Impregnation physical model

Basically, impregnation has been rationaKzed following two physical models. For the sake of clarity, their presentation starts with the simplest one (wet/diffusional impregnation) that will be further completed to describe dry impregnation. 

In this chapter, the adsorption of catalyst precursors on oxidic surfaces will be discussed in terms of a chemical-interaction model, i.e. allowing the formation of inner-sphere complexes, but it is important to realize that other models are being advocated in the literature as well on the one hand, one has the physical adsorption models, allowing only the formation of outer-sphere complexes, and on the other, it is proposed that during adsorption one really has a reaction between the precursor and the support to form a new phase or phases. We will meet the latter situation in our discussion of deposition-precipitation (Section 10.3.3), but we will disregard it when discussing impregnation chemistry (Section 10.3.2). 

The major difference between the various GRM models is due to the mechanism of intraparticle diffusion that they propose, namely pore diffusion, siuface diffusion or a combination of both, independent or competitive diffusion. The pore diffusion model assumes that the solute diffuses into the pore of the adsorbent mainly or only in the free mobile phase that impregnates the pores of the particles. The surface diffusion model considers that the intraparticle resistance that slows the mass transfer into and out of the pores proceeds mainly through surface diffusion. In the GRM, diffusion within the mobile phase filling the pores is usually assumed to control intraparticle diffusion (pore diffusion model or PDM). This kind of model often fits the experimental data quite well, so it can be used for the calculation of the effective diffusivity. If this model fails to fit the data satisfactorily, other transport formulations such as the Homogeneous Surface Diffusion Model (HSDM) [27] or a model that allows for simultaneous pore and siuface diffusion may be more successful [28,29]. However, how accurately any transport model can reflect the actual physical events that take place within the porous... 

Physical Measurements. For the electrolyses, a Wenking potentiostat model 70TS1 and a Koslow Scientific coulometer model 541 were used. Voltammetry with wax-impregnated graphite and rotating platinum electrodes was performed as described elsewhere (7, 8). IR and electronic spectra were measured on Perkin-Elmer 225 and Cary 14 instruments. X-band ESR spectra were recorded at room temperature on a JEOL MES-3X spectrometer. Phosphorus-31 NMR spectra were recorded in the pulse mode on a Varian XL-100 instrument at 40.5 MHz using a deuterium lock, or on a Bruker HFX-90 instrument at 36.43 MHz using a fluorine lock. 

Since IBM s offer greater stability than ILM s and greater selectivity and permeability than PM s, it would be useful to be able to model transport processes in these materials and to predict the effectiveness of facilitated transport based on relevant physical properties (RPP). Although it may be necessary to modify the model developed for ILM s in order to completely describe transport processes in IBM s, it is likely that moat of the same RPP s of the system will be Important. The purpose of this section is to point out that measurement of RPP s in IBM s, especially permselective IBM s, may be difficult. Although problems with model development and property measurement exist, carrier Impregnated IBM s can produce rapid and selective separations of gas mixtures. Way and co-workers have incorporated the monoprotonated ethylenediamlne cation into Nafion membranes to achieve the separation of carbon dioxide from methane (25). 

There is a natural limitation to Washburn s law for very short times (i 0). The problem is that the impregnation velocity (which varies as l/ /f) then diverges, which is physically impossible. What restores sanity, to the situation is the inertia of the liquid, which was neglected in Washburn s model. The tube (or the porous medium) is connected to a vessel containing a liquid at rest, which resists sudden movements. Since this early phase of the process occurs for short times as well as for small heights (2 —> 0), we may neglect in equation (5.39) both the viscous friction force Frf and the weight W. We are then left with... 

Huang and Chen [152] propose a dynamic adsorption model to simulate removal of H2S by a fixed-bed packed with copper impregnated activated carbon (lAC). After diffusion into the interior of a pellet, H2S species either may be physical adsorbed on carbon surface or may react with the copper impregnated on the lAC. 

The physical properties of water may be assumed for the impregnating solution. Typical nonuniform catalyst distributions (Chen and Anderson 1973) shown in Figure 1.5 suggest that there definitely exists a resistance to solute removal from the solution since otherwise the profiles should show stepwise distributions if enough solutes are present. Solute removal from the impregnating solution onto the pore wall can be modeled as the sequential events of mass transfer at the liquid-solid interface followed by adsorption onto the pore wall. In many cases, the adsorption/ desorption step is the controlling step. The solute balance for the solid phase in such cases is ... 

---