The nature and changes of active substances at the surface of fresh and aged Cu-Cr impregnated carbon were examined by XPS in the paper. The result shows that at the surface of fresh sample there exist several C~(II)-Cr(V1) compounds (A, B, and C) and a little Cr(III) compound (D). A and B are two kinds of active substances for destroying CNCI, and C is formed from the deterioration of A and R. In a humid environment, both A and B change into C and E, but the deactivation rate of B is higher than that of A. In an acidic environment (e.g., CO?). B is easily reduced to Cr(III)(D), and A is almost not affected. That is, B is very sensitive to an acidic environment; its basicity is stronger than that of A. When the humid air contaminated with CNCI flowed through the impregnated carbon, A and B are deactivated, parts of them change into C. and the others change into Cr(III)(CrCll and CrzO,) Key Words-&&r impregnated carbon, XPS, deactivation, active substance.
1. Introduction
Cu-Cr impregnated carbon has been used to remove toxic agents (e.g., CNCl, HCN) from a gas stream for many years; no other material can completely replace it till today, but its stability is very poor. In some extreme conditions, such as higher humidity, acidic environment, or by usage, it will quickly lose its effectiveness. A lot of studies have been done by many researchers about the deactivation mechanism of the impregnated carbon. But this problem is very complicated. Although some basic additives (e.g., TEDA) can increase its stability[ 1,2,3], today we cannot reasonably explain its deactivation mechanism and how TEDA plays its role. Pytiewski[4] investigated the surface chemistry of Cu-Cr impregnated carbon and pointed out that at the surface of carbon, there exist several Cu-Cr-NH,-containing species: Cu(NH3)2.CrOj, Cu(NH,),.Cr04, Cu,(OH),.CrO,.xNH,.yH,O, and CuOH.NH,.CrO,.
The author claimed that the oxidation states of chromium and copper were Cr(VI) and Cu(II), respectively. Hammarstrom and Sacco[S] pointed out that at the surface of carbon only a little Cu(1) and Cu(0) were detected, and about 80% of chromium existed at the state of Cr(V1). The result of Joseph[2] by XPS was similar to Hammarstrom and Sacco; he found a large shoulder of Cr(II1) in the CRp,, spectrum. Hammarstrom and Saccol6] also investigated the CuOH.NH,.CrO, and CuOH.NH,.CrO,-impregnated BPL charcoal; they assumed that CuOH.NH,.CrO, is the active substance at the charcoal surface for the removal of toxic agents, and they revealed that the deactivation under humid atmosphere is due to the conversion of CuOH.NH,.CrO, into a yellow-green brochantite chromate, Cu,,(OH),.CrO,.