Surface Coordinative Unsaturation

A chromia gel activated only at 150° has no detectable catalytic activity for the hydrogenation of oleflns at room temperatures nor does it chemisorb oxygen or carbon monoxide. Catalytic and adsorptive capacity develops upon pretreating the gel at higher temperatures. After heating to 400°, chromia rapidly chemisorbs oxygen and carbon monoxide at —78° and it leads to rapid hydrogenation of ethylene at -78°. 

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- 

The surface of chromia appears to be an ideal case for study at the present. By activation at increasing temperatures, one can vary the number of active sites from none to some maximum number. At a low density of sites, one can hope that the sites are well separated and noninteracting. One can compare chemisorption of various gases with specific catalytic activities at various levels of site densities and hope to gain information about site heterogeneity. This chapter reports a first approach to this objective. 

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 

That the Cr3+ ion is coordinatively unsaturated is obvious. It must, however, be emphasized that the surface O - ion which is generated is also coordinatively unsaturated and of lower coordination number than that in bulk. In general, then, surface condensation of water generates a metal ion of lowered coordination number which will have the generalized properties of an acid and an oxide ion which will be more basic than an oxide ion in bulk. Peri (33) has considered the similar processes on alumina. 

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It is often useful to consider that sites for chemisorption result from surface coordinative unsaturation, i.e., that atoms at the surface have a lower coordination number than those in bulk. Thus, for example a chromium ion at the surface of chromium oxide has a coordination number less than that of a chromium ion in the bulk. The chromium ion will tend to bind a suitable adsorptive so as to restore its coordination number. An atom in the (100) surface of a face-centered cubic metal has a coordination number of 8 vs 12 for an atom in bulk this, too, represents surface coordinative unsaturation. However, of course, there are sites to which the concept of surface coordinative unsaturation does not apply, for example, Br nsted acid sites. 

As a consequence of the identical crystallographic structure and the very similar ionic radii, MgO and NiO or CoO form uniform solid solutions Mg(i X)MxO (0 < x < 1), where M is Ni or Co (25). Real crystals are often covered by strongly adsorbed water and carbon dioxide, and thermal treatments in vacuo are needed to clean the surfaces and create the surface coordinative unsaturation mentioned previously (in particular the fourfold and threefold coordinated sites are cleaned only at very high temperatures). In this connection, it is evident that the surface chemistry of clean surfaces is primarily determined by the presence of these... 

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... 

Hydrogen goes as two protons to convert two oxide ions to hydroxide ions and the two electrons reduce two Cr + to Cr -. The behavior of carbon monoxide is equivalent. As written, there is no requirement of surface coordinative unsaturation. However, coordinative unsaturation in the oxide ions which are converted to hydroxide ions would favor reductive adsorption. Further, for reasons outlined in Section IV, Cr will be easier to reduce to Cr2+ when it is also coordinatively unsaturated. Further, where this is so, heterolytic dissociative adsorption in the sense of Eq. (7) might subsequently occur at Cr +fcus). 

This is the only form of adsorption which we have suggested that does not involve surface coordinative unsaturation. 

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... 

Calcium addition removes the Ce -CO band, decreases the intensity of the Me -CO band, and shifts it to lower frequencies (cf. spectra 2 and 6, Fig. 2a). Since no appreciable variation in the density of bulk extended defects (Table 1) or Vis range absorption (Fig. 1) is observed, this result can be explained by preferential segregation of calcium cations in the vicinity of surface coordinatively unsaturated sites thus blocking them. 

Surface coordinatively unsaturated centers were studied by the infrared spectroscopy of adsorbed test molecules (CO, NO) using IFS-113V Bruker spectrometer [11, 12],... 

There are various acidic and basic sites on surfaces. Coordinatively unsaturated cations act as electron pair acceptors and are called Lewis acid sites. The Bronsted add sites on catalyst surfaces are hydroxyl groups able to donate protons. The basic sites are, in most cases, oxygen anions, including oxygen from hydroxyl groups. 

Alumina prepared by conventional hydrolysis of aluminum compound precursors is covered by surface hydroxyl groups. High-temperature calcination is needed in order to expose the surface coordination unsaturation of A1 ions. A new method, which involves stoichiometric hydrolysis of an amine-Al alkoxide monomeric complex, can generate alumina with a surface that is covered with far fewer hydroxyls without high-temperature postsynthesis treatment. In this method, the coordination unsaturation site of A1 is protected with an amine throughout the preparation process. The bound amine on the alumina surface can be exchanged with other bases, and the final solid is a Lewis-acid catalyst and catalyzes reactions such as aminolysis of epoxide. The chemistry in the preparation of such an alumina is described. 

Ultrasmall size of primary particles, specificity of stracture formation processes, extremely high dispersity and specific surface, coordination unsaturation of surface atoms of carbon, and the presence of functional groupings on them, all these significantly distinguish detonation diamonds from natural diamonds and static-synthesis diamonds. As the detonation method of UDD synthesis developed, it became clear that ultradisperse diamonds could have not only traditional but also absolutely new applications. 

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