Steric Effects in CO Oxidation on Pt(111) by ... - ACS Publications

Subscriber access provided by IDAHO STATE UNIV

C: Surfaces, Interfaces, Porous Materials, and Catalysis

Steric Effects in CO Oxidation on Pt(111) by Impinging Oxygen Atoms Lead to An Exclusive Hot Atom Mechanism Qisheng Wu, Linsen Zhou, and Hua Guo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b02615 • Publication Date (Web): 29 Mar 2019 Downloaded from http://pubs.acs.org on March 29, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Steric Effects in CO Oxidation on Pt(111) by Impinging Oxygen Atoms Lead to An Exclusive Hot Atom Mechanism

Qisheng Wu,1 Linsen Zhou,1,2 and Hua Guo1,* 1Department

of Chemistry and Chemical Biology, University of New Mexico, Albuquerque, New Mexico 87131, USA

2Institute

of Materials, China Academy of Engineering Physics, Jiangyou 621908, Sichuan, China

*: corresponding author: [email protected]

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Ab initio molecular dynamics calculations are performed to gain insights into the oxidation of CO adsorbed on Pt(111) by impinging O atoms. The calculation results indicate facile reactivity for forming desorbed CO2 products. Interestingly, this Eley-Rideal reaction is found exclusively via a hot-atom mechanism, in which the impinging O atom captured by the Pt surface attacks the carbon center in the CO adsorbate. This can be attributed to the strong steric effect of this reaction as the cone of acceptance for CO oxidation is located at its C end and facing the surface. As a result, direct collisions of incident O atoms onto the O end of the CO adsorbate are non-reactive. Consistent with experimental observations, the CO2 product was found to desorb near the surface normal, with high internal excitation. In addition, several previously unknown reactive and nonreactive channels have been identified.


Page 2 of 26

Page 3 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


I.

Introduction Dynamics of surface reactions have attracted much recent attention, thanks to advances in

both experimental techniques1, 2 and theoretical algorithms.3-5 The new insights gained from these studies help to better understand a wide range of interfacial phenomena in heterogeneous catalysis, plasma chemistry, and materials science. Reactions between gas phase species and surface adsorbates can proceed via the EleyRideal (ER) mechanism.6 The ER mechanism differs from the quintessential LangmuirHinshelwood (LH) mechanism7 in that the impinging species is not thermalized by the surface. While the thermal LH reactions can be readily characterized using statistical theories, ER reactions are strongly influenced by dynamics and the outcome depends on many factors such as the incident energy and angles. In recent years, there has been renewed interest in understanding the dynamics of ER reactions.8-15 The gaseous projectile, typically an atom, can be prepared using a beam with precise control of the kinetic energy and incident angles. The angular, translational, and internal distributions of products can then be probed using time-of-flight and/or spectroscopic methods. These studies have yielded a plethora of information on ER reactions. However, experimental measurements seldom provide direct microscopic details of the dynamics between the reactant and product channels. A deeper understanding of the reaction dynamics thus relies often on theoretical insights. Indeed, theoretical studies of ER reaction dynamics have been carried out by a number of groups and they have greatly advanced our understanding of how these processes take place.16-27 Due to difficulties in electronic structure calculations, earlier studies have often been based on empirical potential energy surfaces (PESs) and/or reduced-dimensional models. While insightful, these studies are not expected to be quantitatively accurate. More recently, the ab initio molecular dynamics (AIMD) approach28 based 3 ACS Paragon Plus Environment


on planewave density functional theory (DFT) has been used to investigate ER dynamics.29-32 The AIMD approach has several distinct advantages. First, the DFT calculations provide an economic and reasonably accurate description of the molecule-surface interaction.33 Second, it avoids the difficult task of constructing PESs, which can be quite challenging for high-dimensional systems.34, 35 Third, the energy exchange with surface phonons is explicitly considered, at least approximately,

as some surface atoms in the unit cell are allowed to move. This is particularly important for ER reactions because of the non-thermal projectile. Fourth, all possible channels are naturally included in an AIMD simulation, which helps to explore alternative reactive and non-reactive pathways that are not anticipated in simple models.29 Finally, nonadiabatic effects due to surface electron-hole pairs can be included in the same AIMD framework by treating them via electronic frictions.36, 37 Because ER reactions are often barrierless and highly exothermic, a classical treatment of the dynamics is sufficient as quantum effects are negligible.38 An interesting issue in ER reactions is that they can take place in two microscopically distinct mechanisms. One involves a single direct collision of the impinging atom with the adsorbate before it reaches the surface, while the other is initiated by hot atoms,39 which are the incident atoms trapped on the surface. The kinetic energy of these hot atoms can be quite significant along the surface because of the large adsorption energy of these species on the surface. However, they are not thermally accommodated as in the LH mechanism. Such a Hot Atom (HA) mechanism has been found to play an important role in several ER reactions.19-21, 23, 25 Most ER reactions studied to date involve only atomic species. When a molecular reactant is involved, the chemical shape of the molecule may lead to significant stereodynamics. In a recent AIMD study of the D + CD3/Cu(111) ER reaction,31 for example, it was shown that the direct ER reaction necessarily involves an inversion of the pyramidal CD3 adsorbate, because the D atom 4 ACS Paragon Plus Environment

Page 4 of 26

Page 5 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


attacks from above, while the HA reaction does not, as the trapped hot D atom always approaches the adsorbed CD3 from below. These two microscopic reaction mechanisms give rise to very different umbrella vibrational excitations in the CD4 product, thus providing a signature of the microscopic reaction mechanism. The focus of this study is the CO oxidation on a metal surface (Pt(111)) by impinging O atoms from the gas phase. While this process differs from the common LH CO oxidation,40-43 it offers a useful proving ground for understanding ER reactions involving a molecular adsorbate, particularly their stereodynamics. Furthermore, it helps to shed light on the possible reaction between CO adsorbates and “ballistic” O atoms formed from O2 dissociation.44-46 Experimentally, Mullins and coworkers used an atomic oxygen beam directed to CO-covered Pt(111), Ir(111), and Ru(001) at various surface temperatures to measure the initial reactivity leading to gaseous CO2 products with angular resolution.47, 48 It was found that the CO2 angular distribution is sharply peaked at the surface normal, and the incident angle of O atoms strongly influence reactivity. Furthermore, surface temperature also has an effect. In a separate experiment, the CO2 product vibration was measured and found to be quite hot.49 This ER reaction has a unique steric constraint. Since CO is bound along the surface normal with its carbon end pointing to the metal surface, a direct collision by the impinging O will most likely engage in interaction with the O end of the adsorbate, thus sterically unfavorable to form CO2, which has a symmetric OCO structure. However, an HA mechanism would be much more favorable because a hot O atom on the surface can readily attack the carbon end of adsorbed CO, leading to the formation of chemisorbed bent CO2 before desorbing. Hence, this reaction offers a unique proving ground to understand steric effects in surface reactions involving molecules.



This ER reaction has previously been investigated theoretically using a model PES by Ree et al.50 Large vibrational excitation in the CO2 product was found from trajectories of O collision with adsorbed CO, shedding light on the dynamics. Unfortunately, the model PES did not consider the HA mechanism, neither did it explore other reactive routes. In this publication, we report an AIMD study of this process. As discussed below, our calculation results clearly suggest that the ER reaction is quite unique in that it proceeds exclusively via the HA mechanism. Furthermore, the CO2 products are vibrationally hot, in agreement with experimental observations. Finally, the AIMD trajectories revealed several other reactive and non-reactive channels that have never been known before. As such, the theoretical investigations reported here shed valuable light on the collisional dynamics between gas phase species and adsorbates. II.

Method All of the DFT calculations were carried out with the Vienna Ab Initio Simulation Package

(VASP version 5.4).51, 52 A vacuum region of 15 Å was adopted to minimize interactions between adjacent slabs arising from periodicity in the perpendicular direction. Wavefunctions of the valence electrons were expanded with plane waves using a cutoff energy of 400 eV, while the core electrons were described with projector augmented wave (PAW) method.53 The Brillouin zone integration was realized using a k-point grid of 8 × 8 × 1. A Methfessel-Paxton smearing54 with a width of 0.1 eV was employed. The geometries were optimized with a force convergence criterion of 0.01 eV·Å−1. The bulk lattice constant of Pt was computationally optimized to be 3.97 Å, in accord with the experiment value of 3.92 Å.55, 56 The adsorption energy is defined to be 𝐸𝑎𝑑𝑠 = 𝐸𝑡𝑜𝑡𝑎𝑙 ― 𝐸𝑠𝑙𝑎𝑏 ― 𝐸𝑔𝑎𝑠, where 𝐸𝑡𝑜𝑡𝑎𝑙, 𝐸𝑠𝑙𝑎𝑏, and 𝐸𝑔𝑎𝑠 are energies of the entire system, the bare Pt(111) surface, and the free gas-phase atom or molecule, respectively.


Page 6 of 26

Page 7 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The structures and energetics of adsorbed CO and O on the Pt(111) surface were first determined using spin-unpolarized DFT. The generalized gradient approximation (GGA) functional of Perdew, Burke, and Ernzerhof (PBE)57 was used. The Pt(111) surface was built from a 2×2 unit cell and four-layer slab with only the bottom layer frozen. Since the PBE functional gives reasonable adsorption energy for CO in agreement with the latest experimental value,58 no correction of dispersion forces was included to avoid overestimating CO adsorption energy. Although the calculations show CO molecule prefers to adsorb on face-centered cubic site (fcc), which is known as the “CO/Pt(111) puzzle” in literature,59 the CO molecule was set to adsorb on top site in this work to be consistent with experimental observations.60 The CO adsorption energy at the top site was calculated to be -1.64 eV, which is in good agreement with both previous experimental and theoretical values.58, 61 The energy difference between the fcc and top sites is not large (0.12 eV), and the CO molecule is stable at the top site. On the other hand, the O atom prefers to adsorb on the fcc site, with an adsorption energy of -4.67 eV. This value is close to our previous theoretical result (-4.50 eV) using the PW91 functional.61 Reaction barriers between stable configurations along the reaction path were determined using the climb image nudged elastic band (CI-NEB) method.62 AIMD simulations were run in conditions as close to the experiment47, 48 as possible. To that end, a CO pre-covered Pt(111) surface (1/4 ML) was equilibrated at a surface temperature (Ts) of 91 K, which is close to the experimental value of 95 K,48 by running the simulations in the NVE ensemble prior to the incidence of the O atom for 2 ps, and 100 snapshots after equilibration were randomly taken as initial configurations for the subsequent O atom collision processes. The position of the impinging O atom was initiated at 6 Å above the Pt(111) surface and randomly distributed in the unit cell for the lateral coordinates, with the initial velocity along the surface 7 ACS Paragon Plus Environment


normal. The incidence energy was set to the experimental value of 0.22 eV.48 One hundred trajectories were propagated for up to 1.0 ps with a time step of 1.0 fs. Because the ground electronic state of atomic oxygen is a triplet, spin-polarized DFT is needed in the reactant channel. This leads to costly DFT calculations and sometimes convergence issues. Near the metal surface, however, the open-shell nature of the O atom becomes overwhelmed by the metal electrons and a spin-unpolarized treatment suffices. In our calculations, spin-polarized DFT was employed during the initial O incidence. When the impinging O atom arrives near the surface, the DFT calculation was switched to be spin-unpolarized to help convergence and reduce computational costs. The switch is made at a point near the surface when the magnetic moment vanishes. This procedure has successfully been used before for impinging hydrogen atoms on metal surfaces, where the same issue arises.31, 63 As shown in Figure S1 in Supporting Information (SI), this approximation is reasonable. Indeed, the total energies were well-conserved within 200 meV for most trajectories. For the CO2 products, a normal-mode analysis (NMA)64, 65 was utilized to obtain vibrational quantum numbers for various modes, as presented in our previous work.31 Prior to the NMA calculations, the same DFT method was employed to obtain a three-dimensional PES for the free CO2 molecule, which was fit with the permutation invariant polynomial-neural network scheme.66 III.

Results and Discussion The LH reaction between adsorbed O and CO species on Pt(111) has a significant barrier

(0.87 eV), as computed using the CI-NEB method. After overcoming this transition state (TS), the incipient CO2 is formed in a metastable chemisorption well, featuring a bent CO2. The bent structure of the chemisorbed CO2 is due to fractional electron transfer to the antibonding orbital of the adsorbate.67 As extensively discussed in our recent work,61 this chemisorbed species is 8 ACS Paragon Plus Environment

Page 8 of 26

Page 9 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


stabilized by a barrier of 0.14 eV towards the shallow physisorption well before desorption. These stationary points along the reaction path are shown in Figure 1. Overall, the ER reaction is thermodynamically favorable with a reaction energy (Erxn) of -4.83 eV. Figure 2a displays a schematic initial geometry of the O atom impinging on CO@Pt(111). Figure 2b shows the distribution of initial lateral positions of the impinging O atom in the 2×2 unit cell with outcomes indicated by different labels. Overall, most of the trajectories (79) result in reactions between the impinging O atoms and adsorbed CO molecules, leading to CO2 products desorbed from surface. This high reactivity is a direct result of the large exothermicity of the barrierless ER reaction. However, several other processes were also observed in the AIMD simulation. A significant percentage (8%) of the impinging O atoms were scattered back to the vacuum without reaction. These non-reactive events occur via scattering with either the adsorbed CO molecule or the Pt(111) surface. In addition, desorbed CO products were also observed with O staying on the surface. Interestingly, the gaseous CO products can be formed by displacement (4 trajectories), in which the impinging O atom transfers sufficient energy to knock out the CO adsorbate. Such displacement processes have also been reported for CO@Cu(111) by incident H atoms.68,

69

Alternatively, some other desorbed CO products are formed by abstraction (2

trajectories), in which the impinging O atom abstracts the C atom from the adsorbed CO, leading to CO desorption. The remaining 7 trajectories were found trapped. Among them, 5 involved reactions but the CO2 translational energy along the vertical direction is too small to result in desorption within the time limit of 1.0 ps, while the other 2 trajectories were non-reactive, producing adsorbed O and CO. The ultimate fates of these trapped trajectories are uncertain because they were not extended beyond 1.0 ps. Exemplary trajectories for all types are shown in



SI (Figures S2 and S3). Some of these processes have been discovered for the first time. Since the statistics for the minor channels are quite poor, we will focus on the ER reactions from now on. To understand the ER dynamics, four representative reactive trajectories are displayed in Figure 3, in which the height of the impinging O atom (denoted as O’) relative to the Pt(111) surface (ZO’), the distance (dC-O’) between C and O’, and the height of the CO moiety (ZCO) are plotted as a function of time. The reaction can be roughly separated into three stages. During the first stage, the impinging O’ atom is accelerated by the attractive interaction towards the Pt(111) surface. This stage ends when O’ reaches its first inner turning point (ITP), which is defined as the point where the vertical component of its velocity changes sign from negative to positive and ZO’

Steric Effects in CO Oxidation on Pt(111) by ... - ACS Publications

Recommend Documents