Oxygen, chemisorption hydrogenation

The only conclusions that can be drawn from our experiments concerning the role of oxygen in these interactions are (1) oxygen chemisorption is faster than the subsequent interaction, and (2) oxygen can react with hydrogen on the surface of nickel in the adsorbed layer at room temperature. 

From thermodynamic considerations it is evident that bulk nickel cannot be oxidized by CO2. However, it is not justified to conclude from this that dissociative chemisorption of CO2 will not occur. Consider, for example, the chemisorption of oxygen or hydrogen which on several metals takes place under conditions where bulk oxides or hydrides are not at all thermodynamically stable. Dissociative adsorption of CO2 has indeed been observed by Eischens and Pliskin (35). 

Low Temperature Oxygen Chemisorption. The same volumetric high vacuum system used for NH chemisorption with the facility for reducing the samples in situ by flowing hydrogen, was used for the study of oxygen chemisorption. The quantity of chemisorbed... 

In the preceding chapter we pointed to electrical conductivity as one of the physical properties of semiconductors which is changed by a chemisorption process and is accessable to measurement. A further possibility for investigating the mechanism of chemisorption is the relation between the work function and the external electric field of the semiconductor as influenced by chemisorption. These effects have been used for the interpretation of the mechanism of chemisorption and heterogeneous catalysis by Suhrmann (42), and have been experimentally demonstrated in chemisorption processes by Ljaschenko and Stepko. These effects shall here be correlated with our concept of the boundary layer formed in the presence of oxygen and hydrogen. 

Equations (30) and (31) could be checked by measurements of the kinetics of the chemisorption of oxygen and hydrogen on oxides. However, we shall first apply these correlations to the kinetics of the chemisorption of oxygen on NiO, and check the results obtained with the experimental observations made by several authors. 

A major problem in noble metal catalyzed liquid phase alcohol oxidations -which is principally an oxidative dehydrogenation- is poisoning of the catalyst by oxygen. The catalytic oxidation requires a proper mutual tuning of oxidation of the substrate, oxygen chemisorption and water formation and desorption. When the overall rate of dehydrogenation of the substrate is lower than the rate of oxidation of adsorbed hydrogen, noble metal surface oxidation and catalyst deactivation occurs. 

Table 5.2 Comparison of particle sizes (nm) estimated by oxygen chemisorption and hydrogen titration of adsorbed oxygen with those obtained by physical techniques.

Fio. 10. Catalytic activity for hydrogenation of 1-hexene on amorphous chromia a.s a function of the temperature of activation. The curve for oxygen chemisorption is repeated from Fig. 7 using the axis at the left. Crosses give rate in molecules hydrogenated per second initially using the axis at the right. 

Hydrogen and oxygen chemisorption measurements were performed in a conventional glass system at 22°C. Before a H2 chemisorption measurement, the catalyst was reduced, or oxidized and reduced, at temperatures and during times to be specified under Results. These temperatures were reached with a heating rate of 5°C min-1. 

Reduction and oxidation took place in flowing hydrogen and oxygen, respectively. After evacuation (10-2 Pa) at the same temperature the cell was cooled down to room temperature and the chemisorption of H2 was started. The H2 used was purified by passing through a Pd diffusion cell. To check for activated chemisorption, in some cases the chemisorption of H2 was already started at the same temperature as that of evacuation and the catalyst was then cooled down to room temperature under H2. Similar procedures were used for oxygen chemisorption. 

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