Chemical Science Research Frontiers 2015 92 Reaction of CO 2 on stepped Cu(997) surface revealed by ambient-pressure X-ray photoelectron spectroscopy Today, society presents a strong demand towards developing novel catalysts that will solve energy and environmental problems. In catalysis research, the importance of Operando spectroscopy has recently begun to be realized. Gaps in pressure and temperature exist between ideal surface science studies (ultrahigh vacuum (UHV) and low/moderate temperature) and real catalytic conditions (above atmospheric pressure and high temperature). Such differences in pressure and temperature may result in distinct reactivity under ambient conditions compared with the case of UHV conditions, reflecting thermodynamic and kinetic effects. Ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) [1] is a powerful Operando spectroscopy that allows us to investigate electronic and chemical states of the adsorbate and substrate under gas atmosphere. The activation of CO 2 is an important topic in the efficient use of CO 2 as a chemical feedstock. Methanol synthesis from CO 2 and H 2 on Cu/ZnO catalysts has been widely studied and is already developed industrially. CO 2 is chemically inert, and thus the interaction of the molecule with metallic Cu surfaces plays an essential role in molecular activation. The previous studies in UHV revealed that stepped Cu surfaces are active in the dissociation of CO 2 [2,3]. However, the reaction mechanism of CO 2 at defect sites such as steps and kinks on the Cu surface, which are abundant on real catalysts, is not yet fully understood. In this study [4], therefore, we investigated the reaction process of CO 2 on the stepped Cu(997) surface at a CO 2 pressure of 0.8 mbar and a temperature of 340 K by AP-XPS. We developed a new AP-XPS system (Fig. 1) at the soft X-ray undulator beamline BL07LSU [5]. AP-XPS measurements were carried out using a differentially pumped electron analyzer (SPECS, PHOIBOS 150 NAP) with an ambient-pressure gas cell. A one-dimensional delay-line detector (DLD) is adopted as the detector in the electron analyzer. The ambient-pressure gas cell is equipped with an Si 3 N 4 window for X-ray transmission and a small aperture (300 μ m in diameter), which is the entrance to the differentially pumped electron analyzer. The whole gas cell is mounted on a manipulator of the load-lock chamber, and is docked to the front of the electron analyzer. In the ambient-pressure gas cell, a sample is placed close (~300 μ m) to the entrance aperture in order to minimize the scattering of photoelectrons by gas-phase molecules. The high performance of our AP-XPS instrument was confirmed by successful measurements of the Au 4 f core level of polycrystalline Au foil in the presence of 20 mbar N 2 gas. Figures 2(a) and 2(b) show a series of O 1 s and C 1 s AP-XPS spectra of Cu(997) at 340 K under a CO 2 pressure of 0.8 mbar as a function of elapsed time. Gas-phase CO 2 peaks in O 1 s and C 1 s spectra were observed at 536.6–536.4 eV and at 292.8–292.6 eV, respectively. A peak of the adsorbate was observed at 531.3 eV in the O 1 s XPS ( t = 376 s), whereas three peaks were observed in the C 1 s XPS ( t = 507 s): at 288.4 eV with a broader shoulder peak at higher binding energy (289.0 eV), and at 284.4 eV. The intensities of XPS peaks at 531.3, 289.0, and 288.4 eV were saturated at t ~ 2000 s, and then a new peak at 529.5 eV appeared in O 1 s XPS spectra. Temporal evolutions of each O 1 s and C 1 s XPS peak are shown in Fig. 3. The XPS peaks at 531.3, 289.0, and 288.4 eV are assigned to carbonate (CO 3 ) species, on the basis of the binding energies and the atomic O/C ratio of 3.1 ± 0.1 estimated from the area intensities of O 1 s and C 1 s XPS peaks. Two different C 1 s peak positions of CO 3 may originate from different adsorption sites of CO 3 on Cu(997), such as steps. The O 1 s XPS peak at 529.5 eV, which appeared after the saturation of CO 3 , is attributed to atomic oxygen. The C 1 s XPS peak at 284.4 eV is assigned to neutral carbon species (C 0 ) such as graphitic carbons and hydrocarbons. Next we discuss the reaction mechanism of CO 2 on the Cu(997) surface under near-ambient conditions. Previous experimental studies in UHV revealed the dissociation of CO 2 on the stepped Cu surfaces [2,3]. The CO 2 dissociation should also proceed on the Cu(997) surface under near-ambient conditions to form atomic oxygen and CO. The formed CO is readily desorbed from the Cu surface at 340 K [3], CO 2 → CO (gas) + O (1) Fig. 1. Schematic of the AP-XPS system. Differentially-pumped electron analyzer Pre-lens section Gas cell Manipulator for gas cell Fast-entry chamber Preparation chamber Analysis chamber Loadlock chamber AP-XPS system at SPring-8 BL07LSU Research Frontiers 2015 93 Then, the produced atomic oxygen reacts with CO 2 to form CO 3 , O + CO 2 → CO 3 (2) The produced CO 3 is stable on the Cu surface at 340 K, as seen in Fig. 2. It should be noted that the estimated saturation coverage of CO 3 is rather small (0.05 molecules per surface Cu atom). This indicates that the formed CO 3 species stay at some specific sites, and probably block further CO 3 formation. After the saturation of CO 3 coverage, the atomic O gradually appears on the surface. It may be because minor reaction sites for the CO 2 dissociation remain on the surface while most of the active sites are covered by CO 3 . The present study clearly shows the formation of CO 3 on the stepped Cu(997) surface, and suggests that CO 3 is a candidate for a reaction intermediate in the CO 2 chemistry on Cu surfaces. References [1] M. Salmeron, R. Schlögl: Surf. Sci. Rep. 63 (2008) 169. [2] S.S. Fu, G.A. Somorjai: Surf. Sci. 262 (1992) 68. [3] I.A. Bönicke et al .: Surf. Sci. 307-309 (1994) 177. [4] T. Koitaya, S. Yamamoto, Y. Shiozawa, K. Takeuchi, R.-Y. Liu, K. Mukai, S. Yoshimoto, K. Akikubo, I. Matsuda, J. Yoshinobu: Topics in Catalysis 59 (2016) 526 . [5] S. Yamamoto et al .: J. Synchrotron Rad. 21 (2014) 352. Fig. 2. A series of (a) O 1s and (b) C 1 s AP-XPS spectra of Cu(997) at 340 K under CO 2 pressure of 0.8 mbar as a function of elapsed time. Only selected spectra from the whole series are shown in the figure. The photon energy was 630 eV. The CO 2 gas was introduced to the gas cell at t = 0 s. The fitting results for the spectra at t = 5474 s (O 1 s ) and t = 5607 s (C 1 s ) are also shown in the figures. Fig. 3. Area intensities of each component in (a) O 1 s and (b) C 1 s spectra in Figs. 2(a) and 2(b) as a function of elapsed time ( p (CO 2 ) = 0.8 mbar, T = 340 K). Susumu Yamamoto*, Takanori Koitaya and Jun Yoshinobu The Institute for Solid State Physics, The University of Tokyo *E-mail: susumu@issp.u-tokyo.ac.jp (a) O 1 s (b) C 1 s 540 538 536 534 532 530 528 Intensity (arb. unit) Binding Energy (eV) 5474 3608 1164 376 Clean 294 292 290 288 286 284 282 5607 3739 1295 507 Clean Intensity (arb. unit) Binding Energy (eV) Time (s) Time (s) CO 3 O atom CO3 C 0 Gas-phase CO 2 Gas-phase CO 2 (a) O 1 s (b) C 1 s Area Intensity (arb. unit) Area Intensity (arb. unit) 0 0 0 5000 10000 15000 20000 1000 2000 3000 4000 1000 2000 3000 4000 5000 6000 0 1000 2000 3000 4000 5000 6000 Time (s) Time (s) CO 3 CO3 (Total) CO3 (288.4 eV) CO3 (289.0 eV) C 0 O atom
home
- JP
- EN