- Email: [email protected]
13C NMR spectra of carbonaceous deposits on silica-supported ruthenium catalysts
CHEMICAL PHYSICS LETTERS
Volume 102, number 2,3
13C NMR SPECTRA OF CARBONACEOUS ON SILICASUPPORTED
18 November 1983
DEPOSITS
RUTHENIUM CATALYSTS
T-M. DUNCAN Bell Laboratories, Mumy Hill, New Jersey 07974. USA and P. WINSLOW and A-T_ BELL Deprtment of ChemicaI Engineering, Unimsity Berkeley* CWifornIiz 94720. USA
of CzrlifortIa at %erkeiey,
Received 12 July 1983; in final form 12 September 1983
The 33C NhfR spectra of carbonaceous residue deposited on silica-supported rotheniurn catalysts during met&m&ion reveal the presence of at least three species. Based on chemical shifts, spin-spin relaxation times, and chemical treatment of the sampIes, these species are interpreted as carbides adsorbed on ruthenium, reactive carbon bonded to silicon, and uz-
reactive aggregates.
1. Introduction It has been proposed that the dissociation of adsorbed carbon monoxide is the initial step in the mechanism of the methanation reaction on group VIII metals [ 1-I] _To date, the presence of adsorbed carbon intermediates is based mainly on indirect evidence. For example, carbon deposited on oxide-supported ruthenium catalysts by the disproportionation of CO to form C and CO2 may be subsequently hydrogenated to form methane [S--8] _Similarly, catalysts pretreated with 13C0 and then exposed to 12C0 and Hz initially yiefd primarily 13CHh [9.10] _The direct characterization of carbon deposits is not practical with spectroscopic techniques traditionally applied to catalysts, such as infrared, although Miksbauer spectroscopy has been used to examine carbides on supported iron catalysts f 11.12]_ We report here the application of 13C nuclear magnetic resonance (NMR) spectroscopy to the study of carbon on a silica-supported ruthenium catalystRecently, it has been demonstrated by WiusIow and Bell [ 131 that carbon deposited from the dissocia0 009-2614/83/0000-0000/$03_00
0 1983 North-Holland
tion of CO on silica-supported ruthenium is present in two reactive forms, designated C, and Cp_The two forms were differentiated by their reactivity with hydrogen at 190°C to yield methane; C, reacted rapidly and was depleted from the catalyst in ==5 s, whereas Cp produced methane less rapidly and required *60 s to be removed. In the initial study [ 131, it was observed that the relative amounts of the two reactive forms were determined by the partial pressures of Dz and CO during methanation, and the forms could be interconverted after deposition. By measuring the 13C NMR spectra of samples prepared with various amounts of C, and Cs, we have identified at least three forms of carbon on the catalyst_ Specifically, the spectra suggest the presence of unreactive carbon deposits and two reactive species: a metal carbide and carbon associated with the silica support.
2. Experimental The preparation of carbon on silica-supported ruthenium has been described previously [ 13]_ Briefly, 163
Vo’olume102. number 2.3
CHEMICAL PHYSKS LETTERS
4.3 wt% ruthenium on silica was reduced in a 10% mixture of D2 in He for 7 h at 4OO”C, which typically resulted in a dispersion of 27%. The catalytic activity was then tested, as follows. The sample was cooled to 190°C and a I _S : I .O mixture of D2 : 13CO was passed through Ihe sample at a rate of S.6 mol 13C0 per mol RIJ per min. After 5 min of steady-state methanation, tfie inlet gas was changed to D2 and 12CO for 30 s, which has been shown by infrared spectroscopy lo replace the adsorbed 13C0 with 12C0 within 5 s 113 J_ Finally. the catalyst was exposed to 1OR D, in He. while the production of 13CD4 was iilonitored with a mass spectrometer. The transient decays of 13CD4 were comparable to those reported previousfy [13]. In the original study [ 131, the c~and /3 forms of carbon on silica-supported ruthenium were measured by titration with D2 immediately after preparation. To test the the long-term stability of the deposited carbon. necessary i‘or estended analyses with 13C NMR, a catalyst sample was cooled to 20°C after the initial merhanation reaction. stored for 3,4 h at 20°C. reheated to 190°C. and finally titrated with D2 as before. Tfie rate of evolution of 13CD4 was unaffected by cooling and reheating the catalyst. 111 this study. three samples of silica-supported ruthenium containing different amounts of C, and Cp were prepared for analysis with 13C NMR spectroscopy. Each sample was first tested for ~let~lanation activity, then reduced in D2 for 2 hat 190°C. To prepare a sample with Cp, a 1 .S : I .O mixture of D2 and 13C0 was again flowed through the catalyst for 5 min at 190°C. foflowed by a ffow of D, and ‘“CO for 30 s. Finally, the sample was cooled rapidly to 20°C and sealed_ A sample containing C, in addition to C was prepared with the same procedure, except the cttafyst was purged with He at 190°C for 2 min before sealing the reactor. The third sample of silica-supported ruthenium was processed to contain little or no reacrive carbon; the sample was sealed after the reduction in D2 that followed the initial ntetltanation test. For reference, it was necessary to measure the 13C NMR spectrum of 13CO adsorbed on silica-supported ruthenium_ A fourth catalyst sample was reduced in D2 at 4OO”C, cooied to 20°C. saturated with 13C0 and then purged with He for 5 min to rentove the gas-phase and physically adsorbed 13C0, The catalyytic activity of the sample was not measured and therefore 164
18 November 1983
the sample contained no carbonaceous intermediates or byproducts of the methanation reaction. The catalyst samples each contained x0.35 g of silica-supported ruthenium and were prepared and sealed in U-tubes consisting of 10 mm diameter glass sample sections at the base and glass+opper seals on the sides. After preparation, the reactors were permanently sealed by pinching-off both copper tubes to form cold welds. The seals were leak-free to UHV conditions after being coated with epoxy. The 13C NMR experiments were performed on a Bruker CXP-ZOO spectrometer, operating at 50.34 MHz. Spectra were measured by Fourier transfo~ing the accumulation of typically 24000 free-induction decays. The spin-lattice (rI) and spin-spin (Tz) relaxation times were determined from the results of saturation-recovery and spin-echo pulse sequences, respectively [14,15]. The 13C NMR spectra are plotted on the u scale for chemical shifts, relative to tetr~ethylsilane (TMS), such that downfield lies to the left, and the spectral intensities were calibrated with samples of SIC and ZrC.
3. Results Fig. 1 shows the l3C NMR spectra of silica-supported ruthenium catalysts prepared with various intermediates of the met~tanation reaction_ Spectrum A was measured on a sample that was prepared in a manner shown to yield predominantly 13Cp and adsorbed 12C0. The principal peak in the spectrum is a lorentzian lineshape centered at -15 ppm relative to TMS with a half-width at half-height of 1.10 kHz_ The Tl of the peak at -15 ppnt is 4.7 s, the Tz is 0.8 ms and the integrated intensity indicates that the ratio of 13C to Ru is O-20, In comparison, the integrated 13CD4 signal observed by mass spectrometry suggests the Cp to Ru ratio is 0.11, or about half the amount detected by NMR. The spectrum also contains evidence of broad features in the range -400 to -100 ppm, which account for =lO% of the integrated intensity. When the catalyst was treated to enhance the amount of 13Cp, the intensities of the down~eld components increase, as shown in spectrum B, The 13C NMR spectrum contains at least three overlapping features vaguely discernible in fig. 1, but readily dif-
Volume 102, number 2.3
18 November 1983
CHEMICAL PHYSICS LETTERS
double the amount detected by mass spectrometry. The 13C remaining on the catalyst after treatment with D, at 190°C for 2 h yields spectrum C shown in fig_ 1_ The amount of carbon detected is 4.4 X 1018 13C spins, or a 13C-to-Ru ratio of 0.050. The slightly asymmetric spectrum has an isotropic shift of --8O ppm and a T2 of x0-2
ms.
The spectrum of adsorbed
13C0, centered at -217 ppm, is not characteristic of an axially symmetric powder pattern. The infrared spectra indicate that CO is adsorbed in terminal sites [13], but spectrum D is narrower than that of Ru~(CO)Q [17], which suggests that 13C0 diffuses between sites and/or librates anisotropically at a rate on the order of the linewidth (10 hHz). The ratio of adsorbed 13CO to Ru as determined by the integrated intensity of spectrum D is 0.22. The sharp peak at -124 ppm in spectrum D is interpreted to be gaseous or physically adsorbed CO, [18]. The linewidth of the CO,(,) peak places an upper limit on the spectral broadening caused by susceptibility effects.
_i‘i,.,..
4_ Discussion of results
FREQUENCY,
in
ppm,
relative
to
TMS
Fig. 1. The ‘jC NMR spectra at 20°C of carbon species on silks-supported ruthenium catalysts prepared by (A) reaction of 13C0 : Da at 190°C followed by 12C0 : D2, (B) A followed by purge with He, (Cl A followed by reduction in D2 for 2 h at 190°C and (D) saturation with 13C0. ferentiated on the basis of T1 and T2_ For example, the narrow feature at -15 ppm has a T1 of 4.5 s and a T2 of 0.6 ms. The extreme downfield component, centered at a-350 ppm has the shortest Tl, 1 .O s, and the longest T2, 1.1 ms. A broad band in the range -200 to -50 ppm has a comparable T, of al.7 s, but a T2 at least five times as short, =0.2 ms. The approximate relative areas of each component, obtained by deconvoluting the spectra with the method used for 13C0 adsorbed on alumina-supported rhodium [I 61, are 35%, 35% and 30% for the downfield, mid-range and narrow peaks, respectively_ The ratio of the total amount of 13C to Ru is 0.19 which is again about
The spectra of adsorbed 13C0 and 13C02 have features that allow one to detect their presence in the spectra of the carbide samples. For example, the maximum in the spectrum of adsorbed 13C0 is at -270 ppm, which would be evident if 13C0 was present in the Cp and reduced samples (spectra A and C) and is conveniently located between the two downfield features in the Co sample (spectrum B)_ 13C0, is weakly adsorbed on the catalyst and can be detected by the presence of a sharp peak at -124 ppm. Therefore, in agreement with the conclusions of transient-response studies [13], spectra A, B and C are of non-oxygenated 13C. The 13C NMR spectra of the c.rrbonaceous deposits may be interpreted by considering the chemical shifts, which are correlated to the electronic configuration of the carbon atoms, spin-spin relaxation times (T& which are determined by 13C-13C internuclear dipolar couplings *, and the linewidths. The linewidth
’ It is assumed that the spinecho eqreriment refocuses dipolar couplings between 13C and other nuclei in the catalyst (e-g. 2D, 29Si, 99Ru and ‘o’Ru) since these spin systems are chemically and/or isotopiCany dilute, and thus the home nuckar couphngs are less than the heteronuclcar couplings to rsc. 165
Volume 102, number 2.3
CHEMICXL PHYSICS LETTERS
is determined by the heteronuclear and homonuclear dipolar couplings, chemical shift inhomogeneity and anisotropy, and may be attenuated by motional effects. The relatively narrow peak at -15 ppm is the dominant feature in the spectrum of the sample treated to produce Cp, and therefore is assigned to Cp. The nature of the Cs species can be determined by a process of elimina:ion. as follows. A spatially fixed C-D bond would result in a dipolar coupling of 2.6 kHz and it is conceivable that the broadening may be reduced to the observed linewidth of 1.I kHz. but not eliminated_ by anisotropic motion. If C, were bonded to deuterium. then it follows that samples prepared with 11, should result in C-H bonds. which would increase the broadening by about a factor of four. However. the width of the peak at -15 ppm is unchanged by substituting H2 for Dz in the preparation [19)_ and therefore the Cp is not bonded to hydrogen, consistent with the results of transient-response studies [13]. It is unlikely that the C, species exists as a two-dimensional overlayer of carbon (such as graphite) since the T2 of the peak indicates that the minimum C-C internuclear distance would be 2.0 A. Carbon bonded to ruthenium cannot be differentiated from carbon bonded to silicon solely on the basis of nuclear dipolar couplings, which are weak and undetectable_ Rather, the relative viability of these two situations is better evaluated by comparison of the observed isotropic shift (-15 ppm) to the shifts of model compounds_ For example, the isotropic shifts of binary transition-metal carbides (-600 to -200 ppm) [20] and non-hydrogenated carbon in metal clusters (-450 to -300 ppm) [2 1 ] are considerably downfield from the C, peak and thus it is unlikely that the Cp species is bonded to ruthenium_ The remaining interpretation is that the Cp is bonded to silicon. which is consistent with the chemical shift of Sic. -21 ppm [22]_ The broad peak centered at -350 ppm appears only in the spectrum of the sample prepared with C, and is in the range expected for transition metal carbides [2 I] _For example, the s&fold coordinated carbides in Ru&(CO)~; and Ru10C2(CO)$ have isotropic shifts of -459 and -457 ppm, respectively [23]. It would be expected that, since the coordination number of a carbide on a surface site is less than that of a carbide in a metal cage, the isotropic shift becomes less negative, analogous to trends observed 166
18 November 1983
in the coordination of CO [21]. Thus, we assign the absorption in the range 400 to -200 ppm to be C,, which is interpreted to be multiply coordinated carbide species on ruthenium_ The line broadening of ~6 kHz greatly exceeds 5’ and is attributed to chemical shift anisotropy and inhomogeneity caused by distributions in site geometries and coordination numbers. Spectrum C of the unreactive carbon is similar in shape to the feature in spectrum B that extends from a shoulder at ---200 ppm to the location of the Ca absorption_ Since the T2 values of these two peaks are also similar, it proposed that the peaks correspond to a single species_ This interpretation suggests that purging the catalyst with He to convert Cp to C, also produces unreactive carbon deposits_ This is conceivable since it has been reported previously that reactive carbon on fused iron catalysts may be deactivated by treatment in He [24] _There are at least two possibilities for the unreactive species. The isotropic shift, L3C-13C dipolar coupling and general shape are similar to that of graphite [20] _However, the shape of the spectrum also resembles that of carbon in tungsten carbide 1201, a crystal structure composed of hexagonal layers of tungsten and carbon, which is also the structure reported for RuC [25].
5. Summary The L3C NMR spectra of silica-supported ruthenium catalysts prepared with 13C-enriched carbonaceous deposits suggest the presence of at least three different forms of carbon: a carbide multiply coordinated to rutheniurn, carbon dispersed onto the silica, and unreactive carbon aggregates_ The initial separation and interpretation of the overlapping NMR peaks is based on the spectral changes that accompany predetermined chemical modifications of the catalyst surface. However, it is demonstrated that the peaks in the 13C NMR spectra may be separated by NMR techniques that differentiate species on the basis of spin-spin and spin-lattice relaxation times. Thus, in principle, a methanation catalyst may be analyzed with 13C NMR spectroscopy to determine the absolute population of the various forms of carbon on the surface.
Volume 102. number 2,3
CHEMICAL PHYSICS LETTERS
References 111 f2f [3f [4] 15 ] [6] [7] [S] [P] [lo]
[ll] [12]
A-T- Bell, Catal. Rev, Sci. Eng 23 (1981) 203, CH. Bartholomew,Cataf. Rev. ScL Eng. 24 (1982) 67. E.L. Muetterties and J_ Stem, C&m_ Rev. 79 (1979) 479_ V. Ponec, Catal. Rev. Sci. Eng. 18 (1978) 151. G.G. Low and A-T. Bell, J. Catal. 57 (1979) 397. J-G. Ekerdt and A-T- Bell, J. CataL 58 (1974) 170. J-A. Rabo, A.P. Risch and M-L. Poutsma, J. CataL 53 (1978) 295. N-M. Gupta, VS. Kamble,K.A. Rao and R-hi. Iyer, J. Catal. 60 (1979) 57. P. Biloerr,J-N. HeBe and W.M.H. Sachtler, J. CataL 58 (1979) 95. Y_ Kobti, H_ Yamasaki, S. N&o, T_ OnJsbiand JC.Tamaru, 3. Chem. Sot. Faraday Trans. I 78 (1982) 1473. J-A. Dumesic and H. Topsoe,Advan. Catal. 26 (1977) 121. J-A. Amelse, J-B. Butt and L.H. Schwartz, J. Phys. Chem. 82 (1978) 558.
1131 A-T. Bell and P. Winslow, AKhE Annual Meeting, Los Angeles (1982) paper 25e; P. Wiiow and A-T. Bell, J. Cat&, to be pub&&d.
18 November 1983
[ 141 CI. Slichter, F’rhmiplesof magnetic resonance, 2nd Ed. (Springer, Berlin, 1978). [I!!] T.M. Duncan and C, Dybowski, Surface ScL Rept. 1 (1981) 157. 1161 T-M. Duncan, J-T. Yates Jr. and R-W, Vaqhan, J. Chem. Phys. 73 (1980) 975. [17] J.W. G&son and R-W. Vaughan, J. Chem. Phys. 78 (1983) 5384. 1181 J-B. Stothers, Carbon-13 NMR spectroscopy (Academic Press, New York, 1972). [lP] TM. Duncan, P_ Winslow and A-T_ Bell, manuscript in prepefi0a
1205 T-M. Duncan, manuscript in preparation. 1211 M. Tachikawa and E-L. Muetterties, J%ogr_Inorg. Chem. 28 (1981) 203. [22 ] G.R. Hofzmarr, PC. Lauterbur, J-H. Anderson and W. Koth, J. Chem. Phys. 25 (1956) 172. 1231 C.-h1.T. Hayward,J_R. Shapley, MR. Churchill, C. Bueno and A-L. Rheingold, J. Am. Chem. Sot. 104 (1982) 7347. 1241 H. hlatsumoto and CO. Bennett, J. Catal. 53 (1978) 331. [25] Cl’_ Kempter and M-R. Nadler, J_ Chem. Phys. 33 (1960) 1580; CF. Kempter, J. Chem. Phys. 41(1964) 1515.
167
Volume 102, number 2,3
13C NMR SPECTRA OF CARBONACEOUS ON SILICASUPPORTED
18 November 1983
DEPOSITS
RUTHENIUM CATALYSTS
T-M. DUNCAN Bell Laboratories, Mumy Hill, New Jersey 07974. USA and P. WINSLOW and A-T_ BELL Deprtment of ChemicaI Engineering, Unimsity Berkeley* CWifornIiz 94720. USA
of CzrlifortIa at %erkeiey,
Received 12 July 1983; in final form 12 September 1983
The 33C NhfR spectra of carbonaceous residue deposited on silica-supported rotheniurn catalysts during met&m&ion reveal the presence of at least three species. Based on chemical shifts, spin-spin relaxation times, and chemical treatment of the sampIes, these species are interpreted as carbides adsorbed on ruthenium, reactive carbon bonded to silicon, and uz-
reactive aggregates.
1. Introduction It has been proposed that the dissociation of adsorbed carbon monoxide is the initial step in the mechanism of the methanation reaction on group VIII metals [ 1-I] _To date, the presence of adsorbed carbon intermediates is based mainly on indirect evidence. For example, carbon deposited on oxide-supported ruthenium catalysts by the disproportionation of CO to form C and CO2 may be subsequently hydrogenated to form methane [S--8] _Similarly, catalysts pretreated with 13C0 and then exposed to 12C0 and Hz initially yiefd primarily 13CHh [9.10] _The direct characterization of carbon deposits is not practical with spectroscopic techniques traditionally applied to catalysts, such as infrared, although Miksbauer spectroscopy has been used to examine carbides on supported iron catalysts f 11.12]_ We report here the application of 13C nuclear magnetic resonance (NMR) spectroscopy to the study of carbon on a silica-supported ruthenium catalystRecently, it has been demonstrated by WiusIow and Bell [ 131 that carbon deposited from the dissocia0 009-2614/83/0000-0000/$03_00
0 1983 North-Holland
tion of CO on silica-supported ruthenium is present in two reactive forms, designated C, and Cp_The two forms were differentiated by their reactivity with hydrogen at 190°C to yield methane; C, reacted rapidly and was depleted from the catalyst in ==5 s, whereas Cp produced methane less rapidly and required *60 s to be removed. In the initial study [ 131, it was observed that the relative amounts of the two reactive forms were determined by the partial pressures of Dz and CO during methanation, and the forms could be interconverted after deposition. By measuring the 13C NMR spectra of samples prepared with various amounts of C, and Cs, we have identified at least three forms of carbon on the catalyst_ Specifically, the spectra suggest the presence of unreactive carbon deposits and two reactive species: a metal carbide and carbon associated with the silica support.
2. Experimental The preparation of carbon on silica-supported ruthenium has been described previously [ 13]_ Briefly, 163
Vo’olume102. number 2.3
CHEMICAL PHYSKS LETTERS
4.3 wt% ruthenium on silica was reduced in a 10% mixture of D2 in He for 7 h at 4OO”C, which typically resulted in a dispersion of 27%. The catalytic activity was then tested, as follows. The sample was cooled to 190°C and a I _S : I .O mixture of D2 : 13CO was passed through Ihe sample at a rate of S.6 mol 13C0 per mol RIJ per min. After 5 min of steady-state methanation, tfie inlet gas was changed to D2 and 12CO for 30 s, which has been shown by infrared spectroscopy lo replace the adsorbed 13C0 with 12C0 within 5 s 113 J_ Finally. the catalyst was exposed to 1OR D, in He. while the production of 13CD4 was iilonitored with a mass spectrometer. The transient decays of 13CD4 were comparable to those reported previousfy [13]. In the original study [ 131, the c~and /3 forms of carbon on silica-supported ruthenium were measured by titration with D2 immediately after preparation. To test the the long-term stability of the deposited carbon. necessary i‘or estended analyses with 13C NMR, a catalyst sample was cooled to 20°C after the initial merhanation reaction. stored for 3,4 h at 20°C. reheated to 190°C. and finally titrated with D2 as before. Tfie rate of evolution of 13CD4 was unaffected by cooling and reheating the catalyst. 111 this study. three samples of silica-supported ruthenium containing different amounts of C, and Cp were prepared for analysis with 13C NMR spectroscopy. Each sample was first tested for ~let~lanation activity, then reduced in D2 for 2 hat 190°C. To prepare a sample with Cp, a 1 .S : I .O mixture of D2 and 13C0 was again flowed through the catalyst for 5 min at 190°C. foflowed by a ffow of D, and ‘“CO for 30 s. Finally, the sample was cooled rapidly to 20°C and sealed_ A sample containing C, in addition to C was prepared with the same procedure, except the cttafyst was purged with He at 190°C for 2 min before sealing the reactor. The third sample of silica-supported ruthenium was processed to contain little or no reacrive carbon; the sample was sealed after the reduction in D2 that followed the initial ntetltanation test. For reference, it was necessary to measure the 13C NMR spectrum of 13CO adsorbed on silica-supported ruthenium_ A fourth catalyst sample was reduced in D2 at 4OO”C, cooied to 20°C. saturated with 13C0 and then purged with He for 5 min to rentove the gas-phase and physically adsorbed 13C0, The catalyytic activity of the sample was not measured and therefore 164
18 November 1983
the sample contained no carbonaceous intermediates or byproducts of the methanation reaction. The catalyst samples each contained x0.35 g of silica-supported ruthenium and were prepared and sealed in U-tubes consisting of 10 mm diameter glass sample sections at the base and glass+opper seals on the sides. After preparation, the reactors were permanently sealed by pinching-off both copper tubes to form cold welds. The seals were leak-free to UHV conditions after being coated with epoxy. The 13C NMR experiments were performed on a Bruker CXP-ZOO spectrometer, operating at 50.34 MHz. Spectra were measured by Fourier transfo~ing the accumulation of typically 24000 free-induction decays. The spin-lattice (rI) and spin-spin (Tz) relaxation times were determined from the results of saturation-recovery and spin-echo pulse sequences, respectively [14,15]. The 13C NMR spectra are plotted on the u scale for chemical shifts, relative to tetr~ethylsilane (TMS), such that downfield lies to the left, and the spectral intensities were calibrated with samples of SIC and ZrC.
3. Results Fig. 1 shows the l3C NMR spectra of silica-supported ruthenium catalysts prepared with various intermediates of the met~tanation reaction_ Spectrum A was measured on a sample that was prepared in a manner shown to yield predominantly 13Cp and adsorbed 12C0. The principal peak in the spectrum is a lorentzian lineshape centered at -15 ppm relative to TMS with a half-width at half-height of 1.10 kHz_ The Tl of the peak at -15 ppnt is 4.7 s, the Tz is 0.8 ms and the integrated intensity indicates that the ratio of 13C to Ru is O-20, In comparison, the integrated 13CD4 signal observed by mass spectrometry suggests the Cp to Ru ratio is 0.11, or about half the amount detected by NMR. The spectrum also contains evidence of broad features in the range -400 to -100 ppm, which account for =lO% of the integrated intensity. When the catalyst was treated to enhance the amount of 13Cp, the intensities of the down~eld components increase, as shown in spectrum B, The 13C NMR spectrum contains at least three overlapping features vaguely discernible in fig. 1, but readily dif-
Volume 102, number 2.3
18 November 1983
CHEMICAL PHYSICS LETTERS
double the amount detected by mass spectrometry. The 13C remaining on the catalyst after treatment with D, at 190°C for 2 h yields spectrum C shown in fig_ 1_ The amount of carbon detected is 4.4 X 1018 13C spins, or a 13C-to-Ru ratio of 0.050. The slightly asymmetric spectrum has an isotropic shift of --8O ppm and a T2 of x0-2
ms.
The spectrum of adsorbed
13C0, centered at -217 ppm, is not characteristic of an axially symmetric powder pattern. The infrared spectra indicate that CO is adsorbed in terminal sites [13], but spectrum D is narrower than that of Ru~(CO)Q [17], which suggests that 13C0 diffuses between sites and/or librates anisotropically at a rate on the order of the linewidth (10 hHz). The ratio of adsorbed 13CO to Ru as determined by the integrated intensity of spectrum D is 0.22. The sharp peak at -124 ppm in spectrum D is interpreted to be gaseous or physically adsorbed CO, [18]. The linewidth of the CO,(,) peak places an upper limit on the spectral broadening caused by susceptibility effects.
_i‘i,.,..
4_ Discussion of results
FREQUENCY,
in
ppm,
relative
to
TMS
Fig. 1. The ‘jC NMR spectra at 20°C of carbon species on silks-supported ruthenium catalysts prepared by (A) reaction of 13C0 : Da at 190°C followed by 12C0 : D2, (B) A followed by purge with He, (Cl A followed by reduction in D2 for 2 h at 190°C and (D) saturation with 13C0. ferentiated on the basis of T1 and T2_ For example, the narrow feature at -15 ppm has a T1 of 4.5 s and a T2 of 0.6 ms. The extreme downfield component, centered at a-350 ppm has the shortest Tl, 1 .O s, and the longest T2, 1.1 ms. A broad band in the range -200 to -50 ppm has a comparable T, of al.7 s, but a T2 at least five times as short, =0.2 ms. The approximate relative areas of each component, obtained by deconvoluting the spectra with the method used for 13C0 adsorbed on alumina-supported rhodium [I 61, are 35%, 35% and 30% for the downfield, mid-range and narrow peaks, respectively_ The ratio of the total amount of 13C to Ru is 0.19 which is again about
The spectra of adsorbed 13C0 and 13C02 have features that allow one to detect their presence in the spectra of the carbide samples. For example, the maximum in the spectrum of adsorbed 13C0 is at -270 ppm, which would be evident if 13C0 was present in the Cp and reduced samples (spectra A and C) and is conveniently located between the two downfield features in the Co sample (spectrum B)_ 13C0, is weakly adsorbed on the catalyst and can be detected by the presence of a sharp peak at -124 ppm. Therefore, in agreement with the conclusions of transient-response studies [13], spectra A, B and C are of non-oxygenated 13C. The 13C NMR spectra of the c.rrbonaceous deposits may be interpreted by considering the chemical shifts, which are correlated to the electronic configuration of the carbon atoms, spin-spin relaxation times (T& which are determined by 13C-13C internuclear dipolar couplings *, and the linewidths. The linewidth
’ It is assumed that the spinecho eqreriment refocuses dipolar couplings between 13C and other nuclei in the catalyst (e-g. 2D, 29Si, 99Ru and ‘o’Ru) since these spin systems are chemically and/or isotopiCany dilute, and thus the home nuckar couphngs are less than the heteronuclcar couplings to rsc. 165
Volume 102, number 2.3
CHEMICXL PHYSICS LETTERS
is determined by the heteronuclear and homonuclear dipolar couplings, chemical shift inhomogeneity and anisotropy, and may be attenuated by motional effects. The relatively narrow peak at -15 ppm is the dominant feature in the spectrum of the sample treated to produce Cp, and therefore is assigned to Cp. The nature of the Cs species can be determined by a process of elimina:ion. as follows. A spatially fixed C-D bond would result in a dipolar coupling of 2.6 kHz and it is conceivable that the broadening may be reduced to the observed linewidth of 1.I kHz. but not eliminated_ by anisotropic motion. If C, were bonded to deuterium. then it follows that samples prepared with 11, should result in C-H bonds. which would increase the broadening by about a factor of four. However. the width of the peak at -15 ppm is unchanged by substituting H2 for Dz in the preparation [19)_ and therefore the Cp is not bonded to hydrogen, consistent with the results of transient-response studies [13]. It is unlikely that the C, species exists as a two-dimensional overlayer of carbon (such as graphite) since the T2 of the peak indicates that the minimum C-C internuclear distance would be 2.0 A. Carbon bonded to ruthenium cannot be differentiated from carbon bonded to silicon solely on the basis of nuclear dipolar couplings, which are weak and undetectable_ Rather, the relative viability of these two situations is better evaluated by comparison of the observed isotropic shift (-15 ppm) to the shifts of model compounds_ For example, the isotropic shifts of binary transition-metal carbides (-600 to -200 ppm) [20] and non-hydrogenated carbon in metal clusters (-450 to -300 ppm) [2 1 ] are considerably downfield from the C, peak and thus it is unlikely that the Cp species is bonded to ruthenium_ The remaining interpretation is that the Cp is bonded to silicon. which is consistent with the chemical shift of Sic. -21 ppm [22]_ The broad peak centered at -350 ppm appears only in the spectrum of the sample prepared with C, and is in the range expected for transition metal carbides [2 I] _For example, the s&fold coordinated carbides in Ru&(CO)~; and Ru10C2(CO)$ have isotropic shifts of -459 and -457 ppm, respectively [23]. It would be expected that, since the coordination number of a carbide on a surface site is less than that of a carbide in a metal cage, the isotropic shift becomes less negative, analogous to trends observed 166
18 November 1983
in the coordination of CO [21]. Thus, we assign the absorption in the range 400 to -200 ppm to be C,, which is interpreted to be multiply coordinated carbide species on ruthenium_ The line broadening of ~6 kHz greatly exceeds 5’ and is attributed to chemical shift anisotropy and inhomogeneity caused by distributions in site geometries and coordination numbers. Spectrum C of the unreactive carbon is similar in shape to the feature in spectrum B that extends from a shoulder at ---200 ppm to the location of the Ca absorption_ Since the T2 values of these two peaks are also similar, it proposed that the peaks correspond to a single species_ This interpretation suggests that purging the catalyst with He to convert Cp to C, also produces unreactive carbon deposits_ This is conceivable since it has been reported previously that reactive carbon on fused iron catalysts may be deactivated by treatment in He [24] _There are at least two possibilities for the unreactive species. The isotropic shift, L3C-13C dipolar coupling and general shape are similar to that of graphite [20] _However, the shape of the spectrum also resembles that of carbon in tungsten carbide 1201, a crystal structure composed of hexagonal layers of tungsten and carbon, which is also the structure reported for RuC [25].
5. Summary The L3C NMR spectra of silica-supported ruthenium catalysts prepared with 13C-enriched carbonaceous deposits suggest the presence of at least three different forms of carbon: a carbide multiply coordinated to rutheniurn, carbon dispersed onto the silica, and unreactive carbon aggregates_ The initial separation and interpretation of the overlapping NMR peaks is based on the spectral changes that accompany predetermined chemical modifications of the catalyst surface. However, it is demonstrated that the peaks in the 13C NMR spectra may be separated by NMR techniques that differentiate species on the basis of spin-spin and spin-lattice relaxation times. Thus, in principle, a methanation catalyst may be analyzed with 13C NMR spectroscopy to determine the absolute population of the various forms of carbon on the surface.
Volume 102. number 2,3
CHEMICAL PHYSICS LETTERS
References 111 f2f [3f [4] 15 ] [6] [7] [S] [P] [lo]
[ll] [12]
A-T- Bell, Catal. Rev, Sci. Eng 23 (1981) 203, CH. Bartholomew,Cataf. Rev. ScL Eng. 24 (1982) 67. E.L. Muetterties and J_ Stem, C&m_ Rev. 79 (1979) 479_ V. Ponec, Catal. Rev. Sci. Eng. 18 (1978) 151. G.G. Low and A-T. Bell, J. Catal. 57 (1979) 397. J-G. Ekerdt and A-T- Bell, J. CataL 58 (1974) 170. J-A. Rabo, A.P. Risch and M-L. Poutsma, J. CataL 53 (1978) 295. N-M. Gupta, VS. Kamble,K.A. Rao and R-hi. Iyer, J. Catal. 60 (1979) 57. P. Biloerr,J-N. HeBe and W.M.H. Sachtler, J. CataL 58 (1979) 95. Y_ Kobti, H_ Yamasaki, S. N&o, T_ OnJsbiand JC.Tamaru, 3. Chem. Sot. Faraday Trans. I 78 (1982) 1473. J-A. Dumesic and H. Topsoe,Advan. Catal. 26 (1977) 121. J-A. Amelse, J-B. Butt and L.H. Schwartz, J. Phys. Chem. 82 (1978) 558.
1131 A-T. Bell and P. Winslow, AKhE Annual Meeting, Los Angeles (1982) paper 25e; P. Wiiow and A-T. Bell, J. Cat&, to be pub&&d.
18 November 1983
[ 141 CI. Slichter, F’rhmiplesof magnetic resonance, 2nd Ed. (Springer, Berlin, 1978). [I!!] T.M. Duncan and C, Dybowski, Surface ScL Rept. 1 (1981) 157. 1161 T-M. Duncan, J-T. Yates Jr. and R-W, Vaqhan, J. Chem. Phys. 73 (1980) 975. [17] J.W. G&son and R-W. Vaughan, J. Chem. Phys. 78 (1983) 5384. 1181 J-B. Stothers, Carbon-13 NMR spectroscopy (Academic Press, New York, 1972). [lP] TM. Duncan, P_ Winslow and A-T_ Bell, manuscript in prepefi0a
1205 T-M. Duncan, manuscript in preparation. 1211 M. Tachikawa and E-L. Muetterties, J%ogr_Inorg. Chem. 28 (1981) 203. [22 ] G.R. Hofzmarr, PC. Lauterbur, J-H. Anderson and W. Koth, J. Chem. Phys. 25 (1956) 172. 1231 C.-h1.T. Hayward,J_R. Shapley, MR. Churchill, C. Bueno and A-L. Rheingold, J. Am. Chem. Sot. 104 (1982) 7347. 1241 H. hlatsumoto and CO. Bennett, J. Catal. 53 (1978) 331. [25] Cl’_ Kempter and M-R. Nadler, J_ Chem. Phys. 33 (1960) 1580; CF. Kempter, J. Chem. Phys. 41(1964) 1515.
167