Article pubs.acs.org/JPCC
Adsorption of CO2 on Graphene: A Combined TPD, XPS, and vdW-DF Study Kaori Takeuchi,† Susumu Yamamoto,*,† Yuji Hamamoto,‡ Yuichiro Shiozawa,† Keiichiro Tashima,§ Hirokazu Fukidome,§ Takanori Koitaya,† Kozo Mukai,† Shinya Yoshimoto,† Maki Suemitsu,§ Yoshitada Morikawa,‡ Jun Yoshinobu,† and Iwao Matsuda† †
The Institute for Solid State Physics, The University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa, Chiba 277-8581, Japan Department of Precision Science and Technology, Graduate School of Engineering, Osaka University, 2-1, Yamada-oka, Suita, Osaka 565-0871, Japan § Research Institute of Electrical Communication, Tohoku University, 2-1-1 Katahira, Aobaku-ku, Sendai, Miyagi 980-8577, Japan ‡
S Supporting Information *
ABSTRACT: The adsorption of CO2 molecules on monolayer epitaxial graphene on a SiC(0001) surface at 30 K was investigated by temperature-programmed desorption and X-ray photoelectron spectroscopy. The desorption energy of CO2 on graphene was determined to be (30.1−25.1) ± 1.5 kJ/mol at low coverages and approached the sublimation energy of dry ice (27−25 kJ/mol) with increasing the coverage. The adsorption of CO2 on graphene was thus categorized into physisorption, which was further supported by the binding energies of CO2 in core-level spectra. The adsorption states of CO2 on graphene were theoretically examined by means of the van der Waals density functional (vdW-DF) method that includes nonlocal correlation. The experimental desorption energy was successfully reproduced with high accuracy using vdW-DF calculations; the optB86b-vdW functional was found to be most appropriate to reproduce the desorption energy in the present system. CO2 capture applications.7−9 The use of graphene powder samples impedes obtaining atomic- and molecular-level pictures of CO2 adsorption on graphene due to their complex and inhomogeneous nature. Knowledge of the adsorbed states of CO2 on graphene is of fundamental importance to clarify their roles in any surface reactions on graphene and to obtain the design guidelines for catalysts with improved reactivities. In addition, CO2 on graphene is a model system in surface physics to study the nature of weak interactions between inert materials. Theoretical calculations of the adsorption of CO2 molecules on graphene have been carried out at a different level of theory.10−21 The lack of experimental information on the adsorption of CO2 on the well-defined graphene surface makes the evaluation of computational accuracy of calculated adsorption energies difficult. In this study we investigated the adsorption of CO2 molecules on the well-defined graphene surface, monolayer epitaxial graphene on a SiC(0001) surface, at 30 K by combining temperature-programmed desorption (TPD), Xray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculation with the vdW-DFs. The desorption energies of CO2 on graphene are experimentally determined to
1. INTRODUCTION Graphene has been used as a support for catalytic materials since it is inherently a sheet with monatomic thickness that can provide the largest surface area for chemical reactions. Recently, the graphene nanosheet (GNS) was adopted to support nanometer-sized metal clusters that function as active sites in various catalytic reactions. The Pt or Pt alloy catalysts supported on GNS exhibited higher catalytic activities than catalysts supported on carbon black for various reactions such as methanol oxidation reaction, ethanol oxidation reaction, oxygen reduction reaction, and CO oxidation reaction.1−5 Furthermore, graphene-based photocatalysts attract strong attention for the activation of CO2 molecules, the causative agent of a greenhouse effect, which chemically transforms CO2 to hydrocarbon or alcohol molecules that can be used as chemical energy by exposing to visible light.6 Knowledge of the reaction mechanism in catalytic reactions is essential for the development of efficient catalysts. Since catalytic reactions proceed in an inhomogeneous complex system via various reaction intermediates, it is necessary to reveal each of the individual elementary steps to comprehend the whole picture of catalytic reactions. One of the most fundamental elementary steps in catalytic reactions is the adsorption of reactant molecules on catalyst surfaces. The adsorption of CO 2 molecules on graphene has been experimentally investigated mainly on graphene powders for © 2017 American Chemical Society
Received: November 11, 2016 Revised: December 21, 2016 Published: January 13, 2017 2807
DOI: 10.1021/acs.jpcc.6b11373 J. Phys. Chem. C 2017, 121, 2807−2814
Article
The Journal of Physical Chemistry C
coverage in the multilayer regime and depends on the growth mode of adsorbates. Next, each coverage of CO2 on graphene was determined by comparing the TPD peak area of CO2 with that of CO2 with the coverage already determined by XPS (i.e., θ = 0.07). The TPD peak area was obtained by numerical integration of TPD spectrum after subtracting a linear background in the temperature range of 70−105 K. The coverage of CO2 on graphene is given by fractional coverage, the number of CO2 molecules per surface carbon atom on graphene (3.82 × 1015 cm−2 at θ = 1). It should be mentioned that the saturation coverage of CO2 on graphene is roughly estimated to be θ = 0.20 (7.71 × 1014 cm−2) from the density of bulk dry ice (ρ= 1.565 g/cm3).27 The DFT calculations were carried out using the STATE code28,29 with norm-conserving pseudopotentials.30 The planewave basis set was used with an energy cutoff of 64 Ry (400 Ry) for wave functions (charge density). The graphene sheet was modeled with a periodically repeated 2 × 2 unit cell of freestanding graphene with a vacuum layer of 30 Å, and 12 × 12 × 1 k-points were sampled in the Brillouin zone. Since the weak adsorption of CO2 on graphene cannot be well-described within the generalized gradient approximation such as Perdew− Burke−Ernzerhof (PBE)31 functional, van der Waals interaction was taken into account using the van der Waals density functional (vdW-DF) method32 with an efficient algorithm for self-consistent calculations.33,34 Three types of vdW-DFs were considered: the first version of vdW-DF (vdW-DF1),32 the optimized Becke86b35 exchange combined with the vdW-DF1 correlation32 (optB86b-vdW),36 and the revised Becke86b35 exchange combined with the second version of vdW-DF correlation37 (rev-vdW-DF2).38 While PBE treats the exchangecorrelation term only semilocally, the other functionals include nonlocal correlation, thereby enabling us to describe the weak adsorption of CO2 on graphene.
be about 25−30 kJ/mol as a function of coverage. The adsorption of CO2 on graphene is thus categorized into physisorption, which is further supported by the binding energies of CO2 XPS spectra. The experimentally determined desorption (adsorption) energies serve as a benchmark for evaluation of computational accuracy of various vdW-DFs and enable us to determine the most appropriate vdW-DFs to describe the weak interactions between CO2 and graphene.
2. EXPERIMENTAL AND THEORETICAL METHODS Monolayer epitaxial graphene was grown on a SiC(0001) substrate by annealing at 1873 K for 1 min under the Ar atmosphere of 1 atm. The monolayer epitaxial graphene was evaluated with Raman spectroscopy (see the Supporting Information). After the growth and Raman characterization of graphene, the graphene sample was installed into an ultrahigh vacuum (UHV) chamber with a base pressure of
