(498a) Bimetallic Silver Catalysts for the Reformate-Assisted Hydrocarbon Selective Catalytic Reduction of NOx

Introduction

Alumina supported Ag catalysts have been demonstrated to possess high activity for the hydrocarbon selective catalytic reduction of NO_x (HC-SCR). However, the rates are relatively low below 300°C.  While the addition of H₂ to the exhaust has been shown to improve low-temperature performance,[1]rates are still too low.[2] Platinum-group metals (PGM) are known to have high activities at low temperature; however, selectivities are poor and the temperature window is narrow.[3] The goal of research described in this paper was to investigate the influence of adding small amounts of Pt, Pd, and Rh to Ag/γ-Al₂O₃. 

Materials and Methods

A series of catalysts containing Pt, Pd or Rh, and Ag on Al₂O₃ was prepared using the incipient wetness method. The metals were added via co-impregnation with aqueous metal salt solutions. The target loadings were 2 wt% for Ag and 0.5% for Pt, Pd or Rh.  We focused on performance at temperatures between 200 and 400°C, and atmospheric pressure.  The feed composition consisted of 600 ppm NO, 800 ppm C₃H₆, 1600 ppm CO, 1600 or 3200 ppm H₂, 10% O₂, 4% H₂O, 8% CO₂, and Ar as balance. A NO_x analyzer was employed to measure the NO and NO₂ content. A Varian micro-GC was used to measure the amounts of CO, CO₂, H₂, C₃H₆, N_2, O₂, and N₂O in the effluent.

Results and Discussion

The NO_x conversions with 0.16% or 0.32% H₂ are displayed in Figure 1. In the absence of H₂, NO_x light-off does not occur until 400^oC. [4] For Ag/Al₂O₃, the addition of H₂ caused the light-off temperature to decrease by 200^oC and the maximum NO_x conversion occurred at 400^oC. The behavior of this catalyst is in agreement with reports by Burch et al., who observed a 200^oC decrease in light-off for NO_x reduction when H₂ was added (0.72%).⁴ Doubling the H₂ increases the NO_x conversion due to increased conversion of the C₃H₆.The conversion of C₃H₆ is key because the bulk of NO_x reduction activity comes from reactions between the hydrocarbon and inlet NO_x over the catalyst in the presence of O₂. For the bimetallic catalysts at 0.16% H₂, NO_x conversion is enhanced on Ag-Pd/Al₂O₃ and Ag-Rh/Al₂O₃ at 200^oC, but not on Ag-Pt/Al₂O₃. At 300^oC, Ag-Pd/Al₂O₃ and Ag-Pt/Al₂O₃ show an increase in NO_x conversion, and Ag-Rh/Al₂O₃ does not change. CO may be inhibiting the conversion of NO_x for Ag-Rh/Al₂O₃ between 200^oC and 300^oC, whereas the C₃H₆ oxidation is taking precedence over CO oxidation on Ag-Pd/Al₂O_3. At 400^oC, NO_x conversion for Ag-Pd/Al₂O₃ does not change, increases slightly on Ag-Pt/Al₂O₃ and increases significantly on Ag-Rh/Al₂O₃. The increase in NO_x conversion for Ag-Rh/Al₂O₃ is likely due to the absence of CO. CO is likely completely consumed before the C₃H₆, so additional NO_x reduction is occurring with the presence of the C₃H₆. When 0.32% H₂ is added, it is observed that at 200^oC there is an improvement in NO_x conversion. A significant drop is observed at 300^oC, likely due to promotion of CO oxidation at this temperature. CO effectively competes with C₃H₆ for oxygen at 300^oC. The amount of H₂ seems to play a role in the degree of oxidation, as this behavior is not observed with 0.16 % H₂ added. The NO_x conversions recover at 400^oC because CO is completely gone, allowing any remaining C₃H₆ to react with the oxygen and initiate the SCR reaction.

Figure  SEQ Figure \* ARABIC 1: The NO_x consumption rates for a series of Al₂O₃-supported Ag-M catalysts (M=% Pt, Pd or Rh). Conditions: 600 ppm NO, 800 ppm C₃H₆, 1600 ppm CO, 1600 or 3200 ppm H₂, 10% O₂, 4% H₂O, 8% CO₂, and Ar as balance. Target loadings were 2 wt% Ag and 0.5% Pt, Pd or Rh.

Significance

New EPA standards for NO_x emissions that are scheduled to go into effect in 2010 for on-road vehicles and 2015 for off-road vehicles will require an 80% reduction in emitted NO_x compared to 2005 levels.[5] Achieving these standards will require new after-treatment systems and catalysts. NO_x SCR with hydrocarbons and H₂ has proven to be a promising strategy, especially if the reformate can be produced on-board the vehicle so that a single tank can be used.

[2] Breen et al., Applied Catalysis B: Env., 70 (2007), 36-44.