Hydrogen inhibition

The finding of the low-temperature activation of propane on zeolite Ga/HZSM-5 indicates a bifunctional reaction mechanism 179,181. The highly dispersed gallium oxide species in close vicinity to the Bronsted acid sites promote the initial activation of propane. Derouane et al. 179,181 further showed that hydrogen inhibits the activation of propane. This effect was explained by a competitive adsorption of hydrogen on the gallium species or even by a reduction of Ga to Ga species. 

In order to explain this hydrogen effect it can be supposed that i) the hydrogen coverage in normal conditions is not sufficient to maintain the catalyst in the adequate reduced state, ii) the excess of hydrogen inhibits the formation of carbonaceous deposits (and the modification of the catalyst) or the strong adsorption of some reagents and products... 

Hiittingcr and Merdes give a comprehensive description of the models proposed in the literature for the carbon-steam reaction. Basically, there are two models of the reaction mechanism the oxygen exchange model and the hydrogen inhibition model. The equations involved are ... 

The oxygen exchange model is based on equations 2 (reversible -kif and k[b-) and 3, the traditional hydrogen inhibition model is based on equations 2 (irreversible -only kir), 3... 

Pffi) =------Ph2 > hydrogen inhibition model (second version) (6.3.)... 

According to Huttingcr and Merdes, it is not possible to determine which is the dominating hydrogen inhibiting mechanism by looking at the reaction rate because the equations are identical, with exception of the second version of the hydrogen inhibition model. 

The experiments show that the presence of hydrogen inhibits the steam gasification reaction, as presented in Fig. 5. 

Hydrogen inhibits the steam gasification reaction. The char gasification reaction with steam and hydrogen can be modelled based on Langmuir-Hinshelwood kinetics. The model fits well the results. 

Htittinger, K.J. Merdes, W.F. (1992). The carbon-steam reaction at elevated pressure formations of product gases and hydrogen inhibitions, Carbon, Vol. 30, No. 6, pp. 883-894. 

Barrio, M., Gpbel, B, Risnes, H., Henriksen, U., Hustad, J.E. Sprensen, L.H, Steam gasification of wood char and the effect of hydrogen inhibition on the chemical kinetics. This conference. 

In this model, only hydrogen inhibits the water-gas reaction. The rate-limiting step is the desorption of the carbon-oxygen surface complex, which has an activation energy of 250-300 kJ/mol. 

By adding a third reaction, a hydrogen-inhibition model is proposed ... 

In this case, the first reaction (reaction R-4.26) is not reversible. Hydrogen inhibition is attributed to the formation of the C(H)2 complex. The reaction rate becomes... 

This approach was used successfully to describe the gasification of several Australian coal chars by Roberts and Harris.27,28 In the second hydrogen-inhibition model, the third reaction (reaction R-4.28) is replaced by another reaction (reaction R-4.29) as... 

Table I. Activation of RhCl(PPh J3 Catalyzed Alkene Hydrogenations Inhibited by Triphenylphosphine Using PS-S03Ag.a J J...

It was also found in this study that the hydrogen inhibition of the rate decreases as the temperature increases, and disappears completely for temperatures higher than 700°C or more. 

Figure 10.39. Fit of the kinetic model compared to experimental data in aldol hydrogenation. Inhibition effect of formaldehyde on the reaction kinetics included. Abbreviations, Aldol=TMP-aldol (A), FA=formaldehyde, TMP=trimethylpropane (AiH2).

With regards to the apparent kinetic constant, there is experimental evidence that hydrogen inhibits the reaction kinetics significantly, whereas carbon monoxide does not. From this evidence, the apparent kinetic constant is an expression of the Langmuir-Hinshelwood type [32] ... 

The model captures the decreased but sustained activity over platinum promoted sulfated zirconia in the presence of hydrogen by eliminating all contributions to the activity by the S sites and decreasing the activity over the S2 sites by 70%. The deactivation constant for the S2 sites was set to zero to match the experimental data. These results from the model are consistent with a two active site concept where hydrogen inhibits that activity of one type of active sites more than the other type of active site. 

Gaseous hydrogen inhibits the hydrogenation reaction of ethylene there exists therefore a blocking effect (Soga et al., 1977a). 

The reaction proceeds along the same elementary steps as with the Fe catalyst outlined in the earlier text and again successful modeling of the kinetics in terms of the rate parameters for the elementary steps could be achieved (52). Similarly as with Fe, dissociative chemisorption of nitrogen is rate limiting, and the sticking coefficient on pure Ru at room temperature was found to be extremely low (only 10 (53). However, contrary to Fe, adsorbed hydrogen inhibits N2 ad-... 

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