Adsorption on active sites

Water exerts both a deactivating and inhibiting influence on Cu and Fe samples, while the reaction over Co is only inhibited. The deactivation of Fe- and Cu-ZSM-5 is clearly due to migration and the sintering of the active component in H2O atmospheres [34]. The Co-ZSM-5 catalyst is much more hydrotheimally stable in wet gas conditions [34,35]. The inhibition by water can be accounted for in a similar way as for CO via competitive adsorption on active sites, like in selective NO reduction studies [34]. For N2O decomposition this yields an expression like eq. (12). At 793 K Kn amounts to about 0.7 kPa". ... 

Methanol played a role not only on selectivity but also on activity. With the methanol-lean solution the conversion was higher than with the commercial, methanol-rich solution. This confirms that methanol competes with formaldehyde for adsorption on active sites in the zeolite cavities. The ratio between isomers was not very different from that achieved at pH 2 under homogeneous conditions. 

There are numerous indications in the literature on catalyst deactivation attributed to over-oxidation of the catalyst (3-5). In the oxidative dehydrogenation of alcohols the surface M° sites are active and the rate of oxygen supply from the gas phase to the catalyst surface should be adjusted to that of the surface chemical reaction to avoid "oxygen poisoning". The other important reason for deactivation is the by-products formation and their strong adsorption on active sites. This type of... 

The complications begin when one of the products (the HX molecules particularly) can influence the reaction rate, not only by adsorption on active sites blocking a fraction of them but by forming new active centres of a different nature. Then the parameter k is no longer a constant, but changes with the composition of the reaction mixture. This possibility has received only limited attention until now, but could explain some unusual empirical rate equations which have been found for the dehydration of alcohols on oxide catalysts [8,69]. As has been outlined in... 

Water may perturb directly the activity of the catalysts by competitive adsorption on active sites. In that case, the inhibition will be a function of the adsorption strength. It was reported in previous papers [7,8] that the inhibition by oxygenated compounds is moderate. Water caused... 

It is the need for improvements in the HDS and HDN processes or the development of new methods for the removal of sulfur and nitrogen from fuels that has attracted the interest of inorganic and organometallic chemists. Their investigations have been directed toward understanding how organosulfur and organonitrogen compounds bind in transition metal complexes as models for their adsorption on active sites of catalyst surfaces such studies have also provided... 

Irreversible adsorption on active-sites (less than 90% yield) For basic compounds use end-capped, base-deactivated, sterically protected, high coverage, or polymeric reversed-phase for acidic compounds use endcapped or pol3mieric packing acidify mobile phase. 

Figure 4 illustrates the fi actional loss in active sites with carbon content due to AN coke via SSD and BIT coke via SPD. Also shown is the relative activity curve derived from a power-law fit [2] of the data of Fig. 1. As can be seen, BIT coke deactivates considerably less active sites than does AN coke at the same carbon content, due to its lesser selectivity for adsorption on active sites. In both cases, appreciable residual catalytic activity exists when the fraction of remaining active sites becomes extremely small, in line with the high activity of the very low activation energy sites, as shown in Fig. 3.

Both steam and carbon dioxide inhibit the reaction rate, probably due to adsorption on active sites. For the investigated conditions, the oxidation activity of ceria can be neglected. 

A catalysed reaction is a result of several steps involving adsorption on active sites, reaction between sites and desorption fi om sites. Following Langmuir-Hinselwood-Hougen-Watson (LHHW) approach, the reaction rate has the form ... 

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