Chromium oxides, deposition

Satisfactory yields of simple aldehydes are also usually obtained when the vapour of the primary alcohol is dehydrogenated by passage over a heated catalyst of copper-chromium oxide deposited on pumice (Expt 5.75). 

Michorczyk P, Ogonowski J, Zenczak K (2011) Activity of chromium oxide deposited on different silica supports in the dehydrogenation of propane with CO2—a comparative study. J Mol Catal A Chem 349 1-12... 

By passing the alcohol vapour over a copper - chromium oxide catalyst deposit on pumice and heated to 330°, for example ... 

Chromium Oxide-Based Catalysts. Chromium oxide-based catalysts were originally developed by Phillips Petroleum Company for the manufacture of HDPE resins subsequendy, they have been modified for ethylene—a-olefin copolymerisation reactions (10). These catalysts use a mixed sihca—titania support containing from 2 to 20 wt % of Ti. After the deposition of chromium species onto the support, the catalyst is first oxidised by an oxygen—air mixture and then reduced at increased temperatures with carbon monoxide. The catalyst systems used for ethylene copolymerisation consist of sohd catalysts and co-catalysts, ie, triaLkylboron or trialkyl aluminum compounds. Ethylene—a-olefin copolymers produced with these catalysts have very broad molecular weight distributions, characterised by M.Jin the 12—35 and MER in the 80—200 range. 

The triggering mechanism for the corrosion process was localized depassivation of the weld-metal surface. Depassivation (loss of the thin film of chromium oxides that protect stainless steels) can be caused by deposits or by microbial masses that cover the surface (see Chap. 4, Underdeposit Corrosion and Chap. 6, Biologically Influenced Corrosion ). Once depassivation occurred, the critical features in this case were the continuity, size, and orientation of the noble phase. The massive, uninterrupted network of the second phase (Figs. 15.2 and 15.21), coupled... 

Tin Free Steel—Electrolytic Chromium-Coated. A less expensive substitute for tinplate, electrolytic chromium coated-steel, has been developed and is designated TFS-CT (tin free steel-chromium type) or TFS-CCO (tin free steel-chromium-chromium oxide) (19). This material can be used for many products where the cathodic protection usually supplied by tin is not needed. A schematic cross section is shown in Figure 2. Electrolytic, chromium-coated steel is made by electro-lytically depositing a thin layer of metallic chromium on the basic tin mill steel, which is in turn covered by a thin passive coherent layer of chromium oxide. 

Chromium oxide is deposited by the decomposition of chromium acetyl acetonate, Cr(C5H702)3, in the 520-560°C temperature range.[ 1 It can also be deposited by the decomposition of the carbonyl in an oxidizing atmosphere (CO2 or H2O), at low pressure (< 5 Torr).Dl... 

The kinetics of chromium (III) oxide deposition from solution during the leaching of skeletal copper has been studied, and a linear rate was found that is not affected by chromate concentration but decreases with increasing hydroxide concentration [55], The total amount deposited was greater as the chromate concentration increased. 

After heating, the EB is mixed with superheated steam and fed to the first stage reactor. Both the first and second stage reactors are packed with a catalyst of metal oxide deposited on an activated charcoal or alumina pellets. Iron oxide, sometimes combined with chromium oxide or potassium carbonate, is commonly used. 

Oxyhalides are another type of volatile inorganic precursors, which have been used in only a few studies, however. For instance, timgsten oxyfluo-ride (WO F ) and H2O have been used as precursors in the deposition of WO3 [48], while Cr02Cl2 together with CH3OH as an oxygen source have been used in the deposition of chromium oxide [49]. 

Natural, minable deposits of chromium oxide are not known. In addition to pigment grade, chromium oxide producers usually also offer a technical grade for applications based on properties other than coloration. These include ... 

These results indicate that alumina acts on CrgOg phase to prevent its clustering and segregation with high coordinate Cr + ions. This dispersive effect of the support provides a suitable environment for the formation on the surface of low coordinate chromium ions (ref. 2). However this effect of alumina tends to depress when the content of chromium exceeds 30 % at 410°C. This result seems to Indicate a saturation of the surface sites of the support which interact with the chromium (surface of alumina covered with a layer of chromium oxide). Then an excess of Cr O deposited leads to its clustering and crystallization. Consequently the coordination of chromium ions changes from tetrahedral (low coordination) to octahedral (high coordination). 

Certain types of catalyst uranium oxide and chromium oxide may be used as a promoter. This is reported to give a higher resistance to catalyst poisoning by sulfur components and a lower tendency to form carbon deposits. 

In the discussion of the subject Balandin mentions (15) that Fischer previously postulated that methylene radicals may be produced as an intermediate in the formation of hydrocarbons by his method (116). This mechanism of carbon deposition on platinum supported on oxides of nickel and chromium (oxidized nichrome) through the intermediate formation of methylenes was thought by Balandin to be similar to the mechanism of dehydrogenation over this type of catalyst in that both occur on the boundaries of platinum-nickel and of platinum-chromia and were brought in agreement by him with his multiplet theory (26). 

The dehydrogenation catalyst must be sufhciently active to allow for very short contact times and the use of low temperatures, to minimize thermal cracking reactions. Carbon deposits are eliminated by heatihg in the presence of a gas containing oxygen. -This means that the catalyst must be thermally stable to avoid being deactivated during the oxidation of the deposits. The best catalysts contain alamina and chromium oxide, but these cannot be employed in the presence of steam. Operations are conducted at a temperature between 550 and 700 C, and low pressure, less than 0.1.10 Pa absolute. 

We briefly describe here the results obtained from studies of copper, nickel and chromium overlayers deposited on polystyrene, polyvinyl alcohol, polyvinyl methyl ether, polyethylene oxide, polyvinyl acetate and polymethyl methacrylate. Using X-ray photoemission spectroscopy we measured significant variations in the core binding energies and lineshapes as we varied both the metal and the substrate atoms. These changes can be related to both differences between the intrinsic properties of the metal atoms as well as to differences in the interactions with the substrates. In the following sections we describe the details of... 

A nickel-chrome spinel, NiO.CrjOg, has been made artificially in small green crystals. The green colour of emerald, serpentine, possibly sapphire, and other minerals is due to the presence of compounds of chromium, which also occur in rubies, corundums, and spinels. It has been shown that the peculiar spectra of ruby and diamond are due to chromium oxide which has been compelled to vibrate in an abnormal manner, leading to the production of narrow absorption bands. Traces of chromium have been detected in the soot produced by the combustion of coal obtained from deposits at Li ge. Compounds of chromium are not known to play any part in the economy of plants or animals. 

Precipitation-deposition can be used to produce catalysts with a variety of supports, not only those that are formed from coprecipitated precursors. It has been employed to prepare nickel deposited on silica, alumina, magnesia, titania, thoria, ceria, zinc oxide and chromium oxide.36 It has also been used to make supported precious metal catalysts. For example, palladium hydroxide was precipitated onto carbon by the addition of lithium hydroxide to a suspension of... 

PS can also be formed chemically without the application of a current by simply immersing a silicon sample in a HF solution containing some oxidation agents such as nitric acid or chromium oxide. The oxidants can be the redox couples, which do not deposit onto the surface after the reduction, or metal ions which deposit onto the surface after the reduction. The PS formed under such a condition are often referred to as stain films or chemical PS. As a characteristic feature, the OCP of the silicon during formation of chemical PS in the presence of an oxidant is several hundred millivolts more positive than without addition of an oxidation agent. The formation of chemical PS can further be modified by illumination. ... 

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