Manganese dioxides composite

The crystal stmcture of ramsdeUite [12032-73-4] is similar to that of P Mn02 except that double chains of MnO octahedra are cross-Unked to adjacent double chains through the sharing of oxygen atoms located at the corners. RamsdeUite and pyrolusite are the only manganese dioxide phases where the composition approaches the stoichiometric Mn02 formula. Heating ramsdeUite to 250°C transforms it to pyrolusite. 

Fig. 1. Schematic representation of a battery system also known as an electrochemical transducer where the anode, also known as electron state 1, may be comprised of lithium, magnesium, zinc, cadmium, lead, or hydrogen, and the cathode, or electron state 11, depending on the composition of the anode, may be lead dioxide, manganese dioxide, nickel oxide, iron disulfide, oxygen, silver oxide, or iodine.

Cathode Reaction. There are many different types of manganese dioxide (18), having varying activity in batteries. The only type suitable for alkaline batteries is y-Mn02, the mineral form of which is nsutite. The chemical composition of has been described (19) by the general formula... 

Ramsdellite is thermodynamically unstable toward a transformation into the stable ft -modification. Hence, it is rarely found in natural deposits. Natural ramsdellite has a stoichiometry close to the composition of Mn02 and can be considered another true modification of manganese dioxide. Attempts to synthesize ramsdellite in the laboratory usually lead to materials of questionable composition and structural classification. It is very likely that synthetic ramsdellite materials are more or less well-crystallized samples of the y-modification that will be described in more detail below. 

Depending on the composition of the active materials and on the manganese dioxide type employed, the OCV of freshly manufactured zinc-carbon cells with salt electrolyte varies between 1.55 and 1.85 V. It decreases during discharge and formation of the variable-composition mass. Upon prolonged storage of undischarged batteries, their OCV also decreases. 

The composition of the particles is related to that of the source rocks. Quartz sand [composed of silica (silicon dioxide)], which makes up the most common variety of silica sand, is derived from quartz rocks. Pure quartz is usually almost free of impurities and therefore almost colorless (white). The coloration of some silica sand is due to chemical impurities within the structure of the quartz. The common buff, brown, or gray, for example, is caused by small amounts of metallic oxides iron oxide makes the sand buff or brown, whereas manganese dioxide makes it gray. Other minerals that often also occur as sand are calcite, feldspar and obsidian Calcite (composed of calcium carbonate), is generally derived from weathered limestone or broken shells or coral feldspar is an igneous rock of complex composition, and obsidian is a natural glass derived from the lava erupting from volcanoes see Chapter 2. 

Nitric acid was known to alchemists in ancient times. Cavendish in 1784 synthesized the acid by applying an electric spark to humid air. Earlier in 1776, Lavoisier determined that the acid contained oxygen. In 1798, Milner prepared nitric acid from ammonia along with nitrogen oxides by oxidation of ammonia vapor over red-hot manganese dioxide. In 1816, Gay-Lussac and Berthollet established its composition. 

The name Leclanche cell is given to the familiar primary system consisting of a zinc anode, manganese dioxide cathode and an electrolyte of ammonium chloride and zinc chloride dissolved in water. The alternative designation zinc-carbon ceir is broader and includes the so-called zinc chloride system which, due to a different electrolyte composition, is characterized by a different discharge mechanism. The Leclanche cell may be written as... 

Qualitative and quantitative analysis show that a catalyst undergoes no change in mass or chemical composition. However, it may undergo a physical change. Thus, granular manganese dioxide (Mn02) used as a catalyst in the thermal decomposition of potassium chlorate is left as a fine powder at the end of the reaction. 

Kuwabata S, Kishimoto A, Tanaka T, Yoneyama H. Electrochemical synthesis of composite films of manganese dioxide and polypyrrole and their properties as an active material in lithium secondary batteries. J Electrochem Soc 1994 141 10-15. 

Gemeay AH, Nishiya H, Kuwabata S, Yoneyama H. Chemical preparation of manganese dioxide/poly-pyrrole composites and their use as cathode active materials for rechargeable lithium batteries. J Electrochem Soc 1995 142 4190-4195. 

Cupric oxide reacts with CO at temperatures which are substantially higher than those at which manganese dioxide is effective. The use of copper oxide in the combustion of CO in gas analysis is well known. By itself, it is not an effective catalyst for the oxidation of CO at ordinary temperatures. It may, however, greatly increase the activity of other oxides when used in mixtures of the proper proportion. The early work of Hofmann (23) indicated that a previously prepared surface of CuO will oxidize a mixture of CO and air, but that the rate can be increased by a factor of 3 if the copper is moistened with a little alkali. The catalytic activity is further increased if a little iridium is incorporated in the oxide. It was thought that the oxidation of CO depended upon the formation of an unstable copper peroxide of the composition, Cu203 or Cu02, which reacted with CO to form C02 and CuO. The principle was proposed for a gas generator cell of the type O/Cu/alkali/Cu/CO. The reaction was, however, too slow to be of practical importance. 

---