Titanium-chromium oxides

Titanium/chromium oxide (Ti/Cr203 + Ti02) composite electrodes, fabricated by a ceramic method [168], may be used for oxidation of organic compounds. The chromium is oxidized to Cr03, which chemically oxidizes the substrate. The hfetime of the electrode may be improved by doping with Sb203. 

HDPE resias are produced ia industry with several classes of catalysts, ie, catalysts based on chromium oxides (Phillips), catalysts utilising organochromium compounds, catalysts based on titanium or vanadium compounds (Ziegler), and metallocene catalysts (33—35). A large number of additional catalysts have been developed by utilising transition metals such as scandium, cobalt, nickel, niobium, molybdenum, tungsten, palladium, rhodium, mthenium, lanthanides, and actinides (33—35) none of these, however, are commercially significant. 

LLDPE resias are produced ia iadustry with three classes of catalysts (11—14) titanium-based catalysts (Ziegler), metallocene-based catalysts (Kaminsky and Dow), and chromium oxide-based catalysts (Phillips). 

A wide variety of greens ranging from blue to yellow in shade ate based on cobalt in combination with chromium, aluminum, titanium, nickel, magnesium, antimony, or zinc. These are brighter than the chromium oxides. 

Titanium, vanadium or chromium oxides activated with chlorine-free organo-aluminum compounds, triethyl- or triisobutyl aluminum, have also been used as catalysts [285],... 

In polymerization by one-component catalysts [chromium oxide catalyst (75), titanium dichloride 159) at ethylene concentrations higher than 1 mole/liter and temperatures below 90°C the transfer with the monomer is a prevailing process. The spontaneous transfer, having a higher activation energy, plays an essential role at higher temperatures and lower concentrations of the monomer. 

The pigment properties of chromium oxides can be modified by precipitation of hydroxides (e.g., of titanium or aluminum), and subsequent calcining. This treatment changes the color to yellow-green, and decreases the flocculation tendency [3.54], Aftertreatment with organic compounds (e.g., alkoxylated alkylsulfon-amides) is also used [3.55]. 

Chromium(VI) oxide is strongly acidic. vanadium(V) oxide is amphoteric, titanium(IV) oxide is inert, and scandium(IH) oxide is basic with some amphoteric properties. 

Silicon, titanium, chromium, and aluminum oxides should be relatively acidic, with isoelectric points near 4.0, 6.0, 7.0, and 7.0, respectively. Iron oxide should be intermediate, with an isoelectric point near 9.0. Parks predicted the isoelectric point of nickel oxide to be of the order of 10-12. Other investigators [20] have reported the isoelectric point of nickel oxide to be near 9.4. 

Common chemical properties The alkali metals are so chemically reactive that they are never found free in nature. Sodium and potassium react explosively with water to produce hydrogen gas. The alkaline earth metals are not quite as reactive as the alkali metals. The alkali metals react with water but not explosively. The transition metals are generally the least reactive of all the metals. However, when they combine with other elements, they form a large variety of colored compounds. Chromium oxide is green, titanium oxide and zinc oxide are white, manganese oxide is purple, and iron oxide is ochre. 

The adsorption of formic acid and acetic acid leads to the formation of car-boxylate groups on aluminas (194, 295-299), titanium dioxides, (134, 135b, 176, 194, 300, 301), chromium oxide (134, 302, 303), zinc oxide (298, 304-306), and magnesium oxide (299, 304, 306). The corresponding dissociative chemisorption step most probably takes place on acid-base pair sites of the type... 

The corrosion resistance of steel can be greatly increased by alloying with chromium to form the stainless steels. Figure 12 shows the effect of increasing chromium content on the corrosion rate of steel. At 12-14% Cr there is a dramatic decrease in corrosion rate. The corrosion resistance is due to the formation of a thin adherent layer of chromium oxide on the steel surface [23]. The steel will remain stainless provided the oxide layer remains intact or can be rapidly repaired, i.e. the steel is exposed to oxidising conditions. The precipitation of chromium carbide at grain boundaries will cause disruption of this oxide film (See Sect. 3.2.5) and hence localised corrosion. Precipitation of chromium carbide can be reduced by alloying with elements which form carbides more readily than chromium, e.g. titanium, niobium, and tantalum. 

An increasing number of colored inorganic pigments are FDA-compliant. Historically, yellow iron oxide, red iron oxide, black iron oxide, zinc ferrite, burnt umber, raw and burnt sienna, channel carbon black, chromium oxide green, ultramarine blue, cobalt blue and copper chrome black have enjoyed FDA—compliant status, under 21 CFR 178.3297, Colorants for Polymers . More recently, the FDA has been successfully petitioned with regard to nickel titanium yellow, chrome titanium yellow, and cobalt green under 21 CFR 170.39, Threshold of Regulation for Substances Used in Food-Contact Articles . 

There are also catalysts known in which ACs are formed during the direct interaction between the transition metal compound and the monomer without participation of special cocatalysts (so-called one-component catalysts). Examples of one-component catalysts are chromium oxide catalysts, lower halides of transition metals (titanium dichloride) and catalysts prepared by precipitating organometallic compounds of transition elements on oxide carriers... 

Crystals have fascinating properties. A clear, colorless quartz crystal is pure silicon dioxide (Si02). But a crystal that is colorless in its pure form may exist as a variety of colored gemstones when tiny amounts of transition element compounds, usually oxides, are present. Amethyst (purple), citrine (yellow-brown), and rose quartz (pink) are quartz crystals with transition element impurities scattered throughout. Blue sapphires are composed of aluminum oxide (AI2O3) with the impurities iron(II) oxide (FeO) and titanium(IV) oxide (Ti02). If trace amounts of chromium(III) oxide (Cr203) are... 

Processing of bauxite to produce alumina produces volumes of red muds, which contain principally water, iron oxide, silica, and the oxides of titanium, chromium, vanadium, and aluminum. The solids in this mixture eventually settle to a relatively high solids content sludge, so that a moderately sized holding pond may be used for many years. 

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