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Langmuir 2006, 22, 3166-3173
Structure Sensitivity in the Oxidation of CO on Ir Surfaces Wenhua Chen,† Ivan Ermanoski,† Timo Jacob,‡ and Theodore E. Madey*,† Department of Physics and Astronomy and Laboratory for Surface Modification, Rutgers, The State UniVersity of New Jersey, Piscataway, New Jersey 08854, and Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14 195 Berlin-Dahlem, Germany ReceiVed NoVember 23, 2005. In Final Form: January 20, 2006 We report results on the catalytic oxidation of carbon monoxide (CO) over clean Ir surfaces that are prepared reversibly from the same crystal in situ with different surface morphologies, from planar to nanometer-scale facets of specific crystal orientations and various sizes. Our temperature-programmed desorption (TPD) data show that both planar Ir(210) and faceted Ir(210) are very active for CO oxidation to form CO2. Preadsorbed oxygen promotes the oxidation of CO, whereas high coverages of preadsorbed CO poison the reaction by blocking the surface sites for oxygen adsorption. At low coverages of preadsorbed oxygen (e0.3 ML of O), the temperature Ti for the onset of CO2 desorption decreases with increasing CO coverage. At high coverages of preadsorbed oxygen (>0.5 ML of O), Ti is 1700 K and subsequent cooling in O2 to 300 K.
2. Methodology 2.1. Experiment. Most of the data reported here are temperatureprogrammed desorption (TPD) spectra that are measured in an ultrahigh vacuum (UHV) chamber with a base pressure of ∼1 × 10-10 Torr, which has been described elsewhere.17 This chamber is equipped with a quadrupole mass spectrometer (QMS) capable of collecting up to 12 masses simultaneously, Auger electron spectroscopy (AES), and low-energy electron diffraction (LEED). Scanning tunneling microscopy (STM) images are acquired in a separate UHV chamber containing a variable-temperature STM, AES, and LEED.18 The Ir(210) crystal is spot-welded onto two Re ribbons that are attached to Mo rods for support and resistive heating. The temperature of the sample is measured with a W-5%Re/W26%Re thermocouple spot-welded to the back of the crystal. Sample heating is achieved by electron bombardment from a W filament behind the Ir(210) crystal as well as resistive heating from dc current passing through the sample. A linear heating rate of 5 K/s is used for all TPD measurements. Carbon monoxide (CO), hydrogen (H2), and oxygen (O2) are of research purity and are used without further purification. All gases are dosed from the background at a sample temperature of 300 K. All exposures shown are in Langmuir (1L ) 10-6 Torr‚s ) 1.3 × 10-4 Pa‚s) and are uncorrected for ion gauge sensitivity. In this work, two different types of catalytic surfaces are prepared in situ from the same Ir(210) crystal: clean planar Ir(210) and clean faceted Ir(210). Clean planar Ir(210) is prepared by cycles of flashing the sample to 1700 K in O2 (5 × 10-8 Torr) followed by flashing the sample to 1700 K in UHV. Clean faceted Ir(210) is obtained through two steps. In the first step, oxygen-covered faceted Ir(210) is prepared by heating clean planar Ir(210) in O2 (5 × 10-8 Torr) at a temperature g600 K; the facets are three-sided nanoscale pyramids exposing two different kinds of faces (one (110) face and two {311} faces on each pyramid) (Figure 1). For annealing temperatures >1200 K, facets form as the crystal cools below ∼1150 K in O2. The facet size increases with increasing annealing temperature, and the average facet size 〈l〉 ranges from 5 to 14 nm when the annealing temperature increases above 600 K.16,18 In the second step, clean faceted Ir(210) is generated by removing surface oxygen from oxygen-covered faceted Ir(210) via reaction with H2 (5 × 10-9 Torr) at ∼400 K while facets retain their original structure and size. The cleanliness of the surfaces is verified by AES and TPD, and the surface structure is monitored by LEED and STM. The LEED pattern from clean planar Ir(210) shows (1 × 1) structure, and the LEED pattern from clean faceted Ir(210) gives rise to extra LEED spots associated with the presence of facets.15,17 The spot sizes in LEED patterns from the faceted surfaces change with the facet size: the larger the facet size, the sharper the LEED spots.15 2.2. DFT Calculations. To characterize the adsorption sites of oxygen on Ir(210), density functional theory (DFT) pseudopotential (17) Chen, W.; Ermanoski, I.; Wu, Q.; Madey, T. E.; Hwu, H. H.; Chen, J. G. J. Phys. Chem. B 2003, 107, 5231-5242. (18) Ermanoski, I.; Kim, C.; Kelty, S. P.; Madey, T. E. Surf. Sci. 2005, 596, 89-97.
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Figure 2. Oxygen uptake curve obtained on the basis of the integrated area under the TPD spectra of O2 from Ir(210) following adsorption at 300 K. The inset contains the TPD spectra (5 K/s) of O2 following the adsorption of 0.5 L of O2 (∼0.3 ML of O) and 3 L of O2 (∼0.7 ML of O) on Ir(210) at 300 K. slab calculations were preformed using the CASTEP code.19 Throughout the calculations, optimized ultrasoft pseudopotentials20 and the generalized gradient approximation (GGA) exchangecorrelation functional suggested by Perdew, Burke, and Ernzerhof (PBE)21 were used. The surface was represented by a 16-layer slab with 13 Å vacuum. For each system, the bottom three layers were fixed at the calculated bulk-crystal structure (lattice constant a0 ) 3.899 Å), and the remaining Ir atoms and the adsorbates were allowed to relax freely. For the oxygen-adsorbed system, the cutoff energy was converged to 340 eV, and the Brioullin zone (BZ) sampling of the (1 × 1) unit cell was converted to an 10 × 8 Monkhorst-Pack mesh.22
3. Results 3.1. Adsorption and Desorption of O2 on Ir(210). The clean Ir(210) surface shows a (1 × 1) LEED pattern with sharp spots and low background intensity. Upon adsorption of oxygen at 300 K, the (1 × 1) LEED pattern persists but with an increase in the diffuse background intensity, which implies that the asdosed oxygen surface is disordered. Our detailed studies of adsorption and desorption of O2 on Ir(210) have been reported elsewhere.15 Here we mention only briefly the main results that are relevant for our studies of CO oxidation on Ir(210). Figure 2 shows the oxygen uptake curve, based on the integrated area under the TPD spectra of O2 from Ir(210), and two selected TPD spectra of O2 from Ir(210) as the inset; here one monolayer (ML) is defined to be the saturation coverage at ∼80 L dosed on the Ir(210) at 300 K. Oxygen adsorbs dissociatively on Ir(210) at room temperature. TPD data reveal the existence of two binding states of oxygen on Ir(210). For exposures e0.5 L (e0.3 ML of O), O2 desorbs in a single peak from the high-binding-energy state (A), and the peak position shifts to lower temperature as the exposure increases. With increasing exposures higher than 0.5 L (>0.3 ML of O), an additional O2 desorption peak associated with the low-binding-energy state (B) appears at lower temperatures. Notably, adsorbed oxygen does not desorb from the surface below 700 K for exposures e3 L (∼0.7 ML of O). 3.2. Adsorption and Desorption of CO from Ir(210). Upon adsorption of CO at 300 K, no additional LEED beams are observed compared to the LEED pattern from the clean Ir(210) surface taken at the same incident electron beam energy. However, (19) Segall, M. D.; Lindan, P. L. D.; Probert, M. J.; Pickard, C. J.; Hasnip, P. J.; Clark, S. J.; Payne, M. C. J. Phys.: Condens. Matter 2002, 14, 2717-2743. (20) Vanderbilt, D. Phys. ReV. B 1990, 41, 7892-7895. (21) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 38653868. (22) Monkhorst, H. J.; Pack, J. D. Phys. ReV. B 1976, 13, 5188-5192.
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Figure 3. TPD spectra of CO following the adsorption of 0.1 L (∼0.1 ML), 0.75 L (∼0.6 ML), and 3 L (∼1 ML) of CO on clean planar Ir(210) and clean faceted Ir(210) at 300 K. The clean faceted Ir(210) is prepared by flashing Ir(210) in O2 (5 × 10-8 Torr) to 1700 K and subsequent cooling in O2 to 300 K, followed by reaction with H2 at 400 K; the average facet size 〈l〉 is 14 nm.16
an increase in the diffuse background intensity is observed, which implies that the as-dosed CO surface is disordered. CO adsorbs molecularly on both planar and faceted surfaces. Molecular adsorption of CO has also been reported on other iridium surfaces (e.g., Ir(110),23,24 Ir(111),23,25,26 Ir(100),27 Ir(755),26 and polycrystalline Ir28). We have measured TPD spectra of CO following adsorption on clean planar Ir(210) and clean faceted Ir(210) at 300 K; all adsorbed CO desorbs completely from the surfaces below ∼650 K. Figure 3 directly compares three TPD spectra of CO from clean planar Ir(210) with those from clean faceted Ir(210), with an average facet size of 14 nm,16 at different CO exposures. It is evident that CO spectra from the two surfaces are very similar in terms of spectra profile, peak position, and shoulder appearance. However, there exists one notable difference in the CO spectra from the two surfaces: the CO spectra from the planar surface contain a “tail” on the low-temperature side compared to those from the faceted surface, and the tail becomes more pronounced with increasing CO exposure. The onset of CO desorption from the planar surface occurs at a temperature ∼100 K lower than that from the faceted surface for a CO saturation exposure of 3 L. The difference in CO spectra between the planar and faceted surface indicates the existence of extra adsorption states for CO with lower binding energies on planar Ir(210), but the amount of CO in these states is less than 10% adsorbed CO. 3.3. Oxidation of CO on Ir(210). When CO and oxygen are coadsorbed on clean planar Ir(210) and clean faceted Ir(210) at 300 K and then heated, the formation of carbon dioxide (CO2) is observed, indicating that both planar and faceted Ir(210) are active for the oxidation of CO to form CO2. We have prepared two different coadsorption systems of CO and oxygen: CO/O/ Ir(210) (Ir is predosed with O2 first and then exposed to CO) and O/CO/Ir(210) (Ir is predosed with CO first and then exposed to O2). We present our CO oxidation results for clean planar Ir(210) and clean faceted Ir(210) in the following two sections. Note that all data shown are for CO/O/Ir(210) except the data in Figure 6b for O/CO/Ir(210). 3.3.1. Oxidation of CO on Planar Ir(210). The TPD spectra given in Figures 4 and 5 illustrate the thermal desorption of reactively formed CO2 after different exposures of CO dosed onto planar Ir(210) preexposed to 0.2 L of O2 (0.1 ML of O), (23) Marinova, Ts. S.; Chakarov, D. V. Surf. Sci. 1989, 217, 65-77. (24) Lyons, K. J.; Xie, J.; Mitchell, W. J.; Weinberg, W. H. Surf. Sci. 1995, 325, 85-92. (25) Lauterbach, J.; Boyle, R. W.; Schick, M.; Mitchell, W. J.; Meng, B.; Weinberg, W. H. Surf. Sci. 1996, 350, 32-44. (26) Hagen, D. I.; Nieuwenhuys, B. E.; Rovida, G.; Somorjai, G. A. Surf. Sci. 1976, 57, 632-650. (27) Kisters, G.; Chen, J. G.; Lehwald, S.; Ibach, H. Surf. Sci. 1991, 245, 65-71. (28) Hooker, M. P.; Grant, J. T. Surf. Sci. 1977, 62, 21-30.
0.5 L of O2 (0.3 ML of O), 1.5 L of O2 (0.6 ML of O), and 3 L of O2 (0.7 ML of O), respectively. The integrated areas under the TPD spectra of CO2 and CO are also included in the Figures. For 0.1 ML of O (Figure 4a), at low CO exposure (0.09 L), the temperature Ti for the onset of CO2 desorption is ∼460 K and shifts to lower temperature with increasing CO exposure. The CO2 peak temperature also shifts to lower temperature as CO exposure increases. Moreover, the precovered oxygen is totally consumed for CO oxidation, and the amount of CO2 formed is limited by the amount of adsorbed oxygen (Figure 4b). With increasing precoverage of oxygen to 0.3 ML of O (Figure 4c), more CO2 is formed as seen by the increase in the area of CO2 spectra, but the amount of desorbed CO2 is still limited by the amount of adsorbed oxygen (Figure 4d). In addition, as CO exposure increases, Ti for CO2 desorption moves to lower temperature. At high oxygen precoverages of 0.6 ML of O (Figure 5a) and 0.7 ML of O (Figure 5c), Ti is
