Surfaces coordinative unsaturated

For Ru(OOOl) the corresponding reaction energy scheme is shown in Figure 1.7 [4]. The relative energies of the different reaction intermediates, Cjds or CHads, may strongly depend on the type of surface and metal. When for different surfaces or metals the relative interaction with Hads increases Cads may for instance become more stable than CH. This is found for more coordinative unsaturated surfaces or more reactive metals. 

Figure 13. Dependence of ethane hydrogenolysis TOF and apparent activation energy on Pt particle size. TOFs decrease by two orders of magnitude over the size range, while the apparent activation energy increases. Coordinatively unsaturated surface atoms in small particles have a higher reactivity and subsequently a smaller barrier for hydrogenolysis than highly coordinated surface atoms of larger particles. TOFs were measured at 20 Torr C2H6, 200 Torr H2, and 658 K [16]. (Reprinted from Ref [16], 2006, with permission from American Chemical Society.)...

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. 

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). 

Coordinatively unsaturated surface pairs abstract hydrogen ions from the a position of pyridine as shown in Scheme 5. The [Py] can then further react with excess Py to give bipyridyl radical anions (259). As usual, these negatively charged species can then react with oxygen to give superoxidic anion radicals. 

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. 

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... 

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-... 

We have considered loss of water by condensation processes which do not change the ON of Cr +. However, loss of water by condensation involving surface OH groups will, in general, generate coordinatively unsaturated surface species as shown schematically in Fig. 1. The reaction is... 

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. 

ESR spectra as well as additional experiments with very pure alumina (steaming, treatment with diluted acids, interaction with hydrogen sulfide) indicate that at least two factors can influence hydrogenation activity of USY zeolites obtained via steaming contamination of parent material with Fe ions and/ or formation of new active phase of altimina inside supercages. The catalytic sites are probably coordinatively unsaturated surface species, which are poisoned by carbonaceous deposits formed as the result of prolonged interaction with olefins. 

Although they are not as widely used as pyridine, substituted pyridines have found their own place in the characterization of hydroxyl groups. It was proposed (470) that, for steric reasons, 2,6-dimethylpyridine (DMP, lutidine) does not interact with Lewis acid sites and is thus a proton-specific probe. Later, it was demonstrated that DMP (247,256,471-473) (as well as 2,4,6-trimethyl pyridine or collidine (474)) stiU forms coordination bonds with coordinatively unsaturated surface cations. This bond is only weakened by the steric interference of the methyl groups with the surface but not prevented. In any case, the steric hindrance leads to preferential interaction ofDMP with hydroxyl groups (471-473). DFT calculations suggest that the proton transfer is promoted by the stabilization of the lutidinium ion on the deprotonated site, rather than by the intrinsic acidity of the acid site itself (475). 

Atomically clean surfaces can be prepared in vacuum to allow for the fundamental characterization of intrinsic surface properties such as dangling bonds associated with coordinatively unsaturated surface atoms. Ultimately, both the local and long-range electronic structure of the surface can be investigated. It is this electronic structure which fundamentally controls the chemical reactivity of the surface, and of which we know relatively little. 

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