(67c) Catalytic Activity of Arsenic Poisoned SCR Catalysts

Due to the variety of pollutants emitted from coal-fired power plants there has been increased effort to optimize catalysts that can limit multiple pollutants.  One example being researched is selective catalytic reduction (SCR) catalysts, which are currently used for the reduction of nitrogen oxides (NO_X) through the injection of ammonia.  Recently these catalysts have also been examined for their ability to oxidize mercury (Hg), thereby limiting Hg emissions.  However modern mixed-oxide catalysts used to carry out these reactions are susceptible to poisoning and deactivation over time due to the numerous species present in the flue gas such as arsenic.  Due to this limitation of current SCR catalysts, novel SCR catalysts are currently being investigated. While the activity and mechanism of deactivation has been studied on traditional V-Mo-Ti oxide catalysts to an extent, it is unknown what effect the arsenic species might have on novel SCR catalysts, as well as how severely Hg oxidation activity is affected.  This study examines the mechanism through which arsenic can poison commercial and novel catalysts.  The fresh and poisoned catalysts can then be compared to analyze the optimal conditions for NO_Xreduction and Hg oxidation.

In this study, copper-exchanged small pore zeolite SSZ-13 is compared to a commercial SCR catalyst with respect to its NO_X reduction and Hg oxidation activities after arsenic deactivation.  An arsenic deactivated sample is synthesized by exposing the catalysts to a simulated flue gas stream that is created by combusting methane, creating an environment representative of the one present in a coal-fired boiler.  Prior to combustion, the stream is premixed with arsine (AsH₃), which is converted to arsenic oxide (As₂O₃) as it passes through the oxygen-rich flame. The catalysts are characterized before and after exposure to arsenic using XPS, XRD, and SEM/EDX, to determine the specific mechanism of arsenic adsorption on the surface of the catalysts. NO_X and Hg activity are monitored with analyzing the reactor effluent with mass spectrometry and PS Analytical Sir Galahad Hg analyzer, respectively.   The gas stream composition is varied to include SO­₂, NO, NO₂, and HCl to study the effect that other flue gas components play in arsenic adsorption and catalyst activity.  In this way, the bi-functional activity of the commercial and zeolite SCR catalysts can be compared before and after arsenic deactivation.