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  • 16558 Phys. Chem. Chem. Phys., 2012, 14, 16558–16565 This journal is c the Owner Societies 2012

    Cite this: Phys. Chem. Chem. Phys., 2012, 14, 16558–16565

    Site-dependent catalytic activity of graphene oxides towards oxidative dehydrogenation of propanewz Shaobin Tanga and Zexing Cao*b

    Received 27th April 2012, Accepted 19th June 2012

    DOI: 10.1039/c2cp41343d

    Graphene oxides (GOs) may offer extraordinary potential in the design of novel catalytic systems

    due to the presence of various oxygen functional groups and their unique electronic and

    structural properties. Using first-principles calculations, we explore the plausible mechanisms for

    the oxidative dehydrogenation (ODH) of propane to propene by GOs and the diffusion of the

    surface oxygen-containing groups under an external electric field. The present results show that

    GOs with modified oxygen-containing groups may afford high catalytic activity for the ODH of

    propane to propene. The presence of hydroxyl groups around the active sites provided by

    epoxides can remarkably enhance the C–H bond activation of propane and the activity

    enhancement exhibits strong site dependence. The sites of oxygen functional groups on the GO

    surface can be easily tuned by the diffusion of these groups under an external electric field, which

    increases the reactivity of GOs towards ODH of propane. The chemically modified GOs are thus

    quite promising in the design of metal-free catalysis.

    1. Introduction

    Graphene and other low-dimensional sp2-hybridized carbon

    nanomaterials have attracted considerable attention owing to

    their outstanding structural and electronic properties and

    potential applications in nanoscale electronics.1–3 These nano-

    carbon materials and carbon-based composites, as metal-free

    catalysts, also show novel activity in facilitating synthetic

    transformations.4–9 For example, carbon nanotubes4 (CNTs)

    and fullerenes5 have recently been shown to exhibit fascinating

    catalytic activity towards the oxidative dehydrogenation (ODH)

    of n-butane and hydrogenation of nitrobenzene, respectively.

    There is a continued interest in searching for catalysts consisting

    of inexpensive materials, but retaining high activity, in metal-

    free catalysts.10,11

    The structural and chemical modification of nanocarbon

    materials as catalysts, such as introduction of defects and

    chemical functional groups, can offer diverse active sites and

    enhance catalytic activity and selectivity. Graphene oxides (GOs),

    the derivatives of graphene modified by oxygen-containing

    functional groups, have emerged as a new class of carbon-

    based nanoscale materials with broad application.12–17 Our

    recent density functional calculations showed that the presence

    of oxygen groups in GOs can improve the interactions of

    nitrogen oxides and ammonia with graphene due to the

    formation of hydrogen bonds and covalent bonds between

    molecules and the surface.16,17 The recent experimental studies

    by Bielawski and co-workers18 revealed that GOs can serve as

    a high performance catalyst (called a carbocatalyst) for oxidation

    of alcohols and hydration of alkynes in the absence of metals. It

    was assumed that the plausible catalytic mechanisms18,19 distinctly

    differ from those in Suzuki–Miyaura coupling reactions20 and

    methanol electro-oxidation,21,22 induced by GOs impregnated

    by palladium nanoparticles, because the active centers are

    carbon atoms for GOs, but the latter is a supported metal.

    The oxidative dehydrogenation of alkanes, which is an

    exothermic reaction overall, is an attractive alternative to the

    conventional dehydrogenation. However, many current ODH

    catalysts have limited activity and/or poor selectivity.23 On the

    contrary, the high reactivity of GOs towards ODH of alkane

    could be expected due to the presence of the various oxygen-

    containing active sites. Despite considerable research for GOs,

    the chemical structure of active sites and catalytic mechanisms

    of GOs as metal-free catalysts for C–H bond activation are

    still unclear, both experimentally and theoretically. Herein the

    efficient catalytic activity of GOs for the ODH of propane to

    propene, the effect of surrounding oxygen groups on catalysis,

    and the detailed mechanisms were explored using density

    functional theory (DFT).

    a Key Laboratory of Organo-Pharmaceutical Chemistry of Jiangxi Province, Gannan Normal University, Ganzhou 341000, China

    b State Key Laboratory of Physical Chemistry of Solid Surfaces and Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China. E-mail: zxcao@xmu.edu.cn

    w This article was submitted as part of a collection on Computational Catalysis and Materials for Energy Production, Storage and Utilization. z Electronic supplementary information (ESI) available: optimized structures, spin densities, and relative energy profiles for formation of C3H7 on other GO models. See DOI: 10.1039/c2cp41343d

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  • This journal is c the Owner Societies 2012 Phys. Chem. Chem. Phys., 2012, 14, 16558–16565 16559

    2. Models and methods

    The density functional theory (DFT) calculations were per-

    formed with the DMol3 package24 using the Perdew, Burke,

    and Ernzerhof (PBE) exchange–correlation functional.25 The

    double numerical plus polarization function (DNP) basis set

    and a real-space cutoff of 4.5 Å were used. The whole atomic

    configuration was allowed to relax until all of the force

    components on any atom were less than 10�3 au. The total

    energy was converged to 10�5 au with spin-polarized calcula-

    tion. The periodic boundary conditions with a supercell of

    4 � 4 graphene unit cells composed of 32 carbon atoms were employed in the calculations. A vacuum region of 12 Å was

    considered to separate the layer and its images in the direction

    perpendicular to graphene plane. The 2D Brillouin zone was

    sampled by 7 � 7 � 1 k-points within the Monkhorst–Pack scheme. We used the linear/quadratic synchronous transit

    (LST/QST)26 methods to determine the reaction pathways

    and barriers.

    The structural features of GOs as promising functional materials

    have been extensively investigated, both experimentally27–33

    and theoretically.34–44 It is widely accepted that hydroxyl and

    epoxy groups are the two major functional groups on the GO

    surface, although some new oxidation species, such as ketone,

    carbonyl, ether, and other groups, have been reported. Based

    on theoretical calculations,34–44 various structural models of

    GO were proposed. It was found that the oxygen functional

    groups prefer to aggregate together. The energetically favor-

    able atomic configuration of GO has also been supported

    by X-ray photoelectron spectroscopy (XPS)39 and nuclear

    magnetic resonance (NMR) simulations40 as well as molecular

    dynamics (MD)41,42 simulations. However, the complete struc-

    ture of GO remains elusive due to the random distribution of

    hydroxyl and epoxy groups, as well as different preparation

    conditions. Accordingly, only the hydroxyl and epoxide func-

    tional groups were considered in our computational models

    for GOs here.

    3. Results and discussion

    3.1 ODH of propane on GOs with only epoxides

    The oxidative dehydrogenation of propane on GOs with

    only epoxides is first investigated. Based on the previous

    studies,36–38 the oxygen groups of GOs energetically prefer

    to aggregate on the graphene plane. Fig. 1a shows that two

    nearest-neighbor epoxy groups at the same side are adsorbed

    on graphene, named GO1. There are other possible binding

    sites of two epoxy groups for the initial GO candidates apart

    from GO1, such as (ac, ed), (ed, bf) and (ed, fg) shown in

    Fig. 1a. The total energy calculations show that the structure

    of GO1 is energetically more favorable than other configura-

    tions with two epoxides located at ac and ed sites (or ed and fg).

    For the ed and bf sites, the atomic arrangement of two epoxy

    groups is similar to GO1. However, epoxides with active

    centres close to the periodic boundary may affect the inter-

    action of GO with C3H8, and here only GO1 is chosen as our

    initial structure accordingly.

    Adsorption of propane on GO1 is slightly exothermic by

    1.3 kcal mol�1 (Fig. 1a). Such weak physical adsorption may be

    attributed to the electrostatic attraction between H (H1) in a

    methylene grou