CO oxidation and the inhibition effects of carboxyl-modification and copper-clusters on multi-walled carbon nanotubes
An inhibition of CO oxidation on catalytically active pristine multi-walled carbon nanotubes (MWCNT) in the presence of selected pollutant constituents in flue gas streams was studied. We simulated an interaction between the active MWCNT and the contaminants in an O₂-rich CO oxidation atmosphere of: (i) an acidic wet flue gas environment modelled by using MWCNT grafted with carboxyl (–COOH) groups; and (ii) a polluted environment formed by trace metal copper particles and other contaminant constituents such as polycyclic aromatic hydrocarbons (PAH), volatile organic compounds (VOC), and phosphorous (P), by using a 2 copper cluster as a model pollutant. The model copper pollutant was in the form of a copper cluster species of chemical formulae [(PPh3)CuH]6·0.75THF, and its simulated modification of the active MWCNT was modelled by using MWCNT doped with these cluster species. In the case of pristine, unmodified MWCNT exposed to reaction gas mixture, MWCNT were catalytically active from ~150 °C, achieving close to complete CO oxidation from approximately 230 °C. In an acidic environment where the MWCNT’s surface was modified with –COOH groups, the material behaved as an adsorbent of CO molecules without converting them into CO₂ in the presence of O₂. Low concentrations of dispersed Cu particles by themselves (not in the form of copper cluster species) doped on the carboxyl-modified MWCNT prepared by conventional method demonstrated activity in the CO oxidation. In the case of copper cluster species pollutant, the model copper cluster was found to have formed CuCO₃ during the CO oxidation reaction at temperatures below 330 °C but decomposed above 400 °C to release CO₂ product.