Oxygen adsorbed chromium

Schlatter et al. found that their data with copper chromite agrees better with 0.7 order for CO concentrations (53). For crystals of nickel oxide and chromium oxide, Yu Yao and Kummer have found that the kinetics depend on CO or hydrocarbon around 0.55 order and depend on oxygen around 0.45 order (79). Hertl and Farrauto found evidence that CO adsorbs on copper as a carbonyl group, and adsorbs on chromium oxide as a unidentate carbonate. They found that the kinetics depends on CO to the first order, and depends on oxygen to the zero order (80). 

Here NT is the center containing a reduced chromium ion and A is the oxygenated compound that is adsorbed on the active center formed. 

Thermal reduction at 623 K by means of CO is a common method of producing reduced and catalytically active chromium centers. In this case the induction period in the successive ethylene polymerization is replaced by a very short delay consistent with initial adsorption of ethylene on reduce chromium centers and formation of active precursors. In the CO-reduced catalyst, CO2 in the gas phase is the only product and chromium is found to have an average oxidation number just above 2 [4,7,44,65,66], comprised of mainly Cr(II) and very small amount of Cr(III) species (presumably as Q -Cr203 [66]). Fubini et al. [47] reported that reduction in CO at 623 K of a diluted Cr(VI)/Si02 sample (1 wt. % Cr) yields 98% of the silica-supported chromium in the +2 oxidation state, as determined from oxygen uptake measurements. The remaining 2 wt. % of the metal was proposed to be clustered in a-chromia-like particles. As the oxidation product (CO2) is not adsorbed on the surface and CO is fully desorbed from Cr(II) at 623 K (reduction temperature), the resulting catalyst acquires a model character in fact, the siliceous part of the surface is the same of pure silica treated at the same temperature and the anchored chromium is all in the divalent state. 

Despite their importance in olefin polymerization reactions, little attention has been paid to the nature of the adsorbed oxygen species on supported chromium oxide systems. 

Oxygen is adsorbed by the divalent catalyst with a brilliant flash of chemiluminescence, converting the chromium back to its original orange hexavalent state (15,17,41,42). The ease with which this reversal occurs suggests that there is little rearrangement during reduction at 350°C. 

However, a second component shifted to higher binding energy was observed for thin films of the compound adsorbed onto aluminum that had been cleaned by ion bombardment and then exposed to oxygen. Finally, Linde [18] has suggested that polyamic acids of pyromellitic dianhydride and oxydianiline do not react with films formed by y-APS adsorbed onto metal substrates such as aluminum and chromium because the silane is adsorbed through the amino groups. 

From the standpoint of a theory recently published (2) the initial oxidation rate of a metal is often established by escape of electrons from the metal to the oxide. Hence, the oxidation resistance of chromium is expected to be governed not by a low potential acting on cations in the oxide, but by a high negative field established first by oxygen ions adsorbed on the metal and later by oxygen ions adsorbed on the oxide and by an increasing num-... 

On the theoretical side also, importance must be attached to oxides and sulfides because these present many problems which do not arise with metallic catalysts. The function of the oxygen and sulfur atoms in such lattices is a matter of prime importance, and structures produced by partial removal of these elements from their compounds must have very special properties. It would be of great interest to know, for example, in what positions the hydrogen atoms split off during ring-closure of aliphatic hydrocarbons are adsorbed on the catalyst it seems highly probable that the oxygen atoms in the chromium oxide or molybdenum oxide are involved and that they contribute an essential part to the specific effects of these catalysts. 

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