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J. Phys. Chem. 1996, 100, 14962-14968
Adsorption and Reactions of Formic Acid on (2×2)-NiO(111)/Ni(111) Surface. 1. TPD and IRAS Studies under Ultrahigh Vacuum Conditions Athula Bandara, Jun Kubota, Akihide Wada, Kazunari Domen, and Chiaki Hirose* Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan ReceiVed: March 14, 1996; In Final Form: June 14, 1996X
The adsorption and decomposition of formic acid on (2×2)-NiO(111) surface have been studied using temperature-programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRAS) under ultrahigh vacuum (UHV) condition. Formic acid dissociated to surface formate at 163 K for low exposure and molecular adsorption occurred at higher exposure of formic acid. Absorption bands observed at 2858, 1570, 1360, and 778 cm-1 on the IRA spectra were assigned to the C-H stretching, O-C-O asymmetric stretching, O-C-O symmetric stretching, and O-C-O deformation modes of adsorbed formate, respectively. Cumulative consideration of vibrational frequencies, assignment of the bands, and the selection rule of IRAS revealed that the formate is in bidentate configuration but is tilted sideways to the surface. The adsorbed formate decomposed through two reaction pathways on raising the temperature; dehydrogenation producing H2 and CO2 occurred at 340, 390, and 520 K and dehydration producing CO occurred at 415 and 520 K. The temperature-dependent IRA spectra reflected the thermal desorption behavior of adsorbed formate.
Introduction The adsorption and reactions of molecules on the surfaces of oxide have attracted interest in the areas of surface science and catalysis. To date, several works on the characterization of adsorbates on well-defined metal oxides have been carried out but information associated with the bonding and orientation of adsorbates is limited. The major reason for this shortcoming has been the difficulty of using the technique of electron spectroscopy for the analysis of such insulating materials and the difficulty has been recently circumvented by growing a wellordered thin oxide layers on a single crystal-metal substrate.1-14 The growth of crystalline NiO(100) layers on Ni(100), Ni(110), and Ni(111)1,15 substrates has been reported. Neddermeyer and co-workers reported the synthesis of thin film of NiO(111) on Au(111) substrate, and the scanning tunneling electron microscopic study proved the growth of oxygenterminated NiO(111) films during surface oxidation process.16 Rohr et al.17 reported the preparation of NiO(111) films on Ni(111) substrate and found that the surface undergoes reconstruction to (2×2) structure upon heating by the elimination of surface hydroxyl groups. A sketch of the resulting surface with anion and cation termination is shown in Figure 1. The interaction of carboxylic acids with metal oxides is the key factor in such catalytic reactions as water-gas-shift reactions, methanol synthesis, and selective oxidation since carboxylate intermediates formed by the dissociative adsorption of carboxylic acids are regarded as the intermediates of these reactions. To date, the interaction of formic acid with single crystalline surfaces of metal oxides such as NiO(100),12, 18 ZnO(1000),19 MgO(100),10 TiO2(100),20 and ZrO2(100)21 has been investigated by several research groups. These studies have indicated that formic acid either adsorbs dissociatively to form formate and hydroxyl groups even at low temperatures or physisorbs initially and dissociation to surface formate takes place upon subsequent heating. The presence of surface formate has been confirmed X
Abstract published in AdVance ACS Abstracts, August 1, 1996.
S0022-3654(96)00783-6 CCC: $12.00
Figure 1. A sketch of the (2 × 2) reconstructed nickel- and oxygenterminated NiO(111) surfaces.
by vibrational spectroscopy such as HREELS and infrared spectroscopy. Troung et al.12 used HREEL spectroscopy to study the adsorption of formic acid on NiO(100) films grown on Mo(100) and found that the molecule adsorbs associatively at 90 K, heterolytic dissociation to formate occurs upon annealing to >200 K, and recombinative desorption starts at 375 K on further heating. The study indicated the monodentate configuration of adsorbed formate while Wulser and Langell18 reported a tilted geometry for the formate adsorbed on NiO(100)/Ni(100) surface on the basis of the observation of higher frequency for the O-C-O asymmetric stretching mode of formate on the HREEL spectra. As for the decomposition of surface formate at elevated temperature, Henderson et al.20 recently observed dehydration reaction to occur on the decomposition of formate on TiO2(100) surface with trace amount of formaldehyde and Peng and Barteau22 observed dehydration products as the decomposition products of formate on MgO(100) surface. Vohs and Barteau23 and Ludviksson24 on the other hand observed CO2, H2, and CO as the decomposition products of formate on ZnO(0001) surface. Thus, the decomposition of formate on © 1996 American Chemical Society
Reactions of Formic Acid on (2×2)-NiO(111)/Ni(111) Surface metal oxides proceeds by either dehydrogenation giving the products of H2 and CO2 (eq 1) or dehydration giving the products of H2O and CO (eq 2).
HCOOH f H2 + CO2
(1)
HCOOH f H2O + CO
(2)
Recently, Wu and Goodman25 investigated the adsorption of formic acid on thick (∼30 monolayers, ML) (1×1)-NiO(111) layers prepared on Mo(110) substrate by TPD and HREELS and observed that both the dehydrogenation and the dehydration reactions occurred at 560 K and the adsorbed formate had either monodentate or bridging configuration on the surface. The initial motivation of the present study was the application of infrared reflection absorption spectroscopy (IRAS) in the region where the thickness of the oxide layer is thin enough,
