Catalyst surface-wetting

Assuming A and B are volatile reactants, let us define as the fraction of the catalyst surface wetted by a moving liquid film, and (l-n ) the fraction non-wetted by a liquid layer,, as mentioned before, the fraction non-wetted with dry pores, and finally... 

Gomez-Sainero et al. (11) reported X-ray photoelectron spectroscopy results on their Pd/C catalysts prepared by an incipient wetness method. XPS showed that Pd° (metallic) and Pdn+ (electron-deficient) species are present on the catalyst surface and the properties depend on the reduction temperature and nature of the palladium precursor. With this understanding of the dual sites nature of Pd, it is believed that organic species S and A are chemisorbed on to Pdn+ (SI) and H2 is chemisorbed dissociatively on to Pd°(S2) in a noncompetitive manner. In the catalytic cycle, quasi-equilibrium ( ) was assumed for adsorption of reactants, SM and hydrogen in liquid phase and the product A (12). Applying Horiuti s concept of rate determining step (13,14), the surface reaction between the adsorbed SM on site SI and adsorbed hydrogen on S2 is the key step in the rate equation. 

The mercury penetration approach is based on the fact that liquid mercury has a very high surface tension and the observation that mercury does not wet most catalyst surfaces. This situation holds true for oxide catalysts and supported metal catalysts that make up by far the overwhelming majority of the porous commercial materials of interest. Since mercury does not wet such surfaces, the pressure required to force mercury into the pores will depend on the pore radius. This provides a basis for measuring pore size distributions through measurements of the... 

Figure 5.28. In situ wet-ETEM of real-time catalytic hydrogenation of nitrile liquids over novel Co-Ru/Ti02 nanocatalysts, (a) Fresh catalyst with Co-Ru clusters (arrowed at C). The support is marked, e.g., at u. (b) Catalyst immersed in adiponitrile liquid and H2 gas in flowing conditions growth of hexamethylene diamine (HMD) layers (at the catalyst surface S in profile, arrowed) at 81 °C, confirmed by composition analysis and mass spectrometry, (c) ED pattern of HMD in (b) in liquid environments. Further growth is observed at 100 °C. The studies show that wet-ETEM can be used to design a catalytic process (after Gai 2002). (d) Scaled up reactivity data for novel Co-Ru/Ti02 nanocatalysts confirming wet-ETEM studies of high hydrogenation activity of the nanocatalyst (2). Plots 1 and 3 are the data for Raney-Ni complexes and Ru/alumina catalysts, respectively.

The catalyst wetting efficiency of the external catalyst surface can be calculated at atmospheric pressure using the correlation of El-Hisnawi et al. (1981 Wu, 1996) ... 

Die difference from the real value (lm) is mainly due to the approximation made about the mass transfer coefficient as well as the complete wetting of the catalyst, as the actual wetting efficiency is 88%. Furthermore, the problem is more complicated because under incomplete wetting, the gas reactant reaches the catalyst surface more easily than the unwetted part, as Horowitz et al. found out experimentally. 

For example, the most noteworthy disadvantage of catalytic wet oxidation is the severe catalyst deactivation (Larachi el al., 1999). Hamoudi el al. (1998, 1999) systematically studied the deactivation of Mn02/Ce02 catalyst during wet catalytic oxidation of phenol and the catalyst-surface modifications. It was observed that deactivation was induced mainly by the formation of carbonaceous deposits on the catalyst surface. Ohta et al. (1980) reported that the size of the catalyst particles affected the stabilization of catalytic activity. For granular particles of supported copper oxide, the catalytic activity was decreased after each inn, even after six successive experiments. In contrast, for larger particles the catalytic activity was stabilized after the first three runs. 

Wetting of the external catalyst surface is desired as to delay catalyst fouling. 

The most commonly used method to synthesize the PA is the Shirakawa method. In this method a smooth surface wetted by the Ziegler-Natta catalyst is exposed to the acetylene gas. A film of PA (generally c-PA) is produced on the smooth surface. The c-PA is converted to the f-PA by heating. The process of doping also converts the c-PA to the r-PA. 

Several forms of incomplete catalyst wetting were visually observed and reported in previous studies. These observations include i) dry areas on a portion of the catalyst surface... 

Bid = Biot number for gas-solid mass transfer at the inactively wetted catalyst surface based on catalyst particle volume,... 

Ai, iws = concentration of dissolved gaseous reactant A in the liquid phase on the inactively wetted catalyst surface, mole cm 3. 

---