NMR Study of the Chemisorption and Decomposition of Methylamine

May 10, 2007 - Nuclear magnetic resonance (NMR) studies of the interaction of methylamine with the rhodium surface in a Rh/SiO2 catalyst at temperatur...
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J. Phys. Chem. C 2007, 111, 7783-7794

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NMR Study of the Chemisorption and Decomposition of Methylamine on Rh/SiO2 Henrik von Schenck,†,‡ Neil Kumar,§ Christopher A. Klug,*,§,| and John H. Sinfelt⊥ Department of Chemical Engineering, Stanford UniVersity, Stanford, California 94305-5025, and Department of Materials Physics, The Royal Institute of Technology, Stockholm, Sweden ReceiVed: September 11, 2006; In Final Form: December 20, 2006

Nuclear magnetic resonance (NMR) studies of the interaction of methylamine with the rhodium surface in a Rh/SiO2 catalyst at temperatures in the range of 253-298 K reveal two principal features in both the 13C and 15 N spectra. One feature is a resonance occurring at a frequency very close to the resonance frequency for methylamine gas. This feature is attributed to methylamine molecules adsorbed without dissociation. Because the methylamine is not removed from the catalyst surface by evacuation to 10-6 Torr, its interaction with the surface is reasonably strong. The silica support does not retain methylamine after such an evacuation, and therefore, the adsorbed methylamine is associated with the rhodium. The other feature in both the 13C and 15 N spectra is a very broad resonance occurring downfield of the resonance for the nondissociatively adsorbed methylamine. It is attributed to partially dehydrogenated surface species designated by the generalized formula (CN)Hx. Incipient scission of carbon-nitrogen bonds is observed at room temperature. As the temperature is increased above room temperature, the extent of scission of the carbon-nitrogen bonds increases. Ammonia, methane, and higher-carbon-number hydrocarbons then appear in the gas phase. The nondissociatively adsorbed methylamine is continuously transformed into the species (CN)Hx, thereby releasing hydrogen on the surface. The hydrogen is consumed in the production of the ammonia and methane observed in the gas phase. Ammonia is formed much more readily than methane or the other hydrocarbons, with the result that the species remaining on the surface are rich in carbon relative to nitrogen.

Introduction Nuclear magnetic resonance (NMR) spectroscopy has been shown to be an incisive probe for the investigation of molecules adsorbed on catalyst surfaces and, in selected cases, for the study of the catalysts themselves.1-22 The applicability of NMR spectroscopy in studies with real catalysts under conditions other than those typical of ultrahigh-vacuum apparatus (pressures on the order of 10-10 Torr) is a particularly important attribute of this technique. Studies employing solid-state NMR methods have been conducted on the chemisorption of ethylene and acetylene on catalysts consisting of small nanoscale metal clusters or crystallites dispersed throughout the pores of a typical support material.15,16 Conclusions were drawn with regard to the structures of the chemisorbed species,15,16 and kinetic data for simple reactions of these species were obtained. For example, activation energies for the scission of the carbon-carbon bond on clusters of several different metals were determined from NMR measurements of the extent of scission occurring in a given time period over a range of temperatures.16,17,20 Kinetic information was also obtained for simple surface reactions involving the formation and dissociation of carbon-hydrogen bonds.19,21,22 Studies of the kinetics of simple reactions of * Corresponding author. Tel.: 202-767-3239. E-mail: [email protected]. mil. † The Royal Institute of Technology. ‡ Current Address: COMSOL AB, Tegne ´ rgatan 23, SE-111 40 Stockholm, Sweden. § Stanford University | Current address: Chemistry Division, Naval Research Laboratory, Washington, D.C. 20375. ⊥ P.O. Box 364, Oldwick, New Jersey 08858.

chemisorbed hydrocarbon species of known structure contribute to our general understanding of the kinetics of more complex catalytic reactions of hydrocarbons. In particular, the results of NMR studies of acetylene and ethylene chemisorbed on supported metal clusters have been of interest in relation to a large body of work on the catalytic hydrogenolysis of ethane on metals.23-25 The work reported in the present article extends these NMR investigations of adsorbed molecules to methylamine, thus broadening the scope of the work to include the surface chemistry of carbon-nitrogen and nitrogen-hydrogen bonds. Although investigations of the interactions of methylamine and other amines with metal surfaces, including heterogeneous catalysis studies, have been reported by various groups,26-43 it does not appear that NMR spectroscopy has received attention in the work reported to date. The application of NMR spectroscopy in studies of adsorbed methylamine is a logical sequel to its use in the studies on adsorbed ethylene and acetylene, differing primarily from the earlier work in its inclusion of the nuclide 15N in such investigations. A comprehensive investigation of the reactions of methylamine over a wide range of supported metal catalysts by Sinfelt et al.33-36 provided the stimulus for the current NMR studies. The catalytic studies were part of a long-term research program concerned with the catalytic specificity of metals for reactions involving the activation of carbon-hydrogen, carbon-carbon, carbon-nitrogen, and carbon-halogen bonds.24,44,45 The work was conducted with mixtures of methylamine and hydrogen, which are known to exhibit two major kinds of reactions, hydrogenolysis and disproportionation.28,29,31

10.1021/jp0659251 CCC: $37.00 © 2007 American Chemical Society Published on Web 05/10/2007

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CH3NH2 + H2 f CH4 + NH3

(hydrogenolysis)

(1)

2CH3NH2 f (CH3)2NH + NH3

(disproportionation)

(2)

Whereas hydrogenolysis occurred on all of the metal catalysts investigated, it was overshadowed by the disproportionation reaction on a number of metals (especially palladium).33 Of the eight metals studied, including the 4d metals ruthenium through palladium and the 5d metals rhenium through gold, rhodium was the most active catalyst for the hydrogenolysis reaction yielding methane and ammonia. No significant amount of disproportionation occurred on rhodium.33 Because the simplest distribution of products (consisting of only methane and ammonia, with negligible dimethylamine or other products) was obtained with a rhodium catalyst, we decided to begin the NMR studies of adsorbed methylamine with such a catalyst. Other work particularly relevant to the present NMR investigation includes that of Kemball in the 1950s on the exchange reactions of deuterium with the hydrogen atoms in ammonia,47 methane,48 and methylamine46 on evaporated metal films and later studies by various workers on the adsorption of methylamine on metal single crystals.37-43 Considered as a whole, this work supports the hypothesis that the initial reaction occurring in the dissociative chemisorption of methylamine on a number of metals (including Pt, Pd, and Rh) is a dehydrogenation reaction yielding a hydrogen-deficient surface entity, (CN)Hx, with the carbon-nitrogen bond still intact. The dehydrogenation occurs at temperatures somewhat lower than those required for scission of the carbon-nitrogen bond. Moreover, on platinum, palladium, and rhodium surfaces, Kemball’s work on the aforementioned exchange reactions led to the conclusion that nitrogen-hydrogen bonds are more readily broken than carbon-hydrogen bonds in the dissociative chemisorption of ammonia, methane, and methylamine on these surfaces. Thus, when methylamine chemisorbs dissociatively on platinum, palladium, or rhodium, the initial dehydrogenation yielding the surface species (CN)Hx is attributed to dissociation of N-H rather than C-H bonds. The present NMR investigation provides further characterization of the surface species formed in the adsorption of methylamine on a rhodium surface at temperatures in the range of 253-298 K, with particular emphasis on the differentiation of dissociatively adsorbed from nondissociatively adsorbed species. The reactions of these surface species when the temperature is increased above 298 K were also investigated and are discussed in the context of previously reported results on the catalytic hydrogenolysis of methylamine on rhodium. Points of contrast with results of previously published work on rhodium single crystals under ultrahigh-vacuum conditions are also considered. Materials and Methods Samples. The Rh/SiO2 catalyst, prepared at Exxon Research and Engineering Co., had a metal dispersion of 52% as determined by hydrogen chemisorption. The dispersion is defined as the percentage of the metal atoms in the Rh clusters that are present in the surface. The metal loading of the catalyst was 5% by weight, and the surface area (BET) of the silica support was 750 m2/g. The average size of the Rh clusters was roughly 2 nm. Adsorption Apparatus and Procedure. The adsorption of methylamine on a sample was performed using a gas handling apparatus consisting of a glass furnace tube of 0.6-L volume connected to a glass manifold of 1.1-L volume that was attached

to a turbopump station and sources of ultrapure hydrogen, oxygen, and methylamine gases. Prior to exposure to methylamine, samples were treated using the following steps: First, the sample was heated to 573 K during evacuation (10-6 Torr) for 24 h, after which it was exposed to three cycles of alternating 10-min flows of ultrapure hydrogen and oxygen, between which the apparatus was evacuated to 10-3 Torr. The procedure was completed by flowing hydrogen over the sample for 15 min and then reevacuating to 10-6 Torr and cooling to the desired temperature for methylamine adsorption, e.g., room temperature or 77 K. In the case of the Rh/SiO2 catalyst, the Rh at this point is said to exist in the form of “clean” metal clusters. Samples used in the NMR experiments were exposed to 13CH315NH2 (99% 13C, 99% 15N, Isotec Inc.). Typical initial methylamine pressures were 10 Torr. Following equilibration, a sample was sealed in a small glass tube of roughly 1-cm3 volume, usually following reevacuation to 10-6 Torr to remove weakly adsorbed methylamine and excess methylamine gas. NMR Methodology. All NMR experiments were carried out at 9.40 T using a Chemagnetics console (Fort Collins, CO) and home-built NMR probes. Measurements at 77 K were performed by immersing the NMR probes in liquid nitrogen using a Janis cryostat designed to fit inside the 89-mm bore of a superconducting magnet. All 13C and 15N spectra were obtained from the Fourier transform of the second half of a spin-echo using a pulse sequence with pulse separation delay time τd. When multiple species existed, a deconvolution of the spectra was used to obtain the relative amounts of the various species. The 13C and 15N chemical shift reference scales were set using glycerol (CH doublet centered at 74.7 ppm and CH2 triplet centered at 65.2 ppm) and ammonium sulfate (0 ppm), respectively. Spin-lattice relaxation times, T1, were measured using the saturation recovery technique, i.e., by varying the repetition time, Trep, of the spin-echo pulse sequence. For the case of a single T1, the signal size, S, as a function of Trep, was fit using the relation S(Trep) ) S(∞)[1 - exp(-Trep/T1)]

(3)

When the data could not be reasonably well fit to eq 3, we attempted a fit using two T1 values, which required a total of four parameters: T1a; T1b; S(∞); and β, the ratio of the two components. The error bars associated with these four-parameter fits tended to be larger. Spin-spin relaxation times, T2, were measured by varying the total spin-echo evolution time, 2τd, and fitting the resulting data to the expression S(2τd) ) S(0) exp(-2τd/T2)

(4)

In general, echo amplitudes were measured for the purpose of extracting T1 and T2. However, echo amplitudes reflect the total signal from all nuclei of the sample, i.e., all 13C or all 15N. In cases where multiple species with different relaxation behaviors existed and the different species have different chemical shifts, e.g., chemisorbed CHy fragments versus CH4 gas, relaxation times as a function of spectral frequency or, equivalently, chemical shift, were measured via the Fourier transform of the time-domain data. Indeed, quantitative determinations of the relative amounts of various surface species required that both relaxation times, T1 and T2, be used to calculate the maximum signal for each species via extrapolation to the ideal case of Trep ) ∞ and τd ) 0. (To minimize this correction, spectra used for these determinations were obtained using τd < 50 µs and Trep ) 8 s.) Relaxation times also serve as indicators of the

Chemisorption and Decomposition of CH3NH2 on Rh/SiO2 bonding environment of particular nuclei, sometimes allowing for distinctions between species showing similar chemical shifts. Spin-echo double-resonance (SEDOR) spectroscopy is a powerful tool for the measurement of heteronuclear dipolar couplings, e.g., 13C-15N and 15N-1H. The dipolar coupling is proportional to the inverse cube of the internuclear distance r, and therefore, its measurement allows for the monitoring of changes in local structure, e.g., changes due to carbon-nitrogen bond scission and dehydrogenation. The experiment involves two parts. In the first part, a reference signal, S0, is obtained using a spin-echo pulse sequence. In the second part, a spinecho signal is again obtained for the observed nucleus (e.g., 13C), but an additional π pulse is applied to the second nucleus at a time τ, measured from the end of the π/2 pulse of the spinecho.49 The effect of this additional pulse, the so-called dephasing pulse, is to reduce the signal for those nuclei that are coupled to the second group of spins. This reduced or dephased signal is labeled S. The SEDOR fraction is then calculated from the change in the observed signal size and is plotted as a function of τ, with time τd between the two pulses of the spin-echo held fixed. The SEDOR fraction is described by the equation SEDOR fraction ) (S0 - S)/S0) 1 - cos(2Dτ/3)

(5)

The dipolar coupling constant, D, is a function of both the internuclear distance r and the orientation of the internuclear vector with respect to the applied magnetic field. When one performs an average over all possible orientations of the internuclear vector, the so-called powder average, the resulting theoretical curve has a rise time proportional to the magnitude of the dipolar coupling, and the SEDOR fraction ultimately reaches a plateau of 1.0 for a single coupled spin pair, i.e., the maximum dephasing is 100%. Experimentally, one often observes an overall scaling of the SEDOR fraction by a factor of R, where 0 e R e 1, because of imperfect dephasing pulses. In cases where multiple species exist, spectra obtained from the Fourier transforms of the second halves of the echo signals S0 and S allow for the calculation of the SEDOR fraction as a function of frequency. Furthermore, in the 13C-15N SEDOR experiments of this work, these spectra are particularly useful for determining the chemical shifts of species with intact carbon-nitrogen bonds. Results Interaction of Methylamine with the Support. The ultimate objective of this work was to investigate the interaction of methylamine with the surfaces of supported Rh metal clusters. However, it was first necessary to investigate the interaction of methylamine with the support. In a series of experiments, an adsorption/evacuation/readsorption sequence was used to determine whether any methylamine irreversibly adsorbs on the support. When the SiO2 support was exposed to methylamine at room temperature, the amount of methylamine adsorbed was 1.12 × 10-3 mol per gram of support. Following evacuation at room temperature, a second exposure to methylamine gave an adsorption of 1.13 × 10-3 mol per gram of support. Therefore, we conclude that no methylamine remains irreversibly adsorbed on SiO2 at room temperature. This result is crucial because it implies that, when a Rh/SiO2 catalyst is exposed to methylamine, evacuation at room temperature will remove all methylamine adsorbed on the support, leaving only methylamine interacting

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Figure 1. Room-temperature 13C spectra of [13C, 15N]-methylamine adsorbed on silica at 298 K: (a) without a subsequent evacuation step (amount adsorbed ) 3.3 × 10-4 mol/g) and (b) after evacuation to 10-6 Torr (amount adsorbed ) 3.0 × 10-6 mol/g). The spectra were obtained by averaging 13 312 and 19 456 spin-echoes, respectively (repetition time ) 4 s, π/2 pulse length ) 10.5 µs).

with the Rh clusters. If Al2O3 rather than SiO2 had been chosen as the support for the Rh, this simple result would not have been observed. Thus, when a typical Al2O3 support with a surface area of 200 m2/g was exposed to methylamine at room temperature, 9.4 × 10-4 mol of methylamine adsorbed per gram of support. For a determination of the amount of methylamine irreversibly adsorbed on the Al2O3, the sample was evacuated to 10-6 Torr for approximately 12 h at room temperature to remove weakly adsorbed molecules. A second exposure to methylamine gas was then performed, and this gave an adsorption of 6.0 × 10-4 mol of methylamine. This suggests that 3.4 × 10-4 mol of methylamine remained on the support after evacuation, still a significant amount. Given the relatively high boiling point of methylamine, 267 K, a second reevacuation was carried out at 373 K, and a third exposure to methylamine gas at room temperature was then performed. This gave an adsorption of 7.2 × 10-4 mol of methylamine, indicating that 2.2 × 10-4 mol of methylamine still remained irreversibly adsorbed. This is comparable in magnitude to the 2.5 × 10-4 mol of surface Rh atoms present in 1 g of catalyst containing 5 wt % Rh with a dispersion of 52%. Clearly, there is a strong interaction between methylamine and alumina, most likely between the acidic sites of alumina and the NH2 group of methylamine, similar to the strong adsorption of methylamine on Brønsted acid sites of zeolites observed by Dumesic et al.50 For the purposes of our investigation here, it is clear why SiO2 is preferable to Al2O3 as a support for Rh. Before we proceeded to investigate methylamine interactions with the Rh/SiO2 catalyst, we performed a series of NMR measurements on samples where methylamine was adsorbed on silica. Figure 1a shows room-temperature 13C spectra for a sample where 3.3 × 10-4 mol of [13C,15N]-methylamine was adsorbed on 1 g of SiO2 and the sample was subsequently sealed without evacuation, i.e., the weakly adsorbed methylamine remained. A single narrow absorption line centered at 26 ppm, the expected chemical shift for methylamine gas, is observed. The measured 13C spin-lattice relaxation time, T1, and spinspin relaxation time, T2, are 0.20 ( 0.02 s and 4.0 ( 0.4 ms, respectively, consistent with a highly mobile intact methylamine molecule weakly interacting with the support. A second sample was prepared where the initial amount of methylamine adsorbed was identical to the first sample but this

7786 J. Phys. Chem. C, Vol. 111, No. 21, 2007 sample was reevacuated prior to sealing, i.e., the weakly adsorbed methylamine should have been removed. The intensity of the 13C signal for the evacuated sample, shown in Figure 1b, is less than 1% of the intensity of the signal of the unevacuated sample, corresponding to about 3.0 × 10-6 mol of methylamine per gram of SiO2. This is roughly only about 1% of the number of moles of Rh in the surfaces of the metal clusters in 1 g of a Rh/SiO2 catalyst with a Rh loading of 5% and a Rh dispersion of 52%. Thus, with substantial strong adsorption of methylamine on the Rh in the Rh/SiO2 catalyst, one can reasonably expect that any contribution to the NMR signal from methylamine on the SiO2 will not be a problem. Further NMR measurements for the unevacuated sample included room-temperature 15N spectra, 77 K 13C and 15N spectra, and 13C-15N SEDOR spectroscopy at both room temperature and 77 K. The room-temperature 15N spectra show a single narrow line centered at -6 ppm, the expected 15N chemical shift, with a line width of 16 ppm. (All line widths are given in terms of the full width at half-maximum, fwhm.) The measured 15N T1 and T2 values are 0.020 ( 0.005 s and 1.5 ( 0.1 ms, respectively, again consistent with a mobile species. At low temperature, the absorption lines for both 13C and 15N were considerably broader than in the corresponding room-temperature spectra. The 13C line shape broadens to 55 ppm, and the -6 ppm peak in the 15N spectrum increases in width to approximately 120 ppm. The spin relaxation data for both nuclei suggest multiple components. The 15N T1 data obtained from echo amplitudes are best fit using two components. Of the total 15N nuclei, 81% ( 15% have a T1 value of 12.2 ( 3.0 s, and the remainder have a much shorter T1 value of 0.6 ( 0.2 s. Because 15N relaxation is dominated by mobility, the increased T1 values indicate the formation of a rigid structure. The 13C T1 data obtained from echo amplitudes are also best fit using two components, although the difference in T1 values is considerably smaller than for 15N. For this reason, the relative amounts of the two components and the T1 values were more difficult to determine. The majority of the 13C nuclei, 60% ( 20%, have a T1 value of 0.17 ( 0.09 s, and the remaining nuclei have a T1 value of 0.09 ( 0.03 s. In contrast to the 15N results, the 13C spin-lattice relaxation times do not increase with decreasing temperature, in agreement with relaxation being dominated by methyl rotation, a motion that is not frozen in at 77 K. To further investigate the range of relaxation times observed, an inversion recovery experiment for 13C was carried out. In this measurement, a π inversion pulse precedes a pair of spin-echo pulses with fixed separation, and the time, Tdelay, between the π inversion pulse and the spin-echo pulses is varied. The total repetition time of the sequence is chosen to be long compared to the longest T1 value of the sample. The series of spectra from this experiment suggest that at least two components contribute to the 13C line shape, both with similar average chemical shifts. The broader of these two components has a longer T1 value. In summary, the presence of multiple relaxation components for both 15N and 13C at 77 K but not at room temperature suggests that, whereas the molecules are highly mobile at room temperature and therefore each nucleus sees an average environment on the NMR time scale, at 77 K, this mobility is significantly reduced and the methylamine molecules experience long-lived interactions either with each other or with the support, leading to a range of local environments for the 15N and 13C nuclei. SEDOR measurements of the 13C-15N dipolar coupling were performed at both room temperature and 77 K. At room temperature, there was no observed SEDOR dephasing, con-

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Figure 2. SEDOR data at 77 K for 13C in [13C, 15N]-methylamine adsorbed on silica (O) and Rh/SiO2 (b). The abscissa is the time between a π/2 pulse for the observed nuclide (13C) and a π pulse for the dephasing nuclide (15N). Data are shown for several SEDOR experiments on the same two samples for which spectra are shown in Figures 1a and 4. The number of spin-echoes averaged in the SEDOR experiments ranged from 1024 to 5120. Pulse lengths for π/2 13C and π 15N pulses were 8 and 20 µs, respectively, and repetition times were 1 s (SiO2) and 2 s (Rh/SiO2). The solid and dashed curves are calculated SEDOR fractions for carbon-nitrogen distances in single bonds (1.47 Å) and triple bonds (1.15 Å).

sistent with large mobility and complete averaging of the 13C-15N dipolar coupling (recall the dependence of the dipolar coupling on the orientation of the internuclear vector with respect to the applied magnetic field). Results from 13C observed nucleus, 15N dephasing nucleus SEDOR measurements at 77 K are shown as the open circles in Figure 2, along with several calculated curves. The solid circles are the results of similar measurements for the Rh/SiO2 catalyst, which will be considered later. For the moment, we will simply ignore them. A vertical scaling factor, R, of 0.89 was used for all curves. As mentioned earlier, this scaling factor reflects imperfections due to the inability to completely invert the 15N spectrum, i.e., the weak 15N π pulse inverted only 89% of the 15N spins. Although the data are generally consistent with a carbon-nitrogen single bond length of 1.50 ( 0.03 Å, there are significant deviations from the theoretical curve calculated for a single isolated carbonnitrogen spin pair with this separation. In an effort to improve the quality of the fit, we considered a number of factors. Both 13C and 15N spectral and relaxation measurements suggest the presence of at least two species at 77 K. We therefore attempted to fit the data by allowing a mixture of two carbon-nitrogen bond lengths. A slightly improved fit was obtained for a mixture of 85% CsN single bond (1.47 Å) plus 15% CtN triple bond (1.15 Å). However, given that both the 13C and 15N roomtemperature spectra contain only one narrow line, at the respective methylamine shift, we can rule out the presence of more than an extremely small amount of nitrile species. A second approach to fitting the data was to consider the effects of intermolecular 13C-13C couplings due to close interactions between methylamine molecules at 77 K, e.g., condensation or clustering. The heteronuclear dipolar coupling for a carbonnitrogen spin pair separated by 1.5 Å is 900 Hz. The distance between two carbon nuclei that leads to a homonuclear dipolar coupling of the same size is 2.0 Å. Furthermore, homonuclear couplings of 200 and 100 Hz correspond to carbon-carbon distances of 3.4 and 4.2 Å, respectively. (The 13C T2 value at 77 K was measured to be approximately 850 µs, consistent with a 13C-13C dipolar coupling on the order of 250 Hz.) The effect of such homonuclear couplings was tested by SEDOR calcula-

Chemisorption and Decomposition of CH3NH2 on Rh/SiO2 tions performed for a carbon-nitrogen spin pair with a second 13C spin coupled to the carbon spin of the 13C-15N pair, with the relative orientation of the two internuclear vectors being varied. Results suggest that the presence of 13C-13C homonuclear dipolar couplings on the order of 100 Hz or greater do indeed affect the shape of the resulting SEDOR curve in a way consistent with the observed deviations. However, we were ultimately unable to arrive at a satisfactory fit, most likely because of our inability to accurately model the distribution of orientations of the two internuclear vectors and the simplification of using only three spins. As a third approach, we were able to obtain excellent fits to the SEDOR data by allowing for a Gaussian-weighted distribution of carbon-nitrogen distances centered at 1.5 Å. However, the width of this distribution, roughly 0.5 Å, was clearly unphysically large, giving an unreasonable spread of bond lengths. Ultimately, we believe that the inability to fit the SEDOR data using a single carbonnitrogen distance is primarily caused by the presence of significant intermolecular dipolar couplings due to the condensation or clustering of methylamine molecules. The presence of a mixture of at least two surface species with slightly different bond lengths might also be a factor, although the difference in the carbon-nitrogen bond lengths must be relatively small. In summary, at room temperature, a significant amount of methylamine adsorbs on silica. However, methylamine is removed from silica by evacuation at room temperature. The NMR results show that methylamine adsorbs intact, is highly mobile, and interacts very weakly with silica at room temperature. At 77 K, both spectral and relaxation NMR data indicate the presence of a mixture of methylamine species, i.e., methylamine molecules in a range of environments. At this temperature, it is likely that the surface layer condenses, forming methylamine aggregates. Such aggregates are known to be stable as a result of effective hydrogen bonding between molecules.51 The range of environments might then correspond to differences in the intermolecular interactions within these aggregates. Whereas the SEDOR data are generally consistent with a carbon-nitrogen single bond, the presence of these methylamine aggregates at 77 K and the resulting intermolecular dipoledipole couplings are believed to cause the poor agreement between the SEDOR data and the theoretical curves calculated for an isolated 13C-15N pair. Interaction of Methylamine with Rh/SiO2. Prior to NMR experiments for labeled methylamine adsorbed on the Rh/SiO2 catalyst, a series of adsorption/evacuation/readsorption experiments was performed at room temperature to determine the amount of methylamine that irreversibly adsorbs on the catalyst. It has already been shown that more than 99% of the methylamine adsorbed on silica at room temperature is removed by evacuation to 10-6 Torr. We therefore assume that, for the Rh/ SiO2 samples, all irreversibly adsorbed methylamine molecules are interacting with the Rh metal clusters. Interestingly, such adsorption appears to extend beyond an amount that one would expect for the completion of a chemisorbed monolayer on the metal surface. In previous studies involving acetylene and ethylene adsorbed on supported group VIII metal catalysts, the maximum amount of hydrocarbon irreversibly adsorbed at room temperature typically corresponded to about 0.25 mol per mole of surface metal atoms.20-22 This amount was deemed reasonable for the formation of a monolayer in the adsorption of an intact small hydrocarbon molecule. However, in this study, irreversible methylamine adsorptions as high as 0.5 mol per mole of surface metal atoms were observed at room temperature, depending on the initial gas pressure. Such high irreversible

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Figure 3. Spectra (13C at 77 K) of [13C, 15N]-methylamine adsorbed on Rh/SiO2 after annealing of the sample at different temperatures prior to the measurements of the spectra. The Rh/SiO2 was initially exposed at 77 K to a quantity of methylamine amounting to 0.25 mol per mole of surface Rh atoms. The spectra, represented by the solid lines, were obtained by averaging 1024 spin-echoes (pulse delay ) 8 s, π/2 pulse length ) 4.2 µs). The dotted lines are spectral components obtained by deconvolution of the spectrum measured after the annealing period at 273 K.

adsorptions on the metal suggest significant decomposition of methylamine upon adsorption and/or formation of methylamine multilayers. To minimize any decomposition or multilayer formation in the initial adsorption process, a sample was prepared by exposing a clean catalyst at 77 K to an amount of [13C,15N]methylamine gas corresponding to 0.25 mol per mole of surface rhodium atoms. The methylamine condensed in the sample tube (methylamine melting point, 180 K), which was subsequently sealed without evacuation and maintained at 77 K. Because of the condensation process, the methylamine was not likely to be interacting with the metal initially. To allow the methylamine to equilibrate with the catalyst, this sample was maintained at 190 K, slightly above the methylamine melting point of 180 K, for 7 days. A 13C spectrum was then obtained at 77 K and again after each of two subsequent 24-h annealing periods at 253 and 273 K. The spectra are shown as the solid lines in Figure 3. Taking into account T1 and T2 effects, the total integrated intensities of these spectra in Figure 3 are approximately the same. After the anneal at 190 K, the spectrum is similar to that found for methylamine on SiO2, i.e., a single peak at the methylamine shift of 26 ppm. However, the line width is significantly greater, 95 versus 55 ppm. The 13C spin lattice relaxation data are again best fit with two components, the major one, approximately 70% ( 15%, with a T1 value of 0.10 ( 0.03 s, and the other with a T1 value of 1.3 ( 0.3 s, the latter being considerably longer than either of the T1 values observed for methylamine on SiO2: 0.17 and 0.09 s. After the anneal at 253 K, a small tail to the left of the main peak at 26 ppm can be seen, indicating the formation of a new species. Following the 273 K anneal, the spectrum is considerably more asymmetric, consistent with additional formation of such a new species. We obtain a reasonable fit to this spectrum from a sum

7788 J. Phys. Chem. C, Vol. 111, No. 21, 2007 of a Lorentzian line of width 110 ppm centered at 26 ppm (slightly broader than the line observed at 190 K) and a Gaussian line of width 310 ppm centered at 200 ppm (see the dotted lines in Figure 3). The Gaussian line accounts for roughly 35% of the total intensity of the spectrum. We interpret the foregoing results as follows: After the annealing period at 190 K, intact molecules of methylamine are present in the form of multilayer aggregates38 on the surfaces of the rhodium clusters. Some of the methylamine molecules are in direct contact with the rhodium, whereas others are not. Thus, we consider two different kinds of methylamine molecules in the aggregates. They are expected to have similar chemical shifts but different line widths and relaxation times, with the molecules in direct contact with the rhodium having a greater line width and a longer relaxation time because of the direct effect of the metal. The observed width of the 26 ppm line in the 13C spectrum is therefore due to contributions from the two kinds of intact methylamine and is expected to be a function of the relative amounts of the two kinds present. After the annealing periods at 253 and 273 K, the new species that appears is identified as a partially dehydrogenated surface entity, (CN)Hx. This identification is based partly on the known downfield shift of 13C absorption lines of imines and nitriles relative to amines52 and partly on the basis of investigations of other workers on the adsorption and reactions of methylamine on group VIII metals by methods other than NMR spectroscopy.28,37-39,42,43 The broad line in the 13C spectrum at 200 ppm that is associated with this species indicates strong interaction with the metal. The strong interaction of methylamine with group VIII metals is well-known.37-39,42,43 Moreover, in previous studies of acetylene and ethylene on supported platinum clusters, where the stable room-temperature species were strongly adsorbed vinylidene and ethylidyne, the 13C line widths were similar to those in the present work.15,20-22 A sample in which [13C, 15N]-methylamine was contacted with the Rh/SiO2 catalyst at 298 K was also prepared. The amount of methylamine taken up by the catalyst, as determined from a measurement of the change in gas pressure in the adsorption apparatus after contact of the methylamine with the catalyst, was 0.4 mol per mole of surface rhodium atoms. Before the sample was sealed off from the apparatus, it was evacuated to 10-6 Torr to remove weakly adsorbed species, and therefore, the true final adsorption was presumably lower. Measurements of 13C and 15N spectra were made at both 77 and 298 K. The 15N spectrum at 77 K includes a strong background signal arising from the natural-abundance 15N in the liquid nitrogen temperature bath. A determination of the true 15N spectrum at 77 K therefore requires removal of the background signal from the measured spectrum. The spectra shown in Figure 4 are the 13C and 15N spectra at 298 K (solid lines). Also shown (dotted lines) are separate Lorentzian and Gaussian lines obtained by deconvoluting the spectra. Consistent with the locations of such lines in Figure 3, the 13C spectrum in Figure 4 exhibits a line centered at 26 ppm and a broad line centered at about 200 ppm. However, there is an additional broad and weaker line centered around 400 ppm that will be considered later in this article. In the 15N spectrum in Figure 4, there is a line at -6 ppm corresponding to the 26 ppm line in the 13C spectrum. There is also a broad line in the 15N spectrum centered around 200 ppm, but no evidence was found for an additional broad line such as the 400 ppm line in the 13C spectrum. The fraction of the total spectral intensity associated with the 26 ppm line in the 13C spectrum or the -6 ppm line in the 15N spectrum is roughly the same, about 0.60-0.65. In the 13C spectrum in Figure 4,

von Schenck et al.

Figure 4. Room-temperature 13C and 15N spectra (solid lines) of [13C, 15 N]-methylamine adsorbed on Rh/SiO2 at 298 K and the resolution of the spectra into components (dotted lines). After an initial adsorption of 0.4 mol per mole of surface Rh atoms, the sample was evacuated to 10-6 Torr at 298 K prior to the measurements of the spectra. The 13C spectrum was acquired by averaging 3072 spin-echoes (pulse delay ) 8 s, π/2 pulse length ) 2.7 µs), and 4096 spin-echoes (pulse delay ) 4 s, π/2 pulse length ) 4.4 µs) were averaged in obtaining the 15N spectrum.

the widths of the 26 ppm line and the broad line at 200 ppm are 20 and ∼325 ppm, respectively. In the 15N spectrum, the -6 ppm line has a width of 15 ppm, whereas the width of the broad line at 200 ppm is roughly 400 ppm. When the spectra obtained at 77 K are examined for comparison, it is found that the 26 ppm line in the 13C spectrum and the corresponding -6 ppm line in the 15N spectrum are broader, with line widths of 70 and 150 ppm, respectively. In contrast, it is found that the widths of the broad lines at 200 ppm are essentially unchanged. With regard to the strongly chemisorbed species associated with the broad lines in the spectra, some results of spin-lattice relaxation measurements are of interest. Relaxation measurements for the broad lines at both 298 and 77 K are complicated by the spectral overlap, and we were not able to obtain satisfactory fits to the saturation recovery data using simple models with one or two T1 values. However, in the case of the 13C spectra, for example, we were able to determine for the broad line at 200 ppm ranges of T1 values from 0.2 to 0.5 s at 298 K and from 1 to 2 s at 77 K. This rough inverse temperature dependence is consistent with the broad line being associated with species strongly bound to the metal surface and being relaxed via the metal conduction electrons.53,54 Ranges of relaxation times associated with the broad line in the 15N spectra exhibited roughly the same kind of variation with temperature, again being consistent with surface species strongly bound to the metal clusters. In contrast, the lines at 26 and -6 ppm in the 13C and 15N spectra are associated with species less strongly bound to the metal that are highly mobile at room temperature. To obtain further insight into the structure and bonding of the surface species, we attempted to use SEDOR measurements

Chemisorption and Decomposition of CH3NH2 on Rh/SiO2

Figure 5. Room-temperature 13C and 15N spectra (solid lines) for the methylamine on Rh/SiO2 sample of Figure 4 after a 3-h annealing period at 350 K following a similar annealing period at 325 K. Resolution of each of the spectra into components (dotted lines) is also shown. The experimental parameters for each spectrum were the same as for those shown in Figure 4.

to determine carbon-nitrogen and nitrogen-hydrogen dipolar couplings. A series of measurements was performed at 77 K: (i) 13C observed nucleus, 15N dephasing nucleus; (ii) 15N observed nucleus, 13C dephasing nucleus; (iii) 15N observed nucleus, 1H dephasing nucleus. In all cases, a significant amount of dephasing was observed over a broad range of frequencies. However, quantitative analysis of the SEDOR data was difficult. For example, whereas the 77 K 13C and 15N spectra are dominated by features that we associate with irreversibly adsorbed intact methylamine (located at 26 and -6 ppm, respectively), a plot of the SEDOR fraction as a function of τ measured at these frequencies in a 13C observed nucleus, 15N dephasing nucleus experiment (see the data represented by the solid circles in Figure 2) only qualitatively matches the theoretical prediction for a C-N single bond length of 1.5 Å. Indeed, there are significant deviations from the theoretical curve. The data are not very different from the data obtained for methylamine weakly adsorbed on the silica support itself (open circles in Figure 2), suggesting that intermolecular dipole-dipole couplings present in condensed methylamine aggregates played a significant role. For the catalyst samples, significant intermolecular dipole-dipole couplings might be present for molecules adsorbed as a multilayer. Analysis of the SEDOR fraction data for the broad downfield line was complicated by the short T2 value of the species associated with this line, which leads to low-intensity signals at echo evolution times longer than a few hundred microseconds. (In the case of 15N observed nucleus experiments, the significant overlap with the 15N signal from liquid nitrogen was an additional complication.) Although it was difficult to extract accurate quantitative bond length information from the SEDOR data, such data found application in experiments to monitor reactions of surface species, where SEDOR spectroscopy was used primarily to monitor the disappearance of species with strong carbon-

J. Phys. Chem. C, Vol. 111, No. 21, 2007 7789

Figure 6. Room-temperature 13C and 15N spectra (solid lines) for the methylamine on Rh/SiO2 sample of Figure 4 after a 3-h annealing period at 425 K subsequent to a succession of similar annealing periods at 325, 350, 375, and 400 K. Resolution of each of the spectra into components (dotted lines) is also shown. The experimental parameters for each spectrum were the same as for those shown in Figure 4.

nitrogen dipolar couplings, i.e., to monitor carbon-nitrogen bond scission. Reactions of Methylamine Adsorbed on Rh/SiO2. A study was made of the reactions of methylamine adsorbed on Rh/ SiO2 with the sample prepared by exposing the catalyst to [13C, 15N]-methylamine at room temperature in the adsorption apparatus and then evacuating the apparatus to 10-6 Torr. Carbonnitrogen scission and subsequent reactions of the surface species resulting from such scission were monitored by performing NMR measurements at 77 and 298 K after each of a succession of 3-h annealing periods at temperatures between 325 and 450 K in the order 325, 350, 375, 400, 425, and 450 K. SEDOR measurements at 77 K were used to distinguish hydrogendeficient surface species with intact carbon-nitrogen bonds from surface species formed via scission of such bonds. It is difficult to determine quantitatively the fraction of adsorbed methylamine that has undergone scission of carbon-nitrogen bonds from SEDOR measurements alone, because of substantial differences in the relaxation times, both T1 and T2, of the various species, combined with the need to use relatively long total evolution times (on the order of 1 ms) to achieve substantial SEDOR dephasing. However, by carrying out separate T1 and T2 experiments at room temperature for 13C and 15N, the extents of formation of various species could be determined from the relative integrated intensities of the various spectral features in the room-temperature spectra whose assignments are aided by the SEDOR results obtained at 77 K. The room-temperature 13C and 15N spectra after the annealing periods at 350 and 425 K are presented in Figures 5 and 6, respectively. Also shown in these figures are calculated spectra that were obtained by deconvoluting the total spectra into components corresponding to various surface species. A summary of the results from these deconvolutions is provided in Tables 1 and 2.

7790 J. Phys. Chem. C, Vol. 111, No. 21, 2007

von Schenck et al.

TABLE 1: Disposition of 13C among Various Species Produced in the Chemisorption and Decomposition of [13C, 15N]-Methylamine on Rh/SiO 2 molecular speciesa

chemisorbed radicalsa

T (K)

CH3NH2

CH4

∑CnHm, n > 1

CHy

(CN)Hx

298 325 350 375 400 425 450

60 59 47 26 13 9 3

0 2 11 15 17 19 21

0 0 0 16 23 27 30

9 10 16 20 19 18 14

31 29 26 23 28 27 32

a

Uncertainties in the values are approximately (5%.

TABLE 2: Disposition of 15N among Various Species Produced in the Chemisorption and Decomposition of [13C, 15N]-Methylamine on Rh/SiO 2 molecular speciesa

chemisorbed radicalsa

T (K)

CH3NH2

NH3

(CN)Hx

298 325 350 375 400 425 450

65 65 43 20 5 0 0

0 4 27 50 60 70 80

35 31 30 30 35 30 20

a

Uncertainties in the values are approximately (10%.

After the annealing period at 325 K, there is evidence for the formation of methane and ammonia: a new narrow line at -7 ppm in the 13C spectrum and a new narrow line at -22 ppm in the 15N spectrum. The spectra obtained after the annealing periods at 350 and 425 K (Figures 5 and 6) show these lines very clearly. In addition, the broad low-field line centered around 400 ppm in the 13C spectra in Figures 5 and 6 is somewhat clearer than the corresponding line in Figure 4. We attribute this line to monocarbon surface fragments CHy on the Rh clusters. Carbon resonating around 400 ppm has been assigned to carbidic species in a study of CO hydrogenation on Ru,55 and similar low-field broad lines have been observed for 13CH species on supported Pt catalysts.56 At this point, one y might wonder about the absence of a line in the 15N spectrum for surface fragments NHy. Perhaps such fragments are hydrogenated to ammonia as soon as they are formed from (CN)Hx via carbon-nitrogen scission. As the annealing temperature is increased, we also observe additional narrow lines in the 13C spectra at shifts ranging from 40 to -6 ppm. We assign these features to methylene and methyl carbons of additional gaseous product species CnHm, n > 1, formed from CHy fragments on the surface. Reactions of carbon fragments forming Cn, n > 1, carbon species have been observed when reacting methane over Rh, Pt, and Ru.57-59 The formation of these products in our experiments is a natural consequence of the lack of sufficient hydrogen to produce only methane from the monocarbon surface species. Under these circumstances, some of the monocarbon species combine to form higher-carbon-number species. For the highest annealing temperatures, we observed some additional features including peaks of weak intensity ( 1

CHy

(CN)Hx

298 350 400 450

3 4 5 3

0 3 6 12

0 0 8 13

10 9 17 8

87 84 64 64

a Surface residue was formed by the decomposition of chemisorbed [13C, 15N]-methylamine on Rh/SiO2 at 450 K and 10-6 Torr. b Uncertainties in the values are approximately (5%.

TABLE 4: Disposition of 13C in the System after the Interaction of [13C, 15N]-Methylamine with a Surface Residue Formed from Unlabeled Methylaminea molecular speciesb

chemisorbed radicalsb

T (K)

CH3NH2

CH4

∑CnHm, n > 1

CHy

(CN)Hx

298 350 400 450

100 61 21 2

0 11 10 13

0 0 14 21

0 5 23 16

0 23 32 48

a Surface residue was formed by the decomposition of unlabeled chemisorbed methylamine on Rh/SiO2 at 450 K and 10-6 Torr. b Uncertainties in the values are approximately (5%.

of the total 13C intensity is associated with methane and higher hydrocarbons with roughly 50% of the total 15N nuclei appearing as ammonia. Virtually all of the methylamine reacts. The remaining carbons are associated with adsorbed surface species such as (CN)Hx, (C2N)Hx, and CHy. A goal of these experiments was to determine whether a carbonaceous overlayer would interact with additional methylamine admitted into the system. The formation of 13CH4 in the sample where the overlayer was prepared using labeled methylamine indicates that the overlayer is reactive, i.e., carbon fragments from the overlayer are hydrogenated to form methane. Furthermore, the presence of a preformed overlayer did not materially hinder the decomposition of subsequently added methylamine, as shown by the data for the other sample.

7792 J. Phys. Chem. C, Vol. 111, No. 21, 2007 However, it does appear that formation of ammonia, methane, and higher hydrocarbons was less extensive for the samples with preformed overlayers. This might reflect the lower average hydrogen population of the surface for these latter samples, i.e., heating under vacuum removed hydrogen. Discussion At temperatures in the range of 190-253 K, methylamine is largely adsorbed intact on the rhodium clusters in the Rh/SiO2 catalyst. At 253 K, there is evidence of incipient formation of a new surface species. The evidence consists of the appearance of a small tail on the downfield side of the main NMR absorption line for the adsorbed methylamine in either the 13C or 15N spectrum. The broadened 13C and 15N lines can each be resolved into two separate lines, the original relatively narrow line and a very broad second line overlapping the original line on the downfield side. The broad line is attributed to the partially dehydrogenated surface species (CN)Hx with the carbonnitrogen bond still intact. Hwang, Kong, and Schmidt,38 in a study of the adsorption and decomposition of methylamine on the (111) plane of a rhodium single crystal by temperature-programmed desorption (TPD) and Auger electron spectroscopy (AES) in an ultrahighvacuum (UHV) apparatus, observed desorption of molecular hydrogen at a temperature of 320 K. This indicated that dehydrogenation of the methylamine on the surface was already occurring in the vicinity of room temperature. Moreover, the work of Kemball and Wolf46 on the exchange of deuterium with the hydrogen atoms in methylamine on evaporated films of platinum and palladium showed that the reaction was occurring at temperatures as low as 225-250 K. Because only two of the five hydrogen atoms in methylamine exchanged with the deuterium, the data indicated that the exchange involved only the hydrogen atoms bonded to the nitrogen. Presumably, the exchange, which occurred stepwise, proceeded through the chemisorbed radical CH3NH bonded to the metal surface through the nitrogen atom. It seems probable that rhodium would behave very similarly to platinum and palladium in activating the N-H bonds in methylamine more readily than the C-H bonds, because it catalyzes the exchange of deuterium with the hydrogen atoms in ammonia much more readily than it catalyzes the exchange with the hydrogen atoms in methane.47,48 We note that the exchange reaction of deuterium with the hydrogen atoms in ammonia is observed on a rhodium film at a temperature as low as 255 K,47 which is close to the temperature at which the broad NMR absorption lines are first observed in the 13C and 15N spectra in the present work. However, the exchange of deuterium with the hydrogen atoms in methane requires substantially higher temperatures on such a rhodium film, 410-490 K.48 Thus, in the present work, it seems reasonable to conclude that the chemisorbed radical CH3NH, the composition of which corresponds to x ) 4 in the generalized formula (CN)Hx, is the first species formed when methylamine begins to decompose on rhodium clusters at temperatures in the vicinity of 250 K. As the temperature is increased, the degree of dehydrogenation of the surface species (CN)Hx increases, as indicated by a decreasing value of x. When methylamine is contacted with the rhodium clusters at 298 K, and the system is subsequently evacuated to 10-6 Torr, NMR studies of the species present on the rhodium clusters or in the vapor space in the sealed sample cell after a series of heat treatments at progressively higher temperatures provide

von Schenck et al. clear evidence of the scission of carbon-nitrogen bonds. First, there is a broad NMR absorption line in the 13C spectrum attributable to surface fragments CHy. This line overlaps the broad line associated with the surface species (CN)Hx on the downfield side, but is resolvable from it. Second, the appearance of ammonia and methane in the system, as observed after the heat treatment at 325 K, shows unequivocally that C-N bonds have been broken. The quantities of ammonia and methane increase as the temperature of the heat treatment is increased over the range from 325 to 450 K. Ammonia is observed to form much more readily than methane. For scission of the C-N bond to occur in (CN)Hx species chemisorbed on a metal surface, it has been proposed that bonding to the metal must occur through both the carbon and nitrogen atoms. It has also been argued that at least one of these atoms must form more than one bond with the metal.31 If these criteria are adopted, one concludes that the value of x in the species (CN)Hx undergoing C-N scission must have an upper limit of 2. Similar criteria have been proposed for the scission of C-C bonds in alkanes on metal surfaces.24,60,61 Continuing this line of reasoning, we note that the dehydrogenation of methylamine prior to the scission of its C-N bond releases a considerable amount of hydrogen on the surface. The carbon and nitrogen surface fragments CHy and NHz formed during the C-N bond scission consume the hydrogen in the formation of methane and ammonia, with most of the hydrogen going to the latter. As surface sites are vacated during these reactions, the nondissociatively adsorbed methylamine on the surface simultaneously undergoes dehydrogenation, thereby releasing more hydrogen on the surface and replenishing the (CN)Hx species undergoing C-N scission. The process continues until the nondissociatively adsorbed methylamine is no longer present, as indicated by the disappearance of the primary 13C and 15N absorption lines associated with that species in the NMR spectra. This situation is closely approached after the heat treatment at 450 K. After the heat treatments at 375 K and higher temperatures, features appear in the 13C NMR spectra indicating that other hydrocarbons, most likely ethane and propane, are formed in addition to methane. The kind of reaction leading to these products is reminiscent of the hydrocarbon chain growth occurring in Fischer-Tropsch synthesis.62 At temperatures of 375-450 K in the present work, the concentration of carbon fragments CHy relative to that of hydrogen on the surface presumably becomes high enough to permit reactions involving the combination of such fragments to become competitive with the hydrogenation of the fragments yielding methane. Studies of Meitzner, Mykytka, and Sinfelt33,35,36 of the reaction of methylamine with hydrogen on the rhodium clusters in a Rh/SiO2 catalyst in a steady-state flow reactor yielded ammonia and methane as products in the same range of temperatures as required for their formation in the present work. This correspondence provides support for the view that the catalytic reaction in the flow system proceeds through a partially dehydrogenated surface intermediate such as the surface species (CN)Hx identified in the present NMR investigation. However, the flow reactor studies with methylamine-hydrogen mixtures did not yield any hydrocarbons other than methane, except possibly in trace amounts. This was undoubtedly due to higher concentrations of hydrogen on the surface in those studies. The concentration of hydrogen relative to that of the carbon fragments CHy on the rhodium clusters in that work presumably was high enough at all times to ensure that hydrogenation was the only significant reaction of the fragments.

Chemisorption and Decomposition of CH3NH2 on Rh/SiO2 When the results of the present NMR studies of the decomposition of methylamine on supported rhodium clusters in a sealed sample cell are compared to results of methylamine decomposition on the (111) face of a rhodium single crystal under ultrahigh-vacuum (UHV) conditions, some striking differences are found. The UHV studies of Hwang, Kong, and Schmidt,38 to which reference was made earlier, showed that scission of the carbon-nitrogen bond occurred much less readily than it does in the present work, and virtually no ammonia or methane was formed. Bond scission was not observed at temperatures lower than about 820 K. Moreover, when it was observed, the products were nitrogen and surface carbon. The surface concentration of hydrogen at temperatures higher than about 320 K was probably negligible in that work, with the result that a partially dehydrogenated surface intermediate such as (CN)Hx underwent further dehydrogenation to the surface intermediate CN in preference to scission of the C-N single bond. If one visualizes the carbon-nitrogen bond in chemisorbed CN as a multiple bond (i.e., CdN or CtN) instead of a single bond with the carbon and/or nitrogen forming multiple bonds with the metal surface, the resistance to scission is readily understood. In the present work, a relatively high surface concentration of hydrogen might suppress the dehydrogenation of (CN)Hx to CN in the sealed sample cell, with the result that (CN)Hx then undergoes significant C-N scission with formation of ammonia and methane at a temperature as low as 325 K. The overall adsorption layer remaining on the rhodium clusters after the final heat treatment at 450 K has a N/C atomic ratio of about 0.4. The H/C atomic ratio in the overall layer is lower than 2. These values compare with N/C and H/C atomic ratios of 1 and 5 in methylamine. In experiments where the sample cell is heated to 450 K while being continuously evacuated to 10-6 Torr, the surface hydrogen concentration and the H/C ratio in the overall adsorption layer are undoubtedly lower than they are in the experiments with the sealed cell. Quantitative estimates of N/C and H/C atomic ratios in the resulting overall layer could not be made in these experiments. However, the adsorption of additional methylamine on a sample containing such a layer on the rhodium clusters, followed by heat treatments in a sealed sample cell of the kind already discussed, produced data of some interest. The data showed that both the original adsorption layer and the added methylamine participated significantly in the reactions that occurred during the heat treatments. This information was obtained by performing two separate experiments, one in which only the starting adsorption layer was labeled with 13C and 15N and another in which only the added methylamine was labeled in this way. The results showed that the adsorption layer, although possibly being almost completely devoid of hydrogen, was still reactive. Moreover, they provided good support for the general scheme described earlier in this section for the decomposition of methylamine adsorbed on clean rhodium clusters as a result of subsequent heat treatments. Acknowledgment. H.v.S. thanks the Sweden-America Foundation for a scholarship and the Ernst Johnson foundation for financial support. Acknowledgement is made to the donors of the Petroleum Research Fund administered by the American Chemical Society for funding this research. References and Notes (1) Duncan, T. M.; Yates, J. T., Jr.; Vaughan, R. W. J. Chem. Phys. 1980, 73, 975. (2) De Menorval, L. C.; Fraissard, J. P. Chem. Phys. Lett. 1981, 77, 309.

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