Adsorbed Water Molecules on a K-Promoted Catalyst Surface Studied

It is likely that the Raman transitions are observed only as a result of the coupling to the K Rydberg states on the surface. Thus, we observe no line...
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Langmuir 2003, 19, 5756-5762

Adsorbed Water Molecules on a K-Promoted Catalyst Surface Studied by Stimulated Micro-Raman Spectroscopy Frans Olofson, Shahriar Badiei, and Leif Holmlid* Reaction Dynamics Group, Department of Chemistry, Go¨ teborg University, SE-412 96 Go¨ teborg, Sweden Received January 28, 2003. In Final Form: May 8, 2003 Results are presented from a micro-Raman (confocal Raman) study of H2O adsorption on a K-promoted FeO (styrene production) catalyst. On such a material, Rydberg states of the K promoter atoms are formed as shown in previous studies. Due to the huge polarizability of the Rydberg species, stimulated Raman processes are observed, as expected. The stimulated Raman process is shown to give Raman gain at the positions of the plasma lines from the He-Ne laser. The main plasma line supported Stokes band from H2O(ν2) or a K-water complex at 1639 cm-1 and a nearby non-Raman band at 1884 cm-1 are shown to vary almost identically under changes of the vapor pressure of water and the sample temperature, confirming their origin from adsorbed H2O. The band intensities also depend strongly on the amount of K in the catalyst sample, which shows that the K atoms enhance the Raman scattering via Rydberg state formation. Several of the enhanced Stokes bands from the surface are interpreted as bending and stretching transitions in Rydberg complexes (K+-OH2)-e-. The Gaussian 98 program package was used to help assign the observed Raman lines. The Raman gain method should be very useful for spectroscopy at alkali-promoted catalysts in general.

1. Introduction The alkali metal atoms on an alkali-promoted heterogeneous catalyst surface form long-lived Rydberg states at the surface by thermal excitation.1-7 Such states8,9 carry an excitation energy of several electronvolts, that is, several hundred kilojoules per mole (up to 419 kJ mol-1 for K and 376 kJ mol-1 for Cs), and it is very likely that they take part in the reaction steps at the catalyst surface. The most long-lived Rydberg states are circular states with large angular momentum quantum numbers, and their lifetimes increase rapidly with increasing excitation energy.8,9 A number of studies have been published on the formation of promoter metal clusters in Rydberg states at catalytically active surfaces.4,5,10 A recent review describes the Rydberg desorption and reaction processes of the alkali promoter atoms at catalyst surfaces.11 Ordinary optical spectroscopic techniques are preferred to study Rydberg states at catalyst surfaces because the catalysts are normally used at high-pressure and hightemperature conditions. Under such conditions, surface science techniques are not applicable and information from such experiments is often not very useful. Because the polarizability of Rydberg species is very large as a result of the loosely bound Rydberg electron in an orbital with * Corresponding author: Phone no. +46-31 7722832, Fax no. +46-31 7723107, E-mail [email protected]. (1) Lundin, J.; Engvall, K.; Holmlid, L.; Menon, P. G. Catal. Lett. 1990, 6, 85. (2) Engvall, K.; Holmlid, L.; Menon, P. G. Appl. Catal. 1991, 77, 235. (3) Engvall, K.; Holmlid, L. Appl. Surf. Sci. 1992, 55, 303. (4) Engvall, K.; Kotarba, A.; Holmlid, L. Catal. Lett. 1994, 26, 101. (5) Holmlid, L. Z. Phys. D 1995, 34, 199. (6) Engvall, K.; Kotarba, A.; Holmlid, L. J. Catal. 1999, 181, 256. (7) Kotarba, A.; Baranski, A.; Hodorowicz, S.; Sokolowski, J.; Szytula, A.; Holmlid, L. Catal. Lett. 2000, 67, 129. (8) Rydberg States of Atoms and Molecules; Stebbings, R. F., Dunning, F. B., Eds.; Cambridge University Press: Cambridge, U.K., 1983. (9) Gallagher, T. F. Rydberg Atoms; Cambridge University Press: Cambridge, U.K., 1994. (10) Kotarba, A.; Engvall, K.; Pettersson, J. B. C.; Svanberg, M.; Holmlid, L. Surf. Sci. 1995, 342, 327. (11) Holmlid, L.; Menon, P. G. Appl. Catal. A 2001, 212, 247.

a very large radius,9 Raman spectroscopy is probably the best method for such studies. Among the new Raman techniques, micro-Raman with its high spatial resolution seems ideal for the study of heterogeneous materials such as catalysts. Stimulated Raman processes, that is, Raman processes where the Stokes and anti-Stokes intensities are so high that these waves give further processes in the system,12-14 have been observed at K-promoted catalyst surfaces.15-17 These processes are efficient as a result of the large polarizability of the Rydberg species formed from the promoter K atoms. An important problem for many catalysts is the destruction of their function by thick carbon deposits, socalled coking. In the case of the iron oxide catalyst used for styrene production, the addition of water vapor to the reactant gases is used to prevent such deposits.18 This works well for the life span of the catalyst of 1-2 years. The catalyst temperature in the process is around 900 K. The K promoter does not react directly with the water, but functions as a promoter during this period, both for the main catalyzed reaction and for the reaction between carbon on the surface and water. It is, thus, important to understand the basic nonreactive interaction between the alkali promoter and the added water. There exists a large number of studies of H2O coadsorbed with alkali on surfaces, mainly on metal surfaces, thus not directly applicable to the iron oxide catalyst studied here. In the recent review of water adsorbed on surfaces by Henderson,19 most of the water-alkali coadsorption studies are (12) Demtro¨der, W. Laser Spectroscopy, Basic Concepts and Instrumentation, 2nd ed.; Springer: Berlin, 1996. (13) Svanberg, S. Atomic and Molecular Spectroscopy, 2nd ed.; Springer: Berlin, 1992. (14) White, J. C. In Tunable lasers, Topics in Applied Physics, 2nd ed.; Mollenauer, L. F., White, J. C., Pollock, C. R., Eds.; Springer: Berlin, 1992; Vol. 59. (15) Holmlid, L. Langmuir 2001, 17, 268. (16) Holmlid, L. Astrophys. J. 2001, 548, L249. (17) Holmlid, L. Phys. Rev. A 2001, 63, 013817. (18) Meima, G.; Menon, P. G. Appl. Catal. A 2001, 212, 239. (19) Henderson, M. A. Surf. Sci. Rep. 2002, 46, 1.

10.1021/la034142t CCC: $25.00 © 2003 American Chemical Society Published on Web 06/12/2003

Micro-Raman of a K-Promoted Catalyst

concerned with metal surfaces and just a few are related to nonmetal surfaces such as graphite or metal oxides.20 The techniques used for these latter studies are usually temperature programmed desorption and photoelectron spectroscopies, thus not directly comparable to the present study. In some cases, vibrational spectroscopy like highresolution electron energy loss spectroscopy was used. It is typical for the metal surface studies that alkali reacts with water above a certain temperature (below 0 °C), as described, for example, in ref 21. Thus, the information from metal surfaces is not applicable to the case of interest here, where no such reaction is known to take place, possibly as a result of the chemical state of K on the surface that is either bonded (possibly to oxidic sites) or, alternatively, not in the reactive atomic ground state. The same type of reaction is also observed on most metal oxide studies reviewed in ref 19. Only in one case, NiO(100),22 was no such reaction observed, but no vibrational spectra were obtained. Here, we report what we believe is the first micro-Raman spectroscopic study of an alkali-doped catalyst surface, using Stokes Raman scattering with a small continuouswave (CW) He-Ne laser to investigate the interaction between the catalyst and water vapor. As will be demonstrated in the results, the spectroscopic processes are not ordinary Raman but stimulated Raman scattering, also in the form of Raman gain. There are two main problems treated, one concerning the identification of the Raman lines and a non-Raman band as a result of adsorbed H2O on the sample surface in the form of a K-water complex and the assignment of the transitions, and one concerning the verification of the stimulated Raman mechanism for the generation of the observed signal. 2. Experimental The experiments are carried out in a vacuum chamber with a pressure of 10-2 bar down to approximately 10-6 mbar. The apparatus is the same as that in other experiments15,16 but with the addition of a microscope objective. The sample studied, a commercial iron oxide catalyst, is placed inside a folded Ta foil with a flat surface cut in the exposed part. It is held in the vertical direction by two arms, which are connected to an alternating current power supply. A heating current of up to 40 A can be fed through the Ta foil, giving a temperature of 1100 K (only for calibration purposes, not used with the experimental samples). In the nonpyrometric region, a linear interpolation to room temperature is used to not overestimate the temperatures used. The catalyst initially contains 8 wt % K and is used in the largescale production of styrene from ethyl benzene in the chemical industry at 900 K with a feed of water vapor to prevent coking.18 Water vapor is introduced into the vacuum chamber by a leak valve, usually at a rate of 3 × 10-2 mbar dm3 s-1 to a pressure of e1 × 10-4 mbar. By means of micrometer screws, the sideways position of the sample in front of the microscope and also the distance to the microscope are varied. The laser is a CW 10-mW He-Ne laser (Melles-Griot) with plane polarized light at 632.8 nm, which is brought to the sample through a semitransparent glass mirror (beam splitter), a glass window in the vacuum wall, and finally the microscope objective (40×). The objective is constructed for an image distance of >160 mm, and a negative lens (f ) -200 mm) is usually placed in front of the laser to improve the focus of the microscope. The nominal working distance between the sample surface and the objective lens front is 0.42 mm. The diameter of the laser spot is approximately 8 µm. The light from the sample is taken out through the semitransparent mirror. It is focused by a positive lens (f ) 50 mm) on the monochromator slit. The image plane is a few (20) Foster, M.; Furse, M.; Passno, D. Surf. Sci. 2002, 502, 102. (21) Nakamura, M.; Ito, M. Surf. Sci. 2002, 502, 144. (22) Reissner, R.; Radke, U.; Schulze, M.; Umbach, E. Surf. Sci. 1998, 402, 71.

Langmuir, Vol. 19, No. 14, 2003 5757 centimeters in front of this lens, and the focusing and lateral sample motion can be followed there. Two different monochromators have been employed for these studies. One of them was used for the identification of the water lines and for the nonRaman band studies. It is a Spex 500M (ISA Inc.) with a focal length of 0.5 m, a holographic grating with 1800 lines/mm, and a photomultiplier in pulse counting mode. Entrance and exit slit widths of the monochromator were optimized at 35 and 50 µm, respectively. Another spectrometer, a Spex Triax 320 with higher sensitivity, was used for the higher resolution identification of the stimulated Raman mechanism. It has a focal length of 0.32 m, two different gratings with 1200 and 600 lines/mm, the second one holographic, and a photo multiplier with a very low background count rate. In front of the monochromator, a holographic “supernotch” filter with an optical density at the laser line measured to at least 4.2 (Kaiser Optical Systems, Inc.) strongly suppresses the laser light for the Raman measurements.

3. Results and Discussion Two main themes are covered below, one concerning the identification of the origin of the Raman lines and a non-Raman band as a result of adsorbed H2O on the sample surface and the assignment of the transitions, and the second concerning the verification of the mechanism for the generation of the observed Raman signal. 3.1. Laser Plasma Lines. At the low signal levels used, allowed (nonlasing) emission lines of Ne and He from the laser plasma, so-called laser plasma lines, are observed simultaneously with the Raman lines. This is notable especially when the Raman filter is used. The scattering of these lines from the sample cannot easily be removed in the present setup without one further filter or extra monochromator to filter the laser beam. In principle, a subtraction of the contribution from the plasma lines should be possible, using an appropriate weighting with the wavelength because the Rayleigh scattering varies as λ-4. Unfortunately, such a procedure does not work in the present case because the scattering intensity from the sample seems to increase with the wavelength. This is further described below. Thus, a reliable subtraction of the plasma lines has not been found. Another method has been found to identify the Raman scattering signal. By making small changes in the conditions, for example, by heating the sample slightly, the lines solely due to the scattering of the plasma lines from the laser should be easily subtractable because they will not change, whereas the Raman lines should stand out, increasing or decreasing depending on the variation of the density of the scattering molecules in the sample with temperature. The processes used are temperature variations, polarization variations (assuming almost randomly polarized plasma lines), and moving the sample out from focus (assuming a common decrease in the scattering of the plasma lines after retracting the sample). These methods give similar answers, showing that some of the lines observed are scattered with a common change in the behavior, whereas other lines either increase or decrease in a more complex behavior. In Figure 1, the spectrum found before a small temperature increase and the difference spectrum ∆I(∆T) found by subtraction are compared, T being the sample temperature. It is observed that many lines stand out clearly because they are sensitive to changes in the sample. 3.2. Stimulated Raman Scattering. The Raman scattering shows some peculiar features, which indicate that the scattering is stimulated Raman scattering:12-14 (a) a relatively large volume of the sample compared to the size of the laser focus is involved in the scattering (see the following), (b) a saturation is sometimes observed, (c) an angular dependence of the scattering is observed, giving

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Figure 2. Stimulated micro-Raman spectrum of the catalyst sample with adsorbed H2O at 620 K. The lower trace shows the measured plasma lines from the He-Ne laser, scaled for subtraction from the measured spectrum to give the upper trace. The Raman filter was not used in the lower trace. A gain on the order of 10 can be observed for most of the line positions at long wavelengths.

The laser has the wavenumber ωL, and the He and Ne lines agree with Stokes (S) or anti-Stokes (AS) transitions with wavenumber ωS or ωAS. This means that in the general stimulated Raman process

2ωL ) ωS + ωAS Figure 1. Stimulated micro-Raman spectrum with adsorbed H2O in the upper panel at a sample temperature of 620 K. Difference spectrum ∆I(∆T) for two temperatures (620 and 510 K) in the lower panel. The Raman filter was used. A positive line means that the signal has increased with the sample temperature. The water vapor pressure was approximately 10-4 mbar.

different scattering cones for different transitions, and (d) the lines observed are much sharper than those expected. The width of the vibration-rotation bands expected in the Raman scattering is large for water molecules in the gas phase and also for liquid water. The half-width of the bending vibration band in liquid water23 is >100 cm-1. The width of the rotational distribution in the gas phase at room temperature for this band is on the order of 500 cm-1, whereas its total width in the spectra shown here is only 10-40 cm-1. This shows that another process generates the signal. The line positions observed in general agree with He and Ne lines, but the intensity distribution after scattering from the sample is changed strongly. In Figure 2, the lower plot shows the directly observed plasma lines from the laser. In the upper plot, a spectrum of the sample with adsorbed water is shown, after subtraction of the plasma line spectrum shown below. This subtraction is scaled so that a few lines even become negative, to make a conservative estimate of the process observed. It is directly seen from the figure that the lines observed in the upper plot are at approximately the same positions as the plasma lines but that they have a considerably higher intensity. The actual gain is generally on the order of a factor 10 for lines close to the laser line. Thus, it is concluded that the Raman scattering process is a Raman gain process.14,24 This process can be described as follows. (23) Venyaminov, S. Y.; Prendergast, F. G. Anal. Biochem. 1997, 248, 234. (24) Andrews, D. L. Lasers in Chemistry, 3rd ed.; Springer: Berlin, 1997.

the plasma lines ωS or ωAS will induce the transformation of more laser photons to pairs ωS + ωAS. Because there exists a phase-matching requirement for the different waves, the Stokes and anti-Stokes waves will not necessarily leave the sample in the same direction and may not be observed simultaneously. We conclude that the signal intensity enhancement observed is likely due to Raman gain in the boundary layer of the sample. This means also that the observed line positions are close to the He and Ne plasma line positions and that the true Raman bands are only “highlighted” at such positions. In fact, it seems that the observed lines often are biased in wavelength to either side, and this is proposed to be due to the structure of the true Raman bands. 3.3. Stokes Scattering. A typical micro-Raman spectrum after the adsorption of water vapor in the catalyst sample is shown in the upper panel in Figure 1. A few of the Stokes peaks are identified as due to H2O, and the assignments of the bands (S) are given in Table 1. No anti-Stokes lines are assigned. It is possible to obtain spectra also without the Raman interference filter in front of the monochromator, as seen in Figure 3. The observed spectra without the Raman filter are due to the scattering of light with an unchanged wavelength but still at very high intensities far from the laser line. We suggest that they are due to phase-shifted light,25 and one of the observed peaks of this type at 720 nm (1884 cm-1) in Figure 3 will be studied in detail here. It is shown below to be due to water at the surface. Also in Table 1 a few strong transitions observed without the Raman filter, here summarized as non-Raman bands (nR), are included. It is likely that the Raman transitions are observed only as a result of the coupling to the K Rydberg states on the surface. Thus, we observe no lines that clearly or likely originate from the surface structure of iron oxide materials, such as Fe2O3, KFeO2, etc., that the catalyst and its surface layer are composed of. (25) Holmlid, L. Submitted for publication.

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Table 1. Assignment of the Main H2O-Related Linesa typeb S, nR S, nR S, nR S S S nR a

observed wavenumber (cm-1) 197-211 426 640-650 818 1578 1639 (1606-) 1884 ( 50

wavenumber (cm-1)

assignment ν(K+-OH

stretchc

2) ν(K+-OH2) bendc

205-276;39 216 (calcd) 363 (calcd)

libration ν2(H2O adsorbed) ν4(K+-OH2); ν2(H2O liquid) ν2[H2O* A ˜ (1B1)] or 2ν2[H2O* B ˜ (1A1)]

peak at 73537 or 80038 1595 (g) 1663 (calcd); 1643.523,51 1850;40 1050 + 921 ) 192140

The uncertainty of the Stokes lines is (10 cm-1. b S means Stokes bands, nR non-Raman bands.25

Figure 3. Spectra with and without the Raman interference filter at a sample temperature 600 K, showing that informative spectra can be observed without the Raman filter in place (upper curve).

The assignment of the observed bands in the table is based on quantum mechanical calculations using the Gaussian 98 program package.26 The method used is the second-order Møller-Plasset perturbation theory.27 A relatively large basis set, the 6-311+G(3df,3pd) of Pople and co-workers,28-33 was used for the calculations. The inclusion of a larger number of diffuse and polarization functions for the construction of the basis set is necessary for a better description of the wave functions of the molecules with heavy atoms, as is the case here. The optimized geometry shows that K+-OH2 belongs to the C2v symmetry group. The frequencies obtained are compared with the experimental results in Table 1. The strongest positive line at 1639 cm-1 is assigned to the water bending vibration in the complex K+-OH2 and possibly to the bending vibration in liquid water23 (see the following). It is not very likely that the environment (26) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, Revision A.9; Gaussian, Inc.: Pittsburgh, 1998. (27) Møller, C.; Plasset, M. S. Phys. Rev. Lett. 1934, 46, 618. (28) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1971, 54, 724. (29) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2257. (30) Hariharan, P. C.; Pople, J. A. Mol. Phys. 1974, 27, 209. (31) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta. 1973, 28, 213. (32) McLean, A. D.; Chandler, G. S. J. Chem. Phys. 1980, 72, 5639. (33) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys. 1980, 72, 650.

c

Tentative assignments.

on the catalyst corresponds to liquid water at the prevailing conditions of 600 K and 10-4-mbar water vapor pressure, even if the energy of the bonding to the actual sample surface is not known. However, the agreement with the calculated vibration in the complex is quite good. The main reason the observed Raman lines, and especially the largest line at 1639 cm-1, are not assigned to water but to the K+-OH2 complex is the variation with K content in the catalyst sample. During the heating of the sample, the K is slowly desorbed, which is a main problem with this type of catalyst.11,18 This means that the Raman signal decreases in time over many days of use in the experiments. We have studied this type of K loss from catalysts in many different types of experiments1-6,10,11,34-36 and found that it is the main irreversible change taking place in the vacuum during the experiments. A loss of oxygen from the surface layer is also often apparent, but this process is not very fast at the low temperatures used here. In the experiments, the oxygen loss, which is visible as a change of the sample color from reddish brown to gray, can be inhibited by a small air (or possibly water) leak into the vacuum chamber. In the present experiments, the sample did not change its appearance probably because of the low temperatures used. Other surface components such as Fe ions will not be depleted by desorption, and a loss of oxygen, which might give a reduction of the iron on the surface, does not seem to take place. When the Raman signal became too small in the present experiments, the sample was replaced by a new catalyst sample from the same batch. A typical example of the resulting signal changes is shown in Figure 4. There, the signal at 1639 cm-1 increases by a factor of 60 from an old to a new sample with practically no other change of conditions (3 days later). Thus, we conclude that at least this Raman line is due not only to water but also to K. The assignment to the water bending vibration in the complex K+-OH2 is, thus, strongly supported. Assuming that the other Raman lines observed are at He and Ne positions but with a higher intensity due to the Raman gain effect, the lines should lie within reasonable bands for adsorbed molecules on the surface. Other processes in water may give transitions in good agreement with the observed lines. The libration in ice is a broad band between 600 and 900 cm-1, peaking at 735 cm-1.37 The same process in liquid water may give a combination peak with bending at 2127.5 cm-1 23, thus with 488 cm-1 due to the libration. In other studies, libration is stated to give a band at 800 cm-1.38 We assign the observed Stokes transition at 818 cm-1 as due to libration in water, probably in the form of clusters on the catalyst surface. The low-frequency modes in the K+-water complex can be estimated with the help of the frequency at 305 cm-1 (34) A° man, C.; Holmlid, L. Appl. Surf. Sci. 1992, 62, 201. (35) A° man, C.; Holmlid, L. Appl. Surf. Sci. 1993, 64, 71. (36) Engvall, K.; Holmlid, L.; Kotarba, A.; Pettersson, J. B. C.; Menon, P. G.; Skaugset, P. Appl. Catal. 1996, 134, 239. (37) Cox, P. Astron. Astrophys. 1989, 225, L1. (38) Chakarov, D. V.; O ¨ sterlund, L.; Kasemo, B. Langmuir 1995, 11, 1201.

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Figure 5. Time variation of the peaks at 1639 cm-1 (Stokes Raman) and 1884 cm-1 (non-Raman) after the start of heating the sample to 620 K with a high water content, initially in equilibrium with water vapor at a low pressure and room temperature. Note that both bands vary in approximately the same way. The heating current was adjusted intermittently, which gave the more rapid variations in the signals.

Figure 4. Two spectra showing a spectrum from a K-depleted sample, in the upper plot, and from a fresh sample, in the lower plot. The conditions are almost identical, with a sample temperature 500-600 K (higher for the depleted sample). The lower spectrum is measured 3 days after the upper one.

measured in the Na+-water complex,39 giving a value of 276 cm-1. Calculations give instead 216 cm-1 for the stretching K+-water vibration, while the bending of this bond is at 363 cm-1. A second complex vibrational motion (oxygen translation) is at 351 cm-1. The calculated stretch is close to the lowest frequency observed here, whereas the two other vibrations may give contributions at the observed line at a wavenumber of 426 cm-1. 3.4. Water Vapor Pressure. The direct relation between water vapor pressure, sample temperature, and the intensities of the two main bands at 1639 and 1884 cm-1 has been studied. It is observed that the heating of the sample in a high vacuum to high temperatures, on the order of 800 K, reduces the intensity of the lines and that water vapor introduction into the chamber increases their intensity. Thus, the origin of these lines is certain. In Figure 5, an experiment is shown that involves the heating of a sample to 600 K that initially was in equilibrium with the water vapor in the chamber at room temperature. At the start of the run, the water vapor admission was interrupted, the heating was started, and the intensities of the bands were followed in time. The signal of the main Stokes band at 1639 cm-1 increases during the heating as a result of an increased rate of diffusion from the bulk. Small time variations in the heating give time variations in the signal. The signal at 1884 cm-1 is followed simultaneously, and this signal varies in the same way as the water line at 1639 cm-1. Thus, this non-Raman band also corresponds to water adsorbed on the catalyst surface. An assignment of this band is given in Table 1,

indicating reasonable agreement with vibrational transitions in electronically excited water molecules. This identification is only tentative but based on the existing information about the formation of K Rydberg species at the surface. Because all electronically excited states of H2O have a considerable Rydberg character,40 it is not unlikely that a facile transfer of the excitation energy to H2O in the Rydberg (K+-OH2)-e- complexes takes place. 3.5. Distance Dependence. By varying the distance between the sample and the microscope, further information can be obtained about the origins of the various bands. In Figure 6, the distance dependence of the Stokes 1639 cm-1 band is shown. The focusing and acceptance angles of the microscope imply that a scattered (ordinary Raman) signal from the surface will change rapidly with distance to the microscope, not due to changes in the laser intensity at the surface but primarily due to the viewing restrictions. The variation of the signal with distance is slower than might be expected from the size of the focus. This indicates a nonlinear, almost saturated behavior even at low light intensities at the sample surface. Such a behavior is expected for stimulated Raman scattering.12-14

(39) Schulz, C. P.; Haugsta¨tter, R.; Tittes, H.-U.; Hertel, I. V. Z. Phys. D 1988, 10, 279.

(40) Wang, H. T.; Felps, W. S.; McGlynn, S. P. J. Chem. Phys. 1977, 67, 2614.

Figure 6. Variation of the 1639 cm-1 band with distance between the microscope front and the sample at room temperature. The nominal surface is at 0.

Micro-Raman of a K-Promoted Catalyst

Figure 7. Spectra without the Raman filter, focusing on the surface (upper curve) and at a large distance from the surface (lower curve) at a sample temperature of 600 K. The band at 2067 cm-1 is probably due to the scattering of a laser plasma line, while the band at 1884 cm-1 is a non-Raman band from H2O on the surface.

Figure 8. The difference spectrum ∆I(∆z) with the Raman filter, moving the sample at room temperature from focus to a position 0.18-mm backward. To compensate for the difference in light collection, the spectrum in the distant position has a multiplier of 1.3 in the subtraction. A positive value for a line means that the signal is larger with the sample in the focus; thus, the signal is from the sample surface. The dominating line is the water complex bending line at 1639 cm-1. Note the good agreement with the positive lines in Figure 1.

In Figure 7, two spectra are shown that are taken without the Raman filter, one with the focus at the surface and one at a large distance from the surface. Of the observed bands, only the non-Raman band at 1884 cm-1 disappears completely at a large distance. This indicates that its origin definitely is at the surface, despite its not completely understood formation mechanism. In other cases, especially for a band at 2067 cm-1, the signal is unchanged also at a large distance from the surface. Thus, this peak may be due to a laser plasma line. A more complete overview of the different lines observed is found in Figure 8, where the difference between two spectra taken at different sample distances ∆I(∆z) is shown, with a positive line showing that the signal increases in the laser focus. Note that this figure is very similar to Figure 1, where instead the change ∆I(∆T) is shown, T being the sample temperature. Thus, almost the same lines are observed as emanating from the sample surface in both cases.

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3.6. Rydberg States. Thermal excitation of the K promoter atoms to Rydberg states K* is possible in the present case, as shown previously.15 This excitation is due to desorption from covalently bonded states on the surface, as discussed in ref 6 for another catalyst surface, and proven for the somewhat less complex system of K atoms at a graphite surface.41 A review 11 of the experiments with the present type of catalyst is also published. The main reason for the thermal desorption to Rydberg states is that the adsorbed state of K, which correlates with desorbed ground-state K atoms, is not stable on the surface. Thus, very little flux can desorb from the surface to the K ground state outside the surface. Instead, almost all K atoms desorb to K* Rydberg states. The K Rydberg states can form electronically excited clusters at the surface.5,10,42 They can also interact with adsorbed molecules such as H2O, forming Rydberg complexes presumably of the form (K+-OH2)-e-, with the Rydberg electron encircling the entire complex. Already in the ground state, the electron from the alkali atom in such a complex is spread out and delocalized into a “surface Rydberg state”.43 Because also the electronically excited states of water40,44,45 are well-described as Rydberg states, or with a dominant contribution from Rydberg states, the system K-OH2 is likely to keep the outermost electron (from the K atom) in a Rydberg orbital independent of its excitation state. Thus, excited states formed by K Rydberg atoms and water are very likely in the form (K+-OH2)-e-, as described here. 3.7. Implications for the Catalysis Process. The observed behavior of water on the catalyst surface and its complex formation with K on the surface is of importance, especially in the actual industrial use of the catalyst. In the process, water vapor is fed to the catalyst to remove hydrocarbon and carbon residues. In this way, the formation of thick inactivating carbon layers on the catalyst surface, so-called coking, is inhibited. The K promoter is thought to take part in these reactions between water and the residues, not only in the main reaction of ethylbenzene to styrene and hydrogen. However, how the K promotion process is taking place is not well understood, as also is the case for the promoter function of alkali in general. In principle, two possibilities exist for a reaction mechanism involving the K* Rydberg states. In the first suggested mechanism, the promotion is through the formation of K* Rydberg states on the areas covered by carbon as a first step. That such an effect exists has been shown in a large number of studies of K Rydberg species formation on carbon layers on metals.41,46-48 K* interacts with the water molecules, bringing them into a more energetic and, thus, much more reactive Rydberg state. Evidence is here provided for the formation of a Rydberg complex between K and water, which is the complex that may react with the carbon residue. The second possible mechanism for the activation of water also involves the formation of K-water Rydberg complexes, but it does not involve a carbon residue as the place for formation of the K* Rydberg states. Instead, K* Rydberg species are formed as part of the regular K (41) Holmlid, L. J. Phys. Chem. A 1998, 102, 10636. (42) Wang, J.; Engvall, K.; Holmlid, L. J. Chem. Phys. 1999, 110, 1212. (43) Barnett, R. N.; Landmann, U. Phys. Rev. Lett. 1993, 70, 1775. (44) Warken, M. J. Chem. Phys. 1995, 103, 5554. (45) Theodorakopoulos, G.; Petsalakis, I. D.; Buenker, R. J.; Peyerimhoff, S. D. Chem. Phys. Lett. 1984, 105, 253. (46) Holmlid, L.; Pettersson, J. B. C.; Hansson, T.; Wallin, E. Rev. Sci. Instrum. 1994, 65, 2034. (47) Holmlid, L. Chem. Phys. 1998, 230, 327. (48) Svensson, R.; Holmlid, L. Phys. Rev. Lett. 1999, 83, 1739.

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promoter mechanism on the catalyst surface. The K* formation process has been studied in detail on catalyst samples for a few different types of alkali-promoted catalysts.1-6,10,11,34-36 The recent studies by Kotarba et al. have proved a direct relation between the catalytic activity of the sample and the formation of promotor Rydberg species,7,49,50 such that catalytic inactive materials, like S-poisoned ammonia catalysts, do not form Rydberg species while active K-promoted catalysts do form K Rydberg species. The K* formed in this reaction mechanism interacts with water, giving activated water molecules that react with the residues on the surface, as in the first mechanism proposed.

4. Conclusions We have observed the stimulated micro-Raman scattering in the form of Raman gain due to the weak laser plasma lines in a system of H2O adsorbed on a K-promoted iron oxide catalyst. The interaction between K and H2O is likely to take the form of (K+-OH2)-e- Rydberg complexes. We have assigned vibrational frequencies on the basis of quantum chemical calculations and also the direct variation of the line intensities with the amounts of water and K atoms that exist on the catalyst surface. Novel non-Raman bands are also observed that are due to other scattering processes in water molecules in the sample.

(49) Kotarba, A.; Adamski, G.; Sojka, Z.; Witkowski, S.; DjegaMariadassou, G. Stud. Surf. Sci. Catal., Part A 2000, 130, 485 (International Congress on Catalysis, 2000, Part A). (50) Kotarba, A.; Dmytrzyk, J.; Narkiewicz, U.; Baranski, A. React. Kinet. Catal. Lett. 2001, 74, 143. (51) Itoh, T.; Sasaki, Y.; Maeda, T.; Horie, C. Surf. Sci. 1997, 389, 212.

Acknowledgment. Our sincere thanks to P. Govind Menon, from whom the catalyst samples were obtained. This study was supported by the Swedish Research Council (VR). LA034142T