Identifying the Thermal Decomposition Mechanism of Guaiacol on Pt

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Identifying the Thermal Decomposition Mechanism of Guaiacol on Pt(111): An Integrated X-ray Photoelectron Spectroscopy and Density Functional Theory Study Alyssa J. R. Hensley, Claudia Wöckel, Christoph Gleichweit, Karin Gotterbarm, Christian Papp, Hans-Peter Steinrück, Yong Wang, Reinhard Denecke, and Jean-Sabin McEwen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10006 • Publication Date (Web): 29 Nov 2017 Downloaded from http://pubs.acs.org on December 21, 2017

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The Journal of Physical Chemistry

Identifying the Thermal Decomposition Mechanism of Guaiacol on Pt(111): An Integrated X-ray Photoelectron Spectroscopy and Density Functional Theory Study Alyssa J. R. Hensleya, Claudia Wöckelb, Christoph Gleichweitc, Karin Gotterbarmc, Christian Pappc, Hans-Peter Steinrückc, Yong Wanga,d, Reinhard Deneckeb*, and Jean-Sabin McEwena,d,e,f* a

The Gene & Linda Voiland School of Chemical Engineering and Bioengineering, Washington

State University, Pullman WA 99164 b

Wilhelm-Ostwald-Institute, University of Leipzig, Leipzig D-04103

c

Department of Chemistry and Pharmacy, Friedrich-Alexander-Universität Erlangen-Nürnberg,

Erlangen D-91054 d

Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, WA, 99352

e

Department of Physics and Astronomy, Washington State University, Pullman, WA 99164

f

Department of Chemistry, Washington State University, Pullman, WA 99164

*Corresponding authors: Reinhard Denecke; +49-341-9736451 (phone), [email protected] Jean-Sabin McEwen; +1-509-335-8580 (phone), [email protected]

Abstract Using a concerted effort from both experiment and theory, we determine the thermal decomposition mechanism for guaiacol on Pt(111), a reaction of interest in the area of bio-oil upgrading. This works serves as a demonstration of the power of combining in situ temperature programmed x-ray photoelectron spectroscopy (TPXPS) and density functional theory (DFT) to

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elucidate complex reaction mechanisms occurring on heterogeneous surfaces. At low temperature (230 K), guaiacol was found to chemisorb with the aromatic ring parallel to the Pt(111) surface with five distinct carbon species and three oxygen species. As the temperature was increased, TPXPS showed several significant changes to the surface species. The increase in the species associated with the decomposition of the functional groups of guaiacol is followed by their subsequent disappearance and an increase in the non-aromatic carbon signal. Based on an energetic analysis of the various mechanisms using DFT, along with the comparison of the experimentally and theoretically derived core level binding energies, we determined that guaiacol’s decomposition mechanism occurs via the dehydrogenation of both the methyl and hydroxyl functional groups, followed by demethylation of the CH2 or CH group to form 1,2benzoquinone. Further heating to above 375 K likely breaks the aromatic ring and results in the rapid formation and desorption of CO, accounting for the disappearance of all O 1s signal above 450 K. These results show that a knowledgeable application of TPXPS and DFT can result in the quantitative identification of surface species during complex reactions, providing insight useful for the design of future heterogeneous surfaces.


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1. Introduction The quantitative determination of surface species under reaction conditions during heterogeneous reactions is a major challenge to be overcome in the pursuit of ever more active and selective heterogeneous catalysts. The molecular level quantification of surface species is made difficult due to the dynamic nature of the surface under reaction conditions, both in terms of the surface’s effect on adsorbates and the adsorbates’ effect on the surface.1-3 Overcoming this challenge requires combining advanced, surface sensitive experimental and theoretical techniques, which together can provide the information necessary to determine reaction pathways and design new heterogeneous catalysts. Despite the large amount of information obtainable from spectroscopic surface sensitive experimental techniques (such as x-ray absorption near-edge spectroscopy (XANES), x-ray emission spectroscopy (XES), x-ray photoelectron spectroscopy (XPS) or infrared spectroscopy (IR)), interpreting the data can be difficult for complex surface reactions due to contributions from bulk species (i.e. the subsurface and gas phase species) and/or the limited energy resolution and sensitivity of the equipment. For example, the C 1s core level shifts in XPS between different hydrocarbon fragments on a metal surface can be too small to be resolved even with a laboratory-based x-ray monochromotor, despite the high spectral resolution of 300 meV or better obtainable with such systems. The contribution from the bulk species can be eliminated by using techniques such as sum frequency generation vibrational spectroscopy (SFG),4,

5

infrared

reflection-absorption spectroscopy (IRRAS),6 and x-ray photoelectron spectroscopy (XPS, note that ambient-pressure XPS is needed when experiments are not performed under vacuum). As for the spectral resolution, this can be improved for XPS using synchrotron radiation, which has the advantage of both high intensity and high resolution (180 meV or less for C 1s); thus providing



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unprecedented insights into the elementary reaction processes occurring on heterogeneous surfaces.7-19 However, for larger, more complex systems, spectroscopy features (such as the C 1s) are much harder to deconvolute, and a close correlation between theory and experiment is necessary to correctly interpret the experimental spectra.18, 19 Herein, we combine synchrotron-based temperature-programmed XPS (TPXPS) and density functional theory (DFT) with van der Waals corrections to quantitatively determine the elementary reaction pathways for the thermal decomposition of guaiacol on Pt(111). Guaiacol was chosen as it is often used as a model compound for the design of hydrodeoxygenation (HDO) catalysts for bio-oil upgrading,20-24 and Pt(111) was chosen because of the vast amount of data that is already available on it for various hydrocarbon species.10,

13

While the high-

resolution, synchrotron-based XPS allows us to distinguish different surface species by their particular core level binding energies, it is in most cases not possible to identify and assign molecular species unambiguously. Thus, combining the benefits of XPS with DFT calculations of reaction pathways, the molecular surface species involved, and their core level binding energies has allowed us to quantify the species present and their decomposition pathways as the surface temperature is increased. This quantitative analysis of surface species as a function of surface temperature goes well beyond the majority of combined theory and experimental studies, which typically use experimental results to provide the starting point for theoretical models with little to no model validation via a direct comparison between theory and experiment.25-28 In addition to the mechanistic information obtained here, we show how sensitive synchrotron-based XPS is to the nature of the C-C, C-H, O-H, and C-O bonds within an aromatic compound during its decomposition on a metal surface. Overall, this work shows how such a quantitative approach can be used to determine the adsorbed surface species as a function of the reaction temperature,


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thereby providing new physical insight into (1) the elementary reaction pathways for the decomposition of a complex, heterogeneous system, i.e. guaiacol adsorbed on Pt(111), and (2) a quantitative methodology for the in situ analysis of surface species in heterogeneous chemical reactions.

2. Methods 2.2 Experimental Details The adsorption and temperature programmed reaction experiments were performed at the third-generation synchrotron facility BESSY II in Berlin. A transportable UHV apparatus (adequately described elsewhere29) equipped with a hemispherical electron energy analyzer (Omicron EA 125 U7 HR) was utilized at beamline U49/2 PGM1. The surface normal of the Pt(111) surface was pointing towards the electron analyzer (normal emission). A bifilar tungsten coil was installed in the back of the crystal for indirect heating during the XPS measurements; alternatively, two tungsten rods directly spot-welded to the sides of the crystal can be used for direct current heating. Cycles of argon sputtering and subsequent heating to 1050 K were used to clean the crystal; the cleanliness was assured via XPS and surface order was checked by low energy electron diffraction (LEED). Guaiacol was purchased from Sigma Aldrich and vacuum distilled previous to pump-freeze-thaw cycles. The molecules were introduced to the UHV chamber via a microcapillary array doser utilizing the vapor pressure at approximately 35 °C. The photon beam was hitting the sample only during measurements, and the sample surface was scanned in order to minimize beam damage in a given area. For the C 1s (O 1s) XPS measurements, the photon energy was set to 380 eV (650 eV) for maximum surface sensitivity, leading to an overall resolution of 250 meV (370 meV). A



linear heating ramp of 0.5 Ks-1 was used for TPXPS experiments, with spectra being taken every 10 K; the acquisition time per spectrum was below 10 s. The carbon coverage was calibrated to the adsorption of CO as described elsewhere18. The XPS data were handled by UNIFIT 201630, 31

. A Shirley function with a 2nd order polynomial function was used for the description of the

background. Peak fitting was performed with a convolution of Gaussian and Lorentzian functions. For all the spectra of one experiment and a particular core level, the Gaussian and Lorentzian peak widths of all peaks were kept constant. All binding energies were referenced to the Fermi level. Additionally, temperature programmed desorption (TPD) measurements were taken in parallel with the TPXPS measurements to observe evolving gas phase species and they were consistent with our TPXPS findings.

2.2 Computational Details The theoretical calculations presented in this work were performed using the Vienna Ab Initio Simulation (VASP) package.32, 33 The core electrons were treated using VASP's projector augmented waves (PAW)34, 35 and the valence electrons were treated using the optB88-vdW36, 37 exchange-correlation functional to account for van der Waals corrections. The supercells were sampled using a (3 × 3 × 1) Monkhorst-Pack38 mesh and the kinetic energy cutoff for the plane wave basis set was set at 400 eV. The Methfessel-Paxton39 (N = 1) smearing method was used with a smearing width of 0.1 eV to improve convergence, and the total energy was extrapolated to zero Kelvin. The total energy and interatomic forces were converged to 10-4 eV and 0.03 eV/Å for all groundstate optimizations, with a conjugate-gradient algorithm used to find the energy minimizing structures. All calculations were spin polarized and dipole corrections were used in order to avoid dipole interactions between repeating supercells. Such parameters are identical to


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those presented in our previous work.40-43 The surfaces were modeled using p(4x4) supercells in order to minimize the lateral interactions between the repeating supercells due to VASP’s implementation of periodic boundary conditions. The near surface structures were studied with van der Waals corrections, and five metal layers with the bottom three layers kept fixed during optimization. This number of metal layers was found to be sufficient to converge the adsorption energies. All other atoms, including the adsorbates atop the metal slab, were allowed to relax in order to find the optimum structural configuration. The oxygen functional group configuration for adsorbed guaiacol used in these studies was the most energetically stable gas phase configuration of guaiacol.42 In addition to these parameters, the repeating supercells in the zො direction were separated by either ~14 or 18 Å of vacuum when studying either the horizontal or vertical adsorption of guaiacol, respectively. The bulk fcc Pt unit cell was optimized and the calculated lattice constant was found to be 3.986 Å, which agrees well with previous work.18, 44 All structures were visualized using VESTA.45 We modeled numerous reactions for guaiacol’s thermal decomposition on Pt(111) in order to gain insight into the decomposition mechanism. The favorability of each of these mechanisms was evaluated using the reaction energy as defined by: Erxn =Eproducts -Ereactants

(1)

where Eproducts and Ereactants are the total energies of the products and reactants, respectively. All co-adsorption sites for the H, OH, OCH3, CH3, CH2, and CH fragments for all of the mechanisms studied in this work were chosen based on their most favorable adsorption sites on clean Pt(111) found in previous work.46 When an additional hydrogen was used for a reaction, which was not present in the reactants, the reaction energy was calculated by adding the adsorption energy of an isolated hydrogen atom on Pt(111) in the most favorable top site46 using Equation 1. This energy



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was calculated to be -0.44 eV, relative to 1/2 H2. One has to stress that H in both on-top and hollow sites has very similar calculated binding energies; experimentally, hollow sites seem to be preferred at high coverages.46 Brønsted-Evans-Polanyi (BEP) relations determined by Lee et al.21 were used to estimate all activation barriers presented here except for the ring opening reaction. This transition state was calculated using the climbing image nudged elastic band (CINEB)47 method using energy and force tolerances of 10-5 eV and 0.03 eV/Å. Additionally, a damped second order equation of motion was used to relax the structures along the minimum energy pathway (MEP). The transition state structure was confirmed by calculating its vibrational frequencies and determining that only a single, unique imaginary normal mode eigenvector was present.48 These parameters are identical to those used in our previous work.49 The intermediate structures resulting in the most energetically favorable mechanism were further investigated by comparisons to the experimentally obtained TPXPS results. The core level binding energy (CLBE) is a measure of how tightly bound an atom’s core electrons are. This is measured experimentally using XPS on certain core orbitals (here C 1s and O 1s). Using DFT, the relative CLBE shifts between two systems are defined as: ECLS =ൣEsystem ሺnc -1ሻ-Esystem ሺnc ሻ൧-ൣEreference ሺnc -1ሻ-Ereference ሺnc ሻ൧

(2)

where nc is the total number of core electrons in the system, Ei(nc-1) is the total energy of the adsorbed system with a single core electron removed from the atom of interest, and Ei(nc) is the total energy of the adsorbed system with all core electrons present in the system. As the absolute, calculated CLBEs for a single system are not comparable to experiment, it is these theoretically calculated CLBEs energy shifts which are comparable to the shifts found by XPS.50 In this work, we have calculated the C 1s and O 1s CLBE shifts for atoms in guaiacol on Pt(111) using the final state approximation50. The reference systems used here are benzene on Pt(111) for the C 1s


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peaks with an experimental binding energy of 284.4 eV18 and atomic oxygen adsorbed on Pt(111) for the O 1s peaks with an experimental binding energy of 529.9 eV51, 52.

3. Results and Discussion 3.1 Temperature Programmed X-ray Photoelectron Spectroscopy Guaiacol was introduced into the UHV chamber at crystal temperatures of 170 and 230 K. The adsorption was followed in situ by measuring the O 1s and C 1s core levels in frequent sequence during dosing of guaiacol. At 170 K (see Figure S1 in the Supporting Information), weakly bound guaiacol molecules result in specific spectral features whose intensity increases as long as guaiacol is being dosed. Upon subsequent heating to 240 K, the signals rapidly decrease by about 60 % due to desorption of weakly bound molecules (Figure S2). The missing saturation and the low desorption temperature identify the majority species at 170 K as physisorbed species. In fact, their core-level binding energy values are very close to the ones of chemisorbed guaiacol, in good agreement with calculations of physisorbed guaiacol species (see Figure S3S5). Simultaneous with the overall signal decrease, new features appear and the spectra at 230 K reach a shape similar to the ones displayed in Figure 1. Exposure to guaiacol directly at 230 K mainly yields a saturated layer of chemisorbed guaiacol with a coverage of around 0.17 ML (where ML has been defined as 1 molecule per 1 Pt surface atom). This is slightly smaller than the 0.18 ML saturation coverage found for benzene on Pt(111), but is larger than the 0.1 ML saturation coverage found for anisole on Pt(111).18, 26 However, one has to mention the difficulties in determining saturation as weakly bound species and potential dissociation products occur. Thus, a comparison contains some uncertainties. The resulting O 1s and the C 1s spectra after saturation are shown in Figure 1. The presence and



position of the peaks were confirmed by repeated experiments. As shown below, some residual physisorbed molecules cannot be excluded.

Figure 1. Guaiacol saturation spectra at 230 K on Pt(111) for the O 1s (bottom) and C 1s (top) core levels with results of a peak fitting procedure (see text for details). The raw data is shown in filled black circles, while the fit results are shown as smooth lines. The inset in the top graph shows a comparison of C 1s saturation spectra of guaiacol at 230 K (filled black squares) and of benzene at 200 K (filled grey circles) on Pt(111), normalized to the total spectral intensity (i.e. the C coverage). The molecular coverages are 0.18 ML for benzene and 0.17 ML for guaiacol. Mind the difference in carbon atom count per molecule (6 in benzene, 7 in guaiacol).

The O 1s spectrum in Figure 1 (bottom) essentially shows a major signal at ~533 eV with a smaller contribution at a lower binding energy. Peak fitting reveals three contributions at 533.1, 10 ACS Paragon Plus Environment

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532.4, and 530.6 eV. The peak at 533.1 eV is already present at 170 K and is closely related to the peak at 533.4 eV (Figure S1). Thus, the peak at 533.1 eV seems to belong partly to remains of the physisorbed species. Since the O 1s peak at 530.6 eV increases at higher temperatures (see Figure 2), it must be assigned to a guaiacol dissociation product. As this peak is weakly present at 170 K (Figure S1), this suggests that physisorbed, chemisorbed, and some decomposed guaiacol are all present at temperatures as low as 170 K. In the C 1s spectrum in Figure 1 (top), two major signals are found at ~284.4 and ~286.2 eV. Peak fitting (already taking into account parameters obtained from higher temperatures) reveals the existence of a total of five species. The signal at ~284.4 eV is composed of two peaks at 284.3 (with a vibrationally split component at 284.7 eV) and 283.9 eV; the signal at 286.2 eV contains two peaks at 286.5 and 285.9 eV. In between there is another component at 285.15 eV. The signals at 284.3, 286.5, and 285.9 eV are partly remains of features present already at 170 K (Figure S1) in that binding energy range. The presence of two major peaks in the C 1s spectrum here for adsorbed guaiacol is similar to that observed for anisole on Pt(111) with a benchtop XPS source.26 A visual comparison of the two data sets shows that the noise in the C 1s spectra is significantly lower for our synchrotron-based XPS measurements allowing us to perform a much more detailed peak fit, and highlighting the greater precision of our synchrotron-based XPS measurements over traditional benchtop XPS measurements. By comparison to the benzene C 1s saturation spectrum at 200 K from ref.18 (see Figure 1 insert), we assign the peak at 284.3 eV (with its satellite) to the four ring C atoms of guaiacol that are bonded to H. The O-functionalized aromatic compound shows an additional C 1s signal at higher binding energies, which should be due to carbon atoms next to oxygen. To determine the absolute surface coverages, the spectra are normalized to the carbon coverage derived from



comparison to the CO data. Taking the number of carbon atoms into account, the molecular saturation coverages are 0.17 ML for guaiacol and 0.18 ML for benzene, reflecting the different space requirements on the surface. The temperature dependent reaction is followed by TPXPS, as shown in Figure 2. After exposure to guaiacol at 230 K, the surface is indirectly heated with a linear ramp of 0.5 Ks-1. XPS spectra were recorded in steps of approximately 10 K. Upon heating, the C 1s signal at ~286.2 eV and the two major O 1s signals decrease and finally disappear above ~430 K. In contrast, the remaining C 1s peak at 284.4 eV does not disappear, but broadens and shifts to slightly lower binding energies at ~430 K.


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Figure 2. Waterfall presentations (top) and color-coded intensity plots (bottom) of the TPXPS experiments after guaiacol adsorption on Pt(111) at 230 K for the (left) C 1s spectra and (right) O 1s spectra.

The TPXPS spectra were further analyzed by peak fitting. Over the studied temperature range, some binding energy shifts of the components were permitted, as suggested by the visual inspection of Figure 2 (in the following discussions, the components are still labelled by their binding energy values at 230 K). The quantitative analysis of the C 1s and O 1s peak intensities with temperature is shown in Figure 3; the identification and assignment of the various surface species present during the decomposition of guaiacol on Pt(111) was based on the theory results presented below. The total C and O intensities in Figure 3 show a very similar behavior up to ~380 K: Between 230 and 300 K, there is a significant drop in peak intensity of about 20 %, which is assigned to desorption of physisorbed or weakly-bound molecules, whose core-level 13 ACS Paragon Plus Environment


binding energies coincide with those of the chemisorbed ones. Between 300 and 380 K, the total O and C 1s intensities remain relatively constant, before decreasing above 400 K. The O 1s signal disappears finally above 430 K, suggesting the desorption of some O-containing products. The C 1s signal reaches another stable value around 430 K, suggesting that some of the C species desorb and some remain present at higher temperatures. Besides desorption, changes in photoelectron diffraction caused by the different surface species would be an explanation for varying intensities, amounting to possibly ±10 %. A more detailed analysis of the changes to the C 1s and O 1s peaks in Figure 3 with temperature is presented in Section 3.4.


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Figure 3. Quantitative fit results of the TPXPS of guaiacol on Pt(111) as shown in Figure 2. The C 1s fit is shown on top and the O 1s fit is shown on the bottom.

3.2 Assignment of TPXPS Peaks 3.2.1 Guaiacol Adsorption In order to connect the observed changes in the TPXPS results to changes in the surface molecular structures of guaiacol, we first searched for the most stable adsorption configurations of guaiacol on Pt(111) by theoretical calculations. The evaluated flat-lying and upright-standing structures are shown in Figures S3 and S4, respectively; the corresponding adsorption and distortion energies, and a summary of the geometric parameters for all studied guaiacol adsorption sites are given in Table S1. We use the same adsorption site nomenclature as in our



previous work.42 The comparison of the different sites shows the B30o-BB site with a binding energy of -2.35 eV as most favorable geometry (see Figure 4); this site is similar to the one for benzene, phenol, anisole, and guaiacol on Pt(111)18, 21, 22, 53-56, and for phenol and guaiacol on Pd(111)40, 42. Additionally, the binding energy of guaiacol on Pt(111) calculated here agrees well with previous work examining guaiacol’s adsorption on Pt(111).21, 22, 56 In a next step, we calculated the C 1s and O 1s CLBEs for guaiacol on the B30o-BB site on Pt(111), in order to identify the molecular structures that produce the spectral features in the experimental XPS results. The resulting CLBEs for seven carbons atoms (C1-C7) and the two oxygen atoms (O1 and O2) are shown in Figure 4, with the experimental binding energies shown as horizontal lines for comparison. Both experimental and theoretical values are subject to similar errors, on the order of ±0.1 eV 7. For the sake of clarity, the error bars are not shown in the figure.


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Figure 4. Comparison of the experimental C 1s and O 1s spectra for guaiacol on Pt(111) at 230 K (left) to the theoretically calculated CLBEs for guaiacol adsorbed on Pt(111) in the B30o-BB site (right). The calculated values (symbols) are labeled C1-C7 and O1, O2, as indicated for guaiacol. (top) C 1s CLBEs; (bottom) O 1s CLBEs. Experimentally observed binding energies from the peak fitting shown in Figure 1 are shown as horizontal lines; C 1s region: 284.3 eV (solid), 286.5 eV (dash dot), 285.9 eV (short dash), 285.15 eV (long dash), and 283.9 eV (dot); O 1s region: 533.1 eV (solid), 532.4 eV (long dash), and 530.6 eV (dot).

The comparison between the calculated values for intact guaiacol (symbols) and the experimentally observed XPS peaks (horizontal lines) results in the identification of three of the five C 1s signals and one of the three O 1s signals observed experimentally at 230 K: The C 1s peak at 284.3 eV belongs to the aromatic carbon atoms bonded to hydrogen (Csp2), the peak at 285.9 eV to the aromatic carbons singly bonded to oxygen (Csp2-O), and the peak at 286.5 eV to 17 ACS Paragon Plus Environment


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the methyl carbon singly bonded to oxygen (O-CH3). On the other hand, the O 1s peak at 533.1 eV can be associated with the oxygen atoms in chemisorbed guaiacol on Pt(111) (i.e. Csp2-O). These assignments are summarized in Table 1. The other C 1s peaks at 285.15 and 283.9 eV and O 1s peaks at 532.4 and 530.6 eV must then be due to decomposition products.

Table 1. Experimental C 1s and O 1s binding energies for surface species along with their corresponding assignment based on DFT calculated binding energies. The cited references are for experimental XPS studies of systems with similar C 1s and O 1s binding energies to those described in detail here. For more details on the binding energy descriptions, please see text in Section 3.2. Core Level

Binding Energy (eV)

Description

Related Studies

C 1s

283.9

Csp3 (hydrocarbon fragments)

Methyl/Pt Surfaces10, 15; Phenol/Pt(111)57; Phenol/Rh(111)58

C 1s

284.3

Csp2 (aromatic carbon)

Benzene/Pt Surfaces18; Anisole/Pt(111)26; Phenol/Pt(111)57; Phenol/Rh(111)58

C 1s

285.15

Csp2=O and

Phenol/Pt(111)57

Csp3-O (ring carbon) Anisole/Pt(111)26; Phenol/Pt(111)57; Phenol/Rh(111)58 Methanol/Pd(111)59, 60; Methanol/Cu(110)61; Methanol/Ag(111)62

C 1s

285.9

Csp2-O (ring carbon)

C 1s

286.5

O-CH3

O 1s

530.6

Csp2=O

Phenol/Pt(111)57; Phenol/Rh(111)58

O 1s

532.4


Phenol/Pt(111)57; Methanol/Cu(110)61; Methanol/Ag(111)62; Methanol/Pt(110)63

O 1s

533.1


Phenol/Rh(111)58

3.2.2 Reactions of Guaiacol’s Functional Groups We have broken up the theoretical examination of guaiacol’s decomposition into a first and second stage. The first stage reactions involve only changes to the functional groups and the second stage involve ring breaking. As all the remaining unknown peaks observed in TPXPS 18 ACS Paragon Plus Environment

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occur already at low temperatures (T < 375 K), the assignment of these unknown peaks was done by comparison with the C 1s and O 1s CLBEs of the first stage guaiacol decomposition species. The favorability of a given elementary reaction was determined by calculating its reaction energy using Equation 1. Furthermore, we used the BEP relationships developed by Lee et al.21 to estimate the activation energy barrier for each first stage reaction (Equations S1-S4). Numerous elementary reactions were examined in this work and are shown in detail in the Supporting Information (Schemes S1-S2 and Figures S6-S10). The most likely mechanism for guaiacol’s first stage decomposition reaction is shown in Scheme 1. It is consistent with those proposed by Lee et al.21 and Lu et al.22 for the HDO of guaiacol on Pt(111) (see Figure S7 in the Supporting Information), and is similar to that found by Lu et al.23 on Ru(0001). Based on our detailed analysis, two initial reactions have similar activation energies, namely the dehydrogenation of either the hydroxyl or the methyl species (Scheme 1 structures H1 and M1, respectively). The subsequent reactions of the H1 species involve the further stepwise dehydrogenation of the methyl group (H2, H3), followed by the demethylation of the methoxy functional group (B). For the M1 species, the subsequent reactions involve demethylation of methoxy functional group (M2), followed by dehydrogenation of the hydroxyl group (B). As can be seen from Scheme 1, the decomposition reactions following the formation of species H2 and M2 have higher activation energies and are significantly more endothermic than the initial reactions. This suggests that the initial decomposition of guaiacol will occur rapidly to the H2 and M2 species, and that further decomposition will require higher temperatures. The overall energetic favorability for the removal of the methyl group shown in Scheme 1 qualitatively agrees with experimental results for the HDO of anisole on a Pt/Al2O3 catalyst64 and guaiacol on a Pt/C catalyst20, 65.



Scheme 1. Most energetically favorable reaction mechanisms for the first stage thermal decomposition of guaiacol horizontally adsorbed on a metal catalyst surface. Greater bonding between the guaiacol decomposition fragments and the catalyst surface, due to guaiacol’s decomposition and hybridization change, is denoted with an M. The species are labeled as: G for guaiacol; H1-H3 for the hydroxyl dehydrogenated mechanism species; M1-M2 for the methyl dehydrogenated mechanism species; and B for the 1,2-benzoquinone species. The values shown are the reaction energies and BEP estimated activation energies (in parenthesis).21

Comparing the theoretically calculated CLBEs for the C 1s and O 1s guaiacol fragments (Scheme 1) to the TPXPS results (Figure 3) allows us to determine the remaining unknown peaks in the experimental results for the guaiacol surface reaction as shown in Figure 5.


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Figure 5. The CLBEs for the first stage decomposition of guaiacol on Pt(111) according to Scheme 1. The calculated values (symbols) are labeled C1-C7 and O1, O2, as indicated for guaiacol (G). (a) C 1s CLBEs; (b) O 1s CLBEs. Experimentally observed binding energies are shown as horizontal lines; C 1s region: 284.3 eV (solid), 286.5 eV (dash dot), 285.9 eV (short



dash), 285.15 eV (long dash), and 283.9 eV (dot); O 1s region: 533.1 eV (solid), 532.4 eV (long dash), and 530.6 eV (dot).

First, for the 285.15 eV peak, Figure 5a shows that the C 1s atoms that most closely align with this peak (long dashed line) can be associated with both the C sp2=O and Csp3-O species. The formation of such functional groups occur when either the hydroxyl group is dehydrogenated or the methoxy group is demethylated (e.g. species H1 and M2), causing the ring to lose its aromaticity. For example, the M2 structure in Figure 5 sees corresponding CLBE shifts for C5 and C6 by ~-0.4 and -0.6 eV, respectively, compared to guaiacol (species G). Similar shifts are seen for the other guaiacol decomposition structures, e.g. H1, H2, B+CH2, and B+CH. Second, for the 283.9 eV peak (dotted line), Figure 5a shows that the C 1s species that most closely align with this peak can be associated with ring carbons that have lost their aromaticity or the surface methyl fragment CH (Csp3). For all decomposition species except M1, the C1 and C4 carbons, which are the non-oxygen bonded ring carbons’ adjacent to the oxygen functional groups, see a shift in their CLBEs to lower binding energies compared to the Csp2 species in guaiacol (Figure 5a). This change in the C1 and C4 carbons’ CLBEs is consistent with the change in hybridization at these carbons due to the changes in the oxygen functional groups. Additionally, the surface methyl fragment CH also has a CLBE value near the 283.9 eV peak, while the CH2 fragment has a CLBE value significantly lower at ~283.1 eV which is not observed experimentally. An examination of the literature shows that from 275 – 500 K the surface CH species is the dominant methyl derived species on Pt(111), which suggests that any CH2 fragments present on Pt(111) due to the demethylation of guaiacol quickly decompose to CH.10, 15, 66, 67 Overall, the 283.9 eV peak is likely associated with both non-aromatic ring carbons and possibly surface CH species in a hollow site (Csp3). 22 ACS Paragon Plus Environment

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For the O 1s peaks, the analysis in Figure 5b suggests that the peak at 532.4 eV (long dash) most closely aligns with the Csp3-O species. This peak assignment is true when the oxygen species is bonded to either ring carbons or to both ring and methyl carbons. For example, structure M1 shows that the methyl carbon bonds strongly to the Pt surface upon the loss of one of its hydrogen, and that the methoxy oxygen (O2) has a similar binding energy to that seen for the hydroxyl oxygen (O1) in structure M2, where the neighboring ring carbon (C6) binds strongly to a surface Pt atom (as seen in the rotation of the M2 ring and shortening of the C6-Pt bond by ~0.2 Å relative to structure M1) to compensate for the formation of the adjacent Csp2=O species. For the 530.6 eV peak (dotted line), Figure 5b shows that the O 1s species that most closely aligns with this peak can be associated with the Csp2=O species. As the hydroxyl group is dehydrogenated and the methoxy group is demethylated, there is a significant shift in the associated oxygen’s CLBEs to lower binding energies as compared to adsorbed guaiacol. Based on the comparisons to the calculated CLBEs for the possible guaiacol decomposition species on Pt(111), the experimental C 1s peaks were assigned to Csp2 (284.3 eV), Csp2-O (285.9 eV), O-CH3 (286.5 eV), Csp3-O (285.15 eV), Csp2=O (285.15 eV), and Csp3 (283.9 eV). Furthermore, the O 1s peaks were assigned to Csp2-O (533.1 eV), Csp3-O (532.4 eV), and Csp2=O (530.6 eV). These assignments are summarized in Table 1, and can be used to identify the likely temperature dependent guaiacol decomposition mechanism.

3.2.3 Complete Decomposition of Guaiacol on Pt(111) Three possible mechanisms for the complete decomposition of guaiacol on Pt(111) starting from the 1,2-benzoquinone intermediate B (in Scheme 1) are shown in Scheme 2, including the breaking of the carbon ring and desorption of all oxygen containing species. A 23 ACS Paragon Plus Environment


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crucial step of these second stage mechanisms is the breaking of the aromatic ring; its viability was tested by calculating the activation energy for the ring breaking reaction shown in Scheme 2 (reaction B to R2). This energy barrier was found to be 0.41 eV (Figure S12), which shows that ring breaking is feasible for guaiacol’s decomposition. Other possible ring breaking mechanisms were also examined (see Scheme S3 and Figure S11), but only the one shown in Scheme 2 was found to be energetically feasible. Notably, the breaking of the aromatic ring observed here is expected to occur below 450 K, which is consistent with results for anisole on Pt(111) that show ring opening occurring at 450 K with a calculated C-C cleavage barrier within the aromatic ring of ~1 eV.26 The three second-stage guaiacol decomposition mechanisms studied here are: partial hydrogenation followed by decarbonylation (HYD, labeled as RHx (x=1-3) in Scheme 2); esterification (EST, labeled as REx (x=1-3) in Scheme 2) followed by the formation and desorption of CO2; and direct decarbonylation (DDC, labeled as RDx (x=1,2) in Scheme 2). The likelihood of the HYD and EST mechanisms to occur depends on the presence of surface hydrogen species, which can only come from the first stage decomposition steps (since H2 is not co-fed during our experiment in contrast to the technically used HDO process). TPD shows that H2 desorption from Pt(111) occurs between 100 and 500 K, with major desorption peaks at ~230 and ~310 K68, 69 for H2 alone, and ~350 K for H2 produced from alcohol decomposition.24, 57, 70, 71 We estimated the first nearest neighbor lateral interactions for hydrogen and 1,2-benzoquinone (B) and found them to be attractive with a value of -0.01 eV (Figure S13 and Table S3). As the decomposition of each adsorbed guaiacol is predicted to produce three surface hydrogen atoms, complete H2 desorption is expected to not occur until 450-500 K;24,

57, 70, 71

also, as there are

attractive lateral interactions between hydrogen and the dominant guaiacol surface fragment (1,2-


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benzoquinone), the HYD and EST mechanisms are likely feasible as each only requires one surface hydrogen species.

Scheme 2. Most energetically favorable reaction mechanisms for the second stage thermal decomposition of 1,2-benzoquinone (B) intermediate. Enhanced bonding between the guaiacol decomposition fragments and the catalyst surface, due to guaiacol’s decomposition and hybridization change, is denoted with an M. The species are labeled as: R2 for the ring broken species; RD1-RD2 for the direct decarbonylation mechanism species; RE1-RE3 for the esterification mechanism species; RH1-RH3 for the hydrogenation-decarbonylation mechanism species. The values shown are the reaction energies. The activation energy for the ring-breaking step (B to R2) was calculated here and is given in parenthesis (see Figure S12 for the minimum energy pathway). No other activation energies are shown due to the lack of BEP relations for such reactions.

Based on the reaction energies shown in Scheme 2, the DDC (to RD2) and HYD (to RH3) mechanisms are both highly exothermic, while the EST (to RE3) mechanism is overall mildly endothermic with the formation of the ester group intermediate being the only exothermic reaction in this mechanism. From the experimental results (Figure 3), the second stage 25 ACS Paragon Plus Environment


decomposition above 380 K occurs rapidly with the disappearance of the O 1s peaks. This suggests that the deoxygenation of the guaiacol fragments must be highly exothermic upon the breaking of the aromatic ring, which occurs in the DDC and HYD mechanisms; because the EST mechanism is slightly endothermic, it appears unlikely to occur. Due to the similarity in energetic favorability between the DDC and HYD mechanisms and the rapid oxygen removal seen in TPXPS, the CLBEs for the final species in each of these two second stage mechanisms (RD2 and RH3) were compared to the TPXPS results in order to determine the ring breaking mechanism for guaiacol on Pt(111) and account for the disappearance of the O 1s signal observed experimentally above 430 K (Figure 3). The CLBEs for the final species in the DDC, HYD, and EST second stage guaiacol decomposition mechanisms (Scheme 2, structures RD2, RH3, and RE3), along with the initial, ring broken species (Scheme 2, structure R2), were compared to the TPXPS results, as shown in Figure 6. The same reference energies were used here as discussed above for the CLBE comparison of the first stage guaiacol decomposition species. Out of the final species, the CLBE of RH3, that is, the species resulting from the HYD mechanisms has the closest agreement with experiment. For the RD2 species (DDC mechanism), the Csp2 and Csp3 atoms shift by ~1 eV to lower CLBEs upon cleavage of some C-C bonds to form surface CO, predicting a peak at ~283 eV (see Figure 6 C1-C4 in species RD2), which is not present in the experiment. For the RE3 species (EST mechanism) a similar shift of C1-C4 species is predicted, again in contradiction to experiment. In addition, the very high CLBEs calculated for the adsorbed CO2 species (289 and 534 eV for C 1s and O 1s, respectively) render this mechanism quite unlikely. On the other hand, the CLBE of the RH3 (HYD) agrees well with the experimental results, showing that the removal of the CO from the guaiacol fragment does not result in a significant shift of the Csp2


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and Csp3 atoms to lower CLBEs (see Figure 6 C1-C4 in species RH3). This result agrees with the reaction energies predicted from theory which show that the HYD mechanism is the most exothermic of the three proposed second stage decomposition mechanisms.

Figure 6. The CLBEs for the final species in the DDC, HYD, and EST proposed second stage decomposition mechanisms for guaiacol on Pt(111) according to Scheme 2. The calculated values (symbols) are labeled C1-C7 and O1, O2, as indicated for the ring broken species (R2). (a) C 1s CLBEs; (b) O 1s CLBEs. Experimentally observed binding energies are shown as horizontal lines; C 1s region: 284.3 eV (solid), 286.5 eV (dash dot), 285.9 eV (short dash), 285.15 eV (long dash), and 283.9 eV (dot); O 1s region: 533.1 eV (solid), 532.4 eV (long dash), and 530.6 eV (dot).

It should be noted that in the above comparison, the C 1s and O 1s CLBEs associated with surface CO molecules predict peaks at ~286.5 and ~533.0 eV, which are not observed 27 ACS Paragon Plus Environment


experimentally at temperatures above 375 K (see Figure 6, C5, C6, O1, and O2 in species RD2 and RH3). This is likely due to the rapid desorption of CO upon formation. Indeed, when examining the TPD experiments for CO or guaiacol on Pt(111) one can see that CO desorbs at temperatures ranging from 325 to 475 K, depending on the surface coverage of CO.17, 24, 72-74 Furthermore, we estimate the first nearest neighbor lateral interactions for CO and 1,2benzoquinone to be repulsive with a value of 0.01 eV (Figure S13 and Table S3), so that the adsorbed CO is repelled by surface guaiacol fragments. Finally, the CO forming reactions are highly exothermic (Scheme 2). As such, the disappearance of the O 1s signal is likely caused by the formation and quick desorption of CO following the ring breaking reaction at temperatures above 375 K.

3.3 Analysis of TPXPS Intensities With the connection of the TPXPS peaks to specific surface species, the quantitative intensity information (Figure 3) can be used to follow the reaction progress as the temperature increases. Based on the assignments in Section 3.2 that are summarized in Table 1, the Csp2 (284.3 eV), Csp2-O (285.9 eV/533.1 eV), O-CH3 (286.5/533.1 eV), Csp3-O (285.15 eV/532.4 eV), and Csp2=O (285.15 eV/530.6 eV) functional groups are all present at 230 K. Starting from the O 1s data in Figure 3 at 230 K, the experimental intensity ratio of the contributions at 530.6, 532.4, and 533.1 eV is roughly 1:2:1. Since guaiacol contributes an O 1s intensity at 533.1 eV (Figure 5b, O1 and O2), the presence of the 530.6 and 532.4 eV peaks show that dissociation products must already be present on the surface at 230 K. While for almost all the intermediates in Figure 5, the calculation yields O 1s contributions at low binding energies. As such, only H1, H2, M1, and M2 are expected to have intensity around 532.4 eV, along with low enough barriers for these 28 ACS Paragon Plus Environment

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species to be formed at low temperatures. Since the peaks 530.6 and 532.4 eV have an intensity ratio of 1:2 at 230 K, H1 and H2 alone (which both would yield a 1:1 ratio) are not able to describe the situation. Considering the additional presence of an M1 species, a ratio of G:H1(H2):M1 = 1:2:1 would roughly describe the situation, yielding an O 1s intensity ratio (for the peaks at 530.6, 532.4, and 533.1 eV) of 2:3:3 (or 1:1.5:1.5 when reduced) in reasonable agreement with the experimental results. From Figure 3, the C 1s peak intensity ratios at 230 K are determined as 1:15:3:6:3 for peaks at 283.9, 284.3, 285.15, 285.9, and 286.5 eV. The relatively high intensity for the 286.5 eV peak suggests that both G and H1 must be present at 230 K as these are the two species with the O-CH3 functional group. Translating the 1:2:1 ratio for G:H1(H2):M1 to the C 1s data results in an intensity distribution of 2:14:5:4:3. While there is general agreement between the theoretically predicted and experimentally observed C 1s peak intensities, there seems to be a discrepancy for the relative intensities of the 285.15 and the 285.9 eV peaks. This discrepancy results from the uncertainty of assigning the H1 C5-C7 to one of two experimentally observed peaks (considering the error bars), but does not affect the overall assignment. To summarize, besides adsorbed guaiacol, both of the H1/H2 and M1 intermediates are present at 230 K. As the temperature is increased to 325 K, there is a decrease of the C 1s peaks at 284.3, 285.15, 285.9, and 286.5 eV and the O 1s peak at 533.1 eV (Figure 3), with the peaks at 286.5 and 533.1 eV completely disappearing. As the latter are associated with O-CH3 and Csp2-O species, respectively, their disappearance indicates that no intact guaiacol or H1 species remain on the surface at 325 K. Also, at 325 K, the O 1s peak at 532.4 eV begins to decrease, simultaneously with an increase of the 530.6 eV peak. This suggests that increasing the temperature above 325 K results in the conversion of the Csp3-O oxygen atoms to Csp2=O oxygen



atoms, which occurs upon dehydrogenation of the hydroxyl group and demethylation of the methoxy group (Scheme 1 H2, M2, and B species). Species H3 can be excluded, since one would expect an O 1s peak at ~533.1 eV from Figure 5, which has disappeared in the experiment by 325 K. With the intensities of the 532.4 and 530.6 eV peaks being roughly 3:2 and an increase of the signal at 530.6 eV, the surface species are likely some mixture of H2, M2, and B at 325 K. As the surface temperature is further increased to 350 - 375 K, the TPXPS shows a broad maximum of the C 1s and O 1s peaks at 285.15 and 530.6 eV, respectively. As both peaks are associated with the Csp2=O carbon and oxygen atoms, the maximum in these signals suggest that the final intermediate of the first stage mechanisms shown in Scheme 1, 1,2-benzoquinone (B), with two Csp2=O species per molecule, has its maximum coverage at 375 K. With the transition toward the B species, we expect an increase in the C 1s intensities at 285.15 and at 283.9 eV, as is indeed seen in Figure 3. This transformation towards B+CH continues until 350 - 375 K. Heating the surface further results in the breaking of the aromatic ring in 1,2benzoquinone (B) to form surface hydrocarbon fragments and CO. The surface carbon fragments lose their aromaticity, resulting in both the increase in the species associated with the 283.9 eV peak (Csp3 ) and decrease in the aromatic carbon species associated with the 284.3 eV peak (Csp2 ). Utilizing the quantitative intensity information from Figure 3, one would expect only C 1s signals in the range of 283 to 284 eV after CO desorption at 450 K. In fact, the remaining 530.6 eV intensity would coincide with the small C 1s signal at 285.9 eV. These contributions would be related to the R2 intermediate. Once oxygen is nearly gone at 500 K, the intensities of 284.3 and 283.9 eV are about equal. This is in agreement with the predicted RH3 product. Overall, based on the combined theoretical and experimental analysis of the TPXPS data for guaiacol on Pt(111), we propose the most likely thermal decomposition mechanism for 30 ACS Paragon Plus Environment

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guaiacol on Pt(111), as shown in Scheme 3. This study demonstrates the power of combining detailed theoretical and experimental surface science techniques for the elucidation of elementary reaction mechanisms on metal surfaces, allowing for the more accurate determination of catalytic active sites and, therefore, the optimization of heterogeneous catalysts.

Scheme 3. Proposed mechanism for the thermal decomposition of guaiacol on Pt(111). Greater bonding between the guaiacol decomposition fragments and the catalyst surface, due to guaiacol’s decomposition and hybridization change, is denoted with an M. The species have identical labels to those presented in Schemes 1 and 2.

5. Conclusions The adsorption and thermal decomposition of guaiacol on Pt(111) was studied using experiment, TPXPS, and theory, DFT-based theoretical models. The TPXPS peak fitting showed that the C 1s spectra resulted in five peaks at 283.9, 284.3, 285.15, 285.9, and 286.5 eV while the O 1s spectra had three peaks at 533.1, 532.4, and 530.6 eV. A thorough comparison of the TPXPS and DFT-based CLBEs allowed us to assign the C 1s and O 1s peaks to Csp2 (284.3 eV), Csp2-O (285.9 eV and 533.1 eV), O-CH3 (286.5 eV), Csp3-O (285.15 eV and 532.4 eV), Csp2=O (285.15 eV and 530.6 eV), and Csp3 (283.9 eV). As the temperature was increased after guaiacol adsorption on Pt(111), changes in both the C 1s and O 1s spectra indicated decomposition of the adsorbed species. Using the most stable



guaiacol adsorption site, with the plane parallel to the surface, we studied several possible thermal decomposition routes and compared the resulting CLBEs to those measured using TPXPS in order to identify the thermal decomposition mechanism for guaiacol. Based on this comparison, we found that the adsorption of a saturated layer of guaiacol on Pt(111) at 230 K does not occur without the dissociation of some of the surface species through the dehydrogenation of the hydroxyl and methoxy functional groups. Increasing the surface temperature to 325 K results in the decomposition of all surface guaiacol via the demethylation of the methoxy functional group. Additional heating to 375 K resulted in a maximum surface coverage of the 1,2-benzoquinone intermediate before ring breaking occurred, leading to the formation of surface carbon fragments and the fast desorption of CO until 450 K. Overall, these results show that the surface species present during reaction can be effectively quantified and identified for complex systems, such as the decomposition of phenolics, using a knowledgeable application of TPXPS and DFT. This type of collaborative approach has immense potential in the identification of reaction pathways and design variables for heterogeneous catalytic processes.

Supporting Information The supporting information contains: measured carbon coverage with temperature, guaiacol adsorption sites with theoretically calculated energetic and geometric parameters, all studied mechanisms for guaiacol decomposition, a study of the effect of the guaiacol configuration on the reaction energy, the mechanisms studied using vertically adsorbed guaiacol, theoretically calculated energetic and geometric parameters for guaiacol decomposition products,


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minimum energy pathway for ring breaking in the 1,2-benzoquinone adsorbate, and the theoretically calculated CLBEs.

Acknowledgements The work at Washington State University was primarily funded by the U. S. Department of Energy (DOE), Office of Basic Energy Sciences, Division of Chemical Sciences, Biosciences and Geosciences under Award Number DE-SC0014560. Additional institutional funds were provided to J.S.M. from the Voiland School of Chemical Engineering and Bioengineering. C.P., C.G., K.G., and H.P.S. thank the German Science Foundation DFG for support within its Excellence Cluster for “Engineering of Advanced Materials”. Support by the staff of BESSY II and by Michael Weiß, as well as financial support by HZB, is gratefully acknowledged by C.W and R.D. The Pacific Northwest National Laboratory is operated by Battelle for the U. S. DOE.

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TOC Graphic 82x39mm (300 x 300 DPI)

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Figure 1. Guaiacol saturation spectra at 230 K on Pt(111) for the O 1s (bottom) and C 1s (top) core levels with results of a peak fitting procedure (see text for details). The raw data is shown in filled black circles, while the fit results are shown as smooth lines. The inset in the top graph shows a comparison of C 1s saturation spectra of guaiacol at 230 K (filled black squares) and of benzene at 200 K (filled grey circles) on Pt(111), normalized to the total spectral intensity (i.e. the C coverage). The molecular coverages are 0.18 ML for benzene and 0.17 ML for guaiacol. Mind the difference in carbon atom count per molecule (6 in benzene, 7 in guaiacol). 84x101mm (150 x 150 DPI)


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Figure 2. Waterfall presentations (top) and color-coded intensity plots (bottom) of the TPXPS experiments after guaiacol adsorption on Pt(111) at 230 K for the (left) C 1s spectra and (right) O 1s spectra. 172x114mm (150 x 150 DPI)



Figure 3. Quantitative fit results of the TPXPS of guaiacol on Pt(111) as shown in Figure 2. The C 1s fit is shown on top and the O 1s fit is shown on the bottom. 99x119mm (150 x 150 DPI)


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Figure 4. Comparison of the experimental C 1s and O 1s spectra for guaiacol on Pt(111) at 230 K (left) to the theoretically calculated CLBEs for guaiacol adsorbed on Pt(111) in the B30o-BB site (right). The calculated values (symbols) are labeled C1-C7 and O1, O2, as indicated for guaiacol. (top) C 1s CLBEs; (bottom) O 1s CLBEs. Experimentally observed binding energies from the peak fitting shown in Figure 1 are shown as horizontal lines; C 1s region: 284.3 eV (solid), 286.5 eV (dash dot), 285.9 eV (short dash), 285.15 eV (long dash), and 283.9 eV (dot); O 1s region: 533.1 eV (solid), 532.4 eV (long dash), and 530.6 eV (dot). 203x239mm (150 x 150 DPI)



Scheme 1. Most energetically favorable reaction mechanisms for the first stage thermal decomposition of guaiacol horizontally adsorbed on a metal catalyst surface. Greater bonding between the guaiacol decomposition fragments and the catalyst surface, due to guaiacol’s decomposition and hybridization change, is denoted with an M. The species are labeled as: G for guaiacol; H1-H3 for the hydroxyl dehydrogenated mechanism species; M1-M2 for the methyl dehydrogenated mechanism species; and B for the 1,2-benzoquinone species. The values shown are the reaction energies and BEP estimated activation energies (in parenthesis).21 294x112mm (150 x 150 DPI)


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Figure 5. The CLBEs for the first stage decomposition of guaiacol on Pt(111) according to Scheme 1. The calculated values (symbols) are labeled C1-C7 and O1, O2, as indicated for guaiacol (G). (a) C 1s CLBEs; (b) O 1s CLBEs. Experimentally observed binding energies are shown as horizontal lines; C 1s region: 284.3 eV (solid), 286.5 eV (dash dot), 285.9 eV (short dash), 285.15 eV (long dash), and 283.9 eV (dot); O 1s region: 533.1 eV (solid), 532.4 eV (long dash), and 530.6 eV (dot). 358x441mm (150 x 150 DPI)



Scheme 2. Most energetically favorable reaction mechanisms for the second stage thermal decomposition of 1,2-benzoquinone (B) intermediate. Enhanced bonding between the guaiacol decomposition fragments and the catalyst surface, due to guaiacol’s decomposition and hybridization change, is denoted with an M. The species are labeled as: R2 for the ring broken species; RD1-RD2 for the direct decarbonylation mechanism species; RE1-RE3 for the esterification mechanism species;RH1-RH3 for the hydrogenation-decarbonylation mechanism species. The values shown are the reaction energies. The activation energy for the ring-breaking step (B to R2) was calculated here and is given in parenthesis (see Figure S12 for the minimum energy pathway). No other activation energies are shown due to the lack of BEP relations for such reactions. 339x182mm (150 x 150 DPI)


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Figure 6. The CLBEs for the final species in the DDC, HYD, and EST proposed second stage decomposition mechanisms for guaiacol on Pt(111) according to Scheme 2. The calculated values (symbols) are labeled C1C7 and O1, O2, as indicated for the ring broken species (R2). (a) C 1s CLBEs; (b) O 1s CLBEs. Experimentally observed binding energies are shown as horizontal lines; C 1s region: 284.3 eV (solid), 286.5 eV (dash dot), 285.9 eV (short dash), 285.15 eV (long dash), and 283.9 eV (dot); O 1s region: 533.1 eV (solid), 532.4 eV (long dash), and 530.6 eV (dot). 263x297mm (150 x 150 DPI)



Scheme 3. Proposed mechanism for the thermal decomposition of guaiacol on Pt(111). Greater bonding between the guaiacol decomposition fragments and the catalyst surface, due to guaiacol’s decomposition and hybridization change, is denoted with an M. The species have identical labels to those presented in Schemes 1 and 2. 578x140mm (150 x 150 DPI)


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Identifying the Thermal Decomposition Mechanism of Guaiacol on Pt

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