Lead oxide crystal modification

While attempting to use platinum in fuel cells, it has been demonstrated that its surface exhibits important electrocatalytic activities toward the oxidation of organic compounds. However, this effect can sometimes be enhanced by the use of bimetallic surfaces [1-10]. The physical mixture and the electronic interaction of the alloy components lead to a modification in the interaction between the adsorbate and the substrate in an electrocatalytic reaction. As a consequence of the structural changes at the single crystal surfaces during the electrochemical activation (examined with in situ STM) [11], it has been demonstrated that most of the catalysts are constituted by randomly oriented islands [12-14]. 

It is generally assumed that the lead dioxide crystal comprises zones of ideal structures, Pb24048 (octahedrons) with no protons, and strongly protonated zones, Pb2404o(OH)g [90,91]. These protons form O—H O bonds with OH-type sensitivity. A correlation has been found between the oxide non-stoichiometry and the presence of protons in the crystal structure. The minimum Pb content in the two Pb02 modifications corresponds to a stoichiometric coefficient of 1.83. The latter corresponds to the maximum hydrogen amount that can be incorporated into the Pb02 crystal lattice [90,91]. 

The crystal modification of the lead oxide depends on the conditions of its manufacture. At temperatures of oxidation below 470 °C (as is the case with ball mill PbO), the tetragonal polymorph is formed. If oxidation is performed in the temperature range of 460—470 °C through the Barton process, 60—70 wt% of the lead oxide formed is tetragonal (a-PbO), 15—20 wt% is orthorhombic (p-PbO) and the remaining balance is amorphous lead oxide. 

Lead oxide PbO (plumbous oxide) can exist in two different modifications. Red lead oxide PbO precipitates in tetragonal crystals. Yellow lead oxide precipitates in rhombic crystals. Yellow Iccid oxide is also called Litharge (= Stone Silver ) this is because this oxide precipitates as slags when plumbiferous silver is refined. According to Atkins Physical Chemistry, these modifications of lead oxide have the following thermodynamic standard values ... 

TEMPO-mediated oxidation. With regenerated and mercerized celluloses, the oxidation leads to water-soluble p-l,4-linked poly glucuronic acid sodium salt (cellouronic acid, CUA) quantitatively [16]. In contrast, with native celluloses having the cellulose I crystal structure, the cellulose slurries maintain the slurry states even after TEMPO-mediated oxidation. These modified celluloses form water-insoluble fibers [17]. This has enabled modification of the surface of cellu-losic fibers. 

The recent discovery that nitric oxide (NO) is a signaling molecule ubiquitous in tissue has raised the question that one of the pathways contributing to superoxide toxicity in vivo might be the formation of the highly reactive peroxynitrite anion (ONOO") produced by spontaneous reaction of NO with superoxide (77). It has been shown that perox5mitrite is a substrate of SOD (78). The interaction of SOD with peroxynitrite leads to a permanent modification of the enzyme at Tyr-108. The structural determination of the peroxynitrite-modified Cu2Zn2SOD has been conducted on monoclinic crystals (79). The structure confirms that peroxynitrite permanently modifies the Tyr-108 side chain with formation of 3-nitroty-rosine. The modification does not alter active site residues and the enzyme remains fully active. 

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