Comparative Study on the Reaction Pathways of 2-Chloropropanoic

Article pubs.acs.org/JPCC

Comparative Study on the Reaction Pathways of 2‑Chloropropanoic Acid on Cu(100) and O/Cu(100) Zi-Xian Yang, Shang-Wei Chen, Szu-Han Lee, Tai-You Chen, and Jong-Liang Lin* Department of Chemistry, National Cheng Kung University, 1 Ta Hsueh Road, Tainan, Taiwan, Republic of China S Supporting Information *

ABSTRACT: CH3CHCOOH and CH3CHCOO have been theoretically predicted to be important in the decarboxylation or decarbonylation of propanoic acid on Pd(111). In the present study, we explore the possibility to prepare these two intermediates on Cu(100) and oxygen-predosed Cu(100) (O/Cu(100)), with CH3CHClCOOH as the precursor, and to investigate their adsorption geometries and reactions on the surfaces using X-ray photoelectron spectroscopy, reflection− absorption infrared spectroscopy, temperature-programmed reaction/desorption, and calculations of density functional theory. CH3CHClCOO and CH3CHCOO are suggested to be formed on Cu(100) at 250 K by heating the CH3CHClCOOH adsorption layers. However, CH3CHCOO can promptly recombine with the H from the deprotonation of CH3CHClCOOH to generate CH3CH2COO. The adsorbed propanoate decomposes mainly into H2, CO, and CO2 at a temperature higher than ∼380 K. On O/Cu(100), CH3CHClCOO is the predominant species found in the dissociation of CH3CHClCOOH at 250 K. The preadsorbed O disables the CH3CHCOO hydrogenation to form CH3CH2COO due to the lack of adsorbed H. This intermediate is not stable above ∼400 K and decomposes into H2, H2O, CO, and CO2, probably with a small amount of acrolein or methylketene. The CH3CHCOO is shown theoretically to be bonded on Cu(100) via the unsaturated carbon and one of the oxygen atoms.

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cm−1.6 On oxygen-dosed Pd(111), deprotonation of propanoic acid is enhanced to produce propanoate at 170 K.7 Further reaction of this intermediate forms CO2 and H2O near 380 K, as well as H2 and CO at a temperature higher than 400 K. The surface oxygen atoms increase the stability of the propanoate by ∼25 K.7 On Pt(111) and Ni(100), propanoic acid primarily dissociates into propanoate.2,8 This intermediate was reported to form CO on Ni(100). Recently, a model system of propanoic acid/Pd(111) has been calculated for the mechanisms involving decarbonylation, decarboxylation, and reactive deoxygenation using microkinetics modeling and density functional theory.9 This research is related to renewable energy of biomass. CH3CH2CO and CH3CHCOOH are predicted to be the surface intermediates in the pathways losing CO. CH3CH2COO, CH3CHCOO, and

INTRODUCTION Adsorption and reactions of carboxylic acids on well-defined metals have been extensively studied especially for formic acid and acetic acid. Coordination compounds with carboxylate ligands were common, which motivated the exploration of the adsorption structure and spectroscopic characteristics of carboxylates on metal surfaces.1,2 Carboxylates are considered to play a role in the reactions of CO and H2 over group VIII metal catalysts to form methanol and ethanol.3−5 Carboxylic acids have been used as probing molecules to investigate the surface properties of metals and metal oxides.3 Propanoic acid molecules form hydrogen-bonded catemers on Pd(111) at 170 K.6 Surface propanoates emerge upon heating the propanoic acid adlayer to 240 K and then decarboxylate to form CO2 at 355 K. Other reaction products of CO (470 K) and H2 (330 K, 415 K) also evolve in the reaction of propanoic acid/Pd(111). Furthermore, acetylide (CCH) species bound with the C−C bond roughly parallel to the surface is evidenced by an electron energy-loss peak at 700 © XXXX American Chemical Society

Received: September 22, 2016 Revised: December 9, 2016 Published: December 12, 2016 A

DOI: 10.1021/acs.jpcc.6b09624 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

energy analyzer, was estimated to be better than 0.3 eV. A 620 eV photon energy was used for the measurements of C1s, O1s, and Cl2p. The photoelectrons were collected at an angle of 50° from the surface normal. All of the XPS spectra presented here were first normalized to the photon flux by dividing the recorded XPS signal by the photocurrent derived from a gold mesh situated in the beamline. The binding energy scale in all of the spectra was referenced to a well-resolved spin−orbit component of the bulk Cu 3p3/2 peak at 75.10 eV. The size of the X-ray beam used was approximately 2 × 2 mm2. In the study of CH3CHClCOOH decomposition as a function of temperature, the spectra were measured at different positions on Cu(100), which were obtained by moving the crystal. The X-ray photoelectron spectra obtained were fitted with Gaussian−Lorentzian functions based on a nonlinear leastsquares algorithm after Shirley background subtraction. In our theoretical cluster model calculations for the optimized bonding geometries of CH3CHCOO and its infrared frequencies, two slabs with a total of 25 Cu atoms fixed at their lattice positions were used. All of the calculations were performed in the framework of density functional theory using the program package Cerius2-DMol3, in which the generalized gradient approximation with Perdew and Wang exchange-correlation functional (GGAPW91) was employed. A double-numeric quality basis set with polarization functional (DNP) was used for the all-electron calculations including relativistic effect for the core electrons. The geometry optimization convergence threshold for energy change, maximum force, and maximum displacement between optimization cycles were 0.00001 Ha, 0.001 Ha/Å, and 0.0005 Å, respectively. No scaling factor has been used for the computational frequencies reported in this article. The mode assignments for the frequencies calculated were based on the animated molecular vibrations.

CH3CHCOOH are predicted to appear in the pathways losing CO 2 . We attempted to prepare the intermediates of CH3CHCOO and CH3CHCOOH, using CH3CHClCOOH as precursor, on Cu(100) and oxygen-precovered Cu(100) surfaces, and to investigate the surface reactions of these intermediates. Little is known about CH3CHCOOH and CH3CHCOO on metal surfaces. In this paper, we report the formation of surface intermediates CH 3 CHClCOO, CH3CH2COO, and CH3CHCOO in the decomposition of CH3CHClCOOH on Cu(100) and O/Cu(100) and their reactions.

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EXPERIMENTAL SECTION AND COMPUTATIONAL METHODS The temperature-programmed reaction/desorption (TPR/D) and reflection−absorption infrared spectroscopy (RAIRS) experiments were performed in an ultrahigh vacuum (UHV) apparatus equipped with an ion gun for sputtering, a differentially pumped mass spectrometer for TPR/D, and a cylindrical mirror analyzer for Auger electron spectroscopy. The chamber was evacuated by a turbomolecular pump and an ion pump to a base pressure of approximately 2 × 10−10 Torr. The quadrupole mass spectrometer used for TPR/D studies was capable of detecting ions in the 1−300 amu range and of being multiplexed to acquire up to 20 different masses simultaneously in a single desorption experiment. In TPR/D experiments, the Cu(100) surface was positioned ∼1 mm from an aperture 3 mm in diameter leading to the mass spectrometer, and a heating rate of 2 K/s was used. The Cu(100) single crystal (1 cm in diameter) was mounted on a resistive heating element and could be cooled with liquid nitrogen and heated to 1100 K. The surface temperature was measured by a chromel−alumel thermocouple inserted into a hole on the edge of the crystal. Cleaning of the surface by cycles of Ar+ ion sputtering and annealing was done prior to each experiment until no impurities were detected by Auger electron spectroscopy. In the RAIRS study, the IR beam was taken from a Bruker FTIR spectrometer and focused at a grazing incidence angle of 85° through a KBr window onto the Cu(100) in the UHV chamber. The reflected beam was then passed through a second KBr window and refocused on a mercury−cadmium− telluride (MCT) detector. The entire beam path was purged with a Balston air scrubber for carbon dioxide and water removal. All the IR spectra were taken at a temperature about 115 K, with 1000 scans and 4 cm−1 resolution. The presented spectra have been ratioed against the spectra of a clean Cu(100) surface recorded immediately before dosing. O2 (99.998%) was obtained from Isotec. CH3CHClCOOH (98%) and CH3CH2COOH (98%), purchased from SigmaAldrich, 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 30 L (1 L = 1 langmuir = 10−6 Torr × s) of O2. It was estimated that the oxygen coverage (θO) for the oxidized Cu(100) used in this study was ∼0.2 monolayer (ML). The previous study showed that a long-range order started to develop at θO = 0.34 ML, and at a saturation coverage of θO = 0.48 ML, a (√2 × 2√2)−R45° structure was formed.10 The measurements of X-ray photoelectron spectroscopy (XPS) were carried out at the wide range spherical grating monochromator beamline (WR−SGM) at the National Synchrotron Radiation Research Center, Republic of China. Total instrumental resolution, including the beamline and

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RESULTS AND DISCUSSION Thermal Stability of CH3CHClCOOH on Cu(100). We first investigated the thermal stability of 2-chloropropanoic acid on Cu(100). Shown in Figure 1 are the temperatureprogrammed desorption spectra of the acid molecules on Cu(100), represented by its most abundant ion of 27 amu. It is found that 2-chloropropanoic acid begins to desorb (∼200 K) at 0.5 L. The desorption amount increases linearly with exposure, as shown in the inset. Below 0.5 L, the acid molecule decomposes on Cu(100) at the adsorption temperature (∼120 K) or during the heating course after adsorption; therefore, no molecular desorption occurs. 0.5 L is considered to be the exposure that renders the first saturated adsorption layer of 2chloropropanoic acid on Cu(100), and a higher exposure causes the adsorption of multilayer molecules. Spectroscopic Studies for the Surface Reaction Intermediates of CH3CHClCOOH on Cu(100). X-ray photoelectron spectroscopy has been employed to follow the dissociation of the functional −COOH and C−Cl of CH3CHClCOOH on Cu(100). Figure 2 shows the XP spectra measured, following 0.8 L CH3CHClCOOH adsorption on Cu(100) and subsequent, progressive surface annealing to the temperatures indicated. We start the binding-energy analysis from the 200 K spectra. The O1s spectrum can be fitted with three peaks located at 531.3, 532.7, and 533.7 eV. Previously, the respective O1s binding energies have been found at 532.4 eV (broad) and 532.9 eV for the acetic acid on Cu(110) and the iodoacetic acid on Cu(100).11,12 Upon deprotonating the B


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−COOH on the basis of their corresponding C1s and O1s intensities. The C1s peak area ratio of −COOH/−COO is roughly 1.0/2.5, being similar to that of O1s at ∼1.0/2.0. The C1s binding energy for the −COOH of acetic acid on Cu(110) has been reported at 290.1 eV, in contrast to the 288.2 eV for the −COO of the adsorbed acetate.11 In the 200 K spectrum of Figure 2, the C1s peak of 286.2 eV is from the C−Cl group as observed in the chlorinated h y d r o ca r b o n s . 1 6 , 1 7 B e s i d e s , t h e CH 3 m o i et y o f CH3CHClCOOH itself or its reaction intermediates has a contribution to the C1s emission at 284.9 eV. The Cl2p spectrum reveals two chlorine states of C−Cl (200.0 and 201.6 eV for the 2p3/2 and 2p1/2 components) and adsorbed Cl atom (Cl(a): 198.2 and 199.8 eV). The area ratio of C−Cl/Cl(a) is roughly 2.2/1.0. Continuously heating the surface to 250 K causes the decrease of both the −COOH and C−Cl, in parallel to the increase of −COO and Cl(a), through −COOH deprotonation and C−Cl bond cleavage. At 250 K, the −COOH becomes relatively small, and the ratio of C−Cl/ Cl(a) is close to 1.0. In terms of these results, CH3CHClCOO and CH3CHCOO are suggested to be formed at 250 K. From 250 to 400 K, the trend in the decomposition of 2chloropropanoic acid on Cu(100) continues, leading to near depletion of the −COOH and C−Cl groups, as evidenced by the disappearance of the C1s peaks of 286.2 (C−Cl) and 289.7 (−COOH) eV at 400 K. At this temperature, the C1s spectrum only shows two peaks at 284.7 and 287.8 eV, which can be assigned to alkyl and carboxylate (−COO) carbons, respectively. It is found that the carbon features in the 400 K spectrum are very similar to those of propanoate adsorbed on Cu2O(100) in terms of the C1s peak positions at 285.2 and 287.9 eV and their relative peak intensity. 18 Indeed, CH3CH2COO formation is further confirmed in the study of 2-chloropropanoic acid on Cu(100) with vibrational spectroscopy shown later. Furthermore, the presence of the C1s peaks at 284.7 and 287.8 eV in the 250 K photoemission spectrum suggests that CH3CH2COO may already exist at this temperature. In Figure 2, the intensities of 284.7, 287.8, and

Figure 1. Temperature-programmed desorption spectra of CH3CHClCOOH on Cu(100).

acetic acid on Cu(110) to form acetate, the O1s peak is shifted to 531.6 eV.11 Acetate on hydroxylated NiO(111) has an O1s binding energy of 531.2 eV.13 Accordingly, the deconvoluted peaks at 532.7 and 531.3 eV for the 200 K spectrum are ascribed to −COOH and −COO, respectively. The other 533.7 eV peak could be due to residual adsorbed water. O1s of 533.2 eV has been reported for H2O on Cu(100) and Ni(110).14,15 Four deconvoluted peaks are obtained at 284.9, 286.2, 287.8, and 289.7 eV after curve-fitting to the C1s spectrum (Figure 2). The two highest binding energy peaks, 287.8 and 289.7 eV, are due to −COO and −COOH functional groups, respectively.11 Furthermore, the amount of −COO is larger than that of

Figure 2. X-ray photoelectron spectra of 0.8 L CH3CHClCOOH on Cu(100). C


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Note that at 250 K the XPS study has revealed the possible formation of CH 3 CHClCOO, CH 3 CHCOO, and/or CH3CH2COO. The infrared spectral features further change upon heating the surface to 300 and 400 K, but no infrared peaks are detected at 650 K, presumably due to desorption of reaction products. The set of the peaks (1307, 1368, 1413, and 1467 cm−1) observed at 400 K is identified to be adsorbed CH3CH2COO, which is confirmed by CH3CH2COOH adsorption shown later. Figure 4 shows the temperature-

531.3 eV in the 550 K spectrum decrease considerably, reflecting further reaction of the CH3CH2COO intermediate and evolution of products containing C and/or O. Residual carbon (284.5 eV) is left on the surface after heating the Cu(100) to 980 K. In the temperature-dependent infrared study shown later, a similar surface decomposition behavior of CH3CHClCOOH/Cu(100) has been also observed. This resemblance indicates that the X-ray irradiation and the induced secondary electrons during the X-ray photoelectron measurements are not important in the observed thermal decomposition of CH3CHClCOOH on Cu(100). Figure 3 shows the reflection−absorption infrared spectra measured, following 0.8 L CH3CHClCOOH adsorption on

Figure 4. Reflection−absorption infrared spectra of 0.1 and 0.5 L CH3CHClCOOH on Cu(100).

Figure 3. Reflection−absorption infrared spectra of 0.8 L CH3CHClCOOH on Cu(100).

dependent reflection−absorption infrared spectra of 0.1 and 0.5 L CH3CHClCOOH on Cu(100). In the 0.5 L case, the infrared absorption behaviors from 250 to 400 K are similar to those of 0.8 L exposure. However, the 1469 cm−1 peak is clearly observed at 250 K in the 0.1 L spectrum, and the characteristic propanoate absorptions are fully revealed at 300 K. Figure 5 shows the infrared spectra measured after adsorption of 1.0 L propanoic acid on Cu(100), with subsequent, consecutive surface heating to 335, 400, and 660 K. CH3CH2COO on Pd(111) has been generated by heating the adsorbed propanoic acid layer to 240 K.6 Similarly, deprotonation of acetic acid on Cu(100) occurs between 210 and 240 K, and the resulting acetate is stable up to 500 K.11 Infrared spectra of propanoate salts, such as Na, Ni, and Ba propanoates, have been measured and analyzed.19−21 The infrared absorptions of the 400 K CH3CHClCOOH (0.8 L) and 335 K CH3CH2COOH on Cu(100) are listed in Table 1 and compared to those of the nickel propanoate. It is found that these two surface spectra are similar to the infrared absorptions of the nickel propanoate; therefore, they are attributed to CH3CH2COO. Accordingly, the observed 1307 cm−1 is assigned to ω(CH2), 1368 cm−1 to δs(CH3), 1413 cm−1 to νs(COO), and 1467 cm−1 to νas(COO). Since no

Cu(100) and subsequent, progressive surface annealing to the temperatures indicated. In the 200 K spectrum, the absorption peaks of 1635 and 1752 cm−1 manifest the presence of carboxyl groups (−COOH), which agrees with the XPS result (Figure 2), and their multiple interaction states. Carboxyl groups interacting with each other through hydrogen bonding or with metal surfaces may shift the CO stretching vibration (ν(C O)) toward lower frequencies. In the case of the adsorbed layer of propanoic acid on Pd(111), the observed ν(CO) of 1660 cm−1 has been attributed to hydrogen-bonded catemers previously.6 We have calculated the infrared frequencies and vibrational modes for a CH3CHClCOOH molecule, as shown in Table S1, to assist in the analysis of the 200 K spectrum in Figure 3. Therefore, the observed peaks are accordingly assigned as follows: 941 cm−1 to γ(CH3) + δ(C−H), 1076 cm−1 to ν(C−C) + γ(CH3), 1220 cm−1 to δ(C−H), and 1456 cm−1 to δas(CH3). Because the XPS result (Figure 2) has shown the −COO existence at 200 K, the 1411 cm−1 peak is assigned to νs(COO). Propanoate on O/Pt(111), Pd(111), and Ni(100) has been reported to have a νs(COO) of ∼1400 cm−1.2,6,8 In the 250 K spectrum of Figure 3, the CO stretching bands disappear, with two main peaks located at 1415 and 1458 cm−1. D


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infrared absorptions (Figure 3). It can be concluded that the main reaction sequence of CH3CHClCOOH on Cu(100) is CH3CHClCOOH → CH3CHCOO + H → CH3CH2COO. Evolution of the Thermal Products of CH3CHClCOOH/ Cu(100). Shown in Figure 6 are the temperature-programmed

Figure 5. Reflection−absorption infrared spectra of 1.0 L CH3CH2COOH on Cu(100).

antisymmetric COO stretching frequency near 1550−1670 cm−1 is observed for the propanoate on Cu(100) from dissociation of CH3CHClCOOH and CH3CH2COOH, the propanoate is adsorbed with highly symmetric, bridging, or bidentate configuration.2,7,8 The spectra of Figures 3 and 4 in the fingerprint region clearly show, together with the XPS result, the reaction process of CH 3 CHClCOOH → CH 3CH2 COO. Above 2000 cm −1 , relatively small CH x stretching peaks are also observed, as shown in Figure S1(A) and Figure S2(B) for 0.8 L CH3CHClCOOH and 1.0 L CH3CH2COOH on Cu(100) at 200, 250, and 300 K. The CHx stretching frequencies of CH3CH2COO on Cu(100) appear at 2945 and 2976 cm−1, which are consistent with the previously reported 2941 and 2973 cm−1 for barium propanoate.21 Both the XPS and RAIRS results confirm the formation of propanoate from 2-chlropropanoic acid reaction on Cu(100). Moreover, there is no H2 evolution found at ∼300 K, where coupling of surface H atoms occurs, in the temperaturepro grammed reaction/d esor ption experim en ts o f CH3CHClCOOH/Cu(100) shown later.22 This means that th e H atom s g en erated from deprotonation o f CH3CHClCOOH, occurring below 250 K, on Cu(100) can react with its decomposition intermediates. It is proposed that the CH3CH2COO is from reaction of H + CH3CHCOO, which takes place on Cu(100) below 300 K, according to the appearing temperature of the characteristic propanoate’s

Figure 6. Temperature-programmed reaction/desorption spectra of 0.5 L CH3CHClCOOH on Cu(100).

reaction/desorption spectra from 0.5 L CH3CHClCOOH decomposition on Cu(100). First of all, H2 desorption at ∼300 K is not detected.22 Since the XPS study (Figure 2) has shown that most of the CH 3 CHClCOOH adsorbates deprotonate at 250 K, forming −COO groups, the surface H atoms generated from the O−H bond scission (deprotonation) are expected to recombine to evolve H2 near 300 K. However, this is not the case. The H reacts with the surface intermediate CH3CHCOO to form propanoate below 300 K, which further decomposes into H2 (m/z 2), CO (m/z 28), and CO2 (m/z 44) between ∼380 and 650 K, probably with a tiny amount of C2H4 (m/z 26, 27), on Cu(100) with adsorbed Cl atoms. Although CO2 and C2H4 fragmentation has a contribution to the intensity at 28 amu, they cannot account for the strong m/z 28 desorption peak. HCl (m/z 36) is also formed, possibly from recombination of surface H and Cl. The evolution of these products is reaction-limited because the desorption

Table 1. Comparison of the Infrared Frequencies (cm−1) of CH3CH2COO

a

CH3CHClCOOH/Cu(100), 300 K

CH3CH2COOH/Cu(100), 335 K

(CH3CH2COO)2Ni·4H2O(s)b

modea,b

1307 1368 1413 1467

1303 1361 1417 1465

1309 1377 1421 1467 1569

ω(CH2) δs(CH3) νs(COO) δas(CH3) νas(COO)

ν: stretching; ω: wagging. bRef 21. E


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The Journal of Physical Chemistry C temperatures of these species on copper surfaces are below 400 K. Since, in the X-ray photoelectron (Figure 2) and infrared absorption (Figure 3) studies, no other surface intermediates from CH3CHClCOOH decomposition on Cu(100) are observed, except for CH3CH2COO, the reaction products of H2, C2H4, CO, and CO2 are originated from the CH3CH2COO reaction. Their widely spread desorption features, from ∼370 to 650 K, could be due to the interaction between propanoate adsorbates and/or to adsorbate-induced electronic variation of the copper surface. In Figure 6, no desorption features appear in the 56 amu trace, indicating that no reaction product of acrolein (CH2CHCHO) or methylketene (CH3CHC O) is formed in the decomposition of propanoate on Cu(100). Evolution of acrolein from Cu2O(100) and from the (0001)-Zn polar surface of ZnO in the decomposition of propanoic acid has been reported previously.18,23 Reaction of propanoic acid on {001}-faceted TiO2 produces methylketene instead.24 On Pd(111), the propanoate generated from decomposition of propanoic acid dissociates into H2 (330 K, 410 K), CO (470 K), and CO2 (355 K).6 Shown in Figure S2 are the temperature-programmed desorption spectra of 2-chloropropanoic acid on oxygenprecovered Cu(100). No CH3CHClCOOH desorption is observed at an exposure ≤0.5 L, revealing its thermal instability on O/Cu(100). Infrared Spectroscopic Study for the Surface Reaction Intermediates of CH3CHClCOOH on O/Cu(100). From the CH3CHClCOOH/Cu(100) study, we have known that formation of the CH3CH2COO intermediate is due to recombination of CH3CHCOO and H. The adsorbed H is from deprotonation of the acid molecule. In order to isolate CH3CHCOO on the surface, the adsorbed H must be removed or transformed. Formation of adsorbed H from deprotonation of CH3CHClCOOH on Cu(100) can be prevented by preadsorption of O atoms.7,10 In this study, O coverage at ∼0.2 monolayer was used. We believe that higher oxygen coverages would have a similar result. Figure 7 shows the temperature-dependent reflection− absorption infrared spectra of 0.8 L CH3CHClCOOH on O/ Cu(100). The infrared features in the 190 K spectrum are similar to those of CH3CHClCOOH on Cu(100) at 200 K (Figure 3), except for the absence of the 1635 cm−1 peak. The loss of this peak is likely due to the suppression of the hydrogen-bonding interactions between the −COOH groups, by the preadsorbed oxygen atoms. At 250 K (Figure 7), the carboxyl groups responsible for the 1751 cm−1 vanish, and the spectral pattern is close to that of CH3CHClCOOH on Cu(100) at the same temperature. The CH3CHClCOOH on O/Cu(100) can desorb or dissociate by O−H bond cleavage at 250 K. Continuously heating the surface to 380 K causes a different infrared absorption behavior, with the peaks at 1367, 1416, 1468, and 1573 (broad) cm−1. At this temperature, the C−Cl bond of CH3CHClCOOH on Cu(100) cannot remain intact. H 2 O evolution at low temperatures following CH3CHClCOOH adsorption on O/Cu(100) is observed in the TPR/D experiment (Figure S3), with a broad feature at ∼200 K. On O/Cu(100), the acid hydrogen of CH3CHClCOOH is transferred to the adsorbed O, forming OH groups, which finally desorb as H2O at lower temperatures.7,10,25,26 In addition, as shown later in the TPR/D study (Figure 10), the reaction products, except for water desorbing at lower temperatures, have not been formed and desorbed prior to 400 K. These experimental results suggest that the

Figure 7. Reflection−absorption infrared spectra of 0.8 L CH3CHClCOOH on O/Cu(100).

surface intermediate of CH3CHCOO exists after dissociation of the O−H and C−Cl bond of CH3CHClCOOH on O/ Cu(100), and the infrared change from 250 to 380 K reflects the chemical reaction of CH3CHClCOO → CH3CHCOO. The 250 K spectrum with the peaks at 1367, 1412, and 1458 cm−1 is attributed to CH3CHClCOO. The 1573 cm−1 band observed in the 380 K spectrum, which can originate from asymmetrically bonded CH3CHCOO on the surface, disappears upon heating the surface to 440 K, with formation of two peaks at 1410 cm−1 (broad) and 1464 cm−1 (small) probably due to propanoate. No infrared absorptions can be detected after the surface is heated to 650 K. Figure S1(C) shows the additional spectra from the relatively weak CHx stretching absorptions above 2000 cm−1 detected for 0.8 L CH3CHClCOOH/O/Cu(100) at 190, 250, and 380 K. The spectral variation from 190 to 380 K can be correlated to the chemical process as stated. Figure 8 shows the temperaturedependent infrared spectra of 0.1 and 0.5 L CH3CHClCOOH on O/Cu(100). In the temperature range of 250−380 K, the spectral absorptions are generally the same in the cases of 0.1, 0.5, and 0.8 L. However, three resolved peaks at 1573, 1595, and 1614 cm−1 are observed, instead of a broad feature, in the 0.1 L spectrum, showing multiple adsorption states of the CH3CHCOO intermediate on O/Cu(100). We have calculated the optimized CH3CHCOO with asymmetric geometry on Cu(100) and its vibrational absorptions in the framework of DFT theory, as shown in Figure 9 and Table 2. The CH3CHCOO is bonded to the surface through the CH approximately at an atop site and through one O of the carboxylate near a bridge site. This η2CH3CHCOO bonding geometry results in two inequivalent C−O bonds, with lengths at 1.234 (1C−1O) and 1.357 (1C−2O) Å. The 1C−2C is slightly shorter than the 2C−3C, which is typical for a C−C single bond. The frequency for the 1 C−1O stretching vibration has been calculated to be 1572 cm−1. Table 2 shows the infrared peak frequencies measured after 0.8 L CH3CHClCOOH adsorption on O/Cu(100), F


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followed by progressively flashing the surface to 250 and 380 K and the calculated surface η 2-CH3 CHCOO vibrational frequencies (1360−1572 cm−1) for comparison. More detailed theoretical η2-CH3CHCOO/Cu(100) frequencies (600−3048 cm−1) are provided in Supporting Table S2. Experimentally, the broad nature of the 1573 cm−1 peak in Figure 7 could be due to inhomogeneous chemisorption of the CH3CHCOO intermediate or slightly different interactions between CH3CHCOO adsorbates. Thermal Reaction of CH3CHCOO on O/Cu(100). Figure 10 shows the temperature-programmed reaction/desorption

Figure 8. Reflection−absorption infrared spectra of 0.1 and 0.5 L CH3CHClCOOH on O/Cu(100).

Figure 10. Temperature-programmed reaction/desorption spectra of 0.5 L CH3CHClCOOH on O/Cu(100).

spectra of 0.5 L CH3CHClCOOH on O/Cu(100). H2, H2O, CO, and CO2 with tiny C2H4 are generated, being originated from decomposition of the CH3CHCOO. The weak m/z 56 feature at 420 K may be due to a small amount of acrolein or methylketene. The intriguing implication for the minor acrolein or methylketene is that it is a possible formation route for these two species from CH3CHCOO reaction on O/Cu(100). As a contrast, formation of acrolein from Cu 2 O(111) and methylketene from TiO2 in the propanoic acid reactions has been reported previously.18,24

Figure 9. Calculated η2-CH3CHCOO bonding geometry on Cu(100) and the structural parameters.

Table 2. Infrared Absorptions (cm−1) of CH3CHClCOOH on O/Cu(100) after Progressively Heating the Surface to 250 and 380 K and of the Calculated η2-CH3CHCOO/Cu(100)a

a

CH3CHClCOOH (250 K) → CH3CHClCOO

mode

1367 1412 1458

δs(CH3) νs(COO) δas(CH3)

CH3CHClCOOH (380 K) → CH3CHCOO

calculated η2-CH3CHCOO/ Cu(100)

mode

1367 1416a 1468 1573a

1360 1441 1449 1572

δs(CH3), δ(C2−H) δas(CH3) δas(CH3) ν(CO)

a: broad. G


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The Journal of Physical Chemistry C Scheme 1

Scheme 2

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CONCLUSION In the reaction of 2-chloropropanoic acid on Cu(100), as shown in Scheme 1, CH3CHClCOO and CH3CHCOO are suggested to be formed at 250 K. The CH3CHCOO can promptly recombine with the H from deprotonation of CH3CHClCOOH, generating CH3CH2COO, which decomposes above ∼380 K to form H2, CO, CO2, and a tiny amount of C2H4. With preadsorption of oxygen atoms on Cu(100), the main surface reaction pathway (Scheme 2) is CH3CHClCOOH → CH3CHClCOO → CH3CHCOO, and the hydrogenation of CH3CHCOO to form CH3CH2COO is suppressed. The CH3CHCOO is stable up to ∼400 K and decomposes into H2, H2O, CO, and CO2, with a tiny amount of CH2CH− CHO, CH3−CHCO, and/or C2H4.

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electron spectra in the National Synchrotron Radiation Research Center, the Republic of China.

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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.6b09624. Figure S1 shows the infrared spectra of CH3CHClCOOH and CH3CH2COOH on the copper surface in CHX stretching region. Figure S2 shows the TPD spectra of CH3CHClCOOH on O/Cu(100). Figure S3 shows the evolution of H2O, at lower temperatures, from CH3CHClCOOH on O/Cu(100). Table S1 shows the calculated infrared frequencies of a 2chloropropanoic acid molecule. Table S2 shows the calculated η2-CH3CHCOO/Cu(100) infrared frequencies from 600 to 3048 cm−1 (PDF).

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail (Lin, J.-L.): [email protected]. Phone: 886 6 2757575 ext. 65326. 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). We thank Dr. Chia-Hsin Wang and Dr. Yaw-Wen Yang for their assistance in obtaining the photoH


Article

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Comparative Study on the Reaction Pathways of 2-Chloropropanoic

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