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

The influence of reducing conditions on transient redox behavior of a serie s of noble metal (NM)-loaded Ce0.68Zr0.32O2 and CeO2 materials was investig ated. When H-2 was used as a reducing agent, significant dynamic oxygen sto rage (H-2-OSC) values, with H2O formation, were measured at room temperatur e (RT). This is attributed to H-2 spilled over the support followed by titr ation of this adsorbed hydrogen by O-2. The phenomenon requires the presenc e of metal to activate H-2, and was observed for Rh, Pd, and Pt, all of whi ch show high efficiency in this regard. Furthermore, there appears to be a direct relationship between this process and surface area: Decreasing the s urface area of samples diminishes RT H-2-OSC proportionally. The higher the rmal stability of ceria-zirconia relative to ceria means that H-2-OSC remai ns significant even after severe redox aging. Over Pt/CeO2, the H-2-OSC val ues measured at 373-773 K depend on surface area, indicating that the reduc tion is a surface-related process, whereas for ceria-zirconia-supported sam ples deeper reduction/vacancy creation becomes evident as the temperature i s increased, the dynamic H-2-OSC being therefore independent of sample surf ace area. When CO is used as reducing agent, complicated dynamic CO-OSC beh avior is observed, with surface reduction, CO storage, and CO desorption be ing 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. (C) 2001 Academic Press.