NH3–NO Coadsorption System on Pt(111). II. Intermolecular

Article pubs.acs.org/JPCC

NH3−NO Coadsorption System on Pt(111). II. Intermolecular Interaction Andrea Cepellotti,†,# Angelo Peronio,†,‡,§,▲ Stefano Marchini,†,§,¶ Nasiba Abdurakhmanova,§,○ Carlo Dri,†,§ Cristina Africh,§ Friedrich Esch,§,◊ Giovanni Comelli,†,‡,§ and Maria Peressi*,†,∥,⊥ †

Department of Physics, Università degli Studi di Trieste, via Alfonso Valerio 2, 34127 Trieste, Italy Center of Excellence for Nanostructured Materials (CENMAT), Università degli Studi di Trieste, via Alfonso Valerio 2, 34127 Trieste, Italy § IOM-CNR Laboratorio TASC, Area Science Park, s.s. 14 km 163.5, Basovizza, 34149 Trieste, Italy ∥ IOM-CNR DEMOCRITOS Theory@Elettra Group, Sincrotrone Trieste, Area Science Park, s.s. 14 km 163.5, Basovizza, 34149 Trieste, Italy ⊥ Consorzio Interuniversitario Nazionale per la Scienza e la Tecnologia dei Materiali (INSTM), Unità di ricerca di Trieste, piazzale Europa 1, 34128 Trieste, Italy ‡

S Supporting Information *

ABSTRACT: Coadsorption of ammonia and nitric oxide on the (111) surface of platinum causes the mutual stabilization of the two adsorbed species, arranged in an ordered 2 × 2 mixed layer. Furthermore, their interaction leads also to stable, isolated triangular units, which we observe on the surface after annealing to 345 K. Having provided in the preceding article (10.1021/jp406068y) a detailed structural description of the NH3−NO mixed layer, we focus here on the stabilizing intermolecular interactions. By combining scanning tunneling microscopy (STM) experiments and density functional theory (DFT) calculations, we identify the isolated triangular units as formed by one NH3 and three NO molecules, and we characterize them in terms of structure, energetics, and charge rearrangement. Eventually, we investigate the nature of the chemical bond between the coadsorbed NH3 and NO both in the mixed layer and in the isolated triangular units, pointing out the essential role of the surface mediation in inducing attractive dipole−dipole interactions and the presence of hydrogen bonds.

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INTRODUCTION Nitric oxide (NO) reduction to N2 with NH3 is selectively enhanced by platinum-based catalysts. This selectivity has been tentatively related to the formation of NH3−NO coadsorption complexes on the catalyst surface.1 Indeed, different surface science techniques have shown evidence of a relevant interaction between these two species upon coadsorption on the (111) surface of platinum.1−3 In the preceding article4 we addressed NH3−NO/Pt(111) coadsorption at the atomic scale, finding that a mixed 2 × 2 ordered adlayer forms. We characterized it in terms of adsorption structure, energetics, and charge rearrangement by combining scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. We found that NH3 adsorbs on-top and NO on fcc-hollow5 sites, giving rise to a configuration that maximizes mutual interactions. This structure is 0.29 eV/unit cell more stable than the constituents individually adsorbed on the same sites they occupy in the mixed adlayer. The interaction that stabilizes the adlayer could play a crucial role in the selective catalytic reduction (SCR) process, because in such a configuration NH3 and NO molecules are intermixed and stabilized in a favorable geometry, ready to react with each © 2013 American Chemical Society

other. It was indeed found that on various metals, including Pt, the resulting N2 stems mainly from the interaction of one molecule of NH3 with one of NO.6−10 The surface has clearly a fundamental role in mediating the intermolecular interactions, as pointed out by near-edge X-ray absorption fine structure spectroscopy (NEXAFS) measurements.1 Indeed, the coadsorption of NO and NH3 increases the N−O bond length in nitric oxide, leading to a surface-mediated donor−acceptor picture: the weakening of the N−O bond is due to an enhanced electron back-donation into the NO π* antibonding resonance, with NH3 acting as the electron donor. On the other side, the peculiar geometry of this mixed adlayer points toward the existence of hydrogen bonds contributing to the stabilization of the ordered structure: the hydrogen atoms lie in an almost linear configuration between two more electronegative atoms, the nitrogen of NH3 and the oxygen of NO. Received: June 19, 2013 Revised: September 4, 2013 Published: October 4, 2013 21196

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resembling the structures already reported in Figure 3c of the preceding article4 (see inside the white dotted circles). Imaged by STM, the isolated structures show a regular triangular shape always pointing toward the [1̅ 1̅2] direction, with a higher (brighter) central part surrounded by three lower (less bright) lobes, as shown in Figure 1, thus suggesting the

However, the nature of the intermolecular interaction that stabilizes the complex is not completely elucidated. In this paper we address this point in detail, by combining further STM experiments and DFT calculations. More specifically, we show that isolated structures with well-defined shape and composition survive after the progressive desorption of the adsorbates upon annealing. These structures are identified as formed by one NH3 molecule surrounded by three NO molecules, and a comprehensive characterization of their adsorption geometry and energetics is provided. The thermal stability of these triangular units confirms the existence of an attractive NH3−NO intermolecular interaction, which includes dipole−dipole and hydrogen-bonding contributions, both originating from a relevant charge rearrangement induced by the adsorption on the platinum surface.

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METHODS Experimental Section. The details concerning the STM experiments can be found in the preceding article.4 Theoretical. DFT calculations were performed with the plane-wave-based suite QUANTUM ESPRESSO,11 as detailed in the preceding article.4 To get more insight concerning the intermolecular bonding, for some selected structures we have also computed the maximally localized Wannier functions (MLWFs).12 Essentially they consist of a change of the basis set, passing from Bloch functions built on plane waves, not localized in real space, to a superposition of localized Wannier functions. An enhanced understanding of chemical coordination and bonding properties can be obtained by analyzing the shape, the symmetry, and the position of the centers of the MLWFs. The latter property, in particular, provides information about the location of an electron pair. The technical details of this calculation are presented in the Supporting Information. Information about the kinetics of chemisorption and diffusion was obtained by computing the activation barriers for these processes by means of the climbing-image nudged elastic band (NEB) method.13,14 A well-known problem of standard exchange-correlation functionals in DFT is that they neglect van der Waals interactions, which are in some cases fundamental for a correct description of the bonding. To verify the reliability of our approach for the NH3−NO coadsorbed structures, we performed some tests with different exchange-correlation functionals and with/without van der Waals corrections, for three well-known hydrogen-bonded molecular dimers related to the system under investigation, namely, H2O···H2O, H2O··· NH3, and NH3···NH3. The comparison of our tests with available reference data from more sophisticated computational approaches, reported in the Supporting Information, shows that the available van der Waals corrections do not improve the accuracy of the calculated interaction energy, suggesting to proceed the investigation of the NH3−NO coadsorbed aggregates with a standard GGA-PBE approach.

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Figure 1. NH3−3NO triangular units on Pt(111). (a) Experimental STM image after a brief annealing at 345 K of the NH3−NO mixed layer. Bottom: a detail (b) of the former image with the corresponding DFT-calculated STM image (c) and a ball model of the calculated geometry (d). Image dimensions: (a) 6.6 nm × 3.4 nm; (b) 1.8 nm × 1.8 nm; (c) and (d) 1.1 nm × 1.1 nm. Experimental image parameters: V = 15 mV, I = 10 nA, T = 5 K. For the DFT-calculated STM image an ILDOS iso-value of 1.3 × 10−2 nm−3 was used.

presence of one molecule in the middle and three at the vertices. The apparent height of the central feature is between 87 and 95 pm, compatible with NH3 in the mixed layer. Isolated structures with a truncated triangular shape (as if a molecule at one vertex was lacking) are rarely observed, indicating the general stability of the complete triangular structure. The unique orientation of the triangular shapes in the detected images, together with the knowledge of the surface orientation and of the initially adsorbed species, allow their composition and structure to be inferred. Only three possibilities are compatible with the observed orientation of the triangular shape, in analogy to those discussed for the 2 × 2 mixed adlayer (Figure 4, bottom row, in the preceding article4): (i) three molecules on fcc-hollow and the middle one on an ontop site, or (ii) three molecules on-top and the middle one on an hcp-hollow site, or (iii) three molecules on hcp-hollow and the middle one on an fcc-hollow site. Structures with three NH3 and one NO in the middle could be reasonably excluded. In fact, the isolated structure appears in the experimental samples after annealing at high temperature, and it is known both from TDS experiments and from our total-energy calculations that NH3 has a much weaker adsorption energy with respect to NO (by about 1 eV).4 Roughly, a structure with three NH3 and one NO would be about 2 eV less stable than a structure with three NO and one NH3; therefore, the stable triangular units should have a stoichiometric ratio in favor of NO. Structures where

NH3−3NO ISOLATED TRIANGULAR UNIT

Starting from the coadsorbed layer characterized in the preceding article,4 a progressive desorption of NH3 and NO takes place upon annealing up to 345 K. Remarkably, after the annealing, on the surface remains a low coverage of isolated stable structures with a well-defined regular shape, closely 21197

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Table 1. Calculated Structural Parameters, Charge Transfers, and Cohesion Energies for NH3 and NO Adsorbed Separately on Pt(111) and in the NH3−NO Structuresa parameter

unit

adsorbed constituents

NH3−3NO unit

NH3

hN dN−H αPt−N−H Δnb dipole work function

pm pm deg electrons D eV

216 103 107.5 −0.34 2.18 2.8

221 103 110.2 −0.34

NH3−NO layer 217 102 109.7 −0.33

NO

hN dN−O αO−N−ẑ Δnσc Δnπd dipole work function

pm pm deg electrons electrons D eV

134 120 0.0 −0.54 +0.68 −0.19 5.9

130 122 3.4 −0.55 +0.43

123 126 0.0 −0.57 +0.80

NH3−NO

Ecoh GGA Ecoh LDA dH−O αO−H−N dipole work function

eV/cell eV/cell pm deg D eV

−0.11 −0.39 223 158.5 1.20 5.2

−0.29 −0.50 229 (230) 159.7 (164.7) 1.29 3.9

a Columns refer to NO and NH3 individually adsorbed in 2 × 2 unit cells (constituents), to the same molecules coadsorbed in the triangular NH3− 3NO isolated unit in a 4 × 4 cell, and to the same molecules coadsorbed in the 2 × 2 mixed layer. Symbols denote: h height of an atom with respect to the average height of surface atoms; d distance between two atoms; α angle between three atoms; Δn difference of Löwdin charges with respect to the gas-phase molecules frozen in the adsorbed geometry; Ecoh cohesion energy. The dipole moment per unit cell is positive if the positive charge is outwards. The work function of the clean Pt surface is 5.7 eV. The values for dH−O and αO−H−N before allowing the 2 × 2 layer to relax from the separate adsorption geometry are reported in brackets. bNH3 n is the sum of projected charges on NH3 atomic orbitals. cNO nσ is the sum of projected charges on 2s and 2pz atomic orbitals of N and O, z being the surface normal. dNO nπ is the sum of projected charges on 2px and 2py atomic orbitals of N and O.

(namely, the N−O distance dN−O and the height hN of N with respect to the average height of the surface Pt atoms) are in between the values for the 2 × 2 α-NO and mixed NH3−NO layers. At variance with the N−O distance, which slightly elongates upon coadsorption, the characteristic distances of NH3 do not change. Considering the intermolecular parameters, the H−O distance reduces with respect to the mixed adlayer because the O atom is attracted toward a hydrogen atom and does not have symmetry constraints as in the mixed adlayer. In correspondence, the NO axis is slightly tilted toward NH3. Energetics. The GGA adsorption energy of the triangular unit, calculated with respect to the constituent molecules in the gas phase (eq 1 of the preceding article4), is −6.28 eV per 4 × 4 unit cell. More specifically, the adsorption energy of each species, individually desorbed from the unit, is −1.11 eV (NH3) and −1.72 eV (NO), to be compared with −0.99 eV (NH3 at a coverage of 1/16 ML) and −1.71 eV (NO at 1/4 ML) for the individually adsorbed molecules.4 At variance with NH3, which is clearly stabilized by 0.12 eV upon coadsorption, the NO adsorption energy is almost unchanged. The adsorption energy of the whole unit cannot be directly compared with that of the 2 × 2 mixed layer since the number of adsorbed molecules is different. Rather, we define the cohesion energy Ecoh of the triangular unit, in analogy to eq 2 of the preceding article,4 as the difference between the adsorption energy of the unit itself and those of ammonia and nitric oxide separately adsorbed in the same adsorption sites

ammonia is not adsorbed on-top are also unlikely since the ontop site is largely favored with respect to the others, by about 0.7 eV or even more. These considerations strongly support the hypothesis of a stable complex with the structure (i), with three NO on fcc-hollow sites and one NH3 on the on-top site in the middle (see Figure 1d). To characterize the triangular unit, we performed DFT calculations using a supercell with a 4 × 4 surface periodicity, with three NO on fcc-hollow sites and one NH3 on the on-top site arranged as shown in Figure 1d. The NO coverage is therefore 3/16 ML,15 in a structure resembling the 2 × 2 α-NO but lacking one molecule out of four. The simulated STM image is reported in Figure 1c. Apart from the different resolution, the qualitative features are the same as in the experimental images in Figure 1a and 1b. The ammonia molecule corresponds to the brightest (highest) circular spot in the middle, and the three surrounding NO molecules appear lower. Around the triangular unit, both in the simulated and in the experimental STM maps, a region of depleted charge density can be observed. In the simulations, all the qualitative features do not change with the applied bias, in agreement with the experimental data. Therefore, the comparison between observed and calculated STM images definitely validates the proposed structure for the NH3−3NO triangular unit. Adsorption Geometry. In Table 1 we summarize the most significant structural parameters and other relevant data obtained from DFT calculations for the triangular unit, in comparison with the 2 × 2 mixed layer and the constituent molecules, whose data are reported from the preceding article.4 The main structural parameters for NO in the triangular unit 21198

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Table 2. Löwdin Valence Charges (In Electrons) For Different Atoms in the Adsorption Structures on Pt(111) Studied in this Work: NH3 and NO Separately Adsorbed in 2 × 2 Layers (Upper Part), the NH3−NO Mixed 2 × 2 Layer (Middle Part), and the NH3−3NO Triangular Unit (Lower Part)a

Ecoh = (E(NH3 + 3NO/Pt4 × 4) + E(Pt4 × 4)) − (E(NH3/Pt4 × 4) + E(3NO/Pt4 × 4))

Since there are three NH3−NO bonds both in the triangular unit and in the 2 × 2 unit cell of the mixed layer, the cohesion energy for the two cases is directly comparable and, divided by three, gives the strength of the NH3−NO bond in the two structures. The GGA (LDA) cohesion energy per unit turns out to be −0.11 eV (−0.39 eV), smaller than the corresponding value of −0.29 eV (−0.50 eV) per formula unit of the 2 × 2 mixed adlayer. This corresponds to a rather weak intermolecular interaction of −0.04 eV (−0.13 eV) per bond, which does not fully explain the observed stability of the triangular unit. Indeed, as shown in Figure 1, units are found undissociated on the surface after an annealing at 345 K, which causes the desorption of most of the NH3 and NO molecules. Purely energetic arguments cannot therefore fully explain the observed stability of the triangular unit. To assess the role of kinetics, we considered the energy barrier required for the detachment of one NO molecule from the triangular unit and its diffusion on the surface. The GGA activation barrier for this process, obtained from an NEB calculation, is 0.30 eV, slightly higher than the barrier of 0.26 eV for the diffusion of an α-NO molecule individually adsorbed. The former diffusion barrier, enhanced by the presence of NH3, is definitely larger than the estimated strength of the intermolecular bond (Ecoh/3 = −0.04 eV), suggesting thus an important role of kinetics in the observed stability of the triangular unit. In any case it should be noted that the calculated diffusion barrier of NO is much weaker than the adsorption energy of both NH3 and NO, whereas it is experimentally observed that some triangular units are still undissociated when the majority of NH3 and NO molecules have already desorbed. This points toward a possible role of the kinetic prefactors of the various processes involved, whose estimate however is beyond the scope of the present investigation. Electronic Properties. The details of the electronic charge distribution are studied by assigning the electrons to the various atomic orbitals via the Löwdin analysis. The resulting atomic charges are compared between the coadsorbed system and the sum of the separate components, not allowing the positions of the nuclei to relax. The main results are summarized in Table 1, and further details are reported in Table 2. Upon adsorption, alone or coadsorbed with nitric oxide, the NH3 molecule transfers to the surface the same amount of electronic charge (0.34 electrons), always acting as a donor. The NO molecule, instead, acts as a donor or acceptor depending on the adsorption configuration. A common feature of the adsorbed NO configurations (individually or coadsorbed with NH3) is a donation/back-donation mechanism. The NO σ orbitals lose almost the same amount of charge (0.54 or 0.57 electrons), but the occupation of the π and π* orbitals strongly depends on the adsorption configuration. As a result, NO shows a weak donor behavior in the isolated triangular unit, transferring 0.12 electrons to the surface, whereas it behaves as a weak acceptor when adsorbed individually or in the 2 × 2 mixed adlayer. Due to the different stoichiometry, the adsorbed NH3−3NO unit donates a total of 0.70 electrons to the surface, much more than the 2 × 2 mixed adlayer (0.10 electrons per formula unit). In the 4 × 4 surface cell of the NH3−3NO triangular unit there are 16 Pt atoms of five nonequivalent

NH3 and NO individual 2 × 2 layers adsorbed structure

constituents

Δn

PtNH3

9.95

9.92

+0.03

PtNO

9.95

9.92

+0.03

NH3

N H total

5.56 0.66 7.54

5.83 0.68 7.87

−0.27 −0.02 −0.34

NO

N O total

4.78 4.80 6.04 5.92 10.82 10.68 NH3−NO 2 × 2 mixed layer

+0.02 +0.12 +0.14

atom surface

adsorbed structure

constituents

Δn

PtNH3

10.03

9.96

+0.07

PtNO

10.00

9.96

+0.04

NH3

N H total

5.54 0.66 7.52

5.82 0.68 7.86

−0.28 −0.02 −0.33

NO

N O total

4.85 4.86 6.27 6.03 11.12 10.89 NH3−3NO triangular unit

−0.01 +0.24 +0.23

atom surface

adsorbed structure

constituents

Δn

PtNH3

10.03

9.96

+0.07

PtNO 1 PtNO 2 Ptfree 1 Ptfree 2

10.00 10.00 9.97 10.02

9.96 9.96 9.96 9.96

+0.04 +0.05 +0.02 +0.06

atom surface

NH3

N H total

5.53 0.66 7.51

5.81 0.68 7.85

−0.28 −0.02 −0.34

NO

N O total

4.80 6.13 10.93

4.97 6.08 11.05

−0.17 +0.05 −0.12

a

For comparison, the second column (constituents) reports the charges of the clean surface and of the molecules in the gas phase, frozen in the same geometry they have in the adsorbed system; the last column (Δn) lists the corresponding differences. The reported Pt atoms are the atom bound to NH3 and the three equivalent atoms bound to NO. For the NH3−3NO triangular unit we report instead the atom bound to NH3, the two nonequivalent ones bound to NO, and the other two nonequivalent surface atoms, see text. The uncertainty of the calculated Löwdin charges is in the order of 0.02 electrons.

kinds. One atom (PtNH3) is directly bound to NH3, whereas two nonequivalent types (PtNO 1 with multiplicity 6 and PtNO 2 with multiplicity 3) are bound to NO. The remaining two nonequivalent types (Ptfree 1 and Ptfree 2) are not directly bound to the adsorbates and have a multiplicity of 3 each. All 21199

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H···Y−Z, where the dots denote the bond. In the present case, X is the N atom of ammonia and Y and Z the O and the N atoms of nitric oxide, respectively (N−H···O−N bond). References 18 and 19 provide a precise method for characterizing a hydrogen bond in terms of energetics and structural and electronic properties, on the basis of specific criteria that are reported in the Supporting Information. We used DFT simulations to verify the applicability of most of these criteria, and in the following we report our main findings for the NH3−NO mixed coadsorbed structures, both the 2 × 2 adlayer and the triangular unit. a. Forces Involved in the H···O Bond (Criterion 1). A hydrogen bond includes electrostatic, covalent, and dispersion contributions. Information about the electrostatics involved in the N−H··· O−N bonding is provided by the analysis of the Löwdin electronic charges (Table 2). The strong electronegativity difference between N and H atoms makes the hydrogen atom in ammonia positively charged, in the gas phase as well as in the case of adsorption and coadsorption with NO. The N−H pair is also globally positively charged in all the adsorption configurations. Considerations based on electronegativity cannot be easily applied to nitric oxide since N and O have similar electronegativities. However, the calculated Löwdin electronic charges indicate that O is always negatively charged when NO is adsorbed or coadsorbed. Moreover, coadsorption enhances the negative charge accumulation on the O atom by about 0.2 electrons. We can therefore conclude that there is an electrostatic charge−charge attractive interaction between N− H (positive) and O (negative). The electrostatic interaction, accompanied by the directionality of the bond, is the main responsibility of the rotation barrier of NH3 along the Pt−N axis. Indeed, a NEB calculation estimates a barrier of 100 meV in the 2 × 2 layer, at variance with individual NH3 adsorption, where it is basically zero. A model of a rigid rotator with frozen point-like charges on H, N, and O atoms accounts for a significant part of this effect. We can further analyze the charge rearrangement in the coadsorbed systems by looking at its spatial distribution. We report in Figure 2 the charge density difference20 Δρ(r) on a plane perpendicular to the surface and containing the N−H··· O−N bonds in the adsorbed triangular unit. This quantity can be compared with a similar map obtained for the hydrogenbonded H2O···H2O dimer. Comparing the region around the H···O bridge we note some similarities: accumulation of charge in the regions between the atoms, depletion of charge on the hydrogen atom, polarization on the oxygen atom, same order of magnitude of charge rearrangement. The covalent character of the intermolecular bond is highlighted by the MLWF analysis. The comparison of the amplitude isosurface contours of a MLWF associated to the H···O bond (one for each bond, Figure 3) with the analogous one in the isolated water dimer (Figure 14 of reference 21) shows remarkable similarities. More quantitative information comes from the location of its center (Figure 4), which is not in the middle between O and H but closer to O, consistent with the accumulation of electrons on the O atom and indicating therefore a partially ionic character of the H···O bond. At the same time, the location along a well-defined orientation (toward the H atom) indicates its partially covalent, directional nature. As justified in the Supporting Information, we chose not to explicitly consider the dispersion contribution in our study.

the surface and subsurface Pt atoms receive a non-negligible electronic charge, in particular, but not exclusively, those below ammonia. The charge rearrangement upon coadsorption is reflected in the work function of the Pt(111) surface, which is 5.2 eV with one triangular unit adsorbed in a 4 × 4 surface cell, i.e., reduced by 0.5 eV with respect to a clean Pt(111) surface, consistently with the electronic transfer from the adsorbate to the metal. Although care should be taken in comparing work functions at different surface coverages, the same result can be roughly obtained from a proper weighted average of the work functions of the clean surface and of the separately adsorbed molecules. Surface-Mediated NH3−NO Interaction. Following the analysis of the charge distribution in the coadsorbed systems, insight can be gained into the nature of the surface-mediated NH3−NO interaction by identifying some relevant contributions. Dipole−Dipole Interaction. DFT predicts that the intrinsic dipoles of both NH3 and NO molecules in the gas phase have their negative pole on the nitrogen atom16 and values of 1.47 D (NH3) and 0.17 D (NO). Without the influence of the substrate, they should therefore repel each other when aligned in the coadsorption complex. Upon individual adsorption, the NH3 dipole is strengthened up to 2.18 D, always pointing outward from the surface. Instead, the dipole related to NO is reversed, amounting to −0.19 D (Table 1). In the case of coadsorption, in general, a dipole moment cannot be attributed to each molecule individually. However, the Löwdin charge analysis reported in Table 1 suggests that the orientation of the molecular dipoles is the same of the individually adsorbed molecules. In particular, the charges pertaining to NH3 and to the underlying Pt atom are only minimally affected by coadsorption; therefore, we can reasonably attribute to the coadsorbed NH3 the same dipole of the individually adsorbed species. The charge rearrangement of NO is more significant; however, the balance between the electronic charge gained by the O atom and the charge lost by N with respect to the gas-phase molecule originates a dipole pointing toward the surface, leading thus to an attractive interaction with the opposite dipole of NH3. To estimate the order of magnitude of this effect, while attributing to the coadsorbed NH3 a dipole of −2.18 D, we consider located on NO all the remaining dipole moments of the coadsorbed systems (Table 1). With this choice, the NO dipole moment is estimated to be 1.29 D − 2.18 D = −0.89 D in the 2 × 2 layer and (1.20 D − 2.18 D)/3 = −0.33 D in the triangular unit. The corresponding estimate for the dipole− dipole interaction energy Edd = (4πε0)−1dNH3dNOr−3 is in turn −35 meV (2 × 2 layer) and −13 meV (triangular unit), i.e., about one-third of the estimated bond energy Ecoh/3, which is −100 and −40 meV, respectively. The coadsorption of NO and NH3 on the Pt(111) surface cannot thus be explained in terms of dipole−dipole interaction only. Indeed, the adsorption geometry itself points toward the existence of hydrogen bonds. Hydrogen Bond. The hydrogen bond is defined by the International Union of Pure and Applied Chemistry (IUPAC) as: an attractive interaction between a hydrogen atom f rom a molecule or a molecular f ragment X−N in which X is more electronegative than H, and an atom or a group of atoms in the same or a dif ferent molecule, in which there is evidence of bond formation.18,19 In general, the hydrogen bond is depicted as X− 21200

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Figure 4. Side and perspective view of the location of the MLWF centers involving the adsorbates. In the neighborhood of NH3 there are three MLWFs pertaining to the N−H bonds and one for the Pt−N bond. In the region of the nitric oxide there are three MLWFs representing the bonds with three surface Pt atoms, three pertaining to the O···H bonds, and one to the N−O bond.

becoming 420 meV in the mixed adlayer and 419 meV in the isolated triangular unit. Coadsorption leads thus to a small, but well-defined, red shift. Moreover, new vibrational modes coupling NH3 and NO are found (see Table 3 of the preceding article4). c. N−H···O Angle (Criterion 3). The N−H···O angle in the mixed coadsorbed configurations is about 160° (Table 1), close to the ideal value of 180°. The reliability of DFT-GGA for the description of the hydrogen bond in connection with the bond directionality and the angle involved is discussed in ref 22. d. Energetics (Criterion 6). We have previously discussed that the GGA (LDA) estimate of the strength of the H···O bond in the NH3−NO coadsorbed structures is about 0.10 eV (0.17 eV) in the 2 × 2 mixed adlayer and about 0.04 eV (0.13 eV) in the triangular unit. Albeit small, these values are well beyond our numerical uncertainty on energy differences and are compatible with those normally found in hydrogen bonds.18 They are also greater than the thermal energy at experimental conditions, which is 15 meV at most in our STM experiments (170 K) and 26 meV in the EELS experiments reported in ref 2 (300 K). Summarizing, we conclude that the bond between H and O in the NH3−NO coadsorbed configurations on Pt(111) shows the characteristic features of a hydrogen bond. Some elements, such as the small red shift of the N−H stretching mode, the N− H bond length not increasing upon coadsorption, and the N− H···O angle smaller than 180°, suggest that this bond is rather weak.

Figure 2. Upper panel: electron density difference due to the NH3− NO interactions in the 2 × 2 mixed layer. Lower panel: electron density difference due to intermolecular interaction in the water dimer. Red/blue indicates electron accumulation/depletion; iso-density lines are drawn every 10−3|e|/a30.

Figure 3. Amplitude isosurface contour of the MLWF associated to the O···H bond in the NH3−NO 2 × 2 coadsorbed layer. The MLWF is polarized toward the hydrogen atom. The function is real-valued, and the red/blue contours indicate the positive/negative isosurfaces at . Only the relevant atoms are shown. ±4a−3/2 0

b. N−H Bond (Criteria 2 and 4). The N−H is a covalent bond, as commonly found in other systems and confirmed by charge and Wannier function analysis in our case. The H atom in the ammonia in the coadsorbed configurations is positively charged (it has 0.66 electrons), whereas the N atom has 5.53 or 5.54 electrons, i.e., about half an electron more than its pseudovalence (see Table 2). The N−H bond is therefore polarized. The N−H bond length in the adsorbed NH3 does not change upon NO coadsorption. To investigate the effect of coadsorption on the N−H stretching modes, we also studied the related vibrational frequencies. There are three modes associated to N−H vibrations: one symmetric stretching mode with energy of 412 meV in the individually adsorbed ammonia that decreases to 406 meV in the 2 × 2 mixed adlayer and to 405 meV in the isolated triangular unit; two degenerate antisymmetric stretching modes at 430 meV in NH3/Pt,

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CONCLUSIONS Following the characterization of the ordered 2 × 2 NH3−NO mixed layer adsorbed on the Pt(111) surface, we have performed further investigations that combine atomically resolved STM imaging and state-of-the-art DFT calculations to assess the nature of the bonding mechanism that mutually stabilizes NH3 and NO on the surface. Experiments provided further evidence of the existence of a relevant NH3−NO intermolecular interaction: upon annealing, a progressive desorption of the adsorbates takes place, and isolated stable structures of regular triangular shape survive at low coverage. We have unambiguously identified and 21201

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The Journal of Physical Chemistry C

Article

(4) Peronio, A.; Cepellotti, A.; Marchini, S.; Abdurakhmanova, N.; Dri, C.; Africh, C.; Esch, F.; Peressi, M.; Comelli, G. The NH3−NO Coadsorption System on Pt(111): I. Structure of the Mixed Layer. J. Phys. Chem. C 2013, DOI: 10.1021/jp406068y. (5) Fcc-hollow sites have another hollow site under them, and hcphollow ones have instead an atom of the second layer. (6) Otto, K.; Shelef, M.; Kummer, J. T. Studies of Surface Reactions of Nitric oxide by Nitrogen-15 Isotope Labeling. I. Reaction between Nitric Oxide and Ammonia over Supported Platinum at 200−250°. J. Phys. Chem. 1970, 74, 2690−2698. (7) Otto, K.; Shelef, M.; Kummer, J. T. Studies of Surface Reactions of Nitric Oxide by Isotope Labeling. II. Deuterium Kinetic Isotope Effect in the Ammonia-Nitric Oxide Reaction on a Supported Platinum Catalyst. J. Phys. Chem. 1971, 75, 875−879. (8) Otto, K.; Shelef, M. Studies of Surface Reactions of Nitric Oxide by Isotope Labeling. IV. Reaction between Nitric Oxide and Ammonia over Copper Surfaces at 150−200°. J. Phys. Chem. 1972, 76, 37−43. (9) Otto, K.; Shelef, M. Studies of Surface Reactions of NO by Isotope Labeling. Z. Phys. Chem. 1973, 85, 308−322. (10) Lombardo, S.; Esch, F.; Imbihl, R. The NO + NH3 Reaction on Pt(100): Steady State and Oscillatory Kinetics. Surf. Sci. 1992, 271, L367−L372. (11) 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. (12) Marzari, N.; Mostofi, A. A.; Yates, J. R.; Souza, I.; Vanderbilt, D. Maximally Localized Wannier Functions: Theory and Applications. Rev. Mod. Phys. 2012, 84, 1419−1475. (13) Henkelman, G.; Uberuaga, B. P.; Jónsson, H. A Climbing Image Nudged Elastic Band Method for Finding Daddle Points and Minimum Energy Paths. J. Chem. Phys. 2000, 113, 9901−9904. (14) Henkelman, G.; Jónsson, H. Improved Tangent Estimate in the Nudged Elastic Band Method for Finding Minimum Energy Paths and Saddle Points. J. Chem. Phys. 2000, 113, 9978−9985. (15) One monolayer (ML) corresponds to one adsorbate per surface atom. (16) The orientation of the gas-phase NO dipole has been experimentally confirmed only recently; see ref 17. (17) Gijsbertsen, A.; Siu, W.; Kling, M. F.; Johnsson, P.; Jansen, P.; Stolte, S.; Vrakking, M. J. J. Direct Determination of the Sign of the NO Dipole Moment. Phys. Rev. Lett. 2007, 99, 213003. (18) Arunan, E.; Desiraju, G. R.; Klein, R. A.; Sadlej, J.; Scheiner, S.; Alkorta, I.; Clary, D. C.; Crabtree, R. H.; Dannenberg, J. J.; Hobza, P.; et al. Defining the Hydrogen Bond: an Account (IUPAC Technical Report). Pure Appl. Chem. 2011, 83, 1619−1636. (19) Arunan, E.; Desiraju, G. R.; Klein, R. A.; Sadlej, J.; Scheiner, S.; Alkorta, I.; Clary, D. C.; Crabtree, R. H.; Dannenberg, J. J.; Hobza, P.; et al. Definition of the Hydrogen Bond (IUPAC Recommendations 2011). Pure Appl. Chem. 2011, 83, 1637−1641. (20) The reported charge density difference is given by

characterized these units as formed by one NH3 molecule and three NO ones. The common origin of the stability of the mixed coadsorbed structures has been elucidated by accurate DFT calculations: it is an attractive, surface-mediated NH3−NO interaction which includes a dipole−dipole contribution and a directional contribution, the latter having the characteristic features of a weak hydrogen bond.

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ASSOCIATED CONTENT

S Supporting Information *

Details about the maximally localized Wannier functions analysis, the van der Waals contribution in hydrogen bonded dimers of interest for the system under investigation, and the definition of the hydrogen bond according to the IUPAC recommendations.18,19 This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +39 040 2240242. Fax: +39 040 2240601. Present Addresses

Theory and Simulations of Materials (THEOS), École Polytechnique Fédérale de Lausanne, Station 12, 1015 Lausanne, Switzerland. ▲ Institut für Experimentelle and Angewandte Physik, Universität Regensburg, 93040 Regensburg, Germany. ¶ Grünecker, Kinkeldey, Stockmair & Schwanhäusser, Leopoldstr. 4, 80802 München, Germany. ○ Nanoscale Science Department, Max Planck Institut für Festkö rperforschung, Heisenbergstr. 1, 70569 Stuttgart, Germany. ◊ Department of Chemistry, Technische Universität München, Lichtenbergstr. 4, 85748 Garching, Germany. #

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support from Commissariato del Governo di Trieste through Fondo Trieste and from CNR through the ESF FANAS project NOMCIS. C.D. acknowledges financial support from MIUR through Futuro in Ricerca − FIRB 2010 project no. RBFR10FQBL. Computational resources were partly obtained through Italian SuperComputing Resource Allocation (ISCRA) grants of the Consorzio Interuniversitario CINECA and partly within the agreement between the University of Trieste and CINECA. A.C. acknowledges a scholarship from Elettra − Sincrotrone Trieste S.C.p.A. and Collegio Universitario per le Scienze “Luciano Fonda”. N.A. is grateful to ICTP for supporting her through the TRIL program. A.P. thanks Joseph Wright for its kind and competent help during the typesetting of the manuscript.

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Δρ(r) = ρNH + NO/Pt (r) + ρPt (r) − ρNH /Pt (r) − ρNO/Pt (r) 3

3

The clean-surface term must be added to compensate the double subtraction of the metal density by the ρNH3/Pt(r) and ρNO/Pt(r) and ρNO/Pt(r) terms. The patterns are calculated with the atoms frozen in the configuration of the 2 × 2 NH3−NO mixed adlayer, to avoid spurious differences due to the small changes of the equilibrium atomic positions in the case of separate adsorption. (21) Marzari, N.; Souza, I.; Vanderbilt, D. An Introduction to Maximally-Localized Wannier Functions. Psi-k Newsl. 2003, 57, 129− 168. (22) Ireta, J.; Neugebauer, J.; Scheffler, M. On the Accuracy of DFT for Describing Hydrogen Bonds: Dependence on the Bond Directionality. J. Phys. Chem. A 2004, 108, 5692−5698.

REFERENCES

(1) Esch, F.; Greber, T.; Kennou, S.; Siokou, A.; Ladas, S.; Imbihl, R. The formation of a NO−NH3 Coadsorption Complex on a Pt(111) Surface: a NEXAFS Study. Catal. Lett. 1996, 38, 165−170. (2) Gland, J.; Sexton, B. A. Observation of an NH3−NO Complex on the Pt(111) Surface. J. Catal. 1981, 68, 286−290. (3) Burgess, D.; Cavanagh, R.; King, D. S. NO/NH3 coadsorption on Pt(111): Kinetic and Dynamical Effects in Rotational Accommodation. Surf. Sci. 1989, 214, 358−376. 21202

dx.doi.org/10.1021/jp406069q | J. Phys. Chem. C 2013, 117, 21196−21202