The extent of both NO x removal and NH 3 oxidation in the selective reduction of NO x by NH 3 over a sample of a commercial V 2 O 5 -WO 3 /TiO 2 catalyst that had been used for 20,000 h at an NH 3 -SCR deNO x process in a domestic heavy oil-fired power plant has been determined using an on-line IR-based system coupled with a modified White cell and the catalyst has been characterized to ascertain the reason why it possessed visibly different behaviors in the deNO x reaction, compared to a fresh sample. The on-site-used catalyst gave not only much higher deNO x activity at all temperatures less than 400°C than that indicated for a fresh catalyst but also a greatly increased NH 3 oxidation reaction. SEM-EDX measurements with samples of the used and fresh catalysts represented the presence of a huge amount of sulfur in the former sample, very consistent with sulfur amounts determined by a C/S analyzer; however, this did not create the measured enhancement in the lowtemperature deNO x and NH 3 oxidation performances. ICP measurements indicated a significant increase, by 1% as a basis of V 2 O 5 , in an amount of vanadium species after the on-site use and such vanadium species came from heavy oils used to generate electricity at the power plant and were in the form of polycrystalline V 2 O 5 nanoparticles as acquired by XRD measurements. All these results propose that the increased V 2 O 5 content might be associated with the noticeable difference in SCR activity and NH 3 oxidation between both of the samples.
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