J. Phys. Chem. B 2000, 104, 10265-10270
10265
The Defect-Mediated Mechanism of the High-Temperature Oscillatory NO + CO Reaction on Pt{100} As Revealed by Real-Time in-situ Vibrational Spectroscopy J. H. Miners† and P. Gardner*,‡ Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6 D-14195, Germany, and Department of Chemistry, Faraday Building, UMIST, Manchester M60 1QD, U.K. ReceiVed: February 4, 2000; In Final Form: May 22, 2000
At a partial pressure of 10-7 mbar, the reaction between NO and CO on Pt{100} exhibits oscillatory behavior in two distinct temperature regimes. Oscillations in the high-temperature regime (380-411 K) are accompanied by a phase transition from the (1 × 1) surface to the hex surface. Using infrared reflection-absorption spectroscopy (IRAS) and a novel method of data acquisition, we show that during the oscillation cycle, the only molecular species present on the surface is atop CO, adsorbed on the (1 × 1) phase at very low coverage (∼0.03-0.007 ML). Furthermore, the minimum in the CO coverage coincides with the maximum reaction rate, as measured by the partial pressure of CO2. From a comparison of these data with previously published LEED and PEEM studies of the same system, it can be seen that the high-reaction-rate branch of the oscillatory cycle coincides with the maximum area of the surface in the hex phase. This is in contrast to previously proposed mechanisms, which assume that the (1 × 1) surface is the active phase. Since the hex surface is inactive for NO dissociation, we conclude that defects on the hex surface, created during the (1 × 1)-hex phase transition and known to be active for NO dissociation, are responsible for the high-reaction-rate branch. Removal of these defects by annealing provides the means by which the reaction returns to the low-rate branch of the cycle. This annealing process also accounts for the observation that the period of oscillation decreases with temperature.
1. Introduction Kinetic oscillations in heterogeneous catalysis were rediscovered nearly 30 years ago, during studies of CO oxidation on polycrystalline Pt surfaces at atmospheric pressure.1-3 More recent studies, in particular of the CO + O2 and CO + NO reactions, have focused on single-crystal surfaces where the full range of modern surface science techniques can be applied. Kinetic oscillations in the CO + NO reaction on Pt{100} were first reported by Singh-Boparai and King.4 Since then, four separate regions of existence have been identified.5-8 Further work by Veser et al.9-12 determined that the lower-temperature oscillatory regime exists for the range of partial pressure ratios 0.8 < PNO:PCO < 2.5 and that only the unsynchronized local pattern-forming oscillations are observed, whereas the uppertemperature regime exists in the range 0.8 < pNO:pCO < 1.8 and exhibits sustained oscillations. Fink et al. have mathematically modeled the reaction and have reproduced both the steady-state behavior and the lowertemperature kinetic oscillations by assuming island formation and a vacant site requirement for NO dissociation.8 The driving forces are considered to be the surface explosion and the subsequent availability of vacant sites for NO dissociation. Veser et al.10 subsequently noted, however, that the model did not adequately describe the oscillations in the higher-temperature regime, where their predictions were in contradiction with their experimental results of an inverse relationship between temperature and the period of oscillation. * Corresponding author. E-mail:
[email protected]. Tel: ++44 161 200 4463. † Fritz-Haber-Institut der Max-Planck-Gesellschaft. ‡ UMIST.
More recently, Hopkinson and King13 have formulated an alternative model which predicts essentially the same behavior in the lower existence region but not in the upper one, where the reaction mechanism is very similar to that of the oscillatory CO + O2 reaction on the same surface. The driving force is considered to be a combination of the surface phase transition, the existence of two rate branches, and the hysteresis associated with the change in coverage of the (1 × 1) phase. The essential steps in their proposed mechanism for the high-temperature oscillatory regime are as follows: the higher sticking coefficient of CO, compared to NO, leads to a buildup of CO coverage on the hex surface and a local lifting of the reconstruction. The (1 × 1) islands initially grow via the trapping of CO from the surrounding hex surface. As the (1 × 1) islands grow, direct adsorption from the gas phase begins to play a more prominent role, and since PNO > PCO, NO adsorption dominates. NO immediately dissociates and reacts with the CO, resulting in O-covered (1 × 1) islands. These are not stable, and the reconstruction reforms. This results in a reduction of the reaction rate, and so, the CO coverage on the hex phase increases again. This model suffers, however, from an inability to describe the temperature-related behavior of the oscillatory frequency, similar to that of Fink et al.8,14 The weakness in these models stems largely from a lack of direct information concerning the nature and concentration of the species adsorbed on the surface during the reaction. Infrared spectroscopy offers the possibility of remedying this situation at least in part. Infrared reflection-absorption spectra have been measured for such systems under globalsi.e., nontemporally resolvedsconditions.15-17 Spectra with temporal resolution were, however, recorded in the transmission mode by Schu¨th and Wicke on supported polycrystalline platinum under 1 atm
10.1021/jp000465b CCC: $19.00 © 2000 American Chemical Society Published on Web 10/12/2000
10266 J. Phys. Chem. B, Vol. 104, No. 44, 2000 of CO + NO.18 They followed the real-time variation in the peak position and intensity of the atop CO band during measurements of the high-temperature oscillation. When a single crystal is used, there is the problem of obtaining sufficient sensitivity with good temporal resolution to perform measurements at many different points on the oscillation cycle. A further requirement is the ability to relate the periodic changes in the concentrations of the adsorbed species to those of the gas-phase reactants and products. Although considerable success has been recently achieved by Lauterbach et al., who used conventional rapid scan techniques to investigate the CO + O2 reaction, it is clear that increases in S/N cannot be obtained without degrading the temporal resolution.19 We have recently developed a new method that overcomes these problems and gives detailed information on the coverage and local environment of the ad species and the partial pressures of the reactants and products throughout the oscillation cycle.20 A similar technique has subsequently been used by Magtoto and Richardson.7 Our method involves applying a small temperature modulation to the sample and continuously recording interferograms over a period of many oscillatory cycles. Interferograms recorded at equivalent points in the cycle are then added to create a single data set representing a single oscillatory period. The temporal resolution of the resulting set of spectra is twice the data acquisition time required for the original interferograms. Throughout the measurement, the partial pressures of the reactants and products are measured simultaneously, and the phase of these signals related to the infrared data of the adsorbates is synchronized again via the applied temperature modulation. The method is applicable to any periodically repeating process, provided that the phase remains exactly constant or can be held exactly constant. In a previous report,20 we described the application of this technique to the low-temperature oscillatory regime, where there is good agreement with the model of Fink et al.8,14 In this paper, we use the method to study the high-temperature regime. 2. Experimental Section The experimental setup has been previously described.20 It consists of a commercial FTIR spectrometer (Bio-Rad FTS60A/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. Spectra were recorded using a liquid-nitrogen-cooled narrow-band MCT detector. The Pt{100} crystal was mounted on a liquid-nitrogen-cooled cold finger 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 computer-controlled temperature control unit. The crystal was cleaned prior to each experiment via repeated cycles of sputtering at 750 K, followed by annealing at 1000 K. After the normal sputtering and annealing cycles, the crystal was cooled to a temperature 10 K above the existence range of the oscillatory regime to be studied. Prior to initiating the oscillations, we prepared the system in the following way. NO was admitted to the chamber and left to stabilize to a pressure of 5 × 10-7 mbar over a period of several hours. (There is no feedback system on the gas inlet valve, so some small longterm drift in pressure was observed during this period.) CO was then introduced such that the pNO:pCO ratio becomes 1.33. After
Miners and Gardner
Figure 1. Effect of a temperature modulation of 2.5 K on the reaction rate of NO + CO on the Pt{100} hex in the upper temperature oscillatory regime with pNO:pCO ) 1.3 and ptot ) 7 × 10-7 mbar.
a period of pressure stabilization (∼30 min), during which the partial pressures of both gases were continuously monitored with a mass spectrometer, the crystal was heated to 700 K and gradually cooled to a temperature ∼10 K above the range of the oscillatory regime to be studied. This ensures that the system is of the high-rate cooling branch of the temperature-dependent hysteresis.8 A temperature modulation of the desired period and amplitude (60 s and 2 K) was applied to the sample to initiate and sustain the oscillations, which are damped at these low pressures due to the loss of macroscopic synchronicity.12,21 The application of an external global coupling mechanism gives rise to a regular oscillation and is considered not to change the mechanism of the reaction. The mean temperature was gradually reduced until the natural frequency of the system was in resonance with the applied temperature oscillation. This was marked by a large increase in the amplitude of the CO2 partial pressure oscillation. Although this procedure is often referred to as forcing the oscillations, extensive studies of periodic and random temperature perturbations of this system have shown that it can only be forced to oscillate at frequencies very close to the natural frequency, i.e., this system acts as a narrow bandpass filter.21 The fact that the oscillation was unforced was verified by stopping the applied temperature modulation to observe that no significant changes in the form and period of the gas-phase partial pressure traces, as measured by the mass spectrometer, had taken place (see Figure 1). The modulation was then reapplied, and after a second period of stabilization (∼10-20 min), in which the amplitude and peak shape of the oscillation as measured by the partial pressure of CO2 had become completely reproducible, the infrared data acquisition was started. During the oscillations, the partial pressures of NO, CO + N2, and CO2 are continuously monitored with the QMS. Simultaneously, the FTIR spectrometer continuously records a succession of interferograms over as many oscillatory periods as required. The infrared spectrometer gives a trigger pulse each time a spectrum is acquired, which is fed into a PC. This records the spectrum number, the temperature of the sample, and the time at which the spectrum was obtained. Another PC simultaneously records the temperature of the sample and the data from the mass spectrometer. Thus, each data point in the mass spectrum and each FTIR spectrum can be uniquely identified in terms of the time and temperature at which they were recorded. Spectra from equivalent points in the cycle are then
Oscillatory NO + CO Reaction on Pt{100}
J. Phys. Chem. B, Vol. 104, No. 44, 2000 10267
Figure 2. Spectrum of the CO + NO reaction in the high-temperature oscillatory regime acquired over 7 oscillatory cycles with a spectral resolution of 2 cm-1 and representing the addition of 1024 interferograms.
combined and averaged to give a set of spectra that represent a single period of oscillation. 3. Results To establish the optimum spectroscopic parameters for the time-resolved spectra of the oscillatory reaction, we obtained a high-resolution high S/N time integrated spectrum. The spectrum, recorded at 2 cm-1 resolution and averaged over seven oscillatory cycles representing the addition of 1024 interferograms, is shown in Figure 2. As can be seen, just a singleabsorption band indicative of the atop CO is observed. As expected, molecular NO is not observed. Because of the low adsorbate coverage under reaction conditions for obtaining usable S/N, the time-resolved spectra were recorded at 8 cm-1 resolution and represent the combination of 280 interferograms collected over 20 oscillatory cycles, yielding a temporal resolution of 4 s. Detailed analysis of the time-resolved data is shown in Figure 3. This relates the peak position, the integrated absorbance, and the half-width of the atop C-O stretch for one oscillation in the high-temperature regime, with the partial pressures of CO2 (44 amu), NO (30 amu), and CO + N2 (28 amu). From Figure 3, it can be seen that the position of the linear CO band varies from 2071 to 2077 cm-1 during the course of the oscillation and is in-phase with the integrated absorption. In theory, the frequency of this band should provide information concerning the nature of the CO species, but this is not straightforward since there is still some debate in the literature concerning the frequency of CO adsorbed on the hex and (1 × 1) phases.22-24 Isotopic dilution studies identified two different types of adsorbed CO molecules having different singleton frequencies of 2065 and 2080 cm-1.22,23 It was originally suggested that the CO on the hex surface had the higher singleton frequency, exhibiting a band at ∼2080-2084 cm-1, and that the CO on the (1 × 1) surface had the lower singleton frequency but gave rise to a band at 2080-2091 cm-1 during island growth. (The higher frequency of the CO on the (1 × 1) surface compared with the singleton frequency was attributed to the strong dipole coupling, which results in a high local CO coverage during island growth.) Subsequently, however, after a molecular-beam study of this system,25 Kim et al. argued that CO on a hex surface could not have been observed under the conditions and that the band initially at 2080 cm-1 was CO adsorbed on nucleated (1 × 1) islands.24 They were able to show
Figure 3. Temperature modulation, the partial pressures of CO2, NO, and CO + N2, and the integrated absorbance, peak position, and fwhm of the atop C-O infrared absorption band for one oscillation of the reaction in the high-temperature regime.
that initial adsorption on the hex phase gave rise to a band at 2069 cm-1. They saw this band grow and shift to ∼2076 cm-1, where it was accompanied by a new band at 2080 cm-1. The first band at 2069-2076 cm-1 was assigned to CO on a hex surface and the second at 2080 cm-1 to CO on a (1 × 1) surface. More specifically, this latter band was attributed to nucleating (1 × 1) islands, i.e., 4 CO molecules on approximately 10 Pt atoms, giving a local coverage of 0.4 ML. If this assignment were correct, it would suggest that all the CO observed during the oscillation is adsorbed on the hex surface. An alternative assignment can be made if the hysteresis in the CO frequency is considered as a function of temperature. It can be clearly demonstrated that as the temperature of a CO-covered (1 × 1) surface is increased, the frequency of CO decreases to ∼2070 cm-1 before the (1 × 1)-hex phase transition occurs.22,26 If the nucleation and annihilation of (1 × 1) are assumed to occur at the same critical coverage, the appearance of a frequency between 2070 and 2076 cm-1 may represent adsorption on small nucleating (1 × 1) islands prior to significant island growth. This is consistent with a recent Monte Carlo simulation of the nucleation process.27 The kinetics of nucleation could not be determined in the molecular beam study, and so, there is no contradiction between this assignment and the flux-dependent island growth determined by Hopkinson et al.25 On the bases of our own hysteresis data22,26 and the molecular beam data,25 we conclude that the band at 2080 cm-1 observed by Kim et al.24 is due to CO associated with the onset of (1 × 1) island growth rather than nucleation and that the band observed during
10268 J. Phys. Chem. B, Vol. 104, No. 44, 2000
Figure 4. CO2 partial pressure traces and LEED spot intensity recorded for a free oscillation in the high-temperature regime (from Fink et al.8).
the oscillation is CO on just nucleated (1 × 1) islands with a local coverage below that required for large scale growth, i.e.,
