ZnO(101Ì0) toward Methanol Partial

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High Catalytic Activity of Pd1/ZnO(10-10) toward Methanol Partial Oxidation: A DFT+KMC study Takat Rawal, Shree Ram Acharya, Sampyo Hong, Duy Le, Yu Tang, Franklin (Feng) Tao, and Talat Shahnaz Rahman ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04504 • Publication Date (Web): 03 May 2018 Downloaded from http://pubs.acs.org on May 3, 2018

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0) toward Methanol Partial Oxidation: A DFT+KMC study High Catalytic Activity of Pd1/ZnO(10 Takat B. Rawal1, Shree Ram Acharya1, Sampyo Hong1,2, Duy Le1, Yu Tang3, Franklin Feng Tao3*, and Talat S. Rahman1,4,* 1

Department of Physics, University of Central Florida, Orlando, FL, 32816

2

Division of Physical Sciences, Brewton-Parker College, Mount Vernon, Georgia, 30445

3

Departments of Chemical and Petroleum Engineering and Chemistry, University of Kansas, Lawrence,

KS 66047 4

Donostia International Physics Center, Donostia-San Sebastian, 20018, Spain

*Corresponding authors; email [email protected] and [email protected]

Abstract We perform density functional theory (DFT) calculations of the energetics for several pathways associated with methanol partial oxidation (MPO) reaction on singly-distributed Pd on ZnO (Pd1/ZnO) and use them in kinetic Monte Carlo (KMC) simulations for elucidating reaction rates. We compare these results for Pd1/ZnO with those obtained for the same set of reactions on a 32-atom Pd16Zn16 nanocluster. Our KMC simulations show that Pd1/ZnO offers high, temperature dependent, selectivity (~93%) for H2 production and a moderate one (~76%) for CO2, in good agreement with experiment (which reports 90% and 85%, respectively). On the other hand, Pd16Zn16 yields no selectivity for H2 but almost perfect, temperature-independent selectivity (~100%) for CO2 and H2O leading to full oxidation of methanol. The high activity of Pd1/ZnO for MPO can be credited to the singly-distributed Pd sites and to the Pdmodified geometric and electronic structures of the neighboring Zn sites, and its high H2 selectivity may be related to the abundant supply of H atoms resulting from methanol decomposition on the surface. Pd loading has a decisive impact on adsorption and dissociation of methanol and oxygen. With higher Pd loadings, the activity of the Zn site alters in such a way that it provides weaker binding to methanol and stronger to O2, thereby resulting in facile O2 dissociation. Singly distributed Pd atoms not only serve as a more stable binding site for methanol than does Pd in Pd16Zn16, but also induces spontaneous CO2 formation and nearly spontaneous dissociation of H2O. In an alternate but slower pathway for production of CO2 involving HCOO* intermediate on Pd1/ZnO, the rate-limiting step is dissociation of H2COO*, followed by decomposition of HCOO* into CO2* and H*. Key Words: Single-site-catalyst, methanol-partial-oxidation, DFT+KMC, reaction rates, reactionpathways, Pd-Zn nanoalloy, Pd-ZnO catalyst

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1. Introduction Methanol partial oxidation (MPO) is an industrially important reaction for production of H2, an excellent source of clean energy in fuel-cell technology. The overall reaction is CH3OH + ½ O2 → CO2 + 2H2, proceeding through several intermediate steps ultimately leading to the production of H2 and CO2 as desirable products, which compete with the undesirable counterparts H2O and CO, respectively. Cu-based catalysts exhibit high selectivity1,2, but they suffer from poor thermal stability3 owing to their pyrophoric nature in contact with air and sintering of metal ions at temperatures beyond 270 0C4. On the other hand, Pd/ZnO catalysts are more stable than Cu/ZnO in oxidative environments5. Furthermore, Pd/ZnO catalysts possess high selectivity for H2 and CO21, 6-8 making them suitable catalysts for methanol-based fuel cell applications9. Metal atoms on metal-oxide surfaces are known to form various phases – monometallic or bimetallic nanoclusters or even singly-dispersed atom sites, each having well-defined local bonding structures10. Formation of the singly-dispersed metal atom sites is particularly attractive as it offers a local environment distinct from the ones afforded by the larger bimetallic system or the oxide surface alone. Consequently, the reaction pathways on the singly-dispersed catalytic sites can be quite distinct from those found in the larger nanostructures11,12. Not surprisingly, these sites have been reported13-16 to offer high activity for various chemical reactions. If, ideally, only the metal atom sites are the active center for the catalytic process, they will help preserve homogeneity of reactions, and thereby increase selectivity for the desired product. Single-atom based catalysts are thus highly desirable not only for maximizing catalyst efficiency but also optimizing activity and selectivity. The catalytic activity of single-atom catalysts is, however, strongly affected by the local environments and the local electronic structures of the active sites17-20 and the need to ensure their stability. Pd/ZnO catalyst systems for MPO reaction have been investigated experimentally6, example, Cubeiro and Fierro

6, 21

21,22

. For

showed that low Pd loading on ZnO surface, i.e., 1% Pd/ZnO, yields high

selectivity for H2. Although these experiments have provided insights into the differential activity for various Pd loadings on ZnO, an in-depth understanding of the activity of single Pd atom loading on ZnO is still lacking. In addition, it is an important to understand why the activity of Pd/ZnO changes when Pd loading is increased. When a small number of Pd atoms are dispersed on ZnO, the resulting chemical activity for MPO might stem from either the single atomic sites or small Pd-Zn nanoclusters or a combination of the two, or even from the Zn sites surrounding the Pd atoms. With increasing Pd content on ZnO there is the possibility of forming PdZn nanoclusters or alloy whose active sites may have different activity from those of isolated Pd and Zn sites of the Pd1/ZnO system. Indeed, experimental surface microscopic techniques have confirmed the formation of PdZn nanoparticles and alloy in Pd/ZnO 2 ACS Paragon Plus Environment

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catalyst systems23,24. Epitaxially grown Pd particles embedded in layers of amorphous ZnO materials tend to form PdZn nanoparticles with a ratio of 1:123. A PdZn alloy with the same ratio of 1:1 was also identified25,26. These PdZn nanoparticles were found to be structurally and thermally stable in temperature range 473–873 K23. In addition, the (111) surface of the PdZn nanoalloy is the most stable27-29 , thus providing an alternate system with which we may compare the activity of Pd1/ZnO. In this study, we compare the catalytic activity of Pd1/ZnO(1010) and Pd16Zn16 nanocluster as model catalyst systems for MPO reaction. The reactions involve the several complex reaction pathways including decomposition of CH3OH, one of the key elementary steps in MPO. Ensuing questions are: what are the active sites for the reactions? Why does low Pd loading yield high selectivity for CO2 and H2? How can molecular oxygen be stabilized on the ZnO surface for further oxidation steps? Why does the Pd-Zn nanoalloy enhance the formation of H2O? What is an optimal pathway for MPO reaction to produce H2 and CO2? In this work, we address these questions from insights from the results of DFT calculations in combination with kinetic Monte Carlo simulation. Apart from the energetics for different reaction pathways, we also compare the adsorption properties of molecules on three systems: Pd1/ZnO, Pd16Zn16, and pristine ZnO. Dispersion of single Pd on ZnO is found to be critical not only for stabilizing the molecular oxygen on the surface that is essential for further oxidation steps but also for providing the active center to produce atomic hydrogens via C-H scission as well as holding the CO molecule for reaction with atomic oxygen. Finally, we contrast the selectivity for desired products (CO2 and H2) vs. undesired products (CO and H2O). The paper is organized as follows: we present our model systems in section 2, computational details in section 3, and present and discuss our results in section 4 and 5, and present our conclusions in section 6.

2.

Model Systems Our model systems of pristine ZnO(1010), Pd1/ZnO(1010) with an surface O atom substituted by

a Pd atom, and the Pd16Zn16 nanocluster are shown in Figure 1. As clear from Figure 1, each of the three systems provides a distinct local environment for MPO. Among the four low-index surfaces – (0001), (0001 ), (112 0), and (101 0) of the hexagonal wurtzite ZnO crystals with space group C6v, the non-polar (1010) surface has the lowest surface energy (it is ~1/2 that of the polar surfaces, (0001) and (0001)30, and slightly smaller than that of (1120)). The ZnO(1010) surface is formed by the cleavage of the same number of Zn and O-bonds at the surface, reducing the coordination of Zn and O atoms from four-fold to three-fold. These atoms form characteristic rows of ZnO dimers separated by trenches running along (1210) direction31. The pristine ZnO(1010) is constructed using our calculated equilibrium lattice constants of a=3.282Å and c=5.308Å 3 ACS Paragon Plus Environment

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which are in agreement with experiment (a=3.25Å and c=5.206Å)32,33 within 1% and 2%, respectively. Our supercell consists of a (4x4) unit cell to model ZnO(1010) and a 16 Å vacuum along z-direction between ZnO(1010) slabs to avoid spurious electrostatic interaction between the periodic images. To model a singly-dispersed Pd atom on ZnO(1010), we remove an O atom from the top O layer of the pristine ZnO surface and place a Pd atom in the oxygen vacant site. The non-equivalent Zn sites are labelled as Zn1, Zn2, Zn3, Zn4, and Zn5 (see Figure 1b). As a matter of fact, a Pd atom can also occupy a Zn vacancy site. However, our calculations show that the formation energy of the Zn vacancy (4.38 eV) is greater than that of the O vacancy (3.23 eV). We thus proceed with Pd substitution for an O vacancy on ZnO(1010) as the model system. Our calculated Pd-Zn distance for singly-distributed Pd atom on ZnO is 2.43 Å, which is in good agreement with an experiment (see Table S1). Our assumption is in accord with the inference from recent experiment (see Section S9 in supplementary information) by one of us on the stability of the Pd1/ZnO surface. Furthermore, our ab initio molecular dynamics (MD) simulations also find the Pd1/ZnO system to be thermally stable. We find neither significant geometrical distortions nor diffusion of single Pd atom to other surface or subsurface sites. The outcome of our ab initio MD simulation can be found in the supporting information (Figure S1a). We construct a Pd16Zn16 model nanocluster using a PdZn alloy (with 1:1 ratio) in the form of CuAu (L10) type tetragonal (and distorted) face-centered cubic crystal structure with the lattice parameters (a=4.11 Å and c=3.35Å, reported in Ref.34). The selection of atoms within a radius of 5 Å results in a bimetallic structure, which consists of a total of 16 Zn and 16 Pd atoms. This sub-nanometer cluster has six (111) facets, and it is kept in a box with size of 25Å x 25Å x 25Å. We consider one of the representative (111) facet (see Figure 1c) which exposes four Zn atoms and four Pd atoms. The nonequivalent top sites are labeled as Zn(1), Zn(2) and Pd(1), Pd(2) and Pd(3) (see Figure 1c). We consider the top and bridge sites as probable adsorption sites for of each molecule of interest. To gain insights into the finite-temperature stability of the Pd16Zn16 nanocluster, we performed the ab-initio molecular dynamics simulation. Our simulations indicate that the nanocluster undergoes no significant geometrical distortions or reconstruction up to 700 K. The total energy variation and temperature fluctuations for the nanocluster for the simulation time (16 ps) is reported in the supporting information (Figure S1b). The snapshots from the ab-initio MD simulation for the Pd16Zn16 system at T=550 K-700 K are displayed in Figure S2, which clearly suggest that the nanocluster may not segregate.


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Figure 1. A schematic representation of (a) pristine ZnO(1010), (b) Pd1/ZnO(1010), and (c) Pd16Zn16. In (a) and (b), the top and bottom panels show the top and side views, respectively. In (a), Zn atoms in the top layer and the layer below it are represented by Zn(T) and Zn(ST), respectively. In (b), one O atom in the top layer is substituted by a Pd atom. Gray, blue, and red balls represent Zn, Pd and O atoms, respectively.

3.

Computational Details

3.1 DFT Calculations We perform ab-initio calculations based on DFT as implemented in the VASP code35. Interactions between ionic cores and valence electrons are treated using the projector-augmented wave (PAW) method36. For electron-electron interaction, we adopt the generalized-gradient approximation with Perdew, Burke, and Ernzerhof (PBE) functional37. We use cutoff energy of 400 eV for the plane wave expansion. For geometry optimization, we relax all atoms in the supercell until the total energy is converged to 10-5 eV and the force on each atom reduces below 0.01 eV/Å. For integration over the Brillouin zone of pristine ZnO and Pd1/ZnO, we employ the (4x3x1) k-point mesh with irreducible kpoint generation according to the Monkhorst-Pack scheme38. We use only one k-point for the Pd16Zn16 nanocluster. For ab-initio molecular dynamics simulations for Pd1/ZnO and Pd16Zn16 systems, the atomic motion was described by using the Nose-Hoover thermostat dynamics39,40 for a canonical ensemble. The time step of 1fs was used for integrating the Newton’s equations of motion and the simulation was run for 16 ps. To calculate the formation energy for O and Zn vacancies of ZnO surface, we use the following

relations: E

E E E and E

E E E, , respectively, where Ef

is the formation energy, EN represents the total energy of the pristine ZnO system with N total number of 5 ACS Paragon Plus Environment

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atoms, E and E represent the energy of the ZnO system with O and Zn vacancies, respectively (where nZn and nO represent the number of missing Zn and O atoms, respectively), and E and E, denote the energy of O2 in gas phase and the energy per n atom of the bulk zinc. These calculations indicate that the formation of O vacancy is more favorable than that of Zn, thus rationalizing our choice of substitution of an O atom by a Pd atom in the model system. The adsorption energy (Ead) is calculated using Ead = E(Adsorbate/Slab or nanocluster) – E(Adsorbate) – E(Slab), where E(Adsorbate/Slab or Adsorbate/nanocluster), E(Adsorbate) and E(Slab or nanocluster)) represent the total energies of adsorbate/slab [or adsorbate/nanocluster], adsorbate and slab [or nanocluster] systems, respectively. Here, slab represents either pristine ZnO or Pd1/ZnO. The reaction energy (∆E) is calculated as E(FS) – E(IS), where FS and IS stand for final and initial states, respectively. The vibrational frequencies are calculated using the finite difference method. We use the Nudged Elastic Band (NEB) method41 to calculate the initial minimum energy pathways (MEPs) and then use the Climbing-Image NEB (CI-NEB) method42 for determining the final MEPs with a saddle point.

3.2 KMC Simulation We use the standard ab-initio KMC approach43-46 to perform KMC simulation for which the kinetic parameters are derived from DFT calculations. Details on the algorithms of our KMC code can be found elsewhere43. The rate constants (R) for reaction, desorption and diffusion are calculated using the following relation:

R = A exp ( −Ea /k BT ) ,

(1)

where A, Ea, kB, and T are the pre-exponential factor (a.k.a. prefactor), activation energy, Boltzmann constant, and absolute temperature, respectively. The rate constants ( R ′ ) for adsorption of reactant species are calculated using the following relation:

R′ =

s% P exp ( −E a /k BT ) , σ 2πMk BT

(2)

where s% , σ , P, and M are the sticking coefficient, site density, pressure and mass of the reactant species, respectively. We have chosen the sticking coefficients for each reactant species to be unity. We include reverse process for every elementary reaction (except for the desorption process). This inclusion assures detailed balance in our KMC simulations. The pre-exponential factors for the elementary processes of Pd1/ZnO system are calculated using the simple vineyard approximation47:


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N

∏υ

IS ( or FS ) i

i =1

,

N −1

∏υ

(3)

TS i

i =1

IS (or FS ) FS where υi and υi are the set of vibrational frequencies of the equilibrium and saddle point

configurations, respectively. For the elementary steps of Pd16Zn16 system, we use the standard prefactor (1013 s-1). As shall be seen later, most of the calculated prefactors for the Pd1/ZnO system are in fact in the order of 1013 except only for a few processes. Thus, the standard prefactor used for the nanoparticle system should be a reasonable alternative. If RH2, RH2O, RCO, and RCO2 represent the reaction rates at steady state of the product molecules, the selectivity for desired products (H2 and CO2) is calculated using S

!" !" #!"

and

S$

!%

(4)

!% #!%

If not otherwise specified, in our KMC simulations reported here, the mesh consists of 40x30 grid and up to 2x1010 KMC iterations are performed for temperatures in range 243 0C to 352 0C. After 109 iterations, steady-state statistics is collected to obtain the reaction rate and the selectivity. The pressure for adsorption of methanol and oxygen from their gas phase are 2x10-4 and 1x10-4 bar, respectively (feed ratio 2:1). The catalytic sites on Pd1/ZnO surface are by nature inhomogeneous and every Zn site is distinct from the others (Figure 1a). However, we do not use a site-specific grid in our KMC simulations. Rather, we used a homogeneous grid which treats all sites, both Pd and Zn sites, alike. Thus, our KMC grid is more or less a mathematical grid for solving the master equation for the entire reaction network made up of the elementary steps. One consequence is that site-specific local statistics is not available from our KMC simulations.

4.

Results

4.1 Adsorption of reactants and products on Pd1/ZnO, Zn16Pd16, and pristine ZnO We begin with the results for adsorption properties of reactants (CH3OH and O2) and products (CO, CO2, H2 and H2O) on Pd1/ZnO, Pd16Zn16 and ZnO. In Table 1, we summarize the energetics and the selected bond lengths for molecular adsorption at various inequivalent top and bridge sites, as shown in Figure. For simplicity, we discuss only the key sites of these model systems. Figure 2 shows the preferred adsorption geometry for the reactant and product molecules on Pd1/ZnO, while those on Pd16Zn16 are provided in Figure S3 and S4, and those on pristine ZnO are summarized in Figure S5.


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Figure 2. DFT-optimized structures of adsorbed molecules on Pd1/ZnO: a) CH3OH, b) O2, c) CO, d) CO2, e) H2 and e) H2O. Top and bottom panel shows the top and side views, respectively. Gray, red, light blue, black, pink, and green balls represent the Zn, O, Pd, C, O (of adsorbed molecules), and H atoms, respectively.

4.1.1 Methanol We find that CH3OH adsorbs preferentially at the Zn4 site on Pd1/ZnO (see Figure 1b and Figure 2a), with an adsorption energy of -1.02 eV, which is almost the same as our calculated value (-0.97 eV) for pristine ZnO(1010) ( in good agreement with previous study (-1.12 eV) by Pala et al.48). The small difference in adsorption energy between ours and theirs (0.1 eV) can be ascribed to usage of different DFT functionals (PW91 vs. PBE) for electron-electron interaction. Overall, our results suggest that CH3OH adsorption at Zn sites is significantly modified by the presence of a Pd atom. Our calculations further show that CH3OH adsorbs less strongly at a singly-distributed Pd site (Ead= -0.58 eV). Thus, on Pd1/ZnO (and on pristine ZnO) the energetically favorable adsorption site is Zn. On the other hand, on Pd16Zn16, CH3OH preferentially adsorbs at the Pd(3) site (see Figure 1c and Figure S4a) with an adsorption energy of -0.40 eV. Note that at the Zn(1) site on the nanoparticle it adsorbs weakly with an energy of 0.21 eV, almost ~5 times weaker than the adsorption at Zn4 site of Pd1/ZnO. (The difference might be attributed to the fact that the surface Zn sites of Pd1/ZnO are more cationic than those in Pd16Zn16, as we shall see.) At Pd-Zn bimetallic site, CH3OH adsorbs weakly with an energy of -0.10 eV, indicating that the adsorption strength is ~6 times weaker than that of Pd1/ZnO (see Table 1). Our results thus indicate that the higher the Pd loading the weaker the CH3OH adsorption (ZnO ≥ Pd1/ZnO > Pd16Zn16). Such characteristic may have an impact on CH3OH conversion. Indeed, an earlier experimental study6 has shown that the catalytic activity is strongly related to Pd loading and that the higher content of Pd causes the reduction of the catalytic activity of Pd/ZnO. Similarly, other studies49,50 showed that for 15 wt% Pd/ZrO2 CH3OH conversion reached to 18% whereas for 1% Pd/ZrO2 it amounted to 82%, indicating that smaller loading of Pd on the oxide substrate gives rise to higher CH3OH conversion.


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Table. 1 Comparison of adsorption energies of MPO reactants and products at various sites on Pd1/ZnO, Pd16Zn16, and pristine ZnO. For clarity, specific adsorption sites are provided in parenthesis for each system (the sites are depicted in Figure 1). For pristine ZnO, all molecules adsorb on either on top of Zn atoms or on bridge sites in the top layer. The energies and distance (d) are expressed in eV and Å, respectively.

Adsorbates

Sites

Pd1/ZnO

Pd16Zn16

Pristine ZnO

-1.02 (Zn4) d(O-Zn4)=2.08 d(O-C)=1.44 d(O-H)=1.04 -0.58 d(O-Pd)=2.28 d(O-C)=1.45 d(O-H)=0.97

-0.21 (Zn(1)) d(O-Zn(1))=2.38 d(O-C)=1.44 d(O-H)=0.99 -0.40 (Pd(3)) d(O-Pd(3))=2.33 d(O-C)=1.45 d(O-H)=0.97 -0.10 (Pd(3)-Zn(2)) d(O-Zn(2))=2.58 d(O-Pd(3))=3.17 d(O-C)=1.44 d(O-H)=0.99 -1.13 (Zn(1)-Zn(1)) d(O-Zn(1))=1.92 d(O-O)=1.42

-0.97 (Zn(T)) d(O-Zn(T))=2.06 d(O-C)=1.43 d(O-H)=1.04 --

Reported values in literatures Pd(111)

CH3OH

Zn

Pd

Pd-Zn

O2

Zn-Zn

-0.63 (Pd-Zn2) d(O-Zn2)=2.41 d(O-Pd)=2.42 d(O-C)=1.46 d(O-H)=0.98 +0.13 (Zn4-Zn5) d(O-Zn4)=2.11 d(O-Zn5)=2.11 d(O-O)=1.31

Pd-Pd

Pd-Zn

CO

Zn

-0.83 (Pd-Zn2Zn4) d(O-Pd)=2.04 d(O-Zn2)=2.22 d(O-Zn4)=2.10 d(O-O)=1.36 -0.30 (Zn5) d(C-Zn)=2.15 d(O-C)=1.14

-0.79 Pd(1)-Pd(2) d(O-Pd(1))=2.05 d(O-Pd(2))=2.06 d(O-O)=1.37 -1.35 (Pd(3)-Zn(2)) d(O-Pd(3))=2.02 d(O-Zn(2))=2.03 d(O-O)=1.48

-0.25 (Zn(1)) d(C-Zn(1))=2.03 d(O-C)=1.15

PdZn(111) -0.3251

ZnO -0.4051 -1.1052 -0.9953

-0.37 54 -0.2555

-0.1851

-0.0256

+0.37 d(O-Zn(T))=2.15 d(O-O)=1.27 -0.2357

--

-0.35 (Zn(T)) d(C-Zn(1))=2.16 d(O-C)=1.14


-0.3258

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CO2

H2

Pd

-1.43 d(C-Pd)=1.91 d(O-C)=1.16

Zn

--

Pd

-0.18 d(O-Pd)=2.43 d(O-C)=1.17

Pd-Zn

-0.12 (Pd-Zn2) d(O-Zn2)=2.13 d(C-Pd)=2.04 d(O-C)=1.22 -0.12 (Zn4) d(H-Zn4)=2.91 d(H-H)=0.76 -0.53 d(H-Pd)=1.82 d(H-H)=0.82

Zn

Pd

H2O

Zn

Pd

-0.64 (Zn2) d(O-Zn2)=2.26 d(O-H)=1.02 -0.57 d(O-Pd)=2.30 d(O-H)=0.98

-1.53 (Pd(3)) d(C-Pd(1))=1.91 d(O-C)=1.16 -0.01 (Zn(2)) d(O-Zn(2))=3.51 d(O-C)=1.17 -0.10 (Pd(3)) d(O-Pd(3))=2.55 d(O-C)=1.17 -0.06 (Pd(3)-Zn(2)) d(O-Zn(2))=2.03 d(C-Pd(3))=2.13 d(O-C)=1.21 -0.04 (Zn(2)) d(H-Zn(2))=3.29 d(H-H)=0.75 -0.21 (Pd(2)) d(H-Pd(2))=1.77 d(H-H)=0.87 -0.24 (Zn(1)) d(O-Zn(1))=2.26 d(O-H)=1.02 -0.42 (Pd(3)) d(O-Pd(3))=2.35 d(O-H)=0.98

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--

-1.3659 -1.4460

-1.1161

-0.5362 -0.8363

-0.15 d(O-Zn(T))=2.86 d(O-C)=1.18 --

--

-0.3162

-0.13 d(H-Zn(T))=2.91 d(H-H)=0.76 --

-0.58 (Zn(T)) d(O-Zn(1))=2.08 d(O-H)=1.04 --

-0.2451

-0.3751

-0.29 55

4.1.2 Oxygen On Pd1/ZnO, O2 preferentially adsorbs at Pd-Zn2 site (see Figures 1b and 2b), with an adsorption energy of -0.83 eV, with its axis parallel to the surface with one O atom sitting at the Pd-Zn2 site and the other bonded to Zn atom in the top layer. At the Zn4-Zn5 site (see Figure 1b), O2 does not bind at all (Ead=+0.13 eV). In contrast, on Pd16Zn16, O2 bind comparatively strongly at the Pd(3)-Zn(2) site with an energy of -1.35 eV. At Zn(1)-Zn(1) (Figure 1c), it adsorbs with an energy of -1.13 eV. On the other hand, O2 does not bind at all on pristine ZnO (Ead=+0.37 eV). These comparisons (see also Table 1) clearly indicate that O2 adsorption strength follows the following trend: Pd16Zn16 > Pd1/ZnO >> ZnO, which is opposite to that for CH3OH. It indicates that the more the Pd loading, the stronger the O2 adsorption at the Zn site. As shall be described later, in Pd1/ZnO the Zn atom next to Pd becomes less cationic than the 10 ACS Paragon Plus Environment

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Zn atoms which are further away. Similarly, Zn in Pd16Zn16 is expected to be even less cationic as there each O is replaced with Pd. Therefore, O2 should interact more strongly with the less cationic Zn species, presumably owing to better availability of electrons at Zn sites. The various adsorption characteristics of O2 have remarkable impact on O2 dissociation. Our DFT calculations show that the activation energy for O2 dissociation on Pd16Zn16 (0.36 eV) is much lower than that (1.1 eV) on Pd1/ZnO. Obviously, the stronger adsorption lead to easier O-O bond breaking. In particular, as the Pd content increases, the binding strength for O2 increases and the activation energy for O2 dissociation decreases. It is thus evident that Pd plays as a modifier of Zn’s chemical and catalytic properties. While defect-free Zn is completely inactive for O2 adsorption and dissociation, Zn surfaces with increasing Pd becomes more favorable for O2 adsorption and dissociation.

4.1.3 Carbon Dioxide and Hydrogen (Desired Products) CO2 can be formed via reactions such as that between O* and CO* or the abstraction of H from HCOO* intermediate64-65. Before looking into these reaction pathways, we first examine the adsorption property of CO2 on all three model systems. On Pd1/ZnO, CO2 weakly adsorbs at the Pd site (Figure 2d) with an adsorption energy of -0.18 eV via O-Pd bonding. The distance d(O-Pd) is 2.42Å. In this configuration, the orientation of adsorbed CO2 is almost vertical. It can also adsorb with parallel configuration (i.e. with respect to the surface) with same adsorption energy (Ead=-0.18 eV) via formation of O-Pd bond. On pristine ZnO, CO2 weakly adsorbs on top of a Zn(T) site (c.f. Figure 1a and Figure S5d) with Ead=-0.15 eV in parallel orientation with respect to the surface. On Pd16Zn16, it adsorbs at a Pd(3) site (c.f. Figure 1c and Figure S4d) with Ead=-0.10 eV in slightly tilted configuration via the formation of OPd bond. The above comparison thus allows us to conclude that while all these surfaces offer weak CO2 adsorption, the binding is somewhat stronger on Pd1/ZnO. On Pd1/ZnO, H2 adsorbs at the Pd site (Figure 2e) with an adsorption energy of -0.53 eV. On the other hand, it weakly adsorbs at a Zn4 site with Ead=-0.12 eV. Pristine ZnO also offers a similar adsorption to H2 at the Zn site with Ead=-0.13 eV. On the other hand, on Pd16Zn16 H2 binds at Pd(2) site (see Figure 1c and Figure S4e) with an energy of -0.21 eV, indicating that the adsorption for H2 on Pd16Zn16 is weaker than that on Pd1/ZnO. Thus, the single Pd atom offers slightly higher stabilization of H2 than the other two systems. The stronger Pd-H bond on Pd1/ZnO as compared to that on Pd16Zn16 could be attributed to the enhanced anionic nature of the Pd site in the former.

4.1.4

Water and Carbon Monoxide (Undesired Products) On Pd1/ZnO, H2O preferentially adsorbs at the Zn2 site (Figure 1b and Figure 2f) with an

adsorption energy of -0.63 eV. The second preferred site is singly-distributed Pd atom with Ead=-0.57 eV. 11 ACS Paragon Plus Environment

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On the other hand, on Pd16Zn16, it adsorbs at the Pd(3) site (Figure 1c and Figure S4f) with Ead=-0.42 eV. The adsorption strength is even weaker (Ead=-0.24 eV) when it adsorbs at Zn(1) site (Figure 1c and Figure S3f). To the pristine ZnO, it binds at the Zn(T) site (Figure S5f) with Ead=-0.58 eV. Thus, the Zn sites of Pd1/ZnO and ZnO offer stronger adsorption than the Zn sites of Pd16Zn16. Such difference in adsorption properties among those Zn sites in these systems can be understood in terms of the local environment and the electronic structure. The adsorbed H2O does not form hydrogen bond owing to the absence of O atom in Pd16Zn16 cluster whereas there is formation of hydrogen bonds (1.54 Å and 1.71 Å) in H2O/ZnO and H2O/Pd1/ZnO, thus resulting in weaker binding of H2O on the cluster than on the other two systems. The formation of hydrogen bonds also affects the overall adsorption characteristics of H2O. Indeed, the helium-TDS data66 reveals a sharp, well-defined desorption peak at 367 K (Ead=-1.02 eV) for the higher coverage of H2O on the surface. Another undesired product in MPO is CO, which must be converted either into CO2 or other less toxic carbonates. It is therefore important to determine its favorable adsorption site and energetics. We find that on Pd1/ZnO, CO preferentially adsorbs at the singly-distributed Pd site, with an adsorption energy of -1.43 eV. At the Zn5 site (Figure 1b and Figure 2c), it weakly adsorbs with Ead=-0.30 eV. It does not adsorb at a Pd-Zn2 site, showing unsuitability of a bridging configuration. Not surprisingly, during structural relaxation, CO at the Pd-Zn2 site spontaneously migrates to the single Pd site. Similarly, CO also migrates spontaneously from Zn4-Zn2 bridge site to the Pd site, indicating that CO is stabilized by singly-dispersed Pd atom, which is also the active site for reaction between O* and CO* as shall be discussed later. Note that the above conclusion of CO migration from the ZnO surface to Pd atom has already been suggested in the analysis of temperature programed desorption (TPD) data65. On Pd16Zn16, CO binds strongly at the Pd(3) site (Figure 1c and Figure S4c) with an adsorption energy of -1.53 eV. At Zn(1) site, it weakly adsorbs with Ead=-0.25 eV. On the other hand, on pristine ZnO, the binding strength for CO (Ead=-0.35 eV), in agreement with experiment (-0.36 eV)67, is the weakest of the three model systems (see Table 1). The above results indicate that CO needs to overcome large desorption energy barriers of 1.43 eV (Pd1/ZnO), and of 1.53 eV (Pd16Zn16) on the respective model catalysts. In contrast, it only needs 0.35 eV for desorption from pristine ZnO. As CO binds more strongly onto a surface with more Pd content, Pd loading on ZnO plays a decisive role for CO2 formation via O*+CO*→CO2*, thereby increasing the selectivity for CO2.

4.2 Co-adsorption and Vibrational Properties In Table 2, we summarize our results for the adsorption energetics of molecules, intermediates and products, either singly adsorbed or co-adsorbed with other intermediates at the energetically favorable 12 ACS Paragon Plus Environment

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sites, and the vibrational frequencies of adsorbates on Pd1/ZnO. Our results show that the CH3OH does not gain energy when co-adsorbed with O*, but gains 0.1 eV when co-adsorbed with OH*. On the other hand, CH3O* gains energy significantly, by 0.75 eV and 1.41 eV, when co-adsorbed with O* and OH*, respectively. Similarly, CH2O* also gains energy by 0.27 eV and 0.14 eV, when co-adsorbed with O* and OH*, respectively. Adsorption of formyl (CHO) is also found to be stronger when it is co-adsorbed with O* and OH* (see Table 2). Thus, these results indicate that co-adsorption plays an important role for enhancing the binding strength of the intermediates.

Table 2. Adsorption and co-adsorption energies and vibrational frequency of reactants, intermediates and products of MPO at the energetically favored sites on Pd1/ZnO. Preferred

Adsorption

Co-adsorption

Frequency

adsorption

energy (eV)

energy (eV)

(cm-1)

site CH3OH

3758, 3059, 2984, 2935, 1464,

(gas)

1453, 1429, 1334, 1137, 1058, 1005, 301

O2(gas)

1560

CO(gas)

2128

CO2(gas)

2363, 1317, 626, 621

H2(gas)

4350

H2O(gas)

3847, 3734, 1586

CH2O

2858,2833,1763,1488,1222,11 43

O*

Pd-Zn2-Zn4

-4.06

-5.63 (with H*)

445, 295, 293

H*

Osurf

-2.68

-3.95 (with O*)

3562, 753, 662

OH*

Pd-Zn2

-2.80

-4.08 (with H*)

3735, 763, 442, 384, 319, 115

H2O*

Zn2

-0.64

3727, 2872, 1577, 883, 524, 422, 215, 152, 63

O2*

Pd-Zn2

-0.83

CO*

Pd

-1.43

CO2*

Pd

-0.18 (weakly

1013, 411, 357, 263, 212, 121 -1.45 (with H*)

2036, 378, 275, 239, 58, 40 2363, 1316, 611, 610, 89, 50,


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adsorbed) H2*

Pd

-0.53

CH3OH*

Zn4

-1.02

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35, 28, 3 3304, 1196, 837, 279, 231, 157 -0.86 (with O*)

3074, 3029, 2973, 2440, 1487,

-1.14 (with OH*)

1453, 1451, 1426, 1145, 1143,

-1.08 (with H2O*)

1047, 1008, 308, 222, 149, 145, 116, 79

CH3O*

Pd-Zn2

CH2O*

Pd-Zn2

CHO*

Zn2-Osurf

-2.37

-1.00

-2.84

-3.03 (with O*)

3001, 2969, 1917, 1442, 1432,

-3.69 (with OH*)

1421, 1136, 1106, 1042, 408,

-2.67 (with H2O*)

276, 140, 99, 71, 40

-1.27 (with O*)

3000, 2902, 1493, 1299, 1149,

-1.14 (with OH*)

1022, 639, 415, 292, 226, 197,

-1.26 (with H2O*)

106

-3.21 (with O*)

2990, 1528, 1342, 1026, 989,

-3.88 (with OH*)

533, 286, 239, 118

-2.85 (with H2O*) HCOO*

Pd-Zn2-Zn4

-2.92

-3.25 (with H*)

2954, 1558, 1356, 1289, 997, 716, 305, 274, 253, 130, 104, 98

H2COO*

Pd-Zn2-Zn4

--

2951, 2925, 1456, 1325, 1186, 1088, 1055, 852, 615, 409, 344, 288, 239, 165, 140

CH2OH*

Pd-Zn2

Not stable

CHOH*

Pd-Zn2

Not stable

4.3 Dehydrogenation of Methanol There is a consensus68-72 that, on solid catalyst surfaces, the dehydrogenation of CH3OH to CO and H occurs via methoxy (CH3O) as the first intermediate, and followed by stepwise hydrogen abstraction to formaldehyde (CH2O), formyl (CHO) and CO. We thus considered these reaction pathways for the dehydrogenation of CH3OH whose energetics are summarized in Figure 3. On Pd1/ZnO, the dissociation of CH3OH* into CH3O* and H* via O-H bond breaking is slightly endothermic (∆E=+0.07 eV and Ea=0.14 eV). On the other hand, C-H bond breaking of the resulting CH3O* is energetically favorable (∆E=-0.13 eV, Ea=0.56 eV). The minimum energy pathways for the reactions CH3OH*CH3O*+H* and CH3O*CH3O*+H* will be shown later (Figure 5). Further


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decomposition of CH2O* into CHO* and H* is again slightly endothermic (∆E=0.11 eV, Ea=0.79 eV). Finally, the activation of C-H bond breaking is favorable (CHO*→CO*+H*; ΔE=-0.23 eV, Ea=0.13 eV). On Pd16Zn16, the dissociation of CH3OH* into CH3O* and H* via O-H bond breaking is exothermic (∆E=-0.34 eV and Ea=0.35 eV), while the dissociation of CH3O* into CH2O* and H* is endothermic (∆E=+0.59 eV, Ea=1.33 eV). The decomposition of CH2O* into CHO* and H* is slightly endothermic (∆E=+0.50 eV, Ea=1.17eV). Finally, the process of C-H bond breaking of CHO* is highly exothermic (CHO*→CO*+H*; ∆E=-1.54 eV, Ea=0.42 eV). Interestingly, the barriers for these reactions at different sites are not the same. The energetics profiles for the dehydrogenation of the species: CH3OH, CH2O and CHO are provided in the supplementary information (Figure S6-S8). On pristine ZnO, the dissociation of CH3OH* into CH3O* and H* via O-H bond breaking is endothermic (∆E=+0.44 eV and Ea=0.60 eV). In addition, the C-H bond breaking of CH3O* is not energetically favorable (∆E=+0.89 eV, Ea=1.35 eV). The further decomposition of CH2O* into CHO* and H* is exothermic (∆E=-0.26 eV, Ea=1.56 eV). On the other hand, the process of C-H bond breaking of CHO* is not favorable (CHO*→CO*+H*; ΔE=+0.63 eV, Ea=2.78 eV). As shown in Figure 3, the location of the transition states at the higher potential energy profile clearly suggest that pristine ZnO has poor reactivity toward the successive dehydrogenation of CH3OH. As summarized in Figure 3, the activation energy barriers for each reaction step involved in dehydrogenation of CH3OH on Pd1/ZnO are lower than those for the same processes on Pd16Zn16 and on pristine ZnO. In addition, the potential energy associated with formation of CH2O* from CH3O* dehydrogenation on Pd1/ZnO is less uphill than that on both Pd16Zn16 and pristine ZnO (The efficient production of CH2O* species is essential since, as shall be shown later, its reaction with O adatom is highly exothermic with relatively smaller barrier for the case of Pd1/ZnO than that for the case of Pd16Zn16, thus suggesting an important role of this species for the partial oxidation). As the potential is downhill, the barrier for the forward process, i.e. CH3O*CH2O*+H*, will be smaller than that for the backward process, i.e. CH2O*+H*CH3O*, thus the former process may be favored over the latter. Considerations below of the kinetics of the entire reaction network indicate that Pd1/ZnO is optimal for dehydrogenation of CH3OH although the eventual (intermediate) dissociation reaction of CHO* into CO* and H* is more favored on Pd16Zn16 than on Pd1/ZnO. Indeed, Cubeiro et al.6, 21 show that 1% Pd/ZnO greatly facilitates the dehydrogenation of CH3OH, producing CO as the intermediate species. Moreover, Chin et al.73 show that with the increase of Pd content the formation of CO decreases owing to the formation PdZn alloy. This is related to the reduction of the adsorption strength of CH3OH with increasing Pd content, as described earlier in section 4.1.1.


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Figure 3. Potential energy profile for the successive dehydrogenation of CH3OH to produce atomic hydrogens on three model systems. The red, green, and blue bars represent the initial and final states for the reaction steps on Pd1/ZnO, Pd16Zn16, and pristine ZnO, respectively. The gray, pink, and black bars represent the corresponding transition states, respectively.

4.4 Formation Energetics of Products We now turn to gauge the chemical activity of Pd1/ZnO, Pd16Zn16, and pristine ZnO toward the key selective reactions for formation of both desired products (CO2 and H2) and undesired products (CO and H2O). The energetics for the formation of these products are described in section 4.4.1 and 4.4.2.

Figure 4. DFT-calculated energetics for formation of a) CO2 and b) H2 on: Pd1/ZnO, Pd16Zn16 and pristine ZnO. The gray, pink, and black bars represent the transition states for the reactions on those systems, respectively.

4.4.1 Formation of CO2 versus CO


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In Figure 4(a), we show the potential energy profile for the formation of CO2(g) on Pd1/ZnO, Pd16Zn16, and pristine ZnO, via the reaction between CO* (from dehydrogenation of CH3OH, as described earlier in section 4.3) and O*. On Pd1/ZnO, the reaction is spontaneous and strongly exothermic (∆E=1.89 eV (Table 3)), followed by desorption with a barrier of 0.18 eV [which is 7.9 times smaller than the barrier (1.43 eV, see Table 3) for CO desorption on this system]. One may thus expect that CO2 formation on Pd1/ZnO will be both thermodynamically and kinetically preferred. In contrast, there is a barrier of 0.49 eV (Table 4) for the formation of CO2* on Pd16Zn16 (slightly exothermic by 0.07 eV (Table 4)), followed by a small barrier (0.1 eV, see Table 4) for the conversion of CO2* to free CO2. On this nanoalloy, CO desorption has a barrier of 1.53 eV (Table 4). CO2 formation is thus energetically favored over that of CO also on Pd16Zn16. On pristine ZnO, the barrier for reaction between CO* and O* (exothermic by ∆E =-4.7 eV, as shown in Figure 4a) is comparatively high (1.0 eV in Figure 4a), suggesting a relatively low probability for the process. Note that the value of ∆E for the reaction between CO* and surface (lattice) O is only -0.21 eV, while that between CO* and O* (adsorbed O) is -1.89 eV, thus indicating that the latter reaction is thermodynamically favored. In addition, the latter process is spontaneous whereas the former is estimated to have a barrier of 0.8 eV or higher, indicating that the reaction between CO* and O* is also kinetically favored over that between CO* and lattice O by a large margin.

4.4.2 Formation of H2 versus H2O In Figure 4(b), we show potential energy profiles for formation of H2(g) on Pd1/ZnO, Pd16Zn16, and pristine ZnO. On Pd1/ZnO, the energy barrier (0.91 eV in Table 3) for the reaction H*+H*→H2* is higher than that on the other two systems: Pd16Zn16 and pristine ZnO. The barrier for the backward process is 0.57 eV, and for desorption it is 0.53 eV. In contrast, on Pd16Zn16, H2 is more likely to decompose rather than desorb as the barrier for the reaction H*+H*→H2* is 0.66 eV (Table 4), much higher than that (0.1 eV, see Table 4) for the backward process (i.e. H2*→H*+H*). In other words, the barrier for the dissociation of H2* on Pd16Zn16 turns out to be ~5.7 times smaller than the barrier (0.57 eV) for the same process on Pd1/ZnO, indicating H2* dissociation to be energetically more favorable over H2* formation on Pd16Zn16. Overall, these results suggest a higher selectivity for H2 formation on Pd1/ZnO as compared to that of Pd16Zn16. On the other hand, on Pd1/ZnO, the reaction H*+OH*→H2O* is not energetically favored over the dissociation of H2O. Since, as shown in Table 3, the barrier for the formation of H2O* is 0.63 eV, which is much larger than the barrier (0.08 eV) for the backward reaction H2O*→H*+OH*. The barrier for H2O* dissociation is even smaller than the barrier (0.64 eV) for desorption of H2O*. This clearly 17 ACS Paragon Plus Environment

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indicates that H2O* dissociation is energetically and kinetically favored. In contrast, on Pd16Zn16 the formation of H2O* from H* and OH* needs to overcome a barrier of 0.68 eV, which is slightly smaller than that (0.72 eV) for the dissociation of H2O*. Thus, the H2O* on Pd1/ZnO is likely to dissociate into H* and OH* instead of desorption to free H2O while on Pd16Zn16 the formation of H2O is preferred, suggesting the reactivity of Pd1/ZnO toward the formation of H2O(g) is significantly lower than that of Pd16Zn16. Note that the presence of H2O significantly hinders the formation of CO* on Pd/ZnO64. On Pd1/ZnO, H2 does not chemically bind to the Zn sites as the distance between H and Zn atom is 2.91 Å and the H-H internal bond is 0.76 Å. At the Zn site, H2 interacts weakly with the surface as can be understood from the adsorption energy of -0.12 eV. Moreover, H2 adsorption on surface O is unstable and instantly decomposes into H atoms, one of which adsorbs at the top of surface O and the other at the Zn site.

4.5 Elementary Reaction Processes for MPO We present the calculated energetics and rate constants for various elementary reaction steps used in our KMC simulations in Table 3 for Pd1/ZnO system and in Table 4 for Pd16Zn16. The minimum energy pathways for the reactions between the selected carbon-contained intermediate species, which are produced from the successive dehydrogenation of CH3OH, with O* on Pd1/ZnO are provided in Figure S9-S12 while those for the reactions on Pd16Zn16 are displayed in Figure S13-S16. The profiles for the dissociation of H2COO* and HCOO* are given in Figure S13-S14. Similarly, the energetics profiles for the reactions between the intermediate species with OH* on Pd1/ZnO are given in Figure S19-S22 while those for the reactions on the nanocluster are shown in Figure S23-S24. We proceed now to discuss the selected reaction steps in Table 3 for the Pd1/ZnO model catalyst. Table 3. A list of elementary processes used in KMC simulation of MPO for the Pd1/ZnO system. Reactions steps

∆E (eV)

R1

CH3OH(g)→CH3OH*

-1.02

R2

O2(g)+*→O2*

-0.83

R3 R4 R5 R6

O2*+*→O*+O* CO*→CO(g)+* CO*+O*→CO2*+* CO2*→CO2(g) +*

0.71 1.43 -1.89 0.18

Activation Prefactor (s-1) energy (eV) [For forward process] 0.0 1 (Sticking Probability) 0.0 1 (Sticking Probability) 1.10 0.959×1014 1.43 0.1×1014 0.0 0.1×1014 0.18 0.1×1014 18

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Rate constant at 257 0C (s-1) Forward Reverse process process

1.01e+03 1.25e+00 1.00e+13 2.32e+11

2.86e+09 1.00e+13 6.73e-05 1.00e+13

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R7 R8 R9 R10 R11 R12 R13 R14 R15 R16 R17 R18 R19 R20 R21 R22 R23 R24 R25 R26 R27 R28 R29 R30 R31

H*+H*→H2*+* H2*→H2(g)+* H*+OH*→H2O*+* H2O*→H2O(g)+ * CH3OH*+*→CH3O*+H* CH3OH*+O*→CH3O*+OH* CH3OH*+OH*→ CH2O*+H2O* CH3O*+*→CH2O*+H* CH3O*+O*→CH2O*+OH* CH3O*+OH*→CH2O*+H2O* CH2O*+*→CHO*+H* CH2O*+O*→CHO*+OH* CH2O*+OH*→CHO*+H2O* CH2O*+O*→H2COO*+* H2COO*+*→HCOO*+H* CHO*+O*→HCOO*+* HCOO*+*→COO*+H* CHO*+*→CO*+H* CHO*+O*→CO*+OH* CHO*+OH*→CO*+H2O* H*+*→*+H* O*+*→*+O* CO*+*→*+CO* OH*+*→OH*+* O*+H*→OH*

0.34 0.53 0.55 0.64 0.07 -0.99 0.55

0.91 0.53 0.63 0.64 0.14 0.28 0.55

0.429×1015 0.1×1014 0.120×1013 0.1×1014 0.932×1013 0.945×1013 0.1×1014

5.40e+04 1.53e+08 1.89e+07 1.53e+07 5.35e+11 2.86e+10 1.01e+08

6.62e+07 1.00e+13 1.88e+12 1.00e+13 2.31e+12 2.89e+01 1.00e+13

-0.48 -0.62 -0.36 0.05 -1.80 -0.65 -0.31 -2.89 -1.77 1.04 -0.78 -0.77 -0.10

0.56 0.87 0.69 0.79 0.16 0.43 0.71 1.62 1.04 1.47 0.13 0.14 0.98 1.35 1.25 1.15 1.15 0.79

0.323×1013 0.411×1014 0.133×1013 0.356×1014 0.749×1014 0.136×1015 0.295×1014 0.152×1015 0.213×1013 0.589×1014 0.805×1016 0.775×1014 0.675×1015 1013 1013 1013 1013 1013

8.17e+07 1.25e+05 5.38e+06 6.64e+05 3.52e+11 1.24e+09 3.54e+06 1.91e-02 3.56e+03 4.41e-01 6.59e+11 5.35e+11 1.25e+04 5.51e+00 4.46e+01 3.61e+02 3.61e+02 6.70e+05

3.56e+03 2.90e-01 2.88e+03 1.89e+06 1.56e-05 1.54e+03 5.40e+03 1.06e-28 2.95e-13 1.24e+09 5.40e+04 5.40e+04 1.54e+03

-0.69

3.64e-01

Table 4. A list of elementary processes used in KMC simulation of MPO for Pd16Zn16 nanocluster system. Reactions steps

R(1) R(2) R(3) R(4) R(5) R(6) R(7) R(8) R(9) R(10)

CH3OH(g) + *→CH3OH* O2(g)+*→O2* O2*+*→O*+O* CO*→CO(g)+* CO*+O*→CO2*+* H*+H*→H2*+* H2*→H2(g)+* H*+OH*→H2O*+* H2O*→H2O(g)+ * CH3OH*+*→CH3O*+H*

∆E (eV)

-0.40 -1.48 -1.99 1.53 -0.07 0.56 0.56 -0.04 0.42 -0.21

Activation energy (eV) [For forward process] 0.0 0.0 0.36 1.53 0.49 0.66 0.56 0.68 0.42 0.60 19


Rate constant at 257 0C (s-1) Forward process

Reverse process

5.36e+09 1.26e-01 3.53e+08 8.17e+07 8.17e+07 2.87e+06 1.53e+09 3.54e+07

4.45e-09 1.00e+13 8.17e+07 1.02e+07 1.00e+13 6.63e+06 1.00e+13 4.37e+05

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R(11) R(12) R(13) R(14) R(15) R(16) R(17) R(18) R(19) R(20) R(21) R(22) R(23)

CH3OH*+O*→ CH3O*+OH* CH3OH*+OH*→ CH2O*+H2O* CH3O*+*→CH2O*+H* CH3O*+O*→CH2O*+OH* CH3O*+OH*→ CH2O*+H2O* CH2O*+*→CHO*+H* CH2O*+O*→CHO*+OH* CH2O*+OH*→CHO*+H2O* CHO*+*→CO*+H* CHO*+O*→CO*+OH* CHO*+OH*→CO*+H2O* H*+O*→OH*+* CO2*→CO2(g)+*

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-0.74

0.0

1.00e+13

1.89e+06

-0.26

0.21

1.24e+11

5.37e+08

0.46 -0.38 1.14

0.70 1.33 1.30

4.37e+06 8.24e+00 1.54e+01

6.60e+10 2.91e-03 3.52e+11

0.16 -1.11 -0.36 -0.85 -1.62 -0.29 0.70 0.10

0.37 0.72 0.76 0.44 0.41 0.54 0.56 0.10

4.35e+09 2.87e+06 1.24e+06 1.01e+09 1.88e+09 1.24e+08 8.17e+07 1.24e+12

1.24e+11 2.36e-04 6.67e+02 1.90e+01 3.60e-06 2.88e+05 1.87e+14 1.00e+13

Methoxy In successive dehydrogenation of CH3OH, the first intermediate produced is either methoxy (CH3O*) or its isomer hydroxymethyl (CH2OH*). On Pd1/ZnO, the CH2OH* path does not proceed since at the Pd-Zn2 site (see Figure 1c) it is not stable. Our DFT results thus indicate that the CH3O* path is the dominant one over the CH2OH*. So, rather than C-H bond activation, the process begins with O-H bond scission. As described earlier, the activation energy is 0.14 eV for decomposition of CH3OH via the O−H bond-breaking channel leading to CH3O*. This reaction, represented by R11 in Table 3, is slightly endothermic by 0.07 eV. On the other hand, CH3O* can be formed by the other reaction pathway (R12 in Table 3) and is exothermic by ∆E=-0.99 eV. Note that the formation of CH3O* via CH3OH* → CH3O*+H* and CH3OH*+O*→CH3O*+OH* is also suggested by prior study74 on Cu/ZnO. The former pathway is very common for dehydrogenation of CH3OH. Indeed, this path was reported by several studies for CH3OH steam reforming on Pd/ZnO5, 64, Pt1/ZnO75, Cu/ZnO/Al2O376 catalysts, and for simple dehydrogenation of CH3OH on Ti1/ZnO77. Indeed, a previous DFT study by Abedi et al.78 shows that on ZnO(1010) the decomposition of CH3OH via O-H bond-breaking route has a lower barrier of 0.6 eV than that via O-C bond-breaking one which faces a high barrier of 2.8 eV. The former route of O-H cleavage becomes easier since the O atom of the CH3OH* is bonded to a surface Zn+2 cation via its lone electron pair, and the H atom forms a hydrogen bond across the trench to a surface O2- anion. In fact, O-C bond of CH3OH was also found to be thermally stable at ~600 K on Pd(111)68.


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Figure 5. Minimum energy pathways for decomposition of a) CH3OH* and b) CH3O* on Pd1/ZnO. In b) LS represents the local minimum state.

Formaldehyde Formaldehyde (CH2O*) is also an intermediate formed in MPO. The interaction between CH2O and Pd/ZnO is strong, characterized by an adsorption energy of −1.0 eV. As summarized in Table 3, CH2O* can be formed via three reaction pathways: i) R14, ii) R15 and iii) R16, all of which are exothermic by -0.48 eV, -0.62 eV, and -0.36 eV, respectively. Previous studies have shown that the successive decomposition of CH3OH leads to CH2O* on 8, 79,80

Pd/ZnO

, Cu/ZnO74, and Cu/ZnO/Al2O381. It was also reported that the reaction between CH3O* and

O* yields CH2O* on Pd/ZnO64. In an alternative route, CH2O* can be produced by the hydrolysis of CH3O* via reaction R16 (see Table 3). If all these pathways (R14, R15, and R16) are active and thermodynamically favorable, then CH3OH conversion will be accelerated. Our DFT results, indeed, shows that all pathways for formation of CH2O* on Pd1/ZnO are exothermic with moderate barriers (see Table 3).

Formyl Formyl (CHO*) is another intermediate formed in MPO. The interaction between CHO and Pd1/ZnO is quite strong, characterized by an adsorption energy of −2.84 eV. It can be formed in the following ways: i) CH2O* dissociation (R17) which is slightly endothermic (∆E=+0.05 eV); ii) reaction between CH2O* and O* (R18) which is highly exothermic (∆E=-1.8 eV); and iii) reaction between CH2O* and OH* (R19) which is also exothermic (∆E=-0.65 eV). A prior study has shown that CHO* species are produced in the successive decomposition of CH3OH on ZnO surface.

Formate Formate (HCOO*) is one of the key intermediates possibly formed in CH3OH conversion, e.g., on Pd/ZnO64, Pt/ZnO82, and Cu/ZnO/Al2O376. Surface HCOO* species can be formed in different ways: 21 ACS Paragon Plus Environment

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(i) reaction between CHO* and O*, ii) dissociation of H2COO*, and (iii) decomposition of formic acid (HCOOH). The reaction between CHO* and O* must be immediately followed by reaction between CH3O* and O* (leading to H2), which has been proposed by an experimental study82,83. Here, we have considered only two reaction pathways for formation of HCOO* which will be described below. Our DFT results show that HCOO* binds on Pd1/ZnO through both O atoms of HCOO* such that one O is bonded with Zn and the other with the singly-distributed Pd atom with an adsorption energy of −2.68 eV. As summarized in Table 3, surface HCOO* species can be formed in various ways: i) H2COO* dissociation (R21) which is highly exothermic (∆E=-2.89 eV); ii) the reaction between CHO* and O* (R22) which is also exothermic (∆E=-1.77 eV). For H2COO* dissociation to occur, it needs to overcome a barrier of 1.62 eV. On the other hand, the reaction between CHO* and O* to form HCOO* faces relatively smaller barrier (1.04 eV) than that (1.62 eV) for H2COO* dissociation. Thus, the dissociation of H2COO* is the rate-limiting step for the efficient production of HCOO* whose dissociation yields CO2* and H*.

HCOO* vs CO* Path for CO2 Formation In this study, we have considered two possible key reaction routes for formation of CO2 via i) oxidation of CH2O*, followed by decomposition of resultant intermediate: CH2O*+O*→H2COO*→ HCOO*+H* → 2H*+CO2*, and ii) successive dehydrogenation of CH2O* (CH2O* → CHO* → CO*), followed by oxidation that ultimately produces CO2*. Although the first step in those two routes have the competing barriers (0.79 eV for R17 and 0.71 eV for R20, see Table 3), barriers of successive steps of the former are significantly larger than those of latter routes leading to complete dominancy of CO2* formation via the second route. Since the barriers for R21 and R23 are 1.62 eV and 1.47 eV, respectively, which are much larger than those barriers of 0.13 eV and 0.0 eV for R24 and R5, respectively. Thus, the formation of CO2 via HCOO* path is hindered whereas that via CO* path is energetically favored. These results suggest that the rate-limiting steps in the latter could be dissociation of H2COO*, followed by decomposition of HCOO* into CO2* and H*.

4.6 Reaction Kinetics on Pd1/ZnO and Pd16Zn16 We summarize the results of our KMC simulations for steady-state coverage of those species for various temperatures in Figure 6. As shown, the coverage of H and CO is equal at 283 0C. As the temperature increases, the CO coverage decreases. There is no trace of H2O on the surface during simulation.


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Figure 6. Steady-state coverage of H and CO.

Our KMC results of the selectivity for the products of MPO, namely H2, H2O, CO2 and CO, are summarized in Figure 7. The formation of these products on Pd1/ZnO is in good agreement with experimental findings6, 21. As shown in the Figure 7(a), the selectivity for H2 is much higher than that of H2O: at 243 0C, the selectivity for H2 is ~93% whereas that for H2O is ~7%. With increasing temperature, however, H2 selectivity tends to decrease to 86% at 263 0C, to 69% at 360 0C, while H2O selectivity increases substantially to 31%. This rise in H2O selectivity is linked to enhanced OH formation via O + H recombination. On the other hand, the selectivity for CO2 at 76% is not as high as that for H2. This drops further to 36% at 360 0C, at which point CO has a higher selectivity. Thus, our KMC simulation indicates that selectivity is strongly dependent on the temperature, yet Pd1/ZnO offers high selectivity of over 90% for H2 and fairly high near 80% for CO2. In a similar vein, preliminary experimental data from two of us (YT and FFT) shows that single-dispersed Pd on ZnO catalyst yields the high selectivity of 90% and 85% for H2 and CO2, respectively (at 290 0C). Our results may be compared with experimental findings of Cubeiro et al.21 who reported that the 1% Pd/ZnO catalyst yields H2 and CO2 products with high selectivity of 78% and 80%, respectively, during CH3OH partial oxidation at 350 0C.Thus, our calculated maximum selectivity for H2 and CO2 (93% and 76%, respectively) are in good agreement with experiment (90% and 85%, respectively). However, our predicted CO2 selectivity deviates from measured ones at high temperatures. As the temperature rises, H2O formation rises and CO2 formation drops. The remarkable drop of CO2 selectivity is in fact a result of competition for O species consumption during CO2 and H2O formation steps.


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Figure 7. Selectivity for products: a) H2 vs. H2O and b) CO2 vs. CO on Pd1/ZnO model catalyst as a function of temperature.

Figure 8. Selectivity for products: a) H2 vs. H2O and b) CO2 vs. CO on Pd16Zn16 model catalyst as a function of temperature.

For Pd16Zn16, as shown in the Figure 8(a), the selectivity for H2O (~100%) does not change with temperature in the range of study. This can be rationalized on the basis of spontaneous dissociation of adsorbed H2 (reverse of reaction R(6) in Table 4) rather than its desorption (R(7)) and the availability of OH* (initially from the spontaneous oxidation of CH3OH*, R(11)) that reacts with O* producing H2O* (R(8)) that desorbs (R(9)). As shown in Figure 8(b), the selectivity for CO2 is ~100%. The significantly high barrier (1.53 eV) for desorption of CO* (R(4)) and the small barrier (0.1 eV) for desorption of CO2* after its formation (Ea=0.49 eV) through CO* oxidation (R(5)) explains the higher CO2 selectivity. Our results above show that the formation of CO is inhibited by atomic Pd sites (at low temperature only), particularly on the PdZn nanocluster. In fact, Chin et al.73 showed that with the


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increase of Pd content the formation of CO decreases owing to the formation PdZn alloy. In the light of our results, we propose that the decrease of CO production in experiments is in fact a result of the formation of PdZn nanoparticles or alloy in the catalyst.

5

Discussion

5.1 Rationale for Higher Selectivity

Figure 9. Evolution of coverage of molecules during KMC simulations conducted at 283 0C: (a) Pd1/ZnO and (b) Pd16Zn16.

Table 5. Comparison of rates (molecules/second) for various reaction paths. The rates are calculated from KMC simulations performed at 283 0C. Here, negative rate means that the reverse process has a higher rate than the forward rate. Reactions CH3OH*→CH3O* + H* CH3OH* + O*→CH3O* + OH* CH3OH* + OH*→CH3O* + H2O* CH3O*→CH2O* + H* CH3O* + O*→CH2O* + OH* CH3O* + OH*→CH2O* + H2O* CH2O*→CHO* + H* CH2O* + O*→CHO* + OH* CH2O* + OH*→CHO* + H2O* CHO*→CO* + H*

Pd1/ZnO 3223 12 3 3233 3 3 3017 97 124 3949

Pd16Zn16 -524 2760 639 2911 0.4 -372 2800 16 9 2257 25


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CHO* + O*→CO* + OH* CHO* + OH*→CO* + H2O* CO* + O*→CO2* H* + OH*→H2O* H* + H*→H2*

-711 -0.5 1999 1987

56 64 2883 5500 1

5088

For rate constants of elementary processes in Tables 3 and 4 we note that for Pd1/ZnO dehydrogenation of CH3OH and its intermediates (CH3O, CH2O, CHO) has much higher forward rate than their oxidation (forward) counterparts. On the contrary, for Pd16Zn16, oxidation of CH3OH has a much higher (forward) rate than its dehydrogenation counterpart. Although the intermediates of CH3OH have higher dehydrogenation than oxidation rates, the reverse dehydrogenation rates are always much greater than the forward ones. Our KMC simulation results presented in Table 5 clearly show that dehydrogenation of CH3OH for Pd1/ZnO is by far faster than the oxidation pathway producing hydrogen atoms at a rate of 13,422 atoms per second whereas OH species are only produced at a rate of 693 molecules per second. The consequence is quite drastic. There are plenty of hydrogen atoms but there are insufficient numbers of OH species on ZnO which are needed for H2O formation. As a result, H2 formation is very fast but H2O formation is very slow. In comparison, for Pd16Zn16 because of the slow CH3OH dehydrogenation, H atoms are produced at a much slower rate of 7,444 atoms per second than that on Pd1/ZnO. (This difference in dehydrogenation of CH3OH can be seen in Figure 3) Owing to the active oxidation of CH3OH, OH molecules are produced at a rate of 2,491 molecules per second, which is much faster than that on Pd1/ZnO. Now when it comes to H2O and H2 formation on PdZn, surprisingly H + OH reaction is active (5,500 molecules/s) but the reaction between H* and H* is virtually deactivated (1 molecule/second). This drastic difference between the two reactions is not due to the population of reactants but due to the difference in the rates of the two. The forward and reverse rate constants for reaction between H* and OH* are 2.87e+06 s-1 and 6.63e+06 s-1, respectively, whereas those for H*+H* are 8.17e+07 s-1 and 1.00e+13 s-1, respectively (see Table 4). Thus, the reverse reaction between H* and H* is 5 times faster than the forward reaction (H2 formation). It is quite insightful to see why this happens. In Figure 4b the relative positioning of IS, TS, and FS is shown. The FS of H2 on the Pd16Zn16 cluster is very close to TS, thus indicating that H2 is prone to dissociation. Interestingly, in Table 1, H2 preferred adsorption site is Pd(2) with an adsorption energy of only ~0.2 eV. In comparison, H2 on Pd1/ZnO adsorbs at the same elemental Pd site with Ead=-0.53 eV. Clearly, the Pd site of Pd1/ZnO offers much stronger adsorption to H2 than Pd on Pd16Zn16. If the adsorption energy for H2 at Pd(2) were as strong as that for the molecule at Pd1/ZnO, H2 would have been quite stable. Such energetics along with different adsorption characteristics may be an important factor for a drastic difference in the selectivity of these two model catalyst systems for H2 and H2O as products. 26 ACS Paragon Plus Environment

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The strong temperature dependence of H2, and particularly CO2, selectivity stems from the recombination rates of H and O to form OH species. The OH production rate is 1,987 molecules/s at 283 0

C but it increases to 2,656 molecules/s (by a factor 1.33). Note that with almost the same ratio (1.5) H2O

selectivity increases in the same temperature range. Moreover, the dissociation of O2 is extremely fast on Pd16Zn16 at a rate of 98,430 molecules/s, whereas it is only 1,418 molecules/s on Pd1/ZnO. This process is another factor for active oxidation on Pd16Zn16 as it provides O atomic species needed for CO2 formation. Thus, CO2 formation is quite fast on Pd16Zn16. On the other hand, CO desorption rate is 495 molecules/s for Pd1/ZnO but only 0.3 molecules/s for Pd16Zn16. The drastic difference in CO desorption rates of the two systems stems from the competition with CO oxidation and somewhat from the energy barrier difference (1.43 vs 1.53 eV). The outcome of the contrasting preferred pathways of the two systems can be clearly seen from the evolution of the coverage in Figure 9. In case of Pd16Zn16, O species has the highest coverage throughout reaction, whereas it is H species in the case of Pd1/ZnO. At higher temperatures there exists a discrepancy between our calculated selectivity of H2 and particularly CO2 for Pd1/ZnO and the observed ones in the small Pd loading experiments. On the one hand, our calculations clearly show that at lower temperatures the system would show quite a high selectivity (see Figure 7) and thus the discrepancy could be just an issue of the temperature scale. On the other hand, the measured selectivity in experiments could have contributions from active sites other than singly-distributed Pd atoms whereas our results are just for singly distributed Pd and nearby Zn sites. Previous experiments showed that only with a small loading of Pd high selectivity for H2 could be obtained and at high loading CO formation decreases. From our calculations, it is quite clear that when PdZn nanoparticle forms, formation of H2 and CO are inhibited. Therefore, the observation can be explained by the formation of Pd atom sites at low loadings but the formation of PdZn nanoparticle with increased Pd loadings. This explanation is quite convincing considering the KMC results that CO formation increases quite substantially at higher temperature in the presence of singly-distributed atomic Pd sites. Singly-distributed Pd atoms form isolated Pd-Zn localities in which Pd forms a localized geometry of PdZn3 (one Pd atom is bonded to three Zn atoms). The Zn atoms bind both to Pd and O. On the other hand, Pd atom on PdZn nanoparticle forms a structure with multiple Pd-Zn alloy bonds, lacking O species as their nearest neighbors. This local geometry of Zn sites has a decisive impact on methanol adsorption and dissociation, as hydrogen bonding plays an important role in the adsorption and reaction of CH3OH. The local structure of Zn on PdZn nanoparticle cannot provide hydrogen bonding and thus adsorption and reaction is not so favorable. On the contrary, the local geometry formed by Zn in Pd1/ZnO is excellent for adsorption and dissociation of CH3OH as it has O and Pd as neighbors. The hydrogen 27 ACS Paragon Plus Environment

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bonding between H of CH3OH and lattice O (see Figure 2a) leads to weakening of the O-H bond of CH3OH, and thus facilitates its dissociation into CH3O* and H*. The binding affinity of O species to the catalyst systems also has direct influence on the propensity for oxidation of reaction intermediates and in turn the selectivity for final products in MPO. As our calculations indicated that with the increase of Pd contents, the binding affinity for O2 increases and the barrier for O2 dissociation decreases. It is thus evident Pd plays as a modifier of Zn’s activity for oxidation. Therefore, the catalytic activity (and the selectivity) of Pd/ZnO catalysts for CH3OH oxidation can be tuned with Pd contents. The principle of varying the size of metal atoms, in general, to tune the reactivity of the catalysts may be applicable to a wide variety of materials: metal surfaces84-87, oxide surface88-90, metal-oxide-supported metal nanostructures91-93, sulphide-supported metal nanoparticles94, and single-atom catalysts20, 95,96. In sum, Pd-Zn sites of Pd16Zn16 nanocluster are excellent for O-O bond breaking (O2 dissociation) but is moderate for O-H bond breaking (CH3OH dehydrogenation). Thus, Pd16Zn16 is excellent for full oxidation of methanol. The Pd-Zn sites of Pd1/ZnO, on the contrary, are excellent for O-H bond breaking but moderate for O-O bond breaking. Therefore, it naturally leads to partial oxidation of methanol with high selectivity for H2 (at lower temperatures). Thus, our DFT+KMC simulations offer a contrasting picture for the two systems. Lastly, an alternate pathway for production of CO2 involving HCOO* intermediate on Pd1/ZnO is virtually inactive because of very slow rate (1.5 molecules/second). The rate-limiting step is dissociation of H2COO*, followed by decomposition of HCOO* into CO2* and H* because of their very high energy barriers (1.4 ~1.6 eV).

5.2. Examination of Electronic Structure In order to gain insights into the origin of difference in catalytic activity of Pd1/ZnO with that of Pd16Zn16, we present the calculated density of states (DOS) in Figure 10. The DOS of the low-coordinated Pd atom (indicated by green-dotted circle in the inset, Figure 10a) in Pd16Zn16 is drastically different from that in Pd1/ZnO. The former is distributed broader, mainly from -3 eV to -1 eV from the Fermi level having many peaks indicating wider-energy-range hybridization arising from the multi-bonding nature in the nanocluster. In contrast, that of the singly dispersed Pd on ZnO (solid curve) in Figure 10a exhibits a narrow distribution with only three peaks for bonding with Zn below, with the major DOS distributed between -2 eV and -0.5 eV, closer to the Fermi level than that of Pd atom in Pd16Zn16. The signature peak of the frontier state, below the Fermi energy, located at ~0.5 eV is dominantly dx2-y2 in character. The most pronounced peak at -1 eV below the Fermi level is dominantly of dxy character, and is mixed with dx2-y2. These results indicate that the singly-dispersed Pd atom bears unique electron-rich features (with 28 ACS Paragon Plus Environment

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active electrons) near the Fermi energy. This feature must be responsible for providing stronger bonding to H2 (strong enough to stabilize but not too strong to dissociate it) because the Pd site without such a feature on PdZn nanocluster only provides a very weak adsorption. Note that the presence of the frontier states near the Fermi energy have been proposed to be critical for the reactivity of some other hybrid solid materials94, 97. Therefore, Pd1/ZnO can be expected to provide an enhanced reactivity which is quite different from that of Pd16Zn16.

Figure 10. Density of states of catalytically active (a) Pd atoms of Pd16Zn16 (dotted trace) and Pd1/ZnO (red solid trace) and Pd16Zn16 (green dotted); and (b) Zn atoms of Pd1/ZnO (red solid), of Pd16Zn16 (dashed), and of ZnO (blue dotted). (Both Pd and Zn atoms of Pd1/ZnO and Pd16Zn16 are indicated in inset).

Zn sites in Pd1/ZnO and Pd16Zn16 nanocluster also exhibit characteristics in DOS, as modified by Pd atoms. For example, while Zn in the former system has more or less similar features to that of pristine ZnO deep in the band, it shows a significant DOS between -1 and -2 eV. On the other hand, the DOS of Zn in the nanocrystal is strikingly different from that in Pd1/ZnO, with the major DOS shifted to the lower energy and its band width remarkably spread due to the formation of Pd-Zn alloy bonds. (Note that the height of the band is remarkably high.) On the basis of Bader analysis, we find that Zn atoms in Pd16Zn16 are much less cationic, being charged only about +0.3e whereas the Zn atom bonded to the Pd atom in Pd1/ZnO is charged +0.85e or +1.0e. The other Zn atoms bonded to O atoms in Pd1/ZnO are charged about +1.3e. Thus, Zn bonded to Pd is less cationic than those bonded to O. In comparison, single Pd on ZnO is charged -0.36e gaining electrons from surrounding Zn atoms. It is quite clear that Zn will become more cationic (or deficient of electrons) when it forms more Zn-O bonds and becomes less cationic when it makes more Zn-Pd bonds. Clearly O2 will favor (more strongly bind with) less cationic Zn site such as those on PdZn as they can provide more electrons. This trend simply proves the remarkable impact of Pd on the local electronic structure of Zn atoms on ZnO surface. It has been shown that the modified 29 ACS Paragon Plus Environment

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electronic structure of singly-dispersed sites and neighboring sites is one of the key factors for the enhanced catalytic activity99. In sum, the high activity and selectivity of Pd1/ZnO for MPO can be credited to singly-distributed Pd sites and to the Pd-modified geometric and electronic structures of its Zn sites.

6. Conclusions By means of DFT calculations, we have examined and compared the energetics of the adsorption of reactants (CH3OH, O2) and products (CO, H2O, H2, CO2) of MPO reaction and those of the selected elementary reactions on Pd1/ZnO(1010) with those facilitated by Pd16Zn16 and pristine ZnO. We find that Pd1/ZnO shows a remarkable, excellent activity as compared to Pd16Zn16 and pristine ZnO. The higher catalytic activity of Pd1/ZnO can be credited to the critical influence of the local environment (or geometry) and electronic structure of the active Zn sites modified by single Pd, and those of the active single Pd and Pd-Zn sites, thus resulting in optimal activity. Our ab-initio kinetic Monte Carlo simulations clearly reveal that for partial pressures of 1x10-4 bar for O2 and 2x10-4 bar for CH3OH and temperature of 283 0C, the selectivity for H2 is ~79% whereas that for H2O is merely 21%. For the same conditions, however, we find that the selectivity for CO2 is only 62% whereas it is 38% for CO. In contrast, the Pd16Zn16 system offers high selectivity of ~100% for H2O and CO2, thereby suggesting the full oxidation of CH3OH under the same conditions. Our comparative study of adsorption properties of molecules on the three systems, taken together with calculated energetics and kinetics for several reaction pathways on Pd1/ZnO and Pd16Zn16 model systems and the reaction rates from KMC simulation, indicate that the Pd1/ZnO efficiently catalyzes MPO reaction and offers high selectivity for H2. In contrast, the Pd16Zn16 nanocluster may efficiently catalyze the full oxidation of CH3OH. Our study thus provides a new avenue for designing single precious-metalatom catalysts based on the Pd1/ZnO model that will result in enhanced activity and selectivity for MPO for hydrogen energy. Our findings may also provide guidelines for future experiments on single-atombased catalysts, and our computational framework may encourage others to follow the same. Acknowledgments We are thankful to Lyman Baker for helpful comments and reading of the manuscript in its initial stages. The theoretical calculations (performed at UCF) were supported by DOE grant DE-FG02-07ER15842. Preliminary experimental data (taken at KU) which motivated the project were supported by DOE grant DE-SC0014561 and NSF Career Award grant CHE 1462121. We acknowledge the STOKES Advanced Research Computing Center at the University of Central Florida, the National Energy Research Scientific


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Computing Center (NERSC), and the Extreme Science and Engineering Discovery Environment (XSEDE) for high performance computational resources.

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