In Situ Studies of Catalytic Reactions with Fourier TransformâInfrared

Chapter 13

Downloaded by NORTH CAROLINA STATE UNIV on October 12, 2012 | http://pubs.acs.org Publication Date: January 23, 1992 | doi: 10.1021/bk-1992-0482.ch013

In Situ Studies of Catalytic Reactions with Fourier Transform—Infrared Reflection—Absorption Spectroscopy Island Formation and Stability of Ultrathin Layers at High Pressure Friedrich M . Hoffmann and Mark D. Weisel Exxon Research and Engineering Company, Annandale, NJ 08801 Time-resolved Fourier Transform-Infrared Reflection Absorption Spectroscopy (FT-IRAS) can be used to follow in-situ catalytic reactions on single crystal surfaces at high pressure. This is illustrated with studies of the CO dissociation and methanation reactions on Ru(001), where the formation of adsorbate islands is observed in-situ at high pressure. Vibrational frequencies, intensities and lineshapes allow us to determine both local and total coverage of surface adsorbates, and to estimate island domain sizes. In the case of Cu-Ru surfaces, we are able to estimate domain sizes of copper islands and investigate their stability at high pressure. Time-resolved measurements allow us to investigate the kinetic implications of island formation for the CO disproportionation and methanation reactions over Ru(001). Lateral interactions and island formation of adsorbates on solid surfaces are important factors which determine the kinetics of surface reactions. The application of surface sensitive probes to well-defined surfaces has greatly contributed to the understanding of these interactions at low pressures. Island formation in catalytic reactions at higher pressures, on the other hand, remains largely unexplored due to the pressure limitation of most surface science probes (typically 10" Torr). Recently, Fourier Transform-Infrared Reflection Absorption Spectroscopy (FT-IRAS) has been applied to study catalytic reactions at higher pressures on polycrystalline foils (7) and single crystal surfaces (2-4) of bridging the pressure gap between surface science and catalysis at high pressures. Here, we review some examples, which demonstrate the in-situ observation of island formation during catalytic reactions. IRAS has been utilized in the past to study intermolecular interactions between adsorbed molecules. Early infrared studies of CO adsorption on metal surfaces have revealed large frequency shifts as a function 5

0097-6156/92/0482-0202$06.00/0 © 1992 American Chemical Society In Surface Science of Catalysis; Dwyer, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.


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of surface coverage as discussed in detail earlier (5). Two effects contribute to this shift. First, the sensitivity of the molecule with respect to its "chemical" or "electronic" environment, i.e. the adsorption site on the metal and/or the presence of a coadsorbed atom or molecule which electronically modifies the adsorption site. Second, the intermodular interaction within the adsorbate layer. The latter causes coverage dependent frequency shifts both as a result of vibrational coupling and of electronic interaction among neighboring molecules. Therefore, frequency shiftsfrom intermolecular interaction are a sensitive probe of the intermolecular spacing in an adsorbate layer. This fact has been exploited in several U H V studies of C O island formation (6,7) and C O compression phases (8,9). Experimental Description 1 0

The experiments were performed in a multilevel U H V chamber ( l x l O Torr), shown in Figure 1, which was equipped for Infrared Spectroscopy, L E E D , Auger Spectroscopy, Thermal Desorption Mass Spectroscopy and workfunction measurements (Kelvin probe). Infrared Spectroscopy was performed at a high pressure level, which could be isolated from the U H V chamber with a gate valve. This permits insitu FT-IRAS at gas pressures from 10" torr to 1000 torr. Reaction products can be measured either with a Gas Chromatograph or by IR gas-phase spectra. Infrared spectra were obtained with a Perkin Elmer 1800 FTIR and a wide band M C T detector in single reflection with an angle of incidence of 80°. Typically, 100 scans at 4 cm" resolution were added in a total measurement time of 50 seconds and ratioed against the background from the clean surface. In the "fast" time-resolved mode, spectra were obtained at 300 msec per scan and a spectral resolution of 8 cm" . A typical experiment proceeded in the following manner. The Ru crystal was cleaned and characterized for surface cleanliness according to procedures described elsewhere in detail (7). Subsequently, adlayer films (e.g. Cu) were prepared by insitu evaporation and characterized for surface coverage and cleanliness. After the film preparation, the sample was translated to the high-pressure reactor level. After an IR background spectrum of the clean crystal was obtained, the surface was exposed to the reaction gases by increasing the pressure to the desired value. A l l the reactant gases were of high purity; however, CO was further purified by flowing the gas through a liquid nitrogen trap to remove residual carbonyl (particularly Ni(CO) ). Following the high pressure IR experiments, the sample was cooled to room temperature. Then the high pressure cell was evacuated to < 10" Torr (typically in < 2 minutes) and the crystal was reintroduced to the U H V chamber. There, postreaction Multiple Mass Thermal Desorption Spectroscopy, L E E D and Auger Spectroscopy spectroscopy were performed to characterize the state of the surface after reaction. 10

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Island Formation and the Stability of Adlayers The fact that lateral interactions within molecular adlayers result in vibrational frequency shifts can be used to determine the intermolecular spacing within an adsorbed C O layer (5). This allows us to determine surface coverages in-situ during reaction at high pressures. The case of CO/Ru(001) is in this respect ideal, since CO

In Surface Science of Catalysis; Dwyer, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

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Figure 6. The formation of copper islands demonstrated with vibrational spectra of CO adsorbed on a Cu-Ru(OOl) surface (0 = 0.45) which has been annealed successively to higher temperature (75): (a) Randomly dispersed copper after evaporation at 85 K ; (b) Formation of 3D- and 2D- copper aggregates after annealing to 250 K; (c-d) Annealing above room temperature produces pseudomorphic 2D-copper islands exposing large domains of bare ruthenium substrate. Cu

In Surface Science of Catalysis; Dwyer, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

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In Situ Studies of Catalytic Reactions with Fourier TransformâInfrared

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In Situ Studies of Catalytic Reactions with Fourier TransformâInfrared