Facile Fischer-Tropsch Chain Growth from CH2 Monomers Enabled

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Facile Fischer-Tropsch Chain Growth from CH2 Monomers Enabled by the Dynamic CO Adlayer Lucas Foppa, Marcella Iannuzzi, Christophe Copéret, and Aleix Comas-Vives ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00239 • Publication Date (Web): 03 Jun 2019 Downloaded from http://pubs.acs.org on June 5, 2019

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Facile Fischer-Tropsch Chain Growth from CH2 Monomers Enabled by the Dynamic CO Adlayer Lucas Foppa,a Marcella Iannuzzi,b Christophe Copéreta and Aleix Comas-Vives*a,c aDepartment

of Chemistry and Applied Biosciences, ETH Zürich, Vladimir-Prelog-Weg 1-5, CH-8093 Zürich, Switzerland. of Physical Chemistry, University of Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland. cDepartment of Chemistry, Universitat Autònoma de Barcelona, 08193 Cerdanyola del Vallès, Catalonia, Spain. bInstitute

Abstract: Numerous chain growth mechanisms, namely CO insertion and carbide, and active sites (flat and stepped surfaces) have been proposed to explain how hydrocarbons are formed from syngas during the Fischer-Tropsch reaction on Ru catalysts, particularly active and selective towards long-chain products. While these pathways are supported by DFT calculations, computational models often considered surfaces at rather low adsorbate coverage. A systematic comparison of chain growth mechanisms including the CO adlayer present on the catalyst’s surface under reaction conditions is however not available due to the challenging representation of co-adsorbate interactions in the DFT models. Here, we show that the high coverage of chemisorbed CO on the metal surface favors the carbide mechanism on flat surfaces by using ab initio molecular dynamics simulations, which introduce the complex adlayer effects at the reaction temperature of 200 °C. At the considered CO and H coverages (0.50-0.72 and 0.24 monolayer, respectively) hydrocarbon formation involves CH2 monomers forming ethylene and propylene as primary products, consistent with the selectivity observed in experiments. Such mechanism only becomes the most feasible in the presence of the CO adlayer. Indeed, in the absence of co-adsorbed CO, methane may be formed instead on the flat surface and the first C-C bond occurs preferentially on stepped surfaces via CH and CH2 monomers with a higher free-energy barrier (55 kJ.mol-1) compared to the coupling of two CH2 species at high CO coverage on the flat surface (20 kJ.mol-1). Therefore, the CO adlayer strongly modulates the nature of chain growth monomers and active sites, and in fine drives the formation of hydrocarbons during Ru-catalyzed Fischer-Tropsch. Overall, these results show how adsorbate-adsorbate interactions dictate reaction mechanisms operating in adlayers, ubiquitous in heterogeneous catalysis. Keywords: Fischer-Tropsch Synthesis, Density Functional Theory, Ruthenium, Ab Initio Molecular Dynamics, Chain Growth Mechanism Due to the challenging identification of reaction intermediates 1. Introduction under reaction conditions (typically 200°C, 10bar), first The Fischer-Tropsch Synthesis (FTS)1-4 catalyzed by Fe, Co principles modeling based on density functional theory and Ru metal nanoparticles (NPs) is one of the key reactions of (DFT) has emerged as a powerful tool to distinguish the gas-to-liquid technology5,6 and allows the conversion of potential FTS reaction mechanisms and active site syngas (CO/H2 mixture) produced from CH4, to liquid hydrocarbons. The distribution of products, ranging from C1 (methane) up to C30+ (waxes), depends on chain growth and termination steps,4,7-9 being Ru-based catalysts highly efficient towards the formation of long-chain hydrocarbons.3 While FTS mechanisms on Ru have been extensively investigated,4,10,11 the chain growth mechanism remains highly debated. The two main proposals, the so-called CO insertion and carbide mechanisms, involve C-C bond formation from adsorbed R-CHx + CO vs. RCHx + CHx monomers, respectively, where R is the growing alkyl chain (Figure 1a, in green and blue, respectively). While the CO insertion mechanism is supported by isotopic transient experiments demonstrating that chain initiation is slower than chain growth (and adsorbed CO must thus act as a monomer),12,13 the carbide mechanism has been historically associated with CH2 monomers, since the use of CH2N2, presumably leading to the increase in CH2 surface concentration, was shown to promote chain growth in (lowpressure) experiments.14,15 Furthermore, the exact nature of the active sites is also disputed, some studies favoring NP’s stepedge sites (e.g. B5 or B6)16-18 present on stepped surfaces and others proposing surface sites present on terraces as the most actives (Figure 1b). structures.19-21

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Figure 1. (a) Schematic representation of the proposed CO insertion and carbide mechanisms for chain growth in Rucatalyzed FTS. The competing methanation route is also displayed. (b) Schematic representation of proposed FTS active sites on Ru NP catalysts: terraces and step-edges at low and high coverage of adsorbates. It should be noted that the structures shown in (b) do not correspond to the structural models used in this work (shown themselves in Figure 2, vide infra). Early computational studies concentrated on modelling FTS on Ru surfaces at low adsorbate coverage (2 1-n-alkenes; condition (ii): FTS polymerization involves C2 surface species and co-fed 13C H is incorporated in alkene products as 13CH 13CH 2 4 3 2 (CH2)nCH=CH2, for n = 0, 1, 2, 3; and condition (iii): a lower amount of C2 products compared to what is expected from the Anderson-Schulz-Flory (ASF) distribution is obtained. The ASF distribution assumes that the chain-growth probability ratio between the rates of propagation and termination for a specific chain length - does not depend on the chain length. The mechanism proposed in Scheme 1 is consistent with all aforementioned conditions, since the formation of linear alkenes is favored compared to CH4 and always occurs prior to that of alkanes or branched hydrocarbons (cond. i), ethylene may initiate the chain growth simply after the 1,2-H shift reaction and its C-C connectivity is kept throughout the cycle (cond. ii), and the rate of termination is related to the rate of alkene desorption (de-coordination), faster for C3> than C2 (cond. iii).72 Regarding the active site for the chain growth, our results suggest that, on the one hand, the formation of H2C=CH2 is much more energetically feasible on flat surfaces compared to

We evaluated here the CO insertion and the carbide chain growth mechanisms for hydrocarbon formation in the FischerTropsch Synthesis on Ru flat and stepped surfaces at high CO coverages using ab initio molecular dynamics simulations that allowed the introduction of complex adlayer effects at the reaction temperature of 200°C. Our results show that the CO adlayer modifies the relative stability of CHx (x=0,1,2,3) species compared to low coverage. In particular, under the CO and H coverages considered in the model (0.50-0.72 and 0.24 monolayer, respectively), CH2 is favored as the most stable monomer. Furthermore, co-adsorbed CO species suppress the formation of methane by inducing the segregation of CHx and H surface speces in different domains on the surface (the formers and latters preferring, respectively, CO and H domains), while lowering the free-energy barriers for C-C bond formation reactions on flat surfaces, consistent with Ru selectivity towards long-chain hydrocarbons. Conversely, the CO adlayer increases the free-energy barriers for chain growth on stepped surfaces compared to low coverages. The most favorable mechanism identified under the considered CO and H coverages is the carbide pathway involving the coupling of two CH2 species on the flat surface and producing adsorbed ethylene (H2C=CH2) as the first C2 product. This adsorbed ethylene is then converted into ethylidene (H3C–CH) via a 1,2-H shift intramolecular reaction and subsequent chain growth proceeds via the reaction of this species with another CH2, resulting in the formation of adsorbed propylene (H3C– CH=CH2). Within this mechanism, chain termination can occur by the desorption of the olefin and is therefore faster for C3 compared to C2. This pathway is consistent with the formation of olefins as primary reaction products, with the product distribution and with isotope labeling experiments. The CO adlayer therefore controls the nature of chain growth monomers and active sites and drives the formation of hydrocarbons during Ru-catalyzed Fischer-Tropsch reaction. More generally, these results show how complex interactions among mobile adsorbates in adlayers, neglected in ultra-high vacuum or DFT models at low coverage but captured here by ab initio molecular dynamics simulations, influence reaction mechanisms occurring at high coverage, showing the importance of developing models that integrate information as close as possible from experimental conditions in heterogeneous catalysis.

AUTHOR INFORMATION Corresponding Author *[email protected]

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ASSOCIATED CONTENT Supporting Information Supplementary figures (Figure S1-S4), computational details (description of initial structures for the AIMD simulations and the unit cells for the model surfaces used, electronic structure, molecular dynamics and metadynamics calculations) as well as the trajectories of the simulations (.xyz and movie .mpg files) are available in SI.

ACKNOWLEDGMENT The authors thank the Swiss National Foundation (Ambizione project PZ00P2_148059), the Holcim Foundation, the Spanish MEC and the European Social Fund (RyC-2016-19930) and ETH (Research Grant ETH42 14-1) for financial support.

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