Gas phase adsorption of ammonia using nano TiO2-activated carbon composites - Effect of TiO2 loading and composite characterization

TiO2-AC composites with different TiO2 loadings were synthesized by sol-gel method and evaluated for adsorption of gaseous ammonia using a continuous flow packed-bed column. The titanium phase in the TiO2-AC composites was identified as anatase using X-ray diffraction (XRD). The equilibrium adsorption capacity of AC at room temperature (22 °C) increased from 2.48 to 3.22, 5.48, 7.07, and 7.73 mg NH3 g−1 by 10, 20, 30, and 40% TiO2 loadings, respectively. This enhanced capacity originated from the addition of Lewis acid sites (TiO2 nano-particles) to the porous structure of AC. Using transmission electron microscopy (TEM), the size of TiO2 nano-particles were determined as 2-10 nm. Scanning electron microscopy (SEM) showed significant accumulation of TiO2 on the external surface of AC at loadings higher than 30%. Surface characterization and room temperature adsorption capacities singled out the optimum TiO2 loading as 30%. The ammonia adsorption capacity of TiO2 nano-particles in TiO2-AC (30%) composite was higher than that of commercial TiO2 nano-particles (40 nm), due to the smaller size of TiO2 nano-particles deposited on AC. The addition of TiO2 to AC changed the multilayer adsorption of ammonia to a monolayer adsorption. The TiO2-AC composites had better performance when compared to the reported values for AC composites with oxide of other metals such as Fe, Co, Cr, Mo, and W. The Langmuir-Freundlich expression accurately described the TiO2-AC (30%) adsorption isotherms developed for ammonia concentrations in the range of 50-500 ppmv and temperatures in the range of 22-280 °C.