CO Oxidation over Strained Pt(100) Surface: A DFT Study - The

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CO Oxidation over Strained Pt(100) Surface: a DFT Study Fuzhu Liu, Chao Wu, Guang Yang, and Shengchun Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b04511 • Publication Date (Web): 08 Jun 2015 Downloaded from http://pubs.acs.org on June 16, 2015

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The Journal of Physical Chemistry

CO Oxidation Over Strained Pt(100) Surface: a DFT Study Fuzhu Liu, † Chao Wu,*, ‡ Guang Yang,§ and Shengchun Yang*,†,⊥ †

School of Science, Key Laboratory of Shaanxi for Advanced Materials and Mesoscopic Physics,

State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi’an, 710049, People's Republic of China. ‡

Frontier Institute of Science and Technology, Xi'an Jiaotong University, Xi’an, 710054

§

Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education &

International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an, China 710049 ⊥

Collaborative Innovation Center of Suzhou Nano Science and Technology, Suzhou Academy of

Xi’an Jiaotong University, 215000, Suzhou, People's Republic of China.

KEYWORDS: Pt(100) surface, Strain, Co-adsorption, Carbon monoxide oxidation, and DFT

ABSTRACT The oxidation of CO on strained Pt(100) surface was studied using periodic density functional theory (DFT). Unlike the uniform response of global properties (e.g. d-band

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center) to strain, the localized nature of adsorption leads to complex site-dependent and adsorbate-dependent responses, invalidating the generally believed statement of "tension strengthens binding". Moreover, the complex responses of reaction energetics to strain require direct study of the reaction under strain rather than extrapolating the known behaviors of individual adsorbates under strain or reaction energetics on unstrained surfaces. We show that the tensile strain lowers the reaction barrier of CO oxidation over the Pt(100) surface. This work provides a theoretical basis of utilizing strain to improve the Pt catalysts with a higher tolerance toward CO poisoning.

A number of important chemical reactions catalytically occur on transition metal surfaces. Various methods, including alloying/dealloying,1 surface masking with heteroatoms,2 morphology control, or defect engineering3, have been developed to improve their catalytic performance by modifying the surface structure. In almost all the methods, surface strain of the catalysts is either introduced and/or varied through mechanisms like local deformation of a single metal phase,4 epitaxial metal overlayers,4-6 defects7, and lattice mismatch between core and shell.8-9 The geometric and electronic structures of surface atoms are perturbed, and consequently, surface catalytic reactions are affected.10-12 In the last decade, an emerging field of strain engineered catalysis, aiming at regulating the catalytic performance of transition metals through strain, has attracted a lot of attention.3, 7, 13-14,15-18 Strasser and coworkers observed high catalytic activity of dealloyed Pt-Cu core-shell nanoparticles in oxygen-reduction reaction (ORR), which is attributed to surface strain induced by lattice mismatch of the core and shell metals.1 Yang group reported that icosahedral platinum alloy (Pt-Au/Ni/Pd) nanoparticles presented higher ORR activity than octahedral ones, as the tensile strain in the former facilitated the reaction while the compressive strain in the latter hindered it.3


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Several theoretical studies have successfully provided a concise and insightful description of the strain effects on catalysis by correlating reactivity and strain via the d-band center shift of the catalyst.3 Nørskov et al. demonstrated that, for Ru(0001), tensile strain leads to surface lattice expansion and up-shift of its d states, in accordance with the enhanced CO dissociation reactivity.4 Yu et. al. further showed that the continuous shift of the d-band center away from the Fermi level triggers the weakening of O adsorption on the compressed Pt(111) and Pt skins of Pt3Co(111) alloy compared with the unstrained surface.19 Gradually, "tension strengthens binding" summarized from theoretical predictions become a widely accepted trend.20 However, this rule is not always observed.21-23 For example, Liu group found that the response of chemisorption of CO on the Au(001) and K(001) surfaces to external strain is site-dependent:24 for either metal surface, two oppositely going linear relationships between the CO adsorption energy and strain exist for the on-top and hollow sites, respectively. Similarly, Groß et al. discovered that for the same type of site on low-index surfaces of different metals like Pd5 and Cu6, the H adsorption energy and tensile strain exhibit opposite linear relationships. In principle, strain adds an additional dimension of tunability on top of the traditional catalytic variables including the types of metals, surfaces, and surface sites, whose different responses to strain may be utilized to regulate the catalytic reactions. In this work, as an example, we try to solve or at least mitigate the well-known problem of "CO poisoning" over Pt utilizing strain. CO poisoning takes place in many catalytic systems, such as polymer electrolyte fuel cells and direct methanol fuel cells,12 which is caused by strong adsorption of CO on Pt surfaces, blocking the active sites and reducing the catalytic activity of Pt. Thus, an important and urgent issue is: how to readily remove CO from Pt surfaces or how to improve the tolerance of CO poisoning at Pt surfaces? Based on the idea of surface strain engineering, several


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techniques such as local deformation,7 epitaxial metal overlayers25-27 have been developed to regulate the binding strength of CO. However, a number of questions about the atomistic behaviors of the relevant adsorbates still remain unclear. (1) How does strain affect multiple species involved in CO oxidation (e.g. O, CO) at multiple sites? (2) Is CO-Pt binding strength the only key parameter to control? (3) For the co-adsorption of CO and O, required during CO oxidation, how does the adsorption of one species affect the other? (4) Whether the tolerance of CO poisoning at Pt surfaces can be improved by strain? Here we use the system of CO oxidation over the Pt(100) surface as an example model to study the strain effects. First, using the periodic density functional theory (DFT), we systematically investigate the adsorption behaviors of key species (e.g. O and CO) at multiple sites (e.g. top, bridge, and four-fold hollow sites) under compressive and tensile strains (-3% to 3%). Next, we check if the response of co-adsorption to strain can be expressed as a linear combination of the responses of each involved species. Finally, we probe the variation of the energetics of CO oxidation under strain, thus verifying if the higher tolerance of CO poisoning at Pt surfaces can be achieved through surface strain engineering. The spin unrestricted density functional theory (DFT) calculations were carried out using the Dmol3 program package.28-29 Generalized gradient approximation (GGA) with the Perdew and Wang-91 (PW91) formulation of the exchange-correlation functional was employed.30-31 The self-consistent PW91 density was determined by iterative diagonalization of the Kohn-Sham Hamiltonian.32 The valence electron wavefunctions were expanded into a set of atomic orbitals composed of the double numerical plus d-functions (DND) basis set. Brillouin-zone integrations were performed on a grid of (8×8×1) Monkhorst-Pack k-point mesh. The width of the Fermi smearing of the Kohn-Sham states was set to kBT = 0.005 Hartree. The Pt(100) surface was


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modeled by using a (2×2) five-layer slab supercell with the bottom three layers frozen. Each slab was separated by a vacuum of 15 Å to minimize the interactions between images. The convergence criteria for geometry optimization were set to 10-5 Hartree, 0.002 Hartree/Å, 0.005 Å for energy, force, and displacement, correspondingly. Compressed and tensile strain ranging from -3% to 3% was imposed in parallel to the surface plane by changing the lattice constant of the unstrained Pt(100) surface. This approach has been shown to yield accurate estimates of adsorption energies on strained transition metal surfaces.11 The calculated equilibrium lattice constant of 3.924 Å for Pt was in agreement with the experimental value of 3.920 Å.33 O2 and CO molecules were relaxed by placing them individually in a rectangular cuboid box with a vacuum space of at least 15 Å in all three directions to minimize image interactions. The bond lengths were 1.224 Å (O2) and 1.140 Å (CO), very close to the experimental values of 1.210 Å19 and 1.120 Å,4 respectively. To investigate the minimum energy pathway for O adatom diffusion and CO oxidation on the strained Pt(100) surface, linear synchronous transit/quadratic synchronous transit (LST/QST) approaches were invoked.34 To interpret the strain effects on binding strength, the d-band center model was employed.35-36,37 In addition, the adsorption energy was defined below. ∆Ead = E (ads/ metal) − E (metal) − E (ads)

Where E (ads/ metal) , E(metal) , and E (ads) are the energies of the adsorbate-slab system, the slab, and the adsorbate, correspondingly. Adsorbates and surfaces mainly interact through mixing the valence orbital of the adsorbates and the d states of the surfaces. The latter is concisely represented by the d-band center model with the d-band center energy denoted as εd. 38 The d-band center model has been widely used to rationalize the catalytic activity trends of transition metals and alloys,4,


19, 39-40

which is also

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employed here to explain how strain modifies the electronic structure of the Pt(100) surface and subsequently its interactions with adsorbates. The d-band center (εd) of the Pt(100) surface rises linearly with the increasing tensile strain (Figure. 1). As the atoms are moved away from their stable positions by the tensile strain, the overlaps of the d states among atoms are reduced and the d-band width becomes narrower. Subsequently, the d-band shifts upwardly,15, 20 which is evidenced in the electron density plots10 (Figure 1, insets). The εd shift range is ~ 0.2 eV from -3% to 3% strain, suggesting a fairly wide tunability of the surface electronic structure, thus it is interesting to see how the adsorbates (e.g. O and CO) respond to this wide εd change.

Figure 1. The d-band center of the Pt(100) surface as a function of surface strain. The solid line only serves as a guide for eye. Top insets: top views of the electron density plots of the -3% compressed (left), equilibrium (middle), and 3% stretched (right) Pt(100) surfaces. The color bar from red to blue indicates the reduction of the electron density. Bottom inset: top view of the Pt(100) surface. The top (t), bridge (b), and fourfold (4f) sites are marked by the dark red circles. Dark and light gray circles represent the sublayer and toplayer Pt atoms, correspondingly. Strain may cause various responses among different adsorbates. The stronger binding of O atom, CO, and NO molecules over stretched Ru(0001) surface were observed experimentally,7, 41 while the reduction in binding strength of O atom on compressed Pt(111) was rationalized as a


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result of the shift of d-band center away from the Fermi level.19 The adsorption sites on the Pt(100) surface are defined in the bottom inset of Figure 1. For O adatom, our calculations show that the 4f site is the most stable site, the b site takes second place, and the t site is less stable(Table S1), confirming the previous studies.42,43 As illustrated in Figure 2a, with the increase of the tensile strain, the O adsorption energy difference between the 4f and b sites decreases from 0.29 eV at -3% strain to 0.14 eV at 3% strain. For the b site, the O adsorption energy and the tensile strain (or the lattice constant expansion) relationship is monotonic: more exothermic adsorption corresponds to larger tensile strain, which needs two linear segments with similar slopes for compressed and stretched regions to describe accurately. For the 4f site, the flat line in the compressive strain range reflects the insensitiveness of the 4f site to compression. Usually, O adatoms prefer a large coordination number when adsorbed on a metal surface.42 For the 4f site, tensile strain counteracts the lateral contraction caused by the O adsorption, thus weakens the adsorption. For the b site, tensile strain promotes the O adatom's bridging coordination to the Pt atoms, where the distance between the O and Pt atoms decreases from 2.027 Å to 2.024 Å, strengthening the adsorption from -3.639 eV to -3.678 eV (Table S1). The very different responses of O adatom at the 4f and b sites to strain might serve as a handle for strain engineered catalysis.


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Figure 2. O adsorption energy at the 4f and b sites on the Pt(100) surface versus strain. (a) O adsorption energy at the 4f and b sites. Insets: top views of O adsorption configurations. The t site O is unstable and migrates to the 4f site (indicated by the black arrow in the bottom left inset). (b) CO adsorption at the t, 4f, and b sites. Insets: top views of CO adsorption configurations. The solid lines only serve as a guide for eye. The color coding of the circles is as in Figure 1. Next, we studied the CO adsorption on the Pt (100) surface at the 4f, t, and b sites (Figure 2b and Table 1), where the C-Pt coordination is favored and the b site is the most stable binding position, in agreement with the previous study.12 Overall, the CO adsorption is much weaker than that of the O adsorption by ~ 2.3 eV. Calculations reveal that the CO adsorption energy at different sites changes very differently with strain. All sites show slight rises in adsorption energies (less exothermic) to compressive strain, suggesting a weakened adsorption. For the 4f and t sites on a stretched surface, the adsorption energy increases along with the rising tensile strain. Particularly, for the latter (the t site), the tensile strain introduces evident nonlinear response of the adsorption energy. However, an opposite trend appears for the b site, quite different from the previous report of Au(001) and K(001) systems,24 which may be caused by the different d-states of the metal surfaces at the Fermi level.36 The calculation results also reveal that the Pt(100) surface, like the stepped Cu(211) and Ni(211) surfaces, exhibits complex CO binding response to external strain, emphasizing the over simplicity of the rule of "tension strengthens binding".20


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Table 1. CO adsorption energy (Ead) and the C-O bond length (d(C-O)) on the Pt(100) surface. 4f site

b site

t site

Strain Ead (%)

Ead (eV)

d(C-O) (Å)

d(C-O) d(C-O) (Å)

Ead (eV)

(eV)

(Å)

-3

-1.509

1.189

-1.589

1.169

-1.200

1.152

-2

-1.512

1.190

-1.608

1.170

-1.207

1.153

-1

-1.517

1.189

-1.624

1.171

-1.210

1.153

0

-1.516

1.189

-1.638

1.171

-1.217

1.153

1

-1.502

1.190

-1.644

1.171

-1.241

1.153

2

-1.481

1.190

-1.646

1.171

-1.212

1.153

3

-1.466

1.188

-1.650

1.172

-1.199

1.154

The CO oxidation over the lateral strained Pt(100) surface follows the LangmuirHinshelwood (L-H) mechanism,44 which requires the co-adsorption of CO and O. We focused on the co-adsorption energy (ECO+O(ε)) as a function of tensile strain (ε) on the Pt(100) surface (eq(1)). ECO+O(ε) = E0 + k × ε (1) E0 represents the co-adsorption energy of CO and O at zero strain and k is the slope. ECO+O can be obtained from either direct DFT calculations of co-adsorbed configurations or by summing up the individual adsorption energies of CO molecule and O atom over the strained surface. The directly calculated co-adsorption energy (Figure 3a, the black line and squares) becomes more exothermic linearly with the rising tensile strain. The summed-up co-adsorption energy can be computed using the most stable adsorption site of each individual adsorbate, i.e., the b site for


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CO and the 4f site for O, respectively (Figure 3a, the dashed blue line and triangles). For the summed-up co-adsorption energy and strain, the opposite trend is observed. The values of E0's are markedly different by over ~ 0.4 eV, with the directly calculated E0 being the higher one. Apparently, the adsorption energy of co-adsorbed species is not a simple linear combination of that of each adsorbate at its most stable site. In fact, on Pt(100), the most stable co-adsorption configuration of CO and O before oxidation occupies two neighboring b sites. At the presence of CO, the O adatom is pushed away from its most stable 4f site by repulsive lateral interactions. It implies that O adatoms could be easily relocated with the help of CO molecules.45-47 Therefore, selecting adsorption energy at the b site for O adatom instead of that at the 4f site is more reasonable for the summing-up scheme. This time, the summed-up co-adsorption energy (Figure 3a, the red line and dots) presents the same trend as the directly calculated ones. Furthermore, the lateral interactions of co-adsorbed CO and O can be extracted (Figure 3b). A rough estimation can be done (the dashed red line) by directly subtracting the summed-up coadsorption energy using the b site adsorption energies for both CO and O (Figure 3a, the red line and dots) from the directly calculated ones. The lateral interactions are ~ 0.2 eV repulsive and they drop slightly as the tensile strain rises. A more accurate way ("the indirect subtraction") to obtain the lateral interactions is by using the frozen structures under strain with either CO or O removed, whose energies are used to extract the lateral interactions (Figure 3b, the dashed black line and dots). Both curves in Figure 3b have the same trend and quantitatively close (differ by ~ 0.03 eV). These results suggest that the trend of co-adsorption energy can be quantitatively reproduced by the linear combination of the adsorption energy of individual components at the near co-adsorption configuration, rather than the most stable configuration by itself.


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Figure 3. Co-adsorption of CO and O on Pt(100) surface with tensile strain. (a) Co-adsorption energies (ECO+O(ε)) versus strain. ECO+O(ε) are calculated using co-adsorbed configuration (Directly calculated, black), using both b site adsorption energies for CO and O (Summedup(b+b), red), and using 4f site and b site adsorption energies for CO and O (Summed-up(4f+b), blue), respectively. (b) Extracted CO and O lateral interactions versus strain. Directly subtracted (E1CO+O - E2CO+O, red) and indirectly subtracted (black). To get the latter, two steps are involved. (1) The individual adsorption energy is calculated by freezing the relaxed co-adsorption structures and removing one adsorbate (keeping the other) at a time. (2) The lateral interaction is obtained by subtracting these energies from the total co-adsorption energy. Note: The coadsorption configuration of CO and O on the Pt(100) surface is presented in Figure S1. At least at low O coverage, O2 automatically dissociates on low-index Pt surfaces such as (111), (100), (321).48-50 Although O adsorption is much stronger than CO adsorption, their coadsorption has exemplified the change of favorable binding site of O. Thus the mobility of O


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adatoms over strained surface may be crucial in many reactions including CO oxidation.42 The diffusion of an O adatom from one 4f site to a nearby 4f site is investigated under different strains. Similar to H diffusion on the TiN(100) surface, there are two typical migration pathways for O adatoms to move over the surface: the 4f-4f and 4f-b-4f paths (Figure 4a).51 For unstrained surface, the diffusion barrier via the 4f-4f path is 0.275 eV, higher than that of the 4f-b-4f path by 0.048 eV. A local minimum appears in the latter when the O adatom moves onto the b site, thus lowering the barrier than directly crossing the b site. The minimum energy paths of O diffusion under strains are plotted in Figure 4b, details see Table S2. A distinct decrease in transition barrier by ~ 0.165 eV is observed when the tensile strain increases from -3% to 3%, consistent with the tensile strain induced weakening of O adsorption at the 4f site mentioned above. However, for the co-adsorption case, the most stable configuration has CO and O both adsorbed at two neighboring b sites and the relocation of the latter from the 4f to b site is necessary (Figure 4), thus an easier diffusion of O atom could facilitate the reaction of CO oxidation under tensile strain.


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Figure 4. Diffusion pathways of an O adatom on the Pt(100) surface. (a) On the unstrained surface, diffusion pathways from a 4f site to its neighboring 4f site (black) directly or via the b site (red). Insets: top views of the diffusion paths. Dashed-open, pink-filled, and solid-open circles represent the initial, transition (TS), and finial states, respectively. The color coding of other circles is as in Figure 1. (b) Minimum energy paths (4f-b-4f) on the -3%, 0%, and 3% strained Pt(100) surface. The lack of unanimous response of local properties (e.g. adsorbates at various sites) to the perturbation introduced to global properties (e.g. strain or the d-band center shift) is expected to alter the energetics of a reaction path. Figure 5 sketches the potential energy profiles of CO oxidized by O adatoms via the L-H mechanism44 on the strained Pt(100) surfaces. The activation energy barrier from the initial state to the transition state for the stretched surface (3%) is below that of the unstrained one by ~ 0.29 eV, while the barrier of the compressed surface (-3%) is higher by ~ 0.57 eV. For the transition state of the stretched surface (3%), the O atom moves


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slightly to the 4f site compared with the originally favored b site in the initial state, whereas the CO molecule tilts about 19° relative to the vertical configuration. The distance between the C atom and the O adatom is 2.712 Å which is smaller than the initial state by 0.328 Å (Figure 5, inset). The big variation in reaction energetics is the collective results of different responses of the adsorbed O and CO to strain, invalidating the simple statement that all adsorptions are strengthened by tensile strain. In addition, our results are in line with the experimental and theoretical observations that the activity of nanoparticles with tensile strained surface is higher than that of compressed surface.3, 7

Figure 5. Energy profiles of CO oxidation on the Pt(100) surface. IS, TS, and FS refer to the initial, transition, and final states, respectively. The activation energies are labeled. Inserts: the geometry of the key states during the CO oxidation on the -3%, 0%, and 3% strained Pt(100) surface. The color coding of the circles is as in Figure 1. In summary, we have studied CO oxidation over the Pt(100) surface under surface strain (3% to 3%) using the periodic DFT calculations. Through investigating this model system, we hope to utilize strain to help solve the problem of CO poisoning over Pt. Our main observations are summarized as: (i) in contrast to the single linear relationship between the upward shift of the d-band center and the increasing strain (from compressive to tensile), the adsorption energies of


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key reactants (e.g. O adatom and CO molecule) exhibit bi- or multi-segment correlations to strain. These complex site-dependent responses of different adsorbates to strain are rooted in the localized nature of adsorption events. For example, the adsorption strengths of O adatom and CO molecule on their most stable binding sites present opposite changes over the same strained surfaces; (ii) for co-adsorption of CO and O, the lateral interactions relocate the O away from its most stable binding site, thus linearly combining the information of each involved adsorbate at its most stable binding site does not qualitatively reproduce the co-adsorption; (iii) the O diffusion barrier decreases on the tensile strained surface, which may facilitate the CO oxidation; (iv) the activation energy of CO oxidation on the strained Pt (100) surface is evidently reduced compared to unstrained surface, which is desired for curing the CO poisoning; (v) the complex responses of reaction energetics to strain require direct study of the reaction under strain rather than extrapolation of known behaviors of individual adsorbates under strain or reactions on unstrained surfaces. We have shown that the tensile strain can be utilized to enhance the reactivity of CO oxidation over the Pt(100) surface, thus strain is a useful handle to remedy or even solve the CO poisoning problem.

ASSOCIATED CONTENT Supporting Information Adsorption energy of O atom and O2, the bond length of O2 are provided in Table S1; Table S2 sum the energy difference and diffusion barrier of O atom under strained surface. Figure S1 show the configuration of O and CO co-adsorption on Pt surface. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author *Email: [email protected], [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is supported by National Natural Science Foundation of China (No. 51271135, 21203143), the program for New Century Excellent Talents in university (No. NCET-12-0455), the Fundamental Research Funds for the Central Universities, and the project of Innovative Team of Shanxi Province (No. 2013KCT-05).

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Table of Contents Graphic


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CO Oxidation over Strained Pt(100) Surface: A DFT Study - The

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