J. Phys. Chem. C 2010, 114, 16669–16676
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On the Role of a Cobalt Promoter in a Water-Gas-Shift Reaction on Co-MoS2 Yan-Yan Chen,† Mei Dong,† Jianguo Wang,*,† and Haijun Jiao*,†,‡ State Key Laboratory of Coal ConVersion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi 030001, People’s Republic of China, and Leibniz-Institut fu¨r Katalyse e.V. an der UniVersita¨t Rostock, Albert-Einstein-Strasse 29a, 18059 Rostock, Germany ReceiVed: July 20, 2010; ReVised Manuscript ReceiVed: August 24, 2010
The role of a Co promoter in a water-gas-shift reaction on Co-MoS2 has been investigated on the basis of density functional theory computation. On the basis of the computed adsorption energy of the reaction intermediates and H2O dissociation barriers, the active catalyst is the Mo edge with 25% Co substitution and 25% sulfur coverage, while the S edge with 25% Co substitution and 50% sulfur coverage is not active. On the basis of the computed reaction barriers, the redox mechanism (CO + H2O f CO + O + 2H; CO + O + 2H f CO2 + H2) is the preferable reaction path, and the rate-determining step is the second step dissociation of OH into surface O and H, while the reaction path from carboxy (CO + OH f COOH; COOH f CO2 + H) is not favored due to its high dissociation barrier. In addition, formate (HCOO) is a side product from gas phase CO2 and surface H and does not participate directly in the reaction mechanism. Detailed comparisons reveal that the Co promoter is not an active center in H2O dissociation and CO oxidation but changes the adsorption configuration of the reaction intermediates and reduces the reaction barriers. The Co promoter plays the role of a textual promoter in creating more active sites and accelerating the reaction rate. 1. Introduction Molybdenum-based catalysts have been widely used as CO hydrogenation (HYD) and hydrodesulfurization (HDS) catalysts, and they also play an important role in water-gas-shift (WGS) reaction (CO + H2O f CO2 + H2); this is because a WGS reaction can provide hydrogen for CO HYD.1–7 A WGS reaction is one of the important reactions in the synthesis of hydrocarbons and alcohols from synthesis gas.8–10 Moreover, synthesis gas frequently contains sulfur-containing compounds as impurities, and supported metal catalysts require a severe sulfur cleanup, whereas molybdenum-based catalysts are known to be much more sulfur-resistant. Because of the broad applications and importance of the WGS reaction, many experimental1–7 and theoretical11,12 studies have been conduced to elucidate the reaction mechanism, to identify the catalytically active species, and to reveal the role of promoters and supports over MoS2-based catalysts. Both redox and associative mechanisms have been proposed for the WGS reaction. In a redox mechanism, water is activated to atomic oxygen, which oxides CO to CO2.13–17 Hou et al.2 and Lund18 claimed the redox mechanism, wherein the catalyst surface is alternately oxidized by water and then reduced by carbon monoxide over MoS2/Al2O3. In an associative mechanism, surface formate (HCOO) is formed from surface CO and OH and then decomposes into CO2 and H2.19–22 A third mechanism, involving an active surface COOH intermediate, was proposed on Cu (111)23 using a density functional theory (DFT) calculation and on Au (111)24 using near-edge X-ray adsorption fine structure, infrared adsorption spectroscopy, and DFT calculations. Many researchers investigated the WGS reaction on Co-MoS2 catalysts due to their high activity and sulfur tolerance, and nonsulfided Co-Mo catalysts exhibit very little * To whom correspondence should be addressed. E-mail: iccjgw@ sxicc.ac.cn (J.W.) and
[email protected] (H.J.). † Chinese Academy of Sciences. ‡ Leibniz-Institut fu¨r Katalyse e.V. an der Universita¨t Rostock.
WGS activity.25 Using a microkinetic model, Lund26 studied the WGS reaction over sulfide Co-MoS2/Al2O3 catalysts and found that Co promotes the catalyst activity at a low CO/H2O ratio. The higher performance of the WGS reaction on presulfided Co-Mo/MgO-Al2O3 catalyst was attributed to the optimal formation of active Co-Mo sulfides.27 On Co-Mo carbide catalysts, the amorphous Co-Mo oxycarbide was claimed to be responsible for WGS reaction.28 Despite these studies, the role of Co as a promoter in the WGS reaction is not well understood. For example, does Co actively involve in the elementary steps or is it only a textual promoter for creating more active sites? Atomic-scale images of a Co-MoS cluster on a gold surface by using scanning tunneling microscopy by Lauritsen et al.29 showed that cobalt atoms locate at the shorter S edges of a truncated hexagonal MoS2 cluster, while theoretical investigations on different models suggested that the S edge,30,31 Mo edge,32 and bulk structure33 are the preferred cobalt locations. However, these studies did not consider the effect of sulfidation conditions on the relative stabilities of different structures. Using a single sheet periodic model, Sun et al.34 found that cobalt prefers to incorporate into the S edge of MoS2 under typical sulfidation conditions and that the promoter atoms tend to be uncovered. The structure of Co-MoS2 under reductive conditions is unclear. In this study, a periodic DFT study on the WGS reaction on Co-MoS2 catalyst was performed. The adsorption of reactants, products, and intermediates has been investigated. The reaction pathways, including redox mechanism, HCOO- and COOHmediated mechanisms involving H2O dissociation, CO reaction with OH, and CO2 formation, have been studied. The experimental results26 on Co-MoS2 and computational results12 on the pure MoS2 surface are used for comparison.
10.1021/jp106751a 2010 American Chemical Society Published on Web 09/09/2010
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J. Phys. Chem. C, Vol. 114, No. 39, 2010
Chen et al. The adsorption energy Eads was calculated using eq 1, where Etot(ads/slab) is the total energy for the slab with absorbates in its equilibrium geometry, Etot(slab) is the total energy of the bare slab, and Etot(ads) is the total energy of the free adsorbates in the gas phase. According to eq 1, a negative Eads value corresponds to an exothermic adsorption, and the more negative the Eads is, the stronger the adsorption is.
Eads ) Etot(ads/slab) - Etot(ads) - Etot(slab)
(1)
To determine the sulfur coverage, relative energies of the surfaces at different sulfur coverage are calculated using similar methods in the literature.30,32,34,42,43 The free energy change for adding n sulfur atoms to the reference surface at temperature T, pH2S, and pH2 is obtained from the energetics of the reaction: S* + nH2S ) S*2 + nH2, where S* stands for the reference surface and S*2 stands for the different sulfided surface. The Gibbs energy of this reaction from eq 2 allows one to determine the stable surfaces under experimental conditions. For H2, in the gas phase, µ ) E0(H2) + G0(T) + RT ln(pH2/p0), while in the solid state, µ ) E0. Therefore, the Gibbs energy is approximated by eq 3, where ∆E0 is the standard energy change at 0 K, and ∆G0 is the temperature correction for free energy change from 0 to T K, and 575 K is used in this work.
∆G ) {µ(S*2) + n[µ(H2)]} - [µ(S*) + µ(H2S)] Figure 1. Crystal structure of the hexagonal MoS2(100) surface: (a) ideal bulk and (b) sulfur reconstructed surface (green for Mo atoms and yellow for S atoms).
(2) ∆G ) ∆E0 + n∆G0(T) + nRT ln(pH2 /pH2S)
(3)
2. Methods Bulk MoS2 forms a hexagonal crystal lattice with a layer type structure described by weakly binding sulfide layers parallel to the (001) net plane, and (100)12,35 is considered as the reactive surface. Figure 1a shows the ideal bulk model including four S-Mo-S rows in the z-direction and four S-Mo-S units in the y-direction. Figure 1b shows the most stable configurations where the two nonequivalent sheets are terminated by sulfur, one sulfur atom per surface Mo under H-rich conditions, that is, for H2S/H2 ratios
