Impact of Rh Oxidation State on NOx Reduction Performance of Multi-Component Lean NOx Trap (LNT) Catalyst

摘要

Typical Lean NOx Trap (LNT) catalyst composition includes precious metal components (Pt, Pd, and/or Rh), responsible for NO oxidation during lean operation and NOx reduction during rich operation. It was found that redox history of commercial LNT catalyst plays a significant role on deciding its NOx conversion under Lean/Rich cyclic condition. Further test had shown that fully formulated LNT catalyst being pre-reduced had shown much better NO reduction activity during the temperature-programmed reduction (TPRx) of NO than the same LNT catalyst being oxidized. The following study with Rh-only and Pt-only catalyst had demonstrated that Rh plays a key role on the large variation of the NO reduction function due to oxidation state change over LNT catalyst. Kinetic analysis of the NO reduction was performed in an attempt to elucidate the underlying mechanistic relationship, where it was found that NO reduction over reduced Rh can be well described by an Arrhenius equation with first-order dependence on NO concentration while the oxidized catalyst had been changing its surface redox state during NO reduction. The activation energy of the NO reduction process over reduced fully formulated LNT catalyst was found to be ～ 180±14kJ/mol, which is consistent with Rh-only catalyst but very different from Pt-only catalyst. The observed apparent activation energy of NO reduction on LNT catalyst was independent of the reductant used or the degree of hydrothermal aging either from field-aging or lab aging. These findings are consistent with NO dissociation being the rate-limiting step in the NO reduction process. The hydrothermal aging, redox state as well as the reductant type would only change the total number of sites available, active sites accessible as well as the surface coverage, respectively.