Metal oxides, catalytic etching

Metal oxides, 31 78-79, 89, 102, 123, 157-158, 191, 32 199-121 see also Amorphous metal oxides Sulfate-supported metal oxides specific oxides adsorbed oxygen on, 27 196-198 binary, surface acidity, 27 136-138 catalytic etching, 41 390-396 coordination number, 27 136 electrocatalysts, 40 127-128 Fe3(CO)i2 reaction with, 38 311-314 Lewis acid-treated, 37 169-170 multiply-valent metals, electrocatalytic oxidations, 40 154-157 superacids by, 37 201-204 surface acidity, methods for determining, 27 121... 

In sum, according to one postulated mechanism of catalytic etching, particularly of platinum, etching occurs via the transport of volatile metal oxide species. It has always been understood, even by the model s proponents, that it does not fully explain the observations. It has been repeatedly suggested that other volatile species may exist, or that other unaccounted-for processes are responsible for the observed behavior. Indeed, as discussed below, several recent studies suggest that other volatile species and other unaccounted-for processes may very well explain some or all catalytic etching. 

The SiC Schottky diodes and capacitors that have been processed by the authors were processed on either 6H or 4H substrates (n-type, about 1 x 10 cm ) with a 5-10- m n-type epilayer (2-6 x lO cm" ) [123, 124]. A thermal oxide was grown and holes were etched for the metal contacts. In the case of the Schottky sensors, the SiC surface was exposed to ozone for 10 minutes before deposition of the contact metal. This ozone treatment produces a native silicon dioxide of 10 1 A, as measured by ellipsometry [74, 75]. The MISiC-FET sensors (Figure 2.9) were processed on 4H-SiC, as previously described [125]. The catalytic metal contacts consisted of 10-nm TaSiyiOO-nm Pt, porous Pt, or porous Ir deposited by sputtering or by e-gun. 

The concept of mechanical fixation of metal on carbon makes catalytic applications at high temperatures possible. These applications require medium-sized active particles because particles below 2nm in size are not sufficiently stabilised by mechanical fixation and do not survive the high temperature treatment required by the selective etching. Typical reactions which have been studied in detail are ammonia synthesis [195, 201-203] and CO hydrogenation [204-207]. The idea that the inert carbon support could remove all problems associated with the reactivity of products with acid sites on oxides was tested, with the hope that a thermally wellconducting catalyst lacking strong-metal support interactions, as on oxide supports, would result. 

The materials mentioned in this section have in common that they were derived from intermetallic compounds, which acted as precursors for the active species. The selective etching of the compounds leads to Raney-type catalysts, whereas the controlled oxidation results in catalysts that consist of supported transition metals on oxides. Both systems have the common advantage that the transition metal of the intermetallic compound can be obtained in a finely divided state, resulting in a high specific activity. However, the catalytic properties such as selectivity and deactivation behavior often resemble to those of the underlying transition elements. A different approach is the use of intermetallic compounds with the ability to form hydrides. But also in this case, the catalytically active species is not the intermetallic compound, but its hydride, which possesses different properties, for example, the highly active hydridic hydrogen atoms as well as a different crystal and electronic structure. 

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