Hg BINDING ON Pd BINARY ALLOYS AND OVERLAYS

摘要

Coal-fired power plants are the major source of mercury worldwide and reducing the emissions of mercury is a major environmental concern since mercury is considered to be one of the most toxic metals found in the environment. Additionally, mercury is reported as a hazardous air pollutant (HAP) by The Clean Air Act (CAA) of 1990. Currently within the United States there are over five hundred 500-megawatt coal-fired power plants. The amount of energy produced from coal is predicted to increase 3% by 2030. In 2005, The United States Environmental Protection Agency (USEPA) adopted the Clean Air Mercury Rule (CAMR) to reduce the release of mercury from coal-fired power plants ultimately reducing US Hg emissions by 70% in 2018. In February 2008, this rule was vacated by the Courts and power plants are removed from the CAA list of sources of hazardous air pollutants; however, roughly half of the states still have Hg emissions controls in place for coal-fired power utilities. The vast majority of the mercury released from coal combustion is elemental mercury. Noble metals such as palladium, gold, silver and copper have been proposed to capture elemental mercury. Density functional theory (DFT) calculations are carried out to investigate mercury interaction with Pd binary alloys and overlays in addition to pure Pd, Au, Ag and Cu surfaces using a projected augmented wave method with the Perdew-Wang generalized gradient approximation. It has been determined that Pd has the highest mercury binding energy in comparison to other noble metals. In addition, Pd is found to be the primary surface atom responsible for improving the interaction of mercury with the surface atoms in both Pd binary alloys and overlays. Deposition of Pd overlays on the top of Au and Ag enhance the reactivity of the surface by shifting the d-states of surface atoms up in energy. Strong mercury binding causes a significant overlap between s- and p- states of Pd and the d-state of mercury.