Chromia surface coordinative unsaturation

It is clear that we cannot give such a specific discussion of the generation of surface coordinative unsaturation on amorphous chromia. However, similar considerations must be involved and we may suspect that various types of site pairs of coordinatively unsaturated Cr + and 0 -can be formed. 

In all of Section V to this point, what has been said with respect to chromia could be transferred with little change to alumina. However, data in the literature suggest that Eq. (3), the reaction which generates surface coordinatively unsaturated sites, is more difficult on... 

As indicated in Section IV, one might worry that the surface of chromia would become oxidized upon drying in air at 100-110° and that reduction by hydrogen of the oxidized form contributes in some special way to the formation of surface coordinatively unsaturated Cr3+. A chromia gel prepared by the urea method (21) which was dried at room... 

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. 

A series of papers was published by Vansant and coworkers dealing with the gas-phase deposition and thermal transformation of Cr(acac)3 to chromia on the surface of alumina and silica supports. Cr(acac)3 binds to the hydroxyl-terminated alumina surface by hydrogen bonding and/or a donor-acceptor interaction with coordinatively unsaturated Al + ions as outlined in Figure 23 . ... 

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. 

Experimental counterparts for the above computations are rare because the surface of a metal oxide does not usually contain exposed unsaturated metal sites. Only very recently have coordinatively unsaturated sites (cus) been identified on an oxide surface RuO2(110) can be seen to bind CO to Ru cus by scanning tunneling microscopy.239 In 19 Burwell proposed that dehydroxylated chromia (Cr203) contains cus, and that the Cr3+(cus) and 02-(cus) ion pairs chemisorb H2 nondissociatively below - 13(TC, most probably at Cr via polarization adsorption at the ion pairs.239 Pulses of D2 at -196°C completely and rapidly displace adsorbed... 

For chromia and zinc oxide catalysts, which have coordinatively unsaturated surfaces, alkenes can adsorb and produce a p-allyl anion. Double-bond migration on these surfaces is achieved by hydrogen abstraction to form the allyl species to which the hydrogen can be reintroduced in a different position. 

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