Oxides, bulk properties

Given the modest change in the W Liii white line, we had reason to believe that the bulk properties of the WO3 were hardly affected. In fact, the monitoring of the W Lj edges confirmed this idea. The two extreme cases, the fully oxidized WZ and PtWZ(acac) after reduction at 723 K, are compared to three different reference materials. 

Praliaud, H., Mikhailenko, S., Chajar, Z. et al. (1998) Surface and bulk properties of Cu—ZSM-5 and Cu/A1203 solids during redox treatments. Correlation with the selective reduction of nitric oxide by hydrocarbons, Appl. Catal. B, 16, 359. 

All subsequent preparations of Cf metal have used the method of choice, that is, reduction of californium oxide by La metal and deposition of the vaporized Cf metal (Section II,B) on a Ta collector 10, 30, 32, 45, 91, 97, 120). The apparatus used in this work is pictured schematically in Fig. 16. Complete analysis of Cf metal for cationic and anionic impurities has not been obtained due to the small (milligram) scale of the metal preparations to date. Since Cf is the element of highest atomic number available for measurement of its bulk properties in the metallic state, accurate measurement of its physical properties is important for predicting those of the still heavier actinides. Therefore, further studies of the metallic state of californium are necessary. 

In general terms, building a defined nano-architecture in oxide-type materials further extends the concept of nanocatalysis, e.g. when the electrons are confined, and physical and chemical properties are not scalable from the bulk properties. Studies have been made mainly on clusters/metal particles in the... 

Correlations between catalytic activity and a variety of bulk properties of semiconductors have been reported (i) the average band gap of III-V and II-VI semiconductors and activity towards hydrogenation of isopropanol (ii) enthalpy of oxides and their activity towards oxidation of propylene and (iii) number of d-electrons (and crystal field stabilization energy) or 3rf-metal oxides and their activity towards N2O decomposition. The last correlation, due to Dowden (1972), is important since it provides a connection between heterogeneous catalysis and coordination chemistry of transition-metal compounds. A correlation between the catalytic activity of transition-metal sulphides towards hydrodesulphurization of aromatic compounds and the position of the transition metal in the periodic table has been made by Whittingham ... 

The bulk properties of a material are best obtained from large single crystals. However, measurements on single crystals of zinc oxide of desirable purity and size are not yet available. Some work, however, has been done on small pure crystals and larger impure crystals. 

Considering the method of preparation of these ZnO samples, these results correspond to what one would expect on the basis of the adsorption model. The samples were sintered or evaporated at some high temperature, and then cooled to room temperature in air. As discussed in Section IV, 1, adsorption will occur until the rate of electrons crossing the surface barrier is pinched off to zero at room temperature. If the temperature is now lowered below room temperature, no electron transfer will be possible between the surface level and the bulk of the solid due to this high surface barrier. Thus the surface levels will be isolated and unable to affect the conductivity, which will therefore reflect bulk properties of the zinc oxide. 

Fluorescence detection, because of the limited number of molecules that fluoresce under specific excitation and emission wavelengths, is a reasonable alternative if the analyte fluoresces. Likewise, amperometric detection can provide greater selectivity and very good sensitivity if the analyte is readily electrochemically oxidized or reduced. Brunt (37) recently reviewed a wide variety of electrochemical detectors for HPLC. Bulk-property detectors (i.e., conductometric and capacitance detectors) and solute-property detectors (i.e., amperometric, coulo-metric, polarographic, and potentiometric detectors) were discussed. Many flow-cell designs were diagrammed, and commercial systems were discussed. 

The above processes involve separation based either on bulk properties (for example, size, density, shape, etc.) directly or by subtle control of the chemistry of the narrow interfacial region between the mineral particle and the aqueous solution in which it is suspended. In the processing of certain ores, such as those of uranium, gold or oxidized copper, chemical alteration of the minerals may be required to recover the valuable metals. These techniques are not discussed here, except to include those aspects which are directly related to surfaces and interfaces. 

Figure 21 provides an example of the use of ESCA to define an oxidation state of a freshly reduced palladium-on-carbon hydrogenation catalyst exposed to the air. The metallic palladium peaks (Fig. 21a) are quite evident, indicating no bulk oxidation occurred. There is a strong peak for carbon, probably due to adsorbed CO2 from the air. The presence of a small amount of PdO is suggested at 337 eV in Fig. 21B. This peak is a shoulder on the palladium 3 5/2 peak and most likely represents a surface layer of oxide on the palladium. This information could not be conveniently obtained by XRD because small palladium (or PdO) crystallites cannot diffract X rays. Furthermore, XRD measures bulk properties and would not see surface oxides even if the crystallite sizes were sufficiently large to be XRD sensitive. We can therefore expect to see more frequent use of ESCA or other surface sensitive techniques to monitor the surface of catalytic materials.

Indeed, lattice parameters of both the copper and the zinc oxide were found to depend on the catalyst composition. The lattice extension of copper was attributed to alpha brass formation upon partial reduction of zine oxide, and an attempt was made to correlate the lattice constant of copper with the decomposition rate of methanol to methyl formate. Furthermore, the decomposition rate of methanol to carbon monoxide was found to correlate with the changes of lattice constant of zinc oxide. Although such correlations did not establish the cause of the promotion in the absence of surface-area measurements and of correlations of specific activities, the changes of lattice parameters determined by Frolich et al. are real and indicate for the first time that the interaction of catalyst components can result in observable changes of bulk properties of the individual phases. Frolich et al. did not offer an interpretation of the observed changes in lattice parameters of zinc oxide. Yet these changes accompany the formation of an active catalyst, and much of this review will be devoted to the origin, physicochemical nature, and catalytic activity of the active phase in the zinc oxide-copper catalysts. 

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