Article pubs.acs.org/JPCC
Influence of Step Defects on Methanol Decomposition: Periodic Density Functional Studies on Pd(211) and Kinetic Monte Carlo Simulations Sen Lin,*,† Jianyi Ma,*,‡,§ Linsen Zhou,∥ Caijin Huang,† Daiqian Xie,∥ and Hua Guo*,‡ †
Research Institute of Photocatalysis, Fujian Provincial Key Laboratory of Photocatalysis−State Key Laboratory Breeding Base, Fuzhou University, Fuzhou 350002, China ‡ Department of Chemistry and Chemical Biology, University of New Mexico, Albuquerque, New Mexico 87131, United States § Institute of Atomic and Molecular Physics, Sichuan University, Chengdu, Sichuan 610065, People’s Republic of China ∥ Institute of Theoretical and Computational Chemistry, Key Laboratory of Mesoscopic Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China ABSTRACT: Methanol decomposition on noble metal surfaces is an important industrial process and prototype for understanding heterogeneous catalysis. Despite many advances, the role played by surface defects and structural sensitivity is still not fully understood. In this work, methanol decomposition on a stepped palladium surface, Pd(211), is investigated using periodic density functional theory (DFT). The activation barriers and thermochemistry for relevant elementary steps leading to the final decomposition products CO and H2 are obtained. Similar to the previous theoretical results on flat Pd surfaces, the initial C−H bond scission is preferred on Pd(211) because it has a lower barrier than those for the initial O−H and C−O scissions. It was also found that the barriers for the C−H or O−H bond scissions are lowered at the step sites. Finally, kinetic Monte Carlo simulations on a realistic Pd surface reproduce the temperature-programmed desorption spectrum for methanol decomposition but only when modified DFT data are used. These simulations show that most of the reaction occurs at under-coordinated sites. loss spectrum (HREELS),7−9,11 X-ray photoelectron spectroscopy (XPS), 14,17 and secondary ion mass spectroscopy (SIMS).14,15,17 An alternative pathway involves the formation of CH2OH*, which is stoichiometrically identical to methoxyl. Its further decomposition via formaldehyde eventually reaches the same CO* product. The CH2OH* intermediate has been found in SIMS experiments.10,17 In addition, the intense peak at 705 cm−1 in the electron energy loss spectrum observed by Davis and Barteau13 is most likely due to adsorbed CH2OH*.19 In addition to these two pathways, there was also a suggestion that the reaction might involve C−O bond cleavage,10 but later studies proved it unlikely12 or viable only under specific conditions.17 Theoretically, the earlier plane-wave density functional theory (DFT) work of Zhang and Hu found that the O−H bond scission on Pd(111) is energetically preferred to the C−O bond scission.21 This theoretical result is consistent with experimental observations,12,17 thus ruling out the C−O bond cleavage as a predominant pathway at low coverage. The preference in the O−
1. INTRODUCTION Methanol is considered to be an important fuel for future transportation needs, either in direct fuel cells1 or for on-board generation of H2 for proton-exchange membrane fuel cells via, for example, steam reformation.2,3 Methanol has several unique advantages as a hydrogen fuel carrier.4 It is environmentally friendly, with no sulfur or nitrogen and biodegradable, and has a large H/C ratio. Its liquid form at room temperature also allows it to leverage the current transportation and dispersion infrastructure for existing fossil fuels. In the past decades, many noble metals, such as copper, platinum, and palladium, have been used to catalyze methanol decomposition (CH3OH → CO + 2H2) and/or steam reforming (CH3OH + H2O → CO2 + 3H2).5 Among them, palladium is considered to be an effective catalyst for the decomposition reaction and received much experimental6−20 and theoretical19,21−26 attention. The decomposition of methanol on noble metal surfaces has several possible pathways.21,27,28 One possibility is the formation of the methoxyl species (CH3O*) via the O−H bond cleavage in adsorbed CH3OH* species. Further dehydrogenation leads to formaldehyde (CH2O*) and formyl (CHO*) species and finally to CO*. There is some evidence in support of the surface methoxyl species, which includes high-resolution electron energy © 2012 American Chemical Society
Received: October 25, 2012 Revised: November 29, 2012 Published: December 10, 2012 451
dx.doi.org/10.1021/jp310600q | J. Phys. Chem. C 2013, 117, 451−459
The Journal of Physical Chemistry C
Article
H and C−H bond scissions was studied by Schennach et al.,19 by Jiang et al.,24,25 and by Gu and Li.26 Their DFT results indicated that the C−H bond scission has a lower barrier. In particular, the recent work by Jiang et al.25 on methanol decomposition on Pd(100) has helped to identify the three stoichiometric “CH3O” species observed in a previous temperature-programmed desorption (TPD) experiment7 as CH3O*, CH2OH*, and CH3O* trapped at defect sites. In addition, the dehydrogenation of methoxyl and formyl species on Pd was investigated by Rösch and coworkers.22,23 These theoretical studies strongly suggested that the decomposition of methanol on Pd surfaces prefers the pathway: CH3OH* → CH2OH* → CH2O* → CHO* → CO*. This mechanism differs from that on Cu,28 which involves CH3O*, but is similar to the one on Pt.27,29−31 We note in passing that the reverse process, namely, methanol synthesis, has also been investigated using plane-wave DFT.32−35 Whereas the reaction pathways for methanol decomposition on Pd have been largely mapped out, almost all previous DFT studies have focused on low Miller index facets with no defect. On real metal surfaces, the surface is decorated with various defects such as steps, kinks, and vacancies,36 which often offer stronger adsorption and lower reaction barriers. These defect sites are prevalent in nanocrystals in real catalysts.33,37 Theoretically, it has been shown that the more open Pd(100) face yields lower reaction barriers for methanol decomposition than those on the close-packed Pd(111) face.25 Similar observations have been reported by Cao et al. for methanol decomposition on the Pt(211) surface.38 Comparison of calculated barriers on defect-free surfaces with experimental TPD data indicates substantial underestimation of the adsorption energy and overestimation of the reaction barriers by the theoretical results. For example, the calculated adsorption energy of methanol on defect free Pd(111) is −0.25 to −0.39 eV,21,24,26 which is significantly smaller than the experimental value of −0.51 eV.9 This discrepancy is likely the result of various defects on real surfaces, as pointed out by several authors.39,40 These observations are consistent with the established paradigm that the surface defect sites play an important role in heterogeneous catalysis.41,42 Despite the common knowledge, however, very few theoretical treatments of the defect-dominated catalysis have been reported. In this work, we focus on DFT studies of methanol decomposition on Pd(211), which serves as a model for step defects, and simulate the TPD process with a kinetic Monte Carlo (KMC) model that includes a phenomenal treatment of defects. This publication is organized as follows. The computational details are described in Section II. The next section (Section III) presents and discusses the results in the context of methanol decomposition. The final section (Section IV) concludes.
tested to be converged. The Fermi level was smeared using the Methfessel−Paxton method with a width of 0.1 eV.50 We also used a vacuum spacing of 16 Å to avoid interactions between adsorbates and slab images in the z direction. The lattice parameter of bulk Pd crystal was calculated to be 3.952 Å after bulk optimization, which is in good agreement with the previously reported results.26 We have studied the adsorption of various pertinent species in methanol decomposition on the Pd(211) surface. The adsorption energy was calculated as follows: Eads = E(adsorbate + surface) − E(free molecule) − E(free surface). The climbing image-nudged elastic band (CI-NEB) method51,52 was used to determine the transition states with the conventional energy (10−4 eV) and force (0.05 eV/Å) convergence criteria. Stationary points were confirmed by normal-mode analysis using a displacement of 0.02 Å and an energy convergence criterion of 10−6 eV. In addition, the vibrational frequencies were used to compute zero-point energy (ZPE) corrections. 2.2. Kinetic Monte Carlo Method. KMC is a useful theoretical method to study the chemical reaction kinetics on surfaces.53−56 It can be considered as a coarse-grained simulation technique that follows the time evolution of adsorbates on the surface lattice. For the system we are interested in this work, all possible elementary processes are first tabulated. Each process (i) is assigned a rate constant ki: ⎛ E≠ ⎞ ki = A exp⎜⎜ − i ⎟⎟ ⎝ kBT ⎠
(1)
E≠i
where is the energy barrier for the ith process, T is the temperature, and kB is the Boltzmann constant. These elementary processes include desorption, diffusion, and reaction. To simplify the simulation, we treated desorption as irreversible processes, and only diffusion of H*, CH2O*, and CH3OH* was considered. All forward and reverse reaction processes were also included, and the barriers and exothermicities were obtained from the original and scaled DFT values. For all elementary processes studied here, a pre-exponential factor of A = 1012 was assumed, as it has been shown not to significantly affect the results.57 Additionally, hydrogen-bond interactions between CH3OH, CH2OH, CHOH, and COH species were considered using a simple model. It is reasonable to assume that hydrogen-bond interactions exist if the distance of two species is
