Adsorption of Acetonitrile, Benzene, and Benzonitrile on Pt(111

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Adsorption of Acetonitrile, Benzene and Benzonitrile on Pt(111): Single Crystal Adsorption Calorimetry and Density Functional Theory Armin Shayeghi, Stephan Krähling, Peter Hörtz, Roy L. Johnston, Christopher James Heard, and Rolf Schaefer J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05549 • Publication Date (Web): 07 Sep 2017 Downloaded from http://pubs.acs.org on September 24, 2017

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The Journal of Physical Chemistry

Adsorption of Acetonitrile, Benzene and Benzonitrile on Pt(111): Single Crystal Adsorption Calorimetry and Density Functional Theory A. Shayeghi,

∗,†,‡

S. Krähling,

†

P. Hörtz,

†

R. L. Johnston,

Schäfer

¶

C. J. Heard,

§

and R.

†

†Eduard-Zintl-Institut, Technische Universität Darmstadt, Alarich-Weiss-Straÿe 8, 64287 Darmstadt, Germany ‡Vienna Center for Quantum Science and Technology, Faculty of Physics, University of Vienna, Boltzmanngasse 5, A-1090 Vienna, Austria ¶School of Chemistry, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom §Department of Applied Physics, Chalmers University of Technology, SE-41296 Gothenburg, Sweden E-mail: [email protected]

Abstract

Introduction

Single crystal adsorption calorimetry on Pt(111) surfaces allows us to study the inuence of dierent functional groups on heats of adsorption. At zero coverage we nd 103 kJ/mol, 194 kJ/mol and 325 kJ/mol for acetonitrile, benzene and benzonitrile, respectively. This leads to the idea of summing up the heats of adsorption of acetonitrile and benzene, assuming that the total heat of adsorption of benzonitrile is due to the combined eect of isolated functional groups. This idea is discussed in the light of the recent literature regarding nitrile adsorption on metal surfaces and further investigated theoretically by density functional theory. In order to gure out the importance of dispersive eects on the heats of adsorption, van der Waals corrected calculations are performed considering dierent binding modes and surface reconstructions.

A precise knowledge of the interactions between adsorbed molecules and surfaces is one of the key steps towards a deep understanding of heterogeneous catalysis. Much work has been done to investigate a large variety of molecules on dozens of dierent surfaces. One way to address such complex problems is to build ideal model systems in order to reduce complexity. More complex systems can then be studied in experiments. 1 Currently, the direct investigation of the interaction between adsorbates and well-dened surfaces under ultra high vacuum (UHV) conditions is possible with a large variety of analytical methods. These range from XRay Photoelectron Spectroscopy (XPS) to High Resolution Electron Energy Loss Spectroscopy (HREELS) and Scanning Tunneling Microscopy (STM). 2,3 Another promising tool is the so-called Single Crystal Adsorption Calorimetry (SCAC), developed by the group of King 48 and further improved by the groups

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of Campbell, 918 Freund 19,20 and Schäfer. 2123 This technique is able to provide detailed information on heats of adsorption as well as other important kinetic parameters, such as sticking coecients. By measuring heats of adsorption directly, no initial assumptions on the adsorption processes are necessary as is also the case for methods such as temperature programmed desorption (TPD). Some molecules, such as carbon monoxide, have been studied widely and therefore the inuence of surface material, crystal orientation, surface temperature and coverage is well known. 24 Benzene (BZ) has also been studied in detail. 12,21 It is a molecule of technological interest and a feasible model system to study with theoretical methods. In the case of Pt(111) surfaces BZ is known to bind via four di- σ and two π bonds and is oriented parallel to the surface. 2527 By combining the study of BZ with one of acetonitrile (AN) and benzonitrile (BN) we arrive at a point where it is plausible to discuss the inuence of functional groups and van der Waals (vdW) eects on the heats of adsorption. The question arises if the nitrile and phenyl groups act independently in the case of BN adsorption or if only one functional group is particularly responsible for binding to the surface? If both groups interact independently this would reect known group additivity methods in thermochemistry like the Benson group increment theory or the UNIFAC method. 28 Such an approach for heats of adsorption would have to be checked for a large variety of molecules, which is far beyond the scope of this paper. We rather focus on what happens if we replace one hydrogen atom of BZ by a nitrile group, both from an experimental and theoretical point of view. The nitrile group contains a second π electron system which can interact with that of BZ. Furthermore, it is capable of binding to the surface on its own as it is seen in the case of AN with dierent binding modes and orientations. 29,30 Here, we study BN, BZ and AN with Single Crystal Adsorption Calorimetry (SCAC) at T = 300 K. The results are discussed together with other studies regarding nitrile adsorption and are supported by theoretical results ob-

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tained by a detailed investigation of dierent binding modes and sites at the Density Functional Theory (DFT) level including vdW corrections, with dierent stepped surfaces also considered. Combined with previous studies this approach may help to develop a fundamental understanding of adsorbate geometries of organic π electron systems on platinum surfaces.

Experimental & Theoretical Methods Experiments

The measurements have been performed in a previously described SCAC apparatus designed and built in our labs. 21 It consists of two UHV chambers (≈ 2 × 10−10 mbar). One chamber is used for sample preparation, Auger Electron Spectroscopy (AES) as well as a Low Energy Electron Diraction (LEED) and the other for the actual calorimeter. Only a brief overview of the experimental setup is given here, more details can be found elsewhere. 22,23 An eusive molecular beam is generated with a piezoelectric driven gas doser targeting the sample surface. 22,31 The sample is a 2 µm thin Pt(111) foil (Mateck, Germany), spot-welded between two Ni-sheets for mechanical support. It is cleaned with standard UHV cleaning techniques like Ar-ion sputtering followed by an annealing step in order to remove surface defects introduced by sputtering. Sample cleanliness is controlled by AES and LEED. Measurements of heats of adsorption at 300 K are performed using a pyro-electric detector based on a β -PVDF foil pressed to the back of the Pt(111) sample. Adsorbing molecules cause a temperature rise leading to a pyro-electric current which is amplied and recorded. A diode laser is used to calibrate the calorimeter. The number of adsorbed and reected molecules is determined by a modied KingWells method with a quadrupole mass analyzer detecting all molecules which do not stick permanently to the surface. 32 The surface coverage θ (with the unit ML) is here dened as the number of adsorbate molecules divided by the


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number of surface atoms (for Pt(111) 1.5051015 cm−2 ). To determine the molar heat of adsorption from the calorimeter signal corrections for (i) the energy dierence between a gas ux and a gas volume at the measurement temperature and (ii) the conversion of adsorption energy to enthalpy, as shown by Lytken et al., have to be applied. 3335 The determination of the monolayer coverage is based on the measurement of the sticking coecients. For example, for BZ the sticking probability drops to zero at 300 K as one monolayer is completed. 12,21 However, the determination of the monolayer coverage is complicated by the fact that typically only a small number of molecules adsorb permanently close to the completion of a monolayer. This is particularly true for BZ and AN. This is due to the fact that molecules also adsorb at the sample mount and other components of the apparatus. As a consequence the mass spectrometer signals corresponding to the adsorbed and reected molecules showed a strong background signal which necessitates a baseline correction to determine the mass spectrometer signals correctly. Therefore, the evaluation of the number of the adsorbed and reected molecules according to the King-Wells method is not unambiguous. Additionally, one has to discriminate between transiently adsorbed molecules, which are relevant for the measured heats signals, and molecules which adsorb permanently at the surface, which determine the absolute value of the coverage. Therefore, short-term (apparent) and long-term (netto) sticking coecients have to be determined. Averaging several measurements indicated that both sticking probabilities could be determined with an uncertainty of 10-20%. The monolayer coverage could then be obtained from a precursor-mediated sticking model (Kisluik model) 36 tted to the data of the long-term sticking probabilities. Thus, the derived values for the monolayer coverage are uncertain as well to at least 10%. Furthermore, the heats of adsorption also give some indication for the monolayer coverage, as a constant value of the heat indicates the completion of a monolayer of adsorbed molecules. The presented values for the monolayer coverage of BZ,

AN and BN has been obtained by taking the sticking probabilities and the heats of adsorption into account and these values should be taken with an uncertainty of not less than 10%. Additionally, settings of the gas doser diers for the three molecules. In case of BZ the vapor pressure is high enough to directly introduce the gas. In case of AN and BN freeze-pump-thaw cycles were used to generate a clean lling of the gas doser. The cleanliness of the introduced gas was checked with the quadrupole mass detector.

Theory Calculations were performed using the periodic formalism of DFT, within the QuantumEspresso package, PWscf. 37 Ultrasoft RabeRappe-Kaxiras-Joannopoulus (RRKJ) pseudopotentials 38,39 were employed for the treatment of core electronic states, while the valence was explicitly modelled, with 10, 4, 5 and 1 electrons for platinum, carbon, nitrogen and hydrogen atoms, respectively. Non-linear core corrections are included in the N and C pseudopotentials, while the Pt pseudopotential contains a semi-core d state in the valence. The wavefunctions and charge density functions are represented by expansions to 476 eV and 3.8×103 eV, respectively. The generalised gradient exchange correlation (xc) functional as developed by Perdew, Burke and Ernzerhof (PBE) were used. 40 For calculations with vdW corrections, the exchangeconsistent vdW-DF method of Berland and Hyldgaard is applied. 41 This method is based on the non-local correlation formulism of Dion et al. 42,43 and has proven accurate for modelling both metallic systems and the adsorption of small organic molecules. 44 The closed packed platinum surface was modelled as a Pt(111) slab. Nine vacuum layers were introduced above the surface, in order to minimise spurious image-image interactions in the z direction, while three metal layers were used to represent the surface. Test calculations have shown there to be a minor eect of increasing the number of layers to ve increasing the adsorption energy by less than 0.05 eV. K-point sampling was performed employing to a 5 ×5×1


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grid for the 2×2 supercell, a 3×3×1 grid for the 3×3 supercell, and at the gamma point only for the 4×4 supercell. The Methfessel-Paxton smearing scheme was employed to rst order, 45 to smear the Fermi-Dirac distribution, and aid the convergence of metallic electronic states, with a smearing width of 0.14 eV. To model stepped surfaces, a range of model Pt terminations are considered, including the Pt(211), Pt(533) and Pt(321) structures. During local geometry optimisation, the top two surface layers were allowed to relax in all three directions, in addition to adsorbate molecules. Electronic self consistency was considered to be achieved at a maximum dierence of 1.4×10−4 eV between steps. Geometries were considered to be relaxed when consecutive total energies and forces have a maximum dierence of less than 1.4×10−3 eV and 2.6 ×10−2 eV Å−1 between steps, respectively. It should be noted that our theoretical method considers energy rather than enthalpy, which is strictly appropriate only for adsorption at 0 K in the absence of entropic contributions, which are inherent in the experimental data. The dierence between enthalpy and energy, however, is 2-3 kJ/mol for our experimental conditions. The adsorption energy is therefore 2-3 kJ/mol smaller than the adsorption enthalpy, which is negligible when compared to the experimental errors and the uncertainties of our theoretical predictions.

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Figure 1: Integral and dierential heats of ado sorption −∆Hm of BZ on Pt(111) at room temperature. The dierential heats decrease linearly to 80 ± 6 kJ/mol at the monolayer coverage of the surface of θ = 0.165 ML while the integral heat at this coverage is 129 ± 5 kJ/mol. The latter decrease linearly to this value but with a smaller slope than the dierential heats. with earlier measurements from our group, 21 of 200 kJ/mol and the group of Campbell, 12 of 197 kJ/mol. The dierential heats decrease linearly to 80 ± 6 kJ/mol at the monolayer coverage of θ = 0.165 ML while the integral heat at this coverage is 129 ± 5 kJ/mol. Following the investigation of BZ it is natural to look at the nitrile group as the second functional group of BN before considering the complete BN molecule. The nitrile group is modeled by the simplest nitrile AN. The heat of adsorption of AN is shown in Fig. 2. At zero coverage it is approximated to be 103 kJ/mol. It drops rather fast with increasing coverage until it reaches a value of 66 ± 5.5 kJ/mol at θ = 0.06 ML. Reaching the monolayer coverage of θ = 0.19 ML at 300 K the dierential heat drops to 36 ± 4 kJ/mol while the integral heat is 49 ± 3 kJ/mol. The molar heats of adsorption of BN on Pt(111) are shown in Fig. 3. The initial heat at zero coverage of 325 kJ/mol is signicantly higher than the heat obtained for BZ. Following a sigmoidal curve with increasing coverage, the dierential heat drops to 70 ± 7 kJ/mol at the monolayer coverage of θ = 0.14 ML, while the

Results & Discussion SCAC Results

Benzene is one of the most extensively studied molecules in the eld of SCAC and therefore the results obtained and presented here may be seen as a benchmark for our setup. The heat of adsorption of BZ on Pt(111) is shown in Fig. 1 as a function of surface coverage θ. The coverage is here dened as the number of adsorbate molecules divided by the number of Pt surface atoms. The initial heat obtained by extrapolation of the measured data to zero coverage is 194 kJ/mol, in good agreement


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integral heat shows a value of 169 ± 5 kJ/mol. At even higher coverages the formation of a multilayer takes place. This is evident from the sticking probabilities but also from the heats of adsorption which nearly constant values for the adsorption enthalpy for >0.14. In the coverage range >0.14 we obtain a dierential heat of 55 kJ/mol which is in remarkably good agreement with the heat of vaporisation of BN,. 46 This supports our determination of the monolayer coverage and it demonstrates that the formation of multilayers is accompanied with heats of adsorption close to the heat of vaporisation. However, the formation of multilayers at room temperature is quite unexpected for such a small value of the adsorption enthalpy. Taking rst order desorption kinetics into account one arrives at residence times much shorter than the time between two gas pulses if typical values of the pre-exponential factor are considered. Therefore, no multilayer formation should be observed at room temperature. The emergence of multilayers point either to a constraint transition state for BN desorption, i.e. the prefactor must be much smaller than 10 10 1/s, or an additional activation barrier has to be overcome before desorption takes place. Looking at the heats of adsorption of BZ we note that the obtained results are in good agreement with literature results. This shows that our SCAC setup provides data in reasonable agreement with other experiments. In case of BZ, the high value of the heat of adsorption can be explained by the formation of four diσ - and two π -bonds between BZ and the Pt surface. Starting at this point it is even more striking that the measured heat for BN is higher than that of BZ. By simply summing the initial heats of BZ and AN we obtain a value of 297 kJ/mol which is close to the measured heat of BN of 325 kJ/mol. Thus, the natural question is whether this is due to simultaneous adsorption of both functional groups?

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Figure 2: Integral and dierential heats of ado sorption −∆Hm of AN on Pt(111) at room temperature. Reaching the monolayer coverage of θ = 0.19 ML at 300 K the dierential heat drops to 36 ± 4 kJ/mol while the integral heat is 49 ± 3 kJ/mol. As in the case of BZ the slope of the integral heat is smaller than in the case of the dierential heats while both curves show similar trends.

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Figure 3: Integral and dierential heats of ado sorption −∆Hm of BN on Pt(111) at room temperature. The initial heat at zero coverage of 325 kJ/mol is signicantly higher compared to BZ. Following a sigmoidal curve with increasing coverage the dierential heat drops to 70 ± 7 kJ/mol at the monolayer coverage of θ = 0.14 ML while the integral heat shows a value of 169 ± 5 kJ/mol. The sigmoidal curve shape is much less pronounced for the integral heat than for the dierential heats.

Adsorption There is no literature data for the heat of adsorption of AN and BN measured with SCAC available to the best of our knowledge. There-


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siently adsorbed molecules. 21 They adsorb for a short period of time while releasing the heat of adsorption into the sample and desorb later on. The rather high initial heat could be explained by the presence of a small number of surface defects. Unfortunately, there are only a few studies about BN on Pt(111) under UHV conditions. Near-Edge X-Ray Absorption Spectroscopy (NEXAFS) shows that benzonitrile is lying at and parallel to the surface upon adsorption. 52 Another study investigating the reaction of styrene and nitrogen to form BN by Yin et al. is available. 53 In their TPD and Infrared Reection Absorption Spectroscopy (IRRAS) experiments a rather weakly bound adsorption state desorbing at 87 K is found. Only weak IRRAS signals are obtained in the C-N region. Therefore, the authors conclude that BN is lying at on the surface. However, it should be noted that a rehybridisation of the nitrile group would lead to a deviation from the planar molecular geometry. Surface Enhanced Raman Spectroscopy (SERS) was used to study BN adsorbed from solution onto Ptelectrodes. Here, the binding of BN to the surface is achieved through the lone pair of the nitrogen atom. 54 This type of binding is also observed for BN on silica-supported platinum via IR spectroscopy. 55 In contrast AN and BN have been studied in detail on Pd- and Ni- surfaces. 4749,51,56,57 Therefore, we discuss the binding of AN and BN to dierent surfaces, such as Ni and Pd and compare them with the binding modes observed on Pt. Adsorption through the free electron pair of nitrogen to polycrystalline Ni surfaces has been observed with XPS methods, in addition to the rehybridisation type binding for BN. 48 Rehybridisation seems to be smaller in the case of BN than for AN. The authors conclude that there is no interaction between the π system of BN and the surface. This conclusion is supported by TPD and especially IRRAS measurements of BN on Ni(111) as well as from XPS data. 51,57 In summary, we expect that the rehybridisation of the nitrile group is higher on Ni surfaces than on Pt surfaces as Ni is a stronger electron

fore, only a comparison with values obtained with TPD measurements are possible. The following discussion will also include what is known about the binding modes of AN and BN on various transition metal surfaces from spectroscopy and theoretical studies. Nitriles are interesting, as they oer the possibility to coordinate through the nitrogen lone pair in addition to several binding modes which are based on (partial) rehybridisation of the CN triple bond in the case of chemisorption. In the language of organometallic chemists we can name these as η 2 (C, N ) in the case of a sp2 hybridisation and η 4 (C, N ) in case of a sp3 hybridisation of the carbon and nitrogen atom. Acetonitrile has been studied on a large variety of single and polycrystalline surfaces. Sexton et al. used TPD experiments of AN on Pt(111) and observed two distinct binding modes corresponding to two dierent adsorption modes. 29,30 The rst, which has only been observed for higher coverages with a desorption temperature of 210 - 240 K could be assigned to a η 2 (C, N ) state while a simple Redhead analysis with a pre-exponential factor of ν = 1 × 1013 1/s results in a binding enthalpy or heat of adsorption of 57.5 - 66.5 kJ/mol. 29 This is in good agreement with our TPD results for larger θ. The second state with a desorption temperature of 310 - 340 K results in a heat of adsorption of 85.0 - 93.5 kJ/mol and corresponds to a η 4 (C, N ) state. Despite the fact that the coverage is not known here the values t quite well to what we nd experimentally for small θ. The binding of AN to Pt surfaces has been studied by TPD, XPS/UPS, EELS and work function measurements. 4751 It was found that the molecule binds to the surface under rehybridisation leading to Pt-N and Pt-C bonds. 29,30 In summary, AN is predicted to adsorb in an 2 η (C, N ) state while a small fraction is adsorbed with higher heats of adsorption in a η 4 (C, N ) mode, probably due to surface defects. However, the question arises why we are able to observe an adsorption of a quite large amount of AN at room temperature while having only few surface defects as shown by clean LEED patterns. One can argue that this is due to tran-


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putationally at a coverage of θ = 0.06, which lies in the mid range probed by the current experiments. The most stable adsorption geometries are shown in Fig. 4. It has been stressed that in chemisorption reactions, such as BZ upon Pt(111), the binding is a balanced combination of vdW and covalent bonding. 44,58 As a result, the vdW component to the adsorption energy could be comparable to the covalent component even for such species that are strongly adsorbed. The adsorption energies and geometries are therefore determined for each system using a vdW-corrected GGA functional. As a well-studied adsorption system, it has been established that the coplanar, "bridge30" adsorption mode is preferred upon Pt(111) for BZ, 26,5962 and is captured both with and without vdW corrections to the exchangecorrelation (xc) functional. We therefore only consider this mode here. On adsorption, the binding energy is found to be 179 kJ/mol (experimental integral heat of adsorption: 170.2 ± 6.9 kJ/mol). This value agrees well with the predictions at θ = 0.06 made by Campbell et al. 12 of 183 kJ/mol and earlier measurements of our group of 178 kJ/mol. 21 In addition, this is in excellent agreement with the adsorption energy recently calculated by Liu et al. using a vdw-corrected DFT method. 58 The geometry of BZ is distorted from the gas-phase structure upon chemisorption, with hydrogen atoms pushed away from the surface due to the partial rehybridisation of the BZ molecule towards sp3 . On adsorption, the C-C bonds of benzene encounter two dierent Pt local environments, leading to an asymmetry in the nal C-C bond lengths. Two C-C bonds lie directly atop Pt atoms. Four C-C bonds lie above bridging sites between adjacent Pt atoms. This dierence in adsorption environment leads to two equilibrium C-C bond lengths: 1.43 Å for the atop sites, and 1.48 Å for the bridging sites. This behaviour can be rationalised by comparison to the adsorption of alkenes upon transition metal (111) surfaces. It has been shown that the main adsorption modes for ethylene on Pt(111) are the bridging and atop modes. 63 The bridging mode involves a formal reduction of bond order

donor into the empty π ∗ orbital of the nitrile group. Also a higher temperature leads to different adsorption mechanisms. The rehybridisation should be lower in the case of BN because of the phenyl group. Following the discussed results of other groups we propose that BN binds to the Pt(111) surface with a partial rehybridisation leading to a bent molecule. Therefore, an interaction between the nitrile group as well as the phenyl ring with the surface may be possible. If this is combined with a possible reconstruction of the surface, both groups may contribute signicantly to the binding and the heats of adsorption. 1 In order to get a detailed picture of the dierent binding modes and their possible contributions to heats of adsorption state of the art vdW-corrected DFT calculations are performed.

Theoretical Results Adsorbate Structures

The adsorption of the three molecules, BZ, AN and BN on platinum surfaces is modeled com-

Figure 4: Tilted and top-down views of the most stable adsorption geometries of BZ, AN and BN on Pt(111). Atom colours are: greyplatinum, black-carbon, white-hydrogen, bluenitrogen.


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from two to one, and a concomitant increase in bond length which is greater than for the atop mode, in alignment with the behaviour of benzene upon Pt(111). The preferred adsorption geometry of AN was predicted by Markovits and Minot using the PBE functional, 64 at a coverage of θ = 0.25 and an adsorption energy of 53.6 kJ/mol over Pt(111) was found. Including a C-N bond which is coplanar with the surface, with rehybridisation of the nitrile carbon from sp to sp 2 , leads to a distortion of the CH 3 group away from the surface. Recent work by Pa²ti et al. 65 has extended this work to include Pt(100) surfaces and a range of coverages, predicting a heat of adsorption of 77.2 kJ/mol at θ = 0.09. The preference of coplanar Pt and AN, with the adsorbate in a bridge-bound conguration and a rehybridisation of the AN bonding is recovered consistently. In this work, several binding modes to Pt(111) are considered, taking vdW corrections into account: the adsorption of sp-hybridised AN in a di-σ mode, the adsorption of sphybridised AN in a π mode, and the endon adsorption of AN at the N atom (atop). These molecules are adsorbed at a coverage of θ = 0.06. It is observed that the π mode has no stable absorption to the surface, while the binding energies of the atop and di- σ modes are 110.1 kJ/mol and 131.3 kJ/mol, respectively, which are higher than found in the current experiments. The preference for the mode parallel to the surface, as found in previous studies is recovered. For the di- σ mode, the C-C bond length is extended from 1.45 to 1.49 Å, due to rehybridisation, while the C-N bond increases from 1.16 to 1.25 Å. This is in excellent agreement with the results of Pa²ti et al. 65 The C-N bond is almost perfectly parallel to the surface, with C-Pt and N-Pt bond lengths of 2.07 and 2.03 Å, respectively. Interestingly, the bond lengths to platinum atoms are longer for the di-σ mode than for the atop mode (1.98 Å), which has a lower adsorption energy. Qualitatively, this result agrees with Markovits and Minot, 64 and with the work of Pa²ti and colleagues. 65 The former study observing that the atop mode adsorbs with 77% the energy of

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the di-σ mode, as compared with 84% in the current work. For BN, several possible adsorption geometries are available, due to the possibility to adsorb either via the phenyl or nitrile groups. In this work, we consider three adsorption modes: binding at the C-N group, binding at the phenyl group and perfectly coplanar binding at both C-C and C-N groups. In addition to these modes, two orientations of the molecule on the Pt(111) surface are considered: one in which the C-N group is aligned along the Pt-Pt long bridge (shown in Fig. 4), and another, in which the group is aligned along the (111) axis (not shown). Of these six calculations, three stable local minima were located. These modes are two phenyl-bound structures, and one in which the cyano group adsorbs, leaving the phenyl group free. The highest adsorption energy is 163.2 kJ/mol at θ = 0.06, and shows adsorption through the phenyl group to be favourable over binding through the cyano group, as shown in Fig. 4. The adsorption energies are lower than for BZ, which can be attributed to the fact that BN requires an unfavourable distortion which twists the cyano group away from the surface. As the phenyl ring binds to the surface, a signicant rehybridisation occurs, in which all hydrogen atoms (and, in the case of BN, the CN group) point away from the surface. The main energetic driving force for adsorption is the electronic redistribution on binding phenyl. Forcing the system to readopt a planar sp 2 hybridisation in order to additionally bind CN to Pt, induces an unfavorable strain, and therefore, no such minimum is found. While the rehybridisation of BN is less extensive than AN, it is large enough to deny CN adsorption, and as a result, we do not nd the co-binding expected from the SCAC energetic results. Overall, the adsorbate geometries are in good agreement with the existing computational literature where available, and the adsorption energy of benzene is accurately reproduced with respect to the current experimental results. However, the benzonitrile adsorption energy is signicantly underestimated, and thus the sum-rule for the adsorption energies of the three molecules, predicted


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by the current SCAC experiments, is not observed in the calculations. The most striking dierence for BN adsorption is that the vdWDFT calculations suggest a bent mode, in which the cyano group does not interact strongly with the surface. Such an interaction, representing the acetonitrile part of the adsorption energy, would be expected, to full the sum-rule. In the following sections, we examine the role of adsorbate coverage and surface defects on adsorption geometries and energies, and determine whether the predicted sum-rule is reproducible under dierent surface conditions. In addition, it has been recently noted in a computational study of Sautet et al., that for various hydrocarbons, the adsorption energies may vary signicantly upon the addition of a vdW correction, while the geometric and electronic structure of the adsorbed system is negligibly aected. 66 It is therefore interesting to compare the results obtained with PBE-DFT, and determine the extent of the vdW eect for our systems of study, with particular attention paid to the geometry of the BN adsorption mode. It has been shown that the current generation of vdW-corrected DFT functionals have limitations to their transferability in calculating adsorption energies, 66 with diering accuracies for dierent adsorption types. A possible explanation for the discrepancy found in this work is that while the pi-bonding of benzene and phenyl groups are accurately represented, the behaviour of the CN group is less accurately treated by our choice of functional.

relevant to the current experiments. The resulting adsorption energies are 68.6 kJ/mol and 179.6 kJ/mol, respectively. Thus, the adsorption energy is found to decrease signicantly with increasing coverage. We conclude that the absolute adsorption energy in the dilute limit is well characterised by the vdW functional, and the qualitative behaviour of adsorbate interactions is captured. The coverage dependence of AN is additionally tested. It is found, when decreasing the cell from the 4×4 to the 2×2 cell, which correspond to coverages of θ = 0.06 and 0.25, respectively, the adsorption energy changes negligibly, with a value of 131.3 kJ/mol for both. This is interesting, as it does not reproduce the trend found with SCAC. Because the molecule has localized covalent bonding to a small number of surface atoms, it may be that the vdW correction does not fully account for the lateral interactions, while overbinding the molecule to the surface. It is interesting to note, however, that the study of Pa²ti et al. also shows a complicated coverage dependence for AN adsorption in the stable mode, which decreases by only 5.8 kJ/mol from θ = 1/9 to θ = 1/3, with a less stable intermediate coverage of θ = 0.25 and a decrease of 12.6 kJ/mol showing an overall weak coverage dependence. For BN, cells of 3×3 and 4×4 are considered, corresponding to coverages of θ = 0.11 and 0.06, respectively. This relatively small change in coverage already shows a dramatic decrease in adsorption energy from 163.2 kJ/mol to less than 9.7 kJ/mol. Additionally, calculations at an intermediate coverage of 0.08 (corresponding to a 4×3 cell), lead to an adsorption energy of 85.0 kJ/mol. Apparently, the formation of multilayers is prevented by frozen congurations explaining the low heats of adsorption as obtained at a coverage 0.11. In fact, the adsorption mode remains unchanged at the high coverage, exhibiting bonds between C and Pt, while the cyano group is directed away from the surface, through rehybridisation. As with BZ, the coverage dependence of the adsorption energy is strong, and lateral repulsions drive weaker binding without aecting the structure of the adsorbate layer.

Coverage Dependence

The eect of lateral adsorbate-adsorbate effects are investigated by determining the coverage dependence of adsorption energies for BZ on Pt(111). Experimentally, a systematic decrease in integral adsorption enthalpy occurs on increasing coverage, from 180.6 kJ/mol at θ = 0.02, to 130.3 kJ/mol at θ = 0.17. BZ is theoretically investigated by being deposited upon Pt(111) slabs with 2 ×2 and 4×4 symmetries, corresponding to coverages of θ = 0.25 and 0.06, respectively. These coverages are chosen to represent the range from high coverage, to one


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ened on defective surfaces, due to the fact that, as has been previously reported, 67 the molecule prefers the undercoordinated step sites to the planar regions which make up (111) surfaces. Moving from (111) to (211) to (321), the adsorption energy increases from 179.6 kJ/mol to 231.7 kJ/mol, to 272.3 kJ/mol. Coverage dependence of the BZ adsorption energy was tested for defective surfaces, using the Pt(211) termination as an example. It is found that increasing coverage from θ = 0.04 to 0.17, decreases the adsorption energy strongly, from 231.7 kJ/mol to 96.5 kJ/mol. Thus lateral repulsion between nearby BZ molecules is signicantly enhanced for the defective surface. For AN, the defective surface has little eect on the adsorption strength on the plane regions, in correspondence with the result for Pt(111). The adsorption energy of AN upon the Pt(321) surface is 141.9 kJ/mol, corresponding to an enhancement of only 9.7 kJ/mol over the pristine Pt(111) surface. On the less distorted (211) and (533) surfaces, it adsorbs with 122.6 kJ/mol and 134.2 kJ/mol, respectively. The possibility that for AN, adsorption modes which are unfavourable upon the pristine (111) surface become favourable on rougher surfaces was tested with the (533) surface. AN was adsorbed in the fourfold hollow of the step, the (111)-like plane, and an atop mode. The adsorption energies of the hollow and the plane were nearly identical, at 134.2 J/mol and 127.5 kJ/mol, respectively, while the atop mode did not converge to a stable adsorption mode. It is concluded that undercoordination of Pt does not drive a change in the preference for the σ mode. The adsorption energies of BN to the surface are summarized in Table 1, and show an in-

Figure 5: Tilted and top-down views of the various surface terminations used in this work. Overall, the coverage dependence found in experiment is reproduced qualitatively for BZ and BN, while the adsorption energy of AN does not show any signicant dependence on coverage, in contrast with the current experimental results. This discrepancy may be a result of the limitations of our DFT supercell approach. The role played by defects in the surface on adsorption energy is a possible source of discrepancy, and may be explored theoretically, by considering a range of surface terminations. Surface Terminations

The Pt(211), Pt(533) and Pt(321) terminations are constructed as models of defective adsorption sites (see Fig. 5). Adsorption of BZ, AN and BN is considered both on the terrace (111)like region and the step regions of each termination. Reasonably low coverages are considered for each slab, in which one adsorbate molecule is adsorbed in each supercell. For Pt(111), the supercell gives a surface coverage of θ = 0.06. For Pt(533), this coverage is 0.06. For Pt(211), the 4×2 slab gives a coverage of 0.04, while on Pt(321) the coverage is approximately 0.06. The adsorption of BZ is signicantly strength-

Table 1: Adsorption energies in kJ/mol (eV), of BN over Pt terminations with coverage [θ ] and adsorption modes.

Site 111 [0.06] 533 [0.06] 211 [0.04] 321 [0.06]


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Plane 163.2 (1.69) 147.7 (1.53) 182.5 (1.89) 234.6 (2.43)

Edge 239.4 (2.48) 176.7 (1.83) 289.7 (3.00)

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crease in stability, from the pristine (111) surface to the rough (321) surface, while the edge site is found to be preferred in general to the plane region. A particularly strong adsorption is available to surfaces of sucient roughness to allow for both the phenyl ring and the CN group to adsorb, which is observed both for the (533) and (321) surfaces. In extremely defective surfaces, stronger adsorption than BZ is possible, as the strain of rehybridisation is no longer required to bind both functional groups coincidentally. In Fig. 6, adsorption geometries to the various surfaces are shown. BZ and AN are found to have maximal coverages of θ = 0.165 and θ = 0.19 on the (111) surface in experiment, while BN exhibits a value of 0.14. Theoretically, it is found that at θ = 0.11, BN adsorbs on Pt(111) at less than 9.7 kJ/mol, suggesting a monolayer coverage θ of around 0.11. For a Pt(211) surface, the exclusion zone occupied by a BN molecule is around ve Pt atoms in the most stable conguration, suggesting an upper limit of θ = 0.2. Coverage dependence is calculated over the Pt(211) surface, employing 4 ×2 and 2×1 cells which correspond to coverages of θ = 0.04 and 0.17. The adsorption mode most similar to that over Pt(111), in which the phenyl moiety, but not the CN group, is adsorbed to the surface, is employed. It is observed that BN may adsorb at both coverages, and is found to show a notable coverage dependence, with a variation from 143.9 kJ/mol at θ = 0.04, down to 99.4 kJ/mol at θ = 0.17. This variation is a decrease of 44%, and is therefore smaller than that of the experiment (77% over the same range), but is in qualitative agreement. Overall, it is found that the pristine Pt(111) surface is the least stable of the terminations considered for all species, and that signicant adsorption energy gains may be made through surface reordering. For the most defective surface considered, the adsorption energy is in line quantitatively with the experimental calorimetric result, and for BN, new adsorption modes may become stable, while AN shows no such change in structural preference. These results are rationalised according to the ability to ad-

Figure 6: Views of the optimal adsorption geometry of AN and BN on Pt(211), Pt(321) and Pt(533) surfaces, with the coverages indicated. Platinum atoms at step edges are denoted in green as a guide to the eye. Top-down and tilted views are given for each system.


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sorb to energetically unfavourable surface sites, which gain from additional coordination. As the surface becomes more defective, all three species show increased adsorption energy, but signicantly, there is a crossing point at which BN begins to out-compete BZ. Our conditions indicate a rather small number of defects, i.e. the deviation of the experimental values from theory is only to some extent due to defects. Perhaps, the correct description of vdW eects is crucial for a better agreement.

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desorb spontaneously. This is in good agreement with the vdW results. For atop and diσ modes, the binding energies are 52.1 kJ/mol and 62.8 kJ/mol, respectively. These values are signicantly less than the vdW calculations, implying that more than 50% of their adsorption energy is made up of vdW interactions with the surface. Furthermore, the ratio of adsorption energies is 0.83, which compares with 0.84 for vdW, and 0.77 in the prior GGA work of Markovits and Minot. 64 Regarding structures, the atop mode exhibits identical C-C and CN bonds lengths to the vdW result (1.44 and 1.16 Å), while the N-Pt bond is extended very slightly, from 1.98 to 2.00 Å. The adsorption mode is slightly bent away from the vertical. For the di-σ mode, the lengths of the C-Pt and N-Pt bonds are increased by 0.01 Å each, from 2.07 and 2.03 (vdW), to 2.08 and 2.04 Å (nonvdW), respectively, due to the weaker adsorption. The internal bond lengths, C-N and C-C are unchanged. Benzonitrile is found to adsorb through the phenyl ring, with a rehybridisation of the molecule, which reorients the CN group away from the surface. This structure is similar to that found with the vdW-DF method. The adsorption energy is 103.2 kJ/mol, which is a considerable reduction from that found with the vdW correction.

PBE functional

For comparison, and to determine the extent to which the vdW correction aects the structures and energetics of adsorbates, the PBE xc functional is employed. It is found that BZ adsorbs to the surface at low coverage ( θ = 0.06) with a signicantly lower adsorption energy (126.5 kJ/mol) than for the vdW-corrected system (179.6 kJ/mol). This may be expected, due the lack of dispersive contributions to the adsorption energy. The adsorption geometry, however, is almost identical. The C-Pt bond lengths are slightly increased with respect to the vdW values. For carbon atoms which share a Pt atom with another carbon atom, the C-Pt bond length is 2.21 Å, while carbon atoms which do not share a Pt atom, have a bond length of 2.17 Å. C-C bond lengths are negligibly shorter, at 1.43 and 1.47 Å. This similarity is noted by Liu et al., 58 who conclude that the covalency of the chemical bonds almost completely determines the adsorption geometry. In this work, we observe the eect noted by Sautet et al., that while the adsorption energies vary signicantly upon the addition of a vdW correction, the structure may change very little. The addition of a vdW correction does little to change the adsorption mode preferences of acetonitrile, benzene or benzonitrile, but strengthens the adsorption overall. The bent mode for BN remains the most stable geometry, and thus the sum-rule for adsorption energies is not reproduced. For the acetonitrile molecule, both the atop and the di-σ modes are determined to be stable minima, while the π mode is found to

Conclusions VdW-corrected DFT calculations are employed to determine the stable adsorption mode, and energetics of the three studied molecules, benzene (BZ), acetonitrile (AN) and benzonitrile (BN). Excellent agreement with experiment is found for BZ in both structure and geometry, showing that the novel functional is able to accurately reproduce chemisorption through C-C π systems. Furthermore, coverage dependence found in current SCAC experiments is qualitatively recovered for both BZ and BN. Overall, the vdW-correction represents an improvement of the quantitative comparison to experiment, particularly for benzene, but falls short of agreeing with the SCAC ndings of a sum-rule


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(2) Vickermann, J. C. Surface Analysis: The principal techniques ; John Wiley & Sons, Ltd, 1999.

in adsorption energies for acetonitrile, benzene and benzonitrile. Chemisorption of the type found for acetonitrile to platinum surfaces is not well described, showing over-binding and a lack of coverage dependence. This varying accuracy across adsorption types is a known issue for vdW-DFT methods. In future, the development of vdW functionals which are suciently exible to treat multiple types of adsorption simultaneously and accuracy is evidently needed. We nd that the molecule is bent, diminishing the role of the CN group in surface adsorption. Exemplary defective surfaces are, however, found to allow for adsorption through both functionalities, and exhibit the adsorption strengths found in experiment. However, from experiment al point of view the inuence of defects is probably only important for small coverages. Apparently, the adequate description of vdW eects may be crucial for a sucient agreement between experimental and theoretical results. For our calculations, in the case of AN the vdW eects are responsible for 50% of the binding energy, for BN we still found a contribution of 37%. In contrast, the vdW inuence for BZ amounts only to 30%. Therefore, vdW eects seem to be most important for AN (and BN). This dierences in importance of vdW corrections may explain theoretical discrepancies and eventually point to a lack of theory for an accurate description of AN and BN on Pt(111) model surfaces.

(3) Kolasinski, K. W. Surface Science: Foundations of Catalysis and Nanoscience ; John Wiley & Sons, Ltd, 2009. (4) Brown, W. A.; Kose, R.; King, D. A. Femtomole adsorption calorimetry on singlecrystal surfaces. Chem. Rev. 1998, 98, 797832. (5) Borroni-Bird, C. E.; Al-Sarraf, N.; Andersoon, S.; King, D. A. Single crystal adsorption microcalorimetry. Chem. Phys. Lett. 1991, 183, 516. (6) Borroni-Bird, C. E.; King, D. A. An ultrahigh vacuum single crystal adsorption microcalorimeter. Rev. Sci. Instrum. 1991, 62, 2177. (7) Dixon-Warren, S.; Kovar, M.; Wartnaby, C.; King, D. A. Pyroelectric single crystal adsorption microcalorimetry at low temperatures: oxygen on Ni100. Surf. Sci. 1994, 307-309 Part A, 16. (8) Stuck, A.; Wartnaby, C. E.; Yeo, Y. Y.; Stuckless, J. T.; Al-Sarraf, N.; King, D. A. An improved single crystal adsorption calorimeter. Surf. Sci. 1996, 349, 229 240. (9) Stuckless, J. T.; Starr, D. E.; Bald, D. J.; Campbell, C. T. Metal adsorption calorimetry and adhesion energies on clean single-crystal surfaces. Phys. Rev. B 1997, 56, 13496.

Acknowledgments The computations described in this paper were performed using the University of Birmingham's BlueBEAR HPC service, which provides a High Performance Computing service to the University's research community.

(10) Stuckless, J. T.; Starr, D. E.; Bald, D. J.; Campbell, C. T. Calorimetric measurements of the energetics of Pb adsorption and adhesion to Mo(100). J. Chem. Phys. 1997, 107, 5547.

References

(11) Stuckless, J. T.; Frei, N. A.; Campbell, C. T. A novel single-crystal adsorption calorimeter and additions for determining metal adsorption and adhesion energies. Rev. Sci. Instrum. 1998, 69, 2427.

(1) Somorjai, G. A. Introduction to Surface Chemistry and Catalysis ; 1994; pp John Wiley & Sons, Inc.


13


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

(12) Ihm, H.; Ajo, H. M.; Gottfried, J. M.; Bera, P.; Campbell, C. T. Calorimetric Measurement of the Heat of Adsorption of Benzene on Pt (111). J. Phys. Chem. B 2004, 108, 1462714633.

Page 14 of 19

Campbell, C. T.; Schauermann, S.; Freund, H.-J. An improved single crystal adsorption calorimeter for determining gas adsorption and reaction energies on complex model catalysts. Rev. Sci. Instrum. 2011, 82, 024102.

(13) Ajo, H. M.; Ihm, H.; Moilanen, D. E.; Campbell, C. T. Calorimeter for adsorption energies of larger molecules on single crystal surfaces. Rev. Sci. Instrum. 2004, 75 .

(21) Schieÿer, A.; Hörtz, P.; Schäfer, R. Thermodynamics and kinetics of CO and benzene adsorption on Pt(111) studied with pulsed molecular beams and microcalorimetry. Surf. Sci. 2010, 604, 20982105.

(14) Diaz, S.; Zhu, J.; Shamir, N.; Campbell, C. T. Pyroelectric heat detector for measuring adsorption energies on thicker single crystals. Sens. Actuators B 2005, 107, 454.

(22) Schieÿer, A.; Schäfer, R. Versatile piezoelectric pulsed molecular beam source for gaseous compounds and organic molecules with femtomole accuracy for UHV and surface science applications. Rev. Sci. Instrum. 2009, 80, 086103.

(15) Gottfried, J. M.; Vestergaard, E. K.; Bera, P.; Campbell, C. T. Heat of Adsorption of Naphthalene on Pt(111) Measured by Adsorption Calorimetry. J. Phys. Chem. B 2006, 110, 17539.

(23) Hörtz, P.; Schäfer, R. A compact lowtemperature single crystal adsorption calorimetry setup for measuring coverage dependent heats of adsorption at cryogenic temperatures. Rev. Sci. Instrum. 2014, 85, 074101; In eq (7) a negative sign is missing on "RTsample". However, the analysis has been done with the correct equation.

(16) Lytken, O.; Lew, W.; Campbell, C. T. Catalytic reaction energetics by single crystal adsorption calorimetry: hydrocarbons on Pt(111). Chem. Soc. Rev. 2008, 37, 2172. (17) Lew, W.; Lytken, O.; Farmer, J. A.; Crowe, M. C.; Campbell, C. T. Improved pyroelectric detectors for single crystal adsorption calorimetry from 100 to 350 K. Rev. Sci. Instrum. 2010, 81, 024102.

(24) Hörtz, P.; Ru, P.; Schäfer, R. A temperature dependent investigation of the adsorption of CO on Pt (111) using low-temperature single crystal adsorption calorimetry. Surf. Sci. 2015, 639, 66.

(18) Karp, E. M.; Silbaugh, T. L.; Campbell, C. T. Energetics of adsorbed CH3 on Pt (111) by calorimetry. J. Am. Chem. Soc. 2013, 135, 5208.

(25) Saeys, M.; Reyniers, M.-F.; Marin, G.; Neurock, N. Density functional study of benzene adsorption on Pt (111). J. Phys. Chem. B 2002, 106, 7489.

(19) Fischer-Wolfarth, J.-H.; Farmer, J. A.; Flores-Camacho, J. M.; Genest, A.; Yudanov, I. V.; Rösch, N.; Campbell, C. T.; Schauermann, S.; Freund, H.-J. Particlesize dependent heats of adsorption of CO on supported Pd nanoparticles as measured with a single-crystal microcalorimeter. Phys. Rev. B 2010, 81, 241416.

(26) Morin, C.; Simon, D.; Sautet, P. Densityfunctional study of the adsorption and vibration spectra of benzene molecules on Pt(111). J. Phys. Chem. B 2003, 107, 29953002. (27) Gao, W.; Zheng, W.; Jiang, Q. Dehydrogenation of benzene on Pt(111) surface. J. Chem. Phys. 2008, 129, 164705.

(20) Fischer-Wolfarth, J.-H.; Hartmann, J.; Farmer, J. A.; Flores-Camacho, J. M.;


14

Page 15 of 19

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


solids. J. Phys. Chem. Solids 1957, 3, 95 101.

(28) Benson, S. W.; Cruickshank, F. R.; Golden, D. M.; Haugen, G. R.; O'Neal, H. E.; Rodgers, A. S.; Shaw, R.; Walsh, R. Additivity rules for the estimation of thermochemical properties. Chem. Rev. 1969, 69, 279324.

(37) Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G. L.; Cococcioni, M.; Dabo, I. et al. QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials. J. Phys.: Condens. Matter 2009, 21, 395502.

(29) Sexton, B. A.; Avery, N. R. Coordination of acetonitrile (CH3CN) to platinum (111): Evidence for an η 2 (C, N) species. Surf. Sci. 1983, 129, 2136. (30) Sexton, B. A.; Hughes, A. E. A comparison of weak molecular adsorption of organic molecules on clean copper and platinum surfaces. Surf. Sci. 1984, 140, 227 248.

(38) Rappe, A. M.; Rabe, K. M.; Kaxiras, E.; Joannopoulos, J. D. Optimized Pseudopotentials. Phys. Rev. B. 1990, 41, 1227 1230. (39) Rappe, A. M.; Rabe, K. M.; Kaxiras, E.; Joannopoulos, J. D. Optimized Pseudopotentials. Phys. Rev. B. 1991, 44, 13175 13176.

(31) Proch, D.; Trickl, T. A highâintensity multi-purpose piezoelectric pulsed molecular beam source. Rev. Sci. Instrum. 1989, 60, 713716.

(40) Perdew J. P., Burke K. and Ernzerhof M., Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, 3865 3868.

(32) King, D. A.; Wells, M. G. Molecular beam investigation of adsorption kinetics on bulk metal targets: Nitrogen on tungsten. Surf. Sci. 1972, 29, 454482. (33) Lytken, O.; Drescher, H. J.; Kose, R.; Gottfried, J. M. Adsorption Calorimetry on Well-Dened Surface. Springer Series In Surface Science 2013, 51, 3555.

(41) Berland, K.; Hyldgaard, P. Exchange functional that tests the robustness of the plasmon description of the van der Waals density functional. Phys. Rev. B. 2014, 89, 035412.

(34) Silbaugh, T. L.; Campbell, C. T. Energies of Formation Reactions Measured for Adsorbates on Late Transition Metal Surfaces. J. Phys. Chem. C. 2016, 120, 2516125172.

(42) Dion, M.; Rydberg, H.; E, E. S.; Langreth, D. C.; Lundqvist, B. I. Van der Waals density functional for general geometries. Phrs. Rev. Lett. 2004, 92, 246101.

(35) Lytken, O.; Lew, W.; Harris, J. J. W.; Vestergaard, E. K.; Gottfried, J. M.; Campbell, C. T. Energetics of Cyclohexene Adsorption and Reaction on Pt(111) by Low-Temperature Microcalorimetry. J. Am. Chem. Soc. 2008, 130, 1024710257; In this paper a typographical error is crept in due to a negative sign missing in eqs (5) and (8) on "1/2 RT source". This has been pointed out in ref. 34.

(43) Lee, K.; Murray, E. D.; Kong, L.; Lundqvist, B. I.; Langreth, D. C. Higheraccuracy van der Waals density functional. Phys. Rev. B. 2010, 82, 081101. (44) Berland, K.; Cooper, V. R.; Lee, K.; Schröder, E.; Thonhauser, T.; Hyldgaard, P.; Lundqvist, B. I. van der Waals forces in density functional theory: a review of the vdW-DF method. Rep. Prog. Phys. 2015, 78, 066501.

(36) Kisliuk, P. The sticking probabilities of gases chemisorbed on the surfaces of


15


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

Page 16 of 19

Journal of Physical Chemistry C 2012, 116, 11930019306.

(45) Methfessel, M.; Paxton, A. T. HighPrecision Sampling for Brillouin-Zone Integration in Metals. Phys. Rev. B. 1989, 40, 36163621.

(54) Mrozek, M. F.; Wasileski, S. A.; Weaver, M. J. Periodic Trends in Electrode-Chemisorbate Bonding: Benzonitrile on Platinum-Group and Other Noble Metals As Probed by SurfaceEnhanced Raman Spectroscopy Combined with Density Functional Theory. J. Am. Chem. Soc. 2001, 123, 1281712825.

(46) Evans, F. W.; Skinner, H. A. The heats of combustion of organic compounds of nitrogen Part 2.-n-Propyl, isopropyl and phenyl cyandies. Trans. Faraday Soc. 1959, 55, 255. (47) Kishi, K.; Ikeda, S. Adsorption of acetonitrile on evaporated nickel and palladium lms studied by X-ray photoelectron spectroscopy. Surf. Sci. 1981, 107, 405416.

(55) Oranskaya, O. M.; Semenskaya, I. V.; Filimonov, V. N. Infrared study of the bonding of adsorbed nitriles to metals. React. Kinet. Catal. L. 1976, 5, 135139.

(48) Nakayama, T.; Inamura, K.; Inoue, Y.; Ikeda, S.; Kishi, K. Adsorption of benzonitrile and alkyl cyanides on evaporated nickel and palladium lms studied by XPS. Surf. Sci. 1987, 179, 4758.

(56) Murphy, K.; Azad, S.; Bennett, D. W.; Tysoe, W. T. Adsorption, decomposition and isomerization of methyl isocyanide and acetonitrile on Pd (111). Surf. Sci. 2000, 467, 19.

(49) Friend, C. M.; Stein, J.; Muetterties, E. L. Coordination chemistry of metal surfaces. 2. Chemistry of acetonitrile and methyl isocyanide on nickel surfaces. J. Am. Chem. Soc. 1981, 103, 767772.

(57) Myers, A. K.; Benziger, J. B. Eect of substituent groups on the interaction of benzene with nickel (111). Langmuir 1989, 5, 12701288.

(50) Friend, C. M.; Muetterties, E. L.; Gland, J. L. Vibrational studies of acetonitrile and methyl isocyanide adsorbed on nickel (111) and nickel (111)-carbon surfaces. J. Phys. Chem. 1981, 85, 3256 3262.

(58) Liu, W.; Tkatchenko, A.; Scheer, M. Modeling adsorption and reactions of organic molecules at metal surfaces. Acc. Chem. Res. 2014, 47, 33693377. (59) Hammer, B.; Norskov, J. Theoretical Surface Sciene and Catalysis- Calculations and Concepts. Adv. Catal. 2000, 45, 71 129.

(51) Kishi, K.; Okino, Y.; Fujimoto, Y. XPS studies of the adsorption of CH3CN and C6H5CN on the Ni (111) surface. Surf. Sci. 1986, 176, 2331.

(60) Saeys, M.; Reyniers, M. F.; Marin, G. B.; Neurock, M. Density functional study of benzene adsorption on Pt(111). J. Phys. Chem. B 2002, 106, 74897498.

(52) Johnson, A. L.; Muetterties, E. L.; Stöhr, J. Orientation of Complex Molecules Chemisorbed on Metal Surfaces: Near-Edge X-ray Absorption Studies. J. Am. Chem. Soc. 1983, 105, 7183.

(61) Morin, C.; Simon, D.; Sautet, P. Chemisorption of benzene on Pt(111), Pd(111), and Rh(111) metal surfaces: A structural and vibrational comparison from rst principles. J. Phys. Chem. B 2004, 108, 56535665.

(53) Yin, J.; Krooswyk, J. D.; Hu, X.; Meyer, R. J.; Trenary, M. Formation of Benzonitrile from the Reaction of Styrene with Nitrogen on the Pt(111) Surface. The


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(62) Ayishabi, P. K.; Lakshmikanth, K. G.; Chatanathodi, R. Chemisorption of benzene on Pt (111) surface: A DFT study with van der Waals interaction. Chem. Phys. Lett. 2015, 637, 182188. (63) Heard, C. J.; Siahrostami, S.; Gronbeck, H. G. Structural and Energetic Trends of Ethylene Hydrogenation over Transition Metal Surfaces. J. Phys. Chem. C. 2016, 120, 9951003. (64) Markovits, A.; Minot, C. Theoretical study of the acetonitrile ip-op with the electric eld orientation: adsorption on a Pt (111) electrode surface. Catal. Lett. 2003, 91, 225. (65) Pa²ti, I. A.; Markowit¢, A.; Gavrilov, N.; Mentus, S. V. Adsorption of Acetonitrile on Platinum and its Eects on Oxygen Reduction Reaction in Acidic Aqueous Solutions-Combined Theoretical and Experimental Study. Electrocatalysis 2016, 7, 235248. (66) Gautier, S.; Steinmann, S. N.; FleuratLessard, P.; Sautet, P. Molecular adsorption at Pt(111). How accurate are DFT functionals? Phys. Chem. Chem. Phys. 2015, 17, 2892128930. (67) Camarillo-Cisneros, J.; Liu, W.; Tkatchenko, A. Steps or Terraces? Dynamics of Aromatic Hydrocarbons Adsorbed at Vicinal Metal Surfaces. Phys. Rev. B. 2015, 115, 086101.


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Figure 7: TOC image


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Graphical TOC Entry

Single crystal adsorption calorimetry allows us to study the inuence of dierent functional groups on heats of adsorptions. The experimental heat of adsorption of benzonitrile at zero coverage is 325 kJ/mol while it is 194 kJ/mol and 103 kJ/mol for benzene and acetonitrile, respectively. This leads to the idea of summing up the latter two values assuming isolated functional groups of benzonitrile causing its heat of adsorption which is investigated in more detail with density functional theory.


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Adsorption of Acetonitrile, Benzene, and Benzonitrile on Pt(111

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