J. Phys. Chem. 1993, 97, 1258-1261
1258
Infrared Reflection-Absorption Spectroscopy of Adsorbates on a Pt ( 100) Surface during CO Oxidation Seongsik Hongt and Hugh H. Richardson'** Physics and Chemistry Departments, Condensed Matter and Surface Science Program, Clippinger Research Laboratories, Ohio University, Athens, Ohio 45701 - 2979 Received: June 30, 1992; In Final Form: December 28, 1992
The infrared spectra of adsorbates and partial pressures of CO, 0 2 , and C02 taken in situ during C O oxidation on a Pt( 100) surface are reported. At 400 and 430 K three bands are observed in the infrared spectrum. The first band at 1630 cm-l is assigned to a complex involving adsorbed C O and adsorbed oxygen. This complex is a precursor to C02 formation. The second and third bands at -2087 and 1880 cm-l are assigned to C O adsorbed on top and bridge sites ordered in a ( d 2 X d 2 ) R 4 S 0 adlayer. At 460 K the [CO-01 complex band is not seen in the infrared spectrum because the rate of reaction of the complex is much faster than the rate of complex formation. Small islands of adsorbed C O (-10 A on an edge) are inferred from the infrared spectrum in the reaction rate region of high C02 production. The variation of the [CO-O] complex band with temperature and C O island size is presented. The [CO-01 complex band sharpens with decreasing temperature and increasing C O island size.
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The investigation of chemical reaction with model catalysts has given a fundamental conception of how molecules interact with catalysts to form products.] The oxidation reaction of CO to C02 on group VI11 metals is one of the most thoroughlystudied heterogeneous catalytic reactions to date.2-26 This catalytic reaction shows many complicated behaviors, including selfsustained kinetic o~cillations,~-5~7-~ 1 . 1 3 ~kinetic ~ phase transitions,21,22 dissipative structure~,~-53~-l I and gaseous products with excess translational and vibrational energy.16J7J9,27q28 Very little, however, is known about the reaction intermediateson the surface. In this Letter we present the first spectroscopicmeasurement of a reaction intermediate formed during the reaction of CO and O2on a Pt( 100)surface. These results are taken in situ by probing the surfaceadsorbatesof a continuousflow reactor with reflectionabsorption Fourier transform infrared spectroscopy. The behavior of the intermediate and island growth of an adlayer of CO on Pt( 100) surface maintained at different temperatures is presented. The Pt crystal is situated in an ultrahigh-vacuum (UHV) continuous flow reactor and CO is introduced with increasing partial pressure into a constant flow of 02.A stationary state is achieved, and data are collected on the adsorbates and the gas-phase composition. The flow rate of CO is increased to yield another stationary state where data collection is initiated again. This procedure is continued until an unstable stationary point is obtained where time-resolved data are collected with no change in the flow rates of the reactant gases. The samplechamber in a fast-flow condition is directly pumped with a turbomolecular pump, and only a small portion of the gases is leaked into the main chamber for mass analysis. The pumping speed in the fast-flow experiments is 60 L/s. In the slow-flow experiments, all of the gas is pumped into the main chamber for mass selective analysis. The mass currents from CO, 02, and C02 gases eluted from the sample chamber are traced by the quadrupole mass spectrometer housed in the main chamber. The rate of C02 production is related to the parent peak of C02 in the mass spectrum of the eluted gases. The ion gauges are cut off during data collection because the hot filaments also catalyze CO oxidation. Infrared reflection-absorption spectra of the Pt sample during CO oxidation are collected with a Mattson Sirius 100 spectromTo whom correspondence should be addressed. Physics Department. f Chemistry Department.
t
0022-3654/93/2097-1258%04.00/0
eter. The partial pressure of CO is varied, and the rate of C02 production is correlated with the adsorbed molecules. The Pt surface is cleaned with Ar ion bombardment, annealing, and reaction at 500 K with pure 0 2 gas. The oxygen pressure is Torr. Infrared spectra of CO on Pt( 100) at 300 K give nearly identical spectra to previously published infrared spectra of CO on a clean Pt(100) surface at 300 K.29*30Our apparatus and methods will be described in more detail e l ~ e w h e r e . ~ ~ Figure 1 correlates the peak absorbance of a vibrational band at 1630 cm-I (complex band) and a vibrational band at 2089 cm-1 (C0,d band (top site)) to the sample chamber pressures of CO, 02,and C02. The temperature of the Pt sample is 400 K, and the data are collected under slow-flow conditions. The pumping speed is 0.088 L/s. CO gas is introduced at spectrum zero and the partial pressure slowly increased. The 0 2 partial pressure increases because CO displaces 0 2 from the UHV chamber walls. At spectrum 15, the flow rate of CO is not changed, but the partial pressure and infrared measurements show a spontaneousevolution of the reaction to spectrum 18. The point of maximum C02 production is labeled as point B in Figure 1. Figure 2 shows the three-dimensional time-evolved infrared reflection-absorption spectra of the Pt( 100) surface at 400,430, and 460 K as the partial pressureof CO is varied. The 0 2 pressure in the test chamber is initially constant with the flow from the 0 2 leak valve held constant during data collection. The initial 0 2 pressure at 400, 430, and 460 K is 5.4 X 5.23 X lW5, and 5.20 X Torr, respectively. The background CO partial pressure is less than 4% of the initial total pressure. Data acquisition is started when CO gas is admitted into the test chamber. ThepartialpressureofCOisincreasedata ratebetween Torr min-I. The spectra are numbered, 4X and 1.O X and the time in minutes is displayed on the z axis. The data are collected under fast-flow conditions. At 400 K the infrared absorbance spectra (taken with 4-cm-1 resolution and 100 scans) exhibit three characteristic bands. The first band at 1630 cm-1 is observed when CO gas is admitted into the test chamber at an increasing rate of -4 X lo-* Torr min-I. This band is tentatively assigned to a reaction complex between adsorbed CO and adsorbed oxygen. In this interpretation the loss of adsorbed oxygen leads to the disappearance of the 1630-cm-l band. Hereafter, the 1630-cm-I band is referred to as the [CO-O] complex band.
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0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 7, 1993 1259
Letters
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Figure 1. Partial pressures of CO, 02,and CO2 correlated with the peak height of the complex band and the C0,d band (top site). The platinum temperature is 400 K, and the data are collected with a 0.088 L/s pumping speed. WAVENUMBERS ( 1 /CM)
One alternative assignment is that the band at 1630 cm-I is from CO adsorbed in a 3- or 4-fold site on the surface.32 In this case adsorbed oxygen modifies the surfaceso that CO is adsorbed on the less favorable 3- or 4-fold site. The stretching frequency for CO on a high symmetry site is expected to be a t a lower frequency than CO adsorbed on the top or bridge site. Another possibility is that the 1630-cm-I band is from CO adsorbed on or near a surface contaminant. Surfacecontamination is detected in the infrared spectrum as a decrease in the band area of the saturated C0,d band. The intensity of the saturated C0,d band compared to theoretical estimates shows that the amount of contaminated surface is low. Hence, it is unlikely that the 1630-cm-I band is from CO adsorbed on or near a surface contaminant. The second and third bands are a t 2087 and 1880 cm-I. These bands have been previously assigned to CO adsorbed on top and bridge sites respectively in a (d2Xd2)R45O ordered 0verlayer.2~ The bands at 2087 and 1880 cm-1 do not shift with increasing coverage which suggests that CO grows on the surface as islands of a (d2Xd2)R4S0 ordered overlayer. The adlayer of CO can only be removed by a manual decrease in the CO partial pressure (spectra 34-45). At 430 K the same three bands are also observed. The intensity of the band from the [CO-O] complex is greatly reduced, and at 460 K, the vibrational band from the [CO-01 complex is not detected in the infrared spectrum. The change in the surface concentration of the [CO-O] complex with respect to time is given in eq I where 8co is the surface coverage of adsorbed CO d[CO-O]/dt = k,8,,0,
- k,[CO-01
(1)
and 80 is the surface coverage of adsorbed oxygen. The rate constant kq is a first-order rate constant which is related to the desorption of the [CO-O]complex into COz gas. kj is a secondorder rate constant which relates back to the rate that CO molecules at the perimeter of the CO adlayer find adsorbed oxygen.
'loo
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5 2 ~
1800 1WAMNUMBERS (1/CM)
iioo-
Figure 2. Three-dimensionaltime-evolvedinfrared reflection-absorption spectra of the Pt(100) surface at 400, 430, and 460 K as the partial pressure of CO is varied.
The steady-state concentration of the [CO-O]complex is just [ k 3 / k 4 ] 8 C ~A ~ osteady-state . concentration of the [CO-01 complex can be maintained because in the flow system new reactants are being introduced at the same rate that products are being removed. A large temperature dependence is expected for k4 with a much smaller temperature dependence for kj. The temperature dependence in the infrared spectrum can be analyzed in terms of a larger increase in k4 relative to k3 with increasing temperature. The steady-state concentration of the [CO-O] complex a t 460 K is reduced below the detection limit of the Fourier transform infrared spectrometer and is not detected in the infrared spectrum. C 0 2 gas production is high in the region where the [CO-O] complex band is observed and low in the region where infrared bands from the CO adlayer are observed (see Figure 1). An infrared band from a reaction intermediate involving adsorbed CO and adsorbed oxygen is direct spectroscopic confirmation of
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1260 The Journal of Physical Chemistry, Vol. 97, No. 7 , 1993
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Figure 3. (A) A 25 A on an edge island of adsorbed CO. Adsorbed CO is shown as the filled circles, adsorbed oxygen as the partially filled circles, and platinum atoms ( - 3 A in diameter) as the larger nonfilled circles. (B) A 10 A on the edge island of adsorbed CO.
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O,(gas) CO(ad)
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+ O(ad)
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k,, k-,
. d
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the Langmuir-Hinshelwd mechanism. This mechanism can be diagramed as follows:
2000 1800 1 600 WAVENUMBERS (1/CM)
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[CO-O](ad) k,
(3) 29
The intensity of the [CO-O] complex band, in the region of high C 0 2 production, saturates and then decreases even though the rate of C 0 2 production increases. If k4 is constant, then the [CO-O] complex band intensity should increase with increasing C 0 2 production. This is clearly not the case. However, the bandwidth of the [CO-O] complex band does change with intensity (see Figure 4 and following discussion). This suggests that the rate constant for the conversion of the complex into C02 gas does change with coverage. It is possible that the apparent lack of correlation between the intensity of the 1630-cm-i band and C 0 2production rate is caused by a coverage dependence in
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k4.
At 400 K it is apparent from Figure 1 that, in the region of high C02 production, the infrared spectrum has only a band from the [C W ] complex. There are no infrared bands observed for CO adsorbed on top or bridge sites in an ordered overlayer structure. This observation limits the possible size of the CO cluster on the surface. If the CO cluster size is 25 A on an edge with an overlayer structure of ( 4 2 X 4 2 ) R 4 5 ' , then there are an equal number of adsorbed CO on the border of the cluster as there are in the interior of the cluster. This cluster is shown in Figure 3a. Only adsorbed CO at the perimeter of the cluster can complex with adsorbed oxygen. Adsorbed CO in the interior of the cluster will give bands at 2087 and 1880 cm-I. Because a vibrational band is only observed from the [CO-O] complex, a CO cluster size of 10 A on an edge is expected in the region of high C02 production. This cluster is shown in Figure 3b. When the partial pressureof CO is increased to a critical point, ordered overlayergrowth of CO occurs. Islands of adsorbed CO grow in size and eventually cover the surface. At this point the production of C02 is inhibited by the adlayer of CO, and the low CO2 rate branch of the catalytic reaction is observed. 'Lowering the partial pressure of CO reverses this process. Oxygen is adsorbed on the surface, and the CO adlayer is removed by reacting with adsorbed oxygen. An infrared band from the [CO-O] complex is again observed with the disappearance of the infrared bands from the CO adlayer. Figure 4 shows the changes in the frequency and intensity of the [CO-O] complex band with temperature and overlayer
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2, 2000 1 800 1600 1400 WAVENUMBERS (1/CM) Fipre4. Variation in the [CO-O] complex band with temperature (400, 430, and 460 K) and CO island growth (bands at -2087 and -1880 cm-I). -
0 2200
growth. In the region where the CO adlayer is not detected, the [CO-O] complex band is broader and the peak frequency is higher.
Letters
The Journal of Physical Chemistry, Vol. 97, No. 7, 1993 1261
As the intensity of the band from CO adlayer increases,the [COO] complex band shifts to a lower frequency and the bandwidth narrows. In general, there are two possible explanations for this behavior. The first possible interpretation is that the broad [COO] complex band observed is from variations in the local environment of the [CO-O] complex. These variations depend upon the island size of the CO adlayer. When the island size increases, the [CO-O] complex is localized into nearly identical environments at the boundary of the CO island. The [CO-O] complex band narrows and shifts to a lower frequency. The second possible interpretation is that the bandwidth is not from heterogeneous broadening but from homogeneous broadening. In thiscase thevibrationallifetimeofthe [ C W ] complex is shortened by reaction to form gaseous C02. The initial band for the [CO-O] complex is broad because the rate of reaction is greater when the island size is smaller. When the islands of CO grow, the [CO-O] complex band narrows because the rate of reaction decreases. At 430 K, the [CO-O] complex band is broader because the rate of reaction increases with temperature. By 460 K the rate of reaction of the [CO-O] complex is so fast that an infrared band from the complex is not observed. This interpretation can be tested by estimating the effect that a reasonable rate of reaction has upon the [CO-O] complex bandwidth. The minimum binding energy of the [CO-O] complex is -20 kJ/mol. This is the binding energy of CO2 on platinum.I2 The binding energies of adsorbed CO and adsorbed oxygen are 120-170 and 170-200 kJ/mol respectively.I2 Assuming a frequency factor of 1013 s-I and using a binding energy of 20 kJ/mol yields a value for k4 of 2 X 1O1Os-l at 400 K. The lifetime of the excited vibrational state ( 7 ) is the reciprocal of kq and is related to the bandwidth of the complex by ~'[CO-OI = [ ~ ? F c T ] - ~ . When k4 is 2 X 1Olo s-I, then T is 50 ps and the bandwidth is 0.1 cm-I. The contribution of lifetime broadening to the observed bandwidth is negligible compared to the observed bandwidth. Hence, the narrowing of the [CO-O] complex band with CO island growth cannot be from a shortening of the vibrational lifetime of the [CO-O] complex from reaction. In this Letter we present infrared spectra of adsorbates and partial pressures of CO, 02, and C02 taken in situ during CO oxidation on a Pt( 100) surface. At 400 and 430 K, three bands are observed. The first band is at 1630 cm-I and is tentatively assigned to a complex involving adsorbed CO and adsorbed oxygen. The second and third bands are at 2087 and 1880 cm-I. These bands are assigned to CO on top and bridge sites ordered in a (d2Xd2)R45O adlayer, respectively. At 460 K the [COO] complex band is not observed because the rate of desorption of the complex is much faster than the rate of complex formation. The steady-state surface concentration of the [CO-O] complex at 460 K is below the detection limit of the Fourier transform
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infrared spectrometer. In the CO partial pressure region of high C 0 2 production, small clusters of adsorbed CO (- 10 A on an edge) are inferred from the infrared spectrum. The infrared spectrum shows only a band from the [CO-O] complex. Larger clusters give infrared bands from both the CO adlayer and from the [CO-O] complex. The variation of the [CO-O] complex band with temperature and CO island size is reported. The [COO] complex band narrows with decreasing temperature and CO adlayer growth.
References and Notes (1) Sault, A. G.; Goodman, D. W. Adu. Chem. Phys. 1989,76, 153. (2) Ertl, G. Science 1991, 254, 1750. (3) Rotermund, H. H.; Engel, W.; Jakubith, S.; von Oertzen, A. Ertl, G. Ultramicroscopy 1991,36, 164. (4) Sander, M.; Imbihl, R.; Ertl, G. J . Chem. Phys. 1991. 95, 6162. (5) Rotermund, H. H.; Jakubith, S.; von Oertzen, A.; Ertl, G. Phys. Reu. Lerr. 1991,66,3083. (6) Hoffmann, F. M.;Weisel, M. W.; Peden, C. F. H. J . Eleciron Specrrosc. Relat. Phenom. 1990, 54,55,779. (7) Rotermund, H.H.;Jakubith, S.;Kubala, S.;von Oertzen, A,; Ertl, G. J . Elecrron Soectrosc. Relat. Phenom. 1990. 52. 8 11. (8) Jakubitii, S.;Rothermund, H. H.; Engel, W.; von Oertzen, A,; Ertl, G . Phys. Rev. Lerr. 1990, 65,3013. (9) Rotermund, H. H.; Engel, W.; Kordesch, M.; Ertl, G . Nature 1990, 343,355. (IO) Ehsasi, M.; Seidel, C.; Ruppender, H.; Drachsel, W.; Block, J. H.; Christmann. K. Surf Sci. Lert. 1989. 210. L198. (1 1) Rotermund; H. H.; Jakubith,'S.; von Oertzen, A.; Ertl, G. J . Chem. Phys. 1989,91,4942. (12) Berlowitz, P. J.; Peden, C. H. F.; Goodman, D. W. J . Phys. Chem. 1988,92,5213. 113) Burrows. V.A. J . Elecrron SDectrosc. Relat. Phenom. 1987.45.41. (14) Burrows; V. A.; Sundaresari, S.; Chabal, Y. J.; Christman, S. B. Surf. Sei. 1987, 180, 110. (15)Raz0n.L. F.;Schmitz,R.A.Caral.Reu.-Sci. Eng. 1986,28,89and references therein. (16) Bernasek. S. L.: Leone. S. R. Chem. Phvs. Left. 1981.84. 401. (17) Mantell, D.A.; Ryali, S.B.; Halpern, B:L.; Haller, G: L.;'Fenn, J. B. Chem. Phys. Lerr. 1981,81, 185. (18) Campbell, C. T.; Ertl, G.; Kuipers, H.; Segner, J. J . Chem. Phys. 1980,73, 5862. 119) Becker. C. A.: Cowin. J. P.: Wharton. L.: Auerbach, D. J. J . Chem. Phys. i977,67,3394. (20) Langmuir, I. Trans. Faraday Soc. 1921, 17,621. (21) Mai, J.;vonNeissen, W.;Blumen,A. J.Chem. Phys. 1990,93,3685, (22) Ziff, R. M.; Gulari, E.; Barshad, Y. Phys. Reu. Lett. 1986,56,2553. (23)Moller, P.; Wetzl, K.; Eiswirth, M.; Ertl, G. J . Chem. Phys. 1986, 85. 5328. (24)Imbihl, R.; Cox, M. P.; Ertl, G.; Muller, H.; Brenig, W. J . Chem. Phys. 1985,83,1578. (25) Thiel, P. A.; Behm, R. J.; Norton, P. R.; Ertl. G. J . Chem. Phys. 1983,78,1448. (26) Norton, P. R. Surf. Sci. 1974. 44,624. (27)Matsushima, T. Surf. Sci. 1983,127,403. (28) Matsushima. T. J . Carol. 1983,83,446. (29) Gardner, P.; Martin, R.; Tushaus, M.; Bradshaw, A. M. J . Elecrron Specrrosc. Relat. Phenom. 1990, 54f 55,619. (30)Crossley, A.; King, D. A. Surf. Sci. 1980,95, 131. (31)Hong, S.;Richardson, H. H. In preparation. (32)Ellis, T. H.; Kruus, E. J.; Wang, H. Surf. Sci. 1992,273,73. .
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