ZrO2 (111) Surface: A

Oct 29, 2009 - atoms to form first CH2O(a) and then CHO(a), and ultimately produces CO(a) plus 4H(a). The proposed overall reaction to produce gaseous...
0 downloads 0 Views 2MB Size
Download PDF
20139

2009, 113, 20139–20142 Published on Web 10/29/2009

Mechanism of CH2 Steam Reforming on a Rh/ZrO2(111) Surface: A Computational Study Han-Jung Li and Jia-Jen Ho* Department of Chemistry, National Taiwan Normal UniVersity, 88, Section 4, Tingchow Road, Taipei, Taiwan 116 ReceiVed: July 30, 2009; ReVised Manuscript ReceiVed: September 2, 2009

A periodic density-functional theory (DFT) calculation was performed to examine the steam-reforming reaction of CH2 on a Rh/ZrO2(111) surface. After loss of one hydrogen atom from H2O(a), OH(a) tends to associate with CH2(a) to form an intermediate, CH2OH(a), on the Rh cluster. This intermediate loses consecutive hydrogen atoms to form first CH2O(a) and then CHO(a), and ultimately produces CO(a) plus 4H(a). The proposed overall reaction to produce gaseous CO and H2 is calculated to be exothermic by 2.14 eV. Hydrogen is abundant as a component of water, biomass, and fossil hydrocarbons1 but is scarcely present in its elemental form (H2). Because of increasing problems due to environmental pollution, exploring alternative resources becomes an active issue in our daily life.2 As H2 is considered to be a highly desirable fuel to meet the various needs,3,4 its synthesis becomes essential. A common source is from the methane-reforming reaction,5 which comprises three major stages: steam reforming (1), partial oxidation (2), and CO2 reforming (3).

CH4 + H2O f CO + 3H2

(1)

CH4 + 1/2O2 f CO + 2H2

(2)

CH4 + CO2 f 2CO + 2H2

(3)

Of the ratios H2/C among these three reactions, process 1 has the greatest (H2/C ) 3), whereas partial oxidation (2) (H2/C ) 2) in the middle has a ratio much greater than that for the analogous partial oxidation of heptane (H2/C ) 1.15) or a highly condensed polyaromatic molecule such as coronene (H2/C ) 0.25).1 The ratio H2/C of the latter process (CO2 reforming) is only 1, the smallest among the three. For this reason, steam reforming of hydrocarbons such as in process 1 is considered to be a highly significant process for the manufacture of hydrogen.6,7 Among all hydrocarbon fragments during steam reforming, CH2 is generally suggested to be a common adsorbing intermediate,8-11 which could further react with the adsorbed H2O to carry out steam reforming, and it would be interesting to understand the possible reaction pathways of this process. Incidentally, the adsorbed CH2 would not be so reactive to recombine barrierlessly to form ethylene as was expected in the gas phase; instead, it would take more than 1 eV of barrier (1.36 eV in our later discussed Rh/ZrO2 system) to perform the ethylene recombination on a common metal/metal oxide surface. In this work, we undertook a detailed investigation of the mechanism of CH2 steam reforming on a Rh/ZrO2(111) surface for these two reasons. ZrO2 is known to have a reducible nature, * Corresponding author. E-mail: [email protected]. Phone: (886)-229309085. Fax: (886)-2-29324249.

10.1021/jp9072894 CCC: $40.75

Figure 1. Top and side views of p(3×2) slab models for (a) ZrO2(111) and (b) Rh/ZrO2(111) surfaces. The gray, red, and green spheres represent Zr, O, and Rh, respectively.

to be stable at high temperatures, and to be commonly employed as an effective catalyst support for various reactions.12,13 Noble metal Rh is known to activate the C-H bond effectively as well as to be active and selective in forming the products, relative to Pt, Pd, and Ru of similar metals loaded on a catalyst support for surface reactions.7 We therefore performed a DFT calculation for the steam reforming of CH2 on the surface Rh/ ZrO2(111)-(3×2) and addressed the chemical behavior of the adsorbed intermediates, the reaction barriers of possible paths, and the potential-energy surface. All calculations were performed with the DFT plane-wave method utilizing the Vienna ab initio simulation package (VASP).14-17 The nudged elastic-band (NEB)18-20 method was applied to locate transition structures; the minimum-energy paths (MEPs) were constructed accordingly. The detailed description of the computation methods are described in the Supporting Information. The p(3×2) lateral cells of clean ZrO2(111) and Rh/ZrO2(111) surfaces are modeled as periodically repeated slabs shown in Figure 1. The geometries of doped Rh atoms on the ZrO2(111) surface at Rh coverages of θ ) 1/6 ML to 2/3 ML are shown in Figure 2 (including top view and side view); the corresponding average adsorption energies and bond lengths are listed in Table 1. For one Rh atom adsorbed on a ZrO2(111) surface  2009 American Chemical Society

20140

J. Phys. Chem. C, Vol. 113, No. 47, 2009

Letters

Figure 2. Top and side views of a Rh/ZrO2(111) surface at rhodium coverages of 1/6, 1/3, 1/2, and 2/3 ML. The gray, red, and green spheres represent Zr, O, and Rh, respectively.

TABLE 1: Variation of Average Adsorption Energies and Rh-Rh Bond Lengths with Rh Coverage on ZrO2(111) Surfaces coverage (ML) θ ) 1/6 θ ) 1/3 θ ) 1/2 θ ) 2/3 a

geometry

Eads/eV

dRh-Rha/Å

triangular quadrilateral

-2.39 -2.75 -3.40 -3.54

2.41 2.48 2.51

The values are average bond lengths.

(θ ) 1/6 ML), the most stable site is at the hollow between three Zr atoms; the calculated adsorption energy for this structure is -2.39 eV. For two adsorbed Rh atoms (θ ) 1/3 ML), a dimer structure is energetically more stable, with a larger average adsorption energy (-2.75 eV per each Rh atom); this Rh dimer bridges between one Zr atom and one O atom of the ZrO2(111) surface. The bond length between two Rh atoms is 2.41 Å, indicating the existence of metal-metal interaction. At θ ) 1/2 ML (three coadsorbed Rh atoms on the ZrO2 surface), the preferred structure has a planar triangular cluster with an average adsorption energy of -3.40 eV/Rh and an average Rh-Rh bond length of 2.48 Å. At θ ) 2/3 ML (four Rh atoms), a quadrilateral structure parallel to the surface is energetically stable, with an averaged adsorption energy -3.54 eV/Rh and averaged Rh-Rh bond distance of 2.51 Å, similar to the predicted Rh-cluster structure on the CeO2(111) surface.21 The average adsorption energy of Rh on the ZrO2(111) surface is smaller than that on the CeO2 analogue (-3.85 eV) by 0.31 eV. In this Letter, we chose four Rh atoms to represent a possible small Rh accumulation on the surface 3×2 ZrO2(111), depicted in Figure 1b. The calculated potential-energy surface for the reforming reaction of CH2 and H2O molecules on the 4Rh/ZrO2(111) surface is drawn in Figure 3. We find that coadsorbates CH2 and H2O tend to be adsorbed onto a rhodium cluster via carbon and oxygen atoms, respectively; the oxygen atom has two O-H bonds parallel to the surface, and the carbon atom is on a bridge

site between two Rh atoms, depicted in Figure 4-(1). These two molecules interact with the rhodium cluster through their lonepair electrons with bond lengths of C · · · Rh ) 2.03 Å and O · · · Rh ) 2.12 Å; the calculated adsorption energy of coadsorbates CH2 and H2O is -5.86 eV, listed in Table 2. The first step of dehydrogenation of this coadsorbate might proceed through dissociation of one O-H bond of water with a modest barrier of 0.29 eV (via TS2 in which the O · · · H bond is stretched to 1.44 Å, shown in Figure S1 of the Supporting Information) and endothermic by 0.12 eV, to form a coadsorbed intermediate complex CH2(a) + OH(a) (Figure 4-(2)). An alternative is via the scission of one C-H bond of CH2 that is exothermic by 0.42 eV with a barrier of 0.33 eV (TS1), forming coadsorbed CH(a) + H2O(a) (Figure 4-(3)). The former process is kinetically preferable; it might be followed immediately by one of three possible pathways: (i) scission of the other O-H bond of H2O to form CH2(a) + O(a) (Figure 4-(4)) via TS3 (barrier 1.76 eV); (ii) dehydrogenation of CH2 to form CH(a) + OH(a) (Figure 4-(6)) via TS4 (barrier 0.80 eV); (iii) forming a CH2OH molecular adsorbate (Figure 4-(5)) via TS5 (barrier 0.32 eV). The latter has the least energy barrier and might be the most favorable. This CH2OH(a) forms a ring structure (exothermic by 0.26 eV) (Figure 4-(5)) adsorbed on a Rh cluster via its C and O atoms (adsorption energy -3.65 eV) with bond lengths of C · · · Rh ) 1.99 Å and O · · · Rh ) 2.09 Å. It might undergo dissociation of the C-H bond via TS6 (C · · · H elongated to 1.65 Å, barrier 1.85 eV), forming CHOH(a) ((8), exothermic by 0.73 eV), or it might proceed with another dehydrogenation via the scission of the O-H bond (subject to a barrier of 1.33 and 0.11 eV exothermic), forming species CH2O(a) (7). The latter process is more likely. Species (7) might undergo further dehydrogenation via the scission of a C-H bond with a barrier of 0.74 eV (TS8, C · · · H bond stretched to 1.41 Å) to form the CHO(a) species (9), which is 0.70 eV exothermic. The detachment of a H atom from CHO(a) requires 0.83 eV to cross the barrier (TS9) and to form CO(a). At this final stage, for the remaining adsorbed hydrogen atoms to recombine and then become desorbed to form 2H2(g) together with the desorption of CO(a) to form CO(g) is 5.28 eV endothermic. Our calculated results indicate that the most favorable pathway in this CH2 steam-reforming reaction is as follows: (1) f TS2 f (2) f TS5 f (5) f TS7 f (7) f TS8 f (9) f TS9 f CO(a) + 4H(a) f CO(g) + 2H2(g), with the overall reaction 2.14 eV exothermic. On comparison with a similar reaction on the surface Rh/ CeO2(111),21 the energy of coadsorption of species CH2 and H2O on the Rh/ZrO2(111) surface is -5.86 eV, less than for the Rh/CeO2(111) system (-6.17 eV). The barrier of the first O-H bond scission of water on the Rh/ZrO2 surface is 0.29 eV, much less than that for the Rh/CeO2 counterpart (0.73 eV), and also less than that for some pure metallic surfaces (the barriers of first scission of H2O on the Pt(111), Au(111), Ni(111), and Pd(111) surfaces are 0.68, 1.80, 0.96, and 1.09 eV, respectively).22-24 Our result shows that the Rh/ZrO2 system has a greater catalytic effect for scission of the first O-H bond of water, but for the C-H bond scission, the dissociation barrier on a Rh/ZrO2 surface is greater than that on a Rh/CeO2(111) surface. We found also that, in the Rh/ZrO2 system, after loss of one hydrogen atom from H2O, OH preferentially associates with CH2 to form a CH2OH intermediate on the surface, whereas for the Rh/CeO2 counterpart the CH2 and H2O preferably lose one hydrogen atom from each species and then associate to form the intermediate CHOH on the Rh/CeO2 surface. These results are explained with an interaction between the surfaces and the

Letters

J. Phys. Chem. C, Vol. 113, No. 47, 2009 20141

Figure 3. Calculated potential-energy diagram for the reaction CH2 + H2O on a Rh/ZrO2(111) surface.

TABLE 2: Adsorption Energies and Optimized Bond Lengths for Coadsorbed CH2 + H2O as Well as Their Relevant Dissociated Intermediates on the Rh/ZrO2(111) Surface (Optimized Structures Are Shown in Figure 4) species

Eads/eV

dC-Rha/Å

dO-Rh/Å

CH2(a) + H2O(a) (1) CH2(a) + OH(a) (2) CH(a) + H2O(a) (3) CH2(a) + O(a) (4) CH2OH(a) (5) CH(a) + OH(a) (6) CH2O(a) (7) CHOH(a) (8) CHO(a) (9) CO(a) (10)

-5.86 -8.36 -8.22 -9.95 -3.65 -10.94 -2.43 -5.91 -4.08 -3.13

2.03 2.00 1.97 2.01 1.99 1.94 2.04 2.00 2.11 2.01

2.12 1.93 2.16 1.73 2.09 1.94 1.93 2.12 1.99

a

Figure 4. The optimized geometries of possible dissociated intermediates of the CH2 + H2O reaction on the Rh/ZrO2(111) surface. The gray, red, green, black, and white spheres represent Zr, O, Rh, C, and H, respectively.

adsorbed molecules. The adsorption energy of H2O on the Rh/ ZrO2(111) (in Figure 4-(1)) surface is -0.72 eV but -0.63 eV

The values are average bond lengths.

for the Rh/CeO2 system, indicating a stronger interaction between adsorbed H2O and the Rh/ZrO2 surface, which results in a much smaller barrier for O-H scission on Rh/ZrO2 relative to Rh/CeO2 (0.29 vs 0.73 eV). In contrast, CH2 has a smaller adsorption energy on the Rh/ZrO2 surface (-4.22 eV) than on the Rh/CeO2 surface (-4.89 eV), indicating a weaker interaction of CH2 on the Rh/ZrO2 surface. The barrier for C-H bond scission is thus much smaller on the Rh/CeO2 surface (0.12 vs 0.80 eV), which results in preferential formation of fragment CH on the Rh/CeO2 surface before the association with OH to form the intermediate CHOH, whereas for CH2(a) to remain on the Rh/ZrO2 surface and to combine directly with OH to form the intermediate CH2OH is favored. Acknowledgment. National Science Council of Republic of China (NSC 96-2113-M-003-007-MY3) supported this work; National Center for High- performance Computing provided computer time.

20142

J. Phys. Chem. C, Vol. 113, No. 47, 2009

Supporting Information Available: Information on the computational methods and figure showing the transition state structures of the CH2 + H2O reaction on the Rh/ZrO2(111) surface. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Navarro, R. M.; Penˇa, M. A.; Fierro, J. L. G. Chem. ReV. 2007, 107, 3952. (2) Duan, S.; Senkan, S. Ind. Eng. Chem. Res. 2005, 44, 6381. (3) Whittingham, M. S.; Savinell, R. F.; Zawodzinski, T. Chem. ReV. 2004, 104, 4243. (4) Holladay, J. D.; Wang, Y.; Jones, E. Chem. ReV. 2004, 104, 4767. (5) Wang, S.-G.; Cao, D.-B.; Li, Y.-W.; Wang, J.; Jiao, H. J. Phys. Chem. B 2006, 110, 9976. (6) Haryanto, A.; Fernando, S.; Murali, N.; Adhikari, S. Energy Fuels 2005, 19, 2098. (7) Vaidya, P. D.; Rodrigues, A. E. Chem. Eng. J. 2006, 117, 39. (8) Xiao, H.; Xie, D. Surf. Sci. 2004, 558, 15. (9) Van Santen, R. A.; De Koster, A.; Koerts, T. Catal. Lett. 1990, 7, 1.

Letters (10) Hindermann, J. P.; Hutchings, G. J.; Kiennemann, A. Catal. ReV. Sci. Eng. 1993, 35, 1. (11) Wang, S.-G.; Liao, X.-Y.; Hu, J.; Cao, D.-B.; Li, Y.-W.; Wang, J.; Jiao, H. Surf. Sci. 2007, 601, 1271. (12) Bi, J. L.; Hsu, S. N.; Yeh, C. T.; Wang, C. B. Catal. Today 2007, 129, 330. (13) Grau-Crespo, R.; Herna´ndez, N. C.; Sanz, J. F.; De Leeuw, N. H. J. Phys. Chem. C 2007, 111, 10448. (14) Kresse, G.; Hafner, J. Phys. ReV. B 1993, 47, 558. (15) Kresse, G.; Hafner, J. Phys. ReV. B 1994, 49, 14251. (16) Kresse, G.; Furthmuller, J. Comput. Mater. Sci. 1996, 6, 15. (17) Kresse, G.; Hafner, J. Phys. ReV. B 1996, 54, 11169. (18) Mills, G.; Jo´nsson, H.; Schenter, G. K. Surf. Sci. 1995, 324, 305. (19) Henkelman, G.; Uberuaga, B. P.; Jo´nsson, H. J. Chem. Phys. 2000, 113, 9901. (20) Ulitsky, A.; Elber, R. J. Chem. Phys. 1990, 92, 1510. (21) Li, H.-J.; Chen, H.-L.; Peng, S.-F.; Ho, J.-J. Chem. Phys. 2009, 356, 141. (22) Phatak, A. A.; Delgass, W. N.; Ribeiro, F. H.; Schneider, W. F. J. Phys. Chem. C 2009, 113, 7269. (23) Cao, Y.; Chen, Z.-X. Surf. Sci. 2006, 600, 4572. (24) Michaelides, A.; Hu, P. J. Am. Chem. Soc. 2001, 123, 4235.

JP9072894