JOURNAL OF CHEMICAL PHYSICS
VOLUME 112, NUMBER 19
15 MAY 2000
Ammonia adsorption by hydrogen bond on ice and its solvation Hirohito Ogasawara, Noriko Horimoto, and Maki Kawaia) RIKEN (The Institute of Physical and Chemical Research), 2-1, Hirosawa, Wako 351-0198, Japan
共Received 26 January 2000; accepted 21 March 2000兲 Regarding the solvation of molecules to water, the adsorption of molecules on the water surface has mostly been considered. Here we provide spectroscopic evidence for the adsorption and solvation behavior of ammonia on the ultra thin ice film surface formed on Ru共001兲 by the use of infrared reflection absorption and thermal desorption spectroscopies. Here we prove that the solvation of ammonia involves two steps. They are the hydrogen bond adsorption and the transfer into bulk. The hydrogen bonding adsorbed ammonia on ultra thin ice film in a NH3 form is evidenced for the first time. Upon heating, bulk transfer upon a conversion to a NH⫹ 4 form is observed for this hydrogen bonded species, however it is not for bilayer and multilayer species. © 2000 American Institute of Physics. 关S0021-9606共00兲71019-1兴
monia adsorption was studied. Adsorption of ammonia via a hydrogen bond is evidenced by infrared reflection absorption spectroscopy 共IRAS兲 for the first time. The adsorbed ammonia molecules via hydrogen bond are in a NH3 form. The combination of IRAS and thermal desorption spectroscopy 共TDS兲 evidences that the solvation occurs via two steps: the hydrogen bonding adsorption and the transfer into bulk, where ammonia molecules are in a NH⫹ 4 form. The experiments were carried out in an ultra high vacuum chamber which was equipped with a three-grid retarding field analyzer for low energy electron diffraction 共LEED兲 and Auger electron spectroscopy 共AES兲, and a quadrupole mass filter for thermal desorption spectroscopy 共TDS兲. The base pressure was ⬍1⫻10⫺10 Torr. A resolution of 4 or 8 cm⫺1 and p-polarized light were used for IRAS measurement. Notably, s-polarized light gave no absorption peak for both ice and adsorbate, indicating that the dipole selection rule of IRAS at a metal surface operates at this surface. All the spectra reported here were recorded at the surface temperature of 38 K. Details of the apparatus have been published elsewhere.9,10 The Ru共001兲 clean surface was prepared by Ar-ion bombardment, annealing, oxidation, reduction and flashing cycles. The structure and cleanliness were confirmed by LEED and IRAS of a ( 冑3⫻ 冑3)-CO-covered surface. The heating rate (  ) for TDS and IRAS annealing sequences was ⬇1.5 K/s. The cooling rate was ⬇1 K/s. We prepared the well-crystallized and monohydride terminated ice surface on Ru共001兲. The ice film was grown below 100 K. The deposition rate was ⬇ 17 – 16 bilayer/s. After the deposition, the film was annealed at ⬇125 K. The water coverage was estimated from TDS assuming that the ice multilayer grows after the saturation of the bilayer growth and that the sticking coefficient of water is independent from the coverage.11,12 Figure 1 shows a schematic drawing of the structure of monohydride terminated ice. In bulk, each water molecule has two hydrogen bonds at oxygen sites and two hydrogen bonds at hydrogen sites. At the surface, one hydrogen bond at a hydrogen site is missing. This missing hydrogen bond is confirmed by IRAS for monohydride terminated six bilayers
The transport of molecules across the air–water interface is an important step of a chemical reaction in nature: chemical reactions at sea and lake water surfaces and in the clouds involve such a process. So far, the transport of molecules across a gas–water interface has been studied based on the thermodynamic studies, where the changes of enthalpy and entropy upon solvation and vaporization have been considered.1–4 The previous studies have shown that the change upon solvation and that upon the vaporization are almost the same for an ammonia molecule. This macroscopic property of solvation is explained by accepting the existence of ‘‘surface bound state.’’1 In this model, the solvation occurs via two steps. The first step is the adsorption of ammonia on the liquid surface, where the adsorbed ammonia molecules are trapped in the ‘‘surface bound state,’’ and at the second step, they are transferred into the bulk. Does conversion of ammonia from NH3 to NH⫹ 4 occur before or after the bulk transfer? At the moment, there is no direct evidence for this problem, because there has been little microscopic evidence for this ‘‘surface bound state’’ so far. The spectroscopic study of a ‘‘surface bound state’’ will give an answer to this question. It is necessary to prepare a well-defined and contamination-free water surface, because the lack of information about structure and chemical composition makes this field complex and confusing. An ultra thin ice film grown on a metal surface under ultra high vacuum5 is a good candidate for this purpose. A similarity between liquid water surface and ice water surface has been shown by vibrational study of an air–water interface,6–8 where a hydrogen bond is missing at both surfaces. Therefore, a study of molecules adsorbed on a well defined ice surface also provides valuable information about them on a liquid water surface. In the present study, we studied the transfer of ammonia across the gas–water interface. We prepared wellcrystallined ultrathin ice film on a Ru共001兲 surface, and ama兲
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FIG. 1. The sketched structure of monohydride terminated ice surface and adsorption structure of ammonia on monohydride terminated ice. Open large circles indicate oxygen atoms in the first layer, shaded large circles indicate oxygen atoms in the second layer, filled small circles indicate hydrogen atoms, and open large circles indicate nitrogen atoms.
of ice film 关Fig. 2共a兲兴 where single peak at 3695 cm⫺1 is observed for surface O–H vibration. A blue shift compared to the hydrogen bonded O–H vibration 关strong and broad absorption around 3400 cm⫺1 as seen in Fig. 2共a兲兴 indicates that this species does not have a hydrogen bond, i.e., the hydrogen bond is missing at the hydrogen site. Notably, the position of this peak is the same as that observed for the surface O–H vibration peak at the liquid water surface,8 indicating that the hydrogen bond at the hydrogen site is also missing at the water surface. This missing hydrogen bond plays an important role at the adsorption of ammonia on an ultra thin ice film. Figure 2 also shows coverage dependent IRAS of ammonia adsorbed on monohydride terminated ice. In Fig. 2共a兲, broad peaks observed at around 3400 and 1620 cm⫺1 are assigned to vibration of bulk ice. When ammonia is introduced to this surface, the intensity of surface the O–H peak observed at
FIG. 2. Series of IRAS spectra of ammonia (NH3 ) on monohydride terminated ice at 38 K: at 共a兲 without ammonia, at 共b兲 coverage of 0.11 ML, at 共c兲 coverage of 0.22 ML and at 共d兲 coverage of 0.33 ML. In 共e兲, the difference spectrum between 共a兲 and 共d兲 is shown. Thickness of ice is six bilayers. Spectral resolution is 8 cm⫺1 .
Ogasawara, Horimoto, and Kawai
3695 cm⫺1 decreases, indicating ammonia molecules are bound at the missing hydrogen bond sites. Here two types of adsorption forms are possible. One is ionic bond adsorption with HO⫺ – NH⫹ 4 interaction. The other is hydrogen bond adsorption with HOH–NH3 interaction. Judging from the dipole selection rule and frequency of symmetric N–H scissors mode 共umbrella mode兲, the adsorbed ammonia molecules are in a NH3 form indicating they are hydrogen bound to ice. Such a hydrogen bond adsorption of ammonia was also reported for ammonia adsorption on silica.13 In Fig. 2共d兲, the missing hydrogen bond sites are completely terminated by adsorbed ammonia molecules. We defined this situation as monolayer saturation coverage of ammonia. Monolayer ammonia shows an umbrella mode at 1088 cm⫺1 , the frequency of which did not change with the coverage of adsorbed ammonia. The difference spectra between Figs. 2共a兲 and 2共d兲 are shown in Fig. 2共e兲. The negative peak at 3695 cm⫺1 corresponds to the vibration of surface O–H in Fig. 2共a兲, which is hydrogen bound to ammonia in Fig. 2共d兲. The positive absorption peaks are due to adsorbed ammonia vibration. Asymmetric N–H scissors modes are observed at 1625 and 1685 cm⫺1 . In the gas phase, these asymmetric N–H scissors modes are degenerated. The observed lift of degeneracy evidences deformation of adsorbed ammonia molecule is no longer in the C 3 v symmetry. When ammonia molecules are strongly adsorbed on the substrate, these values are observed below 1600 cm⫺1 , for instance on Ru共001兲.14,15 Such a high frequency for asymmetric N–H scissors modes was reported for ammonia adsorbed on Ag surfaces where the metal–ammonia bond is weak.16,17 In N–H stretching region, peaks at 3248, 3321, 共symmetric character兲 and 3404 cm⫺1 共asymmetric character兲 are resolved. The appearance of three bands does not correspond to three types of N–H bond but Fermi resonance. The first overtone frequency of asymmetric scissors mode 共⬇3300 cm⫺1 ) is close to N–H vibrational mode. It causes Fermi resonance and reveals the appearance of two bands at 3248 and 3321 cm⫺1 . To confirm the assignments, heavy ammonia adsorption on ice is also studied. Figure 3 shows a series of IRAS for heavy ammonia adsorption on 13 bilayers ice. The adsorption behavior of ammonia does not depend on the thickness of ice. Due to the isotope shift, the N–D stretch modes show downward frequency shift and are observed at 2400 共symmetric character兲 and 2535 cm⫺1 共asymmetric character兲, as seen in Fig. 3. Fermi resonance structure is not as prominent as seen in Fig. 2. The observed weak coupling between these modes would be due to the less anharmonic nature of the N–D stretch modes. From the dipole selection rule of IRAS, the appearance of an asymmetric vibration peak shows that the molecular axis of ammonia is not parallel to the surface normal. The lift of degeneracy indicates that adsorbed ammonia molecules are distorted upon the tilt. Strong dipole–dipole coupling between adsorbed ammonia molecules on metal surfaces has been considered as an origin of tilt and evidenced by electron energy loss spectroscopy from the coverage dependent vibrational frequency of the adsorbate.15,18 This could be an origin of the tilt of ammonia adsorbed on the ice surface. Upon annealing, an ultra thin ice film partially melts and an ultra thin ice film surface becomes liquidlike.19 Thus, an
J. Chem. Phys., Vol. 112, No. 19, 15 May 2000
FIG. 3. Series of IRAS spectra of heavy ammonia (ND3 ) on monohydride terminated ice at 38 K: at 共a兲 without ammonia, at 共b兲 coverage of 0.16 ML, at 共c兲 coverage of 0.33 ML and at 共d兲 coverage of 0.67 ML. In 共e兲, the difference spectrum between 共a兲 and 共d兲 is shown. Thickness of ice is 13 bilayers. Spectral resolution is 8 cm⫺1 .
ammonia molecule can have a chance of solvation. Figure 4 shows the thermal desorption spectrum of m/e⫽17 for adsorbed ammonia on monohydride terminated ice surface. A large peak at 155 K is mainly due to the desorption of ice. At the ammonia coverage of 4 ML, the multilayer and bilayer ammonia desorptions20,21 are observed at 112 and 125 K, respectively. However, there is no pronounced structure due to the monolayer desorption, indicating that the hydrogen bonded adsorbed ammonia molecules cannot desorb from the ice. The behavior of hydrogen bonded ammonia upon heating is well presented by heat and quench experiment in IRAS; here bulk transfer of the molecule converting to a NH⫹ 4 form is evidenced. Figure 5共a兲 shows IRAS for monolayer ammonia adsorbed at 38 K and annealing to the indicated temperatures. As mentioned before, surface hydrogen atoms are completely terminated by ammonia molecules at
FIG. 4. TDS spectra (m/e⫽17 which involves signal from NH3 and OH ions兲 of ammonia (NH3 ) on monohydride terminated ice prepared by adsorption at 38 K. Thickness of ice is six bilayers. The heating rate (  ) was ⬇1.5 K/s.
Ammonia adsorption on ice
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FIG. 5. Series of IRAS spectra of ammonia (NH3 ) on monohydride terminated ice at 38 K; at 共a兲 without ammonia, at 共b兲 coverage of 0.33 ML prepared by adsorption at 38 K, at 共c兲 coverage of 0.33 ML prepared by heating to 120 K. In 共d兲, 0.33 ML of additional ammonia molecules are added. In 共e兲, the difference spectrum between 共a兲 and 共d兲 is shown. The inset shows TDS spectra (m/e⫽17 which involves signal from NH3 and OH ions兲 of ammonia (NH3 ) on monohydride terminated ice prepared by adsorption at 38 K. Thickness of ice is six bilayers. Spectral resolution is 4 cm⫺1 .
38 K. The peak for the umbrella mode is observed at 1080 cm⫺1 . Upon heating, no pronounced desorption peak for a monolayer ammonia molecule is observed below 140 K where an ice film starts to desorb 共see Fig. 4兲. However, significant change is observed in IRAS for monolayer ammonia adsorbed on ice after heating to 120 K 关Fig. 5共b兲兴. The peak for the umbrella mode disappears and the peak for surface O–H reappears. This means that ammonia molecules are evacuated from the surface without desorption. In fact, bulk transfer of an ammonia molecule is evidenced by the appearance of a broad peak around 1470 cm⫺1 which is assignable to the asymmetric deformation mode of ammonium ion (NH⫹ 4 ). A transfer into bulk upon ionization is evidenced by the appearance of a peak at 1200 cm⫺1 . This is assignable to ammonia molecules adsorbed on Ru共001兲. The position of the peak agrees well with that for ammonia and water coadsorbed on a metal surface.22 Thus, NH⫹ 4 species diffuse in ice at 120 K, and some of them adsorb on Ru共001兲 as a result of diffusion. Notably, similar experiments at a thicker ice film does not reveal this ammonia adsorbed on Ru共001兲 because duration of annealing is not sufficient for the diffusion. As a result of the bulk transfer, we have the missing hydrogen bonds at the surface again. When additional ammonia molecules are introduced onto this surface at 38 K, these ammonia molecules reveal hydrogen bonding adsorption at missing hydrogen bond sites. As a whole, peaks for hydrogen bonding adsorbed ammonia at 1080 cm⫺1 , for adsorbed ammonia at 1200 cm⫺1 and for ammonium ion ⫺1 are observed in Fig. 5共c兲. Upon heat(NH⫹ 4 ) at 1470 cm ing to 120 K, these additional ammonia molecules are also transferred into bulk. Therefore, the hydrogen bound ammonia corresponds to so-called ‘‘surface bound state,’’1 and the hydrogen bound adsorption is the first step to bulk transfer.
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In conclusion, monolayer ammonia molecules are hydrogen bound at missing hydrogen bond sites of an ultra thin ice film. The adsorbed ammonia molecules are in a NH3 form. The dipole selection rule and the lift of degeneracy support that the adsorbed ammonia molecules are tilted and deformed. Upon heating, these hydrogen bound species are transferred into bulk ice layer, and bilayer and multilayer species are not. This is the first direct evidence of the ‘‘surface bound state.’’ Financial support for this work was provided in part by the Grant-in-Aid for Scientific Research on Priority Areas ‘‘Molecular Physical Chemistry’’ from the Ministry of Education, Science, Sports and Culture of Japan. D. J. Donaldson, J. Phys. Chem. A 103, 62 共1999兲. D. J. Donaldson, J. A. Guest, and M. C. Goh, J. Phys. Chem. 99, 9313 共1995兲. 3 P. Davidovits, J. H. Hu, D. R. Worsnop, M. S. Zahniser, and C. E. Kolb, Faraday Discuss. 100, 65 共1995兲. 4 D. S. Karpovich and D. J. Ray, J. Phys. Chem. B 102, 649 共1998兲. 5 P. A. Thiel and T. E. Madey, Surf. Sci. Rep. 7, 211 共1987兲. 6 J. R. Scherer, M. K. Go, and S. Kint, J. Phys. Chem. 78, 1304 共1974兲. 1 2
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