Heterogeneous Character of Oxides

The heterogeneous character of oxides is due to (i) irregularity in the surface crystallographic structure (different types of crystal planes, growth steps, crystal edges and corners. Fig. 8.2) (ii) presence of various functional groups (M-0 , M-O-M, M=0, 

The complex picture of oxide surfaces can be studied from both qualitative and quantitative point of view by spectroscopic techniques, conventionally infrared (FT-IR) but also Raman and NMR spectroscopies [11, 13-15], spectrophotometric 

Starting from the strongest ones. This assumption can hold only if large differences among the adsorption constants are involved [40, 41], 

Following the approach above described, the energy distribution of several acidic simple metal oxides has been determined by using ammonia as probe [42]. The obtained total munber of acid sites (nmax = Sinmax,i) allowed to disclose the fol- 

Enthalpy of adsorption (kj/mol) dlAI203 TIOZ HseO ni4bS09 BZr02 MwOS 

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Performed experiments proved the heterogeneous character of oxidation of the PP plaques containing HAS by profiling nitroxide formation using ESRI method. 

In semi-crystalline materials, the heterogeneous character of oxidation can be described by preferential oxidation of the amorphous regions with subsequent oxidative degradation of the crystalline regions. 

Chain Decay of Hydroperoxyl Groups of PP Oxidative Degradation of Macromolecules Heterogeneous Character of Polymer Oxidation... 

The method was used for the study of the chemisorption of hydrogen on nickel and on zinc oxide, and Keier and Roginskil could demonstrate the heterogeneous character of the surfaces of these adsorbents 309). They could also demonstrate the heterogeneous character of the surface of charcoal for the chemisorption of hydrogen 310). 

Heterogeneity of the surfaces of oxide catalysts such as chromium oxide, zinc oxide, and zinc-chromium oxide has been postulated by H. S. Taylor (75) on the basis of adsorption studies. In the author s view, Taylor s experimental observations may be also explained without assuming a heterogeneous character of the oxide surfaces. 

S = selectivity, X = (mmol substrate converted)/(mmol substrate initially present) and E = efficiency of oxidant use, measured as (meq O in oxidation products / mmol H2O2 consumed). Control experiments prove the truly heterogeneous character of the catalysis. Maximally 0.3% of the total W was found in solution, which is by far insufficient to account for the observed activity. H2O2 was analyzed via cerimetry. TON = mmol (epoxide + solvolysis products) / mmol W. Maximum TON value is 300. in 4.5 ml MeOH. 

With microwave-assisted technology, Porcheddu and colleagues [11] prepared benzimidazoles via one-pot C-H activation of tertiary amines/oxidative cycliza-tion with 1,2-phenylenediamine in the presence of Pd/C (Scheme 14.10). To ascertain the heterogeneous character of the catalytically active species in this process, further leaching studies were performed. Both Sheldons hot filtration test and atomic absorption spectroscopy analysis (ICP-MS) of the filtrate detected no significant quantities of leached Pd in solution, suggesting that the reaction was a heterogeneous process. 

Another study examined the acidity and basicity of bulk Ga203 by NH3 and SO2 adsorptions microcalorimetry performed at 150°C. As alumina, Ga203 is amphoteric, with heats higher than 100 kJ/mol for both NH3 and SO2 adsorption, respectively [186]. The amphoteric character of bulk gallium oxides and strong heterogeneity of the surface acidic and basic sites were proved also by Petre et al. [179] using microcalorimetry of pyridine adsorption at 150°C and CO2 adsorption at 30°C. 

These rather complicated effects may probably result in a rather heterogeneous distribution of heats of those chemisorptions on the surfaces of oxides and of salts where the surrounding ions suffer changes of character and charge simultaneously. Other crystallographic faces will also give variations in the strength of these chemisorptions (see Sec. VIII). 

Because carbon black is the preferred support material for electrocatalysts, the methods of preparation of (bi)metallic nanoparticles are somewhat more restricted than with the oxide supports widely used in gas-phase heterogeneous catalysis. A further requirement imposed by the reduced mass-transport rates of the reactant molecules in the liquid phase versus the gas phase is that the metal loadings on the carbon support must be very high, e.g., at least lOwt.% versus 0.1-1 wt.% typically used in gas-phase catalysts. The relatively inert character of the carbon black surface plus the high metal loading means that widely practiced methods such as ion exchange [9] are not effective. The preferred methods are based on preparation of colloidal precursors, which are adsorbed onto the carbon black surface and then thermally decomposed or hydrogen-reduced to the (bi)metallic state. This method was pioneered by Petrow and Allen [10], and in the period from about 1970-1995 various colloidal methods are described essentially only in the patent literature. A useful survey of methods described in this literature can be found in the review by Stonehart [11]. Since about 1995, there has been more disclosure of colloidal methods in research journals, such as the papers by Boennemann and co-workers [12]. 

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