Surfaces coordinatively unsaturated sites

Degassing ZrOa at high temperatures leads to the appearance of an abnormal absorption and photoluminescence spectra which could be attributed to the formation of surface sites in low coordination or coordinatively unsaturated surface sites (see Section lV.A.2.b) (101-104). Moreover, Zr02... 

The nature of the metal and the conditions used for the reaction can also have an effect on this competitive adsorption. In addition, since these strongly adsorbed species seem to prefer to adsorb initially on the more coordinately unsaturated surface sites, the extent of inhibition can also depend on the morphology of the catalyst surface. - ... 

Another ion exchange procedure involves the interaction of a metal acetylacetonate (acac) with an oxide support. Virtually all acetylacetonate complexes, except those of rhodium and ruthenium, react with the coordinatively unsaturated surface sites of 7 alumina to produce stable catalyst precursors. On thermal treatment and reduction these give alumina supported metal catalysts having relatively high dispersions. 38 Acetylacetonate complexes which are stable in the presence of acid or base such as Pd(acac)2, Pt(acac)2 and Co(acac)3, react only with the Lewis acid, Al" 3 sites, on the alumina. Complexes which decompose in base but not in acid react not only with the Al 3 sites but also with the surface hydroxy groups. Complexes that are sensitive to acid but not to base react only slightly, if at all, with the hydroxy groups on the surface. It appears that this is the reason the rhodium and ruthenium complexes fail to adsorb on an alumina surface. 38... 

The above is formally equivalent to the picture of a coordinatively unsaturated surface (CUS) put forward by Burwell et al. (8) in their discussion of chromia. The acid-base formalism does have the advantage of drawing attention to the analogy of acid and base catalyzed reactions. If a hydrocarbon undergoes reaction at these sites via loss of a proton to the oxide site, the reaction should be analogous to a base catalyzed reaction if it undergoes reaction via the loss of a hydride to the zinc site or addition of a proton from the oxide site, the reaction should be analogous to an acid catalyzed reaction. This view, which we find useful, is implicit in the discussion that follows. 

Lewis acid sites may be formed following dehydroxylation of zeolite surface in H-form. At sufficiently high temperatures two Bronsted acid sites can drive off a water molecule and leave behind a coordinatively unsaturated Al site, as illustrated in Figure 13.16 [32]. Here not only the resulting tri-coordinated Al but also the tri-coordinated positively charged Si can act as a Lewis acid. Furthermore dehydroxylation may be followed by framework dealumination, leading to cationic extra-framework species like AlO AlfOHij that can act as Lewis acids [33-37]. 

The cracking of alkylbenzenes can be treated as a case of aromatic electrophilic substitution (for recent views on this type of reaction see ref. 241) where the attacking agent is either a proton from a surface Br0nsted site or a coordinatively unsaturated surface cation acting as a Lewis site (cf. ref. 238)... 

Che et al. observed the formation of surface Q species by EPR upon dosing 170-enriched O2 molecules on pyridine-pretreated CaO (250), MgO, and SrO (148). For the three oxidic systems the splitting of the 2p r orbitals by the surface crystal field has been measured from the gzz component of the g tensor, and a dependence on the polarizing tendency of the surface cation (q/r) was found (148). It was inferred that the donor sites producing the Oj molecular ions are the coordinatively unsaturated surface O2- ions. Fs-centers in CaO have been investigated by EPR (251). 

In conclusion, CO is an excellent probe molecule for the properties of coordinatively unsaturated surface Zn2+ sites, both isolated sites or those grouped into 2D patches. It is also emphasized that the prismatic faces also expose coordinatively unsaturated oxygen ions their presence, however, is indirectly detected only via their influence on the electrostatic field at the Zn2+ center. 

Thus, the adsorption of CO on active ZrOa catalysts led to the formation of various types of adsorption species of CO having different reactivities toward Ha, and these species were found to play a significant role in the hydrogenation of CO. Moreover, it is likely that CO is adsorbed on the active surface sites of low coordination and that an electron transfer from the other surface sites to this CO species leads to the formation of the dimeric adsorbed species (CO)a. These dimeric species react, step by step, with CO molecules from the gas phase to from a relatively stable cyclic polymer species of (CO)s and then (CO) ", Such adsorbed CO species easily react with hydrogen and are also activated through the dissociative adsorption of hydrogen on surface sites of low coordination or the coordina-tively unsaturated surface sites on the catalyst. 

We have proposed (12, 21) that the active sites which develop during activation are coordinatively unsaturated surface ions formed by loss of water. This idea is, of course, not new, but advances in the theory of inorganic chemistry permit us to employ the concept with greater pre-... 

In Sections III-V, we speculated about the nature of active sites on chromia and the relations of such speculations to chemisorption and heterogeneous catalytic reactions. In particular, we suggested that many types of active sites would involve coordinatively unsaturated surface (cus) ions of Cr3+ and 0 - and that the following types of chemisorption might occur at such sites simple coordinative adsorption at Cr3+(cus), adsorption of generalized acids at 02-(cus), heterolytic dissociative adsorption at pair sites of Cr +(cus) and 02-(cus), and reductive adsorption. In addition, we considered the possibility of ligand displacement adsorption which does not depend upon (cus) ions. 

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