The influence of reducing conditions on transient redox behavior of a series of noble metal (NM)-loaded Ce0.68Zr0.32O2 and CeO2 materials was investigated. When H-2 was used as a reducing agent, significant dynamic oxygen storage (H-2-OSC) values, with H2O formation, were measured at room temperature (RT). This is attributed to H-2 spilled over the support followed by titration of this adsorbed hydrogen by O-2. The phenomenon requires the presence of metal to activate H-2, and was observed for Rh, Pd, and Pt, all of which show high efficiency in this regard. Furthermore, there appears to be a direct relationship between this process and surface area: Decreasing the surface area of samples diminishes RT H-2-OSC proportionally. The higher thermal stability of ceria-zirconia relative to ceria means that H-2-OSC remains significant even after severe redox aging. Over Pt/CeO2, the H-2-OSC values measured at 373-773 K depend on surface area, indicating that the reduction is a surface-related process, whereas for ceria-zirconia-supported samples deeper reduction/vacancy creation becomes evident as the temperature is increased, the dynamic H-2-OSC being therefore independent of sample surface area. When CO is used as reducing agent, complicated dynamic CO-OSC behavior is observed, with surface reduction, CO storage, and CO desorption being detected. For the fresh (oxidized) high-surface-area Pt/Ce0.68Zr0.32O2 catalyst, appreciable CO-OSC is detected only at and above 473 K. When the catalyst is prereduced at 500 K, CO-OSC is observed at 373 K. In contrast, no appreciable CO-OSC was detected over law-surface-area samples below 673 K. Compared with Pd and Pt, which exhibit similar behavior, the presence of Rh promotes support reduction by CO.

Effects of the nature of the reducing agent on the transient redox behaviour of NM/Ce0.68Zr0.32O2 (NM= Pt, Pd and Rh).

HICKEY, JAMES NEIL;FORNASIERO, Paolo;KASPAR, JAN;
2001

Abstract

The influence of reducing conditions on transient redox behavior of a series of noble metal (NM)-loaded Ce0.68Zr0.32O2 and CeO2 materials was investigated. When H-2 was used as a reducing agent, significant dynamic oxygen storage (H-2-OSC) values, with H2O formation, were measured at room temperature (RT). This is attributed to H-2 spilled over the support followed by titration of this adsorbed hydrogen by O-2. The phenomenon requires the presence of metal to activate H-2, and was observed for Rh, Pd, and Pt, all of which show high efficiency in this regard. Furthermore, there appears to be a direct relationship between this process and surface area: Decreasing the surface area of samples diminishes RT H-2-OSC proportionally. The higher thermal stability of ceria-zirconia relative to ceria means that H-2-OSC remains significant even after severe redox aging. Over Pt/CeO2, the H-2-OSC values measured at 373-773 K depend on surface area, indicating that the reduction is a surface-related process, whereas for ceria-zirconia-supported samples deeper reduction/vacancy creation becomes evident as the temperature is increased, the dynamic H-2-OSC being therefore independent of sample surface area. When CO is used as reducing agent, complicated dynamic CO-OSC behavior is observed, with surface reduction, CO storage, and CO desorption being detected. For the fresh (oxidized) high-surface-area Pt/Ce0.68Zr0.32O2 catalyst, appreciable CO-OSC is detected only at and above 473 K. When the catalyst is prereduced at 500 K, CO-OSC is observed at 373 K. In contrast, no appreciable CO-OSC was detected over law-surface-area samples below 673 K. Compared with Pd and Pt, which exhibit similar behavior, the presence of Rh promotes support reduction by CO.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11368/1725445
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