Article pubs.acs.org/JPCC
Theoretical Study of Syngas Hydrogenation to Methanol on the Polar Zn-Terminated ZnO(0001) Surface Ya-Fan Zhao,†,‡ Roger Rousseau,‡ Jun Li,*,†,§ and Donghai Mei*,‡ †
Department of Chemistry and Laboratory of Organic Optoelectronics and Molecular Engineering of the Ministry of Education, Tsinghua University, Beijing 100084, China ‡ Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, Washington 99352, United States § Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352, United States ABSTRACT: Methanol synthesis from syngas (CO/CO2/H2) hydrogenation on the perfect Zn-terminated polar ZnO(0001) surface has been investigated using periodic density functional theory calculations. Our results show that direct CO2 hydrogenation to methanol is unlikely because, in the presence of surface atomic H and O, the highly stable formate (HCOO) and carbonate (CO3) readily produced from CO2 with low barriers of 0.11 and 0.09 eV will eventually accumulate and block the active sites of the ZnO(0001) surface. In contrast, methanol synthesis from CO hydrogenation is thermodynamically and kinetically feasible on the perfect ZnO(0001) surface. CO can be consecutively hydrogenated into formyl (HCO), formaldehyde (H2CO), and methoxy (H3CO) intermediates, leading to the final formation of methanol (H 3 COH). The reaction route via hydroxymethyl (H2COH) intermediate, a previously proposed species on the defective O-terminated ZnO(0001̅) surface, is kinetically inhibited on the perfect ZnO(0001) surface. The rate-determining step in the consecutive CO hydrogenation route is the hydrogenation of H3CO to H3COH. We also find that this final hydrogenation step is pronouncedly facilitated in the presence of water by lowering the activation barrier from 1.02 to 0.55 eV.
1. INTRODUCTION Methanol is used as a basic industrial chemical in the production of many other chemical products and in direct methanol fuel cells as well as CO2 fixation.1,2 Methanol synthesis from syngas, a mixture of CO2, CO, H2, and H2O generated from chemical transformation of natural gas, coal, and biomass, is therefore one of the most important catalytic processes in chemical industry. Efficient production of methanol and other higher-alcohols from syngas has been investigated for nearly a century. Currently, methanol is commercially produced by converting syngas at 200−300 °C and 50−120 atm using Cu/ZnO-based catalysts.3 It is wellknown that ZnO itself can hydrogenate CO into methanol under high temperature and high pressure conditions.4−6 The addition of Cu into ZnO-based catalysts is to reduce the reaction barriers, so that methanol synthesis could occur at lower pressure conditions.7,8 Although methanol synthesis from syngas has been industrialized for a long time, the reaction mechanism, especially elementary reaction steps and key reaction intermediates, is still not well understood. For example, it is still controversial whether CO2 or CO is the main carbon source for methanol because of accompanying forward and reverse water−gas shift (WGS) reactions on ZnO catalysts during methanol synthesis.1 Bowker et al. investigated the reactivity of methanol synthesis over ZnO powder under ultra-high vacuum conditions and suggested that methanol is generated from the hydrogenation of CO2 via formate © 2012 American Chemical Society
(HCOO), dioxomethylene (H 2 COO), and methoxy (H3CO).9 Methanol synthesis presumably occurs on ZnO catalysts with oxygen vacancies.6,9 It is assumed that small amounts of anion surface defects on the ZnO surface, which are produced thermally, are essential and responsible for CO2 adsorption and hydrogenation to methanol.6 Tabatabaei et al. found that CO hydrogenation does not produce methanol on ZnO catalysts,6 whereas the bidentate HCOO, which is predominately produced after CO2 adsorption, is the reaction intermediate for forward and reverse WGS reactions with the monodentate HCOO as the reaction intermediate for methanol synthesis.6 On the basis of careful characterization of five different well-defined ZnO crystal structure samples using N2 adsorption, X-ray diffraction (XRD), and extended X-ray absorption fine structure spectroscopy (EXAFS) techniques, Wilmer et al. found that methanol synthesis from CO hydrogenation on ZnO catalysts is structure-sensitive at high pressure conditions.10 They also suggested that the polar ZnO surfaces might be highly relevant to methanol synthesis. Recently, Kurtz et al. studied the reactivity of methanol synthesis over polycrystalline ZnO powder catalysts.5 They concluded that CO, rather than CO2, is the carbon source for methanol production. Furthermore, they observed that trace Received: November 16, 2011 Revised: July 10, 2012 Published: July 10, 2012 15952
dx.doi.org/10.1021/jp211055s | J. Phys. Chem. C 2012, 116, 15952−15961
The Journal of Physical Chemistry C
Article
2. COMPUTATIONAL DETAILS All calculations were carried out using the CP2K package.22,23 The optimized lattice constants for ZnO bulk are a(b) = 3.3171 Å and c = 5.2917 Å, which are in good agreement with the experimental values of a(b) = 3.2493 Å, c = 5.2054 Å, and previous DFT calculations.11,24,25 The GGA-PBE functional26 and the pseudopotentials of Geodecker, Teter, and Hutter27,28 were used in the calculations. The polar Zn-terminated ZnO(0001)-(4 × 4) surface is modeled with four ZnO double layers. A vacuum layer of 15 Å is used to avoid the unphysical interactions between the surface slabs. The DZVP basis sets for Zn and O atoms in the ZnO(0001) slab and TZVP basis sets29 for adsorbates on the ZnO(0001) surface were used. The cutoff energy of the auxiliary planewave basis was set as 300 Ry. The BFGS algorithm with SCF convergence criteria of 1.0 × 10−6 au was used in geometry optimizations. All calculations are spin unrestricted. DFT calculations of polar metal oxide surfaces are complicated by long-range electrostatic interactions because of the stacking sequence of Zn and O layers in the slabs representing polar ZnO surfaces.30,31 This induces an artificial dipole layer within the slab. This can be seen clearly in the projection along the direction of the surface normal of the Hartree potential, VH, as shown in Figure 1. VH for the surface
amounts of CO2 presented in the syngas mixture dramatically decrease the activity of methanol synthesis.5 Theoretical studies of methanol synthesis from syngas on various structure orientations of ZnO surfaces have been reported previously.5,7,8,11−16 Using a hybrid QM/MM embedded method, French et al. studied CO2 hydrogenation to methanol on the ZnO(0001̅) surface,12 following the reaction mechanism proposed by Chinchen et al.17 To elucidate how HCOO is converted to H3COH, which clearly involves multiple hydrogenation steps, H2COO and H2COOH were proposed as the possible intermediates, although both species were never identified by experiment. Rossmüller et al. studied CO hydrogenation on the ZnO(0001̅) surface with an oxygen vacancy using density functional theory (DFT) calculations.16 With an embedded cluster model for the ZnO(0001̅) surface, they found that the hydrogenation of CO to formyl (HCO), then to formaldehyde (H2CO) and hydroxymethyl (H2COH), is the most likely reaction route for methanol synthesis. The reaction route via HCOO and H3CO intermediates is unlikely because of higher activation barriers needed for further transformations.16 Recently, Kiss et al. investigated the CO hydrogenation to methanol on the defective ZnO(0001̅) surface using ab initio molecular dynamic methods based upon the metadynamics approach for sampling reaction free energies.13 Various reaction routes in the complex reaction network were explored on the free energy surface landscapes. They found that different charge states of oxygen vacancy defects (F-centers) play an important role in determining the reaction intermediates and pathways of methanol synthesis on the defective ZnO(0001̅) surface. Of two possible hydrogenation routes of formaldehyde (H2CO) leading to methanol formation, the reaction path via H2COH is energetically more favorable than the path via H3CO species at the defective F− active site.13 Though it has been proved by experiment that Zn-terminated ZnO(0001) is more reactive,10,18−20 so far only few theoretical studies on the this surface were carried out.11,21 Chuasiripattana et al. reported the adsorption structures and energetics of the possible reaction intermediates involved in methanol synthesis and WGS reactions on the ZnO(0001) surface.11 The detailed theoretical investigation of the reaction pathways for syngas hydrogenation to methanol on the Zn-terminated ZnO(0001) surface is not available yet. To understand methanol synthesis over Cu/ZnO catalysts on the molecular level, fundamental insight into both CO and CO2 hydrogenation chemistry on the well-defined single crystal ZnO surfaces is crucial. In this context, the current work is aimed toward filling the gap in the understanding of hydrogenation chemistry on the ZnO by examining the reactivity on the Zn-terminated (0001) surface. In the present work, methanol synthesis from CO/CO2 hydrogenation on the polar defect-free ZnO(0001) surface is investigated using periodic DFT calculations. The article is organized as follows. The computational procedures are given in section 2. In section 3.1, we present the results of adsorption energies and optimized structures of possible reaction intermediates involved in CO and CO2 hydrogenation toward methanol. Then, the calculated reaction energies and activation barriers of elementary reaction steps are discussed for CO and CO2 in sections 3.2 and 3.3, respectively, with a comparison in section 3.4. Finally, the effects of water on the methanol formation are explored in section 3.5. The conclusions drawn from our calculations are given in section 4.
Figure 1. Hartree potential of the Zn-terminated ZnO(0001) surface.
slab shows a strong linear (dipolar term) dependence in the vacuum region as a result of the vastly different work functions of the top and bottom layers of the slab. Likewise, there is a pronounced polarization within the slab. To eliminate the internal dipole moment of the polar ZnO(0001) surface,32,25 each surface oxygen atom at the bottom of the slab was saturated with a pseudohydrogen atom having a positive charge of +0.5 |e|, which is compatible with previous DFT studies.11,19 This compensates for the asymmetry in the potential and removes the internal polarization within the slab, as is evident by the flatter VH for the H capped slab shown in Figure 1. As such, all calculations reported in this work are performed on the periodic slab that is passivated with pseudohydrogen atoms. The adsorption energy of the reaction intermediates on the ZnO(0001) surface, Eads, was calculated as: 15953
dx.doi.org/10.1021/jp211055s | J. Phys. Chem. C 2012, 116, 15952−15961
The Journal of Physical Chemistry C Eads = Eadsorbate + ZnO(0001) − (Eadsorbate + EZnO(0001))
Article
(1)
where Eadsorbte+ZnO(0001) is the total energy of the adsorbate with the ZnO(0001) surface slab; EZnO(0001) is the total energy of the bare Zn-terminated ZnO(0001) surface slab; and Eadsorbate is the total energy of the adsorbate in vacuum. Zero-point energy (ZPE) might significantly affect the thermodynamics and kinetics.5 We therefore also included the ZPE correction based on harmonic vibrational frequency analysis. Transition states in the reaction pathway were located using the IT-NEB method with the maximum force convergence of 0.001 hartree/bohr.33 Test calculation showed that the energy change was negligible (
