ZrO2(101) Surface: A

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CO Reforming of CH on the Tetragonal ZrO(110) Supported Pt Yanxin Wang, and Hongwei Gao J. Phys. Chem. B, Just Accepted Manuscript • Publication Date (Web): 02 May 2017 Downloaded from http://pubs.acs.org on May 6, 2017

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The Journal of Physical Chemistry

Mechanism of CO2 Reforming of CH4 on a Pt4/ZrO2(101) Surface: A Density Functional Theory Study Yanxin Wang, Hongwei Gao∗ Key Laboratory of Plant Resources and Chemistry in Arid Regions, Xinjiang Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Urumqi 830011, China ____________________________________________________________________ ABSTRACT: The reaction mechanism for CO2 reforming of CH4 on the tetragonal ZrO2(101) supported Pt4 is investigated using density functional theory (DFT) method with the periodic slab model. We search and explore two elementary reaction pathways for CO2/CH4 reforming and finally determine the most energetically favorable route through the potential energy surface analysis. The key intermediate and the rate-determining step are also identified. Our results indicate that the ZrO2 support plays an essential role for the activity of the catalyst in CO2/CH4 reforming reaction and it provides an unique adsorption site for CO2, the key for the formation of carbonate and formate species, which could not be realized on Pt4 cluster and some other supports. _____________________________________________________________________ 1. INTRODUCTION CO2/CH4 reforming reaction (CO2 + CH4 → 2CO + 2H2) is a significant reaction to

∗

Corresponding author: Tel.: +86-991-3858319; Fax : +86-991-3858319; E-mail: [email protected]

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produce pure CO or synthesis gas with a low H2/CO ratio (1:1), which can be used to produce long-chain hydrocarbons in Fischer-Tropsch synthesis and bulk chemicals such as acetic acid, dimethyl ether, and oxoalcohols. The reaction may also offer a practical means to reduce the emission of two greenhouse gases, CH4 and CO2.1-7 The key to CO2/CH4 reforming is to develop a catalyst with high reaction activity and good carbon deposition resistance.8-10 Ni and Pt have been usually used as catalysts in the CO2/CH4 reforming reaction.11-26 Ni catalysts are more economical than noble metal catalysts but suffer from causing a high rate of coke formation.11-16 However, the Pt catalyst is a reasonable compromise in terms of relative low price and strong resistance to coking.16-18 Lercher et al.19 concluded that the catalysts which could form carbonates on the support (Pt/Al2O3, Pt/TiO2 and Pt/ZrO2) showed a much higher activity compared to the catalysts which could not form carbonates on the support (Pt/SiO2). Many studies have confirmed that ZrO2 is a highly effective support for the Pt catalysts rather than the more common Al2O3 and TiO2, because of its higher activity as well as better stabilization and promotion effect on the Pt catalyst.16,20-24 Seshan et al.16 have studied CO2/CH4 reforming using ZrO2 as a support, as well as TiO2 and Al2O3. They found that, ZrO2 was the most effective among these three supports, resulting in a stable catalyst at 600℃ and 1:1 CH4:CO2 feed ratio. Therefore, Pt supported on a ZrO2 catalyst (Pt/ZrO2 catalyst) has been widely applied to CO2/CH4 reforming. The reaction mechanisms for CO2 reforming of CH4 have been investigated

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previously on various catalysts. Mark et al.27,28 and Erdohelyi et al.29 suggest an Eley Rideal type mechanism in which methane is adsorbed on the metal (Rh) and decomposed to H2 and adsorbed carbon. The carbon reacts directly with CO2 from the gas phase to CO. Early experimental study found CHO and CH2O as intermediates in the CO2 reforming systems on Ni-based catalysts, and both CHx (CH4 → CH → C → CO)16 and CHxO (CH4 → CHx → CHxO → CO)30,31 pathways were proposed. When CO2 reforming reaction occurs on oxide-supported metal catalysts, it is generally agreed that the CHx species are formed on the metal, and however, it is not clear whether CO2 is activated on the support or if the metal is involved. Qin et al.32 suggest that CO2 dissociates on the metal to form M-CO and M-O. This is further confirmed by Bodrov et al.33 for CO2/CH4 reforming over a Ni foil. Keulen et al.,25 in contrast, propose a bifunctional mechanism for CO2 reforming over Pt/ZrO2 by the TAP Investigations, in which CO2 is activated on the support to dissociate CO and adsorbed O, whereas methane decomposes to adsorbed C and H on the metal. Lercher and co-workers34 observed the formation of carbonate and formate species on the ZrO2 support using I.r. spectroscopy and the presence of CHx species and hydrogen on the metal (Pt) via pulse experiments. Although the previous researches provide important insight on the reaction mechanism for CO2/CH4 reforming, the key intermediates and the main reaction mechanism are still unclear on Pt/ZrO2 catalyst. Theoretical calculation based on density functional theory study (DFT) are a crucial tool for searching optimal reaction pathway and exploring reaction mechanism at an atomic level.35-38 DFT has been used to investigate the CH4/CO2 reforming on

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pure Ni surface and Ni/MgO catalyst,39,40 however, to our best knowledge, the mechanism of CO2/CH4 reforming on Pt/ZrO2 catalyst has been seldom reported. In this paper, a systematic study using DFT method was performed to investigate different plausible reaction mechanisms of CO2/CH4 reforming on tetragonal ZrO2-supported Pt4, aiming to identify the most energetically favorable route, the key intermediate, the rate-determining step and the role of ZrO2 support in this process.

2. COMPUTATIONAL METHOD AND MODEL The DFT calculations were performed using the Cambridge Sequential Total Energy Package

(CASTEP)

environment.41 The

module

implemented

generalized

gradient

in

Material

approximation

Studio (GGA)

(MS) with

v8.0 the

Perdew-Burke-Ernzerhof (PBE) exchange correlation functional was used in all calculations.42 Ionic cores were described by ultrasoft pseudopotential43 and the Kohn–Sham one-electron states were expanded in a plane wave basis with 450 eV cutoff energy. The tetragonal ZrO2 (101) surface has been used to study stability and nucleation of Ptn clusters on it in our previous studies.44 In the present study, we use the same size of unit cell to model the tetragonal ZrO2 (101) surface. The ZrO2 (101) surface was modeled via (2×2) supercell with a thickness of three O-Zr-O trilayers (nine atomic layers) separated by a 15 Å vacuum region. Brillouin zone integration was approximated by a sum over special k-points chosen using the Monkhorst-Pack method,45 and which were set up to 2 × 3 × 1. The convergence criteria for structure optimization and energy calculation has reached the ultra-fine quality including the

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SCF tolerance of 5.0 × 10-7 eV/atom, the energy of 5.0 × 10-6 eV/atom, maximum force of 0.01 eV/Å, maximum displacement of 5.0 × 10-4 Å, respectively. Spin polarization was considered in all calculations. During optimization, the top three atomic layers including the adsorbates are allowed to fully relax, whereas the bottom six layers are remained restrained in their bulk positions. The Transition states (TS) were searched by using complete linear synchronous transit (LST) or quadratic synchronous transit (QST) methods.46 LST maximization was firstly performed, along with an energy minimization in the directions conjugating to reaction pathway. Then, QST maximization was performed exploring the TS approximation. From that point, another conjugate gradient minimization was carried out. The cycle had been repeated until a stationary point was located. The convergence criterion for transition states search was set to 0.25 eV/Å of per atom for the root-mean-square force. The adsorption energy (Eads ) in this paper is defined as follows: Eads = Eadsorbate/substrate − Eadsorbate − Esubstrate

(1)

where Eadsorbate/substrate is the total energy of the adsorbate-substrate system in the equilibrium state. Eadsorbate and Esubstrate are the total energies the free adsorbate and the substrate, respectively. Therefore, the negative value of of Eads corresponds to the energetically favorable adsorption.

3. RESULTS 3.1. Tetragonal ZrO2(101) and Pt4/ZrO2(101) Surfaces. Among three crystal

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forms of ZrO2, the cubic, tetragonal and monoclinic structures, the tetragonal form is selected in the paper because the monoclinic polymorph has little practical applications,47 whereas the high temperature polymorphs (tetragonal and cubic) exhibit excellent mechanical, thermal, chemical and dielectric properties.48,49 Zr atoms are eightfold coordinated in tetragonal phase and the calculated lattice parameters of bulk tetragonal ZrO2 (t-ZrO2) are a = b = 3.616 Å , c = 5.115, which agrees well with the experimental values of a = b = 3.64 Å , c = 5.270.50 In this study, we explore the t-ZrO2(101) surface as the support because it is the most stable facet of t-ZrO2.51 The top and side views of the t-ZrO2(101) surface are shown in Figure 1a, in which two kinds of Zr atoms (denoted as Zra and Zrb) and four kinds of O atoms (denoted as Oa, Ob, Oc, and Od) are exposed. For supported Pt cluster, the Pt4 cluster size was chosen since it is large enough to capture both the contact and non-contact parts with the ZrO2 surface and, meanwhile, is small enough for reasonable computational cost. Moreover, the tetrahedron structure was used because of its better stability than the planar rhombus cluster.52,53 The most stable adsorption structures of Pt4 cluster on the t-ZrO2(101) surface are presented in Figure 1b. As shown in Figure 1b, Pt1, Pt2 and Pt3 atoms directly interact with the surface via Pt1-Oc (1.99 Å), Pt2-Oa (2.06 Å), Pt3-Oc (1.98 Å) and three Pt-Zra bonds (2.90, 2.83 and 2.94 Å), while Pt4 atom is located at the top vertex away from the support surface. 3.2. Adsorption Geometries and Energies. Adsorption of reactants, all possible intermediates, and products were investigated in CO2/CH4 reforming reaction over

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Pt4/ZrO2 catalyst. The stable geometries of these species are schematically depicted in Figure 2 and the calculated adsorption energies, preferred adsorption sites and corresponding geometric parameters of them are listed in Table 1. The detailed descriptions of the key adsorbed species (CH4, CO2 and HCO) are discussed in this section and the descriptions of other adsorbed species are shown in the supporting information. 3.2.1. CH4 Adsorption. CH4 preferentially adsorbs at the top site of Pt4 atom. The average C-H bond length elongates from 1.095 Å in free CH4 molecule to 1.22 Å of adsorbed CH4, resulting in the adsorbed CH4 being more reactive and easily dissociated. The distances of C-Pt4 is 2.743 Å with an adsorption energy of 0.02 eV, indicating the weak interaction between CH4 and Pt4/ZrO2. 3.2.2. CO2 Adsorption. The most stable adsorption site for CO2 on the ZrO2 surface is on the top site of surface Oc atom with an adsorption energy of -3.85 eV. CO2 strongly interacts with ZrO2 surface via C-Oc bond (1.436 Å), making an enormous difference to the initial position of surface Oc atom and the geometry of CO2. When CO2 binds with the surface Oc atom, Zra-Oc and Zrb-Oc bond lengths elongate from the original 2.097 and 2.014 Å to 2.189 and 2.290 Å, respectively, while C1-O-C2 angel of CO2 decreases from 180° (in gaseous form) to 131.21° and meanwhile, the distances of C1-O and C2-O increase from 1.180 and 1.185 Å to 1.251 and 1.280 Å, respectively. Obviously, CO2 species tends to combine with Oc atom to form a carbonate intermediate 3.2.3. HCO Adsorption. HCO prefers to bind with the surface Oc atom through

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its C atom with the C-Oc bond length of 1.312 Å. The strong interaction between C and Oc atoms makes Oc atom rise by 0.686 Å relative to its original position. The Oc atom movement immensely weakens the Oc bond strength with its left and right side Zr atoms (Zra and Zrb, respectively), resulting in the scission of Zra-Oc bond (3.068 Å) and a longer length of Zrb-Oc bond (2.229 Å). It is distinct that HCO species tends to combine with Oc atom to form a formate intermediate, in which C-H, C-O and C-Oc bond lengths equal to be 1.110, 1.250 and 1.312 Å, respectively. The formate intermediate links with ZrO2 surface via the Zrb-Oc bond, which is tilted by 59.2° to the surface normal. 3.3. CH Formation. Having acquired the preferred adsorption sites for important possible species, we then explored the reaction mechanism involved in CH4/CO2 reforming on Pt4/ZrO2 surface. The calculated activation barriers (Ea) and reaction energies (∆H) for every reaction step are listed in Table 2. CH formation arise from the sequential dissociation of CH4 adsorbed on the top site of Pt4 atom. The corresponding potential energy surface is shown in Figure 3 and the optimized adsorption configurations of the reactants, transition states (TS) and products for this process are illustrated in Figure 4. 3.3.1. CH4 Dissociates into Co-Adsorbed CH3 and H. The dissociation of adsorbed CH4 into CH3 and H1 proceeds through a transition state (TS1), in which both C and H1 atoms link with the Pt4 atom with bond lengths of 2.143 Å and 1.628 Å, respectively, and meanwhile the C-H1 bond is stretched to 1.764 Å. Then, H1 atom transfers to the bridge site of Pt3-Pt4, forming H1-Pt3 (1.699 Å) and H1-Pt4 (1.774 Å)

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bonds, while the remaining CH3 still binds with Pt4 atom with a shorter bond length of 2.058 Å. This process is endothermic by 0.34 eV, with an activation barrier of 1.05 eV. 3.3.2. CH3 Dissociates into Co-Adsorbed CH2 and H. When adsorbed CH3 dissociates into CH2 and H2, the activated H2 atom of CH3 shifts to the bridge site of Pt3-Pt4, forming two H2-Pt bonds with bond lengths of 1.833 and 1.694 Å, and the remaining CH2 fragment moves to the bridge site of Pt2 and Pt4 atoms with two C-Pt bonds of 2.030 and 2.019 Å. This step proceeds via a transition state (TS2), in which the C-H2 bond is stretched to 1.959 Å. The dissociation reaction is endothermic by 0.36 eV, with a high activation barrier of 1.81 eV. 3.3.3. CH2 Dissociates into Co-Adsorbed CH and H. For the dissociation of CH2 on Pt4, the activated H3 atom goes away from C atom, leading a longer C-H3 distance of 1.740 Å, and binds with Pt4 atom, forming a new Pt4-H3 bond (1.423 Å) as seen in TS3. Subsequently, the H3 atom transfers to the bridge site of Pt3-Pt4 atoms, and the H3-Pt3 and H3-Pt4 bonds are 1.737 and 1.759 Å, respectively. Throughout the whole dissociation process of CH2, the remaining CH fragment always remains at the bridge site of Pt2-Pt4 bond. The dissociation reaction is largely endothermic by 1.02 eV, with an activation barrier of 1.89 eV. 3.4. From CH to Final Products. Beginning with CH species and CO2, (CO2 is adsorbed on the top site of surface O3 atom of support, forming the carbonate intermediate), two possible reaction pathways (pathway 1 and pathway 2) occur for the formation of products (CO and H2); pathway 1 is CH reacting with adsorbed CO2, forming the formate species, followed by its dissociation to gaseous CO and OH, and

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pathway 2 is the direct C-H bond scission of CH to C and H atoms followed by C oxidation to CO. 3.4.1. Reaction Pathway 1. The potential energy surface for reaction pathway 1 is shown in Figure 5 and the optimized adsorption configurations of the reactants, transition states (TS) and products for this process are illustrated in Fig. 6. This reaction pathway involves two steps. For the first step, starting from the co-adsorbed CH and carbonate, the C-O bond adjacent to CH species is first broken and then O atom transfers to C atom of CH fragment to form a HCO species, leading to the formation of an intermediate (IM). In IM, a new C-O bond is formed (1.208 Å) and the site of surface Oc atom has risen by 0.43 Å as compared to that in the primary carbonate, making the lengths of C-O2 and C-Oc bonds shorten to 1.233 and 1.290 Å, respectively. This process proceeds through a transition state (TS4), in which the C-O1 bond is stretched to 1.966 Å and it is endothermic by 1.21 eV, overcoming a barrier of 1.72 eV. Subsequently, the C-H bond of the HCO species is broken and stretched to 2.023 Å by overcoming a small barrier of 0.26 eV (TS5), and then the detached H atom approaches to the C atom of CO2 in the support, leading to the formation of a formate, in which the new C-H bond length is 1.105 Å. This reaction process is very facile with the reaction energy of -2.34 eV. In the last step, for the remaining formate species on the support, the HCO fragment of it goes away from Oc atom with C-O bond elongating to 2.115 Å, and then moves and rotates until it almost locates parallel to the ZrO2 surface, as seen in TS6. Subsequently, the O atom of HCO in TS6 migrates to the surface Oc atom, forming a OH species with the O-H bond length of

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0.98 Å. At the same time, the rest CO from HCO moves to the top site of surface Zra atom, in which CO keeps perpendicular to the ZrO2 surface with O-Zr distance of 2.618 Å. The CO molecule may desorb quickly due to its weak adsorption energy (0.04 eV) with respect to the support. The reaction for formate decomposition into OH and CO is endothermic by 1.08 eV, with an activation barrier of 3.47 eV. The OH groups can either recombine and desorb as water or react further with CHx (x = 1-4) to form CO and hydrogen (steam reforming). 3.4.2. Reaction Pathway 2. The potential energy surface of reaction pathway 2 is presented in Figure 7 and the optimized adsorption configurations of the reactants, transition states (TS) and products for this process are illustrated in Figure 8. CO formation might arise from the combination of adsorbed C and O atoms which comes from the direct CH dissociation and CO2 decomposition, respectively. For the adsorbed CH species on the Pt4 cluster, it can once again dissociates into C and H via TS7, in which the C-H bond is elongated to 1.733 Å. The left C atom remains at the bridge site of Pt3 and Pt4 atoms, while the H atom shifts to the top site of Pt3 with H-Pt3 bonds of 1.644 Å. This step is endothermic by 0.66 eV, with an activation barrier of 2.45 eV. When CO2 adsorbed on the support as carbonate decomposes into gaseous CO and adsorbed O, the activated O atom goes away from C atom and approaches to Pt2 atom with C-O and Pt2-O distances of 2.344 Å and 1.930 Å, respectively, and meanwhile, the left CO species also separates itself from the surface Oc atom and finally locates almost parallel to the ZrO2 surface, as seen in TS8. Subsequently, the O atom migrates to Zra atom, forming a Zra-O bond (2.055 Å),

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while the CO molecule is further away from the support and desorbs quickly due to the weaker adsorption energy (0.04 eV) with the surface. This step has a very large activation barrier of 4.37 eV and it is slightly endothermic by 0.17 eV. Finally, the O atom locating on the ZrO2 surface tend to migrate to the adsorbed C atom at Pt2-Pt4 bridge site of Pt4 cluster to form CO via TS9, in which the Zra-O bond has broken and the O-C distance shortens to 2.640 Å. The final formed CO locates at the bridge site of Pt2-Pt4 forming two C-Pt bonds with bond lengths of 1.953 and 2.041 Å. The reaction barrier and energy for this step are 5.39 and -1.29 eV.

4. DISCUSSION Figure 9 presents the potential energy surfaces of the two reaction pathways (red line for pathway 1 and black line for pathway 2) for CO2 reforming of CH4 reaction. As shown in Figure 9, CH4 is favored to dissociate to CH3 with a low barrier of 1.05 eV and then transforms to CH by sequential dehydrogenation needing to overcome the higher energy barriers of 1.81 and 1.89 eV for each step. In pathway 2 (black line), the dehydrogenation of CH to C is very difficult for its kinetically quiet high energy barrier (2.45 eV), and then the process of adsorbed C combining with O (generating by CO2 dissociation) to produce CO is also less favorable because of its much higher barrier of 5.39 eV. Furthermore, the desorption of the formed CO to free gaseous form seems hard due to its strong binding with Pt4 cluster (-1.21 eV), indicating that CO prefers to accumulate on the Pt surface and thus hind the continues reactions. In contrast, for pathway 1, the reaction between CH and the carbonate species to

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generate the adsorbed CO and the formate species requires overcoming a small energy barrier of 1.72 eV, much lower than that of CH direct dissociation, demonstrating that the formate intermediate formation is more preferred. This results is in good agreement with the experimental facts by Lercher and co-workers,34 who reported formate species present on the support detected by i.r. spectra during the CH4/CO2 reforming reaction. Subsequently, the formate is decomposed to gaseous CO and an OH group which remains on the surface with a slightly higher barrier of 3.47 eV. According to foregoing analysis, it can be concluded that the pathway 2 proves to be the most favorable route for CO2/CH4 reforming and the rate-determining step for that is the decomposition of formate to CO and OH. We also investigate the reaction mechanism for CO2/CH4 reforming when CH4 and CO2 both adsorption on Pt4 following two above-mentioned pathways. Following pathway 1, CH reacts with CO2 to generate the adsorbed CHO and CO requiring overcoming energy barrier of 3.86 eV, much higher than that for CO2 adsorbed on the support. The potential energy surface as well as optimized adsorption configurations of the corresponding reactants, transition states (TS) and products for this step is shown in Figure S1. Following pathway 2, CO2 could not decompose to CO and O on Pt4 cluster which is well consistent with the experiment result that CO2 does not dissociate on Pt, even though dissociation did occur on Rh.34 Based on above results, it can be found that CO2 is activated on the support, whereas CH4 is activated on Pt, and these two activated species react with each other on the Pt-ZrO2 boundary by the bifunctional mechanism, which has been proposed by previous studies.19,25,34 In

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addition, it was reported that Pt-based catalysts, such as Pt/ZrO2 which could form carbonates on the support showed a much higher activity compared to the catalysts (Pt/SiO2) which could not form carbonates on the support.19 These results clearly indicate that the ZrO2 support plays an essential role for the activity of the catalyst in CO2/CH4 reforming reaction and it provides an unique adsorption site for CO2, the key for the formation of carbonate and formate species, which could not be realized on Pt4 cluster and some other supports. The finding will provide guidance for choosing highly efficient catalysts to catalyze CO2/CH4 reforming reaction.

5. CONCLUSIONS In this study, we investigate the mechanism for CO2 reforming of CH4 on the tetragonal ZrO2(101) supported Pt4 using density functional theory (DFT) calculations together with the periodic slab model. Two elementary reaction pathways for CO2/CH4 reforming have been searched and explored, and the most energetically favorable route is finally identified through the potential energy surface analysis. The preferable pathway includes the sequential dissociation of CO2/CH4 to CH species and CH reacting with the carbonate species to form the formate species, followed by its dissociation to gaseous CO and OH. It can be found that the formate species is the key intermediate and the rate-determining step is the decomposition of formate to CO and OH. Our results indicate that the ZrO2 support plays an essential role for the activity of the catalyst in CO2/CH4 reforming reaction and it provides an unique adsorption site for CO2, the key for the formation of carbonate and formate species, which could

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not be realized on Pt4 cluster and some other supports.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. The description of adsorption structures of some species involved in the CO2/CH4 reforming on Pt4/ZrO2 and potential energy surface for initial reaction step of CH and CO2 as well as optimized adsorption configurations of the corresponding reactants, transition states (TS) and products for this step on Pt4 surface of Pt/ZrO2.

ACKNOWLEDGMENTS This work was financially supported by Recruitment Program of Global Experts, and the Director Foundation of XTIPC, CAS, Grant No. 2015RC011. This work was also financially supported by Natural Science Foundation of Xinjiang, China, Grant No. 2016D01A073.

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Table 1. Calculated adsorption energies and key geometrical parameters (Å) of adsorption species involved in CO2/CH4 reforming reaction on Pt4/ZrO2. species

Eads (eV)

sites

dC-Pt (Å)

dC-O (Å)

CH4

0.02

top

2.743

CH3

-1.61

top

2.037

CH2

-4.16

bridge

2.011, 2.084

CH

-5.58

bridge

1.899, 1.892

C

-5.65

bridge

1.899, 1.862

CO2

-3.85

top

1.436

HCO

-4.32

top

1.312

0.04

top

CO-ZrO2 CO-Pt4

-1.21

bridge

dC-Zr (Å)

2.589 1.953, 2.041

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Table 2. Possible elementary reactions involved in CO2/CH4 reforming together with the activation energies (Ea) and reaction energies (∆H). reactions

transition state

Ea (eV)

△H (eV)

R1

CH4 → CH3 + H

TS1

1.05

0.34

R2

CH3 → CH2 + H

TS2

1.81

0.36

R3

CH2 → CH + H

TS3

1.89

1.02

R4

CH + CO2 → IM

TS4

1.72

1.21

R5

IM → HCO + CO

TS5

0.26

-2.34

R6

HCO → CO + H

TS6

3.47

1.08

R7

CH → C + H

TS7

2.45

0.66

R8

CO2 → CO + O

TS8

4.37

0.17

R9

C + O → CO

TS9

5.39

-1.29

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Figure Captions Figure 1. Schematic structure model (top and side views) for (a) the t-ZrO2(101) surface, (b) Pt4 cluster supported on t-ZrO2(101) surface. Red, white-blue and blue balls stand for O, Zr and Pt atoms, respectively. Figure 2. Most stable adsorption configuration of possible species involved in CO2/CH4 reforming on Pt4/t-ZrO2(101) surface. Gray and white balls stand for C and H atoms, respectively. See Figure 1 for color coding of other atoms. Figure 3. Potential energy surface for CH formation from sequential decomposition of CH4 on Pt4/t-ZrO2(101) surface.(CH4(gas) and CH4(ads) denote gaseous and adsorbed CH4, respectively.) Figure 4. Optimized adsorption configurations of the reactants, transition states (TS) and products for CH formation from sequential decomposition of CH4 on Pt4/t-ZrO2(101) surface. See Figure 2 for color coding. Figure 5. Potential energy surface for reaction pathway 1 about final products formation from CH on Pt4/t-ZrO2(101) surface. Figure 6. Optimized adsorption configurations of the reactants, intermediates (IM) transition states (TS) and products for reaction pathway 1 about final products formation from CH on Pt4/t-ZrO2(101) surface. See Figure 2 for color coding. Figure 7. Potential energy surface for reaction pathway 2 about final products formation from CH on Pt4/t-ZrO2(101) surface. Figure 8. Optimized adsorption configurations of the reactants, intermediates (IM) transition states (TS) and products for reaction pathway 2 about final products

23



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formation from CH on Pt4/t-ZrO2(101) surface. See Figure 2 for color coding. Figure 9. Potential energy surfaces for reaction pathway 1 and reaction pathway 2 involved in CO2/CH4 reforming on Pt4/t-ZrO2(101) surface. (CH4(g) and CH4(a) denote gaseous and adsorbed CH4, respectively.)

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

29



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Figure 6

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Figure 7

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Figure 8

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Figure 9

33


ZrO2(101) Surface: A

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