Carbon−Nitrogen Bond Formation from the Reaction of Ammonia with

The reaction of NH3 with C2 molecules on a Pt(111) surface was investigated using temperature programmed desorption, X-ray photoelectron spectroscopy ...
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17088

J. Phys. Chem. C 2007, 111, 17088-17093

Carbon-Nitrogen Bond Formation from the Reaction of Ammonia with Dicarbon on the Pt(111) Surface Rongping Deng and Michael Trenary* Department of Chemistry, UniVersity of Illinois at Chicago, 845 West Taylor Street, Chicago, Illinois 60607-7061 ReceiVed: July 19, 2007; In Final Form: August 31, 2007

The reaction of NH3 with C2 molecules on a Pt(111) surface was investigated using temperature programmed desorption, X-ray photoelectron spectroscopy (XPS), and reflection absorption infrared spectroscopy. Surface C2, which was prepared by dissociative adsorption of acetylene (C2H2) at 750 K, reacts with NH3 to form C-N bonds as revealed by the desorption of HCN and C2N2 at 570 and 650 K, respectively. The formation of C-N bonds was also detected with XPS when the NH3/C2 layer was annealed from 85 to 200 K through the appearance of a chemically shifted C 1s peak at 286.5 eV, which is well-resolved from the unreacted C 1s peak at 284.3 eV. Comparison of calculated and experimental RAIRS data indicates that C-N bond formation involves an HCCNH2 surface intermediate.

1. Introduction The reaction of ammonia with carbon on platinum surfaces is highly relevant to the industrial process of HCN synthesis from CH4 and NH3 over Pt gauze catalysts.1,2 Previously, we have used surface science methods to show that carbon and nitrogen atoms, generated in various ways, undergo a coupling reaction at 500 K to form surface CN.3,4 Carbon-nitrogen bond formation was reported in the reaction of NO with carbidic carbon on Pt(111)5 and on Rh(331).6 HCN formation in the reaction of NO with ethylene (C2H4) on Rh(111) was reported by van Hardeveld et al.7,8 They also reported both HCN and C2N2 formation in the reaction of atomic N with C2H4 on Rh(111).9 In the latter case, the surface reactions were thought to involve only carbon atoms or CHx species but not C2Hx species. However, given that surface carbon atoms can couple to form C2 molecules on Pt(111),10,11 it is also important to investigate C-N coupling reactions involving surface C2. More generally, the reactivity of various forms of surface carbon is of broad importance to heterogeneous catalysis. In methane synthesis over nickel catalysts, disproportionation of CO produces CO2 and carbidic carbon, which is then hydrogenated to methane.12 A kinetic study of selectivity in cobaltcatalyzed Fischer-Tropsch synthesis of hydrocarbons indicated that surface carbon promotes hydrocarbon chain growth.13 Carbon species formed either from the disproportionation of CO or from the dehydrogenation of hydrocarbons form graphite layers on metal surfaces at high temperatures,14-17 while at lower temperatures, these carbons can readily react with other coadsorbed species. High-resolution electron energy loss spectroscopy (HREELS) studies showed that hydrocarbon species, CHx, were produced from the hydrogenation of C on Ni(100)18 and Ru(001).19 Smirnov et al. have reported that fulminate (CNO) and isocyanate (NCO) can form on Pt(111) by the reaction of isolated carbon atoms with nitric oxide (NO).5 We recently found that dissociative adsorption of acetylene (C2H2) on Pt(111) at 750 K forms C2 molecules (dicarbon) on * Corresponding author. Tel.: (312) 996-0777; fax: (312) 996-0431; e-mail: [email protected].

the surface.20 This form of surface carbon was found both to be stable over a wide temperature range and to readily react with hydrogen to form the ethylidyne (CCH3) species. Through a combination of H2 temperature programmed desorption (TPD) peak areas, reflection absorption infrared spectroscopy (RAIRS) intensities, and C/Pt ratios determined with Auger electron spectroscopy, we were able to establish that more than 79% of the acetylene-derived carbon was in the form of C2 molecules, with the rest in the form of surface carbon atoms. Although C atoms constitute only a small fraction of the surface carbon, if they had a significantly higher reactivity than the C2 molecules, then they might still contribute to the results reported here. However, a comparison of our previous study of the C-N coupling reaction involving C atoms3,4 with the present data indicates that the results presented here are not influenced by the small C atom coverage. The acetylene-derived carbon was converted to an unreactive graphitic form only after repeated heating to 750 K or annealing to higher temperatures. The C2/ Pt(111) system produced from acetylene in this way provides an opportunity for studying the reactivity of ammonia with an unusually well-characterized form of surface carbon. Also, given that ammonia desorbs without appreciable dissociation on a clean Pt(111) surface,21 the ability of coadsorbed species to activate N-H bond breaking in NH3 is of considerable interest. 2. Experimental Procedures The TPD and RAIRS experiments were performed in a ultrahigh vacuum chamber that has been described in detail elsewhere.22 This chamber was maintained at a base pressure under 1 × 10-10 Torr during the experiments and was equipped with a Balzers QMG112 quadrupole mass spectrometer for TPD and with a Bruker IFS-66 v/S FTIR spectrometer for RAIRS. A heating rate of 2 K/s was used in the TPD experiments. To obtain optimum sensitivity in the RAIRS experiments, an InSb detector with a tungsten IR source was used in the 2200-4000 cm-1 range, and a HgCdTe detector with a SiC IR source was used in the 800-2200 cm-1 range. Each IR spectrum was recorded with a resolution of 4 cm-1 and 1024 scans.

10.1021/jp075668f CCC: $37.00 © 2007 American Chemical Society Published on Web 10/18/2007

Reaction of Ammonia with Dicarbon on Pt(111) Surface

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Figure 1. Temperature programmed desorption of hydrogen (H2) (a), hydrogen cyanide (HCN) (b), and cyanogen (C2N2) (d) after a 0.3 L NH3 exposure to a Pt(111) surface with 0.2 ML of preadsorbed carbon. For comparison, hydrogen cyanide (c) and cyanogen (e) were also monitored for a surface covered with 0.2 ML of carbon only.

Figure 2. C 1s XPS spectra recorded at 85 K after annealing to the indicated temperatures for a 0.2 ML carbon layer prepared by acetylene exposure to the Pt(111) surface at 750 K without (a) and with (b) coadsorbed ammonia. The spectra were recorded with step sizes of 0.05 eV in (a) and 0.1 eV in (b).

The X-ray photoelectron spectroscopy (XPS) results were obtained in a second chamber with a base pressure of ∼1 × 10-10 Torr equipped with a VG CLAM2 hemispherical analyzer and a dual anode X-ray source, although only Mg KR radiation was used here. The XPS spectrometer was calibrated with the Pt 4f7/2 peak at a binding energy of 71.2 eV. This second chamber has been described in detail elsewhere.23 The platinum crystal preparation and cleaning procedures24 followed standard methods. A survey XPS spectrum was obtained and is given in the Supporting Information, which shows only Pt peaks. The acetylene (99.6%) gas was purchased from BOC Gases and was subjected to further purification by several freeze-pump-thaw cycles before use. Ammonia (99.9992%) and hydrogen (99.9999%) were purchased from Matheson Gas Products. Gas exposures were performed by backfilling the chamber. In these experiments, a submonolayer of carbon was prepared by exposing the Pt(111) surface to acetylene at 750 K. The crystal was quickly cooled down to 85 K after acetylene exposure to avoid carbon aggregation. Unless otherwise specified, all experiments were performed on samples with a 0.3 L (1 L ) 1 × 10-6 Torr s) NH3 exposure at 85 K to the 0.2 ML carbon covered Pt(111) surface.

570 K and an unresolved shoulder with tailing to higher temperatures that extends to above 700 K. Cyanogen desorbs as a broad peak centered at 650 K. The higher temperature range of the hydrogen desorption correlates with that of HCN and C2N2, suggesting reaction limited desorption associated with the decomposition of a surface intermediate. Increasing the NH3 exposure to 0.5 L (not shown) did not change the amount of HCN and C2N2 desorption because only the NH3 molecules in the first layer react with C2, while NH3 molecules in the multilayer desorb from the surface without reaction. An earlier study showed that multilayer NH3 desorbs from the Pt(111) surface at temperatures below 200 K,21 and we found that multilayer NH3 begins to form after a 0.2 L exposure.25 The results of a control TPD experiment in which HCN and C2N2 were monitored for a C2/Pt(111) surface without coadsorbed NH3 are also shown in Figure 1. Clearly, the desorption of HCN and C2N2 only from the NH3/C2/Pt(111) surface is a direct indication of a C-N coupling reaction following activation of the N-H bonds of the ammonia by the C2 molecules. 3.2. XPS Results. Figure 2 shows C 1s XPS spectra after annealing at the indicated temperatures for the C2/Pt(111) surface with and without coadsorbed NH3. Spectra in the N 1s region were also obtained and are shown in the Supporting Information, but the signal is weaker and less informative. The C2 molecules are characterized by a single peak at 284.3 eV that does not change in position or shape with annealing in the absence of NH3 (Figure 2a). However, the two carbon atoms are expected to be nonequivalent based on DFT calculations26 and could show different chemical shifts. Although two C 1s peaks are not resolved here, it might be possible to resolve them using highresolution synchrotron-based instruments.27 There is no change in the C 1s peak after ammonia adsorption at 85 K, as shown in Figure 2b. After the surface was annealed to 200 K for 60 s, a new C 1s peak at ∼286.5 eV appeared. No further change occurred upon annealing to 300 K for 60 s, whereas after annealing to 550 K, the higher binding energy peak became less intense. The dashed line at 286.5 eV is not based on fitting

3. Results 3.1. TPD Results. Figure 1 shows hydrogen (H2, 2 amu), hydrogen cyanide (HCN, 27 amu), and cyanogen (C2N2, 52 amu) desorption from the Pt(111) surface after a 0.2 ML carbon pre-adsorbed surface was exposed to 0.3 L NH3 at 85 K. Desorption of mass 41 (acetonitrile, CH3CN or methylisocyanide, CH3NC) was monitored but not observed. On the clean surface, only a few percent of the ammonia molecules dissociated, leading to hydrogen desorption in the range of 300400 K that was only slightly higher than that due to adsorption from the H2 background.25 In contrast, hydrogen desorption from the NH3/C2 layer (Figure 1a) takes place in a broad range of 400-700 K with a peak at 465 K. HCN desorbs with a peak at

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Figure 3. C 1s XPS spectra for a monolayer of CH3CN and the 300 K annealed NH3/C2 coadsorption layer on Pt(111).

the spectra in Figure 2 but is drawn to best represent the position of the higher binding energy peak in the spectra after annealing to 200, 300, and 550 K. It also corresponds to the position of the fitted peak shown in Figure 3, which is the same as the spectrum shown in Figure 2 following annealing to 300 K. The C 1s XPS results for the NH3 + C2 reaction product are compared with those of acetonitrile in Figure 3, obtained after exposing 3.0 L acetonitrile to the Pt(111) surface at 85 K and annealing to 200 K to desorb the multilayer. As a stable molecule containing a C-C-N unit, it might be expected to show some similarities to surface intermediates formed from NH3 + C2. Other studies have shown that acetonitrile desorbs without dissociation from the Pt(111) surface,28-30 which implies that the spectrum shown in Figure 3 is of a monolayer of intact acetonitrile molecules. The acetonitrile C 1s peak at 284.9 eV is assigned to the CH3 carbon with the 286.6 eV peak assigned to the CN carbon, which is consistent with previous XPS results for acetonitrile on both Pt(111)28 and W(100).31 The comparison with the NH3/C2/Pt(111) results indicates that one carbon atom of the C2 molecule forms a CN bond, whereas the other carbon does not. The fact that the methyl carbon of CH3CN has a binding energy of 284.9 eV, whereas the reacted C2 molecule has a different, lower binding energy peak at 284.3 eV, indicates that this carbon has not been hydrogenated to a CH3 group. In particular, the comparison in Figure 3 shows that the reaction product is not in fact acetonitrile but is only similar to it. The XPS results do not provide any other information about the presence or absence of C-H or N-H bonds in the product of the NH3 + C2 reaction. 3.3. RAIRS Results. Further information on the nature of the surface intermediates formed in the thermal evolution of the NH3/C2/Pt(111) system is provided by the RAIRS results in Figure 4, which were obtained after the C2/Pt(111) surface was exposed to 0.3 L NH3 at 110 K and then annealed to the indicated temperatures. Only peaks due to ammonia are seen in the 110 K spectrum. Because of strong adsorbate-adsorbate interactions as well as interactions between monolayer and multilayer NH3 molecules, the ammonia peaks can display a range of frequencies and shapes, as has been discussed previously.32,33 Nevertheless, the peaks at 1240 and 1622 cm-1 are readily assigned to the δs(NH3) and δas(NH3) modes, respectively, and the peaks in the range of 3000-3400 cm-1 are all due to symmetric and asymmetric ν(N-H) modes of intact NH3

Figure 4. RAIRS of the NH3/C2 coadsorption layer on Pt(111) after annealing to the indicated temperatures. All spectra were recorded at 110 K.

molecules with varying degrees of hydrogen bonding. The 200 K anneal produces a monolayer of ammonia, which has a simpler spectrum with strongly red-shifted δs(NH3) and δas(NH3) modes and a single ν(N-H) peak at 3252 cm-1. Two new peaks at 1381 and 3363 cm-1 begin to develop at this temperature, are clearly visible after the 300 K anneal, and are still present in the 400 K spectrum. The peak at 2964 cm-1 is due to the CH species34 and is attributed to the hydrogenation of surface carbon atoms and was observed at 2959 cm-1 when a mixed C2 + C layer was exposed to hydrogen and annealed to 400 K.20 Note that the intensity scale is enlarged for the spectra taken after annealing to 300 K and above, as the peaks are quite weak as compared to the 110 and 200 K cases. The 1188 cm-1 peak in the 300 K spectrum is assumed to be the δs(NH3) mode of residual ammonia on the surface. Thus, the only indications from RAIRS of the surface intermediate produced by the C-N coupling reaction are the rather weak peaks at 1381 and 3363 cm-1. No peaks are observed after annealing to 550 K, implying that the remaining hydrogen is contained in species without strong RAIRS peaks; only the TPD results showing hydrogen desorption up to 700 K reveal the presence of hydrogen-containing species on the surface at 550 K and above. Some indication of the nature of the surface species present after the 550 K anneal is provided by the RAIRS results after a subsequent 40 L H2 exposure at 110 K followed by a 300 K anneal for 30 s. This procedure permits the identification of surface species through their hydrogenation products. In addition to the peak at 2964 cm-1 due to the CH species, there are peaks at 1338, 2796, and 2887 cm-1 assigned to the δs(CH3) fundamental, 2×δas(CH3) overtone, and νs(C-H) fundamental of ethylidyne (CCH3) that is produced by the hydrogenation of surface C2.20,35 These results indicate that C2 molecules and C atoms are present on the surface after the 550 K anneal. The peaks at 1381 and 3363 cm-1 are also detectable but with low

Reaction of Ammonia with Dicarbon on Pt(111) Surface

Figure 5. RAIRS for a 0.3 L NH3 exposure at 85 K followed by annealing to 300 K for the clean surface and for carbon coverages of 0.08 and 0.2 ML. All spectra were recorded at 85 K.

intensity after the hydrogenation reaction. Other surface species that we have previously shown can be detected through their hydrogenation products include N atoms, CN molecules, and C2N2 molecules, which can be hydrogenated to NH, CNH2, and H2NCCNH2, respectively.25,36 The characteristic features of these species are not observed, which provides important insight into the reaction mechanism. Figure 5 compares the spectrum presented in Figure 4 of the NH3/C2/Pt(111) system at a carbon coverage of 0.2 ML after a 300 K anneal with the corresponding spectrum for a lower carbon coverage of 0.08 ML. Also shown for comparison is a spectrum of NH3 on the clean surface under the same conditions in which an ammonia peak at 1167 cm-1 is more intense than in the other cases, implying that the C2 blocks ammonia adsorption to some extent. A sharp and intense peak at 3310 cm-1 due to the NH species is also clearly seen, which is reduced in height and shifted to 3313 cm-1 for 0.08 ML of preadsorbed carbon and is not observed at all for the 0.2 ML carbon case. Since the NH species is produced from NH3 dissociation, which occurs more readily on some,37,38 but not all,39 stepped Pt surfaces than on Pt(111), we assume that the NH is produced at the step sites of our Pt(111) crystal. It therefore appears that preadsorbed C2 blocks the step sites needed for NH3 dissociation to NH. The fact that peaks at 2962 and 1338 cm-1 due to the CH and CCH3 species are seen for 0.08 ML but not for 0.2 ML of carbon is probably due to the availability of surface hydrogen from ammonia dissociation in the former case but not in the latter. The peaks at 1381 and 3363 cm-1 are present with similar intensities for both 0.08 and 0.2 ML of carbon, indicating that they do not scale with the carbon coverage. 4. Discussion The TPD experiments showing HCN and C2N2 desorption, along with the XPS results showing a reaction at 200 K, not only definitively establish CN bond formation from NH3 + C2 but also provide insight into the mechanism. There are basically three alternative pathways in the C-N coupling reaction:(1) the C-C bond may be broken as the C-N bond is formed; (2) the N atom may attach to the end of a CC molecule without breaking the C-C bond; or (3) the N atom may insert between the carbon atoms to form a C-N-C unit. The first possibility seems unlikely as the HCN desorption seen here is at a higher

J. Phys. Chem. C, Vol. 111, No. 45, 2007 17091 temperature than when HCN is adsorbed directly onto the surface,40,41 in which case the highest temperature peak is below 500 K. Thus, if surface CN was formed at 200 K in the C-N bond forming reaction detected with XPS, then we would expect HCN desorption below 500 K. Instead, the HCN desorption seen here occurs in the temperature range of 500-700 K with a peak at 570 K. This observation also argues against a C-N coupling reaction between ammonia and the small coverage of carbon atoms that are also present along with the C2 molecules. The cyanogen desorption at 650 K is at a similar temperature to what has been seen in other cases where it is basically due to recombinative desorption of two CN molecules,40,42,43 although the desorption kinetics is somewhat complicated. CN is likely produced in the same decomposition step that produces HCN. Therefore, the C2N2 desorption at 650 K merely serves to indicate that surface CN was produced at a lower temperature. The possibility of N atom insertion into the C-C bond would produce a C-N-C unit similar to what exists in methylisocyanide, CNCH3. Therefore, similar surface intermediates that occur in the adsorption and decomposition of methylisocyanide might be expected. The thermal desorption of HCN following CNCH3 adsorption on Pt(111) occurs at 500-520 K, and no C2N2 desorption was observable. Furthermore, the RAIRS results for CNCH3 on Pt(111) and its decomposition products are quite different from what is observed here. Methylisocyanide decomposes by way of a methylaminocarbyne (CNHCH3) intermediate, which is also produced from dimethylamine, and the spectroscopic characterization of the common intermediates was discussed in the context of dimethylamine decomposition on Pt(111).44 These considerations can largely rule out the insertion mechanism. Therefore, the mechanism most likely involves the initial formation of a N-C-C unit, and the issue then becomes the number and location of the hydrogen atoms in the surface intermediate. In principle, RAIRS can provide this information. The IR spectrum for the 300 K annealed sample in Figure 4 shows two weak peaks at 1381 and 3363 cm-1, which can be associated with a CN-containing intermediate. This result is consistent with the XPS measurement in which CN bond formation was detected when the NH3/C2/Pt(111) surface was annealed to temperatures of 200 K and above. The 3363 cm-1 peak is too high for a C-H stretch and can therefore be reliably assigned to an N-H stretch. The lower frequency peak at 1381 cm-1 could be assigned to a CH3 deformation mode or a C-C/ C-N stretch. Many CN-containing molecules and the intermediates formed during their decomposition on Pt(111) have been well-characterized with surface vibrational spectroscopy. These molecules include hydrogen cyanide (HCN), acetonitrile (CH3CN), methylisocyanide (CH3NC), azomethane (CH3N2CH3), ethylenediamine (NH2CH2CH2NH2), methylamine (CH3NH2), and dimethylamine ((CH3)2NH).23,30,44-47 The IR spectrum observed here does not match what was observed for any of these molecules. However, with the exception of acetonitrile, which does not decompose on Pt(111), none of these molecules contains the C-C-N structural unit of the proposed intermediate. Following the method described in detail elsewhere,48 density functional theory (DFT) based on the Pt7 cluster model of the surface shown in Figure 7 was used to identify stable intermediates and to calculate their RAIR spectra. We used the B3LYP functional49 with a triple-ζ 6-311(d,p) basis set for the C, N, and H atoms and the LANL2DZ basis set for the Pt atoms. The distance between the neighboring Pt atoms was frozen at 2.775 Å, the Pt-Pt distance in the bulk metal. We found that an

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Figure 7. Illustration of the proposed NH3 + C2 reaction scheme on Pt(111).

Figure 6. Comparison of the experimental RAIR spectrum of the NH3 + C2 reaction product with spectra calculated with DFT for HCCNH2 and H2CCNH bonded to a Pt7 cluster model of the Pt(111) surface.

HCCNH2 intermediate is stable on the surface but that it is calculated to have only two vibrational modes of appreciable surface IR intensity: an N-H stretch at 3316 cm-1 and a C-N/ C-C stretch at 1429 cm-1. The calculation yields frequencies of the C-H stretch and NH2 scissors modes of 3029 and 1562 cm-1, respectively, but also yields intensities for these modes that are quite low. Among the spectra calculated for various possible intermediates, including HCCNH2, H2CCNH, CCNH, and NCCH3, the one for HCCNH2 best matches the experimental spectrum. A comparison of the calculated and experimental spectra is shown in Figure 6. An H2CCNH surface intermediate was proposed by Friend and Serafin in their study of acetonitrile decomposition on W(100).31 In their XPS experiment, a C-N bond was determined to still be present for acetonitrile annealed to 400 K. On the other hand, according to the HREELS data and the results of an H/D isotopic substitution experiment, they showed that the observed peaks at 3376 and 1400 cm-1 can be assigned to the N-H stretch and the doubly bonded C-N stretch, respectively. Furthermore, the intermediate was formed through isomerization rather than through hydrogenation, implying that it should have the same number of H atoms, three, as the initially deposited acetonitrile. Therefore, they proposed the formation of H2CCNH from the isomerization of CH3CN. Our calculation shows that such an intermediate, while stable, would have an intense peak due to a doubly bonded C-N stretch at 1684 cm-1, as shown in Figure 6. Its absence in our experimental spectra argues against such an assignment here. Although this intermediate might adopt a structure with the CdN bond parallel to the surface, thus possibly rendering the CdN stretch too weak to observe, the calculated spectra shown in Figure 6 are based on geometry-optimized structures, which argues against this possibility. Instead, we propose that a HCCNH2 intermediate forms through the transfer of an H atom from NH3 to C as the C-N bond is formed, as illustrated in Figure 7. Since the observed intermediate decomposes below 550 K and the desorption of HCN and C2N2 were observed above 550 K, additional intermediates are expected on the surface, which were not observed with RAIRS. Some quantitative estimates of the coverages of the surface species involved in the NH3 + C2 reaction can be obtained by combining information from TPD, XPS, and RAIRS. The formation of HCN and C2N2 from the proposed HCCNH2

intermediate at temperatures above 500 K and their desorption at temperatures up to 700 K are consistent with hydrogen desorption, which also extends up to 700 K. The XPS data for the 550 K anneal in Figure 2b show that the C 1s peak at 286.5 eV for the C-N bond becomes less intense, which correlates with the onset of HCN desorption. Although the combined peak area of the two C 1s peaks drops by about 11% after the 550 K anneal, the ratio of the area of the 286.5 eV peak to that of the 284.3 eV peak drops from 0.58 for the 300 K anneal to 0.39 after the 550 K anneal. The fact that the ratio is not 1:1 at 300 K is an indication that HCCNH2 is not the only species on the surface, although photoelectron diffraction effects may make coverage estimates of the different species only semi-quantitative. Upon annealing to 550 K, the C 1s peak at 286.5 eV, corresponding to the carbon bound to nitrogen, becomes weaker, while the other carbon peak at 284.3 eV remains about the same. Assuming that the two carbon atoms of C-C-N give rise to C 1s peaks of equal intensity, other carbon species in addition to the carbon atom not bound to nitrogen in the C-C-N unit must contribute to the lower binding energy peak at 284.3 eV. If we subtract the C-C-N contribution from the 284.3 eV peak, the remaining peak area provides a way to estimate the coverage of surface carbon present in the form of C atoms and C2 molecules. In this way, we estimate that upon annealing to 550 K, about 56% of the surface carbon is in the form of C-C-N, while the remaining 44% of the surface carbon is in the forms of C and C2. We calculate that the latter is equivalent to 0.08 ML of carbon. This surface carbon can be hydrogenated to form the CCH3 and CH species, as shown in Figure 4, indicating that both C atoms and C2 molecules were present. By comparing the IR intensity of the δs(CH3) peak at 1338 cm-1 of ethylidyne with that of a saturation coverage layer of ethylidyne,20 the coverage of carbon in the form of C2 molecules following the 550 K anneal is estimated to be ∼0.09 ML or 45% of the total carbon initially deposited on the surface. From our previous work with acetylene-derived carbon, we expect only a small coverage of C atoms relative to C2 molecules.20 Thus, the C2 coverage derived from the IR data is essentially the same as the value estimated from the XPS data. However, the C2 coverage after the 550 K anneal may be greater than the amount after the C-N coupling reaction takes place at lower temperatures. This is suggested by the spectrum in Figure 4 following the 400 K anneal in which a 2964 cm-1 peak is observed due to CH, indicating C atoms on the surface, without appreciable peaks due to CCH3 indicative of surface C2. In contrast, in our earlier study, we found that when H2 is dosed onto a C2 + C covered surface at 85 K and then annealed to 400 K, the CH stretch of ethylidyne (CCH3) is more intense than the CH stretch of methylidyne (CH).20 There are two possible explanations for this result. The first is that for the 400 K anneal spectrum in Figure 4, the C2 molecules are largely bound to N and are hence precluded from being hydrogenated to CCH3, but upon annealing to 550 K, there is some decomposition of the CCN species to produce C2 and N atoms, with the latter desorbing as N2. The second possibility is that there is simply insufficient H to

Reaction of Ammonia with Dicarbon on Pt(111) Surface produce both CH and CCH3 at 400 K. This seems somewhat more likely as the coverage estimates from RAIRS and XPS would then be consistent. 5. Conclusion A C-N coupling reaction was demonstrated by the desorption of HCN and C2N2 from the NH3/C2/Pt(111) surface at temperatures above 500 K. The XPS and RAIRS experiments indicate that the C-N coupling step takes place at temperatures as low as 200 K. Since this temperature is well below the onset of HCN desorption, a stable surface intermediate containing a CN bond must be present over a wide temperature range. Definitive identification of this intermediate is hampered by the presence of only two weak vibrational features in the RAIR spectrum, although comparison with a spectrum for HCCNH2 calculated using DFT gives reasonably good agreement with the experimental spectrum. This study extends our previous work in which we showed that surface C2 can be readily hydrogenated on Pt(111) to also show that C2 can activate the NH bonds of coadsorbed ammonia to form CN bonds on the surface. The CN coupling reaction is relevant to processes such as the synthesis of HCN and other CN-containing molecules over platinum catalysts. Further studies of this sort should provide better understanding of the role of surface carbon in catalysis on transition metal surfaces. Acknowledgment. This work was supported by the National Science Foundation under Grant CHE-0714562. Supporting Information Available: Additional XPS spectra. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Satterfield, C. N. Heterogeneous Catalysis in Practice, 2nd ed.; McGraw Hill: New York, 1991. (2) Schmidt, L. D.; Hickman, D. A. Surface chemistry and engineering of HCN synthesis. In Catalysis of Organic Reactions; Kosak, J. R., Johnson, T. A., Eds.; Marcel Dekker, Inc.: New York, 1994; p 195. (3) Herceg, E.; Trenary, M. J. Am. Chem. Soc. 2003, 125, 15758. (4) Herceg, E.; Trenary, M. J. Phys. Chem. B 2005, 109, 17560. (5) Smirnov, M. Y.; Gorodetskii, V. V. Catal. Lett. 1993, 19, 233. (6) DeLouise, L. A.; Winograd, N. Surf. Sci. 1985, 154, 79. (7) van Hardeveld, R. M.; Schmidt, A. J. G.; van Santen, R. A.; Niemantsverdriet, J. W. J. Vac. Sci. Technol., A 1997, 15, 1642. (8) van Hardeveld, R. M.; Schmidt, A. J. G. W.; Niemantsverdriet, J. W. Catal. Lett. 1996, 41, 125. (9) van Hardeveld, R. M.; van Santen, R. A.; Niemantsverdriet, J. W. J. Phys. Chem. B. 1997, 101, 7901. (10) Smirnov, M. Y.; Gorodetskii, V. V.; Cholach, A. R.; Zemlyanov, D. Y. Surf. Sci. 1994, 311, 308.

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