Impact of Edge Oxidation State on Red-Ox Barriers during

Feb 6, 2017 - A computational investigation of the oxidative dehydrogenation (ODH) of hydrocarbons over model edge-oxidized carbon nanotubes (CNTs) pr...
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Impact of Edge Oxidation State on Red-Ox Barriers during Hydrocarbon Oxydehydrogenation over Carbon Nanotube Catalysts: A Theoretical Study Oleksiy V. Khavryuchenko*,‡,† and Benjamin Frank§ ‡

Research and Development Department, TMM LLC, Volodymyrska Street 49a, Kyiv 01601, Ukraine Centre for Research in Molecular Modeling, 7141 Sherbrooke Street West, Montréal, H4B1R6, Quebec, Canada § BasCat - UniCat BASF Joint Lab, Hardenbergstraße 36, D-10623 Berlin, Germany †

S Supporting Information *

ABSTRACT: A computational investigation of the oxidative dehydrogenation (ODH) of hydrocarbons over model edge-oxidized carbon nanotubes (CNTs) providing different degrees of CO/C−OH surface termination (i.e., reduction) was performed. The pool of gaseous species is assumed to contain the initial reagents (hydrocarbons, O2) and volatile intermediates (R•, HO2•, HO•, H2O2). The barriers of both the H-abstraction from the substrate molecules by surface carbonyl groups as well as of the reoxidation of hydroxyl groups sensitively respond to the red-ox state of the model cluster. Accordingly, the surface state of CNT catalysts under ODH steady-state reaction conditions represents an intermediate degree of catalyst reduction. This conclusion agrees with previously published kinetic and in situ XPS studies of carbon-catalyzed ODH.

1. INTRODUCTION The chemical utilization of natural gas demands efficient catalysts for selective activation of light alkanes. Here, the oxidative dehydrogenation (ODH) is discussed as an alternative to the direct dehydrogenation.1,2 Although ODH is typically investigated over transition metal oxides a pursuit for metal-free catalysis indicated a catalytic potential of carbon nanotubes (CNTs).3 Electron-rich CO groups located at the edges of CNTs and other carbons mimic the catalytic function of active sites on transition metals oxides.3,4 We recently reported computational results of the reaction of light hydrocarbons and ethylbenzene (EB), respectively, with flat graphene-like carbon models terminated by CO groups as well as of the reoxidation of hydrogenated clusters by molecular oxygen.5 Compared to the metal oxide based systems, e.g., VOx/SiO2,6 the spin-activity of the carbon clusters plays an essential role in the reaction mechanism of ODH. The spin state M of the catalyst defines the product of the first H abstraction, i.e., a carbon-centered radical in the case of the highest stable multiplicity state (Mmax) or a surface ether in the case of M < Mmax. Moreover, the carbon cluster enables a strong interaction between active sites due to the conductivity of its framework,5 which is a major difference if compared to transition metal oxide-based catalysts. However, it remained unclear if all these regularities are valid for the curved carbon structures, which form the backbone of a real CNT-based catalyst. Here, we present the results of quantum chemical (QC) simulations of the reactions of ethane, EB, as well as their radicals, respectively, with ultrashort (US) CNTs. The reoxidation of partially reduced US-CNTs with O2, HO2•, HO•, and H2O2 is © 2017 American Chemical Society

also considered. As the reoxidation is typically much faster than the dehydrogenation steps, only limited insight is available for this process over metal oxide-based catalysts.7 Since hydrocarbon radicals are repelled from the carbon cluster after formation, the gas phase during ODH contains both hydrocarbon molecules and radicals. Also, O-containing species can easily desorb from the surface of CNTs, thus intermediate products of O2 reduction (HO2•, HO•, and H2O2) should be included into the investigation.8 These species stochastically interact with the active sites on the CNT surface. On the other hand, carbon clusters with a different degree of reduction, i.e., (partial) conversion of quinone groups into phenol groups, can exhibit a different reactivity due to the electronic correlation between O-containing sites via the carbon backbone. Therefore, in order to establish reactivity correlations for CNTs in ODH, it is mandatory to check the reactions of all gas-phase species with all forms of CNTs. In a reasonable approach, we focused on extreme regions of the reduction scale, namely, the fully oxidized and reduced clusters, respectively, further interpolating data on intermediate states of reduction. The following tasks were defined: 1. Does the reactivity of US-CNTs toward hydrocarbons differ from that of flat carbon clusters, and what is the impact of the tube dimensions? 2. Are the regularities related to spin properties, which have been formulated for flat carbon clusters (pseudodegeReceived: January 3, 2017 Revised: February 1, 2017 Published: February 6, 2017 3958

DOI: 10.1021/acs.jpcc.7b00068 J. Phys. Chem. C 2017, 121, 3958−3962

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The Journal of Physical Chemistry C

saturated with H atoms. For the investigation of the impact of the degree of reduction on the reactivity of the CNT models, a series of US-CNTs HxZ3−8Q8 (with x = 1−8) and HxA3− 8Q8 (with x = 1−4) were considered. All z-terminated tubes are spin-active (Figure 2).

neracy of several spin states and difference in chemical reactivity between the highest stable spin state and lower ones),5 valid for CNT models? 3. How does the reactivity of CNTs in ODH depend on the red-ox state of the CNTs?

2. COMPUTATIONAL METHODS AND MODELS Quantum chemical calculations were performed on model clusters in the framework of density-functional theory (DFT), with B3LYP/TZVP(aux-def2-TZVP)9−11 by the ORCA package12,13 with Grimme’s vdW correction.14 Atom-pairwise dispersion correction to the DFT energy with Becke-Johnson damping15,16 was also checked for selected systems demonstrating no significant difference (Tables SB1, SC1). Spin density distributions were visualized with the UCSF Chimera package.17 Six zigzag-edge (z) terminated US-CNT models were considered, namely Z2-8Q8, Z3-8Q8, Z4-8Q8, Z5-8Q8, Z210Q10, and Z2-12Q12 (Figure 1). Here, the number after Z

Figure 2. Side and top view of spin density distributions over Z212Q12. (a,c) multiplicity 13 (Mmax), (b,d) singlet state.

The catalyst is considered as a mixture of independent edgeoxidized z- and a-terminated CNTs, while the pool of gaseous species contains both the initial reactants (hydrocarbon and O2) as well as volatile intermediates. The highly global reaction model implies that each H abstraction occurs independently from other elementary reactions, which take place at different sites of the catalyst, and the reaction products return to the pool of gaseous reactants, from which they can react with any other CNT with arbitrary red-ox state. This opens a matrix of 14 catalyst US-CNTs (in all stable multiplicity states) per 7 gaseous reagents. Since this implied extreme computational costs, we had to exclude intermediate states of reduction.

3. RESULTS AND DISCUSSION The reaction of z-terminated CNTs with C2H6 was chosen as a probe for the reactivity of tubes varying in diameter and length. The regularities for flat carbon clusters5 can be confirmed for the US-CNTs: the reaction in the Mmax state leads to an ethyl radical, while lower multiplicity states favor the formation of surface ethers.18 For five of six z-terminated CNT models, the barriers of the first H abstraction (Table SB1) fit into the range of values found for abstraction of the first H atom by flat carbon clusters (60−109 kJ/mol),5 and this value converges to 70 kJ/ mol upon increase of CNT length (for exclusion of the Z210Q10 case, refer to Supplementary B). The typical profiles of the reaction of hydrocarbon molecules with Z3-8Q8 are demonstrated in Figure SB1. The reactivity of CNT models is based on the presence of quinone groups on a conductive support, i.e., the aromatic carbon framework. The data in Table SB1 demonstrate that the reactivity is independent of the tube length. The impact of the CNT diameter is expressed very slightly, since the barrier increases slowly with increasing diameter. Notably, however, the thermal stability of CNTs strongly depends on their size. The smaller it is (both in length and diameter), the less stable

Figure 1. Structures of US-CNTs and one typical transition state structure for reaction of US-CNT with ethane. Color scheme: carbon (grey), oxygen (red), hydrogen (white).

indicates the length (in number of aromatic rings), whereas the numbers before and after Q identify the width (in number of aromatic rings) and the quantity of quinone groups, respectively. The armchair-edge (a) terminated US-CNT A38Q8 is three aromatic rings long, eight aromatic rings wide, and carries eight quinone groups grafted on one edge of the tube. In each case, the opposite edges of the respective model CNTs are 3959

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The Journal of Physical Chemistry C under oxidative conditions it is.19 Therefore, multiwalled (MW) CNTs behave like composite materials: the highly reactive thin tubes inside the multiwall composition expose catalytically active sites at the tip of the open tube, while they are protected from combustion by an outer shell of thicker and less reactive tubes. On the other hand, surface defects within the less curved carbon planes of the outer CNTs also contribute to the overall activity of the MWCNT catalyst, which are better described by flat models.5 One should emphasize that O atoms are significantly deflected from the aromatic moiety (Figure 1), most probably due to electrostatic repulsion between closely grafted quinone groups (the same was observed for edge-oxidized flat clusters).5 This effect is much more pronounced than that of the graphene sheet curvature, hence, quinone groups on both flat and curved carbon frameworks have the same reactivity due to the same extent of the aromatic system deformation. O2 and HO2• are the most abundant oxidizing agents in the gas phase. HO• radicals are extremely reactive and reoxidize any hydrogenated CNT (and hydrocarbon) in any multiplicity state without barrier. Instead, H2O2 fails to abstract H atoms from any hydrogenated CNT. Its decomposition is not efficiently catalyzed by quinone groups (with a calculated barrier of ∼169 kJ/mol), while the reaction H2O2 + O2 → 2 HO2• in the gas phase has a barrier of only ΔE‡ = ∼120 kJ/ mol. Thus, although the regeneration of the HO2• radical is much more probable, H2O2 decomposition on impurities or defects in CNT materials should be considered.20 Another important issue that can be derived from the computational data is the red-ox state of CNTs in the catalytic cycle. It is typically observed that freshly produced CNTs require an oxidative activation before turning into a stationary regime.21 On the other hand, preoxidized CNTs initially show a superior conversion, which usually drops within a few minutes on-stream to the level of steady-state.4 This nicely evidences that activation and passivation of carbon-based catalysts in ODH is related to the oxidation state of the active sites. Regarding our model calculations on HxZ3-8Q8, the partially reduced z-terminated CNTs are increasingly passivated toward H atom abstraction from both ethane and EB substrate molecules (Figure 3a), although the impact of the red-ox state on the abstraction of the first and the second H atom is essentially different. The barriers of reoxidation with both O2 and HO2• demonstrate the opposite trend (Figure 3b), again with the radical species being less susceptible to changes in the C−OH/CO surface ratio. The intersection of these two trends for ethane ODH occurs at a degree of reduction of approximately 20−30%, where both parts of the catalytic cycle have an activation barrier in the range of 125−150 kJ/mol. The consecutive conversion of the ethyl radical to ethylene on both z- and a-terminated CNT models occurs with lower barrier of 90−110 kJ/mol, thus not affecting the general kinetics of ODH. Therefore, the intermediate degree of catalyst reduction represents the stationary state of CNT catalysts in the ODH of light alkanes, while the initial stage of the process, i.e., its equilibration, must be regarded as a stoichimetric reaction between alkanes and excess quinone groups on the CNT surface. Similarly, the equilibration of a steady-state oxidation state, which sensitively depends on alkane chain length during the oxidation of C2−C4 alkanes, has been experimentally observed over a MoVTeNb mixed oxide catalyst.22 Here it was found that the longer the hydrocarbon chain, the lower is the oxidation state due to an increasing number of electrons to be

Figure 3. Reaction barriers (a) of the dehydrogenation of C2H6, C2H5•, EB, and PhCH•CH3 with HxZ3-8Q8 and (b) of the reoxidation of HxZ3-8Q8 with O2 and HO2•. Circles denote the energies of unsuccessful reactions (barrier was not overcome after d(O−H) decreased to 0.9 Å).

provided by the catalyst for selective and/or unselective substrate conversion. To correlate these findings with real catalytic data, we reevaluated a recent report on ethane ODH over MW-CNTs.23 Notably, the apparent activation energy is significantly lower than the barriers calculated for HxZ3-8Q8; however, a formal kinetic analysis revealed rate orders with respect to ethane and O2 of 0.54 and 0.16, respectively. If we fit the kinetic data generated using the power rate law by a red-ox type model (eq 1) we obtain rate constants for ethane dehydrogenation and catalyst reoxidation, which allow us to estimate the degree of catalyst reduction at a certain feed composition (Table SA1). rate =

k redpC2H6 × koxpO2 n k redpC2H6 + koxpO2 n

(1)

Depending on the rate order of O2 for reoxidation n (0.5 or 1) and within a typical range of reaction conditions (feed ratio C2H6:O2 between 2 and 1), this model reveals a highly oxidized surface with a C−OH/CO surface ratio of only 7−28%, well matching the range of operation predicted by DFT. Moreover, a kinetic study on EB ODH over CNTs reporting rate orders 3960

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The Journal of Physical Chemistry C with respect to EB and O2 of 0.53 and 0.32, respectively,24 similarly predicts a more reduced catalyst surface under steady state conditions (C−OH/CO surface ratio of 20−42%, Table SA2). For this substrate, DFT delivers a steady state at around 30−40% of active sites being in reduced state (Figure 3), in good agreement with experimental findings. The partial reduction of the carbon surface is experimentally confirmed by in situ XPS studies during n-butane ODH,3 where similarly large peaks of both CO and C−OH species were detected in steady state. Finally, the difference in experimentally measured activation energies for ethane and EB ODH (100 and 75 kJ/ mol, respectively),23,24 which can in parts be related to the different strength of aliphatic and benzylic C−H bonds, is reflected in our calculations even when considering different degrees of surface reduction in steady state. Noteworthy, the effects of the C−OH/CO ratio at the edge of HxZ3-8Q8 are similar to its negative charging, i.e., computational analysis of anion-radicalic models. As shown in Table SB6, for Z2-8Q8, Z3-8Q8, and Z2-10Q10 clusters, the negatively charged clusters show increased activation energies for the H abstraction from ethane. We refer this to the analogous electronic structure of hydrogenated HxZ3-8Q8 and its anion-radicals. Schematically, the hydrogenation of a CO group can be interpreted as the cleavage of its π-bond with one electron being involved into the formation of the O−H bond, while the other one enters the pool of delocalized electrons of the carbon backbone, which is analogous to the acception of additional electrons in anion-radicalic clusters. These calculations indicate that nucleophilicity is an important but not the decisive factor for selective ODH. Finally, it should be stressed that most of the reactions are not localized at the same active site. Many types of intermediates easily desorb and can undergo further transformations at different sites. Thus, mass transport is a crucial aspect of ODH over carbon catalysts. Considering the abundance of O2 and HO2• radicals in the gas phase homogeneous reactions may significantly contribute to the overall reaction network of ODH. These pathways in most cases lead to nondesirable products such as CO and CO2; however, the selective C−H bond cleavage by highly reactive radicalic species cannot be excluded.

(reduced to number of quinone groups on graphene edge), as long as thermal stability of CNTs is preserved. The analysis of the activation barriers for the reaction of partially hydrogenated HxZ3-8Q8 clusters with reducing and oxidizing species from the gas phase demonstrates that the catalytic cycle under ODH steady state conditions occurs at an intermediate red-ox state, in agreement with experimental findings in recent literature. It is anticipated that CNT doping both with heteroatoms or metal clusters, as well as application of voltage to the catalyst, should affect the selectivity of the reaction.

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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/acs.jpcc.7b00068. Additional information including the computational protocols and Cartesian coordinates of the clusters can be viewed at https://figshare.com/articles/jp7b00068_SI_D_C2H6_zip/ 4644367. Supplementary A: details of kinetic modeling; Supplementary B: details of DFT modeling; Supplementary C: DFT raw data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; tel.: +4917698210067. ORCID

Oleksiy V. Khavryuchenko: 0000-0001-5056-3993 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Annette Trunschke and Prof. Dr. Robert Schlögl (Fritz-Haber-Institut Berlin) for fruitful discussions and computational resources. No external funding was used.

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REFERENCES

(1) Cavani, F.; Ballarini, N.; Cericola, A. Oxidative Dehydrogenation of Ethane and Propane: How far from Commercial Implementation? Catal. Today 2007, 127 (1−4), 113−131. (2) Håkonsen, S. F.; Holmen, A. Oxidative Dehydrogenation of Alkanes. In Handbook of Heterogeneous Catalysis; Ertl, G., Knözinger, H., Schüth, F., Weitkamp, J., Eds.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2008; pp 3384−3400. (3) Zhang, J.; Liu, X.; Blume, R.; Zhang, A.; Schlögl, R.; Su, D. S. Surface-Modified Carbon Nanotubes Catalyze Oxidative Dehydrogenation of N-Butane. Science 2008, 322 (5898), 73−77. (4) Frank, B.; Zhang, J.; Blume, R.; Schlögl, R.; Su, D. S. Heteroatoms Increase the Selectivity in Oxidative Dehydrogenation Reactions on Nanocarbons. Angew. Chem., Int. Ed. 2009, 48 (37), 6913−6917. (5) Khavryuchenko, O. V.; Frank, B.; Trunschke, A.; Hermann, K.; Schlö gl, R. Quantum-Chemical Investigation of Hydrocarbon Oxidative Dehydrogenation over Spin-Active Carbon Catalyst Clusters. J. Phys. Chem. C 2013, 117 (12), 6225−6234. (6) Rozanska, X.; Fortrie, R.; Sauer, J. Oxidative Dehydrogenation of Propane by Monomeric Vanadium Oxide Sites on Silica Support. J. Phys. Chem. C 2007, 111 (16), 6041−6050. (7) Frank, B.; Fortrie, R.; Hess, C.; Schlögl, R.; Schomäcker, R. Reoxidation Dynamics of Highly Dispersed VOx Species Supported on γ-Alumina. Appl. Catal., A 2009, 353 (2), 288−295.

4. CONCLUSIONS In summary, the catalytic behavior of CNT models in hydrocarbon ODH is analogous to that of flat graphene-like clusters:5 the conjugation of quinone groups terminating the edge of conductive carbon systems makes them strongly correlated, thus even the reaction of a single quinone group affects the state and properties of the whole graphene sheet within the real carbon catalyst. Product of hydrocarbon reaction with the O-terminated graphene edge independently of its curvature extent depends on the overall spin state of the tube and reactant; in the highest stable spin state, a hydrocarbon radical is evolved, while in lower stable states the surface ethers are formed, blocking reactive centers and, presumably resulting in nonselective oxidation. Therefore, the ODH over CNTs is a subject to spin catalysis,25 and impact of external magnetic field on selectivity of the reaction is expected. The CNT length is not affecting the reaction barriers, while the reactivity slightly drops with its increasing diameter. Therefore, larger quantity of thinner CNTs would be more catalytically effective than smaller number of wide ones 3961

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The Journal of Physical Chemistry C (8) Conte, M.; Chechik, V. Spin Trapping of Radical Intermediates in Gas Phase Catalysis: Cyclohexane Oxidation over Metal Oxides. Chem. Commun. 2010, 46 (22), 3991−3993. (9) Becke, A. D. Density-functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98 (7), 5648−5652. (10) Schäfer, A.; Horn, H.; Ahlrichs, R. Fully Optimized Contracted Gaussian Basis Sets for Atoms Li to Kr. J. Chem. Phys. 1992, 97 (4), 2571−2577. (11) Schäfer, A.; Huber, C.; Ahlrichs, R. Fully Optimized Contracted Gaussian Basis Sets of Triple Zeta Valence Quality for Atoms Li to Kr. J. Chem. Phys. 1994, 100 (8), 5829−5835. (12) Neese, F. ORCA 2.9.0. (13) Neese, F. The ORCA Program System. WIREs Comput. Mol. Sci. 2012, 2 (1), 73−78. (14) Grimme, S. Semiempirical GGA-Type Density Functional Constructed with a Long-Range Dispersion Correction. J. Comput. Chem. 2006, 27 (15), 1787−1799. (15) Grimme, S.; Ehrlich, S.; Goerigk, L. Effect of the Damping Function in Dispersion Corrected Density Functional Theory. J. Comput. Chem. 2011, 32 (7), 1456−1465. (16) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A Consistent and Accurate Ab Initio Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94 Elements H-Pu. J. Chem. Phys. 2010, 132 (15), 154104. (17) Pettersen, E. F.; Goddard, T. D.; Huang, C. C.; Couch, G. S.; Greenblatt, D. M.; Meng, E. C.; Ferrin, T. E. UCSF Chimera–a Visualization System for Exploratory Research and Analysis. J. Comput. Chem. 2004, 25 (13), 1605−1612. (18) With one exception for Z2-8Q8 in quintet, where ethylene spontaneously forms in one stage. (19) Chiang, I. W.; Brinson, B. E.; Huang, A. Y.; Willis, P. A.; Bronikowski, M. J.; Margrave, J. L.; Smalley, R. E.; Hauge, R. H. Purification and Characterization of Single-Wall Carbon Nanotubes (SWNTs) Obtained from the Gas-Phase Decomposition of CO (HiPco Process). J. Phys. Chem. B 2001, 105 (35), 8297−8301. (20) Hart, A. B.; McFadyen, J.; Ross, R. A. Solid-Oxide-Catalyzed Decomposition of Hydrogen Peroxide Vapour. Trans. Faraday Soc. 1963, 59 (0), 1458−1469. (21) Delgado, J. J.; Chen, X.-W.; Frank, B.; Su, D. S.; Schlögl, R. Activation Processes of Highly Ordered Carbon Nanofibers in the Oxidative Dehydrogenation of Ethylbenzene. Catal. Today 2012, 186 (1), 93−98. (22) Heine, C.; Hävecker, M.; Sanchez-Sanchez, M.; Trunschke, A.; Schlögl, R.; Eichelbaum, M. Work Function, Band Bending, and Microwave Conductivity Studies on the Selective Alkane Oxidation Catalyst MoVTeNb Oxide (Orthorhombic M1 Phase) under Operation Conditions. J. Phys. Chem. C 2013, 117 (51), 26988− 26997. (23) Frank, B.; Morassutto, M.; Schomäcker, R.; Schlögl, R.; Su, D. S. Oxidative Dehydrogenation of Ethane over Multiwalled Carbon Nanotubes. ChemCatChem 2010, 2 (6), 644−648. (24) Zhang, J.; Su, D.; Zhang, A.; Wang, D.; Schlögl, R.; Hébert, C. Nanocarbon as Robust Catalyst: Mechanistic Insight into CarbonMediated Catalysis. Angew. Chem., Int. Ed. 2007, 46 (38), 7319−7323. (25) Khavryuchenko, O. V.; Khavryuchenko, V. D.; Su, D. Spin Catalysts: A Quantum Trigger for Chemical Reactions. Chin. J. Catal. 2015, 36 (10), 1656−1661.

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