J. Phys. Chem. B 1997, 101, 9995-10003
9995
In Situ 1H NMR Investigation of Hydrogen Adsorption on a Cu/MgO Catalyst M. A. Chesters, K. J. Packer,* H. E. Viner, and M. A. P. Wright† Department of Chemistry, UniVersity of Nottingham, Nottingham NG7 2RD, U.K. ReceiVed: May 8, 1997; In Final Form: July 4, 1997X
Hydrogen adsorption on a Cu/MgO catalyst is investigated using in situ 1H NMR. The adsorption is largely homogeneous, demonstrating reasonable agreement with the Langmuir isotherm and only small variations in the frequency shift and spin-lattice relaxation time with increasing coverage. The catalyst is investigated under flowing gas conditions, and the hydrogen/deuterium exchange kinetics are studied. The activation energy for exchange is measured to be 96 kJ mol-1. The 1H variable-temperature NMR probe developed here and used for the in situ adsorption isotherm and the flowing gas experiments is also described.
1. Introduction This paper describes a study of dissociative hydrogen chemisorption on a Cu/MgO catalyst using 1H NMR. In order to reduce experiment times and improve signal-to-noise ratios, which can be a limitation in studies of such catalysts,1,2 we have used a probe which allows for large samples (3-4 g of catalyst). This probe has recently been used in a study of hydrogen chemisorption on the EuroPt-1 Pt/SiO2 catalyst,3,4 but since its design has not yet been presented, it is described as part of this paper. The in situ catalyst sample can be connected to either a conventional vacuum apparatus or a flowing gas system. This allows the development of the NMR spectrum to be followed throughout the adsorption isotherm (10-3-103 Torr) or under conditions of flowing reactant gas of a controlled composition. The flowing gas aspect is an important step toward conditions representative of an actual catalytic reaction, as recognized by others,5,6 although elevated pressures are limited by the glass construction of the sample holders. The interaction of hydrogen with copper is of interest because of the activity of copper in the methanol synthesis reaction, and this has stimulated a number of recent NMR studies.7-9 Early work on hydrogen adsorption on copper black detected a single resonance peak with a coverage-independent frequency shift arising from the adsorbed hydrogen.10 However, more complex spectra have been presented in recent studies of hydrogen adsorption on Cu/Al2O3, indicating a more complicated adsorption process.9 In this study of hydrogen adsorption on a Cu/MgO catalyst, we show that the NMR spectrum of adsorbed hydrogen is influenced by the interaction with the metal surface and that the frequency shift of the resonance peak is modified slightly by the nature of the adsorption site. We apply the flowing gas method in order to compare the spectra obtained under flowing gas and under vacuum, and we study the hydrogen-isotopeexchange reaction on Cu/MgO with quantitative NMR, leading to an analysis of the hydrogen desorption kinetics. We have found that the interpretation of the NMR spectra is aided by a knowledge of the surface species expected, and this can be obtained from a correlation with single-crystal experiments, for example.1,3 Our approach is that an understanding of the in situ adsorption isotherm in terms of the surface species populated is a prerequisite for other studies. Hence, our 1H † Present address: Transport Research Laboratory, Old Wokingham Rd, Crowthorne, Berkshire RG11 6AU, U.K. X Abstract published in AdVance ACS Abstracts, October 15, 1997.
S1089-5647(97)01548-4 CCC: $14.00
NMR results are correlated with the volumetric adsorption isotherm throughout this work. 2. Experimental Section 2.1. Catalyst Preparation. Batches of 10% (by weight) Cu/MgO catalyst were prepared by stirring the support material (Timstar, calcined in air at 973 K for 24 h) in solution with copper nitrate. An equimolar quantity of 1 M KOH was added, dropwise with continuous stirring, via a syringe held below the surface of the liquid. The resulting suspension was filtered and washed in hot water (≈353 K) several times, producing a blue gel which was dried overnight at 378 K, crushed, and then calcined in air at 973 K for 24 h. Reduction was carried out in a flowing gas stream (150 cm3 min-1) as follows. The sample was heated to 473 K in flowing helium and then reduced in hydrogen for 4 h at this temperature, by which stage it was red in color. The relatively low reduction temperature was necessary to avoid sintering, which occurs readily for copper,10 but should ensure complete reduction of the metal for a MgO-supported sample.11 Finally, the gas was switched to helium and a temperature of 423 K maintained for 15 h, before cooling in the gas stream. We have observed that a drastic reduction in chemisorption capacity follows treatment in hydrogen above 523 K, although the sample in its oxidized form after preparation survives calcination in air at 973 K. BET analysis of nitrogen adsorption on the reduced catalyst at 77 K gives a total surface area of 35 m2 g-1. The metal surface area was estimated from the amount of N2O dissociated after 12 h of exposure at 363 K. Under these conditions, the maximum takeup of oxygen atoms occurs without bulk oxidation of the copper.12-14 Assuming the adsorption stoichiometry to be one surface oxygen atom per two copper surface atoms gives a dispersion of 6% and a mean particle size of 20 nm, assuming cubic particles, and a metal surface area of 3.3 m2 g-1 catalyst, taking the density of copper atoms as 1.7 × 1019 m-2.13 Approximately 3-4 g of sample was transferred to a glass NMR sample holder and was subject to the following rereduction treatment at 473 K prior to adsorption experiments: 1 h of evacuation, 1 h of reduction in 200 Torr of hydrogen with several gas changes, and 1 h of evacuation (≈10-5 Torr) to desorb the hydrogen. A standard glass high-vacuum system (described elsewhere1) was employed for this reduction procedure and for the volumetric adsorption studies. A flowing gas NMR sample holder is shown in place in the NMR probe in Figure 1. The sample cavity is 22-mm o.d. by 30-mm length (volume ≈ 7 cm3), with the sample held in place © 1997 American Chemical Society
9996 J. Phys. Chem. B, Vol. 101, No. 48, 1997
Figure 1. Variable-temperature 100-MHz 1H NMR probe developed for in situ studies of supported catalysts under high vacuum or flowing gas conditions.
at each end by a thin layer of glass wool and a porous glass frit. Gas flows in through the center of the sample and back via the catalyst bed. Each leg of the sample holder is sealed below the base of the probe by a Swagelok Quick Connect and a Youngs tap, both of which are sealed when the rereduced sample is transferred from the vacuum system to the NMR probe. The Youngs tap is required, as the Quick Connect incorporates a small volume of air when the connection to the flow system is made. This is removed by several pump/purge cycles before allowing gas to flow over the sample. For adsorption isotherm measurements, a simpler version of the sample holder is used, with a single leg terminated in a balland-socket joint for in situ connection to the vacuum system. 2.2. NMR. All 1H NMR experiments were carried out at 100 MHz using a Bruker MSL 100 spectrometer in conjunction with a wide-bore (120-mm air bore) 2.4-T Oxford Instruments superconducting magnet. The NMR probe (Figure 1) is based on a Bruker 70-mm probe shell, with the diameter increased to 120 mm at the top to accommodate the large sample. Construction is of aluminum and PTFE where possible to minimize background 1H signals. A small signal is detected from the glass sample holder, but the glass Dewars do not give rise to any signal at the sensitivity used here.
Chesters et al. The coil is a solid copper wire (16 SWG) solenoid of length 40 mm and diameter 24 mm, which completely encloses the glass sample holder. The high voltage tuning capacitor built into the existing shell has a range of 3-30 pF. Therefore, for series resonance at 100 MHz, an inductance in the range 84844 nH is required; hence, the coil is wound with 5 turns (≈350 nH), allowing the tuning capacitor to be used in its midrange.15 Matching (50 Ω) is provided by two fixed ceramic capacitors (American Technical Ceramics, 10 pF) and an adjustable vacuum capacitor (2-10 pF, Voltronics) wired in parallel. This network results in a measured quality factor (Q) of approximately 500 (ω/∆ωFWHH) for the circuit resonance peak at 100 MHz. The probe achieves a 90° pulse length of 3 µs using 300 W of rf power, with a ringdown time of 8 µs. By doping a D2O sample with water, it was established that the probe is able to detect less than 1019 protons within the 7-cm3 sample volume and gives line widths of 150 Hz for the 1H resonance of H2O. For Cu/MgO samples, a recycle time of 100 ms was sufficient for quantitative work, due to the fast spin-lattice relaxation observed for all signals (section 3.2). Typically 4096 scans were acquired, giving a signal-to-noise ratio of 25 for the adsorbed hydrogen signal at 2 µmol of H2 g-1 (section 3.2), with a total acquisition time of ≈7 min. Variable-temperature operation (150-400 K) is achieved by passing gas of the appropriate temperature over the sample. Temperature control is by a Bruker BVT-1000 controller, via a thermocouple near the sample and a heater below the base of the probe. Tests showed that the temperature recorded at the center of a flow sample agreed to within 2 K of that measured by the temperature controller. Excessive heating or cooling of the probe body and magnet bore is avoided by purging with dry air. 2.3. Flowing Gas System. The flowing gas system is constructed from 0.25-in. stainless steel tubing, except where 0.125-in. tubing, being flexible, is used to allow connection to the sample holder. Hydrogen, nitrogen, and deuterium gases (BOC chemical physics grade) were passed through Messer Griesheim Oxysorb cartridges in order to remove residual water (10 Torr), peak c is observed, arising from gas-phase hydrogen, with an area directly proportional to the hydrogen partial pressure. The integrated area of peak b correlates linearly with the coverage predicted by the Langmuir volumetric isotherm at each hydrogen partial pressure. By calibrating the integrated area, using this predicted coverage, we obtain the flowing NMR isotherm (Figure 9). The correlation between the NMR and volumetric isotherms in Figure 9, and between the NMR spectra obtained under flowing and vacuum conditions, shows that the hydrogen adsorption characteristics of the catalyst are unchanged under flowing gas conditions. Equilibrium is established between the adsorbed and gas phases on a time scale comparable with that of the volumetric adsorption isotherm. Figure 9 also contains three points that represent the integrated area of the Cu-H resonance peak following a switch to a gas composition with a lower hydrogen partial pressure. The points fall close to the adsorption isotherm, confirming the reversibility of the adsorption process. The flowing technique therefore enables the selection of any hydrogen coverage by controlling the partial pressure, with no hysteresis apparent within the experimental error. 4.2. Hydrogen/Deuterium-Exchange Reaction. The hydrogen-isotope-exchange reaction was monitored by recording NMR spectra after switching the gas flow from a partial pressure of 100 Torr of hydrogen to 100 Torr of deuterium (section 2.3). Upon switching to deuterium, the gas-phase peak, c, is greatly reduced in amplitude almost immediately, as shown in the background-subtracted spectra of Figure 10. Peak b reduces
H Adsorption on a Cu/MgO Catalyst
J. Phys. Chem. B, Vol. 101, No. 48, 1997 10001
Figure 11. Rate of loss of adsorbed hydrogen peak after switching flowing gas stream from 100 Torr of hydrogen to 100 Torr of deuterium (see text). Temperatures: (1) 320, (×) 305, (2) 295, (*) 285, (b) 280, ([) 270, and (9) 260 K.
Figure 10. Background-subtracted 1H NMR spectra recorded (a) under 100 Torr flowing hydrogen and (b-h) at the elapsed time indicated after switching to 100 Torr of deuterium: (b) 87, (c) 670, (d) 1090, (e) 1520, (f) 2620, (g) 3240, and (h) 4010 s.
in intensity over a longer time scale as adsorbed hydrogen atoms are replaced by deuterium, which is not detected at this NMR frequency. Background subtraction shows that the peak arising from the support protons, a, is unaffected by the exchange reaction. The invariance of peak a shows that exchange does not occur on the surface of the MgO, although this has been measured with an activation of 10 kJ mol-1 in other work.36 The lack of activity of the MgO support here may arise from the relatively low evacuation temperature used prior to the exchange studies (523 K) or the calcination prior to the precipitation step in the catalyst production process. Boudart et al. have reported that the activity of MgO for exchange falls “dramatically” and irreversibly after treatment in air at high temperatures.36 However, the NMR spectrum shows unambiguously that, for this Cu/MgO catalyst, the copper surface plays the major, and probably only, role in the exchange process. 4.3. Analysis of the Exchange Rate. The decay in the integrated area of peak b measures the decrease in hydrogen coverage, θH, and hence the loss of hydrogen from the copper surface. We assume that the exchange proceeds via the associative desorption of atoms on the copper surface, in a Langmuir-Hinshelwood mechanism. Also, we assume that after switching the gas stream to deuterium, the adsorption of hydrogen onto the surface of the catalyst is insignificant compared with the adsorption of deuterium. Hydrogen will desorb as both H2 and HD from the metal surface. This process can be described by a single rate equation if we assume that activation energies for desorption of H2 and HD are not significantly different:
-
dθH ) 2kθH2 + kθHθD dt
(3)
Figure 12. Arrhenius plot: (9) rate constants, k, derived from Figure 11, (‚‚‚) fit to low-temperature data, (*) rate of reaction due to higher activation energy process, ln[k - A exp(-E1/(RT))] (by rearranging eq 5 and taking logs), (-‚-) fit to *, and (s) overall rate of reaction calculated via eq 5 using the parameters obtained from the two fits.
where the first part of the sum describes the second-order desorption of H2 molecules and the second part describes the desorption of hydrogen as HD. θD represents the deuterium coverage (which is zero at t ) 0). Since the total coverage remains constant throughout the experiment, θT ) θH + θD, eq 3 can be solved to give
-
(
)
( )
θH 1 0.5 ln ) kt - ln θT θH + θT θT
(4)
Figure 11 demonstrates that eq 4 gives an excellent description of the measured decrease in θH with time, recorded at several temperatures. θT was taken from the 100 Torr adsorption isobar and varied between 20 and 25 µmol of H2 g-1 over the temperature range studied. The rate constants, k, obtained from the gradients in Figure 11 vary between 1.6((0.1) × 10-6 µmol-1 g s-1 at 260 K and 4.4((0.5) × 10-4 µmol-1 g s-1 at 320 K. The Arrhenius plot of the rate constants (Figure 12) shows that a single activation energy is not sufficient to describe the temperature variation of the isotope-exchange rate. However, considering two competing processes provides a reasonable description of the data. If the rate constant is given by
10002 J. Phys. Chem. B, Vol. 101, No. 48, 1997
( )
k ) A exp
( )
-E1 -E2 + B exp RT RT
Chesters et al.
(5)
then neglecting the higher activation energy process at low temperatures allows us to obtain E1 ) 40.8 kJ mol-1 and A ) 240 µmol-1 g s-1 from the gradient and intercept of the data below 280 K. With these parameters known, by rearranging eq 5 and taking the logarithms, the values E2 ) 96.9 kJ mol-1 and B ) 2.9 × 1012 µmol-1 g s-1 are obtained. These fits are shown in Figure 12 together with the function ln k evaluated via eq 5 using the values of the parameters E1, E2, A, and B. The value of E2 is consistent with a previous measurement of the activation energy of exchange on copper foil (96 kJ mol-1).35 It is also within the range of desorption energies reported from Cu/SiO2 and copper films (84-103 kJ mol-1),16,21,40 which is as expected considering that the exchange process requires hydrogen to be desorbed from the surface in order to be replaced by deuterium and that the desorption step is likely to have the highest activation energy. Due to the limited temperature range studied, the uncertainty in the preexponential factor (the intercept in Figure 12) is high. However, the value of B is within the normal range expected for surface reactions.21,26 As yet, we do not have a detailed model to explain the second desorption route. In section 3.1, the deviation of adsorption from Langmuir behavior at low coverage was attributed to a minority of strongly bound sites, and it is tempting to assign the low-temperature exchange to these sites. Although having a high heat of adsorption, these sites could conceivably require a lower energy for desorption than the majority if the activation barrier to adsorption was much lower (Ed ) Ea + qst). The observation of two activation energies is believed to be genuine, as the change in gradient in Figure 12 is reproducible and the measurement of exchange via the reduction in the signal intensity of adsorbed hydrogen is uniquely free from complications, for example by alternative reactions on the support or the sample holder. One remaining possibility is that the adsorbed hydrogen is removed by a side reaction with trace oxygen impurities to form water. From experience, we know that contamination by water invariably gives rise to an NMR peak close to 0 ppm. No such signal is observed to grow in the background-subtracted spectra (Figure 10), so if formed, water must be desorbed at a comparable rate. This would not be surprising given the low partial pressure of water in the gas stream. However, the level of oxygen impurity would have to be at least 100 times worse than the 0.1 ppm quoted for the oxysorb cartridges used, in order to be capable of removing the adsorbed hydrogen quickly enough, given the flow rate used in these experiments. We feel this is unlikely but are not currently able to exclude it as a possibility, which underlines the importance of maintaining scrupulous control of the catalyst environment in these studies. 5. Conclusions The NMR probe described here has improved our ability to study adsorption on catalyst samples in terms of the signal-tonoise ratios and acquisitions times achieved. In addition, it has enabled us to extend our studies from the in situ adsorption isotherm, which we continue to believe is a prerequisite for more complex experiments, to studying catalyst sample under flowing gas conditions. The benefit of this in situ approach to flowing gas experiments was illustrated in section 4.2, where it was shown unambiguously that in this work it is the metal surface and not the support which is active for deuterium exchange on Cu/MgO.
In the NMR spectrum, the MgO support gives rise to a broad (32-kHz FWHM) Gaussian line. The integrated area implies an 82% coverage of the support surface in protons in spite of the high-temperature calcination of the support. Admitting hydrogen to the sample results in a Knight shifted peak (≈+85 ppm) arising from chemisorbed hydrogen and a hydrogen gas peak (+3.5 ppm). The frequency shift and T1 for the adsorbed species are nearly constant with increasing coverage, consistent with the good agreement with a Langmuir isotherm at 295 K and supporting the view that a single type of adsorption site is being populated. However, the isosteric heat of adsorption is low (28 kJ mol-1) and falls at high coverage. The adsorption isotherm is reproduced under flowing gas conditions by varying the partial pressure of hydrogen and monitoring the intensity of the adsorbed hydrogen NMR peak. Adsorption is reversible and equilibrium is established with the flowing gas stream over a similar time scale to the volumetric experiment. The rate of the hydrogen/deuterium-exchange reaction is investigated at temperatures between 260 and 320 K. The loss of hydrogen atoms from the copper surface at constant pressure of deuterium is described by a combined second-order desorption of H2 and HD. An activation energy of 96.9 kJ mol-1 is measured for the isotope-exchange reaction above 280 K, with another process, having an activation energy of 40.1 kJ mol-1, significantly below this temperature. Acknowledgment. The flowing gas system was originally designed by Dr. David Lennon (University of Glasgow), who we would also like to thank for his constructive and enthusiastic comments. The expertise of Mr. N. Barnes, Mr. M. Beasley, Mr. A. Buckland, and Mr. R. Cartwright was essential in the construction of the probe and flowing gas system. Financial support of the EPSRC (H.E.V.) and Nottingham University (M.A.P.W.) is also acknowledged. References and Notes (1) Chesters, M. A.; Packer, K. J.; Lennon, D.; Viner, H. E. J. Chem. Soc., Faraday Trans. 1995, 91, 2191. (2) Sheng, T. C.; Gay, I. D. J. Catal. 1981, 71, 119. (3) Chesters, M. A.; Packer, K. J.; Viner, H. E.; Wright, M. A. P.; Lennon, D. J. Chem. Soc., Faraday Trans. 1996, 92, 4709. (4) Wright, M. A. P. Ph.D. Thesis, University of Nottingham, 1996. (5) Hunger, M.; Horvath, T. J. Chem. Soc., Chem. Commun. 1995, 1423. (6) Goguen, P.; Haw, J. F. J. Catal. 1996, 161, 870. (7) Dennison, P. R.; Packer, K. J.; Spencer, M. S. J. Chem. Soc., Faraday Trans. 1 1989, 85, 3537. (8) Bendada, A.; Chinchen, G.; Clayden, N.; Heaton, B. T.; Iggo, J. A.; Smith, C. S. Catal. Today 1991, 9, 129. (9) Cobb, J. B. C.; Bennett, A.; Chinchen, G. C.; Davies, L.; Heaton, B. T.; Iggo, J. A. J. Catal. 1996, 164, 268. (10) Ito, T; Kadowaki, T. Jpn. J. Appl. Phys. 1975, 14, 1673. (11) Pritchard, J.; Catterick, T.; Gupta, R. K. Surf. Sci. 1975, 53, 1. (12) Soerensen, K. J.; Cant, N. W. Catal. Lett. 1995, 33, 117. (13) Scholten, J. J. F.; Konvalinka, J. A. Trans. Faraday Soc. 1965, 65, 2465. (14) Osinga, Th. J.; Linsen, B. C.; Van Beek, W. P. J. Catal. 1967, 7, 277. (15) Serway, R. A. Physics for Scientists and Engineers, 2nd ed.; Saunders College: 1986; pp 757-759. (16) Sandoval, M. J.; Bell, A. T. J. Catal. 1993, 144, 227. (17) Dus, R. Prog. Surf. Sci. 1993, 42, 231. (18) Anger, G.; Winkler, A.; Rendulic, K. D. Surf. Sci. 1989, 220, 1. (19) Alexander, C. S.; Pritchard, J. J. Chem. Soc., Faraday Trans. 1 1972, 68, 202. (20) Greuter, F.; Plummer, E. W. Solid State Commun. 1983, 48, 37. (21) Roberts, D. L.; Griffin, G. L. J. Catal. 1988, 110, 117. (22) Michelson, H. A.; Rettner, C. T.; Auerbach, D. A. Surface Reactions; Maddix, R. J., Ed.; Springer Series in Surface Sciences No. 34; Springer Verlag: New York, 1994; pp 184-237. (23) Cadenhead, D. A.; Wagner, N. J. J. Phys. Chem. 1968, 72, 2775. (24) Shield, L. S.; Russel, W. W. J. Phys. Chem. 1960, 64, 1592.
H Adsorption on a Cu/MgO Catalyst (25) Beebe, R. A.; Low, G. W.; Wildner, E. L.; GoldWasser, S. J. Am. Chem. Soc. 1935, 57, 2527. (26) Pavlenko, N. V.; Tripol’skii, A. I.; Goldets, G. I. Kinet. Catal. 1987, 28, 384. (27) Campbell, J. M.; Campbell, C. T. Surf. Sci. 1991, 259, 1. (28) Abragam, A. Principles of Nuclear Magnetism; Clarendon: Oxford, 1986. (29) Schreiber, L. B.; Vaughan, R. W. J. Catal. 1975, 40, 226. (30) Peri, J. B. J. Phys. Chem. 1966, 70, 2937. (31) Hall, W. K.; Leftin, H. P.; Cheselske, F. J.; O’Reilly, D. E. J. Catal. 1963, 2, 506. (32) Wu, X.; Gerstein, B. C.; King, T. S. J. Catal. 1990, 121, 271. (33) Slichter, C. P. Principles of Magnetic Resonance; 3rd ed.; Springer-Verlag: New York, 1990.
J. Phys. Chem. B, Vol. 101, No. 48, 1997 10003 (34) Rudaz, S. L.; Ansermet, J. P.; Wang, P. K.; Slichter, C. P.; Sinfelt, J. H. Phys. ReV. Lett. 1985, 54, 71. (35) Mikovsky, R. J.; Boudart, M.; Taylor, H. S. J. Am. Chem. Soc. 1954, 76, 3814. (36) Boudart, M.; Delbouille, M. A.; Derouane, E. G.; Indovina, V.; Walters, A. B. J. Am. Chem. Soc. 1972, 94, 6622. (37) Yakerson, V. I.; Subbotin, A. N.; Gudkov, B. S.; Tkachenko, O. P.; Sarmurzina, R. G. Kinet. Catal. 1994, 35, 732. (38) Dmitriev, R. V.; Subbotin, A. N.; Gudkov, B. S.; Yakerson, V. I.; Kiperman, S. L.; Minachev, Kh. M. Bull. Acad. Chem. Sci. USSR DiV. Chem. Sci. 1988, 37, 2430. (39) Bravetti, D.; Cimino, A.; Indovina, V.; Inversi, M.; Jacono, M. L.; Moretti, G. Gazz. Chim. Ital. 1991, 121, 225. (40) Pritchard, J. Trans. Faraday Soc. 1963, 59, 437.
