EFFECT OF AGING ON THE BEHAVIOR OF COPPER-CHROMIUM COMPOUNDS SUPPORTED ON ACTIVATED CARBON

Copper-chromium compounds deposited on activated carbon by the impregnation technique are used for the removal of low boiling point gases from air. Their efficiency decreased markedly after exposure to humid air. Hexavalent chromium and divalent copper salts are the precursor of the supported species. Metallic silver is also added to the impregnation solution in the preparation of whetlerite-type materials. Since Cr(+6), Cu(+2) compounds can be exothermally reduced in an inert atmosphere by the carbon substrate, the thermal behavior of whetlerite-type adsorbents was investigated by means of differential scanning calorimetry (DSC). For unaged impregnated samples, all copper species are in the oxidation state +2, whereas chromium is found to have oxidation states +6 and +3. This fact is attributed to a partial reduction of the hexavalent chromium precursor during preparation of the supported phase. Exposure of the impregnated carbon to weathering conditions (50-degrees-C, 90% relative humidity) leads to a segregation of copper from the copper-chromium complexes and to a progressive consumption of the Cr(+6) fraction left on the carbon. The efficiency of the impregnated samples for the removal of cyanogen chloride from an air stream has been measured using a dynamic adsorption system. The breakthrough time of CNCl for samples weathered at different periods of time is directly linked to the amount of hexavalent chromium associated with copper species. After consumption of the total amount of Cr(+6) compounds, the efficiency of the impregnated carbon is equivalent to that of nonimpregnated carbon. A carbon only impregnated with copper behaves similarly to a nonimpregnated one for the removal of CNCl. Hence it is the association of Cr(+6) with Cu(+2) species which leads to an efficient destruction of CNCl in dynamic adsorption conditions.