Activity–Composition Relationships of Fe–Ni–Cu Ternary Nanoparticles Supported on Al₂O₃ as Three-Way Catalysts for NO Reduction

Abstract

Ternary Fe x Ni y Cu 1−x−y (x, y: molar fractions) metal nanoparticles supported on Al 2 O 3 were prepared by H 2 -reduction treatment at 900 °C and investigated as three-way catalysts free of precious metals (Rh, Pd, and Pt). As-prepared nanoparticles consisted of nearly homogeneous alloys, whereas their surfaces were partially oxidized. Further oxidation occurred upon exposure to a reaction gas mixture (NO−CO−C 3 H 6 −O 2 −H 2 O) at lower temperatures of ≤400 °C. At higher temperatures, however, Cu and Ni regenerated the active metallic states and reconstructed alloy nanoparticles, whereas most Fe was fully oxidized and formed a spinel-like Fe−Ni oxide phase. A contour map analysis revealed that the NO reduction activity and the fraction of metallic states were strongly dependent on the metal composition (x and y). Among the compositions investigated in the ternary system, near-equimolar catalysts (0.2 ≤ x ≤ 0.33, 0.2 ≤ y ≤ 0.33) exhibited the highest activity and the highest fraction of metallic states. The regenerability and stability of metallic-state Cu and Ni species were improved by the copresence of Fe−Ni oxide, which was found to promote the oxidative adsorption of C 3 H 6 as a carboxylate on the Al 2 O 3 surface. A subsequent reaction with NO formed N 2 , which converted the carboxylate to CO and NCO species close to the perimeter of the metal−support interface. As oxidative C 3 H 6 adsorption consumes oxygen, this provides a more reductive surface environment, thereby enhancing the stability of the active metallic state.

Cr–Fe–Ni–Cu Quaternary Nanostructure as a Substitute for Precious Metals in Automotive Three-Way Catalysts

“…Considering the 1:2 ratio of NiFe 2 O 4 , the remaining Ni may be in the form of CuNi alloy, as shown in Figure . Previously, we reported a similar structural change that occurred in the ternary FeNiCu catalyst when exposed to the reaction gas mixture . Based on these characterization results, the local structural change under the catalytic reaction condition is schematically inferred, as illustrated in Scheme .…”

“…The Cu metal surface favors nondissociative NO adsorption, but it also catalyzes NO reduction by dimerizing two NO molecules to form ONNO, followed by the N–O bond cleavage to form N 2 . , The Cu metal surface’s activity can further be modified by a combination with other transition metals. A NiCu alloy is one such case showing superior catalytic performance for NO reduction, as revealed by several research groups. ,,− Tanaka et al and Asakura et al ,, reported the higher catalytic activity of NiCu compared to its monometal counterparts. Papanikolaou and Stamatakis successfully demonstrated the NiCu surface’s catalytic activity in the NO–CO reactions using density functional theory combined with microkinetic modeling.…”

“…Papanikolaou and Stamatakis successfully demonstrated the NiCu surface’s catalytic activity in the NO–CO reactions using density functional theory combined with microkinetic modeling. We investigated various ternary 3 d transition metal nanoparticles supported on Al 2 O 3 as NO reduction catalysts in a stoichiometric NO–CO–C 3 H 6 –O 2 –H 2 O reaction. , In the presence of high-concentration H 2 O, a ternary FeNiCu catalyst was discovered to have higher TWC activity and stability than unary and binary Cu-based metal nanoparticles, which is associated with the regenerability of metallic species under the reaction atmosphere. The FeNiCu catalyst as prepared by H 2 reduction treatment at 900 °C consisted of single alloy nanoparticles, whereas the surface was oxidized under a low-temperature TWC atmosphere (≤300 °C).…”

“…Therefore, further studies were focused on Cr x FeNiCu with different numbers of Cr molar fractions (0 ≤ x ≤ 0.15) to optimize the metallic composition. To reveal the role of Cr in the ternary catalysts, X-ray diffraction (XRD), X-ray absorption fine structure (XAFS) analysis, and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) with energy-dispersive X-ray spectrometry were applied to characterize the nanostructure and chemical state of each metal, compared with the ternary FeNiCu catalysts in our previous studies. , …”

Tandem Base-Metal Oxide Catalyst for Automotive Three-way Reaction: MnFe2O4 for Preferential Oxidation of Hydrocarbon

“…Various types of PGM-free catalysts have been proposed so far [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26], however, the low NO reduction activities are the serious problems in base metal oxide catalysts. For example, although NiFe 2 O 4 showed the highest NO reduction activity among base metal oxide-based TWC catalysts, the rate of NO reduction was only 1/8 of that of Rh/ZrO 2 [27].…”