Bimetallic Silver Catalysts for the Reformate-Assisted Selective Catalytic Reduction of NOx.

Abstract

The objective in this dissertation was to investigate a strategy to improve the low- temperature performance of Ag/Al2O3 for hydrocarbon selective catalytic reduction of NOx. Two approaches were studied. The first approach was to add H2 into the reactant stream. The second approach was to impregnate Pd, Pt, or Rh onto the Ag surface. Specifically, the two approaches were combined to maximize the catalytic performance. A series of bimetallic catalysts were synthesized and a full factorial design was conducted to determine the conditions in which the low temperature performance of the Ag/Al2O3 catalyst would be maximized. These factors included (1) HC/NOx ratio; (2) H2/CO ratio; (3) second metal loading; (4) second metal type; and (5) temperature. The NOx conversion and N2 selectivity generally decreased as the loading of the second metal increased. The NOx conversion was affected by second metal type specifically at 300°C, in which the order of performance was Pd < Pt < Rh. In response to the detrimental effect of second metal loading, the loading order was switched. The loading order independently did not significantly affect the NOx conversion or N2 selectivity. However, the loading order did affect NOx conversion and N2 selectivity performance with regards to Pd. It was hypothesized that the detrimental effect of the second metal on the NOx conversion was caused by the unselective combustion of the C3H6 to CO2. With regard to the N2 selectivity, the platinum group metal catalysts are known to produce N2O. It was believed that the PGMs did not interact with the Ag. In regards to the loading order, the interaction between Ag and Pd was significant. The NOx conversion and N2 selectivity improved when Pd was added before Ag. In summary, the addition of the PGMs did not improve the NOx conversion or N2 selectivity even in the presence of H2. The presence of the PGMs enhanced the unselective combustion of the hydrocarbon to CO2 and increased production of N2O, inhibiting both the conversion and selectivity. Switching the loading order resulted in an improvement in performance for the Pd bimetallic catalysts.