Silver/ion exchanger nanocomposites as low-temperature redox-catalysts for methanal oxidation

Metal/granulated ion exchanger materials composed of silver nanoparticles distributed near the external surface of anion- or cation-exchanger have been synthesized via redox deposition procedure, anion-exchanger composites being in its strongly basic (OH−) form while the cation-exchanger one being in its neutral (Na+) form. These composites have been applied as catalytic materials for room temperature oxidation of aldehyde pollutants (formaldehyde in water or acetaldehyde in ethanol) by dissolved oxygen. Conversion level of aldehyde is 80-95% with respect to its initial concentration for anion-exchange composites. Reaction rate depends on the oxidized surface state of silver nanoparticles. In the absence of dissolved oxygen the overall consumption of formaldehyde is limited by the amount of oxidation products on the silver surface, the redox process does not occur at all for reduced state of the silver surface. HCOH conversion in the presence of dissolved oxygen leads expectedly to formation of CO (gas) and CO2/CO32−. This conclusion follows from consumption of OH− counter-ions inside strongly basic anion exchanger during kinetic or dynamic experiments. If this loss is compensated by the OH-ion transport from the external solution into the ion-exchanger the composite is able to serve as efficient catalyst of HCOH oxidative removal from a series of its solutions. Otherwise, if the external solution is not sufficiently basic or OH-ions are excluded from the exchanger because of its cation type the aldehyde oxidation process stops rapidly. These observations imply that hydroxide ions inside the matrix are consumed by their reaction with intermediate products of the oxidation reaction. Increase of the hydroxide-ion concentration inside matrix by means of its NaOH pretreatment returns the activity of the catalyst to its initial level. Thus, silver/anion-exchanger composites can be considered as renewable redox catalysts for oxidation of aldehyde impurities from water or ethanol.