Surface reaction with carbon dioxide

Surfaces functionalized with primary amine groups should normally be further reacted directly after they are produced, as the amine groups rapidly lose reactivity when kept in normal aqueous buffer conditions [31]. This is believed to occur by carbamate formation via reaction with carbon dioxide, and also via oxidation phenomena. 

The pH of the electrolyte does not only have an effect on the passivation potential, but also on the passivation current density, because both the metal dissolution kinetics and the solubility of hydroxides depend on pH. Figure 6.16 shows that the passivation current density of iron becomes smaller at higher pH. This has been explained by a lowering of the solubility of ferrous hydroxide, which precipitates at the surface. Since both the passivation potential and the passivation current density decrease with increasing pH, spontaneous passivation of iron becomes possible in basic, aerated media. This explains why steel reinforcements in concrete (pH >13) resist corrosion well as long as chemical reactions with carbon dioxide from air (carbonation of concrete) do not modify the alkalinity. 

The adsorbed oxygen atom on the copper surface is removed by reaction with carbon monoxide and provides a pathway for the formation of the carbon dioxide needed in the main reaction. 

Methane reforming with carbon dioxide proceeds in a complex sequence of reaction steps involving the dissociative adsorption/reaction of methane and COj at metal sites. Hydrogen is generated during methane dissociation In the second set of reactions CO2 dissociates into CO and adsorbed oxygen. The reaction between the surface bound carbon (from methane dissociation) and the adsorbed oxygen (from CO2 dissociation ) yields carbon monoxide. A stable catalyst can only be achieved if the two sets of reactions are balanced. 

Epoxy resin formulations containing aliphatic amines will blush and provide an oily surface under very humid conditions. This is due to a reaction of the amine primary hydrogen atoms with carbon dioxide. Resistance to blushing is more important for coatings than for... 

Table 1(c) on the formation or removal in vacua of carbon dioxide by reaction of the surface oxides with carbon in the metal shows the results of these calculations. The reactions are feasible for tungsten and iron but not for zirconium and magnesium. Chromium presents an intermediate case with an equilibrium pressure of 10-12-46 at 800°C., 10-9,88 at 1000°C., and 10 768 at 1200°C. The reverse reaction is feasible for zirconium and magnesium and for chromium at low temperatures. From a kinetic viewpoint the probability that this reaction will occur is small compared to the reaction to form carbon monoxide gas. In this case zirconium will act as a getter for carbon dioxide, while tungsten, iron, and chromium will be relatively inert to carbon dioxide molecules. 

From the results of both pulse reaction and adsorption experiments, it could be confirmed that Ni has a strong affinity with methane, while alkali promoters with carbon dioxide. The retardation of coke deposition on KNiCa/ZSI catalyst must be ascribed to the abundantly adsorbed CO2 species. This explanation is similar to the suggestion of Horiuchi et al. [5], showing that the surface of the Ni cat2ilyst with basic metal oxides was labile to CO2 adsorption, while the surface without them was labile to CH4 adsorption. Since coke deposition was mainly caused by methane decomposition, the catalyst surface covered with adsorbed CO2 or reactive oxygen species from the dissociation of CO2 would suppress coke deposition. The addition of alkaline promoters also seemed to greatly suppress the activity of supported Ni catalyst for the direct decomposition of methane. 

The reaction order with respect to time was determined by the differential method. A fractional order (1.3) is obtained for the catalytic reaction on both doped samples. However, as in the case of the same reaction on pure oxides, the initial reaction rate does not depend upon the pressure of either reagent (order zero). Since these results are similar to those obtained on pure samples, NiO(200°) and NiO(250°), we believe that the order with respect to time is, as in the former case, apparent and that it results from the inhibition of surface sites by carbon dioxide, the reaction product. The slowest step of the reaction mechanism on doped oxides should occur, therefore, between adsorbed species. 

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