Spectroscopic Identification of Surface Intermediates in the

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Spectroscopic Identification of Surface Intermediates in the Decomposition of Methylamine on Ru(001) Yuan D. Ren, Dominic A. Esan, Iradwikanari Waluyo, Joel D Krooswyk, and Michael Trenary J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b02092 • Publication Date (Web): 13 Apr 2017 Downloaded from http://pubs.acs.org on April 15, 2017

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The Journal of Physical Chemistry

Spectroscopic Identification of Surface Intermediates in the Decomposition of Methylamine on Ru(001)

Yuan Ren, Dominic Esan, Iradwikanari Waluyo, Joel Krooswyk, and Michael Trenary*

Department of Chemistry, University of Illinois at Chicago, 854 West Taylor Street, Chicago, IL 60607-7061, USA *E-mail: [email protected]

Abstract The thermal decomposition of methylamine on Ru(001) was studied with reflection absorption infrared spectroscopy (RAIRS) and temperature programmed reaction (TPR). After the multilayer methylamine desorbs at 150 K, the RAIR spectra of the remaining monolayer methylamine undergo small changes due to structural rearrangements but do not indicate any chemical changes until 250 K. The results are in agreement with recent theoretical investigations indicating that CH3 dehydrogenation to produce HxCNH2 species occurs before N-H and C-N bond scission. Experimental spectra of 13C- and 15N-substituted methylamine combined with DFT calculations of HxCNHy fragments attached to a Ru19 cluster to simulate the RAIR spectra, including isotopic shifts, clarified the spectral assignments.

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1. Introduction The surface chemistry of amines is relevant to several important technologies. For example, it is related to the challenge of removing nitrogen-containing compounds from petroleum feedstocks in a catalytic process known as hydrodenitrogenation.1 Organic amines are also used in corrosion prevention and for adhesion.2 Knowledge of the chemical interaction between organic amines and metal surfaces can facilitate understanding of the fundamental reactions underlying these applications. Methylamine is the simplest organic amine and is therefore a prototype for more complicated amines.3 Despite its relative simplicity, the decomposition pathway can begin with the breaking of a C-N, C-H, or N-H bond, which have dissociation energies of 356.1 ± 2.1, 392.9 ± 8.4, and 425 ± 8.4 kJ/mol, respectively.4 Experimental identification of the surface intermediates produced following the first bond-scission step requires use of a technique with a high degree of molecular specificity. Even then, unambiguous identification of surface intermediates can be difficult based on experimental evidence alone and often requires additional information that can only be obtained from theoretical calculations. As many of the elementary steps in decomposition reactions are the reverse of coupling steps, an understanding of the surface chemistry of methylamine can provide a better understanding of C-N coupling reactions that are related to a variety of heterogeneous catalytic processes such as the catalytic synthesis of HCN and amines.5 Methylamine has been studied extensively with a variety of surface techniques on different metal surfaces, such as Ni(111)6-10 and Ni(100)9-10, Pt(111)11-12 and Pt(100)13, Fe14, Cr(100) and Cr(111)9, Pd(111)15, Rh(111)16, W(100)17-18, Mo(100)19, and Ru(001)20-23. On Ni(111), Gardin et al.6 reported that methylamine dehydrogenates to form adsorbed HCN, which further 2 ACS Paragon Plus Environment

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decomposes without desorption to produce C and N adatoms. On Pt(111), Jentz et al.24 reported that C-H dehydrogenation occurs to form CNH2 and that the C-N bond remains intact as all CN is eventually removed from the surface through desorption of either HCN or C2N2. In contrast, on Pt(100) Thomas et al.13 found that some C-N bond breaking occurs. On Pd(111), Chen et al.15 reported that N-H dehydrogenation is favored and that the C-N bond does not break. On the Si(100)-2×1 surface, Mui and coworkers found that the reaction pathway resembles the precursor-mediated dissociative adsorption of ammonia and that N-H dehydrogenation is favored over N-C bond cleavage.25 Although the latter is thermodynamically favored, the former has a significantly lower activation barrier, which is suggested as the reason for the selectivity.25 In addition to the many previous experimental studies, theoretical studies of the adsorption and reactions of methylamine have been performed for Si(100)25-26, Ni(111)27-28, Mo(100)29, and Pd(111)30. On the Ni(111) surface, Olivia et al.27 used density functional theory (DFT) calculations to show that C–H cleavage is slightly preferred over N–H cleavage, which confirms the experimental findings. On Mo (100), Lv et al.29 not only found that coadsorbed C, N, and O atoms reduce the reactivity of the Mo surface, but also proposed a reaction mechanism featuring C-H cleavage followed by C-N cleavage. On Ru(001), thermal decomposition of methylamine has been studied using high resolution electron energy loss spectroscopy (HREELS) and temperature programmed reaction spectroscopy (TPRS).20-21, 23 Johnson et al.20 reported that methylamine decomposition is initially activated by the dehydrogenation of the methyl group, as indicated by assigning HREELS peaks to µ-η2-H2CNH2. They further concluded that most of the µ-η2-H2CNH2 dehydrogenated to form η1-(C)-HCNH2, and µ-CNH2 intermediates. Further annealing caused dehydrogenation of the amino group of HCNH2 and CNH2 to form µ3-η2-HCNH, µ-CNH and η1-(C)-CNH, which 3 ACS Paragon Plus Environment


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further dehydrogenated to µ3-η2-CN. The CN finally undergoes C-N cleavage to form C and N adatoms, with the latter recombining and desorbing as N2 to leave only C on the surface. They also concluded that µ-η2-H2CNH2 can follow a minor pathway in which the C-N bond breaks to form short-lived CH2 and NH2 intermediates, which eventually convert to CH and NH3. However, the resolution of HREELS is not nearly high enough to clearly distinguish between the many proposed surface intermediates. Additional motivation for the present work is provided by the recent DFT study of Lv et al. in which a detailed decomposition pathway for methylamine on Ru(001) was proposed.31 Their calculations generally support the conclusions reached by Johnson et al.,20 except that they found C-N bond scission to occur in the HCNH2 and HCNH species, rather than in H2CNH2 and CN. The present study goes beyond previous work in exploiting the higher resolution of RAIRS to resolve closely spaced peaks and to also measure small spectral shifts associated with 13C and 15

N substitution. We have also repeated the DFT calculations of Lv et al.31 , but have used the

calculated results to simulate the vibrational spectra of possible intermediates for direct comparison to our experimental RAIR spectra. All evidence indicates that methylamine adsorbs at low temperatures on Ru(001) as depicted in Figure 1 through the nitrogen lone pair such that it retains a plane of symmetry perpendicular to the surface. This means that just as for the gas phase molecule, all the normal modes can be classified as either symmetric (A′) or asymmetric (A″) with respect to the plane. However, because the surface spectra lack rotational fine structure, they are simpler than the gas phase spectra, and the peaks are sharper than for the solid and liquid phases. The unique nature of the RAIR spectra provides an opportunity for more thorough characterization of adsorbed methylamine through analysis of its high resolution vibrational spectrum. For the decomposition process, comparison of measured and calculated shifts upon 13C 4 ACS Paragon Plus Environment

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and 15N substitution allows δ(NH2), δ(CH3), and ν(CN) modes of hypothesized intermediates to be distinguished. In previous studies we have used such comparisons to definitively identify CNH2 as a stable intermediate on Pt(111).32 Using the same approach here, we find that the assignment by Johnson et al.20 of an HREELS peak at 1620 cm–1 to ν(CN) of CNH2 is better assigned to δ(NH2) and that like CNH2 on Pt(111), the CN bond of CNH2 on Ru(001) has a bond order between one and two. 2. Experimental The experiments were performed in an ultra-high vacuum (UHV) chamber with a base pressure of 1×10-10 Torr. The chamber is equipped with PHI 15-120 low energy electron diffraction (LEED) optics, a PHI 10-155 cylindrical mirror analyzer for Auger electron spectroscopy (AES), and a Hiden HAL201/3F quadrupole mass spectrometer for temperatureprogrammed reaction spectroscopy (TPRS). The chamber is 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 IR source. An LN2 -cooled InSb detector with a low wavenumber cutoff of 2100 cm–1 and tungsten IR source were used to obtain spectra with better signal-to-noise ratios at higher frequencies. Each RAIR spectrum was obtained with 1024 scans and 4 cm–1 resolution. The sample mounting and cleaning procedures are described in detail elsewhere.33 Methylamine (H3CNH2) was dosed onto the surface by backfilling the chamber. After dosing, the annealing experiments were performed once the chamber pressure had decreased to 2×10-10 Torr. Methylamine hydrochloride (H3CNH2·HCl) was purchased from TCI America. 15N-labeled 5 ACS Paragon Plus Environment


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methylamine hydrochloride (H3C15NH2·HCl) and 13C-labeled methylamine hydrochloride (H313CNH2·HCl) were purchased from Cambridge Isotope Laboratories. Methylamine (H3CNH2), 15

N-labeled methylamine (H3C15NH2), and 13C-labeled methylamine (H313CNH2) were prepared

from the hydrochlorides according to the method described by Szammer et al.34 and purified by several freeze-pump-thaw cycles before introduction into the chamber. The purity of all gases was checked periodically by mass spectrometry. 3. Computational details The theoretical calculations, including geometry optimization and vibrational frequency analysis, were performed using the Gaussian 0935 DFT package with the B3LYP hybrid functional.36-41 We used the same method as described in Waluyo et al.42 The C, N, and H atoms were described with a 6-311G(d,p) basis set, while the core electrons of the Ru atoms were represented by the Hay-Wadt pseudopotential and the valence shells were described using a double-zeta basis set.43 The calculated vibrational frequencies were scaled by a factor of 0.9682,44 partially to account for the absence of anharmonicity in the calculations. Lorentzian broadening was used to simulate the 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, with the z direction defined as perpendicular to the surface in accordance with the surface selection rule of RAIRS, and the full-width at half-maximum (FWHM) is set as 15 cm–1. The GaussView 5.0.8 visualization program45 was used to visualize normal modes for frequency assignments. The DFT calculations were performed for H3CNH2, H2CNH2, H3CNH, HCNH2, H2CNH, CNH2, and HCNH attached to a Ru19 cluster with the Ru-Ru distances fixed at 2.77 Å, while the C, N, and H atoms were unconstrained and were allowed to optimize. The optimized geometric 6 ACS Paragon Plus Environment

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parameters are presented in the Supporting Information as Table S1, which generally agrees with the results of Lv et al.31 4. Results 4.1. TPRS Figure 2 shows TPR results following 1.0 L exposures of H312C14NH2 (left panel) and H313C14NH2 (right panel) to Ru(001) at 90 K. Although the parent mass of methylamine (H312C14NH2 ) is 31 amu, its strongest signal is for the H2CNH2 fragment at mass 30, which is therefore used to monitor methylamine desorption (and m/z = 31 was used to monitor H313CNH2). Our TPR spectra generally agree with the previously reported results of Johnson et al.20 A peak at 755 K for m/z = 28 matches that for N (m/z = 14), indicating that it is due to recombinative desorption of N2. Furthermore, the absence of CH4 (m/z = 16) desorption indicates that CHx from C-N bond scission stays on the surface instead of desorbing as CH4. The 129 K desorption peak for methylamine (m/z = 30) and its fragments is attributed to multilayer desorption, indicating that a 1.0 L exposure is more than sufficient to saturate the monolayer. There are weak desorption features ranging from 200 to 320 K for m/z = 30 and 28 due to the H2CNH2 and CNH2 fragments of methylamine, which reveal that undissociated methylamine can be present on the surface up to these temperatures. The m/z = 28 peak at 455 K is due to desorption of CO that had adsorbed from the background. The desorption of H2 (m/z = 2) shows two distinct peaks around 330 and 360 K. Studies of hydrogen desorption from clean Ru(001) shows that H2 desorbs over a broad range, depending on coverage, from 250 to 470 K46 and at the highest hydrogen coverages consists of a main peak at 370 and a shoulder at ~ 420 K.47 Thus both H2 peaks observed here are consistent with desorption-limited kinetics. The reason we can resolve two peaks may be due to our lower heating rate of 2 K/s, whereas Feulner 7 ACS Paragon Plus Environment


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et al. used 20 K/s. Johnson et al.20 used a heating rate of 15 K/s, and their H2 TPD results for methylamine on Ru(001) displayed a peak shape suggesting unresolved components at ~ 340 and ~ 380 K. The cleavage of the C-N bond must occur to produce the NH3 (m/z = 17) that desorbs at 339 K and the absence of other CN-containing molecules, such as HCN (m/z= 27) and C2N2 (m/z =52), indicates the complete dissociation of the C-N bond of the dehydrogenation products of methylamine. Our TPR results are in agreement with those of Johnson et al.,20 although small differences in peak shapes and temperatures can be attributed to the differences in coverage and heating rates. Therefore we assume that the conclusions that they reached from analysis of their data for both H3CNH2 and H3CND2 can be used to interpret our RAIRS results. From relative areas of the TPRS peaks for hydrogen and ammonia for a saturation coverage of methylamine, they estimated that the coverage of irreversibly adsorbed methylamine was only 0.15 ML, indicating that a substantial fraction of the initially adsorbed molecules desorbed without dissociation. For low coverages of methylamine, desorption of the parent molecule was observed at 330 K, while for higher coverages a broader desorption profile was observed with an additional peak at 240250 K. The observations of ammonia desorption in the range of 300 to 400 K and hydrogen desorption up to temperatures of 460 K, indicates that dehydrogenation and C-N bond scission occur after desorption of undissociated methylamine. This then implies that RAIRS peaks associated with undissociated methylamine should be expected for temperatures of 300 K or lower. As TPRS doesn’t provide information on reactions at temperatures below where products desorb, the RAIR spectra need to be scrutinized for the appearance of peaks associated with the products of methylamine dissociation as well as for those of undissociated methylamine. 4.2. RAIRS of Adsorbed Methylamine 8 ACS Paragon Plus Environment

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Figure 3 shows RAIR spectra following exposure at 90 K to 5.0 L of H312C14NH2, H313C14NH2, and H312C15NH2, respectively, and after heating to the indicated temperatures, holding at those temperatures for 60 s, and then cooling back to 90 K where the spectra were acquired. The spectrum at 90 K is characteristic of the multilayer. Annealing to 220 K leads to desorption of the multilayer, which causes some peaks to disappear and the remaining ones to become sharper, indicating a more ordered monolayer structure. In particular, the two N-H stretch peaks at 3197 and 3291 cm–1 in the 90 K spectrum are replaced by a single N-H stretch at 3244 cm–1 and the loss of the δ(NH2) peak at 1616 cm–1. Based on the TPRS results, it can be assumed that the spectra for annealing temperatures up to 220 K correspond to intact methylamine molecules and that the peaks can be assigned based on the literature for the vibrational spectra of methylamine in the gas, liquid, and solid phases. Methylamine has 15 normal modes and in the vibrational spectroscopy literature the corresponding fundamentals are numbered ν1 to ν9 for those of Aʹ symmetry, and ν10 to ν15 for the Aʺ vibrations. These are listed in Table 1 along with the peaks positions of the multilayer (90 K) spectrum in Figure 3. The actual spectra observed for methylamine in the gas, liquid and solid phases contain many more peaks than the fundamentals and the exact assignments have been extensively discussed in the literature. In considering the RAIR spectra of H3CNH2, we focus mainly on the N-H and C-H stretch regions. The presence of two N-H stretch peaks for the multilayer indicates that the molecules are randomly oriented with respect to the surface normal so that both νsymm(NH2) (ν1)(3197 cm–1) and νasymm(NH2) (ν10 ) (3291 cm–1) are allowed by the surface dipole selection rule. Assignment of the two N-H stretches to the ν1 and ν10 fundamentals follows the assignment of Johnson et al.,20 but in the study of solid methylamine, Durig et al.48 assigned IR peaks at 3260 and 3191 9 ACS Paragon Plus Environment


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cm–1 to the ν1 fundamental of non-hydrogen bonded and hydrogen bonded N-H bonds, respectively, an assignment that is a better match to our observed peaks at 3291 and 3197 cm–1. In this way, the multilayer RAIR spectra are closer to the IR spectra of solid methylamine than implied by Table 1. After desorption of multilayer molecules, the spectra indicate that the remaining ones bond to the surface as shown in Figure 1 making only ν1 (the symmetric NH2 stretch) surface IR allowed. Furthermore, hydrogen bonding for the multilayer causes a red-shift of ν1 (3197 cm–1) relative to both its value in the gas phase (3361 cm–1) and its value (3244 cm–1) when bonded to the surface. Although ν1 for the monolayer appears as a single sharp peak for H312C14NH2, and H313C14NH2, it clearly consists of two components for H312C15NH2. The origin of these two components may be analogous to what is seen in liquid and solid methylamine where the overtone of ν4 (δ(NH2)) is in Fermi resonance with ν1. Wolff has assigned peaks at 3290 and 3207 cm–1 in liquid methylamine to ν1 and 2ν4 of hydrogen-bonded (associated) molecules, whereas in crystalline methylamine, he has assigned peaks at 3260 and 3200 cm–1 to 2ν4 and ν1, respectively, without explanation for the reversed assignments.49 In condensed phases, the presence of hydrogen-bonding can greatly complicate assignment of vibrational peaks, and distinctly different N-H bonds can be identified depending on whether or not they participate in hydrogen bonding to neighboring molecules. With methylamine adsorbed as shown in Figure 1, hydrogen bonding between two molecules can be ruled out and the red shift of ν1 from its value in the gas phase must be attributable entirely to bonding to the surface. We further assume that for adsorbed methylamine, ν1 and 2ν4 also form a Fermi resonance pair; the two components are presumably too close to be distinguished for H312C14NH2 and H313C14NH2, but slightly different isotopic shifts for ν1 and 2ν4 in H312C15NH2 allow separate peaks to be resolved.


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In the C-H stretch region, more peaks are observed than the three fundamentals. However, this is not only a feature of the adsorbed molecule as eight C-H stretch peaks are reported for methylamine in the solid state and isolated in an Ar matrix.48 The extra peaks originate both from intermolecular (factor group splitting) and intramolecular (overtones and combinations enhanced through Fermi resonances) interactions. Wolff lists four peaks in the C-H stretch region for crystalline methylamine, with the two lowest frequency ones at 2879 and 2800 cm–1 due to a Fermi resonance between 2ν6 and ν3, where ν6 is the symmetric CH3 deformation and ν3 the symmetric CH3 stretch.49 Wolff also disputes the assignments of Durig et al.48 of the two higher frequency peaks at 2961 and 2942, which he assigns instead to the ν11 and ν2 fundamentals. Although we follow Wolff’s assignments in Table 1, the ν11 fundamental should not be symmetry allowed for the assumed adsorption geometry of Figure 1. If we assume that the strongest C-H stretch peak, seen at 2911 cm–1 for H312C14NH2 and H312C15NH2 in the 220 K spectrum of Figure 3, corresponds to a fundamental, it must be assigned to ν2, with the assumption that it has been red-shifted relative to its value in the solid through its interaction with the Ru surface. A similar redshift of ν11 from 2961 in the solid to 2942 cm–1 for the molecule chemisorbed on the surface is assumed to occur. The value of 2784 cm–1 is notably low for a CH3 symmetric stretch, but such low C-H stretch values are an intrinsic feature of certain molecules, including amines, and are known as Bohlmann bands. McKean and Ellis have considered this issue in detail by analyzing methylamines containing HD2C groups, where in the case of HD2CNH2 two C-H stretches were observed, associated with weak and strong C-H bonds, with the weak C-H bond corresponding to a molecular configuration where it is trans to the nitrogen lone pair.50 The fact that such a weakened C-H stretch occurs for the molecule chemisorbed on the Ru(001) surface indicates that 11 ACS Paragon Plus Environment


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the intramolecular interaction between the lone pair and the trans C-H bond is not affected much by bonding to the surface via the lone pair. With the C-H bond trans to the nitrogen lone pair (the C-H bond drawn in the plane of the page in Figure 1), it would be directed away from the metal surface so that this would be opposite to cases where C-H bonds are softened through their interaction with a metal surface.51 As in the C-H stretch region, the 220 K anneal leads to sharper and better resolved peaks in other spectral regions. The broad feature with maximum intensity at 1007 cm–1 in the 90 K spectrum evolves into three distinct peaks at 970, 991, and 1008 cm–1, while the peak at 1045 cm–1 disappears. Durig et al. assigned peaks in this region to two fundamentals: ν8, the C-N stretch, and ν9, the NH2 wag. In the gas, liquid, and solid phases, ν8 occurs at 1040, 1048, and 1051 cm–1, while ν9 has a wider range of 875, 955, and 795 cm–1, respectively. The high sensitivity of ν9 to the methylamine phase implies that ν9 for the adsorbed molecule might be quite different from that of other phases. Furthermore, for solid methylamine, Durig et al.48 list not only fundamentals, but additional peaks at 1005, 998, and 913 cm–1, with the 1005 cm–1 peak assigned to 2ν15, the first overtone of the intramolecular torsional mode, and the other two peaks unassigned. The peaks at 1182 and 1161 cm–1 match the ν7 and ν14 fundamentals at 1182 and 1172 cm–1 in solid methylamine, although the latter would not be allowed for the assumed geometry as it is of Aʺ symmetry. 4.3. RAIRS of Methylamine Decomposition After the 250 K anneal, the RAIRS peaks associated with adsorbed methylamine decrease in intensity and decrease even further after the 300 K anneal. While the N-H stretch peak is still visible at 3246 cm–1 for the 250 K anneal, it is not visible in the 300 K spectrum. No RAIRS peaks were detectable for annealing temperatures above 300 K. However, the TPRS 12 ACS Paragon Plus Environment

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results of Figure 2 showing H2, NH3, and N2 desorption above 300 K reveal that intermediates must remain on the surface. As is often the case with RAIRS, it is common for intermediates to have no visible peaks above 900 cm–1. There are several indications from the spectra for annealing temperatures of 300 K and below that methylamine decomposition produces one or more additional surfaces species. This is most apparent in the spectral range of 1400-1700 cm–1. In Figure 3, the peak that appears at 1474 cm–1 in the 220 K spectrum is replaced by peaks at 1450 and 1500 cm–1 at 250 K for H312C14NH2. With the 300 K anneal the 1450 cm–1 peak remains, the 1500 cm–1 peak shifts to 1552, and a new peak appears at 1658 cm–1. The intensities of these peaks relative to the C–H stretch peaks grows with annealing temperature such that the 1552 cm–1 peak is the largest one in the 300 K spectrum. Vibrations of several different functional groups can give rise to vibrations in this region and comparison of measured and calculated isotopic shifts can help in spectral assignments. The lack of a shift of the 1658 cm–1 peak with 13C substitution, but to 1654 cm–1 with 15N substitution, makes assignment to a δ(NH2) scissors mode most reasonable, although the frequency is at the upper range expected for such a mode. The 1500 cm–1 peak in the 250 K spectra of Figure 3 shifts to 1495 cm–1 with 13C substitution, but to 1492 cm–1 with 15N substitution, implying that it is a mode with both C–H and N–H bend character. This is in marked contrast to the 1474 cm–1 peak in the 220 K spectra of Figure 3, which shifts to 1472 with 13C, but undergoes no shift with 15N substitution, confirming its assignment to the δas(CH3) mode of methylamine. Somewhat different isotopic shifts are seen for the 1552 cm–1 peak for the 300 K spectra, which shifts to 1550 cm–1 with 13C, but to 1545 with 15N substitution, indicating that the mode consists mainly of N–H bending character. All of the peaks are in the range expected for a C=N stretch, but none show the large shifts for both 13C and 15N substitution expected for such a mode.



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Further insights can be gained through comparisons with simulated spectra based on DFT calculations of hypothesized intermediates attached to a Ru19 cluster. To assess the extent to which calculated frequencies and isotopic shifts match the experimental values, results for a known species, methylamine, are compared in Table S2. Both Johnson et al.20 and Lv et al.31 concluded that the first step in methylamine decomposition on Ru(001) is the breaking of a C–H bond to produce H2CNH2. Figure 4 shows simulated spectra for H3CNH2, H2CNH2, HCNH2, and CNH2 and the calculated isotopic shifts are given in Table 2. In the C-H stretch region, the simulated spectrum for H3CNH2 shows only the two surface-IR allowed fundamentals, whereas at least six distinct peaks or shoulders are seen in the experimental spectra. The simulated νs(NH2) fundamental is at 3355 cm–1, whereas the experimental value is at 3244 cm–1. The δ(NH2) peak is calculated to be at 1581 cm–1, whereas the experimental peak, which is only seen for the multilayer, is at 1616 cm–1. These comparisons imply that differences of a few percent between the calculated and experimental frequencies are to be expected. Unfortunately, the frequency differences between the different moieties are also only a few percent. For example, comparing the simulated spectra for H3CNH2 and H2CNH2 shows that the calculated νs(NH2) and δ(NH2) frequencies differ by only 0.5% and 1%, respectively. On the other hand, the simulated spectra (Figures S4 and S5) indicate that while the ν(NH) stretch of H3CNH is very close to νs(NH2) of both H2CNH2 and H3CNH2, the N–H bending mode of H3CNH is at 1328 cm–1, which is significantly lower than the values of 1500, 1552, and 1658 cm–1 for the peaks seen in the 250 and 300 K spectra of Figure 3. A reasonable hypothesis then is that these three peaks are due to the δ(NH2) modes of H2CNH2, HCNH2, and CNH2. In addition to comparing frequencies, comparison of isotopic shifts in the simulated and experimental spectra provides further aid in spectral assignments. For each species listed in 14 ACS Paragon Plus Environment

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Table 2, there is a larger shift in the δ(NH2) mode frequency for 15N than for 13C substitution. The extent to which the peaks shift for the 13C isotopologue depends on how close the δ(NH2) frequency is to the frequency of a mode involving displacement of the carbon atom. For example, the minimum calculated shift is for H13CNH2 where the CH bend frequency is at 1190 cm–1 (with the δ(NH2) at 1549 cm–1), whereas for CNH2, with the largest calculated shift, the CN stretch is calculated to be at 1355 cm–1 (with the δ(NH2) at 1567 cm–1). For H2CNH2, the calculated value for δ(CH2) is 1415 cm–1, whereas in the 250 K experimental spectra the peak at 1450 cm–1 is likely δ(CH2). Since the experimental δ(CH2) and δ(NH2) frequencies are closer than the calculated ones, one would expect the experimental δ(NH2) peak to shift more with 13C substitution, as is the case. In the C-H and N-H stretch regions there are still peaks due to adsorbed H3CNH2 in the 250 and 300 K spectra. Although the calculated spectrum for H2CNH2 shows an N-H stretch 17 cm–1 higher than for the N-H stretch of H3CNH2, the observed peak at 3246 cm–1 in the 250 K spectrum is only 2 cm–1 higher than in the 220 K spectrum, implying that both are due to νs(NH) of H3CNH2. The simulated spectra in Figure 4 indicate that H3CNH2, H2CNH2, and HCNH2 all give strong peaks in the region near 1000 cm–1. In the case H3CNH2, the 220 K spectrum reveals three distinct peaks at 970, 991, and 1008 cm–1, whereas the simulated spectra for the structure shown in Figure 1 reveals only a single peak at 970 cm–1. However, the calculations also indicate that this peak has contributions from two surface IR allowed fundamentals, the CN stretch and NH2 wag, both of Aʹ symmetry. Although these modes are not resolved for H312C14NH2 and H313C14NH2, for H312C15NH2 separate peaks due to ν(CN) and ω(NH2) are seen at 962 and 984 cm–1 in the calculated spectra. This is due to both a different separation and a different intensity distribution for the modes of 15N-substituted methylamine. As the differences in the calculated 15 ACS Paragon Plus Environment


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spectra for the different possible species with modes in this spectral range are smaller than the differences between the calculated and experimental spectra for methylamine, this region does not provide unambiguous information on the identity of the intermediates. 5. Discussion 5.1. RAIRS of Adsorbed Methylamine The thermal desorption results imply that RAIR spectra from the initial adsorption at 90 K up to an annealing temperature of 300 K should contain peaks due to intact methylamine, with possible additional peaks due to decomposition products. To distinguish peaks of methylamine from other possible moieties, it is first necessary to account for the complexity of the methylamine spectrum itself. In particular, it is necessary to establish if the multiple well resolved C-H stretch peaks can all be attributed to methylamine, or if they are indicative of additional species. This was not an issue in the HREELS data of Johnson et al. 20 where the experimental resolution was insufficient to reveal the complexity observed here. At the simplest level, if we start with the geometry depicted in Figure 1, then only transitions with vibrational final states of A′ symmetry will be surface IR allowed. The simulated spectra contain only fundamental transitions, and only two C-H stretches are predicted. Although factor group splitting can occur in the solid state when there is more than one molecule per unit cell, Durig et al.48 did not find this to be significant in their spectra. Instead, they attributed extra peaks to overtones and/or combinations. Similarly, Gray and Lord noted that for gas phase methylamine the C-H stretch region is quite complex due to overtones and combinations in Fermi resonance with the C-H stretch fundamentals.52 The relatively high frequencies of the three CH3 deformation modes of gas phase methylamine of 1430, 1473, and 1485 cm–1, along with the NH2 twist mode at 1455 cm–1, implies that there are four overtones between 2860 and 2970 cm–1, all 16 ACS Paragon Plus Environment

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of which are surface IR allowed by symmetry. Several symmetry-allowed combination bands would also fall in this range. Given the expected enhancement of these transitions by Fermi resonance with the C-H stretch fundamentals, the large number of resolved C-H stretch peaks can all be attributed to methylamine adsorbed as shown in Figure 1. However, because the Fermi resonances for solid methylamine have not been treated in detail in the literature and because the spectra are sufficiently different for the adsorbed molecule, there is no simple way to definitively assign all the peaks. Detailed analyses of similar Fermi resonances in the C-H stretch region for alkoxides on surfaces has been carried out by Uvdal and coworkers.53-55 Such a treatment is beyond the scope of the present study. 5.2. Methylamine dissociation products The theoretical study by Lv et al.31 provides the binding energies and geometries of methylamine and various intermediates in its decomposition as well as the activation energies of 22 different elementary steps. The predicted pathway involves sequential dehydrogenation of the CH3 group to produce H2CNH2, HCNH2, and CNH2. They note that dehydrogenation of the amino groups of HCNH2 and CNH2 to form HCNH and CNH would also be likely. Breaking the N-H bond of CNH forms CN, which then undergoes C-N bond scission to yield adsorbed C and N atoms. They also note that as a minor route, the C-N bond of H2CNH2 can also break to form short-lived H2C and H2N groups, which would then dehydrogenate to also produce atomic carbon and nitrogen. Our observation of NH3 and N2 desorption in Figure 2 at 339 and 755 K, respectively, indicates that some C-N bond scission occurs, but since no RAIRS peaks are observable for annealing temperatures above 300 K, it is not possible to correlate these desorption peaks with a particular surface species. Although some C-N bond scission may occur below 300 K, there are no features in the RAIR spectra that can be uniquely assigned to any of 17 ACS Paragon Plus Environment


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the possible CHx or NHy species that would be produced by C-N bond scission. Despite providing detailed information on the pathway, Lv et al.31 do not make specific predictions that can be directly compared with experiments. To know which intermediates might be detected spectroscopically, it would be useful to have predictions of their coverages versus annealing temperatures. This sort of information can be provided by kinetic Monte Carlo methods. In the case of ethylene on Pt(111), such methods show that some energetically stable C2Hx intermediates would have negligible coverages based on the kinetics of their formation and consumption reactions.56 To provide a more direct comparison of our RAIR spectra to theory, we have carried out our own DFT calculations to produce simulated RAIR spectra of the possible intermediates. The calculations only yield the vibrational fundamentals and ignore any effects of anharmonicity and as such the simulated spectra are not expected to exactly match the experimental spectra. Our RAIRS results and the HREELS results of Johnson et al.20 provide complementary information: we used isotopic substitution with 13C and 15N, they used H3CNH2, CH3ND2, and CD3ND2; our resolution was 4 cm–1, the FWHM of their elastically scattered peak was is the range of 60-80 cm–1; our spectra were limited to the range above about 900 cm–1, they report peaks as low as 340 cm–1. The biggest difference is their higher sensitivity, which allowed them to detect peaks for annealing temperatures above 300 K, whereas no peaks are evident in our spectra above this temperature. Although it is not readily apparent how they distinguish genuine peaks from noise and how they assign peaks to a particular species, they present tables of peak positions and assignments for adsorbed methylamine; the products of successive C-H bond breaking in methylamine, H2CNH2, HCNH2, and CNH2; and the products of HCNH2 decomposition by successive breaking of the N-H and C-H bonds, HCNH, CNH, and CN. They


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also identify NH3 formed after breaking the C-N bond in H2CNH2 and hydrogenation of the NH2 fragment. Their assignments are largely guided by analogy with organometallic complexes containing related ligands, although such complexes are seldom characterized with vibrational spectroscopy. At the time of their study, it was not feasible to carry out the sort of calculations performed by Lv et al.,31 or to use DFT to calculate simulated spectra. These newer computational results, along with our observed shifts with 13C and 15N substitution, can be used to reassess some of the assignments of Johnson et al.20 In comparing the tabulated HREELS results with our spectra, Table 3 shows that the greatest discrepancies pertain to the C-N stretches. This is most pronounced for aminocarbyne, CNH2, which they assume is bridge bonded to two Ru atoms. They assign peaks at 1545 and 1620 to δ(NH2) and ν(CN), respectively, whereas our simulated spectra show these modes to have frequencies of 1567 and 1355 cm–1. The weak peak we observe with RAIRS at 1658 cm–1 and assign to δ(NH2) of CNH2 shifts to 1654 cm–1 with 15N substitution but does not shift for the 13

C isotopologue, making it incompatible with a CN stretch. In previous work on Pt(111), we

have thoroughly characterized 12C14NH2 , 13C14NH2 and 12C15NH2 both computationally and with RAIRS. Unlike in this case, on Pt(111) CNH2 has intense ν(CN), δ(NH2) and νs(NH2) peaks at 1329, 1566, and 3370 cm–1, respectively, values that are quite close to those in our simulated spectra for CNH2 at a three-fold bridge site on Ru(001). On Rh(111), ν(CN), δ(NH2), and νs(NH2) peaks of CNH2 from H3CNH2 decomposition were observed with RAIRS at 1311, 1562 and 3367 cm–1, respectively.57 In the same study, CNH2 was ruled out as the species responsible for peaks at 1380 and 1564 cm–1 observed with HREELS after heating azomethane to 350 K.57 In contrast, a later RAIRS study of azomethane on Rh(111) did attribute a 1564 cm–1 peak to δ(NH2) of CNH2.58 Simulated spectra for CNH2 at the two-fold bridge and at the three-fold hollow sites



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of Pt(111) show that ν(CN) is much lower for bonding at the hollow site, but even for bridge bonded CNH2, ν(CN) is much lower than δ(NH2). In addition, the assignment by Johnson et al.20 of a peak at 3175 cm–1 to νs(NH2) of CNH2 is both incompatible with its value on Pt(111) as well as with our simulations for CNH2 on Ru(001). Nevertheless, these previous studies of CNH2 on Pt(111) and Rh(111) as well as our calculations for CNH2 on Ru(001) all indicate that the peak at 1658 cm–1 that we observe is too high for δ(NH2) of CNH2, so that our assignment to this species must be considered somewhat tentative. Similar to the case of CNH2, the value of ν(CN) (952 cm–1) from our simulated spectra of HCNH2 is at odds with the assignment of a HREELS peak at 1420 cm–1 to ν(CN) of HCNH2. In this case, the different values reflect the different bonding configuration assumed by Johnson et al.20 Lv et al.31 found that HCNH2 bonds to the Ru surface at a bridge site through the C atom, with the CN bond tilted by 74.4° from the surface normal implying that there may be some interaction between the N atom and the Ru surface. They calculated a C-N bond length of 1.48 Å for HCNH2, a length characteristic of a C-N single bond and fully consistent with our calculated value of ν(CN). Johnson et al.20 assumed that HCNH2 bonds to the Ru surface in a manner analogous to how HCN(Me)2 bonds to an Os3 cluster complex, where the C-N bond length was determined by X-ray diffraction to be only 1.27-1.29 Å, a value consistent with a C=N double bond and a correspondingly higher value of ν(CN). For both CNH2 and HCNH2, it appears that in the surface species there is a much greater rehybridization to give a CN bond with more single-bond character than would be expected based on organometallic analogs. The assignments of Johnson et al.20 for two other species can also be compared with the calculated results. Lv et al.31 found that HCNH bonds in a µ3-η2 fashion with the CN bond nearly parallel to the surface with a CN bond length of 1.37 Å, indicating a bond order between 1 and 2.


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The corresponding calculated ν(CN) value is 1286 cm–1, whereas Johnson et al.20 assigned an HREELS peak at 1450 cm–1 to ν(CN) of HCNH, which they assumed to have a µ3-η2 structure, in analogy to the structure of H3CNH in organometallic complexes. For the CNH species, Lv et al.31 found it to be bound through the carbon atom at the hcp three-fold hollow site with the CN axis nearly parallel to the surface normal. For this species, we calculate a ν(CN) value of 1452 cm–1. Johnson et al.20 assigned a peak at 1660 cm–1 to ν(CN) of a bridge bonded CNH. They also assigned a peak at 2280 cm–1 to ν(CN) of an η1 CNH species. Finally, they assigned a peak at 1670 cm–1 to a µ3-η2 structure of CN, which is essentially the same structure obtained by Lv et al.,31 with a calculated CN bond length of 1.25 Å. Although we did not calculate ν(CN) for adsorbed CN, the short bond length 1.25 Å implies a CN triple bond, implying that ν(CN) should be in the range of 2100 to 2200 cm–1. We do not observe any peaks with RAIRS that could be assigned to HCNH, CNH or CN. 6. Conclusions The use of RAIRS along with 13C and 15N substitution provides new insights into the previously proposed decomposition pathway of methylamine on Ru(001). Molecular adsorption at 90 K leads to a spectrum that is similar to that of solid methylamine. The RAIRS peaks of monolayer methylamine, produced by annealing to 220 K, persist to 300 K. Annealing to 250 K leads to a new peak at 1500 cm–1 that is assigned to the δ(NH2) mode of H2CNH2 and after annealing to 300 K, peaks appear at 1552 and 1658 cm–1 that are assigned to δ(NH2) of HCNH2 and CNH2, respectively. These assignments are supported through comparison to observed and calculated shifts upon 13C and 15N substitution. Furthermore, these isotopic shifts preclude assigning any vibration above 1400 cm–1 to a C=N stretch, in contrast to earlier assignments. The results are consistent with a decomposition pathway that primarily proceeds by sequential loss of



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H from the CH3 group of methylamine. No spectroscopic evidence of stable HCNH, CNH, or CN intermediates was observed. AUTHOR INFORMATION Corresponding Author *Phone: (312) 996-0777. E-mail: [email protected]. Notes The authors declare no competing financial interest. Supporting Information Calculated structures and structural parameters of various species on the Ru19 cluster model; additional TPR results for CO, N2, and NH3; simulated RAIR spectra of H3CNH2, H2CNH2, HCNH2, CNH2, H3CNH, H2CNH, HCNH, CNH and their 13C and 15N isotopologues; tables of calculated wavenumbers of the modes of H3CNH2 and shifts with substitution by 13C and 15N. Acknowledgements This work was supported by a grant from the National Science Foundation (CHE-1464816).


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–1 Table 1. Comparison of vibrational assignments, in cm units, of H3CNH2 adsorbed on Ru(001)

Vibrational H3CNH2 H3CNH2 H3CNH2 / H3CNH2 / Ru(001) mode, gasa solidb Ru(001)c (this work)d fundamental no. νas(NH2), ν10 3427 3331 3255 3291 νs(NH2), ν1 3361 3260 3200 3197 νas(CH3), ν11 2985 2942 2920 2943 νs(CH3), ν2, ν3 2961, 2820 2887, 2793 2865 2886, 2796 δ(NH2), ν4 1623 1651, 1636 1560 1616 δas(CH3), ν5 1473 1492, 1467 N/R 1478 δs(CH3), ν6 1430 1441 1425 1457 ωt(NH2), ν13 1455 1353 ρr(CH3), ν14 1195 1182 1190 1182 ρw(CH3), ν7 1130 1182 1190 1147 ν(CN), ν8 1044 1048 1025 1045 ωw(NH2), ν9 780 955 980 1007 a Gray et al.52 The values of 1455 and 1195 cm–1 were not directly observed for ν13 and ν14 of H3CNH2. b During et al.48 c Johnson et al.20 The values listed are for the saturated first layer. d Values listed are for the multilayer spectrum of Figure 3.

Table 2. Comparison of the experimental and calculated frequencies and isotopic shifts of the δ(NH2) modes for four species. 13 C, 14N C isotopologue frequencies ( cm–1) shifts ( cm–1)

15

Exp.

Calc.

Exp.

Calc.

Exp.

Calc.

H3CNH2

1616

1581

0

0

-5

-4

H2CNH2

1500

1566

-5

0

-8

-4

HCNH2

1552

1549

-2

0

-7

-3

CNH2

1658

1567

0

0

-4

-9

δ(NH2)

12

N isotopologue shifts ( cm–1)



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Table 3. Comparison of ν(CN) values for different species obtained here with values reported in the literature.

Moeity and Method CNH2/Ru(001), HREELSa CNH2/Ru19, DFTb CNH2/Pt(111), RAIRSc HCNH2/Ru(001), HREELSa HCNH2/Ru19, DFTb HCNH/Ru(001), HREELSa HCNH/Ru19, DFTb CNH/Ru(001), HREELSa CNH/Ru19, DFTb a Johnson et al.20 b This study. c Chatterjee et al.32

ν(CN)/ cm–1 1620 1355 1323 1420 952 1450 1286 1660 1452


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Figure 1. Adsorption geometry of monolayer methylamine on Ru(001). The molecule is bonded to the surface through the nitrogen lone pair with a plane containing the C and N atoms and one H atom perpendicular to the surface giving the adsorbed molecule Cs symmetry.



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Figure 2. TPR spectra obtained after exposing Ru(001) to 1.0 L of (left) H312C14NH2 and (right) H313C14NH2 at 90 K. The heating rate was 2 K/s.


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Figure 3. Experimental RAIR spectra obtained after exposing Ru(001) to 5.0 L of H312C14NH2 (black), H313C14NH2 (blue), and H312C15NH2 (red) at 90 K, followed by heating to 220, 250, and 300 K. All spectra were taken at 90 K. The low and high frequency regions were acquired using MCT and InSb detectors, respectively. 31 ACS Paragon Plus Environment


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Figure 4. Simulated spectra for H3CNH2, H2CNH2, HCNH2, and CNH2 based on DFT calculations using a Ru19 cluster model of the Ru(001) surface.


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Spectroscopic Identification of Surface Intermediates in the

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