Lead oxides, deposition

The mechanism of poisoning automobile exhaust catalysts has been identified (71). Upon combustion in the cylinder tetraethyllead (TEL) produces lead oxide which would accumulate in the combustion chamber except that ethylene dibromide [106-93-4] or other similar haUde compounds were added to the gasoline along with TEL to form volatile lead haUde compounds. Thus lead deposits in the cylinder and on the spark plugs are minimized. Volatile lead hahdes (bromides or chlorides) would then exit the combustion chamber, and such volatile compounds would diffuse to catalyst surfaces by the same mechanisms as do carbon monoxide compounds. When adsorbed on the precious metal catalyst site, lead haUde renders the catalytic site inactive. 

Experiments continued to find an appropriate form of lead that could at the same time prevent the formation of oxide deposits. Ethylene was found to combine with lead to form tetraethyllead (TEL), a stable compound that satisfied this requirement. 

The deposits formed in internal combustion engines by high-octane petrols may be classed as ashes they consist of mixtures of lead oxides, bromides and sulphates derived from the anti-knock additives and, of course, exert their main corrosive effect on the parts operating at the highest temperature the exhaust valves and the sparking-plug electrodes. 

An interesting case of ash attack is encountered with valves in engines powered by high octane fuels containing lead compounds. These compounds are deposited from the gases as mixtures of lead oxide, sulphate and bromide, and can cause serious scale-fluxing effects with high-alloy valve steels. 

Localized pre-boiler scale and corrosion debris deposits. Combination of New phosphate, iron, copper, and silica deposition Old re-deposited debris Transport of Fe, Cu, Ni, Zn, Cr oxides to HP boiler section, leading to deposition, fouling, and possible tube failures Transport of minerals and debris including malachite, ammonium carbamate, basic ferric ammonium carbonate Precipitation in FW line of phosphates, iron, and silicates... 

Using a "home made" aneroid calorimeter, we have measured rates of production of heat and thence rates of oxidation of Athabasca bitumen under nearly isothermal conditions in the temperature range 155-320°C. Results of these kinetic measurements, supported by chemical analyses, mass balances, and fuel-energy relationships, indicate that there are two principal classes of oxidation reactions in the specified temperature region. At temperatures much lc er than 285°C, the principal reactions of oxygen with Athabasca bitumen lead to deposition of "fuel" or coke. At temperatures much higher than 285°C, the principal oxidation reactions lead to formation of carbon oxides and water. We have fitted an overall mathematical model (related to the factorial design of the experiments) to the kinetic results, and have also developed a "two reaction chemical model". 

We propose that the complicated dry oxidation of bitumen can be represented as the sum of contributions from two classes of oxidation reaction. One class of reactions is the partial oxidation that leads to deposition of coke and formation of "oxygenated bitumen", with very little production of carbon oxides and water. This class of reactions is concisely summarized by... 

Melroy and co-workers88 recently reported on the EXAFS spectrum of Pb underpotentially deposited on silver (111). In this case, no Pb/Ag scattering was observed and this was ascribed to the large Debye-Waller factor for the lead as well as to the presence of an incommensurate layer. However, data analysis as well as comparison of the edge region of spectra for the underpotentially deposited lead, lead foil, lead acetate, and lead oxide indicated the presence of oxygen from either water or acetate (from electrolyte) as a backscatterer. 

A. Eftekhari, pH sensor based on deposited film of lead oxide on aluminum substrate electrode. Sens. Actuators B. 88, 234—238 (2003). 

There was no accumulation of metals in either the anolyte or catholyte circuits when a spike of metals was fed with the M28 propellant to simulate particles from antiresonance rods. AEA attributes this success to the use of the catholyte-to-anolyte recycle and the anolyte purge operation. Lead, present in M28 propellant as lead stearate (approximately 0.5 weight percent), was oxidized to lead oxide (Pb02) and did not accumulate in solution. Lead oxide was found on the electrode surfaces and as a deposit in the bottom of the cell cavities (AEA, 2001d). A demonstration test successfully removed the lead oxide using an offline formic acid wash of the cell electrode cavities. This is the planned approach for removing accumulating lead oxide. No lead material balance was provided. 

The PE separators have excellent microporous structure for electrolyte flow with minimal lead particle deposits excellent ductility, strength, and toughness for envelopability and plate puncture resistance excellent oxidation, chemical and thermal resistance to resist premature deterioration and good manufacturability with high production efficiency and relatively low raw material cost, which reduces overall manufacturing costs. The PE pocket sepa-... 

Alkyl lead compounds are extremely effective gasoline antiknock agents. By decomposing to form lead oxide compounds during the gasoline combustion process, lead alkyls interrupt the rapid chain scission reactions which lead to combustion knock. Also, lead alkyls help to prevent exhaust valve seat wear and may minimize octane requirement increase. However, unless utilized in conjunction with lead scavengers such as 1,2-dichloromethane, lead deposits can accumulate within the gasoline combustion chamber. 

A variation of the CD process for PbSe involved deposition of a basic lead carbonate followed by selenization of this film with selenosulphate [64]. White films of what was identified by XRD as 6PbC03-3Pb(0H)2-Pb0 (denoted here as Pb—OH—C) were slowly formed over a few days from selenosulphate-free solutions that contained a colloidal phase and that were open to air (they did not form in closed, degassed solutions). CO2 was necessary for film formation—other than sparse deposits, no film formation occurred of hydrated lead oxide under any conditions attempted in this study. Treatment of these films with selenosulphate solution resulted in complete conversion to PbSe at room temperature after 6 min. The selenization process of this film was followed by XRD, and it was seen to proceed by a breakdown of the large Pb—OH—C crystals to an essentially amorphous phase of PbSe with crystallization of this phase to give finally large (ca. 200 nm) PbSe crystals covered with smaller (15-20 nm) ones as well as some amorphous material. 

Lead deposition, undeipotential deposition, 1313 Lead oxide, as electrocatalyst, 1287 in lead acid battery, 1287 Levich, V. G 1140, 1468, 1516 Levich equation, 1141, 1234 Librational entropy, 914, 915 Librational motion of adsorbed ions, 928 Librator fluctuation model, 1516 Libratory motion, 915... 

Studies of the formation, chemical composition, and properties of deposits have shown that they consist of partially oxidized organic material, including more or less nitrogen, sulfur, and phosphorus. Compounds of iron, silicon, calcium, and other metals are present in small quantity, together with substantial amounts of lead oxides, sulfates, and halides from combustion of the antiknock fluid. The effects of these deposits are both physical and chemical in nature they may physically interfere with lubrication, heat transfer, gas flow, operation of valves and spark plugs chemically, they may bring about corrosion and oxidation. 

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