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J. Phys. Chem. C 2008, 112, 3042-3048
NMR Study of the Chemisorption and Surface Chemistry of Methylamine on Pd/SiO2 Henrik von Schenck,‡,| Neil Kumar,† Christopher A. Klug,*,†,§ and John H. Sinfelt⊥ Department of Chemical Engineering, Stanford UniVersity, Stanford, California 94305-5025, Department of Materials Physics, The Royal Institute of Technology, Stockholm, Sweden, and P. O. Box 364, Oldwick, New Jersey 08858 ReceiVed: August 17, 2007; In Final Form: NoVember 15, 2007
Results of an investigation of the interaction of methylamine with a Pd/SiO2 catalyst utilizing nuclear magnetic resonance (NMR) spectroscopy are presented. Two sets of experiments were conducted. In one set, a sample of catalyst was initially exposed to 13CH315NH2 at 77 K and then subjected to a lengthy equilibration period at 190 K. A 13C spectrum at 77 K obtained on the sample at this point exhibited a single symmetric line at a frequency very close to the resonance frequency for methylamine in the absence of the catalyst. The line is attributed to methylamine adsorbed without dissociation. In 13C spectra obtained at 77 K on the sample after each of a series of subsequent annealing periods at successively higher temperatures in the range 253-298 K, the line exhibited a gradual broadening on the downfield side. Deconvolution of the spectra resolved the original symmetric line from a very broad downfield line attributed to partially dehydrogenated surface species designated by the formula (CN)Hx. In the second set of experiments, 13CH315NH2 was adsorbed on a sample of catalyst at 298 K. The sample was then annealed at successively higher temperatures in the range 325-450 K. Deconvolution of the 13C spectra obtained at 298 K after the various annealing periods revealed extensive formation of dimethylamine, reaching a maximum after the annealing period at 400 K. Ammonia and methane were first detected in the 15N and 13C spectra, respectively, after the annealing period at 375 K. During the annealing periods at 425 and 450 K, the dimethylamine decomposed almost completely, leaving only methane, ammonia, and residual surface species.
Introduction The interaction of amines, primarily methylamine, with metal surfaces has been investigated by various workers.1-16 The investigations have been conducted by exposing a metal surface to the amine alone or to mixtures of the amine and hydrogen. The investigations with mixtures include studies of the catalytic conversion of methylamine on supported metal catalysts in steady-state flow reactors.6-9 In the present paper we report results of studies using nuclear magnetic resonance (NMR) spectroscopy to characterize the surface species present on the palladium metal clusters in a Pd/SiO2 catalyst at temperatures in the range of 253-298 K subsequent to lower temperature exposure of the catalyst to methylamine labeled with 13C and 15N nuclides. Also included are results of studies of reactions of the surface species when the temperature is increased above 298 K. A comparison is made of the findings of this investigation with recently reported findings of a similar investigation on the rhodium metal clusters in a Rh/SiO2 catalyst,17 and the findings of both investigations are considered in relation to results of catalytic studies with mixtures of methylamine and hydrogen on similar palladium and rhodium surfaces. Whereas methane and ammonia are the only products observed with a rhodium surface in such catalytic * Corresponding author. Telephone 202-767-3239. E-mail: klug@ nrl.navy.mil. † Stanford University. ‡ The Royal Institute of Technology. ⊥ P. O. Box 364, Oldwick, NJ 08858. | Current address: COMSOL AB, Tegne ´ rgatan 23, SE-111 40 Stockholm, Sweden. § Current address: Chemistry Division, Naval Research Laboratory, Washington, DC 20375.
studies, dimethylamine is the predominant product obtained on palladium. This dramatic example of catalytic specificity provided the incentive for the present NMR study of the interaction of methylamine with the palladium clusters in a Pd/SiO2 catalyst. Materials and Methods Catalyst. The Pd/SiO2 catalyst used in this investigation consisted of small clusters of palladium dispersed on a silica support. It was prepared at Exxon (now Exxon Mobil) Research and Engineering Company. The Pd content was 5% by weight and the surface area of the silica support was 750 m2/g. The palladium metal dispersion, defined as the percentage of the palladium metal atoms in the catalyst that are present in the surfaces of the palladium metal clusters, was 49% (as determined by carbon monoxide chemisorption). The average size of the metal clusters estimated from the dispersion was approximately 2 nm. Apparatus and Procedure. Samples for the NMR experiments were prepared by exposing about 1 g of catalyst to a known volume of 13CH315NH2 gas (99% 13C, 99% 15N, Isotech, Inc.) at a pressure initially in the approximate range of 7-20 Torr. Prior to such exposure, the catalyst was placed in a quartz furnace tube of 0.6 L volume connectible at one end to a glass manifold of 1.1 L volume. The manifold was in turn connectible to various gas sources and to a turbo pump station for evacuation of the system when the other end of the furnace tube was closed off. With the furnace tube in place in the furnace, the catalyst was first heated to 573 K during evacuation to 10-6 Torr. The evacuation was then continued for 24 h at 573 K. Next the catalyst was subjected to three cycles of alternating 10-min flows of ultrapure hydrogen and oxygen, with evacuation of the
10.1021/jp076637n CCC: $40.75 © 2008 American Chemical Society Published on Web 02/05/2008
NMR Study of Methylamine on Pd/SiO2
J. Phys. Chem. C, Vol. 112, No. 8, 2008 3043
apparatus to 10-3 Torr between cycles. In the final step of the procedure hydrogen was passed over the catalyst for 15 min, and this was followed by reevacuation to 10-6 Torr. The catalyst was exposed to 13CH315NH2 either before or after transfer of the catalyst from the furnace tube (via a side arm of the tube) to an evacuated glass NMR sample vial of about 1 cm3 volume. An exposure at 77 K was made after transfer of the catalyst to the sample vial by simply condensing 13CH315NH2 from the furnace tube (and its side arm) into the catalyst-loaded vial immersed in liquid nitrogen, after which the vial was sealed off from the system for an NMR experiment. An exposure of the catalyst to 13CH315NH2 at 298 K was made directly in the furnace tube, after which the sample was evacuated to 10-6 Torr to remove any weakly adsorbed species. The catalyst sample was then transferred to an evacuated NMR sample vial that was subsequently sealed off from the side arm of the furnace tube. With this procedure, the state of evacuation of the sample was maintained for the NMR experiment. 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 Fourier transforms of the second halves of the spin echo signals. The typical dwell time was 2 µs. 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. The signal size, S, as a function of Trep was fit using the relation:
S(Trep) ) S(∞)[1 - exp(-Trep/T1)]
(1)
Spin-spin relaxation times, T2, were measured by varying the total spin echo evolution time 2τd (where τd is the pulse separation delay time) and fitting the resulting data to:
S(2τd) ) S(0)exp(-2τd/T2)
(2)
In general, echo amplitudes were measured for the purposes of extracting T1 and T2. However, echo amplitudes reflect the total signal from all nuclei of the sample, e.g., all 13C or all 15N. In cases where multiple species with different relaxation behaviors existed and the different species had different chemical shifts, relaxation times as a function of spectral frequency were obtained by Fourier transforming the time domain data. Indeed, quantitative determinations of the relative amounts of various surface species required use of both relaxation times, T1 and T2, to calculate the maximum signal for each species via extrapolation to the ideal case of Trep ) ∞ and τd ) 0. (To minimize the uncertainty of such extrapolations, spectra used for these determinations were obtained using τd < 50 µs and Trep ) 8 s.) In addition to their usefulness in this regard, relaxation times also serve as indicators of the bonding environment of particular nuclei, sometimes allowing for distinctions between species showing similar chemical shifts. Spin-echo double-resonance (SEDOR) is a powerful tool for the measurement of heteronuclear dipolar couplings, e.g., 13C-15N. The dipolar coupling (D) is proportional to the inverse cube of the internuclear distance r, and therefore its measure-
ment allows for monitoring changes in local structure. 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 spin echo signal is again obtained for the observed nucleus (e.g., 13C) but an additional π-pulse is applied to the second nucleus (e.g., 15N) at a time τ measured from the end of the π/2-pulse of the spin echo.18 The effect of this additional pulse, the so-called dephasing pulse, is to reduce the signal for those observed nuclei that are coupled to the dephasing nuclei. This reduced or dephased signal is labeled S. The SEDOR fraction, (S0 - S)/S0, is then calculated from the change in the observed signal size and is plotted as a function of τ with the time τd between the two pulses of the spin echo held fixed. From such data, the dipolar coupling, D, can be obtained from the relation:
SEDOR fraction ) (S0 - S)/S0 ) 1 - cos(2Dτ/3) (3) Internuclear distances can in turn be obtained from values of D. SEDOR has been successfully applied to identify surface species through bond lengths for simple molecules adsorbed on metal clusters, e.g., ethylene and acetylene adsorbed on supported platinum clusters.19-23 In the present work, however, short spin-spin relaxation times in combination with very broad widths of the signals associated with the surface species made it very difficult to obtain quantitative information on SEDOR dephasing. Only qualitative conclusions could be drawn from a comparison of a reference signal (S0) with a dephased signal (S). Results The interaction of methylamine with the high surface area silica support of the Pd/SiO2 catalyst has been described in a previous NMR study of methylamine adsorption on a Rh/SiO2 catalyst.17 Methylamine adsorbed on the SiO2 at room temperature was completely removed by evacuation to 10-6 Torr, indicating weak interaction. Since methylamine interacts strongly with group VIII metals,10-15 but only very weakly with silica, it was concluded that all irreversibly adsorbed species formed when a Rh/SiO2 catalyst was contacted with methylamine were associated with the Rh metal clusters in the catalyst.17 In the present work on methylamine adsorption on Pd/SiO2, we conclude similarly that all irreversibly adsorbed species are associated with the Pd metal clusters. Preliminary Adsorption Experiments. Prior to the NMR experiments on methylamine adsorbed on the Pd/SiO2 catalyst, experiments comprising an initial adsorption at room temperature followed by an evacuation to 10-6 Torr and a subsequent readsorption were conducted to determine the amount of methylamine irreversibly adsorbed on the catalyst. In previous studies involving acetylene and ethylene adsorbed on group VIII metal clusters, the maximum ratio of irreversibly adsorbed molecules to surface metal atoms was typically about 0.25.23-25 This was deemed reasonable for monolayer (ML) coverage by an intact small hydrocarbon molecule. However, in the present study, ratios as high as 0.8 have been observed for methylamine molecules irreversibly adsorbed on the Pd/SiO2 catalyst at room temperature, depending on the gas pressure during equilibration. This corresponds to strong adsorption much in excess of that required for monolayer formation on the metal surface. Methylamine, however, is known to form multilayers due to effective hydrogen bonding between molecules.10,13,26 NMR Experiments. Two sets of NMR experiments were conducted in the present investigation of methylamine adsorption
3044 J. Phys. Chem. C, Vol. 112, No. 8, 2008
Figure 1. 13C NMR spectra at 77 K of [13C,15N]-methylamine adsorbed on Pd/SiO2 at 77 K, after each of a series of subsequent annealing periods at successively higher temperatures. The spectra were acquired using 1024 scans, a 90° pulse length of 4.2 µs, and a repetition time of 8 s.
and reaction on Pd/SiO2. In one, the catalyst was initially exposed at 77 K to a dose of 13CH315NH2 such that the ratio of the number of methylamine molecules in the dose to the number of surface palladium atoms in the catalyst sample was equal to 0.25. The dose was chosen to be about the amount expected for monolayer coverage of the palladium. After condensation of the methylamine in an NMR sample vial, the vial was sealed and brought to a temperature of 190 K (note that the melting point of methylamine is 180 K). The sample was then maintained at 190 K for a period of 7 days. Subsequent to this equilibration period, the sample was cooled to 77 K for a 13C NMR spectrum that exhibited a single symmetric line centered at a chemical shift of 26 ppm. The sample was then subjected to a series of three annealing periods of 24 h duration at successively higher temperatures, 253, 273, and 298 K. Following each of these annealing periods, a 13C NMR spectrum was obtained at 77 K. The spectra are shown in Figure 1. A line centered at 26 ppm is clearly observed, but one notes the onset of line broadening on the downfield side, the extent of broadening increasing with each successive annealing period as the annealing temperature is increased from 253 to 298 K. For the line observed after the annealing period at 253 K, the full width at half-maximum (fwhm) is approximately 80 ppm, and spin-lattice relaxation data are best fit with two components of roughly equal weighting, one with T1 ) 0.20 ( 0.06 s and the other with T1 ) 5.4 ( 0.5 s. The presence of two components suggests that two local environments for 13C nuclei exist, possibly reflecting whether the methylamine is strongly associated with the metal surface. Similar results were obtained for methylamine adsorbed on a Rh/SiO2 catalyst.17 The developing downfield tail of the line as the annealing temperature is increased indicates the progressive formation of new surface species. A deconvolution of the 77 K spectrum obtained after the annealing period at 298 K into two components (one a Lorentzian and the other a Gaussian) is shown in Figure 2. The broad Gaussian component is centered at approximately 260 ppm with a width (fwhm) of about 350 ppm. We estimate that roughly 25% of the total spectral intensity is contained in the broad component. Results of spin-lattice relaxation experiments suggest a distribution of T1 values for this component ranging approximately from 0.1 to 0.5 s. After the annealing period at 298 K, the 13C NMR experiments at 77 K were supplemented
von Schenck et al.
Figure 2. Resolution of the spectrum of Figure 1 obtained after the annealing period at 298 K (solid line) into components (dotted lines).
with similar experiments at 298 K. The width of the 26 ppm line decreases from 80 to 20 ppm when the temperature of the NMR experiments is increased from 77 to 298 K, indicating increased mobility of the species associated with the line. In contrast, the width of the broad component centered at 260 ppm is unchanged by this temperature increase, remaining at about 350 ppm. The spin-lattice relaxation data at 298 K are simpler than the data at 77 K in the respect that both the 26 ppm line and the broad line centered at about 260 ppm are characterized by single relaxation times, 2.3 ( 0.4 and 0.08 ( 0.02 s, respectively. The changes in spin-lattice relaxation times with increasing temperature are similar to those obtained for methylamine adsorbed on a Rh/SiO2 catalyst and are consistent with the increased mobility of the species associated with the 26 ppm line and the strong interaction with the metal for the species associated with the broad line.17 The foregoing results on Pd/SiO2 exposed initially to 13CH 15NH at 77 K and subsequently annealed at a series of 3 2 successively higher temperatures exhibit a trend of variation of the 13C spectrum with change of annealing temperature similar to that reported previously by us for the same sequence of experiments with a Rh/SiO2 catalyst.17 In that work, a relatively narrow line at 26 ppm was resolved and assigned to methylamine molecules adsorbed without dissociation on the Rh metal clusters, possibly with multilayer regions being present. A broad feature downfield from this line was also resolved and assigned to surface species designated (CN)Hx, formed via partial dehydrogenation of methylamine molecules on the metal clusters, with an unspecified number x of hydrogen atoms remaining. The inclusion of CN in parentheses indicates that the carbon and nitrogen remain bonded to one another. Bonding to the metal surface occurs via this (CN) unit, and further discussion of this is deferred until later in the paper. The assignment was based in part on the known downfield shift of 13C NMR lines of imines and nitriles relative to amines27 and partly on the basis of conclusions of others derived from investigations on the adsorption and reactions of methylamine on group VIII metals by methods other than NMR spectroscopy.3,10,13-16,28 In the present work with Pd/SiO2 catalyst we resolve similar spectral lines by deconvolution of 13C spectra
NMR Study of Methylamine on Pd/SiO2
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TABLE 1: Percentage Disposition of 13C among Various Species Produced in the Low Temperature (77 K) Adsorption of [13C, 15N]-Methylamine on Pd/SiO2 after Each of a Series of Annealing Periods at Successively Higher Temperaturesa
a
T [K]
molecular species CH3NH2
chemisorbed radicals (CN)Hx
190 253 273 298
100 95 90 75
0 5 10 25
The uncertainties in the values are approximately (5%.
into Lorentzian and Gaussian components. An example of the lines resolved by such a deconvolution has already been noted in Figure 2. We assign the resolved lines to species present on the Pd metal clusters that are similar in kind to species believed to be present on Rh metal clusters in the earlier work17 on Rh/SiO2. Thus, the 26 ppm line is assigned to nondissociatively adsorbed methylamine molecules (possibly including some multilayer entities), and the broad line is assigned to surface species (CN)Hx. Results on the amounts of the two kinds of species present on the Pd clusters as a function of annealing temperature, as derived from deconvolutions of the spectra of Figure 1, are given in Table 1. We proceed now to a presentation of results of the second of the two sets of NMR experiments mentioned earlier. For these experiments the Pd/SiO2 catalyst was exposed initially to 13CH 15NH at 298 K. The amount of methylamine taken up 3 2 by the catalyst, as determined from the change in gas pressure in the adsorption apparatus after contact of the methylamine with the catalyst, was 0.8 mol per mole of surface Pd atoms; i.e., the ratio of the number of molecules adsorbed to the total number of Pd atoms in the surfaces of the metal clusters was 0.8. From what has been stated earlier, we expect that this extent of uptake of methylamine by the catalyst exists entirely in the form of species strongly bound to the Pd metal clusters upon equilibration of the methylamine with the catalyst, the strong adsorption not being limited to a single layer (presumably because of a particularly strong hydrogen bonding interaction of “extra” methylamine with that in the first adsorption layer). Upon completion of the adsorption, the system was evacuated to 10-6 Torr. After the catalyst was transferred from the furnace tube to an NMR sample vial, the vial was sealed off from the furnace tube and maintained at 298 K for a 3 h period. It was then subjected to a series of 3-h annealing periods at successively increasing temperatures ranging from 325 to 450 K. Each annealing period was followed by measurements of 13C and 15N spectra at 298 K. The spectra, including the spectra obtained after the initial adsorption of the methylamine at 298 K, are shown in Figures 3 and 4. The species originally present after the adsorption, i.e., nondissociatively adsorbed methylamine and the species (CN)Hx, undergo reactions to yield new species during the annealing periods at the higher temperatures. The new species include various surface radicals and the product molecules dimethylamine, ammonia, and methane. The reactions were followed by deconvolutions of the spectra and determinations of the relative integrated intensities of the various spectral lines resulting from the deconvolutions (taking relaxation effects into account). A deconvolution of the 13C spectrum obtained after the annealing period at 375 K is presented as a typical example in Figure 5. The deconvolutions were less informative for the 15N spectra than those for the 13C spectra. In particular, the 15N spectral lines for methylamine and dimethylamine could not be resolved in a satisfactory manner.29 Hence, relative
Figure 3. Room temperature 13C NMR spectrum of [13C,15N]methylamine chemisorbed at 298 K on Pd/SiO2, and additional 13C NMR spectra obtained after each of a series of annealing periods at successively higher temperatures. The spectra were acquired using 3072 scans, a 90° pulse length of 2.7 µs, and a repetition time of 8 s. (a) Spectral features in the region 100 to 900 ppm (b) Spectral features in the region -50 to 100 ppm.
Figure 4. Room temperature 15N NMR spectrum of [13C,15N]methylamine chemisorbed at 298 K on Pd/SiO2, and additional 15N NMR spectra obtained after each of a series of annealing periods at successively higher temperatures. The spectra were acquired using 4096 scans, a 90° pulse length of 4.4 µs, and a repetition time of 4 s. (a) Spectral features in the region 50 to 1000 ppm of the 15N NMR spectra obtained after the chemisorption at 298 K and after the annealing period at 450 K. Spectra are scaled to show the extremely broad peak centered at around 270 ppm (500 ppm fwhm). (b) Spectral features in the region -60 to 50 ppm.
amounts of these species were obtainable from the 13C spectra only. The variation of the dispositions of the 13C and 15N, respectively, among various carbon-containing and nitrogen-
3046 J. Phys. Chem. C, Vol. 112, No. 8, 2008
von Schenck et al.
Figure 5. Resolution of the spectrum of Figure 3 obtained after the annealing period at 375 K (solid line) into components (dotted lines).
TABLE 2: Percentage Disposition of 13C among Various Species Produced in the Room Temperature (298 K) Chemisorption of [13C, 15N]-Methylamine on Pd/SiO2, and after Each of a Series of Subsequent Annealing Periods at Successively Higher Temperaturesa T [K]
CH3NH2
298 325 350 375 400 425 450
83 80 64 23 13 0 0
molecular species (CH3)2NH 0 6 20 59 62 10 2
CH4
chemisorbed radicalsb
0 0 0 3 11 69 74
17 14 16 15 14 21 24
a The uncertainties in the values are approximately (5% b Includes (CN)Hx, species with a CNCN or CNC skeletal arrangement of carbon and nitrogen atoms, and CHy.
TABLE 3: Percentage Disposition of 15N among Various Species Produced in the Room Temperature (298 K) Chemisorption of [13C, 15N]-Methylamine on Pd/SiO2, and after Each of a Series of Subsequent Annealing Periods at Successively Higher Temperaturesa T [K] 298 325 350 375 400 425 450
molecular species CH3NH2 + (CH3)2NH 81 83 83 70 42 3 2
NH3
chemisorbed radicalsb
0 0 0 13 42 80 77
19 17 17 17 16 17 21
a The uncertainties in the values are approximately ( 10% b Includes (CN)Hx, species with a CNCN or CNC skeletal arrangement of carbon and nitrogen atoms, and NHz.
containing species, as the annealing periods accumulated in the order of increasing annealing temperature, are shown in Tables 2 and 3. For the temperatures 425 and 450 K in Tables 2 and 3, the total number of hydrogen atoms in the various species per molecule of methylamine originally present is slightly higher than five, possibly due to some hydrogen not being removed from the Pd prior to methylamine adsorption. After the annealing period at 325 K, the 13C spectrum at 298 K differs very little from that obtained after the adsorption of the methylamine at 298 K. Deconvolution of either spectrum gives a relatively narrow line at 26 ppm and a very broad line
(350 ppm fwhm) centered at about 260 ppm. However, after the annealing period at 350 K, two separate lines begin to appear in the spectrum at 298 K in the frequency region of the original very broad line centered at 260 ppm. As we see in Figure 3(a), one of these lines is centered at about 210 ppm and the other at about 390 ppm. Moreover, spectral deconvolution indicates that the 26 ppm line in the region of the spectrum shown in Figure 3(b) now contains an appreciable contribution from a component centered at 39 ppm and assigned to dimethylamine. Nondissociatively adsorbed methylamine has been converted to dimethylamine, presumably via (CN)Hx as a reaction intermediate on the surface. Table 2 shows that 64% of the 13C in the sample appears in methylamine as compared to 20% in dimethylamine, corresponding to a mole ratio of the latter to the former of 0.16. After the annealing period at 375 K, there is little change in the appearance of the broad lines centered at 210 and 390 ppm in Figure 3(a), but, in the region of the spectrum in Figure 3(b), there is a much greater change. Spectral deconvolution indicates that the dimethylamine line at 39 ppm is now much more intense than the methylamine line at 26 ppm. Attention was directed earlier to this particular deconvolution (shown in Figure 5 as an illustrative example). From Table 2 we see that only 23% of the 13C now appears in the methylamine with 59% now appearing in dimethylamine, giving a dimethylamine to methylamine mole ratio of 1.3. In addition to the large increase in this ratio observed after the annealing period at 375 K, there is a new feature at -7 ppm appearing in the spectrum in Figure 3(b) that is assigned to methane. Upon completion of the annealing period at 400 K, there is still not much change in the broad spectral lines in Figure 3(a), but again there is substantial change in the region of the spectrum shown in Figure 3(b). The methane line at -7 ppm is now several fold more intense and the spectral deconvolution of the overlapping methylamine and dimethylamine lines indicates that the former is largely eclipsed in favor of the latter. After the annealing period at 425 K, the broad spectral lines in Figure 3(a) finally exhibit some degree of change, becoming appreciably more intense. In the spectral region in Figure 3(b) the change is huge, with scission of carbon-nitrogen bonds leading to almost complete disappearance of spectral features associated with the methylamine and dimethylamine and to a corresponding precipitous increase in the intensity of the line due to methane. Finally, after completion of the annealing period at 450 K, the 13C spectrum exhibits only the broad lines in Figure 3(a) and the relatively narrow line due to methane in Figure 3(b). With regard to the 15N spectra obtained in these studies, we observe a relatively narrow line at -6 ppm in the two lowermost spectra in Figure 4(b) that can be attributed almost wholly to methylamine, since the corresponding 13C spectra in Figure 3(b) indicate the presence of only small amounts of dimethylamine. For the 15N spectra obtained after the annealing periods at 375-425 K, spectral deconvolutions gave a line in the vicinity of -5 ppm that is due mostly to dimethylamine. They also gave a narrow line at -19 ppm assignable to ammonia. After the final annealing period at 450 K, the only features remaining in the 15N spectrum are the line for ammonia and a very broad line (500 ppm fwhm) centered at about 270 ppm. The latter feature is associated with surface species that are designated in Table 3 simply as chemisorbed radicals, which presumably include species beyond those represented by the formula (CN)Hx in Table 1. To probe the presence of carbon-nitrogen bonds, a SEDOR study (with 13C as the observed nucleus and 15N as the dephasing nucleus) as a function of annealing temperature was conducted
NMR Study of Methylamine on Pd/SiO2
Figure 6. Results of a SEDOR study after an annealing period at 375 K for a sample with a similar preparation and annealing history to the sample corresponding to the spectra show in Figure 3. The 13C NMR reference spectrum (S0) and dephased spectrum (S) were obtained without and with the application of a π 15N dephasing pulse, respectively. The spectra were acquired at 77 K using 5120 scans (13C 90° pulse length ) 7 µs, 15N 180° pulse length ) 12 µs, τd ) 600 µs, τ ) 500 µs, and repetition time ) 1 s).
for a sample with a similar preparation to the sample whose spectra are shown in Figures 3 and 4. The data obtained following an annealing period at 375 K are shown in Figure 6. S and S0 represent the signals with and without application of the 15N dephasing pulse. In the spectral region between -10 and 60 ppm, the decrease in signal from S0 to S brought about by the dephasing is most pronounced around 26 ppm (the position of the methylamine line in a 13C spectrum), a consequence of the presence of the carbon-nitrogen bond in the methylamine molecule. The smaller decrease in signal at 39 ppm (the position of the dimethylamine line in the 13C spectrum) compared to the decrease at 26 ppm is likely due to the substantial carbon-carbon homonuclear coupling (∼500 Hz) relative to the carbon-nitrogen heteronuclear coupling (∼900 Hz) i.e., the SEDOR fraction is no longer given simply by eq 3. The broad component centered at about 210 ppm in the 13C spectrum disappears completely as a result of the dephasing, providing support for its assignment to surface species with the carbon-nitrogen bond intact, as in (CN)Hx. In Figure 6 no information is obtainable on the effect of the dephasing pulse on the signal S0 expected for the broad feature at 400 ppm because the signal S0 itself in this spectral region could not be observed (a consequence of the failure to excite the entire width of the 13C spectrum of interest in the experiment). Discussion In the present work, nuclear magnetic resonance spectroscopy has been utilized to investigate chemical changes that occur over a range of temperatures in methylamine adsorbed on the palladium surface of a Pd/SiO2 catalyst. At temperatures in the range 253-298 K some of the adsorbate present exclusively as intact methylamine molecules at lower temperatures (presumably bonded to palladium via the lone electron pair of the nitrogen atom) undergoes transformation to new surface species characterized by a broad 13C NMR absorption line. The line is observed downfield from the relatively narrow line for the nondissociatively adsorbed methylamine molecules. The downfield shift is consistent with a chemical change in the methylamine molecule in which there is a partial loss of hydrogen from the molecule, but no accompanying scission of the
J. Phys. Chem. C, Vol. 112, No. 8, 2008 3047 carbon-nitrogen bond that would produce surface fragments such as CHy and NHz. Evidence for the formation of a partially dehydrogenated methylamine entity on the surface, represented in the present work by the formula (CN)Hx, has been supplied much earlier by Kemball and Wolf.28 In their study of the exchange reaction of methylamine with deuterium on evaporated films of palladium and platinum, they observed that the hydrogen atoms bonded to the nitrogen atom undergo exchange at temperatures around 250 K. The hydrogen atoms bonded to the carbon atom do not undergo exchange with deuterium so readily, presumably requiring higher temperatures for the occurrence of the reaction.30 On the basis of the results of the exchange studies of Kemball and Wolf, the species initially formed in the present work at temperatures of 253-273 K, following lower temperature exposure of the Pd/SiO2 catalyst to methylamine, is very likely the surface radical CH3NH (corresponding to x ) 4 in the formula (CN)Hx). Over the range of temperatures from 190 to 298 K, the results of the present work on the interaction of methylamine with a Pd/SiO2 catalyst are qualitatively similar in nature to those reported in our study with a Rh/SiO2 catalyst referred to earlier. However, at higher temperatures, a major difference is observed. On the Pd/SiO2 catalyst, the surface chemistry of interest centers on the formation of dimethylamine, which is found to be very extensive at temperatures in the vicinity of 375-400 K. In contrast, no dimethylamine formation is detected on the Rh/SiO2 catalyst, the only reaction in that case being the scission of the carbon-nitrogen bond in the surface species (CN)Hx. The scission yields CHy and NHz surface fragments, the former subsequently being converted to methane and to other hydrocarbons CnHm with n > 1, and the latter being converted to ammonia. The markedly different behavior of Pd/SiO2 and Rh/SiO2 catalysts regarding conversion of methylamine to dimethylamine in these surface chemistry investigations parallels the findings of Sinfelt et al.6-9 in earlier catalytic studies with methylamine-hydrogen mixtures in a flow-reactor system. The observation is therefore not dependent on the presence or absence of extraneous hydrogen in the system, although rates of dimethylamine formation on Pd/SiO29 are affected. In considering the mechanism of formation of dimethylamine, which can presumably be present as a nondissociatively adsorbed species or as a gaseous species, or both, in the work reported here, we note that a surface intermediate with the carbon-nitrogen skeleton C-N-C must play a role, irrespective of the details3,5 of how it is formed. There may be more than one such species, each with a different number of hydrogen atoms attached to the C-N-C skeletal unit. For simplicity, however, we refer to a single species [C-N-C], the brackets signifying a surface species with an unspecified number of hydrogen atoms. The broad downfield NMR absorption lines obtained by deconvoluting 13C or 15N spectra obtained after annealing periods in which dimethylamine is formed very likely have significant contributions arising from the species [C-N-C]. Another contribution could arise from a species such as [C-N-C-N], if the latter is a precursor in the formation of [C-N-C]. Formation of [C-N-C-N] would occur via the bimolecular reaction of two [C-N] species,5 where [C-N] is a partially dehydrogenated methylamine entity designated earlier by the formula (CN)Hx. Scission of the appropriate C-N bond in [C-N-C-N] yields [C-N-C]. After annealing periods at 350 K and higher temperatures, the separate absorption lines arising at about 210 and 390 ppm in the region of the broad line in the 13C spectra may reasonably be attributed to the species [C-N] and [C], respectively. Both of these are formed via the
3048 J. Phys. Chem. C, Vol. 112, No. 8, 2008 scission of a carbon-nitrogen bond in the species [C-N-C], while [C] is also formed from [C-N] by such scission. After the annealing periods at 425 and 450 K, the substantial increases in the intensities of the 210 and 390 ppm lines in the 13C spectra are consistent with the nearly complete disappearance of the dimethylamine accompanying these increases. The broad line remaining in the 15N spectrum after the 450 K annealing period would then also be largely attributable to the species [C-N], if a species such as [N] no longer remains as a result of being hydrogenated to ammonia, the latter being present either on the palladium surface or in the gas phase. In seeking to rationalize the markedly different behavior of palladium and rhodium with regard to the conversion of methylamine to dimethylamine, we note that the strengths of the metal-carbon and metal-nitrogen bonds would be expected to be significantly higher for rhodium than for palladium.31 Theoretical calculations by Au, Liao, and Ng32 and by Liao and Zhang33 provide quantitative information related to the strengths of the metal-carbon bonds for rhodium and palladium. Higher metal-carbon and metal-nitrogen bond strengths provide a rationale for a higher rate of scission of the carbon-nitrogen bond on rhodium,31 which leads to the production of methane and ammonia at the expense of dimethylamine. Acknowledgment. H.v.S. would like to thank the SwedenAmerica Foundation for a scholarship and the Ernst Johnson foundation for financial support. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for funding this research. References and Notes (1) (2) (3) 107. (4) 398. (5) (6) 513.
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