Article pubs.acs.org/JPCC
Brønsted−Evans−Polanyi Relationship for Transition Metal Carbide and Transition Metal Oxide Surfaces Francesc Viñes,† Aleksandra Vojvodic,‡,§ Frank Abild-Pedersen,‡ and Francesc Illas*,† †
Departament de Química-Física & Institut de Química Teòrica i Computacional (IQTCUB), Universitat de Barcelona (UB), C/Martí i Franquès 1, E-08028 Barcelona, Spain ‡ SUNCAT Center for Interface Science and Catalysis, SLAC, National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States § Department of Chemical Engineering, Stanford University, Stanford, California 94305, United States ABSTRACT: The splitting of O2 on transition metal, transition metal carbide, and transition metal oxide surfaces is analyzed in the framework of Brønsted−Evans−Polanyi (BEP) relationships. It is shown that these hold for all three types of substrates, thus giving support to the idea of universality behind these useful relationships. Moreover, comparison of the BEP relationships for the three substrates suggests a significantly higher catalytic activity on metal carbides and rutile metal oxides.
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INTRODUCTION The empirical Brønsted−Evans−Polanyi (BEP) relationships,1,2 relating the activation energy for a given reaction or elementary step with its corresponding reaction energy in a linear manner, provide a way to estimate the kinetic behavior of a chemical reaction from simple thermodynamic data. This useful tool has spread through various disciplines, ranging from biochemistry to applications in heterogeneous catalysis. For the latter, BEP relationships evolved from the empirical mainstay3−5 to a more theoretically grounded basis thanks to the incorporation of a large database coming from Density Functional Theory-based (DFT) calculations. In fact, during the past decade, a significant number of theoretical studies reported the fulfillment of BEP linear relationships for several low Miller-index metal (Pt, Cu, Ag, Au, Pd, Fe, Mo, Ru, Rh, Os, Ir, Co, Ni, Tc, Nb, and Zr) surfaces6−16 and, in the last years, for nanoparticles as well.10,17 Most of the earlier studies focused on BEP relations in bond breaking reactions of simple diatomic molecules,6 for example, N2 dissociation, a very crucial step in ammonia synthesis.9 Establishing such relations is of enormous importance in heterogeneous catalysis, and it provides a powerful screening tool when such bond scission shows to be the overall rate-limiting step.15 More recently, such studies have extended to include processes of formation and disaggregation of triatomic molecules,14,16,17 and also the breaking of C−X (X = C, N, O) bonds in small organic molecules and dehydrogenation reactions.11,13,14 Despite that excellent BEP relations are obtained by mixing different kind of reactions, substrates, and surface active sites,6,11,14 it has been suggested that slightly © 2013 American Chemical Society
better adjustments appear when making a distinction on reaction types,14 the series of the d transition metal substrates,15 or by differentiating pristine surfaces from defect sites.6,9 Distinctions like this became critical in a recent study on the applicability of BEP relations on a different class of materials, the transition metal oxides (TMO).18 Here, it was shown that it was mandatory to distinguish between different kinds of surfaces, rutile-like (110) surfaces for binary oxides MO2 (M = Ti, Mo, Ru, Ir, Pt) and perovskite ABO3 (001) surfaces with BO2 termination (A = Sr, La and B = Sc, Ti, V, Cr, Mn, Fe, Ru, Co, Ni, and Cu), different surface atoms involved in the activation, and the nature of the bond scission, including H2, N2, O2, NO, CO, Br2, Cl2, I2, H2O, OH, HCl, HBr, and HI scissions. In these cases, the BEP relations seem to hold, but they appear to be cumbersome without a well-defined overall trend. It has long been questioned whether the BEP relations could be extended to materials beyond metals. In other words, are they really universal? In the present study, we will address these questions by focusing on well-defined transition metal carbide (TMC) and transition metal oxide (TMO) surfaces. To facilitate the comparison between the different substrates, only metal sites are considered, which constitute the common denominator among materials under scrutiny. Note that such a restriction could miss peculiarities on the catalytic activity of oxide and carbide surface sites but, on the other hand, enables Received: December 22, 2012 Revised: February 6, 2013 Published: February 6, 2013 4168
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The reaction considered in the present study is O 2 dissociation, which is a key step in many heterogeneously catalyzed oxidation reactions. On the TMC(001) , TM = Ti, V, Nb, Zr, Hf, Ta, and Mo, surfaces in the NaCl structure, the reaction is mediated by the surface metal cations.23,29−32 In this study, the following TMC(001) surfaces have been considered: ScC, TiC, VC, ZrC, NbC, δ-MoC, and TaC. For comparison of the BEP data for the same elementary reaction on close-packed M(111), M = Mo, Cu, Ru, Rh, Pd, Ag, Pt, and Au, and stepped M(211), M = Pd, Pd, Ag, Pt, and Au, transition metal surfaces. The data for metal surfaces have been extracted from the existing literature.6,11,18 Data for the rutile metal oxides MO2(110) have been taken from the literature,18 but only in cases where the reaction is mediated entirely by the coordinately unsaturated sites (cus) on top of a 5-fold coordinated metal atom on the stoichometric surface. This ensures consistency with all other data points shown. Details of the calculation procedures can be found in ref 18. Figure 1 shows the different surface orientations involved in the calculations.
one to quantify ensemble and ligand effects on similar surface metal sites. The selection of such materials is not arbitrary; on the contrary, it comes from the well-known technological and industrial interest in these materials. In fact, both TMC and TMO exhibit a unique combination of physical and chemical properties, which emerge from their mixed covalent, ionic bonding between the cations and the anions.19−21 TMC and TMO are in a sense intermediate materials between metals and ceramics, as they display simple crystallographic structures, in many cases good electrical and thermal conductivities, and are ultrahard and refractory materials.19 Moreover, TMC and TMO are important materials that have found widespread application in catalysis. The TMC have been targeted as possible low-cost alternative materials to replace scarce Ptgroup metals in heterogeneous catalysis, displaying similar or even better catalytic activity and selectivity than the latter for a diversity of reactions.22−25 In addition, TMC exhibit a high tolerance to poisoning by sulfur.22 The TMO are used as catalysts in a large number of chemical processes: The water− gas-shift reaction, CO oxidation, and a large number of reactions that convert hydrocarbons into other chemicals. In summary, it is reasonable to assume that both TMCs and TMOs will be important materials in the development of highly reactive and selective catalysts for the future.
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COMPUTATIONAL METHODS The ΔETS and ΔEdiss data for metal, carbide, and oxide substrates6,11,18 have been obtained carrying out DFT periodic Kohn−Sham calculations with the DACAPO code.26 The slab surface models are as described in previous work,6,11,18 and the calculations have been carried out using the revised version of the Perdew−Burke−Ernzerhof (RPBE) exchange-correlation functional.27 Vanderbilt ultrasoft pseduopotentials were used to describe the core electrons.28 A plane-wave basis-set with a kinetic energy cutoff of at least 340 eV was employed. The transition states were determined using the fixed-bond length method, and the bond breaking was constrained to the metal− metal plane for the oxides and carbides. Nature of the minima and transition states were corroborated with frequency analyses.
Figure 1. Top view of the different surfaces of the considered materials.
Here, it is worth mentioning that in order to avoid unwanted side-reactions, the reactions on the MO2(110) and TMC(001) surfaces have been constrained to the (1̅10) plane through the surface metal atoms. For the carbides, we have found that during O2 dissociation a strong O−C bond is formed, which naturally leads to a strong deviation from the BEP relation obtained for dissociation in the metal−metal plane. The reason is, of course, an increased stability of carbon monoxide on the carbide surfaces, especially for group IV TMC.33−36 Combining data sets from previous studies together with the calculations performed here, the database used in this work includes 14, 7, and 5 data points for metals, TMC, and rutile MO2 metal oxides, respectively. The above-described set of data is used to obtain BEP plots relating the energy of activation ΔETS and the reaction energy ΔEdiss. Definitions of the energies are shown in Figure 2, and the data are shown in Figure 3. In all cases, the transition state energy and the dissociative chemisorption energy have been calculated relative to H2O and H2 in the gas phase.37 For each of the four series, corresponding to different types of materials and structures, an overall linear regression arises without discriminating among the nature (coordination and species of the neighboring atoms) of the metal surface atoms. The slope, ordinate intersection, and mean absolute errors, with respect to a perfectly linear BEP trend, are displayed in Table 1. From either the plots in Figure 3 or the coefficients for the first-order fits in Table 1, it is clear that all surfaces show highly linear trends. A closer look at Figure 3 reveals that the BEP correlation for stepped (211) metal surfaces is well below the M(111) surfaces. This observed increase in reactivity of M(211) as compared to M(111) surfaces has been explained in detail elsewhere,6 and we will not repeat it here. Instead, we now focus on the TMC(001)
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RESULTS AND DISCUSSION In the present study, we provide evidence of BEP relationships that holds true for the simple bond scission reaction of O2 taking place on well-defined metal sites on transition metal, on TMC in NaCl structure, and on TMO surfaces in the rutile structure. The data entering into these BEP relationships have been compiled from a number of DFT-based calculations on TMC and TMO surfaces together with data from previous studies. In addition to the established BEP relationships on metals and oxide surfaces, we will show the existence of a welldefined BEP relationship for the O2 dissociation on a series of TMC surfaces. Furthermore, the comparison of the complete set of results for metals, rutile oxide structures, and TMCs reveals that both the TMC and the rutile oxide surfaces exhibit lower values for the activation energies than metals in the entire region of the reaction enthalpy. When compared to the closepacked and stepped transition metal surfaces, the reduction in activation energy for the carbides is ∼0.25 eV relative to the stepped surface, and for the oxides the reduction is ∼0.50 eV. Furthermore, a much larger slope of the fitted BEP line is observed for the oxides. This suggests that both oxides and carbides should be very active catalysts for splitting reactions. 4169
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TMC(100) surfaces is ∼0.25 eV smaller than the intercept for the M(211) surfaces. For the MO2(110) surfaces, the slope is significantly larger than that for the metal and TMC(001) surfaces. As was found for the TMC surfaces, the MO2 series has a ΔETS-intercept term, which is ∼0.5 eV smaller than the one for the M(211) surfaces. These relations strongly suggest that for splitting reactions, TMC(001) and MO2(110) are more active than metals in the energy regime −1 eV < ΔEdiss < 1 eV, which, according to the Sabatier principle, is viewed as the catalytically relevant region. It is tempting to argue that such enhanced activity of TMC and MO2 has its origin in a combination of ensemble and ligand effects of oxygen and carbon atoms in the crystal and electronic structure of their parent early transition metal compounds. The observed differences between the BEP relationships for metals and TMC(001) and MO2(110) metal compounds have important consequences in catalysis. In fact, in cases where the rate-limiting step is the breaking of a chemical bond, the reduction on the energetic costs is likely to induce an equally large reduction in the working temperatures at which the reactions take place, as seen previously, for example, in oxidative dehydrogenation processes on oxide catalyts.38 Industrially relevant reactions, where the breaking of diatomic molecules with intrinsic strong bonds is important, are numerous. It includes the Fischer−Tropsch process, where the strong bond in the CO molecule has to be split to allow subsequent C−C coupling of carbon containing species, the NO decomposition reaction, and the ammonia synthesis where splitting of N2 is rate-limiting. In this Article, we have only considered the stoichiometric termination of the carbide and oxide surfaces. However, the various working conditions required for industrial reactions might influence the surface termination of the catalyst and hence its ability to break and form specific molecular bonds. Because of the complex and flexible nature of the carbide and oxide surfaces, novel reaction mechanisms could be possible, and this could potentially lead to unique reactivity and selectivity patterns.
Figure 2. Illustration of the potential energy diagram, where ΔETS and ΔE diss are the transition state energy and the dissociative chemisorption energy, respectively. All energies are given relative to gas-phase H2O and H2.
Figure 3. Calculated transition state energies, ΔETS, as a function of dissociative chemisorption energies, ΔEdiss, of O2 on TMC(001) (red), M(111) (green), M(211) (blue), and MO2(110) (black) surfaces. Solid lines represent the averaged BEP linear fit to the calculated data, following the same color code. The dashed line illustrates the dissociation line, that is, ΔETS = ΔEdiss.
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CONCLUSIONS The results presented in this work indicate that BEP relationships hold for the simple reaction of O2 dissociation on a number of different surfaces. We have found that the activity of the metal sites on TMC(001) and rutile MO2(110) is much higher than that on close-packed and corrugated metal surfaces. These observations allow us to extend the idea of universality, first developed for reactions catalyzed by metal surfaces6−9,17 and, more recently, extended to oxide surfaces,18 also to include metal carbides. More importantly, the comparison of the energy profile and corresponding BEP relationships for the reactions considered allows one to emphasize that transition metal carbides and rutile metal oxides should display a much higher catalytic activity.
Table 1. All of the Fitting Values for the Considered Systems Displayed in Figure 1a substrate
α
β/eV
MAE/eV
M(111) M(211) MO2(110) TMC(001)
0.64 0.58 0.84 0.57
1.69 0.89 0.42 0.65
0.09 0.26 0.35 0.28
For each individual fit, the slope, α, and the independent term, β, are given together with the mean absolute error, MAE. a
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surfaces and the MO2(110) surfaces. Their BEP trends (Figure 2) are found to be linear, and the regression coefficients obtained show that the fits for the carbides and oxides are as good as for the metals. Note that these relationships arise from a series of data in both the regions of endothermic reaction, ΔEdiss > 0, and in the region corresponding to exothermic reaction, ΔEdiss < 0; hence, they are comparable to the data for metal substrates. For the TMC, the extracted slope is similar to the one for the close-packed metal surfaces. However, the largest and, to some extent, surprising difference between the two series lies in the ΔETS-intercept term, which for the
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS F.V. thanks the Spanish Ministerio de Ciencia e Innovación (MICINN) for a postdoctoral Juan de la Cierva grant (JCI4170
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2010-06372). Financial support has been provided by Spanish MICINN grant FIS2008-02238 and in part by Generalitat de Catalunya (grants 2009SGR1041 and XRQTC). F.I. acknowledges additional support through 2009 ICREA Academia award for excellence in research. The Barcelona Supercomputing Centre has generously provided computational time. A.V. and F.A.-P. gratefully acknowledge support from the U.S. Department of Energy under contract number DE-AC02-76SF00515.
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