Adsorption and Reactivity of Pyridine Dicarboxylic Acid on Cu(111

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Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

Adsorption and Reactivity of Pyridine Dicarboxylic Acid on Cu(111) Maryam Abyazisani,† Jonathan Bradford,† Nunzio Motta,†,‡ Josh Lipton-Duffin,†,‡ and Jennifer MacLeod*,†,‡ †

School of Chemistry, Physics and Mechanical Engineering and ‡Institute for Future Environments, Queensland University of Technology (QUT), Brisbane, Queensland 4001, Australia

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S Supporting Information *

ABSTRACT: A detailed understanding of the reaction of a range of molecules on surfaces will be key to developing targeted strategies for on-surface synthesis. Here, we studied the deprotonation and decarboxylation reactions of 3,5-pyridinedicarboxylic acid (PDC) on Cu(111) using synchrotron radiation photoelectron spectroscopy (SRPES), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and density functional theory (DFT). PDC partially deprotonates upon deposition on Cu(111) at room temperature and adsorbs with the plane of its aromatic ring inclined at an average of ∼45° with respect to the surface. By heating to 100 °C, the deprotonation of the molecule increases. When the PDC is partially deprotonated the plane of its aromatic ring adopts a more upright orientation with respect to the surface. Additional heating to 160 °C causes complete deprotonation, upon which the molecule returns to a more planar molecular adsorption geometry. By examining both the N 1s core level and the NEXAFS results, we ascribe these changes in adsorption to a reaction-induced change in the predominant molecule−surface interaction, which is driven by the nitrogen lone pair prior to deprotonation, and by the carboxylate groups after deprotonation of the −COOH groups. These interaction channels and adsorption geometries are supported by our DFT calculations. Heating above 200 °C induces decarboxylation of the molecule; by observing the rate of reaction over a range of fixed temperatures, we extract an activation energy of 1.93 ± 0.17 eV for the decarboxylation reaction. Around this temperature we also begin to observe the ring opening of the molecule, suggesting that if PDC is to be used as a building block for on-surface synthesis of polymers, careful control of temperature is necessary for obtaining decarboxylation and covalent coupling of the molecule without destroying the aromatic core.

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INTRODUCTION On-surface covalent polymerization has grown into an active research domain1−4 owing to promising applications in nanotechnology and molecular electronics.5,6 A variety of surface-confined reactions performed under ultrahigh vacuum (UHV) conditions have been translated from established solution-chemistry methodologies. In particular, the Ullmann reaction for C−C coupling of aryls7 has been thoroughly studied on surfaces, and many of the details of the process are understood.8 However, this reaction inevitably produces halogen byproducts which tend to remain chemisorbed on the surface, resulting in spatial restriction of the polymer as well as in catalyst poisoning through a reduction of active sites on the surface. To address this problem, alternative reactions that do not produce any byproducts or that have volatile byproducts are actively being pursued. Typical examples include the Glaser reaction,9,10 Bergman cyclization,11 and dehydrogenation.12 More recently, on-surface decarboxylation has also been explored as a “clean” reaction.13 Decarboxylative coupling is appealing since the byproducts, H2 and CO2, are unlikely to remain adsorbed on the surface under reaction conditions; CO2 is volatile, and molecular hydrogen remains adsorbed on Cu(111) only until 330 K.14 On-surface covalent coupling via decarboxylation was demonstrated for the first time when poly-2,6-naphthalene was © XXXX American Chemical Society

formed using 2,6-naphthalenedicarboxylic acid as a precursor.13 Scanning tunneling microscopy (STM) confirmed that molecules first undergo deprotonation upon annealing, followed by decarboxylation with further increase of the reaction temperature, and subsequent coupling to form polynaphthalenes. More recently, decarboxylative coupling of 1,3,5-benzenetribenzoic acid (BTB) was used to fabricate a 2D network.15 To date, studies of decarboxylative coupling have focused only on benzene-based precursors. However, incorporation of heteroatoms in the aromatic ring is a powerful approach for tuning the physical properties of polymers in a controlled way.16−19 For example, nitrogen substitutions have been used to modify the electronic properties of nanographenes.20,21 The presence of nitrogen in the precursor may also have implications for the adsorption geometry of the monomer.22,23 Interaction between the substrate and the nitrogen lone pair can lead to adsorption with the molecular plane tilted or vertical with respect to the surface in small molecules.24−27 On the other hand, the π orbitals also present an interaction channel between the molecule and the surface and can result in Received: May 22, 2018 Revised: June 30, 2018

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DOI: 10.1021/acs.jpcc.8b04858 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

desorbed from the surface with heating to 100 °C (Figure S1). We additionally performed a beam damage test of PDC on Ag(111) and found that a 486 eV beam caused deprotonation of the PDC (Figure S2). Although the same test was not performed on Cu(111), we assume a similar effect is present and hence that the level of deprotonation of any sample may be dependent on the exposure to the beam. To minimize this effect, we moved the sample frequently so that we were not exposing any one region for more than one spectrum. Every spectrum took 30 s to acquire. The PDC was deposited onto the room-temperature Cu(111) substrate from a Knudsen cell (Eberl-MBE Komponenten) held at 135 °C. The sample was annealed at different temperatures in order to trigger the deprotonation and decarboxylation reactions (shown in Figure 1b). The presented SRPES and NEXAFS data were acquired in seperate experiments. For SRPES expriments, the sample was annealed at 100, 160, 240, and 270 °C, with each step performed for 10 min. For NEXAFS expriments, the sample was annealed at 100, 160, and 280 °C, with each step performed for 5 min. SRPES spectra were collected with beam energies of 486 eV for C 1s and N 1s regions and 908 eV for the O 1s region. The pass energy of the analyzer was set to 10 eV for an overall energy resolution of 0.29 eV. For quantitation, survey spectra were recorded using a beam energy of 1487 eV with the pass energy set to 20 eV. All PES spectra were analyzed using CasaXPS software.37 Spectra were calibrated with respect to the Fermi level and were fitted using a Shirley background. For time-resolved experiments, the sample was kept at fixed temperatures (220, 230, and 240 °C, all ±1 °C) and C 1s spectra were sequentially collected using a beam energy of 486 eV over a period of 30 s per spectrum. Lineshape fitting of the C 1s region for each of the spectra permits a determination of the proportion of reacted molecules and was used to find the activation energy of the decarboxylation reaction. An estimate for the uncertainty of the fitted species was obtained by applying a Monte Carlo method to the fitted intensities. NEXAFS spectra were collected using linearly polarized light at three different incidence angles with respect to the surface plane: glancing incidence at θ = 20°, the so-called “magic” (tilt-independent) angle at θ = 55°, and normal incidence at θ = 90°. All spectra were analyzed using the QANT software package.38 NEXAFS spectra were double normalized against the incoming photon flux determined by measuring the current, I0, collected from a gold mesh in the path of the beam and against spectra collected over the same energy range and at the same angle from a clean Cu(111) substrate. DFT calculations were performed using the projectoraugmented wave (PAW) pseudopotentials in Quantum ESPRESSO v6.2.139 under the generalized gradient approximation (GGA) with Perdue−Burke−Ernzerhof (PBE) parametrization for the exchange-correlation functional.40 The exchange correlation was modified by adding an ab initio nonlocal van der Waals correlation contribution (vdW-DF), which is known to improve the description of the long-range dispersive forces of the standard GGA.41−43 After relaxing the bulk Cu face-centered cubic (FCC) cell, the Cu(111) surface was modeled using an eight-layer slab, holding the bottom three layers fixed and allowing the top five layers to relax using a 12 × 12 × 1 k-point grid. The top three layers were then used to model the absorption of PDC in a 5 × 5 supercell at highsymmetry sites in a variety of starting geometries, allowing the top two copper layers to relax along with the molecule while

flat adsorption of pyridine under some circumstances, e.g., electrochemical conditions.28 The addition of carboxylic groups to small molecule precursors add some complication to this scenario, since the copper−carboxylate bond strength is comparable to copper−copper bonding of the surface,29 and therefore, the carboxylate group can also influence adsorption geometries.30 Moreover, additional effects on adsorption are known to relate to molecular coverage,31 number of COOH groups,32 and/or availability of metal adatoms.29 This is a matter of some importance for on-surface coupling, since the adsorption geometry of the monomer has been shown to critically affect molecular reactions on surfaces.33 A surface-catalyzed reaction requires proximity of the molecular reaction site to the metal surface, and consequently, the presence of nitrogen in an aromatic ring can result in an adsorption-based reaction barrier. For example, dibromobenzene fully dehalogenates upon room-temperature adsorption on Cu(110),34,35 whereas only one of the two C−Br bonds of dibromopyridine is broken at room temperature on Cu(100); this difference is attributed to the adsorption geometry induced by the presence of the nitrogen in the aromatic ring, which orients the nitrogen into the surface, leaving the unreacted C− Br oriented out of the surface.36 In this work, we aim to contribute to the understanding of on-surface decarboxylation by studying the reaction of 3,5pyridinedicarboxylic acid (PDC) on Cu(111). Studying this molecule gives us the opportunity to understand how decarboxylation progresses in a nitrogen-containing molecule. It also enables us to understand the interplay between the different parts of the molecule as well as its interaction with surface. The changes in chemistry and adsorption geometry with annealing were investigated through synchrotron radiation photoelectron spectroscopy (SRPES), near-edge Xray absorption fine structure (NEXAFS) spectroscopy, and density functional theory (DFT), and the kinetics of decarboxylation were studied by measuring the decarboxylation rate at different temperatures.

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EXPERIMENTAL METHODS All experiments were carried out under UHV conditions (base pressures of 2 × 10−10 mbar) at the Soft X-ray beamline at the Australian Synchrotron. Cu(111) and Ag(111) single crystals were cleaned by repeated cycles of Ar+ sputtering (E = 1 keV) and annealing (∼400 °C) until no impurities were detected by SRPES. 3,5-Pyridinedicarboxylic acid (PDC) (98%, Alfa Aesar, shown schematically in Figure 1a) was thoroughly degassed at 120 °C in UHV. Results from Ag(111) are not reported in the main manuscript, but selected results are included in the SI. Briefly, PDC deposited onto Ag(111) held at −130 °C adsorbed intact, then partially deprotonated, and finally

Figure 1. (a) Molecular structure of 3,5-pyridinecarboxylic acid (PDC). (b) Decarboxylation reaction schematic for PDC molecule. B


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Figure 2. C 1s, O 1s, and N 1s spectra for PDC deposited onto Cu(111) at RT and annealed up to 270 °C. Line plot with black circle markers represents the acquired data, and solid gray lines show the envelope of the fitted peaks. Fitted components are color- coded and detailed in the text.

holding the bottom layer fixed in the geometry from the bare metal slab relaxation. A vacuum gap of at least 10 Å (more for the more planar starting geometries) between the slabs was used to prevent communication of the molecules in the top layer with the Cu atoms of the bottom layer. The k-point sampling for these geometry optimizations was restricted to the gamma point.

molecules.51 Two additional contributions arise from carbonyl (CO at 532.3 eV) and hydroxyl (−OH at 533.6 eV)44,45 (see Figure 2b). Additionally, for a very precise fit of the spectra one small feature at 532.5 eV is required. The ratio of deprotonated to intact molecules determined from the O 1s core level is 0.86, which is reasonably close to the ratio derived from the C 1s core level (0.75). The spacing between the −COO peak and the carbonyl is 1 eV, which is consistent with previous reports.44 This is indicative of the simultaneous presence of deprotonated and intact carboxylic groups, in agreement with previously reported studies for carboxylated precursors that adsorb partially deprotonated onto different Cu surfaces.15,44,46,51 Following room-temperature deposition, two main regions of intensity are observed in the N 1s core level: a feature at lower binding energy that we attribute to the nitrogen in the first layer and a higher binding energy feature consistent with multilayer adsorption24 in which molecules interact with each other through hydrogen bonding between the carboxyl/ carboxylate group and nitrogen. The peak at lower BE can be deconvolved into two components, suggesting the existence of different adsorption configurations,52 which imply different orientations of the nitrogen with respect to the surface. Figure 3 shows the calculated adsorption geometries and energies for the configurations suggested for PDC on Cu(111) when (a) intact, (b) singly deprotonated, and (c) completely deprotonated. In Figure 3a, we show the minimum-energy adsorption geometry for a single molecule in isolation, which is inclined slightly from the plane of the surface and stabilized by an adsorption energy of −0.92 eV. The peak at 399.8 eV is assigned to the nitrogen atom for molecules chemisorbed to the surface via the lone pair,24,52 consistent with the calculated geometry for the intact molecule, and the peak at 399.1 eV can be assigned to non-surface-adsorbed nitrogen, consistent with a monopodal adsorption geometry of PDC via carboxylate−Cu bonding with the nitrogen oriented away from the surface. The calculated geometry corresponding to this state is shown in

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RESULTS SRPES Study of the Annealing of 3,5-Pyridinedicarboxylic Acid/Cu(111). Figure 2a−c shows the photoemission spectra of the C 1s, O 1s, and N 1s core levels of PDC on Cu(111) deposited at RT, annealed to 100 °C, and annealed to 270 °C. The C 1s spectra for the as-deposited sample exhibit two well-separated peaks centered at 285.2 and 289.3 eV which we assign to the aromatic ring and carboxylic group (−COOH) in the PDC molecule, respectively. The binding energy (BE) splitting between the carboxyl and the aromatic peaks, 4.1 eV, is in agreement with the previously reported values for carboxylated phenyls.30,44−49 The peak assigned to the aromatic ring contains two contributions due to the two inequivalent types of carbon atoms in the ring: two carbons adjacent to nitrogen, C−N (285.8 eV), and three carbons that bond only to other carbons, C−C (285.4 eV).50 The ratio of these components is fixed at 2:3 based on the stoichiometric ratio between C−C and C−N in the pyridine.This is in contrast to previous work, wherethe five carbon atoms in the pyridine ring have been treated as indistinguishable.36 Three additional components contribute to the spectrum: the carboxylate group (−COO−) at a BE of 288.1 eV, C−Cu at a BE of 284.1 eV, and finally the π−π* shakeup transition of the aromatic system at 291.7 eV.44 The carboxylate signature implies that PDC has already partially deprotonated upon adsorption on Cu(111) at room temperature. This is in agreement with the O 1s spectrum for the as-deposited sample, where the peak at 531.3 eV originates from the two equivalent oxygen atoms in the −COO associated with deprotonated C


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configuration of the molecule changes upon completion of the deprotonation reaction: the integrated intensity of the peak assigned to the chemisorbed nitrogen at 399.5 eV decreases in conjunction with an increase in peak intensity at 398.8 eV, reflecting that the carboxylate-dominated geometry is the favored geometry compared to the chemisorbed nitrogen geometry for deprotonated PDC. This is consistent with a bipodal adsorption configuration via carboxylate−Cu bonding, as shown in Figure 3c, which corresponds to an adsorption energy of −5.67 eV. Deprotonated trimesic acid (TMA) on Cu(100) has also been reported to adsorb with the carboxylate group oriented into the surface.53 The higher binding energy peaks at 401.4 and 402.1 eV have almost disappeared following desorption of the multilayer, as expected for second-layer species. Annealing the sample to 270 °C causes the peak at 531.1 eV in the O 1s to vanish, confirming that the carboxylate has been fully cleaved from the molecule. Decarboxylation is also indicated by the C 1s region, where the peak at 288.1 eV has vanished as well, as shown in Figure 2a. There is an obvious decrease in the intensity of the peak corresponding to the pyridine ring as a result of annealing at 270 °C, in conjunction with the appearance of two peaks at 284.1 and 286.3 eV. One possible explanation for these new states is that PDC undergoes ring opening due to scission of a C−N bond. It has similarly been reported that annealing pyridine at 260 °C on Ni(111) leads to decomposition of the molecule.52 The N 1s core level spectra shown in Figure 2c reveal that after annealing PDC at 270 °C the two molecular adsorption peaks are no longer observable, while three new peaks appear: the peak at 398.6 eV, assigned to nitrogen in a fully decarboxylated molecule, and two peaks at 398.1 and 400.5 eV, which are attributed to broken molecules which arise from N−C bond scission. The thermal stability of this product was studied by annealing the sample to 480 °C. The C 1s and N 1s spectra suggest that after this high-temperature treatment the PDC has fully decomposed (see Figure S3). The two N 1s peaks assigned to the decomposed molecule at 270 °C

Figure 3. Calculated adsorption geometries and energies for configurations for PDC on Cu(111) when (a) intact, (b) singly deprotonated, and (c) completely deprotonated.

Figure 3b and corresponds to an adsorption energy of −3.18 eV. After annealing the sample to 100 °C for 10 min the integrated intensity of the phenyl peak is reduced by about 50%, suggesting that the multilayer has desorbed, leaving only one monolayer. This desorption produces a shift to lower binding energy by 0.4 eV for all peaks. The shift arises from enhanced core-hole screening for the surface monolayer. In addition, the carboxyl peak at 289.2 eV almost vanishes, indicating that deprotonation has nearly completed at this point. However, we cannot exclude the scenario in which some fraction of molecules in the first layer were completely deprotonated immediately upon adsorption, and that desorption of the multilayer reveals the previously attenuated signal from these molecules. The corresponding O 1s spectrum corroborates the complete deprotonation of the PDC: the hydroxyl and carbonyl peaks have disappeared, and the only remaining oxygen peak at 531.1 eV corresponds to −COO−. This is consistent with reported XPS studies on other carboxylic acids on copper.15,44 Deconvolution of N 1s spectra reveals that the

Figure 4. Stacked plots of the C 1s region for PDC on Cu(111) with the sample held at a constant temperature of 225 (a), 230 (b), and 235 °C (c). Each spectrum was collected over ∼30 s. D


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The Journal of Physical Chemistry C dramatically increase after full decomposition of the molecule at 480 °C, in agreement with other works.36 Those studies reported that decomposition of the molecule produces fragments such as CH2, (CH)n,52 and (CN)2,36 suggesting that the decomposed molecule will leave behind diminished and nonstoichiometric on-surface residue, consistent with our measurements. Activation Energy for the Decarboxylation Reaction of PDC. The decarboxylation reaction was further studied to examine the reaction kinetics. The Arrhenius equation can be applied to determine the activation energies for thermally activated reactions54,55 k = Ae−Ea / kBT

where k is the reaction rate, A is the pre-exponential factor which is dependent on the rate of transfer of energy to a decomposition site, Ea is the activation energy, kB is Boltzmann’s constant, and T is the temperature. By assuming that A and Ea are independent of temperature,56,57 linearization of the equation yields ln(k) =

−Ea ij 1 yz jj zz + ln(A) kB k T {

such that the activation energy can be calculated from the slope of a plot of ln(k) versus 1/T. To determine the decarboxylation rate, spectra were acquired continuously over the C 1s region with the sample held at a fixed temperature.58 The spectra could then be deconvolved to determine how the spectral intensity in the −COO− region varied with time. Data sets acquired at 225, 230, and 235 °C are shown in Figure 4. From the fitted components of the C 1s region, the proportion of reacted molecules can be defined as 1 − (3ICOO/ 2IC−C). A plot of the proportion of reacted molecules as a function of time for each of the three data sets is shown in Figure 5a. The plots are linear, suggesting zero-order kinetics, and the slope of the linear fit for each data set gives the decarboxylation rate at the respective temperature. The plot of the natural logarithm of the obtained rates as a function of 1/ T, shown in Figure 5b, allows calculation of the activation energy for decarboxylation directly from the slope. In this case, Ea = 1.93 ± 0.17 eV. NEXAFS Study of the Annealing Treatment of 3,5Pyridinedicarboxylic Acid on Cu(111). The adsorption geometry of the molecule on the surface was further investigated using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Typical NEXAFS spectra for an asdeposited multilayer film are shown along with indicative peak fitting in Figure 6. The carbon K-edge has two distinct spectral components that can be associated with the PDC molecule: the signal originating from the aromatic ring, apparent between 283 and 287 eV, and the signal from the carboxylic groups, apparent between 287 and 291 eV. NEXAFS of pyridine has previously been studied with a combined experimental and theoretical approach,59 and the contributions to the spectral structure are well understood. The split peak centered at 285 eV arises from C 1s → 1π* transitions, with the splitting attributable to the chemical difference between the proximity of carbon atoms to the nitrogen atom in the pyridine ring. The ortho carbons give rise to the higher energy component at 285.5 eV, and the carbons in the meta and para positions give rise to the lower

Figure 5. (a) Decarboxylation rate and (b) Arrhenius plot for the decarboxylation reaction of PDC on Cu(111) derived from analysis of PES spectra of the C 1s region at 225, 230, and 235 °C, where k is the reaction rate determined from a.

energy component at 284.8 eV. NEXAFS of small carboxylated aromatics have also been previously studied experimentally and theoretically,60 allowing us to interpret the spectral region between 287 and 289 eV. Here, the peaks at 288.3 and 291 eV can be attributed to C 1s → π* transitions originating from the carboxylic carbon. At higher energies, the spectral features arise from C 1s → σ* transitions. The primary spectral component at the nitrogen K-edge (399.5 eV) can be assigned to an N 1s→ 1π* transition, with a high-energy shoulder (400.8 eV) arising from a vibrationally assisted N 1s → 2π* transition. The peak at 402.4 eV occurs due to a N 1s → 3π* transition.59 At the oxygen K-edge, welldefined peaks arising from carbonyl O 1s → π* transitions are evident at 532.1 and 534.3 eV, with an extended structure due to both carbonyl- and hydroxyl-related transitions extending through the region above 535 eV.60 Previous work on carboxylated aromatics at metal surfaces suggests that neither the carbon- nor the oxygen-related π* resonances show a significant dependence on the chemical state of the carboxylic/carboxylate group.30,47 By taking NEXAFS data through our annealing range, we obtained data that directly address this point. Figure 6 shows the carbon and oxygen K-edges for PDC films annealed to different temperatures. We find that the primary resonances are not significantly changed. In the carbon K-edge, the aromaticE


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respect to the low-energy split peak can be seen in the deprotonated molecule, but we cannot unequivocally ascribe this to deprotonation. In the carboxylic/carboxylate region, the peaks at 288.3 and 291 eV are relatively unchanged in position and shape as the molecule goes from intact to partially deprotonated to deprotonated. A small shoulder peak at ∼289 eV loses intensity with reaction; this peak is related to the carbon in the carboxylic/carboxylate group60 and may be sensitive to the deprotonation. The spectral contributions above 290 eV are associated with σ* orbitals.60 A small peak at ∼290 eV is present at RT, disappears after annealing to 100 °C, and is restored after annealing to 155 °C. Data taken at 55° radiation incidence through the same annealing steps appear to show the same trend but are much noisier so do not unequivocally support this behavior. At the oxygen K-edge, the primary carbonyl-related resonances at 532.1 and 534.3 eV stay constant in position and shape as the molecule deprotonates. Spectral changes, mostly in the form of intensity loss, occur throughout the region above 536 eV with deprotonation of the molecule. Since there are hydroxyl-related contributions in this range, this spectral loss is expected.60 In both the carbon and the oxygen spectra, the molecule is clearly changed after annealing to 280 °C. In oxygen, this corresponds to the loss of all signal intensity, as expected for a decarboxylated molecule. In carbon, this corresponds to the loss of well-defined spectral features associated with the aromatic ring and the carboxylic region. The sharp double peak indicative of the pyridine ring has been replaced by a broad, poorly defined resonance that we ascribe to molecular remnants following the disintegration of the pyridine ring. Through consideration of the selection rules for the electric dipole transitions associated with photon absorption, NEXAFS spectra acquired at different incident radiation angles can be analyzed to determine the orientation of molecular orbitals with respect to the surface. The formalism for this process is described by Stöhr,61 with the geometry dependence of the intensity of a given NEXAFS transition described by the following relation for a 3-fold-symmetric surface and a polarization factor of 1, as is typical for an undulator beamline ÅÄÅ ÑÉÑ 1 I(α , θ ) = AÅÅÅÅ1 + (3 cos2 θ − 1)(3 cos2 α − 1)ÑÑÑÑ ÅÇ ÑÖ 2 where A is a constant, θ is the angle between the incident radiation and the sample plane, and α is the angle between the sample normal and the transition dipole moment of the molecular orbital, which for a π* orbital is perpendicular to the plane of the aromatic ring or −COO−/COOH group. We acquired NEXAFS spectra at three different incident angles: glancing incidence at θ = 20°, magic (tilt-independent) angle at θ = 55°, and normal incidence at θ = 90°. For planar adsorption of an aromatic molecule like PDC, the selection rules dictate that that intensity associated with π* transitions should vanish for normal incidence radiation and should be maximized as θ → 0°. The insets in Figure 6a−c clearly show that for all three absorption edges measured a pronounced angular dependence (dichroism) is observed. Similar considerations can be made for the σ* orbitals,61 but we focus here on the π* transitions since these transitions are clearly defined by sharp peaks and can be used to fully characterize the adsorption geometry of the molecule. Table 1 summarizes the calculated adsorption geometries for the PDC molecule in a submonolayer film, a multilayer film, and thorough annealing at 100 (leading to partial deprotona-

Figure 6. NEXAFS spectra collected at the carbon (a), nitrogen (b), and oxygen (c) K-edges for an as-deposited multilayer sample (sample 1) with θ = 20°. Indicative peak-fitting analyses are shown through the multicoloured components ascribed to each spectrum. (Inset) Angular dependence of each set of NEXAFS spectra is shown. The inset of (a) shows the experimental geometry and the definition of the angle θ, which relates the orientation of the electric field polarization to the sample surface.

related feature stays at a constant position. A slight reduction in the relative intensity of the high-energy split peak with F


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Table 1. NEXAFS Average Adsorption Angles (relative to the surface plane) for Submonolayer and Multilayer PDC on Cu(111)a pyridine as-deposited submonolayer as-deposited multilayer annealed to 100 °C (partially deprotonated) annealed to 155 °C (fully deprotonated)

carboxylic/carboxylate

C K-edge

N K-edge

calcd

± ± ± ±

25.2°

45° ± 3° 52° ± 5° 31° ± 10°

31° 41° 58° 24°

3° 1° 2° 7°

60.0° 24.9°

C K-edge

O K-edge

calcd

± ± ± ±

17.2°/16.9°

44° ± 1° 51° ± 7° 30° ± 20°

36° 44° 44° 25°

3° 1° 1° 4°

65.1°/59.4° 40.5°/38.4°

Following annealing to 100 °C, the uncertainty on the inclination angles calculated from the carbon K-edge increase but the uncertainties associated with the nitrogen and oxygen edges remain relatively low. The partially deprotonated molecule assumes a more upright geometry in the aromatic ring (from ∼52° to ∼ 58°), whereas data from the oxygen K-edge suggest that the carboxylic/carboxylate groups may be slightly out-ofplane with respect to the ring, with an inclination of only 44°. However, the carbon K-edge/nitrogen K-edge data put the inclination of the ring and carboxylic/carboxylate in agreement with one another, suggesting that any distortion of the molecule is minimal. This is borne out by the calculation results for a singly deprotonated PDC molecule, which is found to have a minimum adsorption energy when the deprotonated group points down into the substrate, leaving the ring tilted at an angle of 60° with respect to the surface normal and with the carboxylic/carboxylate essentially aligned with the aromatic ring.

a

tion) and 155 °C (full deprotonation). Similar results were obtained on a second sample, described in Table S1 in the SI. In the submonolayer film, the molecules adsorb at a relatively low inclination with respect to the surface (∼35°), whereas in the multilayer film the molecules assume a more upright geometry (∼45°). In both sub-ML and multilayer films the inclination of the aromatic ring and the carboxylic group agree within uncertainty and to high precision, so we assume that the molecule remains untwisted in its intact form. Annealing the film to 155 °C produced a completely deprotonated PDC layer. NEXAFS measurements from all edges provide a consistent result, with calculated adsorption tilt geometries from ∼25° to ∼30°. The uncertainties on all calculated values are quite large (4−20°), which may arise from inhomogeneities in the film due to adsorption at step edges and defects. These results are in partial agreement with the calculated adsorption geometry of a fully deprotonated molecule, where the ring is found to tilt by ∼25°. In the calculated geometry the carboxylate groups twist out of plane to form backbonds to the substrate, but our experimental data do not reveal this twist, which may be obscured by the large uncertainties and/or by the existence of a minority adsorption phase, as suggested by the spectral signature of surfaceadsorbed nitrogen in the SRPES data (Figure 2b). As shown in Figure 7, spectra collected after annealing to 280 °C are indicative of a broken molecule.

Figure 7. Evolution of the carbon (a) and oxygen (b) K-edges with annealing of the PDC film. RT corresponds to an as-deposited multilayer, 100 °C to a film of partially deprotonated molecules, 155 °C to a film of fully deprotonated molecules, and 280 °C to the remnants of a broken, decarboxylated molecule (carbon K-edge) and to near-complete desorption of all oxygen from the surface (oxygen kedge). All spectra were collected at θ = 20°.

based calculation of the transition state involved in the decarboxylation of biphenyl-4-carboxylic acid on Cu(111).15 This difference can be rationalized in the context of the variable adsorption geometry of PDC on Cu(111). As evidenced in our study, the PDC molecule adopts a semiupright configuration during deprotonation, and a similar dependence of adsorption geometry with state of reaction could occur during decarboxylation. If the adsorption geometry reduces the proximity of the molecular reaction site to the catalytic metal surface or places the −COO− in some other unfavorable geometry, we would expect to see a corresponding increase in the reaction barrier as compared to the case of planar adsorption. The DFT-calculated structure for biphenyl-4-carboxylic acid suggests that it is adsorbed relatively flat on Cu(111), putting the carboxylic group close to the Cu surface. This is consistent with the experimental decarboxylation activation barrier for PDC on Cu(111), Ea = 1.93 ± 0.17 eV, exceeding the one calculated for biphenyl-4carboxylic acid on Cu(111), Ea = 1.63 eV. As shown in Figure 4, annealing time is a determining factor for the state of reaction of the molecule. PES data suggest that the molecule is partially decomposed after annealing for 10 min at 270 °C, whereas the NEXAFS data show the molecule is fully decomposed after annealing for 5 min at 280 °C. This molecular fragmentation result suggests that ring opening

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DISCUSSION We learn two important things from our SRPES data: (1) the PDC molecule assumes different adsorption geometries on the surface, with the nitrogen coordinated both into and out of the surface, even as it deprotonates completely, and (2) the process of molecular fragmentation is competitive with the process of decarboxylation. The former finding indicates that additional adsorption geometries may be energetically similar to the ones identified in Figure 3. The latter finding suggests that obtaining a polymer product from PDC may be challenging, since fragmentation may occur before the onsurface diffusion of decarboxylated moieties that is required to form oligomers/polymers. Similar competitive activation/ fragmentation reactions have been observed in other heteroatom-containing molecules,62,63 where they can preclude the formation of the desired polymer product.64 From our kinetic study, we obtain a reaction barrier that is higher than the free energy barrier predicted from a DFTG


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The Journal of Physical Chemistry C could onset at temperatures below 270 °C and may be a significant factor in samples that are held for a long time at temperatures in the range where decarboxylation occurs. The NEXAFS-observed changes in adsorption geometry of the PDC could arise from two different sources: (1) the coverage of the molecule on the surface and (2) the chemical state of the molecule. The coverage clearly has an impact on the adsorption geometry: the sub-ML and multilayer films of unreacted molecules have different adsorption geometries, due to intermolecular interactions within the film. This effect has previously been observed for other systems, e.g., benzene on Pd(111), which evolves from a flat adsorption geometry at low coverage (

Adsorption and Reactivity of Pyridine Dicarboxylic Acid on Cu(111

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