SO2 adsorption

SO2 adsorbed on copper oxide bed forming CuSO. Bed is regenerated with H2 or H2—CO mixture giving concentrated SO2 stream. Bed is reduced to Cu, but reoxidized for SO2 adsorption. 

In the absence of SO2, adsorption of NO, CsHe and O2 involved the steps formulated in Eqs. 10, 11 and 2. The competitive adsorption of SO2 at anion vacancies (Eq.22) reduces the formation of both nitrosyl and nitrate species (Eqs. 10 and 8). An inverse relation between the... 

The CsHe desorption was essentially inhibited in the presence of SO2 because sulfur species can react with Fe O radical to form a relatively stable Fe SOs Fe (see Eq. 23), resulting in a significant decline in the density of available adsorption sites for CsH . Simultaneously, the scarcity of a-02 surface species (Fe 02") due to a competitive SO2 adsorption (Eq. 22) leads to a decrease in both rates of propene oxidation and carbonaceous species (CO and CO2) formation. 

DeBerry and Sladek W followed up on the work of Lowell by measuring the rates of SO2 adsorption on the oxides selected by Lowell. Cerium oxide was found to have one of the highest rates. 

To show the alumina effect quantitatively, a series of catalysts was made in which the amount of alumina in the matrix was varied from 25 to 100% by adding alumina sol to a 25% alumina, silica-alumina slurry. These catalysts were formulated with REY molecular sieve. The results for SO2 removal are shown in Figure 3 where SO2 removal (corrected for unit factor) increases with increasing alumina. Our conclusion that alumina was important for SO2 adsorption also confirmed the results of Blanton and Flanders at Chevron (22). The non-linearity of the relationship implies an antagonistic effect between silica and alumina. The silica-alumina antagonism will be discussed relative to deactivation subsequently. 

Bulk boron oxide was found to be much more acidic than basic [168], When SO2 adsorption microcalorimetry was used, no basic sites were observed, but some phy-sisorption occurred. Ammonia and pyridine adsorption microcalorimetry were used to characterize the acidity of B2O3. Boron oxide displays an initial heat for NH3 adsorption of 80 kJ/mol and can adsorb irreversibly a large amount of ammonia. The number of active sites determined by pyridine adsorption and the corresponding integral heats were found to be much lower than those determined by using ammonia. 

As can be seen in Table 13.2, the heats of NH3, pyridine, CO2 or SO2 adsorption clearly show that these molecules are chemisorbed on all aluminas (heats of adsorption higher than 100 kJ/mol) in spite of the different origins of AI2O3 and different pretreatment and adsorption temperatures used. 

Another study examined the acidity and basicity of bulk Ga203 by NH3 and SO2 adsorptions microcalorimetry performed at 150°C. As alumina, Ga203 is amphoteric, with heats higher than 100 kJ/mol for both NH3 and SO2 adsorption, respectively [186]. The amphoteric character of bulk gallium oxides and strong heterogeneity of the surface acidic and basic sites were proved also by Petre et al. [179] using microcalorimetry of pyridine adsorption at 150°C and CO2 adsorption at 30°C. 

The amphoteric indium oxide can be considered as more basic than acidic when comparing the adsorption heats and irreversible adsorbed amounts, which are clearly higher for SO2 adsorption than for ammonia adsorption [40,47]. The heats of NH3 adsorption decreased continuously with coverage, while the SO2 adsorption heat remained constant over a wide range of coverage. 

The AlGaPON samples were used as catalysts of the Knoevenagel condensation reaction and the authors [211] found that the -NH2 groups present at the surface of the samples were the basic sites responsible for the condensation properties of the catalysts. The catalytic performances of the studied samples increased with their basic character observed by SO2 adsorption microcalorimetry. 

Figure 9.20 displays the differential enthalpy of SO2 adsorption at 353 K as a function of the probe uptake on samples with various vanadium contents and on pure Y-AI2O3. 

Differences are also noticed in the values of the surface pH and amounts of preadsorbed water. The pH values for the exhausted samples after subsequent SOj and HjS adsorption runs are much lower than those after HjS adsorption followed by SOj adsorption. This suggests differences in the surface reaction products. These differences are also reflected in the amount of water adsorbed after the first runs in the breakthrough tests. After SOj adsorption much more water is preadsorbed before the next run than after adsorption of HjS. This once again indicates differences in the chemistry of inorganic phase. After SO2 adsorption it is likely that still some oxides able to adsorb water are present (hydrophilic surface) whereas reactions with HjS and deposition of sulfur [12,14] almost totally "screen" active centers for water adsorption. 

Fig. 20. Models of TiO2(100)lx3 (top) and TiO2(100)lxl (bottom) along with the proposed SO2 adsorption sites corresponding to, from left to right, chemisorbed SO2, SO32-, and SO42- -like species [105].

When water is present in the gas stream, it reacts with the SO, and O2 to produce sulfuric acid on the carbon surface, and can subsequently desorb. The overall SO adsorption capacity is enhanced due to its solubility in the water film that forms on the carbon surface. Conversely, active sites for SO2 capture are simultaneously reduced by water coverage. In general, the SO2 adsorption characteristics of an activated carbon are dependent upon its physical form, the pore structure, the surface area, and the surface chemistry. Similarly, both temperature and contact time also affect the efficiency of the process. The temperature for practical application is usually between ambient and 200°C, with ambient to 50°C being favored due to the decreasing solubility of SO2 in water at higher temperatures. 

The two-stage process was licensed by Mitsui Mining Company (MMC) in Japan in 1982, and by 1993 a modified form of the process was installed in four commercial plants in Japan and Germany [58]. The granular carbon or activated coke used in this process has a surface area of initially 150 to 250 mVg, which is much lower than that of commercial activated carbons. It is produced from a bituminous coal and a pitch binder. Low surface area carbons have been found to be the most effective in this process they are cheaper than high surface area activated carbons, they retain their SO2 adsorption capacity more efficiently on repeated cycling, and their relatively low porosity contributes to strength and abrasion resistance. 

The mechanism of sulphate formation is still under debate. Recently, a kinetic model has been proposed for alumina sulphation [13]. Two different types of alumina surface sites could be involved in the elementary stages. The first one would be an oxygen atom on the surface that would favor SO2 adsorption and then sulphation. The second one would allow the dissociation of oxygen. After the formation of superficial sulphates on the surface of the alumina, a nucleation of core sulphates, via a slow process, causes formation of irreversible chemical species. 

Kisamori, S., Mochida, I., and Fujitsu, H. (1994). Roles of surface oxygen groups on poly(acrylonitrile)-based active-carbon fibers in SO2 adsorption. Langmuir, 10, 1241-5. 

Basic nitrogen species present on the surface of activated carbons or carbon fibers, like in the case of H2S, were found to enhance the sulfur dioxide uptake. Polyacrylonitrile (PAN)-based activated carbon fibers are examples of good adsorbents for SO2 removal [82, 92]. Although role of nitrogen present in the carbon matrix was not emphasized by Lee and coworkers [93] in their studies of SO2 adsorption on PAN-based activated carbon fibers, [93] Kawabuchi and coworkers noticed a significant increase in the sorption capacity when activated carbon fibers were modified with pyridine and basic nitrogen functionalities were introduced to the surface [93]. Pyridine provided basic functionality, which increased catalytic removal of SO,. . 

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