Chemisorption temperature dependence

Do not infer from the above discussion that all the catalyst in a fixed bed ages at the same rate. This is not usually true. Instead, the time-dependent effectiveness factor will vary from point to point in the reactor. The deactivation rate constant kj) will be a function of temperature. It is usually fit to an Arrhenius temperature dependence. For chemical deactivation by chemisorption or coking, deactivation will normally be much higher at the inlet to the bed. In extreme cases, a sharp deactivation front will travel down the bed. Behind the front, the catalyst is deactivated so that there is little or no conversion. At the front, the conversion rises sharply and becomes nearly complete over a short distance. The catalyst ahead of the front does nothing, but remains active, until the front advances to it. When the front reaches the end of the bed, the entire catalyst charge is regenerated or replaced. 

Henero E, Feliu JM, Blais S, Jerkiewicz G. 2000. Temperature dependence of CO chemisorption and its oxidative desorption on the Pt(lll) electrode. Langmuir 16 4779-4783. 

Works [40, 91] surveyed y versus temperature for deactivation of 02( Aj ) on quartz at 350- 900 K. The obtained temperature dependencies were in the Arrhenius form with the activation energy of 18.5kJ/mole. A conclusion was drawn up about the chemisorption mechanism of singlet oxygen deactivation on quartz surface. A similar inference was arrived at by the authors of work [92] relative to 02( A ) deactivation (on a surface of oxygen-annealed gold). 

We report the discovery of a new Pd-Sn catalyzed hydrogenolysis reaction to produce thiol product in high yields. The relationship between catalyst activity and surface characterization (chemisorption, ESCA, and in situ temperature-dependent XRD,) has aided om understanding of the reasons why these catalysts are sulfur resistant, extremely active, and activated at certain temperatures and pressures. The predominant mode of deactivation appears to be the formation of Pd-CN species rather than the formation of Pd-S species on the surface of the catalyst. 

The temperature dependence of the extent of adsorption was not interpreted, except that the results were considered to be consistent with the magnetic measurements of Selwood (see Section II,C) which indicate that the number of carbon-metal bonds between adsorbed species and the surface increases threefold between 120°and 200°C due to extensive dissociative chemisorption. The authors proposed that two forms of chemisorbed benzene exist at the nickel surface, (i) an associatively adsorbed form which can be displaced by further benzene, and which may be w- or hexa-dissociatively adsorbed form that requires the presence of hydrogen to bring about its removal from the surface. 

The experiments discussed above were all carried out with total pressures below 10-4 Torr. However, Hori and Schmidt (187) have also reported non-stationary state experiments for total pressures of approximately 1 Torr in which the temperature of a Pt wire immersed in a CO—02 mixture was suddenly increased to a new value within a second. The rate of C02 production relaxed to a steady-state value characteristic of the higher temperature with three different characteristic relaxation times that are temperature dependent and vary between 3 and 100 seconds between 600 and 1500 K. The extremely long relaxation time compared with the inverse gas phase collision rate rule out an explanation based on changes within the chemisorption layer since this would require unreasonably small sticking coefficients or reaction probabilities of less than 10-6. The authors attribute the relaxation times to characteristic changes of surface multilayers composed of Pt, CO, and O. The effects are due to phases that are only formed at high pressures and, therefore, cannot be compared to the other experiments described here. 

Infrared spectroscopy was used to gain a better understanding of the mechanism of photocatalytic oxidation of TCE (Fan and Yates, 1996). IR spectroscopy also was used to determine intermediates formed from the reaction. Chemisorption of oxygen onto the TiOz surface plays an important role in the oxidation of TCE. The reaction was also temperature dependent. TCE is more easily degraded in the gas phase than in the aqueous phase. For this reason, a process that strips TCE from the groundwater and then treats the vapor containing TCE can be used. 

The remaining vacuum pumps to be discussed in this chapter fall into a group which remove gas particles from systems by sorption effects such as adsorption, chemisorption/gettering and implantation. They tend to be used on systems where any contamination of the vacuum by pump fluids, lubricants, etc. must be avoided. However, those pumps that remove gas particles exclusively by temperature-dependent gas adsorption on molecular sieves or A1203 (adsorption pumps) will not be discussed. 

Chemisorption of inhibitor molecules on metals is slow and involves interaction forces stronger than the forces in physisorption. A coordinate type of bond between the metal and the inhibitor is thought to be present with charge transfer from the inhibitor to the metal.55 An opposing view is that a chemical bond is not necessarily present in chemisorption of an inhibitor on the metal surface.56 In some cases the temperature dependence shows higher inhibition efficiency and higher activation energy than physisorption. 

When chemisorption takes place, the rate may be diffusion-controlled or reaction-controlled. The former mode Is expected when all arriving molecules are rapidly scavenged by the reaction. Reaction-controlled adsorption has a kinetics typical for chemical processes, with an activation energy and an Arrhenius type of temperature dependence. 

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