(608c) Analysis of the Adsorption Mechanism of Hg in SCR Catalyst: V2O5

Over 32% of anthropogenic mercury emissions in the United States are from the flue gas emitted by coal plants. In 2004, 44.2 tons of mercury was emitted to the atmosphere just from U.S. coal-fired power plants. The environment and health effects associated to the emission of Hg have pushed the power plants to use control technologies for removing mercury from the flue gas. It is possible to rely in part on existing control devices in place for NOx, particulate matter, and SOx to assist in the oxidization and subsequent capture of mercury throughout the quench zone of combustion flue gases. One such co-benefit is the vanadium-based selective catalyst reduction (SCR) unit, which is used primarily for the reduction of NO and NO2 to N2 and water vapor by passing ammonia across the catalyst. This catalyst may also act as an oxidizing catalyst for Hg0 to Hg2+. In addition, it has been suggested that the catalyst is also responsible for oxidizing SO2 to SO3.

The primary focus of this work is on the potential oxidation of mercury over the SCR unit. The mechanism for mercury oxidation across the vanadium oxide catalyst is currently unclear with multiple reaction pathways being possible. Density functional theory calculations have been carried out to model the kinetic and thermodynamic behavior of the Hg on the V2O5(001)

Thermodynamic stability characterization of the different V2O5 surfaces is carried out to determine which surface is more abundant at the conditions of the SCR, which further provides indication to which adsorption sites are more active to initiate the mercury oxidation process. The different oxygen atoms on the V2O5 (001) surface are distinguished by the coordination number (number of V atoms that coordinated with oxygen). It has been hypothesized that the most active sites for mercury oxidation are the one-coordinated oxygen atoms in the V2O5 (001) surface. However, the binding energies and density of states (DOS) calculations of Hg on the other surface oxygen atoms, two- and three-coordinated sites, are also carried out to fully understand the mechanism of mercury binding. Additionally, water vapor is present in the oxidizing environment of combustion flue gases so that hydrated surfaces are also investigated for mercury reactivity.

Finally, complementing the theoretical analysis of Hg adsorption and oxidation on V2O5, samples of crystalline vanadium oxide is characterized and tested for mercury reactivity in a packed-bed reactor within a simulated flue gas. An electron ionization quadrupole mass spectrometer is used to directly measure the extent of oxidized mercury in the gas stream after the packed-bed. The catalyst was characterized using XPS before and after exposure to flue gas to determine the nature of the surface.