TiO2 Model

Unfortunately, this precludes a direct comparison of the reactivity of molecular oxygen in the carbon monoxide oxidation reaction as a function of gol...
15 downloads 0 Views 188KB Size
Download PDF
J. Phys. Chem. B 2006, 110, 20337-20343

20337

Reactivity of Molecularly Chemisorbed Oxygen on a Au/TiO2 Model Catalyst James D. Stiehl, Jinlong Gong, Rotimi A. Ojifinni, Tae S. Kim, Sean M. McClure, and C. Buddie Mullins* The UniVersity of Texas Austin, Department of Chemical Engineering and Texas Materials Institute, 1 UniVersity Station CO400, Austin, Texas 78712-0231 ReceiVed: May 5, 2006; In Final Form: July 31, 2006

We present results of an investigation into the reactivity of molecularly chemisorbed oxygen with CO on a Au/TiO2 model catalyst at 77 K. We previously discovered that exposing the model catalyst sample to a radio-frequency-generated plasma jet of oxygen results in co-population of both atomically and molecularly chemisorbed oxygen species on the sample. We tested the reactivity of the molecularly chemisorbed oxygen by comparing the CO2 produced from a sample populated with both species to the CO2 produced from a sample that has been cleared of molecularly chemisorbed oxygen employing collision-induced desorption. Samples that are populated with both species consistently result in greater CO2 produced than samples with only atomic oxygen. We interpret this result to indicate that molecularly chemisorbed oxygen on the sample can directly participate in the CO oxidation reaction. The reactivity of molecularly chemisorbed oxygen has been investigated for five different gold coverages (0.5, 0.75, 1, 1.25, and 2 ML), and we observe that there is a greater fractional difference in the CO2 produced (difference between sample populated with both molecularly and atomically adsorbed oxygen and sample populated solely with atomically adsorbed oxygen) for the 1 ML Au coverage than for the other coverages for equivalent oxygen plasma-jet exposures. However, it is not possible to unambiguously conclude that this observation is directly related to a particle size effect on the chemistry since the absolute O2,a and Oa content on the various surfaces is different for all the coverages studied because of the plasma-jet technique that we employed for populating the surfaces with oxygen. Unfortunately, this precludes a direct comparison of the reactivity of molecular oxygen in the carbon monoxide oxidation reaction as a function of gold coverage and hence particle size.

Introduction Questions regarding the mechanistic details of the CO oxidation reaction on gold-based catalysts have driven the research of many scientists since the outset of this unexpected discovery by Haruta.1 That gold, a metal with a reputation for being inert in its bulk form,2 could be involved in catalyzing a reaction, which inherently necessitates the activation of an oxygen species, was very surprising. Some specific issues that remain active subjects of research regarding CO oxidation on Au catalysts are the particle size effect observed for the reaction, the oxidation state of the gold, CO adsorption, moisture effects, support effects, catalyst pretreatment, and, what is of interest in this study, the active oxygen species involved in the reaction.1,3-5 Despite all the work that has been done concerning the CO oxidation reaction on gold catalysts, as of yet there seems to be no satisfactory picture of the reaction mechanism,5 including identification of the active oxygen species. One possible candidate for the active oxygen species in catalytically relevant chemistry is atomically adsorbed oxygen (Oa). Atomic oxygen has been shown to be highly reactive toward CO when preadsorbed on single-crystal gold samples6-10 and also on Au model catalysts.11-13 One of the first studies of the reactivity of Oa on gold was performed by Outka et al. on a Au(110) single crystal.6 Outka et al. populated a Au(110) single-crystal sample with atomic oxygen by cracking molecular oxygen on a hot platinum filament near the Au surface. They * To whom correspondence should be addressed. E-mail: mullins@ che.utexas.edu.

observed prompt evolution of CO2 from the oxygen precovered sample on exposure to an effusive source of CO over the temperature range 275-440 K. Recent work by Gottfried et al. on CO oxidation with preadsorbed oxygen atoms on Au(110)(1 × 2) has revealed that the reaction takes place at temperatures below 100 K with an activation energy of ∼0.6 eV.8,9 Koel and co-workers also showed that atomically adsorbed oxygen on Au(111) was active for CO oxidation at temperatures ranging from 250 to 375 K.7 They populated their samples with atomic oxygen by exposing the sample to ozone. Gong et al. also recently studied the reactivity of Oa on a Au(111) single crystal at 77 K and observed prompt CO2 production over a large range of O-atom coverages.10 Bondzie et al.11 as well as Mullins and co-workers12,13 studied the reactivity of preadsorbed oxygen atoms on Au/TiO2 model catalysts. Bondzie et al. observed prompt reaction of CO with the atomic oxygen overlayer at room temperature. Mullins and co-workers extended this work down to temperatures as low as 65 K. Although all of these studies have shown conclusively that atomically adsorbed oxygen is active for CO oxidation, characterization of atomically adsorbed oxygen as the key reactant in the CO oxidation reaction would require dissociation of O2 on the catalyst surface prior to reaction. However, oxygen has not been observed to dissociate on any gold surface. This result, as well as results from theory discussed below, casts doubt on whether atomic oxygen is a key player in the observed catalytic activity of supported gold nanoclusters. Another alternative for the reaction mechanism for the CO oxidation reaction involves the reactivity of molecularly chemi-

10.1021/jp062766c CCC: $33.50 © 2006 American Chemical Society Published on Web 09/21/2006

20338 J. Phys. Chem. B, Vol. 110, No. 41, 2006 sorbed oxygen (O2,a). Some useful evidence motivating a reaction mechanism involving a molecularly chemisorbed oxygen species has come from the theoretical community. In agreement with experiment, recent theoretical work regarding oxygen adsorption on many gold systems has indicated that the activation barrier for dissociation of oxygen is on the order of ∼1 eV and therefore not likely to occur under reaction conditions commonly employed. Some interesting results have been obtained from studying defective gold surfaces. Steps and defects have been shown to be suitable sites for molecular chemisorption of oxygen. Mavrakakis and co-workers have also shown that a strained gold surface will result in conditions suitable for molecular chemisorption of oxygen.14 Lopez and Nørskov also report calculations that show molecular adsorption of oxygen on a 10-atom gold cluster and reveal that it is as reactive as atomically adsorbed oxygen for CO oxidation.15 Of more relevance to catalysis, calculations of supported gold clusters have also indicated that O2,a can be active in the reaction. Liu et al. calculated that molecularly chemisorbed oxygen at the Au-TiO2 interface will readily react with CO to form CO2.16 Molina et al. also presented similar results showing that CO will react with molecularly chemisorbed oxygen at the AuTiO2 interface with an activation barrier of 0.15 eV.17 Sanchez et al. presented DFT results in which they determined that CO will react spontaneously with molecularly chemisorbed oxygen on Au8 clusters supported on MgO.18 Despite all this theoretical evidence for a molecularly chemisorbed intermediate, no experimental evidence exists identifying a molecularly chemisorbed oxygen species under relevant reaction conditions on catalyst samples. Investigations of gas-phase anionic gold clusters, Aun(n ) 2-20), have also been very useful in understanding the chemistry of small gold clusters.19-23 Oxygen has been observed to interact with gold cluster anions with an odd number of electrons, and UPS measurements20,21 on 2- and 4-atom gold cluster anions have revealed fine structure in the spectra consistent with an oxygen-oxygen bond, indicating molecular adsorption of oxygen on these clusters. Socaciu and co-workers also presented evidence suggestive of reaction of CO with a molecularly chemisorbed oxygen species on Au2-.22 In our previous work we determined that exposing a Au/ TiO2 model catalyst to an oxygen plasma jet resulted in the population of molecularly chemisorbed and atomically chemisorbed oxygen.24 We even observed this phenomenon on a Au(111) single crystal, although to a much smaller extent (possibly related to a smaller number of defects in comparison to the supported gold clusters). In this study we present results of reactivity measurements of the molecularly chemisorbed oxygen on Au/TiO2 model catalyst samples. We present experimental evidence that suggests that molecularly chemisorbed oxygen can react directly with CO to form CO2 on titania-supported gold clusters in the 2-5 nm range. We studied the reaction on five different gold coverages (0.5, 0.75, 1.0, 1.25, and 2.0 ML Au), which were given equivalent exposures to an oxygen plasma jet, and we present evidence for reaction of molecularly chemisorbed oxygen on all surfaces studied. A brief account of some of this work has been published previously.25 Experimental Section All experiments reported in this paper were performed in a molecular beam surface scattering apparatus that has been described in detail elsewhere.24-27 Briefly, the apparatus consists

Stiehl et al. of an ultrahigh vacuum (UHV) surface scattering chamber with a base pressure of ∼1 × 10-10 Torr and a quadruply differentially pumped molecular beam source chamber. The scattering chamber is equipped with an Auger electron spectrometer (AES), low-energy electron diffraction optics (LEED), a quadrupole mass spectrometer (QMS), an ion gun, a quartz crystal microbalance (QCM), and a custom-built gold deposition system. The TiO2(110) single crystal (10 mm × 10 mm × 0.5 mm thick) used in this study is mounted on a tantalum plate (16.25 mm × 12.5 mm × 1 mm), and a Au(111) single crystal is mounted on the face of the tantalum plate opposite the TiO2 crystal in a 0.5 mm deep recess 0.5 in. in diameter. Both samples are held in place with two 0.5 mm diameter tungsten wire clips. A 0.1 mm thick gold foil is sandwiched between the tantalum plate and the TiO2 crystal for improved thermal contact.28 Two tantalum wires are spot-welded to the tantalum plate to allow for computer-controlled, resistive heating of the sample. The tantalum heating wires are attached to two copper posts mounted on the sample probe that are in thermal contact with a liquid nitrogen reservoir. A type K thermocouple is spot-welded to the top edge of the tantalum plate for measurement of temperature. Gold was vapor deposited on the TiO2(110) single crystal using a procedure described in detail by Goodman et al.29,30 A custom-built gold doser was used to evaporate gold on the TiO2(110) sample by resistively heating a small piece of gold attached to a tungsten filament. The gold deposition rate was determined using a QCM, and various gold coverages were obtained by varying the deposition time. The typical gold deposition rate used was ∼0.1 Å/s. In this study, 1 ML of gold refers to a single atomic layer of close-packed gold (1.387 × 1015 atoms/cm2), which has a thickness of approximately 2.35 Å. Lai et al. performed an extensive study of the characterization of vapor-deposited gold on TiO2(110),30 and we expect our samples to closely resemble theirs in regard to gold particle size and distribution for the various gold coverages employed in this investigation. The molecular beam source chamber is equipped with an alumina, radio frequency (rf) plasma-jet nozzle31-33 (200 µm) used for dosing oxygen on the surfaces studied. Gas mixtures of 8% oxygen in argon are used for generating atomic oxygen in the RF plasma-jet source. Both 16O2 (99.93%) and 18O2 (99.4%) gas mixtures are used in this study. As explained in more detail later, employing both isotopes of oxygen allows us to increase the ratio of molecularly chemisorbed oxygen to atomically adsorbed oxygen of a giVen isotope. An oxygen dissociation fraction of ∼40%, as determined via time-of-flight techniques, is achieved.32,33 The flux of oxygen atoms in the plasma jet was ∼2.6 × 1014 atoms/cm2‚s and determined by the exposure time necessary to achieve a 0.23 ML oxygen atom coverage on the Au(111) single crystal.10 Oxygen coverages on the model catalyst samples reported in this study are defined as the coverage of oxygen relative to saturation for the gold coverage of interest and were obtained by integration of thermal desorption spectra of O2. The oxygen coverage on the catalyst samples was varied by controlling the O-atom beam exposure time. Pure CO was dosed through the 200 mm nozzle with the RF power off. A CO beam intensity of ∼9 × 1013 molecules/ cm2‚s was typically employed. The apertures which shape the molecular beams result in a beam spot of ∼3 mm; this is smaller than the dimensions of the sample, thus minimizing direct exposure of the probe assembly other than the crystal face. Following exposure of the samples to the oxygen plasma jet, the sample is known to be populated with molecularly chemi-

Reactivity of O2,a on Au/TiO2

Figure 1. Kr collision-induced desorption measurement of molecularly chemisorbed oxygen from a 2 ML Au/TiO2 sample at 77 K. Sample was first exposed to the oxygen plasma jet at 77 K to give a ∼60% relative O-atom coverage followed by impingement of a ∼1 eV kineticenergy beam of Kr (beam starts at t ) 5 s).

sorbed oxygen.24 To ensure that the sample is solely populated with atomic oxygen when desired, either the sample could be heated to 300 K to thermally desorb the oxygen or the molecularly chemisorbed oxygen could be removed via collisioninduced desorption.24,34-35 Collision-induced desorption of molecularly chemisorbed oxygen was accomplished by bombarding the oxygen-covered surface with an energetic (∼1 eV) beam of Kr or with a lower energy (∼0.5 eV) beam of Ar. The high kinetic-energy beams of Kr, or Ar, are generated by expanding a dilutely seeded Kr (2%), or Ar (2%), in helium gas mixture through the 200 mm nozzle with the rf power off. Figure 1 shows a typical example of a Kr collision-induced desorption experiment. For the experiment shown in Figure 1, a 2 ML Au/TiO2 sample was exposed to the oxygen plasma jet at 77 K to achieve a 60% relative Oa coverage. Following exposure to the plasma jet, the ∼1 eV Kr beam was impinged on the sample (t ) 5 s) and the evolution of mass 32 from the sample was monitored with the QMS. On impingement of the Kr beam, a rapid rise is observed in the mass 32 signal, which decays to the background level as the O2,a is removed from the surface. Although annealing and collision-induced desorption were both suitable for removal of molecular oxygen from the sample, they did result in differing reactivities of the atomic oxygen overlayer. Figure 2 shows some results that reveal that annealing the sample can drastically affect the CO oxidation activity of the atomic oxygen overlayer. Figure 2a shows the CO2 produced from impingement by CO on a 2 ML Au/TiO2 sample that was exposed to the oxygen plasma jet at 77 K to achieve a ∼50% relative O coverage (CO beam starts at t ) 5 s). The sample was neither heated nor collision-induced desorption employed to clear the surface of O2,a following exposure to the plasma jet. Figure 2b shows the CO2 produced from a sample that was given the same oxygen exposure as the sample shown in Figure 2a but cleared of O2,a via Kr collision-induced desorption prior to exposure to a CO beam. There is a decrease in the reactivity of the sample which we attribute to the absence of reactive O2,a. We will show later that the Kr CID has an immeasurable effect on the reactivity of the atomic oxygen overlayer. Figure 2c shows the CO2 produced from (i) a sample that was given the same exposure as the samples shown in Figure 2a and 2b, (ii) collision-induced desorption performed to clear the sample of O2,a, and (iii) a sample annealed to 300 K prior to exposure to the CO beam at 77 K. The CO2 produced

J. Phys. Chem. B, Vol. 110, No. 41, 2006 20339

Figure 2. CO2 production from a 2 ML Au/TiO2 sample at 77 K (a) following exposure of the sample to a plasma jet of oxygen to give a ∼50% relative O-atom coverage, (b) following exposure of the sample to a plasma jet of oxygen to give a ∼50% relative O-atom coverage and Kr collision-induced desorption to remove molecularly chemisorbed oxygen, and (c) following exposure of the sample to a plasma jet of oxygen to give a ∼50% relative O-atom coverage, Kr collision-induced desorption to remove molecularly chemisorbed oxygen, and anneal to 300 K.

in Figure 2c is noticeably smaller than the CO2 produced in the experiments shown in Figure 2a and 2b, indicating that annealing drastically affects the reactivity of the atomic oxygen overlayer. Results In our previous study regarding identification of molecularly chemisorbed oxygen on Au/TiO2 following exposure to an oxygen plasma jet we noted that the amount of molecularly chemisorbed oxygen relative to the amount of atomically adsorbed oxygen on the sample can be small (∼10% O2,a to Oa ratio for a 2 ML Au/TiO2 with 60% relative O coverage).24 For the purposes of this study, it was desirable to increase this ratio in order to enhance the probability of measuring the reactivity of the molecular oxygen since the adsorbed atomic oxygen is known to be quite reactive. To increase the ratio of molecularly chemisorbed oxygen to atomically adsorbed oxygen of a given isotope, we developed a procedure (described immediately below) employing both isotopes of oxygen which allowed us to enhance this ratio. This procedure was employed for all the experiments reported in this study: (i) the sample (Ts ) 77 K) was exposed to a plasma jet formed using 16O2 (exposure of ∼2.6 × 1014 atoms/cm2‚s for 30 s), (ii) the sample was heated to 300 K to desorb 16O2,a, and after cooling to 77 K (iii) the 16O -covered sample was exposed to a plasma jet formed using a 18O (exposure of ∼2.6 × 1014 atoms/cm2‚s for 15 s). By 2 preadsorbing 16Oa on the sample we could limit the available sites for 18Oa adsorption and thus increase the ratio of 18O2,a to 18O on the sample. The presence of preadsorbed atoms does a not inhibit population of the molecularly chemisorbed oxygen state. Figure 3 shows the 18O2,a/18Oa values (ratio is calculated by dividing the area of the mass 36 signal observed during a collision-induced desorption measurement of molecular oxygen from the TiO2 and the gold particles by two times the mass 36 area plus the mass 34 area observed during recombinative, thermal desorption of oxygen from the gold particles) for four different gold coverages after preparing the samples following the procedure described above (b). Also shown in Figure 3, for comparison, is the ratio that is obtained without predosing the sample with 16O atoms (2). As can be seen in Figure 3, the

20340 J. Phys. Chem. B, Vol. 110, No. 41, 2006

Figure 3. (2) 18O2,a/18Oa as a function of Au coverage following exposure of the catalyst sample to the oxygen plasma jet (∼2.6 × 1014 atoms/cm2‚s for 15 s). (b) 18O2,a/18Oa as a function of Au coverage following procedure described in text. (9) Fraction of TiO2 covered by gold as a function of gold coverage (calculated using characterization data from ref 25).

ratios for each gold coverage increase by following the procedure described above. Finally, the fractional area of the TiO2 that is expected to be covered by gold (9) is also shown in Figure 3 (calculated from data of Lai et al.30). In general, the ratio of molecular to atomic oxygen decreases because the amount of 18O2,a that can adsorb on the TiO2 surface is greatly reduced as the Au coverage increases and the amount of 18Oa that can adsorb on Au surfaces is greatly increased with increasing Au coverage, thus reducing the ratio. It was not possible to calculate the 18O2,a to 18Oa ratio for the 0.5 ML Au/ TiO2 sample because we could not resolve the oxygen recombinative desorption feature from the gold particles. Once a sample was prepared following the procedure described above, two experiments were required to determine if any of the 18O2,a on the sample reacted. One experiment consisted of exposing the as-prepared sample to a beam of C16O and monitoring the C16O18O produced. For comparison, another experiment was performed by following the same sample preparation procedure with the exception that the 18O2,a was removed via collision-induced desorption prior to exposure to a beam of C16O. The first experiment determines the amount of C16O18O produced from a sample that is populated with both molecular and atomic oxygen. The second experiment provides a measure of the amount of C16O18O produced from a sample that has an equivalent coverage of atomic oxygen as the first experiment but has no molecularly chemisorbed oxygen. The greater yield of C16O18O in the first experiment in comparison to the second experiment can be attributed to the enhanced reaction rate due to the presence of 18O2,a on the sample. Figures 4, 5, and 6 show three examples of experiments performed following the procedure described above on 1, 1.25, and 2.0 ML Au/TiO2 samples. The 18O2,a to 18Oa ratio for the three Au coverages of 1.0, 1.25, and 2 ML are 0.75, 0.30, and 0.25, respectively, as shown in Figure 3. Figure 4 shows the mass 46 (C18O16O) produced during experiments representative of both CO oxidation scenarios discussed above from a 1 ML Au/TiO2 sample. The dashed curve in Figure 4 shows the mass 46 production from samples with both 18Oa and 18O2,a species present on the sample, and the solid curve shows the mass 46 production from surfaces that have been cleared of 18O2,a via Kr collision-induced desorption. Both reactions show behavior typical of the CO oxidation reaction with preadsorbed oxygen

Stiehl et al.

Figure 4. C16O18O production at 77 K from a 1 ML Au/TiO2 sample populated with both 18Oa and 18O2,a (dashed curve) and 18Oa only (solid curve) upon exposure to a pure CO beam at 10 s. Inset shows average values and uncertainties of C16O18O produced from a series of three identical measurements such as the ones shown.

Figure 5. C16O18O production at 77 K from a 1.25 ML Au/TiO2 sample populated with both 18Oa and 18O2,a (dashed curve) and 18Oa only (solid curve) upon exposure to a pure CO beam at 10 s. Inset shows average values and uncertainties of C16O18O produced from a series of three identical measurements such as the ones shown.

atoms reported previously.8,9 However, comparison of the C16O18O production curves in Figure 4 reveals that the samples populated with both of the oxygen species consistently result in more C16O18O production than the samples solely populated with atomic oxygen species. The average values and uncertainties for the experiments are shown in the inset of Figure 4. Figures 5 and 6 show similar results to those observed in Figure 4. For the three gold coverages shown in Figures 4, 5, and 6 (1.0, 1.25, and 2.0 ML), average differences of 41 ( 2%, 30 ( 2.8%, and 18.9 ( 4.5% in the C16O18O production are observed, respectively, between the two experiments. The values of fractional difference presented above (and in Figure 7, discussed below) are calculated by subtracting the area of the mass 46 signal observed from a Au/TiO2 sample populated with only atomically adsorbed oxygen exposed to a CO beam from the area of the mass 46 signal evolved during CO impingement on a sample with both molecularly and atomically adsorbed oxygen and then dividing this difference by the mass 46 area associated with both molecular and atomic oxygen. It should be noted that C16O2 is produced during the experiments from reaction of C16O with 16Oa and that the C16O2 produced is the same, within experimental uncertainty, during all comparable experiments.

Reactivity of O2,a on Au/TiO2

Figure 6. C16O18O production at 77 K from a 2 ML Au/TiO2 sample populated with both 18Oa and 18O2,a (dashed curve) and 18Oa only (solid curve) upon exposure to a pure CO beam at 10 s. Inset shows average values and uncertainties of C16O18O produced from a series of three identical measurements such as the ones shown.

Figure 7. Fractional C16O18O difference for five gold coverages studied in this investigation (0.5, 0.75, 1.0, 1.25, and 2.0 ML).

Experiments such as the ones shown in Figures 4-6 were performed on five different gold coverages (0.5, 0.75, 1.0, 1.25, and 2.0 ML). Figure 7 shows the results of the fractional C16O18O difference for all gold coverages studied. The ratios of 18O2,a to 18Oa for these experiments are shown in Figure 3. As can be seen in Figure 7, a 1 ML Au covered sample results in the greatest fractional difference, 41 ( 2%, for the oxygen exposures and gold coverages studied in this investigation. Discussion The results shown in Figures 4-7 indicate that the samples that are populated with both molecular and atomic oxygen result in greater formation of C16O18O than samples which are solely populated with atomic oxygen. The difference in the C16O18O production can therefore be attributed to the presence of 18O2,a on the sample. However, it is not apparent if all of the difference that is observed is actually related solely to reaction of C16O molecules with molecularly chemisorbed oxygen species. When the C16O reacts with 18O2,a, there will be an atom, 18O, left behind which is reactive and can further react with an incoming C16O. If the atom that is left behind promptly reacts with another C16O molecule, then only one-half of the difference shown in

J. Phys. Chem. B, Vol. 110, No. 41, 2006 20341 Figure 5 can be attributed to the direct reaction of molecularly chemisorbed oxygen. Another scenario for the fate of the remaining oxygen atom is that it adsorbs to the sample and merely increases the population of 18Oa on the sample. However, this scenario seems unlikely since, as we have shown previously,12 an increase in the oxygen-atom population in the coverage regimes shown in Figures 4-7 actually results in a decrease in the CO2 yield. Another aspect of the study that needs to be addressed regarding the reactivity of the molecular oxygen species is the effect of collision-induced desorption on the reactivity of the atomic overlayer. Thus far it has been assumed that the collisioninduced desorption experiments do not alter the reactivity of the adsorbed oxygen atoms. There is some evidence that this assumption is valid from the experiments that have been discussed above. As mentioned earlier, production of the mass 44 (C16O2) species during the reactivity experiments is the same within experimental uncertainty. This result indicates that the reactivity of an annealed atomic overlayer is not affected by the Kr collision-induced desorption. We attempted to address this issue further by performing some different control experiments to show that the reactivity of the atomically adsorbed oxygen overlayer is not affected by collision-induced desorption. One experiment involves use of a lower energy beam of Ar (∼0.5 eV) to perform collisioninduced measurement. The experiment shown in Figure 4 was repeated using Ar collision-induced desorption in place of Kr collision-induced desorption to determine if a lower energy beam resulted in a similar reactivity difference for the two samples (not shown). An average C16O18O production difference of 44 ( 4% was observed when substituting Ar for Kr. This value is similar to the value measured (41 ( 2%) using the more energetic Kr beam for collision-induced desorption. This result suggests that Kr collision-induced desorption does not measurably affect the reactivity of the atomic oxygen overlayer. To further address the effect of Kr collision-induced desorption on the reactivity of Oa, an experiment was performed on 16O -covered Au/TiO in which (i) the sample was exposed to a 2 the plasma jet at 77 K, (ii) the sample was heated to 170 K to desorb 16O2,a and after cooling to 77 K (iii) Kr collision-induced desorption was performed (as expected, no desorption of 16O2,a was observed), and (iv) the CO2 production was measured. For comparison, a similar experiment was conducted, with the exception that step iii was not performed. The difference in the CO2 production between these two experiments was found to be negligible (