Poison mass transfer effects

Alkene hydrogenation was also suggested to test for mass transfer effects during liquid-phase hydrogenations236,237. The method is based on the linear poisoning of hydrogen addition to alkenes (cyclohexene and apopinene) by CS2. When the active sites of Pd or Pt catalysts are titrated with CS2 the decrease in rate is linear unless mass transfer limitations occur. 

The effect of impurities on fuel cells, often referred to as fuel cell contamination, has been identified as one of the most important issues in fuel cell operation and applications. Studies have shown that the component most affected by contamination is the MEA [3]. Three major effects of contamination on the MEA have been identified [3,4] (1) the kinetic effect, which involves poisoning of the catalysts or a decrease in catalytic activity (2) the conductivity effect, reflected in an increase in the solid electrolyte resistance and (3) the mass transfer effect, caused by changes in catalyst layer structure, interface properties, and hydrophobicity, hindering the mass transfer of hydrogen and/or oxygen. 

In general, PEM fuel cell contamination effects are classified into three major categories (1) kinetic effect (poisoning of the catalyst sites or decreased catalyst activity) (2) ohmic effect (increases in the membrane and ionomer resistances, caused by alteration of the proton transport path) and (3) mass transfer effect (mass transport problems caused by changes in the structure of CLs and GDLs, and in the ratio between their hydrophilicity and hydro-phobicity). Of these, the kinetic effect of the electrocatalysts on both anode and cathode sides is the most significant. 

Contamination effects of impurities on a PEMFC may be classified into three categories (1) kinetic effects, caused by adsorbing onto the catalyst surface and poisoning active sites on both the anode and cathode catalyst layers (2) mass transfer effects, due to changes in the structure, pore size, pore size distribution, and hydrophobicity/hydrophilicity of the catalyst layers or gas... 

In the kinetic control regime (where the overall effectiveness factor t = 1), the rate is directly proportional to the concentration of active sites, L, which is incorporated into the rate constant. In the regime of internal (pore) diffusion control, the rate becomes proportional to and when external diffusion controls the rate there is no influence of L, i.e., there is a zero-order dependence on L. This can be seen by examining equations 4.47 and 4.68. This observation led to the proposal by Koros and Nowak to test for mass transfer limitations by varying L [62]. This concept was subsequently developed further by Madon and Boudart to provide a test that could verify the absence of any heat and mass transfer effects as well as the absence of other complications such as poisoning, channeling and bypassing [63]. 

Many industrial processes are mass-transfer limited so that reaction kinetics are irrelevant or at least thoroughly disguised by the effects of mass and heat transfer. Questions of catalyst poisons and promoters, activation and deactivation, and heat management dominate most industrial processes. 

In the previous section the effects of poisons on reaction rates were related to the active component surface, while the influence of mass transfer was disregarded. It has long been recognized, of course, that the overall rate and selectivity of chemical reactions in porous systems involves the coupling of chemical reactions with convective and diffusive mass transfer processes. Beginning with the pioneering work of Thiele (67), an entire discipline has evolved in which model systems are used to... 

The orthogonal collocation polynomial approximation using a single parameter trial function was employed to solve equations (l)-(3), In addition to the solution for time concentration and activity profiles, effectiveness factors representing the combined effect of mass transfer resistance and poisoning in terms of pellet surface conditions were computed according to... 

The main advantages of a batch reactor are as follows. It is simple and allows rapid measurements. Many experiments can be performed in a short period of time. It is convenient when using pure, expensive, corrosive, or high boiling temperature chemicals. Its use is recommended if the catalyst is sensitive to traces of poisons since there is no accumulation effect. In principle, by varying the stirring conditions it is possible to investigate the influence of heat and mass transfer processes. 

The overall effectiveness factor of a catalyst pellet can be characterized by the ratio of the observed reaction rate to the rate in the absence of poisoning or external mass transfer resistance. It is expressed in the form of a power-law kinetic model for benzene hydrogenation as... 

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