10.1021/cs5020157.s001 Yuki Nagao Yuki Nagao Yunosuke Nakahara Yunosuke Nakahara Takahiro Sato Takahiro Sato Hironori Iwakura Hironori Iwakura Shoya Takeshita Shoya Takeshita Saki Minami Saki Minami Hiroshi Yoshida Hiroshi Yoshida Masato Machida Masato Machida 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 American Chemical Society 2015 Rh reactive aldehyde species C 3H oxidation ZrP 2O ZrO 2 Efficient Automotive Catalyst YPO 2015-03-06 00:00:00 Journal contribution https://acs.figshare.com/articles/journal_contribution/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/2189014 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.