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Dehydration Pathway to the Dissociation of Gas-Phase Formic Acid on Pt(111) Surface Observed via Ambient-Pressure XPS Beomgyun Jeong, Hongrae Jeon, Ryo Toyoshima, Ethan J. Crumlin, Hiroshi Kondoh, Bongjin Simon Mun, and Jaeyoung Lee J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07735 • Publication Date (Web): 28 Dec 2017 Downloaded from http://pubs.acs.org on December 30, 2017

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The Journal of Physical Chemistry

Dehydration Pathway for the Dissociation of Gas-Phase Formic Acid on Pt(111) Surface Observed via Ambient-Pressure XPS Beomgyun Jeong,1-3 Hongrae Jeon,2,4† Ryo Toyoshima,5 Ethan J. Crumlin,6 Hiroshi Kondoh,5 Bongjin Simon Mun,2,3* and Jaeyoung Lee2,4* 1

Advanced Nano-Surface Research Group, Korea Basic Science Institute, Daejeon 34133, Korea.

2

Ertl Center for Electrochemistry and Catalysis, Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, Korea.

3

Department of Physics and Photon Science, GIST, Gwangju 500-712, Korea.

4

Electrochemical Reaction and Technology Laboratory, School of Earth Science and Environmental Engineering, GIST, Gwangju 500-712, Korea.

5

Department of Chemistry, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan.

6

Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.

†

Current affiliation: Non-Proliferation System Research Division, Korea Atomic Energy Research Institute, Daejeon 34057, Korea.

*[email protected], [email protected]

Abstract While model studies of surface science under ultrahigh vacuum (UHV) have made significant contributions to understanding electrochemistry, many issues related to electrochemical phenomena still remain unanswered due to the extreme environmental differences between UHV and actual electrochemical conditions. Electrochemical formic acid

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(HCOOH) oxidation is one such example. While the dehydration step in the indirect oxidation pathway (HCOOH → H2O + COad → 2H+ + 2e- + CO2) is observed in the electrochemical oxidation of formic acid on Pt(111) surface, the surface science studies conducted in UHV condition reported the complete HCOOH dissociation to H2 and CO2 on Pt(111) surface with no adsorbed CO at room temperature. A dehydration mechanism may also exist in gas-phase HCOOH dissociation in some conditions different from UHV, but it has not been demonstrated with a surface science method due to pressure limitations. Using ambient pressure X-ray photoelectron spectroscopy (AP-XPS), we observed the dehydration mechanism of gas-phase HCOOH in unprecedented high pressure environment for the first time. This study is a demonstration of reconciling the disagreement between electrocatalysis and surface science by bridging the environment gap.

1. Introduction Formic acid (HCOOH) is an electro-oxidizable C1 molecule. Since HCOOH has a simple molecular structure and oxidation mechanism (HCOOH → 2H+ + 2e- + CO2),1 it shows a faster oxidation rate than other molecules and it is the starting point for understanding electro-oxidation of other organic molecules bearing more carbon atoms. In the studies to harness electrical energy from HCOOH oxidation with a fuel cell, the electrocatalyst is an important component for power performance. Platinum (Pt) exhibits the best performance as a catalyst for HCOOH oxidation.2 However, carbon monoxide (CO), an intermediate species in the indirect pathway (HCOOH → COad + H2O → 2H+ + 2e- + CO2), adsorbs strongly on active sites of Pt surface and decreases the reaction rate (CO poisoning).3–5 Over the years, CO poisoning of polycrystalline Pt surface in HCOOH electro-oxidation has been well identified with in situ IR spectroscopy.6–8 Although CO poisoning of Pt(111) is relatively slow compared to the other crystal facets, dipping the surface in 0.1 M HCOOH aqueous (aq) solution for two minutes is enough to complete the surface poisoning.9 In the indirect pathway of the HCOOH oxidation, there is the non-electrochemical reaction that is called dehydration (HCOOH → COad + H2O). Since this reaction step does not require an electrochemical activation energy, it might be possible to study on the CO generation from the dehydration step in heterogeneous catalysis of gas-phase HCOOH dissociation (HCOOH → H2 + CO2) instead of electrochemical HCOOH oxidation (HCOOH → 2H+ + 2e- + CO2). This idea was attempted in several studies by measuring the amount of


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COad on the Pt(111) surface that was exposed to HCOOH molecules using a temperature programmed desorption method.10–12 Interestingly, they reported that no evidence of dehydration (HCOOH → H2O + COad) was found and instead only dehydrogenation (HCOOH → H2 + CO2) took place on Pt(111) surface.10–12 These experimental studies are consistent with the theoretical investigation reporting that CO forms more easily at stepped or defect sites of Pt surface in contrast to the ideal Pt(111) terrace in HCOOH electro-oxidation.13 This discrepancy in regard to the CO formation during the HCOOH oxidation between surface science and electrochemistry is presumably due to the extreme difference in experimental environment, i.e., UHV vs liquid phase. It is suspected that there is a window of pressure regime where COad is formed on the Pt(111) single-crystal surface during gas-phase HCOOH dissociation, but surface science studies to explore such a higher pressure condition have not been carried out due to the requirement of UHV condition for most surface analyses. To overcome this so-called pressure gap and explore the nature of HCOOH oxidation on Pt surface, ambient pressure XPS (AP-XPS) has been used. X-ray photoelectron spectroscopy (XPS) is a surface analysis tool for studying chemical adsorbates on a conducting surface, and it has been normally operated under UHV condition. Lately, with the combination of differential pumped electron transport system and high intensity soft X-rays produced from a synchrotron light source, in situ XPS operation under elevated pressure conditions has becomes possible.14–16 In this study, we observed CO adsorption on Pt(111) surface when exposed to gaseous HCOOH at the near ambient pressure conditions. Our finding is consistent with the electrochemistry observation, yet it has never been convincingly reported so far using UHV analyses. This study is a demonstration of AP-XPS reconciling the disparity in results obtained in the fields of electrocatalysis and surface science.

2. Experimental details The AP-XPS experiments for the Pt(111) sample were performed at the BL-13A of Photon Factory in Tsukuba, Japan. The base pressure of the preparation chamber was ~1.5×10-8 Torr and that of the main chamber was from 4.5×10-9 to 2×10-8 Torr when XPS was conducted in UHV conditions. To prepare a clean and ordered Pt(111) single-crystal surface, repeated cycles of Ar+ sputtering, heating on O2 at 10-6 Torr pressure, and annealing in UHV were performed. The cleanness and ordering of the surface was confirmed with survey XPS and LEED. Narrow region XPS were collected in Pt 4f, C 1s, and O 1s regions for investigating



the surface reaction of HCOOH. We used different photon energies for different regions (180 eV for Pt 4f, 390 eV for C 1s, and 650 eV for O 1s) for the same kinetic energy of photoelectrons (~100 eV) and obtained the same probing depth as a result. When we changed the photon energy for collecting XPS of a different region, valence band (VB) was also collected at the same time for using the Fermi level as the binding energy (BE) reference. A metal leak valve was used for dosing HCOOH (98.0 % JIS special grade of Wako pure chemical industries Ltd) at RT. AP-XPS for polycrystalline Pd was carried out at BL 9.3.2 of the Advanced Light Source in the Lawrence Berkeley National Laboratory. The Pd surface was cleaned by repeated cycles of Ar sputtering and annealing in the UHV procedure. The photon energies were the same as the energies of the experiment conducted in the Photon Factory. We calibrated the BE scale of each spectrum with the Fermi level of the VB spectrum that was collected together with the XPS data. The Doniac-Sunjic profile was used for fitting the Pt 4f and C 1s core level spectra, and the Voigt profile was used for the O 1s spectra. We first fitted the XPS spectra collected in 10 mTorr HCOOH condition without constraints because it was easier to fit the spectra with larger COad components. Then, we utilized the previously determined parameters as the initial guess for fitting the spectra of the 100 L HCOOH condition. We constrained the area ratio of COad components for the consistency in the CO coverage for different spectral regions. For consistent comparison on the change of the Pt 4f spectrum, the asymmetry factor and BE positions of some components were set to be the same for different conditions.

3. Results and discussion Following the cleaning process, we confirmed that a clean and well-ordered Pt(111) surface was obtained using the LEED pattern. No trace signal was observed in the C 1s and O 1s regions of the survey spectrum collected in UHV conditions (Fig. S1a). The Pt 4f spectrum shows two peak components located at the binding energies of 70.5 eV (PtS, Pt surface state) and 71.0 eV (PtB, Pt bulk state) (Fig. 1a). The PtS component provides another indication of a clean and well-ordered surface. Despite the 10 L (Langmuir = 10-6 Torr·s) HCOOH dose, no signal was detected over the noise of the C 1s and O 1s regions in the survey spectrum (Fig. S1b). This observation is consistent with the previous studies on temperature-programmed desorption (TPD) of HCOOH which is pre-adsorbed onto Pt(111) surface with exposures lower than 1.2 L at a low temperature between 80 K and 100 K.8,12 It was reported that


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HCOOH is only dissociated to H2 and CO2 via the dehydrogenation pathway (HCOOH → H2 + CO2) on Pt(111).10–12 CO2 is completely desorbed from the surface while H2 is at the desorption onset at RT.12 Some H atoms may still have remained on the Pt(111) surface but H is not detected with XPS due to its very low photoelectron cross-section. Therefore, no adsorbate detectable with XPS is left on the surface. However, when the Pt(111) surface was exposed to 100 L HCOOH, we could observe clear signals in the C 1s and O 1s regions of the survey spectrum (Fig. S1c) indicating that some surface adsorbates were formed. We collected the narrow region spectra (Pt 4f, C 1s, and O 1s) to investigate the surface adsorbates (Figs. 1a-c). In the C 1s region (Fig. 1b(ii)), two peaks at the binding energies of 286.0 eV and 286.7 eV newly appeared. This peak profile coincides with the typical spectral feature of adsorbed CO onto Pt(111) surface that is well-known from previous detailed studies.17–21 According to these studies, the 286.0 eV peak can be attributed to the COad at the bridge site between two Pt atoms (bridge, COB) and the 286.7 eV peak is attributed to the COad directly on top of each Pt atom (on-top, COT) (Fig. 1b).17,19 The peak at 284.0 eV is attributed to the pre-existing carbon impurities that remained even after cleaning the surface as shown in Fig. 1b(i). However, this residual carbon did not show any change during the process of CO adsorption from HCOOH dehydration. Therefore, this small amount of carbon appears to be uninvolved in the reactions. Not only in the C 1s but also in the O 1s region, two peaks located at 531.0 eV (COB) and 532.8 eV (COT) were observed (Fig. 1c), which is also consistent with the previous studies on CO adsorption on Pt(111).17,19 In the dehydration of HCOOH, H2O is produced together with CO; therefore, adsorbed water is a possible source that can explain the higher binding energy component. However, since H2O is desorbed at temperatures higher than 150 K,22,23 a peak component attributable to water cannot be detected in the O 1s spectrum at RT (300 K). In the Pt 4f region, the intensity of the surface state (PtS) decreased and instead, new peaks corresponding to COad appeared at 71.5 eV (COB) and 72.1 eV (COT) (Fig. 1a(ii)). The attribution of the deconvoluted components is consistent with the previous study on the CO/Pt(111) overlayer.19



Fig. 1 (a) Pt 4f, (b) C 1s and (c) O 1s XP spectra collected at room temperature for (i) as cleaned, (ii) 100 L HCOOH dosed, and (iii) under 10 mTorr HCOOH environment. After the ex situ XPS measurement for the Pt(111) surface that was exposed to HCOOH by 100 L, we cleaned out the Pt(111) surface again and conducted in situ AP-XPS under the HCOOH pressure of 10 mTorr. For the in situ AP-XPS, the chamber is only pumped through the analyzer aperture while the HCOOH molecules are continuously supplied into the main chamber through a leak valve. We can adjust the flow rate of HCOOH vapor using the leak valve to create a dynamic equilibrium at the pressure of 10 mTorr. In this condition, the exposure is no longer finite but rather is continuous, and the HCOOH molecules are constantly impinging onto the surface in a higher pressure environment. The COad components (on-top and bridge) are enhanced in intensity in all regions whereas the PtS component in Pt 4f is almost suppressed due to the increased CO coverage (Fig. 1(iii)). The Pt 4f spectrum collected under 10 mTorr HCOOH pressure in this study is similar to the one collected under 50 mTorr CO pressure in the previous study of Toyoshima et al. In their study, PtS completely disappears under the CO pressure of 50 mTorr (√19-13CO) while the peak area of PtS is almost the same to the one of the COad components at 20 L CO exposure (c(4×2)-2CO). In our results for the 10 mTorr HCOOH pressure, PtS shows a slight intensity which is significantly smaller than the intensity of the COad components (Fig. 1a(iii)). Therefore, we speculate that CO is continuously generated at the surface with the coverage


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close to √19-13CO. Under the HCOOH pressure of 10 mTorr, the peak area ratio of COB and COT components (COB/COT) measured in the C 1s spectrum becomes larger than one (~1.5), different from the ratio measured for Pt 4f (~1.2). Kinne et al. previously observed the ratio discrepancy for c(4×2)-2CO/Pt(111).17 In that overlayer structure, the peak area of COB is larger than that for COT in the C 1s spectrum, while the actual CO coverage of the bridge sites is equal to that of the on-top sites.17 They explained the mismatch between the spectral area ratio and the actual coverage of the COad components with the photoelectron diffraction effect. Bondino et al. investigated this effect for the same overlayer structure in detail with XPS measurements and theoretical modeling.24 They showed that the intensity modulation of the C 1s components by surface orientation depends on the adsorption site.24 Although Bondino et al. did not show an intensity modulation of COad components in O 1s region, we expect that there should be some sensitivity difference between the COB and COT components in the O 1s region as well as in the C 1s region. In fact, in our O 1s region spectrum, the COT component is larger than the COB component which appears inconsistent with the C 1s and Pt 4f spectra. In addition, in the previous study of Toyoshima et al for CO/Pt(111), the O 1s XPS showed that the COT component had a larger peak area than the COB component while the C 1s XPS displays the opposite.19 This behavior is consistent with our result collected under the HCOOH pressure of 10 mTorr. Therefore, the detection sensitivity of COB appears to be lower than that of COT in the O 1s region while the opposite is true in the C 1s region. We observed the CO adsorption on the Pt(111) surface at a high pressure of HCOOH, and the CO molecules appear to be generated via the dehydration of HCOOH (HCOOH → CO + H2O) on the Pt(111) surface. However, the CO molecules can also be produced by the background CO which can be produced by the photodissociation of HCOOH by soft X-rays,25 or catalytic dissociation of formic acid at the inside surface of the stainless steel chamber.8,10 Therefore, we investigated the influence of these artificial effects on the generation of the background. Based on the order of magnitude calculation, the estimated rate of photodissociation is negligible and cannot affect the XPS results on the experimental timescale (The details of the calculation are explained in the Supporting Information). To investigate the effect of the background CO from the inner surface of the chamber, we conducted a HCOOH dosing experiment for the polycrystalline Pd surface. It is well-known that CO molecules are very well adsorbed onto the Pd surface similar to Pt,26 but the CO-poisoning rate of Pd surface in the electro-oxidation of HCOOH is extremely slow



compared to Pt.27–29 While the immersion of the Pt(111) surface in 0.1 M HCOOH (aq) solution for two minutes is sufficient for full poisoning,9 a notable poisoning of Pd surface requires the dipping of the surface in 10 M HCOOH (aq) solution for hours.30

Fig. 2. C 1s XP spectra of polycrystalline Pd surface exposed to (a) carbon monoxide (CO) and (b) formic acid (HCOOH). Using a Pd(poly) surface that shows a very slow CO-poisoning in HCOOH oxidation compared to the Pt(111) surface, we can determine the effect of the background CO on the surface adsorbates from the comparison of the C 1s spectrum collected for the CO and HCOOH environments. If background CO has a significant influence on the surface species, we would observe a similar peak profile in the C 1s region whether Pd surface is in the HCOOH or CO environment considering the susceptibility of the Pd surface to CO adsorption from a direct CO exposure. The C 1s spectra of a Pd surface that were collected under HCOOH and CO environments are shown in Figs. 2a and 2b, respectively. In the C 1s spectra of the CO-exposed Pd surface (Fig. 2a), a component at 286.0 eV (COad) is developed with the 284.2 eV component (C=C). In contrast, the C 1s spectra of the HCOOH-exposed Pd surface show very different profiles (Fig. 2b). In these spectra, the 289 eV (COOH) and 284.6 eV (C-C or C-H) components, which could be formed from HCOOH, dominate the 286.0 eV


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component (COad). This result suggests that the dosed HCOOH vapor governs the adsorbed chemical species on surfaces rather than background CO. The observation of COad on Pt(111) at a higher HCOOH exposure but not at a lower HCOOH exposure could be understood in several ways. One possible reason is the small number of the step sites, which practically exist on the Pt(111) surface. Considering the report stating that the CO poisoning rate of the Pt surface in electrochemical HCOOH oxidation increases with step density,31 these step sites may also be active toward the dehydration pathway in gas-phase HCOOH oxidation. At a higher pressure, the rate of HCOOH dehydration may be high enough to produce a significant number of CO molecules and they could diffuse to the (111) terrace sites in the experimental time similar to electrochemical oxidation.31 Another possible reason is the adsorbate-induced surface restructuring at a high gas pressure. This phenomenon was observed in an ambient pressure STM study,32 and surface restructuring could alter the surface properties to make the surface prone to CO adsorption. The most likely adsorbates that may induce such a surface restructuring are hydrogen atoms. These can be produced by the dehydrogenation of the HCOOH molecules and the dissociative adsorption of hydrogen molecules at room temperature. The last possible reason we could think of is the HCOOH dimer that could exist at the higher density of HCOOH vapor. According to a DFT simulation of CO formation on Pt(111) surface, the presence of HCOOH dimer accounts for the facile CO poisoning on the Pt(111) surface.33 For a higher HCOOH pressure, there would be greater numbers of the HCOOH dimers and accordingly the probability of CO formation on Pt(111) could be higher. To elucidate which hypothesis is correct, a further study by using some different analytical methods is required. We may need an ambient pressure STM for the investigation of the restructuring under an elevated pressure and some IR spectroscopic method could provide a way to measure the amount of the HCOOH dimer in gas-phase HCOOH to evaluate whether the HCOOH dimer is the origin of the dehydration mechanism. We investigated the temperature dependence of the C 1s, O 1s, and Pt 4f spectra of the Pt(111) surface at 100 L HCOOH exposure by controlling the surface temperature to RT, 100 °C, 200 °C, and 300 °C. At 100 °C, the PtS component is enhanced (Fig. 3a) while the intensities of the CO peaks diminish (Figs. 3b and 3c). At the temperatures over 100 °C, a broad sub-peak appears at approximately 71.5 eV in the Pt 4f region, suggesting the existence of surface contamination. Although the sub-peak located at 71.5 eV is usually identified as due to O/Pt (Fig. 3a),34 it develops with the inorganic carbon component (284 eV in C 1s



region, Fig. 3b), and there is no corresponding peak in the O 1s region developing with increasing temperature (Fig. 3c). Thus, we assigned this sub-peak to the carbon-adsorbed Pt (C-Pt in Fig. 3). As the surface temperature rises from the RT to 300 °C, it is clear from all core level spectra that the CO peaks completely disappear, which is most likely due to CO desorption or CO2 formation (CO + CO → C + CO2). The reaction of the CO molecules is supported by the development of an inorganic carbon peak at 284 eV (Fig. 3b). The inorganic carbon keeps growing even after the CO peaks completely disappear. It seems that some residual HCOOH molecules produced carbon on the heated Pt(111) surface.

Fig. 3. (a) Pt 4f, (b) C 1s and (c) O 1s XP spectra at (i) room temperature, (ii) 100 °C, (iii) 200 °C, and (iv) 300 °C after 100 L HCOOH dosed into the XPS main chamber.

4. Conclusion Although CO poisoning on the Pt(111) surface due to the indirect electro-oxidation of formic acid (HCOOH → H2O + COad → CO2 + 2H+ + 2e-) is generally accepted, CO formation has not been observed in the oxidation of gaseous HCOOH. This inconsistency is one of the examples showing that the surface characterization of electrocatalysts in UHV ACS Paragon Plus Environment

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conditions is incompatible with electrochemical analysis. However, using AP-XPS, we observed CO adsorption on the Pt(111) surface at the near ambient pressure conditions and demonstrated that it is due to the dehydration of HCOOH, which is the same as the reaction mechanism for the CO poisoning of Pt(111) surface in electro-oxidation of HCOOH. This study could be considered as an example of the validity for the use of AP-XPS for understanding the electrocatalysis phenomena by bridging the environment gap between electrocatalysis and surface science.

Supporting Information The Supporting Information includes the survey spectra of cleaned, 10 L HCOOH-exposed, and 100 L HCOOH-exposed Pt(111) surface; and the order of magnitude estimation of HCOOH photodissociation rate by soft X-rays to see whether photodissociation can explain the CO formation on Pt(111).

Acknowledgement The authors are grateful to Dr. Toru Shimada for his valuable help during the course of the experiment at Photon Factory, KEK. This work was supported by the GIST Research Institute (GRI) in 2017. B.

S. Mun

would like to acknowledge

the supports from SRC (C-AXS,

NRF-2015R1A5A1009962) and Korea Basic Science Institute Grant (E36800). This research used resources of the Advanced Light Source, which is a DOE Office of Science User Facility under contract no. DE-AC02-05CH11231.

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