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

Abstract

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 NO x purification (deNO x ) efficiency under slightly lean conditions (14.6 < A/F ≤ 15.3) as compared with conventional Rh/ZrO2 catalysts. The deNO x 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 deNO x 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 deNO x performance of this catalyst in a slightly lean region.

Lean NOx reduction over Rh/ZrP2O7 catalyst under steady-state and perturbation conditions

“…In both cases, Rh/ZrP 2 O 7 showed a higher NO x conversion activity compared with Rh/ZrO 2 . These results of powdered catalysts are consistent with those of cordierite honeycomb coated catalysts in our previous study [15]. This suggests that the higher deNO x performance is the intrinsic property of the Rh/ZrP 2 O 7 catalyst.…”

“…More interestingly, this unusual redox behavior of the Rh catalysts causes a positive impact on their lean NO x purification (deNO x ) performance. One typical example can be seen for the Rh catalyst supported on ZrP 2 O 7 , which exhibited the highest NO x conversion under slightly lean condition (A/F ≤ 15.0) among various phosphate materials in our previous study [15].…”

“…ZrP 2 O 7 was prepared by the coprecipitation method as reported in our previous paper [15]. Aqueous http://dx.doi.org/10.1016/j.cattod.2016.05.020 0920-5861/© 2016 Elsevier B.V. All rights reserved.…”

“…The catalytic activity and stability of catalysts is strongly dependent on the interactions at the metal-support interface. We have previously studied various metal phosphates, such as AlPO 4 , ZrP 2 O 7 , and LaPO 4 , as thermally stable supports for Rh catalysts [11][12][13][14][15][16][17][18][19]. These materials produce optimum interactions with the Rh species via Rh O P bonding and enable high dispersion of Rh anchored onto the phosphate unit on the surface [13,14,17].…”

Effect of Rare Earth Additives on the Catalytic Performance of Rh/ZrO2 Three-Way Catalyst

“…For example, Tanabe et al [4] reported that Rh interacts strongly with Nd 2 O 3 , resulting in Rh sintering suppression by the formation of Rh-O-Nd bond. Machida et al [5][6][7] developed metal phosphate materials such as tridymiteAlPO 4 and ZrP 2 O 7 as a robust support material producing optimum interactions with Rh species, owing to the high thermal stability and the high dispersion of Rh species anchored onto the surface. Kawabata et al [8,9] reported that Rh catalyst supported on La-containing ZrO 2 showed high performance for the removal of NOx and hydrocarbons.…”