CO2 + O on Ag(110): Stoichiometry of Carbonate Formation, Reactivity

Apr 5, 2001 - Surface carbonate species CO3(a) forms when exposing oxygen-covered Ag(110) surface to CO2. The stoichiometry of this reaction has been ...
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J. Phys. Chem. B 2001, 105, 3878-3885

CO2 + O on Ag(110): Stoichiometry of Carbonate Formation, Reactivity of Carbonate with CO, and Reconstruction-Stabilized Chemisorption of CO2† Xing-Cai Guo and Robert J. Madix* Department of Chemical Engineering, Stanford UniVersity, Stanford, California 94305-5025 ReceiVed: September 16, 2000; In Final Form: February 23, 2001

Surface carbonate species CO3(a) forms when exposing oxygen-covered Ag(110) surface to CO2. The stoichiometry of this reaction has been a subject of study for 20 years. Here, using a combined approach of TPRS (temperature programmed reaction spectroscopy), LEED (low energy electron diffraction), and STM (scanning tunneling microscopy), we establish the 1:1 stoichiometry, in contrast to conclusions reached by others. Further, we find that carbonate reacts readily with CO, resulting in a chemisorbed CO2 state. The reactivity of carbonate is as high as that of atomic oxygen under the same conditions. The chemisorbed CO2 desorbs at 485 K, distinct from physisorbed CO2 which desorbs at 130 K. The (1 × 2) reconstruction induced by carbonate formation appears essential to CO2 chemisorption. Direct CO2 exposure at 300 K on Ag(110)(1 × 2) partially covered by chemisorbed CO2 leads to additional CO2 adsorption, whereas on clean Ag(110)-(1 × 1) no uptake is observed at this temperature. As a result of these reactions, background CO inherent in ultrahigh vacuum converts carbonate into chemisorbed CO2 within a few minutes in UHV or with large CO2 exposures (>30 L). STM images we have acquired of pure chemisorbed CO2 on Ag(110)(1 × 2) bear strong resemblance to those previously assigned to carbonate by others; we reinterpret these STM features as due to chemisorbed CO2. Mobility and disorder of chemisorbed CO2 are also observed. At a sufficiently long time or large CO exposures chemisorbed CO2 is depleted from the surface, which reverts to clean Ag(110)-(1 × 1).

1. Introduction The reactions of CO2 and oxygen adsorbed on Ag(110) were first investigated 20 years ago.1 Though no CO2 adsorption was found on the clean Ag(110) at 160 K, on the basis of temperature programmed reaction spectroscopy (TPRS) surface carbonate, CO3(a), was suggested to form when exposing oxygen precovered Ag(110) to CO2 at 300 K. The following reaction steps were proposed to occur on the surface:

300 K:

CO2(a) + O(a) f CO3(a)

(1)

485 K:

CO3(a) f CO2(a) + O(a)

(2)

where CO2(a) desorbs immediately upon carbonate decomposition, and O(a) desorbs associatively as O2 around 590 K. The stoichiometry of the reactions was probed using isotope-labeled oxygen 18O2, which, according to reactions 1 and 2, should yield C16O16O18O(a) and product yield ratios, C16O16O: C16O18O ) 1:2 and 18O: 16O (in dioxygen) ) 1:2 if all three CsO bonds in C16O16O18O(a) react equivalently. Experimentally, only a very slight excess of 18O was observed to desorb, and this result was attributed to a small fraction of preadsorbed oxygen which remained unreacted with CO2. These results suggested that (A) a surface carbonate species with three equivalent CsO bonds is indeed formed, and (B) almost all of the desorbed CO2 and O2 result from carbonate decomposition, i.e., the stoichiometry of reaction 2 is correct. Furthermore it was suggested that the stoichiometry of reaction 1 was correct. However, this conclusion was in fact based on an implicit premise that the amount †

Part of the special issue “John T. Yates, Jr. Festschrift”. * Corresponding author. E-mail: [email protected]. Fax: 650-723-9780.

of preadsorbed O on the surface in whatever form remains the same before and after CO2 exposure and that all of it desorbs at the 590 K peak. In the following three years more detailed studies of the same system were carried out in this laboratory using LEED,2 HREELS,3 and XPS/UPS.4 The formation of the surface carbonate species was confirmed by surface spectroscopic studies. It was observed that carbonate formation transformed a p(3 × 1)-O LEED pattern to p(2 × 1), and finally to p(1 × 2) symmetry.2 Subsequently there appeared many studies of the same system from other laboratories.5-17 Backx et al.6 contested the 1:1 stoichiometry in CO3(a) formation. These authors found that the amount of CO2 obtained from CO titration of preadsorbed O was “twice” (no actual value given) the amount of CO2 desorbing from the decomposition of carbonate after the reaction of CO2 with the same dosage of preadsorbed oxygen. This result indicated that one CO2 molecule was adsorbed per two oxygen atoms on the surface, or CO2:O ) 1:2. They proposed that half of the preadsorbed oxygen went into the “subsurface” and could not be detected by thermal desorption. In fact, no subsurface oxygen could be detected by any of the techniques (AES, LEED, or EELS) used in their experiments. Furthermore, the results are questionable because their surface was cleaned by Ar ion sputtering only, without any oxygen treatment to clean off S and C impurities. This cleaning procedure can lead to uncertainty in the peak areas in oxygen adsorption/desorption cycles, as reported earlier by the same authors.18 Support for the 1:1 stoichiometry came from the XPS and TPD studies by Campbell and Paffett.7 The authors compared the O(1s)-XPS peak areas after 1500 L CO2 exposure on an

10.1021/jp003328n CCC: $20.00 © 2001 American Chemical Society Published on Web 04/05/2001

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O-adlayer at 0.085 ML with that obtained after flashing the surface to 500 K to desorb CO2. The CO2:O ratio was found to be 1.5:1. Obviously, this ratio is closer to 1:1 than to 1:2, but only the stoichiometry of reaction 2 was probed. The results do not have direct bearing on reaction 1. The 1:1 stoichiometry was challenged again 12 years later by Stensgaard et al.12 with the benefit of beautiful STM images; coverage values were obtained by the counting of features attributed to adsorbed oxygen (before reaction) and carbonate (after reaction with CO2). To ensure complete reaction of all preadsorbed oxygen atoms, extremely large CO2 exposures were used in three experiments: (1) 700 L for O-precoverage of 0.09 ML, (2) 4200 L for O-precoverage of ∼0.23 ML, and (3) more than 10000 L for O-precoverage of ∼0.31 ML. After the CO2 exposures, bright spots (“protrusions”) appeared in the STM images. The authors conclude that “In all cases, the ratio of carbonate ions formed to oxygen atoms consumed from added rows is close to 1:2.” From the given number of protrusions, the following ratios were derived for each experiment: (1) 1:2.1, (2) 1:2.4, and (3) 1:3.1. It was argued “that the protrusions are indeed the carbonate species and that the fluctuations observed in the approximate 1:2 ratio are related to the imperfection of the accompanying reconstruction of the surface.” To account for the missing half of the preadsorbed oxygen the authors proposed that oxygen desorbed during CO2 exposure at 300 K, i.e., reaction 1 was modified as follows:

1 CO2(g) + 2 O(a) f CO3(a) + O2(g) 2

(4)

No experiments directed toward proving this postulate were reported. In the same year, STM studies of the same system were also reported from two other laboratories.13,14 Similar protrusions were recorded and designated as carbonate species accordingly. In one experiment a 100000 L CO2 exposure was used.13 The state of “missing” oxygen remained unknown. Clearly, the reactions of CO2, O and carbonate on Ag(110) are far from understood, and the controversy over the reaction stoichiometry is not settled. In this paper the 1:1 stoichiomentry is established. Surprising effects of long measurement time and/ or large CO2 exposures are demonstrated that include the discovery of a strongly chemisorbed state of CO2. Finally it is found that the protrusions reported in previous STM studies appear not to be carbonate, but CO2 chemisorbed on a p(2 × 1) reconstructed Ag(110) surface.

Figure 1. (a) Temperature programmed desorption spectrum of oxygen from Ag(110) after 30 L O2 exposure at 300 K (2 K/s heating rate). (b) Temperature programmed reaction spectra from Ag(110) after exposure to 30 L O2 and 30 L CO2 at 300 K. CO2 from carbonate decomposition desorbs at 485 K (2 K/s heating rate). The O2 peak area is the same as in the blank experiment, to within 6%.

peaks. Final cleaning was achieved using oxygen adsorptiondesorption cycles (1 × 10-6 Torr for 10 min at 300 K and flashing the sample to 700 K) until no CO or CO2 desorbed from the surface. After the excessive oxygen exposures the chamber pressure usually increased to 1 × 10-9 Torr, with the major background gases being H2, H2O, CO, and CO2. STM images showed that clean Ag(110) surface had large terraces several thousand angstroms wide. High purity gases were purchased from Praxair without further purification: O2 (99.999%), CO (99.99%), and CO2 (99.9%). The purity of gases introduced to the chamber was checked with the mass spectrometer. O2 was delivered to the surface via a capillary array doser, and CO2 via a needle doser. CO was exposed with an ambient pressure to ensure a uniform exposure. Exposure was expressed in Langmuir (1 L ) 10-6 Torr s). The enhancement factor in doser exposure was not corrected. All gas exposures were carried out at 300 K. A nearly linear heating rate of 2 K/s was used in the TPRS experiments. Both LEED and STM measurements were conducted at 300 K. 3. Results and Discusssion

2. Experimental Section Experiments were carried out in an ultrahigh vacuum (UHV) apparatus (typical base pressure ) 2 × 10-10 Torr) equipped with STM (SPM 100 by RHK Technology), LEED/AES (PRI 179), and TPRS techniques. The apparatus consists of two chambers: a sample preparation chamber and an STM chamber. Sample cleaning, LEED/AES and TPRS were conducted in the preparation chamber. Sample transfer between the two chambers took less than 5 min. A QMS from Balzers Instruments (QME 200) was used for TPRS measurements. The Ag(110) crystal was aligned to within 0.5° of the (110) plane using Laue backscattering, and mechanically polished with diamond paste down to 0.25 µm. The crystal was mounted on a sample holder to expose a surface of 8.5 mm in diameter. The surface was cleaned by Ar ion bombardment (2 µA, 500 eV, 15 min at 300 K), oxygen treatment (1 × 10-7 Torr at 500 K), and annealing at 800 K. After the cleaning procedure no S or C impurities could be seen in the Auger electron spectra. However, AES is not sensitive to C due to overlap with the Ag

3.1. TPRS Measurements. 3.1.1. Proof of the 1:1 Stoichiometry. As mentioned in the Introduction, the premise of the original suggestion on a 1:1 reaction stoichiometry in carbonate formation was that the amount of preadsorbed oxygen remains the same before and after CO2 exposure and that all of it will eventually desorb at the 590 K dioxygen peak. A simple blank experiment suffices to prove this premise (Figure 1). First, clean Ag(110) surface was exposed to 30 L oxygen at 300 K, resulting in an oxygen desorption peak at 585 K (Figure 1a). The peak area for the same oxygen exposure was reproducible for many successive adsorption-desorption cycles, in contrast to the results of Backx et al.,18 who found that the TPD peak area continued to increase following the same exposure in adsorption-desorption cycles. They proposed that more than half of adsorbed oxygen atoms went into “subsurface” and did not desorb around 585 K. However, the reproducibility of the TPD areas in our experiments indicate that all of adsorbed oxygen atoms have desorbed at 585 K. In fact, when we cleaned the Ag(110) surface as they did (i.e. by ion sputtering only, without

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oxygen treatment), we also observed a progressive increase in the amount of oxygen desorbing with each successive dose, but we also saw the evolution of CO and CO2, which they did not monitor. This result suggests that the missing oxygen atoms actually are consumed by clean-off reactions with surface carbon impurities. Figure 1b shows the TPRS spectra from the Ag(110) surface which was first exposed to 30 L O2 at the same positions using the same doser as the blank experiment and then exposed to 30 L CO2 using a separate doser from a separate gas line. As expected,1,2 CO2 from carbonate decomposition desorbs around 485 K, and O2 desorbs around 585 K. Importantly, the O2 peak area is the same as in the blank experiment shown in Figure 1a to within 6%. If half of the preadsorbed oxygen were to desorb during CO2 exposure, as proposed by Stensgaard et al.,12 the O2 peak area in Figure 1b would be half of that in Figure 1a. This is clearly not the case. Reaction 4 is therefore not correct; to within experimental error there is no loss of adsorbed oxygen in the reaction to form carbonate. Combining the results of Figure 1 with those of the previous isotope experiments, we can prove rigorously that the stoichiometry of carbonate formation is 1:1. In the experiment of Figure 1 the CO2 exposure is relatively low, and there may be some atomic oxygen remaining on the surface after 30 L CO2 exposure at 300 K. Thus, we have

300 K: iCO2(a) + jO(a) f iCO3(a) + (j-i-k)O(a) + kO(u) (5) 485 K: iCO3(a) f iCO2(a) + iO(a)

(6)

where i, j, and k are integers. O(a) stands for adsorbed atomic oxygen which desorbs around 585 K. O(u) stands for an unknown oxygen state undetectable with desorption methods. The overall reaction is the sum of reactions 5 and 6,

iCO2(a) + jO(a) f iCO2(a) + (j-k)O(a) + kO(u) (7) Our first experiment measures jO(a), and the second experiment measures (j-k)O(a). The two are equal according to our results, i.e., j ) j-k. Therefore, k ) 0. The unknown oxygen state, O(u), is nonexistent. Reaction 5 becomes

iCO2(a) + jO(a) f iCO3(a) + (j-i)O(a)

(8)

For a complete stoichiometric reaction, all of adsorbed oxygen atoms result in carbonate formation; to within experimental error this was shown to be the case in the previous isotope experiments at higher CO2 exposures. With no unreacted oxygen left on the surface, j-i ) 0, or j ) i. Reaction 8 becomes

iCO2(a) + iO(a) f iCO3(a)

(9)

CO2(a) + O(a) f CO3(a)

(1′)

or

3.1.2. The Stability of Carbonate with Time in UHV. Atomresolved STM measurements may take an appreciable time to achieve useable results. Generally, the times spent between exposure of surfaces to gases and the recording of the images are not reported in STM work. For reactive systems, of course, there may be changes that occur in this time interval. In this study we prepared the carbonate as described above, 30 L O2 exposure followed by 30 L CO2 at 300 K. The chamber was then evacuated, and the crystal was positioned for TPRS, taking less than 3 min. The temperature was ramped, and products

Figure 2. (a) Temperature programmed reaction spectra from Ag(110) after exposure to 30 L O2 and 30 L CO2 at 300 K. The data were collected less than 3 min after CO2 exposure. (b) Temperature programmed reaction spectra from Ag(110) after exposure to 30 L O2 and 30 L CO2 at 300 K. The data were collected 50 min after CO2 exposure at a background pressure of 1 × 10-9 Torr. The absence of oxygen desorption indicates that only CO2 exists on the surface at 300 K.

were monitored, yielding the TPR spectrum shown in Figure 2a. In the second experiment we prepared the surface exactly the same way. After the CO2 exposure the crystal was turned away from the doser, but before starting the TPRS measurement we deliberately waited for 50 min. During this time the crystal was positioned to face the viewport window, the mass spectrometer was off, and the ion gauge indicated a pressure of 1 × 10-9 Torr. The TPR spectrum recorded after this 50 min waiting is shown in Figure 2b. The CO2 peak appears at the same temperature, slightly diminished in magnitude, but the O2 peak is absent. No oxygen desorbs up to 800 K. As discussed in the last section, thermal desorption does indeed give the measure of the amount of oxygen present at 300 K, either as adsorbed oxgen or released from carbonate decomposition. The absence of O2 desorption peak in the TPR spectrum indicates that CO3(a) has, at 300 K, become CO2(a) which desorbs at 485 K. This form of CO2(a) is clearly distinct from physisorbed CO2(a) on Ag(110), which desorbs at 130 K.3 We therefore designate it as “chemisorbed CO2(a)”. The cause of the conversion from carbonate to chemisorbed CO2 will be shown later. What we have demonstrated here is that carbonate no longer exists 50 min after its formation in UHV. The length of time required for its disappearance may vary, but the effect is real. 3.1.3. The Effect of Large CO2 Exposure on the Surface Carbonate. Extremely large CO2 exposures (up to 100,000 L) have been used in previous STM measurements with the intention to react all oxygen atoms. Thus the effect of large CO2 exposure in UHV on the carbonate was explored. The oxygen exposure was fixed at 30 L at room temperature, and the subsequent CO2 exposure was varied. The TPRS peak areas measured for CO2 and O2 are shown in Figure 3a-c. For low CO2 exposures (e30 L, Figure 3a), the amount of oxygen released, which consists of both oxygen incorporated into carbonate and unreacted oxygen atoms on the surface, remains constant. The amount of CO2 desorption from carbonate decomposition increases with increasing CO2 exposure. Beyond 30 L CO2 exposure, however, the amount of oxygen evolved begins to decline, whereas the amount of CO2 continues to increase, as shown in Figure 3b. The cause of oxygen

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Figure 4. A typical background mass spectrum taken at 7 × 10-10 Torr after surface cleaning, O2 and CO2 exposures. Major background gases in a UHV chamber are H2, H2O, CO, and CO2.

preadsorbed oxygen desorbs at the usual temperature without any changes in line-shape due to water adsorption.1 CO2 is the gas being dosed. Background CO2 cannot contribute to additional reactions other than those caused by CO2 exposed from the doser. Background CO remains as a possible reactant. In general, background CO is generated during CO2 exposure by the ion pump and ion gauge, etc. It has been observed that CO reacts effectively with preadsorbed oxygen on Ag(110), and the oxidation product CO2 desorbs upon formation,1 i.e., at 300 K: Figure 3. Peak areas of oxygen and CO2 in TPR spectra versus CO2 exposures on Ag(110) precovered with constant oxygen coverage. The data are shown in expanded ranges: (a) 0 to 30 L, (b) 0 to 300 L, and (c) 0 to 4500 L. The amount of oxygen remains constant below 30 L CO2 exposure, thereafter it declines with increasing CO2 exposure, down to zero above 3000 L CO2 exposure. The amount of CO2 increases continuously and reaches saturation above 1500 L.

depletion will be discussed later. The amount of CO2 desorbing continues to increase even though the amount of oxygen, and hence the amount of carbonate, decreases. This trend continues with continued CO2 exposure up to 1500 L, as seen in Figure 3c. The amount of CO2 approaches saturation above 1500 L, while oxygen is completely depleted above 3000 L. What remains on the surface is solely CO2 with no oxygen of any form, the same state that was found after waiting in UHV. However, in this case the CO2 coverage is doubled. Thus, for large CO2 exposures, not only has all the carbonate initially formed been converted to CO2, but also more CO2 adsorbs on the otherwise clean surface. Therefore, large CO2 exposure does not necessarily increase the extent of carbonate formation. It may convert carbonate to chemisorbed CO2, and lead to more CO2 adsorption. 3.1.4. ReactiVity of Carbonate with CO. The results of the two experiments reported above appear to be due to a reaction with background gases inside the UHV chamber. Typical major background gases in a UHV chamber are H2, H2O, CO, and CO2, as shown in Figure 4 for a typical mass scan at 7 × 10-10 Torr after surface cleaning, oxygen treatment, and CO2 exposure. It has been found previously that hydrogen does not adsorb on an oxygen-covered Ag(110) surface even at 160 K and that no reaction occurs between O(a) and H2(g) between 160 and 1000 K.1 Water adsorbs and desorbs on oxygen-covered Ag(110) surface at 300 K, undergoing only isotope exchange reactions;

O(a) + CO(g) f CO2(g)

(10)

The reactivity of CO with carbonate on Ag(110) can be tested by deliberately exposing pure CO gas to carbonate-covered Ag(110). Figure 5a displays the results of such an experiment. 30 L oxygen exposure was followed by 30 L CO2 exposure, leading to a carbonate-covered Ag(110) surface. The carbonate-covered surface was then treated with variable CO exposures. After each CO exposure TPRS was carried out to measure the amount of remaining oxygen. The decrease in the remaining oxygen with increasing CO exposure indicates that oxygen in the carbonate species is consumed and that reaction does occur between carbonate and CO. The evolution of CO2 was also monitored, and the spectra are shown in Figure 5b. The evolution peak temperature for CO2 shifts slightly to low temperature as CO exposure increases, and a low temperature shoulder emerges, whereas the peak area remains constant within the range of CO exposure employed. The constant yield of CO2 shows that the amount of carbonate prepared is the same for each measurement and that the original CO2 remains adsorbed on the surface after the conversion. Therefore, the following reaction is believed to occur:

300 K: CO3(a) + CO(g) f CO2(a) + CO2(g) (11) At 1 L CO exposure almost all preadsorbed oxygen atoms are consumed by the oxidation reaction, leaving a pure adlayer of CO2(a) on an otherwise clean Ag(110) surface. For comparison, results from CO reactions with preadsorbed atomic oxygen are also shown in Figure 5a. The same decreasing trend is observed in the amount of oxygen remaining versus

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Figure 6. (a) Thermal desorption spectrum of chemisorbed CO2(a) prepared by carbonate conversion (30 L O2, 30 L CO2, and 1 L CO at 300 K). Only chemisorbed CO2 is present on the surface at 300 K. (b) Thermal desorption spectrum of CO2(a) following 1500 L CO2 exposure on the CO2-precovered surface at 300 K. The 71% increase in desorption peak area indicates additional CO2 has adsorbed on the surface without the presence of oxygen or carbonate.

Figure 5. (a) Amount of remaining oxygen versus CO exposure to carbonate-covered Ag(110). Data are obtained from the TPRS measurements. The TPR spectra for CO2 are shown in (b). The decline in the remaining oxygen amount shows the reactivity of carbonate toward CO oxidation. Results from CO reactions with preadsorbed atomic oxygen are also shown in (a) as filled circles for comparison. Firstorder kinetics fit both curves.

CO exposure. This suggests that carbonate is as reactive as atomic oxygen for CO oxidation on Ag(110) at 300 K. It is clear that the conversion from carbonate to chemisorbed CO2(a) during waiting in UHV or large CO2 exposure is actually caused by the reaction between carbonate on Ag(110) and background CO in the UHV chamber. 3.1.5. CO2 Adsorption by Direct CO2 Exposure. Further adsorption of CO2 on the Ag(110) surface can be achieved without the presence of O(a). Shown in Figure 6a is a thermal desorption spectrum of chemisorbed CO2(a) prepared by carbonate conversion as described above (30 L O2, 30 L CO2, and 1 L CO at 300 K). In this case, only chemisorbed CO2(a) is present on the surface at 300 K. Next such a CO2-precovered surface was exposure to 1500 L CO2 at 300 K, yielding the thermal desorption spectrum displayed in Figure 6b. The desorption peak area increased by 71%. Clearly additional CO2 adsorbed on the surface though there was no oxygen or carbonate present. Therefore, CO2 adsorption does not require the presence of reactive oxygen. We will show below that CO2 adsorption is actually related to a surface reconstruction induced by carbonate formation. 3.1.6. The Effect of Large CO Exposure. The experiments on carbonate reactivity described above show that the evolution

Figure 7. Temperature programmed reaction spectra following carbonate reaction with CO at 300 K for a CO exposure of (a) 1.2 L, (b) 2.4 L, and (c) 3.6 L. The initial conditions were the same those shown in Figure 5b. Carbonate was prepared with 30 L O2 and 30 L CO2 at 300 K. As CO exposure is doubled, there is a 46% loss of CO2. A 64% loss occurs at 3.6 L. No oxygen desorption was detected in any of the spectra (not shown).

of CO2 shifts to lower temperatures as the CO exposure increases (see Figure 5b). The temperature shifts and changes in peak shape are more pronounced with larger CO exposures (Figure 7). No oxygen desorption was detected in any of the spectra (not shown). At a CO exposure of 1.2 L a low temperature shoulder appears on the CO2 peak at 460 K. As CO exposure is doubled, the major peak at 485 K almost disappears, and the low temperature shoulder develops into a peak, with a 46% loss of CO2. The loss continues to 64% with an even higher CO exposure at 3.6 L. Eventually all chemisorbed CO2(a) can be depleted by CO exposure. CO does not adsorb on Ag(110) at 300 K, and therefore no substitutional displacement occurs here. The cause for CO2 depletion is not yet understood. 3.1.7. Effects InVolVed in PreVious STM Experiments. As we mentioned in the Introduction, three groups have previously carried out STM studies and imaged what they believed to be carbonate. However, in light of the effects reported here, and the STM images discussed below, the “carbonate” in their images appears to have been chemisorbed CO2(a). The protrusions counted in the STM experiments of Stensgaard et al.12

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Figure 8. LEED patterns of the Ag(110) surface with a schematic showing surface preparation procedures and what is on the surface. (a) Clean (1 × 1), (b) streaky (n × 1) after 30 L O2 exposure (n ) 3-4), (c) sharp (1 × 2) after 36 L CO2, (d) sharp (1 × 2) after 1 L CO exposure, (e) diffuse (1 × 2) after 600 L CO2 exposure, (f) diffuse (1 × 1) after 120 L CO exposure.

therefore appear to be chemisorbed CO2(a). Furthermore, it is very unlikely that the stoichiometries they calculated have any bearing on the reaction between preadsorbed oxygen and CO2, for the reasons enumerated above. 3.2. LEED Measurements. LEED measurements were performed at 300 K with an electron beam energy ranging from 187 to 199 eV. LEED patterns are displayed in Figure 8 along with a schematic showing the surface preparation procedures for each LEED measurement. Figure 8a is a (1 × 1) pattern of the clean Ag(110) surface. After a 30 L O2 exposure the surface exhibits a streaky (n×1) LEED pattern (Figure 8b), which is due to mobile, imperfect (n×1) oxygen domains (n ) 3 and 4 as verified by STM images below). Exposure of 36 L CO2 leads to the appearance of a (1 × 2) LEED features (Figure 8c). As we found in the TPRS measurements above, such a CO2 exposure is sufficient to react with most of the adsorbed oxygen atoms, converting them to carbonate species. Clearly carbonate formation leads to a (1 × 2) surface reconstruction, as proposed previously.10 The (1 × 2) reconstruction is stabilized by adsorbed carbonate. Thermal decomposition of carbonate into CO2 and O(a) lifts the reconstruction, the surface reverting to (1 × 1). The (1 × 2) surface structure remains following the carbonate conversion to chemisorbed CO2(a) by 1 L CO exposure (Figure 8d). In this case, no adsorbed atomic oxygen is left on the surface; only chemisorbed CO2 remains. This demonstrates that chemisorbed CO2 resides on the (1 × 2) reconstructed Ag(110) surface. The fact that no CO2 adsorption occurs on the (1 × 1) nonreconstructed Ag(110) suggests that the (1 × 2) reconstruction is essential for CO2 chemisorption. Additional CO2 can be adsorbed on the (1 × 2) reconstructed surface with increased CO2 exposures (600 L), resulting in a diffuse (1 × 2) LEED pattern (Figure 8e). This result suggests that the overall surface

Figure 9. STM image and stacking ball model of the clean Ag(110) surface. The STM image was taken at 300 K at a tunneling current of 0.12 nA and a sample bias of -0.7 V. The (1 × 1) unit cell (4.09 × 2.89 Å2) is indicated.

structure remains as (1 × 2), but the CO2 is not perfectly ordered. The (1 × 2) structure finally disappears upon the depletion of all adsorbed CO2(a) by large CO exposures (120 L), and the surface exhibits a diffuse (1 × 1) LEED pattern (Figure 8f). This result suggests that CO2 chemisorbed on the (1 × 2) surface may be destabilized by lifting the surface reconstruction. 3.3. STM Measurements. 3.3.1. Structure of Clean and Oxygen-CoVered Surfaces. The clean and oxygen-covered Ag(110) surfaces have been studied previously with STM.19 The STM images observed in this work for these surfaces are shown for reference to those observed for CO2 adsorption. Figure 9 displays an STM image and a stacking ball model of the clean Ag(110) surface. The (1 × 1) unit cell (4.09 × 2.89 Å2) is indicated. The atom-resolved STM image shows some point defects (adatoms and vacancies) on an otherwise perfect (110) terrace. A typical area of the surface consists of terraces several thousand Angstroms wide separated by mono- or multiatomic steps. When the clean Ag(110) surface was exposed to 30 L oxygen, the STM image shown in Figure 10a was obtained; the corresponding model is shown in Figure 10b. The -Ag-O- rows seem to be quite mobile, appearing as zigzag

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Figure 10. STM image and added -Ag-O- row model of the oxygen-covered Ag(110) surface. Oxygen was exposed at 300 K for 30 L. STM images was taken at 300 K at tunneling a current of 382 fA and a sample bias of +193 mV. The spacing between the rows is 8 ∼ 12 Å, or 3 ∼ 4 lattice unit. The p(4 × 1) and p(3 × 1) unit cells are indicated.

-Ag-O- rows in a single image, and exhibiting displacements in consecutive STM images (not shown). The spacing between the rows is 8 ∼ 12 Å, or 3 ∼ 4 lattice unit. The p(4 × 1) and p(3 × 1) unit cells are indicated on the model. These images are consistent with the observed streaky (n × 1) LEED pattern. Similar structures have been observed previously by others with STM.19 3.3.2. Structure of Chemisorbed CO2. An STM image of the Ag(110) surface after exposures of 30 L O2 and 1500 L CO2 is displayed in Figure 11a. The image was recorded 1 h 52 min after the exposure. As shown by our TPRS measurements, this preparation procedure leads to chemisorbed CO2 on the surface without any carbonate or O(a). This expectation was reconfirmed by a TPRS measurement immediately following the STM observation. The round protrusions in the image, as well as in the images shown below, resemble those seen in previous STM experiments,12-14 strongly suggesting that the protrusions are actually chemisorbed CO2(a), not carbonate species. The underlying (1 × 2) reconstruction suggested by LEED measurements is clearly visible in this STM image. A structural model for the chemisorbed CO2(a) on top of the (1 × 2) reconstructed Ag(110) surface is therefore proposed and depicted in Figure 11b. The top portion of the schematic shows some

Guo and Madix

Figure 11. STM image of the Ag(110) surface after exposures of 30 L O2 and 1500 L CO2. The image was recorded 1 h 52 min after the exposures at 300 K at tunneling a current of 392 pA and a sample bias of +282 mV. Only chemisorbed CO2 is on the surface as shown by afterward TPRS. The bright round protrusions are assigned to chemisorbed CO2, and light gray features are added Ag rows. A structural model is depicted at the bottom for the chemisorbed CO2(a) on top of the (1 × 2) reconstructed Ag(110) surface. The (3 × 2) or shifted (3 × 2) unit cells are indicated. A “triplet” structure is circled.

(3 × 2) units based on our STM images as well as those recorded previously by others.12,14 The lower-left part shows shifted (3 × 2) units which are frequently observed (see below and ref 12). The so-called “triplet” structures observed early12 are shown in the lower-right part of the figure. The disorder in different domains as well as the mobility along the Ag rows (see below) may render a (1 × 2) LEED pattern instead of the (3 × 2). 3.3.3. Mobility of Chemisorbed CO2. The mobility of chemisorbed CO2(a) is illustrated by the STM images shown in Figure 12a. In the image acquired at a scanning speed of 2 min/frame, a slow moving CO2(a) appears as a round protrusion, whereas a fast moving CO2(a) shows up as a short line one pixel wide. CO2 appears to be moving along the added Ag rows. Similar observations have been made for S on Re(0001) where simulated images of diffusing S reproduce the exact line features.20 The mobility along the [1 1h 0] direction makes the transient (3 × 2) order invisible in the LEED pattern; the (1 × 2) order is observed instead. As discussed above TPRS measurements show that additional CO2 can be adsorbed on the Ag(110) surface covered with CO2 resulting from the conversion of carbonate (cf. Figure 6); this chemisorbed CO2(a) at high coverage was imaged by STM. The surface was prepared with the following exposure sequence at 300 K: 30 L O2 (preadsorbed atomic oxygen), 30 L CO2 (creates carbonate), 1 L CO (convert to chemisorbed CO2), and 660 L

CO2 + O on Ag(110)

J. Phys. Chem. B, Vol. 105, No. 18, 2001 3885 (1) There is no loss of oxygen of any form during carbonate formation, giving evidence to the premise of the 1:1 proposal in CO2:O stoichiometry. (2) All carbonate is converted to chemisorbed carbon dioxide with time after its formation in UHV. (3) Large CO2 exposures do not necessarily increase the extent of reaction for carbonate formation. Such exposure decreases the amount of oxygen released, converts carbonate to chemisorbed CO2, and leads to more CO2 adsorption. (4) Experiments with pure CO show that carbonate is converted to chemisorbed CO2 by reacting with background CO during waiting or large CO2 exposures. Carbonate is as reactive as atomic oxygen for CO oxidation on Ag(110) at 300 K. (5) Further CO2 exposure on the CO2-converted surface leads to additional CO2 chemisorption, indicating that CO2 adsorption can occur directly on the reconstructed surface. (6) With varying CO exposures it is found that chemisorbed CO2 is depleted by large CO exposures. (7) LEED measurements demonstrate that chemisorbed CO2 resides on the (1 × 2) reconstructed Ag(110) surface and the reconstruction is important for CO2 adsorption. (8) STM images of pure chemisorbed CO2 are obtained which resemble those seen in previous STM experiments but were assigned to carbonate. (9) A structural model with (3 × 2) and shifted (3 × 2) unit cells is proposed for the chemisorbed CO2 on the Ag(110)(1 × 2) surface. Observed disorder and mobility of CO2(a) explain the appearance of a (1 × 2) LEED pattern. Note Added in Proof. Fig. 8 does not adequately show the additional two fold features that appear in frames (c), (d) and (e). Though fainter than the brightest diffraction spots, they are clearly visible. Acknowledgment. We gratefully acknowledge the National Science Foundation (NSF CHE 9820703) for support of this work.

Figure 12. (a) STM image of low coverage CO2 on Ag(110). The surface was prepared by exposures of 30 L O2 and 1500 L CO2 at 300 K. Only chemisorbed CO2 is on the surface as shown by TPRS. The image was taken at 300 K at tunneling a current of 483 pA, a sample bias of +303 mV, and a scanning speed of 2 min/frame. As indicated, the bright round protrusions are assigned to chemisorbed CO2. As indicated, slow moving CO2(a) appears as a round protrusion, whereas fast moving CO2(a) shows up as a short line of one pixel wide. (b) STM image of high coverage CO2 on Ag(110). The surface was prepared by exposures of 30 L O2, 30 L CO2, 1 L CO, and 660 L CO2 at 300 K. Only chemisorbed CO2 is on the surface as shown by afterward TPRS. The image was taken at 300 K at tunneling a current of 343 pA, a sample bias of +440 mV, and a scanning speed of 30 s/frame. The (3 × 2) and shifted (3 × 2) unit cells are indicated.

CO2 (adsorb additional CO2). An STM image of such a surface is shown in Figure 12b. The image was recorded at a scanning speed of 30 s/frame. As seen from the image, the local structure of chemisorbed CO2(a) (small round protrusions) is again (3 × 2) or shifted (3 × 2), as indicated. Even at such a high coverage, chemisorbed CO2(a) appears to be quite mobile along the added Ag rows, as seen in an STM movie recorded at 20 s/frame (data not shown). Summary Using a combination of TPRS, LEED, and STM techniques, we have shown that

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