DFT Study of Hg Oxidation across Vanadia-Titania SCR Catalyst under Flue Gas Conditions

Abstract

The density functional theory was used to analyze the thermodynamic stability and reactivity of the vanadia-titania catalyst below monolayer regime with the purpose of having a good representation of a commercial SCR catalyst (V 2 O 5 (<2 wt %)−TiO 2 ). The objective of this paper is to understand the reactivity of this catalyst in Hg oxidation. The SCR catalyst is modeled as a tetrahedrally coordinated divanadate unit supported on a 3-layer TiO 2 (001) slab to represent a catalyst with low vanadia loadings. Under flue gas conditions, the interaction of water with this surface has been studied as a function of pressure and temperature using ab initio thermodynamic calculations, showing that water coverage is temperature-dependent. Adsorbed water acts as a Lewis base, donating electrons to the TiO 2 (001) surface support, which increases the negative charge and reactivity of the oxygen atoms of the vanadia dimer. The reactivity of the vanadia dimer toward Hg oxidation is analyzed through the adsorption energies of Hg, HgCl, HCl, and H 2 O. Surfaces with high water coverage showed higher reactivity toward HgCl, which has the highest adsorption energy, followed by HCl. The adsorption energies of Hg suggest a negligible interaction with the vanadia dimer. Lateral interactions between neighboring adsorbed flue gas components on the vanadia dimer were studied, suggesting that having H 2 O or HgCl adsorbed on a neighboring oxygen atom increases the adsorption energies of Hg and HCl respectively. Temperature, pressure, and entropic effects were taken into account to study the reactivity of these surface interactions under flue gas conditions. Based on these results, it is proposed that the oxidation of Hg to HgCl 2 follows a Langmuir−Hinshelwood mechanism, represented as a cycle where HgCl and HCl interact without poisoning the surface. The proposed steps during the formation of HgCl 2 are the adsorption and dissociation of HCl, adsorption of HgCl, formation of HgCl 2 , and its desorption from the surface. ■ INTRODUCTIONThe U.S. Environmental Protection Agency (EPA) proposed the Mercury and Air Toxics Standards (MATS) in December 2011, which require U.S. natural gas and coal-fired power plants to install air pollution control devices to prevent 91% of the Hg present in flue gas from being released.1 Rising atmospheric levels of gaseous Hg have caused public concern because Hg has been shown to have negative neurological effects, affecting memory, attention, language, and visual skills.2 In the U.S., 40% of the total coal-fired generating capacity is generated in power plants equipped with selective catalytic reduction (SCR) units . Oxidized Hg is highly soluble in aqueous solutions, making its removal by conventional wet flue gas desulfurization processes possible. 5−7A widely employed material in commercial SCR units is titaniasupported vanadium and tungsten oxides, that is, V 2 O 5 −WO 3 / TiO 2 . 8,9 The active vanadia phase, V 2 O 5 , is responsible for NO x reduction 10 and can also catalyze Hg 0 oxidatio...

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