Spectroscopic Identification of Surface Intermediates in the

Article pubs.acs.org/JPCC

Spectroscopic Identification of Surface Intermediates in the Dehydrogenation of Ethylamine on Pt(111) Iradwikanari Waluyo, Joel D. Krooswyk, Jun Yin, Yuan Ren, and Michael Trenary* Department of Chemistry, University of Illinois at Chicago, 845 West Taylor Street, Chicago, Illinois 60607, United States S Supporting Information *

ABSTRACT: Reflection absorption infrared spectroscopy, temperature-programmed desorption, and density functional theory (DFT) have been used to study the surface chemistry and thermal decomposition of ethylamine (CH3CH2NH2) on Pt(111). Ethylamine adsorbs molecularly at 85 K, is stable up to 300 K, and is partially dehydrogenated at 330 K to form aminovinylidene (CCHNH2), a stable surface intermediate that partially desorbs as acetonitrile (CH3CN) at 340−360 K. DFT simulations using various surface models confirm the structure of aminovinylidene. Upon annealing to 420 K, undesorbed aminovinylidene undergoes further dehydrogenation that results in the scission of the remaining C−H bond and the formation of a second surface intermediate called aminoethynyl with the structure CCNH2, bonded to the surface through both C atoms. The assignment of this intermediate species is supported by comparison between experimental and simulated spectra of the isotopically labeled species. Further annealing to temperatures above 500 K shows that the C−N bond remains intact as the desorption of HCN is observed. W(100)-(5 × 1)-C surfaces14 have shown that acetonitrile is the main desorption product. From a spectroscopic perspective, since ethylamine has a CCN backbone, it is interesting to characterize the surface intermediates formed through the dehydrogenation of ethylamine and compare them with other CCN- or CNC-containing intermediates previously identified or proposed. Gardin and Somorjai used high-resolution electron energy loss spectroscopy (HREELS) to characterize the structure of the intermediate formed in the partial dehydrogenation of ethylamine on Ni(111). They proposed an intermediate species with the structure CH3CNH, bonded to the surface through the C and N atoms with the C−N axis parallel to the surface.13 Pearlstine and Friend proposed the same surface intermediate for ethylamine dehydrogenation on W(100)-(5 × 1)-C,14 but they did not provide spectroscopic evidence for it. From a broader perspective, the characterization of surface species containing a CCN unit is also relevant to other systems, such as acetaldehyde oxime, which can be electrochemically reduced on Pt nanoparticles to form ethylamine as well as dehydrated in solution to form acetonitrile.15 In a previous study,16 we have demonstrated using RAIRS and density functional theory (DFT) calculations that ethylamine on Pt(111) is partially dehydrogenated at 330 K to form a stable surface intermediate called aminovinylidene (CCHNH2), which has a markedly different structure from

1. INTRODUCTION Heterogeneously catalyzed hydrogenation and dehydrogenation reactions involving surface C−N bonds have been studied on a variety of single-crystal transition-metal surfaces. Most notably, a class of stable surface intermediates called aminocarbynes with the general formula CNRR′ (R, R′ = H or CH3) has been characterized on Pt(111) using reflection absorption infrared spectroscopy (RAIRS). The simplest aminocarbyne with the formula CNH2 can be formed on Pt(111) through either the hydrogenation of surface CN or the decomposition of azomethane (CH 3 NNCH 3 ) and methylamine (CH3NH2).1−3 Methylaminocarbyne (CNHCH3) is formed through either the hydrogenation of methyl isocyanide (CH3NC) or the partial dehydrogenation of dimethylamine ((CH3)2NH).4−6 Finally, dimethylaminocarbyne (CN(CH3)2) has been shown to be the product of the partial dehydrogenation of trimethylamine ((CH3)3N).7 All of these aminocarbyne-type surface species are bonded to the surface through the terminal C atom with the C−N bond perpendicular to the surface, and the C−N bond has an intermediate character between that of a single and a double bond. In contrast, a NH2CCH intermediate bonded to the surface through the C atoms with the CC bond parallel to the surface was proposed to form from the reaction between NH3 and surface C2 on Pt(111).8 Ethylamine dehydrogenation is of particular interest as a model system for the formation of nitriles and/or imines from the catalytic dehydrogenation of amines, which generally is carried out under aerobic conditions.9−12 Studies of surfacecatalyzed dehydrogenation of ethylamine on Ni(111)13 and © 2013 American Chemical Society

Received: December 15, 2012 Revised: January 31, 2013 Published: February 21, 2013 4666

dx.doi.org/10.1021/jp312373a | J. Phys. Chem. C 2013, 117, 4666−4679

The Journal of Physical Chemistry C

Article

Figure 1. Experimental RAIR spectra obtained after exposing Pt(111) to 0.3 L of (a) CH3CH214NH2, (b) CH3CH215NH2, and (c) CD3CD214NH2 at 85 K, followed by heating to 300, 330, and 420 K. All spectra were measured at 85 K. For (a) and (c), the low and high frequency regions were acquired using MCT and InSb detectors, respectively. Spectra in (b) were obtained using an MCT detector.

aminovinylidene. Nevertheless, the combined experimental and theoretical results provided definitive evidence for aminovinylidene as the surface intermediate.16 In the present work, we present more extensive experimental results from RAIRS and temperature-programmed desorption (TPD) to study the adsorption and thermal decomposition of ethylamine on Pt(111) as well as results from DFT simulations to characterize the structure of the intermediate species formed on the surface. From the RAIR spectra, we show that, at temperatures above 400 K, a second stable surface intermediate with an intact NH2 group and a C−N bond is formed, which eventually desorbs as HCN. The formation of a second surface

the intermediate proposed by Gardin and Somorjai. Although it has a CCN unit, similar to the CH3CNH intermediate formed through ethylamine dehydrogenation on Ni(111)13 and the NH2CCH species formed through the reaction between NH3 and C2 on Pt(111),8 aminovinylidene has an aminocarbyne-like structure with the terminal C atom bonded to the surface and the C−C bond more or less perpendicular to the surface. In addition, there is a delocalization of π orbitals across the CCN unit. While the identification of surface-adsorbed aminocarbynes has been supported by a wealth of literature on the formation of analogous aminocarbyne species in organometallic complexes,17,18 no such analogue has been reported for 4667



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estimated from the average of fwhm of other peaks in the spectra. Molden software30 was used to visualize normal modes for frequency assignments.

intermediate upon heating to higher temperatures has also been reported for the dehydrogenation of dimethylamine and trimethylamine on Pt(111).6,7 This is in direct contrast to the study by Gardin and Somorjai, which suggested that, after the desorption of acetonitrile, only surface C and N atoms remain on Ni(111) at temperatures above 350 K.13 On the basis of observations from experimental RAIRS and TPD data as well as DFT simulation results, we propose the structure of the second intermediate as CCNH2 (aminoethynyl), which is bonded to the surface through both C atoms.

4. EXPERIMENTAL RESULTS 4.1. RAIRS. 4.1.1. Adsorbed Ethylamine. Figure 1a−c displays the RAIR spectra of the sample after it was exposed to 0.3 L (1 L = 1 × 10−6 Torr s) of CH3CH214NH2, CH3CH215NH2, and CD3CD214NH2 at 85 K followed by heating to 300, 330, and 420 K. The sample was held at the target temperature for 30 s before it was cooled down to 85 K, the temperature at which all RAIR spectra were obtained. For each isotopologue of ethylamine, comparison between the initial spectra at 85 K and after heating to 300 K shows peaks that are higher and narrower, suggesting a more ordered overlayer. The observation of similar peaks in the spectra obtained before and after heating to 300 K indicates that the molecularly adsorbed ethylamine remains stable up to 300 K. The peak assignments for molecularly adsorbed CH3CH214NH2 on Pt(111) after the 300 K anneal, summarized in Table 1, are

2. EXPERIMENTAL DETAILS The experiments were performed in an ultrahigh vacuum (UHV) chamber with a base pressure of 1 × 10−10 Torr. The chamber is equipped with optics for low energy electron diffraction, a cylindrical mirror analyzer for Auger electron spectroscopy, and a Hiden HAL201/3F quadrupole mass spectrometer for TPD. TPD experiments were performed with a heating rate of 2 K/s. The chamber is also coupled to a Bruker IFS-66v/s Fourier transform infrared (FTIR) spectrometer for RAIRS measurements. The incident and reflected IR beams enter and exit the UHV chamber through differentially pumped, O-ring sealed KBr windows. RAIR spectra between 800 and 4000 cm−1 were obtained using a liquid nitrogen (LN2)-cooled MCT (HgCdTe) detector and a SiC source. A LN 2-cooled InSb detector with a low wavenumber cutoff of 1900 cm−1 was used in conjunction with a W source to obtain spectra with better signal-to-noises ratio at higher frequencies. Each RAIR spectrum was obtained with 1024 scans and 4 cm−1 resolution. The Pt(111) single crystal was mounted on a LN2-cooled sample holder and was cleaned using a standard procedure that has been described previously.1 Ethylamine (CH3CH2NH2, >99.0%) and deuterium-labeled ethylamine (CD3CD2NH2, 98%) were purchased from Fluka and Cambridge Isotope Laboratories, respectively; both gases were used without further purification. 15N-Labeled ethylamine (CH3CH215NH2) was synthesized according to the method described by Szammer et al.19 and purified with repeated freeze−pump−thaw cycles. Further details about the synthesis of 15N-labeled ethylamine can be found in the Supporting Information of ref 16. The purity of all gases was checked using mass spectrometry.

Table 1. Vibrational Frequencies (cm−1) for Ethylamine (CH3CH214NH2) Dissolved in Liquid Kr (trans and gauche Conformations), on Ni(111), and on Pt(111) liquid Kr (ref 31)

mode

3. COMPUTATIONAL DETAILS Theoretical calculations, including geometry optimization and vibrational frequency analysis, were performed using the Gaussian 0320 and Gaussian 0921 DFT packages with the B3LYP hybrid functional.22−27 The C, N, and H atoms were described with a 6-311G(d,p) basis set, while the core electrons of the Pt atoms were represented by the Hay−Wadt pseudopotential and the valence shells were described using a double-ζ basis set.28 The calculated vibrational frequencies were scaled by a factor of 0.9682,29 partially to account for the absence of anharmonicity in the calculations. Lorentzian broadening was used to simulate IR spectra with the peaks centered at the scaled harmonic frequencies. The integrated intensity of each peak is equal to the square of the z component of the dipole derivatives, defined as the direction perpendicular to the surface in accordance with the surface selection rule of RAIRS, and the full-width at half-maximum (fwhm) is equal to the fwhm of the corresponding experimental peak. If no corresponding experimental peak is observed, the fwhm is

Ni(111) at 150 K (ref 13)

Pt(111) at 300 K (this work)a

trans

gauche

NH2 wag CCN sym stretch CCN asym stretch

795 881

777 891

760 880

not observed not observed

1052

1082

1080

1038 (1031)

CH3 rock CH2 wag CH3 sym deformation CH3 asym deformation NH2 scissors CH2 sym stretch CH3 sym stretch CH2 asym stretch CH3 asym stretch NH2 asym stretch

1116 1347 1374

1116 1395 1374

1140 1360 not observed

1052 1145 1344 1383

1453

1467

1440

1450 (1447)

1620 2913

1620 2842

1540 2680

not observed 2828 (2828)

2874

2874

2960

2879 (2879)

2929

2929

not observed

2939 (2939)

2962

2977

3040

2973 (2973)

3331

3331

3200

not observed

a

(1048) (1141) (1344) (1383)

Values in parentheses are from 15N-labeled ethylamine.

inferred from comparisons with the previously reported HREEL spectrum of ethylamine on Ni(111)13 and the experimental spectra of the trans and gauche conformers of ethylamine dissolved in liquid Kr.31 Our vibrational frequency assignments in general also agree with those of the ethylamine moiety in the experimental RAIR spectra of adsorbed 1-(1naphthyl)ethylamine on Pt(111).32 Comparison between our RAIR spectrum of ethylamine on Pt(111) after the 300 K anneal with the HREEL spectrum of ethylamine on Ni(111) reveals that there is relatively good agreement for some of the vibrational modes, such as the CH3 4668



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rock at 1145 cm−1 for Pt(111) and 1140 cm−1 for Ni(111) and the CH3 asymmetric deformation mode at 1450 cm−1 for Pt(111) and 1440 cm−1 for Ni(111). The fact that these peaks shift with 15N substitution indicates that the corresponding modes involve some N-atom displacement. We observe two peaks at 1344 and 1383 cm−1 that are unshifted upon 15N substitution. On the basis of the comparison with the trans conformer of ethylamine in liquid Kr, these peaks can be assigned to the CH2 wag and CH3 symmetric deformation, respectively. Gardin and Somorjai observed a peak at 1360 cm−1 that they assigned to the CH2 wag,13 but they did not observe a peak corresponding to the CH3 symmetric deformation, possibly due to the inherently poorer resolution of HREELS compared to RAIRS. In addition, while Gardin and Somorjai observed three peaks assigned to various CH2 and CH3 stretching modes,13 we can clearly resolve four peaks in the 2900−3000 cm−1 region after the 300 K anneal, all of which are unshifted upon 15N substitution. For 14N ethylamine, two relatively intense peaks are observed at 1038 and 1052 cm−1. Upon 15N substitution, these peaks are redshifted by 7 and 4 cm−1, respectively, which indicates N atom involvement in the corresponding modes. The CCN asymmetric stretching frequencies for trans- and gaucheethylamine in liquid Kr have been reported to be 1052 and 1082 cm−1, respectively.31 Therefore, it is reasonable to assign the 1052 cm−1 peak to the CCN asymmetric stretching mode. The CCN symmetric stretching modes for ethylamine in liquid Kr are observed at 881 and 891 cm−1 for the trans and gauche conformations, respectively.31 It is unlikely that the 1038 cm−1 peak arises from this mode due to the large frequency difference. Therefore, this peak is also assigned to the asymmetric CCN stretch, presumably from a second ethylamine structure on the surface. Alternatively, the splitting of the CCN asymmetric stretch peak could be due to the adsorption of two molecules per unit cell. As expected, while 15N substitution results only in small frequency shifts, the spectrum of molecularly adsorbed CD3CD214NH2 shows large frequency shifts as well as dramatic changes in the relative peak intensities. For the deuterated species, only four peaks of reasonably strong intensities are observed, all of which are located below 1200 cm−1. While CH2 and CH3 stretches feature prominently in the spectra of CH3CH214NH2 and CH3CH215NH2, CD2 and CD3 stretches would be expected in the 2100−2200 cm−1 spectral region, which is not shown in Figure 1c due to overlap with the peak due to CO adsorbed from the background. Table 2 summarizes the frequency assignments for deuterated ethylamine compared

to the experimental frequencies of the trans and gauche conformers of gas-phase CD3CD2NH2.33 From the comparison with the experimental spectra of gas-phase CD3CD2NH2, the peak at 1174 cm−1 can be attributed to either CD2 wag or CD2 scissors modes depending on the conformation, while the 1049 cm−1 peak can be unambiguously assigned to the CD3 symmetric deformation mode. The peak at 941 cm−1 could be due to the CCN asymmetric stretch, which was observed for gas-phase CD3CD2NH2 at 902 and 980 cm−1 for the trans and gauche conformers, respectively. No peak around 1090 cm−1 is observed in the experimental spectra of gas-phase CD3CD2NH2; however, calculations show that there should be another CD3 deformation peak at 1073 cm−1 for the gauche conformer.34 Therefore, the 1090 cm−1 peak is tentatively assigned to the CD3 asymmetric deformation mode. 4.1.2. Aminovinylidene. Upon heating the sample to 315 K, the peaks associated with molecularly adsorbed ethylamine are reduced in intensity, and new peaks assigned to aminovinylidene16 first appear (see the Supporting Information). These new peaks reach their maximum intensities at 330 K for regular and 15N-labeled ethylamine and at 340 K for the Dlabeled species. For all three isotopologues, the peaks between 3300 and 3500 cm−1 are all assigned to N−H stretch modes. For the 14N- and 15N-labeled aminovinylidene, the peak at 2940 cm−1 is assigned to the C−H stretch. In addition, the peak at 1601 cm−1 for regular aminovinylidene (redshifted to 1593 cm−1 for the 15N-labeled species) arises from the NH2 scissors mode. The strong peak at 1463 cm−1 is only shifted by 2 cm−1 upon 15N substitution, and from comparison with simulated IR spectra, it is assigned to a C−C−N asymmetric stretch with some contributions from C−H and N−H bending modes. Finally, the 1283 cm−1 peak is due to a mix of N−H bend, C− C−N asymmetric stretch, and C−H bend. Table 3 summarizes the frequency assignments for 14N- and 15N-labeled aminovinylidene. Table 3. Vibrational Frequencies (cm−1) for 14N- and 15NLabeled Aminovinylidene on Pt(111) mode peak 1, NH bend + CCN asym stretch + CH bend peak 2, CCN asym stretch + CH bend + NH bend peak 3, NH2 scissors peak 4, CH stretch peak 5, NH2 sym stretch peak 6, NH2 asym stretch

Table 2. Vibrational Frequencies (cm−1) for Deuterated Ethylamine (CD3CD214NH2) in the Gas Phase (trans and gauche Conformations) and on Pt(111)

CCN asym stretch CD3 sym deformation CD3 asym deformation CD2 scissors CD2 wag a

trans

gauche

Pt(111) at 300 K (this work)

902 1056

980 1058

941 1049

(1140)

(1073)

1090

not observed 1176

1174 not observed

1174

shift

1283

1275

−8

1463

1461

−2

1601 2940 3434 3488

1593 2939 3430 3479

−8 −1 −4 −9

For D-labeled aminovinylidene, the peak at 3475 cm−1 corresponds to an N−H stretching mode while the peaks at 2411, 2565, and 2609 cm−1 are assigned to various N−D stretching modes. The observation of an N−D bond, which is not present in the ethylamine precursor, indicates an H/D exchange. Unlike the spectra of regular and 15N-labeled aminovinylidene, which show sharp, easily resolved peaks, the spectrum of the D-labeled aminovinylidene contains a series of closely spaced peaks below 1700 cm−1 that give the appearance of a broad, asymmetrical peak with a maximum at 1441 cm−1 and shoulders at 1462 and 1500 cm−1. As discussed in our previous work,16 the experimental spectrum of D-labeled aminovinylidene can only be reproduced upon the summation of the simulated spectra of three variations of the species:

gas phase (ref 33)a mode

CCH14NH2 CCH15NH2

Values in parentheses are from calculations. 4669



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Figure 2. Experimental RAIR spectra obtained after 0.3 L CH3CH214NH2 (top), 0.3 L CH3CH214NH2 coadsorbed with 4 L D2 (middle), and 0.3 L CD3CD214NH2 (bottom) on Pt(111) was heated to (a) 330 K and (b) 420 K.

1515, 2564, and 3475 cm−1. The latter two are clearly from N− D and N−H stretches, respectively, and peaks at the same frequencies can be found in the spectrum of deuterated aminovinylidene. After the 420 K anneal, the spectrum of coadsorbed D2 and CH3CH214NH2 shows peaks at 1422, 1539, 1591, 3366, 3435, and 3505 cm−1, similar to the spectrum of annealed CH3CH214NH2. We can also observe weaker peaks at 1515, 2560, and 3470 cm−1, all of which are found in the spectrum of annealed CD3CD214NH2. From the comparison with DFT simulation results, which will be presented in the Discussion section, we conclude that these additional peaks for both the 330 and 420 K anneals are from the species with partially deuterated amine groups. 4.2. TPD. Figure 3 shows the TPD data following a 0.3 L exposure of CH3CH214NH2 to Pt(111) at 85 K. The corresponding data for CH3CH215NH2 and CD3CD214NH2 are in the Supporting Information. Although ethylamine has a mass of 45, its largest signal is for the CH2NH2 fragment at mass 30. The observation of these peaks at 331 K along with a small m/z = 27 peak from either the HNC or HCN fragment of ethylamine suggests the molecular desorption of ethylamine, which was also observed for Ni(111).13 By analyzing the fragmentation pattern, mass 41 is confirmed to be from the desorption of acetonitrile at ∼340 K as the product of the partial dehydrogenation of ethylamine, which has also been observed at 350 K on Ni(111).13 Interestingly, acetonitrile desorption is also observed at 274 and 310 K, well before the molecular desorption of ethylamine. The mass 41 peaks are shifted to mass 42 following adsorption of 15N-ethylamine and occur with essentially the same peak temperatures and intensity ratios. For CD3CN at mass 44 formed from CD3CD2NH2, the small peak at 274 is absent, but the 310 and 340 K peaks seen for CH3CN are shifted to 317 and 352 K. Gardin and Somorjai observed a similar lower temperature peak for mass 41, but they assign it to a fragment of the ethylamine precursor.13 However, this is unlikely to be the case on Pt(111) as no corresponding

CCDND2, CCDNDH, and CCDNHD. The frequency assignments for deuterated aminovinylidene based on DFT calculations are presented in the Discussion section. 4.1.3. Aminoethynyl. Heating the sample to a higher temperature results in the decreased intensities of the peaks assigned to aminovinylidene and the appearance of a new set of peaks assigned to a second intermediate species that becomes dominant at 420 K. For regular and 15N-labeled ethylamine, the retention of peaks between 3300 and 3600 cm−1 indicates that N−H bonds are still present. In addition, the peak at 1591 cm−1 (shifted to 1578 cm−1 for the 15N species) falls within the range of an NH2 scissors mode. Upon 15N substitution, the peaks at 1422 and 1539 cm−1 are redshifted by 9 and 8 cm−1, respectively, indicating that they are due to modes that strongly involve the N atom. For the D-labeled species, the N−H and N−D bonds of the D-labeled aminovinylidene are preserved as evidenced by the 2460 and 3470 cm−1 peaks. In the lower frequency region, two prominent peaks are observed at 1474 and 1514 cm−1. However, as in the case of the D-labeled aminovinylidene, peak assignment of the D-labeled 420 K intermediate is nontrivial based only on the experimental spectra. Upon heating to temperatures higher than 500 K, all intermediate species have either desorbed or decomposed into surface species with no observable IR peaks. 4.1.4. H/D Exchange in the Formation of Aminovinylidene and Aminoethynyl. To investigate the effect of H/D exchange, we performed an experiment in which 4 L of D2 was coadsorbed with 0.3 L of CH3CH214NH2, and the sample was subsequently annealed to 330 and 420 K. The resulting RAIR spectra compared with the spectra of CH3CH214NH2 and CD3CD214NH2 annealed to the same temperatures are shown in Figure 2. After the 330 K anneal, the spectrum of coadsorbed D2 and ethylamine predominantly shows peaks characteristic of aminovinylidene produced by annealing CH3CH214NH2 to 330 K, namely, the peaks at 1283, 1463, 1601, and 3488 cm−1. However, there are additional weaker peaks at 1228, 1380, 4670



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methyl group as well as a C−H bond from the methylene group. H2 desorption at 340 K was also observed in the dehydrogenation of dimethylamine on Pt(111) resulting in the formation of methylaminocarbyne,6 while for ethylamine on Ni(111), it was observed at 380 K.13 In addition to the strong H2 peak at 340 K, there is another H2 desorption that appears as a shoulder at 380 K. For dimethylamine dehydrogenation on Pt(111), it was concluded that H2 desorption between 340 and 425 K is from the dissociation of the N−H bond of methylaminocarbyne.6 Since the NH2 group remains intact in the 420 K intermediate of ethylamine on Pt(111), the H2 desorption at 380 K is likely from C−H bond dissociation, which is supported by the fact that no C−H stretching peak is observed in the RAIR spectra after the sample was annealed to 420 K. H2 desorption peaks at higher temperatures, including the peak at 517 K and potentially another peak at ∼450 K, are likely related to the dehydrogenation of the 420 K intermediate species to HCN. Desorption of NH3 (m/z = 17) is observed at 337 K, and the analogous mass 18 from 15NH3 is also observed from 15Nlabeled ethylamine. Although CN bond dissociation from alkylamines has been observed on Ru(0001),37 Ni(111),13 W(100)-(5 × 1)-C,14,38 and Cr surfaces,39 the CN bond is thought to remain intact on Pt(111).2,6,7,40 In addition, the desorption of HCN above 500 K is strong evidence that the C−N bond remains intact even at higher temperatures. Therefore, it is unlikely that the NH3 is from C−N bond scission, and instead, it is possibly caused by an NH3 impurity in the ethylamine gas. In order to quantify the amount of NH3, we performed TPD of NH3 on Pt(111) at varying exposures and found that the amount of desorbed NH3 from ethylamine corresponds to less than 0.03 monolayers (see the Supporting Information). To determine the source of the desorbed ammonia, we carefully checked the purity of the CH3CH214NH2, CH3CH215NH2, and CD3CD214NH2 gases using mass spectrometry, and small amounts of ammonia were detected as an impurity in all three gases.

Figure 3. TPD data obtained after exposing Pt(111) to 0.3 L of CH3CH214NH2 at 85 K. The heating rate was 2 K/s.

peaks are observed at the same temperatures for masses 30 and 45. Acetonitrile has been observed to desorb from clean Pt(111) at ∼220 K for monolayer coverage.35 However, a coverage-dependent TPD study by Sexton and Avery has shown that, at very low coverages, CH3CN desorbs from clean Pt(111) with a single peak at 340 K.36 As the coverage increases, the acetonitrile monolayer adsorbs to clean Pt(111) in two different states, resulting in the shift of the 340 K peak to 310 K and the appearance of a low temperature desorption peak at 210 K.36 In our TPD data, we observed an acetonitrile desorption peak at 310 K, but the 274 K peak has a higher temperature than expected for the low-temperature desorption of acetonitrile from clean Pt(111), indicating that it could be a reaction-limited desorption. The peaks at 274 and 310 K could be due to an alternative decomposition pathway occurring at lower temperatures that does not result in the formation of stable surface intermediates. On the other hand, the 340 K peak occurs from the desorption of the aminovinylidene intermediate. The fact this peak has the same temperature as the desorption of a low coverage of acetonitrile from clean Pt(111) indicates that acetonitrile desorption from ethylamine dehydrogenation is a desorption-limited process. Mass 27 desorption above 500 K is likely from the desorption of either HCN or its less stable isomer HNC. RAIR spectra show that the surface species retains N−H bonds up to 420 K, suggesting that HNC may be the desorption product. However, HNC is unstable in the gas phase, and it may rapidly isomerize to HCN. As there is no experimental means to distinguish the two species, they are both interchangeably referred to as HCN. Since RAIR spectra show the formation of aminovinylidene (CCHNH2) between 315 and 340 K, the desorption of H2 (m/ z = 2) at 340 K can be ascribed to the dehydrogenation of the

5. DISCUSSION 5.1. Molecularly Adsorbed Ethylamine on Pt(111). Methylamine and ethylamine on Ni(111)13 as well as methylamine, dimethylamine, and trimethylamine on Pt(111)2,6,7 have all been reported to adsorb to the metal surface through the nitrogen lone pair, so this is likely to be the case also for ethylamine on Pt(111). As we have mentioned previously, ethylamine is molecularly adsorbed at 300 K, since the peaks found in the 85 and 300 K spectra can clearly be assigned to the same species. Gardin and Somorjai observed a peak at ∼3200 cm−1 assigned to an NH2 asymmetric stretching mode.13 However, experimental IR spectra of ethylamine dissolved in liquid Kr31 as well as results from simulations indicate that the intensity for this mode is very weak.31,34 In addition, a peak from the NH2 scissors mode, observed at 1540 cm−1 for ethylamine on Ni(111),13 is not observed in our spectra. This mode could be weak if the NH2 plane is oriented nearly parallel to the surface. Gardin and Somorjai proposed that ethylamine on Ni(111) is adsorbed in the trans conformation (i.e., C−C bond perpendicular to the surface with the methyl group pointing away from the surface) instead of the gauche conformation based on steric reasons as well as the argument that the CH3 rocking mode at 1140 cm−1 is closer to the value for gas-phase ethylamine in the trans conformation (1119 cm−1) than in the 4671



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gauche conformation (1016 cm−1).13 Although we also observe a CH3 rocking peak at 1145 cm−1 for ethylamine on Pt(111), the observed frequency shift of 4 cm−1 as a result of 15N substitution indicates that this mode is likely mixed with a C−N stretching mode, as Gardin and Somorjai also suggested,13 which is also supported by results from ab initio molecular orbital calculations.33 In addition, they speculated based on their HREEL spectra that the molecular adsorption of ethylamine on Ni(111) occurs only up to 270 K and that both C−H bonds of the methylene group are already dissociated at 300 K, resulting in a species bonded to the surface through the C atom.13 However, as clearly shown in Figure 1, the RAIR spectra of all isotopologues indicate that molecularly adsorbed ethylamine on Pt(111) remains stable up to 300 K. In fact, peaks that correspond to ethylamine molecules that remain on the surface after the 315 K anneal are similar to those at 85 and 300 K, only with reduced intensities due to the lower coverage of ethylamine. From Table 1, comparison between the experimental RAIR spectra of ethylamine on Pt(111) with the spectra of ethylamine in liquid krypton also shows better agreement with the trans compared to the gauche conformer. The bonding of ethylamine to the surface via the nitrogen lone pair would also involve too much steric hindrance for the gauche conformer. On the basis of these reasons, we also propose that molecular ethylamine is adsorbed on Pt(111) as the trans conformer. 5.2. Formation of Aminovinylidene at 330 K and Desorption of Acetonitrile. RAIR spectra show that the partial dehydrogenation of ethylamine to form aminovinylidene starts to occur at ∼315 K. At this temperature, the reduced intensity of the peaks assigned to ethylamine and the appearance of peaks due to aminovinylidene indicate the coexistence of both species on the surface (see the Supporting Information). At 330 K, aminovinylidene becomes the only adsorbed species on the Pt(111) surface. For the formation of aminovinylidene to occur, all methyl C−H bonds and one methylene C−H bond must be dissociated, leading to the H2 desorption peak at 340 K in the TPD data shown in Figure 3. On Ni(111), the formation of a CH3CNH intermediate with the C−N bond parallel to the surface was proposed to form through the dehydrogenation of the methylene group and one N−H bond.13 The TPD data presented in Figure 3 clearly shows the desorption of acetonitrile (m/z = 41) at 340 K after ethylamine is partially dehydrogenated. Acetonitrile (CH3CN) has been reported as the dehydrogenation product of ethylamine on Ni(111)13 and W(100)-(5 × 1)-C.14 From the literature, it is well known that metastable isomers of C2H3N such as ketene imine (H2CCNH) and ethynamine (HC≡CNH2), both of which are molecules of interest in the study of interstellar space, rapidly tautomerize to the more stable CH3CN.41−43 Therefore, it is extremely likely that aminovinylidene undergoes a similar tautomerization reaction on the surface to form the desorbing acetonitrile. As we have previously mentioned, the assignment of the 330 K intermediate to aminovinylidene is based on the excellent agreement with the results from DFT calculations, particularly with isotopic substitutions. In order to investigate the effect of different adsorption sites and size of the Pt cluster, we performed DFT calculations using the four aminovinylidene models shown in Figure 4. In all models, the Pt−Pt distances were fixed at 2.75 Å, while except for model 1B, the C, N, and

Figure 4. Side view (left) and top view (right) of aminovinylidene on four Pt surface models used in the DFT calculations. For all models, the Pt atoms were fixed with Pt−Pt distances of 2.75 Å. For models 1A, 1C, and 1D, the C, N, and H atoms were unconstrained and were allowed to optimize. For model 1B, the C−C bond was constrained so that it remained perpendicular to the surface.

H atoms were unconstrained and were allowed to optimize. In the simplest model 1A, the aminovinylidene species is bonded to two Pt atoms to simulate two-fold bridge-site bonding. In models 1B and 1C, aminovinylidene is bonded to the three-fold hollow site of the triangular face of a Pt4 tetrahedron. However, in model 1B, the C−C axis is constrained so that it remains perpendicular to the surface, while in model 1C, the optimized geometry results in an 11° tilt of the C−C bond from the surface normal. In model 1D, the aminovinylidene species is also bonded to a three-fold hollow site, but the Pt cluster is expanded to 19 atoms so that the 3 Pt atoms to which aminovinylidene is bonded are saturated with bonding to other Pt atoms to eliminate edge effects. In their DFT study of the chemisorption of CHx and C2Hy species on Pt(111), Jacob and Goddard suggested that a 35 atom Pt cluster was necessary to accurately describe all adsorption sites.44 However, the calculation of such a large Pt cluster is computationally expensive; therefore, we limited the size of our cluster to 19 Pt atoms to reduce the required computational time while still ensuring that the edge effect is avoided. The parameters for the optimized aminovinylidene of the four models are shown in Table 4. In general, C−C and C−N bond lengths are in between the values for single (1.53 Å for C−C and 1.46 Å for C−N) and double (1.34 Å for CC and 1.21 Å for CN) bonds,45 indicating that there is a 4672



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intensity ratios between peak 2 (1464 cm−1) and peak 3 (1601 cm−1), only models 1B and 1C can reproduce the experimental value well. In the NH2 stretch region, there are more discrepancies between the experimental and theoretical spectra. In particular, the separation between the symmetric and asymmetric NH2 stretch peaks is 54 cm−1 in the experimental spectrum, but the calculated values range from 116 to 216 cm−1. For the intensity ratio between peak 6 and peak 5, the values for models 1B and 1D are in good agreement with the experimental value, but those for models 1A and 1C are not. Both the frequencies and relative intensities of the NH2 stretching peaks seem to be strongly dependent on small variations in the geometry of the amine group, which in turn, is affected by the tilt of the C−C axis from the surface normal. Of course, other factors encountered in experimental conditions such as coverage and intermolecular interactions cannot be exactly replicated in the theoretical simulations. From this DFT study, it is clear that bonding at the three-fold hollow site gives better agreement with the experiment compared to bonding at the two-fold bridge site. In addition, by comparing models 1C and 1D, we can see that, by increasing the size of the Pt cluster, the relative intensities of peaks 5 and 6 are improved but the relative intensities of peaks 2 and 3 are worsened. Overall, all four models, including the simplest model 1A, can still reasonably reproduce the important features in the experimental spectrum. In our previous report,16 we briefly mentioned that the simulated spectrum of deuterated aminovinylidene was calculated from three species: Pt4CCDND2, Pt4CCDNDH, and Pt4CCDNHD (shown in Figure 6a). The latter two species have inequivalent positions for the N−H and N−D bonds and therefore different spectra. The absence of an NH2 scissors mode peak between 1550 and 1650 cm−1 precludes the inclusion of the Pt4CCDNH2 species. Figure 6b displays the simulated IR spectra of each of these species, the summed simulated spectrum, and the experimental spectrum of deuterated aminovinylidene. The optimized geometry from model 1C in Figure 4 was used because it is the least computationally demanding model that shows good agreement with the experimental spectrum. The frequency assignments of the calculated peaks compared to the experimental values are shown in Table 5. The peak at 3475 cm−1 in the experimental spectrum corresponds to the N−H stretch of species 2B while the 2565 cm−1 peak can be assigned to the N−D stretch of 2C. The N− H and N−D stretching peaks from 2C and 2B, respectively, are not observed in the experimental spectrum due to their very weak intensities. The symmetric and asymmetric ND 2 stretching modes of 2A give rise to calculated peaks at 2491 and 2640 cm−1, respectively, which correspond to the 2411 and 2609 cm −1 peaks in the experimental spectrum. The experimental peak at 1441 cm−1 is predominantly due to the asymmetric C−C−N stretching mode of 2A; however, it also contains contributions from a mixture of a C−C−N asymmetric stretch and an N−H bend of 2B. In the experimental spectrum, the C−D stretching peak is not observed probably because it has a weak intensity and/or it overlaps with the CO stretch peak from the background. In addition, the series of peaks below 1300 cm−1 in the calculated spectra is not observed experimentally due to the low signal-tonoise ratios in that region. The formation of the CCDNHD and CCDNDH isotopologues of aminovinylidene from CD3CD2NH2 would be

Table 4. Comparison between the Different Models (Shown in Figure 4) Used in DFT Simulations of Aminovinylidene with the Corresponding Experimental Valuesa model C−C bond length C−N bond length C−C tilt from surface normal position of peak 1 (cm−1) peak 1/peak 2 intensity ratio peak 2/peak 3 intensity ratio separation between peaks 5 and 6 (cm−1) peak 6/peak 5 intensity ratio a

1A

1B

1C

1D

expt

1.36 Å 1.38 Å 0° 1235 0.28 2.34 116

1.40 Å 1.36 Å 0° 1226 0.48 1.20 216

1.40 Å 1.36 Å 11° 1246 0.52 1.24 118

1.41 Å 1.32 Å 16° 1300 0.35 0.78 164

1283 0.41 1.33 54

0.46

2.31

0.49

3.27

4.85

Peaks numbers are defined in Table 3.

delocalization of π orbitals along the C−C−N unit. Figure 5 shows the simulated IR spectra of the four aminovinylidene

Figure 5. Experimental RAIR spectrum of aminovinylidene formed by heating 0.3 L CH3CH214NH2 on Pt(111) to 330 K (top) compared to the simulated IR spectra of aminovinylidene on the four Pt surface models pictured in Figure 4. For clarity, only calculated peaks that have corresponding experimental peaks are labeled.

models compared to the experimental spectrum. It is clear that, for all four models, the DFT simulations reproduced the frequencies of the 1463 and 1601 cm−1 peaks from experiment very well. However, there is a larger discrepancy between the calculated and experimental frequencies for the 1283 cm−1 peak, with the best agreement from model 1D with a 17 cm−1 frequency difference and the largest deviation from model 1B with a 57 cm−1 difference. The relative intensities of peak 1 (1283 cm−1) to peak 2 (1464 cm−1) of models 1B, 1C, and 1D are also in reasonably good agreement with the experimental value, although they are less so for model 1A. However, for the 4673



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Table 5. Comparison between Experimental and Calculated Vibrational Frequencies (cm−1) for Deuterated Aminovinylidene mode

speciesa

calcd

CCN asym stretch + N−H bend CCN asym stretch + N−H bend CCN asym stretch NDH bend + CCN asym stretch NHD bend + CCN asym stretch C−D stretch ND2 sym stretch N−D stretch

2C 2B 2A 2B 2C 2A, 2B, 2C 2A 2B 2C 2A 2C 2B

1362 1427 1435 1457 1491 2270 2491 2548 2576 2640 3500 3531

ND2 asym stretch N−H stretch

exptl 1362 1441 1462 1500 not observed 2411 not observed 2565 2609 not observed 3475

a

See Figure 6a for the aminovinylidene species used in the calculations.

not observed in Figure 2a after annealing to 330 K coadsorbed D2 and CH3CH2NH2 indicates that an ND2 group is not formed. From Figure 6b, it is evident that the strongest peaks from 2B and 2C are found at 1459 and 1492 cm−1, respectively, which correspond to the 1462 and 1500 cm−1 peaks in the experimental spectrum. However, the strongest H/D exchange peak in Figure 2a is observed at 1515 cm−1 with two weaker peaks at 1228 and 1380 cm−1. Figure 7 shows the simulated IR spectra of species 2B and 2C after the C−D bond has been

Figure 6. (a) Three species of deuterated aminovinylidene used in DFT calculations using the optimized geometry of model 1C in Figure 4. (b) Comparison between the experimental spectrum of deuterated aminovinylidene formed by heating 0.3 L CD3CD214NH2 on Pt(111) to 330 K with the simulated IR spectrum obtained by summing the spectra calculated from the three species shown in (a). The three individual simulated spectra (A−C) were given equal weights in the summation.

expected to result in the desorption of the CD2HCN isotopologue of acetonitrile. Mass 43 (CD2HCN) desorption was indeed observed in addition to mass 44 from CD3CN (see the Supporting Information); however, it is a relatively minor desorption product. The observation of CD3CN as the dominant desorption product indicates that further H/D exchange likely occurs on the surface as aminovinylidene is tautomerized to acetonitrile. This is supported by the observation that D2 desorption occurs at a slightly higher temperature (358 K) than acetonitrile desorption (352 K). As shown in Figure 2a, the coadsorption of D2 and CH3CH214NH2 results in the formation of nondeuterated aminovinylidene (CCH14NH2) and a smaller amount of the deuterated species upon annealing to 330 K. By comparing the frequencies of the peaks that arise from the H/D exchange with those in Table 5, we can determine that the N−D and N−H stretch peaks at 2564 and 3475 cm−1 correspond to species 2C and 2B, respectively, both of which have partially deuterated amine groups. The fact that peaks at 2411 and 2609 cm−1 are

Figure 7. Comparison between the experimental spectrum of aminovinylidene formed by heating 0.3 L CH3CH214NH2 coadsorbed with 4 L of D2 on Pt(111) to 330 K with the simulated IR spectrum obtained by summing the spectra calculated from (A) nondeuterated and (B, C) partially deuterated aminovinylidene. 70% of spectrum A, 15% of spectrum B, and 15% of spectrum C were added to reproduce the relative peak intensities of the experimental spectrum. For clarity, only the spectral region between 1000 and 1800 cm−1 is shown. 4674



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replaced with a C−H bond (i.e., Pt 4 CCHNDH and Pt4CCHNHD), compared with the experimental spectrum. The strongest peak from 2C, which corresponds to a mix of NHD bend and CCN asymmetric stretch, has been shifted to 1508 cm−1. Meanwhile, the strongest peak from 2B, which is assigned to a mix of NDH bend and CCN asymmetric stretch, is shifted to 1472 cm−1; this peak overlaps with the 1467 cm−1 peak from the calculated spectrum of regular aminovinylidene. In addition, there are small peaks at 1216 and 1376 cm−1, which agree with the weak peaks at 1228 and 1380 cm−1 in the experimental spectrum. By summing 70% of the spectrum from nondeuterated aminovinylidene and 15% each of Pt4CCHNDH and Pt4CCHNHD, the experimental spectrum (shown only for the region below 1800 cm−1) can be reproduced with excellent agreement. From this coadsorption experiment, we can conclude that the H/D exchange preferentially occurs at the amine group, which explains the observation of N−D bonds upon annealing CD3CD2NH2. However, it is interesting that a fully deuterated aminovinylidene (i.e., CCDND2) can be produced from the decomposition of CD3CD2NH2, but the coadsorption of D2 and CH3CH2NH2 can only result in partially deuterated aminovinylidene. H/D exchange of ethylamine on zeolite NaYsupported transition-metal catalysts has been studied using NMR and mass spectrometry, and it was concluded that the main product for a Pt/NaY catalyst is CH3CH2NHD,46 which supports our findings for the coadsorption experiment. The preferential H/D exchange of amine over alkyl groups on Pt catalysts has also been observed for diethylamine46 and aniline.47 On the other hand, H/D exchange of alkylamines on Ru catalysts has been found to occur at the methylene group.46 5.3. Formation of Aminoethynyl at 420 K. Gardin and Somorjai suggested based on their TPD and HREELS data that only atomic C and N remain on the surface after annealing ethylamine on Ni(111) to higher than 450 K.13 However, as shown in Figure 1, we found that annealing ethylamine on Pt(111) to 420 K results in the formation of a second stable surface intermediate. As we have mentioned previously, the NH2 group is still intact at this temperature as evidenced by the peaks between 3300 and 3600 cm−1 assigned to NH2 stretching modes and the peak at 1591 cm−1 due to the NH2 scissors mode. The observation of H2 desorption around 380 K indicates that the remaining aminovinylidene on the surface is further dehydrogenated, likely through the C−H bond. This is supported by the fact that no C−H stretch is observed in the RAIR spectra. In addition, the desorption of HCN above 500 K is evidence that the C−N bond is still intact. On the basis of these observations, we propose the structure shown in Figure 8a as the 420 K intermediate. This species is called aminoethynyl based on an analogous surface species, CCH, designated ethynyl by Jacob and Goddard.44 To achieve good agreement between the calculated and experimental spectra, the C−N bond length was constrained to 1.32 Å while the other parameters other than the Pt−Pt distances were allowed to optimize. This value for the C−N bond length and an optimized C−C bond length of 1.45 Å indicate that, similar to aminovinylidene, there is a delocalization of π orbitals across the CCN unit of aminoethynyl. The resulting simulated spectra for both the 14N- and 15N-labeled aminoethynyl are compared with the experimental spectra in Figure 8b. For the 14N-labeled species, the peaks at 1422 and 1591 cm−1 in the experimental spectrum are well reproduced by the calculated peaks at 1420

Figure 8. (a) Proposed structure of aminoethynyl as the 420 K intermediate species in the thermal decomposition of ethylamine on Pt(111). The C−N and C−C bond lengths are 1.32 Å and 1.45 Å, respectively. (b) Comparison between the experimental spectra of aminoethynyl formed by heating 0.3 L of CH3CH214NH2 (top panel) and CH3CH215NH2 (bottom panel) on Pt(111) to 420 K with the calculated spectra of Pt4CC14NH2 (top panel) and Pt4CC15NH2 (bottom panel) using the geometry shown in (a).

and 1600 cm−1, which are assigned to a C−N stretch and an NH2 scissors mode, respectively. In the NH2 stretch region, the calculated peaks at 3400 and 3516 cm−1, the latter of which has a very low intensity, correspond to the experimental peaks at 3435 and 3505 cm−1, assigned to the NH2 symmetric and asymmetric stretching modes, respectively. The frequency assignments and comparison between experimental and calculated frequencies for the 14N and 15N species of aminoethynyl are summarized in Table 6, which shows that there is good agreement between the experimental and calculated values of the frequency shifts as a result of 15N substitution. The calculated spectra of the three species of deuterated aminoethynyl (Pt4CCND2, Pt4CCNDH, and Pt4CCNHD, shown in Figure 9a) and the summed calculated spectrum are compared with the experimental spectrum of the 420 K intermediate of CD3CD2NH2 in Figure 9b. The most intense peak in the experimental spectrum is observed at 1474 cm−1, which corresponds to the calculated peak at 1452 cm−1 assigned to the C−N stretch from 3A. This species also gives 4675



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experimental spectrum. The 1515 cm −1 peak in the experimental spectrum correlates to the 1492 and 1494 cm−1 peaks assigned to the C−N stretching modes from 3B and 3C, respectively. 3B and 3C give nearly identical spectra below 1800 cm−1, but the relative intensities of the ND and NH stretch peaks are reversed due to the inequivalent positions of the H and D atoms. Since the experimental spectrum shows that the 1515 cm−1 peak has 40% of the intensity of the 1474 cm−1 peak, we can conclude that the fully deuterated CCND2 species is the main product. Therefore, the summed spectrum shown in Figure 9b was generated by adding 70% of Pt4CCND2 and 15% each of Pt4CCNDH and Pt4CCNHD. The frequency assignments for the experimental and calculated peaks are shown in Table 7. For the 420 K intermediate from

Table 6. Experimental and Calculated Vibrational Frequencies (cm−1) for the 14N- and 15N-Labeled Species of Aminoethynyl mode C−N stretch expt theory NH2 scissors expt theory NH2 sym stretch expt theory NH2 asym stretch expt theory

CC14NH2

CC15NH2

shift

1422 1420

1413 1412

−9 −8

1591 1600

1578 1588

−13 −12

3435 3400

3427 3394

−8 −6

3505 3516

3494 3504

−11 −12

Table 7. Frequency (cm−1) Assignments for Deuterated Aminoethynyl from DFT Calculations and Experiment mode

speciesa

calcd

exptl

NDH bend + C−N stretch NHD bend + C−N stretch C−N stretch

3B 3C 3A 3B 3C 3A 3C 3B 3A 3B 3C

1372 1376 1452 1492 1494 2460 2512 2540 2604 3444 3480

1370

ND2 sym stretch N−D stretch ND2 asym stretch N−H stretch a

1474 1515 2460 2561 2629 3433 3470

See Figure 9a for the aminoethynyl species used in the calculations.

the coadsorption of D2 and CH3CH214NH2, shown in Figure 2b, the peaks that arise from the H/D exchange are observed at 1515, 2560, and 3470 cm−1. Comparison with Table 7 reveals that these peaks can easily be assigned to the CCNDH and CCNHD species. This is consistent with the assignment of the partially deuterated aminovinylidene species described in the previous section. Aminoethynyl contains a CNH2 moiety found both in aminocarbyne, formed on Pt(111) through the dehydrogenation of methylamine and azomethane2 and the hydrogenation of surface CN,3 and in diaminoethylene (H2NCCNH2), formed through the hydrogenation of cyanogen (C2N2) on Pt(111).48,49 The RAIR spectrum of diaminoethylene has a C− N stretch peak at 1425 cm−1 and an NH2 scissors mode peak at 1600 cm−1, which are comparable to the corresponding peaks at 1422 and 1591 cm−1 for aminoethynyl shown in Figure 8. There are two weaker peaks at 1539 and 3366 cm−1 in Figure 8, which are redshifted by, respectively, 8 and 7 cm−1 upon 15N substitution. These cannot also belong to CCNH2 and so must be due to a different intermediate. The 1539 cm−1 peak can reasonably be assigned to an NH2 scissors mode. It is notable that the 3366 cm−1 peak is close to the symmetric NH2 stretch peak of aminocarbyne observed at 3363 cm−1. Therefore, we tentatively assign the 1539 and 3366 cm −1 peaks to aminocarbyne. The fact that the NH2 scissors peak at 1539 cm−1 has a lower frequency than the previously published value for aminocarbyne at 1566 cm−1 could be due to intermolecular interactions with adjacently adsorbed aminoethynyl. In addition, although a C−N stretch peak should be observed at 1323 cm−1 for aminocarbyne, it is weak compared to the NH2 scissors mode and therefore would not be expected to be seen

Figure 9. (a) Three species of deuterated aminoethynyl used in DFT calculations using the optimized geometry shown in Figure 8a. (b) Comparison between the experimental spectrum of deuterated aminoethynyl formed by heating 0.3 L CD3CD214NH2/Pt(111) to 420 K with the simulated IR spectrum obtained by summing the spectra calculated from the three species shown in (a). 70% of spectrum A, 15% of spectrum B, and 15% of spectrum C were added to reproduce the relative peak intensities of the experimental spectrum.

an ND2 symmetric stretch peak at 2460 cm−1 and a very weak ND2 asymmetric stretch peak at 2604 cm−1, which correspond to the 2460 and 2629 cm−1 peaks, respectively, in the 4676



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C axis perpendicular to the surface and delocalized π orbitals across its CCN unit, is analogous to the delocalized π orbitals across the CNC unit of methylaminocarbyne, which is formed through the dehydrogenation of dimethylamine6 or hydrogenation of methyl isocyanide4,5 on Pt(111). At 340 K, aminovinylidene tautomerizes and desorbs as acetonitrile, a well-known dehydrogenation product of ethylamine. Starting at 360 K, the remaining aminovinylidene on the surface is further dehydrogenated to form aminoethynyl (CCNH2), which becomes the dominant species at 420 K. Aminoethynyl is bonded to the surface through both C atoms and it contains a CNH2 moiety, similar to aminocarbyne formed through the dehydrogenation of methylamine and hydrogenation of surface CN on Pt(111). Upon heating to higher temperatures, aminoethynyl undergoes a further decomposition process in which the C−C bond and one N−H bond are dissociated, to desorb either HCN or HNC. For the deuterated species (shown in Figure 10b), a similar thermal decomposition mechanism occurs. However, the D atoms from the C−D dissociation exchange with one or both H atoms of the amine group, creating a mixed deuterated aminovinylidene species containing ND2, NDH, and NHD groups, with the latter two giving different RAIR spectra due to the inequivalent positions of the H and D atoms. Above 400 K, the completely deuterated aminoethynyl CCND2 becomes the dominant species with smaller fractions of the partially deuterated CCNHD and CCNDH species. The preferential H/D exchange at the amine group is observed by coadsorbing D2 and CH3CH214NH2, in which the formation of a partially deuterated aminovinylidene and aminoethynyl with an NHD/ NDH group is observed while the C atom remains nondeuterated.

here. For deuterated aminocarbyne (CND2), the most intense peak is the CN stretch peak at 1358 cm−1. This peak is not observed in the spectrum of CD3CD214NH2 after the 420 K anneal because presumably it is below the noise level in that region. In Figure 1a, the RAIR spectrum obtained after the 330 K anneal contains a weak N−H stretch peak at 3342 cm−1, shifted by 8 cm−1 upon 15N substitution, which cannot be assigned to aminovinylidene. This peak is notably close to the N−H stretch peak at 3347 cm−1, also shifted by 8 cm−1 upon 15N substitution, assigned to surface HNC, which was suggested to form at 200−250 K as an intermediate in the formation of aminocarbyne from HCN on Pt(111).1 Therefore, we propose that there is a minor ethylamine decomposition channel in which surface HNC is formed at ∼330 K, followed by the formation of aminocarbyne at ∼420 K. At higher temperatures, both aminocarbyne and aminoethynyl decompose to HCN. The observed 5 cm−1 frequency difference between the N−H stretch peak of HNC in this study with that in the previous study as well as the differences in the temperatures at which HNC and CNH2 are formed could be due to the effect of different coadsorbates.

6. CONCLUSIONS The proposed mechanism for the thermal decomposition of ethylamine on Pt(111) is illustrated in Figure 10. Ethylamine is

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ASSOCIATED CONTENT

S Supporting Information *

RAIR spectra of CH3CH214NH2 and CH3CH215NH2 at 85 K and after annealing to 300, 315, 330, 360, 420, 450, and 500 K; RAIR spectra of CD3CD214NH2 at 85 K and after annealing to 300, 320, 330, 370, 420, 450, and 500 K; TPD data of CH3CH215NH2 and CD3CD214NH2; TPD of coverage-dependent NH3; TPD data of masses 43 and 44 from CD3CD214NH2; and simulated IR spectrum of unconstrained aminoethynyl. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (312) 996-0777; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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Figure 10. Proposed scheme for the thermal decomposition of (a) ethylamine and (b) deuterated ethylamine on Pt(111). Only the major surface species formed in the decomposition are shown.

ACKNOWLEDGMENTS We thank Matthew J. O’Connor and Prof. Daesung Lee for their help with the synthesis of CH3CH215NH2. This work was supported by a grant from the National Science Foundation (CHE-1012201). This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant number OCI-1053575.

adsorbed molecularly as the trans conformer on Pt(111) through the nitrogen lone pair from 85 to 300 K. Starting at 315 K, the C−H bonds of the methyl and methylene groups are activated, and ethylamine is partially dehydrogenated to form aminovinylidene (CCHNH2), which becomes the major surface species at 330 K. The structure of aminovinylidene, with its C− 4677



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