Article pubs.acs.org/JPCC
Reactions of CH2CHBr and CH3CHBr2 on Cu(100) and O/Cu(100) Jong-Liang Lin,*,† Hong-Ping Lin,† Che-Ming Yang,† Tai-You Chen,† Szu-Han Lee,† and Chao-Ming Chiang*,‡ †
Department of Chemistry, National Cheng Kung University, 1 Ta Hsueh Road, Tainan, Taiwan, Republic of China Department of Chemistry, National Sun Yat-Sen University, 70 Lienhai Road, Kaohsiung, Taiwan, Republic of China
‡
S Supporting Information *
ABSTRACT: Temperature-programmed reaction/desorption (TPR/D) and reflection−absorption infrared spectroscopy (RAIRS) have been employed to study the reactions of CH2 CHBr and CH3CHBr2 on Cu(100) and O/Cu(100). In the TPR/D study, CH2CHCHCH2 is the sole product detected from the reaction of CH2CHBr adsorbed on Cu(100) and featured by complex, coverage-dependent thermal desorption profiles (∼220−380 K). The preadsorbed oxygen can modify the evolution behavior of 1,3-butadiene from the CH2CHBr reaction but has no influence on the main 1,3-butadiene formation at 265 K. Moreover, the surface oxygen participates in the CH2CHBr reaction, forming an intermediate of >CCO, as well as additional products of H2O, C2H2, CO, and CO2, presumably via H-abstraction. New reaction pathways, which are otherwise not observed in the TPR/D study, are opened when CH2CHBr impinges on Cu(100) at high temperatures. At 500 K, H2, C2H2, and C2H4 are generated from the incident CH2CHBr molecules upon Cu(100). The reaction of adsorbed CH3CHBr2 on Cu(100) only forms CH3CHCHCH3 in TPR/D experiments. This product can be generated at the surface temperature as low as 120 K. Preadsorbed oxygen on Cu(100) can increase the 2-butene formation to 190 K, the peak temperature. An additional product of CH3CHO is also formed, but its amount is small. Apparently, preadsorbed oxygen on Cu(100) has different effects on the reaction pathways for the adsorbed CH2CHBr and CH3CHBr2.
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INTRODUCTION Halogenated compounds play important roles in atmospheric chemistry, and are often used as precursors to form hydrocarbon fragments on metal surfaces in model heterogeneous catalyst systems. For example, vinyl iodide (CH2CHI) can decompose on Pt(111) at a temperature ≤160 K to form an adsorbed vinyl (CH2CH) group, which is considered to be a viable intermediate in catalytic hydrogenation of acetylene and dehydrogenation of ethylene.1 The vinyl group further reacts on the surface below 450 K, forming ethylidyne (CH3C) and ethylene.1 Preadsorption of hydrogen is found to enhance the formation of ethylidyne in the reaction of CH2CHI on Pt(111).2 On Ag(111), decomposition of CH2CHI generates 1,3-butadiene (CH2CHCHCH2) at ∼250 K, due to self-coupling of two vinyl groups.3 In the case of CH2CHBr on Cu(100), the reaction starts with CBr bond scission at 160 K, followed by 1,3-butadiene evolution below 350 K.4 The vinyl species from the CH2CHBr dissociation on Cu(100) adopts a tilted orientation, with the CC bond pointing away from the surface normal by an angle of 50−56°.4 Ethylidene (CH3CH) on Pt(111) has been prepared from the facile CI bond cleavage of 1,1-diiodoethane (CH3CHI2).5 This hydrocarbon fragment is the intermediate in the dehydrogenation process from ethylene to ethylidyne.2,5 In addition to the surface species of CH2CH © XXXX American Chemical Society
, CH3C, and CH3CH on Pt(111), the presence of other species, such as HC, H2C, and HCC, has been reported, together with their vibrational frequencies.6 At the 80 K dosing temperature of CH3CHI2, ethylidene groups form on Ag(111) and dimerize into adsorbed trans-2-butene (CH3 CHCHCH3).7 Desorption of the surface trans-2-butene exhibits a sharp peak at ∼140 K and a broad state at ∼184 K.7 Self-coupling reaction of hydrocarbon intermediates occurs readily on Cu, Ag, or Au, for example, CH2CH and CH3CH, but it is not the case on Pt.6 In this research, we first focus on the effects of surface temperature and oxygen on the reaction of CH2CHBr on Cu(100). It has been known that dimerization of vinyl species on Cu(100) to form 1,3-butadiene is the only reaction pathway taking place in the TPR/D experiments of CH2CHBr with a linearly ramped surface temperature from ∼100 K.4 We are interested in exploring the reaction channels of vinyl groups on Cu(100) possibly occurring at high temperatures. The experiments are performed with the surface temperature held at 300, 400, or 500 K while dosing CH2CHBr and monitoring the possible gaseous products. For a surface species Received: June 2, 2017 Revised: July 24, 2017 Published: July 25, 2017 A
DOI: 10.1021/acs.jpcc.7b05397 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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were corrected with a rubberband method and gaseous water peaks were deducted. O2 (99.998%) was obtained from Matheson. CH2CHBr (98%, Sigma-Aldrich) and CH3CHBr2 (99%, Acros) were subjected to several cycles of freeze− pump−thaw before introduction of its vapor into the vacuum chamber. The oxygen-precovered Cu(100) surface was prepared by exposing a clean Cu(100) surface at 500 K to O2. It was estimated that 30 L of O2 (1 L = 1 langmuir = 10−6 Torr × s) could render an oxygen coverage (θ0) of ∼0.2 monolayer (ML).11 It has been shown that a long-range order begins to occur at 0.34 ML and 0.48 ML leads to a (√2 × √2)R45° oxygen overlayer.11 In the present study, 15 and 30 L of O2 were used to prepare the oxygen-precovered Cu(100). The oxygen coverage from the former exposure was assumed to be ∼0.1 ML.
with two reaction pathways of different energy barriers, the reaction rate ratio of the high-energy route to the low-energy route (Rh/Rl) would increase with increasing surface temperature. For example, with the activation energies of 15.0 and 20.0 kcal/mol, Rh/Rl at 300 K is 0.62, assuming an Arrhenius-form rate constant (k = Ae−Ea/RT) and the same preexponential factor; however, this ratio is raised to 0.75 at 500 K. Therefore, the high-energy pathway is prone to occur at a higher surface temperature. In addition to the kinetics standpoint, dynamic factors upon surface collisions of molecules may also affect the product distribution, such as the energetic state of impinging molecules (translational and internal energies), incident angle, molecular orientation, and collision site.8−10 The other motivations for this research are to directly compare the reactions of CH2CHBr and CH3CHBr2 on the same surface of Cu(100) and to examine the effect of adsorbed oxygen on these reactions. These two brominated compounds with potentially different reaction kinetics in the CBr bond dissociation are worth a detailed investigation. In the present study, it is found that there are indeed reaction channels forming H2, C2H2, and C2H4, other than the coupling of vinyl groups, when Cu(100) at a high temperature is exposed to CH2CHBr. The TPR/D results show that reactions of CH2CHBr and CH3CHBr2 on Cu(100) generate CH2 CHCHCH2 and CH3CHCHCH3, respectively, and the formation rate of CH3CHCHCH3 is higher than that of CH2CHCHCH2. The reactions of the two brominated hydrocarbons are affected in different ways in the presence of oxygen atoms on Cu(100).
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RESULTS AND DISCUSSION CH2CHBr Reaction on Cu(100): The Oxygen Effect. The adsorption and reaction of CH2CHBr on Cu(100) using the TPR/D technique have been reported previously.4 In the present study, we also only observe the evolution of 1,3butadiene from vinyl bromide, similar to the previous TPR/D outcome.4 Figure 1 shows the formation of CH2CHCH CH2 from CH2CHBr/Cu(100), with a complex, coveragedependent desorption behavior. At 0.1 L of CH2CHBr, the CH2CHCHCH2 product evolves at ∼315 K and quickly becomes saturated at higher exposures. This hightemperature desorption state is ascribed to 1,3-butadiene at surface defect sites.4 From 0.4 to 1.5 L, there are two
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EXPERIMENTAL SECTION The ultrahigh vacuum (UHV) apparatus to conduct the experiments of temperature-programmed reaction/desorption (TPR/D) and reflection−absorption infrared spectroscopy (RAIRS) was equipped with an ion gun for sputtering, a differentially pumped quadrupole mass spectrometer (QMS) for TPR/D, and a cylindrical mirror analyzer for Auger electron spectroscopy (AES). A base pressure, approximately 2 × 10−10 Torr, of the chamber was maintained by a turbomolecular pump and an ion pump. The QMS used for multiplex TPR/D studies was capable of detecting ions within 300 amu and acquiring up to 20 different masses simultaneously in a single desorption experiment. The Cu(100) single crystal disk, 1 cm in diameter, was mounted on a resistive heating element and could be cooled with liquid nitrogen and heated to 1100 K. In typical TPR/D experiments, the Cu(100) surface was positioned ∼1 mm from a sampling aperture 3 mm in diameter in front of the mass spectrometer. A heating rate of 2 K/s was selected. A chromel−alumel thermocouple junction was inserted into a hole on the edge of the crystal to measure the surface temperature. Cycles of Ar+ ion sputtering and annealing were carried out for copper surface cleaning prior to each experiment until no impurities were detected by AES. In the RAIRS study, the IR beam from the FTIR spectrometer was focused, at a grazing incidence angle of 85° through a KBr window, onto the Cu(100). The reflected beam, through a second KBr window, was refocused on a mercury−cadmium− telluride detector. The entire optical beam path was purged with an air scrubber to remove carbon dioxide and water. All of the IR spectra were measured at 115 K, with 1000 scans, 4 cm−1 resolution. The presented spectra have been ratioed against the spectra of a clean Cu(100) surface recorded immediately before gas dosing. In some infrared spectra, the nonlinear baselines
Figure 1. Temperature-programmed reaction/desorption spectra of CH2CHCHCH2 from CH2CHBr reaction on Cu(100). The inset shows the change of 1,3-butadiene evolution due to the preadsorption of oxygen atoms. The O/Cu(100) surface was prepared by exposing the clean Cu(100) held at 500 K to 15 L of O2. B
DOI: 10.1021/acs.jpcc.7b05397 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C discernible, additional desorption states at ∼265 and 289 K. However, these two states seem to merge together and become a broad desorption peak (∼265 K) at higher exposures. The previous CH2CHBr/Cu(100) study suggested that the coverage-dependent 1,3-butadiene evolution is due to both desorption-limited and vinyl recombination-limited mechanisms.4 It has been shown that Br atoms on Cu(100) can be removed in CuBr form above 800 K.4 In the reaction of CH2 CHI on Ag(111), the 1,3-butadiene evolution is determined by the CH2CH coupling which occurs at ∼250 K.3 The presence of preadsorbed oxygen on Cu(100) can change the desorption behavior of 1,3-butadiene resulting from CH2 CHBr. As shown in the inset of Figure 1, the two hightemperature 1,3-butadiene states (289 and 315 K) are largely suppressed, likely due to the site-blocking effect of the preadsorbed O that prevents the butadiene desorption from the blocked defect sites. However, the 1,3-butadiene feature at 265 K is not affected or is only slightly affected by the presence of oxygen. This result reveals that the effect of oxygen on the main recombination of CH2CH groups to form 1,3-butadiene is not significant. The preadsorbed oxygen not only modifies the 1,3-butadiene evolution but also reacts with the vinyl group derived from CH2CHBr dissociation on Cu(100). Figure 2 compares the TPR/D spectra of CH2CHBr, CH2CHCHCH2, C2H2, H2O, CO, and CO2 from CH2CHBr (1.0 L) on Cu(100) and oxygen-precovered Cu(100) following 15 and 30 L O2 exposures. At 1.0 L of CH2CHBr, no parent molecule desorption from the bare Cu(100) surface is observed (Figure 2a), indicating all of the adsorbed CH2CHBr molecules dissociate on the surface. The preadsorbed oxygen atoms exert a steric and/or electronic effect to suppress the CH2CHBr dissociation and therefore cause the direct desorption of CH2CHBr at ∼195 K. The larger the O2 exposure, the more surface CH2CHBr molecules desorb (Figure 2a). In contrast, the amount of 1,3-butadiene from the CH2CHBr reaction diminishes with the increase of oxygen exposure (Figure 2b). The oxygen preadsorption results in additional products of C2H2, H2O, CO, and CO2 in the CH2CHBr surface reaction. These products originate from reaction between the surface O and vinyl. In the m/z 26 spectra of Figure 2c, the detected ion intensity for the bare Cu(100) (0 L of O2) between ∼250 and 350 K is due to the fragment of the 1,3-butadiene formed. The small state at ∼160 K could be due to residual ethylene. At 15.0 L of O2 (Figure 2c), the peaks at 195 and 265 K are due to CH2CHBr and CH2CHCHCH2 desorption, respectively. Acetylene (C2H2) evolution appears at 330 K and becomes broader at 30.0 L of O2. In parallel to the acetylene evolution, H2O also evolves at 330 K in the case of 15.0 L of O2 (Figure 2d). The onset temperature of water desorption (∼250 K) is similar to that of 1,3-butadiene. The adsorbed O can abstract H from the CH2CH, forming C2H2 and OH and thus leading to the evolution of acetylene and water at the same temperature (330 K). Note that recombination of OH groups on Cu(100) occurs at ∼220 K.12 H2O evolution is split into two states at 295 and 356 K as the O2 exposure is increased to 30 L (Figure 2d). Multiple hydrogen abstraction from the surface bound CH2CH by the adsorbed oxygen atoms can occur and leads to the evolution of CO and CO2 at temperatures higher than 500 K (Figure 2e and f). The m/z 28 intensities of Figure 2e are derived from both CO and CO2. CO2 alone cannot account for the m/z 28 peaks.
Figure 2. Temperature-programmed reaction/desorption spectra of CH2CHBr, CH2CHCHCH2, C2H2, H2O, CO, and CO2 from Cu(100) and oxygen-precovered Cu(100) at 1.0 L of CH2 CHBr.
Characterization of the Surface Species of CH2CHBr on Cu(100) and O/Cu(100). Reflection−absorption infrared spectroscopy has been employed to investigate the adsorption geometry of vinyl bromide and its reaction intermediates on Cu(100) and O/Cu(100), as shown in Figures 3 and 4. The infrared peaks detected in the systems of 1.5 L of CH2 CHBr/Cu(100) and 0.7 and 8.5 L of CH2CHBr/O/ Cu(100) at 120 K are listed in Table 1 and compared to the infrared absorptions of CH2CHCl in the vapor phase reported previously.13 Only a small, broad peak at 918 cm−1 is observed in the 120 K spectrum of 1.5 L of CH2CHBr/ Cu(100). No molecular CH2 CHBr desorption from Cu(100) occurs at 1.5 L exposure. The molecular desorption of CH2CHBr from Cu(100) as a function of exposure is shown in Supporting Information Figure S1. As compared to the reported CH2CHCl infrared study, the 918 cm−1 band can be assigned to the ω(CH2) and/or ω(CH) (out-of-plane C
DOI: 10.1021/acs.jpcc.7b05397 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Table 1. Comparison of the Infrared Frequencies (cm−1) of Vinyl Chloride and Bromide CH2CHBr/Cu(100) (120 K) CH2CHCla (vapor)
modeb
902
ω(CH2)
947 1039 1281 1377 1614 a
ω(CH) ρ(CH2) δ(CCH) δ(CH2) ν(CC)
CH2CHBr/O/Cu(100) (120 K)
1.5 L
0.7 L
8.5 L 901
918
901 922 937
937 1005 1252 1367 1595
Reference 13. bω, wagging; ρ, rocking; δ, bending; ν, stretching.
CH2CHBr on Cu(100) has been reported to occur in the temperature range 140−180 °C, evidenced by the work function change.4 Therefore, the temperature-dependent infrared change shown in Figure 3a is related to the CBr bond breakage in CH2CHBr. However, infrared absorptions of the vinyl group following the CBr bond dissociation of CH2 CHBr on Cu(100) are not detected. This could be due to weak infrared absorption coefficients of the vinyl and/or due to its adsorption geometry on Cu(100). Vinyl groups adsorbed on Ni(100) and Pt(111) have been measured with high-resolution electron energy-loss spectroscopy.1,15 In Figure 3b, there are three resolved peaks at 901, 922, and 937 cm−1 in the 120 K spectrum of 0.7 L of CH2CHBr on O/Cu(100), which is prepared by exposing the Cu(100) at 500 K to 15 L of O2, reflecting that the presence of the adsorbed oxygen slightly changes the adsorption geometry of the CH2CHBr and/or interacts with the CH2CHBr. From 120 to 210 K, the surface CH2CHBr gradually diminishes and eventually disappears as a consequence of its decomposition. However, in contrast to the case of clean Cu(100) (the 180 K spectrum, Figure 3a), some of the CH2CHBr molecules still exist on the surface at 180 K, as shown by the small, broad band of ∼920 cm−1, suggesting that the adsorbed oxygen suppresses the CH2 CHBr dissociation. Figure 4 shows the temperature-dependent RAIR spectra of 8.5 L of CH2CHBr on O/Cu(100), which forms multilayers on O/Cu(100) (15 L O2). At 120 K, the observed peak frequencies are similar to those of CH2CHCl (Table 1) in the vibrational modes of ω(CH2), ω(CH), ρ(CH2), δ(CCH), δ(CH2), and ν(CC), indicating that the adsorbed CH2 CHBr molecules may adopt random orientations on the surface. Upon progressively heating to 160 K, the infrared change with the decrease of the 1595 cm−1 peak (ν(CC)) is a result of molecular desorption from the multilayers, leaving the first layer of CH2CHBr on O/Cu(100) with the main absorption at 937 cm−1, similar to the 120 K spectrum of 0.7 L of CH2CHBr/O/Cu(100) (Figure 3b). A partial dissociation of the adsorbed CH2CHBr molecules cannot be completely ruled out at 160 K. From 160 to 200 K, the adsorbed CH2 CHBr molecules undergo further desorption (Figure 2a) and dissociation, forming CH2CH(ad). No infrared peaks are measured in the range of 200−450 K. Note that CH2CH CHCH2, C2H2, and H2O evolve from reaction of CH2 CHBr on O/Cu(100) between 200 and 400 K (Figure 2). In Figure 4, the infrared peak at 2031 cm−1 in the 500 K spectrum is attributed to >CCO (ketenylidene) intermediate due to
Figure 3. Temperature-dependent reflection−absorption infrared spectra of CH2CHBr on Cu(100) and O/Cu(100).
Figure 4. Temperature-dependent reflection−absorption infrared spectra of CH2CHBr on O/Cu(100), showing formation of >C CO intermediate at 500 K.
CH bending) modes of CH2CHBr. No CC stretching absorption at ∼1600 cm−1 is observed in this case. This infrared result suggests that the CH2CHBr adopts an adsorption geometry with the CC parallel or near parallel to the surface, according to the surface dipole selection rule (only the vibrational modes with nonzero dynamic dipole moment perpendicular to the surface can be detected in RAIRS).14 The previous study of near-edge X-ray absorption fine structure on CH2CHBr/Cu(100) at 0.8 monolayer has suggested a CH2CHBr adsorption geometry with the CC tilted away from the surface by 32°.4 In Figure 3a, the 918 cm−1 band decreases in intensity and disappears upon heating the surface to 150 and 180 K, respectively. The CBr bond scission of D
DOI: 10.1021/acs.jpcc.7b05397 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 5. Ion intensities of m/z 2, 26, 28, 39, and 108 as a function of time from CH2CHBr impinging on Cu(100) at 300, 400, and 500 K. The 0 s marks the starting time when the CH2CHBr vapor is introduced into the ultrahigh vacuum chamber. After this point, the chamber pressure of CH2CHBr is maintained at 1 × 10−7 Torr.
Effect of Surface Temperature on the Reaction of CH2CHBr Impinging on Cu(100). In performing the TPR/D experiments, CH2CHBr is adsorbed on the Cu(100) held at a low temperature of ∼120 K. The reaction of CH2 CHBr on Cu(100) occurs during the surface heating process at a rate of 2 K/s, i.e., 2CH2CHBr(ad) → 2CH2CH(ad) + 2Br(ad) → CH2CHCHCH2(g) + 2Br(ad).4 However, in addition to the typical Langmuir−Hinshelwood mechanism, other reaction channels, such as the Eley−Rideal type, might occur as CH2CHBr molecules striking against the Cu(100) at high temperatures. Here the influence of surface temperature on the reaction of CH2CHBr was performed with the Cu(100), at 300, 400, and 500 K, positioning in front of the QMS sampling aperture (1 mm apart, as shown in Supporting Information Scheme S1), while the CH2CHBr vapor was introduced into the UHV chamber and kept at 1 × 10−7 Torr. The QMS was used to monitor the species desorbing directly from the surface upon the impact of CH2CHBr on Cu(100). Figure 5 shows the surface-temperature-dependent and timedependent ion intensities of m/z 108, 39, 28, 26, and 2 measured in this temperature effect study. It is found that CH2CHCHCH2, CH2CH2, CHCH, and H2 can be immediately formed at these surface temperatures. Except for the 1,3-butadiene, the other three products have not been found in the normal TPR/D experiments for CH2CHBr/ Cu(100). The zero point of the abscissa axis in each panel of Figure 5 represents the starting time when CH2CHBr is introduced into the UHV chamber, with the copper crystal at 300, 400, and 500 K, respectively. The chamber pressure of CH2CHBr is carefully controlled and held at 1 × 10−7 Torr.
the reaction of vinyl and oxygen on Cu(100). The >CCO has been proposed as the intermediate, with a strong infrared absorption of 2020 cm−1 (ν(CCO)), in the acetone oxidation on oxygen-precovered Ag(111).16,17 In the partial oxidation of acetic acid on the Au/TiO2 catalyst, ketenylidene is also formed, as evidenced by its characteristic frequency at 2040 cm−1.18 Moreover, the theoretical calculation based on the framework of density functional theory points out that >C CO on Cu(100) exhibits an antisymmetric CCO stretching frequency at 2029 cm−1.19 In a previous reaction study of styrene (C6H5CHCH2) on O/Ag(111), phenylketene (C6H5CHCO) has been reported to be formed at ∼550 K.20 Other surface species, such as HCCH, HC C, CH2CCHCH2, and CH3CHCC, are not considered to be responsible for the 2031 cm−1 peak in Figure 4. The CC stretching frequencies of HCCH and HCC on Cu(100) have been measured to be 1312 and 1432 cm−1, respectively, instead of ∼2031 cm−1.21 For the latter two species bearing a cumulated double bond (C), they may have a CCC stretching frequency around 2000 cm−1; however, their existence on metal surfaces has never been reported. Besides, no CHx absorptions have been detected with the 2031 cm−1 peak in Figure 4. In the present study for the reaction of vinyl bromide on O/Cu(100), it is reasonable to propose that the >CCO intermediate is formed and contributes to the CO and CO2 evolution at temperatures ≥500 K found in Figure 2. Therefore, its characteristic 2031 cm−1 peak is no longer detectable in the 550 K spectrum of Figure 4. E
DOI: 10.1021/acs.jpcc.7b05397 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C At the same pressure, the impinging rate of CH2CHBr on the Cu(100) would be the same, regardless of the surface temperatures. For the CH2CHBr (Figure 5a), its desorption increases with the increasing surface temperature. This could be due to residence-time decrease of the surface CH2CHBr molecules at higher temperatures, causing less CH2CHBr decomposition and therefore more CH2CHBr molecular desorption.22 The inset in Figure 5a shows the relative yields of CH2CHBr at 15 s in the three different cases. Figure 5b shows the production of 1,3-butadiene from CH2CHBr decomposition on Cu(100). At 500 K, the intensity of m/z 39 is small, in parallel to the high yield of CH2CHBr desorption. As compared to the 500 K case, the intensities of m/z 39 measured at 300 and 400 K are relatively high. In the 300 K trace, the ion intensity does not rise abruptly near the 0 s mark but seems to increase linearly up to ∼15.0 s. This may be related to the limited desorption rate of CH2CHCH CH2 generated from CH2CHBr reaction on Cu(100) at 300 K. Note that there still is 1,3-butadiene desorption at temperatures higher than 300 K in the TPR/D of CH2 CHBr/Cu(100) (Figure 1). That is, a portion of the 1,3butadiene molecules formed on the 300 K surface may not desorb immediately. Therefore, there is no sudden rise of the m/z 39 intensity near 0 s in the 300 K trace of Figure 5b. The relative 1,3-butadiene formation yields at 15 s are shown in the inset of Figure 5b. Although decomposition of CH2CHBr to produce 1,3-butadiene on Cu(100) at 500 K is suppressed, other reaction channels are opened via CH bond scission to form H2, C2H2, and C2H4. Figure 5c shows the H2 formation, with the highest yield at 500 K, as opposed to the formation of 1,3-butadiene. The H2 relative yields obtained from the 15.0 s intensities are shown in the inset. Following the CH bond cleavage of the vinyl group (CH2CH(ad) → C2H2(g) + H(ad)), hydrogenation of the vinyl group can occur (CH2CH(ad) + H(ad) → C2H4(g)), in addition to H2 formation (H(ad) + H(ad) → H2(g)). Formation of the C2H2 and C2H4 from the reactions of CH2CHBr on Cu(100) at 300, 400, and 500 K is shown in Figure 5d and e, with their relative yields at 15.0 s. In the estimation of the relative yields from the measured ion intensities, the contributions from other products have been deducted. For example, the contributions of CH2CHBr, CH2CHCHCH2, and CH2CH2 to the intensity of m/z 26 have been taken into account for the C2H2 yield. As shown in Figure 1, recombination of CH2CH groups on Cu(100) occurs below 300 K. The CH2CH groups have not yet dehydrogenated at the recombination temperature and therefore no products of H2, C2H2, and C2H4 are produced in the TPR/D experiment of CH2CHBr/Cu(100). Adsorption and Reactions of CH3CHBr2 on Cu(100) and on O/Cu(100). Figure 6 shows the TPR/D spectra of CH 3 CHCHCH 3 and intact CH 3 CHBr 2 following CH3CHBr2 adsorption on Cu(100). 2-Butene is the only product observed in the reaction of adsorbed CH3CHBr2 on Cu(100). As shown in Figure 6a, no CH3CHBr2 desorption is observed at an exposure CCO, possibly through Habstraction. However, the CH3CH(ad) can recombine with the O to produce CH3CHO, albeit the amount is small.
Table 2. Comparison of the Infrared Frequencies (cm−1) of CH3CHBr2 CH3CHBr2a (vapor)
mode
966 1045 1070 1172 1260 1383 1443
ν(C−C) ρ(CH3), δ(CH) ρ(CH3), ν(C−C) δ(CH), ρ(CH3) δ(CH), ν(C−C) δs(CH3) δas(CH3)
a
on Cu(100) (1.5 L, 115 K)
on O/Cu(100) (1.5 L, 115 K) 963
1377 1438
1375 1443
Reference 23.
and disappears at 170 K. According to the previous vibrational study of 2-butene on Ag(111), the 973 cm−1 is attributed to trans-2-butene.7 This result suggests that the 2-butene desorption observed in the 1.5 L TPR/D spectrum of CH3CHBr2/Cu(100) (Figure 6b) is desorption-limited, at least for the 175 and 236 K states. In the previous study of the CH3CHI2/Ag(111), the 2-butene product evolving at 140 and 184 K has been shown to be desorption-determined.7 In the 115 K spectrum of Figure 9b, the three peaks appearing at 963, 1375, and 1442 cm−1 can be ascribed to adsorbed CH3CHBr2. The different relative peak intensities of 1.5 and 3.0 L of CH3CHBr2 at 115 K in Figure 9 reflect the change in the CH3CHBr2 adsorption orientation at different coverages. In Figure 9b, the CH3CHBr2 is diminished in the 170 K spectrum and no longer observable at 195 K. From 140 to 195 K, the characteristic 2-butene peak (973 cm−1) is not observed in Figure 9b, indicating that most of the 2-butene molecules generated from CH3CHBr2 reaction on Cu(100) desorb promptly at higher coverages. Figure 10 shows the RAIR spectra of 1.5 L of CH3CHBr2 on O/Cu(100). The peaks (963, 1375, and 1442 cm−1) belonging to CH3CHBr2 are present in
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CONCLUSIONS The reaction pathways of CH2CHBr and CH3CHBr2 on Cu(100) and O/Cu(100) are shown in Scheme 1. On Cu(100), CH2CHCHCH2, due to recombination of the surface vinyl groups, is the sole product detected from the reaction of the adsorbed CH2CHBr. The preadsorbed oxygen can modify the 1,3-butadiene evolution behavior, without significantly changing the main recombination of CH2CH groups at 265 K. Besides, the surface atomic oxygen can react with the vinyl groups, forming >CCO surface intermediates and the products of H2O, C2H2, CO, and CO2. H
DOI: 10.1021/acs.jpcc.7b05397 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
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Scheme 1
Impingement of CH2CHBr onto Cu(100) at high temperatures (500 K, for example) opens up new reaction pathways to form C2H2, C2H4, and H2, involving dehydrogenation and hydrogenation of the CH2CH. The reaction of CH3CHBr2 adsorbed on Cu(100) only forms the coupling product of CH3CHCHCH3. The preadsorption of oxygen decreases the 2-butene formation rate, resulting in the main desorption at a higher temperature (190 K) and can also form an additional product of CH3CHO. On Cu(100), the rate of 1,3-butadiene formation from CH2CHBr is lower than that of 2-butene from CH3CHBr2. The preadsorbed oxygen has different influences on the reaction pathways for the adsorbed CH2 CHBr and CH3CHBr2 on Cu(100).
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b05397. Figure S1 showing the temperature-programmed desorption spectrum of CH2CHBr on Cu(100) and Scheme S1 showing the scaled configuration used in the study of the effect of surface temperature on the reaction of CH2CHBr impinging on Cu(100) (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. Phone: 886 6 2757575 ext. 65326. *E-mail:
[email protected]. Phone: 886 7 5252000 ext. 3939. ORCID
Jong-Liang Lin: 0000-0002-1276-5479 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was financially supported by the Ministry of Science and Technology of the Republic of China (MOST 1052113-M-006-002).
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REFERENCES
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DOI: 10.1021/acs.jpcc.7b05397 J. Phys. Chem. C XXXX, XXX, XXX−XXX
