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J. Phys. Chem. B 2004, 108, 14270-14275
A CO2 Surface Molecular Precursor during CO Oxidation over Pt{100}† J. H. Miners,‡ P. Gardner,§ A. M. Bradshaw,‡,⊥ and D. P. Woodruff*,| Fritz-Haber-Institut der Max-Plank-Gesellschaft, Faradayweg 4-6 D-14195, Germany, Department of Chemistry, Faraday Building, UMIST, Manchester M60 1QD, U.K., and Department of Physics, The UniVersity of Warwick, CoVentry CV4 7AL, U.K. ReceiVed: January 15, 2004; In Final Form: March 5, 2004
Using different isotopologues of the reactant gases CO and O2, infrared reflection absorption spectroscopy (IRAS) has been used to investigate the transient surface species on the Pt{100} surface under reaction conditions which was first shown to give rise to an absorption band around 1630 cm-1 by Hong and Richardson (J. Phys. Chem. 1993, 97, 1258). The results show that this band cannot be attributed to a C-O stretching frequency of the CO from the gas-phase incorporated into a CO-O surface complex, such as that identified as the transition state in recent density-functional theory (DFT) calculations of the Pt{111}/CO + O2 and Pt{100}/CO + NO reactions. The IRAS results are consistent, however, with a surface O-C-O species of low symmetry in which the IR band is due to a C-O stretching mode involving an O atom arising from the molecular O2, and estimates of the desorption energy of this species show it is chemisorbed. This surface intermediate may also be involved in the CO + NO oxidation reaction over Pt{100}, but the steady-state coverage at the higher reaction temperature would preclude its observation in IRAS. The results suggest that further DFT calculations exploring alternative reaction paths may be of value.
1. Introduction The CO oxidation reaction on Pt{100} has attracted considerable interest, in part because under appropriate conditions it exhibits temporal oscillations in the reaction rate and spatial pattern formation associated with the two states of the surface between which these oscillations occur.1 Despite the extensive investigations which have resulted from this interest, however, information regarding the nature and coverage of the surface species during reaction conditions is relatively sparse, mainly because most surface spectroscopies which are capable of providing this information on surfaces cannot be used at the elevated pressures of the reaction conditions. Optical probes are the exception, and infrared reflection absorption spectroscopy (IRAS) is both applicable and highly informative. IRAS was first applied to this problem on a Pt foil by Burrows et al.2 who measured the variation in the intensity of a C-O stretching band, in the frequency range characteristic of atop adsorption, concurrently with the rate of CO2 production, while Schu¨th and Wicke3 also investigated the variation in the intensity of the C-O stretching bands characteristic of both atop and bridging adsorption sites on an oxide-supported Pt catalyst under oscillatory conditions. More recently, Hong and Richardson undertook a series of IRAS measurements under steady-state conditions on Pt{100} (initially in the “hex” clean surface reconstruction), in which a partial pressure of 7 × 10-5 mbar O2 was applied and the CO partial pressure was then gradually increased.4 At 400 K they observed that the rate of CO2 production initially increases approximately linearly with the †
Part of the special issue “Gerhard Ertl Festschrift”. * Corresponding author: D.
[email protected]. Fritz-Haber-Institut der Max-Plank-Gesellschaft. § UMIST. | The University of Warwick. ⊥ Present address: Max-Planck-Institut fu ¨ r Plasmaphysik, Boltzmannstrasse 2, D-85748 Garching, Germany. ‡
increasing CO partial pressure, then reaches a plateau, and finally drops rapidly to zero. Coinciding with this final fall in reactivity, IRAS showed the growth of adsorption bands at 1880 cm-1 and 2087 cm-1 associated with the C-O stretching vibrations of bridged and atop CO, respectively. This drop in reactivity can thus be attributed to the creation of a surface with a high CO coverage which blocks sites for O2 dissociative adsorption and thereby poisons the surface, in a manner similar to that described by Ertl et al.5 Interestingly, at both 400 and 430 K, they also observed the development of an IRAS band at around 1630 cm-1, which grew in intensity as the applied CO partial pressure and CO2 production rate grew, but which dropped rapidly to zero when the reaction rate fell and the C-O bands appeared. At 460 K, the 1630 cm-1 absorption band was no longer observed and features due to atop and bridged CO were observed immediately after the (initial) application of a pressure of ∼1 × 10-7 mbar CO. Hong and Richardson tentatively assigned the absorption band at 1630 cm-1 as being due to the presence of some kind of (CO-O) complex, a precursor to CO2 formation.4 This interpretation has been somewhat clouded, however, by an IRAS study of the NO + CO reaction over Pt{100} by Magtoto and Richardson.6,7 They performed a series of experiments at 470 and 500 K in which they first exposed the reconstructed Pt{100} hex surface to a constant partial pressure of NO and waited for all the adsorbed NO to dissociate, a procedure which results in an oxygen-covered surface.8 Subsequent application of a partial pressure of CO then resulted in an absorption band being observed at 1630 cm-1 which they initially assigned to the same (CO-O) complex which is observed in the CO + O2 reaction.6 Notice that in these two distinct experiments involving NO + CO and CO + O2 reaction, the starting point in both cases was a surface covered in atomic oxygen (from NO or O2 dissociation) which was exposed to increasing partial pressures of CO, albeit in the presence of
10.1021/jp0497918 CCC: $27.50 © 2004 American Chemical Society Published on Web 04/24/2004
CO2 Precursor during CO Oxidation over Pt{100} different (NO and O2) oxidizing gases, so a similar surface reaction intermediate could well be involved in both experiments. Subsequently, however, Magtoto and Richardson7 conducted further experiments with NO + C18O and 15N18O + CO; they found that the use of C18O results in no shift in the 1630 cm-1 band, whereas using 15N18O shifted the band to 15551545 cm-1. On this basis they finally concluded that the 1630 cm-1 band seen in these NO + CO experiments was due to a surface NO species and not to the (CO-O) complex. Of course, a N-O stretching band on this surface in the range 1600-1640 cm-1 is well-known,9-12 but one might not have anticipated that there would be a significant coverage of undissociated NO on the surface at these high temperatures, even under reaction conditions. The conclusion of this work that the 1630 cm-1 band in the NO + CO reaction is associated with adsorbed NO led the authors to raise the possibility that this same interpretation may apply to the CO + O2 reaction of the same surface, presumably due to some contamination. While this interpretation of the CO + O2 results in terms of contamination would be rather surprising, it does underline the doubt as to the true identity of this absorption band in the CO + O2 reaction over Pt{100}. The problem is also of interest in the light of more recent theoretical density functional theory (DFT) calculations of the microscopic details of the CO + O2 reaction over Pt{111}13,14 and the CO + NO reaction over Pt{100}.15 In both cases the transition state identified involves the creation of a weakly adsorbed bent CO2 species with one O atom and the C atom in off-atop sites (such that this C-O axis is essentially parallel to the surface) while the second O atom produces a C-O axis somewhat tilted from the surface normal. As the 1630 cm-1 vibrational band falls within the expected range of a CdO stretching frequency, such a species might account for the IR band reported by Hong and Richardson. In an attempt to clarify this matter further, we have conducted new IRAS experiments in which we have investigated the influence of several different isotopologues of the reactants on the observed vibrational frequency. Our results provide clear support for the existence of an adsorbed O-C-O precursor to CO2 production, but also show that the IR data are clearly not consistent with the specific transition state identified by the DFT calculations. 2. Experimental Details and Results The experimental setup has been described previously.12 Briefly, it consists of a commercial FTIR spectrometer (Biorad FTS-60A/896), modified for operation under low vacuum (∼1 × 10-3 mbar), interfaced to a UHV chamber equipped with rear-view LEED optics, a movable quadrupole mass spectrometer, and facilities for argon ion sputtering. IRAS spectra were recorded using a liquid-nitrogen-cooled narrow-band MCT detector. The nominal spectral range of this detector is 8003500 cm-1, though it is only truly stable for values greater that 950 cm-1. The Pt{100} crystal was mounted on a liquidnitrogen-cooled coldfinger and could be resistively heated such that temperatures between 85 and 1500 K could be maintained to a stability of 0.05 K. The temperature was measured using a K-type thermocouple spot-welded directly to the top of the crystal, the signal from which is fed back into the computercontrolled temperature control unit. The sample gases used in the experiments were of the highest commercially available purity: 99.997% for CO (Linde), and 99.999% for O2 (MesserGriesheim). The partial pressures quoted in each experiment have been corrected for the different ion gauge sensitivities of the respective gases, i.e., SCO/SN2 ) 1.0 and SO2/SN2 ) 0.8 were
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Figure 1. Infrared reflection absorption spectra recorded from a Pt{100} surface (initially in the reconstructed “hex” phase) held at 400 K in a partial pressure of 5 × 10-5 mbar O2. During the course of recording the spectra, a CO partial pressure was introduced and raised incrementally from 5 × 10-7 to 3 × 10-6 mbar.
used as correction factors. The UHV chamber routinely achieved a base pressure of 1 × 10-10 mbar. Effects due to chamber history and the possibility of contamination with N-containing species were minimized by baking at 120 °C for 5 days, followed by flushing the chamber with 10-6 mbar oxygen for 2 days, prior to making the measurements. The crystal was cleaned prior to each experiment by repeated cycles of argon ion sputtering at 750 K, followed by annealing at 1000 K. The procedure used for the IRAS experiments reported here was as follow. The Pt{100}sample (with the surface initially in the reconstructed “hex” phase) was kept at 400 K and a background spectrum acquired, prior to exposure to a constant partial pressure of O2 in the range (1-5) × 10-5 mbar, as monitored by a Viscovac VM 212 spinning rotor gauge, which has the advantage over an ion gauge that it does not cause molecular dissociation or fragmentation. The gas pressure was left to stabilize over an hour, leading to an exposure of at least 0.4 mbar s O2 which should ensure that the “hex” reconstruction is lifted over the entire surface and a saturation oxygen coverage in the range 0.44 < Θ < 0.6 ML was reached.16-19 The CO partial pressure was then gradually increased. Note that the partial pressures of CO quoted here are based on prior calibration of the leak valve settings using CO as the inlet gas and an ion gauge to measure the pressure; this procedure avoided the need to have the ion gauge operating during the surface reaction, thereby removing one potential source of contamination. The initial experiments were undertaken to establish that we could reproduce the results of Hong and Richardson;4 the results, shown in Figures 1 and 2, for experiments with O2 partial pressure of 5 × 10-5 mbar and 1 × 10-5 mbar, respectively, show the behavior reported by these authors. Notice, as also seen in the data of Hong and Richardson, that the exact frequency of the nominal 1630 cm-1 band
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Miners et al.
Figure 2. Infrared reflection absorption spectra recorded from a Pt{100} surface (initially in the reconstructed “hex” phase) held at 400 K in a partial pressure of 1 × 10-5 mbar O2. During the course of recording the spectra a CO partial pressure was introduced and raised incrementally from 1 × 10-7 to 5 × 10-7 mbar.
decreases as the intensity grows, but does not increase again when the intensity falls as the CO appears on the surface. This behavior is very unusual; coverage-dependent frequency shifts in IRAS are most commonly dominated by dynamic dipole coupling of adsorbed molecules with their dipole moments essentially perpendicular to the surface, leading to a (reversible) increase in frequency with increasing coverage. In addition, there are so-called chemical shifts associated with electronic interaction between adsorbed species which can produce shifts of either sign, but in this case, too, the chemical shifts associated with interactions between the same species are reversible in the coverage of this species. The shifts seen in the 1630 cm-1 band must therefore result from electronic interaction between the species giving rise to this band and the coadsorbed atomic O or CO, the coverages of which change monotonically (down and up, respectively) through the sequence of spectra of Figures 1 and 2. Indeed, as the CO is visible only in the final few spectra (and has a very high cross-section in IRAS), we may infer that the frequency shift is related to the decreasing coverage of atomic oxygen. As may be seen by comparison of Figures 1 and 2, the maximum intensity of the 1630 cm-1 band decreases with decreased partial pressure of oxygen, and indeed with O2 partial pressures lower than 1 × 10-5 mbar O2 the band became difficult to observe. There also appears to be a weak dependence on O2 partial pressure of the frequency at which the band develops, the value being lower at reduced partial pressure of oxygen. Although spectra were recorded over the much wider frequency range of 800 cm-1 to 3500 cm-1, no bands other than those shown in Figures 1 and 2 were observed. To try and elucidate the nature of the surface species giving rise to the 1630 cm-1 band, further experiments were performed using the isotopologues 13C16O, 12C18O, and 18O2. The results of these experiments are shown in Figures 3, 4, and 5. All of these experiments were conducted with an oxygen partial pressure of 1 × 10-5 mbar, so the resulting IRAS data can be compared most directly to the spectra of Figure 2 (recorded using 12C16O and 16O2) in which the band of interest first appears
Figure 3. Infrared reflection absorption spectra recorded from a Pt{100} surface (initially in the reconstructed “hex” phase) held at 400 K in a partial pressure of 1 × 10-5 mbar O2. During the course of recording the spectra a 13CO partial pressure was introduced and raised incrementally from 5 × 10-8 to 2 × 10-7 mbar.
at 1637 cm-1. In the experiments using the different isotopologues of CO (see Figures 3 and 4), only small shifts in the observed frequency are seen, with the band appearing initially at 1620 cm-1 with 12C18O and at 1622 cm-1 with 13C16O band, although in both cases much larger shifts are seen in the atop C-O stretching band to 2034 cm-1 as expected. Note that the higher-frequency shoulder seen on the atop CO bands in these spectra is due to a fractional content of 12C16O, the frequency being lower than in Figures 1 and 2 due to the reduced dynamic dipole coupling of this minority isotopologue; the frequency of 2065 cm-1 is consistent with a previous IRAS study of CO isotopic mixtures on Pt{100},20 and the disproportionately high intensity of this band (the 12C16O impurity content of the nominal 12C18O is
