposition experiment. Under this condition, a uniformly thickness alumina washcoat on metallic wire mesh was prepared under 10 V for 10 min.In order to improve the washcoat adhesion, wire meshes were dipped in HCl solution for wiping off the dirts and enhancing the cohesion between the wire mesh and washcoat. When the concentration of aluminum isopropoxide was 0.27 g·L^-1, the sample exhibited excellent adhesion onto the support in thermal shock, losing only 5.0 wt.% after 13 times of thermal shock. The ability of thermal resistant and vibration resistant was improved by adding aluminum powders and higher calcinations temperature, and the phase structure of washcoat doesn't change due to increase temperature. SEM showed that alumina washcoat was even and no chap. XRD revealedγ-Al₂O₃ was main monocrystalline presented in the coating. In addition, after the samples were immersed in lanthanum nitrate solution, the loss of washcoat decreased and it was 44.0 wt.% for 35 min in ultrasonic test.(2) After depositing Al₂O₃ washcoat, the wrie-mesh honeycomb was manufactured by stacking alternatively corrugated and plain wire meshes. The calculation results showed that the wire-mesh honeycomb had more open frontal area (OFA) and geometric surface area (GSA) than cordierite monolith with 50 cpsi. The GSA of wire-mesh honeycomb decreased 25% relative to thin wall cordierite monolith with 400 cpsi. But the pressure drop of wire-mesh honeycomb didn't increase due to the increasing of GSA. The manufacture process of wire-mesh honeycomb was simply. Metallic wire mesh was adopted to pack the honeycomb and it didn't need to solder the joining, which could reduce the price of wire-mesh honeycomb.(3) The selective catalytic reduction of NO_x by C₃H₆ was investigated over Pd/Al₂O₃, Pd/TiO₂/Al₂O₃, Pd/CeZr/TiO₂/Al₂O₃ wire-mesh honeycomb prepared under lean-burn conditions. The results showed that 0.23%- Pd/CeZr/TiO₂/Al₂O₃ wire-mesh honeycomb catalyst exhibited high catalytic activity at low temperature over a broad temperature range and N₂O was not detected in measurable quantities in the outlet flow and other byproducts containing nitrogen were not detected. The results of temperature-programmed desorption experiments in N₂ and in C₃H₆-O₂-N₂ mixture suggested that there were two storage site of ad-NO_x on Pd/CeZr/TiO₂/Al₂O₃ wire-mesh honeycomb catalyst, one was TiO₂ and another was CeZr. Except of the storage site, TiO₂ and CeZr also participated in selective catalytic reduction reaction. The effect of Pd content, oxygen, space velocity and reaction time on the catalytic activity of Pd/CeZr/TiO₂/Al₂O₃ wire-mesh honeycomb catalyst was further investigated.(4) Comparing with pellet catalyst, cordieilte monolith catalysts with 50 and 400 cpsi, wire-mesh honeycomb catalyst exhibited highest activity of removal NO_x at low temperature. In unsteady-state experiment, conversion of NO_x over wire-mesh honeycomb catalyst was quickly stabilized in short time after temperature elevation, though that of ceramic monolith catalyst was slowly increased and it should be taken more time for reaching a steady-state conversion. Furthermore, the light-off temperature of propene was lowest.In conclusion, the firm alumina washcoat on wire mesh honeycomb was prepared by electrophoretic deposition. Pd supported on wire-mesh honeycomb showed better activity for selective catalytic reduction of NO_x by propene than ceramic monolithic catalysts under lean burn condition at low temperature, which provide the theoretical and experimental basis for the application of wire-mesh honeycomb in automotive exhaust treatment.