Surface partially covered with adsorbed gases

Because the synthesis reactions are exothermic with a net decrease in molar volume, equiUbrium conversions of the carbon oxides to methanol by reactions 1 and 2 are favored by high pressure and low temperature, as shown for the indicated reformed natural gas composition in Figure 1. The mechanism of methanol synthesis on the copper—zinc—alumina catalyst was elucidated as recentiy as 1990 (7). For a pure H2—CO mixture, carbon monoxide is adsorbed on the copper surface where it is hydrogenated to methanol. When CO2 is added to the reacting mixture, the copper surface becomes partially covered by adsorbed oxygen by the reaction C02 CO + O (ads). This results in a change in mechanism where CO reacts with the adsorbed oxygen to form CO2, which becomes the primary source of carbon for methanol. 

In isotherms of Type I, the amount of gas adsorbed per given quantity of adsorbent increases relatively rapidly with partial pressure and then much more slowly as the surface becomes covered with gas molecules. To represent the variation of the amount of adsorption per unit area or unit mass with partial pressure, Freundlich proposed the following equation... 

Here M and T represent methylcyclohexane and toluene in the gas phase, and Ttt represents adsorbed toluene. The first step in the above reaction sequence represents the adsorption of methylcyclohexane with subsequent reaction to form toluene, while the second step is the desorption of toluene from the surface. Very likely the first step represents a series of steps involving partially dehydrogenated hydrocarbon molecules or radicals. However, at steady-state conditions the rates of the intermediate steps would all be equal, and the kinetic analysis is, therefore, not complicated by this factor. To account for the near zero-order behavior of the reaction, it was suggested that the active catalyst sites were heavily covered with... 

Adsorption has been assumed to be the rate-determining process during early oxide-film formation. When a clean surface is exposed to an oxidizing gas, each molecule impinging on the surface may either rebound or adsorb. The fraction, a, that remains adsorbed on the metal surface should be constant for a constant temperature and oxygen partial pressure. Therefore, under these conditions a constant reaction rate is expected. However, the value of a is markedly lower on those parts of the surface covered with a monolayer or oxide nuclei. Thus, as adsorption or oxide nucleation proceeds, the reaction rate is expected to decrease accordingly until complete coverage of the surface by oxide has been achieved, when a much lower rate is expected to be observed. 

Another model to consider is the reaction of adsorbed molecules of A with energetic molecules of B from the gas phase. The reaction rate is assumed proportional to the fraction of the surface covered by A and the collision frequency for B, which is proportional to the partial pressure of B ... 

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