Phenyl Species Formation and Preferential Hydrogen Abstraction in

Srdjan Kisin, Jelena Božović Vukić, Paul G. Th. van der Varst, Gijsbertus de With, and Cor E. Koning ... Junseok Lee, Daniel B. Dougherty, and John...
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J. Phys. Chem. B 2006, 110, 9939-9946

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Phenyl Species Formation and Preferential Hydrogen Abstraction in the Decomposition of Chemisorbed Benzoate on Cu(110) Junseok Lee, Daniel B. Dougherty, and John T. Yates, Jr.* Department of Chemistry, Surface Science Center, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15260 ReceiVed: February 1, 2006; In Final Form: March 27, 2006

The adsorption and decomposition of benzoic acid on the Cu(110) surface has been investigated using temperature-programmed reaction (TPR) spectroscopy and scanning tunneling microscopy (STM). The benzoate species is found to exist in two conformationssa phase containing upright species at monolayer saturation and a phase containing many lying-down species at lower coverages. Thermal decomposition begins to occur near 500 K, yielding benzene and CO2. It is found that phenyl species, generated preferentially from the lying-down benzoate species, efficiently abstract H atoms from undecomposed benzoate species to produce benzene in a rate-controlling process with an activation energy of about 29 kcal/mol. Using deuterium-atom substitution at the 4-C position on the benzoate ring it is found that the hydrogen-abstraction reaction is selective for 2,3 and 5,6 C-H bonds. This observation indicates that the mobile phenyl species is surface bound and preferentially attacks C-H bonds which are nearest the Cu surface and bind the benzoate species as either an upright species or a tilted species.

Introduction Knowledge of the chemisorption and decomposition of organic molecules on metal surfaces forms a basis for the control of many surface processes, including those involving the selfassembly of monolayer films. The carboxylic acids constitute one such type of surface-active molecule. Carboxylic acids have been relatively well studied because of their importance in industrial catalytic processes,1 biomolecular sciences,2 and nanotechnology.3 Carboxylic acids are known to release the acidic hydrogen when chemisorbed on some metal surfaces at room temperature, leaving adsorbed carboxylate on the surface.4-7 For the most simple carboxylic acid, formic acid, many studies have been done on clean and oxidized metal surfaces because the surface formate species is considered to be a reaction intermediate in the water-gas shift8 and methanol synthesis reactions on copper-based catalysts.9 Adsorption of formic acid on Cu,10,11 Pt,12 and Ru13,14 surfaces showed the molecule readily produces a stable formate and decomposes autocatalytically to produce H2 and CO2. On Ni surfaces, CO desorption has also been observed as well as H2 and CO2, suggesting that there is C-O bond cleavage in the formate.15,16 On the other hand, on clean Ag(110) and Ag(111) surfaces deprotonation of the acidic hydrogen is not found to occur, leading to the formation of adsorbed carboxylic acid species.17 On the Cu(110) surface, after deprotonation, the carboxylate is bound to two close-packed Cu atoms with the plane of the carboxylate moiety being oriented parallel to the 〈11h 0〉 azimuth.18-21 Temperature-programmed reaction (TPR) studies show that the carboxylate species is very stable at room temperature. Upon heating, carboxylate decomposition is observed. Formate decomposition on Cu(110) produces CO2 and H2 near 470 K. Hydrogen formation from formate is believed to proceed via C-H bond dissociation followed by adsorption of atomic hydrogen on the surface and a fast H(a) + H(a) recombination process.22 * To whom correspondence should be addressed. E-mail: [email protected].

There are several studies of benzoic acid adsorption indicating that bonding similar to formate occurs on Cu(110), and STM studies show that a series of coverage-dependent ordered structures form as the coverage changes.6,23-26 Decomposition of benzoate species on Cu(110) has been observed, but no detailed model was presented.25 In this paper, we report the decomposition mechanism of benzoate species chemisorbed on the Cu(110) surface using TPR spectroscopy and STM. We show that the major products are benzene and CO2. Evidence for phenyl species generation followed by a selective hydrogen-abstraction reaction by phenyl species is given. Experimental Section The TPR experiments described in this paper have been carried out in an ultrahigh vacuum (UHV) system with a base pressure below 1 × 10-10 mbar, as described elsewhere.21 The Cu(110) single crystal was 10 mm in diameter and 2 mm thick and could be cooled to 81 K and electrically heated to 900 K. It was cleaned by Ar+ bombardment, followed by annealing in vacuum at 800 K. The pressure in the UHV system rises by about 2 × 10-10 mbar during dosing of benzoic acid. All exposures were carried out at a crystal temperature of 200 K to reduce possible adsorption of water and other gases except for the hydrogen desorption experiment. For TPR measurements, a UTI 100C quadrupole mass spectrometer (QMS) was used at 70 eV ionization energy. To prevent the spurious electron bombardment of the crystal from the ionizer of the QMS, a crystal bias of -75 V has been applied. The ionization region of the QMS was enclosed by a stainless steel shield with an aperture of 6 mm diameter. During TPR measurements, several species including CO2 and benzene have been monitored simultaneously by multiplexing. The benzoic acid (C6H5COOH) as supplied was 99.5% pure (Aldrich). The benzoic acid-4-d (IsoScience, 98%), benzoic acid-O-d (Aldrich, 98%), and benzoic acid-d5 (Aldrich, 98%) were also used. To obtain

10.1021/jp0606863 CCC: $33.50 © 2006 American Chemical Society Published on Web 05/05/2006

9940 J. Phys. Chem. B, Vol. 110, No. 20, 2006 reproducible dosing for benzoic acid and to raise the vapor pressure, a high-temperature dosing system has been employed.21,27 Benzoic acid vapor was admitted to the UHV chamber through a heated effusive beam doser. Molecular sieve material in a trap between the storage bulb for crystalline benzoic acid and the turbo pumped gas handling line was used to remove water and volatile contaminants. STM experiments were performed in a different UHV system consisting of a sample preparation chamber and an analytical chamber housing a commercial STM (Omicron LT-STM). The two chambers were isolated from one another by a gate valve except during sample transfer. A rectangular Cu(110) crystal (MaTeck, 8 mm × 5 mm × 1.5 mm) was mounted on a commercial Ta sample plate (Omicron) with a 7 mm × 4 mm rectangular geometry milled from its center to allow effective radiative heating from the rear. The crystal was cleaned by Ar+ sputtering and annealing to 800 K in the preparation chamber (base pressure ≈ 1 × 10-9 Torr). The only minor contaminant visible by AES was carbon. Subsequent STM observations showed that its concentration was ∼1013 cm-2. Benzoic acid was dosed onto the Cu(110) surface held at 300 K in the preparation chamber through a heated stainless steel tube ∼3 cm from the sample. The benzoic acid was admitted to the tube through an all-metal leak valve from an independently pumped and heated glass ampoule. During the dose, the pressure in the chamber never rose above 1.3 × 10-8 Torr and recovered almost immediately after closing the leak valve. The sample was transferred to the analytical chamber (base pressure < 1 × 10-10 Torr) quickly after dosing. Annealing experiments were performed by transferring the sample back into the preparation chamber and heating it radiatively using a pyrolytic boron nitride heater mounted behind the crystal on the end of the sample manipulator. Temperature was monitored with a K-type thermocouple attached to the manipulator near the sample that had a known temperature offset of ∼100 K. When the desired temperature was attained, the heater was quickly turned off, resulting in annealing steps of 10-30 s. The sample was then transferred to the STM chamber, where it was allowed to cool to room temperature for imaging. All STM images were obtained with tungsten tips in constant current mode. The sample was held at -2 V with respect to the tip during imaging and the current set point (0.2-2 nA) was varied to attain stable imaging conditions at the desired resolution.

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Figure 1. Temperature-programmed reaction (TPR) spectra of benzoate on Cu(110) at saturated monolayer coverage. The heating rate is 2 K/s. The major products are CO2 and benzene with the peak temperature around 590 K. There is virtually no desorption of the benzoic acid (122 amu) parent mass and its major cracking product at 105 amu.

Results

Figure 2. Thermal desorption spectra of the hydrogen molecule and its isotopomers from Cu(110) covered with a monolayer of C6H5COOD. The heating rate is 2 K/s.

A. Observation of Desorption Processes for Chemisorbed Benzoate Species. Temperature-programmed reaction (TPR) spectra for a saturated monolayer coverage of benzoate on Cu(110) are shown in Figure 1. In this paper, we define 1 ML as the saturated benzoate coverage. Benzene and CO2 are major products exhibiting a maximum desorption rate at around 590 K. The onset of desorption is near 500 K. The ratio of C6H5+/ C6H6+ ) 0.26 is very close to the value that is expected for gas-phase benzene fragmentation in the mass spectrometer.28 The absence of a product at 105 amu indicates that benzoic acid and benzoate do not desorb as major products in this process when the coverage is 1 ML. At elevated temperature, however, the hydrogen atom from the decomposition reaction can recombine with benzoate to yield benzoic acid even though the probability is very low. In fact, we could observe a very tiny amount of benzoic acid desorption (105 amu) at the same desorption peak temperature as CO2 and benzene. In Figure 1 this tiny amount of benzoic acid desorption is invisible at the

mass spectrometer gain employed. No other major desorbing species is found except benzene, CO2, and their mass spectrometer cracking products for an initial coverage of 1 ML. Similar results have been found for formate decomposition on Cu(110) where CO2, H2, and a very small amount of formic acid have been observed.29 At higher benzoic acid coverages above 1 ML, the desorption of the intact benzoic acid molecule is observed beginning near 250 K with the peak temperature around 270 K due to the desorption of a multilayer of benzoic acid as described in our previous study.21 To investigate the possibility of acidic hydrogen contribution to desorption products following adsorption of a monolayer, benzoic acid-O-d (C6H5COOD) was adsorbed to monolayer coverage and the desorption of H2, HD, and D2 was monitored as shown in Figure 2. Only a very tiny D2 desorption peak is observed above the noise level at 200 K. The result shows that there is no mixing of the dissociated deuterium atoms with the ring hydrogen atoms to produce HD.

Decomposition of Chemisorbed Benzoate on Cu(110)

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Figure 3. Coverage-dependent TPR spectra of CO2 and benzene from benzoate chemisorbed on Cu(110). The temperature range between the two vertical dashed lines indicates the increase in the peak temperature for both CO2 and benzene between the lowest and highest coverage. The inset shows the integration of peak area for each of the molecules. The dashed line is a guide to the eye. The heating rate is 2 K/s.

Figure 3 shows the coverage dependence of the TPR spectra of CO2 and benzene up to 1 ML of the initial benzoate coverage. The TPR spectra for both of the desorbing product molecules show that the peak shape is asymmetric and the peak temperature increases with increasing coverage from 580 to 595 K. It is reported that adsorption of the CO2 molecule can hardly occur on the clean Cu(110) surface even at 110 K.30 Also, the desorption temperature of the chemisorbed benzene molecule is near 300 K on the clean Cu(110) surface.31 On this basis, we assign the simultaneous desorption of benzene and CO2, beginning near 500 K, to decomposition of the adsorbed benzoate species. As shown in the inset of Figure 3, the desorption yield increases as the initial coverage of benzoate increases and reaches a constant value at the saturation coverage. The magnitude of the desorption signal in thermal desorption measurements made in a rapidly pumped UHV system is directly proportional to the kinetic rate of desorption.32 Therefore, a comparison of the coverage-dependent desorption peak shape for the two products of benzoate decomposition may be used to directly compare the relative rates of desorption of the two products, rbenzene/rCO2, as a function of decreasing coverage during thermal desorption as shown in Figure 4. We normalized the desorption peaks to the peak temperature for better comparison. At low coverages (0.15 ML), the peak shape and hence the desorption kinetics of benzene and CO2 are almost identical as shown in Figure 4a; at high coverage (1 ML), the peak shapes for the two products diverge somewhat from each other as shown in Figure 4b. The rate ratios of rbenzene/rCO2, obtained from the raw data, are plotted in Figure 4c. Due to signal scatter only the ratio between 545 and 620 K is considered meaningful. If the rate of benzene and CO2 liberation was identical throughout the temperature and coverage range of the TPR experiment, the ratio of the desorption rates would be constant. Figure 4c shows that at an initial benzoate coverage of 0.15 ML, rbenzene/rCO2 drops uniformly from ∼0.35 to ∼0.15. More importantly, for an initial coverage of 1 ML of benzoate, the ratio of rates rbenzene/rCO2 rises initially as the thermal decomposition/desorption occurs, which may be seen by the dark points in Figure 4c. In the later stages of product liberation

Figure 4. TPR spectra normalized to peak maximum showing the difference in peak shape and peak temperature at two different initial coverages of benzoate: (a) 0.15 and (b) 1 ML. (c) The ratios of rbenzene/ rCO2 obtained from raw data are also shown. The regime between 545 and 620 K shows ratio information with acceptable signal/noise quality.

Figure 5. Plot of ln of the desorption rate versus 1/T at two different initial benzoate coverages (open circles, CO2; filled circles, benzene).

the ratio of rates falls in the manner seen for the 0.15 ML experiments. The activation energy of decomposition of the benzoate has been calculated by using the Habenschaden-Ku¨ppers method33 in Figure 5, where we plot the log of the desorption rate versus 1/T and use the initial slope of the line to calculate the activation energy. At 0.15 ML, an activation energy of 29 ( 2 kcal/mol is obtained for both of the products where we used only the initial desorption temperature range to ensure that there is no

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Figure 6. Coverage-dependent TPR yield ratio (benzene/CO2). Each desorption yield is taken from the inset of Figure 3. Dashed curve is a guide to the eye.

significant change in the coverage. The activation energy is slightly lower than the activation energy of 31.9 kcal/mol for formate decomposition on Cu(110).22 At 1 ML, the activation energy obtained for CO2 production is almost the same as the value obtained at low coverage. However, the benzene desorption trace for 1 ML of benzoate gives an activation energy of 57.4 kcal/mol, which is much higher than the other cases. This effect may be easily seen in Figure 4b, where the benzene desorption is retarded with respect to CO2 desorption, which tells us there is a coverage-dependent kinetic step influencing the benzene production during the decomposition of benzoate. Benzoate itself is not observed in TPR spectra. It is expected that this species will not desorb intact because the bonding energy between the two oxygen atoms in the carboxylate group and surface copper atoms is known be relatively high.34 Upon heating, the weakest bond, probably in this case the C-C bond that connects the CO2 and phenyl moieties, dissociates to produce CO2 and the phenyl species initially. The activation energy of benzoate decomposition is much smaller than the bonding energy (85 kcal/mol) of the carboxylate group to the copper atoms.34 B. Observation of Product Yields for Benzoate Decomposition. The ratio of the total benzene/CO2 product yield as a function of the initial benzoate coverage is shown in Figure 6. The benzoate coverages have been found to be proportional to exposure from the heated beam doser used in these experiments in previous work.21 The coverage is calibrated assuming the saturation of benzoate is at 1 ML, which would yield a c(8 × 2) structure as observed at saturation coverage by others.24 As the benzoate initial coverage increases, the fraction of the benzene product decreases relative to CO2. This is supported by the inset in Figure 3 where the attenuation of the benzene yield at higher coverages relative to continued CO2 production is evident. C. STM Observation of the Effects of Benzoate Decomposition. The saturated c(8 × 2) overlayer of benzoate is shown in the STM image of Figure 7a immediately after its formation at room temperature. The STM measurements are made at room temperature in Figures 7 and 8. Annealing to ∼510 K produces no visible changes as shown in Figure 7b. No structure other than the c(8 × 2) overlayer can be found on the surface. Annealing the same sample to 610 K however produces the

Figure 7. Sequence of STM images during an annealing experiment: (a) starting surface with saturated c(8 × 2) structure everywhere (1250 Å × 1250 Å, -1.89 V, 0.33 nA), (b) surface after annealing to 510 K showing no change (1000 Å × 1000 Å, -1.89 V, 0.32 nA), (c) after annealing to 610 K showing coexistence of disordered c(8 × 2) and β-phase domains (1000 Å × 1000 Å, -1.89 V, 0.32 nA), (d) after another annealing step at 610 K showing very few clear molecular features (1000 Å × 1000 Å, -2V, 0.55 nA).

dramatic changes shown in Figures 7c and 8b. The c(8 × 2) has become disordered, and domains of a new ordered structure are also clearly present.35 Figure 8b shows a high-resolution STM image of one of the new ordered domains. This so-called β phase of benzoate on Cu(110) was first observed by Frederick et al.25 and has a very different arrangement of benzoate species than the c(8 × 2) layer before annealing. Specifically, it is known from STM and RAIRS25 investigation to consist not only of the upright benzoate species visible by STM in Figure 8b, but also of an array of benzoate species with their molecular planes nearly parallel to the Cu surface. Even for only slight reductions from saturation coverage, the ordered structures formed contain mixtures of both flat-lying and upright benzoate species. Continued annealing at 610 K results in the surface shown in Figure 7d. Very few molecular features are visible in the image. Figure 8c shows a higher resolution image of the sample at this stage. The boxed area in this figure highlights four protrusions characteristic of the lowest coverage ordered structure of benzoate on Cu(110).25 These features are ordered flat-lying benzoate species at low coverage. Otherwise the image is somewhat “noisy”, indicating the presence of highly mobile species on the surface. Successive annealing steps in the decomposition regime show expected successive reductions in surface benzoate concentration. Importantly, the reduction is accompanied by dramatic rearrangements of the benzoate species remaining on the surface. As the coverage is lowered by benzoate decomposition and desorption of benzene and CO2, surface benzoate species reorganize into the most energetically favorable ordered overlayer which involves formation of flat-lying benzoate species.24,25 This rearrangement is associated with the change in TPR peak shape described in the previous section. D. Production of Benzene from Benzoate Decompositions Study of Isotopic Products from Benzoate-4-d Decomposition. Decomposition of chemisorbed benzoate on Cu(110)

Decomposition of Chemisorbed Benzoate on Cu(110)

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Figure 8. Higher resolution sequence of STM images during annealing: (a) starting c(8 × 2) surface (300 Å × 300 Å, -1.89 V, 0.32 nA), (b) image of a β-phase domain after the first annealing step at 610 K (247 Å × 247 Å, -1.89V, 0.29 nA). The features in the image correspond to alternating rows of paired and single upright benzoates as observed in ref 25. (c) Image showing remnant of the lowest coverage ordered phase after a second annealing step at 610 K (153 Å × 153 Å, -2V, 0.55 nA). The four protrusions in the box are the only stable molecular features present in the image and correspond to lying-down benzoate species.

Figure 9. TPR spectra of benzene from benzoate-4-d/Cu(110). The 79 amu is, in this case, the benzene-d molecule (C6H5D). The heating rate is 2 K/s, and a saturated benzoate coverage is used for this experiment.

produces CO2 and a phenyl species as the C-C bond joining the phenyl ring to the carboxylate moiety is broken. The lineof-sight mass spectrometry does indeed detect a 77 amu product ion (Figure 1) which could be assigned to desorbing phenyl species, but the magnitude of this desorption signal can be quantitatively explained as being a mass spectrometer cracking product from the benzene product (78 amu) of the benzoate species’ thermal decomposition. It is well established that a radical species such as a phenyl radical abstracts a hydrogen atom from organic species containing C-H bonds.36 Thus, benzene production likely originates from phenyl species which encounter C-H bonds on neighboring benzoate species which have not decomposed. To study the stereochemical aspects of the attack of phenyl species on undecomposed benzoate species, the benzoate-4-d species was studied. In this case, if the upright benzoate species is involved, the 4-carbon atom and its associated D moiety will be far removed from the Cu(110) surface, compared to the other C-H bonds in the upright benzoate species. Figure 9 shows the isotopic distribution of benzene products in the mass spectrometer analysis of the desorbing benzene as well as other important products. We observed desorption of C6H5D (79 amu) and CO2 as major desorbing species. Mass at 78 amu is also observed representing the phenyl cation fragment of the parent C6H5D ion (79 amu) in the mass spectrometer. It is evident that the major species detected is the benzene molecule, not the phenyl

radical, because the ionic product distribution closely resembles the gas-phase mass spectrum of benzene. In addition, the result in Figure 9 indicates that the source of the hydrogen atom for the benzene formation is not the hydrogen at the 4(para) position of the phenyl ring in the neighboring benzoate. If the source of the hydrogen were from the C-H bond at the 4 position, it would have yielded C6H4D2 (80 amu) via the reaction C6H4D + D in this experiment. Desorption of the 80 amu species is shown in Figure 9. A careful analysis of the relative integrated intensity of the 80 amu product to the 79 amu desorption product shows that its contribution only slightly exceeds that of the expected 6.6% contribution due to the natural abundance of 13C at 6 positions in the desorbing heavy benzene. This slight excess at 80 amu could be due to C6H4D2 but would amount to only 1.6 mol % of the desorbing benzene. Since in the benzoate-4-d isotopomer the label exists on 20% of the carbon-hydrogen bond positions in the phenyl ring, we conclude that abstraction of a D atom from the 4 position of adsorbed benzoate species by phenyl species is a minor process. Biphenyl (156 amu) desorption is also shown in Figure 9. The intensity of the desorption peak of biphenyl is very small compared to the benzene desorption intensity. Even though it is small, formation of biphenyl indicates there is a phenyl generation step during decomposition and a small fraction of the phenyl species recombine to produce desorbing biphenyl. Note that there is no biphenyl desorption around 400 K. In other experiments where the phenyl species is generated on a clean Cu(111) surface, the desorption of biphenyl is observed at ∼400 K.37 This means that prior to the decomposition of benzoate, phenyl species are not present on the surface. If the surface recombination mechanism of phenyl species applies, we would expect to see an appreciable amount of biphenyl desorption at the same temperature as CO2 desorption as the C-C bond breaks. The observation of a very small yield of biphenyl implies that another efficient reaction involving phenyl species occurs before the phenyl species encounters another phenyl species on the surface. E. Production of Benzene from Benzoate Decompositions Study of Isotopic Products from C6H5COO and C6D5COO Species. A mixture of C6H5COOH and C6D5COOH has been used to further verify the details of the reaction mechanism. The H-abstraction process by phenyl species has been investigated using a mixture of C6H5COO(a) and C6D5COO(a) species chemisorbed on the Cu(110) surface. Adsorption of a gas mixture of C6H5COOH and C6D5COOH resulted in a mole ratio of 1.0:0.6 as judged by desorption studies of the multilayer21 of benzoic acid. Figure 10 shows the benzene product yield from the mixture of benzoic acid isotopomers as well as the cracking pattern of the idealized benzene gas-phase mixture consisting of C6H6 and

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C6H6:C6H5:C6H4 ) 1:0.28:0.06

(2)

C6H5D:C6H5:C6H4:C6H4D:C6H3D ) 1:0.28:0.017:0.235:0.04 (3) C6HD5:C6D5:C6D4:C6D4H:C6D3H ) 1:0.047:0.017:0.14:0.026 (4) C6D6:C6D5:C6D4 ) 1:0.17:0.04

Figure 10. Comparison of the mass cracking pattern between integrated TPR feature and the idealized gas mixture of C6H6 and C6D6 in the ratio of 1.0:0.6. The intensities are normalized to 78 amu. The ratio of 1:0.6 between C6H6 and C6D6 is assumed from the amount of multilayer desorption from a mixture of C6H5COOH and C6D5COOH.

C6D6 in the ratio 1.0:0.6. The intensity of each mass was obtained by integrating the peak area of individual TPR spectra. The gas-phase mass spectrum of the mixture of C6H6 and C6D6 is shown for the initial 1.0:0.6 reactant ratio. Note that the mass intensities are normalized to the mass at 78 amu to compare the relative amount of each isotopomer. If there is no benzene formation reaction driven by hydrogen abstraction by the phenyl species during the decomposition process, the experimental product yield would resemble the ideal benzene gas-phase mixture mass spectrum. However, deviations can be seen at several masses, especially at 79 and 83 amu, which represent C6H5D and C6HD5, respectively. Excluding a severe isotopic intermixing in the mass spectrometer ionizer, these benzene isotopomers can only come from the surface by C6H5 (C6D5) + D-C or H-C reactions. In addition, the 84 amu yield is much lower than in the idealized benzene gas mixture and the 81 amu yield is present in the desorbing benzene while absent in the idealized gas mixture. These results indicate that hydrogen abstraction by phenyl species occurs following decomposition of some benzoate species to produce phenyl radicals. To verify a radical-mediated benzene production mechanism, we consider the phenyl radicals (C6H5• and C6D5•) that are generated during the benzoate decomposition and calculate the expected product yield distribution. Assuming the initial reactant ratio as x/(1 - x), each phenyl radical can have xH and (1 x)D abstraction sources to make benzene. Including the initial reactant ratio, the product distribution desorbing from the surface can be obtained as follows

x2C6H6 + x(1 - x)C6H5D + x(1 - x)C6D5H + (1 - x)2C6D6 (1) The above product distribution, however, does not include mass cracking inside the mass spectrometer ionizer or different probabilities for H abstraction from different ring positions on the benzoate species. Thus, we need to consider the cracking pattern of each mass in order to simulate the final product yield distribution. The cracking pattern of each benzene molecule is obtained from the mass spectra database.28 We consider the following cracking ratios in the mass spectrometer ionizer for each of the benzene molecules.

(5)

In reactions 3 and 4 there are two radical generation processes, i.e., one from breaking the C-H bond and the other from breaking the C-D bond. For the C6H4 species, two sources (C6H5• and C6H4D•) are considered. We used one assumption that the C-H and C-D bond dissociation probability to make diradical species from phenyl species is the same as for the parent molecule. Starting from distribution 1 and considering the cracking ratios for each benzene molecule, we are able to calculate the actual isotopomer product yield distribution in the TPR spectra as shown in Figure 11. We can see a very close match between the experimental and calculated mass product yields. The slight discrepancies may be due to the assumptions made or to additional reactions in the mass spectrometer which we could not take into consideration. This result strongly confirms the phenyl radical-mediated reaction mechanism involving phenyl species abstraction of ring hydrogen atoms from neighboring benzoate species. Discussion A. Mechanistic Understanding from These Studies. Two fundamental features of the surface decomposition of chemisorbed benzoate species on Cu(110) have been discovered in this work: (1) a fully covered benzoate surface is composed of standing-up benzoate species, likely produced as a result of repulsive intermolecular forces in the overlayer at high coverages, while a partially covered surface possesses many lyingdown benzoate species (Figure 8); (2) the yield of benzene compared to CO2 increases as the surface coverage decreases during the surface reaction (Figures 4 and 6). These findings indicate that the conformation of chemisorbed benzoate is critically important in governing its dissociative reaction to produce benzene. Benzoate in the lying-down configuration is more likely to produce phenyl species leading to benzene production. The strong surface interaction of the lying-down benzoate species leads to facile C-C bond scission and concomitant phenyl species generation. The activation energy for decomposition from the lying-down benzoate species (low coverage) is about 29 kcal/mol, but for upright benzoate species (high coverage) it is about 57 kcal/mol (Figure 5). We find that generation of benzene from the interaction of phenyl radical species with benzoate species is conformationally governed by the orientation of the benzoate species. Thus, for hydrogen abstraction from the C-H bonds of benzoate, the 4 position is strongly disfavored (Figure 9, showing low C6H4D2 yield from benzoate-4-d species). It is likely that phenyl species abstraction of hydrogen from C-H bonds of benzoate occurs preferentially by reaction pathways in which the phenyl species is near the surface. For all conformations of benzoate, if the plane of the benzene ring is either tilted (lying down species) or normal to the surface, the 4 position will be at the largest distance from the surface and therefore be disfavored for hydrogen abstraction involving a colliding phenyl species. The π-bound adsorbed phenyl species (generated preferentially from lying-down benzoate species) move across the surface seeking

Decomposition of Chemisorbed Benzoate on Cu(110)

J. Phys. Chem. B, Vol. 110, No. 20, 2006 9945 is insufficient stoichiometrically. This indicates that there are sinks for the hydrogen atoms evolved by the carboxylic acid O-H bond scission during carboxylate formation. It may be possible for subsurface H atoms to form on Cu(110) as has been seen for atomic H adsorption.38,39 Summary of Results

Figure 11. TPR product yield distribution considering phenyl radical H or D atom abstraction from neighboring benzoate species, corrected for mass spectrometer fragmentation processes. To obtain the product ratio, the cracking of parent molecule (mass m) to (m - 1) and (m 2) has been considered. The intensities are normalized to 78 amu, and the initial reactant ratio of 1:0.6 is used as in Figure 10.

Figure 12. Schematic of the decomposition reaction of the benzoate species adsorbed on the Cu(110) surface. The upright benzoate species at higher coverage changes its geometry to the tilted species. The phenyl radical species generated from C-C bond scission preferentially attacks C-H bonds at the 2,3 and 5,6 positions of neighboring benzoate species, as colored red. CO2 and benzene are the major decomposition products.

accessible C-H bonds on undecomposed benzoate species for hydrogen abstraction, and the 2,3 and 5,6 C-H bonds will be more easily abstracted due to their proximity to the surface as a result of the attachment of benzoate to the surface via the carboxylate moiety. Figure 12 shows schematically the favored benzoate geometry for C-C bond scission and phenyl species generation (left side); it also shows (right side) the favored C-H bonds (designated as red H atoms) on a tilted or standing-up benzoate species which undergo facile H-atom abstraction by a mobile phenyl species which moves in close contact with the Cu(110) substrate. B. Unresolved Issues. 1. Other Surface Reactions. If we assume that all benzoate species yield CO2 upon thermal decomposition, then the increasing relative yield of benzene during TPR from the saturated benzoate layer must indicate that other organic products than phenyl species and benzene are initially produced in the surface decomposition reaction. We identified small coverages of carbon and oxygen on the surface by AES following benzoate decomposition. These products, either remaining on the surface or desorbing, have not been investigated in this work. The only other observed product is biphenyl, which is a minor one. 2. Fate of the Acidic Hydrogen. The evolution of a tiny amount of D2 and HD from C6H5COOD near 200 K (Figure 2)

The surface chemistry of benzoic acid on Cu(110) has been studied by TPR and STM. The following results have been found: (1) the benzoate species is formed by O-H bond scission when benzoic acid is adsorbed on Cu(110). This species exists in two conformations: a lying-down species at low coverages converting to upright species as the coverage is increased. The full coverage of benzoate involves only upright species; (2) carbon dioxide and benzene are the major volatile product species observed to be evolved when the chemisorbed benzoate species begins to thermally decompose at about 500 K. The relative yield of benzene increases, compared to CO2, as the coverage of benzoate decreases during thermal decomposition, and the activation energy for benzene production decreases from about 57 to about 29 kcal/mol as the benzoate coverage decreases, allowing lying-down benzoate species to form; (3) the lying-down benzoate species undergoes facile C-C bond scission compared to upright benzoate species. Production of lying-down benzoate species is therefore responsible for the increased selectivity toward benzene at lower coverages; (4) phenyl species are produced when C-C bond scission occurs preferentially for lying-down benzoate species. These reactive species are likely weakly bound in a π-type structure and are highly mobile; (5) phenyl species efficiently abstract H atoms from C-H bonds in undecomposed benzoate species to produce benzene, which desorbs immediately. The dimerization of phenyl species to produce biphenyl is a very minor reaction process; (6) the H-abstraction process is selective for C-H bonds located closest to the Cu surface bonding the benzoate species. Almost no H abstraction by phenyl species occurs from the 4-C position in benzoate. This observation is consistent with involvement of the mobile phenyl species, bound weakly as a π-type complex near the surface which preferentially abstract hydrogen from C-H bonds at the 2,3 and 5,6 positions of both upright and tilted adsorbed benzoate species. Acknowledgment. This work was supported by a grant for the W. M. Keck Foundation to the W. M. Keck Center for Molecular Electronics and by a NEDO grant from Japan. We also thank Mr. Oleksandr Kuzmych for assistance with certain measurements. References and Notes (1) Rodriguez, J. A.; Goodman, D. W. Surf. Sci. Rep. 1991, 14, 1. (2) Hasselstro¨m, J.; Karis, O.; Weinelt, M.; Wassdahl, N.; Nilsson, A.; Nyberg, M.; Pettersson, L. G. M.; Samant, M. G.; Sto¨hr, J. Surf. Sci. 1998, 407, 221. (3) Hatzor, A.; Weiss, P. S. Science 2001, 291, 1019. (4) Poulston, S.; Bennett, R. A.; Jones, A. H.; Bowker, M. Phys. ReV. B 1997, 55, 12888. (5) Poulston, S.; Jones, A.; Bennett, R. A.; Bowker, M. Surf. Sci. 1997, 377, 66. (6) Frederick, B. G.; Ashton, M. R.; Richardson, N. V.; Jones, T. S. Surf. Sci. 1993, 292, 33. (7) Baumann, P.; Bonzel, H. P.; Pirug, G.; Werner, J. Chem. Phys. Lett. 1996, 260, 215. (8) Campbell, C. T.; Koel, B. E.; Daube, K. A. J. Vac. Sci. Technol. A 1987, 5, 810. (9) Klier, K. AdV. Catal. 1982, 31, 243. (10) Hayden, B. E.; Prince, K.; Woodruff, D. P.; Bradshaw, A. M. Surf. Sci. 1983, 133, 589.

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