First-Principles Investigation of AdsorbateâAdsorbate Interactions on

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A first-principles investigation of adsorbate-adsorbate interactions on Ni(111), Ni(211), and Ni(100) surfaces Mingxia Zhou, and Bin Liu Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 08 May 2017 Downloaded from http://pubs.acs.org on May 9, 2017

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Industrial & Engineering Chemistry Research

A first-principles investigation of adsorbateadsorbate interactions on Ni(111), Ni(211), and Ni(100) surfaces Mingxia Zhou,1 and Bin Liu1* Department of Chemical Engineering, Kansas State University, Manhattan, KS, 66506

KEYWORDS: Adsorbate-adsorbate interaction, surface coverage, density functional theory, water-gas shift reaction

ACS Paragon Plus Environment

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ABSTRACT Periodic density functional theory calculations were employed to investigate the co-adsorption patterns between CO and key water-gas shift reaction (WGSR) intermediates, i.e., H2O, H, OH, O, COOH, on Ni(111), Ni(100), and Ni(211) single crystal surfaces. It has been shown that although the nature of these adsorbate pair interactions are predominantly repulsive, as commonly assumed in the literature, the interaction pattern for each adsorbate pair can be complicated by facet lattice structures and potential intermolecular hydrogen bonds, which play an unsubtle role in influencing the WGSR redox and carboxyl pathways. This investigation can enrich our descriptions of the adsorbate lateral interactions for surface chemistry modeling, and will also provide theoretical insights into tuning the properties of nanoscale nickel catalysts by manipulating the surface structure and morphology for hydrogen fuel production.


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1. INTRODUCTION Hydrogen is considered as one of the most significant chemical and energy sources,1,2 and is obtained industrially through reforming, assisted by the water-gas shift reaction (WGSR), which proceeds according to Equation (1), H O() + CO() ⇌ CO() + H() , ∆H ° = −41.1 kJ/mol.

(1)

The mechanistic insights into key WGSR reaction intermediates are gained as a result from density functional theory (DFT) calculations.3-5 On transition metals, the carboxyl pathway is the dominant pathway on platinum6 and copper,7 while the redox pathway can also be competitive on nickel.8 A general reaction sequence consisting of 8 elementary steps (R1-R8), involving 8 reaction intermediates, i.e., CO, H2O, OH, O, H, COOH, CO2, and H2, are identified: () + ∗ ↔ ∗

(R1)

() + ∗ ↔ ∗

(R2)

∗ + ∗ ↔ ∗ + ∗

(R3)

∗ + ∗ ↔ ∗ + ∗

(R4)

∗ + ∗ ↔ () + 2 ∗

(R5)

∗ + ∗ ↔ ∗ + ∗

(R6)

∗ ↔ () + ∗

(R7)

2 ∗ ↔ () + 2∗

(R8)

According to the above mechanism, the active sites (on transition metals) are responsible for CO chemisorption, as well as water activation. In recent years, convincing evidence has been accumulated suggesting that oxide supports are very effective in oxidizing CO at the metal-oxide interfacial regions.9,10 Regarding metal catalyzed WGSR, Grenoble et al11 showed that a volcano-type relationship exist to predict WGSR activity using CO adsorption energy as the descriptor, where copper is shown to be the most active.11 Nevertheless, nickel’s role in WGSR should be considered relevant due to its significance as an industrial catalyst for hydrogen


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production through reforming12 and earth abundance. It is well recognized that nickel also promotes a number of side reactions, such as methanation and hydrogenolysis, both of which would lower hydrogen selectivity.8,12,13 This makes understanding catalytic behavior on nickelbased material an even more deserving topic. Adsorbate lateral interactions directly influence the chemisorptions and reaction energy barriers of reaction intermediates.14-17 According to the mean field microkinetic modeling of WGSR activity on Ni nanoparticle surface, Zhou et al.18 found that the H2 production rate follows the order of Ni(111) < Ni(100) < Ni(211), a qualitatively anticipated behavior. However, in the absence of later interactions, the coverage of strong binding reaction species, such as CO, can be also severely overestimated. Here, we hypothesized that adsorbate-adsorbate lateral interactions may vary on different single crystal facets, also as a function of particle shape and size. Furthermore, the level of impact could also alter the catalyst selectivity behavior. Hence, it is necessary to elucidate the nature of adsorbate-adsorbate interactions on different catalyst facets. In a broader context, the different behaviors of how adsorbate molecules interact with its neighbors in a local environment would offer insights into development of the capability of modeling the heterogeneity of a catalyst surface. In order to quantitatively describe the CO-CO lateral interaction, which is repulsive, empirical correlations, in the forms of polynomial functions of CO coverage ( ! ) on copper Equation (2),7 and platinum Equation (3),6 have been developed by fitting to the DFT-calculated CO binding energies.

"#

!∗ ( ! )

"#

!∗ ( ! )

= 6.81 ! − 3.14

!

− 0.53)*

= −1.78 + 0.0065 exp(4.79 ! ) + 0.031135 ! exp (4.79 ! )

(2) (3)

The binding energies of other adsorbate species, such as O, H, OH, and COOH, can then be approximated through a simple correction using either Equations (4) or Equation (5), as a function of ! . "#( ! ) = "#0 +

1 1 + exp [3(0 − ! )]

"#( ! ) = "#0 + 1exp(3 ! )


(4) (5)

4

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where a, b, θ0, are fitting parameters, and BE0 is the binding energy of the adsorbate calculated without the presence of CO. More recent work has suggested that the binding energy of an adsorbate, such as CO, remains relatively constant below a certain threshold coverage (0 ). Beyond this coverage, the influence of lateral interaction becomes pronounced quickly.14,16,17 This pattern can be generalized by the piece-wise integral binding energy ("# 567 ), as shown in Equations (6),

"# 567 ( ! ) = 8

"#0 ! , "#0 ! + :( ! − ∗ ) ,

!

!

≤ 0 > 0

(6)

where "# 567 ( ! ) = ! "# , and : (in eV/ML) is a measure of the interaction strength between adsorbates, known as the interaction parameters. In this paper, periodic DFT calculations were first performed to determine the CO chemisorption structures and adsorption energies over a range of surface coverage on Ni(111), Ni(100), and Ni(211) facets. The adsorption structures and corresponding binding energies for H2O, OH, O, H and COOH co-adsorbed at the 4/9 ML coverage were then obtained. Finally, the adsorbate-adsorbate interactions were considered explicitly for the analysis of WGSR pathways for hydrogen production on different nickel single crystal facets.

2. COMPUTATIONAL METHODS Periodic density functional theory (DFT) calculations were performed using Vienna Ab initio Simulation Package (VASP)19,20 with the GGA-PBE exchange-correlation functional21. The electron-ion interaction was described by the projector-augmented wave (PAW) method,22 with plane wave energy cutoff being 385 eV. The Brillouin-zone was sampled with a 4 × 4 × 1 k-point mesh based on the Monkhorst-Pack scheme.23 The electronic occupancies were determined by the Methfessel-Paxton scheme,24 with the width of smearing being 0.2 eV. The self-consistent iterations were converged to 1 × 10-6 eV, and the geometry optimizations were stopped until the residual force is smaller than 0.02 eV/Å. A p(3 × 3), an orthogonal (3 × 3), and an orthogonal (1 × 3) supercell with 3 layers with the bottom two layers fixed were use to model the Ni(111), Ni(100), and Ni(211) facet with the


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vacuums between successive slabs are about 24 Å, 15 Å, and 18 Å, respectively. All calculations were performed with spin polarization to account for the net magnetic moment of nickel. The optimal configuration of CO co-adsorption at a 1/9, 2/9, 1/3, 4/9, 5/9, 2/3, 7/9, 8/9, and 1 ML coverages were determined through geometrical optimizations. In this study, we adopted the conventional definition for surface coverage, hence, the corresponding number of adsorbed CO molecules is 1 – 9 on each facet. The interactions of CO with H2O, OH, O, H, and COOH were considered specifically at the 4/9 ML coverage, using the optimized CO configuration (at 4/9 ML coverage) as a starting point. The adsorbate molecule was then introduced to the open site for further optimization. At the low coverage limit (i.e., 1/9 ML), the binding energy ("#? ) of adsorbate @, representing H2O, H, OH, O, and COOH, is calculated using Equation (7), "#? = #?ABC

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