pubs.acs.org/Langmuir © 2010 American Chemical Society
Density Functional Investigations of Methanol Dehydrogenation on Pd-Zn Surface Alloy Yucheng Huang†,‡ and Zhao-Xu Chen*,† †
Institute of Theoretical and Computational Chemistry, Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, People’s Republic of China, and ‡School of Chemistry and Material Science, Anhui Normal University, Wuhu 241000, People’s Republic of China Received February 10, 2010. Revised Manuscript Received April 9, 2010
Methanol dehydrogenation on Pd(111) and various Pd-Zn surface alloy films supported on Pd(111) have been investigated using density functional method in combination with periodic slab models. Calculations show that compared to Pd(111) the interaction between CH3O and the films is enhanced, whereas that for CH2O and CHO is weakened. Zn in top layer facilitates the CH3O stability. At variance, the subsurface Zn reduces the interaction of CH2O and CHO with the substrate significantly. Addition of Zn promotes the O-H breaking of CH3OH and the dehydrogenation of CHO but hinders the dehydrogenation of CH3O and CH2O. Comparison shows that the thirdlayer Zn atoms have essentially no effect on the reactions. Our calculations demonstrate that the experimentally observed 360 K desorption peak cannot be originated from CH2O adsorbed at flat Pd-Zn alloy surfaces, and it is very likely that CH2O combines preferentially with some species before decomposing into CHO during methanol steam reforming if CH2O is an intermediate. Finally, we show that the newly proposed relationship between the energy of the initial states and transition states exhibits better correlation than the classical BEP relation.
1. Introduction With fossil fuels becoming rare everyday, renewable energy attracts more and more attention. As one of the most promising substitutions, hydrogen energy has many merits such as high efficiency, condensed density, and zero emission, etc. Methanol is an ideal energy carrier which can produce hydrogen in situ for onboard fuel cells. Methanol dehydrogenation (CH3OH = CO þ 2H2), partial oxidation (2CH3OH þ O2 = 2CO2 þ 4H2), and steam reforming (CH3OH þ H2O = CO2 þ 3H2, MSR) are considered as possible channels for hydrogen production.1-3 Over the past two decades or so, there have been considerable interests in the adsorption and reaction of methanol over various metals.4-13 Thereinto, as an effective catalyst for methanol synthesis, decomposition, and steam reforming, palladium has received much attention with intensive publications concerning *Corresponding author. E-mail:
[email protected]. (1) Peppley, B.; Amphlett, J.; Kearns, L.; Mann, R. Appl. Catal., A 1999, 179, 21. (2) Alejo, L.; Lago, R.; Pena, M. A.; J. Fierro, L. G. Appl. Catal., A 1997, 162, 81. (3) Liu, S.; Takahashi, K.; Uematsu, K.; Ayabe, M. Appl. Catal., A 2004, 277, 265. (4) Wang, G. C.; Zhou, Y. H.; Morikawa, Y.; Nakamura, J.; Cai, Z. S.; Zhao, X. Z. J. Phys. Chem. B 2005, 109, 12431. (5) Hofmann, P.; Schindler, K. M.; Bao, S.; Fritzche, V.; Ricken, D. E.; Bradshaw, A. M.; Woodruff, D. F. Surf. Sci. 1994, 304, 74. (6) Wachs, I. E.; Madix, R. J. Surf. Sci. 1978, 76, 351. (7) Chen, Z. X.; Neyman, K. M.; Lim, K. H.; R€osch, N. Langmuir 2004, 20, 8068. (8) Greeley, J.; Mavrikakis, M. J. Am. Chem. Soc. 2004, 126, 3910. (9) Houtman, C.; Barteau, A. Langmuir 1990, 6, 1558. (10) Kerkar, M.; Hayden, A. B.; Woodruff, D. P.; Kadodwala, M.; Jones, R. G. J. Phys.: Condens. Matter 1992, 4, 5043. (11) Outka, D. A.; Madix, R. J. J. Am. Chem. Soc. 1987, 109, 1708. (12) Fukui, K.; Motoda, K.; Iwasawa, Y. J. Phys. Chem. B 1998, 102, 8825. (13) Barros, R. B.; Garcia, A. R.; Ilharco, L. M. J. Phys. Chem. B 2004, 108, 4831. (14) Hartmann, N.; Esch, F.; Imbihl, R. Surf. Sci. 1993, 297, 175. (15) Jiang, R. B.; Guo, W. Y.; Li, M.; Fu, D. L.; Shan, H. H. J. Phys. Chem. C 2009, 113, 4188. (16) Schennach, R.; Eichler, A.; Rendulic, K. D. J. Phys. Chem. B 2003, 107, 2552.
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reactions of methanol on different Pd surfaces such as Pd(110),14 Pd(111),7,15-18 Pd(100),19 Pd19 cluster,20 and nanoparticle Pd79.21 On Pd methanol preferentially undergoes stepwise decomposition and eventually changes into CO and H2. Note that low CO concentrations have a detrimental effect on the performance of fuel cells.22 Therefore, a suitable catalyst which exhibits higher activity and selectivity toward CO2 rather than CO was most important for portable hydrogen production. Since Iwasa et al. discovered that Pd/ZnO catalyst has many advantages over the traditional Cu catalyst toward MSR reaction,23,24 a series of experimental25-33 and theoretical studies7,34-39 have been carried out to investigate the thermal stability of the catalyst, (17) Zhang, C. J.; Hu, P. J. Chem. Phys. 2001, 115, 7182. (18) Desai, S. K.; Neurock, M.; Kourtakis, K. J. Phys. Chem. B 2002, 106, 2559. (19) Christmann, K.; Demuth, J. E. J. Chem. Phys. 1982, 76, 6308. (20) Neurock, M. Top. Catal. 1999, 9, 135. (21) Yudanov, I. V.; Matveev, A. V.; Neyman, K. M.; R€osch, N. J. Am. Chem. Soc. 2008, 130, 9342. (22) Choudhary, T. V.; Goodman, D. W. Catal. Today 2002, 77, 65. (23) Iwasa, N.; Takezawa, N. Top. Catal. 2003, 22, 215. (24) Takewaza, N.; Iwasa, N. Catal. Today 1997, 36, 45. (25) Jeroro, E.; Vohs, J. M. J. Phys. Chem. C 2009, 113, 1486. (26) Jeroro, E.; Vohs, J. M. Catal. Lett. 2009, 130, 271. (27) Jeroro, E.; Vohs, J. M. J. Am. Chem. Soc. 2008, 130, 10199. (28) Stadlmayr, W.; Penner, S.; Kl€ozer, B.; Memmel, N. Surf. Sci. 2009, 603, 251. (29) Karim, A.; Conant, Travis.; Datye, A. J. Catal. 2006, 243, 420. (30) Tamt€ogl, A.; Kratzer, M.; Killman, J.; Winkler, A. J. Chem. Phys. 2008, 129, 224706. (31) Kratzer, M.; Tamt€ogl, A.; Killmann, J.; Schennach, R.; Winkler, A. Appl. Surf. Sci. 2009, 255, 5755. (32) Karim, A. M.; Conant, T.; Datye, A. K. Phys. Chem. Chem. Phys. 2008, 10, 5584. (33) Weirum, G.; Kratzer, M.; Koch, H. P.; Tamt€ogl, A.; Killmann, J.; Bako, I.; Winkler, A.; Surnev, S.; Netzer, F. P.; Schennach., R. J. Phys. Chem. C 2009, 113, 9788. (34) Chen, Z. X.; Neyman, K. M.; Gordienko, A. B.; R€osch, N. Phys. Rev. B 2003, 68, 075417. (35) Chen, Z. X.; Neyman, K. M.; R€osch, N. Surf. Sci. 2004, 548, 291. (36) Lim, K. H.; Chen, Z. X.; Neyman, K. M.; R€osch, N. J. Phys. Chem. B 2006, 110, 14890. (37) Neyman, K. M.; Sahnoun, R.; Inntam, C.; Hengrasmee, S.; R€osch, N. J. Phys. Chem. B 2004, 108, 5424.
Published on Web 04/26/2010
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Figure 1. Illustration of Pd-Zn surface alloy models. Light and deep blue spheres denote Zn and Pd atoms, respectively.
mechanism of alloy formation and the reaction mechanism, etc. The strong enhancement of catalytic activity toward MSR reaction on Pd/ZnO was revealed to be owing to the formation of PdZn alloys with the atomic ratio of 1:1.24 However, using lowenergy electron diffraction (LEED), temperature-programmed desorption (TDP), and high-resolution electron energy loss spectroscopy (HREELS), Jeroro and Vohs25-27 investigated adsorption and reaction of methanol and formaldehyde on the two-dimensional PdZn alloys on a Pd(111) surface as a function of Zn content. They found that Zn atoms incorporated into the Pd(111) surface dramatically decrease the dehydrogenation activity, and the activation energy for the dehydrogenation reactions increases with increasing Zn coverage. In particular, at Zn coverage as low as 0.03-0.06 ML, a TPD peak at ∼360 K appears, whereas with the increase of Zn coverage, this peak diminishes. On the basis of the observations, the authors speculate that the 1:1 PdZn alloy may have no activity toward MSR, whereas Pd(111) surface deposited with 0.03-0.06 ML Zn may be most active. The prediction that Pd(111) deposited with very low coverage of Zn exhibits significant activity toward MSR (i.e., selectively producing CO2) seems puzzling and intriguing, when one considers that MSR reaction exclusively yields CO on pure metal Pd and a large portion of Pd surface will not be covered or doped with Zn at such low coverage if Zn is distributed equally on the surface (thus a large amount of CO will be produced, which contradicts the experiment). To understand the role of Zn on the activity of Pd(111), we recently carried out a systematic study of methanol dehydrogenation (CH3OH f CH3O f CH2O f CHO f CO) on a series of Pd-Zn surface alloys using density functional theory (DFT). Parallel calculations were also performed on Pd(111) for comparison. This paper is arranged as follows. In the next section, we describe models and computational details. In section 3, we present the adsorption of methanol and its derived intermediates and the decomposition mechanisms of various species on different substrates. Discussion and conclusions are given in sections 4 and 5, respectively.
2. Models and Computational Details Experimental and theoretical evidence shows that the 1:1 surface alloy is the most stable structure.28,35 Schennach et al.33 demonstrate that bilayer 1:1 Pd-Zn alloy was formed when submonolayer of Zn is deposited on Pd(111), and the PdZn bilayers are energetically more stable than single PdZn layer. The recent experiment27 indicates that even a very small amount of Zn (0.03-0.06 ML coverage) at the top layer can alter the reactivity of Pd(111) substrate significantly, and it was believed that Zn atoms are on the top layer of the surface. The latter contradicts the previous theoretical investigations of surface segregation of Zn on PdZn alloys.35 According to that study, diffusion of Zn into inner layer is energetically favorable when Zn (38) Koch, H. P.; Bako, I.; Schennach, R. Surf. Sci. 2010, 604, 596. (39) Huang, Y. C.; Chen, Z. X.; Ding, W. P. 2010, unpublished.
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is deposited onto Pd substrate because of stronger Zn-Pd interaction. Furthermore, our recent work of CO adsorption on Pd-Zn surface alloy reveals39 that the subsurface Zn atoms have more pronounced influence on the reactivity of Pd-Zn surface alloys than those Zn atoms on the top-layer, indicating that the top-layer Zn atoms alone may not be able to exhibit such significant modification on the dehydrogenation activity as observed in the experiment work. Indeed, our test calculations show that on Pd(111) doped with 1/16 Zn on the top-layer the binding energy of formaldehyde is essentially the same as on pure Pd(111). These results indicate that the location of the doped Zn is still bewildered. With all the above information in mind, we constructed a series of models in which the total Pd-Zn ratio is 1:1 with varied content of Zn on the top layer to examine the effect of Zn on methanol decomposition. We distributed 16 Zn atoms between the top two layers, keeping the total atomic ratio of Pd:Zn to be 1:1 using a (4 4) surface unit cell. We considered four alloy films: Pd15Zn1/Pd1Zn15/Pd(111), Pd14Zn2/Pd2Zn14/Pd(111), Pd12Zn4/ Pd4Zn12/Pd(111), and Pd8Zn8/Pd8Zn8/Pd(111). Hereafter, we denoted these surface alloy films as 1/15/Pd(111), 2/14/Pd(111), 4/12/Pd(111), and 8/8/Pd(111), respectively. Here m/n/Pd(111) means that there are m Zn atoms (thus 16-m Pd atoms) on the top layer and n Zn atoms (thus 16-n Pd atoms) on the subsurface. Figure 1 shows the models of surface alloys considered in this paper. We adopted a vacuum spacing of 11 A˚ to separate periodic slabs. The bottom two layers composed of Pd atoms were kept at the theoretical equilibrium bulk positions (corresponding to a lattice parameter of 3.954 A˚, compared to the experimental value of 3.89 A˚) while the top two layers that contain both Pd and Zn atoms were fully relaxed during optimization. Periodic plane-wave DFT calculations combined with minimum mode following saddle point searches using the climbing nudged elastic band (cNEB)40,41 were carried out to explore dehydrogenation reaction pathways of methanol on surface alloy films. Ion-electron interactions were modeled within the framework of the projector augmented wave method.42,43 The generalized gradient approximation with the Perdew-Wang 91 functional44 was used to describe electron correlation. The geometry of all stationary points was located with the conjugate-gradient algorithm and was considered converged when the force on each ion smaller than 0.03 eV/A˚. A 2 2 1 MonkhorstPack k-point mesh45 was used to integrate over the Brillouin zone. All calculations were performed using the Vienna ab initio simulation package (VASP).42,46 Binding energies of an adsorbate, Eb, were calculated with Eb= Esub þ Eads - Eads/sub. Here Esub and Eads are the total energies of the bare slab and the free (40) 9901. (41) (42) (43) (44) (45) (46)
Henkelman, G.; Uberuaga, B. P.; Jonsson, H. J. Chem. Phys. 2000, 113, Henkelman, G.; Jonsson, H. J. Chem. Phys. 2000, 22, 9978. Bl€ochl, P. E. Phys. Rev. B 1994, 50, 17953. Kresse, G.; Furthm€uller J. Comput. Mater. Sci. 1999, 6, 15. Perdew, J. P.; Wang, Y. Phys. Rev. B 1992, 45, 13244. Monkhorst, H. J.; Pack, J. D. Phys. Rev. B 1976, 13, 5188. Kresse, G.; Hafner, J. Phys. Rev. B 1994, 49, 14251.
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Table 1. Binding Energies (in eV) and the Most Stable Configuration and Structural Parameters (in A˚ and deg) for Intermediates Involved in Methanol Dehydrogenation over Pd(111) and Pd-Zn Surface Alloysa species
substrate
configuration
Eb
dC-O
dO-Zn
dO-Pd/C-Pdb
R
Pd(111) η -(O) 0.30 1.45 2.39 50 0.26 1.45 2.22 50 1/15/Pd(111) η1-(O) 0.27 1.45 2.22 62 2/14/Pd(111) η1-(O) 0.26 1.45 2.20 50 4/12/Pd(111) η1-(O) 1 0.24 1.45 2.22 46 8/8/Pd(111) η -(O) Pd(111) η3-(O) 1.91 1.44 2.16/2.18 0 CH3O 2.10 1.43 1.97 2.33/2.39 6 1/15/Pd(111) η3-(O) 2.16 1.44 1.97 2.31/2.42 4 2/14/Pd(111) η3-(O) 4/12/Pd(111) η3-(O) 2.40 1.44 1.97 2.28/2.31 1 2.57 1.44 2.03 2.57 3 8/8/Pd(111) η3-(O) Pd(111) η2-(C,O) 0.50 1.31 2.07/2.16 80 CH2O 0.34 1.30 2.12 2.18 74 1/15/Pd(111) η2-(C,O) 0.36 1.30 2.12 2.19 80 2/14/Pd(111) η2-(C,O) 2 0.43 1.31 2.10 2.15 69 4/12/Pd(111) η -(C,O) 0.49 1.33 2.09 2.16 76 8/8/Pd(111) η2-(C,O) 2.78 1.27 2.17/2.07 72 CHO Pd(111) η2-C-η1-O 2 1 2.19 1.27 2.13 2.04/2.12 67 1/15/Pd(111) η -C-η -O 2.24 1.27 2.11 2.05/2.13 70 2/14/Pd(111) η2-C-η1-O 2.30 1.28 2.09 2.08/2.09 71 4/12/Pd(111) η2-C-η1-O 2.38 1.28 2.07 2.12/2.13 74 8/8/Pd(111) η2-C-η1-O a R is the angle defined between the O-C bond axis and the surface normal. b For CH3O dO-Pd/C-Pd refers to dO-Pd; for CH2O and CHO it denotes dC-Pd.
CH3OH
1
adsorbate; Eads/sub is the energy of the substrate covered with the adsorbate.
3. Results 3.1. Adsorption of Methanol, Methoxide, Formaldehyde, and Formyl. The binding energies and structural parameters of the most stable configurations for species involved in methanol dehydrogenation over various surface alloy films and Pd(111) are listed in Table 1. In the following, we will briefly describe the most relevant features of these adsorption complexes. Methanol. It is generally believed that methanol adsorbs via the lone pair of electrons of the oxygen to metallic surfaces,47-49 which is also observed here. Previously reported binding energies on Pd(111) range from 0.31 to 0.39 eV.7,15,16,18 Our calculated value, 0.30 eV, agrees with these results. On the alloy films, methanol prefers the top site of Zn atom. The O-Zn distance is about 2.20 A˚, indicating that methanol interacts weakly with the substrates. The C-O bond axis tilts away from the surface normal by 46°-62° (defined as R in Table 1) to let the lone pair of electron of the oxygen atom better interact with the surface. The calculated binding energies, ∼0.26 eV, on various alloy films, are smaller than 0.30 eV on Pd(111), in agreement with recently theoretical result38 of 0.25 eV on (2 1) PdZn surface alloy on Pd(111) and the experimental observation27 (TPD peak shifted from 160 K on Pd(111) to 140 K on Zn deposited Pd(111) surface). The peak at 140 K corresponds to a binding energy of 0.36 eV using Redhead analysis.50 The difference between our theoretical result and the empirical estimation is slightly higher due to the fact that the DFT method cannot commendably describe the dispersive forces. Methoxide. Methoxide is an important intermediate during MSR process.22,27 Previous slab model calculations showed that hollow sites (fcc) are favored on Pd(111) with a binding energy ranging from 1.67 to 1.73 eV.7,15,18 We obtain a value of 1.91 eV, which is slightly higher than the previous results owing to reduced lateral effect at lower coverage (1/16). In the most stable configurations on surface alloy films, CH3O resides at pseudo-hollow (47) (48) (49) (50)
Mavrikakis, M.; Barteau, M. A. J. Mol. Catal. A: Chem. 1998, 131, 135. Bowker, M.; Madix, R. J. Surf. Sci. 1980, 95, 190. Sexton, B. A.; Hughes, A. E. Surf. Sci. 1984, 140, 227. Redhead, P. A. Vacuum 1962, 12, 203.
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site and the C-O axis tends to be parallel to the surface normal with the corresponding value of R close to 0° (Table 1). We calculated 2.10, 2.16, 2.40, and 2.57 eV for 1/15/Pd(111), 2/14/ Pd(111), 4/12/Pd(111), and 8/8/Pd(111) surfaces, respectively, indicating that the binding energy increases with the top-layer Zn content. Koch et al.38 calculated the binding energy of methoxide on (2 1) PdZn surface alloy on Pd(111) to be 2.24 eV, which is somewhat lower than ours on 8/8/Pd(111), likely due to the different coverages. Obviously, all the calculated binding energies on the films are higher than that on pure Pd(111) surface. This demonstrates that CH3O is more stable on the alloy films, in agreement with the previous conclusion that O atom prefers sites with as many Zn atoms as possible.7,36 The most stable adsorption configuration is η3-(O) mode in which CH3O binds to the surface via the O atom over a pseudo-hollow site. The O-Zn distance is 1.97 A˚ while the O-Pd is around 2.30 A˚, indicating that O-Zn bond plays the key role in stabilizing CH3O. With the increase of Zn on the top layer, the most stable configuration changes to η2-(O) mode in which the O atom bonds to Zn and Pd (or Zn) atoms simultaneously. The O-Zn and O-Pd distances reach 2.03 and 2.57 A˚, respectively, on 8/8/Pd(111) accompanied by adsorption site change from a hollow position to a bridgelike site. Formaldehyde. Formaldehyde is another important intermediate in methanol decomposition and synthesis.23,27 It adsorbs on transition metal surfaces in two bonding modes, η1-(O) and η2-(C, O). The latter is favored on Pd(111) surface by recent theoretical7,15,16,18 and experimental studies.27 We compute a binding energy of 0.50 eV on Pd(111), in nice agreement with the recent theoretical values ranging from 0.45 to 0.63 eV as well as those determined from the TDP experiment (0.52 eV).51 On different surface alloy films, we locate η2-(C, O) to be the most stable configuration with the C bound to Pd and O to Zn. The O-Zn distance decreases from 2.12 A˚ on 1/15/Pd(111) to 2.09 A˚ at 8/8/Pd(111). Correspondingly, the binding energy increases from 0.34, 0.36, 0.43 to 0.49 eV from 1/15/Pd(111) to 8/8/Pd(111). The binding energy of 0.49 eV on 8/8/Pd(111) is essentially equal to the previous value.38 Formyl. Previous theoretical studies resulted in the adsorption energy at Pd(111) in the range of 2.21-2.53 eV,7,15,18 which is (51) Davis, J. L.; Barteau, M. A. J. Am. Chem. Soc. 1989, 111, 1782.
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Table 2. Reaction Enthalpies (in kJ/mol) and Energy Barriers (in eV) of Dehydrogenation Reaction of Methanol, Methoxide, Formaldehyde, and Formyl over PdZn/Pd(111) Surface of Various Zn Concentrations reactions
Pd(111)
1/15/Pd(111)
2/14/Pd(111)
ΔH Ea ΔH ΔH Ea ΔH0 Ea,0 ΔH0 Ea,0 ΔH0 24 1.01 32 0.94 29 11 0.78 17 0.71 14 -23 0.66 31 0.83 42 CH3O f CH2O þ H -40 0.45 15 0.61 26 -72 0.53 -1 0.62 -8 CH2O f CHO þ H -84 0.38 -12 0.46 -19 CHO f CO þ H -86 0.44 -85 0.39 -109 -98 0.28 -95 0.24 -118 a Values in parentheses are results based on 1:1 bulk alloy taken from ref 36. without ZPE correction with ZPE correction CH3OH f CH3O þ H
lower than our result of 2.78 eV, likely due to different coverages. The binding energies, 2.19, 2.23, 2.30, and 2.38 eV from 1/15/ Pd(111) to 8/8/Pd(111), vary regularly with the surface Zn concentration, which will be discussed in section 4. The O-Zn bond becomes shorter with more Zn atoms deposited on surface, which is consistent with the variation tendency of the binding energy. Compared with Pd(111), the binding energies on the films are lower, indicating that introduction of Zn weakens the adsorbate-substrate interaction. The reported binding energy, 1.73 eV on PdZn(111) based on 1:1 PdZn bulk alloy,36 is much lower than our value (2.38 eV) on 8/8/Pd(111) film. Two reasons may account for the discrepancy. First is the coverage (1/4 vs 1/16). The second is adsorption modes. Our binding energy on the 8/8/Pd(111) film refers to the η2-C-η1-O configuration, where the C atom sits over a Pd-Pd bridge site and the O atom is oriented on top of an adjacent Zn atom on PdZn surface alloys. This configuration which has been confirmed by vibrational analysis is the most stable one we determined, whereas ref 36 calculated the η2-(C,O) mode which was not the most stable adsorption complex on present models. In fact, the η2-C-η1-O configuration was also considered as the most stable configuration of CHO on Pd(111) and Pt(111) surfaces.15,18 3.2. Dehydrogenation Mechanisms. In this subsection, we present dehydrogenation pathways of O-H bond breaking of methanol and subsequent C-H bond breaking of methoxide, formaldehyde, and formyl. Table 2 lists the reaction enthalpy and energy barrier of each dehydrogenation step over different surfaces. All the initial states (ISs), final states (FSs), and transition states (TSs) are characterized to be either local minima or first-order saddle points on potential energy surfaces (PES). It is natural to imagine that methanol dehydrogenation involves two reaction pathways. One is the O-H bond breaking which produces methoxide, and the other is the C-H bond scission that yields hydroxymethyl. On the basis of the calculated results, Schennach et al.16 concluded that the cleavage of methanolic C-H bond is preferred. Guo et al.15 demonstrated that the energy barrier of the C-H breaking is slightly lower than that of the O-H breaking. Experimentally, both methoxide and hydroxymethyl were detected on Pd(111).52-55 Static secondary ion mass spectrometry, X-ray photoelectron spectrometry, and pulsed field desorption mass spectrometry measurements56 demonstrate the preferential pathway of methanol decomposition as CH3OH f CH3O f CH2O f CHO f CO. In this work, we follow this proposed route. This choice is also based on the (52) Gates, J. A.; Kesmodel, L. L. J. Catal. 1983, 83, 437. (53) Yang, H.; Whitten, J. L. Langmuir 1995, 11, 853. (54) Chen, J. J.; Jiang, Z. C.; Zhou, Y.; Chakraborty, B. R.; Winograd, N. Surf. Sci. 1995, 328, 248. (55) Levis, R. J.; Jiang, Z.; Nicholas, W. J. Am. Chem. Soc. 1989, 111, 4605. (56) Kruse, N.; Rebholz, M.; Matolin, V.; Chuah, G. K.; Block, J. H. Surf. Sci. 1990, 238, L457.
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Ea Ea,0 0.94 0.72 0.85 0.65 0.70 0.55 0.24 0.10
4/12/Pd(111) ΔH ΔH0 32 20 47 32 -34 -44 -103 -112
8/8/Pd(111)
Ea Ea,0 0.79 0.58 0.90 0.70 0.56 0.42 0.32 0.16
ΔH ΔH0 -25 (-12)a -37 60 (61) 45 (-) -10 (-4) -24 (-18) -76 (-71) -87 (-80)
Ea Ea,0 0.91 0.69 1.13 (1.17) 0.93 (0.96) 0.55 (0.81) 0.40 (0.66) 0.39 (0.46) 0.24 (0.26)
following considerations. First, the experimental studies56 support the O-H breaking as an initial step for methanol decomposition. Second, methoxide is demonstrated to be an initial product of methanol dehydrogenation on Zn/Pd(111) surface in a very recent work27 with which we aim at comparing our theoretical results to investigate how the incorporated Zn modify the reactivity of Pd(111) toward methanol decomposition. Dehydrogenation of Methanol. The O-H bond breaking of CH3OH starts from the approaching of the H atom to the surface. During this process, the O-H bond gradually increases and reaches about 1.6 A˚ in TS (Figure 2). The CH3O group occupies the top site of the Zn atom through the O atom, and the atomic H stays at an adjacent Pd-Zn(Pd) bridgelike site. As reported before,7,36 atop Zn and Pd-Zn(Pd) bridge sites are not most stable positions for CH3O and H species, respectively. Thus, these two species eventually move to a pseudo-fcc hollow site and a 3-fold Pd hollow site, respectively (Figure 2). Because there is no 3-fold hollow site formed by three Pd atoms on 8/8/Pd(111), the dissociated H atom locates at a bridgelike site. The reported energy barrier of O-H bond scission of CH3OH on Pd(111) disperses widely. Schennach et al.16 and Zhang et al.17 obtained almost the same result of ∼0.8 eV, while Ni et al.57 and Guo et al.15 calculated 1.07 and 1.45 eV, respectively. Our calculated energy barrier, 1.01 eV, is intermediate between previously reported values and close to that of ref 57. The calculated barriers on the films are 0.94, 0.94, 0.79, and 0.91 eV from 1/15/ Pd(111) to 8/8/Pd(111) and are slightly lower than that on Pd(111). Zero-point energy correction usually reduces the barrier by ∼0.2 eV (Table 2). The O-H bond elongates from 0.98 A˚ in ISs to 1.67, 1.68, 1.64, and 1.46 A˚ in TSs, respectively. The calculated reaction energies, about 30 kJ/mol (endothermic, Table 2), are close to that on Pd(111), except for the 8/8/Pd(111) model which releases 25 kJ/mol, comparable to the result on PdZn(111).36 Note that methanol is weakly bound system and our PW91 functional fails to describe the dispersive interaction, leading to an underestimate of the binding energy. If such interaction is properly considered, one would expect that the hydrogenation barrier of methanol would increase slightly because the IS becomes more stable. Dehydrogenation of Methoxide. As shown in Figure 3, H abstraction from methoxide initiates with the C-O bond titling toward the surface. In the course of the reaction, the O atom moves from the hollow site (bridgelike site on 8/8/Pd(111)) to top position of Zn. Concomitantly, one of the methylic H gets closer to the surface and interacts with a Pd atom. In the TSs, the C-H length increases from ∼1.10 A˚ in ISs to 1.75, 1.73, 1.68, and 1.65 A˚ on different surface alloy films. The corresponding energy barriers are 0.83, 0.85, 0.90, and 1.13 eV, respectively (Table 2). These (57) Ni, Z. M.; Mao, J. H.; Pan, G. X.; Xu, Q.; Li, X. N. Acta Phys.-Chim. Sin. 2009, 25, 876.
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Figure 2. Dehydrogenation of methanol via O-H scission on various surface alloys. Value in parentheses is the O-H length in each transition state. Light and deep blue spheres denote Pd and Zn atoms. For clarity, only two layers of substrates are displayed.
barriers are inversely proportional to the C-H length in the TSs. The energy barrier on Pd(111) we calculated, 0.66 eV, is comparable to the value of ref 15, which is lower than those on the four alloy surfaces, indicating that incorporation of Zn hinders the C-H bond scission of methoxide. The energy barrier of 1.13 eV on 8/8/Pd(111) is essentially the same as 1.17 eV on PdZn(111), indicating that the metal atoms beyond the third layer have no significant influence on the energetics of surface reactions. In the FSs, CH2O exhibits a top-bridge-top (tbt) configuration (Figure 3). The H atom resides at 3-fold Pd hollow sites on 1/15/Pd(111), 2/14/Pd(111), and 4/12/Pd(111) and at a Pd-Pd bridgelike site on the 8/8/Pd(111) surface. The calculated reaction energy is endothermic by 31, 42, 47, and 60 kJ/mol from 1/15/ Pd(111) to 8/8/Pd(111). At variance, this step is calculated exothermic by 23 kJ/mol on Pd(111). Therefore, thermodynamically addition of Zn also disfavors the C-H bond breaking of CH3O. Dehydrogenation of Formaldehyde. We choose the most stable η2-(C,O) configuration as the ISs for formaldehyde dehydrogenation. The reaction begins with the tilting of the molecular plane toward the surface, which shortens the distance between the H atom and the surface and thus increases the interaction between them. In the TSs, the C-H lengths are extended to 1.56, 1.68, 1.65, and 1.62 A˚ from 1/15/Pd(111) to 8/8/Pd(111) alloy surfaces (Figure 4). In FSs, the produced formyl is at Pd2Zn hollow site, forming a η2-C-η1-O configuration; the dissociated H atom moves to a nearby hollow site except for 8/8/Pd(111) film on which the H sits at a bridgelike site. This step needs to overcome a barrier of 0.62, 0.70, 0.56, and 0.55 eV on each film (Table 2). All these barriers are higher than 0.53 eV (0.61 eV in ref 15) on Pd(111), indicating that alloying inhibits formaldehyde dehydrogenation kinetically. Dehydrogenation of formaldehyde is calculated thermodynamically neutral with the absolute value of reaction heat less than 10 kJ/mol except for 4/12/Pd(111) surface where a value of -34 kJ/mol is computed. On Pd(111) this step is predicted to release 72 kJ/mol. Hence, Zn atoms incorporated into the Pd(111) surface decreases the dehydrogenation activity of the substrate. Because of more stable final state of CHO on 8/8/Pd(111) than on 1:1 PdZn(111) 10800 DOI: 10.1021/la100619q
Huang and Chen
Figure 3. Dehydrogenation of methoxide on various surface alloys. Light and deep blue spheres denote Pd and Zn atoms, respectively.
Figure 4. Dehydrogenation of formaldehyde on various surface alloys. Light and deep blue spheres denote Pd and Zn atoms, respectively.
alloy surface, the activation energy, 0.55 eV on the former, is lower than 0.81 eV on PdZn(111).36 Like methanol, formaldehyde also binds to the substrate weakly, and the dispersive interaction is poorly described by the PW91 functional. Inclusion of the dispersive force would stabilize CH2O, which will make dehydrogenation of methoxide less endothermic and the H abstraction of formaldehyde more difficult. Dehydrogenation of Formyl. Figure 5 displays the dehydrogenation of formyl on various films. The process is initiated by the titling of the C-H bond toward the surface, accompanied by the concomitant rising up of the oxygen atom, which enhances/ weakens the H/O interaction with the substrate. In TSs the CO group locates at a Pd-Pd bridge site, and the H atom occupies a 3-fold hollow site. The C-H distance ranges from 1.21 A˚ (TS44 on 8/8/Pd(111)) to 1.53 A˚ (TS24 on 2/14/Pd(111)). After the TS, the CO group moves to a 3-fold hollow site on 1/15/Pd(111), 2/14/ Pd(111), and 4/12/Pd(111) films. On 8/8/Pd(111) the CO situates at a Pd-Pd bridge site and the H at an adjacent Pd-Pd bridge position. Langmuir 2010, 26(13), 10796–10802
Huang and Chen
Article
Figure 6. Binding energies versus the number of surface Zn or subsurface Zn on various films.
Figure 5. Dehydrogenation of formyl on various surface alloys. Light and deep blue spheres denote Pd and Zn atoms, respectively.
Formyl dehydrogenation is a strong exothermic process, >76 kJ/mol on films, compared to a value of -86 kJ/mol on Pd(111). This comparison shows that addition of Zn does not reduce the exothermicity (in fact, it tends to increase the exothermicity, Table 2). The calculated energy barrier ranges from 0.24 to 0.39 eV, compared to 0.44 eV on Pd(111), indicating that Zn promotes the CHO dehydrogenation. In addition, the low barrier demonstrates that this process is quite feasible on these substrates.
4. Discussion Because of weak interaction, the binding energy of CH3OH does not vary significantly (Table 1). Compared with Pd(111), the binding energy of CH3O increases, showing that introduction of Zn enhances the adsorbate-substrate interaction. Furthermore, from Pd(111) which can be regarded as 0/0/Pd(111) to 1/15/ Pd(111) and finally to 8/8/Pd(111), the binding energy of CH3O increases steadily. Figure 6a displays the relationship between the binding energy of CH3O and the Zn number on the top layer, which clearly shows that with the increase of surface Zn content the stability of methoxide increases. The adsorption site of CH3O changes from fccPd3 (meaning the fcc site is composed of three Pd atoms) on pure Pd(111) to fccPd2Zn on 1/15/Pd(111), 2/14/Pd(111), and 4/12/Pd(111) to fccZn2Pd on 8/8/Pd(111) (Table 1). The ensemble effect, which refers to changes in the properties of a site caused by changes of the chemical composition, is mainly responsible for the enhanced adsorbate-substrate interaction, consistent with previous finding that methoxide prefers sites with as many Zn as possible.7,36 On 1/15/Pd(111), 2/14/Pd(111), and 4/12/Pd(111), the adsorption sites have the same ensemble (fccPd2Zn) and the bond length of O-Zn is essentially the same (1.97 A˚). The binding energy increase of methoxide on these surfaces should be ascribed to the ligand effect which becomes stronger with the decrease of the subsurface Zn because of an upshift of d-band center to Fermi level (thus, a stronger adsorbate-substrate interaction, see the following). This ligand effect facilitates the Pd-O interaction as evidenced by the decrease of the shortest O-Pd bond (Table 1). For CH2O and CHO, their binding energies behave differently from 0/0/ Pd(111) to 8/8/Pd(111). From 0/0/Pd(111) to 1/15/Pd(111), the binding energy drops from 0.5/2.78 to 0.34/2.19 eV for CH2O/ CHO (Table 1), indicating that the surface Zn is unfavorable Langmuir 2010, 26(13), 10796–10802
for the adsorbate-substrate interaction. However, from 1/15/ Pd(111) to 8/8/Pd(111), the binding energy increases from 0.34/ 2.19 to 0.49/2.38 eV, seemingly implying that the surface Zn favors the interaction. The ensemble effect is not responsible for the variation of the binding energies of CH2O and CHO. To explain this phenomenon, we mention our recent study on CO adsorption at Pd-Zn surface alloy.39 There we show that the subsurface Zn lowers the d-band center of surface Pd while the top-layer Zn atom tends to shift up slightly the center. We found that the subsurface Zn exhibits more pronounced weakening effect on the adsorbate-substrate interaction than the top-layer Zn atoms. In other words, the ligand effect of subsurface Zn surpasses that of surface Zn. On the basis of this finding, we can expect that the binding energy of CH2O and CHO would reduce with the increase of the subsurface Zn because the d-band center decreases. In other words, from Pd(111) to 8/8/Pd(111), 4/12/ Pd(111), 2/14/Pd(111), and 1/15/Pd(111), the number of subsurface Zn increases from 0 to 8, 12, 14, and 15 per unit cell, the calculated binding energy will go down. This prediction is faultlessly confirmed by Figure 6b. Jeroro and Vohs27 observed two desorption peaks at 210 and 360 K when dosing CH2O onto Zn deposited Pd(111) surface. The estimated binding energies using Redhead analysis are 0.5 eV for 210 K and 1.0 eV for 360 K. The 210 K peak can be assigned to CH2O in η2-(C,O) mode in which the O bonds to Pd or Zn and the C to Pd. However, the 360 K desorption peak, whose binding energy doubles the value we calculated on all the films and Pd(111), cannot be explained by any model constructed in the present paper. It should be pointed out that Zn was believed only present in the topmost layer, and there was no subsurface Zn in the experiment work.27 Considering this, we also studied adsorption of CH2O on Pd(111) with Zn on the top layer. In addition, the influence of surface and subsurface oxygen on the adsorption of formaldehyde is also examined. No calculated binding energy exceeds 0.6 eV, which is far from 1.0 eV which corresponds to the 360 K TPD peak. Thus, new models are needed to understand the TPD experiment of CH2O. Figure 7 shows the overall PES of dehydrogenation of CH3OH on various alloy surfaces. For comparison, the results on Pd(111) are also presented. The calculated PES reveals that introduction of Zn, on the one hand, reduces the barrier of the O-H bond cleavage of CH3OH and C-H rupture of CHO. On the other hand, the C-H bond breaking of CH3O and CH2O becomes harder on the alloy surfaces, in agreement with experimental results.27 Previously, the PES is reported for methoxide decomposition on 1:1 DOI: 10.1021/la100619q
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Figure 7. Potential energy surfaces of methanol dehydrogenation on Pd(111) and various Pd-Zn films supported on Pd(111) surface. For the structures represented by TSmn, please refer to Figures 2-5.
PdZn(111), PdZn(221), Cu(111), and Pd(111).58 In general, the two PESs agree with each other well. For example, the reported barriers for CH3O f CH2O f CHOf CO process are 1.17, 0.81, and 0.46 eV on 1:1 PdZn(111) compared to 1.13, 0.55, and 0.39 eV obtained on 8/8/Pd(111) surface in the present work. Both predicted the same trend with dehydrogenation of methoxide being the most difficult step. From PdZn(111) to Pd(111), the barrier of CH3O dehydrogenation decreases ∼0.8 eV in ref 58, which also agrees with the present finding that with the decreases of surface Zn content the dehydrogenation barrier of CH3O decreases. Still, the calculated PES shed some light on the mechanism of methanol steam reforming catalyzed by Pd/ZnO. According to the PES, dehydrogenation of CHO to CO is quite easy no matter whether there is Zn or not on the surfaces. However, experimentally only a trace amount of CO is produced in the MSR on Pd/ ZnO. Since CH2O has been identified as an intermediate in MSR, thus two possibilities may account for the trace amount of CO: (i) CHO undertakes a much more feasible reaction to become another species that serves as precursors for CO2 formation. This possibility is less likely because dehydrogenation of CHO only needs to overcome a very low barrier. (ii) CH2O combines with some coadsorbed species such as surface O or hydroxyl groups, resulting in formate which ultimately decomposes to CO2 and H2. This combination of CH2O with other coadsorbate must dominate its dehydrogenation. Otherwise, a large amount of CO will be produced from CHO since the conversion of CO to CO2 via water-gas shift reaction is experimentally excluded.59 Thus, studies on reaction of CH2O with surface coadsorbates are required, which is under way. Recently, Loffreda et al. found that while the classical BEP relationship may not apply, there is a nice correlation between the energy of ISs and the energy of TSs.60 Figure 8a shows the plot of the activation energies of C-H bond scission as a function of its corresponding dehydrogenation reaction enthalpies. The linear correlation coefficient is 0.85. Figure 8b correlates the energy of (58) Neyman, K. M.; Lim, K. H.; Chen, Z. X.; Moskaleva, L. V.; Bayer, A.; Reindl, A.; Borgmann, D.; Denecke, R.; Steinrueck, H.-P.; R€osch, N. Phys. Chem. Chem. Phys. 2007, 9, 3470. (59) Dagle, R. A.; Platon, A.; Palo, D. R.; Datye, A. K.; Vohs, J. M.; Wang, Y. Appl. Catal., A 2008, 342, 63. (60) Loffreda, D.; Delbecq, F.; Vigne, F.; Sautet, P. Angew. Chem., Int. Ed. 2009, 48, 8978.
10802 DOI: 10.1021/la100619q
Huang and Chen
Figure 8. Relationship between the energy barriers of C-H bond scission and the reaction energies (a) and between the energy of initial states and transition states (b) on Pd(111) and various surface alloys.
transition states (ETS) of C-H bond scission and the energy of the corresponding initial states (EIS). A correlation coefficient of 1.00 is evidently finer than the classic BEP relation, indicating the newly proposed relationship is also applicable to our system and may be extended to other cases.
5. Conclusions Methanol dehydrogenation to CO and H on various Pd-Zn surface alloys and Pd(111) has been explored using periodic DFT calculations. Based on the results and analyses, the following conclusions can be drawn. 1. Valence saturated species like CH3OH and CH2O bind weakly to the substrates while CH3O and CHO interact with the substrates strongly. The variation of binding energy on different substrates can be well rationalized with distribution of Zn: the top-layer Zn enhances the interaction of species like CH3O that binds to the substrate via the oxygen atom. For such species the binding energy increases with the increase of the surface Zn. For those adsorbates that adsorb on the substrate mainly though the C-Pd interaction, the binding energy decreases with the increase of subsurface Zn atoms. 2. Addition of Zn reduces the activation energy of O-H/C-H bond breaking of CH3OH/CHO whereas it raises the energy barriers of dehydrogenation of CH3O and CH2O. Because of weak interaction, CH3OH prefers desorption, rather than dehydrogenation. For CH2O, desorption and dehydrogenation are expected to have similar possibilities. For CH3O and CHO, they are stable enough on the substrate to undergo dehydrogenation. 3. Adsorption and reactions on 8/8/Pd(111) resemble the corresponding ones on PdZn(111), indicating that Zn atoms beyond the third layer have essentially no influence on the reactions at the top layer. The observed 210 K peak of CH2O can be assigned to CH2O adsorbed on Pd-Zn or Pd-Pd tbt site. To explain the 360 K peak, other models should be invoked. Acknowledgment. This work was supported by Natural Science Foundation of China No. 20973090, 973 Program 2009CB623504 and Provincial Science Fund for Excellent Young Scholars of High Schools No. 2010SQL024.
Langmuir 2010, 26(13), 10796–10802
