Abstract
The effect of Pd loading on the redox characteristics of a ceria support was examined using in situ Pd K-edge XAS, Ce L3-edge XAS and in situ X-ray diffraction techniques. Analysis of the data obtained from these techniques indicates that the onset temperature for the partial reduction of Ce(IV) to Ce(III), by exposure to H2, varies inversely with the loading of Pd. Whilst the onset and completion temperatures of the reduction of Ce(IV) to Ce(III) are different, both samples yield the same maximal fraction of Ce(III) formation independent of Pd loading. Furthermore, the partial reduction of Ce is found to be concurrent with the reduction of PdO and demonstrated that the presence of metallic Pd is necessary for the reduction of the CeO2 support. Upon passivation by room temperature oxidation, a full oxidation of the reduced ceria support was observed. However, only a mild surface oxidation of Pd was identified. The mild passivation of the Pd is found to lead to a highly reactive sample upon a second reduction by H2. The onset of the reduction of Pd and Ce has been demonstrated to be independent of the Pd loading after a mild passivation with both samples exhibiting near room temperature reduction in the presence of H2.
Highlights
The effect of Pd loading on the redox characteristics of a ceria support was examined using in situ Pd K-edge X-ray absorption spectroscopy (XAS), Ce L3-edge XAS and in situ X-ray diffraction techniques
It has been suggested that the support enhances the activity of noble metal catalysis due to strong metal support interactions (SMSIs).[16,17]
In order to understand the reduction properties of palladium ions and their influence on the reactivity of a ceria support, it is necessary to study both the short and long range structures of the system and to obtain element specific information probing the supported Pd metal ions. This is accomplished by combining suitable, complimentary structural methods. This includes X-ray absorption spectroscopy (XAS), which comprises X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), an element specific technique and useful to determine short-range geometric and electronic structural information
Summary
The effect of Pd loading on the redox characteristics of a ceria support was examined using in situ Pd K-edge XAS, Ce L3-edge XAS and in situ X-ray diffraction techniques. Chemical interactions, such as redox reactions, complicate these interactions and often influence the catalytic performance and reducibility of the loaded metal.[30,31,32] Among the various PGMs, the use of Pd on ceria has been well documented Paper such as in automotive exhaust catalysis,[33] steam reforming,[34] methanol synthesis,[35] abatement of methane,[36] C–H bond activation[37] and in various catalytic oxidation processes.[38,39,40,41] In order to understand the reduction properties of palladium ions and their influence on the reactivity of a ceria support, it is necessary to study both the short and long range structures of the system and to obtain element specific information probing the supported Pd metal ions. We report the use of in situ Pd K-edge, Ce L3-edge, and XRD techniques to determine the effect of the concentration of palladium on the reactivity of the Pd/CeO2 system
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