Ethanol Decomposition on Co(0001): C−O Bond Scission on a Close

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Ethanol Decomposition on Co(0001): C-O Bond Scission on a Close-Packed Cobalt Surface Cornelis J. Weststrate,*,† Hendrik J. Gericke,† Martinus W. G. M. Verhoeven,† Ionel M. Ciobîc a,‡ Abdool M. Saib,z and J. W. (Hans) Niemantsverdriet† †

Schuit Institute of Catalysis, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands, Sasol Technology Netherlands B.V., Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, Netherlands, and z Sasol Technology (Pty) Ltd., P.O. Box 1, Sasolburg 1947, South Africa ‡

ABSTRACT Recently there has been a renewed interest in Co-catalyzed FischerTropsch synthesis (FTS) from natural gas, coal, and biomass, because it offers a realistic alternative to crude oil as a source of transportation fuels. Efforts to understand the FT mechanism on the atomic level have mainly focused on theoretical methods, whereas experimental surface science results have only had little impact on the understanding of the mechanism. An essential step in any FT mechanism is scission of the C-O bond. On a flat Co(0001) surface direct dissociation of the CO molecule is practically impossible at FTS conditions. We have found for the first time experimentally that the C-O bond can be broken at 350 K even on the relatively inert Co(0001) surface if a CxHy group and a hydrogen atom are attached to the C-end of the C-O moiety. SECTION Surfaces, Interfaces, Catalysis

ischer-Tropsch (FT) synthesis, discovered and developed by Fischer and Tropsch in the 1920s and 1930s,1,2 is a versatile process in which synthesis gas (H2 and CO) is converted into long-chain hydrocarbons and water by a catalyst that contains either Fe or Co as the active component. The hydrocarbon product is further processed into high-quality transportation fuels. Synthesis gas can be produced from a variety of carbon-containing feedstocks, such as coal, natural gas, or biomass (yielding coal-to-liquid, gasto-liquid, and biomass-to-liquid fuels, respectively). The high crude oil price and the demand for clean fuel have stimulated further development of the FT process. This has led to an increase in the scientific efforts to fundamentally understand the FT reaction and to use this knowledge to improve the process.3 In recent studies, sophisticated X-ray spectroscopies and high-resolution microscopy have been used to study real/ realistic supported Co catalysts.4-8 These studies have led to a better understanding of the deactivation mechanisms of FT catalysts4,7 and how the size of the catalyst particles influences activity and reactivity.6,8 From the side of theoretical chemistry, efforts have been focused on understanding the FT mechanism, which is a surface-catalyzed polymerization reaction of C1 species.9-12 In a recent review article, the different mechanisms are discussed and compared.13 Surface science studies, which have been essential to understand the ammonia synthesis reaction14,15 and the three-way exhaust catalyst,14,16 are rather scarce for cobalt surfaces. In most of the earlier studies, polycrystalline Co foils have been used. The available single crystal studies on Co(0001) mainly deal with the adsorption and desorption of CO and hydrogen or with

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surface reconstructions, while in some studies nonbasal planes of Co were used.17-24 The cobalt nanoparticles in a real catalyst have a facecentered cubic (fcc) crystal structure.25 Such particles exhibit mainly (111) and (100) planes,26 and the sites that can dissociate the CO molecule directly are present in small concentrations at defect sites. In our experiments, we use a hexagonal close-packed (hcp)-Co(0001) surface, a structural analog of the fcc-Co(111) surface present on a real Co catalyst particle. An essential step in any FTreaction scheme is the scission of the C-O bond. Dissociation of the CO molecule does not occur on the close-packed hcp-Co(0001) surface,17,22 and theory predicts a sizable activation barrier of 220 kJ/mol for the process.10 Experimental and theoretical studies have identified certain step sites that are able to dissociate the CO molecule with a much lower activation barrier.9,20 Experimentally, the activity of such sites for CO dissociation has been confirmed on a stepped Ru surface.27 Some authors have suggested that C-O bond scission is facilitated by partial hydrogenation of the CO molecule prior to decomposition.12 Recent theoretical results show that the CO bond is easily broken when a methyl group and a hydrogen are attached to the C-end of the CO molecule.10 We have studied the decomposition of ethanol, to see whether C-O bond scission is facilitated by the presence of a methyl group at the C-end of the C-O moiety.

Received Date: April 27, 2010 Accepted Date: May 18, 2010 Published on Web Date: May 24, 2010

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DOI: 10.1021/jz100537h |J. Phys. Chem. Lett. 2010, 1, 1767–1770

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Figure 1. O1s and C1s spectra after heating an ethanol-saturated surface to the indicated temperatures. The O1s spectrum of 0.1 ML Oad (obtained by dosing O2) and the C1s spectrum of 0.06 ML adsorbed acetylene molecules are shown in orange, for comparison. The image in the center shows the results of a temperature-programmed X-ray photoelectron spectroscopy (TP-XPS) experiment (individual spectra seen in top view), in which the sample was continuously heated (0.5 K/s) while measuring XPS spectra (10 K/spectrum). The sequence clearly shows the breaking of the C-O bond of the ethoxy moiety into atomic oxygen (529.26 eV) and acetylene (283.3 eV), which occurs around 350 K.

The high-resolution photoemission beamline I311 at MAXlab (Lund, Sweden) and the superESCA beamline at ELETTRA (Trieste, Italy) were utilized to study the decomposition mechanism of ethanol on the hcp-Co(0001) surface. Figure 1 shows the O1s and C1s photoemission spectra after heating an ethanol-saturated surface to the indicated temperatures. The two center panels show a top view of the C1s and O1s spectral regions, measured in real time during a slow heating. Between 160-330 K, the surface is covered with a species that we identified as ethoxy (CH3-CH2-O), based on the spectral shape of the C1s spectrum (see Supporting Information for details). According to density functional theory (DFT) calculations, this species is bound through the oxygen atom, with the C-O bond perpendicular to the surface (see inset Figure 2). It is thermodynamically more stable than the other candidate, ethanal (CH3-CHdO). Ethoxy stays bound to the surface and remains unchanged up to 300 K. Between 300-360 K, the ethoxy moiety decomposes, which is accompanied by desorption of ethanal between 300-340 K (see Supporting Information). We therefore believe that ethoxy first decomposes into a surface-bound ethanal species, which is bound to the surface through both the oxygen and the CH-carbon, with the CH3 group away from the surface (see the work of Saeys et al.10 and Figure 2). A small amount of the ethanal that is produced desorbs, while the rest decomposes, into atomic oxygen (0.1 ML) and adsorbed C2Hx species. The activation barrier for ethoxy decomposition was estimated to be around 70 kJ/mol, assuming an Arrhenius temperature dependence and absence of interactions between the surface species.28 A small amount of CO is seen as well (0.014 ML), either formed via a minor decomposition pathway, but most likely due to adsorption from the CO-containing background gas.


Figure 2. Energy diagram showing the relative stability of different intermediates during ethanol decomposition on Co(0001). The reaction path observed in the experiment is indicated with a dotted line. Ethoxy, which is bound to the surface through the oxygen atom, is more stable than adsorbed ethanal, which is bound with the CdO bond parallel to the surface. It can also be seen that C2H2 (acetylene) and CH3-C (ethylidyne) are the most stable C2Hx species.

Acetylene was identified as the major C-containing product of ethoxy decomposition, based on the C1s binding energy of the main peak that was found after annealling to 370 K.29 DFT also shows that adsorbed acetylene is the most stable C2Hx species (Figure 2), in line with our assignment. Furthermore, some ethylene (C2H4) desorption was found between 300450 K (Supporting Information), formed by hydrogenation of adsorbed acetylene. The C1s spectrum at 370 K also contains minor peaks due to adsorbates other than acetylene. These additional features are assigned to other C2Hx species,

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ethylidyne (C-CH3) being a likely candidate. According to temperature-programmed desorption (TPD) (see Supporting Information), the C2Hx fragments loose all their hydrogen below 440 K, leaving atomic carbon (282.6-283.6 eV) and some polymeric carbon (284.8-285.2 eV) behind. The spectrum of the carbon that is left undergoes some changes between 450-550 K, which are assigned to conversion of polymeric to atomic carbon. Half of the remaining carbon is removed between 550 and 620 K by reaction with atomic oxygen, forming CO, which desorbs upon formation (see TPD in the Supporting Information). The concentration of the remaining carbon after heating to 630 K is 0.1 ML. Our results show for the first time experimentally that C-O bond scission is possible on the Co(0001) surface, which is the most common surface present on a Co catalyst particle. It is interesting to note here that methanol, which forms a methoxy species (CH3O) upon adsorption at 160 K, also decomposes around 350 K, but exclusively into CO and hydrogen.30 Our preliminary results for 1-propanol, on the other hand, also show C-O bond scission.This implies that C-O bond scission on Co(0001) readily occurs when a CxHy group is attached to the CO molecule, while hydrogen atoms attached to the C-end of CO do not lead to dissociation of the C-O bond.10,12 We thus identified a key ingredient that facilitates the C-O bond scission. From our experimental study, it is not clear whether the presence of an additional hydrogen at the C-end is a crucial ingredient, but recent theory calculations by the group of Saeys showed that the presence of this hydrogen atom lowers the activation barrier considerably.10 Interestingly, on the close-packed surfaces of elements close to Co in the periodic table such as Ni, Rh, and Ir, C-C bond breaking is the preferred pathway.31-33 Our theory simulations (Figure 2) show that breaking of the C-O bond instead of the C-C bond is thermodynamically more favorable on Co(0001), as the products that we found, C2H2 þ O, are more stable than CH þ CO. For the FT mechanism, our work implies that C-O bond breaking is feasible on most parts of the catalyst particle surface if a CxHy-O moiety can be formed, for example by addition of CO to a CHx fragment, an FT mechanism that is usually referred to as the CO-insertion mechanism. The initial CHx species has to be generated on a step site where direct CO dissociation can take place. Carbon then needs to be hydrogenated to form a mobile CHx species, which can react with CO on other regions of the catalyst particle surface. Our work indicates that, for this mechanism, the endothermic (unlikely) CO insertion step rather than the C-O dissociation step determines the feasibility of this mechanism.10

ACKNOWLEDGMENT Sasol Technology is acknowledged for funding this project. The authors thank Prof. J. N. Andersen for beamtime at beamline I311 (MAX-lab, Lund). ELETTRA is acknowledged for beamtime at the SuperESCA beamline. The help of the technical staff at ELETTRA (Trieste, Italy) and at TU/e Eindhoven is also greatly appreciated.

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SUPPORTING INFORMATION AVAILABLE Experimental

procedures and supplementary experimental data (TPD, C1s spectrum of ethanal). This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION Corresponding Author:

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*To whom correspondence should be addressed. E-mail: [email protected].


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Ethanol Decomposition on Co(0001): C−O Bond Scission on a Close

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