Room Temperature Carbonylation of IronâPhthalocyanines Adsorbed

Article pubs.acs.org/JPCC

Room Temperature Carbonylation of Iron−Phthalocyanines Adsorbed on a Single Crystal Metal Surface: An in Situ SFG Investigation at Near-Ambient Pressure M. Corva†,‡ and E. Vesselli*,†,‡ †

Physics Department, University of Trieste, via Valerio 2, I-34127 Trieste, Italy IOM-CNR Laboratorio TASC, Area Science Park, S.S. 14 km 163.5, I-34149 Basovizza (Trieste), Italy

‡

ABSTRACT: We report the experimental spectroscopic evidence for the room temperature carbonylation, at equilibrium with the carbon monoxide gas phase, of iron phthalocyanines adsorbed on the (111) termination of the iridium single crystal. The adsorption process occurs at CO pressures above the mbar yield. Interestingly, heme and hemelike catalytic, carrier, and enzymatic biomolecules interact with the gaseous reactants at partial pressures that are similar to the values that we observed on this model system, a 2D layer of single-atom metallorganic catalysts preassembled under ultrahigh vacuum conditions. A simple Langmuir description of the adsorption mechanism yields a CO-Fe binding energy of 0.1− 0.3 eV, depending on the assumptions about the preexponential factor. The internal vibrational structure of the adsorbed iron phthalocyanines is also investigated.

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enough to provide optimal candidates for model SACs.6,8,9 However, while nature performs catalytic conversion reactions at ambient and near-ambient pressure of the reactants,2 a thorough atomic-level characterization of the involved reaction mechanisms is necessary to yield insight into the structural basis of function in heme-based proteins,10 calling for model conditions in order to exploit surface- and single-atom sensitive techniques. The latter belong to the approach typical of surface science that generally relies on ultrahigh vacuum (UHV) conditions. Several studies concentrating on the structural, electronic, magnetic, self-metalation, and reactivity properties of porphyrins and phthalocyanines with atomic level resolution have been reported, limited however to extreme environments like UHV and/or cryogenic temperatures.8,9,11−16 Just to report a few examples, and with particular insight into the aspects of chemical reactivity, CO and NO were found to bind to cobalt and iron metalloporphyrins on Ag(111) and Au(111);12,17 similar binding configurations have been found also in the case of adsorption of ammonia, chlorine, hydrogen, and oxygen on Ni, Zn, Mn, Fe, and Al porphyrins;8 molecular oxygen can be cleaved by manganese porphyrins adsorbed on the same surface;18 the properties of oxygen adsorption on a FePc can be tuned by means of controlling the bonding with an Ag(110) substrate.19 We extend here the investigation to the reactivity at

INTRODUCTION We exploit the surface sensitivity of infrared-visible sumfrequency generation (IR−vis SFG)1 vibrational spectroscopy to investigate in situ the carbonylation process of a single layer of iron phthalocyanine (FePc) molecules adsorbed on a supporting Ir(111) single crystal metal surface. We provide evidence for the formation of the CO−Fe bond above the mbar yield, at room temperature, thus at pressures that resemble the reactant’s partial pressure of some biomolecules.2 With this study we investigate the chemical reactivity of single-atom catalysts (SACs) in a 2D model layer, in which the biomimetic metallorganic bricks of the assembly resemble the reaction centers that nature exploits in several biomolecules where the catalytic reactivity is indeed depending even on a single metal atom center,3,4 like for example in heme prosthetic groups, where the active metal is bonded to N sites.2 As an example, hemoglobin and myoglobin are proteins belonging to this kind of compounds, developed by vertebrates in order to sustain aerobic life. The effective reactivity of these biomolecules is ultimately defined by a more complex network of interactions that fall outside the scope of this introductive and general discussion.5 However, the idea of investigating the properties of stabilized SACs is nevertheless intriguing, thus offering biomimetic approaches to the development of a novel family of applicative catalysts.6,7 Among a large variety of metallorganic templates and bricks, porphyrin and phthalocyanine (Pc) molecules can accommodate a single central metal atom in their tetrapyrrole cage. They are able to self-assemble in ordered 2D film structures when deposited at surfaces, stable © XXXX American Chemical Society

Received: May 27, 2016 Revised: September 14, 2016

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DOI: 10.1021/acs.jpcc.6b05356 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. IR−vis SFG vibrational spectra of a FePc monolayer adsorbed on Ir(111) measured at room temperature under UHV conditions (bottom) and upon exposure to 8 mbar CO (top): (a) stretching modes in the porphyrazine (P) and benzenic (B) rings; (b) C−H stretching region. Data (black lines) and the results of the least-squares fitting (blue lines) are shown; color-filled curves represent deconvoluted intensity modulations with respect to the nonresonant background (see text for further details) [λvis = 532 nm; ppp polarization].

room temperature and near-ambient pressure (NAP) conditions, in particular up to the 10 mbar scale.

crucible temperature was monitored by means of a thermocouple and the evaporation rate was calibrated by means of a quartz microbalance. After normalization to the impinging visible and IR photon fluxes, the raw SFG spectra in the low energy range (1480− 1760 cm−1) were also normalized for the effective contribution of air absorption in the laser free path. This was obtained thanks to the SFG signal from a GaAs crystal that generates an intense nonresonant reference, independent from the IR wavelength, because of its zincblende, noncentrosymmetric structure.21 The SFG spectra were then analyzed by leastsquares fitting to an established parametric, effective expression of the nonlinear second order susceptibility,20,22−26 in order to correctly reproduce the observed line shape and to account for the resonant IR−vis vibronic transitions and for the nonresonant background component:

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METHODS Infrared-visible sum frequency generation vibrational spectroscopy measurements were performed at the Physics Department of the University of Trieste in a dedicated setup described elsewhere.20 The Ir(111) sample was mounted by means of Ta wires that allowed resistive heating up to 1300 K in ultrahigh vacuum (UHV) and up to 800 K in the reactor (K-type thermocouple). The system allows transferring the sample between the high-pressure cell and the UHV environment without breaking the vacuum. The reactor is equipped with a gas handling system in order to control the background carbon monoxide pressure, in the present case under static conditions (no gas flow) for the measurements. The reactor was operated up to NAP conditions, thus allowing a precise control of the CO pressure from 10−9 up to more than 10 mbar. The inlet of the infrared and visible beams, as well as the outlet of the SFG signal, are provided by UHV-compatible BaF2 windows. The excitation source (EKSPLA) delivers a 532 nm visible beam and tunable IR radiation in the 1000−4500 cm−1 range and, together with the detection system, yields an ultimate energy resolution better than 6 cm−1. In the present study all spectra were collected in the ppp polarization configuration (SFGvisible-infrared). The Ir(111) surface was cleaned by standard cycles of Ar+ sputtering and annealing in UHV, alternated with oxygen treatments, in the preparation chamber (background pressure 5 × 10−11 mbar). Surface order was checked by LEED, yielding a sharp (1 × 1) pattern with extremely low background. The FePc molecules, purity better than 98%, were purchased from TCI Europe (I0783-1G, 132-16-1). The powder was inserted in a quartz crucible that was heated in UHV by thermal contact with a resistively heated tantalum filament. After an initial outgassing treatment, the molecules were evaporated from the crucible on the sample surface kept at room temperature. The

ISFG(ωIR ) ∝ ANRes + I visIIR (ωIR )

∑ k

ωIR

Ak eiΔφk − ωk + i Γk

2

ANres and Ak account for the amplitudes of the nonresonant and kth-resonant contributions, respectively, Δφk is the phase difference between the kth-resonant and nonresonant signals, ωk is the energy position of the line, and Γk its Lorentzian broadening that is related with the dephasing rate, which stems from the energy lifetime and from the elastic dephasing of the excited vibronic state.27 The fitting algorithm yielded an accuracy on the output line shape parameters of ±5° for Δφ, ±2 cm−1 for ω, and ±2 cm−1 for Γ. We also recall that the bestfitting set of parameters is known to be nonunique in homodyne SFG, since 2N‑1 equivalent sets can be obtained when N resonances are present in the spectrum. However, provided one optimal set is extracted from the data by means of the least-squares fitting procedure, the remaining equivalent sets can be easily obtained mathematically.26 In the manuscript’s figures, we plot the normalized SFG signal intensity (black) together with the best fit (blue lines) according to the B


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Table 1. Lineshape Parameters of the Observed Vibronic Features, Obtained upon Deconvolution of the IR−Vis SFG Spectra According to the Effective Second Order Susceptibility Reported in the Texta reference

a

‑1

‑1

energy (cm )

Δφ (deg)

1529 1597 (1603)

230 (210) 290 (330)

10 9

2004 2079 3055 3075 3093

225 30 130 235 (280) 300

15 6.0 6.5 6.0 12

Γ (cm )

‑1

energy (cm )

system

1508, 1524 1601 1602 1977−2034 2013−2083

FePcs/Ag29 FePcs/Ag29 H2Pcs/Ir28 CO/Fe30−32 CO/Ir20 FePcs/Ag29 H2Pcs/Ir28

3065

assignment P, porphyrazine B, benzene rings CO/Fe on top CO, internal stretch H1, CH stretch H2, CH stretch H3, CH stretch

Values in parentheses refer to the carbonylated molecules.

multilayer regime (1508 and 1601 cm−1) and one peak was detected instead for the monolayer (1524 cm−1). The features were assigned to the in-plane stretching of the porphyrazine (P, low energy) and benzene (B, high energy) groups. The contribution of the latter vanishes on Ag(111) in the monolayer regime since the benzenic rings of the molecule are perfectly planar on this silver surface.29 On the basis of the above information we therefore associate the peaks at 1529 and 1597 cm−1 to the P and B stretching modes, respectively, also in agreement with HREELS measurements for H2Pcs/ Ir(111).28 In the high energy region, we are instead able to distinguish three resonances (Figure 1, panel b, bottom) that can be associated with three nonequivalent C−H stretching modes (H1, H2, and H3). In the case of previous observations on similar systems,28,29 however, due to the lower energy resolution of the techniques, there is no evidence for a fine structure like the one we observe. In our case, the spectral contributions are best fitted by resonances centered at 3055, 3075, and 3093 cm−1, respectively. The observed chemical shifts are likely ascribed to both substrate−molecule and molecule−molecule interactions, while the different Δφk values of the resonances (see Table 1) may be due to different geometric orientations of the C−H bonds with respect to the metal surface. Carbonylation of Adsorbed FePcs. Upon exposure of the FePc monolayer adsorbed on Ir(111) to a carbon monoxide background at near-ambient pressure conditions (8 mbar), little changes are observed in the vibrational fingerprint of the metallorganic molecules (Figure 1, panels a and b, top). In the low energy region, a small blue-shift is observed for the B feature (6 cm−1). No changes in the Lorentzian broadening are observed, being related to the dephasing rate of the excited state of the resonant second order emission process. The B resonance also diminishes in intensity and its phase varies by 40°, consistently with a realignment of the benzenic rings of the molecule. A small phase shift (20°) is observed also for the P resonance. Almost no changes are instead observed in the C−H stretching region, as expected since the external part of the benzenic rings should be less influenced by the adsorption of CO at the metal center. Only a phase shift of 45° is observed for the H2 resonance. Turning to the C−O stretching region, while obviously no features are present under UHV conditions (Figure 2), a peak progressively grows in intensity upon exposure of the FePc monolayer to a carbon monoxide environment at stepwise growing pressure values. Measurements at selected CO pressures in static and at equilibrium conditions (Figure 2) at room temperature indicate that CO

above function that accounts for all the interfering cross-terms. In Figure 1, we also plot (color-filled curves) the intensity of each resonance and its interference with the nonresonant background by calculating, with the parameters obtained from the fitting procedure, the following quantity: ISFG, k(ωIR ) I visIIR (ωIR )

∝ ANRes +

ωIR

Ak eiΔφk − ωk + i Γk

2

These plots can directly put in evidence the amplitude and the relative phase for each of the resonances. Further details can be found in our previous work.20

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RESULTS AND DISCUSSION Adsorption of FePcs. A thick (few monolayers) film of FePcs was grown by chemical vapor deposition on the Ir(111) clean surface, kept at room temperature under UHV conditions. Subsequent annealing above the multilayer desorption temperature (600 K) yielded a saturated layer of molecules adsorbed on, and directly interacting with, the metal surface. There is indeed previous literature evidence that H2Pc molecules deposited on the Ir(111) termination are flat-lying at the surface, as observed by STM imaging and accordingly described by ab initio calculations.28 If this were the case also for FePc molecules, no vibrational signal would be expected since charge images at the metal surface of the in-plane oscillating dipoles would cancel the signal. The molecular plane is however slightly distorted. In Figure 1 (bottom) we plot the measured SFG signal in the 1480−1760 and 3020−3130 cm−1 ranges, panels a and b, respectively. The development of evident vibrational fingerprints in the spectra witnesses a deformation of the molecule upon adsorption. This phenomenon can be ascribed to a pinning of the molecules via the central metal atom (Fe) to the metallic surface, more strongly than in the case of the nonmetallized H2Pc molecule,28 and in analogy to what observed on a Ag(110) surface,19 thus yielding an “umbrella” shape of the molecular plane. In the low energy region (Figure 1, panel a, bottom), we observe two distinct features at 1529 and 1597 cm−1 (see Table 1 for details about the line shape parameters). A third feature is present at lower energy (not visible), as expected from literature data about H2Pc adsorption on this surface.28 However, this feature could not be measured since, below 1450 cm−1, our laser intensity drastically drops. The tail of this feature has however been considered for the best fitting of the data. In the case of FePcs adsorbed on Ag(111),29 the vibrational spectra of both the molecular multilayer and the single layer were measured by means of HREELS. Two features were observed in the C


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and lineshapes are obtained, while the C−O stretching resonance intensity vanishes. In Figure 3, both the resonant (bottom) and the nonresonant (top) SFG signal amplitudes in the C−O stretching

Figure 3. Amplitude of the nonresonant (top) and resonant (bottom) contributions to the IR−vis signal in the C−O stretching region at room temperature as a function of the background carbon monoxide pressure; the dashed line represents the best fit according to a first order Langmuir isotherm model.

region are plotted as a function of the stepwise increasing CO background pressure (isothermal uptake), while keeping the sample at room temperature. While the nonresonant amplitude diminishes by about 15%, the peak at 2004 cm−1 progressively gains intensity. The nonresonant signal originates from the nonresonant part of the nonlinear molecular response, from the metal surface, and from the metal bulk.33,34 A change of its intensity during the CO uptake indicates therefore a substantial contribution from the molecular layer to the constant SFG background. Concerning the resonant signal amplitude, instead, we assume here that it is proportional to the CO coverage. This is known to be a nonreliable quantity in this sense, since it depends on the CO dipole moment that is strongly influenced by lateral interactions, and on changes in the Raman and infrared matrix elements. However, a metal Pc molecular lattice is known to consist in a square structure, where the metallic centers (and consequently the adsorbed CO molecules) measure a nearest neighbor distance of about 1.5 nm, yielding a CO−CO distance of about 1 order of magnitude larger than in the case of CO adsorption on a single crystal metal surface. Therefore, the CO−CO dipole interactions can be reasonably neglected in the present case, thus supporting our first-order approximation. Moreover, the resonant signal amplitude may be assumed as proportional to the CO concentration at the surface since no phase changes are observed during the uptake. A simple fit of the resonant amplitudes with a first order Langmuir isotherm model (dashed line in Figure 3),35 yields a Fe−CO binding energy of 0.30 ± 0.05 eV, roughly but straightforwardly obtained by assuming an initial sticking coefficient equal to one and a standard pre-exponential factor of 1013 Hz. A lower initial sticking coefficient may be plausible, and pre-exponential factors may vary within a range spanning over several orders of magnitude. Indeed, kinetic modeling of

Figure 2. IR−vis SFG vibrational spectra collected in situ in the C−O stretching region upon exposure of a FePc monolayer on Ir(111) to a carbon monoxide background at room temperature; in the bottom part of the figure, the spectrum obtained upon saturation of the clean Ir(111) surface is reported as a reference (intensity divided by 5). Data (black dots) and the results of the least-squares fitting (blue lines) are shown [λvis = 532 nm; ppp polarization].

binds to the molecules above the mbar yield, giving origin to a C−O stretching feature at 2004 cm−1. This stretching frequency is in line with data reported for CO adsorption on Fe in ontop configuration,30 as well as in iron carbonyls.31,32 The Ir(111) metal surface is completely passivated, since ontop CO at Ir terminal atoms would yield a very intense feature at 2079 cm−1 that is instead not observed. A saturated CO/ Ir(111) monolayer was prepared as a benchmark (Figure 1, bottom, intensity divided by five), in agreement with a previous SFG investigation of CO and CO2 adsorption on Ir(111) at near-ambient pressure conditions.20 The CO adsorption mechanism on the FePc molecules is found to be reversible, and the CO residence time at the Fe center is short at room temperature. Indeed, by pumping out the carbon monoxide gas in few minutes and recovering the UHV conditions (not shown), the initial P, B, H1, H2, and H3 vibrational features D


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at NAP conditions in CO atmosphere. A deep insight into the bonding mechanism between ligand and reaction center has already been considered as a key point in order to both understand reactivity and selectivity of catalytic biomolecules and to engineer biomimetic reaction centers,10,41,42 thus shedding light onto the atomic level reaction mechanisms involved in novel, promising synthetic organometallic (electro-) catalytic systems.7,39,43,44 Our study demonstrates that experimental techniques like IR−vis SFG working at near-ambient pressure conditions may pave the way to the in situ and operando investigation of the properties of these systems.

the heme pocket in myoglobin yields a classical attempt frequency for CO of 1012 Hz that is reduced down to 109 Hz when considering the CO motion to be synchronized with the iron displacement from the heme plane (reducing factor of 150), the CO orientation, and the nonadiabaticity of the electronic spin change in Fe (reducing factor of 6).36,37 With these latter assumptions, we obtain a Fe−CO binding energy value of 0.10 ± 0.05 eV, thus confining our estimate in the 0.1− 0.3 eV range. Pre-exponential factors and entropy contribute also to the discrimination between preferential adsorption of CO with respect to the O2 ligand in natural biomolecules.37 With evident caution due to the different chemical and structural environments, it is noteworthy to mention that while we get a saturation CO partial pressure of 10 mbar, in the case of hemoglobin and myoglobin molecules the working O2 pressure is of the order of 10 mbar at 300 K,2 while CO is more strongly bound, yielding saturation partial pressures of the order of 0.1−1 mbar.38 Since a large variety of biomolecules host catalytic reaction centers based on single-atom metallic structures, this represents the driving reason for a growing research effort in the direction of the stabilization and characterization of model biomimetic single-atom catalysts.6,39 Accordingly, there are a number of reports in the literature that focus on the interaction of simple gas phase molecules (like CO and NO) with adsorbed metal porphyrins and phthalocyanines.8,9 However, in both scanning tunneling microscopy and photoelectron spectroscopy approaches the samples are generally kept at cryogenic temperature in order to stabilize both the macromolecules and the weakly bound adsorbates. Evidence for both mono- and dicoordination has been obtained in this way for the carbonylation of cobalt and iron porphyrins, 11 and a spectroscopic fingerprint for CO and NO binding to iron phthalocyanines was reported.12 In this context, the oxidation state of the metal plays a relevant role and deserves a separate discussion, even if it cannot be directly determined with the SFG technique in our case. It is known that the iron atom in the FePc molecule is in the Fe(II) state.12 Upon adsorption of the molecule on a metal surface like Au, a broad multiplet structure of the Fe 2p3/2 line is observed in photoelectron spectroscopy, at cryogenic temperature and UHV conditions, due to the open shell structure of the iron ion.12 Donation of electron density from CO to the metal and back-donation from the metal d orbitals into antibonding ligand orbitals with π* character occurs upon ligand adsorption. The Fe−CO bond has coordinate character with both electrons involved in the direct CO−Fe donation originating from CO. Consequently, a narrowing of the Fe 2p spectroscopic line is observed. Moreover, the FePc coupling to the metal surface is reduced upon CO adsorption due to what is known as trans effect, which is relevant for heme-like complexes. Indeed, the bonding between the ligand and the Fe ion induces a redistribution of the Fe 3d valence orbitals and affects also the Fe spin state of the molecule.40 Similarly, in the case of Co porphyrins, the Co(II) state of the free CoTPP reduces to Co(I) upon adsorption of the molecule on a Cu(111) surface due to the coupling of the Co d-states and the Cu substrate.11 The monocarbonyl ligation does not change the Co oxidation state, although the electronic structure is slightly modified. An analogous situation is observed also for CO ligation to both Co− and Fe−TPPs on an Ag surface,17 thus yielding a reasonable extension of this reasoning also to our system while waiting for future measurements of the Fe electronic structure

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CONCLUSIONS We have provided experimental evidence obtained in situ at near-ambient pressure for the carbonylation of iron phthalocyanine molecules supported on the Ir(111) metal surface. We found that the internal vibrational structure (stretching of the porphyrazine and benzenic structures) of the molecule is almost not modified by the formation of the CO−Fe bond. A C−O stretching feature develops at 2004 cm−1 upon exposure of a FePc monolayer to CO above the mbar regime, being the uptake reversible. A complete FePc monolayer adsorbed on Ir(111) completely passivates the underlying metal surface with respect to CO adsorption.

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AUTHOR INFORMATION

Corresponding Author

*(E.V.) E-mail: [email protected]. Telephone: +39-040-3756442. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.C. and E.V. acknowledge financial support from MIUR through the project Futuro in Ricerca FIRB 2010 No. RBFR10J4H7. Fruitful discussion with A. Morgante is gratefully acknowledged. This work is dedicated to, and in memory of, L. Rimoldi: without his contribution, the preparation chamber of the SFG setup would not have become reality.

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