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Oxygen and carbon monoxide interactions on Rh(110) studied by real-time X-ray photoemission spectroscopy
surface science ELSEVIER
Surface Science 385 (1997) 376-385
Oxygen and carbon monoxide interactions on Rh(110) studied by real-time X-ray photoemission spectroscopy A. B a r a l d i a,,, S. Lizzit a, D. C o c c o a, G. C o m e l l i a,b G. P a o l u c c i a, R. R o s e i a,b 9 M. Kiskinova ~ a Sincrotrone Trieste, Padriciano 99, 34012 Trieste, Italy b Dipartimento di Fisica, Universit~ degli Studi di Trieste, Via A. Valerio 2, 34127 Trieste, ltaly Received 20 October 1996; accepted for publication 7 April 1997
Abstract
The interaction of CO with a (2 x 1)p2mg oxygen layer on ( 1 x 1)-Rh(110) and a c(2 x 8) oxygen layer o n ( 1 x 4)-Rh(110) has been studied by synchrotron radiation real-time X-ray photoelectron spectroscopy, low energy electron diffraction and mass spectrometry. The structure and composition of the surface layer were continuously monitored during the reactioa performed at 200, 290 and 350 K. The intensity of the components of the O ls spectra reflected the changes in the oxygen and CO coverages during the reaction. The intensity decrease of the O ls peak at binding energies <530.3 eV, corresponding to the adsorbed oxygen, was used as a measure of the CO oxidation rate. The emergence and increase of a second O Is peak at binding energies > 531.0 eV identified the CO uptake. The changes in the CO 2 partial pressure during titration of the oxygen layers with CO were also monitored by a mass spectrometer. The observed difference in the reactivity of the (2 x 1)p2mg and c(2 x 8) layers was related to the structural differences of the two surfaces: the ( 1 x 4) reconstruction creates adsorption sites where oxygen is more strongly bonded and less reactive. The effect of the structural changes on the reaction rate was observed during the titration reaction at 350 K. © 1997 Elsevier Science B.V. Keywords: Photoelectron spectroscopy; Rhodium; Surface chemical reaction; Synchrotron radiation photoelectron spectroscopy
1. Introduction
As one of the simplest catalytic reactions, the CO oxidation allows us to study the fundamental issues of the catalytic phenomenon and use this knowledge in the search for the better and cheaper anti-pollution catalysts, Rh along with Pt is at present one of the commonly used catalysts in the exhaust gas automobile converters. This has motivated many studies of CO oxidation on single * Corresponding author. E-mail: [email protected] 0039-6028/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. 1)1I S0039-6028 ( 97 ) 00264-1
crystal Rh surfaces [1-10]. It has already been proved that the CO oxidation on Rh proceeds via the Langmuir-Hinshelwood mechanism between adsorbed O and CO. Comparing the behaviour of R h ( l l l ) , Rh(100) and R h ( l l 0 ) surfaces it has been shown that the reaction becomes structure sensitive in the low CO and high oxygen coverage regime [4,7]. Among the most important factors controlling the kinetic parameters are the oxygen and CO adsorption bond strengths, which vary with the adsorbate coverage and substrate structure, adsorbate surface structure, formation of mixed or separated ordered phases by the coadsorbed species in the course of the reaction, etc.
A. Baraldi et aL/ Surface Science 385 (1997) 376-385
Recent studies on Rh(110) surfaces have noted the importance of the surface structure, as determined by the adsorbed oxygen on the CO oxidation rate [5,10]. The great variety of O-reconstructed phases and the existence of an O-unreconstructed structure have made the R h ( l l 0 ) surface very attractive for systematic studies of the correlation between adsorbate structure and reaction rate. At elevated temperatures oxygen induces (l x n) missing-row type reconstructions of the Rh(110) surface, where each nth [1 [0] row is missing (n can have values from 2 to 5) [5, 11 13]. In all cases oxygen occupies the fcc three-fold sites between the top and second Rh layer in a zigzag arrangement along the [1[0] ( 1 × 1 ) and ( 1 x 2) troughs. With increasing oxygen coverage from 0.5 monolayer (ML), corresponding to a (2 x 2)p2mg structure on a (1 x 2) surface, to 0.8 ML, corresponding to a c(2 x 10) structure on a (1 x 5) surface, the number of the missing [1 [0] rows decreases. This results in a decrease of the oxygen adatoms along the (1 x 2) troughs and an enhancement of the population along the ( 1 x 1 ) troughs, where the O bonding configuration is the same as for the unreconstructed 0-(2 x 1)p2mg structure formed at T < 300 K. Two studies of the CO oxidation rate performed by titration of oxygen from different surface structures with CO at room temperature have revealed that oxygen adsorbed along the (1 x 2) troughs is the least reactive [5,10]. Scanning tunnelling microscopy studies of the reactivity of the oxygen reconstructed surfaces have evidenced a high anisotropy in the reaction propagation: it progresses along the [110] direction without affecting neighbouring structures across the (1 x 2 ) troughs [10]. Both studies, however, do not provide any information about the changes in the coadsorbed layer, concerning CO and oxygen coverage and adsorption state variations during the titration reactions. This knowledge is of fundamental importance for understanding the factors controlling the reaction rate. Here we report on the different reactivity of the ( 2 x 1)p2mg and c ( 2 x 8 ) oxygen overlayers studied by titration with CO at different temperatures. The emphasis is on tracing the variations in the adsorbed layer during CO titration using
377
real-time X-ray photoelectron spectroscopy (RT-XPS). The shape and intensity of the collected O l s spectra were used to identify the changes occurring during the reaction in the oxygen and CO bonding configurations and coverages. The decay in the intensity of the oxygen O l s signal was used as a measure of the titration rate which evidences the structural effect on the oxygen reactivity. The evolution of the LEED patterns during the reaction was also monitored and correlated with the observed changes in the titration rate. Qualitative correlations with the CO z partial pressure changes measured by a mass spectrometer under the same titration conditions confirm the structural effect on the oxygen reactivity.
2. Experimental The experiments were performed at the SuperESCA beamline of the E L E T T R A synchrotron radiation facility in Trieste. The experimental chamber is equipped with a VSW Class 150 16-channels electron energy analyser, low energy electron diffraction (LEED) and mass spectrometer. The beamline provides photons in the 180 1200 eV energy range with a resolving power up to 104 [14]. In order to acquire XPS spectra at a fast rate during the reactions we used the following experimental configurations: ( 1 ) the photon beam was impinging the sample at very grazing angle ( 8 5 from the surface normal) with the electron energy analyser at 45 ° from the sample normal: (2) the beamline and analyser were operated at an overall resolution of 0.5 eV. After the fitting procedure this energy resolution was sufficient to monitor energy shifts of 0.1 eV. This experimental set-up resulted in a photoelectron signal that allowed an O l s spectrum to be acquired in 12 s with a photon energy of 651 eV. The base pressure in the experimental chamber with the photon beam hitting the sample was 8 x 10-1~ mbar. Under these experimental conditions the cleaning off effects due to the presence of hydrogen in the residual gas (5 x 10 ~1 mbar) are negligible. The cleaning procedure of the Rh sample involved cycles of Ar + bombardment and
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annealing in oxygen atmosphere [5]. The final annealing to 1400 K, needed in order to remove any residual oxygen, was substituted by a reduction in hydrogen atmosphere (1 × 10-7mbar, 700 K). Sample cleanliness was checked by XPS. The oxygen overlayers were prepared by exposing the clean Rh surface, kept at constant temperature, to pure oxygen according to the following recipes: 10 L [ 1 L (langmuir) = 10 -6 Torr-s -1] at 270 K for forming a saturated (2 × 1)p2mg; 8 L at 570 K for a c(2 × 8) layer. For coverage calibration we used the intensity of the O ls spectra corresponding to the (2× 1)p2mg oxygen layer which contains 1 ML oxygen [5]. One monolayer equals the atom density of the (1 × 1)-Rh(ll0) surface and is the coverage unit used in the paper. By exposing these oxygen overlayers to a constant CO pressure, ranging from 2 . 5 × 10 -9 to 5 × 10-8mbar, while keeping the sample at a fixed reaction temperature the reactivity of the O-c (2 × 8) and 0-(2 × 1)p2mg layers was probed. In order to monitor the changes in the surface layer, XPS spectra of the O 1s region were continuously acquired during the titration reaction.
3. Results
The reactivity of the O-(2×l)p2mg and O-c(2 × 8) layers, representative of unreconstructive and reconstructive oxygen adsorption, was studied by titration with CO at three substrate temperatures: 200 K, 290 K and 350 K. Fig. 1 shows selected O ls spectra recorded at 200 K during titration of the (a) 0-(2 × 1) and (b) O-c(2 × 8) surfaces. The emergence and growth of a distinctive second component at higher binding energy in the O ls spectra indicates the onset of CO coadsorption. CO and oxygen can be easily distinguished by the large difference in the core level binding energies: O ls<530.3 eV for atomic O and 531.9 and 530.8 eV for on-top and bridge bonded CO, respectively [15-17]. In order to differentiate the CO and O-related components in the O 1s spectra and to identify the changes of the CO and O adsorption state, the spectra were fitted using Doniach-Sunjic functions. The CO-O ls
and O-O 1s peak integrals were used as a measure of the O coverage reduction and CO uptake versus reaction time. The intensity variations of O- and CO-O Is components are outlined by the images on the left side of Fig. 1. Distinctive features in Fig. 1 are: (i) the decrease of the oxygen O ls intensity accompanied by a shift to lower binding energies (indicated by the dashed line trough the peak's maxima); and (ii) the CO-related O ls component at binding energy of ~ 533.0 eV which is by ~ 1.0 eV higher than the binding energy of the on top CO on an O-free surface (531.9eV) [15]. Similar sets of data were obtained for the other two temperatures. In all cases CO coadsorbs initially in sites characterized by an O ls binding energy higher than that of the on-top CO on an O-free surface. Fig. 2 shows the initial and final O ls spectra measured at the end of each titration set when the reaction had ceased. At 350 K only a tiny feature induces a broadening on the low energy side of the final CO-related O 1s spectrum. The fraction of oxygen remained after titration of the c(2×8) initial structure was always larger. Notable features are: (i) the difference in the CO-O ls binding energy coadsorbed with O at 200 K and that adsorbed at higher temperatures, when the major part of oxygen has already reacted; and (ii) the shift of the O-O ls peak to lower binding energies with decreasing oxygen coverage. The coexistence of differently coordinated CO, uneffected on-top (531.9eV) and bridge-bonded (530.8 eV), adsorbed on an oxygen-affected sites (530.25 eV) along with residual oxygen (529.65eV) can result in a rather broad O ls spectrum (e.g. the spectrum at 290 K in Fig. 2b). The actual shape of the spectrum is determined by the relative contribution of the CO-O ls and O ls peaks to the O ls spectra. For the same temperature the coverage of the different adsorbed species varies with the substrate surface structure, as evidenced by the fitting components of the spectra in Fig. 2. The evolution of the LEED patterns observed during the titration reaction can be summarized as follows. The O-related (2 × 1)p2mg structure fainted and almost disappeared at 200 K. At 290 K it was replaced by a CO-(2 × 1)p2mg structure,
A. Baraldi et al. / Surface Science 385 (1997) 376 385
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A. Baraldi et al. / Surface Science 385 (1997) 376-385
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whereas at 350 K there was a conversion to a very weak and streaky c(2 x 4) structure. In the latter case the intensity variations of the fractional (0, 1/2) spots with electron energy were indicative for initiation of a (1 x 2 ) reconstruction with decreasing oxygen coverage. Note that a c(2 x 4) structure was observed when ~0.5-0.75 ML of CO is adsorbed on a (1 x 2)-Rh(110) surface [15]. Coexistence of both bridge and on-top CO species is the reason for the broad CO part of the O ls spectra fitted with two components as shown in Fig. 2. The c(2 x 8) structure at 200 and 290 K converts into a (1 x 4), indicating that the longrange oxygen order on this reconstructed surface was destroyed during titration. At 350 K the fractional I/4 spots of the reconstructed surface
became very diffuse and weak indicating partial lifting of the reconstruction upon removal of oxygen. Table 1 summarizes the CO and O coverages and LEED patterns measured after 10 L CO exposure. After these exposures the reaction on the 0-(2 × 1)p2mg surface had practically levelled off at all temperatures. As far as the O-c(2 x 8) surface is concerned at 290 K the reaction continued at a very slow rate and levelled off after ~ 20 L CO exposure leaving ~0.2 ML oxygen behind. Note that the adsorptive capacity of the surfaces has not changed substantially in the presence of oxygen. At 200 K the final total CO + O coverage is even more than the initial oxygen coverage, that is, > 1 ML on the (1 × 1) surface. This means that the cease of the reaction is not simply determined by the coverage of the interacting species. With increasing the reaction temperature the final coadsorbate coverage decreases due to more vigorous removal of oxygen and thermal desorption of the less strongly bonded CO molecules in the oxygenaffected adsorption sites. Fig. 3 shows the oxygen coverage decay as a function of CO exposure for the 0-(2 x 1)p2mg and O-c(2 x 8) surfaces obtained at different temperatures. The initial slopes of the oxygen decay plots and the following variations during the progress of the reaction are different for the two surfaces and change substantially with reaction temperature. Note that at 350 K there is a slope discontinuity in the oxygen decay plot for the O-c(2 × 8) surface (see the insert) which distinguishes this plot from the other plots in Fig. 3, characterized by more monotonous slope changes. For the sake of comparison Fig. 4 displays the changes of the COz signal measured mass spectrometrically under the same experimental conditions. Since the plots in Fig. 4 are obtained by simple monitoring of the partial pressure changes they cannot be used for quantitative evaluations; only qualitative correlations with the data in Fig. 3 can be made. In particular, it is notable that the CO2 plots at 350 K in Fig. 4 follow a more complicated route indicative of abrupt changes in the CO2 production rate between 1 and 2 L of CO exposure. This correlates with the slope break observed in the corresponding oxygen decay plots in Fig. 3.
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Table 1 LEED patterns, CO and oxygen coverages measured after 10 L of CO exposure at different temperatures for the (2 × l)p2mg and c(2 × 8) oxygen structures on Rh(110) Oxygen structure
Temperature (K)
CO coverage (after 10 L of CO)
Oxygen coverage (after 10 L of CO)
LEED pattern
(2 × 1)p2mg
200 290 350
0.62+_0.01 0.94_+0.01 0.78 ±0.01
0.57 _+0.01 0.09+_0.01 0.10 ±0.01
Diffuse (2 × 1)p2mg (2 x I )p2mg Weak c( 2 × 4)
c(2x8)
200 290 350
0.47 ±0.01 0.42±0.01 0.67+0.01
0.57 ±0.01 0.36+0.01 0.10_+0.01
Diffuse ( 1 x 4) ( 1 z 4) Weak and diffuse (1 x4)
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Fig. 5 s h o w s t h e C O u p t a k e p l o t s f o r t h e 0 - ( 2 × 1 ) p 2 m g a n d O - c ( 2 × 8) surfaces. T h e t e m p e r a t u r e effect o n t h e a d s o r p t i o n r a t e is o b v i o u s l y m o r e p r o n o u n c e d in t h e case o f t h e c ( 2 x 8) surface, w h i c h , as s h o w n in T a b l e 1 also e x h i b i t s a lower adsorptive capacity.
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4. Discussion T h e o b s e r v e d differences in the t i t r a t i o n r a t e o n unreconstructed 0-(2 x 1)p2mg and reconstructed
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A. Baraldi et al. / Surface Science 385 (1997) 376-385
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O - c ( 2 x 8 ) R h ( l l 0 ) surfaces can be tentatively correlated with the structural sensitivity reported in the literature for the CO oxidation reaction on R h ( l l 0 ) at high temperatures and high oxygen pressures, when oxygen is the dominating surface species [4]. As briefly described in the introduction, under these conditions the R h ( l l 0 ) surface undergoes different reconstructions determined by the oxygen coverage. The driving force for reconstruction is the energy gain in chemisorption energy (strengthening of the R h - O bond in the present case) which compensates for the energy loss from rearrangement of the substrate atoms. The R h - O bonding strength and bonding configuration along with surface diffusion are the main factors controlling the reactivity of oxygen. Despite that, the changes in the substrate structure can also affect the adsorption rate and adsorption state of CO and the coupling configurations required to create the transition state to CO2 formation. In
the present study we have evidenced that the structural effects on the titration rate can be linked chiefly to the different reactivity of the adsorbed oxygen. Let us first consider the apparent difference in the initial oxygen titration rates manifested by the initial slopes of the oxygen decay curves for the (2 x 1)p2mg and c(2 x 8) surfaces in Fig. 3. The initial titration rate can be described using the simple relationship r = v e x p ( - e/RT) SP¢oO o. Comparing the oxygen coverage decay curves in Fig. 3 with the CO uptake plots in Fig. 5 it becomes obvious that there is no correlation between the temperature effect on the initial titration rates and the corresponding CO adsorption rates. For example the initial CO adsorption rate is almost the same at 200 and 350 K on both the (1 x l) and (l x 4 ) surfaces, whereas the titration rates are distinctively different. In contrast there is an apparent faster decrease in the CO adsorption rate on the c(2 × 8) surface with increasing reaction temperature from 200 to 290 K, whereas the difference between the titration rates between the two surfaces remains the same. This indicates that within this temperature range the onset of oxygen titration requires very low CO coverage, so that the initial titration rate remains practically unperturbed by the changes in the CO sticking coefficient. Since the experiments are performed at the same CO partial pressure, the experimentally obtained ratio, r~z×t)/re~z×8) in the temperature range 200-350 K should be determined by the initial oxygen coverage and the kinetic parameters, v and E. The initial oxygen coverage, 0o is 1 and 0.75 ML, respectively, which results in r(2 × 1 ) / r c ( 2 × 8) = 1.33, assuming the same v and E for both surfaces. At 200 K the simplest oxygen decay plots are obtained. The oxygen plots for the (2 × 1)p2mg surface at 200 K show about twice a steeper initial slope, that is, rt2 x 1)/ret2×8)=2. In order to explain the experimental value of 2 we should assume that in addition to the oxygen coverage term the v and/or E also are different for the (2 × 1)p2mg and c(2 x 8) surfaces. Assuming that one of the latter is constant we evaluated that either the activation energy for the c ( 2 × 8 ) surface is --~0.7kJ larger or the pre-exponential factor is -~ 1.5 smaller than those of the (2 x 1) surface.
A. Baraldi et al. / Surface Science 385 (1997) 376 385
However, it is likely that both parameters vary, but the present data are insufficient for more thorough evaluations. Another approach to justify the experimental r~2x l~/rcla×8)=2 ratio at 200 K can be to take into account only the coverage of the oxygen species which are reacting at 200 K. Considering the difference in the reactivity of the oxygen located along the (1 x 1) and (1 x 2) through we assume that at 200 K the oxygen adsorbed along the (1 x 2) through is not reactive. Then we can use as an oxygen coverage term in the rate equation only the oxygen coverage along the ( 1 x 1) troughs, which is 0.5 M L for the c(2 x 8) and 1 M L for the (2 x 1 )p2mg surface. These oxygen adatoms have the same bonding configuration, and are expected to have similar reactivity. Using 0 o = 0 . 5 M L instead of the total oxygen coverage of 0.75 ML for the c(2 x 8) structure results in a twice lower reaction rate at 200 K, as experimentally observed. This approach also works for explaining the difference in the initial reaction rates at 290 K, where the initial titration rates and the ratio between them r12× 1)/rc~2 x 8) = 1.95 is very similar to that at 200 K. At 350 K the difference between the initial titration rates of the two surfaces increases, the rt2× l ) / r c l 2 × s ) ratio becomes more than 3. This is due to the strong temperature-induced enhancement of the initial titration rate for the (2 x 1 )p2mg surface, whereas that of the c ( 2 x 8 ) surface remains very similar in the 200-350 K temperature range. It is clear that at this temperature the simple approach which relates the difference in the initial titration rate to the oxygen coverage along the ( 1 x 1 ) troughs does not work, The above considerations confirm that the difference in the initial titration rate is indeed associated with the structural factor which determines the oxygen adsorption state and reactivity. The oxygen adatoms, which are more deeply embedded in the fcc three-fold sites along the ( 1 x 2) troughs of the ( 1 x 4) reconstructed surface, are nonreactive [ 13]. As manifested by the data in Fig. 3 this species is likely to be the main fraction of the residual oxygen at 200 and 290 K. Structural effect on the CO and oxygen mobility and on the transition state configuration exists as well, which
383
seems to become important at 350 K. It is likely that the anisotropic (1 x 4) surface imposes more constraint on the adspecies surface diffusion so that the reduction of oxygen remains slower at temperatures below the (1 x 4 ) to (1 x 1) phase transitions. With progressing of the reaction the slopes of the decay plots in Fig. 3 vary in a different manner. The simplest decay plots obtained at 200 K show that the reaction on both surfaces levels off after the initial almost linear decay. According to the data ~0.45 M L of oxygen from the (2 x 1)p2mg surface and ~0.20 ML from the c(2 x 8) surface have reacted at 200 K. Obviously the reaction at 2 0 0 K is impeded by the coadsorbed CO. Inspection of the O ls data shows that a large fraction of the remaining oxygen has changed its adsorption state, as manifested by the shift to a lower binding energy. If we consider the evolution of the O bonding configuration with coverage in the oxygen adlayers on R h ( l l 0 ) , this shift indicates conversion from three-fold to bridge bonded oxygen, the preferred configuration at low coverages, when no ordered structure is formed [ 17,18]. This is in agreement with the extinction of the fractional spots in the (2 x 1)p2mg and c(2 x 8) patterns related to the oxygen order. We suggest that at 200 K the reaction takes place between the first arriving CO molecules which coadsorb weakly sharing substrate atoms with oxygen. They are mobile and possibly no site sensitive, which facilitates attaining the favourable distance between C and adsorbed O in order to form COz. CO2 leaves and liberates adsorption sites where the next impinging CO molecules can stick. These CO molecules are more strongly bonded and distant from the remaining oxygen adatoms. Parallel surface process is a change of the oxygen adsorption site upon reduction of the oxygen density, which undoubtedly influences the delicate balance between the rate controlling parameters. The cessation of the reaction means that in the dense coadsorbed layer formed at 200 K an activation barrier to CO2 formation exists. This activation barrier can be overcome at higher temperatures, as evidenced by the oxygen decay plots at 290 and 350 K, when the reacting species become more
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.4. Baraldi et al. / Surface Science 385 (1997) 376-385
mobile and can more easily attain the required interaction configuration. There is a substantial difference between the (2 x 1)p2mg and c(2 x 8) surfaces concerning the evolution of the oxygen titration rate with reaction time at different reaction temperatures. Fig. 3 shows that for the (2 x 1)p2mg surface the shape of oxygen decay plots at 290 and 300 K remains similar to that at 200 K, the initial slope gradually decreases and levels off. The main difference is that at higher temperatures the levelling off occurs at much lower oxygen coverage. In contrast, the shapes of the oxygen plots for the c(2 x 8) surface at 290 and 350 K differ from that at 200 K: at 290 K a monotonous continuous decrease of the reaction rate was observed, whereas at 350 K there is a discontinuity between 1 and 2 L CO exposure, indicating a sharp enhancement of the titration rate. It is notable that in the latter case after the break the rate becomes comparable with that of the (2 x 1)p2mg surface. Abrupt changes can be seen in the corresponding CO2 plots for the c(2 x 8) surface at 350 K (see Fig. 4). The break points correlate with the onset of the transition from the ( 1 × 4) to unreconstructed ( 1 × 1) surface evidenced by LEED. This major structural change during the reaction results in a reaction mechanism similar to that followed by the 0-(2 x 1)p2mg surface. This is because by partial lifting of the (1 x 4) reconstruction the concentration of the less reactive oxygen adatoms along the (1 × 2) through is reduced. Finally, let us discuss the effect of oxygen on the CO bonding configuration and adsorption rate. The most distinguishable effect, a large upward shift of the C O - O I s core level peak, was observed at 200 K when CO is coadsorbed with a large amount of oxygen. In these dense layers the O ls binding energy changes are determined by initial state (repulsive interactions and changes in the coupling between the CO and substrate orbitals) and final state relaxation effects. Since both CO and O are classified as electronegative species, the measured higher O ls binding energy for CO in these dense layers is expected considering dominance of the initial state effects, which implies weakening of the CO adsorption energy as well [19]. The C O - O ls binding energy appears to be
affected by the coadsorbed oxygen rather than by the closely located other CO molecules, because in a dense CO adlayer of 1 ML the C O - O ls binding energy of the on-top CO does not exceed 532 eV [15]. At higher reaction temperatures, when oxygen is cleaned off, the C O - O ls binding energies are similar to the one measured for an O-free surface [15,16]. The strong oxygen effect on the C O - O 1s energy supposes that the coadsorbed CO and O do not separate to form large CO or O islands. The lower adsorption rate of CO on the c(2 x 8) surface can be ascribed to the morphology of the (1 × 4) surface. When CO is adsorbed on the [ 1 [0] rows, the oxygen atoms in the fcc sites and even in the long bridge sites are nearly in the surface plane so that the blocking effect and the O - C O repulsion are weakened. On the ( 111 ) facets the C atom of the CO molecule should sit almost in the same plane with O sharing substrate atoms. This suggests that the CO adsorption onto the facets is hindered resulting in the observed lower adsorptive capacity of the partially reduced O-c(2 x 8) surface, where the oxygen along the (1 x 2) troughs remains. The interaction between CO and O in the fcc three-fold sites along the [110] rows requires the two species to be close to each other. The C - O distance in the adsorbed CO2 molecule with a O - C - O bond angle of 133 ° was reported to be 1.22 ,~ [20]. This means that a favourable CO configuration for reacting with the O located along the [1 f0] rows can be attained if CO is in the bridge site onto the rows next to the oxygen adatom. The projected distance between CO and O in this configuration will be 1.58 A and the O - C - O angle 102 °.
5. Conclusions
The impact of the surface reconstruction induced by oxygen on the oxygen titration rate by CO has been clearly manifested in this comparative study of the reactivity of the 0 - ( 2 × 1)p2mg and O-c(2 × 8) layers on Rh(110). The obtained results are a good starting basis for understanding the mechanism of the reported structural sensitivity of
A. Baraldi et al. / Surface Science 385 (1997) 376-385
CO oxidation on Rh surfaces. The main findings about the evolution of the adlayer surface composition and structure and the rate of the oxygen titration at different temperatures can be summarized as follows: (1) At reaction temperatures in the 200-350 K range the oxygen titration rate is higher for the unreconstructed O-(2x 1)p2mg surface than for the reconstructed O-c(2 × 8) surface. The initial titration rate for the O-c(2×8) surface showed weak temperature dependence in the whole temperature range, whereas the initial titration rate O-(2x 1)p2mg surface increases about twice at 350 K. The difference in the apparent reactivity of the two surfaces at temperatures _<290 K can be satisfactorily explained assuming that the initial titration rate depends only on the concentration of oxygen adsorbed along the (1 × 1) troughs, whereas the oxygen adsorbed along the ( 1 × 2) troughs is practically nonreactive. At 350 K the enhanced reactivity of the (2 × 1)p2mg surface was tentatively attributed to the structural impact on the CO and O mobility. (2) The substrate structure determined by the adsorbed oxygen remains intact at temperatures <290 K. At 350 K partial lifting of the (1 x4) surface reconstruction occurs with reducing oxygen concentration. This structural change results in an abrupt enhancement of the titration rate, which confirms the correlation between the oxygen reactivity and its bonding configuration: a large fraction of nonreactive oxygen initially located along the (1 x 2) troughs converts into more reactive species when lifting the (1 × 4) reconstruction.
385
References [1] S.B. Schwartz, L.D. Schmidt, G.B. Fisher, J. Phys. Chem. 90 (1986) 6194. [2] B.A. Gurney, J. Lee, J.S. Villarrubia, W. Ho, J. Chem. Phys. 87 (1987) 6710. [3] M. Bowker, Q. Guo, R. Joyner, Surf. Sci. 280 (1993) 50. [4] M. Bowker, Q. Guo, Y. Li, R. Joyner, Catal. Lett. 18 (1993) 119. [5] G. Comelli, V.R. Dhanak, M. Kiskinova, G. Paolucci, K.C. Prince, R. Rosei, Surf. Sci. 260 (1992) 7, Surf. Sci. 269/270 (1992) 360. [6] S.H. Oh, G,B. Fisher, J.E. Carpenter, D.W. Goodman, J. Catal. 100 (1986) 360. [7] C.H.F. Peden, D.W. Goodman, D.S. Blair, P.J. Berlowitz, G.B. Fisher, S.H. Oh, J. Phys. Chem. 92 (1988) 1563. [8] W.M. Daniel, J.M. White, Int. J. Chem. Kinet. 17 (1985) 413. [9] A. Baraldi, L. Gregoratti, G. Comelli, V.R. Dhanak, M. Kiskinova, R. Rosei, Appl. Surf. Sci. 99 (1996) 1. [10] F.M. Leibsle, P.W. Murray, S.M. Francis, G. Thornton, M. Bowker, Nature 363 (1993) 706. [11] V.R. Dhanak, G. Comelli, G. Cautero, G. Paolucci, M. Kiskinova, K.C. Prince, R. Rosei, Chem. Phys. Lett. 188 (1992) 237. [12] V.R. Dhanak, K.C. Prince, R. Rosei, P.W. Murray, F.M. Leibsle, M. Bowker, G. Thornton, Phys. Rev. B 49 (1994) 5585. [13] M. Gierer, H. Over, G. Ertl, H. Wohlgemuth, E. Schwarz, K. Christman, Surf. Sci. 297 (1993) L73. [14] A. Baraldi, M. Barnaba, B. Brena, D. Cocco, G. ComeUi, S. Lizzit, G. Paolucci, R. Rosei, J. Electron Spectrosc. Relat. Phenom. 67 (1994) 211. [15] V,R. Dhanak, A. Baraldi, G. Comelli, G. Paolucci, M. Kiskinova, R. Rosei, Surf. Sci. 295 (1993) 287, [16] A. Baraldi, G. Comelli, S. Lizzit, D. Cocco, G. Paolucci, R. Rosei, Surf. Sci. 367 (1996) L67. [17] G. Comelli, A. Baraldi, S. Lizzit, D. Cocco, G. Paolucci, R. Rosei, M. Kiskinova, Chem. Phys. Lett. 261 (1996) 253. [18] D. Alf6, P. Rudolf, M. Kiskinova, R. Rosei, Chem. Phys. Lett. 211 (1993) 220. [19] M.P. Kiskinova, in: B. Delmon, J.T. Yates (Eds.), Studies of Surface Science and Catalysis, Vol. 70, Elsevier, Amsterdam, 1992. [20] H.-J. Freund, R.P. Messmer, Surf. Sci. 172 (1986) I.
Surface Science 385 (1997) 376-385
Oxygen and carbon monoxide interactions on Rh(110) studied by real-time X-ray photoemission spectroscopy A. B a r a l d i a,,, S. Lizzit a, D. C o c c o a, G. C o m e l l i a,b G. P a o l u c c i a, R. R o s e i a,b 9 M. Kiskinova ~ a Sincrotrone Trieste, Padriciano 99, 34012 Trieste, Italy b Dipartimento di Fisica, Universit~ degli Studi di Trieste, Via A. Valerio 2, 34127 Trieste, ltaly Received 20 October 1996; accepted for publication 7 April 1997
Abstract
The interaction of CO with a (2 x 1)p2mg oxygen layer on ( 1 x 1)-Rh(110) and a c(2 x 8) oxygen layer o n ( 1 x 4)-Rh(110) has been studied by synchrotron radiation real-time X-ray photoelectron spectroscopy, low energy electron diffraction and mass spectrometry. The structure and composition of the surface layer were continuously monitored during the reactioa performed at 200, 290 and 350 K. The intensity of the components of the O ls spectra reflected the changes in the oxygen and CO coverages during the reaction. The intensity decrease of the O ls peak at binding energies <530.3 eV, corresponding to the adsorbed oxygen, was used as a measure of the CO oxidation rate. The emergence and increase of a second O Is peak at binding energies > 531.0 eV identified the CO uptake. The changes in the CO 2 partial pressure during titration of the oxygen layers with CO were also monitored by a mass spectrometer. The observed difference in the reactivity of the (2 x 1)p2mg and c(2 x 8) layers was related to the structural differences of the two surfaces: the ( 1 x 4) reconstruction creates adsorption sites where oxygen is more strongly bonded and less reactive. The effect of the structural changes on the reaction rate was observed during the titration reaction at 350 K. © 1997 Elsevier Science B.V. Keywords: Photoelectron spectroscopy; Rhodium; Surface chemical reaction; Synchrotron radiation photoelectron spectroscopy
1. Introduction
As one of the simplest catalytic reactions, the CO oxidation allows us to study the fundamental issues of the catalytic phenomenon and use this knowledge in the search for the better and cheaper anti-pollution catalysts, Rh along with Pt is at present one of the commonly used catalysts in the exhaust gas automobile converters. This has motivated many studies of CO oxidation on single * Corresponding author. E-mail: [email protected] 0039-6028/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. 1)1I S0039-6028 ( 97 ) 00264-1
crystal Rh surfaces [1-10]. It has already been proved that the CO oxidation on Rh proceeds via the Langmuir-Hinshelwood mechanism between adsorbed O and CO. Comparing the behaviour of R h ( l l l ) , Rh(100) and R h ( l l 0 ) surfaces it has been shown that the reaction becomes structure sensitive in the low CO and high oxygen coverage regime [4,7]. Among the most important factors controlling the kinetic parameters are the oxygen and CO adsorption bond strengths, which vary with the adsorbate coverage and substrate structure, adsorbate surface structure, formation of mixed or separated ordered phases by the coadsorbed species in the course of the reaction, etc.
A. Baraldi et aL/ Surface Science 385 (1997) 376-385
Recent studies on Rh(110) surfaces have noted the importance of the surface structure, as determined by the adsorbed oxygen on the CO oxidation rate [5,10]. The great variety of O-reconstructed phases and the existence of an O-unreconstructed structure have made the R h ( l l 0 ) surface very attractive for systematic studies of the correlation between adsorbate structure and reaction rate. At elevated temperatures oxygen induces (l x n) missing-row type reconstructions of the Rh(110) surface, where each nth [1 [0] row is missing (n can have values from 2 to 5) [5, 11 13]. In all cases oxygen occupies the fcc three-fold sites between the top and second Rh layer in a zigzag arrangement along the [1[0] ( 1 × 1 ) and ( 1 x 2) troughs. With increasing oxygen coverage from 0.5 monolayer (ML), corresponding to a (2 x 2)p2mg structure on a (1 x 2) surface, to 0.8 ML, corresponding to a c(2 x 10) structure on a (1 x 5) surface, the number of the missing [1 [0] rows decreases. This results in a decrease of the oxygen adatoms along the (1 x 2) troughs and an enhancement of the population along the ( 1 x 1 ) troughs, where the O bonding configuration is the same as for the unreconstructed 0-(2 x 1)p2mg structure formed at T < 300 K. Two studies of the CO oxidation rate performed by titration of oxygen from different surface structures with CO at room temperature have revealed that oxygen adsorbed along the (1 x 2) troughs is the least reactive [5,10]. Scanning tunnelling microscopy studies of the reactivity of the oxygen reconstructed surfaces have evidenced a high anisotropy in the reaction propagation: it progresses along the [110] direction without affecting neighbouring structures across the (1 x 2 ) troughs [10]. Both studies, however, do not provide any information about the changes in the coadsorbed layer, concerning CO and oxygen coverage and adsorption state variations during the titration reactions. This knowledge is of fundamental importance for understanding the factors controlling the reaction rate. Here we report on the different reactivity of the ( 2 x 1)p2mg and c ( 2 x 8 ) oxygen overlayers studied by titration with CO at different temperatures. The emphasis is on tracing the variations in the adsorbed layer during CO titration using
377
real-time X-ray photoelectron spectroscopy (RT-XPS). The shape and intensity of the collected O l s spectra were used to identify the changes occurring during the reaction in the oxygen and CO bonding configurations and coverages. The decay in the intensity of the oxygen O l s signal was used as a measure of the titration rate which evidences the structural effect on the oxygen reactivity. The evolution of the LEED patterns during the reaction was also monitored and correlated with the observed changes in the titration rate. Qualitative correlations with the CO z partial pressure changes measured by a mass spectrometer under the same titration conditions confirm the structural effect on the oxygen reactivity.
2. Experimental The experiments were performed at the SuperESCA beamline of the E L E T T R A synchrotron radiation facility in Trieste. The experimental chamber is equipped with a VSW Class 150 16-channels electron energy analyser, low energy electron diffraction (LEED) and mass spectrometer. The beamline provides photons in the 180 1200 eV energy range with a resolving power up to 104 [14]. In order to acquire XPS spectra at a fast rate during the reactions we used the following experimental configurations: ( 1 ) the photon beam was impinging the sample at very grazing angle ( 8 5 from the surface normal) with the electron energy analyser at 45 ° from the sample normal: (2) the beamline and analyser were operated at an overall resolution of 0.5 eV. After the fitting procedure this energy resolution was sufficient to monitor energy shifts of 0.1 eV. This experimental set-up resulted in a photoelectron signal that allowed an O l s spectrum to be acquired in 12 s with a photon energy of 651 eV. The base pressure in the experimental chamber with the photon beam hitting the sample was 8 x 10-1~ mbar. Under these experimental conditions the cleaning off effects due to the presence of hydrogen in the residual gas (5 x 10 ~1 mbar) are negligible. The cleaning procedure of the Rh sample involved cycles of Ar + bombardment and
378
A. Baraldi et al. / Surface Science 385 (1997) 376-385
annealing in oxygen atmosphere [5]. The final annealing to 1400 K, needed in order to remove any residual oxygen, was substituted by a reduction in hydrogen atmosphere (1 × 10-7mbar, 700 K). Sample cleanliness was checked by XPS. The oxygen overlayers were prepared by exposing the clean Rh surface, kept at constant temperature, to pure oxygen according to the following recipes: 10 L [ 1 L (langmuir) = 10 -6 Torr-s -1] at 270 K for forming a saturated (2 × 1)p2mg; 8 L at 570 K for a c(2 × 8) layer. For coverage calibration we used the intensity of the O ls spectra corresponding to the (2× 1)p2mg oxygen layer which contains 1 ML oxygen [5]. One monolayer equals the atom density of the (1 × 1)-Rh(ll0) surface and is the coverage unit used in the paper. By exposing these oxygen overlayers to a constant CO pressure, ranging from 2 . 5 × 10 -9 to 5 × 10-8mbar, while keeping the sample at a fixed reaction temperature the reactivity of the O-c (2 × 8) and 0-(2 × 1)p2mg layers was probed. In order to monitor the changes in the surface layer, XPS spectra of the O 1s region were continuously acquired during the titration reaction.
3. Results
The reactivity of the O-(2×l)p2mg and O-c(2 × 8) layers, representative of unreconstructive and reconstructive oxygen adsorption, was studied by titration with CO at three substrate temperatures: 200 K, 290 K and 350 K. Fig. 1 shows selected O ls spectra recorded at 200 K during titration of the (a) 0-(2 × 1) and (b) O-c(2 × 8) surfaces. The emergence and growth of a distinctive second component at higher binding energy in the O ls spectra indicates the onset of CO coadsorption. CO and oxygen can be easily distinguished by the large difference in the core level binding energies: O ls<530.3 eV for atomic O and 531.9 and 530.8 eV for on-top and bridge bonded CO, respectively [15-17]. In order to differentiate the CO and O-related components in the O 1s spectra and to identify the changes of the CO and O adsorption state, the spectra were fitted using Doniach-Sunjic functions. The CO-O ls
and O-O 1s peak integrals were used as a measure of the O coverage reduction and CO uptake versus reaction time. The intensity variations of O- and CO-O Is components are outlined by the images on the left side of Fig. 1. Distinctive features in Fig. 1 are: (i) the decrease of the oxygen O ls intensity accompanied by a shift to lower binding energies (indicated by the dashed line trough the peak's maxima); and (ii) the CO-related O ls component at binding energy of ~ 533.0 eV which is by ~ 1.0 eV higher than the binding energy of the on top CO on an O-free surface (531.9eV) [15]. Similar sets of data were obtained for the other two temperatures. In all cases CO coadsorbs initially in sites characterized by an O ls binding energy higher than that of the on-top CO on an O-free surface. Fig. 2 shows the initial and final O ls spectra measured at the end of each titration set when the reaction had ceased. At 350 K only a tiny feature induces a broadening on the low energy side of the final CO-related O 1s spectrum. The fraction of oxygen remained after titration of the c(2×8) initial structure was always larger. Notable features are: (i) the difference in the CO-O ls binding energy coadsorbed with O at 200 K and that adsorbed at higher temperatures, when the major part of oxygen has already reacted; and (ii) the shift of the O-O ls peak to lower binding energies with decreasing oxygen coverage. The coexistence of differently coordinated CO, uneffected on-top (531.9eV) and bridge-bonded (530.8 eV), adsorbed on an oxygen-affected sites (530.25 eV) along with residual oxygen (529.65eV) can result in a rather broad O ls spectrum (e.g. the spectrum at 290 K in Fig. 2b). The actual shape of the spectrum is determined by the relative contribution of the CO-O ls and O ls peaks to the O ls spectra. For the same temperature the coverage of the different adsorbed species varies with the substrate surface structure, as evidenced by the fitting components of the spectra in Fig. 2. The evolution of the LEED patterns observed during the titration reaction can be summarized as follows. The O-related (2 × 1)p2mg structure fainted and almost disappeared at 200 K. At 290 K it was replaced by a CO-(2 × 1)p2mg structure,
A. Baraldi et al. / Surface Science 385 (1997) 376 385
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A. Baraldi et al. / Surface Science 385 (1997) 376-385
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whereas at 350 K there was a conversion to a very weak and streaky c(2 x 4) structure. In the latter case the intensity variations of the fractional (0, 1/2) spots with electron energy were indicative for initiation of a (1 x 2 ) reconstruction with decreasing oxygen coverage. Note that a c(2 x 4) structure was observed when ~0.5-0.75 ML of CO is adsorbed on a (1 x 2)-Rh(110) surface [15]. Coexistence of both bridge and on-top CO species is the reason for the broad CO part of the O ls spectra fitted with two components as shown in Fig. 2. The c(2 x 8) structure at 200 and 290 K converts into a (1 x 4), indicating that the longrange oxygen order on this reconstructed surface was destroyed during titration. At 350 K the fractional I/4 spots of the reconstructed surface
became very diffuse and weak indicating partial lifting of the reconstruction upon removal of oxygen. Table 1 summarizes the CO and O coverages and LEED patterns measured after 10 L CO exposure. After these exposures the reaction on the 0-(2 × 1)p2mg surface had practically levelled off at all temperatures. As far as the O-c(2 x 8) surface is concerned at 290 K the reaction continued at a very slow rate and levelled off after ~ 20 L CO exposure leaving ~0.2 ML oxygen behind. Note that the adsorptive capacity of the surfaces has not changed substantially in the presence of oxygen. At 200 K the final total CO + O coverage is even more than the initial oxygen coverage, that is, > 1 ML on the (1 × 1) surface. This means that the cease of the reaction is not simply determined by the coverage of the interacting species. With increasing the reaction temperature the final coadsorbate coverage decreases due to more vigorous removal of oxygen and thermal desorption of the less strongly bonded CO molecules in the oxygenaffected adsorption sites. Fig. 3 shows the oxygen coverage decay as a function of CO exposure for the 0-(2 x 1)p2mg and O-c(2 x 8) surfaces obtained at different temperatures. The initial slopes of the oxygen decay plots and the following variations during the progress of the reaction are different for the two surfaces and change substantially with reaction temperature. Note that at 350 K there is a slope discontinuity in the oxygen decay plot for the O-c(2 × 8) surface (see the insert) which distinguishes this plot from the other plots in Fig. 3, characterized by more monotonous slope changes. For the sake of comparison Fig. 4 displays the changes of the COz signal measured mass spectrometrically under the same experimental conditions. Since the plots in Fig. 4 are obtained by simple monitoring of the partial pressure changes they cannot be used for quantitative evaluations; only qualitative correlations with the data in Fig. 3 can be made. In particular, it is notable that the CO2 plots at 350 K in Fig. 4 follow a more complicated route indicative of abrupt changes in the CO2 production rate between 1 and 2 L of CO exposure. This correlates with the slope break observed in the corresponding oxygen decay plots in Fig. 3.
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A. Baraldi et aL / Surface Science 385 (1997) 376-385
Table 1 LEED patterns, CO and oxygen coverages measured after 10 L of CO exposure at different temperatures for the (2 × l)p2mg and c(2 × 8) oxygen structures on Rh(110) Oxygen structure
Temperature (K)
CO coverage (after 10 L of CO)
Oxygen coverage (after 10 L of CO)
LEED pattern
(2 × 1)p2mg
200 290 350
0.62+_0.01 0.94_+0.01 0.78 ±0.01
0.57 _+0.01 0.09+_0.01 0.10 ±0.01
Diffuse (2 × 1)p2mg (2 x I )p2mg Weak c( 2 × 4)
c(2x8)
200 290 350
0.47 ±0.01 0.42±0.01 0.67+0.01
0.57 ±0.01 0.36+0.01 0.10_+0.01
Diffuse ( 1 x 4) ( 1 z 4) Weak and diffuse (1 x4)
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Fig. 3. Oxygen decay plots obtained from the integrated O--O 1s intensity versus CO exposure for different reaction temperatures. 0 , 0-(2 x l)p2mg layer; and ©, O-c(2 x 8) layer.
Fig. 5 s h o w s t h e C O u p t a k e p l o t s f o r t h e 0 - ( 2 × 1 ) p 2 m g a n d O - c ( 2 × 8) surfaces. T h e t e m p e r a t u r e effect o n t h e a d s o r p t i o n r a t e is o b v i o u s l y m o r e p r o n o u n c e d in t h e case o f t h e c ( 2 x 8) surface, w h i c h , as s h o w n in T a b l e 1 also e x h i b i t s a lower adsorptive capacity.
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Fig. 4. CO2 partial pressure as a function of CO exposure during titration at different temperatures: (top) 0-(2 × l)p2mg; and (bottom) O-c(2 × 8) layers.
4. Discussion T h e o b s e r v e d differences in the t i t r a t i o n r a t e o n unreconstructed 0-(2 x 1)p2mg and reconstructed
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A. Baraldi et al. / Surface Science 385 (1997) 376-385
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O - c ( 2 x 8 ) R h ( l l 0 ) surfaces can be tentatively correlated with the structural sensitivity reported in the literature for the CO oxidation reaction on R h ( l l 0 ) at high temperatures and high oxygen pressures, when oxygen is the dominating surface species [4]. As briefly described in the introduction, under these conditions the R h ( l l 0 ) surface undergoes different reconstructions determined by the oxygen coverage. The driving force for reconstruction is the energy gain in chemisorption energy (strengthening of the R h - O bond in the present case) which compensates for the energy loss from rearrangement of the substrate atoms. The R h - O bonding strength and bonding configuration along with surface diffusion are the main factors controlling the reactivity of oxygen. Despite that, the changes in the substrate structure can also affect the adsorption rate and adsorption state of CO and the coupling configurations required to create the transition state to CO2 formation. In
the present study we have evidenced that the structural effects on the titration rate can be linked chiefly to the different reactivity of the adsorbed oxygen. Let us first consider the apparent difference in the initial oxygen titration rates manifested by the initial slopes of the oxygen decay curves for the (2 x 1)p2mg and c(2 x 8) surfaces in Fig. 3. The initial titration rate can be described using the simple relationship r = v e x p ( - e/RT) SP¢oO o. Comparing the oxygen coverage decay curves in Fig. 3 with the CO uptake plots in Fig. 5 it becomes obvious that there is no correlation between the temperature effect on the initial titration rates and the corresponding CO adsorption rates. For example the initial CO adsorption rate is almost the same at 200 and 350 K on both the (1 x l) and (l x 4 ) surfaces, whereas the titration rates are distinctively different. In contrast there is an apparent faster decrease in the CO adsorption rate on the c(2 × 8) surface with increasing reaction temperature from 200 to 290 K, whereas the difference between the titration rates between the two surfaces remains the same. This indicates that within this temperature range the onset of oxygen titration requires very low CO coverage, so that the initial titration rate remains practically unperturbed by the changes in the CO sticking coefficient. Since the experiments are performed at the same CO partial pressure, the experimentally obtained ratio, r~z×t)/re~z×8) in the temperature range 200-350 K should be determined by the initial oxygen coverage and the kinetic parameters, v and E. The initial oxygen coverage, 0o is 1 and 0.75 ML, respectively, which results in r(2 × 1 ) / r c ( 2 × 8) = 1.33, assuming the same v and E for both surfaces. At 200 K the simplest oxygen decay plots are obtained. The oxygen plots for the (2 × 1)p2mg surface at 200 K show about twice a steeper initial slope, that is, rt2 x 1)/ret2×8)=2. In order to explain the experimental value of 2 we should assume that in addition to the oxygen coverage term the v and/or E also are different for the (2 × 1)p2mg and c(2 x 8) surfaces. Assuming that one of the latter is constant we evaluated that either the activation energy for the c ( 2 × 8 ) surface is --~0.7kJ larger or the pre-exponential factor is -~ 1.5 smaller than those of the (2 x 1) surface.
A. Baraldi et al. / Surface Science 385 (1997) 376 385
However, it is likely that both parameters vary, but the present data are insufficient for more thorough evaluations. Another approach to justify the experimental r~2x l~/rcla×8)=2 ratio at 200 K can be to take into account only the coverage of the oxygen species which are reacting at 200 K. Considering the difference in the reactivity of the oxygen located along the (1 x 1) and (1 x 2) through we assume that at 200 K the oxygen adsorbed along the (1 x 2) through is not reactive. Then we can use as an oxygen coverage term in the rate equation only the oxygen coverage along the ( 1 x 1) troughs, which is 0.5 M L for the c(2 x 8) and 1 M L for the (2 x 1 )p2mg surface. These oxygen adatoms have the same bonding configuration, and are expected to have similar reactivity. Using 0 o = 0 . 5 M L instead of the total oxygen coverage of 0.75 ML for the c(2 x 8) structure results in a twice lower reaction rate at 200 K, as experimentally observed. This approach also works for explaining the difference in the initial reaction rates at 290 K, where the initial titration rates and the ratio between them r12× 1)/rc~2 x 8) = 1.95 is very similar to that at 200 K. At 350 K the difference between the initial titration rates of the two surfaces increases, the rt2× l ) / r c l 2 × s ) ratio becomes more than 3. This is due to the strong temperature-induced enhancement of the initial titration rate for the (2 x 1 )p2mg surface, whereas that of the c ( 2 x 8 ) surface remains very similar in the 200-350 K temperature range. It is clear that at this temperature the simple approach which relates the difference in the initial titration rate to the oxygen coverage along the ( 1 x 1 ) troughs does not work, The above considerations confirm that the difference in the initial titration rate is indeed associated with the structural factor which determines the oxygen adsorption state and reactivity. The oxygen adatoms, which are more deeply embedded in the fcc three-fold sites along the ( 1 x 2) troughs of the ( 1 x 4) reconstructed surface, are nonreactive [ 13]. As manifested by the data in Fig. 3 this species is likely to be the main fraction of the residual oxygen at 200 and 290 K. Structural effect on the CO and oxygen mobility and on the transition state configuration exists as well, which
383
seems to become important at 350 K. It is likely that the anisotropic (1 x 4) surface imposes more constraint on the adspecies surface diffusion so that the reduction of oxygen remains slower at temperatures below the (1 x 4 ) to (1 x 1) phase transitions. With progressing of the reaction the slopes of the decay plots in Fig. 3 vary in a different manner. The simplest decay plots obtained at 200 K show that the reaction on both surfaces levels off after the initial almost linear decay. According to the data ~0.45 M L of oxygen from the (2 x 1)p2mg surface and ~0.20 ML from the c(2 x 8) surface have reacted at 200 K. Obviously the reaction at 2 0 0 K is impeded by the coadsorbed CO. Inspection of the O ls data shows that a large fraction of the remaining oxygen has changed its adsorption state, as manifested by the shift to a lower binding energy. If we consider the evolution of the O bonding configuration with coverage in the oxygen adlayers on R h ( l l 0 ) , this shift indicates conversion from three-fold to bridge bonded oxygen, the preferred configuration at low coverages, when no ordered structure is formed [ 17,18]. This is in agreement with the extinction of the fractional spots in the (2 x 1)p2mg and c(2 x 8) patterns related to the oxygen order. We suggest that at 200 K the reaction takes place between the first arriving CO molecules which coadsorb weakly sharing substrate atoms with oxygen. They are mobile and possibly no site sensitive, which facilitates attaining the favourable distance between C and adsorbed O in order to form COz. CO2 leaves and liberates adsorption sites where the next impinging CO molecules can stick. These CO molecules are more strongly bonded and distant from the remaining oxygen adatoms. Parallel surface process is a change of the oxygen adsorption site upon reduction of the oxygen density, which undoubtedly influences the delicate balance between the rate controlling parameters. The cessation of the reaction means that in the dense coadsorbed layer formed at 200 K an activation barrier to CO2 formation exists. This activation barrier can be overcome at higher temperatures, as evidenced by the oxygen decay plots at 290 and 350 K, when the reacting species become more
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mobile and can more easily attain the required interaction configuration. There is a substantial difference between the (2 x 1)p2mg and c(2 x 8) surfaces concerning the evolution of the oxygen titration rate with reaction time at different reaction temperatures. Fig. 3 shows that for the (2 x 1)p2mg surface the shape of oxygen decay plots at 290 and 300 K remains similar to that at 200 K, the initial slope gradually decreases and levels off. The main difference is that at higher temperatures the levelling off occurs at much lower oxygen coverage. In contrast, the shapes of the oxygen plots for the c(2 x 8) surface at 290 and 350 K differ from that at 200 K: at 290 K a monotonous continuous decrease of the reaction rate was observed, whereas at 350 K there is a discontinuity between 1 and 2 L CO exposure, indicating a sharp enhancement of the titration rate. It is notable that in the latter case after the break the rate becomes comparable with that of the (2 x 1)p2mg surface. Abrupt changes can be seen in the corresponding CO2 plots for the c(2 x 8) surface at 350 K (see Fig. 4). The break points correlate with the onset of the transition from the ( 1 × 4) to unreconstructed ( 1 × 1) surface evidenced by LEED. This major structural change during the reaction results in a reaction mechanism similar to that followed by the 0-(2 x 1)p2mg surface. This is because by partial lifting of the (1 x 4) reconstruction the concentration of the less reactive oxygen adatoms along the (1 × 2) through is reduced. Finally, let us discuss the effect of oxygen on the CO bonding configuration and adsorption rate. The most distinguishable effect, a large upward shift of the C O - O I s core level peak, was observed at 200 K when CO is coadsorbed with a large amount of oxygen. In these dense layers the O ls binding energy changes are determined by initial state (repulsive interactions and changes in the coupling between the CO and substrate orbitals) and final state relaxation effects. Since both CO and O are classified as electronegative species, the measured higher O ls binding energy for CO in these dense layers is expected considering dominance of the initial state effects, which implies weakening of the CO adsorption energy as well [19]. The C O - O ls binding energy appears to be
affected by the coadsorbed oxygen rather than by the closely located other CO molecules, because in a dense CO adlayer of 1 ML the C O - O ls binding energy of the on-top CO does not exceed 532 eV [15]. At higher reaction temperatures, when oxygen is cleaned off, the C O - O ls binding energies are similar to the one measured for an O-free surface [15,16]. The strong oxygen effect on the C O - O 1s energy supposes that the coadsorbed CO and O do not separate to form large CO or O islands. The lower adsorption rate of CO on the c(2 x 8) surface can be ascribed to the morphology of the (1 × 4) surface. When CO is adsorbed on the [ 1 [0] rows, the oxygen atoms in the fcc sites and even in the long bridge sites are nearly in the surface plane so that the blocking effect and the O - C O repulsion are weakened. On the ( 111 ) facets the C atom of the CO molecule should sit almost in the same plane with O sharing substrate atoms. This suggests that the CO adsorption onto the facets is hindered resulting in the observed lower adsorptive capacity of the partially reduced O-c(2 x 8) surface, where the oxygen along the (1 x 2) troughs remains. The interaction between CO and O in the fcc three-fold sites along the [110] rows requires the two species to be close to each other. The C - O distance in the adsorbed CO2 molecule with a O - C - O bond angle of 133 ° was reported to be 1.22 ,~ [20]. This means that a favourable CO configuration for reacting with the O located along the [1 f0] rows can be attained if CO is in the bridge site onto the rows next to the oxygen adatom. The projected distance between CO and O in this configuration will be 1.58 A and the O - C - O angle 102 °.
5. Conclusions
The impact of the surface reconstruction induced by oxygen on the oxygen titration rate by CO has been clearly manifested in this comparative study of the reactivity of the 0 - ( 2 × 1)p2mg and O-c(2 × 8) layers on Rh(110). The obtained results are a good starting basis for understanding the mechanism of the reported structural sensitivity of
A. Baraldi et al. / Surface Science 385 (1997) 376-385
CO oxidation on Rh surfaces. The main findings about the evolution of the adlayer surface composition and structure and the rate of the oxygen titration at different temperatures can be summarized as follows: (1) At reaction temperatures in the 200-350 K range the oxygen titration rate is higher for the unreconstructed O-(2x 1)p2mg surface than for the reconstructed O-c(2 × 8) surface. The initial titration rate for the O-c(2×8) surface showed weak temperature dependence in the whole temperature range, whereas the initial titration rate O-(2x 1)p2mg surface increases about twice at 350 K. The difference in the apparent reactivity of the two surfaces at temperatures _<290 K can be satisfactorily explained assuming that the initial titration rate depends only on the concentration of oxygen adsorbed along the (1 × 1) troughs, whereas the oxygen adsorbed along the ( 1 × 2) troughs is practically nonreactive. At 350 K the enhanced reactivity of the (2 × 1)p2mg surface was tentatively attributed to the structural impact on the CO and O mobility. (2) The substrate structure determined by the adsorbed oxygen remains intact at temperatures <290 K. At 350 K partial lifting of the (1 x4) surface reconstruction occurs with reducing oxygen concentration. This structural change results in an abrupt enhancement of the titration rate, which confirms the correlation between the oxygen reactivity and its bonding configuration: a large fraction of nonreactive oxygen initially located along the (1 x 2) troughs converts into more reactive species when lifting the (1 × 4) reconstruction.
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