Chemisorption pore diffusion

The catalytic reaction can be subdivided into pore diffusion and chemisorption of reactants, chemical surface reaction, and desorption and pore diffusion of products, the number of steps depending upon the nature of the catalyst and the catalytic reaction. 

The reaction kinetics approximation is mechanistically correct for systems where the reaction step at pore surfaces or other fluid-solid interfaces is controlling. This may occur in the case of chemisorption on porous catalysts and in affinity adsorbents that involve veiy slow binding steps. In these cases, the mass-transfer parameter k is replaced by a second-order reaction rate constant k. The driving force is written for a constant separation fac tor isotherm (column 4 in Table 16-12). When diffusion steps control the process, it is still possible to describe the system hy its apparent second-order kinetic behavior, since it usually provides a good approximation to a more complex exact form for single transition systems (see Fixed Bed Transitions ). 

When chemisorption is involved, or when some additional surface chemical reaction occurs, the process is more complicated. The most common combinations of surface mechanisms have been expressed in the Langmuir-Hinshelwood relationships 36). Since the adsorption process results in the net transfer of molecules from the gas to the adsorbed phase, it is accompanied by a bulk flow of fluid which keeps the total pressure constant. The effect is small and usually neglected. As adsorption proceeds, diffusing molecules may be denied access to parts of the internal surface because the pore system becomes blocked at critical points with condensate. Complex as the situation may be in theory,... 

Physical adsorption equilibrium is achieved rapidly since no activation energy is required as in chemisorption. An exception here is adsorption in small pores where diffusion can limit the adsorption rate. 

Figure 2.1 Physical (1,2,6,7) and chemical (3-5) steps involved in the following heterogeneously catalysed model reaction A + B —> C. For sake of simplification the surface reaction (4) is supposed to occur between chemisorbed A and nonadsorbed B molecules. 1, Diffusion of A (la) and B (lb) molecules from the homogeneous phase to the external surface of catalyst particle. 2, Diffusion of A (2a) and B (2b) molecules along the pores. 3, Chemisorption of A on the active site. 4, Reaction between chemisorbed A and nonadsorbed B with formation of C chemisorbed on the active site. 5, Desorption of C from the active site. 6, Diffusion of C (6c) out of the pore. 7, Diffusion of C (7c) from the pore mouth to the homogenous phase...

In the gas-phase, benzene shows a single line,77 78 and can yield useful information regarding the diffusion/transport properties. Benzene trapped within pores in glasses and silica gels too yields results, about pore size and adsorbed versus liquid-phase conditions.79 Chemisorption on alumina-supported platinum catalysts leads to disclosure as to how and where the benzene molecules are located, via FT NMR.80... 

The Study of ammonia adsorption was carried out in a flow adsorption microcalorimeter under dynamic conditions. Some of the parameters from these experiments are collected in Table 2.. In all cases the amount desorbed was much smaller than the adsorbed one, and so was the absolute value of the heat evolved. This clearly points out that ammonia adsorption consists of two different components. One is related to chemisorption (irreversible adsorption) and the other one (more labile or reversible) to physisorption in the pores. Another important feature about these experiments is that heat was still released long afterwards the NH3 uptake was negligible. This heat evolution, already reported in other experimental systems [4,7,8], is due to diffusion of adsorbed ammonia from low energy sites to higher energy sites having low accessibility. The lack of further uptake of NH3 may be due to irreversibly adsorbed molecules on the borders of micropores blocking NH3 towards the end of the adsorption process. 

As pointed out by Ruthven [3], the rates of adsorption and desorption in porous adsorbents are usually controlled by the rate of diffusion within the pore network, more than by the kinetics of adsorption-desorption. This is especially true in chromatography, where adsorbents are carefully prepared to exhibit only moderately strong energy of physisorption and no chemisorption. Thus, it is important to consider diffusion within the pore networks existing in the columns. 

For non-porous catalyst pellets the reactants are chemisorbed on their external surface. However, for porous pellets the main surface area is distributed inside the pores of the catalyst pellets and the reactant molecules diffuse through these pores in order to reach the internal surface of these pellets. This process is usually called intraparticle diffusion of reactant molecules. The molecules are then chemisorbed on the internal surface of the catalyst pellets. The diffusion through the pores is usually described by Fickian diffusion models together with effective diffusivities that include porosity and tortuosity. Tortuosity accounts for the complex porous structure of the pellet. A more rigorous formulation for multicomponent systems is through the use of Stefan-Maxwell equations for multicomponent diffusion. Chemisorption is described through the net rate of adsorption (reaction with active sites) and desorption. Equilibrium adsorption isotherms are usually used to relate the gas phase concentrations to the solid surface concentrations. 

After drying and reduction, the Pd-Ag/C catalysts are composed of bimetallic Eilloy nanoparticles ( 3 nm). CO chemisorption coupled to TEM and XRD analysis showed that that, for catalysts 1.5% wt. in each metal, the bulk composition of the alloy is close to 50% in each metal, whereas the surface is 90% in Ag and 10% in Pd [9]. Mass transfer limitations can be detected by testing the same catalyst with various pellet sizes [18]. Indeed, if the reactants diffusion is slow due to small pore sizes, the longer the distance between the pellet surface and the metal particle, the larger the influence of the difiusion rate on the apparent reaction rate. Pd-Ag catalysts with various pellet sizes were thus tested in hydrodechlorination of 1,2-dichloroethane. Results were compared to those obtained with a Pd-Ag/activated charcoal catalyst. Fig. 4 shows the evolution of the effectiveness factor of the catalysts, i.e. the ratio between the apparent reaction rate and the intrinsic reaction rate, as a function of the pellet size. The intrinsic reaction rate was considered equal to the reaction rate obtained with the smallest pellet size. When rf = 1, no diffusional limitations occur, and the catalyst works in chemical regime. When j < 1, the observed reaction rate is lower than the intrinsic reaction rate due to a slow diffusion of the reactants and products and the catalyst works in diffusional regime [18]. 

Adsorbents with narrow pores, even in the case of physisorption, may exhibit an increased energy of interaction, partial irreversibility, and a reduced diffusion-controlled rate. Although the distinction between physi- and chemisorption is not always straightforward, electron exchange leading to chemical bonds is an exclusive feature of chemisorption. 

Inert sampling could be done when desired. Zr, W and Ni were determined by XRF, Ti and Cr by neutron activation analysis (NAA), Mg by AAS, C with a Leco carbon analyzer and Cl by potentiometric titration. FTIR in diffuse reflectance mode was used to follow the chemisorption and to detect possible decomposition of the reactant. Scanning electron microscopy with an energy dispersive spectrometer (SEM/EDS) was used to determine element concentrations through the particles. The specific surface area and pore volume were determined by means of nitrogen adsorption and condensation with Micromeritics ASAP 2400 equipment. Detailed experimental conditions used in the characterization are in Ref. 16. 

At the same time, in the active centres in solids interactions between solid and liquid phases occur, having the character of chemical bonds. This type of adsorption is therefore called chemisorption. The chemisorption process is slower than physical adsorption, as it is preceded by diffusion of the substance to the outer surface of the adsorbent and diffusion through the adsorbent pores to the surface (to the active centres). 

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