Article pubs.acs.org/JPCC
Ni-Doping Effects on Carbon Diffusion and Oxidation over Mo2C Surfaces 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, Pudong, Shanghai 201210, China S Supporting Information *
ABSTRACT: Spin-polarized periodic density functional theory calculations have been performed to study the adsorption, diffusion, and oxidation of carbon on the Mo-terminated β-Mo2C(001) surface as well as on a model Ni-doped β-Mo2C(001) surface with a surface Ni:Mo ratio of 1:2. The most stable adsorption sites for O and CO were found to be similar on the two surfaces, whereas those for C are different in that C prefers to adsorb at the step interface on the model Ni-doped surface. The adsorption energies for all three species were found to be less negative on the Nidoped surface. The energy barriers and reaction energies for the diffusion and oxidation of carbon on the above β-Mo2C(001) surfaces were calculated. On the pure β-Mo2C(001) surface, C diffusion from its most stable adsorption site has a much smaller energy barrier of ∼1.0 eV than C oxidation of ∼2.6 eV, with both processes being quite endothermic. Upon Ni doping, the lowest energy barrier for C diffusion from its most stable adsorption site remains ∼1.0 eV, whereas the lowest energy barrier for C oxidation is ∼1.6 eV, much lower than that of ∼2.6 eV on the pure β-Mo2C(001) surface. The energy barrier difference between C diffusion and oxidation of ∼0.6 eV on the Ni-doped surface is much smaller than that of ∼1.6 eV on the pure β-Mo2C(001) surface, and this can be beneficial for preventing carbon deposition and increasing CO selectivity. tion.41,46−49 Thus, it is highly desirable to develop Ni-based DRM catalysts with high catalytic activity and strong resistance to coke formation. In searching for more efficient Ni-based DRM catalysts, researchers have considered many supported Ni nanomaterials for inhibiting coke formation and the sintering of the active Ni nanoparticles.8−10,23,50−52 Zhang and coworkers recently prepared monolithic catalysts with supported hydrotalcite-like films on Al wires by in situ growth, which result in significant reduction in coke formation with higher resistance to sintering in DRM, and these were attributed to the hierarchical structure, high dispersion of Ni, more basic sites, and strong metal− support interaction.44 Zhang and coworkers further prepared the Ni−MgO−Al2O3@m-SiO2 modular catalysts with dual confinements by combining the Ni−MgO−Al2O3 mixed oxide nanoplates and the mesoporous SiO2 coating, which display high catalytic activity and stability with improved resistance to coking and sintering in DRM.45 In addition, Mo2C-based catalysts have been extensively studied after York et al. showed that these catalysts behave similarly as Pt catalysts for the reforming reactions while being much less susceptible to coking than Ni catalysts.53 The majority of these studies have been carried out over unsupported Mo2C catalysts. Although unsupported β-Mo2C catalysts were shown to have high
1. INTRODUCTION Because of the increasing energy demand and the rising environmental concern, the efficient production of clean energy has become of paramount importance. As a greenhouse gas, CO2 is also considered to be an alternative, renewable, and economical carbon feedstock, and its efficient utilization may lead to the reduction in greenhouse gas emission.1−3 Catalytic dry reforming of methane (DRM, CH4 + CO2 → 2H2 + 2CO) is a promising technology for the efficient utilization of CO2 on an industrial scale to reduce CO2 emission.1−18 It may also lead to more efficient utilization of CO2-rich natural gas by reducing the cost for separating CO2 from natural gas prior to further processing. In addition, DRM enables the production of synthesis gas (syngas) with a low H2/CO molar ratio of ∼1, which is more suitable for certain Fischer−Tropsch processes for producing fuels and chemicals.10−12 Although DRM is a highly endothermic reaction with a reaction enthalpy of ∼286 kJ/mol at 298 K, the heat required by this reaction may come from nuclear or renewable energy sources. For the above reasons, DRM has been extensively studied with experimental and theoretical tools in the past few decades with the aim of commercialization.5−9,18−40 Group VIII transition metals except Os show catalytic activity toward DRM.10,40−45 Among them, only the relatively inexpensive Ni-based catalysts were considered to be commercially viable due to their excellent catalytic activity and relatively low prices. The main problem with existing Nibased catalysts is their deactivation due to coke forma© 2013 American Chemical Society
Received: May 27, 2013 Revised: August 13, 2013 Published: August 19, 2013 18936
dx.doi.org/10.1021/jp405209c | J. Phys. Chem. C 2013, 117, 18936−18946
The Journal of Physical Chemistry C
Article
Figure 1. Top view (a) and side view (b) of the pure β-Mo2C(001) surface with key structural parameters shown in angstroms (c). VC, HC, HMo1, and HMo2 are three-fold adsorption sites for adsorbates. Blue balls for the first-layer Mo atoms bonding with two carbon atoms (Mo2C1st first ); dark cyan balls for the first-layer Mo atoms bonding with one carbon atom (Mo1C1st first ); cyan balls for the second-layer Mo atoms bonding with two carbon 1C1st atoms (Mo2C1st second); light-green balls for the second-layer Mo atoms bonding with one carbon atom (Mosecond); and black balls for the first layer carbon atoms (Cfirst).
2. COMPUTATIONAL DETAILS Periodic density functional theory (DFT) calculations were performed using the Vienna ab initio simulation package (VASP)62−64 with the Perdew−Burke−Ernzerhof (PBE)65 exchange-correlation functional and the plane-wave basis sets. The projector-augmented wave (PAW)66,67 method developed by Blöchl was employed to describe the interaction between the atomic cores and the electrons. For the plane-wave basis sets, a kinetic energy cutoff of 400 eV and an augmentation charge cutoff of 650 eV were found to give converged results for the surface calculations. Spin-polarized calculations were performed throughout this work. The hexagonal closed-packed (hcp) form of Mo2C phase (βMo2C) was used in our calculations to optimize the bulk lattice constants, where the Mo atoms form an hcp array and the C atoms occupy half of the octahedral interstitial sites. The 5 × 5 × 6 Monkhorst-Pack k-point grid68 was used. The bulk lattice constants were calculated to be a = b = 3.03 Å and c = 4.71 Å, which are in good agreement with the experimental values of a = b = 3.011 Å and c = 4.771 Å.69 The Mo-terminated Mo2C(001) surface was simulated by five layers of alternating Mo and C atoms. Increasing the number of layers to seven leads to changes of HMo > HC. The average C−Mo and O−Mo bond distances were calculated to be 2.03 and 2.04 Å, respectively. For CO, the most favorable adsorption site is the tilted VC site (Figure 3c), with C located at the VC site and O located at the bridge site between Mo1C1st first and Mo2C1st . The calculated adsorption energy of CO is −3.21 eV first and the average C−Mo and O−Mo distances are 2.21 and 2.39 Å, respectively, which are ∼0.2 and ∼0.35 Å longer than those
Figure 2. Top view (a) and side view (b) of the Ni-doped βMo2C(001) surface with key structural parameters shown in angstroms (c). VC1, VC2, HC1, HMo1, and HMo2 are three-fold adsorption sites, Hollow1 and Hollow2 are four-fold interface sites, and SE1 and SE2 are three-fold step edge sites. Color codes for the Mo and C atoms are the same as those in Figure 1 and dark orange balls are Ni atoms. Mo atoms at the interface are labeled as Mo2C1st 1‑step 2C1st and Mo1C1st 1‑step, and Mo1‑step′ are the Mo atoms in the neighboring unit cell. Ni(1−4) are different Ni atoms in the unit cell, and Ni(1−2)′ are Ni atom in the neighboring unit cell.
Geometries were optimized until the residual force on each atom was
