Adsorption of CO on Palladium

Adsorption of CO on a palladium electrode is interesting because this metal is known to behave very differently from platinum. Breiter could conclude from electrochemical measurements that most of the adsorbed CO species were occupying more than one site. Similarly, spectroscopic investigations 

FIGURE 35. EMIRS spectra of various mixtures of CO/ CO adsorbed on Pt in 1 M H2SO4 as a function of % CO. After Bewick et 

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Figure 6.3. The heat of adsorption of CO on palladium and nickel surfaces depends on coverage (CO/Pd data from Conrad et ah, 1974, CO/Ni from Stuckless et al., 1993).

To answer these questions, two examples are considered the adsorption of CO on platinum and the adsorption of CO on palladium electrodes in acid media. 

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). 

CO to a Pd(l 1 l)-like thick film and a Pd monolayer supported on Ta(110) [30,31], The spectrum for a thick palladium film is in very good agreement with that observed for adsorption of CO on a single-crystal Pd(lll) surface. The features at 11 and 8 eV correspond to emissions fi om the 4o and (In + 5o) levels of CO, respectively [30,31], In the photoemission spectrum for the Pd monolayer the 4o and (In + 5o) peaks of CO appear at higher binding energy than in the ectrum for the Pd(lll)-like film, and there is also an extra shake-up satellite ( s peak) around 13.6 eV. The spectrum for CO on the Pd monolayer matches the ectrum seen for CO on Cu(lll) [30,31], where the bonding interactions between the admolecule and metal substrate are much weaker than on Pd(l 11). 

Figure 4. Heat of adsorption of CO on supported palladium (300K).

The adsorption of CO on nickel has received extensive study since large area nickel films are comparatively simple to prepare. The nickel system therefore offered the possibility of producing a clean surface on a metal crystallizing with a face centered cubie structure. The results obtained on this surface could then be compared with those measured on tungsten, which crystallizes with a body centered,cubic structure. Much of the earlier work carried out for CO adsorption on the elements nickel, palladium, and platinum was reviewed by Gundry and Tompkins in 1960 135). 

As an introductory example we take one of the key reactions in cleaning automotive exhaust, the catalytic oxidation of CO on the surface of noble metals such as platinum, palladium and rhodium. To describe the process, we will assume that the metal surface consists of active sites, denoted as We define them properly later on. The catalytic reaction cycle begins with the adsorption of CO and O2 on the surface of platinum, whereby the O2 molecule dissociates into two O atoms (X indicates that the atom or molecule is adsorbed on the surface, i.e. bound to the site ) ... 

The same group has looked into the conversion of NO on palladium particles. The authors in that case started with a simple model involving only one type of reactive site, and used as many experimental parameters as possible [86], That proved sufficient to obtain qualitative agreement with the set of experiments on Pd/MgO discussed above [72], and with the conclusion that the rate-limiting step is NO decomposition at low temperatures and CO adsorption at high temperatures. Both the temperature and pressure dependences of the C02 production rate and the major features of the transient signals were correctly reproduced. In a more detailed simulation that included the contribution of different facets to the kinetics on Pd particles of different sizes, it was shown that the effects of CO and NO desorption are fundamental to the overall behavior... 

Fig. 10. Spectra of CO adsorbed on palladium samples Pd-105, Pd-45, and Pd-15. The height of the arrow gives the intensity of the band at 2260 cm-1 after adsorption of N2 on the samples.

Low pressure studies Adsorption of CO. The experiments were performed in an ultra-high vacuum system described previously (1). The data obtained on palladium particles with a size smaller than 2 nm or larger than 3 nm will be discussed in turn. 

The obvious decrease in the number of electron-acceptor sites with palladium deposition on silica-alumina strongly suggests an interaction between the metal and these sites. Turkevich (28) first demonstrated that palladium behaves like an electron-donor toward tetracyanoethylene we suppose that it can be the same toward an electron-acceptor site of a solid support. In that hypothesis, palladium should have a partial positive charge on the second class of supports. This is actually observed by the adsorption of CO. This adsorbate can be considered as a detector of the electronic state of palladium. The shift toward higher frequencies of the CO band reflects a decrease in the back donation of electrons from palladium to CO. Thus, palladium on silica-alumina or HY is electron-deficient compared with the silica- or magnesia-supported metal. Moreover, the shift of CO vibration frequency is roughly parallel to the increase of activity thus, these two phenomena are connected. We propose that the high activity of palladium on acidic oxides is related to its partial electron deficiency. 

C.2.6. PM-IRAS of CO on Nanoparticle Model Catalysts. For completeness, we mention that investigations of CO adsorption on palladium nanoparticle catalysts were also carried out by PM-IRAS. The observed adsorbate species essentially agree with those observed by SFG, and References 175,306,307,453) provide more information. 

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