Article pubs.acs.org/JPCC
Formation of Benzonitrile from the Reaction of Styrene with Nitrogen on the Pt(111) Surface Jun Yin,† Joel D. Krooswyk,‡ Xiaofeng Hu,‡ Randall J. Meyer,† and Michael Trenary*,‡ †
Department of Chemical Engineering, University of Illinois at Chicago, 810 South Clinton Street, Chicago, Illinois 60607, United States ‡ Department of Chemistry, University of Illinois at Chicago, 845 West Taylor Street, Chicago, Illinois 60607, United States S Supporting Information *
ABSTRACT: Temperature-programmed reaction spectroscopy (TPRS) and reflection absorption infrared spectroscopy (RAIRS) were used to investigate the adsorption and reaction chemistry of styrene with clean and nitrogen covered Pt(111). The RAIR spectra indicate that the adsorption geometry of styrene is altered by both coverage effects and by the presence of nitrogen atoms on the surface. For an annealed monolayer of styrene on the clean Pt(111) surface, the most intense peaks correspond to out-of-plane bending vibrations indicating that the molecular plane is oriented parallel to the surface. For monolayer styrene on the nitrogen-precovered surface and for an unannealed monolayer of styrene, the in-plane and out-of-plane vibrations have comparable intensities indicating a more random orientation of the molecular plane. Upon heating, styrene reacts with the coadsorbed nitrogen atoms to form benzonitrile, which desorbs at 380 K. When benzonitrile is directly adsorbed on the Pt(111) surface, TPRS and RAIRS reveal that it desorbs at 350 K, indicating that its desorption at 380 K when formed from the reaction of styrene with nitrogen is reaction limited.
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Pd(100),30 Pt(110),31 and Cu(111).32,33 Here we report on the reaction of styrene (C6H5CHCH2) with adsorbed nitrogen atoms on the Pt(111) surface. In addition to HCN desorption as evidence for carbon−nitrogen coupling, desorption of benzonitrile is also observed. A specific motivation for studying the reactions of nitrogen with styrene on Pt(111) are recent reports of the formation of 2-phenylaziridine, benzyl nitrile, and benzonitrile from the reaction of either NH or N on Au(111).34,35 As Deng et al.34 note, there is a great deal of interest in using heterogeneous catalysts to synthesize aziridines as these compounds, with their reactive three-membered C(NH)C rings, are useful reagents in the synthesis of a variety of important compounds. As the NH molecule is isoelectronic to an O atom, the addition of NH to a CC double bond to form an aziridine is analogous to the addition of O to CC double bonds to form the COC threemembered rings in epoxides. This suggests that surfaces on which epoxides form should also promote the formation of aziridines. The surface chemistry of styrene on Ag surfaces has been extensively studied in an effort to provide insights into alkene epoxidation by silver catalysts.36−39 These studies have focused on silver because it is the only metal that catalyzes olefin epoxidation with viable selectivity, and styrene is often a preferred alkene for surface science studies because its high desorption temperature compared to olefins with lower
INTRODUCTION The surface chemistry associated with the making and breaking of carbon−nitrogen bonds underlies many processes in heterogeneous catalysis. These include the synthesis of HCN over Pt gauze catalysts1 and removal of nitrogen from petroleum feedstocks by hydrodenitrogenation using principally catalysts based on the oxides of Mo, Co, and Ni.2 We have studied several cases of C−N coupling reactions on Pt(111) that lead to the desorption of HCN, including the reactions of adsorbed N with C atoms,3 ethylene,4 and acetylene5 and through the reactions of C2 with ammonia.6 In other studies, we have generally found that the decomposition of molecules that already contain CN bonds leads predominantly to the desorption of HCN, indicating that once CN bonds are formed, they generally do not break on Pt(111). This behavior has been observed by us and others for HCN,7,8 azomethane,9,10 cyanogen,11,12 methyl amine,9,13,14 dimethyl amine,15 trimethyl amine,16 and methyl isocyanide17 on Pt(111). The surface chemistry seen for Pt(111) extends to some metals but not others. For example, azomethane decomposes on the Pd(111)18 and Rh(111)19,20 surfaces without breaking C−N bonds to desorb only HCN and H2. On Ag(111)21 and Cu(111),22 azomethane desorbs without dissociation, whereas on Cu(110)22,23 the N−N bond breaks to give adsorbed methyl nitrene. In contrast, on Mo(110)24 azomethane decomposes via C−N bond cleavage. In the presence of oxygen, the C−N bonds can break as seen for azomethane on Rh(111).25 In compounds containing C, N, and O, the C−N bonds are readily cleaved as seen for NCO on Pt(111),26,27 Rh(111),28,29 © 2012 American Chemical Society
Received: May 22, 2012 Revised: August 9, 2012 Published: August 22, 2012 19300
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described by Herceg et al.52 The presence of the oxygen-free p(2 × 2)-N layer was verified with LEED and AES. Ammonia (99.9992%) and oxygen (99.998%) were purchased from Matheson Trigas and used without further purification. 15NH3 (Cambridge Isotope Laboratory, 98%) was also used without further purification for preparing a 15N layer. Styrene (Alfa Aesar, 99.5%) and benzonitrile (Sigma-Aldrich, 99.0%) were used after several freeze−pump−thaw cycles, and the purity was measured by mass spectrometry. For some experiments, styrene and benzonitrile were introduced into the chamber using a homemade directed doser, while other experiments were performed by backfilling the chamber.
molecular weight allows for easier observation of its partial oxidation behavior before desorption occurs.36 Recent work has shown that the partial oxidation of hydrocarbons to form various products including styrene epoxide can also occur on Au(111).40−42 The formation of 2-phenylaziridine on Au(111) thus confirms the expectation that a surface that catalyzes the formation of an epoxide41 will also promote the formation of the analogous aziridine. Although this line of reasoning suggests that 2-phenylaziridine should also form from the reaction of NH with styrene on silver surfaces, no such studies have yet been reported, possibly because of the difficulty of preparing N or NH-covered silver surfaces. As the procedure for preparing N and NH-covered Pt(111) surfaces is well established, it is of interest to determine the extent to which the surface chemistry observed on Au(111) can be extended to Pt(111). While we find that the reaction to form benzonitrile on Pt(111) is quite similar to its formation on Au(111), the aziridine does not form. Benzonitrile and its surface chemistry are of interest as it is commercially produced through ammoxidation of toluene using vanadium-based catalysts43−45 and is an important starting compound in the synthesis of pharmaceuticals. For this reason, we have also obtained temperature-programmed reaction spectroscopy (TPRS) and reflection absorption infrared spectroscopy (RAIRS) data on the adsorption and reactions of benzonitrile on Pt(111), which are useful in the interpretation of data on the formation of benzonitrile from styrene and nitrogen. The vibrational results are compared to those of a surface-enhanced Raman spectroscopy (SERS) study of benzonitrile on a Pt electrode surface.46 The adsorption and reactions of benzonitrile have also been studied on Si(100),47 Au(100),48 Cu(111),49 and Ni(111).50
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RESULTS AND DISCUSSION TPRS Results. A prior study by Ranke and Weiss with ultraviolet and X-ray photoelectron spectroscopies (UPS and XPS) found that styrene chemisorbs strongly on Pt(111) below room temperature, polymerizes at about 400 K, undergoes further dehydrogenation upon heating, and then forms a graphite layer by 800 K.53 This finding is supported by the TPRS data shown in Figure 1a for hydrogen desorption (m/z =
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EXPERIMENTAL SECTION The experiments were performed in an ultrahigh vacuum (UHV) chamber with a base pressure of ∼8 × 10−11 Torr. The chamber is equipped for performing Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), TPRS, and RAIRS experiments. All TPRS results were obtained with a Hiden mass spectrometer (HAL 3F-RC) with a mass range up to 200 amu using a linear heating rate of 2 K/s. The temperature was measured with a K-type (chromel/alumel) thermocouple, and the analog thermocouple voltage was sent to the spectrometer where it was digitized and recorded in real time along with the ion current as an auxiliary channel. The thermocouple voltages were converted to temperature using a standard conversion table. The TPRS data were acquired after exposure to the surface at 87 K. The RAIR spectra were obtained with a commercial Fourier transform infrared (FTIR) spectrometer (Bruker IFS 66v/S). The IR beam enters and exits the UHV chamber through differentially pumped O-ring-sealed KBr windows and passes through a polarizer before reaching the infrared detector. An MCT (HgCdTe) detector and a SiC IR source were used. The spectra were acquired using 1024 scans for both background and sample spectra at a resolution of 4 cm−1 at a temperature of 87 K. In cases where the sample was annealed to a temperature above 87 K, it was cooled back to 87 K before a spectrum was acquired. The Pt(111) surfaces were cleaned and judged free of impurities before each experiment by a standard procedure described previously.51 A p(2 × 2) nitrogen layer was prepared through reaction of ammonia with chemisorbed O2 as
Figure 1. H2 TPRS data following a 2 L exposure of styrene to: (a) clean Pt(111), (b) 14N/Pt(111), and (c) 15N/Pt(111).
2) following a 2 L exposure of styrene to the clean Pt(111) surface. Although Figure 1a shows that there are clearly multiple desorption channels, the most prominent peak between 400 and 440 K correlates with the polymerization temperature whereas the second major peak between 560 and 800 K corresponds to dehydrogenation of the polymer to form graphite. On the basis of the electron escape depth, Ranke and Weiss53 estimated that styrene has a saturation coverage on Pt(111) at 295 K that is 83% of a physisorbed monolayer, which they state has a surface density of 1.66 × 1014 cm−2. This then implies a coverage of 0.09 ML, with one monolayer (ML) defined as the density of Pt atoms on the (111) surface (1.50 × 1015 cm−2). As noted below, our 2 L styrene exposure corresponds approximately to the saturated chemisorbed layer, 19301
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Pt(111) when coadsorbed with nitrogen.54 The 380 K peak is then assumed to be due to benzonitrile. Furthermore, we monitored m/z = 14, and this channel also shows a 380 K peak indicating that the species desorbing at this temperature contains nitrogen. The formation of benzonitrile is definitively confirmed by the results for the 15N-covered surface where the 380 K peak now appears only for the m/z = 104 channel. As also revealed by the H2 desorption results in Figure 1, there is some variability in styrene coverage from one exposure to the next. A small difference in the initial styrene coverage is the likely reason that the 220 and 310 K peaks in Figure 2 have the opposite intensity ratio for the 14N- and 15N-covered surfaces. The larger signal for the nominal 2-L styrene exposure to the clean surface (it is scaled by a fifth in Figure 2) relative to the 2L exposures to the nitrogen-covered surfaces is partly a reflection of the fact that the m/z = 103 and 104 desorption occurs as a single peak in the former case but is distributed over multiple peaks in the latter case, but it also appears that the initial styrene coverage was higher on the clean surface. Conclusions about the surface chemistry should not be affected by the different initial styrene coverages evident in Figure 2. In an effort to better understand the benzonitrile formation reaction on Pt(111), the TPRS data shown in Figure 3 were
which we assume corresponds to an absolute coverage of 0.09 ML as determined by Ranke and Weiss.53 The surface chemistry is strongly altered, as indicated by the hydrogen desorption results, for styrene adsorbed on the nitrogen (14N and 15N) covered surfaces as shown in parts b and c of Figure 1. Although no difference in H2 desorption for styrene interacting with the two nitrogen isotopes would be expected, results for both are shown to illustrate the degree of reproducibility of the data and for comparison with the TPRS results for other desorption products. In both cases, the first dehydrogenation channel is greatly suppressed and the second is shifted to lower temperatures by about 40 K. Figure 2 shows TPRS data for m/z = 103 and m/z = 104 following a 2-L styrene exposure to clean Pt(111) and to
Figure 2. TPRS data for m/z = 103 and m/z = 104 following a 2-L exposure of styrene to: (a) clean Pt(111), (b) 14N/Pt(111), and (c) 15 N/Pt(111).
Pt(111) covered with 14N and 15N. The parent ion of styrene corresponds to m/z = 104, whereas m/z = 103 corresponds both to the styrene fragment missing one hydrogen atom and to the parent ion of benzonitrile. Molecular styrene desorbs as a single peak at around 220 K. As shown in the Supporting Information, a 4-L styrene exposure to the clean surface yields a lower temperature peak at about 180 K in addition to a peak at 212 K, with the former associated with multilayer desorption and the latter to desorption of monolayer styrene. As no multilayer desorption is seen for a 2-L exposure, saturation of the monolayer evidently occurs for exposures between 2 and 4 L. This indicates that although styrene strongly interacts with Pt(111), it only partially dissociates even on the clean surface. Molecular desorption of styrene also occurs from the nitrogencovered surfaces at 220 K but the amount desorbing at this temperature is considerably less than from the clean surface. For the 14N-covered surface, the m/z = 103 channel shows two peaks at 310 and 380 K, whereas only the 310 K peak occurs for the m/z = 104 channel. This suggests that the 310 K peak is due to the desorption of molecular styrene that is inhibited from dissociation by the coadsorbed nitrogen. A previous study showed that ethylene also undergoes less dissociation on
Figure 3. Comparison of TPRS data for m/z = 27 and m/z = 103 after (a) exposing the clean Pt(111) surface to benzonitrile using a directed doser (background pressure increased to 2 x10−10 Torr for 20 s) and (b) after exposing the 14N/Pt(111) surface at 87 K to 2 L of styrene.
obtained for directly adsorbed benzonitrile and are compared with benzonitrile formed from the reaction of styrene with nitrogen. In this case, the HCN product at m/z = 27 is monitored in addition to m/z = 103. The doser was used for the benzonitrile exposure where the chamber pressure rose from 8 × 10−11 to 2 × 10−10 Torr during the 20 s that the doser valve was open. Figure 3a shows the desorption of a saturation monolayer of benzonitrile on Pt(111), with a peak at 350 K and a small tail that extends to 390 K. Similar to styrene, benzonitrile undergoes partial dissociation and hydrogen desorption (Supporting Information) was observed over a wide range of temperatures. At a sufficiently high exposure, 19302
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10−10 Torr). The 87 K spectrum of Figure 4 displays CH stretch peaks at 3014, 3031, 3062, and 3090 cm−1 as well as features at 910, 995, 1086, 1412, 1450, 1495, and 1631 cm−1. The peaks at 910, 995, and 1086 cm−1 are assigned to the CH2 wagging mode of the vinyl group and the CH bends of the vinyl group and phenyl ring, respectively, while those at 1412, 1450, and 1495 cm−1 can be assigned to phenyl ring stretches. Finally, a peak assigned to the vinyl CC stretch is seen at 1631 cm−1. These features are generally similar to those seen for styrene adsorption on Ag(111)36,37 and are summarized in Table 1. Pronounced changes in the spectra
multilayer desorption of benzonitrile at around 200 K was observed. The desorption of HCN (m/z = 27) following adsorption of benzonitrile occurs as a broad peak between 500 and 800 K. The reaction of styrene with nitrogen produces HCN with a similar profile in the high temperature region suggesting a common reaction pathway. In addition, m/z = 28 corresponding to HC15N was monitored following styrene reaction with a 15 N-covered surface, which supports the assignment of the high temperature desorption product to HCN. The desorption of HCN could be due to formation of C−N bonds from benzonitrile decomposition products at high temperature (i.e., benzonitrile completely dissociates and CN coupling follows), but HCN may also be due to cleavage of the phenyl ring to liberate CN, which could then be hydrogenated to HCN (i.e., the CN bond is never broken). Following styrene adsorption, the lower temperature region of Figure 3b shows that m/z = 27 and m/z = 103 have similar desorption profiles implying that in this region the m/z = 27 peaks are due to the C2H3 fragment of styrene, which is confirmed by observation of a m/z = 27 fragment for styrene desorption from the clean surface. The reaction to form benzonitrile from styrene and nitrogen evidently occurs at 380 K (reaction-limited desorption) as benzonitrile desorbs at a lower temperature when it is directly adsorbed onto the clean Pt(111) surface. As shown in the Supporting Information, the dominant pathway for removal of the nitrogen that does not form benzonitrile is through the desorption of ammonia rather than of N2. The formation of ammonia in this case is similar to what we recently reported for the reaction of acetylene with the N/Pt(111) surface.5 RAIRS Results. Styrene on Pt(111). Figure 4 shows RAIR spectra for multilayer styrene on clean Pt(111), which was prepared using the doser to expose the surface (50 s exposure, causing the chamber pressure to rise from 8 × 10−11 to 8 ×
Table 1. Vibrational Frequency (cm−1) Assignments for Styrene Multilayers
a
mode assignment
Ag(111)a
Ag(111)b
Pt(111)c
N/ Pt(111)c
W (CH2) δ (CH) CH bend of CHCδPhenyl (CH) δPhenyl (CH) δ (CH2) ν (C−C ring) ν (C−C ring) ν (C−C ring) ν (CC) νPhenyl (CH) νPhenyl (CH) νPhenyl (CH) νPhenyl (CH)
910 980 994 1086 1158 1412 1452 1496
910 975 994
910
910
995 1086 1183 1412 1450 1495
995 1084 1389 1413 1450 1495 1576 1631 3014 3029 3062 3090
1632
1411 1449 1494 1575 1630
1631 3014 3031 3062 3090
Reference 36. bReference 37. cThis work.
occurred when the surface was annealed to 160 K. As the TPRS results indicate that 160 K is below even the multilayer desorption temperature, the changes in the RAIR spectra for the 160 K anneal are attributed to reordering in the monolayer. However, to facilitate comparison with the frequencies reported for multilayer styrene on Ag(111), the frequencies for styrene on Pt(111) in Table 1 are for the unannealed surface. Although the CH stretch frequencies that we observe are assigned to phenyl CH stretches, the asymmetric CH2 stretch of the vinyl group also occurs in this range. As the inset for the annealed surface shows, the increased sharpness of the peaks allows resolution of separate components so that the two peaks formerly at 910 and 995 cm−1 become four peaks at 908, 912, 990, and 1005 cm−1. In the ring stretch region the peaks at 1450 and 1495 cm−1 reverse their relative intensities, with the former more intense for the monolayer. After annealing to 160 K, distinct CH stretches are no longer observed for the monolayer. The vinyl CC stretch at 1631 cm−1 is also less intense after the anneal. The changes in the spectra after annealing to 160 K can be attributed to differences in orientation of the styrene molecules. As the out-of-plane C H bends in the 900−1010 cm−1 region would be most intense for the molecular plane parallel to the surface, the results imply that styrene lies flat on the surface for the annealed monolayer but assumes a more random orientation prior to annealing. After further annealing to 250 K, the only peaks seen are in the CH stretch region at 2903 and 2933 cm−1. The TPRS data indicates that molecularly adsorbed styrene completely desorbs from clean Pt(111) by 220 K, implying that the CH stretches
Figure 4. A multilayer RAIR spectrum of styrene on Pt(111) obtained after dosing styrene for 50 s with a background pressure increase from 8 × 10−11 to 8 × 10−10 Torr with the crystal at 87 K followed by a spectrum obtained by annealing to 160 K for 30 s and a spectrum of the styrene decomposition product obtained by annealing to 250 K for 30 s. 19303
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at 2903 and 2933 cm−1 are due to one or more styrene dissociation products. Styrene on N/Pt(111). RAIR spectra for multilayer styrene adsorbed on nitrogen covered Pt(111), analogous to those of Figure 4, are plotted in Figure 5. Following adsorption at 87 K,
obtained after directly dosing benzonitrile onto the clean surface. Benzonitrile. A monolayer of benzonitrile on Pt(111) was prepared as for the TPRS results of Figure 3 by using a 20-s exposure from the doser and the RAIR spectra are shown in Figure 6. The 87 K spectrum reveals only a low frequency peak
Figure 5. RAIR spectra after dosing a multilayer of styrene (50 s with a background pressure increase from 8 × 10−11 to 8 × 10−10 Torr) onto the N/Pt(111) surface at 87 K and following annealing to the indicated temperatures.
Figure 6. RAIR spectra after dosing a monolayer of benzonitrile (50 s with a background pressure increase from 8 × 10−11 to 2 × 10−10 Torr) onto the clean Pt(111) surface at 87 K and following annealing to the indicated temperatures for 30 s.
the spectra in Figures 4 and 5 are quite similar but with a few extra peaks resolved in the phenyl ring stretching region from 1400 to 1500 cm−1 in Figure 5. Unlike in Figure 4, there is little change with annealing to 150 K indicating little change in orientation of the molecules. The persistence of the peak at 1495 cm−1 reveals that molecular styrene is still present even after annealing to 200 and 250 K. This is consistent with the TPRS data showing a second molecular desorption peak at 310 K. The two positive peaks seen at 1199 and 2091 cm−1 are due to the δs(NH3) and ν(CO) modes of NH3 and CO. Ammonia and carbon monoxide were adsorbed from the background and were present in the reference spectra and were then displaced by the adsorption of styrene. The NH3 had been used to prepare the nitrogen layer and a small background pressure of CO is always present in the chamber. Since the intrinsic intensity of the C−O stretch is very high, the small size of this feature implies an insignificantly low CO coverage. No RAIRS peaks were observed after annealing the styrene/ N/Pt(111) surface to temperatures above 250 K though the TPRS results show that the final stage of molecular desorption of styrene gives rise to the peak at 310 K. In addition, while species containing H, C, and N are shown through H2 and HCN desorption to remain on the surface to temperatures as high as 800 K, these species are undetectable with RAIRS. Although TPRS implied that benzonitrile desorption at 380 K is reaction limited, this is only slightly above the desorption temperature of benzonitrile from the directly dosed clean surface. This suggests benzonitrile might be detectable with RAIRS after it forms but before it desorbs. To establish what should be observed with RAIRS if benzonitrile was present after annealing the styrene/N/Pt(111) surface, a set of spectra were
at 853 cm−1 due to a CH bending mode as well as a small broad CN triple bond stretch at 2223 cm−1. The relatively featureless spectrum seen in Figure 6 after dosing benzonitrile onto Pt(111) is somewhat surprising as the molecule has 33 normal modes, 30 of which have IR active fundamentals. However, a similar spectrum was observed with HREELS for a submonolayer coverage of benzonitrile on Au(100)48 where only a weak CH stretch at 3066 cm−1 was observed in the spectral range above 800 cm−1, whereas the most intense peak was an out-of-plane C−H bending mode at 741 cm−1. This is consistent with the molecular plane of benzonitrile being oriented parallel to the surface on Pt(111) and Au(100). The CN triple-bond stretch is not strictly forbidden for this geometry but would be weak if observable at all. Some modest changes are observed in Figure 6 with annealing. The CN stretch at 2223 cm−1 gains some intensity but is almost undetectable after heating to 450 K. This is consistent with the small benzonitrile desorption peak seen at 350 K in Figure 3. As the spectral region below 1000 cm−1 is prone to artifacts due to the IR detector cutoff at around 800 cm−1, no significance can be attributed to the changes with annealing seen in this region. A distinct new peak does appear at 1617 cm−1 for the 350 K anneal, which could be due either to a reoriented molecule as there is a strong peak at 1598 cm−1 for liquid benzonitrile55 or to a benzonitrile dissociation product. To further characterize the vibrational spectrum of benzonitrile, a RAIR spectrum for a benzonitrile multilayer was also obtained and is given in the Supporting Information along with a table comparing peak positions for multilayer coverages on Au(100)48 and Si(100)47 19304
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and for liquid benzonitrile.55 The main conclusion reached from Figure 6 is that even if some benzonitrile remained on the surface after its formation from the reaction of styrene with nitrogen it would be difficult to detect with RAIRS as it does not yield any strong and characteristic peaks. In particular, although the CN triple bond stretch at 2223 cm−1 is highly characteristic of a nitrile and can often be quite intense and occurs in a region where there are no styrene peaks, it is very weak for monolayer benzonitrile. As shown in the Supporting Information, the CN stretch at 2232 cm−1 is one of the two most intense peaks in the spectrum for the multilayer, where a range of orientations are likely present.
containing CC bonds. Specifically, an N atom might add to the double bond of a molecule of the form RCHCH2 to make the corresponding RCN nitrile. The simplest possibility would be the formation of acetonitrile (CH3CN) from propene (CH3CHCH2), which is a reaction we have not investigated. However, we do find that acetylene and ethylene react on the p(2 × 2)-N/Pt(111) surface to produce HCN. As HCN desorption is observed following the decomposition of a variety of molecules containing CN bonds, by itself its observation does not offer much insight into the generality of the mechanism by which benzonitrile is formed from styrene and nitrogen. One reason this reaction might not be applicable to unsaturated aliphatic hydrocarbons is that styrene presents a case where the CC bond is preserved to higher temperature by virtue of the molecule attaching to the surface by the flatlying phenyl ring. Thus by 250 K, propene will convert to propylidene,58 making the CC bond unavailable for reaction with nitrogen, which based on the benzonitrile formation temperature of 380 K would be expected to occur at higher temperatures. The fact that benzonitrile is also formed from nitrogen on Au(111) at a similar temperature of 385 K35 suggests a similar formation mechanism. This is despite the fact that styrene molecularly desorbs without decomposition from clean Au(111) but undergoes complete dehydrogenation on clean Pt(111). This indicates that as long as molecularly adsorbed styrene is stable up to 380 K it can form benzonitrile, regardless of what happens at higher temperatures. The similar benzonitrile formation temperatures on Pt(111) and Au(111) are in contrast to the lower binding energies of N on the two metals as revealed by the lower N2 desorption temperatures on Au(111) of 425 or 375 K, depending on how the N or NHx layers were prepared,34,35 vs 450 K on Pt(111).52 Similarly, the binding energy of N on Au(211) was calculated to be 2.26 eV lower than on Pt(211).59 This suggests that the rate-limiting step to benzonitrile formation does not involve N atom addition to the CC bond but rather the dissociation of a nitrogen-containing intermediate.
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DISCUSSION The formation of benzonitrile from nitrogen and styrene on Pt(111) shares some similarities with the analogous reaction on Au(111). Styrene reacts with N atoms on Au(111) to form not only benzonitrile but also benzyl nitrile (C6H5CH2CN, m/z = 117) and 2-phenylaziridine (C6H5CHNHCH2, m/z = 119).35 The N/Au(111) surface was prepared by electron bombardment of a condensed ammonia layer followed by heating to 300 K, which TPRS showed completely desorbs the hydrogen that is produced either directly from electron-induced dissociation of ammonia or from the thermal decomposition of any NH or NH2 species that might also be present. The remaining nitrogen desorbs as N2 at 375 K. The reaction yields for styrene on N/Au(111) were 54% benzonitrile, 32% 2-phenylaziridine, and 14% benzyl nitrile. These yields were compared with those from a Au(111) surface containing predominantly NHx (x = 1,2) prepared from the reaction of ammonia with oxygen, which changed the product mix to 63% 2-phenylaziridine, 22% benzonitrile, and 6% benzyl nitrile. Ethylbenzene (9%) was also formed by reaction with surface hydrogen. The results on Au(111) show that the selectivity toward the different reaction products is quite different for N vs NHx. In contrast, the only nitrogen-containing reaction product other than HCN detected from the reaction of styrene with N on Pt(111) was benzonitrile. Given that the yield for 2-phenylaziridene was higher on the NHx-covered Au(111) surface, we also studied styrene on a hydrogenated N/Pt(111) surface. Although previous work52 indicated that up to 70% of the N atoms of the p(2 × 2)-N/Pt(111) surface could be converted to NH, we did not observe any desorption products for m/z > 104 from the reaction of NH with styrene. Thus, in contrast to Au(111), benzonitrile is the only product of the C−N coupling reaction (again, other than HCN) of styrene with either N or NH on Pt(111). From the stoichiometry of benzonitrile formation from nitrogen and styrene, three hydrogen atoms and a carbon atom must also be accounted for. Previous work has shown that both CH3 and CH2 would dissociate on Pt(111) well below the benzonitrile formation temperature.56,57 Therefore, formation of hydrogen, which would immediately desorb, and atomic carbon likely accompany benzonitrile formation. Deng and Friend35 speculated that this could then combine with surface N and H on Au(111) to form the HCN desorption product that they observe. However, in our case the similarity of the HCN desorption results following the direct adsorption of benzonitrile and from the reaction of styrene with nitrogen indicates that the HCN desorption in the latter case is also from the decomposition of benzonitrile. The reaction of the CC double bond of styrene with N to form benzonitrile might suggest that other nitriles could be formed from N atoms on Pt(111) from other hydrocarbons
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CONCLUSIONS Styrene adsorbs molecularly on Pt(111) at 87 K and upon heating follows two paths, one of which leads to desorption at 220 K and the other to complete dehydrogenation by 800 K. Annealing a monolayer of styrene deposited at 87 Kto 160 K leads to a RAIR spectrum with narrower peak widths and with the most intense peaks associated with out-of-plane vibrations. The spectrum is indicative of a well-ordered monolayer of styrene with the molecular plane oriented parallel to the surface. When adsorbed on the p(2 × 2)-N/Pt(111) surface, the RAIRS spectra indicate a less-ordered structure as well as a reduction in the extent of dehydrogenation at low temperatures, which enables molecular styrene to remain undissociated to higher temperature as revealed by a new molecular desorption peak at 310 K. At a slightly higher temperature benzonitrile is formed as indicated by a reaction-limited desorption peak at 380 K. Comparison with RAIRS and TPRS of directly adsorbed benzonitrile supports the conclusion of reaction limited desorption at 380 K and indicates that the HCN desorption at higher temperatures from the styrene/N/ Pt(111) system is likely due to decomposition of the benzonitrile product. The similarity of the conditions for benzonitrile formation from coadsorbed styrene and nitrogen on the Au(111) and Pt(111) surfaces suggests a common reaction mechanism. However, the generally higher reactivity of 19305
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The Journal of Physical Chemistry C
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Pt(111) prevents formation of 2-phenylaziridine and benzyl nitrile that are observed on the Au(111) surface.
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ASSOCIATED CONTENT
S Supporting Information *
RAIR spectra for multilayer benzonitrile on Pt(111) along with a table of spectral assignments and TPRS data for multilayer styrene on clean Pt(111) and H2 desorption from benzonitrile dissociation are provided as a supplement. This information is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the U.S. National Science Foundation Grant Nos. CHE-1012201 and CBET-0730937. REFERENCES
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