J. Phys. Chem. C 2009, 113, 193–203
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Modeling Hydrogen Sulfide Adsorption on Mo-Edge MoS2 Surfaces under Solid Oxide Fuel Cell Conditions Natasha M. Galea, Eugene S. Kadantsev, and Tom Ziegler* Department of Chemistry, UniVersity of Calgary, UniVersity DriVe 2500, Calgary, Alberta T2N 1N4, Canada ReceiVed: NoVember 26, 2007; ReVised Manuscript ReceiVed: October 17, 2008
We examine the degree to which sulfur-based fuel contaminants, such as hydrogen sulfide (H2S), can adsorb on the molybdenum sulfide-based anode of a solid oxide fuel cell (SOFC) under normal SOFC conditions. Our examination takes into account multiple adsorption/desorption events involving H2S and the fuel (H2). By means of a kinetic model that allows us to approximate the rate of adsorbed oxygen formation based on experimental O2- anion consumption at the triple-phase-boundary (TPB), we also consider the reaction mechanisms associated with the formation and desorption of H2O(g), SO2(g), and S2(g). Preferred adsorption sites, energies, transition states, and kinetic barriers are calculated for the resulting species, *SHx, *OHx, *SOx, and *S2 (x ) 0-2). We have concluded that at typical SOFC operating conditions, the level of adsorbed sulfur on the MoS2 anode surface will not exceed 25% (one S surface atom for every one Mo surface atom). 1. Introduction The high efficiency of a solid oxide fuel cell (SOFC), as compared to traditional power generation systems, makes it a promising source for cheap and environmentally friendly energy. However, limitations are currently imposed on the use of SOFC technologies by the high costs of purification of conventional anode fuels, which contain contaminants such as hydrogen sulfide (H2S). These contaminants degrade fuel cell performance by blocking the adsorption sites on the anode catalyst, disallowing further adsorption of the anode fuel. Recently, the research group of Chuang published data concerning hightemperature SOFC experiments utilizing a molybdenum sulfide (MoS2) anode catalyst, an yttria-stabilized-zirconia (YSZ) electrolyte, and a Pt cathode, with syngas anode fuel (40% H2 and 60% CO) containing 5000 ppm of the pollutant H2S (H2S/ H2 fuel ratio of 5000 ppm/0.4 atm).1 Based on experiments at 800-900 °C, Chuang demonstrated that the performance of the SOFC was better using H2S-containing anode fuel than using pure H2. Therefore, it seemed natural to consider MoS2, from a theoretical point of view, as a possible sulfur-tolerant SOFC anode catalyst. MoS2 catalysts consist of MoS2 units containing Mo4+ and S2- ions. A layered structure is formed by the hexagonal arrangement of Mo and S stacked together to give S-Mo-S sandwiches (MoS2) coordinated in a trigonal prismatic fashion, held together by weak VDWs forces. The active phase of the MoS2 particles is coordinatively unsaturated (CUS) Mo atoms located at the edge of the crystallite, allowing further adsorption of sulfur-based species. Therefore, the number of sulfur vacancies or CUS sites is related to catalytic activity. The adsorption and desorption processes of H2S on a molybdenum sulfide (MoS2) surface, studied here in connection with SOFC anode poisoning, are also an integral part of the hydrodesulfurization (HDS) reactions catalyzed by MoS2. As compared to the SOFC, HDS systems operate at much lower temperatures (400-700 K) and at much higher H2S/H2 fuel pressure ratios (larger amount of H2S present in the HDS system). It has been deduced from STM experiments2 and theoretical3 calculations that the morphology of the MoS2 surface * Corresponding author. E-mail:
[email protected].
is sensitive to different H2S/H2 pressures. Based on typical HDS reaction temperatures, single edge MoS2 triangular morphology is observed at typical H2S/H2 pressure ratios of 500 (H2S:H2 ) 500:1, 500/1 ) 500). At lower H2S/H2 ratios, such as 0.07, which is even higher than that observed during typical SOFC conditions (H2S/H2 fuel ratio of 10 ppm/1 atm, 10 ppm ) 10-5 atm), a MoS2 hexagonal morphology is experimentally observed.2 The presence of MoS2 hexagons implies that both Mo(101j0) and S- (1j010) edges have comparable stability. On the basis of the theoretically calculated chemical potential of sulfur (µS) during H2S/H2 adsorption and desorption reactions on the MoS2 surface, Schweiger et al. have demonstrated that at the lowest H2S/H2 partial pressure anode fuel ratios that are possible while still retaining a stable MoS2 surface, the 50%S Moedge is the most stable of the two edges.4 Upon reaching even lower H2S/H2 partial pressure ratios (µS < -1.3 eV), the overall reaction conditions are so strongly reducing that gas-phase sulfur is more preferable to adsorbed sulfur, reducing the MoS2 surface to metallic body-centered cubic Mo and destabilizing the entire surface. Within this destabilized region, at much lower values of µS (
