Dispersion reduction temperature effect

Rolek et al tried to distinguish between viscosity, temperature and composition in their influence on the effects of molybdenum disulphide dispersions in oils. They used a series of white oils, mineral oils, and the same mineral oils with some polar additives removed. The results were not entirely clear, but they supported Tsuya s findings (see below) on the effect of viscosity. However, they also seemed to indicate a more specific effect of temperature and the presence of polar additives. It seemed that there was a specific inhibiting effect of polar additives in suppressing any friction reduction by the molybdenum disulphide. In addition they identified a temperature effect distinct from its effect on viscosity, and suggested that this might be related to a transition temperature, possibly associated with desorption of polar compounds. 

H2 chemisorption. Both Rh/R-Ti02 and Rh/A-Ti02 show a decrease in H2 chemisorption when the reduction and evacuation temperature is increased, while at the same time the slope of the chemisorption vs. In t curve decreases. The decrease in H2 chemisorption is of course due to the gradual transition of the Rh particles into the SMSI state. Whatever the explanation for this state, an electronic interaction between metal particles and support or a covering of the metal particles by the support, in this SMSI state the metal particles are unable to adsorb H2. The decreased slope of the H/Rh-ln t curve can be explained in several ways, such as slow H2 chemisorption on Rh because of an activated process, dependence on metal dispersion, or an effect related to the support. The experiments in which H2 chemisorption was started around 200°C proved that the time dependence is indeed due to a slow adsorption at room temperature, but the experiment with Rh/Si(>2 showed that there is no kinetic limitation in the H2 chemisorption on the metal part of the catalyst. In accordance with this conclusion, no effect of rhodium dispersion on the time dependence of the H2 chemisorption was observed for catalysts in the normal state (cf. Figure 1 curves A, B and F). 

The effect of reduction temperature on the chemisorption of CO on Rh/Ti02 catalysts has been studied by IR spectroscopy (56,57). The low-temperature reduced catalysts evidence the presence of the three typical adsorption forms of CO on Rh, i.e., linear (2070 cm-1), dicarbonyl (2030 and 2100 cm-1), and bridged (1800 cm-1). As the metal dispersion increases, the proportion of dicarbonyl form, characteristic of small particles (65) or nonzero valent Rh ions (66), increases. As shown in Fig. 5 (56), an increase in reduction temperature to 517 K causes a more pronounced decrease in the amount of multisite bridged form. At higher reduction temperatures (623 K) there is a substantial decrease of both the... 

Table 31 Effect of reduction temperature on the average crystallite size and dispersion of metallic copper and the amount of metallic and ionic copper species and the activity of CuO-ZnO-AhOs catalyst (CZA-1) (Reproduced from ref. 244 with permission)...

Chemicals (demulsifiers) are normally used to reduce the interfacial tension. Chemical effectiveness is enhanced by mixing, time, and temperature. Adequate mixing and sufficient time are required to obtain intimate contact of the chemical with the dispersed phase. A certain minimum temperature is required to ensure the chemical accomplishes its function. Both viscosity reduction and effectiveness of chemical are dependent on the attainment of a certain minimum temperature. It may well be that the increase in chemical effectiveness is a result of the decrease in viscosity of the oil phase. 

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