Article pubs.acs.org/JPCC
CO2 Chemisorption and Its Effect on Methane Activation in La2O3‑Catalyzed Oxidative Coupling of Methane Changqing Chu,† Yonghui Zhao,† Shenggang Li,*,†,‡ and Yuhan Sun†,‡ †
CAS Key Laboratory of Low-Carbon Conversion Science and Engineering, Shanghai Advanced Research Institute, Chinese Academy of Sciences, 100 Haike Road, Shanghai 201210, China ‡ School of Physical Science and Technology, ShanghaiTech University, 319 Yueyang Road, Shanghai 200031, China S Supporting Information *
ABSTRACT: Density functional theory and coupled cluster theory calculations were carried out to study the formation of the carbonate species on La2O3 catalyst using the cluster model and its effect on subsequent CH4 activation. Physisorption and chemisorption energies as well as energy barriers for the reaction of CO2 and La2O3 clusters, and the reaction of CH4 with the CO32− site on the resulting clusters, were predicted. Our calculations show that CO2 chemisorption at the La3+−O2− pair sites is thermodynamically and kinetically very favorable due to the strong basicity of the O2− site on La2O3, which leads to the formation of the La3+−CO32− pair sites. In addition, CH4 activation at the La3+−CO32− pair sites is similar to that at the La3+−O2− pair sites, which results in the formation of the bicarbonate species and the La−CH3 bond, although the La3+−CO32− pair sites are much less reactive with CH4 in terms of both thermodynamics and kinetics. Further thermodynamical calculations show that the CO32− species in these clusters dissociate between 500 to 1250 K, with half of them completely dissociated at 873 K, consistent with the experimental observation. Our studies suggest that the CO32− site is unlikely to be the active site in La2O3catalyzed oxidative coupling of methane, and CO2 as a major byproduct is likely to act as a poison to the La2O3-based catalysts especially at modest reaction temperature.
1. INTRODUCTION Efficient conversion of methane into fuels and value-added chemicals are of increasing importance due to the surging interest in nonconventional natural gas such as shale gas. Oxidative coupling of methane (OCM)1,2 is a promising technology for directly converting natural gas into C 2 hydrocarbons especially ethylene. Over the past three decades, a large number of OCM catalysts have been developed, such as the well-known Li/MgO3 and La2O3-based catalysts.4−6 However, the commercialization of the OCM process is still hindered by the limited yield of C2 hydrocarbons.7 The OCM reaction is widely acknowledged to occur via the Lunsford mechanism,8,9 where CH3• radicals are formed by CH4 activation on the catalyst surface followed by their coupling to form C2H6 and the further conversion of C2H6 to C2H4. Although significant experimental and theoretical studies have been carried out to elucidate the detailed catalytic mechanism, there remain debates on such fundamental issues as the nature of the active oxygen species, and the oxygen radical (O•−),1010 the superoxo radical (O2•−),11−14 the peroxide (O22−),15,16 and the oxide (O2−)17−20 have all been proposed. A number of recent studies on the Li/MgO catalyst21−23 suggest that the Mg2+−O2− pair sites located at the corners and steps of the catalyst surface are the actual sites for CH4 activation. For the La2O3-based catalysts, previous © 2016 American Chemical Society
experimental and theoretical studies also suggest the heterolytic splitting of the CH3−H bond at the La3+−O2− pair sites as the rate-limiting step in CH4 activation.17−20 The OCM reaction is usually operated at high reaction temperature (>750 °C), so the catalyst surface can be expected to be very complex.24 In addition, a significant amount of byproducts such as CO and CO2 are formed, and the byproduct CO2 has been considered poisonous to the strongly basic OCM catalysts,7 due to the formation of carbonates.25−27 Xu et al. studied the effects of adding CO2 into the feed on the K2O- and SrO-promoted La2O3/ZnO catalysts.28 They found a sharp decrease in CH4 conversion and C2 production above a certain CO2 partial pressure for all the catalysts examined, which was attributed to the formation of carbonates on the catalyst surface. Xu et al. also studied the effects of adding CO2 on the SrO-promoted La2O3/CaO catalyst.29 Using Fouriertransform infrared (FT-IR) spectroscopy, they showed the easy formation of carbonates upon the interaction of CO2 with the catalyst surface, and the decrease in the catalytic activity with CO2 introduction into the feed at low reaction temperatures, but little change in the catalytic activity at high reaction Received: October 26, 2015 Revised: December 15, 2015 Published: January 11, 2016 2737
DOI: 10.1021/acs.jpcc.5b10457 J. Phys. Chem. C 2016, 120, 2737−2746
Article
The Journal of Physical Chemistry C
Figure 1. La(CO3)(OH), La3O3(CO3)(OH), and La2nO3n−1(CO3) (n = 1−3) with B3LYP/aVDZ bond distances in Å and relative energies in kcal/ mol at 0 K.
prepared a rod-shaped La2O2(CO3) catalyst with better thermal stability and C2 selectivity than La2O3,32 which was attributed to the isolation of the active sites by CO32− leading to their better dispersion. In this study, we carried out DFT and CCSD(T) calculations to study the formation of carbonates by the reaction of CO2 with the O2− site on the La2O3 clusters as the catalyst models and its effect on the subsequent activation of CH4. In addition, we also examined the thermal stability of the carbonates based on the calculated CO2 chemisorption energies.
temperatures. Lacombe and co-workers used in situ diffuse reflectance infrared spectroscopy (DRIFT) to characterize the La2O3 catalyst surface under O2 flow at increasing temperatures30 and identified transient carbonate species on the catalyst surface between 500 to 700 °C. They also carried out CH4/CD4 isotope exchange experiments18 and found that the formation of carbonates at the basic O2− site inhibited CH4 dissociation at this site. Sato et al. showed that the carbonate species on La2O3 decomposed above 500 °C by temperatureprogrammed desorption (TPD) of adsorbed CO2.27 Cornu et al. combined DFT calculations and DRIFT experiments to study CO2 adsorption on the (100), (111), and (110) surfaces of MgO, as well as the MgO(100) surface with defects,31 and found that the formation of carbonates were preferable on the MgO(100) surface with defects. Very recently, Hou et al.
2. COMPUTATIONAL METHODS Equilibrium geometries and vibrational frequencies were calculated at the DFT level with the B3LYP functional.33 Geometry optimizations for the minima were carried out with 2738
DOI: 10.1021/acs.jpcc.5b10457 J. Phys. Chem. C 2016, 120, 2737−2746
Article
The Journal of Physical Chemistry C
Figure 2. Potential Energy Surface (ΔE0K, kcal/mol) for the reaction of LaO(OH) with CO2 and CH4. RC: reactant complex, TS: transition state, IM: intermediate. Values in black: B3LYP/aVDZ; red: CCSD(T)/aVTZ.
the Berny algorithm.34 For transition state optimizations, the synchronous transit-guided quasi-Newton (STQN) method was usually employed.35 For initial geometries sufficiently close to the transition states, the Berny algorithm was used instead. The geometry optimizations were performed in the redundant internal coordinates.36,37 The aug-cc-pVDZ basis set for H, C, O,38,39 and the (14s13p10d8f6g)/[10s8p5d4f3g] basis set for La in segmented contraction40 based on the small-core (28 core electrons: 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 ) effective core potential (ECP)41 were used for geometry optimization and frequency calculations; this combination of basis sets will be denoted as aVDZ. For the reactions of CO2 and CH4 with LaO(OH) and La2O3, single point energies were calculated at the CCSD(T)42−46 level with the aug-cc-pVTZ basis set for H, C, and O, and the above ECP basis set in the generalized contraction for La;47 this combination of basis sets is denoted as aVTZ. The DFT calculations were performed with the Gaussian 09 program package.48 The CCSD(T) calculations were performed with the MOLPRO 2012.1 program package.49,50 Molecular visualization was performed using the AGUI graphics program from the AMPAC program package.51 The calculations were performed on our Xeon-based Lenovo computing cluster and the computing cluster at Shanghai Advanced Research Institute.
La2O2(CO3) arises from the reaction of CO2 with La2O3. The CO32− group also acts as a bidentate ligand by replacing the bridge O2− atom in La2O3 and forming a La−O bond with each of the two La atoms. The two La−O bonds between La3+ and CO32− are slightly longer than the bridge La−O bonds by
