Cu (100)

Article pubs.acs.org/JPCC

Adsorption and Reactions of ICH2CN on Cu(100) and O/Cu(100) Jong-Liang Lin,* Che-Wei Kuo, Che-Ming Yang, Yi-Shiue Lin, Tz-Shiuan Wu, and Pei-Yu Chao Department of Chemistry, National Cheng Kung University, 1, Ta Hsueh Road, Tainan, 701 Taiwan, Republic of China S Supporting Information *

ABSTRACT: Monitoring surface species and their bonding structures in link to specific chemical processes has long been an active, important subject in heterogeneous catalysis. In this article, with employment of temperature-programmed reaction/desorption, reflection− absorption infrared spectroscopy, Auger electron spectroscopy, and X-ray photoelectron spectroscopy in combination with density functional theory computation, we present three CH3CN formation channels from reaction of CH2CN generated by ICH2CN dissociative adsorption on Cu(100) and first spectroscopic evidence for CHCN on single crystal surfaces. The CH3CN formation mechanisms are dependent on CH2CN adsorption geometries. At lower coverages, CH2CN is adsorbed with the C−C−N approximately parallel to the surface. Reaction of these adsorbates produces CH3CN via first- and second-order kinetics, with the largest desorption rates occurring at 213 K and ∼400 K, respectively. At or near a saturated first-layer coverage, decomposition of ICH2CN forms C-bonded CH2CN (−CH2CN), which then transforms to N-bonded −NCCH2 with tilted orientation. Disproportionation of the −NCCH2 generates CH3CN at ∼324 K. Thermal products of H2, HCN and (CN)2 evolving at higher temperatures are originated from the CHCN dissociation. On oxygen-precovered Cu(100), reaction of CH2CN forms new surface intermediates of vertical −NCO and −CCO, in addition to perturbed CH3CN desorption. In the conditions studied, formation of H2, HCN, and (CN)2 is terminated due to the presence of preadsorbed O. −NCO and −CCO on O/Cu dissociate at ∼525 and 610 K, respectively, into CO and CO2.

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(HREELS).13 A CH2CN adsorption structure, as shown in Scheme 1a with a metal−carbon bond and flat−lying C−C−N

INTRODUCTION In heterogeneous catalysis of transition metals and their oxides, understanding of the surface processes such as reaction pathways and kinetics at molecular level is necessary in order to control the practical issues of activity and selectivity and to further successfully design highly efficient catalysts.1−4 The buildup of the knowledge for the chemistry occurring at interfaces relies on the key factors of identifying surface intermediates and monitoring their chemical processes, which is bonding structure related. The connection between adsorbate’s bonding geometry and reaction pathway can be established with the employment of sensitive surface analytical techniques for well-defined surfaces under an ultrahigh vacuum condition. The versatile transformation of organic and organometallic molecules containing a CN group has been well-recognized and utilized in surface functionalization, solid-based sensors, and heterocatalysis.5−8 Industrially, hydrogenation of nitriles is a common process to produce amines.9,10 CH2CN (cyanomethyl) bonded on the metal surfaces of SiO2-supported Pt and Ru catalysts has been proposed as the key intermediate in the hydrogenation of acetonitrile.11 In the early study of CH3CN thermal reaction on W(100), Friend and Pearlstine suggested a mechanism involving formation of CH2CN, which can abstract surface hydrogen atoms to evolve CH3CN above 400 K.12 On the less-reactive Ag(110) surface, Madix et al. reported the reversible adsorption of CH3CN at ∼166 K.13,14 However, as the surface was covered with oxygen atoms acting as Brönsted base sites to react with the acidic H of CH3CN, CH2CN was detected with high-resolution electron energy-loss spectroscopy © 2013 American Chemical Society

Scheme 1. Possible Bonding Structures of CH2CN on Surfaces

skeleton allowing π donation from the CN group to the surface, was proposed to account for the HREEL peaks detected at 695 cm−1 (tw(CH2)), 850 cm−1 (ω(CH2)), and 1060 cm−1 (γ(CH2)). CH2CN may adopt different adsorption structures (Scheme 1), which can be surface- and coverage-dependent, therefore showing various reaction pathways. In Scheme 1b, the terminal N and C atoms of CH2CN are attached to the surface. The structure of Scheme 1c has a metal−carbon bond, but unlike Scheme 1a, the CCN is not parallel to the surface, Received: January 28, 2013 Revised: September 9, 2013 Published: September 23, 2013 19916

dx.doi.org/10.1021/jp400968d | J. Phys. Chem. C 2013, 117, 19916−19926

The Journal of Physical Chemistry C

Article

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. 16O2 (99.998%) and 18O2 (>99 atom %) were obtained from Isotec. ICH2CN (98%), purchased from Aldrich, was 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 (θ0) 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 θ0 = 0.34 ML, and a (√2 × √2)R45° structure was formed at a saturation coverage of θ0 = 0.48 ML.28 Photoemission measurements were carried out at the wide range spherical grating monochromator beamline (WR−SGM) at the National Synchrotron Radiation Research Center of ROC. Total instrumental resolution, including the beamline and energy analyzer, was estimated to be better than 0.3 eV. The photoelectrons were collected at an angle of 50° from the normal surface. 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 Cu2p3/2 peak at 75.10 eV. The size of Xray beam used was approximately 2 × 2 mm2. In the study of ICH2CN 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 least-squares algorithm after Shirley background subtraction. In our theoretical cluster−model calculations for the optimized bonding geometries of surface species and their 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 generalized gradient approximation proposed by Perdew and Wang (GGAPW91) was employed for the exchange−correlation functional. 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 were based on the animated vibrations of the corresponding bands.

lacking an interaction of the CN surface. Scheme 1d shows the N-bonded CH2CN with two cumulated double bonds. In terms of the reported CH2CN-containing metal complexes, CH2CN can be bonded to various metal centers, such as Hg, Rh, Co, Ni, La, Pt, Cu, Ag, and Au, and so forth.15−26 For most of the metal complexes, CH2CN is C-metalated (M−CH2CN), with the CN as terminal noncoordinated group. However, in the crystal structure of [Cp2*LaCH2CN]2, the two Cp2*La units are connected by the bridging CH2CN groups (M−CH2CN− M).22 These bonding characteristics are similar to the CH2CN adsorption structures shown in Scheme 1b and c. Attachment of halogenated acetonitrile (XCH2CN) on Si(100)−2×1 has been reported to show the multiple bonding possibilities.27 BrCH2CN and ClCH2CN can undergo C−X bond scission on the silicon surface, producing Si−NCCH2, which is the same as that of Scheme 1d. In the previous study of CH3CN/O/Ag(110) system, Madix et al. observed the CH3CN evolution at ∼450 K and suggested two reaction pathways for this result, that is, 2CH2CN(a) → CH3CN(g) + CHCN(a) (direct coupling) as well as CH2CN(a) → CHCN(a) + H(a), CH2CN(a) + H(a) → CH3CN(g) (sequential dehydrogenation and hydrogenation).13,14 However, no spectroscopic evidence of adsorbed CHCN was provided to strongly support the mechanistic validity. Besides, coveragedependent experiments were not performed for the reaction kinetics. Studies of CH2CN on single crystals are rare. In view of the possibility for the multiple bonding structures shown in Scheme 1, adsorption and reactions of CH2CN are worthy to be further thoroughly explored. In the investigation of ICH2CN on Cu(100), aiming to produce surface CH2CN after the facile C−I bond dissociation and to prevent the interference of reactive oxygen, we found that CH2CN indeed adopts coverage-dependent bonding geometries, including parallel CH2CN and tilted −CH2CN and −NCCH2, and shows different formation kinetics for CH3CN, leaving CHCN on the surface responsible for the thermal products formed at higher temperatures. With the presence of surface oxygen, new intermediates of −NCO (isocyanate) and −CCO (ketenylidene) were detected.

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EXPERIMENTAL SECTION All TPR/D, AES, and 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 15 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 Bomem FTIR spectrometer and focused at a grazing

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RESULTS AND DISCUSSION Adsorption and C−I Bond Dissociation of ICH2CN on Cu(100). Temperature-programmed desorption was investigated first to understand the adsorption and thermal stability of ICH2CN on Cu (100). Figure 1 shows the TPD spectra, 19917



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Figure 1. Temperature-programmed desorption spectra of ICH2CN on Cu(100) at the exposures indicated. The inset shows the relative desorption yield as a function of exposure.

represented by ICH2CN+ (m/z 167), after adsorption of ICH2CN at ∼180 K. At an exposure of 1.0 L or less, no ICH2CN molecules desorbing from the surface are measured. From 1.5 to 7.0 L, a molecular desorption state appears at ∼200 K. In the inset showing the relative desorption yield against exposure, it is found from extrapolation that ICH2CN desorption starts at ∼1.2 L and then the yield increases linearly with exposure, which is typical for molecular desorption from multilayers on a surface. No ICH2CN desorption below 1.2 L indicates that ICH2CN dissociates on the surface at 180 K or during the heating course in the TPD experiment. The previous studies regarding the chemistry of alkyl iodides on copper single crystals indicated that C−I bond scission is the primary reaction step, which occurs below 180 K, resulting in the formation of adsorbed iodine atoms and alkyl groups.29−31 Indeed, the separate X-ray photoelectron spectroscopic study of ICH2CN/Cu(100), as shown in supporting Figure S1, confirms the C−I breakage at 180 K. Figure S1 shows the I4d XP spectra taken after ICH2CN adsorption (2.0 L) on Cu(100) at 180 K, followed by annealing to 210 K. There are four resolved peaks in the 180 K spectrum that can be attributed to ICH2CN molecules (50.8 and 52.5 eV) and iodine atoms (49.5 and 51.2 eV) on Cu(100).32,33 The C−I bonds dissociate completely at 210 K. Spectroscopic Evidence of Coverage-Dependent CH2CN Adsorption Structures. After knowing the primary C−I dissociation step of ICH2CN, RAIRS was employed to monitor the surface species present due to the bond scission and their adsorption structures in terms of the characteristic infrared frequencies and the surface dipole selection rule. Several ICH2CN coverages, lower (0.3 and 0.7 L) and higher (3.0 and 7.0 L) than the first saturated adsorption layer, were investigated. Figure 2 and Supporting Figure S2 show the temperature-dependent infrared spectra for 0.7 and 0.3 L of ICH2CN adsorbed on Cu(100). In Figure 2, there are two peaks at 837 and 1369 cm−1 in the 180 and 210 K spectra. Since the C−I bond of ICH2CN on Cu(100) is unstable at 210 K, the 837 and 1369 cm−1 bands are impossibly due to intact

Figure 2. Reflection−absorption infrared spectra taken after dosing 0.7 L of ICH2CN on Cu(100) at 180 K, followed by flashing the surface to the temperatures indicated.

ICH2CN. According to the peak frequencies, CH2CN and CHCN are considered to be the responsible intermediates.34 Theoretical computation has been performed to assist in identification of the surface species. Figure 3 and Table 1 show

Figure 3. Theoretically predicted CH2CN and CHCN bonding structures on Cu(100), with two different (top and side) viewing angles.

the theoretically predicted CH2CN and CHCN structures on Cu(100) and their fundamental infrared absorptions, respectively. In the process for obtaining the optimized structures, all of the coordinates, including the bond lengths and angles and the atomic heights and positions relative to the surface, were allowed to be varied. In terms of the calculated frequencies of CH2CN and CHCN, it can be concluded that the two experimental bands at 837 and 1369 cm−1 belong to CH2CN 19918



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Table 1. Comparison of the Theoretically Obtained Infrared Frequencies (cm−1) and Structural Parameters for CH2CN and CHCN on Cu(100) 2

IR

mode

length and angle (Å, deg)

504 553 837 1026 1067 1415 2082 3062 3138

δ(CCN) tw(CH2) ω(CH2) νs (CCN) tw(CH2) ρ(CH2) δ(CH2) νas (CCN) νs (CH2) νas (CH2)

d(C2−H): 1.102 d(C2−H): 1.102 d(C2−C1): 1.400 d(C1−N): 1.192 d(C2−Cus): 2.027 ∠HC2H: 113.5 ∠HC2C1: 115.2 ∠HC2C1: 115.5 ∠C2C1N: 170.9

502 553 981 1094 1842 3166

δ(CCN) δ(CH) δ(HCC) νs (CCN) νas (CCN) ν(CCH)

d(C2−H): 1.086 d(C2−C1): 1.362 d(C1−N): 1.223 d(C2−Cus): 1.599 ∠HC2C1: 123.0 ∠C2C1N: 162.4

1

C H2C N

C2HC1N

Figure 4. Reflection−absorption infrared spectra taken after dosing 7.0 L of ICH2CN on Cu(100) at 180 K, followed by flashing the surface to the temperatures indicated.

Table 2. Changes of ICH2CN Infrared Frequencies (cm−1) with Cu(100) Surface Temperature

and are assignable to CH2 wagging and scissoring modes, respectively. In this optimized CH2CN structure, the CCN skeleton is almost parallel to the surface, with the C−C and C− N bond lengths at 1.40 and 1.19 Å (Table 1). For comparison, the bond lengths of C−C, CC, and CC are reported to be typically at 1.50, 1.35, and 1.20 Å, respectively, and those of C− N, CN, and CN are at 1.47, 1.29, and 1.16 Å.35 Due to the limitation by the dipole selection rule for RAIRS, which states that a fundamental transition is only allowed for those vibrational modes with a nonzero dynamic dipole moment perpendicular to surface, the two CCN stretching modes (ν(CCN)) are unable to be measured experimentally. In Figure 2, the CH2CN peaks are shifted from 837 and 1369 cm−1 to 879 and 1377 cm−1 as the temperature is increased from 210 to 240 K. This result is ascribed to the change of CH2CN in adsorption structure. At 375 K, the CH2CN peaks disappear in response to its reaction on the surface at elevated temperatures, shown later in the TPR/D study. Furthermore, no infrared bands are detected at higher temperatures. In the optimized adsorption structure of CHCN shown in Figure 3b, the CH moiety is bonded at bridge site, being different from the atop position of CH2CN through CH2. The CCN skeleton of the CHCN is approximately parallel to the surface, with the C−C and C−N bond lengths at 1.36 and 1.22 Å, respectively. The bond distances suggest the existence of delocalized π system over the C−C−N backbone.36 The CH2CN and CHCN structures (Figure 3) were obtained, without considering the lateral effect among adsorbed molecules, and may be more appropriate for lower surface concentrations. Figure 4 and Figure S3 show the change in the infrared absorptions of 7.0 and 3.0 L of ICH2CN on Cu(100) with temperature. The peaks observed in the 180 K spectrum of Figure 4 are listed in Table 2 and are compared to the infrared absorptions of ICH2CN in liquid phase reported previously.37 Due to the close infrared resemblance, the set of the peaks at 1405, 2246, 2950, and 3020 cm−1 can be attributed to adsorbed intact ICH2CN molecules. Since C−I breakage of ICH2CN takes place at 180 K, appearance of the extra peaks at 839, 1375, and 2205 cm−1, not belonging to ICH2CN is not surprising.

ICH2CN(l)a

modea

594 802 944 1100 1155 1408

ν(C−I) ρ(CH2) ν(C−C) tw(CH2) ω(CH2) δ(CH2)

2245

ν(C−N)

2960 3024

νs(CH2) νas(CH2)

a

7.0 L ICH2CN,b Cu(100), 180 K

7.0 L ICH2CN,b Cu(100), 270 K

839

887

1375 1405 2205 2246 2950 3020

1381 2191

Reference 37. bThis work.

First of all, flat-lying CH2CN may contribute to the absorption at 839 cm−1. In Figure 4, the adsorbed ICH2CN molecules do not exist when the surface is heated to 210 K, a result of ICH2CN desorption from multilayers and further dissociation of ICH2CN. The latter process leads to the growth at 2191 cm−1. This peak together with two relatively small ones at 887 and 1381 cm−1 continue to increase in intensity up to 270 K. Except for the difference in intensity, the 2191 cm−1 band in the 270 K spectrum becomes narrower. The spectral change from 210 to 270 K is a sign of CH2CN bonding variation. The presence of the relatively strong 2191 cm−1 peak reveals that the CCN skeleton of CH2CN at high coverages is not parallel to the surface. Note that the ν(CN) of CH2CN on Ag(110) has been reported to be at 2075 cm−1.13,14 In Figure 4, the set of the peaks of 887, 1381, and 2191 cm−1 disappears at 330 K. C-bonded or N-bonded CH2CN (−CH2CN or −NC CH2) (Scheme 1c and d), without the CCN being parallel to the surface, should be responsible for the 2191 cm−1 band assignable to CN or NCC stretching vibration.34 Figure 5a shows the theoretical adsorption structures and relative energies for the two CH2CN structures bonded at specific surface sites (atop, bridge, and hollow sites). In these calculations the CH2 was fixed at atop, bridge, or hollow site; otherwise a parallel CH2CN geometry would be obtained 19919



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instead. In Figure 5a, ΔE denotes the structure energy relative to that of the parallel CH2CN (Figure 3a). The calculated −CH2CN, irrespective of the bonding site, adopts nearly vertical orientation with the ΔE at 26 kcal·mol−1. Table S1 shows the theoretical infrared frequencies and structural parameters for −CH2CN at the three specific surface sites. The calculated CN stretching frequencies of the −CH2CN are in the range of 2072−2105 cm−1. As shown in Figure 5b, the −NCCH2, fixed at the different bonding sites, is also predicted to be adsorbed perpendicularly, but being more stable than the −CH2CN species. In particular, the energy difference between the bridging −NCCH2 and −CH2CN amounts to ∼20 kcal·mol−1. As shown in Table S2 for the −NCCH2 structural and spectral information, the NCC stretching frequency of −NCCH2 is more sensitive to the adsorption site (2072, 1977, and 1879 cm−1, respectively). In terms of the calculated stability of −CH2CN and −NCCH2 and their frequencies, the experimentally observed band at 2191 cm−1 (Figure 4) can be preferentially attributed to atop or bridging −NCCH2.

Figure 5. Theoretically predicted −CH2CN and −NCCH2 bonding structures at atop, bridge, and hollow sites on Cu(100). ΔE (kcal· mol−1) represents the energy relative to that of parallel CH2CN (Figure 3a).

Figure 6. Temperature-programmed reaction/desorption spectra of ICH2CN on Cu(100). 19920



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However, under the condition taking the interaction between adsorbates into account, the possibility for the presence of −CH2CN cannot be completely ruled out. This point is clearly addressed later in the XPS study. Besides, the molecular orientation of adsorbed −CH2CN or −NCCH2 at higher coverages may not be completely normal to the surface. For example, the band at 887 cm−1 in the 270 K spectrum of Figure 4 could be due to δ(CCN) + ρ(CH2) of −NCCH2. In the vertical orientation of −NCCH2, this mode is not expected to be detected with RAIRS according to the surface dipole selection rule. Effect of CH2CN Adsorption Structure on Its Reaction Pathways and Kinetics. From the infrared study in combination with theoretical computation, it is established that CH2CN is adsorbed in parallel or tilted form depending on ICH2CN coverage. Now, we show that variation in CH2CN adsorption orientation on Cu(100) results in distinct reaction kinetics for CH3CN desorption. Figure 6 shows the exposuredependent TPR/D spectra represented by the ions of 27 amu for HCN, 41 amu for CH3CN, 2 amu for H2, and 52 amu for (CN)2 after adsorption of ICH2CN at 180 K. In the CH3CN evolution at lower coverages (≤0.7 L), the low-temperature desorption channel (213 K) is characteristic of first-order kinetics, that is, no dependence of the peak temperature on coverage.38 However, the additional CH3CN peak changes from 410 to 381 K, showing a second-order reaction kinetics.38 The CH3CN features at 213 K or ∼400 K are reaction-limited, because in our separate CH3CN/Cu(100) study the surface CH3CN molecules below a monolayer coverage desorb at 168 K, similar to the case of Ag(110). In Figure 6, as the exposure is increased to 1.0 L (near a saturated first layer coverage) or higher, the CH3CN evolution is different from that observed at lower coverages. The 213 K state is highly suppressed and the predominant desorption occurs in a broader temperature range (∼275−550 K). Note that the state at ∼203 K, from 1.5 to 7.0 L, is due to ICH2CN molecular desorption (Figure 1). The desorption pattern for the other products of HCN, H2, and (CN)2 also shows a change from 0.7 to 1.0 L. Note that no desorption of ethylamine, a possible hydrogenation product of ICH2CN on Cu(100), was observed. Besides, no N−H stretching mode (∼3400 cm−1) was detected in the RAIRS study. The CH3CN desorption behavior is well-correlated to the observed infrared changes of ICH2CN/Cu(100) as a function of exposure and temperature. In the 0.7 L case, the evolution of CH3CN at 213 K corresponds to the vanishing peaks of 837 and 1369 cm−1, and that at 381 K corresponds to the disappearance of the peaks at 879 and 1377 cm−1 (Figure 2) due to the parallel CH2CN. On the other hand of 7.0 L, the 324 K channel is associated with the set of peaks at 887, 1381, and 2191 cm−1 (Figure 4), belonging to tilted CH2CN. Figure 7 shows the Auger electron spectra taken after adsorption of 3.0 L of ICH2CN on Cu(100) at 180 K, followed by annealing to 980 K. Increasing the temperature for the ICH2CN covered surface results in the desorption of unreacted ICH2CN molecules and formation of the carbon-containing products of CH3CN, HCN, and (CN)2. In the 980 K spectrum, the relatively strong carbon signal, in addition to the barely detectable N and I, indicates that carbon atoms are left on the surface after evolution of the reaction products. Study of Photoelectron Emission on the CH2CN Structure and Detection of CHCN. In the chemical process of ICH 2 CN/Cu(100) or CH 3 CN/O/Ag(110) forming

Figure 7. Auger electron spectra taken after dosing 3.0 L of ICH2CN on Cu(100) at 180 K, followed by flashing the surface to 980 K.

CH 3 CN, although 2CH 2 CN → CH 3 CN + CHCN is considered to be a rational overall reaction, the presence of CHCN has not been confirmed with surface vibrational spectroscopy including HREELS and RAIRS.13,14 In this section, the XPS result provides decisive information to further elucidate the reaction steps of CH 2 CN on Cu(100), supplementary to RAIRS. Figure 8 shows the temperature-

Figure 8. X-ray photoelectron emission of N1s after dosing 2.0 L of ICH2CN on Cu(100), followed by flashing the surface to the temperatures indicated.

dependent XP spectra after adsorption of 2.0 L ICH2CN on Cu(100). The 180 K spectrum can be fitted with two peaks at 397.9 and 399.2 eV, indicating the presence of two Ncontaining species, that is, ICH2CN and CH2CN as suggested by the I4d spectra of Figure S1. Upon heating to 210 K, the 397.9 eV peak increases in intensity at the expense of the peak of 399.2 eV. The reduction of the signal is attributed to ICH2CN desorption and dissociation by C−I breakage, which also means that the N1s binding energy (BE) of multilayer ICH2CN is located at 399.2 eV. Furthermore, as shown in Figure S1, ICH2CN does not exist on the surface at 210 K; therefore the presence of two N1s states reflects that the 19921



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recombination to form (CN)2 between 800 and 1000 K.42−45 Note that the 1s BE for adsorbed nitrogen atoms on copper films has been reported to be 396.5 eV,42 and they can recombine to evolve N2 from Cu(100) and Cu(111) at a temperature >700 K.43−45 No N2 desorption was observed in the ICH2CN decomposition on Cu(100) in the present study. To support the possibility for the formation of CxNyHz in the reaction of CH2CN on Cu(100), we have studied the reaction of 2,4-dibromopyridine on Cu(100) and indeed found the reaction products of H2, HCN and (CN)2 and N1s signals of ∼398.3 and 400.5 eV at high temperatures. This result will be published elsewhere. Formation Mechanisms of CH 3CN from CH 2 CN Reaction on Cu(100). It is found that the 213 K CH3CN desorption state at lower ICH2CN coverages shows first-order kinetics (Figure 6) and originates from CH2CN intermediate with CCN almost parallel to the surface (Figures 2 and 3). A two-step reaction of CH2CN → CHCN + H (slow) and CH2CN + H → CH3CN (fast) is a possible mechanism.39 In the study of Si(100)−2×1 with doubly-occupied dimers (H− Si−Si−H), H2 evolution shows a first-order behavior.46 The mechanistic models proposed for the first-order H2 formation may be applicable to the present CH2CN reaction on Cu(100) to generate CH3CN at 213 K, including (a) concerted desorption of pre-paired H atoms, (b) recombination of a delocalized H with a localized H, and (c) 1,2-hydrogen atom shift to from a three-center dihydride-like structure.46 The CH3CN desorption channel may involve intimate, flat-lying CH2CN adsorbates. The rate-determining step could be due to hydrogen excitation of CH2CN to form a H-delocalized state or due to concerted desorption of pre-paired CH2CN (NCCH2··· H···CHCN). The first-order CH3CN formation kinetics may also involve diffusion of CH2CN to specific sites, such as surface defects, that activates CH2CN. At lower ICH2CN exposures, CH3CN desorption also occurs around 400 K with second-order kinetics. Since no H2 is produced at ∼400 K, this CH3CN formation channel can be ascribed to bimolecular disproportionation of flat-lying CH2CN: 2CH2CN → CH3CN + CHCN. At the ICH2CN coverage near a monolayer, tilted −NCCH2 is responsible for the CH3CN formation at ∼324 K. Without concomitant H2 evolution, this CH3CN state is thought to result from 2 −NCCH2 → CH3CN + CHCN. Effect of Preadsorbed Oxygen on the Chemistry of CH2CN on Cu(100). Figure 9 shows the TPR/D spectra of 0.7 L ICH2CN/O/Cu(100), representing the formation of CH3CN (m/z 41), H2O (m/z 18), CO2 (m/z 44), NO (m/z 30), and CO and N2 (m/z 28). No ICH2CN molecular desorption was observed. As compared to the case of 0.7 L ICH2CN/Cu(100) (Figure 6), the products of HCN, H2, and (CN)2 are not found; CH3CN desorption is affected in the peak shape and temperature by pre-adsorbed oxygen. The presence of O on Cu(100) may change the electronic property of the metal surface and the adsorption structure of CH2CN and may interact directly with neighboring CH2CN, resulting in the complicated CH3CN desorption behavior. Here, we emphasize the CH2CN reaction pathways other than CH3CN formation due to pre-adsorbed O atoms. H2O desorption is peaked at three temperatures of 222, 385, and 500 K. The H2O evolving at 220 K is consistent with the previous report that OH groups on Cu(100) recombine to generate H2O.47 In the present study, the OH groups are likely to be generated from H-abstraction of CH2CN by O atoms. However, the desorption

CH2CN fragments have two different bonding structures. The 399.2 eV signal is attributed to C-bonded CH2CN (−CH2CN) which is tilted away from the surface without the interaction of the terminal CN group with the surface and thus has the same N1s BE as that of ICH2CN. On the other hand, the N1s peak at 397.9 eV is ascribed to −NCCH2, with the N atom directly attached to the surface that causes the BE shift. The presence of two CH2CN structures nicely explains the broadened 2191 cm−1 peak observed in the 210 K spectrum of Figure 4. Besides, the 2191 cm−1 peak becomes narrower and continues to increase intensity from 210 to 270 K (Figure 4), showing the growth of the −NCCH2 species. In Figure 8, the N1s spectral features continue to change with temperature. At 280 K, the −CH2CN signal decreases significantly in company with the formation of a small, new peak at 396.9 eV and peak shift from 397.9 eV to ∼398.4 eV with enhanced intensity. As suggested by the infrared result (Figure 4), the ∼398.4 eV peak is also attributed to −NCCH2. With the N atom directly bonded to the surface, the N1s binding energy of −NCCH2 is sensitive to the surface change from 180 to 280 K. The transformation from −CH2CN to −NCCH2 is consistent with the theoretical result that −NCCH2 is thermodynamically more favored. The appearance of the small peak at 396.9 eV indicates that CH2CN starts to dissociate on the surface. Heating the surface to 360 K causes two major changes: decrease of −NCCH2 and growth of the 396.9 eV peak. Besides, the total peak area is diminished by ∼24% as compared to that of 280 K, which corresponds to the CH3CN evolution peaked at ∼324 K (Figure 6). We deduced that, the CH3CN comes from −NCCH2 reaction on Cu(100). Since no H2 desorption from recombination of H atoms is observed prior to 360 K in the CH2CN decomposition,39 the 396.9 eV peak is attributed to CHCN according to the reaction of 2 −NCCH2 → CH3CN + CHCN. Note that, in Figure 4, the −NCCH2 infrared peak (2191 cm−1) is no longer detected at 330 K. Therefore the N1s peak at ∼397.9 eV in the 360 K spectrum is not deemed to be −NCCH2 but is attributed to flat−lying CH2CN. This species continues to be consumed in the formation of CH3CN as the surface is heated to 420 K. Further raising the temperature to 580 K induces considerable decline in the CHCN peak, but interestingly with the peak area of ∼397.9 eV enhanced. Decomposition of residual CH2CN to form CH3CN should proceed progressively in the temperature range of 420−580 K; therefore, the enhanced intensity at ∼397.9 eV must be due to new surface species, likely from the reaction of CHCN. Moreover, the CHCN decomposition may also lead to the desorption of H2 and HCN between 420 and 580 K (Figure 6). The area of the ∼397.9 eV peak decreases continuously from 580 to 980 K, agreeing with the formation of H2, HCN, and (CN)2 at high temperatures. In the XP analysis of N-doped graphene prepared by chemical vapor deposition, nitrogen defects designated as pyridinic, pyrrolic, and graphitic nitrogens, with the 1s BE at 398.2, 400.1, and 401.7 eV, respectively, have been reported.40,41 Accordingly, the predominant ∼397.9 eV peak in the 580 K spectrum can be attributed to CxNyHz (H and N-containing carbon islands or clusters) from CHCN reaction. Since the H2 and HCN are desorbed up to ∼810 K (Figure 6), the surface species responsible for the N1s peaks at ∼397.9 and 400.1 eV in the 980 K spectrum do not possess H and are suggested to be N-containing carbon clusters. However, the ∼397.9 eV signal could be also due to adsorbed CN groups, in terms of the previous studies showing N1s at 398.2 eV for CN on copper surfaces and CN 19922



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Figure 9. Temperature-programmed reaction/desorption spectra of 0.7 L ICH2CN on oxygen-precovered Cu(100). Figure 10. Reflection−absorption infrared spectra taken after dosing 0.7 L ICH2CN on oxygen-precovered Cu(100) at 180 K, followed by briefly heating the surface to the temperatures indicated.

channels at higher temperatures could be due to the reaction between the pre-adsorbed oxygen and CH2CN reaction intermediates. CO2 desorbs predominantly at 514 K, with a 603 K shoulder. NO is detected at 854 K, indicating that the integrity of the CN moiety is no longer retained in the reaction between CH2CN and O. The trace of 28 amu ion shows three peaks at 539, 615, and 837 K. The first two peaks have contribution from CO desorption because the CO2 fragmentation cannot totally account for the intensity of m/z 28 detected.35,36 The 837 K channel is due to N2 desorption. In the previous TPR/D study for the reaction of cyanogen ((CN)2) on O/Cu(100), N2 and NO were generated at high temperatures through the intermediate of adsorbed N.43−45,48,49 Detection of the Surface Intermediates from the Reaction of CH2CN and O on Cu(100). Figure 10 shows the temperature-dependent RAIR spectra of 0.7 L ICH2CN on O/ Cu(100) at 180 K. The IR absorption pattern from 180 to 270 K is similar, with relatively weak peaks at 1383, 2151, and 2195 cm−1. The last two CN characteristic stretching bands reveal that not all of the CH2CN fragments have their CCN backbone parallel to the surface, and the CH2CN has at least two different bonding structures. The 1383 cm−1 band is due to CH2 scissoring vibration of CH2CN. The CH2CN infrared features in the presence of O are different from those observed on clean Cu(100) in this temperature range (Figure 2), presumably due to the variation in its adsorption structure. Although CH3CN desorption occurs at 180−250 K (Figure 9), no striking infrared change is observed. This result can be explained by the presence of CH2CN with vibrational modes that are RAIR insensitive and/or are below our detection limit. In the 300 K spectrum of Figure 10, a small and sharp peak appears at 2156 cm−1 and grows up to 460 K. At 460 K, a new peak of 2027 cm−1 emerges. The absorption at 2156 cm−1 is no longer observed in the 540 K spectrum. The 2027 cm−1 band continues to grow at 500 K, but it diminishes with increasing temperature and finally vanishes at 580 K. To assist in the analysis for the bands at 2156 and 2027 cm−1, 18 O2 was used to prepare an 18O/Cu(100) surface, with subsequent ICH2CN adsorption. Figure 11 compares the infrared spectra of 16O/Cu(100) and 18O/Cu(100), showing

Figure 11. Comparison of the infrared spectra obtained from ICH2CN reaction on 16O- and 18O-precovered Cu(100).

the isotopic shifts of 2156 cm−1→2139 cm−1 and 2027 cm−1→ 1998 cm−1. Obviously, the species responsible for the bands contain oxygen. Five surface species of −NCO (isocyanate), −OCN (cyanate), −ONC (isofulminate), −CNO (fulminate), and −CCO (ketenylidene) have been calculated for the optimized structures, relative energies, and stretching frequencies, which are shown in Figure 12. The calculated oxygen isotopic shifts play a pivotal role in determining the correct species for the observed 2156 cm−1 and 2027 cm−1 bands. −NCO is the most stable structure among the C, N, and Ocontaining species and is predicted to be adsorbed in upright orientation at bridge site. The −NCO bonding structure is consistent with the previously theoretical −NCO/Cu(100) results.48,49 The calculated difference in νas(NCO) of −NC16O and −NC18O is 15 cm−1, which matches well the observed shift 19923



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Our RAIRS and TPR/D results show several oxygen-related CH2CN reaction pathways on O/Cu(100) including dehydrogenation even at 210 K to evolve H2O, breakage of the C−CN bond possibly by the attack of the O atom at the nitrile carbon (an electrophilic center) to form −NCO, and replacement of the N of CCN by O to form −CCO. Note that, in the previous study of CH3CN/O/Ag(110), −NCO and −CCO were not considered to be the reaction intermediates.13,14

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CONCLUSION Adsorption and chemistry of CH 2 CN from ICH 2 CN dissociation on Cu(100) and O/Cu(100) have been investigated using TPR/D, RAIRS, AES, and XPS in combination with theoretical computations. It is found that variation in the bonding structure of CH2CN can lead to CH3CN evolution at different temperatures. At lower coverages on Cu(100), recombination of intimate, flat-lying CH2CN fragments produces CH3CN at 213 K with first-order kinetics. The parallel CH2CN also generates CH3CN (a second-order channel) by disproportionation at ∼400 K. Near or at a saturated first-layer coverage, the N-bonded, tilted −NCCH2 species is responsible for the CH3CN desorption at 324 K. CHCN is formed from the CH2CN reaction, which undergoes further decomposition to produce H2, HCN, and (CN)2 at higher temperatures. In the conditions studied on O/Cu(100), two new surface intermediates of −NCO and −CCO with vertical orientation are formed, which are unstable at ∼525 and 610 K, respectively. The presence of pre-adsorbed O varies the CH2CN reaction behavior, including perturbation of the CH3CN desorption behavior and termination of H2, HCN and (CN)2 formation, but with the products of H2O, CO, CO2, NO and N2.

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Figure 12. Theoretically predicted adsorption structures of −NCO, −OCN, −ONC, −CNO, and −CCO on Cu(100) and their infrared absorptions. ΔE represents the relative energy in kcal·mol−1.

ASSOCIATED CONTENT

S Supporting Information *

I4d X-ray photoelectron spectra of ICH2CN/Cu(100) (Figure S1); infrared spectra of ICH2CN (0.3 and 3.0 L) on Cu(100) (Figures S2 and S3); calculated infrared absorptions and structural parameters of −CH2CN and −NCCH2 on Cu(100) (Tables S1 and S2). This material is available free of charge via the Internet at http://pubs.acs.org.

(17 cm−1) from 2156 cm−1 to 2139 cm−1 in Figure 11. Therefore, the 2156 cm−1 band is ascribed to −NCO. The νas(NCO) frequency of −NCO on Cu(100) prepared by cyanogen oxidation or HNCO dissociation has been reported to be coverage-dependent, in the range of 2162−2210 cm−1.50 In the present study, the infrared change of −NCO as a function of crystal temperature correlates to the CO and CO2 desorption around 525 K observed in Figure 9. The experimental peaks at 2027 and 1998 cm−1 in Figure 11 are attributed to −CC16O and −CC18O, respectively, in accordance to the calculated νas(CCO) frequencies and the oxygen isotopic shift (27 cm−1). The −CCO on Cu(100) is theoretically suggested to be adsorbed at the hollow site, with vertical orientation. As revealed by Figures 9 and 10, the disappearance of −CCO corresponds to the CO and CO2 formation at ∼610 K. −CCO has not been reported as the reaction intermediate in the previous investigations of (CN)2 and HCN on oxidized copper surfaces.43−45,50 In the acetone decomposition on O/ Ag(111), −CCO has been proposed to account for the observed 2026 cm−1 peak.51 Recently, partial oxidation of acetic acid on gold nanoparticles supported on TiO2 also shows the formation of −CCO/Au.4 However, in the these two reports reaction of −CCO on Ag and Au was not explored. To the best of our knowledge, Cu(100) and Ag(111) are the only single crystal surfaces studied to show the adsorption of −CCO until now.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 886 6 2757575 ext. 65326. Fax: 886 6 2740552. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Science Council of the Republic of China (Grant NSC 101-2738-M-006-004). We are grateful to Drs. Liang-Jen Fan and Yaw-Wen Yang at the National Synchrotron Radiation Research Center of ROC for their assistance in obtaining XPS data.

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