Effect of Thermal Aging on Local Structure and Three-Way Catalysis of Cu/Al₂O₃

Abstract

Cu supported on Al2O3, prepared by impregnation, was thermally aged at different temperatures, and the influence of thermal aging on the local structure, redox behavior of Cu, and catalytic activity for a stoichiometric NO–CO–C3H6–O2 reaction was investigated. Crystalline CuO was mainly formed on Al2O3 after thermal aging at ≤700 °C, whereas aging at higher temperatures induced Cu2+ incorporation into tetrahedral (Td-Cu2+) rather than octahedral (Oh-Cu2+) sites of γ-Al2O3. Despite its lower surface area, thermally aged Cu/Al2O3 with Td-Cu2+ sites showed higher catalytic performance for the stoichiometric NO–CO–C3H6–O2 reaction compared with the as-prepared catalyst, especially for NO reduction. Td-Cu2+ was reduced to Td-Cu+ during reaction with CO and/or C3H6, and NO could be reduced in subsequent reoxidation of Td-Cu+ to Td-Cu2+ by NO. This redox behavior is more probable on Td-Cu rather than crystalline CuO, resulting in enhancement of NO reduction during the three-way catalyst reaction.

Comparative study of structure-catalytic activity relationship for Ni–Cr–Al–O and Cu–Ni–Cr–Al–O spinel oxide solid solutions

“…Similar behavior can be widely found for other Cu-based oxide catalysts and is related to the NO reduction mechan-ism as discussed below. 22) When the temperature ramping experiment was repeated, the light-off temperatures at 50 % conversion agreed within an error of «15°C, indicating a sufficiently degree of reproducibility. This is in accordance with a high phase stability of the quaternary spinel oxide solid solution under the reaction atmosphere.…”

Enhanced Catalytic NO Reduction in NO–CO–C3H6–O2 Reaction Using Pseudo-Spinel (NiCu)Al2O4 Supported on γ-Al2O3

“…The XRD patterns of the thermally aged Ni–Cu/Al 2 O 3 samples with different Ni loadings are shown in Figure . The diffraction peaks of crystalline NiO or CuO are not visible in any sample profile, which indicates the incorporation of Ni 2+ and Cu 2+ ions in the γ-Al 2 O 3 framework through thermal aging; however, the peaks assignable to the pseudo-spinel structure were observed instead, in addition to those of the γ-Al 2 O 3 support . γ-Al 2 O 3 is known to possess cation-defective structures with the formula (M 1– x Al x )­[M 2– y Al y ]­O 4 , where (M) and [M] represent metal cation vacancies at tetrahedral and octahedral sites, respectively; − both Ni 2+ and Cu 2+ ions can be incorporated into these Al 3+ defect sites to form pseudo-spinel (NiCu)­Al 2 O 4 .…”

“… ,, The importance of the local Cu structure and oxidation state for facilitating NO reduction was proposed in our previous study in which T d -Cu + ions that were formed via the reduction of T d -Cu 2+ effectively reduced NO to N 2 . This took place during the subsequent reoxidation of Cu + to Cu 2+ that was facilitated by NO . Considering that the number of Cu active sites on Cu/Al 2 O 3 and Ni–Cu/Al 2 O 3 is similar (Table ), the enhancement of the NO reduction activity possibly involves the presence of Ni in pseudo-spinel (NiCu)­Al 2 O 4 , which increases the proportion of T d -Cu in all the Cu active sites.…”

“…The surface modification of γ-Al 2 O 3 (165 m 2 g –1 ; JRC-ALO8, Catalysis Society of Japan) by Cu and/or Ni was conducted by a conventional impregnation method using aqueous solutions of Cu­(NO 3 ) 2 and Ni­(NO 3 ) 2 (Wako Pure Chemical Industries). The optimal Cu loading on γ-Al 2 O 3 was found to be 8 wt %, and the catalyst showed a higher activity in a stoichiometric NO–CO–C 3 H 6 –O 2 reaction than the catalyst in our previous study . Therefore, the Cu loading was fixed at 8 wt % for all samples with different Ni loadings of 0, 2, 4, 8, and 16 wt %.…”

“…The decrease can be attributed to thermal sintering that causes significant catalyst deactivation. In our recent study, the activity of Cu supported on γ-Al 2 O 3 for NO reduction in a stoichiometric NO–CO–C 3 H 6 –O 2 reaction was found to be enhanced by thermal aging at 900 °C, during which a solid solution of pseudo-spinel CuAl 2 O 4 was formed on the γ-Al 2 O 3 surface . Owing to the change in structure from crystalline CuO to pseudo-spinel CuAl 2 O 4 , Cu 2+ reduction was enhanced and the conversion of NO at 600 °C increased from 13% to greater than 60%, along with the complete purification of CO and C 3 H 6 ; this indicated the importance of the local Cu structure and its redox behavior for catalytic NO reduction in a three-way catalytic reaction.…”