Rh/ZrP<sub>2</sub>O<sub>7</sub> as an Efficient Automotive Catalyst for NO<sub><i>x</i></sub> Reduction under Slightly Lean Conditions

The three-way catalyst performances of honeycomb-coated Rh catalysts were studied on several metal phosphate supports (AlPO<sub>4</sub>, YPO<sub>4</sub>, ZrP<sub>2</sub>O<sub>7</sub>, and LaPO<sub>4</sub>) using various simulated exhausts with different air-to-fuel ratios (<i>A</i>/<i>F</i>). These supports significantly improved the NO<sub><i>x</i></sub> purification (deNO<sub><i>x</i></sub>) efficiency under slightly lean conditions (14.6 < <i>A</i>/<i>F</i> ≤ 15.3) as compared with conventional Rh/ZrO<sub>2</sub> catalysts. The deNO<sub><i>x</i></sub> activity exhibited the following sequence of increasing the mean electronegativity of the supports: ZrO<sub>2</sub> < YPO<sub>4</sub> < LaPO<sub>4</sub> < AlPO<sub>4</sub> < ZrP<sub>2</sub>O<sub>7</sub>. Although both NO–CO and NO–C<sub>3</sub>H<sub>6</sub> reactions contributed to the deNO<sub><i>x</i></sub> process, the latter reaction was more favored on Rh/ZrP<sub>2</sub>O<sub>7</sub> than on Rh/ZrO<sub>2</sub>. Partially oxidized C<sub>3</sub>H<sub>6</sub> was adsorbed on Rh/ZrP<sub>2</sub>O<sub>7</sub> in the form of reactive aldehyde species, in contrast to the less-reactive carboxylate species adsorbed on Rh/ZrO<sub>2</sub>. Furthermore, Rh oxide was more easily reduced to the active metallic state on ZrP<sub>2</sub>O<sub>7</sub> compared with Rh/ZrO<sub>2</sub> when the atmosphere was changed from lean (<i>A</i>/<i>F</i> > 14.6) to rich (<i>A</i>/<i>F</i> < 14.6). Metallic Rh formed on ZrP<sub>2</sub>O<sub>7</sub> was only slowly oxidized on exposure to excess O<sub>2</sub>, whereas Rh on ZrO<sub>2</sub> was readily oxidized to less-active Rh<sub>2</sub>O<sub>3</sub>. The high activity of Rh/ZrP<sub>2</sub>O<sub>7</sub> toward C<sub>3</sub>H<sub>6</sub> oxidation via aldehyde species as well as the resistance of metallic Rh against reoxidation are considered to be possible reasons for the enhanced deNO<sub><i>x</i></sub> performance of this catalyst in a slightly lean region.