Oxygen adsorption, char

Another school of thought believes that the efficacy of boric oxide in afterglow suppression is related to its high ionization energy/or electron affinity active sites for oxygen adsorption on the char surface may be deactivated by boric oxide via electron transfer. It was reported that boric oxide increases the oxidation temperature of crystalline carbon from 700°C to 800°C.109... 

With appropriate choices of kinetic constants, this approach can reproduce the NSC experimental data quite well. Park and Appleton [63] oxidized carbon black particles in a series of shock tube experiments and found a similar dependence of oxidation rate on oxygen concentration and temperature as NSC. Of course, the proper kinetic approach for soot oxidation by 02 undoubtedly should involve a complex surface reaction mechanism with distinct adsorption and desorption steps, in addition to site rearrangements, as suggested previously for char surface combustion. 

Step one is, oxygen diffusion in the porous system of the particle inwards to the char combustion front and the reaction site, (2) adsorption of oxygen to the active sites on the intraparticle char phase, (3) oxidation reaction with carbon, and (4) desorption of... 

Adsorption isotherms were determined in an automated volumetric gas adsorption apparatus (Autosorb 1, Quantachrome Co.). Adsorption of nitrogen was performed at 77 K. Before measurements, samples were outgassed at 672 K for at least 8 hr in vacuum. Where some bumoff of the char was desired, the reactions were performed in an Online Instruments TG-plus thermogravi metric analyzer. The reactions were performed in a mixture of helium and oxygen, flowing at a rate of about 220 cc/min. Samples of 30-50 mg were dispersed on a circular platinum pan with a large flat surface and raised sides, resulting in a particle beds of about 1 mm thickness. Temperatures between 573-748 K... 

The porosity of the cellulose chars was studied using nitrogen adsorption at 77K. The results for the fresh cellulose char and (he char burned off to differing extents in oxygen are shown in Figure 5. 

The surface areas of chars prepared from cellulose samples at different HTTs were determined by application of the Dubinin-Po-lany equation to CO2 adsorption at room temperature and compared with the area occupied by surface oxides calculated from oxygen chemisorption at 230 C. The results shown in Figure 25 indicate that cellulosic chars have large surface areas that vary according to the HTT, and peak at about 550 °C. The surface oxides formed by chemisorption occupy only a portion of the total surface area, and the chemisorption also shows a peak for chars formed at about 550 °C, corresponding to the temperature of smoldering combustion. 

Adsorption of NO and its reduction on chars was studied by Izquierdo and Rubio [102]. The removal process was designed to work at the temperature range 373-473 K in the presence of oxygen and moisture. The removal capacity was between 100 and 150mg/g with around 30% of NO conversion. The conversion of NO via direct reduction with the carbon surface occurred as follows [102] ... 

The effect of potassium in the form of potassium carbonate or potassium silicate on reduction of NO, on coal chars was also investigated [108-111]. The best materials were prepared by pyrolysis of coal at 1300 K with high KOH/coal ratio [108]. On these adsorbents, at temperature smaller than 473 K, physical adsorption is predominant while the true NO, reduction by char occurs at T> 473 K with formation of Nj and COj. The results indicated that a material with the high surface area should be used to promote adsorption of NO, and potassium remaining in chars catalyzes NO reduction in the presence of oxygen [109]. 

Another important catalytic reaction crucial for environmental remediation is reduction of NO , When activated carbons are used as removal media, the elimination process includes adsorption combined with either oxidation or reduction, with carbon acting as the reducing agent and perhaps even as a catalyst [332,333], Oxidation usually leads to the formation of nitric acid, whereas N2 is the product of NO c reduction. It was found that surface chemistry affects NO removal performance, and an optimal amount of oxygen-functional groups on the surface of char is needed [334-337],... 

Lizzio, A.A., and DeBarr, J.A., Effect of surface area and chemisorbed oxygen on the sulphur dioxide adsorption capacity of activated char. Fuel, 75(13), 1515-1522(1996). 

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