Investigation of deactivation mechanisms of ASC whetlerite charcoal

ASC whetlerite, prepared by the impregnation of CWS grade activated charcoal with ammoniacal carbonate solutions of copper(II), chromium(VI), and silver salts, is an effective material for the removal of many low-molecular-weight gases (e.g., cyanogen chloride and hydrogen cyanide) from air streams. Whetlerite loses its effectiveness when exposed to extremes in temperature and/or humidity or by excessive periods of usage. Associated with this deactivation/deterioration process are changes in metal oxidation state(s) and impregnant morphology. This investigation examines the behavior of the metal impregnants on ASC whetlerite when subject to certain extreme conditions and, as a result, develops an understanding of the deactivation mechanism(s) that occur. The deterioration of whetlerite due to thermal deactivating conditions as well as that due to exposure to hydrogen involves the decomposition of the supported CuOHNH₄CrO₄ (basic copper ammonium chromate) species. This complex chromate decomposes into a CuOCuCr₂O₄ (copper-chromite) species upon thermal treatments up to 490 K. At temperatures above 490 K the CuO (cupric oxide) sinters away from the chromite and is subsequently reduced to Cu¹⁺-containing and Cu⁰ species. When exposed to high humidities for long periods of time (i.e., more than 2 days) it is hypothesized that the whetlerite surface experiences a decrease in pH. This results in the supported CuOHNH₄CrO₄ species converting to Cu₄(OH)₆CrO₄ (brochantite chromate). The brochantite chromate gradually decomposes into less complex crystalline compounds such as CuO, Cu₂Cr₂O₄, and Cr₂O₃ which are not effective for destroying irritating gases.