J. Phys. Chem. C 2007, 111, 7741-7747
7741
Methanol Decomposition and Oxidation on Ir(111) C. J. Weststrate,*,† W. Ludwig, J. W. Bakker, and A. C. Gluhoi Leids Instituut Voor Chemisch Onderzoek, UniVersiteit Leiden, P.O. Box 9502, Einsteinweg 55, 2333 CC Leiden, The Netherlands
B. E. Nieuwenhuys Leids Instituut Voor Chemisch Onderzoek, UniVersiteit Leiden, P.O. Box 9502, Einsteinweg 55, 2333 CC Leiden, The Netherlands, and Technische UniVersiteit EindhoVen, Schuit Institute of Catalysis, P.O. Box 5600 MB, EindhoVen, The Netherlands ReceiVed: January 22, 2007; In Final Form: March 30, 2007
The adsorption, decomposition, and oxidation of methanol (CH3OH) has been studied on Ir(111) using temperature-programmed desorption and high-energy resolution fast XPS. Molecular methanol desorption from a methanol-saturated surface at low temperature shows three desorption peaks, around 150 K (R), around 170 K (β1), and around 220 K (β2), respectively. The R peak is assigned to methanol adsorbed on top of the first, chemisorbed layer, whereas β1 and β2 are both assigned to methanol directly coordinated to the metal surface atoms (chemisorbed). The CH3OHad responsible for the β2 desorption peak appears as a separate component in the C 1s core level spectra. A part of the initially adsorbed methanol decomposes into COad and Had around (or even below) 175 K. Intermediate CHxO species of CH3OH decomposition were not observed. The formation of a small amount of CHxad indicates that (Hx)C-O(H) bond scission occurs as well. Temperature-programmed desorption experiments confirm that CHxad species form, as evidenced by a hightemperature (500 K) H2 formation peak due to decomposition of CHad. The presence of Oad causes a downward shift in the C 1s and O 1s BEs of molecularly adsorbed methanol, but the desorption barrier for molecular methanol desorption is not significantly influenced by the presence of Oad. A stable reaction intermediate, most probably methoxy (CH3Oad), was observed in the presence of Oad, between 160 and 220 K. It is an intermediate in the formation of both formate (HCO2ad) and COad, which occurs around 220 K. Formate decomposes around 350 K into CO2 (g) and Had (which reacts with the remaining oxygen to H2O), whereas the COad reacts with Oad around 400 K.
I. Introduction
II. Experimental Section
The interaction of small alcohols with catalytically active surfaces is of considerable interest for the chemical industry, as alcohols are important precursors for the manufacturing of a number of products. The production of H2 from small alcohols (methanol and ethanol) is an attractive method for small scale H2 production, for example in a fuel-cell driven car. The alcohols are relatively easy to store, and the problems associated with H2 storage are circumvented in this way. Catalytic decomposition of methanol is a crucial step in the direct methanol fuel cell (DMFC), so a fundamental understanding of the interaction of methanol with catalytically active materials, such as Ir,1 is important to improve the DMFC. A number of low-pressure studies have been performed to study the surface chemistry of methanol on various single-crystal surfaces of Pt, Pd, and Rh.2-19 Other authors have studied methanol decomposition on Au, Ag, and Cu20-31 single-crystal surfaces. Methanol on Ir surfaces has, to our knowledge, only been studied in an electrochemical environment.32-34 Here we report our results concerning methanol adsorption, decomposition, and oxidation on Ir(111) under UHV conditions.
The vacuum system used for the temperature-programmed desorption (TPD) measurements, with a base pressure better than 5 × 10-10 mbar, is equipped with a sputter ion gun for sample cleaning, LEED optics, an Auger/XPS system, and a differentially pumped quadrupole mass spectrometer (QMS). The sample is placed in front of a 2 mm wide hole, at a distance of ∼2 mm. As a result, the contribution of signal from the heating wires and edges of the sample is reduced, and the desorbing species that are observed by the QMS originate predominantly from the sample surface. High-energy resolution fast X-ray photoelectron spectroscopy (XPS) measurements35,36 were performed at the SuperESCA beamline of ELETTRA, the synchrotron radiation facility in Trieste, Italy. The vacuum system, with a base pressure of ∼1 × 10-10 mbar, is equipped with a sputter ion gun for sample cleaning, a mass spectrometer, and LEED optics. High purity methanol (BioSolve, 99.8%) was further cleaned by several freeze-pump-thaw cycles before use. Exposures are reported in Langmuir (1 × 10-6 Torr s), calculated using uncorrected ion gauge readings. It was found that the exposures necessary to obtain a certain coverage were somewhat different in the two different vacuum systems. However, in the comparison of the results obtained in the two systems the methanol saturated surface was used, so that the error in the exposures is eliminated.
* To whom correspondence should be addressed. E-mail: kees-jan.
[email protected]. † Present address: Dept. of Synchrotron Radiation Research, Lund University, Sweden.
10.1021/jp070539k CCC: $37.00 © 2007 American Chemical Society Published on Web 05/10/2007
7742 J. Phys. Chem. C, Vol. 111, No. 21, 2007 The Ir single crystal (diameter 1 cm, thickness 2 mm) was cut and polished within 0.1° from the desired (111) orientation. It was cleaned using Ar+ sputtering (3 kV) and annealing cycles (∼1400 K) followed by oxygen treatments. The oxygen was removed either by flashing to 1400 K or by heating in the presence of H2. The cleanliness of the sample was checked by XPS, showing no oxygen and carbon contamination in the O 1s and C 1s core level regions. The C 1s spectra were measured with a photon energy of 400 eV, and for the O 1s spectra, a photon energy of 650 eV was used. Temperature-programmed XPS (TP-XPS) measurements37 were performed using heating rates of 0.11 and 0.16 K s-1, respectively. The different core level regions were measured in a separate experiment; that is, during one experiment either O 1s or C 1s could be measured. The position of the sample was changed after some time during the measurement, but no significant differences were observed between the spectra obtained at the two different positions. Hence, changes induced by the X-ray beam (beam damage) were not very significant. The XP spectra were evaluated, after subtraction of a linear background, by fitting the peaks with a Doniach-Sˇ unjic´ function convoluted with a Gaussian function.38 The BE values are reported with respect to the Fermi level, which was measured between the different measurements using the same excitation energy as the measured spectra. The adsorption of H2, CO, and O2 on Ir(111) has been studied previously.39-41 It was found that CO desorbs between 400 and 600 K and that the CO saturation coverage at room-temperature equals 7/12 ML. This value was used to normalize the C 1s signal. This calibration procedure gives a rough estimate of the surface coverage of the different species, as photoelectron diffraction can have a significant effect on the observed signal intensity. For oxygen adsorption on Ir(111), a saturation coverage of 0.5 ML was observed after exposure to O2 at 300 K.42-44 Recombinative desorption of Oad was found in a broad temperature region, between 700 and 1200 K. During our experiments in the presence of Oad the exposure to O2 was done at 300 K. This resulted in a well-ordered Oad overlayer, showing a (2 × 2) LEED pattern at saturation coverage (0.5 ML), in line with literature reports.44 We used this Oad-covered surface to normalize the O 1s signal. Hagedorn et al.45 reported H2 desorption from a Had covered Ir(111) surface, between 150 and 350 K. We performed reference desorption experiments for H2, CO, and O2. The desorption temperatures that we observed are in good agreement with the literature values. Water desorption (after dosing H2O at 100 K) was studied as well. The main H2O desorption peak was observed around 175 K, and a small (∼10%) H2O desorption peak was observed around 220 K. III. Adsorption and Decomposition on the Clean Surface 1. Thermal Desorption. Figure 1a shows the thermal desorption spectra that were obtained after exposing the clean Ir(111) surface to methanol at 90 K (10 L). The molecular methanol desorption (m/e ) 31) trace above 150 K shows two distinct peaks, around 170 (β1) and 220 K (β2), respectively. Both desorption peaks are assigned to (chemisorbed) methanol that is directly coordinated to the metal surface atoms. On Rh(111), two different molecular desorption peaks were observed as well, around 170 and 210 K, respectively.18 On Pt(111), on the other hand, only a single molecular desorption peak was observed, around 190 K.9 The origin of the two peaks is discussed in more detail in section III 3.
Weststrate et al.
Figure 1. Panel a: Temperature-programmed desorption TPD (3 K s-1) and TP-XPS (0.16 K s-1) results [panel b] obtained during heating of a methanol-covered surface (∼10 L, 90 K) in vacuum.
For exposures higher than 5 L, an additional desorption feature was observed around 140 K (labeled R), which is assigned to desorption of methanol adsorbed on top of the chemisorbed methanol species.13 Decomposition of methanol takes place as well, as can be concluded on the basis of the appearance of H2 and CO desorption [Figure 1a]. The H2 (m/e ) 2) desorption trace has been corrected for molecular methanol desorption (using m/e ) 31), which also appears in the m/e ) 2 signal due to cracking in the QMS. The H2 desorption trace shows a large peak around 320 K, at the temperature where Had recombination occurs on Ir(111).45 A small H2 desorption peak is also observed around 520 K. This H2 desorption feature is assigned to decomposition of CHxad species that are formed by (Hx)C-O(H) bond breaking on the surface. The XPS results (see the next section) confirm that this indeed takes place, as CHxad species appear in the C 1s core level spectra above ∼200 K (see Figure 2). (Hx)CO(H) bond scission as a minor methanol decomposition pathway was also reported on Pd(111) and on Pt(110).6,10,12,46 Desorption of other products, like formaldehyde (m/e ) 30) or methane (m/e ) 16), was not observed. Possible water formation due to (Hx)C-O(H) bond scission could not be studied in detail, due to the fact that the m/e ) 18 signal contains contributions from the residual gas and from methanol decomposition in the QMS ionizer. 2. X-ray Photoelectron Spectroscopy. The interaction of methanol with Ir(111) was also studied using XPS. Both the C 1s and O 1s core level regions were measured during methanol uptake at 90 K and during subsequent heating (0.16 K s-1). The C 1s and O 1s binding energies (BE) of the different components that were observed during the different experiments are listed in Table 1. During uptake at 90 K (not shown) the C 1s spectrum shows a single, broad peak at 285.9 eV, which shifts to 286.1 eV with
Methanol Decomposition and Oxidation on Ir(111)
J. Phys. Chem. C, Vol. 111, No. 21, 2007 7743
Figure 2. TP-XPS during heating (0.16 K s-1) of a methanol-covered surface (10 L, 90 K): the left panel shows several C 1s core level spectra, whereas in the right panel, a top view of the C 1s core level region is shown as a function of temperature.
TABLE 1: C 1s and O 1s Binding Energies of the Different Species Observed during XPS Measurements Using Methanol and Oxygena species
C 1s BE (eV)
phys. CH3OH CH3OHad (I) CH3OHad (II) COad CHxad Oad CH3OHad (on ox.) methoxy formate
286.9 286.1-285.9 285.8-285.7 286.3 283-284
a
285.7 285.4 287.2
O 1s BE (eV) 533.1 532.7 532.7 532.2 529.9 532.1 532.1 (?) 531.3
des./dec. T (K) 140 170 220 500 300, 520 170, 220 210 350
The desorption/decomposition temperature is reported as well.
increasing coverage. We were not able to clearly resolve the different components due to vibrational splitting, unlike Wiklund et al.30 In the O 1s spectrum, a single peak is observed at 532.7 eV. After an exposure of ∼3 L, an additional component is observed in both the C 1s (at 286.9 eV) and O 1s (533.1 eV) spectra. During heating this species disappears again around 140 K (see Figure 2). This corresponds to the R desorption peak, and it is, therefore, assigned methanol adsorbed (“physisorbed”) on top of the first, chemisorbed layer.13 Figure 2 shows the results that were obtained during heating of the surface after exposure to methanol at a low surface temperature. The figure only shows the C 1s spectral region, as the O 1s spectral region did not provide additional information. The left panel shows several individual spectra taken at specific temperatures, whereas the right panel shows a top view of the C 1s spectral region as a function of temperature. The intensities of the different species shown in Figure 1b were obtained by fitting the individual spectra with four different components: Physisorbed methanol, CH3OH(I), CH3OH(II), COad, and CHxad (BE values of these species can be found in Table 1). Figure 3 shows the spectrum taken at 175 K in which three different fit components are shown. The peaks for the two forms of molecular methanol are rather broad. This is caused by vibrational broadening due to the presence of C-H bonds in the adsorbate molecule.47-49 In our measurements, the peaks due to vibrational splitting are not clearly resolved, and we were not able to extract useful information (like number of C-H bonds per molecule) from this. The only information that was
Figure 3. C 1s spectrum taken at 175 K during heating of an adsorbed methanol layer (10 L, 90 K). Three different fitting components used to evaluate the C 1s core level spectra are shown.
obtained from the spectra is the intensity of the different C 1s components. The intensity obtained for “physisorbed” methanol is not included in Figure 1b. In the C 1s spectrum taken at 150 K, only molecular (chemisorbed) methanol (286.1 eV) is observed. A small amount of COad seems to be present as well (286.2 eV), which is responsible for the second maximum at the high BE side. In the region between 150 and 200 K, two maxima develop gradually, and at 175 K, two peaks are observed, at 286.3 and 285.8 eV, respectively. The component with a maximum at 286.3 eV is assigned to COad. This assignment was checked by dosing CO to the surface, which indeed revealed a C 1s BE of 286.3 eV. The presence of COad is indicative of methanol decomposition around 175 K. The other component, with a maximum at 285.8 eV, continues to shift to lower BE values upon further heating, and around 200 K, it is observed around 285.7 eV, clearly distinguishable from the COad peak. It disappears around 225 K upon further heating. This coincides with the second methanol desorption peak (β2) that was observed during the TPD experiments. The atomic composition of this species cannot be determined directly from the XP spectrum. A detailed discussion about the assignment of this species is provided in section III 3.
7744 J. Phys. Chem. C, Vol. 111, No. 21, 2007 Around 200 K, the formation of CHxad species is observed, indicating that (Hx)C-O(H) bond scission occurs. The CHxad species appear between 283.2 and 284.2 eV in the C 1s spectrum.10-12 These species undergo a change around 300 K. We tentatively assign this change to the formation of CHad species.50-52 This assignment is confirmed by experiments in which ethanol was used. During these measurements on Ir(111) CHad species were observed between 283.2 and 284.2 eV during ethanol decomposition, which proceeds via C-C bond scission, yielding CHxad species and COad.53 It was found that CHad forms around 300 K and decomposes around 500 K. The H2 desorption peak found around 520 K in the experiments using methanol is, therefore, assigned to decomposition of CHad which was formed via C-O bond scission of CH3OHad. Decomposition of methanol via C-O bond breaking is a minor decomposition pathway, and we cannot exclude that the observed C-O bond breaking activity is caused by defect sites on the surface rather than by the (111) surface itself. 3. Assignment of the 285.8-285.7 eV Species. The assignment of the species at 285.8-285.7 eV requires a detailed look at the experimental results. The temperature at which this species disappears coincides with the molecular methanol desorption peak around 220 K (β2). As only COad and the 285.8-285.7 eV species are present on the surface above 200 K, the 285.8285.7 eV species should be responsible for the molecular methanol desorption peak at 220 K. This suggests that it should be assigned to methanol that is chemisorbed intact. The C 1s BE value found for the 285.8-285.7 eV species, on the other hand, is outside the BE range found for molecular methanol during methanol uptake at 90 K (286.1-285.9 eV). Furthermore, the ∆BE between the 285.8-285.7 eV species and the molecular methanol peak is similar to the ∆BE between CH3OHad and CH3Oad in the presence of Oad (0.3-0.4 eV in both cases, see section IV 5). The latter observation suggests that the 285.8-285.7 eV species should be assigned to CH3Oad (or another CHxO intermediate). An argument against assignment to methoxy is the observation that the COad peak at 200 K is already fully developed, and its intensity does not increase anymore upon further heating, indicating that methanol decomposition is completed at 200 K. This shows that the 285.8-285.7 eV species is not a precursor for COad formation. A model that would explain both the β2 methanol desorption peak and the presence of a CH3Oad that is not the precursor for COad formation is rehydrogenation of CH3Oad around 220 K, forming molecular methanol which immediately desorbs. Such a mechanism, via CH3Oad + Had f CH3OH (g), was reported for methanol desorption from Pd(100) by Christmann et al.13,14 on the basis of the observation that the high-temperature methanol desorption peak (labeled β2 in their article) exhibits second order desorption behavior, i.e., a decreasing peak temperature with increasing coverage. We performed thermal desorption measurements as well, to see whether this model could explain our β2 desorption peak. Figure 4a shows molecular methanol desorption from Ir(111) as a function of exposure (i.e., CH3OHad coverage). The β2 desorption peak clearly shows first order desorption behavior; that is, the temperature at which the desorption maximum occurs is the same for all methanol precoverages. Interestingly, the low-temperature desorption peak (β1) does show second order behavior. We explain this by repulsive (dipole-dipole) interactions between the adsorbed methanol molecules on the surface. Furthermore, we performed several experiments in which D2 was used in an attempt to form deuterated methanol moieties.
Weststrate et al.
Figure 4. Molecular methanol desorption (3 K s-1) as a function of exposure [panel a]. Panel b shows the results of an experiment in which the surface was exposed to methanol at 200 K and subsequently exposed to D2 at 100 K (0.5 K s-1).
In the experiment shown in Figure 4b, the surface was exposed to CH3OH at 200 K to ensure that only the β2 desorption state is populated. D2 was subsequently dosed at 100 K. In this experiment, only a small amount (max. ∼20%) of m/e ) 33 (CH3OD) was observed as compared to the amount of m/e ) 32 (CH3OH) [see Figure 4b]. The desorption traces of D2, HD, and H2 (not shown) revealed that Dad was present in a large excess on the surface compared to Had. When CH3Oad rehydrogenation would take place, the large excess of Dad would result in the preferential formation of CH3OD. This is not reflected in the ratio between m/e ) 32 and 33. This ratio does not show a strong dependence on the methanol exposure that was used. The small amount of CH3OD formed is assigned to exchange reactions inside the mass spectrometer. When the reaction would be responsible for the β2 desorption peak one would expect a much higher contribution of m/e ) 33, and its relative amount would increase with decreasing CH3OH dose. Based on these observations, we conclude that the 285.8285.7 eV species should be assigned to a molecular form of methanol [CH3OH(II)], with a higher desorption temperature and a different C 1s BE than CH3OH(I). Houtman and Barteau18 came to a similar conclusion after studying methanol adsorption/decomposition from Rh(111). Their desorption experiments using deuterium (similar to our experiments) revealed that the high-temperature methanol desorption peak around 210 K is caused by desorption of molecular methanol rather than by re-hydrogenation of intermediate species like methoxy (CH3Oad). IV. Adsorption and Decomposition in the Presence of Oad We also studied the effect of Oad on the surface chemistry of CH3OHad. The surface was exposed to O2 (300 K, 20 L) until a saturation coverage (0.5 ML) of Oad was obtained. Methanol was subsequently dosed at ∼100 K, until saturation was reached. 1. Thermal Desorption. The thermal desorption traces of m/e ) 18 (H2O), 31 (CH3OH), and 44 (CO2) are shown in Figure 5a. Molecular desorption in the presence of Oad occurs at the same temperatures as desorption from the clean surface. The H2O desorption trace shows two distinct peaks, around 230 K and around 350 K, respectively. The CO2 desorption trace also shows a peak around 350 K and another peak around 400 K. Note that the desorption of CO2 around 350 K is accompanied by a H2O desorption peak. The CO2 peak around 400 K, on the other hand, is not accompanied by a H2O desorption peak.
Methanol Decomposition and Oxidation on Ir(111)
Figure 5. TPD and TP-XPS of an Oad saturated (0.5 ML, 20 L O2 at 300 K), methanol post-dosed surface (10 L, 140 K). Panel a shows the TPD traces (3 K s-1), and panel b shows the corresponding TP-XPS results (0.11 K s-1).
Desorption of other products, like formaldehyde or formic acid, was not observed. CO desorption was also absent, showing that there was enough Oad present to oxidize all COad formed during methanol decomposition. 2. X-ray Photoelectron Spectroscopy. Temperature-programmed XPS measurements provide more information about the nature and concentration of surface species, which can be used to understand the desorption spectra reported in the previous part. Figure 6 shows the results of the TP-XPS measurements. In the top of panel a, several typical O 1s core level spectra are shown, whereas in the bottom part of panel a, an overview of the O 1s core level region is shown as a function of temperature. In panel b, the corresponding C 1s spectra are shown, in a similar fashion. The BE values for the different components that were found are reported in Table 1. The spectra were fitted using the following fitting components: CH3OHad (on ox.), CH3Oad, HCO2ad, COad, Oad, and CHxad. Some of the peaks used in the fitting are shown in Figure 7. The result of the fitting procedure are shown in Figure 5b. Note that the peaks used to fit the spectra are rather broad, due to vibrational splitting (see section III 2). CH3OHad in the presence of preadsorbed Oad appears at a lower BE than CH3OHad on the initially clean surface, both in the C 1s (-0.4 eV) and in the O 1s spectrum (-0.6 eV).2 As the molecular desorption in the presence of Oad is similar to that in the absence of Oad, we assign the shift to an influence of Oad on the photoemission process rather than to distortion of the adsorbed methanol molecule. The oxygen adatoms are negatively charged, and the negative charge on the Oad atoms can have an influence on the energy of the core levels in the adsorbed methanol, an initial state effect. In a similar way, the energy of the final state can be lowered by stabilization of the positively charged ion formed in the photoemission. We cannot
J. Phys. Chem. C, Vol. 111, No. 21, 2007 7745 distinguish between these two processes based on our experimental results, and theoretical calculations would be necessary to quantify the different contributions. At 140 K both molecular methanol and Oad are present on the surface. A part of the CH3OHad desorbs molecularly around 160 K, and this is accompanied by an apparent increase of the Oad signal. We explain this apparent increase by photoelectron diffraction; that is, the adsorbed methanol influences the signal coming from the oxygen atoms. During methanol uptake on an oxygen-saturated surface at 140 K, which was followed using XPS (results not shown here), the opposite effect was observed; that is, the apparent Oad signal decreased from 0.5 to 0.3 ML due to the presence of coadsorbed methanol. Around the same temperature (160 K), a new species is observed in the C 1s spectrum, with a BE of 285.4 eV (-0.3 eV with respect to CH3OHad in the presence of Oad). The O 1s BE of this species could not be determined exactly, due to the presence of multiple components in the same energy range in this temperature region. We tentatively suggest that the O 1s component due to CH3Oad has a binding energy that is similar to that of adsorbed methanol. This species is a precursor for the formation of the other methanol decomposition products (COad and formate), and it is, therefore, assigned to a partial decomposition product of methanol. The XPS data do not provide the atomic composition of the intermediate, and possible candidates include methoxy (CH3Oad), formaldehyde (H2Cd O), or formyl (HCdO). Most literature reports on (111) surfaces propose methoxy as an intermediate.8,18,54 The assignment of this species to methoxy deviates from the assignment of the 285.8-285.7 eV component (see section III 3, which has a similar ∆BE with respect to CH3OHad in the absence of Oad). However, there are several significant differences between the thermal behavior of the 285.8-285.7 eV species and the methoxy, which resulted in a different assignment: First of all, there is a difference in the fact that the species in the absence of Oad is not a precursor for methanol decomposition, whereas the species in the presence of Oad is a precursor for both COad and formate formation. Furthermore, literature reports for Pt(111) indicated that methoxy is only stable in the presence of Oad,8,54 similar to what we find on Ir(111). The methoxy species disappears again around 220 K. Two different pathways are identified: (I) decomposition into COad (and Had/H2O) and (II) reaction with Oad to form formate (and some H2O as well). The overall reactions are described in eqs 1 and 2
2CH3Oad + 3Oad f 2COad + 3H2O(g)
(1)
CH3O + 2 Oad f HCO2ad + H2O(g)
(2)
As can be seen from the equations, Oad is consumed in both processes and its concentration, therefore, drops around 220 K. This is accompanied by a H2O desorption peak. Formate appears around 220 K on the surface, and is observed at BEs of 287.2 (C 1s) and 531.3 eV (O 1s), respectively. We also looked at the decomposition of formic acid on the same surface, which yielded exactly the same C 1s and O 1s components during heating. Furthermore, the thermal behavior of the species formed during formic acid decomposition (most probably formate) was identical. This confirms our assignment to HCO2ad. Formate decomposes around 350 K, and as a result both H2O (g) and CO2 are observed. This is accompanied by a decrease of the Oad concentration. During decomposition of formic acid (not shown), a H2 desorption peak was observed
7746 J. Phys. Chem. C, Vol. 111, No. 21, 2007
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Figure 6. Two spectra taken during heating of a mixed Oad/CH3OHad overlayer. The different fitting components used to fit the C 1s spectra are included.
showed that HCO2ad also forms when only Oad is available. On Pd(111), formate can also be produced via reaction between Oad and CH3OHad.17 Formate on Ir(111) is also significantly more stable than on Pt(111) and Pd(111), where it decomposes around 280 K.3,17 V. Summary and Conclusions
Figure 7. TP-XPS (0.11 K s-1) of an Oad-saturated (0.5 ML, 20 L O2 at 300 K), methanol post-dosed surface (10 L, 140 K). The top part of panel a shows several individual O 1s spectra, whereas the bottom part shows a top view of the O 1s core level region as a function of temperature. Panel b shows the C 1s spectra in a similar fashion.
instead at this temperature, as in that case Oad was not present to react with the Had formed during HCO2ad decomposition. This shows that Oad does not play a direct role in the HCO2ad decomposition mechanism (for example by abstracting a hydrogen atom), but only reacts with the HCO2ad decomposition product. Above 350 K, only Oad, COad, and a small amount of CHxad species (which were formed around 200 K) remain on the surface. COad and Oad react around 400 K, and this explains the second CO2 desorption peak. The formation of HCO2ad was also observed on Pt(111),3 but only when O2ad was present on the surface. On Ir(111), we
In this study, the interaction of methanol with Ir(111) is reported. Both temperature-programmed desorption and X-ray photoemission spectroscopy have been used to study the adsorption, decomposition, and oxidation of methanol. Chemisorbed methanol desorbs molecularly in two distinct desorption peaks, around 170 (β1) and 220 K (β2), respectively. At low temperature, the formation of a second adsorption layer is observed, which desorbs around 140 K. The methanol responsible for the β2 desorption peak appears as a separate peak in the C 1s core level spectrum. Thermal desorption experiments, including experiments in which isotopes were used, indicated that this component is a molecular form of methanol, rather than an intermediate in methanol decomposition, like CH3Oad. Decomposition of methanol into COad and Had already occurs around or below 175 K. The presence of CHxad around 200 K indicates that a small part of the methanol decomposes via C-O bond scission. The presence of CHxad species was also observed during the TPD experiments, in which H2 desorption occurred around 500 K due to CHad decomposition. It cannot be excluded that defect sites are responsible for the C-O bond breaking activity. The methanol adsorption energy is not significantly influenced by the presence of 0.5 ML Oad on the surface, as evidenced by the CH3OHad desorption temperature. Both the C 1s and the O 1s core level spectra exhibit a downward shift (-0.4 and -0.6
Methanol Decomposition and Oxidation on Ir(111) eV, respectively) as a result of the presence of Oad. This is explained by an influence of (negatively charged) oxygen adatoms on the stability of the initial and/or final state of the photoemission process. Decomposition in the presence of Oad proceeds via a methoxy (CH3Oad) intermediate, which forms around 160 K. This CH3Oad intermediate reacts around 220 K with Oad, forming both formate (HCO2ad) and COad. Formate decomposes around 350 K, into CO2 (g) [and Had, which reacts with Oad to form H2O]. The remaining Oad and COad react around 400 K, forming CO2. A small amount of CHxad was observed in these experiments as well. There was no indication of the formation of other gaseous products, like formaldehyde, methane, or formic acid. Acknowledgment. The authors acknowledge ELETTRA and the European Union for financial support to perform measurements at the SuperESCA beamline of ELETTRA. S. Lizzit, L. Petaccia, and A. Baraldi are acknowledged for their help during the beamtime. R. C. V. van Schie is acknowledged for excellent technical support. C.J.W. acknowledges The Netherlands Technology foundation STW, the applied science division of NWO, and the technology programme of the Ministry of Economic Affairs, for financial support under Project Number UPC.5037. References and Notes (1) Reddington, E.; Sapienze, A.; Gurau, B.; Viswanathan, R.; Saragapani, S.; Smotkin, E. S.; Mallouk, T. E. Science 1998, 280, 1735. (2) Attard, G. A.; Chibane, K.; Ebert, H. D.; Parsons, R. Surf. Sci. 1989, 224, 311. (3) Endo, M.; Matsumoto, T.; Kubota, J.; Domen, K.; Hirose, C. Surf. Sci. 1999, 441, L931. (4) Desai, S. K.; Neurock, M.; Kourtakis, K. J. Phys. Chem. B 2002, 106, 2559. (5) Gibson, K. D.; Dubois, L. H. Surf. Sci. 1990, 233, 59. (6) Wang, J.; Masel, R. I. J. Am. Chem. Soc. 1991, 113, 5850. (7) Kizhakevariam, N.; Stuve, E. M. Surf. Sci. 1990, 286, 246. (8) Akhter, S.; White, J. M. Surf. Sci. 1986, 167, 101. (9) Sexton, B. A.; Rendulic, K. D.; Hughes, A. E. Surf. Sci. 1982, 121, 181. (10) Morkel, M.; Kaichev, V. V.; Rupprechter, G.; Freund, H.-J.; Prosvirin, I. P.; Bukhtiyarov, V. I. J. Phys. Chem. B 2004, 108, 12955. (11) Chen, J.-J.; Jiang, Z.-C.; Zhou, Y.; Chakraborty, B. R.; Winograd, N. Surf. Sci. 1995, 328, 248. (12) Levis, R. J.; Zhicheng, J.; Winograd, N. J. Am. Chem. Soc. 1989, 111, 4605. (13) Christmann, K.; Demuth, J. E. J. Chem. Phys. 1982, 76, 6308. (14) Christmann, K.; Demuth, J. E. J. Chem. Phys. 1982, 76, 6318. (15) Holroyd, R. P.; Bowker, M. Surf. Sci. 1997, 317-319, 786. (16) Solymosi, F.; Berko´, A.; To´th, Z. Surf. Sci. 1993, 285, 197. (17) Davis, J. L.; Barteau, M. A. Surf. Sci. 1988, 197, 123. (18) Houtman, C.; Barteau, M. A. Langmuir 1990, 6, 1558. (19) Parmeter, J. E.; Jiang, X.; Goodman, D. W. Surf. Sci. 1990, 240, 85.
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