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Thermodynamics of alkanethiol selfassembled monolayer assembly on Pd surfaces Gaurav Kumar, Timothy Blair Van Cleve, Jiyun Park, Adri C.T. van Duin, J. Will Medlin, and Michael J. Janik Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b04351 • Publication Date (Web): 16 May 2018 Downloaded from http://pubs.acs.org on May 16, 2018
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Thermodynamics of alkanethiol self-assembled monolayer assembly on Pd surfaces
Gaurav Kumar,1 Timothy Van Cleve,2, Jiyun Park,1 Adri van Duin,1 J. Will Medlin,2, Michael J. Janik1 1 – The Pennsylvania State University, University Park, PA, 16802, USA 2 – University of Colorado, Boulder, CO, 80309, USA Abstract We investigate the structure and binding energy of alkanethiolate SAMs on Pd (111), Pd (100), and Pd (110) facets at different coverages. Dispersion-corrected density functional theory calculations are used to correlate the binding energy of alkane thiolates with alkyl chain length and coverage. The equilibrium coverage of thiolate layers strongly prefers 1/3 ML on the Pd (111) surface. The coverage of thiolates varies with chemical potential on Pd (100) and Pd (110), increasing from 1/3 ML to ½ ML on (100) and ¼ to ½ ML on (110) as the thiol chemical potential is increased. Higher coverages are driven by attractive dispersion interactions between the extended alkyl chains, such that transitions to higher coverages occur at lower thiol chemical potentials for longer chain thiolates. Stronger adsorption to the Pd (100) surface causes the equilibrium Wulff construction of Pd particles to take on a cubic shape when saturated with thiols. The binding of H, O, and CO adsorbates is weakened as the thiolate coverage is increased, with saturation coverages causing unfavorable binding of O and CO on Pd (100) and weakened binding on other facets. Temperature dependent CO DRIFTS experiments are used to corroborate the weakened binding of CO in the presence of thiolate SAMs of varying surface density. Preliminary results of multi-scale modeling efforts on the Pd-thiol system using a reactive force field, ReaxFF, are also discussed.
1. Introduction Self-assembled monolayers (SAMs) provide a controlled way to make surfaces with specific chemical functionalities.1-2 A SAM typically consists of highly ordered molecules with an inorganic head group covalently bonded to a solid substrate at one end. On the other end, the organic tails act as
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stabilizers due to van der Waals (vdW) interactions among them.3 Due to their high stability, SAMs provide molecular-scale control over surface chemistry and are used for organic electronics,4-5 lithography,6 corrosion resistance,7 molecular recognition,8 and controlled nanoparticle growth.9 Although metal nanoparticles modified by organic ligands are long proposed as promising candidates for catalytic applications10-12, it is only in the last decade that catalysis on surface-modified metal nanoparticles have been extensively explored.13-17 Thiol SAMs are a widely used class of SAMs with sulfur as the inorganic head group.18 Thiolatecoated Pd particles have been used for hydrogen sensing applications,19 etch resistance,20 and altering the magnetic properties of the nanoparticle.21 Palladium is also a widely used hydrogenation catalyst. Thiol coatings on Pd catalysts can selectively block certain surface sites or alter the binding orientation of reacting species, thereby driving the reaction toward the selective pathway. For example, thiol SAMs enhance selectivity in reactions involving molecules with multiple functionalities.22-23 Thiol SAMs on Au surfaces have been the most extensively studied.24-27 On Au (111), scanning tunnel microscopy (STM) and density functional theory (DFT) studies have revealed a predominant 1/3 ML covered (√3 x√3) 30 surface structure of alkane thiolates with a thiol nearest neighbor distance of 4.99 Å.27-29 On Au (100), however, thiols have been observed to form denser structures with a thiol-thiol nearest neighbor distance of 4.40 Å.27-29 The structure and stability of thiol-SAMs on Pd surfaces are yet to be resolved.30 X-ray photoelectron spectroscopy (XPS) has been used to determine the nature of the sulfur, showing multiple peaks.31-32 The XPS peaks are assigned to sulfur in the thiolate phase (attached to the alkyl chain and Pd surface), sulfide phase (incorporated in the Pd lattice), thiol phase (weakly adsorbed hydrogen capped thiol), and disulfide phase (dimerized thiolate). Although the sulfide phase can make the sulfur coverage higher, thiolates on Pd surfaces have been found to have a coverage close to 1/3 ML.33 We have previously shown that non-covalent interactions in thiol SAMs can be used for catalytic site selection, thus improving reaction selectivity.34 On thiol-coated Pd surfaces, furfuryl alcohol
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selectively hydrodeoxygenates to form methyl furan, with selectivity increasing with an increase in thiol chain length. Long chain alkanethiols are expected to poison the Pd surface completely, rendering it catalytically
inactive.35
However,
a
CO
DRIFTs
(Diffuse
Reflectance
Infrared
Fourier
Transform Spectroscopy) study performed on Pd surfaces coated with thiols of different chain length revealed that long chain thiols preferentially bind to the terrace sites, pushing CO to bind to an atop site signifying step site binding. Additionally, we used DFT to calculate the binding energetics of thiol SAMs on Pd (111) and Pd (221) surfaces to show that long-chain alkanethiols preferentially occupy the terrace sites, leaving steps sites open for reaction chemistry. These open step sites explain catalytic activity in Pd nanoparticles coated with long-chain alkanethiols. Reaction selectivity can be controlled by altering the binding orientation of the reactive intermediates,34 or by exposing specific surface facets that can perform selective chemistry.36 Facet dependent selective chemistry is often performed by synthesizing nanocrystals of specific shapes that reveal only the facets selective to the desired reaction. However, SAMs are promising candidates for modifying surfaces of metal polymorphs for selective catalysis. Therefore, to use thiolSAMs to tailor Pd catalysts with precise control over selectivity, there is a need for a better understanding of the assembly of the thiol molecules on different Pd surface facets. Despite significant advancements in the field, there are remaining challenges that need to be resolved for SAMs to advance in these applications. These include, but are not limited to, a reliable understanding of surface morphology, surface mobility, and thermal and chemical stability. Compared to the extensive literature on the properties of SAMs on Au, there is little information on thiolate SAMs on Pd (100) and (110) surfaces with most of the prior work focusing on elucidating the SAM structure on Pd (111).30, 37 Pd (100) and Pd (110) surfaces are expected to be exposed to a significant extent on nanoparticles. Herein, we use vdW-corrected density functional theory to investigate the assembly of alkane thiolates on Pd (111), Pd (100), and Pd (110) surfaces. We calculate binding energies of alkane thiolates as a function of alkyl chain length and surface coverage. We deconvolute the covalent and the non-
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covalent contributions of the binding energy to show that the non-covalent interactions between the alkyl chains dictate binding energy difference as thiol chain length and coverage is changed on a Pd surface. We also investigate the effect of thiol chemical potential on the SAM structure and examine how thiolate chemical potential can alter the Pd nanoparticle equilibrium morphology. 2. Methods 2.1. Electronic structure method: The surface stability of thiolates on Pd surfaces was studied using electronic structure calculations. All DFT calculations were performed using the Vienna Ab initial Simulation Package (VASP)38-39 code. The ion-core interactions were represented using the projector augmented-wave (PAW) method,40 with the exchange and correlation energy estimated using the Perdew, Burke, and Ernzerhof (PBE) functional.41 During the self-consistent energy calculations, the Brillouin zone was sampled using a Monkhorst-Pack grid.42 A Pd FCC bulk structure was used with a lattice constant of 3.953 Å. A plane wave basis set with an energy cutoff of 400 eV was used. Spurious interslab interactions were minimized by using a large vacuum region. All structures were relaxed such that the forces on all the atoms are lower than 0.05 eV Å-1. A stricter convergence criterion of 0.02 eV Å-1 resulted in marginal energetic or structural change. Detailed information about the parameters used for energy minimization and the optimal k-point mesh is provided in the supporting information. 2.2. Dispersion interactions: On thiolate-coated Pd surfaces, the sulfur head group of the thiolates bonds covalently to the Pd surface. However, the alkane thiolates also contain organic alkyl moieties that are stabilized by vdW interaction. The non-local dispersion effect of such organic molecules are not captured well by standard GGA functionals, and thus the binding energy can be significantly underestimated.34 Herein, we use the vdW-corrected DFT+D3 method43-44 to account for the non-local dispersion interactions. Herein, we use the vdW-corrected DFT+D3 method43-44 to account for the nonlocal dispersion interactions. The Grimme’s method used in this study has been previously compared to other van der Waals corrections like optPBE, optB88 and optB86b on Pd (111) surface.45 All the different dispersion corrections lead to similar optimized geometries and substantially increase the binding strength
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of functionalized molecules like furfural. As shown in Section 3, for long-chain alkane thiolates, the dispersion interaction can account for a major part of the overall binding energy. To study the binding behavior of complex organic structures, the importance of accounting for the non-local dispersion has also been shown in previous theoretical works.46-47 2.3. Surface models: Adsorption of alkane thiolates was studied on three Pd low index facets: Pd (111), Pd (100), and Pd (110). Although the close-packed Pd (111) and Pd (100) surfaces account for nearly all terrace sites and up to 70% of the total surface area of the Pd polymorph,48 we also used Pd (110) to investigate the adsorption behavior of the thiolates over more open-packed (less Pd atoms per unit area) and under coordinated facets. All the Pd surface slabs were modeled using at least four atomic layers with at most two bottom layers frozen. Various thiolate coverage was modeled using slabs of different unit cell sizes, and the k-point grid was adjusted accordingly. Detailed information about the unit cell sizes, k-point sampling, and the number of thiolates per unit cell are provided in the supporting information. 2.4. Thiolate binding energy: We calculate the adsorption energy of alkane thiolates on Pd (111), Pd (100), and Pd (110) surfaces at various coverages and for different alkyl chain length of thiol. We define the adsorption/binding energy of the thiolate, Eads as: =
+
/" #
−
/ %
(1)
where, EPd/clean, Ethiolate, and EPd/thiolate are the DFT calculated energies of bare Pd surface, isolated gas phase thiolate molecule, and the thiolate-adsorbed surface respectively. Nthiolate represents the number of thiolate molecules in the surface unit cell. The adsorption energy per thiolate is defined such that a positive value would represent favorable binding. The thiolate (R-S) in the gas phase is used as the reference state for the calculation of adsorption energies. Although there is much debate on the fate of thiol hydrogen and the choice of the reference state when considering binding energetics,49-52 gas phase
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thiolate provides a consistent and convenient reference for us to compare thiolate binding energies across different coverages, facets, and alkyl chain lengths. On C3 thiolate coated Pd (111), Pd (100), and Pd (110) surfaces, we calculate the adsorption energy of hydrogen, oxygen, and carbon monoxide. The adsorption energy for hydrogen, oxygen, and carbon monoxide on a thiolate coated Pd surface is defined as:
/& = /
+ ' &( − &/ /
/) = /
/*) = /
(2)
+ ' )( − )/ /
(3)
+ *) − &/ /
(4)
where, &/ / , )/ /
, *)/ /
are the energies of C3 thiolate coated Pd surface
with adsorbed H, O, and CO respectively. &( , )( , and *) are the energies of the gas phase H2, O2, and CO molecules. 2.5. Surface free energy change of thiolate binding: The surface free energy of different Pd facets is defined as the energy required per unit area to form the surface from bulk Pd atoms. We write the surface free energy of a bare Pd surface as: ," # = '-
./
0 123 " − 41 5 6
(5)
where, ," # is the surface free energy of a Pd facet, 123 " is the energy of the mirrored surface slab, 41 5 is the bulk energy of a Pd atom,
is the number of Pd atoms in the mirrored surface slab, and
7 4 is the surface area of one side of the mirrored slab. Surface energy of the thiolate coated surface, as a function of the thiolate chemical potential ,89 ), can be written as: ,89 ) =
0 / -./
−
− 9 6 + ," #
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(6)
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where,
/
and
are the energies of the thiolate-coated surface and the bare surface
respectively, 7 4 is the area of the surface slab, and is the number of thiolate molecules per unit cell. To calculate
/ ,
we assume that the thiolates lose all the configurational entropy upon
adsorption, and only include vibrational energetics. 9 , the chemical potential of the gas phase thiolate molecule (R-S), can be described in terms of the chemical potentials of thiol (R-SH) and H2 as below:
9 = 9 − 9&( '
(7)
In addition to the zero-point vibrational energy, we also account for the gas phase entropy of thiols and H2. The total chemical potential for the gas phase molecules can be written as: 9 :; 9&( = + ?
@A
− BC + DE
(8)
where, EDFT is the energy obtained from an electronic structure calculation, EZPVE is the zero-point vibrational energy, T is the temperature of the system, and Stot is the total vibrational and configurational entropy of the system. Even at very high pressures, the pV term is relatively small, and therefore, is neglected. 2.6. Wulff Shapes: Any crystal preferentially transforms itself into a shape that lowers the overall surface energy. Wulff construction is a method to predict the equilibrium shapes of nanoparticles based on the relative surface energy of different facets.53-54 Using the surface free energy data of Pd coated with thiolates of different chain lengths at various chemical potentials, we use Wulffmaker55 to predict the equilibrium crystal geometries of Pd nanoparticles. 2.7. Multi-scale modeling using a ReaxFF potential: When compared to quantum methods, empirical force field methods are computationally inexpensive, and therefore, allow for investigating a phenomenon over higher length and time scales. To elucidate the nature of thiol binding on a multifaceted nanoparticle, we use a ReaxFF potential. ReaxFF is a reactive potential based on the bond-order
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conservation principle and includes polarizable charge descriptions for Coulomb, covalent and van der Waals interaction between atoms. For a detailed description of the development and application of the ReaxFF method, readers are referred to a recent review article.56 We train a ReaxFF potential to DFT data, and use it together with a hybrid GCMC/MD method to investigate the structure of a thiolate covered Pd particle. The details of the force-field training method are provided in the Supplementary Information. 2.8. Catalyst Preparation and CO DRIFTS: Relative CO affinity on Al2O3-supported Pd catalysts modified with different thiols was experimentally measured using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). 1-Adamantane (AT), 1-hexanethiol (C6), and 1octadecanethiol (C18) were deposited onto a Pd/Al2O3 catalyst using an established procedure.57-59 Samples were reduced for 1h at 160 °C in 25 % H2/Ar, then cooled to 50 °C before introducing pure CO to the reactor system. After 5 minutes of exposure, CO was evacuated with a vacuum pump to a base pressure of
