Active surface centers

The interpretation of the C.E. by a superimposition of reactions occurring at different active surface centers is compatible with the fact that many multicomponent catalysts exhibit a C.E. but no C.E. is found when very pure substances have been subjected to different thermal pretreatments (17). This implies the possibility that many active centers are due to impurities and that their numbers may change with the pretreatment of the catalyst, e.g., by means of aggregation, volatilization, etc. As an illustration, data for the decomposition of N2O on MgO, prepared from synthetic and from natural magnesites, and data for the para-ortho hydrogen conversion on pure metals and on alloys are presented in Tables II and III. 

In many cases a compensation effect is indicated on comparison of the rates of the same reaction at the same type of catalyst after the latter has undergone special pretreatments or has been changed in its composition. In such cases the occurrence of a compensation effect may be expected if different kinds of active surface centers act simultaneously as sites of the catalytic reaction and if the proportions of these different active centers (involving different activation energies) are shifted by means of special pretreatments of the catalyst or by changes in its composition. 

Chemically, each silicon atom is surrounded by four oxygen atoms in the form of a tetrahedron. The surface of silica contains (a) siloxane groups (Si — O — Si ), (b) silanol groups (Si —OH), (c) water hydrogen-bonded to the silanol groups, and (d) nonsorbed capillary or bulk water. The silanol groups (about 8 Limol/m ) represent adsorption-active surface centers... 

The catalytic performance of 03 to produce carbon oxides in MSR can, to some extent, be rationalized using a concept of Tatibouet et al. [35], which was originally applied to the selective oxidation of methanol and is based on the influence of the strength of acidic and basic oxide surface sites on product formation. According to this concept, the formation of carbon oxides (and preceding formate species) reqnires the presence of redox-active surface centers as well as both weak acidic and rather strong basic sites. 363 is a reducible/amphoteric oxide with... 

Route 1 can be practiced easily. Its main disadvantage has already been described for polystyrene and includes the possibility of creating different sites and the difficulty of determining their structure. Murrell [69] discussed four possible surface complexes as a result of the reaction between phosphinated silica with [Rh(CO)2Cl]2. The uniformity of the catalytically active surface centers can be guaranteed by route 2, which involves the reaction of a preformed complex bearing silicon-substituted ligands [69] (eq. (14)). 

C. Collective properties of the solid and active surface centers ... 

In the case of nonmodified silica gels, adsorption of the test substances by the stationary phase is the decisive retention mechanism for chromatographic separation. Selective interactions of the sample molecules to be separated take place at the active surface centers of the silica gel. Forces that affect interactions include hydrogen bonding, dipole-dipole, and electrostatic interactions. The intensity of these forces depends on the following factors ... 

It has been suggested that one might explain the spread in adhesive force by reference to the energy inhomogeneity of the surface and the presence of active surface centers. Particles falling in such regions are held particularly firmly, so that it is most difficult to remove the last few particles. 

In order to explain the number of inductive loops, Schweickert and CO-workers postulated a catalytic iron dissolution mechanism occurring at kink sites, the number of these active surface centers depending exponentially on the electrode potential. 

While the MMA study focused on the bulk polymerization induced by the surface, only the surface species has been under consideration for Y MPS. Also, polymerized FMMA can be removed from the surface by dissolving into MMA and consequent renewal of the active surface centers. On the other hand, Y MPS forms cross-linked polymers that cover the surface permanently. 

His researches and those of his pupils led to his formulation in the twenties of the concept of active catalytic centers and the heterogeneity of catalytic and adsorptive surfaces. His catalytic studies were supplemented by researches carried out simultaneously on kinetics of homogeneous gas reactions and photochemistry. The thirties saw Hugh Taylor utilizing more and more of the techniques developed by physicists. Thermal conductivity for ortho-para hydrogen analysis resulted in his use of these species for surface characterization. The discovery of deuterium prompted him to set up production of this isotope by electrolysis on a large scale of several cubic centimeters. This gave him and others a supply of this valuable tracer for catalytic studies. For analysis he invoked not only thermal conductivity, but infrared spectroscopy and mass spectrometry. To ex-... 

Theories neglect that catalysts usually have limited turnover numbers due to destructive side reactions. This may not be so obvious in analytical experiments but it has severe consequences for large scale applications. A simple calculation can illustrate this problem if a redox polymer with a monomer molecular weight of 400 Da and a density of 1 g cm " is considered with all redox centers addressable from the electrode and accessible to the substrate with a turnover number of 1000, then, to react 1 nunol of substrate at a 1 cm electrode surface, at least 5 pmol of active catalyst centers corresponding to 2 mg of polymer, or a dry film thickness of 20 pm are required. This is 20 times more than the calculated optimum film thickness for rather favorable conditions... 

In this figure, the activation energies of N2 dissociation are compared for the different reaction centers the (111) surface structure ofan fee crystal and a stepped surface. Activation energies with respect to the energy of the gas-phase molecule are related to the adsorption energies of the N atoms. As often found for bond activating surface reactions, a value of a close to 1 is obtained. It implies that the electronic interactions between the surface and the reactant in the transition state and product state are similar. The bond strength of the chemical bond... 

Thermal reduction at 623 K by means of CO is a common method of producing reduced and catalytically active chromium centers. In this case the induction period in the successive ethylene polymerization is replaced by a very short delay consistent with initial adsorption of ethylene on reduce chromium centers and formation of active precursors. In the CO-reduced catalyst, CO2 in the gas phase is the only product and chromium is found to have an average oxidation number just above 2 [4,7,44,65,66], comprised of mainly Cr(II) and very small amount of Cr(III) species (presumably as Q -Cr203 [66]). Fubini et al. [47] reported that reduction in CO at 623 K of a diluted Cr(VI)/Si02 sample (1 wt. % Cr) yields 98% of the silica-supported chromium in the +2 oxidation state, as determined from oxygen uptake measurements. The remaining 2 wt. % of the metal was proposed to be clustered in a-chromia-like particles. As the oxidation product (CO2) is not adsorbed on the surface and CO is fully desorbed from Cr(II) at 623 K (reduction temperature), the resulting catalyst acquires a model character in fact, the siliceous part of the surface is the same of pure silica treated at the same temperature and the anchored chromium is all in the divalent state. 

The types of macrocycles most studied in which the active metal center is believed to be retained include Co, Fe, Ru porphyrins and related macrocycles. In these studies the optimal pyrolysis temperature is often reported to be between 400-800 °C. Above these temperatures, the active site begins to be destroyed, and activity decreases.49 An array of characterization techniques have been used to support these claims. XPS analysis has demonstrated that at the highest activity of samples, the surface composition of metal and nitrogen is also at its highest.78,96 Above the optimal treatment temperature, nitrogen and metal begin to disappear from the surface. Furthermore, Mossbauer spectroscopy and XAS have been used to... 

Specific, surface confined reactions not only directly involve catalysis but also the built-up of sdf-assembled multilayers (see Fig. 9.1 (3)) with co-functionalities for more complex (bio-) catalytic systems such as proteins or the directed deposition of active metals. Furthermore, SAM on flat substrates can be used for the study and development of e.g. catalytic systems, but are not useful for large scale applications because they have very limited specific surface. Here, nanoparticle systems covered with 3D-SAMs are the ideal solution of combining the advantages of high surface area, defined surface composition and accessibility of proximal active catalytic centers. 

The fulvene route was also successfully employed in the preparation of a compound, which can be regarded as one of the most advanced molecular models for a catalytically active titanium center on a silica surface. When Cp Ti(C5Me4CH2) was reacted with the monosilylated silsesquioxane precursor 12 in refluxing toluene a color change from deep purple to amber was observed. Crystallization afforded a bright-yellow material, which was subsequently shown to be the novel mo o(pentamethyleyclopentadienyl) titanium(IV) silsesquioxane complex 126 (69% yield). Its formation is illustrated schematically in Scheme 42. 

Not all the dislocations outcropped on a crystal surface are active growth centers. In Fig. 6.6, the distribution of growth hillocks on a (111) face of an as-grown Ba(N03)2 crystal is compared with the distribution of dislocations revealed by X-ray... 

Simultaneous Occurrence of Reactions on Surface Centers Involving Different Activation Energies... 

---