Infrared Spectroscopy Characterization of Surfaces

Schematic representation of the experimental apparatus used in our laboratory for reflection-absorption infrared spectroscopy (RAIRS) under ultrahigh.  The sample can be transferred between the infrared position and the main chamber, where other surface spectroscopies can be used.  A high-pressure enclosing cell is presently being developed in order to be able to acquire RAIRS spectra in-situ under atmospheric catalytic conditions (see below).

We have over the years employed this RAIRS setup for the identification of a number of surface intermediates during hydrocarbon conversion reactions [F. Zaera and H. Hoffmann, J. Phys. Chem. 95 (1991) 6297; C. J. Jenks, B. E. Bent, N. Bernstein and F. Zaera, Surf. Sci. Lett. 277 (1992) L89; C. J. Jenks, B. E. Bent, N. Bernstein and F. Zaera, J. Am. Chem. Soc. 115 (1993) 308; F. Zaera and N. Bernstein, J. Am. Chem. Soc. 116 (1994) 4881; T. V. W. Janssens and F. Zaera, Surf. Sci. 344 (1995) 77; F. Zaera, T. V. W. Janssens and H. Öfner, Surf. Sci. 368 (1996) 371; N. R. Gleason, C. J. Jenks, C. R. French, B. E. Bent and F. Zaera, Surf. Sci. 405 (1998) 238; F. Zaera and D. Chrysostomou, Surf. Sci., 457 (200) 71; T. V. W. Janssens and F. Zaera, Surf. Sci., 501 (2002) 1; ] as well as for other surface work [F. Zaera, Surf. Sci. 255 (1991) 280; F. Zaera, J. Liu and M. Xu, J. Chem. Phys. 106 (1997) 4204; Z. Ma, I. Lee, J. Kubota and F. Zaera, J. Mol. Catal. A: Chem. 216 (2004) 199; I. Lee and F. Zaera, J. Phys. Chem. B 109 (2005) 2745].  Below we provide an example of this type of data, in this case for the thermal chemistry of acrolein on Pt(111) [J. C. de Jesús and F. Zaera, J. Mol. Catal. A 138 (1999) 237]. 

RAIRS spectra for acrolein on Pt(111) as a function of annealing temperature.  Several surface species are identifiable in these data, including the ketene and/or acrolein dimer responsible for the two peaks at 1698 and 1725 cm-1 in the 280 K trace.  An initial dimerization of acrolein is also suggested by its flat cis conformation on the surface at 90 K.

Kinetic measurements for ethylidyne formation from ethylene on Pt(111) both in the presence and in the absence of H2.  The conversion was determined by scaling the integrated RAIRS intensity for the umbrella mode of ethylidyne to that obtained after reaction completion.  The linearity of these semi-logarithmic plots indicates that the reaction is first order, and the similarity of the data in the two types of experiments show that the rate of ethylidyne formation is independent of the hydrogen coverage.

On metals the IR analysis has the extra advantage that it follows the so-called surface selection rule: only vibrational modes with dynamic dipoles with a non-zero component perpendicular to the surface can be detected.  As a consequence, the relative intensities of the different vibration bands of a given adsorbate can be used to determine its adsorption geometry [F. Zaera, in The Encyclopedia of Chemical Physics and Physical Chemistry, J. Moore and N. Spencer, editors, Institute of Physics Publishing (UK), 2001 Vol. 2, pp. 1563-1581].  The figure below illustrates this point for the case of ethyl bromide on Pt(111) [F. Zaera, H. Hoffmann and P. R. Griffiths, J. Electron Spectrosc. Relat. Phenom. 54/55 (1990) 705], where a collective rearrangement of the molecules is seen, from flat-lying adsorption at low coverages to a standing-up configuration at saturation [C. J. Jenks, B. E. Bent, N. Bernstein and F. Zaera, J. Am. Chem. Soc. 115 (1993) 308; C. J. Jenks, B. E. Bent, and F. Zaera, J. Phys. Chem., 104, 3017-3027 (2000)]. As an interesting extension of geometry determinations using RAIRS, recently we were able to identify a number of conformational phase transitions in a thin solid film of 1,3-diiodopropane (DIP) condensed over Pt(111) [C. R. French and F. Zaera, Chem. Phys. Lett. 309 (1999) 321]. 

Data from 20 and 100% of a monolayer of molecular ethyl bromide on a Pt(111) surface illustrating the use of the RAIRS surface selection rule for the determination of adsorption geometries.  While at low coverages only the asymmetric deformation of the terminal methyl group is seen, at saturation the feature correspondent to the symmetric deformation is the one observed instead.  This means that a flat adsorption geometry prevails at low coverages but that a collective rearrangement of the adsorbates to a standing-up configuration takes place at about half saturation.

In recent years we have been working on extending the use of RAIRS to non-vacuum environments.  In one instance, we are developing a retractable cell capable of enclosing our solid sample and isolating it from the UHV environment of the main spectroscopy chamber in order to be able to expose it to the atmospheric pressures typical of most catalytic processes.  Sodium chloride windows have been retrofitted to the walls of this cell in order to perform RAIRS in situ during the high-pressure exposures.  The Figure below shows an application of this setup for the characterization of the stability of ethylidyne monolayers on Pt(111) in the presence of hydrogen gas. 

RAIRS of an ethylidyne monolayer on Pt(111) as a function of exposure to 1 Torr of hydrogen gas at 320 K.  The vibrational data was acquired in situ in the presence of the gas by using the high-pressure cell described in the text.  The peak at 1339 cm-1, which corresponds to the umbrella mode of the terminal methyl group in ethylidyne, persists throughout this treatment, but the growth of additional peaks in the C–H stretching region close to 3000 cm-1 indicates the formation of new species on the surface.

We have also arranged a separate setup to carry out RAIRS experiments in air to characterize self-assembled monolayers.  In the figure below we display an example of the data obtained in these studies.  In this case, a monolayer of ferrocene-terminated hydrocarbon chains was adsorbed via a thiol group to a gold film.  The resulting layer retains most of the features of the individual molecular units, except for the loss of the end carboxylic protecting group attached to the thiol end.  On the other hand, the relative intensities of the different modes vary with respect to those measured in the isolated compound, indicating a tilted geometry for the alkyl chains and flat-oriented ferrocene rings.  These experiments are also performed in the attenuated total reflection (ATR) mode [Amir A. Yasseri, Dennis Syomin, Vladimir L. Malinovskii, Robert S. Loewe, Jonathan S. Lindsey, Francisco Zaera, and David F. Bocian J. Am. Chem. Soc. 126 (2004) 11944; Amir A. Yasseri, Dennis Syomin, Robert S. Loewe, Joydev K. Laha, Jonathan S. Lindsey, Francisco Zaera, and David F. Bocian, J. Am. Chem. Soc. 126 (2004) 15603; Lingyun Wei, Dennis Syomin, Robert S. Loewe, Jonathan S. Lindsey, Francisco Zaera, and David F. Bocian, J. Phys. Chem. B 109 (2005) 6323].

RAIRS data contrasting the vibrational spectra of a ferrocene-terminated C16-chain-long thiol by itself versus when self-assembled on a gold surface.  The good match seen for most peaks between the two traces indicates the molecular nature of the adsorption.  In addition, the predominance of the out-of-plane over the in-plane ferrocene C–H deformation modes in the spectrum of the monolayer attests to their flat adsorption geometry.

The polarization dependence of the IR absorption in adsorbed species also allows for the discrimination of signals between adsorbed and gas or liquid phase molecules.  Several schemes have been devised by us and others to take advantage of this feature [H. Hoffmann, N. A. Wright, F. Zaera and P. R. Griffiths, Talanta 36 (1989) 125].  One important development in our lab has been the development of a setup for the characterization of adsorbates in-situ in the liquid-solid interface [J. Kubota, Z. Ma and F. Zaera, Langmuir 19 (2003) 3371].  The picture below shows the appropriate components for this setup.  This equipment has been used to carry out our studies on the adsorption of organic solvents [Zhen Ma and Francisco Zaera, Catal. Lett. 96 (2004) 5] and on the adsorption of chiral modifiers [J. Kubota and F. Zaera, J. Am. Chem. Soc. 123 (2001) 11115; Z. Ma, J. Kubota and F. Zaera, J. Catal. 219 (2003) 404; W. Chu, R. J. LeBlanc, C. T. Williams, J. Kubota and F. Zaera, J. Phys Chem. B 107 (2003) 14365; Z. Ma and F. Zaera, J. Phys. Chem. B 109 (2005) 406].

RAIRS data contrasting the vibrational spectra of a ferrocene-terminated C16-chain-long thiol by itself versus when self-assembled on a gold surface.  The good match seen for most peaks between the two traces indicates the molecular nature of the adsorption.  In addition, the predominance of the out-of-plane over the in-plane ferrocene C–H deformation modes in the spectrum of the monolayer attests to their flat adsorption geometry.

CO titration infrared characterization of palladium catalysts supported on high surface area supports, on sol-gel prepared alumina (left) and 25% zirconia/75% alumina (right) materials.  Carbon monoxide is adsorbed at 150 K on catalysts previously treated at 700 K with either hydrogen (reduced samples, top traces) or oxygen (oxidized samples, bottom traces).  The fact that the vibrational stretching frequency of the C–O bond is highly sensitive to the electronic details of the adsorption renders these spectra quite useful for the determination of the nature of the catalytic sites.  In this case, the incomplete reduction of the reduced samples is indicated by the features about 2140 and 2170 cm-1 that accompany the main Pd metal feature at 2100 cm-1 in both top traces.  Perhaps more importantly, the differences between the two bottom spectra point to the potential role of zirconia as a stabilizer of palladium oxide particles.

 Last modified July 18, 2005