Palladium particles, electron deficiency

Of course, such a chemical probing would be possible only if we can prove, by means of other (mainly physical) techniques, that the existence of electron-deficient palladium in supported palladium is possible. Therefore, the organization of this section is as follows First, we discuss the results of XPS studies of electronic properties of small Pd particles deposited on various supports. Then we examine other evidence for the existence of positively charged Pd species using other techniques, such as electron spin resonance (ESR) and infrared (IR) spectroscopy of adsorbed CO. Finally, catalytic consequences of the appearance of positively charged species in the Pd/support catalysts will be demonstrated. 

The conclusion that palladium particles in zeolites may carry a partial positive charge follows from the IR study of CO adsorption. This adsorbate can be considered to be a probe of the electronic state of palladium. Namely, the shift toward higher frequencies of the CO linear band (for Pd°-CO it appears at <2100 cm ) reflects a decrease in the back donation of electrons from Pd to CO. Along with such an interpretation, Figueras et al. (138) detected the presence of electron-deficient Pd species in Pd/ HY but not in Pd/Si02. More recently, Lokhov and Davydov (139) confirmed the presence of positively charged Pd species apart from Pd° in reduced (at 300°C) Pd/Y samples and ascribed a 2120- to 2140-cm"1 band to Pd+-CO complexes (Fig. 7). Similarly, Romannikov et al. (140) report that adsorption of CO on Pd/Y samples reduced at 300°C produces IR bands at >2100 cm 1 ascribed to Pd+-CO and Pdzeolite protons, because the IR band of the zeolite O-H group decreases when CO is released and increases when CO is added to the cluster (141, 142). 

This process is often described not in terms of Fermi-level quantities but in terms of total charge if the metal particle is electron deficient, it has fewer electrons available for the Blyholder mechanism. Initially, the expression referred to the experimental fact that catalysts of platinum or palladium in zeolites, when compared to the same metals on oxides, often seemed to behave like their neighbors in the periodic system, iridium and rhodium. 

The existence of metal cations close to small metal particles in RU/AI2O3 has been noted," but they are not seen in Rh/AlaOs. They are however commonly seen with palladium catalysts, and electron-deficient palladium and Pd + cations are assigned unusually high catalytic activity in hydrocarbon reactions (Figure 2. lOB). (2) Such species (and similar platinum could arise in acidic... 

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