We have previously shown that oxide semiconductors (for example, TiO 2 , NiO, etc.) exhibit significant catalytic effects when heated at about 350-500C. This is due to the formation of a vast number of highly oxidative holes at high temperatures. Then, we applied the present finding for complete removal of volatile organic compounds (VOCs). In the present investigation, we focused on Cr 2 O 3þx (0 < x < 1) in an attempt to lower the operation temperature of VOC decomposition. We present here how the low-temperature operation proceeds with Cr 2 O 3þx and also elucidate why Cr 2 O 3þx is colored green, on the basis of the temperature dependences of electrical conductivity and Seebeck potential of Cr 2 O 3þx . The green color of Cr 2 O 3þx is found to arise from Cr vacancies in Cr 2 O 3þx , and the vacancies are responsible for the formation of a narrow conduction band or a deep acceptor level, resulting in the low-temperature operation of VOC decomposition.

Chloro-based volatile organic compounds (VOCs) are known to exert noxious effects on the environment, and their destruction by catalytic combustion, for example, accompanies a production of hydrochloric acid (HCl) that damages the catalyst. We have been interested in complete removal of VOCs by our system based on thermally activated oxide semiconductors (i.e., catalysts) such as Cr 2 O 3 , TiO 2 , NiO, -Fe 2 O 3 . In the present investigation, we have fundamentally studied the decomposition process of dichloromethane (CH 2 Cl 2 : DCM) and trichloroethylene (CHCCl 3 ; TCE) on the basis of the mass-and Raman spectra in an attempt to identify the formation temperature of HCl. Then, we found that the decomposition of DCM and TCE starts at about 100 and 200 C, respectively; whereas HCl is abruptly formed at a critical temperature of about 350 C in both compounds. Based on this result, we have optimized the operation temperature below 300 C for Cr 2 O 3 -impregnated honeycomb systems and achieved the complete removal of DCM and TCE that accompanies no formation of HCl.