ARTICLE pubs.acs.org/JPCC
Sum Frequency Generation Vibrational Spectroscopy and Kinetic Study of 2-Methylfuran and 2,5-Dimethylfuran Hydrogenation over 7 nm Platinum Cubic Nanoparticles Cesar Aliaga,†,§ Chia-Kuang Tsung,†,§ Selim Alayoglu,†,§ Kyriakos Komvopoulos,‡ Peidong Yang,†,§ and Gabor A. Somorjai*,†,§ †
Department of Chemistry and ‡Department of Mechanical Engineering, University of California, Berkeley, California 94720, United States § Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ABSTRACT: Sum frequency generation vibrational spectroscopy and kinetic measurements obtained from gas chromatography were used to study the adsorption and hydrogenation of 2-methylfuran (MF) and 2,5-dimethylfuran (DMF) over cubic Pt nanoparticles of 7 nm average size, synthesized by colloidal methods and cleaned by ultraviolet light and ozone treatment. Reactions carried out at atmospheric pressure in the temperature range of 20 120 °C produced dihydro and tetrahydro species, as well as ring-opening products (alcohols) and ring-cracking products, showing high selectivity toward ring opening throughout the entire temperature range. The aromatic rings (MF and DMF) adsorbed parallel to the nanoparticle surface. Results yield insight into various surface reaction intermediates and the reason for the significantly lower selectivity for ring cracking in DMF hydrogenation compared to MF hydrogenation.
1. INTRODUCTION Furan and its derivatives extracted from biomass (cellulose) through depolymerization and subsequent reactions may be used to obtain aromatic substituted rings, which can be further hydrogenated to produce tetrahydrofuran (THF), methyltetrahydrofuran (MTHF), and dimethyltetrahydrofuran (DMTHF) that may be used as fuels.1,2 Hydrogenation reactions occur in the gas phase at relatively low temperatures (below 100 °C) and atmospheric pressure in the presence of catalysts such as platinum. 2-Methylfuran (MF) and 2,5-dimethylfuran (DMF) are fivemember aromatic heterocyclic molecules of the furan family with one of the electron lone pairs of the oxygen atom delocalized over the π-system in the ring. Studies of furan adsorption on different single-crystal surfaces (e.g., Cu, Pt, Pd, and Ag) have shown that furan adsorbs to metal surfaces through the heterocyclic π-cloud in a nearly flat configuration and may also involve some cracking of the ring.4 9 Catalytic studies of MF and DMF hydrogenation have been carried out in an acetic acid solution over Adams platinum consisting of platinum(IV) oxide hydrate, which converts to metallic platinum (Pt black) during the reaction.10 It was observed that ring cleavage during DMF hydrogenation yields DMTHF and 2-hexanol, whereas MF hydrogenation produces 2-pentanol (80 90%) and MTHF (balance). Most spectroscopic studies of catalytic reactions on metal surfaces have been performed either on conventional-support nanocatalysts using infrared (IR) and Raman spectroscopy or metal single crystals. However, because the shape and size of the r 2011 American Chemical Society
metal nanoparticles affect chemical reaction turnover rates and product distributions, in the present work, reaction studies were carried out on nanoparticles of controlled size, shape, and crystalline orientation. Sum frequency generation vibrational spectroscopy (SFGVS) is particularly suitable for such studies because its high sensitivity and surface specificity allow for the investigation of monolayers of nanomaterials and the detection of surface intermediates in situ. Hydrogenation of DMF and MF was carried out over 7 nm Pt nanoparticles to examine the formation of various products (Figure 1). To obtain clean surfaces, the organic capping layer of the Pt nanoparticles deposited on fused silica by the Langmuir Blodgett (LB) technique was removed by ultraviolet (UV) light and ozone cleaning.3 The main objective of the present study was to investigate the effects of catalyst structure and molecular conformation on the reaction mechanism and selectivity. To accomplish this goal, in situ SFGVS and gas chromatography (GC) experiments were performed to identify the surface species produced in the temperature range of 20 120 °C.
2. EXPERIMENTAL PROCEDURES 2.1. Materials. 2-Methylfuran (99%, Aldrich), 2,5-dimethylfuran (99%, Aldrich), 2-methyltetrahydrofuran (>99%, Sigma-Aldrich), Received: November 29, 2010 Revised: February 7, 2011 Published: March 25, 2011 8104
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Figure 3. TEM micrograph of UV-ozone-cleaned 7 nm Pt cubic nanoparticles deposited on a carbon grid by the LB method.
Figure 1. Suggested mechanisms of MF and DMF hydrogenation. In both systems, ring opening and ring cracking occur simultaneously during hydrogenation. MF is the only molecule that produces different aliphatic alcohols, depending on the ring cleavage location. The mechanism of furan hydrogenation21 is also shown for comparison and as a reference model.
Figure 2. Schematic of the SFGVS reaction cell.
2,5-dimethyltetrahydrofuran (96%, mixture of cis and trans, Aldrich), 1-pentanol (>99%, Sigma-Aldrich), 2-pentanol (98% Aldrich), and 2-hexanol (99%, Aldrich) were subjected to several freeze pump thaw cycles prior to use. Ultrahigh-purity hydrogen (Praxair) was used in all gas-phase hydrogenation experiments. 2.2. High-Pressure Reaction Cell. Experiments were carried out in a temperature-controlled high-pressure cell (Figure 2) consisting of a stainless steel chamber on top of which an equilateral fused silica prism (20 20 mm) with a LB film of nanoparticles deposited onto its bottom face was attached through a Kalrez O-ring and clamped using a Teflon piece. Product gases in the reaction cell were constantly mixed by a
recirculation pump. Kinetic data were acquired periodically by sampling the reaction mixture and analyzing the relative gasphase composition in a flame ionization detector of a GC apparatus (HP 6898, Hewlett Packard) equipped with a 0.2% Carbowax 1500 packed column. The desired temperature was maintained by resistive heating of the reactor wall. 2.3. Nanoparticle Synthesis and Characterization. A detailed description of the techniques of nanoparticle synthesis and characterization used in this study can be found elsewhere.3,11 14 Briefly, to synthesize Pt nanoparticles capped with a poly(vinylpyrrolidone) (PVP) layer, 0.1 mmol of ammonium hexachloroplatinate (IV), 1.5 mmol of trimethyl(tetradecyl)ammonium bromide, and 2 mmol of PVP were added to 20 mL of ethylene glycol in a 50 mL three-necked flask at room temperature. The stock solution was first heated to 80 °C in a Glass-Col electromantle (60 W, 50 mL) with a Cole Parmer temperature controller (Diqi-sense) and then evacuated at this temperature for 20 min under magnetic stirring to remove the water and oxygen. Subsequently, the flask was heated to 180 °C at a rate of 10 °C/min and maintained at this temperature for 1 h under Ar. Upon the completion of the reaction, excess acetone was added at room temperature to form a cloudy black suspension, which was then separated by centrifugation at 4200 rpm for 10 min, and the black product was collected by discarding the colorless supernatant. Precipitated Pt nanocrystals were washed twice by precipitation/dissolution consisting of nanocrystal redispersion in 7.5 mL of ethanol by sonication and nanocrystal precipitation by the addition of 37.5 mL of hexane. After nanoparticle LB deposition onto the bottom face of the equilateral fused silica prism (SFGVS experiments) or the surface of a silicon wafer (GC experiments), the nanoparticles were UVozone treated to remove the PVP capping layer, as described previously.3 The removal of the capping layer was verified by observing the disappearance of the C H stretch mode in the SFGVS spectrum acquired before each experiment. The nanoparticle shape and size were examined with a transmission electron microscope (TEM). Figure 3 shows a typical TEM micrograph obtained before the reaction showing that the UVozone-treated nanoparticle layer consists of Pt nanocubes of average size equal to 7 nm. Scanning electron microscopy images of nanoparticle layers deposited on silicon wafers obtained before and after reaction confirmed that the nanoparticle distribution and average size did not change during the reaction. 2.4. Sum Frequency Generation Vibrational Spectroscopy. An active/passive mode-locked Nd:YAG laser (Leopard 8105
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Figure 4. Temperature dependence of MF hydrogenation over UVozone-cleaned 7 nm Pt cubic nanoparticles deposited on a silicon substrate by the LB method (10 torr MF and 100 torr H2). Reaction products are MTHF (b), 1-pentanol (2), 2-pentanol (1), and cracking products (9). The reaction is selective toward ring opening, leading to the production of 1-pentanol. However, when the temperature is raised above 95 °C, the selectivity changes to the other ring-opening product 2-pentanol.
D-20, Continuum) with a 20 ps pulse width and a 20 Hz repetition rate was used in all the SFGVS studies. The fundamental output at 1064 nm was passed through an optical parametric generation/amplification stage where a tunable IR beam (2700 4000 cm 1) and a second harmonic visible (VIS) beam (532 nm) were generated. The IR (100 μJ) and VIS (100 μJ) beams were spatially and temporally overlapped at the bottom surface of the fused silica prism where the nanoparticles had been deposited at angles of incidence equal to 42 and 65°, respectively, with respect to the surface normal. All the experiments were carried out in ppp polarization combination. The generated SFG signal was then collected and sent to a photomultiplier tube. The signal-to-noise ratio was further enhanced by a gated integrator, while the IR beam was scanned through the spectral region of interest. Additional information about the SFGVS system used in this study can be found elsewhere.15 20
3. RESULTS AND DISCUSSION 3.1. Methylfuran Hydrogenation. MF hydrogenation over UV-ozone-cleaned 7 nm Pt nanoparticles deposited as a LB film on a silicon wafer was carried out under conditions of 10 torr MF, 100 torr H2, and 650 torr Ar in the temperature range of 40 120 °C. Figure 4 shows reaction product distributions versus temperature obtained from GC measurements. 2-Methyltetrahydrofuran (MTHF), 1-pentanol, 2-pentanol, and cracking products were detected by GC. Due to the very low conversion, it was not possible to determine the nature of the cracking products. At 40 °C, the reaction shows ∼98% selectivity toward the ring-opening product 1-pentanol. At about 95 °C, the selectivity reverses, and the main product is the ring-opening product 2-pentanol. The relative concentration of the hydrogenated ring (MTHF) remains very low throughout the temperature range, similar to furan hydrogenation over an Adams platinum catalyst where the only reaction product detected was the ring-opening product butanol.10 There is also similarity with a previous study21 of furan hydrogenation over Pt nanoparticles of average size in the range of 1 7 nm, where the cracking product propylene and the ring-opening product butanol were
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Figure 5. SFGVS spectra collected during MF hydrogenation over UVozone-cleaned 7 nm Pt cubic nanoparticles deposited on a silicon substrate by the LB method in the temperature range of 80 120 °C (10 torr MF, 100 torr H2, and balance Ar). The spectra show molecular vibrations corresponding to the methylene symmetric stretch [CH2(s)] at 2845 cm 1, the methyl symmetric stretch [CH3(s)] at 2865 cm 1, the methylene asymmetric stretch [CH2(a)] at 2928 cm 1, and the methyl asymmetric [CH3(a)] stretch at 2980 cm 1. Schematics show dominant molecular adsorption species at different temperatures (oxygen (red b), carbon (gray b), hydrogen (white O)).
the dominant products, with the relative percentages of THF and DHF remaining at very low levels throughout the temperature range. For furan hydrogenation on Pt(100) and Pt(111), the dominant product up to 90 °C is THF, which is the saturated ring product; however, a complete reversal in selectivity occurs at higher temperatures, and butanol becomes the dominant product (∼90%).21 These results suggest that the mechanism of MF hydrogenation does not involve the formation of the saturated ring before the production of the cracking species and that hydrogenation, ring opening, and ring cracking occur simultaneously. At 120 °C, the relative concentration of the cracking products is 10%, which is low compared to furan hydrogenation, where the dominant species is the cracking product propylene.21 Figure 5 shows SFGVS spectra of MF hydrogenation over UVozone-cleaned 7 nm Pt nanoparticles for 10 torr MF and 100 torr H2 in the temperature range of 20 120 °C. It is noted that the cleaned Pt nanoparticles possess a high concentration of platinum oxide, which reduces to metallic platinum when the temperature is raised to 80 °C. Thus, the spectrum corresponding to 20 °C is red-shifted relative to all the other spectra because the SFGVS signal is mainly from the oxide interface. Four molecular vibration peaks are observed at 80 °C in the region of 2700 3300 cm 1. These peaks are assigned to methylene symmetric stretch [CH2(s)] at 2855 cm 1 (appearing as a shoulder), methyl symmetric stretch [CH3(s)] at 2866 cm 1, methylene asymmetric stretch [CH2(a)] at 2928 cm 1, and methyl asymmetric stretch [CH3(a)] at 2980 cm 1. The absence of the C H aromatic stretch of MF at 3145 cm 1 suggests that the molecule lies flat on the nanoparticle surface. This has been previously observed with a variety of aromatic molecules (e.g., benzene, pyridine, naphthalene, and toluene) adsorbed to a metal surface.22,23 As the temperature is raised to 100 and 120 °C, the amplitude of the CH2(s) stretch increases and exceeds that of the CH3(s) stretch, suggesting a higher surface concentration of CH2 groups that may be originating from either 8106
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Figure 6. Temperature dependence of DMF hydrogenation over UVozone-cleaned 7 nm Pt cubic nanoparticles deposited on a silicon substrate by the LB method (10 torr DMF and 100 torr H2). Reaction products are DMDHF (9), DMTHF (b), 2-hexanol (2), and cracking products (1). The reaction is selective toward ring opening, leading to the production of 2-hexanol.
Figure 7. SFGVS spectra collected during DMF hydrogenation over UV-ozone-cleaned 7 nm Pt cubic nanoparticles deposited on a silicon substrate by the LB method in the temperature range of 80 120 °C (10 torr DMF, 100 torr H2, and balance Ar). The spectra show molecular vibrations corresponding to the methylene symmetric stretch [CH2(s)] at 2851 cm 1, the methyl symmetric stretch [CH3(s)] at 2869 cm 1, the methylene asymmetric perturbed stretch [CH2(a,p)] at 2900 cm 1, the methylene asymmetric stretch [CH2(a)] at 2925 cm 1, and the methyl asymmetric stretch [CH3(a)] at 2977 cm 1. Schematics show dominant molecular adsorption species at different temperatures (oxygen (red b), carbon (gray b), hydrogen (white O)).
Figure 8. (a) SFGVS spectra of MTHF adsorbed on UV-ozone-cleaned 7 nm Pt cubic nanoparticles deposited on a silicon substrate by the LB method collected at 20 and 120 °C (10 torr MTHF, 100 torr H2, and balance Ar). The four dominant molecular vibrations are assigned to the methylene symmetric stretch [CH2(s)] at 2845 cm 1, the methyl symmetric stretch [CH3(s)] at 2862 cm 1, the methylene asymmetric perturbed stretch [CH2(a,p)] at 2900 cm 1, and the methylene asymmetric stretch [CH2(a)] at 2920 cm 1. (b) SFGVS spectra of DMTHF adsorbed on UV-ozone-cleaned 7 nm Pt cubic nanoparticles deposited on a silicon substrate by the LB method collected at 20, 80, and 120 °C (10 torr DMTHF, 100 torr H2, and balance Ar). The spectra contain the same four dominant molecular vibrations shown in (a) and an additional peak corresponding to the methyl asymmetric stretch [CH3(a)] at 2980 cm 1. Schematics show molecular adsorption configurations of MTHF and DMTHF in the 20 120 °C temperature range (oxygen (red b), carbon (gray b), hydrogen (white O)).
ring hydrogenation or the ring-opening products 1-pentanol and 2-pentanol. In addition, the dominance of the CH2(s) stretch over the CH2(a) stretch at 100 and 120 °C indicates an oxygenbound standing up MTHF species.24 The relative increase in amplitude of the CH2(a) stretch and the fact that it does not correspond to the perturbed stretch [CH2(a,p)] (centered at 2900 cm 1 and attributed to interaction with the Pt surface) suggests that the CH2(a) stretch is mainly due to alcohols adsorbed in a standing up position introducing gauche defects,25 which is common with aliphatic chains longer than four carbon atoms. 3.2. Dimethylfuran Hydrogenation. DMF hydrogenation was carried out under conditions identical to those of MF
hydrogenation. Figure 6 shows reaction product distributions versus temperature obtained from GC measurements. 2,5Dimethyltetrahydrofuran (DMTHF), 2,5-dimethyldihydrofuran (DMDHF), 2-hexanol, and a very small amount of cracking products were detected. Similar to MF hydrogenation, it was not possible to determine the nature of cracking products produced from DMF hydrogenation using either GC or mass spectrometry measurements due to the very low overall conversion. Also similar to MF hydrogenation, at 40 °C the reaction shows a high (70%) selectivity toward ring opening that leads to the production of 2-hexanol. However, there is also 20% selectivity toward the formation of DMDHF, the partially hydrogenated intermediate (Figure 1). This was not observed with MF, where only 8107
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2851 cm 1, CH3(s) stretch at 2869 cm 1, CH2(a,p) stretch at 2900 cm 1 (attributed to electronic interaction with the Pt surface), and CH3(a) stretch at 2977 cm 1.24 Similar to MF hydrogenation, the C H aromatic stretch of DMF centered at 3145 cm 1 is not visible, again suggesting that the aromatic ring is adsorbed parallel to the nanoparticle surface.21 Increasing the temperature to 100 and 120 °C decreased the amplitude of the CH3(s) stretch relative to that of the CH2(s) stretch, implying a higher concentration of CH2 groups, which may be originating from DMDHF, DMTHF, or 2-hexanol. In addition, the increase in temperature above 80 °C intensified significantly the CH3(a) stretch, initially assigned to a higher surface concentration of 2-hexanol. Control experiments were performed to confirm the origin of the intense CH3(a) peak. SFGVS spectra of pure MTHF and pure DMTHF were collected from the Pt nanoparticle surface in the presence of hydrogen. The SFGVS spectra of hydrogenated MTHF (Figure 8a) show four dominant vibrations centered at 2845 cm 1 [CH2(s)], 2862 cm 1 [CH3(s)], 2900 cm 1 [CH2(a, p)], and 2920 cm 1 [CH2(a)]. At 120 °C, the SFGVS spectrum shows a clear dominance of the CH2(s) peak, suggesting an oxygen-bound standing up ring, while a CH3(a) peak is not observed. The SFGVS spectra of the fully hydrogenated DMTHF (Figure 8b) contain the same dominant peaks with the spectra of MTHF and an additional stretch at 2980 cm 1 [CH3(a)] that is only visible at 80 °C, i.e., when the surface is metallic. At 80 and 120 °C, the CH2(s) peak dominates the CH3(s) peak, although not as noticeably as in the case of MTHF, suggesting that DMTHF is more tilted toward the surface, exposing the methyl vibrations. These control experiments indicate that the CH3(a) stretch appearing during DMF hydrogenation (Figure 7) is due to the presence of DMTHF at the surface, not 2-hexanol as previously believed. In addition, when only pure 2-hexanol adsorbed to the nanoparticle surface in the presence of hydrogen, the SFGVS spectrum (not shown here) did not contain a CH3(a) stretch. Vibrational frequencies for the different modes are summarized in Table 1. Since the intensity of the SFGVS signal depends on both molecular orientation and surface concentration, it may be inferred that either DMTHF has a significant presence at the surface of the catalyst at 120 °C or its molecular orientation is responsible for the enhancement of the intensity of the CH3(a) stretch. The presence of DMTHF at the surface should also correspond to an oxygen-bound standing up species, because of the dominance of the CH2(s) stretch over the CH2(a) stretch throughout the temperature range. Reaction intermediates and products of MD and DMF hydrogenation detected in this study are listed in Table 2 together with those of furan hydrogenation22 for comparison.
the fully hydrogenated species (MTHF) was detected throughout the temperature range (Figure 4). Moreover, there is less selectivity toward cracking in DMF hydrogenation than in MF hydrogenation. At 120 °C, for example, the relative concentrations of the cracking products in MF and DMF hydrogenation are 10 and 5%, respectively. A plausible explanation for the observed less cracking during DMF hydrogenation is the presence of an extra methyl group, which forces the aromatic ring further away from the catalytic surface, thereby reducing the probability of a reaction. Previous studies26,27 have shown a decrease in reaction turnover during hydrogenation of methyl-substituted aromatic molecules with increasing number of methyl groups due to a steric effect. Further supporting evidence is provided by another study of furan hydrogenation over Pt nanoparticles of sizes between 1 and 7 nm, where the reaction selectivity toward ring cracking commenced at ∼100 °C, with the cracking-product propylene being the dominant species at higher temperatures, an intrinsic characteristic of Pt nanoparticle systems since this phenomenon is not encountered with Pt single crystals. The significant ring cracking may be attributed to the absence of protective methyl groups in the furan ring. The presence of an extra methyl group in DMF may also explain the partially hydrogenated species (DMDHF) detected during DMF hydrogenation (Figure 6), as opposed to only MTHF detected during MF hydrogenation (Figure 4). Figure 7 shows SFGVS spectra of DMF hydrogenation over UV-ozone-cleaned 7 nm Pt nanoparticles for 10 torr DMF, 100 torr H2, and 650 torr Ar in the temperature range of 20 120 °C. At 20 °C, the spectrum is again slightly red-shifted from all the other spectra due to the high concentration of platinum oxide in the cleaned nanoparticles. In addition, the CH2(a) stretch at 2925 cm 1 vanishes when the temperature is raised to 80 °C. This stretch may be associated with DMDHF, a partially hydrogenated species detected in the gas phase by GC at low temperatures. Four dominant stretches can be seen in the spectrum obtained at 80 °C, namely, CH2(s) stretch at Table 1. Vibrational Peak Assignments for MF, DMF, MTHF, and DMTHF at the Surface of Platinum Nanoparticles under Reaction Conditions wavenumber (cm 1) CH3(s)
CH3(a)
2866
2980
2855
DMF
2869
2979
2845 2854
2925
2901
MTHF
2862
npa
2845
2920
2895
DMTHF
2865
2985
2845
2926
2900
species MF
a
CH2(s)
CH2(a)
CH2(a,p)
2928
npa
np = not present.
Table 2. Reaction Intermediates and Products Detected during Hydrogenation of Furan, 2-Methylfuran (MF), and 2,5-Dimethylfuran (DMF) furan21
2-methylfuran (MF)
2,5-dimethylfuran (DMF)
flat laying aromatic ring intermediates products
standing up hydrogenated ring propylene
cracking products
cracking products
butanol
1-pentanol
2-hexanol
tetrahydrofuran (THF)
2-pentanol
2,5-dimethyltetrahydrofuran (DMTHF)
dihydrofuran (DHF)
2-methyltetrahydrofuran (MTHF)
2,5-dimethyldihydrofuran (DMDHF)
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4. CONCLUSIONS MF and DMF hydrogenation was carried out over 7 nm Pt cubic nanoparticles. SFGVS and GC studies were performed to detect intermediates and products, respectively, under reaction conditions. SFGVS results indicate that the aromatic rings adsorb parallel to the nanoparticle surface. Various surface reaction intermediates were detected for each hydrogenation case. MF hydrogenation produces both ring-opening products (1-pentanol and 2-pentanol) and a hydrogenated ring (MTHF). The only dominant surface species during DMF hydrogenation appears to be a hydrogenated ring (DMTHF). This finding is supported by the results of a control experiment carried out with pure DMTHF. GC measurements indicate that both MF and DMF hydrogenation show high selectivity toward ring-opening reaction, as evidenced by the high relative concentration of aliphatic alcohols over the entire temperature range. Thus, in contrast to previous studies of furan hydrogenation, minimal cracking was observed for both MF and DMF hydrogenation. This difference is attributed to electronic and steric hindrance effects introduced by extra methyl group(s) in MF and DMF. A trend for the percentage of cracking products to decrease was observed with increasing number of methyl groups. ’ AUTHOR INFORMATION Corresponding Author
*Tel. 510-642-4053; fax 510-643-9668; e-mail somorjai@ socrates.berkeley.edu.
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(14) Zhang, Y.; Grass, M. E.; Kuhn, J. N.; Tao, F.; Habas, S. E.; Huang, W.; Yang, P.; Somorjai, G. A. J. Am. Chem. Soc. 2008, 130, 5868–5869. (15) Bratlie, K. M.; Flores, L. D.; Somorjai, G. A. Surf. Sci. 2005, 599, 93–106. (16) Kung, K. Y.; Chen, P.; Wei, F.; Rupprechter, G.; Shen, Y. R.; Somorjai, G. A. Rev. Sci. Instrum. 2001, 72, 1806–1809. (17) Yang, M.; Tang, D. C.; Somorjai, G. A. Rev. Sci. Instrum. 2003, 74, 4554–4557. (18) Shen, Y. R. The Principles of Nonlinear Optics; Wiley: Hoboken, NJ, 2003. (19) Shen, Y. R. Annu. Rev. Phys. Chem. 1989, 40, 327–350. (20) Shen, Y. R. Nature 1989, 337, 519–525. (21) Kliewer, C. J.; Aliaga, C.; Bieri, M.; Huang, W.; Tsung, C.-K.; Wood, J. B.; Komvopoulos, K.; Somorjai, G. A. J. Am. Chem. Soc. 2010, 132, 13088–13095. (22) Gland, J. L.; Somorjai, G. A. Surf. Sci. 1973, 38, 157–186. (23) Rauscher, H.; Jakob, P.; Menzel, D.; Lloyd, D. R. Surf. Sci. 1991, 256, 27–41. (24) Yang, M.; Somorjai, G. A. J. Am. Chem. Soc. 2004, 126, 7698–7708. (25) Guyot-Sionnest, P.; Hunt, J. H.; Shen, Y. R. Phys. Rev. Lett. 1987, 59, 1597–1600. (26) V€olter, J.; Hermann, M.; Heise, K. J. Catal. 1968, 12, 307–313. (27) Bahaman, M. V.; Vannice, M. A. J. Catal. 1991, 127, 251–266.
’ NOTE ADDED AFTER ASAP PUBLICATION This manuscript was originally published on the Web on March 25, 2011, with an error to the spelling of Chia-Kuang Tsung's name. The corrected version was reposted on April 1, 2011.
’ ACKNOWLEDGMENT This work was supported by the Director, U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Contract No. DE-AC0205CH11231. One of the authors (K.K.) also acknowledges funding for this work provided by the UC Berkeley-KAUST Academic Excellence Alliance (AEA) Program. ’ REFERENCES (1) Roman-Leshkov, Y.; Barrett, C. J.; Liu, Z. Y.; Dumesic, J. A. Nature 2007, 447, 982–985. (2) Zhao, H.; Holladay, J. E.; Brown, H. M.; Zhang, Z. C. Science 2007, 316, 1597–1600. (3) Aliaga, C.; Park, J. Y.; Yamada, Y.; Lee, H. S.; Tsung, C.-K.; Yang, P.; Somorjai, G. A. J. Phys. Chem. C 2009, 113, 6150–6155. (4) Gui, J. Y.; Stern, D. A.; Lu, F.; Hubbard, A. T. J. Electroanal. Chem. 1991, 305, 37–55. (5) Hlavathy, Z.; Tetenyi, P. Surf. Sci. 2007, 601, 2026–2031. (6) Knight, M. J.; Allegretti, F.; Kr€oger, E. A.; Polcik, M.; Lamont, C. L. A.; Woodruff, D. P. Surf. Sci. 2008, 602, 2524–2531. (7) Loui, A.; Chiang, S. Appl. Surf. Sci. 2004, 237, 555–564. (8) Sexton, B. A. Surf. Sci. 1985, 163, 99–113. (9) Solomon, J. L.; Madix, R. J.; St€ohr, J. J. Chem. Phys. 1991, 94, 4012–4023. (10) Smith, H. A.; Fuzek, J. F. J. Am. Chem. Soc. 1949, 71, 415–419. (11) Huang, W.; Kuhn, J. N.; Tsung, C.-K.; Zhang, Y.; Habas, S. E.; Yang, P.; Somorjai, G. A. Nano Lett. 2008, 8, 2027–2034. (12) Kuhn, J. N.; Huang, W.; Tsung, C.-K.; Zhang, Y.; Somorjai, G. A. J. Am. Chem. Soc. 2008, 130, 14026–14027. (13) Lee, H.; Habas, S. E.; Kweskin, S.; Butcher, D.; Somorjai, G. A.; Yang, P. Angew. Chem., Int. Ed. 2006, 45, 7824–7828. 8109
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