Rh/ZrP2O7 as an Efficient Automotive Catalyst for NOx Reduction under Slightly Lean Conditions

The three-way catalyst performances of honeycomb-coated Rh catalysts were studied on several metal phosphate supports (AlPO4, YPO4, ZrP2O7, and LaPO4) using various simulated exhausts with different air-to-fuel ratios (A/F). These supports significantly improved the NOx purification (deNOx) efficiency under slightly lean conditions (14.6 < A/F ≤ 15.3) as compared with conventional Rh/ZrO2 catalysts. The deNOx activity exhibited the following sequence of increasing the mean electronegativity of the supports: ZrO2 < YPO4 < LaPO4 < AlPO4 < ZrP2O7. Although both NO–CO and NO–C3H6 reactions contributed to the deNOx process, the latter reaction was more favored on Rh/ZrP2O7 than on Rh/ZrO2. Partially oxidized C3H6 was adsorbed on Rh/ZrP2O7 in the form of reactive aldehyde species, in contrast to the less-reactive carboxylate species adsorbed on Rh/ZrO2. Furthermore, Rh oxide was more easily reduced to the active metallic state on ZrP2O7 compared with Rh/ZrO2 when the atmosphere was changed from lean (A/F > 14.6) to rich (A/F < 14.6). Metallic Rh formed on ZrP2O7 was only slowly oxidized on exposure to excess O2, whereas Rh on ZrO2 was readily oxidized to less-active Rh2O3. The high activity of Rh/ZrP2O7 toward C3H6 oxidation via aldehyde species as well as the resistance of metallic Rh against reoxidation are considered to be possible reasons for the enhanced deNOx performance of this catalyst in a slightly lean region.