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Characterization of hydrogen on silica-supported rhodium with 1H NMR spectroscopy
Volume 137, number 1
CHEMICAL PHYSICS LETTERS
29 May 1987
CHARACTERIZATION OF HYDROGEN ON SILICA-SUPPORTED RHODIUM WITH ‘H NMR SPECTROSCOPY T.W. ROOT’ and T.M. DUNCAN AT&T Bell Laboratories, Murray Hill, NJ 07974, USA
Received 3 October 1986; in final form 3 April 1987
This study describes the application of selective labeling with NMR to examine hydrogen adsorbed on silica-supported rhodium. Hydrogen previously reported absent from ‘H NMR spectra is detected indirectly through its interaction with an observed state of hydrogen adsorbed on rhodium particles. This interaction has a time constant of z 0.1 to 1.Oms at 295 K, which is much faster than exchange between hydrogen adsorbed on rhodium and neighboring silica hydroxyl groups.
1. Introduction At 300 K and above, hydrogen exists on silica-supported Rh catalysts in at least three forms: hydrogen atoms dissociatively adsorbed on rhodium particles, silica hydroxyl groups, and water. Hz exchanges rapidly with hydrogen on the metal particles, dissociating on adsorption. Exchange of hydrogen with the silica hydroxyl groups is very slow in the absence of metal particles or adsorbed water; the exchange is catalyzed by either of these. When this exchange occurs at the metal particles, it is termed “spillover”, and is proposed to be important in determining the catalytic properties of the surface. Studies with isotopic labeling suggest that spillover is slow; the time constant for D2 to exchange with surface OH groups is about 10 min at 300 K [ 1] when rhodium is present. In the absence of a catalyst, D2 exchange with surface hydroxyl groups is much slower. ‘H nuclear magnetic resonance (NMR) spectra of hydrogen adsorbed on oxide-supported rhodium [ l-81 contain two salient features: an intense peak near 3 ppm and an upfield peak in the range - 100 to - 160 ppm. These two peaks are interpreted to be surface hydroxyls (and water), and hydrogen on rhodium [ l-81. The upfield peak is not detected in the absence of rhodium and has a short spin-lattice ’ Present address: Department of Chemical Engineering, University of Wisconsin, Madison, WI 53706, USA.
relaxation time ( T, ) of typically 10 to 40 ms, which is indicative of an efficient relaxation mechanism such as provided by the conduction electrons of the rhodium. The upfield peak shifts toward 0 as the HZ pressure is increased [ 7,8], consistent with rapid exchange between the gas phase and the metal surface, on the NMR time scale (i.e. less than 1 ms). Quantitative analysis based on the integrated intensity of the upfield peak [ 1 ] has shown that it accounts for only 30% of the hydrogen chemisorbed on the supported rhodium particles, as determined from uptake measurements. This study illustrates the application of NMR to monitor the exchange, either chemical or magnetic, between the adsorbed states of hydrogen. We describe a method to excite nuclear spin states of one species selectively, thus labeling this species so that its exchange to other species may be observed.
2. Spin labeling with NMR Manipulation of nuclear spins with rf irradiation has long been recognized as a tool to increase the utility of NMR spectroscopy. Bloembergen, Purcell and Pound [ 91 first reported the possibility of selectively saturating a portion of a sample, and Forstn and Hoffman [ lo] conducted an early study of chemical exchange using selective saturation. More recently, there has been a resurgence in interest in
0 009-2614/87/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
57
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1
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PHYSICS
selective pulse sequences for spectral simplification, as reflected in the work of Morris and Freeman [ 111. Jeener et al. [ 12 ] developed pulsed two-dimensional NMR techniques that use spin labeling to study exchange. Current work in our laboratory involves the use of selective excitations to generate spin labels in formic acid [ 131 and hydrogen [ 141 adsorbed on surfaces, and using these labels to monitor processes occurring within the adsorbates. The advantages and motivation of spin labeling with NMR can be described by analogy with isotopic spin labeling. For example, to observe spillover, gas phase Hz may be replaced with Dz, which concomitantly replaces the H on the rhodium particles. Exchange between H in hydroxyl groups and D on the rhodium particles may then be observed spectroscopically by changes in the peak intensities in infrared or NMR spectra, or by monitoring the isotopic ratio in the gas phase with mass spectrometry. Similar isotopic methods have been effective in studies of other catalytic processes. However, the time required to introduce an isotopic label is mass-transfer limited in porous samples, which precludes studies of processes with time constants less than = 1 s. Furthermore, isotopic labels can only be introduced via the gas phase, which limits the selectivity. Spin labeling with NMR spectroscopy, like isotopic substitution, labels the nucleus of a species in a particular subgroup, but it does so via the spin state of the nuclear magnetic dipole (e.g. parallel or antiparallel with the external magnetic field). However, as will be shown, whereas isotopic labeling requires seconds NMR labels may be introduced in x 20 us. Exchange between a spin-labeled species and an unlabeled species appears as a decrease in the intensity of the NMR peak of the unlabeled exchanging species. Two types of exchange will produce this effect in the NMR spectrum: (1) movement of atoms between labeled sites and unlabeled sites (chemical exchange), and (2) mutual exchange of magnetic spin states (spin diffusion). These two processes can be differentiated by the temperature dependence of the rate of exchange; spin diffusion is not thermally activated for temperatures above 10 IL In a previous investigation of adsorbate dynamics [ 131, we used attenuated pulses to achieve excitation only within a narrow bandwidth about the rf frequency. This technique requires pulses approxi58
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29 May 1987
mately 100 us long to irradiate only one peak in a hydrogen sample selectively. Since that is longer than the spin-spin relaxation times (T,) of both hydrogen peaks (vide infra), the resulting dephasing during the pulse will produce saturation rather than inversion and greatly reduce the dynamic range of the observations. Therefore we have applied instead a faster two-pulse sequence described below. For adsorbed hydrogen, in which the principal peaks are not coincident, selective labeling may be achieved by a pair of 90 ; Xpulses separated by a delay 7,, for 7I -G T,,T,. The nuclear magnetization after a 90;-7,-90:, pulse sequence may be expressed in terms of the density matrix, p, and is a function of the offset frequency Aw, as given in p(Aw;
TV)
x exp(
=exp( fircZ,) exp(iAoz,Z,) -T~/T~)
exp( - linZX) p(Aw; 0)
x exp( finZ,) exp( -iAor,Z,)
exp( -iixZX) . (1)
For a system in a pure Zeeman state at equilibrium, p(Ao; 0) =I,, and therefore p(Aw; 7,) = [Zr cos(Aw7,) +Z, sin(Awz,)] xexp(-z,lT2).
(2)
The x component in eq. (2) decays to zero with a time constant Tf and the z component decays to Z, with time constant T,. For adsorbed hydrogen, as is the case for most solids, T$ < T,. Thus, at a time 7223TT after the 90:, pulse, the remaining magnetic coherence resides in the z component and the intensity is a periodic function of the offset frequency, P(Aw;
71;
7.2)
=Z, cos(Aw 7,) exp(-7,/T,)
exp( -r2/T,)
.
(3) Thus the 90:-7,-90!, sequence can be used to invert the magnetization of one of a pair of peaks (a and b) selectively by irradiating at the resonance of one peak (Aw,=O) and setting the delay to maintain 7, =~c/Aw~. The behavior after this selective excitation can be used to characterize exchange between the two peaks by sampling the z magnetization at 72 with a 90” pulse or spin echo, when the
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CHEMICALPHYSICSLETTERS
correlation time for exchange lies in the range T2<~,
295 K. After cooling the reactor, one of its ports was sealed mechanically, as described elsewhere [ 171. H2 was adsorbed onto the catalyst at 295 K, the uptake at 10.8 Torr corresponded to a H-to-Rh ratio of 0.78, which indicates a dispersion of 78%, assuming a 1:1 ratio of surface Rh to H. Finally, the remaining arm of the u-tube was pinched off to seal the catalyst under 1.0 Torr of H,. The silica used in this study is Degussa Aerosil380. RhC13*3HZ0 was obtained from Alfa Products and H,(99.9999%) was obtained from Scientific Gas Products. The ‘H NMR experiments were performed on a Bruker CXP-200 spectrometer, operating at 200.13 MHz. Spectra were collected using echoes and add/ subtract sequences to reduce artifacts from probe ringdown. T, and T2 magnitudes were determined using inversion-recovery and spin-echo (90:-z180.:-z-observe) sequences. The ‘H NMR spectra are plotted on the 6 scale for chemical shifts, relative to TMS, so positive shifts correspond to lower field (i.e. C6H6 is at 7.3 ppm.).
4. Results The ‘H NMR spectrum of H2 on silica-supported rhodium is shown in fig. 1. The peak at 3 ppm, assigned to OH groups in previous studies [ l-81, represents 94% of the integrated intensity, has a T, =2.5 s, and T2= 135 ps at 295 K. The upfield peak
J_i
3.Experimental procedures A 5OhRh/silica catalyst was prepared by silica impregnation to incipient wetness with an aqueous solution of RhCl,, followed by drying in air at room temperature. After drying, the catalyst was packed into glass u-tube reactors described previously [ 171, and reduced in flowing H2 at 473 IS for 6 h. The catalyst was then purged with flowing He and cooled to
x IO
t
0
-50 -100 -150 FREQUENCY, in ppm, RELATIVE TO TMS
50
Fig. 1. ‘HNMR spectraof a silica-supported rhodiumcatalystat 300 K with a Hz pressureof I Torr. The spectrumis the accumulationof = 1000 scansacquiredat 10 s intervals. 59
Volume 137, number 1
CHEMICAL PHYSICS LETTERS
29 May 1987
I
T=293K
oc 100
/L’$
I
1nl*
I
I
10ms 100 EXCHANGE
ms
I
I
IS
10s
J
DELAY
Fig. 3. Intensity of the peak at - 104 ppm, relative to the OH peak in a standard spectrum (e.g. fig. t ) plotted as a function of T*,the delay after the 90;~r,-90 !, sequence.
b
150
I
I
100 50 FREOUENCY.
I
0 8” &vn,
i
1
,
-50 -400 -150 RELATIVE TO TMS
d -200
Fig. 2. Representative ‘H NMR spectra for which the peak at B 3 ppm is selectively inverted with a [90;-2,-904 ,-rz-observe] sequence, plotted at various intervals of 7*,
at - 104 ppm, assigned to hydrogen
associatively adsorbed on rhodium particles, has a T1 = 77 ms and jr, = 87 p at 295 K. The half-widths of both peaks are about 2.2 kHz, thus T$ = 72 us. The system is irradiated with a 90:-r,-909, sequence with r, =23.3 J.B, to investigate exchange between the peak at 3 ppm and the peak at - 104 ppm. In principle, either peak may be inverted, but the OH peak was chosen because its longer T,provides more persistent spin labels. We conducted the experiment at 295 K, where hydrogen spillover is slow, and observed the spectra shown in fig. 2, which
represent various intervals r2 after selective excitation. The minimum interval before artifact-free observation is the time necessary for any residual magnetization in the xy plane to dephase and therefore depends on the individual T; magnitudes. As described above, the peak at 3 ppm remains inverted until r2x T,. During this period, a decrease is observed in the upfield peak, shown in fig. 3 by a plot of the intensity as a function of the time after the selective inversion. Multiple-quantum filtering [ 16 ] 60
shows that the spin system after T, has only Z, order, thus demonstrating that multiple-quantum transitions during the preparation sequence are negligible, presumably owing to weak internuclear couplings. Doubling the delay between the two pulses in the preparation sequence allows the OH spins to precess through 360 O,thereby removing one of the two possible excitations produced by this sequence, i.e. inversion. With z1 =46.6 us, the hydroxyl spins returned to the t-Z, axis and the hydrogen peak at - 104 ppm showed the same decrease in intensity as seen when the OH peak was inverted (fig. 3). Additional studies at different temperatures [ 18 ] show that the rate of this exchange is independent of temperature in the range 250 to 320 K.
5. Discussion The decrease in the upfield peak is not caused by exchange with the support OH peak at 3 ppm, since the decrease does not depend on whether the OH spins have been inverted or rotated back to their original orientation. This is consistent with the 10 min time constant for OH exchange at 295 K reported earlier [ I]. The exchange suggests the presence of previously unobserved, recondite hydrogen whose magnetization dephases in less than 20 us. Interrupted temperature-programmed desorption studies [3] have shown that the NMR peak at 3 ppm may
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CHEMICAL PHYSICS LETTERS
be separated into broad (65 ppm) and narrow ( -C10 ppm) components which are chemically distinct. The recondite species detected here would have a linewidth of at least 250 ppm, and hence be very difficult to observe directly in the presence of the much larger OH peak. The relative amount of hydrogen in this second state may be estimated from the decrease in intensity in the peak at - 104 ppm by invoking conservation of magnetization. From the previous discussion, we conclude that the recondite hydrogen is saturated by the preparation pulses and thus begins the evolution period with zero net magnetization. The decrease in magnetization observed for the state at - 104 ppm is not exponential, so we will characterize it here by the time for half of the observed decrease to occur, which is z 1 ms. Exchange continues to occur to 30 ms, at which time the effects of spin-lattice relaxation will contribute and the spin system is no longer adiabatic. Therefore, both the characteristic exchange time and the total amount of the decrease may be underestimated. Since the peak at - 104 ppm decreases by 50-6OW at 295 K or lower temperatures before spin-lattice relaxation becomes a factor, the population of unobserved hydrogen must be at least l-l .5 times as large as the detected state. The recondite hydrogen is characterized by two features: it has a T2< 20 its and it exchanges with the detectable hydrogen adsorbed on the metal. Furthermore, the exchange is predominantly spin diffusion rather than chemical exchange, because the rate of exchange is temperature independent. The time constant of the exchange is z 1 ms, which indicates that the exchanging state is at equilibrium with the hydrogen on the rhodium particles in any chemisorption measurement. This additional state therefore contains some of the hydrogen previously unobserved by NMR but known from chemisorption measurements to be on or near the rhodium particles. Since the integrated intensity of the peak at - 104 ppm corresponds to z 30% of the adsorbed hydrogen, these two states then account for at least 60-759/o of the hydrogen adsorbed on the metal. The short T2 indicates that the recondite species is strongly coupled to a zero-quantum relaxation source such as other protons or a motion with a large lowfrequency spectral density. Immobile hydrogen would have a T, less than 10 ps owing to the internuclear
29 May 1987
dipolar interaction. This undetected species may represent a strongly bound hydride-like form inside the rhodium particle, although rhodium forms no bulk hydride [ 191. Alternatively, the short T2 may be related to the particular electronic structure of the rhodium particles. Further data are necessary to understand fully this newly detected adsorbed hydrogen species.
6. Summary Exchange between the adsorbed states of hydrogen on silica-supported rhodium is investigated with a NMR spin labeling technique sensitive to processes occurring from x0.1 ms to z 1 s. The data indicate that hydrogen on rhodium particles interacts with another state of adsorbed hydrogen approximately as large, with a time constant of 1 ms at 295 K. This second state of hydrogen has a very short T2(< 20 us) and is adsorbed on or near the metal particles. The NMR peak corresponding to this second state of hydrogen is predicted to be very broad and weak, and therefore difficult to observe directly. Estimates indicate that this peak may account for much or all of the discrepancy between the hydrogen indicated by volumetric uptake and by the intensity of the uptield peak.
References [l]T.-C. ShengandLD.Gay, J.Catal. 77 (1982) 53. [2] T.M. Apple, P. Gajardo and C. Dybowski, J. Catal. 68 (1981) 103. [ 31 T.M. Apple and C. Dybowski, Surface Sci. 121 (1982) 243. [4] S.J. DeCanio, J.B. Miller, J.B. Michel and C. Dybowski, J. Phys. Chem. 87 (1983) 4619. [ 51J.B. Miller, S.J. DeCanio, J.B. Michel and C. Dybowski, J. Phys. Chem. 89 (1985) 2592. [6] J.C. Conesa, P. Malet, G. Munuera, J. Sanz and J. Soria, J. Phys. Chem. 88 (1984) 2986. [7] J. Sanz and J.M. Rojo, J. Phys. Chem. 89 (1985) 4974. [ 81 J. Sanz, J.M. Rojo, P. Malet, G. Munuera, M.T. Blasco, J.C. Conesa and J. Soria, J. Phys. Chem. 89 (1985) 5427. [ 91 N. Bloembergen, E.M. Purcell and R.V. Pound, Phys. Rev. 73 (1948) 679. [lo] S. Forsen and R.A. Hoffman, J. Chem. Phys. 39 (1963) 2892. [ 111 G.A. Morris and R. Freeman, J. Magn. Reson. 29 (1978) 433.
61
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[ 121 J. Jeener, B.H. Meier, P. Bachman and R.R. Ernst, J. Chem. Phys. 71 (1979) 4546. [ 131 T.W. Root and T.M. Duncan, J. Catal. 102 (1986) 109. [ 141 T.W. RootandT.M.Duncan,Bull.Am.Phys. Sot. 31(1986) 534. [ 15 ] M. Goldman and L. Shen, Phys. Rev. 144 ( 1966) 32 1. [ 161 A. Wokaun and R.R. Ernst, Chem. Phys. Letters 52 (1977) 407.
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[ 17 ] T.M. Duncan, P. Winslow and A.T. Bell, J. Catal. 93 (1985) 1.
[ 181 T.W. Root and T.M. Duncan, in preparation. [ 191 N.A. Galaktionowa, Hydrogen-metal systems databook (Ordentlich, Holon, 1981) p. 172.
CHEMICAL PHYSICS LETTERS
29 May 1987
CHARACTERIZATION OF HYDROGEN ON SILICA-SUPPORTED RHODIUM WITH ‘H NMR SPECTROSCOPY T.W. ROOT’ and T.M. DUNCAN AT&T Bell Laboratories, Murray Hill, NJ 07974, USA
Received 3 October 1986; in final form 3 April 1987
This study describes the application of selective labeling with NMR to examine hydrogen adsorbed on silica-supported rhodium. Hydrogen previously reported absent from ‘H NMR spectra is detected indirectly through its interaction with an observed state of hydrogen adsorbed on rhodium particles. This interaction has a time constant of z 0.1 to 1.Oms at 295 K, which is much faster than exchange between hydrogen adsorbed on rhodium and neighboring silica hydroxyl groups.
1. Introduction At 300 K and above, hydrogen exists on silica-supported Rh catalysts in at least three forms: hydrogen atoms dissociatively adsorbed on rhodium particles, silica hydroxyl groups, and water. Hz exchanges rapidly with hydrogen on the metal particles, dissociating on adsorption. Exchange of hydrogen with the silica hydroxyl groups is very slow in the absence of metal particles or adsorbed water; the exchange is catalyzed by either of these. When this exchange occurs at the metal particles, it is termed “spillover”, and is proposed to be important in determining the catalytic properties of the surface. Studies with isotopic labeling suggest that spillover is slow; the time constant for D2 to exchange with surface OH groups is about 10 min at 300 K [ 1] when rhodium is present. In the absence of a catalyst, D2 exchange with surface hydroxyl groups is much slower. ‘H nuclear magnetic resonance (NMR) spectra of hydrogen adsorbed on oxide-supported rhodium [ l-81 contain two salient features: an intense peak near 3 ppm and an upfield peak in the range - 100 to - 160 ppm. These two peaks are interpreted to be surface hydroxyls (and water), and hydrogen on rhodium [ l-81. The upfield peak is not detected in the absence of rhodium and has a short spin-lattice ’ Present address: Department of Chemical Engineering, University of Wisconsin, Madison, WI 53706, USA.
relaxation time ( T, ) of typically 10 to 40 ms, which is indicative of an efficient relaxation mechanism such as provided by the conduction electrons of the rhodium. The upfield peak shifts toward 0 as the HZ pressure is increased [ 7,8], consistent with rapid exchange between the gas phase and the metal surface, on the NMR time scale (i.e. less than 1 ms). Quantitative analysis based on the integrated intensity of the upfield peak [ 1 ] has shown that it accounts for only 30% of the hydrogen chemisorbed on the supported rhodium particles, as determined from uptake measurements. This study illustrates the application of NMR to monitor the exchange, either chemical or magnetic, between the adsorbed states of hydrogen. We describe a method to excite nuclear spin states of one species selectively, thus labeling this species so that its exchange to other species may be observed.
2. Spin labeling with NMR Manipulation of nuclear spins with rf irradiation has long been recognized as a tool to increase the utility of NMR spectroscopy. Bloembergen, Purcell and Pound [ 91 first reported the possibility of selectively saturating a portion of a sample, and Forstn and Hoffman [ lo] conducted an early study of chemical exchange using selective saturation. More recently, there has been a resurgence in interest in
0 009-2614/87/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
57
Volume 137, number
1
CHEMICAL
PHYSICS
selective pulse sequences for spectral simplification, as reflected in the work of Morris and Freeman [ 111. Jeener et al. [ 12 ] developed pulsed two-dimensional NMR techniques that use spin labeling to study exchange. Current work in our laboratory involves the use of selective excitations to generate spin labels in formic acid [ 131 and hydrogen [ 141 adsorbed on surfaces, and using these labels to monitor processes occurring within the adsorbates. The advantages and motivation of spin labeling with NMR can be described by analogy with isotopic spin labeling. For example, to observe spillover, gas phase Hz may be replaced with Dz, which concomitantly replaces the H on the rhodium particles. Exchange between H in hydroxyl groups and D on the rhodium particles may then be observed spectroscopically by changes in the peak intensities in infrared or NMR spectra, or by monitoring the isotopic ratio in the gas phase with mass spectrometry. Similar isotopic methods have been effective in studies of other catalytic processes. However, the time required to introduce an isotopic label is mass-transfer limited in porous samples, which precludes studies of processes with time constants less than = 1 s. Furthermore, isotopic labels can only be introduced via the gas phase, which limits the selectivity. Spin labeling with NMR spectroscopy, like isotopic substitution, labels the nucleus of a species in a particular subgroup, but it does so via the spin state of the nuclear magnetic dipole (e.g. parallel or antiparallel with the external magnetic field). However, as will be shown, whereas isotopic labeling requires seconds NMR labels may be introduced in x 20 us. Exchange between a spin-labeled species and an unlabeled species appears as a decrease in the intensity of the NMR peak of the unlabeled exchanging species. Two types of exchange will produce this effect in the NMR spectrum: (1) movement of atoms between labeled sites and unlabeled sites (chemical exchange), and (2) mutual exchange of magnetic spin states (spin diffusion). These two processes can be differentiated by the temperature dependence of the rate of exchange; spin diffusion is not thermally activated for temperatures above 10 IL In a previous investigation of adsorbate dynamics [ 131, we used attenuated pulses to achieve excitation only within a narrow bandwidth about the rf frequency. This technique requires pulses approxi58
LETTERS
29 May 1987
mately 100 us long to irradiate only one peak in a hydrogen sample selectively. Since that is longer than the spin-spin relaxation times (T,) of both hydrogen peaks (vide infra), the resulting dephasing during the pulse will produce saturation rather than inversion and greatly reduce the dynamic range of the observations. Therefore we have applied instead a faster two-pulse sequence described below. For adsorbed hydrogen, in which the principal peaks are not coincident, selective labeling may be achieved by a pair of 90 ; Xpulses separated by a delay 7,, for 7I -G T,,T,. The nuclear magnetization after a 90;-7,-90:, pulse sequence may be expressed in terms of the density matrix, p, and is a function of the offset frequency Aw, as given in p(Aw;
TV)
x exp(
=exp( fircZ,) exp(iAoz,Z,) -T~/T~)
exp( - linZX) p(Aw; 0)
x exp( finZ,) exp( -iAor,Z,)
exp( -iixZX) . (1)
For a system in a pure Zeeman state at equilibrium, p(Ao; 0) =I,, and therefore p(Aw; 7,) = [Zr cos(Aw7,) +Z, sin(Awz,)] xexp(-z,lT2).
(2)
The x component in eq. (2) decays to zero with a time constant Tf and the z component decays to Z, with time constant T,. For adsorbed hydrogen, as is the case for most solids, T$ < T,. Thus, at a time 7223TT after the 90:, pulse, the remaining magnetic coherence resides in the z component and the intensity is a periodic function of the offset frequency, P(Aw;
71;
7.2)
=Z, cos(Aw 7,) exp(-7,/T,)
exp( -r2/T,)
.
(3) Thus the 90:-7,-90!, sequence can be used to invert the magnetization of one of a pair of peaks (a and b) selectively by irradiating at the resonance of one peak (Aw,=O) and setting the delay to maintain 7, =~c/Aw~. The behavior after this selective excitation can be used to characterize exchange between the two peaks by sampling the z magnetization at 72 with a 90” pulse or spin echo, when the
Volume137,number1
29 May 1987
CHEMICALPHYSICSLETTERS
correlation time for exchange lies in the range T2<~,
295 K. After cooling the reactor, one of its ports was sealed mechanically, as described elsewhere [ 171. H2 was adsorbed onto the catalyst at 295 K, the uptake at 10.8 Torr corresponded to a H-to-Rh ratio of 0.78, which indicates a dispersion of 78%, assuming a 1:1 ratio of surface Rh to H. Finally, the remaining arm of the u-tube was pinched off to seal the catalyst under 1.0 Torr of H,. The silica used in this study is Degussa Aerosil380. RhC13*3HZ0 was obtained from Alfa Products and H,(99.9999%) was obtained from Scientific Gas Products. The ‘H NMR experiments were performed on a Bruker CXP-200 spectrometer, operating at 200.13 MHz. Spectra were collected using echoes and add/ subtract sequences to reduce artifacts from probe ringdown. T, and T2 magnitudes were determined using inversion-recovery and spin-echo (90:-z180.:-z-observe) sequences. The ‘H NMR spectra are plotted on the 6 scale for chemical shifts, relative to TMS, so positive shifts correspond to lower field (i.e. C6H6 is at 7.3 ppm.).
4. Results The ‘H NMR spectrum of H2 on silica-supported rhodium is shown in fig. 1. The peak at 3 ppm, assigned to OH groups in previous studies [ l-81, represents 94% of the integrated intensity, has a T, =2.5 s, and T2= 135 ps at 295 K. The upfield peak
J_i
3.Experimental procedures A 5OhRh/silica catalyst was prepared by silica impregnation to incipient wetness with an aqueous solution of RhCl,, followed by drying in air at room temperature. After drying, the catalyst was packed into glass u-tube reactors described previously [ 171, and reduced in flowing H2 at 473 IS for 6 h. The catalyst was then purged with flowing He and cooled to
x IO
t
0
-50 -100 -150 FREQUENCY, in ppm, RELATIVE TO TMS
50
Fig. 1. ‘HNMR spectraof a silica-supported rhodiumcatalystat 300 K with a Hz pressureof I Torr. The spectrumis the accumulationof = 1000 scansacquiredat 10 s intervals. 59
Volume 137, number 1
CHEMICAL PHYSICS LETTERS
29 May 1987
I
T=293K
oc 100
/L’$
I
1nl*
I
I
10ms 100 EXCHANGE
ms
I
I
IS
10s
J
DELAY
Fig. 3. Intensity of the peak at - 104 ppm, relative to the OH peak in a standard spectrum (e.g. fig. t ) plotted as a function of T*,the delay after the 90;~r,-90 !, sequence.
b
150
I
I
100 50 FREOUENCY.
I
0 8” &vn,
i
1
,
-50 -400 -150 RELATIVE TO TMS
d -200
Fig. 2. Representative ‘H NMR spectra for which the peak at B 3 ppm is selectively inverted with a [90;-2,-904 ,-rz-observe] sequence, plotted at various intervals of 7*,
at - 104 ppm, assigned to hydrogen
associatively adsorbed on rhodium particles, has a T1 = 77 ms and jr, = 87 p at 295 K. The half-widths of both peaks are about 2.2 kHz, thus T$ = 72 us. The system is irradiated with a 90:-r,-909, sequence with r, =23.3 J.B, to investigate exchange between the peak at 3 ppm and the peak at - 104 ppm. In principle, either peak may be inverted, but the OH peak was chosen because its longer T,provides more persistent spin labels. We conducted the experiment at 295 K, where hydrogen spillover is slow, and observed the spectra shown in fig. 2, which
represent various intervals r2 after selective excitation. The minimum interval before artifact-free observation is the time necessary for any residual magnetization in the xy plane to dephase and therefore depends on the individual T; magnitudes. As described above, the peak at 3 ppm remains inverted until r2x T,. During this period, a decrease is observed in the upfield peak, shown in fig. 3 by a plot of the intensity as a function of the time after the selective inversion. Multiple-quantum filtering [ 16 ] 60
shows that the spin system after T, has only Z, order, thus demonstrating that multiple-quantum transitions during the preparation sequence are negligible, presumably owing to weak internuclear couplings. Doubling the delay between the two pulses in the preparation sequence allows the OH spins to precess through 360 O,thereby removing one of the two possible excitations produced by this sequence, i.e. inversion. With z1 =46.6 us, the hydroxyl spins returned to the t-Z, axis and the hydrogen peak at - 104 ppm showed the same decrease in intensity as seen when the OH peak was inverted (fig. 3). Additional studies at different temperatures [ 18 ] show that the rate of this exchange is independent of temperature in the range 250 to 320 K.
5. Discussion The decrease in the upfield peak is not caused by exchange with the support OH peak at 3 ppm, since the decrease does not depend on whether the OH spins have been inverted or rotated back to their original orientation. This is consistent with the 10 min time constant for OH exchange at 295 K reported earlier [ I]. The exchange suggests the presence of previously unobserved, recondite hydrogen whose magnetization dephases in less than 20 us. Interrupted temperature-programmed desorption studies [3] have shown that the NMR peak at 3 ppm may
Volume 137, number 1
CHEMICAL PHYSICS LETTERS
be separated into broad (65 ppm) and narrow ( -C10 ppm) components which are chemically distinct. The recondite species detected here would have a linewidth of at least 250 ppm, and hence be very difficult to observe directly in the presence of the much larger OH peak. The relative amount of hydrogen in this second state may be estimated from the decrease in intensity in the peak at - 104 ppm by invoking conservation of magnetization. From the previous discussion, we conclude that the recondite hydrogen is saturated by the preparation pulses and thus begins the evolution period with zero net magnetization. The decrease in magnetization observed for the state at - 104 ppm is not exponential, so we will characterize it here by the time for half of the observed decrease to occur, which is z 1 ms. Exchange continues to occur to 30 ms, at which time the effects of spin-lattice relaxation will contribute and the spin system is no longer adiabatic. Therefore, both the characteristic exchange time and the total amount of the decrease may be underestimated. Since the peak at - 104 ppm decreases by 50-6OW at 295 K or lower temperatures before spin-lattice relaxation becomes a factor, the population of unobserved hydrogen must be at least l-l .5 times as large as the detected state. The recondite hydrogen is characterized by two features: it has a T2< 20 its and it exchanges with the detectable hydrogen adsorbed on the metal. Furthermore, the exchange is predominantly spin diffusion rather than chemical exchange, because the rate of exchange is temperature independent. The time constant of the exchange is z 1 ms, which indicates that the exchanging state is at equilibrium with the hydrogen on the rhodium particles in any chemisorption measurement. This additional state therefore contains some of the hydrogen previously unobserved by NMR but known from chemisorption measurements to be on or near the rhodium particles. Since the integrated intensity of the peak at - 104 ppm corresponds to z 30% of the adsorbed hydrogen, these two states then account for at least 60-759/o of the hydrogen adsorbed on the metal. The short T2 indicates that the recondite species is strongly coupled to a zero-quantum relaxation source such as other protons or a motion with a large lowfrequency spectral density. Immobile hydrogen would have a T, less than 10 ps owing to the internuclear
29 May 1987
dipolar interaction. This undetected species may represent a strongly bound hydride-like form inside the rhodium particle, although rhodium forms no bulk hydride [ 191. Alternatively, the short T2 may be related to the particular electronic structure of the rhodium particles. Further data are necessary to understand fully this newly detected adsorbed hydrogen species.
6. Summary Exchange between the adsorbed states of hydrogen on silica-supported rhodium is investigated with a NMR spin labeling technique sensitive to processes occurring from x0.1 ms to z 1 s. The data indicate that hydrogen on rhodium particles interacts with another state of adsorbed hydrogen approximately as large, with a time constant of 1 ms at 295 K. This second state of hydrogen has a very short T2(< 20 us) and is adsorbed on or near the metal particles. The NMR peak corresponding to this second state of hydrogen is predicted to be very broad and weak, and therefore difficult to observe directly. Estimates indicate that this peak may account for much or all of the discrepancy between the hydrogen indicated by volumetric uptake and by the intensity of the uptield peak.
References [l]T.-C. ShengandLD.Gay, J.Catal. 77 (1982) 53. [2] T.M. Apple, P. Gajardo and C. Dybowski, J. Catal. 68 (1981) 103. [ 31 T.M. Apple and C. Dybowski, Surface Sci. 121 (1982) 243. [4] S.J. DeCanio, J.B. Miller, J.B. Michel and C. Dybowski, J. Phys. Chem. 87 (1983) 4619. [ 51J.B. Miller, S.J. DeCanio, J.B. Michel and C. Dybowski, J. Phys. Chem. 89 (1985) 2592. [6] J.C. Conesa, P. Malet, G. Munuera, J. Sanz and J. Soria, J. Phys. Chem. 88 (1984) 2986. [7] J. Sanz and J.M. Rojo, J. Phys. Chem. 89 (1985) 4974. [ 81 J. Sanz, J.M. Rojo, P. Malet, G. Munuera, M.T. Blasco, J.C. Conesa and J. Soria, J. Phys. Chem. 89 (1985) 5427. [ 91 N. Bloembergen, E.M. Purcell and R.V. Pound, Phys. Rev. 73 (1948) 679. [lo] S. Forsen and R.A. Hoffman, J. Chem. Phys. 39 (1963) 2892. [ 111 G.A. Morris and R. Freeman, J. Magn. Reson. 29 (1978) 433.
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[ 121 J. Jeener, B.H. Meier, P. Bachman and R.R. Ernst, J. Chem. Phys. 71 (1979) 4546. [ 131 T.W. Root and T.M. Duncan, J. Catal. 102 (1986) 109. [ 141 T.W. RootandT.M.Duncan,Bull.Am.Phys. Sot. 31(1986) 534. [ 15 ] M. Goldman and L. Shen, Phys. Rev. 144 ( 1966) 32 1. [ 161 A. Wokaun and R.R. Ernst, Chem. Phys. Letters 52 (1977) 407.
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[ 17 ] T.M. Duncan, P. Winslow and A.T. Bell, J. Catal. 93 (1985) 1.
[ 181 T.W. Root and T.M. Duncan, in preparation. [ 191 N.A. Galaktionowa, Hydrogen-metal systems databook (Ordentlich, Holon, 1981) p. 172.