Water desorption from catalyst

To demonstrate the potential available, simulations were carried out for the oxidation of carbon monoxide on a palladium shell catalyst with water desorption from 3A zeolite as a heat sink, based on experimentally validated model parameters for the individual steps (Figure 16). The calculations indicated that the reaction cycle time could be lengthened by a factor of 10, to a total 20 minutes, in comparison to a simple regenerative process with a similar amount of inert material instead of adsorbent in the fixed bed and for the same threshold for temperature deviation from the initial value. 

Durable changes of the catalytic properties of supported platinum induced by microwave irradiation have been also recorded [29]. A drastic reduction of the time of activation (from 9 h to 10 min) was observed in the activation of NaY zeolite catalyst by microwave dehydration in comparison with conventional thermal activation [30]. The very efficient activation and regeneration of zeolites by microwave heating can be explained by the direct desorption of water molecules from zeolite by the electromagnetic field this process is independent of the temperature of the solid [31]. Interaction between the adsorbed molecules and the microwave field does not result simply in heating of the system. Desorption is much faster than in the conventional thermal process, because transport of water molecules from the inside of the zeolite pores is much faster than the usual diffusion process. 

Figure 16 Simulated temperature profiles along a reactor with and without "desorptive" cooling at various times for the oxidation of CO on a Pt catalyst with water vapor desorption from 3A zeolite in a fixed bed comprising equal proportions of catalyst and adsorbent. The solid curves give the simple regenerative behavior and the dotted curves describe the desorptively cooled case. Initial reactor temperature is 125°C, initial adsorbent loading 0.12 kg/kg, inlet CO-concentration 0.2 mol/l, gas loading 6000 h-1.

The reaction of photo-induced sulphur desorption from the surfaces of the metal oxide-supported (rutile and anatase Ti02, SrTiOs, ZnO, Fe203 and Sn02) Au nanoparticles in water at room temperature has also been studied [209]. It was found to be driven by an upward shift of the Fermi energy of the metal oxide-loaded Au nanoparticles with irradiation. It has also been demonstrated that this phenomenon is applicable to the low-temperature cleaning of sulphur-poisoned metal catalysts. 

The above catalyst was submitted to a preoxidation step prior to disposal in the environment. A batch of 500 g was placed in a furnace which was heated from room temperature to 873 K or 1273 K (5Kmin ), and kept at the desired temperature for 5 hours (atmospheric pressure). During this procedure, besides water desorption, coke was eliminated as carbon oxides and sulfur was partially converted into SO2. No losses of other elements were observed. After cooling (inside the furnace) the treated catalyst was placed into clean tin boxes... 

The mechanism of the catalyzed shift reaction for both copper- and iron-based catalysts remains controversial. Two types of mechanism have been proposed adsorptive and regenerative. In the former, the reactants adsorb on the catalyst surface, where they react to form surface intermediates such as formates, followed by decomposition to products and desorption from the surface. In the regenerative mechanism, on the other hand, the surface undergoes successive oxidation and reduction cycles by water and carbon monoxide, respectively to form the corresponding hydrogen and carbon dioxide products of the WGS reaction. 

Part of the waste heat is generated initially in localized hot spots that can lead to transient evaporation of water, and may be the starting point for the formation of pinholes. An increased local tanperature will also support desorption of adsorbed reactive intermediates from catalyst grains. In general, deviations from equilibrium are expected to be larger when higher power is drawn from the cell. 

Figure 13.9 Reaction scheme for Ci molecule oxidation on a Pt/C catalyst electrode, including reversible diffusion from the bulk electrolyte into the catalyst layer, (reversible) adsorption/ desorption of the reactants/products, and the actual surface reactions. The different original reactants (educts) and products are circled. For removal/addition of H, we do not distinguish between species adsorbed on the Pt surface and species transferred directly to neighboring water molecule (H d, H ) therefore, no charges are included (H, e ). For a description of the individual reaction steps, see the text.

After the catalyst was saturated with carbon dioxide, a temperature programmed desorption (TPD) was carried out by heating the sample in helium (40 cm3min 1) from room temperature to 873 K (10 Kmin 1). The mass spectrometer was used to follow water (mass 18), carbon monoxide (mass 28), carbon dioxide (mass 44) and oxygen (mass 32). 

Figure 2.1 Experimental set ups for temperature programmed reduction, oxidation and desorption. The reactor is inside the oven, the temperature of which can be increased linearly in time. Gas consumption by the catalyst is derived from the change in thermal conductivity of the gas mixture it is essential to remove traces of water, etc. because these would affect the thermal conductivity measurement. The lower part shows a TP apparatus equipped with a mass spectrometer.

In the majority of impurity removal processes, the adsorbent functions both as a catalyst and as an adsorbent (catalyst/adsorbent). The impurity removal process often involves two steps. First, the impurities react with the catalyst/adsorbent under specified conditions. After the reaction, the reaction products are adsorbed by the catalyst/adsorbent. Because this is a chemical adsorption process, a severe regeneration condition, or desorption, of the adsorbed impurities from the catalyst/adsorbent is required. This can be done either by burning off the impurities at an elevated temperature or by using a very polar desorbent such as water to desorb the impurities from the catalyst/adsorbent. Applications to specific impurities are covered in the followings section. The majority of industrial applications involve the removal of species containing hetero atoms from bulk chemical products as purification steps. 

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