Microwave-enhanced degradation of phenol over Ni-loaded ZnO nanorods catalyst

Abstract

Nickel was loaded onto hydrothermally-grown ZnO nanorods on cordierite substrates and tested as catalysts in microwave-enhanced degradation of phenol from its aqueous solution (100ppm) at 70°C. Effects of metal loadings (1, 10 and 20mM impregnation solutions) on the degradation of phenol in aqueous solution was investigated. The catalyst performances were monitored based on phenol degradation, product distributions and carbon dioxide (CO2) evolutions. Based on the type of the catalysts, two different mechanistic pathways for the decomposition were observed—through catechol and/or hydroquinone as intermediates. It was found that the 10mM nickel loaded sample catalyzed the degradation through one pathway with hydroquinone as the benzenediol intermediate formed, while the 20mM nickel impregnated sample catalyzed the reaction through two pathways, producing catechol as well as hydroquinone by products. These differences in reaction pathways were attributed to the variation in the composition of the nickel compounds and surface structures between the two catalysts. Furthermore, the effect of hydrogen peroxide (H2O2) as an oxidant was explored. It was found that although addition of H2O2 led to an increase in the degree of phenol degradation, it also led to enhanced catalyst leaching. There was also an increase in CO2 evolution due to the addition of H2O2. It was observed that 20mM nickel-loaded sample without the addition of H2O2 exhibited optimum performance in terms of phenol degradation and CO2 evolution with no drawback of catalyst leaching. Catalytic microwave enhanced degradation is an effective means for the removal of dissolved organic compounds from wastewater.