Effect of particle size on resistance to chloride for hydrodechlorination of trichloroethylene in water

2022-8
Karahan, Selin
Contamination of groundwater by chlorinated compounds is an important environmental hazard to humanity because worldwide drinking water demand is generally supplied by groundwater. Chlorinated pollutants, especially trichlorethylene (TCE), are released to the environment due to its frequent use in industrial applications. This highly toxic chemical poses a serious threat to human health and the environment. In this regard, cleaning of groundwater from TCE is of great importance. Catalytic hydrodechlorination (HDC) is a desirable approach for remediation of water contaminated with chlorinated compounds. It is an elimination-based catalytic remediation technique where toxic chlorinated compounds in water are transformed to chloride-free hydrocarbons with hydrogen over a catalyst. Studies conducted so far have concluded that platinum (Pt) and palladium (Pd)-based catalysts show high catalytic activity for HDC of chlorinated compounds. However, during HDC, the overall performance of the catalyst is mainly affected by (i) adsorption of the reactants and the reaction medium on catalyst surface, (ii) simultaneous reactions including dechlorination, hydrogenation, and hydrodechlorination, and (iii) inhibition due to the unavoidable reaction product HCl. Of particular note is the inability of HCl to desorb from the active sites that leads to complete loss of the catalytic activity and irreversible catalyst deactivation by the chloride poisoning. This effect, however, could be alleviated by catalysis engineering. In this study, the objective is to undertake an approach to investigate the effect of particle size of Pt nanoparticles (Pt NPs) on resistance to chloride poisoning for HDC of TCE in water. Although there are existing studies which examined particle size effect on catalytic activity, studies on HCl inhibition are scarce. By fine-tuning the particle size distribution of catalysts, the inhibition problem of HDC catalysts can be circumvented. The study includes synthesizing Pt nanoparticles with three different particle sizes (3.0, 5.8 and 60.9 nm) by colloidal synthesis method, performing activity experiments in a semi-batch reactor, carrying out characterization studies and performing kinetic measurements. The results demonstrate that Pt NPs were synthesized successfully as evidenced by transmission electron microscopy. For 20 ppm initial TCE concentration, almost 100% conversion was achieved in all experiments. As expected, the fastest reaction kinetic was observed at the smallest particle size. Rate constants were found as 4.2 h-1, 1.3 h-1 and 0.4 h-1 for Particle size 1 (small), 2 (medium) and 3 (large), respectively. The initial turnover frequency (TOF) was calculated based on initial rate and molar concentration of Pt in the batch reactor. TOF data (0.077 (Particle 1), 0.023 (Particle 2) and 0.007 (Particle 3) mol TCE/(mol Pt.s)) decreased with increasing particle size. The highest resistance to chloride poisoning were also observed at the smallest particle size. The heat and mass transfer limitations were also investigated. Kinetic experiments performed within the scope of this study revealed insight into structure sensitivity of reactions involved in HDC of TCE in water at atmospheric conditions.

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Citation Formats
S. Karahan, “Effect of particle size on resistance to chloride for hydrodechlorination of trichloroethylene in water,” M.S. - Master of Science, Middle East Technical University, 2022.