J. Phys. Chem. C 2007, 111, 14207-14214
14207
Attenuated Total Reflection Fourier Transform Infrared Spectroscopic Investigation of CO Adsorption on Hydrated Ru/Al2O3 Adam S. Baird,† Katherine M. Kross,† Diana Gottschalk,† Erin A. Hinson,‡ Neil Wood,† and Kathryn A. Layman*,† Department of Chemistry and Center for Materials Science, MSC 4501, James Madison UniVersity, Harrisonburg, Virginia 22807, and Department of Chemistry, Western Carolina UniVersity, Cullowhee, North Carolina 28723 ReceiVed: May 1, 2007; In Final Form: June 18, 2007
The adsorption of CO on hydrated 5 wt % Ru/Al2O3 produced νCO absorbance features at ∼2048, 1992, and 1924 cm-1 that are red-shifted by 50-116 cm-1 from those seen in the absence of water (2020-2040, 2080, and 2140 cm-1). This red-shift most likely arises from dipole-dipole interaction between coadsorbed CO and water molecules since (1) the exact frequency of the νCO absorbance feature depends upon the amount of coadsorbed water and (2) the presence of flowing liquid water further red-shifts the frequencies. These νCO absorbance features are uncorrelated, since the relative intensities of the νCO absorbances at 2049, 1992, and 1924 cm-1 depend on the amount of coadsorbed water and CO on the surface. Temperature programmed desorption done with TGA-MS indicated three different high-temperature CO2 desorption peaks. These CO2 peaks (T ≈ 350, 400, and 550 °C) are most likely the result of the oxidation of adsorbed CO reacting with surface adsorbed water (COads + H2Oads f H2 + CO2) and/or the disproportionation of CO (2CO f Cads + CO2). These high-temperature CO2 desorption peaks suggest that CO strongly adsorbs to hydrated 5 wt % Ru/Al2O3 catalysts. This is corroborated by the fact that intensities of the νCO absorbance features do not decrease in the presence of flowing liquid water.
Introduction CO adsorption,3-6 methanol reforming,4 and the water gas shift reaction (WGSR)4 on Pt/Al2O3 catalysts have been investigated to learn more about the processes that occur in catalyzed aqueous-phase carbohydrate reforming (ACR) of renewable biomass.4,7-11 Reaction kinetics studies and catalytic activity measurements for the ACR of ethylene glycol over silica-supported metal catalysts indicate that the rate of CO2 production (at 438 K) was highest on supported Pt, Ru, and Ni catalysts.7,10 However, only silica- and alumina-supported Pt exhibit high selectivity for H2;4,7,10-13 supported Ni and Ru have a high selectivity for alkane production.7,10 The observed differences in the selectivity of the supported Pt and Ru catalysts may be due to the influence of water on CO adsorption for these catalysts. Water has been shown to affect the selectivity and activity of catalysts used in the WGSR and CO oxidation. Through the correlation of kinetics measurements with in situ ATR-FTIR spectroscopic data, He et al.4 concluded that increasing the partial pressure of water leads to a higher rate of CO removal from the Pt surface via the WGSR. Lee et al.14 observed that CO strongly adsorbed to Ru(001) in the presence of water indicating that water may play a role in methanation by assisting CO bond cleavage.14 After dosing a Ru(001) single crystal pre-exposed to CO with 1 monolayer (ML) of D2O, Nakamura et al.15 observed a 103 cm-1 red-shift in the linear-bonded νCO absorbance feature (from 2027 to 1924 cm-1) confirming that CO strongly interacts with Ru in the * Author to whom correspondence should be addressed. Phone: 540568-1656. Fax: 540-568-7081. E-mail:
[email protected]. † James Madison University. ‡ Western Carolina University.
presence of water. The frequency of the linear-bonded νCO absorbance feature in the presence of 1 ML of D2O is similar to those reported for CO adsorption on Ru(001) single-crystal electrodes16-20 and Ru thin film21 electrodes in aqueous solution. However, Solymosi et al.22 did not observe a significant shift in the linear-bonded νCO absorbance features at 2025, 2075, and 2140 cm-1, when 5 wt % Ru/Al2O3 was exposed to 0.5 Torr of CO at 300 K in the presence of 0.1 Torr of H2O. Since the investigation by Solymosi et al.22 is the only reported vibrational study of the adsorption CO on supported Ru catalysts in the presence of water and their results differ from those found for Ru crystals, this work used ATR-FTIR to investigate the adsorption of CO on hydrated 5 wt % Ru/Al2O3. ATR-FTIR is an ideal methodology for these studies because it selectively probes solid-liquid and gas-solid interfaces, while enabling the simultaneous investigation of the adsorbate layer and changes in metal oxidation states.4-6,23-35 In contrast to the investigations reported by Nakamura et al.15 and Solymosi et al.,22 5 wt % Ru/Al2O3 was hydrated prior to CO adsorption at room temperature to more accurately measure the interactions under use conditions. Experimental Methods Gases. The helium (UHP), hydrogen (UHP), 1 mol % oxygen in helium (UHP), and carbon monoxide (UHP) were obtained from Robert’s Oxygen Gas Company and dried by flowing through a moisture trap prior to use. Impurities in the carbon monoxide were trapped by using a pentane-liquid nitrogen bath. 13CO (Cambridge Isotope Laboratories, 99% 13C, 1 is blended with a band observed for another CO species, making it difficult to observe at the resolution used in these experiments. The νCO absorbances at 1992 and 1924 cm-1 increase as the sample is exposed to more CO. Since the 1992 cm-1 band is blended by the other bands, it is difficult to accurately determine its relative intensity compared to the 1924 cm-1 band, but the differences in relative intensity for these bands presented in Figures 8-10 indicate that they arise from different sites. The 1924 cm-1 feature is the same as the frequency reported by Nakamura et al.15 for CO adsorption on the atop sites on hydrated Ru(001) and probably arises from the same atop site. The assignment of the 1992 cm-1 band is still ambiguous. This feature may be similar to the 2008 cm-1 feature observed by Nakamura et al.,15 suggesting that 2008/1992 cm-1 are also from similar sites. The frequency difference for this feature could result from slight differences in this bonding site caused by interactions between the Ru and the substrate. Another possibility is that the 1992 cm-1 feature arises from the formation of a Ru(CO)Xm (X ) H2O) species, in agreement with Chen, Zhong, and White.43 Origin of the Observed Red-Shift of CO Absorbance Features. The 50-116 cm-1 red-shift may be explained by (1) a decrease in the number of absorption sites leading to fewer CO-CO interactions, (2) creation of a well-oriented dipole field by coadsorbed water, (3) the formation of more strongly bonded CO species in the presence of water, and (4) a change in adsorption geometry. Dipole-dipole coupling of neighboring carbon monoxide molecules, especially at high coverage, is known to blue-shift the observed νCO absorbance feature of linear-bonded CO. However, since the dipole moment of CO is only 0.112 D,54 it is very unlikely that a lower coverage of adsorbed CO molecules could fully account for the aforementioned 50-116 cm-1 red-shift. Although the density of CO on the surface is expected to decrease because water is occupying some of the surface sites, the reduced dipole-dipole coupling between neighboring CO molecules is probably not the main cause of the observed red-shift in the presence of water. A strong dipole-dipole interaction between coadsorbed CO and water molecules requires that the water molecules organize to form a uniform over-layer (or tetramer clusters); this uniform
J. Phys. Chem. C, Vol. 111, No. 38, 2007 14213 water structure generates a well-oriented dipole field (dipole moment of water ) 1.87 D54) that then couples with the CO dipole moment. Since the TGA indicated that as much as 2% of the sample mass was surface adsorbed water, there is plenty of water adsorbed on the surface to form the uniform overlayer. This leads to the conclusion that a dipole-dipole interaction between coadsorbed CO and water molecules is a primary cause for the observed 50-116 cm-1 red-shift for CO adsorption frequencies on hydrated Ru/Al2O3 compared to those reported for CO adsorption on Ru/Al2O3 in the absence of water. This conclusion is in agreement with those proposed by Nakamura et al.,15 and Kizhakevariam, Jiang, and Weaver.55 Furthermore, 30 cm-1 red-shifts have been reported for CO adsorption on 2-5 wt % Pt/Al2O33,4 and Pt(100)56,57 in the presence of coadsorbed water. While a dipole-dipole interaction is expected to be stronger than van der Waals forces, another possibility is that water electronically shields the dynamic dipole moment of CO. This is in agreement with Ehlers et al. and Hoffman et al. where large red-shifts (∼30-40 cm-1) in the νCO absorbance feature were reported for CO adsorption on Pt(111)58 and Ru(001)59 in the presence of coadsorbed Xe and other polarizable molecules. The usual explanation for the red-shift observed for νCO in transition metal complexes is π back-bonding between the metal atom and the CO π* orbital. By using this model, the red-shift observed for CO bonded to Ru in the presence of water could be explained by assuming the interactions between the Ru and coadsorbed water increases the electron density on the Ru, which then increases the CO back-bonding. This weakening of the CO bond would be consistent with the conclusion by Lee et al.14 that water may play a role in methanation by assisting CO bond cleavage.14 Although the observed red-shift and the TGA data for the desorption from the catalyst surface are consistent with the CO being more strongly bonded to the Ru in the presence of the coadsorbed surface water, additional information will be needed to establish if this is the case. Additional investigations (e.g., density-field theory calculations) are also needed to investigate changes in CO adsorption geometry in the presence of water, and the influence of these changes on the frequency of the νCO absorbance features. Conclusion The ATR-FTIR of CO adsorbed on hydrated 5 wt % Ru/ Al2O3 indicated νCO absorbance features at ∼2048, 1992, and 1924 cm-1. These features are red-shifted by 50-116 cm-1 from those seen in the absence of water (2140, 2080, and 20202040 cm-1). The origin of the red-shift most likely arises from dipole-dipole interaction between coadsorbed CO and water molecules as indicated by the following observations: (1) the exact frequency of these νCO absorbance features depends on the amount of coadsorbed water and (2) the presence of flowing liquid water further red-shifts the frequency of the νCO absorbance features. The relative intensities of the νCO absorbances are not correlated and depend on the amount of coadsorbed water on the surface indicating that each results from a different surface site. This result is in agreement with TGA-MS results that indicate three different high-temperature CO2 desorption peaks at T ≈ 350, 400, and 550 °C. These high-temperature CO2 desorption peaks also suggest that CO strongly adsorbs to hydrated 5 wt % Ru/Al2O3 catalysts. This is corroborated by the fact that intensities of the νCO absorbance features do not decrease in the presence of flowing liquid water. Since the frequency of the νCO absorbance features depends on the amount of coadsorbed water, our results indicate that analysis of in situ
14214 J. Phys. Chem. C, Vol. 111, No. 38, 2007 vibrational spectra that involve CO as a reactant, intermediate, or product requires careful consideration of the amount of coadsorbed water on oxide-supported Ru catalysts. Since the observed ∼100 cm-1 red-shift indicates that the CO bond is weakened in the presence of water, we conclude, in agreement with Lee et al.,14 that water may play a role in methanation by assisting CO bond cleavage. Acknowledgment. This research was supported by a Research Corporation Cottrell College Science Award (CC6552) and by a supplemental award from the Camille and Henry Dreyfus Scholar/Fellow Program for Undergraduate Institutions (SL-05-001). The PANalytical X’Pert Pro θ/θ XRD utilized in this research was funded by the National Science Foundation (DMR-0315345). Acknowledgment is also given to the National Science Foundation (DMR-0353773 and CHE-0353807) for undergraduate summer research stipends. BET surface area measurements were conducted in the laboratory of S. Ted Oyama at Virginia Polytechnic Institute and State University with the assistance of Pelin Hacarlioglu and Hankwon Lim. The authors would also like to acknowledge the Department of Chemistry and Center for Material Science at James Madison University for start-up funds and unlimited access to departmental equipment. Furthermore, we thank the Central Microscopy Research Facility, its director Kenneth C. Moore at the University of Iowa, and the National Science Foundation (CHE 0320387) for providing the opportunity to analyze our samples using XPS. Finally, we extend our gratitude to Jonas Baltrusaitis and Vicki H. Grassian for their time and effort in collecting the XPS data. References and Notes (1) Liao, M.-S.; Cabrera, C. R.; Ishikawa, Y. Surf. Sci. 2000, 445, 267282. (2) Dulaurent, O.; Nawdali, M.; Bourane, A.; Bianchi, D. Appl. Catal. A 2000, 201, 271-279. (3) Bourane, A.; Dulaurent, O.; Bianchi, D. Langmuir 2001, 17, 54965502. (4) He, R.; Davda, R. R.; Dumesic, J. A. J. Phys. Chem. B 2005, 109, 2810-2820. (5) Ortiz-Hernandez, I.; Willams, C. T. Langmuir 2003, 19, 29562962. (6) Ferri, D.; Burgi, T.; Baiker, A. J. Phys. Chem. B 2001, 105, 31873195. (7) Davda, R. R.; Shabaker, J. W.; Huber, G. W.; Cortright, R. D.; Dumesic, J. A. Appl. Catal. B 2003, 43, 13-26. (8) Utaka, T.; Takeguchi, T.; Kikuchi, R.; Eguchi, K. Appl. Catal. A 2003, 246, 117-124. (9) Rosso, I.; Galletti, C.; Saracco, G.; Garrone, E.; Specchia, V. Appl. Catal. B 2004, 48, 195-203. (10) Huber, G. W.; Shabaker, J. W.; Dumesic, J. A. Science 2003, 300, 2075-2077. (11) Valenzuela, M. B.; Jones, C. W.; Agrawal, P. K. Energy Fuels 2006, 20, 1744-1752. (12) Shabaker, J. W.; Davda, R. R.; Huber, G. W.; Corthwright, R. D.; Dumesic, J. A. J. Catal. 2003, 215, 344-352. (13) Shabaker, J. W.; Huber, G. W.; Davda, R. R.; Corthwright, R. D.; Dumesic, J. A. Catal. Lett. 2003, 88, 1-8. (14) Lee, H.-I.; Koel, B. E.; Daniel, W. M.; White, J. M. J. Catal. 1982, 74, 192-195. (15) Nakamura, M.; Ito, M. Surf. Sci. 2001, 490, 301-307. (16) Ikemiya, N.; Senna, T.; Ito, M. Surf. Sci. 2000, 464, L681-L685. (17) Lin, W. F.; Christensen, P. A.; Hamnett, A. J. Phys. Chem. B 2000, 104, 6642-6652.
Baird et al. (18) Lin, W. F.; Christensen, P. A.; Hamnett, A. J. Phys. Chem. B 2000, 104, 12002-12011. (19) Yajima, T.; Uchida, H.; Watanabe, M. J. Phys. Chem. B 2004, 108, 2654-2659. (20) Jin, J. M.; Lin, W. F.; Christensen, P. A. J. Electroanal. Chem. 2004, 563, 71-80. (21) Zheng, M.-S.; Sun, S.-G. J. Electroanal. Chem. 2001, 500, 223232. (22) Solymosi, F.; Rasko, J. J. Catal. 1989, 15, 107-119. (23) Burgi, T.; Wirz, R.; Baiker, A. J. Phys. Chem. B 2003, 107, 67746781. (24) Burgi, T.; Baiker, A. J. Phys. Chem. B 2002, 106, 10649-10658. (25) Burgi, T.; Bieri, M. J. Phys. Chem. B 2004, 108, 13364-13369. (26) Ferri, D.; Burgi, T. J. Am. Chem. Soc. 2001, 123, 12074-12084. (27) Ferri, D.; Burgi, T.; Baiker, A. Chem. Commun. 2001, 1172-1173. (28) Keresszegi, C.; Ferri, D.; Mallat, T.; Baiker, A. J. Phys. Chem. B 2005, 109, 958-967. (29) Ebbesen, S. D.; Mojet, B. L.; Lefferts, L. Langmuir 2006, 22, 10791085. (30) Burgener, M.; Tyszewski, T.; Ferri, D.; Mallat, T.; Baiker, A. Appl. Catal. A 2006, 299, 66-72. (31) Burgi, T. J. Catal. 2005, 229, 55-63. (32) Caravati, M.; Grunwaldt, J.-D.; Baiker, A. Appl. Catal. A 2006, 298, 50-56. (33) Caravati, M.; Meier, D. M.; Grunwaldt, J.-D.; Baiker, A. J. Catal. 2006, 240, 126-136. (34) Keresszegi, C.; Burgi, T.; Mallat, T.; Baiker, A. J. Catal. 2002, 211, 244-251. (35) Keresszegi, C.; Ferri, D.; Mallat, T.; Baiker, A. J. Catal. 2005, 234, 64-75. (36) Nyholm, R.; Martensson, N. J. Phys. C 1980, 13, L279-284. (37) Wu, Q.-H.; Thissen, A.; Jaegermann, W. Solid State Ionics 2004, 167, 155-163. (38) Wu, Q.-H.; Thissen, A.; Jaegermann, W. Appl. Surf. Sci. 2005, 252, 1801-1805. (39) Hoffmann, F. M. J. Chem. Phys. 1989, 90, 2816-2823. (40) Harrick, N. J. Internal reflection spectroscopy; Interscience Publishers: New York, 1967. (41) Stanislaus, A.; Evans, M. J. B.; Mann, R. F. J. Phys. Chem. 1972, 76, 2349-2352. (42) Zaki, M. I.; Knozinger, H. Mater. Chem. Phys. 1987, 17, 201215. (43) Chen, H.-W.; Zhong, Z.; White, J. M. J. Catal. 1984, 90, 119126. (44) Dalla Betta, R. A. J. Phys. Chem. 1975, 79, 2519-2525. (45) Davydov, A. A.; Bell, A. T. J. Catal. 1977, 49, 332-344. (46) Guerra, C. R.; Schulman, J. H. Surf. Sci. 1967, 7, 229-249. (47) Hadjiivanov, K. I.; Lavalley, J. C.; Lamotte, J.; Mauge, F.; SaintJust, J.; Che, M. J. Catal. 1998, 176, 415-425. (48) Kobayashi, M.; Shirasaki, T. J. Catal. 1973, 28, 289-295. (49) Nawdali, M.; Bianchi, D. Appl. Catal. A 2002, 231, 45-54. (50) Maroto-Valiente, A.; Cerro-Alarcon, M.; Guerrero-Ruiz, A.; Rodriguez-Ramos, I. Appl. Catal. A 2005, 283, 23-32. (51) Dulaurent, O.; Chandes, K.; Bouly, C.; Bianchi, D. J. Catal. 2000, 192, 273-285. (52) Mazzieri, V. A.; L’Argentiere, P. C.; Coloma-Pascual, F.; Figoli, N. S. Ind. Eng. Chem. Res. 2003, 42, 2269-2272. (53) Pourbaix, M. Atlas of Electrochemical Equilibria in Aqueous Solutions; National Association of Corrosion Engineers: Houston, TX, 1974. (54) CRC Handbook of Chemistry and Physics, 72 ed.; CRC Press: Boston, MA, 1991. (55) Kizhakevariam, N.; Jiang, X.; Weaver, M. J. J. Chem. Phys. 1994, 100, 6750-6764. (56) Yee, N. C.; Chottiner, G. S.; Scherson, D. A. J. Phys. Chem. B 2005, 109, 7610-7613. (57) Yee, N. C.; Chottiner, G. S.; Scherson, D. A. Langmuir 2005, 21, 10256-10259. (58) Ehlers, D. H.; Esser, A. P.; Spitzer, A.; Luth, H. Surf. Sci. 1987, 191, 466-478. (59) Hoffmann, F. M.; Lang, N. D.; Norskov, J. K. Surf. Sci. Lett. 1990, 226, L48-L50.
