Adsorption at defect sites

Abstract This review is a summary of supported metal clusters with nearly molecular properties. These clusters are formed hy adsorption or sirnface-mediated synthesis of metal carbonyl clusters, some of which may he decarhonylated with the metal frame essentially intact. The decarhonylated clusters are bonded to oxide or zeolite supports by metal-oxygen bonds, typically with distances of 2.1-2.2 A they are typically not free of ligands other than the support, and on oxide surfaces they are preferentially bonded at defect sites. The catalytic activities of supported metal clusters incorporating only a few atoms are distinct from those of larger particles that may approximate bulk metals. 

The smallest molecule in the class is methanethiol, which has been studied at several surfaces as an indicator for the behaviour of larger thiols. On all metals, except gold, adsorption results in the dissociation of the S H bond. Yates and co-workers suggested47 that the S-H bond of methanethiol is also stable on silver, dissociation being catalysed by adsorbed sulfur present at defect sites. However, this contradicts Jaffey and Madix s results,56 which showed S-H bond cleavage below 350 K in their study of ethanethiol on Ag(lll). 

As mentioned above, it is reasonable to assume that this tetrahedral V species forms at defect sites (hydroxyl nests) in the zeolite framework, but is stabilized by this interaction in a well defined environment through V-O-Si bonds. As indicated by the characterization data, the local coordination of vanadium must be different from that found for well dispersed vanadium sites on silica. This stabilization probably limits the unselective metal-bonded propane or propylene adsorption, in agreement with the role of adsorbate bonding on the selection of partial and total oxidation pathways of ethane on vanadium supported on silica (76) and in agreement with IR evidence (Fig. 

At low coverage and 0.1 V vs SCE on a gold electrode, adsorbed CO exhibits three bahds (2040, 1970 and 1870 cm ), as can be seen in Fig. 18. The difference in wavenumber is too high to be attribued to adsorption on defect sites. Thus the three bands are related to three types of coordination for CO on polycrystalline gold. Tentatively, the 1870, 1970 and 2040 cm bands have been assigned to threefold, twofold and on-top adsorbed CO [61]. 

The different reactivity of MgO versus CaO surfaces predicted by cluster calculations [95] has been recently confirmed by metastable impact electron spectroscopy (MIES) experiments on CO2 adsorption on MgO [100] and CaO [101] as well as by synchrotron based photoemission spectroscopy on the same systems [102,103] while the CaO surface exposed to CO2 is highly reactive and becomes terminated by carbonate complexes, on the MgO surface CO2 chemisorption does not take place at regular sites but only at defect sites, presumably the O " ions at the step or comer sites. 

Film breaking it has been suggested that the passive film is continuously subjected to breakdown and repair (Vetter and Strehblow, 1970 Sato, 1971 Sato et al., 1971). The local breakdown events would be caused by mechanical stresses at defect sites or by electrostriction effects. In the absence of aggressive ions such as chloride, rapid repassivation takes place, whereas the presence of chloride could prevent repassivation of locally depassivated surfaces and thus cause pitting. This view of pitting considers that passivity breakdown itself is not caused by chloride, but is inherent to the nature of passive films. In this mechanism, adsorption on the passive film surface is not an important factor, but chloride adsorption on the metal surface remains a necessary step in the process of repassivation inhibition (and salt film formation). 

According to Vitanov et a/.,61,151 C,- varies in the order Ag(100) < Ag(lll), i.e., in the reverse order with respect to that of Valette and Hamelin.24 63 67 150 383-390 The order of electrolytically grown planes clashes with the results of quantum-chemical calculations,436 439 as well as with the results of the jellium/hard sphere model for the metal/electro-lyte interface.428 429 435 A comparison of C, values for quasi-perfect Ag planes with the data of real Ag planes shows that for quasi-perfect Ag planes, the values of Cf 0 are remarkably higher than those for real Ag planes. A definite difference between real and quasi-perfect Ag electrodes may be the higher number of defects expected for a real Ag crystal. 15 32 i25 401407 10-416-422 since the defects seem to be the sites of stronger adsorption, one would expect that quasi-perfect surfaces would have a smaller surface activity toward H20 molecules and so lower Cf"0 values. The influence of the surface defects on H20 adsorption at Ag from a gas phase has been demonstrated by Klaua and Madey.445... 

Also, under continuous CO oxidation conditions, alkaline media exhibit a much higher activity than acidic media. Markovic and co-workers observed a shift of about 150 mV of the main oxidation wave, and a pre-wave corresponding to CO oxidation at potentials as low as 0.2-0.3 V [Markovic et al., 2002]. Remarkably, the hysteresis that is so prominently observed in the diffusion-controlled CO oxidation wave in acidic media (see Fig. 6.9), is no longer present in alkaline media. Markovic and co-workers also attribute the high activity of alkaline media to a pH-dependent adsorption of OH ds at defect/step sites. 

---