Article pubs.acs.org/IECR
Ethanol Activation on Closed-Packed Surfaces Jonathan E. Sutton and Dionisios G. Vlachos* Catalysis Center for Energy Innovation and Center for Catalytic Science and Technology, Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, Delaware 19716, United States S Supporting Information *
ABSTRACT: Density functional theory (DFT) calculations on close-packed surfaces of Co, Ni, Pd, Pt, Rh, and Ru are conducted to generate insights into the adsorption and activation of ethanol. Metals with stronger C binding energies are expected to be more active, relative to those with weaker C binding energies. Single metals preferentially dehydrogenate before C−C and C−O cracking reactions are sufficiently facile, and early dehydrogenation reactions are likely more kinetically relevant than the cracking reactions. Metals with high O binding energies, especially relative to their C binding energies, are projected to be more selective to C−O scission, whereas those with lower O binding energies should be more selective to C−C scission. Finally, Brønsted−Evans−Polanyi (BEP)-type correlations are developed.
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was made36 to elucidate trends in how the adsorption and activation energies vary as the structure of the molecule changes, which is a technique that has recently been shown to provide valuable insights into the overall decomposition mechanism.47,53,54 Ferrin et al.55 combined linear scaling relations (LSRs)56−60 with a single Brønsted−Evans−Polanyi (BEP) relation (with C−C and C−O cracking reactions from a single BEP correlation) to map out the reaction and activation energies of the C−C and C−O scission reactions on a variety of single transition metals and to construct low-cost microkinetic models for transition metals. Such an approach also enables the construction of high-throughput microkinetic models to enable the mapping of complex activity and selectivity trends for both pure metals and metal alloys.52,61 We recently demonstrated, for C−C cracking in ethanol, that the common approach of employing a BEP relation based on a single metal introduces systematic errors into estimated activation energies when applied to other metals, i.e., a single metal BEP correlation does not extrapolate well.60 In the interest of minimizing such errors as well as enabling more-accurate estimation of activation energies for all types of reactions appearing in the mechanism, it is vital that additional BEP relations be developed. To summarize, we have three principal aims in this work. First, we wish to employ DFT to identify common features and qualitative differences in the energetically most favorable decomposition pathways on close-packed and potentially prevalent facets of Co, Ni, Pd, Pt, Rh, and Ru. Second, we examine the trends in the minimum adsorption and activation energies to identify qualitative descriptors for activity and selectivity and provide general insights into the activation of ethanol, a surrogate for higher primary alcohols (e.g., 1-
INTRODUCTION Over the past decade, ethanol has received attention both as a potential carbon-neutral fuel source (e.g., for direct ethanol fuel cells) and as a feedstock for value-added chemicals.1−3 Furthermore, ethanol itself is often considered to be a surrogate for larger biomass-derived molecules. As such, the catalytic activation of ethanol has been well-studied, both theoretically and experimentally. Experimental studies include both supported catalyst4−17 and surface science studies.18−30 Density functional theory (DFT) studies of ethanol decomposition (including oxidation and reforming) have been carried out on various transition-metal surfaces.31−49 Despite the amount of work done on elucidating the mechanism of ethanol decomposition and reforming, there has been limited theoretical work done on elucidating the impact of the metal (and potentially, metal alloys) on the selectivity and activity trends of prototypical catalytic reactions. Analysis of such trends could provide valuable insights into principles for designing catalysts with improved activity and selectivity. Two theoretical methods could be employed for this purpose: DFT and microkinetic modeling (MKM). Both methods have historically been applied to one metal surface at a time. We are only aware of a few DFT studies that have simultaneously compared the adsorption and activation trends of ethanol on a variety of metals within the same computational setup.36,50,51 Mavrikakis et al. considered only the stability of two adsorbed conformations of ethylene oxide.50 Tereshchuk and da Silva studied the adsorption but not activation of ethanol.51 The work of Wang et al.36 is closest in scope to our work (including demonstrating that the decomposition barriers decrease as the d-band center moves toward the Fermi level). The reactions considered were limited to, at most, three dehydrogenation reactions (two C−H and one O−H), a C−C cracking reaction, and a C−O cracking reaction. Recent combined DFT and microkinetic modeling studies47,52 suggest that the mechanism for oxygenate decomposition on Pt and likely other metals is significantly more complex, proceeding through many dehydrogenations before undergoing C−C cracking very late in the sequence. In addition, no attempt © XXXX American Chemical Society
Special Issue: Scott Fogler Festschrift Received: November 2, 2014 Revised: December 12, 2014 Accepted: December 15, 2014
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DOI: 10.1021/ie5043374 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research butanol) and potentially polyols. Finally, we develop BEP correlations that should be useful for future high-throughput kinetic models of ethanol activation on transition metals.
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METHODOLOGY Density Functional Theory Calculations. The SIESTA DFT code62 was used to calculate adsorption and activation energies for a variety of ethanol derivatives on six close-packed transition-metal surfaces: Co(0001), Ni(111), Pd(111), Pt(111), Rh(111), and Ru(0001). We employed Troullier− Martins norm-conserving scalar relativistic pseudo-potentials,63 a double-ζ plus polarization (DZP) basis set, and the Perdew− Burke−Ernzerhof GGA functional.64 Spin polarization was included for calculations with Co and Ni. In all cases, 3 × 3 unit cells with four metal layers and 15 Å vacuum slabs were used with a 3 × 3 × 1 Monkhorst−Pack k-point mesh. The bottom two layers were frozen in their bulk positions. A force tolerance of 0.1 eV/Å was used as the convergence criterion. Spot checking of at least 20 stable adsorbates for each metal at a finer k-point mesh (5 × 5 × 1) and stricter force tolerance (0.05 eV/ Å) was done to quantify the level of convergence. All systematic deviations were
