S and

Research Article pubs.acs.org/acscatalysis

Development of a ReaxFF Reactive Force Field for Fe/Cr/O/S and Application to Oxidation of Butane over a Pyrite-Covered Cr2O3 Catalyst Yun Kyung Shin,*,† Hyunwook Kwak,‡,§ Alex V. Vasenkov,‡,∥ Debasis Sengupta,‡ and Adri C.T. van Duin*,† †

Department of Mechanical and Nuclear Engineering, Pennsylvania State University, University Park, Pennsylvania 16802, United States ‡ CFD Research Corporation, Huntsville, Alabama 35806, United States S Supporting Information *

ABSTRACT: We developed a ReaxFF force field for Fe/Cr/O/S, which is parametrized against data from quantum mechanical (QM) calculations. Using this force field, we studied the Cr-oxide catalyzed oxidation reaction of butane at 1600 K. Our simulation results demonstrate that the active oxygen species on the oxide surface play an important role in the conversion of butane. Dehydrogenation of butane, which is found to be catalyzed by oxygen species on the oxide surface, initiates the reaction and generates butane radicals and surface OH groups. The radical intermediates are associated with the oxygen atoms to form C−O bonds or make double bonds when neighboring carbon atoms are dehydrogenated, forming light alkenes. On the clean Croxide, the major oxidation product is CH2O. The presence of iron pyrite (FeS2), a common inorganic component in coal-derived fuels and a major slagging component, on Cr-oxide accelerates the complete oxidation of butane forming CO2 and CO. Surface reconstruction by iron pyrite is probably responsible for the change of the catalytic behavior. Reoxidation of the reduced oxide surface can occur through removal of surface H2O and adsorption of gaseous molecular oxygen at the vacancy sites on the clean Cr-oxide. On the other hand, on the modified Cr-oxide, it is found that a considerable amount of SOH molecules are released from the surface. These results can provide the detailed mechanisms for the catalytic oxidation of alkane and product distributions in Cr-oxide catalyst and give, for the first time, atomistic-scale insight in the complex surface chemistry of these catalysts under realistic operating conditions. KEYWORDS: Cr-oxide, alkane, oxidation, metal oxide catalyst, pyrite

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INTRODUCTION The dehydrogenation and oxidative dehydrogenation of alkane on catalysts play an important role in the petrochemical industry. The noble-metal-based catalysts (Pt and Pd) and transition-metal oxide catalysts (V, Mo, Co, and Cu) have been successfully used for combustion reactions of organic compounds.1−5 Pd-based catalysts show the best activity in methane combustion, and vanadium oxide is among the most © 2015 American Chemical Society

active and selective metal oxides for oxidation reactions of light alkanes including propane and butane. Supported vanadium oxide catalysts have been extensively studied for the oxidative dehydrogenation of light alkanes. In particular, butane can be Received: August 12, 2015 Revised: October 28, 2015 Published: October 30, 2015 7226

DOI: 10.1021/acscatal.5b01766 ACS Catal. 2015, 5, 7226−7236

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ACS Catalysis

Using the developed Fe/Cr/O/S ReaxFF force field, the oxidation of butane on a clean Cr2O3 surface and a modified pyrite/Cr2O3 surface under combustion condition is studied to understand the catalytic activity of the surface chromium oxide. Of particular interest are the mechanism of complete oxidation of butane and its products. We examine how the modified oxide surface by pyrite changes the catalytic activity and selectivity.

activated on vanadium TiO2-supported catalysts, but only carbon oxides are formed.6,7 Vanadium oxide surfaces, which are reduced during redox reactions, have been proposed as the active sites.4,8 Chromium oxide is one of the major constituents of the protective oxides growing on many alloys and has been known to improve the high-temperature oxidation resistance and mechanical properties of chromium-oxide-forming steels. In addition, Cr-oxide is able to catalyze several chemical reactions, e.g., dehydrogenation, oxidation, and polymerization.9−12 Particularly, Cr-oxide is a highly efficient catalyst for full combustion.13 Chromia/alumina catalysts were used for the dehydrogenation of butane to give butenes in industry at reaction temperatures of 500−600 °C.14,15 It has been known that the active oxygen species play an important role in the catalytic activities of metal oxide catalysts in the oxidation process.5 The surface oxygen species on chromium oxide exist as terminal (Cr−O) or bridging oxygen (Cr−O−Cr) in the absence of O 2 , and peroxo oxygen (Cr−O−O) in O 2 environment. Because the reactivity of oxygen is strongly dependent on the local environment, such as neighboring metals and Cr−O bond distance, the information on molecular structure of the surface oxide catalysts is of great interest. The discovery of potential pathways of selective activation of alkanes is not only an exciting field but also a very open problem. In particular, molecular-level identification of the nature of the elementary reactions is still challenging and remains mainly unknown. Thus, fundamental research on this aspect is necessary, with respect to how the active oxygen species on the catalyst surface participate in the selective abstraction of hydrogen from alkane and in the oxygen insertion to the dehydrogenated alkane. Many theoretical studies have addressed the mechanisms of dehydrogenation and oxidation on predefined single surface models. However, single surface models of oxide catalysts are structurally equivalent and are not able to capture the chemistry of the complicated catalytic reactions, which will probably provide energetically favorable pathways and more realistic distributions of oxidation products. It is necessary to identify reactivity of oxygen species involved in the reactions to understand the realistic catalytic oxidation reactions taking place in complicated systems, in which more than single oxygen species react concurrently with reactant molecules. The study of chemical reactions of the iron pyrite (FeS2) has recently become popular in the fields of surface science and catalysis. Iron pyrite is known as one of the dominant minerals in coal and a major source of wall slagging and deposition. The oxidation of pyrite can also result in the acidification and destruction of living organisms. In particular, sulfur has been known as an inhibitor of possible surface reactions. When sulfur is released from the iron pyrite through the oxidation, it tends to combine with other multivalent metals present in the system. Oxidation of iron pyrite can also generate Fe-oxides, which will cause deposits in piping and machinery. Even small amounts of iron can be detrimental to catalysis and consequently, change the catalytic behavior such as productivity and selectivity. There is a complex chemistry that is still not yet fully understood. Thus, it is important to identify the effect of pyrite on alkane oxidation under catalytic oxidation environment. In this paper, we begin with a brief description of ReaxFF force field and the first-principles method used to build the ReaxFF training set for Fe/Cr/O/S. Specific details of the parametrization of the Fe/Cr/O/S force field are highlighted.

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COMPUTATIONAL METHODS ReaxFF Method. The ReaxFF force field 16,17 was developed to help bridge the gap between quantum mechanical (QM) methods and experiment. Unlike nonreactive potentials, ReaxFF is a bond-order-dependent force field with instantaneous connectivity for the chemical bonds depending on the atomic local environment. Thus, ReaxFF is reliable to examine the reaction process and describe the energetics for various reaction intermediates. ReaxFF overall system energy is described by physically meaningful many-body empirical potential terms. The partial contributions include bond-orderdependent energy terms such as bond, angle, and torsion, which disappear upon bond dissociation, and nonbonded interaction terms such as van der Waals and Coulomb interactions. ReaxFF employs a distance-corrected Morsepotential for the van der Waals energy to properly describe the short-range interactions. For the Coulomb interaction, ReaxFF uses the electronegativity equalization method (EEM) to determine the atomic charges and takes into account the shielding between two atoms at short distances. For the details of the energy formulas, the reader is referred to refs 16 and 17. All force field parameters describing energy terms are optimized against the QM data and/or literature values using a singleparameter based parabolic extrapolation method. QM Calculations. First-principles DFT calculations were performed using plane-wave basis first-principles code, VASP18 combined with projector augmented wave (PAW)19 potential, and Perdew−Burke−Ernzerhof (PBE)20 exchange−correlation potential. Calculations were performed with spin polarization to describe properly the magnetic structure of the alloy systems. For lattice optimization of ordered bulk crystals, we applied an energy cutoff of 500 eV with 0.20−0.25 Å−1 of spacing between k-points in reciprocal space. For all other calculations, we applied energy cutoff of 400 eV with 0.25−0.35 Å−1 of spacing between k-points in reciprocal space. The reciprocal space integration was carried out over the set of points generated according to the Monkhorst−Pack scheme.21 To take the strong correlation effect into account for the transition metal, which is not properly described with the simple delocalized exchange−correlation functional from traditional DFT (e.g., LDA or GGA), the DFT+U method was employed. DFT+U calculations performed in our work are based upon the rotationally invariant form of Hubbard U potential developed by Dudarev and co-workers,22 which is often denoted as the effective U potential, Ueff. To find the best Ueff that describes the strong correlation in the metal oxide systems, we applied different Ueff values upon CrxOy crystals (e.g., CrO2, Cr2O3, and Cr3O4) and identified the Ueff values for Cr which would give the closet agreement with experiment for the known systems. The oxidation energies was estimated by calculating the enthalpy of formation based on DFT+U formalism with varying the effective U potential in range of 0−4 eV. The optimal effective U potential for chromium was determined to be 1.84 eV. 7227

DOI: 10.1021/acscatal.5b01766 ACS Catal. 2015, 5, 7226−7236

Research Article

ACS Catalysis DFT calculations of the gas phase molecules were performed with the Jaguar software package23 using B3LYP functional with the LACV3P+G** effective core potential24,25 for Cr atom and 6-311+G** basis set for O and H atoms. ReaxFF MD Simulations. ReaxFF MD simulations were performed to investigate the catalytic effect of Cr-oxide on butane oxidation in three systems: (1) C4H10/O2, (2) Cr2O3/ C 4 H 10 /O 2 , and (3) pyrite/Cr 2 O 3 /C 4 H 10 /O 2 . First, we performed the oxidation simulations in small systems of less than 1000 atoms as shown in Figure S1 and compared the reactions observed during the simulations. Then we investigated the oxidation products and mechanisms of the product formation in large-scale systems of 54400 (Cr2O3/C4H10/O2) and 60288 atoms (pyrite/Cr2O3/C4H10/O2) at 1600 K. First, we carried out NVT-MD heating simulations from 800 to 2000 K in three small systems of 20 C4H10 molecules with 200 O2, with 150 O2/Cr-oxide cluster and with 100 O2/Cr-oxide/pyrite clusters in a periodic box of 40 × 40 × 40 Å3. The systems were equilibrated at 800 K under NVT ensemble for 10 ps, using Nose−Hoover thermostat with a relaxation constant of 100 fs. During the equilibration simulations, the interactions of butane/O2, butane/Cr-oxide and butane/pyrite were switched off. Following the equilibration at 800 K, the systems were heated up to 2000 K. The NVT ensemble was applied for 20 ps with a relaxation constant of 100 fs during which the temperature of the system was maintained at a desired temperature, e.g., 1000, 1200, 1400, 1600, 1800, and 2000 K. Atomic charges of the system were updated every step. To investigate the oxidation products and the oxidation mechanisms in the catalytic oxidation reactions, we carried out the NVT-MD simulations at large scales. The Cr2O3/C4H10/O2 system consists of 1280 C4H10, 9600 O2, and α-Cr2O3 clusters of 17 280 atoms in a periodic cubic box of 160.0 × 160.0 × 160.0 Å3. For the oxidation in the presence of pyrite, pyrite clusters of 12 288 atoms were inserted into a system consisting of 1280 C4H10, 6400 O2, and α-Cr2O3 clusters of 17 280 atoms (Figure 1a,c). Following the equilibration and heating procedures (1600 K) as stated in the simulation of small systems, NVT simulations were performed for data analysis at 1600 K for 150 ps using a time step of 0.1 fs. We note that for the present work, we set no bond interactions between Cr/C and Cr/H pairs and set dummy force field parameters for relevant valence angle parameters. Thus, we do not observe bond-formation and bond-breaking between butane and Cr metal. In fact, dehydrogenation of alkanes can be carried out on metal as well as oxide catalysts. However, metal-C and metal-H bonds typically have lower strengths comparing to O−C and O−H bonds, and during our simulation, we do not observe metallic Cr-species, thus justifying the neglect of Cr/C and Cr/ H bonds. In addition, we performed NVT-MD simulations for the systems of pyrite/C4H8/O2 and pyrite/C4H8 to clarify the effect of pyrite on the oxidation and dehydrogenation of butane. The system for the oxidation consists of 80 C4H10, 600 O2, and pyrite clusters of 1152 atoms and for the study of dehydrogenation the system has 80 C4H10 and pyrite clusters of 1152 atoms in a periodic cubic box of 80.0 × 80.0 × 40.0 Å3. Following the same procedures as described above for the oxidation simulations, we collected the data for analysis for 1 ns at 1600 K for the oxidation simulation and 400 ps at 2000 K for the dehydrogenation simulation.

Figure 1. Snapshots of the clean Cr-oxide (a, b), modified Cr-oxide surface with pyrite (c, d) and pyrite (e, f) at 1600 K.

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RESULTS AND DISCUSSION Parameterization of ReaxFF Force Field. The energy− volume relationship of metals and alloys are of great importance for predicting phase stability of the crystal structures and structural phase transitions under pressure and temperature. Accordingly, the equations of state of single crystals of Cr, Cr-oxide, Fe−S alloys and Fe−Cr alloys are trained against QM data over a wide range of volume. The lattice parameters and cohesive energies of Cr for three different crystal structures (bcc, fcc and a15) are reported in Table 1. The cohesive energies of bcc and a15 Cr in ReaxFF Table 1. ReaxFF and QM Values for Lattice Parameter (a, Å), Cohesive Energy (Ec, kcal/mol), and Surface Energy per Atom (Es, kcal/mol) of Pure Cr in bcc, fcc and a15, and αCr2O3 Structures structure

property

ReaxFF

QM

bcc

a Ec a Ec a Ec Es

2.8516 93.7 3.6290 92.6 4.5603 91.1 40.4 27.1 5.7 17.4

2.8620 93.9 3.6180 85.2 4.5376 91.9 38.5 28.0 6.4 12.1

fcc a15 bcc Cr(100) (110) α-Cr2O3(0001) (1012) 7228

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ACS Catalysis (93.7 and 91.1 kcal/mol, respectively) agree well with the QM data (93.9 and 91.9 kcal/mol, respectively) and the experimental value (94.6 kcal/mol for bcc Cr).26 The ReaxFF energy difference of fcc from bcc Cr, 1.1 kcal/mol is smaller than the QM value, 9.3 kcal/mol. Thus, the stability of the fcc lattice with respect to the bcc lattice is overestimated. We note that this is very close to the one reported using EAM potential (92.7 kcal/mol for fcc) in ref 27; however, the value still deviates from the energy differences reported by ab initio calculations.28,29 The lattice parameter of bcc Cr is smaller than those of fcc and a15 Cr. Figure S2 shows the relative energy of Cr crystals as a function of volume with the bcc Cr taken as standard state. As one can see, the equations of state for single crystal structures are well reproduced in ReaxFF, except for fcc. We also include surface free energies to the training set in order to enable the force field to describe important surface phenomena such as segregation and adsorption. The ReaxFF surface energies calculated per surface atom of bcc Cr (Table 1) are 40.4 kcal/mol for Cr(100) and 27.1 kcal/mol for Cr(110). This indicates that the Cr(110) surface is more closely packed and more stable than the Cr(100) surface. These values are in good agreement with the QM data. To extend the metal force field to oxide, we performed the QM calculations for condensed-phase Cr-oxide systems (CrO2, Cr2O3, and Cr3O4) and gas-phase molecules (Cr(OH)2 and CrO3). It has been suggested that various volatile gaseous species of chromium oxyhydroxide species (e.g., CrO2(OH)2, CrO(OH)3, CrO2(OH)) are transformed from Cr2O3 in the presence of water vapor, and thus, Cr−O−H species are involved in the degradation process of Cr2O3.30,31 The CrO3 and Cr(OH)2 species are possibly one of those volatile gaseous species at high temperature. The dissociation energy of the Cr− O single bond was calculated by constraining the Cr−O bond distance of Cr(OH)2 using B3LYP method with LACV3P+G** basis set. To obtain the correct bond dissociation energy, we also calculated the triplet state at a large bond distance (6.2 Å). As shown in Figure 2a, the ReaxFF bond dissociation energy profile is in good agreement with the QM data. Furthermore, we calculated Mulliken charges for this molecule and used these to optimize the EEM charge parameters such as electronegativity, hardness, and shielding, and ReaxFF reproduces the atomic charge distributions well. The energy profile for the valence angle distortion of the O−Cr−O angle (Figure 2b) was also well reproduced in ReaxFF. For condensed phase Cr-oxide, we calculated heats of formation for multiple oxidation states such as CrO2, Cr2O3, and Cr3O4. As can be seen in Figure 3, ReaxFF slightly underestimates the stability of CrO2; however, the overall equations of state for these oxides are in agreement with the QM data, enabling ReaxFF to describe oxidation reactions on metal oxide surfaces. We also included the energetics of S in bulk Cr and various surfaces into the training set. When sulfur atoms are added to the octahedral interstitial and metal vacancy sites, the ReaxFF heats of formation are positive, 4.5 and 0.6 kcal/mol, respectively. These are in good agreement with the QM values, 4.6 and 0.5 kcal/mol, respectively. On the other hand, the binding energies of S on Cr surfaces are negative, showing that S prefers to adsorb at surface binding sites (Table 2). On a given crystal face, S is most stable on a hollow site with binding energies of −77.4 kcal/mol on Cr(100) and −61.0 kcal/mol on Cr(110). Our results indicate that the most likely adsorption site of S on Cr(100) and (110) is a hollow site, with the bridge

Figure 2. (a) Bond dissociation energy for Cr−O single bond in ReaxFF and QM (B3LYP/LACV3P+G**), as well as atomic charge distributions in Cr(OH)2 calculated by ReaxFF (black) and Mulliken charge distributions by QM (red). (b) Valence angle energies for O− Cr−O angle.

and metal sites being less favored. This adsorption strength trend is in agreement with the QM values. The force field for Fe−Cr was directly derived from the QM data, including heats of formation for condensed phase B2 FeCr, B32 FeCr, D03 Fe3Cr and D03 FeCr3 systems (Figure 3), and FexCry systems in low Cr concentrations. The heats of formation of the Fe−Cr alloys presented in Figure 4 are calculated with bcc Fe and bcc Cr as reference states. The most interesting part is the low-Cr region. DFT calculations32,33 predict negative values for the formation enthalpy for the range of low Cr concentrations while positive values as the Cr concentration increases. Thus, Cr steels are stable in low Cr concentrations while the clustering is energetically preferable at higher Cr concentrations, as first reported experimentally in bcc Fe−Cr alloys at low Cr concentrations ( C2H4 > C3H6 > C4H6. Although C4H8 is the major olefin product, the comparable amounts of butane are also converted to C2H4, producing practically larger amount of C2H4 species than C4H8. On the other hand, the pyritemodified Cr-oxide selectively produces C4H8 (C4H8 ≫ C3H6 > C2H4 > C4H6), suppressing the formation of lighter alkene, C2H2, while slightly enhancing C4H8 formation. When dehydrogenation is occurred only with pyrite, C4H8 is a major product first, which is similar to the modified Cr-oxide, and it converts to butadiene (C4H6) later (C4H8 ≃ C4H6 > C2H4 > C3H6). A clear difference between the clean Cr-oxide and the pyrite involving systems may support the experimental observation, which shows the high performance for selective dehydrogenation on metal sulfide.48 While the pyrite oxidation process under O2 environment continues increasing the content of FeOx, as predicted in the heats of formation of the Fe-oxide lower than the Fe-sulfide (Figure 6), the content of sulfur in the pyrite fragments becomes low. As a result, the loss of sulfur in pyrite converts gradually the sulfide surface to the oxide surface with broad Feoxide patches. As shown in Figure 12, the formation of complete oxidation products such as H2O and CO2 is considerably retarded in comparison to the oxidation on both the clean and the modified Cr-oxide. The CO2 species is

Figure 13. Conversion of butane to olefins observed from the NVTMD simulations at 1600 K on (a) Cr2O3, (b) Cr2O3/pyrite, and (c) pyrite surfaces.

formed in the mechanism similar to the CO generation in that gaseous sulfur and oxygen species play a role as a reductant and an oxidant, respectively. The recovery of the reduced Fe-oxide surface is made by the formation of SOH. In order to understand the mechanism of dehydrogenation and olefin formation by pyrite, oxygen free atmosphere was introduced to the system. As pyrite is known as the source of gaseous sulfur compounds in the combustion, the first reaction is that S2 species is released from the pyrite at 1400 K. The first dehydrogenation of butane by S2 takes place at 1800 K. The initiation temperature is higher than the dehydrogenation temperature on the oxidized pyrite and the Cr-oxide surfaces. This may indicate that butane is activated more readily at the oxide metal surface than by the reaction with S2. We also note that addition of S2 to the butane radical induces a C−S bond formation and, consequently, a C−C bond rupture, forming C1and C3-fragments. The pyrite fragment as well as gaseous sulfur species participates in the further dehydrogenation of the radicals, forming various olefins, e.g., C4H8, C4H6, C3H6 and C2H4. However, continuous dehydrogenation and C−S bond 7234

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Figure 14. Energy barriers for the dehydrogenation at various compositions of the Cr-oxide surface.

significantly higher if the reaction is carried out in the presence of pyrite. This indicates that the activity of the modified Croxide catalyst is improved and the conversion of oxidation intermediates to CO and CO2 is accelerated. However, this modified catalyst is shown to be inefficient for the evolution of H2 (no H2 released). In conclusion, the analysis of product distributions shows that the presence of pyrite changes the selectivity of Cr-oxide catalyst from CH2O to CO2. The catalytic behavior of the Cr-oxide is probably related to the surface structure of the catalyst and the ease of reduction and reoxidation of the catalyst.

formation give cleavage predominantly between carbon atoms, resulting in the decomposition to C2H4 and carbon sulfides in the gas phase. As shown in Figure S9, the content of sulfur in pyrite becomes low because pyrite decomposes to mainly S2 first, and subsequently H2S by hydrogenation. In the present work, we have observed that the modified Croxide surface which is subsequently sulfided by pyrite is even more active for oxidation than pyrite alone or the clean Croxide and is as selective for butane dehydrogenation as the pyrite. In heterogeneous catalysis, it is still an open question whether the active sites on the surface are altered not only geometrically but also electronically when it is exposed to another species of the different catalytic activity. Partial blocking of the Cr-oxide surface significantly improves the activity and selectivity toward oxidation. In order to understand whether the Cr-oxide surface is altered electronically by the addition of pyrite, we investigated the atomic charges at the active site and the energy barriers for the dehydrogenation at various Cr-oxide surfaces, which are partially substituted by Fe and S. Given that reactions that require a single site are gently affected only for the activity of the active site, we have generated four small fragments that have a terminal oxygen atom surrounded by different neighborsa clean Cr-oxide, a Cr-oxide that the first oxygen neighbors of the Cr atom are substituted by sulfur, a Cr-oxide that the second neighbors of the Cr atom are substituted by Fe, and a Cr-oxide that the first and second neighbors are substituted by S and Fe, respectively. As shown in Figure 14, the atomic charge of the terminal oxygen is not affected by its neighbors and is similar in all fragments (−0.41 ∼ −0.43e) although the atomic charge of the Cr atom decreases, responding to the neighboring sulfur. However, despite the very small electronic modification of the active oxygen by its neighbors, the energy barriers for the dehydrogenation of butane decrease from 5.2 to 1.7 and 3.4 kcal/mol when the active site is surrounded by Fe neighbors while sulfur does not play an essential role in the dehydrogenation on the oxide surface. Products of Butane Oxidation. Figure 11 shows the product distributions for the first 150 ps in the oxidation of butane over the clean Cr2O3 and the modified pyrite/Cr2O3 surface. Clean Cr2O3 catalyst produces no CO but small amount of CO2. Experimental results reported by Pradier et al.13 suggest that Cr2O3, one of the most active oxides, presents the highest selectivity of CO2 while no CO is produced. However, in this work, CH2O is the major oxidation product, which is formed by the partial oxidation. In contrast to the clean Cr-oxide results, the yields of CO and CO2 are

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CONCLUSIONS We developed the ReaxFF force field for Fe/Cr/O/S and studied the catalytic complete oxidation of butane on Cr2O3 and pyrite/Cr2O3 catalysts. We demonstrated that the active oxygen species on the Cr-oxide surface are very reactive and readily interact with butane. The activation of butane occurs through hydrogen abstraction by active oxygen atoms such as terminal (Cr−O), bridging (Cr−O−Cr), and peroxo (Cr−O− O) oxygen atoms, forming butane free radicals and surface OH groups. The radical intermediate forms a C−O bond with a nearby oxygen atom at the terminal or the peroxo oxygen sites. The structural properties including the distribution of active oxygen sites probably play an important role in determining the catalyst activity. Reoxidation of the reduced catalyst surface can occur by forming oxygen vacancies through the removal of surface-H2O at the oxygen sites and oxidation intermediates (e.g., CH2O). Then gaseous molecular oxygen can be adsorbed at the vacancy sites of the surface. The oxidation reactions on the modified Cr-oxide with pyrite proceed in a similar manner to that proposed for the reactions over the clean oxide surface. However, the modified Cr-oxide presents different catalytic properties compared to the clean Cr2O3 surface. Reconstruction of its surface structure affects the catalytic activity, greatly enhancing the yield of complete oxidation products (CO and CO2). Furthermore, a large number of SOH molecules are released from the oxide surface, forming vacancies on the oxide surface. These structural properties probably make the modified Cr-oxide an excellent catalyst for oxidation and dehydrogenation reactions. The advantage over previously studied single surface models of oxide catalysts is that this model can capture the chemistry of the complicated catalytic reactions via structurally nonequivalent surface oxygen types. Thus, complex reactions can proceed following energetically favorable pathways and finally reproduce more realistic distributions of oxidation products. 7235

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The lack of theoretical study in the catalytic activity of Cr2O3 makes this study the unique source of information on the catalytic oxidation of alkanes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.5b01766. Details of the MD simulation results given in Figures S1−S9; ReaxFF reactive force field parameters for Cr/O/ Fe/S/C/H (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Present Addresses §

(H.K.) Schrödinger, Inc., Cambridge, Massachusetts 02142, United States ∥ (A.V.V.) Multi Scale Solutions, Lexington, Kentucky 40511, United States Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by DoE-grant DE-FE0005867. We wish to thank the PSU Institute for CyberScience Advanced Cyberinfrastructure group and the National Energy Research Scientific Computing Center (NERSC) and Oak Ridge National Laboratory Center for Nanophase Materials Sciences (ORNL CNMS) for providing the computational resources.

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DOI: 10.1021/acscatal.5b01766 ACS Catal. 2015, 5, 7226−7236