Quantum Chemical Molecular Dynamics Study of the WaterâGas Shift

Article pubs.acs.org/JPCC

Quantum Chemical Molecular Dynamics Study of the Water−Gas Shift Reaction on a Pd/MgO(100) Catalyst Surface Farouq Ahmed,† Ryuji Miura,‡ Nozomu Hatakeyama,‡ Hiromitsu Takaba,‡ Akira Miyamoto,‡ and Dennis R. Salahub*,† †

Department of Chemistry and Institute for Sustainable Energy, Environment and Economy, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4 ‡ New Industry Creation Hatchery Center, Tohoku University, 403, 6-6-10, Aoba, Aramaki, Aoba, Sendai 980-8579, Japan S Supporting Information *

ABSTRACT: The water−gas shift (WGS) reaction on a Pd/ MgO(100) catalyst surface was studied using the tight bindingquantum chemical molecular dynamics (TB-QCMD) method. Molecular adsorption of CO was observed. In contrast, we observed that H2O adsorption occurs first molecularly but the molecule then dissociates on the surface. The resultant hydroxyl group reacts with preadsorbed CO to form an OCOH intermediate and a single H atom. This process is relevant as the initial hydroxylation step, and it is part of the catalyzed hydrolysis mechanism. During the molecular dynamics simulation the OCOH intermediate inverted into an H−CO2 like molecule and finally HCO2 decomposed to CO2 and H. Later on, the resultant H interacts with the previously dissociated single H atom (H released from the H−OH dissociation) and forms the WGS product H−H molecule. It was observed that the CO2 desorbed from the supported Pd cluster while the H2 molecule remains attached to the Pd cluster during the simulation. The geometries and dissociation energies of water molecules were obtained and the type of adsorption assessed. Chemical changes, changes in electronic and adsorption states, and structural changes were also investigated through TB-QCMD calculations, which indicate that the metal-oxide interface plays an essential role in the catalysis, helping in the dissociation of water and the formation of the OCOH intermediate. The present study indicates that the MgO(100) support has a strong interaction with the Pd catalyst, which may cause an increase in Pd activity as well as enhancement of the metal catalyst dispersion, hence, increasing the rate of the WGS reaction. Furthermore, from the molecular dynamics and electronic structure calculations, we have identified a number of consequences for the interpretation and modeling of the WGS reaction.

■

INTRODUCTION The water−gas shift (WGS) reaction, CO + H2O = H2 + CO2, provides a method for extracting the energy from toxic CO by converting it into usable H2 along with CO2, which is an important reaction in the fuel cell, automotive and chemical industries.1,2 More recently, use of the WGS reaction has increased due to its emerging applications for on-board purification and production of H2 for fuel cell vehicles. The reformed fuel contains 1−10% of CO, which degrades the performance of the metal electrodes utilized in the fuel cell systems. This is the reason why the water−gas shift reaction plays a key role in obtaining clean hydrogen for both fuel cells and other industrial applications. The WGS reaction has generated considerable interest due not only to its importance in terms of emissions control but also because its relative simplicity makes it an ideal, tractable heterogeneous catalysis problem. To obtain a fundamental knowledge of the factors that determine the activity of heterogeneous catalysts is a challenge for modern science because many of these systems are very complex in nature. In principle, when a molecule adsorbs on the surface of a heterogeneous catalyst, it can interact with a large number of © 2013 American Chemical Society

bonding sites. It is known that the chemical properties of these bonding sites depend strongly on the chemical environment around them. Thus, there can be big variations in chemical reactivity when going from one region to another on the surface of a heterogeneous catalyst. A main objective is to understand how the structural and electronic properties of a surface affect the energetics for adsorption processes and the paths for dissociation and chemical reactions. Nowadays, advances in instrumentation and experimental procedures have allowed a large series of detailed works on the surface chemistry of heterogeneous catalysts.3,4 In recent years, several studies have been undertaken to understand the mechanism of the WGS reaction, the catalytic behaviors and rational design of catalysts. Several experimental studies were carried out using surface science methods associated with catalysis and in-situ characterization of catalysts.5−8 In contrast, a few theoretical studies regarding the WGS mechanism9−12 were carried out using density functional theory, Received: November 5, 2012 Revised: February 14, 2013 Published: February 18, 2013 5051

dx.doi.org/10.1021/jp310946x | J. Phys. Chem. C 2013, 117, 5051−5066

The Journal of Physical Chemistry C

Article

On the other hand, among the precious metals, Pd is a relatively inexpensive and environmentally benign metal with well-known physical and chemical properties. Currently, precious metals like Pt-, Au-, and Cu-based catalysts have received much attention. Changing these to Pd-based formulations, both pure and supported on metal oxides, yields some of the most active and promising catalysts for the water−gas shift reaction; however, their performance is not fully understood and is highly dependent on the synthesis conditions or the nature of the oxide support. However, besides the factor of cost, the deactivation of these precious metal catalysts was also observed under the working conditions of the WGS reaction,37,38 where the strong interaction between CO and the metal sites eventually leads to a loss of metal surface area. For this reason, we embarked on a project to obtain a deeper understanding of the molecular events involved in the WGS reaction such that a more systematic and theory-guided approach may be used to design and select catalysts more efficiently. Additionally the choice of the support plays a very important role in the development of novel Pd catalysts. Metal-oxide interfaces also play a key role in a large set of technologically important applications such as metal-oxide contacts in microelectronics and photovoltaic devices, gas sensors, coatings for electrochemical devices, and oxide-supported, transition-metal catalysts. Moreover, nanoparticles of supported metal oxides have been shown to be very active for several reactions such as CO oxidation or the low-temperature WGS (WGS) reaction.39−42 The catalysis of the WGS reaction over MgO(100)supported metal catalysts has recently been the subject of numerous investigations. The growth of Pd on an MgO(100) single crystal has been studied by various analytical techniques, and the results show that the Pd grows as three-dimensional (3D) particles with different size distributions as a function of temperature and coverage.43−47 The interaction between Pd and MgO has been investigated theoretically,48,49 and a chemical interaction between Pd atoms and the MgO surface was predicted.50−53 It was observed that small Pd clusters supported on either thin MgO(100) films or single crystal MgO(100) reveal size effects for CO oxidation.54,55 For instance, the Pd/ MgO(100) system has been investigated as a model metal/ metal-oxide system.56,57 However, to date, information on the interface between Pd particles or atoms and an MgO surface from experiments is not available. This is because MgO is an electronic insulator having a band gap of 7.7 eV,58 which makes electron spectroscopic studies difficult because of the surface charging problem caused by incident electrons on the sample surface. However, to the best of our knowledge, there are no studies regarding quantum chemical molecular dynamics, which are able to simulate every step of the mechanism of the WGS reaction. Conversely, more studies are needed to collect detailed information on the electronic structure of Pd/MgO(100). Because of the wide range of feasibility, we have investigated the WGS reaction using a Pd cluster supported on an MgO(100) surface in this study. However, the results of several of these previous investigations suggest that the WGS reaction largely occurs via four specific mechanisms: (1) the redox mechanism,59−62 (2) the formate mechanism,63,64 (3) the associative mechanism,65,66 and, more recently, (4) the carbonate mechanism.67,68 In our present study we investigate which mechanism occurs in a quantum chemical molecular dynamics study. A kinetic model was proposed for the WGS reaction but no theoretical studies using quantum chemical molecular dynamics

molecular dynamics, and the kinetic Monte Carlo method. In many cases, these experimental studies have shown interesting and unique phenomena, but experimental studies of the individual steps during the WGS reaction are difficult.13 With recent improvements, computational studies are capable of providing qualitative insights into the surface catalysis.14 Moreover, theory is needed to unravel the basic interactions behind these phenomena and to provide a general framework for the interpretation of experimental results. Ideally, theoretical calculations based on quantum-chemical calculations have evolved to the point that one should be able to predict patterns in the activity of catalytic surfaces. Although WGS is a well-established industrial process, alternate catalysts are sought for fuel cell and automotive applications. Catalyst selection for the WGS reaction has, until recently, been based on trial-and-error screening of potential catalysts due to a lack of fundamental understanding of the catalyst’s functioning. In such applications, a successful WGS catalyst will have to possess high activity as well as good structural stability under all reaction conditions. However, existing catalysts do not meet these stability requirements, and many research programs have been undertaken in order to discover more stable and more active catalysts. There has been a substantial effort to elucidate the basic reaction pathways and catalytic characteristics of these systems to facilitate improvement to their design. Generally, Cu-based catalysts are used for industrial operations of the WGS at relatively low temperature (470−520 K).15,16 Currently, Cu-based catalysts, both pure and supported on metal oxides, fall among the more active and promising catalysts for this reaction, although their performance is not fully understood and is highly dependent on the synthesis conditions or the nature of the oxide support.6 However, there are problems in using these systems for automotive applications,17 which may result in condensation of water and subsequent deactivation of the catalysts. Extensive studies have been carried out to search for new WGS catalysts in both theory and experiment to replace the commercial Cu-based catalyst.18−22 Hence, advanced WGS catalysts that include high activity and stability become a real need for the development of automotive and fuel cell technologies. It was found that the drawbacks of Cu-based catalysts have been overcome by the use of precious metal-based systems, supported and unsupported Pt and Au,18−20,23 and their alloys.21,22 First-principles calculations describe the microscopic origins of the remarkable enhancement of the catalytic activity of gold nanoclusters adsorbed on a perfect MgO(100) surface or in the gas phase.24 It was also found that the Au/N-TiO2 (110) system was more active for the WGS reaction than undoped AuTiO2 materials,23,25 which is even better than Cu, the most active noble-metal catalyst. On the contrary, bulk metallic gold itself is not active as a WGS catalyst; it has difficulty in dissociating water, but can easily perform the subsequent steps of the WGS reaction. Calculation shows that titania is efficient for the dissociation of water, but binds O, formates, and carbonates too strongly. Moreover, supported platinum catalysts have received much interest during the past decade because of their WGS activity at low temperatures.26−33 The WGS reaction has been investigated over Pt nanoparticles (NPs) dispersed over various oxide supports: Al2O3, CeO2, ZrO2, CeO2/ZrO2, and TiO2.27−33 However Pt-based catalysts are the subjects of debate. Previous experimental34,35 and theoretical studies29,36 indicated that Pt(111) has problems in adsorbing and dissociating the water molecule and is not a good WGS catalyst. The dissociation of water on Pt(111) was calculated to be an endothermic process. 5052



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MD simulations was repeated to calculate the time development of the system. MD consists of the numerical, step-by-step, solution of the classical equations of motion. In the TB-QCMD an electronic structure calculation was performed by solving the Schrödinger equation (HC = εSC; H, C, ε, and S refer to the Hamiltonian matrix, eigenvectors, eigenvalues, and overlap integral matrix, respectively) with the diagonalization condition (CTSC = I; I refers to the identity matrix). In the TB-QCMD, a single-ζ Slater type basis set was employed and long-range Columbic interactions were computed by the Ewald method. Here the diagonal Hamiltonian Hrr is equal to the negative of the ionization potential (−Ir) of each atomic orbital. To determine the off-diagonal elements of H, Hrs, the corrected distancedependent Wolfsberg−Helmholz formula (eq 2) was used;

have yet been reported in the literature. To elucidate the molecular dynamics of the WGS reaction mechanism, a new theoretical work covering the adsorption, activation barriers, the dynamics of the dissociation of H2O, and comparison of changes in electronic structure is necessary. We have developed a new and powerful theoretical tool TB-QCMD to gain further insight into the dominant pathways on a catalytic surface as reactants become products and for the quantitative determination of the dynamics of the reaction. The purpose of the study is to determine the elementary steps of the WGS reaction and simulate the dynamics of individual steps of this reaction. The goal of this present research is to develop a comprehensive predictive model for the WGS reaction that is based solely on a detailed mechanism along with theories of surface−molecule interactions. Quantum Chemical Molecular Dynamics Method. We have used our in-house code (TB-QCMD)69−76 to investigate the electronic and structural change of a supported Pd cluster as well as the diffusion characteristics of Pd on the metal oxide support with periodic boundary conditions. The TB-QCMD method is based on a self-consistent charge (SCC) tight binding theory (TB-QC), which belongs to our in-house program “New Colors”69−76 and a classical molecular dynamics (MD) program “New-Ryudo”.69−76 TB-QCMD enables one to perform quantum chemical molecular dynamics calculations by reflecting the binding energy and atomic charges based on quantum chemistry calculations. The dynamics of atoms were carried out using the following potential function, which was employed to consider the ionic, covalent, and van der Waals interactions among atoms. U=

⎡ Z Z e2

∑∑⎢ i

+

i j

⎢ j > 1 ⎣ rij

Hrs =

K rs = {1 + krs(1 − Δ4 ) + Δ2 } × exp[−δrs{rrs − drs}]

(3)

and

Δ=

Hrr − Hss Hrr + Hss

(4)

Here, Srs is the overlap integral matrix element and d is the summation of the orbital radii. Moreover, κ and δ are parameters for the chemical bonding interactions. In the TB-QCMD simulator, the total energy, E, is expressed by the following equation,

⎛ ai + aj − rij ⎞⎤ ⎟⎟⎥ + f0 (bi + bj) exp⎜⎜ ⎝ bi + bj ⎠⎥⎦

occ

E=

∑ nkεk + ∑ ∑ k=1

i

j>1

zizje 2 R ij

+

⎛ aij − R ij ⎞ ⎟⎟ exp⎜⎜ ⎝ bij ⎠

∑ ∑ bij i

j>1

(5)

j>1

(rij − r0)]}

(2)

where

∑ ∑ Dij{exp[−2βij(rij − r0)] − 2 exp[−βij i

1 K rsSrs(Hrr + Hss) 2

On the right-hand side of eq 5, the first term is the summation of the eigenvalues for all of the occupied molecular orbital’s (nk and εk are the number of electrons which occupy the kth molecular orbital and its energy, respectively), the second term represents the long-range Coulombic interaction (Rij is the interatomic distance, Z is the atomic charge, and e is the elementary electric charge), and the third term corresponds to the short-range exchange repulsion energy (aij and bij are constants for each atom pair). The first term on the right-hand side of eq 5 is rewritten as follows,

(1)

The first term corresponds to the Coulomb potential, and the second term corresponds to the short-range exchange repulsion potential ( f 0 is a constant for unit adjustment, a is the size, and b is the stiffness), which gives a good account of the repulsive interactions arising from the overlap of electronic clouds. The third term in eq 1 corresponds to the Morse potential, which represents covalent interactions, where Dij is the bond energy, βij is the form factor, and r0 is the bond length at minimum energy. Using this potential determined by the above-mentioned scheme, a classical MD simulation was performed. This system can solve the equations of motion for a large set of atoms.77,78 The Verlet algorithm79 is employed to integrate the equations of motion. Moreover, the temperature scaling method implemented in the system is similar to the Woodcock algorithm.80 Hydrogen is the lightest atom in the periodic table so that the vibrational frequency of hydrogen is very high even at room temperature. So MD simulations were carried out with a 0.1 fs integration time-step for the equations of motion. Using tight binding (TB) calculations, Zi and Dij in eq 1 were determined for the conformation of atoms at certain steps (when we have establish the desired interaction between adsorbate and metal cluster) of the molecular dynamics. Further MD simulation develops a new atomic configuration to update Zi and Dij parameters. This combination of quantum calculations and

occ

occ

occ

∑ nkεk = ∑ ∑ nk(Ckr)2 Hrr + ∑ ∑ ∑ nkCkrCksHrs k=1

k=1

r

k=1

r

s

(6)

where the first and second term on the right-hand side refer to the monatomic contribution to the binding energy and the diatomic contribution to the binding energy, respectively (nk is the number of electrons occupied in the k-th MO). A binding energy calculated from the second term of eq 6 is used for the determination of the Dij parameter in eq 1. First-Principles Parametrization in Tight-Binding Quantum Chemical Calculation. In order to set the Hamiltonian matrix H and overlap integral matrix S in our TBQCMD simulator, exponents of a Slater-type atomic orbital (AO), denoted as ζr and valence state ionization potentials (VSIPs) for the 1s AO of H atoms, 2s, 2p AOs of C, O, and Mg atoms; 4s, 4p, and 4d AOs for Pd atoms were optimized. The 5053



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Figure 1. Computational model of Pd/MgO(100) catalyst system which was used in the TB-QCMD calculation: (a) Pd bulk model containing 32 atoms, (b) MgO bulk model containing 64 (32 Mg and 32 O) atoms were used in the validation of parametrization (Tables 1 and 2), (c) the top view of Pd/MgO(100) surface, and (d) the side view of Pd/MgO(100) surface.

latter were used for the diagonal element of H (Hrr or Hss in eq 4). The relationship between Hrr and VSIP of the r-th AO of the i-th atom (Iir) is described as Hrr = −Iir. These were represented by polynomial functions of the atomic charges. The ζr and Hrr were calculated by the polynomial functions of atomic charges described below by eqs 7 and 8, respectively.

optimizations were performed using the Vosko, Wilk, and Nusair (VWN) local density approximation functional,82 while the generalized gradient approximation (GGA) with Perdew− Burke−Ernzerhof (PBE) exchange-correlation functional83 was adopted for energy calculations. The PBE functional has been widely used for transition-metal systems and it shows good, though not perfect, performance for geometries (bond length), bond dissociation energies (binding energies), and activation energies (typical errors of 0.01−0.03 Å, 3−5 kcal/mol and 5−10 kcal/mol, respectively).84 Hybrid functionals, like B3LYP, should be avoided for metallic systems because of the wellknown pathological behavior of Hartree−Fock exchange for metals. The clusters we are studying have sufficiently closespaced energy levels that, we believe, caution is advised. Although hybrids perform very well for organic molecules and even for (some) organometallics, we prefer to stay at the GGA level. While the GGA functionals are not perfect, PBE has been widely used for solid-state systems and it should perform well in the present case. The accuracy of DFT predictions of reaction energy barriers85−87 has been assessed in a number of benchmark studies. GGA and meta-GGA functionals have absolute errors in the 5−10 kcal/mol range. While they still systematically underestimate activation energies, GGA and meta-GGA functionals usually yield transition state structures in good agreement with accurate post-HF correlation methods.88,89 Moreover, GGAs correctly lower the dissociation energies and reduce the overbinding errors to 5−10 kcal/mol.90

5

ζr = a0 +

∑ ak(Zi)k k=1

(7)

5

Hrr = b0 +

∑ bk(Zi)k k=1

(8)

In eqs 7 and 8, Zi corresponds to the atomic charge on the atom i. The parameters regarding ζr, that is, a0, a1, a2, a3, a4, and a5 in eq 7 and regarding Hrr, that is, b0, b1, b2, b3, b4, and b5 in eq 8, were determined so as to reproduce the binding energies and electronic structures of a H2O/CO/Pd/MgO(100) system obtained by DFT, which are summarized in Tables S1 and S2, respectively (see Supporting Information). Density Functional Theory (DFT) Method: for the Validation of the TB-QCMD Method. DFT calculations were performed to determine the parameters in TB-QCMD. The DMol3 code81 and double numerical basis sets with polarization functions (DNP) were employed. The parameters used in Tables S1 and S2 (Supporting Information) were tuned by comparison with DFT results (atomic charges, bond populations, atomic orbital populations and binding energies). The geometry 5054



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spin-magnetic moment94 and enhances the reactivity of small clusters. When adsorbed on alkaline supports, such as magnesia, surface defects serve as strong trapping centers for the clusters that maintain their open valence shells and finite spin moments.95 Model catalytic experiments and ab initio simulations indicated that even a single Pd atom adsorbed at a surface color center (FC) of the magnesia support can be catalytically active for CO oxidation with direct CO2 formation from either molecularly adsorbed oxygen or from a Pd− carbonate complex.96 The Pd13 cluster is a cuboctahedron with eight triangular faces and six square faces, which can stimulate the WGS reaction more efficiently than any other cluster. The Pd13 clusters are of special interest for the study of adsorption phenomena, because there are 3-fold as well as 4-fold hollow sites due to the existence of (111) and (100) microfacets that are distributed throughout the surface. The top layer of the MgO oxide surface and the metal nanoparticles were allowed to relax fully with the adsorbates. The Pd cluster is weakly bound and essentially can move freely on the oxide surface. The adsorption of intermediates involved in the WGS reaction (CO, H2O) was investigated. In all cases, both atop and high-symmetry sites were considered. In the calculations, the adsorbates were allowed to relax in all dimensions. Three-dimensional periodic boundary conditions were applied in the TB-QCMD simulation. A CO molecule was placed in the vacuum region of the unit cell above the surface. An initial distance between the CO and H2O molecule and top region of the Pd cluster was fixed to 5.97 Å and the initial velocity of the CO and H2O toward the supported Pd cluster was set to 1000 m/s. We have chosen the average velocity of the CO molecule (1000 m/s) from the Maxwell−Boltzmann distribution at 300 K. The lengths of the unit cell shown in Figures 1b were fixed to a = 16.84 Å, b = 12.63 Å, c = 41.47 Å, and α = β = γ = 90° during the simulation. As a first step we have stabilized the target model by carrying out an MD simulation up to 50000 steps. Before performing the simulation of the WGS reaction a single point tight binding quantum chemical calculation was carried out with the optimized structure. The atomic charges and Morsetype 2-body interatomic potentials were updated at different intervals where we have found the most up-to-date interaction between adsorbate and metal cluster during the MD simulation. After the calculation, again this model was transferred for a single point tight binding quantum chemical calculation and the MD simulation was resumed. A total of 31660 steps of MD simulations were performed with an integration time step of 0.1 fs to perform the water−gas shift reaction. Validation of Parameters Used in TB-QCMD Method. The accuracy of the parameters used in this calculation is evident in the comparison of the atomic charges, atomic orbital populations, bond populations, and total binding energies of the H2O/CO/Pd/MgO(100) system obtained by TB-QCMD and DFT. To validate the parameters shown in Tables S1 and S2 (Supporting Information), a Pd bulk model containing 32 atoms (Figure 1a) and a MgO bulk model (Figure 1b) containing 64 atoms (32 Mg atoms, 32 O atoms) were prepared. The cell parameters of the Pd bulk model were fixed during the calculations, which are a = b = c = 7.78 Å, and α = β = γ = 90°. Similarly the cell parameters of the MgO bulk model were also fixed during the calculations, which are a = b = c = 8.4224 Å, and α = β = γ = 90°. Using DFT, we have performed a geometry optimization of this model (Figure 1a,b). Hirshfeld atomic charges, found through density functional theory (DFT), were compared with atomic charges obtained by TB-QCMD. Atomic

Although spin in not explicitly treated in our TB-QCMD method, which ignores electron−electron interactions, spin effects are included through the parameters. DFT calculations were performed with unrestricted Kohn−Sham (UKS) and the fitting procedure was done with these spin-polarized results. The ionic cores were described by effective core potentials (ECP). A local basis cutoff of 5.0 Å in real space was employed. All the parameters for the TB-QCMD code were determined through first-principles DFT calculations. Parameters (δ, κ, a, and b) were set so as to reproduce the binding energy, bond distances, and bond angles of Pd−Pd (bulk model), Mg−O (bulk model), C−O, and H−O−H (H2O). The charge populations were analyzed by the Hirshfeld method. Binding energies, Ebind described by eq 9, were calculated as the energy difference between the total energy of the optimized H2O/CO/Pd/ MgO(100) system and the summation of total energies of the individual atoms present in the system. In eq 9, A, B, C, D, and E refer, respectively, to the total number of H, C, O, Mg, and Pd atoms. Adsorption energies, Eads, described by eq 10, were calculated as the energy difference between the total energy of the optimized CO/Pd/MgO (100) catalyst surface and the summation of the total energies of the isolated Pd/MgO(100) catalyst surface and the CO molecule. The adsorption energy of H2O was also calculated by the same procedure, by eq 11. E bind = E H2O/CO/Pd/MgO − (A ·E H + B ·EC + C·EO + D·E Pd + EMg ) Eads(CO) = ECO/Pd/MgO − (E Pd/MgO + ECO)

(9) (10)

Eads(H 2O) = E H2O/CO/Pd/MgO − (ECO/Pd/MgO + E H2O) (11)

Preparation of Calculation Model. The water−gas shift reactions were simulated by applying the TB-QCMD method to a target model of the Pd/MgO(100) supported catalyst system shown in Figure 1c,d as top and side views, respectively. The MgO(100) surface was represented by slabs generated from a super cell made of 4 × 3 unit cells, four layers thick. In this target model, a MgO(100) support consists of 96 Mg and 96 O atoms in the unit cell. The top two layers of the bulk surfaces were allowed to relax along with the adsorbates, while the bottom two layers were kept fixed at the calculated bulk lattice positions. From the TB-QCMD data on the structure of the MgO(100) crystal, we define the distance between a Mg ion and the nearest O ion to be 2.11 Å and the Mg−Mg distance is found to be 2.98 Å. It is generally accepted that the activity of particles in chemical reactions on various supports is strongly size dependent. Nanometer-scale metal particles on oxides often have novel properties physically and chemically, which are of wide applications in many areas, especially in catalysis. A cluster consisting of 13 Pd atoms was positioned on top of the MgO(100) surface. The pronounced chemical activity of small metal clusters is due to a combination of several factors, with their relative contributions strongly depending on cluster size and elemental composition.91,92 For transition-metal clusters, the highest occupied valence orbital is generally close to or within a manifold of d-derived states, and the average position and the width of such a “d band” dictates many of the characteristics of adsorption of molecules by means of covalent bonding.93 In gasphase Pd clusters, the closed-shell 4d10, 5s0 atomic configuration opens by means of significant s−d hybridization, which induces a 5055



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The calculated binding energies of O−H were −105.28 kcal/ mol using our TB-QCMD code and −104.16 kcal/mol calculated using DFT. The bond length of H−H calculated using TB-QCMD (0.738 Å) is close to that of DFT (0.749 Å) and the binding energy of H−H through TB-QCMD is −104.21 kcal/mol and through DFT is −103.38 kcal/mol. Thus, the geometry and the binding energy of bulk Pd, Mg−O, H−H, and O−H were in good agreement with those calculated using DFT. This demonstrates that TB-QCMD coupled with the firstprinciples parametrization is effective for describing the geometry and energy of various fragments related to the WGS reactions. We have employed a very short time step of 0.1 fs in order to reproduce the chemical reactions in the WGS reaction under the high velocity condition of adsorbate atoms. Our TBQCMD simulation has a timing advantage compared to the firstprinciples molecular dynamics simulation. The temperature in the simulation was controlled by scaling the atomic velocities to maintain a substrate temperature of 300 K, with the exception of the emitted molecules and fixed atoms.

charges of Pd (in the Pd bulk model) obtained by TB-QCMD were close to those found through DFT. Here TB-QCMD binding energy in Pd bulk was −2901.6 kcal/mol whereas the DFT value of −2895.3 kcal/mol. Table 1 shows that the atomic Table 1. Atomic Charge of Individual Atoms Obtained by DFT and TB-QCMD atomic charge element MgO Pd CO H2O

Mg O Pd C O H H O

DFT

TB-QCMD

0.41 −0.41 0.02 0.13 −0.15 0.1505 0.1505 −0.3010

0.38 −0.39 0.10 0.17 −0.17 0.1514 0.1514 −0.3028

■

charges (TB-QCMD) of the different atomic species such as Mg and O (in each atomic layer of the MgO), Pd, C, O (CO) and H, O (H2O) were close to those obtained by DFT. Table 2 shows

RESULTS AND DISCUSSIONS Preferred Adsorption Site of CO on Pd13/MgO(100). We have carried out a systematic TB-QCMD simulation considering different initial positions and different initial adsorption sites of CO with respect to the Pd/MgO(100) catalyst. The preferred adsorption site of CO on the MgO(100)-supported Pd13 catalyst depends on three factors: the metal cluster, the crystallographic faces, and the CO coverage in the metal cluster. The Pd13 cluster consists of three layers, with four atoms in the first layer, five atoms in the second layer, and another four atoms in the third layer. As a consequence, bridge or on-top site adsorption is possible on the top layer of the Pd13 cluster. In the present simulation, CO adsorption was carried out on two different adsorption sites, such as bridge and on-top sites. Figure 2a shows (initial to final position) the snapshots of CO adsorption on the bridge site, where CO adsorbed on the bridge site at 710 fs where two strong (−42.16 and −30.31 kcal/mol) bonds existed between Pd and C (shown in Figure 2a). We have carried out several hundred steps of MD simulation to investigate the change of bond energy that formed between Pd−C, but could not find any significant changes during the simulation, hence, confirming the adsorption of CO on the Pd cluster. Similarly, Figure 2b considers different initial positions of CO in the gas phase, such as above an on-top site. It was observed that at 780 fs, CO adsorbed at the on-top site where only one bond (−11.35 kcal/ mol) existed between Pd and C. Comparison of Pd−C binding energy at the on-top site with that of the bridge site indicates the greater probability of CO2 desorption from the on-top site by breaking the Pd−C bond. The energies are expressed relative to the clean surface and a free CO molecule in the gas phase. Up to now, there are different conclusions existing regarding the nature of the active Pd catalyst. Extraordinarily high catalytic activity of the supported Pd catalyst for CO oxidation arises from the reaction of CO adsorbed on the step, edge and corner sites on metallic Pd particles. The interface between Pd and the MgO oxide support is responsible for the high CO oxidation activity. Considering the above phenomena as well as the expected simulation target of this study, we have introduced CO above the on-top site of the supported Pd cluster and we investigate the reaction dynamics, adsorption, dissociation, and desorption in this section. Because of the industrial significance of the water−gas shift reaction, many researchers have investigated the reaction

Table 2. Comparison of the Bond Populations of Mg−O, Pd− Pd, Pd−O, Pd−C, and C−O at the Adsorbed State Obtained by DFT and TB-QCMD atomic pair

DFT

TB-QCMD

Mg−O Pd−Pd Pd−O Pd−C C−O H−O

0.33 0.27 0.30 0.21 0.78 0.70

0.30 0.26 0.29 0.15 0.70 0.69

the comparisons of the bond populations of Mg−O, Pd−Pd, Pd−O, Pd−C, C−O, and OH of the H2O/CO/Pd/MgO(100) model, which agreed well with those obtained through DFT. Table 3 compares the ratios of atomic orbital populations for individual atoms, where the ratios of atomic orbital populations for each valence orbital obtained by TB-QCMD was close to that obtained by DFT. Table 3. Comparison of Atomic Orbital Population of Individual Atoms of This System Obtained by DFT and TBQCMD method

element

s%

p%

d%

DFT TB-QCMD DFT

Pd Pd Mg O Mg O C O C O H O H O

5.40 5.43 38.43 25.82 38.07 26.48 46.36 28.50 45.19 27.83 100.0 26.93 100.0 26.31

2.40 2.75 61.57 74.18 61.91 73.51 53.63 71.49 54.81 72.17 0.00 73.07 0.00 73.69

92.20 91.81

TB-QCMD DFT TB-QCMD DFT TB-QCMD

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Figure 2. Determination of the preferred adsorption sites of CO on the Pd13/MgO(100) surface. Relaxed atomic configuration displaying several stages in the simulation of the adsorption of CO on the top facet of a Pd13 cluster supported on MgO(100), and the subsequent reaction with gaseous CO (a) Bridge site adsorption of CO: snapshots (i−iv) shows the different steps (0 to 710 fs) of the TB-QCMD simulation (b) On top site adsorption of CO: snapshots (i−iv) shows the different steps (0 to 780 fs) of the TB-QCMD simulation.

Figure 3. Relaxed atomic configurations displaying several steps of the adsorption of CO on the top facet of a Pd13 cluster supported MgO(100). Snapshots shows the TB-QCMD simulation results of different steps (0 to 780 fs): (a) initial model of CO adsorbed on Pd/MgO catalyst, (b) snapshot at 330 fs where CO approaches to the Pd cluster, (c) interaction of CO with Pd cluster, (d) adsorption of CO on the Pd13 cluster.

mechanism and developed models to reflect the behavior of the reaction over common industrial catalysts. We have observed the below-mentioned formate mechanism-taking place during our TB-QCMD simulation. We have summarized different steps of this reaction in the following discussion. 5057

CO(gas) − CO(ads)

(1)

H 2O(gas) − H 2O(ads)

(2)

H 2O(ads) − OH(ads) + H(ads)

(3)



Article

Figure 4. Relaxed atomic configuration displaying several stages in the simulation of the coadsorption of H2O on the top facet of a Pd13 cluster supported on MgO(100), and the subsequent reaction H2O with gaseous CO and Pd cluster: (a) introduction of H2O at the same initial position of CO on the gas phase, (b) H2O approaches to supported Pd cluster, (c) interaction of CO with H2O molecule; note the preferential orientation of the H2O and partial proton sharing, CO induced proton transfer resulting in formation of a hydroperoxyl like group and a hydroxyl, (d) finally adsorption of H2O on Pd13/ MgO(100).

CO(ads) + OH(ads) − OCOH(ads)

(4)

OCOH(ads) − CO2 (gas) + H(ads)

(5)

2H(ads) − H 2(gas)

(6)

because of their potential application.98,99 It is found that, for example, H2O adsorbs molecularly on clean Pd(100), Pd(110), and Pd(111) surfaces at low temperature100−102 and dissociates partially on an MgO(100) thin film.103 It is well-known that metal clusters deposited on metal oxides may have intriguing properties due to finite size effects, surface effects, or support effects. For instance, the Pd/MgO(100) system has been investigated as a model metal/metal-oxide system. All these distinctive characteristics of Pd clusters/particles on metal oxides stimulated us to explore the details of water adsorption. In a second step of our calculation, adsorption of H2O was investigated by introducing an H2O molecule in the gas phase above the CO/Pd/MgO (100) model at 780 fs, the aforementioned position where the CO molecule was previously adsorbed on the surface, which was discussed in Figure 4a. The adsorption energy of H2O through TB-QCMD was found to be −93.01 kcal/mol whereas the adsorption energy of H2O on TiO2 is estimated to be about 30.0 kcal/mol.104 H2O is adsorbed dissociatively on the titania surface, with an adsorption energy of 36.89 kcal/mol.105 The adsorption energy of H2O in a carbon nanotube is −111.61 kcal/mol.106 The adsorption energy of H2O obtained in this study is different from the above-mentioned value, which shows the effect of size, shape, and adsorption site of the Pd clusters. A snapshot at 840 fs (Figure 4b) shows that the H2O molecule approaches the Pd cluster. Later on a snapshot at 930 fs (Figure 4c) shows that the H2O molecule interacts with the previously adsorbed CO. Finally a snapshot at 1070 fs (Figure 4d) shows H2O adsorbed on the Pd cluster. The interaction between water and metal surfaces are dominated by a chemical bonding formed between the lone pair of the water molecules and the surface electronic states. As a result, the water-surface bond is rather localized, mostly in the contacting regions. Our results provide much insight into the fundamental water−metal interactions at the atomic to electronic scales. We have ascertained that an adsorbed CO molecule serves as an “attractor” of H2O to its vicinity, with the coadsorbed molecules forming a complex strongly bound to the Pd cluster. Hence, at the interface between the Pd cluster and the MgO(100) surface, peripheral sites show a high propensity to bind both H2O and CO.

In the formate mechanism, adsorbed water dissociates into an adsorbed hydroxyl group and adsorbed atomic hydrogen, that is, eq 3. The hydroxyl group then combines with adsorbed carbon monoxide to form adsorbed formate, that is, eq 4, which eventually decomposes into carbon dioxide and hydrogen via eq 5, yielding the WGS product, CO2. Finally, two atomic H combined to form H2, that is, eq 6. A Langmuir−Hinshelwood model has described the reaction mechanism over noble metals, where molecularly adsorbed CO reacts with hydroxyl (OH−) coming from the dissociative adsorption of HO−H on the surface and the formation of an OCOH intermediate is seen at the supported cluster interface. Adsorption of CO. The first step of the WGS reaction implies a successive adsorption of gas-phase CO that was shown in Figure 3. In Figure 3a, a snapshot at 0 fs shows the initial structure of the assigned model for the TB-QCMD simulation, where the CO is located above the top layer of the cluster. The distance between CO and the top region of the Pd cluster is equivalent to 5.97 Å. During the simulation a snapshot at 330 fs (Figure 3b) shows that the CO molecule approaches to the Pd cluster whereas a snapshot at 480 fs (Figure 3c) shows that CO interacts with the Pd cluster and finally at 780 fs (Figure 3d) CO is adsorbed. As the interactions between CO with the cluster take place, the binding energy becomes slightly debilitated compared to the original CO dissociation energy (−262.19 kcal/mol) but finally CO is adsorbed almost molecularly during the WGS reaction. This leads to the plausible assumption that the electronic structure of the free CO molecule is only slightly modified upon adsorption, that is, molecular adsorption of CO. The TB-QCMD adsorption energy of CO at the top site was −27.59 kcal/mol whereas the experimental adsorption energy of virtually the same molecular system was −30.0 kcal/mol.97 Adsorption of H2O. Many fundamental studies have been carried out on the adsorption of H2O on solid surfaces, principally on metal single crystals and ordered metal oxides, 5058



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Figure 5. Relaxed atomic configuration displaying several steps of the dissociation of H2O and formation of CO + OH intermediate on the top facet of a Pd13 cluster supported on MgO(100) (a) shows the adsorbed H2O strongly interacts with Pd cluster and started to dissociate at 1070 fs, (b) at1130 fs adsorbed H2O dissociates more frequently, and (c) at 1300 fs OH− part of the dissociated H2O interacts with the C atom of CO and formed CO + OH intermediate. In the coadsorbed configurations the O−H bond of the H2O molecule pointing toward the nearest oxygen atom of the CO elongates to 1.11 Å (d) at 1380 fs the C−O (O of O−H) bond becomes incredibly strong and bond length of C−O (O of O−H) becomes 1.049 Å.

Figure 6. Relaxed atomic configuration displaying several steps of the decomposition of CO2−H into CO2 and H atom on the top facet of a Pd13 cluster. (a) Snapshot at 1457 fs shows that the dissociated H atom started to interact with the previously dissociated H atom (H from H−OH dissociation), (b) at 1497 further dissociation take place between CO2−H, as a results H−H bond becomes stronger during the simulation, (c) at 1587 fs it was evidence from the H−H binding energy that there is very strong bond formed between H−H atoms where the respective bonding energy between H−H was −63.25 kcal/mol, and (d) rurther TB-QCMD simulation shows that at 1786 fs complete dissociation of the O−H take place and, hence, the H−H bond formed has the binding energy equivalent to H2 molecule.

Dissociation of H2O into OH− and H+. Similar to the CO chemisorption, the chemisorption of water is also an exothermic process and the released energy can be used to overcome the energy barriers for the dissociation of water, eq 3, the formation of the OCOH intermediate, eq 4, and its decomposition into CO2 and atomic H, eq 5. The markedly larger binding energies of the coadsorbed complex (compared to the individual adsorbates) reflect a synergistic effect, expressed through the occurrence of the aforementioned partial proton sharing and proton-transfer processes. Finally, in the formate mechanism adsorbed water dissociates into an adsorbed OH− and atomic H+, that is, eq 3. As soon as H2O is adsorbed (1060 fs) on the MgO(100) supported Pd cluster, adsorbed H−O−H strongly interacts with adsorbed CO and the Pd cluster and starts to dissociate at 1070 fs (Figure 5a). Figure 5b shows that (1130 fs) adsorbed H2O dissociates more frequently due to the interaction

with the preadsorbed CO molecule, hence, the interaction with the supported Pd cluster. At this stage, the O−H bond, which is dissociated, increases to the bond length of 1.09 Å, whereas the initial bond length was 0.973 Å. However, another OH bond remained constant (0.99 Å) up to this step of calculation. In addition the initial bond energy of the two O−H bonds of H2O was about −105.28 kcal/mol, whereas initially one bond becomes −46.47 kcal/mol. The assumption of the above properties confirms the dissociation of H2O into H+ and OH−. Furthermore, the dissociation of H2O into a surface hydroxyl and an adsorbed hydrogen atom was identified as the rate-limiting step. It has been found that strong metal-support interaction lowers the barrier of water dissociation. The strong MgO−Pd interaction is able to tune the electronic structure of MgO and therefore the activity toward water dissociation.107 It seems that 5059



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Figure 7. Here (a), (b), (c), and (d) show the snapshots of CO2 desorption from the Pd cluster. Relaxed atomic configuration display several steps of the desorption of O−C−O molecule and H2 molecule remain attached to the surface on the top facet of a Pd13 cluster supported on MgO(100): (a) snapshot at 2086 fs shows that O−C−O molecule closed with the Pd cluster and on track to dissociate from the Pd cluster; (b) snapshots at 3076 fs shows the CO2 like molecule that formed started to desorbed from the Pd cluster; and (d) snapshot at 3166 fs shows the O−C−O like molecule that formed completely desorbed into the gas phase.

resulting in adsorbed CO2 and atomic hydrogen, that is, eq 5. A snapshot at 1457 fs shows (Figure 6a) the dissociated H atom on track to interact with the previously dissociated H atom (originating from the dissociation of H2O). In the next step, at 1497 fs (Figure 6b), further dissociation occurs between CO2− H; as a result, bond formation occurs between the two H atoms. Later on the H−H bond becomes stronger during the simulation. At 1587 fs (Figure 6c), it was evident from the H−H bonding energy that there is a very strong bond formed between the two H atoms where the particular bonding energy of H−H was −63.24 kcal/mol. Besides this it is observed that the bond length of O−H increases significantly at the time of interaction with the adsorbed CO and the Pd cluster. Simultaneously, the O atom of O−H (originating from H2O) strongly bonded with the C atom of CO and formed the O−C−O bond. The fast step in this process is the scavenging of atomic O by CO to desorbed CO2. Later on, at 1786 fs (Figure 6d), total dissociation of the O−H takes place where the binding energy of O−H becomes nearly zero, and hence, the H−H bond that formed has the binding energy equivalent to an H2 (−99.61 kcal/mol). Formation and Desorption of the CO2 Molecule. For a snapshot at 2086 fs (Figure 7a), we have observed the O−C−O molecule (formed after decomposition of the CO2H complex), which was bonded with the Pd cluster trying to dissociate from that cluster. A snapshot at 3076 fs (Figure 7b) and another at 3116 fs (Figure 7c) also show the CO2-like molecule that formed on track to desorbing from the Pd cluster. A snapshot at 3166 fs (Figure 7d) shows the O−C−O-like molecule completely dissociated and desorbed from the catalyst into the gas phase. Our study shows that the desorption energy of CO2 from the medium size particles is about −15.71 kcal/mol. It was concluded from the previous study that the reactivity of the Pd clusters depends on their sizes and shapes. Moreover, the desorption energy also depends on the size, shape, and morphology of the Pd cluster and varies strongly with surface coverage. For particles smaller than 4−5 nm and at very low coverage, desorption energy generally increases as their size decreases. The supporting oxide also plays an important role in the reaction kinetics and desorption. Here the formation of the

Pd13/MgO(100) is able to dissociate water with a minimum barrier. During the process, both the Pd cluster and MgO participate in the reaction directly. The estimated barrier for water dissociation is 17.89 kcal/mol. Let us analyze our results more carefully by comparison with some earlier reported results for water dissociation. The dissociation of water on the (111) surfaces of Rh and Ni have been previously examined by DFT methods.108 The barriers for water dissociation on the above two surfaces have been reported to be 21.21 and 20.52 kcal/mol respectively for Rh and Ni,108 whereas our result for the barrier of water dissociation is 17.89 kcal/mol. Formation of OCOH Intermediate. The surface hydroxyls that formed during the dissociation of H2O were then readily consumed by the adsorbed CO resulting in a formate intermediate. Here the dissociation of H2O (HO− and H+) and OCOH intermediate formation took place in a very short time. Figure 5c,d shows the further dissociation of the O−H bond and the formation of the OCOH intermediate took place simultaneously. At 1300 fs (Figure 5c) we have found that the HO− part of the dissociated H2O interacts with the C atom of CO and formed the OCOH intermediate. In the coadsorbed configurations the O−H part of the H2O molecule pointing toward the nearest oxygen atom of the CO elongates to 1.11 Å, with the distance between the two oxygens on the two sides of the proton being close to 2.5 Å (partial proton sharing). Later on at 1380 fs (Figure 5d) the C and O (O of O−H) bond becomes very close to each other and the bond length of C−O (O of O− H) reduces to 1.049 Å and as a result the bond energy increases to −192.31 kcal/mol, which confirms the bond formation between C and O (O of O−H). In addition the initially adsorbed CO bond length and bond energy remain almost constant. Formation of the complex involves the partial sharing of a proton between the coadsorbed molecules, which in certain adsorption configurations is fully transferred to the H2O molecule, leading to an OCOH like complex. Decomposition Reaction of the OCO−H Complex and Formation of the H−H Molecule. In this step the OCO−H complex that formed is decomposed into OCO and H. During the simulation the adsorbed hydroxyl oxidizes adsorbed CO 5060



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length, and bond population of Pd−C also confirm the formation of a strong Pd−C bond. Here, as the simulation proceeds the binding energy of Pd−C increases and finally a strong bond (−39.29 kcal/mol) forms between Pd and C at 780 fs. But at the same time a very small increase in bond length between C and O in CO is observed. Further simulation shows that CO and H2O interact to form CO2, which tends to desorb, resulting in the significant decrease in Pd−C binding energy. The change in binding energy is denoted as a green line in Figure 8. Similar changes are also observed in cases of bond length and bond population, which are denoted as red and broken blue lines in Figure 8. Here the CO bond length increases from 1.135 to 1.21 Å and the bond population is reduced to 0.91 atomic units from 1.13 atomic units. From the above inspections it is concluded that CO almost molecularly adsorbs on the supported Pd/ MgO(100) catalyst. Small changes in CO bond length at the adsorption state causes the large changes in bond energy between C and O, which might be the influence of the H2O molecule. Figure 9 shows the changes in binding energy, bond length, and bond population of the two OH bonds (H−O−H) present

CO2 molecule depends on the frequent dissociation of H2O. In this simulation it was found that after the dissociation of two O− H bonds (H−O−H), a strongly bound H−H formed that does not desorb during the reaction. In the present study, we have found that the H2 molecule that originates from the dissociation of H2O is strongly bound with the Pd cluster. TB-QCMD calculations showed that the desorbed O−C−O (CO2) molecule supports all the properties of a carbon dioxide molecule (Figure 4i). In the gas phase, the bond angle of O−C−O molecule was found around 170°−175°, whereas experimentally CO2 is linear. The two bond lengths between C−O (O of H2O) and C−O (O of CO) in CO2 were 1.16 and 1.17 Å, respectively, whereas the experimental value is 1.14 Å. Bond energies of the two bonds were about −211.81 and −209.64 kcal/mol, respectively. From the energy description of the reaction mechanism, we have observed that reactants are converted into products in the WGS reaction. As the reactants approach each other along a reaction path, the potential energy increases as some bonds are distorted and repulsive interactions are enhanced. The highest point on the reaction coordinates corresponding to that time period wherein profound molecular reconfigurations occur. Here a snapshot at 780 fs represents the initial position of the reactant. There is an intermediate stage of reaction at 1457 fs, at which chemical bonds are partially cleaved and formed. This stage (1457 fs) is at higher energy and unstable, a transition state. Here, in the reaction of carbon monoxide with water to form carbon dioxide and hydrogen, both reactants react to form a transition state. The transition state is readily converted into products (1606 fs), carbon dioxide (CO2) and hydrogen (H2), with the release of energy, as the reaction is exothermic. A reaction channel of the exothermic reaction of the CO H2O complex (formed on the top facet of the supported Pd13) with gaseous CO, occurring via the Langmuir−Hinshelwood mechanism with an energy barrier of 12.90 kcal/mol, is found. Changes in Binding Energy, Bond Length, and Bond Population of Different Bonds During Simulation. Figure 8 shows the changes in binding energy, bond length, and bond population of the CO during the simulation. At the initial adsorbed state (780 fs) CO adsorbed molecularly but it is then slightly dissociated due to the interaction with H2O. However, CO association occurs again and remains as the molecular form during the simulation. Moreover changes in bond energy, bond

Figure 9. Comparison of the changes in binding energies, bond lengths, and bond populations of H2O dissociation into OH− and H+. Here we have shown the change of H−OH bond during the simulation. Blue broken (ball) line indicates the change of bond population, red solid (diamond) line indicates change of bond length, and green solid (triangle) line indicates the change of bond energy during the simulation (bond length and bond populations were set in the same Y-axis because of same range of values in Y-axis even though they have different units; second Y-axis was used to demonstrate bond energy in kcal/mol).

in the H−O−H molecule. By measuring the bond length, binding energy, and bond population of two O−H bonds, we can affirm the dissociation of H2O and, if it dissociates, which OH bond is broken. Initially H2O adsorbed almost molecularly at 1050 fs (Figure 4d) where the two OH bond energies are −104.67 and −86.12 kcal/mol. But after a very short time it was observed that (1060 fs) one of the O−H bond dissociates. We have observed that at 1130 fs the H−O−H dissociates and forms a hydroxyl (OH−) molecule and a hydrogen (H+) atom. The dissociation of HO-H was also confirmed by the changes of bond length and bond population of the O−H bond. The bond energy calculation shows that the OH bond energy becomes close to zero whereas another OH bond remained constant during the simulation. Similar changes are also observed in cases of bond length and bond population. Here the bond length of OH increases from 0.993 to 2.23 Å and the bond population is reduced to −0.11 atomic units whereas the initial value was 0.661 atomic units at the adsorption state. In fact, the dissociation of H2O into OH− and H+ is a crucial step for the WGS reaction. In

Figure 8. Comparison of the change of binding energies, bond lengths and bond populations of C−O bond during the simulation time. Blue dashed (ball) line indicates the change of bond population, red solid (diamond) line indicates change of bond length, and green solid (triangle) line indicates the change of bond energy during the simulation (bond length and bond populations were set in the same Y-axis because of same range of values in Y-axis even though they have different units; second Y-axis was used to demonstrate bond energy in kcal/mol). 5061



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case of a high coverage of CO, there is no room for H2O to dissociate and no reaction can occur. Changes in binding energy, bond length, and bond population of the OCOH intermediate formation are shown in Figure 10.

Figure 11. Comparison of the changes in binding energies, bond lengths and bond populations of decomposition of CO2−H during simulation. Here we have shown the change of O−H bond (COO−H complex) during the simulation. Blue broken (ball) line indicates the change of bond population, red solid (diamond) line indicates change of bond length, and green solid (triangle) line indicates the change of bond energy during the simulation (bond length and bond populations were set in the same Y-axis because of same range of values in Y-axis even though they have different units; second Y-axis was used to demonstrate bond energy in kcal/mol).

Figure 10. Comparison of the changes in binding energies, bond lengths and bond populations of CO + OH intermediate formation during simulation. Blue broken (ball) line indicates the change of bond population, red solid (diamond) line indicates change of bond length, and green solid (triangle) line indicates the change of bond energy during the simulation (bond length and bond populations were set in the same Y-axis because of same range of values in Y-axis even though they have different units; second Y-axis was used to demonstrate bond energy in kcal/mol).

The bonding between OC and O (O of OH) is shown which confirms the formation of the OCOH intermediate. At 1020 fs, there is no bonding between C and O (O of OH). But at 1050 fs CO begins to form a bond with OH and at 1070 fs the bond energy of C−O becomes −32.11 kcal/mol and at 1300 fs it becomes −198.60 kcal/mol, which is close to the bond energy of CO in CO2. Finally, at the time of CO2 desorption the bond energy becomes closer to the standard bond energy of the C−O bond in CO2. Simultaneously, the bond length and the bond population also changed during the simulation, which also confirms the bond formation between C−O (O of OH) shown in Figure 10. Here the bond length of CO decreases to 1.13 Å from the 2.38 Å, and the bond population increases to 1.13 from −0.015 at the adsorption state. Figure 11 shows the changes in the binding energy, bond length and bond population of C−O (O of OH−) during the simulation. In this step the COOH complex that formed decomposed to the CO2 molecule and an H atom. In that case the O−H bond becomes weaker and the C−O (O of OH) bond becomes stronger. Finally, we have found the complete dissociation of the O−H bond and the formation of an O−C− O molecule and an H atom. It was found that initially at 780 fs a very strong bond existed between the O−H molecules, but finally, when CO2−H decomposed the OH bond energy becomes close to 0.0 kcal/mol. Changes in bond length and bond populations were also observed step-by-step. Here the bond length of OH increases to1.33 Å from 0.97 Å and the bond population is reduced to −0.10 from 0.674 atomic units at the adsorption state. Figure 12 shows the interactions of two dissociated H atoms and the formation of the H−H bond. Snapshots at 1300 and 1380 fs show that there is a very small interaction between two individual H atoms. Surprisingly at 1457 fs we found that a very strong H−H bond created which is equivalent to −41.57 kcal/ mol. Later on at 1606 fs we have found a stronger H−H bond energy, which is almost close to the value of H−H bond energy

Figure 12. Comparison of the changes in binding energies, bond lengths and bond populations of the H−H bond during simulation. Blue broken (ball) line indicates the change of bond population, red solid (diamond) line indicates change of bond length, and green solid (triangle) line indicates the change of bond energy during the simulation (bond length and bond populations were set in the same Y-axis because of same range of values in Y-axis even though they have different units; second Y-axis was used to demonstrate bond energy in kcal/mol).

(−86.91 kcal/mol) in molecular form. The bond length and bond population also show similar results for the H−H bond, which confirm the formation of molecular H2 on the Pd cluster. It was found that the binding energy between Pd−H2 increases during the simulation and a strong bond (−54.29 kcal/mol) formed between Pd and H2 at about 1786 fs. Similar changes are also observed for the bond length and bond population, which are denoted as black and blue lines, respectively, in Figure 12. Analysis of Partial Density of States. Figure 13 shows the partial density of states (PDOS) of the different individual steps of the reaction. To figure out the contribution of each type of atom and the contributions from atomic orbital to the DOS, we have plotted the partial densities of states of the different orbital’s (s, p, and d) from the Pd, C, O, and H. As the CO molecules interact with the Pd cluster the valence electrons of the Pd overlap with the lowest unoccupied molecular orbital (LUMO). The more negative the LUMO band energy, the more likely it is for the electron to be trapped in the LUMO. Among the above molecules, the O atom has the lowest LUMO energy, and as it interacts with the Pd atom, it binds strongly with the valence 5062



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the catalytic performance of the metal.109 Moreover the support also plays an important role in the reaction kinetics through the diffusion and capture, by the Pd particles, of the reactant molecules physisorbed on the support.110 The present study also indicates that the MgO(100) support has strong interactions with the Pd catalyst, which may cause metal catalyst dispersion, enhancement of the catalytic activity of Pd and hence an increase in the rate of the WGS reaction. In our system the large binding energies between Pd and O (in the Pd/MgO system) as well as donation-back-donation make the structure catalytically more active. Furthermore oxygen atoms from the uppermost layer of the MgO surface seem to interact strongly with the closest Pd atoms.111 As a result, interaction between Pd and MgO reduced the bond population of Pd−Pd and increased the bond population of Pd−O. Therefore, it was suggested from our analysis that Pd anchored with MgO by the Pd−O bonds and prevented it from diffusing. Hence, the catalyst shows good catalytic activity. The maximum Pd−O bond energy of Pd13 on the supports was found to be 78.75 kcal/mol. This binding energy of Pd−O bonds leads to the activity of Pd on supporting metal oxides. Pd/MgO provides a clear manifestation of the role of the metal/support interaction in modifying the behavior of the metal surface. Furthermore, the contributions of atomic charges were considered to explain the metal−support interactions. It is concluded from Figure 14 that charge transfer initially takes place

Figure 13. Comparison of the partial density of states (PDOS) at different steps of WGS reaction.

electrons of the Pd. The oxygen molecule is more reactive toward the Pd atoms and is an important factor in understanding the different adsorption states. The calculated partial densities of states around the valence and conduction bands of the system are shown in Figure 13. The projected density of states shows (Figure 13a) the overlap of C-2p with Pd-4d, which suggests there is a charge transferred from the empty d orbital of Pd to C. In Figure 13b there is overlap with C-2p with O-2p, which suggests the interaction of CO with the H2O molecule. Figure 13c shows the overlap of C-2p with O-2p, which also suggests the interaction of the C atom with the O atom (O atom of H2O molecule). Determination of Metal−Support Interaction. For most metal/oxide systems, the interaction and charge transfer between metal particles and oxides is very important in catalytic reactions. There has been much debate in the literature regarding the activity and stability of supported Pd catalysts. Quantum chemical molecular dynamics calculations indicate that, the metal-oxide interface plays an essential role in the catalysis, helping in the dissociation of water and the formation of an OCOH intermediate, which decomposes to yield CO2 and H. It was found that the metal−support interactions are an important factor affecting the oxidation state, the structure and

Figure 14. Analysis of the metal support interaction through atomic charges.

from the MgO(100) support to the interacting Pd atom of the Pd cluster and the atomic charges of those Pd atoms become more negative. Moreover, during the simulation, electron transfer takes place from the interacting Pd atom to the neighboring upper layer Pd atom. It was explained in the Giordano et al.111 study that when an oxide support provides charge to the metal cluster, the transferred electrons are localized at the interacting atom and the nearest neighbors. As a result, the interacting lower layer of Pd becomes electron deficient and hence there is an easy electron transfer from the MgO(100) support to the lower layer of the Pd cluster. Moreover, TB-QCMD atomic charges agreed well with those obtained through DFT (Hirshfeld atomic charge, using GGA/PBE exchange-correlation functional), validating the TBQCMD method. Table 4 explains the transfer of atomic charges between metal and support during the TB-QCMD simulation. Initially, in the Pd13 cluster, the average atomic charge of Pd is 5063



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H2O adsorption occurs first molecularly on the Pd cluster but it is then dissociatively adsorbed after interacting with CO. Both the activity and the selectivity of metal catalysts are crucial for the H2O dissociation. It is also observed that H2O dissociation is the key steps for this reaction. In this simulation we found a strongly bound H2 (H) atom, which remains attached with the Pd cluster. The support also plays an important role in the reaction mechanism through the diffusion and capture, by the Pd particles, of the reactant molecules physisorbed on the support. This is the first TB-QCMD-based simulation of the dynamics of CO oxidation, H 2 O dissociation, formation of OCOH intermediate, and desorption of CO2 on the gas phase. Finally we have successfully corroborated the speculations of experimental results. These elementary reactions are cross-linked to many other catalytic processes, and understanding them can provide general insight into heterogeneous catalysis.

Table 4. Distribution of Atomic Charges at Different Atoms and Proof of Metal−Support Interactions element

molecular system

avg atomic charge

Pd

Pd-13 cluster Pd/MgO(100) MgO(100) Pd/MgO(100) MgO(100) Pd/MgO(100)

0.000 −0.033 0.423 0.417 −0.423 −0.417

Mg O

about 0.0. After deposition of the Pd13 cluster on an MgO(100) surface, the average atomic charge of the upper surface Pd atoms in the Pd cluster becomes −0.0325e, that is, the atomic charge from the MgO support is transferred to the Pd cluster. Similarly, the average atomic charges of Mg and O in the MgO(100) surface is approximately +0.423e and −0.423e, respectively. But the Pd13 cluster deposition on the MgO(100) surface modifies the average atomic charge of Mg and O as +0.417e and −0.417e, respectively. We have also calculated the total charge transfer from oxide to metal. In that case we observed an overall charge transfer from MgO to the Pd cluster of about 1.28e for 13 Pd atoms. In such a case we have observed a strong metal support interaction as atomic charge transfer takes place from the MgO support to the Pd cluster and back-donation takes place from Pd to MgO. This indicates the possibility of a strong metal−support interaction between the Pd cluster and the MgO(100) support. To estimate the possible electronic charge rearrangement and charging effects in the coadsorbed system, we focused on the specific adsorption complex shown for Pd13. For this particular set of nuclear positions we have calculated the difference between the electronic charge density of the whole system and the sum of the densities of the separate components (the Pd13 cluster and the CO H2O species, respectively, keeping the geometries as found for the adsorption system). This procedure allows us to assess the charge redistribution occurring by joining the Pd cluster and the coadsorbed species. We find that electronic charge depleted in the region of the Pd13 cluster accumulates at the location of the adsorbed CO molecule. Underlying the waterinduced increased catalytic activity of Pd nanoclusters toward the oxidation of CO is the formation of a complex between the adsorbed molecules involving partial proton sharing. We believe that these findings could provide the impetus for future theoretical and experimental investigations of the effect of water on nanocatalytic processes.

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ASSOCIATED CONTENT

S Supporting Information *

Additional tables of data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +403 220 3720. Fax: +403 210 8655. E-mail: dennis. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS D.R.S. is grateful to NSERC-Canada for continued support through Discovery Grants and a CIAM (Interamericas Research on Materials) grant.

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REFERENCES

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CONCLUDING REMARKS There has been vigorous discussion of the nature of the active form of Pd and an ongoing debate about the relative importance of different reaction mechanisms. The purpose of this study is to provide a critical analysis of the WGS reaction on precious metalbased catalysts. A universal mechanism was presented which shows that the dominant process is a function of the catalytic system; with the choice of catalyst, the temperature, and the composition of the reaction mixture all having an influence on the pivotal reaction intermediates. In this article, based on a line of theoretical studies involving TB-QCMD using CO, H2O, and an MgO(100) supported Pd cluster as a prototype system, we have successfully examined the individual steps of the WGS reaction. In the present work, the oxidation of 4d transition metal palladium clusters and palladium’s effect on the reactivity in CO oxidation and H2O dissociation reactions are discussed. It is observed that CO adsorption occurs almost molecularly, while 5064



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