Metal–support interaction in Pd/CeO₂catalysts: Fourier-transform infrared studies of the effects of the reduction temperature and metal loading. Part 1.—Catalysts prepared by the microemulsion technique

Abstract

The effect of the temperature of reduction by H, of Pd/CeO, catalysts on the accessibility of the metal to carbon monoxide adsorption has been estimated from a study of the integrated intensity of the v(C0) absorption bands.No adsorption occurred when the reduction temperature was higher than 673 K ; the behaviour of the catalysts, which present crystallites of the same morphology, appeared to be independent of the Pd loading (0.5, 2.5, 5 wt.%). The adsorption could be completely recovered when the catalysts were first reoxidized at 723 K and subsequently reduced by H, at 423 K : this characterized the occurrence of a strong metal-support interaction (SMSI). Study of the v(C0) absorption bands corresponding to CO adsorption on the (100) and ( 111) planes tended to show only a weak selectivity of the (100) planes in the SMSI phenomenon. The effect of the reduction on t h e ceria support was established from t h e study of methanol and tetracyanoethylene adsorption ; surface ceria reduction was enhanced owing to t h e presence of palladium metal.

Pd/Mg(Al)O catalysts obtained from hydrotalcites: investigation of acid–base properties and nature of Pd phases

“…In contrast, the ratio between linearly and bridged bonded species is high for PdMgAlx(ac) samples, as generally observed for polycrystalline Pd systems supported on various oxides [38][39][40][41]. In fact, the relative concentration of linear and bridged CO chemisorption sites is particle-size dependent, the former increasing at the expense of the latter on decreasing the Pd particle sizes [38,39].…”

Chemical and Nanostructural Aspects of the Preparation and Characterisation of Ceria and Ceria-Based Mixed Oxide-Supported Metal Catalysts

“…In the earlier studies, most of which have dealt with catalysts reduced at temperatures not higher than 773 K, electronic metal/support interaction effects (133), metal decoration (64,78,219,221,227), and NM-Ce alloying phenomena (324,325) have been suggested to play a role. More recently, the characteristics of the metal/support interaction effects occurring in these catalysts have been reviewed, and a model aimed at interpreting the chemical and nano-structural changes occurred in NM/Ce0 2 on increasing the reduction temperature has been proposed (117).…”

“…Accordingly, on M/Ceria and related catalysts, similar effects might be expected to occur. In fact, many authors (28,41,64,70,73,74,78,82,131,133,137,138,179,196,219,221,224,227,258) have speculated on the existence of some kind of strong metal/support interaction effect in ceria-supported metal catalysts. This phenomenon is relevant by itself, but also because of its obvious implications in the characterisation of metals supported on ceria and related oxides.…”

“…Thus, a stabilized rhodium hydrosol with an average particles size of 5 nm was used in the preparation of a Rh(l%)/Ce0 2 catalyst (182). Likewise, a series of Pd/Ce0 2 catalysts with 0.5, 2.5 and 5 wt.% have been prepared from microemulsion of metallic palladium, further destabilized by addition of tetrahydrofuran (78).…”

Part II: Oxidative Thermal Aging of Pd/Al2O3 and Pd/CexOy-ZrO2 in Automotive Three Way Catalysts: The Effects of Fuel Shutoff and Attempted Fuel Rich Regeneration

“…The interaction of Pd with CeO2 in intimate contact enhances the reducibility of the support [36], and positively influences the redox state of the active metal [37][38][39][40]. For fresh and aged Pd/CZO, the TPR peaks observed between 100 °C and 120 °C are attributed to the reduction of Ce 4+ to Ce 3+ assisted by Pd.…”