Partial pressure, aluminum oxidation

The chemical composition of a newly formed groundwater is initially determined by rainwater (sometimes also by river water) that becomes exposed to increased partial pressure of CO2 (from the microbially mediated oxidation of organic matter in the soil horizon) after infiltration into the soil. The CO2-enriched water dissolves minerals such as aluminum silicates, CaC03, and CaMg(C03)2. 

A small amount of copper in the mixed oxide facilitates the hydrogenation of the nickel oxide. As discussed in Chapter 12, reduction of bulk nickel oxide was facilitated by the presence not only of copper but also of silver. It would seem, then, that if silver were present in the coprecipitated nickel-aluminum oxide, it should also increase the rate of nickel oxide hydrogenation. High nickel areas were obtained when the partial pressure of water in the reducing gas was kept low.3>9 The presence of a small amount of aluminum oxide in the reduced nickel particles distorts the crystallites and produces more of the catalytically active surface defect sites.5... 

Estimate the equilibrium partial pressures of oxygen for the oxidation of chromium and aluminum at lOOO C. 

Common failures are because of oxidation, carburization, metal dusting, sulfidation, chlorination, and nitridation. The most common high-temperature degradation mode is oxidation, and the protection against oxidation is achieved by the formation of chromium oxide scale. Small amounts of aluminum or silicon may improve the resistance of chromia alloy. The attack by chlorine and sulfur depends strongly on the partial pressure of oxygen in the system. 

The Standard Oil process has the longest history. Working with ethylene pressures around 70 bar, it represents a medium-pressure process. The polymerization is started by one of the usual commercial hydroforming catalysts partially reduced M0O3 aluminum oxide, activated by sodium or lithium aluminum hydride. The ethylene is dissolved to 5-10% in xylene and polymerized to practically a 100% yield at temperatures below 200°C but still above the melting point of poly(ethylene). This solution polymerization enables the catalyst surfaces to be kept predominantly free and active. 

The more or less simultaneously developed Phillips process likewise uses a xylene solution, but at lower pressures (30-50 bar) and temperatures (150°C) and with a partially reduced chromic oxide on aluminum oxide or aluminum silicate as supporting agent. The catalyst, which is used in relatively large quantities (10% of ethylene content), is activated by heating and filtered off after an almost 100% yield of poly(ethylene) has been obtained. The poly(ethylene) precipitates from the cooled solution the solvent, before distillation, still contains low-molecular-weight poly-... 

In the second reaction stage, where hydrogenation of the mono-olefins and desulfurization are carried out, molybdenum and cobalt catalyst on aluminum oxide carriers are used. The reaction temperature in this hydrogenation stage is between 280 and 350 °C, with a partial hydrogen pressure of 15 to 25 bar, and a total pressure of around 45 to 65 bar. 

As the reaction conversion increases, the effects of coadsorbed potassium and aluminum oxide become apparent. " The partial pressure of ammonia in the reactor loop versus time for the 0.15 ML K/Fe(100), clean Fe(lOO), 0.25 ML K/25%... 

To realize fully the promotional effects of potassium and aluminum oxide, the promoters must be added at different times during the preparation of the active catalyst. " In Fig. 4.27, the partial pressure of ammonia in the reaction loop versus the time of reaction for clean Fe(lOO), restructured Alj,O /Fe(100), and restructured 0.25 MLK/25% Al O /Fe(100) where the potassium was added after the restructuring procedure is plotted (see the figure for conditions). The restructured Al ,Oy/Fe(100) surface with 0.25% Al O is about one order of magnitude more active than the clean Fe(lOO) surface when the surface areas of the samples are taken into account. The addition of 0.25 MLK to a restructured Al O /Fe(100) surface increases the activity of the restructured surface twofold at an ammonia partial pressure of 20torr. Thus the restructured 0.25 MLK/25% Alj Oy/Fe(100) surface is now 20 times more active than the clean Fe(lOO) surface. The only way to achieve this level of enhancement is to restructure the surface in the presence of aluminum oxide alone, and then to add the potassium. 

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