J. Phys. Chem. B 2000, 104, 1235-1240
1235
Scanning Tunneling Microscopy Study of Formate Species Synthesized from CO2 Hydrogenation and Prepared by Adsorption of Formic Acid over Cu(111) T. Fujitani,*,†,‡ Y. Choi,§ M. Sano,§ Y. Kushida,§ and J. Nakamura*,|,§ National Institute for Resources and EnVironment, Tsukuba, Ibaraki 305-8569, Japan, and Institute of Materials Science, UniVersity of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan ReceiVed: June 18, 1999; In Final Form: December 2, 1999
The adsorption structure of formate species on Cu(111), prepared by two different methods, was studied using ultrahigh vacuum scanning tunneling microscopy (UHV-STM), that is, the synthesis by the hydrogenation of CO2 at atmospheric pressure and the adsorption of formic acid on an oxygen-precovered Cu(111). Linear chains of formate molecules were imaged by the hydrogenation of CO2 by STM at low formate coverage with the distance to the nearest neighbor of the formate species estimated to be 5.0 ( 0.2 Å, twice that of the nearest Cu-Cu neighbor. The adsorption phase of formate thus grows linearly at the initial stage by an anisotropic attractive interaction between the formate species. The ordered structure of the formate species changed in the order of p(2 × 4), c(2 × 8), (7 × 7), p(2 × 3), (5 × 5), and c(2 × 4) with increasing formate coverage, indicating that various ordered structures appeared corresponding to the small change in the formate coverage. All the formate structures, except for (7 × 7) and (5 × 5), consisted of the same chain as observed at low coverage, with the distance between the formate chains decreasing as the formate coverage increased. A repulsive interaction thus acts between the formate chains, resulting in a lowering of the adsorption energy of the formate species as reported in the literature. On the other hand, (4 × 4) and (3 × 7/2) structures were observed upon exposing the Cu(111) surface to formic acid at 2 and 15 L, respectively, showing no chain structure of the formate species. At low formate coverage, no molecular image was observed, indicating that the formate species diffuses more rapidly than the scanning of the STM tip. Thus, the adsorption structure of formate on Cu(111) was found to be different depending on the preparation method. It is suggested that the difference in the adsorption structure strongly influences the rate constant of the previously reported formate decomposition.
1. Introduction A high-pressure reactor of about 1 Torr to 50 atm connected to an ultrahigh vacuum (UHV) apparatus is crucial for studying catalytic reactions on an atomic level using surface science techniques. The high-pressure studies have successfully clarified the kinetics, mechanism, and reaction intermediates of major catalytic reactions.1 However, few scanning tunneling microscopy (STM) studies have been reported using a UHV-STM connected to the reactor,2,3 although very important information is expected for the local structure and island formation of catalytic intermediates. We recently developed a UHV-STM system for studying catalytic intermediates synthesized at atmospheric pressure. We have studied the methanol synthesis from CO2 and H2 on Cu(111) and Zn/Cu(111) model catalysts at high pressure to clarify the role of ZnO in Cu/ZnO catalysts. It was found that the Zn deposited on Cu(111) significantly promoted methanol synthesis activity, suggesting that an active site was created on Cu(111) by the deposition of Zn.4-6 Since the formate species is known to be an important intermediate for methanol synthesis because of hydrogenation of CO2,7,8 we first studied * To whom correspondence should be addressed. † Fax: +81-298-521374. Phone: +81-298-588173. E-mail fujitani@ nire.go.jp. ‡ National Institute for Resources and Environment. § University of Tsukuba. | E-mail:
[email protected].
the formation process of formate from CO2 and H2 to examine the role of Zn in the reaction mechanism using X-ray photoelectron spectroscopy (XPS) and infrared reflection-absorption spectroscopy (IRAS). It was clearly demonstrated that the rate of formate synthesis over Zn-deposited Cu(111) was the same as that on Zn-free Cu(111), indicating that the formate species was synthesized on the copper surface and that Zn had no promotional effect on the formate synthesis.2,9,10 Formate species on Cu(110) and Cu(100) have been studied using both experimental and theoretical approaches.11-14 Surface formate is known to be adsorbed on the top site of copper atoms in a bidentate structure with the molecular plane perpendicular to the surface. The adsorption structure of the formed formate species from formic acid has also been studied using STM.15-17 Formic acid exposure on Cu(110) preadsorbed with
